Synthesis of biogenic SnO2 nanoparticles and evaluation of thermal, rheological, antibacterial and antioxidant activities

Powder Technology 270 (2015) 312–319

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Synthesis of biogenic SnO2 nanoparticles and evaluation of thermal, rheological, antibacterial and antioxidant activities M. Meena Kumari, Daizy Philip ⁎ Department of Physics, Mar Ivanios College, Thiruvananthapuram 695 015, India

a r t i c l e

i n f o

Article history: Received 24 June 2014 Received in revised form 17 October 2014 Accepted 19 October 2014 Available online 29 October 2014 Keywords: SnO2 nanoparticles Photoluminescence Thermal conductivity Viscosity Antioxidant Antibacterial activity

a b s t r a c t Quantum confined SnO2 nanoparticles with tunable band gap are synthesized at room temperature using biogenic method. The crystalline and morphological changes during heat treatment of the synthesized samples are discussed using XRD and TEM analysis. Better visible emission intensity is observed at an excitation wavelength of 265 nm for the nanoparticles. Nanofluids with concentrations of SnO2 nanoparticles up to 25 weight percentage are used for the thermal conductivity and rheological studies. A thermal conductivity enhancement of 24 % is observed within the selected range whereas the viscosity is found to be independent of shear rate for all chosen concentration of SnO2/ethylene glycol nanofluids. The in vitro antioxidant study of samples treated at two different temperatures using DPPH assay provide the possibility of improving radical scavenging activity by varying the preparation conditions. The increased presence of SnO2 nanoparticles enhances the antibacterial activity of the nanoparticles. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Semiconductor nanocrystals are of significant interest and focus in the last decades with respect to their technological applications. The electrical, optical and magnetic properties of such particles are highly dependent on their particle size [1,2]. A number of metal oxide nanostructures exhibiting attractive performances have been synthesized and reported recently [3–9]. Tin oxide is an n-type metallic oxide semiconductor that crystallizes in tetragonal rutile structure with a bulk band gap of 3.6 eV at room temperature [10,11]. When the size of tin oxide nanoparticles (TONPs) is less than or compared to their exciton Bohr radius, due to quantum confinement, their properties differ considerably from that of bulk material [2]. The anomalous variation in the size and shape dependent properties of TONPs makes it a promising candidate in wide range of applications like optoelectronics, sensing, laser and solar cells [2,12,13]. The other factors that influence the incorporation of TONPs in various applications include low degree of agglomeration and homogenous arrangement of particles which greatly rely on the method of synthesis [2]. Synthesis of high-quality TONPs using cheap materials and simple preparation routes are yet to be explored. Several methods reported for the synthesis of nanocrystalline tin oxide either requires a sophisticated apparatus or are carried out in a specific environment [12,14–18]. Here we have adopted a biogenic wet synthesis route using fruit extract of Pomegranate, rich in phenolic contents [19–21]. The protocol is advantageous over other methods as it ⁎ Corresponding author. Tel.: +91 471 2530887; fax: +91 471 2530023. E-mail addresses: [email protected], [email protected] (D. Philip).

http://dx.doi.org/10.1016/j.powtec.2014.10.034 0032-5910/© 2014 Elsevier B.V. All rights reserved.

