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Feb 10, 2014 - nanoparticles, the aminosilanes are interesting because they can ... These findings indicated the important effects of aminosilane ...

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Synthesis and Characterization of Silver Nanoparticles Produced with a Bifunctional Stabilizing Agent André L. Nogueira,* Ricardo A. F. Machado, Alan Z. de Souza, Flávia Martinello,† César V. Franco,‡ and Gabriel B. Dutra§ Department of Chemical Engineering, Federal University of Santa Catarina (UFSC), Campus Universitário Trindade, Caixa Postal 476, 88040-900, Florianópolis, Santa Catarina, Brazil ABSTRACT: Among the different chemical compounds used to prevent the aggregation and to control the size of silver nanoparticles, the aminosilanes are interesting because they can simultaneously act as stabilizing and coupling agents. The aim of this study was to investigate the effects of different concentrations of an aminosilane on the synthesis of silver nanoparticles. The functionalized nanoparticles were characterized using UV−vis spectrophotometry, transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray diffraction (XRD), and zeta (ζ) potential. Antibacterial assays were also performed. According to the results, increasing the concentration of the aminosilane produced smaller, less dispersed, and more stable silver nanoparticles. Besides the effective antibacterial activity verified in all the concentrations tested, a significant influence on the kinetics of bacteria annihilation was also observed when aminosilane was used in a concentration dependent fashion. These findings indicated the important effects of aminosilane concentrations in controlling the size and stability of the colloids, as well as the rate of silver ions releasing from nanoparticles.

1. INTRODUCTION The development of metallic nanomaterials has been an issue of great interest among researchers and companies in past decades due to the special properties that these materials exhibit when they are in the nanometric scale, such as electrical,1,2 catalytical,3 antimicrobial,4−6 sensorial,7,8 and optical.9 Metallic nanoparticles are commonly obtained through reduction reactions of metal ions of a salt. During these reactions, when the concentration of metallic atoms achieves a supersaturation level, the aggregation process takes place in the system, and small clusters or nuclei are formed (self-nucleation process). Once this process begins, the clusters grow rapidly producing metal nanocrystals.10 Because of the small size of nanocrystals, the ratio between the number of atoms on the surface and in the interior is high. As a consequence, the nanocrystals are thermodynamically unstable and are prone to aggregation.11,12 If there is no protection to prevent such aggregation, the nanocrystals grow indefinitely and produce an unstable colloidal system with later precipitation of the nanoparticles. Therefore, the use of stabilizing agents in the synthesis of metal nanoparticles is of major importance to prevent their aggregation and to control their growth.13 Different chemical compounds have been used to prevent nanoparticle aggregation through the steric hindrance mechanism, such as poly(vinyl pyrrolidone),14−17 poly(vinyl alcohol),4 polystyrene,18 polyaniline,19 dodecanethiol,20 dodecylamine,2 poly(amido amine) dendrimers,7 and organic−inorganic bifunctional molecules such as aminosilanes.21−23 Due to the interaction between the atoms of silver (Ag) and nitrogen (N) through coordination bonds,2,15,21,22,24,25 stabilizing agents that contain nitrogen atoms in their compositions prevent efficiently the aggregation of silver nanoparticles. Consequently, a long-term stable product may be produced. Among the stabilizing agents which contain nitrogen atoms, molecules of aminosilanes are especially interesting due to their bifunctionality. The amine © 2014 American Chemical Society

functional group, presented in one extreme of the molecule, is responsible for protecting the silver nanoparticles against aggregation. The silicate functional group, presented at the other extreme of the molecule, should be responsible for the anchorage of the silver nanoparticles on different types of substrates.24 Silver nanoparticles which are stabilized and simultaneously functionalized by aminosilanes are suitable to be used in a variety of industrial processes without any chemical treatment. For instance, the aqueous dispersion of colloidal silver stabilized by aminosilane can be directly applied in textile processes (e.g., foulard baths) to produce antibacterial fabrics. The same colloidal dispersion can be used to obtain a silica-based powder additive to be applied in the production of antibacterial polymers. Due to the potential applications of such colloidal dispersions in textile and polymer processing industries, our goal was to investigate the effect of different concentrations of an aminosilane in the synthesis and properties of silver nanoparticles.

