Shape-directing role of cetyltrimethylammonium bromide in the preparation of silver nanoparticles Zaheer Khan a,⇑ , Shaeel Ahmed AL-Thabaiti b , Abdullah Yousif Obaid b , Ziya Ahmad Khan b , Abdulrahman A.O. Al-Youbi b a Department of Chemistry, Jamia Millia Islamia, Central University, New Delhi 110 025, India b Department of Chemistry, Faculty of Science, King Abdul Aziz University, P.O. Box 80203, Jeddah 21413, Saudi Arabia article info Article history: Received 8 June 2011 Accepted 6 October 2011 Available online 22 October 2011 Keywords: Shape directing UV–visible spectrum Tyrosine Silver nanoparticles abstract We report a simple chemical reduction method for the synthesis of different colored silver nanoparticles, AgNP, using tyrosine as a reducing agent. Effects of cetyltrimethylammonium bromide, CTAB, and tyro- sine concentrations are analyzed by UV–visible measurements and scanning electron microscopy (SEM) to evaluate the mode of AgNP aggregation. The position and shape of the surface resonance plasmon absorption bands strongly depend on the reaction conditions, i.e., [CTAB], [tyrosine], and reaction time. Sub-, post-, and dilution-micellar effects are accountable for the fast and slow nucleation and growth pro- cesses. Spectrophotometric measurement also shows that the average size and the polydispersity of AgNP increase with [CTAB] in the solution. CTAB acted as a shape-directing agent. Ó 2011 Elsevier Inc. All rights reserved. 1. Introduction The literature is replete with the investigations of the use of bio- molecules (polysaccharides, vitamins, enzymes/proteins, amino acids, and plant extracts) as the reducing-, stabilizing-, and capping agents for carrying out the syntheses of gold and silver sols [1]. Chemical reduction is the most frequently applied method for the preparation of nanosized metal silver with different morphologies and sizes in water [2]. Lee and his coworkers [3,4] reported a biolog- ical to biomimetic chemical reduction synthesis of single-crystalline gold and silver nanoplates using the extract of unicellular green algae Chlorella vulgaris as reducing-cum-shape-directing agent. Reduction ability of protein(s) came directly from the amino resi- dues rather than an enzyme-mediated process, and protein side groups, –NH 2 , –COOH, –OH, –SH, and CH 3 S, were most likely responsible to the complexation and reduction of Ag + ions. Different peptides such as a gelatin-polypeptide [5], valine-, tryptophan-, tyrosine-based oligo- and/or short peptides [6], a simple bifunctional tripeptide (Asp-Asp-Tyr-OMe) [4], and glutathione (c-Glu-Cys-Gly-) [7] have been used as reducing/stabilizing/capping agents for the preparation of gold, silver, and gold–silver bimetallic advanced nanomaterials. Mandal and his coworkers reported that the average particle size of gold nanoparticles and reaction rate de- pend on the number of tyrosine moieties present in the peptide mol- ecules [6c]. It has been suggested that the side chain of peptides (glutamic acid-, indole-, hydroxyl-, carboxyl-, and sulfur-side chain residues) were involved in the reduction process and control the shape and size of resulting silver nanomaterials at room tempera- ture. Polypeptides containing a higher number of peptide repeats were found to be more effective in controlling nanoparticle growth. For example, CuSe nanosnakes have been fabricated by using bovine serum albumin polypeptides as bio-templates at room temperature [8]. Gold nanoparticles were most likely stabilized through the interaction of amino groups of peptides with the gold surface. Structural unit of proteins, i.e., amino acids, find a number of applications in metabolism, nutrition, fortification of seeds, and biochemical research. It has been recognized that amino and car- boxyl functional groups of amino acids undergo chemical transfor- mations while the side chain remains intact. Amino acids with aromatic side chain were oxidized more rapidly than the alkyl side chain amino acids. Amino acids also play an important role in deter- mining the morphologies of the noble metal nanocomposites. Gold–silver nanocomposites of different shapes from gold nanorods in aqueous solutions were prepared [9] (depending on the value of pH, I-, dumbbell-, and sphere-shaped gold–silver nanocomposites in the presence of glycine, glutamic acid, glutamine and arginine, corn-shaped nanocomposites in methionine, pearl-necklace shaped in lysine). Various researchers have investigated the ability of acidic and basic side chain amino acids (aspartic acid, lysine, tyrosine, and tryptophan) to act as reducing and stabilizing agents for the synthe- sis of gold nanostructures at room temperature [10]. Despite the extensive studies carried out on the preparation and characteriza- tion of gold nanoparticles and their composites using peptides 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.10.014 ⇑ Corresponding author. Fax: +91 11 2698 0229. E-mail address: [email protected](Z. Khan). Journal of Colloid and Interface Science 367 (2012) 101–108 Contents lists available at SciVerse ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
