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DOI: 10.1002/adma.200903590

Supporting Information

Gating of Redox Currents at Gold Nanoelectrodes Via DNA Hybridization

By Gang Liu, Chunfeng Sun, Di Li, Shiping Song, Bingwei Mao,* Chunhai Fan,* and

Zhongqun Tian

Experimental section

Reagents

All oligonucleotides were synthesized and purified by Sangon Inc. (Shanghai, China).

Hexaamminerutheniumchloride (Ru(NH3)63+, RuHex), potassium ferricyanide (Fe(CN)6

3-) and

6-mercapto-1-hexanol (MCH) were obtained from Sigma. Tris(hydroxymethyl)aminomethane

was from Cxbio Biotechnology Ltd, and HJ-8105 electrophoretic paint was from Hawking

Corporation LTD. All solutions were prepared with MilliQ water (18 MΩ·cm-1).

Fabrication of gold nanoelectrodes

Gold nanoelectrodes were fabricated from gold wires of 0.25 mm diameter via an

electrochemical etching method as previously reported.[1, 2] The setup for electrochemical

etching is shown in Figure S1. In brief, a gold ring of ~8 mm in diameter made from 1-mm

gold wire was placed onto the surface of the solution. During the etching course the gold ring

was placed on the surface of the etching solution, usually 1/2~3/4 height of the gold ring was

immersed, which served as both the reference and auxiliary electrode. A gold wire of 0.25

mm in diameter (99.99% purity) was first flame-annealed and then immersed in the center of

the ring. We used a TSMV60-1S Vertical Stages from Zolix Instruments Co. to control the

immersing of the gold wire. The length immersed is ~2-3 mm.

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The etching solution was prepared with fuming hydrochloric acid and ethanol (Sigma). The

etching solution consisted of equal amount of hydrochloric acid and ethanol and etching was

performed under a DC voltage of 2.2 V. The fabrication process was automatically stopped

when the etching current decreased to a preset value of 50 µA. The shape of the tip was

examined using a scanning electron microscopy (LEO 1530 VP SEM) operated at an

accelerating voltage of 5 kV and a working distance of 4 mm.

Figure S1. The setup for the electrochemical etching of gold wires.

Figure S2. SEM image for a cone-shaped nanotip electrode without coating of electrophoretic

paint. Inset is an amplified image.

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The gold nano tip was then coated with electrophoretic paint as previously described by

White and coworkers.[2-4] A potentiostat from Sange was employed to supply a constant

voltage (Setup shown in Figure S3). The anode was a steel ring, and the etched gold nanotip

was connected to the cathode. Gold nanotips were coated several times depending on the

utility, and each coating step takes 40 seconds. After each coating step, nanotips were heated

under 105 for 30 min.

Figure S3. The setup for coating with electrophoretic paint.

Electrochemical measurements

All electrochemical measurements were performed with a CHI 650 electrochemical

workstation (CH Instruments Inc., Austin). A conventional three-electrode configuration was

employed all through the experiment, which involved a gold working electrode, a platinum

wire auxiliary electrode, and an Ag/AgCl reference electrode. Cyclic voltammetry (CV) was

conducted at a scan rate of 50 mV/s), and square wave voltammotric (SWV) measurements

were taken at a frequency of 5 Hz. The electrolyte was 25 mM PB containing 5 mM

Fe(CN)63- and 250 mM NaCl. A typical CV curve for ferricyanide at gold nanoelectrodes are

shown in Figure S4.

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-0.3 0.0 0.3 0.6

-2

-1

0

I, n

A

E, V (vs. Ag/AgCl)

Figure S4. A typical CV curve of 5 mM Fe(CN)63- at the bare gold nanoelectrodes.

Electrochemical characterization of nanoelectrodes

Before being used for DNA sensor, each gold nanoelelctrode was characterized in a

solution of 0.1 M KCl with CV. The effective radius (r0) of each electrode was calculated

from the steady-state limiting current by using 10 mM Ru(NH)63+ as a redox probe. Since the

analytical solution for cone-shaped electrodes is not available, we employed a simplified

assumption by regarding electrodes as of hemispherical geometry.

i0=2πnFDC*r0 (1)

where i0 is the steady-state limiting current, C* and D are the bulk concentration (10 mM) and

diffusion coefficient (0.548×10-5 cm2 s-1) of Ru(NH)63+, respectively, n is the number of

electrons transferred per molecule, and F is the Faraday’s constant. In this study, we

employed nanoelectrodes with r0 in the range of 50-150 nm. A note of caution is that r0 is

only used as a guidance which does not reflect the real geometry of the cone-shaped nanotip

due to the simplified assumption of hemispherical geometry.

Fabrication of DNA probe-modified nanoelectrodes and hybridization at the surface

The thiolated ssDNA capture probe (1) were immobilized at gold nanoelectrodes by

incubating the electrodes to the immobilization solution containing 5 µM of (1) and 500 nM

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MCH at room temperature for 2 h. DNA immobilization buffer contains 25 mM sodium

phosphate (pH 7.4), 25 mM NaCl and 50 mM MgCl2.

The (1)-modified nanoelectrodes were immersed in the hybridization buffer containing a

series concentrations of target DNA (2) or (2)-tagged Au NPs for 1 h, which were then rinsed

with buffer. DNA hybridization buffer: 25 mM sodium phosphate (pH 7.4), 25 mM NaCl, and

100 mM MgCl2.

DNA detection at macroelectrodes

Millimeter sized disk electrodes (2 mm in diameter) were employed as a control.

Electrodes were modified DNA probe (1) with a series of surface densities. The modification

protocol was adopted from our previous report,[5] i.e., by elaborate controlling DNA

concentration, ionic strength and incubation time. Subsequently target DNA (2) of 1 µM was

hybridized to the ssDNA probe DNA at the surface. These electrodes were evaluated by using

the ferricyanide probe and with CV and SWV as described in the nanoelectrode part. The

results are shown in Figure S5.

Figure S5. SWV peak currents before and after hybridization with 1 µM target DNA at

macroelectrode, with a series of surface densities of probe (1). A) 1.17×1013 molecular/cm2;

B) 9.5×1012 molecular/cm2; C) 4.7×1012 molecular/cm2; D) 1.78×1012molecular/cm2.

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References

[1] B. Ren, G. Picardi, B. Pettinger, Rev. Sci. Instrum. 2004, 75, 837.

[2] J. J. Watkins, J. Y. Chen, H. S. White, H. D. Abruna, E. Maisonhaute, C. Amatore, Anal.

Chem. 2003, 75, 3962.

[3] J. J. Watkins, B. Zhang, H. S. White, J. Chem. Educ. 2005, 82, 712.

[4] B. Zhang, Y. H. Zhang, H. S. White, Anal. Chem. 2004, 76, 6229.

[5] J. Zhang, S. Song, L. Wang, D. Pan, C. Fan, Nat. Protoc. 2007, 2, 2888.