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Mechanism and Kinetics of J‑Aggregation of Thiacyanine Dye in the Presence of Silver Nanoparticles Bojana Laban,† Vesna Vodnik,‡ Miroslav Dramićanin,‡ Mirjana Novaković,‡ Nataša Bibić,‡ Sofija P. Sovilj,§ and Vesna M. Vasić*,‡ †

Department of Chemistry Science, University of Priština, Kosovska Mitrovica, Serbia Vinča Institute of Nuclear Sciences, University of Belgrade, P.O. Box 522, Belgrade, Serbia § Faculty of Chemistry, University of Belgrade, P.O. Box 118, 11158 Belgrade, Serbia ‡

S Supporting Information *

ABSTRACT: The aim of the present work was to elucidate the binding mechanism and kinetics of anionic cyanine dye 3,3′-disulfopropyl-5,5′-dichlorothiacyanine sodium salt (TC) J-aggregation on the surface of silver nanoparticles (AgNPs, particle size ∼6 nm). The hybrid J-aggregate−AgNPs assembly was characterized by TEM analysis, UV−vis spectrophotometry, and fluorescence measurements. In the elucidation of TC binding on the surface of AgNPs, they were considered as macromolecules with several binding sites and TC dye was considered as a ligand. Scatchard and Hill analysis revealed that TC binding was a random process rather than cooperative, with ∼200 bonded TC molecules per AgNP and a binding constant Ka = 4.8 × 107 M. The TC−AgNP assembly exerted concentration-dependent fluorescence quenching properties. The linearity of the Stern−Volmer relation, accounting for both static and dynamic quenching, indicated that only one type of quenching occurred, suggesting that AgNPs quenched the fluorescence of TC with an extraordinarily high Stern− Volmer constant (KSV) in the range of 108 M−1. Additionally, the kinetics of J-aggregation of TC in the presence of AgNPs was studied using a stopped−flow technique. Kinetic measurements were performed as a function of the TC and AgNP concentration, yielding sigmoidal kinetic curves. The concentration dependence of the parameters of the kinetic curves indicated that J-aggregate formation on the AgNP surface occurred via a two-step process; the first was adsorption of the initial dye layer, followed by the growth of consecutive layers.

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INTRODUCTION Cyanine dyes belong to an important class of organic molecules which have significant applications in the photographic industry as spectral sensitizers because of their strong absorption in the visible region and fluoresce properties.1,2 Their ability to form complex polymolecular formations such as dimers, H- and Jaggregates3−6 makes them suitable for developing novel optoelectronic devices consisting of a photoactive dye, metals, macromolecules, or nanoparticles.7−12 J-aggregates are highly ordered nanoclusters of noncovalently bonded organic molecules13−15 which exhibit characteristic narrow optical absorption peaks (J-band), red-shifted from the monomer band. This band is a result of exciton delocalization over the molecular building blocks of the aggregates16 owing to strong intermolecular van der Waals-like attractive forces between the molecules, arranged in a brick-stone structure. The J-aggregation of cyanine dyes usually occurs in the presence of metal ions,17,18 nanoparticles,19−25 or macromolecules7,8,26 and strongly depends on the structure of the dye and also on the environment, particularly micelle and microemulsion formation, pH, ionic strength, concentration, solvent polarity, electrolyte, and temperature parameters.27 Noble metal nanostructures consisting of Au or Ag NPs have been studied extensively because of their unique optical © 2014 American Chemical Society

properties arising from localized surface plasmon resonance in the visible region. Modification of the surface of metal NPs leads to changes in their electronic properties.28−32 The possibility of controlled modification of these properties provides greater opportunities for their applications in the electronic, metallurgical, chemical, biological, and pharmaceutical industries.3,33 The J-aggregation of cyanine dyes of various structures mediated by Au and Ag NPs has been extensively studied using various experimental techniques as well theoretically.19,25,28,29,34−38 The modulation and enhancement of the chromophore’s optical characteristics are obtained due to the electronic coupling of the dye exciton to the polarization of the metal NPs.39−42 As reported earlier, 3,3′-disulfopropyl-5,5′-dichlorothiacyanine dye (TC) forms J-aggregates on the surface of AuNPs and AgNPs with sharp absorption dip and peak at 481 nm, respectively.22 Recently, we performed a detailed study of the kinetics and adsorption mechanism of TC on citrate-capped AuNPs using various experimental techniques.43,44 J-aggregate formation characterized by the sharp absorption dip near the Received: July 17, 2014 Revised: September 10, 2014 Published: September 11, 2014 23393