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Journal of Colloid and Interface Science 367 (2012) 101–108
Contents lists available at SciVerse ScienceDirect
Journal of Colloid and Interface Science
www.elsevier .com/locate / jc is
Shape-directing role of cetyltrimethylammonium bromide in the preparationof silver nanoparticles
Zaheer Khan a,⇑, Shaeel Ahmed AL-Thabaiti b, Abdullah Yousif Obaid b, Ziya Ahmad Khan b,Abdulrahman A.O. Al-Youbi b
a Department of Chemistry, Jamia Millia Islamia, Central University, New Delhi 110 025, Indiab Department of Chemistry, Faculty of Science, King Abdul Aziz University, P.O. Box 80203, Jeddah 21413, Saudi Arabia
a r t i c l e i n f o
Article history:Received 8 June 2011Accepted 6 October 2011Available online 22 October 2011
We report a simple chemical reduction method for the synthesis of different colored silver nanoparticles,AgNP, using tyrosine as a reducing agent. Effects of cetyltrimethylammonium bromide, CTAB, and tyro-sine concentrations are analyzed by UV–visible measurements and scanning electron microscopy (SEM)to evaluate the mode of AgNP aggregation. The position and shape of the surface resonance plasmonabsorption bands strongly depend on the reaction conditions, i.e., [CTAB], [tyrosine], and reaction time.Sub-, post-, and dilution-micellar effects are accountable for the fast and slow nucleation and growth pro-cesses. Spectrophotometric measurement also shows that the average size and the polydispersity of AgNPincrease with [CTAB] in the solution. CTAB acted as a shape-directing agent.
� 2011 Elsevier Inc. All rights reserved.
1. Introduction
The literature is replete with the investigations of the use of bio-molecules (polysaccharides, vitamins, enzymes/proteins, aminoacids, and plant extracts) as the reducing-, stabilizing-, and cappingagents for carrying out the syntheses of gold and silver sols [1].Chemical reduction is the most frequently applied method for thepreparation of nanosized metal silver with different morphologiesand sizes in water [2]. Lee and his coworkers [3,4] reported a biolog-ical to biomimetic chemical reduction synthesis of single-crystallinegold and silver nanoplates using the extract of unicellular greenalgae Chlorella vulgaris as reducing-cum-shape-directing agent.Reduction ability of protein(s) came directly from the amino resi-dues rather than an enzyme-mediated process, and proteinside groups, –NH2, –COOH, –OH, –SH, and CH3S, were most likelyresponsible to the complexation and reduction of Ag+ ions. Differentpeptides such as a gelatin-polypeptide [5], valine-, tryptophan-,tyrosine-based oligo- and/or short peptides [6], a simplebifunctional tripeptide (Asp-Asp-Tyr-OMe) [4], and glutathione(c-Glu-Cys-Gly-) [7] have been used as reducing/stabilizing/cappingagents for the preparation of gold, silver, and gold–silver bimetallicadvanced nanomaterials. Mandal and his coworkers reported thatthe average particle size of gold nanoparticles and reaction rate de-pend on the number of tyrosine moieties present in the peptide mol-ecules [6c]. It has been suggested that the side chain of peptides
ll rights reserved.
).
(glutamic acid-, indole-, hydroxyl-, carboxyl-, and sulfur-side chainresidues) were involved in the reduction process and control theshape and size of resulting silver nanomaterials at room tempera-ture. Polypeptides containing a higher number of peptide repeatswere found to be more effective in controlling nanoparticle growth.For example, CuSe nanosnakes have been fabricated by using bovineserum albumin polypeptides as bio-templates at room temperature[8]. Gold nanoparticles were most likely stabilized through theinteraction of amino groups of peptides with the gold surface.
Structural unit of proteins, i.e., amino acids, find a number ofapplications in metabolism, nutrition, fortification of seeds, andbiochemical research. It has been recognized that amino and car-boxyl functional groups of amino acids undergo chemical transfor-mations while the side chain remains intact. Amino acids witharomatic side chain were oxidized more rapidly than the alkyl sidechain amino acids. Amino acids also play an important role in deter-mining the morphologies of the noble metal nanocomposites.Gold–silver nanocomposites of different shapes from gold nanorodsin aqueous solutions were prepared [9] (depending on the value ofpH, I-, dumbbell-, and sphere-shaped gold–silver nanocompositesin the presence of glycine, glutamic acid, glutamine and arginine,corn-shaped nanocomposites in methionine, pearl-necklace shapedin lysine). Various researchers have investigated the ability of acidicand basic side chain amino acids (aspartic acid, lysine, tyrosine, andtryptophan) to act as reducing and stabilizing agents for the synthe-sis of gold nanostructures at room temperature [10]. Despite theextensive studies carried out on the preparation and characteriza-tion of gold nanoparticles and their composites using peptides
102 Z. Khan et al. / Journal of Colloid and Interface Science 367 (2012) 101–108
and amino acids, the use for silver in a similar study is rare [11]. Tothe best of our knowledge, no such investigation has so far been re-ported for the reduction of Ag+ ions by tyrosine.
