Surface-enhanced Raman scattering (SERS) is a highly sensitive and versatile analytical technique that can be implemented on an optical fiber platform for use in challenging environments. This work has sought to address a major factor limiting the use of optical fibers for SERS analytical applications, namely the silica Raman background generated inside the fiber can make it difficult to detect the target analyte. Two different approaches were investigated to address this problem. Firstly, double clad fiber (DCF) was found to increase the collection of Raman scattered signal from the analyte, giving up to twelve-fold improvement in the signal-to-background ratio (SBR). Secondly, a prototype microfilter was manufactured by femtosecond laser machining and attached directly to the DCF tip. Its performance in rejecting background signal was then evaluated. When taking the lengths of the optical fibers into account, the filtered DCF microprobe delivers 7.0 SBR.cm, while the bare DCF probe provided 3.0 SBR.cm. Therefore, the microfilter assembly more than doubled the performance of the SERS probe and, with further optimization in future, it shows great promise for ultra-compact SERS and Raman optical fiber probes.
Since its discovery in the 1970s, surface-enhanced Raman scattering (SERS) has attracted interest as a sensitive technique for detecting a wide range of analytes. However, SERS is a complex physicochemical phenomenon and tends to suffer from poor levels of reproducibility, which has hindered its translation into practical applications. Here we confirm that the low wavenumber pseudoband arising from the interaction between the edge filter and the elastically scattered light from the laser excitation can be used to perform "hotspot" normalisation of SERS spectra. Together with judicious use of resonant Raman scattering and/or careful control of the surface chemistry, this breakthrough in spectral data processing can address many of the challenges encountered when developing a quantitative SERS-based assay.
Numerous potential biomedical sensing applications of surface-enhanced Raman scattering (SERS) have been reported, but its practical use has been limited by the lack of a robust sensing platform. Optical fiber SERS probes show great promise, but are limited by the prominent silica Raman background, which requires the use of bulky optics for filtering the signal collection and excitation delivery paths. In the present study, a SERS microprobe has been designed and developed to eliminate the bottlenecks outlined above. For efficient excitation and delivery of the SERS signal, both hollow core photonic crystal fiber and double clad fiber have been investigated. While the hollow core fiber was still found to have excessive silica background, the double clad fiber allows efficient signal collection via the multi-mode inner cladding. A micro filtering mechanism has been designed, which can be integrated into the tip of the optical fiber SERS probe, providing filtering to suppress silica Raman background and thus avoiding the need for bulky optics. The design also assists in the efficient collection of SERS signal from the sample by rejecting Rayleigh scattered light from the sample. Optical fiber cleaving using ultra-short laser pulses was tested for improved control of the fiber tip geometry. With this miniaturized and integrated filtering mechanism, it is expected that the developed probe will promote the use of SERS for minimally invasive biomedical monitoring and sensing applications in future. The probe could potentially be placed inside a small gauge hypodermic needle and would be compatible with handheld portable spectrometers.
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