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Colloids and Surfaces B: Biointerfaces 117 (2014) 528–533

Contents lists available at ScienceDirect

Colloids and Surfaces B: Biointerfaces

jo ur nal ho me p ag e: www.elsev ier .com/ locate /co lsur fb

ctivity of catalytic silver nanoparticles modulated by cappinggent hydrophobicity

eralathan Janani, Priscilla Stevenson, Anbazhagan Veerappan ∗

epartment of Chemistry, School of Chemical and Biotechnology, SASTRA University, Thanjavur, Tamil Nadu, India

r t i c l e i n f o

rticle history:eceived 5 October 2013eceived in revised form 25 February 2014ccepted 4 March 2014vailable online 19 March 2014

eywords:-Acyl tyramineilver nanoparticlesatalysisydrophobicity

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 micro-scopic analysis shows that NATA exists initially as larger aggregates in alkaline aqueous solution. Theaddition of AgNO3 dissociates these larger aggregate and subsequently promotes the formation of self-assembled NATA and AgNPs. Characterization of AgNPs using UV–vis spectroscopy, scanning electronmicroscope and transmission electron microscope revealed that the hydrophobic acyl chain length ofNATA does not influence the particle size, shape and morphology. All NATA-AgNPs yielded relativelyidentical values in full width at half-maximum (FWHM) analysis, indicating that the AgNPs preparedwith NATA are relatively polydispersed at all tested acyl chain lengths. These nanoparticles are able toefficiently catalyze the reduction of 4-nitro phenol to 4-amino phenol, 2-nitro aniline to 1,2-diamino ben-zene, 2,4,6-trinitro phenol to 2,4,6-triamino phenol by NaBH4 in an aqueous environment. The reductionreaction rate is determined to be pseudo-first order and the apparent rate constant is linearly dependenton 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 rolein 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 cap-ping agent hydrophobicity, and shows different catalytic activity with respect to the alkyl chain lengthof the capping agent. As discussed, the ability to modulate the metal nanoparticles catalytic property, bymodifying the capping agent hydrophobicity represents a promising future for developing an efficientnanocatalyst without altering the size, shape and morphology of the nanoparticles.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

In modern science, metal nanoparticles (NPs) have beenxploited in various fields of chemistry and physics. This is dueo the fact that the NPs often have totally different propertiesrom their bulk counterparts. For example, nanosized gold (Au)hows excellent catalytic activity, whereas the bulk Au is inert1,2]. Among the many metal NPs, silver (Ag) and Au NPs receivedpecial attention due to their profound application in catalysis,hotonics, sensors, antibacterial, medicine, etc. These properties

re very sensitive to particle sizes, shapes and morphology of thePs [1,2]. Because of the nanosize, metallic NPs often congregateue 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: anbugv@gmail.com, anbazhagan@scbt.sastra.edu

A. Veerappan).

ttp://dx.doi.org/10.1016/j.colsurfb.2014.03.008927-7765/© 2014 Elsevier B.V. All rights reserved.

agglomeration, stabilizing agents such as citric acid, ascorbic acid,sodium dodecyl sulfate, sodium oleate, and polymers containingthiol, carboxyl as well as amino groups are widely used [3]. Thesestabilizers or capping agents most often dominate the chemical andphysical properties of the NPs [4]. Although, these capping agentsprevent the NPs from aggregation, they can also influence the activ-ity of the NPs. For example, the catalytic activity can be decreased bycovering the surface of the catalyst [5–7]. Particularly for catalysis,the total surface area of the NPs is believed to play an important rolein the catalytic reaction [8]. Since the NP surfaces are protected bythe capping agent, it is imperative to investigate which property ofthese capping agents are important to create the right environmentfor the NPs stabilization and to obtain efficient catalysis.

