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Biofabrication, characterization, and possible bio-reduction mechanism of platinum nanoparticles mediated by agro-industrial waste and their catalytic activity
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JIEC-2119; No. of Pages 7 Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx
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Biofabrication, characterization, and possible bio-reduction mechanism of platinum nanoparticles mediated by agro-industrial waste and their catalytic activity Preeti Dauthal, Mausumi Mukhopadhyay * Department of Chemical Engineering, S.V. National Institute of Technology, Surat 395-007, Gujarat, India
A R T I C L E I N F O
Article history: Received 25 March 2014 Received in revised form 2 July 2014 Accepted 9 July 2014 Available online xxx Keywords: Punica granatum Biofabrication Platinum Nanoparticle Reduction
A B S T R A C T
The present study showed biofabrication of platinum nanoparticles (Pt-NPs) using agro-industrial waste Punica granatum’s peel extract. Appearance of the broad spectrum from visible to the ultraviolet region, confirmed the biofabrication of Pt-NPs. Pt-NPs were spherical, within size range of 16–23 nm. XRD suggested the fabrication of crystalline Pt-NPs with (111) plane in predominant orientation. The negative z potential value of colloidal Pt-NPs revealed high stability. FTIR confirmed the role of hydroxyl and carbonyl groups of polyphenolic compounds of peel extract for biofabrication. The reduction of anthropogenic pollutant, 3-nitrophenol, by NaBH4 using colloidal Pt-NPs established it as an efficient ‘‘green catalyst.’’ ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction Today, transition metal nanoparticles, particularly platinum nanoparticles (Pt-NPs) are subject to intensive research. The emerging catalytic applications of Pt-NPs in variety of reactions such as hydrogenation [1], oxidation [2], reduction [3], and for the synthesis of organic dyes [4] has attracted the focus of researchers. The biofabricated nanoparticles for their eco-friendly properties are the preferred option for variety of applications. The development of simple and environment-friendly methods for controlled synthesis of Pt-NPs is therefore, important, not only for the fascinating utilization of Pt-NPs but also for the demands of green chemistry. In recent years, significant efforts have been made toward fabrication of nanoparticles using biogenic resources, which also represents a growing connection between biotechnology and nanotechnology [5]. This approach for nanoparticle fabrication shows several benefits concerning biocompatibility, thermal and chemical stability, high efficiency, fast process, cost efficiency, and eco-friendly nature [6,7]. However, till date plant-based synthesis of Pt-NPs has been reported only by leaf extract of Ocimum sanctum
* Corresponding author. Tel.: +91 261 2201645; fax: +91 261 2227334, 2201641. E-mail addresses: [email protected], [email protected] (M. Mukhopadhyay).
[8], Diospyros kaki [9], Cacumen platycladi [10], and Anacardium occidentale [11]. Few other biological resources are also reported for biological synthesis of Pt-NPs such as horse spleen apoferritin [12] and honey [4]. Recently, use of Punica granatum (P. granatum) peel has been reported for cost-effective and environmentally benign synthesis of Ag [13] and Au-NPs [14]. Earlier, leaf [15] and fruit [16] of P. granatum were also utilized for biofabrication of Au and Ag-NPs. P. granatum peel is one of the most valuable by-products of the food and agriculture industry, which is mainly composed of ellagic tannins, punicalagin, gallic acid, ellagic acid, and quercetin [17,18]. It is also shown antioxidant [19,20], antimutagenic [21], and chemo-preventive potential [22]. An extensive literature survey revealed that there is no report available for the biosynthesis of Pt-NPs using P. granatum peel. In the present study, antioxidant potential of P. granatum peel is utilized for biofabrication of Pt-NPs. The present study involves biofabrication of Pt-NPs using agroindustrial waste P. granatum peel as the bio-reducing agent. Further, catalytic activity of biofabricated colloidal Pt-NPs is also investigated for reduction of anthropogenic pollutant 3nitrophenol (3-NP) using sodium borohydride (NaBH4) as a hydrogen or electron donor. Nowadays, nitrophenols are extensively used as raw materials in pharmaceutical, dye, and insecticide industries [23]. Since nitrophenols are readily soluble and stable in water [24] they are abundantly present in agricultural and industrial waste water. Further, they are perilous to public
http://dx.doi.org/10.1016/j.jiec.2014.07.009 1226-086X/ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: P. Dauthal, M. Mukhopadhyay, J. Ind. Eng. Chem. (2014), http://dx.doi.org/10.1016/j.jiec.2014.07.009
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JIEC-2119; No. of Pages 7 P. Dauthal, M. Mukhopadhyay / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx
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health as they possess mutagenic and carcinogenic properties harmful for humans [25]. It is therefore, necessary to eliminate nitrophenols from industrial waste water. The reduction of 3-NP to 3-aminophenol (3-AP) is therefore, of significant importance. Use of eco-friendly biofabricated colloidal Pt-NPs as a catalyst in this direction is of enormous value in environmental and industrial aspect. Biofabricated Pt-NPs are characterized by UV–visible spectrophotometer (UV–visible), dynamic light scattering (DLS), X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), and Fourier transform infrared spectroscopy (FTIR) techniques. Further, the reduction of 3-NP is monitored by UV–visible spectrophotometer. Experimental Materials Chemicals, including hexachloroplatinic acid (H2PtCl6.6H2O) 99.9%, 3-nitrophenol (C6H5NO3) 98%, and sodium borohydride (NaBH4) 98% were procured from HiMedia Pvt. Ltd, Mumbai, India. Peels of the P. granatum fruit were collected from the local market of Surat, Gujarat, India. Double-sterilized Milli-Q water was used throughout the experiments. Preparation of the P. granatum peel extract and biofabrication of colloidal Pt-NPs A total of 30 g of fresh peel of P. granatum was extracted with 120 mL of distilled water at 60 8C for 10 min and filtered. The solution was decanted and stored at 4 8C for further use. About 100 mL of aqueous P. granatum peel extract was added to 400 mL of 1 103 M H2PtCl6.6H2O solution. The mixture was maintained at 90 8C in a sealed flask for 30 min under shaking conditions on a rotary shaker (500 rpm). The reduced Pt-NPs were sonicated for 10 min to separate Pt-NPs from the biomolecules present in P. granatum peel extract. After sonication, Pt-NPs were purified by repeated centrifugation at 14,000 rpm for 10 min and the pellets were washed thrice with distilled water to remove the impurities. Control reactions, in which H2PtCl6.6H2O solution and peel extract was kept in a separate conical flask, under the same reaction conditions. Evaluation of catalytic activity of biofabricated colloidal Pt-NPs In order to find out the catalytic activity of biofabricated colloidal Pt-NPs (Pt-NPs dispersed in P. granatum peel extract) three typical reactions were carried. In the first reaction, 1 mL of 1 103 M 3-NP was mixed with 0.5 mL of water. In the second reaction, 0.5 mL of 1 M NaBH4 was added to the first reaction mixture. In the third reaction, 1 mL of colloidal Pt-NPs was mixed with reaction mixture obtained from the second reaction. All the three reactions were monitored by UV–visible spectrophotometer (DR 5000, HACH, USA). Temperature and concentration dependent catalytic activity of biofabricated colloidal Pt-NPs were also carried out for 3-NP reduction. Catalytic activity of P. granatum peel extract and bulk H2PtCl6.6H2O was also evaluated for 3-NP reduction.
