Surface enhanced Raman scattering (SERS) spectroscopy, a powerful contemporary tool for studying low-concentration analytes via surface plasmon induced enhancement of local electric field, is of utility in biochemistry, material science, threat detection, and environmental studies. We have developed a simple, fast, scalable, and relatively low-cost optical method of fabricating and characterising large-area, reusable and broadband SERS substrates with long storage lifetime. We use tightly focused, intense infrared laser pulses to write gratings on single-crystalline, Au (111) gold films on mica which act as SERS substrates. Our single-crystalline SERS substrates compare favourably, in terms of surface quality and roughness, to those fabricated in poly-crystalline Au films. Tests show that our SERS substrates have the potential of detecting urea and 1,10-phenantroline adulterants in milk and water, respectively, at 0.01 ppm (or lower) concentrations. Our SERS substrates can be optimised to detect very low concentrations of adulterants, pollutants, explosives and other analytes even on ppb level. The complete set-up can be made hand-held or portable for effective in-field detection.We have developed innovative techniques based on direct laser writing and characterisation of reusable, large-area SERS substrates (in the form of gratings), with a long storage lifetime, for a broad λ-range, in Au films. Our direct laser writing technique is simple, fast, scalable, and relatively low-cost. We fabricated gratings on poly- and single-crystalline Au films deposited on mica and glass substrates, respectively. Substrates fabricated on single-crystalline, Au (111) gold films on mica are found to be superior for SERS applications compared to poly-crystalline substrates. We anticipate that our fabrication and characterisation methods may pave the way for fabrication of other optical elements.