Title Electrochemical determination of silver sols for

sensor developments

Name Yuanyangs Wang

This is a digitised version of a dissertation submitted to the University of Bedfordshire.

It is available to view only.

This item is subject to copyright.

Electrochemical determination of silver sols

for sensor developments

by

Yuanyang Wang

A thesis submitted to the University of Bedfordshire in accordance with

requirements for the degree of Master of Science by Research

LIRANS Institute of Research in the Applied Natural Sciences

University of Bedfordshire

250 Butterfield

Great Marlings

Luton, Bedfordshire

LU2 8DL UK

June, 2009

i

Electrochemical determination of silver sols

for sensor developments

by

Yuanyang Wang

A thesis submitted to the University of Bedfordshire in accordance with

requirements for the degree of Master of Science by Research

LIRANS Institute of Research in the Applied Natural Sciences

University of Bedfordshire

250 Butterfield

Great Marlings

Luton, Bedfordshire

LU2 8DL UK

June, 2009

ii

ABSTRACT

Characterisation of an electrochemical method for measuring silver sols

(aqueous suspensions of nanometer-sized particles) is described. Such

particles are receiving much attention by others as valuable components in

the development of biosensing systems. The work was centred on the use of

screen-printed three-electrode devices to measure the concentration of silver

sols by a sequence of processes: (i) dissolution of the silver particles to form

silver ions; (ii) accumulation of silver on the working electrode; and (iii)

stripping of the accumulated silver. The silver sol concentration was related to

the observed stripping peak (peak height or peak area).

Carbon electrodes were used throughout and the influence of carbon type,

electrode format/arrangement and dissolution potential were examined. A

number of interesting observations were made and conclusions arrived at: (a)

the arrangement of working, reference and counter electrodes was important

and a preferred arrangement was indicated; (b) electrode material and/or

format were important – but further work would be necessary to identify

whether one or both factors were particularly important; (c) the choice of

dissolution potential was crucial – and further work needs to be carried out to

ensure that a sufficiently stable reference electrode can be arrived at; and (d)

preliminary evidence is presented that indicates that silver contamination of

the screen-printed silver electrodes was a limiting factor that needed to be

corrected/mitigated in order to arrive at robust/reproducible measurement

devices.

iii

CONTENTS

TITLE PAGE i

ABSTRACT ii

CONTENTS iii

LIST OF TABLES v

LIST OF FIGURES vi

ACKNOWLEDGEMENT viii

Chapter 1: INTRODUCTION 1

1.1 Electrochemical measurement of silver ion 1

1.2 Cyclic voltammetry 3

1.3 The nature and properties of nanoparticulates 5

1.4 Other electroanalytical applications of silver nanoparticles 8

Chapter 2: MATERIALS AND METHODS 12

2.1 Materials and equipment 12

2.1.1 Chemicals 12

2.1.2 Electrochemical Workstation 12

2.1.2.1 Cyclic voltammetry 13

2.1.2.2 Steps and sweep voltammetry 14

2.1.3 Microscopy 15

2.2 Electrochemical Sensors 16

2.2.1 Calculation method of electrode areas 17

2.3.1.1 Calculation of LIRANS-type sensor working electrode area 17

2.3.1.2 Calculation of NPL-type sensor working electrode areas 19

2.2.2 Sensor measurements 21

2.3 Data presentation 22

iv

Chapter 3: RESULTS 24

3.1 Geometric areas of electrodes 24

3.2 Cyclic voltammetry 29

3.2.1 Cyclic voltammetry of silver ions 29

3.2.2 Cyclic voltammetry of silver sol 34

3.3 Steps and sweep voltammetry 38

3.3.1 Steps and sweep measurements of silver sol 38

3.3.1.1 Comparison of drop from bulk and bulk methods 40

3.3.1.2 Drop from bulk and drop measurements (high concentration) 44

3.3.2 Effect of step 1 (dissolution) potential on silver sol measurements 51

3.3.3 Comparison of NPL and LIRANS–type sensors as to the effect of

dissolution (step 1) potential 54

3.3.4 Comparison of silver sol and supernatant as to effect of dissolution (step

1) potential 60

3.3.5 Comparison of silver sol and silver nitrate as to effect of dissolution (step

1) potential 65

3.3.6 Comparison of alternative electrode arrangement as to the effect of the

dissolution (step 1) potential 71

Chapter 4: SUMMARY AND CONCLUSIONS 77

4.1 Electrochemistry of silver ions on carbon electrodes 77

4.2 Measurement of silver sols 78

4.2.1 Steps and sweep measurement of silver sols 79

4.2.2 Effect of step 1 (dissolution) potential on measurement of silver sols 79

REFERENCES 83

APPENDIX 90

v

LIST OF TABLES

Chapter 3: RESULTS 24

3.1 Working electrode dimensions of LIRANS-type sensors estimated from pixel co-

ordinates and reference to ruler images 25

3.2 Pixel co-ordinates and calculated electrode areas of NPL-type sensors

calculated using geometry (equations 2.1 – 2.5) and reference to ruler images 28

3.3 Details of parameters for silver nitrate and blank determinations using cyclic

voltammetry 29

3.4 Silver stripping peak parameters determined using cyclic voltammetry 33

3.5 Sequence of drop from bulk and bulk measurements of silver sol 41

3.6 Summary of drop and drop from bulk measurements of silver sol 46

3.7 Measurement sequence to determine the effects of step 1 (dissolution) potential

on silver sol signals 53

3.8 Comparison of dissolution potential and sensor type for the measurement of

silver sols 56

3.9 Estimated switch potentials for NPL and LIRANS-type sensors 59

3.10 Comparison of dissolution potential for steps and sweep measurements of silver

sol and supernatant 61

3.11 Test solutions prepared for comparison of steps and sweep measurements of

silver sol and silver nitrate 67

3.12 Sequence of step 1 potentials used when comparing NPL and alternative

electrode arrangements for using NPL-type sensors 72

3.13 Estimated switch potentials for NPL and alternate arrangement of electrodes on

NPL-type sensors 76

vi

LIST OF FIGURES

Chapter 1: INTRODUCTION 1

1.1 A typical cyclic voltammogram recorded for a reversible single electrode transfer

reaction 4

1.2 Electrochemical measurement of silver sol: dependence of the silver stripping

peak on the silver accumulation time 11

Chapter 2: MATERIALS AND METHODS 12

2.1 Screen captured image of cyclic voltammetry window in GPES 4.9 13

2.2 Screen captured image of sweeps and step voltammetry window in GPES 4.9 14

2.3 VCS-1032 power stereo microscope with Novaflex double-headed optical fibre

illumination system 15

2.4 Scanned images of the sensor types employed in this work 16

2.5 Schematic showing the electrode arrangement on LIRANS-type sensors 18

2.6 Schematic showing the electrode arrangement on NPL-type sensors 20

2.7 Schematic showing, in an exaggerated way, a generalized electrode with the

defining co-ordinates 20

2.8 Schematic showing, typical step change behavior when characterizing

electrochemical signals (y) as a function of the step 1 potential (E) 22

Chapter 3: RESULTS 24

3.1 Photomicrograph images of LIRANS-type sensors 26

3.2 Photomicrograph images of NPL-type sensors 27

3.3 Cyclic voltammetry of silver nitrate on LIRANS and NPL-type sensors 31

3.4 Comparison of silver nitrate voltammetry on LIRANS and NPL-type sensors 32

3.5 Silver sol voltammetry on NPL-type sensor 36

3.6 Silver sol voltammetry on NPL-type sensor 37

vii

3.7 Example of steps and sweep potential sequence and resultant current

responses 39

3.8 Comparison of silver stripping peak heights obtained using bulk and drop from

bulk methods 42

3.9 Comparison of silver stripping peak areas obtained using bulk and drop from

bulk methods 43

3.10 Dependence of steps and sweep voltammetry peak data as a function of the

concentration of silver sol using drop method 47

3.11 Dependence of steps and sweep voltammetry peak height as a function of the

concentration of silver sol 48

3.12 Dependence of steps and sweep voltammetry peak area as a function of the

concentration of silver sol 49

3.13 Combined steps and sweep measurements of silver sol drops 50

3.14 Effect of dissolution (step 1) potential on silver sol measurement 53

3.15 Comparison of NPL and LIRANS-type sensors as to the effect of the dissolution

(step 1) potential 57

3.16 Comparison of NPL and LIRANS-type sensors as to the effect of the dissolution

(step 1) potential 58

3.17 Absorbance curves of silver sol, supernatant, and blank (deionised water) 62

3.18 Comparison of silver sol and supernatant using steps and sweep

measurements 63

3.19 Effect of dissolution (step 1) potential on the steps and sweep measurement of

silver sol 68

3.20 Effect of dissolution (step 1) potential on the steps and sweep measurement of

silver nitrate 69

3.21 Scanned image of an NPL-type sensor 72

3.22 Steps and sweep measurement of silver sol as a function of dissolution (step 1)

potential and electrode arrangement 73-74

viii

ACKNOWLEDEMENT

I wish to express my special thanks to my supervisor, Dr Barry Haggett for his

supervision, valuable comments, constant support and patience. This thesis

could not have been written without his help, enthusiasm, dedication and

encouragement. Much is owed to him.

Many thanks to Robert Porter, Mateusz Szymanski and the National Physical

Laboratory for their invaluable discussions, comments and supply of materials.

I am grateful to Professor Tiantian Zhang, for her interest and support

throughout the whole study.

I very much appreciate all the kindness, help and friendship from Professor

David Rawson, Gowri, Simon, Mo, Yurong, Tiziana and others in the LIRANS

Institute for their words of encouragement.

Finally, I wish to thank my entire family for their prayers and financial support.

Without their love and support, this thesis would not have been possible.

1

CHAPTER 1 INTRODUCTION

Silver sols are suspensions of silver nanoparticles. Such particles are receiving

increasing attention as components in (bio-)sensing devices since they have a range of

attractive properties (e.g. high surface area, efficient electron conduction, bactericidal,

antifungal, antiviral, etc.), e.g. Birch et al. (2005) and Porter and Szymanski (2009).

One of the attractive properties is the ease with which it is possible to measure silver

ions.

1.1 Electrochemical measurement of silver ions

Dilleen et al. (1998) described the electrochemical determination of silver in

photographic solutions using fixed-volume single-use sensors. The electrodes were

printed using carbon and dielectric inks on an alumina substrate. Square-wave

voltammetry was used to measure silver ions in a typical photographic fixer

(ammonium thiocyanate, 3 M; ammonium thiosulphate, 1 M; and sodium sulphite, 0.4

M). They reported the formation of strong complexes in the presence of excess

ammonium thiocyanate:

144 ])SCN()NH(Ag[)SCN()NH(Ag +−−++ ⎯→⎯++ nm

nmnm (1.1)

nmnm

nmnm e −−+− ⎯→⎯+ ])SCN()NH(Ag[])SCN()NH(Ag[ 4

14 (1.2)

−+−− +⎯→⎯ enmnm

nmnm

144 ])SCN()NH(Ag[])SCN()NH(Ag[ (1.3)

(N.B. The above equations are believed to be correct versions of those that appeared in

Dilleen et al. where additional charge signs were included.)

The linear range was 0.6 – 1.3, 0.1 – 0.8 or 0.6 – 6 mg l-1 depending on the

2

measurement arrangement (beaker or thin-layer cell). Silver stripping peaks were

shown at about -550 mV versus the carbon pseudo-reference electrode (except when

copper was present – in which case the peak was shown at ∼ -650 mV).

Other groups have reported electrochemical methods for measurements of silver ions.

Johnson and Allen (1973) illustrated the new technique of stripping voltammetry with

collection using silver accumulation on a glassy carbon electrode and reported the

determination of 0.1 µM Ag+. Hunag et al. (1994) described the pre-concentration

and measurement of silver ions using a carbon paste electrode that incorporated

2,9-dichloro-1,10-phenanthroline. The method was reported to be linear over the

range 8 × 10–10 – 5 × 10–7 M. Won et al. (2003) described the determination of silver

ions using a carbon paste electrode containing N,N'-diphenyl oxamide with anodic

stripping voltammetry. With a deposition time of ten minutes, the logarithmic linear

range was 5.0 nM – 0.1 µM with a detection limit of 0.7 nM. Won et al. (1995)

described the pre-concentration and measurement of silver ions using a carbon paste

electrode that incorporated glyoxal bis(2-hydroxyanil). The detection limit for Ag+

was reported to be 10-10 M with a 40 minute accumulation time. Švancara et al. (1996)

described the pre-concentration and measurement of silver ions using a carbon paste

electrode employing tricresyl phosphate modified with heptylsulfonic acid. With an

accumulation time of 15 minutes, the limit of detection was reported to be 2.5 pM.

Majidi et al. (2007) reported a carbon ceramic electrode modified with 5-(4-dimethyl

amino benzelyden)-rhodanin for the determination of silver ions using differential

pulse anodic stripping voltammetry. With a ten minute accumulation time, a linear

response was obtained in the range 10-13 to 10-7 M with a limit of detection of 4.5

×10-14 M.

