Highly photoluminescent carbon dots: a multifunctional platform for pH sensing, nano thermometry, and mercury(ii) detection

Abstract

Carbon dots (CDs), a class of heavy-metal-free fluorescent nanomaterials, have attracted significant attention due to their excellent optical properties, chemical stability, low toxicity, and biocompatibility. In this study, highly photoluminescent N-CDs with small sizes and exceptional quantum yields (up to 90%) were successfully synthesized using citric acid and tri-(2-aminoethyl)amine via a hydrothermal method. The synthesized N-CDs exhibit a single emission band at 450 nm under 380 nm excitation and dual emission bands at 460 nm and 581 nm under 258 nm excitation. Notably, their fluorescence emission spectra display strong temperature dependence and pH-responsive behaviour, rendering them highly versatile as nano thermometric devices. Specifically, their emission intensity demonstrates remarkable sensitivity across the temperature range of 298–343 K, enabling precise thermal measurements. Moreover, these N-CDs function effectively as pH sensors, further expanding their utility. Additionally, the fluorescence of the N-CDs is rapidly and selectively enhanced in the presence of Hg²⁺ ions at room temperature, without requiring any surface modification. This enhancement exhibits a linear relationship within the concentration range of 0–10 μM, with a detection limit as low as 0.46 μM. These findings highlight the multifunctional nature of the synthesized N-CDs, which hold great promise for applications in pH sensing, nano thermometry, and the specific detection of mercury ions.