Chemosphere 83 (2011) 545–551

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Chemosphere

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Potential environmental influence of amino acids on the behaviorof ZnO nanoparticles

Rodrigo Molina a,d,1, Yasser Al-Salama b, Kerstin Jurkschat c,d, Peter J. Dobson a,d,1, Ian P. Thompson a,d,⇑a Department of Engineering Science, University of Oxford, Oxford OX1 3PJ, UKb Faculty of Agriculture, Al-Furat University, Deir Ezzor, Syriac Department of Materials, University of Oxford, Oxford OX1 3PH, UKd Begbroke Directorate, University of Oxford Science Park, Oxford OX5 1PF, UK

a r t i c l e i n f o a b s t r a c t

Article history:Received 28 April 2010Received in revised form 1 December 2010Accepted 5 December 2010Available online 8 January 2011

Keywords:NanoparticlesAgglomerationAmino acidsZeta potentialZnOMetal oxides

0045-6535/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.chemosphere.2010.12.020

⇑ Corresponding author at: Department of EngineOxford, Oxford OX1 3PJ, UK. Tel.: +44 01865 283789/

E-mail address: ian.thompson@eng.ox.ac.uk (I.P. T1 Tel.: +44 01865 283783; fax: +44 01865 374992.

The fate of nanomaterials when they enter the environment is an issue of increasing concern and thus it isimportant to know how they interact with natural organic molecules since this may have a significantimpact on the particles’ behavior. Because of our poor knowledge in this regard, the interaction of ZnOnanoparticles with amino acids of contrasting surface charge, including Histidine (HIS), Glycine (GLY),Aspartic acid (ASP) and Glutamic acid (GLU) which occur commonly in natural habitats, such as the plantroot zone, was investigated over a range of pH conditions and concentrations. The addition of the indi-vidual amino acid led to significant changes in nanoparticle colloidal zeta potential stability, particle sizedistribution and the extent of agglomeration. Variations in pH resulted in considerable changes in nano-particle surface charge and hydrodynamic size. In general, the particle size distribution decreased as theamino acid concentration increased, with more acidic conditions exacerbating this effect. In addition,increased concentrations of amino acids resulted in more stable nanoparticles in aqueous suspensions.Histidine had the greatest effect on colloidal stability, followed by Glycine, Aspartic acid and finally Glu-tamic acid. This study illustrates how nanoparticle behavior may change in the presence of naturallyoccurring amino acids, an important consideration when assessing the fate of nanoparticles in the envi-ronment. Additionally, utilization of amino acids in industrial processes could reduce particle agglomer-ation and it could lead to a way of employing more sustainable reagents.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

ZnO nanoparticles are widely used in plastics, ceramics, lubri-cants, paints, coatings and adhesives and in the personal careindustry and annual production rates are increasing. At the endof their life cycle, most of these products are currently disposedto landfills. As a result, nanoparticles, if in a ‘‘free condition’’ mayenter the environment and potentially bio-accumulate throughthe food chain and end up in higher-level organisms (Royal Society,2004). Therefore, it is important to understand the environmentalfate of nanoparticles and their potential impact on ecosystems. Inparticular, it is important to determine the interaction of nanopar-ticles with their immediate surroundings since this will influencetheir fate and risk. However, there are few published studies inves-tigating the interaction between nanoparticles and plants, and of

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ering Science, University of3; fax: +44 01865 374992.hompson).

these, contradictory observations have been reported. (Lin andXing, 2008; Navarro et al., 2008; Molina et al., 2010).

Another important issue that has so far been neglected is nano-particle agglomeration, in particular in the context of exposure inthe environment and its effects when assessing nanotoxicity. Nano-particle agglomeration is likely to be affected by particle shape, size,surface area and charge, as well as the adsorption properties of thenanomaterial in question. In addition, factors such as pH, ionicstrength, water hardness and the presence of organic matter canpotentially alter nanoparticle aggregation, which in turn may influ-ence their toxicity and reactivity (Handy et al., 2008).

