Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 238–244
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Phoenix dactylifera L. leaf extract phytosynthesized gold nanoparticles; controlled synthesis and catalytic activity Mervat F. Zayed a,⇑, Wael H. Eisa b a b
Chemistry Department, Faculty of Science, Menoufia University, Egypt Spectroscopy Department, Physics Division, National Research Center (NRC), Egypt
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A Phoenix dactylifera is an excellent
sink for biosynthesis of the gold nanoparticles. Carbohydrates, flavonoids, tannins and phenolic acids present in P. dactylifera acts as reducing and capping agents. The as-synthesized Au colloids exhibited good catalytic activity for the degradation of 4-nitrophenol.
a r t i c l e
i n f o
Article history: Received 25 August 2013 Received in revised form 8 October 2013 Accepted 19 October 2013 Available online 1 November 2013 Keywords: Green synthesis Gold nanoparticles Phoenix dactylifera 4-Nitrophenol
a b s t r a c t A green synthesis route was reported to explore the reducing and capping potential of Phoenix dactylifera extract for the synthesis of gold nanoparticles. The processes of nucleation and growth of gold nanoparticles were followed by monitoring the absorption spectra during the reaction. The size and morphology of these nanoparticles was typically imaged using transmission electron microscopy (TEM). The particle size ranged between 32 and 45 nm and are spherical in shape. Fourier transform infrared (FTIR) analysis suggests that the synthesized gold nanoparticles might be stabilized through the interactions of hydroxyl and carbonyl groups in the carbohydrates, flavonoids, tannins and phenolic acids present in P. dactylifera. The as-synthesized Au colloids exhibited good catalytic activity for the degradation of 4-nitrophenol. Ó 2013 Elsevier B.V. All rights reserved.
Introduction Gold nanoparticles have been considered as important area of research due to their diverse applications in various fields including drug delivery, tissue/tumor imaging, photothermal therapy, catalysis, water purification, surface-enhanced Raman Scattering detection and optoelectronics [1–7]. Researchers make a great effort in synthesis of gold nanoparticles by a variety of chemical and physical methods [8–16]. However, these methods not only
⇑ Corresponding author. Tel.: +20 48 2210369. E-mail address: [email protected] (M.F. Zayed). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.10.092
use expensive and toxic reagents as reducing and stabilizing agents, but it is very likely that residual unreacted toxic chemicals and by-products make gold nanoparticles so produced unsuitable for use in biomedical applications. As a result, the search for environment friendly molecules for the synthesis of nanoparticles has gained a great deal of effort. Synthesis of nanoparticles using biological systems, such as microorganisms and plants, has gained a great importance in recent years due to using of less harmful, biodegradable and costeffective reagents. Moreover, nanoparticles synthesized with biological base are interesting, predominantly because they exhibit the best compatibility with biomolecules [17]. Although gold
M.F. Zayed, W.H. Eisa / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 238–244
nanoparticles synthesis using plant extracts has already been reported with various plants [18–25], there is still a lot of attention paid to this field because of the high potential of plants in producing nanoparticles with different sizes and shapes, as well as the broad diversity of plant metabolites that may aid in the reduction. Phoenix dactylifera L. (Palmae) commonly known as the date palm has been cultivated in the Arab world for centuries [26] and its cultivation has now spread to Middle East, parts of Central and South America, and Southern Europe [27,28]. It is considered as one of the most important commercial crops; the fruits of date palm constitute a substantial part of the diet in the Arabian world. In addition to its dietary use, the leaves are used in traditional medicine to treat intestinal hemorrhage, diarrhoea, jaundice and diabetes [29–31]. It is reported to possess antioxidant, antimicrobial, antidiabetic and antilipaemic activities [32–34]. However, there are no reports till date that documents its potential in nanobiotechnology to synthesize nanoparticles and thereby evaluating its chemocatalytic applications. Nitrophenols are among the most common organic waste water pollutants widely used in the chemical industry. 4-Nitrophenol (4NP) is an important chemical that is being used as a precursor or intermediate for the preparation of pesticides, insecticides, herbicides, explosives, synthetic dyes and pharmaceuticals. 