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Biomimetic synthesis of lotus leaf extract-assisted silver nanoparticles and shape-directing role of cetyltrimethylammonium bromide
Journal of Molecular Liquids 220 (2016) 795–801
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Biomimetic synthesis of lotus leaf extract-assisted silver nanoparticles and shape-directing role of cetyltrimethylammonium bromide Abou Talib a, Hui-Fen-Wu a,b,c,d,e,⁎ a
Doctoral Degree Program in Marine Biotechnology, National Sun Yat-Sen University, Taiwan Department of Chemistry, National Sun Yat-Sen University Kaohsiung, 70, Lien-Hai Road, 80424, Taiwan Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Taiwan d School of Pharmacy, College of Pharmacy, Kaohsiung Medical University, Kaohsiung 800, Taiwan e Institute of Medical Science and Technology, National Sun Yat-Sen University, 80424, Taiwan b c
a r t i c l e
i n f o
Article history: Received 14 December 2015 Accepted 23 April 2016 Available online xxxx Keywords: Lotus leaf Biomimetic synthesis Silver nanoparticles Morphology Adsorption
a b s t r a c t We report a simple biomimetic chemical reduction method for the preparation of orange-red color silver sol in absence and presence of cetyltrimethylammonium bromide (CTAB), at room temperature. Silver nitrate and lotus leaf extract were taken as the metal precursor and reducing agent. As the reaction proceeds, a typical plasmon absorption band in 400 to 550 nm range appears for the silver nanoparticles and the intensities increase with the time and [reactants]. UV–visible spectroscopy, transmission electron microscopy (TEM) and selected areas electron diffraction (SAED) were used to characterize the silver sol. TEM images show that the silver sol consists of aggregated, cross-linked and inter-connected arrangement of spherical and truncated nanoplates, and irregular silver nanoparticles. The dependency of position and shape of the absorption band on CTAB concentration infers that it acts as a shape-directing, cross-linking and stabilizing agent during the growth of different shape and size of silver nanoparticles. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Metals nanoparticles have earned so much attention of researchers by virtue of their unique optical and electronic properties [1–5]. Synthesis and characterization of advanced nanomaterials of silver and gold using various, physical, biochemical, and chemical, methods have been the subject of large number of studies in the last two decades [6–13]. The most common chemical route for fabrication of metal nanoparticles is the reduction of aqueous solution of metal salt as precursor, executed by a suitable reducing agent [14]. In most of the cases, a further appropriate stabilizing agent (toxic chemicals, polymers, organic solvents, ligands, dendrimers, copolymers, surfactants, phospholipids and Gemini surfactants) is required to prevent agglomeration or further growth of the particles [15–20], which may carry some environmental and biological risks. Out of these stabilizers, surfactant aggregates, micelles, reverse micelles and macroemulsions, will get an edge over other stabilizers [21]. Biosynthetic methods to synthesize advanced noble metal particles by employing either biological microorganisms (bacteria, fungi, yeast and fruits) or plant extracts have emerged as an alternative reductant of the toxic chemical synthetic procedures [22–26]. Extracts from bio⁎ Corresponding author at: Doctoral Degree Program in Marine Biotechnology, National Sun Yat-Sen University, Taiwan. E-mail address: [email protected] (Hui-Fen-Wu).
http://dx.doi.org/10.1016/j.molliq.2016.04.100 0167-7322/© 2016 Elsevier B.V. All rights reserved.
organisms may act as reducing, capping, and stabilizing agents in Ag nanoparticles synthesis. Although the extensive studies have been performed on the synthesis and the anti-microbial activities of Ag nanoparticles using plants leaf and fruits extract, the use of surfactant(s) as the stabilizing agents is very rare, in the similar studies [27]. The role of surfactants in the green biosynthesis of Ag sol is not yet understood. Surfactants can serve as a capping or protecting agent for nanoparticles in order to stabilize them in solutions of higher ionic strength. The kinetics and morphology of Ag nanomaterials can be altered, depending on the nature of the reducing agents and the surfactant. Amphiphilic nature of surfactant having both polarities form aggregates, such as micelles, in such a way that hydrophobic tails form the core of the aggregate and the hydrophilic heads are in contact with the surrounding liquid. Synthesis of multi-branched gold nanoparticles was reported by Sau et al. [28] in the presence of CTAB, for the very first time. Chen et al. [29] also reported Ag plate seeded synthesis of gold monopods, bipods, tripods, and tetrapods using the cationic CTAB surfactant as a capping agent. It is therefore imperative to investigate the Ag-nanoparticles formation in presence of lotus leaf extract with a view to having an insight into the role of surfactants. Lotus leaf contains high concentrations of phytochemicals, compounds produced by plants to defend themselves against bacterial and fungal infections. The substances found in the lotus leaf extract include alkaloids, flavonoids and tannins [30]. The isoquinoline alkaloid in lotus leaf has sedative and antispasmodic
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resembles that of polyhedral truncated triangular nanoplates in absence and presence of CTAB.
