fractionation of surface-modified gold nanorods using gas-expanded liquids

Fractionation of Surface-Modified Gold Nanorods Using Gas-Expanded LiquidsGregory 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 theirunique optical, chemical, and photothermal properties, which are dictated by the rod size and aspect ratio. In this study, goldnanorods (GNRs, aspect ratio of 3−4) were synthesized using a high-yield, aqueous, solution-based method, followed by asurface modification reaction to facilitate their dispersion in organic media. A gas-expanded liquid (GXL) precipitation techniquewas 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-selectivenanoparticle precipitation that is easily controlled by the CO2 antisolvent partial pressure. This work demonstrated effectiveGNR size fractionation and a 73% reduction in the number of residual 4-nm-diameter spherical seed nanoparticles remainingafter synthesis with a single precipitation and redispersion. The GNR dispersibility and precipitation was monitored byultraviolet-visible (UV−vis) absorbance spectroscopy and found to be dependent on the solvent choice and GNR ligand. CO2-expanded cyclohexane provided the greatest dispersibility of GNRs stabilized by 18-carbon-chain-length ligands, which weredispersible at pressures up to roughly 30 bar (0.44 mol fraction CO2), compared to a lower pressure, on the order of 24 bar forCO2-expanded toluene (∼0.21 mol fraction CO2) and n-hexane (0.31 mol fraction CO2). Varying the hydrophobic stabilizingligand chain length also impacted nanorod dispersibility in CO2-expanded toluene, where 12-carbon-chain-length dodecanethiolligands yielded nanorod dispersion/precipitation at CO2 pressures much greater than those for the 18-carbon octadecanethiolligands. This work is the first application of a GXL solvent medium for the processing, purification, and size fractionation ofnonspherical particles, which has led to a greater understanding of gold nanorod dispersibility and demonstrated the feasibility ofGXLs as a green solvent medium for post-synthesis nanorod processing.

1. INTRODUCTIONGold nanorods (GNRs) have unique size- and aspect-ratio-dependent properties,1−3 which are ideal for sensing andelectronic applications,4 thin-film optical limiters,5 as well asbiomedical contrast agents and therapeutics.6 GNRs are alsoefficient at transforming absorbed radiation energy into heat(photothermal activity), making them useful in nanomedicineas hyperthermia agents.7,8 The shape, stiffness, and aspect ratiosof rod-shaped particles make GNRs ideal filler materials forapplications ranging from hydrophobic polymers in compositeand thin-film applications to biomedical therapies anddiagnostics.9−12 Many of these applications require mono-disperse populations of GNRs with hydrophobic surfacechemistries in order to promote matrix compatibility, improvedstability, and chemical functionalities. Currently, there are nohigh-yield solution-based synthesis procedures that affordmonodisperse populations of hydrophobic GNRs (dispersiblein organic media); thus, ligand exchange reactions and post-synthesis size fractionation are required.Cetyltrimethylammonium bromide (CTAB) is the most

widely used shape-directing cationic surfactant employed forthe synthesis of nonspherical gold nanoparticles (in particular,GNRs).3,13−15 Murphy and co-workers have demonstrated thattailoring the synthesis conditions affords high yields and varyingsizes of hydrophilic GNRs with minimal growth of sphericaland nonrod shaped nanoparticles.2,15 However, post-synthesisprocessing must be employed to facilitate surface modification

and lower toxicity,16 either by removal of CTAB, polymercoating,17,18 replacement of CTAB with polymers,18 or acidtreatment.19 Removal of excess CTAB dispersed in solution istrivial and can be achieved by centrifugation and redispersion inneat solvent (water).8,16 Removal of CTAB bound to thenanorod surface without compromising the GNR stability is afar more daunting task and is often circumvented by polymerencapsulation.17 The CTAB bilayer provides electrostatic andsteric repulsion forces, which facilitate stable GNR dispersions;thus, excessive CTAB removal leads to irreversible aggregation.Maintaining sufficient dispersion forces during the surfacemodification process to prevent irreversible agglomeration is asignificant barrier to the hydrophobization of GNRs. Moreover,conventional ligand exchange reactions commonly employedfor 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-mediatedprocess to synthesize GNRs in toluene.23 Hydrophobic amineswere used as phase transfer catalysts for Au ions and 6.1-nmseed nanoparticles, as well as functioning as the reducing and

