Yimin Fang

Plasmonic imaging of electrochemical oxidation of single nanoparticles.

Measuring electrochemical activities of nanomaterials is critical for creating novel catalysts, for developing ultrasensitive sensors, and for understanding fundamental nanoelectrochemistry. However, traditional electrochemical methods measure a large number of nanoparticles, which wash out the properties of individual nanoparticles. We report here a study of transient electrochemical oxidation of single Ag nanoparticles during collision with an electrode and voltammetry of single nanoparticles immobilized on the electrode using a plasmonic-based electrochemical current microscopy. This technique images both electrochemical reaction and size of the same individual nanoparticle, enabling quantitative examination of size-dependent electrochemical activities at single nanoparticle level. The imaging capability further allows detection of the reaction kinetics of each individual nanoparticle and analysis of the average behaviors of multiple nanoparticles. The average kinetics and size dependence can be accurately described by the Tafel equation, but there is a large variability between different nanoparticles, which underscores the importance of single nanoparticle analysis.

Plasmonic Imaging of Electrochemical Reactions of Single Nanoparticles

Electrochemical reactions are involved in many natural phenomena, and are responsible for various applications, including energy conversion and storage, material processing and protection, and chemical detection and analysis. An electrochemical reaction is accompanied by electron transfer between a chemical species and an electrode. For this reason, it has been studied by measuring current, charge, or related electrical quantities. This approach has led to the development of various electrochemical methods, which have played an essential role in the understanding and applications of electrochemistry. While powerful, most of the traditional methods lack spatial and temporal resolutions desired for studying heterogeneous electrochemical reactions on electrode surfaces and in nanoscale materials. To overcome the limitations, scanning probe microscopes have been invented to map local electrochemical reactions with nanometer resolution. Examples include the scanning electrochemical microscope and scanning electrochemical cell microscope, which directly image local electrochemical reaction current using a scanning electrode or pipet. The use of a scanning probe in these microscopes provides high spatial resolution, but at the expense of temporal resolution and throughput. This Account discusses an alternative approach to study electrochemical reactions. Instead of measuring electron transfer electrically, it detects the accompanying changes in the reactant and product concentrations on the electrode surface optically via surface plasmon resonance (SPR). SPR is highly surface sensitive, and it provides quantitative information on the surface concentrations of reactants and products vs time and electrode potential, from which local reaction kinetics can be analyzed and quantified. The plasmonic approach allows imaging of local electrochemical reactions with high temporal resolution and sensitivity, making it attractive for studying electrochemical reactions in biological systems and nanoscale materials with high throughput. The plasmonic approach has two imaging modes: electrochemical current imaging and interfacial impedance imaging. The former images local electrochemical current associated with electrochemical reactions (faradic current), and the latter maps local interfacial impedance, including nonfaradic contributions (e.g., double layer charging). The plasmonic imaging technique can perform voltammetry (cyclic or square wave) in an analogous manner to the traditional electrochemical methods. It can also be integrated with bright field, dark field, and fluorescence imaging capabilities in one optical setup to provide additional capabilities. To date the plasmonic imaging technique has found various applications, including mapping of heterogeneous surface reactions, analysis of trace substances, detection of catalytic reactions, and measurement of graphene quantum capacitance. The plasmonic and other emerging optical imaging techniques (e.g., dark field and fluorescence microscopy), together with the scanning probe-based electrochemical imaging and single nanoparticle analysis techniques, provide new capabilities for one to study single nanoparticle electrochemistry with unprecedented spatial and temporal resolutions. In this Account, we focus on imaging of electrochemical reactions at single nanoparticles.

Optical Imaging of Phase Transition and Li-Ion Diffusion Kinetics of Single LiCoO2 Nanoparticles During Electrochemical Cycling

Understanding the phase transition and Li-ion diffusion kinetics of Li-ion storage nanomaterials holds promising keys to further improve the cycle life and charge rate of the Li-ion battery. Traditional electrochemical studies were often based on a bulk electrode consisting of billions of electroactive nanoparticles, which washed out the intrinsic heterogeneity among individuals. Here, we employ optical microscopy, termed surface plasmon resonance microscopy (SPRM), to image electrochemical current of single LiCoO2 nanoparticles down to 50 fA during electrochemical cycling, from which the phase transition and Li-ion diffusion kinetics can be quantitatively resolved in a single nanoparticle, in operando and high throughput manner. SPRM maps the refractive index (RI) of single LiCoO2 nanoparticles, which significantly decreases with the gradual extraction of Li-ions, enabling the optical read-out of single nanoparticle electrochemistry. Further scanning electron microscopy characterization of the same batch of nanoparticles led to a bottom-up strategy for studying the structure-activity relationship. As RI is an intrinsic property of any material, the present approach is anticipated to be applicable for versatile kinds of anode and cathode materials, and to facilitate the rational design and optimization toward durable and fast-charging electrode materials.

Detection of charges and molecules with self-assembled nano-oscillators

Detection of a single or small amount of charges and molecules in biologically relevant aqueous solutions is a long-standing goal in analytical science and detection technology. Here we report on self-assembled nano-oscillators for charge and molecular binding detections in aqueous solutions. Each nano-oscillator consists of a nanoparticle linked to a solid surface via a molecular tether. By applying an oscillating electric field normal to the surface, the nanoparticles oscillate, which is detected individually with ∼0.1 nm accuracy by a plasmonic imaging technique. From the oscillation amplitude and phase, the charge of the nanoparticles is determined with a detection limit of ∼0.18 electron charges along with the charge polarity. We further demonstrate the detection of molecular binding with the self-assembled nano-oscillators.

