Fabrication of low cost highly structured silver capped aluminium nanorods as SERS substrate for the detection of biological pathogens

We report the fabrication of low cost highly structured silver (Ag) capped aluminium (Al) nanorods (NRs) as surface enhanced Raman spectroscopy (SERS) substrate utilising the glancing angle deposition technique. The nano-capping of silver onto the Al NRs can concentrate the local electric field within the minimal volume that can serve as hotspots. The average size of the Ag nanocaps was 50 nm. The newly proposed nanoporous Ag capped Al NRs as SERS substrate could detect the Raman signal of rhodamine 6G (R6G) up to 10(-15) molar concentration. The significant enhancement in the Raman signal of 10(7) was achieved for Ag capped Al NRs considering R6G as a probe molecule. Using the developed SERS substrate, we recorded Raman spectra for Escherichia coli bacteria with its concentration varying from 10(8) colony forming units per ml (CFU ml(-1)) up to 10(2) CFU ml(-1). All the reported Raman spectra were acquired by a portable handheld Raman spectrometer. Hence, this newly proposed low cost, effective SERS substrate can be used commercially for the onsite detection of clinical pathogens. The 3D finite difference time domain simulation model was performed for Ag capped Al nanostructure to understand the generation of hotspots. The simulated results show excellent agreement with the experimental results. We fabricated uncapped Ag nanorods of similar dimensions and performed the experimental measurements and simulations for comparison. We found a significant enhancement in Ag capped Al NRs compared to the long Ag NRs. The description of the Raman signal enhancement has been elaborated.

Automated summary

Low cost highly structured silver capped aluminium nanorods were fabricated as a surface enhanced Raman spectroscopy substrate using glancing angle deposition, achieving Raman signal detection for Rhodamine 6G down to 10(-15) molar concentration. The developed SERS substrate successfully recorded Raman spectra for Escherichia coli bacteria with varying concentrations, demonstrating significant signal enhancement compared to uncapped silver nanorods.