Flake-like structure of SrTiO3 nanoparticles dispersed on graphene oxide: A selective and sensitive electrochemical sensor for determination of chloramphenicol in milk and honey samples

The use of Chloramphenicol (CAP), a potent antibiotic with broad-spectrum capabilities in food-producing animals has been restricted by the European Union and several other countries due to its severe side effects. Thus, CAP must be detected quickly and sensitively. In this investigation, the preparation of SrTiO3 nanoparticles was carried out utilizing a hydrothermal technique. The as-synthesized strontium titanate was decorated on the graphene oxide (SrTiO3/GO) using an ultrasonication method. An electrochemical sensor was developed by employing a modified electrode consisting of SrTiO3/GO, which can accurately detect CAP in food samples. The synergistic effect of SrTiO3 and GO could improve the peak current response. Remarkably, the SrTiO3/GO-modified glassy carbon electrode has a LOD and sensitivity of 6.08 µM nM and 2.771 µA·μM-1·cm-2, respectively. This modified electrode was evaluated in food samples and had an outstanding reaction with a high percentage of recovery, which makes it a potential electrocatalyst for CAP detection.

Hydrangea-like TiO2/Bi2MoO6 porous nanoflowers triggering highly sensitive electrochemical immunosensing to tumor marker

Rapid and sensitive detection of carcinoembryonic antigen (CEA) is of great significance for cancer patients. Here, molybdenum (Mo) was doped into bismuth oxide (Bi2O3) by one-pot hydrothermal method forming porous tremella Bi2MoO6 nanocomposites with a larger specific surface area than the spherical structure. Then, a new kind of hydrangea-like TiO2/Bi2MoO6 porous nanoflowers (NFs) was prepared by doping titanium into Bi2MoO6, where titanium dioxide (TiO2) grew in situ on the surface of Bi2MoO6 nanoparticles (NPs). The hydrangea-like structure provides larger specific surface area, higher electron transfer ability and biocompatibility as well as more active sites conducive to the attachment of anti-carcinoembryonic antigen (anti-CEA) to TiO2/Bi2MoO6 NFs. A novel label-free electrochemical immunosensor was then constructed for the quantitative detection of CEA using TiO2/Bi2MoO6 NFs as sensing platform, showing a good linear relationship with CEA in the concentration range 1.0 pg/mL ~ 1.0 mg/mL and a detection limit of 0.125 pg/mL (S/N = 3). The results achieved with the designed immunosensor are comparable with many existing immunosensors used for the detection of CEA in real samples.

A MOF nanozyme-mediated acetylcholinesterase-free colorimetric strategy for direct detection of organophosphorus pesticides

Although some simple and rapid colorimetric methods have been developed to detect organophosphorus pesticides (OPs), the difficult extraction and easy denaturation of acetylcholinesterase (AChE) are still drawbacks needing to be overcome. Here, we propose a MOF nanozyme-mediated AChE-free colorimetric strategy for the direct detection of OPs. In the presence of OPs (pirimiphos-methyl as a model), the intense blue of oxidized 3,3',5,5'-tetramethylbenzidine (TMB) becomes light due to the quenching effect of OPs towards hydroxyl radicals (˙OH) that are generated by the decomposition of H2O2 catalyzed by the Cu4Co6 ZIF nanozyme with excellent peroxidase (POD)-like activity. The developed colorimetric sensor exhibits assay performance and offers a universal and promising analysis strategy for detecting OPs in practical samples.

Advances of Electrochemical and Electrochemiluminescent Sensors Based on Covalent Organic Frameworks

Covalent organic frameworks (COFs), a rapidly developing category of crystalline conjugated organic polymers, possess highly ordered structures, large specific surface areas, stable chemical properties, and tunable pore microenvironments. Since the first report of boroxine/boronate ester-linked COFs in 2005, COFs have rapidly gained popularity, showing important application prospects in various fields, such as sensing, catalysis, separation, and energy storage. Among them, COFs-based electrochemical (EC) sensors with upgraded analytical performance are arousing extensive interest. In this review, therefore, we summarize the basic properties and the general synthesis methods of COFs used in the field of electroanalytical chemistry, with special emphasis on their usages in the fabrication of chemical sensors, ions sensors, immunosensors, and aptasensors. Notably, the emerged COFs in the electrochemiluminescence (ECL) realm are thoroughly covered along with their preliminary applications. Additionally, final conclusions on state-of-the-art COFs are provided in terms of EC and ECL sensors, as well as challenges and prospects for extending and improving the research and applications of COFs in electroanalytical chemistry.