Recently, much attention has been directed toward synthesiz-

ing metal NPs using biological important compounds [9,10]. In thiswork, we report the preparation of silver nanoparticles (AgNPs)using a homologous series of N-acyl tyramines (NATA). NATAbelong to an important class of endogenous signaling molecules,

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ormed by an amide linkage with long chain fatty acids and 4-ydroxyphenethylamine (tyramine) [11]. It has been shown thathe phenolic acids with one or more phenolic hydroxyl groups areble to donate electrons and reduce Au3+ and Ag+ to their corre-ponding nanoparticles [12]. In our work, the phenolic OH groupnd the long fatty acid chains present in NATA not only facilitatehe reduction of Ag+ to Ag0, but also prevents the AgNPs fromgglomeration. By making use of N-acylation with different acylhain length fatty acids, different hydrophobicity was introducedo the capping agent, without modifying the polar functional group.ilver nanoparticles derived from N-acyl tyramine were charac-erized using UV–vis spectroscopy, scanning electron microscopeSEM) and transmission electron microscope (TEM). The obtainedata clearly show that the size, shape and morphology are iden-ical, irrespective of the long hydrophobic fatty acid acyl chain of-acyl tyramine. However, with increasing acyl chain length, theeta potential of the NATA-capped silver nanoparticles shows aignificant shift from −29.9 to −33.5 mV, indicating that the NPsre well separated from one another and are strongly stabilized atonger acyl chain lengths.

The hydrophobic acyl chain surrounding the NPs is known tonfluence the inter-particle distance depending on the acyl chainength [13]. Thus, we postulate that the hydrophobicity introducedy the N-acyl chain of tyramine (capping the nanoparticles) may

nfluence the chemical activity of the AgNPs. As a proof of con-ept, we tested the catalytic activity of AgNPs synthesized with-acyl tyramine using the standard reduction reaction, 4-nitro phe-ol to 4-amino phenol [14], as well as with 2-nitro aniline and,4,6-trinitrophenol. The obtained result shows that the rate ofeduction of nitroaromatic compound is significantly influenced byhe capping agent hydrophobicity, suggesting the importance of theydrophobic effect in modulating the activity of silver nanoparti-les.

. Materials and methods

.1. Materials

Tyramine, decanoyl chloride, lauroyl chloride, myristoyl chlo-ide, palmitoyl chloride and steroyl chloride were purchased fromigma. Silver nitrate, sodium borohydride (NaBH4), 4-nitrophenol4-NP), 2-nitro aniline and picric acid were purchased from Merck.ll other chemicals and solvents used in this work are of analyticalrade obtained from the local supplier.

.2. Preparation of silver nanoparticles

N-Acyl tyramines (Fig. S1) have been used as an in situ reduc-ng and capping agent for preparing the stable silver nanoparticles.ATA have been synthesized via the reaction of tyramine with theppropriate acid chloride, essentially according to the procedureescribed for N-acyl ethanolamine [15]. In a typical procedure toynthesize silver nanoparticles, 0.2 mM of N-acyl tyramine was dis-olved in 10 mM of sodium hydroxide (NaOH) and was vigorouslytirred for about 30 min. Then, 0.25 mM of silver nitrate (AgNO3)ere added into the above solution. The resulting solutions were

ently stirred for 2 h. The solution was initially colorless and iturned to pale yellow color after 2 h. On standing for 24 h, theale yellow color solution changed to a bright golden yellow color,

ndicating the complete reduction of Ag+ ion to Ag0 nanoparticles.lternatively, AgNPs was synthesized using sodium borohydride

NaBH4) as the reducing agent and NATA as the capping agent.ddition of NaBH4 results in an immediate reduction of Ag+ ion

o Ag0 nanoparticles, as evident from the appearance of the brightolden yellow color.

