Environ. Sci. Technol. 2010, 44, 1260–1266
Impact of Environmental Conditions (pH, Ionic Strength, and Electrolyte Type) on the Surface Charge and Aggregation of Silver Nanoparticles Suspensions AMRO M. EL BADAWY,† TODD P. LUXTON,‡ RENDAHANDI G. SILVA,§ KIRK G. SCHECKEL,‡ MAKRAM T. SUIDAN,† AND T H A B E T M . T O L A Y M A T * ,‡ Department of Civil & Environmental Engineering, University of Cincinnati, Cincinnati, OH; U.S. Environmental Protection Agency, Office of Research and Development, Cincinnati, OH; and Shaw Environmental Inc., Cincinnati, OH
Received July 31, 2009. Revised manuscript received January 5, 2010. Accepted January 5, 2010.
The impact of capping agents and environmental conditions (pH, ionic strength, and background electrolytes) on surface charge and aggregation potential of silver nanoparticles (AgNPs) suspensions were investigated. Capping agents are chemicals used in the synthesis of nanoparticles to prevent aggregation. The AgNPs examined in the study were as follows: (a) uncoated AgNPs (H2-AgNPs), (b) electrostatically stabilized (citrate and NaBH4-AgNPs), (c) sterically stabilized (polyvinylpyrrolidone (PVP)-AgNPs), and (d) electrosterically stabilized (branched polyethyleneimine (BPEI)-AgNPs)). The uncoated (H2-AgNPs), the citrate, and NaBH4-coated AgNPs aggregated at higher ionic strengths (100 mM NaNO3) and/or acidic pH (3.0). For these three nanomaterials, chloride (Cl-, 10 mM), as a background electrolyte, resulted in a minimal change in the hydrodynamic diameter even at low pH (3.0). This was limited by the presence of residual silver ions, which resulted in the formation of stable negatively charged AgCl colloids. Furthermore, the presence of Ca2+ (10 mM) resulted in aggregation of the three previously identified AgNPs regardless of the pH. As for PVP coated AgNPs, the ionic strength, pH and electrolyte type had no impact on the aggregation of the sterically stabilized AgNPs. The surface charge and aggregation of the BPEI coated AgNPs varied according to the solution pH.
1. Introduction Numerous applications have been reported for silver nanoparticles (AgNPs) in areas such as electronics, biosensing, and surface-enhanced Raman spectroscopy (SERS) (1-3). * Corresponding author address: USEPA Office of Research and Development, National Risk Management Laboratory, 26 West Martin Luther King Drive, Cincinnati, OH 45224; phone: 513-487-2860; fax: 513-569-7879; e-mail:
[email protected]. † Department of Civil & Environmental Engineering, University of Cincinnati. ‡ U.S. Environmental Protection Agency, Office of Research and Development. § Shaw Environmental Inc. 1260
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Because of their antibacterial properties (4), AgNPs are also used in medical equipment and textiles as well as being incorporated into various consumer products such as cosmetics, fabrics, and plastics (5-7). Because of their wide use, it is more than likely that AgNPs will be released to the environment either during manufacturing, consumer use, and/or end-of-life management. The principal concern with the release of AgNPs in the environment is their potential toxic impact on ecosystems and microorganisms (4, 6). Once released into the environment, the mobility, bioavailability, and toxicity of AgNPs in any ecosystem are, largely, affected by colloidal stability. Colloidal stability is a function of many factors including the type of capping agent, the surrounding environmental conditions, such as pH, ionic strength, and the background electrolyte composition (8-10). An extensive number of capping agents have been investigated to enhance NPs’ suspension stability. Capping agents are chemicals (such as polymers and surfactants) used in the synthesis of AgNPs to prevent their aggregation through electrostatic repulsion, steric repulsion or both. In the case of silver, the most prevalent capping agents are citrate, sodium borohydride (NaBH4), and polyvinylpyrrolidone (PVP) (11). However, the mechanism and functional groups involved in colloid stabilization differ with capping agents, which may lead to varying particle size and stability. For example, some capping agents are electrostatic stabilizers while others are steric stabilizers (7). As a consequence of colloidal interactions, mobility and toxicity may differ. In toxicological studies, the particle size and surface charge may have a significant impact