LAB ON FIBER SERS OPTRODES BY NANOSPHERE LITHOGRAPHY

We report on the engineering of repeatable surface enhanced Raman scattering (SERS) optrodes realized through nanosphere lithography on optical fiber (Figure 1). In particular, the Lab-on-Fiber SERS optrode consists of polystyrene nanospheres in close-packed arrays (CPA) configuration covered by a thin film of gold onto the optical fiber tip. The SERS surfaces were fabricated by using a nanosphere lithography approach that already demonstrated to be able to produce highly repeatable patterns on the fiber tip [1]. Starting from the promising pristine results, in order to engineer and optimize the SERS probes, we first evaluated and compared the SERS performances in terms of Enhancement Factor (EF) pertaining to different patterns with different nanospheres diameters and gold thicknesses. To this aim, the EF of SERS surfaces with a pitch of 500, 750 and 1000 nm, and gold films of 20, 30 and 40 nm have been retrieved, adopting the SERS signal of a monolayer of biphenyl-4-thiol (BPT) as a reliable benchmark (Figure 2). The analysis allowed us the identification of the most promising SERS platform: for the samples with nanospheres diameter of 500 nm and gold thickness of 30 nm, we measured values of EF of 4×105, which is comparable with state-of-the-art SERS EF achievable with highly performing colloidal gold nanoparticles [2]. The reproducibility of the SERS enhancement was thoroughly evaluated. Finally, in order to determine the most suitable optical fiber probe, in terms of excitation/collection efficiency and Raman background, we selected several commercially available optical fibers and tested them towards a BPT solution used as benchmark. A fiber probe with pure silica core of 200 µm diameter and high numerical aperture (i.e. 0.5) was found to be the most promising fiber platform providing the best trade-off between high excitation/collection efficiency and low background (Figure 3). Ongoing activities are devoted to use our Lab-on-Fibre SERS optrodes for cancer cell identification with the ambitious aim to achieve a highly advanced in-vivo sensor.