Article pubs.acs.org/JPCC
Metal-Enhanced Fluorescence of Pseudoisocyanine J‑Aggregates Formed in Layer-by-Layer Assembled Films Alexander V. Sorokin,*,† Alexander A. Zabolotskii,‡ Nikita V. Pereverzev,† Irina I. Bespalova,† Svetlana L. Yefimova,† Yury V. Malyukin,† and Alexander I. Plekhanov‡ †
Institute for Scintillation Materials, STC “Institute for Single Crystals”, NAS of Ukraine, 60 Lenin Avenue, 61001 Kharkov, Ukraine Institute of Automation and Electrometry, SB of RAS, 1 Academician Koptug Avenue, 630090 Novosibirsk, Russia
‡
S Supporting Information *
ABSTRACT: A formation of pseudoisocyanine (PIC) dye Jaggregates in the polyelectrolyte film by the layer-by-layer (LbL) assembly method has been studied. It has been shown that this process leads to significant J-band widening and fluorescence quenching as a result of increasing static disorder. To enhance the J-aggregate fluorescence properties, the effect of J-aggregate interaction with plasmon resonances of gold nanoparticles has been used. It is found that the maximal 8-fold fluorescence enhancement for PIC J-aggregates in the LbL films could be achieved at 16 nm distance between Au nanoparticles (NPs) and the J-aggregates. Plasmon influence on the J-aggregate fluorescence has been analyzed using a two-level system in the local plasmon field approximation. The model gives a good correlation with the experimental results and could be used for further studying the exciton−plasmon interaction in J-aggregates.
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There are two main ways to form J-aggregates in polymer films: spin-coating11−16 and layer-by-layer assembly (LbL).17−19 Unfortunately, both ways cause a significant decrease in Jaggregate fluorescence quantum yield.15,16,19 So, special efforts should be made to increase the fluorescence quantum yield of Jaggregates formed in polymer films. A very attractive way to improve the optical properties of Jaggregates is using the effect of the metal-enhanced fluorescence (MEF) as a result of fluorophore interaction with a surface plasmon resonance of noble metal nanoparticles (NPs).20−22 Exciton−plasmon coupling in “J-aggregate−metal nanoparticle” complexes is under great attention due to novel hybrid electronic states appearing.23−29 However, a huge amount of works are addressed to absorption spectra changes with main attention to the case of the maximal exciton− plasmon interaction which is achieved at minimal distance between the J-aggregates and metal nanoparticles.23−29 This case corresponds to a strong fluorescence quenching and could not be used for our purpose. Recently, we have reported the first observation of the J-aggregate fluorescence enhancement due to the exciton−plasmon interaction.30 Up to now it was the single MEF study for J-aggregates found in the literature. Using silver NPs coated with the polymer LbL film of varied thickness we obtained 2-fold fluorescence enhancement of thiacyanine Jaggregates in a water solution. The theoretical description of
INTRODUCTION Well-ordered molecular nanoclusters called J-aggregates attract great attention due to their unique optical properties, distinctly different from those of the individual molecules constituting the aggregate: narrow absorption band, high oscillator strength, giant third-order susceptibility, resonant fluorescence, etc.1−4 Such optical properties of J-aggregates are explained by the strong interactions between the molecules within the aggregates.1−4 The resulting delocalization of electronic excitations over certain molecules on the chain leads to the formation of collective eigenstates for all molecules, the exciton state (Frenkel exciton formation).1−5 Depending on a type of molecular packing within the aggregate chain, one can observe a blue-shifted exciton band (H-band, the “face-to-face” arrangement), a red-shifted band (J-band, the “face-to-tail” arrangement), or both J- and H-bands due to the “herringbone”-type molecular packing.1−4 The distinct feature of J-aggregates is a close correlation between J-aggregate excitonic properties and structure that opens up possibilities for the manipulation of J-aggregate optical characteristics by changing the condition of nanocluster formation.1−4 Jaggregates have proved themselves as a perspective material for a number of applications such as photography, nonlinear optical devices, optical memory, and some others.1−8 Low photostability in solutions resulting in photodegradation and photoreorganization processes is a considerable disadvantage of J-aggregates.9,10 One of the ways to overcome this problem is using solid samples of J-aggregates especially in the form of polymer films suitable for many applications.11−13 © 2014 American Chemical Society
