Facile Synthesis of Silver Nanoparticles Using a Novel Benzenethiol Derivative: Addition Effect of Cationic Surfactants

We propose a new type of preparation method of silver nanoparticles (AgNps) by using a triazine benzenethiol derivative such as (4,6-dimethoxy-1,3,5-triazin-2-yl)-4-mercaptobenzoate (TBSH) as a reductant in the presence of a cationic surfactant such as cetyltrimethylammonium chloride (CTAC) or cetyltrimethylammonium bromide (CTAB). The formation amount and stability of AgNps in the presence of CTAC is compared with those in CTAB system; the addition of CTAC play important roles for the formation of AgNps. The optical properties of as prepared AgNps were characterized by using UV-vis, FTIR, and X-ray photoelectron (XPS). Transmission electron microscopy (TEM) images of as-prepared AgNps distinctly show the morphology of AgNps with average size of 30 nm. CTAC-concentration dependence of FTIR spectra indicates that the combination of rigid and compact network of TBSH and CTAC capped on AgNps significantly affects the size and shape of AgNps and also exhibit the presence of enhanced gauche conformers below the critical micellar concentration (cmc ∼ 0.6 mM) and trans conformers at cmc and above cmc of CTAC. Based on the present results, we propose a model of molecular structure and stability of AgNps. [DOI: 10.1380/ejssnt.2015.440]


INTRODUCTION
In the research field of nanomaterials, the recent exponential growth of "nano silver" or colloidal silver nanoparticles is due to their distinct optical, electrical, magnetic and fascinating catalytic properties [1][2][3].In particular, AgNps have been successfully used in many fields such as catalysis, sensing, nanophotonics, optoelectronics, biosensing and molecular diagnostics [4][5][6][7][8][9][10].Advanced research led to their simple, facile and high performance fabrication with manipulated morphology and size [11][12][13].Substantial research is underway to achieve monodispersity and stability of silver nanoparticles without perturbing the desired properties [14,15].Alkane thiols are widely explored as reductive stabilizer of metal nanoparticles [16][17][18].However, much attention has not been paid to the formation of AgNps using short-chain benzene thiol derivatives.The presence of short methylene chains and symmetrical distances, electrons moves faster through delocalized system than saturated system and conjugated molecules are rigid compared with saturated flexible molecules.These unique properties of TBSH attributed to relative ease of ligands (SH) bounded with the nanoparticles surface within short span of time and with superior stability by compact and rigid network.TBSH is synthesized readily and obtained in good yield.For synthesis of TBSH, we used DMT-MM which is commercially available.We are able to investigate novel functions by using compounds which possess chromophores and molecular recognition sites instead of methoxy group for further applications.
From this view point, we incorporated 1,3,5-triazine moiety with mercaptobenzoate as a thiolphenol derivative.Triazine unit is a remarkable unit which interacts with other organic moiety through coordination bond, hydrogen bond, electrostatic charge transfer and aro-matic stacking with applications in host guest chemistry, catalysis, anion recognition, sensors, electronics and magnetism [19].To increase the stabilization, aggregation of nanoparticles can be avoided by the addition of cationic or anionic surfactants, such as CTAC, CTAB and SDS [20][21][22].The surfactant coating on nanoparticles changes their aggregation behavior due to interparticle potential.Therefore, different types of surfactants, depending upon their molecular structures, may tune the interparticle interactions to different extent and hence have distinctive tendency to prevent the nanoparticle aggregation [23].The most interesting aspect of this present study is to investigate the unique functionality formed by the TBSH and cationic surfactant network as combine stabilizer capped on the nanoparticles.The effect of surfactant on nanoparticles would occupy the nanoparticle growth site, thus reducing the growth rate of nanoparticles or a full coverage of stabilizer.
Adding a new dimensions in nano material chemistry, we developed the striking route to synthesize silver nanoparticles by using a new type of trainzine benzene thiol (4,6-dimethoxy-1,3,5-triazin-2-yl)-4-mercaptobenzoate (TBSH) as reductive stabilizer in combination with cationic surfactant.Moreover, we compared the role of two surfactants, such as cetyltrimethylammonium chloride (CTAC) and cetyltrimethylammonium chloride (CTAB), having same hydrocarbon chain length but head groups containing different halide ions.
In case of CTAB having bromide as a counter ion, the micelles are much bigger in size as compared to their counterpart CTAC micelles.Based on the present results, we propose a model of molecular structure and stability of silver nanoparticle (AgNps).

