Article pubs.acs.org/JPCC
Synthesis and Catalytic Properties of Silver Nanoparticle−Linear Polyethylene Imine Colloidal Systems Kelly de O. Santos,† Welman C. Elias,† Aline M. Signori,† Fernando C. Giacomelli,‡ Hong Yang,§ and Josiel B. Domingos*,† †
LaCBio - Laboratory of Biomimetic Catalysis, Chemistry Department, Universidade Federal de Santa Catarina, Campus Trindade, Florianópolis - SC, 88040-900, Brazil ‡ Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, Rua Santa Adélia 166, Santo André - SP, 09210-170, Brazil § Department of Chemical Engineering, University of Rochester, Gavett Hall 206, Rochester, New York 14627, United States S Supporting Information *
ABSTRACT: The excellent catalytic properties of colloidal metal nanoparticles (M-NPs), such as good selectivity, efficiency, and recyclability, have attracted great interest in academic and industrial research. However, new M-NP stabilizers/supports still need to be developed and their performance needs to be better understood. Herein, we report an approach for effectively combining a high-throughput method using linear polyethylene imine (LPEI) with in situ screening and multivariate optimization of the synthesis conditions to produce highly catalytically stable Ag-NPs. Selected Ag-NP/stabilizers were able to efficiently catalyze the p-nitrophenol (Nip) reduction by NaBH4 in water with a rate constant normalized to the surface area of the nanoparticles per unit volume (k1) up to 1.66 s−1 m−2 L. A full kinetic analysis based on the Langmuir model indicates that the Nip molecules have a much stronger adsorption affinity than BH4− ions for the Ag-NP surface and all species are likely adsorbed and accommodated on the surface before they take part in any reaction.
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an important aspect in terms of catalysis.10,11 Polymers also have the advantages of possessing many multivalent sites, allowing the incorporation of various types of functional groups.12 Despite all the potential benefits of using polymeric ligands, there is still not enough research emphasis placed on this research area.13 It is not obvious what the optimal amounts and proportions of functional groups are required to create the right environment for the nanoparticle stabilization and, in the case of catalysis, high activity. Recently, we have shown that a simple one-step and systematic derivatization of the branched polyethylene imine (PEI) scaffold with alkyl and ethanolic groups led to a structural diversity that greatly affected the stabilization and catalytic property of silver nanoparticles (Ag-NPs).13 The optimization of stabilizer was achieved through the use of an innovative methodology, which consists of using a high-throughput in situ screening in a 96-well plate platform, through following the surface plasmon resonance (SPR) band of formed Ag-NPs by UV−vis spectroscopy.13 When a linear polymer is used as a protective agent, modification of the functional groups can provide a specific reactive environment around metal nanoparticles that controls
INTRODUCTION Catalytic systems based on colloidal metal nanoparticles (M-NPs) exhibit exceptional properties such as their ability to accelerate reactions that do not occur under normal conditions, excellent selectivity, and recyclability.1−3 New applications are also found in many different areas, ranging from fuel cells4 to catalytic converters5 and to photocatalytic devices.6 Given the pivotal roles they play in the production of chemicals, the field of M-NP catalysts will certainly continue to expand in the coming years.7,8 Colloidal metal nanoparticles have been prepared using a wide range of methodologies.9 However, in all of the methods, a noble metal precursor must be reduced to its metallic form first and then the formed metallic atom cluster must be able to form a stable sol. To inhibit the irreversible coagulation of particles, a stabilizing agent is usually required. These stabilizing agents include both low molecular weight organic molecules, such as surfactants, and polymers. Polymer ligands stabilize nanoparticles through the steric effects of their framework, from the electrostatic repulsion, in the case of polyelectrolytes, or through a combination of both, known as “electrosteric stabilization”. Contrary to small molecule ligands, which typically need to form very strong affinities with nanoparticle surface sites to ensure nanoparticle stability, polymers enable weak binding to the particle surface, © 2012 American Chemical Society
Received: September 9, 2011 Revised: January 27, 2012 Published: February 14, 2012 4594
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of each of the 96 different combinations of reaction mixtures were diluted in water to 1.6 mL in deep 96-well plates to provide a LPEI concentration of 0.06 mmol L−1. The silver salt precursor (AgNO3) was added to aliquots of this solution, the samples were then incubated for 10 min, and hydroquinone (HQ), the reducing agent, was added to give a total volume of 0.150 mL. Polymer, AgNO3, and HQ concentrations were varied systematically, but the final LPEI concentration was kept at 0.04 mmol L−1. In one typical set of experiments, for example, [AgNO3]/[PEI] was 20 and [AgNO3]/[HQ] was 10. The UV−vis spectra were acquired in the range from 300 to 800 nm in transparent 0.300 mL 96-well plates (NUNC) with a microtiter plate reader (Molecular Devices Spectramax Plus 384). Multivariate Analysis in the Synthesis of LPEI-AgNPs. Optimization of the variables such as concentrations of LPEI (F1 or F12), AgNO3, and HQ for the formation of the silver nanoparticles was carried out using a two-level full factorial experimental design with two axial and four central points.19 The values were given high (+), low (−), and central (0) points for each factor, and summarized in Table 1 and in the
the particle size in the synthesis. In this regard, linear polyethylene imine (LPEI) is a very good candidate, since polyamine polymers have the highest density of amine groups14 and only secondary amino groups on the chain, providing a simpler local macromolecular environment in aqueous solution than the branched ones.15 Thus, the main objective of the current work is to systematically modify the LPEI framework and to create new ligands of polymer stabilizers for the preparation of catalytically active Ag-NPs by utilizing the highthroughput in situ screening methodologies13,16,17 and the intrinsic LPEI properties. Moreover, using a new experimental design tool,18 the optimization of the parameters that contribute to the NP formation (primarily the metal salt precursor, reducing agent, and stabilizer concentrations) is carried out based on a straightforward multivariate analysis. The LPEI-Ag-NP composites were characterized by transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), electrophoresis (ELS), and dynamic light scattering (DLS). Their catalytic activity in the reduction reaction of p-nitrophenol (Nip), using NaBH4 as the reducing agent, was studied and their kinetics were fully analyzed.
