Electrocatalytic Amplification Research Articles

Enhanced signal to noise ratio of single entity electrochemistry signal of platinum nanoparticles using passive silver ultramicroelectrode

AbstractThe single‐entity electrochemistry (SEE) of electrocatalytic platinum (Pt) single nanoparticles (NPs) on a less electrocatalytic silver (Ag) ultramicroelectrode (UME) surface was investigated using the electrocatalytic amplification method. Two characteristic types of current responses—current staircases and blips (or spikes)—were observed during single NP collision experiments, depending on the applied potential at the Ag UME. Notably, at applied potentials of 0.13 and 0.17 V, the Ag UME becomes passive due to the formation of a delicate oxide layer, resulting in a highly stable background current. This leads to an enhanced signal‐to‐noise (S/N) ratio, attributed to the low background current, when using Ag UME compared to commonly used UMEs such as Au, C, Ni, and Hg for the SEE of Pt NPs. The exceptionally low background current can provide a significant advantage for detailed observation of SEE signals and further mechanistic studies based on the current response.

Understanding Reaction Kinetics of Iron Oxide (α- Fe2O3) Reduction with Single Entity Measurements

Emitting minimal amount of CO2 during the production of steel is one of the major challenges that can be overcome by producing iron using electrowinning method. The reaction of interest is at the cathode where the direct reduction of Iron oxide (α-Fe2O3) to iron takes place. In this study, we are investigating the electrochemical conversion of hematite (α-Fe2O3) Nanoparticles to iron in an extremely alkaline medium (50 wt% NaOH) using Single Entity Electrochemistry (SEE).Single Entity Electrochemistry is the study of single atom, molecule or particle colliding with the surface of the electrode usually by diffusion or migration. For these experiments, we are developing alkaline stable carbon-fiber (C)/Nickel (Ni) ultra-microelectrode (UME), prepared using multiple pathways such as plasma-focused ion beam.A spike in current transient during chronoamperometry (CA) experiments is observed when a hematite particle collides stochastically with the UME and gets reduced in the process. Each spike is the reduction event of one hematite nanoparticle to iron. Individual reduction events, that last 0.2-0.3 s and resulting in 0.1-0.2 nC of charge being passed were observed at Ni and C microelectrodes, this shows good agreement with the average particle radius of 100 nm. The shape of the current transient was distinct from previously reported transients for metal deposition at cathode1-3. However, A. Allanore, et.al. model was used to support a mechanism for the direct oxide reduction process at nanoscale particles4.The results presented here illustrate the reaction kinetics and the mode of reduction propagation in a single nano-sized hematite particle in alkaline conditions. The kinetics of hematite reduction in alkaline medium is considerably slow as observed from the slope of current spikes in SEE experiments. Furthermore, considering Allanore’s model, the reduction propagation occurs from the outer surface to the inner core.(1) Luo, L.; White, H. S. Electrogeneration of single nanobubbles at sub-50-nm-radius platinum nanodisk electrodes. Langmuir 2013, 29 (35), 11169-11175.(2) Xiao, X.; Fan, F.-R. F.; Zhou, J.; Bard, A. J. Current transients in single nanoparticle collision events. Journal of the American Chemical Society 2008, 130 (49), 16669-16677.(3) Bard, A. J.; Zhou, H.; Kwon, S. J. Electrochemistry of single nanoparticles via electrocatalytic amplification. Israel Journal of Chemistry 2010, 50 (3), 267-276.(4) Allanore, A.; Lavelaine, H.; Valentin, G.; Birat, J.; Delcroix, P.; Lapicque, F. Observation and modeling of the reduction of hematite particles to metal in alkaline solution by electrolysis. Electrochimica Acta 2010, 55 (12), 4007-4013.

