Using G-Quadruplex/Hemin To “Switch-On” the Cathodic Photocurrent of p-Type PbS Quantum Dots: Toward a Versatile Platform for Photoelectrochemical Aptasensing

FeNi-MIL-88B-based electrochemiluminescence immunosensor for ultra-sensitive detection of CD44 protein via dual-quenching strategy

BACKGROUND

Cluster of Differentiation 44 (CD44) is considered an important biomarker for various cancers, and achieving highly sensitive detection of CD44 is crucial, which plays a significant role in tumor invasion and metastasis, providing essential information for clinical tumor diagnosis. Commonly used methods for analysis include fluorescence spectroscopy (FL), photoelectrochemical analysis (PEC), electrochemical analysis (EC), and commercial ELISA kits. Although these methods offer high sensitivity, they can be relatively complex to perform experimentally. Electrochemiluminescence (ECL) has gained widespread research attention due to its high sensitivity, ease of operation, effective spatiotemporal control, and close to zero background signal.

RESULTS

In this work, a sandwich-type ECL immunosensor for detecting CD44 was constructed using luminol as a luminophore. In this sensing platform, bimetallic MOFs (Pd@FeNi-MIL-88B) loaded with palladium nanoparticles (Pd NPs) were used as a novel enzyme mimic, exhibiting excellent catalytic performance towards the electroreduction of H2O2. The hybrids provided a strong support platform for luminol and antibodies, significantly enhancing the initial ECL signal of luminol. Subsequently, core-shell Au@MnO2 nanocomposites were synthesised by gold nanoparticles (Au NPs) encapsulated in manganese dioxide (MnO2) thin layers, as labels. In the luminol/H2O2 system, Au@MnO2 exhibited strong light absorption in the broad UV-vis spectrum, similar to the black body effect, and the scavenging effect of Mn2+ on O2•-, which achieved the dual-quenching of ECL signal. Under the optimal experimental conditions, the immunosensor demonstrated a detection range of 0.1 pg mL-1 - 100 ng mL-1, with a detection limit of 0.069 pg mL-1.

SIGNIFICANCE

Based on Pd@FeNi-MIL-88B nanoenzymes and Au@MnO2 nanocomposites, a dual-quenching sandwich-type ECL immunosensor for the detection of CD44 was constructed. The proposed immunosensor exhibited excellent reproducibility, stability, selectivity, and sensitivity, and provided a valuable analytical strategy and technical platform for the accurate detection of disease biomarkers, and opened up potential application prospects for early clinical treatment.

Self-powered photoelectrochemical immunosensing platform for sensitive CEA detection using dual-photoelectrode synergistic signal amplification

Self-powered photoelectrochemical (PEC) sensing, as an emerging sensing mode, can effectively solve the problems such as weak anti-interference ability and poor signal response of individual photoanode or photocathode sensing. In this work, an ITO/Co-CuInS2 photocathode and ITO/WO3@CdS photoanode based self-powered cathodic PEC immunosensor was developed, which integrated dual-photoelectrode to synergistic amplify the signal for highly sensitive and specific detection of carcinoembryonic antigen (CEA). The self-powered PEC sensor could drive electrons transfer through the difference in Fermi levels between the two photoelectrodes without an external bias voltage. The photoanode was introduced to amplify the photoelectric signal, and the photocathode was only designed for the construction of sensing interfaces. The proposed sensor quantitatively determined the target CEA with the detection limit of 0.23 pg/mL and a linear correlation confine of 0.1 pg/mL ∼100 ng/mL. The constructed immunosensing platform exhibited high sensitivity, satisfactory stability and great biological detection selectivity, providing a feasible and effective strategy for the manufacture of new self-powered sensors in high-performance PEC bioanalytical applications.

Rolling circle amplification assisted CRISPR/Cas12a dual-cleavage photoelectrochemical biosensor for highly sensitive detection of miRNA-21

BACKGROUND

MicroRNA-21 has been determined to be the only microRNA overexpressed in 11 types of solid tumors, making it an excellent candidate as a biomarker for disease diagnosis and therapy. Photoelectrochemical (PEC) biosensors have been widely used for quantification of microRNA-21. However, most PEC biosensing processes still suffer from some problems, such as the difficulty of avoiding the influence of interferents in complex matrices and the false-positive signals. There is a pressing need for establishing a sensitive and stable PEC method to detect microRNA-21.

RESULTS

Herein, a nicking endonuclease-mediated rolling circle amplification (RCA)-assisted CRISPR/Cas12a PEC biosensor was fabricated for ultrasensitive detection of microRNA-21. The p-p type heterojunction PbS QDs/Co3O4 polyhedra were prepared as the quencher, thus the initial PEC signal attained the "off" state. Furthermore, the target was specifically identified and amplified by the RCA process. Then, its product single-stranded DNA S1 activated the cis- and trans-cleavage abilities of CRISPR/Cas12a, leading to almost all of the PbS QDs/Co3O4 polyhedra to leave the electrode surface, the p-n semiconductor quenching effect to be disrupted, and the signal achieving the "super-on" state. This pattern of PEC signal changed from "off" to "on" eliminated the interference of false-positive signals. The proposed PEC biosensor presented a satisfactory linear relationship ranging from 1 fM to 10 nM with a detection limit of 0.76 fM (3 Sb/N).

SIGNIFICANCE AND NOVELTY

With innovatively synthesized PbS QDs/Co3O4 polyhedra as the effective quencher for PEC signal, the CRISPR/Cas12a dual-cleavage PEC biosensor possessed excellent selectivity, stability and repeatability. Furthermore, the detection of various miRNAs can be realized by changing the relevant base sequences in the constructed PEC biosensor. It also provides a powerful strategy for early clinical diagnosis and biomedical research.

