Superparamagnetic Nanostructures for Split-Type and Competitive-Mode Photoelectrochemical Aptasensing

Magnetic beads and GO-assisted enzyme-free signal amplification fluorescent biosensors for disease diagnosis

Cancer detection is still a major challenge in public health. Identification of oncogene is the first step toward solving this problem. Studies have revealed that various cancers are associated with miRNA expression. Therefore, the sensitive detection of miRNA is substantially important to solve the cancer problem. In this study, let-7a, a representative substance of miRNA, was selected as the detection target. With the assistance of magnetic beads commonly used in biosensors and self-synthesized graphene oxide materials, specificity and sensitivity detection of the target gene let-7a were achieved via protease-free signal amplification. The limit of detection (LOD) was as low as 15.015pM. The fluorescence signal intensity showed a good linear relationship with the logarithm of let-7a concentration. The biosensor could also detect let-7a in complex human serum samples. Overall, this fluorescent biosensor is not only simple to operate, but also strongly specificity to detect let-7a. Therefore, it has substantial potential for application in the early diagnosis of clinical medicine and biological research.

S-induced Phase Change Forming In2 O3 /In2 S3 Heterostructure for Photoelectrochemical Glucose Sensor

In the past several decades, Photoelectrochemical (PEC) sensing still remains a great challenge to design highly-efficient semiconductor photocatalysts via a facile method. It is of much importance to design and synthesize various novel nanostructured sensing materials for further improving the response performance. Herein, we present an In2 O3 /In2 S3 heterostructure obtained by combining microwave assisted hydrothermal method with S-induced phase change, whose energy band and electronic structure could be adjusted by changing the S content. Combining theoretical calculation and spectroscopic techniques, the introduction of sulfur was proved to produce multifunctional interfaces, inducing the change of phase, oxygen vacancies and band gap, which accelerates the separation of photoexcited carriers and reduces their recombination, improving the electronic injection efficiency around the interface of In2 O3 /In2 S3 . As anticipated, an enhanced glucose response performance with a photocurrent of 0.6 mA cm-2 , a linear range of 0.1-1 mM and a detection limit as low as 14.5 μM has been achieved based on the In2 O3 /In2 S3 heterostructure, which is significant superior over its pure In2 O3 and S-doped In2 O3 counterparts. This efficient interfacial strategy may open a new route to manipulate the electrical structure, and energy band structure regulation of sensing material to improve the performance of photoelectrodes for PEC.

Insights into Chemical Bonds for Eliminating the Depletion Region and Accelerating the Photo-Induced Charge Efficient Separation toward Ultrasensitive Photoelectrochemical Sensing

The empty-space-induced depletion region in photoelectrodes severely exacerbates the recombination of electron-hole pairs, thereby reducing the photoelectrochemical (PEC) analytical performance. Herein, the chemical bond that can suppress the potential barrier and overcome the high energy barrier of out-of-plane Ohmic or Schottky contact is introduced into the PEC sensor to eliminate the depletion region and dramatically promote the separation of electron-hole pairs. Specifically, three-dimensional (3D) hierarchically wheatear-like TiO2 (HWT) nanostructures featuring a large surface area to absorb incident light are crafted as the substrate. The facile carbonized strategy is further employed to engineer the Ti-C chemical bond, serving as the touchstone. The average PL lifetime of HWT-C (4.14 ns) is much shorter than that of the 3D HWT (8.57 ns) due to the promoting effect of the chemically bonded structure on carrier separation. Consequently, the 3D HWT-C covalent photoelectrode (600 μA/cm2) exhibits a 3.6-fold increase in photocurrent density compared with the 3D HWT (167 μA/cm2). Ultimately, the model analyte of the tumor marker is detected, and the linear range is 0.02 ng/mL-100 ng/mL with a detection limitation of 0.007 ng/mL. This work provides a basic understanding of chemical bonds in tuning charge separation and insights on strategies for designing high-performance PEC sensors.