4-NP is highly soluble and stable in water so it stays a very long time in the soil and ground water without degradation [35]. This accumulation rises the environmental risk due to the carcinogenic activities of 4-NP [36,37]. Many processes have been employed for the removal of nitrophenols [38–40] but these methods are energy consuming and use organic solvents. However, GNPs are used as a catalyst to degrade 4-NP to 4-aminopenol as an easy, rapid and energy saving operation [41–44]. On the other hand, 4-aminophenol is considered as an environmentally safe end product because it is an important intermediate for the manufacture of analgesic and antipyretic drugs. In this paper we describe a rapid, simple, economic and environmentally benign method for synthesis of gold nanoparticles using leaves extract of P. dactylifera as reducing and capping agent without adding chemicals (see Scheme 1). We also investigated the effects of reaction conditions such as the extract quantity and contact time on the rate of synthesis, size, shape and size
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distribution of the gold nanoparticles. Furthermore, we demonstrated its chemocatalytic potential in reduction of 4-NP to 4-aminophenol and the size-dependence of the catalytic activity of gold nanoparticles. Experimental Chemicals All the reagents purchased were of analytical grade and used as received. Chloroauric acid (HAuCl43H2O) was obtained from Sigma–Aldrich Co. 4-Nitrophenol was purchased from LOBA CHEMIE, India. Sodium borohydride (NaBH4) was provided by Merck Co. All aqueous solutions were prepared using deionized double distilled water. Preparation of plant extract Fresh leaves of P. dactylifera were collected from Shebin El-kom city, Egypt and had been washed thoroughly with de-ionized water. The leaves (20 g) were extracted by maceration with 70% ethanol/water (100 mL). The extraction process was done at room temperature for 7 days, and then the solution was filtered and stored at 4 °C for further experiments. Synthesis of gold nanoparticles Gold nanoparticles were prepared by adding varying volumes of the plant extract (100, 200 and 400 lL) to 10 mL of aqueous chloroauric acid solution (0.4 mM) at room temperature. A control experiment was done without addition of chloroaurate ions. The biotransformation of chloroaurate ions to gold nanoparticles was periodically monitored using UV–vis spectrophotometer. Characterization of gold nanoparticles The UV–vis spectra measurements were recorded on a PG Instruments T80+ UV–vis double beam spectrophotometer (PG Instruments, United Kingdom). The morphologies and sizes of the
Scheme 1. P. dactylifera extract assisted phytosynthesis of Au nanoparticles.
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as-prepared samples were performed using transmission electron microscope (TEM) of the type JEOL-JEM-1011. A Jasco FT/IR 6100 spectrometer was employed to demonstrate the chemical nature of the synthesized composites in the range of 4000–400 cm1. The gold concentration measurement was carried out using atomic absorption spectrometer Varian SpectrAA (220) with graphite furnace accessory and equipped with deuterium arc background corrector. Catalytic activity of P. dactylifera stabilized Au nanoparticles in degradation of 4-nitrophenol For studying the catalytic activity of as-prepared gold nanoparticles, the reduction of 4-NP to 4-aminophenol by NaBH4 is performed as a probe reaction. The effect of nanoparticle size on the speed of catalytic reduction is studied using the gold colloids prepared with three different quantities of P. dactylifera extract (100, 200 and 400 lL). The method reported by Narayanan and Sakthivel [43] was applied here but with using smaller volumes of gold nanoparticles. In standard quartz cell with a 1-cm path length, 2.77 mL of water was mixed with 25 lL (102 M) of 4-NP solution and 200 lL of freshly prepared NaBH4 solution (101 M). Thereafter, 50 lL of gold solution was added to the above mixture. The same atomic concentrations of Au nanoparticles (40 ppm) were applied in all experiments of the catalytic reduction. After the addition of gold colloid, reduction is ascertained by recording the UV–vis spectra. Results and discussions Time-dependent UV–vis spectra and kinetics of nanoparticles formation When light strikes the gold nanoparticles, they get excited and show a strong absorption band in the visible region. This takes place when the frequency of the electromagnetic field is resonant with the coherent electron motion what is known as ‘‘surface plasmon resonance (SPR) absorption’’ [45]. This feature makes the UV– vis spectroscopy is the most frequently used technique to judge the success of Au nanoparticles production. The UV–vis spectrum shown in Fig. 1, explains the impact of different volumes (100, 200 and 400 lL) of P. dactylifera extract on biosynthesis of gold nanoparticles. The SPR band can be easily observed on the visible range for all the spectra. This SPR band was recorded at 552 nm for the sample prepared with the aid of
Fig. 1. The UV–vis spectra of the Au nanoparticles as a function of P. dactylifera extract volumes.
