Effect of NiAl alloy microparticles deposited in flexible SERS substrates on the limit of detection of rhodamine B molecules

Abstract

Flexible-SERS (FSERS) substrates were fabricated by depositing Ni₆₄Al₃₆(NiAl)-alloy-microparticles and/or spherical Ag-NPs (sizes of 10–40 nm) on recycled plastics, which had an aluminum layer on their surface. First, FSERS substrates made of Al + Ag-NPs and an area of 1 cm² were used to detect rhodamine B (RhB) molecules. The limit-of-detection (LOD) for RhB was 8.35 × 10⁻²² moles (∼503 molecules), and the enhancement factor (EF) was 3.11 × 10¹⁵. After adding NiAl-microparticles to the substrate, the LOD decreased to 8.35 × 10⁻²⁴ moles (∼5 molecules) and the EF was increased to 2.05 × 10¹⁷. Such EF values were calculated with respect to substrates made only with Al + NiAl-alloy (without Ag-NPs), which did not show any Raman signal. Other FSERS substrates were made with graphene-layer + Ag-NPs or graphene-layer + NiAl-alloy + Ag-Nps, and the best LOD and EF values were 8.35 × 10⁻²² moles and 6.89 × 10¹⁵, respectively. Overall, combining the Ag-NPs and NiAl-alloy microparticles allowed for the zeptomole detection of RhB. This was possible due to the formation of Ag aggregates around the alloy microparticles, which enhanced the number of hotspots. If no alloy is present in the FSERS substrates, the detection of RhB is lowered. Overall, we presented a low-cost FSERS substrate that does not require expensive Au films or Au-NPs (as previously reported) to detect RhB at the zeptomole level.