Portable Visual Photoelectrochemical Biosensor Based on a MgTi2O5/CdSe Heterojunction and Reversible Electrochromic Supercapacitor for Dual-Modal Cry1Ab Protein Detection

Reversible electrochromic supercapacitors (ESCs) have attracted considerable interest as visual display screens. The use of ESCs in combination with a photoelectrochemical (PEC) biosensor promises to improve the detection efficiency. Herein, a visual PEC biosensor is developed by introducing a circuit module between a PEC-sensing platform (PSP) and a reversible ESC for Cry1Ab protein detection. In PSP, a type II MgTi2O5/CdSe heterojunction effectively drives charge separation by their cross-matched band gap structures, generating an amplified photocurrent. Next, the circuit module is designed to connect the PSP and ESC, realizing the signal conversion from photocurrent to voltage. ESC, as a visual display screen, produces reversible color changes with different voltages. As the concentration of Cry1Ab increases, the photocurrent decreases due to the specific binding between the aptamer and Cry1Ab in PSP, while the color of the reversible ESC changes from green to blue. To improve the integrity of the device, a portable PEC biosensor is further constructed via three-dimensional printing for dual-modal Cry1Ab protein detection, thus collecting both PEC and visual signals. The linear ranges are 0.3-3000 ng mL-1 for PEC mode and 1-1000 ng mL-1 for visual mode. This work presents a portable, efficient, sensitive, and visualized detection system, providing an important reference for practical visualization applications.

Polarity-Specific and Pyrimidine-over-Purine Adaptive Triplex DNA Recognition by a Near-Infrared Fluorogenic Molecular Rotor

Triplex DNA structures have displayed a wide range of applications including nanosensing, molecule switching, and drug delivering. Therefore, it is of great importance to effectively recognize triplex DNA structures by a simple and highly selective manner. Herein, we found that a near-infrared fluorogenic probe of NIAD-4 with a molecular rotor (MR) merit can selectively recognize triplex DNA structures over G-quadruplex, i-motif, and duplex structures (Tri-over-QID selectivity), which is competent over the widely used MR probe of thioflavin T (ThT). Furthermore, NIAD-4 exhibits as well a high selectivity toward the 'pyrimidine-type' triplex structures (Y:R-Y type) with respect to the 'purine-type' triplex structures (R:R-Y type) (a Y-over-R selectivity). Interestingly, NIAD-4 recognizes the Y:R-Y triplex structures by a polarity-dependent manner. The 3' end triplet is the preferential binding field of NIAD-4 with respect to the 5' end one (a 3'-over-5' selectivity) as the 3' end triplet is more stable than the 5' end one in the Hoogsteen hydrogen bond. It is expected that the adaptive stacking interaction between NIAD-4 and the 3' end triplet favors the Tri-over-QID, Y-over-R, and 3'-over-5' selectivities since this MR probe has three rotating shafts matching well with the triplet in topology. Such a high selectivity of NIAD-4 opens a new route in designing sensors with DNA structures switching between triplex, i-motif, and G-quadruplex structures.

HCR/DNAzyme-triggered cascaded feedback cycle amplification for self-powered dual-photoelectrode detection of femtomolar HPV16

For high-performance dual-photoelectrode assay, developing a pair of photoactive materials with well-matched band structure and the design of a powerful sensing strategy are highly desirable. Herein, the Zn-TBAPy pyrene-based MOF and BiVO4/Ti3C2 Schottky junction were employed as photocathode and photoanode to form an efficient dual-photoelectrode system. The integration of the cascaded hybridization chain reaction (HCR)/DNAzyme-assisted feedback amplification with DNA walker-mediated cycle amplification strategy realizes femtomolar HPV16 dual-photoelectrode bioassay. Through the activation of the HCR cascaded with the DNAzyme system in the presence of HPV16, plentiful HPV16 analogs are generated that leads to exponential positive feedback signal amplification. Meanwhile on the Zn-TBAPy photocathode, the NDNA hybridizes with the bipedal DNA walker followed by circular cleavage by Nb.BbvCI NEase, producing a dramatically enhanced PEC readout. The achieved ultralow detection limit of 0.57 fM and a wide linear range of 10-6 nM-103 nM showcase the excellent performance of the developed dual-photoelectrode system.

