1 Introduction. Keywords: Silver nanoparticles, cyanine dye, J-aggregation

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1 Nanospectroscopy 2015; 1: Research Article Open Access Bojana B. Laban, Vesna Vodnik, Vesna Vasić* Spectrophotometric observations of thiacyanine dye J-aggregation on citrate capped silver nanoparticles Abstract: The J-aggregation of anionic cyanine dye, 5,5 disulfopropyl-3,3 dichloro - thiacyanine sodium salt (TC) in the presence of 2x10-3 M KCl and citrate capped silver nanoparticles (AgNPs, diameter ~10nm) was investigated. The influence of added salt (KCl), as well as AgNPs and TC concentration on the intensity of the absorption bands with the maxima at 464 and 481 nm, characteristic of the J-aggregation of the dye in solution and on AgNPs surface, respectively, was studied. The spectrophotometric and fluorescence spectra confirmed that AgNPs induced the decomposition of J-aggregates formed in solution on the account of their formation on NPs surface. The obtained results enabled the evaluation of number of TC molecules per AgNPs participating in J-aggregate formation. The experimental results yielded about 320 molecules of TC per AgNPs in at least three layers in a slanted orientation. Keywords: Silver nanoparticles, cyanine dye, J-aggregation DOI /nansp Received July 21, 2014; accepted February 4, Introduction Nanoparticles (NPs) of noble metals have been studied extensively because of their unique properties, arising from localized surface plasmon resonance in the UV-Vis region [1-3]. As a result, NPs have potential applications in many fields, e.g. in catalysts, sensors, microelectronics, photonic and optoelectronic devices [4-6]. In particular, *Corresponding author: Vesna Vasić: Vinča Institute of Nuclear Sciences, University of Belgrade, P.O. Box 522, Belgrade, Serbia, evasic@vin.bg.ac.rs Bojana B. Laban: Department of Chemistry, Science University of Priština, KosovskaMitrovica, Serbia Vesna Vodnik: Vinča Institute of Nuclear Sciences, University of Belgrade, P.O. Box 522, Belgrade, Serbia AgNPs have been focused on due to their remarkble properties, such as good conductivity, catalytic and antibacterial effects [7]. The aggregation of cyanine dyes, has been studied by various groups [8], which are usually present in solution in the monomer and dimer forms. This aggregation occurs due to the strong intermolecular Van der Waals attractive forces between dye molecules [9]. It is well known that the addition of metal ions or NPs in solution induce the formation of J-aggregates of cyanine dyes [10, 11]. The J-aggregates are organized molecular assemblies composed of a large number of dye molecules, which are characterised by the sharp absorption band (J-band), redshifted with respect to the monomer band [12]. Various factors influence J-aggregation, such as solution acidity, temperature, concentration of added inorganic salts, presence of macromolecules and surfactants [8, 13-15]. The formation of J-aggregates on the surface of spherical Ag and AuNPs [4, 16, 17] has been investigated by numerous experimental and theoretical studies. Theoretical and experimental investigations of optical properties of noble metal NPs coated with a dye J-aggregate monolayer suggested that they are strongly dependent on the geometry of the dye and optical properties of the core and the shell materials [16]. The adsorption of dye molecules onto metal surface leads to different photophysics and photochemistry properties than those found in single molecules or on individual metal surfaces [18]. Recently, we studied the mechanism of J-aggregation of 5,5 -disulfopropyl-3,3 -dichloro-thiacyanine dye (TC) in the presence of various capped Au and AgNPs [11, 19, 20]. The results demonstrated that the first step of the reaction was the sorption of the dye on NPs surface, followed by rapid formation of the J-aggregate. Moreover, the quenching of the dye fluorescence on the NPs surface occurred. In this paper, we report the results of the continuation of our study of the interaction between citrate capped AgNPs and TC Bojana B. Laban et al., licensee De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License.