2. Experimental 2.1. Materials and methods
Fig. 1. Spectra of lotus leaf extract (=5.0 cm3; ■) and its reaction product with Ag+ ions (4.0 × 10−4 mol dm−3) as a function of time (=30 (●), 60 (▲) and 90 min (▼)) at 30 °C.
The double distilled (first time from alkaline KMnO4), CO2 free and deionized, water was used as a solvent. Silver nitrate (AgNO3) (Merck 99%), and cetyltrimethylammonium bromide (CTAB) (BDH, England, 99%) were used to prepare their stock solutions without further purification. Spectral properties were studied by UV–Vis Spectroscopy (Lambda-25, Perkin Elmer) using a standard quartz cuvette having the path length of 1 cm. Transmission electron microscopy (TEM, Jeol, Japan) was used to clarify the size and shape of Ag nanoparticles (AgNPs). Samples were dried on a formwar for the TEM analysis. Crystallinity was studied using X-ray Diffraction (XRD) (Phillips, The Nederland) technique. For XRD analysis, samples were coated on a glass coverslip and dried under ambient conditions.
properties, which may cause indigestion. Both flavonoids and alkaloids are powerful antioxidants. Here, we report a simple biomimetic room temperature method (lotus + Ag+ reaction) for the synthesis of multi-branched interconnected silver nanoparticles whose shape
Fig. 2. Spectra of silver nanoparticles as a function of time under different experimental conditions. Reaction conditions: [Ag+] = 4.0 × 10−4 mol dm−3, [leaf extract] = 10.0 (A) and 15.0 cm3 (B), temperature = 30 °C.
Fig. 3. Spectra of silver nanoparticles as a function of time. Reaction conditions: [Ag+] = 8.0 (A) and 12.0 × 10−4 mol dm−3 (B), [leaf extract] = 5.0 cm3, temperature = 30 °C.
A. Talib, Hui-Fen-Wu / Journal of Molecular Liquids 220 (2016) 795–801
2.2. Preparation of leaf extract Fresh 10 g of lotus leaf were washed with tape water followed by distilled water to remove dust particles, and subsequently boiled for 20 min with 250 cm3 distilled water in an Erlenmeyer flask at room temperature. The solution was cooled to room temperature and then filtered with the Whatman paper No. 1. The filtrate was used as a reducing - as well as stabilizing agent for the preparation of AgNPs. 2.3. Synthesis and characterization of AgNPs In a typical experiment, when AgNO3 (5.0 cm3, 0.01 mol dm−3) was allowed to react with 5.0 cm3 of the leaf extract (total volume = 50 cm3 adjusted with deionized water for the green biomimetic reduction process), a readily distinguishable yellow-brownish color appeared as the reaction proceeded, indicating the AgNPs formation [22,27]. In order to confirm the nature of the yellow-brownish color mixture, the spectra were recorded on a UV–visible spectrophotometer at different time intervals with varying concentrations of [Ag+], [leaf extract] and [CTAB]. 2.4. Kinetic measurements The reaction mixture (silver nitrate and H2O) was added in a twonecked flask fitted with a spiral double-walled condenser and a glass stopper. The reaction was started after the addition of measured amount of the leaf extract solution (reaction volume was always 50 cm3). The progress of reaction was followed by measuring the absorbance of brownish colored Ag sol at 550 nm (neither Ag+ ions and leaf
797
extract nor products show any absorbance at this wavelength) on the spectrophotometer. A controlled dynamic pH-meter fitted with a combination electrode was used to measure the pH of the working solution. 3. Results and discussion 3.1. Morphology of silver nanoparticles in absence of CTAB The color change (from pale yellow, yellow, orange to brownishyellow) of the AgNPs occurred as a function of the reaction time indicates the morphology alteration of AgNPs at different reaction stages. Therefore, UV–visible spectra recorded during the synthesis and the observed results are depicted graphically in Figs. 1–4, for the different experimental conditions. Inspection of these data indicates that shape of the spectra, position of band and intensity strongly depend on the experimental conditions, i.e., reaction time (Fig. 1), [leaf extract] (Fig. 2) and [Ag+] ions (Fig. 3). The curve reflects the typically observed time dependency, with a fast initial intensity increase followed by a slow increase. All the spectra have one broad shoulder and/or broad band in common, located at 500 nm. With increase in the reaction time, the shoulder at 500 nm shifts towards a longer wavelength for 550 nm (Fig. 1). In addition to the red shift, the width of the band is also increased. The wavelength shift with an increase in the reaction time might be attributed to an increase in particle and/or shape anisotropy. The position of the band at higher wavelength depends on the sharpness of the corners of triangles [3,31]. From Mie s̉ theory [32], small single metallic nanocrystals should exhibit a single surface Plasmon resonance band depending on their shapes. In this study, the appearance of a broad
Fig. 4. TEM images of silver nanoparticles. Reaction conditions: [Ag+] = 4.0 × 10−4 mol dm−3, [leaf extract] = 5.0 cm3, temperature = 30 °C.