ZS90, Malvern, UK). TEM and selected area electron diffraction (SAED) pattern data were obtained by using TEM (CM-200, Philips, UK). TEM image was recorded at 100 kV accelerating voltage with resolution of 2.4 A´˚ . XRD pattern of Pt-NPs on the glass substrate was recorded by using XRD (X’Pert Pro, PANalytical, Holland) operated at a voltage of 45 kV and current of 35 mA with Cu–Ka radiation (K = 1.5406 A´˚ ). The scanning range (2u) was selected from 20 8 to 80 8 at 0.045 8/min continuous speed. The crystallite size of the Pt-NPs was calculated using Scherrer’s formula. The natures of elements were identified by EDX (INCA X-sight, Oxford Instruments, UK) coupled with scanning electron microscopy (SEM) (JSM-6380LV, JEOL, Japan). In order to identify the phytochemicals responsible for bio-reduction and stabilization of Pt-NPs, FTIR analysis of peel extract before and after bioreduction and biogenic Pt-NPs were carried out using FTIR (Magna550, Nicolet, USA). All spectra was taken in the mid-IR region of 600–3600 cm1.
Results and discussion UV–visible spectroscopy analysis for biofabrication of Pt-NPs The biofabrication of colloidal Pt-NPs was confirmed by UV– visible spectroscopy analysis. The H2PtCl6.6H2O solution (pale yellow) showed an absorption peak at around 260 nm in its UV– visible spectrum due to the ligand-to-metal charge-transfer transition between Pt4+ and Cl ions [26], displayed in Fig. 1. As the bio-reduction reaction was carried out, colloidal Pt-NPs were formed simultaneously with a change in pale yellow color of H2PtCl6.6H2O solution into light brown. The absorption peak present at 260 nm disappeared and was replaced by a broad continuous absorption spectrum, which gradually increases in intensity from visible to the ultraviolet region, suggested Pt4+ ions were completely reduced to Pt0 [27]. Control reaction mixture recovered as pale yellow in color, with no light-brown, being observed suggested Pt-NPs were formed only in presence of P. granatum peel extract. DLS and z potential analysis Fig. 2a revealed that the Z-average diameter of the biofabricated Pt-NPs was 30 nm with a polydispersity index (PDI) 0.270. The corresponding average z potential value of 15.7 mV confirmed the stability of Pt-NPs in colloidal solution (Fig. 2b). This low value of z potential of biofabricated nanoparticles was attributed to the additional influence by the electric charge of bio-organics present in peel extract. Various process parameters such as biomaterial dosage, temperature, and pH (change the electric charge of bioorganics), which might further affect their capping and stabilizing
Characterization of Pt-NPs Biofabrication of Pt-NPs was monitored first by visual inspection and then by using UV–visible spectrophotometer (DR 5000, HACH, USA). Baseline correction was made with P. granatum peel extract. Hydrodynamic size distribution and z potential of the colloidal Pt-NPs were determined by using DLS (Zetasizer Nano
Fig. 1. UV–visible spectra of aqueous H2PtCl6.6H2O and colloidal Pt-NPs.
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Fig. 2. Colloidal Pt-NPs (a) particles size and (b) corresponding z potential distribution.
abilities. This was subsequently affecting the z potential value of nanoparticles [28]. Similar results were also reported earlier for different stable biofabricated nanoparticles synthesized using different plant resources [29–31]. This negative value of z potential indicated high electrical charge on the surface of Pt-NPs, which causes strong repellent forces between particles and prevents aggregation of nanoparticles. This resulted in higher stability of colloidal Pt-NPs. The negative z potential value could be due to the capping of polyphenolic compounds present in P. granatum peel extract [32].
Fig. 3. TEM Images of (a–b) biosynthesized Pt-NPs, inset represented SAED image of Pt-NPs.
XRD analysis of Pt-NPs TEM analysis of Pt-NPs Further, TEM analysis was performed in order to determine the shape and size of biofabricated Pt-NPs (Fig. 3a and b). Some clustering of the particles was also apparent in TEM image. TEM images showed particles were spherical with relatively uniform size range of 16–23 nm. The basic reason behind the size difference in TEM and DLS analysis was attributed to the different instrument response to particle number, volume, mass, optical property, and sample preparation method and also due to the polydispersity of the sample [32]. In DLS analysis, size measurement was based on the phenomenon of Brownian motion. DLS method measured the hydrodynamic diameter of particles in colloidal solution, which was slightly higher than the size measured by other techniques due to the various forces of interaction works in the colloidal solution such as van der Waals force of attraction [33]. The DLS instrument was known to measure the shell thickness of a capping agent enveloping the nanoparticles along with the actual size of the metallic core. However, in TEM analysis dry forms of synthesized particles were directly analyzed under an electron microscope. So in scientific scenario for the estimation of exact particle size, TEM was the appropriate technique [24]. These nanoparticles were also analyzed by electron diffraction directly on the microscope. The SAED pattern (inset picture in Fig. 3a) shows bright circular-concentric rings resulting from the random orientation of crystal planes, suggested crystalline nature of biofabricated Pt-NPs. In the SAED image, four rings were observed, which represents the diffraction planes (111), (200), (220), and (311), respectively.