Self-assembled p-n Ag2O@Bi2O2S nanoflower heterojunctions for sensitive photoelectrochemical immunoassay

A new photoelectrochemical immunoassay based on self-assembled p-n Ag2O@Bi2O2S nanoflower heterojunction was designed and developed for quantitative monitoring of prostate-specific antigen (PSA) in biological fluids. Primarily, self-assembled p-n Ag2O@Bi2O2S nanoflower heterojunctions were served as the photoactive materials and coated onto the surface of electrodes. Subsequently, the glucose oxidase (GOx) was bound to the detection antibody (mAb2) labeled gold nanoparticles (Au NPs) and then were employed to accomplish a sandwich-like immunoreaction to generate H2O2 on a microplate incubated with monoclonal anti-PSA antibodies. In the presence of PSA, the product (H2O2) was catalyzed by the substrate, which was used as an electron sacrificial agent to improve signal conversion and capture of photogenerated electrons. Under optimum conditions, a wide linear range of 0.01-50 ng mL-1 and a low detection limit of 5.3 pg mL-1 were accomplished with the sensor, exhibiting an excellent photocurrent response. Moreover, the proposed sensor revealed satisfactory reproducibility, high selectivity, and acceptable accuracy for the real sample testing. Importantly, our work provides a novel strategy for high sensitivity detection of disease-associated biomarkers for the early diagnosis of cancers.

Multifunctional Photoelectrochemical Biosensor Based on ZnIn2S4/ZnS QDs@Au-Ag-Reversed Photocurrent of Cu-Metal-Organic Framework Coupled with CRISPR/Cas-12a-Shearing for Assay of Dual Targets

False positives and negatives in bioanalytical assays remain a persistent problem. Herein, a multifunctional photoelectrochemical (PEC) biosensor based on ZnIn₂S₄ (ZIS)/ZnS quantum dots (QDs)@Au-Ag-reversed photocurrent of Cu-metal-organic framework (MOF) coupled with CRISPR/Cas-12a-shearing was innovatively developed for assay of dual targets. First, Cu-MOF as a good PEC material shows cathodic photocurrent. Then, numerous ZIS/ZnS QDs were assembled to the Au-Ag nanoparticles (NPs) to prepare a stable and highly amplified signal probe, which can just match the energy level of Cu-MOFs and realized the polarity-reversed photocurrent of Cu-MOF for the first time. As the empty-core nanostructure of Au-Ag NPs has a high specific surface area and low material density, the bimetallic nanocrystal can much increase the reaction rate and improve the redox efficiency. When target CEA-produced cDNA opened the hairpin DNA (HP1 DNA) on the electrode, the ZIS/ZnS QDs@Au-Ag signal probe was conjugated to the electrode via DNA hybridization, achieving a significantly reversed PEC current for CEA detection. Moreover, the specific binding of kanamycin/aptamer generated the acDNA (activator), which can activate the trans-cleavage activity of the CRISPR-CAS12a system on ssDNA, so the signal probe was sheared and caused the obvious decrease of PEC signal for kanamycin detection. The newly developed ZIS/ZnS QDs@Au-Ag NPs displayed excellent PEC properties and reversed photocurrent to MOF and were combined with the unique CRISPR-Cas12a system to achieve sensitive detection of dual targets, which can open a new polarity-reversed PEC sensing platform for rapid and accurate analysis of multiple targets and can effectively avoid false positives results in clinical testing.

Rapid and Sensitive Detection of Sulfamethizole Using a Reusable Molecularly Imprinted Electrochemical Sensor

Efficient methods for monitoring sulfonamides (SAs) in water and animal-source foods are of great importance to achieve environmental safety and protect human health. Here, we demonstrate a reusable and label-free electrochemical sensor for the rapid and sensitive detection of sulfamethizole based on an electropolymerized molecularly imprinted polymer (MIP) film as the recognition layer. To achieve effective recognition, monomer screening among four kinds of 3-substituted thiophenes was performed by computational simulation and subsequent experimental evaluation, and 3-thiopheneethanol was finally selected. MIP synthesis is very fast and green, and can be in situ fabricated on the transducer surface within 30 min in an aqueous solution. The preparation process of the MIP was characterized by electrochemical techniques. Various parameters affecting MIP fabrication and its recognition response were investigated in detail. Under optimized experimental conditions, good linearity in the range of 0.001-10 μM and a low determination limit of 0.18 nM were achieved for sulfamethizole. The sensor showed excellent selectivity, which can distinguish between structurally similar SAs. In addition, the sensor displayed good reusability and stability. Even after 7 days of storage, or being reused 7 times, higher than 90% of the initial determination signals were retained. The practical applicability of the sensor was also demonstrated in spiked water and milk samples at the nM determination level with satisfactory recoveries. Compared to relevant methods for SAs, this sensor is more convenient, rapid, economical, and eco-friendly, and had comparable or even higher sensitivity, which offered a simple and efficient method for SA detection.

