Stepwise Protonation of Three-Dimensional Covalent Organic Frameworks for Enhancing Hydrogen Peroxide Photosynthesis.

Three-dimensional covalent organic frameworks (3D COFs), recognized for their tailorable structures and accessible active sites, offer a promising platform for developing advanced photocatalysts. However, the difficulty in the synthesis and functionalization of 3D COFs hinders their further development. In this study, we present a series of 3D-bcu-COFs with 8 connected porphyrin units linked by linear linkers through imine bonds as a versatile platform for photocatalyst design. The photoresponse of 3D-bcu-COFs was initially modulated by functionalizing linear linkers with benzo-thiadiazole or benzo-selenadiazole groups. Furthermore, taking advantage of the well-exposed porphyrin and imine sites in 3D-bcu-COFs, their photocatalytic activity was optimized by stepwise protonation of imine bonds and porphyrin centers. The dual protonated COF with benzo-selenadiazole groups exhibited enhanced charge separation, leading to an increased photocatalytic H2O2 production under visible light. This enhancement demonstrates the combined benefits of linker functionalization and stepwise protonation on photocatalytic efficiency.

Bimodal Oxidation Electrochemiluminescence Mechanism of Coreactant-Embedded Covalent Organic Frameworks via Postsynthetic Modification.

Electrochemiluminescence (ECL) efficiency is determined by charge transfer between coreactants and emitters in coreactant systems, which are usually limited by their slow intermolecular charge transfer. In this study, a covalent organic framework (COF) with aldehyde residue was synthesized, and then coreactants were covalently integrated into the skeleton through the postsynthetic modification strategy, resulting in a crystalline coreactant-embedded COF nanoemitter (C-COF). Compared to the pristine COF with an equivalent external coreactant, C-COF exhibited an extraordinary 1008-fold enhancement of ECL intensity due to the rapid intrareticular charge transfer. Significantly, with the pH increase, C-COF shows protonation-induced ECL enhancement for the first ECL peaked at +1.1 V and an opposite trend for the second ECL at +1.4 V, which were attributed to the antedating oxidation of coreactant in framework and COF self-oxidation, respectively. The resulting bimodal oxidation ECL mechanism was rationalized by spectral characterization and density functional theory calculations. The postsynthetic coreactant-embedded nanoemitters present innovative and universal avenues for advancing ECL systems.

Photocurrent Polarity Reversal Induced by Electron-Donor Release for the Highly Sensitive Photoelectrochemical Detection of Vascular Endothelial Growth Factor 165.

Photocurrent polarity reversal is a switching process between the anodic and cathodic pathways and is critical for eliminating false positivity and improving detection sensitivity in photoelectrochemical (PEC) sensing. In this study, we construct a PEC sensor with excellent photocurrent polarity reversal induced by ascorbic acid (AA) as an electron donor with the energy level matching the photoactive material zirconium metal-organic framework (ZrMOF). The ZrMOF-modified electrode demonstrates cathodic photocurrent in the presence of O2 as an electron acceptor, while the anodic photocurrent is generated in the presence of AA, achieving photocurrent polarity reversal. By the in situ release of AA from AA-encapsulated apoferritin modified with DNA 2 (AA@APO-S2) as a detection tag in the presence of trypsin after the recognition of hairpin DNA-modified indium tin oxide to the reaction product of aptamer/DNA 1 with the target protein and the following rolling cycle amplification for introducing the detection tag to the sensing interface, the reversed photocurrent shows an enhanced photocurrent response to the target protein, leading to a highly sensitive PEC sensing strategy. This strategy realizes the detection of vascular endothelial growth factor 165 with good specificity, a wide linear range, and a low detection limit down to 5.3 fM. The actual sample analysis offers the detection results of the proposed PEC sensor comparable to those of commercial enzyme-linked immunosorbent assay tests, indicating the promising application of the photocurrent polarity reversal-based PEC sensing strategy in biomolecule detection and clinical diagnosis.

Electron Donor Coordinated Metal-Organic Framework to Enhance Photoelectrochemical Performance.

