Sensing of iron(III)-biomolecules by surfactant-free fluorescent copper nanoclusters

Surfactant-free copper nanoclusters (sf-CuNCs) synthesized in the absence of external stabilizer, could be promising fluorescence biosensors because of the presence of relatively exposed surfaces that may ease coupling with biomolecules. This article shows that sf-CuNCs are effective fluorescence sensor of iron(III)-biomolecules: hemin, cytochrome C (cyt C), and ferritin. Although, sf-CuNCs are found to be weaker sensor of aqueous Fe3+ (Stern-Volmer constant (KSV) 3.0 × 103 M−1 and limit of detection (LOD) up to 2 μM) compared to other capped CuNCs, however, the sensing ability markedly increases in the presence of Fe(III)-biomolecules. KSV (LOD) are 5.3 × 104 M−1 (0.8 μM), 9.0 × 105 M−1 (68 nM), 1.6 × 106 M−1 (16.50 nM), respectively for cyt C, hemin and ferritin. We carried out detailed mechanistic investigations of the quenching process using time-resolved fluorescence, zeta potential and transmission electron microscopy (TEM) measurements. We found that fluorescence quenching is associated with complexation of Fe3+ with CuNCs which changes zeta potential of the sf-CuNCs and leads to aggregation of CuNCs. Moreover, fluorescence quenching, zeta potential and aggregation of CuNC can be reversed by addition of vitamin C which is a very strong chelator and reducing agent of Fe(III). Vitamin C reduces Fe(III) to Fe(II) and promotes disassembly of CuNCs-aggregates to regenerate fluorescence intensity. Since, the sensing is primarily governed by complex formation with Fe3+, absence of ligands on the surface of sf-CuNCs may be detrimental to quenching to aqueous Fe3+ but favors binding with the biomolecules. Keywords: Copper nanocluster, Proteins, Fluorescence, Iron sensing, TE