Dyads of Au nanoparticles and carbon nanodots confined in plasmonic nanoslits for fluorescence enhancement.

Considerable efforts have been made to enhance the fluorescence of carbon nanodots (CNDs) through surface engineering or functionalization. However, these methods often alter the intrinsic properties of the CNDs. Although only a few studies have reported such plasmonic enhancement for CNDs, the rational engineering of plasmonic platforms to modulate light-matter interactions has become a powerful alternative for enhancing emitter performance without altering the emitter. Herein, we offer a simple strategy in which CNDs and gold nanoparticles (AuNPs) are first hybridized and then immobilized within the cavities of a gold nanoslit array to enhance the fluorescence signal of CNDs. For a better understanding of the plasmonic effects, finite difference time domain (FDTD) simulation was conducted for different sizes of AuNPs inside the Au nanoslit. The light intensity and fluorescence measurements correlate with the simulated results and show a fluorescence enhancement factor of 3.7 ± 0.2 for the CND-AuNP dyads confined in the nanoslit, defined relative to pristine CNDs measured on glass under identical illumination and collection conditions (2.2 ± 0.1 relative to the same dyads outside the slit). Controls on CNDs alone with and without the nanoslit, on bare AuNPs and on the empty array establish that dyad formation and slit confinement contribute sequentially, and that the measured enhancement exceeds the product of the two separately measured contributions by a factor of 1.4 ± 0.1, which identifies the architecture as cooperative rather than merely cumulative. The excitation-wavelength dependence of the enhancement, together with the spectral positions of the CND absorption and emission bands relative to the plasmon resonances of the hybrid, indicates that the amplification is dominated by an increased excitation rate in the plasmonic near field, with a smaller emission-side (plasmon-exciton) contribution; the ∼7 nm metal-to-metal gap defined by the self-assembled monolayer and the CND itself keeps the emitters out of the strong non-radiative quenching regime. Comparison with the flat-gold-film reference measured previously on the same platform shows that the glass reference used here is the conservative choice. This study presents a plasmonic platform with the potential to significantly enhance the optical performance of emitters for biosensing and biomedical applications.