does not require any specific environment and is possible to prepare nanoparticles (NPs) of nearly homogenous size and shape even at room temperature. The nanostructure of TONPs could also be manipulated by temperature treatment [22]. Based on the previous investigations on the effect of heat treatment on NPs synthesized using other chemical routes [2,10,16,23–26], we also attempted a subsequent discussion on the samples annealed at different temperatures. Optical band gap measurement and photoluminescence study are useful techniques for the detection of structure defects and impurities in the nanostructures and there are several reports on the band gap shift and luminescence dependent on the particle size of synthesized NPs [23,27–29]. In the present study, we intend to account on visible emission enhancement of TONPs synthesized at room temperature. The high value of thermal conductivity and other distinctive features like rheological property of nanofluids dependent on the properties of base fluid and suspended NPs makes it a potential applicant in microelectronics, advanced cooling systems and energy supply [5,6, 30]. Yu et al. [6] investigated the thermal conductivity and viscosity of ethylene glycol (EG)-based ZnO nanofluid while a similar investigation on SnO2/EG nanofluid has been done by Mariano et al. [31].In our present work, we have investigated the thermal conductivity and viscosity of as synthesized TONPs and suggest a Newtonian behaviour of prepared nanofluid. The high surface to volume ratio of nanostructures makes it viable candidates to act as free radical scavengers than their bulk counterparts [32]. The antioxidant assessment of nanomaterials has become a crucial study in pharmaceutical science as well as nanotechnology [9]. There are previous reports on the free radical scavenging by ZnO [32] and

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NiO [7] NPs. Also, Das et al. [9] has reported on the antimicrobial and antioxidant activity of CuO nanoparticles. All these encouraged our present work where we studied the antioxidant and antibacterial activity of biosynthesized TONPs to get an insight about the behaviour and interaction of these materials with biomolecules inside a living system.

milliliters of DPPH is taken as control and Ascorbic acid (10 mg/mL DMSO) is used as reference. The DPPH scavenging activity is calculated using the equation [7]

2. Materials and methods

In order to examine the antibacterial activity, SnO2 NPs are tested against the bacterial strain: Escherichia coli. In a typical experiment, petriplates containing 20 mL Muller Hinton medium are seeded with 24 h culture of bacterial strain E. coli. Wells of approximately 10 mm are bored using a well cutter and 25 μL, 50 μL and 100 μL of sample are added to the well from a stock concentration of 0.1 g/mL. The plate is then incubated at 37 °C for 24 h. The antibacterial activity is assayed by measuring the diameter of the inhibition zone formed around the well. Gentamycin is used as a positive control.

Tin (IV) chloride (SnCl4.xH2O) used as the precursor for the synthesis of tin oxide NPs and Ethylene glycol (EG) are purchased from SigmaAldrich. Ripe pomegranate seeds are crushed to get 1 ml of fresh concentrated juice. The juice is then made up to 50 mL using deionised (DI) water and filtered. The filtrate (pg) is used for the preparation of NPs.

% inhibition ¼

control−test  100 control

ð1Þ

2.1. Synthesis of SnO2 nanoparticles 3. Characterization Twenty millilitres of 0.1 M aqueous solution of tin (IV) chloride is mixed with 5 mL of pg and allowed to stir continuously for 5 min. The resulting turbid sol is kept undisturbed, and the precipitate formed is centrifuged and washed repeatedly using DI water and finally using acetone. The washed precipitate is dried at room temperature to obtain SnO2 nanoparticles (Sn5) and is characterized. The experiment is repeated using 10 (Sn10) and 20 mL (Sn20) of pg in order to make out the effect of varying quantities of extract on the precursor solution. Parts of the sample Sn20 are then annealed for 1 h at 300, 600 and 900 °C separately and cooled to room temperature to investigate on the heat treatment effects on the SnO2 NPs. 2.2. Thermal conductivity and viscosity measurements SnO2/EG nanofluids of different weight percentage are prepared by uniformly dispersing TONPs in EG at definite volumes using a sonicator. The nanofluids so prepared are used for thermal conductivity andviscosity measurements. Thermal conductivity measurement of TONPs is carried out using transient short hot-wire (SHW). The experimental setup involves alumina coated platinum wire as hot-wire which is mounted on a Teflon cap. In the measurement system, the temperature is monitored by the 2 thermo couples located at its welding spots. EG of known thermophysical properties is used as a standard fluid to calibrate the measurement of hot-wire. To avoid temperature variations the hot wire cell is placed in a thermostatic bath kept at 303 K. The thermal conductivity is measured from the temperature variation of the wire and heat generation. The samples at different weight percentage 10%, 15%, and 25% are chosen to evaluate its enhancement in thermal conductivity of the base fluid EG. The rheological behaviour of SnO2/EG nanofluids is analyzed by directly measuring the viscosity for 10, 15 and 25 weight percentage of Sn20 as a function of shear rate in the range 5–20 rotations per minute (rpm) at a constant temperature 303 K. The shear rate is varied by applying controlling torques and normal force specified for the rheometer. 2.3. Antioxidant and antibacterial activity tests The radical scavenging activity of SnO2 nanoparticles is determined using DPPH assay. In a typical procedure, different concentrations (12.5-200 μg/mL) of NPs are made up to 40 μL with DMSO and 2.96 mL DPPH (0.1 mM) solution is mixed to it. DPPH is used as the radical source and SnO2 as the radical scavenger. The deep violet color of DPPH radical solution becomes colorless in presence of NPs. The reaction mixture is incubated in dark condition at room temperature for 20 min. After 20 min, the concentration of radical is monitored by the decrease in absorbance percentage of the mixture at 517 nm. Three