Electrocatalytic amplification of coreactant electrochemiluminescence using redox mediators

Electrochemiluminescence (ECL) has emerged as a sensitive analytical technique with a wide range of applications. While recent studies have begun to explore the role of redox mediators to facilitate ECL systems, this work extends these foundational insights by developing a comprehensive theoretical framework that supports, generalizes, and rationalizes mechanistic pathways borrowed from molecular electrocatalysis. This advancement is demonstrated through the electrocatalytic amplification of coreactant ECL within the [Ru(bpy)3]2+/TPrA system, utilizing a water-soluble redox-active Ir(III) complex as an electrocatalyst. Our investigation unveils previously uncharted mechanisms that enable the enhancement of Ru complex ECL at low electrode potentials, crucially without requiring the direct oxidation of [Ru(bpy)3]2+ or TPrA at the electrode, thus offering a deeper understanding of ECL activation through moelcular electrocatalysis. Through an integrated approach combining electrochemical analysis, spectroscopic investigation, and finite element modeling, we elucidate how the redox mediator critically modulates ECL efficiency by controlling the kinetics of radical species production and decay. The mediation by the Ir(III) complex not only governs the generation of TPrA radicals and excited [Ru(bpy)3]2+ states, thereby enhancing ECL intensity, but also the conditions under which ECL can be quenched. Further exploration of the mediator's redox characteristics provides predictive insights into ECL behavior, underscoring the mediator's redox potential as pivotal in determining ECL onset, peak potential, and intensity. This refined understanding paves the way for tailoring ECL systems for enhanced performance in analytical, imaging, and biomedical applications through the judicious selection of redox mediators.

Electrocatalytic Reduction of Benzyl Bromide during Single Ag Nanoparticle Collisions.

Previous reports of the electrocatalytic activity of Ag nanoparticles (AgNPs) toward the reduction of organic halides have been limited to measurements of immobilized nanoparticle ensembles. Here, we have investigated the electrochemical reduction of benzyl bromide (PhCH2Br) occurring at single AgNPs (4.2 to 37 nm radius) in methanol, where the effects of nanoparticle size on catalytic behavior can be more thoroughly examined and rigorously quantified. AgNP collisions at a 6.3 μm radius Au ultramicroelectrode (UME) result in measurable electrocatalytic amplification currents from the reduction of PhCH2Br, where collision events are indicated by a sudden step increase in the reduction current recorded in the current-time trace. The dependence of the height of these steps on the applied potential allowed for an analysis of reaction kinetics based on the Butler-Volmer model, resulting in an estimation of the standard rate constant (k0) as a function of AgNP size. Measured values of k0 range from 4.0 × 10-4 to 8.0 × 10-4 cm/s on AgNPs with radii of 14, 29, and 37 nm, whereas k0 was found to be 6.2 × 10-4 cm/s at a 12.3 μm radius Ag disk UME. The results indicate that the kinetics of PhCH2Br reduction are independent of AgNP size and are similar to the reaction kinetics observed at a Ag UME. The frequency of observed particle collisions was found to be dependent on particle size, where 14 nm radius AgNPs resulted in the highest-frequency collisions. The potential- and size-dependent interactions of AgNPs with the Au UME are discussed in terms of the DLVO theory.

Catalytically synthesized Prussian Blue nanozymes as labels for electrochemical DNA/RNA sensors

We propose catalytically synthesized nanozymes based on Prussian Blue (PB) and azidomethyl-substituted poly (3,4-ethylenedioxythiophene) (azidomethyl-PEDOT) as novel electrocatalytic labels for DNA/RNA sensors. Catalytic approach allowed to synthesize highly redox and electrocatalytically active Prussian Blue nanoparticles functionalized with azide groups that enable ‘click’ conjugation with alkyne-modified oligonucleotides. Both competitive and sandwich-type schemes were realized. As the sensor response the direct (mediator-free) electrocatalytic current of H2O2 reduction can be measured, which is proportional to the concentration of the hybridized labeled sequences. The current of H2O2 electrocatalytic reduction is only 3–8 times increased in the presence of the freely diffusing mediator catechol, which indicates high efficiency of direct electrocatalysis with the elaborated labels. Electrocatalytic amplification of the signal allows robust detection of (63–70)-base target sequences with concentrations below 0.2 nM in blood serum within an hour. We believe, the use of advanced Prussian Blue based electrocatalytic labels sets new avenues for point-of-care DNA/RNA sensing.

Observation and Analysis of Staircase Response of Single Palladium Nanoparticle Collision on Gold Ultramicroelectrodes.

