Vitamin B₆ Cofactor-Conjugated Fluorescent Polymeric Nanoparticles for Nanomolar Detection of Cu(II) and Fe(II) and Their Application in Edible Items

In the present study, vitamin B 6 cofactor pyridoxal 5′-phosphate (PLP)-conjugated yellow-emitting polymeric nanoparticles (NPs) were synthesized, characterized, and applied for the sensing of metal ions. The branched polyethyleneimine (PEI) was cross-linked by adding formaldehyde and self-aggregated to form the cyan-blue-emitting PEIF NPs. The covalent conjugation of PLP over the surface of PEIF NPs via forming an imine linkage between the −NH 2 group present over PEIF and the −CHO group of PLP formed the yellow-emitting PEIFPLP NPs. The spectroscopic (ultraviolet–visible (UV–vis), fluorescence, Fourier-transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS)) and high-resolution transmission electron microscopy (HRTEM) analyses were performed to characterize the PEIFPLP NPs. The PEIFPLP NPs showed good photostability, and the fluorescence emission at 552 nm (λ ex = 450 nm) was stable over a wide pH range from 2 to 12. In the metal ions’ sensing experiments, the fluorescence emission of PEIFPLP NPs was quenched by Cu(II) and Fe(II) among the other examined metal ions such as Ca(II), Mg(II), Cd(II), Mn(II), Co(II), Ni(II), Zn(II), Fe(III), Pt(II), Pb(II), Cr(III), Hg(II), Al(III), and Cu(I). The static quenching occurred due to the electron transfer from the excited PEIFPLP NPs to the partially filled d-orbital of Cu(II) and Fe(II). The PEIFPLP NPs showed good linearity between 2.4 × 10 –6 to 1.9 × 10 –5 and 7.4 × 10 –6 to 2.2 × 10 –5 M with a nanomolar detection limit of 17 and 31 nM for Cu(II) and Fe(II), respectively. Further, the analytical applicability of the developed PEIFPLP NPs was validated by quantifying Cu(II) and Fe(II) ions in fruits, vegetables, and environmental water samples.