Silver-decorated cylindrical nanopores: Combining the third dimension with chemical enhancement for efficient trace chemical detection with SERS

We report on the facile fabrication of efficient porous alumina membrane-based SERS substrates that avoid the cumbersome stages of chemical surface modification of the pores and premixing/infiltration of nanoparticles with analytes. The design relies on higher light transmission through the SERS substrates by widening the cylindrical pore diameter to 355 nm and in situ growth of uniform silver nanoparticles on the inner walls. Electromagnetic simulations confirm that the interaction of excitation light with the nanoparticles along the pore walls can be maximized in such a membrane when the nanoparticles are placed within the first 14 μm of the pore depth. For common benchmark Raman analytes such as benzenethiol and Rhodamine 6G, nanomolar detection limits are readily obtained without any additional chemical surface functionalization and/or additional premixing and preconcentration of metal nanoparticles and analytes. Moreover, a high enhancement of 10 ⁶ and a micromolar detection limit are achieved for nonresonant, Raman-stealthy perchloric acid molecules. Quantum mechanical calculations of perchloric acid bound to nanostructured silver clusters with different sizes and binding sites suggest that the maximum chemical enhancement is achieved for molecules located at the tips of the (111) planes of silver lattices, which are abundantly available on the nanoparticles grown in this study.