Preparation of thermo-responsive electrospun nanofibers containing rhodamine-based fluorescent sensor for Cu2+ detection

A series of Cu2+-sensing nanofibers has been successfully prepared by electrospinning of poly[(N-isopropylacrylamide)-co-(N-hydroxymethyl acrylamide)-co-(4-rhodamine hydrazonomethyl-3-hydroxy-phenyl methacrylate)] [poly(NIPAAm-co-NMA-co-RHPMA), PNNR] random copolymers. These PNNR copolymers were synthesized by free radical copolymerization of three monomers, thermo-responsive NIPAAm, chemically crosslinkable NMA and Cu2+-sensing RHPMA, with the composition of RHPMA in the range of 2.4–16.3 wt%. In acidic environments, the PNNR copolymers showed highly selective and sensitive recognition and displayed “ON-OFF” fluorescence toward Cu2+ both in solution and in solid state (thin films and nanofibers). From the quantitative analysis via Stern-Volmer plots, PNNR nanofibers exhibited comparable Stern-Volmer constants as those of PNNR solutions in the order of 104 M−1, which are much higher than those of PNNR thin films. The enhanced sensitivity of PNNR electrospun nanofibers is attributed to their higher surface area compared to dip-coating films. The PNNR nanofibers also exhibited an on/off switchable sensing behavior in response to temperature change due to the hydrophilic-hydrophobic transition of PNIPAAm. In addition, the binding of PNNR with Cu2+ is chemically reversible both in solution and in nanofibers with the treatment of Na4EDTA.