II. EXPERIMENTAL
All starting materials of analytical grade were commercially available (Wako Purer Chemistry Industries Ltd.) and were used without further purification.Double distilled water was used throughout the experiments.
The AgNps solution was prepared by the following procedures.10 ml of aqueous CTAB (1.3 mM) or CTAC (0.3-2.0 mM) solution, adjusted to pH 10 by using Na 2 CO 3 (50 mM) under stirring.To this solution AgNO 3 (50 mM, 80 µL) was added.After that TBSH (2.5 mM, 50 µL in THF) was added and then the sample solutions containing CTAB were irradiated for 7 hours and those containing CTAC were irradiated for 5 minutes by Hg-lamp.Absorption and FTIR spectra were recorded on a Hitachi U-2900 spectrophotometer and a Perkin Elmer (Spectrum One-B) Multiscope FTIR spectrophotometer, respectively.Hg lamp used was UVL-400HA, RIKO medium pressure lamp.X-ray photoelectron spectroscopy (XPS) was obtained on an ESCA-1000 Shimadzu electron spectrometer.Transmission electron microscopic (TEM) images were recorded on a JEOL JEM-3010 VII operating at 300 kV.A drop of dilute solution containing a suspension of as prepared AgNps was placed on a copper grid with a carbon foil and the solvent evaporated under reduced pressure at room temperature in an incubator for one week.Approximately 200 particles were counted and then size distribution of AgNps was determined.

III. RESULTS AND DISCUSSIONS
Figure 1 shows the UV-vis absorption spectra of TBSH-Ag-Surfactant systems after UV irradiation.The formation of AgNps was attributed to the reducing capacity of the terminal thiol groups of TBSH under UV irradiation.We investigated the role of two cationic surfactants systems, cetyltrimethylammonium bromide (CTAB) and cetyltrimethylammonium chloride (CTAC), having the same hydrocarbon chain length of C18 but different head groups due to the presence of different counter ions (CTAB-Br − and CTAC-Cl − counter ions).The spectra show a maximum absorbance at 419 and 430 nm for  CTAC and CTAB assignable to be characteristic of silver surface plasmon resonance (SPR), respectively.Both CTAB and CTAC have different cmc values in the same system, for uniform comparison, above cmc (1.3 mM) were used at alkaline pH 10.The pH of the reaction medium plays a key role.After doing several trials it was found that acidic pH suppresses the silver nanoparticle formation due to over nucleation and within short span of time resulted in fast agglomeration and large size of nanoparticles.Whereas, at alkaline pH (9 or 10) the nucleation process is moderate and resulted in smaller size of silver nanoparticles with higher stability.When CTAB was used as a capping agent, the reaction mixture turned from transparent to turbid with the addition of AgNO 3 and TBSH.After 7 hours of UV irradiation the color of the turbid sol changed to blackish yellow and no clear SPR band were found (Fig. 1(A)).This change could be ascribed to the formation of large quantity of silver bromide (AgBr) from AgNO 3 (Ag + ) and CTAB (Br − ).In this system, CTAB did not work well as a capping agent, due to the formation of large quantity of water insoluble AgBr precipitates, hindered the reduction of Ag + ions to Ag 0 by TBSH.This is also supported by the fact that the solubility of AgBr in CTAB (described by the constant of solubility equilibrium, K sp = 7.7×10 −13 ) is much lower than that of AgCl (K sp = 1.56 × 10 −10 ) in CTAC at 25 • C. In case of CTAC, the reaction mixture after addition of AgNO 3 and TBSH were transparent before UV irradiation after irradiation of 5 minutes turned to yellow with no turbidity (Fig. 1  ions, due to small size and low polarizibility always solvated by water molecules and more disperse, hence TBSH easily reduced Ag + ions to zero valent Ag 0 .The absorption spectra were followed as a function of time after UV irradiation (Fig. 2(B)).