The crystallographic phase of the as prepared as well as annealed samples is determined using the XRD patterns obtained from XPERTPRO diffractometer. The size profile and morphology of the synthesized nanoparticles is elucidated from TEM images recorded using Tecnai G2 30 transmission electron microscope. FTIR spectra of pg and synthesized NPs recorded using IR Prestige-21 Schimadzu spectrometer help to recognize the functional groups in pg extract responsible for the reduction/stabilization of TONPs and also to ascertain the formation of SnO2 NPs. PerkinElmer Lambda-35 UV-Visible spectrophotometer is used to optically probe the synthesized TONPs and account on its quantum confinement. The photoluminescence spectra for an excitation at 265 nm are obtained on a Flurolog III spectrofluorometer with samples placed in non-fluorescent quartz cuvette. The thermal conductivity measurement is carried out using transient hot-wire technique and the rheological property of nanofluids are estimated using BrookfieldDV III ultraprogrammable rheometer. The radical scavenging activity of TONPs is elucidated spectrophometrically. 4. Results and discussion 4.1. Structural analysis The diffraction pattern in the XRD spectrum shown in Fig. 1(a) and (b) can be indexed to the tetragonal rutile crystalline phase of tin dioxide (JCPDS No: 41–1445) of as prepared and annealed samples which are in agreement with the earlier reports [10,13,14,17]. The broad peaks of the raw powders (Sn5, Sn10 and Sn20) suggest the formation of particles with small crystalline size prepared at room temperature using different quantities of pg. The diffraction peaks get narrower and stronger when annealed at 300, 600 and 900 °C as an indication of high order crystallinity and increased average crystallite size [14,16,25]. The most intense peak observed for the sample annealed at 600 °C can be attributed to its highest degree of crystallinity, which can be further analyzed using TEM images. The crystallite sizes of the samples synthesized are estimated using Debye Scherrer equation based on (211) reflection [13]. The particle size calculated for Sn5, Sn10 and Sn20 are found to be 2.7, 2.6 and 2.5 nm, respectively. The decrease in particle size observed for samples Sn5 to Sn20 elucidates the necessity of increased quantity of pg fruit juice to prevent agglomeration of SnO2 nanoparticles. On annealing, the atoms from the grain boundary diffuse to the grain leading to an increase in the crystallite size [2,11] as evident from the calculated average particle size for Sn203 (5.2 nm), Sn206 (21 nm) and Sn209 (40 nm). The effect of lattice strain on the full width half maximum (FWHM) of the XRD peaks for Sn20, Sn203, Sn206 and Sn209 have been studied using Hall-Williamson equation [2,11,13]. The lines fitted by plotting βcosθ/λ vs Sinθ/λ as seen in Fig. 1(c) are nearly horizontal which almost

