transparent and flexiblepH sensor

Martti Kaempgenmax-planck-institute for solid state research, stuttgart, germany

group Siegmar Roth

nt06 june 18 – 23, 2006, nagano

outline

I. preparation and performance of transparent and flexible cnt networksII. preparation of transparent and flexible cnt/polyaniline pH sensorsIII. performance

a. optical pH responseb. potentiometric pH response

IV. quick summary

outline

I. preparation and performance of transparent and flexible cnt networksII. preparation of transparent and flexible cnt/polyaniline pH sensorsIII. performance

a. optical pH responseb. potentiometric pH response

IV. quick summary

preparation of cnt suspension

1. raw materialsticky, dusty, maybe toxic,

not soluble.

3. stable cnt suspension

1% sodium dodecyl sulfate (sds)

+ ultrasonic treatment

2. processingsuspending in aqueoussolution of a surfactantwith aid of ultrasound.

surfactant forms Micelles on bothcnt bundles and individual CNTs,yielding in a stable suspension.

45Å

CNTAxis

alignment of sds on cnt surface(C. Richards et al., Science 300, 775, 2003)

strong tendency to bundle(picture from Smalley group, I guess)

preparation of thin cnt networks

1a) adsorption 1b) spraying

CNT Suspension

Air (~ 1bar)

CNT Suspension

Substrate

- thin films only- homogenous covering

- large area- all densities- less homogenous

for very diluted networks

- thin films only- sophisticated process- requires high temperature

=> not suitable fortransparent and flexible substrates

2) cvd growth

1c) removing surfactant

a) simple procedure bydipping sample into pure water…

b) … and blowing dry.

cnt networks for flexible conductive coatings

this particular sample:transmission: T = 98 %4-probe-resistivity :before folding = 20 kOhmafter folding = 70 kOhm

M.Kaempgen et al., Applied Surface Science, 252 (2) 2005, 425.

flexibility test: sheet conductivity remain even after heavy mechanical treatment

note:the multimeter is used only to demonstrate a conductivity, which is in fact a 2 probe measurement.

comparison of cnt material

sheet resistivity vs. transmission

0 10 20 30 40 50 60 70 80 90 100

0,0

1,0

2,0

3,0

4,0

5,0

6,0

full range

~100 µm

~50 µm

ITO

SWCNT

CVD MWCNT

Res

istiv

ity [k

Ohm

]

Transparency [%]

M.Kaempgen et al., Applied Surface Science, 252 (2) 2005, 425.

conclusions • swcnts are favorable over mwcnts• the longer the better (fewer tube/tube contacts)• performance of ito cannot be reached currently

potential applications

EMI FPD TS EMS ESD1

10

100

1000

100000

1000000D

ispl

ays

EMI S

hiel

ding

Elec

tros

tatic

Dis

sipa

tion

Touc

h Sc

reen

son

PETC

atho

de R

ay T

ubes

CNT Coating: T = 90 %

Targ

et R

esis

tivity

[Ohm

/sq]

Applications forTransparent Conductive Coatings

=> cnt networks combine flexibility, transparency, and sufficient conductivity.They already fulfill the requirements for various applications where the high performance of ito is not needed.

potential applications

M.Kaempgen et al., Applied Surface Science, 252 (2) 2005, 425.

also, ito replacement demonstrated successfully in

a) solar cellse.g. A.D. Pasquer et al., APL, 87, 203511, 2005

b) oledse.g. C.M. Aguirre et al., APL, 88, 183104 ,2006

1

10

100

1000

10000

n-type InP Nanowire [7]

Pentacene (thin film) [6]

Individual SWCNT [1]

Thin CNT Network [3]

Rubrene [4]

Pentacene (crystal) [5]

crystalline Silicon [2] M

obili

ty (c

m2 /V

s)

a-Silicon [3]

e.g. transparent flexible transistor

E. Artukovic, M. Kaempgen et al. Nano Lett. 2005, 5, 757.

-30 -20 -10 0 10 20 300

2

4

6

8

10

12

14

16

18

20

Sour

ce-D

rain

Cur

rent

(nA)

Gate Voltage (V)

Before bending During bending After bending

on silicon

flexible transparentcnt network transistor

mobility is competitive to organic TFTsreversible response upon bending

outline

I. preparation and performance of transparent and flexible cnt networksII. preparation of transparent and flexible cnt/polyaniline pH sensorsIII. performance

a. optical pH responseb. potentiometric pH response

IV. quick summary

pure cntO2 (P.G. Collins, 2000)