Received: September 1, 2011Revised: March 14, 2012Accepted: March 18, 2012Published: March 19, 2012

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stabilizing agents. Size control was achieved for aspect ratios upto 11 by varying the synthesis conditions; however, largespheres and other irregular shapes were synthesized in additionto the low yield of GNRs. Subsequent post-synthesis processingwould be required to isolate monodisperse populations ofGNRs, 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 hydrophobicallystabilized GNRs. Pastoriza-Santos et al. coated hydrophilicCTAB-stabilized GNRs with a silica shell (varying thicknessesin the range of 12−58 nm), enabling their suspension inchloroform;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 orphotothermal activity). Mitamura et al. hydrophobized GNRsusing 3-mercaptopropyltrimethoxysilane (MPS) and subse-quent tethering to octadecyltrimethoxysilane (ODS) throughthe hydrolysis of the Si−OR groups.24 This proceduredemonstrates a simple and successful approach to thehydrophobization of CTAB-capped GNRs, with minimalchanges to ligand shell thickness. Surface modification ofaqueous dispersed CTAB-capped GNRs is advantageous,because of the demonstrated higher yield and monodispersityof GNRs produced; however, it is not size or shape-selectiveand undesired seed or other shaped nanoparticles will also bemodified and remain with the GNRs. Size-selective fractiona-tion of hydrophobically stabilized GNRs and the removal ofseed particles (and other non-rod-shaped particles) has thepotential to minimize the breadth of the longitudinal surfaceplasmon resonance (SPR) peak and decrease the intensity ofthe transverse SPR peak, resulting in improved opticalproperties 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-selectivefractionation.25,26 Carbon dioxide (CO2) is the primary gasemployed in GXL processes, because of the abundance, inertnature, and high solubility in organic solvents. When CO2 isadded to an organic solvent, the mole fraction of CO2 increasesin 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 forhydrophobic nanoparticles dispersed in nonpolar solvents,providing control over size-selective precipitation of alkane-thiol-modified, sub-10-nm spherical nanoparticles of gold,silver, platinum, and quantum dots.20,21,27−29 Once precipi-tated, the nanoparticles can be redispersed in neat solvent andthe supernatant recycled for new synthesis/reuse followingdepressurization. The GXL technique is ideal for nanoparticleisolation, compared to recursive liquid−liquid solvent/anti-solvent techniques (e.g., chloroform/ethanol), because theantisolvent composition is easily controlled with pressure andthe original solvents can be recovered through simplydepressurization, resulting in zero solvent waste.26 It alsoeliminates energy- and time-intensive centrifugation because ofthe enhanced transport properties of the GXL media,2,5 whichare less invasive, and facilitates nanoparticle redispersion.Furthermore, centrifugation processes are not easily controlledor easily scaled up.In this work, we use an adapted procedure developed by

Mitamura et al.30 to hydrophobize CTAB-capped GNRs anddisperse them in various organic solvents. The GNR synthesis

was not optimized to achieve maximum shape uniformity, butwas tuned to yield large volumes of GNRs (i.e., excess seedparticles and large spheres were present in the sample butGNRs were the predominant nanoparticles). More-uniformshape distributions of GNRs are achievable if NaCl is addedduring synthesis (NaCl concentrations typically vary from 1 to4 times the gold salt concentration in the growth solution). Ingeneral, size polydispersity and the presence of non-rod-shapedparticles is unavoidable and post-synthesis size fractionation isoften required.2 We investigated the dispersibility and sizefractionation of GNRs (aspect ratio of 3−4) in varying CO2-expanded solvents including cyclohexane, toluene, and n-hexane, as well as the impact of ligand chain length on GNRdispersibility in CO2-expanded toluene. GNRs stabilized by 18-carbon-long ligands exhibited the highest dispersibility in CO2-expanded cyclohexane and GNRs stabilized by 12-carbon-ligand lengths proved to have greater dispersibility in CO2-expanded toluene, compared to 18 carbon ligands. Thefractionation demonstrated an improvement in GNR mono-dispersity and decrease in excess seed concentration with asingle pass precipitation. Further recursive precipitation andprocess optimization is possible to enhance the fractionationresults.

