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The evolution of photoelectrochemical (PEC) bioanalysis has resulted in substantial progress in its analytical performance and biodetection applications. The aim of this review is to provide a panoramic snapshot of the state of the art in this dynamically developing field, with special emphasis on PEC DNA analysis, immunoassay, enzymatic biosensing and cell-related detection. The future prospects in this area are also evaluated and discussed. This work will serve as a useful source to inform the interested audience of the latest developments and applications in the field of PEC bioanalysis. [ABSTRACT FROM AUTHOR]

With reduced background and high sensitivity, photoelectrochemistry (PEC) may be applied as an intracellular nanotool and open a new technological direction of single‐cell study. Nevertheless, the present palette of single‐cell tools lacks such a PEC‐oriented solution. Here a dual‐functional photocathodic single‐cell nanotool capable of direct electroosmotic intracellular drug delivery and evaluation of oxidative stress is devised by engineering a target‐specific organic molecule/NiO/Ni film at the tip of a nanopipette. Specifically, the organic molecule probe serves simultaneously as the biorecognition element and sensitizer to synergize with p‐type NiO. Upon intracellular delivery at picoliter level, the oxidative stress effect will cause structural change of the organic probe, switching its optical absorption and altering the cathodic response. This work has revealed the potential of PEC single‐cell nanotool and extended the boundary of current single‐cell electroanalysis. [ABSTRACT FROM AUTHOR]

With reduced background and high sensitivity, photoelectrochemistry (PEC) may be applied as an intracellular nanotool and open a new technological direction of single‐cell study. Nevertheless, the present palette of single‐cell tools lacks such a PEC‐oriented solution. Here a dual‐functional photocathodic single‐cell nanotool capable of direct electroosmotic intracellular drug delivery and evaluation of oxidative stress is devised by engineering a target‐specific organic molecule/NiO/Ni film at the tip of a nanopipette. Specifically, the organic molecule probe serves simultaneously as the biorecognition element and sensitizer to synergize with p‐type NiO. Upon intracellular delivery at picoliter level, the oxidative stress effect will cause structural change of the organic probe, switching its optical absorption and altering the cathodic response. This work has revealed the potential of PEC single‐cell nanotool and extended the boundary of current single‐cell electroanalysis. [ABSTRACT FROM AUTHOR]

Abstract: A novel hemoglobin (Hb) electrode has been presented through adsorbed Hb onto mesoporous tungsten oxide via one-step electrodeposition. Positions of Soret absorption band showed Hb retained its natural configuration on the resulting films. Electrochemical impedance spectroscopy and cyclic voltammetry were employed to study the electrochemical behavior of adsorbed Hb. The immobilized Hb showed a pair of stable and well-defined peaks at about −284 and −174mV at 100mVs−1 in pH 6.0 buffers and displayed excellent electrocatalytical responses to the reduction of H2O2, and trichloroacetic acid. The presence of meso-WO3 film greatly enhances the protein loadings, accelerates interfacial electron transfer and improves thermal stability of the adsorbed protein. [Copyright &y& Elsevier]

Based on the comprehensive analyses of the experimental results of the electrochemical methods, in situ UV-Vis absorption spectra, in situ electron spin resonance (ESR), and attenuated total-internal reflection (ATR) as well as the calculation of UV-Vis absorption data by PM3 Semi-Empirical method, a reaction mechanism for the redox processes of dobutamine was presented. When the anodic sweep is carried out, dobutamine firstly undergoes a free radical reaction with one-electron and one-proton to form semi-quinone free radicals, which will continuously convert to its corresponding quinone form by further electrochemical oxidation reaction. The formed quinone cannot only undergo a cyclization process by chemical reaction to produce a new compound, which can be reduced at more negative potential, but also be reduced to form dobutamine again when subsequent cathodic sweep is followed. The cyclization rate is depended upon pH values, and it increases with the increase of pH. In neutral medium, the corresponding oxidation form of the cyclization reaction product is easy to convert to melanin. [Copyright &y& Elsevier]