S-Scheme Porphyrin Covalent Organic Framework Heterojunction for Boosted Photoelectrochemical Immunoassays in Myocardial Infarction Diagnosis

Cardiac troponin I (cTnI) is an extremely sensitive biomarker for early indication of acute myocardial infarction (AMI). However, it still remains a tough challenge for many newly developed cTnI biosensors to achieve superior sensing performance including high sensitivity, rapid detection, and resistance to interference in clinical serum samples. Herein, a novel photocathodic immunosensor toward cTnI sensing has been successfully developed by designing a unique S-scheme heterojunction based on the porphyrin-based covalent organic frameworks (p-COFs) and p-type silicon nanowire arrays (p-SiNWs). In the novel heterojunction, the p-SiNWs are employed as the photocathode platform to acquire a strong photocurrent response. The in situ-grown p-COFs can accelerate the spatial migration rate of charge carriers by forming proper band alignment with the p-SiNWs. The crystalline π-conjugated network of p-COFs with abundant amino groups also promotes the electron transfer and anti-cTnI immobilizing process. The developed photocathodic immunosensor demonstrates a broad detection range of 5 pg/mL-10 ng/mL and a low limit of detection (LOD) of 1.36 pg/mL in clinical serum samples. Besides, the PEC sensor owns several advantages including good stability and superior anti-interference ability. By comparing our results with that of the commercial ELISA method, the relative deviations range from 0.06 to 0.18% (n = 3), and the recovery rates range from 95.4 to 109.5%. This work displays a novel strategy to design efficient and stable PEC sensing platforms for cTnI detection in real-life serums and provides guidance in future clinical diagnosis.

Ultrasensitive alkaline phosphatase activity assay based on controllable signal probe production coupled with the cathodic photoelectrochemical analysis

A highly sensitive and selective split-type perovskite-based photoelectrochemical (PEC) platform was developed for measuring alkaline phosphatase (ALP) activity in milk and serum samples. ALP in the test sample hydrolyzed 2-phosphate sesquimagnesium salt hydrate (AAPS) in a 96-microwell plate to produce ascorbic acid (AA), a PEC electron donor. The resulting AA, which could preferentially annihilate the photogenerated holes, indirectly reflects ALP activity. The PEC used a cetyltrimethylammonium bromide (CTAB)-functionalized CH₃NH₃PbI₃ (CTAB@CH₃NH₃PbI₃) film as the cathode to monitor the controlled AA production. Due to the excellent photoelectric characteristics of the CH₃NH₃PbI₃ perovskite and the split-type assay, excellent sensitivity and selectivity for ALP detection were obtained. Under the optimum experimental conditions, ALP activity with a limit of detection (LOD) of 2.6 × 10^-4 U/L in a linear dynamic range of 10^-3 ∼ 10² U/L was obtained. With its sensitive, rapid, and high-throughput detection capabilities, this split-type and label-free PEC platform has great potential for use in food and biomedical analysis.

Trap remediation of CuBi2O4 nanopolyhedra via surface self-coordination by H2O2: an innovative signaling mode for cathodic photoelectrochemical bioassay

This work conveys a new philosophy of surface self-coordination mediated trap remediation for innovative cathodic photoelectrochemical (PEC) signal transduction. Initially, the surface trap states of CuBi₂O₄ nanopolyhedra resulting from dangling bonds can function as charge carrier recombination centers, which suppress the carrier separation efficiency and result in a low photocurrent output. Particularly, hydrogen peroxide (H₂O₂) spontaneously interacts with the uncoordinated Cu(II) on the surface of CuBi₂O₄, enabling efficient elimination of dangling bonds and remedy of trap states, thereby outputting intensified photocurrent readout. Exemplified by Flap endonuclease 1 (FEN1) as a model target, a tetrahedron DNA (THD)-based strand displacement amplification (SDA) was introduced to manipulate the formation of hemin impregnated G-quadruplex (G-quadruplex/hemin) DNAzyme and the resultant catalytic reduction for H₂O₂. In addition, a highly efficient and ultra-sensitive PEC sensing platform was achieved for FEN1 detection with a wide linear range from 1.0 fM to 100.0 pM and a detection limit of 0.3 fM (S/N = 3). This work not only establishes a new idea of cathodic PEC signal transduction, but also offers an efficient biosensing platform for FEN1.

Insight into a Target-Induced Photocurrent-Polarity-Switching Photoelectrochemical Immunoassay for Ultrasensitive Detection of Escherichia coli O157:H7

Sensitive, portable methods of detection for foodborne pathogens hold great significance for the early warning and prevention of foodborne diseases and environmental pollution. Restricted by a complicated matrix and limited signaling strategies, developing a ready-to-use sensing platform with ultrahigh sensitivity remains challenging. In this work, near-infrared (NIR) light-responsive AgBiS₂ nanoflowers (NFs) and Cu₂O nanocubes (NCs) were introduced to construct a novel target-induced photocurrent-polarity-switchable system and verified for the development of an all-in-one, ready-to-use photoelectrochemical (PEC) immunosensor. NIR-responsive n-type AgBiS₂ NFs and p-type Cu₂O NCs producing anodic and cathodic photocurrents were conjugated with monoclonal (MAb₁) and polyclonal antibodies (PAb₂), respectively. Using a sandwich-type immunocomplex bridged by Escherichia coli O157:H7, an efficient photocurrent-polarity-switching PEC system was formed on a paper-based working electrode (PWE). Owing to the spatial separation of the photogenerated carriers and the elimination of false-positive/negative signals by the polarity-switchable photocurrent, the proposed NIR PEC immunoassay for E. coli O157:H7 exhibits a considerably low detection limit of 8 colony-forming units/milliliter (CFU/mL) with a linear range from 25 to 5 × 10⁷ CFU/mL. The platform includes a PWE with an automatic cleaning function and a portable PEC analyzer with smartphone-compatible Bluetooth capability, thus achieving point-of-care testing of E. coli O157:H7. The sensor was applied to the analysis of pork samples artificially contaminated with E. coli O157:H7, and the detection results were in good agreement with the plate counting method, a gold standard in the field. This work aimed to investigate the photoelectric activity of the NIR-responsive p/n-type composites and to provide a new signal-reversal route for the construction of an all-in-one ready-to-use PEC immunosensor for the detection of low-concentration biomolecules.

