dr. alan o’riordan -...

www.tyndall.ie/nanotech

Discrete gold nanowires as

electrochemical nanosensor

devices

Dr. Alan O’Riordan

Nanotechnology Group

Tyndall National Institute


Elements of a Nanosensor

Nanosensors

At the nanoscale:

• Sensor elements are of similar length scales to

analytes

• Lower limits of detection

• Increased sensitivity & selectivity


Why Nanosensors

Sensing at the nanoscale:

• Many nanomaterials or nanostructured

devices have unique or enhanced properties

compared similar materials at bulk scale

• Fabrication of devices with improved

performance

– Lower limits of detection

– Increased sensitivity & selectivity

– Faster analysis

– Higher S/N ratios

• Several start up companies in market place

based of semiconducting nanowire – FET

technologies, e.g., Vista Therapeutics


planar diffusion LIMITED

peak shaped response

faster mass transport

shorter response times in analysis

radial diffusion

steady state response

Micro

electrode

Vs.

Why Nanoelectrochemistry?

60 µm

30 µm

1 mM

0

0.530 µm

30 µm

1 mM

0

0.5

60 µm

30 µm

1 mM

0

0.530 µm

30 µm

1 mM

0

0.5

1 mM

0.5

0 mM

250 μm

50 μm

Micro-square Electrode: w = 190 μm

10 μm

Nanowire

electrode

- NW

100 m

+ - - - - -

+ + + +

- +

+

- - +

-

- +


Voltammetry: Cyclic and Square Wave

Time (s)

Pote

nti

al (V

vs

ref)

Square Wave Voltammetry

Time (s)

Pote

nti

al (V

vs

ref)

Cyclic Voltammetry

• Three electrode cell, working counter and reference electrodes

• Source voltage (positive or negative) and measure current

• Sweep or potential step appraoches


Nanowire Fabrication - E-beam lithography

• Nanowire devices fabricated by Hybrid Lithography Process:

• Nanowire structures created by E-beam Lithography and metal deposition:

Ti/Au (5/50 nm)

• Micron-scale interconnections fabricated by optical lithography and metal

deposition: Ti/Au (10/ 200 nm)


PCB packaged nanowires

• Typical NW width ~ 100 ± 6 nm

• Typical NW height ~ 50 ± 0.6 nm

• NW length ~ 40 μm

• Highly reproducible structures

Dawson, K. O'Riordan, A., Journal of Physics: Conference Series 2011, 307 (1), 012004.


Electrical/electrochemcial Characterisation

• Ohmic behaviour and low resistances observed

• Typical voltammogram for a gold nanoelectrode in sulphuric acid

• Low currents consistent with size of electrode

• Nanowires exhibited steady-state voltammetry kinetics measurements

CV in H2SO4


Electron Transfer at Nanowire Electrodes-Oxidation

• Rate determining step is rate of electron transfer between redox analyte and nanowire.

• Range of k0 values determined for different nanowires in 1 mM FcCOOH; average 1.02

0.4 cm/s

• Higher values ~2 orders of magnitude higher than reported values

• Also determined for k0 for oxidation of Ferrocyanide at nanowire; average 0.95

0.19 cm/s

kOX = kET

Fit K0, , imt

K0 = FACk0/imt

diffusion


Electron Transfer at Nanowire Electrodes-Reduction

• Reduction of Ru(NH3)63+ at nanowire electrodes was found to be excellently

described by Butler-Volmer kinetics

• An average k0 value of 1.2 cm/s was determined

• Values consistent with those reported

Dawson, K. O’Riordan, A.; et al. J. Phys. Chem. C 2012, 116 (27), 14665-14673.

Fit K0, , imt


Arrays of Nanowire Electrodes

15 mm300 μm 10 μm 10 μm

(a) (c)(b)

• Single nanowire electrodes 100 nm wide

• Nanowire metal stack: 5 nm/50 nm Ti/Au

• On-chip Au counter electrodes located in the centre of device

• Silicon Nitride passivation layer ~ 500 nm thick

NW


WECEPRE

WE CE

RE

• Unknown peak observed at ~0.1 volts during

forward scan

• Nanowires sufficiently sensitive to detect Ag

ions diffusing from commercial electrode

• Redesign with on-chip quasi-reference

electrode

Diffusing through Ag/AgCl reference

300 μm 10 μm 10 μm

(a) (c)(b)

