"On-Off-On" Photoelectrochemical/Visual Lab-on-Paper Sensing via Signal Amplification of CdS Quantum Dots@Leaf-Shape ZnO and Quenching of Au-Modified Prism-Anchored Octahedral CeO2 Nanoparticles

Recent advances in microfluidic-based photoelectrochemical (PEC) sensing platforms for biomedical applications

Photoelectrochemical (PEC) techniques seamlessly combine electrochemical and spectroscopic principles, offering a powerful platform for the detection of biomarkers and biological molecules in clinical and biomedical settings. This review provides a comprehensive overview of microfluidic PEC probes, emphasizing their potential for ultrasensitive detection through enhanced light absorption and charge transfer processes. Key advantages of microfluidic PEC include real-time monitoring of biological processes, non-invasive detection, and the possibility of multiplexing when integrated with various quantification modalities. However, the practical implementation of PEC faces challenges such as bulky setup, matrix interference, and stability of PEC-active materials. Also, this paper discusses the intricate mechanisms of PEC sensing, highlighting the roles of nanomaterials in enhancing microfluidic PEC systems. Additionally, the limitations inherent in PEC material selection, including stability and bandgap engineering, are critically discussed. Solutions such as doping and the development of composite materials are proposed to address these issues. Through presented examples of PEC applications in biomedical fields, this review elucidates the future potential of PEC-based methods as reliable and effective tools for diagnostic applications. Additionally, this review proposes the most effective probes for future investigations to develop commercial devices.

Selective and stable visible-light-prompted scavenger-free photoelectrochemical strategy based on a ternary ErVO4/P@g-C3N4/SnS2 nanocomposite for the detection of lead ions in different water samples

Lead ions (Pb2+) are heavy metal environmental pollutants that can significantly impact biological health. In this study, the synthesis of a ternary nanocomposite, ErVO4/P@g-C3N4/SnS2, was achieved using a combination of hydrothermal synthesis and mechanical grinding. The as-fabricated photoelectrochemical (PEC) sensor was found to be an ideal substrate for Pb2+ detection with high sensitivity and reliability. The ErVO4/P@g-C3N4/SnS2/FTO was selected as the substrate because of its remarkable and reliable photocurrent response. The Pb2+ sensor exhibited a low detection limit of 0.1 pM and a broad linear range of 0.002-0.2 nM. Moreover, the sensor exhibited outstanding stability, selectivity, and reproducibility. In real-time applications, it exhibited stable recovery and a low relative standard deviation, ensuring reliable and accurate measurements. The as-prepared PEC sensor was highly stable for the detection of Pb2+ in different water samples. This promising characteristic highlights its significant potential for use in the detection of environmental pollutants.

Overview of the Design and Application of Dual-Signal Immunoassays

Immunoassays have been widely used for the determination of various analytes in the fields of disease diagnosis, food safety, and environmental monitoring. Dual-signal immunoassays are now advanced and integrated detection technologies with excellent self-correction and self-validation capabilities. In this work, we summarize the recent advances in the development of optical and electrochemical dual-signal immunoassays, including colorimetric, fluorescence, surface-enhanced Raman spectroscopy (SERS), electrochemical, electrochemiluminescence, and photoelectrochemical methods. This review particularly emphasizes the working principle of diverse dual-signal immunoassays and the utilization of dual-functional molecules and nanomaterials. It also outlines the challenges and prospects of future research on dual-signal immunoassays.

High performance photoelectrochemical immunosensing platform based on front-illuminated Mo:BiVO4 photoelectrodes for procalcitonin assay

The outstanding photoactive materials are the imperative for the construction of a front-illuminated photoelectrochemical (PEC) biosensor, which is crucial step for improving the detection sensitivity. Yet, the weak and unstable initial PEC signals of the photoelectrodes have limited evidently the detection performance. Herein, a front-illuminated "on-off" PEC immunosensor was constructed based on Mo:BiVO4 as photoactive matrix and Au/CeO2 as signal quencher for sensitive detection of procalcitonin (PCT). Systematic studies reveal that the Mo doped BiVO4 can increase the charge carrier density of BiVO4, leading to much higher initial signal under front illumination than back illumination. Moreover, Mo:BiVO4 was directly grown on conducting substrates, which effectively overcomes the loose combination of sensing substrate ensuring good electrical contact and continuity. Upon coupling with Au/CeO2 as signal quencher, the initial photocurrent signal can be significantly quenched. As a result, the proposed PEC immunosensor presents a wide linear range from 10 fg mL-1 to 50 ng mL-1 with a detection limit of 2.45 fg mL-1. Impressively, this study will open a new avenue for the construction of highly efficient and stable photoelectrode, as well as extend the application of PEC biosensor for biomarkers detection in early disease diagnosis.