2. EXPERIMENTAL SECTION 2.1. Reagents. All the reagents were used as received without further purification. Silver nitrate (AgNO3) was purchased from Cennabras (purity >99%), sodium borohydride (NaBH4) was ́ purchased from Casa da Quimica (CAQ; Nuclear brand, purity >98%) and the diaminofunctional silane (N-(β-aminoethyl)-γaminopropyltrimethoxysilane, active material 85% w/w) was purchased from Dow Corning. They were used as the metal precursor, reducing agent, and stabilizing agent, respectively. Deionized water, produced in a reverse osmosis unit acquired from Quimis, was used as the reaction medium. Received: Revised: Accepted: Published: 3426

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2.2. Reactions. The synthesis reactions were carried out at room temperature in a 2.0 L capacity reactor (PARR Model 4843, Hastelloy), containing an internal coil for heat exchange and a controllable stirring system. A thermo-cryostat bath was connected to the internal coil to control the temperature of the reactions at 20 °C. The stirring rate was maintained at 600 rpm in all experimental runs. The reactor was filled with 0.675 L of deionized water, followed by addition of the stabilizing agent and the silver nitrate, which were stirred for 10 min to ensure homogeneity. Finally, 0.075 L of a 46.4 mmol/L aqueous solution of sodium borohydride maintained in an ice bath was added to the system at a molar flow of approximately 0.1 mmol/min. A glass buret was used for this procedure. After the addition of the reducing agent solution, the system was maintained under stirring for 10 min to ensure the complete consumption of the reagents. The silver nanoparticles produced were stored in amber glass containers, and they were not submitted to any separation or purification process before characterization or utilization in the antibacterial assays. In order to evaluate the influence of the stabilizing agent in the production of the silver nanoparticles, concentrations ranging from 2.7 to 21.6 mmol/L were used in the experimental runs. This range was selected due to its significant influence in the colloidal stability, in addition to the size and shape of the silver nanoparticles produced under such concentrations. All the reactions were carried out considering a stoichometric ratio of 1:0.5 between the AgNO3 and NaBH4, respectively. The amounts of both reagents were calculated to provide a concentration of 9.3 mmol/L AgNO3 and 4.64 mmol/L NaBH4 inside the reactor, taking into account the total reaction volume of 0.75 L. It is important to highlight that all the reactions were performed in triplicate to ensure the reproducibility of the results. 2.3. Characterization. The silver nanoparticles produced in the reactions were characterized by UV−vis spectrophotometry (Hitachi U1900), transmission electron microscopy (TEM; Jeol JEM-1011), selected area electron dispersion (SAED; Jeol JEM-1011), X-ray diffraction (XRD; Philips X’Pert), and zeta (ζ) potential (Malvern Zetasizer Nano Series). Antibacterial assays were also carried out according to the standard method ASTM E2149-01. The reaction samples were diluted in deionized water (ratio 0.1:7.5) before being characterized by UV−vis spectrophotometry, with wavelength varied from 300 to 700 nm. A glass cuvette with a square aperture (optical path of 10 mm) and a step size of 1 nm was used for the analyses. They were carried out in triplicate, and the absorbance spectra displayed in the results represent the average of the three measurements. The TEM analysis was performed by using 10 drops of the concentrated colloidal dispersions, which were carefully dripped over copper grids (300 mesh) coated with carbon. The grids were dried overnight at room conditions before being analyzed by the microscope with a resolution of 100 kV. Magnifications of 150000× and 300000× were used to generate the images. The software ImageJ (public domain) was used to evaluate the images and determine the particle size distributions (PSDs), the average sizes, and their respective standard deviations. Selected area electron diffraction (SAED) images were acquired during TEM analysis and were used to support the X-ray diffraction measurements. A powder sample containing the silver nanoparticles was used to perform the X-ray diffraction analysis. The sample was prepared according to the following procedure: 50 mL of the