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.

Zinc-Air Battery-Assisted Self-Powered PEC Sensors for Sensitive Assay of PTP1B Activity Based on Perovskite Quantum Dots Encapsulated in Vinyl-Functionalized Covalent Organic Frameworks

The self-powered sensors have attracted widespread attention in the analysis field due to a huge demand of point-of-care testing (POCT) in the early diagnosis of diseases. However, the output voltage of the reported self-powered sensors is always small, resulting in a narrow linear detection range and low assay sensitivity. Herein, a self-powered photoelectrochemical (PEC) sensor with zinc-air batteries as a power source was developed for activity assay of protein tyrosine phosphatase 1B (PTP1B) based on perovskite quantum dots encapsulated in the vinyl-functionalized covalent organic framework (COF-V). CsPbBr₃ nanocrystals were stabilized by the confinement effect of the COF-V cage without aggregation, and the resulting CsPbBr₃@COF-V composite was used as the cathodic photoelectric material to construct the zinc-air battery with a large open-circuit voltage (OCV, 1.556 V). Before PTP1B activity assay, an auxiliary peptide-polyamidoamine-phosphopeptide (P2-PAMAM-P1) hybrid was introduced into the photocathode via thiol-ene click reaction between the thiol group on the P1 and the vinyl group on the COF-V. The steric hindrance effect of the P1-PAMAM-P2 hybrid inhibited the PEC performance of the photocathode, resulting in a small OCV of the zinc-air battery. When the PTP1B existed, PTP1B-catalyzed dephosphorylation of tyrosine on P1 facilitated the cleavage process of P1 by chymotrypsin, leading to the removal of the P2-PAMAM-P1 hybrid from the photocathode and consequently the enhancement of the OCV. Therefore, the activity of PTP1B was sensitively detected. The developed self-powered PEC sensor showed superior performance for PTP1B activity assay (broad linear response range, 0.1 pM to 10 nM and low detection limit, 0.032 pM) due to the large output voltage of the constructed zinc-air battery and has great potential in POCT of protein phosphatase-related diseases and the discovery of protein phosphatase-targeted drugs.

Photoelectrochemical Detection of Exosomal miRNAs by Combining Target-Programmed Controllable Signal Quenching Engineering

MicroRNAs extracted from exosomes (exosomal miRNAs) have recently emerged as promising biomarkers for early prognosis and diagnosis. Thus, the development of an effective approach for exosomal miRNA monitoring has triggered extensive attention. Herein, a sensitive photoelectrochemical (PEC) biosensing platform is demonstrated for exosomal miRNA assay via the target miRNA-powered λ-exonuclease for the amplification strategy. The metal-organic framework (MOF)-decorated WO₃ nanoflakes heterostructure is constructed and implemented as the photoelectrode. Also, a target exosomal miRNA-activatable programmed release nanocarrier was fabricated, which is responsible for signal control. Hemin that acted as the electron acceptor was prior entrapped into the programmed control release nanocarriers. Once the target exosomal miRNAs-21 was introduced, the as-prepared programmed release nanocarriers were initiated to trigger the release of hemin, which enabled the quenching of the photocurrent. Under the optimized conditions, the level of exosomal miRNAs-21 could be accurately tracked ranging from 1 fM to 0.1 μM with a low detection limit of 0.5 fM. The discoveries illustrate the possibility for the rapid and efficient diagnosis and prognosis prediction of diseases based on the detection of exosomal miRNAs-21 and would provide feasible approaches for the fabrication of an efficient platform for clinical applications.

