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.

Loading of Au/Ag Bimetallic Nanoparticles within and Outside of the Flexible SiO2 Electrospun Nanofibers as Highly Sensitive, Stable, Repeatable Substrates for Versatile and Trace SERS Detection

“…Among of them, the diffraction peak at 23.1° belongs to the amorphous SiO 2 phase, while the peaks at 2θ = 38.1°, 44.3°, 64.3°, 77.4°, and 81.5°are assigned to the (111), (200), (220), (311) and (222) crystal planes of face-centered cubic (fcc) Au (JCPDS, No. 04-0784), respectively [ 7 , 23 ]. After the TA-APTES modification, the diffraction peaks of T-A@SiO 2 @Au-20 nanofibers do not show significant changes, indicating that the modification TA-APTES on the nanofiber’s surfaces has no effect on the crystal structure of SiO 2 @Au nanofibers.…”

“…The second step occurs from 100 to 350 °C; the sample loses 7.7% of its mass, which can be attributed to the dehydration of TEOS, the decomposition of PVP side chains and HAuCl 4 . The last step with a significant mass loss occurs from 350 to 500 °C—the weight loss is about 50.1%, which is assigned to the decomposition of the PVP skeleton and the condensation reaction of TEOS [ 23 , 27 ]. Note that there is almost no mass loss for the SiO 2 @Au precursor nanofibers after 500 °C, suggesting the complete formation of SiO 2 @Au nanofibers.…”

“…It can be found from this figure that all SERS spectra show a high degree of uniformity in both the position and intensity of peaks. There are several main SERS peaks at 733, 1327, 1444 and 1576 cm −1 , corresponding to the vibrational absorption of adenine, guanine, saturated lipids and amides in proteins [ 23 ]. At the same time, as shown in Figure 10 b, the peak intensity at 733 cm −1 remains stable, and the relative standard deviation (RSD) is calculated to be 6.1%, indicating the good homogeneity or repeatability of the as-prepared Ag@T-A@SiO 2 @Au-20 substrates.…”

“…The setup of electrospinning device was described in our reported work [ 23 ]. SEM images were obtained by using a scanning electron microscope (FEI Nova NanoSEM 450, Thermo Fisher Scientific, Waltham, MA, USA), at acceleration voltage of 30 kV, a working distance of 5 mm, and a spot size of 3.0 nm.…”

“…Tang et al employed electrospinning and pyrolysis to successfully synthesize silica nanofibrous membranes containing Ag nanoparticles, which showed good SERS activity for enrofloxacin [ 22 ]. Our group prepared Ag@polydopamine@SiO 2 nanofiber mats through electrospinning and surface chemical modification for the label-free SERS detection of bacteria [ 23 ]. However, the above studies only reported the preparation of SiO 2 nanofibers loaded with single noble metal nanoparticles for SERS detection, and the analytes are specific.…”