Gold Nanoparticles

Commercially available AuNRs (A12-25-750, Nanopartz, Loveland, CO, USA) suspended in water and coated with cetyltrimethylammounium bromide (CTAB) were functionalized with MUTAB, a cationic ligand that covalently attaches to the gold surface via a sulfur bond. Excess CTAB was removed by centrifugation of the stock solution for 10 min at 7500 rpm. The AuNRs were then resuspended in 1 mg/mL MUTAB in Millipore H₂O (>1 MΩ) and placed overnight in a water bath at 35 °C. Excess MUTAB was removed by centrifugation at 7500 rpm for 10 min and replaced with Millipore H₂O for storage. We estimate that the final concentration of free MUTAB in solution after this centrifugation step was less than 0.01 mg/mL. The MUTAB-AuNR solutions were positively charged (ζ = 35 ± 5 mV) at pH 7.2 according to zeta-potential measurements (Malvern Zen 3600). Further analyses of the MUTAB-AuNRs in the buffer conditions of the experiments (20 mM HEPES, 20 mM NaCl in Molecular Biology Grade H₂O) by UV/vis spectroscopy (Figures S17 and S18) and by transmission electron microscopy (TEM) (Figure S19) are included in the Supporting Information.

Commercially available citrate-coated gold nanospheres (AuNPs) with a nominal diameter of 50 nm were purchased from BBI solutions (Cardiff, U.K.). Sizing performed by the manufacturer using TEM showed that their actual size is 48 ± 4 nm (Batch # 16659, ∼75 pM concentration based on the mass of gold per milliliter used for the synthesis). For the control experiments shown in Figures S7–S9, citrate-AuNPs were functionalized with MUTAB using the same procedure as described above for the AuNRs. The zeta-potential of the AuNPs went from negatively charged (ζ = −35 ± 5 mV) in the presence of citrate, to positively charged once functionalized with MUTAB (ζ = 40 ± 5 mV), confirming the ligand exchange reaction occurred.

Gold nanowires (Au nanowires, diameter ∼70 nm, length 1–10 μm) were synthesized using a modified three-step seeding synthesis originally developed for AuNRs.⁷¹ Seed particles were prepared by a rapid reduction of HAuCl₄ (gold precursor) using NaBH₄ (reducing agent). These particles were then grown in a solution containing HAuCl₄, ascorbic acid, and CTAB. The presence of ascorbic acid reduces the gold salt from its Au(III) to a stable Au(I) state. Further addition of seeds to this solution induces an autocatalytic reaction of Au(I) that enlarges the seed particles. While this method would typically yield nanowires with an aspect ratio smaller than 10, aspect ratios larger than 100 can be achieved when the reaction takes place in an acidic environment (pH ∼ 2) and less seeds are added to the growth solution. CTAB on the as-synthesized Au nanowires was replaced with MUTAB. The CTAB-Au nanowires sedimented at the bottom of the vial after 1–2 h at room temperature without mixing or shaking. Carefully, CTAB was removed out of solution without removing the nanowires. An equal volume of MUTAB (1 mg/mL in Millipore H₂O) was added to fill the volume left by removed CTAB. The solution was mixed using a micropipette and placed overnight in a water bath at 35 °C. The MUTAB-Au nanowires were then resuspended in solution by gently shaking the vial, and then left untouched for 1–2 h at room temperature to allow sedimentation. Finally, excess MUTAB was removed out of solution (carefully without removing the nanowires), and an equal volume of Millipore H₂O was added to fill the volume left by excess MUTAB.