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pH Sensor using CdSe/ZnSQuantum Dots

P. Kumar, A. Prakash, and S. Maikap ,

Thin Film Nano Tech. Lab., Departmentof Electronic Engineering Chang Gung

University, Tao-Yuan, Taiwan

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Introduction A very important measurement in many liquid chemical

processes (industrial, pharmaceutical, manufacturing, food

production, etc.) is that of pH: the measurement of hydrogen

ion concentration in a liquid solution.

The common pH scale extends from 0 (strong acid) to 14

(strong basic), with 7 (neutral) representing pure water

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pH measurementpH can be measured by

pH strips-not much precise

Colorimetric methods

Potentiometric measurement (Most common )

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Potentiometric electrode

pH measurement usingpotentiometric electrode is basicallymeasuring the voltage producedbetween two electrodes (reference

and glass electrode) immersed inthe liquid solution replaced bycombination pH electrodecontaining both reference and

glass membrane electrode in asingle cylinder

Temperature sensor

Silver (AgCl) wire

Electrolyte

Internal buffer

Refererence element

Junction

Glass membrane

TypicalCombinationElectrode

Voltameter

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Advanced solid state device Chemical sensitive solid state device are of two

types:-

Chemical sensitive field effect transistor o ISFET, ENFET, GASFET, BIOFET etc.

Chemical sensitive capacitor

o EIS (Electrolyte-Insulator-Semiconductor)

potentiostat

Workingelectrode

Back contact

Electrolyte

Si

Insulator

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Advantages of solid state devices

These devices are virtually unbreakable so no risk of

broken glass

Have very short response time

Device is rugged enough to clean sensing surface

Device can be stored dry while glass electrode need to

store in aquous soln to prevent dehydration

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p-type Si wafer

SiO 2 deposition (40nm)

Hydroxilation in piranha soln.

Silanization using PTS

Adsortion of chaperonin

CdSe/ZnS QD SAM

Fabrication

Si Substrate

SiO 2 (40nm)

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AFM image of CdSe QuantumDots array fabricated on EIS structurewith cross section area of 800nmx800nm.

3.9nm

High magnified AFM image (Tappingphase) of Quantum Dot array with scanarea 100 x 100nm showing size rangeof quantum dots is 3-4nm.

Physical Characterization

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Results & Discussion

. . . ..

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.

.

pHpHpHpHpHpH

N o r m a

l i z e

d c a p a c

i t a n c e

Substrate bias voltage

Linear Region

CdSe/ZnS ModifiedEIS Sensor

CV curve of bare EIS sensor (Right) and CdSe/ZnS quantum Dotsmodified EIS sensor (Left) at different pH buffer solutions

. . . .

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Linear Region

Bare EIS sensor

N o r m a

l i z e

d C a p a c

i t a n c e

Substrate Bias Voltage

pHpHpHpHpHpH

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pH Sensitivity

pH V

ysensitivit

Sensitivity of EIS sensor calculated using the equationbelow :-

Comparative sensitivity and linearity of bareSiO 2 and CdSe/ZnS quantum dots modifiedEIS structure

.

.

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.

.

S : mV/pHL : . %

S : . mV/pHL : . %

SiO EIS sensor

CdSe/ZnS QD modified EIS sensor Linear fit of data

B i a s v a r

i a t i o n a

t

l i n e a r r e g

i o n

( v )

pH

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Bare EIS

sensor

QD modifiedEIS sensor

Schematic Mechanism

EFV

H+

H+

H+

H+

H+

H+

H+

Without electrolyte pH 2pH 12

SiO 2SiO 2SiO 2

H+

H+

H+

H+

H+

H+

p-Sip-Sip-Si

- --- -- -- --- -- -

EFEv

Ec

Ec

EFEv

Ec

EFEv

OH-

OH-

OH-

OH-

OH-E

FV

0.76eV1.10eV

Without electrolyte pH 2 pH 12

SiO 2 SiO 2SiO 2

H+

H+

H+

H+H+

H+

H+

H+ H+

H+

H+

H+

H+

H+

p-Si

p-Sip-Si

-- --- -- --- --- -- -

EcEF

Ev

Ec

EFEv

Ec

EFEv

OH-

OH-

H+

H+

H+

OH-

OH-

EC

EV

H+

H+

1.21eV1.75eV

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Concap response

-

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pH

pH

pHpH

pH

pH

pH

pH

pH

V o

l t a

g e

( v )

Time (second)

pH

pH

CdSe/ZnS QDmodified EISstructure;f= Hz

Concap response of CdSe/ZnS Quantum dots modifiedEIS sensor shows the stabilty and repeatability of device

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Conclusion Effect of surface modification on EIS response was investigated bycomparing pH sensitivity of bare EIS sensor and CdSe/ZnS quantumdots modified EIS sensor

pH sensor with SiO 2 structure has sensitivity ~35mv/pH while after modification with CdSe/ZnS quantum dot using chaperonin proteinsensitivity increases upto ~53.30mv/pH

Improvement in pH sensitivity after SiO 2 modification with CdSe/ZnSquantum dots and high surface area of CdSe/ZnS quantum dot arraymay allow to functionalize with biomolecules for biosensor applications.

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Acknowledgement

This work was supported by Chang Gung Hospital,Taoyuan, Taiwan

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Thank you for yourkind attention

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MOSFET to EIS

S

Substrate

Vdr

Vgs

S DS

Insulator coating

Insulator coating

Substrate

Gate oxideReference

Vdr

Vgs

D

potentiostat

Workingelectrode

Back contact

Electrolyte

Si

Insulator

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SiO 2 concap response

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.

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pH

pHpH

pH

pH

pH

pHpH

pH

pH

V o

l t a g e

( V )

Time (Sec.)

pH

- . . . . . . . . . .

. E-

. E-

. E-

. E-

. E-

. E-

. E-

. E-

. E-

. E-

. E-

. E-

pHpH

pHpHpHpH

C _ _ .

c a p

a c

i t a n c e

voltage

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ReferenceMeasuring charge trap occupation andenergy level in CdSe/ZnS quantum dots usinga scanning tunneling microscope

M. R. Hummon, A. J. Stollenwerk, and V. Narayanamurti, P. O. Anikeeva, M. J. Panzer, V. Wood,

and V. Bulovi