Colloids and Surfaces A: Physicochem. Eng. Aspects 339 (2009) 60–67

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Colloids and Surfaces A: Physicochemical andEngineering Aspects

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Fabrication of Ag nanoparticles by �-irradiation: Application tosurface-enhanced Raman spectroscopy of fungicides

A. Torreggiania, Z. Jurasekovab, M. D’Angelantonioa, M. Tambaa,J.V. Garcia-Ramosb, S. Sanchez-Cortesb,∗

a Istituto ISOF (CNR), Via P. Gobetti, 101, 40129-Bologna, Italyb Instituto de Estructura de la Materia, CSIC, Serrano, 121, 28006-Madrid, Spain

a r t i c l e i n f o

Article history:Received 13 August 2008Accepted 29 January 2009Available online 6 February 2009

Keywords:Silver colloidsSERS spectroscopy�-RadiolysisNanoparticles

a b s t r a c t

This paper is concerned with the surface enhanced Raman scattering (SERS) activity of silver col-loids obtained by a radiolytic method. Ag nanoparticles were successfully prepared by �-radiolysisof Ag+ aqueous solution containing t-BuOH or i-PrOH at room temperature without the addition ofaggregating or stabilizing substances. The metal colloids were characterised by UV/vis spectroscopyand scanning electron microscopy. Many experimental conditions were tested (i.e. Ag+ concentration,dose and •OH scavenger alcohol) in order to obtain the best controlled size of nanoparticles as wellas the high stability of colloidal silver with time. The use of relatively low irradiation doses and Ag+

concentrations allowed to obtain very stable suspensions of Ag nanoparticles without adding anycolloid stabiliser, a source of further spurious bands in the Raman spectra. The suitability of the �-irradiated colloids in SERS spectroscopy was tested by using thiram, a known fungicide. Micro-SERS

Fungicideand SERS spectra of good quality were achieved at very low concentration of adsorbate, without theoverlapping of impurities normally present in conventional citrate colloidal suspensions of Ag nanopar-

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. Introduction

Surface-enhanced Raman scattering (SERS) has represented areat advance in the field of the Raman spectroscopy during theeventies since the inherent weakness of the Raman signal can beubstantially increased by six or more orders of magnitude [1]. SERSas been successfully applied in the study of poorly soluble com-ounds in water, since very low concentrations are required, withhe additional advantage of the fluorescence quenching occurringn the metal surface [2]. The application of SERS requires the usef metal surfaces fulfilling the surface-plasmon resonance (SPR)ondition in the region of the laser light employed for Raman exci-ation [3–5]. This implies the use of mainly Ag, Au and Cu having aanostructured morphology. In fact, it is crucial the absence on theurface of impurities, since SERS technique is so sensitive that veryow adsorbate concentrations can also be detected giving spuriousands in the SERS spectra [6].

Most SERS active substrates are metal nanoparticles prepared byhemical reduction using an excess of reductants, such as hydrox-lamine hydrochloride or trisodium citrate. Other reagents can bedded to colloidal solution either for aggregating the colloidal par-

∗ Corresponding author. Tel.: +34 91 5 61 68 00; fax: +34 91 5 64 55 57.E-mail address: imts158@iem.cfmac.csic.es (S. Sanchez-Cortes).

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927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2009.01.018

© 2009 Elsevier B.V. All rights reserved.

ticles or stabilising them. However, these colloids display severaldisadvantages which limit their application, such as the existenceof impurities resulting from the oxidation residual species, thecounter ions of the employed salts [6,7] and their unstability insuspension. For these reasons, the efforts in developing methods togenerate stable Ag substrates without the interference of inherentimpurities are of interest.

In addition, small metal clusters are promising because of theiroptical, electronic and catalytic properties and are useful in widefields [8–10], including biological labelling [11,12] and photography.

Besides the use of conventional chemical and photochemicaltechniques, �-radiolysis appears to be a suitable method to formmetal colloids in solution [13–16]. The �-irradiation has importantadvantages as compared to the chemical reduction method:

(a) It does not require the addition of reducing agents, because thereduction is performed by radical species formed after interac-tion of ionising radiation with the solvent.

b) The production of undesired oxidation products from the reduc-ing agents is avoided.

(c) The progressive extent of the reduction is accurately controlledby monitoring the absorbed dose.

d) Reducing species are uniformly distributed in the solution.(e) The overall process is performed at room temperature.

