Self-assembly approach to optical fiber sensors

The realization of optical fiber tip sensors is attractive for wide a range of strategic application fields including environmental monitoring, life science, food monitoring, safety and security. In recent years, significant advances have been achieved in the development of novel fabrication processes devoted to patterning the tip of optical fibers [1-2]. The most conventional methods used to produce micro- and nanostructures on the optical fiber tip, such as electron beam or focusing ion beam lithography, demonstrated their efficacy in precision and size control, but all of them are inherently time consuming and require complex and expensive fabrication procedures with a relatively low throughput. By contrast, self-assembly approaches provide a much simpler, faster and inexpensive alternative to nanolithography in creating micro- and nanostructures in ordered fashion. Our studies aim to attain advanced nanostructured optical fiber tip sensors by exploiting simple and low-cost fabrication processes suitable to be employed in massive production of technologically advanced devices. Here we propose two different fabrication approaches based on self-assembly: breath figure patterning and nanosphere lithography. The first approach consists in the preparation of a honeycomb-like microstructured polymeric film directly on the optical fiber tip, obtained in a few seconds by a simple solution casting under controlled conditions. The successive metal deposition leads to metallo-dielectric honeycomb patterns which are sensitive to the surrounding refractive index changes, demonstrating their potentialities for chemical and biological sensing applications. In the second approach, polymer microspheres are assembled at the air/water interface and then transferred on the fiber tip. By applying to the fiber further treatments like particle size reduction, metal coating and sphere removal, different periodic structures are conveniently realized, from regularly distributed metallic-dielectric sphere arrays to differently shaped metallic patterns with dimensional features down to a submicron scale. Finally, as proof of concept, we demonstrate that the realized patterns are able to work as efficient Surface Enhanced Raman Spectroscopy (SERS) fiber probes, legitimizing the self-assembly approach as valid option for the realization of ultra-sensitive tools for in vivo molecular recognition.