Dawson, K. Wahl, A.; Barrett, C.; Sassiat, N.; Quinn , A.J.; O’Riordan, A.; . Electrochimica Acta, 2013, submitted


Structural and Electrical Characterisation

• Nanowire dimensions: width 99 ± 3 nm, height ~ 50 nm

• Nanowires and nanowire arrays displayed Ohmic electrical

responses

• Average resistance for single nanowire was ~ 918 ± 55 Ω

• CVs exhibited stead-state sigmoidal behaviour 10,000 mV s-1

• Highly reproducible across different wafers and fabrication

runs. ~8% variation (n = 73 devices)

200 nm


Arrays of Nanowire Electrodes

• Nanowire electrodes arrays (2, 3, 4, 5 and 6 nanowires)

• Nanowire stack structure: 5 nm/50 nm Ti/Au

• Interelectrode distance in arrays ~ 2 μm

• On-chip Au counter electrodes located in the centre of device

• Silicon Nitride passivation layer ~ 500 nm thick

15 mm300 μm 10 μm 10 μm

(a) (c)(b)

NW

300 μm 10 μm 10 μm

(a) (c)(b) (b) (c)

NWs


d

δ

r

Diffusional Independence: Approach using δ

• Nernst diffusion layer is distance to bulk concentration from electrode.

• In arrays neighbouring diffusion profiles should not overlap.

• Estimated thickness of diffusion layer for single nanowire was ~ 850 nm

for w = 100 nm

• Nanowire Arrays designed with 2 μm spacing d > 2δ

Bulk

δ

4


Electrical Characterisation

• 2-point I-V measurements to confirm functionality

• Ohmic behaviour and low resistances observed. Decreasing resistance with increasing numbers of nanowires

• Resistance decreasing linearly as expected.

• Low currents consistent with size of electrode

6 NWs

1 NW


• Voltammetric characterization for nanowire electrode arrays with increasing number of nanowire elements in 1 mM FcCOOH in 10 mM PBS at 5 mV s-1.

• Increase in the average steady state current values (n=30 per array type) recorded at 5 mV s-1 for increasing numbers of nanowires in arrays (navy data point).

• Expected current values for these nanowire arrays extrapolated from the single nanowire current average are included in grey diffusional overlap ?

Electrochemical Characterisation

Dawson, K; Wahl, A.; Barrett, C.; Sassiat, N.; Quinn, A.J., O’Riordan, A.; Electrochimica Acta, 2012, published online 10.1016/j.electacta.2012.09.105


• Representation of a plane normal to the electrodes

• Boundaries 1, 3 & 19 : the bulk concentration, C*

• Boundaries 2, 8, 13 & 18 : flux = 0

• No conditions are applicable at boundaries 5, 10 & 15

• Boundaries 4, 6, 7, 9, 11, 12, 14, 16, & 17 : concentration at the

electrode surface with respect to the time step of the electrolysis

Diffusion Domain Approach

CR(t) =

(a)

1

3

19

1617

15141312 18

11

10987

6

542

(b)


slow scan rates,

closer electrodes

slow scan rates, well

separated electrodes

fast scan rates, well

separated electrodes

slow scan rates,

much closer electrodes

δ = diffusion layer thickness

r = critical dimension

d = interelectrode distance

(i) δ << r planar diffusion

insulating

substrate

nanowire

electrode δ

d r

(iv) δ >> d planar diffusion

(iii) r < δ ≤ d planar vs. radial

diffusion

(ii) δ = r < d radial diffusion

Independent

Overlap

Diffusion Processes at Nanowire Electrode Arrays


Sweep Voltammetry: Diffusional Independence

• Target very high scan rates 5000 mV.s-1

• Enables rapid analysis and data capture

• Increase in faradaic peak current, ip, of 5.26 nA for 15 m spaced arrays compared with

3.12 nA for the 5 m spaced arrays, an increase of ~60%.