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peak around. 500 nm may be attributed to the characteristic of faceted particles and triangular silver nanoplates with size ca. 23 to 45 nm (Fig. 4a and b). The TEM image indicates (Fig. 4b) the aggregation and/ or deposition of particles onto the surface of sphere-shaped particles. It further depicts the arrangement of AgNPs aggregates in a symmetric manner to form a beautiful necklace-like structure. A large fraction of triangles having round corners can be seen in TEM images of the resulting silver nanoparticles (Fig. 4b). Selected area electron diffraction ring patterns also suggest that the AGNPs are pure single crystals (Fig. 4C). It is important to know the exact nature of the silver particles formed and this can be achieved by measuring the XRD patterns of the samples. The XRD pattern measured in this case resulted in four intense peaks i.e. 111,200,220,311 (Fig. 4d). The four intense peaks observed in the spectrum are in accordance to the Braggs's reflection patterns of silver nanocrystals reported elsewhere [33]. The TEM analyses corroborate well with the results drawn from the corresponding spectra (Figs. 1 to 3). A faint thin layer of another material was also visualized on the surface of resulting NPs in the TEM images which might be due to the stabilizing and capping action of the organic constituents (polyphenols, alkaloids, steroids, saponins and tannins [34]). 3.2. Morphology of silver nanoparticles in presence of CTAB It is a well documented fact that advanced noble metal NPs can be synthesized by using a suitable long hydrocarbon tail and less bulky head group surfactant [20,21]. The nature of the head group of the surfactant plays an important role in determining stability, and the morphology of the nanoparticles due to there unique interaction with nanoparticle surface in a particular solution. The termination of the nanoparticle growth is controlled by the diffusion and the attachment rates of surfactants on the nanoparticle surface. Due to the negative charge present on the surface of silver nanoparticles, CTAB could be the most ideal shape navigating as well as stabilizing agent. The effect of [CTAB] was studied at two different fixed [Ag+] (4.0 × 10−4 and 12.0 × 10−4 mol dm−3), [leaf extract] (5.0 cm3), and temperature 30 °C. The [CTAB] varied from 2.0 × 10−4 to 8.0 × 10−4 mol dm−3. The absorbance increased with reaction time, and the position and shape of band was changed significantly with increasing [CTAB] and/or [Ag+] (Figs. 5 and 6). In presence of [Ag+] = 4.0 × 10−4 mol dm−3 and [CTAB] = 2.0 × 10−4 mol dm−3, absorption spectra of the silver nanoparticles shows a sharp band at 425 nm (Fig. 5A). On the other hand, the shape of the spectra entirely changed at higher [CTAB] (4.0 × 10−4 mol dm−3). There is a broad shoulder begins to develops in the whole visible region instead of a peak (Fig. 5B). However, the spectrum of the AGNPs at higher [Ag+] = 12.0 × 10−4 mol dm−3 with the same [CTAB] as those of Fig. 5, shows the appearance of different color at different time intervals with a broad absorption in the range of 425 to 500 nm (Figs. 5 and 6). These results also demonstrate that the CTAB catalytic effect takes place not only above the CMC but also below it (i.e., micellar as well as pre-micellar catalyses are observed) [35]. Surprisingly, as the [CTAB] increased from 12.0 × 10−4 mM, drastic alterations in optical and physical properties of silver nanoparticles was observed as evident from Fig. 6. These alterations might be due to interaction of surfactants with the nanoparticles, thus changing the dielectric properties and/or interfering during the nucleation of nanoparticles. The combined effect of both the process can be change in the optical behavior and shape of the silver nanoparticles. [36]. The position of reactants and extent of water penetration into the bulk of micellar structure has a major impact on the reactivity. The water potential is lower at the interface than in the bulk due to the enhanced ionic concentration of the micellar region [37–39]. The lotus leaf extract contains large numbers of bioactive molecules like polyphenols, alkaloids, flavanoids, steroids, saponins and tannins. Therefore, addition of the lotus leaf constituents into the Stern layer of CTAB micelles via electrostatic and hydrophobic interactions can also be considered as the prime reason for shape alteration and optical
Fig. 5. Spectra of silver nanoparticles as a function of time in presence of CTAB. Reaction conditions: [Ag+] = 4.0 × 10−4 mol dm−3, [leaf extract] = 5.0 cm3, [CTAB] = 2.0 (A) and 4.0 × 10−4 mol dm−3 (B), temperature = 30 °C.
