INTRODUCTION. Particles with the core/metal nanoshell structure are of increasing interest to researchers due to their unique properties, mostly to their optical ...
ISSN 1061933X, Colloid Journal, 2011, Vol. 73, No. 3, pp. 340–344. © Pleiades Publishing, Ltd., 2011. Original Russian Text © M.E. Kartseva, O.V. Dement’eva, M.A. Filippenko, V.M. Rudoy, 2011, published in Kolloidnyi Zhurnal, 2011, Vol. 73, No. 3, pp. 334–339.
Anisotropic Particles with Different Morphologies of Silver Nanoshell: Synthesis and Optical Properties M. E. Kartseva, O. V. Dement’eva, M. A. Filippenko, and V. M. Rudoy Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russia Received August 3, 2010
Abstract—Hydrosols of spindleshaped composite particles with the core of iron(III) oxyhydroxide and sil ver shell are synthesized by enlarging metal seeding nanoparticles adsorbed on the surface of the cores in the solution containing silver nitrate and mild reducing agent. It is revealed that the character of the growth of shells on gold seeding particles greatly depends on the type of reducing agent. When using ascorbic acid, seed ing particles grow primarily in the direction normal to the core surface due to the blocking of some of the par ticle faces by ions present in the solution. As a result, the forming shell is characterized by a fairly nonuniform structure. At the same time, when using formaldehyde, the growth of seeding nanoparticles proceeds pre dominantly in the lateral direction to form first an islandlike film, then a continuous thin metal shell on the core surface. It is demonstrated that the position of localized surface plasmon resonance of such structure can be fine tuned to the preset wavelength by controlled variations in the thickness of Ag shell with very small step (up to 1 nm). DOI: 10.1134/S1061933X11030045
INTRODUCTION Particles with the core/metal nanoshell structure are of increasing interest to researchers due to their unique properties, mostly to their optical properties, which broaden a wide range of applications [1, 2]. A distinctive feature of these composite particles is the possibility of controlling the position of localized sur face plasmon resonance (LSPR) by changing the shape of the core, the ratio between the core size and shell thickness, and the nature of the shell. An analysis of published data demonstrated that the majority of them describe structures composed of the spherical dielectric core and the shell of precious metal, mostly gold [3–5]. At the same time, particles with silver shells that possess a substantially larger absorption cross section have drawn much less atten tion of researchers, since the formation of this shell is a more complex process and is sensitive to the action of external factors [6–9]. Recently, interest has developed regarding the use of anisotropic particles as cores, which can be explained rather simply; i.e., this makes it possible to substantially widen the range of LSPR tuning based on the shape of the particle [1, 10]. This possibility was demonstrated experimentally in [10] by synthesizing hydrosols of socalled nanorice, i.e., composite parti cles with spindleshaped cores of iron(III) oxyhydrox ide and gold shell.According to the authors of [1, 10, 11], the optical properties of similar plasmon struc tures, particularly nanorice, can be represented as a result of the hybridization of the plasmons of metal
nanorod and nanocavity in the form of an ellipsoid inside of a solid metallic volume. Works devoted to the synthesis of silver shell on the spindleshaped cores of oxide or iron(III) oxyhydrox ide, FeOOH, have been published in recent years [12, 13]. The algorithm used in [12] included the following stages: (1) the synthesis of oxide cores with preset sizes, (2) the formation of thin SiO2 shells on these cores followed by the activation of shell surfaces with SnCl2, and (3) the fabrication of the metal shell by chemical precipitation. However, regardless of the fact that the data of electron microscopy are evidence of the formation of continuous (although not too uni form) Ag shell on particles, the spectral characteristics of the thus prepared structures leave much to be desired. The same can be said about the composite FeOOH/Ag particles synthesized in [13] by the seeded growth of silver nanoparticles adsorbed on the surface of anisotropic FeOOH cores modified with poly(eth ylene amine) layer. In this work, we studied the possibility of the syn thesis of composite particles with anisotropic core of iron oxyhydroxide and uniform silver shell, which makes it possible to fine tune the position of the LSPR in the range of 500–1100 nm. These particles are undoubtedly interesting from the viewpoint of their use in medicine, particularly as sensitizers in pulse laser photothermal therapy of malignant tumors.