To enhance photoelectrochemical (PEC) performance, additional electron donor/acceptor is generally required to inhibit the electron-hole recombination. However, the enhancement is limited due to the long-distance diffusion. Herein a self-supplying electron strategy is designed for PEC enhancement by coordinating an electron donor 1.4-diazabicyclo[2.2.2]octane (Dabco) in metal-organic framework (MOF). The intrareticular photoelectron transfer mechanism in mixed-ligand MOF (m-MOF) is experimentally revealed and verified by density functional theory calculations. The presence of Dabco efficiently inhibits the electron-hole recombination due to the self-supplying electrons and longer electron lifetime in the framework, and thus leads to 23.2-fold enhancement of photocurrent. As proof of concept, a simple PEC method is constructed with the designed m-MOF to demonstrate its application in sensitive bioanalysis. This work provides a new avenue for improving the PEC performance of nanomaterials.

Framework-promoted charge transfer for highly selective photoelectrochemical biosensing of dopamine.

Traditional photoelectrochemical (PEC) systems with inorganic semiconductors as photoactive materials generally involve effortless recombination of electron-hole pairs, which greatly limit the detection sensitivity. The arrangement of multiple components with tunable bandgaps provides an effective way to accelerate charge transfer. In this work, a framework material with adjustable structure was used to promote the charge transfer in the PEC process. The framework was constructed with 9,10-di(p-carboxyphenyl)anthracene (DPA) ligands as the light collector to coordinate with Zn2+ nodes, which formed an electronegative metal-organic framework (ZnMOF), and showed good conductivity and PEC performance due to the π-π stacking of DPA and the intrareticular charge transfer. Based on the band and charge matching of dopamine (DA) with ZnMOF, the ZnMOF modified electrode as a biosensor showed excellent PEC response to DA with good selectivity, thus realized sensitive detection of DA ranging from 0.03 to 10 µM with a detection limit of 17.7 nM. The biosensor could be used to monitor the release of DA from PC12 cells and evaluate the stimulation of K+ to DA release. The conductive framework material provided an approach to develop highly selective sensing platform for trace bioanalysis.

In-situ synthesis of 3D Cu2O@Cu-based MOF nanobelt arrays with improved conductivity for sensitive photoelectrochemical detection of vascular endothelial growth factor 165.

Construction of novel photoelectrochemical (PEC) materials with unique structures can effectively improve the photoelectric conversion efficiency. Here, a self-supported Cu2O@Cu-MOF/copper mesh (CM) nanobelt arrays with high specific surface area, high orientation, and high photoelectric conversion performance is obtained by in-situ grown strategy. Such PEC aptasensor is constructed based on the Cu2O@Cu-MOF/CM combined with rolling circle amplification and enzymatic biocatalytic precipitation for vascular endothelial growth factor 165 analysis. This strategy achieves excellent cooperative signal amplification, which greatly improves the detection sensitivity. The PEC aptasensor exhibited a wide calibration ranged from 10 to 1 × 108 fM with a detection limit down to 2.3 fM (S/N = 3). The construction of semiconductor@MOFs has developed the potential application of MOFs in photoelectrochemical and found a reliable path for ultrasensitive detection of biomarkers.

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

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

A photoelectrochemical aptasensor based on p-n heterojunction CdS-Cu2O nanorod arrays with enhanced photocurrent for the detection of prostate-specific antigen.

A sensitive photoelectrochemical (PEC) aptasensor was constructed for prostate-specific antigen (PSA) detection using an enhanced photocurrent response strategy. The p-n heterostructure CdS-Cu2O nanorod arrays were prepared on Ti mesh (CdS-Cu2O NAs/TM) by a simple hydrothermal method and successive ionic-layer adsorption reactions. Compared with the original CdS/TM, the synergistic effect of p-n type CdS-Cu2O NAs/TM and the internal electric field realizes the effective separation of photoinduced electron-hole pairs and improves the PEC performance. In order to construct the aptasensor, an amino-modified aptamer was immobilized on CdS-Cu2O NAs/TM to serve as a recognition unit for PSA. After the introduction of PSA, PSA was specifically captured by the aptamer on the PEC aptasensor, which can be oxidized by photogenerated holes to prevent electron-hole recombination and increase photocurrent. Under optimal conditions, the constructed PEC aptasensor has a linear range of 0.1-100 ng·mL-1 and a detection limit as low as 0.026 ng·mL-1. The results of aptasensor detection of human serum indicate that it has broad application prospects in biosensors and photoelectrochemical analysis.

