Colloids and Surfaces B: Biointerfaces 117 (2014) 528–533
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Activity of catalytic silver nanoparticles modulated by capping agent hydrophobicity Seralathan Janani, Priscilla Stevenson, Anbazhagan Veerappan ∗ Department of Chemistry, School of Chemical and Biotechnology, SASTRA University, Thanjavur, Tamil Nadu, India
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Article history: Received 5 October 2013 Received in revised form 25 February 2014 Accepted 4 March 2014 Available online 19 March 2014 Keywords: N-Acyl tyramine Silver nanoparticles Catalysis Hydrophobicity
a b s t r a c t In this paper, a facile in situ method is reported for the preparation of catalytic silver nanoparticles (AgNPs) using N-acyl tyramine (NATA) with variable hydrophobic acyl length. Scanning electron microscopic analysis shows that NATA exists initially as larger aggregates in alkaline aqueous solution. The addition of AgNO3 dissociates these larger aggregate and subsequently promotes the formation of selfassembled NATA and AgNPs. Characterization of AgNPs using UV–vis spectroscopy, scanning electron microscope and transmission electron microscope revealed that the hydrophobic acyl chain length of NATA does not influence the particle size, shape and morphology. All NATA-AgNPs yielded relatively identical values in full width at half-maximum (FWHM) analysis, indicating that the AgNPs prepared with NATA are relatively polydispersed at all tested acyl chain lengths. These nanoparticles are able to efficiently catalyze the reduction of 4-nitro phenol to 4-amino phenol, 2-nitro aniline to 1,2-diamino benzene, 2,4,6-trinitro phenol to 2,4,6-triamino phenol by NaBH4 in an aqueous environment. The reduction reaction rate is determined to be pseudo-first order and the apparent rate constant is linearly dependent on the hydrophobic acyl chain length of the NATA. All reaction kinetics presented an induction period, which is dependent on the N-acyl chain length, indicating that the hydrophobic effects play a critical role in bringing the substrate to the metal nanoparticle surface to induce the catalytic reaction. In this study, however, the five catalytic systems have similar size and polydispersity, differing only in terms of capping agent hydrophobicity, and shows different catalytic activity with respect to the alkyl chain length of the capping agent. As discussed, the ability to modulate the metal nanoparticles catalytic property, by modifying the capping agent hydrophobicity represents a promising future for developing an efficient nanocatalyst without altering the size, shape and morphology of the nanoparticles. © 2014 Elsevier B.V. All rights reserved.
1. Introduction In modern science, metal nanoparticles (NPs) have been exploited in various fields of chemistry and physics. This is due to the fact that the NPs often have totally different properties from their bulk counterparts. For example, nanosized gold (Au) shows excellent catalytic activity, whereas the bulk Au is inert [1,2]. Among the many metal NPs, silver (Ag) and Au NPs received special attention due to their profound application in catalysis, photonics, sensors, antibacterial, medicine, etc. These properties are very sensitive to particle sizes, shapes and morphology of the NPs [1,2]. Because of the nanosize, metallic NPs often congregate due to the high surface energy and van der Waals forces. To prevent
∗ Corresponding author. Tel.: +91 4362 264101x657; fax: +91 4362 264120. E-mail addresses: [email protected], [email protected] (A. Veerappan). http://dx.doi.org/10.1016/j.colsurfb.2014.03.008 0927-7765/© 2014 Elsevier B.V. All rights reserved.
agglomeration, stabilizing agents such as citric acid, ascorbic acid, sodium dodecyl sulfate, sodium oleate, and polymers containing thiol, carboxyl as well as amino groups are widely used [3]. These stabilizers or capping agents most often dominate the chemical and physical properties of the NPs [4]. Although, these capping agents prevent the NPs from aggregation, they can also influence the activity of the NPs. For example, the catalytic activity can be decreased by covering the surface of the catalyst [5–7]. Particularly for catalysis, the total surface area of the NPs is believed to play an important role in the catalytic reaction [8]. Since the NP surfaces are protected by the capping agent, it is imperative to investigate which property of these capping agents are important to create the right environment for the NPs stabilization and to obtain efficient catalysis. Recently, much attention has been directed toward synthesizing metal NPs using biological important compounds [9,10]. In this work, we report the preparation of silver nanoparticles (AgNPs) using a homologous series of N-acyl tyramines (NATA). NATA belong to an important class of endogenous signaling molecules,
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formed by an amide linkage with long chain fatty acids and 4hydroxyphenethylamine (tyramine) [11]. It has been shown that the phenolic acids with one or more phenolic hydroxyl groups are able to donate electrons and reduce Au3+ and Ag+ to their corresponding nanoparticles [12]. In our work, the phenolic OH group and the long fatty acid chains present in NATA not only facilitate the reduction of Ag+ to Ag0 , but also prevents the AgNPs from agglomeration. By making use of N-acylation with different acyl chain length fatty acids, different hydrophobicity was introduced to the capping agent, without modifying the polar functional group. Silver nanoparticles derived from N-acyl tyramine were characterized using UV–vis spectroscopy, scanning electron microscope (SEM) and transmission electron microscope (TEM). The obtained data clearly show that the size, shape and morphology are identical, irrespective of the long hydrophobic fatty acid acyl chain of N-acyl tyramine. However, with increasing acyl chain length, the zeta potential of the NATA-capped silver nanoparticles shows a significant shift from −29.9 to −33.5 mV, indicating that the NPs are well separated from one another and are strongly stabilized at longer acyl chain lengths. The hydrophobic acyl chain surrounding the NPs is known to influence the inter-particle distance depending on the acyl chain length [13]. Thus, we postulate that the hydrophobicity introduced by the N-acyl chain of tyramine (capping the nanoparticles) may influence the chemical activity of the AgNPs. As a proof of concept, we tested the catalytic activity of AgNPs synthesized with N-acyl tyramine using the standard reduction reaction, 4-nitro phenol to 4-amino phenol [14], as well as with 2-nitro aniline and 2,4,6-trinitrophenol. The obtained result shows that the rate of reduction of nitroaromatic compound is significantly influenced by the capping agent hydrophobicity, suggesting the importance of the hydrophobic effect in modulating the activity of silver nanoparticles. 2. Materials and methods 2.1. Materials Tyramine, decanoyl chloride, lauroyl chloride, myristoyl chloride, palmitoyl chloride and steroyl chloride were purchased from Sigma. Silver nitrate, sodium borohydride (NaBH4 ), 4-nitrophenol (4-NP), 2-nitro aniline and picric acid were purchased from Merck. All other chemicals and solvents used in this work are of analytical grade obtained from the local supplier. 2.2. Preparation of silver nanoparticles N-Acyl tyramines (Fig. S1) have been used as an in situ reducing and capping agent for preparing the stable silver nanoparticles. NATA have been synthesized via the reaction of tyramine with the appropriate acid chloride, essentially according to the procedure described for N-acyl ethanolamine [15]. In a typical procedure to synthesize silver nanoparticles, 0.2 mM of N-acyl tyramine was dissolved in 10 mM of sodium hydroxide (NaOH) and was vigorously stirred for about 30 min. Then, 0.25 mM of silver nitrate (AgNO3 ) were added into the above solution. The resulting solutions were gently stirred for 2 h. The solution was initially colorless and it turned to pale yellow color after 2 h. On standing for 24 h, the pale yellow color solution changed to a bright golden yellow color, indicating the complete reduction of Ag+ ion to Ag0 nanoparticles. Alternatively, AgNPs was synthesized using sodium borohydride (NaBH4 ) as the reducing agent and NATA as the capping agent. Addition of NaBH4 results in an immediate reduction of Ag+ ion to Ag0 nanoparticles, as evident from the appearance of the bright golden yellow color.
