Silver or gold deposition onto magnetite nanoparticles

Artificial Cells, Nanomedicine, and Biotechnology, 2015; Early Online: 1–7 Copyright © 2015 Informa Healthcare USA, Inc. ISSN: 2169-1401 print / 2169-141X online DOI: 10.3109/21691401.2015.1019672

Silver or gold deposition onto magnetite nanoparticles by using plant extracts as reducing and stabilizing agents

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Araz Norouz Dizaji*, Mehmet Yilmaz* & Erhan Piskin Department of Chemical Engineering, Bioengineering Division, Hacettepe University and Biyomedtek/Nanobiyomedtek, Beytepe, Ankara 06800, Turkey Ban et al. 2005, Cho et al. 2006, Lin et al. 2001). Superparamagnetic iron oxide nanoparticles (SPIONs) in the form of maghemite (g-Fe2O3) or magnetite (Fe3O4) have attracted much interest, and they have been employed in potential applications such as magnetic drug delivery, targeting and imaging of cancer cells, hyperthermia for the treatment of solid tumors, and magnetic resonance imaging (MRI) (Sun et al. 2008, Gao et al. 2009, Gupta and Gupta 2005, Laurent et al. 2008, Lu et al. 2007, McBain et al. 2008, Wu et al. 2008). The biomedical applications of SPIONs strongly depend upon their stability in physiological solutions and the capability of their surfaces to be chemically functionalized. In order to enhance their stability, dispersibility, and functionality compared to naked magnetic nanoparticles, SPIONs have been coated with organic molecules such as macrocyclic surfactants and polymers or inorganic shells (Gao et al. 2009, Gupta and Gupta 2005, Laurent et al. 2008, Lu et al. 2007, Sun et al. 2008, Wu et al. 2008). Despite extensive studies on polymer or silica-coated magnetic nanoparticles, there are limited reports on the coating of SPIONs with metallic shells. Gold (Au) and silver (Ag) are ideal and attractive metallic materials for deposition, due to their low reactivity, high chemical stability, and biocompatibility. In addition, they can easily be functionalized by binding to the amine or thiol terminal groups of organic molecules. This treatment allows the linkage of functional ligands, which may make the materials suitable for catalytic and optical applications (Lu et al. 2007). Moreover, Au and Ag depositions also render plasmonic properties to SPIONs. The presence of metallic content could provide a platform for optical absorption and emission caused by the collective electronic response of the metal to light, and could also make these compatible and adaptable particles good candidates for sensor technology (Wu et al. 2007). Meanwhile, the deposit of Ag or Au protects the magnetic core against oxidation, drastically reducing the magnetic properties such as coercivity or blocking temperature (Ban et al. 2005, Lin et al. 2001). Several methods for the synthesis of Ag or Au-deposited magnetic particles have been reported, including g-ray

Abstract In this paper, we describe an environmentally friendly procedure to produce silver (Ag) or gold (Au)-deposited magnetite nanoparticles by using plant extracts (Ligustrum vulgare) as reducing and stabilizing agents. Firstly, magnetite nanoparticles (∼6 nm) with superparamagnetic properties – SPIONs – were synthesized by co-precipitation of Fe 2 and Fe 3 ions. Color changes indicated the differing amounts of Au and Ag ions reduced and deposited on to the SPIONs when the plant extracts were used. UV-vis and transmission electron microscope (TEM) with energy dispersive X-ray (EDX) apparatus confirmed the metallic deposition. Magnetic saturation decreased when the amount of the metallic deposition increased, which was measured by vibrating sample magnetometry (VSM). Due to the molecules coming into contact with – and even remaining on – the surface of the nanoparticles after aggressive washing procedures, the Ag/Au-deposited SPIONs were stable, and almost no agglomeration was observed for months. Fourier Transform Infrared (FTIR) spectra depicted that functional groups such as carboxylic and ketone groups, which are most probably responsible for the reduction and stabilization of Ag/Aucarrying magnetite nanoparticles, originated from the plant extract. The proposed route was facile, viable, and reproducible, and it should be stressed that nanoparticles do contain only safe biomolecules as stabilizing agents on their surfaces. Keywords: green synthesis, Ligustrum vulgare plant extract, superparamagnetic iron oxide nanoparticles, tunable gold and silver deposition

