Addressable TiO2 Nanotubes Functionalized Paper-Based Cyto-Sensor with Photocontrollable Switch for Highly-Efficient Evaluating Surface Protein Expressions of Cancer Cells

Photoelectrochemical sensor for histone deacetylase Sirt1 detection based on Z-scheme heterojunction of CuS-BiVO4 photoactive material and the cyclic etching of MnO2 by NADH

A novel photoelectrochemical (PEC) biosensor was constructed for histone deacetylase Sirt1 detection based on the Z-Scheme heterojunction of CuS-BiVO4 and reduced nicotinamide adenine dinucleotide (NADH) induced cyclic etching of MnO2 triggered by Sirt1 enzyme catalytic histone deacetylation event. Based on the Z-Scheme heterojunction, the photoactivity of the CuS-BiVO4 was improved greatly due to the highly effective separation of the photogenerated electron-hole pairs. In the presence of MnO2 nanosheets on the CuS-BiVO4/ITO electrode surface, the photocurrent decreased due to the inhibition effect of MnO2. However, this inhibition effect was eliminated by the incubation of MnO2/CuS-BiVO4/ITO with NADH, where NADH was produced in the deacetylation process of acetylated peptide catalyzed by Sirt1 with NAD+. The formed NADH etched MnO2, resulting in an increased photocurrent. In this process, NADH was oxidized to produce NAD+, which further involved the deacetylation process. Based on this cycle, the photocurrent of the biosensor was improved greatly and the sensitive and selective detection of Sirt1 was achieved. The biosensor presented a wide linear range from 0.005 to 10 nM with the low detection limit of 3.38 pM (S/N = 3). In addition, the applicability of the developed method was evaluated by investigating the effect of sodium butyrate and perfluorohexane sulfonate on Sirt1 activity.

Constructing DNAzyme-driven three-dimensional DNA nanomachine-mediated paper-based photoelectrochemical device for ultrasensitive detection of miR-486-5p

As a unique class of dynamic nanostructures, biomimetic DNA walking machines that exhibit geometrical complexity and nanometre precision have gained great success in photoelectrochemical (PEC) bioanalysis. Despite certain achievements, the slow reaction kinetics and low processivity severely restrict the amplification efficiency of the DNA walker-mediated biosensors. Herein, by taking advantage of efficient DNA rolling machines, a three-dimensional (3D) DNA nanomachine-mediated paper-based PEC device for speedy ultrasensitive detection of miR-486-5p was successfully constructed. To achieve it, a novel In2S3/SnS2 sensitized heterojunction was firstly in-situ grown on the Au-modified paper fibers and implemented as the photoanode with effective separation of photogenerated carriers to achieve an enhanced initial photocurrent. Subsequently, the copper hexacyanoferrate(II)-modified CuO nanosphere was introduced as a multifunctional signal regulator via the competitive capture of electron donors and photon energy with the photoelectric layer for efficiently quenching the PEC signal. With the introduction of targets, the DNAzyme-driven DNA nanomachine with editable motion modes was gradually activated and it could continuously cleave the tracks DNA labeled quenching probes, finally achieving the recovery of PEC signal. As a proof of concept, the elaborated paper-based PEC device presented a wide linear range from 0.1 fM to 100 pM and a detection limit of 35 aM for miR-486-5p bioassay. This work provides an innovative insight to the exploitation of DNA nanobiotechnology and nucleic acid signal amplification strategy.

A highly sensitive TiO2-based molecularly imprinted photoelectrochemical sensor with regulation of imprinted sites by Photo-deposition

Molecularly imprinted photoelectrochemical sensors (MIPES) have gained significant attention in the detection field due to their high selectivity and accuracy. However, their sensitivity still needs improvement. Here we developed a TiO2-based MIPES (TiO2 NRs/NiOOH/rMIP) to detect ciprofloxacin (CIP). We identified the photoactive sites of TiO2 by NiOOH photo-deposition and anchored the imprinted sites on the photoactive sites by complexation between CIP and NiOOH. By regulating the imprinted sites, the photocurrent difference before and after the addition of CIP increases and the detection sensitivity of CIP is improved. Moreover, a PN heterojunction is formed between TiO2 and NiOOH, which enables rapid transfer of photoexcited holes and electrons to different semiconductors under the built-in electric field. This leads to improved photoactivity of TiO2 and further increases the sensitivity of MIPES. Compared with sensors prepared by the traditional electro-polymerization CIP and Molecularly imprinted polymers (TiO2 NRs/NiOOH/eMIP), TiO2 NRs/NiOOH/rMIP as constructed in this work displays higher sensitivity, wider linear detection range, and lower limit of detection (LOD). Additionally, TiO2 NRs/NiOOH/rMIP shows good selectivity, stability, and recovery rate, and has a promising application prospect in the actual detection of antibiotics.

