Silver-functionalized g-C3N4 nanohybrids as signal-transduction tags for electrochemical immunoassay of human carbohydrate antigen 19-9

Silver nanoparticles in electrochemical immunosensing and the emergence of silver-gold galvanic exchange detection

Nanoparticle-based electrochemical immunosensors demonstrate high sensitivity toward biomarker detection due to the large surface area of the nanoparticles and their ability to amplify the signal of the target molecule. Additionally, they have a fast response time, relatively lower cost, and can be easily miniaturized for point-of-care applications. Among noble metals, silver nanoparticles (AgNPs) have been extensively used in electrochemical sensors due to their unique properties, such as catalytic activity and excellent electrical conductivity. This Feature Article describes six approaches for incorporating AgNPs in electrochemical platforms, featuring the most recent developments in the silver-gold galvanic exchange-based detection strategy. With a few exceptions, many of these detection methods use AgNP oxidation into Ag+ ions, followed by electrodeposition of Ag+ ions onto the working electrode as zero-valent Ag metal and a final stripping step using a voltammetric technique. Combining these steps provides desirable low detection limits and good sensitivity for various biomarkers. A few other methods involved the reduction of Ag+ ions and depositing them as Ag metal onto the electrode using a reagent mixture so that the striping analysis could be performed. Typically, this reagent mixture includes Ag+ ions, a reducing agent, or an enzyme substrate. Besides, AgNPs have also been directly used to modify the surface of electrodes to facilitate kinetically favored redox-mediated electrochemical reactions. In addition to Ag detection methods, this report will also provide recent examples to illustrate how the size and shape of AgNPs impact the detection limits and sensitivity of an electrochemical assay. Finally, we discuss recent developments in lab-on-a-chip type immunosensors designed explicitly for Ag-based metalloimmunoassay detection, and we envision that this article will provide a comprehensive summary of the operational principles and new insights into such immunoassay systems.

Highly sensitive photoelectrochemical immunosensor for detecting cancer marker CA19-9 based on a new SnSe quantum dot

A sandwich-type photoelectrochemical (PEC) immunosensor was constructed on a screen-printed electrode (SPE) using gold-coated tin selenide quantum dots (Au-SnSe QDs) to determine the carbohydrate antigen 19 9 (CA19-9). Water-soluble Au-SnSe QDs were prepared by coating low-cost SnSe QDs, prepared by reacting tin(II) 2-ethyl hexanoate with selenium ions (HNaSe) without needing to add an external capping agent (SnSe QDs). SnSe-based QDs were characterized using high-resolution transmission electron microscopy (HR-TEM) and dynamic light scattering (DLS). DSP (dithio-bis (succinimidyl propionate)) as a linker was attached on Au@SnSe QDs and conjugated with CA19-9 monoclonal antibodies (Ab₂-DSP-Au@SnSE QD). For capture probe assembling, an Au nano-layer was electrochemically deposited on a SPE by HAuCl₄ reduction using 12 cycles of cyclic voltammetry (0 to - 1.4 V) at the scan rate of 50 mV s^-1, then covered by self-assembly of DSP and covalent conjugation of CA19-9 Ab₁. Our developed PEC immunosensor showed a significant photoelectrochemical response, recorded using chronoamperometry (0.3 V), for the presence of CA19-9 antigen in serum samples under light irradiation, with a detection limit (LOD) of 0.0011 U mL^-1 and a dynamic range of 0.005-100 U mL^-1. The recovery of CA19-9 determination from serum samples was 101 to 113%.

Two-Dimensional Graphitic Carbon Nitride (g-C3N4) Nanosheets and Their Derivatives for Diagnosis and Detection Applications

The early diagnosis of certain fatal diseases is vital for preventing severe consequences and contributes to a more effective treatment. Despite numerous conventional methods to realize this goal, employing nanobiosensors is a novel approach that provides a fast and precise detection. Recently, nanomaterials have been widely applied as biosensors with distinctive features. Graphite phase carbon nitride (g-C₃N₄) is a two-dimensional (2D) carbon-based nanostructure that has received attention in biosensing. Biocompatibility, biodegradability, semiconductivity, high photoluminescence yield, low-cost synthesis, easy production process, antimicrobial activity, and high stability are prominent properties that have rendered g-C₃N₄ a promising candidate to be used in electrochemical, optical, and other kinds of biosensors. This review presents the g-C₃N₄ unique features, synthesis methods, and g-C₃N₄-based nanomaterials. In addition, recent relevant studies on using g-C₃N₄ in biosensors in regard to improving treatment pathways are reviewed.

