One-step immunoassay without washing steps for influenza A virus detection using ISFET

Micro- and nano-devices for electrochemical sensing

Electrode miniaturization has profoundly revolutionized the field of electrochemical sensing, opening up unprecedented opportunities for probing biological events with a high spatial and temporal resolution, integrating electrochemical systems with microfluidics, and designing arrays for multiplexed sensing. Several technological issues posed by the desire for downsizing have been addressed so far, leading to micrometric and nanometric sensing systems with different degrees of maturity. However, there is still an endless margin for researchers to improve current strategies and cope with demanding sensing fields, such as lab-on-a-chip devices and multi-array sensors, brain chemistry, and cell monitoring. In this review, we present current trends in the design of micro-/nano-electrochemical sensors and cutting-edge applications reported in the last 10 years. Micro- and nanosensors are divided into four categories depending on the transduction mechanism, e.g., amperometric, impedimetric, potentiometric, and transistor-based, to best guide the reader through the different detection strategies and highlight major advancements as well as still unaddressed demands in electrochemical sensing.

Graphical Abstract

Strategies to simplify operation procedures for applying labeled antibody-based immunosensors to point-of-care testing

Point-of-care testing (POCT) is an ideal testing format for the rapid and on-site detection of analytes in patients, and facilitates disease diagnosis and monitoring. Molecular recognition elements are required for the specific detection of analytes, and biosensors that use antibodies as the molecular recognition elements are called immunosensors. Traditional immunosensors such as sandwich enzyme-linked immunosorbent assay (ELISA) require complicated procedures to form immunocomplexes consisting of detection antibodies, analytes, and capture antibodies. They also require long incubation times, washing procedures, and large and expensive specialized equipment that must be operated by laboratory technicians. Immunosensors for POCT should be systems that use relatively small pieces of equipment and do not require special training. In this review, to help in the construction of immunosensors for POCT, we have summarized the recently reported strategies for simplifying the operation, incubation, and washing procedures. We focused on the optical and electrochemical detection principles of immunosensors, compared the strategies for operation, sensitivity, and detection devices and discussed the ideal system. Combining detection devices that can be fabricated inexpensively and strategies that enable simplification of operation procedures and enhance sensitivities will contribute to the development of immunosensors for POCT.

Electrochemical One-Step Immunoassay Based on Switching Peptides and Pyrolyzed Carbon Electrodes

Switching peptides were designed to bind reversibly to the binding pocket of antibodies (IgG) by interacting with frame regions (FRs). These peptides can be quantitatively released when antigens bind to IgG. As FRs have conserved amino acid sequences, switching peptides can be used as antibodies for different antigens and different source animals. In this study, an electrochemical one-step immunoassay was conducted using switching peptides labeled with ferrocene for the quantitative measurement of analytes. For the effective amperometry of the switching peptides labeled with ferrocene, a pyrolyzed carbon electrode was prepared by pyrolysis of the parylene-C film. The feasibility of the pyrolyzed carbon electrode for the electrochemical one-step immunoassay was determined by analyzing its electrochemical properties, such as its low double-layer capacitance (C_dl), high electron transfer rate (k_app), and wide electrochemical window. In addition, the factors influencing the amperometry of switching peptides labeled with ferrocene were analyzed according to the hydrodynamic radius, the number of intrahydrogen bonds, dipole moments, and diffusion coefficients. Finally, the applicability of the electrochemical one-step immunoassay for the medical diagnosis of the human hepatitis B surface antigen (hHBsAg) was assessed.

Recent advances in enzyme-enhanced immunosensors

Among the products for rapid detection in different fields, enzyme-based immunosensors have received considerable attention. Recently, great efforts have been devoted to enhancing the output signals of enzymes through different strategies that can significantly improve the sensitivity of enzyme-based immunosensors for the need of practical applications. In this manuscript, the significance of enzyme-based signal transduction patterns in immunoassay and the central role of enzymes in achieving precise control of reaction systems are systematically described. In view of the rapid development of this field, we classify these strategies based on the combination of immune recognition and enzyme amplification into three categories, namely enzyme-based enhancement strategies, combination of the catalytic amplification of enzymes with other signal amplification methods, and substrate-based enhancement strategies. The current focus and future direction of enzyme-based immunoassays are also discussed. This article is not exhaustive, but focuses on the latest advances in different signal generation methods based on enzyme-initiated catalytic reactions and their applications in the detection field, which could provide an accessible introduction of enzyme-based immunosensors for the community with a view to further improving its application efficiency.