2 Spectrophotometric observations of thiacyanine dye J-aggregation on citrate capped silver nanoparticles 55 2 Methods 2.1 Chemicals Silver nitrate (AgNO 3 ), sodium borohydride (NaBH 4, 99%), potassium chloride (KCl) were purchased from Aldrich and were used as received. Thiacyanine dye (5,5 disulfopropyl-3,3 dichloro - thiacyanine sodium salt, TC, Scheme 1) was purchased from Hayashibara Biochemical Laboratories, Okayama, Japan. Aqueous TC stock solution (5.0 x 10-5 M), containing 6 x 10-3 M KCl was prepared with Milli-Q water. All solutions unless mentioned were prepared with Milli-Q water. Cl Scheme 1: TC dye structure S N Synthesis of Ag NPs S N SO 3 - Cl Na + SO 3 - The aqueous solution of AgNPs was synthesized by NaBH 4 reduction of AgNO 3 in the presence of sodium citrate as a stabilizing reagent. Briefly, 20 ml (1 mm) aqueous solution of AgNO 3 and 1.6 ml (38.8 mm) sodium citrate solution was mixed and vigorously stirred for 10 min. Then, 0.4 ml (21 mg) fresh aqueous solution of NaBH 4 was added dropwise under continuous stirring. A stable yellow Ag solution was left overnight and then cooled (4 o C, in the dark). The concentration of AgNPs (1.6 x 10-8 M) in stock dispersion was determined using eq.1: where c M represents AgNO 3 molar concentration, M Ag - Ag molar mass, r average particle size radius (5 nm), ρ Ag density (10.49 g cm -3 ) and N o Avogadros number. The working solutions were prepared by appropriate dilution of stock dispersion. (1) 2.3 Apparatus Absorption spectra of colloidal solutions were measured using a Perkin Elmer Lambda 35 UV Vis spectrophotometer using a quartz cuvette with a path length of 1 cm. Transmission electron microscopy (TEM) measurements were performed on Hitachi H-7000 and JEOL 100CX microscope instruments at an operating voltage of 100 kv. The samples for TEM observations were prepared by putting a small amount of the colloidal solution on the carbon-supported copper grids, followed by drying in air. For each sample, the size of more than 200 particles in the TEM images was measured to obtain the average particles size. Photoluminescence emission spectra were measured using a Fluorolog-3 Model FL3-221 spectroflurometer (HORIBA Jobin-Yvon). The spectra were collected over a range of nm (excitation, 350 nm). Excitation and emission monochromators were double grating with dispersion of 2.1 nm/mm (1200 grooves/mm) using a xenon lamp source and a TBX detector in a right angle configuration with a 1 cm path length cuvette. Since the intensity is dependent on the power settings, the obtained data were compared within the same set of spectra. 3 Results 3.1 TEM and spectrophotometric characterization of J-aggregates The mean particle size of the prepared citrate capped AgNPs was obtained by fitting the data from the TEM image (Fig.1) to Gaussian distribution function. The results are presented in the inset of Fig. 1. The analysis of the presented data shows that the shape of NPs was nearly spherical and the average particle diameter was found to be ~10 nm. The addition of 1 x 10-5 M TC to the colloid dispersion containing 2.2 x 10-9 M AgNPs resulted in the partial aggregation of dye coated AgNPs (Fig. 1b). The similar behaviour was also observed in the case of J-aggregate formation of TC on the surface of borate capped AuNPs with a particle diameter of 6 nm [19]. The interaction between AgNPs and TC was further followed by spectrophotometry. As can be seen in Fig. 2 line 1, colloidal solution of 2.2 x 10-9 M citrate capped AgNPs has an intense surface plasmon resonance band centered at 390 nm. This band is characteristic of a yellow color dispersion and its position depended strongly on the size and shape of the particles as well as on the coating

3 56 B.B. Laban et al. a) b) Fig. 1: TEM a) micrograph of AgNPs with corresponding particle size distribution (PSD, Inset); b) micrograph of AgNPs in presence of TC dye. on the surface and the dielectric constant of the medium. Moreover, such an intensive absorption of electromagnetic radiation in the visible region occurs when the frequency of the incident light is resonant with the collective oscillation of the free conduction band electrons of AgNPs [21] M) the new absorption band appeared, characterized by the sharp maximum at 481 nm, which was largely redshifted (53 nm) with respect to the monomer absorption (Fig.2, line 3). Similar spectral behaviour of TC was found in the presence of metal ions [13]. However, in the presence of AgNPs, these spectral changes are characteristic of the formation of TC dye J-aggregates on the surface of NPs [20]. The J-aggregate formation could be explained by Frenkel exciton formed through the coupling of transition dipoles on molecules without the formation of chemical bonds between them [23]. However, the absorption spectra of AgNPs dispersion and AgNPs TC dye assembly overlapped and the questions regarding the formation of the AgNPs aggregation could not be answered. 3.2 Spectrophotometric and fluorescence properties of AgNPs TC dye assembly in the presence of KCl Fig.2: Absorption spectra of AgNPs (1, 2.2 x 10-9 M), TC (2, 1 x 10-5 M) and their mixture (the same final concentrations) in the presence of KCl(3,2 x 10-3 M). As previously shown [22], TC dye (Fig.2, line 2) is present as an equilibrated mixture of monomers and dimers in aqueous solution, which are characterized by two absorption bands. The former, with the maximum at 428 nm, is usually assigned to the TC dye monomer (TC - ), since the latter, with the absorption maximum at 409 nm, is assigned to the dimer (TC 2 2- ). After mixing solutions of AgNPs (2 ml, 9.7 x 10-9 M) and TC dye (1 ml, 3 x 10-5 M) in the presence of KCl (2 x The aggregation of TC in aqueous solution is usually dependent on the presence of metal ions [13], which have the crucial role in the formation of J-aggregates. The specificity of metal ions is not only to affect the rate of J-aggregation, but also to increase the equilibrated concentration of J-aggregates when the equilibrium is reached [13]. KCl supports the formation of TC J-aggregates, as found in previous publications [22]. In the present work, the formation of TC J-aggregates was investigated in the presence of 1 x 10-3 M and 2 x 10-3 M KCl. As an example, Fig.3 represents the absorption spectra of TC AgNPs assembly as the function of AgNPs concentration in the solution containing 2 x 10-3 M KCl.

