Article pubs.acs.org/IECR
Fractionation of Surface-Modified Gold Nanorods Using GasExpanded Liquids Gregory Von White II, Matthew Grant Provost, and Christopher Lawrence Kitchens* Department of Chemical and Biomolecular Engineering, Clemson University, Clemson, South Carolina 29634, United States S Supporting Information *
ABSTRACT: Gold nanorods (GNRs) have found widespread applications in nanocomposites and thin films, because of their unique optical, chemical, and photothermal properties, which are dictated by the rod size and aspect ratio. In this study, gold nanorods (GNRs, aspect ratio of 3−4) were synthesized using a high-yield, aqueous, solution-based method, followed by a surface modification reaction to facilitate their dispersion in organic media. A gas-expanded liquid (GXL) precipitation technique was used to effectively size-fractionate the GNRs with CO2 as a green antisolvent for hydrophobic nanorods dispersed in toluene, hexane, and cyclohexane. The advantages of using GXL media lie in the tunable solvent properties that enable size-selective nanoparticle precipitation that is easily controlled by the CO2 antisolvent partial pressure. This work demonstrated effective GNR size fractionation and a 73% reduction in the number of residual 4-nm-diameter spherical seed nanoparticles remaining after synthesis with a single precipitation and redispersion. The GNR dispersibility and precipitation was monitored by ultraviolet-visible (UV−vis) absorbance spectroscopy and found to be dependent on the solvent choice and GNR ligand. CO2expanded cyclohexane provided the greatest dispersibility of GNRs stabilized by 18-carbon-chain-length ligands, which were dispersible at pressures up to roughly 30 bar (0.44 mol fraction CO2), compared to a lower pressure, on the order of 24 bar for CO2-expanded toluene (∼0.21 mol fraction CO2) and n-hexane (0.31 mol fraction CO2). Varying the hydrophobic stabilizing ligand chain length also impacted nanorod dispersibility in CO2-expanded toluene, where 12-carbon-chain-length dodecanethiol ligands yielded nanorod dispersion/precipitation at CO2 pressures much greater than those for the 18-carbon octadecanethiol ligands. This work is the first application of a GXL solvent medium for the processing, purification, and size fractionation of nonspherical particles, which has led to a greater understanding of gold nanorod dispersibility and demonstrated the feasibility of GXLs as a green solvent medium for post-synthesis nanorod processing. and lower toxicity,16 either by removal of CTAB, polymer coating,17,18 replacement of CTAB with polymers,18 or acid treatment.19 Removal of excess CTAB dispersed in solution is trivial and can be achieved by centrifugation and redispersion in neat solvent (water).8,16 Removal of CTAB bound to the nanorod surface without compromising the GNR stability is a far more daunting task and is often circumvented by polymer encapsulation.17 The CTAB bilayer provides electrostatic and steric repulsion forces, which facilitate stable GNR dispersions; thus, excessive CTAB removal leads to irreversible aggregation. Maintaining sufficient dispersion forces during the surface modification process to prevent irreversible agglomeration is a significant barrier to the hydrophobization of GNRs. Moreover, conventional ligand exchange reactions commonly employed for spherical nanoparticles20−22 do not work for GNRs, resulting in irreversible agglomeration. Few researchers have successfully synthesized or dispersed GNRs in organic solvents (chloroform, toluene, n-hexane, etc.).1,17,23,24 Recently, Chandran et al. used a seed-mediated process to synthesize GNRs in toluene.23 Hydrophobic amines were used as phase transfer catalysts for Au ions and 6.1-nm seed nanoparticles, as well as functioning as the reducing and
1. INTRODUCTION Gold nanorods (GNRs) have unique size- and aspect-ratiodependent properties,1−3 which are ideal for sensing and electronic applications,4 thin-film optical limiters,5 as well as biomedical contrast agents and therapeutics.6 GNRs are also efficient at transforming absorbed radiation energy into heat (photothermal activity), making them useful in nanomedicine as hyperthermia agents.7,8 The shape, stiffness, and aspect ratios of rod-shaped particles make GNRs ideal filler materials for applications ranging from hydrophobic polymers in composite and thin-film applications to biomedical therapies and diagnostics.9−12 Many of these applications require monodisperse populations of GNRs with hydrophobic surface chemistries in order to promote matrix compatibility, improved stability, and