Single enzyme electroanalysis

Revisiting the catalytic activity of single horseradish peroxidase clusters through electrochemical collision technique: Effect of electrolyte and substrate

Horseradish peroxidase (HRP) is a versatile biosensing label and signal reporter owing to its broad-spectrum catalytic ability. In present work, we characterized HRP's catalytic performance with various substrates using electrochemical collision technique and analyzed the associated electron transfer processes. Different electrolyte solutions greatly affected enzyme dispersibility and zeta potential, thereby impacting HRP collision dynamics in single H2O2 substrate system. The maximum turnover number (kcat) for single HRP molecules was calculated to be 3.611 ± 0.149 × 103 s-1 in 0.85 % NaCl and 2.967 ± 0.286 × 103 s-1 in 0.1 M PBS solution, reflecting differences in cluster size induced by the electrolyte conditions. More severe agglomeration of HRP molecules was observed in double-substrate systems, where the hydrophilic mediator (K4Fe(CN)6) and lipophilic mediator (ABTS) served as electron donors and signal reporters. The calculated kcat value of single HRP molecules in ABTS-H2O2 was 7.6 times higher than that in K4Fe(CN)6-H2O2. This difference is attributed to mediators' solubility, lipophilicity, and HRP's affinity for different substrates, with HRP demonstrated stronger affinity for ABTS-H2O2 substrates, which realized more efficient electron transfer and compensated for the low diffusion coefficient of ABTS. This work provides a comprehensive analysis of the effects of electrolytes and substrates on HRP collision and catalytic behavior, offering valuable insights for the advanced design of HRP-based biosensors and diagnostic platforms.

Single Entity Electrocatalysis

Making a measurement over millions of nanoparticles or exposed crystal facets seldom reports on reactivity of a single nanoparticle or facet, which may depart drastically from ensemble measurements. Within the past 30 years, science has moved toward studying the reactivity of single atoms, molecules, and nanoparticles, one at a time. This shift has been fueled by the realization that everything changes at the nanoscale, especially important industrially relevant properties like those important to electrocatalysis. Studying single nanoscale entities, however, is not trivial and has required the development of new measurement tools. This review explores a tale of the clever use of old and new measurement tools to study electrocatalysis at the single entity level. We explore in detail the complex interrelationship between measurement method, electrocatalytic material, and reaction of interest (e.g., carbon dioxide reduction, oxygen reduction, hydrazine oxidation, etc.). We end with our perspective on the future of single entity electrocatalysis with a key focus on what types of measurements present the greatest opportunity for fundamental discovery.

Ultrasensitive detection of dopamine using Au microelectrodes integrated with mesoporous silica thin films

An electrochemical method was developed for ultrasensitive and selective detection of dopamine in human serum using mesoporous silica thin film modified gold microelectrodes. Vertically aligned mesoporous silica thin films were deposited onto Au microelectrodes by electrochemically assisted self-assembly (EASA). The mesochannels have uniform pore sizes of 2.1 nm in diameter and a negatively charged wall surface. Cyclic voltammetry reveals effective charge permselectivity through the negatively charged mesoporous channels. By using differential pulse voltammetry, the mesoporous silica thin film modified Au microelectrode can be employed for the ultrasensitive detection of dopamine with a detection limit as low as 0.084 μM. In addition, thanks to the electrostatic and steric effects of the silica mesochannels, excellent anti-interference and anti-fouling properties of the electrochemical sensors are demonstrated.

An Electrochemical Perspective on Reaction Acceleration in Microdroplets

Analytical techniques operating at the nanoscale introduce confinement as a tool at our disposal. This review delves into the phenomenon of accelerated reactivity within micro- and nanodroplets. A decade of accelerated reactivity observations was succeeded by several years of fundamental studies aimed at mechanistic enlightenment. Herein, we provide a brief historical context for rate enhancement in and around micro- and nanodroplets and summarize the mechanisms that have been proposed to contribute to such extraordinary reactivity. We highlight recent electrochemical reports that make use of restricted mass transfer to enhance electrochemical reactions and/or quantitatively measure reaction rates within droplet-confined electrochemical cells. A comprehensive approach to nanodroplet reactivity is paramount to understanding how nature takes advantage of these systems to provide life on Earth and, in turn, how to harness the full potential of such systems.

Impact electrochemistry for biosensing: advances and future directions

Impact electrochemistry allows for the investigation of the properties of single entities, ranging from nanoparticles (NPs) to soft bio-particles. It has introduced a novel dimension in the field of biological analysis, enhancing researchers' ability to comprehend biological heterogeneity and offering a new avenue for developing novel diagnostic devices for quantifying biological analytes. This review aims to summarize the recent advancements in impact electrochemistry-based biosensing over the past two to three years and provide insights into the future directions of this field.