Highly sensitive photoelectrochemical detection of bleomycin based on Au/WS2 nanorod array as signal matrix and Ag/ZnMOF nanozyme as multifunctional amplifier.

An ultrasensitive photoelectrochemical (PEC) biosensor was constructed based on gold nanoparticles (Au NPs)/tungsten sulfide nanorod array (WS2 NA) photoelectrode as the PEC matrix and silver nanoparticles/flake-like zinc metal-organic framework (Ag/ZnMOF) nanozyme with the peroxidase mimetic enzyme property for sensitive detection of bleomycin (BLM). In particular, Au/WS2 and Ag/ZnMOF were linked by thiolate DNA1 and DNA2 strand, respectively, and the Au/WS2-Ag/ZnMOF probe was prepared via hybridization reaction between the two DNAs. The introduction of Ag/ZnMOF in the probe offers two functions: i) the steric hindrance effect can effectively impede electron transport and reduce the photocurrent; ii) Ag/ZnMOF nanozyme can also be used as mimic peroxidase to effectively catalyze 3,3-diaminobenzidine (DAB) to produce the relevant precipitation, which will further reduce photocurrent and eliminate false positive signals. When BLM exists, BLM with Fe2+ as irreversible cofactor can specifically recognize and cleave of the 5'-GC-3' active site of DNA2, resulting in reduced precipitation deposited on the electrode and recovery of PEC signal. The highly sensitive PEC biosensor exhibits a the linear strategy from 0.5 nM to 500 nM with a detection limit down to 0.18 nM. Further, the unique strategy was conducted in biological samples for BLM detection with satisfactory consequence, offering available and efficient pathway for disease diagnosis.

Photoelectrochemical determination of trypsin by using an indium tin oxide electrode modified with a composite prepared from MoS2 nanosheets and TiO2 nanorods.

A photoelectrochemical (PEC) method has been developed for sensitive detection of trypsin. It is based on the use of a composite consisting of MoS2 nanosheets and TiO2 nanorods (MoS2-TiO2). The material has a high specific surface area, superior electrical conductivity, excellent biocompatibility and good band gap matching. The composite was synthesized by a one-pot method using TiO2 as a template. This results in a uniform distribution of the MoS2 nanosheets (<5 layers) in the composite. If the composite, placed on an indium tin oxide (ITO) electrode, is coupled to apoferritin, the photocurrent response decreases due to the insulating effect of the protein. Trypsin, in acting as an alkaline protease, decomposes the apoferritin. This results in the recovery of the PEC signal. Attractive features of this PEC method include (a) a superior PEC signal, (b) sensor stability, (c) simple operation, and (d) the lack of any additional modifications of the biosensor. This warrants high sensitivity, reproducibility, repeatability and practicality. The ITO sensor has a linear response in the 1 to 1000 ng·mL-1 trypsin concentration range and a 0.82 ng·mL-1 detection limit. The assay was applied to the determination of trypsin in spiked serum samples and gave satisfactory results. Graphical abstract Schematic presentation of an indium tin oxide (ITO)/MoS2-TiO2 sensor for detecting trypsin. The PEC signal was decreased after immobilization of apoferritin (APO) on the modified ITO. Trypsin catalytically hydrolyzes APO specifically and induces the PEC signal to recover.

Fluorometric turn-on detection of ascorbic acid based on controlled release of polyallylamine-capped gold nanoclusters from MnO2 nanosheets.

A fluorometric turn-on assay is described for ascorbic acid (AA). It is based on the controlled release of polyallylamine-stabilized gold nanoclusters (polyallylamine-AuNCs) from MnO2 nanosheets. In an aqueous solution of near-neutral pH value, the positively charged capped AuNCs are adsorbed on the surface of the negatively charged MnO2 nanosheets. The adsorption leads to the quenching of the fluorescence of the AuNCs. However, in the presence of AA, MnO2 is reduced to Mn2+. This causes the destruction of the MnO2 nanosheets. As a result, the fluorescence of the polyallylamine-AuNCs at 615 nm is recovered. This method for determination of AA is inexpensive, sensitive, and selective. It works in the 0.01 to 200 µM concentration range and has a 3.2 nM detection limit (for S/N = 3). Graphical abstract Gold nanoclusters (AuNCs) and polyallylamine can form polyallylamine-AuNCs to enhance the orange fluorescence of AuNCs. MnO2 nanosheets can absorb polyallylamine-AuNCs, and this results in fluorescence quenching of polyallylamine-AuNCs. Ascorbic acid (AA) can reduce MnO2 nanosheets, in this results in the fluorescence recovery of polyallylamine-AuNCs.