iointerfaces 117 (2014) 528–533 529

2.3. Characterization

Optical absorbance of the synthesized AgNPs was monitoredby 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 (JEOL-JSM 6701-F, Japan). The samples was placed over a carbon tape anddried. Prior to imaging, the samples were coated with a thin layerof platinum in an auto fine coater. TEM image was performed onthe NATA-AgNPs using JEM 1011, JEOL, Japan. A drop of the NATA-AgNPs was placed on a carbon-coated copper grid and dried priorto the measurement. Fourier transfer infra-red (FT-IR) spectra ofNATA and NATA-AgNPs were recorded using a PerkinElmer FT-IRspectrometer with 1 cm−1 resolution. NATA-AgNPs solutions werecentrifuged at 10,000 rpm for 30 min. The obtained residue wasfreeze-dried by lyophilization and grounded with KBr prior to theFT-IR measurement. Zeta potential of the prepared nanoparticleswas determined by using a Malvern Zetasizer instrument (version6.20).

2.4. Catalysis

Typically, 1 mM of 4-NP and 15 mM of NaBH4 were added into aquartz cuvette containing 3 mL of water. 10 mL of 0.25 mM AgNPswas centrifuged at 10,000 rpm for 20 min and the residue was dis-solved in 1 mL of water. 50 �L of the suspension is added directlyinto the cuvette to start the reduction reaction. The intensity ofthe absorption peak at 404 nm in UV–vis spectroscopy was used tomonitor the process of the conversion of 4-NP to 4-aminophenol (4-AP). To rule out the possibility of formation of metal oxide layer andcatalytic surface poisoning, the samples were carefully degassedbefore adding NaBH4.

3. Results and discussion

In order to investigate the effect of capping agents on the phy-siochemical properties of AgNPs, we synthesized the following fivedifferent acyl chain length stabilized AgNPs, N-decanoyl tyramine(N10TA-AgNPs), N-lauroyl tyramine (N12TA-AgNPs), N-myristoyltyramine (N14TA-AgNPs), N-palmitoyl tyramine (N16TA-AgNPs),N-steroyl tyramine (N18TA-AgNPs), by reducing silver ions in aque-ous solutions of N-acyl tyramine (NATA) as presented in Section2. The physicochemical properties of AgNPs such as stability, size,morphology and chemical activity were characterized.

Because of the lengthy acyl chain, NATAs are sparingly solublein an aqueous environment; however, the addition of NaOH fairlyincreased its solubility. When the NATA–NaOH–AgNO3 aqueoussolution was incubated at room temperature, a milky colloidal solu-tion showed up immediately. While keeping it for 24 h, the milkycolloidal solution turned into transparent yellow color, indicatingthat the Ag+ ions were reduced to AgNPs (Fig. 1A). SEM imageswere recorded at each stage of AgNPs formation (Fig. 1B). It is visi-bly clear that the NATA exists initially as larger aggregates. Additionof AgNO3 dissociates these larger aggregates of NATA and it finallyresults in the formation of self assembled structure of NATA, whichstabilizes the AgNPs (Fig. 1B). The UV–vis spectra of the AgNPs pre-pared with the same AgNO3 concentration, but by utilizing differentacyl chain lengths NATA are shown in Fig. 2. Irrespective of the acylchain length, the characteristic surface plasmon resonance (SPR)peak of AgNPs was observed around 420–425 nm. The dispersity ofAgNPs was evaluated by comparing the full width at half-maximum

(FWHM) from the UV–vis spectra [16]. All the samples show essen-tially identical FWHM (83–102 nm). This indicates that the AgNPsprepared with NATA are relatively polydisperse at all tested acylchain lengths, and there is no change in absorption maximum and

530 S. Janani et al. / Colloids and Surfaces B: Biointerfaces 117 (2014) 528–533

Fig. 1. (A) Representative preparation of NATA-AgNPs. (B) Corresponding SEMi–2

Fgd

caNNac

Fm

mages, measured at each stage of the preparation. Top – alkaline NATA, middle NATA with AgNO3, and bottom – NATA reduced and capped AgNPs obtained after4 h.

WHM between the different NATA-AgNPs. These data clearly sug-est that the hydrophobic acyl chain of NATA does not alter theispersity of AgNPs.

