Article pubs.acs.org/JPCC
Purification of Gold Organosol by Solid Reagent Mainak Ganguly,† Anjali Pal,‡ and Tarasankar Pal*,† †
Department of Chemistry and ‡Department of Civil Engineering, Indian Institute of Technology, Kharagpur 721302, India S Supporting Information *
ABSTRACT: Straight chain amines of variable chain length offer steric stabilization to gold nanoparticles (AuNPs), and thereby, gold organosol is produced in nonpolar solvent. We have found that dodecylamine (DDA) becomes a preferred ligand over other amines, for extraction, stabilization, and preparation of gold organosol in hexane. In the same context, straight chain thiols also provide steric stabilization to the AuNPs and compete with amines. But, the extracting power of thiols is dramatically reduced while the size of AuNPs is increased, and amines accomplish success for variable sizes of AuNPs. Organosol system always remains admixed with excess free ligands, and it is true for amines also. Hence, free amines (unbound) conjecture false information and pose a problem during further course of investigation. Excess unbound ligands in the organosol system limit application of AuNPs in the organic medium. A simple heterogeneous separation technique is reported here to prepare amine stabilized organosol devoid of unbound (free) amine. A greenish blue solid copper stearate [Cu(St)2] has been introduced as the best suited reagent that endorses quantitative removal (97.3 ± 0.12%) of free amine. The solid reagent [Cu(St)2] in turn produces a violet solid. The greenish blue to violet color change of the solid reagent indicates selective removal of excess amine from the organosol keeping the properties of amine stabilized organosol unaltered.
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INTRODUCTION Gold nanoparticles are interesting for their characteristic optical property visualized due to strong interaction of the visible light >via resonant excitation of collective oscillations of the conduction electrons of the particles.1 Ligand stabilized organosol has become the most announced word now a days especially for gold organosol syntheses. Two approaches are usually followed to obtain organosol: (i) the most popular one is to transfer metal ions from aqueous solution to organic solvent by a suitable phase transfer agent followed by reduction;2 (ii) the other approach is the transfer of a preformed metal nanoparticles into organic solvent from aqueous medium with suitable phase transfer agent.3 Anyway, metal nanoparticles because of their high surface energy need effective stabilizers, that is, capping agents of varying properties, and they include anions,4 cations,5 surfactants,6 micelles,7 polymers,8 macrocycles,9 thiols,10 and amines.11 Straight chain thiols or amines offer steric stabilization to the preformed AuNPs. Electrostatically stabilized gold hydrosol often aggregates irreversibly where as steric stabilized AuNPs may be obtained as powder. The powder form of AuNPs after being redispersed possesses unchanged size indicating extraordinary stability.12 An useful popular method was reported by Leff et al.2b as a modification of the famous Brust protocol12 where stable gold organosol was made with and without surfactant simply replacing thiol by amine. There is an interesting report by Negishi et al. for obtaining highly stable thiolated gold clusters.13 Klabunde et al.14 reported digestive ripening for obtaining highly monodispersed AuNPs from a polydisperse colloidal suspension of AuNPs using thiol ligands of variable chain length. © 2012 American Chemical Society
In a two-phase method, monolayer protected gold nanoparticles have been prepared where tetraoctylammonium bromide (TOABr), a quaternary ammonium salt, is used as the phase transfer reagent. There AuNPs persistently remain associated with TOABr as an impurity. A simple Soxhlet extraction protocol has been used to eliminate the impurity.15 In another protocol, fractional crystallization16,17 is also used for purification. Size dependent solubility criterion has been considered by Chemseddine and Weller18 to separate and purify cadmium sulfide particles. Aerosol decomposition process19 has also been reported where effects of foreign inorganic salts, temperature, and variable precursors have been considered for separation of nanoparticles. Clarke et al. have reported that thiol capped ultrasmall gold nanoparticles can be purified employing fractional crystallization taking the help of supercritical ethane.20 Size-selective fractionation of nanoparticles at an application scale using CO2 gas-expanded