Introduction Magnetic nanomaterials, generally smaller than 50 nm, exhibit magnetic properties that are very different when compared to the bulk material (Cho et al. 2006, Laurent et al. 2008, Lu et al. 2007, Sun et al. 2008). The decrease in the particle size leads to an increase in the reactivity of magnetic nanoparticles, and their magnetic properties are dominated by surface effects and superparamagnetism (Lu et al. 2007,

*Equally contributed. Correspondence: Erhan Piskin, Department of Chemical Engineering, Bioengineering Division, Hacettepe University and Biyomedtek/Nanobiyomedtek, Beytepe, Ankara 06800, Turkey. Tel:  90 312 2977400. Fax:  90 312 2992124. E-mail: [email protected] (Received 3 February 2015; accepted 11 February 2015)

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2 A. N. Dizaji et al. radiation (Kinoshita et al. 2005), laser ablation (Kawaguchi et al. 2007), sonochemical reaction (Wu et al. 2007), layerby-layer electrostatic deposition (Spasova et al. 2005), wet chemical reduction (Lo et al. 2007, Xu et al. 2007, Bao et al. 2009, Chin et al. 2009, Han et al. 2011, Hien Pham et al. 2008, Lyon et al. 2004, Leo and áJustin Gooding 2012, Liu et al. 2008, Robinson et al. 2010, Shin et al. 2009, Wang et al. 2005, 2011), photochemical reduction (Oliva et al. 2006), micelle methods (Mandal et al. 2005, Cho et al. 2005, 2006, Kouassi et al. 2007, Lin et al. 2001), and galvanic replacement reaction (Park et al. 2007, Wang et al. 2008, Ban et al. 2005). However, the challenge is the synthesis of metal-deposited magnetic materials in controllable ways. Furthermore, the use of toxic chemicals in the process of nanoparticle synthesis is the major bottleneck that limits the involvement of nanoparticles in some critical industries such as nutrition and medicine. In order to overcome this restriction, the biological synthesis of these particles has been suggested as a new approach and has attracted a great deal of attention in recent years. However, there are limited reports on green synthesis procedures to produce Ag-deposited (Ag@ SPION) and Au-deposited (Au@SPION) nanostructures. Preparation of Ag-coated magnetite nanoparticles can be achieved in two steps, according to Mandal et al. (2005) glucose is used as a reducing agent to cause the immigration of Ag ions onto the surface of SPIONs and to create a Ag shell. Another approach to prepare Ag and Au shells was reported by Chin et al. (2009). In this method, SPIONs are modified by dopamine, and Ag nanoparticle seeds are used to coat the magnetite core. HAuCl4 and AgNO3 are added to glucose and SPION-containing media. Then, shells start to appear by growth of the immobilized seeds on the surface of the SPIONs. These studies reveal that the coating of Ag and Au layers is insufficient in surface plasmonic resonance properties and needs to be optimized at every step of the procedure. In this study, the synthesis of Ag@SPIONs and Au@SPIONs via an efficient one-pot green synthesis procedure has been reported. To achieve this objective, the extract of the plant Ligustrum vulgare was used as a reducing and stabilizing agent for Ag@SPIONs and Au@SPIONs. L. vulgare is widely used in yards as hedges and as decorative material (Swearingen et al. 2002). The leaves of L. vulgare have immunomodulatory, cardioprotective, antibacterial, and antidiabetic effects, and are used as a folk medicine in the prevention or treatment of diseases in southern Europe. Besides, the cytotoxic activity of L. vulgare against HeLa cells prove that it can be a potential candidate as an antiproliferative agent, and the presence of flavonoid, phenylpropanoid, and terpenoid (mainly secoiridoid) groups support its use as a potential pharmacological agent.

Materials and methods Preparation of plant extract Fresh leaves of L. vulgare were collected, washed, and homogenized using a mortar. A quantity of 10 g of the plant’s leaves was heated until boiling point in 1 L of deionized (DI) water. To obtain a transparent plant extract, the extract was filtrated by injection through a 0.20 mm microfilter. The

transparent plant extract was stored in the refrigerator for further use.