A dual-mode "signal-on" split-type aptasensor for bisphenol A via target-induced hybridization chain reaction amplification

Herein, a dual-mode detection system was constructed for efficient and accurate detection of bisphenol A (BPA) with the assistance of the BPA-induced hybridization chain reaction (HCR). The captured DNA (cDNA) was first modified on the surface of magnetic spheres modified with gold nanoparticles and polydopamine and then hybridized with the BPA aptamer to form double-stranded DNA (dsDNA). In the presence of the BPA target, the BPA aptamer was released from the surface of the magnetic sphere. The free cDNA triggered a HCR to construct a DNA duplex. Methylene blue (MB), as a bifunctional probe, was intercalated into the double-stranded DNA to amplify the photocurrent (IPEC) of the CdS-modified electrode and generate an electrochemical current (IEC) at the same time. Under the optimized conditions, the PEC and EC signal responses of the system were linear to the logarithm of BPA concentration in the range of 1.0 × 10-10 M to 1.0 × 10-5 M. The detection limits were found to be 1.27 × 10-11 M and 3.0 × 10-11 M using the PEC and EC methods, respectively. The constructed dual-mode biosensor exhibited good performance for real sample analysis, demonstrating its promising potential for practical applications. In addition, this dual-mode detection strategy provides more accurate and reliable detection results.

Paper-Supported Photoelectrochemical Biosensor for Dual-Mode miRNA-106a Assay: Integration of Luminescence-Confined Upconversion-Actuated Fluorescent Resonance Energy Transfer and CRISPR/Cas13a-Powered Cascade DNA Circuits

Near-infrared (NIR)-responsive bioassays based on upconversion nanoparticle (UCNP) incorporating high-performance semiconductors have been developed by researchers, but most lack satisfactory ultrasensitivity for exceedingly trace amounts of target. Herein, for the first time, the CRISPR/Cas13a system is combined with cascade DNA circuits, fluorescent resonance energy transfer (FRET) effect, and luminescence-confined UCNPs-bonded CuInS2/ZnO p-n heterostructures-functionalized paper-working electrode to construct dual-signal-on paper-supported NIR-irradiated photoelectrochemical (PEC) (NIR-PEC) and upconversion luminescence (UCL) bioassay for high-sensitive quantification of miRNA-106a (miR-106a). By constructing an ideal FAM-labeled aminating molecular beacon (FAM-H2) model, a relatively good FRET ratio between the UCNP and FAM (≈85.3%) can be achieved. In the existence of miR-106a, the hairpin-structure FAM-H2 was unwound, bringing about the distance increase of UCNP and FAM and the restraint of FRET. Accordingly, both the NIR-PEC signal and the UCL intensity gradually recovered distinctly. Unlike conventional single-mode PEC sensors, with NIR excitation, the designed dual-mode sensing system could implement minimized misdiagnose assay and quantitative miR-106a determination with low detection limits, that is, 76.54 and 51.36 aM for NIR-PEC and UCL detection, respectively. This work not only broadens the horizon of application of the CRISPR/Cas13a strategy toward biosensing but also constructs a new structure of the UCNP-semiconductor in the exploration of efficient NIR-responsive tools and inspires the construction of a no-misdiagnosed and novel biosensor for dual-mode liquid biopsy.

Polarity Conversion of the Ag2S/AgInS2 Heterojunction by Radical-Induced Positive Feedback Polydopamine Adhesion for Signal-Switchable Photoelectrochemical Biosensing

