(340as) Development of a SERS Biodegradable Platform Using Electrospun Soy Protein Isolate Fibers for the Detection of Food Analytes

Research Interests: my research interests lie in developing Surface-Enhanced Raman spectroscopy biodegradable sensors that can be used to detect harmful analytes present in foods as a food safety tool.

Abstract

Introduction:

Surface-enhanced Raman spectroscopy (SERS) sensors have become a rapid, reliable, and efficient analytical technique for the detection of a wide range of analytes in food safety, medical and environmental fields. The applications of SERS have increased with the incorporation of noble metals such as gold, silver, copper, aluminum, and synthetic materials such as graphene, polyvinyl alcohol, and polydimethylsiloxane. However, the increase of synthetic materials in these devices raises the concern of their deposition in the environment. Due to this, the quest for replacing synthetic materials with natural materials that can be sustainable for the environment has been gaining a place. Therefore, natural polymer materials like zein, chitosan, or cellulose have been used to fabricate SERS sensors.

Electrospun nanofibers have become an alternative to be used as a SERS platform due to their high surface area that can catch noble metal nanoparticles that can enhance the weak signals in Raman spectroscopy. Electrospinning is a technique that enables the fabrication of fibers from a polymer solution by applying an electrostatic field on a drop of that solution.

Soy protein is one of the most abundant plant proteins globally, consisting of globulins classified as 2S, 7S, 11S, and 15S. Soy protein isolate (SPI), soy protein concentrates, and soy flour are the most common products obtain from soy protein. SPI is obtained from the de-hulled, and de-fatted soybean flakes after oils and fats are separate from soybeans. Electrospun fibers from SPI are possible by applying a denaturation treatment such as heat or alkaline solution to enable fibers' production to unfold its globular structure.

This research aims to fabricate a surface-enhanced Raman spectroscopy (SERS) biodegradable sensor using Soy Protein Isolate (SPI) nanofibers as a platform. SPI nanofibers are functionalized with noble metal nanoparticles to increase SERS signals.

Methods:

First, SPI fibers were fabricated using the electrospinning technique. Then, two solutions of SPI in water were prepared as follow: 10 wt% SPI + 5 wt% polyethylene oxide (PEO, Mw: 0.1 MDa) + 1 wt% NaOH and 12 wt% SPI + 5 wt% polyethylene oxide (PEO, Mw: 0.1 MDa) + 1 wt% NaOH were mixed at a temperature of 60 ºC for 3 hours.

The electrospinning parameters were set up for a voltage of 15 kV, 20 kV, and 27 kV and at a working distance of 10 cm from the spinneret to the collector. A dispensing needle of 0.381 mm inner diameter was used for fibers production. Fibers were collected onto aluminum foil for scanning electron microscopy (SEM) micrographs to study nanoparticles' distribution. Fibers were also collected onto glass slides to determined their surface wettability with water contact angle measurements.

Fiber mats were crosslinked using 25 wt% vapor glutaraldehyde (GLA) inside a desiccator for 24 and 48 hours. The decoration of nanoparticles was performed with a drop deposition technique onto the fiber mats. Three consecutive 2 µL drops were placed onto the fiber mats, waiting one hour to let dry between nanoparticle drops. Fiber mats were decorated with 60-nm gold nanostars, and other fiber mats were decorated with 20-nm gold-shelled silver nanoparticles. Nanoparticles were deposited onto nanofibers exposed with GLA during 24 and 48 hours.

Results:

The globular structure of soy protein hinders the possibility of electrospun it, so that is necessary to unfold the spherical structure by adding an alkaline solution to enable the production of fibers (Shankar, Seyam, & Hudson, 2013; Vega-Lugo & Lim, 2008). Polymer solutions with viscosities close to 0.1-2.0 Pa·s assure that fibers can be formed during electrospinning (Zhang et al., 2018). SPI alone with water (10 wt% SPI and 12 wt% SPI in water with 1 wt% NaOH) does not enable the fabrication of nanofibers in electrospinning due to its low viscosity (below 0.1-2.0 Pa·s) and the low interaction of polymer chains. The addition of PEO increased the viscosity of SPI solutions to the polymer viscosity window that can enable the formation of fibers.

The applied voltage during electrospinning had an impact on the morphology of fibers. As the applied voltage increased, the fiber diameters decreased. A bigger drop at the needle tip is formed when low voltages are applied, leading to thicker fibers than when higher voltages form a smaller drop and thinner fibers. Fiber mats fabricated with 15 kV in electrospinning created a bigger surface area that allowed the nanoparticles' distribution better than fibers created with 20 kV and 27 kV. Fiber diameters were between 80-130 nm for the 10 wt% SPI + 5 wt% PEO (0.1 MDa) solution while fiber diameters were between 125-160 nm for 12 wt% SPI + 5 wt% PEO (0.1 MDa) solution. Water contact angle measurements showed that a higher concentration of SPI in solution (12 wt% + 5 wt% PEO) had a higher hydrophobicity.

The 12 wt% + 5 wt% PEO solution was chosen for crosslinking with GLA and for nanoparticles decoration. SPI+PEO nanofiber mats were exposed to 25 wt% vapor glutaraldehyde to increase the hydrophobicity of fibers' surface for decoration with nanoparticles. If nanoparticles are widely spread onto fiber mats, this can weaken the SERS signals, which is why GLA is use for enhancing the distribution of nanoparticles onto the fibers. SEM micrographs showed that nanofibers morphology did not swell or deformed when nanoparticles deposition was placed. Even after the third drop, fibers kept their shape very well without substantial changes. Furthermore, the exposition of nanofibers to vapor GLA had an impact on the distribution of nanoparticles. Fibers exposed longer to vapor glutaraldehyde (48 hours) showed that nanoparticles cluster tightly compared to fibers exposed to glutaraldehyde for less time (24 hours). Finally, the use of two different nanoparticles shapes is to compare the enhancement signals in SERS. Gold nanostars are expected to have better SERS signals compared to the gold-shelled-silver nanoparticles due to the edges in the nanostar shapes.

Conclusions: These results demonstrate the effect of SPI concentrations in solution and the impact on the morphology of fibers, creating thicker nanofibers as SPI concentration is higher. The addition of PEO enables the fabrication of nanofibers during electrospinning as it increases its viscosity. The applied voltage during electrospinning also directly impacts the fiber's morphology by creating thinner fibers as applied voltage increases. A higher concentration of SPI also showed to have a greater surface hydrophobicity. The crosslinking of the fiber's mats increases the surface hydrophobicity of the fibers, and more prolonged exposure to GLA keeps the nanoparticles together; this will lead to a better enhancement of SERS signals.

Future research: Experiments in progress consists of evaluating the impact of the different nanoparticle shapes and concentrations in SERS measurements. The final goal is to optimize the functionalization process with nanoparticles of the SPI+PEO nanofibers using glutaraldehyde as a crosslinker and different concentrations of nanoparticles.

References

Shankar, A., Seyam, A.-F. M., & Hudson, S. M. (2013). Electrospinning of soy protein fibers and their compatibility with synthetic polymers [Article]. Journal of Textile and Apparel, Technology and Management, 8(1).

Vega-Lugo, A.-C., & Lim, L.-T. (2008). Electrospinning of Soy Protein Isolate Nanofibers. J. Biobased Materials and Bioenergy, 2, 223-230-.

Zhang, H., Xi, S., Han, Y., Liu, L., Dong, B., Zhang, Z., ... Liu, J. (2018). Determining electrospun morphology from the properties of protein-polymer solutions. Soft Matter, 14(18), 3455–3462. https://doi.org/10.1039/c7sm02203d