3

1.2 Cyclic voltammetry

Cyclic voltammetry or CV is a type of electrochemical measurement where the applied

potential is changed with time. In a cyclic voltammetry experiment the working

electrode potential is ramped linearly versus time but the working electrode's potential

ramp is inverted when the applied potential reaches a limit. This inversion can

happen multiple times during a single experiment. The current at the working electrode

is plotted versus the applied voltage to give the cyclic voltammogram trace. Cyclic

voltammetry is generally used to study the electrochemical properties of an analyte in

solution (Anon 2009a) and in particular, it can be used to investigate and understand

electrochemical reactions (Anon 2009b). A typical cyclic voltammo- gram recorded

for a reversible single electron transfer reaction (equations 1.4 and 1.5) is shown, Fig.

1.1.

+−+ ⎯→⎯+ 23 FeFe e (1.4)

−++ +⎯→⎯ e32 FeFe (1.5)

In this case, the bulk solution contains only a single electrochemical reactant (Fe3+).

As shown the forward sweep begins with an oxidizing potential (+0.2 V), there is no

current flow since there is no ferrous in the solution and the potential is too oxidizing

to convert any ferric ions to ferrous ions. As the potential is made less positive (more

reducing) the rate of the reaction x.1 increases, the concentration of Fe3+ on the surface

of electrode goes down and the concentration of ferrous ions at the electrode surface

goes up. The current is dependent on the rate of reaction x.1 times the concentration

of Fe3+ on the surface, so the current will surge to a peak (cpI ) and then go down until

4

the [Fe3+] on the electrode surface becomes zero, then the current will be stable and the

value of the current is depends mainly on the [Fe3+] in the bulk. When the scan is

reversed, the electrode becomes more oxidizing the electron flow is to the electrode

with the increasingly positive electrode potential from the ferrous ions generated at the

electrode surface. The rate of reaction x.2 increases and [Fe2+] on the electrode

surface reduces - so the current will reach a peak (apI )then goes down to about zero (as

the [Fe2+] in the bulk is near zero). Cyclic voltammetry can be used to investigate

much more complicated reaction schemes then simple example shown (Anon 2009b,

2009c).

Fig. 1.1 A typical cyclic voltammogram recorded for a reversible single electrode transfer reaction +−+ ⎯→⎯+ 23 FeFe e . The initial concentration of ferrous ions is zero and the scan starts at

+0.2 V (Anon 2009c).

5

1.3 The nature and properties of nanoparticulates

Nanoparticles are sized between 1 and 100 nanometers in diameter – which is

approaching molecular dimensions – but even individual macromolecules are usually

not referred to as nanoparticles. Therefore, nanoparticles bridge dimension scales

between molecular structures and bulk materials. An important difference between

bulk material and nanoparticles is that, no matter how big a bulk material is, it has

stable physical properties, but this is not true for nanoparticles. When the size of a

material is of nanoscale the percentage of surface atoms becomes big and the

properties change with size, either or both bulk and surface properties of a

nanomaterial may change with size. In nanomaterials surface properties, such as

energy levels, electronic structure, and reactivity can be quite different from inside

states and that gives different material properties (Anon 2009d, 2009e, Kathleen 2002).

The special properties of nanoparticles and other nanostructured materials have made

them actually or potentially useful in a wide rage of applications. Nanoparticles can

be used to deliver drug, to sustain drug effect in target tissue. Nanoparticles can be

the delivery system for anti-cancer agents to tumors because after intravenous injection,

nanoparticles have the tendency to accumulate in tumors. Nanoparticles can also be

used in brain drug targeting (Rajeev 2007).

Carbon nanotubes are the subject of a very active research area within nanomaterials.

Nanoparticles of carbon – rods, fibers, tubes with single walls or double walls, open or

closed ends, and straight or spiral forms – have been synthesized in the past 10 years.

Carbon nanotubes have been shown to have unique properties, stiffness and strength

6

higher than any other material. The applications of carbon nanotubes are varied, such

as in electrical circuits (Akturk 2007), as paper batteries (Troy 2007), toughness

material (Zhang 2005, Dalton 2003), integrated memory circuit (Tseng 2004),

semiconducting (Ding 2009), solar cells (Anon 2007), electrically conductive films

(Simmons 2007) and loudspeaker (Colin 2008).

Gold nanoparticles are very important sensing materials. They have unique size and

shape dependent optical properties, high extinction coefficients, super quenching

capability and molecular-recognition properties. In consequence, gold nanoparticles

are used in wide areas, such as electronics and nanotechnology (Ramachandra Rao et

al. 2000). Gold nanoparticles bioconjugated with DNA have been demonstrated for

sensitive and selective detection of analytes, such as mercury (II) ions, platelet-derived

growth factor and adenosine triphosphate (Lin et al. 2009).

There is an effort to incorporate silver nanoparticles into a wide range of medical

devices, including but not limited to bone cement (Anon 2009f), surgical instruments

and masks (Anon 2009g) and as the cathode in a silver-oxide battery (Anon 2009h).

Silver nanoparticles have an antimicrobial effect that lasts longer than ionic silver.

Samsung has created and marketed a material called Silver Nano, that includes silver

nanoparticles on the surfaces of household appliances (Anon 2009i).

A colloid is a type of chemical mixture in which a solid substance is dispersed evenly

throughout another liquid substance (Anon 2009j). The particles of the dispersed

substance are only suspended in the mixture, but in a solution, the particles are

completely dissolved. The particle size in colloids is between 1 and 1000 nanometers

7

in diameter, so suspensions of nanoparticles (1-100 nanometer in diameter) form a

subset of colloids. The particles in a colloid are small enough to be dispersed evenly

and maintain a homogeneous appearance, and large enough to scatter light and not

dissolve (Anon 2009k).

1.4Analytical applications of silver nanoparticles

Xu et al. (2004) described a biosensor prepared by co-immobilisation of horseradish

peroxidase, methylene blue and silver nanoparticles in a sol-gel on a glassy carbon

electrode. The silver provided a surface for the enzyme immobilisation and a

conductive glassy carbon electrode. The sensor responded to hydrogen peroxide with

a linear range from 1 nM to 1 mM and a detection limit of 0.4 nM.

Dai et al. (2006) reported the use of gold, silver and palladium nanoparticles supported

separately on glassy carbon microspheres. The prepared microspheres were then

bound to the surface of glassy carbon electrodes using multiwalled carbon nanotubes.

Surfaces could be prepared using microspheres modified with just one metal

nanoparticle – or with combinations of differently modified microspheres. Electrodes

could be prepared in this way with combination of metals that were not effected by

alloying, co-deposition or formation of bimetallic species. Such devices were

reported to have advantages over and above those of metal macrodisc electrodes and

even of other nanoparticle electrodes. Šljukić et al. (2007) used this approach to

produce a device for measurement of bromide with a detection limit of 3 µM.

Zhao et al. (2006) reported the use of colloidal silver nanoparticles co-entrapped with

8

hemoglobin in a sol-gel matrix on the surface of a glassy carbon electrode. Direct fast

electron transfer between hemoglobin and the electrode was observed and the catalytic

ability to reduce nitrite was used as a biosensor to measure nitrite with a linear range of

0.2 – 0.6 µM and a detection limit of 34 nM.

Fanjul-Bolado et al. (2007) described alkaline phosphatase-catalyzed deposition of

silver for electrochemical detection of virulence nucleic acid determinants present on

the genome of the human pathogen Streptococcus pneumoniae. 3-Indoxyl phosphate

was used as substrate for the enzyme and a product was a compound able to reduce

silver ions in solution to a metallic deposit in the vicinity of the enzyme label. Anodic

stripping voltammetry was used to measure the deposited silver. The silver

measurements were reported to be fourteen times more sensitive than the direct

electrochemical detection of indigo carmine.

Frederix et al. (2007) described the use of gold and silver nanoparticles in optical

biosensing based on the observation that their absorption spectra were effected by the

dielectric constant of the surrounding material. Antibodies were attached to silver

nanoparticles that were themselves attached to a quartz surface. Antigen binding was

measured via changes in light absorption that could be very large.

Taheri et al. (2009) described preparation of a gold electrode – sol-gel – silver

nanoparticle device that was used to measure cyanide with a linear range from 1.5 ×

10-6 to 2.1 × 10-4 M and with a limit of detection of 1.4 × 10-8 M.

Porter and Szymanski (2009), Szymanski and Porter (2009) and Porter (2009)

9

described the use of silver nanoparticles (diameter = 40 nm) to develop novel

electrochemical immunoassays. The nanoparticles were linked to antibodies and

concentration of target analytes determined by measurement of the amount of silver

label. Silver was measured by bringing the particles into contact with a

screen-printed carbon electrode subjected to the following regime (Szymanski and

Porter, 2009):

step 1 dissolution of the sol by application of an oxidizing potential (+0.6 V) to

the working electrode;

−+ +⎯→⎯ eAgAg0 (1.6)

144 ])SCN()NH(Ag[)SCN()NH(Ag +−−+−++ ⎯→⎯++ nm

nmnm (1.7)

step 2 nucleation of silver on the working electrode (-1.6 V);

step 3 accumulation of silver on the working electrode (-1.2 V); and

nmnm

nmnm e −−+−+−−+ ⎯→⎯+ ])SCN()NH(Ag[])SCN()NH(Ag[ 4

14 (1.8)

stripping of silver from the working electrode by applying a linear sweep from -1.2

to +0.1 V:

−+−−+−−+ +⎯→⎯ enmnm

nmnm

144 ])SCN()NH(Ag[])SCN()NH(Ag[ (1.9)

(N.B. Equations 2.5 – 2.7 are written as shown – but it is supposed that these were

propagated errors and the equations should have read the same as 2.1 – 2.3). The

effects of ammonium thiocyanate concentration, dissolution potential and

accumulation time on measurements of silver sol (800 µl ml-1) were also reported:

• Stripping peaks were acutely dependant on the thiocyanate concentration –

increasing up to ∼0.8 M before becoming much less dependant – though the

signals continued to increase up to the highest reported concentration (2 M).

• With respect to the dissolution (step 1) potential, at an applied potential of ∼

10

+0.4 V there was a step change in the measured silver stripping signal from

3.5 – 5.5 µC (potentials less than +0.4 V) to ∼ 9.5 µC (potentials greater than

+0.4 V).

• The stripping signal was acutely dependant on the accumulation time (step 3).

The signal increased roughly linearly between 0 and 10 s and then levelled off

so that the silver peak was approximately independent of accumulation time

between 20 and 40 s, Fig. 1.1.

0

4

8

12

0 10 20 30 40

silver accumulation time /s

silv

er s

tripp

ing

char

ge /µ

C

Fig. 1.2 Electrochemical measurement of silver sol: dependence of the silver stripping peak on the silver accumulation time. Adaptation of figure 17 in Porter and Szymanski (2009) using data provided by the authors in their Table 7.

The work of Szymanski and Porter was used as the basis for the work reported in this

thesis. Various aspects of their electrochemical measurement régime (equations 2.4 –

2.9) were studied in detail.

12

CHAPTER 2 MATERIALS AND METHODS

2.1 Materials and equipment

2.1.1 Chemicals

Ammonium thiocyanate stock solution (5 M) was prepared by dissolving ammonium

thiocyanate (76.12 g; Aldrich, 321798) in water (200 ml). Silver sol (EM.SC40, 40

nm; BBI, Cardiff, Appendix) was a gift from the National Physical Laboratory (NPL)

and was used as received. Test sols were prepared in situ unless otherwise indicated.

Silver nitrate (0.1 N; Fluka, 85235) was also used as received. Fresh aqueous

solutions were prepared, shortly before their use, with 18 MΩ water (UHQ, Elga Ltd.,

High Wycombe).

2.1.2 Electrochemical Workstation

An electrochemical workstation (Autolab PGSTAT10 or PGSTAT20, Eco Chemie BV)

running GPES 4.9 software (Eco Chemie BV) was used for all measurements.

Electrodes were connected to the interface via a spring-loaded connector containing

gold-plated contact pins (gift from Kodak Ltd.).

13

2.1.2.1 Cyclic voltammetry

Cyclic voltammograms were recorded using cyclic voltammetry (staircase) normal

mode, Fig. 2.1. All potentials were with respect to the carbon pseudo-reference

electrodes (not the open circuit potential).

Fig. 2.1 Screen captured image of cyclic voltammetry window in GPES 4.9. The ‘Edit procedures’ window shows the first page of parameters.

Page 2 of the Edit procedure window was not used in the work reported here. The ‘Data presentation’ window shows a resultant voltammogram.

14

2.1.2.2 Steps and sweep voltammetry

Steps and sweep measurements were carried out in steps and sweeps mode, Fig. 2.2.

No preconditioning was employed and, unless otherwise indicated, measurement

parameters were as indicated by NPL:

step 1, +0.4 V for 15 s with a 150 ms time interval;

step 2, -1.6 V for 5 s with a 100 ms time interval;

step 3, -1.2 V for 55 s with a 100 ms time interval; and

linear sweep from -1.2 to +0.5 V at scan rate 1 V s-1 with potential step 10 mV.

Fig. 2.2 Screen captured image of sweeps and step voltammetry window in GPES 4.9.

15

2.1.3 Microscopy

Sensor images were obtained using a low power stereo microscope (VCS-10132,

World Precision Instruments) equipped with a digital camera (Pixera) and image

capture software (PXView Finder, version 2.1.1.2, Pixera Corporation). Illumination

was provided by a double-headed optical fibre illumination system (Novaflex, World

Precision Instruments), Fig. 2.3.

Fig. 2.3 VCS-1032 power stereo microscope with Novaflex double-headed optical fibre illumination

system.