Molecular Electrocatalytic Processes in Carbon Nanopipettes

Conductive nanopipettes have been recognized as powerful multifunctional platforms for electrochemical sensing applications in confined spaces. However, the electron-transfer processes of many biological analytes (i.e., enzymes or proteins) are slow and coupled with chemical reactions, which have not been well elucidated in conductive nanopipettes. In this Letter, both experimental and simulation methods are used to study electron-transfer processes coupled to chemical reactions (EC mechanism) in carbon nanopipettes (CNPs). It is demonstrated that the electroactive species can serve as redox mediator to help oxidize and reduce the nonelectroactive analytes of interest in the solution and produce noticeable catalytic current signals. Besides, glutathione was directly measured by using ferrocenemethanol as the redox mediator in the CNPs. The elucidated EC processes in CNPs would offer a new opportunity to measure nonelectroactive analytes in biological fields.

Sensitive quantification of mercury ions in real water systems based on an aggregation-collision electrochemical detection

Nanoparticle impact electrochemistry (NIE) is an emerging electroanalytical technique that has been utilized to the sensitive detection of a wide range of biological species. So far, the NIE based trace ion detection is largely unexplored due to the lack of effective signal amplification strategies. We herein develop an NIE-based electrochemical sensing platform that utilizes T-Hg2+-T coordination induced AgNP aggregation to detect Hg2+ in aqueous solution. The proposed aggregation-collision strategy enables highly sensitive and selective detection. A dual-mode analysis based on the change in impact frequency and oxidative charge of the anodic oxidation of the AgNPs in NIE allows for more accurate self-validated quantification. Furthermore, the current NIE-based sensor demonstrates reliable analysis of Hg2+ of real water samples, showing great potential for practical environmental monitoring and point-of-care testing (POCT) applications.

The Modification and Applications of Nanopipettes in Electrochemical Analysis

Nanopipette, which is fabricated by glasses and possesses a nanoscale pore in the tip, has been proven to be immensely useful in electrochemical analysis. Numerous nanopipette-based sensors have emerged with improved sensitivity, selectivity, ease of use, and miniaturization. In this minireview, we provide an overview of the recent developments of nanopipette-based electrochemical sensors based on different types of nanopipettes, including single-nanopipettes, self-referenced nanopipettes, dual-nanopipettes, and double-barrel nanopipettes. Several important modification materials for nanopipette functionalization are highlighted, such as conductive materials, macromolecular materials, and functional molecules. These materials can improve the sensing performance and targeting specificities of nanopipettes. We also discuss examples of related applications and the future development of nanopipette-based strategies.

Electrochemical-Shock Synthesis of Nanoparticles from Sub-femtoliter Nanodroplets

ConspectusNanoparticles have witnessed immense development in the past several decades due to their intriguing physicochemical properties. The modern chemist is interested not only in methods of synthesizing nanoparticles with tunable properties but also in the chemistry that nanoparticles can drive. While several methods exist to synthesize nanoparticles, it is often advantageous to put nanoparticles on a variety of conductive substrates for multiple applications (such as energy storage and conversion). Despite enjoying over 200 years of development, electrodeposition of nanoparticles suffers from a lack of control over nanoparticle size and morphology. There have been heroic efforts to address these issues over time. With an understanding that structure-function studies are imperative to understand the chemistry of nanoparticles, new methods are necessary to electrodeposit a variety of nanoparticles with control over macromorphology and also microstructure.This Account details our group's efforts in overcoming challenges of classical nanoparticle electrodeposition by electrodepositing nanoparticles from water nanodroplets. When a nanodroplet full of metal salt precursor is incident on the electrode biased sufficiently negative to drive electroplating, nanoparticles form at a fast rate (on the order of microseconds to milliseconds). We start with the general nuts-and-bolts of the experiment (nanodroplet formation and methods for electrodeposition). The deposition of new nanomaterials often requires one to develop new methods of measurement, and we detail new measurement tools for quantifying nanoparticle porosity and nanopore tortuosity within single nanoparticles. We achieve nanopore characterization by using Focused Ion Beam milling and Scanning Electron Microscopy. Owing to the small size of the nanodroplets and fast mass transfer (the contents of a femtoliter droplet can be electrolyzed in only a few milliseconds), the use of nanodroplets also allows the electrodeposition of high entropy alloy nanoparticles at room temperature.We detail how a deep understanding of ion transfer mechanisms can be used to expand the library of possible metals that can be deposited. Furthermore, simple ion changes in the dispersed droplet phase can decrease the cost per experiment by orders of magnitude. Finally, electrodeposition in aqueous nanodroplets can also be combined with stochastic electrochemistry for a variety of interesting studies. We detail the quantification of the growth kinetics of single nanoparticles in single aqueous nanodroplets. Nanodroplets can also be used as tiny reactors to trap only a few molecules of a metal salt precursor. Upon reduction to the zerovalent metal, electrocatalysis at very small metal clusters can be probed and evaluated with time using steady-state electrochemical measurements. Overall, this burgeoning synthetic tool is providing unexpected avenues of tunability of metal nanoparticles on conductive substrates.