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Table 1. Factor Levels in the Experimental Designs
EXPERIMENTAL SECTION In order to optimize the synthesis of LPEI-Ag-NPs, our approach involves three main steps: (i) functionalization of LPEI; (ii) high throughput screening of functionalized polymer and selection of the two most efficient on the stabilization of Ag-NPs; and (iii) multivariate optimization of the metal salt precursor, reducing agent, and stabilizer concentrations on the formation of the Ag-NPs. All of these steps are detailed below, along with the physical and catalytic characterization of the LPEI-Ag-NPs. All reagents and solvents were purchased from commercial sources and used as received. Ultrapure water (resistivity of 18.2 mΩ3 cm), degassed by ultrasonic treatment, was used in all experiments. All glassware was washed with concentrated nitric acid and rinsed copiously with deionized water prior to use. Functionalization of LPEI. A polymer stock solution was prepared by dissolving linear PEI (25 kDa, Polyscience) in dimethylsulfoxide (DMSO) to give a final concentration of 0.86 mg mL−1 (20 mmol L−1, in monomer residues). A freshly prepared solution mixture of LPEI and N,N-diisopropylethylamine in DMSO was then obtained, to give final concentrations of 1.25 mmol L−1 each. To 0.4 mL of this solution, 0.3 mL of 2-chloroethanol and 0.3 mL of 1-bromobutane (or 1bromooctane) solutions (0−0.83 mmol L−1, 0−0.5 equiv per monomer residue of LPEI) were added, under vigorous stirring using micro stirring bars (2 mm × 5 mm, Sigma-Aldrich), in 96-well 2 mL polypropylene plates (Axigen, USA). Stirring was continued for 5 days at room temperature to generate a library of 96 different ligand polymers. For the scaled-up synthesis performed after the initial screening, the selected Ag-NP stabilizers were obtained by functionalizing the selected polymer with (i) 0.4 equiv of 2-chloroethanol (named F1) and (ii) 0.4 equiv of 2-chloroethanol and 0.5 equiv of 1-bromobutane (named F12). The synthesis was performed in amber bottles under the same experimental synthetic conditions in the 96-well plates, and the difference was the final volume, which increased from 1 to 116 mL. The final LPEI concentration was 20 mmol L−1 (in monomer residues). High-Throughput in situ Library Screening. The initial library screening was performed spectrophotometrically following in situ the formation of Ag-NPs. First, aliquots (0.192 mL)
level
[LPEI] (mmol L−1)
[AgNO3] (mmol L−1)
[HQ] (mmol L−1)
−2 −1 0 1 2
0.040 0.080 0.120 0.160 0.200
0.050 0.088 0.125 0.163 0.200
0.050 0.138 0.225 0.313 0.400
experimental design section of Table S1 (Supporting Information). The HQ, AgNO3, and LPEI concentration-level ranges were chosen on the basis of preliminary experiments and taking into account the sensibility of the spectrophotometric assays. The response values ψ obtained through eq 4 were used as the analytical response. The evaluation of the results of the factorial design was carried out using analysis of variance (ANOVA) at the 95% confidence level. These experiments were carried out in a quartz cell and the UV−vis spectra in the range between 300 and 800 nm were acquired in situ on a spectrophotometer installed with a xenon flash lamp and a thermostatted cell holder (Varian Cary 50). Typically, AgNO3 was added to a predetermined volume of a LPEI (F1 or F12) aqueous solution, and this solution incubated for 10 min before the addition of the reducing agent (HQ), to give a total volume of 3 mL. The final concentrations of LPEI, AgNO3, and HQ for each multivariate experiment are shown in Table S2 of the Supporting Information. A reaction time of 180 min was set to ensure the formation of NPs. All the experiments were repeated at least once. Experimental data was processed using the Statistica 8.0 computer program.20 Characterization of LPEI-AgNP Colloids. Besides UV− visible spectroscopy, the LPEI-Ag-NPs were characterized by transmission electronic microscopy (TEM) and scattering techniques. For TEM analysis, one or two drops of reaction mixture were deposited on a carbon film on a 400-square mesh copper grid, and the grid was left to dry naturally. Electron micrographs were taken on an FEI TECNAI F-20 field emission microscope at an accelerating voltage of 200 kV. The particle size analysis was conducted by analyzing at least 150 particles, and the size distribution was determined from the maximum length of the particles. The theoretical specific surface area of the Ag-NPs was estimated from the TEM 4595
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analysis and the density of bulk silver (ρ = 10.5 g cm−3). The dynamic (DLS) and electrophoretic (ELS) light scattering techniques were performed to probe the average size (RH) and zeta potential (ζ) of the nanoparticles. The experiments were performed using a Zetasizer Nano ZS instrument (Malvern Instruments, U.K.). The samples were measured at a constant temperature of 25 ± 1 °C. The hydrodynamic radii (RH) of the Ag-NPs were determined from the measured intensity correlation functions g2(t) converted to distribution of relaxation times and further to size distributions using the Stokes−Einstein (eq 1). k Tq2 RH = B τ 6πη