NH3, NO2 (J.Li, 2003) cnt networksNH2-R (J.Kong, 2001, T)

HO-R (T.Someya, 2003, T)

metal coating (Pd)H2, (J.Kong, 2001)

CH4 (Y.Lu, 2004), cnt networks

non conducting polymerPolyethylenimine/NO2 (P.Qui, 2003, T)

Polyethylenimine/CO2 (A. Star, 2004, T)Nafion/H2O (A. Star, 2004, T)

biomoleculesProteins (R.J.Chen, 2001)

Enzymes, (J.J.Gooding, 2003, …)DNA (C.V. Nguyen, 2002)

conducting polymerPolythiophen, -pyrrol- aniline

polymer coating

immobilizationof biomolecules

Nafion/tyrosinase/phenol (Q.Zhao, 2005)Polyaniline/GOx/Glucose (P.Soundarrajan, 2003)

Polypyrrol/GOx/Glucose (M.Gao, 2003)

sensing materialPolyaniline sulfonic acid/NH3,

(E.Bekyarova,2004) cnt networks

CH4

NH2-RNO2

e--donorsR-OH

e--acceptorsO2

strategies for cnt sensors

cntcnt

H2

polymercoating

Enhanced selectivity through surface modifications:

NH3cnt

The electronic properties of in particular semiconducting CNTs depend strongly on the interaction with the environment (adsorbed molecules, substrate).Sensing is based on interaction with electron donors/acceptors but is not selective for pure CNTs => Surface modification required.

cntpolymercoating

biomolecules

A

B H+

NH3doping

charging

structure of polyaniline

EB

-2H+

-2A-

+2H+

+2A-

-2e-

-2A-

-4H+

+2e-

+2A-

+4H+

LE

ES

PG

+2e-

-2A-

-2e-

+2A-

colourless

green

purple

NNNHNH

n

blue

Reduktion

Oxidation

A- A-

n

HHN+N+NHNH

NHNHNHNH

n

n

Reduktion

Oxidation

NNNN

n

H HNN

+NN+

A-A-

-2H+

-2A-

+2H+

+2A-

pH

structure, colour, and electronic properties strongly depend on pH value.

LE - Leuco-EmeraldinES – Emeraldin SaltEB – Emeraldin BasePG - Pernigraniline

NNN NH H

HHN

+NN N+

H H

HHN

+NN N+

H H

HHNNN N

H H

+2H+-2H+

Emeraldin Base

Emeraldin Salt

Bipolaron

Polaron

Lattice

+ 500 mV

- 300 mV

Blue

quinoid

Green

EMF vs SC

E

device preparation

pure bulk polyaniline is greasy and sticky=> a hardly processable material

how to put polyaniline onto cnts? electrochemical deposition:polymer grows only onto the cnt networks (no deposition on the substrate)

potentiostat

coun

ter e

lect

rode

refe

renc

e el

ectro

de

workingelectrode

thickness of polymer coatingvaries with immersion depth

sem characterization

the function of the cnt network is both, electrical contact and mechanical support.

devices

possible devices:

a) Transparent films for optical pH measurements

b)flexible tip-like shapefor demanded potentiometricpH measurements

in contrast:commercial pH sensors made from glass are big, stiff, brittle and need

more volume of the liquid.cnt/pani-pH-sensor(on a insulated wire)

reference electrode (sce)

pH=1 pH=13

simple preparation: spraying / adsorbing + electrochemical deposition=> use of almost any user defined substrate

outline

I. preparation and performance of transparent and flexible cnt networksII. preparation of transparent and flexible cnt/polyaniline pH sensorsIII. performance

a. optical pH responseb. potentiometric pH response

(linearity, selectivity, reproducibility, signal stability)IV. quick summary

optical pH response

pH=1 pH=13

0 2 4 6 8 10 12 14

0,5

1,0

1,5

2,0

2,5

820 nm

585 nm

Abs

orba

nce

[a.u

.]

pH [-log c(H+)]200 400 600 800 1000 1200

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

pH = 7

Polaron-π∗π-chinoid

π-Polaron

π-π*

pH = 13

Abs

orba

nce

[a.u

.]