dx.doi.org/10.1021/jp507086g | J. Phys. Chem. C 2014, 118, 23393−23401

The Journal of Physical Chemistry C

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TEM measurements were carried out using Hitachi H-7000 and JEOL 100CX microscope instrument at an operating voltage of 100 kV. More than 200 NPs were measured in each sample to obtain the average particle size. The absorption spectra of colloidal solutions were measured using a PerkinElmer Lambda 35 UV−vis spectrophotometer with a quartz cuvette (path length 1 cm). Fluorescence measurements were performed using a Fluorolog-3 model FL3−221 spectrofluorimeter (HORIBA Jobin-Yvon). Excitation and emission monochromators were a double grating design, with a dispersion of 2.1 nm/mm (1200 grooves/mm), blazed at 350 nm for excitation and 420−600 nm for emission. A xenon lamp provided excitation and a Horiba TBX-04 PMT detector was used for the emission measurements in a right angle configuration with a 1 cm path cuvette. Kinetic measurements were carried out using a Hi-Tech stopped flow accessory from T2g Scientific. The accessory (20 ms dead time) with 1 cm path length thermostated quartz cuvette (25 μL) was connected to a spectrophotometer and used for reaction rate measurements. The rate of reaction was evaluated from the change of absorbance at the exciton resonance of 481 nm. The values reported are averages of at least five runs under equal experimental conditions. Measurements at different temperatures were performed using a thermostat with a drift error of ±0.1 °C.

exciton resonance was obtained only in the presence of small borate-capped AuNPs and depended strongly on the NP size and concentration. The kinetic curves fitted the double exponential form, which postulated a kinetic model of two consecutive steps, the first being dye adsorption and the second J-aggregation.24 On the contrary, the concentration-dependent J-aggregate exciton band for AgNPs appeared as a peak with a maximum at 481 nm.45 In the present study, we continue our investigations into the mechanism of TC J-aggregate formation in the presence of AgNPs. The detailed absorption and fluorescence spectroscopy measurements were performed with the aim of elucidating the TC adsorption mechanism and also to determine the adsorption parameters. In addition, the kinetics of J-aggregation were followed and the reaction rate profile is discussed in terms of AgNPs and TC concentration.

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EXPERIMENTAL METHODS Chemicals. Silver nitrate (AgNO3), potassium chloride (KCl), and sodium borohydride (NaBH4) were commercial products (Aldrich) of the highest purity and were used as received. Thiacyanine dye (TC; 3,3′-disulfopropyl -5,5′dichlorothiacyanine sodium salt) was purchased from Hayashibara Biochemical Laboratories, Okayama, Japan. The chemical structure of TC dye is shown in Scheme 1. All

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Scheme 1. TC Dye Structure

RESULTS AND DISCUSSION TEM and optical characterization of AgNPs in the presence of TC dye. The TEM study of AgNPs confirmed the presence of spherical particles with an average diameter dav = 6 ± 0.91 nm, as shown in Figure 1a. When the samples were prepared in the presence of TC dye, AgNPs showed a tendency to aggregate (Figure 1b). Similar behavior 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.44 The reason for this behavior was attributed to TC molecule adsorption on the AgNP surface in J-aggregate form, which are involved in the interlinking of AgNPs. Since both the TC dye and AgNPs are negatively charged, adsorption of the dye on the NPs is only possible if the dye molecules are oriented toward the surface of AgNPs via their thiazole moieties, which carry a partial positive charge, rather than the negatively charged sulfate groups.27 In this particular case, the orientation of the dye molecules on the surface of AgNPs appears to be a key factor in driving J-aggregation; second, adsorption of the dye on the NP induces the coalescence of particles and a broadening of the size distribution of the particles with respect to the size distribution of the initial Ag colloid, as seen in Figure 1b. The initial Ag colloid dispersion exhibited an intense surface plasmon resonance (SPR) peak at ∼387 nm (Figure 2, line 1). Its position is dependent on the size and shape of the NPs as well as on the dielectric constant of the medium.47 The formation of TC dye J-aggregates on the surface of the AgNPs was followed by the appearance of a new peak at 481 nm (Figure 2, lines 2−4) due to the coupling of J-aggregate excitons and polarization in the AgNPs.20,22 This polarization coupling to AgNPs involved only the intraband contribution from the surface plasmon of the metal core, unlike interband and intraband contributions in the case of AuNPs.39 It is interesting to note that the coalescence of the NPs was not evident from the absorption spectra, since a lack of background intensity was observed between 600 and 700 nm