Dual-Modal Biosensor for Staphylococcus aureus Detection Based on a Porphyrin-Based Porous Organic Polymer FePor-TPA with Excellent Peroxidase-like, Catalase-like, and Photoelectrochemical Properties

Bacterial infections seriously harm human health and cause many severe diseases, which triggered urgent demands to exploit specific and sensitive biosensor strategies for Staphylococcus aureus detection. Here, a colorimetric and photoelectrochemical dual-mode biosensor for S. aureus assay based on FePor-TPA was constructed. 2D FePor-TPA thin film and its bulk powder (FePor-TPA) were synthesized by in situ growth on ITO and a solvothermal condition, respectively, both of which exhibited excellent peroxidase-like and catalase-like activity, originating from their metalloporphyrin linkers. Benefiting from the in situ growth on ITO electrodes, the 2D FePor-TPA thin film also possessed a more ordered stacking mode and in turn exhibited good electrical conductivity, stable initial photocurrent, and high sensitivity to O2. As for bulk FePor-TPA, its porous structure and high specific surface area make it a possible scaffold to load an amount of AuNPs, the rabbit anti-Staphylococcus aureus Rosenbach tropina antibody (Ab2), and GOx for constructing the signal probe (GOx/Ab2@Au@FePor-TPA) and realizing catalytic amplification. With these satisfactory features in mind, the 2D FePor-TPA thin film and its bulk powder (FePor-TPA) were utilized to construct a dual and signal-on bioplatform for sensitively and selectively detecting S. aureus, which, as far as we know, has not been reported.

Photoelectrochemical sensors based on paper and their emerging applications in point-of-care testing

Point-of-care testing (POCT) technology is urgently required owing to the prevalence of the Internet of Things and portable electronics. In light of the attractive properties of low background and high sensitivity caused by the complete separation of excitation source and detection signal, the paper-based photoelectrochemical (PEC) sensors, featured with fast in analysis, disposable and environmental-friendly have become one of the most promising strategies in POCT. Therefore, in this review, the latest advances and principal issues in the design and fabrication of portable paper-based PEC sensors for POCT are systematically discussed. Primarily, the flexible electronic devices that can be constructed by paper and the reasons why they can be used in PEC sensors are expounded. Afterwards, the photosensitive materials involved in paper-based PEC sensor and the signal amplification strategies are emphatically introduced. Subsequently, the application of paper-based PEC sensors in medical diagnosis, environmental monitoring and food safety are further discussed. Finally, the main opportunities and challenges of paper-based PEC sensing platforms for POCT are briefly summarized. It provides a distinct perspective for researchers to construct paper-based PEC sensors with portable and cost-effective, hoping to enlighten the fast development of POCT soon after, as well as benefit human society.

Paper-based bipolar electrode electrochemiluminescence sensors for point-of-care testing

In the past few years, point-of-care testing (POCT) technology has crossed the boundaries of laboratory determination and entered the stage of practical applications. Herein, the latest advances and principal issues in the design and fabrication of paper-based bipolar electrode electrochemiluminescence (BPE-ECL) sensors, which are widely used in the POCT field, are highlighted. After introducing the attractive physical and chemical properties of cellulose paper, various approaches aimed at enhancing the functions of the paper, and their underlying principles are described. The materials typically employed for fabricating paper-based BPE are also discussed in detail. Subsequently, the universal method of enhancing BPE-ECL signal and improving detection accuracy is put forward, and the ECL detector widely used is introduced. Furthermore, the application of paper-based BPE-ECL sensors in biomedical, food, environmental and other fields are displayed. Finally, future opportunities and the remaining challenges are analyzed. It is expected that more design concepts and working principles for paper-based BPE-ECL sensors will be developed in the near future, paving the way for the development and application of paper-based BPE-ECL sensors in the POCT field and providing certain guarantee for the development of human health.

Synthesis of luminescent chitosan-based carbon dots for Candida albicans bioimaging

In this work, we used chitosan as a raw material to synthesize carbon dots using fast microwave carbonization. We studied the influence of the synthesis time, doping agent, and the molar ratio between the reactants on the quantum yield of carbon dots. Chitosan-based carbon dots displayed stable blue fluorescence emission with excitation-dependent behavior and quantum yield values ranging from 1.16 to 7.07 %. ANOVA results showed that the interaction factor between the doping agent and the molar ratio of the reactants was a significant combination to produce carbon dots with higher quantum yield. The presence of the doping agent improved the carbon dots optical properties by obtaining higher fluorescence intensity values. Confocal laser microscope images showed that the carbon dots internalized in the Candida albicans cellular membrane, exhibiting blue, green, and red emissions, acting as a promising agent for bioimaging.

2D carbon materials based photoelectrochemical biosensors for detection of cancer antigens

Cancer is a leading cause of death globally and early diagnosis is of paramount importance for identifying appropriate treatment pathways to improve cancer patient survival. However, conventional methods for cancer detection such as biopsy, CT scan, magnetic resonance imaging, endoscopy, X-ray and ultrasound are limited and not efficient for early cancer detection. Advancements in molecular technology have enabled the identification of various cancer biomarkers for diagnosis and prognosis of the deadly disease. The detection of these biomarkers can be done by biosensors. Biosensors are less time consuming compared to conventional methods and has the potential to detect cancer at an earlier stage. Compared to conventional biosensors, photoelectrochemical (PEC) biosensors have improved selectivity and sensitivity and is a suitable tool for detecting cancer agents. Recently, 2D carbon materials have gained interest as a PEC sensing platform due to their high surface area and ease of surface modifications for improved electrical transfer and attachment of biorecognition elements. This review will focus on the development of 2D carbon nanomaterials as electrode platform in PEC biosensors for the detection of cancer biomarkers. The working principles, biorecognition strategies and key parameters that influence the performance of the biosensors will be critically discussed. In addition, the potential application of PEC biosensor in clinical settings will also be explored, providing insights into the future perspective and challenges of exploiting PEC biosensors for cancer diagnosis.

