Carbon quantum dot synthesis of a Ag/PCQD composite for the visual detection of Hg2+ in dH2O solution and seawater samples

Abstract

In this study, carbon quantum dots (PCQDs) were synthesized via a simple, green, single-step ultrasonic method from palm mane with a fluorescence quantum yield (QY) of 38.5%. A stable silver nanoparticle/carbon quantum dot (Ag/PCQD) composite was prepared using PCQDs and silver nitrate with a color change from brown to red-brown. When Hg²⁺ from Hg(ClO₄)₂ was introduced to the Ag/PCQD composite in dH₂O solution and seawater samples, a color change from red-brown to colorless was observed. The interactions between the PCQDs, Ag/PCQDs and Ag/PCQDs after the addition of Hg²⁺ were investigated using TEM and XPS analyses. The analyses showed that the formation of a silver–mercury (AgHg) amalgam could be the working mechanism of this system. Moreover, we found that the Ag/PCQDs can also detect Hg²⁺ from HgCl₂, Hg(NO₃)₂, Hg(OAc)₂ and CH₃HgCl (organic mercury species) with good detection performance in dH₂O solution and seawater samples. It is vital to develop a sensing method that can detect Hg²⁺ in seawater samples. Therefore, an efficient sensing platform for detecting Hg²⁺ in pure water solution and seawater was constructed using the Ag/PCQD composite prepared from palm mane biomass. When Hg²⁺ from Hg(ClO₄)₂ was added to dH₂O and 3.5% saline solution, the calculated detection limits were found to be 35.4 and 14.3 nM, respectively. The detected concentration exceeds the Hg²⁺ concentrations commonly found in seawater (∼300–400 nM). Furthermore, it is possible to improve the on-site analysis of complex seawater samples with colorimetric response without the need for complex equipment. Our strategy provides a highly innovative, rapid and low-cost method for the detection of Hg²⁺ in complex water matrices and seawater samples.