solutions were prepared using triply distilled water from a Millipore Milli-Q water system. An aqueous stock solution of TC dye (5 × 10−5 M) was prepared by dissolving the solid TC in water containing 1 mM KCl, which is necessary to assist in Jaggregate formation. The dye working solutions were prepared by appropriate dilution of the TC stock solution immediately before use. Synthesis of AgNPs. Silver hydrosols were prepared by chemical reduction of silver ions with NaBH4, as described elsewhere.30,46 Briefly, 250 mL of AgNO3 solution (5 × 10−4 M) was bubbled with argon in order to remove all the oxygen. Under vigorous stirring, 13.2 × 10−3 M NaBH4 was added to the solution and left for 1 h in an argon atmosphere before use. The concentration of AgNPs (c) in the stock dispersion (5 × 10−8 M) was approximately determined using the equation: c=

c MMAg 4 3 r πρNo 3

(1)

where cM represents the AgNO3 molar concentration, MAg is the Ag molar mass, r is the average particle size radius, ρ is the Ag density (10.49 g cm−3), and No is Avogadro’s number. The AgNP working solutions were made by appropriate dilution of stock dispersion. Instrumentation. Transmission electron microscopy (TEM) was used to estimate the average size of AgNPs. 23394



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intensity of the absorption peak at 481 nm depended strongly on AgNPs as well as on the TC concentration. The dependence of spectral changes vs. TC or AgNPs concentration enabled us to determine the extinction coefficient of J-aggregates at 481 nm. As shown in some published papers, the concentration of aggregates was defined in terms of the concentration of aggregated monomeric TC dye molecules.7,8,26 Moreover, the spectrophotometric measurements of J-aggregate formation included both absorption and light scattering and therefore were more appropriately referred to as extinction measurements. Consequently, the value for εJ/ monomer of 1.08 × 105 M−1 dm3 cm−1 was evaluated from the plateau of the sigmoidal curves presented in Figure 2, inset, as the mean value for the complete conversion of 1.6 × 10−6, 5 × 10−6, and 7 × 10−6 M TC molecules into J-aggregates. Binding Mechanism of TC to AgNPs. The obtained value of εJ/monomer was further used for the calculation of Jaggregates and equilibrium TC concentrations from spectrophotometric data in the analysis of the binding mechanism of TC to AgNPs, as previously described for the J-aggregation of cyanine dyes in the presence of gelatin.7 For this purpose, the absorbance of the TC−AgNP assembly at 481 nm was measured, keeping the concentration of AgNPs at 1 × 10−8, 2.5 × 10−8, or 5 × 10−8 M. In all cases, the TC concentration varied from 1.6 × 10−6 to 1.6 × 10−5 M. The saturation binding curves (Figure 3a) representing the concentration of Jaggregates vs the equilibrium concentration of dye in the presence of AgNPs evaluated from the spectrophotometric data. The methods developed for the description of the interaction between biological macromolecules and ligands48 were applied to the results presented in Figure 3a in order to evaluate the binding mechanism. In our case, AgNPs were considered as macromolecules with several binding sites and the TC dye was considered as the ligand. Scatchard analysis,49,50 as a method of linearizing data from a saturation binding experiments, was applied in order to determine the binding constants. This is a method for analyzing data from freely reversible ligand/ receptor binding interactions according to the relation:

Figure 1. TEM analysis of AgNPs in the absence (a) and presence of TC dye (b). Inset: corresponding particle size distributions (PSD).

(TC bound /mol NPs)/TCeq = NK a − K a(TC bound /mol NPs)

(1)

where (TCbound/mol NPs)/TCeq represents the ratio of concentrations of bound ligand per mol AgNPs (TCbound/mol NPs) to unbound ligand (TCeq), N is the number of binding sites, and Ka is the apparent microscopic association (affinity) constant. For each site where the ligand binds reversibly according to mass-action principle, eq 1 is a straight line (Figure 3b) with the slope representing −Ka; the intercept at the x axis represents the total number of binding sites, N. Here, TCeq is the concentration of free TC molecules which are in equilibrium with bound ligands. The results of the Scatchard analysis are listed in Table 1. Additionally, the semiempirical equation developed from the Hill relationship was used7,51,52 to evaluate the cooperativity of binding:

Figure 2. Absorption spectra of 2.5 × 10−8 M AgNPs (1); 2.5 × 10−8 M AgNPs in the presence of various TC concentrations: (2) 1.6 × 10−6, (3) 5 × 10−6, and (4) 1.4 × 10−5 M; aqueous solution of 1 × 10−5 M TC dye (5). Inset: dependence of absorbance at 481 nm on the AgNP concentration in the presence of various TC concentrations: (a) 1.6 × 10−6, (b) 5 × 10−6, and (c) 7 × 10−6 M.