2. EXPERIMENTAL SECTION2.1. Synthesis of Surface Modified GNRs. The materials

and GNR synthesis methods are described in the SupportingInformation. The GNR synthesis was adapted from a CTABseed-mediated growth procedure by Sau et al.13 The surfacemodification process that enabled the CTAB displacement andGNR redispersion in toluene was adapted from the work ofMitamura et al.24 In short, 3-mercaptopropyltrimethoxysilane(MPS) in ethanol (0.30 mL of 0.02 M) was added to 30.0 mLof aqueous GNRs, followed by vigorous mixing for at least 30min. 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 stirbar for at least 4 h. After mixing, the deep purple colortransferred from the upper aqueous phase to the lowerchloroform phase. The ODS-stabilized GNRs in chloroformwere removed from the biphasic mixture and octadecanethiol inchloroform (1.0 mL of 0.01 M) was added as a co-stabilizingligand. GNR dispersions prepared without the addition of a co-stabilizing agent (alkanethiol) oxidized during water purifica-tion and irreversibly aggregated during washing/centrifugationwith ethanol.The octadecanethiol/ODS-stabilized GNRs were washed by

adding 15.0 mL of water and vortex mixing for ∼30 s. Thecloudy white supernatant, containing water-soluble ligands, wasremoved and the process was repeated. Next, the GNR solutionwas diluted with ethanol (2:1 ratio ethanol to GNR solution)and centrifuged at 5000 rpm for 5 min to precipitate the GNRsand decant any excess dispersing ligands. The precipitatedGNRs were dried with nitrogen and resuspended in neatsolvent (cyclohexane, toluene, or n-hexane). The stabledispersion of GNRs was sonicated for 5 min. Figures S1 andS2 in the Supporting Information show transmission electronmicroscopy (TEM) images and respective histograms of thelength, width, and aspect ratios of the synthesized CTAB-stabilized GNRs and the octadecanethiol/ODS-stabilizedGNRs. GNRs were also stabilized with dodecyltrimethoxysilane(DDS) and dodecanethiol co-stabilizing ligand, replacing ODS

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and octadecanethiol during modification. For all, there was nodiscernible difference is GNR size from TEM.2.2. Characterization. Ultraviolet−Visible (UV−vis)

Light Spectroscopy of GNRs in GXLs. UV−vis spectroscopyof GNRs dispersed in GXLs was performed on a Varian Cary50 spectrophotometer. The sample cell used during exper-imentation was a custom stainless steel pressure cell equippedwith two sapphire windows (12.7 mm thick/windows, 15.8 mmpath length), a pressure transducer, and an inlet valve for CO2

addition/removal. Neat solvent (toluene, n-hexane, or cyclo-hexane) was used for baseline correction of all UV−vis spectra.For each experiment, 4.0 mL of GNR dispersion was added tothe pressure cell prior to pressurization. CO2 was delivered tothe cell under pressure using a Teledyne Isco 500 HP syringepump. Spectra were collected 20−30 min after pressurization,allowing the system to reach equilibrium pressure and aconstant UV−vis absorbance. Analysis of the UV−vis spectrafor GNRs in GXLs included a second background compensa-tion using the absorbance value at a wavelength of minimumabsorbance. The absorbance maximum ratio and normalization

is to account for concentration differences, and a correction forvolume expansion, which is due to increased CO2 composi-tion.27

The volume expansion coefficients (V/V0) were calculatedfor predetermined CO2 pressures in toluene, cyclohexane, andn-hexane, using the Patel−Teja equation of state (PT-EOS).31

The PT-EOS was chosen because it allows adjustment of thecritical compressibility factor, and experimental/modeling dataare available for comparison.31−33 All UV−vis spectra werecorrected for volume expansion using a polynomial trend linefit to the calculated volume expansion coefficients for allpressures. The acentric factor and critical properties (ω, Pc, Tc,Zc) for CO2 (0.707, 73.8 bar, 304.1 K, 0.309),

34 toluene (0.262,41.06 bar, 591.8 K, 0.264),35 n-hexane (0.225, 507.6 K, 30.25bar, 0.266),34 and cyclohexane (0.210, 553.6 K, 40.73 bar,0.273)36 were readily found in the literature and used todetermine the volume expansion coefficients. The binaryinteraction parameters (kij, lij) used in the PT-EOS were alsoobtained from the literature for CO2−toluene (0.090, 0.000),CO2−n-hexane (0.138, 0.074),34 and CO2−cyclohexane

Figure 1. Calculated volume expansion coefficients (V/V0) for CO2-expanded (A) toluene, (B) n-hexane, (C) cyclohexane at 25 °C, and (D) CO2composition in each solvent, as a function of partial pressure, as determined using the Patel−Teja Equation of State (PT-EOS). [The volumeexpansion coefficients for CO2-expanded toluene are compared to the experimental work presented by Houndonougbo et al.35 at 30 °C andMukhopadhyay et al.36 at 25 °C, respectively.]

