Raman microscopy to monitor gold (Au) covered silicon nanowire for SERS biosensing

Significance of the Topic

Surface enhanced Raman scattering (SERS) offers ultrahigh sensitivity for molecular detection in various fields such as diagnostics, environmental monitoring and trace chemical analysis. Integrating SERS substrates based on gold-coated silicon nanowires with Raman microscopy enables label-free biosensing with low-cost, scalable fabrication and compatibility with commercial instruments.

Objectives and Study Overview

This study aims to monitor each step of the surface modification process on gold-coated silicon nanowires (Au/SiNWs) deposited on glass slides for the development of a biotin-mediated biosensor for avidin detection. A combination of optical microscopy, scanning electron microscopy (SEM) and Raman spectroscopy is employed to characterize morphology and chemical changes throughout functionalization.

Used Instrumentation

• Thermo Scientific DXR3 Raman Microscope with 50× objective and 532 nm laser
• OMNIC Spectral Software for spectral acquisition and processing
• Scanning electron microscope (SEM) for top and side view imaging

Methodology

• Growth of silicon nanowires by plasma-enhanced chemical vapor deposition (PECVD) on patterned glass slides
• Deposition of a 150 nm gold film by evaporation
• Surface functionalization with cysteamine self-assembled monolayer (SAM) at 70 °C overnight
• Conjugation of N-hydroxysuccinimide ester–biotin in phosphate-buffered saline (PBS)
• Immersion in 1 μM avidin solution for 30 minutes to enable receptor–ligand binding
• Raman spectra collected over 50–3000 cm⁻¹ range with 5 mW laser power, 1 s exposure and 200 accumulations

Main Results and Discussion

SEM imaging confirms uniform Au coverage on disordered SiNW arrays, which enhances plasmonic excitation and light absorption. Optical microscopy reveals color shifts corresponding to each functionalization step: pristine Au/SiNWs, cysteamine SAM, biotin grafting, and avidin binding. Raman spectra show ~20× signal enhancement in Au/SiNW structures compared to planar Au. Progressive increases in intensity between 1280–1450 cm⁻¹ correlate with cysteamine, biotin and avidin modifications. A distinct peak at ~1380 cm⁻¹ confirms specific avidin immobilization only after biotin functionalization, demonstrating selective receptor–ligand interaction.

Benefits and Practical Applications

• Scalable, low-temperature fabrication directly on common glass slides
• Compatibility with commercial Raman microscopes for in situ monitoring
• High sensitivity and specificity through plasmonic enhancement
• Potential for multiplexed, label-free biosensors in medical diagnostics and environmental testing

Future Trends and Potential Uses

Advancements may include integration of SERS substrates with electrochemical detection, development of portable Raman devices for point-of-care testing, multiplex analyte detection arrays, and implementation of machine learning algorithms for spectral analysis and pattern recognition.

Conclusion

The combined use of Raman microscopy and microscopy-based imaging provides a comprehensive characterization of multi-step surface modifications on Au/SiNW SERS substrates. The nanowire morphology significantly amplifies plasmonic effects, and sequential Raman monitoring confirms specific avidin–biotin binding. This platform demonstrates a robust approach for constructing high-performance, label-free biosensors.

References

1. Convertino V, Mussi L, Maiolo L. Disordered array of Au covered Silicon nanowires for SERS biosensing combined with electrochemical detection. Scientific Reports. 2016;6:25099.