Spatiotemporal Confinements of Distance-Dependent Emitters for Enhancing Plasmonic Signals

Surface plasmon resonance (SPR) is a common technique used for the real-time tracing of various analytes through refractive index-dependent resonance shifts. However, many plasmonic biosensors do not meet the clinical detection requirements for ultra-low concentration and low refractive index biomarkers. To address this challenge, researchers have explored unique labeling and interface modification strategies. One common strategy is utilizing fluorescence with plasmonic structures and enhancing the fluorescence intensity. However, these studies primarily focused on plasmon-enhanced fluorescence intensity, leaving the influence of fluorophores on reflection-/absorption-based plasmonic resonance shifts unexplored. Herein, we introduce a technique for amplifying the resonance shift of a plasmonic metasurface by confining the interdistance of fluorescence emitters. By adjusting nanospaces (∼4 to 20 nm), we couple surface plasmons with fluorescence in the near-field, achieving interdistance-dependent resonance shift behavior. This approach results in a 4.5-fold signal enhancement in the resonance shift for detecting conjugated proteins from complex matrices. In this regard, we utilize a plasmonic metasurface and distinct fluorescent emitters (FITC, Texas Red, streptavidin-quantum dot (QD) 525, and streptavidin-QD 625) with diverse excitation and emission assets. We also experimentally demonstrate a spectral blue shift of the plasmonic resonance through resonant coupling between QDs and surface plasmons, in contrast to the conventionally observed red shift. To hurdle the cost- and fabrication-related challenges in metasurfaces, we recycle off-the-shelf digital versatile discs (DVDs) into plasmonic metasurfaces due to their intrinsic nanograting structures, thereby significantly minimizing the cost down to $1.5. Moreover, we collect spatiotemporal signals using a palm-sized platform (5 cm × 10 cm x 1 cm) within 15 min that would be easily adapted into any settings possible. Consequently, this strategy paves the way for creating novel configurations and arrangements on a metasurface sensor to couple with fluorescence molecules while boosting the sensor's analytical performance that would be potentially integrated with biosensing applications in disease diagnostics.