Catalytic Activity Study. The catalytic activities of F1-AgNPs and F12-Ag-NPs were evaluated using the reduction reaction of p-nitrophenol (Nip) to p-aminophenol (Amp) in a quartz cell with 3 mL of final volume in water at 15 °C. First, the concentration of reducing agent (NaBH4) was changed from 0 to 40 mmol L−1 for F1 and from 0 to 130 mmol L−1 for F12, while keeping the concentrations at 0.05 mmol L−1 for Nip and 0.025 mmol L−1 for Ag-NP (based on the mole amount of silver) in the case F1 was used, and at 0.05 mmol L−1 for Nip and 0.05 mmol L−1 for Ag-NP in the case of F12. Second, the Nip concentration was varied from 0 to 0.08 mmol L−1 for F1 and 0 to 0.07 mmol L−1 for F12 while keeping the Ag-NP and NaBH4 concentrations at 0.025 and 25 mmol L−1 for F1 and at 0.05 and 100 mmol L−1 for F12, respectively. Finally, the concentration of the catalyst (Ag-NPs) was varied from 0 to 0.06 mmol L−1 for F1, keeping the Nip and NaBH4 concentrations at 0.03 and 25 mmol L−1, respectively. For F12, the concentration of the catalyst was varied from 0 to 0.07 mmol L−1, keeping the Nip and NaBH4 concentrations at 0.015 and 80 mmol L−1, respectively. Reactions were started after addition of NaBH4 and monitored by the decreasing absorbance at 400 nm on a spectrometer with a thermostatted cell holder.
(1)
where kB is the Boltzmann constant, T is the absolute temperature, η is the viscosity of the solvent, τ is the relaxation time related to the diffusion movement of the nanoparticles, and q is the scattering vector given by q=
4πn ⎛⎜ θ ⎞⎟ sin ⎝2⎠ λ
(2)
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where n is the refractive index of the solvent (nwater = 1.33), λ is the wavelength of the incident beam (λ = 633 nm), and q is the scattering angle (θ = 173°). Herein, the intensity size distributions were converted to volume-weighted size distributions by considering the Rayleigh scattering of spherical particles. The average zeta potential (ζ) of the nanoparticles was determined by measuring their electrophoretic mobility (UE), and the values were converted to ζ-potential (mV) through the Henry equation:
ζ=
3ηUE 2εf (ka)
RESULTS AND DISCUSSION LPEI Derivatization Gives Rise to Efficient Ag-NP Stabilizers. Using a high-throughput system16,24 for the LPEI derivatization, the “parallel” synthesis25 as well the screening for the best functional group combinations, in the stabilization of the Ag-NPs formed in situ, were performed in the same 96-well format. The formation of the Ag-NPs was followed based on the SPR band in the visible region and evaluated through eq 4.13 ψ=
(3)
where ε is the dielectric constant of the medium and f(ka) is the Henry function, which was calculated to be 1.5 through the Smoluchowski approximation. SAXS experiments were performed on the SAXS1 beamline of the Brazilian Synchrotron Light Laboratory (LNLS − Campinas, SP, Brazil). The polymer solutions were loaded into a temperature-controlled vacuum flow-through cell composed of two mica windows separated by 1 mm, normal to the beam.21 The collimated beam (λ = 1.55 Å) crossed the sample through an evacuated flight tube and was scattered to a Pilatus 300K 2D detector (Dectris). The 2D scattering patterns were collected after an exposure time of 120 s. In order to cover the desired q range (from 0.07 to 1.2 nm−1) where n is about 1 for X-ray, the sample-to-detector distance was set to 1589.2 mm (silver behenate was used for sample-todetector distance calibration). In all cases, the 2D images were found to be isotropic and they were normalized by the sample transmission. The above procedure was undertaken using the FIT2D software developed by A. Hammersley.22 Furthermore, the resulting I(q) vs q scattering curves were corrected by the subtraction of the scattering from the pure solvent and then placed on an absolute scale using water as the standard. The I(q) vs q scattering profile of the Ag-NPs could be fitted using the form factor of homogeneous spheres. The fitting procedures and other analysis were performed using the SASfit software, which makes use of the least-squares fitting approach to minimize the chi squared (χ2) parameter. The SASfit software package was developed by J. Kohlbrecher, and it is available free of charge.23
A max λ maxfwhh
(4)
