Materials Science and Engineering C 43 (2014) 21–26
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A comparative study of the effect of α-, β-, and γ-cyclodextrins as stabilizing agents in the synthesis of silver nanoparticles using a green chemistry method Javier Suárez-Cerda a, Gabriel Alonso Nuñez b, Heriberto Espinoza-Gómez c, Lucía Z. Flores-López a,⁎ a b c
Centro de Graduados e Investigación, Instituto Tecnológico de Tijuana, Apartado Postal 1166, Tijuana, B. C., Mexico Centro de Nanociencia y Nanotecnología de la UNAM, CNyN, Km. 107 Carretera Tijuana-Ensenada, C.P. 22860 Ensenada, B. C., Mexico Facultad de Ciencias Químicas e Ingeniería, UABC, Calzada Universidad 14418 Parque Industrial Internacional, C.P. 22390 Tijuana, B.C., Mexico
a r t i c l e
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Article history: Received 11 February 2014 Received in revised form 1 May 2014 Accepted 1 July 2014 Available online 10 July 2014 Keywords: Silver nanoparticles Green chemistry method Stabilizing agents Cyclodextrins
a b s t r a c t This paper describes the effect of different types of cyclodextrins (CDs) in the synthesis of silver nanoparticles (Ag-NPs), using an easy green chemistry method. The Ag-NPs were obtained using an aqueous silver nitrate solution (AgNO3) with α-, β-, or γ-CDs (aqueous solutions) as stabilizing agents, employing the chemical reduction method with citric acid as a reducing agent. A comparative study was done to determine which cyclodextrin (CD) was the best stabilizing agent, and we found out that β-CD was the best due to the number of glucopyranose units in its structure. The formation of the Ag-NPs was demonstrated by analysis of UV–vis spectroscopy, atomic force microscopy (AFM), scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS) and transmission electron microscopy (TEM). SEM–EDS showed the formation of a cluster with a significant amount of silver, for β-CDAg-NPs, spherical agglomerates can be observed. However, for α-, γ-CD, the agglomerates do not have a specific form, but their appearance is porous. TEM analysis shows spherical nanoparticles in shape and size between ~0.5 to 7 nm. The clear lattice fringes in TEM images and the typical selected area electron diffraction (SAED) pattern, showed that the Ag-NPs obtained were highly crystalline with a face cubic center structure (FCC). © 2014 Elsevier B.V. All rights reserved.
1. Introduction In recent decades, scientists have shown an increasing interest in the study, preparation and application of nanoparticles. The main reason is that noble metal nanoparticles, as gold [1,2], silver [3,4], copper [5], platinum [6,7], and palladium [8,9], have many potential applications in fields as electronics [10], optics [11,12], biomedicine [13,14], and catalysis [15–18]. In the nanoscale, chemical and physical properties of inorganic nanoparticles are highly dependent on factors such as size and shape for two main reasons. The first one is that nano-materials have, relatively, a larger surface area for the same mass of similar material, which makes them more chemically reactive. The second one is that, below 50 nm, the laws of classical physics behave differently from those of the same material on a larger scale [19,20]. Furthermore, the nanoparticles synthesis in a colloidal solution requires the use of methods to obtain a precise control, on nanoparticule size and shape. These methods have achieved a set of monodisperse particles, with a very specific particular property. The literature reports different synthetic routes to obtain metal nanoparticles. In the case ⁎ Corresponding author. Tel.: +52 6646233772; fax: +52 6646234043. E-mail address: lzfl[email protected] (L.Z. Flores-López).
http://dx.doi.org/10.1016/j.msec.2014.07.006 0928-4931/© 2014 Elsevier B.V. All rights reserved.
of nanoclusters of transition metals we can cite: chemical reduction of a metallic salt [21,22]; thermal decomposition, photochemical or sonochemical [23,24]; ligand reduction and ligand displacement from organometallic precursors: metal vapor synthesis and electrochemical synthesis [25,26]. Chemical reduction of transition metal precursor salts, in the presence of chemical stabilizing and reducing agents, is one of the most common methods for the preparation of nanoparticles. This method has the advantage of reproducibility and capacity to obtain monodisperse colloids, with a narrow distribution in particle size [27–30]. The chemical reduction method has been successfully used for the preparation of silver nanoparticles (Ag-NPs), which may present important biomedical properties, such as: antimicrobial, antiviral, antifungal, etc. This is why Ag-NPs synthesis is of great importance and interest [14,31,32]. In recent years due to environmental issues, biodegradable and environmental friendly materials for further industrial application, have greatly increased. The main key in green synthesis of metal nanoparticles has been factors such as: solvent choice, the use of an environmentally benign reducing agent, and the use of a non-toxic material for nanoparticle stabilization [33]. Cyclodextrins (CDs) are a group of such eco-friendly chemicals used as a stabilizing agent in the preparation of Ag-NPs.