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2.3. Characterization Optical absorbance of the synthesized AgNPs was monitored by a UV–vis spectrophotometer (Thermo Scientific Evolution 201) between a wavelength of 350 and 800 nm at a resolution of 1 nm. NATA-AgNPs morphology was characterized using a SEM (JEOLJSM 6701-F, Japan). The samples was placed over a carbon tape and dried. Prior to imaging, the samples were coated with a thin layer of platinum in an auto fine coater. TEM image was performed on the NATA-AgNPs using JEM 1011, JEOL, Japan. A drop of the NATAAgNPs was placed on a carbon-coated copper grid and dried prior to the measurement. Fourier transfer infra-red (FT-IR) spectra of NATA and NATA-AgNPs were recorded using a PerkinElmer FT-IR spectrometer with 1 cm−1 resolution. NATA-AgNPs solutions were centrifuged at 10,000 rpm for 30 min. The obtained residue was freeze-dried by lyophilization and grounded with KBr prior to the FT-IR measurement. Zeta potential of the prepared nanoparticles was determined by using a Malvern Zetasizer instrument (version 6.20). 2.4. Catalysis Typically, 1 mM of 4-NP and 15 mM of NaBH4 were added into a quartz cuvette containing 3 mL of water. 10 mL of 0.25 mM AgNPs was centrifuged at 10,000 rpm for 20 min and the residue was dissolved in 1 mL of water. 50 L of the suspension is added directly into the cuvette to start the reduction reaction. The intensity of the absorption peak at 404 nm in UV–vis spectroscopy was used to monitor the process of the conversion of 4-NP to 4-aminophenol (4AP). To rule out the possibility of formation of metal oxide layer and catalytic surface poisoning, the samples were carefully degassed before adding NaBH4 . 3. Results and discussion In order to investigate the effect of capping agents on the physiochemical properties of AgNPs, we synthesized the following five different acyl chain length stabilized AgNPs, N-decanoyl tyramine (N10TA-AgNPs), N-lauroyl tyramine (N12TA-AgNPs), N-myristoyl tyramine (N14TA-AgNPs), N-palmitoyl tyramine (N16TA-AgNPs), N-steroyl tyramine (N18TA-AgNPs), by reducing silver ions in aqueous solutions of N-acyl tyramine (NATA) as presented in Section 2. The physicochemical properties of AgNPs such as stability, size, morphology and chemical activity were characterized. Because of the lengthy acyl chain, NATAs are sparingly soluble in an aqueous environment; however, the addition of NaOH fairly increased its solubility. When the NATA–NaOH–AgNO3 aqueous solution was incubated at room temperature, a milky colloidal solution showed up immediately. While keeping it for 24 h, the milky colloidal solution turned into transparent yellow color, indicating that the Ag+ ions were reduced to AgNPs (Fig. 1A). SEM images were recorded at each stage of AgNPs formation (Fig. 1B). It is visibly clear that the NATA exists initially as larger aggregates. Addition of AgNO3 dissociates these larger aggregates of NATA and it finally results in the formation of self assembled structure of NATA, which stabilizes the AgNPs (Fig. 1B). The UV–vis spectra of the AgNPs prepared with the same AgNO3 concentration, but by utilizing different acyl chain lengths NATA are shown in Fig. 2. Irrespective of the acyl chain length, the characteristic surface plasmon resonance (SPR) peak of AgNPs was observed around 420–425 nm. The dispersity of AgNPs was evaluated by comparing the full width at half-maximum (FWHM) from the UV–vis spectra [16]. All the samples show essentially identical FWHM (83–102 nm). This indicates that the AgNPs prepared with NATA are relatively polydisperse at all tested acyl chain lengths, and there is no change in absorption maximum and
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Fig. 3. Hydrophobic acyl chain length versus zeta potential of NATA-AgNPs.
Fig. 1. (A) Representative preparation of NATA-AgNPs. (B) Corresponding SEM images, measured at each stage of the preparation. Top – alkaline NATA, middle – NATA with AgNO3 , and bottom – NATA reduced and capped AgNPs obtained after 24 h.
FWHM between the different NATA-AgNPs. These data clearly suggest that the hydrophobic acyl chain of NATA does not alter the dispersity of AgNPs. In general, AgNPs have been widely prepared by chemical methods using sodium borohydride (NaBH4 ) as a reducing agent. In order to test the effect of NaBH4 on the preparation of NATA-AgNPs, we synthesized AgNPs by adding 10 mM NaBH4 to NATA–NaOH–AgNO3 aqueous solution. Addition of the reducing agent immediately shows yellow color at room temperature, indicating the formation of AgNPs. The absorbance bands were detected
Fig. 2. UV–vis spectra of silver nanoparticles prepared with different N-acyl tyramine.
at 420–425 nm (Fig. S3), similar to the one prepared without NaBH4 . Although NATA reduces Ag+ ions to AgNPs, the presence of NaBH4 accelerates the rate of AgNPs formation. The FWHM calculated from Fig. S3 are in the broad range of 78–121 nm, indicating the formation of relatively polydisperse AgNPs for all tested acyl chain lengths. It is clear from FWHM, the formation of relatively narrow dispersed AgNPs is most favored in the reduction without NaBH4 . Also noteworthy that the AgNPs prepared solely with NATA did not show any change in the absorption maximum in the course of one month (less than 1 nm). However, over the time, the AgNPs prepared with the NaBH4 reduction method showed visible aggregates. These aggregates deposit in the bottom of the test tube, suggesting that the particles are relatively unstable. Thus, for the subsequent studies, we used the nanoparticles prepared solely with NATA. The morphology and the particle size distributions of these synthesized AgNPs are shown in Fig. S2. SEM image shows the presence of polydisperse nanoparticles with size ranging from 30 to 58 nm (Fig. 1B). As characterized by TEM, NATA-AgNPs shows spherical shape irrespective of the length of hydrophobic acyl chain (Fig. S2A, B, C, D, E). The particle size calculated using TEM image is in the range of 33 ± 5 nm for all the acyl chain lengths. Even though the hydrophobicity of NATA differs due to the hydrophobic acyl chain length, the morphology and sizes of NATA-AgNPs remain unaltered. Zeta potential measurements were explored to understand the surface chemistry of the AgNPs and the effect of their interaction with NATA. Fig. 3 shows the zeta potential of NATA-AgNPs. Upon increasing the acyl chain length, zeta potential shifts from −29.9 to −33.5 mV. The negative sign indicates the alkaline nature of the pH [16]. The significant difference in the zeta potential for different NATA can be attributed to the repulsion caused by the AgNPs due to the presence of anionic charges on the surface. As inferred from the zeta potential, the longer the acyl chain length, higher the repulsion between the individual nanoparticles. A previous study showed that an increase in an acyl chain length will lead to an increase in the inter-particle distance of the nanoparticles [13]. By analyzing these data, it is clear that the longer acyl chain of NATA provides more stability to nanoparticles, most likely by modulating the inter-particle distance. A FTIR studies of pure NATA and NATA reduced AgNPs showed an unaltered carbonyl stretching vibration of the amide group at 1638 cm−1 (Fig. 4). This indicates that the amide group was not involved in the formation and stabilization of AgNPs. In case of pure NATA, a sharp peak with a shoulder responsible for N H and O H stretching vibrations is observed at 3301 cm−1 . These features are completely broadened in the case of NATA-AgNPs,
