Chemically Roughened Solid Silver: A Simple, Robust and Broadband SERS Substrate

Shavini Wijesuriya, Krishna Burugapalli, Ruth Mackay, Godwin Chukwuebuka Ajaezi and Wamadeva Balachandran

Surface-enhanced Raman spectroscopy (SERS) substrates manufactured using complex nano-patterning techniques have become the norm. However, their cost of manufacture makes them unaffordable to incorporate into most biosensors. The technique shown in this paper is low-cost, reliable and highly sensitive. Chemical etching of solid Ag metal was used to produce simple, yet robust SERS substrates with broadband characteristics. Etching with ammonium hydroxide (NH₄OH) and nitric acid (HNO₃) helped obtain roughened Ag SERS substrates. Scanning electron microscopy (SEM) and interferometry were used to visualize and quantify surface roughness. Flattened Ag wires had inherent, but non-uniform roughness having peaks and valleys in the microscale. NH₄OH treatment removed dirt and smoothened the surface, while HNO₃ treatment produced a flake-like morphology with visibly more surface roughness features on Ag metal. SERS efficacy was tested using 4-methylbenzenethiol (MBT). The best SERS enhancement for 1 mM MBT was observed for Ag metal etched for 30 s in NH₄OH followed by 10 s in HNO₃. Further, MBT could be quantified with detection limits of 1 pM and 100 µM, respectively, using 514 nm and 1064 nm Raman spectrometers. Thus, a rapid and less energy intensive method for producing solid Ag SERS substrate and its efficacy in analyte sensing was demonstrated.

Figure 6. Raman and SERS spectra of MBT recorded using 514 nm and 1064 nm Raman spectrometers: (a) 514 nm and 1064 nm Raman spectra of solid MBT; and (b,c) 514 nm and 1064 nm SERS spectra respectively, for 1 mM MBT on 30 s NH₄OH + 10 s HNO₃ Ag substrate compared to Raman spectrum of solid MBT and the base Ag substrate (blank).

Raman spectra for the different Ag substrates were recorded using two different Raman spectrometers. First was a benchtop Renishaw InVia confocal Raman microscope (Wotton-under-Edge, Gloucestershire, UK) equipped with 514 nm laser having maximum power output of 50 mW. The samples were scanned with a 20 × 0.4 NA objective, 12.68 µm laser spot size, 10% power, 10 s acquisition time, 4 cm⁻¹ resolution and spectra recorded using Wires 3.3 software. Second was a portable StellarNet Inc. (Tampa, FL, USA) 1064 nm Raman spectrometer with diode laser having maximum power output of 499 mW. The samples were scanned using a fibre optic probe at a working distance of 7.5 mm, 158 µm laser spot size, ~250 mW laser power, 5000 ms acquisition time, 8 cm⁻¹ resolution and spectra recorded using SpectraWiz software.