Photoelectrochemical sandwich immunoassay of brain glycogen phosphorylase based on methyl orange-sensitized TiO2 nanorods.

The photoelectrochemical immunoassay of glycogen phosphorylase BB (GPBB) was studied. A methyl orange/TiO2 nanorod heterojunction was constructed on a fluorine-doped tin oxide electrode by hydrothermal synthesis, calcination, and chemical adsorption. A sandwich immune structure consisting of GPBB as the first antibody, GPBB, and a CdS@mesoporous silica-ascorbic acid (AA)-GPBB as secondary antibody composite was constructed on each of the selected well surfaces of a 96-well microplate. By adding mercaptoethylamine to structurally destroy the secondary antibody composite and release the electron donor AA, the amplification of photocurrent, and thus the "off-on" photoelectrochemical biosensing of GPBB were realized. The use of the 96-well microplate provides good reproducibility of the assembled immune structures and eliminates the possible effect of the photogenerated hole-induced protein oxidation on the photocurrent. The relevant electrodes and materials were characterized by electrochemistry, UV-vis diffuse reflectance spectra, Fourier transform infrared spectroscopy, X-ray diffractometer, scanning electron microscopy/energy dispersive spectroscopy, transmission electron microscopy and BET method. Under the optimal conditions, the photocurrent was linear with the logarithm of GPBB concentration from 0.005 to 200 ng mL-1 and with a limit of detection of 1.7 pg mL-1 (S/N = 3). Satisfactory results were obtained in the analysis of real serum samples. A sandwich immune structure consisting of GPBB first antibody, GPBB, and a CdS@mesoporous silica-ascorbic acid (AA)-GPBB secondary antibody composite was constructed on each of the selected well surfaces of a 96-well microplate. By adding mercaptoethylamine to structurally destroy the secondary antibody composite and release the electron donor AA, the amplification of photocurrent, and thus the "off-on" photoelectrochemical biosensing of GPBB were realized.

Understanding the Dynamic Potential Distribution at the Electrode Interface by Stochastic Collision Electrochemistry.

The potential distribution at the electrode interface is a core factor in electrochemistry, and it is usually treated by the classic Gouy-Chapman-Stern (G-C-S) model. Yet the G-C-S model is not applicable to nanosized particles collision electrochemistry as it describes steady-state electrode potential distribution. Additionally, the effect of single nanoparticles (NPs) on potential should not be neglected because the size of a NP is comparable to that of an electrode. Herein, a theoretical model termed as Metal-Solution-Metal Nanoparticle (M-S-MNP) is proposed to reveal the dynamic electrode potential distribution at the single-nanoparticle level. An explicit equation is provided to describe the size/distance-dependent potential distribution in single NPs stochastic collision electrochemistry, showing the potential distribution is influenced by the NPs. Agreement between experiments and simulations indicates the potential roles of the M-S-MNP model in understanding the charge transfer process at the nanoscale.

Tracking the Electrocatalytic Activity of a Single Palladium Nanoparticle for the Hydrogen Evolution Reaction.

The nanoparticle-based electrocatalysts' performance is directly related to their working conditions. In general, a number of nanoparticles are uncontrollably fixed on a millimetre-sized electrode for electrochemical measurements. However, it is hard to reveal the maximum electrocatalytic activity owing to the aggregation and detachment of nanoparticles on the electrode surface. To solve this problem, here, we take the hydrogen evolution reaction (HER) catalyzed by palladium nanoparticles (Pd NPs) as a model system to track the electrocatalytic activity of single Pd NPs by stochastic collision electrochemistry and ensemble electrochemistry, respectively. Compared with the nanoparticle fixed working condition, Pd NPs in the nanoparticle diffused working condition results in a 2-5 orders magnitude enhancement of electrocatalytic activity for HER at various bias potential. Stochastic collision electrochemistry with high temporal resolution gives further insights into the accurate study of NPs' electrocatalytic performance, enabling to dramatically enhance electrocatalytic efficiency.

