Redox magnetohydrodynamics (RMHD) microfluidics is coupled with dark-field microscopy (DFM) to offer high-throughput single-nanoparticle (NP) differentiation in situ and operando in a flowing mixture by localized surface plasmon resonance (LSPR) and tracking of NPs. The color of the scattered light allows visualization of the NPs below the diffraction limit. Their Brownian motion in 1-D superimposed on and perpendicular to the RMHD trajectory yields their diffusion coefficients. LSPR and diffusion coefficients provide two orthogonal modalities for characterization where each depends on a particle's material composition, shape, size, and interactions with the surrounding medium. RMHD coupled with DFM was demonstrated on a mixture of 82 ± 9 nm silver and 140 ± 10 nm gold-coated silica nanospheres. The two populations of NPs in the mixture were identified by blue/green and orange/red LSPR and their scattering intensity, respectively, and their sizes were further evaluated based on their diffusion coefficients. RMHD microfluidics facilitates high-throughput analysis by moving the sample solution across the wide field of view absent of physical vibrations within the experimental cell. The well-controlled pumping allows for a continuous, reversible, and uniform flow for precise and simultaneous NP tracking of the Brownian motion. Additionally, the amounts of nanomaterials required for the analysis are minimized due to the elimination of an inlet and outlet. Several hundred individual NPs were differentiated from each other in the mixture flowing in forward and reverse directions. The ability to immediately reverse the flow direction also facilitates re-analysis of the NPs, enabling more precise sizing.
Single particle electrochemical oxidation of poly(vinylpyrrolidone)-capped silver nanoparticles at a microdisk electrode was investigated as a function of particle shape (spheres, cubes, and plates) in potassium nitrate and potassium hydroxide solutions. In potassium nitrate, extreme anodic potentials (≥1500 mV vs. Ag/AgCl (3 M KCl)) were necessary to achieve oxidation, while lower anodic potentials were required in potassium hydroxide (≥900 mV vs. Ag/AgCl (saturated KCl)). Upon oxidation, silver oxide is formed, readily catalyzing water oxidation, producing a spike-step current response. The spike duration for each particle was used to probe effects of particle shape on the oxidation mechanism, and is substantially shorter in nitrate at the large overpotentials than in hydroxide solution. The integration of current spikes indicate initial oxidation to Ag(I) in a mixed-valance complex. In both electrolytes, the rate of silver oxidation strongly depends on silver content of the nanoparticles, rather than the shape-dependent variable–surface area. The step height, which reflects rate of water oxidation, also tracks the silver content more so than shape. Results were compared to those from less-protected citrate-capped particles and suggest that contributions of the polymer capping ligand to kinetic barriers to oxidation are negligible under these conditions.
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