Efficient tuning of the polarity of photoactive nanomaterials is of great importance in improving the performance of photoelectrochemical (PEC) sensing platforms. Herein, polarity of the Ag2S/AgInS2 heterojunction is converted by radical-induced positive feedback polydopamine (PDA) adhesion, which is further employed to develop a signal-switchable PEC biosensor. In the nanocomposites, Ag2S and AgInS2 achieve electron-hole separation, exhibiting a strong anodic PEC response. Under the irradiation of light, the Ag2S/AgInS2 heterojunction is able to produce superoxide radical and hydroxyl radical intermediate species, leading to the polymerization of dopamine (DA) and the subsequent adhesion of PDA onto the Ag2S/AgInS2 heterojunction (Ag2S/AgInS2@PDA). By constructing a new electron-transfer pathway with PDA, the polarity of the Ag2S/AgInS2 heterojunction is converted, and the PEC response changes from anodic to cathodic photocurrents. In addition, since the photoreduction activity of PDA is stronger than that of the Ag2S/AgInS2 heterojunction, more superoxide radical can be produced by Ag2S/AgInS2@PDA once PDA is generated, thereby promoting the generation of PDA. Consequently, a positive feedback mechanism is established to enhance the polarity conversion of the Ag2S/AgInS2 heterojunction and amplify the responding to DA. As a result, the bioanalytical method is capable of sensitively quantifying DA in 10 orders of magnitude with an ultralow limit of detection. Moreover, the applicability of this biosensor in real samples is identified by measuring DA in fetal bovine serum and compared with a commercial ELISA method. Overall, this work offers an alternative perspective for adjusting photogenerated carriers of nanomaterials and designing high-performance PEC biosensors.

A dual-model immobilization-free photoelectrochemical/visual colorimetric bioanalysis based on microemulsion self-assemblies mediated multifunctional signal amplification strategy

Herein, a novel silver ion-loaded gold microemulsion assemblies (Au/Ag+ MAs) mediated multifunctional signal amplification strategy was proposed to construct a sensitive immobilization-free photoelectrochemical (PEC)/colorimetric biosensor for carcinoembryonic antigen (CEA) detection. Through the sandwiched reaction among CEA, the CEA aptamer (DNA1) loaded on the Au nanoparticles (NPs) functionalized iron oxide (Fe3O4) nanospheres and another CEA aptamer (DNA2) immobilized on Au/Ag+ MAs, a complex is formed and acquired by magnetic separation. Then, Au/Ag+ MAs of the complex are disassembled into Au NPs and Ag+ ions driven by an acetone response, and the obtained demulsification solution is transferred to the cadmium sulfide/cadmium telluride (CdS/CdTe) photoactive composites modified electrode. Based on the multiple inhibition functions (blocking effect of oleylamine; energy transfer effect of Au NPs; and electron snatching effect of Ag+), the photocurrent of the electrode decreases obviously, resulting in the ultrasensitive detection of CEA (a detection limit of 16 fg mL-1). Interestingly, the ion-exchange reactions between CdS/CdTe composites and Ag+ ions generate silver sulfide/silver telluride (Ag2S/Ag2Te) composites, and a color change of composites can be distinguished directly, leading to a quick visual detection of CEA. Compared with the traditional single-modal assay for CEA, such dual-modal PEC/colorimetric assay is a more accurate and reliable due to different mechanisms and independent signal conversion. This work will offer a new perspective for the applications of various self-assemblies in PEC bioanalysis.

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.

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.

AuNPs/CdS QDs/CeO2 ternary nanocomposite coupled with scrollable three-dimensional DNA walker mediated cycling amplification for sensitive photoelectrochemical miRNA assay

Herein, a novel ternary nanocomposite (AuNPs/CdS QDs/CeO₂) with excellent photoelectrochemical (PEC) performance was synthesized as signal probe to construct a near-zero background biosensor for sensitive miRNA-182-5p detection, by integrating with a scrollable three-dimensional (3D) DNA walker mediated cleavage cycling amplification. Impressively, the formation and rolling of scrollable 3D DNA walker triggered by target could realize dynamic, rapid and specific digestion of hairpin DNA on electrode with the aid of Exonuclease III (Exo III), which thus exposed abundant binding sites for assembling stable DNA labeled AuNPs/CdS QDs/CeO₂ nanoprobes. Thanks to the formation of type-II heterojunction (between CeO₂ and CdS QDs) and Schottky junction (generated by CeO₂ and AuNPs), an ideal photoelectric conversion efficiency accompanied with stunningly improved photocurrent was thus acquired for significantly improving the detection sensitivity. It turned out that the detection limit (LOD) of biosensor was ultralow (31 aM). Significantly, the proposed PEC biosensor would exhibit great potential for the composite as a splendid indicator and provide an avenue for constructing the sensing platform with excellent sensitivity and ultralow background.

