Plasmon Enhanced Fluorescence with Aggregated Shell-Isolated Nanoparticles Igor O. Osorio-Roma ́ n, †,§ Ariel R. Guerrero, † Pablo Albella, ‡ and Ricardo F. Aroca* ,† † Materials and Surface Science Group, Department of Chemistry and Biochemistry, University of Windsor, Windsor, Ontario, Canada, N9B 3P4 ‡ Experimental Solid State Group, Department of Physics, Imperial College London, SW7 2AZ, London, United Kingdom § Departamento de Química Inorga ́ nica, Facultad de Química, Pontificia Universidad Cató lica de Chile, 7820436, Santiago, Chile * S Supporting Information ABSTRACT: Shell-isolated nanoparticles (SHINs) nanostructures pro- vide a versatile substrate where the localized surface plasmon resonances (LSPRs) are well-defined. For SHINEF, the silver (or gold) metal core is protected by the SiO 2 coating, which is thicker than the critical distance for minimum quenching by the metal. In the present work, it is shown that an increase in the SHINEF enhancement factor may be achieved by inducing SHIN aggregation with electrolytes in solution. The proof of concept is demonstrated using NaCl as aggregating agent, although other inorganic salts will also aggregate SHIN nanoparticles. As much as a 10- fold enhancement in the SHINEF enhancement factor (EF) may be achieved by tuning the electrolyte concentrations in solution. The SHINEF experiments include the study of the aggregation effect controlling gold SHIN’s surface concentration via spraying. Au-SHINs are sprayed onto layer-by-layer (LbL) and Langmuir-Blodgett (LB) films, and samples are fabricated using fluorophores with low and also high quantum yield. S HINERS 1 and SHINEF 2 are acronyms used for plasmon enhanced scattering and fluorescence observed with shell- isolated nanoparticles (SHINs). For scattering, the coating of the metal core is made as thin as possible (∼2 nm). However, for surface enhanced fluorescence (SEF) 3 or metal enhanced fluorescence (MEF), 4 there is a critical fluorophore-metal distance for maximum enhancement. 3,5 When an incident field E 0 impinges on a metallic nanostructure, an enhanced local field E loc is observed. 6 The ratio leads to a local enhancement factor | E| = |(E loc /E 0 )|. The plasmonic origin of enhancement leads to surface enhanced fluorescence (SEF) proportional to |E| 2 , and surface enhanced Raman scattering (SERS) proportional to | E| 4 . 7,8 In a previous report, 9 it was shown that recording SHINERS and SHINEF for a low quantum yield molecule and Ag-SHINs in solution can provide direct experimental evidence for the predicted local enhancement dependence of the measured spectroscopic signal. The latter measurements in solution of molecules adsorbed onto mainly isolated Ag-SHINs were characterized for very modest enhancement factors (EF). In this work, we look at the increase of the EF by aggregating SHIN nanoparticles. The effect of aggregation with Ag and Au colloids is well-known in the literature; 10,11 in particular, interparticle junctions in aggregated nanoparticles serve as hot- spots for field enhancement in nanometric spatial locations. 12 Aggregated nanoparticles are shown experimentally, and when the fluorophore is at the appropriate distance from the surface, efficient SEF is observed, where enhancement factors are in the range of 15-750. 13 Recently, self-assembled aggregates of Ag nanoparticles produced by introducing polyacrylamide into Ag colloids have been shown to enhance fluorescence of a high quantum yield fluorescein isothiocyanate isomer by more than 27-fold. 14 We explore here SHIN’s aggregation, first in solution to demonstrate the increase in the EF solely due to the formation of aggregates. Further, we look at the aggregation effect using spraying techniques on layer-by-layer (LbL) and Langmuir-Blodgett (LB) samples of high and low quantum yield molecules. In addition, finite-difference time-domain (FDTD) computations qualitatively illustrate the near field and far field enhancement trend, as SHIN’s dimer gap is decreased. ■ EXPERIMENTAL SECTION Crystal violet (CV, total dye content 90%) was purchased from Fisher Scientific. Malachite green and Eosin-Y (total dye content 50%), 3-aminopropyltrimethoxysilane (APTMS), and tetraethylorthosilicate (TEOS, 98%) were purchased from Sigma-Aldrich and used as received. Octadecyl Rhodamine B (R18) was obtained from Invitrogen. Unless otherwise specified, solutions are aqueous and the water employed is Received: July 1, 2014 Accepted: September 16, 2014 Published: September 16, 2014 Article pubs.acs.org/ac © 2014 American Chemical Society 10246 dx.doi.org/10.1021/ac502424g | Anal. Chem. 2014, 86, 10246-10251