Nanomaterials and paper-based electrochemical devices: merging strategies for fostering sustainable detection of biomarkers

In the last few decades, nanomaterials have made great advances in the biosensor field, thanks to their ability to enhance several key issues of biosensing analytical tools, namely, sensitivity, selectivity, robustness, and reproducibility. The recent trend of sustainability has boosted the progress of novel and eco-designed electrochemical paper-based devices to detect easily the target analyte(s) with high sensitivity in complex matrices. The huge attention given by the scientific community and industrial sectors to paper-based devices is ascribed to the numerous advantages of these cost-effective analytical tools, including the absence of external equipment for solution flow, thanks to the capillary force of paper, the fabrication of reagent-free devices, because of the loading of reagents on the paper, and the easy multistep analyses by using the origami approach. Besides these features, herein we highlight the multifarious aspects of the nanomaterials such as (i) the significant enlargement of the electroactive surface area as well as the area available for the desired chemical interactions, (ii) the capability of anchoring biorecognition elements on the electrode surface on the paper matrix, (iii) the improvement of the conductivity of the cellulose matrix, (iv) the functionality of photoelectrochemical properties within the cellulose matrix, and (v) the improvement of electrochemical capabilities of conductive inks commonly used for electrode printing on the paper support, for the development of a new generation of paper-based electrochemical biosensors applied in the biomedical field. The state of the art over the last ten years has been analyzed highlighting the various functionalities that arise from the integration of nanomaterials with paper-based electrochemical biosensors for the detection of biomarkers.

Biofabricated functionalized graphene quantum dots (fGQDs): Unravelling its fluorescence sensing mechanism of human telomerase reverse transcriptase (hTERT) antigen and in vitro bioimaging application

Lung cancer (LC) is a deadly malignancy that is posing a serious threat to human health. Therefore, early detection of LC biomarkers is the key to reducing LC-related fatalities. Herein, we present the first fluorescent-based selective detection of LC biomarker human telomerase reverse transcriptase (hTERT) using polyethyleneimine (PEI) functionalized graphene quantum dots (fGQDs). One-pot in situ synthesis of amine-functionalized GQDs was accomplished by hydrothermal carbonization of biowaste-derived cellulose and PEI. Synthesized fGQDs were characterized by various analytical techniques. Synthesized fGQDs not only exhibited enhanced fluorescence life-time but also excellent stability in the different solvents compared to bare GQDs. The surface activation of hTERT-Ab by carbodiimide chemistry (EDC-NHS) resulted in stacking interactions with fGQDs, involving adsorption-desorption as well as competitive mechanisms. The higher inherent affinity of hTERT-Ag (hTERT antigen) for hTERT-Ab (hTERT antibody) resulted in complex formation and recovery of fGQD fluorescence. As a result, this fluorescence sensing demonstrated a greater linear detection range (0.01 ng mL-1 to 100 µg mL-1) as well as a notable low detection limit (36.3 pg mL-1). Furthermore, the fabricated immunosensor (Ab@fGQDs) has excellent stability and performance in real samples, with an average recovery of 97.32%. The results of cytotoxicity and cellular bioimaging study in A549 cells show that fGQDs can be used for additional nanotherapeutics and biological applications.

Optical cytosensors for the detection of circulating tumour cells

Blood analysis is an established approach to monitor various diseases, ranging from heart defects and diabetes to cancer. Among various tumor markers in the blood, circulating tumor cells (CTCs) have received increasing attention due to the fact that they originate directly from the tumors. Capturing and detecting CTCs represents a promising approach in cancer diagnostics and clinical management of cancers. CTCs in blood progress to self-seeding a tumour or initiating a new lesion mass. Cytosensors are biosensors intended to identify CTCs in a blood sample of cancer patients and provide information about the cancer status. Herein, we firstly discuss different detection methods of state-of-the-art optical cytosensors, including colorimetry, fluorescence, surface plasmon resonance, photoelectrochemistry and electrochemiluminescence. Then we review the significant advances made in implementing biorecognition elements and nanomaterials for the detection of cancer cells. Despite great progress in optical cytosensors, and their integration with smartphones, they have still only been explored to prototype stages. Much more effort is needed to fulfil their potential in modern cancer diagnostics and in monitoring the state of disease for cancer patients.

Orange-emissive N,S-co-doped carbon dots for label-free and sensitive fluorescence assay of vitamin B12

N,S-CDs with orange fluorescent emission were synthesized via a hydrothermal method using o-phenylenediamine and thiourea. A novel fluorometric method for the determination of VB12 based on the IFE was established.

