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Adsorption-mediated water treatment leaves adsorbents as secondary pollutants in the environment. However, photocatalysis aids in decomposing the contaminant into its nontoxic forms. In this context, we demonstrate an adsorption−photocatalysis pairing in Au− CeO 2 nanocomposites for a total methylene blue (MB) removal from water. We synthesized Au−CeO 2 through the citrate (cit) reduction method at different Au loading and studied its adsorption capacity with kinetics and thermodynamic models. We observe that the high adsorption capacity of Au−CeO 2 is primarily because of the presence of Ce 3+ states in CeO 2 and citrate ligands on Au NPs. The Ce 3+ states interact and transfer their electrons to supported Au NPs, rendering a negative charge over Au. The negatively charged Au surface and the carboxyl (−COO − ) group of citrate ligands mediate an electrostatic interaction/adsorption of cationic MB. The total removal of MB is expedited under white light and lasers. A control experiment with Au NPs shows less adsorption−photocatalysis. The size of Au NPs and Au−CeO 2 interfacial interaction is responsible for the surface plasmon resonance spectral position at 550−600 nm. Linear sweep voltammetry (LSV) and plasmonic field simulation show surface plasmon-driven photocatalysis in Au−CeO 2 . LSV shows a 3-fold higher photocurrent density in Au− CeO 2 than colloidal Au NPs under white light. The simulated electric field intensity in Au−CeO 2 is maximum at SPR excitation and the closest interfacial separation (d = 0 nm). The plasmon-driven photocatalysis in colloidal Au NPs is mainly due to the interaction of hot electrons with the adsorbed MB molecule. Notably, near-field light concentration, hot electrons, and interfacial charge separation are responsible for excellent MB removal in the Au−CeO 2 nanosystem. The total MB removal through adsorption− photocatalysis pairing is 99.3% (Au−CeO 2 ), 30.7% (Au NPs), and 13% (CeO 2 ).

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Nitrite level estimation in soil and water bodies is critical to monitor the ecosystem of our environment and agricultural yield. Herein, we report a carbon nanodots (C-dots) and neutral red (NR)-based photometric and fluorescence mode sensing scheme using which trace detection of nitrite in water and soil has been done. The proposed scheme utilizes a handheld compact sensing platform developed on a smartphone. For the present sensing studies, the C-dots and NR act as donors and acceptors, respectively, in the frequency resonance energy transfer (FRET) process. The presence of nitrite in the C-dot–NR mixture affects the FRET process, which causes a decrease in the absorption wavelength conditions at 560 nm while increasing the fluorescence intensity at 563 nm upon excitation with a source of wavelength 375 nm. These variations have been correlated to quantify the concentration of the nitrite level in the medium. With the designed sensing system, variations of nitrite concentration from 0.1 to 3.0 μg/mL have been measured with a high degree of accuracy. The designed sensor has been implemented to estimate the nitrite level of infield water and soil samples, and the experimental results are compared with the laboratory standard tools. With the advantages of being low-cost, field portable, and relatively convenient to handle, it is anticipated that the proposed smartphone sensor could emerge as a potential avenue for onsite assessment of other parameters of water and soil as well.

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