Determination of iron(II) and iron(III) via static quenching of the fluorescence of tryptophan-protected copper nanoclusters.

"Tryptophan-coated blue fluorescent copper nanocluster (CuNC@Trp) was prepared by a strategy where Trp acts as both the reducing and capping agent. The fluorescence of the CuNC, with excitation/emission peaks at 340/405 nm, is selectively quenched by iron(II) and iron(III) ions. Studying the mechanism of this interaction revealed that Fe2+ and Fe3+ ions can make a ground state complex with the protecting ligand which can result in quenching of the cluster emission. Structural and optical properties of the modified CuNC were investigated by ESI-MS, DLS, TEM, UV-vis and photoluminescence. The effects of pH value and temperature, time of interaction, and cluster volume were optimized. Under optimized conditions, the probe response is linear in concentration range of 10-1000 μM for Fe(II) and Fe(III) with the relative standard deviations of 0.13 and 0.14% (n = 5) respectively. The respective limits of detection are 3.0 and 2.2 μM. The method was successfully used for determination of trace amount of both ions in spiked water, blood and iron supplement tablets. The results were in good agreement with those obtained by the ICP-AES method." Graphical abstractThe scheme represents the synthesis of CuNC@Trp at basic conditions and at elevated temperature. The emission of the cluster decreases due to static quenching of fluorescence by iron ions.