In general, AgNPs have been widely prepared by chemi-al methods using sodium borohydride (NaBH4) as a reducinggent. In order to test the effect of NaBH4 on the preparation of

ATA-AgNPs, we synthesized AgNPs by adding 10 mM NaBH4 toATA–NaOH–AgNO3 aqueous solution. Addition of the reducinggent immediately shows yellow color at room temperature, indi-ating the formation of AgNPs. The absorbance bands were detected

ig. 2. UV–vis spectra of silver nanoparticles prepared with different N-acyl tyra-ine.

Fig. 3. Hydrophobic acyl chain length versus zeta potential of NATA-AgNPs.

at 420–425 nm (Fig. S3), similar to the one prepared without NaBH4.Although NATA reduces Ag+ ions to AgNPs, the presence of NaBH4accelerates the rate of AgNPs formation. The FWHM calculated fromFig. S3 are in the broad range of 78–121 nm, indicating the for-mation of relatively polydisperse AgNPs for all tested acyl chainlengths. It is clear from FWHM, the formation of relatively narrowdispersed AgNPs is most favored in the reduction without NaBH4.Also noteworthy that the AgNPs prepared solely with NATA didnot show any change in the absorption maximum in the course ofone month (less than 1 nm). However, over the time, the AgNPsprepared with the NaBH4 reduction method showed visible aggre-gates. These aggregates deposit in the bottom of the test tube,suggesting that the particles are relatively unstable. Thus, for thesubsequent studies, we used the nanoparticles prepared solely withNATA.

The morphology and the particle size distributions of these syn-thesized AgNPs are shown in Fig. S2. SEM image shows the presenceof polydisperse nanoparticles with size ranging from 30 to 58 nm(Fig. 1B). As characterized by TEM, NATA-AgNPs shows sphericalshape irrespective of the length of hydrophobic acyl chain (Fig. S2A,B, C, D, E). The particle size calculated using TEM image is in therange of 33 ± 5 nm for all the acyl chain lengths. Even though thehydrophobicity of NATA differs due to the hydrophobic acyl chainlength, the morphology and sizes of NATA-AgNPs remain unaltered.

Zeta potential measurements were explored to understand thesurface chemistry of the AgNPs and the effect of their interactionwith NATA. Fig. 3 shows the zeta potential of NATA-AgNPs. Uponincreasing the acyl chain length, zeta potential shifts from −29.9to −33.5 mV. The negative sign indicates the alkaline nature of thepH [16]. The significant difference in the zeta potential for differ-ent NATA can be attributed to the repulsion caused by the AgNPsdue to the presence of anionic charges on the surface. As inferredfrom the zeta potential, the longer the acyl chain length, higher therepulsion between the individual nanoparticles. A previous studyshowed that an increase in an acyl chain length will lead to anincrease in the inter-particle distance of the nanoparticles [13]. Byanalyzing these data, it is clear that the longer acyl chain of NATAprovides more stability to nanoparticles, most likely by modulatingthe inter-particle distance.

A FTIR studies of pure NATA and NATA reduced AgNPs showedan unaltered carbonyl stretching vibration of the amide group at1638 cm−1 (Fig. 4). This indicates that the amide group was notinvolved in the formation and stabilization of AgNPs. In case of

pure NATA, a sharp peak with a shoulder responsible for N Hand O H stretching vibrations is observed at 3301 cm−1. Thesefeatures are completely broadened in the case of NATA-AgNPs,

S. Janani et al. / Colloids and Surfaces B: B

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ig. 4. Typical FTIR spectra of pure NATA (black line) and NATA capped AgNPs (blueine). (For interpretation of the references to color in this figure legend, the readers referred to the web version of the article.)

lausibly due to the intermolecular hydrogen-bonded network ofH and NH groups of the NATA with the surface of the nanopar-

icles. This also suggests that the phenolic -OH group might benvolved in the formation of NATA-AgNPs. Therefore, it is expectedhat the NATAs self-assemble around the NPs and stabilize the NPs,hich might facilitate the disaggregation of the larger aggregates

f NATAs, finally resulting in the formation of transparent yellowgNPs (Fig. 1).