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CDs are cyclic oligosaccharides produced by selective enzymatic synthesis. There are three types of CDs with similar structures, consisting of six, seven, or eight glucose monomers (ring-shaped, glucopyranose units) called alpha (α), beta (β) or gamma (γ) cyclodextrin (CD), respectively (Fig. 1). They are also known as cycloamyloses, cyclomaltoses and Schardinger dextrins [34]. They were discovered in 1891, and produced by contamination of Bacillus macerans during an enzymatic degradation of related compounds (dextrins) and starch [35]. CDs have a hydrophobic cavity and a hydrophilic exterior. The specific coupling of the glucose monomers gives each CD a rigid molecular structure, with a “cavity” of predetermined volume. This “internal cavity” (hydrophobic in nature), is a key structural feature of CDs, since it provides the ability to form complexes with other molecules of very different nature. Nevertheless, these molecules must have a size compatible with the internal cavity of the CD, in order to form a stable “inclusion complex” (Fig. 1) [36]. In 1942, the structures of α and β-CDs were determined by X-ray crystallography [37]. In 1948, the X-ray structure of γ-CD was understood and it was also recognized that CDs can form inclusion complexes [38]. The main interest in CDs lies in their ability to form inclusion complexes with several compounds [39–41]. The X-ray structures show that in CDs the secondary hydroxyl groups (C2 and C3) are located on the wider edge of the ring, and the primary hydroxyl groups (C6) on the other edge. The apolar (C3 and C5) hydrogens and ether-like oxygens are on the inside of the torus-like molecules. Therefore, this molecule with a hydrophilic outside able to dissolve in water and an apolar hydrophobic cavity, is described as a “micro heterogeneous environment” [42]. As a result, the particular structural characteristics present in CDs make them excellent candidates as stabilizing agents, using the chemical reduction method in the preparation of metal nanoparticles. Some works refer to β-CD as a stabilizing agent or as a reducing agent on the synthesis of Ag-NPs [29,43]. However, there are no reported papers on the synthesis of Ag-NPs using α- or γ-CDs.
In this research, the synthesis of Ag-NPs has been carried out through a single step, in an aqueous medium, with no organic solvents. This represents an innovating and easy green chemistry method, able to be an eco-friendly and economical one; since CDs are water soluble carbohydrate polymers. Furthermore, we report a comparative study over the effect of CDs size, on the size and shape of the synthetized Ag-NPs; in order to determine which CD of the three is the best stabilizing agent. 2. Materials and methods 2.1. Materials Silver nitrate AgNO3 ACS 99.9 + % (metal basis), was supplied by Alfa Aesar, Johnson Matthey Company (AgNO3). The reactions were carried out with the three existing CDs α, β, or γ respectively. α(C36H60O30, 972.84 g/mol) ≥ 98%, β(C42H70O35, 1,134.98 g/mol) ≥ 97% and γ (C48H80O40, 1297.12 g/mol) ≥ 98% -CDs were supplied by Sigma Aldrich. All other reagents were of laboratory grade. The solutions were prepared with distilled water. Careful handling of the glassware is important to avoid a possible contamination. 2.2. General method for the preparation of Ag-NPs CD (α, β, or γ) aqueous solution (10 mL, 0.01 M) was added to AgNO3 aqueous solution (10 mL, 0.002 M) and stirred at 0 °C for 15 min. Subsequently, citric acid aqueous solution (10 mL, 0.02 M) was added drop wise and stirred at room temperature overnight. The reaction mixture was then poured into a test tube and the separation phase was carried out using a centrifuge (4000 rpm/10 min). The precipitate was extracted with a micropipette and was vacuum filtered through a nylon membrane 0.2 μm. The cake formation on the surface of the nylon membrane acts as a second filter for the supernatant solution. It was then washed with ethanol and water.
Fig. 1. α, β, and γ-CDs sizes.
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Fig. 2. General reaction for the formation of Ag-NPs stabilized with CD.
