l-Tryptophan-capped carbon quantum dots for the sensitive and selective fluorescence detection of mercury ion in aqueous solution

l -Tryptophan-capped carbon quantum dots ( l -CQDs) were facilely synthesized through “green” methodology, and the obtained material was utilized as a sensitive and selective fluorescence sensor for mercury ion (Hg 2+ ) in pure aqueous solutions. Carboxyl-functionalized CQDs were first green synthesized by a one-step hydrothermal route, and l -tryptophan was then attached to CQDs via direct surface condensation reaction in aqueous solution at room temperature. The as-synthesized l -CQDs had an average size of ca. 5 nm with a good dispersity in water, and exhibited a favorable selectivity for Hg 2+ ions over a range of other common metal cations in aqueous solution (10 mM PBS buffer, pH 6.0). Upon the addition of Hg 2+ , a complete fluorescence quenching (ON–OFF switching) of l -CQDs was evident from the fluorescence titration experiment, and the fluorescence detection limit of Hg 2+ was calculated to be 11 nM, which indicated that the obtained environmentally friendly l -CQDs had sensitive detection capacity for Hg 2+ in aqueous solution.