High-Throughput Detection of Multiple Contaminants Based on Portable Photoelectrochromic Sensor Chip

With the increasing concerns about the environment and food safety, it is necessary to develop portable, low-cost, and high-throughput biosensors for the simultaneous detection of multiple contaminates. However, traditional photoelectrochemical (PEC) biosensors lack the ability of multiplexed assays due to the inherent mechanism limitation. Also, specialized instruments are necessary for most PEC biosensors. In this work, a portable high-throughput sensor chip has been successfully developed. By introducing electrochromic materials, the detection is based on color change instead of electric signals, which reduces the limitation of instruments. This designed sensor chip is composed of three parallel sensing channels fabricated by laser etching. Each channel is modified with TiO₂/3D-g-C₃N₄ composites with excellent PEC activity and electrochromic material Prussian blue (PB). Under light illumination, photoinduced electrons generated by TiO₂/3D-g-C₃N₄ are injected into PB, and blue PB is reduced to colorless Prussian white. Three organic contaminates, ochratoxin A, lincomycin, and edifenphos, can be simultaneously detected because the binding of these molecules with aptamers affects the electron transfer and the corresponding color changes. This portable and high-throughput sensor chip provides a convenient choice for multiplexed assays with good sensitivity and accuracy.

Label-free photoelectrochemical immunosensor for aflatoxin B1 detection based on the Z-scheme heterojunction of g-C3N4/Au/WO3

Aflatoxin B1 (AFB1) is the most toxic mycotoxin, is widely found in foods and animal feeds, and can pose a serious threat to our lives. A label-free photoelectrochemical (PEC) immunosensor was fabricated for the sensitive detection of AFB1. A Z-scheme heterojunction of gold nanoparticles (Au NPs) loaded on graphitic carbon nitride sheet and tungsten trioxide sphere composite (g-C₃N₄/Au/WO₃) acted as the highly sensitive platform. The g-C₃N₄/Au/WO₃ is capable, not only of immobilizing antibodies via Au NPs, but also enhancing the separation of electron-hole pairs due to its good energy band matching efficiency. The mechanism of photo-generated electron/hole transfer on g-C₃N₄/Au/WO₃ was explored using scavengers to eliminate active components. On this basis, an electron transfer pathway for the immunosensor was deduced. The PEC immunosensor displayed a linear concentration range from 1.0 pg mL^-1 to 100 ng mL^-1 and a low detection limit of 0.33 pg mL^-1 (S/N = 3) for AFB1. Good reproducibility, stability, and specificity provide a solid foundation for the practical application of this immunosensor.

Photoelectrochemical detection of human epidermal growth factor receptor 2 (HER2) based on Co3O4-ascorbic acid oxidase as multiple signal amplifier

A sensitive photoelectrochemical (PEC) sensor based on hexagonal carbon nitride tubes (HCNT) as photoactive material was prepared for the detection of human epidermal growth factor receptor 2 (HER2). Magnetic Fe₃O₄ nanospheres (MNs) modified with anti-HER2 antibodies were employed for highly efficient capture of HER2 from serum sample, and Co₃O₄ nanoparticles (Co₃O₄ NPs) modified with ascorbic acid oxidase (AAO) as well as HER2 aptamer were used for signal amplification. When the aptamer-Co₃O₄-AAO probe was captured onto the electrode surface through the specific binding of the aptamer with HER2, the photocurrent intensity decreased. This was because Co₃O₄ NPs competed with HCNT for consumption of the excitation energy. As a consequence AAO catalyzed the oxidation of the electron donor (AA), and the aptamer-Co₃O₄-AAO probe increased the steric hindrance at the electrode surface, leading to significant photocurrent intensity decrease, thus realizing multiple signal amplification. Based on this signal amplification strategy, at 0 V (vs Ag/AgCl), the PEC sensor shows a wide linear response ranging from 1 pg mL^-1 to 1 ng mL^-1 with a low detection limit of 0.026 pg mL^-1 for HER2. Importantly, the prepared PEC sensor was applied for detection of HER2 in human serum samples with recoveries between 98.8 and 101%. Sensitive photoelectrochemical sensor based on Co₃O₄ nanoparticles modified with ascorbic acid oxidase for signal amplification is reported.

Research progress on the applications of paper chips

Due to the modern pursuit of the quality of life, science and technology have rapidly developed, resulting in higher requirements for various detection methods based on analytical technology. Herein, the development, fabrication, detection and application of paper-based microfluidic chips (μPAD) are summarized. We aim to provide a comprehensive understanding of paper chips, and then discuss challenges and future prospects in this field.

Integrated paper-based 3D platform for long-term cell culture and in situ cell viability monitoring of Alzheimer's disease cell model

Reactive oxygen species including superoxide anion, hydrogen peroxide (H₂O₂) and hydroxyl radicals, as a conflicting class of biological metabolites in living organism, act crucial effect on Alzheimer's disease (AD). In this work, a facile integrated platform composed of a paper-based three-dimension (3D) cell culture system and an electrochemical sensor was developed for the construction of AD cell model in third dimensional structure and in situ cell viability monitoring by H₂O₂ released from PC12 cells cultured on paper scaffold were divided into three groups containing control group, amyloid beta peptide 25-35 (Aβ_25-35) group and Aβ_25-35+curcumin (Aβ_25-35+cur) group, respectively. In addition, the paper-based 3D platform displayed excellent properties, such as sensitivity, selectivity, reproducibility and stability. The levels of H₂O₂ expressed in PC12 cells of the three groups were monitored through a paper-based 3D platform. The viability of cells cultured on the 96-well plate was measured by 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) assay. Results of this paper-based platform are consistent with those of MTT, both displaying improved cell viability and decreased H₂O₂ production in Aβ_25-35+cur group compared to Aβ_25-35 group, which indicates that curcumin has effective cytoprotection. The paper-based 3D platform provides a convenient, economic and universal platform for in situ cell activity monitoring by key small molecules released from living cells.