Visual/Photoelectrochemical Off-On Sensor Based on Cu/Mn Double-Doped CeO2 and Branched Sheet Embedded Cu2O/CuO Nanocubes

An integrated dual-signal bioassay was devised to fulfil thrombin (TB) ultrasensitive detection by integrating visualization with the photoelectrochemical technique based on G-quadruplex/hemin. During the process, branched sheet embedded copper-based oxides prepared with illumination and alkaline condition play a vital role in obtaining the desirable photocurrent. The switchover of photoelectrochemical signal was realized by the adjustable distance between electron acceptor G-quadruplex/hemin and interface materials due to dissociation of the Cu/Mn double-doped cerium dioxide (CuMn@CeO₂)/DNA caused by the addition of TB. Then, CuMn@CeO₂ transferred onto visual zones triggered catalytic reactions under the existence of 3,3',5,5'-tetramethylbenzidine and hydrogen peroxide, making a variation in color recognized by the naked eye and providing visual prediction. Under optimized conditions, this bioassay protocol demonstrated wide linear ranges (0.0001-50 nM), high selectivity, stability, and reproducibility. More importantly, the proposed visual/photoelectrochemical transduction mechanism platform exhibits a lower background signal and more reliable detection results, which also offers an effective way for detecting other proteins and nucleic acids.

Ultrasensitive photoelectrochemical immunosensor based on Dual-Photosensitive electrodes

In the study, a photoelectrochemical (PEC) immunosensor based on dual-photosensitive electrodes was developed for cardiac troponin I (cTnI) detection. The sensing photocathode with biometric functions was prepared by CuInS₂ and narrow band gap semiconductor In₂S₃ as the counter electrode. In this way, the separation of photoanode and biometric events was realized, and the ability of stability of the immunosensor could be effectively improved. Moreover, the attraction to the photogenerated electrons (e^-) from photoanode would be increased by the abundant holes (h⁺) of photocathode, under the radiation of light. This tremendously improves the photoelectric response, which further improves the sensitivity of the immunosensor. The controllable-synthesis uncomplicated photoelectric material not only accords with the principle of simplicity of electrode modification but also makes the immunosensor more conducive to the practical application. Additionally, even in the case of zero bias voltage, the constructed PEC immunosensor can operate with high efficiency, namely, self-powered. The immunosensor could provide the quantitative readout photocurrent to a concentration of cTnI in the range of 0.10 pg/mL to 1.00 μg/mL and the detection limit was 0.0113 pg/mL under the optimal experimental conditions. With favorable performance in terms of anti-interference, stability, specificity and reproducibility, this immunosensor will provide new prospects for general PEC bioanalysis development.

Dual-Engine Powered Paper Photoelectrochemical Platform Based on 3D DNA Nanomachine-Mediated CRISPR/Cas12a for Detection of Multiple miRNAs

This work proposed a novel double-engine powered paper photoelectrochemical (PEC) biosensor based on an anode-cathode cooperative amplification strategy and various signal enhancement mechanisms, which realized the monitoring of multiple miRNAs (such as miRNA-141 and miRNA-21). Specifically, C₃N₄ quantum dots (QDs) sensitized ZnO nanostars and BiOI nanospheres simultaneously to construct a composite photoelectric layer that amplified the original photocurrent of the photoanode and photocathode, respectively. Through the independent design and partition of a flexible paper chip to functionalize injection holes and electrode areas, the bipolar combination completed the secondary upgrade of signals, which also provided biological reaction sites for multitarget detection. With the synergistic participation of a three-dimensional (3D) DNA nanomachine and programmable CRISPR/Cas12a shearing tool, C₃N₄ QDs lost their attachment away from the electrode surface to quench the signal. Moreover, electrode zoning significantly reduced the spatial cross talk of related substances for multitarget detection, while the universal trans-cleavage capability of CRISPR/Cas12a simplified the operation. The designed PEC biosensor revealed excellent linear ranges for detection of miRNA-141 and miRNA-21, for which the detection limits were 5.5 and 3.4 fM, respectively. With prominent selectivity and sensitivity, the platform established an effective approach for trace multitarget monitoring in clinical applications, and its numerous pioneering attempts owned favorable reference values.

Black phosphorous quantum dots for signal-on cathodic photoelectrochemical aptasensor monoitoring amyloid β peptide

In this paper, a quantitative cathodic photoelectrochemical aptasensor is described by using black phosphorous quantum dots (BPQDs) as photoactive material and assisted by heme as electron acceptor for sensing of amyloid β peptide (Aβ). Specifically, BPQDs were synthesized by solvothermal method and characterized by various techniques. The as-prepared BPQDs were assembled on the transparent indium tin oxide electrode, and the positively charged poly-l-lysine (PLL) was then absorbed onto BPQDs via electronic interaction. Subsequently, the aptamer as the specific recognition element for Aβ oligomer was introduced on the BPQDs-PLL modified electrode. After bound with heme to form Aβ-heme complex, Aβ oligomer was simultaneously captured by the aptamer on the electrode, resulting in an enhanced photocurrent response. Under the optimized conditions, the present PEC sensor reveals a good linear response to Aβ peptide ranging from 1.0 fM to 100 nM with a detection limit of 0.87 fM. The present signal-on cathodic PEC bioassay possesses the potential to create a new paradigm in amplified PEC assays that could provide outstanding performance for bioanalysis.

Surface polarization of BiOI to boost the photoelectrochemical signal transduction for high performance bioassays

The surface hydroxylation induced polarization (SHIP) is disclosed as an effective tactic to promote the cathodic photoelectrochemical (PEC) communication of bismuth oxyiodide with doxorubicin (Dox) by as large as three...

