Colloids and Surfaces A: Physicochem. Eng. Aspects 280 (2006) 116–124

Effect of ethylene glycol-bis(2-aminoethylether)-N,N,N′,N′-tetraacetic acid(EGTA) on the growth, stabilization and morphology of silver nanoparticles

Shweta Hegde a, Sudhir Kapoor b,∗, Satyawati Joshi a, Tulsi Mukherjee b

a Department of Chemistry, Pune University, Pune 411007, Indiab Radiation & Photochemistry Division, Chemistry Group, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India

Received 20 September 2005; received in revised form 22 January 2006; accepted 24 January 2006Available online 2 March 2006

Abstract

The effect of ethylene glycol-bis(2-aminoethylether)-N,N,N′,N′-tetraacetic acid (EGTA) in stabilizing different shapes of silver nanoparticleshave been examined by electronic absorption spectroscopy and transmission electron microscopy. The silver nanoparticles were prepared by twomethods, i.e. �-irradiation and chemical reduction method. Two types of effects of EGTA were identified which lead to the formation of truncatedtriangular silver nanoplates and chain—like silver aggregates respectively. Time-dependent infrared attenuated total reflectance (ATR-FTIR) studiesss©

K

1

enatatosdhntSpmmt

0d

howed that the nature of adsorption of EGTA on the silver nanoparticle surface influences the shape of the nanoparticles. Pulse radiolysis studieshowed the mechanism of formation of the initial silver nanoclusters. 2006 Elsevier B.V. All rights reserved.

eywords: EGTA; Silver nanoparticles; Shape

. Introduction

Nanomaterials with size and shape-dependent physical prop-rties provide a number of opportunities for diversified andovel applications [1–5]. In particular, metal nanoparticlesre very attractive for research in nanotechnology because ofheir appealing features. The special properties include sizend shape-dependent optical, electronic and catalytic proper-ies [6–8]. Hence, they have various applications in the fieldf catalysis, as surface enhanced Raman spectroscopy (SERS)ubstrates to study the orientation of various molecules, in nano-evices and opto-electronic applications [9,10]. Presently, thereas been a large focus on the shape selective synthesis of metalanoparticles, especially gold and silver metal nanoparticles ashey show shape dependent surface plasmon absorption band.o far, a number of surfactants such as CTAB, SDS, etc. andolymers such as PVP have been identified to induce the for-ation of various shapes in aqueous solutions [11–13]. Severalechanisms such as site selective adsorption and rate of aggrega-

ion have been proposed for the formation of these nanoparticles

[14,15]. However, there have been very few reports, where lig-ands have been utilized for the stabilization of different shapesof metal nanoparticles [16–19].

Silver nanoparticles are being used as biological sen-sors, in drug delivery systems and as a potential bacterici-dal [20,21]. Ethylene glycol-bis(2-aminoethylether)-N,N,N′,N′-tetraacetic acid (EGTA) (Scheme 1.) forms strong chelates withmany di- and trivalent cations and is widely used in biologi-cal sciences [22,23]. The behavior of enzymes, nucleic acids,lectins and other biomolecules can be greatly affected by thepresence of EGTA and its concentrations. In view of its greatimportance, it is necessary to study the interaction of EGTAwith metal nanoparticles. Other polyaminocarboxylic acids likeEDTA, HEDTA, etc. have been explored widely in the field ofnanosciences [19,24] to prepare silver nanoparticles of differ-ent shapes and aggregates. But as far as our knowledge goesthis is the first report where EGTA has been used in the syn-thesis of silver nanoparticles. In this work, we report the effectof ethylene glycol-bis(2-aminoethylether)-N,N,N′,N′-tetraaceticacid (EGTA) in the formation of different shape of Ag nanopar-ticles by utilizing the complexation properties of EGTA with the

∗ Corresponding author. Tel.: +91 22 25590298; fax: +91 22 25505151.E-mail address: sudhirk@apsara.barc.ernet.in (S. Kapoor).

silver ions. Transmission electron microscopy was employed toconfirm the presence of silver aggregates of various shapes andsizes.