Transparent Microcrystalline Cellulose/Polyvinyl Alcohol Paper as a New Platform for Three-Dimensional Cell Culture

Multilayered and stacked cellulose paper has emerged as a promising platform for construction of three-dimensional (3D) cell culture because of its low cost, good biocompatibility, and high porosity. However, its poor light transmission makes it challenging to directly and clearly monitor cell behaviors (e.g., growth and proliferation) on the paper-based platform using an optical microscope. In this work, we developed a transparent microcrystalline cellulose/polyvinyl alcohol (MCC/PVA) paper with irregular pores through dissolution and regeneration of microcrystalline nanocellulose, addition of a porogen reagent (NaCl), and subsequently dipping in PVA solutions. The transparent MCC paper displays high porosity (up to 90%), adjustable pore size (between 23 and 46 μm), large thickness (from 315 to 436 μm), and high light transmission under water (>95%). Through further modification of the transparent MCC paper with PVA, the obtained transparent MCC/PVA paper shows enhanced mechanical properties (dry and wet strengths), good hydrophilicity (with a contact angle of 70.8°), and improved biocompatibility (cell viability up to 90%). By stacking and destacking multiple layers of the transparent MCC/PVA paper, it has been used for both two-dimensional and three-dimensional cell culture platforms. The transparent MCC/PVA paper under water enables both direct observation of cell morphology by an optical microscope via naked eyes and fluorescence microscope after staining. We envision that the developed transparent MCC/PVA paper holds great potential for future applications in various bioanalytical and biomedical fields, such as drug screening, tissue engineering, and organ-on-chips.

Paper-based electrochemiluminescence determination of streptavidin using reticular DNA-functionalized PtCu nanoframes and analyte-triggered DNA walker

A paper-based electrochemiluminescence (ECL) biosensor characterized by the signal amplification of reticular DNA-functionalized PtCu nanoframes (DNA-PtCuTNFs) and analyte-triggered DNA walker was developed for sensitive streptavidin assay. Silver microflower functionalized paper-based sensing platform was prepared to fix the hairpin strand (S1). With addition of the streptavidin, plenty of DNA walkers consisting of the walking strands (S2) labeled with biotin and streptavidin were established, which protected S2 from digestion via the terminal protection mechanism. The sequential introduction of the DNA walker and capture probe initiated the hairpin structure opening of S1 and strand displacement reaction (SDR) happening, causing the S2 release. Subsequently, S1 hybridized with S3. The free S2 further hybridized with adjacent S1 to trigger the next cycle. After multiple cycles, the DNA-PtCuTNFs, the fire-new signal enhancer, with remarkable peroxidase activity, were successfully attached onto the paper electrode via metal-catalyst-free click chemistry. Based on the SDR of the DNA walker and the catalysis of DNA-PtCuTNFs, a significantly boosted ECL signal of luminol was obtained. Under the optimal conditions, the developed sensor for streptavidin assay exhibited a low detection limit of 33.4 fM with a linear range from 0.1 pM to 0.1 μM. Graphical abstract.

Direct-readout photoelectrochemical lab-on-paper biosensing platform based on coupled electricity generating system and paper supercapacitors

A direct-readout photoelectrochemical (PEC) lab-on-paper device based on coupled an electricity generating system and paper supercapacitors was established for highly sensitive detection of adenosine triphosphate (ATP). Concretely, CdSe quantum dots (QDs) decorated ZnO networks assembled sensing surface provided outstanding photoelectric properties, on which glucose oxidase (GOx) labeled aptamer was subsequently immobilized via the hybridization chain reaction. With analytes present, specific recognition was stimulated by aptamer, resulting in labeled GOx released. Such released GOx could flow to electrochemical cell to conduct electrochemical redox reactions, which could effectively produce electricity that was stored by capacitor I. Sequentially, photoactive material produced an outstanding voltage due to the decrease of steric hindrance on the sensing interface, which was utilized for charging an external capacitor II. The two instantaneous current was acquired along with the discharge of capacitor I and II by digital multimeter (DMM) readout, respectively. The summational current values performed an increment in pace with the addition of target ATP concentration with the dynamic working range from 10 nM to 3 μM and a detection limit of 6.3 nM attained. Significantly, the signal amplified strategy utilizing as-generated electricity from electrochemical redox reactions were isolated from the photoelectrodes, which was beneficial for amplifying the signal response in the PEC matrices and the development of more efficient signal performance.

Photoelectrochemical aptasensor for thrombin based on Au-rGO-CuS as signal amplification elements

A photoelectrochemical platform for thrombin determination was developed based on Au-rGO-CuS as multiple signal amplification elements. CuInS₂ QDs was used to sensitize burr-shape TiO₂ (b-TiO₂) to obtain a strong photocurrent. Under the specific recognition between aptamer and thrombin, a sandwichlike structure was formed and the Au-rGO-CuS-labeled aptamer (S2@Au-rGO-CuS) was immobilized on the electrode surface. This induced a sharp decrease in photocurrent. The phenomenon is mainly due to the fact that CuS NPs can competitively consume the light energy and electron donor with CuInS₂/b-TiO₂. The rGO can increase the amount of CuS NPs and the Au NPs can accelerate charge transferring which depress the recombination of photogenerated electrons and holes in CuS to further enhance the competitive capacity of CuS. The sandwichlike structure has a steric hindrance effect. Therefore, the S2@Au-rGO-CuS has a multiple signal amplification function for thrombin determination. Under optimal conditions, the PEC aptasensor exhibited a wide linear concentration range from 0.1 pM to 10 nM with a low detection limit of 30 fM (S/N = 3) for thrombin. Besides, the designed aptasensor performed well in the assay of human serum sample, indicating good potential for the determination of thrombin in real samples. Graphical abstract.