Murphy and her coworkers prepared the silver nanoparticleshaving different morphology (nanorods, nanowires and short nano-rods) in the presence cetyltrimethylammoniun bromide and NaOH.In the absence of CTAB, spheroidal nanorods were unstable and re-verted to spheres within 10 min. In the absence of seed, silver ionreduction by ascorbic acid in the presence of CTAB yielded only afew rods, which varied in the aspect ratio [12a]. These researchersalso prepared spheroidal and rod-like gold nanoparticles througha seed-mediated growth in the presence of rod-shaped micelle ofCTAB [12b]. The role of micellar template is to direct the particlegrowth along one dimension and finally stabilize the short nano-rods [12c]. Sau and Murphy [12d] described a solution-based chem-ical route to the production of a number of gold nanoparticles (fromrod-, rectangle-, hexagon-, cube-, triangle-, and star-like outlines tobranch) at room temperature in the presence of a single surfactant,CTAB in aqueous solution. Various investigators too (Chen et al.[13], Kuo and Huang [14], Chen et al. [15] and Bakshi [16]) reportedthe preparation of multi-branched and/or multi-pods (monopods,bipods, tripods, and tetrapods) metallic gold nanoparticles byseed-growth synthesis using surfactant (anionic, cationic, andGemini cationic) as a capping and/or shape-directing agents. In allsyntheses of branched metal nanostructures, a surfactant, espe-cially CTAB, is required as a shape-directing agent by preferentialadsorption on specific crystal planes.
Recently, Eastoe and his coworkers [17] reported a new kineticmodel for two-step surfactant (a single chain hydrophilic surfactant,dodecyltrimethylammonium bromide in water, and the double-chain oleophilic surfactant, didodecyldimethylammonium bromidein low-dielectric organic solvents) adsorption onto solid surfacesfrom solution, which incorporates both surfactant diffusion andattachment terms. Therefore, the choice of an appropriate surfactantis highly important to obtain metallic nanoparticles. For lack ofinformation on similar studies using Ag+ ions, we report herein theresults of the title investigation using tyrosine (4-hydroxyphenyl-alanine, a non-essential amino acid with a polar side group, involvedin many protein oxidations, being one of the most easily oxidizedamino acids). Molecular products of tyrosine oxidation includedityrosyl units, which are involved in protein cross-linking and dop-aquinone units as the substrate in the presence of cetyltrimethylam-monium bromide for the first time. This paper should stimulatefurther applications of CTAB and tyrosine in engineering nanome-ter-sized particles, contributing to understanding of shape-controlled synthesis of metal nanoparticles for desired functionalproperties.
2. Experimental section
2.1. Materials
Silver nitrate (AgNO3), tyrosine (HOPhCH2CH(NH2)COOH),sodium hydroxide (NaOH), hydrochloric acid (HCl), and cetyltri-methylammonium bromide were obtained from Aldrich and usedas received. Ultrapure water (double distilled, first time fromalkaline permanganate) was used for all aqueous preparations. Allglassware was washed by ultrasonication in a mixture of ultrapurewater and non-ionic detergents and dried prior to use. Tyrosine isslightly soluble in water, solubility increases in hydrochloric acidand in sodium hydroxide solutions at 25 �C. Therefore, stock solu-tions of tyrosine (0.01 mol dm�3) were prepared by adding therequired amount in 25 ml of double distilled water and dilutedwith 0.01 mol dm�3 HCl and NaOH solutions to the required vol-ume and stored overnight.
