Polyhydroquinone-graphene composite as new redox species for sensitive electrochemical detection of cytokeratins antigen 21-1

A novel electrochemical sensor based on process-formed laccase-like catalyst to degrade polyhydroquinone for tumor marker

Methods to improve the sensitivity of electrochemical sensors based on catalytic reactions generally require adscititious or pre-modified catalysts, which make the sensitive detection of sensors extremely challenging. This is because the activity of the catalyst is susceptible to the storage and modification process, such as aggregation during storage or loss of active sites during multi-step modification, which impairs the performance of the sensor. To solve this thorny issue, a novel electrochemical sensor based on a process-formed laccase-like catalyst was constructed for sensitive detection of tumor markers. Cu²⁺-polydopamine (CuPDA) combined with antibody (Ab₂) were employed as copper-containing immunoprobe, which released Cu(Ⅱ) ions under acidic stimulation. Cu(Ⅱ) ions coordinate with the self-assembly cationic diphenylalanine-glutaraldehyde nanospheres (CDPGA) to form a laccase-like catalyst, which had stronger catalytic activity than laccase. The freshly formed catalyst was immediately used to degrade the polyhydroquinone-reduced graphene oxide (PHQ-rGO) composite, resulting in a significant reduction in the current signal. The PHQ-rGO composite plays dual roles of signal substance and substrate on the sensing interface. The proposed electrochemical sensor demonstrated wide linearity for the determination of a model analyte, human epididymis protein 4 (HE4), from 1 pg mL^-1 to 100 ng mL^-1, and the detection limit was as low as 0.302 pg mL^-1 (S/N = 3), which had good consistency with that of electrochemiluminescence method. This process-formed catalyst approach will have potential reference significance for the construction of other sensors.

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.

π-Self-Assembly of a Coronene on Carbon Nanomaterial-Modified Electrode and Its Symmetrical Redox and H2O2 Electrocatalytic Reduction Functionalities

The structure-electroactivity relationship of graphene has been studied using coronene (Cor), polyaromatic hydrocarbon (PAH), and a subunit of graphene as a model system by chemically modified electrode approach. In general, graphene and PAH do not show any redox activity in their native form. Herein, we report a simple electrochemical approach for the conversion of electro-inactive coronene to a highly redox-active molecule (Cor-Redox; E°' = 0.235 ± 0.005 V vs Ag/AgCl) after being adsorbed on graphitic carbon nanomaterial and preconditioned at an applied potential, 1.2 V vs Ag/AgCl, wherein, the water molecule oxidizes to dioxygen via hydroxyl radical (^•OH) intermediate, in acidic solution (pH 2 KCl-HCl). When the same coronene electrochemical experiment was carried out on an unmodified glassy carbon electrode, there was no sign of faradic signal, revealing the unique electrochemical behavior of the coronene molecule on graphitic nanomaterial. The Cor-Redox peak is found to be highly symmetrical (peak-to-peak potential separation of ∼0 V tested by cyclic voltammetry (CV)) and surface-confined (Γ_Cor-Redox = 10.1 × 10^-9 mol cm^-2) and has proton-coupled electron-transfer (∂E°'/∂pH = -56 mV pH^-1) character. Initially, it was speculated that Cor is converted to a hydroxy group-functionalized Cor molecule (dihydroxy benzene derivative) on the graphitic surface and showed the electrochemical redox activity. However, physicochemical characterization studies including Raman, IR, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), redox-site selective oxidation probe, cysteine (for dihydroxy benzene), radical scavenger ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl, TEMPO), and scanning electrochemical microscopy (SECM) using ferricyanide redox couple have revealed that coronene cationic radical species like electroactive molecule is formed on graphitic material upon the electrochemical oxidation reaction at a high anodic potential. It has been proposed that ^•OH generated as an intermediate species from the water oxidation reaction is involved in the coronene cationic radical species. Studies on coronene electrochemical reaction at various carbon nanomaterials like multiwalled carbon, single-walled carbon, graphite, graphene oxide, and carbon nanofiber revealed that graphitic structure (without any oxygen functional groups) and its π-π bonding are key factors for the success of the electrochemical reaction. The coronene molecular redox peak showed an unusual electrocatalytic reduction of hydrogen peroxide similar to the peroxidase enzyme-biocatalyzed reduction reaction in physiological solution.