100 lL of extract. Increasing the volume of the P. dactylifera extract to 200 and 400 lL shifted the SPR band to 544 and 538 nm, respectively. The blue shift of the SPR band is due to a size dependent phenomenon called quantum confinement i.e. smaller Au nanoparticles were formed. In addition, the SPR band got narrower and sharper under the action of increasing extract volumes. The increase in intensity could be due to increasing number of nanoparticles formed as a result of reduction of Au3+ ions presented in the aqueous solution. Time-dependent UV–vis spectra were recorded to shed light on the processes of nucleation and growth of gold nanoparticles reduced under the action of different P. dactylifera extract volumes. For comparison sake, the time-dependent UV–vis spectra were taken at regular time intervals as well as keeping the concentrations of metal ions the same in each system. The nucleation and growth processes of Au nanoparticles can be monitored by following the behavior of the surface plasmon resonance (SPR) as a function of the time elapsed after the addition of P. dactylifera extract (see Fig. 2). The UV–vis spectra showed increased absorption with increasing time in the spectral region (500–600 nm). The increase in intensity could be due to increasing number of nanoparticles formed as a result of reduction of Au3+ ions presented in the aqueous solution. Three more remarkable results could be addressed here. First, the reaction rate increases with increasing the P. dactylifera extract volumes, i.e. shorter incubation times were required at higher extract volumes; for 100 lL of extract the incubation time was 40 min; for 200 lL the incubation time was 15 min whereas for 400 lL of extract the incubation time was only 3 min. Secondly, as the reaction process continued, the SPR band kept increasing in intensity, narrowing in the FWHM and its position in the spectrum shifted to longer wavelengths. The color of the growth solution gradually evolved into blue-purple. Thirdly, the UV–vis spectrum of the sample prepared with 100 lL of extract was dominated with an absorption tail extended to the near IR regions. This tail was obviously limited by increasing the volume of the extract in the reduction reaction. The Appearance of the absorption tail in the spectrum may be either because of the size distribution of the particles and/or formation of non-spherical gold nanoparticles with increasing aspect ratio. The combination of both the above phenomenon is also possible [46]. In the light of the spectral results, the reaction pathway for producing Au nanoparticles passes through the following successive stages: reduction of the soluble tetrachloroauric acid by P. dactylifera extract, nucleation of metallic gold, and growth of individual nuclei. Upon addition of the P. dactylifera extract to aqueous solution of tetrachloroauric acid the Au3+ species are reduced to metallic gold. The concentrations of metallic gold in solution increase, reaching the super saturation conditions and finally the critical concentration to nucleate. Thereafter, spontaneous nucleation takes place and many nuclei are formed with time. These nuclei in the growth solution are quasispherical in shape, with polydisperse distribution, which is collected from the low intensity and wide FWHM of the SPR band of the initial growth solution. As the reaction go on, the nuclei grow by the deposition of metallic gold to give birth to spherical nanoparticles of uniform size. This is confirmed by the spectral evolution of the growth solution as the intensity of the SPR band increases considerably while its FWHM narrows and its position is red shifted. The particle size of the gold nanoparticles depended strongly on the amount of P. dactylifera extract. It is well stated that, in order to obtain monodispersed metal nanoparticles, rapid nucleation in a short period of time is important; that is, almost all ionic species have to be reduced rapidly to metallic species simultaneously, followed by conversion to stable nuclei so as to be grown [47].
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Fig. 3. FTIR spectra of P. dactylifera and P. dactylifera-stabilized gold nanoparticles.
P. dactylifera extract and the Au nanoparticles capped with P. dactylifera extract and displayed in Fig. 3. It has been reported that the whole date plant of P. dactylifera (including pits and leaves) contains carbohydrates, alkaloids, steroids, flavonoids, saponins and tannins [34]. The phenolic profile of the plant revealed presence of mainly cinnamic acids, flavonoid glycosides, flavanols, four free phenolic acids and nine bound phenolic acids [48–51]. The vibrational stretching of OH group of P. dactylifera was appeared as an intensive band at 3415 cm1. This band undergoes a shift to 3398 cm1 and become broader upon interaction with gold salt. P. dactylifera spectrum shows a sharp peak at 1632 cm1 due to the stretching vibrations of the AC@O group [52]. The reduction of gold salt into gold nanoparticles caused the 1632 cm1 IR absorption band of P. dactylifera to be splitted into two peaks at 1651 and 1613 cm1. These evidences indicate that during the formation of Au nanoparticles, there is coordination between Au3+ ions and the oxygen atoms of AOH groups and/or AC@O groups. On the light of FTIR analysis, it can be stated that the hydroxyl and carbonyl groups present in carbohydrates, flavonoids, tanins and phenolic acids of P. dactylifera may be accountable for the reduction of the Au ions and stabilization of Au nanoparticles.
TEM analysis
Fig. 2. The UV–vis spectral evolution of the growth solution of gold nanoparticles under the action of (a) 100, (b) 200 and (c) 400 lL of P. dactylifera extract.
Hence, at the smallest extract volume (100 lL) larger particles were generated, most likely due to a slower nucleation rate, while at a highest extract volume (400 lL) faster nucleation produced monodispersed and smaller-sized particles. FT-IR analysis To better understand the reaction mechanism between P. dactylifera extract and gold salt, FTIR spectra were recorded for
TEM micrographs were taken to assess the size, size distribution and shape of the green synthesized gold nanoparticles with the aid of P. dactylifera extract. The micrographs reveal that the Au nanoparticles are surrounded by a thin layer of biomaterial which appears to be characteristic of Au nanoparticles prepared in plant extracts. Using low volume of the plant extract will result in irregular shapes (mixture of triangles, tetragonals and nanorods) as well as wide size distribution of the reduced gold nanoparticles (see Fig. 4a). In presence of excess extract to reduce the chloroaurate ions, the biomolecules strongly shaped the nanoparticles on the spherical shape (see Fig. 4b and c). These observations may reflect the fact that the P. dactylifera extract is considered as reducing and stabilizing agent in synthesis of gold nanoparticles. The quantitative measurements of the TEM micrographs were executed through performance of particle size histograms together with their Gaussian fits as displayed in Fig. 4a0 –c0 . Average particle size was determined from the peak of the Gaussian curve. Three different sizes of Au nanoparticles were synthesized by tuning the volume of P. dactylifera extract from 100 to 400 lL. The largest particle size is 45 nm for the lowest volume of the extract (100 lL)
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Fig. 4. TEM micrographs of gold nanoparticles prepared with (a) 100, (b) 200 and (c) 400 lL of P. dactylifera extract. The corresponding histograms were represented by (a0 ), (b0 ) and (c0 ).
