Herein, the reproducibility and a double validation of on-body measurements provided by new wearable potentiometric ion sensors (WPISs) is presented. Sweat collected during sport practice was first analyzed using the developed device, the pH-meter, and ion chromatography (IC) prior to on-body measurements (off-site validation). Subsequently, the accuracy of on-body measurements accomplished by the WPISs was evaluated by comparison with pH-meter readings and IC after collecting sweat (every 10–12.5 min) during sport practice. The developed device contains sensors for pH, Cl–, K+, and Na+ that are embedded in a flexible sampling cell for sweat analysis. The electrode array was fabricated employing MWCNTs (as an ion-to-electron transducer) and stretchable materials that have been exhaustively characterized in terms of analytical performance, presenting Nernstian slopes within the expected physiological range of each ion analyte (Cl–, 10–100 mM; K+, 10–10 mM; and Na+, 10–100 mM and pH, 4.5–7.5), drift suitable for midterm exercise practice (0.3 ± 0.2 mV h–1), fast response time, adequate selectivity for sweat measurements, and excellent reversibility. Besides that, the designed sampling cell avoids any sweat contamination and evaporation issues while supplying a passive sweat flow encompassing specifically the individual’s perspiration. The interpretation of ion concentration profiles may permit the identification of personal dynamic patterns in sweat composition while practicing sport.

In this paper, we present a simple and inexpensive method for the fabrication of high-performance graphene-based heaters on different large-scale substrates through the laser photothermal reduction of graphene oxide (laser-reduced graphene-oxide, LrGO). This method allows an efficient and localized high level of reduction and therefore a good electrical conductivity of the treated films. The performance of the heaters is studied in terms of steady-state temperature, power consumption, and time response for different substrates and sizes. The results show that the LrGO heaters can achieve stable steady-state temperatures higher than 200 °C when a voltage of 15 V is applied, featuring a time constant of around 4 s and a heat transfer coefficient of ~200 °C cm2/W. These characteristics are compared with other technologies in this field, demonstrating that the fabrication approach described in this work is competitive and promising to fabricate large-scale flexible heaters with a very fast response and high steady-state temperatures in a cost-effective way. This technology can be easily combined with other fabrication methods, such as screen printing or spray-deposition, for the manufacturing of complete sensing systems where the temperature control is required to adjust functionalities or to tune sensitivity or selectivity.

This work presents a microfluidic paper-based analytical device (μPAD) for glucose determination using a supported metal-organic framework (MOF) acting as a peroxidase mimic. The catalytic action of glucose oxidase (GOx) on glucose causes the formation of HO, and the MOF causes the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by HO to form a blue-green product with an absorption peak at 650 nm in the detection zone. A digital camera and the iOS feature of a smartphone are used for the quantitation of glucose with the S coordinate of the HSV color space as the analytical parameter. Different factors such as the concentration of TMB, GOx and MOF, pH and buffer, sample volume, reaction time and reagent position in the μPAD were optimized. Under optimal conditions, the value for the S coordinate increases linearly up to 150 μmol·L glucose concentrations, with a 2.5 μmol·L detection limit. The μPAD remains stable for 21 days under conventional storage conditions. Such an enzyme mimetic-based assay to glucose determination using Fe-MIL-101 MOF implemented in a microfluidic paper-based device possesses advantages over enzyme-based assays in terms of costs, durability and stability compared to other existing glucose determination methods. The procedure was applied to the determination of glucose in (spiked) serum and urine. Graphical abstract Schematic representation of microfluidic paper-based analytical device using metal-organic framework as a peroxidase mimic for colorimetric glucose detection with digital camera or smartphone and iOS app readout.

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