2.2. Preparation of silver sols
In the first set of experiments, a solution of silver nitrate(10 cm3, 0.01 mol dm�3) was added to a series of tyrosine–HClsolutions (5–20 cm3, 0.01 mold m�3) and CTAB (2.5 cm3,0.01 mol dm�3, total vol. 25 cm3) at 25 �C. The most interestingfeatures of the present observations were that the reaction mixturebecame turbid, and a white precipitate appeared immediately. Theturbidity increased as the [tyrosine] increased at constant [CTAB].Appearance of turbidity may be due to the insoluble AgCl forma-tion in the presence of HCl. In the second set of experiments, thesame volumes of tyrosine–NaOH solutions were added to a solu-tion containing same volumes of silver nitrate and CTAB solutions.A perfect transparent color (pale yellow, yellow, brownish yellow,gray and wine red) appeared as the reaction proceeded (instead ofwhite turbidity) under different experimental conditions, i.e.,[CTAB], [tyrosine], [Ag+], and reaction time, suggesting the forma-tion of silver nanoparticles. Therefore, all the experiments wereperformed with the tyrosine–NaOH solution. Preliminary observa-tions showed that a 1:2 (tyrosine: Ag+ ions) stoichiometric ratiowas suitable to the formation of prefect transparent pale yellow-colored silver sols (vide infra).
2.3. Characterization of silver nanoparticles
UV–visible spectroscopy is one of the best widely used tech-niques for the structural characterization of silver nanoparticles[35]. The formations of the silver nanoparticles were monitoredby using a UV/vis spectrophotometer (UV-260 Shimadzu, with1 cm quartz cuvettes) to follow the appearance of the surface res-onance plasmon (SRP) band in the vicinity of 325–550 nm, charac-teristic of silver nanoparticles having different morphologies (size,shape, and the size distributions) [2a,2b,11,18]. Scanning electronmicroscope (SEM) (QUANTA FEG 450, FEI Company, Eindhoven,The Netherlands) was used to determine the morphologies of thesilver particles. Samples were prepared by placing a drop ofworking solution on a carbon-coated standard copper grid (300mesh) operating at 80 kV. An Accumet, Fisher Scientific digitalpH meter 910 fitted with a combination electrode was used forpH measurements.
3. Results and discussion
3.1. Effect of CTAB concentrations on the position of SRP band
The choice of an appropriate stabilizer (surfactant, solid matrix,ligand, and polymer), especially surfactant, is highly important toobtain monodisperse nanoparticles and not all surfactants provide1D nanostructures. A closely packed surfactant film on {100} or{110} crystal planes completely passivates these planes from fur-ther nucleation and hence directs the crystal growth at {111} crystalplanes. This can be achieved with a surfactant having unbranchedlong hydrocarbon tail and fewer bulky head groups [12–16,19,20].Strongly hydrophobic cationic double-tail Gemini surfactants andlipids are the best shape-directing and better capping agents. In or-der to the see the role of cationic CTAB surfactant, a series of exper-iments were carried out under different [CTAB] (range: from1.0 � 10�4 mol dm�3 to 40.0 � 10�4 mol dm�3) at fixed [Ag+](40.0 � 10�4 mol dm�3) and [tyrosine] (20.0 � 10�4 mol dm�3) at23 �C. Visual observation showed that the typical color of silvernanoparticles changed from pale yellow, yellow, orange, light red,wine-red to purple-red, which indicated the nanoparticle morphol-ogy altering with [CTAB] at different reaction stage and wasconfirmed by UV–visible spectroscopy (Table 1). The observed re-sults (spectra at different time intervals) are depicted graphically
Table 1Effects of [CTAB] on the color and position of silver nanoparticles.a
Z. Khan et al. / Journal of Colloid and Interface Science 367 (2012) 101–108 103
in Figs. 1–4 for the different [CTAB]. Inspection of these dataindicates that resulting violet color shows a broad-band absorptionwith a band maximum at around 450 nm at [CTAB] = 1.0� 10�4 mol dm�3 (Fig. 1; inset), which is attributed to the character-istic SRP excitation of spherical nanoparticles [21]. However, thespectrum of the silver nanoparticles prepared at [CTAB] P 2.0� 10�4 mol dm�3 shows the appearance of different color at differ-ent time intervals with a broad absorption at 400–475 SRP band(Figs. 1–3), indicating that large silver nanoparticles began to formand also suggesting that during this stage, the silver nanoparticleshad a wide size distribution [22]. When the reaction time proceededfor P3–4 h, the color changed from yellow, wine red to violet, andthe absorption spectrum (Figs. 1 and 2) also exhibited a small SRPband at 400 nm along with a broad shoulder at the higher wave-length region (ca. 550 nm), which remained unchanged in terms of
peak position and its nature with [CTAB], which could be due tothe presence very small particles. The UV–visible absorption spec-trum of the silver nanoparticles prepared with [CTAB] = 14.0� 10�4 mol dm�3 shows a broad band at around 425–475 nm. Inter-estingly, it was observed that the position of the band shifted withtime (red shift of total 50 nm within 4 h of the reaction time; winered color) and a sharp peak developed at 475 nm (the shape of theabsorption band gradually became sharper with a wide bandwidthand exhibited a red shift with [CTAB], Table 1). The presence of var-ious absorption bands indicates the existence of AgNP of variousshapes, sizes with wide distribution [23]. With an increase in [CTAB],the peak at 400 nm is shifted to a higher wavelength. The shift of theplasmon peak to higher wavelengths may happen due to variousreasons.