properties of silver nanoparticles. On the other hand, the polyhydroxyl groups of leaf constituents confront with the CTAB. Also, sub-micellar aggregates and ion-pairs are formed between the positive head group of CTAB and the reactants. As a result of above two phenomenon, the surface area of the leaf constituents and Ag+ ions depletes which, in turn, accelerates the reduction of Ag ions into the metallic silver [40]. In accordance with the aforementioned comments, the reduction of Ag+ ions to Ag0 and the transformation of oligomeric Ag clusters, a possible mechanism was proposed in which Higenamine, an important constituent of lotus leaf extract, plays pivotal role. As displayed in Scheme 1, the process of nucleation initiates the reduction of Ag+ ions into the metallic silver in presence of hydroxy groups of Higenamine. The complexation of the formed Ag0 atoms with Ag+ ions yields Ag2 + ions, which further dimerize to yield yellow-color silver sol [41].
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Fig. 6. Spectra of silver nanoparticles as a function of time in presence of CTAB. Reaction conditions: [Ag+] = 12.0 × 10−4 mol dm−3, [leaf extract] = 5.0 cm3, [CTAB] = 2.0 (A), 4.0 (B) and 8.0 × 10−4 mol dm−3 (C), temperature = 30 °C.
Fig. 7 depicts the TEM images of AgNPs prepared by the lotus leaf extract reduction of Ag+ ions in the presence of CTAB. Initially, it is apparent that the as-prepared structures are roughly spherical (size = 35 to
70 nm), nanodisks and /or truncated triangles with diameter of ca. 90 nm, wide dispersed (Fig. 7A). UV–visible spectra with one strong peak in fact correspond to the spherical with some irregular
Scheme 1. Mechanism to the reduction of Ag+ ions to the silver nanoparticles.
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Fig. 7. TEM images of silver nanoparticles in presence of CTAB. Reaction conditions: [Ag+] = 4.0 × 10−4 mol dm−3, [leaf extract] = 5.0 cm3, temperature = 30 °C.
morphology of resulting silver nanoparticles. The TEM image also indicates the non-linear inter particle interaction (cross-linking), aggregation and/or deposition of tiny particles onto the surface of sphereshaped particles and also reveals that these silver nanoparticles aggregate beautifully in an unsymmetric manner to form branches-like structure (Fig. 7B). The bodies have a very dark contrast, indicating rich three-dimensional (3D) structures which are likely formed by selforganizing two-dimensional (2D) flower-like nanocrystals [42]. When the electron diffraction is carried out a limited number of crystals one observes only some spots of diffraction distributed on concentric circles. A typical SAED pattern is also shown in Fig. 7C and it clearly exhibits diffraction rings with the interplanar spacing that can be indexed ({110} (1), {111} (2), {133} (3), {200} (4), {211} (5), {220} (6), {222} (7), {311} (8), {331} (9) and {422} (10)) according to the pure facecentered cubic (fcc) Ag structure and are consistent with the literature (JCPDS, File No. 4-0787) [43–45]. 4. Conclusions The impact on biological synthesis using plant extract was found to be pivotal in the present study. Due to involvement of plethora proteins involved in stabilization as well as reduction of silver ions to nanoparticles, the stability of the nanoparticles was found to be exceptionally high. However, the key problem associated with bio-synthesis of silver nanoparticles was the polydispersity. In presence of low concentration of CTAB, the optical properties of the nanoparticles were augmented as evident from the sharpness of the peak. CTAB along with proteins present in the leaf extract modified the shape of silver nanoparticles to give mono-dispersity. Finally, we conclude the significance of positively charged CTAB along with capping proteins in order to tune the morphology of silver nanoparticles.