Homogeneous Electrochemical Immunoassay Using an Aggregation-Collision Strategy for Alpha-Fetoprotein Detection

Homogeneous immunoassays represent an attractive alternative to traditional heterogeneous assays due to their simplicity and high efficiency. Homogeneous electrochemical assays, however, are not commonly accessed due to the requirement of electrode immobilization of the recognition elements. Herein, we demonstrate a new homogeneous electrochemical immunoassay based on the aggregation-collision strategy for the quantification of tumor protein biomarker alpha-fetoprotein (AFP). The detection principle relies on the aggregation of AgNPs induced by the molecular biorecognition between AFP and AgNPs-anti-AFP probes, which leads to an increased AgNP size and decreased AgNP concentration, allowing an accurate self-validated dual-mode immunoassay by performing nanoimpact electrochemistry (NIE) of the oxidation of AgNPs. The intrinsic one-by-one analytical capability of NIE as well as the participation of all of the atoms of the AgNPs in signal transduction greatly elevates the detection sensitivity. Accordingly, the current sensor enables a limit of detection (LOD) of 5 pg/mL for AFP analysis with high specificity and efficiency. More importantly, reliable detection of AFP in diluted human sera of hepatocellular carcinoma (HCC) patients is successfully achieved, indicating that the NIE-based homogeneous immunoassay shows great potential in HCC liquid biopsy.

Nanoconfined Electrochemical Collision and Catalysis of Single Enzyme inside Carbon Nanopipettes

Revealing the electrocatalytic features of single redox enzyme is significant to both fundamental biological processes and practical catalysis and sensing applications. Herein, we directly reveal the electrocatalytic current from a single enzyme inside the carbon nanopipettes via electrochemical collision strategies, based on the increased activity at nanoscale confinement. Besides the staircase current steps from surface blockage, discrete H₂O₂ oxidation and reduction current transients catalyzed by a single enzyme are also displayed and analyzed. The carbon nanopipette would increase the catalytic activities of enzymes and lead to a detectable current response, thus opening a new way to investigate the fundamental enzymatic mechanisms at the single enzyme level.

Enabling practical nanoparticle electrodeposition from aqueous nanodroplets

The rapid rise of technology in the modern world has led to an increased demand for energy. Consequently, it is essential to increase the efficiency of current energy-producing systems due to the poor activity of their catalysts. Nanoparticles play a significant role in energy storage and conversion; however, electrodeposition of nanoparticles is difficult to achieve due to surface heterogeneities, nanoparticle diffusion layer overlap, and the inability to electrodeposit multi-metallic nanoparticles with stoichiometric control. These problems can be solved through nanodroplet-mediated electrodeposition, a technique where water nanodroplets are filled with metal salt precursors that form stable nanoparticles when they collide with a negatively-biased electrode. Further, this method has demonstrated control over nanoparticle size and morphology, displaying a wide variety of applications for the generation of materials with excellent catalytic properties. Historically, the cost of nanodroplet-mediated electrodeposition experimentation is prohibitive because practitioners use 0.1 M to 0.5 M tetrabutylammonium perchlorate (TBAP) dissolved in the oil phase (∼10 mL). Such high concentrations of electrolytes have been used to lower ohmic drop and provide ions to maintain charge balance during electrodeposition. Here, we show that supporting electrolyte is not necessary for the oil phase. In fact, one can use a suitable salt (such as lithium perchlorate) in the aqueous phase to achieve nanoparticle electrodeposition. This simple change, grounded in an understanding of ion transfer, drives down the cost per experiment by nearly three orders of magnitude, representing a necessary step forward in enabling practical nanoparticle electrodeposition from water nanodroplets. This approach is a promising procedure for future cost-effective energy conversion systems relying on electrocatalytic nanoparticles.

Capturing the signal of weak electricigens: a worthy endeavour

Recently several non-traditional electroactive microorganisms have been discovered. These can be considered weak electricigens; microorganisms that typically rely on soluble electron acceptors and donors in their lifecycle but are also capable of extracellular electron transfer (EET), resulting in either a low, unreliable, or otherwise unexpected current. These unanticipated electroactive microorganisms represent a new chapter in electromicrobiology and have important medical, environmental, and biotechnological relevance. As such, it is essential to continue the momentum of their discovery. However, their study poses unique challenges due to their low current output. Capturing their signal necessitates novel approaches including unconventional electrode choice, the use of sensitive electrochemical techniques, and modifications of conventional experiments that use bioelectrochemical systems (BES).