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Fig. 4. Typical FTIR spectra of pure NATA (black line) and NATA capped AgNPs (blue line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
plausibly due to the intermolecular hydrogen-bonded network of OH and NH groups of the NATA with the surface of the nanoparticles. This also suggests that the phenolic -OH group might be involved in the formation of NATA-AgNPs. Therefore, it is expected that the NATAs self-assemble around the NPs and stabilize the NPs, which might facilitate the disaggregation of the larger aggregates of NATAs, finally resulting in the formation of transparent yellow AgNPs (Fig. 1). On the basis of the above experimental results, a formation mechanism of the AgNPs is proposed and is shown in Fig. 5. NATA suspended into aqueous solution self-assembles into larger aggregates with an inner hydrophobic backbone and outer hydrophilic phenolic hydroxyl groups ( OH). Recently, the reported crystal structure of N-acyl dopamine showed that the N-lauryl dopamine is packed head-to-head (and tail-to-tail) in stacked bilayers [17]. The presence of the catechol group, allows O H· · ·O hydrogen bonds between the head groups of opposite leaflets. In addition, the amide groups of adjacent are also involved in hydrogen bonds. The carbonyl oxygen atoms of adjacent molecules point in the opposite direction and provide an appropriate juxtaposition of
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the amide carbonyl and N H moieties of adjacent molecules to form N H· · ·O C type hydrogen bonds. Since tyramine [11] and dopamine [17] are structurally differing by only one OH group, thus, it can be speculated that the NATAs may exist in similar packing structures. Thus, the larger aggregates of NATA obtained during the hydration can be a result of the head-to-head packing and the hydrogen bond between the amide groups. However, the packing of NATA can be rationalized only after the structures of NATAAgNPs are known. NATA contains a phenolic group which can be “switched on” as a reducing agent under alkaline conditions [12]. In the presence of Ag+ ions, a NATA–Ag+ complex is formed through the electrostatic interaction of Ag+ ions with the phenolic group. The ionization of the phenolic group enables facile electron transfer from the phenolate ion to the silver cations, resulting in the formation of silver nanoparticles. In order to stabilize the AgNPs, NATA self assembles around NPs through the phenolate group and stabilizes through N H· · ·O C type hydrogen bonds (Fig. 5). The significant differences in the zeta potential of the N10TA-AgNPs, N12TA-AgNPs, N14TA-AgNPs, N16TA-AgNPs, and N18TA-AgNPs were further investigated by catalyzing the reduction of 4-NP into 4-AP by NaBH4 . This reaction was chosen because it can be easily monitored by UV–vis spectroscopy [14]. Addition of NaBH4 to 4-NP shifts the absorption peak from 317 nm to 404 nm, due to the formation of 4-nitrophenolate ion [18]. In absence of the catalyst, the peak remains undisturbed at about 404 nm, indicating that the reduction did not take place. However, addition of 50 L of an aqueous suspension of NATA-AgNPs to 4-nitrophenolate ion gradually diminishes the yellow color of the reaction mixture in a time-dependent manner. As shown in Fig. 6A, the 4-NP absorption peak at 404 nm decreases and a new absorption peak, due to the formation of 4-AP appears at 293 nm. Since the amount of nanoparticles added is very small, the absorption spectra of 4-NP are hardly affected by the AgNPs. The catalytic reduction of 4-NP in presence of different NATAAgNPs was associated with an induction time t0 in which no reduction takes place (Fig. 6B). This induction period is typical for a heterogeneous catalytic process and commonly related to (i) an activation or restructuring of the metal surface by 4-NP before the reaction could start [11] and (ii) the diffusion-controlled adsorption of substrates onto the NPs surface [19]. Noteworthy, the induction times are different for different NATA-AgNPs (Fig. 6B). It
Fig. 5. Schematic representation of N-acyl tyramine reduced and stabilized silver nanoparticles. Dotted lines indicate the possible hydrogen bond network.
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Table 1 Catalytic activity of NATA-AgNPs catalytic system for nitroaromatic reduction. Catalyst
N10TA-AgNPs N12TA-AgNPs N14TyA-AgNPs N16TA-AgNPs N18TA-AgNPs
4-Nitro phenol
2-Nitro aniline
2,4,6-Trinitro phenol
t0 (s)
k (×103 ) (s−1 )
t0 (s)
k (×103 ) (s−1 )
t0 (s)
k (×103 ) (s−1 )
460.2 319.8 90 19.8 10.2
3.89 5.89 6.57 9.97 13.1
160.2 49.8 40.2 30.0 19.8
1.12 4.80 6.47 7.47 12.7
330.0 250.2 70.2 40.2 30.0
1.5 1.55 2.35 2.45 2.92
was observed that the induction time is lesser in N18TA-AgNPs compared to the lower acyl chain length NATA-AgNPs (Table 1), indicating that the reduction reaction was initiated more quickly in case of N18TA-AgNPs. In order to test the above observed phenomenon, we further investigated the catalytic reduction of other nitro compounds such as 2-nitro aniline and 2,4,6-trinitro phenol. Strikingly, similar behavior was observed for the reduction of 2,4,6trinitrophenol to 2,4,6-triamino phenol (Fig. S4) and 2-nitro aniline to 1,2-diamino benzene (Fig. S5). The rate constants obtained for the reduction of all the tested nitro compounds are reported in Table 1. The obtained data shows that the hydrophobic acyl chain of NATA plays a significant role in reducing the nitroaromatic compounds. It has been noted that the reaction for the reduction of 4-NP catalyzed by NATA-AgNPs was almost complete within 10 min in all cases. Since the concentration of NaBH4 was much higher than that of 4-NP, the reduction kinetics can be described by pseudo firstorder rate law with regard to 4-NP alone. Therefore, the reaction kinetics can be described as ln(At /A0 ) = −kt, where k is the apparent
Fig. 6. (A) Variation in UV–vis spectra for the reduction of 4-NP in presence of NATAAgNPs and (B) time-dependent absorption of 4-nitrophenolate ion at 404 nm, with an induction period of t0 .
