Polyelectrolyte Layer-by-Layer Assembly To Control the Distance between Fluorophores and Plasmonic Nanostructures

Abstract

In the past several years we have demonstrated the metal-enhanced fluorescence (MEF) and the significant changes in the photophysical properties of fluorophores in the presence of metallic nanostructures and nanoparticles. MEF is largely dependent on several factors, such as chemical nature, size, shape of the nanostructure, and its distance from the interrogating fluorophore. Herein, we elucidate the potential of layer-by-layer (LbL) assembly to understand the distance dependence nature of MEF from sulforhodamine B (SRB) assembled on the plasmonic nanostructured surfaces [in the form of Silver Islands films (SIFs)]. The varied proximity of fluorophores from the SIF surfaces was controlled by constructing different numbers of alternate layers of poly(styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH). An anionic laser dye SRB could be electrostatically attached to the positively charged PAH layer. Orientation of the SRB probe molecule adsorbed in PSS/PAH-layered assembly was determined by polarized absorption spectroscopy. The observed tilt angle of the probe transition dipole moment with respect to the surface normal was 40°. Our results show that MEF is indeed distance-dependent. Accordingly, we observed a maximum of a ~6-fold increase in the fluorescence intensity from a monolayer of the SRB at a distance of ~9 nm from the metal-nanostructured surface, with the enhancement decreasing down to ~1.5-fold at about a 30 nm separation distance. Consistently, the minimum lifetimes were about 4-fold shorter than those on glass slides without silver, with the lifetimes being about nearly the same for 15 layers of the PSS/PAH assembly. The intensity-time decays were analyzed with a lifetime distribution model to underpin the distance effect on the metal-fluorophore interaction in the nanometric range. The present study provides improved understanding of the interaction between plasmonic nanostructures and fluorophores and, more importantly, their distance dependence nature, where we used a robust, easy, and inexpensive alternate electrostatic LbL assembly as a bottom-up nanofabrication technique to control the probe distance from the surface.

The Distance-Dependent Fluorescence Enhancement Phenomena in Uniform Size Ag@SiO2@SiO2(dye) Nanocomposites

“…The maximum MEF enhancement factor of 4.3 was obtained when Ag@SiO 2 -X@SiO 2 (FITC) nanocomposites had the thinnest first silica shell of 8 nm and the average separation distance between the surface of silver nanopartilces and fluorescein moities was 11 nm, which was consistent with reported values. 5,13,15 As the thickness of the first silica shell was increased to 32 nm, the localized surface plasmon from the silver nanoparticles could no longer affect the fluorescence intensity of the fluorescein moities. The enhancement is believed to be due to concentrated local electromagnetic field in the vicinity of fluorescein moieties by the localized surface plasmon resonance at the surface of silver nanoparticles which enhances the excitation rate of fluorescein moieties and eventually leads to an increase in the emitted fluorescence intensity.…”

“…Ray and co-workers employed layer-by-layer (L-b-L) techniques of poly(styrene sulfonate) (PSS) and poly-(allylamine hydrochloride) (PAH) to study the distance dependent nature of metal-enhanced fluorescence from sulforhodamine B (SRB) assembled on the silver islands films and a maximum of a ~6 fold increase in the fluorescence intensity from a monolayer of the SRB at a distance of ~9 nm from the silver island surface was observed. 5 Aslan and co-workers utilized a simple and rapid wet-chemical technique for the deposition of silver triangles and silver nanorods on conventional glass substrates. They found that these new surfaces are a significant improvement over traditional silver island films for applications in metal-enhanced fluorescence.…”

Polymer-based Photonic Crystal Cavity Sensor for Optical Detection in the Visible Wavelength Region

“…To evaluate the performance of the optical sensor, we used the layer-by-layer (LbL) method, which is a technique to form multilayers comprising polymer monolayers on a substrate surface by the electrostatic interaction between a polycation and a polyanion. 22,23 When using this method, changes in the air-hole radius and the surrounding refractive index of the 2D-PhC are expected (schematically shown in Supporting Information 3). To form multilayers, a polycation solution (3 mg/mL PAA) and a polyanion solution (3 mg/mL PSS) were alternately introduced onto the PhC cavity surface and dried using an air blower.…”

Fluorescence enhancement in large-scale self-assembled gold nanoparticle double arrays

“…This increases its emission and influences its quantum yield, shortens the fluorophore life time, and improves its photostability by reducing the chances of photo-bleaching. [12][13][14]18,19,[23][24][25][26][27] The increase in decay rate occurs because the excited fluorophores do not just couple to the free space modes. In close vicinity to the nanoparticle, they can also couple to the plasmon mode.…”

“…1,[12][13][14][15]22 As a result and considering the concerted action of all processes, there is an optimal distance for the fluorophore from the particle's surface to maximize the fluorescence. 12,16,19,21,26,30 Therefore, it is essential to control the distance between the fluorophores and the particles surface. 12,14,16,19,21,26,31 In order to achieve such distance control, the use of different types of spacers has been reported such as silica, 22,24,[32][33][34][35] polymers, 7,19,21,[36][37][38][39] and biomolecules.…”