aqueous colloidal silver was mixed with 0.5 g of a powder silica (purchased from Grace Davison, type Syloid 244 FD, with an average diameter of 5.0 μm and purity of 99.0%) and let sit for 1 day; the supernatant became colorless and was removed with a syringe; the wet silica containing the silver nanoparticles was dried at room conditions for 1 week; then the powder sample was analyzed. The measurements were performed with a diffraction angle ranging from 30 to 90°, Cu Kα radiation (λ = 1.5406 Å), 40 kV and 30 mA, and a step size of 0.02°. The ζ potential measurements were carried out with no changes in the original pH of the colloidal dispersions, which were evaluated using a microprocessed pHmeter (Anailon, AN2000). Prior to measurements, the reaction samples were diluted in deionized water (ratio 1:20). The ζ potential analysis was carried out in triplicate to ensure the reproducibility of the results. Finally, the antibacterial activity of the silver nanoparticles produced under different concentrations of the bifunctional stabilizing agent was evaluated applying the standard method ASTM E2149-01: Determining the Antimicrobial Activity of Immobilized Antimicrobial Agents under Dynamic Contact Conditions. According to this method, the Gram-positive microorganism Staphylococcus aureus (American Type Culture Collection (ATCC) No. 6538) was cultivated in sterile nutrient broth for 24 h before the test. The culture was diluted in buffer solution until it reached an absorbance of 0.28 ± 0.01 at 475 nm (measured spectrophotometrically). After that, the culture solution was properly diluted in sterile buffer solution (1000 times) to obtain a final concentration of (1.5−3.0) × 105 CFU/mL (CFU, colony-forming units), and it was used to prepare six samples in duplicate for analysis. The reference sample was just composed by the buffer solution (0.025 L) and the bacterial solution (0.025 L). The other five samples were prepared as the reference sample, plus 12.5 μL of the colloidal silver dispersions (exactly as produced in the reactor) in order to evaluate their effect on the bacterial growth. The addition of such a small volume of colloidal dispersion reduced 4000-fold the product concentration, compared with the original product obtained in the reactions. The dilution ratio used was defined after initial pilot experiments. It was observed that 100% of the bacteria were killed rapidly when higher concentrations of silver nanoparticles were used (not shown). If the bacteria were all killed in a short time, it would be very difficult to study the different kinetic profiles of bacteria annihilation. All the samples were shaken and aliquots were collected at 0, 1, 2, 3, and 4 h after the substances were mixed. Serial dilutions were performed equally with these samples, and then they were plated in Petri dishes containing tryptone glucose extract agar (Difco Laboratories). The Petri dishes were incubated for 18−24 h at 36 ± 2 °C and the colonies formed were counted to determine the reduction of the bacteria according to eq 1. The antibacterial assays were performed in duplicate. reduction of bacteria (%) =

B−A ·100 B

(1)

where A is the CFU/mL for the sample containing the silver nanoparticles after a specified contact time and B is the CFU/mL at 0 contact time for the sample used to determine A before the addition of the silver nanoparticles.

3. RESULTS AND DISCUSSION 3.1. Transmission Electron Microscopy (TEM). Transmission electron microscopy (TEM) is a powerful characterization technique used to evaluate the morphology of metal 3427

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Figure 1. TEM images (100 kV; magnifications of 150000× and 300000× in the left panels and right panels, respectively; inside left panels: SAED patterns) and respective PSDs (histograms) of the silver nanoparticles produced with different concentrations of stabilizing agent: (A and B) sample a, 2.7 mmol/L; (C and D) sample b, 5.4 mmol/L; (E and F) sample c, 10.8 mmol/L; (G and H) sample d, 16.2 mmol/L; (I and J) sample e, 21.6 mmol/L.

When TEM was used to analyze the first and second set of experiments, good reproducibility of the PSDs was observed. Following, the images captured in the first set of experiments, as a representation of the results obtained, are presented in Figure 1A,C,E,G,I. For each sample, two images are demonstrated. The left panels present the images captured with a

nanoparticles. In this study, this technique was applied to observe the geometry and determine the particle size distribution (PSD) of the silver nanoparticles produced with different concentrations of the aminosilane. The products obtained in the synthesis of silver nanoparticles with 2.7, 5.4, 10.8, 16.2, and 21.6 mmol/L aminosilane were identified as samples a, b, c, d, and e, respectively. 3428