(Figure 2, lines 2−4). However, the solutions remained clear with no evidence of particle precipitation. Moreover, the slight red shift (2−7 nm) and broadening of the SPR band depended on the TC concentration (Supporting Information, Figure 1S). These changes were also influenced by the plasmon coupling of NPs in close proximity and by the local refractive index of the TC medium surrounding the AgNPs. Absorbance at 481 nm was also followed as a function of the AgNP concentration, keeping the TC concentration constant (Figure 2, inset). The present results clearly indicated that the

ln[TCeq ] = − (1/αH)ln{N /(TC bound /mol NPs) − 1} + ln Kd

(2)

where αH represents the Hill coefficient and Kd is the ligand concentration occupying half of the binding sites, which is also 23395



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The saturation curves were also analyzed by fitting to a hyperbola (Figure 3a) yielding the maximal concentration of bonded TC molecules (Nmax) and the binding constant Ka (this value corresponds to the reciprocal value of the ligand concentration occupying half of the binding sites). Moreover, the number of bonded molecules per AgNP (N) was calculated by dividing Nmax by the AgNP concentration. The obtained values are also given in Table 1 and agreed well with those obtained by the Scatchard and Hill analyses. It is generally believed that the ability of dyes to aggregate on the surface of metal NPs relies primarily on electrostatic attractions between the ionic molecular building blocks of the aggregates and the charged nanoparticles.23,27 However, the results of spectrophotometric study suggest that the binding mechanism of TC to AgNPs is a random process rather than cooperative, i.e., the binding of a ligand to one binding site did not alter the affinity of another binding site. The values for Ka and number of binding sites per mole of AgNPs obtained using the three methods described above (∼200) are in the range of experimental error. In general, these results are not in accordance with previous work,7 which showed cooperative binding of a similar cyanine dye on gelatin or biopolymers containing different binding sites. On the contrary, it may be concluded in our case that TC binds only to one type of site on the AgNP surface. TC dye is negatively charged and is usually assumed to engage in electrostatic interactions via the end sulfonate groups. However, it can also utilize partially positively charged sulfur from the thiazole ring. On the other hand, metal-catalyzed hydrolysis of borohydride ions and the association of negative charges with the AgNPs take place during AgNP formation, as was shown in our previous publication.29 This process involves electron injection from borohydride (BH4−) and borate (BO33−) ions into the NPs. They can temporarily stabilize AgNPs by adsorption onto the surface, providing Coulomb repulsion between NPs and preventing their aggregation. In some previous studies, the dye molecule was approximated as a rectangular box with dimensions of 2.5 × 1.5 × 0.5 nm.23,24,45 The conclusion was made that TC molecules were adsorbed in slanted orientation on the NP surface, indicating that more than one layer of TC was needed to reach the optimal exciton signal of TC J-aggregates. Fluorescence Quenching of TC on AgNPs. The study of fluorescence spectra in the presence of AgNPs indicated that the TC−AgNP assembly displayed fluorescence quenching properties. Two series of experiments were performed, keeping the TC concentration constant (0.5 × 10−5 and 1.0 × 10−5 M), whereas the concentration of AgNPs varied from 0 to 5 × 10−8 M. As an example, the fluorescence change in 0.5 × 10−5 M TC in the presence of various AgNP concentrations is presented in Figure 4. For comparison, the absorption and fluorescence

Figure 3. (a) Dependence of J-aggregate formation on equilibrium TC concentration. (b) Scatchard and (c) Hill analysis of saturation graphs. Concentration of AgNPs: (1) 1 × 10−8, (2) 2.5 × 10−8, and (3) 5 × 10−8 M.

