Detection of Silver Nanoparticles by Electrochemically Activated Galvanic Exchange

Abstract

Here we report on the seemingly simple process of galvanic exchange (GE) between electrogenerated AuCl₄^- and silver nanoparticles (AgNPs). The results were obtained in the specific context of using AgNPs as labels for bioassays in paper fluidic devices. Results obtained from a combined electrochemistry and microscopy study indicate that the GE process results in recovery of only ∼5% of the total equivalents of Ag present in the system. This low value is a consequence of two factors. First, after an initial fraction of each AgNP undergoes GE, a Au shell forms around the remaining AgNP core preventing further exchange. Second, to simulate a true biological fluid, the experiments were carried out in a Cl^--containing buffer. Consequently, some Ag⁺ formed during GE precipitates as AgCl, and it also serves to block additional GE. Following optimization of the GE process, it was possible to detect AgNP label concentrations as low as 2.6 fM despite these limitations.

Figure 1.

SEM micrograph of the MμB–AgNP conjugate (a) on a carbon-coated Ni grid and (b) dried onto a carbon/Au working electrode prior to electrochemical activation.

Figure 2.

SEM micrographs of MμB–AgNP conjugates imaged on the carbon/Au working electrode after one GE cycle. (a) Low-resolution view. (b, c) Expanded views of the regions highlighted by the red and blue boxes, respectively, in part a.

Figure 3.

(a) TEM micrograph of a portion of a MμB–AgNP conjugate after one GE cycle. The white arc approximates the edge of the MμB; the red circle highlights one associated NP, and the white circle highlights a pair of closely spaced NPs. (b) EDX element map of the NPs in the white circle in part a. It reveals a Ag core (red) surrounded by a Au shell (green).

Figure 4.

(a) SEM micrograph of MμB–AgNP conjugates on the carbon/Au working electrode after oxidation of Au. The orange rectangle highlights salt around the MμB. (b) Element map of the MμB–AgNP conjugates in part a. The blue and green arrows point to the same MμB and Au nodule, respectively, in part a.

Figure 5.

SEM micrographs of MμB–AgNP conjugates after two GE cycles imaged on the carbon/Au working electrode. (a) Low-resolution view. (b, c) Expanded views of the regions highlighted by the red and blue boxes, respectively, in part a.

Figure 6.

Histogram of the average Ag charge measured at 12 different working electrodes after one, two, and three GE cycles. The MμB–AgNP conjugates were dried on the carbon/Au working electrode and rehydrated in 150 μL of BCl to give a final AgNP concentration of 45 fM.

Figure 7.

(a) Anodic stripping voltammograms (ASVs) obtained by drying the MμB–AgNP conjugates onto the carbon/Au working electrode, rehydrating in 150 μL of BCl, and then carrying out the ASV protocol described in the text. The concentrations listed in the legend represent the total concentration of AgNPs in the 150 μL volume. The scan rate was 50 mV/s. (b) Expanded view of the ASVs for the lowest two AgNP concentrations in part a. (c) Plot of charge, determined by integrating the ASVs in part a, as a function of AgNP concentration. Each data point represents five replicate measurements obtained using independently prepared electrodes. Outliers were eliminated using the Grubb’s test with a 95% confidence level. (d) Expanded view of the linear range in part c. The lowest detectable concentration is 2.6 fM AgNPs. The data were obtained using two GE cycles.

Scheme 1