Surface Enhanced Raman Scattering (SERS) – Expanding the Limits of Conventional Raman Analysis

Importance of the Topic

The ability to detect trace-level analytes in complex matrices is critical across forensic, food safety, environmental, and industrial applications. Conventional Raman spectroscopy offers fingerprint identification but is limited by low scattering efficiency and fluorescence interference. Surface Enhanced Raman Scattering (SERS) dramatically amplifies Raman signals, enabling detection down to parts-per-billion. Integrating SERS with portable analyzers expands field-based, rapid, and cost-effective analysis capabilities.

Objectives and Study Overview

This white paper introduces the principles of SERS and presents Metrohm Raman’s printed SERS (P-SERS) substrates coupled with the handheld Mira DS Raman spectrometer. Key goals include:

• Explaining the SERS enhancement mechanism
• Detailing the manufacture and advantages of P-SERS strips
• Demonstrating applications in forensic drug screening and food contaminant detection

Methodology and Instrumentation

SERS enhancement arises when target molecules adsorb onto gold or silver nanoparticles, boosting the local electromagnetic field by factors up to 10⁷. Metrohm’s P-SERS substrates use inkjet printing to deposit uniform Ag or Au nanoparticles onto flexible membranes, creating low-cost, stable test strips. Samples can be applied by pipetting, dipping, or swabbing. Measurements are performed with the Mira DS handheld Raman spectrometer, featuring:

• A compact design with Orbital Raster Scan technology
• An integrated library of over 10,000 reference spectra
• Touchscreen-controlled automatic identification software

Main Results and Discussion

Several case studies illustrate the performance of P-SERS with Mira DS:

• Heroin detection in street samples: All 18 crude heroin specimens yielded clear SERS spectra, overcoming fluorescence from adulterants.
• Melamine in milk: After acid precipitation and centrifugation, 100 ppm melamine in milk produced distinct peaks; sensitivity suggests potential for lower limits.
• Malachite green: Part-per-billion detection demonstrated in aqueous solutions, confirming suitability for aquaculture monitoring.
• Malathion pesticide: 10 ppm malathion gave unambiguous SERS signatures, indicating relevance for produce safety and water quality testing.

Benefits and Practical Applications

P-SERS with handheld Raman offers:

• High sensitivity at ppm–ppb levels
• Rapid, non-destructive analysis in under a minute
• Portability for in-field screening
• Low per-test cost and ease of use without extensive sample cleanup

Future Trends and Potential Applications

Emerging developments include:

• Integration of on-strip chromatographic separation to reduce matrix effects
• Tailored nanoparticle chemistries for broader analyte affinity
• Combining P-SERS with smartphone-based detectors for decentralized testing
• Applications in environmental monitoring, point-of-care diagnostics, and anti-counterfeiting

Conclusion

Printed SERS substrates coupled with the Mira DS handheld Raman system significantly extend the analytical reach of conventional Raman spectroscopy. This platform delivers fast, sensitive, low-cost detection of narcotics, food contaminants, and pesticides, offering a practical solution for laboratories and field operators.

References

• Hoppmann EP, Yu WW, White IM. Highly sensitive and flexible inkjet printed SERS sensors on paper. Methods. 2013;63(3):219–224.
• Yu WW, White IM. Inkjet-printed paper-based SERS dipsticks and swabs for trace chemical detection. Analyst. 2013;138(4):1020–1025.
• Yu WW, White IM. Chromatographic separation and detection of target analytes from complex samples using inkjet printed SERS substrates. Analyst. 2013;138(13):3679–3686.
• NIH Publication No. 04-446; FDA IA 99-30; EPA-HQ-OPP-2009-0317 Docket; CDC Wonder.