927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved.

oi:10.1016/j.colsurfa.2006.01.041

S. Hegde et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 280 (2006) 116–124 117

Scheme 1. Ethylene glycol-bis(2-aminoethylether)-N,N,N′,N′-tetraacetic acid(EGTA).

To elucidate the mechanism various factors like growth kinet-ics, formation, stabilization and subsequent aggregation of silverclusters were studied using pulse radiolysis technique [25–33].Time-dependent ATR-FTIR studies were also used for explain-ing the mechanism of stabilization and aggregation of thesesilver clusters in the presence of EGTA.

2. Experimental

Materials: silver perchlorate, silver nitrate, ethylene glycol-bis(2-aminoethylether)-N,N,N′,N′-tetraacetic acid (EGTA) andsodium borohydride were obtained from Fluka and used asreceived. All other chemicals were HPLC, AR or GR grade.IOLAR grade N2 gas (purity >99.999%) used for purging solu-tions was obtained from Indian Oxygen Limited.

2.1. Preparation of Ag nanoparticles by γ-irradiation

Fresh aqueous solutions containing 1.0 × 10−4 mol dm−3

AgClO4 (AgClO4 was used to remove any possibilityof counterion effect) were prepared in the presence andabsence of EGTA. Two different concentrations of EGTA,5.0 × 10−4 mol dm−3 and 2.0 × 10−4 mol dm−3 were used.Propan-2-ol (1.0 mol dm−3) was added to the solutions as anOH• radical scavenger. The pH of the solution was adjusted to1nfa

2u

tbv1Nwa

2

k

used to maintain pH of 10.2. Pulse radiolytic studies were car-ried out by irradiating solutions in rectangular quartz cells of1 cm optical path length. Electron pulses of 50 ns or 2 �s, froma 7 MeV linear electron accelerator were employed. The detailsof the LINAC have been given elsewhere [35]. An aerated5.0 × 10−2 mol dm−3 KSCN solution was used for dosimetryand (SCN)2

•− radical was monitored at 475 nm. The absorbeddose per pulse was calculated assuming G� for (SCN)2

•− rad-ical to be 2.6 × 10−4 m2 J−1[36]. The dose employed in thepresent study, using 50 ns electron pulses, was typically 16 Gyper pulse. To study the reaction of e−

aq with silver complexes

of EGTA, 1.0 mol dm−3 propan-2-ol in N2-bubbled solution(pH 10.2) was used as scavenger for OH• radicals according toreaction (1)

H•/OH• + (CH3)2CHOH → H2/H2O + (CH3)2C•OH

(1)

The yields of hydrated electrons and (CH3)2C•OH radicalsformed during the pulse are G(e−

aq) and G(H•) + G(OH•), respec-tively. The G(e−

aq), G(H•) and G(OH•) values were taken as

0.28, 0.062 and 0.28 �mol J−1, respectively, from the literature[G(e−

aq) = 2.8 × 10−7 mol J−1, G(H•) = 0.62 × 10−7 mol J−1,

G(OH•) = 2.8 × 10−7 mol J−1] [37,38]. The rates of reactionswere determined by carrying out the experiments with at leasttfor

cwtkA

2

UrsFSdcn4ppwtoscn

0.2 with NaOH. The solution was de-aerated by bubbling withitrogen prior to �-irradiation (dose rate 60 Gy/min) providedrom a 60Co source. Optical absorption spectra were recordedfter each irradiation.

.2. Preparation of Ag nanoparticles by chemical reductionsing NaBH4 for surface modification studies

The silver nanoparticle solution was prepared by reduc-ion of aqueous silver nitrate (AgNO3) solution with sodiumorohydride (NaBH4) following a procedure similar to a pre-ious report [34]. Briefly, 0.0169 g of AgNO3 was dissolved in00 ml of distilled water. The solution was purged with nitrogen.aBH4 (0.010 g) was then added to the solution. The solutionas shaken vigorously for 5 min. The resulting silver hydrosol

ppeared to be pale yellow.