Collision (or impact) of single palladium nanoparticles (Pd NPs) on gold (Au), copper (Cu), nickel (Ni), and platinum (Pt) ultramicroelectrodes (UMEs) were investigated via electrocatalytic amplification method. Unlike the blip responses of previous Pd NP collision studies, the staircase current response was obtained with the Au UME. The current response, including collision frequency and peak magnitude, was analyzed depending on the material of the UME and the applied potential. Adsorption factors implying the interaction between the Pd NP and the UMEs are suggested based on the experimental results.

(Digital Presentation) Catalytic Interruption Mitigates Edge Effects in the Characterization of Heterogeneous, Insulating Nanoparticles

Single entity electrochemistry encompasses a broad collection of techniques that are used to detect and characterize single, freely diffusing analytes in solution1,2. A particular scheme in this field, “blocking” nanoimpacts, has expanded the scope of electrochemistry to the study of redox-inert materials. Here, as a particle adsorbs, it prevents charge exchange between the microelectrode with a freely diffusing electroactive redox mediator. The interpretation of these blocking results, however, is often complicated by “edge effects,” in which enhanced mass transport to the edges of microelectrodes leads to uneven current distributions3,4. To overcome this problem we introduce here the use of electrochemical amplification5. That is, we employ electrocatalytic amplification to drive a current increase, moving the rate limiting step in current generation away from the electrode surface, reducing the geometric impact of the electrode’s edges. Finite element simulations indicate that the rapid chemical kinetics introduced by this approach contributes to the amplification of the electronic signal to restore analytical precision and reliability detect and characterize the heterogeneity of nanoscale electro-inactive materials. Using this approach, which we have termed “electrocatalytic interruption,” we achieve significantly improved precision in the determination of particle size distributions.6 References Quinn, B. M.; van’t Hof, P. G.; Lemay, S. G. Time-Resolved Elec-trochemical Detection of Discrete Adsorption Events. J. Am. Chem. Soc. 2004, 126 (27), 8360–8361. https://doi.org/10.1021/ja0478577.Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001.Compton, R. G.; Banks, C. E. Understanding Voltammetry (Third Edition); World Scientific, 2018.Deng, Z.; Elattar, R.; Maroun, F.; Renault, C. In Situ Measurement of the Size Distribution and Concentration of Insulating Particles by Electrochemical Collision on Hemispherical Ultramicroelectrodes. Anal. Chem. 2018, 90 (21), 12923–12929. https://doi.org/10.1021/acs.analchem.8b03550.Bonezzi, J.; Boika, A. Deciphering the Magnitude of Current Steps in Electrochemical Blocking Collision Experiments and Its Implica-tions. Electrochimica Acta 2017, 236, 252–259. https://doi.org/10.1016/j.electacta.2017.03.090.Chung, J.; Hertler, P.; Plaxco, K.W.; Sepunaru, L. J. Am. Chem. Soc. 2021, https://doi.org/10.1021/jacs.1c04971. Figure 1

Electrochemical Detection and Analysis of Various Current Responses of a Single Ag Nanoparticle Collision in an Alkaline Electrolyte Solution.

A single silver (Ag) nanoparticle (NP) collision was observed and analyzed in an alkaline solution using the electrocatalytic amplification (EA) method. Previously, the observation of a single Ag NP collision was only possible through limited methods based on a self-oxidation of Ag NPs or a blocking strategy. However, it is difficult to characterize the electrocatalytic activity of Ag NPs at a single NP level using a method based on the self-oxidation of Ag NPs. When using a blocking strategy, size analysis is difficult owing to the edge effect in the current signal. The fast oxidative dissolution of Ag NPs has been a problem for observing the staircase response of a single Ag NP collision signal using the EA method. In alkaline electrolyte conditions, Ag oxides are stable, and the oxidative dissolution of Ag NPs is sluggish. Therefore, in this study, the enhanced magnitude and frequency of the current response for single Ag NP collisions were obtained using the EA method in an alkaline electrolyte solution. The peak height and frequency of single Ag NP collisions were analyzed and compared with the theoretical estimation.

