Hydrothermally reduced graphene oxide as a sensing material for electrically transduced pH sensors

Advances in pH Sensing: From Traditional Approaches to Next-Generation Sensors in Biological Contexts

pH has been considered one of the paramount factors in bodily functions because most cellular tasks exclusively rely on precise pH values. In this context, the current techniques for pH sensing provide us with the futuristic insight to further design therapeutic and diagnostic tools. Thus, pH-sensing (electrochemically and optically) is rapidly evolving toward exciting new applications and expanding researchers' interests in many chemical contexts, especially in biomedical applications. The adaptation of cutting-edge technology is subsequently producing the modest form of these biosensors as wearable devices, which are providing us the opportunity to target the real-time collection of vital parameters, including pH for improved healthcare systems. The motif of this review is to provide insight into trending tech-based systems employed in real-time or in-vivo pH-responsive monitoring. Herein, we briefly go through the pH regulation in the human body to help the beginners and scientific community with quick background knowledge, recent advances in the field, and pH detection in real-time biological applications. In the end, we summarize our review by providing an outlook; challenges that need to be addressed, and prospective integration of various pH in vivo platforms with modern electronics that can open new avenues of cutting-edge techniques for disease diagnostics and prevention.

Stability of Graphene/Nafion Composite in PEM FC Electrodes

Ensuring the stable operation of proton exchange membrane fuel cells is conducive to their real-world application. A promising direction for stabilizing electrodes is the stabilization of the ionomer via the formation of surface compounds with graphene. A comprehensive study of the (electrochemical, chemical, and thermal) stability of composites for fuel cell electrodes containing a modifying additive of few-layer graphene was carried out. Electrochemical stability was studied by cycling the potential on a disk electrode for 5000 cycles. Chemical stability was assessed via the resistance of the composites to H2O2 treatment using ion-selective potentiometry. Thermal stability was studied using differential thermal analysis. Composites were characterized by UV-Vis spectroscopy, Raman spectroscopy, EDX, and SEM. It was shown that graphene inhibits Nafion degradation when exposed to heat. Contrariwise, Nafion is corrosive to graphene. During electrochemical and chemical exposure, the determining change for carbon-rich composites is the carbon loss (oxidation) of the carbon material. In the case of carbon-poor composites, the removal of fluorine and sulfur from the Nafion polymer with their partial replacement by oxygen prevails. In all cases, the F/S ratio is stable. The dispersity of Nafion in a sample affects its chemical stability more than the G/Nafion ratio does.

Construction of an electrochemical pH sensor using one-pot synthesis of a molybdenum diselenide/nitrogen doped graphene oxide screen-printed electrode

In this study, a one-pot synthesis of a molybdenum diselenide/nitrogen-doped graphene oxide (MoSe2/NGO) composite was demonstrated and used for the fabrication of an electrochemical pH sensor. The MoSe2/NGO composite was characterized using powder X-ray diffraction, infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis, scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and Brunauer-Emmett-Teller analysis. The electrochemical behavior at different pH values was determined by recording the open-circuit potential. When applied for pH detection, the MoSe2/NGO modified screen-printed electrode (SPE) showed good linearity with a sensitivity of 61.3 mV pH-1 over a wide pH range of 2-14. In addition, the pH sensor exhibited a remarkably stable response, high reproducibility, and selectivity. The sensor was used to measure the acidity or alkalinity of real food and beverage samples. The results for these samples showed a relative error of less than 10% compared with the results obtained with the commercial pH meter. The portable sensor produced by screen printing electrodes paves the way for the development of simple, cost-effective, real-time, and robust pH sensors for the pH analysis of various sample matrices for clinical diagnostics, biosensing, and cost-effective applications.

A highly sensitive and long-term stable wearable patch for continuous analysis of biomarkers in sweat

Although increasing efforts have been devoted to the development of non-invasive wearable or stretchable electrochemical sweat sensors for monitoring physiological and metabolic information, most of them still suffer from poor stability and specificity over time and fluctuating temperatures. This study reports the design and fabrication of a long-term stable and highly sensitive flexible electrochemical sensor based on nanocomposite-modified porous graphene by simple and facile laser treatment for detecting biomarkers such as glucose in sweat. The laser-reduced and patterned stable conductive nanocomposite on the porous graphene electrode provides the resulting glucose sensor with an excellent sensitivity of 1317.69 μAmM-1cm-2 with an ultra-low limit of detection (LOD) of 0.079 μM. The sensor can also detect pH and exhibit extraordinary stability to maintain more than 91% sensitivity over 21 days in ambient conditions. Taken together with a temperature sensor based on the same material system, the dual glucose and pH sensor integrated with a flexible microfluidic sweat sampling network further results in accurate continuous on-body glucose detection calibrated by the simultaneously measured pH and temperature. The low-cost, highly sensitive, and long-term stable platform could facilitate and pave the way for the early identification and continuous monitoring of different biomarkers for non-invasive disease diagnosis and treatment evaluation.