Ultrasensitive Photoelectrochemical Detection of MicroRNA on Paper by Combining a Cascade Nanozyme-Engineered Biocatalytic Precipitation Reaction and Target-Triggerable DNA Motor

Developing efficient strategies for sensitive detection of microRNAs, the noncoding bioactive molecules and well-established biomarkers, has aroused great interests due to its great potential values in genetic and pathological analyses. Herein, a highly selective and disposable paper-based photoelectrochemical (PEC) sensor was rationally designed for sensing microRNA based on simple self-assembly of a target-triggerable DNA motor and nanozyme-catalyzed multistage biocatalytic precipitation reaction. Specifically, a brand-new type II heterojunction of TiO₂-CeO₂ nanotubes decorated with carbon fiber paper (CFP) was first prepared, which gave an enhanced photoreactive surface and realized fast electron transport and extraction, markedly accelerating photoelectric conversion efficiency of the sensor. For achieving target detection, cascade nanozyme centers of the CeO₂ and Au nanoparticles modified by cyclodextrin were drafted, greatly decreasing the photocurrent intensity and achieving an ultralow background signal. With target introduction, the DNA motor was activated and automatically moved along the predesigned route driven by an endonuclease cleavage reaction, resulting in more substrate probe digestion and nanozyme release from CFP. Consequently, the repressive inner enhancement mechanism was gradually renewed with constant advancement of the enzymatic reaction and walker probe walking progressively, eventually allowing multiple enzymatic factor output in each target import. As a proof-of-concept application, the developed PEC sensor successfully performed detection of miRNA-141, showing a low detection limit of 0.6 fM, and was further applied to real sample bioassays with satisfying results. This work proposes promising strategies to boost the catalytic cascade DNA-motor adhibition in biological samples analysis and also exhibits potential capability in detection of other targets.

Photoelectrochemical detection of circulating tumor cells based on aptamer conjugated Cu2O as signal probe

In this work, a sensitive and reliable photoelectrochemical (PEC) biosensor was proposed based on hexagonal carbon nitride tubes (HCNT) as photoactive material for detection of circulating tumor cells (CTCs). Magnetic Fe₃O₄ nanospheres (MNs) and Cu₂O nanoparticles (Cu₂O NPs) were utilized for highly efficient magnetic capture of CTCs and for signal amplification, respectively. First, anti-epithelial cell adhesion molecule (EpCAM) antibody was linked onto MNs for capture and enrichment of CTCs. With the captured MCF-7 coated onto the electrode, the photocurrent intensity of HCNT was decreased due to the steric hindrance derived from MCF-7. Then, when the Cu₂O-aptamer probe was bound onto the CTC surface, the photocurrent intensity was further decreased because Cu₂O NPs competed with HCNT for absorption of exciting light and the aptamer molecules increased the steric hindrance, which leads to significantly decreased photocurrent response, thus realizing dual signal amplification. Using the breast cancer cell MCF-7 as a model, the proposed PEC biosensor displays good performances with a linear range from 3 to 3000 cell mL^-1 and limit of detection down to 1 cell mL^-1. The HCNT-based PEC biosensor shows good performance for detection of CTCs, which may have potential applications in cancer diagnostics and therapeutics.

A rapid, adaptative DNA biosensor based on molecular beacon-concatenated dual signal amplification strategies for ultrasensitive detection of p53 gene and cancer cells

The cancer diagnosis with single level of biomarkers suffers from limitation of insufficient accuracy. Hence, developing sensitive, rapid and adaptative analytical strategies for double-level biomarkers are essential for improving the accuracy of clinical cancer diagnosis at early stage. Herein, a DNA biosensor was established based on the catalytic hairpin assembly-mediated Y-junction nicking enzyme assisted signal amplification (CHA-YNEASA) circuits, where the two circuits were concatenated by molecular beacon (MB). In absence of target, both the CHA and YNEASA circuits were effectively hindered because of MB's outstanding ability to control signal background. In presence of target, the initiated CHA circuits made enzyme recognition sequences in close proximity to the assisted sequences to open MB, leading to further trigger the YNEASA circuits. Due to the unique design of dual signal amplification strategies, CHA-YNEASA circuits significantly shorten the reaction time, and improve signal-to-background ratio as well as facilitate the analysis process. It was demonstrated that a high sensitivity with limit of detection (LOD) of 0.9 pM for p53 gene detection was obtained just within 23 min by the proposed DNA biosensor. Moreover, mismatched p53 gene at nucleic acid level was effectively discriminated and strong anti-interference capability was achieved. Noticeably, the DNA biosensor was adaptative for designing a cytosensor at cell level using hairpin DNA, containing MUC1 aptamer and initiation strand of CHA-YNEASA circuits, as switch based on modularity principle. The cytosensor is able to measure MUC1 positive breast cancer cells (MCF-7) with the LOD as low as 100 cells/mL. Excellent specificity for MUC1 negative cells, and good anti-interference capability in 10% fetal bovine serum (FBS) were observed by the cytosensor. Therefore, the proposed DNA biosensor is a sensitive, rapid, adaptative platform for detection of double-level biomarkers, offering novel strategy applied for clinical cancer diagnosis.