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Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Biomimetic synthesis of lotus leaf extract-assisted silver nanoparticles and shape-directing role of cetyltrimethylammonium bromide Abou Talib a, Hui-Fen-Wu a,b,c,d,e,⁎ a
Doctoral Degree Program in Marine Biotechnology, National Sun Yat-Sen University, Taiwan Department of Chemistry, National Sun Yat-Sen University Kaohsiung, 70, Lien-Hai Road, 80424, Taiwan Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Taiwan d School of Pharmacy, College of Pharmacy, Kaohsiung Medical University, Kaohsiung 800, Taiwan e Institute of Medical Science and Technology, National Sun Yat-Sen University, 80424, Taiwan b c
a r t i c l e
i n f o
Article history: Received 14 December 2015 Accepted 23 April 2016 Available online xxxx Keywords: Lotus leaf Biomimetic synthesis Silver nanoparticles Morphology Adsorption
a b s t r a c t We report a simple biomimetic chemical reduction method for the preparation of orange-red color silver sol in absence and presence of cetyltrimethylammonium bromide (CTAB), at room temperature. Silver nitrate and lotus leaf extract were taken as the metal precursor and reducing agent. As the reaction proceeds, a typical plasmon absorption band in 400 to 550 nm range appears for the silver nanoparticles and the intensities increase with the time and [reactants]. UV–visible spectroscopy, transmission electron microscopy (TEM) and selected areas electron diffraction (SAED) were used to characterize the silver sol. TEM images show that the silver sol consists of aggregated, cross-linked and inter-connected arrangement of spherical and truncated nanoplates, and irregular silver nanoparticles. The dependency of position and shape of the absorption band on CTAB concentration infers that it acts as a shape-directing, cross-linking and stabilizing agent during the growth of different shape and size of silver nanoparticles. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Metals nanoparticles have earned so much attention of researchers by virtue of their unique optical and electronic properties [1–5]. Synthesis and characterization of advanced nanomaterials of silver and gold using various, physical, biochemical, and chemical, methods have been the subject of large number of studies in the last two decades [6–13]. The most common chemical route for fabrication of metal nanoparticles is the reduction of aqueous solution of metal salt as precursor, executed by a suitable reducing agent [14]. In most of the cases, a further appropriate stabilizing agent (toxic chemicals, polymers, organic solvents, ligands, dendrimers, copolymers, surfactants, phospholipids and Gemini surfactants) is required to prevent agglomeration or further growth of the particles [15–20], which may carry some environmental and biological risks. Out of these stabilizers, surfactant aggregates, micelles, reverse micelles and macroemulsions, will get an edge over other stabilizers [21]. Biosynthetic methods to synthesize advanced noble metal particles by employing either biological microorganisms (bacteria, fungi, yeast and fruits) or plant extracts have emerged as an alternative reductant of the toxic chemical synthetic procedures [22–26]. Extracts from bio⁎ Corresponding author at: Doctoral Degree Program in Marine Biotechnology, National Sun Yat-Sen University, Taiwan. E-mail address: [email protected] (Hui-Fen-Wu).
http://dx.doi.org/10.1016/j.molliq.2016.04.100 0167-7322/© 2016 Elsevier B.V. All rights reserved.
organisms may act as reducing, capping, and stabilizing agents in Ag nanoparticles synthesis. Although the extensive studies have been performed on the synthesis and the anti-microbial activities of Ag nanoparticles using plants leaf and fruits extract, the use of surfactant(s) as the stabilizing agents is very rare, in the similar studies [27]. The role of surfactants in the green biosynthesis of Ag sol is not yet understood. Surfactants can serve as a capping or protecting agent for nanoparticles in order to stabilize them in solutions of higher ionic strength. The kinetics and morphology of Ag nanomaterials can be altered, depending on the nature of the reducing agents and the surfactant. Amphiphilic nature of surfactant having both polarities form aggregates, such as micelles, in such a way that hydrophobic tails form the core of the aggregate and the hydrophilic heads are in contact with the surrounding liquid. Synthesis of multi-branched gold nanoparticles was reported by Sau et al. [28] in the presence of CTAB, for the very first time. Chen et al. [29] also reported Ag plate seeded synthesis of gold monopods, bipods, tripods, and tetrapods using the cationic CTAB surfactant as a capping agent. It is therefore imperative to investigate the Ag-nanoparticles formation in presence of lotus leaf extract with a view to having an insight into the role of surfactants. Lotus leaf contains high concentrations of phytochemicals, compounds produced by plants to defend themselves against bacterial and fungal infections. The substances found in the lotus leaf extract include alkaloids, flavonoids and tannins [30]. The isoquinoline alkaloid in lotus leaf has sedative and antispasmodic
796
A. Talib, Hui-Fen-Wu / Journal of Molecular Liquids 220 (2016) 795–801
resembles that of polyhedral truncated triangular nanoplates in absence and presence of CTAB.