first-order rate constant, t is the reaction time. At and A0 are the absorbance of 4-NP at time t and 0, respectively. As the absorbance intensity of 4-NP and its concentrations are proportional to each other in the medium, the ratio of absorbance (At /A0 ) could be equal with the ratio of concentration of 4-NP (Ct /C0 ). A linear relation between ln(Ct /C0 ) versus reaction time (t) was observed after the induction period (t0 ) (Fig. 6B). Thus, the determination of the apparent rate constant from the slope of the linear correlation of ln(At /A0 ) with time t is straight forward. The rate constant obtained directly from the slope of the linear part of the kinetic trace is given in Table 1. For all the tested nitro compounds, 4-NP, 2-nitro aniline and 2,4,6-trinitro phenol, the rate constant is indeed proportional to the hydrophobic acyl chain length of the NATA. As summarized from Table 1, the N18TyA-AgNPs presents a higher catalytic activity than other lower acyl chain length-stabilized AgNPs. In this study, the catalytic system has same size, shape and same composition, differing only in terms of the hydrophobic acyl chain length. Thus, we postulate that the hydrophobicity of the stabilizing agent strongly influences the chemo-catalytic potential of the NATA-AgNPs. Silver nanoparticles-catalyzed reduction reaction mechanisms have been postulated by the inherent hydrogen adsorption and desorption [14]. Several studies have shown that AgNPs act as an electron relay in the reduction reaction [20], i.e. AgNPs adsorb hydrogen from the NaBH4 and efficiently release them during the reduction reaction. Thus, it is clear that the surface of the AgNPs is expected to play an important role in the catalytic reduction reaction and act as a hydrogen carrier for the reduction reaction. Since the NPs surfaces are stabilized by NATA, they were expected to play a significant role in determining the rate of the reduction reaction. It is well known that the fatty acid acyl chains are responsible for determining the size of the detergent micelles and thickness in case of lipid bilayers [21]. A change of one methyl group is expected to increase the hydrophobic core size by 2.5 A˚ [22]. As a result, the hydrophobic cores formed by N10TA, N12TA, N14TA, N16TA and ˚ respectively. From this approxN18TA as ∼25, 30, 35, 40 and 45 A, imation, it is expected that the hydrophobic effect and the van der Waals interactions between the NATA are greater for longer hydrocarbon chains. These forces are able to draw in the substrate from the aqueous environment, similar to enzymes, and thereby increase the local substrate concentration near the nanoparticle surface. Thus, we postulate that the higher hydrophobicity provided by N18TA is able to draw in substrates more easily than the less hydrophobic NATAs. As a result a faster reduction reaction is expected with N18TA-AgNPs. Alternatively, the difference in the catalytic activity can be explained by considering the different diffusion rates of the reactant on the AgNPs surface [19]. This is supported by observing the induction time, t0 , dependence on the length of acyl chains in NATAs (Table 1). In this regard, the higher induction time was observed for N10TA-AgNPs versus the N18TA-AgNPs, indicating a slower diffusion of the substrates on the N10TA-AgNPs surfaces. Since NATA is amphiphilic in nature with a lipophilic acyl chain moiety, it has a higher tendency to form micelles with a hydrophobic core in water. When the hydrophobic core is smaller, it will result in the formation of less compact colloidal particles, and as a result, a slower diffusion of the reactant,
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which will lead to a slower reaction. This is consistent with the observed catalytic activities, since the N18TA-AgNPs systems offer little resistance to diffusion compared to lower acyl chain length NATA. In addition, nanoparticles surface assemblies can also exhibit rectified quantized charging in aqueous solutions containing bulky and hydrophobic ions [23,24]. Thus, it can be postulated that higher hydrophobic N18TA can initiate a well-defined quantized charging to the nanoparticle surface assemblies compared to the lower acyl chain length NATA. As a result, the addition of a reducing agent more easily activates the metal surface of N18TA-AgNPs compared to N10TA-AgNPs and thereby the stored electrons would easily be transferred from electron-donating molecule (BH4 − ) to electron-accepting molecules (nitro compound). In summary, the present work clearly indicates that the hydrophobic effects play an important role in determining the activity of the catalytic silver nanoparticles. Thus, the ability to modulate the catalytic property by modifying the capping agent hydrophobicity represents a promising future for developing a nanocatalyst without changing the size, shape and morphology of the nanoparticles. 4. Conclusions The present study describes a new and simple method for preparing AgNPs in the presence of a homologous series of hydrophobic N-acyl tyramines in alkaline aqueous solution. It was observed that NATA acted both as a reducing agent and as a capping agent. As evident from SEM images, the formation of AgNPs results in the self-assembly of NATA, which results in capping the nanoparticles. Interestingly, the hydrophobicity of the acyl chains does not affect the shape, size and morphology of the AgNPs. However, zeta potential shows significant shifts with respect to the hydrophobicity of the NATA, indicating that the AgNPs are more stable in the presence of a highly hydrophobic N18TyA. The catalytic activity of NATA-AgNPs for the reduction of nitroaromatics is linearly dependent on the acyl chain length of NATA. It is found that N18TA-AgNPs shows a higher catalytic activity than the other NATAs. Strikingly, the more hydrophobic N18TA-AgNPs show less dependence on the induction period, suggesting that the hydrophobic effects play a critical role in bringing the substrate to the metal nanoparticle surface to induce the catalytic reaction. The approach of preparing catalytically active metal nanoparticles by modulating the hydrophobicity of the capping agent, represents a promising future to develop nanocatalysts without worrying about the size, shape and morphology. Moreover, the use of biological compounds as reducing and capping agents render the synthesized AgNPs as biocompatible.
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Acknowledgments This work was supported by research grants from Department of Science and Technology, Government of India (SB/FT/LS-217/2012) and Prof. T.R. Rajagopalan Research Fund, SASTRA University. We thank central research facility (R&M/0021/SCBT-007/2012-13), SASTRA University for the infrastructure. We thank Ms. H. Pearson, University of Mainz, Germany, Dr. Philip Anthony and Dr. S. Aravind, SASTRA University for critically reading the manuscript. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfb. 2014.03.008. References [1] S. Linic, P. Christopher, H. Xin, A. Marimuthu, Acc. Chem. Res. 46 (2013) 1890. [2] C. Burda, X. Chen, R.R. Narayanan, M.A. El-Sayed, Chem. Rev. 105 (2005) 1025. [3] M. Tejamaya, I. Römer, R.C. Merrifield, J.R. Lead, Environ. Sci. Technol. 46 (2012) 7011. [4] J.S. Bradley, in: G.E. Schmid (Ed.), Clusters and Colloids: From Theory to Applications, VCH, Weinheim, 1994, p. 459. [5] K. Kuroda, T. Ishida, M. Haruta, J. Mol. Catal. A: Chem. 298 (2009) 7. [6] Y. Mei, Y. Lu, F. Polzer, M. Ballauff, M. Drechsler, Chem. Mater. 19 (2007) 1062. [7] K. Esumi, R. Isono, T. Yoshimura, Langmuir 20 (2004) 237. [8] A.M. Signori, O. Santos Kde, R. Eising, B.L. Albuquerque, F.C. Giacomelli, J.B. Domingos, Langmuir 26 (2010) 17772. [9] K.B. Narayanan, N. Sakthivel, Adv. Colloid Interface Sci. 156 (2010) 1. [10] A.K. Mittal, Y. Chisti, U.C. Banerjee, Biotechnol. Adv. 31 (2013) 346. [11] P.H. Yu, A.A. Boulton, Can. J. Biochem. 57 (1979) 1204. [12] P.R. Selvakannan, A. Swami, D. Srisathiyanarayanan, P.S. Shirude, R. Pasricha, A.B. Mandale, M. Sastry, Langmuir 20 (2004) 7825. [13] K. Abe, T. Hanada, Y. Yoshida, N. Tanigaki, H. Takiguchi, H. Nagasawa, M. Nakamoto, T. Yamaguchi, K. Yase, Thin Solid Films 327–329 (1998) 524. [14] P. Hervés, M. Pérez-Lorenzo, L.M. Liz-Marzán, J. Dzubiella, Y. Lu, M. Ballauff, Chem. Soc. Rev. 41 (2012) 5577. [15] M. Ramakrishnan, V. Sheeba, S.S. Komath, M.J. Swamy, Biochim. Biophys. Acta 1329 (1997) 302. [16] J.P. Wilcoxon, B.L. Abrams, Chem. Soc. Rev. 35 (2006) 1162. [17] S.T. Reddy, P.K. Tarafdar, R.K. Kamlekar, M.J. Swamy, J. Phys. Chem. B 117 (2013) 8747. [18] S. Harish, J. Mathiyarasu, K.L.N. Phani, Catal. Lett. 128 (2009) 197. [19] J. Zeng, Q. Zhang, J. Chen, Y. Xia, Nano Lett. 10 (2010) 30. [20] K. Mallick, M. Witcomb, M. Scurrell, Mater. Chem. Phys. 97 (2006) 283. [21] C. Tanford, The Hydrophobic Effect: Formation of Micelles and Biological Membranes, John Wiley & Sons, New York, 1980. [22] A.G. Therien, C.M. Deber, J. Biol. Chem. 277 (2002) 6067. [23] S. Chen, R. Pei, T. Zhao, D.J. Dyer, J. Phys. Chem. B 106 (2002) 1903. [24] S. Chen, R. Pei, J. Am. Chem. Soc. 123 (2001) 10607.