the microscopic dissociation constant.53 The linearity of eq 2 was tested for some N values close to those obtained by the Scatchard analysis. The obtained parameters of eq 2 are also presented in Table 1. However, the value of the Hill coefficient αH was close to 1 (Table 1), suggesting the noncooperative binding. The values N and Ka were close to those obtained by the Scatchard analysis. In this case, the Hill equation, as a relationship between the concentration of a compound adsorbing to binding sites and the fractional occupancy of the binding sites, is equivalent to the Langmuir equation.53

Table 1. Apparent Association Constants (Ka), Total Number of Binding Sites (N) and Hill Coefficient (αH) for the Binding of TC Molecules on the AgNP Surface Scatchard analysis

a

Hill analysis a

AgNPs (M)

Ka (M)

N

Ka (M)

5 × 10−8 2.5 × 10−8 1 × 10−8

(2.1 ± 0.5) × 106 (2.2 ± 0.7) × 10−6 (1.5 ± 0.1) × 106

206 194 180

(2.1 ± 0.2) × 106 (2.5 ± 0.2) × 106 (1.1 ± 0.1) × 106

Analysis of hyperbola N

αH

Ka (M)

Nmax (M)

N

212 212 200

1.09 1.10 1.41

(2.1 ± 0.7) × 106 (2.9 ± 0.1) × 106 (1.4 ± 0.1) × 106

10.60 × 10−6 5.00 × 10−6 1.7 × 10−6

186 220 174

b

The reciprocal of Kd obtained from the intercept of eq 2. bThe reciprocal value of ligand concentration occupying half of the binding sites 23396



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spectra of AgNPs and TC in aqueous solution are presented in the inset of Figure 4.

In our previous investigations, we showed that TC exhibited strong fluorescence in aqueous solution with a maximum at ∼485 nm,43,45 since AgNPs do not exert any fluorescence in this wavelength region.56 The quenching of TC fluorescence occurred, but a change in the fluorescence band position in the presence of AgNPs was not observed. However, this cannot be the direct confirmation of the absence of significant molecular interactions under the prevailing experimental conditions,57 since it may be expected that the fluorescence of a bound TC molecule is subjected to complete quenching due to the TC interaction with AgNPs, such that only unbound TC molecules emit fluorescence. In this case, a change in the TC fluorescence band position could not be observed. This result is also in accordance with our previous studies of TC fluorescence quenching in the presence of borate and citrate-capped AuNPs of various sizes.43,44 Moreover, the fluorescence quenching occurred regardless of whether Jaggregates of TC were formed on the NP surface.44 It was also observed earlier that the surface plasmon acts as an efficient energy acceptor even at a distance of 1 nm between the dye molecule and the NP surface.34,35,58 Since AgNPs are larger than 5 nm, energy rather than electron transfer dominates the quenching mechanism.37,55 Kinetic Analysis of J-Aggregate Formation. It has been found that J-aggregation exhibits a sigmoidal-type curve17,18 or a nonsigmoidal type kinetic curve7,8 depending on the dye structure, dye concentration, pH, and temperature. Sigmoidal curves are characteristic for reversible autocatalytic aggregation of dyes, pigments or other molecules,8,18,59 while nonsigmoidal type kinetic curves are analyzed in terms of a time-dependent rate constant and a simple exponential dependence.8,60 The typical dependence of J-aggregate concentration over time is presented in Figure 6 for the colloid dispersion containing 1 ×

Figure 4. Change in the fluorescence spectra of 5 × 10−6 M TC (1) upon the addition of AgNPs: (2) 0.025 × 10−8, (3) 0.15 × 10−8, (4) 0.50 × 10−8, and (5) 1.00 × 10−8 M. Inset: absorption spectra of 5 × 10−8 M AgNPs (1), 1.6 × 10−5 M TC (2) and the fluorescence spectrum of 1.6 × 10−5 M TC (3) in aqueous solution.

The Stern−Volmer relation accounting for both static and dynamic (collisional) fluorescence quenching54 is generally written as I0/I = 1 + KSV[Q]

(3)

where I0 and I are the fluorescence intensities of TC in the absence and presence of AgNPs, [Q] is the quencher concentration, and KSV = KS + KD is the Stern−Volmer quenching constant (KS and KD are the static and dynamic quenching constants, respectively). The overlapping linear Stern−Volmer plot for the experimental data of two sets of measurements (the slope 2.35 ± 0.10, R2 = 0.9935) was obtained and is presented in Figure 5, together with the residual

Figure 6. Typical kinetic curve for the formation of J-aggregates of 1.4 × 10−5 M TC in the presence of 1 × 10−8 M Ag and 1 × 10−3 M KCl at 25 °C. Points represent the experimental data; solid line is the fit using eq 3. Inset: dependence of reaction rate on time. Figure 5. Stern−Volmer plot of F0/F vs concentration of AgNPs for 5 × 10−6 M TC (circle) and 1 × 10−5 M TC (squares). Inset: residual of F0/F.

