J Nanopart Res (2010) 12:337–345 DOI 10.1007/s11051-009-9620-3
RESEARCH PAPER
From silver nanoparticles to nanostructures through matrix chemistry Omar Ayyad Æ David Mun˜oz-Rojas Æ Judith Oro´-Sole´ Æ Pedro Go´mez-Romero
Received: 29 October 2008 / Accepted: 1 March 2009 / Published online: 19 March 2009 Ó Springer Science+Business Media B.V. 2009
Abstract Direct in situ reduction of silver ions by a biopolymer such as agar, without any other reducing nor capping agent is shown in this article to lead either to nanoparticles (typically 12(2) nm in an optimized case) or to more complex nanostructures depending on the reaction conditions used. This approach takes advantage of the porous polymer lattice acting as a template and leads to hybrid Ag–Agar materials with long-term synergic stability. Silver acts as an antibacterial agent for agar whereas the biopolymer prevents agglomeration of the inorganic nanoparticles leading to a stable nanocomposite formed by a thermoreversible biopolymer from which silver nanoparticles can eventually be recovered. Keywords Agar gel Silver nanoparticles Hybrid nanostructures
Electronic supplementary material The online version of this article (doi:10.1007/s11051-009-9620-3) contains supplementary material, which is available to authorized users. O. Ayyad D. Mun˜oz-Rojas P. Go´mez-Romero (&) Centro de Investigacio´n en Nanociencia y Nanotecnologı´a (CIN2), CSIC-ICN, Campus de la U.A.B., Bellaterra 08193, Spain e-mail:
[email protected] J. Oro´-Sole´ Instituto de Ciencia de Materiales de Barcelona ICMAB (CSIC), Campus de la U.A.B., Bellaterra 08193, Spain
Introduction Controlling the size and shape of metal nanoparticles is the main declared goal of a growing number of papers in the recent literature (Murray et al. 2000; Sun and Xia 2002; Jin et al. 2003; Kim et al. 2003; Wiley et al. 2005; Qu et al. 2006; Xiong and Xia 2007; Park et al. 2007; Mott et al. 2007; Habas et al. 2007; Zhang et al. 2007; Xue et al. 2005; Ganesan et al. 2007; Huber et al. 2007; Lim et al. 2007; Wiley et al. 2007). The interest behind this area of research is multiple, from fundamental questions related with the unique properties of nanoparticulate matter (Rao et al. 2004) to practical approaches applied to many different fields from catalysis (Narayanan and El-Sayed 2004; Daniel and Astruc 2004; Takagi et al. 2006) to bioactivity (Elechiguerra et al. 2005; Salata 2004; Morones et al. 2005). In this context, different procedures have been devised for the preparation of monodisperse nanoparticles in solution, with silver or gold as paradigmatic examples (Sun and Xia 2002; Wiley et al. 2005; Wiley et al. 2007), in most cases involving the use of capping agents. Also, the shape of the nanoparticles grown can be controlled through the control of reaction conditions. But in addition to reproducible procedures, the field could benefit from new approaches which could open the way to the design of new and more complex nanostructured materials.
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In our laboratory, we are developing one such general approach in the form of what we call matrix chemistry (Go´mez-Romero 2001), that is, the use of solid matrices, normally polymeric, to carry out the synthesis and growth of materials, an approach that has led to interesting Ag@PPy nanostructures (Mun˜oz-Rojas et al. 2008a, b). In contrast with solution chemistry, with freely diffusing reacting ions, a polymeric matrix provides a constrained medium with controllable functionalities which allow for both thermodynamic as well as kinetic control of the resulting products. Nucleation and growth of metal nanoparticles is a specific type of reaction, particularly well suited for this approach. In addition to the matrix chemistry control, the trapping of molecular species (TorresGo´mez and Go´mez-Romero 1998, Go´mez-Romero and Torres-Go´mez 2000), clusters (Go´mez-Romero and Lira-Cantu´, 1997, Lira-Cantu´ and Go´mezRomero 1998), or nanoparticles within the polymer network provides a way to stabilize and hold them in a convenient support (Zhu et al. 1998; Yin et al. 1998; Chen et al. 1998; Dirix et al. 1999; Akamatsu et al. 2000; Mbhele et al. 2003; He et al. 2003; Huang and Yang 2004; Porel et al. 2005a, b; Hornebecq et al. 2003; Dong et al. 2007; Radziuk et al. 2007). Furthermore, the type of polymer to be used is open going from dense polymers (Walker et al. 2001; Zhang and Han 2003; Singh and Khanna 2007; Deshmukh and Composto 2007; Costanzo and Beyer 2007; Oliveira et al. 2005; Oliveira et al. 2006) to highly porous gels (Kattumuri et al. 2006; Mohan et al. 2006; Muthuswamy et al. 2007; Wang et al. 2007; Mohan et al. 2007), thus providing nanocavities of different sizes. Indeed, several recent papers have reported very interesting work on this line, with silver or gold nanoparticles dispersed in different polymers (Walker et al. 2001; Zhang and Han 2003; Singh and Khanna 2007; Deshmukh and Composto 2007; Costanzo and Beyer 2007; Oliveira et al. 2005; Oliveira et al. 2006; Kattumuri et al. 2006; Mohan et al. 2006; Muthuswamy et al. 2007; Wang et al. 2007; Mohan et al. 2007). Our goals go beyond the mere production of nanoparticles, though, and aim at harnessing the microstructural complexity of the matrices for the growth and control of structures at all levels from the nano to the macro scales. Here, we report the synthesis of silver nanoparticles and nanostructures grown in a highly porous agar
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gel, a biocompatible polysaccharide widely used in electrophoresis applications (Akerman 1999; Kusukawa et al. 1999; Cole and Tellez 2002) which constitutes an inexpensive, environment-friendly medium for this purpose. Despite the large number of papers reporting silver or gold nanoparticles, this is, to the best of our knowledge, a first attempt to systematically search for and harness the complex structure of the polymer itself for the development of more elaborate nanostructures.