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were used (Figure 1G,I) demonstrated some nanoparticles with geometrical shapes different from spherical. As shown in Figure 1G, the use of 16.2 mmol/L aminosilane led to the formation of a few nanoparticles with a triangular shape (sizes smaller than 30 nm). When this concentration was increased to 21.6 mmol/L, the TEM images of the colloidal silver (Figure 1I) revealed an increase in the number and size of the triangular nanoparticles (sizes smaller than 50 nm). In addition, the formation of a few nanoparticles with a rod shape and low height/diameter ratio (sizes smaller than 20 nm) was also observed. According to Jiang et al.,29 there are two possible reasons that could explain the coexistence of spherical and triangular nanoparticles in some systems. The first could be related to the presence of nuclei or silver clusters with different shapes (e.g., Ag42+, Ag84+,Ag3+, or Ag3). In this case, the trimeric clusters can serve as nuclei for the addition of newly formed silver atoms and eventually lead to the formation of triangular nanoplates. On the other hand, the clusters Ag42+ and Ag84+ might benefit the formation of nanoparticles with spherical shapes.30 The second reason could be related to the essential structure of the silver seeds (face-centered cubic, fcc), which usually shows singlecrystal, singly twinned, or multiply twinned structures, and all of these may coexist in a typical synthesis. The population of seeds differently structured is determined by the statistical thermodynamics of the free energies of the different species in combination with kinetic factors regarding the generation and addition of metal atoms to a nucleus.10 As most of the silver nanoparticles produced in our study were spherical (i.e., the shape with the smallest surface area to minimize the total interfacial free energy), it is reasonable to consider that a thermodynamically controlled growth prevailed in the system. Furthermore, the use of NaBH4, a strong reducing agent,26 contributed to an autocatalytic and isotropic growth, leading to the formation of silver nanoparticles with spherical shape. Over the years researchers have proposed different growth mechanisms for silver nanoparticles with triangular and other specific shapes based on the use of kinetically controlled synthesis (e.g., use of soft templates,31 aging,14 irradiation,32 thermal treatment,33 weak reduction agents,29 and chemicals for substantial slow down of the reduction rate34). The introduction of selective capping agents (e.g., polymeric molecules such as poly(vinyl pyrrolidone) (PVP))35 is an alternative strategy that can be used to control the shape of silver nanoparticles. Once any strategy was used to promote a kinetically controlled synthesis, the production of the anisotropic silver nanoparticles observed in Figure 1G,I (mainly Figure 1I) should be assigned to the interaction of the large number of aminosilane molecules with the silver species. A specific study about the formation of anisotropic silver nanoparticles using N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane, as stabilizing agent, would be necessary to propose a growth mechanism for such nanoparticles. 3.2. UV−Vis Spectrophotometry. UV−vis spectrophotometry is a reliable and reproducible technique that can be used to accurately characterize the metal nanoparticles. Although it does not provide direct information regarding the particle sizes, the surface plasmon bands (absorbance spectra) are influenced by the size and shape of the nanoparticles, as well as the dielectric constant of the surrounding media.20,36−38 Parts A and B of Figure 2 present the absorbance spectra of the silver nanoparticles measured 1 and 30 days, respectively, after the reactions of the first set of experiments. Figure 2 also shows the images of the diluted dispersions of colloidal silver (0.1 mL of

magnification of 150000× (scale of 100 nm), while the right panels present the images with a magnification of 300000× (scale of 50 nm). The pattern of the selected area electron diffraction (SAED) of each sample is shown inside the respective image. The images revealed that the sizes of the silver nanoparticles decreased by raising the concentration of aminosilane from 2.7 to 21.6 mmol/L. The PSDs, presented as histograms in Figure 1B,D,F,H,J, were determined according to an appropriate treatment of the images using ImageJ software. It is important to notice that approximately 10 images were used to determine the PSD of each reaction. Therefore, thousands of nanoparticles were counted (Table 1). Table 1. Size Information Obtained from TEM Images of the First Set of Experiments stabilizing agent concn (mmol/L)

no. nanoparticles counted

av diam (nm)

std dev (nm)

2.7 5.4 10.8 16.2 21.6

2313 4477 3881 4703 7794

25.21 20.01 16.06 15.51 13.79

13.61 10.04 7.52 6.94 7.23

As observed in the histograms, and also with the results of the average size and standard deviation shown in Table 1, raising the concentration of the stabilizing agent made the size of the nanoparticles smaller and the PSD narrower. According to Zhang et al.,14 Chou and Lai,27 Seoudi et al.,20 and Chou et al.,26 the more stabilizing agent is added to the system, the smaller the silver colloids are. During the initial stage of a typical synthesis of metal nanoparticles, metal atoms aggregate into small clusters (i.e., nuclei) via self (or homogeneous) nucleation. These clusters grow fast and then generate the nanocrystals.10 As nanometric particles are subject to Brownian motion, they often collide with each other and tend to associate themselves via weak bonds (agglomeration) or strong bonds (aggregation) to form larger particles.28 This process takes some time, and must be under the control of the stabilizing agent to avoid the production of an unstable colloidal system. Throughout the growth process, the total surface area of the nanoparticles decreases until the colloidal system achieves a stable condition. At this point, which strongly depends on the concentration of the stabilizing agent, the sizes of the nanoparticles are defined. When higher concentrations of the aminosilane were used in the system, the growth time of the silver nanoparticles was reduced. The larger number of aminosilane molecules covered a larger surface area of the nanoparticles. These covered areas worked as diffusion barriers for the addition of atoms, and also hindered the aggregation of the growing species by steric effects.14 Therefore, silver nanoparticles with smaller and less dispersed sizes were produced. On the other hand, when lower concentrations of stabilizing agent were used, the nanoparticles kept growing until the total surface area decreased sufficiently to be well covered by the limited number of aminosilane molecules. This extended growth process resulted in silver nanoparticles with larger sizes and a broader PSD. According to the TEM images shown in Figure 1A,C,E,G,I, the majority of the silver nanoparticles produced in the synthesis presented a spherical shape. Some large aggregates with uneven geometry were observed when low concentrations of the stabilizing agent were used (Figure 1A,C). The careful evaluation of the images taken when higher concentrations of the aminosilane 3429