Invoking Cathodic Photoelectrochemistry through a Spontaneously Coordinated Electron Transporter: A Proof of Concept Toward Signal Transduction for Bioanalysis

Most of the cathodic photoelectrochemical (PEC) bioassays rely on electron accepting molecules for signal stimuli; unfortunately, the performances of which are still undesirable. New signal transduction strategies are still highly expected for the further development of cathodic photoelectrochemistry as a potentially competitive method. This work represents a new concept of invoked cathodic photoelectrochemistry by a spontaneously formed electron transporter for innovative operation of the sensing strategy. Specifically, the hexacyanoferrate(II) in solution easily self-coordinated with CuO nanomaterials and formed electron transporting copper hexacyanoferrate (CuHCF) on the surface, which endowed improved carrier separation for presenting augmented photocurrent readout. Exemplified by the T4 polynucleotide kinase (T4 PNK) and its inhibitors as targets, a homogenous cathodic PEC biosensing platform was achieved with the distinctive merits of label-free, immobilization-free, and split-mode readout. The mechanism revealed here provided a totally different perspective for signal transduction in cathodic photoelectrochemistry. Hopefully, it may stimulate more interests in the design and construction of semiconductor/transporter counterparts for exquisite operation of photocathodic bioanalysis.

Rational Utilization of Photoelectrochemistry of Photosystem II for Self-Powered Photocathodic Detection of MicroRNA in Cells

The photoanode, photosystem II (PSII)/hierarchical inverse opal (IO) TiO₂, is coupled to the complementary photocathode, PbS quantum dots (QDs)/DNA probes, which is then integrated into a two-compartment photoelectrochemical (PEC) cell to achieve a self-powered system to enable photocathodic detection of microRNA-10b from HeLa cells. In such a system, all of the PSII catalytic products, i.e., electrons, protons, and O₂, were rationally utilized and could overcome the general issue of varied O₂ levels in photocathodic detection. The correlation between the target-triggered formation of the DNA complexes and the catalytic reduction of the dissolved O₂ makes possible the steady microRNA-10b detection with good sensitivity and selectivity. This work has unveiled the ability of PSII to construct self-powered detecting devices and shed light on its application in new arenas.

Intercalated doxorubicin acting as stimulator of PbS photocathode for probing DNA-protein interactions

Label-free and turn-on DNA-binding protein detection based on the doxorubicin (Dox)-intercalated DNA as a signal stimulator in cathodic photoelectrochemistry is reported. The double-stranded DNA (dsDNA) acted as the matrix accommodating the intercalative Dox and allowed its effective photoelectrochemical (PEC) communication with the PbS quantum dots (QDs) for realizing cathodic photocurrent readout. In the presence of the target of the vascular endothelial growth factor (VEGF), the dsDNA was prevented from being digested by the exonuclease III (Exo III), allowing the anchor of Dox to perform as activation stimuli of the photocurrent. The VEGF can be detected in the linear range from 1.5 pM to 100 nM, with an impressively low detection limit of 0.49 pM. This study hints the prospect of DNA intercalated architectures as innovative signaling transduction elements for wide and versatile cathodic PEC bioassays. Effective signaling molecules that are conducive to probe-related cathodic PEC bioassays using DNA as the recognition or signification elements are scarce but very demanding. Herein, the doxorubicin intercalated in duplex DNA functions as an efficient signal stimulator of PbS-consisted photocathode, and thus hints the versatility of the strategy for various targets through cathodic photoelectrochemistry.

Novel Anti-Interference Strategy for a Self-Powered Sensor: Mediator-Free and Biospecific Photocathode Interface

As a new electrochemical sensing concept, a self-powered sensor shows a good application prospect in the field of analysis. However, it is still a great challenge to improve the anti-interference capability of sensors through reasonable design. In this study, we investigated the difference between the single photoanode and photocathode self-powered sensor and combined the advantages of these two aspects to fabricate a mediator-free self-powered aptasensor based on the dual-photoelectrode system, which combined the biological events from the photocathode. The biological events occurred at the photocathode could avoid the interference caused by the generated hole oxidation of reducing small molecules in the real sample on the photoanode surface, which was helpful to enhance the anti-interference capability of the sensor. Moreover, due to the sufficient Fermi level differentiation between two photoelectrodes, the redox mediator was not necessary. This could avoid the redox reaction caused by the introduction of extra electron donors or electron acceptors occurring before the photoelectrical behavior, thus improving the accuracy of the sensor. According to the influence of the generated biological conjugate on the external circuit, electron transmission between interfaces, and the obstruction of visible light irradiation, the sensitive and accurate detection of the analytical model was achieved. This work provided a proof-of-concept for the establishment of a mediator-free dual-photoelectrode self-powered sensing platform with high sensitivity and strong anti-interference performance.

An aptasensor for the detection of Pb2+ based on photoinduced electron transfer between a G-quadruplex-hemin complex and a fluorophore

Due to the threat to health of heavy metal contamination, simple and rapid detection methods for heavy metals are an urgent needed in environment protection and food safety. In this work, we have developed a fluorescent aptasensor for the 'turn-off' model detection of Pb²⁺ . The key feature of the aptasensor is that the dye-labelled nucleic acid strand can be folded into a G-quadruplex structure in the presence of Pb²⁺ . This conformational change induces photoinduced electron transfer (PET) between a G-quadruplex-hemin complex and 6-carboxyrhodamine X (ROX), which results in fluorescence quenching of ROX. We systematically investigated the interaction mechanism between Pb²⁺ and the aptasensor and the effects of several environmental parameters were also studied. Under the optimum conditions, the proposed method exhibited a good liner relationship between the concentration of Pb²⁺ and fluorescence quenching efficiency in the range 25-500 nM and the limit of detection was 1.02 nM. In addition, this method also manifested good performance in spiked lettuce samples with satisfactory recoveries of 87.10-109.6%. This target-induced PET platform can be further expanded to other heavy metal analysis.

