Preparation of Ag@PDA@SiO2 electrospinning nanofibrous membranes for direct bacteria SERS detection and antimicrobial activities

Abstract

In this paper, a convenient method to fabricate flexible and free-standing surface-enhanced Raman scatting (SERS) substrates for direct bacteria detection without aptamer bonding is presented. SiO2 nanofibers were prepared via electrospinning and calcination by using tetraethyl orthosilicate as the precursor. Subsequently, it was coated with polydopamine (PDA) by self-polymerization. Finally, Ag@PDA@SiO2 nanofibrous membranes were obtained through in situ growth of Ag nanoparticles in Tollens’ reagents. The as-prepared Ag@PDA@SiO2 composite nanofibrous membranes were characterized by techniques of scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction, energy-dispersive x-ray spectroscopy and thermo gravimetric analysis. The flexibility of the as-prepared nanofibrous membranes were verified simply through manual folding. Small molecule probes of 4-mercaptophenol (4-MPh) and 4-mercaptobenzoic acid (4-MBA) were chosen to investigate the SERS sensitivity of the as-prepared Ag@PDA@SiO2 as free-standing substrates. Furthermore, Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus), as typical strains of Gram-negative and Gram-positive bacteria, were performed to directly SERS detection by dropping bacteria suspension onto the fibrous membranes without any previous treatment, such as aptamer combination. In addition, the antimicrobial properties of the as-prepared Ag@PDA@SiO2 electrospinning nanofibrous membranes were tested by inhibition zone and turbidity methods. The results show that Ag nanoparticles with an average diameter of 50 nm are uniformly deposited on the surface of electrospinning nanofibers, and the as-prepared Ag@PDA@SiO2 nanofibrous membranes are flexible. As SERS substrates, it shows a rather high detection limitation of 10−11 mol l−1 for 4-MPh and 4-MBA. More importantly, this substrate can be applied for bacteria label-free SERS detection, i.e., complicated procedures are avoided. Meanwihile, the as-prepared Ag@PDA@SiO2 nanofibrous membranes exhibit excellent antibacterial properties. Thus, it has application prospects in trace bacteria detection and water purification.