Experimental part Materials Silver nitrate (99?%) and agar powder were purchased from Aldrich and were used as received without any further purification. Deionized water was used for preparation of all silver nitrate solutions, agar gel, and organic-inorganic matrices. In situ preparation of silver nanoparticles in the organic matrix The preparation of silver nanoparticles in the organic gel was achieved by three approaches referred to as method A, method B, and method C throughout the text. In method A, 0.6 and 1 g agar powder (3 and 5% w/v, respectively) were dispersed in 20 ml aqueous silver nitrate solution (0.1 mM–0.5 M). Each mixture was heated over a hot plate in a Pyrex beaker until a temperature of 90–95 °C was reached, and then stirred for 5 min at that temperature. The gel matrix was formed upon cooling down to room temperature. In method B, 6% w/v agar solution was prepared as in method A by dispersing 1.2 g agar powder in 20 ml deionized water rather than silver nitrate solution and heating to 90–95 °C for 5 min. The heating was stopped by removing the suspension from the hot plate and, with continuing stirring, 20 ml of a AgNO3 solution (at room temperature) of various concentrations were then added directly to the hot agar solution, the mixture was stirred for 40 s and then left to cool down to room temperature. The final organic-inorganic matrix had a 3% w/v agar concentration. Method C (impregnation) used 5% w/v pure agar gel that was prepared in deionized water as described in method B, letting it cool down and
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become a gel. The formed gel was then soaked in 10 ml of silver nitrate solutions of different concentrations (0.1 mM–0.5 M) and left in the refrigerator (*7 °C) for 30 days before characterization. It should be noted that all the three methods lead to hybrid Ag–Agar gels which are stable for many months when kept from drying out in open air. Contrary to pristine agar gels, Ag–Agar hybrid gels kept in plastic bags at room temperature did not suffer mold or bacterial growth even after 1 year.
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nanostructure as those of the dried samples is observed. HRTEM images were captured using JEOL JEM 2011 electron microscope, operating at 200 kV.
