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Despite its ultrasensitive detection capability, surface-enhanced Raman spectroscopy (SERS) faces challenges as a quantitative biochemical analysis due to the significant dependence of local field enhancement on nanoscale geometric variations. Significant efforts have been devoted to develop SERS calibration methods by introducing Raman tags as internal standards. Raman tags undergo similar SERS enhancement, and ratiometric signals for target analytes can be generated with reduced SERS enhancement variations. However, using Raman tags still faces challenges for label-free applications, including spatial competition between the analyte and tags in hotspots, spectral interference, limited long-term stability due to laser-induced photo-degradation. We demonstrate that electronic Raman scattering (ERS) signals from metallic nanostructures at hotspots can serve as the internal calibration standard to enable quantitative SERS analysis and improve biostatistical analysis. ERS is omnipresent in any plasmonic construct and shown as a broad continuous background in SERS measurements. Both ERS and SERS processes experience the |E|4 local enhancements during the excitation and inelastic scattering transitions. We demonstrate that ERS-calibrated SERS signals are insensitive to variations from different hotspots and thus can enable more accurate quantitative SERS analysis. For validation, we performed label-free SERS analysis of living biological systems using four different cancer cell lines cultured on SERS devices and their drug responses. Remarkably, after ERS calibration, the statistical scatter plots are more similar to the intrinsic biological properties of cancer subtype categorization and their known drug responses. Therefore, we envision that ERS calibrated SERS can find crucial opportunities in label-free molecular profiling of complicated biological systems.
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