# Spatiotemporal Confinements of Distance-Dependent Emitters for Enhancing Plasmonic Signals

## TL;DR

This paper introduces a low-cost method to enhance plasmonic biosensors by controlling the distance between fluorescent emitters and plasmonic surfaces, improving detection sensitivity for biomarkers.

## Contribution

A novel technique using nanoscale emitter confinement to amplify plasmonic resonance shifts through near-field coupling with fluorescence.

## Key findings

- Confining fluorophores within 4–20 nm of a plasmonic surface increases resonance shift by 4.5-fold.
- A blue shift in plasmonic resonance was observed due to coupling with quantum dots, unlike the typical red shift.
- Recycled DVDs were used to create low-cost plasmonic metasurfaces at under $1.50.

## Abstract

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.