where ψ is the response, which is a figure of merit that takes into consideration the maximum absorbance (Amax), which relates to the yield of Ag-NPs formed;26,27 the wavelength at Amax (λmax), which is relevant to the size of the Ag-NPs;27−30 and the full width at half-height (fwhh), which is associated with the size distribution of Ag-NPs.26,27 In this equation, the best response is obtained when the Amax value is maximized and the λmax and fwhh values are minimized, indicating the formation of small narrow Ag-NPs. Our previous work13 showed that there was a synergistic effect between alkyl (butyl or octyl) and ethanolic functional groups, when grafted on branched PEI, for the stabilization of Ag-NPs. This means that the combination of these reagents had a greater effect than the sum of the effects of each individual reagent. Similar behavior is observed with LPEI, and changes in the properties of the stabilizers occurred as a function of the amount of derivatization reagent. Figure 1 show the 3D graphs obtained from the library for the functionalization of LPEI with 1-bromobutane, 1-bromoctane, and 2-chloroethanol. The regions that had the highest response were evident in the graphs. The butyl group inhibited the stabilization of Ag-NPs, and at a high concentration of 1-bromobutane, even with an increase in the concentration of 2-chloroethanol, ψ did not increase significantly. On the other hand, we observed the synergistic effect of ethanolic and octyl groups and the composition of the mixture of derivatives was crucial in obtaining a good response. From these experiments, two of the best combinations for the stabilization of Ag-NPs 4596
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changing one variable at a time while the others are held constant at a selected level. This approach is time-consuming and labor-intensive, requiring multiple experiments to be performed.31−33 It also neglects any possible interactions among the variables. In this study, a 23 full factorial design with six axial points and four replications at the center point19 was employed for the optimization of the concentrations of polymer, salt precursor, and reducing agent, key parameters in the formation of M-NPs. Thus, a total number of 18 experiments were required for each polyelectrolyte: F1 and F12. Central points were used to estimate the pure error and curvature in the model (runs 15−18 of Table S2 in the Supporting Information).34 The polymers, salt precursor, reducer concentrations, and response for all multivariate experiments are summarized in Table S2 (Supporting Information). The optimization used for this work was the so-called response surface model: a two-level factorial design expanded to a central composite design.35 When the number of independent variables is small, overlying response surfaces and choosing optimum conditions constitute a simple and usually highly effective method. The analytical response (ψ) used for plotting the response surfaces was estimated from eq 4. At a confidence level of 95%, the results considering the analysis of variance (ANOVA, Table S3 in the Supporting Information), for the multivariate optimization for the F1AgNP system, demonstrate that only the coefficients [LPEI] and [AgNO3] are statistically significant, as shown by the analytical equation below. This equation gives the regression coefficients and their standard errors, and further illustrates the relationship between these variables and the analytical response (ψ): ψ = − 50.5(± 6.5) + 441.9(± 64.1)x1 − 1696.0(± 261.6)x12 + 518.5(± 75.8)x2 − 1797.7(± 297.7)x2 2
where x1 and x2 are the concentrations for LPEI and AgNO3. This equation has a ratio of mean square lack of fit to mean square pure error of 4.08, smaller than the 95% significant F5,3,95% value of 9.01 and a determination coefficient (R2) of 0.88, indicating that the results obtained are reliable and the model does not suffer from lack of fit. Figure 2 shows the response surfaces according to the calculation of this equation. In the 3-D graph, the normalized response is drawn as a function of two variables and the shape reflects the interactions and curvatures, or the lack of it, for the variables. The best conditions for the F1-AgNP stabilization (polymer, salt precursor, and reducer agent concentrations) were determined by taking into account the three graphs. From Figure 2a and b, it can be observed that HQ did not have a major influence on the formation of Ag-NPs for the concentration range studied, though LPEI and salt precursor concentrations did have an effect. The response values increased with the increase in LPEI and salt precursor concentrations, reaching a saddle point; i.e., the response may increase or decrease when it moves away from the point. In our case, the response is slightly influenced by the concentration of HQ. Figure 2c shows the synergistic effect between the polymer and salt precursor concentrations, which can be attributed to a positive interaction between silver nitrate and LPEI. In this case, the response reached a maximum, and would decrease when it moved away from the stationary point.
Figure 1. 3D screening graphs from the library for preparation of AgNPs with 1-bromobutane and 2-chloroethanol (A) and 1-bromoctane and 2-chloroethanol (B). The equivalent numbers were varied from 0 to 0.5 for each reagent per monomer residue of PEI. The final concentrations were 0.04 mmol L−1 for PEI, 0.8 mmol L−1 for AgNO3, and 0.08 mmol L−1 for HQ.
