Abstract

Overexposure to metals can induce adverse health and environmental effects, thus monitoring metal concentrations is crucial. While current detection techniques are highly sensitive, they come at elevated costs, which limit their global use. As such, cheap and accessible sensors, providing both sensitivity and selectivity, are in high demand. With high surface area-to-volume ratios, tunable fluorescence and surface chemistries, nanoparticles are under investigation for development as metal sensors. However, many challenges exist including photobleaching, toxicity and a lack of selectivity.
Owing to their low cytotoxicity, water-dispersibility and photostability, carbon dots have emerged as interesting alternatives. These amorphous carbon-based particles are ~10 nm in diameter and mainly composed of carbon, oxygen and hydrogen. While they have been investigated in metal sensing applications, the focus remains primarily on the application rather than the fundamental understanding of the mechanism of carbon dot-metal interactions in solution.
In this work, we study the synthesis of carbon dots using several cheap and accessible precursors resulting in a surface decorated with functional groups such as amines, carboxylic acids and thiols. Following extensive purification protocols aimed at removing impurities that could bind to metal cations, we evaluate how these surface groups impact metal-carbon dot interactions. We demonstrate that some of our systems evidence sensitivity to Pb2+ and Hg2+ and exploit our knowledge of charge density and hard-and-soft acid-base theory to explain these findings and the underlying mechanism. This work provides a better understanding of metal-carbon dot interactions, which can allow us to design more sensitive and selective optical probes.