Materials Research Bulletin 45 (2010) 989–992
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Hydroxy propyl cellulose capped silver nanoparticles produced by simple dialysis process L. Francis a, A. Balakrishnan b, K.P. Sanosh c, E. Marsano a,* a
University of Genova, Department of Chemistry and Industrial Chemistry, via Dodecaneso 31, 16146 Genova, Italy Laboratoire SIMaP – GPM2, Grenoble-INP/UJF/CNRS BP46, 38042 Saint Martin d’He`res cedex, France c Department of Innovation Engineering, University of Lecce, via per Monteroni, 73100 Lecce, Italy b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 January 2010 Received in revised form 15 March 2010 Accepted 12 April 2010 Available online 18 April 2010
Silver (Ag) nanoparticles (6 nm) were synthesized using a novel dialysis process. Silver nitrate was used as a starting precursor, ethylene glycol as solvent and hydroxy propyl cellulose (HPC) introduced as a capping agent. Different batches of reaction mixtures were prepared with different concentrations of silver nitrate (AgNO3). After the reduction and aging, these solutions were subjected to ultra-violet visible spectroscopy (UVS). Optimized solution, containing 250 mg AgNO3 revealed strong plasmon resonance peak at 410 nm in the spectrum indicating good colloidal state of Ag nanoparticles in the diluted solution. The optimized solution was subjected to dialysis process to remove any unreacted solvent. UVS of the optimized solution after dialysis showed the plasmon resonance peak shifting to 440 nm indicating the reduction of Ag ions into zero-valent Ag. This solution was dried at 80 8C and the resultant HPC capped Ag (HPC/Ag) nanoparticles were studied using transmission electron microscopy (TEM) for their particle size and morphology. The particle size distribution (PSD) analysis of these nanoparticles showed skewed distribution plot with particle size ranging from 3 to 18 nm. The nanoparticles were characterized for phase composition using X-ray diffractrometry (XRD) and Fourier transform infrared spectroscopy (FT-IR). ß 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Metals A. Polymers A. Nanostructures B. Chemical synthesis C. X-ray diffraction
1. Introduction Nano-silver particles are widely used in wound healing [1–4] applications due to its positive effects through antimicrobial properties, reduction in wound inflammation, and modulation of fibrogenic cytokines. Literature show two types of approaches commonly reported for the synthesis of nano-silver: (a) by using inorganic reducing agents such as sodium borohydride [5], glucose [6], sodium formaldehyde sulfoxylate [7], etc. and organic solvents such as ethanol [8], N,N-dimethyl formamide [9], ethylene glycol [10,11], which have been successfully implemented to reduce silver salts into metallic silver and (b) by capping silver nanoparticles using polymers such as poly(vinylalcohol) [12– 14], polystyrene [15], polymethylmethacrylate [16], poly(vinylpyrolidone) [17,18], etc. The latter technique has gained popularity due to the fact that the polymers used could be easily dissolved in inorganic/organic solvents and nano-silver are embedded or encapsulated in polymer. This makes the particle size to be controlled well within the desired regime, prevents agglomeration and finds wide applications in thin film casting. For opto-
* Corresponding author. Tel.: +39 010 3538727; fax: +39 010 3538727. E-mail address:
[email protected] (E. Marsano). 0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2010.04.007
electronics and electronics applications, using different polymers embedded with nano-silver could be useful. However, most polymers that are reported for these applications are either not biocompatible or bioadsorbable, which makes it ineffective for antiseptic applications like wound healing. HPC is a natural polymer widely used in drug delivery [19] and its presence on nano-silver could be beneficial since it is bioadsorbable [20]. Although capping of HPC is reported on gold [19,21], there are hardly any reports of its capping on Ag. Our experience shows that unlike gold, direct reduction of Ag salts like AgNO3 by HPC is difficult. Hence, the present study successfully employs hydroxy propyl cellulose (HPC) combined with ethylene glycol and reduces AgNO3 using a simple dialysis technique to synthesize HPC capped nanosilver particles. Compared to conventional capping technique, this dialysis method eliminates the use of acetone [22] for removing unreacted solvent, thereby making it cost effective. The results show that, the use of HPC as a capping agent produces nano-silver which has good colloidal state in liquid phase. This can be beneficial in wound healing applications since nano-silver in the colloidal state is highly antimicrobial but also non-toxic to humans [23]. The other advantage of this technique lies in the fact that the presence of HPC at the solid/liquid interface does not interfere with the silver diffusion on the surface as a result the particles nucleate
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Table 1 Different batches of reaction mixtures for synthesizing HPC/Ag nanoparticles. Batch
HPC (g)
Ethylene glycol (g)
Silver nitrate (mg)
Water (g)
1 2 3 4
0.5 0.5 0.5 0.5
25.3 25.3 25.3 25.3
50.0 100.0 150.0 250.0
5.0 5.0 5.0 5.0
to a similar size. This property was evident as the particles were narrowly distributed from 3 to 18 nm particle size range. 2. Experimental Fig. 2. Schematic representation of HPC capping mechanism on Ag.