Synthesis of SPIONs The synthesis of SPIONs was carried out according to Kang et al. (1996). In brief, 0.670 g of FeCl2.4H20 (Sigma, Germany) and 1.913 g of FeCl3.6H2O (Sigma, Germany) (Fe 2/Fe 3  ½) were dissolved in 5 ml of DI water. To remove the O2 dissolved in the solution, the solution was purged with a stream of N2 for 30 min. Approximately 60 ml of 2 M NaOH (Sigma, Germany) was added to the ferrous and ferric solution dropwise with high mechanical stirring (Heidolph Type RZR1, 2000 rpm, Germany) and was purged with N2. The color of the solution initially changed to dark brown, and finally to dark black. SPIONs were removed from the basic solution by applying a strong magnet and were then washed three times with distilled water to remove the non-magnetic components. They were stored at ambient conditions for further use.

Synthesis of Ag@SPIONs and Au@SPIONs using the extract of L. vulgare This section details the use of SPIONs as seed material for deposition of the Au and Ag metallic structure. In the typical deposition procedure, 0.3 ml of SPION solution with a density of 19 mg/ml was mixed with an appropriate amount of 10 mM of AgNO3 (Sigma, Germany) or HAuCl4 (Sigma Aldrich, Germany) solution and plant extract. Ag@ SPIONs and Au@SPIONs were separated from the AgNPs or AuNPs using a magnet. This was followed by a procedure of washing with DI water three times, to ensure that there were no non-magnetic nanoparticles such as AgNPs or AuNPs (Figure 1). The parameters selected to determine the ability of the plant extract to reduce and accumulate the Ag and Au ions onto the surface of SPIONs were the volumetric ratio of the Ag or Au solution to the SPION solution and the amount of plant extract (Table I).

Characterization of SPIONs, Ag@SPIONs and Au@SPIONs Spectral analysis for the development of nanoparticles at different reaction conditions was performed using UV–vis spectroscopy (Jasco, V-530 spectrophotometer, USA). In most cases, Ag@SPIONs and Au@SPIONs resulted in distinctive peaks in the range of the visible region of the electromagnetic spectrum (300–800 nm). A transmission electron microscope (TEM) JEM- 1200EX, JEOL (USA) was employed for the analysis of size and morphology of the SPIONs, Ag@SPIONs and Au@SPIONs that were prepared. Approximately 5 ml of the sample was dropped onto a copper grid, making a thin film of sample on the grid, and then left to dry at room temperature for 15 min. The presence of elemental Ag and Au on the SPIONs was determined using energy dispersive X-ray analysis (EDX) with a Zeiss Evo 50 (USA) instrument. More than 100 nanoparticles were analyzed via ImageJ software to determine the average particle size of the SPIONs, Ag@SPIONs, and Au@ SPIONs (Table I). The as-prepared solutions of the SPIONs, Ag@SPIONs and Au@SPIONs were dropped onto clean glass surfaces


Silver or gold deposition onto magnetite nanoparticles by using plant extracts 3

Figure 1. Steps in the synthesis and purification of Ag@SPIONs and Au@SPIONs.

and dried at 40°C under vacuum for X-ray powder diffraction measurements (Rigaku Ultima-IV, USA). Crystallite sizes of all the nanoparticles produced were evaluated with the Scherrer formula using full-width at half-maximum (FWHM) values. FTIR spectroscopic measurements were carried out to determine the biofunctional groups that were involved in the synthesis of Ag@SPIONs and Au@SPIONs. Samples for the FTIR were prepared by drying the untreated plant extract, Ag@SPIONs, and Au@SPIONs without any further procedure. The samples were measured by Thermo Scientific FTIR-SMART ITR (USA) in the attenuated total reflection mode with diamond crystal, using the spectral range of 4000–500 cm 1 and a resolution of 4 cm 1. All the samples were analyzed twice, to account for the potential preparation effects.

