Coaxial electrospinning of Au@silicate/poly(vinyl alcohol) core/shell composite nanofibers with noncovalently immobilized gold nanoparticles to prepare flexible, freestanding, and highly sensitive SERS substrates amenable to large-scale fabrication

Abstract

In this study, coaxial electrospinning was used to prepare novel core/shell composite nanofiber films for use as substrates in surface-enhanced Raman spectroscopy (SERS). The shell layer was composed of Au@silicate nanohybrids, i.e., Au nanoparticles (AuNPs) immobilized in silicate nanosheets via electrostatic attraction. The core layer consisted of polyvinyl alcohol (PVA), which functioned as the mechanical support of the composite nanofibers. By using coaxial electrospinning to form the Au@silicate/PVA core/shell composite nanofibers, the Au@silicate nanohybrids were uniformly dispersed on the PVA nanofibers. Transmission electron microscopy revealed that the AuNPs immobilized on the silicate nanosheets had particle sizes of 25–30 nm and uniform spherical shapes. Furthermore, the core/shell composite nanofiber film had a three-dimensional network structure owing to its formation via random deposition. Compared with the uniaxially electrospun nanofibers, the core/shell composite nanofibers required a significantly lower AuNP content to achieve the same SERS sensitivity. Furthermore, the Au@silicate nanohybrid shell layer increased the ultimate tensile strength by a factor of almost 1.5. In SERS experiments, the uniaxially electrospun nanofiber had an enhancement factor (EF) of 4.1 × 104, whereas the coaxially electrospun nanofiber had an EF of 1.7 × 105. It was demonstrated that our nanofiber can be used for environmental, food safety, and biomedical applications, as it has a detection limit of 10-7 M and EF of 2.5 × 105 for Direct Blue 200 (an industrial dye) and detection limits lower than 10-6 M for paraquat (a pesticide) and adenine (a biomolecule). Therefore, Au@silicate/PVA core/shell composite nanofibers can be used to prepare portable, flexible, freestanding, and stable SERS substrates that are amenable to large-scale fabrication. Furthermore, films prepared using Au@silicate/PVA core/shell composite nanofibers are less susceptible to the “coffee ring” effect observed in conventional droplet coatings, which improves the reproducibility and uniformity of SERS analysis. The developed SERS substrate has a wide range of applications, and the findings of this study are expected to improve sensor technologies for environmental, food safety, and biomedical applications.