Results and discussion
UV-vis spectroscopic measurements were performed using a Cary 5 (Varian) spectrophotometer, the samples were characterized 24 h after preparation. For UV/Vis spectra comparison, the same thickness of different silver–agar gel disks with an optical path length of 3.5 mm was prepared in a mold for this purpose. The TEM images were taken on a JOEL1210 transmission electron microscope operated at an acceleration voltage of 120 keV. For TEM studies, samples were dried at 50 °C and ground, dispersed in absolute ethanol, and a drop was then put onto a conventional Cu grid with carbon membrane and allowed to evaporate slowly under ambient conditions. An ultra narrow window Energy Dispersive Xray (EDX) spectrometer attached to the HRTEM was used to determine the chemical identity of the particles. The silver nanoparticles size distribution was analyzed and size histograms were made based on TEM images using DigitalMicrograph software. To rule out a possible gel nanostructure modification during sample preparation for TEM, several samples were also prepared by cutting a very thin film from the gel which was placed on an Al grid and left to dry at room temperature. Supporting information Fig. S1 shows one of such samples in which the same type of
The formation of the gel was carried out following conventional procedures by heating agar powders in water at 90–95 °C and letting cool down to room temperature. As detailed in the experimental procedure, the incorporation of silver ions into the gel was carried out in two major different ways, either during the formation of the gel (in situ reaction) or ‘‘a posteriori’’ by soaking the already formed gels in AgNO3 solutions. Through these procedures, it was possible to impregnate the gels with Ag? ions prior to their reduction to metallic silver nuclei, which must take place by slow reaction of the ions with reducing functional groups (R–CH2OH) from the agar gel itself. This was ascertained by the progressive change in color from the whitish translucent pristine gel to yellow, to orange, or to red-brown depending on the final concentration of silver nanoparticles (Fig. 1a–g), which in turn depends on the initial AgNO3 concentration. The specific structures obtained are thought to be directly dependent upon the microstructure of the particular gel formed, thus yielding an inverse template of the gel pore mesostructure. In a first series of reactions (method A), the in situ materials were formed using a silver nitrate solution to which solid agar was added, subsequently heating the mixture to 90 °C. In principle, the results were satisfactory and increasing concentrations of AgNO3 resulted in larger final concentrations of silver nanoparticles, as evidenced both by the darker color of the gels (Fig. 1a–g) and the increasing absorbance
Fig. 1 Left: Silver–agar nanostructured gels prepared using method A (see text) and loaded with different amounts of silver nanoparticles. The final color saturation is directly proportional to the amount of reduced silver, which in turn depends on the
initial AgNO3 concentration: a 0.1 mM; b 0.5 mM; c 1 mM; d 5 mM; e 10 mM; f 50 mM; g 100 mM; Right: 5 wt% agar gels prepared by impregnation (method C, see text) in (h) 1 mM and (i) 100 mM AgNO3 solutions
Characterization
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of the characteristic UV–vis band centered at ca. 430 nm, which arises from the surface plasmon absorption of nanosized silver particles (Fig. 2). With increasing the silver nitrate concentration, the absorbance peak increased and slightly red shifted from 423 nm, 425 nm, 429 nm and 433 nm for a silver nitrate concentration of 50 mM, 100 mM, 300 mM, and 500 mM, respectively. As shown in (Fig. 2), the absorbance is directly proportional to the silver nanoparticles concentration. Hence, the increasing absorbance with silver nitrate concentration indicates a higher concentration of silver nanoparticles. Higher concentration of metal nanopaticles may lead to the red shift of surface plasmon resonance band due to the multipoles interaction among the nanoparticles (Liu et al. 1998), and to some extent also to an increase in their size. Indeed, the analysis of these samples by transmission electron microscopy (TEM) showed the presence of nanometric silver (Fig. 3a), although their broad size distribution and a variety of particles (2–64 nm) were detected (Fig. 3a1). This polydispersity was most likely due to the fact that the reduction of silver by agar was taking place from the beginning, before gel formation and in a continuous way at all temperatures. As a matter of fact, the samples with higher Ag? concentration turned brownish before the gel was formed. A high 3 AgNO Concentration
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Fig. 2 UV–vis spectra of 5% gels prepared following method A with AgNO3 solutions of increasing concentration. Although only qualitatively, it can be appreciated that the hybrid gels prepared from AgNO3 solutions having a higher concentration present a higher load of silver nanoparticles, as reflected by the increased absorption. The relatively high peak widths are due to the polydispersity intrinsically associated to method A
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resolution TEM (HRTEM) study (see Fig. 4) reveals that Ag nanoparticles are indeed embedded in the gel matrix. Figure 4 shows a representative HRTEM photograph with ca. 5 nm nanoparticles. Direct measurement of the interspacing of the fringes (using DigitalMicrograph Software) gives an average value ˚ , corresponding, within experimental error, to of 2.3 A ˚ {111} fcc Ag. Dim lattice the value of 2.359 A fringes can be observed in the vicinity of some nanoparticles (like the one highlighted in the figure for example). These could correspond to zones in which further growth of silver was taking place. In order to avoid this continuing nanoparticle growth, we devised an improved procedure (method B) in which cold silver nitrate solutions were added to the forming agar gel at 90 °C in order to favor a more instantaneous reaction at lower temperatures within an incipient gel matrix. Representative TEM images of narrowly dispersed silver nanoparticles