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Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
Aqueous Gold Nanoparticle Solutions for Improved Efficiency in Electrogenerated Chemiluminescent Reactions Pilar Perez-Tejeda,* Elia Grueso, Ana Marin-Gordillo, Concepcion Torres-Marquez, and Rosa M. Giraĺ dez-Peŕ ez Department of Physical Chemistry, Faculty of Chemistry, University of Seville, 41012 Sevilla, Spain
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ABSTRACT: The efficiency of the high-energy electrogenerated chemiluminescence (ECL) tris(2,2′-bipyridine)ruthenium(II)/oxalate reaction was determined in the presence of citrate-coated gold nanoparticle (AuNPs) aqueous solutions. The ECL efficiency (ΦECL) for this reaction is enhanced by the addition of increasing amounts of AuNPs, followed by a sharp decrease when [AuNPs] is higher than 1.6 × 10−8 M. To explain this ΦECL/[AuNPs] trend, photoluminescent (PL) quenching, transmission electronic microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS) measurements were also performed as well as redox potentials for the [Ru(bpy)3]3+/2+ couple. Two facts were noteworthy: First, there is an increase in ΦECL, which accompanies an energy transfer effect by coupling between the Ru(II) complex ECL emission band and the AuNPs surface plasmon resonance (SPR)-induced band. This coupling, which we have called the ECL-SPR effect, enabled the quantifying of a coefficient (KECL‑SPR, ECL-SPR effectiveness) for the improved ΦECL of the tris(2,2′-bipyridine)ruthenium(II)/ oxalate reaction, which turned out to be about 3.5 times higher than the quenching constant for the [Ru(bpy)3]2+/Au@citrate system. This ECL-SPR effectiveness is the basis for applying the ECL-SPR coupling effect in a given reaction. Specifically, it measures the degree to which an ECL reaction has been improved. Second, a metal nanostructure is generated in the solution due to a sufficiently strong electrostatic binding between the Ru(II) complex and the AuNPs, which is revealed through TEM and EDS measurements and the redox potential trend. KEYWORDS: electrogenerated chemiluminescence, Au@citrate/[Ru(bpy)3]2+ binding, metal nanostructure, emission−SPR coupling, ECL-SPR effectiveness
1. INTRODUCTION It has been established that photoluminescent (PL) emission of a dye can readily be quenched when the dye and a noble metal nanoparticle (NP) are sufficiently close (Fö rster resonance energy transfer (FRET)1−4 or surface energy transfer (SET)5,6 models). These energy transfer processes depend on the size and shape of the NP, the dye’s molecule− NP distance as mentioned above, and the overlap of the dye’s emission with the NP’s absorption spectrum, in such a way that when the dye molecule is at a certain distance from a nanoparticle, the interaction of both species can be suitable, and light emission from the dye can generate a local NP surface plasmon resonance (SPR) excitation; as a result, the dye’s emission can be amplified by the action of the local SPRinduced excitation.7,8 © XXXX American Chemical Society
Overall, the improvement of PL by a metal nanostructure uses effective coupling between excited dyes and the localized SPR in metal nanostructures to enhance PL emission. A singularity that highlights the plasmon bands of metal nanostructures is regarded as the improvement of the electromagnetic field, which converts them into suitable passive antennas to transfer light energy to apt molecules located at a certain distance. Such a peculiarity has been used in many fields, such as solar energy storage and conversion,9−12 optical biosensing,13−16 and surface-enhanced Raman spectroscopy (SERS).17−21 Received: July 29, 2018 Accepted: August 7, 2018
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DOI: 10.1021/acsanm.8b01323 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Nano Materials Scheme 1. Au@citrate/[Ru(bpy)3]2+ System
solutions. The noteworthy findings were that ECL measurements can display the emission−SPR coupling effect, and it is also possible to quantify its effectiveness. We have been defined a coefficient to measure the efficacy of the NPs system in enhancing a given ECL reaction. This coefficient, the ECLSPR effectiveness, is the basis for applying the ECL-SPR coupling effect in a given reaction. Specifically, it measures how much an ECL reaction has been improved.
In a broad sense, the enhancement of PL by dye emission− SPR nanostructure coupling has contributed to the development of nanotechnology.9−22 Recently, the study of the plasmon−molecule interaction is proving to be of great interest.4,8,23−26 For example, studies can be highlighted that evaluate the conditions (critical dye concentration and oscillator strength) for reaching the strong coupling limit between NP plasmons (silver nanodisks) and organic dyes (rhodamine-6G), using scattering and absorption spectroscopy.8 Also, it has just been confirmed that the coupling energy for AgNPs plasmon−Alexa Fluor 488 exciton is extremely distance dependent. A strong coupling is detected even at a considerable distance from the FRET limit of 10 nm.4 The use of the SERS technique for monitoring the kinetics of chemical reactions could also be highlighted. For example, the reduction of the 4-nitrobenzenethiol (NBT) to 4-aminobenzenethiol (ABT) by sodium borohydride in the presence of a single NP core of 80 nm with attached small particles.26 Based on the foregoing results, the focus of plasmon− molecule couplings is of interest because the understanding of these interactions can help address important issues related to the potential applications of nanomaterials and nanostructures. The aim of this study is to obtain a deeper insight into the plasmon−molecule interactions using a high-energy electrogenerated chemiluminescent (ECL) reaction in colloidal gold solutions. ECL is a process in which species that are created in an electrode subsequently give place to high-energy electrontransfer reactions in the solution, producing at least one species in its excited state.27−37 After about 50−60 years of study, ECL has now become a powerful tool of research which is applied in a wide range of areas.31−39 More specifically, it should be noted that studies related to the increase in ECL due to coupling between plasmons and dye molecules were applied with the improvement in the detection of different species, which had and still have a great impact,15,18,38,39 thereby contributing to the development of ECL reactions as an analytical technique. Nevertheless, to the best of our knowledge, there are no studies in which the magnitude of the effect of the coupling is quantified, which is one of the main findings presented here. In this study, the ECL reaction, [Ru(bpy)3]2+ + C2O42−, in the presence of AuNPs (5 nm core diameter, stabilized by citrate ions) (see Scheme 1) aqueous solutions was carried out. In addition, photoluminescent (PL) quenching of the [Ru(bpy)3]2+* species by AuNPs, transmission electronic microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) measurements were done and redox potentials of the [Ru(bpy)3]3+/2+ couple were determined in several NPs
2. EXPERIMENTAL SECTION 2.1. Materials. All chemicals (commercial gold colloid solution, [Ru(bpy)3]Cl2, NaCl, Na2C2O4, Na2PO4H, and NaPO4H2) were analytical grade, purchased from Sigma-Aldrich and used without further purification. The commercial citrate gold colloid solution contained about 0.01% (2.94 × 10−4 M) HAuCl4 suspended in 0.01% tannic acid, 0.04% trisodium citrate, 2.6 × 10−4 M potassium carbonate, and 0.02% sodium azide. From [HAuCl4] solution, 8.0 × 10−8 M AuNPs was calculated under the assumption that the particles are hard spheres with a 5.0 nm core diameter and taking 17.81 Å3 as the volume of an Au atom. All solutions were made with deionized water from a Millipore Milli-Q system, having a conductivity