Single‐Entity Electrochemical Detection of As‐Prepared Metallic and Dielectric Nanoparticle Stochastic Impacts in a Phosphonium Ionic Liquid

AbstractNanoparticles (NPs) are of considerable interest, owing to their enhanced catalytic activity and optical properties. Ionic liquids (ILs) are employed as a medium to prepare small (<20 nm in diameter) and relatively low dispersity (typically, ±1–2 nm) NPs. While ensemble techniques (e. g., UV/Vis) can provide a great deal of information, increasingly, single‐entity electrochemistry (SEE) has been used to provide physical insight into individual NP catalysis and dynamics. Herein, Pt NP impact current spikes for particles prepared in the IL phase and based on electrocatalytic amplification (ECA) of the borohydride oxidation reaction (BOR) were recorded during i‐t measurements. Additionally, direct oxidative LiBH4 nanocrystal (NC) impact signals were observed herein for the first time. By quantifying the charge transferred during the stochastic collision events, the radius of the NCs (rNC) was calculated (∼5.5 nm) and compares favorably with TEM images. Critically, small additions (1 % v/v) of a molecular, polar organic solvent (tetrahydrofuran, THF) disrupt the ILs supramolecular fluidic nature, which enhances NC impact frequency and the aggregation of Pt NPs.

Stochastic Collision Electrochemistry from Single Pt Nanoparticles: Electrocatalytic Amplification and MicroRNA Sensing.

Single-particle collisions have made many achievements in basic research, but challenges still exist due to their low collision frequency and selectivity in complex samples. In this work, we developed an "on-off-on" strategy based on Pt nanoparticles (PtNPs) that catalyze N2H4 collision signals on the surface of carbon ultramicroelectrodes and established a new method for the detection of miRNA21 with high selectivity and sensitivity. PtNPs catalyze the reduction of N2H4 on the surface of carbon ultramicroelectrodes to generate a stepped collision signal, which is in the "on" state. The single-stranded DNA paired with miRNA21 is coupled with PtNPs to form the complex DNA/PtNPs. Because PtNPs are covered by DNA, the electrocatalytic collision of N2H4 oxidation is inhibited. At this time, the signal is in the "off" state. When miRNA21 is added, the strong complementary pairing between miRNA21 and DNA destroys the electrostatic adsorption of DNA/PtNP conjugates and restores the electrocatalytic performance of PtNPs, and the signal is in the "on" state again. Based on this, a new method for detecting miRNA21 was established. It provides a new way for small-molecule sensing and has a wide range of applications in electroanalysis, electrocatalysis, and biosensing.

Electrochemical Amplification Methods in Observation of Single Nanoparticle Collisions

We describe the electrochemical detection of single nanoparticle (NP) collision at a less catalytic electrode. We could observe the various single NP collision through the enhanced current by electrocatalytic redox recycling. The current responses were caused by random individual events as a function of time rather than the usual continuous current caused by an ensemble of a large number of events. The overall current transient such as spike-like or staircase, changed depending on the reaction mechanism of collided NP. The height and frequency of the peak current are proportional to the applied potential and the particle concentration.Single iridium oxide (IrOx) NP collision at a less catalytic electrode surface such as the NaBH4-treated Pt or the bare Au ultramicroelectrode (UME) was investigated. The overall current transient consists of repeated current spikes, superimposed on a current decay to reach a steady-state value.Collisions of Ag NPs with a more electrocatalytic Au or Pt surface have been observed. Depending on the applied potential to the UME, the current response of Ag NPs collision onto the UME resulted in various shape changes. A staircase decrease, a blip decrease, and a blip increase of the hydrazine oxidation current were obtained at an applied potential of 0.33 V, 0.80 V, and 1.3 V, respectively.The current response of ascorbic acid stabilized Cu single NP collision on Au UME surface was observed. Here, the glucose oxidation electrocatalyzed by oxidized Cu NP was used as the indicating reaction. In this system, the NP collision signals were obtained simultaneously by both direct particle electrolysis and electrocatalytic amplification.

Study on Collided Ag or Pt Single Nanoparticle’s Current Peak Shape Changing Depending on Applying Potential

Electrocatalytic amplification method was used for analyzing electrochemical properties about collided nanoparticles. Silver nanoparticles (Ag NPs) and platinum nanoparticles (Pt NPs)’s characteristics about hydrazine oxidation reaction were investigated. Those NPs have distinct electrocatalytic activation for hydrazine oxidation reaction. For detecting these NPs, gold (Au) ultramicroelectrode (UME) was used as working electrode which has appropriate catalytic activities comparing with both NPs. When applied potential was above 0 V, Pt NPs showed positive electrocatalytic characteristic, then collided Pt NPs produced amplified transient current signal. In contrast, Ag NPs produced decreased transient current signal when colliding onto Au UME applied above 0.25 V which potential range Ag NPs showed negative electrocatalytic characteristics. But the transient current signals by each collided NPs showed shape shifting as changing applied potential. When applying relatively low potential range, current signals produced by each NPs were staircase shape which means the collided NPs sustain stable state on Au UME after collision events. After changing high potential range, the current signals were shifting to blip or spike shape. As analyzing current peak shape changing phenomenon, electrocatalytic reaction mechanisms by each NPs could be understood by this method.

