Fabrication of Printable Colorimetric Food Sensor Based on Hydrogel for Low-Concentration Detection of Ammonia

With the increasing market share of ready-to-cook foods, accurate determination of the food freshness and thus food safety has emerged as a concern. To commercialize and popularize food sensing technologies, food sensors with diverse functionalities, low cost, and facile use must be developed. This paper proposes printable sensors based on a hydrogel-containing pH indicator to detect ammonia gas. The sensors were composed of biocompatible polymers such as 2-hydroxyethyl methacrylate (HEMA) and [2-(methacryloyloxy)ethyl] trimethylammonium chloride (MAETC). The p(HEMA-MAETC) hydrogel sensor with bromothymol blue (BTB) demonstrated visible color change as a function of ammonia concentration during food spoilage. Furthermore, polyacrylonitrile (PAN) was added to improve transport speed of ammonium ions as the matrix in the sensors and optimized the viscosity to enable successful printing. The color changed within 3 min at ammonia concentration of 300 ppb and 1 ppm, respectively. The sensor exhibited reproducibility over 10 cycles and selective exposure to various gases generated during the food spoilage process. In an experiment involving pork spoilage, the color change was significant before and after exposure to ammonia gas within 8 h in ambient conditions. The proposed sensor can be integrated in bar codes and QR codes that are easily mass produced

Thermoelectric properties of thermally reduced graphene oxide observed by tuning the energy states

Reduced graphene oxide (rGO) possesses a similar electronic structure to graphene but can be synthesized on a larger scale. Hence, rGO is considered as an attractive alternative to graphene. Here we report the carrier transport properties of thermally reduced graphene oxide (TrGO) as a function of reduction temperature. The transfer curve of a field effect transistor fabricated with TrGO exhibited ambipolar properties, and the charge neutrality point of TrGO was shifted from negative to positive as the reduction temperature increased. Furthermore, as revealed in Arrhenius plots of the carrier densities and carrier mobilities, TrGO behaved as a metallic conductor at all reduction temperatures. To investigate the effect of reduction temperature on the thermoelectric properties of TrGO, the Seebeck coefficients of the fabricated TrGOs were calculated from the transfer curve using Mott's equation for metallic materials. All samples showed ambipolar carrier transport. At V-g = 0 V, the Seebeck coefficient switched sign from negative to positive as the reduction temperature became higher, indicating that electron and hole carrier transport dominates at higher and lower reduction temperature, respectively. The calculated Seebeck coefficients at zero gate bias were compared with the measured coefficients in TrGO bulk films. The thermoelectric properties of the measured and calculated coefficients showed similar trends with increasing reduction temperature, and the charged carrier transport (i.e., the energy states) of TrGO can be tuned by varying the reduction temperature without doping with impurities

Tunable Temperature Response of a Thermochromic Photonic Gel Sensor Containing N-Isopropylacrylamide and 4-Acryloyilmorpholine

In this study, thermochromic photonic gels were fabricated using 2-hydroxyethyl methacrylate (HEMA) as a hydrogel building block, and 4-Acryloyl morpholine (ACMO) and N-isopropylacrylamide (NIPAAM) as thermoresponsive monomers with different critical solution temperature behaviors. Rapid photopolymerization of opal-templated monomer mixtures of varying ACMO contents formed five individual thermochromic inverse opal photonic gels integrated on a single substrate. With temperature variation from 10 °C to 80 °C, the changes in reflective colors and reflectance spectra of the respective thermochromic gels were noted, and λpeak changes were plotted. Because NIPAAM exhibits a lower critical solution temperature (LCST) at 33 °C, the NIPAAM-only gel showed a steep slope for dλpeak/dT below 40 °C, whereas the slope became flatter at high temperatures. As the ACMO content increased in the thermochromic gel, the curve of dλpeak/dT turned out to be gradual within the investigated temperature range, exhibiting the entire visible range of colors. The incorporation of ACMO in NIPAAM-based thermochromic gels therefore enabled a better control of color changes at a relatively high-temperature regime compared to a NIPAAM-only gel. In addition, ACMO-containing thermochromic gels exhibited a smaller hysteresis of λpeak for the heating and cooling cycle

Colorimetric Humidity Sensor Using Inverse Opal Photonic Gel in Hydrophilic Ionic Liquid

We demonstrate a fast response colorimetric humidity sensor using a crosslinked poly(2-hydroxyethyl methacrylate) (PHEMA) in the form of inverse opal photonic gel (IOPG) soaked in 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM+][BF4−]), a non-volatile hydrophilic room temperature ionic liquid (IL). An evaporative colloidal assembly enabled the fabrication of highly crystalline opal template, and a subsequent photopolymerization of PHEMA followed by solvent-etching and final soaking in IL produced a humidity-responsive IOPG showing highly reflective structural color by Bragg diffraction. Three IOPG sensors with different crosslinking density were fabricated on a single chip, where a lightly crosslinked IOPG exhibited the color change response over entire visible spectrum with respect to the humidity changes from 0 to 80% RH. As the water content increased in IL, thermodynamic interactions between PHEMA and [BMIM+][BF4−] became more favorable, to show a red-shifted structural color owing to a longitudinal swelling of IOPG. Highly porous IO structure enabled fast humidity-sensing kinetics with the response times of ~1 min for both swelling and deswelling. Temperature-dependent swelling of PHEMA in [BMIM+][BF4−] revealed that the current system follows an upper critical solution temperature (UCST) behavior with the diffraction wavelength change as small as 1% at the temperature changes from 10 °C to 30 °C

Low-Power All-Organic Electrophoretic Display Using Self-Assembled Charged Poly(<i>t</i>‑butyl methacrylate) Microspheres in Isoparaffinic Fluid

The increasing demands for display devices with low power consumption and outdoor readability have stimulated comprehensive research into full-color reflective displays that employ color-tunable photonic crystal technologies. Although the recently developed crystalline colloidal arrays (CCAs) of the charged microspheres have shown the outstanding color tunability, the practical application is limited because the use of highly polar liquid medium such as water is required to maintain the charges on the surface of microsphere, whereas it is not suitable for long-term use in an electric field. Herein, a self-assembled CCA from charged poly­(<i>t</i>-butyl methacrylate) microspheres was successfully fabricated, which was stabilized by the charged inverse micelles of sodium di-2-ethylhexyl-sulfosuccinate in a nonpolar isoparaffinic fluid. A charged all-organic CCA was found to exhibit full-color tunability with a 1000-fold reduction in the power consumption (∼6 μW cm^–2) under a direct current voltage bias of 4 V in comparison to that in an aqueous system, which is a promising feature for a low-power-consumption display device