Electrochemical Detection and Sizing of Colloidal ZnO Nanoparticles

Particle Impact Electrochemistry

Experiments involving collisions between a single entity and the electrode surface have become an active area of research. The electrochemical contribution of individual nanoparticles (NPs), enzymes, and other entities, such as aggregates or agglomerates, can be determined using particle impact experiments. Destructive nanoimpact experiments of materials, such as Ag, and the electrocatalytic amplification (ECA) are used to detect the NP/electrode interactions. This review covers the seminal work, critical theoretical studies, and some recent applications. The applications to electrocatalysis include measurements of electron transfer rate constants on individual nanoparticles. Applications in analytical chemistry have allowed the detection of nonelectroactive species by detecting the collisions of soft materials, e.g. micellar suspensions and proteins have increased the technique's analytical possibilities. With ECA, NPs can be used as tags for the electrochemical detection of bioanalytes such as DNA, proteins, and liposomes. The theory of ECA collisions, including frequency of collision and the size of the electrochemical current transients, are also covered. For nanoimpacts, the charge measured during a NP electrolysis, such as Ag NP, is used to detect the NP. Measurements of NP diameter are possible, but limitations to this analysis are covered. The electron transfer studies to the electrolysis of Ag and of metal oxides are discussed. Finally, key experimental instrumentations are discussed, including instrumentation techniques for the small currents inherent to single NP measurement. The effect of filtering, instrumentations rise time, and sampling frequency are also covered.

Single Entity Behavior of CdSe Quantum Dot Aggregates During Photoelectrochemical Detection

We demonstrate that colloidal quantum dots of CdSe and CdSe/ZnS are detected during the photooxidation of MeOH, under broad spectrum illumination (250 mW/cm²). The stepwise photocurrent vs. time response corresponds to single entities adsorbing to the Pt electrode surface irreversibly. The adsorption/desorption of the QDs and the nature of the single entities is discussed. In suspensions, the QDs behave differently depending on the solvent used to suspend the materials. For MeOH, CdSe is not as stable as CdSe/ZnS under constant illumination. The photocurrent expected for single QDs is discussed. The value of the observed photocurrents, > 1 pA is due to the formation of agglomerates consistent with the collision frequency and suspension stability. The observed frequency of collisions for the stepwise photocurrents is smaller than the diffusion-limited cases expected for single QDs colliding with the electrode surface. Dynamic light scattering and scanning electron microscopy studies support the detection of aggregates. The results indicate that the ZnS layer on the CdSe/ZnS material facilitates the detection of single entities by increasing the stability of the nanomaterial. The rate of hole transfer from the QD aggregates to MeOH outcompetes the dissolution of the CdSe core under certain conditions of electron injection to the Pt electrode and in colloidal suspensions of CdSe/ZnS.

Semiconducting Nanoparticles: Single Entity Electrochemistry and Photoelectrochemistry

Semiconducting nanoparticles (SC NPs) play vital roles in several emerging technological applications including optoelectronic devices, sensors and catalysts. Recent research focusing on the single entity electrochemistry and photoelectrochemistry of SC NPs is a fascinating field which has attained an increasing interest in recent years. The nano-impact method provides a new avenue of studying electron transfer processes at single particle level and enables the discoveries of intrinsic (photo) electrochemical activities of the SC NPs. Herein, we review the recent research work on the electrochemistry and photoelectrochemistry of single SC NPs via the nano-impact technique. The redox reactions and electrocatalysis of single metal oxide semiconductor (MOS) NPs and chalcogenide quantum dots (QDs) are first discussed. The photoelectrochemistry of single SC NPs such as TiO₂ and ZnO NPs is then summarized. The key findings and challenges under each topic are highlighted and our perspectives on future research directions are provided.