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 (fp), 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 fp. 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.

Atom-by-atom electrodeposition of single isolated cobalt oxide molecules and clusters for studying the oxygen evolution reaction

We report an electrodeposition protocol for preparing isolated cobalt oxide single molecules (Co1Ox) and clusters (Co n Oy) on a carbon fiber nanoelectrode. The as-prepared deposits are able to produce well-defined steady-state voltammograms for the oxygen evolution reaction (OER) in alkaline media, where the equivalent radius (rd) is estimated by the limiting current of hydroxide oxidation in accordance with the electrocatalytic amplification model. The size of isolated clusters obtained from the femtomolar Co2+ solution through an atom-by-atom technique can reach as small as 0.21 nm (rd) which is approximately the length of Co-O bond in cobalt oxide. Therefore, the deposit was close to that of a Co1Ox single molecule with only one cobalt ion, the minimum unit of the cobalt-based oxygen-evolving catalyst. Additionally, the size-dependent catalysis of the OER on Co n Oy deposits shows a faster relative rate on the smaller cluster in terms of the potential at a given current density, implying the single molecular catalyst shows a superior OER activity.

The medium effect on electrodissolution of adsorbed or suspended Ag nanoparticles

Electrochemical behaviour of six types of Ag nanoparticles (three stabilised by citrate and three others stabilised by polymers) was studied on carbon screen-printed electrode and glassy carbon rotating disc electrode. A stationary electrode was used as a support for adsorbed nanoparticles, whereas the rotating disc electrode was applied for experiments in nanoparticles suspension. Peak shaped cyclic voltammograms obtained for immobilised nanoparticles indicate their electro-dissolution and electrodeposition of silver in the backward scan. The given peak shape and its position on potential scale depend both on the type of nanoparticles and the supporting electrolyte. When aggregation of Ag nanoparticles was detected by UV–vis spectroscopy, splitting of an anodic peak corresponding to electrodissolution of silver in neutral phosphate buffer or NaOH solution is seen. Injection of a fraction of microgram of Ag nanoparticles was detected in chronoamperometric experiments on rotating disc electrode. However, in the presence of H2O2, the effect of nanograms of silver was seen on voltammetric curve indicating electrocatalytic amplification.

Handheld Aptasensor for Sandwiched Detection of Chloramphenicol

Chloramphenicol(CAP) is an antifungal agent approved for use in aquarium. However, CAP residue imparts serious adverse effect on human health as well as the ecology. Thus, keeping track of CAP in aquatic products and its environmental metabolites are of great importance. Herein, we developed a novel and neat electrochemical aptasensor for the detection of simulated CAP residual in a sandwich fashion. The screen-printed carbon electrode was employed as the sensing platform and electrodeposited with peculiar nanogolds to enhance the electrochemical performance. For the specific recognition of such small target, a pair of chloramphenicol-binding aptamers, the primary Apt1 and the secondary Apt2, were tailored as an artificial single-chained antibody couple, of which the thiolated Apt1 stood on the coated working substrate to capture the analyte, while the biotinylated Apt2 rallied the signal output. In the presence of CAP, Apt1 at the lower place cooperated with the upper Apt2 pinched CAP together. After that, the reporter avidin-conjugated horseradish peroxidase associated atop the mounted biotin labels, resulting in an electrocatalytic amplification. In whole, this modular biosensing construction would set a paradigm not just for practicable checkpoint quarantine or point-of-care testing, but potentially adaptable for technological fusion with “Internet Plus” and “Internet of Things”.

Observation of Single Nanoparticle Collisions with Green Synthesized Pt, Au, and Ag Nanoparticles Using Electrocatalytic Signal Amplification Method.