Target-induced formation of multiple DNAzymes in solid-state nanochannels: Toward innovative photoelectrochemical probing of telomerase activity

Solid-state nanochannels have great potentials in the vibrant field of photoelectrochemical (PEC) bioanalysis. This work herein demonstrates the innovative use of DNA-decorated nanoporous anodic alumina (NAA) nanochannels for sensitive PEC bioanalysis of telomerase (TE) activity. Specifically, telomerase primer sequences (TS) were initially immobilized within the NAA nanochannels and then extended by TE in the presence of deoxyribonucleoside triphosphates (dNTPs). The as formed single-strand DNA was then directed to hybrid with many partially matched single-strand assisting DNA (aDNA), leading to the formation of multiple DNAzymes by the unmatched parts and the subsequent DNAzyme-stimulated biocatalytic precipitation (BCP) within the nanochannels. Because the inhibited signals of the photoelectrode could be correlated with TE-enabled TS extension, an innovative nanochannels PEC bioanalysis could be realized for probing TE activity. This work features the ingenious use of DNA-associated nanochannels for PEC bioanalysis of TE activity. Given the versatile functions of DNA molecules, the extension of this strategy easily allows for addressing numerous other targets of interest. Also, we envision this work could inspire more interest for the further development of nanochannels PEC bioanalysis.

Facile, Rapid, and Low-Cost Electrophoresis Titration of Thrombin by Aptamer-Linked Magnetic Nanoparticles and a Redox Boundary Chip

An aptamer-linked assay of a target biomarker (e.g., thrombin) is facing the challenges of long-term run, complex performance, and expensive instrument, unfitting clinical diagnosis in resource-limited areas. Herein, a facile chip electrophoresis titration (ET) model was proposed for rapid, portable, and low-cost assay of thrombin via aptamer-linked magnetic nanoparticles (MNPs), redox boundary (RB), and horseradish peroxidase (HRP). In the electrophoresis titration-redox boundary (ET-RB) model, thrombin was chosen as a model biomarker, which could be captured within 15 min by MNP-aptamer 1 and HRP-aptamer 2, forming a sandwich complex of (MNP-aptamer 1)-thrombin-(HRP-aptamer 2). After MNP separation and chromogenic reaction of 3,3',5,5'-tetramethylbenzidine (TMB) within 10 min, an ET-RB run could be completed within 5 min based on the reaction between a 3,3',5,5'-tetramethylbenzidine radical cation (TMB^•+) and l-ascorbic acid in the ET channel. The systemic experiments based on the ET-RB method revealed that the sandwich complex could be formed and the thrombin content could be assayed via an ET-RB chip, demonstrating the developed model and method. In particular, the ET-RB method had the evident merits of simplicity, rapidity (less than 30 min), and low cost as well as portability and visuality, in contrast to the currently used thrombin assay. In addition, the developed method had high selectivity, sensitivity (limit of detection of 0.04 nM), and stability (intraday: 3.26%, interday: 6.07%) as well as good recovery (urine: 97-102%, serum: 94-103%). The developed model and method have potential to the development of a point-of-care testing assay in resource-constrained conditions.

A paper-based chemiluminescence immunoassay device for rapid and high-throughput detection of allergen-specific IgE

The fast and precise detection of potential allergen-specific immunoglobulin E (sIgE) is imperative for the diagnosis and appropriate treatment of allergic diseases. In this study, we have successfully fabricated a novel paper-based immunoassay device for the detection of sIgE in allergic diseases. We used Can f 1, one of the main dog allergens, as a model allergen to detect sIgE in human sera. To achieve excellent performance, the experimental parameters were optimized. Further, we extended this device for potential applications in the clinical diagnosis of allergic diseases: worthwhile clinical performance in the detection of allergens was achieved as compared to that achieved by commercial enzyme-linked immunosorbent assay (ELISA) kit. Therefore, it was proven that this strategy has the advantages of high-throughput, rapid, sensitive, and highly accurate detection of trace amounts of sIgEs. Furthermore, by simply changing the antigen and antibody, this device could be used for the high-throughput detection of other allergens, so as to achieve multiallergen detection and appropriate desensitization therapy, thereby making it promising in the determination of allergic diseases in clinics.

Beta-cyclodextrin-functionalized CdS nanorods as building modules for ultrasensitive photoelectrochemical bioassay of HIV DNA

Nowadays, acquired immunodeficiency syndrome has become a formidable danger to human health, and its early diagnosis is urgent need with the increasing quantity of patients around the world. Herein, we first synthesized beta-cyclodextrin-functionalized CdS nanorods (β-CD@CdS NRs) with high stability and desirable photo-electricity activity, and served as easy-to-assemble building modules to design a novel photoelectrochemical biosensor for human immune deficiency virus (HIV) DNA detection by coupling with catalytic hairpin assembly (CHA)-mediated biocatalytic precipitation and the host-guest interaction between adamantine (ADA) and β-CD. In the presence of HIV DNA, CHA process was triggered with the aid of hairpin DNA1 and ADA-labelled hairpin DNA2, and then generated large amounts of G-quadruplex, which could be formed hemin/G-quadruplex DNAzyme to catalyze 4-chloro-1-naphthol to generate insoluble precipitation on photoelectrode surface, followed by the decreased photocurrent response due to the corresponding stereo-hindrance effect. Under optimized conditions, this biosensor exhibited wide linear dynamic range (10 fM - 1 nM) and low detection limit of 1.16 fM, as well as high sensitivity, excellent stability, and satisfactory feasibility in human-serum samples. Moreover, the prepared β-CD@CdS NRs could be applied to the construction of other advanced sensing platform, showing great prospect in clinical diagnostics.