liquids also exists in literature.21 Sometimes fractional crystallization and other standard purification techniques (i.e., precipitation, extraction, chromatography, electrophoresis, centrifugation, or dialysis) often become ineffective12,22−24 for the purification of AuNPs due to comparable solubility of both the nanoparticles and the impurities. To overcome such a problem of purification, Sweeney et al. adopted a diafiltration protocol, a very convenient and useful procedure to purify size selective gold nanoparticles.25 A report by Bai et al. describes rapid separation and purification of typical colloidal nanoparticle including Au, Ag, and CdSe by ultracentrifugation in a nonhydroxylic organic Received: December 15, 2011 Revised: March 30, 2012 Published: April 3, 2012 9265
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SPECTRUM 2000 instrument. The fluorescence property of the molecule was measured using a Perkin-Elmer fluorescence spectrometer (Model LS 55) interfaced with personal computer. Particle morphology was examined using a field emission scanning electron microscope (Supra 40, Carl ZEISS Pvt. Ltd.). Compositional analysis of the sample was done by an energy-dispersive X-ray microanalyzer (Oxford ISI 300) attached to the scanning electron microscope. X-ray photoelectron spectroscopy (XPS) analysis was carried out with a VG Scientific ESCALAB MK II spectrometer (UK) equipped with Mg Kα excitation source (1253.6 eV) and a five-channeltron detection system. Transmission electron microscopy (TEM) analysis was performed with an instrument H-9000 NAR, Hitachi, using an accelerating voltage of 300 kV. The sample was prepared by placing the drop of the solution on a carbon coated copper grid. For distribution histogram of particles, we have considered 340 particles. Preparation of Gold Hydrosol. An aliquot of 1.0 mL of 10−2 M HAuCl4 solution was taken in a conical flask, and the solution was diluted to 46 mL by triple distilled water. Sodium borohydride (10−2 M) was prepared in 10−2 M NaOH solution. Immediately after the preparation of borohydride solution, 3 mL of it was added to HAuCl4 solution and shaken vigorously for 10 min. A red solution was obtained indicating the formation of gold hydrosol (hydrosol-A) with a characteristic plasmon band maximum at 509 nm. In a likewise procedure, gold hydrosol was synthesized using 10−1 M sodium borohydride, and the plasmon absorption maxima were 519 nm (hydrosol-B, Supporting Information, Figure S1). Synthesis of [Cu(St)2]. The precursor [Cu(St)2] was synthesized by a reported method using copper acetate.34 Methanolic solution of stearic acid and copper acetate (5:1) were mixed together, and a greenish blue precipitate was obtained (insoluble in methanol). The precipitate was washed thoroughly with methanol in order to make it free from the starting copper acetate or stearic acid (both are soluble in methanol). The precipitate was dried and characterized by powder XRD pattern, melting point, TLC, and the FTIR analysis to confirm neat [Cu(St)2]. Preparation of Gold Organosol. An aliquot of 5 mL of red gold hydrosol was taken in a screw capped test tube. Then, 2 mL of acetone and 100 μL of DDA were introduced successively into the test tube. Instantly, 5 mL of distilled hexane was poured into the test tube. The test tube was then capped and shaken vigorously for ∼3 min. The screw capped test tube was allowed to stand for ∼5 h for aging. Two layers were distinctly visible in the test tube. Upper red layer contains DDA capped AuNPs in hexane, and the transparent bottom layer was aqueous acetone. Red color of the hexane layer and colorless water−acetone layer speak about the evolution, stabilization, and separation of gold organosol as the upper hexane layer. Similar procedure was followed for other amines and thiols of different chain length. The amines such as HA, DA, TDA, HDA and thiols of various chain length, namely, OT, DT, DDT, HDT, and so forth were employed for the study. TEM images and a size distribution histogram of organosol capped with different straight chain amines and thiols have been shown in Figure S2 of the Supporting Information. Removal of Free Amine from Gold Organosol. Gold organosol in hexane was reserved in a well stoppered centrifuge tube. To it, 8 mg of [Cu(St)2] was introduced slowly as a fine powder as a solid extracting agent, and the organosol was shaken well for ∼15 min. Then, the solution was centrifuged at 6000 rpm for 5 min. It was observed that the greenish blue color of [Cu(St)2] disappeared, and in turn, a violet precipitate
