Single Molecule Techniques Can Distinguish the Photophysical Processes Governing Metal-Enhanced Fluorescence

Abstract

Plasmonic metal nanoparticles can impact the behavior of organic molecules in a number of ways, including enhancing or quenching fluorescence. Only through a comprehensive understanding of the fundamental photophysical processes regulating nanomolecular interactions can these effects be controlled and exploited to the fullest extent possible. Metal-enhanced fluorescence (MEF) is governed by two underlying processes, increased rate of fluorophore excitation, and increased fluorophore emission, the balance between which has implications for optimizing hybrid nanoparticle–molecular systems for various applications. We report groundbreaking work on the use of single molecule fluorescence microscopy to distinguish between the two mechanistic components of MEF, in a model system consisting of two analogous boron dipyrromethene (BODIPY) fluorophores and triangular silver nanoparticles (AgNP). We demonstrate that the increased excitation MEF mechanism occurs to approximately the same extent for both dyes, but that the BODIPY with the higher quantum yield of fluorescence experiences a greater degree of MEF via the increased fluorophore emission mechanism and higher overall enhancement, as a result of its superior ability to undergo near-field interactions with AgNP. We foresee that this knowledge and methodology will be used to tailor MEF to meet the needs of different applications, such as those requiring maximum enhancement of fluorescence intensity or instead prioritizing excited-state photochemistry.

Improved imaging and preservation of lysosome dynamics using silver nanoparticle-enhanced fluorescence

“…Nevertheless, while it is true that the scattering component of the extinction spectrum of AgNP tends to be greater for larger nanostructures of the same shape, it is the overlap between the emission spectrum of the dye and the nanoparticle scattering that is essential for MEF. In our case, the synchronous scattering (λEx = λEm) spectrum of AgNP (Dragan et al, 2013;Hodgson et al, 2020;Knoblauch et al, 2020) shows that the scattering component of the AgNP extinction well overlaps with the emission of BODIPY (Fig. S1, Supplementary Information).…”

“…1B, green traces). Previously, we demonstrated that the fluorescence of green-emitting probes based on the BODIPY platform can experience MEF by large triangular silver nanoplates (∼ 80 nm), for which the surface plasmon band is centred at 660 nm (Dogantzis et al, 2020; Golian et al, 2021; Hodgson et al, 2020). Indeed, large, non-spherical nanoparticles are known to exhibit increased scattering (Knoblauch et al, 2020; Lakowicz, 2005; Lakowicz et al, 2008).…”

“…MEF holds great promise as a tool to augment spectroscopic and fluorescence-based applications (Bouhelier et al, 2004; Ekgasit et al, 2004b; Hodgson et al, 2020; J. Kneipp et al, 2006; K. Kneipp et al, 2006; Lakowicz et al, 2003; Petryayeva and Krull, 2011).…”

“…Surface plasmons of noble metal nanoparticles are a promising means to enhance the emission intensity of fluorescent probes without the need to synthetically alter their chemical structure (Bouhelier et al, 2004; Ekgasit et al, 2004b, 2004a; Hodgson et al, 2020; Lakowicz et al, 2003; Peterson et al, 2016; Petryayeva and Krull, 2011). Plasmon excitation can be described as the collective oscillation of conduction band electrons on the surface of metal nanostructures following the incidence of a photon of appropriate wavelength (Barnes et al, 2003; Hao and Schatz, 2004; Liz-Marzán, 2006; Moores and Goettmann, 2006; Odom and Schatz, 2011).…”

“…This phenomenon is generally described as metal-enhanced fluorescence (MEF) and arises from near-field interactions between AgNP and a fluorochrome. This interaction ultimately enhances fluorescence by increasing the rate of fluorophore excitation (i.e., the enhanced electromagnetic field resulting from plasmonic excitation of AgNP increases the probability of excitation) and/or by increasing the radiative decay rate, which is proportional to the quantum yield of fluorescence (Hodgson et al, 2020). The latter mechanism occurs by coupling between the plasmon resonance oscillation of AgNP and the oscillating dipole of radiating fluorophores, effectively forming a radiative AgNP-fluorophore complex, often referred to as a "plasmophore".…”