Crystalline nature and purity of biofabricated Pt-NPs were confirmed from the five intense Bragg’s peaks in XRD pattern of PtNPs (Fig. 4) at 2u = 39.438, 45.918, 66.848, 81.368, and 86.098 corresponding to the (111), (200), (220), (311), and (222) diffraction planes, respectively, based on the comparison with the standard data given by JCPDS file no. 04-0802. The broadening of these Bragg’s peaks indicated the biofabrication of particles in nanoscale dimensions. The average particle size (d) of biosynthesized Pt-NPs, was estimated using the Scherrer formula (d = 0.9 l/b cosu, where b is the full width at half-maximum peak height measured at the diffraction angle u), which yielded a value of 21 nm. This is consistent with the particles size of Pt-NPs estimated
Fig. 4. XRD diffractogram obtained for dried Pt-NPs.
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Fig. 5. EDX spectrum of dried Pt-NPs.
using TEM and DLS analysis. The ratio between the intensities of (111) and (200) diffraction peak was observed higher than the usual bulk values which indicated that the biofabricated Pt-NPs oriented in (111) plane. The lattice constant calculated from these patterns were 3.956 A˚ confirmed the fcc structure of Pt-NPs. EDX analysis of Pt-NPs Compositional analysis of biofabricated nanoparticles was carried out using EDX. The presence of Pt-NPs is confirmed by the appearance of optical absorption peak approximately at 2.10 keV. The appearance of optical absorption peak is due to SPR of Pt-NPs (Fig. 5). EDX spectrum also indicated the presence of some traces of carbon and oxygen, which implies the adsorption of organic moieties on the surface of the Pt-NPs [34,35]. Table 1 represented the percent of the different content present in EDX spectrum of dried Pt-NPs.
Fig. 6. FTIR spectra of (a) P. granatum peel extract before and (b) after bio-reduction (c) dried Pt-NPs.
FTIR analysis FTIR spectra of P. granatum peel extract before and after bioreduction of Pt4+ were compared to identify the functional groups involved in biofabrication of Pt-NPs. FTIR spectrum of peel extract (Fig. 6a) represented various peaks present at 3424, 2922, 1729, 1646, 1381, and 1050 cm1 positions attributed to –OH, C–H, C5 5O, C5 5C,–OH, and C–O–C stretching vibration, respectively. From the above peak responses, it was observed that peel extract with aromatic ring and phenolic group attached to different positions of aromatic ring. These observations indicated that the compounds present in peel extract were mostly polyphenolic in nature. Earlier researchers also reported the presence of different polyphenolic compounds such as ellagic acid, gallic acid, and quercetin in P. granatum peel [17,18]. Presence of different absorption bands due to above mentioned stretching vowed for the same. However, in the FTIR spectrum of P. granatum peel extract after bio-reduction (Fig. 6b), the peak present at 3424 got shifted to 3439 cm1 position and became relatively narrow. This observation suggested that the phenolic hydroxyls group of peel extract was utilized in bio-reduction reaction. Also, peak present at
Table 1 Elemental composition of Pt-NPs (%). Element
Weight (%)
Atomic (%)
C O Pt Total
26.11 19.90 53.99 100.00
58.84 33.66 7.50 100.00
1729 cm1 position of peel extract got shifted to 1723 cm1 position and became relatively sharp indicating that the P. granatum peel extract interacts with platinum ion through its phenolic hydroxyls group (–OH) and got oxidized to its quinones having C5 5O group. The proposed oxidized structure of polyphenolic compounds present in P. granatum peel extract (ellagic acid, gallic acid, and quercetin) are shown in Fig. 7a–c. Further, a strong intense peak at 1721 cm1 position in FTIR spectra of dried Pt-NPs (Fig. 6c) compared to curves (a) and (b), also attributed to the binding of C5 5O group with Pt-NPs. The chemistry behind the formation of Pt-NPs represented as in Fig. 8, which showed that the watersoluble polyphenolic compounds and their quinones were responsible for the biofabrication and stabilization of Pt-NPs, respectively. Bio-reduction mechanism 5O groups of polyphenolic compound showed The OH and C5 stronger ability to bind metal ions [36] and also peel of P. granatum was reported with high level of antioxidant polyphenolic compounds [19–21]. Based on the above finding, a schematic representation of a proposed mechanism for biofabrication and stabilization of Pt-NPs using representative polyphenolic compound of P. granatum peel (ellagic acid) was shown in Fig. 8. Pt4+ first chelated ellagic acid through its adjacent phenolic hydroxyls and formed an intermediate platinum complex. Due to the high oxidation–reduction potential of Pt4+, the adjacent phenolic hydroxyls of ellagic acid were inductively oxidized to the corresponding quinones. The Pt4+ was reduced to Pt0 in
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Fig. 7. Mechanism of oxidation of ellagic acid, gallic acid, and quercetin.
the presence of free electron or nascent hydrogen produced during the bio-reduction reaction. These adjoining Pt0 atoms further collided with each other and Pt-NPs formed. The Pt-NPs thus formed was further stabilized by quinones and ellagic acid.
Catalytic activity of biofabricated colloidal Pt-NPs for 3-NP reduction Further, in order to evaluate the catalytic activity of colloidal PtNPs, the reduction of 3-NP by NaBH4 was adopted as a model
Fig. 8. Proposed mechanisms for biofabrication and stabilization of Pt-NPs by ellagic acid.