Single-cell analysis (SCA) could provide in-depth understanding of cell biology yet is of great challenge. Many attempts have been made to develop novel techniques for SCA. Nanopipette-based ion current rectification (ICR) electroanalysis represents an emerging methodology to combat in this arena. In this review, the working rationale of ICR and its key regulation factors are discussed, followed by the sensing strategies established for SCA using illustrative examples. Future perspectives in this field are also proposed. • Ion current rectification (ICR)-nanopipette technique for single-cell analysis (SCA) is reviewed. • The fundamentals, operation rationales, and the state of the art are discussed. • Trends and prospects of this field are presented. [ABSTRACT FROM AUTHOR]

Abstract: Ionic current rectification (ICR) based nanopipettes allow accurate monitoring of cellular behavior in single living cells. Herein, we proposed a 30 nm nanopipette functionalized with G‐quadruplex DNAzyme as an efficient biomimetic recognizer for ROS generation at subcellular level via the changes of current–voltage relationship. Taking advantages of the ultra‐small tip, the nanopipette could penetrate into a single living cell repeatedly or keep measuring for a long time without compromising the cellular functions. Coupled with precision nanopositioning system, generation of ROS in mitochondria in response to cell inflammation was determined with high spatial resolution. Meanwhile, the changes of aerobic metabolism in different cell lines under drug‐induced oxidative stress were monitored continuously. We believe that the ICR‐nanopipette could be developed as a powerful approach for the study of cellular activities via electrochemical imaging in living cells. [ABSTRACT FROM AUTHOR]

Abstract: Ionic current rectification (ICR) based nanopipettes allow accurate monitoring of cellular behavior in single living cells. Herein, we proposed a 30 nm nanopipette functionalized with G‐quadruplex DNAzyme as an efficient biomimetic recognizer for ROS generation at subcellular level via the changes of current–voltage relationship. Taking advantages of the ultra‐small tip, the nanopipette could penetrate into a single living cell repeatedly or keep measuring for a long time without compromising the cellular functions. Coupled with precision nanopositioning system, generation of ROS in mitochondria in response to cell inflammation was determined with high spatial resolution. Meanwhile, the changes of aerobic metabolism in different cell lines under drug‐induced oxidative stress were monitored continuously. We believe that the ICR‐nanopipette could be developed as a powerful approach for the study of cellular activities via electrochemical imaging in living cells. [ABSTRACT FROM AUTHOR]

Conformational fluctuations play a central role in the electron transfer reactions of molecules. Because the fluctuations can be extremely fast in kinetics and small in amplitude, a technique with fast temporal resolution and high conformational sensitivity is needed to follow the transient electron transfer processes. Here we report on an electrochemically controlled plasmonic detection technique capable of monitoring conformational changes in redox molecules with ns response time. Using the technique, we study the electron transfer reaction and the associated conformational gating of a redox protein (cytochrome c). The study reveals that the conformational gating takes place over a broad range of time scales, from microsecond to millisecond. [ABSTRACT FROM AUTHOR]

Abstract: As single biomarker often has inadequate predictive value in clinical examination, we designed a dual-biomarker “AND” logic device that defines the concentration of prostate-specific antigen (PSA) and survivin mRNA as inputs and the increase in faradaic current from intercalation agent Co(phen)3 3+ as output for prostate cancer (CaP) diagnosis on Au nano-film covered gold electrode surfaces. Primary capture antibody (Ab1) was first immobilized on AuNP electrode, then while both PSA and survivin mRNA exist, current of Co(phen)3 3+ observably increased due to the formation of dsDNA structure between survivin antisensoligonucleotide connected secondary detection antibody (Ab2) and survivin mRNA. And when one of the two biomarkers was absent, no obvious current increment was found. Thus the designed “AND” logic gate greatly reduced the possibility of false-positive assay from non-cancerous prostate diseases such as prostatic hyperplasia and prostatitis in diagnosis of CaPs. [Copyright &y& Elsevier]

