Nanoporous Silver Film Fabricated by Oxygen Plasma: A Facile Approach for SERS Substrates

Nanoporous metal films are promising substrates for surfaced-enhanced Raman scattering (SERS) measurement, owing to their homogeneity, large surface area, and abundant hot-spots. Herein, a facile procedure was developed to fabricate nanoporous Ag film on various substrate surfaces. Thermally deposited Ag film was first treated with O2 plasma, resulting in porous Ag/Ag x O film (Ag x O-NF) with nanoscale feature. Sodium citrate was then used to reduce Ag x O to Ag, forming nanoporous Ag film (AgNF) with similar morphology. The AgNF substrate demonstrates 30-fold higher Raman intensity than Ag film over polystyrene nanospheres (d = 600 nm) using 4-mercapto­benzoic acid (4-MBA) as the sensing molecule. Comparing with ordinary Raman measurement on 4-MBA solution, an enhancement factor of ∼6 × 106 was determined for AgNF. The AgNF substrate was evaluated for benzoic acid, 4-nitrophenol, and 2-mercapto­ethanesulfonate, showing high SERS sensitivity for chemicals that bind weakly to Ag surface and molecules with relatively small Raman cross section at micromolar concentration. In addition to its simplicity, the procedure can be applied to various materials such as transparency film, filter paper, hard polystyrene film, and aluminum foil, revealing similar Raman sensitivity. By testing the durability of the substrate, we found that the Ag x O films can be stored in ambient conditions for more than 90 days and still deliver the same SERS intensity if the films are treated with sodium citrate before use. These results demonstrate the advantage of the proposed approach for mass production of low-cost, sensitive, and durable SERS substrates. The transferable nature of these AgNF to different flexible surfaces also allows their easy integration with other sensing schemes.