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3.1.1. �-Radiolysis of Ag+ aqueous solution containing t-BuOHFig. 1 shows the absorption spectra of �-irradiated Ar-purged

solutions containing 2 × 10−4 M AgNO3 and 0.5 M t-BuOH as afunction of the irradiation dose. Irradiated solutions showed an

A. Torreggiani et al. / Colloids and Surfaces

The formation and reactivity of Ag nanoparticles in solution by-irradiation has been studied extensively, but the majority of stud-

es have been directed on the preparation of these particles in theresence of surfactants or polymers as stabiliser of the colloidal par-icles [13,17–20]. Recently, two works have reported the applicationf Ag colloids prepared by �-irradiation as metal substrates for SERS21,22]. However, these colloids have been obtained under experi-

ental conditions very different from ours, that is in the presencef chemical stabilisers and relatively high irradiation doses.

The aim of this work was the preparation of Ag colloids inqueous solutions by the �-irradiation method in the absence ofhemical stabilisers. The fabrication of these Ag nanoparticles wasested at different experimental conditions to check the effect ofhe �-radiation dose, the metal concentration and the presence oflcohols acting as •OH radical scavengers. The Ag colloidal nanopar-icles were then characterised by both UV–visible spectroscopynd scanning electron microscopy (SEM). The SERS efficiency of Aganoparticles prepared by �-radiations was tested and discussedy using a substance with environmental interest, the fungicidehiram. This fungicide is of remarkable environmental interest, dueo its frequent use in agricultural practises and it was already char-cterised by us in previous works [23,24].

. Experimental

AgNO3 (ACS, Sigma–Aldrich), Thiram (Labor Dr. Enhrenstofer),-BuOH (ACS, Sigma–Aldrich), and i-PrOH (suprapure, Fluka) weresed as supplied. AgNO3 solutions (1–10 mM) were freshly pre-ared in water purified with a Millipore (Milli-Q) system.

�-Irradiation was carried out by using a 60Co-GammacellAtomic Energy of Canada Ltd.) at the dose rate of ≈13 Gy/min. Theotal irradiation dose given to the samples was in the 25–1000 Gyange. The exact absorbed radiation dose was determined withhe Fricke chemical dosimeter, by taking G(Fe3+) = 1.61 �mol J−1

25]. In aqueous solutions the formation of two short-lived reduc-ng species, solvated electron (eaq

−) and hydrogen atoms (•H), isbtained together with hydroxyl radical (•OH), a strong oxidizingpecies (Eq. (1)). The values in parentheses represent the radia-ion chemical yields (G) in �mol J−1 taken from Buxton et al. [26].oth eaq

− (E◦ = −2.9 V) and •H (E◦ = −2.4 V) have reduction poten-ials capable of reducing metal ions to lower valences and, finally,o metal atoms.

(1)

In order to avoid metal ion oxidation by •OH (E◦ = +1.9 V), 0.5 M-butanol (t-BuOH) or i-propanol (i-PrOH) was added as •OH radicalcavengers to the silver salt solutions before irradiation. In the pres-nce of t-BuOH, •OH radicals are scavenged efficiently (Reaction2), k2 = 6.0 × 108 M−1 s−1) [26] whereas •H atoms react only slowlyReaction (3), k3 = 2.3 × 105 M−1 s−1) [27]. The tertiary carbon-entred radical obtained is fairly unreactive, with poor reducingroperties (E◦ [(CH2C(CH3)2O,H+/•CH2C(CH3)2OH)] = −0.1 V) [28].

OH + (CH3)3COH → •CH2(CH3)2COH + H2O (2)

H + (CH3)3COH → •CH2(CH3)2COH + H2 (3)

On the contrary, in the case of i-PrOH both •OHnd •H species are scavenged (k4 = 1.9 ×109 M−1 s−1 and5 = 7.4 × 107 M−1 s−1) [26,27] giving rise to a secondary carbon-entred radical, that is a good reducing agent (CH ) •COH (E◦

3 2(CH3)2CO,H+/(CH3)2

•COH] = −1.39 V) [29].

OH + (CH3)2CHOH → (CH3)2•COH + H2O (4)

H + (CH3)2CHOH → (CH3)2•COH + H2 (5)

sicochem. Eng. Aspects 339 (2009) 60–67 61

In order to avoid the fast reaction of O2 with eaq− (k = 1.9 ×

1010 M−1 s−1) [7,26] the solutions were purged of air by bubblingargon and then irradiated at room temperature. All the aqueoussamples were at natural pH. Yellow-light brown coloured Ag col-loids were obtained as a homogeneous solution.