Wavelength λ [nm]

pH = 1

UV/VIS Spectra

- linear response only in mild pH regime (pH ≈ 4-10)- but huge hysteresis=> optical pH sensing is not favorable

0 2 4 6 8 10 12 14-400

-200

0

200

400

600

EMF

[mV

vs. S

CE]

pH [-log(H+)]

Polyaniline coated CNT Network

pureCNT Netzwerk

potentiometric pH response: linearity

Slope: ~58 mV/pH

pH response of pure cnt networkdue to functional groups

R

OOH

R

OO--H+

+H+

R OH RO--H+

+H+

RC

O -H+

+H+

H

RC+

O

CNT/Pani-pH-Sensor(on a insulated wire)

Reference Electrode(SCE)

M.Kaempgen et al., Journal of Electroanalytical Chemistry, 586 (1) 2006, 72-76.

→ excellent linearity

potentiometric pH response: selectivity

0,0 0,5 1,0 1,5 2,0 2,5-180-160-140-120-100-80-60-40

K+

Na+

EMF

[mV

vs. S

CE]

Concentration [mol/l]

Li+

possible interfering ions: Li+, Na+, K+, …

Interfering Ion M+

Selectivity coefficientlog (KH+,M+)

Li+ - 10,0

Na+ - 10,4

K+ - 10,5

a value of -10,5 meanssignal for K+ and H+ is equal when

concentration of K+ = 1010.5 x conc. H+

Matched potential method

M.Kaempgen et al., Journal of Electroanalytical Chemistry, 586 (1) 2006, 72-76

→ excellent selectivity

CNT/Pani-pH-Sensor(on a insulated wire)

Reference Electrode(SCE)

potentiometric pH response: reproducibility

0 20 40 60 80 100100

200

300

400

500

600pure CNT-Network

EMF

[mV

vs. S

CE]

Time [s]

pH = 13pH = 10

pH = 7

pH = 4

pH = 2

0 20 40 60 80 100

-200-100

0100200300400500

pH = 13pH = 10

pH = 7

pH = 4

pH = 2

EMF

[mV

vs. S

CE]

Time [s]

Polyaniline coated CNT-Network

referencesample

3x subsequent dipping [pH = 2 →7 →12 → 4 →10]

M.Kaempgen et al., Journal of Electroanalytical Chemistry, 586 (1) 2006, 72-76.

→ excellent reproducibility

CNT/Pani-pH-Sensor(on a insulated wire)

Reference Electrode(SCE)

potentiometric pH response: signal stability

0 100 200 300-400

-200

0

200

400

600

3.9

11.913.211.010.49.57.17.96.34.9

3.22.1

EMF

[mV

vs. S

CE]

Time [s]

pH=1.1

0 100 200 300-400

-200

0

200

400

600

pH = 1

pH = 13

pH = 7

pH = 4

EMF

[mV

vs. S

CE]

Time [h]

pH = 10

measuring 5 min: measuring 5 hours:drift: 2-10 mV/h

M.Kaempgen et al., Journal of Electroanalytical Chemistry, 586 (1) 2006, 72-76.

CNT/Pani-pH-Sensor(on a insulated wire)

Reference Electrode(SCE)

→ higher drift for extreme pH valuesbut still good

conclusioncnt / polyaniline pH sensor

• simple preparation• use of any user defined substrate• excellent performance(linearity, selectivity, stability, reproducibility)

• optical and electronic investigationson various coatings possible

• for demanding applications

0 2 4 6 8 10 12 14

0,5

1,0

1,5

2,0

2,5

820 nm

585 nm

Abs

orba

nce

[a.u

.]

pH [-log c(H+)]

0 2 4 6 8 10 12 14-400

-200

0

200

400

600

EMF

[mV

vs. S

CE]

pH [-log(H+)]

Polyaniline coated CNT Network

pureCNT Netzwerkdomo arigato for attention!

sayonara!

contact: m.kaempgen@fkf.mpg.de

NNNHNH

n

LB

VB

ΔE 2,1 eV»

ΔE 3,9 eV»

NHNHNHNH

n++

LB

VB

ΔE 1,5 eV»

ΔE 2,9 eV»

ΔE 3,5 eV»

200 400 600 800 1000 12000

1

2

3π-chinoidπ-π*

3,9 eV

Abs

orpt

ion

[a.u

.]

Wavelength λ [nm]

Polyanilinat pH=13

2,1 eV

200 400 600 800 1000 1200

0

1

2

3σ-Polaron

π-Polaron

π-π*

2,9 eV3,5 eV

Abs

orpt

ion

[a.u

.]

