Highly sensitive label-free detection of analytes at different scales using uniform graphene-nanopyramids hybrid SERS system

Surface-enhanced Raman spectroscopy (SERS) has attracted enormous interests as a biological sensing technique. However, the wide application of SERS technique suffers from the limitation of sensitivity, uniformity and universality for the analytes. Herein, a graphene-nanopyramids hybrid SERS system with high uniformity and sensitivity was used to label-freely detect analytes at different scales. Colloidal lithography and modern semiconductor nanofabrication process were employed for the fabrication of wafer-scale highly-ordered array of Au nanopyramids, which ensures the uniformity of the system. The graphene layer not only plays a role of biology-compatible supporting layer for analytes but also further enhances the Raman signals and facilitates the quantitative analysis capability of the hybrid system as an intensity gauge. For nanoscale chemical and protein molecules including Rhodamine 6G (R6G), methylene blue (MB), catalase and lysozyme with concentrations lower than 10⁻¹⁰ M, clear SERS signals can be obtained with analytical enhancement factors (EFs) of ~10⁹ for R6G and MB. For micron-scale analytes, Raman fingerprints of cells, i.e. erythrocyte and leukocyte have been obtained. Moreover, using statistical principal component analysis method, one can easily distinguish different analytes based on their Raman fingerprints. The experiment results of EF are in good agreement with the theoretical values predicted by finite-difference time-domain simulation. Such a wafer-scale, highly uniform and universal system will certainly enable the wider application of SERS in multiscale label-free detection of chemicals, proteins and cells, etc. with high sensitivity and specificity.