Advanced Sensors from <i>Acalypha indica</i> Weed-Derived Carbon Dots for Local Hot-Spot Promoted Nitrite Detection

Nitrite is a harmful environmental pollutant that can contaminate water and soil due to excessive fertilizer use, industrial discharge, and microbial activity with overgrowing global population; this has led to increasing nitrite concentration in soil, eventually affecting human health and the environment. This warrants the development of advanced methods for effective nitrite detection. Hence, the present study introduces an approach for the synthesis of biogenic carbon dots from Acalypha indica weed (WCDs), an unexplored precursor. The synthesized WCDs exhibit high quantum yield, exceptional photostability, and solar absorbance acting as efficient “nanoheaters”. Sensing studies found that WCDs demonstrate highly selective and ultrasensitive nitrite detection performance with limit of detection (LOD) and limit of quantification (LOQ) values of 9 and 27.2 nM, respectively, even in the presence of other anions. Paper-based kits and composite films containing WCDs in the chitosan matrix crosslinked using triformylphluorglucinol (CS-WCD-TFP) were fabricated by integrating WCDs, which significantly improved the practical applicability and accessibility of the method for real-world applications. The LOD and LOQ values with the composite films were found to be 9.7 and 28.7 nM, respectively. Real-sample analysis was performed using several water samples and even raw vegetables. Studies demonstrated excellent recoveries of between 98 and 101% and a minimal standard deviation of ≤ 0.4%. Hence, through these studies, we envisage a fluorescence-based rapid, accurate, sustainable, and cost-effective approach for optosensing of nitrite, which proves to be an excellent alternative to traditional techniques. Additionally, the exceptional local hot-spot effects facilitate significant photothermal conversion, generating localized thermal gradients under solar illumination, substantially accelerating the binding kinetics between the WCDs and nitrite. This substantially decreases the interaction time between the analyte and the sensor from 10 to 1 min, enhancing the optical sensing ability. This study also highlights the untapped potential of solar energy for environmental sensing technologies and green chemistry through carbon dots.