One-pot wet-chemical fabrication of 3D urchin-like core-shell Au@PdCu nanocrystals for electrochemical breast cancer immunoassay

Carbohydrate antigen 15-3 (CA15-3) is an important biomarker for early diagnosis of breast cancer. Herein, a label-free electrochemical immunosensor was built based on three-dimensional (3D) urchin-like core-shell Au@PdCu nanocrystals (labeled Au@PdCu NCs) for highly sensitive detection of CA15-3, where K3[Fe(CN)6] behaved as an electroactive probe. The Au@PdCu NCs were synthesized by a simple one-pot wet-chemical approach and the morphology, structures, and electrocatalytic property were investigated by several techniques. The Au@PdCu NCs prepared worked as electrode material to anchor more antibodies and as signal magnification material by virtue of its exceptional catalytic property. The developed biosensor exhibited a wide linear detection range from 0.1 to 300 U mL-1 and a low limit of detection (0.011 U mL-1, S/N = 3) for determination of CA15-3 under the optimal conditions. The established biosensing platform exhibits some insights for detecting other tumor biomarkers in clinical assays and early diagnosis.

Pt Nanodot Inlaid Mesoporous NaBiOF Nanoblackberry for Remarkable Signal Amplification Toward Biomarker Detection

A new ultrasensitive sandwich-type electrochemical immunosensor has been successfully constructed to quantitatively detect carcinoembryonic antigen (CEA) using blackberry-like mesoporous bismuth-based nanospheres NaBiOF (NBOF NSs) inlaid with Pt nanodots (NDs) (BiPt NSs) as the antibody capture and signal-amplifying probe. The growth of Pt NDs inside the holes of NBOF NSs formed the nanozyme inlay outside NBOF NSs, greatly increasing the specific surface area and exposure of the catalytic active sites by minimizing the particle size of the Pt to nanodot scale. Such a blackberry-shaped heterojunction structure of BiPt NSs was well-suited to antibody capture and improved the catalytic performance of BiPt NSs in reducing H₂O₂, amplifying the signal, and yielding highly sensitive detection of CEA. The use of Au nanoparticle-modified multi-walled carbon nanotubes (Au@MWCNTs) as the electrode substrates significantly enhanced the electron transfer behavior over the electrode surface, further increasing the conductivity and sensitivity of the immunosensor. Remarkably, good compatibility with human body fluid was achieved using the newly developed BiPt-based immunosensor resulting from the favorable biocompatibility and stability of both BiPt NSs and Au@MWCNTs. Benefiting from the double signal amplification strategy and the high biocompatibility, the immunosensor responded linearly to CEA in a wide range from 50 fg/mL to 100 ng/ml with an extremely low detection limit of 3.52 fg/mL (S/N = 3). The excellent detection properties of this new immunosensor were evidenced by the satisfactory selectivity, reproducibility, and stability obtained, as well as the reliable and precise determination  of CEA in actual human blood samples. This work provides a new strategy for the early clinical diagnosis of cancer. Novel blackberry-like mesoporous NaBiOF nanospheres with Pt nanodot inlay were successfully usedto construct a sandwich-type electrochemical immunosensor for the ultra-sensitive detection ofcarcinoembryonic antigen in human blood plasma based on a remarkable signal amplification strategy.

