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Colloids and Surfaces A: Physicochemical andEngineering Aspects
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ffects of solvents on the stability and morphology of CTAB-stabilized silveranoparticles
aheer Khanb,∗, Shaeel Ahmed AL-Thabaiti a, Abdullah Yousif Obaida, Ziya Ahmad Khana,bdulrahman O. Al-Youbia
Department of Chemistry, Faculty of Science, King Abdul Aziz University, P.O. Box 80203, Jeddah 21413, Saudi ArabiaDepartment of Chemistry, Jamia Millia Islamia, Jamia Nagar, New Delhi 110025, India
r t i c l e i n f o
rticle history:eceived 30 June 2011eceived in revised form2 September 2011ccepted 14 September 2011
a b s t r a c t
We report the effects of polar-protic and polar-aprotic solvents on absorption spectra and particle size ofsurfactant stabilized silver nanoparticles. Cysteine and cetylteimethylammonium bromide, CTAB wereused as the reducing and stabilizing agents, respectively. The prepared orange color silver sols possess anunusually narrow Plasmon absorption shoulder at 450 nm. The absorbance and shape of this shoulder areaffected by protic (methanol, ethanol) and aprotic (acetonitrile, DMF, DMSO and 1,4-dioxane) solvents.
The observed results are interpreted in terms of the dielectric constant, boiling point, hydrogen bonding,solubilization and donation of electron density from the silver particles to the solvents. Absorbanceincreases with increasing the dielectric constants of reaction mixture. The particle size decreases withdecreasing the hydrophobic character of protic solvents: particle size = 70, 58 and 39 nm in presence andabsence of solvents (ethanol and methanol), respectively.
olvents
. Introduction
Today, nanometal particles’, ranging from 1 to 100 nm, is anmerging area of nanoscience and nanotechnology. These nanopar-icles have drawn the attention of scientists because of theirumerous applications in the areas of electronics, material sci-nces and medicine at the nano-scale [1,2]. Noble metals likeold, silver, copper etc. have been widely used for synthesis ofanoparticles as these are useful in areas like photography, cataly-is, biological labeling, surface-enhanced Raman scattering (SERS)etection [3,4]. Metal nanoparticles show a surface Plasmon res-nance absorption band in the UV–vis region. The free electronsn the conduction band give arise to the surface plasma band dueo the small particle size of the nanoparticles [5,6]. The shifting ofhese bands give information on the particle size, chemical sur-ounding, adsorbed species on the surface, and dielectric constant7,8]. The properties and applications of nano-materials depend onheir morphologies; thus, the control of their sizes and shapes isssentially requirement, and many methods have been developedo synthesize nanoparticles of various morphologies. The simplest
nd most commonly used method for synthesis of nanoparticles ishe chemical reduction method [9–11].
Formation of long-lived clusters of silver by the chemicalmethod using sodium borohydride as a reducing agent in presenceof poly-anion has been the reported by Henglein and his co-workersfor the first time [12,13] and suggested that UV–vis spectroscopycan be applied to the characterization of metal and semiconductornanoparticles with plasmon resonance lines in the visible range.Henglein also prepared the colloidal silver particles by radiationmethod using 2-propanol, AgClO4 and polyphosphate as reduc-tant, oxidant and stabilizer, respectively, discussed the effects ofadsorbed additives, organic solvent, and some metal cations onthe stability of resulting silver particles. Khilar and his co-workerssuggested that the inter-micellar exchange rate, particle size andplasmon absorption band of silver nanoparticles have been changedby varying the organic solvent, surfactant and organic additives inreverse microemulsions of AOT [14].
