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

Importance of the topic

Surface-enhanced Raman scattering (SERS) on nanostructured noble metals delivers ultrahigh sensitivity for molecular detection in fields such as diagnostics, environmental monitoring and trace analysis. Gold-coated silicon nanowires (Au/SiNWs) represent a low-cost, scalable SERS substrate whose enhanced light absorption and plasmonic response can be tailored for biosensing applications.

Objectives and study overview

This work demonstrates the use of Raman microscopy to monitor each step in the fabrication of a biotin-modified Au/SiNW substrate designed for selective avidin detection. The goal is to correlate morphological and chemical changes during surface functionalization with SERS performance.

Methodology and sample preparation

SiNWs were grown on defined areas of a glass slide by Au-catalyzed plasma-enhanced chemical vapor deposition (PECVD). A 150 nm Au film was then evaporated over the nanowire array. Sequential surface modifications included:

• Immersion in 20 mM cysteamine at 70 °C to form a self-assembled monolayer (SAM).
• Reaction with N-hydroxysuccinimide ester-biotin (NHS-biotin) in PBS at room temperature.
• Exposure to 1 µM avidin solution for 30 minutes, followed by PBS rinsing and drying.

Instrumentation used

• Thermo Scientific™ DXR3 Raman Microscope with 50× objective and 532 nm laser (5 mW).
• OMNIC Spectral Software for data acquisition.
• Scanning electron microscope for top- and side-view imaging of nanowires.

Main results and discussion

Optical images revealed a color shift from green to blue upon biotin functionalization and avidin binding, indicating uniform molecular adsorption. SEM views confirmed Au conformity over SiNWs. Raman spectra showed:

• ~20-fold signal enhancement in the 960–1760 cm⁻¹ range for Au/SiNWs versus planar Au.
• Progressive intensity increase between 1280–1450 cm⁻¹ after each modification step.
• A distinct peak at ~1380 cm⁻¹ corresponding to avidin, persisting after PBS rinsing only in biotin-modified samples, confirming specific receptor-ligand binding.

Benefits and practical applications of the method

The combined use of microscopy and Raman spectroscopy enables real-time, in situ verification of each surface functionalization step. The high SERS activity of Au/SiNWs supports sensitive detection of biomolecules in liquid environments. The fabrication approach is compatible with conventional glass substrates and commercial Raman systems, facilitating adoption in research and QA/QC laboratories.

Future trends and potential applications

Advances may include integration of microfluidic channels for continuous biosensing, multiplexed detection using patterned nanowire arrays, development of portable Raman instruments for point-of-care testing, and extension of the platform to other receptor-ligand pairs and environmental analytes.

Conclusion

This study confirms that Raman microscopy, combined with optical and electron imaging, provides a comprehensive toolkit to monitor and optimize the multi-step assembly of Au/SiNW SERS biosensors. Nanowire morphology enhances plasmonic excitation and signal strength, and functionalization steps are unambiguously validated by characteristic Raman signatures of each chemical layer and target binding.

References

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