Influence of Hydrazine-Induced Aggregation on the Electrochemical Detection of Platinum Nanoparticles

Abstract

To study the catalytic activity of single nanoparticles (NPs) electrochemically, we investigated the applicability of a novel method for nanoparticle detection as a means to immobilize individual NPs. This method consists of analyzing the current steps that can be measured at an ultramicroelectrode (UME) when a colloid of NPs is injected into an electrolyte containing an electroactive species, that is turned over at the NP but not the UME surface. We have measured these current steps for the hydrazine oxidation at Pt NPs landing on a lithographically fabricated Au UME, showing a mean step size comparable to theory and prior measurements. We found a reduced landing frequency with respect to values reported in the literature and those predicted from theory, while the current step distribution showed a long tail of large current steps. This could be explained by the particle aggregation, which would lower the effective NP concentration and therefore lower the landing frequency and would result in higher current steps when aggregates reach the electrode. Cyclic voltammetry (CV) measurements of the Pt-modified Au UME showed a signal characteristic of the presence of Pt, while electron microscopy revealed aggregated NPs, after landings were performed in the presence of hydrazine or hydrogen gas. Conversely, no aggregates were found after particles were injected in absence of such reducing agents, while CV still suggested the presence of Pt, indicating individual particles. The finding, that landing nanoparticles in the presence of hydrazine yields NP aggregates on the surface, means that this particular method is currently not suited for the preparation of individually immobilized particles to facilitate catalysis studies at individual nanoparticles.

Landing and Catalytic Characterization of Individual Nanoparticles on Electrode Surfaces

“…The landing frequency was low, with up to tens of seconds between successive landing events, attributable to a much lowered concentration of free AuNPs in solution due to extensive aggregation. 13 This aggregation was observed qualitatively by the color change of a fairly concentrated AuNP solution upon addition of small amounts of hydrazine from pink to gray, followed by AuNP precipitation. Occasionally, these aggregates blocked the pipet completely, and no landings could be observed.…”

“…Similar discrepancies have been consistently reported before. 3c,d, 13 Although various explanations have been forwarded to account for this discrepancy, the issue is not yet well understood. Finally, it should be noted that at moderately high potentials (between 1.0 and 1.5 V), the landing frequency lies below the average.…”

Addressing Colloidal Stability for Unambiguous Electroanalysis of Single Nanoparticle Impacts

“…This difference was attributed to the ability of the NP impact method to differentiate between irreversible aggregates and reversible agglomerates in solution. 29 Most relevant to the present study, however, is the work of Koper and coworkers, 33 who discovered evidence of aggregation in a NIE system based on electrocatalytic amplification (ECA), a detection strategy first demonstrated by Bard and coworkers. [1][2] The ECA method for sensing individual NP/electrode collisions relies on the detection of a burst of current when a NP strikes an inert UME and catalyzes the oxidation or reduction of active molecules in solution.…”

“…The Pt NPs in this study were synthesized for compatibility with NTA to have a diameter of 50±10 nm as measured by SEM ( Figure S1, Supporting Information). 48 Because N2H4 had already been determined to act as a coagulant for 3-5 nm citrate-capped Pt NPs, 33 we first investigated the effect of sodium phosphate buffer (SPB) on colloidal stability. As shown in Figure 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 time.…”

Electrochemical tracking hydrogen peroxide secretion in live cells based on autocatalytic oxidation reaction of silver nanoparticles

“…The interpretation is as follows. In the beginning, H 2 O 2 removes citrate cap of AgNPs and produces Ag + , which neutralizes the negative charges on AgNPs surface [24]. The colloidal system thus becomes unstable and AgNPs tend to aggregate, leading to the shift and decline of absorption peak [25].…”