Open tip-enhanced V-shaped nanocavity arrays for urolithiasis biomarker osteopontin detection

Surface-enhanced Raman spectroscopy (SERS) is a highly sensitive and selective analytical technique that has shown great potential in bioanalysis. However, due to the relatively large size of protein molecules, steric hindrance can impede target molecules from entering hotspot regions, leaving most analytes at SERS-inactive regions and thereby reducing detection sensitivity and signal reproducibility. In this work, we designed an open, tip-enhanced V-shaped nanocavity array formed by an ordered bilayer of gold nanotetrahedra to enable SERS detection of osteopontin, a biomarker for urinary calculi. A finite element multiphysics simulation was used to elucidate the distributions of electric field enhancements and hydrodynamic processes in solution and air of the nanocavity arrays. The results show that during solvent evaporation, the V-shaped geometry guides and traps osteopontin molecules at the upper and lower tip regions of the nanocavity where the electric field is enhanced, achieving a synergistic effect between localized high field enhancement at the tips and hydrodynamic enrichment. Protein-size-dependent experiments showed that the nanocavity preferentially detects proteins whose dimensions match the cavity scale while exhibiting limited response to larger proteins. Further application to clinical urine sample analysis identified characteristic fingerprint peaks of osteopontin in samples from urolithiasis patients, enabling preliminary discrimination between healthy controls and patients with urinary calculi. Therefore, this tip-enhanced V-shaped nanocavity provides a new approach for the noninvasive detection of urolithiasis biomarkers and offers a valuable solution to the challenge of SERS detection of large molecules.