Table I. The effect of AgNO3 or HAuCl4/SPION ratio and amount of plant extract on nanoparticle size. AgNO3 or HAuCl4/ SPION ratio Plant extract NP size Synthesized NP (mg/mg) (ml) (nm)* Fe3O4 Ag@SPIONs Effect of AgNO3/SPION ratio

Effect of plant extract Au@SPIONs Effect of HAuCl4/SPION ratio

Effect of plant extract

–

–

6.2

1 5 10

25 25 25

ND 21.5 29.6

5 5

10 50

17.5 14.8

1 5 10

25 25 25

ND ND 12.1

5 5

10 50

10.9 10.1

* Determined from TEM images by analyzing more than 100 particle sizes. ND: Not determined. Since there was no peak observed in UV-vis analysis, TEM analyses were not performed for this run.

A vibrating sample magnetometer (Cryogenic Limited PPMS, UK) was used to obtain magnetic measurements of the SPIONs, Ag@SPIONs, and Au@SPIONs at 300 K.

Results and discussion After the addition of Ag or Au solution to the mixture containing SPIONs and plant extract, the color changed to yellowish brown for Ag and to brownish red for Au (Figure 1), in about 5 min. The UV-vis spectra of the Ag@SPIONs and Au@SPIONs under different conditions are shown in Figure 2A–B and in Figure 2C–D, respectively. Reduction of Ag or Au ions onto SPIONs led to an absorption peak because of excitation of surface plasmon (SP), that is, collective oscillations of Drude-like conduction electrons and d-band electrons that are optically excited into the conduction band (Ban et al. 2005). Notice that no peak appeared for the bare SPIONs. In addition, the accumulation of Ag or Au induced a broadening and red-shift of the SP peak due to a collective interaction of the electrons of the interconnected particles (Ban et al. 2005, Chin et al. 2009, Hien Pham et al. 2008, Mahmoudi and Serpooshan 2012, Shin et al. 2009, Xu et al. 2007). In Figure 2A, due to the small amount of AgNO3, there was no peak observed for the first run of Ag@ SPIONs. By increasing the ratio of AgNO3/SPIONs, the peak for Ag deposition was detected at 419 and 425 nm for the second and third runs, respectively. Notice that as a result of greater deposition of Ag ions onto the SPIONs, there was a red-shift and broadening, and increased intensity in the absorbance value of the UV-vis spectra (Chin et al. 2009, Mahmoudi and Serpooshan 2012). Increasing the amount of plant extract from 10 to 50 ml for the same, the AgNO3/ SPION ratio resulted in much more intense peaks (Figure 2B). The plant extract has a crucial role here: the reduction and stabilization of Ag or Au ions onto SPIONs. In the case of Au deposition, similar results were obtained. In the first two runs, no peak for Au@SPIONs was detected due to the low amount of Au ions available to accumulate onto SPIONs. Similarly, by increasing the ratio of HAuCl4/SPION, the peak


4 A. N. Dizaji et al.

Figure 2. Effect of AgNO3/SPION ratio A) (A) 0 (B) 1 (C) 5 (D) 10, Effect of plant extract on SPION coating with Ag B) (A) 0 (B) 10 ml (C) 50 ml, Effect of HAuCl4/SPION ratio C) (A) 0 (B) 1 (C) 5 (D) 10 and, Effect of plant extract on SPION coating with Au D) (A) 0 (B) 10 ml (C) 50 ml (For details see Table I).