Wavelength λ [nm]

Polyanilinat pH=1

1,5 eV

EB

-2H+

-2A-

+2H+

+2A-

NHNHNHNH

n

LE

ES

NNNN

n

PG

colourless

-2e-

-2A-

-4H+

+2e-

+2A-

+4H+OxidationReduction

+2e

-2A-

-2e-

+2A-

ReductionOxidation

NNNHNH

n

nA- A-

HHN

+N

+NHNH

LB

VB

ΔE 3,9 eV»

purple

blue

green

not conductive

not conductive

-2H+

-2A-

+2H+

+2A- LB

VB

ΔE 1,5 eV»

ΔE 2,9 eV»

ΔE 3,5 eV»

metallic

not conductive

pH

partially oxidized state

fully oxidized state

NHNHNHNH

n++

pH

4

8

0

6

10

2

12LB

VB

ΔE 2,1 eV»

ΔE 3,9 eV»

LB

VB

ΔE 2,2 eV»

ΔE 3,5 eV»

LB

VB

ΔE 1,5 eV

ΔE 2,9 eVΔE 3,5 eV»

»

»

semi conductive

fully reduced state

200 400 600 800 1000 12000

1

2

3

3,9 eV

10

Polaron-π∗

2,9 eV

1,5 eV

13

867

1pH=

Abs

orba

nce

[a.u

.]

Wavelength λ [nm]

PolyanilinepH = 13 → 1

2,1 eV

π-π*π-Polaron

π-chinoid

NNN NH H

HHN

+NN N+

H H

HHN

+NN N+

H H

HHNNN N

H H

+2H+-2H+

Emeraldin Base

Emeraldin Salt

Bipolaron

Polaron

Lattice

+ 500 mV

- 300 mV

Blue

quinoid

Green

EMF vs SC

E

Strategies for CNT Sensors

NH4/NO3-Sensor (not selective)J. Kong et al.,

Science 287, 622, 2000

a) Surface not modified b) …attached metal cluster c) …single (bio)molecules d) …entirely coated surface

O2

NO2

NH3

H2S

PH3

CH4

H2

Glucose-Sensor (Transistor)K. Besteman et al.,

Nano Lett. 3 (6) 727, 2003

Enzym/pH-SensorR.J. Chen et al.,

J. Am. Che. Soc. 123, 3838, 2001

CO

Methan Sensor (network)Y. Lu et al

Chem. Phys. Lett.391, 344, 2004 Selective NH4/NO3-SensorP, Qui et al.,

Nano Lett. 3 (3) 347, 2003

CNTCNT CNT

Polymer

Enhanced selectivity through …

Interaction with stronge-donators/acceptors

(not selective)

CNT

CO2-Sensor (network)Y. Lu et al

Chem. Phys. Lett.391, 344, 2004

H2-SensorJ. Kong et al.,

Science 287, 622, 2000

Strategies for CNT SensorsThe electronic properties of (in particular semiconducting) CNTs depend strongly on all kinds of interaction with the environment (e.g. adsorbed molecules, surface chemistry of substrate).

CNT

Interaction with stronge-donators/acceptors

(not selective)

J. Kong et al., Science 287, 622, 2000

Gas sensor

Individual CNT

CNT

Individual CNT

NH3, H2S, CO2, PH3, … (donor molecules)

O2, NO2, SO2, … (acceptor molecules)

Y. Lu et al., Chem. Phys. Lett.391, 344, 2004

CNT

H2, CH4, …

Comparison to ITO

UV/VIS Spectra Resistivity vs. Transmission

Surface Resistance for T = 90%• ITO : ~10 Ohm• CNT Network : ~ 1000 Ohm

0 10 20 30 40 50 60 70 80 90 100

0,0

1,0

2,0

3,0

4,0

5,0

6,0

full range

~100 µm

~50 µm

ITO

SWCNT

CVD MWCNT

Res

istiv

ity [k

Ohm

]

Transparency [%]

Transparency in ITO is limited(band edge and plasma edge).

200 400 600 800 1000 1200

0

20

40

60

80

100quarz glas

bgr

86%73%

91%

CNT Networks74% - 100nm

58% - 800nm

ITOTran

smitt

ance

[%]

Wavelength [nm]

Towards a transparent and flexible All CNT Network Transistor

Lithographically defined CNT Network Electrodes

Channel: Thin NetworkSource:

Thick NetworkDrain:

Thick NetworkGate: Thick Network

Line width: 10µm

Gap wide: 5µm (left)20µm (right)