16

2.2 Electrochemical Sensors

Electrochemical sensors were three-electrode devices produced by screen-printing

carbon and insulating ink on to a polyester substrate (Melinex ST725 or ST328). Inks

were sourced from Gwent Electronic Materials Ltd., Pontypool. Conductive carbon

was either D14 (C10903D14) or D2 (C2000802D2). The insulating ink was D1

(D60202D1). Screen-printing was carried out using a semi-automatic printer (Dek

1760RS, Weymouth) by LIRANS staff (Rashid Kadara, Roberto Andres or Gowri

Dep). NPL-type sensors were printed for, and supplied by, the National Physical

Laboratory (NPL), Fig. 2.4.

Fig. 2.4 Scanned images of the sensor types employed in this work.

(Left) LIRANS-type sensor. Electrode arrangement (from left to right): working, counter and reference. (Right) NPL-type sensor. Electrode arrangement (from left to right): working, reference and counter (NPL arrangement); and reference, working and counter (alternative electrode arrangement).

17

2.2.1 Calculation method of electrode areas

Electrode areas were calculated using photomicrographs obtained using a low power

stereo microscope (section 2.2.3). Jpeg images were compared to reference images of

a ruler – captured using the same microscope settings. Co-ordinates defining the

electrode areas were identified using Microsoft Paint.

2.2.1.1 Calculation of LIRANS-type sensor working electrode area

For LIRANS-type sensors, the electrode working electrode area ALIRANS,WE was

approximated to the electrode width multiplied by the average height of the two sides:

2

)( 21WELIRANS,

llwA +≈ (2.1)

and

)(2/1 3412 xxxxw −+−= (2.2)

213

2131 )()( yyxxl −+−= (2.3)

224

2242 )()( yyxxl −+−=

(2.4)

where w is the width and l1, l2 the lengths of the shorter and longer electrode sides,

respectively and the co-ordinates (x1, y1) to (x4, y4) are those identified in Fig. 2.5.

18

Fig. 2.5 Schematic showing the electrode arrangement on LIRANS-type sensors. The co-ordinates (x1, y1) to (x4, y4) were co-ordinates identified in Microsoft Paint and used

to calculate the working electrode area.

19

2.2.1.2 Calculation of NPL-type sensor working electrode areas

The dimensions of the electrodes on NPL-type sensors were determined in a similar

manner. In this case, however, it was not assumed that opposite edges were parallel.

The areas of each electrode ANPL, were calculated with reference to Figs. 2.6 and 2.7:

DCBNPL ++=A (2.5)

where

)())((

2/1))(())((2/1B 2728

278217121212 yy

yyyyxx

yyxxyyxx −−

−−−−−+−−=

(2.6)

))((2/1C 1771 yyxx −−= (2.7)

])(

))(())[((2/1D

28

27827278 yy

yyxxxxyy

−−−

−−−= (2.8)

and the co-ordinates (x1, y1), (x2, y2), (x7, y7) and (x8, y8) are particular to the

left-hand electrode in Fig. 2.6 – but the calculation can be extended to the other

electrodes by appropriate substitution of co-ordinates.

20

Fig. 2.6 Schematic showing the electrode arrangement on NPL-type sensors. The co-ordinates (x1, y1) to (x12, y12) were co-ordinates identified in Microsoft Paint and

used to calculate the electrode areas.

Fig. 2.7 Schematic showing, in an exaggerated way, a generalized electrode with the defining

co-ordinates. Solid lines define the edges of the electrode. Dashed lines are parallel/orthogonal to the

co-ordinate system. B, C and D were used to subdivide the electrode into regions – the areas of which could be

calculated using geometry (equations 2.6 – 2.8).

21

2.2.2 Sensor measurements

Measurements using the sensors were made in several different ways:

(a) Bulk measurements. Sensors were dipped into test solution contained in

cut-down semi-micro cuvettes (Fisher, FB55147).

(b) Drop from bulk measurements. Sensors were held horizontally with the

electrodes facing up. Drops (20 µl) taken from bulk test solutions were placed on

to the sensors so as to cover the three electrodes (working, counter and reference).

(c) Drop measurements. Sensors were held horizontally with the electrodes facing

up. Test solutions were mixed in situ, over the electrodes, by pipetting the

component parts on to the sensors and mixing gently with the pipette.

Measurements were generally carried out using a single sensor – but “important”

experiments were carried out at least three independent times so as to obtain an

indication of the uncertainty in the results.

22

2.3 Data presentation

Electrochemical data were imported into Microsoft Excel for presentation and data

analysis. Step 1 (dissolution) measurements exhibited a step-change behavior (Fig.

2.8) visualization of which was assisted by using the Solver function in Microsoft

Excel to fit the measured data to a tanh approximation:

y = b × tanh a × (E - d) + c (2.9)

step 1 potential, E

elec

troch

emic

al s

igna

l, y

b

d

c

Fig. 2.8 Schematic showing, typical step change behavior when characterizing electrochemical

signals (y) as a function of the step 1 potential (E). Parameters a, b, c, and d were used to define the tanh function shown, equation 2.9.

23

where a relates to the slope at the step-change (slope is a×b when E=d)

b determines the magnitude of the signal change

c defines the magnitude of pre-step signal

d is the potential (the ‘switch potential’) at the step-change in signal, y

E is the applied (step 1) potential; and

y is the measured signal.

24

CHAPTER 3 RESULTS

3.1 Geometric areas of electrodes

NPL and LIRANS-type sensors were photographed using a low power stereo

Microscope (VCS-10132, World Precision Instruments). Images were captured as

jpeg files, Figs. 3.1 and 3.2. The jpeg files were examined using Microsoft Paint.

For LIRANS-type sensors, Paint was used to identify the co-ordinates that defined the

working electrodes (Chapter 2, section 2.3.1.1) and the mean working electrode size

was 1.89 mm2 with a standard deviation of 0.12 mm2, Table 3.1.

In the case of the NPL-type sensors, the co-ordinates defining all three electrodes

(working, counter and reference) were identified. The co-ordinates were used to

calculate the electrode areas according to equations 2.5 – 2.8. The data (Table 3.2)

showed that sensor D2-1 had left and right-hand electrodes (8.91 and 12.64 mm2) with

areas outside the range of the other nine devices (9.73 – 10.06 and 11.78 – 11.91 mm2)

whereas the size of the dielectric window on the sensor D2-1 (40.29 mm2) was similar

to those of the other sensors (40.50 – 41.23 mm2). These results indicate a problem

with the design of the NPL-type sensors – the areas of the two side electrodes were

very dependent on the relative position of the dielectric print with respect to the carbon

layer.

In subsequent calculations of current and charge densities, the NPL working electrode

area was taken to be 9.91 ± 0.12 mm2 – although Table 3.2 shows that the ‘real’ mean

and standard deviation values may be significantly different.

25

image x1 y1 x2 y2 x3 y3 x4 y4 area /mm2

ruler1 294 1074ruler2 322 992D14-1 448 187 526 131 459 670 532 712 1.82D14-2 476 303 548 247 476 680 552 742 1.75D14-3 385 265 452 221 389 674 460 720 1.83D14-4 445 149 522 84 445 507 521 570 1.88D14-5 462 163 539 114 468 584 545 630 2.07D14-6 417 284 497 223 417 666 492 722 2.00D14-7 514 212 591 153 513 581 588 643 1.86D14-8 458 208 524 163 467 631 524 665 1.71D14-9 527 306 607 247 532 698 608 751 2.01D14-10 548 293 630 237 549 685 624 734 1.99

mean 1.89sd ± 0.12

20090505 Yuanyang table Table 3.1 Working electrode dimensions of LIRANS-type sensors estimated from pixel co-ordinates

and reference to ruler images. 5 mm Interval on ruler. Images ruler1 and D14-1 taken on 30 October 2008. Other Images taken on 10 November 2008.

26

ruler1 D14-1 D14-2

D14-3 D14-4 D14-5

D14-6 D14-7 D14-8

D14-9 D14-10

Fig. 3.1 Photomicrograph images of LIRANS-type sensors. Photomicrograph of ruler (1 mm intervals) shown for comparison. Text beneath each image identifies the JPEG file name of the source image. Images D14-1.jpg and ruler1.jpg were taken on 30 October 2008. The other images were taken on 10 November 2008.

27

ruler2 D2-1 D2-2

D2-3 D2-4 D2-5

D2-6 D2-7 D2-8

D2-9 D2-10 Fig. 3.2 Photomicrograph images of NPL-type sensors. Photomicrograph of ruler (1 mm intervals) shown for comparison. Text beneath each image identifies the JPEG file name of the source image. Image D2-1.jpg was taken on 30 October 2008. The other images were taken on 10 November 2008.

28

image x1 y1 x2 y2 x3 y3 x4 y4 x5 y5 x6 y6 x7 y7 x8 y8 x9 y9 x10 y10 x11 y11 x12 y12

left middle right window

ruler1 294 1074ruler2 322 992D2-1 18 71 273 72 498 72 663 71 809 73 1173 67 29 937 275 936 504 933 668 932 815 929 1173 927 8.91 5.81 12.64 40.29D2-2 231 92 470 92 660 92 807 92 933 92 1222 91 232 833 469 831 668 832 805 831 930 834 1218 831 9.81 5.85 11.89 40.66D2-3 163 146 406 148 597 152 742 152 864 155 1152 155 158 889 397 890 594 890 733 890 860 892 1146 893 9.99 5.84 11.79 40.99D2-4 217 50 456 51 648 54 793 55 920 56 1209 55 217 792 454 792 649 793 789 793 913 794 1204 794 9.84 5.87 11.91 40.92D2-5 200 102 444 104 638 107 782 110 906 109 1195 111 199 846 438 847 633 847 772 846 903 849 1186 851 10.03 5.84 11.82 41.23D2-6 53 95 292 95 490 96 632 96 759 96 1048 95 60 837 292 837 493 837 631 837 763 834 1047 835 9.73 5.78 11.78 40.79D2-7 233 147 473 149 667 152 810 153 935 153 1226 153 227 887 467 890 662 890 803 892 928 892 1216 892 9.93 5.85 11.91 41.09D2-8 157 51 399 54 593 56 737 58 860 57 1150 59 154 795 395 797 587 797 732 797 857 797 1143 799 10.04 5.97 11.90 41.17D2-9 175 86 418 86 613 87 757 87 879 87 1169 85 178 831 420 831 614 829 756 829 883 828 1168 827 10.06 5.91 11.84 40.85D2-10 180 125 421 124 613 126 757 124 880 124 1172 123 184 865 419 863 615 864 752 862 881 863 1167 862 9.79 5.76 11.88 40.50

mean% 9.91 5.85 11.86 40.91sd% ± 0.12 ± 0.06 ± 0.05 ±0.24

mean* 9.81 5.85 11.94 40.85sd* ±0.34 ±0.06 ±0.25 ±0.30

20090505 Yuanyang table

area /mm2

Table. 3.2 Pixel co-ordinates and calculated electrode areas of NPL-type sensors calculated using geometry (equations 2.1 – 2.5) and reference to ruler images.

5 mm Interval on rulers. Images ruler1 and D2-1 taken on 30 October 2008. Images D2-2 to D2-10 and ruler2 taken on 10 November 2008. % Values calculated by excluding the D2-1 results. * Values calculated by including the D2-1 results.

29

3.2 Cyclic voltammetry

3.2.1 Cyclic voltammetry of silver ions

Cyclic voltammetry of silver nitrate was carried out using both NPL and LIRANS-type

sensors. Silver measurements were carried out by mixing (on the sensor) 4 µl silver

nitrate stock solution, 4 µl ammonium thiocyanate stock solution and 12 µl deionised

water (in that order) so that the working, reference and counter electrodes were

covered. Blank measurements were carried out using 4 µl ammonium thiocyanate

stock solution and 16 µl deionised water. The Autolab was used in cyclic

voltammetry mode with step potential of 2 mV; scan rate of 0.1 V s-1; number of scans,

1; and other parameters as given in Table 3.3. A new sensor was used for each

measurement.

order sensor type test solution start potential /V

first vertex potential /V

second vertex potential /V

1 NPL(new) silver nitrate +0.4 -1.6 +0.4 2 NPL(new) blank +0.4 -1.6 +0.4 3 NPL(new) silver nitrate +0.3 -1.6 +0.3 4 NPL(new) blank +0.3 -1.6 +0.3 5 NPL(original) silver nitrate +0.4 -1.6 +0.4 6 NPL(original) blank +0.4 -1.6 +0.4 7 NPL(original) silver nitrate +0.3 -1.6 +0.3 8 NPL(original) blank +0.3 -1.6 +0.3 9 LIRANS silver nitrate +0.4 -1.6 +0.4 10 LIRANS blank +0.4 -1.6 +0.4 11 LIRANS silver nitrate +0.3 -1.6 +0.3 12 LIRANS blank +0.3 -1.6 +0.3

report 13/11/2008 (P134) Table 3.3 Details of parameters for silver nitrate and blank determinations using cyclic voltammetry.

‘Order’ refers to the order in which the measurements were made. NPL (new): NPL-type sensors (D2 carbon, new batch). NPL (original): NPL-type sensors (D2 carbon, original batch). LIRANS: LIRANS-type sensors (D14 carbon).

30

The measured cyclic voltammograms (Fig. 3.3) were also converted into current

density form (i.e. measured current divided by the area of the working electrode), Fig.

3.4.