chemical functionalities. Currently, there are no high-yield solution-based synthesis procedures that afford monodisperse populations of hydrophobic GNRs (dispersible in organic media); thus, ligand exchange reactions and postsynthesis size fractionation are required. Cetyltrimethylammonium bromide (CTAB) is the most widely used shape-directing cationic surfactant employed for the synthesis of nonspherical gold nanoparticles (in particular, GNRs).3,13−15 Murphy and co-workers have demonstrated that tailoring the synthesis conditions affords high yields and varying sizes of hydrophilic GNRs with minimal growth of spherical and nonrod shaped nanoparticles.2,15 However, post-synthesis processing must be employed to facilitate surface modification © 2012 American Chemical Society
Received: Revised: Accepted: Published: 5181
September 1, 2011 March 14, 2012 March 18, 2012 March 19, 2012 dx.doi.org/10.1021/ie201975p | Ind. Eng. Chem. Res. 2012, 51, 5181−5189
Industrial & Engineering Chemistry Research
Article
was not optimized to achieve maximum shape uniformity, but was tuned to yield large volumes of GNRs (i.e., excess seed particles and large spheres were present in the sample but GNRs were the predominant nanoparticles). More-uniform shape distributions of GNRs are achievable if NaCl is added during synthesis (NaCl concentrations typically vary from 1 to 4 times the gold salt concentration in the growth solution). In general, size polydispersity and the presence of non-rod-shaped particles is unavoidable and post-synthesis size fractionation is often required.2 We investigated the dispersibility and size fractionation of GNRs (aspect ratio of 3−4) in varying CO2expanded solvents including cyclohexane, toluene, and nhexane, as well as the impact of ligand chain length on GNR dispersibility in CO2-expanded toluene. GNRs stabilized by 18carbon-long ligands exhibited the highest dispersibility in CO2expanded cyclohexane and GNRs stabilized by 12-carbonligand lengths proved to have greater dispersibility in CO2expanded toluene, compared to 18 carbon ligands. The fractionation demonstrated an improvement in GNR monodispersity and decrease in excess seed concentration with a single pass precipitation. Further recursive precipitation and process optimization is possible to enhance the fractionation results.
stabilizing agents. Size control was achieved for aspect ratios up to 11 by varying the synthesis conditions; however, large spheres and other irregular shapes were synthesized in addition to the low yield of GNRs. Subsequent post-synthesis processing would be required to isolate monodisperse populations of GNRs, for example, recursive solvent/antisolvent (e.g., toluene/ethanol) precipitation combined with centrifugation. Surface modification of CTAB-capped aqueous nanorod dispersions is an alternative approach to obtain hydrophobically stabilized GNRs. Pastoriza-Santos et al. coated hydrophilic CTAB-stabilized GNRs with a silica shell (varying thicknesses in the range of 12−58 nm), enabling their suspension in chloroform;17 however, the silica shell thicknesses were large. Such a thick shell can inhibit desirable properties of the GNRs (for example, refractive-index-dependent plasmon resonance or photothermal activity). Mitamura et al. hydrophobized GNRs using 3-mercaptopropyltrimethoxysilane (MPS) and subsequent tethering to octadecyltrimethoxysilane (ODS) through the hydrolysis of the Si−OR groups.24 This procedure demonstrates a simple and successful approach to the hydrophobization of CTAB-capped GNRs, with minimal changes to ligand shell thickness. Surface modification of aqueous dispersed CTAB-capped GNRs is advantageous, because of the demonstrated higher yield and monodispersity of GNRs produced; however, it is not size or shape-selective and undesired seed or other shaped nanoparticles will also be modified and remain with the GNRs. Size-selective fractionation of hydrophobically stabilized GNRs and the removal of seed particles (and other non-rod-shaped particles) has the potential to minimize the breadth of the longitudinal surface plasmon resonance (SPR) peak and decrease the intensity of the transverse SPR peak, resulting in improved optical properties suited for sensing applications. Gas-expanded liquids (GXLs) are a class of pressure-tunable solvents used for a variety of processes, including extraction, separations, and even nanoparticle synthesis and size-selective fractionation.25,26 Carbon dioxide (CO2) is the primary gas employed in GXL processes, because of the abundance, inert nature, and high solubility in organic solvents. When