Surfactant mediated interaction of vancomycin with silver nanoparticles

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Surfactant mediated interaction of vancomycin with silver nanoparticles Amritpal Kaur, Divya Goyal, Rajesh Kumar ∗ Department of Physics, Panjab University Chandigarh, Chandigarh, 160014, India

a r t i c l e

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Article history: Received 28 September 2017 Received in revised form 2 December 2017 Accepted 7 December 2017 Available online xxx Keywords: Silver nanoparticles Vancomycin Drug loading UV–vis DLS FTIR

a b s t r a c t In this present study, spherical AgNPs have been synthesized by chemical reduction method in the presence of different surfactants i.e. trisodium-citrate and polyvinylpyrrolidone. Further, these synthesised AgNPs have been functionalized with different concentrations of glycopeptide antibiotic (vancomycin). The interaction between AgNPs and antibiotic has been studied using various analytical techniques i.e. UV–vis absorption spectroscopy, Dynamic light scattering, and X-ray diffraction. Further, the nature of bonding between antibiotic and AgNPs has been probed by Fourier transform infrared spectroscopy, which shows the amine bonding between vancomycin and silver nanoparticles surface. The role of different nature of surfactants on attachment and interaction mechanism of drug with AgNPs has been studied. © 2017 Elsevier B.V. All rights reserved.

1. Introduction In the recent years, there is an enhancement in the investigation of antibacterial activities of metal nanoparticles, due to increase in bacterial resistance to some of the classic antibiotics [1,2]. For example, S. aureus (Gram-positive) and Enteroccoci (Gramnegative) shows resistance to various antibiotics like vancomycin, ampicillin, amaikacin, and so forth. Nanoparticles represent one of the most innovative non-invasive approaches for the delivery and targeting of drugs and pharmacologically active substances. Moreover, the drug-loaded nanoparticles provide several advantages with respect to the free drugs, such as protection against degradation and thus promote a suitable, selective and specific targeted therapy and the increase in the patient compliance [3]. Metallic nanoparticles due to some of their attractive properties like high stability and their ability to modify the surface characteristics easily, have been found useful to study drug targeting, drug delivery systems, and enhancement of drug bioavailability. Among all metallic nanoparticles, silver nanoparticles (AgNPs) are one of the most widely studied nanomaterials due to their excellent antimicrobial and antibacterial activities [4]. Silver nanoparticles are also incorporated in various medical supplements, like, catheters and wound dressings to inhibit the growth of pathogens.

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (R. Kumar).

A large number of other applications of silver nanoparticles include water purification, biosensors, cosmetics, bone prostheses, and gene delivery etc. [5–8]. Silver is very much popular since ancient times for its antibacterial properties [9,10]. But due to the solubility characteristics of silver salts and silver metal makes it impractical for various medical applications. Therefore, synthesis of colloidal silver nanoparticles have been a subject of great interest among researchers nowadays [11–13]. Previously many studies have demonstrated that nanoparticles such as solid lipid nanoparticles, dendrimers, liposomes, polymeric nanoparticles, and particularly metal nanoparticles were able to reduce the harmful effects of various antibiotics [14]. Therefore, the main advantages of using drug-coated metal nanoparticles for drug delivery systems are (1) increases the resistance time in the body, (2) its site of action (targeting drug to the specific location), and (3) improvement in the bioavailability by enhancing aqueous solubility. These advantages of metal nanoparticles lead to the reduction in the quantity of a particular drug which is required and also reduces its dosage toxicity. So, the drug-loaded nanoparticles enable the safe drug delivery to a particular location and protection of non-target cells from the severe side effects [15]. In this present study, spherical silver nanoparticles have been synthesized by chemical reduction method by using different protecting agents such as sodium citrate (Na3 C6 H5 O7 ), polypyrrolidine (PVP) and citrate + PVP (PVP/citrate) and sodium borohydride (NaBH4 )

https://doi.org/10.1016/j.apsusc.2017.12.066 0169-4332/© 2017 Elsevier B.V. All rights reserved.

