Facile Synthesis of a MoS2-Prussian Blue Nanocube Nanohybrid-Based Electrochemical Sensing Platform for Hydrogen Peroxide and Carcinoembryonic Antigen Detection

Recent advances in synthesis and biosensors of two-dimensional MoS2

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted tremendous research interests due to their exciting optical properties, large surface area, intercalatable morphologies and excellent electrochemically catalytic activity. Acting as the most typical member in TMDCs family, layer-dependent molybdenum disulfide (MoS₂) with particular direct bandgap of 1.8 eV in monolayer has been widely applied in various biosensors with high sensitivity and selectivity. In this review, the preparation methods of MoS₂, together with MoS₂-based biosensors for detecting cells and biomolecules (such as glucose, DNA and antigens) would be summarized. In addition, the current challenges and future perspectives are outlined for the applications of biosensors based on 2D MoS₂.

Sandwich-type electrochemical immunosensor for carcinoembryonic antigen detection based on the cooperation of a gold-vertical graphene electrode and gold@silica-methylene blue

In this study, a sandwich-type electrochemical (EC) immunosensor was proposed to detect a carcinoembryonic antigen (CEA) based on Au-graphene and Au@SiO₂-methylene blue (MB). The Au nanoparticles (NPs)-vertical graphene (VG) electrode efficiently amplifies the response signal by immobilizing a large amount of the coating antibody (Ab) and is characterized by excellent electrocatalytic activity. The MB nanodot-loaded Au@SiO₂ carriers with core-shell nanostructure and detection Ab were used to construct the Ab-Au@SiO₂-MB label, which improved the sensitivity due to the high EC signal of MB nanodots and the high labeling effect between the detection Ab and MB probe. A novel double-Ab sandwich strategy was developed to further improve the sensitivity and stability based on the same specificity of the coating and detection Abs for the recognition of CEA. Under optimal conditions, the developed EC sensor exhibited a wide linear range from 1 fg mL^-1 to 100 ng mL^-1, with an ultralow detection limit of 0.8 fg mL^-1 (S/N = 3). The feasibility in the clinical application of the EC sensor was verified by the in vitro detection of CEA in human serum.

Folic acid conjugated Prussian blue nanoparticles: Synthesis, physicochemical characterization and targeted cancer cell sensing

In the study, folic acid doped Prussian blue nanoparticles (FA-PB NPs) for theranostic applications were synthesized for the first time. Folic acid was chosen for maintaining nanoparticle stability and also to increase its binding affinity especially for cancer cells. Multifunctional PB NPs were fabricated by one route co-precipitation method to synthesize biocompatible NPs without any further process. Then, FA was doped on the surface of PB NPs. The characterization studies demonstrated that the FA-PB NPs modified sensor surface had large surface area with biocompatible and hydrophilic properties where cancer cells can easily bind. The FA-PB NPs were used for the modification of pencil graphite electrode (PGE) for electrochemical detection of colon cancer cells (DLD-1). Electrochemical impedimetric diagnosis was based on the specific interaction between FA groups on the nanoparticles and FA receptors overexpressed on cancer cells. The voltammetric and impedimetric results showed that the FA-PB NPs based electrode had good sensing performance for the immobilized DLD-1 cells.

2D Materials in Development of Electrochemical Point-of-Care Cancer Screening Devices

Effective cancer treatment requires early detection and monitoring the development progress in a simple and affordable manner. Point-of care (POC) screening can provide a portable and inexpensive tool for the end-users to conveniently operate test and screen their health conditions without the necessity of special skills. Electrochemical methods hold great potential for clinical analysis of variety of chemicals and substances as well as cancer biomarkers due to their low cost, high sensitivity, multiplex detection ability, and miniaturization aptitude. Advances in two-dimensional (2D) material-based electrochemical biosensors/sensors are accelerating the performance of conventional devices toward more practical approaches. Here, recent trends in the development of 2D material-based electrochemical biosensors/sensors, as the next generation of POC cancer screening tools, are summarized. Three cancer biomarker categories, including proteins, nucleic acids, and some small molecules, will be considered. Various 2D materials will be introduced and their biomedical applications and electrochemical properties will be given. The role of 2D materials in improving the performance of electrochemical sensing mechanisms as well as the pros and cons of current sensors as the prospective devices for POC screening will be emphasized. Finally, the future scopes of implementing 2D materials in electrochemical POC cancer diagnostics for the clinical translation will be discussed.