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Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb
Activity of catalytic silver nanoparticles modulated by capping agent hydrophobicity Seralathan Janani, Priscilla Stevenson, Anbazhagan Veerappan ∗ Department of Chemistry, School of Chemical and Biotechnology, SASTRA University, Thanjavur, Tamil Nadu, India
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Article history: Received 5 October 2013 Received in revised form 25 February 2014 Accepted 4 March 2014 Available online 19 March 2014 Keywords: N-Acyl tyramine Silver nanoparticles Catalysis Hydrophobicity
a b s t r a c t In this paper, a facile in situ method is reported for the preparation of catalytic silver nanoparticles (AgNPs) using N-acyl tyramine (NATA) with variable hydrophobic acyl length. Scanning electron microscopic analysis shows that NATA exists initially as larger aggregates in alkaline aqueous solution. The addition of AgNO3 dissociates these larger aggregate and subsequently promotes the formation of selfassembled NATA and AgNPs. Characterization of AgNPs using UV–vis spectroscopy, scanning electron microscope and transmission electron microscope revealed that the hydrophobic acyl chain length of NATA does not influence the particle size, shape and morphology. All NATA-AgNPs yielded relatively identical values in full width at half-maximum (FWHM) analysis, indicating that the AgNPs prepared with NATA are relatively polydispersed at all tested acyl chain lengths. These nanoparticles are able to efficiently catalyze the reduction of 4-nitro phenol to 4-amino phenol, 2-nitro aniline to 1,2-diamino benzene, 2,4,6-trinitro phenol to 2,4,6-triamino phenol by NaBH4 in an aqueous environment. The reduction reaction rate is determined to be pseudo-first order and the apparent rate constant is linearly dependent on the hydrophobic acyl chain length of the NATA. All reaction kinetics presented an induction period, which is dependent on the N-acyl chain length, indicating that the hydrophobic effects play a critical role in bringing the substrate to the metal nanoparticle surface to induce the catalytic reaction. In this study, however, the five catalytic systems have similar size and polydispersity, differing only in terms of capping agent hydrophobicity, and shows different catalytic activity with respect to the alkyl chain length of the capping agent. As discussed, the ability to modulate the metal nanoparticles catalytic property, by modifying the capping agent hydrophobicity represents a promising future for developing an efficient nanocatalyst without altering the size, shape and morphology of the nanoparticles. © 2014 Elsevier B.V. All rights reserved.
1. Introduction In modern science, metal nanoparticles (NPs) have been exploited in various fields of chemistry and physics. This is due to the fact that the NPs often have totally different properties from their bulk counterparts. For example, nanosized gold (Au) shows excellent catalytic activity, whereas the bulk Au is inert [1,2]. Among the many metal NPs, silver (Ag) and Au NPs received special attention due to their profound application in catalysis, photonics, sensors, antibacterial, medicine, etc. These properties are very sensitive to particle sizes, shapes and morphology of the NPs [1,2]. Because of the nanosize, metallic NPs often congregate due to the high surface energy and van der Waals forces. To prevent
∗ Corresponding author. Tel.: +91 4362 264101x657; fax: +91 4362 264120. E-mail addresses: [email protected], [email protected] (A. Veerappan). http://dx.doi.org/10.1016/j.colsurfb.2014.03.008 0927-7765/© 2014 Elsevier B.V. All rights reserved.
agglomeration, stabilizing agents such as citric acid, ascorbic acid, sodium dodecyl sulfate, sodium oleate, and polymers containing thiol, carboxyl as well as amino groups are widely used [3]. These stabilizers or capping agents most often dominate the chemical and physical properties of the NPs [4]. Although, these capping agents prevent the NPs from aggregation, they can also influence the activity of the NPs. For example, the catalytic activity can be decreased by covering the surface of the catalyst [5–7]. Particularly for catalysis, the total surface area of the NPs is believed to play an important role in the catalytic reaction [8]. Since the NP surfaces are protected by the capping agent, it is imperative to investigate which property of these capping agents are important to create the right environment for the NPs stabilization and to obtain efficient catalysis. Recently, much attention has been directed toward synthesizing metal NPs using biological important compounds [9,10]. In this work, we report the preparation of silver nanoparticles (AgNPs) using a homologous series of N-acyl tyramines (NATA). NATA belong to an important class of endogenous signaling molecules,
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formed by an amide linkage with long chain fatty acids and 4hydroxyphenethylamine (tyramine) [11]. It has been shown that the phenolic acids with one or more phenolic hydroxyl groups are able to donate electrons and reduce Au3+ and Ag+ to their corresponding nanoparticles [12]. In our work, the phenolic OH group and the long fatty acid chains present in NATA not only facilitate the reduction of Ag+ to Ag0 , but also prevents the AgNPs from agglomeration. By making use of N-acylation with different acyl chain length fatty acids, different hydrophobicity was introduced to the capping agent, without modifying the polar functional group. Silver nanoparticles derived from N-acyl tyramine were characterized using UV–vis spectroscopy, scanning electron microscope (SEM) and transmission electron microscope (TEM). The obtained data clearly show that the size, shape and morphology are identical, irrespective of the long hydrophobic fatty acid acyl chain of N-acyl tyramine. However, with increasing acyl chain length, the zeta potential of the NATA-capped silver nanoparticles shows a significant shift from −29.9 to −33.5 mV, indicating that the NPs are well separated from one another and are strongly stabilized at longer acyl chain lengths. The hydrophobic acyl chain surrounding the NPs is known to influence the inter-particle distance depending on the acyl chain length [13]. Thus, we postulate that the hydrophobicity introduced by the N-acyl chain of tyramine (capping the nanoparticles) may influence the chemical activity of the AgNPs. As a proof of concept, we tested the catalytic activity of AgNPs synthesized with N-acyl tyramine using the standard reduction reaction, 4-nitro phenol to 4-amino phenol [14], as well as with 2-nitro aniline and 2,4,6-trinitrophenol. The obtained result shows that the rate of reduction of nitroaromatic compound is significantly influenced by the capping agent hydrophobicity, suggesting the importance of the hydrophobic effect in modulating the activity of silver nanoparticles. 2. Materials and methods 2.1. Materials Tyramine, decanoyl chloride, lauroyl chloride, myristoyl chloride, palmitoyl chloride and steroyl chloride were purchased from Sigma. Silver nitrate, sodium borohydride (NaBH4 ), 4-nitrophenol (4-NP), 2-nitro aniline and picric acid were purchased from Merck. All other chemicals and solvents used in this work are of analytical grade obtained from the local supplier. 2.2. Preparation of silver nanoparticles N-Acyl tyramines (Fig. S1) have been used as an in situ reducing and capping agent for preparing the stable silver nanoparticles. NATA have been synthesized via the reaction of tyramine with the appropriate acid chloride, essentially according to the procedure described for N-acyl ethanolamine [15]. In a typical procedure to synthesize silver nanoparticles, 0.2 mM of N-acyl tyramine was dissolved in 10 mM of sodium hydroxide (NaOH) and was vigorously stirred for about 30 min. Then, 0.25 mM of silver nitrate (AgNO3 ) were added into the above solution. The resulting solutions were gently stirred for 2 h. The solution was initially colorless and it turned to pale yellow color after 2 h. On standing for 24 h, the pale yellow color solution changed to a bright golden yellow color, indicating the complete reduction of Ag+ ion to Ag0 nanoparticles. Alternatively, AgNPs was synthesized using sodium borohydride (NaBH4 ) as the reducing agent and NATA as the capping agent. Addition of NaBH4 results in an immediate reduction of Ag+ ion to Ag0 nanoparticles, as evident from the appearance of the bright golden yellow color.