Graphite Carbon Nitride and Its Composites for Medicine and Health Applications

With the progress of science and technology and the improvement of people's living standards, the performance of traditional materials can no longer fully meet the needs of social development. Graphitic phase carbon nitride (g-C₃ N₄ ), as a new type of nanomaterial, has good properties. Its unique graphite like structure and stable thermodynamic characteristics have led an increasing number of researchers to explore its diverse functions and use this as a basis to develop related energy and products for applications in various fields. Among them, applications in the field of medicine health have become popular in recent years. Therefore, this review summarizes the synthesis methods of g-C₃ N₄ and its composites, as well as their applications in food, medicine, environmental monitoring and disease treatment, in the hope of providing references and basis for further expanding the applications of g-C₃ N₄ in large health areas.

Electrochemical biosensors for measurement of colorectal cancer biomarkers

Colorectal cancer (CRC) is associated with one of the highest rates of mortality among cancers worldwide. The early detection and management of CRC is imperative. Biomarkers play an important role in CRC screening tests, CRC treatment, and prognosis and clinical management; thus rapid and sensitive detection of biomarkers is helpful for early detection of CRC. In recent years, electrochemical biosensors for detecting CRC biomarkers have been widely investigated. In this review, different electrochemical detection methods for CRC biomarkers including immunosensors, aptasensors, and genosensors are summarized. Further, representative examples are provided that demonstrate the advantages of electrochemical sensors modified by various nanomaterials. Finally, the limitations and prospects of biomarkers and electrochemical sensors in detection are also discussed. Graphical abstract.

Colorimetric Assay Using Mesoporous Fe-Doped Graphitic Carbon Nitride as a Peroxidase Mimetic for the Determination of Hydrogen Peroxide and Glucose

Iron can enter the electron-rich cavities of graphitic carbon nitride (g-C₃N₄). On account of this phenomenon, Fe-doped g-C₃N₄ (Fe-g-C₃N₄) was prepared as a peroxidase mimetic by using one-step pyrolysis of urea and FeCl₃·6H₂O. Compared to g-C₃N₄, Fe-g-C₃N₄ has a large specific surface area due to the presence of mesopores and cracks, a smaller band gap, and a high loading of Fe in its structure. Thus, Fe-g-C₃N₄ exhibits greater peroxidase activity with a more obvious color change when using 3,3',5,5'-tetramethylbenzidine (TMB) as a substrate in the presence of hydrogen peroxide (H₂O₂). The color of a mixture of TMB and Fe-g-C₃N₄ gradually deepens with increasing concentrations of H₂O₂. Accordingly, a rapid, sensitive, and low-cost colorimetric assay for the detection of H₂O₂ was developed. After optimization, this method boasts a wide linear dynamic range for H₂O₂ detection from 0.005 to 400 μM (r² = 0.9971) with a detection limit of 0.005 μM. Because H₂O₂ is a main product of glucose oxidation by glucose oxidase (GOx), a colorimetric assay for glucose detection was also realized, with a linear dynamic range of 1-1000 μM (r² = 0.9996) and a detection limit of 0.5 μM. These assays were applied to the quantitative detection of H₂O₂ in milk and glucose in human serum, respectively.

A Label-Free Electrochemical Immunosensor for Detection of the Tumor Marker CA242 Based on Reduced Graphene Oxide-Gold-Palladium Nanocomposite

As a tumor marker, carbohydrate antigen 24-2 (CA242) is a highly accurate and specific diagnostic indicator for monitoring pancreatic and colorectal cancers. The goal of this study was to create a novel label-free electrochemical immunosensor using a nanocomposite glassy carbon electrode for the detection of CA242. Graphene oxide (GO) and polyvinyl pyrrolidone were chosen as the dopants for the preparation of a high-performance reduced-GO-gold-palladium (rGO-Au-Pd) nanocomposite. RGO-Au-Pd was characterized using X-ray diffraction and transmission electron microscopy, revealing that the material exhibited superior electrochemical redox activity and electron transfer ability. The effects of the synthesis method, material concentration, reduction cycle, and pH were investigated to optimize the performance of the immunosensor. As a result of the catalytic activity and biocompatibility of rGO-Au-Pd, the prepared CA242 immunosensor displayed a wide linear range of detection from 0.001 U/mL to 10,000 U/mL with a detection limit of 1.54 × 10⁻³ U/mL and a sensitivity of 4.24 μA (log₁₀C_CA242)⁻¹. More importantly, the immunosensor exhibited satisfactory reproducibility and selectivity when detected CA242 in PBS or human serum. The results of our study provide a platform for the development of novel bioassays for use in early cancer diagnosis and promote the application of biosensing technology in the medical field.