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Figure 2. Absorbance spectra of the samples prepared with different concentrations of aminosilane (sample a, 2.7 mmol/L; sample b, 5.4 mmol/L; sample c, 10.8 mmol/L; sample d, 16.2 mmol/L; sample e, 21.6 mmol/L) (A) 1 day after the reactions and (B) 30 days after the reactions.

by increasing the concentration of aminosilane due to the coordination between the Ag+ ions and N atoms. This chemical bond may decrease the potential of Ag+/Ag (EAg+/Ag) and promote the reduction of Ag+. According to Zhang et al.14 and Wang et al.,16 an increase in the reduction rates of silver ions was observed when PVP was used as stabilizing agent. They stated that this behavior was a consequence of the coordination between Ag+ ions and N atoms presented in PVP molecules. Three optical parameters were obtained from the spectra shown in Figure 2B and used to evaluate the effect of aminosilane concentration on the properties of the silver nanoparticles. First, the fwhm (full width at half-maximum) is a numerical parameter that represents the width of an absorbance spectrum and, consequently, provides important information regarding the dispersion of the sizes and shapes of the nanoparticles. Second, the maximum wavelength absorbed (λmax) is the wavelength at which most of the nanoparticles absorb the UV−visible light. Finally, the average wavelength absorbed (λaver), which was obtained by integration of the spectra (eq 2), contains reliable and complete information about the sizes and shapes of all nanoparticles in just one value.

the concentrated colloidal dispersion diluted in 7.5 mL of deionized water) used in the UV−vis spectrophotometric analyses. The different colors demonstrated by the samples are visual evidence of the effects of aminosilane concentration on the morphology of the silver nanoparticles produced. The absorbance spectra of the silver nanoparticles were measured at 7, 14, 21, and 30 days after the reactions, to evaluate the stability of the produced colloidal systems. Samples a (2.7 mmol/L), b (5.4 mmol/L), and c (10.8 mmol/L) presented a pronounced reduction in their absorbances during the first 15 days. After this period, the absorbance spectral measurement of these samples remained unchanged for more than 2 months, indicating that the products reached a stable condition (results not shown). This behavior showed that the concentrations of aminosilane used to synthesize samples a, b, and c were insufficient to produce stable colloidal systems just after the synthesis. The decrease of the absorbances can be related to an excessive growth of some nanoparticles, followed by their precipitation. Although 15 days were sufficient to observe that the absorbances stopped changing, a period of 30 days after the reactions was considered to ensure that the systems had reached a stable condition. Once the absorption spectra of samples d (16.2 mmol/L) and e (21.6 mmol/L) did not present significant variations between the first and 30th days after the synthesis, it was concluded that such concentrations of the aminosilane produced stable dispersions of silver nanoparticles. As reported by Roldán et al.,22 the use of aminosilanes as stabilizing agents can produce stable dispersions of colloidal silver due to the strong interaction between amino groups and the silver surface. According to the Beer−Lambert law, the peak height of a surface plasmon band (absorbance spectrum) is proportional to the concentration of the species in the system.39 If only one plasmon band is obtained, the increase of its intensity (absorbance) is an indication of the reaction advance degree (Ag+ to Ag0 reduction) with the subsequent increment in the number of particles produced.40 In this way, the results about the maximum absorbance (λmax) presented in Figure 3A demonstrated qualitatively that after the “unstable period” (i.e., 30 days after the reactions) the concentration of silver nanoparticles was higher in those products synthesized with more stabilizing agent. The absorbance peaks measured 1 day after the synthesis tended to be slightly higher with increased aminosilane concentrations (Figure 2A). This means that using more stabilizing agent resulted in more concentrated colloidal systems. Frattini et al.21 reported that the formation of silver nanoparticles can be favored