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Fig. 9. UV–visible absorption spectra of (a) 3-NP taken before and after immediate addition of NaBH4 (b) successive UV–visible absorption spectra of the reduction of 3-NP by NaBH4 in the presence of colloidal Pt-NPs (c) plot of ln (3-NPt/3-NP0) against reaction time for colloidal Pt-NPs in the catalytic reduction of 3-NP.
reaction. Fig. 9a represented the UV–visible spectra of 3-NP and yellow color reaction mixture of 3-NP + NaBH4. Addition of colloidal Pt-NPs as a catalyst to the reaction mixture (3NP + NaBH4) causes fading and ultimate bleaching of the yellow color of the reaction mixture. In the representative, UV–visible spectra of the reduction of 3-NP by colloidal Pt-NPs, absorption of 3-NP ion at 390 nm decreases gradually (Fig. 9b). Similar change in the absorption spectra was also obtained earlier for reduction of 3NP to corresponding amino derivatives [37]. The absorbance changes of 3-NP ion at 390 nm were recorded to monitor the kinetics of the reaction.
In this experiment, the catalytic activity of colloidal Pt-NPs in the reduction of 3-NP was explained by Fig. 10, where colloidal PtNPs started the catalytic reduction by relaying electrons from donor BH4 to acceptor 3-NP immediately after the adsorption of both onto the colloidal Pt-NPs surface. As the initial concentration of NaBH4 was very high, it remained essentially constant throughout the reaction. Therefore, for evaluation of the apparent rate constant (kapp), pseudo-first-order kinetics with respect to 3NP was used. The kapp of this catalytic reaction in the presence of the colloidal Pt-NPs was calculated to be 3.2 103 s1, from the plot of ln(3-NPt/3-NP0) versus time t (Fig. 9c). In the absence of any catalyst, the thermodynamically favorable reduction of 3-NP was not observed and the peak due to 3-NP ions at 390 nm remains unaltered for 4 h. In presence of P. granatum peel extract and bulk H2PtCl6.6H2O the peak present at 390 nm was unchanged under the tested time scale. This observation implies that P. granatum peel extract and bulk H2PtCl6.6H2O did not work as the catalyst for this reaction. The effect of the concentration of colloidal Pt-NPs on reduction of 3-NP was studied using varied concentration of colloidal Pt-NPs from 0.5 to 2.5 mL, keeping other parameters constant. From Table 2, it is evident that the kapp for reduction reactions increases proportional to the Pt-NPs concentration. At lower concentration of Pt-NPs (0.5 mL), kapp for reduction of 3-NP was found 1.9 103 s1. However, at higher concentration (2.5 mL), kapp obtained for reduction of 3-NP was 8.4 103 s1. This observation was justified by the fact that the catalysis usually takes place on the surface of nanoparticles [38]. Reduction of 3-NP was also studied at 10–50 8C temperature range. It was observed that the rate of reduction of 3-NP increases with an increase in temperature. The temperature coefficient, Q10 for the catalytic reduction of 3-NP was calculated using Eq. (1), where kapp2: apparent rate constant at temperature T2; kapp1: apparent rate constant at temperature T1; T1, and T2: reaction temperature. From the Eq. (1), Q10 calculated for reduction of 3-NP was increased by a factor of 1.2–2 for every 10 8C rise in temperature. 10 ka p p2 T 2 T 1 Q 10 ¼ (1) : ka p p1 The kapp value for the 3-NP reduction increased from 0.9 103 s1 to 6.7 103 s1 in the investigated temperature range as shown in Table 3. The temperature dependence of the reduction rate was calculated using Arrhenius equation, Eq. (2), where A: frequency factor; Ea: activation energy; R: universal constant; and T: absolute temperature. From the Arrhenius equation, Ea was calculated to be 37.79 kJ mol1 for 3-NP reduction. ka p p ¼ Aexp
Ea : RT
(2)
The present study reported first time the biofabricated colloidal Pt-NPs use as the catalyst for the reduction of 3-NP. There was no data available for direct comparison. Therefore, this study leads to
Fig. 10. Scheme of the reduction of 3-NP by NaBH4 on the surface of colloidal Pt-NPs.
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JIEC-2119; No. of Pages 7 P. Dauthal, M. Mukhopadhyay / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx Table 2 kapp for reduction of 3-NP using different concentration of colloidal Pt-NPs as catalyst at room temperature (28 2 8C) (reaction condition: 1 103 M 3-NP, 1 M NaBH4). Colloidal Pt-NPs (mL)
kapp 103 (s1)
0.5 1 1.5 2 2.5
1.9 3.2 5.6 7.1 8.4
Materials Science (MEMS), IIT Bombay and Electrical Research and Development Association (ERDA), Vadodara for providing necessary research facilities for characterizations of samples. References [1] [2] [3] [4] [5] [6]
Table 3 kapp for reduction of 3-NP at different temperature using biofabricated colloidal Pt-NPs as catalyst (reaction condition: 1 103 M 3-NP, 1 M NaBH4, 1 mL of colloidal Pt-NPs). 3
T (8C)
kapp 10
10 20 30 40 50
0.9 1.7 3.5 4.3 6.7
1
(s
)
[7] [8] [9] [10] [11] [12] [13] [14] [15]
a new insight to the activity of biofabricated Pt-NPs as a catalyst for the reduction of an anthropogenic pollutants 3-NP. Conclusions The results of the study show that agro-industrial waste P. granatum peel can be used for the biofabrication of Pt-NPs. The proposed method requires 30 min to synthesize Pt-NPs. Sphericalshaped crystalline Pt-NPs are synthesized with 16–23 nm particle size, and are well supported by TEM, XRD, and DLS analysis. It is evident from FTIR analysis that the polyphenolic compounds (ellagic acid, ellagic tannins, gallic acid, and quercetin) of P. granatum peel extract are responsible for bio-reduction of Pt4+ to Pt0. The high negative z potential of Pt-NPs also reveals the role of polyphenolic compounds (quinones) for capping and stabilization of biofabricated Pt-NPs. Biofabricated colloidal Pt-NPs exhibits catalytic activity for reduction of anthropogenic pollutant 3-NP. Thus the present study is an eco-friendly and cost-effective approach for catalytic reduction of 3-NP.