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as a reducing agent. These capped silver nanoparticles have been modified with the glycopeptides antibiotic, named as, vancomycin. Vancomycin is an antibiotic from glycopeptide family of antibiotics used to treat a number of bacterial infections caused by gram-positive bacteria [16]. Various methicillin- resistance bacteria such as Staphylococcus aureus (MRSA) and Staphylococcus epidermidis (MRSE) were treated with vancomycin. Vancomycin was recommended intravenously as a first-line treatment for complicated skin infections, bloodstream infections, endocarditis, bone and joint infections, and meningitis caused by methicillin-resistant S. aureus [17]. But in 1988, the first vancomycin-resistance gram positive bacterium was deatected, named Enterococci (VRE). With time, it has been found that the resistance property of this drug may transfer to various other bacteria including Staphylococcus aureus (VRSA) [18]. Hence, numerous antibiotic conjugated nanoparticles such as ciprofloxacin, ampicillin, gentamycin, neomycin and various antibodies-capped particles appeared in the past few decades. The goal of this study was to develop and characterize the vancomycin-loaded silver nanoparticles. Hence, in this present work, we have studied the nature of interaction between vancomycin and silver nanoparticles coated with three different surfactants. Firstly, spherical silver nanoparticles can be obtained with different capping agents via two kinds of mechanism depending upon whether capping agent is ionic or non-ionic or polymer. Here, we have used one ionic (cationic) capping agent trisodium citrate (Na3 C6 H5 O7 ) and the other, polymer surfactant PVP. The first mechanism involves by which, ionic surfactant (Na3 C6 H5 O7 ) interact with silver nanoparticles, is based on electrostatic stabilization mechanism. However, polymers or non-ionic surfactants (like PVP) interact with silver nanoparticles through steric repulsion mechanism [19–22]. Further, the nature of interaction between capped silver nanoparticles and drug was studied by using various characterization techniques, such as, UV–vis optical spectroscopy, FTIR, DLS and XRD.

2. Experimental materials and methods 2.1. Materials Silver nitrate (AgNO3 ), sodium borohydride (NaBH4 ), trisodium citrate (Na3 C6 H5 O7 ), and polyvinylpyrrolidone (PVP 40) (C6 H9 NO)n were used for the synthesis of colloidal silver nanoparticles (AgNPs). Antibiotic named as: vancomycin (C66 H75 Cl2 N9 O24 ) was used for the functionalization of silver nanoparticles. All chemicals were purchased from Sigma Aldrich and were used without any further purification. Double-distilled water was used to prepare all the samples.

2.2. Methods 2.2.1. Synthesis of spherical silver nanoparticles Metal nanoparticles synthesised by chemical reduction methods usually experience nucleation and growth stages. So, various stabilizing agents have been used for the synthesis of spherical nanoparticles. In present study, colloidal silver nanoparticles were synthesised by following the chemical reduction method described in the previous literature, [23] using trisodium citrate (Na3 C6 H5 O7 ) and PVP as capping agents (stabilizing agents) and sodium borohydride (NaBH4 ) as a reducing agent (Fig. 1). The solution of silver nanoparticles (AgNPs) was synthesised by using 1.0 mM AgNO3 as a precursor and 2.0 mM of NaBH4 as a reducing agent, in the presence of capping agents, trisodium citrate (0.5 mM), PVP 0.3% and combination of both Na3 C6 H5 O7 and PVP, by continuous stirring until the colour of solution changes to deep brown or brownish

