Rigid tetrahedral DNA nanostructures on graphene-assisted plasmonic sensor for highly sensitive infrared fingerprinting.

Surface-enhanced infrared absorption spectroscopy is a powerful tool for label-free molecular fingerprinting. However, its quantitative accuracy and sensitivity are often compromised by the inherent roughness of metallic substrates, which leads to heterogeneous probe adsorption and random molecular orientation. Herein, we propose a dual-interface engineering strategy to construct a robust biosensing platform for the highly sensitive detection of microRNA-21. A metal-insulator-metal plasmonic sensor is functionalized with monolayer graphene, serving as an atomically flat buffer layer to shield the underlying gold grain roughness and facilitate uniform bio-functionalization via non-covalent π-π stacking interactions. Furthermore, rigid tetrahedral DNA nanostructures are immobilized on the graphene surface as upright scaffolds, effectively regulating the probe spacing and preventing the "lying-down" orientation common to single-stranded DNA. This synergistic design not only maximizes the overlap between the target biomolecules and the plasmonic near-field hotspots but also significantly improves the signal-to-noise ratio and sensor reproducibility. Consequently, the sensor demonstrates a superior sensing capability detectable down to 1 pM for miRNA-21, representing a 50-fold improvement over the bare metal-insulator-metal (MIM) counterpart. This work highlights the critical role of interfacial architecture in plasmonic sensing and provides a promising strategy for future bioanalytical applications.