Ag nanoparticles were immobilised by direct deposition of 20 �Lof the particle suspension on a glass cover slide, the solvent wasleft to evaporate at room temperature. The dried drop was thenwashed with tri-distilled water several times to remove any saltresidue. Then, 20 �L of a 1.2 × 10−4 M thiram solution in ethanolwas added to the immobilised Ag nanoparticles. An aliquot of tri-distilled water was placed in a glass slide provided with a shallowgroove (2 cm of diameter and 380 �m of depth), and then the coverglass slide containing the dried Ag nanoparticles with thiram wasplaced on the groove with the side containing the dried nanopar-ticles facing downwards the suspension placed in the groove asdescribed elsewhere [30]. In other experiments the Ag nanoparti-cles film containing thiram were washed before placing the coverslide downwards in contact with the water deposited in the well.

UV measurements (200–1100 nm) were carried out at roomtemperature by means of a Perkin Elmer model Lambda 40 spec-trophotometer. The spectra fitting analysis was performed byORIGIN peak fitting (ORIGIN 6.1, ORIGIN Lab. Comp.).

SEM micrographs were obtained in an environmental scanningelectron microscope (ESEM) PHILIPS XL30 with tungsten filamentoperating under high vacuum mode. The acceleration voltage was25 kV. For secondary electrons, the standard Everhard–Thornleydetector was used. Samples coating was accomplished using a high-resolution magnetron sputter coater Polaron SC 7640.

The SERS spectra were recorded in a Renishaw RM2000 micro-Raman. An exciting line at 514.5 nm provided by an Ar+ laser wasused. The resolution was set at 2 cm−1 and the geometry of micro-Raman measurements was 180◦. Optical micrographs shown in thiswork were registered with a 50× Leica optical microscopy coupledto the Raman spectrometer.

3. Results and discussion

3.1. UV/vis spectra of Ag nanoparticles prepared by �-irradiation

Fig. 1. UV–vis spectra of Ar-purged solutions containing 2 × 10−4 M AgNO3 and 0.5 Mt-BuOH after �-irradiation at natural pH; total delivered doses reported in the figure.Inset: time evolution of the 428 nm band absorbance, 100 Gy.

6 A: Physicochem. Eng. Aspects 339 (2009) 60–67

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bsorption maximum at 400–430 nm, not observable before irra-iation, and indicating the radiation-induced formation of silveranoparticles. In fact, it is known that colloidal silver has an intenseurface-plasmon absorption band with a maximum at 390–420 nm31].

The intensity of the 400–430 nm absorption band, as well ashe absorption at longer wavelengths (about 600 nm), increasedontinuously with the irradiation dose. Also the position of theaximum and the band width were both affected by the dose

Fig. 1). Since the position and the peak width of the maximumepends on the silver particle size and particle aggregation [32,33]his behaviour indicates that a wider cluster size distribution andarger particles are obtained by increasing dose. In fact, the absorp-ion at ≈600 nm is attributable to the formation of particle clustersarger than those absorbing at ≈400 nm [34].

The silver sol formation under �-irradiation is due to severalteps of reaction, as results from several studies addressed to thisubject [13,16,35,36]. Metal atoms (Ag0), formed in solution fromhe Ag+ reductions (E◦(Ag+/Ag0(aq)) = −1.8 V) [17] by solvated elec-rons and hydrogen atoms (Reactions (6) and (7)), tend to associateith other ions and coalesce into dimers (Reaction (8)) that pro-

ressively grow into oligomers (Reactions (9) and (10)), followedy larger clusters (Reactions (11) and (12)). Thus, the coagulationf these simple clusters (i.e. Ag2

+•, Ag42+, and others) leads to the

ormation of sub-colloidal particles whose coalescence gives rise tohe colloidal silver formation [17,37].

g+ + e−aq → Ag0 (6)

g+ + H• → Ag0 + H+ (7)

g0 + Ag+ → Ag2+• (8)

g2+• + Ag+ → Ag3

2+• (9)

g2+• + Ag2

+• → Ag42+ (10)

(11)

gm+xx+• + Agn+y

y+• → Agpz+ (12)

The stability of the colloidal solutions on exposure to air wasollowed by UV/vis spectra; in particular, the colloid spectrumbtained by using 100 Gy exposure displays an absorption inten-ity of the band at 427 nm, almost unchanged with time (see insetf Fig. 1). In general, all colloids prepared by using doses up to00 Gy could be preserved for 4 months without appreciable vis-

ble changes of colour or clarity, except for a first stage includinghe first 3 days when the slight increase in the intensity could indi-ate a slow growing of Ag nanoparticles. The high stability of theseilver sols in the absence of chemical stabiliser can be attributedo the adsorption of Ag+ ions on the nanoparticles, leading to therevalence of electrostatic repulsion forces on micelles having suchharges [20].

