Coupling of Molecular Transition with the Surface Plasmon Resonance of Silver Nanoparticles inside the Restricted Environment of Reverse Micelles.

Abstract

Interaction of molecular transitions of two fluorophores—fluorescein (FL) and safranin O (SAF)—with the surface plasmon resonance (SPR) of silver nanoparticles (AgNPs) inside a water/sodium dioctylsulfosuccinate (AOT)/n-heptane reverse micelle (RM) has been studied using ultraviolet–visible and fluorescence spectroscopies. Here, we exploit the natural capacity of a RM to act simultaneously as a template for nanoparticle formation and host the fluorophores. The fluorophores and reducing agent were loaded together into the water pool; thereafter, silver salt was added, and subsequently, spectral modification and size evolution were monitored by steady-state and time-resolved optical spectroscopy. In the FL–AgNP composite, the SPR band of AgNPs undergoes a strong red shift. Moreover, significant modifications of both the fluorescence intensity and lifetime of FL were found when AgNPs formed inside the RM core. On the contrary, in the SAF–AgNP composite, no such effect was noticed, and the composite system retains the original optical characteristics of their constituents (i.e., both the position of molecular transitions and SPR maximum remain unchanged). This differential effect has been rationalized by the dissimilar plasmon–fluorophore coupling in the two systems, controlled by a combination of spatial distribution and spectral detuning of the molecular absorption maxima of the dyes (455 and 530 nm for FL and SAF, respectively) from the SPR band maximum (∼400 nm) of AgNPs.