2.3. Physical characterization 2.3.1. UV–vis spectroscopy The UV–vis spectrum of Ag-NPs was recorded on a Cary 100 Conc UV–Visible Spectrophotometer Varian UV 1002 M151. The synthesis of Ag-NPs was determined following their plasmon absorption bands at their respective λmax values. 2.3.2. Scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS) Scanning electron microscopy (SEM) images were recorded using a JEOL JSM-5300 scanning electron microscope equipped with energydispersive spectrum (EDS) capability. The samples were measured the following day without any further treatment. 2.3.3. Atomic force microscopy (AFM) The analysis testing for AFM was carried out on an Agilent Technologies, SPM 5100 model, in a tapping mode, employing silicon cantilevers at a frequency of 152 KHz with an amplitude of 4.3 V and a N9521A scanner of 100 μm. The AFM was done to observe the surface morphology and size of the resultant Ag-NPs. The samples were dropped onto freshly cleaved mica slices and dried overnight. The samples were measured the following day without any further treatment. 2.3.4. Transmission electron microscopy (TEM) TEM was also used to observe the size, shape and morphology of the resultant nanoparticles. A specimen for the TEM analysis was made by redispersing a small amount of CD-Ag-NPs solution, with isopropyl alcohol by ultrasonification, and casting a drop of suspension onto a carbon coated copper grid. The sample allowed to air dry at room temperature overnight. TEM study was observed on a TEM, JEOL JEM 2100 F Field Emission Electron Microscope at an accelerating voltage of 200 kV and fitted with a CCD camera. The samples were measured the following day without any further treatment.
spectral analysis. This can be possible due to the surface plasmon resonance (SPR). This SPR transition is responsible for the striking yellowish brown (approx. 403 nm) coloration of Ag-NPs. These results agree with those reported previously by other authors [19,29,44]. The UV–vis absorption spectrum of the Ag-NPs is shown in Fig. 3, and it is possible to observe a shift in the maximum wavelength absorption as a function of the type of the CD used as a stabilizing agent. In the case of α-CD, the maximum wavelength appears at 399 nm, whereas the γ-CD was 405 nm. This shift is due to the increase in the polarity of the CDs and their effect on the stabilization of the Ag-NPs. 3.2. SEM–EDS studies SEM–EDS analyses were done to confirm the presence of the Ag-NPs. The Ag-NPs obtained exhibited interesting morphologies depending on the type of CD used. In the case of β-CD-Ag-NPs, spherical agglomerates can be observed. However, for α- and γ-CD, the agglomerates do not have a specific form, but their appearance is porous. The EDS analysis shown in Fig. 4, demonstrates the presence of a significant silver content; this is consistent with the presence of nanoparticles in the SEM micrographs, which present large particle aggregates, and this can be attributed to the delay between sampling and the end of the reaction. 3.3. AFM analysis A comparison of the roughness of the silver films, indicating the degree of coverage by the CD on Ag-NPs was performed by AFM.
3. Results and discussion The method for preparing Ag-NPs using CDs as a protective agent in water is quite simple, but no reports have been found describing the effect of the size of the CD as a stabilizing agent on the synthesis of metal nanoparticles in an aqueous solution at room temperature. The most accepted mechanism for the formation of nanoparticles, including their growth, considers the type of the stabilizing agent used, which influences the shape and size of the nanoparticles (Fig. 2). 3.1. UV–vis spectroscopy analysis Ag-NPs absorb radiation in the visible region of the electromagnetic spectrum (380–450 nm), and then, the formation and stability of Ag-NPs in an aqueous colloidal solution can be confirmed by UV–vis
Fig. 3. UV–vis spectra of Ag-NPs synthetized with α, β, and γ-CD.
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Fig. 4. SEM–EDS analysis of Ag-NPs.
Representative images are shown in Fig. 5, where it can clearly be observed that Ag-NPs synthetized with α-CD have a capped Ag-NPs size between 10 and 50 nm, while the biggest capped Ag-NPs size was found for γ-CD-Ag-NPs (50–100 nm). The capped Ag-NPs formed with the β-CD-Ag-NPs showed a more homogeneous size distribution (25–50 nm). The synthesized Ag-NPs are coordinated with the hydroxyl group of CD molecules preventing their large-scale aggregation. These results are consistent with those previously reported by other authors [27,45].