It is interesting to note that when the CTAB concentration isP20.0 � 10�4 mol dm�3, a broad shoulder begins to develop in thewhole visible region (instead of a peak Figs. 4). The shape of the spec-tra entirely changed at higher [CTAB]. It was also observedthat at higher [CTAB] P 40.0 � 10�4 mol dm�3, the reaction mixture
104 Z. Khan et al. / Journal of Colloid and Interface Science 367 (2012) 101–108
turned milky. This is due to the dilution effect of CTAB micelles. Asshown in all figures (except Fig. 1; inset), the intensity of the SRPband increased with the increase in reaction time, which indicatedthe continued reduction (nucleation, growth, association/adsorp-tion, and stabilization) of the Ag+ ions with tyrosine. The digitalimages of the reaction mixture containing different [CTAB] areshown in Fig. 5 as a function of time, indicating that the typical colorof silver nanoparticles changed from pale yellow, yellow, orange,light red, wine red to purple-red with time, and there is a red shiftand the intensity and shape of the spectra changed with the in-creased [CTAB] (Figs. 2–4). Thus, we may safely conclude that thesize of silver nanoparticles can be controlled not only by the CTABconcentrations but also by the reaction-time alteration. The appear-ance and/or change of color strongly depends on the [CTAB] and de-layed at the higher [CTAB] (Fig. 5) (initially, different color observedbut after some time, all reaction mixtures showed the same color,which is due to the regular aggregation). The position and shape ofthe SRP absorption depend on the particles size, shape, and dielectricconstant of the surrounding medium and surface-adsorbed species.The broadening and red shifting of the absorption band with [CTAB]indicate that initially reduced AgNP grows to form larger particles,and finally, CTAB acts as a shape-directing agent. The above explana-tions are in good agreement with the hypothesis highly, developedby the Murphy et al. [24], Nikoobakht and El Sayed [25], Bakshiand his coworkers [19,20], on the shape-directing role of cationicstrongly hydrophobic double and/or mono tail Gemini and normalsurfactants.
Addition of a small quantity of CTAB gives a pronounced effect onthe path of silver nanoparticles, a concentration of P1.0 �10�4 mol dm�3 being enough to stabilize the silver nanoparticles.Figs. 1–3 demonstrate the CTAB effects not only above but even be-low the cmc; i.e., submicellar and micellar effects are observed. Thecritical micellar concentrations, cmc, of CTAB were determined con-ductometrically at 23 �C, in the presence of AgNO3 and tyrosine un-der different experimental conditions. The cmc values were found tobe 10.2, 9.5 and 8.8 � 10�4 mol dm�3, respectively, for water, Ag+
(40.0 � 10�4 mol dm�3), and tyrosine (20.0 � 10�4 mol dm�3), indi-cating that the submicellar aggregates are formed between CTABand tyrosine [26]. The pre- and post-micellar effects on the growth,stability, and color of silver nanoparticles can be rationalized by con-sidering the distribution and/or solubilization of the reactantsamong the different pseudo-phases present in the reaction medium.
The cmc is lower than in water, suggesting that the active species oftyrosine and/or growing small nanoparticles interact with a positivehead group of CTAB aggregates (monomers, dimers, trimers, etc.).
Aqueous surfactant solution has three following components:surfactant monomers, micellar aggregates, and monomers ab-sorbed as a film at the interface. The surfactant is in dynamic equi-librium among all these components. Surfactant monomers rapidlyjoin and leave micelles, and the aggregation number representsonly an average over time. The distribution of surfactant (D1) be-tween various states of aggregation is controlled by a series of dy-namic association–dissociation equilibria:
D1 þ D1�D2 ð1Þ
D1 þ D2�D3 ð2Þ
Dn�1 þ D1�Dn ð3Þ
The role of CTAB can be sought in the fact that small aggregatesof the CTAB exist below the cmc; these small submicellar aggre-gates can interact physically with the tyrosine forming activeentities [27,28]. The effectiveness of such interaction, therefore,
Z. Khan et al. / Journal of Colloid and Interface Science 367 (2012) 101–108 105
depends on the capping of aggregated CTAB layer around the reac-tants, which governs the ease through which micellar channelscommunicate.
