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A colorimetric probe based on a hybrid sensing system of gold nanoparticles (AuNPs), silver nanoparticles (AgNPs), and thymine (Thy) was developed for easy and rapid detection of copper (II) ions (Cu2+) in solution. The underlying principle of this probe was the Cu2+-triggered aggregation of the nanoparticle components. Color change of the sensing solution (from red to purple) was clearly observed with naked eyes. The experimental parameters, including pH and concentration of tris buffer, thymine concentration and AgNP dilution ratios, were investigated and optimized. Once optimized, the limits of detection were found to be 1, 0.09 and 0.03 ppm for naked eyes, smartphone application and UV-vis spectrophotometer, respectively. Furthermore, determination of Cu2+ was accomplished within 15 min under ambient conditions. For quantitative analysis, the linearity of detection was observed through ranges of 0.09−0.5 and 0.03−0.5 ppm using smartphone application and UV-vis spectrophotometer, respectively, conforming to the World Health Organization guideline for detection of copper at concentrations < 2 ppm in water. This developed hybrid colorimetric probe exhibited preferential selectivity toward Cu2+, even when assessed in the presence of other metal ions (Al3+, Ca2+, Pb2+, Mn2+, Mg2+, Zn2+, Fe3+, Ni2+, Co2+, Hg2+ and Cd2+). The developed procedure was also successfully applied to quantification of Cu2+ in real water samples. The recovery and relative standard deviation (RSD) values from real water sample analysis were in the ranges of 70.14−103.59 and 3.21−17.63%, respectively. Our findings demonstrated a successful development and implementation of the Thy-AuNP-AgNP hybrid sensing system for rapid, simple and portable Cu2+ detection in water samples using a spectrophotometer or a smartphone-based device.

This study develops a simple and low-cost 3D paper-based analytical device (3D PAD) for the detection of available phosphate in soil. Epoxy resin is presented as a new hydrophobic material for low-cost mass production of PADs using the screen-printing method. An optimized concentration of epoxy resin solution is screen printed onto Whatman filter paper no. 1 in an easy one-step process to create hydrophobic patterns on PADs. The epoxy resin is air dried at room temperature, without heating or UV curing. This method delivers high reproducibility, resolution, and stability, and the epoxy resin barrier is compatible with both organic solvents and aqueous solutions. The molybdenum blue method is used in this PAD to measure phosphate in a colorimetric assay. The developed 3D PAD attains a linear range of 0.5-40 mg L -1 , with a limit of detection (LOD) of 0.25 mg L -1 , and a limit of quantitation (LOQ) of 0.83 mg L -1 . The relative standard deviation of intra-day measurements is 1.52-2.46%, and the inter-day standard deviation is 1.89-2.74%, indicating satisfactory reproducibility. This 3D PAD was tested for its ability to detect phosphate in a variety of actual soil samples and the results were validated against spectrophotometric analysis using a paired t -test, which showed high accuracy. In short, the new analytical device described in this study is simple, fast, and inexpensive to make and use, providing a versatile phosphate detection tool for many soil types, even in situations when resources are limited.

This study reports a convenient and low cost paper-based analytical device (PAD) for measuring ammonium in soil. This PAD has been developed using an inexpensive UV resin solution, as a new hydrophobic material, with a screen-printing method to create hydrophobic areas. The paper-based colorimetric device was developed using a modified Berthelot reaction in which salicylate and dichloroisocyanurate are used in order to produce a green compound of 2-2 dicarboxyindophenol in the presence of ammonium. The concentration of ammonium is proportional to the intensity of the resulting green color, as analysed by ImageJ software. A variety of tests was carried out to optimize and evaluate various aspects of the PAD's fabrication and utilization. A linear range was obtained in the range of 10-100 mg L-1 with a limit of detection and limit of quantitation of 0.5 mg L-1 and 1.7 mg L-1, respectively. The relative standard deviation of intra-day measurements was 3.0% and the inter-day precision was 3.2% with good reproducibility. When recovery of ammonium added to samples was evaluated it ranged from 95.5 to 107.5%. The PAD was applied to detect ammonium in a variety of actual soil samples, and the results were validated against spectrophotometric results using a paired t-test, showing good accuracy. A convenient color comparison chart was also created on paper to enable the user to interpret the color results without the need for a color scanner. This developed PAD is fast, easy, inexpensive, and provides an alternative to existing methods for the determination of ammonium in soil.