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the well-defined lattice fringes shown using HRTEM images in Figs. 3(d), 4(c) and 5(c), and the lattice spacing measured corresponds to the growth orientation in the direction of (200) crystallographic plane. The selected area diffraction patterns in inset of Figs. 3(a), 4(d) and 5(d) presents rings that correspond to the rutile phase of tetragonal SnO2 structure [13,16,17,33]. The results obtained using TEM study are in good agreement with those obtained from XRD analysis. To further understand the composition of the sample the Energy Dispersive X-ray (EDX) spectrum of Sn20 obtained from the combined TEM instrument is displayed in Fig. 2(d). The sole presence of Sn and O species in the EDX pattern confirms the high purity of the synthesized samples. The image shows the presence of Cu due to copper grid used for TEM imaging [14,34]. 4.2. Optical studies The optical absorbance spectra of as prepared as well as annealed SnO2 NPs are shown in Fig. 6. The band gap is found to be size dependent and increases with decreasing particle size [13]. The spectra for as synthesized samples from Sn5-Sn20 display a blue shift in band gap when compared to the bulk counterpart of SnO2. The excitonic peaks well evident in the UV–vis spectra correspond to the band gaps within the range 4.37–4.51 eV, which is reasonably greater than the bulk value reported for SnO2, 3.6 eV [1]. This can be attributed to the quantum confinement obtained for synthesized samples using increasing quantity of pg. Also the red shift in band gap (4.34–4.26 eV) with increase in annealing temperatures is obvious for particles having large size [11]. The quantum confinement and a blue shift in band gap observed for nanocrystallites prepared at room temperature having size comparable to the exciton Bohr radius [12] can be well manifested with the size calculated using TEM and XRD analysis. The mean particle size of suspensions can be estimated using a relation derived in the effective mass model based on the blue-shift in band gap energy given as eff

Eg ¼ E g þ

Fig. 1. XRD pattern of SnO2 nanoparticles synthesized (a) at room temperature, (b) at different annealed temperatures and (c) H-W plot for as prepared and annealed samples. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)

nullifies the effect of residual strain on the particle size of prepared samples [2]. The detailed morphology of the synthesized SnO2 NPs is depicted in Figs. 2–5.The samples Sn20 consists of homogenous, almost welldispersed spherical particles with an average size 2.2 nm comparable to the Bohr radius (2.7 nm) of SnO2 atom [12]. Some of the assynthesized particles are partially aggregated to form some irregular shapes whereas the samples annealed at 300 and 600 °C are nonagglomerative, spherical or rectangular in shape with average dimensions 4.2 and 16.2 nm. On further treatment, at 900 °C agglomerates of mean size 42.4 nm are again observed. Hence, it can be noticed that particle size increase with increase in annealing temperature and the optimum annealing temperature for obtaining well separated, highly crystalline samples is 600 °C. Similar results are previously reported [24]. The crystalline nature of the annealed samples is also revealed by

2 2

ħ π

 2μR2

ð2Þ

where Eeff g is the effective band-gap energy,Eg is the bulk band-gap energy, ħ is the Plank's constant divided by 2π, R is the particle radius and μ is the effective reduced mass [18]. The effective mass model is applicable for systems having particle size in the order of exciton Bohr radius and is used for determining the particle size of as-synthesized SnO2 nanoparticles in the present work. The calculated value for Sn5 (2.63 nm), Sn10 (2.58 nm) and Sn20 (2.43 nm) correlates well with the estimated size using TEM and XRD analysis. Generally, there are two emissions observed for semiconductor NPs— excitonic and trapped luminescence. The excitonic emission is sharp and are located near the absorption edge of the particles while the trapped emissions are broad and stokes shifted and often contains multiple luminescent centers [23,35,36]. To investigate the luminescent property of our SnO2 NPs, PL measurement for Sn20 and Sn209 are carried out. PL spectrum obtained using an excitation wavelength of 265 nm is shown in Fig. 7. The visible emissions lower than the band gap of SnO2 nanocrystal cannot be assigned to the direct recombination of conduction band electron with the hole in valence band [37]. Henceforth, the PL bands observed in the visible range at 430,450,470 nm can be attributed the oxygen-related defects introduced during the growth that act as radiative centers for luminescence process [12,34,35]. Analogous earlier reports are there supporting the presence of three bands in the visible range 400–500 nm for all SnO2 products [12]. Tin interstitials or dangling bonds present in the prepared samples might also be the origin of luminescence [37]. The PL spectrum dominated by blue emission for SnO2 NPs is also reported by Li et al. [16].