CO2 is added to an organic solvent, the mole fraction of CO2 increases in the liquid phase (dependent on the CO2 partial pressure) and simultaneously causes the liquid phase volume to expand.25 CO2 has been shown to be an effective antisolvent for hydrophobic nanoparticles dispersed in nonpolar solvents, providing control over size-selective precipitation of alkanethiol-modified, sub-10-nm spherical nanoparticles of gold, silver, platinum, and quantum dots.20,21,27−29 Once precipitated, the nanoparticles can be redispersed in neat solvent and the supernatant recycled for new synthesis/reuse following depressurization. The GXL technique is ideal for nanoparticle isolation, compared to recursive liquid−liquid solvent/antisolvent techniques (e.g., chloroform/ethanol), because the antisolvent composition is easily controlled with pressure and the original solvents can be recovered through simply depressurization, resulting in zero solvent waste.26 It also eliminates energy- and time-intensive centrifugation because of the enhanced transport properties of the GXL media,2,5 which are less invasive, and facilitates nanoparticle redispersion. Furthermore, centrifugation processes are not easily controlled or easily scaled up. In this work, we use an adapted procedure developed by Mitamura et al.30 to hydrophobize CTAB-capped GNRs and disperse them in various organic solvents. The GNR synthesis
2. EXPERIMENTAL SECTION 2.1. Synthesis of Surface Modified GNRs. The materials and GNR synthesis methods are described in the Supporting Information. The GNR synthesis was adapted from a CTAB seed-mediated growth procedure by Sau et al.13 The surface modification process that enabled the CTAB displacement and GNR redispersion in toluene was adapted from the work of Mitamura et al.24 In short, 3-mercaptopropyltrimethoxysilane (MPS) in ethanol (0.30 mL of 0.02 M) was added to 30.0 mL of aqueous GNRs, followed by vigorous mixing for at least 30 min. Next, n-octadecyltrimethoxysilane (ODS) in chloroform (15.0 mL of 0.02M) was added, creating a biphasic mixture, followed by NaOH (0.30 mL of 1.0 M) with vigorous mixing. The biphasic system was mixed vigorously using a magnetic stir bar for at least 4 h. After mixing, the deep purple color transferred from the upper aqueous phase to the lower chloroform phase. The ODS-stabilized GNRs in chloroform were removed from the biphasic mixture and octadecanethiol in chloroform (1.0 mL of 0.01 M) was added as a co-stabilizing ligand. GNR dispersions prepared without the addition of a costabilizing agent (alkanethiol) oxidized during water purification and irreversibly aggregated during washing/centrifugation with ethanol. The octadecanethiol/ODS-stabilized GNRs were washed by adding 15.0 mL of water and vortex mixing for ∼30 s. The cloudy white supernatant, containing water-soluble ligands, was removed and the process was repeated. Next, the GNR solution was diluted with ethanol (2:1 ratio ethanol to GNR solution) and centrifuged at 5000 rpm for 5 min to precipitate the GNRs and decant any excess dispersing ligands. The precipitated GNRs were dried with nitrogen and resuspended in neat solvent (cyclohexane, toluene, or n-hexane). The stable dispersion of GNRs was sonicated for 5 min. Figures S1 and S2 in the Supporting Information show transmission electron microscopy (TEM) images and respective histograms of the length, width, and aspect ratios of the synthesized CTABstabilized GNRs and the octadecanethiol/ODS-stabilized GNRs. GNRs were also stabilized with dodecyltrimethoxysilane (DDS) and dodecanethiol co-stabilizing ligand, replacing ODS 5182
dx.doi.org/10.1021/ie201975p | Ind. Eng. Chem. Res. 2012, 51, 5181−5189
Industrial & Engineering Chemistry Research
Article
Figure 1. Calculated volume expansion coefficients (V/V0) for CO2-expanded (A) toluene, (B) n-hexane, (C) cyclohexane at 25 °C, and (D) CO2 composition in each solvent, as a function of partial pressure, as determined using the Patel−Teja Equation of State (PT-EOS). [The volume expansion coefficients for CO2-expanded toluene are compared to the experimental work presented by Houndonougbo et al.35 at 30 °C and Mukhopadhyay et al.36 at 25 °C, respectively.]
Table 1. Average Length, Width, Aspect Ratio, and Volumea of Octadecanethiol/ODS-Stabilized GNRs and Number Fraction of 4 nm Seeds Obtained at Varying Isolation Conditions in CO2-Expanded Toluene
a
fraction number
isolation pressure (bar)
length (nm) (95% CI)
width (nm) (95% CI)
volume (nm3) (95% CI)
aspect ratio (95% CI)
number fraction of 4 nm seeds
original 1 2 3 4
toluene > n-hexane at lower pressures (