density gradient.26 Recently, a multilayer quasi-continuous density gradient centrifugation method by Steinigeweg et al.27 as well as ions assisted self-assembly by Liu et al.28 have been reported for purification purpose. Normally capping agents possess functional groups such as amines, alcohols, thiols, and phosphines, contributing a wide range of ligand field strength. Weak interaction of dendrimers having amines and alcohol functionalities29 rescues the electronic properties of AuNPs. On the contrary, thiols interact strongly with the metal surface causing considerable charge redistribution.30 Such stabilizers offer resistance to the incoming ligands. Inconvenience is frequently encountered due to unbound free capping agent that always remains in nanoparticle dispersion. To have a neat biological application, Levy et al. indicated the importance of the stable and pure AuNPs devoid of excess capping agents.31 Sometimes excess capping agents react with the probe molecules used in different fields of spectroscopy (viz., fluorescence, Raman spectroscopy) giving false information to the researchers. Again, removal of extra unbound capping agent from a nanocatalyst is also vital to obtain trustworthy results.32 We have adopted a simple heterogeneous extraction procedure for the first time to purify amine stabilized gold organosol employing one solid reagent. Here, the extraction capability of straight chain thiols and amines of variable chain length has also been demonstrated. Broome et al. reported the binding of high molecular weight amine with metal complex in 2:1 fashion and again they studied dodecylamine−cupric acetate binding spectrophotometrically.33 We have introduced an interesting solid reagent greenish blue copper stearate (stearate is the higher homologue of acetate) to trap unbound straight chain amines leaving behind pure amine bound organosol intact. The separated solid is a violet Cu(St)2−amine adduct (St = stearate) that has been characterized. Spectroscopic information suggests that amine protected AuNPs devoid of free amine remain unaffected when free amines are removed from the solution.
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EXPERIMENTAL SECTION Chemicals, Materials, and Equipment. Copper(II) acetate (98%), butylamine (BA, ≥99%), hexylamine (HA, 99%), decylamine (DA, 95%), dodecylamine (DDA, 98%), tetradecylamine (TDA, 95%), hexadecylamine (HDA, 98%), octanethiol (OT, ≥98.5%), decanethiol (DT, 99%), dodecanethiol (DDT, ≥98.5%), hexadecanethiol (HDT, 99%), gold(III) chloride trihydrate (≥99.9%), NaBH4 (99%), and pyrene (99%) were obtained from Sigma-Aldrich. Hexane and acetone were of AR grade and obtained from Sisco Research Laboratory, India. They were distilled before use. Triple distilled water was used throughout the experiment. All the glassware were cleaned with freshly prepared aqua regia, subsequently rinsed with copious amount of distilled water and dried well before use. All UV−vis absorption spectra were recorded in a SPECTRASCAN UV 2600 digital spectrophotometer (Chemito, India), and the solutions were taken in a 1 cm well stoppard quartz cuvette. The spectra were recorded against reference solvent in a reference cell in the double beam spectrophotometer to subtract solvent background. Reflectance spectra were measured using diffuse reflectance spectra (DRS) mode with a Cary model 5000 UV−vis-NIR spectrophotometer. Elemental analyses were done with a Perkin-Elmer CHN 2400. Fourier transform infra red (FTIR) spectral characteristics of the samples were collected in transmittance mode with Perkin-Elmer FTIR spectrometer, 9266
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was obtained. The same solid-state extraction procedure was repeated until the greenish blue colored solid as the precipitate (original color of the added reagent) of [Cu(St)2] was observed, that is, the greenish blue unreacted [Cu(St)2] announced the quantitative removal of free amine from gold organosol. Then, the hexane layer of gold organosol was taken out as a supernatant, and the violet precipitate along with the unreacted [Cu(St)2] (obtained as centrifuged mass) were washed several times with hexane. The violet solid compound is highly soluble in chloroform, but the greenish blue solid [Cu(St)2] is chloroform insoluble. So, the sum total precipitate (with two distinctly different colors) was shaken well in chloroform and filtered. The greenish blue residue was unchanged [Cu(St)2]. The filtrate was dried in vacuum and weighed for the quantitative recovery of free amine. The violet solid was characterized by IR, mass, and elemental analysis.
