Controlling the Luminescence of Carboxyl-Functionalized CdSe/ZnS Core-Shell Quantum Dots in Solution by Binding with Gold Nanorods Monica Focsan, † Ana M. Gabudean, † Adriana Vulpoi, ‡ and Simion Astilean* ,† † Nanobiophotonics and Laser Microspectroscopy Center, and ‡ Nanostructured Materials and Bio-Nano-Interfaces Center, Interdisciplinary Research Institute in Bio-Nano-Sciences and Faculty of Physics, Babes-Bolyai University, 1 M. Kogalniceanu, 400084, Cluj-Napoca, Romania * S Supporting Information ABSTRACT: Plasmonic nanostructures offer promising routes toward artificial control of the photoluminescence properties of various emitters. Here, we investigated the photoluminescence of carboxyl-functionalized CdSe/ZnS core-shell quantum dots (c-QDs) localized near gold nanorods (AuNRs) as a function of c-QDs-AuNRs distance using the cetyltrimethylammonium bromide (CTAB) surfac- tant and Bovine Serum Albumin (BSA) protein layers over coating metal surface as spacer. The direct binding of negatively charged c-QDs to positively charged CTAB (3-4 nm thickness) caused close contact with the metal, resulting in an efficient metal-induced energy transfer (quenching). We found that quenching is modulated by the degree of spectral overlap between the photoluminescence band of c-QDs (620 nm) and longitudinal localized surface plasmon resonance (LSPR) of AuNRs (637 and 733 nm). Deposition of BSA layer over CTAB coated-AuNRs and subsequent decoration with c-QDs yielded an increase in photoluminescence signal when exciting in resonance with the transverse LSPR of AuNRs. On the basis of experimental studies using steady-state and time-resolved fluorescence measurements as well as finite-difference time-domain calculations, we report over 70% quenching efficiency for all investigated AuNRs along with a 4.6-fold in photoluminescence enhancement relative to free c-QDs (39-fold enhancement relative to c-QDs loaded AuNRs) after BSA deposition. ■ INTRODUCTION In the field of nanomaterials research, a key goal is to integrate within the same nanosystem multiple functionalities in view of biosensing and bioimaging applications. 1 Resonant coupling between luminescent semiconductor nanoparticles (quantum dots, QDs) and plasmonic metallic nanoparticles can generate new remarkable optical effects, extending thus the applications field of as-designed nanometer-scale hybrid structures. Because of their broad excitation spectra, size-tunable photolumines- cence emission spectra, and superior photostability against photobleaching, QDs are very appealing in practical biological applications, especially for multiplexed labeling or multiple immunoassays, as an alternative to ionic and molecular fluorophores. 2 On the other hand, due to their unique optical properties related to their localized surface plasmon resonance (LSPR), gold nanoparticles (AuNPs) act as powerful nanoscale optical antennas, 3 as they are able to significantly enhance light absorption or alter the radiative and nonradiative decay rates of nearby located dipoles. 4 In particular, luminescence enhance- ment occurs when the dominant relaxation pathway is radiative decay, and vice versa, the luminescence is quenched when the nonradiative decay represents the dominant mechanism. For instance, QDs were successfully exploited for metal-enhanced fluorescence (MEF), 5,6 as well as for fluorescence resonance energy transfer. 1,7 Furthermore, it has been already demonstrated that several factors influence the plasmon-exciton interaction such as distance between QDs and metal surface, the excitation wavelength, the polarization of excitation, the size of QDs, the geometry of nanoparticles, and the spectral overlap between the luminescence of QDs and the LSPR band of metal nanoparticles. 8-10 The interplay between these factors determines the magnitude of the luminescence enhancement or quenching of QDs. In fact, to control these above-mentioned parameters, a number of experimental methods have been developed, including layer-by-layer (LBL) polyelectrolyte deposition technique, 11 the utilization of hybrid metal@ silica@QDs structures, 12 or the utilization of biomolecules (e.g., DNA molecule, streptavidin, biotin) to adjust the interparticle distance. 13-15 However, despite several exper- Received: February 5, 2014 Revised: October 1, 2014 Article pubs.acs.org/JPCC © XXXX American Chemical Society A dx.doi.org/10.1021/jp501281v | J. Phys. Chem. C XXXX, XXX, XXX-XXX