Highly stable water dispersible calix[4]pyrrole octa-hydrazide protected gold nanoparticles as colorimetric and fluorometric chemosensors for selective signaling of Co(II) ions

•Calix[4]pyrrole octahydrazide (CPOH) has been used as a reducing and stabilizing agent.•One pot method to produce water dispersible and stable (150 days) CPOH-AuNPs.•CPOH-AuNps are selective and sensitive (1 nM to 1 μM) fluorescent sensors for Co(II).•Size 8 nm and stable at neutral pH must find various biological/biomedical applications.

Water dispersible stable gold nanoparticles (AuNps) have been synthesized by using calix[4]pyrrole octa-hydrazide (CPOH) as a reducing as well as stabilizing agent. CPOH-AuNps have been characterized by surface plasmon resonance, particle size analyzer and transmission electron microscopy. CPOH-AuNps are water dispersible, highly stable for more than 150 days at neutral pH with a size of less than 10 nm and zeta potential of 15 ± 2 MeV. Ion sensing property of CPOH-AuNps has been investigated for various metal ions Pb(II), Cd(II), Mn(II), Fe(III), Ni(II), Zn(II), Hg(II), Co(II) and Cu(II) by colorimetry and spectrofluorimetry. Among all the metal ions investigated, only Co(II) ions gives sharp colour change from ruby red to blue and is easily detectable by naked-eye. CPOH-AuNps being fluorescent in nature also shows great sensitivity and selectivity for Co(II) ions. Co(II) ions can be selectively detected at very low concentration level of 1 nM in a facile way of fluorescence quenching.

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