This work describes the tailored design, green synthesis and characterization of noble metal (Pt, Ag and Au) nanoparticles (NPs) using Sapinduss Mukkorossi fruit extract (SMFE) and its signal NP collision signal response, based on the principle of the electrocatatlytic amplication (EA) method. Here, the SMFE can act as both the reducing and the capping agent for the fabrication of noble nanometals. The SMFE-capped NPs was available for the observation of a single NP collision signal. Two general types of current response were observed: a staircase current response for the Pt or Au NPs, and a blip/spike current response for Ag NPs. These results demonstrated that the eco-friendly synthesized SMFE-capped NPs maintained their electrocatalytic activity, therefore they can be used for the single NP experiments and place an arena for future biosensing applications.

Probing Size and Substrate Effects on the Hydrogen Evolution Reaction by Single Isolated Pt Atoms, Atomic Clusters, and Nanoparticles.

We report the catalytic activity of a single, isolated Pt deposit on Bi and Pb supports to probe the size and substrate effects on the electrochemical hydrogen evolution reaction (HER). Deposits were made electrolytically by an atom-by-atom method in a controlled plating; we prepared an individual Pt deposit on Bi and Pb ultramicroelectrodes (UMEs) such as a single isolated atom, clusters containing one to five Pt atoms, and nanoparticles to about 10 nm radius. A steady-state voltammogram on the single Pt deposits is observed by electrocatalytic amplification of the HER, with a negligible contribution by the HER at the substrate UME. A single Pt atom can act as an electrode for the HER, showing a diffusion-limiting current plateau in the voltammogram that can be used to estimate the radius of a single deposit. We simulated the voltammograms of the individual deposits, assuming the Volmer step of the HER is appropriate for a Pt cluster deposit, to obtain kinetic parameters for each deposit. The HER kinetics increases as the particle radius increases from ∼0.2 to ∼4 nm for Bi and Pb substrates and then reaches a limiting plateau. The limiting kinetics on the Bi substrate approaches that of bulk Pt while that on the Pb substrate is much smaller.

MicroRNA detection based on duplex-specific nuclease-assisted target recycling and gold nanoparticle/graphene oxide nanocomposite-mediated electrocatalytic amplification

DNA technology based bio-responsive nanomaterials have been widely studied as promising tools for biomedical applications. Gold nanoparticles (AuNPs) and graphene oxide (GO) sheets are representative zero- and two-dimensional nanomaterials that have long been combined with DNA technology for point-of-care diagnostics. Herein, a cascade amplification system based on duplex-specific nuclease (DSN)-assisted target recycling and electrocatalytic water-splitting is demonstrated for the detection of microRNA. Target microRNAs can form DNA: RNA heteroduplexes with DNA probes on the surface of AuNPs, which can be hydrolyzed by DSN. MicroRNAs are preserved during the reaction and released into the suspension for the digestion of multiple DNA probes. After the DSN-based reaction, AuNPs are collected and mixed with GO to form AuNP/GO nanocomposite on an electrode for the following electrocatalytic amplification. The utilization of AuNP/GO nanocomposite offers large surface area, exceptional affinity to water molecules, and facilitated mass diffusion for the water-splitting reaction. For let-7b detection, the proposed biosensor achieved a limit detection of 1.5 fM in 80 min with a linear detection range of approximately four orders of magnitude. Moreover, it has the capability of discriminating non-target microRNAs containing even single-nucleotide mismatches, thus holding considerable potential for clinical diagnostics.

Chronoamperometric Observation and Analysis of Electrocatalytic Ability of Single Pd Nanoparticle for Hydrogen Peroxide Reduction Reaction.

The current generated by the collision of a single nanoparticle (NP) of palladium (Pd) on a gold (Au) ultramicroelectrode (UME) surface was observed using an electrocatalytic amplification method. The hydrogen peroxide reduction reaction was used for the electrocatalytic reaction because the hydrogen peroxide reduction reaction has no gas-phase product, which would induce rapid signal decay. The electrocatalytic current resulting from a single Pd nanoparticle on the Au UME shows a staircase response with accompanying slow current decay. The applying potential and concentration of hydrogen peroxide were optimized for clear distinction of signal. The height of the current step and signal frequency were analyzed and compared with the theoretical expectation. The analysis of the electrocatalytic activity of single Pd NPs provides insight toward their future application.