Abstract: An electrochemical method for the simultaneous detection of two different tumor markers, carcinoembryonic antigen (CEA) and α-fetoprotein (AFP), in one-pot, using CdS/DNA and PbS/DNA nanochains as labels was developed. Herein, magnetic beads (MBs) as bimolecule immobilizing carriers, were used for co-immobilization of primary anti-CEA and anti-AFP antibodies. The distinguishable signal labels were synthesized by in situ growth of CdS and PbS nanoparticles on DNA chains, respectively, which were further employed to label the corresponding secondary antibodies. A sandwich-type immunoassay format was formed by the biorecognition of the antigens and corresponding antibodies. The assay was based on the peak currents of Cd2+ and Pb2+ dissolved from CdS and PbS nanoparticles by HNO3 using square wave stripping voltammetry. Experimental results show that the multiplexed electrochemical immunoassay has enabled the simultaneous monitoring of CEA and AFP in a single run with wide working ranges of 0.1–100ngmL−1 for CEA and 0.5–200ngmL−1 for AFP. The detection limits reach to 3.3pgmL−1 for CEA and 7.8pgmL−1 for AFP. [Copyright &y& Elsevier]

Abstract: A novel DNAzyme-functionalized Pt nanoparticles/carbon nanotubes (DNAzyme/Pt NPs/CNTs) bioconjugate was fabricated as trace tag for ultrasensitive sandwich DNA detection. The Pt NPs/CNTs were prepared via layer-by-layer (LBL) assembly of the Pt NPs and polyelectrolyte on the carboxylated CNTs, followed by the functionalization with the DNAzyme and reporter probe DNA through the platinum—sulfur bonding. The subsequent sandwich-type DNA specific reaction would confine numerous DNAzyme/Pt NPs/CNTs bioconjugate onto the gold electrode surface for amplifying the signal. In the presence of 3,3′,5,5′ tetramethylbenzidine (TMB) which could be oxidized by the DNAzyme, electrochemical signals could be generated by chronoamperometry via the interrogation of reduction electrochemical signal of oxidized TMB. The constructed DNA sensor exhibited a wide linear response to target DNA ranging from 1.0fM to 10pM with the detection limit down to 0.6fM and exhibited excellent selectivity against even a single base mismatch. In addition, this novel DNA sensor showed fairly good reproducibility, stability, and reusability. [Copyright &y& Elsevier]

Abstract: On the basis of aptamer-based rolling circle amplification (RCA) and magnetic beads (MBs), a highly sensitive electrochemical method was developed for the determination of Ochratoxin A (OTA). Initially, an amino-modified capture DNA was immobilized onto MBs for the following hybridization with an OTA aptamer and a phosphate labeled padlock DNA. In the presence of OTA, the aptamer would dissociate from the bioconjugate, and the padlock DNA would subsequently hybridize with the capture DNA to form a circular template with the aid of the T4 ligase. Next, capture DNA would act as primer to initiate a linear RCA reaction and hence generate a long tandem repeated sequences by phi29 DNA polymerase and dNTPs. Then, two quantum dots (QDs) labeled DNA probes were tagged on the resulted RCA product to indicate the OTA recognition event by electrochemical readout. This strategy, based on the novel design of OTA-mediated DNA circularization, the combination of RCA and double signal probes introduction, could detect OTA down to the level of 0.2pgmL−1 with a dynamic range spanning more than 4 orders of magnitude. The proposed approach is tested to determine OTA in red wines and shows good application potential in real samples. [Copyright &y& Elsevier]

Abstract: A novel microfluidic immunoassay system for specific detection of hemoglobin A1c (HbA1c) was developed based on a three-component shell/shell/core structured magnetic nanocomposite Au/chitosan/Fe3O4, which was synthesized with easy handling feature of Fe3O4 by magnet, high affinity for gold nanoparticles of chitosan and good immobilization ability for anti-human hemoglobin-A1c antibody (HbA1c mAb) of assembled colloidal gold nanoparticles. The resulting HbA1c mAb/Au/chitosan/Fe3O4 magnetic nanoparticles were then introduced into microfluidic devices coupled with a gold nanoband microelectrode as electrochemical detector. After that, three-step rapid immunoreactions were carried out in the sequence of HbA1c, anti-human hemoglobin antibodies (Hb mAb) and the secondary alkaline phosphatase (AP)-conjugated antibody within 20min. The current response of 1-naphtol obtained from the reaction between the secondary AP-conjugated antibody and 1-naphthyl phosphate (1-NP) increased proportionally to the HbA1c concentration. Under optimized electrophoresis and detection conditions, HbA1c responded linearly in the concentration of 0.05–1.5μgmL−1, with the detection limit of 0.025μgmL−1. This system was successfully employed for detection of HbA1c in blood with good accuracy and renewable ability. The proposed method proved its potential use in clinical immunoassay of HbA1c. [Copyright &y& Elsevier]