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RESULTS AND DISCUSSION Aqueous solution of sodium borohydride slowly decomposes, and hence, gold hydrosol, prepared by borohydride, becomes somewhat unstable. Thus, the hydrosol slowly gets aggregated. The borohydride in NaOH medium offers stability to the gold hydrosol system,3b and in turn, it is also stabilized.35 In the presence of NaOH, long-lived OH− (added) endangers its capping capability toward Au(0) when BH4− is decomposed on long-standing. Acetone has been employed here to facilitate the extraction of AuNPs in organic medium in the presence of long chain amine or thiol. Acetone changes the polarity of the hydrosol system and assists the phase transfer process while nonpolar hexane is employed. And without acetone, a large amount of AuNPs stay either in aqueous phase or at the interface of the water−hexane layer. Again, when a lower amount of borohydride is employed, AuNPs become smaller (hydrosol-A). The AuNPs prepared with a higher concentration of borohydride (hydrosol-B) exhibit a red shift of the rich plasmon band maximum indicating aggregation. A similar shift is also observed with a lower concentration of NaBH4 when the reduction of Au(III) ions is done at higher temperature. The red shifting of the plasmon band is due to the removal of adsorbed anions by excess borohydride or by increased thermal agitation, which brings aggregation of AuNPs (Supporting Information, Figure S3). It is observed that dodecylamine (DDA) very efficiently transfers AuNPs from hydrosol-A to the hexane layer, and DDA ligand stabilized organosol is easily produced. Amines with lower chain length such as butylamine, hexylamine cannot extract hydrosol; rather the AuNPs remain in the interface between the two layers as aggregated particles. Amines such as TDA and HDA (solid at 25 °C) with longer chain lengths have low extracting ability as compared to DDA (liquid at 25 °C) probably due to the poor solubility of TDA and HDA in nonpolar hexane. DA (liquid at 25 °C) has also moderately good capacity to extract AuNPs but somewhat less efficient than DDA. This may be due to lower hydrophobicity of DA than DDA resulting in lower extracting efficiency of DA for AuNPs in nonpolar hexane. Long chain thiols are efficient extracting agents when the particle size is small (∼4.5 nm, hydrosol-A). The extraction efficiency of thiols is also good and increases slightly with the increase in chain length of straight chain thiols but for small particles. The gradation of extraction efficiency of thiols follows the following order: OT < DT < DDT < HDT. This order relates to the hydrophobicity of thiols, which helps the dissolution of thiols in nonpolar hexane (Figure 1A). The bar diagram in Figure 1B represents the different extraction capacity of thiols and amines bearing various chain
Figure 1. (A) UV−vis absorption spectra (normalized) of Au(0) hydrosol-A (a) and organosol (b−j) with different capping agents, namely, straight chain thiols and amines obtained from hydrosol-A. Capping agents are: (b) HDA, (c) TDA, (d) DDA, (e) DA, (f) HA, (g) HDT, (h) DDT, (i) DT, and (j) OT. (B) Comparative account to show the extraction efficiency of different straight chain amines and thiols for extraction of Au(0) from hydrosol-A.
lengths for the transformation of hydrosol into organosol for small AuNPs. Thiols cannot extract AuNPs of comparatively larger size (∼10 nm, hydrosol-B, Figure S1 of the Supporting Information) from aqueous medium directly into hexane. This is presumably due to the ‘S’ donor atoms of thiols (in alkaline medium) that remain as thiolate (S−) in polar water−acetone medium. With the increase of size of AuNPs, the heavy AuNPs have a tendency to remain aggregated in presence of S−. The capping capability of thiols for larger AuNPs is not sufficient enough to extract the particles from aqueous medium to hexane. For larger AuNPs, long straight chain amines are a much better extracting agent compared to thiols (Figure 2). In a word, DDA has been found to be the best suited for the extraction of gold hydrosol into hexane from aqueous medium through effective capping of AuNPs whatever the size of AuNPs may be. But, as a consequence of the phase transfer event, the gold organosol contains not only DDA capped AuNPs, but also it inherits excess unbound free amines that often become a serious contaminant. Thus, removal of unbound amines remains as a great challenge. For this purpose, [Cu(St)2] a solid reagent has been introduced, and when it is used, it traps free amines selectively resulting violet nonpolar adduct (Scheme 1). Both [Cu(St)2] and the resulting adduct, that is, [Cu(St)2]− DDA are insoluble in hexane. Elemental analysis of the resulting hexane layers supports the fact. Consequently, amine bound AuNPs (organosol in hexane) remain unaffected. IR spectra of the gold organosol before and after the [Cu(St)2] treatment have been shown in Figure 3. The peaks for free amine at ∼3330 cm−1 due to N−H stretching as well as 9267
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Figure 2. UV−vis absorption spectra (normalized) of Au(0) hydrosolB (a) and organosol (b−i) with different capping agents, namely, straight chain thiols and amines obtained from hydrosol-B. Capping agents are: (b) HDA, (c) TDA, (d) DDA, (e) DA, (f) HDT, (g) DDT, (h) DT, and (i) OT.