Table 1. Average Length, Width, Aspect Ratio, and Volumea of Octadecanethiol/ODS-Stabilized GNRs and Number Fractionof 4 nm Seeds Obtained at Varying Isolation Conditions in CO2-Expanded Toluene

fractionnumber

isolation pressure(bar)

length (nm) (95%CI)

width (nm) (95%CI)

volume (nm3) (95%CI)

aspect ratio (95%CI)

number fraction of 4 nmseeds

original 42.1 (±1.0) 14.7 (±0.4) 5909 (±187) 3.3 (±0.1) 0.151 <6.9 bar 43.6 (±1.0) 14.6 (±0.3) 6036 (±172) 3.3 (±0.1) 0.042 6.9−10.3 40.7 (±1.2) 14.2 (±0.4) 5330 (±192) 3.3 (±0.1) 0.073 10.3−13.8 39.9 (±1.2) 13.7 (±0.5) 4864 (±199) 3.3 (±0.1) 0.274 17.2−20.7 38.4 (±1.5) 13.8 (±0.5) 4750 (±232) 3.1 (±0.1)

aIncluding respective 95% confidence intervals for the mean values



(0.007404, −0.1710).37 The calculated volume expansioncoefficients determined from the PT-EOS compare well topreviously published results. Figure 1 shows the PT-EOS-predicted volume expansion coefficients, compared to exper-imental data measured at 25 °C for CO2 in toluene35 and n-hexane;36 experimental data for CO2 in cyclohexane at 25 °Cwas not found in the literature for comparison.2.3. GNR Isolation Using CO2-Expanded Toluene. The

GNRs were precipitated using the gas-expanded liquid (GXL)technique previously demonstrated by McLeod et al. forspherical nanoparticles.38 A cylindrical glass rod with a spiralgroove was placed inside of a custom stainless steel pressurevessel (rod-shaped) in order to fractionate the GNRs using thepressure-tunable properties of the GXLs (see Figure S3 in theSupporting Information).34 Prior to GNR isolation, 1.0 mL oftoluene was added to the pressure vessel to prevent solventevaporation during fractionation. Next, 0.2 mL of toluene-dispersed GNRs was placed in the first spiral rung of the glassrod and the vessel was sealed. Four different populations ofGNRs were isolated at varying pressure ranges. Table 1 showsthe isolation pressures, average length, width, aspect ratio, andvolume of the GNRs from each population fraction.Octadecanethiol/ODS-modified GNRs were isolated by

initially pressurizing the vessel to 6.9 bar and then allowingthe sample to reach equilibrium (20 min). Next, the cylindricalglass rod was rotated 540° (1.5 turns). This rotation enabledthe first fraction of precipitated GNRs to remain in the firstrung of the rod and transfer of the dispersed GNRs to a rungfurther up the rod. More CO2 was added until the pressurereached 10.3 bar and remained constant for 20 min (isolatingthe second fraction of GNRs in the range of 6.9−10.3 bar).Subsequently, the glass rod was rotated 270° and the cell wasdepressurized at a constant rate of ∼1 bar/min. The first andsecond fraction of GNRs were collected by toluene addition,gentle agitation, and then removal from the glass rod using apipet. The precipitation process was repeated, producing GNRfractions isolated at the pressure intervals listed in Table 1;fraction 1 isolated at 6.9 bar, fraction 2 between 6.9 and 10.3bar, fraction 3 between 10.3 and 13.8 bar, and fraction 4between 17.2 and 20.7 bar. Figure 2 displays representativeTEM images of the nonfractionated GNRs and fractions 1, 2,and 4.