The reduction feature (wave or peak) appearing at about -1.2 V even in the absence of

silver may be attributed either to the ammonium thiocyanate or to some surface

confined process on the working electrode. The reduction feature (wave or peak)

appearing at about -1.4 V was not observed in the absence of silver ions and can be

attributed to reduction of silver ions:

)( Ag)(Ag seaq ⎯→⎯+ −+ (3.1)

Similarly, the oxidation peaks observed at about -0.4 V can be attributed to oxidation

of silver metal since they were not observed in the absence of silver ion:

−+ +⎯→⎯ eaqs )(Ag)( Ag (3.2)

Attributes of the silver stripping peaks are tabulated, Table 3.4. The silver stripping

charge density was significantly higher for the LIRANS-type sensor than for either of

the NPL-type devices. To determine whether this was due to an inherent difference

between the D2 and D14 carbon inks or due to the different electrode layouts it would

be necessary to print similar devices using each of the two carbon materials (D2 and

D14).

31

-125

-75

-25

25

75

-1.6 -1.2 -0.8 -0.4 0 0.4

potential /V vs C

curr

ent /

µA

A

report 13/11/2008 (P135)

-150

-100

-50

0

50

100

-1.6 -1.2 -0.8 -0.4 0 0.4

potential /V vs C

curre

nt /µ

A

B

report 13/11/2008 (P135)

-225

-125

-25

75

175

-1.6 -1.2 -0.8 -0.4 0 0.4

potential /V vs C

curr

ent /

µA

C

report 13/11/2008 (P136)

-125

-75

-25

25

75

-1.6 -1.2 -0.8 -0.4 0 0.4

potential /V vs Ccu

rrent

/µA

D

report 13/11/2008 (P136)

-35

-15

5

25

-1.6 -1.2 -0.8 -0.4 0 0.4

potential /V vs C

curr

ent /

µA

E

report 13/11/2008 (P135)

-40

-20

0

20

-1.6 -1.2 -0.8 -0.4 0 0.4

potential /V vs C

curre

nt /µ

A

F

report 13/11/2008 (P135) Fig. 3.3 Cyclic voltammetry of silver nitrate on LIRANS and NPL-type sensors. (A, B) NPL-type sensors, D2 carbon, new batch.

(C, D) NPL-type sensors, D2 carbon, original batch. (E, F) LIRANS-type sensors, D14 carbon.

[Silver nitrate] /mM: () 0; and (, , ) 0.5. Start potential /V: (A, C, E) +0.4; and (B, D, F) +0.3. [Ammonium thiocyanate] = 1 M.

32

-150

-100

-50

0

50

-1.6 -1.2 -0.8 -0.4 0 0.4

potential /V vs C

curre

nt /µ

A

A

report 13/11/2008 (P135)

-20

-10

0

10

-1.6 -1.2 -0.8 -0.4 0 0.4

potential /V vs C

curre

nt d

ensi

ty /µ

A m

m-2

B

report 13/11/2008 (P135)

Fig. 3.4 Comparison of silver nitrate voltammetry on LIRANS and NPL-type sensors. (A) Current data; and (B) current density data.

() NPL-type sensors, D2 carbon, new batch. () NPL-type sensors, D2 carbon, original batch. () LIRANS-type sensors, D14 carbon. [Silver nitrate] = 0.5 mM; [ammonium thiocyanate] = 1 M.

33

sensor type

start potential

/V

first vertex potential

/V

second vertex

potential /V

peak potential

/mV

peak current density

/µA mm-2

NPL(new) +0.3 -1.6 +0.3 -313 4.68 NPL

(original) +0.3 -1.6 +0.3 -406 4.26

LIRANS +0.3 -1.6 +0.3 -343 6.69 NPL(new) +0.4 -1.6 +0.4 -348 4.39

NPL (original)

+0.4 -1.6 +0.4 -394 4.08

LIRANS +0.4 -1.6 +0.4 -384 5.98 report 13/11/2008 (P134)

Table 3.4 Silver stripping peak parameters determined using cyclic voltammetry. NPL (new): NPL-type (D2 carbon, new batch). NPL (original): NPL-type (D2 carbon, original batch). LIRANS: LIRANS-type sensors (D14 carbon). Working electrode area /mm2: (NPL) 9.90±0.13; and (LIRANS) 1.89±0.12.

34

3.2.2 Cyclic voltammetry of silver sol

Cyclic voltammetry of silver sol was carried out for NPL-type (D2 carbon, original

batch) sensors by dropping 16 µl silver sol stock solution and 4 µl ammonium

thiocyanate stock solution on to the surface of the sensor so that all the electrodes were

covered. The Autolab was used in cyclic voltammetry mode with step potential of 2

mV; scan rate of 0.1 V s-1; number of scans, 1; and cycled between either +0.6 or +1.0

V start and second vertex potential and first vertex potential -1.6 V. The resulting

cyclic voltammograms are shown, Figs. 3.5 and 3.6. A new sensor was used for each

measurement.

The silver sol voltammograms exhibited three features (I, II and III in the figures).

By comparison with the silver nitrate voltammograms:

feature I (reduction peak at about -0.9 V) is likely to be attributable to either the

ammonium thiocyanate or to some surface confined process;

feature II (reduction peak at -1.3 to -1.5 V) is likely to correspond to the plating of

silver ions on to the carbon electrode (equation 3.1); and

feature III (oxidation peak at about -0.2 V) is likely to be attributable to the stripping

of silver from the electrode surface (equation 3.2).

Furthermore, it can be seen that the silver stripping peak was bigger (and more

complex in shape) when the starting potential was +1.0 V rather than +0.6 V (cf. Fig.

3.5 and 3.6). This is as would be expected since in the former case more silver ions

would likely be generated from the silver sol due to the additional time necessary to

scan from +1.0 V to +0.6 V during which additional silver ions could be generated.

35

The more complex stripping behavior observed when the initial potential was + 1.0 V

may be due to the greater quantity of silver plated leading to silver stripping from

silver followed by silver stripping from carbon at a slightly different (more oxidising)

potential:

−+ +⎯⎯ →⎯ eaqs )(Ag)( Ag AE Ag, (3.4)

−+> +⎯⎯⎯ →⎯ eaqs AE )(Ag)( Ag E C, B (3.5)

The silver stripping peaks were presumed to originate from dissolution of the silver

sol:

−+> +⎯⎯⎯ →⎯ eaqsol )(Ag)( Ag V 0.2- E (3.5)

36

-200

0

200

400

600

-1.6 -1.2 -0.8 -0.4 0 0.4 0.8 1.2

potential /V vs C

curre

nt /µ

A

A

I

IIreport30/10/2008 (P128)

III

-2

-1

0

1

2

3

-0.5 -0.3 -0.1

potential /V vs C

curre

nt /µ

A

-30

0

30

60

-1.6 -1.2 -0.8 -0.4 0 0.4 0.8 1.2

potential /V vs C

curre

nt d

ensi

ty /µ

A m

m-2

report30/10/2008 (P128)

B

-0.2

-0.1

0

0.1

0.2

0.3

-0.5 -0.3 -0.1

potential /V vs C

curre

nt d

ensi

ty /µ

A m

m-2

Fig. 3.5 Silver sol voltammetry on NPL-type sensor. (A) Current; and (B) current density data.

[Silver sol] = 800 µl ml-1; [ammonium thiocyanate] = 1 M. Sensor: D2 carbon, original batch. Start potential: +1.0 V; first vertex potential: -1.6 V; and second vertex potential: +1.0 V. Step potential = 2 mV; scan rate = 100 mV s-1.

Features I, II, and III are described in the text. Insets show detail from positive going sweep.

37

-250

-150

-50

50

-1.6 -1.2 -0.8 -0.4 0 0.4

potential /V vs C

curre

nt /µ

A

A

report30/10/2008 (P128)

-3

-2

-1

0

1

2

3

-0.5 -0.3 -0.1

potential /V vs C

curre

nt /µ

A

-25

-15

-5

5

-1.6 -1.2 -0.8 -0.4 0 0.4

potential /V vs C

curre

nt d

ensi

ty /µ

A m

m-2

I

II

IIIB

report30/10/2008 (P128)

-0.4

-0.2

0

0.2

0.4

-0.5 -0.3 -0.1

potential /V vs C

curre

nt d

ensi

ty /µ

A m

m-2

Fig. 3.6 Silver sol voltammetry on NPL-type sensor. (A) Current; and (B) current density data.

[Silver sol] () = 800 µl ml-1; [ammonium thiocyanate] = 1 M. Sensor: D2 carbon, original batch. Start potential: +0.6 V; first vertex potential: -1.6 V; and second vertex potential: +0.6 V. Step potential = 2 mV; scan rate = 100 mV s-1.

Features I, II, and III are described in the text. Insets show detail from positive going sweep.

38

3.3 Steps and sweep voltammetry

3.3.1 Steps and sweep measurements of silver sol

The steps and sweep method in GPES 4.9 software was used (rather then cyclic

voltammetry) to investigate the generation of silver ions from silver sol with

subsequent plating and stripping of silver metal. Unless otherwise indicated, the

parameters used were as follows:

no pretreatment;

step 1, +1.0 V for 15 s with a 150 ms time interval;

step 2, -1.6 V for 5 s with a 100 ms time interval;

step 3, -1.2 V for 55 s with a 100 ms time interval; and

linear sweep from -1.2 to +0.5 V at scan rate 1 V s-1 with potential step 10 mV.

In all cases, each sensor was used for just one measurement. An example of the

resultant potential profile and current response is shown, Fig. 3.7. The inset in the

figure shows a typical silver stripping peak. In the example shown, the stripping peak

occurred at about -50 mV vs. the carbon pseudo-reference electrode.

39

-2

-1

0

1

0 20 40 60 80

time /s

pote

ntia

l /V

vs.

C p

seud

o-re

f

A

report 24/09/2008 (P110)

-200

-100

0

100

0 20 40 60 80

time /s

curre

nt /µ

A

report 24/09/2008 (P110)

B

-4

6

16

75.5 76 76.5

time /s

curre

nt /µ

A

Fig. 3.7 Example of steps and sweep potential sequence and resultant current responses.

Electrochemical signal: (A) potential, and (B) current. The inset figure in graph B shows a detail with silver stripping peak. [Silver sol] /µl ml-1 = 800, [ammonium thiocyanate] = 1 M. Sensors: LIRANS-type, D14 carbon.

40

3.3.1.1 Comparison of drop from bulk and bulk methods

Initial measurements were carried out using drop from bulk and bulk methods (see

section 2.3.2). Test solutions were prepared by mixing silver sol and ammonium

thiocyanate stock solutions in a cut down semi-micro cuvette. A drop (20 µl) was

removed and deposited so as to cover the working, reference and counter electrodes

enabling a steps and sweep measurement (parameters as given in section 3.3.1).

Subsequently a fresh sensor was immersed in the remaining (bulk) test solution and a

further measurement carried out.

The sequence of measurements is given (Table 3.5) and the concentration dependences

of the silver stripping peaks are shown, Figs. 3.8 and 3.9. The bulk method clearly

produced bigger signals than the drop from bulk measurements (compare A and B

scales in Figs. 3.8 and 3.9) but in neither case was there any reason to believe there was

any correlation between the stripping signals and the silver sol concentration.

41

order volume of silver sol stock /µl

volume of ammonium thiocyanate stock /µl

sample method

1 drop from bulk 2

0 1000 bulk

3 drop from bulk 4 50 950 bulk 5 drop from bulk 6 30 970 bulk 7 drop from bulk 8 20 980 bulk 9 drop from bulk 10 40 960 bulk 11 drop from bulk 12 60 940 bulk 13 drop from bulk 14 10 990 bulk 15 drop from bulk 16 70 930 bulk

report 24/09/2008 (P109) and 30/09/2008 (P113) Table 3.5 Sequence of drop from bulk and bulk measurements of silver sol.

Order refers to the order in which the measurements were made.

42

0

1

2

3

4

5

0 10 20 30 40 50 60 70

[silver sol] /µl ml-1

peak

hei

ght /

µA

A

report 15/09/2008 (P105)

0

0.4

0.8

1.2

0 10 20 30 40 50 60 70

[silver sol] /µl ml-1

peak

hei

ght /

µA

B

report 15/09/2008 (P105) Fig. 3.8 Comparison of silver stripping peak heights obtained using bulk and drop from bulk methods.

(A) Bulk method. (Sensors immersed in test solution) (B) Drop from bulk method. (Measurement electrodes covered by drop taken from bulk test

solution.) Symbols represent measured data. Steps and sweep voltammetry: step 1, +1.0 V for 15 s; step 2, -1.6 V for 5 s; step 3, -1.2 V for 55 s; and linear sweep from -1.2 to +0.5 V at 1 Vs-1. Sensors: LIRANS-type, D14 carbon.

43

0

0.2

0.4

0.6

0.8

0 10 20 30 40 50 60 70

[silver sol] /µl ml-1

peak

are

a /µ

C

A

report 15/09/2008 (P105)

0

0.04

0.08

0.12

0.16

0 10 20 30 40 50 60 70

[silver sol] /µl ml-1

peak

are

a /µ

C

B

report 15/09/2008 (P105)

Fig. 3.9 Comparison of silver stripping peak areas obtained using bulk and drop from bulk methods.

(A) Bulk method. (Sensors immersed in test solution) (B) Drop from bulk method. (Measurement electrodes covered by drop taken from bulk

test solution) Symbols represent measured data. Steps and sweep voltammetry: step 1, +1.0 V for 15 s; step 2, -1.6 V for 5 s; step 3, -1.2 V for 55 s; and linear sweep from -1.2 to +0.5 V at 1 Vs-1. Sensors: LIRANS-type, D14 carbon.