2. Experimental 2.1. Materials and methods
Fig. 1. Spectra of lotus leaf extract (=5.0 cm3; ■) and its reaction product with Ag+ ions (4.0 × 10−4 mol dm−3) as a function of time (=30 (●), 60 (▲) and 90 min (▼)) at 30 °C.
The double distilled (first time from alkaline KMnO4), CO2 free and deionized, water was used as a solvent. Silver nitrate (AgNO3) (Merck 99%), and cetyltrimethylammonium bromide (CTAB) (BDH, England, 99%) were used to prepare their stock solutions without further purification. Spectral properties were studied by UV–Vis Spectroscopy (Lambda-25, Perkin Elmer) using a standard quartz cuvette having the path length of 1 cm. Transmission electron microscopy (TEM, Jeol, Japan) was used to clarify the size and shape of Ag nanoparticles (AgNPs). Samples were dried on a formwar for the TEM analysis. Crystallinity was studied using X-ray Diffraction (XRD) (Phillips, The Nederland) technique. For XRD analysis, samples were coated on a glass coverslip and dried under ambient conditions.
properties, which may cause indigestion. Both flavonoids and alkaloids are powerful antioxidants. Here, we report a simple biomimetic room temperature method (lotus + Ag+ reaction) for the synthesis of multi-branched interconnected silver nanoparticles whose shape
Fig. 2. Spectra of silver nanoparticles as a function of time under different experimental conditions. Reaction conditions: [Ag+] = 4.0 × 10−4 mol dm−3, [leaf extract] = 10.0 (A) and 15.0 cm3 (B), temperature = 30 °C.
Fig. 3. Spectra of silver nanoparticles as a function of time. Reaction conditions: [Ag+] = 8.0 (A) and 12.0 × 10−4 mol dm−3 (B), [leaf extract] = 5.0 cm3, temperature = 30 °C.
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2.2. Preparation of leaf extract Fresh 10 g of lotus leaf were washed with tape water followed by distilled water to remove dust particles, and subsequently boiled for 20 min with 250 cm3 distilled water in an Erlenmeyer flask at room temperature. The solution was cooled to room temperature and then filtered with the Whatman paper No. 1. The filtrate was used as a reducing - as well as stabilizing agent for the preparation of AgNPs. 2.3. Synthesis and characterization of AgNPs In a typical experiment, when AgNO3 (5.0 cm3, 0.01 mol dm−3) was allowed to react with 5.0 cm3 of the leaf extract (total volume = 50 cm3 adjusted with deionized water for the green biomimetic reduction process), a readily distinguishable yellow-brownish color appeared as the reaction proceeded, indicating the AgNPs formation [22,27]. In order to confirm the nature of the yellow-brownish color mixture, the spectra were recorded on a UV–visible spectrophotometer at different time intervals with varying concentrations of [Ag+], [leaf extract] and [CTAB]. 2.4. Kinetic measurements The reaction mixture (silver nitrate and H2O) was added in a twonecked flask fitted with a spiral double-walled condenser and a glass stopper. The reaction was started after the addition of measured amount of the leaf extract solution (reaction volume was always 50 cm3). The progress of reaction was followed by measuring the absorbance of brownish colored Ag sol at 550 nm (neither Ag+ ions and leaf
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extract nor products show any absorbance at this wavelength) on the spectrophotometer. A controlled dynamic pH-meter fitted with a combination electrode was used to measure the pH of the working solution. 3. Results and discussion 3.1. Morphology of silver nanoparticles in absence of CTAB The color change (from pale yellow, yellow, orange to brownishyellow) of the AgNPs occurred as a function of the reaction time indicates the morphology alteration of AgNPs at different reaction stages. Therefore, UV–visible spectra recorded during the synthesis and the observed results are depicted graphically in Figs. 1–4, for the different experimental conditions. Inspection of these data indicates that shape of the spectra, position of band and intensity strongly depend on the experimental conditions, i.e., reaction time (Fig. 1), [leaf extract] (Fig. 2) and [Ag+] ions (Fig. 3). The curve reflects the typically observed time dependency, with a fast initial intensity increase followed by a slow increase. All the spectra have one broad shoulder and/or broad band in common, located at 500 nm. With increase in the reaction time, the shoulder at 500 nm shifts towards a longer wavelength for 550 nm (Fig. 1). In addition to the red shift, the width of the band is also increased. The wavelength shift with an increase in the reaction time might be attributed to an increase in particle and/or shape anisotropy. The position of the band at higher wavelength depends on the sharpness of the corners of triangles [3,31]. From Mie s̉ theory [32], small single metallic nanocrystals should exhibit a single surface Plasmon resonance band depending on their shapes. In this study, the appearance of a broad
Fig. 4. TEM images of silver nanoparticles. Reaction conditions: [Ag+] = 4.0 × 10−4 mol dm−3, [leaf extract] = 5.0 cm3, temperature = 30 °C.