yellow. The solutions were kept at low temperature for further characterizations and experiments. 2.2.2. Characterization of synthesised spherical silver nanoparticles The appearance of brownish yellow colour indicates the formation of AgNPs. Furthermore, the synthesis of capped nanoparticles was assured by monitoring the surface plasmon resonance (SPR) of colloidal solution by using UV–vis spectrometer. The optical properties of spherical AgNPs are highly dependent on the nanoparticle size and uniformity. Smaller nanoparticles primarily absorb light with peaks near 400 nm, while larger ones exhibit increased scattering, broader spectral peaks and peak intensities at longer wavelengths [24]. Dynamic light scattering (DLS) was carried out to determine particle size distribution and polydispersity in aqueous solution. DLS is a valuable technique to evaluate particle size and size distribution of nanoparticles in aqueous solution. It measures the fluctuation of the intensity of the scattered light which is caused by particle movement. DLS (Dynamic light scattering) has been carried out by using Zeta analyzer. The FTIR (Fourier-transform infra-red) spectra in the range of 4000–400 cm−1 were recorded by using FTIR spectrophotometer which revels the different functional groups corresponds to different vibrational frequencies. XRD (X-Ray diffraction) pattern was recorded on an X’Pert PRO X-ray diffractometer using Cu K␣1 radiation in the 2 range of 30◦ –80◦ at a scanning rate of 2◦ /min. To record XRD pattern, the films were deposited by dropping colloidal solution of AgNPs on clean glass slides. 2.2.3. Vancomycin loading on citrate and PVP stabilized silver nanoparticles The colloidal solution of silver nanoparticles with all three different capping agents was modified with vancomycin by adding different concentrations of vancomycin (0.1 mM, 0.2 mM, 0.3 mM, 0.4 mM and 0.5 mM). Take 5 ml colloidal solution of citrate-capped silver nanoparticles and add 0.1 mM vancomycin to it and stir it for 15–20 minutes. Similarly, with other concentrations five different samples of vancomycin capped AgNPs (Van@AgNPs) were prepared by adding 0.2 mM, 0.3 mM, 0.4mM and 0.5 mM vancomycin to each 5.0 ml solution of silver nanoparticles, by stirring for the same duration as for the first sample. All the samples kept at room temperature for 24 h. After 24 h incubation, solution was stored in refrigerator for further characterizations (Fig. 2). 3. Results and discussion 3.1. SPR (surface plasmon resonance) studies of AgNPs and Van@AgNPs Fig. 3 shows the UV–vis absorption spectra of vancomycin, bare spherical AgNPs, Van-citrate@AgNPs (vancomycin coated citrateAgNPs), Van-PVP@AgNPs (vancomycin coated PVP-AgNPs) and Van-PVP/citrate@AgNPs (vancomycin coated PVP/citrate-AgNPs). The surface plasmon band for the spherical citrate-capped AgNPs, PVP-capped AgNPs and PVP/citrate-capped AgNPs were observed at 403, 431 and 410 nm respectively, in visible region of UV–vis absorption spectra and vancomycin absorbed in UV region around 290 nm. After the addition of different concentrations (0.1 mM, 0.2 mM, 0.3 mM, 0.4 mM, and 0.5 mM) of vancomycin to the solution of silver nanoparticles, the absorption peak shows significant red shift for Van-citrate@AgNPs and Van-PVP/citrate@AgNPs around 25–43 and 16–25 nm respectively. However, for VanPVP@AgNPs there was a blue shift of about 2 nm. It could be explained on the basis of electronic transitions which take place when UV–vis light is passed through sample.

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Fig. 1. Reaction mechanism for synthesis of AgNPs (a) citrate-AgNPs, (b) PVP-AgNPs, and (c) PVP/citrate-AgNPs.

Fig. 2. Schematic diagram of preparation route for Van@AgNPs.

There are only two possible transitions in visible region one is from non-bonding to antibonding orbital (n → ␲* ) and the other one is from bonding to antibonding orbital (␲ → ␲* ). The interaction mechanism between drug and AgNPs is further discussed in

Section 3.5. According to Wang et al. [25] the nanoparticles with a diameter less than 50 nm were protected by a coordination bond between nitrogen in PVP and Ag, and for the bigger particles both nitrogen and oxygen in PVP coordinated with silver. Thus, lone

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Fig. 3. UV–vis. spectra of (a) vancomycin, (b) bare AgNPs with different capping agents, (c) Van-citrate@AgNPs, (d) Van-PVP@AgNPs and (e) Van-PVP/citrate@AgNPs at different concentrations.