for Au@SPIONs was observed at 520 nm (Figure 2C) (Hien Pham et al. 2008, Lin et al. 2001). A higher amount of plant extract resulted in lower deposition of Au ions and smaller sized Au@SPIONs (notice the red-shift in Figure 2D). It may be concluded that the excess amount of plant extract supported not only the reduction of Au on the SPIONs, but also the formation of Au nanoparticles free of magnetite. The TEM images of the nanoparticles indicated an increase in size after the Ag or Au deposition. Some representative TEM images and related EDX spectra of SPIONs, Ag@ SPIONs, and Au@SPIONs are shown in Figure 3. The image is blurred because the magnetic characteristics of the nanoparticles causes them to aggregate on the grid and interact with the particles of the electron beam (Cho et al. 2005). The size of the bare SPIONs, based on images obtained from the TEM, was approximately 6.2 nm. Depending on the conditions during the reduction procedure, while the size of Ag@ SPIONs ranged from 14.8 to 29.6 nm, the size of Au@SPIONs ranged from 10.1 to 12.1 nm (Table I). As expected from the red-shift in UV-vis spectra, much more deposition of Ag or Au ions on SPIONs were observed in related TEM images (Chin et al. 2009, Ban et al. 2005, Hien Pham et al. 2008). The UV-vis spectra and the corresponding TEM images were highly compatible with each other. In addition, to emphasize the Ag or Au deposition on SPIONs, the related EDX spectra of SPIONs, Ag@SPIONs, and Au@SPIONs are also shown in Figure 3. Due to presence of Ag and Au nanostructures on SPIONs, Ag or Au elements were detected on the spectra. The organic elements such as carbon originated from the organic nature of the plant extract. Furthermore, copper is from the

TEM grid. EDX analysis confirmed the Ag or Au coating on the SPIONs. The elemental existence of iron peaks on the Ag@SPIONs and Au@SPIONs revealed the presence of SPIONs as a magnetic core (Ban et al. 2005, Han et al. 2011, Hien Pham et al. 2008, Lo et al. 2007, Lyon et al. 2004, Shin et al. 2009). Considering both UV-vis and TEM data, we can conclude that the synthesis of nanostructures containing a magnetic core and partially deposited with Ag or Au was sufficiently achieved. The XRD patterns of Ag@SPIONs and Au@SPIONs are depicted in Figure 4. By depositing the SPIONs with Ag or Au nanostructures, the pattern of a-iron is hidden under the pattern of Ag or Au due to the overlap of their diffraction peaks at 2q  44.8, 65.3, and 82.5 (Ban et al. 2005). The diffraction pattern does not show the presence of any known iron oxides either. The average crystallite size of SPIONs, calculated using Scherrer’s formula, is 5.6 nm. The value calculated from the XRD data is very close to the value (6.2 nm) of the mean particle size determined by TEM analysis (Table I). According to the XRD patterns of Ag@SPIONs and Au@SPIONs, the SPION peaks were replaced by peaks of the Ag or Au crystalline structures, which are indications of coverage by the Ag or Au shell. In the case of Ag@SPIONs, XRD peaks were obtained at the 2 angles of 38.45°, 44.48°, 64.69°, and 77.62° for 111, 200, 220, and 311 crystalline planes of cubic Ag (JCPDS 65-2871) (Chin et al. 2009, Shin et al. 2009) respectively. By using Scherrer’s formula for the 2 angles of 38.45° and 77.62°, the crystallite sizes were calculated as 20.9 and 20.1 nm, respectively. This result is highly consistent with the size (21.5 nm) obtained by TEM analysis


Silver or gold deposition onto magnetite nanoparticles by using plant extracts 5

Figure 3. TEM images of (A) SPIONs (C) Ag@SPIONs (AgNO3/SPIONs  5 and 25 ml of plant extract, see Table I) and (E) Au@SPIONs (HAuCl4/ SPIONs  10 and 25 ml of plant extract, see Table I) and their related EDX spectra (B, D, and F) are shown on the right. İnsets are optical images of the relevant nanoparticle solutions.

(Table I). Similarly, in the case of Au@SPIONs, 2 angles of 38.2°, 44.4°, 64.6°, and 77.5° can be indexed to 111, 200, 220, and 311 crystal structures, which are reflections corresponding to FCC bulk gold (JCPDS 4-784) (Ban et al. 2005, Cho et al. 2005, Lin et al. 2001, Lo et al. 2007, Wang et al. 2005).

Figure 4. XRD patterns of Ag@SPIONs (top, AgNO3/SPIONs  5 and 25 ml of plant extract, see Table I) and Au@SPIONs (bottom, HAuCl4/ SPIONs  10 and 25 ml of plant extract, see Table I).