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peak around. 500 nm may be attributed to the characteristic of faceted particles and triangular silver nanoplates with size ca. 23 to 45 nm (Fig. 4a and b). The TEM image indicates (Fig. 4b) the aggregation and/ or deposition of particles onto the surface of sphere-shaped particles. It further depicts the arrangement of AgNPs aggregates in a symmetric manner to form a beautiful necklace-like structure. A large fraction of triangles having round corners can be seen in TEM images of the resulting silver nanoparticles (Fig. 4b). Selected area electron diffraction ring patterns also suggest that the AGNPs are pure single crystals (Fig. 4C). It is important to know the exact nature of the silver particles formed and this can be achieved by measuring the XRD patterns of the samples. The XRD pattern measured in this case resulted in four intense peaks i.e. 111,200,220,311 (Fig. 4d). The four intense peaks observed in the spectrum are in accordance to the Braggs's reflection patterns of silver nanocrystals reported elsewhere [33]. The TEM analyses corroborate well with the results drawn from the corresponding spectra (Figs. 1 to 3). A faint thin layer of another material was also visualized on the surface of resulting NPs in the TEM images which might be due to the stabilizing and capping action of the organic constituents (polyphenols, alkaloids, steroids, saponins and tannins [34]). 3.2. Morphology of silver nanoparticles in presence of CTAB It is a well documented fact that advanced noble metal NPs can be synthesized by using a suitable long hydrocarbon tail and less bulky head group surfactant [20,21]. The nature of the head group of the surfactant plays an important role in determining stability, and the morphology of the nanoparticles due to there unique interaction with nanoparticle surface in a particular solution. The termination of the nanoparticle growth is controlled by the diffusion and the attachment rates of surfactants on the nanoparticle surface. Due to the negative charge present on the surface of silver nanoparticles, CTAB could be the most ideal shape navigating as well as stabilizing agent. The effect of [CTAB] was studied at two different fixed [Ag+] (4.0 × 10−4 and 12.0 × 10−4 mol dm−3), [leaf extract] (5.0 cm3), and temperature 30 °C. The [CTAB] varied from 2.0 × 10−4 to 8.0 × 10−4 mol dm−3. The absorbance increased with reaction time, and the position and shape of band was changed significantly with increasing [CTAB] and/or [Ag+] (Figs. 5 and 6). In presence of [Ag+] = 4.0 × 10−4 mol dm−3 and [CTAB] = 2.0 × 10−4 mol dm−3, absorption spectra of the silver nanoparticles shows a sharp band at 425 nm (Fig. 5A). On the other hand, the shape of the spectra entirely changed at higher [CTAB] (4.0 × 10−4 mol dm−3). There is a broad shoulder begins to develops in the whole visible region instead of a peak (Fig. 5B). However, the spectrum of the AGNPs at higher [Ag+] = 12.0 × 10−4 mol dm−3 with the same [CTAB] as those of Fig. 5, shows the appearance of different color at different time intervals with a broad absorption in the range of 425 to 500 nm (Figs. 5 and 6). These results also demonstrate that the CTAB catalytic effect takes place not only above the CMC but also below it (i.e., micellar as well as pre-micellar catalyses are observed) [35]. Surprisingly, as the [CTAB] increased from 12.0 × 10−4 mM, drastic alterations in optical and physical properties of silver nanoparticles was observed as evident from Fig. 6. These alterations might be due to interaction of surfactants with the nanoparticles, thus changing the dielectric properties and/or interfering during the nucleation of nanoparticles. The combined effect of both the process can be change in the optical behavior and shape of the silver nanoparticles. [36]. The position of reactants and extent of water penetration into the bulk of micellar structure has a major impact on the reactivity. The water potential is lower at the interface than in the bulk due to the enhanced ionic concentration of the micellar region [37–39]. The lotus leaf extract contains large numbers of bioactive molecules like polyphenols, alkaloids, flavanoids, steroids, saponins and tannins. Therefore, addition of the lotus leaf constituents into the Stern layer of CTAB micelles via electrostatic and hydrophobic interactions can also be considered as the prime reason for shape alteration and optical
Fig. 5. Spectra of silver nanoparticles as a function of time in presence of CTAB. Reaction conditions: [Ag+] = 4.0 × 10−4 mol dm−3, [leaf extract] = 5.0 cm3, [CTAB] = 2.0 (A) and 4.0 × 10−4 mol dm−3 (B), temperature = 30 °C.