10−8 M Ag and 1.4 × 10−5 M TC. The concentration of Jaggregates was calculated from the absorbance vs t curves (Figure 2S, Supporting Information). In general, the typical kinetic curves for formation of J-aggregates were slightly sigmoid in shape and could not be successfully fitted to standard first-order, second-order or coupled first-order equations. We defined the concentration of aggregates in terms of the concentration of the aggregated monomeric chromophore. Since no aggregates were present at t = 0, we applied the previously developed stretched exponential function7,8,59,61 to describe J-aggregate formation:

vs independent plot (inset). The linearity of the Stern−Volmer plot indicates that only one type of quenching occurred, as also found with the silver NP-induced quenching of fluorescence of some fluorescent dyes.36,55 The value of KSV = (2.38 ± 0.05) × 108 M−1 was calculated. This suggested that AgNPs quenched the fluorescence of TC with an extraordinarily high Stern− Volmer constant. The very high value of the quenching constant indicates the presence of adsorption of the dye molecule on the AgNP surface,36 and suggests a strong association between the nanoparticles and TC.55 23397


The Journal of Physical Chemistry C C J = C J0 + (C J00 − C J0){1 − exp( −(kappt )n }

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obtained saturation graph can be explained by a series of second- and first-order reactions, with a reversible first step,62 according to the relation:

(3)

Here, C0J and C00 are the concentrations of J-aggregates J immediately after mixing and after the completion of Jaggregation, respectively, kapp is the apparent rate constant of J aggregates formation, and n represents the stretch exponential parameter, i.e., the degree of the sigmoid character. The best fit of the data using eq 3 is presented as the solid line in Figure 6. The values of eq parameters for the corresponding −6 M, n = experimental data are C00 J = (1.12 ± 0.02) × 10 −1 1.11 ± 0.04, and kapp = (4.47 ± 0.09) s . The quality of the fit was very good, with an R2 value 0.9995. For comparison, the kinetic curves were also fitted to a double exponential form. The corresponding fits for the same kinetic curve as presented in Figure 6, along with the corresponding residual plots, indicates that eq 3 fits the data better than the double exponential format (see Supporting Information, Figure 4S). The kinetic curve presented in Figure 6 was also characterized by an inflection point. This has also been found in studies of J-aggregation of some cyanine dyes in the presence of gelatin,7 as well as metal ions.17 The first derivative of the kinetic curve dCJ/dt vs. time has a bell-shaped form with the maximum value at the inflection point (Figure 6, inset). This value represents the maximum rate of J-aggregate formation. Effects of AgNP and TC Concentrations on Kinetics. To study the kinetics of J-aggregation in TC-AgNPs assembly, two series of experiments were performed. In the first series, the concentration of AgNPs was kept constant (1 × 10−8 M Ag), and the concentration of TC was varied from 1.6 × 10−6 M to 1.67 × 10−5 M. The time dependence of the J-aggregate concentration was evaluated from the spectrophotometric data (Figure 2S, Supporting Information). The kinetic parameters (kapp, maximal reaction rate dCJ/dtmax and n) were evaluated by fitting the kinetic curves to eq 3, and are given in Table 1S in the Supporting Information. Figure 7a represents the dependence of kapp on the TC concentration. The shape of the

k+

k2 → AgNPs + TC AgNPsTC → Jagg ← k−

(4)

It is reasonable to assume that the second-order reaction step corresponds to the adsorption of TC on the AgNP surface, and the first-order step corresponds to the formation of J-aggregates from the adsorbed TC molecules.24,43,45 As TC increases, the AgNPs surface is completely occupied and further addition of TC does not affect the rate. In other words, the ratedetermining step has now shifted toward a first-order reaction of the formation of J aggregates. It can be easily shown62 that the following dependence of kapp on the TC concentration can be derived: kapp = k 2[TC]/(K m + [TC])

