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Conventionally, the photoelectrochemistry relies on freely diffusive signal molecules in solution to stimulate the photocurrent output, leading to limited sensing strategies. Herein, we showcase the methylene blue (MB) embedded duplex DNA for efficient signal stimuli and its application for ultrasensitive photoelectrochemical (PEC) bioassay. Specifically, the MB embedded duplex DNA scavenged the photogenerated holes of petal-like BiVO

Most of the photoelectrochemical (PEC) bioassays need to immobilize biomolecules on electrodes, which lead to tedious modification processes, damaged biomolecules, as well as crippled sensitivity/accuracy and low throughput of the performances. To overcome these drawbacks, we now introduce an exquisitely split-mode (which separates the bioreaction (performed in microplates) from the PEC detection (conducted in PEC cell)) cathodic photoelectrochemistry for probing versatile biocatalytic events with high throughput. Specifically, the enzymatically in situ generated 1,2-bezoquinone was covalently attached onto the PbSe quantum dots (QDs) modified indium tin oxide (ITO) (ITO/PbSe) photocathode through the connector of chitosan (CS). And the attached 1,2-bezoquinone acted as an efficient electron acceptor to promote the cathodic photocurrent of the ITO/PbSe electrode, enabling us to probe quinones-generating oxidoreductase (by taking horseradish peroxidase (HRP) as a model) coupled biocatalytic cascades including the alkaline phosphatase (ALP)/HRP and the glucose oxidase (GOx)/HRP cascades. Quantitative probing for ALP activity in a wide linear range of 5.0 × 10−3 to 10 U/L with the detection limit of 1.2 × 10−3 U/L was realized. While a wide linear range of 5.0 × 10−8 to 1.0 × 10−4 moL/L with a quite low detection limit of 1.0 × 10−8 moL/L was obtained for the glucose assay. In addition, this testing protocol was also extended to an immunoassay (taking carcinoembryonic antigen (CEA) as an example) using HRP as a catalytic tracer. The developed bioassays show high sensitivity and good selectivity for CEA detection in the linear range from 0.1 pg/mL to 100 ng/mL with a detection limit of 0.02 pg/mL. Moreover, the proposed detection has distinctive merits because it not only avoids the adverse effects of the surface confined biomolecules for crippling the signal transduction, but also it has enhanced throughput.

A novel and general strategy of split-type immunoassay is developed based on redox chemical reaction modulated photoelectrochemistry of carbon dots (CDs). Specifically, the photocurrent of the CDs sensitized titanium dioxide nanoparticles (TiO2 NPs) modified fluorine doped indium tin oxide (FTO) (that is the FTO/TiO2/CDs) electrode was inhibited obviously by KMnO4 due to the oxidation of surface hydroxyl groups of CDs to electron accepting carbonyls. While the inhibited photocurrent of the KMnO4 treated FTO/TiO2/CDs electrode can be restored by ascorbic acid (AA) because of the regeneration of electron donating hydroxyls to promote electron-hole separation. Take carcinoembryonic antigen (CEA) as a model analyte and alkaline phosphate (ALP) as a catalytic label tracer to hydrolyze ascorbic acid 2-phosphate (AAP) for producing AA, which greatly stimulated the photocurrent of the transducer of KMnO4 treated FTO/TiO2/CDs photoelectrode for signal output. This redox chemical reaction modulated PEC strategy enabled the separation of the immunoreaction from the photoelectrode (that is, a split-type PEC detection), eliminating potential damage of biomolecules during the PEC detection processes and leading to enhanced throughput detection as compared to conventional PEC configurations. A low detection limit of 7.0 fg/mL was achieved for CEA. This convenient, split-type PEC immunoassay with high throughput may be easily extended to other bioaffinity assays for versatile targets.

Conventionally, the photoelectrochemistry relies on freely diffusive signal molecules in solution to stimulate the photocurrent output, leading to limited sensing strategies. Herein, we showcase the methylene blue (MB) embedded duplex DNA for efficient signal stimuli and its application for ultrasensitive photoelectrochemical (PEC) bioassay. Specifically, the MB embedded duplex DNA scavenged the photogenerated holes of petal-like BiVO4 efficiently, and thus greatly augmented the anodic photocurrent output. Taking the miRNA-21 as a model target, whose biorecognition reaction was aided by the rolling circle amplification (RCA) reaction to finally produce an amplified amount of double-stranded DNA (dsDNA) with embedded MB on the photoelectrode's surface, a “label-free” and “signal-on” PEC biosensing platform was implemented with ultra-sensitivity and high selectivity. The proposed strategy could detect miRNA-21 in the concentration range of 5.0 fM to 10 nM, with the detection limit as low as 0.3 fM. This work opens up the utilization of redox substance intercalated duplex DNA for an efficient signal stimulator, which hints the prospect of other more intercalators embedded DNA for versatile biosensing purposes. Importantly, considering the large quantities of bioreactions that involve duplex DNA as reactants/products, the developed signal transduction strategy may further find wide applications in bioanalysis for targeting more analytes.

In this paper, a novel colorimetric biosensor for Hg(2+) and DNA molecules is presented based on Hg(2+) stimulated oxidase-like activity of bovine serum albumin protected silver clusters (BSA-Ag NCs). Under mild conditions, Hg(2+) activated BSA-Ag NCs to show high catalytic activity toward the oxidation of 3,3',5, 5'-tetramethylbenzidine (TMB) using ambient dissolved oxygen as an oxidant. The oxidase-like activity of BSA-Ag NCs was "switched-on" selectively in the presence of Hg(2+), which permitted a novel and facile colorimetric sensor for Hg(2+). As low as 25 nmol L(-1)Hg(2+) could be detected with a linear range from 80 nmol L(-1) to 50 mmol L(-1). In addition, the sensing strategy was also employed to detect DNA molecules. Hg(2+) is known to bind very strongly and specifically with two DNA thymine bases (T) to form thymine-Hg(2+)-thymine (T-Hg(2+)-T) base pairs. The hairpin-structure was disrupted and Hg(2+) ions were released after hybridization with the DNA target. By coupling the Hg(2+) switched-on the oxidase-mimicking activity of BSA-Ag NCs, we developed a novel label-free strategy for facile and fast colorimetric detection of DNA molecules. More important, target DNA can be detected as low as 10 nmol L(-1) with a linear range from 30 to 225 nmol L(-1). Compared with other methods, this method presents several advantages such as the independence of hydrogen peroxide, high sensitivity and good selectivity, avoiding any modification or immobilization of DNA, which holds a great potential of metal NCs for clinical application in biosensing and biotechnology.