Dielectrophoretic Assembly of Electrically Functional Microwires from Nanoparticle Suspensions Kevin D. Hermanson, Simon O. Lumsdon, Jacob P. Williams, Eric W. Kaler, and Orlin D. Velev

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• Movie 1
  Typical microwire growth between two electrodes. High magnification (objective 40x) of a microwire growing inside a suspension of 26 nm diameter gold particles at a field intensity of 300 V/cm. The distance between the electrodes is 4 mm and the film has been modified to 10 times the original speed. Note that the growth occurs by particle aggregation at the tip, leaving behind brighter depletion areas.

• Movie 2
  Breaking and spontaneous re-connection of a microwire. This movie shows the breaking and spontaneous re-connection of a microwire formed from 25 nm diameter gold particles in real time speed using a 4x objective. The voltage is increased from 5 to 25 V, whereupon the wire breaks and spontaneously re-connects due to the high field intensity in the gap between the wire ends.

Supplemental Figure 1. Examples of microwire morphology under different conditions.

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Scanning electron microscopy (SEM) images of dried wires obtained by a JEOL JXA 840 microscope. Image (A) shows the tip of a typical porous wire formed from large gold particles (26 nm diameter) in low electrolyte solution (5 x 10^-4 M NaCl). It highlights the high surface area of these wires and the even growth obtained under these conditions. Image (B), however, shows a much more uneven wire formed in the presence of 1 x 10^-3 M NaCl. In this case large aggregates form and get incorporated in the wires due to the suppressed electrostatic repulsion between particles in higher electrolyte solutions. The thin branched wire shown in image (C) and at higher magnification in image (D) is formed from small gold particles (14 nm diameter) in 5 x 10^-4 M NaCl. The wire appears much smoother than that in image (A) because it is formed from aggregation of smaller particles.

Supplemental Figure 2. Schematic of the bridge mode measurements of microwire resistance.

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Before introducing the gold nanoparticles, the cell is filled with electrolyte solution. Switch K₁ is closed, switch K₂ is turned to the left. The bridge is balanced by the variable resistors R₁ and R₂ (100 k each) until the voltmeter V₀ measures the smallest voltage possible. After this, switch K₁ is turned off, nanoparticles are introduced in the gap and a wire is assembled while monitoring the voltage, V₁, and current (A).

To measure the true resistance of the microwire, the cell is flushed with electrolyte again, K₁ is closed and K₂ is switched to the right. The bridge is balanced again by the variable resistor R₃. At this point, R₃ will be exactly equal to the resistance of the microwire, with the effect of electrolyte conductance subtracted. The resistance of R₃ (and the microwire) is measured by the ohmmeter after flipping the switch K₂ to the right. Subsequent changes in the resistance of the microwire are measured by re-balancing the bridge and measuring the resistance of R₃.

Supplemental Figure 3. Examples of directly assembled electrical circuits from gold nanoparticle suspension.

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As an example of wiring wet circuits, a light emitting diode (LED) is wired through a large gap. (A) One of the electrodes of a commercial LED faces a gap filled with a few droplets of gold nanoparticle suspension and covered by a glass plate; (B) When the AC field is turned on, a wire grows through the gap. The moment of complete electrical connection is seen as the LED starts glowing; (C) Because of the self-repairing properties of the wire, when the voltage is increased further causing wire burn-out, the LED flickers, but does not go out (see Fig. 3 from the paper). Eventually, new metallic fibers grow alongside the first microwire, providing more current to the LED and more light emission.

An example of how the method can be used to form, break and re-form microscopic electrical connections on chip is shown in frames (D)-(G) Microwires are grown in the 18 m gaps between three pairs of electrodes (E), burned open by substituting the gold suspension with pure water (F), and then four pairs of wires are re-assembled in gold suspension (G). Such wiring and re-wiring can in principle be used to store bits of information on the chip. Scale bars (A-C) = 3 mm, (D-G) = 30 m.

Supplemental Figure 4. Schematic of the flow chemiresistance measurements.

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A schematic diagram of the on-line flow cell for chemiresistance measurements. The substrates are encapsulated in a small chamber 170 l in volume. The substrate plate with planar metallic electrodes is rendered hydrophobic with dimethylsilane vapor prior to the experiment to ensure even flow of liquid through the chamber. In all cases the analyte sample is flushed through the chamber for 10 minutes at a rate of 0.35 ml/min before resistance measurements are taken by balancing the bridge as explained above. Subsequently, an unbalanced signal is obtained for chemical compounds such as thiols and cyanide that strongly modify the surface properties of the gold nanoparticles.

Supplemental Table 1. Examples of experimental data from microwire chemiresistance experiments.
SAMPLE	ACTION	Vend (V)	Rcomp (k)
Experiment 1 - Thiol
2.5 x 10-4 M NaCl	Flush and balance bridge	0.02
200 l gold suspension	Wire forms at 80 V and 100 Hz	n/a	n/a
2.5 x 10-4 M NaCl	Balance bridge and measure	0.08	20.0
2.5 x 10-4 M thiol		+ 1.42	22.5
2.5 x 10-4 M NaCl		+ 0.15	22.8
Experiment 2 - Cyanide
1.0 x 10-3 M NaCl / NaOH	Flush and balance bridge	0.08
200 l gold suspension	Wire forms at 85 V and 100 Hz	n/a	n/a
1.0 x 10-3 M NaCl / NaOH	Balance bridge and measure	0.05	4.8
500 ppb cyanide at pH 11		+ 2.85	5.1
1.0 x 10-3 M NaCl / NaOH		+ 0.07	5.1
Experiment 3 - Lysozyme
2.5 x 10-4 M NaCl	Flush and balance bridge	0.02
200 l gold suspension	Wire forms at 105 V and 100 Hz	n/a	n/a
2.5 x 10-4 M NaCl	Balance bridge and measure	0.04	12.1
1 mg/ml lysozyme		0.04	12.1
2.5 x 10-4 M NaCl		0.04	12.1

The ionic conductivity of the analytes is measured prior to experiment and adjusted so that it is constant for all samples. The value of V_end in the table denotes the difference between the balanced voltage (V₀) and the voltage measured after 10 minutes of flushing with the sample (V₁). R_comp is the compensated resistance of the wire, measured after the bridge has been balanced.