Integration of phosphate functionalized Pt/TiO2 and Ru(bpy)32+ sensitization for ultrasensitive assay of adenosine deaminase activity on a novel split-typed PEC aptasensor

To date, it is still a challenge for high-performance photoelectrochemical (PEC) assay of low-abundance adenosine deaminase (ADA) in fundamental research and clinical diagnosis. Herein, phosphate-functionalized Pt/TiO₂ (termed PO₄^3-/Pt/TiO₂) was prepared as ideal photoactive material to develop a split-typed PEC aptasensor for detection of ADA activity, coupled by a Ru(bpy)₃²⁺ sensitization strategy. We critically studied the effects of the PO₄^3- and Ru(bpy)₃²⁺ on the detection signals, and discussed the signal-amplified mechanism. Specifically, hairpin-structured adenosine (AD) aptamer was splited into single chain via ADA-induced catalytic reaction, and subsequently hybridized with complementary DNA (cDNA, initially coating on magnetic beads). The in-situ formed double-stranded DNA (dsDNA) was further intercalated by more Ru(bpy)₃²⁺ to amplify the photocurrents. The resultant PEC biosensor showed a broader linear range of 0.05-100 U L^-1 and a lower limit of detection (0.019 U L^-1), which can fill the blank for analysis of ADA activity. This research would provide some valuable insights for building advanced PEC aptasensors in ADA-related research and clinical diagnosis.

Colorimetric and Photocurrent-Polarity-Switching Photoelectrochemical Dual-Mode Sensing Platform for Highly Selective Detection of Mercury Ions Based on the Split G-Quadruplex-Hemin Complex

Mercury ion (Hg²⁺) is one of the most harmful heavy metal ions with the greatest impact on public health. Herein, based on the excellent catalytic activity toward 3,3',5,5'-tetramethylbenzidine (TMB) and the strong photocurrent-polarity-switching ability to SnS₂ photoanode of the split G-quadruplex-hemin complex, the magnetic NiCo₂O₄@SiO₂-NH₂ sphere-assisted colorimetric and photoelectrochemical (PEC) dual-mode sensing platform was developed for the Hg²⁺ assay. First, the amino-labelled single-stranded DNA1 (S1) was immobilized on NiCo₂O₄@SiO₂-NH₂ and then partly hybridized with another single-stranded DNA2 (S2). When Hg²⁺ was present, the thymine-Hg²⁺-thymine base pairs between S1 and S2 were formed, causing the formation of the split G-quadruplex in the presence of K⁺. After addition of hemin, the split G-quadruplex-hemin complex was obtained and effectually catalyzed the H₂O₂-mediated oxidation of TMB. Thus, the color and absorbance intensity of the TMB solution were changed, resulting in the visual and colorimetric detection of Hg²⁺. The linear response range is 10 pM to 10 nM, and the detection limit is 3.8 pM. Meanwhile, the above G-quadruplex-hemin complex effectively switched the photocurrent polarity of SnS₂-modified indium tin oxide electrode, leading to the sensitive and selective PEC assay of Hg²⁺ with a linear response range of 5 pM to 500 nM and a detection limit of 2.3 pM. Moreover, the developed dual-mode sensing platform provided mutual authentication of detection results in different modes, effectively improving the assay accuracy and confidence, and may have a good potential application in highly sensitive, selective, and accurate determination of Hg²⁺ in environmental fields.