Abstract: In this article, an electrochemically direct stripping approach based on silver nanoparticles (AgNPs) labeled with antibody was proposed. To prepare AgNPs labels, glutathione (GSH) was chemically absorbed on the surface of AgNPs through its free thiol groups. Glutaraldehyde was used as a coupling reagent, its two aldehyde groups reacted with the amino group of GSH absorbed on the AgNPs surfaces and the amino groups of antibodies, respectively. The resulting labeled electrochemical active nanoparticles could recognize the proteins specifically via the immobilized antibody. To prove the electrochemical property of labeled AgNPs, human IgG was used as a model protein sample to be immobilized on a screen printed electrode (SPE) and the goat-anti-human IgG labeled AgNPs was added onto the surface of the SPE, followed by differential pulse voltammetric (DPV) method. With the oxidation of AgNPs labels coupled on the electrodes, the concentration of hIgG could be assayed directly and calculated. The dynamic concentration was in the range of 1–1000ng/mL and the detection limit was 0.4ng/mL (S/N =3). In addition, the presented method was also compared to indirect electrochemical stripping detection by dissolving AgNPs labels with nitric acids and the results showed advantages such as lower detection limit, rapidity and simplicity. [Copyright &y& Elsevier]

Abstract: A new format of solid phase microextraction (SPME), boronate affinity SPME, was proposed for the first time for covalent extraction of cis-diol containing biomolecules. This new SPME format is based on the reversible complex formation between boronic acids and 1,2- and 1,3-cis-diols. The complex formation and dissociation can be facilely controlled by changing pH. An extracting phase of poly-3-aminophenylboronate (polyAPBA) electrochemically deposited on a metal wire was employed to demonstrate the concept of this new methodology. Catechol and riboflavin were used as the test analytes, and the SPME extraction was combined off-line with high-performance liquid chromatographic (HPLC) separation followed by UV absorbance or fluorescence detection. Fundamental aspects, such as selectivity, extraction/desorption equilibrium, linearity, effect of competing compounds, reproducibility and life-time, were first investigated. Then the developed method was applied to beer samples since the content of riboflavin plays an important role in the flavor stability of beverages. Excellent performance of the SPME fibers was observed for both standard and real samples. Particularly, the expected excellent features of the polyAPBA extracting phase were experimentally verified, which include specific selectivity, eliminated matrix effect and manipulable capture/release. The new methodology of SPME can be a promising tool since a lot of 1,2- and 1,3-cis-diol-containing compounds are of great biological importance. [Copyright &y& Elsevier]

Abstract: Gold nanofilm, with a nanoporous morphology, was found to have the unique property to catalyze the oxidization of ascorbic acid (AA) and dopamine (DA), resulting in the improved electrochemical behavior of the two species and a negative shift of the oxidization potential of AA for about 300mV. The catalytic oxidization of gold nanofilm to AA is mainly ascribed to its high surface energy, while the enhanced electron transfer ability of DA on the gold nanofilm is attributed to the strong interaction between DA and the electrode surface. Based on the peak separation of DA and AA, selective detection of DA was achieved on the gold nanofilm modified electrode. Differential pulse voltammetry (DPV) techniques and cyclic voltammetry (CV) were used to investigate the electrochemical behavior of AA and DA on the surface of the electrode. Afterwards, the obtained gold nanofilm was applied in selective detection of dopamine (DA) in the presence of ascorbic acid (AA) at near neutral pH by DPV in the optimized conditions. The linear range for DA was 1.5–27.5μM. The electrode also shows good stability during detection. [Copyright &y& Elsevier]

Abstract: We report a simple and novel method of stirring-only-driven accumulation and electrochemical determination of arsenite (As(III)) with both of the oxidation and reduction peaks associated with As(0)/As(III) using a gold nanofilm electrode in neutral solution. Under stirring, a large amount of As(III) was deposited on the modified electrode and the electrochemical response was greatly amplified. The accumulated As(III) on the electrode showed well-defined redox couple in 0.1M blank phosphate buffer solution (pH 7.0), which could be used for the measurement of As(III). Under optimal conditions, As(III) could be detected in the range from 0.20 to 375ppb with a detection limit of 0.04ppb. In particular, with the use of the reduction peak of As(III) the modified electrode exhibits excellent performance for As(III) determination even in the presence of abundant Cu(II). The regeneration of the electrodes is facile with good reproducibility. The electrochemical system was applied to analyze As(III) in lake water, As(III) spiked tap water and drinking water. [Copyright &y& Elsevier]

