Probing Individual Redox PEGylated Gold Nanoparticles by Electrochemical–Atomic Force Microscopy

Abstract

Electrochemical-atomic force microscopy (AFM-SECM) was used to simultaneously probe the physical and electrochemical properties of individual ~20 nm sized gold nanoparticles functionalized by redox-labeled PEG chains. The redox PEGylated nanoparticles were assembled onto a gold electrode surface, forming a random nanoarray, and interrogated in situ by a combined AFM-SECM nanoelectrode probe. We show that, in this so-called mediator-tethered (Mt) mode, AFM-SECM affords the nanometer resolution required for resolving the position of individual nanoparticles and measuring their size, while simultaneously electrochemically directly contacting the redox-PEG chains they bear. The dual measurement of the size and current response of single nanoparticles uniquely allows the statistical distribution in grafting density of PEG on the nanoparticles to be determined and correlated to the nanoparticle diameter. Moreover, because of its high spatial resolution, Mt/AFM-SECM allows "visualizing" simultaneously but independently the PEG corona and the gold core of individual nanoparticles. Beyond demonstrating the achievement of single-nanoparticle resolution using an electrochemical microscopy technique, the results reported here also pave the way toward using Mt/AFM-SECM for imaging nano-objects bearing any kind of suitably redox-labeled (bio)macromolecules.

Scanning Electrochemical Microscopy of Individual Catalytic Nanoparticles

“…In another novel SECM-type experiment, a conductive atomic force microscopy (AFM) tip was used to measure the size and statistical distribution in grafting density of PEG on the NPs modified with a redoxlabelled ferrocene/polyethylene glycol capping agent. [21] Scanning electrochemical cell microscopy (SECCM) was used for mapping the electrocatalytic activity of the NP ensemble and showed significant differences in activities of similarly sized NPs. [22] Here we employ SECM with a disk-type polished Pt tip to image individual AuNPs and probe ET and HERs at their surfaces.…”

Applications of Scanning Electrochemical Microscopy (SECM) Coupled to Atomic Force Microscopy with Sub-Micrometer Spatial Resolution to the Development and Discovery of Electrocatalysts

“…Thus, the combined AFM-SECM technique using an AFM cantilever with a conductive coating as the tip electrode for SECM has led to recent progress [55][56][57][58][59][60][61][62][63]. A number of approaches have been attempted in order to use the conductive cantilever as the tip electrode, with the objective to confine the active tip area of the AFM cantilever.…”

“…For example, chemical probe molecules were attached to the apex of the tip or to nanoparticles on the substrate to shuttle the electrons between the tip electrode and substrate for the electrochemical reactions (Fig. 8a) [58]. However, this chemical modification method is unsuitable for the evaluation of catalytic reactions using SECM where the chemical probe molecules cannot be used.…”

“…Figs. 8b and c show recent reports in which single nanoparticles (20 nm) were analyzed using the AFM and AFM-SECM techniques [58]. A homemade AFM tip electrode with a diameter of approximately 200 nm was fabricated by melting a gold micro-wire using electric arc heating.…”

Electrochemistry of Metal Nanoparticles and Quantum Dots

“…The scanning electrochemical microscopy technique (SECM), developed by Bard et al [24], uses a tip microelectrode as a local probe providing spatially resolved information on the redox reactivity of surfaces and has been used for measurement on electron transfer dynamics [25] and local deposition of nanoparticles [22,26]. Recent developments involve the use of optical signals of the electrode, namely, surface plasmon resonance [27], and combination with AFM in order to probe individual metal nanoparticles [28]. Spectroelectrochemistry techniques combine spectroscopic and electrochemical methods, thus facilitating the interpretation of electron transfer reactions.…”