on the response of microorganisms. Jiang et al. (12) reported that the use of aggregated nanoparticles affected the toxicity results of these nanoparticles. The high surface area to volume ratio of the NPs results in high reactivity which leads to particle aggregation and settling unless the particles are protected by a capping agent that provides colloidal stability through electrostatic or steric repulsion (7). In the classical Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory colloidal particles are surrounded by a diffuse electrostatic double layer (EDL) and the balance between the van der Waals attraction forces and the electrostatic repulsion forces determines the colloidal stability (8, 13). The magnitude of the electrical charge within and the thickness of the EDL are directly related to solution properties such as pH, ionic strength, and electrolyte ion valence (12). Although numerous studies have investigated the effect of the colloidal surface properties under various environmental conditions on the stability or the aggregation potential of various NPs, there is little information available with regard to AgNPs. In a study by Jiang et al. (12) the aggregate size of (50 mg L-1) TiO2 nanoparticles increased 50-fold upon increasing the solution ionic strength from 1 to 100 mM NaCl (size was analyzed after 5 min). The same study also showed that varying the solution pH resulted in a significant change in the particle surface charge and consequently, the hydrodynamic diameter (HDD) (12). In another study, guar gum adsorbed on the surface enhanced the mobility of nano zerovalent iron (NZVI) in sandy porous media regardless of the solution chemistry (e.g., pH and ionic strength) (14). Saleh et al. (15) reported that uncoated NZVI NPs were immobile in water-saturated sand columns while, triblock copolymercoated NZVI particles were highly mobile. Coating quantum dot nanocrystals with polyethylene glycol suppressed aggregation and stabilized the suspension regardless of the ionic strength (12). Research has also shown that increases 10.1021/es902240k
2010 American Chemical Society
Published on Web 01/25/2010
TABLE 1. Characteristics of the As-Prepared AgNPs particle type
pH
H2-AgNPs 8.9 Citrate-AgNPs 6.9 NaBH4-AgNPs 8.7 4.5 PVP-AgNPsd BPEI-AgNPs 10.5
HDDa (nm) TEM size (nm) size % volume PWb 58 ( 11 19 ( 5 14 ( 1 72 ( 24 10 ( 4
25 10 11 200 11
100 98.4 94.6 83.5 96.3
23.7 8.10 6.5 41.0 2.4
ζ total Ag SPR peak stabilization potentialc (mV) concentration (mg L-1) wavelength (nm) UV abs. mechanism -22 -40 -38 -3.0 8.9
65.1 72.5 70 70 84.5
a Dominant hydrodynamic diameter. b Peak width at half-maximum. nanoparticles. d Purchased from a commercial source.
in cation valence significantly impacts suspension stability. In a study by Chen et al. (9), the critical deposition concentration of fullerene (C60) NPs was reduced from 120 to 4.8 mM due to an increase in the cation valence (Na+ and Ca2+), respectively. Another significant factor impacting suspension stability is the specific surface adsorption of ionic species. Previous work has demonstrated that the point of zero charge (pHPZC) for crystalline anatase TiO2 NPs decreased (lower pH) as a result of Na+ ion adsorption (16). Despite the great concern regarding the environmental impacts of the AgNPs (5), there is lack of information available with regard to the effect of solution chemical properties and the capping agents on the suspension stability of AgNPs. Therefore, the main objective of this study was to investigate stability of AgNPs suspensions by evaluating changes in the HDD and the zeta (ζ) potential as a function of pH, ionic strength, and electrolyte species. AgNPs with varying surface properties were selected to: (a) represent the primary stabilization mechanisms (electrostatic, steric and electrosteric) for AgNPs suspensions and (b) consider the most prevalent capping agents used to enhance AgNPs suspension stability. Electrosteric stabilization occurs when a polyelectrolyte (a polymer whose repeating units bear an electrolyte group that dissociate in aqueous solutions, making the polymers charged) adsorbs on a colloidal particle surface. The stability effect is caused by steric as well as electrostatic repulsion. Steric stabilization is achieved through the adsorption of an uncharged polymer on the nanoparticle surface hindering the particles aggregation through steric repulsion.