while the smallest one is 32 nm for the highest volume of the extract (400 lL). The FWHM of the Gaussian fit was narrowed with increasing extract volume. Thus, a significant improvement in the monodispersity has been achieved using excess of P. dactylifera extract.
Au nanoparticles catalyzed the reduction of 4-nitrophenol to 4aminophenol The reduction of 4-NP by NaBH4 has been chosen as a benchwork reaction for monitoring the catalytic activity of Au nanoparticles. The aqueous solution of 4-NP shows two distinct absorption peaks at 320 and 396 nm (see Fig. 5). The electronic band at 320 is the characteristic band of 4-NP whereas the one at 400 nm
was attributed to the presence of 4-nitrophenolate ions in the system. After immediate addition of freshly prepared aqueous solution of NaBH4, the absorption band at 320 disappeared totally whereas the absorption band at 400 nm grew markedly to up to twice of its original value as displayed in Fig. 5. These spectral changes may be attributed to the conversion of 4-nitrophenol to 4-nitrophenolate ions in basic medium. It is well known that nitro compounds are inert to NaBH4 if it is used alone [43]. This is exactly consistent with our practical visual inspections. In the absence of proper catalyst, the intense yellow color of the 4nitrophenolate solution remained unaltered for several days. Fig. 6 shows the behavior of catalytic conversion of 4-nitrophenol to 4-aminophenol after addition of 50 lL Au nanoparticles using UV–vis spectrophotometer. The process was quantitatively monitored as a successive decrease in the peak height at 400 nm together with the gradual development of the new peak at 300 nm which indicates the formation of 4-aminophenol [44]. These spectroscopic data were corresponded to a change in solution color from yellow to colorless within few minutes. The effect of Au nanoparticle size on catalytic efficiency was studied as well. As expected, the smaller the size of the catalyst the shorter is the time of the reduction process (see Fig. 6). This catalytic reaction was carried out in large excess of NaBH4 concentration as compared with that of 4-NP. Hence, it is convenient to consider the concentration of borohydride ion is constant throughout the experiment. Hence, the reaction rate (Ka) of the reduction is assumed to be only dependent of 4-NP concentration. Therefore, the rate is assumed to follow first order kinetics [53]. The kinetic equation for the reduction can be written as
K a t ¼ ln ðC t =C 0 Þ ¼ ln ðAt =A0 Þ
Fig. 5. UV–vis spectra of 4-NP before and after adding NaBH4 solution.
where Ct and At are the concentration and absorption of 4-NP at time t while C0 and A0 are the concentration and absorption of 4-NP at the starting of the reaction. The rate constant (Ka) can be
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Fig. 6. Successive UV–vis absorption spectra of the reduction of 4-NP by the P. dactylifera-stabilized gold nanoparticles.
calculated from the linear fitting of the graphical representation of ln (At/A0) against the reaction time (t) which is displayed in Fig. 6. The rate constant of 32 nm gold nanoparticles is 3.1 103 s1, which is higher than those of gold nanoparticles with larger sizes, (1.5 103 s1 and 1.12 103 s1 for 36 and 45 nm gold nanoparticles, respectively). It is well known that the surface area of the metals is the most important criteria that determine the efficiency of the metals as catalytic agents [54]. Therefore, particles with smaller sizes have a higher surface-to-volume ratio so they can facilitate the electron relay from the donor to the acceptor. In addition, it needs to be noted that the total number of Au nanoparticles of the 32 nm sample would be larger than those of 36 and 45 nm samples because the size of 32 nm sample is smaller compared with other samples. The calculated rate constant is comparable with previously reported values of glucan and casein-stabilized gold nanoparticles [55,56]. 4. Conclusion P. dactylifera extract is confirmed to be an efficient candidate for bio-fabrication of gold nanoparticles due to its controlled reducing
power as well as presence of capping molecules. The size of colloidal Au nanoparticles could be easily tuned in the nanometer range by adjusting the used volume of P. dactylifera extract solution. Increasing the added extract volume increases the rate of reduction and reduces the particle size as well as their agglomeration. The stabilized gold nanoparticles exhibit size dependent catalytic activity toward degradation of 4-NP. The rate constant of the catalytic reaction was increased with decreasing particle sizes.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Phoenix dactylifera L. leaf extract phytosynthesized gold nanoparticles; controlled synthesis and catalytic activity Mervat F. Zayed a,⇑, Wael H. Eisa b a b
Chemistry Department, Faculty of Science, Menoufia University, Egypt Spectroscopy Department, Physics Division, National Research Center (NRC), Egypt
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A Phoenix dactylifera is an excellent
sink for biosynthesis of the gold nanoparticles. Carbohydrates, flavonoids, tannins and phenolic acids present in P. dactylifera acts as reducing and capping agents. The as-synthesized Au colloids exhibited good catalytic activity for the degradation of 4-nitrophenol.