3. RESULTS AND DISCUSSION

3.1. Nanoparticle Precipitation in GXLs. Figure 3Ashows the UV−vis spectra for octadecanethiol/ODS-modifiedGNRs dispersed in CO2-expanded toluene as a function of CO2partial pressure; which are modified to account for thebackground, GNR deposition onto the pressure cell windows,and solution volume expansion. The decrease in absorbanceintensity with increasing pressure demonstrates GNRsprecipitation with the addition of CO2 antisolvent. Figure 3Bshows the normalized nanorod absorbance intensity at λ = 854nm), as a function of pressure. The precipitation ofoctadecanethiol/ODS-modified GNRs was first observed near5 bar, and complete precipitation had occurred at 24 bar (0.21mol fraction CO2). Comparatively, CO2 pressures exceeding 48bar did not precipitate 4-nm-diameter, spherical, octadecane-thiol-stabilized gold nanoparticles out of toluene. GNRs areexpected to precipitate at lower pressures than small sphericalnanoparticles, because of their larger van der Waals attractiveforces and a stronger interparticle interaction potential.

Nanoparticle dispersibility in solution is directly impacted bythe stabilizing ligand chain length, as well as the solvent−ligandinteractions.27,39 Colloidal phenomena describes the nano-particle ligand contribution to the steric repulsive forces thatmitigate particleparticle attraction forces in solution andprovide favorable interactions with the solvent (osmoticrepulsive forces due to solvation).40 Colloidal modelling ofthe GNR dispersibility in GXL systems has not been previouslyinvestigated. This work provides experimental data for suchmodeling with the effects of GXL organic solvent (toluene, n-hexane, and cyclohexane) and stabilizing ligand length(octadecanethiol/ODS and dodecanethiol/DDS) on GNRdispersibility, which contributes to the fundamental under-standing of nanorod dispersibility.

3.2. Effect of Ligand Length on GNR Dispersibility.Figure 3B shows the normalized maximum absorbance valuesof dodecanethiol/DDS and octadecanethiol/ODS-stabilizedGNRs in CO2-expanded toluene, as a function of the CO2partial pressure. The octadecanethiol/ODS stabilized GNRsbegan to precipitate at ∼5 bar, as indicated by a change in UV−vis absorbance, and were completely precipitated out ofsolution at 24 bar. The shorter dodecanethiol/DDS-stabilizedGNRs began precipitating out of solution at 10 bar andremained stabilized beyond 36 bar. Similar behavior has beenobserved for liquid antisolvent precipitation of 4-nm sphericalgold nanoparticles with octadecanethiol and dodecanethiolstabilizing ligands, where ethanol/toluene and ethanol/hexanewere the antisolvent/solvent pairs. Octadecanethiol-stabilizedgold nanoparticles precipitated at lower ethanol antisolventvolume fractions, compared to the dodecanethiol-stabilizedparticles.39 Anand et al. also demonstrated chain lengthdispersibility dependence for spherical alkanethiol-modifiedsilver nanoparticles in CO2-expanded n-hexane, wheretetradecanethiol-modified silver nanoparticles precipitated atlower CO2 pressures compared to dodecanethiol.27 Dodeca-nethiol has been suggested to be an “optimum” ligand lengthfor spherical nanoparticles, both for increased nanoparticledispersibility and deposition into thin films and orderedarrays.21,41 These results demonstrate this same trend forshort-aspect-ratio GNRs.

3.3. Effect of Solvent on GNR Precipitation. GNRdispersibility was investigated in three different CO2-expandedsolvents: cyclohexane, toluene, and n-hexane. Anand et al.

Figure 2. TEM images of octadecanethiol/ODS-stabilized GNRs thatwere (A) not fractionated and (B) precipitated at ≤6.9 bar, (C) 6.9−10.3 bar, and (D) 17.2−20.7 bar using CO2-expanded toluene.



reported excellent dispersibility of spherical dodecanethiol-stabilized gold nanoparticles in CO2-expanded toluene andcyclohexane that remained in solution at pressures near thevapor pressure of CO2.

