Diabetes is a pathological condition that requires the continuous monitoring of glucose level in the blood. Its control has been tremendously improved by the application of point-of-care devices. Conventional enzyme-based sensors with electrochemical and optical transduction systems can successfully measure the glucose concentration in human blood, but they suffer from the low stability of the enzyme. Non-enzymatic wearable electrochemical and optical sensors, with low-cost, high stability, point-of-care testing and online monitoring of glucose levels in biological fluids, have recently been developed and can help to manage and control diabetes worldwide. Advances in nanoscience and nanotechnology have enabled the development of novel nanomaterials that can be implemented for the use in enzyme-free systems to detect glucose. This review summarizes recent developments of enzyme-free electrochemical and optical glucose sensors, as well as their respective wearable and commercially available devices, capable of detecting glucose at physiological pH conditions without the need to pretreat the biological fluids. Additionally, the evolution of electrochemical glucose sensor technology and a couple of widely used optical detection systems along with the glucose detection mechanism is also discussed. Finally, this review addresses limitations and challenges of current non-enzymatic electrochemical, optical, and wearable glucose sensor technologies and highlights opportunities for future research directions.

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This paper presents the results of the electro-oxidation of pollutants in synthetic textile wastewater containing partially soluble disperse dyes. The experiments were performed in an electrochemical undivided cell reactor using seven different anode materials and 0.1 M NaCl as the supporting electrolyte. With the Ti/Pt-Ir anode, which showed the best performance among all the tested materials, additional experiments were also carried out using 0.05 M Na 2 SO 4 as the supporting electrolyte. Experimental results obtained in the electrochemical reactor, supported by the data obtained during cyclic voltammetry studies, showed that under the conditions of the present research the removal of pollutants was mediated by active chlorine generated by electro-oxidation of chloride ions or by other mediators generated in situ and not by a direct discharge of pollutants at the anode. Under the conditions of free pH evolution 39% removal of chemical oxygen demand was obtained after 40 min of electrolysis. The apparent pseudo-first-order rate constant for the removal of color was equal to 2.54 × 10 -4 s -1 and it increased to 8.23 × 10 -4 s -1 under pH control at the value of 4.5, resulting in 90% removal of color after the passage of 1.9 A h dm -3 . In comparative studies on the chemical oxidation of pollutants by hypochlorite far lower efficiency was obtained.

The steady-state voltammetric behavior for reduction of several polyprotic acids and mixtures of strong and weak mono- and polyprotic acids was studied at platinum microelectrodes. The results demonstrated that over the potential range accessible to reduction of acids in water (up to ∼−1 V vs Ag/AgCl) via a preceding chemical reaction (CE mechanism), the reduction of weak polyprotic acids and mixtures of acids can produce either a single well-defined wave or two waves separated to a different extent, depending on the dissociation constant of each acidic form, on the analytical concentration of each acid, and on the mutual ratio of the acids present at equilibrium in the bulk solutions. The overall reduction mechanism for most of the mixtures examined was interpreted on the basis of a series of CE processes associated to the hydrogen evolution. This interpretation was supported by digital simulation procedures. A theoretical relationship for predicting the steady-state limiting current for any mixture of acidic species, whose dissociation steps are fast, was also derived. This equation proved valid for all those acids with equilibrium constants larger than ∼10-6. On the basis of this theoretical relationship, a simple diagnostic criterion to assess whether or not the reduction process of a mixture of acids is under a kinetic control was also established.

The steady-state voltammetric behavior for the oxidation of aqueous solutions containing the strong bases sodium and barium hydroxide was studied with gold microelectrodes in the absence and in the presence of different concentrations of supporting electrolyte. A well-defined oxidation wave attributed to the oxidation of hydroxide ions to oxygen was observed in all the solutions investigated, regardless of both the nature of the base and the supporting electrolyte employed. However, in solutions with excess electrolyte, the steady-state limiting current was found to depend on the actual concentration of the supporting electrolyte, as the diffusion coefficient of the electroactive species varies with both the ionic strength and viscosity of the medium. Since the hydroxide ion is a negatively charged species, solutions with low or without supporting electrolyte yielded currents enhanced by migration contributions. Theoretical equations for the dependence of steady-state limiting currents with ionic strength were derived; theoretical and experimental data compared satisfactorily. The usefulness of the oxidation wave of hydroxide ions for analytical applications was shown by examining the dependence of steady-state limiting currents on concentration and pH. At a given ionic strength, the steady-state limiting current is proportional to the concentration of hydroxide ion over the range 0.5-5 mM. Moreover, the pH of the basic solutions can be determined by amperometry in place of potentiometry, with the use of an absolute equation.In aqueous solutions, the cathodic and anodic potential limits at noble metals such as platinum and gold are defined by the hydrogen and oxygen evolution processes. 1 These, according to several reviews, 1-5 occur through quite complex reaction pathways. It is also well established that dilute aqueous solutions of acids show a wave for hydronium ion reduction which is separated from the background discharge to an extent that depends, for a given electrode material, on the nature of the acid. 6-16 By analogy, an oxidation wave would be expected from dilute solutions of bases, where free hydroxide ions are available to a considerable extent. Although there are many papers dealing with the anodic voltammetric behavior of noble metals in aqueous solutions of strong bases, we have not found a single article reporting a clear oxidation wave due to hydroxide ion and separated from the oxygen evolution due to the background. This, in part, can be because the anodization of noble metals involves oxide film formation. 1 Recently, 17 during explorative measurements aimed at characterizing, under steady-state conditions, the anodic behavior of gold microelectrodes in aqueous solutions made basic with NaOH, a well-defined wave was observed at very positive potentials (∼1.3 V versus a saturated calomel electrode), just before the background discharge. In excess sodium sulfate or sodium perchlorate,

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