.3. Study of formation and growth of silver nanoparticles

All solutions were prepared just before the experiments andept in the dark to avoid photochemical reactions. NaOH was

hree different concentrations of metal ions, varying by at least aactor of 4. Bimolecular rate constants were derived from plotsf the first-order rates versus concentration. The rate constantseported are generally accurate to ±15%.

EGTA is known to be complexed with Ag+ with a stabilityonstant (log K) as 7.06 [39,40]. The silver ion concentrationas varied from 0.5 × 10−4 mol dm−3 to 2.0 × 10−4 mol dm−3

o determine the rate of reaction. The ratio of [EGTA]/[Ag+] isept constant (5:1) to ensure almost complete complexation ofg+ with EGTA.

.4. Characterization

Absorption measurements were carried out on a ChemitoV 2600 spectrophotometer. The spectra were recorded at

oom temperature using a 1 cm quartz cuvette. The ATR mea-urements were performed with unpolarized radiation using aourier-transform infrared spectrometer (Nicolet Nexus 870,pectra Tech.) equipped with a liquid nitrogen-cooled MCTetector, as the average of 100 scans at 4 cm−1 resolution. Aircle cell having a ZnSe crystal rod was used as the inter-al reflection element and the incidence angle of radiation was5◦. Samples for transmission electron microscopy (TEM) wererepared by putting a drop of the colloidal solution on a cop-er grid coated with a thin amorphous carbon film. Samplesere dried and kept under vacuum in a desiccator before putting

hem in a specimen holder. TEM characterization was carriedut using a PHILIPS CM-200 electron microscope. Particleizes were measured from the TEM micrographs. The parti-le size was calculated by taking an average of at least 100anoparticles.

118 S. Hegde et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 280 (2006) 116–124

3. Results and discussion

Recently, the seed method has been exploited to preparelarger and different shape of metal nanoparticles. Here, we haveattempted a different approach in which metal nanoparticles areprepared in the presence of ligand, which induces the aggrega-tion of particles.

3.1. Effect of amount of dose absorbed and concentrationof EGTA

Fig. 1 shows the effect of EGTA concentration on thesteady state UV–vis absorption spectra obtained after �-irradiation of an N2-bubbled aqueous solution containing1.0 × 10−4 mol dm−3 AgClO4, 1.0 mol dm−3 propan-2-ol at pH10.2. It can be seen that after 5 min of �-irrradition, the charac-teristic surface plasmon band appears at 415 nm. However, in thepresence of 2.0 × 10−4 mol dm−3 EGTA, an asymmetric surfaceplasmon peak appears at 415 nm along with a slight shoulder at570 nm. The appearance of the shoulder band could be due tothe adsorption of EGTA molecules over the silver particle sur-face leading to their aggregation. The effect of adsorption ofEGTA molecules was further confirmed by employing higherconcentration of EGTA. After irradiation of a solution contain-ing 5.0 × 10−4 mol dm−3 EGTA in addition to a band at 415 nm,wnthfEctogppta

Fig. 1. The steady state UV–visible absorption spectra of silver nanoparticlesin a solution cotaining 1.0 × 10−4 mol dm−3 AgClO4, 1.0 mol dm−3 propan-2-ol and EGTA. (a) After 5 min of �-irradiation, with no EGTA ( ) and2.0 × 10−4 mol dm−3 EGTA (—). (b) After 10 min of �-irradiation with noEGTA ( ) and 2.0 × 10−4 mol dm−3 EGTA (—).

F ared by �-irradiation of a solution containing 1.0 × 10−4 mol dm−3 AgClO4,2 ansmission electron micrograph of a single nanoplate from the above sample and(

e observed a broad band in higher wavelength region (resultsot shown). The above observations can be explained as dueo severe aggregation of silver nanoparticles in the presence ofigher concentration of EGTA. Fig. 2 shows the TEM picturesor the �-irradiated sample containing 2.0 × 10−4 mol dm−3

GTA and 1.0 × 10−4 mol dm−3 Ag+ ions. From Fig. 2(a), itan be clearly seen that a mixture of spherical and flat platelikeriangular nanoparticles are obtained. These triangular plates aref approximately 500–700 nm in size. A single truncated trian-ular nanoplate has been shown in Fig. 2(b). The thickness of thelate can be easily measured from the contrast obtained in theicture and is found to be approximately 30 nm. Fig. 2(c) showshe selected area electron diffraction pattern obtained for the tri-ngular nanoplate. The diffraction pattern shows the presence

ig. 2. Transmission electron micrograph of (a) silver nanoparticles prep.0 × 10−4 mol dm−3 EGTA and 1.0 mol dm−3 propan-2-ol for 10 min, (b) trc) Electron diffraction pattern of the Ag nanoplate shown in (b).