27 Hence, facile dispersibility of GNRs inCO2-expanded toluene and cyclohexane was expected. GNRdispersibility in CO2-n-hexane was investigated because thesolvent system is well-characterized and widely used forspherical nanoparticle processing; direct comparisons can bemade for nanoparticle dispersibility as a function of aspect ratioand size.26,28,34,41,42

Figure 4 shows the normalized UV−vis peak maximumabsorbance intensity for octadecanethiol/ODS-stabilized GNRsin each CO2-expanded solvent with GNR dispersibilityincreasing for the different solvents as follows: cyclohexane >toluene > n-hexane at lower pressures (<17 bar). However, atelevated pressures, a shift in GNR dispersibility was observedwith greater dispersibility in CO2 expanded n-hexane thantoluene. For CO2 expanded cyclohexane, precipitation began at∼7 bar and all particles are precipitated at 30 bar. GNRprecipitation from CO2-expanded toluene and n-hexane bothbegan near 5 bar and was complete at 24 bar. Evaluation of theGNR dispersion, as a function of CO2 molar composition inFigure 4B, reveals the trends in dispersibility accounting for theCO2 solubility in the organic solvent. This suggests increaseddispersibility and a wider CO2 mole fraction range for size-dependent fractionation as the organic solvent transitions fromtoluene to n-hexane to cyclohexane. The mole fraction resultsare largely influenced by the increased solubility of CO2 in the

cyclohexane and n-hexane, as compared to toluene (see Figure1D). Of the solvent systems investigated, CO2-expandedcyclohexane provided the widest range in pressure andantisolvent composition. Nanoparticle fractionation at elevatedCO2 compositions also possesses enhanced transport proper-ties that aid in particle fractionation time and efficiency.43

Total interaction energy models40,44,45 have been used topredict nanoparticle dispersibility in solution as a function ofparticle size, ligand shell, and bulk solvent, while trends ofnanoparticle dispersibility can be inferred based on thedifference in ligand and solvent solubility parameters (Δδ)and the Flory−Huggins χ interaction parameters.46 Pei et al.reported toluene to be a “better” solvent than n-hexane forspherical dodecanethiol-stabilized gold nanoparticles, as a resultof the of lower Δδi values for the toluene/dodecanethiol pair,compared to n-hexane/dodecanethiol.46 Evaluation of δ andΔδi for octadecane (15.8 MPa1/2)47 and the GXL media at theGNR precipitation pressures (calculated to be 15.7 MPa1/2,17.6 MPa1/2, and 14.7 MPa1/2 for cyclohexane, toluene, andhexane, respectively based on volume fraction), suggests thatoctadecanethiol ligand is most compatible with cyclohexane,followed by n-hexane, and is least compatible with toluene.Previous work by Shah et al. demonstrated that lower densitiesof supercritical ethane caused a decrease in the steric hindranceof dodecanethiol ligands on the surface of gold and silvernanoparticles, resulting in reversible flocculation and precip-itation.48 The densities of the GXLs at the GNR isolationpressures were determined to be 996, 1090, and 674 kg/m3 for

Figure 3. (A) Normalized UV−vis spectra for octadecanethiol/ODS-modified GNRs dispersed in CO2-expanded toluene with varying pressure. (B)Normalized absorbance intensity at λ = 854 nm with 18- and 12-carbon ligand lengths, as a function of the CO2 partial pressure in toluene. All UV−vis spectra were corrected for dilution effects using the volume expansion coefficients determined with the PT-EOS.

Figure 4. Normalized UV−vis absorbance intensity for ODS/octadecanethiol-modified GNRs dispersed in CO2-expanded cyclohexane toluene, andn-hexane, as a function of CO2 pressure. All UV−vis spectra were corrected for dilution using the volume expansion coefficients determined with thePT-EOS.



cyclohexane, toluene, and n-hexane, respectively, using the PT-EOS. Compounding this comparison is the lower solubility ofCO2 in toluene at comparable pressures for hexane andcyclohexane.The Flory−Huggins χ interaction parameter is a function of

the bulk solvent molar volume and difference in solubilityparameter for the nanoparticle ligand and solvent mixture (eq1). The solvent molar volume was determined using the PT-EOS and the solvent mixture solubility parameter

