Tailoring lab-on-fiber SERS optrodes towards biological targets of different sizes

In this paper, we studied the influence of the geometrical configuration of surface-enhanced Raman scattering (SERS)-active self-assembling gold nanostructures on the interaction with biological targets of different sizes, aiming at large-scope SERS lab-on-fiber optrodes composed of highly efficient SERS substrates integrated onto optical fibre tips. By using nanosphere lithography, we fabricated three types of highly ordered and reproducible SERS-active substrates. To correlate the SERS response to the steric hindrance of the biological target, we experimentally analysed and compared the SERS spectra of three representative biological probes, i.e., ultralow-molecular-weight molecules of biphenyl-4-thiol (BPT, small molecule, 186.27 Da), bovine serum albumin (BSA, medium molecule, 66.5 kDa) and red blood cells (RBCs, diameter <10 ?m, complex target). All SERS-active substrates provided the Raman fingerprint of the biological targets, but the SERS intensity was dependent on the substrate geometry and its specific steric interfacing with biological targets. A full three-dimensional numerical analysis was carried out by means of the finite element method to gain insight into the electric field distribution and hot spot distribution of the fabricated structures. By correlating the electric field distribution with the SERS response, we conclude that the most efficient target-based architectures feature not only "intense" but also "accessible" hot spots. Finally, we optimized a Raman readout system for SERS optrode operation with efficient illumination and collection via an optical fibre.