• Average steady-state current for arrays 5 m & 10 m is 5.9 ± 0.2 & 6.2 ± 0.3 nA, for

arrays 15 m & 20 m 7.0 ± 0.5 & 6.9 ± 0.3 nA, respectively

5 μm

(a)

5 μm

(b)

1 mM

0

0.5

5 μm

(c)

5 μm

(d)

60 µm

30 µm

1 mM

0

0.530 µm

30 µm

Wahl, A.; Dawson, K.; O’Riordan, A . Faraday Discussions 2013, submitted

ip


SWV: Diffusional Overlap Required

• (i) SWV of FcCOOH at a single

nanowire

• (ii) SWV of FcCOOH at a

nanowire array electrode (3

nanowires) diffusionally

overlapped

• (iii) SWV of FcCOOH at a

nanowire array electrode (3

nanowires) diffusionally

independent

Time (s)

Pote

nti

al (V

vs

ref)

(i)

(iii)

(ii)

(i)

(iii)

(ii)

(b)

(a)

5 μm

(a)

5 μm

(b)

1 mM

0

0.5

5 μm

(c)

5 μm

(d)

60 µm

30 µm

1 mM

0

0.530 µm

30 µm

5 μm

(a)

5 μm

(b)

1 mM

0

0.5

5 μm

(c)

5 μm

(d)

60 µm

30 µm

1 mM

0

0.530 µm

30 µm


Key Sensing Areas for Nanowire Sensors

Nanowire Electrodes

Environment:

Trace metals

Bio-sensing:

Glucose

H2O2

Security:

H2O2

DNT

TNT, PETN H2O2


Security: Hydrogen Peroxide Detection

Dawson, K.; Strutwolf, J.; Herzog, G.; Rogers, K.; Arrigan, D.W.M.; Quinn, A.J.;. O’Riordan, A Anal. Chem., 2011, 83, 5535-5540.


Health: Mediated Glucose Detection

• Measurement range : 10 μM – 100 mM Glucose

• LOD~ 3 μM High sensitivity ~ 7.2 mA/mM/cm2

• Blood glucose required detection range: 0.5 mM – 15 mM

• Alternative media e.g. saliva or tears: ~100 μM – 500 μM

Dawson, K.; Baudequin, M.; O’Riordan, A. Analyst, 2011, 136 (21), 4507-4513


Cu2+

SiO2

Cu2+ Cu2+

Cu2+

Cu2+

Cu2+

Cu2+

Cu

SiO2

Cu

Cu2+

Cu

Cu2+

Cu Cu

Cu2+

SiO2

Cu2+

Cu2+

Cu2+

Cu2+

Cu2+

Cu2+

UPD: E = 0.0 V (vs Ag/AgCl) and time

Deposition of a monolayer of Cu onto the gold surface of the

electrode

SV to remove the Cu from the electrode

surface

Au electrode Au electrode Au electrode

Passivation

Interconnection

tracks

+2e-

-2e-

Cu in 0.1M H2SO4

solution

Background subtracted stripping voltammograms of a series of Cu2+ solutions in 0.1M H2SO4 at a gold nanowire electrode for 60s.

(a) (b)

Environmental: Trace Copper Analysis

Wahl A.; Dawson K., Sassiat N.; Quinn A.J.; O’Riordan, A. Journal of Physics: Conference Series, 307, 2011, 012061.


Nitroaromatic species detection

• TNT detection important to homeland security and environmental remediation.

• Faster analysis ~ 30 s per measurement

• Does not require complex pre-treatment or experimental setup

• Detection limits sub 200 ng/ml (at present)

-1.0 -0.8 -0.6 -0.4

Potential (V vs On-chip Pt)

Cu

rren

t

3 NT

2,4 DNT

2,6 DNT

TNT

DNB

2,4 DNT 2,6 DNT

NT

DNB

CH3

TNT

Barry, S.; Dawson, K.; O’Riordan, A . Faraday Discussions 2013, submitted


Simulation - Design for Purpose

Square Wave Voltammetry

1

7

Linear sweep voltammetry

Time (s)

Pote

nti

al (V

vs

ref)

Time (s)

Pote

nti

al (V

vs

ref)

Ox Red Ox Red

Time (s)

Pote

nti

al (V

vs

ref)


Post Docs Students

Research Team

Pierre Lovera Micki Mitchell Karen Dawson Daniel Jones Amelié Wahl Sean Barry Colm Barrett

Phast-ID FP7-ICT-2009-5 (STREP) Robust, affordable photonic crystal sensors for point-of-

care disease diagnostics

CommonSense FP7-SEC-2010-1 (STREP) Development of a Common Sensor Platform for the Detection of IED "Bomb Factories”

E-Brains FP7-ICT-2009-5 (STREP) Best-Reliable Ambient Intelligent Nanosensor Systems by

Heterogeneous Integration

NanoFunction FP7-ICT-2009-5 (NOE) Beyond CMOS nanodevices for adding functionalities to

CMOS

SFI/ 09/RFP/CAP2455

SFI/12/TIDA/I2377

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