S. Hegde et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 280 (2006) 116–124 119

Fig. 3. The UV–vis absorption spectra of silver nanoparticles obtained afteraddition of (a) 2.0 × 10−4 mol dm−3 EGTA and (b) 5.0 × 10−4 mol dm−3 EGTAto the original Ag hydrosol.

of multiply twinned nanoparticles and confirms the formationof Ag crystallites. This particular colloidal sample was storedfor several days to observe if further changes take place in thecolor or its intensity. Surprisingly, we observed that over time,the peak at 415 nm disappeared and a new peak at 580 nm wasobtained (UV spectrum not shown). Going by the result that weobtained above from the TEM analysis, we can conclude thata complete transformation from the spherical to the triangularsilver nanoparticles takes place, which shows a surface plasmonpeak at 580 nm.

In the second method silver nanoparticles were preparedby borohydride reduction. Fig. 3 shows the UV–vis spectraof silver nanoparticles prepared by borohydride reduction. Theoriginal Ag nanoparticles solution (without EGTA) exhibits arelatively narrow band peaking at 395 nm because of the plas-mon resonance of the spherical silver nanoparticles. This bandundergoes drastic dampening and shifts to 523 nm after addition

of 2.0 × 10−4 mol dm−3 of EGTA. The broadening and the redshift of the plasma band can be explained by the developmentof nanoparticle aggregation in the solution. All the spectra weretaken after 5 min of addition of EGTA. With further addition ofEGTA the red shifted band moved towards longer wavelengthaccompanied by further broadening of the surface plasmonabsorption band. An isoabsorption between both the bands wasattained, which indicates that absorption equilibrium occurredafter addition of EGTA. The solutions were preserved to observethe evolution of nanoparticles with elapsed time. The solutionswith larger than 2.0 × 10−4 mol dm−3 of EGTA undergo rapidaggregation with time and within 6 h the solution became color-less and large particles of grayish colour were formed. This couldbe due to the different shape/size of particles being formed (videinfra). Similar observations were recorded by Zang et al. [41].Wang et al. [42], and Prochazka et al. [43], in the presence ofanions and organic molecules. Zang et al. [41] suggested that thenew band at a longer wavelength might be a consequence of thedecrease of the surface electron density over the silver nanopar-ticle surface. However, Prochazka et al. and Muniz-Miranda etal. [44] proposed that the new band resulted from the aggrega-tion of silver nanoparticles. Based on the arguments given byZang and Prochazka, Wang et al. [42] concluded that the changein the absorption spectrum of silver nanoparticles should cor-relate with the decrease in the surface electron density of theaOgn

obtttsE

F 4− re

o

ig. 4. Transmission electron micrograph of silver nanoparticles obtained by BHf 5.0 × 10−4 mol dm−3 EGTA to the original Ag hydrosol.

ggregated silver nanoparticles due to the chemisorbed anions.ur observations discussed above hold true for the explanationiven by Wang et al. [42], where EGTA adsorbs over the silveranoparticles surface.

The results obtained above indicate that there exists a thresh-ld EGTA/Ag ratio at which the silver nanoparticles remain sta-le for longer times. In order to find out more about the nanopar-icle shapes and sizes, TEM pictures were taken for the sample atwo different concentration of the EGTA. Fig. 4(a and b) showshe transmission electron micrographs of the EGTA stabilizedilver nanoparticles. At concentration of 2.0 × 10−4 mol dm−3

GTA, well dispersed nanoparticles were formed having spher-

duction (a) after addition of 2.0 × 10−4 mol dm−3 EGTA and (b) after addition

120 S. Hegde et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 280 (2006) 116–124

ical shape with an average size of 50 nm. At higher concentra-tion of EGTA (5.0 × 10−4 mol dm−3) chain like aggregates of∼=50 nm spherical nanoparticles are formed.