A label-free electrochemical immunosensor based on 11-mercaptoundecanoic acid grafted chitosan and poly(N-methylaniline) for the detection of carcinoembryonic antigen

Carcinoembryonic antigen (CEA) is a cancer marker used for monitoring cancer treatment. Herein, a label-free electrochemical immunosensor for determining CEA concentration composed of the thiolated chitosan (tCHI) and the doped poly(N-methylaniline) (dPNMA) is proposed. The tCHI served as a support matrix for the immobilization of CEA antibodies (anti-CEA) and was prepared by using 11-mercaptoundecanoic acid (MUA) as a grafting agent on chitosan (CHI). The excellent electrical conductivity of the dPNMA was utilized as an electron transfer layer for the proposed immunosensor. The successful preparation of the tCHI was confirmed by the attenuated-total reflection Fourier transform spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Cyclic voltammetry (CV), differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS) were used to illustrate the performance of the proposed immunosensor. The determination of CEA concentration was relied on the decrease in the DPV current response with increasing CEA concentration from the creation of the antigen-antibody immunocomplex. The proposed immunosensor demonstrated a broad concentration range of 0.01 to 30 ng mL^-1 with a low limit of detection (LOD) of 0.01 ng mL^-1. In addition, the present sensor exhibited excellent selectivity, reproducibility, and long-term stability, suggesting its potential use to determine CEA in clinical immunoassay.

Sensitive Detection of the Human Epididymis Protein-4 (HE4) Ovarian Cancer Biomarker through a Sandwich-Type Immunoassay Method with Laser-Induced Breakdown Spectroscopy

Detection of cancer before the appearance of any symptoms is crucial for successful treatment. Early detection is, however, very challenging, particularly for the types of cancer with few or no symptoms at early stages, such as epithelial ovarian cancer (EOC). Developing a user-friendly method that can detect biomarkers with sufficient selectivity, sensitivity, and reproducibility is a promising approach for overcoming the challenges of early detection of EOC. In this study, we report a sandwich-type microparticle immunoassay for sensitive detection of the HE4 biomarker with laser-induced breakdown spectroscopy. Here, we cross-linked elemental particles to a specific functional group of the targeted biomolecules based on a covalent and non-covalent linking chemistry to improve the sensitivity and selectivity of biomarker detection, in which Fe₃O₄ and SiO₂ microparticles were used to conjugate and purify the antibody-antigen in complex media. Simultaneous detection of Fe and Si from a magnetically purified assay significantly improves the HE4 biomarker's detectability, in which HE4 was detected with a limit of detection of 0.0022 pM. We also determined the coupling ratio between HE4 and silica particles using a silicon calibration curve.

Strategies to simplify operation procedures for applying labeled antibody-based immunosensors to point-of-care testing

Point-of-care testing (POCT) is an ideal testing format for the rapid and on-site detection of analytes in patients, and facilitates disease diagnosis and monitoring. Molecular recognition elements are required for the specific detection of analytes, and biosensors that use antibodies as the molecular recognition elements are called immunosensors. Traditional immunosensors such as sandwich enzyme-linked immunosorbent assay (ELISA) require complicated procedures to form immunocomplexes consisting of detection antibodies, analytes, and capture antibodies. They also require long incubation times, washing procedures, and large and expensive specialized equipment that must be operated by laboratory technicians. Immunosensors for POCT should be systems that use relatively small pieces of equipment and do not require special training. In this review, to help in the construction of immunosensors for POCT, we have summarized the recently reported strategies for simplifying the operation, incubation, and washing procedures. We focused on the optical and electrochemical detection principles of immunosensors, compared the strategies for operation, sensitivity, and detection devices and discussed the ideal system. Combining detection devices that can be fabricated inexpensively and strategies that enable simplification of operation procedures and enhance sensitivities will contribute to the development of immunosensors for POCT.