3.4. TEM studies The TEM results show the effect of the CD type on the shape, size and dispersity of Ag-NPs obtained. The TEM images demonstrate that the Ag-NPs obtained were 1–6.5 nm in size and spherical in shape for α-CD and β-CD, while the Ag-NPs prepared with γ-CD show a prismatic shape (Fig. 6). The Ag-NPs produced by this method are stable and comparable in size and polydispersity to those produced using typical methods. The CDs prevented the Ag-NPs growing into larger aggregates. The histograms based on the TEM images illustrated the Ag-NPs average size and size distribution. As seen in Fig. 6, the Ag-NPs obtained with α-CD have a wider size distribution than those synthetized with β-CD, and these in turn with those obtained with γ-CD. The Ag-NPs obtained with α-CD, has a particle size of 1–7 nm (wide size distribution) and a mode of 1.5 nm; 88% of the particles are 1–3.5 nm; the Ag-NPs with β-CD exhibited a smaller size (0.5–4.5 nm) and a narrower size distribution; 95% of the particles are between 0.5 and 3 nm, with a mode of 1 nm; finally, for γ-CD the particle size was 1.5–6.5 nm, with a mode of 3 nm; 91% of the particles are 2–4.5 nm. The nanoparticles obtained by this method have a smaller size and narrower size dispersity than in previous reports [19,44]. From the above mentioned we can conclude that the β-CD is the best stabilizing agent, because these particles have a smaller size. However, the obtained nanoparticles with γ-CD, shows a narrower size distribution.
We also proved that there is a relation between the maximum wavelength absorption obtained from the UV–vis spectra (Fig. 3) and the nanoparticle size distribution histograms (Fig. 6); it can be seen that the maximum wavelength absorption increases with the size of the nanoparticles. Multiple lattice fringes can be clearly observed under high-resolution TEM (Fig. 7). The lattice spacing of the Ag-NPs was 0.208 nm in accordance with the lattice spacing on the (2 0 0) plane of silver. The TEM analyses were obtained counter-clockwise. The d-spacing of the leftmost particle has been measured and the value shows 0.208 nm and 0.148 nm. The d-spacing of the remaining particle is 0.227 nm and 0.144 nm, respectively, which is close to the JCPDS value of 0.205 nm [PDF # 04-0783]. The clear lattice fringes in TEM images and the typical selected area electron diffraction (SAED) pattern with circular rings corresponding to the (1 1 1), (2 0 0), and (2 2 0), planes show that the nanoparticles obtained are highly crystalline (FCC structure). These results are in accordance with those in previous reports [28,45,46]. 4. Conclusions In this paper, we have reported the effect of the size of CDs on the synthesis of Ag-NPs through a simple, safe, and green chemistry method, using the chemical reduction method. The silver nanostructures prepared with this method can be stored at room temperature for nearly 6 months without any visible change or effect on their size. The nanoparticles obtained were characterized by UV–vis, SEM–EDS, AFM and TEM. UV–vis analyses show that the maximum wavelength absorption is directly proportional to the size of the nanoparticles; small nanoparticles show maximum wavelength absorption at high field, whereas with increasing nanoparticle size, the maximum wavelength absorption moves downfield. TEM analysis shows that the nanoparticles obtained are between 0.5 nm and 7 nm (α-CD, 1–7 nm; β-CD, 0.5–4.5 nm; and γ-CD, 1.5– 6.5 nm) in size; the β-CD has the smallest nanoparticle size, and 96.9%
Fig. 5. AFM analysis of Ag-NPs synthesized with different CDs.
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Fig. 6. TEM analysis of Ag-NPs and particle size distribution histograms of Ag-NPs formed with α-, β-, and γ-CD.
of the particles obtained with β-CD are between 0.5–3.5 nm in size, with an average of 1 nm. However, the γ-CD presents a narrowed nanoparticle size distribution; 44.4% of the particles have a size between 2 and 2.5 nm, and also the 95.6% of the particles synthetized with γ-CD has a size between 1.5 and 4.5 nm. On another hand, the α-CD shows a wider size distribution, and 87.8% of the nanoparticles have a size between 1 and 3.5 nm. So we can conclude that (the) β-CD was the best stabilizing agent for the synthesis of small Ag-NPs, whereas γ- CD is better to obtain narrow nanoparticles size distributions. This can be attributed to the number of glucopyranose units in its structure. In addition, TEM images show a clear lattice spacing of silver which is in agreement with a crystalline and face centered cubic structure of Ag-NPs.
Acknowledgments We gratefully acknowledge the support for this research work to the Dirección General de Educación Superior Tecnológica (DGEST Grant 4374.11-P). Also, to Israel Gradilla Martίnez (SEM–EDS) and Francisco Ruiz Medina (TEM) from the Centro de Nanociencias y Nanotecnología de la UNAM; and Pedro Navarro-Vega (AFM) from the Centro de Graduados e Investigación del Instituto Tecnológico de Tijuana.
Fig. 7. Cristal analysis of Ag-NPs.