Long-Term Exposure of Graphene Oxide Suspension to Air Leading to Spontaneous Radical-Driven Degradation

Understanding the environmental transformation and fate of graphene oxide (GO) is critical to estimate its engineering applications and ecological risks. While there have been numerous investigations on the physicochemical stability of GO in prolonged air-exposed solution, the potential generation of reactive radicals and their impact on the structure of GO remain unexplored. In this study, using liquid-PeakForce-mode atomic force microscopy and quadrupole time-of-flight mass spectroscopy, we report that prolonged exposure of GO to the solution leads to the generation of nanopores in the 2D network and may even cause the disintegration of its bulk structure into fragment molecules. These fragments can assemble themselves into films with the same height as the GO at the interface. Further mediated electrochemical analysis supports that the electron-donating active components of GO facilitate the conversion of O2 to •O2- radicals on the GO surface, which are subsequently converted to H2O2, ultimately leading to the formation of •OH. We experimentally confirmed that attacks from •OH radicals can break down the C-C bond network of GO, resulting in the degradation of GO into small fragment molecules. Our findings suggest that GO can exhibit chemical instability when released into aqueous solutions for prolonged periods of time, undergoing transformation into fragment molecules through self-generated •OH radicals. This finding not only sheds light on the distinctive fate of GO-based nanomaterials but also offers a guideline for their engineering applications as advanced materials.

Effect of pH on Adsorption of Tetracycline Antibiotics on Graphene Oxide

Graphene oxide (GO) has good dispersibility and adsorption capacity for antibiotics adsorption, a complex process influenced by many factors. In this work, the adsorption mechanism of GO on tetracycline antibiotics at different pH was studied to address its attenuated effects on the microbial growth. The results showed that the adsorption process of GO on three antibiotics, namely, tetracycline (TC), oxytetracycline (OTC), and chlortetracycline (CTC), followed the pseudo-second-order kinetic model. The maximum adsorption capacities were observed at pH5 which were 133.0 mg/g for TC, 125.4 mg/g for OTC, and 167.0 mg/g for CTC. Furthermore, the reaction was uniform adsorption with a single layer on the surface of GO, and heating was conducive to the reaction. In the microbial growth experiment, the growth of E. coli and B. subtilis senses was optimal at pH5, which was consistent with the adsorption experiment. This study analyzed the effect of pH on the adsorption of antibiotics by GO and provided a theoretical basis for the further application of GO in various aquatic environments.

Defect Density-Dependent pH Response of Graphene Derivatives: Towards the Development of pH-Sensitive Graphene Oxide Devices

In this study, we demonstrate that a highly pH-sensitive substrate could be fabricated by controlling the type and defect density of graphene derivatives. Nanomaterials from single-layer graphene resembling a defect-free structure to few-layer graphene and graphene oxide with high defect density were used to demonstrate the pH-sensing mechanisms of graphene. We show the presence of three competing mechanisms of pH sensitivity, including the availability of functional groups, the electrochemical double layer, and the ion trapping that determines the overall pH response. The graphene surface was selectively functionalized with hydroxyl, amine, and carboxyl groups to understand the role and density of the graphene pH-sensitive functional groups. Later, we establish the development of highly pH-sensitive graphene oxide by controlling its defect density. This research opens a new avenue for integrating micro-nano-sized pH sensors based on graphene derivatives into next-generation sensing platforms.

Removal of Hg2+ from desulfurization wastewater by tannin-immobilized graphene oxide

A novel adsorbent consisting of tannic acid (TA) immobilized on graphene oxide (GO) was proposed and used to remove Hg²⁺ from desulfurization wastewater. The morphology and physicochemical properties of tannin-immobilized graphene oxide (TAIGO) were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The characterization results showed that TA was successfully immobilized on GO, and that new functional groups were introduced on TAIGO. The effects of contact time, adsorbent dose, pH, and ion components on removal efficiency were evaluated. The adsorption process was found to be complete within 15 min, and the removal efficiency increased with increasing adsorbent dosage. The pH value affected the protonation of TAIGO and the form of Hg²⁺ in wastewater. High concentrations of Cl^- and SO₃^2- hindered the adsorption performance, whereas SO₄^2- and cations had a negligible effect. In addition, the excellent economic benefits of TAIGO were analyzed in an economic evaluation, and the Hg²⁺ removal efficiency remained at 88% after three recycles. A pseudo-second-order kinetic model (R = 0.9995) was used to fit the adsorption process, and the oxygen-containing functional groups and chelation reaction played critical roles in adsorption. TAIGO is a low-cost adsorbent with high Hg²⁺ removal efficiency and could be further used in practical desulfurization wastewater.