Background

Surface plasmon resonance (SPR) is a technique used to enhance spectroscopic analysis. SPR works by having the electrons on a thin layer of metal resonate with incoming light (Figure 1). Polarized light is shown on the thin layer which causes the electrons to oscillate. When this oscillation of electrons resonates with the wavelength of incoming light, it creates the plasmon. As molecules interact with the SPR, the detection signal will change in intensity allowing for the kinetics of the chemical interaction to be analyzed. This technique to can be useful in biosensing techniques and spectroscopy1.

The creation of the SPR requires the production of a thin-layer of metal on a grated surface. CDs contain a grated surface that work well for SPR. Using a heat curable polymer, a negative replica of the CD can be obtained. A common polymer for this technique is polydimethylsiloxane (PDMS) as shown in Figure 2. PDMS is selected because it is heat curable and not reactive. The CD is placed in the polymer while it is still in the liquid form thus allowing the polymer to form to the CD. As the polymer is heat-cured, it creates the negative of the CD grating as shown in Figure 3.

Once the grating is obtained, the thin-layer of metal needs to be added. Due to the methyl groups on the PDMS, it is a very stable, nonreactive molecule. To coat the PDMS replica with silver, an electroless deposition method was used. Electroless deposition is similar to electroplating but can be done without the need for an external current. It is effective because it can deposit silver on both conductive and nonconductive materials, such as polymers. The goal for this experiment was to create a PDMS replica with a silver nano-layer using Tollen’s reagent, a known electroless method for depositing silver on glass.

The material presented here is based upon work supported by the National Science Foundation under Award No. EEC-0813570 and EEC-1406296. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Electroless Silver Deposition on PDMS SubstrateKarl Goldsmith1, Andrew Hillier2, Michael Johnson2, Russell Mahmood2

1West Des Moines Valley High School, 2Iowa State University

REFERENCES1Bionavis (2015). How Does Surface Plasmon Resonance Work? http://www.bionavis.com/technology/spr/

2Shin-Won Kang (2010). Application of Soft Lithography for Nano Functional Devices, Lithography, Michael Wang (Ed.), ISBN: 978-953-307-064-3, InTech, DOI: 10.5772/8186. Available from: http://www.intechopen.com/books/lithography/application-of-soft-lithography-for-nano-functional-devices

Thickness Analysis using Spectroscopy

The samples were analyzed using spectroscopy to determine the thickness of the silver deposited based on the time allowed for deposition. Samples were then compared to known values of silver thickness on glass. Based on the data, silver was deposited at a rate of 2.7 nm/sec after the nucleation phase.

Microscopy

Samples were analyzed using optical microscopy and scanning electron microscopy. The top images show SEM analysis of a PDMS sample on the edge and in the middle. The bottom images show optical images of samples at ten second intervals of silver deposition.

SPR Analysis

Samples were analyzed via spectroscopy for the presence of a surface plasmon peak. The image shown has a SPR peak at 1400 nm. SPR peaks will shift if the angle of the light is altered. This shift was present in the analysis.

Methods

Three steps were necessary to create the silver coated PDMS samples: pretreating the polymer, creating the Tollen’s reagent, and silver deposition.

Pretreatment:

Once the PDMS was cured, the samples were pretreated for silver deposition. The samples were

rinsed with DI water and acetone. They were then plasma sensitized and soaked in a tin(II) chloride

solution. These last steps make the fairly inert polymer prepared for silver deposition.

Creating the Tollen’s Reagent

The Tollen’s reagent is comprised of two parts: the silver solution and the reducing agent. The silver solution was 0.5M AgNO3 and 0.8M KOH. The reaction produced silver nitrate which was then reacted with enough NH3 to create a diamine silver complex. The reducing agent was a solution of 0.5M dextrose.

Silver Deposition:

To deposit the silver, the silver solution and reducing agent were mixed at a ratio of 50 ml to 15 ml. The pretreated PDMS sample was placed in the solution for 10-60 seconds. During this time, the silver replaced the tin on the PDMS which created nucleation points of silver and enabled the silver to coat the surface of the PDMS.

ACKNOWLEDGEMENT

I would like to thank Dr. Andrew Hillier for allowing me to work in his laboratories. I would also like to thank Michael Johnson and Russell Mahmood for working with me in the design, implementation, and analysis of these experiments. Finally, I would like to thank the CBiRC RET program for giving me this opportunity to work in a professional research setting.

ConclusionsBased on the results of these experiments, silver was successfully deposited on the PDMS. The pre-treatment with plasma and tin (II) chloride sensitizing worked with the Tollen’s reagent to apply a thin layer of silver on the sample. By comparing the samples to standard values, it was determined that the silver was deposited at a rate of 2.7 nm/sec following nucleation. Optimal thickness of silver for an SPR signal is 50 to 100 nm thus requiring a minimum of 33 seconds of deposition. The optical microscope and SEM images show that the sample was coated. Based on the SEM images, there remains impurities on the sample after deposition. The SEM image of the edge of the sample shows that the silver may not adhere well to the PDMS. An SPR peak was shown at 1400 nm and displayed the characteristic shift as the sample was rotated. While the sample was coated with silver, more work should be done to optimize the process. Further work should be done to improve the adhesion of the silver to the PDMS as well as optimizing the SPR signal.

RESULTS

1000 1100 1200 1300 1400 1500 16000

0.2

0.4

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0.8

1

1.2

1.4

SPR-40 second Ag Deposition

0 degrees

4 degrees

6 degrees

Wavelength (nm)

% T

rans

mitt

ance

2AgNO3 (aq) + 2KOH (aq) Ag2O (s) + 2KNO3 (aq) + H2O (l)

Ag2O (s) + 4NH3 (aq) + 2KNO3 (aq) + H2O (l) 2KOH (aq) + 2[Ag(NH3)2]NO3 (aq)

0 10 20 30 40 50 600.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

f(x) = 2.69527365453771 x − 38.8269219752131R² = 0.984359386495367

Silver Thickness on PDMS

Deposition Time (sec)

Thi

ckne

ss (

nm)

0 10 20 30 40 50 60 70 80 90 1000.0001

0.001

0.01

0.1

1f(x) = 1.23463264795647 exp( − 0.0769382700833891 x )R² = 0.998538064832487

Transmission vs Silver Thickness @ 600 nm

Standard Values

Power (Standard Values)

Exponential (Standard Values)

PDMS Samples

Ag Thickness (nm)

Tra

nsm

issi

on

Nucleation Growth

10 sec 20 sec 30 sec 40 sec 50 sec 60 sec