The nanoparticle agglomeration effects of amino acids wereinvestigated in this study because of their relative abundance inthe environment, in particularly the root zone (i.e. the rhizosphere)of plants. The behavior of nanomaterials in the plant environmentis important since in the case of nanomaterials released into terres-trial habitats since there is strong likelihood they will come in con-tact with plants. In general, amino acids are the second largestcomponent of exudates in the root zone (Juma and McGill, 1986),many plants losing in the region of 50% (w v�1) via the root system.Understanding the influence of amino acids on nanoparticle behav-

546 R. Molina et al. / Chemosphere 83 (2011) 545–551

ior is thus important for predicting their environmental fate andunderstanding how chemical components and processes in theroot habitat influence nanomaterial behavior. Typical amino acidsolutions at naturally relevant pH’s (pH 5.5–8.5) have distinctivecharges. For instance, Histidine is positively charged, Glycine isneutral, and Aspartic acid and Glutamate both have a negativeoverall charge. These four amino acids are representative of thosethat occur naturally in the environment and to which ZnO nano-particles may be exposed to if released, in particularly terrestrialhabitats. In general, these amino acids are found in the environ-ment in organic neutral soils (Bremner, 1950). It is particularlyimportant to understand nanoparticle behavior in colloidal sys-tems under conditions similar to the ones found in nature. Under-standing their behavior in terms of particle size distribution,agglomeration tendencies and surface charge effects is essentialsince they are parameters that can determine the ecological effectsand environmental fate of the nanoparticles.

The colloidal stability of nanoparticles in solution and theirtendency to agglomerate can be considered in the framework ofelectrostatic, steric, and van der Waals forces which can be sum-marized employing the theory of Derjaguin–Landau–Verwey–Overbeek (DLVO) (Derjaguin and Landau, 1941; Verwey andOverbeek (1948)). The agglomeration and colloidal stability of par-ticles, and in this case of nanoparticle dispersions, are determinedby the sum of the attractive and repulsive forces between individ-ual nanoparticles. The attraction between nanoparticles is attrib-uted to the van der Waals forces. The interaction of the electricaldouble layer surrounding each particle is referred to as the electro-static repulsive force. When particles are coated by organics, a ste-ric repulsive force between particles is generated, which is due toan entropic effect resulting from reducing the configurationalfreedom rather than a physical force (Ott and Finke, 2007).

The zeta potential can be described as the potential differencebetween the bulk and the slipping plane, which is dependent onthe electrical double layer thickness and situated some distancefrom the nanoparticle surface. In other words, it is the potentialdifference between the dispersion medium and the stationarylayer of fluid attached to the dispersed particle. Increasing ionicstrength results in the compression of the electrical double layer.Therefore, the zeta potential decreases with increasing ionicstrength. Among the important factors influencing nanoparticlecolloidal stability are the electrical double layer of the zeta poten-tial and its thickness (Morrison and Ross, 2002). An increase in thezeta potential or the double layer thickness causes an increase inthe electrostatic repulsive interaction effects. The surface chargeis controlled by several mechanisms, including surface ionization,ion adsorption, and lattice ion dissolution (Stumm and Morgan,1996), while the thickness of electrical double layer is a functionof solution ionic strength, with an increase in interaction effectsleading to a decrease in double layer thickness.

The dispersion surface charge (zeta potential) and consequentlythe hydrodynamic size can be altered by changing the solution pH.For metal oxides such as ZnO dispersed in water, surface ionizationcontrols their surface charge in the absence of preferential adsorp-tion of soluble ions in solution (Morrison and Ross, 2002). At lowerpH’s, ZnO nanoparticles have a more positive surface charge,whereas at higher pH’s, the surface charge is slightly morenegative.

The main objective of this study was to investigate the interac-tion between ZnO nanoparticles and four distinctive amino acids(Histidine, Glycine, Aspartic acid and Glutamic acid) with contrast-ing surface charge over a range of different pH conditions and ami-no acid concentrations. Specifically, changes in nanoparticlecolloidal zeta potential stability, particle size distribution andagglomeration effects were monitored. In a broader context, theobjective is to develop better insight regarding the factors that

may influence the fate and potential impact of nanoparticles whenthey enter the terrestrial environment.

