Laser-Generated Silica Nanofibers Embedded with Electrospun Gold Nanoparticles: A Novel Platform for Biocompatible Sensing Devices

Sarah Hamza (University of New Brunswick)
Amirkianoosh Kiani (University of New Brunswick)

Introduction

Increasing the overall sensitivity of sensing materials is a key feature in their application. As technology advances, not only is customizing the electrical conductivity of the material of importance, but many applications also require biocompatibility. Increasing the latter tends to result in low-signal-to noise ratios as well as a reduction in the overall conductivity due to the processing techniques, which generally involve the oxidation of the material [1]. The lack of functional stability for implanted sensors has caused their restriction to short-term usage. Unlike pure silicon, porous silicon has shown biocompatibility properties and is thus becoming a more feasible, cost-effective, and available option for use as a sensing material [2]. Its main downside is its reduced conductivity due to the increase in oxygen concentration that occurs during processing. A simple and cost effective technique to generate and customize the conductivity of nanofibrous silicon oxide is proposed to improve its viability in sensing applications requiring a biocompatible environment. Gold embedding through sputtering techniques is suggested in this research as a means of controlling and further imparting electrical properties to laser induced silicon oxide nanofibers. Single crystalline n-type silicon wafers were laser processed using an Nd:YAG pulsed nanosecond laser system at a line spacing of 0.025 mm and at one overlap to generate a porous structure on the surface. As the sputtering time of the samples is increased from four to eight minutes, the conductivities were measured and found to increase accordingly. These findings show agreement between the theory and the experimental results collected for the conductivity.

Materials and Methods

Silicon wafers were processed at a constant power, scanning speed, frequency, and line spacing of 12 W, 100 mm/s, 100 Hz, and 0.025mm respectively. Only one overlap was made on the samples surface and TEM, SEM, and EDX images were acquired to analyze the structure of the substrate. Gold sputtering was conducted for either four or eight minutes on the samples using an Edwards S150A sputter coater. Impedance spectroscopy was used to determine the effects of both gold coating and overlaps on the conductivity of the silicon.

Results

Nanofibrous structures were achieved using a nanosecond pulsed laser system as seen in Fig. 2 (A). The sputtered gold particles can be seen through the TEM image of Fig. 2 (B). Although sputtering shows inconsistency of the overall spread of the gold particles, the conductivity was still able to be controlled by altering the sputtering duration. Due to the small nature of the gold particles, quantum effects play a key role in the conductivity. With small interparticle distances, the gold does not need to be in direct contact with an adjacent conductive particle, and instead can overcome the insulating properties of the silicon oxide. As the interparticle distance of the metal decreases, conductivity of the sample is oppositely affected [3]. Measuring the concentration of the gold from the samples showed agreement with both the conductivity and sputtering time, where increased concentrations and time resulted in higher conductivities. As seen in Fig. 3, the highest conductivity is seen at 8 minutes of gold sputtering, and the lowest when not sputtered. This suggests controllability of the electrical properties of the material.

Discussion and Conclusion

This research offers insight into the ability of controlling the conductivity of porous silicon. Gold is used to enhance the conductivity of the material, since processing causes an increase in the oxygen content, which acts as an insulator. Previous work has proven biocompatibility of porous silicon through invitro testing with both SBF and NIH 3T3 culturing [4]. The method outlined in this research offers an economical and effective way to process silicon into porous and fibrous structures while controlling the electrical conductance of the material.


Fig. 1. Experimental Setup

Fig. 2. Image of fibrous structures generated through laser processing: (A) SEM, (B) TEM.

Fig. 3. Conductivity of samples treated with gold sputtering for zero, four, or eight minutes.

Acknowledgements

This research was funded by the National Sciences and Engineering Research Council (NSERC) Discovery Grant program, the New Brunswick Innovation Foundation (NBIF), and the McCain Foundation.

References

1- Shi, J., & Porterfield, D. M. (2011). Surface modification approaches for electrochemical biosensors. INTECH Open Access Publisher.

2- Jamois, Cécile, et al. "New concepts of integrated photonic biosensors based on porous silicon." Biosensors-Emerging Materials and Applications (2011).

3- Athanassiou, Evagelos K., et al. "Advanced piezoresistance of extended metal-insulator core-shell nanoparticle assemblies." Physical review letters 101.16 (2008): 166804.

4- Colpitts, Candace, et al. "Mammalian fibroblast cells show strong preference for laser-generated hybrid amorphous silicon-SiO2 textures" J. Appl. Biomater. Funct. Mater.15, no 1 (2016):  e84 - e92.

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