To see whether the Br- counter ion are capable of the reactingwith Ag+ ions under our experimental conditions, some experimentswere also performed in the absence of tyrosine (Ag+ + CTAB). Inter-estingly, we did not observe the formation of AgBr (yellow precipi-tate) with increasing [CTAB] from 4.0 � 10�4 to 28.0 �10�4 mol dm�3. Furthermore, bromide ion was also added in a solu-tion of silver ions and CTAB in the form of NaBr ([2.0 �10�4 mol dm�3]). We did not observe the formation of any type ofprecipitate, suggesting that the formation of AgBr(s) was not in-volved even as a side reaction in the present redox system with CTABmicellar media.
Murphy and her coworkers synthesized gold nanorods by aseed-mediated sequential growth process involving the use of cat-ionic surfactant cetyltrimethylammonuim bromide as a shape-directing agent [24a] and suggested that structural and chemicalfactors play an important role in determining the preferentialinteractions between the cationic quaternary ammonium head-groups and growth sites on the side edges and faces (surfactant-containing complexes, [AuBrCTA]+ as well as those involvingAu(I) species, are specifically incorporated into the {100} sideedges, whereas non-complexed ion pairs or Au(0) atoms/clustersare added to the {111}, end faces [24b]).
On the basis of the above description, the possible causes of thedifferent shape of each spectrum may be discussed. The presenceof lone pairs of electrons on hydroxyl and amino groups of tyrosinemust be considered (vide infra). It is certainly possible that tyrosineforms an ion pair with the positive head group of CTAB molecules,which brings a large hydrophobic cetyltrimethyl group in thevicinity of Ag+ ions.
Fig. 6. SEM images of silver nanoparticles at different [CTAB]. Reaction conditions: [tyrosiand 20.0 � 10�4 mol dm�3 (C), temp. = 23 �C.
In fact, there are several processes likely to coexist as follows:(i) the solubilization of reactants, (ii) formation of AgNP (nucle-ation and growth), (iii) adsorption of CTAB onto the surface ofAgNP, (iv) aggregation of Ag particles, and (v) precipitation of theaggregates. For submicellar effects, below the cmc (Figs. 1, 2 and5(1)), the color changes were very fast, probably because mono-mers and dimers of CTAB adsorb more readily with Ag particles.For micellar effects, above the cmc (Fig. 5(2)), the color appearsafter some time, indicating that all processes are rapid, includingprecipitation. Presumably, nucleation and growth (step ii) becomerate limiting due to the non-availability of Ag+ ions in the Sternlayer due to electrostatic repulsion between the CTAB head groupand Ag+ ions. On the other hand, the appearance of color is slow,implying that there may be a barrier to CTAB adsorption onto thesurface of AgNP. The slow adsorption process appears to correlatewith sub- and post-micellar aggregates, and could be related tosurface rearrangement/aggregation, or a lower population ofmonomeric surfactant available for adsorption [17].
According to Mie theory, small single nanocrystals andanisotropic particles should exhibit single- and multiple-surfaceplasmon bands depending on their shape [29]. The larger metalparticles can also exhibit more bands due to the excitation ofquadrupole and higher multipode plasmon excitations [30].Surprisingly, our results (Figs. 1–4) show distinct dipole andquadrupole plasmon resonances. A small peak at ca. 400 nmcan be attributed to the out-of-plane dipole resonance of smallspherical nanoparticles. The shoulder at 450 nm can be attrib-uted to the transverse plasmon resonance and/or multiplasmonexcitation of faceted and anisotropic particles [31] and at475 nm due to longitudinal peak, indicating the possibility of for-mation of nanorods in the system [32]. The SRP spectra alsoshow the possibility of the presence of silver nanostructure of
106 Z. Khan et al. / Journal of Colloid and Interface Science 367 (2012) 101–108
various sizes and shapes. In the dipole approximation, theextinction coefficient, j, and absorbance, A, are determined bythe following equations:
K ¼ 18pNVe3=2m
k� e2
ðe1 þ 2emÞ2 þ e22
ð4Þ
A ¼ j � l2:303
ð5Þ
where j = extinction coefficient, N = number of atoms in the cluster,V = volume, k = wavelength of an absorbing radiation wave,em = complex dielectric function of the cluster in the medium, e1
and e2 = real and imaginary parts of the dielectric functions of themetal material, A = absorbance, and l = path length.
For a single SRP band, the absorption peak position should besize independent within the dipole approximation [33]. Our re-sults clearly indicate the size effect on the SRP band (Figs. 1–4)which may be due to the intrinsic and extrinsic effects [34] andare in good agreement with the observations of other investiga-tors to the size effect on the SRP band [35,36]. According to theEqs. (4) and (5), the absorbance is directly proportional to the con-centration, N/V of the N nanoparticles. The increasing absorbanceand intensity of color with reaction-time in Figs. 1–5 and 7indicate that the concentration of AgNP is increased. Higherconcentration of AgNP may also lead to the red shift of SRP band.To confirm the morphologies of the particles, SEM analysis wascarried out.