Dual-mode photoelectrochemical/electrochemical sensor based on Z-scheme AgBr/AgI-Ag-CNTs and aptamer structure switch for the determination of kanamycin

A "signal-on" dual-mode aptasensor based on photoelectrochemical (PEC) and electrochemical (EC) signals was established for kanamycin (Kana) assay by using a novel Z-scheme AgBr/AgI-Ag-CNTs composite as sensing platform, an aptamer structure switch, and K₃[Fe(CN)₆] as photoelectron acceptor and electrochemical signal indicator. The aptamer structure switch was designed to obtain a "signal-off" state, which included an extended Kana aptamer (APT), one immobilized probe (P1), and one blocking probe (P2) covalently linked with graphdiyne oxide (GDYO) nanosheets. P1, P2, and aptamer formed the double helix structure, which resulted in the inhibited photocurrent intensity because of the weak conductivity of double helix layer and serious electrostatic repulsion of GDYO towards K₃[Fe(CN)₆]. In the presence of Kana, APT specifically bound to the target and dissociated from P1 and P2, and thus, a "signal-on" state was initiated by releasing P2-GDYO from the platform. Based on the sensing platform and the aptamer structure switch, the dual-mode aptasensor realized the linear determination ranges of 1.0 pM-2.0 μM with a detection limit (LOD) of 0.4 pM (for PEC method) and 10 pM-5.0 μM with a LOD of 5 pM (for EC method). The aptasensor displayed good application potential for Kana test in real samples.

In2O3/CdIn2S4 heterojunction-based photoelectrochemical immunoassay of carcinoembryonic antigen with enzymatic biocatalytic precipitation for signal amplification

This work reported a split-type photoelectrochemical (PEC) immunoassay for the detection of carcinoembryonic antigen (CEA) based on target-induced biocatalytic precipitation (BCP) by using In₂O₃/CdIn₂S₄ heterojunctions as the photosensitizers. The synthesized In₂O₃/CdIn₂S₄ heterojunctions improved the efficiency of charge separation and shortened the electron convey path to enhance the photocurrent, thus exhibiting high conductivity and low complexation rates of photogenerated electrons and holes. In the presence of CEA, horseradish peroxidase (HRP) catalyzed 4-chloro-1-naphthol (4-CN) to produce benzo-4-chloro-hexadienone (4-CD) through H₂O₂. Then, 4-CD was deposited onto the surface of In₂O₃/CdIn₂S₄ to reduce the photocurrent and realized the signal amplification. The PEC immunoassay revealed an excellent photocurrent toward target CEA within a wide range of 0.01-50 ng mL^-1 at a low limit of detection of 2.8 pg mL^-1 under the optimum conditions. Multiple switching light excitation tests demonstrated the good reliability and stability of the fabricated PEC biosensor. The accuracy was acceptable in comparison with human CEA enzyme-linked immunosorbent assay (ELISA) kit.

A Novel Signaling Strategy for an Ultrasensitive Photoelectrochemical Immunoassay Based on Electro-Fenton Degradation of Liposomes on a Photoelectrode

A signaling strategy can directly determine the analytical performance and application scope of photoelectrochemical (PEC) immunoassays, so it is of great significance to develop an effective signaling strategy. The electro-Fenton reaction has been extensively used to degrade organic pollutants, but it has not been applied to PEC immunoassays. Herein, we report a novel signaling strategy for a PEC immunoassay based on electro-Fenton degradation of liposomes (Lip) on a photoelectrode. Lip vesicles are coated on Au@TiO₂ core-shell photoactive material, which can prevent ascorbic acid (AA) from scavenging photogenerated holes. In the presence of a target, the immunomagnetic bead labels are converted to Fe³⁺ for electro-Fenton reaction, and hydroxyl radicals generated by the electro-Fenton reaction can degrade the Lip vesicles on the photoelectrode. Because of the degradation of Lip vesicles, photogenerated holes can be scavenged more effectively by AA, leading to an increase in photocurrent. Based on the electro-Fenton-regulated interface electron transfer, the sensitive "signal on" PEC immunoassay of a carcinoembryonic antigen is achieved, which features a dynamic range from 0.05 to 5 × 10⁴ pg mL^-1 and a detection limit of 0.01 pg mL^-1. Our work provides a novel and efficient PEC immunoassay platform by introducing the electro-Fenton reaction into PEC analysis.