Electrochemiluminescence Systems for the Detection of Biomarkers: Strategical and Technological Advances

Electrochemiluminescence (ECL)-based sensing systems rely on light emissions from luminophores, which are generated by high-energy electron transfer reactions between electrogenerated species on an electrode. ECL systems have been widely used in the detection and monitoring of diverse, disease-related biomarkers due to their high selectivity and fast response times, as well as their spatial and temporal control of luminance, high controllability, and a wide detection range. This review focuses on the recent strategic and technological advances in ECL-based biomarker detection systems. We introduce several sensing systems for medical applications that are classified according to the reactions that drive ECL signal emissions. We also provide recent examples of sensing strategies and technologies based on factors that enhance sensitivity and multiplexing abilities as well as simplify sensing procedures. This review also discusses the potential strategies and technologies for the development of ECL systems with an enhanced detection ability.

Paper-Based Bipolar Electrode Electrochemiluminescence Platform Combined with Pencil-Drawing Trace for the Detection of M.SssI Methyltransferase

Herein, a hand-drawing paper-based bipolar electrode (BPE) electrochemiluminescence (ECL) platform for M.SssI methyltransferase (M.SssI MTase) assay was proposed via employing high electrocatalytic Pt@CeO₂ as an ECL co-reaction accelerator and pencil-drawing graphite electric circuits as wires and electrodes. Notably, the introduction of pencil-drawing trace not only simplified the manufacturing process but also reduced the cost and saved fabricating time. Meanwhile, Pt@CeO₂ with good electrocatalytic activity and satisfactory chemical stability was used at the anode of the closed BPE-ECL device to accelerate the oxidation rate of uric acid. Due to the balanced charges of the bipolar electrode, the ECL response of the MnS: CdS@ZnS/S₂O₈^2- system emitted on the cathode was enhanced. In situ growth of gold nanoparticles in the two electrode areas was convenient for DNA immobilization. With the above points in mind, the specific DNA double strands functionalized via Pt@CeO₂ were employed to identify M.SssI MTase. The unmethylated DNA double strands were cut by HpaII endonuclease, resulting in the quenching of the ECL signal. Under the optimal conditions, sensitive detection of M.SssI MTase in a wide linear range of 0.01-100 U·mL^-1 with a satisfactory detection limit of 0.008 U·mL^-1 was realized. The reliable and versatile BPE-ECL tool for the determination of M.SssI MTase with easy-to-operate pencil-drawing traces and independent solution systems provides a new opportunity to develop paper-based devices applied in early disease diagnosis and pathogenesis research.

A Paper-Based Cantilever Beam Mini Actuator Using Hygro-Thermal Response

New technological and scientific advances in the development of sensors and actuators demand the development of new devices to deal with recent problems and challenges in these new and emerging processes. Moreover, paper-based devices have tremendous potential for developing actuators as paper exhibits capillary transport and hygroexpansion due to swelling of the fibers when absorbing water. Therefore, this paper proposes a mini actuator that is based on a hygro-thermal-paper-based cantilever beam that is activated by means of a droplet of an aqueous solution in combination with a circulating electrical current to analyze its response. The contribution of this proposal includes the analysis of the flexural response of the mini actuator when it is tested by using two different solutions: distilled water and a water/alcohol solution. Additionally, four cases related to the droplet volume are studied and a statistical analysis of the bending responses is presented. The results achieved show that that water-alcohol solutions have a lower deviation in comparison with water only. Moreover, it is demonstrated that a specific change in the maximum displacement is obtained according to the volume and the type of solution. Thus, it is suggested that the response of the mini actuator can be tuned using different aqueous solutions.

Photoelectrochemical aptasensor based on La2Ti2O7/Sb2S3 and V2O5 for effectively signal change strategy for cancer marker detection

In this item, a high-efficiency signal "on-off-on" strategy photoelectrochemical (PEC) apatsensor was resoundingly developed for target ultrasensitive analysis. Primarily, the heterojunction formation between Cd: Sb₂S₃ and La₂Ti₂O₇ was contributed to the first "signal-on" state to improve the stability of the PEC platform. Secondly, V₂O₅ nanosphere act as a catalyst for H₂O₂ was used to label on aptamer DNA to consume electron donor for achieving "signal-off" state. Then target analyte was modified on the surface of the PEC platform, and part of V₂O₅ with aptamer DNA would be released from the aptasensor surface, thus, the "signal-on" state was realized again. In this signal "on-off-on" strategy, the PEC performance of perovskite La₂Ti₂O₇ was effectively perfected with Cd: Sb₂S₃ sensitization, and broaden the application of perovskite in PEC sensor field. And the signal attenuation and recovery strategy were distinctly elevated the sensitivity of the aptasensor. In the preferred detection conditions, the proposed PEC sensor for analyte (PSA as an example) analysis revealed a wide sensing range from 1.000 × 10^-5 to 500.0 ng/mL, own a low detection limit of 4.300 fg/mL. This smart response change mode also provide prospect for other target detection, and offer a reference to signal transform for other electrochemical method.