Technological advances in electrochemical biosensors for the detection of disease biomarkers

With an increasing focus on health in contemporary society, interest in the diagnosis, treatment, and prevention of diseases has grown rapidly. Accordingly, the demand for biosensors for the early diagnosis of disease is increasing. However, the measurement range of existing electrochemical sensors is relatively high, which is not suitable for early disease diagnosis, requiring the detection of small amounts of biocomponents. Various attempts have been made to overcome this and amplify the signal, including binding with various labeling molecules, such as DNA, enzymes, nanoparticles, and carbon materials. Efforts are also being made to increase the sensitivity of electrochemical sensors, and the combination of nanomaterials, materials, and biotechnology offers the potential to increase sensitivity in a variety of ways. Recent studies suggest that electrochemical sensors can be a powerful tool in providing comprehensive insights into the targeting and detection of disease-associated biomarkers. Significant advances in nanomaterial and biomolecule approaches for improved sensitivity have resulted in the development of electrochemical biosensors capable of detecting multiple biomarkers in real time in clinically relevant samples. In this review, we have discussed the recent studies on electrochemical sensors for detection of diseases such as diabetes, degenerative diseases, and cancer. Further, we have highlighted new technologies to improve sensitivity using various materials, including DNA, enzymes, nanoparticles, and carbon materials.

A label-free electrochemical immunosensing platform based on PEI-rGO/Pt@Au NRs for rapid and sensitive detection of zearalenone

In this work, we design an immunosensor for zearalenone (ZEN) detection with PEI-rGO/Pt@Au NRs nanocomposite as the modification material. PEI-rGO/Pt@Au NRs nanocomposite have good stability, conductivity and a large specific surface area, so they are chosen as the substrate material for the modified electrode, which is beneficial in improving the detection performance of the sensor. When antibody binds to ZEN, the current signal decreases, and the response signal changes after ZEN incubation, recorded by differential pulse voltammetry (DPV) methods. Under the optimised conditions, the electrochemical response of the constructed immunosensor shows a linear relation to a wide concentration range from 1 pg/mL to 1 × 10⁶ pg/mL with a detection limit of 0.02 pg/mL. Additionally, the proposed electrochemical immunosensor has high selectivity, good stability and great potential for the trace detection of ZEN in real samples.

Sensitive electrochemical immunosensor using a bienzymatic system consisting of β-galactosidase and glucose dehydrogenase

Bienzymatic systems are often used with electrochemical affinity biosensors to achieve high signal levels and/or low background levels. It is important to select two enzymes whose reactions do not exhibit mutual interference but have similar optimal conditions. Here, we report a sensitive electrochemical immunosensor based on a bienzymatic system consisting of β-galactosidase (Gal, a hydrolase enzyme) and flavin adenine dinucleotide-dependent glucose dehydrogenase (FAD-GDH, a redox enzyme). Both enzymes showed high activities at neutral pH, the reactions catalyzed by them did not exhibit mutual interference, and the electrochemical-enzymatic redox cycling based on FAD-GDH coupled with enzymatic amplification by Gal enabled high signal amplification. Among the three amino-hydroxy-naphthalenes and 4-aminophenol (potential Gal products), 4-amino-1-naphthol showed the highest signal amplification. Glucose, as an electro-inactive, stable reducing agent for redox cycling, helped in achieving low background levels. Our bienzymatic system could detect parathyroid hormone at a detection limit of ∼0.2 pg mL-1, implying that it can be used for highly sensitive electrochemical detection of parathyroid hormone and other biomarkers in human serum.

A label-free electrochemical immnunosensor based on signal magnification of oxygen reduction reaction catalyzed by uniform PtCo nanodendrites for highly sensitive detection of carbohydrate antigen 15-3

Developing a highly sensitive immunoassay for tumor biomarkers is particularly important in bioanalysis and early disease diagnosis. In this work, a simple one-pot solvothermal method was developed for controllable synthesis of well-dispersed PtCo alloyed nanodendrites (PtCo NDs) by using l-carnosine as the co-structure-directing agent. The PtCo NDs had a large specific surface area and provided abundant active sites available for electrocatalytic oxygen reduction reaction (ORR). Based on the highly enhanced currents of the ORR, a novel label-free electrochemical immunosensor was fabricated for highly sensitive assay of carbohydrate antigen 15-3 (CA15-3). The sensor showed a wide linear range of 0.1-200 U mL^-1 and a low limit of detection (LOD) down to 0.0114 U mL^-1 (S/N = 3), in turn exploring its application to diluted human serum samples with satisfactory results. This study provides a feasible platform for monitoring other tumor markers in clinical diagnosis.