Figure 3. IR spectra of (a) dodecylamine and (b) gold organosol before free amine removal and (c) gold organosol after free amine.
Scheme 1. Selective Removal of Unbound Amine from Gold Organosol Keeping Amine Bound Gold Organosol Unaffected by [Cu(St)2] Treatment
Figure 4. UV−vis spectra of gold organosol (a) before and (b) after removal of unbound amine by solid [Cu(St)2].
free amine by the solid reagent. The free amine shows an extraordinary binding capability with the solid reagent [Cu(St)2] leaving aside the amine stabilized AuNPs intact. The amine bonded to Au(0) in organosol does not place exchange, instead free amine (unbound) from hexane firmly binds with solid [Cu(St)2] and is thrown out as a violet precipitate. Thus, free amine is quantitatively removed. Free unbound amine (soluble) + Au(0) bound amine (soluble) + [Cu(St)2 ] (greenish blue solid) → Au(0) bound amine (soluble) + amine bound [Cu(St)2 ] (violet solid) ↓ + unreacted [Cu(St)2 ]↓
∼1560 cm−1 due to N−H bending after [Cu(St)2] treatment have been greatly reduced. It guarantees the quantitative removal of free amines from gold organosol via solid phase extraction. UV−vis spectra of DDA capped gold organosol in hexane have been shown in Figure 4. The plasmon band due to AuNPs in hexane, λmax = 515 nm, remains unaltered both before and after the removal of free amine by greenish blue solid [Cu(St)2] (hexane insoluble) powder. Another interesting point is that UV−vis spectra of the DDA capped AuNPs show a ∼240 nm peak due to the bound amine as well as free amine (Figure 4a). This peak is dramatically decreased (Figure 4b) after the addition of solid extractant [Cu(St)2] indicating removal of unbound amine. This observation rationalizes the removal of
([1])
In this context, it is very interesting to mention that thiol passivated gold organosol is destabilized along with successive damping of plasmon band maxima due to AuNPs with the addition of solid [Cu(St)2] (Figure 5). It may be spelled out that, if place exchange reaction between [Cu(St)2] and amine capped AuNPs would have been there, the amine bound to AuNPs would be removed. In that case, amine free Au(0) devoid of any stabilizing agent would not remain in hexane. Hence, the hexane layer would have been colorless due to the precipitation of flocculated Au(0) from the organic solvent. But, this is not observed. After removal of free amine, unaltered plasmon band of AuNPs speaks against the 9268
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Forster resonance energy transfer (FRET).36 This fact was accounted also using organosol of both (with and without free amine) types. We have used a solution of pyrene in hexane. Then, different sets of solutions for gold organosol (first seven sets with and the other seven sets without free amine) containing variable concentrations of Au(0) and fixed concentration of pyrene (2 × 10−6 M) were prepared. In this experiment, we have corrected the background for hexane and used 317 nm excitation wavelength. The extent of quenching of pyrene (λem = 389 nm) with variable concentration of AuNPs was measured. The results from the control experiment, where DDA (in hexane) and pyrene solution were considered (Supporting Information, Figure S4) but without Au(0), confirmed insignificant change in fluorescence intensity of pyrene with the increase of DDA concentration. So, it is concluded that Au(0) is responsible for quenching of the fluorescence of pyrene. The plot of I0/I versus [Au(0)] shows almost the same slope and intercept [I0 = fluorescence intensity of pure pyrene, I = fluorescence intensity after addition of Au(0)] for both the organosol systems. It indicates that, after the removal of free amines, Au(0) concentration is not affected. If there is any real decrease of the concentration of Au(0) from the organosol after [Cu(St)2] treatment, there would be less quenching of fluorescence because of lower concentration of AuNPs (quencher). In that case, the value of I0/I would be lower as well as the slope of I0/I versus [Au(0)] would be smaller than the former. But, the observed fact (same slope and intercept) is an evidence in support of the unchanged Au(0) concentration (Figure 6). The as synthesized DDA capped gold organosol has been subjected to solid phase extraction with [Cu(St)2] for free amine removal. Typically, two sets of gold organosol (0.5 mL each with and without free