44

3.3.1.2 Drop from bulk and drop measurements (high concentration)

NPL reported good correlation between stripping peaks and silver sol concentration –

but their data was obtained using higher concentrations than those shown (Figs. 3.8

and 3.9) and using drops mixed directly on the electrode surfaces. Consequently, the

NPL method was employed to see if their results could be repeated. Silver sol stock,

ammonium thiocyanate stock and water (in that order) were pipetted on to the

electrode surface and mixed in situ (Table 3.6, 24/09 data). The resultant

measurements showed reasonable correlation between silver stripping peaks and sol

concentration, Fig. 3.10.

The NPL drop method appeared to be appropriate for relatively high concentrations of

silver sol (≥ 200 µl ml-1) but was not necessarily appropriate for lower concentrations

of sol due to the small volumes of sol that would need to be dispensed. In

consequence, the NPL approach was repeated in conjunction with drop from bulk

measurements at slightly higher concentrations than originally employed (Table 3.6,

30/09 data). The drop results (Figs. 3.11B and 3.12B) confirmed that reasonable

correlation could be obtained between silver stripping peaks and sol concentration

using the NPL approach. In addition, there was some correlation in the lower

concentration range using the drop from bulk method (Figs. 3.11A and 3.12A).

Comparing the drop and drop from bulk results, both the regression slopes and

intercept values were similar (cf. 0.020 ± 0.003 versus 0.0156 ± 0.0007 µA µl-1 ml;

0.0021 ± 0.0004 versus 0.0023 ± 0.0002 µC µl-1 ml; -0.7 ± 0.4 versus -0.1 ± 0.3 µA;

and -0.00 ± 0.05 versus -0.03 ± 0.10 µC). Combining the available data (Table 3.6)

confirmed that the data (both drop and drop from bulk) were reasonably consistent, Fig.

45

3.13. The indicated limit of detection (three times the standard deviation of the

intercept divided by the slope) was about 30 µl ml-1.

46

date order volume of silver sol stock

/µl

volume of ammonium thiocyanate

stock /µl

volume of deionised

water /µl

concentra-tion of

silver sol /µl ml-1

method

15/09 1 0 1000 0 0 drop from bulk 15/09 2 50 950 0 50 drop from bulk 15/09 3 30 970 0 30 drop from bulk 15/09 4 20 980 0 20 drop from bulk 15/09 5 40 960 0 40 drop from bulk 15/09 6 60 940 0 60 drop from bulk 15/09 7 10 990 0 10 drop from bulk 15/09 8 70 930 0 70 drop from bulk

24/09 1 14 4 2 700 drop 24/09 2 8 4 8 400 drop 24/09 3 4 4 12 200 drop 24/09 4 12 4 4 600 drop 24/09 5 6 4 10 300 drop 24/09 6 10 4 6 500 drop 24/09 7 5 4 11 250 drop 24/09 8 16 4 0 800 drop

30/09 1 14 4 2 700 drop 30/09 2 15 20 65 150 drop from bulk 30/09 3 8 4 8 400 drop 30/09 4 19 20 61 190 drop from bulk 30/09 5 5 4 11 250 drop 30/09 6 9 20 71 90 drop from bulk 30/09 7 16 4 0 800 drop 30/09 8 11 20 69 110 drop from bulk 30/09 9 4 4 12 200 drop 30/09 10 7 20 73 70 drop from bulk 30/09 11 12 4 4 600 drop 30/09 12 13 20 67 130 drop from bulk 30/09 13 6 4 10 300 drop 30/09 14 5 20 75 50 drop from bulk 30/09 15 10 4 6 500 drop 30/09 16 17 20 63 170 drop from bulk

report 24/09/2008 (P109) and 30/09/2008 (P113) Table 3.6 Summary of drop and drop from bulk measurements of silver sol.

Order refers to the order in which the measurements were made.

47

peak height /µA = (0.015±0.001)[silver sol /µl ml-1] + (1.2±0.5)

R2 = 0.98

0

3

6

9

12

15

0 200 400 600 800

[silver sol] /µl ml-1

peak

hei

ght /

µA

report 24/09/2008 (P110)

A

peak area /µC = (0.0024±0.0001)[silver sol /µl ml-1] + (0.03±0.07)

R2 = 0.98

0

0.5

1

1.5

2

2.5

0 200 400 600 800

[silver sol] /µl ml-1

peak

are

a /µ

C

report 24/09/2008 (P110)

B

Fig. 3.10 Dependence of steps and sweep voltammetry peak data as a function of the concentration of

silver sol using drop method. Electrochemical signal: (A) peak height, and (B) peak area. Symbols represent measured data. The lines through the data are regression lines fitted using Microsoft Excel. (N.B In both graphs, the point of 600 µl ml-1 concentration was omitted from calculation of the regression lines.) [Ammonium thiocyanate] = 1 M. Sensors: LIRANS-type, D14 carbon.

48

peak height /µA =(0.020±0.003)[silver sol /µl ml-1]- (0.7±0.4)

R2 = 0.88

-1

0

1

2

3

4

0 50 100 150 200

[silver sol] /µl ml-1

peak

hei

ght /

µA

report 30/09/2008 (P114)

A

peak height /µA =(0.0156±0.0007)[silver sol /µl ml-1] - (0.1±0.3)

R2 = 0.990

-4

0

4

8

12

16

0 200 400 600 800

[silver sol] /µl ml-1

peak

hei

ght /

µA

report 30/09/2008

B

Fig. 3.11 Dependence of steps and sweep voltammetry peak height as a function of the concentration of

silver sol. Method of measurements: (A) drop from bulk, and (B) drop. Symbols represent measured data. The lines through the data are regression lines fitted using Microsoft Excel. [Ammonium thiocyanate] = 1 M. Sensors: LIRANS-type, D14 carbon.

49

peak area /µC =(0.0021±0.0004)[silver sol /µl ml-1] - (0.0002±0.05)

R2 = 0.83

0

0.1

0.2

0.3

0.4

0.5

0 50 100 150 200

[silver sol] /µl ml-1

peak

are

a /µ

C

report 30/09/2008 (P114)

A

peak area /µC =(0.0023±0.0002)[silver sol /µl ml-1] - (0.03±0.10)

R2 = 0.96

0

0.5

1

1.5

2

0 200 400 600 800

[silver sol] /µl ml-1

peak

are

a /µ

C

report 30/09/2008 (P114)

B

Fig. 3.12 Dependence of steps and sweep voltammetry peak area as a function of the concentration of

silver sol. Method of measurements: (A) drop from bulk, and (B) drop. Symbols represent measured data. The lines through the data are regression lines fitted using Microsoft Excel. [Ammonium thiocyanate] = 1 M. Sensors: LIRANS-type, D14 carbon.

50

peak height /µA = (0.0160±0.0004)[silver sol /µl ml-1]- (0.06±0.15)

R2 = 0.98

0

3

6

9

12

15

0 200 400 600 800

[silver sol] /µl ml-1

peak

hei

ght /

µA

report 30/09/2008 (P114)

A

peak area /µC = (0.00235±0.00006)[silver sol /µl ml-1] - (0.01±0.02)

R2 = 0.98

0

1

2

0 200 400 600 800

[silver sol] /µl ml-1

peak

are

a /µ

C

B

report 30/09/2008 (P114)

Fig. 3.13 Combined steps and sweep measurements of silver sol drops.

Electrochemical signal: (A) peak height, and (B) peak area. Drop from bulk data, 15 September 2008. Drop data, 24 September 2008 Drop and drop from bulk data, 30 September. 2008. (N.B. Open symbols represent data

that were omitted from calculation of the regression lines.) [Ammonium thiocyanate] = 1 M. Sensors: LIRANS-type, D14 carbon.

51

3.3.2 Effect of step 1 (dissolution) potential on silver sol measurements

NPL reported that a dissolution step was necessary to dissolve silver sol particles

before silver ions could be plated on to the carbon working electrodes and in the case

that the initial potential was insufficiently oxidizing (less than about +0.4 V) then a

much smaller silver stripping signal was obtained.

In the work reported here, the effect of varying the dissolution (step 1) potential was

investigated by employing the usual steps and sweep parameters but changing the step

1 potential:

no pretreatment;

step 1, in the range from -0.4 to +1.6 V for 15 s with a 150 ms time interval, Table

3.7;

step 2, -1.6 V for 5 s with a 100 ms time interval;

step 3, -1.2 V for 55 s with a 100 ms time interval; and

linear sweep from -1.2 to +0.5 V at scan rate 1 V s-1 with potential step 10 mV.

Each sensor was used for just one measurement.

Drop measurements were carried out using sol concentrations of 800 µl ml-1 (16 µl

silver sol and 4 µl ammonium thiocyanate stock solution) or 200 µl ml-1 (4 µl silver sol,

4 µl ammonium thiocyanate stock solution and 12 µl deionised water), Fig. 3.14.

An increase in signal when the dissolution step potential was very oxidizing was clear

when the sol concentration was 800 µl ml-1 but less clear when the sol concentration

was 200 µl ml-1. Qualitatively similar results were obtained to those obtained by NPL,

52

but the dramatic increase in signal occurred at about +0.8 V vs. the carbon reference

(rather than at about +0.4 V as found by NPL).

order dissolution

potential /V vs. C

concentration of sol

/µl ml-1

order dissolution potential /V vs. C

concentration of sol

/µl ml-1 1 -0.40 800

25 +0.65 800 2 -0.20 800 26 +0.70 800 3 0.00 800 27 +0.75 800 4 +0.10 800 28 +0.80 800 5 +0.20 800 29 +0.90 800 6 +0.30 800 30 +1.00 800 7 +0.40 800 31 +1.30 800 8 +0.50 800 32 +1.60 800 9 +0.60 800 33 0.00 200 10 +0.70 800 34 +0.20 200 11 +0.80 800 35 +0.30 200 12 +0.90 800 36 +0.40 200 13 +1.00 800 37 +0.45 200 14 +1.20 800 38 +0.50 200 15 +1.40 800 39 +0.55 200 16 +1.60 800 40 +0.60 200 17 0.00 800 41 +0.65 200 18 +0.20 800 42 +0.70 200 19 +0.30 800 43 +0.75 200 20 +0.40 800 44 +0.80 200 21 +0.45 800 45 +0.90 200 22 +0.50 800 46 +1.00 200 23 +0.55 800 47 +1.30 200 24 +0.60 800 48 +1.60 200

report 15/09/2008 (P103) Table 3.7 Measurement sequence to determine the effects of step 1 (dissolution) potential on silver sol

signals. Order indicates the order in which the measurements were made.

53

0

0.5

1

1.5

2

2.5

-0.4 0 0.4 0.8 1.2 1.6

potential of dissolution step (step 1) /V vs C

peak

are

a /µ

C

A

report 15/09/2008(P104)

0

4

8

12

16

-0.4 0 0.4 0.8 1.2 1.6

potential of dissolution step (step 1) /V vs C

peak

hei

ght /

µA

report 15/09/2008(P104)

B

Fig. 3.14 Effect of dissolution (step 1) potential on silver sol measurement. (A) Peak area; and (B) peak height. [Silver sol] /µl ml-1: () 200; and (, ) 800.

[Ammonium thiocyanate] = 1 M. Sensors: LIRANS-type, D14 carbon.

54

3.3.3 Comparison of NPL and LIRANS–type sensors as to the effect of

dissolution (step 1) potential

To check whether or not the high potential required to ‘switch on’ silver sol

measurements (section 3.3.2) was aberrant in some way, NPL and LIRANS–type

sensors were compared. As before, the effect of the dissolution (step 1) potential was

observed by employing the usual steps and sweep parameters – but varying the step 1

potential:

no pretreatment;

step 1, in the range from -0.4 to +1.6 V for 15 s with a 150 ms time interval, Table

3.8;

step 2, -1.6 V for 5 s with a 100 ms time interval;

step 3, -1.2 V for 55 s with a 100 ms time interval; and

linear sweep from -1.2 to +0.5 V at scan rate 1 V s-1 with potential step 10 mV.

Each sensor was used for just one measurement.

Analyses were carried out by dropping 16 µl silver sol stock solution and 4 µl

ammonium thiocyanate stock solution (in that order) on to the surface of the sensor.

The dissolution potential dependences of the peak height and peak area are shown (Fig.

3.15) while current and charge density data are also given, Fig. 3.16. To facilitate

visualization of the step changes in signals, the measured data were fitted to a tanh

approximation using the Solver function in Microsoft Excel (section 2.4). Fitted

parameters are given in Table 3.9.

At the silver sol concentration studied (800 µl ml-1) there were clear differences

55

between the NPL and LIRANS-type sensors. The absolute signals (peak height or

peak area) from the NPL-type (D2, new batch) devices were much larger than the

absolute signals from the LIRANS-type (D14) devices. This can be attributed to the

differences in the areas of the two types of working electrode (c.f. 9.90±0.13 versus

1.89±0.12 mm2). When the measured signals were transformed to account for the

geometric area of the working electrodes, then the signals (current density or charge

density) became larger for the LIRANS-type sensors than the NPL-type sensors.

There was clearly a difference between the two types of sensors in the potential at

which the measured signals switched from relatively low to relatively high levels.