Femtomolar Detection of Thrombin in Serum and Cerebrospinal Fluid via Direct Electrocatalysis of Oxygen Reduction by the Covalent G4-Hemin-Aptamer Complex

Thrombin, a serine protease playing a central role in the coagulation cascade reactions and a potent neurotoxin produced by injured brain endothelial cells, is a recognized cardiac biomarker and a critical biomarker for Alzheimer's disease development. Both in vivo and in vitro, its low physiological concentrations and nonspecific binding of other components of physiological fluids complicate electroanalysis of thrombin. Here, femtomolar levels of thrombin in serum and an artificial cerebrospinal fluid (CSF) were detected by the indicator-free electrochemical methodology exploiting the O₂ reduction reaction directly, with no electron transfer mediators, electrocatalyzed by the covalent G4-hemin DNAzyme complex naturally self-assembling upon thrombin binding to the hemin-modified 29-mer DNA aptamer sequence tethered to gold via an alkanethiol linker. Coadsorbed PEG inhibited nonspecific protein binding and allowed the sought signal resolution. The proposed assay exploiting the "oxidase" activity of G4-hemin DNAzyme does not require any coreactants necessary for the traditional peroxidase activity-based assays with this DNAzyme, such as H₂O₂ and redox mediators, or solution deaeration and allows fast, overall 30 min analysis of thrombin in aerated buffer, CSF, and 1% human serum solutions. This pioneer approach exploiting the oxidase activity G4-hemin DNAzyme is simple, sensitive, and selective and represents a new tool for ultrasensitive electrocatalytic assays based on simple and efficient O₂-dependent DNAzyme labels.

A photocurrent-polarity-switching biosensor for highly selective assay of mucin 1 based on target-induced hemin transfer from ZrO2 hollow spheres to G-quadruplex nanowires

Herein, a photocurrent polarity switching platform for highly selective assay of mucin 1 (MUC1) was developed based on target-induced hemin transfer from ZrO₂ hollow spheres (ZrO₂ HSs) to G-quadruplex nanowires (G wires). In this system, SiO₂ spheres were used as templates to synthesize the uniform and mesoporous ZrO₂ HSs. As nanocontainers, ZrO₂ HSs could load hemin in its cavity via pores. Then, the aptamers of MUC1, as bio-gates, blocked the pores of ZrO₂ HSs based on the specific binding of Zr⁴⁺ and the phosphate groups of aptamer. In the presence of MUC1, the aptamer could specifically recognize and bind with MUC1, and then leave away from the surface of ZrO₂ HSs, which resulted in the opening of the bio-gates and releasing of hemin. Assisted with the G wires formed on the Au NPs/In₂S₃/ITO, the released hemin was captured on the electrode through the formation of hemin/G-quadruplex structure, leading to the switch of the photocurrent polarity of the electrode from anodic photocurrent to cathodic photocurrent. The proposed photoelectrochemical biosensor showed outstanding performance for MUC1 assay with high selectivity, wide linear response range (1 fg mL^-1 -10 ng mL^-1) and lower detection limit (0.48 fg mL^-1). And the strategy could be easily extended to a triple-mode detection of MUC1 because the hemin/G-quadruplex structure was widely used in electrochemical and colorimetric methods as a hydrogen peroxide mimetic enzyme, which might provide wide applications in biological or clinical studies.

Cathode-Anode Spatial Division Photoelectrochemical Platform Based on a One-Step DNA Walker for Monitoring of miRNA-21

Photoelectrochemical (PEC) biosensors carried out the whole reaction process in the same solution, which would limit the sensitivity and selectivity of detection in the sensing system. Herein, we reported a promising new cathode-anode spatial division PEC platform based on the two-electrode synergistic enhancement strategy. With the photoanode and photocathode integrated in the same current circuit, the platform exhibited an increased photocurrent response, as well as an improved anti-interference ability led by separating the two electrodes spatially. In this proposal, red light-driven AgInS₂ nanoparticles (NPs) served as the photoanode to build biometric steps and amplify the signal, whereas p-type PbS quantum dots were selected as the photocathode to increase the signal. With the participation of alkaline phosphatase (ALP) labeled on Au NPs-DNA, ascorbic acid 2-phosphate was catalyzed to produce ascorbic acid as an electron donor, resulting in the enhancement of the PEC signal. Interestingly, in the presence of miRNA-21 and T7 Exo, the one-step DNA walker amplification can be triggered to reduce the PEC signal by releasing ALP-Au NP-DNA. The constructed PEC biosensor exhibited a detection limit of as low as 3.4 fM for miRNA-21, which was expected to be applied to early clinical diagnosis. Also, we believe that the proposed cathode-anode spatial division PEC platform can open up a new view for the establishment of other types of PEC biosensors.

Construction of a Dye-Sensitized and Gold Plasmon-Enhanced Cathodic Photoelectrochemical Biosensor for Methyltransferase Activity Assay

DNA methyltransferases may function as important biomarkers of cancers and genetic diseases. Herein, we develop a dye-sensitized and gold plasmon-enhanced cathodic photoelectrochemical (PEC) biosensor on the basis of p-type covalent organic polymers (COPs) for the signal-on measurement of M.SssI methyltransferase (M.SssI MTase). The cathodic PEC biosensor is constructed by the in situ growth of p-type COP films onto a glass coated with indium tin oxide and the subsequent assembly of biotin- and HS-labeled double-stranded DNA (dsDNA) probes onto the COP film via biotin-streptavidin interaction. The dsDNA probe contains the recognition sequence of M.SssI MTase. The COP thin films possess a porous ultrathin nanosheet structure with abundant active sites, facilitating the generation of a high photocurrent compared with the hydrothermally synthesized ones. The presence of DNA methyltransferases can prevent the digestion of restriction endonuclease HpaII, consequently inducing the introduction of gold nanoparticles (AuNPs) to the dsDNA probes via the S-Au bond and the intercalation of rhodamine B (RhB) into the DNA grooves to produce a high photocurrent due to the dye-photosensitized enhancement and AuNP-mediated surface plasmon resonance. However, in the absence of M.SssI MTase, HpaII digests the dsDNA probes, and neither AuNPs nor RhB can be introduced onto the electrode surface, leading to a low photocurrent. This cathodic PEC biosensor possesses high sensitivity and good selectivity, and it can screen the inhibitors and detect M.SssI MTase in serum as well.