amine) have been evaporated to dryness to obtain XPS spectra. In both the cases, peak area for gold [Supporting Information, Figure S5(a)] remains almost same. The separation between the Au4f 5/2 and Au4f 7/2 is 3.7 eV. The broadening of the peaks of gold in the XPS spectra is due to inhomogeneity in the Au environment associated with sample charging37 in presence of waxy solid dodecylamine. Before free amine removal, a large excess of dodecylamine is also responsible for the little hump. However, there happens a ∼67% decrease in the peak area for nitrogen in the XPS spectra of pure organosol (devoid of free DDA) while compared with the organosol containing free DDA [Supporting Information, Figure S5(b)]. The results justify removal of free amine keeping amine stabilized AuNP concentration unchanged. The shift of 0.34 eV for nitrogen 1s peak goes toward higher binding energy side when the free amine is selectively removed.38 Atomic absorption spectroscopy (AAS) also authenticates that the concentration of gold remains unaffected by [Cu(St)2] treatment (Supporting Information). Figure 7 displays the AuNPs in dispersion as hydrosol-A having wide size distribution. To transfer the AuNPs to the hexane layer at the first step, acetone is introduced. There happens destabilization of hydrosol resulting multiparticle aggregation3c in acetone− water mixture. Hence, a red shift of the plasmon band is observed (Supporting Information, Figure S6). Then, long chain amine in hexane is introduced with vigorous shaking. Thus, amine stabilized organosol is obtained where the particles show tight size distribution profile (Figure 8). The phenomenon of digestive ripening10b as an outcome of phase transfer is also to be noted. There happens a tight size distribution in case of DDA stabilized gold organosol, and the particles fall in the 2.5−5.5 nm range. On the other hand, AuNPs obtained after borohydride reduction describe broad size distribution. However, ∼80% particles are smaller for borohydride
Figure 5. Gradual decrease of plasmon band of thiol capped AuNPs in hexane with the addition of [Cu(St)2].
possible place exchange reaction. A violet color is observed as a result of the attachment of free amine with [Cu(St)2] without affecting the 515 nm peak of AuNPs. This spectroscopic evidence vouches the stability of the amine bound AuNPs in hexane and removal of free amine from the organosol. On the contrary, it is worth mentioning that the absorbance of thiol capped orgnosol decreases gradually in steps with the addition of [Cu(St)2]. It indicates that both the bound and unbound thiols are affected by [Cu(St)2]. Hence, heavy aggregates of AuNPs (without thiol stabilizer) are thrown out of hexane as a consequence of place exchange reaction. So, it is concluded that thiol stabilized organosol is destabilized by [Cu(St)2] in hexane and amine bound organosol remains intact (does not loose bound amine) (Scheme 2). Fortunately, a very small amount of Scheme 2. [Cu(St)2] Removes Unbound Amine and Total (Bound and Unbound) Thiol from Organosol
thiol (the molar ratio of total DDT molecules to gold atoms is only 10% as demonstrated by Martin et al.3b) is needed for phase transfer of AuNPs. So, postsynthesis cleaning of thiol capped organosol is really not needed. Again, use of a large excess of DDT does not alter the concentration of AuNPs dispersion in hexane because excess DDT adheres to the wall of the glass vial and/or trapped at the liquid−liquid interface.3b But, in case of amine, cleaning or removal of excess amine is essential, as the molar ratio of total DDA molecules to gold atom (1100% as reported by Leff et al.2b) is generally very high. Selective removal of free amine from amine stabilized organosol has also been explained from fluorescence studies taking pyrene (λem = 389 nm) as a probe molecule. The fluorescence of pyrene is well-known and that is quenched by gold organosol due to 9269
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Figure 6. (A) Effect of variable concentration of Au(0) on the fluorescence spectra of 2 × 10−6 M pyrene. Added concentrations [(0, 4, 6, 8, 12, 16, 20, 24, 28) × 10−6 M] of Au(0) with unbound DDA (a−h) and without unbound DDA (a1−h1), respectively. (B) Plot of I0/I vs [Au(0)] (Stern− Volmer plot) with comparable slope and intercept showing the unchanged concentration of Au(0) both before and after the removal of unbound DDA.
