Significance of Nanofibers for Biosensor Applications

Thin films are often employed in sensors in order to improve their performance. It is well known that the sensitivity of a film is proportional to the surface area of the film per unit mass. Thin films made of nanofibers (NF) have surface area approximately one to two orders of magnitude larger than continuous films and therefore their sensitivities are potentially as large. The large available surface is the advantage that polymer nanofibers have over the existing sensor substrates. Nanofibrous supports with designed hierarchical pore structure architecture can provide a unique environment for biosensing due to controlled fluid delivery, retention, and ability to facilitate direct electron transfer.

Two main nanofiber fabrication techniques were compared: particle coagulation spinning (PCS) and electrospinning [1]. Three types of nanofibrous supports were studied to test their applicability for biosensor packaging: nonporous fibers made of single-wall carbon nanotubes by PCS process, electrospun nanowebs made of conductive polymer nanofibers, and polymer nanofiber/carbon nanotube composite webs. It was concluded that electrospun fibers had better strength, uniformity during fabrication, and gave reproducible results.

Nanofiber films are used as a sensing interface for thickness shear mode (TSM) piezoelectric sensors. TSM sensors coated with nanofiber films made of poly-lactic acid-co-glycolic acid (PLAGA) polymers were studied under various ambient conditions and were reported to possess better sensitivities than their thin film counterparts [2]. For TSM resonators, the resolution varies linearly with the surface area of the sensing interface. Hence, polymer nanofiber would be an ideal material for this purpose.

Recently, efforts were undertaken in the production of nanofibers for electrochemical sensors as well. Metal ions such as Pd, Pt, and Au are coated or electrodeposited onto the surface of the fibers to improve the conductivity and resolution. The literature shows that polymers such as polyaniline, polypyrrole, and polyamic acid have been electrospun and successfully used as sensing interfaces [3]. For electrochemical sensors, the resolution varied in proportion to the conductivity of the interface; in this case, it is the number of electrons transferred that govern the sensitivity. Therefore, nanofibers that have a very high surface area would be idealistic for electrochemical biosensors as well.

Optical sensors are relatively new and not much work has been carried out in this field. From first principles, the sensitivity of an optical sensor would depend on the amount of fluorescence emitted which in turn is proportional to the available area of the interface. Thus by change in morphology, if pores or pockets could be introduced the sensitivity would also increase proportionately. In the case of nanofibers this is inadvertent as the available area by itself is quite large and when porous fibers are used, the further increase is manifold. Thus, nanofibers would be the best-suited materials for fabrication of optical sensors.

Recent progress in biomedical technology has resulted in the development of novel sensor products with new applications. Modern biomedical sensors with advanced microfabrication and signal-processing techniques are becoming more and more accurate and inexpensive [4]. With the burden of accuracy and reliability removed, the focus is now on miniaturization of the bulky instrumentation and development of portable sensors. Also, a lot of work has been previously accomplished in developing various specific target molecules for different analytes that has exhausted all possibilities. So, the focus is again on miniaturization by detection of multianalytes on a single chip.

This idea is the base of the "lab-on-a-chip" projects that are presently being carried out in various laboratories across the world. Although microelectronics has sufficiently minimized the instrumentation and made it portable, the sensing interface is still critical as it forms the heart of the sensor. Therefore, no compromise can be had with regard to the available area of the sensor (which is proportional to the number of active sites). Hence the solution would be to adapt such a morphology that is small yet possesses a large surface area for interaction. The obvious key would be nanofibers. Nanofibers have been recently tested for biomedical applications as well as for fabrication of nanofiber-based immunosensors [5]. Specific target molecules such as antibodies can be tailored on the surface of the nanofibers at high concentrations. It is also possible to attach more than one antibody on the nanofiber mesh which would create a basis for multianalyte detection capability. All said, the nanofibers rule the sensor domain.

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