∫ (Abs(λ)·λ) dλ ∑in Absi ·λi = λaver = n ∑i Absi ∫ Abs(λ) dλ

(2)

where Abs(λ) is the absorbance value measured at a specific wavelength, λ is the wavelength (nm), i is the integer representing the variable position of the wavelength for the absorbance measurement, and n is the integer representing the total number of absorbance measurements (n = 400). On the basis of the light scattering theory of Mie (Mie’s theory), it is possible to establish relationships between the optical properties and the size of the nanoparticles. According to this theory, smaller nanoparticles absorb light at shorter wavelengths, while larger nanoparticles absorb light at longer wavelengths.17,20,29 Mie’s theory also predicts only a single surface plasmon resonance band in the absorbance spectra of spherical nanoparticles. However, anisotropic nanoparticles could give rise to two or more surface plasmon bands, depending on the shape of the nanoparticles.38 Pastoriza-Santos and LizMarzán41 reported that metal nanorods display two distinct dipole resonances due to transversal and longitudinal oscillations. Three years later, Brioudi and Pileni42 used the discrete dipole approximation method to demonstrate that four peaks arise in the surface plasmon band of silver nanoparticles with triangular shape. According to the authors, these peaks are 3430

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Figure 3. Average and standard deviation of the optical properties from UV−vis spectra as a function of the stabilizing agent concentration (measurements carried out 30 days after the reactions): (A) absorbance; (B) full width at half-maximum (fwhm); (C) maximum wavelength, λmax; (D) average wavelength, λaver.

greater than or equal to 10.8 mmol/L. Based on the theoretical calculations of Brioudi and Pileni,42 and the experimental studies of Pastoriza-Santos and Liz-Marzán,41 Sun et al.,33 and Mansouri and Gahder,34 the broadening and red shifting of the absorbance spectra should be assigned to the in-plane dipolar and in-plane quadrupolar resonances of the nanotriangles formed. Moreover, according to the results presented by these authors, the smooth shoulder observed around 350 nm in the plasmon band of sample e should be a consequence of the out-of-plane quadrupolar resonance of the silver nanotriangles. The decrease of the fwhm with raising the concentration of aminosilane from 2.7 to 5.4 mmol/L (Figure 3B) is in agreement to the reduction of the size dispersion (i.e, narrowing of the PSD) of the spherical silver nanoparticles produced in samples a and b. The maximum wavelength (λmax) and the average wavelength (λaver) absorbed, shown in parts C and D of Figure 3, respectively, were also influenced by the variations in the shape of the silver nanoparticles. When just spherical nanoparticles were produced (samples a, b, and c), it was observed that increasing the concentration of aminosilane from 2.7 to 10.8 mmol/L reduced the values of λmax and λaver. This behavior corroborated the TEM results (i.e., size reduction presented in Table 1), and it was in agreement with Mie’s theory for spherical nanoparticles. However, both λmax and λaver started to increase when the concentrations of aminosilane were greater than 10.8 mmol/L. These results can be attributed to the optical effects of the nanotriangles in the surface plasmon bands of the colloidal systems. The values of λmax between 405 and 413 nm indicated that most of the silver nanoparticles produced were spherical.

assigned to the in-plane dipolar, in-plane quadrupolar, out-ofplane dipolar, and out-of-plane quadrupolar resonances. The inplane dipolar resonance occurs at long wavelengths and is more sensible to the size of the triangles than the other resonance modes. The out-of-plane quadrupolar peak appears at the lowest wavelength and is located around 340 nm. The absorbance spectra displayed in Figure 2 show a single peak and, therefore, it is suggested that silver nanoparticles of spherical shape were produced in the synthesis, independently of the concentration of aminosilane. These results are in agreement with the TEM images shown in section 3.1, except that some silver nanoparticles with triangular and rod shapes were observed when higher concentrations of aminosilane were used (samples d and e). However, Deivaraj et al.38 and Pinto et al.43 showed evidence that can explain the absorbance spectra obtained for samples d and e. According to the results reported by these authors, the colloidal systems containing a large number of nanospheres and low quantities of nanotriangles and nanorods presented single absorbance spectra with asymmetry at longer wavelengths (red shift). Therefore, despite the formation of nanotriangles (samples d and e) and nanorods (sample e), the single peak verified in the absorbance spectra of samples d and e can be explained by the low content of anisotropic nanoparticles produced in such colloidal systems. As observed in Figure 2, using higher concentrations of aminosilane broadened and red-shifted the absorbance spectra of samples d and e, mainly the spectrum of sample e. This behavior could be better observed through the plot of the fwhm as a function of the stabilizing agent concentration (Figure 3B). In this case, the fwhm increased for aminosilane concentrations 3431