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Contents lists available at ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
A comparative study of the effect of α-, β-, and γ-cyclodextrins as stabilizing agents in the synthesis of silver nanoparticles using a green chemistry method Javier Suárez-Cerda a, Gabriel Alonso Nuñez b, Heriberto Espinoza-Gómez c, Lucía Z. Flores-López a,⁎ a b c
Centro de Graduados e Investigación, Instituto Tecnológico de Tijuana, Apartado Postal 1166, Tijuana, B. C., Mexico Centro de Nanociencia y Nanotecnología de la UNAM, CNyN, Km. 107 Carretera Tijuana-Ensenada, C.P. 22860 Ensenada, B. C., Mexico Facultad de Ciencias Químicas e Ingeniería, UABC, Calzada Universidad 14418 Parque Industrial Internacional, C.P. 22390 Tijuana, B.C., Mexico
a r t i c l e
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Article history: Received 11 February 2014 Received in revised form 1 May 2014 Accepted 1 July 2014 Available online 10 July 2014 Keywords: Silver nanoparticles Green chemistry method Stabilizing agents Cyclodextrins
a b s t r a c t This paper describes the effect of different types of cyclodextrins (CDs) in the synthesis of silver nanoparticles (Ag-NPs), using an easy green chemistry method. The Ag-NPs were obtained using an aqueous silver nitrate solution (AgNO3) with α-, β-, or γ-CDs (aqueous solutions) as stabilizing agents, employing the chemical reduction method with citric acid as a reducing agent. A comparative study was done to determine which cyclodextrin (CD) was the best stabilizing agent, and we found out that β-CD was the best due to the number of glucopyranose units in its structure. The formation of the Ag-NPs was demonstrated by analysis of UV–vis spectroscopy, atomic force microscopy (AFM), scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS) and transmission electron microscopy (TEM). SEM–EDS showed the formation of a cluster with a significant amount of silver, for β-CDAg-NPs, spherical agglomerates can be observed. However, for α-, γ-CD, the agglomerates do not have a specific form, but their appearance is porous. TEM analysis shows spherical nanoparticles in shape and size between ~0.5 to 7 nm. The clear lattice fringes in TEM images and the typical selected area electron diffraction (SAED) pattern, showed that the Ag-NPs obtained were highly crystalline with a face cubic center structure (FCC). © 2014 Elsevier B.V. All rights reserved.
1. Introduction In recent decades, scientists have shown an increasing interest in the study, preparation and application of nanoparticles. The main reason is that noble metal nanoparticles, as gold [1,2], silver [3,4], copper [5], platinum [6,7], and palladium [8,9], have many potential applications in fields as electronics [10], optics [11,12], biomedicine [13,14], and catalysis [15–18]. In the nanoscale, chemical and physical properties of inorganic nanoparticles are highly dependent on factors such as size and shape for two main reasons. The first one is that nano-materials have, relatively, a larger surface area for the same mass of similar material, which makes them more chemically reactive. The second one is that, below 50 nm, the laws of classical physics behave differently from those of the same material on a larger scale [19,20]. Furthermore, the nanoparticles synthesis in a colloidal solution requires the use of methods to obtain a precise control, on nanoparticule size and shape. These methods have achieved a set of monodisperse particles, with a very specific particular property. The literature reports different synthetic routes to obtain metal nanoparticles. In the case ⁎ Corresponding author. Tel.: +52 6646233772; fax: +52 6646234043. E-mail address: lzfl[email protected] (L.Z. Flores-López).
http://dx.doi.org/10.1016/j.msec.2014.07.006 0928-4931/© 2014 Elsevier B.V. All rights reserved.
of nanoclusters of transition metals we can cite: chemical reduction of a metallic salt [21,22]; thermal decomposition, photochemical or sonochemical [23,24]; ligand reduction and ligand displacement from organometallic precursors: metal vapor synthesis and electrochemical synthesis [25,26]. Chemical reduction of transition metal precursor salts, in the presence of chemical stabilizing and reducing agents, is one of the most common methods for the preparation of nanoparticles. This method has the advantage of reproducibility and capacity to obtain monodisperse colloids, with a narrow distribution in particle size [27–30]. The chemical reduction method has been successfully used for the preparation of silver nanoparticles (Ag-NPs), which may present important biomedical properties, such as: antimicrobial, antiviral, antifungal, etc. This is why Ag-NPs synthesis is of great importance and interest [14,31,32]. In recent years due to environmental issues, biodegradable and environmental friendly materials for further industrial application, have greatly increased. The main key in green synthesis of metal nanoparticles has been factors such as: solvent choice, the use of an environmentally benign reducing agent, and the use of a non-toxic material for nanoparticle stabilization [33]. Cyclodextrins (CDs) are a group of such eco-friendly chemicals used as a stabilizing agent in the preparation of Ag-NPs.
