A mesopore-stimulated electromagnetic near-field: electrochemical synthesis of mesoporous copper films by micelle self-assembly†
Surface-enhanced Raman spectroscopy (SERS) is a powerful analytical method that uses localized surface plasmon resonances (LSPRs) to enhance the Raman cross-section of adsorbed molecules. Nanostructured copper (Cu) has been investigated as a SERS substrate in recent years because it is also a plasmon-supporting metal like gold (Au) and silver (Ag), but Cu is orders of magnitude more abundant in the Earth's crust. Although Cu is more prone to oxidation and tends to generate weaker LSPRs than Au or Ag, the sheer affordability of Cu metal drives the demand for SERS applications where the highest levels of sensitivity are not necessary. In addition, simplifying the fabrication methods for SERS substrates and avoiding costly lithographical techniques are problems to be overcome. In this report, we describe a method to fabricate mesoporous Cu films (MCuFs) using self-assembled block copolymer micelles as pore-directing agents in an electrochemical deposition method. The pores generated by the micelles are relatively large (>20 nm), which enables strong electromagnetic field enhancements via the LSPR. Different electrodeposition conditions such as potentials, times, and micelles molecular weights were tested to study MCuF formation and their effect on the pore size, porous structure, and SERS activity. We found that the samples created with small micelles generated the most robust SERS response. Electromagnetic simulations indicate that small pores are important for generating strong fields and that the presence of interconnected grooves assists in the collection of light into these small pores. The optimal MCuF substrate generated an enhancement factor (EF) and limit of detection (LoD) of 3.8 × 105 and 10−6 M, respectively. The results confirm that MCuFs are efficient for practical SERS applications due to their simple synthesis, high performance, and low cost.