For the NPL (D2) devices the switch potential was at about +440 mV vs. C (D2)

whereas, for the LIRANS (D14) devices the switch potential was about +760 mV vs. C

(D14), Table. 3.9.

56

date order dissolution potential /V vs. C

sensor type

date order dissolution potential /V vs. C

sensor type

9/09 1 -0.40 LIRANS

30/10 1 +0.35 LIRANS 9/09 2 -0.20 LIRANS 30/10 2 +0.35 NPL(new) 9/09 3 0.00 LIRANS 30/10 3 +0.80 LIRANS 9/09 4 +0.10 LIRANS 30/10 4 +0.80 NPL(new) 9/09 5 +0.20 LIRANS 30/10 5 -0.10 LIRANS 9/09 6 +0.30 LIRANS 30/10 6 -0.10 NPL(new) 9/09 7 +0.40 LIRANS 30/10 7 +0.40 LIRANS 9/09 8 +0.50 LIRANS 30/10 8 +0.40 NPL(new) 9/09 9 +0.60 LIRANS 30/10 9 +0.75 LIRANS 9/09 10 +0.70 LIRANS 30/10 10 +0.75 NPL(new) 9/09 11 +0.80 LIRANS 30/10 11 +0.20 LIRANS 9/09 12 +0.90 LIRANS 30/10 12 +0.20 NPL(new) 9/09 13 +1.00 LIRANS 30/10 13 +0.90 LIRANS 9/09 14 +1.20 LIRANS 30/10 14 +0.90 NPL(new) 9/09 15 +1.40 LIRANS 30/10 15 +0.50 LIRANS 9/09 16 +1.60 LIRANS 30/10 16 +0.50 NPL(new) 11/09 1 0.00 LIRANS 30/10 17 +0.25 LIRANS 11/09 2 +0.20 LIRANS 30/10 18 +0.25 NPL(new) 11/09 3 +0.30 LIRANS 30/10 19 +1.30 LIRANS 11/09 4 +0.40 LIRANS 30/10 20 +1.30 NPL(new) 11/09 5 +0.45 LIRANS 30/10 21 +0.70 LIRANS 11/09 6 +0.50 LIRANS 30/10 22 +0.70 NPL(new) 11/09 7 +0.55 LIRANS 30/10 23 +0.10 LIRANS 11/09 8 +0.60 LIRANS 30/10 24 +0.10 NPL(new) 11/09 9 +0.65 LIRANS 30/10 25 +0.85 LIRANS 11/09 10 +0.70 LIRANS 30/10 26 +0.85 NPL(new) 11/09 11 +0.75 LIRANS 30/10 27 +1.00 LIRANS 11/09 12 +0.80 LIRANS 30/10 28 +1.00 NPL(new) 11/09 13 +0.90 LIRANS 30/10 29 +0.30 LIRANS 11/09 14 +1.00 LIRANS 30/10 30 +0.30 NPL(new) 11/09 15 +1.30 LIRANS 30/10 31 +0.60 LIRANS 11/09 16 +1.60 LIRANS 30/10 32 +0.60 NPL(new)

report 30/10/2008 (P123) Table 3.8 Comparison of dissolution potential and sensor type for the measurement of silver sols.

Order indicates the order in which the measurements were made. NPL(new): NPL-type (D2 carbon, new batch); LIRANS: (D14 carbon).

57

0

10

20

30

40

-0.4 0 0.4 0.8 1.2 1.6

dissolution (step 1) potential /V vs C

peak

hei

ght /

µA

A

report 30/10/2008 (P125)

0

1

2

3

4

5

-0.4 0 0.4 0.8 1.2 1.6

dissolution (step 1) potential /V vs C

peak

are

a /µ

C

B

report 30/10/2008 (P125)

Fig. 3.15 Comparison of NPL and LIRANS-type sensors as to the effect of the dissolution (step 1)

potential. Sensor type: (•) NPL, D2 carbon, new batch; and ( ) LIRANS, D14 carbon.

(A) Peak height; and (B) peak area data. Symbols indicate measured data whereas the curves indicate lines of best fit (tanh approximation).

[Silver sol] = 800 µl ml-1; [ammonium thiocyanate] = 1 M. Date of measurement: 9 September 2008; 11 September 2008; and (•) 30 October 2008.

58

0

2

4

6

8

10

-0.4 0 0.4 0.8 1.2 1.6

dissolution (step 1) potential /V vs C

curre

nt d

ensi

ty /µ

A m

m-2

A

report 30/10/2008 (P126)

0

0.3

0.6

0.9

1.2

1.5

-0.4 0 0.4 0.8 1.2 1.6

dissolution (step 1) potential /V vs C

char

ge d

ensi

ty /µ

C m

m-2

B

report 30/10/2008 (P126)

Fig. 3.16 Comparison of NPL and LIRANS-type sensors as to the effect of the dissolution (step 1)

potential. Sensor type: (•) NPL, D2 carbon, new batch; and ( ) LIRANS, D14 carbon. (A) Current density; and (B) charge density data. Points indicate measured data whereas the curves indicate lines of best fit (tanh approximation). [Silver sol] = 800 µl ml-1; [ammonium thiocyanate] = 1 M. Data of measurement: 9 September 2008; 11 September 2008; and (•) 30 October 2008.

59

sensor type

carbon type

electrode arrangement

working electrode

area

current current charge charge peak height peak area current current charge chargedensity density density density

/mm2 /µA /µA mm-2 /µC /µC mm-2 /µA /µA mm-2 /µC /µC mm-2

NPL D2 NPL 9.90 ± 0.13 5.51 0.55 0.56 0.06 +417 +456 33.3 3.34 4.40 0.44LIRANS D14 LIRANS 1.89± 0.12 2.21 1.17 0.29 0.15 +761 +766 14.1 7.47 2.04 1.08

20090427 Yuantang table

switch potential /mV vs C

pre-step post-step

Table 3.9 Estimated switch potentials for NPL and LIRANS-type sensors. Indicated potentials arrived at using SOLVER function in Microsoft Excel to fit tanh approximation to measured data (Figs. 3.15 and 3.16).

60

3.3.5 Comparison of silver sol and supernatant as to effect of dissolution

(step 1) potential

The steps and sweep measurements (Figs. 3.14 – 3.16) indicated that silver stripping

occurred even when the ‘dissolution’ potential (step 1) was insufficiently oxidizing to

produce silver ions from silver sol. This was a surprising observation since it was

supposed that in the absence of sol dissolution there would be negligible silver signal.

It was proposed that the silver sol contained silver ions in addition to sol particles.

UV-VIS spectroscopy showed that the sol particles could be removed from bulk

solution by centrifugation, Fig. 3.17.

The silver sol had an absorption maximum (λmax) at 406 nm. At this wavelength, the

absorbance of the sol was 0.964. After centrifuging (Eppendorf 5415C) twice (2×5

min, 121000 min-1, 1 ml sol solution) the absorbance was 0.021. It was concluded

that ~98% of the sol particles were removed from the supernatant.

Silver sol and the supernatant were tested for the effects of varying the dissolution

(step 1) potential by employing the usual steps and sweep parameters with changing

step 1 potential:

no pretreatment;

step 1, in the range from +0.1 to +1.3 V for 15 s with a 150 ms time interval, Table

3.10;

step 2, -1.6 V for 5 s with a 100 ms time interval;

step 3, -1.2 V for 55 s with a 100 ms time interval; and

linear sweep from -1.2 to +0.5 V at scan rate 1 V s-1 with potential step 10 mV.

61

Each sensor was used for just one measurement.

Steps and sweep measurements were carried out using NPL-type (D2 carbon, new

batch) by dropping 16 µl of either silver sol stock solution or supernatant and 4 µl of

ammonium thiocyanate stock solution on to the surface of the sensor. The dissolution

potential dependences of the peak height and peak area are shown, Fig. 3.18.

order potential /V vs. C

solution order potential /V vs. C

sensor

1 +0.80 sol 14 +0.70 supernatant2 +0.80 supernatant 15 +0.10 sol 3 +0.40 sol 16 +0.10 supernatant4 +0.40 supernatant 17 +1.00 sol 5 +0.20 sol 18 +1.00 supernatant6 +0.20 supernatant 19 +0.30 sol 7 +0.90 sol 20 +0.30 supernatant8 +0.90 supernatant 21 +0.60 sol 9 +0.50 sol 22 +0.60 supernatant10 +0.50 supernatant 23 +0.10 sol 11 +1.30 sol 24 +0.10 supernatant12 +1.30 supernatant 25 +0.65 sol 13 +0.70 sol 26 +0.65 supernatant

report 18/11/2008 Table 3.10 Comparison of dissolution potential for steps and sweep measurements of silver sol and

supernatant. Order indicates the order in which the measurements were made.

62

0

0.4

0.8

1.2

1.6

2

190 280 370 460 550 640 730 820

wavelength, λ /nm

mea

sure

d ab

sorb

ance

18/11/2008

A

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

190 280 370 460 550 640 730 820

wavelength, λ /nm

calc

ulat

ed a

bsor

banc

e

B

18/11/2008

Fig. 3.17 Absorbance curves of silver sol, supernatant, and blank (deionised water). A: Measured absorbance curves. B: Absorbance curves calculated by subtracting blank data

(deionised water). () Silver sol before centrifuging; () supernatant after first centrifuging; () supernatant after second centrifuging; () deionised water for blank sample. Wavelength of calculated peaks (λ /nm) = 200, 406; calculated absorbance shoulder in the silver sol spectrum ~ 280 nm.

63

y = -0.1891x + 6.6322R2 = 0.005

0

10

20

30

40

50

60

0 0.2 0.4 0.6 0.8 1 1.2 1.4

dissolution (step 1) potential /V vs C

peak

hei

ght /

µAA

18/11/2008

y = 0.525x + 0.5033R2 = 0.603

0

1

2

3

4

5

6

7

0 0.2 0.4 0.6 0.8 1 1.2 1.4

dissolution (step 1) potential /V vs C

peak

are

a /µ

C

B

18/11/2008 Fig. 3.18 Comparison of silver sol and supernatant using steps and sweep measurements.

(A) Peak height; and (B) peak area data. ( ) Silver sol; () supernatant. Supernatant was obtained by centrifuging silver sol (2*5 min, 1 ml). Points indicate measured silver sol data whereas the curves indicate lines of best fit (tanh approximation) for the sol and trendline for the supernatant. [Silver sol] = 800 µl ml-1; [ammonium thiocyanate] = 1 M. NPL-type sensors (D2 carbon, new batch), NPL electrode arrangement.

64

Trendlines through the supernatant data indicated a stripping peak current of 6.51 µA

and a stripping peak charge of 5.70 µC at the step potentials. These values were 14.6

and 14.7%, respectively, of the post-step signals. These figures were much higher

than the fraction of sol remaining in the supernatant (~2.2% as indicated by the

spectrophotometric results). This confirmed that the pre-step stripping peaks could

not be due to the sol particles that absorbed at 406 nm, but might be due to silver ions.

65

3.3.6 Comparison of silver sol and silver nitrate as to effect of dissolution

(step 1) potential

Silver sol and silver nitrate measurements were compared as to the effect of the

dissolution (step 1) potential by employing the usual steps and sweep parameters – but

varying the step 1 potential:

no pretreatment;

step 1, 0 or +1.0 V for 15 s with a 150 ms time interval, Table 3.11;

step 2, -1.6 V for 5 s with a 100 ms time interval;

step 3, -1.2 V for 55 s with a 100 ms time interval; and

linear sweep from -1.2 to +0.5 V at scan rate 1 V s-1 with potential step 10 mV.

Each sensor was used for just one measurement.

A silver nitrate stock solution (50 µM) was made by mixing 1 µl 0.1 M silver nitrate

solution and 1999 µl deionised water. Measurements were carried out using silver sol

concentrations in the range 100 to 800 µl ml-1, while the concentrations of silver nitrate

ranged from 5 to 40 µM. All measurements were carried out using the drop method

and the results are shown, Figs. 3.19 and 3.20.

The results for the silver sol were as might be expected. At the lower step 1 potential

(0 V) there was no dependence on the silver sol concentration – indicating that: (a) no

sol was converted into silver ions (equation 3.5); and (b) the silver stripping peaks

observed were due to silver contamination of the carbon electrodes. It would be

necessary to carry out measurements in the absence of added silver in order to confirm

this latter hypothesis.

66

The results for the silver nitrate measurements showed a significant difference between

step 1 potentials of 0 and +1.0 V. This was unexpected since silver nitrate should

contain only silver ions, i.e. Ag+ (aq) and silver contamination of the carbon electrodes

would be expected to affect the intercept rather than the slope of the calibration plots.

The data suggest that there was some kind of dissolution process going on – at least at

the higher step 1 potential, perhaps something like:

−++ +⎯⎯⎯ →⎯ 3V 1.0 C,

3 NO)(Ag)(AgNO aqsol (3.6)

Further work would be needed to provide evidence for or against such a hypothesis.