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[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
Acknowledgments
[34] [35] [36]
Authors wish to acknowledge Sophisticated Analytical Instrument Facility (SAIF), Department of Metallurgical Engineering and
[37] [38]
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Biofabrication, characterization, and possible bio-reduction mechanism of platinum nanoparticles mediated by agro-industrial waste and their catalytic activity Preeti Dauthal, Mausumi Mukhopadhyay * Department of Chemical Engineering, S.V. National Institute of Technology, Surat 395-007, Gujarat, India
A R T I C L E I N F O
Article history: Received 25 March 2014 Received in revised form 2 July 2014 Accepted 9 July 2014 Available online xxx Keywords: Punica granatum Biofabrication Platinum Nanoparticle Reduction
A B S T R A C T
The present study showed biofabrication of platinum nanoparticles (Pt-NPs) using agro-industrial waste Punica granatum’s peel extract. Appearance of the broad spectrum from visible to the ultraviolet region, confirmed the biofabrication of Pt-NPs. Pt-NPs were spherical, within size range of 16–23 nm. XRD suggested the fabrication of crystalline Pt-NPs with (111) plane in predominant orientation. The negative z potential value of colloidal Pt-NPs revealed high stability. FTIR confirmed the role of hydroxyl and carbonyl groups of polyphenolic compounds of peel extract for biofabrication. The reduction of anthropogenic pollutant, 3-nitrophenol, by NaBH4 using colloidal Pt-NPs established it as an efficient ‘‘green catalyst.’’ ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction Today, transition metal nanoparticles, particularly platinum nanoparticles (Pt-NPs) are subject to intensive research. The emerging catalytic applications of Pt-NPs in variety of reactions such as hydrogenation [1], oxidation [2], reduction [3], and for the synthesis of organic dyes [4] has attracted the focus of researchers. The biofabricated nanoparticles for their eco-friendly properties are the preferred option for variety of applications. The development of simple and environment-friendly methods for controlled synthesis of Pt-NPs is therefore, important, not only for the fascinating utilization of Pt-NPs but also for the demands of green chemistry. In recent years, significant efforts have been made toward fabrication of nanoparticles using biogenic resources, which also represents a growing connection between biotechnology and nanotechnology [5]. This approach for nanoparticle fabrication shows several benefits concerning biocompatibility, thermal and chemical stability, high efficiency, fast process, cost efficiency, and eco-friendly nature [6,7]. However, till date plant-based synthesis of Pt-NPs has been reported only by leaf extract of Ocimum sanctum
* Corresponding author. Tel.: +91 261 2201645; fax: +91 261 2227334, 2201641. E-mail addresses: [email protected], [email protected] (M. Mukhopadhyay).
[8], Diospyros kaki [9], Cacumen platycladi [10], and Anacardium occidentale [11]. Few other biological resources are also reported for biological synthesis of Pt-NPs such as horse spleen apoferritin [12] and honey [4]. Recently, use of Punica granatum (P. granatum) peel has been reported for cost-effective and environmentally benign synthesis of Ag [13] and Au-NPs [14]. Earlier, leaf [15] and fruit [16] of P. granatum were also utilized for biofabrication of Au and Ag-NPs. P. granatum peel is one of the most valuable by-products of the food and agriculture industry, which is mainly composed of ellagic tannins, punicalagin, gallic acid, ellagic acid, and quercetin [17,18]. It is also shown antioxidant [19,20], antimutagenic [21], and chemo-preventive potential [22]. An extensive literature survey revealed that there is no report available for the biosynthesis of Pt-NPs using P. granatum peel. In the present study, antioxidant potential of P. granatum peel is utilized for biofabrication of Pt-NPs. The present study involves biofabrication of Pt-NPs using agroindustrial waste P. granatum peel as the bio-reducing agent. Further, catalytic activity of biofabricated colloidal Pt-NPs is also investigated for reduction of anthropogenic pollutant 3nitrophenol (3-NP) using sodium borohydride (NaBH4) as a hydrogen or electron donor. Nowadays, nitrophenols are extensively used as raw materials in pharmaceutical, dye, and insecticide industries [23]. Since nitrophenols are readily soluble and stable in water [24] they are abundantly present in agricultural and industrial waste water. Further, they are perilous to public
http://dx.doi.org/10.1016/j.jiec.2014.07.009 1226-086X/ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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health as they possess mutagenic and carcinogenic properties harmful for humans [25]. It is therefore, necessary to eliminate nitrophenols from industrial waste water. The reduction of 3-NP to 3-aminophenol (3-AP) is therefore, of significant importance. Use of eco-friendly biofabricated colloidal Pt-NPs as a catalyst in this direction is of enormous value in environmental and industrial aspect. Biofabricated Pt-NPs are characterized by UV–visible spectrophotometer (UV–visible), dynamic light scattering (DLS), X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), and Fourier transform infrared spectroscopy (FTIR) techniques. Further, the reduction of 3-NP is monitored by UV–visible spectrophotometer. Experimental Materials Chemicals, including hexachloroplatinic acid (H2PtCl6.6H2O) 99.9%, 3-nitrophenol (C6H5NO3) 98%, and sodium borohydride (NaBH4) 98% were procured from HiMedia Pvt. Ltd, Mumbai, India. Peels of the P. granatum fruit were collected from the local market of Surat, Gujarat, India. Double-sterilized Milli-Q water was used throughout the experiments. Preparation of the P. granatum peel extract and biofabrication of colloidal Pt-NPs A total of 30 g of fresh peel of P. granatum was extracted with 120 mL of distilled water at 60 8C for 10 min and filtered. The solution was decanted and stored at 4 8C for further use. About 100 mL of aqueous P. granatum peel extract was added to 400 mL of 1 103 M H2PtCl6.6H2O solution. The mixture was maintained at 90 8C in a sealed flask for 30 min under shaking conditions on a rotary shaker (500 rpm). The reduced Pt-NPs were sonicated for 10 min to separate Pt-NPs from the biomolecules present in P. granatum peel extract. After sonication, Pt-NPs were purified by repeated centrifugation at 14,000 rpm for 10 min and the pellets were washed thrice with distilled water to remove the impurities. Control reactions, in which H2PtCl6.6H2O solution and peel extract was kept in a separate conical flask, under the same reaction conditions. Evaluation of catalytic activity of biofabricated colloidal Pt-NPs In order to find out the catalytic activity of biofabricated colloidal Pt-NPs (Pt-NPs dispersed in P. granatum peel extract) three typical reactions were carried. In the first reaction, 1 mL of 1 103 M 3-NP was mixed with 0.5 mL of water. In the second reaction, 0.5 mL of 1 M NaBH4 was added to the first reaction mixture. In the third reaction, 1 mL of colloidal Pt-NPs was mixed with reaction mixture obtained from the second reaction. All the three reactions were monitored by UV–visible spectrophotometer (DR 5000, HACH, USA). Temperature and concentration dependent catalytic activity of biofabricated colloidal Pt-NPs were also carried out for 3-NP reduction. Catalytic activity of P. granatum peel extract and bulk H2PtCl6.6H2O was also evaluated for 3-NP reduction.