Various reports has been published regarding the role of aminoacids, such as, glutamine, arginine, lysine, histidine, cysteine,methionine, tyrosine and ascorbic acid) as reducing, capping andstabilizing agent for the preparation of different shaped and sizedsilver and gold nanoparticles [15–18]. The interest in the natureof aggregation of molecules in solvents less polar than water or inmixed solvents (water + co solvents) is driven by both fundamentaland practical considerations [19]. Co solvents, such as, glycerol and
ethanol are often present in drug delivery formulations [20]. The cosolvents are added in order to improve the solubility of the activecompounds and/or to aid in the sensory perception. Thus, the sol-vent quality is a controlling factor for the aggregation of molecules
Fig. 1. Electronic spectra of orange–red color silver nanoparticles at dif-−4 −3
the addition of 5% (v/v) protic solvents (methanol and ethanol)and aprotic solvents (dioxane, acetonitrile, DMF and DMSO). Ini-tially, the absorption maximum was 1.06 at 450 nm. After theaddition of protic solvents, a strong decrease (from 1.06 to 0.69
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Z. Khan et al. / Colloids and Surfaces A: P
21]. The mixture of co solvents with water provides extra degreesf freedom in tailoring the solution properties for specific applica-ions [21,22]. The addition of co solvents causes pronounced effectsn the characteristic length scales of the microstructures [23–25].rganic solvents have been used to synthesize the nanoparticles.owever, the published articles on the effect of co solvents on
he formation and growth of nanoparticles are rather limited. Inhis paper, firstly, we prepared the silver nanoparticles by the usef biologically relevant single amino acid, cysteine. Secondly, dis-ussed the effect of various organic solvents (methanol, ethanol,,4-dioxane, acetonitrile, DMF and DMSO) in different percent onhe shape and size of silver nanoparticles.
. Experimental
Cetyltrimethylammonium bromide (98%, Fluka), cysteine99.9%, BDH), silver nitrate (99.9%, BDH), were used as received.rganic solvents, methanol, ethanol, acetonitrile, 1,4-dioxane, N,N-imethylformamide (DMF) and dimethyl solphoxide (DMSO) usedere of AR grade. Double-distilled water was used for the prepa-
ation of working solutions throughout the experiments. Thebsorption spectrum was recorded in a PerkinElmer UV–vis spec-rometer (Lambda EZ 150) in a 1 cm quartz cuvettes, and the solventackground was subtracted each time. For solvent effect, differ-nt percent of mixed solvents (water + solvent) was prepared andsed as stock. To study the solvent effect UV–vis spectroscopic mea-urements were recorded after necessary solvent correction eachime. Transmission electron microscope (JEOL, JEM-1011; Japan)nd Scanning electron microscope (SEM) (QUANTA FEG 450, FEIompany, Eindhoven, Netherlands) were used to determine theize and shape of the silver nanoparticles.
. Results and discussion
.1. Preparation of silver nanoparticles
Ammonium formate [26], formic acid [27], formamide [28],ethanol [29] and DMF [30,31] are the well known powerful
educing agents for silver salts. Therefore, in order to determinehe role of organic solvents on the stability and/or morphol-gy of silver nanoparticles, the use of organic solvents is arucial problem that we address first. In a typical experiment,CTAB] = 15.0 × 10−4 mol dm−3, [AgNO3] = 20.0 × 10−4 mol dm−3,all solvents] = 20.0 × 10−4 mol dm−3; temp. = 23 ◦C, we did notbserved the formation of any type of prefect transparent color,uggesting that methanol, ethanol, acetonitrile, DMF, DMSO andioxane are not the suitable reductants to the preparation ofrefect transparent yellow color silver nanoparticles under normalxperimental conditions.
For the preparation of silver nanoparticles, a series of experi-ents were carried out under different experimental conditions.n addition of all the reagents (Ag+ ions, cysteine and CTAB),
colorless solution becomes a perfectly transparent yellow wasbserved in the solution, after a certain initial period at 23 ◦C. Ashe reaction proceeds, the color shifts from yellow to dark brown,hrough orange, confirming the reduction of Ag+ ions and the subse-uent formation of silver nanoparticles [12,18]. UV–vis absorptionpectra have proved to be quite sensitive to the formation of silverolloids. The Plasmon absorption peak at 400 nm is the charac-eristic Plasmon absorption peak of silver nanoparticles [12]. Theppearance of a SRP band was observed (wavelength maximum
�max) shifts to longer wavelength) as the reaction time increasesFig. 1). The position of the surface resonance Plasmon (SRP) absorp-ion peak depends on the experimental conditions, i.e., naturef reducing agent, method of preparation, acidity of the reaction
ferent time intervals: Reaction conditions: [CTAB] = 15.0 × 10 mol dm ;[AgNO3] = 40.0 × 10−4 mol dm−3; [cysteine] = 20.0 × 10−4 mol dm−3; temp. = 23 ◦C.(For interpretation of the references to color in this figure legend, the reader isreferred to the web version of the article.)
mixture and absence/or presence of stabilizers and the morphologyof particles (size and shape and the adsorption of nucleophiles orelectrophiles onto the particle surface). Usually, a red shift is asso-ciated with an increase in particle size or with the withdrawal ofelectron density from the surface [12]. Fig. 2 also suggests that thegrowth as well as nucleation (Fig. 2 inset) of the silver nanoparticlesare directly proportional to the reaction time and resulting silver solis stable for a longer time. Size and shape of the particles were deter-mined with the help of transmission electron microscope (TEM).Fig. 3 shows the TEM pictures observed after adding the cysteinein presence of CTAB. The silver nanoparticles are spherical and ofuniform particle size, ca. 12 nm.