A dual-signal self-checking photoelectrochemical immunosensor based on the sole composite of MIL-101(Cr) and CdSe quantum dots for the detection of α-fetoprotein

Designing a photoelectrochemical (PEC) immunosensor that can produce dual photocurrent signals which can refer to each other is a great importance but a big challenge. In this manuscript, a novel dual photocurrent signals immunosensor was constructed for the detection of α-fetoprotein (AFP). Unlike the usual method of using two composite materials to provide cathode and anode photocurrent respectively, this work applies only one compound of MIL-101 (Cr) and CdSe quantum dots (QDs). Thereinto, we found that the photocurrent polarity of MIL-101(Cr) would switch by adjusting applied voltage. And then CdSe QDs was introduced by simple ultrasound mixing to boost the dual photocurrent signals. Furthermore, in the composite of M&C, the electron transfer path between MIL-101(Cr) and CdSe QDs may switch between "Z-type" and "Ⅱ-type" by adjusting voltage. Benefiting by the dual signals, the proposed sensor can not only perform sensitively quantitative detection of α-fetoprotein (AFP), but also can intuitively estimate the accuracy and reliability of the test result by determining whether the corresponding relationship of "cathode photocurrent-analyte concentration-anode photocurrent" is established. The linear ranges of the sensing electrodes as cathode and anode are the same, both from 0.1 to 300 ng mL^-1. The limit of detection (LOD) is 0.082 ng mL^-1 (S/N = 3) when it used as an anode, and the LOD is 0.054 ng mL^-1 (S/N = 3) when it served as cathode. Furthermore, this sensor showed acceptable stability, reproducibility, specificity, and feasibility of detecting AFP in human serum, which has broad development prospects in the early clinical diagnosis.

The controlled growth of conjugated polymer-quantum dot nanocomposites via a unimolecular templating strategy

Size and surface functionality are critically important for organic-inorganic hybrid semiconductive nanocomposites in terms of stable photoelectrochemical properties and superior device performance. The ability of reversible deactivation radical polymerization to control the chain length and dispersity of polymers is herein extended to the tailor-made synthesis of nanocomposites with tunable size, distribution, and surface coating. This is exemplified by the fabrication of cadmium selenide (CdSe) quantum dots (QDs) with uniform sizes from 2 to 10 nm that are intimately coated with poly(3-hexylthiophene) (i.e., CdSe@P3HT).

Formation of a Photoelectrochemical Z-Scheme Structure with Inorganic/Organic Hybrid Materials for Evaluation of Receptor Protein Expression on the Membrane of Cancer Cells

Quantitative analysis of receptor protein expression is essential to give new insights into tumor-related research. Benefitting from their high sensitivity and low background, photoelectrochemical (PEC) platforms are considered as powerful tools for evaluating the expression of receptor proteins. Herein, to reduce the cytotoxicity and facilitate the subsequent assembly, l-cysteine-modified Ag-ZnIn₂S₄ quantum dots (l-Cys AZIS QDs) are prepared and PEC responses under the irradiation of long wavelength light are obtained. To further improve the PEC behavior, iron phthalocyanine (FePc) is employed to form a Z-scheme structure with l-Cys AZIS QDs. The Z-scheme structure based on l-Cys AZIS QDs/FePc hybrid materials exhibits high photo-to-electric conversion efficiency and can be excited with near-infrared range light. Because hyaluronic acid linked to photoactive materials can recognize CD44 expressed on the membrane of cancer cells, cancer cells are immobilized onto l-Cys AZIS QDs/FePc hybrid materials, inducing a decrease of the photocurrent intensity. Consequently, a PEC cytosensor is constructed to quantify cancer cells expressing CD44. The PEC analytical platform is able to determine A549 cells in the range of 2 × 10² to 4.5 × 10⁶ cells/mL, and a detection limit of 15 cells/mL is realized in the case of S/N = 3. In addition, the expression of CD44 in A549 and other five cancer cells is measured with this PEC method. Depending on our data, the expression of CD44 in different cancer cells is distinct, indicating great potential of this method in receptor protein-related studies.