ZS90, Malvern, UK). TEM and selected area electron diffraction (SAED) pattern data were obtained by using TEM (CM-200, Philips, UK). TEM image was recorded at 100 kV accelerating voltage with resolution of 2.4 A´˚ . XRD pattern of Pt-NPs on the glass substrate was recorded by using XRD (X’Pert Pro, PANalytical, Holland) operated at a voltage of 45 kV and current of 35 mA with Cu–Ka radiation (K = 1.5406 A´˚ ). The scanning range (2u) was selected from 20 8 to 80 8 at 0.045 8/min continuous speed. The crystallite size of the Pt-NPs was calculated using Scherrer’s formula. The natures of elements were identified by EDX (INCA X-sight, Oxford Instruments, UK) coupled with scanning electron microscopy (SEM) (JSM-6380LV, JEOL, Japan). In order to identify the phytochemicals responsible for bio-reduction and stabilization of Pt-NPs, FTIR analysis of peel extract before and after bioreduction and biogenic Pt-NPs were carried out using FTIR (Magna550, Nicolet, USA). All spectra was taken in the mid-IR region of 600–3600 cm1.
Results and discussion UV–visible spectroscopy analysis for biofabrication of Pt-NPs The biofabrication of colloidal Pt-NPs was confirmed by UV– visible spectroscopy analysis. The H2PtCl6.6H2O solution (pale yellow) showed an absorption peak at around 260 nm in its UV– visible spectrum due to the ligand-to-metal charge-transfer transition between Pt4+ and Cl ions [26], displayed in Fig. 1. As the bio-reduction reaction was carried out, colloidal Pt-NPs were formed simultaneously with a change in pale yellow color of H2PtCl6.6H2O solution into light brown. The absorption peak present at 260 nm disappeared and was replaced by a broad continuous absorption spectrum, which gradually increases in intensity from visible to the ultraviolet region, suggested Pt4+ ions were completely reduced to Pt0 [27]. Control reaction mixture recovered as pale yellow in color, with no light-brown, being observed suggested Pt-NPs were formed only in presence of P. granatum peel extract. DLS and z potential analysis Fig. 2a revealed that the Z-average diameter of the biofabricated Pt-NPs was 30 nm with a polydispersity index (PDI) 0.270. The corresponding average z potential value of 15.7 mV confirmed the stability of Pt-NPs in colloidal solution (Fig. 2b). This low value of z potential of biofabricated nanoparticles was attributed to the additional influence by the electric charge of bio-organics present in peel extract. Various process parameters such as biomaterial dosage, temperature, and pH (change the electric charge of bioorganics), which might further affect their capping and stabilizing
Characterization of Pt-NPs Biofabrication of Pt-NPs was monitored first by visual inspection and then by using UV–visible spectrophotometer (DR 5000, HACH, USA). Baseline correction was made with P. granatum peel extract. Hydrodynamic size distribution and z potential of the colloidal Pt-NPs were determined by using DLS (Zetasizer Nano
Fig. 1. UV–visible spectra of aqueous H2PtCl6.6H2O and colloidal Pt-NPs.
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Fig. 2. Colloidal Pt-NPs (a) particles size and (b) corresponding z potential distribution.
abilities. This was subsequently affecting the z potential value of nanoparticles [28]. Similar results were also reported earlier for different stable biofabricated nanoparticles synthesized using different plant resources [29–31]. This negative value of z potential indicated high electrical charge on the surface of Pt-NPs, which causes strong repellent forces between particles and prevents aggregation of nanoparticles. This resulted in higher stability of colloidal Pt-NPs. The negative z potential value could be due to the capping of polyphenolic compounds present in P. granatum peel extract [32].
Fig. 3. TEM Images of (a–b) biosynthesized Pt-NPs, inset represented SAED image of Pt-NPs.
XRD analysis of Pt-NPs TEM analysis of Pt-NPs Further, TEM analysis was performed in order to determine the shape and size of biofabricated Pt-NPs (Fig. 3a and b). Some clustering of the particles was also apparent in TEM image. TEM images showed particles were spherical with relatively uniform size range of 16–23 nm. The basic reason behind the size difference in TEM and DLS analysis was attributed to the different instrument response to particle number, volume, mass, optical property, and sample preparation method and also due to the polydispersity of the sample [32]. In DLS analysis, size measurement was based on the phenomenon of Brownian motion. DLS method measured the hydrodynamic diameter of particles in colloidal solution, which was slightly higher than the size measured by other techniques due to the various forces of interaction works in the colloidal solution such as van der Waals force of attraction [33]. The DLS instrument was known to measure the shell thickness of a capping agent enveloping the nanoparticles along with the actual size of the metallic core. However, in TEM analysis dry forms of synthesized particles were directly analyzed under an electron microscope. So in scientific scenario for the estimation of exact particle size, TEM was the appropriate technique [24]. These nanoparticles were also analyzed by electron diffraction directly on the microscope. The SAED pattern (inset picture in Fig. 3a) shows bright circular-concentric rings resulting from the random orientation of crystal planes, suggested crystalline nature of biofabricated Pt-NPs. In the SAED image, four rings were observed, which represents the diffraction planes (111), (200), (220), and (311), respectively.