pair of nitrogen atom in PVP was available for n → ␲* transition. However, in case of citrate molecule ␲ → ␲* transition was more dominating due to absence of lone pairs. Furthermore, it was also observed that with the increase in concentration of vancomycin the absorption peak becomes broader due to formation of Van-Ag complex, which indicates the attachment or loading of vancomycin on the surface of silver. Thus, changes in UV–vis spectra results attributed to the presence of vancomycin on the surface of silver nanoparticles. The shift in peak was due to the attachment of drug molecules to silver surface through citrate and PVP which leads to the formation of silver-drug complex (Scheme 1). Due to the large surface area of AgNPs, more number of drug molecules adsorbed on the silver surface either through covalent or non-covalent binding. That is why absorption peak becomes broader, when vancomycin concentration increases, due to agglomeration. So, the above data concludes that all three citrate-AgNPs, PVP-AgNPs and PVP/citrate-AgNPs act as an effective anchor or carrier for the vancomycin drug. As, antibiotics have several active groups such as amino groups, thiol groups, hydroxyl groups etc., which have very strong affinity to the surface of metallic nanoparticles. The exact orientation of vancomycin on the surface of silver is highly important to maximize the interaction of drug with the bacteria. The anionic part of sodium citrate, which was attached with silver through electrostatic stabilization mechanism, and oxygen atom of PVP acts as a linker either for amine group or thiol group of vancomycin. Among these two

possible mechanisms, which type of bond is formed between AgNP surface and vancomycin was studied by FT-IR spectra.

3.2. FTIR studies of AgNPs and Van@AgNPs The FTIR spectra of vancomycin, bare AgNPs, and Van@AgNPs are shown in Fig. 4. Vancomycin has various functional groups as already shown in Scheme 1. In general, metallic nanoparticles have very strong affinity for both amine and thiol groups [26]. In both, citrate-AgNPs and vancomycin, the bands around 3399 cm−1 , 2127 cm−1 , 1643 cm−1 and 1261 cm−1 , corresponds to stretching frequencies for O H bond of H2 O, alkynene group, C O bond of amides, and C O bond of carboxylic acid, respectively, were observed. But, in case of vancomycin coated silver nanoparticles two additional bands around 1089 and 1061 cm−1 , were observed. Both of these bands attributed to stretching frequencies of C N bond of amine group. Also, slight shift in other functional groups indicates the loading of vancomycin on AgNPs. From the above discussion, it can be evident that it is the hydrogen atom of amine group of vancomycin that binds to the oxygen atom of citrate through hydrogen bonding. Therefore, trisodiumcitrate acts as a linker between silver nanoparticles and vancomycin hydrochloride. Similarly, comparison of PVP-AgNPs (Fig. 4(c)) and Van-PVP@AgNPs (Fig. 4(f)) was done. In this analysis, peaks at 3780 cm−1 (O H stretching vibration), 3406 cm−1 (N H stretching vibration), 2957 cm−1 ( C H symmetrical stretching vibration),

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Scheme 1. (a) Structure of vancomycin and (b) synthesis route of Van@AgNPs.

In case of PVP/citrate-AgNPs (Fig. 4(d)) the vibrational frequencies were observed at 3783 cm−1 (O H stretching vibration), 3387 cm−1 (N H stretching vibration), 1654 cm−1 (C O vibration), 1588 cm−1 (N H bending vibration), 1400 cm−1 ( CH3 vibration) and 1069 cm−1 (C O stretch). Like, Van-PVP@AgNPs, these peaks with slight shift also observed in Van-PVP/citrate@AgNPs (Fig. 4(g)) with few new peaks corresponding to 2960 cm−1 ( CH3 group), 1291 cm−1 (C N vibration), 912 cm−1 (C C vibration), 840 cm−1 (C H vibration), 740 cm−1 (C H vibration) and 678 cm−1 (O H deformation) (Table 1). These shifts in peaks were due to binding of amine group of vancomycin to either oxygen atom of citrate ion or of PVP. As, mentioned in the case of Van-citrate@AgNPs, new peaks correspond to amine bonding. Similarly, in case of PVP, only N atom coordinates with Ag surface and O atom was available for amine bonding with vancomycin. Hence, FTIR studies also confirm the adsorption and loading of vancomycin on the surface of AgNPs.