Figure 9 indicates ∼120 nm blue shift when [Cu(St)2] is converted to its amine adduct. Interestingly, ∼60 nm red shift is observed when the violet compound is dissolved in chloroform. In addition, the GC mass spectrum of the violet adduct shows the ion peak for DDA (185) and stearic acid (283). The peak at 145 (Cu + 2CH3CN) is a proof of the presence of copper (Supporting Information, Figure S9). The elemental analysis (Supporting Information, Table S1) of the violet adduct was done in the CHN analyzer, and copper analysis was done titrimetrically.40 The calculated values of the C, H, N, and Cu percentage fit well with the formula [Cu(St)2(DDA)2] of the violet compound. The violet adduct is nonionic in nature. All these support the proposed formula with two stearate and two amine ligands within the coordination sphere. Broome et al. reported bisdodecylamino−cupric acetate33 as an adduct of copper acetate and DDA. We have used [Cu(St)2], and the formation of bisdodecylamino−cupric stearate is responsible for selective removal process. From the experimental evidence, it can also be stated that 23.05 mg of DDA remains bound with AuNPs in hexane (5 mL) and 57.22 mg of DDA is free, which is quantitatively removed by [Cu(St)2]. Precisely, 4.61 mg ± 0.01 mg is the amount of DDA that remains bound and/or caps 1 mL of Au organosol for spherical AuNPs bearing the diameter of 5 nm. The excess amine (57.22 mg in our experiment) in the organosol system is useless rendering the system complicated and impure for several analyses. This purification tactic can also be carried out using copper acetate in lieu of [Cu(St)2]. Similarly, free amine can be removed using copper acetate also. The problem with copper acetate is that, during shaking, a slight amount of copper acetate dissolves in hexane rendering contamination of gold organosol. That is why, [Cu(St)2] has been introduced for removal of free amine. The percent recovery of free DDA by [Cu(St)2] from gold organosol is 97.3 ± 0.12% (Supporting Information). It is worth mentioning that not only DDA but also other free amines can be selectively eliminated from gold organosol by this novel procedure. There exist two methods for preparation of gold organosol from hydrosol: (a) extraction of preformed gold nanoparticles (AuNPs) from aqueous medium to an organic phase with long chain amines; and (b) extraction of gold chloride into organic solvent with long chain amines and subsequent reduction of
Figure 7. TEM image and size distribution histogram of gold hydrosol-A.
produced AuNPs that are capped by BH4−, a strong nucleophile indeed.39 That is why we observed a blue-shifted plasmon band of AuNPs. Interestingly, size distribution profile is improved for organosol system, and the average particle size become marginally larger, and this is due to multiple aggregations.3c TEM images speak that the diameter of the AuNP before and after removal of free amine from organosol remains intact. The particles are exactly spherical in shape. The particle size histograms before and after free amine removal imply that the average diameter of the particles is not at all affected after the addition of [Cu(St)2]. From the HRTEM image, it is found that before and after free amine separation, lattice fringe also remains the same (lattice fringe spacing of 0.236 nm corresponds to {111} plane of face centered cubic lattice). A question may arise whether the violet [Cu(St)2]−amine adduct contains AuNPs. To rule out the possibility, we have done energy-dispersive X-ray (EDX) analysis, which confirms the absence of gold in the adduct (Supporting Information, Figure S7). It implies that the violet compound is an adduct of [Cu(St)2] and amine only. The violet adduct along with unreacted [Cu(St)2] jointly confirms the quantative removal of the unbound amine. The IR spectrum of the violet product proves the presence of both [Cu(St)2] and DDA (Supporting Information, Figure S8). Carboxylale stretching (1563 and 1406 cm−1), −CH2 stretching (2936 and 2852 cm−1), and C−N stretching (1082 cm−1) can be assigned from the IR spectrum of the violet solid. From reflectance spectral data, absorbance of the [Cu(St)2], and the resulting compound was determined. The spectral profile of 9270
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Figure 8. TEM image of gold organosol (a) before and (b) after free amine separation (produced from hydrosol-A).
Scheme 3. Schematic Diagram indicating that [Cu(St)2]Treatment Is the Best Technique of Free Amine Removal from Gold Organosol
Figure 9. Absorbance vs wavelength spectra of solid [Cu(St)2] (from reflectance measurement), solid Cu(St)2−amine (from reflectance measurement), and Cu(St)2−amine adduct in chloroform.