Crystalline nature and purity of biofabricated Pt-NPs were confirmed from the five intense Bragg’s peaks in XRD pattern of PtNPs (Fig. 4) at 2u = 39.438, 45.918, 66.848, 81.368, and 86.098 corresponding to the (111), (200), (220), (311), and (222) diffraction planes, respectively, based on the comparison with the standard data given by JCPDS file no. 04-0802. The broadening of these Bragg’s peaks indicated the biofabrication of particles in nanoscale dimensions. The average particle size (d) of biosynthesized Pt-NPs, was estimated using the Scherrer formula (d = 0.9 l/b cosu, where b is the full width at half-maximum peak height measured at the diffraction angle u), which yielded a value of 21 nm. This is consistent with the particles size of Pt-NPs estimated
Fig. 4. XRD diffractogram obtained for dried Pt-NPs.
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Fig. 5. EDX spectrum of dried Pt-NPs.
using TEM and DLS analysis. The ratio between the intensities of (111) and (200) diffraction peak was observed higher than the usual bulk values which indicated that the biofabricated Pt-NPs oriented in (111) plane. The lattice constant calculated from these patterns were 3.956 A˚ confirmed the fcc structure of Pt-NPs. EDX analysis of Pt-NPs Compositional analysis of biofabricated nanoparticles was carried out using EDX. The presence of Pt-NPs is confirmed by the appearance of optical absorption peak approximately at 2.10 keV. The appearance of optical absorption peak is due to SPR of Pt-NPs (Fig. 5). EDX spectrum also indicated the presence of some traces of carbon and oxygen, which implies the adsorption of organic moieties on the surface of the Pt-NPs [34,35]. Table 1 represented the percent of the different content present in EDX spectrum of dried Pt-NPs.
Fig. 6. FTIR spectra of (a) P. granatum peel extract before and (b) after bio-reduction (c) dried Pt-NPs.
FTIR analysis FTIR spectra of P. granatum peel extract before and after bioreduction of Pt4+ were compared to identify the functional groups involved in biofabrication of Pt-NPs. FTIR spectrum of peel extract (Fig. 6a) represented various peaks present at 3424, 2922, 1729, 1646, 1381, and 1050 cm1 positions attributed to –OH, C–H, C5 5O, C5 5C,–OH, and C–O–C stretching vibration, respectively. From the above peak responses, it was observed that peel extract with aromatic ring and phenolic group attached to different positions of aromatic ring. These observations indicated that the compounds present in peel extract were mostly polyphenolic in nature. Earlier researchers also reported the presence of different polyphenolic compounds such as ellagic acid, gallic acid, and quercetin in P. granatum peel [17,18]. Presence of different absorption bands due to above mentioned stretching vowed for the same. However, in the FTIR spectrum of P. granatum peel extract after bio-reduction (Fig. 6b), the peak present at 3424 got shifted to 3439 cm1 position and became relatively narrow. This observation suggested that the phenolic hydroxyls group of peel extract was utilized in bio-reduction reaction. Also, peak present at
Table 1 Elemental composition of Pt-NPs (%). Element
Weight (%)
Atomic (%)
C O Pt Total
26.11 19.90 53.99 100.00
58.84 33.66 7.50 100.00
1729 cm1 position of peel extract got shifted to 1723 cm1 position and became relatively sharp indicating that the P. granatum peel extract interacts with platinum ion through its phenolic hydroxyls group (–OH) and got oxidized to its quinones having C5 5O group. The proposed oxidized structure of polyphenolic compounds present in P. granatum peel extract (ellagic acid, gallic acid, and quercetin) are shown in Fig. 7a–c. Further, a strong intense peak at 1721 cm1 position in FTIR spectra of dried Pt-NPs (Fig. 6c) compared to curves (a) and (b), also attributed to the binding of C5 5O group with Pt-NPs. The chemistry behind the formation of Pt-NPs represented as in Fig. 8, which showed that the watersoluble polyphenolic compounds and their quinones were responsible for the biofabrication and stabilization of Pt-NPs, respectively. Bio-reduction mechanism 5O groups of polyphenolic compound showed The OH and C5 stronger ability to bind metal ions [36] and also peel of P. granatum was reported with high level of antioxidant polyphenolic compounds [19–21]. Based on the above finding, a schematic representation of a proposed mechanism for biofabrication and stabilization of Pt-NPs using representative polyphenolic compound of P. granatum peel (ellagic acid) was shown in Fig. 8. Pt4+ first chelated ellagic acid through its adjacent phenolic hydroxyls and formed an intermediate platinum complex. Due to the high oxidation–reduction potential of Pt4+, the adjacent phenolic hydroxyls of ellagic acid were inductively oxidized to the corresponding quinones. The Pt4+ was reduced to Pt0 in
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Fig. 7. Mechanism of oxidation of ellagic acid, gallic acid, and quercetin.
the presence of free electron or nascent hydrogen produced during the bio-reduction reaction. These adjoining Pt0 atoms further collided with each other and Pt-NPs formed. The Pt-NPs thus formed was further stabilized by quinones and ellagic acid.
Catalytic activity of biofabricated colloidal Pt-NPs for 3-NP reduction Further, in order to evaluate the catalytic activity of colloidal PtNPs, the reduction of 3-NP by NaBH4 was adopted as a model
Fig. 8. Proposed mechanisms for biofabrication and stabilization of Pt-NPs by ellagic acid.