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EXPERIMENTAL Materials and Methods The following reagents: FeCl3 ⋅ 6H2O (99.9%), 3aminopropyltriethoxysilane (APTES, 99%), aque ous 80% solution of tetrakis(hydroxymethyl)phospho nium chloride, chloroauric acid trihydrate HAuCl4 ⋅ 3H2O, AgNO3, aqueous 37% formaldehyde solution, ascorbic acid, sodium citrate (Na3C6H5O7 ⋅ 2H2O, > 99%), aqueous 1 M NaOH solution, 28% ammonium hydroxide solution (99.9%), absolute ethanol (all reagents from ACS Reagent, Aldrich), and deionized water (Arium 611, Sartorius AG, Germany) with a conductivity of no more than 0.06 µS/sm were used. Particles were precipitated using a 320R centrifuge (Hettich, Germany). A Sapfir ultrasonic bath was used for particle redispersion. The optical properties of the composite particle were studied with an Evolution 300 doublebeam scanning spectrophotometer (Thermo Electron Corp., UK) in the wavelength range of 250–1100 nm. The sizes and morphology of particles were studied using transmission electron microscopy (TEM) after precipitation from dispersion onto a copper grid coated with carbon. Studies were performed with an EM 300 electron microscope (Philips, The Nether lands) with an accelerating voltage of 160 kV. The proposed procedure for the synthesis of aniso tropic composite FeOOH/Ag particles consists of the following main stages: the synthesis of FeOOH cores, their modification with aminosilane (APTES), the subsequent adsorption of seeding gold nanoparticles, and their enlargement in the solution of mild reducing agent up to the formation of continuous silver shell. Synthesis of SpindleShaped Cores of Iron Oxyhydroxide To synthesize particlescores made of iron(III) oxyhydroxide, we used the modified procedure [14]. The weighed sample (0.5 g) of FeCl3 ⋅ 6H2O was dis solved in 50 ml of deionized water and left to stand in a closed flask at 60°C for 6 h, then rapidly cooled with ice and stored at 4°C. According to TEM data, the average length of synthesized particles was ≈195 nm; maximal diameter was ≈ 50 nm (see Fig. 1a). Surface Modification of Iron Oxyhydroxide Particles with APTES Prior to the modification, it is necessary to transfer FeOOH cores from aqueous to alcohol medium in order to avoid the APTES polymerization in the reac tion system. For this purpose, the hydrosol was centri fuged (10 000 rpm, 15 min) and particles were redis persed in ethanol; this procedure was repeated three times. APTES (25 µl) was added (upon stirring) to10 ml of the thus prepared alcosol. Stirring was continued for COLLOID JOURNAL
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Fig. 1. Fig. 1. Electron micrographs of FeOOH cores: (a) initial and (b) cores with adsorbed seeding gold nano particles.
17 h; afterwards, the system was heated at 65°C for 1 h. FeOOH particles modified with APTES (FeOOH NH2) were separated from the excess of reagents and products by repeated centrifugation/redispersion in absolute ethanol and, at the final stage, were redis persed in deionized water. Synthesis of Seeding Gold Nanoparticles with a Diameter of 1–3 nm Seeding gold nanoparticles were synthesized using the standard procedure [15]. 0.2 M NaOH solution (1.5 ml), tetrakis(hydroxymethyl)phosphonium chlo ride solution with concentration of 9.6 mg/ml (1 ml), and, after 5 min, aqueous 1% HAuCl4 solution (2 ml) were added under stirring to 45.5 ml of deionized water. Reaction mixture immediately became dark brown in color that was evidenced of the formation of ultrafine gold particles (this was also confirmed by the absence of the LSPR band in the absorption spectrum of hydrosol).