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2.3. Characterization Optical absorbance of the synthesized AgNPs was monitored by a UV–vis spectrophotometer (Thermo Scientific Evolution 201) between a wavelength of 350 and 800 nm at a resolution of 1 nm. NATA-AgNPs morphology was characterized using a SEM (JEOLJSM 6701-F, Japan). The samples was placed over a carbon tape and dried. Prior to imaging, the samples were coated with a thin layer of platinum in an auto fine coater. TEM image was performed on the NATA-AgNPs using JEM 1011, JEOL, Japan. A drop of the NATAAgNPs was placed on a carbon-coated copper grid and dried prior to the measurement. Fourier transfer infra-red (FT-IR) spectra of NATA and NATA-AgNPs were recorded using a PerkinElmer FT-IR spectrometer with 1 cm−1 resolution. NATA-AgNPs solutions were centrifuged at 10,000 rpm for 30 min. The obtained residue was freeze-dried by lyophilization and grounded with KBr prior to the FT-IR measurement. Zeta potential of the prepared nanoparticles was determined by using a Malvern Zetasizer instrument (version 6.20). 2.4. Catalysis Typically, 1 mM of 4-NP and 15 mM of NaBH4 were added into a quartz cuvette containing 3 mL of water. 10 mL of 0.25 mM AgNPs was centrifuged at 10,000 rpm for 20 min and the residue was dissolved in 1 mL of water. 50 L of the suspension is added directly into the cuvette to start the reduction reaction. The intensity of the absorption peak at 404 nm in UV–vis spectroscopy was used to monitor the process of the conversion of 4-NP to 4-aminophenol (4AP). To rule out the possibility of formation of metal oxide layer and catalytic surface poisoning, the samples were carefully degassed before adding NaBH4 . 3. Results and discussion In order to investigate the effect of capping agents on the physiochemical properties of AgNPs, we synthesized the following five different acyl chain length stabilized AgNPs, N-decanoyl tyramine (N10TA-AgNPs), N-lauroyl tyramine (N12TA-AgNPs), N-myristoyl tyramine (N14TA-AgNPs), N-palmitoyl tyramine (N16TA-AgNPs), N-steroyl tyramine (N18TA-AgNPs), by reducing silver ions in aqueous solutions of N-acyl tyramine (NATA) as presented in Section 2. The physicochemical properties of AgNPs such as stability, size, morphology and chemical activity were characterized. Because of the lengthy acyl chain, NATAs are sparingly soluble in an aqueous environment; however, the addition of NaOH fairly increased its solubility. When the NATA–NaOH–AgNO3 aqueous solution was incubated at room temperature, a milky colloidal solution showed up immediately. While keeping it for 24 h, the milky colloidal solution turned into transparent yellow color, indicating that the Ag+ ions were reduced to AgNPs (Fig. 1A). SEM images were recorded at each stage of AgNPs formation (Fig. 1B). It is visibly clear that the NATA exists initially as larger aggregates. Addition of AgNO3 dissociates these larger aggregates of NATA and it finally results in the formation of self assembled structure of NATA, which stabilizes the AgNPs (Fig. 1B). The UV–vis spectra of the AgNPs prepared with the same AgNO3 concentration, but by utilizing different acyl chain lengths NATA are shown in Fig. 2. Irrespective of the acyl chain length, the characteristic surface plasmon resonance (SPR) peak of AgNPs was observed around 420–425 nm. The dispersity of AgNPs was evaluated by comparing the full width at half-maximum (FWHM) from the UV–vis spectra [16]. All the samples show essentially identical FWHM (83–102 nm). This indicates that the AgNPs prepared with NATA are relatively polydisperse at all tested acyl chain lengths, and there is no change in absorption maximum and
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Fig. 3. Hydrophobic acyl chain length versus zeta potential of NATA-AgNPs.
Fig. 1. (A) Representative preparation of NATA-AgNPs. (B) Corresponding SEM images, measured at each stage of the preparation. Top – alkaline NATA, middle – NATA with AgNO3 , and bottom – NATA reduced and capped AgNPs obtained after 24 h.
FWHM between the different NATA-AgNPs. These data clearly suggest that the hydrophobic acyl chain of NATA does not alter the dispersity of AgNPs. In general, AgNPs have been widely prepared by chemical methods using sodium borohydride (NaBH4 ) as a reducing agent. In order to test the effect of NaBH4 on the preparation of NATA-AgNPs, we synthesized AgNPs by adding 10 mM NaBH4 to NATA–NaOH–AgNO3 aqueous solution. Addition of the reducing agent immediately shows yellow color at room temperature, indicating the formation of AgNPs. The absorbance bands were detected
Fig. 2. UV–vis spectra of silver nanoparticles prepared with different N-acyl tyramine.
at 420–425 nm (Fig. S3), similar to the one prepared without NaBH4 . Although NATA reduces Ag+ ions to AgNPs, the presence of NaBH4 accelerates the rate of AgNPs formation. The FWHM calculated from Fig. S3 are in the broad range of 78–121 nm, indicating the formation of relatively polydisperse AgNPs for all tested acyl chain lengths. It is clear from FWHM, the formation of relatively narrow dispersed AgNPs is most favored in the reduction without NaBH4 . Also noteworthy that the AgNPs prepared solely with NATA did not show any change in the absorption maximum in the course of one month (less than 1 nm). However, over the time, the AgNPs prepared with the NaBH4 reduction method showed visible aggregates. These aggregates deposit in the bottom of the test tube, suggesting that the particles are relatively unstable. Thus, for the subsequent studies, we used the nanoparticles prepared solely with NATA. The morphology and the particle size distributions of these synthesized AgNPs are shown in Fig. S2. SEM image shows the presence of polydisperse nanoparticles with size ranging from 30 to 58 nm (Fig. 1B). As characterized by TEM, NATA-AgNPs shows spherical shape irrespective of the length of hydrophobic acyl chain (Fig. S2A, B, C, D, E). The particle size calculated using TEM image is in the range of 33 ± 5 nm for all the acyl chain lengths. Even though the hydrophobicity of NATA differs due to the hydrophobic acyl chain length, the morphology and sizes of NATA-AgNPs remain unaltered. Zeta potential measurements were explored to understand the surface chemistry of the AgNPs and the effect of their interaction with NATA. Fig. 3 shows the zeta potential of NATA-AgNPs. Upon increasing the acyl chain length, zeta potential shifts from −29.9 to −33.5 mV. The negative sign indicates the alkaline nature of the pH [16]. The significant difference in the zeta potential for different NATA can be attributed to the repulsion caused by the AgNPs due to the presence of anionic charges on the surface. As inferred from the zeta potential, the longer the acyl chain length, higher the repulsion between the individual nanoparticles. A previous study showed that an increase in an acyl chain length will lead to an increase in the inter-particle distance of the nanoparticles [13]. By analyzing these data, it is clear that the longer acyl chain of NATA provides more stability to nanoparticles, most likely by modulating the inter-particle distance. A FTIR studies of pure NATA and NATA reduced AgNPs showed an unaltered carbonyl stretching vibration of the amide group at 1638 cm−1 (Fig. 4). This indicates that the amide group was not involved in the formation and stabilization of AgNPs. In case of pure NATA, a sharp peak with a shoulder responsible for N H and O H stretching vibrations is observed at 3301 cm−1 . These features are completely broadened in the case of NATA-AgNPs,