On the basis of the above experimental results, a formationechanism of the AgNPs is proposed and is shown in Fig. 5. NATA

uspended into aqueous solution self-assembles into larger aggre-ates with an inner hydrophobic backbone and outer hydrophilichenolic hydroxyl groups ( OH). Recently, the reported crystaltructure of N-acyl dopamine showed that the N-lauryl dopamines packed head-to-head (and tail-to-tail) in stacked bilayers [17].he presence of the catechol group, allows O H· · ·O hydrogen

onds between the head groups of opposite leaflets. In addition,he amide groups of adjacent are also involved in hydrogen bonds.he carbonyl oxygen atoms of adjacent molecules point in thepposite direction and provide an appropriate juxtaposition of

Fig. 5. Schematic representation of N-acyl tyramine reduced and stabilized silver

iointerfaces 117 (2014) 528–533 531

the amide carbonyl and N H moieties of adjacent molecules toform N H· · ·O C type hydrogen bonds. Since tyramine [11] anddopamine [17] are structurally differing by only one OH group,thus, it can be speculated that the NATAs may exist in similar pack-ing structures. Thus, the larger aggregates of NATA obtained duringthe hydration can be a result of the head-to-head packing and thehydrogen bond between the amide groups. However, the packingof NATA can be rationalized only after the structures of NATA-AgNPs are known. NATA contains a phenolic group which can be“switched on” as a reducing agent under alkaline conditions [12]. Inthe presence of Ag+ ions, a NATA–Ag+ complex is formed throughthe electrostatic interaction of Ag+ ions with the phenolic group.The ionization of the phenolic group enables facile electron trans-fer from the phenolate ion to the silver cations, resulting in theformation of silver nanoparticles. In order to stabilize the AgNPs,NATA self assembles around NPs through the phenolate group andstabilizes through N H· · ·O C type hydrogen bonds (Fig. 5).

The significant differences in the zeta potential of theN10TA-AgNPs, N12TA-AgNPs, N14TA-AgNPs, N16TA-AgNPs, andN18TA-AgNPs were further investigated by catalyzing the reduc-tion of 4-NP into 4-AP by NaBH4. This reaction was chosen becauseit can be easily monitored by UV–vis spectroscopy [14]. Addition ofNaBH4 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 thecatalyst, the peak remains undisturbed at about 404 nm, indicatingthat the reduction did not take place. However, addition of 50 �Lof an aqueous suspension of NATA-AgNPs to 4-nitrophenolate iongradually diminishes the yellow color of the reaction mixture in atime-dependent manner. As shown in Fig. 6A, the 4-NP absorptionpeak at 404 nm decreases and a new absorption peak, due to theformation of 4-AP appears at 293 nm. Since the amount of nanopar-ticles added is very small, the absorption spectra of 4-NP are hardlyaffected by the AgNPs.

The catalytic reduction of 4-NP in presence of different NATA-AgNPs was associated with an induction time t0 in which noreduction takes place (Fig. 6B). This induction period is typicalfor a heterogeneous catalytic process and commonly related to

(i) an activation or restructuring of the metal surface by 4-NPbefore the reaction could start [11] and (ii) the diffusion-controlledadsorption of substrates onto the NPs surface [19]. Noteworthy, theinduction times are different for different NATA-AgNPs (Fig. 6B). It

nanoparticles. Dotted lines indicate the possible hydrogen bond network.

532 S. Janani et al. / Colloids and Surfaces B: Biointerfaces 117 (2014) 528–533

Table 1Catalytic activity of NATA-AgNPs catalytic system for nitroaromatic reduction.