properties of silver nanoparticles. On the other hand, the polyhydroxyl groups of leaf constituents confront with the CTAB. Also, sub-micellar aggregates and ion-pairs are formed between the positive head group of CTAB and the reactants. As a result of above two phenomenon, the surface area of the leaf constituents and Ag+ ions depletes which, in turn, accelerates the reduction of Ag ions into the metallic silver [40]. In accordance with the aforementioned comments, the reduction of Ag+ ions to Ag0 and the transformation of oligomeric Ag clusters, a possible mechanism was proposed in which Higenamine, an important constituent of lotus leaf extract, plays pivotal role. As displayed in Scheme 1, the process of nucleation initiates the reduction of Ag+ ions into the metallic silver in presence of hydroxy groups of Higenamine. The complexation of the formed Ag0 atoms with Ag+ ions yields Ag2 + ions, which further dimerize to yield yellow-color silver sol [41].
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Fig. 6. Spectra of silver nanoparticles as a function of time in presence of CTAB. Reaction conditions: [Ag+] = 12.0 × 10−4 mol dm−3, [leaf extract] = 5.0 cm3, [CTAB] = 2.0 (A), 4.0 (B) and 8.0 × 10−4 mol dm−3 (C), temperature = 30 °C.
Fig. 7 depicts the TEM images of AgNPs prepared by the lotus leaf extract reduction of Ag+ ions in the presence of CTAB. Initially, it is apparent that the as-prepared structures are roughly spherical (size = 35 to
70 nm), nanodisks and /or truncated triangles with diameter of ca. 90 nm, wide dispersed (Fig. 7A). UV–visible spectra with one strong peak in fact correspond to the spherical with some irregular
Scheme 1. Mechanism to the reduction of Ag+ ions to the silver nanoparticles.
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Fig. 7. TEM images of silver nanoparticles in presence of CTAB. Reaction conditions: [Ag+] = 4.0 × 10−4 mol dm−3, [leaf extract] = 5.0 cm3, temperature = 30 °C.
morphology of resulting silver nanoparticles. The TEM image also indicates the non-linear inter particle interaction (cross-linking), aggregation and/or deposition of tiny particles onto the surface of sphereshaped particles and also reveals that these silver nanoparticles aggregate beautifully in an unsymmetric manner to form branches-like structure (Fig. 7B). The bodies have a very dark contrast, indicating rich three-dimensional (3D) structures which are likely formed by selforganizing two-dimensional (2D) flower-like nanocrystals [42]. When the electron diffraction is carried out a limited number of crystals one observes only some spots of diffraction distributed on concentric circles. A typical SAED pattern is also shown in Fig. 7C and it clearly exhibits diffraction rings with the interplanar spacing that can be indexed ({110} (1), {111} (2), {133} (3), {200} (4), {211} (5), {220} (6), {222} (7), {311} (8), {331} (9) and {422} (10)) according to the pure facecentered cubic (fcc) Ag structure and are consistent with the literature (JCPDS, File No. 4-0787) [43–45]. 4. Conclusions The impact on biological synthesis using plant extract was found to be pivotal in the present study. Due to involvement of plethora proteins involved in stabilization as well as reduction of silver ions to nanoparticles, the stability of the nanoparticles was found to be exceptionally high. However, the key problem associated with bio-synthesis of silver nanoparticles was the polydispersity. In presence of low concentration of CTAB, the optical properties of the nanoparticles were augmented as evident from the sharpness of the peak. CTAB along with proteins present in the leaf extract modified the shape of silver nanoparticles to give mono-dispersity. Finally, we conclude the significance of positively charged CTAB along with capping proteins in order to tune the morphology of silver nanoparticles.
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