Applying Scherrer’s formula for the 2 angles of 38.45° and 77.62°, crystallite sizes were obtained as 11 and 11.2 nm, respectively. These values are very close to the size (12.1 nm) obtained from TEM images (Table I). To define the magnetic properties of SPIONs, Ag@SPIONs, and Au@SPIONs, VSM sample hysteresis loop charts were evaluated (Figure 5). For all cases, the nanoparticles exhibited superparamagnetic behavior without any coercivity or remanence. The magnetic saturation of SPIONs was 32 emu/g, higher than the corresponding values for Ag@SPIONs and Au@SPIONs. Due to the non-magnetic Ag or Au deposition, a remarkable decrease was detected in the magnetic saturation values of Ag@SPIONs and Au@ SPIONs, with 9 and 18 emu/g, respectively (Jeong et al. 2006, Chin et al. 2009, Hien Pham et al. 2008). Since the thickness of Ag deposition was higher than that of Au, the decrease in magnetic saturation of Ag@SPIONs was higher than that of Au@SPIONs.

6 A. N. Dizaji et al. (not shown here) have confirmed the stability of the core-shell nanostructure over a long period of time (∼6 months) without any shift or broadening in the spectra. Similar results were obtained for the Ag-deposited magnetite nanoparticles.


Conclusion

Figure 5. VSM hysteresis of (A) SPIONs (B) Au@SPIONs (HAuCl4/ SPIONs  10 and 25 ml of plant extract, see Table I) (C) Ag@SPIONs (AgNO3/SPIONs  5 and 25 ml of plant extract, see Table I).

FTIR measurements were carried out to identify the possible functional groups of the biomolecules that are responsible for the reduction and stabilization of the Ag and Au on magnetic nanoparticles. The plant (L. vulgare) used in this study contains flavonoid and phenylpropanoid-based molecules which have several hydroxyl, ketone, and carboxylic groups. Figure 6 shows the FTIR spectra of samples of the untreated plant extract, SPIONs, Ag@SPIONs, and Au@SPIONs. The sample of the untreated plant extract has medium or strong absorption bands at about 3263, 2923, 1592, 1261, 1069 cm 1, corresponding to –OH, aliphatic –CH stretching, C  O stretching frequency, C–O stretching of esters, ethers, and phenols, and symmetric C–O stretching. In case of Ag deposition, the –OH stretching frequency shifts from 3263 to 3306 cm 1 with less intensity. A comparison of the untreated and treated spectra of plant extract samples suggest that in the treated sample, C  O stretching frequency shifts from 1592 to 1634 cm 1 to produce Ag-coated nanoparticles. The C  O stretching peak at 1715 cm 1 is clearly stronger in the treated Ag-coating procedure compared to the untreated one. In the untreated case, it is nearly absent. Thus, based on the FTIR results, we can conclude that carboxylic and ketone groups are highly responsible for the reduction and stabilization of Ag and Au deposition on magnetite nanoparticles. UV-vis data

Figure 6. FTIR spectra of (A) Plant extract (B) SPIONs (C) Ag@SPIONs (AgNO3/SPIONs  5 and 25 ml of plant extract, see Table I) and (D) Au@ SPIONs (HAuCl4/SPIONs  10 and 25 ml of plant extract, see Table I).

Herein, we report the one-pot green synthesis of Ag@SPIONs and Au@SPIONs at room temperature. In this study, we could form a uniform layer of Ag or Au by using the extract of the plant L. vulgare as a reducing and stabilizing agent. The thickness of the metal layer can be manipulated by tuning reaction parameters such as the ratio of AgNO3 or HAuCl4/SPION and the amount of plant extract. The resulting Ag@SPIONs and Au@SPIONs are supermagnetic with good magnetization properties but have lower magnetization value than bare SPIONs due to the layer of Ag or Au. We envision that these kinds of green synthesis products with tunable properties can be used in diagnostic and therapeutic applications.

Acknowledgements MY is supported by a TUBITAK BIDEB fellowship. EP also acknowledges support from the Turkish Academy of Science as a full member. This study was supported by the TUBITAK1003 program (Project Number: 1130864) and also by the EU-FP7-IAPP Project: NanobacterphageSERS.

Declaration of Interest The authors report no declarations of interest. The authors alone are responsible for the content and writing of the paper.

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