were chosen: one was polyelectrolyte F1 with 0.4 equiv of 2chloroethanol, and the other one was polyelectrolyte F12 with 0.4 equiv of 2-chloroethanol and 0.5 equiv of 1-bromooctane. These reaction receipts were used in the scaled-up production. Optimization of Ag-NPs Synthesis with the Selected Stabilizers. A detailed study using F1-LPEI and F12-LPEI derivatives was performed to understand the effects of the polymer, salt precursor, and reducing agent concentrations on the formation of Ag-NPs. Optimization of such parameters has traditionally been carried out using a univariate method, i.e., 4597
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in this case, the results show that all three individual factors and the interactions between [LPEI] and [AgNO3] and between [LPEI] and [HQ] are statistically significant. The results for the regression analysis (R2 = 0.98) of these data are shown in the equation below: ψ = 2.2(± 0.8) − 34.7(± 9.3)x1 + 191.4(± 28.3)x12 + 390.8( ±20.2)x2 2 + 12.7( ± 3.8)x3 − 8.1(± 5.9)x32 − 285.9( ±41.6)x1x2 − 37.2( ±22.1)x1x3
From these factorial analyses, a set of conditions which led to the best response and consequently to an efficient nanoparticle stabilization were found to be 0.12 mmol L−1 of the polymer (for both F1-Ag-NPs and F12-Ag-NPs), 0.14 and 0.20 mmol L−1 of AgNO3 for F1-Ag-NPs and F12-Ag-NPs, respectively, and 0.35 and 0.40 mmol L−1 of HQ for F1-Ag-NPs and F12Ag-NPs, respectively. The UV−vis spectra for the two systems, shown in Figure 3, are typical for nanoparticles with spherical
Figure 3. UV−vis absorption spectra for F1-Ag-NPs (solid line, [LPEI] = 0.12 mmol L−1, [AgNO3] = 0.14 mmol L−1, and [HQ] = 0.35 mmol L−1) and F12-Ag-NPs (dotted line, [LPEI] = 0.12 mmol L−1, [AgNO3] = 0.20 mmol L−1, and [HQ] = 0.40 mmol L−1).
geometry and narrow size distribution (small fwhh).26 Nanoparticles prepared under these conditions showed good stability for months when stored in a refrigerator. Manipulation of these nanoparticles could be carried out under a wide range of experimental conditions without any noticeable precipitation of the NPs or turbidity of the aqueous medium, in contrast to the Ag-NPs prepared in the presence of the nonderivatized LPEI. TEM and Scattering Characterizations of the LPEI-AgNPs. The shape and particle size distribution of F1-Ag-NPs and F12-Ag-NPs were determined by TEM (Figure 4). The particles were prepared under the optimal conditions determined by the multivariate analysis discussed above. For comparison, Ag nanoparticles made under three conditions other than the optimal were also studied by TEM. The mean diameters of the Ag-NPs were obtained through the Gaussian fits of size distribution histograms, and the results are given in Table 2 along with the synthetic conditions. In general, the shape of Ag-NPs formed under the optimal conditions was spherical. The mean diameters and size distribution differed slightly from each other (34 ± 9 nm for F1-Ag-NP and 30 ± 10 nm for F12-Ag-NP). It is worth noting,
Figure 2. Response surfaces as a function of variables for the F1-LPEI system: (A) LPEI and HQ concentrations (indicate silver nitrate level at 0.12 mmol L−1); (B) HQ and silver nitrate concentrations (indicate LPEI level at 0.12 mmol L−1); (C) LPEI and silver nitrate concentrations (indicate HQ level at 0.22 mmol L−1).
Similar analysis of variance was performed for the F12-LPEI system (Table S4 and Figure S1, Supporting Information), but 4598
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Figure 4. TEM micrographs (left) and size distribution histograms (right) for (a) F1-Ag-NPs (0.12 mmol L−1 LPEI, 0.14 mmol L−1 AgNO3, and 0.35 mmol L−1 HQ) and (b) F12-Ag-NPs (0.12 mmol L−1 LPEI, 0.20 mmol L−1 AgNO3, and 0.40 mmol L−1 HQ).
top and bottom left, respectively (a and c). The average hydrodynamic diameters (DH = 2RH) of the nanoparticles were determined to be 37.2 nm for F1-Ag-NPs and 25.8 nm for F12Ag-NPs. Polydispersity of the Ag-NPs could not be determined properly through the cumulant analysis, because both samples had a second distribution of large aggregates (RH ∼ 100 nm), which prevented the use of the cumulant method. Nevertheless, the intensity of light scattering was heavily weighted by the particle molecular weight. Therefore, by taking into account the number of particles, the presence of large aggregates can be neglected, as clearly evidenced by the TEM micrographs. The observed stability of the Ag-NPs is related to their electrostatic stabilization, which can be confirmed by their ζpotential distributions, as shown in Figure 6b and d. The average values were determined as +20.7 ± 11.2 mV for F1-AgNP and +19.6 ± 7.0 mV for F12-Ag-NP. The positive ζpotentials were most certainly related to the stabilization by LPEI. In aqueous solution, the linear polyamine LPEI behaves as a positively charged weak polyelectrolyte, since it is a weak base exhibiting a cationic character. The SAXS measurements were performed in order to probe the size, shape, and polydispersity of the nanoparticles. Figure 7 shows the SAXS profiles of F1-Ag-NPs (a) and F12-Ag-NPs (b). The SAXS scattering intensity I(q) of an isotropic solution of particles embedded in a matrix with a constant electron density,
Table 2. Experimental Conditions and Mean Diameters Obtained by TEM Analysis for the F1-Ag-NPs and F12-AgNPs Prepared with and without the Use of Factorial Design condition
[LPEI] (mmol L−1)
[AgNO3] (mmol L−1)
[HQ] (mmol L−1)
F1 F1.1 F1.2 F1.3 F12 F12.1 F12.2 F12.3
0.12 0.08 0.20 0.12 0.12 0.08 0.20 0.12
0.14 0.08 0.05 0.14 0.20 0.16 0.13 0.20
0.35 0.20 0.18 0.16 0.40 0.31 0.23 0.20
Dm (nm)
fwhh (nm)
± ± ± ± ± ± ± ±
22 43 34 38 25 38 60 29
34 51 31 18 30 38 36 38
9 18 15 16 10 16 25 12
however, that the polydispersity (related to the fwhh values in histograms and the standard deviation) changed substantially for the conditions that were not optimized (Table 2). Furthermore, TEM study indicates that these nanoparticles showed a high level of agglomeration and nonspherical geometries, such as rod and triangle (Figure 5). These TEM results indicate that, besides the polydispersity of F1-Ag-NPs and F12-Ag-NPs, the methodology employed to determine the optimal conditions under which to prepare the Ag-NPs was appropriate. Additional characterization of the Ag-NPs with the light scattering technique is shown in Figure 6. The volume-weighted size distributions for F1-Ag-NP and F12-Ag-NP are shown on the 4599
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Figure 5. TEM micrographs of Ag-NPs prepared under conditions (a) F1.1, (b) F1.2, (c) F1.3, (d) F12.1, (e) F12.2, and (f) F12.3. See Table 2 for details.