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Similar results were obtained by Pastoriza-Santos and Liz-Marzán,41 Solomon et al.,17 and Zielinska et al.4 Evaluating the error bars presented in Figure 3, it was possible to conclude that the results presented good reproducibility. The most difficult condition to reproduce the results was that one using lower concentration of stabilizing agent, probably due to the higher degree of instability of the colloidal system. Comparing the error bars between λmax and λaver, it was verified that the average wavelength absorbed showed better reproducibility, even when anisotropic silver nanoparticles were formed. Besides, the error amplitude for λaver followed the same trend observed by the error bars of the fwhm, i.e., a reduction when the aminosilane concentration was increased. These results showed that this optical parameter could provide more accurate information regarding the morphology of the nanoparticles produced, rather than the maximum wavelength. In an industrial process, once the “fingerprint” of the silver colloids to be produced is determined (i.e., the shape of the absorbance spectra), the absorbance peak, the fwhm, and the average wavelength absorbed (λaver) can be used as quality parameters of the process. In this case, statistical process control techniques can be applied to monitor and evaluate the capability of the process, expressed as a process capability index (Cpk) or as a process performance index (Ppk). 3.3. ζ Potential. The stability of the colloidal silver produced with different concentrations of aminosilane was evaluated by measuring the ζ potential. The samples were analyzed more than 30 days after the reactions to ensure that the colloidal systems reached their stability condition. According to the results shown in Figure 4, the pH of the colloidal dispersions presented a subtle

concentrations of aminosilane ranging from 5.4 to 21.6 mmol/L (samples b−e) presented good stability. Despite the reduction of the absorbance peaks observed in samples b (5.4 mmol/L) and c (10.8 mmol/L) between the first and 30th days after the reactions (Figure 2A and 2B, respectively), the good results of the ζ potential confirmed that these colloidal systems reached a stability condition. However, the reduction of the absorbances was an indication that the amount of stabilizing agent used to prepare samples b and c were insufficient to produce stable colloids just after the synthesis. The appropriate ζ potential values (Figure 4) and the maintenance of the absorbance of samples d and e between the first and 30th days (Figure 2) confirmed that these colloidal systems presented good stability starting from the moment of their production. 3.4. X-ray Diffraction (XRD). X-ray diffraction analysis was performed to ascertain the formation of silver nanoparticles in the reactions and evaluate their crystalline structure. The XRD crystallinity pattern observed in Figure 5 was obtained from

Figure 5. XRD pattern of silver nanoparticles prepared with 5.4 mmol/L aminosilane (sample b).

sample b (5.4 mmol/L aminosilane). As can be seen, the spectrum presented a couple of distinct Bragg reflections corresponding to the sets of lattice planes that could be indexed on the basis of the face-centered-cubic (fcc) structure of silver. According to the result, the peaks located at 38.22, 44.35, 64.48, and 77.50° agree well with the (111), (200), (220), and (311) diffractions shown by Seoudi et al.,20 Chou et al.,26 and Wu et al.9 for face-centered-cubic silver nanoparticles. The SAED crystallographic patterns shown inside the TEM images (Figure 1A,C,E,G,I) were quite similar, and the X-ray diffraction patterns of samples a, c, d, and e should also be very similar to the spectrum presented in Figure 5. 3.5. Antibacterial Activity. Microbiological tests were carried out according to the standard method ASTM E2149-01 to evaluate the influence of the aminosilane concentration on the antibacterial activity of the silver nanoparticles against S. aureus (Gram-positive). The discrete points shown in Figure 6 represent the average results obtained from the duplicates. The b-spline interpolation function, available in the software OriginPro 8.5, was used to generate the smooth lines which highlight the kinetic profiles in Figure 6. The sample “reference” represents the solution without silver nanoparticles and was used as a control in the antibacterial assays. The results shown in Figure 6 demonstrated that the silver nanoparticles produced with different concentrations of aminosilane resulted in different kinetics of bacteria reduction. According to the data, the silver nanoparticles produced with

Figure 4. ζ potential and pH of colloidal dispersions as a function of the stabilizing agent concentration.

decrease from 5.5 to 4.3 by increasing the concentration of aminosilane from 2.7 to 21.6 mmol/L. The ζ potential value close to the isoeletric point (i.e., 0) presented in Figure 4 demonstrates that the colloidal silver produced with the lowest concentration of the stabilizing agent (i.e., 2.7 mmol/L, sample a) was unstable even after the supposed “unstable period”. This result is related to the insufficient amount of aminosilane used in the reaction to properly cover and protect the silver nanoparticles against aggregation. As mentioned previously in section 3.1, the formation of larger spherical nanoparticles and a reasonable number of uneven aggregates (Figure 1A) was indicative of poor stability of the colloidal system. On the other hand, the ζ potential values greater than +40 mV (Figure 4) indicated that the colloidal systems produced with 3432

