Rationally constructed ZnCdS-HDCs@In2S3-HNRs double-hollow heterojunction with promoted light capture capability for photoelectrochemical biosensing

CTAB-Co-MOFs@AuPt NPs as signal probes for the electrochemical detection of carcinoembryonic antigen 15-3

A sensitive electrochemical strategy for carcinoembryonic antigen 15-3 (CA15-3) detection is reported using CTAB-Co-MOFs@AuPt NPs as signal probes. The electrochemical strategy was designed as follows: First, the graphene aerogel@gold nanoparticles (GA@Au NPs) nanocomposites were employed to modify the sensing surface for promoting electron transfer rate and primary antibody (Ab1) immobilization due to GA possesses a large specific surface area, eminent conductivity, and a 3D network structure. Cobalt metal-organic frameworks (CTAB-Co-MOFs) synthesized were then used as a carrier for AuPt NPs and secondary antibody (Ab2) immobilization (notes: labelled-Ab2). With sandwich immunoreaction, the labelled-Ab2 was captured on the surface of the GA@Au NPs nanocomposites. Finally, differential pulse voltammetry (DPV) was employed to register the electrochemical signal of the immunosensor at the potential of - 0.85 V (vs SCE) in phosphate buffer saline (PBS) containing 2.5 mM H2O2. It was verified that the electrochemical reduction signal from Co3+ to Co2+ was recorded. The AuPt NPs could catalyze the reaction of H2O2 oxidizing Co2+ to Co3+, resulting in the amplification of the electrochemical signal. Under the selected conditions, the immunosensor can detect CA15-3 in the range 10 µU/mL to 250 U/mL with a low detection limit of 1.1 µU/mL. In the designed strategy, the CTAB-Co-MOFs were not only employed as carriers for AuPt NPs, but also acted as signal probes. The CTAB-Co-MOFs were investigated including SEM, TEM, XPS, and XRD. The application ability of the immunosensor was evaluated using serum sample, demonstrating the immunosensor can be applied to clinic serum analysis.

MOF-based nanocomposites as transduction matrices for optical and electrochemical sensing

Metal Organic Frameworks (MOFs), a class of crystalline microporous materials have been into research limelight lately due to their commendable physio-chemical properties and easy fabrication methods. They have enormous surface area which can be a working ground for innumerable molecule adhesions and site for potential sensor matrices. Their biocompatibility makes them valuable for in vitro detection systems but a compromised conductivity requires a lot of surface engineering of these molecules for their usage in electrochemical biosensors. However, they are not just restricted to a single type of transduction system rather can also be modified to achieve feat as optical (colorimetry, luminescence) and electro-luminescent biosensors. This review emphasizes on recent advancements in the area of MOF-based biosensors with focus on various MOF synthesis methods and their general properties along with selective attention to electrochemical, optical and opto-electrochemical hybrid biosensors. It also summarizes MOF-based biosensors for monitoring free radicals, metal ions, small molecules, macromolecules and cells in a wide range of real matrices. Extensive tables have been included for understanding recent trends in the field of MOF-composite probe fabrication. The article sums up the future scope of these materials in the field of biosensors and enlightens the reader with recent trends for future research scope.

A ZnIn2S4@ReS2/AgInS2-based photoelectrochemical aptasensor for the ultrasensitive detection of kanamycin

An ultrasensitive photoelectrochemical (PEC) aptasensor was originally designed by using ZnIn2S4/ReS2 as a photoactive material and AgInS2 as a signal amplifier. The signal amplifier AgInS2 was incubated on the terminal of H-DNA (immobilized on the ZnIn2S4/ReS2/FTO surface), leading to an enhanced photocurrent response. Then, due to the introduction of DNA2, the formation of a double-stranded structure caused AgInS2 to keep away from the electrode surface, and the photocurrent was reduced. In the presence of kanamycin, DNA2 was released from the system due to the competition relationship, and a restored photocurrent response was obtained. The combination of ZnIn2S4/ReS2 and AgInS2 accelerated the electron transfer and enhanced the separation efficiency of photogenerated electron-hole pairs, resulting in an improved performance of the PEC aptasensor, which was capable of accurate and sensitive detection of kanamycin in actual samples.