Quantifying Visible-Light-Induced Electron Transfer Properties of Single Dye-Sensitized ZnO Entity for Water Splitting.

Quantifying the photoinduced electron transfer properties of a single entity is of paramount importance for clarifying the link between the photoelectrochemical performance and the specific properties of an individual. Here, we successfully monitored the photoelectrochemical behavior of a single dye-sensitized ZnO entity on a Au ultramicroelectrode with different TiO2 film thicknesses. Due to a trap-limited electron diffusion in TiO2 film, a sub-millisecond photocurrent transient was observed for an individual N719@ZnO associated with single-particle photocatalytic water splitting. Furthermore, a Monte Carlo random walk numerical simulation model was developed to simulate the photoinjected electron transport dynamics and recombination in a nanoparticulate TiO2 film. Our approach allowed the photocatalytic properties of N719 at the single-molecule level to be quantified, and electron diffusivity and electron collection efficiency as a function of the film thickness were estimated by simulation analyses. Excellent agreement was obtained between the experimental results and theoretical simulations, indicating that the underlying photoinduced electron transfer processes can be reliably explored.

Single-Nanoparticle Photoelectrochemistry at a Nanoparticulate TiO2 -Filmed Ultramicroelectrode.

An ultrasensitive photoelectrochemical method for achieving real-time detection of single nanoparticle collision events is presented. Using a micrometer-thick nanoparticulate TiO2 -filmed Au ultra-microelectrode (TiO2 @Au UME), a sub-millisecond photocurrent transient was observed for an individual N719-tagged TiO2 (N719@TiO2 ) nanoparticle and is due to the instantaneous collision process. Owing to a trap-limited electron diffusion process as the rate-limiting step, a random three-dimensional diffusion model was developed to simulate electron transport dynamics in TiO2 film. The combination of theoretical simulation and high-resolution photocurrent measurement allow electron-transfer information of a single N719@TiO2 nanoparticle to be quantified at single-molecule accuracy and the electron diffusivity and the electron-collection efficiency of TiO2 @Au UME to be estimated. This method provides a test for studies of photoinduced electron transfer at the single-nanoparticle level.

Photoelectrocatalysis Synthesis of Ammonia Based on a Ni-Doped MoS2/Si Nanowires Photocathode and Porous Water with High N2 Solubility.

The synthesis of ammonia through photocatalysis or photoelectrochemistry (PEC) and nitrogen reduction reaction (NRR) has become one of the recent research hotspots in the field, where the catalyzed materials and strategies are critical for the NRR. Herein, a Ni-doped MoS2/Si nanowires (Ni-MoS2/Si NWs) photocathode is prepared, where the Si NWs are formed on the surface of a Si slice by the metal-assisted chemical etching method, and the hydrothermally synthesized Ni-MoS2 nanosheets are then cast-coated on the Si NWs electrode. Porous water with high solubility of N2 is prepared by treating a hydrophobic porous coordination polymer with hydrophilic bovine serum albumin for subsequent aqueous dispersing. The relevant electrodes and materials are characterized by electrochemistry, UV-vis spectrophotometry, scanning electron microscopy/energy dispersive spectroscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller method, and zeta potential method. The uses of the Ni-MoS2/Si NWs photocathode and the porous water with high nitrogen solubility for PEC-NRR give a yield of NH3 of 12.0 mmol h-1 m-2 under optimal conditions (e.g., at 0.25 V vs RHE), and the obtained apparent Faradaic efficiency higher than 100% is discussed from the inherent photocurrent-free photocatalysis effect of the photoelectrodes and the suggested classification of three kinds of electrons in PEC, which may have some reference value in understanding and improving other PEC-based processes.

Sandwich photoelectrochemical biosensing of concanavalin A based on CdS/AuNPs/NiO Z-scheme heterojunction and lectin-sugar binding.