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CDs are cyclic oligosaccharides produced by selective enzymatic synthesis. There are three types of CDs with similar structures, consisting of six, seven, or eight glucose monomers (ring-shaped, glucopyranose units) called alpha (α), beta (β) or gamma (γ) cyclodextrin (CD), respectively (Fig. 1). They are also known as cycloamyloses, cyclomaltoses and Schardinger dextrins [34]. They were discovered in 1891, and produced by contamination of Bacillus macerans during an enzymatic degradation of related compounds (dextrins) and starch [35]. CDs have a hydrophobic cavity and a hydrophilic exterior. The specific coupling of the glucose monomers gives each CD a rigid molecular structure, with a “cavity” of predetermined volume. This “internal cavity” (hydrophobic in nature), is a key structural feature of CDs, since it provides the ability to form complexes with other molecules of very different nature. Nevertheless, these molecules must have a size compatible with the internal cavity of the CD, in order to form a stable “inclusion complex” (Fig. 1) [36]. In 1942, the structures of α and β-CDs were determined by X-ray crystallography [37]. In 1948, the X-ray structure of γ-CD was understood and it was also recognized that CDs can form inclusion complexes [38]. The main interest in CDs lies in their ability to form inclusion complexes with several compounds [39–41]. The X-ray structures show that in CDs the secondary hydroxyl groups (C2 and C3) are located on the wider edge of the ring, and the primary hydroxyl groups (C6) on the other edge. The apolar (C3 and C5) hydrogens and ether-like oxygens are on the inside of the torus-like molecules. Therefore, this molecule with a hydrophilic outside able to dissolve in water and an apolar hydrophobic cavity, is described as a “micro heterogeneous environment” [42]. As a result, the particular structural characteristics present in CDs make them excellent candidates as stabilizing agents, using the chemical reduction method in the preparation of metal nanoparticles. Some works refer to β-CD as a stabilizing agent or as a reducing agent on the synthesis of Ag-NPs [29,43]. However, there are no reported papers on the synthesis of Ag-NPs using α- or γ-CDs.
In this research, the synthesis of Ag-NPs has been carried out through a single step, in an aqueous medium, with no organic solvents. This represents an innovating and easy green chemistry method, able to be an eco-friendly and economical one; since CDs are water soluble carbohydrate polymers. Furthermore, we report a comparative study over the effect of CDs size, on the size and shape of the synthetized Ag-NPs; in order to determine which CD of the three is the best stabilizing agent. 2. Materials and methods 2.1. Materials Silver nitrate AgNO3 ACS 99.9 + % (metal basis), was supplied by Alfa Aesar, Johnson Matthey Company (AgNO3). The reactions were carried out with the three existing CDs α, β, or γ respectively. α(C36H60O30, 972.84 g/mol) ≥ 98%, β(C42H70O35, 1,134.98 g/mol) ≥ 97% and γ (C48H80O40, 1297.12 g/mol) ≥ 98% -CDs were supplied by Sigma Aldrich. All other reagents were of laboratory grade. The solutions were prepared with distilled water. Careful handling of the glassware is important to avoid a possible contamination. 2.2. General method for the preparation of Ag-NPs CD (α, β, or γ) aqueous solution (10 mL, 0.01 M) was added to AgNO3 aqueous solution (10 mL, 0.002 M) and stirred at 0 °C for 15 min. Subsequently, citric acid aqueous solution (10 mL, 0.02 M) was added drop wise and stirred at room temperature overnight. The reaction mixture was then poured into a test tube and the separation phase was carried out using a centrifuge (4000 rpm/10 min). The precipitate was extracted with a micropipette and was vacuum filtered through a nylon membrane 0.2 μm. The cake formation on the surface of the nylon membrane acts as a second filter for the supernatant solution. It was then washed with ethanol and water.
Fig. 1. α, β, and γ-CDs sizes.
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Fig. 2. General reaction for the formation of Ag-NPs stabilized with CD.