All-sealed paper-based electrochemiluminescence platform for on-site determination of lead ions

Lab-on-paper (LOP) devices are urgently required for the rapid development of point-of-care diagnoses and environmental assays. Herein, an all-sealed paper-based electrochemiluminescence (ECL) platform was developed to achieve lead ions (Pb²⁺) sensitive analysis via incorporating convenient plastic package technology. Benefiting from transparent plastic encapsulation, the sealed devices effectively avoided the interference of O₂. Meanwhile, myrica rubra-like Pt nanomaterials (MPNs) prepared by an economical and easy-to-operate ultrasound method were employed to catalyze H₂O₂ decomposition. With the help of Pb²⁺-specific DNAzymes, the oligonucleotide probe functionalized via MPNs could be detached from the device in the presence of target, resulting in the reduced ECL intensity. Moreover, the combination of modified paper electrode with functional regions separated by multiple layers of wax enhanced the practicability of the LOP device for rapid detection. Under the optimal conditions, the all-sealed platform achieved wide linear relationship ranging from 0.01 nM to 0.05 μM with a low detection limit of 0.004 nM for sensitive detecting Pb²⁺. It is believed that this platform could provide a robust, simple and versatile strategy for sensitive determination of heavy metal ions, and be applied in on-site contamination analysis in the future.

Novel optoelectronic metal organic framework material perylene tetracarboxylate magnesium: preparation and biosensing

The pursuit for improving photoelectrochemical (PEC) performances of organic materials remains an urgent need. Here, we have proposed an envision of the preparation the metal-organic frameworks (MOFs) with arenes to realize high photo-to-current conversion efficiency and excellent PEC performances. Magnesium 3,4,9,10-perylene tetracarboxylic acid metal-organic frameworks (Mg-PTCA MOFs) were synthesized for the first time. The uniformly distributed and regular-shaped Mg-PTCA MOFs showed a much more stable and higher photocurrent than the single PTCA and its derivatives, which confirmed our hypothesis. A regenerated-biosensor was designed for microRNA analysis based on Mg-PTCA MOFs as a novel photoelectric material, target-triggered three-dimensional DNA Scaffold (3D-Sca) as an efficient signal amplifier, and gold nanoclusters (Au NCs) as quencher. The elaborately designed biosensor achieved ultrasensitive detection for miRNA 21 with a dynamic range from 10 aM to 10 pM and a detection limit of 2.8 aM. This biosensor showed good analytical performance in the extracts of different cancer cells, indicating the possibility for early diagnosis, timely staging assessment, and accurate prognostic judgment for diseases. The recommendable performances of Mg-PTCA MOFs highlight the significance of organic MOFs in PEC sensing.

Hollow performances quenching label of Au NPs@CoSnO3 nanoboxes-based sandwich photoelectrochemical immunosensor for sensitive CYFRA 21-1 detection

In this work, a fire-new "signal-off" type photoelectrochemical (PEC) immunosensor based on bismuth sulfide/iodine doped bismuth oxychloride (Bi₂S₃/I:BiOCl) heterostructure as a platform and Au nanoparticles loaded hollow CoSnO₃ nanoboxes (Au NPs@CoSnO₃) as quenching label was designed, for sensitive detection of CYFRA 21-1. The I:BiOCl with flower-like structure could supply high specific surface area for loading nanometer materials. Then, Bi₂S₃ was formed in-situ by S^2- adsorption on the surface of I:BiOCl by dangling bond of Bi³⁺, but did not change the flower-like structure of I:BiOCl. Then, n-type Bi₂S₃ and p-type I:BiOCl heterostructure showed good photoelectric behavior by providing an additional electric field to accelerate electron-hole separation. Furthermore, the production process of the heterostructure was simple, fast, low temperature, and without complex raw materials. The Au NPs@CoSnO₃ with good photocatalytic activity could strongly compete with Bi₂S₃/I:BiOCl for electron donor of ascorbic acid (AA). Meanwhile, the CoSnO₃ with hollow structure made the quenching effect more significant by the light-scattering effect that enhanced the light absorption capacity and shorten distance of carrier transport. Under optimal conditions, this proposed strategy displayed the low detection limit of 30 fg/mL, with a high linearity range from 100 fg/mL to 100 ng/mL for tumor markers CYFRA 21-1. Simultaneously, it also exhibited excellent specificity and acceptable stability, which might provide a new perspective for the fabrication of other PEC immunosensors with heterostructure simple synthesis and hollow materials.