On the contrary, the precipitation of grey silver metal seen atigher doses indicates the instability of the system, where largeggregates have been formed. In fact, as the dose increases, the con-entration of Ag0 atoms simultaneously produced by consumptionf the stabiliser Ag+ increases, leading to a further coalescence.

As far as the influence of the metal concentration is concerned,t the same �-radiation dose, the peak width of the absorptionand considerably increased by increasing the starting Ag+ con-entration (Fig. 2b), whereas the absorbance of the maximum was

oorly affected (Fig. 2a). This behaviour can be explained by taking

nto account that the Ag+ starting concentration does not affect themount of nanoparticles formed, that is related to the dose expo-ure, but influences the silver particle aggregation mechanism aseduced from Reactions (6)–(12).

Fig. 2. Influence of [AgNO3] on: (a) the absorbance at 428 nm (maximum) and (b) thepeak width of the related band of the colloids obtained from irradiation at differentdoses on Ar-purged AgNO3 solutions at natural pH, in the presence of t-BuOH 0.5 M.

As regards the suitability of these colloids in SERS spectroscopy,silver nanoparticles obtained at the lowest Ag+ concentration and100 Gy of dose is promising because of the higher stability ofnanoparticles with time and the homogeneity in their size distribu-tion. In fact, the profile of the absorption band and its time evolutionunder the above conditions indicate a narrowed size distribution ofAg nanoparticles and a higher stability with time (Fig. 2b, and insetin Fig. 1).

3.1.2. �-Radiolysis of Ag+ aqueous solution containing i-PrOHAbsorption spectra of colloidal silver prepared in the presence

of i-PrOH after 50 Gy irradiation, as a function of the Ag+-startingconcentration, is shown in Fig. 3. An intense narrow and symmetricabsorption band at 408 nm was observed at low starting concen-tration of Ag+ (1 × 10−4 M), indicating a narrow size distribution ofthe silver nanoparticles produced. In contrast, at the highest Ag+

concentration (1 × 10−3 M) a strong decrease in the intensity of theabsorption maximum was associated with a subsequent increaseof the broad absorption at longer wavelengths. This behaviour isdue to the aggregation of silver nanoparticles leading to the for-mation of large aggregates having an irregular shape. This processended 1 day after irradiation with the precipitation of grey silvermetal, whereas the irradiated solution containing the lowest Ag+

concentration was highly stable up to 4 months.

Fig. 4 displays the optical density and peak width of the ≈400 nm

absorption band as a function of AgNO3 concentration. The pres-ence of i-PrOH influences the above parameters in a different waywith respect to t-BuOH (see Fig. 2). In fact, in this case the inten-sity of the maximum absorption seriously decreased by increasing

A. Torreggiani et al. / Colloids and Surfaces A: Phy

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ig. 3. UV–vis spectra of Ar-purged solutions containing different concentrations ofgNO3 (as reported in the figure) and 0.5 M i-PrOH after �-irradiation at natural pH;

otal dose, 100 Gy.

ig. 4. Influence of [AgNO3] on: (a) the absorbance at 408 nm (maximum) and (b) the peakgNO3 solutions at natural pH, in the presence of i-PrOH 0.5 M.

ig. 5. Comparison between UV–vis spectra of (a) Ar-purged and (b) N2O-saturated sbsmax

408 nm = 0.32; (b) Absmax416 nm = 0.03.

sicochem. Eng. Aspects 339 (2009) 60–67 63

the salt concentration, while the corresponding peak width valuesstrongly increased, indicating the coagulation of silver sols and theformation of larger aggregates.

Furthermore, the observed absorbance at ≈400 nm is twice thatobtained at the same Ag+ concentration (in the 1–5 × 10−4 M range)in the presence of t-BuOH (Fig. 2). This behaviour can be explainedby considering the reducing species produced in solution by �-radiolysis. Both in the presence of t-BuOH and i-PrOH, the initiallyformed reducing species, i.e. eaq

− and •H, cause the formation ofsilver atoms via Reactions (6) and (7), while the •OH radicals pro-duce carbon-organic radicals via H atom abstraction from alcohol(Reactions (2) and (4)). The •CH2C(CH3)2OH radical, formed from t-BuOH is not involved in the reduction of silver ions owing to its poorreducing properties. Consequently, in t-BuOH-containing solutionsthe reduction yield of the silver ions should only correspond to thesum of the radiolytic yields of eaq

− (0.27 �mol J−1) and •H atoms(0.062 �mol J−1).