Figure 6. Volume-weighted RH (a and c) and ζ-potential-weighted (b and d) distributions for F1-Ag-NP (top) and F12-Ag-NP (bottom), respectively.
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σ gives quantitative information on the dispersity of the NPs. This fitting approach describes the experimental results reasonably well and led to values of D = 2R = 33.0 nm and σ = 0.24 for F1-Ag-NP and D = 2R = 22.4 nm and σ = 0.29 for F12-Ag-NPs. It is also worth noting that the high quality of the fittings particularly in the low-q range of the SAXS profiles is related to the electrostatic stabilization of the Ag-NPs provided by the linear PEI chains. These data are in agreement with the UV−vis data (Figure 3) and with the TEM studies (Figure 4 and Table 2). It is worth highlighting that the scattering experiments are significantly influenced by the presence of dust, large aggregates, and impurities. Therefore, previous to the SAXS and light scattering measurements, all the samples were filtered into dedusted cuvettes. The filtration procedure generally removes a small fraction of larger particles of the size distribution, displacing the mean average size toward smaller values. Since the sample preparation is distinct, generally scattering and imaging data are not directly correlated. Nevertheless, the TEM, SAXS, and light scattering measurements clearly confirm that the F12-Ag-NPs are smaller and more polydisperse compared to F1-Ag-NP. Finally, the SAXS profiles show that Ag-NPs made under the conditions that were not optimized (Table 2 and Figure 5) had a typical high X-ray scattering in an upward trend in the low-q range. This is clear evidence of the presence of large aggregates or agglomeration of Ag-NPs, indicating again the effectiveness of the methodology used. A representative SAXS profile for F12.3-Ag-NPs is shown in Figure 8.
after normalization with the background scattering of the solvent, is given by I(q) = NP(q)S(q)
(5)
Figure 7. SAXS data (circles) and corresponding curve fittings (lines) for (a) F1-Ag-NPs and (b) F12-Ag-NPs.
where N is the number of particles per unit volume, P(q) is the form factor of an individual particle, and S(q) is related to the particle interference factor. For widely separated systems (as in the current case), S(q) is about 1 and I(q) is due to the form factor P(q) of the scattering objects, correlated to the size and shape. In this study, P(q) of the Ag-NPs was modeled geometrically as a sphere: 2
Figure 8. Experimental SAXS data for F12.3-Ag-NPs.
Catalytic Activity. The catalytic activity of Ag-NPs stabilized with LPEI (F1 and F12-Ag-NPs) was evaluated using the reduction reaction of p-nitrophenol (Nip) with NaBH4. The reduction of Nip is a model reaction which has been widely used for the quantification and comparison of the catalytic activity of different metal nanoparticles immobilized on a variety of supports.36−39 This reaction was monitored by measuring the UV−vis absorption intensity due to Nip, which has a distinct spectral profile with a maximum at 317 nm in water, but in the presence of NaBH4, Nip is converted into p-nitrophenolate ion that has its maximum at 400 nm (Figure S2, Supporting Information).40,41 In the presence of colloidal dispersions of F1-Ag-NPs and F12-Ag-NPs, the peak at 400 nm gradually decreased with time; the yellow color of Nip in an aqueous solution of NaBH4 faded and eventually became
2
I(q) = Vp Δσ P(q , R ) 2 ⎛ 4 3 ⎞2 ⎛ 3[sin(qR ) − qR cos(qR )] ⎞ ⎜ ⎟ ⎜ ⎟ = πR Δσ ⎜ ⎟ ⎝3 ⎠ ⎝ (qR )3 ⎠
(6)
The sample polydispersity was considered using the lognormal distribution for which the probability density function is given by f (R , μ , σ) =
ln(R /μ)2 1 exp − 2π σR 2σ2
(7)
where R is the average radius and parameters μ and σ are the mean and standard deviation of the distribution. The parameter 4601
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completely bleached and a new peak appeared at 310 nm, which is attributed to the absorption of p-aminophenolate (Amp). Interference from oxygen was avoided by purging the solutions with argon, and the effect of ambient light was controlled to the minimum by carrying out the reaction in the dark. An induction time was observed in these reactions (Figure 9). This phenomenon is typical for a heterogeneous catalytic
Figure 10. Plots of the initial rate (v0, black squares) as a function of [NaBH4] for F1-Ag-NPs at [Nip] = 0.05 mmol L−1 and the initial rate (v0, red circles) as a function of [Nip] for F1-Ag-NPs at [NaBH4] = 25 mmol L−1. The surface area (S) in both experiments is 0.0225 m2 L−1, and the reaction temperature was set to 15 °C.