A cathode photoelectrochemical assay of terminal deoxynucleotidyl transferase activity based on Ag-AgI-CNTs composite and surface multisite strand displacement amplification

Photocathode-based assay is anti-interference for real sample detection. Photocathode produces low photocurrent signal and gives rise to poor sensitivity. Herein, a novel cathode photoelectrochemical (CPEC) sensing platform based on Ag-AgI-CNTs as photocathode material and K₃[Fe(CN)₆] as photoelectron acceptor was established. Since [Fe(CN)₆]^3- effectively accepted photoelectrons from Ag-AgI-CNTs, it greatly enhanced the CPEC response. Combining a surface multisite strand displacement amplification (SMSDA) strategy, the CPEC platform was applied for the activity assay of terminal deoxynucleotidyl transferase (TdT). In this proposal, oligo dT primer tethered on CPEC platform was in-situ extended to generate a polyA tail. Then the polyA tail formed a stable multi-point hybrid structure with the adjacent oligo dT. After launching the SMSDA, the CPEC platform was covered by more elongated polynucleotide chains and network, which acutely hampered the photoelectron transfer (eT) between photocathode and electron acceptor and caused a reduced photocurrent. The CPEC sensor possessed a satisfactory linear response from 6 × 10^-5-0.1 U and a low detection limit of 1.1 × 10^-5 U. The strategy offered a more specific and sensitive method for TdT activity assay. It was feasible in the field of TdT-based biochemical research, drug screening, and disease diagnosis.

Porphyrin-Based Covalent Organic Framework Thin Films as Cathodic Materials for "On-Off-On" Photoelectrochemical Sensing of Lead Ions

Currently, cathodic photoelectrochemical (PEC) sensors, which could effectively reduce background interference, are urgently required for ultrasensitive environmental monitoring. Herein, porphyrin-based covalent organic framework (TAPP-COF) thin films were fabricated via a bottom-up growth approach on the liquid/liquid interface and applied as a photocathode material to "on-off-on" PEC sensing of Pb²⁺. Benefitting from the unique charge channels of COFs and the good photoelectric properties of porphyrin, the as-prepared TAPP-COF thin films presented an improved photocathodic current, with a strongly enhanced "signal-on" response with low background. Then, CdSe@SiO₂ quantum dots (QDs), as a quenching agent, were introduced through a hybridization chain reaction (HCR) to obtain a "signal off" PEC response. Afterward, with the introduction of target Pb²⁺, CdSe@SiO₂ QDs were detached from TAPP-COF thin films, and the PEC response transformed into a signal-on state. Benefiting from the multiple-quenching and steric hindrance effect of CdSe@SiO₂ QDs and the photocathodic property of TAPP-COFs, accurate monitoring of Pb²⁺ in a wide detection range from 0.05 to 1000 nM with a lower detection limit of 0.012 nM was realized based on the proposed on-off-on PEC approach. Notably, the methodology provides an efficient platform for ultrasensitive determination of heavy metal ions, which would play a significant role in environmental monitoring and public safety fields.

Nanostructure-based photoelectrochemical sensing platforms for biomedical applications

As a newly developed and powerful analytical method, the use of photoelectrochemical (PEC) biosensors opens up new opportunities to provide wide applications in the early diagnosis of diseases, environmental monitoring and food safety detection. The properties of diverse photoactive materials are one of the essential factors, which can greatly impact the PEC performance. The continuous development of nanotechnology has injected new vitality into the field of PEC biosensors. In many studies, much effort on PEC sensing with semiconductor materials is highlighted. Thus, we propose a systematic introduction to the recent progress in nanostructure-based PEC biosensors to exploit more promising materials and advanced PEC technologies. This review briefly evaluates the several advanced photoactive nanomaterials in the PEC field with an emphasis on the charge separation and transfer mechanism over the past few years. In addition, we introduce the application and research progress of PEC sensors from the perspective of basic principles, and give a brief overview of the main advances in the versatile sensing pattern of nanostructure-based PEC platforms. This last section covers the aspects of future prospects and challenges in the nanostructure-based PEC analysis field.

Chemical-chemical redox cycling amplification strategy in a self-powered photoelectrochemical system: a proof of concept for signal amplified photocathodic immunoassay

A chemical-chemical redox cycling amplification strategy was introduced into a photocathodic immunosensing system. To prove the applicability of the method, a novel self-powered photochemical system by integrating the photoanode and photocathode was designed for protein analysis.

Ultrasensitive DNA Detection Based on Inorganic-Organic Nanocomposite Cosensitization and G-Quadruplex/Hemin Catalysis for Signal Amplification

A novel photoelectrochemical (PEC) aptasensor was fabricated for DNA detection based on the coupling of cosensitization and peroxidase-like catalytic activity. Specifically, the surfaces of branched-TiO₂ nanorods (B-TiO₂ NRs) were modified with Cd²⁺ and S²⁺ to obtain B-TiO₂ NRs/CdS hybrid structures, which were subsequently used as matrices to immobilize hairpin DNA (hDNA) probes. CdTe/TCPP (TCPP = meso-tetra(4-carboxyphenyl)-porphine) used for signal amplification was labeled on the terminal of the hDNA probe. Without the target DNA (tDNA) presence, the immobilized hDNA probe with CdTe/TCPP possessed a hairpin form and was located near the B-TiO₂ NRs/CdS electrode surface, forming a cosensitized structure formation and then generating strong photocurrent with H₂O₂ as the electron donor. During detection, the specific recognition of tDNA by the sensing hDNA probe triggered the formation of the G-quadruplex/hemin DNAzyme, which effectively catalyzed the decomposition of H₂O₂. Meanwhile, cosensitization disappeared when the hDNA probe hybridized with tDNA, further reducing the photocurrent. With a double-signal amplification strategy, the sensing platform designed in this work demonstrated a linear detection ability in the 0.5 fM-5 nM range with a detection limit equal to 0.14 fM. Notably, through encoding in the base sequences of the hDNA and marking it, a versatile PEC platform could be structured for the detection of various DNA targets, which could promise applications in point-of-care diagnostic fields.

Recent Advances in Electrochemical Monitoring of Chromium

The extensive use of chromium by several industries conducts to the discharge of an immense quantity of its various forms in the environment which affects drastically the ecological and biological lives especially in the case of hexavalent chromium. Electrochemical sensors and biosensors are useful devices for chromium determination. In the last five years, several sensors based on the modification of electrode surface by different nanomaterials (fluorine tin oxide, titanium dioxide, carbon nanomaterials, metallic nanoparticles and nanocomposite) and biosensors with different biorecognition elements (microbial fuel cell, bacteria, enzyme, DNA) were employed for chromium monitoring. Herein, recent advances related to the use of electrochemical approaches for measurement of trivalent and hexavalent chromium from 2015 to 2020 are reported. A discussion of both chromium species detections and speciation studies is provided.