Digital Processing for Single Nanoparticle Electrochemical Transient Measurements

We demonstrate the use of digital frequency analysis in single nanoparticle electrochemical detection. The method uses fast Fourier transforms (FFT) of single entity electrochemical transients and digital filters. These filters effectively remove noise with the Butterworth filter preserving the amplitude of the fundamental processes in comparison with the rectangle filter. Filtering was done in three different types of experiments: single nanoparticle electrocatalytic amplification, photocatalytic amplification, and nanoimpacts of single entities. In the individual nanoparticle stepwise transients, low-pass filters maintain the step height. Furthermore, a Butterworth band-stop filter preserves the peak height in blip transients if the band-stop cutoff frequencies are compatible with the nanoparticle/electrode transient interactions. In hydrazine oxidation by single Au nanoparticles, digital filtering does not complicate the analysis of the step signal because the stepwise change of the particle-by-particle current is preserved with the rectangle, Bessel and Butterworth low pass filters, with the later minimizing time shifts. In the photocurrent single entity transients, we demonstrate resolving a step smaller than the noise. In photoelectrochemical setups, the background processes are stochastic and appear at distinct frequencies that do not necessarily correlate with the detection frequency (f_p), of TiO₂ nanoparticles. This lack of correlation indicates that background signals have their characteristic frequencies and that it is advantageous to perform filtering a posteriori. We also discuss selecting the filtering frequencies based on sampling rates and f_p. In experiments electrolyzing ZnO, that model nanoimpacts, a band-stop filter can remove environmental noise within the sampling spectral region while preserving relevant information on the current transient. We discuss the limits of Bessel and Butterworth filters for resolving consecutive transients.

Correlated Anodic-Cathodic Nanocollision Events Reveal Redox Behaviors of Single Silver Nanoparticles

We reported a novel method to real-time monitor the redox behaviors of single Ag nanoparticles (AgNPs) at a Au ultramicroelectrode between oxidizing and reducing pulse potentials using the nanocollision electrochemical method. At fast pulse potentials, the instantaneous anodic-cathodic current transients of a single AgNP were observed for the electrooxidation of AgNP, followed by the electroreduction of the newborn silver oxide (AgO) NP in alkaline media via switching of redox potentials; however, only anodic oxidation signals of individual AgNPs were observed in neutral solution. Through this study, we have revealed the substantial different dynamic nanocollision electrochemical behaviors of single AgNPs on the electrode surface in various media. Our study offers a new view for clearly clarifying in situ tracking of the electron-transfer process of single NPs by correlating electrochemical oxidation and reduction behaviors with the complementary information.

Characterization of LiCoO2 nanoparticle suspensions by single collision events

Transient electrochemical experiments associated with the collisions between hydrothermally synthesized LiCoO₂ (LCO) nanoparticles/aggregates of different sizes and a polarized gold ultramicroelectrode (UME) were used as a new additive-free analytical tool applied to characterize Li ion insertion compounds.

Digital Single Virus Electrochemical Enzyme-Linked Immunoassay for Ultrasensitive H7N9 Avian Influenza Virus Counting

Electrochemistry has been widely used to explore fundamental properties of single molecules due to its fast response and high specificity. However, the lack of efficient signal amplification strategies and quantitative method limit its clinical application. Here, we proposed a digital single virus electrochemical enzyme-linked immunoassay (digital ELISA) for H7N9 avian influenza virus (H7N9 AIV) counting by integration of digital analysis, bifunctional fluorescence magnetic nanospheres (bi-FMNs) with monolayer gold nanoparticles (Au NPs) modified microelectrode array (MA). Bi-FMNs are fabricated by coimmobilizing polyclonal antibody (pAb) and alkaline phosphatase (ALP). At most, one target will be captured per bi-FMNs by controlling the proportion of bi-FMNs to target concentrations (≥5:1). The introduction of digital analysis can solve signal fluctuation and the reliability of single virus detection, enabling the digital ELISA to be sensitively and accurately applied for H7N9 AIV detection with a low detection limit of 7.8 fg/mL, which is greatly promising in single biomolecular detection, early diagnosis of disease, and practical application.