shown in Scheme 1. Such reaction is possible because Nip and NaBH4 concentrations used in these experiments did not affect
Figure 9. Time dependence of the absorption of p-nitrophenolate ions (Nip) at 400 nm. The inset shows the linear fit for the first-order kinetics. ([Nip] = 0.1 mmol L−1; [F1-Ag-NPs] = 0.01 mmol L−1; [NaBH4] = 25 mmol L−1; 15 °C).
Scheme 1. Reaction Mechanism for Monomolecular Surface Reactions
process and commonly related to the time required for the activation of catalyst. All concentrations of NaBH4 were in considerable excess over that of Nip, assuring pseudo-first-order conditions,42,43 as demonstrated by the linear fit of the plot of ln A/A0 (A is the concentration of Nip) versus time (inset of Figure 9). This induction time has been observed by a number of authors with catalysts on different supports and is generally interpreted in terms of the time required for the reactants to diffuse to the surface of the particles. According to Zeng et al., the rate of adsorption of p-nitrophenolate onto the surface of a catalyst is the predominant factor during the induction period.42 Saha et al. alleged that the induction time usually relates to the presence of dissolved oxygen in water reacting at a faster rate with borohydride than nitrophenol;44 however, we can rule out this possibility, since the aqueous stock solutions were carefully degassed and purged with argon before use. Moreover, the presence of an induction time was also related with an initial step involving a reaction with borohydride, such as the transport of a surface-hydrogen species to the metal nanoparticles. The effect of NaBH4 concentration on the reduction reaction of Nip in the presence of F1- or F12-Ag-NPs was studied. As shown in Figure 10, the concentrations of Nip and catalyst were kept constant and the NaBH4 concentration was varied. Increasing the NaBH4 concentration caused the initial reaction rate to increase until saturation. The same behavior was observed in the dependence of the initial reaction rate on the Nip concentration when the saturation occurred at 0.04 mmol L−1 (Figure 10). The observed kinetics profiles presented in Figure 10 can be interpreted as the result of monomolecular surface reactions, as
the reaction rate (i.e., zero-order dependence) and the surface of the nanoparticles was saturated. According to Scheme 1, the reactant R (BH4− at a high concentration of Nip or Nip at a high concentration of BH4−) interacted with the active sites on the catalyst surface C (AgNPs) to form the adsorbed species RC, which underwent reaction to form the final product P (Amp). At high concentrations of BH4− or Nip, the reduction reaction occurred at the surface of catalytic nanoparticles following a monomolecular mechanism and the reaction rate was dependent on the fraction of catalyst surfaces covered by the substrate, as described by the Langmuir model. Therefore, the global reaction law can be expressed by ν = kLS θR
(8)
where kL is the Langmuir rate constant for the formation of the product normalized to S, the surface area of all nanoparticles normalized to the unit volume of the reaction system, and θR is the fraction of catalyst surface covered by the reactant, which can be expressed by θR =
K[R] 1 + K[R]
(9)
Combining eqs 8 and 9, we can obtain
ν=
kLSK[R] 1 + K[R]
(10)
Applying eq 10 to the experimental data for the Nip reduction by Ag-NPs, the rate constant (kL) and the adsorption 4602
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constant (K) can be calculated by means of nonlinear fitting procedures. These values are reported in Table 3. Table 3. Rate Constants and Adsorption Constants of Nip and BH4− system
kLa (mol m−2 s−1)
KBH4b (L mol−1)
KNipb (L mol−1)
F1-Ag-NPs F12-Ag-NPs
(2.6 ±0.4) × 10−5 (1.6 ± 0.1) × 10−6
30.6 ± 8.6 21.6 ± 1.5
(7.0 ± 1.3) × 104 (4.6 ± 0.5) × 104
a
Langmuir rate constant normalized to the total surface area of the nanoparticles per unit of volume. bLangmuir adsorption constant.