a r t i c l e
i n f o
Article history: Received 25 August 2013 Received in revised form 8 October 2013 Accepted 19 October 2013 Available online 1 November 2013 Keywords: Green synthesis Gold nanoparticles Phoenix dactylifera 4-Nitrophenol
a b s t r a c t A green synthesis route was reported to explore the reducing and capping potential of Phoenix dactylifera extract for the synthesis of gold nanoparticles. The processes of nucleation and growth of gold nanoparticles were followed by monitoring the absorption spectra during the reaction. The size and morphology of these nanoparticles was typically imaged using transmission electron microscopy (TEM). The particle size ranged between 32 and 45 nm and are spherical in shape. Fourier transform infrared (FTIR) analysis suggests that the synthesized gold nanoparticles might be stabilized through the interactions of hydroxyl and carbonyl groups in the carbohydrates, flavonoids, tannins and phenolic acids present in P. dactylifera. The as-synthesized Au colloids exhibited good catalytic activity for the degradation of 4-nitrophenol. Ó 2013 Elsevier B.V. All rights reserved.
Introduction Gold nanoparticles have been considered as important area of research due to their diverse applications in various fields including drug delivery, tissue/tumor imaging, photothermal therapy, catalysis, water purification, surface-enhanced Raman Scattering detection and optoelectronics [1–7]. Researchers make a great effort in synthesis of gold nanoparticles by a variety of chemical and physical methods [8–16]. However, these methods not only
⇑ Corresponding author. Tel.: +20 48 2210369. E-mail address: [email protected] (M.F. Zayed). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.10.092
use expensive and toxic reagents as reducing and stabilizing agents, but it is very likely that residual unreacted toxic chemicals and by-products make gold nanoparticles so produced unsuitable for use in biomedical applications. As a result, the search for environment friendly molecules for the synthesis of nanoparticles has gained a great deal of effort. Synthesis of nanoparticles using biological systems, such as microorganisms and plants, has gained a great importance in recent years due to using of less harmful, biodegradable and costeffective reagents. Moreover, nanoparticles synthesized with biological base are interesting, predominantly because they exhibit the best compatibility with biomolecules [17]. Although gold
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nanoparticles synthesis using plant extracts has already been reported with various plants [18–25], there is still a lot of attention paid to this field because of the high potential of plants in producing nanoparticles with different sizes and shapes, as well as the broad diversity of plant metabolites that may aid in the reduction. Phoenix dactylifera L. (Palmae) commonly known as the date palm has been cultivated in the Arab world for centuries [26] and its cultivation has now spread to Middle East, parts of Central and South America, and Southern Europe [27,28]. It is considered as one of the most important commercial crops; the fruits of date palm constitute a substantial part of the diet in the Arabian world. In addition to its dietary use, the leaves are used in traditional medicine to treat intestinal hemorrhage, diarrhoea, jaundice and diabetes [29–31]. It is reported to possess antioxidant, antimicrobial, antidiabetic and antilipaemic activities [32–34]. However, there are no reports till date that documents its potential in nanobiotechnology to synthesize nanoparticles and thereby evaluating its chemocatalytic applications. Nitrophenols are among the most common organic waste water pollutants widely used in the chemical industry. 4-Nitrophenol (4NP) is an important chemical that is being used as a precursor or intermediate for the preparation of pesticides, insecticides, herbicides, explosives, synthetic dyes and pharmaceuticals. 4-NP is highly soluble and stable in water so it stays a very long time in the soil and ground water without degradation [35]. This accumulation rises the environmental risk due to the carcinogenic activities of 4-NP [36,37]. Many processes have been employed for the removal of nitrophenols [38–40] but these methods are energy consuming and use organic solvents. However, GNPs are used as a catalyst to degrade 4-NP to 4-aminopenol as an easy, rapid and energy saving operation [41–44]. On the other hand, 4-aminophenol is considered as an environmentally safe end product because it is an important intermediate for the manufacture of analgesic and antipyretic drugs. In this paper we describe a rapid, simple, economic and environmentally benign method for synthesis of gold nanoparticles using leaves extract of P. dactylifera as reducing and capping agent without adding chemicals (see Scheme 1). We also investigated the effects of reaction conditions such as the extract quantity and contact time on the rate of synthesis, size, shape and size
