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Copper contamination to water bodies is an environmental concern of significant risk due to its harmful effect on aquatic life forms and human beings. Herein, a paper-based colorimetric sensor was fabricated by immobilizing polyethyleneimine-capped silver nanoparticles (PEI-AgNPs) on commercial filter paper. The sensitivity and selectivity of the paper-based sensor's colorimetric response to copper ions in water were investigated. PEI-AgNPs with an average diameter of 12.66 ± 4.07 nm were successfully immobilized on the filter paper by the simple dipping technique, as evidenced by the scanning electron microscopy with energy-dispersive x-ray result. A color change from pale yellow to dark yellow green was noted after exposing the paper sensor in a water sample with copper ions. This colorimetric response was exclusively observed for copper ions only, suggesting the selectivity of the paper sensor towards copper ions. Moreover, ultraviolet-visible spectroscopy results revealed that the detection limit of the paper sensor was observed to about 1.0 ppm. Meanwhile, color analysis on the sensor's digital images revealed the linear response of the sensor with decreasing copper ion concentration down to 1.0 ppm. The selectivity of the sensor was also observed by the color intensity profile of the sensor. This work presents promising results that can be utilized as a reference for developing affordable, fast, portable, and reliable sensing devices for on-site water quality monitoring and other applications.

This paper reports on the synthesis and application of Fe3O4/TiO2 nanocomposite. In situ attachment of TiO2 coating to the surface of the magnetic nanoparticles (Fe3O4) was attained by direct condensation of titanium precursors. Characterization result suggests that the average particle size of the synthesized nanocomposite is 10-15 nm. Also, FT-IR result confirms the presence of TiO2 layer in the surface of the magnetic nanoparticles. Furthermore, the prepared Fe3O4/TiO2 nanocomposite was utilized as an active magnetic nanophotocatalyst for the degradation of cyanide. Results show that even at 5.0 mg of Fe3O4/TiO2 photocatalyst, higher cyanide removal efficiency (91%) was obtained when 60 ppm CN- was incubated with the photocatalyst for 30 minutes. Likewise, it has been demonstrated that the synthesized magnetic nanophotocatalyst can be used to degrade cyanide using sunlight as the natural light source. A 94% cyanide removal efficiency was obtained when the sample was incubated with the synthesized magnetic nanophotocatalyst for 120 minutes under sunlight irradiation. Importantly, the prepared magnetic photocatalyst can be re-used several times (up to 5 cycles) without significant changes in the cyanide removal efficiency.

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