Signal-on cathodic photoelectrochemical aptasensing of insulin: Plasmonic Au activated amorphous MoSx photocathode coupled with target-induced sensitization effect

Cathodic photoelectrochemical (PEC) bioassay is more resistant to reductive interferents, and development of high-performance photocathode is imperatively required in precise monitoring target in complex matrices. In this work, a plasmonic Au activated amorphous MoS_x photocathode (a-MoS_x/Au) was fabricated by sequential electrodeposition. Coupled with a sensitization amplification strategy induced by target-aptamer recognition, an ultrasensitive and high-affinitive signal-on cathodic PEC aptasensor for insulin detection was developed. Under optimum conditions, the sensor exhibits a wide linear range (0.1 pg/mL~100 ng/mL) and an ultralow detection limit (28 fg/mL) even lower than most sensors reported so far. Plasmonic Au activation and target-induced sensitization effect are responsible for high-performance PEC aptasensing of insulin at a-MoS_x photocathode.

Self-powered cathodic photoelectrochemical aptasensor based on in situ-synthesized CuO-Cu2O nanowire array for detecting prostate-specific antigen

A facile and sensitive self-powered cathodic photoelectrochemical (PEC) aptasensor is reported for the detection of prostate-specific antigen (PSA) based on CuO-Cu₂O nanowire array grown on Cu mesh (CuO-Cu₂O NWA/CM) as electrode. The mixed narrow band gaps of the CuO-Cu₂O heterostructure ensured its wide absorption band, effective electron/hole separation, and high photocatalytic activity in the visible region. In addition, nanowires directly grown on the substrate provided high specific surface area and exposed abundant active sites, thus guaranteeing its high photocatalytic efficiency. Therefore, the self-powered sensor exhibited favorable analytical performance with fast response, wide linear ranges of 0.01 to 5 ng/mL and 5 to 100 ng/mL, an acceptable detection limit of 3 pg/mL, and reasonable selectivity and stability. The proposed CuO-Cu₂O NWA/CM can be considered a promising visible light-responsive photoactive material for fabrication of PEC aptasensor with high performance. Graphical abstract a Schematic illustration of construction process of PEC sensing platform based on the CuO-Cu₂O composite for PSA detection. b Schematic mechanism of the operating PEC system.

Cathode Photoelectrochemical Paper Device for microRNA Detection Based on Cascaded Photoactive Structures and Hemin/Pt Nanoparticle-Decorated DNA Dendrimers

In this work, a lab-on-paper cathode photoelectrochemical (PEC) sensing platform was constructed for ultrasensitive microRNA-141 (miRNA-141) assay using cascaded multiple photo-active structures as signal generators and hemin/Pt nanoparticle (Pt NP) trunk-branching-decorated DNA dendrimers as signal reinforcers. Specifically, pyramid-like Cu₂O was first in situ grown on the Au nanoparticle-functionalized tangled cellulose fibers network, followed by the sensitization of trepang-like BiVO₄-Bi₂S₃ heterostructures, forming the cascaded sensitization structures. Then, the DNA dendrimer was introduced into the photocathode sensing interface by coupling the duplex-specific-nuclease (DSN)-induced target recycling reaction with multiple-branched hybridization chain reaction (MHCR). The programmed target recycling procedures propelled using DSN guaranteed the highly amplified transduction of miRNA-141 to the exposed initiator strand, which triggered the cascaded MHCR accompanied by the formation of the DNA dendrimer with unique trunk-branching structures. Finally, the hemin/Pt NP trunk-branching-decorated DNA dendrimer (HPTD) was acquired by the assembly of Pt NPs and hemin on the trunk and branch, respectively. The resulting HPTD with the synergy catalysis of Pt NPs and hemin could efficiently catalyze the decomposition of H₂O₂ for in situ generation of O₂ as the electron acceptor, leading to an enhanced photocurrent response. Based on the target-dependent photocurrent enhancement, ultrasensitive determination of miRNA-141 was realized with persuasive selectivity, high stability, and excellent reproducibility. Thus, the proposed paper-based cathode PEC sensing platform possessed promising application prospect in clinical miRNA diagnosis.

Synergetic Ag2S and ZnS quantum dots as the sensitizer and recognition probe: A visible light-driven photoelectrochemical sensor for the "signal-on" analysis of mercury (II)

A visible-light-driven photoelectrochemical (PEC) sensor has been developed for the "signal-on" analysis of Hg²⁺ by the synergetic combination of low-bandgap Ag₂S and wide-bandgap ZnS quantum dots (QDs). Ag₂S QDs were synthesized with bead-chain-like structure by the self-assembly route and further covalently bound with ZnS QDs to be coated onto the indium tin oxide (ITO) electrodes. It was discovered that the ZnS@Ag₂S-modified electrodes could display the visible-light-driven PEC behavior, of which Ag₂S and ZnS QDs could act as the PEC sensitizer and Hg²⁺-recognition probe, respectively. More importantly, the photocurrent responses of the developed electrodes could be specifically turned on in the presence of Hg²⁺ under the visible-light irradiation, presumably due to that Hg²⁺ might conduct a Zn-to-Hg exchange on ZnS QDs to trigger the formation of HgS/ZnS@Ag₂S heterojunction towards the enhanced electron-hole separation. The as-prepared PEC sensor could facilitate the detection of Hg²⁺ with concentrations ranging from 0.010-1000 nM, with a detection limit of about 1.0 pM. Besides, the feasibility of practical applications of the developed PEC analysis strategy was verified by probing Hg²⁺ in environmental water samples. Such a visible-light-driven PEC detection platform with the unique "turn-on" signal output may promise for the extensive applications for Hg²⁺ evaluation.

A photoelectrochemical aptasensor for the detection of 17β-estradiol based on In2S3 and CdS co-sensitized cerium doped TiO2

In₂S₃ and CdS co-sensitized Ce doped TiO₂ optimized the transmission path of electrons.

