Abstract
In this paper we report on the engineering of repeatable surface enhanced Raman scattering (SERS) optical fiber sensor devices (optrodes), as realized through nanosphere lithography. The Lab-on-Fiber SERS optrode consists of polystyrene nanospheres in a close-packed arrays configuration covered by a thin film of gold on the optical fiber tip. The SERS surfaces were fabricated by using a nanosphere lithography approach that is already demonstrated as able to produce highly repeatable patterns on the fiber tip. 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 nanosphere 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. The analysis allowed us to identify 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. The reproducibility of the SERS enhancement was thoroughly evaluated. In particular, the SERS intensity revealed intra-sample (i.e., between different spatial regions of a selected substrate) and inter-sample (i.e., between regions of different substrates) repeatability, with a relative standard deviation lower than 9 and 15%, respectively. 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 with a BPT solution used as benchmark. A fiber probe with a 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. This work, thus, poses the basis for realizing reproducible and engineered Lab-on-Fiber SERS optrodes for in-situ trace detection directed toward highly advanced in vivo sensing.
Highlights
Raman microscopy (RM) is a non-invasive vibrational spectroscopic technique that takes advantage of the inelastic scattering of light by vibrating molecules [1]
Before surface enhanced Raman scattering (SERS) enhancement evaluation, the localized surface plasmon-polariton resonances (LSPRs) spectral response was investigated as a function of the geometric parameters by measuring the reflectance spectra of the structures, i.e., the far-field light spectrum back-scattered to the detector
We investigated the potential to engineer Lab-on-Fiber SERS optrodes in order to provide advanced and repeatable SERS substrates integrated onto the optical fiber tip
Summary
Raman microscopy (RM) is a non-invasive vibrational spectroscopic technique that takes advantage of the inelastic scattering of light by vibrating molecules [1]. The characteristic bands in Raman spectra are narrow, easy to resolve, and specific to molecular structure, conformation, environment, and interactions with other molecules. RM measurements can be taken without labelling, Sensors 2018, 18, 680; doi:10.3390/s18030680 www.mdpi.com/journal/sensors. Its non-invasive and label-free nature is highly valuable for in vivo imaging [8,9]. The combination of RM and remote sensing through optical fibers realizes the optrode paradigm, which gives rise to an integrated and multiplexed sensing/imaging system for advanced biomedical applications [10,11,12]. Remote Raman sensing in optrode configuration has been validated in important clinical fields like brain surgery [13]
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