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Fig. 2. (a–c) TEM images at different magnifications. (d) EDX pattern of sample Sn20. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)

Fig. 3. (a–b) TEM images at different magnifications. (c) HRTEM. (d) SAED pattern of Sn203. Inset of (a) shows SAED pattern.

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Fig. 4. (a–b) TEM images at different magnifications. (c) HRTEM. (d) SAED pattern of Sn206.

Fig. 5. (a–b) TEM images at different magnifications. (c) HRTEM. (d) SAED pattern of Sn209.

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Fig. 8. FTIR spectra of pomegranate fruit juice and Sn20 tin oxide NPs. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)

Fig. 6. Plot of UV–vis absorbance maximum for (a) as prepared samples Sn5, Sn10 and Sn20. (b) Sn20 annealed at 300 °C, 600 °C and 900 °C. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)

The low-temperature synthesis of samples in aqueous solution can lead to large density of oxygen vacancies and interfacial tin vacancies, thereby resulting in the formation of trapped states which form a series of metastable energy levels within the band gap and can contribute to strong photoluminescence at room temperature [38,39]. A decrease in the intensity of luminescence spectra is observed for sample annealed at 900 °C. The observed attenuation in luminescence can be due to the increased crystalline size resulting in reduction of both the ratio surface area and concentration of oxygen vacancies [23,37]. Oxygen vacancies play an important role in PL emission and hence PL study of NPs can provide valuable information on the quality and purity of prepared samples [23,36].

Fig. 7. PL emission spectra for Sn20 and Sn209 at an excitation wavelength 265 nm. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)

In order to evidence the presence of functional groups, FTIR spectroscopic analysis is carried out for synthesized metal oxide nanoparticles. Fig. 8 shows FTIR spectra of pg fruit extract and the as-synthesized SnO2 NPs. The IR absorption bands at 3412 cm−1, 2360 cm−1, 1630 cm−1 and 1080 cm−1 in the spectrum correspond to the O-H stretching vibration of phenolic hydroxyls [40], stretching vibrations of NH2+ and NH3+ in protein/peptide bonds [41], carbonyl stretching in proteins [40] and COH vibrations of proteins [42] present in pg extract. The still existence of a broad band centered at 3412 cm−1 and 1630 cm−1 in the prepared sample can be assigned to stretching vibration mode of O-H group and the O-H bending band, both associated with the adsorbed water on the SnO2 crystals as the precipitates are not heat treated. The appearance of an intense band positioned at 559 cm− 1 with a shoulder at 661 cm− 1 can be related to the Sn-O-Sn antisymmetric vibrations, which confirm the presence of SnO2 in the crystalline phase [12,16, 34]. The almost complete absence of the NH2 + and NH3 + stretching vibrations in the FTIR spectrum of SnO2 NPs can be attributed to the breakage of amino acid residues of proteins during the reaction and points to the role of proteins present in the pg fruit juice in the reduction/stabilization of NPs.