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Fig. 4. Typical FTIR spectra of pure NATA (black line) and NATA capped AgNPs (blue line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
plausibly due to the intermolecular hydrogen-bonded network of OH and NH groups of the NATA with the surface of the nanoparticles. This also suggests that the phenolic -OH group might be involved in the formation of NATA-AgNPs. Therefore, it is expected that the NATAs self-assemble around the NPs and stabilize the NPs, which might facilitate the disaggregation of the larger aggregates of NATAs, finally resulting in the formation of transparent yellow AgNPs (Fig. 1). On the basis of the above experimental results, a formation mechanism of the AgNPs is proposed and is shown in Fig. 5. NATA suspended into aqueous solution self-assembles into larger aggregates with an inner hydrophobic backbone and outer hydrophilic phenolic hydroxyl groups ( OH). Recently, the reported crystal structure of N-acyl dopamine showed that the N-lauryl dopamine is packed head-to-head (and tail-to-tail) in stacked bilayers [17]. The presence of the catechol group, allows O H· · ·O hydrogen bonds between the head groups of opposite leaflets. In addition, the amide groups of adjacent are also involved in hydrogen bonds. The carbonyl oxygen atoms of adjacent molecules point in the opposite direction and provide an appropriate juxtaposition of
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the amide carbonyl and N H moieties of adjacent molecules to form N H· · ·O C type hydrogen bonds. Since tyramine [11] and dopamine [17] are structurally differing by only one OH group, thus, it can be speculated that the NATAs may exist in similar packing structures. Thus, the larger aggregates of NATA obtained during the hydration can be a result of the head-to-head packing and the hydrogen bond between the amide groups. However, the packing of NATA can be rationalized only after the structures of NATAAgNPs are known. NATA contains a phenolic group which can be “switched on” as a reducing agent under alkaline conditions [12]. In the presence of Ag+ ions, a NATA–Ag+ complex is formed through the electrostatic interaction of Ag+ ions with the phenolic group. The ionization of the phenolic group enables facile electron transfer from the phenolate ion to the silver cations, resulting in the formation of silver nanoparticles. In order to stabilize the AgNPs, NATA self assembles around NPs through the phenolate group and stabilizes through N H· · ·O C type hydrogen bonds (Fig. 5). The significant differences in the zeta potential of the N10TA-AgNPs, N12TA-AgNPs, N14TA-AgNPs, N16TA-AgNPs, and N18TA-AgNPs were further investigated by catalyzing the reduction of 4-NP into 4-AP by NaBH4 . This reaction was chosen because it can be easily monitored by UV–vis spectroscopy [14]. Addition of NaBH4 to 4-NP shifts the absorption peak from 317 nm to 404 nm, due to the formation of 4-nitrophenolate ion [18]. In absence of the catalyst, the peak remains undisturbed at about 404 nm, indicating that the reduction did not take place. However, addition of 50 L of an aqueous suspension of NATA-AgNPs to 4-nitrophenolate ion gradually diminishes the yellow color of the reaction mixture in a time-dependent manner. As shown in Fig. 6A, the 4-NP absorption peak at 404 nm decreases and a new absorption peak, due to the formation of 4-AP appears at 293 nm. Since the amount of nanoparticles added is very small, the absorption spectra of 4-NP are hardly affected by the AgNPs. The catalytic reduction of 4-NP in presence of different NATAAgNPs was associated with an induction time t0 in which no reduction takes place (Fig. 6B). This induction period is typical for a heterogeneous catalytic process and commonly related to (i) an activation or restructuring of the metal surface by 4-NP before the reaction could start [11] and (ii) the diffusion-controlled adsorption of substrates onto the NPs surface [19]. Noteworthy, the induction times are different for different NATA-AgNPs (Fig. 6B). It
Fig. 5. Schematic representation of N-acyl tyramine reduced and stabilized silver nanoparticles. Dotted lines indicate the possible hydrogen bond network.
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Table 1 Catalytic activity of NATA-AgNPs catalytic system for nitroaromatic reduction. Catalyst
N10TA-AgNPs N12TA-AgNPs N14TyA-AgNPs N16TA-AgNPs N18TA-AgNPs
4-Nitro phenol
2-Nitro aniline
2,4,6-Trinitro phenol
t0 (s)
k (×103 ) (s−1 )
t0 (s)
k (×103 ) (s−1 )
t0 (s)
k (×103 ) (s−1 )
460.2 319.8 90 19.8 10.2
3.89 5.89 6.57 9.97 13.1
160.2 49.8 40.2 30.0 19.8
1.12 4.80 6.47 7.47 12.7
330.0 250.2 70.2 40.2 30.0
1.5 1.55 2.35 2.45 2.92
was observed that the induction time is lesser in N18TA-AgNPs compared to the lower acyl chain length NATA-AgNPs (Table 1), indicating that the reduction reaction was initiated more quickly in case of N18TA-AgNPs. In order to test the above observed phenomenon, we further investigated the catalytic reduction of other nitro compounds such as 2-nitro aniline and 2,4,6-trinitro phenol. Strikingly, similar behavior was observed for the reduction of 2,4,6trinitrophenol to 2,4,6-triamino phenol (Fig. S4) and 2-nitro aniline to 1,2-diamino benzene (Fig. S5). The rate constants obtained for the reduction of all the tested nitro compounds are reported in Table 1. The obtained data shows that the hydrophobic acyl chain of NATA plays a significant role in reducing the nitroaromatic compounds. It has been noted that the reaction for the reduction of 4-NP catalyzed by NATA-AgNPs was almost complete within 10 min in all cases. Since the concentration of NaBH4 was much higher than that of 4-NP, the reduction kinetics can be described by pseudo firstorder rate law with regard to 4-NP alone. Therefore, the reaction kinetics can be described as ln(At /A0 ) = −kt, where k is the apparent
Fig. 6. (A) Variation in UV–vis spectra for the reduction of 4-NP in presence of NATAAgNPs and (B) time-dependent absorption of 4-nitrophenolate ion at 404 nm, with an induction period of t0 .