3.2. Effects of polar-protic solvents on the SRP band
Fig. 4 shows absorption spectra of silver–CTAB sol obtained from
Time (hrs)
Fig. 2. Plot showing the effect of time on the growth of silver nanoparticles at400 nm. Reaction conditions were the same as in Fig. 1. Inset: Plot showing theeffect of time on the absorbance for growth processes.
122 Z. Khan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 390 (2011) 120– 125
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ig. 3. TEM images of silver nanoparticles for the same reaction conditions of Fig. 1.
nd 0.19 for methanol and ethanol, respectively) in absorbance androadening of the absorption band is seen; this effect was more pro-ounced than by the addition of ethanol but less pronounced than
or methanol (Fig. 4). This can be attributed to both alcohols act-ng as a nucleophile on the particle surface resulting in decreasen absorbance and broadening of the plasmon peak. The decreasen intensity on addition of various nucleophiles and polymers onhe plasmon band has already been explained by Henglein [12] aseing due to a decrease in the mean-free path of electrons in silverolloids, donates the electron density to the particles via lone pairsf electrons, leading to a decrease in conductivity. As can be seenn Fig. 5 (typical example), as the dielectric constant of protic sol-ents increases, the absorbance increases, which in turn, decreaseshe particle size. This is further supported by the TEM (particle size2 nm) and SEM results in Figs. 3 and 6, which indicate that thearticle size increases and number density increases on addition ofrotic solvents (particle size = 39, 58 and 70 nm for silver–CTAB sol,
n presence of methanol and ethanol, respectively). Inspection ofig. 5 clearly suggests that CTAB–silver sol has higher absorbancen comparison of ethanol– and methanol–silver sols (extrapolatingrom methanol to water by arrow).
It is well known that dielectric constant, hydrogen bonding, boil-ng point and polarity of the protic solvents are related to eachther. As the number of hydrogen bonding, dielectric constant andolarity of the medium increases, the probability of the adsorption
ig. 4. Effect of protic and nonprotic solvents on the electronic spectra of silveranoparticles after 24 h.
Fig. 5. Effect of dielectric constant of protic and nonprotic solvents on theabsorbance of SRP band of silver nanoparticles.
of nucleophiles onto the surface of silver nanoparticles decreases,which leads to the increase in the SPR absorbance of particles(Table 1). Based on their effect on the absorbance, the protic sol-vents can be ordered as: water > methanol > ethanol for the higherabsorbance and lower particles size. This is further supported by theresults, which indicate that absorbance decreases with increasingboiling point of protic solvents, suggesting that hydrophobic char-acter of a particular solvent is an important factor, which governsthe morphology and/or stability of the silver nanoparticles.
3.3. Effects of polar-aprotic solvents on the SRP band
The argument behind the solvents used in this study was notonly to examine their ability to stabilize a nanoparticle surface butmore importantly to provide a functional handle on the nanoparti-cle surface. The addition of aprotic solvents to the reaction mixture([CTAB] = 15.0 × 10−4 mol dm−3; [AgNO3] = 40.0 × 10−4 mol dm−3;[cysteine] = 20.0 × 10−4 mol dm−3) causes decreases the SRP bandabsorbance (Fig. 4). This mode of interaction is indicative of thepresence of free lone-pairs on the solvent that are involved innanoparticle surface coordination. The shape of absorption banddepends on the nature of the solvent, the presence of lone-pairsand in particular on the structure solute molecules. It is well-known that adsorption of the nucleophile to the particle surfaceincreases the Fermi level of the silver particle owing to its dona-
tion of electron density to the particles [12,14]. Figs. 4 and 5 showthat dioxane is more strongly adsorbed than acetonitrile, DMF andDMSO (absorbance increases with dielectric constant; polarity ofthe solvent: dioxane < acetonitrile < DMF < DMSO). In the case of
Table 1Effects of various parameters on the absorbance of silver nanoparticles formation at450 nm for 5% each solvent.
a Reaction mixture containing [CTAB] = 15.0 × 10−4 mol dm−3,[AgNO3] = 40.0 × 10−4 mol dm−3 and [cysteine] = 20.0 × 10−4 mol dm−3 weretaken as the reference.