∑χ = δ − δ δ = ϕ δvRT

( ) with i isolv

solv ligand2

solv

(1)

from volume fraction (ϕ) weighting of the pure componentvalues of δCO2

= 12.1 MPa1/2, δcyclohexane = 16.8 MPa1/2, δtoluene =18.2 MPa1/2, and δn‑hexane = 14.9 MPa1/2.49 Figure 5 presents the

calculated χ as a function of pressure for the three systems,suggesting that the greatest compatibility exists between thenanoparticle ligand and the CO2−cyclohexane solvent. Withincreasing CO2 content, χ decreases for toluene and increasesfor n-hexane. It is difficult to draw specific conclusions from asimple interaction parameter analysis without performing acomplete interparticle interaction energy calculation for theanisotropic particles, particularly assuming a ligand solubilityparameter that neglects entropic penalties.3.4. GNR Fractionation with Varying Pressure Con-

ditions. Figure 2 shows TEM images of octadecanethiol/ODS-stabilized GNRs before fractionation (Figure 2A), andprecipitated from CO2-expanded toluene at 6.9 bar and lower(Figure 2B), between 6.9 and 10.3 bar (Figure 2C), andbetween 17.2 and 20.7 bar (Figure 2D). TEM image analysisshowed that the nonfractionated GNRs possessed an aspectratio (95% confidence intervals) of 3.3 (±0.1), length of42.1(±1.0) nm, and width of 14.7 (±0.4) nm. The largespherical gold nanoparticles present in the population weremeasured to be 27.8 ± 7.2 nm in diameter. The GNRs wereassumed to have a regular hexagonal cross section forsimplicity50 and then their volumes were calculated to beVGNR (nm

3) = (3(3)1/2/8)w2l. The GNR volumes are presentedto account for the polydispersity within both axial directionsand particle volume (mass); these properties dictate theinterparticle van der Waals attractive forces.Figure 6 shows the calculated mean volumes of ODS/

octadecanethiol-modified GNRs that were precipitated atvarying pressures from CO2-expanded toluene. The meanoverall size/volume of the original GNRs (not fractionated)

was calculated to be 5909 nm3, and the large spherical particleshad a volume of 11 249 nm3; the large spheres made up 12% ofthe nanoparticles by number, excluding excess seeds. Isolationof the largest nanoparticles was achieved by precipitation infraction 1 at 6.9 bar, yielding an average volume of 6036 nm3

for GNRs and 11 994 nm3 for the large spheres, which made up7% of the nanoparticle population by number. Fraction 2contained 0.3% large spheres and above 10.3 bar, no largespheres were observed. Increasing the isolation pressure whilemaintaining a constant change in pressure (ΔP = 3.4 bar)demonstrated the fractionation of GNRs. The mean volume ofthe precipitated GNRs was determined to be 5330 nm3 forfraction 2, 4864 nm3 for fraction 3, and 4750 nm3 for fraction 4;this demonstrated a decreasing trend for GNR size with

Figure 5. Flory−Huggins χ interaction parameter for the CO2-expanded solvents, calculated using the PT-EOS.

Figure 6. Calculated (A) volumes, (B) lengths, and (C) widths ofODS/octadecanethiol-modified GNRs isolated under varying pressureconditions using CO2-expanded toluene. Asterisk (*) indicates thatthe error bars shown represent the 95% confidence intervals anddemonstrate significant differences between varying GNR fractionsobtained during isolation.



increasing pressure. Similarly, a decreasing trend in both lengthand width is observed by increasing the CO2 antisolventcomposition (see Figures 6B and 6C).Calculation of the confidence intervals (α = 0.05), denoted

by the error bars in Figure 6, demonstrated significant statisticaldifference among the mean volumes of GNRs isolated at thedifferent CO2 pressures. The data demonstrated with 95%confidence that the mean volume of fractions 2, 3, and 4 werestatistically different than the nonfractionated GNRs. TEManalysis also showed with 95% confidence that the meanvolume of GNR fraction 1 was statistically different from themean volume of fractions 2, 3, and 4. Similarly, the meanvolume of fractions 2 and 3 were statistically different; however,fractions 3 and 4 were statistically the same. The overall trendobserved was decreased GNR size with increased pressure orCO2 composition. It should also be noted that this is an initialfractionationrecursive fractionation will likely result inincreased monodisperse populations.28 What is lost in thisanalysis is the total number of GNRs in each fraction. A roughconcentration can be inferred from the changes in UV−visabsorbance intensity in Figures 3 and 4, which suggests that ahigher concentration of GNRs is precipitated at the lowerpressures (fraction 1). This is true for the 18-carbon ligands;however, the 12-carbon-chain ligands exhibit greater solubilityand a wider fractionation pressure range.The GNR fractionation also demonstrated a decrease in the