2. Experimental Section Silver nanoparticles with four different capping agents as well as one uncoated AgNPs were investigated. Four were synthesized using modified procedures previously reported in the literature. Modifications to the referenced procedures are outlined in Supporting Information. Briefly, these particles were as follows: (a) Hydrogen reduced AgNPs (H2-AgNPs) synthesized according to Evanoff et al. (17) with slight modifications; (b) Citrate reduced AgNPs (citrate-AgNPs) synthesized following a modified method to the one described by Turkevich et al. (18); (c) Borohydride reduced AgNPs (NaBH4-AgNPs) synthesized following Lee and Meisel procedure (3); and (d) Branched polyethyleneimine stabilized AgNPs (BPEI-AgNPs) synthesized according to the procedure reported by Tan et al. (11). The fifth AgNPs was purchased from a commercial source (Nanostructures and Amorphous Materials, Houston, TX, USA) in powder form and stabilized using polyvinylpyrrolidone (PVP-AgNPs) (Supporting Information). The formation of nanosized Ag particles was verified by the presence of a Surface Plasmon Resonance (SPR) peak obtained by UV-Vis spectroscopy, transmission electron microscopy (TEM) (Supporting Information Figure S-1) and dynamic light scattering measurements. 2.1. Experimental Conditions. The hydrodynamic diameter (HDD) of the AgNPs was examined as a function of pH (3, 6, and 9, adjusted using dilute solutions of NaOH
c
470 420 400 410 405
0.4 2.4 0.6 0.5 1.9
NA electrostatic electrostatic steric electrosteric
Measured ζ potential of the as-synthesized silver
and/or HNO3), ionic strength (10 and 100 mM NaNO3), and background electrolyte (NaCl, NaNO3, and Ca(NO3)2). Overall, surface charging properties of the various AgNPs were examined under similar conditions, except for the pH values investigated. Milli-Q water (18MΩ) and reagent grade chemicals were used throughout the experiments. The samples were analyzed following the sequence: (1) preparation of the AgNPs suspensions; (2) adjustment of the ionic strength to the appropriate concentration by adding the required amount of background salt to the as-synthesized or the predispersed AgNPs suspension; (3) pH adjustments; and (4) immediately the samples were analyzed for particle size (HDD), zeta (ζ) potential and the presence of a UV active SPR peak. The AgNPs suspensions used throughout the experiments were used as synthesized and no further purification steps were performed to remove any impurities or residual ions in the as prepared suspensions. A sample duplicate was run with each batch (the batch size was 3 samples and 7 samples for HDD and ζ potential measurements, respectively) to check for variability since there was no measurement of error included in results. The results for the duplicates are presented in Tables S1 and S2 (Supporting Information). As could be seen from these tables there was minimal variability. 2.2. Instrumentation and Analysis. The UV absorbance of the aqueous AgNPs suspensions was measured using a dual beam UV-Vis spectrophotometer (UV-1650PC, Shimadzu Scientific Instruments). The HDD was determined by dynamic light scattering (DLS) using a zetasizer nanoseries (Malvern Instruments) with a 633 nm laser source and a detection angle of 173° (HDD detection range 1 nm to 10 um). The same instrument was also used to measure electrophoretic mobility which was subsequently transformed to ζ potential using the Smoluchowski’s approximation. Transmission electron microscopy (TEM) was used to verify the characteristics (size and shape) of the tested AgNPs suspensions. TEM samples were prepared by depositing a drop of the sample suspension on a carbon coated copper grid. Samples were air-dried at room temperature overnight in a dust-free box. Images were captured using a JEOL-1200 EX TEM (JEOL Inc.) operated at 120 kV.