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distribution of the gold nanoparticles. Furthermore, we demonstrated its chemocatalytic potential in reduction of 4-NP to 4-aminophenol and the size-dependence of the catalytic activity of gold nanoparticles. Experimental Chemicals All the reagents purchased were of analytical grade and used as received. Chloroauric acid (HAuCl43H2O) was obtained from Sigma–Aldrich Co. 4-Nitrophenol was purchased from LOBA CHEMIE, India. Sodium borohydride (NaBH4) was provided by Merck Co. All aqueous solutions were prepared using deionized double distilled water. Preparation of plant extract Fresh leaves of P. dactylifera were collected from Shebin El-kom city, Egypt and had been washed thoroughly with de-ionized water. The leaves (20 g) were extracted by maceration with 70% ethanol/water (100 mL). The extraction process was done at room temperature for 7 days, and then the solution was filtered and stored at 4 °C for further experiments. Synthesis of gold nanoparticles Gold nanoparticles were prepared by adding varying volumes of the plant extract (100, 200 and 400 lL) to 10 mL of aqueous chloroauric acid solution (0.4 mM) at room temperature. A control experiment was done without addition of chloroaurate ions. The biotransformation of chloroaurate ions to gold nanoparticles was periodically monitored using UV–vis spectrophotometer. Characterization of gold nanoparticles The UV–vis spectra measurements were recorded on a PG Instruments T80+ UV–vis double beam spectrophotometer (PG Instruments, United Kingdom). The morphologies and sizes of the
Scheme 1. P. dactylifera extract assisted phytosynthesis of Au nanoparticles.
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as-prepared samples were performed using transmission electron microscope (TEM) of the type JEOL-JEM-1011. A Jasco FT/IR 6100 spectrometer was employed to demonstrate the chemical nature of the synthesized composites in the range of 4000–400 cm1. The gold concentration measurement was carried out using atomic absorption spectrometer Varian SpectrAA (220) with graphite furnace accessory and equipped with deuterium arc background corrector. Catalytic activity of P. dactylifera stabilized Au nanoparticles in degradation of 4-nitrophenol For studying the catalytic activity of as-prepared gold nanoparticles, the reduction of 4-NP to 4-aminophenol by NaBH4 is performed as a probe reaction. The effect of nanoparticle size on the speed of catalytic reduction is studied using the gold colloids prepared with three different quantities of P. dactylifera extract (100, 200 and 400 lL). The method reported by Narayanan and Sakthivel [43] was applied here but with using smaller volumes of gold nanoparticles. In standard quartz cell with a 1-cm path length, 2.77 mL of water was mixed with 25 lL (102 M) of 4-NP solution and 200 lL of freshly prepared NaBH4 solution (101 M). Thereafter, 50 lL of gold solution was added to the above mixture. The same atomic concentrations of Au nanoparticles (40 ppm) were applied in all experiments of the catalytic reduction. After the addition of gold colloid, reduction is ascertained by recording the UV–vis spectra. Results and discussions Time-dependent UV–vis spectra and kinetics of nanoparticles formation When light strikes the gold nanoparticles, they get excited and show a strong absorption band in the visible region. This takes place when the frequency of the electromagnetic field is resonant with the coherent electron motion what is known as ‘‘surface plasmon resonance (SPR) absorption’’ [45]. This feature makes the UV– vis spectroscopy is the most frequently used technique to judge the success of Au nanoparticles production. The UV–vis spectrum shown in Fig. 1, explains the impact of different volumes (100, 200 and 400 lL) of P. dactylifera extract on biosynthesis of gold nanoparticles. The SPR band can be easily observed on the visible range for all the spectra. This SPR band was recorded at 552 nm for the sample prepared with the aid of
Fig. 1. The UV–vis spectra of the Au nanoparticles as a function of P. dactylifera extract volumes.