On the contrary, in the presence of i-PrOH the reducing 1-hydroxy-isopropyl radical (CH3)2C•OH, produced by the •OH attack

on i-PrOH, could contribute to the reduction by reacting with theclusters, such as Ag2

+• (E◦(Ag2+•/Ag2) = −0.62 V) [18,35]

Ag2+• + (CH3)2C•OH → Ag2 + (CH3)2CO + H+ (13)

width of the related band, obtained from irradiation at different doses on Ar-purged

olutions containing 1 × 10−4 M AgNO3 and 0.5 M i-PrOH; total dose, 100 Gy. (a)

6 A: Physicochem. Eng. Aspects 339 (2009) 60–67

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4 A. Torreggiani et al. / Colloids and Surfaces

g2Ag+� Ag3

+• Ag+� Ag4

2+ (14)

On the other hand, although the standard potential of this radicalE◦ = −1.39 V) is not so negative to reduce Ag+ (E◦ = −1.8 V) in one-tep electron reduction, it has been suggested to contribute to theeduction of silver ions by a different mechanism, i.e. through theormation of a complex [Ag(CH3)2COH]+•: [35]

Ag+ + (CH3)2•COH → [Ag(CH3)2COH]+

Ag+−→Ag2

+• + (CH3)2CO + H+ (15)

In fact, the reaction between (CH3)2C•OH and 2Ag+ is thermo-ynamically possible since the reduction potential of (CH3)2C•OH

s more negative than that reported for 2Ag+/Ag2+• couple, i.e. −1.2

s. NHE (normal hydrogen electrode).In order to probe if i-PrOH-derived carbon radical may act as

romoter of silver clusters through the silver ion reduction, a N2O-aturated AgNO3 (1 × 10−4 M) and i-PrOH (0.5 M) solution wasrradiated with 100 Gy. The use of N2O completely converts e−

aq

n •OH, giving rise to a system in which the only reducing species ishe (CH3)2C•OH radical. A low, but well-defined absorption in the00 nm spectral region, was still observed (Fig. 5b), thus support-

ng the involvement of hydroxyalkyl radicals in the Ag+ reduction.herefore, in i-PrOH-containing solutions the reduction yield of theilver ions is equal to the yield of the primary radicals eaq

−, •Hnd •OH (0.61 �mol J−1) and this justifies the almost doubled valuef the absorbance maximum observed in i-PrOH-containing solu-ion respect to that in t-BuOH at the same irradiation dose and Ag+

oncentration (Figs. 2 and 4).With respect to the radiolytic reduction mechanism, probably,

he (CH3)2C•OH radical reduces silver ions leading to the formationf Ag2

+• (Reaction (15)) and, subsequently, of the neutral dimerilver, Ag2 (Reaction (13)). This latter will then act as a nucleationentre for the formation of larger clusters (Reaction (14)).

The low absorption intensity at around 400 nm (Fig. 5b), about0% of that obtained in Ar-purged analogous solution (Fig. 5a),s probably due to a low rate constant of reaction (15) and, ofourse to the low concentration of Ag2

+• primarily formed. In fact,he fast radical-radical reaction (16) (2k16 = 2.4 × 109 M−1 s−1) [37]nvolving (CH3)2C•OH competes more favourable respect to botheactions (13) and (15):

(CH3)2•COH → products (16)

The UV–vis absorption spectra of the prepared silver colloidsere studied in more detail by computer fitting analysis of the

esulting bands, in order to find which are most suitable condi-ions attending to their use in SERS spectroscopy. The peak fittings a method whereby a �2 fit of several individual Lorentzian func-ions to an experimental spectrum is carried out. This analysis is ofarticular value in probing the changes in aggregation induced byifferent factors such as metal ions concentration, etc. The peaktting and the spectral components of the three metal colloidsossessing the best conditions attending their SERS activity areeported in Fig. 6. The UV–vis spectrum of the metallic colloidaluspension obtained by using an Ag+ concentration of 2 × 10−4 M in-BuOH and 100 Gy radiation dose (Fig. 6a) and in i-PrOH and 50 GyFig. 6b) exhibited three and two band components (from 368 to70 nm), respectively. On the contrary, the use of i-PrOH, a minortarting Ag+ concentration (1 × 10−4 M) and a dose of 100 Gy gave

ise to an UV/vis spectrum fitted only by one narrow and symmetriceak at 408 nm (Fig. 6c).