Bi2S3@MoS2 Nanoflowers on Cellulose Fibers Combined with Octahedral CeO2 for Dual-Mode Microfluidic Paper-Based MiRNA-141 Sensors

An effective dual-mode microfluidic paper-based analysis device (μPAD) was proposed via Bi₂S₃@MoS₂ nanoflowers combined with octahedral CeO₂ for ultrasensitive miRNA-141 bioassay. To obtain the amplified electrochemical signal, Bi₂S₃@MoS₂ nanoflowers were first in situ grown onto the surface of cellulose fibers to promote the reduction of H₂O₂. The prism-anchored octahedral CeO₂ nanoparticles with a great catalytic function on the reduction of H₂O₂ were linked up to the functionalized cellulose fibers through the hybridization chain reaction to further enhance the electrochemical signal. By means of the catalysis effect of Bi₂S₃@MoS₂ nanoflowers and octahedral CeO₂ nanoparticles, the obtained signal was amplified, thereby achieving ultrasensitive electrochemical detection of the target. With the help of duplex specific nuclease, the octahedral CeO₂ could be released from the electrochemical detection area and flow to the color channel through capillary action, which could initiate the oxidation reaction of 3,3',5,5'-tetramethylbenzidine in the existence of H₂O₂ to generate a blue visual band, avoiding the error of distinguishing color depth caused by the naked eye and thus improving the accuracy of the visual method. Under the optimal conditions, satisfactory prediction and accurate detection performance were achieved in the range of 10 fM-1 nM and 0.5 fM-1 nM, respectively, by measuring the length of the blue product and the electrochemical signal intensity. The electrochemical/visual detection limits of the proposed μPAD for miRNA-141 were as low as 0.12 and 2.65 fM (S/N = 3). This work provides great potential for the construction of low-cost and high-performance dual-mode biosensors for the detection of biomarkers.

A photoelectrochemical sensor based on an acetylcholinesterase-CdS/ZnO-modified extended-gate field-effect transistor for glyphosate detection

A new photoelectrochemical enzyme biosensor based on an extended-gate field-effect transistor (EGFET) was constructed for the highly sensitive detection of glyphosate based on the inhibition of acetylcholinesterase (AChE) activity by glyphosate. First, a two-step hydrothermal method was used to introduce ZnO and CdS onto an activated indium tin oxide (ITO) electrode to prepare a CdS/ZnO/ITO electrode. Then, AChE was immobilized on CdS/ZnO/ITO with chitosan to obtain an AChE/CdS/ZnO EGFET sensor. Under optimal experimental conditions, the logarithmic value of glyphosate in the range of 1.0 × 10-15-1.0 × 10-11 mol L-1 exhibited a good linear relationship with the photo-drain current response. The detection limit was 3.8 × 10-16 mol L-1 (signal-to-noise ratio = 3). The results show that the AChE/CdS/ZnO EGFET sensor has extremely high sensitivity and good selectivity. Moreover, the sensor was used for the determination of glyphosate in vegetables, demonstrating its application for the real-time detection of samples.

A dual-model "on-super off" photoelectrochemical/ratiometric electrochemical biosensor for ultrasensitive and accurate detection of microRNA-224

