Label-Free SERS Sensing of PFOA at Few-Molecule Levels Using Plasmonic-Active Diatom Photonic Crystal Nanomembranes

Abstract

Perfluorooctanoic acid (PFOA) is a persistent environmental contaminant that poses significant risks to human health, yet its direct, label-free detection at trace levels remains challenging because of its weak Raman fingerprint signals. Reliable detection usually depends on indirect labeling, labor-intensive preconcentration, or specialized signal-amplification strategies. Here, we demonstrate a direct, label-free surface-enhanced Raman scattering (SERS) sensing for trace PFOA detection using plasmonic-active photonic-crystal biosilica nanomembrane based on Coscinodiscus wailesii diatom frustules. Such naturally occurring photonic-crystal scaffolds are periodic porous architectures that support photonic−plasmonic enhancement when integrated with in situ grown silver nanoparticles. Precision inkjet deposition was employed to deliver nanoliter-volume PFOA solutions onto individual frustules, enabling effective analyte enrichment within SERS-active regions. In deionized water, the platform enabled a near-threshold detectable response at 0.01 ppt, corresponding to approximately 1746 PFOA molecules delivered to a single frustule and around 1.18 molecules within each laser spot, indicating few-molecule-level detection under a low-background interference. In river water, a practical detection limit of 10 ppt was achieved, where sensitivity was primarily limited by matrix-induced background and reduced signal-to-noise ratio rather than molecular availability. Spatially resolved SERS mapping, combined with principal component analysis and intensity-distribution analysis, improved the discrimination of weak PFOA-related signals from background variation in both clean and filtered river water samples. These results demonstrate a proof-of-concept PFOA sensing technique using direct, label-free SERS in low-background environmental aqueous samples.