3.3. DLS analysis of AgNPs and Van@AgNPs

Fig. 4. FTIR spectra of (a) Vancomycin, (b) citrate-AgNPs, (c) Van-citrate@AgNPs, (d) PVP-AgNPs, (e) Van-PVP@AgNPs, (f) PVP/citrate-AgNPs and (g) VanPVP/citrate@AgNPs.

1655 cm−1 (C O vibration of amides), 1425 cm−1 (C H deformation and C C stretches vibration) and 1290 (C N vibration) cm−1 , were observed. These vibrations were also observed in case of Van-PVP@AgNPs with slight shift along with some additional vibrations at 1495 cm−1 (C C vibration), 1374 cm−1 (O H vibration), 1321 cm−1 (C N vibration), 1076 (C O vibration) cm−1 , 844 cm−1 (C H out of plane deformation) and 739 cm−1 (C H vibration), which were also present in vancomycin. The shift in the already present peaks and the appearance of new peaks attributed to the functionalization of PVP-AgNPs with vancomycin.

Particle size distribution analysis by DLS has been done for both blank AgNPs and Van@AgNPs. DLS results show that synthesized nanoparticles were polydisperse with all the three protecting agents. Polydispersity index (PDI) basically a measure of broadness of size distribution or degree of agglomeration of sample. In zetasizer software it ranges from 0 to 1. If the value is greater than 1 then the distribution is so polydisperse which is not suitable for DLS. In our samples, it was less than 1 for all samples. Hence, the samples which have more value of PdI are more polydisperse in nature than others and the samples with less PdI are significantly monodisperse or less polydisperse. The Z-average size calculated for citrate-AgNPs was around 46 nm (PdI-0.405) and for Van-citrate@AgNPs was around 56 nm (PdI-0.234) with the increase of 10 nm. The increase in Z-average size attributed to the loading of vancomycin on the surface of AgNPs. This hydrodynamic radius (r) of particles was calculated from these results kB T using Stokes–Einstein equation: D = 6r , where kB Boltzmann’s constant, T is the absolute temperature, ␩ is the viscosity of the medium and D is the diffusion coefficient. It measures the hydrodynamic radius of the particles considering it as a rigid sphere. Similarly, the Z-average size for AgNPs with other surfactants PVP and PVP/citrate has been found about 50 nm (PdI-0.667) and 51 nm (PdI-0.766), respectively. After functionalization with drug the size increases to 61 nm (PdI-0.827) and 62 nm (PdI-0.378) for PVP-AgNPs and PVP/citrate-AgNPs, respectively. So, with all the three capping-agents the increase in Z-average size of around 10–12 nm indicates the presence of vancomycin on AgNPs sur-

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6 Table 1 Comparative FTIR data of AgNPs and Van@AgNPs. Sample

Original peaks

Shifted peaks

New peaks

Citrate-AgNPs and Van-citrate@AgNPs

OH at 3309 cm−1 , at 2126 cm−1 C C group, C O vibration of amides at 1643 cm−1 and C O bond of carboxylic acid at 1261 cm−1 O H stretching vibration 3780 cm−1 , N H stretching vibration at 3406 cm−1 , C H symmetrical stretching 2957 cm−1 , C O vibration of amides at 1655 cm−1 , C H deformation and C C stretches vibration at 1425 cm−1 and C N vibration at 1290 cm−1 O H stretching vibration at 3783 cm−1 , N H stretching vibration at 3387 cm−1 , C O vibration at 1654 cm−1 , N H bending vibration at 1588 cm−1 , CH3 vibration at 1400 cm−1 and C O stretch at 1069 cm−1