obtained using this method are shown in (Fig. 3b, c) along with their size distribution analyses (Fig. 3b1, c1). They correspond to two representative examples with AgNO3 concentrations of 0.55 M and 0.4 M, respectively. It is evident that this method (B) leads to more monodispersed samples with smaller particle sizes than method A (Fig. 3a, a1). One possible explanation of these results is that in method B the gel structure is essentially formed when AgNO3 is added and therefore acts more closely to a restraining medium, preventing continuous nucleation processes or excessive growth of nanoparticles. In order to determine the identity of the nanoparticles, EDX analyses were carried out, one of which is presented in Fig. 3b (inset) confirming the presence of silver nanoparticles in the biopolymer matrix. Finally, in a more radical approach to ensure the incorporation of Ag? ions into a preformed gel matrix, with no thermal activation of the mixture, the impregnation procedure (method C) was carried out by soaking of a 5% w/v pre-formed agar gel with aqueous AgNO3 solutions of different concentrations (Fig. 1h, i). Here again the final color of the gel depends on the initial AgNO3 concentration. Figure 5 shows TEM images of the resulting material after treatment with 0.1 M AgNO3 for 30 days at 7 °C in the refrigerator. In this case, striking nanostructures can be observed, aside from spherical nanopartices. In the low magnification image (Fig. 5a) beautiful formations in the shape of leaves are observed. A
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Total number of particles counted = 340 Mean Diameter = 15.528 nm Standard Deviation = 12.403 nm
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Fig. 3 TEM images of three hybrid gels prepared with a a 0.5 M AgNO3 solution by method A, b and c 0.55 M and 0.4 M AgNO3 solution by method B, respectively, along with (a1), (b1) and (c1) which represents the corresponding silver nanoparicles size distribution of those hybrids. As observed, monodispersity is strongly enhanced by using method B and is higher in (c) than in (b) due to the lower silver nitrate concentration. The inset in (b) shows the EDX pattern of the silver particles, Cu peaks are due to the carbon coated Cu grid
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Fig. 4 HRTEM image of a hybrid 5% agar gel prepared with a 0.1 M AgNO3 solution. As can be observed, Ag nanoparticles ˚ ) are embedded in the gel (ca. 5 nm) ({111} for Ag fcc 2.36A matrix (amorphous). Some of these particles, like the one highlighted, are surrounded by zones with dim lattice fringes which might correspond to further growth of silver
closer look at one of them (Fig. 5b) reveals a mazelike nanostructure grown along apparent nanochannels and pores of the gel. The domain size, shape, and structure of these intricate nanoparticle labyrinths (with a particle size gradient in going from the core to the ‘‘walls’’ of the channel) indicate their formation from water-filled matrix nanopores present in the pristine agar gel which were eventually loaded with Ag? ions. Selected Area Electron Diffraction (SAED) Fig. 5 TEM images of a silver-5% agar hybrid gel prepared by impregnation in a 0.1 M AgNO3 for 30 days at 7 °C according to method C. a Low magnification: striking nanostructures in the shape of leaves are observed; b Close-up: a maze-like nanostructure grown along apparent nanochannels and pores of the gel is revealed along with the presence of spherical Ag nanoparticles of *2 nm. The inset in (a) shows the SAED pattern for the whole image, which can be indexed as fcc Ag
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of these structures confirm that they correspond to fcc Ag (see inset in Fig. 5a). It should be noted that in addition to these peculiar nanostructures, monodisperse small spherical silver nanoparticles (ca. 2 nm) are also detected in these materials within the polymer itself (marked in Fig. 5b). It seems reasonable to expect that the specific silver nanostructures resulting from this impregnation method might be directly dependent upon the internal microstructure of the particular gel formed. In this sense, the silver nanostructure ‘‘picture’’ could be taken as a negative image, i.e., an inverse template of the gel pore mesostructure. It should be noted that in addition to agar providing a convenient support medium for silver nanostructures, the latter provide added stability to the gel avoiding the growth of bacteria typically found in pristine agar gels upon time. Furthermore, thanks to the well-known thermoreversible gelation of agar, the silver nanoparticles and nanostructures obtained, which are stabilized in the gels, can be easily released by disolution of the latter in hot water. Preliminary TEM images of these freshly recovered nanoparticles (in water) show some incipient agglomeration of the particles released. On the other hand, we have found that it is possible to dissolve our Ag–Agar gels at room temperature in solvents such as DMSO. The evolution of thusreleased particles will be the subject of further studies.
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Conclusion The work reported here represents our preliminary account of how to exploit the multiple possibilities provided by a complex gel such as agar for the preparation of hybrid nanocomposites of Ag–Agar in which metallic silver can be obtained in the form of nanoparticles or more complex nanostructures. Furthermore, we have accomplished this using biocompatible hydrogels, in the absence of any additional organic solvents, capping or reducing agents, in an attempt to keep both the procedures and the materials obtained as simple, green, and cheap as possible. Acknowledgements Funding for this research was possible by a grant from the Spanish Ministry of Science and Innovation (MICINN) (CTQ2008-06779-C02-01). We thank CSIC and the European Social Fund for financing through the I3P program (DMR); OA gratefully acknowledges earlier financial support from The Palestinian Ministry of Higher Education (Saudi Committee for the Relief of the Palestinian People) and a loan from Al-Quds University (Jerusalem, Palestine).
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