## 3 Methods  
### Direct EC Detection of AuNPs on the SPCE  
We utilized two pulsed voltammetric techniques, square wave voltammetry (SWV), and DPV, for our analysis. SWV is a special type of DPV [27]. We utilized SWV to identify improved parameters for AuNP size and deposition time. Utilizing these parameters, we performed the subsequent analysis using DPV. A volume of 2 uL containing different concentrations (D0 = 1.4 \(\times\) 10\({}^{8}\), D1 = 1.4 \(\times\) 10\({}^{7}\), D2 = 1.4 \(\times\) 10\({}^{6}\) and D3 = 1.4 \(\times\) 10\({}^{5}\) particles per 2 uL) of different-sized AuNPs (diameter = 5, 10, 20, 40 nm) in DI water was drop-cast on the carbon working electrode of the SPCE and dried in a laminar flow hood until completely dry. A volume of 2 uL was chosen because this volume completely covers but does not extend beyond the surface area of the 3 mm working electrode surface. The droplet on the working electrode was observed to wet the whole working electrode surface up to the electrode insulation material of the SPCE. The control consisted of 2 uL of DI water (no AuNPs) used as a sample for EC detection. Scanning electron microscopy (SEM) was used to verify that the dried AuNP coverage of the working electrode was uniform (image not shown). After the AuNP sample was completely dry, 50 uL of 0.1 M HCl in 5 mM PBS (the supporting electrolyte) was added to the SPCE, completely covering the working, counter, and reference electrodes. A constant voltage of 1.25 V was applied for 120 s (deposition time) to oxidize the AuNPs adsorbed on the surface of the SPCE. This was followed by an SWV scan from 1.25 to 0 V with a pulse size of 50 mV and a frequency of 10 Hz. The SWV EC signal was normalized by the total mass of the particles in the 2 uL volume (such that the Au ion concentration for a given dilution was equal for all the AuNP sizes under consideration). AuNPs (in an HCl solution) at a potential of +1.25 V oxidized to AuCl\({}_{4}\)\({}^{-}\). Subsequently, a potential scan from 1.25 to \(-\)0.2 V resulted in the reduction of AuCl4\({}^{-}\) to Au, with a cathodic peak observed at 0.3 V. We hypothesized that smaller AuNPs should yield a higher cathodic peak given the larger proportion of surface atoms vs. core atoms (in comparison with larger AuNPs with a lower surface-area-to-volume ratio) [31]. We then determined the optimal deposition time by drop-casting 2 \(\mu\)L of 20 nm AuNPs (concentration = 1.4 \(\times\) 10\({}^{6}\) particles in 2 \(\mu\)L DI water) on the SPCE and applying a constant voltage of 1.25 V at four different durations (deposition times = 30, 60, 120, 180 s, performed in 50 \(\mu\)L of 0.1 M HCl solution (supporting electrolyte)). We then performed SWV and measured the peak current. The control (blank) consisted of the peak signal from the SPCE when 2 \(\mu\)L DI water (no AuNPs) was drop-casted and subject to different deposition times (30, 60, 120, 180 s, supporting electrolyte = 50 \(\mu\)L of 0.1 M HCl). The rest of the EC parameters were the same as noted above. DPV was also used for analysis at the optimized AuNP size (20 nm AuNPs) and deposition time (120 s). The DPV settings were as follows: step size = 10 mV, sample period = 0.294 s, pulse size = 50 mV, and pulse time = 0.1 s. DPV was performed at different concentrations of AuNPs in 2 \(\mu\)L of DI water (from D0 = 1.4 \(\times\) 10\({}^{8}\) to D3 = 1.4 \(\times\) 10\({}^{8}\)). The control (blank) consisted of the peak signal when 2 \(\mu\)L of DI water (no AuNPs) was drop-casted on the SPCE (deposition time = 120 s, supporting electrolyte = 50 \(\mu\)L of 0.1 M HCl). For the EC experiments (SWV and DPV), peak current (associated with reducing Au\({}^{3+}\) to Au\({}^{0+}\)) at 0.3 V was measured and recorded. Data were processed using the GraphPad Prism and JMP software (one-way ANOVA, Tukey multiple comparison test). In the figures, an asterisk (*) denotes different probability values as per the GraphPad style (Supporting Information, Table S1).