Real‐Time Monitoring of Phosphorylation Kinetics with Self‐Assembled Nano‐oscillators

Phosphorylation is a post-translational modification that is involved in many basic cellular processes and diseases, but is difficult to detect in real time with existing technologies. A label-free detection of phosphorylation is reported in real time with self-assembled nano-oscillators. Each nano-oscillator consists of a gold nanoparticle tethered to a gold surface with a molecular linker. When the nanoparticle is charged, the nano-oscillator can be driven into oscillation with an electric field and detected with a plasmonic imaging approach. The nano-oscillators measure charge change associated with phosphorylation of peptides attached onto a single nanoparticle, allowing us to study the dynamic process of phosphorylation in real time without antibodies down to a few molecules, from which Michaelis and catalytic rate constants are determined.

Digitizing Gold Nanoparticle-Based Colorimetric Assay by Imaging and Counting Single Nanoparticles

Gold colloid changes its color when the internanoparticle distance changes. On the basis of analyte-induced aggregation or disaggregation behavior of gold nanoparticles (AuNPs), versatile colorimetric assays have been developed for measuring various kinds of analytes including proteins, DNA, small molecules, and ions. Traditional read-out signals, which are usually measured by a spectrometer or naked eyes, are based on the averaged extinction properties of a bulk solution containing billions of nanoparticles. Averaged extinction property of a large amount of nanoparticles diminished the contribution from rare events when the analyte concentration was low, thus resulting in limited detection sensitivity. Instead of measuring the averaged optical property from bulk colloid, in the present work, we proposed a digital counterpart of the colorimetric assay by imaging and counting individual AuNPs. This method quantified the analyte concentration with the number percentage of large-sized AuNPs aggregates, which were digitally counted with surface plasmon resonance microscopy (SPRM), a plasmonic imaging technique recently developed by us and other groups. SPRM was able to identify rare AuNPs aggregates despite their small population and greatly improved the detection sensitivity as demonstrated by two model systems based on analyte-induced aggregation and disaggregation, respectively. Furthermore, besides plasmonic AuNPs, SPRM is also suitable for imaging and counting nonplasmonic nanomaterials such as silica and metal oxide with poor extinction properties. It is thus anticipated that the present digitized assay holds a great potential for expanding the colorimetric assay to broad categories of nonplasmonic nanoparticles.

Intermittent photocatalytic activity of single CdS nanoparticles

Significance Semiconductor photocatalysis holds promising keys to address various energy and environmental challenges. While conventional wisdom suggests a continuous photocatalytic reaction under constant light illumination, in the present article we report the discovery of intermittent photocatalytic activity at single CdS nanoparticle level. The observed intermittent photocatalysis is a photochemical consequence of its intrinsic photoexcitation processes. The latter is also responsible for the well-known fluorescence photoblinking of single-semiconductor quantum dots, a photophysical phenomenon that was discovered in the 1990s. The intermittent photocatalysis (a photochemical process) reported here could be an exciting complement of the beautiful picture of semiconductor photophysics and photochemistry, with significant implications in many application fields from clean energy to pollution treatment. Semiconductor photocatalysis holds promising keys to address various energy and environmental challenges. Most studies to date are based on ensemble analysis, which may mask critical photocatalytic kinetics in single nanocatalysts. Here we report a study of imaging photocatalytic hydrogen production of single CdS nanoparticles with a plasmonic microscopy in an in operando manner. Surprisingly, we find that the photocatalytic reaction switches on and off stochastically despite the fact that the illumination is kept constant. The on and off states follow truncated and full-scale power-law distributions in broad time scales spanning 3–4 orders of magnitude, respectively, which can be described with a statistical model involving stochastic reactions rates at multiple active sites. This phenomenon is analogous to fluorescence photoblinking, but the underlying mechanism is different. As individual nanocatalyst represents the elementary photocatalytic platform, the discovery of the intermittent nature of the photocatalysis provides insights into the fundamental photochemistry and photophysics of semiconductor nanomaterials, which is anticipated to substantially benefit broad application fields such as clean energy, pollution treatment, and chemical synthesis.

Simultaneous optical and electrochemical recording of single nanoparticle electrochemistry

Single nanoparticle collisions have become popular for studying the electrochemical activity of single nanoparticles by determining the transient current during stochastic collisions with the electrode surface. However, if only the electrochemical current is measured, it remains challenging to identify and characterize the individual particle that is responsible for a specific current peak in a collision event; this hampers the understanding of the structure–activity relationship. Herein, we report simultaneous optical and electrochemical recording of a single nanoparticle collision; the electrochemical signal corresponds with the activity of a single nanoparticle, and the optical signal reveals the size and location of the same nanoparticle. Consequently, the structure (optical signal)–activity (electrochemical signal) relationship can be elucidated at the single nanoparticle level; this has implications for various applications including batteries, electrocatalysts, and electrochemical sensors. In addition, our previous studies have suggested an optical-to-electrochemical conversion model to independently calculate the electron transfer rate of single nanoparticles from the optical signal. The simultaneous optical and electrochemical recording achieved in the present work enables direct and quantitative validation of the optical-to-electrochemical conversion model.