Graphene Quantum Dots Decorated Gold-Polyaniline Nanowire for Impedimetric Detection of Carcinoembryonic Antigen

A label-free impedimetric immunosensor based on N, S-graphene quantum dots@Au-polyaniline (N, S-GQDs@Au-PANI) nanowires was fabricated for the quantitative detection of carcinoembryonic antigen (CEA). The N, S-GQDs and Au-PANI were synthesized by a simple hydrothermal pyrolysis and interfacial polymerization, respectively. Subsequently, 2-9 nm N, S-GQDs are successfully decorated onto 30-50 nm Au-PANI nanowires by Au-thiol linkage to serve as the bifunctional probe for amplifying the electrochemical activity as well as anchoring anti-CEA. The N, S-GQDs@Au-PANI nanowires are excellent conducting materials to accelerate the electron transfer, while the formation of CEA antibody-antigen bioconjugates after the addition of CEA significantly increase the charge transfer resistance, and subsequently provides a highly stable and label-free immunoassay platform for the impedimetric detection of CEA. The label-free immunosensor exhibits a wide linear range from 0.5 to 1000 ng mL-1 with a low detection limit of 0.01 ng mL-1. The N, S-GQDs@Au-PANI based immunosensor also shows high selectivity and stability over other cancer makers and amino acids. Moreover, this promising platform is successfully applied to the detection of CEA in human serum samples with excellent recovery of (96.0 ± 2.6)-(103 ± 3.8)%. These results clearly demonstrate a newly developed highly efficient and label-free impedimetric immunosensor for the detection of CEA using N, S-GQDs@Au-PANI nanowires as the biosensing probe, which can pave the gateway for the fabrication of high performance and robust impedimetric immunosensor to detect cancer makers in early stage of cancer diagnosis and therapy.

A MXene-Based Wearable Biosensor System for High-Performance In Vitro Perspiration Analysis

Wearable electrochemical biosensors for sweat analysis present a promising means for noninvasive biomarker monitoring. However, sweat-based sensing still poses several challenges, including easy degradation of enzymes and biomaterials with repeated testing, limited detection range and sensitivity of enzyme-based biosensors caused by oxygen deficiency in sweat, and poor shelf life of sensors using all-in-one working electrodes patterned by traditional techniques (e.g., electrodeposition and screen printing). Herein, a stretchable, wearable, and modular multifunctional biosensor is developed, incorporating a novel MXene/Prussian blue (Ti₃ C₂ T_x /PB) composite designed for durable and sensitive detection of biomarkers (e.g., glucose and lactate) in sweat. A unique modular design enables a simple exchange of the specific sensing electrode to target the desired analytes. Furthermore, an implemented solid-liquid-air three-phase interface design leads to superior sensor performance and stability. Typical electrochemical sensitivities of 35.3 µA mm^-1 cm^-2 for glucose and 11.4 µA mm^-1 cm^-2 for lactate are achieved using artificial sweat. During in vitro perspiration monitoring of human subjects, the physiochemistry signals (glucose and lactate level) can be measured simultaneously with high sensitivity and good repeatability. This approach represents an important step toward the realization of ultrasensitive enzymatic wearable biosensors for personalized health monitoring.

Recent advances in molybdenum disulfide-based electrode materials for electroanalytical applications

The primary objective of this review article is to summarize the development and structural diversity of 2D/3D molybdenum disulfide (MoS₂) based modified electrodes for electrochemical sensors and biosensor applications. Hydrothermal, mechanical, and ultrasonic techniques and solution-based exfoliation have been used to synthesize graphene-like 2D MoS₂ layers. The unique physicochemical properties of MoS₂ and its nanocomposites, including high mechanical strength, high carrier transport, large surface area, excellent electrical conductivity, and rapid electron transport rate, render them useful as efficient transducers in various electrochemical applications. The present review summarizes 2D/3D MoS₂-based nanomaterials as an electrochemical platform for the detection and analysis of various biomolecules (e.g., neurotransmitters, NADH, glucose, antibiotics, DNA, proteins, and bacteria) and hazardous chemicals (e.g., heavy metal ions, organic compounds, and pesticides). The substantial improvements that have been achieved in the performance of enzyme-based amperometry, chemiluminescence, and nucleic acid sensors incorporating MoS₂-based chemically modified electrodes are also addressed. We also summarize key sensor parameters such as limits of detection (LODs), sensitivity, selectivity, response time, and durability, as well as real applications of the sensing systems in the environmental, pharmaceutical, chemical, industrial, and food analysis fields. Finally, the remaining challenges in designing MoS₂ nanostructures suitable for electroanalytical applications are outlined. Graphical abstract • MoS₂ based materials exhibit high conductivity and improved electrochemical performance with great potential as a sensing electrode. • The role of MoS₂ nanocomposite films and their detection strategies were reviewed. • Biomarkers detection for disease identification and respective clinical treatments were discussed. • Future Challenges, as well as possible research development for "MoS₂ nanocomposites", are suggested.

