Nanosphere Lithography on Fiber: Towards Engineered Lab-On-Fiber SERS Optrodes

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.

A self-assembled plasmonic optical fiber nanoprobe for label-free biosensing

“…However, these top-down technologies suffer from several inherent shortcomings, such as high equipment cost, time-consuming and low yields. To overcome these drawbacks, nanosphere lithography technique, with unique features of rapid and large-scale preparation of ordered nanostructure array, is recently used to fabricate the nano-patterned fiber probe 27,28 . This technology enables the effective integration of various nanostructures onto optical fiber with low-cost and facile fabrication.…”

Sensitive and Reproducible Gold SERS Sensor Based on Interference Lithography and Electrophoretic Deposition

“…Several top-down methods, which assure the high sensitivity and reproducibility of SERS substrates, have also been studied, including e-beam lithography [ 18 ], the focused ion beam (FIB) process [ 19 ], nanosphere lithography [ 20 ], and nanoimprint lithography [ 21 ]. However, these approaches have limitations, such as low productivity, high complexity, expensive, low pattern variability, and high-cost mold fabrication.…”

Lab-On-Fiber Technology: A Roadmap toward Multifunctional Plug and Play Platforms

“…Successful integrations onto the optical fibers of plasmonic nanostructures [ 36 , 37 ], photonic crystals [ 38 ], ring resonators [ 39 ], optomechanical microcavities [ 40 ], metallodielectric nanoarchitectures [ 41 , 42 , 43 ], and dielectric block surface-wave resonators [ 44 ] were recently reported. These first proofs of concept provide the basis for the development of multifunctional LOF platforms, such as plasmonic label-free optical fiber nanosensors [ 43 , 45 , 46 ], SERS nanoprobes [ 33 , 47 ], advanced photonic nanoresonators [ 39 , 44 ], ultrahigh-sensitivity acoustic transducers [ 48 ], optical fiber tweezers [ 49 , 50 ], and optical fiber metatips [ 51 ].…”