Z. Khan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 390 (2011) 120– 125 123
Fig. 6. SEM images of silver nanoparticles in absence (a) and presence of 5% each methanol (b) and ethanol (c).
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ioxane as nucleophile and/or additive, the dioxane molecule haswo oxygen atoms. These lone-pairs of oxygen atoms are believedo form ion-pairs with the positive surface of the silver nanopar-icles. As a result the decrease in electron density in the particlesill decrease resulting in the formation of a large number of nuclei
nd corresponding decrease in the absorbance of SRP band. Hydro-en bonding and boiling point of the used aprotic solvents has noystematic effect on the SRP absorbance, indicating that more thanne effect is responsible for the changes in the shape of the SRP
and. These results are in good agreement with the observationsf Henglein et al. [12] (generally, surface Plasmon decreases afterhe adsorption of nucleophiles and/or polymers onto the surface ofilver nanoparticles).
3.4. Co-adsorption effects of CTAB and solvents on the SRP band
Fig. 7 shows the absorbance of a silver sol to which vari-ous amounts of solvents had been added. It can be seen thatthe SRP absorbance is strongly affected in presence of aproticsolvents, although the changes are far less than in methanol. Ini-tially, absorbance decreases with the small amounts of all solvents.At higher amounts, absorbance remains constant with increas-ing ethanol, DMSO, dioxane and methanol (except acetonitrile
and DMF), respectively. These results could be rationalized interms of solubilization/micellization/adsorption/incorporation ofsolvents into the Stern layer of CTAB micelles through electrostaticand hydrophobic interactions. Micelles surrounding the colloidal
124 Z. Khan et al. / Colloids and Surfaces A: Physico
Fig. 7. Plot showing the effect of different amount of solvents on the absorbance ofsilver nanoparticles.
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articles solubilize and control the orientation of substrates. Theossibility of the reactants (cysteine and Ag+ ions) solubiliza-ion/incorporation into the CTAB micelles is diminished but nototally prevent due to the presence of organic solvents into the
icelles. At constant [CTAB], protic and aprotic solvents restricthe solubilization of reactants causing a decrease in concentra-ion; the nucleation rate is thus inhibited. Another way we canay that these solvents (dioxane, ethanol, DMF, DMSO, acetonitrilend methanol) are more hydrophobic than the cysteine, which hasreater tendency of solubilization into the micelles. A comparisonf the effectiveness of used solvents suggests that solubilization,eactivity, and/or adsorption increases with hydrophobic charac-er: dioxane > ethanol > DMF > DMSO > methanol (Fig. 7). In ordero explain the role of aprotic solvents, their possible resonatingtructures could also be considered (Scheme 1).
In Scheme 1, Eqs. (1) and (2) shows the presence of negativeharge on the N and O atoms of acetonitrile and DMF. As a result,hese solvents have greater tendency to form strong ion-pairs withhe positive head group of CTAB and also to adsorb onto the positiveurface of silver nanoparticles. Thus, the involvement of various fac-ors such as inductive effect, steric hindrance and symmetry cannote completely ruled out.
. Conclusions
The following conclusions have emerged from this study: (i)he particle size and plasmon absorption band have been changedy varying the organic solvent. The absorbance decreases withielectric constant of the medium and surface-adsorbed species.ii) Addition of methanol and ethanol molecule results in a increasen particle size. (iii) Addition of acetonitrile, DMF, dioxane and
MSO also decreases the absorbance by decreasing the growth rate.
iv) Higher amounts of solvents do not bring about any significanthange in the absorbance. Various factors namely, hydrophobic,
[
chem. Eng. Aspects 390 (2011) 120– 125
polarity, inductive, steric hindrance, resonance, nucelophilic, etc.are responsible for the optical changes.
Acknowledgment
Financial support to the authors by Deanship of ScientificResearch (MS/12/176), King Abdul Aziz University, is gratefullyacknowledged.
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