Au(III) to obtain organosol. The proposed amine removal technique becomes effective for both types of sol systems as mentioned above. Some literature suggest the cleaning of amine protected AuNPs by alcohol (methanol, ethanol). Upon addition of alcohols to the sol system, the alcoholic solution of amines remains as supernatant, and AuNPs are precipitated.2b,41 The clean, amine protected AuNP precipitate can be redispersed in various organic solvents. But, this procedure is tedious, associated with wastage of solvent, and AuNPs in portion may remain in the supernatant. During precipitation by alcohols (due to change in polarity of the sol system), other components and/or modifiers may also be affected, and the problem of aggregation of AuNPs cannot be ruled out. Sometimes evaporation of solvent phase and redispersion are adopted for cleaning the nanoparticles.42 But for noble metal nanoparticles, flocculation becomes the main problem. Few
reports of washing the precipitate (obtained by direct centrifugation of the sol) before redispersion are also available.43 It is also difficult to obtain solid mass from a stable sol system of very small particle size without affecting the size and shape. Lower particle concentration adds a new dimension to the problem of purification as the collection of the centrifuged mass is not easy in that system. For higher concentration of NPs, aggregation probability is also high. Comparable solubility of nanoparticles and impurity often makes fractional crystallization and other commonly used purification protocol inadequate. The proposed purification tactic disclosed here involves a single step, easy, costeffective, and quantitative technique for amine protected gold organosol purification without affecting the morphology, loss of 9271
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particle concentration, and stability no matter what the sizes of nanoparticles are (Scheme 3). The purification of amine stabilized gold organosol by solid [Cu(St)2] eliminates free amine selectively. Interestingly, the as-obtained gold organosol (devoid of free amine) remains stable for several months (data not shown) at room temperature (25 °C). Thus, the clean gold organosol becomes useful for fluorescence studies, Raman spectroscopy (especially in SERS), catalysis, biochemical processes, or in other fields where excess amine may be a nuisance. To support the contention, differential fluorescence quenching studies of salicylaldehyde using both types of gold organosol systems (with and without free amine) (Figure 10) have
Article
ASSOCIATED CONTENT
* Supporting Information S
Percent recovery of free amine, AAS analysis, TEM images of organosol capped with different straight chain amines and thiols, TEM images of hydrosol-B, UV−vis spectra of hydrosol at different condition, fluorescence spectra of control experiment, UV−vis spectra at different stages of organosol preparation, XPS analysis, EDX analysis, IR spectra, mass spectrum of [Cu(St)2]-amine adduct, and elemental analysis. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are thankful to the UGC, DST, NST, BRNS, and CSIR, New Delhi, India, and the IIT Kharagpur for financial assistance.
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REFERENCES
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Figure 10. Fluorescence spectra of (1) salicylaldehyde (4.4 × 10−4 M), (2) salicylaldehyde (4.4 × 10−4 M) and gold organosol (5.5 × 10−6 M) without free DDA, (3) salicylaldehyde (4.4 × 10−4 M) and gold organosol (5.5 × 10−6 M) with unbound free DDA, and (4) salicylaldeyde (4.4 × 10−4 M) and DDA (5 × 10−3 M) added from outside.
been done. This experiment explicitly speaks the problem of amine contamination for fluorescence studies. Amine forms Schiff base with aldehyde rendering enhancement of fluorescence (fluorophore). Long chain amines such as DDA, if present in excess in organosol, react with salicylaldehyde.44 Thus, the fluorophore (aldehyde) is changed, and the extent of quenching is also altered with large amount of free amine. This work designates a workable place exchange reaction where [Cu(St)2] destabilizes thiol stabilized gold organosol. Thus, naked Au(0) results as a precipitate. However, similar reaction is not observed while amine stabilized gold organosol is treated with [Cu(St)2], which simply removes free unbound amine only keeping the amine stabilized gold organosol intact. So, [Cu(St)2] becomes a unique reagent to remove excess unbound amines, not the unbound thiols.
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CONCLUSIONS A well-characterized reagent, [Cu(St)2], has been rediscovered as a new reagent that selectively removes unbound amine from gold organosol keeping the properties of amine capped AuNPs unaffected. Here, the removal of free long chain amine by the solid reagent becomes a truly demonstrable solid phase extraction process. This work becomes important where the use of pure gold organosol is crucial, namely, in the biochemical field and also in the fields like catalysis and spectroscopy. 9272
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