The same approach was used to calculate the catalytic rate and adsorption constants for the F12-Ag-NP catalytic systems (Figure S3, Supporting Information). As shown in Table 3, the adsorption constant for Nip on the F1-Ag-NPs and F12-AgNPs was much higher than that for BH4−. Moreover, the Langmuir rate constant kL, which determines the reaction rate of the adsorbed molecules, was around 16 times lower for F12Ag-NPs than for F1-Ag-NPs. Thus, the F12-AgNPs were less active than the F1-Ag-NPs in the reduction of Nip. Nonetheless, it is important to note that the assumption for the above kinetics analysis is that all sites on the nanoparticle surface have the same adsorption energy; i.e., a homogeneous surface is assumed, and any spatial correlation between the adsorbed molecules is completely neglected. These are the simplified assumptions of the classical Langmuir model. Another way to compare the catalytic activities of the two systems with others reported in the literature is to determine the k1 value, that is, the catalytic rate constant normalized to the surface area S per unit volume. This rate constant can be determined from a linear plot of the apparent kinetic rate constant (kapp) versus S; i.e., kapp is proportional to the total surface area of all metal nanoparticles, according to eq 11:45−50 −
d[Nip] = kapp[Nip] = k1S[Nip] dt
Figure 11. Plots of the apparent rate constant (kapp) as a function of the surface area of Ag-NPs normalized to the unit volume of the system (S) for F1-Ag-NPs (red circles, [NaBH4] = 25 mmol L−1, [Nip] = 0.03 mmol L−1) and F12-Ag-NPs (black squares, [NaBH4] = 80 mmol L−1, [Nip] = 0.015 mmol L−1) at 15 °C.
form stable Ag-NPs with similar shape, mean size, and degree of dispersity, the catalytic rate constant k1 for F1-LPEI is about 5 times higher than that for F12-LPEI. This small difference in catalytic activity between these two systems is in consonance with the high throughput screening in the selection of the catalytically active LPEI-Ag-NPs. However, contrary to what has been observed for alkyl derivatives of branched PEI,13 the lesser hydrophobic F1-Ag-NPs composite has higher activity. This behavior is most probably due to a difference in the diffusion rates of Nip to the surface of the nanoparticles, which is evidenced when comparing the reaction induction periods at saturation levels of BH4− (Figure S4, Supporting Information).42 The LPEI derivative bearing the lipophilic octyl moiety (F12-LPEI) had always presented longer induction times than the F1-LPEI derivative, which would imply a greater resistance to diffusion and therefore a slower reaction.
(11)
This expression assumes quasi-homogeneous51 conditions of the system and a zero-order dependence of the reaction with respect to NaBH4. Plots of kapp as a function of S for both catalytic systems (F1-Ag-NP and F12-Ag-NP) are shown in Figure 11. In these experiments, the concentration of NaBH4 was kept above the saturation level determined from the experiments shown in Figure 10 and Figure S3 (Supporting Information) (25 mmol L−1 for F1-Ag-NP and 80 mmol L−1 for F12-Ag-NP, respectively), assuring the zero-order conditions of the reaction with respect to NaBH4. It can be observed that the rate constant kapp was indeed proportional to the total surface area of the nanoparticles in the systems; hence, it can be concluded that catalysis takes place on the surface of the nanoparticles. From the angular coefficient of the plots of Figure 11, the values for the catalytic rate constant k1 were found to be 1.66 ± 0.05 s−1 m−2 L for F1-Ag-NPs and 0.37 ± 0.01 s−1 m−2 L for F12-Ag-NPs. The k1 value obtained for F1-Ag-NPs is among the highest reported in the literature for metal nanoparticle catalysts (including silver).13,18,40,45,52−54 The difference in the magnitude of k1 for these systems clearly shows that the composition of the stabilizer has a major influence on the catalytic activity. Although both LPEI derivatives, samples F1 (0.4 equiv of 2-chloroethanol) and F12 (0.4 equiv of 2-chloroethanol and 0.5 equiv of 1-bromooctane), were able to
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CONCLUSIONS The use of the high-throughput method, together with the in situ screening and the multivariate optimization, led to the successful synthesis of Ag-NPs, which have high catalytic efficiency for the reduction of p-nitrophenol by NaBH4 in water, even when compared with other M-NP catalysts. The kinetic analysis of these catalytic systems showed that Nip has a much stronger affinity for the NP surface than BH4− based on the Langmuir model. Also, the Ag-NPs supported with the F1-LPEI derivative showed higher catalytic efficiency when compared with F12-LPEI, in both the Langmuir rate constant (kL) and the rate constant normalized to S (k1) obtained for quasi-homogeneous conditions. The high catalytic efficiency of F1-Ag-NPs is most probably due to the smaller resistance to diffusion of Nip to the surface of the nanoparticles, which is evidenced by the smaller reaction induction periods.
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ASSOCIATED CONTENT
S Supporting Information *
Multivariate optimization experimental designs, concentration data in each optimization experiment, ANOVA tables, response surfaces, variation in UV−visible absorption spectra for the Nip 4603
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reduction reaction, initial rate profiles for the F12-LPEI system, and reaction induction times as a function of NP concentration. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to CNPq and CAPES for financial support for this study. This work was also supported by the Brazilian Synchrotron Light Laboratory (LNLS) under proposal D11A SAXS1 11000. We would also like to thank the Central Laboratory of Electron Microscopy (LCME) at UFSC and CEM at UFABC for allowing access to the Malvern light scattering instrument. H.Y. is also supported by U.S. National Science Foundation.
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