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nanoparticles. In summary, increasing the concentration of this stabilizing agent improved the colloidal stability, narrowed the PSD, and reduced the average size of the silver nanoparticles produced. The majority of the nanoparticles produced in the synthesis presented a spherical shape. However, the TEM images revealed the formation of silver nanotriangles when high enough concentrations of aminosilane were used in the reactions. The microbiological assays showed that the kinetics of bacteria annihilation was influenced by the amount of aminosilane used in the reactions. The silver nanoparticles produced with large amounts of the aminosilane exhibited slower antibacterial action, while the nanoparticles produced with low concentrations of this stabilizing agent presented faster action. These findings demonstrated that, depending on the end use and lifetime of the functionalized product, silver nanoparticles with an optimized releasing rate of silver ions can be synthesized by adjusting the concentration of the aminosilane. The efficient antibacterial activity of the aminosilane-stabilized silver nanoparticles demonstrated in this study highlights their potential for use as an antibacterial additive in a variety of materials (e.g., fabrics, ceramics, and plastic devices).

Figure 6. Antibacterial activity of the silver nanoparticles produced with different concentrations of aminosilane (sample a, 2.7 mmol/L; sample b, 5.4 mmol/L; sample c, 10.8 mmol/L; sample d, 16.2 mmol/L; sample e, 21.6 mmol/L; sample f, reference, i.e., without silver nanoparticles).

higher concentrations of aminosilane presented slower antibacterial actions. On the other hand, the nanoparticles produced with lower concentrations of aminosilane exhibited faster antibacterial actions. Despite these differences, all the samples were able to eliminate the bacteria existing in the inoculum. Sample e (21.6 mmol/L aminosilane) killed 100% of the bacteria after 240 min, while sample b (5.4 mmol/L aminosilane) achieved the same result in 120 min. The results of UV−vis spectrophotometry (Figure 2B) and TEM (Table 1), presented previously, demonstrated that raising the amount of aminosilane increased the concentration of silver nanoparticles, reduced their size, and promoted the formation of nanotriangles. ́ Martinez-Castañ oń et al.44 showed that smaller silver nanoparticles resulted in lower minimum inhibitory concentrations against Escherichia coli (Gram-negative) and S. aureus (Gram-positive). Similar results were reported by Panácek et al.45 for a variety of Grampositive and Gram-negative bacteria. Pal et al.46 demonstrated that silver nanoparticles with triangular shapes presented antibacterial activity greater than nanoparticles with spherical and rod shapes. On the basis of the results reported by such authors,44−46 and taking into account the results of size, shape, and concentration of the nanoparticles discussed in the previous sections, faster antibacterial actions would be expected for the silver nanoparticles produced with higher concentrations of aminosilane. However, the opposite behavior was observed in the results shown in Figure 6. These results suggested that silver nanoparticles more protected against corrosion (i.e., more passivated) were produced by increasing the concentration of aminosilane in the reaction. This means that a larger number of aminosilane molecules were able to cover a large surface area of the nanoparticles and, consequently, reduced the release of silver ions through the oxidation reactions. On the other hand, the silver nanoparticles produced with lower concentrations of aminosilane presented a larger uncovered surface area, which contributed to a more intense oxidation process and higher releasing rate of silver ions. The results presented in Figure 6 demonstrated that the release of silver ions is time dependent, and suggested that the amount of aminosilane molecules attached on the silver surface may act as a barrier to the leaking of silver ions. Therefore, silver nanoparticles with variable releasing rates of silver ions could be produced by adjusting the concentration of the aminosilane.



AUTHOR INFORMATION

Corresponding Author

*Tel.: +55 48 99055683. E-mail: [email protected] Present Addresses †

F.M.: Department of Clinical Analysis, UFSC, Florianópolis, SC, Brazil. ‡ C.V.F.: Department of Chemistry, UFSC, Florianópolis, SC, Brazil. § G.B.D.: Engineering Center for Mobility, UFSC, Joinville, SC, Brazil. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS The authors gratefully acknowledge CNPq (National Council of Scientific and Technological Development, Brazil) for financial support, LCME/UFSC (Central Laboratory of Electronic Microscopy of the Federal University of Santa Catarina, Brazil) for performing the TEM analysis, and TNS Nanotecnologia Ltda, Brazil, for technical support.



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4. CONCLUSIONS This study investigated the effect of different concentrations of an aminosilane on the synthesis of functionalized silver 3433

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