3. Results and Discussion 3.1. Characteristics of the AgNPs Suspensions. The properties of the prepared AgNPs suspensions are summarized in Table 1. Depending on the capping agents used and the final products formed, the pH of the suspension varied from moderately acidic (4.5 for PVP-AgNPs) to basic (10.5 for BPEIAgNPs). To verify the formation of the prepared AgNPs suspension, the SPR peaks are presented in the UV-Vis absorbance spectra (Supporting Information Figure S-1) and TEM images are presented in Supporting Information Figure S2. Variations in the intensity and position of the SPR peaks may reflect varying size (7) which is also reflected by the TEM and DLS measurements (Table 1). The peak width at half-maximum (Table 1) measured by DLS indicates that the VOL. 44, NO. 4, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. The effect of pH and ionic strength on the measured zeta (ζ) potential and hydrodynamic diameter (a) H2-AgNPs, (b) Citrate-AgNPs, (c) NaBH4-AgNPs, (d) PVP-AgNPs, and (e) BPEI-AgNPs. HDD of the synthesized nanoparticles have a wide distribution. This may result in differences between the TEM and DLS measurements. Furthermore, air-dried colloids on TEM grids are probably not representative of the same colloids in aqueous suspensions (19). The PVP-AgNPs had an average HDD in suspension of 200 nm which is larger than the 30 and 72 nm obtained by the TEM for the AgNPs without PVP and the PVP stabilized AgNPs, respectively (Supporting Information Figure S2 d, e). The significantly larger HDD measured may be a result of the following: (a) the adsorption of the PVP on a cluster of particles rather than on individual particles (20); (b) sonication may have not fully dispersed the aggregated particles thus resulting in HDD values in excess of the TEM measurements; and (c) the evaporation of aqueous samples on the TEM grid might have resulted in shrinkage to the PVP molecule and this is similar to what was observed for PVP-C60 molecules (19). The H2-AgNPs have uncoated surfaces because of the mechanism employed during synthesis. The H2-AgNPs were stable as-prepared because of the strong coordination of OHions (formed as a product of the synthesis reaction) (17) resulting in a net negative charge of these NPs (∼-22 mV) as in the case of nanobubles in R-cyclodextrin aqueous solution (21). The NaBH4-AgNPs were electrostatically stabilized by the adsorption of the negatively charged BH4ions on the NPs (22). Citrate-AgNPs were also stabilized electrostatically through the ionization of the polar citrate carboxyl groups on the surface of the AgNPs. The PVP-AgNPs, however, were stabilized through the steric repulsion caused by the adsorption of PVP on the surface. The BPEI-AgNPs were electrosterically stabilized due to the adsorption of the 1262
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BPEI on the particle surface. BPEI is a weak cationic polyelectrolyte that has primary, secondary, and tertiary amine groups in its monomeric unit in the ratio of 1:2:1 (23). Thus, BPEI is ionized by the protonation of the amine groups resulting in the positive charge on the AgNPs surface (11). 3.2. Effect of Ionic Strength on the Measured ζ Potential and HDD. Increasing the ionic strength resulted in an increase in the measured HDD of the H2-AgNPs (uncoated), and the electrostatically stabilized (citrate and borohydride) AgNPs (Figures 1a-c, respectively). An increase in the HDD of the previously identified AgNPs is expected. The electrical double layer (EDL) theory explains a reduction in the thickness of the diffuse double layer with increasing ionic strength. This allows for a greater degree of particle-particle interaction resulting in an increase in the level of aggregation and potential for particle settling. The lack of an effect of ionic strength on the sterically stabilized PVP-AgNPs was also observed (Figure 1d). Previous research has demonstrated that sterically stabilized quantum dots were stable at increased ionic strengths (0.15 M NaCl) (12). Therefore, the changes in the HDD and the measured ζ potential due to variations in ionic strength are expected to be negligible. For the BPEI-AgNPs, ionic strength, and pH had a strong influence on each other, making the isolation of the effect of a single variable difficult. Figure 1e, can be divided into two broad regions based on pH. At pH < 7, addition of background ions had an impact on the HDD of BPEI-AgNPs. However, when pH g 7, the increase in the ionic strength from 10 mM to 100 mM (using NaNO3 as a background) had minimal effect on both the HDD and the measured ζ potential (∼10 mV lower at the 100 mM NaNO3). On the basis of
observations from the sterically stabilized Ag NPs (presented earlier), it appears, in this region, that the NPs were stabilized predominantly through steric repulsion. At pH