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Adsorption of Seeding Gold Nanoparticles on Modified FeOOH Cores After treating gold sol (15 ml) for 2 min in ultra sonic bath, 1.5 ml of 1 M NaCl solution and 1 ml of aminated FeOOH sol were added to the gold sol. The thus prepared colloidal solution was sonicated for another 2 min and left to stand for 3–7 days at 4°C. Afterwards, the dispersion was centrifuged and washed several times with distilled water to remove unad sorbed gold nanoparticles. At the final stage, FeOOH–NH2/Au particles were redispersed in 1 ml of deionized water. TEM images of such particles are shown in Fig. 1b. Note that, according to data of [10], under these con ditions, the degree of the coverage of aminated FeOOH cores with seeding gold nanoparticles is equal to approximately 30%. Formation of Silver Shell Silver shells on FeOOH–NH2/Au particles were obtained by two procedures, i.e., one using ascorbic acid and another using formaldehyde, as reducing agents. Procedure 1. Equal volumes of AgNO3 and ascorbic acid aqueous solutions with concentrations of 5.6 and 2.8 mg/ml, respectively, were added consecutively to FeOOH–NH2/Au hydrozol in 0.1% sodium citrate solution. The process was performed under gentle stir ring for 10 min. Procedure 2. 0.15 mM silver nitrate solution was added to hydrosol containing FeOOH–NH2/Au parti cles and then aqueous 37% formaldehyde solution and 28% NaOH solution were added consecutively under constant stirring. The volume of ammonium hydrox ide solution was chosen so to maintain the pH value in
the 10–11 pH range, since the reduction of silver ions occurs precisely under these conditions on the surface of seeding particles rather than in the bulk of colloidal solution [6]. All reagents were mixed at 50°C; then, heating was stopped and stirring was continued over 10 min. The ratio between the amounts of reagents was determined by the required (calculated) thickness of silver shell, hcalc. RESULTS AND DISCUSSION At first, we used ascorbic acid as a mild reducing agent on the stage of the formation of a silver shell. The extinction spectra of hydrosols of the thus synthesized FeOOH/Ag composite particles with different hcalc values of the shell are presented in Fig. 2a. It can be seen that, beginning with a certain hcalc value, a very broad peak with a maximum in the 700–850nm range appears in the spectra. However, as the shell thickness increases, it turns out that this peak changed in a non trivial manner. Contrary to expectations, when pass ing from hcalc = 20 nm to hcalc = 40 nm, the position of this maximum shifted to the red region, from approx imately 720 to 830 nm. At hcalc ≤ 10 nm, no distinct LSPR peaks appeared at all in the visible or near IR regions. This behavior of spectra can be explained by means of the electron microscopy data on the morphology of silver shells formed using ascorbic acid. A characteris tic TEM image of the FeOOH/Ag composite particle with hcalc = 20 nm synthesized by this procedure is shown in Fig. 3a; here, it can be seen that the shell is very nonuniform. In the first approximation, the structure of the shell can be considered to be a two dimensional aggregate composed of fairly large silver particles, some of which acquire faceting. Moreover, by the TEM data, it can be said that these particles COLLOID JOURNAL
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Fig. 3. Electron micrographs of FeOOH/Ag composite particles synthesized with the use of (a) ascorbic acid and (b) formalde hyde as reducing agents.