Designing Triangular Silver Nanoplates with GSH/GSSG Surface Mixed States as Novel Nanoparticle-based Redox Mediators for Electrochemical Biosensing

Herein, a dual signal-quenched electrochemical (EC) biosensing strategy utilizing surface-engineered trisodium citrate (TSC)-glutathione (GSH)/oxidized glutathione (GSSG)-capped triangular silver nanoplates (Tri-Ag NPs_TSC-GSH/GSSG) as a novel nanoparticle-based redox mediator was explored for biomarker determination. In contrast with conventional redox mediators, Tri-Ag NPs_TSC-GSH/GSSG provided more admirable EC performance along with a lower oxidation potential (∼0.14 V). Taking advantage of the split-type mode, the immune response in a 96-well microplate was independent from EC detection, which could effectively eliminate the biological interference and thereby greatly enhance the sensitivity. As for the surface engineering process of Tri-Ag NPs, it was composed of partial GSH replacement and the formation of the GSH/GSSG surface mixed state. Primarily, the signal response of Ag NPs_TSC-GSH decreased due to the hindrance of GSH on electron transfer. Moreover, varying proportions of GSH/GSSG could further impede the oxidation process of Tri-Ag NPs_TSC-GSH/GSSG and eventually realize efficient dual signal quenching of this system. Notably, the ZIF-67@MIL-88B-GO_x nanocomposite as the label was applied for a cascade reaction system with GSH peroxidase-like activities to form the optimal GSH/GSSG proportion, causing sensitive changes in signal response with a range of different antigen concentrations. On this basis, the fabricated biosensor provided measurable outputs of aflatoxin B1 concentrations in a linear range of 0.0005-50 ng/mL with a low detection limit of 0.61 pg/mL (S/N = 3). All of the results indicated that the novel biosensor could be a promising analytical tool for future biomarker detection.

Recent advances in the selection of cancer-specific aptamers for the development of biosensors

An early diagnosis has the potential to greatly decrease cancer mortality. For that purpose, specific cancer biomarkers have been molecularly targeted by aptamer sequences to enable an accurate and rapid detection. Aptamer-based biosensors for cancer diagnostics are a promising alternative to those using antibodies, due to their high affinity and specificity to the target molecules and advantageous production. Synthetic nucleic acid aptamers are generated by in vitro Systematic Evolution of Ligands by Exponential enrichment (SELEX) methodologies that have been improved over the years to enhance the efficacy and to shorten the selection process. Aptamers have been successfully applied in electrochemical, optical, photoelectrochemical and piezoelectrical-based detection strategies. These aptasensors comprise a sensitive, accurate and inexpensive option for cancer detection being used as point-of-care devices. This review highlights the recent advances in cancer biomarkers, achievements and optimizations made in aptamer selection, as well as the different aptasensors developed for the detection of several cancer biomarkers.

A novel biosensor based on multienzyme microcapsules constructed from covalent-organic framework

Electrochemical biosensors based on enzymes modified electrode are attracting special attention due to their broad applications. However, the immobilization of enzymes on electrode is always an important challenge because it's not conducive to conformational expansion of enzymes and affects the bioactivity of enzymes accordingly. Although the imobilization of enzymes in micropores of crystalline covalent-organic framework (COF) and metal-organic framework (MOF) to construct electrochemical biosensors based on pore embedding can achive good reuslts, their micropores can still not guarantee that the enzyme's conformation is well extended. Herein, a multienzyme microcapsules (enzymes@COF) containing glucose oxidase, horseradish peroxidase and acetylcholinesterase with a 600 nm-sized cavity and a shell of COF was used to construct electrochemical biosensors. The 600 nm-sized cavity ensures free conformation expansion of encapsulated enzymes and the shell of COF with good chemical stablity protects encapsulated enzymes against external harsh environments. And the specific catalytic substrates of the enzymes can infiltrated into the microcapsule through the pores of COF shell. So, the biosensor based on enzymes@COF microcapsules demonstrated preeminent performances as compared with those of enzymes assembled on electrode. The detection limits were 0.85 μM, 2.81 nM, 3.0×10^-13 g/L, and the detection range were 2.83 μM-8.0 mM, 9.53 nM-7.0 μM, 10^-12 g/L-10^-8 g/L for glucose, H2O2 and malathion detection. This work shows that it is feasible to fabricate electrochemical sensors using enzymes@COF microcapsules.