2.3. Physical characterization 2.3.1. UV–vis spectroscopy The UV–vis spectrum of Ag-NPs was recorded on a Cary 100 Conc UV–Visible Spectrophotometer Varian UV 1002 M151. The synthesis of Ag-NPs was determined following their plasmon absorption bands at their respective λmax values. 2.3.2. Scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS) Scanning electron microscopy (SEM) images were recorded using a JEOL JSM-5300 scanning electron microscope equipped with energydispersive spectrum (EDS) capability. The samples were measured the following day without any further treatment. 2.3.3. Atomic force microscopy (AFM) The analysis testing for AFM was carried out on an Agilent Technologies, SPM 5100 model, in a tapping mode, employing silicon cantilevers at a frequency of 152 KHz with an amplitude of 4.3 V and a N9521A scanner of 100 μm. The AFM was done to observe the surface morphology and size of the resultant Ag-NPs. The samples were dropped onto freshly cleaved mica slices and dried overnight. The samples were measured the following day without any further treatment. 2.3.4. Transmission electron microscopy (TEM) TEM was also used to observe the size, shape and morphology of the resultant nanoparticles. A specimen for the TEM analysis was made by redispersing a small amount of CD-Ag-NPs solution, with isopropyl alcohol by ultrasonification, and casting a drop of suspension onto a carbon coated copper grid. The sample allowed to air dry at room temperature overnight. TEM study was observed on a TEM, JEOL JEM 2100 F Field Emission Electron Microscope at an accelerating voltage of 200 kV and fitted with a CCD camera. The samples were measured the following day without any further treatment.
spectral analysis. This can be possible due to the surface plasmon resonance (SPR). This SPR transition is responsible for the striking yellowish brown (approx. 403 nm) coloration of Ag-NPs. These results agree with those reported previously by other authors [19,29,44]. The UV–vis absorption spectrum of the Ag-NPs is shown in Fig. 3, and it is possible to observe a shift in the maximum wavelength absorption as a function of the type of the CD used as a stabilizing agent. In the case of α-CD, the maximum wavelength appears at 399 nm, whereas the γ-CD was 405 nm. This shift is due to the increase in the polarity of the CDs and their effect on the stabilization of the Ag-NPs. 3.2. SEM–EDS studies SEM–EDS analyses were done to confirm the presence of the Ag-NPs. The Ag-NPs obtained exhibited interesting morphologies depending on the type of CD used. In the case of β-CD-Ag-NPs, spherical agglomerates can be observed. However, for α- and γ-CD, the agglomerates do not have a specific form, but their appearance is porous. The EDS analysis shown in Fig. 4, demonstrates the presence of a significant silver content; this is consistent with the presence of nanoparticles in the SEM micrographs, which present large particle aggregates, and this can be attributed to the delay between sampling and the end of the reaction. 3.3. AFM analysis A comparison of the roughness of the silver films, indicating the degree of coverage by the CD on Ag-NPs was performed by AFM.
3. Results and discussion The method for preparing Ag-NPs using CDs as a protective agent in water is quite simple, but no reports have been found describing the effect of the size of the CD as a stabilizing agent on the synthesis of metal nanoparticles in an aqueous solution at room temperature. The most accepted mechanism for the formation of nanoparticles, including their growth, considers the type of the stabilizing agent used, which influences the shape and size of the nanoparticles (Fig. 2). 3.1. UV–vis spectroscopy analysis Ag-NPs absorb radiation in the visible region of the electromagnetic spectrum (380–450 nm), and then, the formation and stability of Ag-NPs in an aqueous colloidal solution can be confirmed by UV–vis
Fig. 3. UV–vis spectra of Ag-NPs synthetized with α, β, and γ-CD.
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Fig. 4. SEM–EDS analysis of Ag-NPs.
Representative images are shown in Fig. 5, where it can clearly be observed that Ag-NPs synthetized with α-CD have a capped Ag-NPs size between 10 and 50 nm, while the biggest capped Ag-NPs size was found for γ-CD-Ag-NPs (50–100 nm). The capped Ag-NPs formed with the β-CD-Ag-NPs showed a more homogeneous size distribution (25–50 nm). The synthesized Ag-NPs are coordinated with the hydroxyl group of CD molecules preventing their large-scale aggregation. These results are consistent with those previously reported by other authors [27,45].