Enhanced electrocatalytic activity of water oxidation in an alkaline medium via Fe doping in CoS2 nanosheets.

The development of new electrocatalysts is critical for efficient water electrolysis to produce hydrogen. In this communication, we report a novel electrocatalyst for the oxygen evolution reaction (OER) which is realized via Fe doping in CoS2 nanosheets (Fe-CoS2). This catalyst shows an overpotential of 302 mV for a current density of 10 mA cm-2, 85 mV less than that for CoS2. In addition, Fe-CoS2 also exhibits high catalytic stability.

Photoelectrochemical determination of the activity of alkaline phosphatase by using a CdS@graphene conjugate coupled to CoOOH nanosheets for signal amplification.

A method is described for photoelectrochemical determination of the activity of alkaline phosphatase (ALP). It employs an indium tin oxide (ITO) electrode modified a CdS quantum dots@graphene (CdS@GR) composite and hexagonal cobalt oxyhydroxide (CoOOH) nanosheets. The CdS@GR nanocomposite was synthesized by assembling the CdS quantum dots onto a GO film to receive a basic photocurrent response of the ITO. This is further improved by covering it with CoOOH nanosheets. Secondly, 2-phospho-L-ascorbic acid (AAP) is added as a substrate for ALP. Its hydrolysis yields ascorbic acid which reduces CoOOH to form cobalt(II) ion. As a result, the CoOOH nanosheets decompose. This is accompanied by a reduction of the photocurrent. The effect was used to design a selective and sensitive assay of determination of the activity of ALP. Under the optimized experimental conditions, response is linear in the 10 to 300 U·L-1ALP activity range. The detection limit is 1.5 U·L-1 at a signal-to-noise ratio of 3. Graphical abstract Indium tin oxide (ITO) was coted with CdS@graphene and CoOOH to obtain a material with superior photoelectrochemical properties. The detection of alkaline phosphatase (ALP) was accomplished by using 2-phospho-L-ascorbic acid (AAP) which is hydrolyzed by ALP to release ascorbic acid (AA) which reduces CoOOH to Co2+.

Ascorbic Acid-Loaded Apoferritin-Assisted Carbon Dot-MnO2 Nanocomposites for the Selective and Sensitive Detection of Trypsin.

A novel "turn-on" fluorescent trypsin detection platform dependant on carbon dot-MnO2 (CD-MnO2) nanocomposites and ascorbic acid-loaded apoferritin (APOAA) was fabricated. The detection mechanism relied on trypsin-catalyzed enzymolysis of APOAA, which released ascorbic acid (AA) as a reducing agent to disintegrate the MnO2 nanosheets, causing the recovery of the fluorescence of CDs. An excellent performance and high sensitivity of trypsin determination were observed with a detection limit (LOD) of 0.3411 ng/mL. This work provides us with a unique strategy for trypsin detection in human serum samples, which reveals the potential applications in clinical detection.

Detection of glutathione based on MnO2 nanosheet-gated mesoporous silica nanoparticles and target induced release of glucose measured with a portable glucose meter.

The authors describe a novel method for the determination of glutathione (GSH). Detection is based on target induced release of glucose from MnO2 nanosheet-gated aminated mesoporous silica nanoparticles (MSNs). In detail, glucose is loaded into the pores of MSNs. Negatively charged MnO2 nanosheets are assembled on the MSNs through electrostatic interactions. The nanosheets are reduced by GSH, and this results in the release of glucose which is quantified by using a commercial electrochemical glucose meter. GSH can be quantified by this method in the 100 nM to 10 µM concentration range, with a 34 nM limit of detection. Graphical abstract Glucose is loaded into the pores of mesoporous silica nanoparticles (MSNs). MnO2 nanosheets are assembled on MSNs through electrostatic interactions. Glutathione (GSH) can reduce the nanosheets, and this results in the release of glucose which is quantified by using a commercial glucose meter.