Table 2Effects of [tyrosine on the color and position of silver nanoparticles.a.
104 [tyrosine] (mol dm�3) Reaction time (h) Color
0.0 1 Clear s4
20.0 1 Yellow4 Yellow
40.0 1 Yellow4 Yellow
60.0 1 Yellow4 Yellow
a [Ag+] = 40.0 � 10�4 mol dm�3; [CTAB] = 20.0 � 10�4 mol dm�3; tem
3.2. Size and shape by SEM images
Typical SEM images for solutions containing 4.0 � 10�4,14.0 � 10�4 and 20.0 � 10�4 mol dm�3 CTAB are shown in Fig. 6.The images show the presence of AgNP of different shapes andsizes. The size of particle is ranging from less than 8.8 to 16.3 nmand has a variety of shapes: sphere, few rod-like, and irregular withbroader size distribution (Fig. 6A). The small particles aggregate toform larger nanostructures (particle size from 35 to 53 nm; Fig. 6B)of different shapes, mainly triangle and polygons with sphericalAgNP. In addition to spherical, nanorod, triangle, and some irregu-larly shaped particles are also observed in Fig. 6C, which may because multiplasmon excitation and hence unexpected broadeningof SRP width at higher [CTAB] (Fig. 4) [32]. From Fig. 6 aggregation,size and polydispersity of particles were found to be increased anddecreased, respectively, with increasing [CTAB]. The SEM analysiscorroborates well with the results drawn from corresponding SRPspectra (Figs. 1–4). Esumi et al. [37] prepared gold nanorods inthe bulk aqueous phase of normal micelles of cetyltrimethylam-monium chloride, CTAC. Pileni [38] suggested that the chlorideions from CTAC could play the role in the nanocrystal growth,and anisotropic nanocrystal formation is more related to selectiveadsorption of ions on different faces of the crystals during thegrowth than to the nature of the micelles. Thus, we may safely con-clude that the morphology of the AgNP changes spherical to aniso-tropic structures followed by an increase in size with an increase in[CTAB], shape-directing properties of cationic surfactant areresponsible to the non-specific growth [12,16,19,20,24] and thesurfactant concentration does not act as a true template for thegrowth of AgNP of definite, shaped and sized [39].
3.3. Effect of tyrosine concentrations on the position of SRP band
It is well known that amino, carboxyl, and side chains of aminoacids undergo chemical transformations in the presence of differentkinds of meal ion oxidants. Amino acids with aromatic side chainwere oxidized more rapidly than their alkyl counterparts. In orderto explain the role of side chain of tyrosine, a series of experimentswere carried out at constant [Ag+] (20.0 � 10�4 mol dm�3), [CTAB](10.0 � 10�4 mol dm�3) and temperature (23 �C) in the presenceof different amounts of tyrosine (range, 10.0 � 10�4 to 60.0 �10�4 mol dm�3). The observed results are summarized in Table 2.The absorption spectra (Fig. 7) of silver colloids show the presenceof broad peak and shoulder at around 410 and 500 nm, respectively,after 4 h with [tyrosine] = 20.0 � 10�4 mol dm�3. The most charac-teristic part of the silver sol is the SRP band observable at ca. 400 nm[39]. The appearance of a broad shoulder at longer wavelength(bathochromic shift) gives information about the increase in size,aggregation, polydispersity, and the size distribution of the silvernanoparticles [40]. At lower and higher concentrations of tyrosine(5.0 � 10�4
6 [tyrosine] P 70.0 � 10�4 mol dm�3), we did not ob-serve the appearance of prefect, transparent, orange-red-colored
Fig. 8. Reaction-time plots for the formation of silver nanoparticles. Reactionconditions: [Ag+] = 40.0 � 10�4 mol dm�3, [CTAB] = 10.0 � 10�4 mol dm�3,temp. = 23 �C.
Z. Khan et al. / Journal of Colloid and Interface Science 367 (2012) 101–108 107
silver sol. As can be seen in Fig. 8, plots of absorbance versus timeclearly indicate that the formation of silver sol has an inductionperiod (nucleation) followed by autocatalysis (growth). The mostimportant and interesting findings of the present observations arethe decrease in the absorbance of silver nanoparticles at higher[tyrosine] concentration (Fig. 8; (J)). This may be due to theadsorption of tyrosine onto the surface of metallic silver particles,which, in turn, increases the Fermi level of particles [18]. (The neu-tral nucleophiles and neutral stabilizing polymers have strongeffect on the plasmon absorption band of silver and/or metal nano-meter particles and donate the electron density to the particles vialone pairs of electrons.)