67

order dissolution (step 1) potential

/V

volume of silver nitrate stock /µl

[silver nitrate]

/µM

volume of silver

sol stock /µl

[silver sol]

/µl ml-1

volume of ammonium thiocyanate

stock /µl

volume of

deionised water

/µl 1 0.0 16 40 0 4 0 2 +1.0 16 40 0 4 0 3 0.0 4 10 0 4 12 4 +1.0 4 10 0 4 12 5 0.0 8 20 0 4 8 6 +1.0 8 20 0 4 8 7 0.0 12 30 0 4 4 8 +1.0 12 30 0 4 4 9 0.0 6 15 0 4 10 10 +1.0 6 15 0 4 10 11 0.0 14 35 0 4 2 12 +1.0 14 35 0 4 2 13 0.0 2 5 0 4 14 14 +1.0 2 5 0 4 14 15 0.0 10 25 0 4 6 16 +1.0 10 25 0 4 6 17 0.0 0 16 800 4 0 18 +1.0 0 16 800 4 0 19 0.0 0 4 200 4 12 20 +1.0 0 4 200 4 12 21 0.0 0 8 400 4 8 22 +1.0 0 8 400 4 8 23 0.0 0 12 600 4 4 24 +1.0 0 12 600 4 4 25 0.0 0 6 300 4 10 26 +1.0 0 6 300 4 10 27 0.0 0 14 700 4 2 28 +1.0 0 14 700 4 2 29 0.0 0 2 100 4 14 30 +1.0 0 2 100 4 14 31 0.0 0 10 500 4 6 32 +1.0 0 10 500 4 6

report 1/12/2008 Table 3.11 Test solutions prepared for comparison of steps and sweep measurements of silver sol and

silver nitrate. ‘Order’ refers to the order in which the measurements were made. (N.B. The whole sequence was repeated.)

68

peak height /µA = (0.053±0.003)([silver sol] /µl ml-1)+ (2.8±1.5)

R2 = 0.96

peak height /µA = (0.001±0.001)([silver sol] /µl ml-1) + (4.0±0.6)

R2 = 0.02

0

10

20

30

40

50

0 200 400 600 800

[silver sol] /µl ml-1

peak

hei

ght /

µA

A

report1/12/2008

peak area /µC = (0.0055±0.0002)([silver sol] /µl ml-1) + (0.3±0.1)

R2 = 0.98

peak area /µC = (0.0002±0.0001)([silver sol] /µl ml-1)+ (0.38±0.07)

R2 = 0.16

0

1

2

3

4

5

0 200 400 600 800

[silver sol] /µl ml-1

peak

are

a /µ

C

B

report1/12/2008

Fig. 3.19 Effect of dissolution (step 1) potential on the steps and sweep measurement of silver sol. Dissolution (step 1) potential /V: (, ∆) 0; and (, ) +1.0.

Measurement date: (, ) 1 Dec. 2008; and (∆, ) repeated on the same day. () Trendline through the data with dissolution potential +1.0 V; () trendline through the

data with dissolution potential 0 V. [Ammonium thiocyanate] = 1 M. Sensors: NPL-type sensors, D2 carbon (new batch). NPL electrode arrangement.

69

peak height /µA = (1.09±0.04)([silver nitrate] /µM) - (1.9±1.0)

R2 = 0.98

peak height /µA = (0.51±0.04)([silver nitrate] /µM) + (3.0±1.0)

R2 = 0.92

-10

10

30

50

0 10 20 30 40

[silver nitrate] /µM

peak

hei

ght /

µA

A

report1/12/2008

peak area /µC = (0.111±0.006)([silver nitrate] /µM)- (0.1±0.1)

R2 = 0.97

peak area /µC = (0.061±0.004)([silver nitrate] /µM) + (0.3±0.1)

R2 = 0.94

-1

0

1

2

3

4

5

0 10 20 30 40

[silver nitrate] /µM

peak

are

a /µ

C

B

report1/12/2008

Fig. 3.20 Effect of dissolution (step 1) potential on the steps and sweep measurement of silver nitrate.

Dissolution (step 1) potential /V: (, ∆) 0; and (, ) +1.0. Measurement date: (, ) 1 Dec. 2008; and (∆, ) repeated on the same day.

() Trendline through the data with dissolution potential +1.0 V; () trendline through the data with dissolution potential 0 V. [Ammonium thiocyanate] = 1 M. Sensors: NPL-type sensors, D2 carbon (new batch). NPL electrode arrangement.

70

The data in Fig. 3.19 and 3.20 were used to estimate the effective silver concentration

of silver in the stock sol (as supplied by BBI). The stock sol was equivalent to 1000

µl ml-1. Using this figure together with the regression slope (0.053 ± 0.003 µA µl-1 ml)

indicated in Fig. 3.19A gives a resultant silver stripping signal of 53 ± 3 µA. Dividing

this result by the regression slope (1.09 ± 0.04 µA µM-1) indicated for silver nitrate

under similar measurement conditions yielded an effective silver concentration of 49 ±

5 µM. A similar calculation with the peak area data yielded 50 ± 4 µM.

71

3.3.7 Comparison of alternative electrode arrangement as to the effect of

the dissolution (step 1) potential

NPL habitually connected sensors so that they were arranged as working, reference,

counter (left to right, Fig. 3.21). The effect of using an alternative arrangement

(reference, working, counter) was investigated with the usual steps and sweep

parameters and changing step1 potential:

no pretreatment;

step 1, in the range from +0.1 to +1.3 V for 15 s with a 150 ms time interval, Table

3.12;

step 2, -1.6 V for 5 s with a 100 ms time interval;

step 3, -1.2 V for 55 s with a 100 ms time interval; and

linear sweep from -1.2 to +0.5 V at scan rate 1 V s-1 with potential step 10 mV.

Each sensor was used for just one measurement.

Measurements were carried out by mixing 16 µl silver sol stock solution and 4 µl

ammonium thiocyanate stock solution using the drop method.

The dissolution potential dependences of the peak height and peak area using the two

different electrode arrangements are shown, Fig. 3.22.

72

Fig. 3.21 Scanned image of an NPL-type sensor. NPL electrode arrangement (from left to right): working, reference and counter. Alternative electrode arrangement (from left to right): reference, working and counter.

order step 1

potential /V vs. C

date order step 1 potential /V vs. C

date

1 +0.100 14 Nov 7 +0.625 19 Nov 2 +0.200 14 Nov 8 +0.650 19 Nov 3 +0.300 14 Nov 9 +0.700 19 Nov 4 +0.400 14 Nov 10 +0.800 19 Nov 5 +0.500 14 Nov 11 +1.000 19 Nov 6 +0.550 14 Nov 12 +1.200 19 Nov 7 +0.600 14 Nov 8 +0.700 14 Nov 13 +0.100 20 Nov 9 +0.800 14 Nov 14 +0.350 20 Nov 10 +0.900 14 Nov 15 +0.500 20 Nov 11 +1.000 14 Nov 16 +0.550 20 Nov 12 +1.300 14 Nov 17 +0.575 20 Nov 18 +0.600 20 Nov 1 +0.100 19 Nov 19 +0.625 20 Nov 2 +0.350 19 Nov 20 +0.650 20 Nov 3 +0.500 19 Nov 21 +0.700 20 Nov 4 +0.550 19 Nov 22 +0.800 20 Nov 5 +0.575 19 Nov 23 +1.000 20 Nov 6 +0.600 19 Nov 24 +1.200 20 Nov

report 14/11/2008 (P141) Table 3.12 Sequence of step 1 potentials used when comparing NPL and alternative electrode

arrangements for using NPL-type sensors. Order indicates the order in which the measurements were made. At each step 1 potential, two measurements were made – one using a sensor connected in the NPL arrangement and one using a sensor connected using the alternative arrangement.

73

0

10

20

30

40

50

60

0.1 0.4 0.7 1 1.3

step 1 potential /V vs C

peak

hei

ght /

µA

A

report 14/11/2008 (P142)

0

2

4

6

8

0.1 0.4 0.7 1 1.3

step 1 potential /V vs C

peak

are

a /µ

C

B

report 14/11/2008 (P142)

Fig. 3.22 Steps and sweep measurement of silver sol as a function of dissolution (step 1) potential and

electrode arrangement. (A) Peak height; and (B) peak area.

Electrode arrangement: (∆◊) NPL; and ( ) alternative. Measurement date: () 14 Nov; (∆) 19 Nov; and (◊ ) 20 Nov. Points indicate measured data whereas the curves indicate lines of best fit (tanh approximation). [Silver sol] = 800 µl ml-1; [ammonium thiocyanate] = 1 M. NPL-type sensors, D2 carbon (new batch). [cont’d.

74

0

2

4

6

8

0.1 0.4 0.7 1 1.3

step 1 potential /V vs C

curre

nt d

ensi

ty /µ

A m

m-2

C

report 14/11/2008 (P143)

0

0.2

0.4

0.6

0.8

1

0.1 0.4 0.7 1 1.3

step 1 potential /V vs C

char

ge d

ensi

ty /µ

C m

m-2

D

report 14/11/2008 (P143)

Fig. 3.22 Steps and sweep measurement of silver sol as a function of the dissolution (step 1) potential

and electrode arrangement…cont’d.] (C) Current density (peak height divided by the working electrode area). (D) Charge density (peak area divided by the working electrode area). Electrode arrangement: (∆◊) NPL; and ( ) alternative. Measurement date: () 14 Nov; (∆) 19 Nov; and (◊ ) 20 Nov. Points indicate measured data whereas the curves indicate lines of best fit (tanh approximation). [Silver sol] = 800 µl ml-1; [ammonium thiocyanate] = 1 M. NPL-type sensors, D2 carbon (new batch).

75

The signals from sensors connected in the NPL arrangement were generally bigger

than signals from sensors connected using the alternative arrangement, Fig. 3.22A and

B. This can be attributed to the fact that the geometric area of the working electrode

was bigger in the NPL as opposed to the alternate arrangement (Table 3.2). When this

factor was taken into account, the signal densities (current per unit area and charge per

unit area) were generally larger using the alternative electrode arrangement, Fig. 3.22C

and D. In either electrode arrangement, the switching potential was similar, Table

3.13. However, the step changes in signal were sharper when the sensors were

connected using the alternative arrangement – probably because the reference potential

was less influenced by the current flowing in the alternate arrangement.

The alternative electrode arrangement would be the preferred option for future use

since: (a) the working electrode area is likely to be more reproducible and (b) the

signal for a given working electrode area was higher.

76

sensor type carbon type

electrode arrangement

working electrode

area

/mm2 before step after step before step after step before step after step before step after step peak height peak area

NPL(new) D2 NPL 9.90 ± 0.13 8.76 44.0 0.84 5.42 0.88 4.41 0.084 0.54 +653 +643NPL(new) D2 alternate 5.84 ± 0.06 2.59 39.8 0.50 4.71 0.43 6.68 0.084 0.79 +646 +638NPL(new)* D2 NPL 9.90 ± 0.13 5.51 33.3 0.56 4.40 0.55 3.34 0.056 0.44 +417 +456LIRANS* D14 LIRANS 1.89 ± 0.12 2.21 14.1 0.29 2.04 1.17 7.47 0.150 1.08 +761 +766NPL+ D2 NPL 9.90 ± 0.13 6.27 44.7 0.82 5.70 0.63 4.52 0.083 0.58 +633 +640

20090429 Yuanyang table

switch potential /mV vs C

fitted current /µA

fitted charge /µC

fitted current density /µA mm-2

fitted charge density /µC mm-2

Table 3.13 Estimated switch potentials for NPL and alternate arrangement of electrodes on NPL-type sensors. Indicated potentials arrived at using SOLVER function in Microsoft Excel to fit tanh approximation to measured data (Fig. 3.22). * Data from Table 3.9, for comparison. + Data from Fig. 3.18, for comparison.

77

CHAPTER 4 SUMMARY AND CONCLUSIONS

Screen-printed devices incorporating carbon working, reference and counter electrodes

were investigated with respect to electrochemical measurements of silver sols. The

work was based on a measurement procedure proposed by collaborators at the National

Physical Laboratory, Teddington. A steps and sweeps method was used as follows:

step 1 an oxidation step to promote dissolution of sol so as to produce silver ions

at the working electrode interface;

step 2 a ‘nucleation’ step to initiate deposition of silver on to the working

electrode;

step 3 an accumulation step to collect a measurable quantity of silver on the

working electrode; and

stripping using linear sweep voltammetry to remove deposited silver from the

working electrode and produce a stripping signal that could be used as an

indication of the concentration of silver sol in the test solution.

4.1 Electrochemistry of silver ions on carbon electrodes

Silver nitrate, rather than silver sol, was used for initial work and cyclic voltammetry

was carried out on two different sensor designs (LIRANS and NPL-type sensors,

section 2.3) printed using two different carbon inks (D2 and D14, Gwent Electronic

Materials Ltd.). Deposition of silver was observed on both types of device. The

78

silver stripping peak potentials were not very reproducible (-406 to -313 mV vs. carbon

pseudo-references; Table 3.4) but there was clearly a difference in the peak current

densities (cf. 4.68, 4.26, 4.39 and 4.08 µA mm-2 for the D2 carbon on NPL-type

sensors with 6.69 and 5.98 µA mm-2 for the D14 carbon on LIRANS-type devices).

It would be necessary to print and test the two types of sensor with both types of

carbon ink in order to ascertain whether the type of ink was more important than the

electrode design with respect to current density – or whether both factors were

important (section 3.2.1).

4.2 Measurement of silver sols

Cyclic voltammetry was also used successfully to observe deposition and stripping of

silver from test solutions containing silver sol particles. The process was believed to

depend on dissolution of metallic silver by application of a sufficiently oxidising

potential to the working electrode before deposition and subsequent stripping (section

3.2.2). With hindsight this might usefully have been demonstrated by recording

cyclic voltammograms where the start potential was insufficient to produce silver ions

(e.g. 0 V) – in which case it would be expected that no silver deposition and no silver

stripping would be observed.