These last conditions seem to be the best to generate metallusters suitable for SERS. In fact, the presence of only one com-onent is indicative of an uniform silver aggregation, whereas

peak fitting routine. UV–vis spectra after �-irradiation of Ar-purged solutions atnatural pH containing: (a) 2 × 10−4 M AgNO3 and 0.5 M t-BuOH, total dose 100 Gy;(b) 2 × 10−4 M AgNO3 and 0.5 M i-PrOH, total dose 50 Gy; (c) 1 × 10−4 M AgNO3 and0.5 M i-PrOH, total dose 100 Gy.

the peak position around 410 nm of a relatively low size dimen-sion of the nanoparticles. In fact, metal particles absorbing at400–430 nm have been reported to have a size range of 10–20 nm[18,38].

3.2. Scanning electron microscopy

The SEM micrographs of Fig. 7 correspond to those obtained byimmobilisation of nanoparticles produced by �-irradiation (50 Gy)of Ar-purged solution containing 2 × 10−4 M AgNO3 and 0.5 M i-PrOH. These micrographs reveal the existence of nanoparticles with

different sizes in good agreement with the curve fitting showed forthis colloid in Fig. 6b, as well as aggregates with several microns ofsize integrated by smaller particles bearing sizes ranging from 50to 500 nm, resulting from the subsequent aggregation of nanopar-ticles upon immobilisation on the glass substrate.

A. Torreggiani et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 339 (2009) 60–67 65

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ig. 7. SEM micrographs with different amplifications of aggregates obtained afterrradiation on a glass substrate.

.3. Surface-enhanced Raman scattering spectra

In order to test the suitability of the irradiated Ag nanopartil-

es in SERS spectroscopy, some SERS experiments were carried outn the Ag colloid which showed to have the highest morphologi-al uniformity, i.e. the colloid obtained by irradiating (100 Gy) anr-purged solution containing 1 × 10−4 M Ag+ and i-PrOH. Fig. 8a

ig. 8. SERS spectra obtained from (a) the �-irradiated Ag colloid (1 × 10−4 M AgNO3,.5 M i-PrOH, 100 Gy of dose) and (b) conventional citrate Ag colloid.

bilizing the Ag colloid (2 × 10−4 M AgNO3 and 0.5 M i-PrOH) obtained after 50 Gy of

shows the control SERS spectrum obtained from this colloid as com-pared to the background spectrum obtained for the conventionalcitrate colloid (Fig. 8b). The latter spectrum is clearly dominated bybands corresponding to citrate and nitrate actually adsorbed on themetal surface [6,7], while the background spectrum of the colloidprepared by irradiation shows weak features corresponding to thesolvent water + i-PrOH, which are non-adsorbed species.

Fig. 9 shows the micro-SERS spectra obtained for the fungi-cide thiram by focusing the laser on three different aggregatesdisplayed in the inset optical micrograph. As can be seen in thisinset figure, the aggregation of Ag nanoparticles upon immobilisa-tion leads to the formation of chain-like aggregates with severalmicrons of length. The measured SERS intensity varies depend-ing on the aggregate where the SERS spectrum is collected dueto the different efficiency of each aggregate morphology in thesurface-enhancement of the electric field (Fig. 9). However, theSERS spectra display the typical Raman bands resulting from theadsorption of thiram on the surface [23,24]. In fact, this moleculeundergoes a breakdown in the S–S bond leading to the formation oftwo dimethyldithiocarbamate (DTC) fragments responsible for theRaman bands observed in the spectra according to the reaction ofFig. 9 placed in the top.

The most intense SERS thiram band can be seen at 1381 cm−1,and corresponds to the ıs(CH3) vibration, which is very much inten-sified due to the perpendicular orientation of DTC ion on the surface,as deduced from the SERS selection rules [4]. The thiram breakdownand the interaction of DTC ion with the metal leads to the formationof the thioureide form (see inset of Fig. 10) which is characterisedby a new band at above 1500 cm−1, not seen in the spectrum ofthe solid DTC. Moreover, the formation of a Ag–S bond leads to theappearance of a band at 185 cm−1 (Fig. 10a). The bands at 1145 and

945 cm−1 are assigned to C–S stretching mode in the DTC ion.