Silver nitrate (AgNO3, Fluka Chemicals Ltd., UK) was used as the starting precursor for the colloidal silver preparation. Reagent grade ethylene glycol (Sigma–Aldrich) was used as solvent and reducing agent for AgNO3. Hydroxy propyl cellulose (HPC, Sigma– Aldrich, Mw = 370,000) was used as a capping agent. Firstly, HPC was completely dissolved in ethylene glycol by stirring. Different amounts of AgNO3 as shown in Table 1 were dissolved separately in 5 ml of distilled water and mixed with polymer solution. The prepared solutions were kept in an oil bath at 55 8C for 6 h and stirred resulting into a brown solution. The solutions were aged for 24 h at room temperature. A definite amount (3.30 g) from each batch solution was subjected to ultraviolet visible spectroscopic analysis (UVS) (Shimadzu 3600 Japan) and optimized. The optimized aged solution was subjected to dialysis through cellulose membranes. The reaction mixture obtained was taken into cellulose membranes such that 70% by volume of the membrane was filled. This membrane was kept in a 2 l measuring cylinder containing water. Water was continuously tapped to the bottom of the cylinder and allowed to exit through the outlet at the top of the cylinder. This dialysis process was carried for 6 h to remove the presence of any unreacted ethylene glycol present in the reaction mixture. The dialysis method employed here has been schematically presented in Fig. 1. The dialysed solution was dried at 80 8C for 6 h. The obtained powders were characterized by X-ray diffraction (XRD) (Philips powder diffractometer Model PW1050 (Ni-filtered Cu Ka1 radiation)) and Fourier transform infrared spectroscopy (FT-IR) (Avatar 380, Thermo Nicolet, Waltham, MA, USA) for their phase composition. Transmission electron microscopy (TEM) (HR-TEM JEOL 2010, Japan) was used to study the particle size and morphology. TEM images were used to determine the particle size distribution (PSD) using an Image Analysis Software (Image J, USA).
Fig. 1. Schematic representation of the experimental setup.
3. Results and discussion The use of polymers to prevent agglomeration and precipitation of the nanoparticles is quite well known. Polymers are also frequently used as stabilizers in the synthesis of metal nanoparticles [24]. The embedding of nano-silver particles in polymer matrix can be advantageous in wound healing especially when nano-silver exhibits strong antimicrobial properties. The use of HPC offers twofold advantages, viz.; (i) apart from glucosidic bonds, HPC offers 6 OH, that is some of the 6 OH groups in the monomer unit of cellulose is replace by hydroxy propyl group(CH3–CH(OH)–CH2 ) to form hydroxylate anions ( O ). This facilitates HPC bonding with Ag. (ii) Secondly, high solubility of HPC in ethylene glycol as well as its use is effectively employed for silver nitrate reduction [25–27]. During the reduction process of AgNO3, it is possible that various ionic states of silver such as Ag+, Ag2+ and Ag32+ can form inside the solution [28]. However, it seems that due to the in situ produced Ag+ ions that is chemically bonded to the HPC, helps the Ag+ ions to get converted into the Ag0. Overall, formation of HPC capped Ag nanoparticles with reactions is shown schematically in Fig. 2. The prepared batch solutions after aging showed absorption bands at about 410 nm (Fig. 3) which varied in wavelength by about 10 nm with respect to amount of AgNO3 loading (Table 1). It is observed that the higher loading of AgNO3 (keeping HPC weight constant), leads to higher and narrower absorption frequency. The increase in frequency can be perhaps understood in terms of higher number of silver ions within the prepared solution. However,
Fig. 3. UV–visible spectroscopic analysis of prepared solutions before dialysis (a–d) and after dialysis (e) for the batch with 250 mg AgNO3.
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Fig. 6. TEM analysis of HPC/Ag nanoparticles. Bar represents 20 nm.