2. Materials and methods

2.1. Reagents

The ZnO nanoparticles (<50 nm) and amino acids (Glycine, L-Glutamic, L-Aspartic and L-Histidine referred to as GLY, GLU, ASPand HIS respectively) were purchased from Sigma–Aldrich (Gilling-ham, UK). The conditions of the reagents intended to mimic aminoacid concentrations found in the environment, in particularly thosein the root zone.

2.2. Analytical techniques

2.2.1. f-Potential of nanoparticlesDynamic light scattering measurements were made to deter-

mine f-potential of suspended ZnO nanoparticles using a ZetasizerNano ZS (Malvern Instruments Ltd., Malvern, UK). f-Potential mea-surements were performed at 25 �C with an equilibration time of1 min in disposable capillary cells (Malvern Instruments Ltd., Mal-vern, UK) in an automatic mode. The Smoluchowski’s model wasemployed and the automatic measurements (10–100 runs) wereperformed with triplicates at a temperature of 25 �C and an equil-ibration time of 1 s

2.2.2. BET specific surface areaThe surface area of the nanoparticles was measured using the

nitrogen adsorption method (5 point isotherm) with a Gemini VISurface Area and Pore Size Analyzer (Micromeritics, Norcross,USA).

2.2.3. Particle size distributionSize distribution of nanoparticles was measured using a CPS

Disc Centrifuge (Model DC24000, CPS Instruments, Inc.), operatedat a speed of 24 000 rpm (maximum g-force >25 000) as suggestedby the manufacturer’s instructions for better accuracy. A sucrosegradient was injected into the spinning disc using 24% (w v�1)and 8% (w v�1) sucrose solutions, and subsequently sealed byinjecting 0.5 mL of dodecane. A polyvinyl chloride (PVC) standardof 0.377 lm in deionised water solution was used for calibration.Samples were prepared for analysis by sonication of 1 g L�1 of par-ticles in Milli-Q™ water (resistivity >18.2 MT cm) for 1 h to facili-tate particle dispersion. Subsequently, 0.1 mL of the suspensionwas injected into the CPS disc centrifuge. The solutions examinedwere adjusted to their respective pH’s 5.5, 7.0 and 8.5 using a buf-fer concentration of HCl and NaOH.

2.2.4. Transmission electron microscopy (TEM)The imaging of nanoparticles was carried out on a JEOL 2010

analytical TEM, equipped with a LaB6 electron gun, operated at200 kV. This instrument has a resolution of 0.19 nm, an electronprobe size down to 0.5 nm and a maximum specimen tilt of ±10�along both axes. The instrument was equipped with an OxfordInstruments LZ5 windowless energy dispersive X-ray spectrometer(EDS) controlled by INCA. It has facilities for point analysis as wellas mapping and line scanning through the SemiStem controller.

2.2.5. pHSuspension pH (INTELLICAL pH standard electrode, liquid elec-

trolyte) was measured using HQ30d Portable Meter (Hach Lange,Manchester, UK).

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2.3. Experimental design

ZnO nanoparticle (1 g L�1 in deionised water) and amino acidwas added to a 40 mL amber glass vial (Fisher, Loughborough,UK) to give a 20 mL suspension. The solutions were adjusted topH values 5.5, 7.0 and 8.5 respectively using HCl and NaOH asappropriated. ZnO nanoparticle charge and particle size distribu-tion were characterized for exposure to four concentrations of ami-no acids [0 (control), 1, 10, 100 mM]. All experiments were carriedout in triplicate for statistical reliability.

3. Results

The results of this study demonstrate that the average rate ofchange of the nanomaterial zeta potential values exposed to allthe amino acids and concentrations was 5.4% as the pH increasedfrom 5.5 to 7.0 and 34.7% as the pH increased from 7.0 to 8.5.(Fig. 1). The statistical calculations were applied directly to theaverage of the triplicate values measured which were comparedto the average zeta potential of the control solutions, respectively.At pH’s where the zeta potential was furthest away from the iso-electric point (the point where a particle has zero net surfacecharge) the particles were more stable in suspension and the de-

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Fig. 1. Effects of pH and different concentration of amino acids on the f-potential of ZnO npresence of the amino acid. (a) Histidine, (b) Glycine, (c) Glutamate and (d) Aspartic aci

tected degree of agglomeration of the nanoparticles was low, atthe same time maintaining a relative high colloidal stability ofthe system. In contrast, in the case of nanoparticles in a less stablesolution with a relative low zeta potential, they would have agreater tendency to agglomerate and form macroagglomerates.This is important in order to assess the actual state of the nanopar-ticles and the stability. The measured isoelectric point for ZnO wasconsistent with what has been reported for ZnO previously(Kosmulski, 2001).