A dual-model "on-super off" photoelectrochemical (PEC)/ratiometric electrochemical (EC) biosensor based on signal enhancing and quenching combining three-dimensional (3D) DNA walker strategy was designed for the ultrasensitive and accurate detection of microRNA-224 (miRNA-224). The "signal on" PEC state was achieved by methylene blue labeled hairpin DNA (MB-DNA) for sensitizing CdS QDs. Then numerous transformational ferrocene labeled DNAs (Fc-DNAs) converted by target-induced 3D DNA walker amplification with the help of Ag nanocubes (NCs) label DNA (Ag-DNA) were introduced to open hairpin MB-DNA. Such configuration change would relocate the sensitizer MB and the quencher Fc, whereas energy transfer placed between Ag NCs and CdS QDs, thereby significantly quenching the PEC signal to obtain "super off" state. Meanwhile, these changes resulted in a decreased oxidation peak current of MB (I_MB) and an increased that of Fc (I_Fc). MiRNA-224 was also detected on basis of the dual-signaling EC ratiometric method for complementary PEC detection. Benefiting from different mechanisms and relatively independent signal transduction, this approach not only avoided interference from difficult assembly but also outstandingly increased sensitivity by distance-controllable signal enhancing and quenching strategies. As a result, the detection ranges of 0.1-1000 fM with a low detection limit of 0.019 fM for PEC, and 0.52 to 500 fM with a low detection limit of 0.061 fM for EC, were obtained for miRNA-224, which opens a new avenue for designing numerous elegant biosensors with potential utility in bioanalysis and early disease diagnosis.

Advanced Signal-Amplification Strategies for Paper-Based Analytical Devices: A Comprehensive Review

Paper-based analytical devices (PADs) have emerged as a promising approach to point-of-care (POC) detection applications in biomedical and clinical diagnosis owing to their advantages, including cost-effectiveness, ease of use, and rapid responses as well as for being equipment-free, disposable, and user-friendly. However, the overall sensitivity of PADs still remains weak, posing a challenge for biosensing scientists exploiting them in clinical applications. This review comprehensively summarizes the current applicable potential of PADs, focusing on total signal-amplification strategies that have been applied widely in PADs involving colorimetry, luminescence, surface-enhanced Raman scattering, photoacoustic, photothermal, and photoelectrochemical methods as well as nucleic acid-mediated PAD modifications. The advances in signal-amplification strategies in terms of signal-enhancing principles, sensitivity, and time reactions are discussed in detail to provide an overview of these approaches to using PADs in biosensing applications. Furthermore, a comparison of these methods summarizes the potential for scientists to develop superior PADs. This review serves as a useful inside look at the current progress and prospective directions in using PADs for clinical diagnostics and provides a better source of reference for further investigations, as well as innovations, in the POC diagnostics field.

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.

An "on-off-super on" photoelectrochemical sensor based on quenching by Cu-induced surface exciton trapping and signal amplification of copper sulfide/porous carbon nitride heterojunction

In this work, we report an "on-off-super on" photoelectrochemical sensor for probing hydrogen sulfide due to its toxicity in water environment by using porous carbon nitride as photoelectric transducers. Synthesized by an alkaline-assisted hydrothermal method, the porous carbon nitride photoanode exhibited a remarkable photocurrent on the initial "on" state. Cu²⁺ immobilized on the surfaces of porous carbon nitride could significantly decrease the charge transfer efficiency and quench the photoelectrochemical signal in the "off" state. In addition, the introduction of S^2- ions could eliminate the influence of Cu-induced surface exciton trapping and amplify the photoelectrochemical signal due to the formation of carbon nitride/copper sulfide heterojunction, thus leading to the achievement of the ''super on'' state and subsequently detection of hydrogen sulfide. More importantly, this photoelectrochemical sensor shows the excellent performance for probing hydrogen sulfide in terms of stability, selectivity, sensitivity and fabrication cost. Enabled by a unique "on-off-super on" strategy, it could serve as a reference for developing the new class of photoelectrochemical sensor.

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.

Paper-Based SERS Sensing Platform Based on 3D Silver Dendrites and Molecularly Imprinted Identifier Sandwich Hybrid for Neonicotinoid Quantification

Real-time monitoring of neonicotinoid pesticide residues is of great significance for food security and sustainable development of the ecological environment. Herein, a paper-based surface-enhanced Raman scattering (SERS) amplified approach was proposed by virtue of multilayered plasmonic coupling amplification. The unique plasmonic SERS multilayer was constructed using three-dimensional (3D) silver dendrite (SD)/electropolymerized molecular identifier (EMI)/silver nanoparticle (AgNP) sandwich hybrids with multiple hotspots and a strong electromagnetic field in nanogaps. Dendritelike 3D silver materials with remarkably high accessible surface areas and the lightning rod effect constituted the first-order enhancement of paper-based sensors. Molecular identifiers coated upon an SD layer as the interlayer were used for target capture and enrichment. Subsequently, AgNPs featuring rough surface and local plasma resonance decorated as the top layer formed the secondary enhancement of the amplification strategy. As the most brilliant part, dendritelike 3D silver coupled with AgNPs has established double Ag layers to accomplish a multistage enhancement of SERS signals based on the superposition of their electromagnetic fields. Owning to the distinctive design of the multiple coupling amplification strategy, the fabricated SERS paper chips demonstrated impressive specificity and ultrahigh sensitivity in the detection of imidacloprid (IMI), with a detection limit as low as 0.02811 ng mL^-1. More importantly, the multiple SERS enhancement paper chip holds great potential for automated screening of a variety of contaminants.