(B)). CTAC counter chloride
The role of CTAC as an additional stabilizer in TBSH-AgNps-CTAC system were further investigated, at three different concentrations of below cmc, at cmc and above cmc.Figure 2(A) shows UV-visible spectra of TBSH-Ag-CTAC systems below cmc (0.3 mM), at cmc (0.6 mM), and above cmc (2.0 mM) of CTAC.After the UV irradiation of 5 minutes the reaction mixture exhibited yellow color due to excitation of surface plasmon resonance band in the visible absorption region.The spectra shows an initial peak at about 255 nm (Ag + ) for all samples.As the time increases the peak of Ag + , centered at 255 nm decreases with a corresponding increase in intensity of zero valent silver (Ag 0 ) with peaks of 407, 410 and 418 nm for below cmc (0.3 mM), at cmc (0.6 mM)and above cmc (2.0 mM) at pH = 10.The sharp absorption peak in this region corresponds to the signature peaks of AgNps.UV-visible absorption spectra explain the factor of timedependent formation of silver nanoparticles.As the time duration increased, the nanoparticles formation and in turn intensity also increased (Fig. 2(B)).
The growth of TBSH-CTAC stabilized AgNps was further confirmed by measuring TEM images (Fig. 3).More- over, the size and shape of as-prepared AgNps was also obtained by processing TEM images for TBSH-CTAC-AgNps system.TEM images of colloidal solution of silver nanoparticles exhibiting spherical shapes and polydisperse in nature.The average particle sizes of below cmc (0.5 mM), at cmc (0.6 mM) and above cmc (2.0 mM) of CTAC were 25.6 ± 6, 26.3 ± 5 nm, and 32.1 ± 6 nm, respectively (Fig. 3(A), (C), and (E)).The histograms were obtained by measuring the size of about 200 particles and their diameter was given in (Fig. 3(B), (D), and (F)).We measured average nanoparticle size against various CTAC concentrations.The average size of the AgNps was in the ranged from 25 to 35 nm and wider size distribution was determined to be 10-58 nm.
Furthermore, XPS spectroscopy was also used to probe the chemical bonding between the sulfur atom of TBSH and AgNps surface.The XPS spectrum (data not shown) in Ag (3d) region of TBSH-CTAC-Ag showed two peaks assignable to spin-orbit coupled energy states J = 5/2 and 3/2.The Ag (3d 5/2 ) and Ag (3d 3/2 ) appeared at 367.7 and 373.9 eV, respectively; this fact confirms the presence of Ag 0 .The XPS spectrum in s(2p 3/2 ) region showed one peak at 162.1 eV.These results are in agreement with the results published for short chain aromatic thiols assembled on silver nanoparticles [19].
FTIR spectroscopy was used to probe the conformational changes and lateral chain-chain interaction of stabilizers with AgNps surface.The formation of new bands and shift in existing bands confirms the presence of binding of thiols to the silver nanoparticles and adsorption of CTAC alkyl chains on the AgNp surface.The FTIR spectra were recorded in the spectral region of 3200-600 cm −1 (Fig. 4) and their frequency assignment was discussed.The spectra of the TBSH-AgNps-CTAC systems below, at and above cmc colloidal solutions are almost similar to one another, however, a slight difference in their peak intensities were found, indicating that the organic molecules have become a part of the silver nanoparticles.
In only TBSH the presence of SH stretching mode at 2549 cm −1 is absent in TBSH-AgNps-CTAC systems.This suggests that SH bond is broken upon binding to the AgNps surface.and carbonyl stretching (C = O) vibrations in TBSH is observed at 1541 cm −1 and 1683 cm −1 and its corresponding TBSH-AgNps-CTAC systems these signals shifted to 1559 cm −1 and 1742 cm −1 , respectively (Fig. 4).The resonance at 2918 and 2850 cm −1 of FTIR spectrum in TBSH and CTAC system corresponds to the characteristic C-H symmetric and asymmetric stretching modes of methylene groups in TBSH and alkyl chain of CTAC.Figure. 4 clearly shown that these intensities shifted to higher frequencies for below cmc (0.3 mM) and lower frequencies at cmc (0.6 mM) and above cmc (1.6-2.0 mM) in TBSH-AgNps-CTAC systems.These peaks observed at 1487, 1464, 961 and 728 cm −1 are assigned to symmetric, asymmetric, stretching, and rocking mode of head group of CTAC (CH 3 − N + ), respectively.
It has been well known that the vibrational modes of symmetric ν s (CH 2 ) and asymmetric ν as (CH 2 ) vibrations are sensitive to the gauche/trans conformer ratio of CTAC capped on AgNps.Figure 5 shows dependence of both vibrational bands of CH 2 stretching symmetric and asymmetric vibrations and provide information about the degree of characteristic conformational order to disorder or gauche to trans conformers ratio in the methylene chain structure in TBSH-AgNps-CTAC systems.It is known that conformational ordered state (trans) is characterized by a symmetric and asymmetric CH 2 stretching mode frequency below 2919 and 2850 cm −1 , while the conformational disorder (gauche) results in a rising of these frequencies by several wave numbers.The observed ν s (CH 2 ) stretching vibration modes below cmc is (2921-2926) cm −1 and for ν as (CH 2 ) at (2851-2856) cm −1 suggesting a higher population of gauche conformers and poor chain packing around AgNps.These frequencies shifted to lower frequency for ν s (CH 2 ) at cmc and above cmc (2920-2916) cm −1 and ν as (CH 2 ) at (2850-2849) cm −1 , confirming the presence of all trans conformers and ordered alkyl chain packing around AgNps.
The FTIR, TEM and XPS analysis indicated the formation process of AgNps with TBSH and CTAC.Finally we can propose (Fig. 6) a possible mechanism and structure of the TBSH-CTAC-AgNps system.Because of the strong affinity of thiols with the AgNps.It immediately formed a strong covalent bond with Ag.The structural integrity of the TBSH and CTAC capped the AgNps maintained.CTAC alkyl chains at cmc and above cmc of CTAC is not significantly perturbed because of the formation of micelles so enhanced trans conformers are present and fairly good chain packing around AgNps surface and present as solid-like state, but below cmc of CTAC there is less extension in the frequencies of long hydrocarbon tail.CTAC below cmc is present as an electrolyte and as liquid-like state.

IV. CONCLUSIONS
We have developed a simple and novel synthetic method of AgNps using TBSH thiol derivative in the CTAC surfactant.TBSH is remarkable powerful reductant for Ag + ions at alkaline pH.In this system, the shape and size distribution of AgNps strongly depended on the reduction potential of the reductant and pH of the reaction media.CTAC as an additional capping agent enhance the complexation and stabilization of TBSH-AgNps.This system is very useful for the preparation of stable AgNps in aqueous solutions.