grow predominantly in the direction normal to the core surface. Apparently, the formation of this defect shell from (interacting) silver nanoparticles is respon sible for both the form of extinction spectra repre sented in Fig. 2a and for their behavior with an increase in shell hcalc. Indeed, as the nominal value of nanoshell thickness increases, the sizes of comprising particles and the degree of their electromagnetic bind ing (the degree of aggregation) should also rise that leads precisely to the batochromic shift of LSPR max imum. The aforementioned anisotropy in the growth of silver particles comprising the shell is also con firmed by the fact that, at fairly large hcalc values, the shape of composite particle as a whole approaches to spherical. Note that the analogous anisotropy in the growth of seeding metal nanoparticles resulted in the formation of morphologically very nonuniform silver shells was observed by us previously when using spher ical particles of polystyrene latex as cores [8]. The dendritelike silver nanoparticles, whose LSPR maximum lies in the 500–900 nm range, are also formed during their synthesis (both seedless and seeded) in the bulk of aqueous solution using ascorbic acid as a reducing agent of Ag+ ions [16–18] in the presence of sodium citrate [17, 18]. According to [17, 18], this effect is related primarily to the predominant adsorption of citrate ions of the faces of growing silver nanoparticles. We cannot also exclude the blocking of some faces by the anions of ascorbic acid and/or by the products of its oxidation. Moreover, the position and form of the resonance band greatly depend on the size and degree of branching of nanoparticles. Evidently, the optical properties of the FeOOH/Ag particles synthesized using ascorbic acid do not satisfy our requirements with regard to tuning the LSPR position to the preset wavelength. However, these COLLOID JOURNAL
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structures can be used as fairly efficient substrates in surfaceenhanced Raman spectroscopy, as was indi cated in [13]. Fundamentally different situation is observed upon the formation of FeOOH silver nanoshells with the use of formaldehyde as a reducing agent of Ag+ ions. First, in this case, two LSPR peaks are recorded in the visible or near IR regions of the spectrum (see Fig. 2b). These peaks seemingly correspond to longitudinal and trans versal plasmon resonance, which are characteristic for elongated particles. Second, as follows from Fig. 2b, for composite particles synthesized using formalde hyde, the regular batochromic shift of both LSPR bands takes place with a decrease in the thickness of silver shell. Analogous spectral changes were observed in [10] upon the synthesis of gold shells on spindle shaped cores of iron(III) oxide (hematite). This char acter of spectra evidences the formation of fairly uni form metal shells on anisotropic FeOOH cores throughout the studied thickness range. Actually, according to the analysis of the TEM data, silver shells synthesized with the use of formalde hyde are continuous and fairly smooth, even at small thicknesses. The microphotograph of composite parti cles with hcalc = 5 nm is shown in Fig. 3b as an example. Apparently, when using formaldehyde, the growth of silver shell catalyzed by the surface of seeding gold nanoparticles proceeds predominantly in the lateral direction to first form an islandlike film, then a con tinuous thin metal shell on the surface of oxide core. Thus, the thickness of the shell can be controllably varied with very small step (up to 1 nm). This makes it possible to rather fine tune the position of LSPR of composite nanoparticles (first, longitudinal plasmon resonance) to the preset wavelength.
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The possibility of this tuning and fairly high inten sity of the peak of longitudinal LSPR in the near IR region (700–1100 nm), i.e., in the window of the max imal transparency of the tissues of living organism, testifies to the promising application of anisotropic FeOOH/Ag particles synthesized using formaldehyde in the diagnostics and treatment of tumors. The point is that the relative contributions of scattering and absorption to the extinction spectra of these particles can be changed by varying their geometry (core sizes and shell thickness). As a result, they can act as con trasting agents (e.g., in optical coherent tomography) and thermosensitizers of laser hyperthermia, new promising method of the therapy of malignant tumors. The fact that, as was demonstrated by preliminary pharmacokinetic study, such particles are capable of prolonged circulation in bloodstreem of tumorbear ing mice without aggregation and can gradually selec tively accumulate in the tumor [19]. In connection with this, we emphasize that poly ethylene glycol modified spherical particles with the SiO2 core and gold shell already demonstrated their efficacy in vivo experiments as sensitizers of laser hyperthermia [20, 21]. Most recently, it was demon strated that analogous structures with silver shells exhibit high thermosensitizing activity in the pulse laser hyperthermia of tumors [22]. ACKNOWLEDGMENTS We are grateful to S.A. Pisarev and V.V. Matveev for their help in electron microscopy study. This work was supported by the Moscow government within the framework of scientifictechnical program “Develop ment and Practical Realization in Public Health Ser vices of New Methods and Means of Preventive Main tenance, Diagnostics, and Treatment of Oncological, Infectious, and Other Dangerous”. REFRERENCES 1. Wang, H., Brandl, D.W., Nordlander, P., and Halas, N.J., Acc. Chem. Res., 2007, vol. 40, p. 53. 2. Brinson, B.E., Lassiter, J.B., Levin, C.S., et al., Lang muir, 2008, vol. 24, p. 14166.
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