3.2. Time-dependent ATR-FTIR studies

To obtain further insight on the interaction of EGTAmolecules with the silver surface and the agglomeration processATR-FTIR studies were carried out. An important advantageof using the infrared ATR method is that the electric fields ofradiation can penetrate only a short distance (much smaller thanradiation wavelength) from the reflecting surface of the ZnSerod into the solution sample. Hence it was possible to over-come the problems related to high absorption coefficient ofthe solvent. Fig. 5 shows the infrared ATR spectra of the sil-ver nanoparticle solution after addition of 2.0 × 10−4 mol dm−3

EGTA recorded at various time intervals. An important fact thatis to be noted is that at the given concentration of EGTA, peaksdue to EGTA could not be detected at all in the absence ofthe silver sol. An almost featureless spectrum is observed afteraddition of 2.0 × 10−4 mol dm−3 EGTA to the silver hydrosol,when recorded after 5 min. However, new bands appear over theregion of 800–1800 cm−1 after 4 days, which gradually increasein intensity over time, concomitantly showing a slight shift for afew bands in this region. The νasym (C O) and the νsym (C O)balctbttss3iir(aim

TI

Sn

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Fig. 5. Time dependent ATR-FTIR spectra after addition of 2.0 × 10−4 moldm−3 EGTA to a silver hydrosol taken at (b) 5 min (c) 30 min (d) 4 days and (e)10 days, (a) spectrum of 1.0 × 10−3 mol dm−3 EGTA in water.

intermolecular attraction and the orientation of the moleculeon the surface of the silver nanoparticles. The gradual increasein the intensity of the bands can be explained by the progressof nanoparticle aggregation at the ZnSe rod/solution interfacevia adhering of the aggregated EGTA capped Ag nanoparticlesto the ZnSe rod. Fig. 5 also shows clearly that as nanoparticleaggregation increases the negative band at 1640 cm−1, whichis attributed to the υbending (H O H) of water also vanishesand bands due to EGTA in that region start appearing. The con-centration of water throughout the solution remains constant.

ands for the free carboxylic acid functionality is seen at 1740nd 1260 cm−1 [45–47]. Whereas, the sodium bound carboxy-ate anion frequencies are seen at 1640 and 1396 cm−1. Thesearboxylic acid sites are the most probable sites for interac-ion with the metal nanoparticle surface. Coordination of car-oxylic acids is usually via the carboxylate group and there arehree common coordination modes: unidentate, chelate biden-ate, and bridging bidentate. These can be distinguished in IRpectra by their differing separations between the carboxylatetretch absorption bands (�υ). Band separations are generally50–500 cm−1 for unidentate binding, 150–180 cm−1 for bridg-ng, and 60–100 cm−1 for chelating [47]. The most evident shiftsn the band frequencies are observed in the lower wavelengthegion where, the strong band at 1094 and 942 cm−1 due to νasymC O C) and the νsym (C O C) bands were shifted to 1085nd 939 cm−1, respectively [46]. The frequencies are tabulatedn Table 1. The perturbation of the CH2 bending (scissoring)

ode for EGTA at 1465 cm−1 gives more information on the

able 1R frequencies in cm−1 for EGTA and EGTA capped Ag nanoparticles

erialumber

EGTA(cm−1)

Ag/EGTA(cm−1)

IR assignement

1740 1740 υasymm (C O) carboxylic acid1676 – υasymm (C O) carboxylate anion1640 1645 υ (C N)1560 1562 υbending (N H)1465 1454 υasymm (CH2)1396 1400 υsymm (C O) carboxylate anion1260 1263 υsymm (C O) carboxylic acid1163 1126 νasym (C O C)1094 1086 νsym (C O C)

S. Hegde et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 280 (2006) 116–124 121

This indicates that the exclusion of the water at the rod/solutioninterface and the gradual aggregation and settling of the nanopar-ticles lead to the formation of a thin film over the crystal rod.Due to this phenomenon we are able to obtain clear bands dueto EGTA after almost complete aggregation takes place. Such aphenomenon has also been observed in the case of phase separa-tion of poly(N-isopropylacrylamide) and subsequent formationof a film on the ATR crystal [48].