A potentiometric immunosensor for enterovirus 71 based on bis-MPA-COOH dendrimer-doped AgCl nanospheres with a silver ion-selective electrode

Herein a new potentiometric immunoassay for the point-of-care detection of enterovirus 71 (EV71) was developed by using a silver (Ag⁺) ion-selective electrode (ISE). Initially, the carboxylated dendrimer-doped AgCl nanospheres were synthesized by the reverse micelle method. Then the synthesized nanospheres were used to label a polyclonal mouse anti-EV71 antibody via a typical carbodiimide coupling method. The immunoreaction was executed on a monoclonal anti-EV71 antibody-coated microplate by using biofunctional AgCl nanospheres as the detection antibody. With a sandwich-type immunoassay format, the carried AgCl nanospheres could be dissolved in the presence of NH₃·H₂O, and the released silver ions were determined with an external silver ion-selective electrode. Under optimal conditions, the shift in the potential increased with the increase in the EV71 concentration, in a wide linear range of 0.3-300 ng mL^-1, with a detection limit of 0.058 ng mL^-1. Intra- and inter-assay relative standard deviations with identical batches were less than 4.15% and 6.15%, respectively. By validating the spiked serum samples, our system shows consistency with the enzyme-linked immunosorbent assay (ELISA) kit.

Highly Sensitive Plasmonic Detection of the Pancreatic Cancer Biomarker CA 19-9

Plasmonic gold (Au) nanotriangular arrays, functionalized with a near infrared (NIR) fluorophore-conjugated immunoassay to Carbohydrate Antigen 19-9 (CA 19-9), a pancreatic cancer biomarker, produce optically tunable substrates with two orders of magnitude fluorescence enhancement. Through nanoscale morphological control, the sensitivities of the plasmonic nanotriangular arrays are controllable, paving the way of such optical platforms for multiplexing. Here, we report a limit of detection (LOD) of 7.7 × 10-7 U.mL-1 for CA 19-9 by using such tunable Au nanotriangular arrays, a great improvement compared to commercially available CA 19-9 immunoassays. The linear dynamic range was from 1 × 10-6 U.mL-1 to 1 U.mL-1, i.e. up to six orders of magnitude. Moreover, high specificity was demonstrated, together with successful validation in serum samples. Their superior tunable sensitivity, along with efforts to combine CA 19-9 with other biomarkers for improved accuracy, open up the possibility for multiplexed NIR-fluorescence enhancement microarrays, for early cancer diagnosis and therapeutic monitoring.

A high sensitive visible light-driven photoelectrochemical aptasensor for shrimp allergen tropomyosin detection using graphitic carbon nitride-TiO2 nanocomposite

Herein, for the first time a visible-light-driven photoelectrochemical (PEC) aptasensor for shrimp tropomyosin determination was fabricated by using graphitic carbon nitride (g-C₃N₄) and titanium dioxide (TiO₂) as photoactive nanomaterials, ascorbic acid (AA) as electron donor and ruthenium (III) hexaammine (Ru(NH₃)₆³⁺) as signal enhancer. The surface of an ITO electrode was first modified with g-C₃N₄, TiO₂, and polyethyleneimine (PEI) and then the amine terminal aptamer_TROP probe was attached to PEI by the use of glutaraldehyde (GA) as cross-linker. After that, Ru(NH₃)₆³⁺ was adsorbed on aptamer to enhance the photocurrent signal. The principle of proposed PEC aptasensor is based on the formation of a selective complex between tropomyosin and immobilized aptamer_TROP probe on the surface of ITO/g-C₃N₄-TiO₂/PEI/aptamer_TROP-Ru(NH3)₆⁺³. After the incubation of tropomyosin with TROP aptamer probe, the photocurrent signal decreased due to releasing adsorbed Ru(NH₃)₆³⁺ on aptamer and preventing AA from scavenging photogenerated holes to the photoactive modified electrode. Under the optimized conditions, the fabricated PEC aptasensor was used for the determination of shrimp tropomyosin in the concentration range of 1-400ngmL^-1 with a limit of detection of 0.23ngmL^-1. The proposed PEC aptasensor exhibited high selectivity, sensitivity, and good stability.