first-order rate constant, t is the reaction time. At and A0 are the absorbance of 4-NP at time t and 0, respectively. As the absorbance intensity of 4-NP and its concentrations are proportional to each other in the medium, the ratio of absorbance (At /A0 ) could be equal with the ratio of concentration of 4-NP (Ct /C0 ). A linear relation between ln(Ct /C0 ) versus reaction time (t) was observed after the induction period (t0 ) (Fig. 6B). Thus, the determination of the apparent rate constant from the slope of the linear correlation of ln(At /A0 ) with time t is straight forward. The rate constant obtained directly from the slope of the linear part of the kinetic trace is given in Table 1. For all the tested nitro compounds, 4-NP, 2-nitro aniline and 2,4,6-trinitro phenol, the rate constant is indeed proportional to the hydrophobic acyl chain length of the NATA. As summarized from Table 1, the N18TyA-AgNPs presents a higher catalytic activity than other lower acyl chain length-stabilized AgNPs. In this study, the catalytic system has same size, shape and same composition, differing only in terms of the hydrophobic acyl chain length. Thus, we postulate that the hydrophobicity of the stabilizing agent strongly influences the chemo-catalytic potential of the NATA-AgNPs. Silver nanoparticles-catalyzed reduction reaction mechanisms have been postulated by the inherent hydrogen adsorption and desorption [14]. Several studies have shown that AgNPs act as an electron relay in the reduction reaction [20], i.e. AgNPs adsorb hydrogen from the NaBH4 and efficiently release them during the reduction reaction. Thus, it is clear that the surface of the AgNPs is expected to play an important role in the catalytic reduction reaction and act as a hydrogen carrier for the reduction reaction. Since the NPs surfaces are stabilized by NATA, they were expected to play a significant role in determining the rate of the reduction reaction. It is well known that the fatty acid acyl chains are responsible for determining the size of the detergent micelles and thickness in case of lipid bilayers [21]. A change of one methyl group is expected to increase the hydrophobic core size by 2.5 A˚ [22]. As a result, the hydrophobic cores formed by N10TA, N12TA, N14TA, N16TA and ˚ respectively. From this approxN18TA as ∼25, 30, 35, 40 and 45 A, imation, it is expected that the hydrophobic effect and the van der Waals interactions between the NATA are greater for longer hydrocarbon chains. These forces are able to draw in the substrate from the aqueous environment, similar to enzymes, and thereby increase the local substrate concentration near the nanoparticle surface. Thus, we postulate that the higher hydrophobicity provided by N18TA is able to draw in substrates more easily than the less hydrophobic NATAs. As a result a faster reduction reaction is expected with N18TA-AgNPs. Alternatively, the difference in the catalytic activity can be explained by considering the different diffusion rates of the reactant on the AgNPs surface [19]. This is supported by observing the induction time, t0 , dependence on the length of acyl chains in NATAs (Table 1). In this regard, the higher induction time was observed for N10TA-AgNPs versus the N18TA-AgNPs, indicating a slower diffusion of the substrates on the N10TA-AgNPs surfaces. Since NATA is amphiphilic in nature with a lipophilic acyl chain moiety, it has a higher tendency to form micelles with a hydrophobic core in water. When the hydrophobic core is smaller, it will result in the formation of less compact colloidal particles, and as a result, a slower diffusion of the reactant,
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which will lead to a slower reaction. This is consistent with the observed catalytic activities, since the N18TA-AgNPs systems offer little resistance to diffusion compared to lower acyl chain length NATA. In addition, nanoparticles surface assemblies can also exhibit rectified quantized charging in aqueous solutions containing bulky and hydrophobic ions [23,24]. Thus, it can be postulated that higher hydrophobic N18TA can initiate a well-defined quantized charging to the nanoparticle surface assemblies compared to the lower acyl chain length NATA. As a result, the addition of a reducing agent more easily activates the metal surface of N18TA-AgNPs compared to N10TA-AgNPs and thereby the stored electrons would easily be transferred from electron-donating molecule (BH4 − ) to electron-accepting molecules (nitro compound). In summary, the present work clearly indicates that the hydrophobic effects play an important role in determining the activity of the catalytic silver nanoparticles. Thus, the ability to modulate the catalytic property by modifying the capping agent hydrophobicity represents a promising future for developing a nanocatalyst without changing the size, shape and morphology of the nanoparticles. 4. Conclusions The present study describes a new and simple method for preparing AgNPs in the presence of a homologous series of hydrophobic N-acyl tyramines in alkaline aqueous solution. It was observed that NATA acted both as a reducing agent and as a capping agent. As evident from SEM images, the formation of AgNPs results in the self-assembly of NATA, which results in capping the nanoparticles. Interestingly, the hydrophobicity of the acyl chains does not affect the shape, size and morphology of the AgNPs. However, zeta potential shows significant shifts with respect to the hydrophobicity of the NATA, indicating that the AgNPs are more stable in the presence of a highly hydrophobic N18TyA. The catalytic activity of NATA-AgNPs for the reduction of nitroaromatics is linearly dependent on the acyl chain length of NATA. It is found that N18TA-AgNPs shows a higher catalytic activity than the other NATAs. Strikingly, the more hydrophobic N18TA-AgNPs show less dependence on the induction period, suggesting that the hydrophobic effects play a critical role in bringing the substrate to the metal nanoparticle surface to induce the catalytic reaction. The approach of preparing catalytically active metal nanoparticles by modulating the hydrophobicity of the capping agent, represents a promising future to develop nanocatalysts without worrying about the size, shape and morphology. Moreover, the use of biological compounds as reducing and capping agents render the synthesized AgNPs as biocompatible.
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Acknowledgments This work was supported by research grants from Department of Science and Technology, Government of India (SB/FT/LS-217/2012) and Prof. T.R. Rajagopalan Research Fund, SASTRA University. We thank central research facility (R&M/0021/SCBT-007/2012-13), SASTRA University for the infrastructure. We thank Ms. H. Pearson, University of Mainz, Germany, Dr. Philip Anthony and Dr. S. Aravind, SASTRA University for critically reading the manuscript. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfb. 2014.03.008. References [1] S. Linic, P. Christopher, H. Xin, A. Marimuthu, Acc. Chem. Res. 46 (2013) 1890. [2] C. Burda, X. Chen, R.R. Narayanan, M.A. El-Sayed, Chem. Rev. 105 (2005) 1025. [3] M. Tejamaya, I. Römer, R.C. Merrifield, J.R. Lead, Environ. Sci. Technol. 46 (2012) 7011. [4] J.S. Bradley, in: G.E. Schmid (Ed.), Clusters and Colloids: From Theory to Applications, VCH, Weinheim, 1994, p. 459. [5] K. Kuroda, T. Ishida, M. Haruta, J. Mol. Catal. A: Chem. 298 (2009) 7. [6] Y. Mei, Y. Lu, F. Polzer, M. Ballauff, M. Drechsler, Chem. Mater. 19 (2007) 1062. [7] K. Esumi, R. Isono, T. Yoshimura, Langmuir 20 (2004) 237. [8] A.M. Signori, O. Santos Kde, R. Eising, B.L. Albuquerque, F.C. Giacomelli, J.B. Domingos, Langmuir 26 (2010) 17772. [9] K.B. Narayanan, N. Sakthivel, Adv. Colloid Interface Sci. 156 (2010) 1. [10] A.K. Mittal, Y. Chisti, U.C. Banerjee, Biotechnol. Adv. 31 (2013) 346. [11] P.H. Yu, A.A. Boulton, Can. J. Biochem. 57 (1979) 1204. [12] P.R. Selvakannan, A. Swami, D. Srisathiyanarayanan, P.S. Shirude, R. Pasricha, A.B. Mandale, M. Sastry, Langmuir 20 (2004) 7825. [13] K. Abe, T. Hanada, Y. Yoshida, N. Tanigaki, H. Takiguchi, H. Nagasawa, M. Nakamoto, T. Yamaguchi, K. Yase, Thin Solid Films 327–329 (1998) 524. [14] P. Hervés, M. Pérez-Lorenzo, L.M. Liz-Marzán, J. Dzubiella, Y. Lu, M. Ballauff, Chem. Soc. Rev. 41 (2012) 5577. [15] M. Ramakrishnan, V. Sheeba, S.S. Komath, M.J. Swamy, Biochim. Biophys. Acta 1329 (1997) 302. [16] J.P. Wilcoxon, B.L. Abrams, Chem. Soc. Rev. 35 (2006) 1162. [17] S.T. Reddy, P.K. Tarafdar, R.K. Kamlekar, M.J. Swamy, J. Phys. Chem. B 117 (2013) 8747. [18] S. Harish, J. Mathiyarasu, K.L.N. Phani, Catal. Lett. 128 (2009) 197. [19] J. Zeng, Q. Zhang, J. Chen, Y. Xia, Nano Lett. 10 (2010) 30. [20] K. Mallick, M. Witcomb, M. Scurrell, Mater. Chem. Phys. 97 (2006) 283. [21] C. Tanford, The Hydrophobic Effect: Formation of Micelles and Biological Membranes, John Wiley & Sons, New York, 1980. [22] A.G. Therien, C.M. Deber, J. Biol. Chem. 277 (2002) 6067. [23] S. Chen, R. Pei, T. Zhao, D.J. Dyer, J. Phys. Chem. B 106 (2002) 1903. [24] S. Chen, R. Pei, J. Am. Chem. Soc. 123 (2001) 10607.
