Random nano-textured surfaces for sensing

Nano-textured surfaces on Si, Cu, and polystyrene (PS) substrates with feature sizes down to 10-20 nm fabricated over large areas by simple maskless plasma and chemical etching (Si and Cu, respectively) or thermally induced PS shrinking are compared for the surface enhanced Raman scattering (SERS) action. Plasma etching of Si wafers was used to create nano-textured surfaces with random pattern of nano-spikes. Their height was scaling linearly with etching time and the tips had an approximately 10-nm-radius curvature. Such surfaces appear black, hence the black-Si (b-Si), and after sputtering of a gold nano-film they perform as efficient surface enhanced Raman scattering (SERS) substrates. The nano-sharp tips of b-Si coated with gold are promising for SERS probing from an inside of cells and bacteria after their membranes are mechanically ruptured when in contact with b-Si. Recently, we have shown an incorporation of b-Si into a micro-fluidic flow channel where biocidal mechanical action of b-Si was used for rupturing of bacteria. Other back-surfaces for SERS were made by chemical oxidation of Cu foil and thermal shrinking of polystyrene (PS) film sputtered with gold. These are examples of highly performing SERS substrates when coated with ~ 100 nm film of gold. These surfaces of black-CuO and black-Au (on PS) are promising for industrial up scaling due to simplicity of fabrication. Randomness of the the black-Si(-CuO,-Au) requires better understanding how different substrates or different areas of the same substrates are performing in SERS. Statistical analysis of SERS mapping from black-surfaces can be used to benchmark different substrates using covalently bound thiophenol which makes self-assembled monolayer (SAM) on gold. We have shown that a simple optical dark-field microscopy can be used to identify the optimum wavelength of excitation for SERS judging by intensity of color in dark-field image. Simple to fabricate substrates with random nano-textured surfaces are available for SERS applications, they can be incorporated into micro-fluidic chips, and used for their biocidal function.