3.3. Effect of EGTA on morphology of Ag nanoparticles

The above results show that there exists a threshold concen-tration of EGTA above which rapid aggregation occurs leadingto turbidity in the solutions. One of the most remarkable propertyof small spherical and monodisperse nanoparticles is their abil-ity to form ordered crystals, Self-organization of nanocrystalson a substrate is not simply entropy-driven. Instead, interactionsbetween nanoparticles and between the nanoparticle and thesubstrate play an important role in determining the packing mor-phology of the superlattices. The surface characteristics of thenanoparticles play a very important role. Borohydride is knownto stabilize Ag nanoparticles dominantly into small, spheri-cal nanoparticles that have a negatively charged surface [34].The stability of the colloid largely depends upon the Coulom-bic repulsion of the charged nanoparticles. As observed in theUV–vis absorption spectra, we have obtained results that areswaarorittEtciiotglaasfmwEopSofl

presence of EGTA during the formation of Ag nanoparticles andwhen added after the formation of Ag nanoparticles assists inthe formation of different shapes of nanoparticles. To elucidatethe reaction mechanism that EGTA interacts with the initial Agclusters, pulse radiolysis experiments were carried out.

3.4. Reaction of e−aq with EGTA

The reaction of e−aq with EGTA was studied in aqueous solu-

tion containing propan-2-ol at pH 10.2 by monitoring the decaykinetics of e−

aq at 700 nm. It was observed that the reactivity ofe−

aq with the above-mentioned ligands was low and an upperlimit for the bimolecular rate constant was calculated to be≤105 dm3 mol−1 s−1.

3.5. Effect of EGTA

Fig. 6 shows the time-resolved absorption spectra of thetransient species obtained at four different times after irra-diation with 50 ns electron pulses, dose 16 Gy per pulse, inN2-bubbled aqueous solutions containing 1.0 × 10−4 mol dm−3

Ag+, 5.0 × 10−4 mol dm−3 EGTA, and 1.0 mol dm−3 propan-2-ol at pH 10.2. The time-resolved absorption spectra weremeasured up to 450 nm as the contribution of the electron tailincreases enormously at higher wavelengths. It was observedt −oe

eTo

A

aa

F

45(a

imilar to those obtained by Wang et al. [42] where anionsere added to the negatively charged Ag nanoparticles. After

ddition of EGTA ions, the borohydride molecules still remainssociated with the nanoparticles. Hence EGTA acts as a counte-ion that strongly adsorbs on the nanoparticle surface. Additionf appropriate concentration of EGTA weakens the Coulobmicepulsion forces inducing rapid coagulation. However, H2BO3

−ons still remain as the major stability-influencing factor leadingo formation of low dimensional chain-like aggregates. Hence,he counterion effect restricts the shape-controlling ability ofGTA. During slow aggregation, the EGTA protected nanopar-

icles interdigitate with each other and arrange themselves in alose orderly fashion. We observed, that the rate of aggregationncreases with increase in the EGTA concentration. Thus, EGTAs able to kinetically control the formation of the nanoparticlesf different shapes or sizes. Whereas, during �-irradiation, con-inuous growth and nucleation takes place leading to continouseneration of Ag nanoparticles. The absence of surrounding ionsike Na+, NO3

− and H2BO3− leads to closer interparticle inter-

ction. As seen from the ATR-FTIR studies the CH2 and CH3ssymmetric bands appearing between 1400 and 1470 cm−1arehifted to a lower wavenumber. This shift in the alkyl grouprequencies indicates that close interaction between the EGTAolecules might take place, along with interaction of the COO−ith the silver nanoparticle surface. The selective adsorption ofGTA molecules on the (1 1 1) plane leads to restricted growthn this plane. Hence, growth along the adjacent (1 1 0) and (1 0 0)lanes takes place leading to formation of triangular nanoplates.imilar shifts in the asymmetric CH2 and CH3 bands have beenbserved when long chain thiol have been self assembled onat gold surfaces [49]. The above results clearly show that the

hat the decay of eaq at 700 nm increased with the concentrationf Ag+(EGTA). The bimolecular rate constant for the reaction of−aq with Ag+(EGTA) was measured by monitoring the decay of−aq at 700 nm and it was found to be 5.0 × 109 dm3 mol−1 s−1.hus, the reduction of Ag+(EGTA) by e−

aq led to the formationf

g+(EGTA) + e−aq → Ag0(EGTA), (4)

