Laser‑Assisted Fabrication of Superhydrophobic PTFE SERS Substrates: A Low‑Cost, Ultra‑Sensitive Platform

Abstract

Surface‑enhanced Raman scattering (SERS) delivers single‑molecule sensitivity, making it an attractive optical sensor for diverse fields. Yet, high‑cost fabrication and complex processing hinder its industrial uptake. Here we report a simple, inexpensive approach that uses CO₂ laser ablation to pattern a commercial PTFE (Teflon) film, creating a microarray of hydrophobic circles surrounded by a superhydrophobic matrix. The resulting surface confines Ag nanoparticles and target molecules to sub‑millimeter “virtual wells,” dramatically concentrating analytes during rapid evaporation (≈10 min). Using methylene blue (MB) and rhodamine 6G (R6G) as probe dyes, we achieve a detection limit of 1 × 10⁻¹⁴ M. Detection of bovine serum albumin (BSA) demonstrates applicability to biological samples. This method delivers a robust, low‑cost (≈¥20) SERS substrate with exceptional sensitivity and repeatability, offering broad commercial potential.

Background

Since its discovery in 1974, SERS has been celebrated for its ability to reveal vibrational fingerprints even in ultra‑dilute solutions. The key to its sensitivity lies in the intense electromagnetic field generated by localized surface plasmons at metal nanostructure junctions—so‑called hot spots. Conventional substrates rely on Ag or Au nanoparticles deposited on glass or silicon, but the inherent hydrophilicity of these supports disperses the particles, weakening hot‑spot formation. Recent work has shown that superhydrophobic surfaces can concentrate solutes into tiny regions during evaporation, enhancing SERS signals and reproducibility. However, many such substrates suffer from analyte loss in their micro‑/nanostructures, and their fabrication is often time‑consuming and costly, limiting real‑world deployment.

In this study, we introduce a laser‑ablation‑assisted technique that transforms a commercially available PTFE film into a patterned, superhydrophobic substrate. By carefully selecting laser parameters (power, speed, step length) and designing a CAD pattern of 0.5‑mm circles, we create a surface that is hydrophobic in the circles and superhydrophobic elsewhere. This dual‑wettability design traps droplets in the circles, concentrating both Ag nanoparticles and analytes during evaporation, thus generating a highly active SERS platform.

Methods and Experiment

Materials

Silver nitrate (99.99 %), polyvinylpyrrolidone (PVP, Mw = 58 000), sodium borohydride (NaBH₄), ethylene glycol (EG), methylene blue (MB), rhodamine 6G (R6G), and bovine serum albumin (BSA) were sourced from Shanghai Aladdin Biochemical and Sigma‑Aldrich. Deionized water (18.25 MΩ·cm) was used throughout. Commercial PTFE sheets (50 × 30 × 5 mm) served as the substrate base.

Ag Nanoparticle Synthesis

Ag nanoparticles were prepared by reducing AgNO₃ in EG at 165 °C, followed by NaBH₄ reduction and PVP stabilization. The colloid was centrifuged and washed repeatedly, then dispersed in water to yield concentrations of 1.19 × 10⁻¹¹ M to 1.19 × 10⁻¹⁵ M.

Laser Engraving of PTFE

After rinsing, the PTFE sheet was engraved with a CO₂ laser (output power 16–24 %, speed 35–75 mm s⁻¹, step length 0.02–0.10 mm) following a CAD layout of 0.5‑mm diameter circles spaced 0.8 mm apart.

Characterization

Surface morphology was examined by SEM (TESCAN MIRA 3 FE). Static water contact angles were measured using a commercial ruler software. Ag nanoparticle deposition was performed by spotting 5 µL of colloid onto the substrate, followed by drying at 70 °C. SERS spectra were collected with a 633 nm He–Ne laser (10 mW) on a Raman spectrograph, acquiring one scan every 20 s.

Results and Discussion

The laser‑engraved PTFE displayed a distinct contrast: untreated circles retained hydrophobicity, while the engraved matrix achieved a static contact angle of 151.8°, satisfying the superhydrophobic criterion. SEM images revealed dense micro‑ and nanostructures on the engraved areas, absent on the pristine surface. During evaporation, droplets on the engraved substrate rapidly collapsed into the circles, reducing the contact area by ≈25 % compared to the original PTFE, as confirmed by high‑speed video.

Ag nanoparticles deposited on the engraved PTFE aggregated tightly within the circles after 70 °C drying, avoiding the coffee‑ring effect typical on the original PTFE. The aggregation area on the engraved substrate was roughly 25 × smaller, increasing the density of hot spots and thus SERS intensity.

Parameter optimization demonstrated that a 0.02 mm step length, 20 % laser power, and 55 mm s⁻¹ speed produced the optimal balance of superhydrophobicity and nanoparticle density. A 0.5‑mm circle diameter maximized analyte enrichment while preventing droplet flattening. An Ag colloid concentration of 1.19 × 10⁻¹² M offered sufficient hot spots without excess material.

Using MB and R6G probes, the engraved PTFE achieved a limit of detection (LOD) of 1 × 10⁻¹⁴ M, with clear spectral peaks even at the lowest concentrations. The substrate also successfully detected BSA down to 0.002 µg mL⁻¹, confirming its suitability for protein analysis.

Compared with unpatterned PTFE, the microarray substrate delivered markedly stronger SERS signals: MB spectra at 1 × 10⁻⁹ M exhibited a dramatic intensity increase, attributable to both higher nanoparticle density and focused analyte concentration within the circles. Mapping experiments confirmed a higher probability of signal acquisition on the engraved PTFE.

Conclusion

We have demonstrated a rapid, low‑cost (≈¥20) method for fabricating a superhydrophobic PTFE SERS substrate via laser ablation. The engineered dual‑wettability design concentrates Ag nanoparticles and analytes into sub‑millimeter wells, yielding a 25‑fold reduction in aggregation area and a 10‑fold increase in SERS intensity compared with unpatterned PTFE. The platform reaches a detection limit of 1 × 10⁻¹⁴ M for small dyes and 0.002 µg mL⁻¹ for BSA, demonstrating broad applicability and commercial potential.

Abbreviations

BSA

Bovine serum albumin

EG

Ethylene glycol

MB

Methylene blue

NaBH₄

Sodium borohydride

PTFE

Teflon (polytetrafluoroethylene)

R.T.

Room temperature

R6G

Rhodamine 6G

SEM

Scanning electron microscope

SERS

Surface‑enhanced Raman spectroscopy