Optimization of Laser Induced Periodic Surface Structures for Surface Enhanced Raman Spectroscopy of Extracellular Vesicles

Since its discovery in 1974 [1], surface enhanced Raman spectroscopy (SERS) has proven to be a viable technique for increasing the usefulness of traditional Raman-based spectroscopic methods, reporting increases in the intensity of the Raman signal up to 10 orders of magnitude [2]–[5]. The signal amplification provided by SERS substrates is primarily attributed to an electromagnetic effect, relying on the ability to couple incident light to optical modes (surface plasmons) on a roughened metallic surface [3], [6]. Here, we propose laser-induced periodic surface structures (LIPSS) as an effective method of nano-structuring stainless steel substrates for SERS. A nanometric gold layer is subsequently deposited for use as a SERS-active surface. Nano-structuring of steel wafers by femtosecond laser pulses is optimised for use in detecting extracellular vesicles (EVs) from a choriocarcinoma cell line (JEG-3), commonly used as a model for placental syncytiotrophoblasts. A variety of parameters are considered - such as writing laser power, polarization, machining speed, and gold layer thickness - each with implications for the LIPSS nano-structure morphology and, in turn, the SERS enhancement. With our experimental setup we have shown that for stainless steel substrates the Raman signal intensity of EVs increases with the machining laser power used to generate LIPSS; illustrated in Fig. 1.