The transient of silver atoms in the presence of EGTA showedn absorption maximum at around 375 nm. As silver atomsre neutral the possibility of Ag0 uncomplexed with EGTA

ig. 6. Transient absorption spectra following 50 ns pulse irradiation ([e−aq] =

.5 × 10−6 mol dm−3) of solution containing 1.0 × 10−4 mol dm−3 AgClO4,

.0 × 10−4 mol dm−3 EGTA and 1.0 mol dm−3 propan-2-ol at pH 10.2. 1.5 �s�), 8 �s (�), 16 �s (�), 40 �s (�). Inset: Decay traces obtained at 270, 310nd 360 nm.

122 S. Hegde et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 280 (2006) 116–124

cannot be ruled out completely. Hence to avoid ambiguity,we refer 375 nm band due to Ag atoms in the presence ofEGTA. It can be seen from Fig. 6 that with time the absorp-tion due to the formation of other transient intermediates buildup at lower wavelengths. The insert of Fig. 6 shows the for-mation of transient at 270 nm (Ag4

2+), 310 nm (Ag2+) and

360 nm (Ag0) [29]. Though, we do not see absorption bandat 270 nm in the transient absorption spectrum, decay tracesclearly show formation of Ag4

2+ at 270 nm. The formation rateof the transient at 310 nm was found to depend on the concen-tration of the silver ion complex. The bimolecular rate constantfor its formation was estimated by monitoring the buildup ofthe transient at 310 nm. The rate constant for the formationas estimated from the pseudo-first-order rate constants, keep-ing the initial ratio of [EGTA]/[Ag+] constant, was found tobe 3.0 × 109 dm3 mol−1 s−1.These results corroborate with thebimolecular rate constants obtained for reduction of silver ionsin aqueous medium and in gelatin [29,31,32]. Thus, it appearsthat the rate determining step for the formation of the dimericsilver ion cluster is the reduction of Ag+(EGTA). It is impor-tant to mention here that formation of Ag0 was not observedwhen an aqueous solution containing 1.0 × 10−4 mol dm−3

Ag+, 5.0 × 10−4 mol dm−3 EGTA, and 1.0 mol dm−3 propan-2-ol at pH 10.2 is bubbled with N2O to scavenge e−

aq. This showsthat Ag+(EGTA) complex does not get reduced by (CH3)2C•OHradicals.

3s

otcEc

F

25(

Fig. 8. The steady state optical absorption spectra obtained just after givinga train of 20, 2 �s electron pulses to N2-bubbled aqueous solution containing1.0 × 10−4 mol dm−3 AgClO4, 5.0 × 10−4 mol dm−3 EGTA and 1.0 mol dm−3

propan-2-ol.

10−5 mol dm−3. Under the experimental conditions, e−aq has

reacted with most of the Ag+ ions present in the solution andthe concentration of free Ag+(EGTA) was low. At 3 �s afterthe pulse, a clear absorption band was observed with λmax at325. This absorption band decayed with a concomitant blueshift towards the UV region. This indicates that the aggrega-tion of silver atoms complexed with EGTA takes place very fastin the absence of excess silver ions and no increase in opticalabsorbance due to the formation of other clusters of silver wasobserved within the time window of our experimental condi-tions. Long-lived clusters were prepared by reducing Ag+ ionswith trains of 2 �s electron pulses operated at the frequency of50 pulses per sec (pps) at the interval of 20 ms. Fig. 8 showsthe steady state optical absorption spectra obtained after irradi-ation of N2-bubbled aqueous solution containing 1.0 mol dm−3

propan-2-ol and 1.0 × 10−4 mol dm−3 AgClO4 in the presenceof 5.0 × 10−4 mol dm−3 EGTA. None of the absorption bandsobserved at short time scales could be observed at these longertime scales. However, we did observe a broad absorption bandat 412 nm, which probably corresponds to larger aggregates ofAgn. It can be seen that in the presence of EGTA the silvernanoparticles remain stable, indicating the stabilizing effect ofEGTA on these silver nanoparticles.