Ultrasensitive photoelectrochemical sensor enabled by a target-induced signal quencher release strategy

In this work, a target-induced signal quencher release strategy was proposed to construct a sensitive photoelectrochemical (PEC) sensor.

Folding Paper-Based Aptasensor Platform Coated with Novel Nanoassemblies for Instant and Highly Sensitive Detection of 17β-Estradiol

Owing to its critical role in the development of female reproductive tissues and as a clinical biomarker, there is an urgent need to develop a rapid and cost-effective method to sensitively detect 17β-estradiol (E2). In this work, a folding aptasensor platform with microfluidic channels for the label-free electrochemical detection of E2 is described. The platform, designed with a delicate folding structure, integrating filter holes, microfluidic channels, reaction chambers, and a three-electrode system, is extremely easy to use. To increase the detection sensitivity and immobilize the aptamer, we synthesized a novel nanoassembly consisting of amine-functionalized single-walled carbon nanotube/new methylene blue/gold nanoparticles (AuNPs) and modified the working electrode with this nanoassembly. The calibration curve obtained from the experimental results exhibited a linear range between 10 pg mL^-1 and 500 ng mL^-1 (R² = 0.993), and a detection limit of 5 pg mL^-1 was achieved (S/N = 3). Furthermore, experiments to detect E2 in clinical serum were conducted, and the results were highly similar to those obtained using a large electrochemical luminescence apparatus. By integrating multiple functional components, adopting novel nanoassemblies, and using a folding structure, this paper-based platform not only has great potential as a simple and convenient integrated device for point-of-care testing of E2, but also as a portable, low-cost, and highly sensitive aptasensor platform capable of detecting many diagnostic biomarkers with the appropriate aptamers.

Signal-switchable lab-on-paper photoelectrochemical aptasensing system integrated triple-helix molecular switch with charge separation and recombination regime of type-II CdTe@CdSe core-shell quantum dots

Herein, a new "on-off-on" signal switch system combined triple helix molecular switch with efficient charge separation and transfer between different sensitization units was designed for the ultrasensitive photoelectrochemical (PEC) determination of prostate-specific antigen (PSA). Concretely, the initial "signal-on" state was obtained via the cascaded sensitization structure consisting of type-II CdTe@CdSe core-shell quantum dots (QDs), CdS QDs, and ZnO nanotubes, which were assembled on Au nanoparticles modified paper fibers with the aid of signal transduction probe (STP). Thereinto, the type-II CdTe@CdSe QDs with hole-localizing core and electron-localizing shell could enable the ultrafast charge transfer and retard the charge recombination, magnifying the initial photocurrent response and preserving the high efficiency of signal-switchable PEC aptasensing system. Subsequently, the PSA aptamer (PSA-Apt) modified with gold nanoparticles (GNPs) was introduced by the hybridization of PSA-Apt with STP and the hairpin configuration of STP changed from closed to open state, forming a triple-helix structure. Hence, the CdTe@CdSe QDs labeled on the terminal of STP moved away from the electrode surface while the GNPs kept attached close to it. The proposed aptasensor turned to "signal-off" state because of the dual inhibition of vanished cosensitization effect and signal quenching effect of GNPs. Upon the target recognition, the triple-helix structure was perturbed with the formation of DNA-protein complex and the recovery of STP hairpin structure, resulting in the second "switch-on" state. Based on the target-induced photocurrent enhancement, the proposed PEC aptasensor was utilized for the determination of PSA with high sensitivity, persuasive selectivity, and excellent stability.

A novel "signal on" photoelectrochemical strategy based on dual functional hemin for microRNA assay

In this work, a novel "signal on" photoelectrochemistry (PEC) biosensor was constructed with dual-functional hemin as a signal quencher and electronic mediator for ultrasensitive target microRNA-141 assay with the assistance of T7 exonuclease (Exo)-initiated target amplification technology.