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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: [email protected]
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)