A new strategy for constructing 3D hybrid graphene/Au/rectangular pyramids PMMA on a flexible SERS substrate for trace molecule detection

Surface enhanced Raman spectroscopy (SERS) has attracted interest in the fields of hazardous substance detection and food safety. However, its limited sensitivity and SERS signal consistency inhibit the widespread use of SERS technology. Herein, a novel approach for the fabrication of a graphene/Au NP/rectangular pyramid PMMA hybrid SERS substrate (Gr/Au/RP PMMA) with high uniformity and sensitivity is presented. The Gr/Au/RP PMMA substrate is machined by indentation firstly, then following wet etching of the copper substrate. The Raman enhancement of Gr/Au/RP PMMA is studied in terms of both electromagnetic enhancement and chemical enhancement. The field strength enhancement of the obtained Gr/Au/RP PMMA substrate is more than 4.9 times greater than that of the Gr/Au/flat PMMA substrate, established using a COMSOL simulation. By actively modulating the machining parameters on a Cu-Gr surface, the number of Gr layers, as well as the Gr/Au/RP PMMA topography, can be controlled, which can effectively influence the performance of the SERS substrate. Moreover, compared with other SERS substrates, including Gr/Au/RP PMMA, Gr/Au/flat PMMA, Au/Gr/RP Cu, Gr/RP PMMA, and Au/RP PMMA, the Raman intensities of R6G were strongest on the Gr/Au/RP PMMA. In addition, the minimum detectable concentrations of paraquat (PQ) and 2,4-d used in situ detection were 10−8 M and 10−6 M, respectively, after being covered with the Gr/Au/RP PMMA flexible substrate. To test its practical applicability with actual fruit surfaces, the Gr/Au/RP PMMA flexible substrate was used for the in situ SERS detection of PQ on apples. Clear Raman peaks of 10−7 M PQ were detected on the apple surface.