3.7. Mixed solutions of MV2+ and silver in the presenceo

grotaiAa

.6. Stabilization of silver clusters in the presence of lowilver ions

Fig. 7 shows the time-resolved absorption spectrum obtainedn reduction of silver ions on pulse irradiation by a 2 �s elec-ron pulse of dose 80 Gy in an of N2-bubbled aqueous solutionontaining 1.0 × 10−5 mol dm−3 Ag+, 5.0 × 10−5 mol dm−3

GTA, and 1.0 mol dm−3 propan-2-ol at pH 10.2. The con-entration of e−

aq produced by a 2 �s electron pulse was

ig. 7. Transient absorption spectra following 2 �s pulse irradiation ([e−aq] =

.8 × 10−5 mol dm−3) of solution containing 1.0 × 10−5 mol dm−3 AgClO4,

.0 × 10−5 mol dm−3 EGTA and 1.0 mol dm−3 propan-2-ol at pH 10.2. 3 �s�), 7 �s (�), 10 �s (�) and 20 �s (�).

f EGTA

In the presence of ligand, the redox potential of aggre-ates might get affected [27,28]. Therefore, the growth andeactivity of silver clusters were also studied in the presencef methyl viologen (MV2+) as a redox probe. Fig. 9 showshe typical profiles of the time evolution of the absorbancet two wavelengths, λ = 700 and 430 nm, obtained on pulserradiation of the same solution, that is, 1.0 × 10−4 mol dm−3

g+, 5.0 × 10−4 mol dm−3 EGTA, 1.0 × 10−3 mol dm−3 MV2+

nd 1.0 mol dm−3 propan-2-ol at pH 10.2. The radical cation,

S. Hegde et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 280 (2006) 116–124 123

Fig. 9. Decay and formation kinetics of MV•+ and silver clusters at 700and 430 nm, following 2 �s electron pulse irradiation, dose = 43 Gy in deaer-ated aqueous solutions at pH 10.2 containing 1.0 × 10−4 mol dm−3 AgClO4,

1.0 × 10−3 mol dm−3 MV2+, 5.0 × 10−4 mol dm−3 EGTA and 1.0 mol dm−3

propan-2-ol.

MV+, was observed at 700 nm where its molar absorptivity[27] is quite intense ε700 = 3460 dm3 mol−1 cm−1. At 430 nmthe silver aggregates have stronger absorption compared toMV+• (ε430 = 1000 dm3 mol−1 cm−1) [27]. It can be seen thaton pulsing the solution containing 1.0 × 10−4 mol dm−3 Ag+,5.0 × 10−4 mol dm−3 EGTA, 1.0 × 10−3 mol dm−3 MV2+ and1.0 mol dm−3 propan-2-ol at pH 10.2 MV+ and Ag aggregatesare produced. In the presence of EGTA there is a slow increase inthe absorption of the silver clusters, which could be due to trans-fer of electrons to Ag aggregates from MV+ with concomitantdecrease in MV+ yields. This indicates that the growth kineticsdecreases marginally in the presence of EGTA. It is pertinent tomention here that in the absence of Ag+ ions the MV+ radicalremains stable under our experimental conditions. This showsthat the potential of the silver aggregates in the presence of EGTAis similar or slightly less than the MV2+/MV+ couple.

4. Conclusions

For the formation of different shapes of nanoparticles, ini-tial nuclei play an important role due to which the growth ofthe particles take place. The above results show that EGTAstabilizes initial clusters of silver. This could be the reasonfor obtaining triangular plate type of nanoparticles when theywere prepared in the presence of EGTA. TEM studies on sil-vEwUoibvn

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