Synergetic-effect-enhanced electrochemiluminescence of zein-protected Au-Ag bimetallic nanoclusters for CA15-3 detection

In this work, a sandwich-type electrochemiluminescence (ECL) system was constructed for the detection of CA15-3. Gold-silver bimetallic nanoclusters (Au-Ag BNCs) with zein as a protective ligand were synthesized, and the excellent ECL performance of this material was demonstrated for the first time. Zein carrying a variety of groups that ligated with Au-Ag BNCs, forming a protective shell of zein, effectively prevented clusters from aggregating or growing into larger nanoparticles. The synergistic effect of the bimetal promotes the ECL emission, making this nanoscale material an ideal ECL probe. GO-PANI, which effectively promoting the production of sulfate radicals of the co-reactant and significantly increasing the ECL strength, was a good sensing platform for antibody immobilization. Consequently, we constructed an ECL sensor with GO-PANI as the sensing platform and Au-Ag BNCs@zein as the ECL probe, with a detection range of 0.001-100 U mL-1 and a detection limit of 0.0003 U mL-1, provided a strong support for the sensor for future CA15-3 detection applications.

Fabrication and nano-engineering of non-/noble metal-coupled plasmonic heterostructures for ultrasensitive photoelectrochemical immunoassays

To achieve reliable and ultrasensitive detection for disease markers in PEC bioanalysis, constructing and nano-engineering of ideal photoelectrodes and signal transduction strategies are of vital importance. Herein, a non-/noble metal coupled plasmonic nanostructure (TiO₂/r-STO/Au) was tactically designed with high-efficient PEC performance. Evidenced by the DFT and FDTD calculations, the reduced SrTiO₃ (r-STO) was found to support the localized surface plasmon resonance due to the sufficiently increased and delocalized local charge in r-STO. Under the synergistic coupling of plasmonic r-STO and AuNPs, the PEC performance of TiO₂/r-STO/Au was found remarkably promoted with reduced onset potential. This merit supported TiO₂/r-STO/Au as a self-powered immunoassay via a proposed oxygen-evolution-reaction mediated signal transduction strategy. With the increase of the target biomolecules (PSA), the catalytic active sites of TiO₂/r-STO/Au would be blocked and result in the decrease of the oxygen evaluation reaction. Under optimal conditions, the immunoassays exhibited an excellent detection performance with a LOD as low as 1.1 fg/mL. This work proposed a new type of plasmonic nanomaterial for ultrasensitive PEC bioanalysis.

Dual cascade DNA walking-induced "super on" photocurrent response for constructing a novel antibiotic biosensing method

The construction of effective methods for the convenient testing of antibiotic residues in real samples has attracted considerable interest. Herein, we designed a dual cascade DNA walking amplification strategy and combined it with the controllable photocurrent regulation of a photoelectrode to develop a novel photoelectrochemical (PEC) biosensing method for antibiotic detection. The photoelectrode was prepared through the surface modification of a glassy carbon electrode with the TiO₂/CdS QDs nanocomposite synthesized by an in situ hydrothermal deposition method. The strong anodic PEC response of the nanocomposite could be well inhibited by the introduction of a silver nanoclusters (Ag NCs)-labeled DNA hairpin onto its surface. Upon the target biorecognition reaction, an Mg²⁺-dependent DNAzyme (MNAzyme)-driven DNA walking was triggered to release another MNAzyme strand-linked streptavidin (SA) complex. As this SA complex could serve as a four-legged DNA walker, its cascade walking on the electrode surface not only released Ag NCs but also caused the linking of Rhodamine 123 with the electrode to realize the "super on" photocurrent output. By using kanamycin as the model analyte, this method showed a very wide linear range from 10 fg mL^-1 to 1 ng mL^-1 and a very low detection limit of 0.53 fg mL^-1. Meanwhile, the simple photoelectrode preparation and the aptamer recognition-based autonomous DNA walking resulted in the convenient manipulation and excellent repeatability. These unique performances determine the great potential of the proposed method for practical applications.

