(729c) Fabrication and Decoration of Zein-Based Electrospun Nanofiber Platforms for SERS Detection

As biosensors become more widely used, concerns about the amount of glass and synthetic polymers used in the fabrication of these biosensors increases and strategies to fabricate more eco-friendly biosensors are being explored. One eco-friendly strategy is to use natural polymers especially from agricultural and food origin. Examples include chitosan-based sensors (Shan et al., 2010) or zein-based sensors (Gezer, Liu, & Kokini, 2016). However, there are not many studies conducted on increasing the sensitivity of these food-grade biosensors.

Electrospun nanofibers are perfect candidates for the production of SERS (Surface Enhanced Raman Spectroscopy) biosensors due to their very high surface area to volume ratio, since this high surface area can accommodate a high concentration of metallic nanoparticles to obtain a high intensity SERS signal. The objectives of this study are to fabricate bead-free zein electrospun fibers with uniform diameter distribution and to successfully decorate the zein nanofibers with gold and silver nanoparticles with enough nanoparticle concentration to obtain necessary enhancement of the SERS signal. First, we studied the effect of the solvent used on the quality and uniformity of the electrospun fibers. 70% (v/v) ethanol and glacial acetic acid were used to prepare zein solutions for electrospinning since they are the two best solvents for zein. Zein solutions were prepared at 30 wt% zein concentration and the electrospun fibers were produced with dispensing tips having an inner diameter of 0.635 mm. 20 kV voltage difference was applied and the distance between the needle and the collecting plate was kept constant at 10 cm. Optical microscopy images of the fibers showed that while zein fibers spun from 70% ethanol had multiple bead formations and fiber breakages, fibers spun from acetic acid solutions had bead-free and continuous structures with a homogenous diameter distribution. For the decoration of the nanofibers, then, acetic acid was chosen as the solvent. Metal nanoparticle decoration was tested on fibers prepared from solutions with three different zein concentration; 30 wt%, 28 wt% and 26 wt% since this corresponds to the optimal viscosity window for electrospinning. Solutions with 30 wt% were electrospun with dispensing tips having 0.635 mm inner diameter and solutions with 28 wt% and 26 wt% solutions were electrospun with 0.381 mm inner diameter tips.

For the decoration of the nanoparticles, two techniques were used; drop deposition technique on the dried fibers, and incorporation of the nanoparticles into the polymer solution before electrospinning. Scanning electron microscopy (SEM) images showed that all fibers without any nanoparticle decoration had smooth surfaces, bead-free structures and homogenous diameter distributions. The fibers spun from 30 wt% solutions had an average diameter of 1µm, whereas nanoscale diameters (200-300 nm) were achieved with 26 wt% and 28 wt% solutions. For the metal decoration 20 nm gold nanoparticles were chosen. In drop deposition technique, 3 µl of gold nanoparticle colloids were placed on nanofiber mats, and dried in desiccators at room temperature. The SEM results showed that while fibers spun from 30 wt% solutions did not experience significant deformations, fibers from 26 wt% and 28 wt% solutions experienced some swelling at fiber junction points due to their smaller diameters and faster water absorption. However, this deformation in thinner nanofibers did not progress further after multiple depositions on the same spot. Chemical crosslinking will be used to further decrease the swelling of nanofibers.

The second way of decoration, incorporation of the gold nanoparticles into the polymer solution prior to the electrospinning step, did not lead to any particles on the surface of the nanofibers, thought to be due to not reaching to enough nanoparticle concentration and to very rapid drying of the fibers. Here, zein-based electrospun nanofibers are shown to be good candidates for biodegradable SERS sensor platforms since it is much easier to produce nano-sized structures with fibers without using any expensive lithographic techniques and since the nanoparticle concentration is easily controllable especially with drop deposition technique.

Work in progress consists of analyzing the effect of crosslinking using glutaraldehyde where our lab has much experience (Turasan, Barber, Malm, & Kokini, 2018), in order to decrease the swelling of nanofibers during drop deposition technique.

To test the effectiveness of the decoration process, SERS measurements will be conducted using both gold and silver nanoparticles at high enough concentrations to create significant proximity between the nanoparticles using the different zein nanofiber formulations. Our laboratory has a decade long experience with SERS (Barber, Gezer, Liu, & Kokini, 2018; Gezer, Brodsky, Hsiao, Liu, & Kokini, 2015; Gezer, Hsiao, Kokini, & Liu, 2016; Gezer, Liu, et al., 2016a; Gezer, Liu, & Kokini, 2016b). The totality of these experiments in this abstract and fabrication rules which emerge, will be presented as part of this paper.

References

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