Electrochemistry/Photoelectrochemistry-Based Immunosensing and Aptasensing of Carcinoembryonic Antigen

Recently, electrochemistry- and photoelectrochemistry-based biosensors have been regarded as powerful tools for trace monitoring of carcinoembryonic antigen (CEA) due to the fact of their intrinsic advantages (e.g., high sensitivity, excellent selectivity, small background, and low cost), which play an important role in early cancer screening and diagnosis and benefit people's increasing demands for medical and health services. Thus, this mini-review will introduce the current trends in electrochemical and photoelectrochemical biosensors for CEA assay and classify them into two main categories according to the interactions between target and biorecognition elements: immunosensors and aptasensors. Some recent illustrative examples are summarized for interested readers, accompanied by simple descriptions of the related signaling strategies, advanced materials, and detection modes. Finally, the development prospects and challenges of future electrochemical and photoelectrochemical biosensors are considered.

An electron donor-acceptor organic photoactive composite with Schottky heterojunction induced photoelectrochemical immunoassay

A signal enhancement photoelectrochemical (PEC) immunoassay system induced by the composite (PTCs@Au) of electron donor-acceptor with Schottky heterojunction was designed. Carcinoembryonic antigen (CEA) was selected as a model target. Initially, the capture anibody (Ab1) was linked to gold nanoparticles electrodeposited on glassy carbon electrode and sealed by bovine serum albumin. Meanwhile, the organic semiconductor (PTCs) with the structure of electron donor-acceptor was synthetized from perylene tetracarboxylic dianhydride (acceptor) and dopamine (donor) via amidation reaction. Then PTCs@Au composite with Schottky heterojunction was formed through gold nanoparticles in situ reduction and functionalization with PTCs. Next, the detection antibody was labeled by PTCs@Au composite (Ab2-PTCs@Au) as an immuno-probe. The PTCs@Au was introduced via sandwich immune reaction leading to enhancement PEC signal without additional electron donor nor acceptor for achieving quantitative detection of CEA under external light. The proposed immunoelectrode showed dynamic ranges of 0.5 fg mL^-1 to 10 pg mL^-1 and 10 pg mL^-1 to 1 μg mL^-1 with the detection limit of 0.17 fg mL^-1. In addition, this PEC strategy with acceptable selectivity and stability can be potentially applied to detect other targets by choosing appropriate target recognition unit.

Highly Selective and Sensitive microRNA-210 Assay Based on Dual-Signaling Electrochemical and Photocurrent-Polarity-Switching Strategies

Highly sensitive and selective microRNA (miRNA) assay is of great significance for disease diagnosis and therapy. Herein, a magnetic-assisted electrochemistry (EC)-photoelectrochemistry (PEC) dual-mode biosensing platform was developed for miRNA-210 detection based on dual-signaling EC and photocurrent-polarity-switching PEC strategies. Porous magnetic Fe₃O₄ octahedra with a large surface area were synthesized by calcining Fe-based metal-organic frameworks. Subsequently, the magnetic photoelectric materials (Fe₃O₄@CdS) were developed by the successive ionic layer adsorption and reaction method in Cd²⁺ and S^2- solutions. Then, the self-assembled DNA nanoprisms contained three thiols/hanging arms that could capture miRNA-210 efficiently and were anchored to the Fe₃O₄@CdS octahedra via the Cd-S bond. When miRNA-210 was present, the double-stranded DNA concatemers [the self-assembled duplex helixes based on a pair of methylene blue (MB)-labeled single-stranded DNAs (AP1 and AP2) through the hybridization chain reaction and then intercalated with adriamycin (Dox) into their grooves] were connected with the Fe₃O₄@CdS-DNA nanoprisms. MB and Dox not only acted as the electrochemical probes but also synergistically switched the photocurrent polarity of the Fe₃O₄@CdS octahedra. Thus, miRNA-210 was assayed sensitively and selectively via the proposed EC-PEC dual-mode biosensing platform. Additionally, the abovementioned recognition steps occurred in a homogeneous system, and the effects of the impurities and interferences on the miRNA-210 assay could be easily avoided by magnetic separation due to the good magnetic properties of Fe₃O₄ octahedra. The proposed EC-PEC dual-mode biosensing platform showed a wide range of potential applications in bioanalysis and early diagnosis of disease.