Tyrosine has three pH sensitive reactions and/or coordinationsites, namely –OH, –COOH, and –NH2. Due to its low solubility,working solutions were prepared in NaOH solution; pH of tyrosinestock solution is 7.8. In order to establish the reactive species of tyro-sine, pH of the reaction mixture containing different concentrationsof tyrosine and/or CTAB was also measured. Interestingly, pH valueswere found to be 7.8 ± 0.4 at [tyrosine] values of 20.0, 40.0, and60.0 � 10�4 mol dm�3 in 10.0 mol dm�3 CTAB. The same resultswere also observed with [CTAB] varying from 8.0 to 14.0 �10�4 mol dm�3 in 40.0 � 10�4 mol dm�3 [tyrosine]. It is not surpris-ing because tyrosine is a weak acid (pKa = 2.2 (COOH); pKb = 9.1(NH3); pKR = 10.0 (OH)). Tyrosine exists in cationic, zwitterionic,and anionic forms in aqueous solution. The concentration of thesespecies strongly depends on the pH of the working reaction mixture.We did not detect the formation of CO2 and ammonium ions as theoxidation products of tyrosine. Under our experimental conditions,zwitterionic species is a major and reactive species of tyrosine,which transfers the proton of phenolic –OH groups to
-CH2CH(NH3+)COO-HO-
(zwitter ionic)
the Ag+ ions [3,6c,10f].Electrostatic, hydrophobic, and hydrogen bonding interactions
seem to play an important role in the solubilization reactants intothe Stern and/or palisade layer of micellar pseudo-phase. Tyrosinegets incorporated hydrophobically into the reaction site. Formationof an ion pair between the –COO� and positive head group of cat-ionic micelles cannot be ruled out completely. It can now be stated
confidently that the electron transfer between tyrosine and Ag+
ions occurs in the Stern layer of CTAB micelles. As a result, differentshape, size, and colored silver nanoparticles were formed (Figs. 3and 5). As the [CTAB] increases, solubilization of tyrosine increasesinto the Stern and/or palisade layer through hydrophobic and elec-trostatic interactions, but, with a fixed total [Ag+], dilution in themicellar pseudo-phase decreases the absorbance and completelydisappeared into the position of SRP band (Fig. 4) [28]. Anisotropyof AgNP may also be due to a difference in the adsorption of tyro-sine and/or CTAB on different faces of the crystals [18,32,38].
3.4. Comparison with related reductant
In order to compare the reactivity of tyrosine, a series of experi-ments were also performed for the reduction of Ag+ ions by alanineand phenylalanine. Under our experimental conditions, [Ag+](20.0 � 10�4 mol dm�3), [CTAB] (10.0 � 10�4 mol dm�3), [alanineand/or phenylalanine] (20.0 � 10�4 mol dm�3), and temperature(23 �C), formation of any color was not observed for 2 h, whereasthe appearance of pale yellow color is very fast with tyrosine(Figs. 7 and 8). Tyrosine has a strong driving force to eject the pheno-lic proton [41]. Thus, the presence of –OH group is responsible for thehigher reactivity of tyrosine, which transfers the proton to Ag+ lead-ing to the formation of the stable prefect transparent silver sols. Wehave also investigated the same effect of –OH group on the reactivityof tyrosine, paracetamol, and acetanilide toward MnO4
- [42,43].
4. Conclusions
In this article, we have successively demonstrated that simplesurfactant like CTAB can also be used for directing the shape andsize of the silver nanoparticles with a suitable reducing agent (tyro-sine). The shape and size of the nanoparticles are depended not onlyon the [CTAB], but also on the time. Pre- and post-micellization alsoplays an important role in the preparation of the following differentcolored silver sols: pale yellow, yellow, and orange, light red, winered to purple red. Our results suggest that the preparation ofnanomaterials having a range of optical properties might be achiev-able through careful control over the concentration of the stabilizerin solution when using suitable reducing agents. The desired mor-phology of silver nanoparticles would be achieved by using suitable[CTAB], which acts as an excellent template to regulate the nano-particle growths. These observations, shape-directing role of stabi-lizers, have also been noted previously by the various investigatorsfor polypeptides [8] and surfactants (anionic, cationic, and cationicGemini) [12–17,24].
Acknowledgment
Financial support to the authors by Deanship of ScientificResearch (MS/12/176), King Abdul Aziz University, is gratefullyacknowledged.
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