It is known that both the intensity and position of all thesebands can change when the DTC is adsorbed on the metal followinga bi-dentate or monodentate configuration (Fig. 10, inset). On theother hand, the interaction mechanism can change depending on

66 A. Torreggiani et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 339 (2009) 60–67

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ig. 9. Raman spectrum of thiram in solid state (a) and micro-SERS spectra of thiraop: reaction scheme of the breakdown of the S–S bond leading to the formation of

he adsorbate-to-metal (A/M) surface ratio. For high A/M values the

nteraction tends to be monodentate, due to the large amount ofTC ions on the surface. In contrast, at lower ratios the interaction

s rather bidentate. The SERS spectrum of thiram obtained on-irradiated colloids undergoes significant changes depending onhe fungicide concentration, analogously to that observed by using

ig. 10. Micro-SERS spectra of thiram on immobilized �-irradiated Ag colloids1 × 10−4 M AgNO3, 0.5 M i-PrOH, 100 Gy of dose) before (a) and after washing (bnd c) the sample with ethanol.

istered from different Ag aggregates as shown the inset optical micrograph (b–d).TC fragments responsible for the Raman bands observed in the spectra.

citrate conventional colloids. Fig. 10 displays the SERS spectra ofthiram on immobilised silver colloids before and after washingthe sample with ethanol. The effect of ethanol was double, givingrise both to an intensity decrease of the signal and a change in therelative intensity and position of bands. The weakness of the SERSsignal after washing the sample is due to the removing of adsorbateafter washing with the solvent. Moreover, the low position of the�(C N) band at 1505 cm−1, the weakness of the ıas(CH3) band at1443 cm−1, and the prominent �(C S) band at 1145 cm−1 observedafter washing (Fig. 10b) suggest a low surface coverage of theAg surface by DTC adsorbed on the surface with a change in theinteraction mechanism on the metal from monodentate to biden-tate [23] as indicated in the inset of Fig. 10. This monodentate tobidentate transition has been observed in Ag colloidal suspensionswhen lowering the fungicide concentration to 10−7 M and has beenattributed to a reorganisation of the adsorbate on the surface atlower concentrations at which a bidentate interaction is possibledue to the higher available space on the metal. Hence, we estimatethat the coverage on the Ag film is similar that that obtained forcolloidal systems at the latter concentration. In conclusion, the useof �-irradiated Ag nanoparticles can allow the study and detectionof the fungicide molecule at so low concentration without theinterference of impurities in the medium in contrast to whathappens on citrate conventional colloids [23].

4. Conclusions

Ag nanoparticles suitable for application in SERS spectroscopywere successfully prepared by �-radiolysis of Ag+ aqueous solutioncontaining t-BuOH or i-PrOH. The absorbance maxima and the size

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A. Torreggiani et al. / Colloids and Surfaces

istribution of particles depend on factors such as irradiation dose,g+ concentration and nature of alcohol employed as scavenger ofOH radicals. Thus, the control of all these experimental conditionsllow to obtain a metal suspension with a controlled mean size. Onef the main advantages of this Ag nanoparticle preparation methods the fact that minimum hindrance from impurities is observedn the SERS spectra. This is an important issue in SERS technique,ince many times the presence of impurities coming from otherpecies present in the medium avoids a correct interpretation ofhe results. The use of relatively low doses in comparison to thosemployed by other authors allowed us to obtain very stable sus-ensions of Ag nanoparticles without adding any colloid stabiliser,hich may introduce further spurious bands in the Raman spectra.

n the presence of i-PrOH an enhancing in Ag reduction is induced byhe reducing ability of the related alcoholic radical, increasing theverall metal reduction efficiency (of the metal reduction). The bestxperimental conditions for radiolitically generation of Ag nanopar-icles with a controlled mean size can be envisaged when a dose of00 Gy and at a metal concentration of 10−4 M are used. The combi-ation of microscopy and SERS allowed to develop a sensitive andore spatially resolved SERS-based method to detect the fungicide

hiram and to carry out studies, even at low concentrations, with-ut the overlapping of impurities normally existent in conventionalolloidal suspensions of Ag nanoparticles.

In conclusion, the present study shows how colloid chemistrynd free-radical chemistry may be combined in an unprecedentedanner giving rise to colloidal metals useful in SERS spectroscopy.

cknowledgements

This work was supported by the bilateral Agreement betweenonsiglio Nazionale delle Ricerche (CNR—Italy) and Consejo Supe-ior de Investigaciones Científicas (CSIC—Spain), by Direccióneneral de Investigación (Ministerio de Educación y Ciencia) projectumber FIS2007-63065, and Comunidad Autonoma de Madridroject number S-0505/TIC/0191 MICROSERES.

eferences

[1] R. Aroca, Surface-enhanced Vibrational Spectroscopy, John Wiley & Sons, Chich-ester, 2006.

[2] D. Jancura, S. Sanchez-Cortes, E. Kocisova, P. Miskovsky, A. Tinti, A. Bertoluzza,Biospectroscopy 1 (1995) 265.