A CdS/AuNPs/NiO Z-scheme heterojunction was prepared on a fluorine-doped tin oxide (FTO) electrode by hydrothermal synthesis of NiO on FTO, electrodeposition of AuNPs on NiO/FTO electrode and then cast-coating of CdS quantum dots. The CdS/AuNPs/NiO/FTO electrode gave a notably increased photocurrent versus NiO/FTO, CdS/FTO, AuNPs/NiO/FTO, CdS/AuNPs/FTO and CdS/NiO/FTO electrodes. The CdS/AuNPs/NiO/FTO electrode was further cast-coated with chitosan to immobilize d-mannose by Schiff base reaction, and concanavalin A (ConA) and then horseradish peroxidase (HRP) were captured on the electrode surface by lectin-sugar binding. 4-Chloro-1-naphthol (4-CN) was oxidized to form an insoluble precipitate catalyzed by HRP in the presence of H2O2, and the presence of precipitate on the photoelectrode inhibited the photocurrent in the presence of holes scavenger ascorbic acid. The relevant electrodes were characterized by electrochemistry, quartz crystal microbalance (QCM), UV-vis spectrophotometry, scanning electron microscopy/energy dispersive spectroscopy, and transmission electron microscopy. The QCM revealed that the collection efficiency (η) of the 4-CN-electrooxidation precipitate on the electrode can be as high as 91.8%. Under the optimal conditions, the decline of photocurrent responded linearly to the common logarithm of ConA concentration from 50 pM to 500 nM, with a limit of detection of 17 pM (S/N = 3). Satisfactory results were obtained in the detection of real soybean samples.

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.

An ultrasensitive photoelectrochemical platform for quantifying photoinduced electron-transfer properties of a single entity.

Understanding the photoinduced electron-transfer process is of paramount importance for realizing efficient solar energy conversion. It is rather difficult to clarify the link between the specific properties and the photoelectrochemical performance of an individual component in an ensemble system because data are usually presented as averages because of interplay of the heterogeneity of the bulk system. Here, we report a step-by-step protocol to fabricate an ultrasensitive photoelectrochemical platform for real-time detection of the intrinsic photoelectrochemical behaviors of a single entity with picoampere and sub-millisecond sensitivity. Using a micron-thickness nanoparticulate TiO2-filmed Au ultramicroelectrode (UME) as the electron-transport electrode, photocurrent transients can be observed for each individual dye-tagged oxide semiconductor nanoparticle collision associated with a single-entity photoelectrochemical reaction. This protocol allows researchers to obtain high-resolution photocurrent signals to quantify the photoinduced electron-transfer properties of an individual entity, as well as to precisely process the data obtained. We also include procedures for dynamic light scattering (DLS) analysis, transmission electron microscopy (TEM) imaging and collision frequency-concentration correlation to confirm that the photoelectrochemical collision events occur at an unambiguously single-entity level. The time required for the entire protocol is ~36 h, with a single-entity photoelectrochemical measurement taking <1 h to complete for each independent experiment. This protocol requires basic nanoelectrochemistry and nanotechnology skills, as well as an intermediate-level understanding of photoelectrochemistry.

Tracking motion trajectories of individual nanoparticles using time-resolved current traces.

Single nanoparticle (NP) electrochemical measurements are widely described, both theoretically and experimentally, as they enable visualization of the electrochemical signal of a single NP that is masked in ensemble measurements. However, investigating the behavior of individual NPs using electrochemical signals remains a significant challenge. Here we report experiments and simulations demonstrating that multiple distinct motion trajectories could be discerned from time-resolved current traces by dynamic Monte Carlo simulations. We show that continuous monitoring and quantification of electrochemical oxidation of individual AgNPs using a low-noise electrochemical measurement platform produce significantly distinguished current traces due to the size-dependent motions of AgNPs. Our findings offer a view of the electrochemical signals of individual NPs that are largely different from that in the literature, and underscore the significance of motion behaviors in single NP electrochemistry.