Aptamer-based electrochemical cytosensors for tumor cell detection in cancer diagnosis: A review

Circulating tumor cells, a type of viable cancer cell circulating from primary or metastatic tumors in the blood stream, can lead to the parallel development of primary tumors and metastatic lesions. Highly selective and sensitive detection of tumor cells has become a hot research topic and can provide a basis for early diagnosis of cancers and anticancer drug evaluation to develop the best treatment plan. Aptamers are single-stranded oligonucleotides that can bind to target tumor cells in unique three-dimensional structures with high specificity and affinity. Aptamer-based methods or signal amplification methods using aptamers show great potential in improving the selectivity and sensitivity of electrochemical (EC) cytosensors for tumor cell detection. This review covers the remarkable developments in aptamer-based EC cytosensors for the identification of cell type, cell counting and detection of crucial proteins on the cell surface. Various EC techniques have been developed for cancer cell detection, including common voltammetry or impedance, electrochemiluminescence and photoelectrochemistry in a direct approach (aptamer-target cell), sandwich approach (capture probe-target cell-signaling probe) or other approach. The current challenges and promising opportunities in the establishment of EC aptamer cytosensors for tumor cell detection are also discussed.

CdTe QD-CeO2 Complex as a Strong Photoelectrochemical Signal Indicator for the Ultrasensitive microRNA Assay

The photoelectrochemical (PEC) signal can be enhanced by constructing sensitization structures containing photoactive materials and appropriate sensitizers. However, usually, the photoactive materials and sensitizers were separated in independent nanostructures, thereby producing long electron-transfer path and large energy loss, which could further result in limited photoelectric conversion efficiency and PEC signals. Herein, we designed a novel sensitization nanostructure simultaneously containing the photoactive material cerium dioxide (CeO₂) and its sensitizer CdTe quantum dots (QDs) as the strong PEC signal indicator (CdTe QD-CeO₂ complex), which prominently enhanced photoelectric conversion efficiency because of the shortened electron-transfer path and reduced energy loss. The proposed CdTe QD-CeO₂ complex was used to construct a PEC biosensor for achieving ultrasensitive determination of microRNA-141 (miRNA-141) coupling with target converting amplification and DNA supersandwich structure amplification. The designed PEC biosensor demonstrated a wide linear range from 0.5 fM to 5 nM with a detection limit of 0.17 fM for miRNA-141. Impressively, this work provided a new and strong PEC signal indicator for the construction of  PEC sensing platform and would extend the application of PEC sensors in bioanalysis and early disease diagnosis.

Microfluidic paper-based photoelectrochemical sensing platform with electron-transfer tunneling distance regulation strategy for thrombin detection

This work reports a microfluidic paper-based photoelectrochemical (μ-PEC) sensing platform for thrombin (TB) detection with electron-transfer tunneling distance regulation (ETTDR) and aptamer target-triggering nicking enzyme signaling amplification (NESA) dual strategies. Specifically, paper-based TiO₂ nanosheets (PTNs) were prepared with an efficient hydrothermal process, serving as the direct pathway for the charge carriers transfer. When CeO₂-labeled hairpin DNA 3 (HP3) was closely located at the PTNs, the CeO₂-PTNs heterostructure was formed, which could great facilitate the photogenerated carries separation of CeO₂. In addition, with the aid of aptamer target-triggering NESA strategy, the input TB could be transducted to numerous output target of DNA (tDNA), achieving the goal of desirable signal amplification. In the presence of TB, the output tDNA could be further hybridized with HP3 and unfold its hairpin loop, which forced the CeO₂ away from the surface of PTNs and vanished the CeO₂-PTNs heterostructure, resulting in the obviously reducing of photocurrent signal. The as-designed sensing platform exhibited a linear range from 0.02 pM to 100 pM with a detection limit of 6.7 fM. Importantly, this μ-PEC sensing platform could not only realize the highly efficient TB detection, but also pave a luciferous way for the detection of other protein in bioanalysis.

A visible-light-driven photoelectrochemical molecularly imprinted sensor based on titanium dioxide nanotube arrays loaded with silver iodide nanoparticles for the sensitive detection of benzoyl peroxide

A novel ultrasensitive photoelectrochemical sensor for benzoyl peroxide (BPO) was constructed under visible light irradiation.

A carbon-dot-based dual-emission probe for ultrasensitive visual detection of copper ions

In this work, we successfully developed an ultrasensitive dual-emission fluorescent sensor (CRB) for instant visual determination of Cu²⁺ by modifying a Cu-sensitive rhodamine B derivative (RB) onto photostable carboxyl modified carbon dots (CDs).