Catalyst 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)

N10TA-AgNPs 460.2 3.89 160.2 1.12 330.0 1.5N12TA-AgNPs 319.8 5.89 49.8 4.80 250.2 1.55N14TyA-AgNPs 90 6.57 40.2 6.47 70.2 2.35

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N16TA-AgNPs 19.8 9.97

N18TA-AgNPs 10.2 13.1

as observed that the induction time is lesser in N18TA-AgNPsompared to the lower acyl chain length NATA-AgNPs (Table 1),ndicating that the reduction reaction was initiated more quicklyn case of N18TA-AgNPs. In order to test the above observed phe-omenon, we further investigated the catalytic reduction of otheritro compounds such as 2-nitro aniline and 2,4,6-trinitro phenol.trikingly, similar behavior was observed for the reduction of 2,4,6-rinitrophenol to 2,4,6-triamino phenol (Fig. S4) and 2-nitro anilineo 1,2-diamino benzene (Fig. S5). The rate constants obtained for theeduction of all the tested nitro compounds are reported in Table 1.he obtained data shows that the hydrophobic acyl chain of NATAlays a significant role in reducing the nitroaromatic compounds.

It has been noted that the reaction for the reduction of 4-NPatalyzed by NATA-AgNPs was almost complete within 10 min inll cases. Since the concentration of NaBH4 was much higher than

hat of 4-NP, the reduction kinetics can be described by pseudo first-rder rate law with regard to 4-NP alone. Therefore, the reactioninetics can be described as ln(At/A0) = −kt, where k is the apparent

ig. 6. (A) Variation in UV–vis spectra for the reduction of 4-NP in presence of NATA-gNPs and (B) time-dependent absorption of 4-nitrophenolate ion at 404 nm, withn induction period of t0.

7.47 40.2 2.45 12.7 30.0 2.92

first-order rate constant, t is the reaction time. At and A0 are theabsorbance of 4-NP at time t and 0, respectively. As the absorbanceintensity of 4-NP and its concentrations are proportional to eachother in the medium, the ratio of absorbance (At/A0) could be equalwith the ratio of concentration of 4-NP (Ct/C0). A linear relationbetween ln(Ct/C0) versus reaction time (t) was observed after theinduction period (t0) (Fig. 6B). Thus, the determination of the appar-ent rate constant from the slope of the linear correlation of ln(At/A0)with time t is straight forward. The rate constant obtained directlyfrom the slope of the linear part of the kinetic trace is given inTable 1. For all the tested nitro compounds, 4-NP, 2-nitro aniline and2,4,6-trinitro phenol, the rate constant is indeed proportional to thehydrophobic acyl chain length of the NATA. As summarized fromTable 1, the N18TyA-AgNPs presents a higher catalytic activity thanother lower acyl chain length-stabilized AgNPs. In this study, thecatalytic system has same size, shape and same composition, dif-fering only in terms of the hydrophobic acyl chain length. Thus, wepostulate that the hydrophobicity of the stabilizing agent stronglyinfluences the chemo-catalytic potential of the NATA-AgNPs.