(5)

where Km = (k2 + k−)/k+ represents the pseudoequilibrium Michaelis constant. Here, k+ and k− are the forward and backward rate constants of the first reaction step in eq 4. However, at low TC concentrations, the rate-determining step can be considered as the formation of an intermediate, i.e., the adsorbed TC molecules on AgNPs. The formation of Jaggregates followed a low first-order rate constant, and kapp tended toward the maximum value of k2 as TC increased. The linearization of eq 5 leads to the expression: 1/kapp = 1/k 2 + K m/k 2 × 1/[TC]

(6)

The plot of 1/kapp vs 1/[TC] represents a straight line and is shown in the inset of Figure 7a. The slope and the intercept gave us the values Km = 9.15 × 10−6 M−1 and k2 = 6.4 s−1. In the second set of experiments, the concentration of TC was kept constant (TC 1 × 10−5 M), and the concentration of AgNPs was varied from 0.5 × 10−8 to 5 × 10−8 M. Figure 8a illustrates the dependence of apparent rate constant and

Figure 7. Dependence of kapp (a) and vmax (b) on the TC concentration; c(Ag) = 1 × 10−8 M, t = 25 °C. Inset: Dependence of 1/kapp vs [TC].

Figure 8. Dependence of kapp(a) and vmax (b) on the AgNP concentration; c(TC) = 1 × 10−5 M, t = 25 °C. 23398



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maximal reaction rate on the concentration of AgNPs. Moreover, the calculated parameters of eq 3 are listed in Table 2S in the Supporting Information. It is important to note that the measurement of kapp in the presence of an extremely low AgNP concentration was uncertain. However, the dependence of kapp vs the AgNP concentration reached a plateau, yielding a value k2 = 3.5 s−1. In both experiments, vmax increased with concentration (Figure 7b and Figure 8b). In general, the stretched exponential function has usually been applied to a relaxation system in which the individual components are not independent and interact with each other.59,61 The obtained parameters kapp in the stretched exponential function represents the “average” relaxation rate for the particular experimental conditions; the physical meaning is that the obtained value is likely to be more or less within ±20%. It is worth noting that the parameter n ranged from 0.90 to 1.11. This means that the sigmoidal degree was not large, indicating a low degree of cooperation in the process of aggregate formation.59

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ASSOCIATED CONTENT

S Supporting Information *

Additional figures and tables of experimental data and results. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: (+381 11) 3408 287. Fax: (+381 11) 244-7207. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Ministry of Education, Science and Technological Development of the Republic of Serbia (Project Nos. 172023 and 172056) for their financial support. The work was done in the frame of COST Action MP1302 Nanospectroscopy.

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CONCLUSION

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In the present work, we report the results of a detailed mechanistic and kinetic study of TC dye J-aggregation in the presence of AgNPs. TEM and spectrophotometric studies confirmed the formation of TC J-aggregates on the AgNP surface, followed by a concentration-dependent change in the dye-AgNP assembly absorption spectra. The value of the Jaggregate extinction coefficient at 481 nm (εJ/monomer 1.08 × 105 M−1 dm3 cm−1) was evaluated from the dependence of spectral changes vs the TC or AgNP concentration. In order to elucidate the mechanism of TC binding on the AgNP surface, the methods developed for the description of the interaction between biological macromolecules and ligands were applied. The analysis of saturation curves representing the dependence of J-aggregate concentration on the free TC concentration, using Scatchard and Hill methods, led to the conclusion that the binding of TC dye to the AgNP surface is a random process, with ∼200 bonded TC molecules per AgNP in a slanted orientation, with a binding constant Ka = 4.8 × 107 M. The study of the fluorescence spectra in the presence of AgNPs indicated that TC-AgNP assembly displayed fluorescence quenching properties. The lack of change in the fluorescence band position in the presence of AgNPs indicated the absence of significant molecular interactions under the prevailing experimental conditions. The linearity of the Stern− Volmer plot indicated that only one type of quenching occurred, yielding a value of the Stern−Volmer constant KSV = (2.38 ± 0.05) × 108 M−1. In the kinetic experiments, J-aggregate formation was described by a stretched exponential function, which is usually applied to systems containing independently relaxing species, each of which decays exponentially over time with a specific relaxation rate kapp. The obtained parameters, kapp and dCJ/dt, were concentration-dependent and confirmed the random mechanism of TC adsorption on AgNPs and J-aggregation of the adsorbed dye. The n values ranged from 0.90 to 1.11, indicating that the sigmoidal degree was not large, with a low degree of cooperation in the process of aggregate formation. 23399



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