An CuInS2 photocathode for the sensitive photoelectrochemical determination of microRNA-21 based on DNA-protein interaction and exonuclease III assisted target recycling amplification

A photocathode is described for the determination of microRNA-21 by using CuInS₂ as an active photocathode material. Exonuclease III assisted target recycling amplification was employed to enhance the detection sensitivity. The TATA-binding protein (TBP) was applied to enhance steric hindrance which decreases the photoelectrochemical intensity. This strategy is designed by combining the anti-interference photocathode material, enzyme assisted target recycling amplification and TBP induced signal off, showing remarkable amplification efficiency. Under the optimized conditions, the detection limit for microRNA-21 is as low as 0.47 fM, and a linear range was got from 1.0 × 10^-15 M to 1.0 × 10^-6 M. Graphical abstract Schematic representation of sensitive photoelectrochemical detection of microRNA-21.CuInS₂ is used as an active photocathode material. Combined Exonuclease III assisted target recycling amplification and TATA-binding protein decreased of photoelectrochemical intensity, the detection limit was 0.47 fM with good selectivity. (miR-21: microRNA-21; CS: chitosan).

A novel photoelectrochemical strategy based on an integrative photoactive heterojunction nanomaterial and a redox cycling amplification system for ultrasensitive determination of microRNA in cells

An ultrasensitive photoelectrochemical (PEC) bioassay for determination of microRNA was proposed based on an integrative photoactive heterojunction nanomaterial to provide the basis of excellent PEC responses and an efficient redox cycling amplification system to improve the detection performances. To establish the bioassay system, the biosensor was firstly modified with Bi₂WO₆@Bi₂S₃ and alkaline phosphatase (ALP). The detection solution was composed of ascorbic acid phosphate (AAP) and ferrocenecarboxylic acid (FcA), where ALP converted AAP into ascorbic acid (AA) to trigger a process of redox cycling amplification by reducing FcA⁺ to FcA, resulting in enhanced photocurrent responses of Bi₂WO₆@Bi₂S₃. In the presence of microRNA 21, it could trigger a hybridization chain reaction via the special designed hairpin DNA to produce a long repeated DNA sequences to inhibit ALP activity. Thus the reduced ALP activity and consequently decreased photocurrent signal could be obtained for detection of microRNA 21. As expected, this bioassay system performed the satisfactory performances for the ultrasensitive detection of microRNA 21 in the range from 1 fM to 1 nM with an experimental detection limit of 0.26 fM and acceptable practical applicability. Collectively, an efficient PEC bioassay for microRNA 21 is established and this strategy can be expanded to detect other microRNAs, even other molecules in cells.

Three-Dimensional TiO2@Cu2O@Nickel Foam Electrodes: Design, Characterization, and Validation of O2-Independent Photocathodic Enzymatic Bioanalysis

This work reports the innovative design and application of a three-dimensional (3D) TiO₂@Cu₂O@nickel foam electrode synergized with enzyme catalysis toward the proof-of-concept study for oxygen-independent photocathodic enzymatic detection. Specifically, a 3D-nanostructured photoelectrode has great potential in the semiconductor-based photoelectrochemical (PEC) biological analysis. On the other hand, using various photocathodes, cathodic PEC bioanalysis, especially the photocathodic enzymatic detection, represents an attractive frontier in the field. Different from state-of-the-art photocathodic enzymatic studies that are oxygen-dependent, herein, we present the ingenious design, characterization, and implementation of 3D TiO₂@Cu₂O@nickel foam photocathodes for the first oxygen-independent example. In such a configuration, the Cu₂O acted as the visible-light absorber, while the TiO₂ shell would simultaneously function as a protective layer for Cu₂O and as a desirable substrate for the immobilization of enzyme biomolecules. Especially, because of the proper band positions, the as-designed photocathode exhibited unique O₂-independent PEC property. Exemplified by glucose oxidases, the as-developed sensor exhibited positive response to glucose with good performance. Because various oxidases could be integrated with the system, this protocol could serve as a universal O₂-independent platform for many other targets. This work is also anticipated to catalyze more studies in the advanced 3D photoelectrodes toward innovative enzymatic applications.

Electrochemical sandwich aptasensor for the carcinoembryonic antigen using graphene quantum dots, gold nanoparticles and nitrogen doped graphene modified electrode and exploiting the peroxidase-mimicking activity of a G-quadruplex DNAzyme

A sandwich-type electrochemical aptasensor has been constructed and applied for sensitive and selective detection of the carcinoembryonic antigen (CEA). The surface of a glassy carbon electrode (GCE) was first modified with nitrogen-doped graphene and then gold nanoparticles and graphene quantum dots electrodeposited on it to obtain an architecture of type GQD/AuNP/NG/GCE. In the next step, the CEA-binding aptamer was immobilized on the modified GCE. Hemin intercalates in the amino-modified hemin aptamer to form a hemin-G-quadruplex (hemin-G4) DNAzyme. The amino modified CEA aptamer II is connected to hemin-G4 by glutaraldehyde (GA) as a linker to produce CEAaptamerII/GA/hemin-G4 (=ApII/GA/DNAzyme). Through a sandwich mode, the ApII/GA/DNAzyme bioconjugates are captured on the modified GCE. Subsequently, the hemin-G4 acts as peroxidase-mimicking DNAzyme and rapidly catalyzes the electroreduction of hydrogen peroxide. The quantitative determination of CEA was achieved by differential pulse voltammetry, best at a working potential of around -0.27 V vs. Ag/AgCl. Under optimized conditions, the assay has a linear response in the 10.0 fg mL^-1 to 200.0 ng mL^-1 CEA concentration range and a lower detection limit of 3.2 fg mL^-1. Graphical abstract Schematic presentation of a sandwich-type electrochemical aptasensor based on nitrogen doped graphene (NG), gold nanoparticles (AuNPs) and graphene quantum dots (GQDs) modified glassy carbon electrode, and the hemin-G4 DNAzyme for femtomolar detection of the carcinoembryonic antigen.