In addition, we estimated that the percentage zeta potential‘‘decrease’’ (rate of change) was affected by increasing the concen-trations of amino acids in the following order: Histidine > Asparticacid > Glycine > Glutamic acid.

The effect of Histidine on nanoparticle size distribution and onparticle surface coating was analysed by TEM. When ZnO nanopar-ticles were treated with two concentrations of Histidine (100 mMand 1 mM respectively) at pH 8.5, electron microscopy observa-tions indicated that particle size distribution (agglomeration) waslower at the higher Histidine concentration (Fig. SM-2).

Another important aspect of this study was to investigatewhether the amino acid coated the nanoparticle and whether thiscoating was responsible for the reduced agglomeration in colloidalsystems containing high amino acid concentrations. The coating ofthe individual nanoparticles by Histidine was confirmed by TEM

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anoparticles. The above figures represent the f-potential of ZnO nanoparticles in thed. Error bars are 1 � SD (n = 3).

Fig. 2. TEM images of ZnO nanoparticles with Histidine. (a) ZnO nanoparticles with a HIS coating in a solution at pH 8.5 and 100 mM HIS. (b) ZnO nanoparticles with a HIS atpH 8.5 in 1 mM HIS. Some (but not all) of the aggregates of very small particles demonstrated a preferential orientation of the primary particles that could possibly point totheir origin from one single crystal.

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Fig. 3. ZnO nanoparticle size distribution at different amino acid concentrations. The above figures represent the particle size distribution of ZnO nanoparticles in thepresence of the amino acid. (a) Histidine, (b) Glycine and (c) Aspartic acid.

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images (Fig. SM-2), reducing the chances of agglomeration. Addi-tionally, the greater surface area of the ZnO nanoparticles(22.6 m2 g�1) meant that there was a large area of contact for inter-action with the amino acid (Fig. 2).

Under all pH values the nanoparticle size distribution decreasedas the amino acid concentration increased. The greatest reductionin particle size distribution was observed in the presence of thegreatest concentration of amino acids (100 mM) (Fig. 3).

Fig. 4. TEM images of ZnO nanoparticles in a solution at a pH 7 with an Aspartic acid concentration of (a) 1 mM ASP and (b) 100 mM ASP.

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Fig. 5. ZnO nanoparticle size distribution of 1 mM Histidine at different pHs.

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TEM images confirmed that the nanoparticle surface was coatedwith Aspartic acid at a concentration of 1 mM and 100 mM (Fig. 4).Smaller agglomeration effects were seen at higher ASP concentra-tions, probably as a result of the effective surface coating of the ASPamino acids.

The nanoparticle diameters decreased as the amino acid con-centration increased for all pH values tested. The smallest nanopar-ticle size distribution was detected at a concentration of 100 mM ofamino acids (e.g. Histidine, Glycine, etc.). For all investigated aminoacids at a concentration of 1 mM (pH 7) the particle size distribu-tion (e.g. agglomeration) increased in the following order: Histi-dine < Glycine < Glutamate< Aspartic acid.

For the case of amino acids at a concentration of 100 mM (pH 7),the particle size distribution followed the same trend (Fig. SM-3).

At low amino acid concentrations (e.g. 1 mM HIS) the nanopar-ticle size distribution increased (at all pH values) in the followingorder: Histidine < Glycine < Glutamic < Aspartic (Fig. 5).

The particle size distribution (agglomeration) of GLY decreasedas the pH became more acidic (8.5–5.5) (Fig. 6). Also, TEM imagesconfirm that the nanoparticle surfaces were coated with GLY at aconcentration of 10 mM (Fig. 7).