100 lL of extract. Increasing the volume of the P. dactylifera extract to 200 and 400 lL shifted the SPR band to 544 and 538 nm, respectively. The blue shift of the SPR band is due to a size dependent phenomenon called quantum confinement i.e. smaller Au nanoparticles were formed. In addition, the SPR band got narrower and sharper under the action of increasing extract volumes. The increase in intensity could be due to increasing number of nanoparticles formed as a result of reduction of Au3+ ions presented in the aqueous solution. Time-dependent UV–vis spectra were recorded to shed light on the processes of nucleation and growth of gold nanoparticles reduced under the action of different P. dactylifera extract volumes. For comparison sake, the time-dependent UV–vis spectra were taken at regular time intervals as well as keeping the concentrations of metal ions the same in each system. The nucleation and growth processes of Au nanoparticles can be monitored by following the behavior of the surface plasmon resonance (SPR) as a function of the time elapsed after the addition of P. dactylifera extract (see Fig. 2). The UV–vis spectra showed increased absorption with increasing time in the spectral region (500–600 nm). The increase in intensity could be due to increasing number of nanoparticles formed as a result of reduction of Au3+ ions presented in the aqueous solution. Three more remarkable results could be addressed here. First, the reaction rate increases with increasing the P. dactylifera extract volumes, i.e. shorter incubation times were required at higher extract volumes; for 100 lL of extract the incubation time was 40 min; for 200 lL the incubation time was 15 min whereas for 400 lL of extract the incubation time was only 3 min. Secondly, as the reaction process continued, the SPR band kept increasing in intensity, narrowing in the FWHM and its position in the spectrum shifted to longer wavelengths. The color of the growth solution gradually evolved into blue-purple. Thirdly, the UV–vis spectrum of the sample prepared with 100 lL of extract was dominated with an absorption tail extended to the near IR regions. This tail was obviously limited by increasing the volume of the extract in the reduction reaction. The Appearance of the absorption tail in the spectrum may be either because of the size distribution of the particles and/or formation of non-spherical gold nanoparticles with increasing aspect ratio. The combination of both the above phenomenon is also possible [46]. In the light of the spectral results, the reaction pathway for producing Au nanoparticles passes through the following successive stages: reduction of the soluble tetrachloroauric acid by P. dactylifera extract, nucleation of metallic gold, and growth of individual nuclei. Upon addition of the P. dactylifera extract to aqueous solution of tetrachloroauric acid the Au3+ species are reduced to metallic gold. The concentrations of metallic gold in solution increase, reaching the super saturation conditions and finally the critical concentration to nucleate. Thereafter, spontaneous nucleation takes place and many nuclei are formed with time. These nuclei in the growth solution are quasispherical in shape, with polydisperse distribution, which is collected from the low intensity and wide FWHM of the SPR band of the initial growth solution. As the reaction go on, the nuclei grow by the deposition of metallic gold to give birth to spherical nanoparticles of uniform size. This is confirmed by the spectral evolution of the growth solution as the intensity of the SPR band increases considerably while its FWHM narrows and its position is red shifted. The particle size of the gold nanoparticles depended strongly on the amount of P. dactylifera extract. It is well stated that, in order to obtain monodispersed metal nanoparticles, rapid nucleation in a short period of time is important; that is, almost all ionic species have to be reduced rapidly to metallic species simultaneously, followed by conversion to stable nuclei so as to be grown [47].
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Fig. 3. FTIR spectra of P. dactylifera and P. dactylifera-stabilized gold nanoparticles.
P. dactylifera extract and the Au nanoparticles capped with P. dactylifera extract and displayed in Fig. 3. It has been reported that the whole date plant of P. dactylifera (including pits and leaves) contains carbohydrates, alkaloids, steroids, flavonoids, saponins and tannins [34]. The phenolic profile of the plant revealed presence of mainly cinnamic acids, flavonoid glycosides, flavanols, four free phenolic acids and nine bound phenolic acids [48–51]. The vibrational stretching of OH group of P. dactylifera was appeared as an intensive band at 3415 cm1. This band undergoes a shift to 3398 cm1 and become broader upon interaction with gold salt. P. dactylifera spectrum shows a sharp peak at 1632 cm1 due to the stretching vibrations of the AC@O group [52]. The reduction of gold salt into gold nanoparticles caused the 1632 cm1 IR absorption band of P. dactylifera to be splitted into two peaks at 1651 and 1613 cm1. These evidences indicate that during the formation of Au nanoparticles, there is coordination between Au3+ ions and the oxygen atoms of AOH groups and/or AC@O groups. On the light of FTIR analysis, it can be stated that the hydroxyl and carbonyl groups present in carbohydrates, flavonoids, tanins and phenolic acids of P. dactylifera may be accountable for the reduction of the Au ions and stabilization of Au nanoparticles.
TEM analysis
Fig. 2. The UV–vis spectral evolution of the growth solution of gold nanoparticles under the action of (a) 100, (b) 200 and (c) 400 lL of P. dactylifera extract.
Hence, at the smallest extract volume (100 lL) larger particles were generated, most likely due to a slower nucleation rate, while at a highest extract volume (400 lL) faster nucleation produced monodispersed and smaller-sized particles. FT-IR analysis To better understand the reaction mechanism between P. dactylifera extract and gold salt, FTIR spectra were recorded for
TEM micrographs were taken to assess the size, size distribution and shape of the green synthesized gold nanoparticles with the aid of P. dactylifera extract. The micrographs reveal that the Au nanoparticles are surrounded by a thin layer of biomaterial which appears to be characteristic of Au nanoparticles prepared in plant extracts. Using low volume of the plant extract will result in irregular shapes (mixture of triangles, tetragonals and nanorods) as well as wide size distribution of the reduced gold nanoparticles (see Fig. 4a). In presence of excess extract to reduce the chloroaurate ions, the biomolecules strongly shaped the nanoparticles on the spherical shape (see Fig. 4b and c). These observations may reflect the fact that the P. dactylifera extract is considered as reducing and stabilizing agent in synthesis of gold nanoparticles. The quantitative measurements of the TEM micrographs were executed through performance of particle size histograms together with their Gaussian fits as displayed in Fig. 4a0 –c0 . Average particle size was determined from the peak of the Gaussian curve. Three different sizes of Au nanoparticles were synthesized by tuning the volume of P. dactylifera extract from 100 to 400 lL. The largest particle size is 45 nm for the lowest volume of the extract (100 lL)
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Fig. 4. TEM micrographs of gold nanoparticles prepared with (a) 100, (b) 200 and (c) 400 lL of P. dactylifera extract. The corresponding histograms were represented by (a0 ), (b0 ) and (c0 ).
