Determining the Absolute Concentration of Nanoparticles without Calibration Factor by Visualizing the Dynamic Processes of Interfacial Adsorption

Previous approaches of determining the molar concentration of nanoparticles often relied on the calibration factors extracted from standard samples or required prior knowledge regarding the geometry, optical, or chemical properties. In the present work, we proposed an absolute quantification method that determined the molar concentration of nano-objects without any calibration factor or prior knowledge. It was realized by monitoring the dynamic adsorption processes of individual nanoparticles with a high-speed surface plasmon resonance microscopy. In this case, diffusing nano-objects stochastically collided onto an adsorption interface and stayed there (“hit-n-stay” scenario), resulting in a semi-infinite diffusion system. The dynamic processes were analyzed with a theoretical model consisting of Fick’s laws of diffusion and random-walk assumption. The quantification of molar concentration was achieved on the basis of an analytical expression, which involved only physical constants and experimental parameters. By using spherical polystyrene nanoparticles as a model, the present approach provided a molar concentration with excellent accuracy

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

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

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