The enhanced photoelectrochemical platform constructed by N-doped ZnO nanopolyhedrons and porphyrin for ultrasensitive detection of brain natriuretic peptide

Nowadays, brain natriuretic peptide (BNP-32) is fundamental to early cardiovascular clinical diagnosis, whose accurate assay is of significance by photoelectrochemistry (PEC) for the low background and high precision. Herein, a novel enhanced PEC platform was built by successive deposition of N-doped ZnO nanopolyhedra (N-ZnO NP) and protoporphyrin IX (PPIX). Specifically, the N-ZnO NP with a narrow bandgap of 2.60 eV was synthesized by direct calcination of zeolitic imidazole framework-8 (ZIF-8), and performed as the substrate to enhance the photocurrents of PPIX (as photosensitizer) whose photoelectron transfer pathway and enhanced PEC mechanism were studied in detail. Under such foundation, a label-free PEC aptasensor was developed by deposition of DNA aptamer onto the PEC platform and then ultrasensitive assay of BNP-32 based on a "signal off" model. The biosensor showed a wide linear range (1 pg mL^-1- 0.1 μg mL^-1) with a limit of detection (LOD) as low as 0.14 pg mL^-1. This doping technique of ZnO nanomaterials provides some valuable guidelines for synthesis of advanced PEC probes in bioanalysis.

Programmable DNA Circuits for Flexible and Robust Exciton-Plasmon Interaction-Based Photoelectrochemical Biosensing

DNA circuits as one of the dynamic nanostructures can be rationally designed and show amazing geometrical complexity and nanoscale accuracy, which are becoming increasingly attractive for DNA entropy-driven amplifier design. Herein, a novel and elegant exciton-plasmon interaction (EPI)-based photoelectrochemical (PEC) biosensor was developed with the assistance of a programmable entropy-driven DNA amplifier and superparamagnetic nanostructures. Low-abundance miRNA-let-7a as a model can efficiently initiate the operation of the entropy-driven DNA amplifier, and the released output DNAs can open the partially hybridized double-stranded DNA anchored on Fe₃O₄@SiO₂ particles. The liberated Au nanoparticles (NPs)-cDNA can completely hybridize with CdSe/ZnS quantum dots (QDs)-cDNA-1 and result in proportionally decreased photocurrent of CdSe/ZnS QDs-cDNA-1. This unique entropy-driven amplification strategy is beneficial for reducing the reversibility of each step reaction, enables the base sequence invariant and the reaction efficiency improvement, and exhibits high thermal stability and specificity as well as flexible design. These features grant the PEC biosensor with ultrasensitivity and high selectivity. Also, instead of solid-liquid interface assembly for conventional EPI-based PEC biosensors, herein, DNA hybridization in the solution phase enables the improved hybridization efficiency and sensitivity. In addition, superparamagnetic Fe₃O₄@SiO₂ particles further ensure the enhancement of the selectivity and reliability of the as-designed PEC biosensor. Particularly, this single-step electrode modification procedure evidently improves the electrode fabrication efficiency, reproducibility, and stability.

TiO2-sensitized double-shell ZnCdS hollow nanospheres for photoelectrochemical immunoassay of carcinoembryonic antigen coupled with hybridization chain reaction-dependent Cu2+ quenching