3.4. TEM studies The TEM results show the effect of the CD type on the shape, size and dispersity of Ag-NPs obtained. The TEM images demonstrate that the Ag-NPs obtained were 1–6.5 nm in size and spherical in shape for α-CD and β-CD, while the Ag-NPs prepared with γ-CD show a prismatic shape (Fig. 6). The Ag-NPs produced by this method are stable and comparable in size and polydispersity to those produced using typical methods. The CDs prevented the Ag-NPs growing into larger aggregates. The histograms based on the TEM images illustrated the Ag-NPs average size and size distribution. As seen in Fig. 6, the Ag-NPs obtained with α-CD have a wider size distribution than those synthetized with β-CD, and these in turn with those obtained with γ-CD. The Ag-NPs obtained with α-CD, has a particle size of 1–7 nm (wide size distribution) and a mode of 1.5 nm; 88% of the particles are 1–3.5 nm; the Ag-NPs with β-CD exhibited a smaller size (0.5–4.5 nm) and a narrower size distribution; 95% of the particles are between 0.5 and 3 nm, with a mode of 1 nm; finally, for γ-CD the particle size was 1.5–6.5 nm, with a mode of 3 nm; 91% of the particles are 2–4.5 nm. The nanoparticles obtained by this method have a smaller size and narrower size dispersity than in previous reports [19,44]. From the above mentioned we can conclude that the β-CD is the best stabilizing agent, because these particles have a smaller size. However, the obtained nanoparticles with γ-CD, shows a narrower size distribution.
We also proved that there is a relation between the maximum wavelength absorption obtained from the UV–vis spectra (Fig. 3) and the nanoparticle size distribution histograms (Fig. 6); it can be seen that the maximum wavelength absorption increases with the size of the nanoparticles. Multiple lattice fringes can be clearly observed under high-resolution TEM (Fig. 7). The lattice spacing of the Ag-NPs was 0.208 nm in accordance with the lattice spacing on the (2 0 0) plane of silver. The TEM analyses were obtained counter-clockwise. The d-spacing of the leftmost particle has been measured and the value shows 0.208 nm and 0.148 nm. The d-spacing of the remaining particle is 0.227 nm and 0.144 nm, respectively, which is close to the JCPDS value of 0.205 nm [PDF # 04-0783]. The clear lattice fringes in TEM images and the typical selected area electron diffraction (SAED) pattern with circular rings corresponding to the (1 1 1), (2 0 0), and (2 2 0), planes show that the nanoparticles obtained are highly crystalline (FCC structure). These results are in accordance with those in previous reports [28,45,46]. 4. Conclusions In this paper, we have reported the effect of the size of CDs on the synthesis of Ag-NPs through a simple, safe, and green chemistry method, using the chemical reduction method. The silver nanostructures prepared with this method can be stored at room temperature for nearly 6 months without any visible change or effect on their size. The nanoparticles obtained were characterized by UV–vis, SEM–EDS, AFM and TEM. UV–vis analyses show that the maximum wavelength absorption is directly proportional to the size of the nanoparticles; small nanoparticles show maximum wavelength absorption at high field, whereas with increasing nanoparticle size, the maximum wavelength absorption moves downfield. TEM analysis shows that the nanoparticles obtained are between 0.5 nm and 7 nm (α-CD, 1–7 nm; β-CD, 0.5–4.5 nm; and γ-CD, 1.5– 6.5 nm) in size; the β-CD has the smallest nanoparticle size, and 96.9%
Fig. 5. AFM analysis of Ag-NPs synthesized with different CDs.
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Fig. 6. TEM analysis of Ag-NPs and particle size distribution histograms of Ag-NPs formed with α-, β-, and γ-CD.
of the particles obtained with β-CD are between 0.5–3.5 nm in size, with an average of 1 nm. However, the γ-CD presents a narrowed nanoparticle size distribution; 44.4% of the particles have a size between 2 and 2.5 nm, and also the 95.6% of the particles synthetized with γ-CD has a size between 1.5 and 4.5 nm. On another hand, the α-CD shows a wider size distribution, and 87.8% of the nanoparticles have a size between 1 and 3.5 nm. So we can conclude that (the) β-CD was the best stabilizing agent for the synthesis of small Ag-NPs, whereas γ- CD is better to obtain narrow nanoparticles size distributions. This can be attributed to the number of glucopyranose units in its structure. In addition, TEM images show a clear lattice spacing of silver which is in agreement with a crystalline and face centered cubic structure of Ag-NPs.
Acknowledgments We gratefully acknowledge the support for this research work to the Dirección General de Educación Superior Tecnológica (DGEST Grant 4374.11-P). Also, to Israel Gradilla Martίnez (SEM–EDS) and Francisco Ruiz Medina (TEM) from the Centro de Nanociencias y Nanotecnología de la UNAM; and Pedro Navarro-Vega (AFM) from the Centro de Graduados e Investigación del Instituto Tecnológico de Tijuana.
Fig. 7. Cristal analysis of Ag-NPs.
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