3309–3399 cm−1 , 2126–2127 cm−1

C N bond of amine group at 1089 and 1061 cm−1

1425–1429 cm−1

C C vibration at 1495 cm−1 , O H vibration at 1374 cm−1 , C N vibration at 1321 cm−1 , C O vibration at 1076 cm−1 , C H out of plane deformation at 844 cm−1 , C H vibration at 740 cm.−1

3387–3375 cm−1 , 1654–1657 cm−1 , 1588–1581 cm−1 , 1400–1393 cm−1 and 1069–1070 cm−1

CH3 group at 2961 cm−1 , C N vibration at 1291 cm−1 , C C vibration at 912 cm−1 , C H vibration at 840 cm−1 , C H vibration at 740 cm−1 and O H deformation at 679 cm−1

PVP-AgNPs and Van-PVP@AgNPs

PVP/citrate-AgNPs and Van-PVP/citrate@AgNPs

Table 2 DLS parameters for AgNPs and Van@AgNPs. Nanoparticles

Z-average (nm)

Polydispersity Index (PdI)

Citrate@AgNPs Van-citrate@AgNPs PVP@AgNPs Van-PVP@AgNPs PVP/citrate@AgNPs Van-PVP/citrate@AgNPs

46 56 50 61 51 62

0.405 0.234 0.667 0.827 0.729 0.378

2 values 38.03◦ , 44.24◦ , 64.25◦ and 77.28◦ shift to 37.51◦ , 43.55◦ , 63.91◦ and 76.84◦ corresponding to (111), (200), (220) and (311) planes, respectively due to drug interaction. The average crystallite size for citrate-AgNPs and van-citrate@AgNPs was found around 15 and 15.4 nm respectively. There was no significant change in crystallite size after the drug addition to silver nanoparticles. The inset peak position corresponding to (111) and crystallite size both were calculated by Gaussian fit. The shift in major peak corresponding to (111) has been shown by inset plot for both PVP and citrate. Hence, XRD results also show the loading and interaction of vancomycin with silver nanoparticles.

face. Hence, DLS results also confirm the loading of vancomycin on surface of silver (Table 2). 3.4. XRD results of bare AgNPs and Van@AgNPs 3.5. Interaction mechanism of drug coated silver nanoparticles X-ray diffraction analysis has been carried out by converting solution of AgNPs and Van@AgNPs into films. The XRD pattern of these films has been shown in Fig. 5. The observed pattern has been found to be in agreement with literature of face-centred cubic crystal structure of silver [27]. The major peaks due of spherical PVP-AgNPs have been found at 2 values of 38.20◦ 44.50◦ , 64.51◦ , and 77.53◦ , corresponding to (111), (200), (220), and (311) crystal planes (Fig. 5(a) (1)). The average crystallite size obtained from XRD data was found to be around 18 nm. This average size was found by undertaking mathematical analysis of Bragg’s reflections by using the Scherrer formula given as: D = 0.9 ␭/␤ cos, where D is the crystallite size, k is constant, ␭ is the wavelength of X-ray radiation, ␤ is the line width and  is the angle of diffraction. The lattice constant (a = 4.091 Å) and the interplanar spacing values (dhkl = 2.364 Å, 2.045 Å, 1.672 Å and 1.233 Å) calculated from XRD spectrum of silver nanoparticles also in agreement with standard silver values. In case of Van-PVP@AgNPs, the peaks shift from 38.20◦ to 37.59◦ , 44.50◦ to 43.63◦ , 64.51◦ to 63.81◦ , and 77.53◦ to 76.91◦ corresponding to (111), (200), (220), and (311) crystal planes (Fig. 5(a) (2)). The average size also changes from 18 to 18.5 nm. This shift in peaks has been attributed to interaction of drug with surface of AgNPs. The shift in d-spacing lines is attributed to change in lattice parameter or stain in crystal structure. The change in electron cloud of silver nanoparticles surface due to the attachment of drug results in the change of lattice parameters and hence contribute towards the shift in d-spacing of XRD line. Similarly, in case of citrate (Fig. 5(b) (1) and (2)), the diffraction peaks for bare AgNPs at