Electrically-Transduced Chemical Sensors Based on Two-Dimensional Nanomaterials

Electrically-transduced sensors, with their simplicity and compatibility with standard electronic technologies, produce signals that can be efficiently acquired, processed, stored, and analyzed. Two dimensional (2D) nanomaterials, including graphene, phosphorene (BP), transition metal dichalcogenides (TMDCs), and others, have proven to be attractive for the fabrication of high-performance electrically-transduced chemical sensors due to their remarkable electronic and physical properties originating from their 2D structure. This review highlights the advances in electrically-transduced chemical sensing that rely on 2D materials. The structural components of such sensors are described, and the underlying operating principles for different types of architectures are discussed. The structural features, electronic properties, and surface chemistry of 2D nanostructures that dictate their sensing performance are reviewed. Key advances in the application of 2D materials, from both a historical and analytical perspective, are summarized for four different groups of analytes: gases, volatile compounds, ions, and biomolecules. The sensing performance is discussed in the context of the molecular design, structure-property relationships, and device fabrication technology. The outlook of challenges and opportunities for 2D nanomaterials for the future development of electrically-transduced sensors is also presented.

Proximity hybridization triggered rolling-circle amplification for sensitive electrochemical homogeneous immunoassay

A new homogeneous electrochemical immunoassay strategy was developed for ultrasensitive detection of carcinoembryonic antigen (CEA) based on target-induced proximity hybridization coupled with rolling circle amplification (RCA). The immobilization-free detection of CEA was realized by the use of an uncharged peptide nucleic acid (PNA) probe labeled with ferrocene (Fc) as the electroactive indicator on a negatively charged indium tin oxide (ITO) electrode. In the presence of a target protein and two DNA-labeled antibodies, the proximate complex formed in homogeneous solution could unfold the molecular beacon, and a part of the unfolded molecular beacon as a primer hybridized with the RCA template to initiate the RCA process. Subsequently, the detection probe modified Fc (Fc-PNAs) hybridized with the long amplified DNA products. The consumption of freely diffusible Fc-PNAs (neutrally charged) resulted in a significant reduction of the Fc signal due to the fact that long amplified DNA/Fc-PNA products were electrostatically repelled from the ITO electrode surface. The reduction of the electrochemical signal (signal-off) could indirectly provide the CEA concentration. Under the optimal conditions, CEA detection was implemented in a wide range from 1 pg mL^-1 to 10 ng mL^-1, with a low detection limit of 0.49 pg mL^-1. The proposed strategy exhibited advantages of good selectivity, high sensitivity, acceptable accuracy, and favorable versatility of analytes. Moreover, the practical application value of the system was confirmed by the assay of CEA in human serums with satisfactory results.

Graphitic-phase carbon nitride-based electrochemiluminescence sensing analyses: recent advances and perspectives

This review describes the current trends in synthesis methods, signaling strategies, and sensing applications of g-C₃N₄-based ECL emitters.

Electrochemical sensor based on novel two-dimensional nanohybrids: MoS2nanosheets conjugated with organic copper nanowires for simultaneous detection of hydrogen peroxide and ascorbic acid

MoS₂nanosheets were conjugated with organic copper nanowires for fabrication of electrochemical hydrogen peroxide and ascorbic acid sensors.

Turn-on Luminescent Probe for Hydrogen Peroxide Sensing and Imaging in Living Cells based on an Iridium(III) Complex-Silver Nanoparticle Platform

A sensitive turn-on luminescent sensor for H2O2 based on the silver nanoparticle (AgNP)-mediated quenching of an luminescent Ir(III) complex (Ir-1) has been designed. In the absence of H2O2, the luminescence intensity of Ir-1 can be quenched by AgNPs via non-radiative energy transfer. However, H2O2 can oxidize AgNPs to soluble Ag+ cations, which restores the luminescence of Ir-1. The sensing platform displayed a sensitive response to H2O2 in the range of 0-17 μM, with a detection limit of 0.3 μM. Importantly, the probe was successfully applied to monitor intracellular H2O2 in living cells, and it also showed high selectivity for H2O2 over other interfering substances.

Ferrocene covalently confined in porous MOF as signal tag for highly sensitive electrochemical immunoassay of amyloid-β

This work provides a universal strategy to attain precise modification of MOF signal tags for sensitive electrochemical detection of proteins.