Fig. 4. XRD profile of HPC/Ag nanoparticles.
broadening of peaks at lower concentration of AgNO3 in the solution could be mainly attributed to the superposition of multi-ionic bands of Ag particles like Ag+, Ag2+ and Ag32+ exhibiting different colloidal states. This means batch solution 4 (Table 1) i.e. solution containing 250 mg AgNO3 seems to exhibit much better colloidal state in liquid phase. After dialysis, 250 mg AgNO3 batch solution showed the same absorption bands shifting from lower wavelength, i.e. it shifts from 410 to 440 nm (Fig. 3). This phenomenon is an indicative of reduction of Ag ions to zero-valent silver (Ag0) thus suppressing the surface plasmon resonance [24]. Hence further characterizations were done on the powder obtained from this batch. Fig. 4 shows the XRD pattern of the HPC capped Ag (HPC/Ag) nanopowder dried at 80 8C. These nano-powders were characterized by a broad peak at (2 0 0) which attributed to the highly reduced crystallite size obtained here. The crystallite size obtained from the XRD patterns using the Scherrer equation [29] was found to be 7 nm, which was in close agreement with the PSD analysis (Fig. 5). Similar XRD pattern were reported in the work carried out by Das et al. [30] where the Ag crystallite size was found to be less than 15 nm. Studies have shown that for Ag crystallite size of 20 nm [24], typical crystalline peaks for Ag appear with no broadening or overlapping as seen in the present case. This means that besides crystallite size, crystallinity also plays an important role in determining the effect of peak broadening. In the present
Fig. 5. PSD analysis of HPC/Ag nanoparticles.
study, the Ag powders synthesized at room temperature could have been to a certain extent amorphous. That is, deviations from ideal crystallinity depends on the finite crystallite size [31] and can be responsible for broadening of the diffraction lines, as seen in the present case. TEM image of the dried nano-Ag particles is shown in Fig. 6. It was seen that the Ag nanoparticles were spherical in shape with a smooth surface morphology and HPC capping indeed prevented agglomeration of Ag nanoparticles. In order to determine the capping mechanism by HPC on Ag, FTIR analysis (Fig. 7) of pure HPC and HPC/Ag were compared by the identification of absorption bands associated with the vibrations of functional groups in the range of 4000–500 cm 1. FT-IR pattern for pure HPC obtained were consistent to the work reported earlier [32]. In both samples, the broad transmission bands at 3600– 3100 cm 1 with a maximum at 3460 cm 1, was assigned to stretching vibrations of the –OH groups in the spectrum of HPC. Further both samples exhibited absorption peaks at 1000– 1200 cm 1 range. The 1130 and 1150 cm 1 peaks correspond to asymmetric C–O–C stretching vibrations of aliphatic ethers [33]. The 1085 and 1050 cm 1 peaks are assigned to asymmetric C–O–C stretching vibrations of cyclic ethers as well as to C–O stretching vibrations in C–OH [33]. The absorption bands corresponding to propyl vibrations were observed at 1650 cm 1. The absorption bands in the range of 2800–3000 correspond to symmetric/
Fig. 7. FT-IR analysis of HPC/Ag nanoparticles.
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asymmetric CH2 stretching attributed to pyranose ring of HPC. However, it was seen that there was a minor broadening and shifting (6–14 cm 1 red shift) of HPC/Ag peaks at 3600–3100 cm 1 and also at 840 cm 1, which suggested the presence of hydrogenbond structures [34] and good interaction was present between the polymer and metal. 4. Conclusion In the present study, the synthesis of Ag nanoparticles capped with HPC by a simple dialysis route is reported. The reduction of AgNO3 in ethylene glycol by HPC is considered to be a key step in formation of HPC/Ag system. The UV-spectra indicate well defined absorption bands for optimized batch solution due to surface plasmon resonance phenomena of Ag nanoparticles stating good colloidal state. The particles diameter of the silver by the current methodology is found to be between 3 and 18 nm. TEM and PSD analysis revealed that the particle dimension is in nanometer regime and the calculation made by the XRD is in proximity with the observations made by TEM and PSD. The shifts in –OH groups during FT-IR analysis showed that HPC was getting capped on Ag nanoparticles. Considering the antimicrobial properties of nano-Ag coupled with excellent biocompatibility of HPC, this system could be used for making thin films for wound healing applications. References [1] J. Tian, K.K. Wong, C.M. Ho, C.N. Lok, W.Y. Yu, C.M. Che, J.F. Chiu, P.K. Tam, Chem. Med. Chem. 1 (2007) 129. [2] W. Yang, C. Shen, Q. Ji1, H. Ani1, J. Wang, Q. Liu1, Z. Zhang, Nanotechnology 20 (2009) 085102. [3] O.C. Farokhzad, A. Khademhosseini, S. Jon, A. Hermmann, J. Cheng, C. Chin, A. Kiselyuk, B. Teply, G. Eng, R. Langer, Anal. Chem. 77 (2005) 5453.
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