4.3. Thermal conductivity and rheological studies Thermal conductivity measurements of three different weight percentage of SnO2/EG nanofluid are done at 303 K. Thermal conductivity ratio (TCR) of Sn20 as a function of weight percentage is shown in Fig. 9(a). As observed, the TCR showed a linear relationship with increase in concentration of metal oxide nanoparticles. A thermal conductivity enhancement of 6%–24% compared to pure base fluid is shown for 10–25 weight percentage increase of Sn20 nanopowder in the nanofluid. Anomalous improvement in thermal conductivity can be accounted by any or all of the four possible mechanisms—Brownian motion of the nanoparticles, molecular level layering of the liquid at the liquid/particle interface, the nature of heat transport in the nanoparticles and the effects of nanoparticles clustering as reported by Keblinski et al. [43]. The shear viscosity for pure EG is independent of shear rate, evidencing a Newtonian behaviour as reported by Mariano et al. [31]. Viscosity is measured as a function of shear rate for different weight percentage of SnO2/EG nanofluid and is shown in Fig. 9(b). For all concentrations, the plot attained a constant value within the time domain 10–20 min, indicating that viscosity is independent of shear rate. Similar reports on nanofluids of Al2O3-propylene glycol by Prasher et al. [44] and ZnO-EG nanofluid by Yu et al. [6] can be referenced to suggest almost Newtonian behaviour of nanofluids in the present study.

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4.4. Antioxidant and antibacterial activity The antioxidant activity of prepared TONPs is validated using DPPH assay. The scavenging effect of Sn20 and Sn206 on the DPPH radical is investigated and the dose–response bar graph showing the results on comparison with the standard is presented in Fig. 10(a). The antioxidant efficiency is found to increase with the increase in sample dosage for both samples. Also using 200 μg/mL of Sn20, more than 50% of the free radical DPPH can be scavenged which is a comparable percentage with that of standard Ascorbic acid (AA). It can be noticed that the annealed sample Sn206 exhibits a lower scavenging activity when compared to the as-prepared sample. The decrease in activity with increase in annealing temperature can be attributed to the decrease in surface area to volume ratio due to increased crystalline size. Similar study on annealed NiO has been reported by Madhu et al. [8].Analogous to the results suggested by Das et al. [9,32], the scavenging activity exhibited by TONPs can also be attributed to the ability of SnO2 NPs to transfer its electron density towards the free radical located at nitrogen atom in DPPH. Antibacterial activity of as prepared SnO2 metal oxide nanoparticles has been mediated through its activity on microorganism E. coli. The sample shows a significant inhibition on the growth of bacteria with respect to the positive control used. The plate in Fig. 10(b) exhibits the growth inhibition on E. coli using Sn20 at different concentrations along with the positive control. The inhibition zone is found to increase with increase in concentration of NPs. Several mechanisms interpreting the antibacterial activity of metal oxides have been previously reported [9,45,46]. We could also conclude upon a similar mechanism suggesting direct interactions of SnO2 NPs that damages the E. coli bacterial membrane owing to the production of oxygen species. Fig. 9. (a) Thermal conductivity ratio v/s mass fraction of sample Sn20. (b) Viscosity as a function of shear rate. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)

5. Conclusions Quantum confined spherical SnO2 nanoparticles are synthesized using different quantities of pg fruit extract at room temperature. The characterization of crystallographic parameters of as prepared and annealed samples are done using XRD, TEM, UV, IR and the photoluminescence intensity shift of annealed samples are discussed. A linear enhancement in thermal conductivity of SnO2/EG nanofluid is presented and the rheological property of nanofluid within the mass fraction 10–25 weight percentage put forward a Newtonian behaviour of nanofluid. The antibacterial and antioxidant activity of the prepared metal oxide nanoparticles proves it to be a potential member of bioactive materials. Even though an effort to elucidate mechanisms responsible for thermal, antibacterial and antioxidant activities of SnO2 nanoparticles is done, thorough investigation should be carried out in future.

Acknowledgment The authors are pleased to acknowledge, NIT, Calicut, for thermal conductivity and viscosity measurements. The help rendered by Biogenix Research center, Thiruvananthapuram, for antibacterial and antioxidant study is thankfully acknowledged. The help in TEM imaging by NIIST, Thiruvananthapuram, is also acknowledged.

References

Fig. 10. (a) Dose–response bar chart for DPPH scavenging activity of SnO2 NPs. (b) Petriplate showing antibacterial activity of Sn20 at different concentrations against Escherichia coli. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)

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