Current methods and prospects of coronavirus detection

SARS-COV-2 is a novel coronavirus discovered in Wuhan in December 30, 2019, and is a family of SARS-COV (severe acute respiratory syndrome coronavirus), that is, coronavirus family. After infection with SARS-COV-2, patients often experience fever, cough, gas prostration, dyspnea and other symptoms, which can lead to severe acute respiratory syndrome (SARS), kidney failure and even death. The SARS-COV-2 virus is particularly infectious and has led to a global infection crisis, with an explosion in the number of infections. Therefore, rapid and accurate detection of the virus plays a vital role. At present, many detection methods are limited in their wide application due to their defects such as high preparation cost, poor stability and complex operation process. Moreover, some methods need to be operated by professional medical staff, which can easily lead to infection. In order to overcome these problems, a Surface molecular imprinting technology (SM-MIT) is proposed for the first time to detect SARS-COV-2 virus. For this SM-MIT method, this review provides detailed detection principles and steps. In addition, this method not only has the advantages of low cost, high stability and good specificity, but also can detect whether it is infected at designated points. Therefore, we think SM-MIT may have great potential in the detection of SARS-COV-2 virus.

Switching-peptides for one-step immunoassay and its application to the diagnosis of human hepatitis B

Herein, we present switching-peptides for a one-step immunoassay, without the need for additional antibody treatment or washing steps to detect antigen-antibody interactions. Fluorescently labeled switching-peptides were dissociated from the immobilized antibody soon after the antigens were bound to the binding pockets. In this study, four different parts of the antibody (IgG) frame regions were chemically synthesized, and these peptides were bound to immobilized antibodies as switching-peptides. We presented the design principle of switching-peptides and used Pymol software, based on the changes in thermodynamic parameters, to study the interaction between antibodies and switching-peptides. The binding properties of switching-peptides were analyzed based on Förster resonance energy transfer between switching-peptides as well as between switching-peptides and antibodies (IgGs) isolated from different animals. The binding constants of the four switching-peptides to antibodies were estimated to be in the range of 1.48-3.29 μM. Finally, the feasibility of using switching-peptides for the quantitative one-step immunoassay was demonstrated by human hepatitis B surface antigen (hHBsAg) detection and statistical comparison of the assay results with those of conventional ELISA. The limit of detection for HBsAg was determined to be 56 ng/mL, and the dynamic range was estimated to be 136 ng/mL-33 μg/mL. These results demonstrate the feasibility of the one-step immunoassay for HBsAg.

Design and Implementation of a pH Sensor for Micro Solution Based on Nanostructured Ion-Sensitive Field-Effect Transistor

pH sensors based on a nanostructured ion-sensitive field-effect transistor have characteristics such as fast response, high sensitivity and miniaturization, and they have been widely used in biomedicine, food detection and disease monitoring. However, their performance is affected by many factors, such as gate dielectric material, channel material and channel thickness. In order to obtain a pH sensor with high sensitivity and fast response, it is necessary to determine the appropriate equipment parameters, which have high processing cost and long production time. In this study, a nanostructured ion-sensitive field-effect transistor was developed based on the SILVACO technology computer-aided design (TCAD) simulator. Through experiments, we analyzed the effects of the gate dielectric material, channel material and channel thickness on the electrical characteristics of the nanostructured field-effect transistor. Based on simulation results, silicon nitride was selected as the gate dielectric layer, while indium oxide was chosen as the channel layer. The structure and parameters of the dual channel ion-sensitive field-effect transistor were determined and discussed in detail. Finally, according to the simulation results, a pH sensor based on the nanostructured ion-sensitive field-effect transistor was fabricated. The accuracy of simulation results was verified by measuring the output, transfer and pH characteristics of the device. The fabricated pH sensor had a subthreshold swing as low as 143.19 mV/dec and obtained an actual sensitivity of 88.125 mV/pH. In addition, we also tested the oxidation reaction of hydrogen peroxide catalyzed by horseradish peroxidase, and the sensitivity was up to 144.26 pA mol⁻¹ L⁻¹, verifying that the ion-sensitive field-effect transistor (ISFET) can be used to detect the pH of micro solution, and then combine the enzyme-linked assay to detect the concentration of protein, DNA, biochemical substances, biomarkers, etc.