Isolation of Sensing Units and Adsorption Groups Based on MOF-on-MOF Hierarchical Structure for Both Highly Sensitive Detection and Removal of Hg2.

Bifunctional materials have attracted ongoing interest in the field of detection and removal of contaminants because of their integration of two functions, but they exhibit commonly exceptional performance in only one of these two aspects. The interaction between the two functional units of the bifunctional materials may compromise their sensing and adsorption abilities. Guided by the concept of domain building blocks (DBBs), a hierarchical metal-organic framework (MOF)-on-MOF hybrid was designed by growing gold nanoclusters (AuNCs)-embedded zeolitic imidazolate framework 8 (AuNCs/ZIF-8) on the surface of Zr-MOF (UiO-66-NH2) for the simultaneous detection and removal of Hg2+. In the hybrid, the amino groups (-NH2) and AuNCs─which were the adsorption groups and sensing units, respectively, were isolated from each other. Specifically, the adsorption groups (-NH2) were assembled in the inner UiO-66-NH2 layer, while the sensing units (AuNCs) were confined in the outer ZIF-8 layer. This hierarchical structure not only spatially hindered the electron transfer between these two units but also triggered the aggregation-induced emission of AuNCs because of the confinement of ZIF-8 on the AuNCs, thus changing the fluorescence of AuNCs from quenching to enhancement. The newly prepared UiO-66-NH2@AuNCs/ZIF-8 hybrid, as expected, showed an ultralow detection limit (0.42 ppb) and a high adsorption capacity (129.9 mg·g-1) for Hg2+. Overall, this work provides a feasible approach to improve the integrated performance of MOF-based composites based on DBBs.

Photoelectrochemical aptasensing with methylene blue filled Ni-MOFs nanocomposite by spatial confinement for microcystin-LR detection.

Microcystin LR (MC-LR) is a hazardous cyanotoxin produced by cyanobacteria during freshwater eutrophication, which can cause liver cancer. Here, a photoelectrochemical (PEC) aptasensor based on methylene blue (MB)-loaded Ni-MOF composite (Ni-MOF/MB) with spatial confinement was constructed for the sensitive detection of MC-LR. Ni-MOF with two-dimensional sheet structure was prepared via a liquid-liquid interface synthesis method with environmental-friendly solvent and milder reaction conditions. Benefiting from the uniform pore size, Ni-MOF acted as reaction platform to anchor the photosensitive molecule MB. The electron donor, ascorbic acid (AA), was produced by alkaline phosphatase (ALP) loaded on DNA strand catalyzing ascorbic acid phosphate. The generated AA was absorbed by Ni-MOF/MB, thereby effectively improving the utilization of AA and avoiding the external environment interferences to enlarge the photocurrent of MB. For analysis, ALP-labeled aptamer can specifically recognize MC-LR by forming a complex to strip from aptasensor, thus leading to a  decreased photocurrent. The developed PEC aptasensor offered a linear range of 10 fM-100 pM with a detection limit of 6 fM. It was successfully employed for detecting MC-LR in farm water and fish meat, and the results were validated by ultrahigh-performance liquid chromatography-mass spectrometry. This method presents a new idea of MOF-limited domain for PEC aptasensing.

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.

Split aptamer-based sandwich-type ratiometric biosensor for dual-modal photoelectrochemical and electrochemical detection of 17ß-estradiol.

BACKGROUND: As one of the most potent environmental estrogens, 17ß-estradiol (E2), which can be enriched into organisms through the food chain and cause harmful biological effects in humans, has been frequently detected in the water environment of the world. High performance liquid chromatography (HPLC) and gas chromatograohy-mass spectrometry (GC/MS) have been widely used for quantification of E2. Despite excellent accuracy, tedious pretreatment and expensive instruments result in their limited application. It is clear that there is an urgent need to establish simple, sensitive and accurate methods for the determination of E2. RESULTS: A split aptamer-based sandwich-type ratiometric biosensor based on split aptamer was developed by coupling photoelectrochemical and electrochemical assays for E2 detection. For analysis, the two fragments of split aptamer recognized E2 by forming sandwich structure, which triggered hybridization chain reaction (HCR) to produce double-stranded DNA (dsDNA) with CdTe quantum dots (QDs) labeled hairpin DNA. The resultant dsDNA can further absorb methylene blue (MB) to sensitize CdTe QDs for an enlarged photocurrent (IPEC) and output a redox current of IMB, and both of them acted as response signals for detection; [Fe(CN)6]3-/4- probe produced redox current of I[Fe(CN)6]3-/4- as reference signal. Using IMB/I[Fe(CN)6]3-/4- and IPEC/I[Fe(CN)6]3-/4- as yardsticks, the developed split aptamer-based sandwich-type ratiometric biosensor provides two linear ranges of 0.1-5000 pg mL-1 for IMB/I[Fe(CN)6]3-/4- and 0.1-10000 pg mL-1 for IPEC/I[Fe(CN)6]3-/4- with detection limits of 0.06 pg mL-1 and 0.02 pg mL-1, respectively. SIGNIFICANCE: These results of the biosensor are benefiting from the coupling of photoelectrochemical (PEC) and electrochemical (EC) assays as well as the unique cooperative recognition mechanism of split aptamer. This method not only enabled the biosensor to be successfully applied to the determination of E2 in lake water, but also broadens the prospects for the realization of sensitive and accurate detection of E2.

