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

Importance of the Topic

Surface Enhanced Raman Scattering (SERS) extends the detection capabilities of conventional Raman spectroscopy from parts-per-thousand to parts-per-billion levels by exploiting the electromagnetic enhancement that occurs when target molecules adsorb onto gold or silver nanoparticles. This advancement is critical for fields requiring trace analysis of chemical residues in forensic investigations, food safety monitoring, environmental testing and anti-counterfeiting applications.

Aims and Study Overview

This white paper from Metrohm Raman introduces a printable SERS (P-SERS) platform designed to enable highly sensitive, rapid and cost-effective Raman analysis. The objectives are to explain the SERS mechanism, describe the development of inkjet-printed nanoparticle substrates, and demonstrate analytical workflows for real-world targets such as narcotics, food adulterants and pesticides.

Methodology and Instrumentation

Metrohm Raman’s approach combines handheld Raman analyzers with P-SERS substrates:

• Mira DS handheld Raman system with laser excitation, Orbital Raster Scan technology and integrated spectral library of over 10,000 entries
• Inkjet-printed silver nanoparticle films on cellulose-based membranes (P-SERS substrates) for uniform enhancement and low cost
• Sample introduction modes including pipetting, dipping, swabbing and on-substrate chromatographic cleanup

The Mira DS captures enhanced Raman signatures within seconds under controlled laser power, integration time and averaging settings.

Major Results and Discussion

• Heroin Detection: 18 illicit street samples containing 1–5 mg of material were dissolved and applied to P-SERS substrates. Characteristic heroin peaks at 531, 625 and 1336 cm⁻¹ were reliably identified despite strong fluorescence from cutting agents.

• Melamine in Milk: Milk spiked to 100 ppm melamine underwent acid-induced protein coagulation and centrifugation. The clear supernatant produced strong SERS signals, demonstrating detection well below regulatory limits.

• Malachite Green: Serial dilutions of this aquaculture fungicide down to parts-per-billion levels yielded distinct spectra, confirming extremely low detection thresholds.

• Malathion: Organophosphate pesticide at 10 ppm was directly deposited on P-SERS substrates, producing unambiguous Raman signatures of the parent compound and highlighting potential for water and produce monitoring.

The experiments demonstrate that P-SERS substrates, when combined with the Mira DS, overcome the weak scattering and fluorescence issues of conventional Raman, enabling rapid field-deployable screening with minimal sample preparation.

Benefits and Practical Applications

• Trace-level sensitivity in field settings without the need for bulky GC-MS or LC-MS systems
• Cost-effective substrates produced via inkjet printing on paper substrates
• Versatile sample introduction formats (pipette, dip, swab, chromatographic strip)
• Automated identification using onboard spectral libraries and software matching
• Reduced analysis time and backlog relief for forensic laboratories and quality control teams

Future Trends and Opportunities

Emerging directions include:

• Customized membrane chemistries for selective adsorption of specific analyte classes
• Integration of microfluidic sample handling with P-SERS strips for automated on-site analysis
• Expansion of spectral libraries with machine-learning-based classification to improve identification accuracy in complex matrices
• Development of multiplexed SERS assays for simultaneous detection of multiple threats or contaminants

Conclusion

Printable SERS substrates coupled with a portable Raman analyzer represent a significant advance in trace chemical detection. Metrohm Raman’s P-SERS technology dramatically lowers cost, simplifies workflows and extends Raman spectroscopy into challenging applications in forensics, food safety and environmental monitoring.

References

1. E. P. Hoppmann, W. W. Yu, and I. M. White, Highly sensitive and flexible inkjet printed SERS sensors on paper, Methods, vol. 63, no. 3, pp. 219–224, 2013
2. W. W. Yu and I. M. White, Inkjet-printed paper-based SERS dipsticks and swabs for trace chemical detection, Analyst, vol. 138, no. 4, pp. 1020–1025, Feb. 2013
3. W. W. Yu and I. M. White, Chromatographic separation and detection of target analytes from complex samples using inkjet printed SERS substrates, Analyst, vol. 138, no. 13, pp. 3679–3686, Jul. 2013
4. NIH Publication No. 04-446
5. FDA IA 99-30
6. EPA-HQ-OPP-2009-0317 Docket
7. CDC Wonder