Abstract: An interesting mode of reactivity of MnO2 nanoparticles modified electrode in the presence of H2O2 is reported. The MnO2 nanoparticles modified electrodes show a bi-direction electrocatalytic ability toward the reduction/oxidation of H2O2. Based on this property, a choline biosensor was fabricated via a direct and facile electrochemical deposition of a biocomposite that was made of chitosan hydrogel, choline oxidase (ChOx) and MnO2 nanoparticles onto a glassy carbon (GC) electrode. The biocomposite is homogeneous and easily prepared and provides a shelter for the enzyme to retain its bioactivity. The results of square wave voltammetry showed that the electrocatalytic reduction currents increased linearly with the increase of choline chloride concentration in the range of 1.0×10−5 –2.1×10−3 M and no obvious interference from ascorbic acid and uric acid was observed. Good reproducibility and stability were obtained. A possible reaction mechanism was proposed. [Copyright &y& Elsevier]

Abstract: The separation of three kinds of aminophenol isomers were achieved within 1min in polyelectrolytes multilayers modified PDMS microchips by layer-by-layer assembly with electrochemical detection (EC). Two polyelectrolytes, poly(dially dimethyl ammonium chloride) (PDDA) and poly(sodium-4-styrene-sulfonate) (PSS) were used to form polyelectrolyte multilayers (PEMs). The surface characteristic of the modified microchip was studied by XPS. The electroosmotic flow (EOF) on PEMs modified PDMS microchips was more stable than that of the native PDMS microchips and the adsorption of samples was greatly reduced on PEMs modified PDMS microchips during the electrophoretic process. The column efficiencies on PEMs modified microchip were increased by 100 times and the signals enhanced by 2 times compared with those of native microchips. The separation conditions such as running buffer pH, running buffer concentration and separation voltage were also optimized. [Copyright &y& Elsevier]

Abstract: A novel film of chitosan-gold nanoparticles is fabricated by a direct and facile electrochemical deposition method and its application in glucose biosensor is investigated. HAuCl4 solution is mixed with chitosan and electrochemically reduced to gold nanoparticles, which can be stabilized by chitosan and electrodeposited onto glassy carbon electrode surfaces along with the electrodeposition of chitosan. Then a model enzyme, glucose oxidase (GOD) is immobilized onto the resulting film to construct a glucose biosensor through self-assembly. The resulting modified electrode surfaces are characterized with both AFM and cyclic voltammetry. Effects of chitosan and HAuCl4 concentration in the mixture together with the deposition time and the applied voltage on the amperometric response of the biosensor are also investigated. The linear range of the glucose biosensor is from 5.0×10−5 ∼1.30×10−3 M with a Michaelis–Menten constant of 3.5 mM and a detection limit of about 13 μM. [Copyright &y& Elsevier]

Abstract: A new strategy for immobilization of horseradish peroxidase (HRP) has been presented by self-assembling gold nanoparticles on chitosan hydrogel modified Au electrode. From a mildly acidic chitosan solution, a chitosan film is electrochemically deposited on Au electrode surface via a negative voltage bias. This process is accompanied by the hydrogen evolution reaction, and the released hydrogen gas made the deposited chitosan film with porous structure, which facilitates the assembly of gold nanoparticles and HRP. The resulting substrates were characterized by atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS). The immobilized HRP displayed an excellent catalytic property to the reduction of H2O2 in the presence of methylene blue mediator. The resulting biosensor (HRP-modified electrode) showed a wide dynamic range of 8.0μM–15mM H2O2, and the linear ranges were 8.0μM–0.12mM and 0.50–12mM, with a detection limit of 2.4μM estimated at a signal-to-noise ratio of 3. Moreover, the biosensor remained about 85% of its original sensitivity after four weeks’ storage. [Copyright &y& Elsevier]

A branched electrode platform was proposed for label-free and reagentless simultaneous tumor markers detection based on different redox substrates. [ABSTRACT FROM AUTHOR]