while the smallest one is 32 nm for the highest volume of the extract (400 lL). The FWHM of the Gaussian fit was narrowed with increasing extract volume. Thus, a significant improvement in the monodispersity has been achieved using excess of P. dactylifera extract.
Au nanoparticles catalyzed the reduction of 4-nitrophenol to 4aminophenol The reduction of 4-NP by NaBH4 has been chosen as a benchwork reaction for monitoring the catalytic activity of Au nanoparticles. The aqueous solution of 4-NP shows two distinct absorption peaks at 320 and 396 nm (see Fig. 5). The electronic band at 320 is the characteristic band of 4-NP whereas the one at 400 nm
was attributed to the presence of 4-nitrophenolate ions in the system. After immediate addition of freshly prepared aqueous solution of NaBH4, the absorption band at 320 disappeared totally whereas the absorption band at 400 nm grew markedly to up to twice of its original value as displayed in Fig. 5. These spectral changes may be attributed to the conversion of 4-nitrophenol to 4-nitrophenolate ions in basic medium. It is well known that nitro compounds are inert to NaBH4 if it is used alone [43]. This is exactly consistent with our practical visual inspections. In the absence of proper catalyst, the intense yellow color of the 4nitrophenolate solution remained unaltered for several days. Fig. 6 shows the behavior of catalytic conversion of 4-nitrophenol to 4-aminophenol after addition of 50 lL Au nanoparticles using UV–vis spectrophotometer. The process was quantitatively monitored as a successive decrease in the peak height at 400 nm together with the gradual development of the new peak at 300 nm which indicates the formation of 4-aminophenol [44]. These spectroscopic data were corresponded to a change in solution color from yellow to colorless within few minutes. The effect of Au nanoparticle size on catalytic efficiency was studied as well. As expected, the smaller the size of the catalyst the shorter is the time of the reduction process (see Fig. 6). This catalytic reaction was carried out in large excess of NaBH4 concentration as compared with that of 4-NP. Hence, it is convenient to consider the concentration of borohydride ion is constant throughout the experiment. Hence, the reaction rate (Ka) of the reduction is assumed to be only dependent of 4-NP concentration. Therefore, the rate is assumed to follow first order kinetics [53]. The kinetic equation for the reduction can be written as
K a t ¼ ln ðC t =C 0 Þ ¼ ln ðAt =A0 Þ
Fig. 5. UV–vis spectra of 4-NP before and after adding NaBH4 solution.
where Ct and At are the concentration and absorption of 4-NP at time t while C0 and A0 are the concentration and absorption of 4-NP at the starting of the reaction. The rate constant (Ka) can be
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Fig. 6. Successive UV–vis absorption spectra of the reduction of 4-NP by the P. dactylifera-stabilized gold nanoparticles.
calculated from the linear fitting of the graphical representation of ln (At/A0) against the reaction time (t) which is displayed in Fig. 6. The rate constant of 32 nm gold nanoparticles is 3.1 103 s1, which is higher than those of gold nanoparticles with larger sizes, (1.5 103 s1 and 1.12 103 s1 for 36 and 45 nm gold nanoparticles, respectively). It is well known that the surface area of the metals is the most important criteria that determine the efficiency of the metals as catalytic agents [54]. Therefore, particles with smaller sizes have a higher surface-to-volume ratio so they can facilitate the electron relay from the donor to the acceptor. In addition, it needs to be noted that the total number of Au nanoparticles of the 32 nm sample would be larger than those of 36 and 45 nm samples because the size of 32 nm sample is smaller compared with other samples. The calculated rate constant is comparable with previously reported values of glucan and casein-stabilized gold nanoparticles [55,56]. 4. Conclusion P. dactylifera extract is confirmed to be an efficient candidate for bio-fabrication of gold nanoparticles due to its controlled reducing
power as well as presence of capping molecules. The size of colloidal Au nanoparticles could be easily tuned in the nanometer range by adjusting the used volume of P. dactylifera extract solution. Increasing the added extract volume increases the rate of reduction and reduces the particle size as well as their agglomeration. The stabilized gold nanoparticles exhibit size dependent catalytic activity toward degradation of 4-NP. The rate constant of the catalytic reaction was increased with decreasing particle sizes.
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