Integrated Photoelectrochemical-SERS Platform Based on Plasmonic Metal-Semiconductor Heterostructures for Multidimensional Charge Transfer Analysis and Enhanced Patulin Detection.

Comprehending the charge transfer mechanism at the semiconductor interfaces is crucial for enhancing the electronic and optical performance of sensing devices. Yet, relying solely on single signal acquisition methods at the interface hinders a comprehensive understanding of the charge transfer under optical excitation. Herein, we present an integrated photoelectrochemical surface-enhanced Raman spectroscopy (PEC-SERS) platform based on quantum dots/metal-organic framework (CdTe/Yb-TCPP) nanocomposites for investigating the charge transfer mechanism under photoexcitation in multiple dimensions. This integrated platform allows simultaneous PEC and SERS measurements with a 532 nm laser. The obtained photocurrent and Raman spectra of the CdTe/Yb-TCPP nanocomposites are simultaneously influenced by variable bias voltages, and the correlation between them enables us to predict the charge transfer pathway. Moreover, we integrate gold nanorods (Au NRs) into the PEC-SERS system by using magnetic separation and DNA biometrics to construct a biosensor for patulin detection. This biosensor demonstrates the voltage-driven ON/OFF switching of PEC and SERS signals, a phenomenon attributed to the plasmon resonance effect of Au NRs at different voltages, thereby influencing charge transfer. The detection of patulin in apples verified the applicability of the biosensor. The study offers an efficient approach to understanding semiconductor-metal interfaces and presents a new avenue for designing high-performance biosensors.

Photo-enhanced electrochemical and colorimetric dual-modal aptasensing for aflatoxin B1 detection based on graphene-gold Schottky contact.

A photo-enhanced electrochemical (PEEC) and colorimetric (CM) dual-modal aptasensor was developed with rGO-AuNP Schottky contact for AFB1 monitoring. The PEEC mode allowed the ultrasensitive quantitation based on the photo-enhanced electroactivity mechanism, while the CM mode offered a rapid threshold-level qualitative assay with a portable colorimeter.

Photoelectrochemical and visual dual-mode sensor for efficient detection of Cry1Ab protein based on the proximity hybridization driven specific desorption of multifunctional probe.

Currently, the development of sensitive and visual strategy for Cry1Ab detection, particularly using a switchable dual-mode detection system based on a single component, remains a great challenge. Here, a photoelectrochemical (PEC) and visual dual-mode sensor was designed for Cry1Ab detection based on a proximity hybridization driven multifunctional probe. In the presence of Cry1Ab, specific desorption of the antibody-DNA conjugate was achieved via sufficient proximity hybridization, leading to the selective release of the multifunctional signal probe, i.e., antibody-labeled single-stranded DNA-gold nanoparticles (Ab1-S1-AuNPs). The released Ab1-S1-AuNPs reduced the photocurrent signal and produced a colored response, thereby achieving PEC and visual dual-mode detection based on a single component. Owing to the different signal generation mechanisms, two independent signals were obtained simultaneously, which provided self-verification to improve reliability and accuracy. Taking advantage of the PEC sensitive detection and visual prediction, the dual-mode sensor achieved efficient detection of the Cry1Ab protein. The developed sensor was successfully used to determine Cry1Ab in corn, wheat, and soil samples with satisfactory results. This method offers a promising biosensing platform for the on-site detection of Cry1Ab protein.

In-depth interpretation of aptamer-based sensing on electrode: Dual-mode electrochemical-photoelectrochemical sensor for the ratiometric detection of patulin.

Electrochemical aptasensors have been extensively used to quantify food contaminants (e.g., mycotoxin) by using high-affinity aptamer for target recognition. Yet, analytical performance of aptasensors using different aptamers can be varied for the same target. Here, four aptamers with different sequences (i.e., A22, A34, A42, and A45) of patulin (PAT) were selected to estimate sensing behaviors at electrodes with electrochemical (EC) and photoelectrochemical (PEC) assays. Synergistic effect of steric hindrance and electron transfer distance was found to significantly affect EC and PEC response for PAT at aptasensors fabricated with A22, A34, A42, or A45. Eventually, A22 emerged to be the optimal aptamer for aptasensing, despite the highest affinity of A42 to PAT. The A22-based EC-PEC dual-mode ratiometric aptasensor offered a linear range of 50 fg mL-1 - 500 ng mL-1 with a detection limit of 30 fg mL-1 for PAT, and it was applied to apple product (i.e., juice, puree) analysis.

All-in-one fabrication of a ratiometric electrochemical aptasensor with tetrahedral DNA nanostructure for fumonisin B1 detection.

Here, we develop an all-in-one strategy for efficient assembly of an electrochemical aptasensor. A multifunctional structure based on a tetrahedral DNA nanostructure (TDN) was synthesized via a one-step annealing process, providing DNA fixation, target recognition, signal amplification and space regulation. Based on the integration of this multifunctional structure, the sensing interface was assembled in one step. A ratiometric aptasensor was constructed by anchoring methylene blue (MB) to the TDN and ferrocene (Fc) on the cDNA. Using the ratio of the currents obtained from Fc and MB as a measure, the developed aptasensor shows excellent analytical performance for fumonisin B1 detection. This strategy is universal and could simplify the fabrication of aptasensors.