4 Spectrophotometric observations of thiacyanine dye J-aggregation on citrate capped silver nanoparticles 57 This is illustrated in Fig.4. On contrary, the concentration of TC J-aggregates formed at AgNPs surface did not change over time. The conclusion can be made that J-aggregation of TC on AgNPs surface is much faster in comparison with the rate of decomposition of TC J-aggregates in solution. Moreover, TC J-aggregates are more stable on AgNPs surface than in solution. This is in accordance with previous findings, that monovalent, divalent and trivalent cations assist nucleus formation due to the coordination of more than one dye molecule [13]. In the presence of AgNPs, K + ions moved from the solution onto the negative AgNPs surface, facilitating the aggregation of the dye on the NPs surface. Fig.3: The influence of AgNPs concentration on the formation J-aggregates on AgNPs surface in the solution containing 2x10-3 M KCl. The absorption spectrum of TC- 1.6 x 10-5 M (line 1) in the absence of AgNPs; The absorption spectra of TC dye in the presence of different concentration of AgNPs: x 10-9 M; x 10-9 M; x 10-9 M; x 10-9 M; x 10-9 M. In the absence of AgNPs, the absorption spectra showed the absorption band with the maximum at 464 nm (Fig.3, line 1). This absorption band corresponded to TC J-aggregates formed in solution [9]. The addition of AgNPs induced the appareance of the absorption band with the maximum at 481 nm due to the J-aggregation of TC on the surface of AgNPs[11]. The absorption band intensity increases as the concentration of AgNPs increases. However, the pure TC dye aggregates formed due to the presence of 2x10-3 M KCl in solution were unstable in the presence of AgNPs, and their concentration decreased with increasing the concentration of AgNPs over time. Fig. 5: Fluorescence spectra of: 1-1 x 10-5 M TC dye in the presence of 2 x 10-3 M KCl; 2 1 x 10-5 M TC dye in the presence of 2.20 x 10-9 M AgNPs; 3 - TC dye in the presence of AgNPs after 20 min. Inset: Fluorescence spectra of 1 x 10-5 M TC dye in the presence of 0.33 x 10-3 M KCl. a) b) Fig. 4:a) Absorption spectra of the dispersion containing 1.6 x 10-5 M TC, 2.17 x 10-9 M AgNPs and 2 x 10-3 M KCl immediately after mixing and after 30 min. b) Dependence of absorbance at 481 nm and 464 nm over time in the presence of different concentrations of KCl. 1 1 x 10-3 M KCl, 2 2 x 10-3 M KCl.