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Fig. 9. UV–visible absorption spectra of (a) 3-NP taken before and after immediate addition of NaBH4 (b) successive UV–visible absorption spectra of the reduction of 3-NP by NaBH4 in the presence of colloidal Pt-NPs (c) plot of ln (3-NPt/3-NP0) against reaction time for colloidal Pt-NPs in the catalytic reduction of 3-NP.
reaction. Fig. 9a represented the UV–visible spectra of 3-NP and yellow color reaction mixture of 3-NP + NaBH4. Addition of colloidal Pt-NPs as a catalyst to the reaction mixture (3NP + NaBH4) causes fading and ultimate bleaching of the yellow color of the reaction mixture. In the representative, UV–visible spectra of the reduction of 3-NP by colloidal Pt-NPs, absorption of 3-NP ion at 390 nm decreases gradually (Fig. 9b). Similar change in the absorption spectra was also obtained earlier for reduction of 3NP to corresponding amino derivatives [37]. The absorbance changes of 3-NP ion at 390 nm were recorded to monitor the kinetics of the reaction.
In this experiment, the catalytic activity of colloidal Pt-NPs in the reduction of 3-NP was explained by Fig. 10, where colloidal PtNPs started the catalytic reduction by relaying electrons from donor BH4 to acceptor 3-NP immediately after the adsorption of both onto the colloidal Pt-NPs surface. As the initial concentration of NaBH4 was very high, it remained essentially constant throughout the reaction. Therefore, for evaluation of the apparent rate constant (kapp), pseudo-first-order kinetics with respect to 3NP was used. The kapp of this catalytic reaction in the presence of the colloidal Pt-NPs was calculated to be 3.2 103 s1, from the plot of ln(3-NPt/3-NP0) versus time t (Fig. 9c). In the absence of any catalyst, the thermodynamically favorable reduction of 3-NP was not observed and the peak due to 3-NP ions at 390 nm remains unaltered for 4 h. In presence of P. granatum peel extract and bulk H2PtCl6.6H2O the peak present at 390 nm was unchanged under the tested time scale. This observation implies that P. granatum peel extract and bulk H2PtCl6.6H2O did not work as the catalyst for this reaction. The effect of the concentration of colloidal Pt-NPs on reduction of 3-NP was studied using varied concentration of colloidal Pt-NPs from 0.5 to 2.5 mL, keeping other parameters constant. From Table 2, it is evident that the kapp for reduction reactions increases proportional to the Pt-NPs concentration. At lower concentration of Pt-NPs (0.5 mL), kapp for reduction of 3-NP was found 1.9 103 s1. However, at higher concentration (2.5 mL), kapp obtained for reduction of 3-NP was 8.4 103 s1. This observation was justified by the fact that the catalysis usually takes place on the surface of nanoparticles [38]. Reduction of 3-NP was also studied at 10–50 8C temperature range. It was observed that the rate of reduction of 3-NP increases with an increase in temperature. The temperature coefficient, Q10 for the catalytic reduction of 3-NP was calculated using Eq. (1), where kapp2: apparent rate constant at temperature T2; kapp1: apparent rate constant at temperature T1; T1, and T2: reaction temperature. From the Eq. (1), Q10 calculated for reduction of 3-NP was increased by a factor of 1.2–2 for every 10 8C rise in temperature. 10 ka p p2 T 2 T 1 Q 10 ¼ (1) : ka p p1 The kapp value for the 3-NP reduction increased from 0.9 103 s1 to 6.7 103 s1 in the investigated temperature range as shown in Table 3. The temperature dependence of the reduction rate was calculated using Arrhenius equation, Eq. (2), where A: frequency factor; Ea: activation energy; R: universal constant; and T: absolute temperature. From the Arrhenius equation, Ea was calculated to be 37.79 kJ mol1 for 3-NP reduction. ka p p ¼ Aexp
Ea : RT
(2)
The present study reported first time the biofabricated colloidal Pt-NPs use as the catalyst for the reduction of 3-NP. There was no data available for direct comparison. Therefore, this study leads to
Fig. 10. Scheme of the reduction of 3-NP by NaBH4 on the surface of colloidal Pt-NPs.
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JIEC-2119; No. of Pages 7 P. Dauthal, M. Mukhopadhyay / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx Table 2 kapp for reduction of 3-NP using different concentration of colloidal Pt-NPs as catalyst at room temperature (28 2 8C) (reaction condition: 1 103 M 3-NP, 1 M NaBH4). Colloidal Pt-NPs (mL)
kapp 103 (s1)
0.5 1 1.5 2 2.5
1.9 3.2 5.6 7.1 8.4
Materials Science (MEMS), IIT Bombay and Electrical Research and Development Association (ERDA), Vadodara for providing necessary research facilities for characterizations of samples. References [1] [2] [3] [4] [5] [6]
Table 3 kapp for reduction of 3-NP at different temperature using biofabricated colloidal Pt-NPs as catalyst (reaction condition: 1 103 M 3-NP, 1 M NaBH4, 1 mL of colloidal Pt-NPs). 3
T (8C)
kapp 10
10 20 30 40 50
0.9 1.7 3.5 4.3 6.7
1
(s
)
[7] [8] [9] [10] [11] [12] [13] [14] [15]
a new insight to the activity of biofabricated Pt-NPs as a catalyst for the reduction of an anthropogenic pollutants 3-NP. Conclusions The results of the study show that agro-industrial waste P. granatum peel can be used for the biofabrication of Pt-NPs. The proposed method requires 30 min to synthesize Pt-NPs. Sphericalshaped crystalline Pt-NPs are synthesized with 16–23 nm particle size, and are well supported by TEM, XRD, and DLS analysis. It is evident from FTIR analysis that the polyphenolic compounds (ellagic acid, ellagic tannins, gallic acid, and quercetin) of P. granatum peel extract are responsible for bio-reduction of Pt4+ to Pt0. The high negative z potential of Pt-NPs also reveals the role of polyphenolic compounds (quinones) for capping and stabilization of biofabricated Pt-NPs. Biofabricated colloidal Pt-NPs exhibits catalytic activity for reduction of anthropogenic pollutant 3-NP. Thus the present study is an eco-friendly and cost-effective approach for catalytic reduction of 3-NP.
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Acknowledgments
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Authors wish to acknowledge Sophisticated Analytical Instrument Facility (SAIF), Department of Metallurgical Engineering and
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