4. Discussion

In this study we investigated the behavior of ZnO nanoparticlesunder conditions likely to mimic those found in the environment,especially around the rhizosphere of plants, which is the areaimmediately surrounding the roots and where amino acid concen-tration can be high. The key parameters determining the nanoma-terial dynamics, and their physical and chemical state werestudied, including pH, surface charge and particle size distribution.The effect of amino acids on ZnO nanoparticles was determinedand correlated to the influence of pH on surface charge (e.g. zetapotential). It was found that pH values closer to the colloidal sys-tem isoelectric point increased the nanoparticle size distribution,

whereas the nanoparticle size distribution decreased as the pH be-came more acidic. The latter influenced agglomeration of the nano-particles via electrostatic repulsion of the charged amino acids andthe specific pH of the solution where the nanoparticles were insuspension.

At the isoelectric point, the electrostatic repulsive force wasdominant over the van der Waals force and agglomeration in-creased. Consequently, the average size distribution of the ZnOnanoparticles was smaller at more acidic pH values in the cases ofHIS and GLY (positive and neutral charge respectively). In contrast,in the case of ASP and GLU which are both negatively charged, theresponse was the opposite. When the pH was close to the isoelectricpoint, the repulsive force weakened due to the low surface charge,

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Fig. 6. ZnO nanoparticle size distribution of 10 mM Glycine at different pHs.

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which resulted in an increase in the ZnO nanoparticle hydrodynamicsize. It was clear from our studies that the particle size distribution(agglomeration effects) for ZnO nanoparticles was correlated totheir respective prevailing pH in the colloidal solution. There wasa relationship between the ZnO nanoparticles behavior and the ami-no acids tested (i.e. Glycine, L-Glutamic, L-Aspartic and L-Histidine)at different concentrations, with nanoparticle size distributiondecreasing as the pH became more acidic (Fig. SM-1) as the nanopar-ticles became less agglomerated.

In addition, the charge of the amino acid affected the nanopar-ticle colloidal stability and particle size distribution (agglomera-tion effects), making the more positively charged amino acidsmore effective at reducing agglomeration than the neutral or neg-ative charged amino acids. The influence of amino acid concentra-tion was also assessed and, in general, the greater theconcentration of amino acid, the lower the zeta potential, an obser-vation which held true over a broad range of pH’s. These resultsprovide unique insights and are important for improving our lim-ited understanding of the effects and fate of nanoparticles for eco-

Fig. 7. TEM images of ZnO nanoparticles in a solution wit

logical and toxicological studies, and to predict the characteristicsof the material when they enter the environment.

Overall, the particle size distribution decreased as the concen-tration of amino acids increased. A similar effect was detected asthe pH became more acidic. In addition, the percentage of zeta po-tential decrease (rate of change) was influenced by an increase inthe concentration of amino acids. This effect was amino acid spe-cific, with the greatest impact detected for Histidine and a succes-sive decrease for the others as indicated > Glycine > Aspartic >Glutamic acid.

5. Conclusions

This is the first time that four representative amino acids of con-trasting charges have been screened and compared for their effectson ZnO nanoparticle agglomeration. Most studies to date have re-ported the behavior of nanomaterials in water (Brar et al., 2010;Scown et al., 2010), with few considering how natural organiccompounds may influence their behavior in the environment.Our observations confirm that natural molecules such as aminoacids have a significant impact on the behavior and in particular,the degree the individual particles interact with each other andagglomerate.

Previously, it was suggested that alterations in the pH have asignificant effect on zeta potential and agglomerate size, whichmay be used as a predictive measure of nanotoxicity (Brar et al.,2010). Our observations confirm that factors such as the root exu-dates and the chemical characteristics of the immediate environ-ment could greatly influence the chemical and physical state ofreleased nanomaterials. This is very helpful information since itgreatly improves our ability to predict the fate of nanomaterials,their potential environmental impact and assess longer term risks.The information may also help to develop protocols for the recov-ery of the materials which have entered the environment, acciden-tally or intentionally.

Acknowledgments

The authors would like to thank Dr. Alison Crossley fromBegbrokeNano, University of Oxford, and Dr. Robert Barnes,

h 10 mM GLY at a (a) pH 5.5, (b) pH 7 and (c) pH 8.5.

R. Molina et al. / Chemosphere 83 (2011) 545–551 551

Department of Engineering Science, University of Oxford, for theirinsight and support.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.chemosphere.2010.12.020.

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