[

[[[

sicochem. Eng. Aspects 339 (2009) 60–67 67

[3] M. Moskovits, M.J. Raman, Spectroscopy 36 (2005) 485.[4] M. Moskovits, Rev. Mod. Phys. 57 (1985) 783.[5] A. Wokaun, Mol. Phys. 56 (1985) 1.[6] S. Sanchez-Cortes, J.V. García-Ramos, J. Raman, Spectroscopy 29 (1998) 365.[7] M.V. Canamares, J.V. García-Ramos, J.D. Gómez-Varga, C. Domingo, S. Sanchez-

Cortes, Langmuir 21 (2005) 8546.[8] Y.P. Sun, J.E. Riggs, H.W. Rollins, R. Guduru, J. Phys. Chem. B 103 (1999) 77.[9] P.V. Kamat, J. Phys. Chem. B 106 (2002) 7729.10] G. Schmid, Clusters and Colloids: From Theory to Application, VCH, Weinlein,

1994.[11] J.W. Slot, H.J. Geuze, J. Cell Biol. 38 (1983) 87.12] J. Belloni, M. Mostafavi, J.L. Marignier, J. Amblard, J. Imaging Sci. 35 (1991)

68.13] A. Henglein, Chem. Rev. 89 (1989) 1861.14] J. Belloni, J. Radiat. Res. 150 (1998) 59.15] S.-H. Choi, S.-H. Lee, Y.-M. Hwang, K.-P. Lee, H.-D. Kang, Radiat. Phys. Chem. 67

(2003) 517.16] S.-H. Choi, K.-P. Zhang, A. Gopalan, K.-P. Lee, H.-D. Kang, Colloids Surf. A: Physic-

ochem. Eng. Aspects 256 (2005) 165.[17] A. Henglein, Ber. Bursenges. Phys. Chem. 81 (1977) 556.18] R. Tausch-Treml, A. Henglein, J. Lilie, Ber. Bunsenges. Phys. Chem. 82 (1978)

1335.19] A. Henglein, J. Phys. Chem. 83 (1979) 2209.20] D.A. Troiskkii, N.L. Sukhov, B.G. Ershov, A.G. Gordeev, Radiat. Chem. 28 (1994)

218.21] S.-H. Choi, H.G. Park, Appl. Surf. Sci. 243 (2005) 76.22] A. Debarre, R. Jaffiol, C. Julien, P. Tchénio, M. Mostafavi, Chem. Phys. Lett. 386

(2004) 244.23] S. Sanchez-Cortes, M. Vasina, O. Francioso, J.V. Garcia-Ramos, Vib. Spectrosc. 17

(1998) 133.24] S. Sanchez-Cortes, C. Domingo, J.V. Garcia-Ramos, J.A. Aznarez, Langmuir 17

(2001) 1157.25] J.W.T. Spinks, R.J. Woods, An Introduction to Radiation Chemistry, 3rd ed., Wiley,

New York, 1990.26] G.V. Buxton, C.L. Greentock, W.P. Helman, A.B. Ross, J. Phys. Chem. Ref. Data 17

(1988) 513 (and references therein).27] M.S. Alam, B.S.M. Rao, E. Jonata, Phys. Chem. Chem. Phys. 3 (2001) 2622.28] J.F. Endicott, in: A.W. Adamson, P.D. Fleischauer (Eds.), Concepts of Inorganic

Photochemistry, Wiley, New York, NY, 1975, pp. 81–142.29] H.A. Schwarz, R.W. Dodson, J. Phys. Chem. 93 (1989) 409.30] G. Fabriciova, S. Sanchez-Cortes, J.V. Garcia-Ramos, P. Miskovsky, J. Raman Spec-

trosc. 35 (2004) 384.31] D.A. Schultz, Curr. Opin. Biotechnol. 14 (2003) 13.32] S.M. Heard, F. Grieser, C.G. Barraclough, J.V. Sanders, J. Colloid Interf. Sci. 93

(1983) 545.33] M. Mostafavi, J.L. Marignier, J. Amblard, J. Belloni, Radiat. Phys. Chem. 34 (1989)

605.34] S. Sanchez-Cortes, J.V. Garcia-Ramos, G. Morcillo, A. Tinti, J. Colloid Interf. Sci.

175 (1995) 358.

35] H. Remita, I. Lampre, M. Mostafavi, E. Balanzat, S. Bouffard, Radiat. Phys. Chem.

72 (2005) 575.36] G.R. Dey, K. Kishore, Radiat. Phys. Chem. 72 (2005) 565.37] M.S. Alam, B.S.M. Rao, E. Janata, Radiat. Phys. Chem. 67 (2003) 723.38] Gmelins Handbuch der anorganischen Chemie, Verlag Chemie, Weinheim, vol.

61 A3, 1971, p. 210.