An ultrasmall chitosan nanosphere encapsulating carbon dots and rhodamine B as a ratiometric probe for the determination of Hg2.

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Citation abstract

Hg 2+ -sensitive carbon dots (CDs) were synthesized by microwave-assisted pyrolysis of citric acid, sodium fluoride, and urea. The CDs as a signal report unit and rhodamine B (RhB) as a reference were then encapsulated in a nanosphere of chitosan assembled by a nonsolvent-induced chitosan colloidal formation and in situ cross-linking to construct a ratiometric probe for Hg 2+ (chitosan-CDs-RhB). Interestingly, without any assistance from acids to improve the solubility of chitosan, the nanosphere containing CDs and RhB had an ultrasmall size of 9.7 nm with only approximately 1.1-nm-thick layers of chitosan enclosing one dot. In order to keep the residual functional groups on the nanosphere from compromising the fluorescence response of CDs to Hg 2+, Co 2+ was used as a fluorescently intact metal ion to saturate the functional groups. The saturated chitosan-CDs-RhB was thus potentially developed for determining Hg 2+ in the fruit bodies and mycelia of edible and medicinal fungi. Limits of detection (LODs) of 2.24, 5.29, and 2.03 μM and recoveries in the ranges 98.3 to 101.8%, 99.5 to 104.6%, and 97.4 to 100.9% were estimated for the determination of Hg 2+ in the fruit bodies of Pleurotus ostreatus, Lentinus edodes, and Hypsizygus marmoreus, respectively. Chitosan-CDs-RhB was further developed as a fluorescent ratiometric probe for quantitatively determining intracellular Hg 2+ in fungal mycelia with a linear calibration curve of R Igreen/Ired = - 0.145c + 1.69 within the range 0.013 to 0.356 μg g -1. Graphical abstract An ultrasmall chitosan nanosphere encapsulating carbon dots and rhodamine B as a ratiometric probe for the determination of Hg 2+.