5 58 B.B. Laban et al. The spectrophotometric data are also in accordance with the fluorescence measurements presented in Fig.5. TC dye (1 x 10-5 M) in the presence of KCl (0.33 x 10-3 M) exhibited fluorescence with a maximum at ~ 485 nm in aqueous solution (Fig. 5, inset). Under these experimental conditions AgNPs did not exhibit fluorescence. After addition of 2 x 10-3 M KCl, a more intense fluorescence maximum at 466 nm appeared, with a shoulder at ~ 485 nm (Fig.5, line 1). This fluorescence band could be ascribed to the TC J-aggregates formed in solution, since the shoulder could be attributed to the fluorescence of dye monomers which are in the equilibrium with the dye aggregates. Moreover, after addition of AgNPs to the dye solution (final concentration being 2.20 x 10-9 M), the fluorescence intensity at 466 nm decreased (Fig. 5, line 2), due to decreasing concentration of TC J-aggregates in solution on the account of their formation on AgNPs surface. These changes are in accordance with the spectrophotometric data presented in Fig Dependence of the formation of J-aggregates on AgNPs and TC concentrations In the first set of experiments, TC concentration was 1.0 x 10-5 M, and concentration of AgNPs varied from 2.2 x 10-9 M 1 x 10-8 M. In addition, 2 x 10-3 M KCl was present in the solution, in order to support the formation of J aggregates. The absorption spectra were recorded after equilibration, using the appropriate concentration of AgNPs dispersion as the reference. The results presented in Fig. 6, a) indicate the strong influence of AgNPs concentration on the formation of J-aggregates, i.e. the intensity of J-aggregate absorption peak. The absorbance at 481 nm increased linearly when the concentration of AgNPs increased, without reaching the maximum for the applied TC and KCl concentrations (inset in Fig. 6, a). In the second set of experiments, the concentrations of AgNPs were constant (1 x 10-8 M and 2.20 x 10-9 M), since TC concentration varied from 1.6 x 10-6 M x 10-5 M. The graph presenting the dependence of absorbance at 481 nm on TC concentration was constructed in order to find out the average number of TC molecules per AgNPs. As obvious from Fig.6, b), the saturation curves were obtained. Moreover, the absorbance increased linearly, reaching the plateau at high concentrations of TC. The number of TC molecules which participate in the J-aggregation on AgNPs surface was evaluated from the intersection between the corresponding lines, which was found at 0.32 x 10-5 M TC for 1 x 10-8 M. The experimental results yielded about 320 molecules of TC per AgNPs. In our previous studies of interaction between Au and AgNPs with TC, we approximated the dye with a rectangular box having the dimensions of 2.5 x 1.5 x 0.5 nm[17] and considered the flat orientation of TC molecules on the surface of NPs as the most probable [11, 19, 20]. Based on this assumption, the calculated number of TC molecules needed to completely cover the NPs surface is about 84, yielding the NP surface coverage of 0.26 TC molecules per nm 2 on the NP surface. These calculations are usually performed by taking into account the mean values of NPs diameter [5, 11]. The obtained value corresponds theoretically to TC concentrations 0.89 x 10-6 M and 0.19 x 10-6 M needed to cover 1 x 10-8 M and 2.17 x 10-9 M AgNPs in a monolayer, respectively. However, the a) b) Fig.6: a) Dependence of J-aggregation on TC concentration in the presence of 1 x 10-5 M TC. AgNPs concentrations: x 10-9 M, x 10-9 M, x 10-9 M, x 10-8 M Inset: absorbance at 481 nm vs. AgNPs concentration; b) Dependence of J-aggregation on TC concentration in the presence of 2.17 x 10-9 M (1) and 1 x 10-8 M (2) AgNPs.

6 Spectrophotometric observations of thiacyanine dye J-aggregation on citrate capped silver nanoparticles 59 repulsive interactions between the negatively charged surface of AgNPs (originated from the citrate ions adsorbed on their surface) and the terminal sulfonate groups of the dye may disable its flat orientation and favour the vertical orientation of TC molecules along the long side of the approximated box on the NPs surface. This orientation allows more than 84 molecules TC to pack on the surface in the first layer, and most likely the TC molecules covering of the surface is in slanted orientations. 4 Discussion The optical absorption properties of the composite AgNPs/TC J-aggregates in aqueous solutions have been widely examined experimentally and theoretically [12, 22]. It was shown that the absorption spectrum of such composite structures exhibits two peaks, which belong to the localized surface plasmon resonance of AgNPs and the electronic excitation of its J-aggregate coating. The theoretical studies indicated that the position and height of the absorption peaks were strongly dependent on the relationships between the inner and outer radii of the core size to the TC shell size in the AgNPs TC dye assembly [4, 24, 25]. However, these observations are applicable for sufficiently dilute suspensions, when inter particle effects may be neglected. In the present paper, the concentration dependence of exciton intensity was examined, using high and low concentrations of TC and citrate capped AgNPs. The obtained results are in good agreement with those obtained for borate capped AgNPs, which suggest that more than one layer of TC molecules form the dye shell of the composite. 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