Ultra-Sensitive, Label-Free Biomolecule Sensing on Spin-Coated Plasmonic Quasicrystal Substrates

Surface-enhanced Raman spectroscopy (SERS) offers rapid, non-invasive detection of biomolecules down to single-molecule levels—but only when the plasmonic substrate is uniform, reproducible, and broadband. Recent work from IIT Delhi and TIFR demonstrates a five-fold plasmonic quasicrystal (PlQC) SERS substrate fabricated via precise PMMA spin-coating and electron-beam lithography (EBL). This platform achieves enhancement factors ≈10¹⁴, attomolar detection of Rhodamine 6G, and nanogram-level cotinine sensing in biofluids, all with <8 % signal variation across centimeter-scale areas.

Why Spin Coating Matters in PlQC Fabrication

  • Perfect PMMA Film: Spin-coat AR632.12 resist at 6000 rpm for 60 s to yield a 100 nm uniform layer.

(a) FESEM images of PlQC at different magnifications, and (b) 3D AFM image of PlQC.

  • High-Resolution Patterning: A defect-free resist layer is essential for precise EBL of the quasicrystal lattice.
  • Scalable Uniformity: Spin coating ensures identical resist thickness across large substrates, critical for reproducibility.

Fabrication Process Overview

  1. PMMA Deposition: Spin-coat 100 nm AR632.12 at 6000 rpm × 60 s; soft-bake to remove solvents.
  2. EBL Pattern Transfer: Write the five-fold quasi-periodic lattice into the PMMA resist.
  3. Reactive Ion Etching: Etch 100 nm Au film through the patterned PMMA to form air-hole PlQC arrays (600 nm pitch, ~92 nm holes).

(a) SERS spectra of R6G recorded at concentrations ranging from µM to aM on PlQC along with blank SERS spectra of PlQC, (b-d) the peak height ratio of different Raman lines with respect to the dominant peak at 1366 cm−1 for different concentrations. Straight lines in red, blue, and green represent the mean values, respectively. This shows the robustness of the PlQC substrate towards specificity and sensitivity down to single molecule level (aM concentration).

Courtesy of Sibashish Chakraborty, Rishabh Vij, Richa Goel, Bhanu Pratap Singh, Kunaal Dhingra, Venu Gopal Achanta & Satish Kumar Dubey, “PlQC based highly sensitive and reproducible novel SERS active substrate for biomolecule detection with high specificity,” Scientific Reports 14, 29186 (2024). DOI: 10.1038/s41598-024-74528-9.

  1. Resist Removal & Cleaning: Strip residual PMMA and clean to expose the plasmonic quasicrystal.

Throughout, Navson’s precision spin coater delivers the uniform resist layers that make high-fidelity EBL and defect-free RIE possible.

Record-Breaking SERS Performance

  • Rhodamine 6G (R6G):
    • Distinct Raman peaks at 1195, 1314, 1366, and 1511 cm⁻¹ detected down to 10⁻¹⁸ M.
    • Enhancement factor (EF) ≈10¹⁴.
  • Cotinine (Nicotine Biomarker):
    • Diagnostic peaks at 1032 & 1052 cm⁻¹ resolved at 1 ng/mL in synthetic urine and saliva—below clinical thresholds.
    • I₁₀₃₂/I₁₀₅₂ ratio ≈1.3 ± 0.24 for reliable identification.
  • Uniformity: Fifteen random spots across 1 × 1 cm² show RSD < 8 %, ensuring reproducibility.
  • Label-Free Specificity: Multiple fingerprint peaks allow unambiguous biomolecule identification without labels.

Applications & Next Steps

  • Clinical Diagnostics: Point-of-care SERS kits for monitoring tobacco exposure via cotinine.
  • Forensics & Food Safety: Ultra-trace detection of drugs, toxins, or contaminants.
  • Environmental Monitoring: Label-free sensing of pollutants at ng/mL–pg/mL levels.

Researchers can scale this PlQC approach to diverse targets by adapting the quasicrystal design and leveraging Navson’s spin-coating protocols.

Conclusion

By integrating PMMA spin coating, precision EBL, and reactive ion etching, the PlQC SERS substrate overcomes longstanding uniformity and sensitivity challenges. The result is a broadband, defect-free plasmonic platform capable of attomolar biomolecule detection across large areas—unlocking practical, label-free sensing for healthcare, security, and environmental applications.

Acknowledgements
Figures and data courtesy of Sibashish Chakraborty, Rishabh Vij, Richa Goel, Bhanu Pratap Singh, Kunaal Dhingra, Venu Gopal Achanta & Satish Kumar Dubey, “PlQC based highly sensitive and reproducible novel SERS active substrate for biomolecule detection with high specificity,” Scientific Reports 14, 29186 (2024). DOI: 10.1038/s41598-024-74528-9.

References
Chakraborty S. et al., “PlQC based highly sensitive and reproducible novel SERS active substrate for biomolecule detection with high specificity,” Scientific Reports 14, 29186 (2024). https://doi.org/10.1038/s41598-024-74528-9

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