Silver nanoparticles-catalyzed reduction reaction mechanismshave been postulated by the inherent hydrogen adsorption anddesorption [14]. Several studies have shown that AgNPs act asan electron relay in the reduction reaction [20], i.e. AgNPs adsorbhydrogen from the NaBH4 and efficiently release them during thereduction reaction. Thus, it is clear that the surface of the AgNPs isexpected to play an important role in the catalytic reduction reac-tion and act as a hydrogen carrier for the reduction reaction. Sincethe NPs surfaces are stabilized by NATA, they were expected to playa significant role in determining the rate of the reduction reaction.It is well known that the fatty acid acyl chains are responsible fordetermining the size of the detergent micelles and thickness in caseof lipid bilayers [21]. A change of one methyl group is expected toincrease the hydrophobic core size by 2.5 A [22]. As a result, thehydrophobic cores formed by N10TA, N12TA, N14TA, N16TA andN18TA as ∼25, 30, 35, 40 and 45 A, respectively. From this approx-imation, it is expected that the hydrophobic effect and the vander Waals interactions between the NATA are greater for longerhydrocarbon chains. These forces are able to draw in the substratefrom the aqueous environment, similar to enzymes, and therebyincrease the local substrate concentration near the nanoparticlesurface. Thus, we postulate that the higher hydrophobicity pro-vided by N18TA is able to draw in substrates more easily thanthe less hydrophobic NATAs. As a result a faster reduction reac-tion is expected with N18TA-AgNPs. Alternatively, the differencein the catalytic activity can be explained by considering the differ-ent diffusion rates of the reactant on the AgNPs surface [19]. Thisis supported by observing the induction time, t0, dependence onthe length of acyl chains in NATAs (Table 1). In this regard, thehigher induction time was observed for N10TA-AgNPs versus theN18TA-AgNPs, indicating a slower diffusion of the substrates onthe N10TA-AgNPs surfaces. Since NATA is amphiphilic in nature

with a lipophilic acyl chain moiety, it has a higher tendency to formmicelles with a hydrophobic core in water. When the hydrophobiccore is smaller, it will result in the formation of less compact col-loidal particles, and as a result, a slower diffusion of the reactant,

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S. Janani et al. / Colloids and Surfac

hich will lead to a slower reaction. This is consistent with thebserved catalytic activities, since the N18TA-AgNPs systems offerittle resistance to diffusion compared to lower acyl chain lengthATA. In addition, nanoparticles surface assemblies can also exhibit

ectified quantized charging in aqueous solutions containing bulkynd hydrophobic ions [23,24]. Thus, it can be postulated that higherydrophobic N18TA can initiate a well-defined quantized charg-

ng to the nanoparticle surface assemblies compared to the lowercyl chain length NATA. As a result, the addition of a reducinggent more easily activates the metal surface of N18TA-AgNPs com-ared to N10TA-AgNPs and thereby the stored electrons wouldasily be transferred from electron-donating molecule (BH4

−) tolectron-accepting molecules (nitro compound). In summary, theresent work clearly indicates that the hydrophobic effects play an

mportant role in determining the activity of the catalytic silveranoparticles. Thus, the ability to modulate the catalytic prop-rty by modifying the capping agent hydrophobicity represents aromising future for developing a nanocatalyst without changinghe size, shape and morphology of the nanoparticles.

. Conclusions

The present study describes a new and simple method forreparing AgNPs in the presence of a homologous series ofydrophobic N-acyl tyramines in alkaline aqueous solution. It wasbserved that NATA acted both as a reducing agent and as a cap-ing agent. As evident from SEM images, the formation of AgNPsesults in the self-assembly of NATA, which results in capping theanoparticles. Interestingly, the hydrophobicity of the acyl chainsoes not affect the shape, size and morphology of the AgNPs. How-ver, zeta potential shows significant shifts with respect to theydrophobicity of the NATA, indicating that the AgNPs are moretable in the presence of a highly hydrophobic N18TyA. The cat-lytic activity of NATA-AgNPs for the reduction of nitroaromaticss linearly dependent on the acyl chain length of NATA. It is foundhat N18TA-AgNPs shows a higher catalytic activity than the otherATAs. Strikingly, the more hydrophobic N18TA-AgNPs show lessependence on the induction period, suggesting that the hydropho-ic effects play a critical role in bringing the substrate to the metalanoparticle surface to induce the catalytic reaction. The approachf preparing catalytically active metal nanoparticles by modulatinghe hydrophobicity of the capping agent, represents a promising

uture to develop nanocatalysts without worrying about the size,hape and morphology. Moreover, the use of biological compoundss reducing and capping agents render the synthesized AgNPs asiocompatible.

[[[

iointerfaces 117 (2014) 528–533 533

Acknowledgments

This work was supported by research grants from Department ofScience 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. Pear-son, 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.

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