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We present herein the most complete characterization of microneedle (MN) potentiometric sensors for pH transdermal measurements for the time being. Initial in vitro assessment demonstrated suitable analytical performances (e.g., Nernstian slope, linear range of response from 8.5 to 5.0, and fast response time) in both buffer media and artificial interstitial fluid (ISF). Excellent repeatability and reproducibility together with adequate selectivity and resiliency facilitate the appropriateness of the new pH MN sensor for transdermal ISF analysis in healthcare. The ability to resist skin insertions was evaluated in several ex vivo setups using three different animal skins (i.e., chicken, pork, and rat). The developed pH MN sensor was able to withstand from 5 to 10 repetitive insertions in all the skins considered with a minimal change in the calibration graph (<3% variation in both slope and intercept after the insertions). Ex vivo pH measurements were validated by determining the pH with the MN sensor and a commercial pH electrode in chicken skin portions previously conditioned at several pH values, obtaining excellent results with an accuracy of <1% and a precision of <2% in all cases. Finally, pH MN sensors were applied for the very first time to transdermal measurements in rats together with two innovative validation procedures: (i) measuring subcutaneous pH directly with a commercial pH microelectrode and (ii) collecting ISF using hollow MNs and then the pH measurement of the sample with the pH microelectrode. The pH values obtained with pH MN sensors were statistically more similar to subcutaneous measurements, as inferred by a paired sample t -test at 95% of confidence level. Conveniently, the validation approaches could be translated to other analytes that are transdermally measured with MN sensors.

The application of a novel Poly(3,4-ethylenedioxythiophene)-Tyrosinase/Sonogel-Carbon electrode (PEDOT-Tyr/SNGC) biosensor to beers and wines analysis is proposed. This biosensor implies a new Sinusoidal Current (SC) electrodeposition method to immobilize the enzyme generating a nanostructure surface. The biosensors were characterized electrochemically, employing cyclic voltammetry and electrochemical impedance spectroscopy. Sensitivity, limit of detection, and correlation coefficients of the linear fitting were 2.40 × 10−4 µA·µM−1, 4.33 µM, and R2 = 0.9987, respectively. Caffeic acid is used as the reference polyphenol. A sampling of nine beers (four lager, three stout, and two non-alcoholic beers), and four wines (three red and one white wine) purchased in a local store was performed. The Polyphenol indeces for beers and wines have been assessed using the proposed biosensor, and the obtained values are in agreement with the literature data. Antioxidant properties of the samples using the 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid (ABTS) radical spectrophotometric method were also evaluated. The correlation between the polyphenol index and the antioxidant capacity was obtained for beers and wines.

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Commercially available gold screen-printed electrodes cured at high temperature were modified or functionalized at the surface with gold nanoparticles, mainly produced by a green synthesis method, and other carbon-based nanomaterials in order to enhance the electron transfer between the sulfhydryl groups of the aminothiols cysteine (Cys), methionine (Met), glutathione (GSH) and homocysteine (hCys) and the electrode. The electrochemical characterization by cyclic and differential pulse voltammetry of some of the modified electrodes showed an improved performance in terms of sensitivity, reproducibility, repeatability, linearity range and limits of detection (12, 1, 0.2 and 1 μmol L -1 for Cys, Met, GSH, hCys, respectively) and quantification (40, 2, 0.5 and 2 μmol L -1 for Cys, Met, GSH, hCys, respectively) as compared to unmodified units. Then, some of these modified devices were successfully applied to the determination of cysteine and methionine in dietary supplements by means of liquid chromatography with amperometric detection.

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