A novel photoelectrochemical immunosensor was constructed to monitor carcinoembryonic antigen (CEA) based on hybridization chain reaction (HCR)-mediated in situ generation of copper nanoparticles (Cu NPs) and subsequent Cu²⁺-dependent quenching reaction, in which titanium dioxide nanoparticles-sensitized double-shell zinc cadmium sulfide hollow nanospheres (TiO₂/DS-ZnCdS)-modified ITO electrode and anti-CEA antibody-modified 96-well plate served as biological recognition and signal detection platforms, respectively. The synergistic effect of TiO₂ NPs and DS-ZnCdS hollow nanospheres contributed to the improvement of photoelectric conversion efficiency, and HCR-mediated signal cascade benefited the enhancement of detection sensitivity. In the presence of CEA, biotin-labelled anti-CEA antibodies were immobilized onto anti-CEA antibody-modified 96-well plate, and triggered HCR process to form long double stranded DNA, which could adsorb a large number of Cu²⁺ ions and then in situ form Cu NPs on double stranded DNA template by a facile reduction reaction. After acid treatment, the dissolved Cu²⁺ ions could significantly reduce the photocurrent response due to the generation of Cu_xS. Under optimal conditions, the immunosensor exhibited a desirable liner range of 1 pg mL^-1 - 50 ng mL^-1 and a low detection limit of 0.1 pg mL^-1, as well as excellent selectivity and stability. Meanwhile, the recoveries of human serum sample analysis ranged from 96.8% to 103.6%, and the relative standard deviation was less than 7.40%, showing a good feasibility in early clinical diagnosis.

DNA-Tetrahedral-Nanostructure-Based Entropy-Driven Amplifier for High-Performance Photoelectrochemical Biosensing

In virtue of the inherent molecular recognition and programmability, DNA has recently become the most promising for high-performance biosensors. The rationally engineered nucleic acid architecture will be very advantageous to hybridization efficiency, specificity, and sensitivity. Herein, a robust and split-mode photoelectrochemical (PEC) biosensor for miRNA-196a was developed based on an entropy-driven tetrahedral DNA (EDTD) amplifier coupled with superparamagnetic nanostructures. The DNA tetrahedron structure features in rigidity and structural stability that contribute to obtain precise identification units and specific orientations, improving the hybridization efficiency, sensitivity, and selectivity of the as-designed PEC biosensor. Further, superparamagnetic Fe₃O₄@SiO₂@CdS particles integrated with DNA nanostructures are beneficial for the construction of a split-mode, highly selective, and reliable PEC biosensor. Particularly, the enzyme- and hairpin-free EDTD amplifier eliminates unnecessary interference from the complex secondary structure of pseudoknots or kissing loops in typical hairpin DNAs, significantly lowers the background noise, and improves the detection sensitivity. This PEC biosensor is capable of monitoring miRNA-196a in practical settings with additional advantages of efficient electrode fabrication, stability, and reproducibility. This strategy can be extended to various miRNA assays in complex biological systems with excellent performance.

Magnetic-Assisted Methylene Blue-Intercalated Amplified dsDNA for Polarity-Switching-Mode Photoelectrochemical Aptasensing

Organic dyes are typically applied as photosensitizers in photoelectrochemical (PEC) cells but have not been reported in polarity-reversal-mode PEC sensors with excellent sensitivity and accuracy. Herein, an elegant and robust PEC biosensor for carcinoembryonic antigen (CEA) has been designed by photocurrent polarity switching of CdTe quantum dots (QDs), which is obtained by embedding methylene blue (MB) into amplified double-stranded DNA (dsDNA) anchored to the superparamagnetic Fe₃O₄@SiO₂. The target-triggered Exo III-assisted cyclic amplification strategy and in situ magnetic enrichment enable the remarkable sensitivity. The extraction of target-analogue single-stranded DNA (output DNA) contributes to high selectivity resulting from the elimination of possible interferences in real samples or matrixes. Particularly, this exclusive polarity-reversal-mode PEC aptasensing can efficiently eliminate the false-positive or false-negative signals, leading to accurate measurements. Moreover, different from the probes and layer-by-layer assembled photoelectric beacons on electrodes in advance, this rational split-type approach is doomed to help the PEC biosensor with additional merits of convenient fabrication, short time consumption, wider linearity, as well as outstanding reproducibility and stability in practical applications. In light of the ability of MB acting as a kind of signal probe in typical electrochemical sensors, certainly, this ingenious design can not only be extended to a wide variety of target monitoring but also provide new ideas for the construction of high-performance electrochemical and PEC biosensors.