An electrochemiluminescence immunosensor based on signal magnification of luminol using OER-activated NiFe2O4@C@CeO2/Au as effective co-reaction accelerator

In this work, a novel, label-free electrochemiluminescence (ECL) immunosensor was constructed for the ultrasensitive detection of carbohydrate antigen 15-3 (CA15-3) by the combined use of NiFe₂O₄@C@CeO₂/Au hexahedral microbox and luminol luminophore. The synthesis of the co-reaction accelerator (NiFe₂O₄@C@CeO₂/Au) was related to the calcination of FeNi-based metal-organic framework (MOF), as well as the ingrowth of CeO₂ nanoparticles and modification of Au nanoparticles. To be specific, the electrical conductivity will be boosted due to the Au nanoparticles, the synergetic effect generated between CeO₂ and calcination FeNi-MOF could offer better activity of oxygen evolution reaction (OER). Herein, the NiFe₂O₄@C@CeO₂/Au hexahedral microbox as a co-reaction accelerator has excellent OER activity and production of reactive oxygen species (ROS), thus increasing the ECL intensity of luminol in a neutral medium without other co-reactants such as H₂O₂. Because of these benefits, the constructed ECL immunosensor was applied to detect CA15-3 as an example under optimum conditions, the designed ECL immunosensor exhibited high-level selectivity and sensitivity for CA15-3 biomarker within a linear response range of 0.01-100 U mL^-1 and an ultralow detection limit of 0.545 mU mL^-1 (S/N = 3), demonstrating its potentially valuable application in the area of clinical analysis.

Novel design constructed In2S3@SnO2 hollow heterojunctions by insufficiently etched MOFs as framework for photoelectrochemical bioanalysis

Compared to sufficiently etched MOFs materials, insufficiently etched MOFs materials tend to display unsatisfactory performance due to their immature structure and have been eliminated from scientific research. Herein, this work reported a novel In₂S₃@SnO₂ heterojunction (In₂S₃@SnO₂-HSHT) materials, which were stably synthesized in high temperature aqueous environment and equipped extraordinary photoelectrochemical (PEC) properties, fabricated by a succinct hydrothermal synthesis method using insufficiently etched MIL-68 as a self-sacrificing template. Compared with the control groups and In₂S₃@SnO₂ heterojunctions with collapse morphology synthesized by sufficiently etched MIL-68 in high temperature aqueous environment, In₂S₃@SnO₂-HSHT synthesized from insufficiently etched MIL-68 as a template had a massively enhanced light-harvesting capability and generated more photoinduced charge carriers due to its well-preserved hollow structure. Therefore, based on outstanding PEC performance of In₂S₃@SnO₂-HSHT, the established PEC label-free signal-off immunosensor to detect CYFRA 21-1, revealing vivid selectivity, stability, and reproducibility. This novel strategy adopted the insufficient chemical etching method neglected by the mainstream chemical etching approaches, which solved the challenge that the stability of the sufficient etched MOFs with hollow structure cannot be maintained under the subsequent high temperature aqueous reaction conditions, and was further applied to the design of hollow heterojunction materials for photoelectrochemical fields.

Novel Ultrasensitive Photoelectrochemical Cytosensor Based on Hollow CdIn2S4/In2S3 Heterostructured Microspheres for HepG2 Cells Detection and Inhibitor Screening

Hepatocellular carcinoma is a life-threatening malignant tumor found around the world for its high morbidity and mortality. Therefore, it is of great importance for sensitive analysis of liver cancer cells (HepG2 cells) in clinical diagnosis and biomedical research. To fulfill this demand, hollow CdIn₂S₄/In₂S₃ heterostructured microspheres (termed CdIn₂S₄/In₂S₃ for clarity) were prepared by a two-step hydrothermal strategy and applied for building a novel photoelectrochemical (PEC) cytosensor for ultrasensitive and accurate detection of HepG2 cells through specific recognition of CD133 protein on the cell surface with the respective aptamer. The optical properties of CdIn₂S₄/In₂S₃ were investigated by UV-vis diffuse reflectance spectroscopy (DRS) and PEC technology. By virtue of their appealing PEC characteristics, the resultant PEC sensor exhibited a wider dynamic linear range from 1 × 10² to 2 × 10⁵ cells mL^-1 with a lower limit of detection (LOD, 23 cells mL^-1), combined by evaluating the expression level of CD133 protein stimulated by metformin as a benchmarked inhibitor. This work opens a valuable and feasible avenue for sensitive detection of diverse tumor cells, holding great potential in early clinical diagnosis and treatment coupled by screening inhibitors.