Article pubs.acs.org/Langmuir
Multidentate Ionic Surfactant Mediated Extraction and Dispersion of Gold Nanoparticles in Organic Solvents Sangbum Han and Ramjee Balasubramanian* Department of Chemistry and Biochemistry, Old Dominion University, Norfolk, Virginia 23529, United States S Supporting Information *
ABSTRACT: Resorcinarenes with three different quaternary ammonium headgroups were synthesized and evaluated for their ability to stabilize gold nanoparticles in organic and aqueous medium. Aqueous dispersions of citrate stabilized gold nanoparticles of dimensions up to 29 nm could be extracted into organic solvents by resorcinarenes functionalized with tetrapyridinium tetrabromide (1), tetratrimethylammonium tetrabromide (2), and tetratributylammonium tetrabromide (3). Such nanoparticles were characterized by TEM, EDS, UV−vis, and IR. Their long-term dispersion stability varied significantly and depended on the nature of the resorcinarene headgroup, and in particular nanoparticles extracted by resorcinarene 1 were stable for several weeks. Nanoparticles passivated by resorcinarenes 1 and 2 were also stable in the presence of thiourea for several hours in both aqueous and organic medium. This is notable as thiourea is known to result in the instantaneous aggregation of citrate stabilized nanoparticles. Remarkably nanoparticles stabilized by resorcinarenes 1 and 2 could be precipitated and redispersed in chloroform without any visible aggregation. The critical parameters controlling the extraction of the nanoparticles into the organic phase have also been evaluated. The resorcinarene surfactant mediated facile phase transfer of gold nanoparticles described here can be readily applied for the stabilization of other citrate stabilized mono- and bimetallic nanoparticles, thus providing opportunities to disperse and stabilize relatively larger nanoparticles in organic solvents using ionic surfactants opening up new applications.
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INTRODUCTION Metal nanoparticles, especially gold, have been subject to intense investigation in recent years and have been applied in wide-ranging areas.1−6 Nanoparticles can be synthesized in water or nonpolar organic solvents and these approaches have their own advantages and disadvantages.7 Applications often require the transfer of nanoparticles synthesized in one phase into the other, and phase transfer methods have been developed for the transfer of nanoparticles from aqueous to organic phase or vice versa.7,8 A variety of surfactants and methods have been employed for the extraction of gold nanoparticles into organic solvents.7,8 The phase transfer of nanoparticles into organic solvents by small molecule ligands have been typically limited to smaller nanoparticles (dimensions 200 nm) and much broader extinction peak (Figure 6c). We also tested the resistance of the nanoparticle dispersions in toluene−acetonitrile mixture, and the extinction spectra of the nanoparticles extracted by 1 did not show any change after the addition of thiourea (Figure 6d). All these experiments unambiguously demonstrate the robust protection provided by resorcinarenes 1 and 2 against the irreversible flocculation of the nanoparticles in both aqueous and organic medium even under adverse conditions.
