Thermo-responsive electrospun nanofibers doped with 1,10-phenanthroline-based fluorescent sensor for metal ion detection

Heavy metal ion pollution is one of the important environmental problems, and many efforts have been devoted to develop highly sensitive sensors for detection of heavy metal ions in environment. In this study, an 1,10-phenanthroline-based fluorescent sensor (F-phen) was synthesized and doped into poly(N-isopropylacrylamide-co-N-hydroxymethyl-acrylamide) (Poly(NIPAAm-co-NMA)) nanofiber films via simple electrospinning technique, where the moieties of NIPAAm and NMA were designed to exhibit thermo-responsive and chemical crosslinking functions, respectively. The electrospun nanofibers prepared from the appropriate composition of NMA in the copolymer (P3) could stabilize their fibrous morphology in aqueous environment. Among different weight ratios of F-Phen doped in nanofibers, P3-3 (3 wt% corresponding to copolymer) showed significant quenching of fluorescence as chelating with Cu2+ ion. The linear Stern–Volmer plot of nanofibers for Cu2+ was observed, and the calculated Stern–Volmer constant (K sv ) was 5.5 × 104 M−1. As compared to dip-coating films, the downward deviation to linear Stern–Volmer plots of thin films was observed, indicating a fraction of F-Phen was inaccessible to the quencher (Cu2+). The enhanced sensitivity of F-Phen-doped electrospun nanofibers is attributed to their higher surface area compared to dip-coating films. In addition, the F-Phen-doped nanofibers also exhibited an on/off switchable sensing behavior. This is due to the hydrophilic–hydrophobic change of PNIPAAm as varying temperature that controlled the metal ions to be accessible to the sensors. All of the results indicate the thermo-responsive electrospun sensing nanofibers not only enhance the sensitivity and have an on–off sensing characteristic, but also provide a simple strategy to design and fabricate effective sensing materials for detection of metal ions.