The observed peak shift and suppress in UV–vis, the shift in XRD reflection lines and appearance of new FTIR peaks are attributed to the interaction between drug and the AgNPs. The interaction mechanism involves the formation of new bonds and the electron transfer between drug and the AgNPs. This leads to the change in electron density of silver nanoparticles. The red and blue shift in UV–vis absorption spectra can be due to four possible transitions (bonding to antibonding orbital) that are n → ␲* , ␲ → ␲* , n → ␴* and ␴ → ␴* . The absorption due to n → ␴* and ␴ → ␴* transitions usually occur in UV range (200–400 nm) because these transitions require very large amount of energy. As silver nanoparticles are coloured samples so n → ␲* and ␲ → ␲* transitions only possible transitions that occurred in these samples in visible range (400–700 nm). In case of PVP-AgNPs blue shift was observed because PVP has a lone pair of electrons which enhances the more chances of transition from n → ␲* orbital which requires higher energy as compared to ␲ → ␲* . The N and O both atoms of PVP are coordinated with silver. Now the N atom has already forms a covalent bond with silver through lone pair. So, the O atom can makes a bond with H of amine of vancomycin through hydrogen bonding. However, in case of citrate-AgNPs, no lone pair was present. So, ␲ → ␲* transition is dominating over n → ␲* transition which require less energy and occurs at higher wavelength. The O atom of anionic part of citrate that coordinates with silver makes a bond with hydrogen of amine group of vancomycin through ␲ → ␲* transition.

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Fig. 5. XRD pattern of (a) PVP-AgNPs and Van-PVP@AgNPs and (b) citrate-AgNPs and Van-citrate@AgNPs. Inset shows the (111) XRD peak (normalised Gaussian fit).

4. Conclusions Silver nanoparticles were synthesized by chemical reduction method in the presence of different capping agents and further functionalized with glycopeptide antibiotic-vancomycin. The change in UV–vis absorption peak position, size, size distribution and X-Ray diffraction pattern of drug loaded AgNPs confirmed the attachment of drug to silver surface. The appearance of new peaks of amine group in the FTIR of drug loaded AgNPs further confirm the attachment and indicates that the amine functional group of the drug is responsible for attachment of drug to AgNPs. As the AgNPs show significant shift with citrate as compared to PVP in UV–vis spectra, it was concluded that citrate-AgNPs show better drug loading properties as compare to PVP-AgNPs. Hence, citrate acts as a better linker between drug and silver nanoparticles than PVP. This Van@AgNPs system may provide an advantageous model system for the development of new effective bactericide. Acknowledgements Financial support from University Grant Commission (UGC) is gratefully acknowledged. We thank the Central Instrumental Laboratory, Panjab University, Chandigarh for FTIR studies. One of the authors (Amritpal Kaur) is thankful to UGC for research fellowship under the MANF scheme. References [1] P. Li, J. Li, C. Wu, Q. Wu, J. Li, Synergistic interaction between silver nanoparticles and membrane-permeabilizing antimicrobial peptides, Nanotechnology 16 (2005) 1912–1917. [2] V.K. Sharma, N. Tatjana, Z. Zboril, Silver colloid nanoparticles: synthesis, characterization, and their antibacterial activity, J. Phys. Chem. B 110 (2006) 16248–16253. [3] E.R. Balmayor, H.S. Azevedo, R.L. Reis, Controlled delivery systems: from pharmaceuticals to cells and genes, Pharm. Res. 28 (2011) 1241–1258. [4] M. Ahamed, M.S. AlSalhi, M.K.J. Siddiqui, Silver nanoparticle applications and human health, Clin. Chim. Acta 411 (2010) 1841–1848. [5] T.M. Tolaymat, A.M. El-Badawy, A. Genaidy, K.G. Scheckel, An evidence-based environmental perspective of manufactured silver nanoparticle in syntheses and applications: a systematic review and critical appraisal of peer-reviewed scientific papers, Sci. Total Environ. 408 (2010) 999–1006.

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