Received: October 11, 2014 Revised: December 16, 2014 Published: December 29, 2014 2743
DOI: 10.1021/jp5102626 J. Phys. Chem. C 2015, 119, 2743−2751
Article
The Journal of Physical Chemistry C
deposition of a negatively charged polymer layer from an aqueous polyanion PSS solution (2 wt %). As PIC dye is the cationic one (Chart 1), the PIC J-aggregate layer was deposited on the PSS layer or directly on the Au NP layer. To control the distance between Au NPs and PIC J-aggregates PDDA and PSS layers were alternated. The J-aggregate layer was coated by the PSS layer to provide isolation from the surrounding air. Each layer deposition was followed by rinsing sprayed distilled water. A scheme of the obtained layered polymer film is presented in Chart 1. Fluorescence spectra of the polymer films were obtained using fluorescent microscope MIKMED-2 var.11 (LOMO, Russia) coupled with microspectrometer USB4000 (OceanOptics, USA) via a homemade fiber-optic adapter attached to the 20X eyepiece. The spectral output of the microspectrometer USB4000 was calibrated using a calibrated tungsten halogen lamp HL-2000-CAL (OceanOptics, USA). To get images and spectra Micro-Fluar type 40X/0.65NA objective (LOMO, Russia) was used. Fluorescence was excited at 450−480 nm and collected in the 520−700 nm spectral range. Absorption spectra of aqueous solution in a 1 mm thick quartz cuvette were registered using a spectrophotometer USB4000 (Ocean Optics, USA) supplied with an incandescent lamp. To obtain absorption spectra of the thin polymer films the fluorescent microscope was used working in transmitted light mode with 10X/0.50NA objective. Fluorescence decay spectra were registered using a FluoTime 200 fluorescence lifetime spectrometer (PicoQuant, Germany) equipped with a 531 nm picosecond pulsed laser diode head. An instrument response function (IRF) width for the whole setup was 100 ps. For both liquid and film samples a solid sample holder was used to provide front face illumination. For decay curves analysis FluoFit software (PicoQuant, Germany) was used. Absolute fluorescence quantum yield was measured using a homemade integrating sphere (diameter of 100 mm), which provides a reflectance >99% over the 300−1000 nm range. As an excitation source, a diode-pumped Nd3+:YAG laser (λexc = 532 nm, 5 mW) was used. The absolute quantum yield was calculated using the two measurement method33 taking into account self-absorption correction34 of the fluorescence spectrum due to a very small Stokes shift characteristic for Jaggregates. The experimental setup was adjusted and tested on standard dyes such as rhodamine 6G (in ethanol, C = 10−5 M, Qlit = 0.9435) and quinine sulfate (in 0.1 M H2SO4, C = 10−4 M, Qlit = 0.5835) resulting in ±5% accuracy which is typical for such a type of setup.33
the created system showed a possibility for 20-fold enhancement at optimal conditions.30 In the present work we have studied the efficiency of fluorescence enhancement for pseudoisocyanine J-aggregates formed in polymer LbL films by gold NPs.
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EXPERIMENTAL SECTION Pseudoisocyanine (1,1′-diethyl-2,2′-cyanine iodide, PIC, Chart 1) dye, anionic polyelectrolyte poly(sodium 4-styrenesulfonate) Chart 1. Scheme of the Layered Polymer Film with the Structural Formula of PIC Dye and Polyelectrolytes
(PSS, average Mw ∼ 70 000 g/mol, powder, Chart 1), cationic polyelectrolyte poly(diallyldimethylammonium chloride) (PDDA, average Mw < 100 000 g/mol, solution 35 wt % in H2O, Chart 1), tetrachloroauric acid (HAuCl4, 99.99% trace metals basis, 30 wt % in dilute HCl), and trisodium citrate dihydrate (Na3C6H5O7·2H2O) were purchased from SigmaAldrich (USA) and used as-received. PIC J-aggregates were prepared by dissolving the dye PIC (0.5 mM) in an aqueous NaCl (0.2 M) solution under moderate heating (