4.2.1 Steps and sweep measurement of silver sols

79

Much of the work focused on use of the steps and sweep approach to measurement of

silver sols. Three different methods of presenting samples to the sensors were

investigated. The first, immersion of sensors into bulk test sols, was not very

practical due to the expense and limited amount of sol that was available. The other

two methods – drop from bulk, and drop – both appeared to give silver stripping peaks

that were linearly related to sol concentration, Fig. 3.10 – 3.13. The drop method,

whereby test sols were mixed in situ over the electrodes, was particularly convenient

for preparing test samples with high sol concentrations (≥200 µl ml-1) – whereas the

drop from bulk measurement approach was useful for preparing low sol concentrations

(10 – 200 µl ml-1). When using a 20 µl drop, the limit of detection was estimated to

be 30 µl ml-1 (calculated from three-times the standard deviation of the linear

regression intercept divided by the regression slope), section 3.3.1.2. In preliminary

work, however, it was noted that bulk measurements gave clearly larger silver

stripping signals than did the drop method (section 3.3.1.1). It would be interesting to

do more work to both characterize and understand the effects of sample size.

4.2.2 Effect of step 1 (dissolution) potential on measurement of silver sols

The step 1 potential clearly had a significant effect on the steps and sweep

measurement of silver sols. The stripping signals were correlated with silver sol

concentration when the potential was sufficiently oxidizing. Conversely, when the

step1 potential was insufficiently oxidizing then sol particles were not turned into

silver ions that could be accumulated on the working electrode surfaces as silver metal

– and consequently the stripping signal was independent of sol concentration, Fig.

3.19.

80

The switch potential was not very reproducible between experiments. Table 3.13

shows values in the range +417 to +653 mV for the NPL-type sensors, but there were

insufficient data to be confident that the switch potential was significantly different

from the LIRANS-type sensors (+761 and +766 mV, Table 3.13). More work would be

necessary to identify whether or not the carbon pseudo-reference electrode potential

was sufficiently stable to provide sensors with reproducible signals.

4.2.3 Silver contamination

It was surprising to observe that substantial stripping signals were obtained using step

1 potentials below the switch potential – and that the signal was independent of the

silver sol concentration, Fig. 3.19. It was concluded that the observed signals were

the result of the carbon working electrodes being contaminated during the fabrication

process. This might be demonstrated by making stripping measurements in the

absence of silver sol. The contamination hypothesis is also consistent with data

comparing the effect of step 1 potential on measurements of silver sol and of the

supernatant obtained after centrifuging silver sol. The supernatant gave signals that

were largely independent of the step 1 potential – and of a similar size to the silver sol

signals at step 1 potentials less than the switch potential, Fig. 3.18.

Use of silver and silver/silver chloride inks is commonplace in sensor screen-printing

but it would be useful to produce sensors taking special care to avoid contamination to

see if background silver peaks could be reduced or eliminated.

81

4.2.4 Other aspects

The effect of the step 1 potential on measurements of silver nitrate solution was

unexpected. Comparisons showed that stripping signals were smaller when the step 1

potential was below, rather than above, the switch potential, Fig. 3.20. It was

expected that with silver nitrate solution the step 1 potential would be largely irrelevant

since its purpose was to generate silver ions from silver sol particles. It would be

reassuring to further investigate the observed effect so as to identify its cause.

4.2.5 Electrode arrangement/sensor format

Two sensor formats (LIRANS and NPL-types) were compared. In addition to the

shape of the electrodes (working, reference and counter) there were also differences in

the carbon inks and the substrate materials – so it was difficult to make many definite

conclusions. Nevertheless, the NPL (rectangular) format was preferred to the

LIRANS (circular) format (Fig. 2.4) since the LIRANS format requires better

registration of the carbon and dielectric layers in order to obtain reproducible

electrodes. With respect to the NPL-type sensor, it is recommended that the central

carbon electrode be used as the working electrode for the following reasons:

(a) Its area is necessarily more reproducible than either of the two side electrodes –

the areas of which are sensitive to registration between the two printed layers

(section 3.1). (Alternatively, the window in the dielectric layer could be widened

so that the size of the edge electrodes was not effected by reasonable uncertainties

in the registration process)

82

(b) Silver stripping signals were bigger and the switch potentials were ‘sharper’

when the central carbon electrode was used as the working electrode (Fig. 3.22C

and D).

83

References

Akturk A. (2007) Terahertz current oscillations in single-walled zigzag carbon

nanotubes. Physical Review Letters 98, 166803. Abstract only accessed via journal

website DOI: 10.1103/physRevlett.98.166803.

Anon (2007) New flexible plastic solar panels are inexpensive and easy to make.

http://www.sciencedaily.com/releases/2007/07/070719011151.htm accessed October

2009.

Anon (2009a) http://en.wikipedia.org/wiki/Cyclic_voltammetry accessed October

2009.

Anon (2009b)

http://www.cartage.org.lb/en/themes/sciences/Chemistry/Electrochemis/Electrochemi

cal/CyclicVoltammetry/CyclicVoltammetry.htm accessed October 2009.

Anon (2009c) Cyclic Voltammetry: The Investigation Of Electrolysis Mechanisms.

http://www.ceb.cam.ac.uk/pages/cyclic-voltammetry-the-investigation-of-electrolysis-

mechanisms.html accessed October 2009.

Anon (2009d) Nanoparticle http://en.wikipedia.org/wiki/Nanoparticle accessed

October 2009.

84

Anon (2009e) Nanomaterial http://en.wikipedia.org/wiki/Nanomaterial accessed

October 2009.

Anon (2009f) Bone cement http://en.wikipedia.org/wiki/Bone_cement accessed

October 2009.

Anon (2009g) Silver nanoparticles http://en.wikipedia.org/wiki/Silver_nanoparticles

accessed October 2009.

Anon (2009h) Silver oxide http://www.duracell.com/Procell/chemistries/silver.asp

accessed October 2009.

Anon (2009i) SAMSUNG electronics introduces Korea’s first “silver sterilization

washing machine”

http://www.samsung.com/us/news/newsRead.do?news_group=corporatenews&news_

type=activitynews&news_ctgry=&news_seq=1822&search_keyword=&from_dt=&to

_dt= accessed October 2009.

Anon (2009j) Colloid. http://www.britannica.com/EBchecked/topic/125898/colloid

accessed October 2009.

Anon (2009k) Colloid http://en.wikipedia.org/wiki/Colloid accessed October 2009.

Birch B.J., Forssten C.S., Kabil A. and Porter R.A. (2005) Method. World

Intellectual Property Organization WO 2005/121792 A1, 22 December.

85

Colin B. (2008) Hot nanotube sheets produce music on demand.

http://www.newscientist.com/article/dn15098-hot-nanotube-sheets-produce-music-on-

demand.html accessed October 2009. New Scientists News, 31 October 2008.

Dai X., Wildgoose G.G. and Compton R.G. (2006) Designer electrode interfaces

simultaneously comprising three different metal nanoparticle (Au, Ag, Pd)/carbon

microsphere/carbon nanotube composites: progress towards combinatorial

electrochemistry. Analyst 131, 1241-1247.

Dalton A. B. (2003) Super-tough carbon-nanotube fibres. Nature 423, 703.

Dilleen J.W., Sprules S.D., Birch B.J. and Haggett B.G.D. (1998) Electrochemical

determination of silver in photographic solutions using fixed-volume single-use

sensors. Analyst 123, 2905-2907.

Ding L., Tselev A., Wang J., Yuan D., Chu H., McNicholas T.P., Li Y. and Liu J. (2009)

Selective growth of well-aligned semiconducting single-walled carbon nanotubes.

Nano Letters 9, 800-805.

Fanjul-Bolado P., Hernández-Santos D., González-García M.B. and Costa-García A.

(2007) Alkaline phosphatase-catalyzed silver deposition for electrochemical detection.

Anal. Chem. 79, 5272–5277. Abstract only via Web of Science.

Frederix F., Friedt J.M., Choi K.H., Laureyn W., Campitelli A., Mondelaers D., Maes G.

and Borghs G. (2003) Biosensing based on light absorption of nanoscaled gold and

silver particles. Anal. Chem. 75, 6894–6900. Abstract only via Web of Science.

86

Hunag S.S., Chen Z.G., Li B.F., Lin H.G. and Yu R.Q. (1994) Pre-concentration and

voltammetric measurement of silver (I) with a carbon paste electrode modified with

2,9-dichloro-1,10-phenanthroline–surfactant. Analyst 119, 1859–1862. Abstract

only via Science Direct.

Johnson D.C. and Allen R.E. (1973) Stripping voltammetry with collection at a

rotating ring-disk electrode. Talanta 20, 305. Abstract only via Science Direct.

Kathleen H. (2002) Nanomaterials: it's a small, small world.

http://www.csa.com/discoveryguides/nano/overview.php accessed October 2009.

Lin Y-W., Liu C-W. and Chang H-T. (2009) DNA functionalized gold nanoparticles

for bioanalysis. Analytical Methods 1, 14-24.

Majidi M.R., Asadpour-Zeynali K. and Hosseini S.A. (2007) Preparation and

electrochemical study of modified carbon ceramic electrode with 5-(4-dimethyl amino

benzelyden)-rhodanin for the determination of silver. Electroanalysis 19, 364-369.

Abstract only via Web of Science.

Marquez K., Ortiz R., Schultze J.W., Marquez O.P., Marquez J. and Staikov G. (2003)

In situ FTIR monitoring of Ag and Au electrodeposition on glassy carbon and silicon.

Electrochimica Acta 48, 711-720. Abstract only via Web of Science.

87

Porter R. (2009) Novel electrochemical immunoassay utilising silver nanoparticles.

Oral presentation at Advances in Biosensors for Point of Care Applications Meeting,

National Physical Laboratory, Teddington, 25 February.

Porter R. and Szymanski M. (2009) Electrochemical detection using nanoparticle

labelled antibodies. World Intellectual Property Organization WO 2009/068862 A1,

4 June.

Rajeev G. (2007) Nanoparticles: a review.

http://www.pharmainfo.net/reviews/nanoparticles-review accessed October 2009.

Ramachandra Rao C. N., Kulkarni G.U., Thomasa P.J., Edwards P.P. (2000) Metal

nanoparticles and their assemblies. Chem. Soc. Rev. 29, 27-35.

Simmons T. (2007) Large area-aligned arrays from direct deposition of single-wall

carbon nanotubes. J. Am. Chem. Soc. 129, 10088–10089.

Šljukić B., Baron R., Salter C., Crossley A. and Compton R.G. (2007) Combinatorial

electrochemistry using metal nanoparticles: From proof-of-concept to practical

realisation for bromide detection. Analytica Chimica Acta 590, 67-73.

Švancara I., Kalcher K., Diewald W. and Vytřas K. (1996) Voltammetric

determination of silver at ultratrace levels using a carbon paste electrode with

improved surface characteristics. Electroanalysis 8, 336-342. Abstract only via

Web of Science.

88

Szymanski M. and Porter R. (2009) Silver nanoparticles as an electrochemical

bio-label. Poster presented at Bio-diagnostics Workshop, National Physical

Laboratory, Teddington, 24 February.

Taheri A., Noroozifar M. and Khorasani-Motlagh M. (2009) Investigation of a new

electrochemical cyanide sensor based on Ag nanoparticles embedded in a

three-dimensional sol-gel. Journal of Electroanalytical Chemistry 628, 48-54.

Abstract only via Web of Science.

Troy N.Y. (2007) Beyond batteries: storing power in a sheet of paper.

http://www.eurekalert.org/pub_releases/2007-08/rpi-bbs080907.php accessed October

2009.

Tseng Y-C. (2004) Monolithic integration of carbon nanotube devices with silicon

MOS technology. Nano Letters 4, 123–127.

Won M.S., Moon D.W. and Shim Y.B. (1995) Determination of mercury and silver at

a modified carbon paste electrode containing glyoxal bis(2-hydroxyanil).

Electroanalysis 7, 1171-1176. Abstract only via Web of Science.

Won M.S., Yeom J.S., Yoon J.H., Jeong E.D. and Shim Y.B. (2003) Determination of

Ag(I) ion at a modified carbon paste electrode containing N,N'-diphenyl oxamide.

Bulletin of the Korean Chemical Society 24, 948-952. Abstract only via Web of

Science.

89

Xu J.Z., Zhang Y., Li G.X. and Jun J.J. (2004) An electrochemical biosensor

constructed by nanosized silver particles doped sol-gel film. Symposium on Fusion

of the Medical Treatment and Materials that Support Healthy, Safe Society, Tsukuba,

Japan, 12-13 March. Abstract only via Web of Science.

Zhang M. (2005) Strong, transparent, multifunctional, carbon nanotube Sheets.

Science 309, 1215–1219.

Zhao S., Zhang K., Sun Y.Y. and Sun C.Q. (2006) Hemoglobin/colloidal silver

nanoparticles immobilized in titania sol-gel film on glassy carbon electrode: Direct

electrochemistry and electrocatalysis. Bioelectrochemistry 69, 10-15. Abstract only

via Web of Science.

90

Appendix

Silver colloid EM.SC40

Silver sol data sheet from bbi website.

http://www.buybbi.com/store/uploads/ProdDocs/EMSC40.pdf accessed October 2009.