We evaluated the role of several other parameters in the extraction process using resorcinarene 2 as it typically leads to the complete extraction of nanoparticles into organic phase. With lesser amount of resorcinarene 2 (0.81 mM instead of 4 mM) gold nanoparticles could only be extracted partially into the organic phase with some remaining in the interface (Figure S7b). Consistent with literature,20,40 hydrochloric acid is required for the effective phase transfer of nanoparticles. Though the precise role of HCl in phase transfer is still unclear, it is believed to neutralize surface charges on nanoparticles41 and in some instances decompose borohydride8 on nanoparticle surfaces and thereby facilitate the adsorption of the added surfactants to nanoparticle surfaces. When HCl was replaced with other weaker organic acids such as acetic acid, only miniscule amounts of gold nanoparticles were extracted (Figure S7c). The cosolvent acetonitrile also played a major role in the extraction process. In the absence of acetonitrile, i.e., when resorcinarene 2 was dissolved in water and added to aqueous colloidal dispersion of gold nanoparticles, the emulsion resulting after the addition of toluene could not be broken even after the addition of concentrated HCl. The emulsion took 2 weeks to partially clear, and nanoparticles were phase transferred into emulsified organic phase (Figure S7d). Yang and co-workers42 have similarly observed the formation of a milky emulsion in the absence of a cosolvent, and in their experiments nanoparticles were not transferred into organic phase. They concluded42 that the cosolvent increases the interfacial contact between citrate stabilized nanoparticles and the surfactant, thereby facilitating the phase transfer. Cosolvents like tetrahydrofuran21 and ethanol42 have been shown to be critical in the extraction and stability of phase transferred nanoparticles. The acetonitrile and toluene used in this extraction process could be readily changed to other solvent pairs. For example, the surfactant could be dissolved in methanol and the nanoparticles could be extracted using chloroform (Figure S7e). In the literature, quaternary ammonium salts have been shown to be unsuccessful in breaching the 10 nm16 or 20 nm17 size limit for phase transfer of nanoparticles into organic solvents. We tested the ability of resorcinarenes 1−3 in extracting larger (23 and 29 nm sized) nanoparticles (Figure S8). While resorcinarenes 1 and 2 were effective in extracting most of the nanoparticles into the organic phase, in the case of resorcinarene 3, nanoparticles were only partially extracted (Figure 3). Also, varying amounts of nanoparticles remained in the interface. While nanoparticles extracted by 1 and 2 were visually stable for months, the nanoparticles extracted by 3 (Figure 3d) were not even stable for 16 h (Figure 3e).43 The TEM analyses of 29 nm nanoparticles extracted by resorcinarenes 1−3 are provided in Figure 4a−c. While nanoparticles extracted by resorcinarenes 1 and 2 are well-dispersed, those extracted by resorcinarene 3 into the organic phase were severely aggregated. The UV−vis spectra of these nanoparticles (Figure 4d) showed that the extinction maxima was slightly red-shifted (∼535 nm) when compared to the smaller 18 nm particles (Figure 1b). Surfactant Headgroup Dependent Stabilization of Nanoparticles. To systematically probe the differences, if any, between the 18 nm gold nanoparticles stabilized by various resorcinarenes, we monitored their long-term dispersion stability. The nanoparticles extracted by resorcinarene 1 showed excellent dispersion stability over several weeks (Figure 5 and Figure S9a) at ambient temperature with minimum
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CONCLUSION In summary, we have developed a simple method for the extraction of citrate stabilized gold nanoparticles into organic solvents using multidentate ionic resorcinarenes 1−3. This approach allows the dispersion of relatively larger gold nanoparticles (up to 29 nm) in organic solvents, hitherto inaccessible by other ionic surfactants.16,17 The phase transfer process and the resulting nanoparticles have been characterized by TEM, EDS, UV−vis, and IR. The long-term dispersion stability of these nanoparticles in organic solvents depended on the nature of the ionic headgroup of the resorcinarene 9068
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surfactants, and the dispersions stabilized by resorcinarene 1 were stable for several weeks under ambient conditions. Further the nanoparticles stabilized by resorcinarenes 1 and 2 were also stable against thiourea induced aggregation44 in both aqueous and organic medium. Resorcinarenes 1 and 2 also endowed the nanoparticles with excellent processability, and they could be precipitated and redispersed in organic solvents without any irreversible aggregation. Though we have demonstrated the ability of resorcinarenes to extract and stabilize gold nanoparticles, this approach can be readily applied to other citrate stabilized mono- and bimetallic nanoparticles 42,45 with implications in a number of applications including catalysis.25
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ASSOCIATED CONTENT
* Supporting Information S
TEM of gold nanoparticles, photographs of extractions, precipitate and redispersed nanoparticles, IR and UV−vis spectra of nanoparticles. 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] (R.B.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Dr. Wei Cao and Mr. Vara Prasad Sheela for help with TEM and EDS measurements. The authors also acknowledge the financial support from Old Dominion University. The authors thank the reviewers of this manuscript for their valuable suggestions and inputs.
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