frequency of spherical seed nanoparticles present in thepopulation of nanoparticles that have precipitated fromsolution. The separation of GNRs from seed nanoparticles ismore evident than separation of the larger non-rod-shapednanoparticles from GNRs, because of the greater disparity inoverall particle volume. The seed nanoparticles were 4.3 ± 1.1nm in diameter and constituted 15% of the total number ofnanoparticles, as determined from TEM image analysis. TheGNRs isolated in fraction 1 showed a 73% improvement,containing as little as 4% seeds by number. As the isolationpressure increased, the seed fraction also increased, with 7% forfraction 2 and 27% for fraction 3. As expected, the smaller-sizegold nanoparticles remained dispersed in solution at pressuresgreater than larger GNRs or large spherical nanoparticles.Although the GNR fractionation and shape separation was

not 100% effective, this work demonstrated a tunable processfor GNR purification and the existence of an opportunity forprocess optimization. The presence of seed particles in theGNR fractions can be explained by coprecipitation of the smallseed particles with the large spherical particles and GNRs;enhanced separation can be achieved by sample dilution toreduce particle−particle interactions and coprecipitation and/or recursive fractionations. Recently, Saunders and co-workersemployed a total-interaction energy model to predict the sizeand size distributions of dodecanethiol-stabilized nanoparticlesthat precipitated out of gas-expanded n-hexane, incorporatingpolydispersity of the original sample population, accounting forthe probability of nanoparticle interactions resulting innanoparticle precipitation.51 By including sample polydispersityand probability of nanoparticle multibody interactions, thetotal-interaction energy model provides excellent agreementwith the experimental results. Saunders did attribute smalldiscrepancies at lower nanoparticle isolation pressures to“contamination” caused by a thin film of small nanoparticlescoating the already-precipitated larger nanoparticles during theliquid removal process of the fractionation procedures. In ourinvestigation, this contamination could have occurred when the

spiral glass rod was rotated. Saunders goes on to point out thatthis phenomenon is pronounced at lower CO2 pressures. Theprecipitation of excess seed nanoparticles at lower CO2pressures can also be described by multiple nanoparticleinteractions and coprecipitation.51 Each of these issues can beaddressed through process optimization with improvedapparatus design, more-dilute concentrations, recursive fractio-nations, narrower pressure intervals, choice of solvent tooperate at higher pressures, and optimization of the ligand−solvent combinations. GXL fractionation of dodecanethiol-stabilized silver nanoparticles demonstrated decreased poly-dispersity in the nanoparticle fractions when the isolationpressure increments were decreased from 3.4 bar to 1.7 bar.26,27

Combined recursive fractionation and smaller isolation incre-ments in pressure will likely lead to more-uniform size andshape distributions of the GNRs and will be included in futureinvestigations. Furthermore, this technique of particle size andshape fractionation may find greater impact on nanoparticlepopulations that possess greater disparity in size and shape.

4. CONCLUSIONSThis work demonstrates the first-ever fractionation of hydro-phobically stabilized gold nanorods (GNRs) using CO2-expanded liquids, which increased the GNR monodispersityand decreased the excess seed concentration by 73% (number)when precipitated at 6.9 bar. Varying both the alkane chainlength and the dispersing solvent directly impacts GNRdispersibility and size fractionation. Ligand chain lengthsconsisting of 18 carbons enabled GNR dispersibility up to 24bar in CO2-expanded toluene, while ligand chain lengths of 12carbons were dispersible to much higher CO2 partial pressures,providing a wider fractionation pressure range. Varying thedispersing solvent (cyclohexane, toluene, or n-hexane) wasshown to alter the precipitation pressures required for completeGNR precipitation. Cyclohexane proved to be the best solventstudied for GNR dispersibility, enabling dispersion up to 30 barof CO2 pressure, providing a wide operating pressure range andthe benefit of enhanced transport properties for particleprecipitation at higher CO2 pressures. Potential for fractiona-tion optimization exists with a scaled-up apparatus, dilute GNRconcentrations, recursive fractionations, narrower pressureintervals, and optimization of the ligand−solvent combinations.Furthermore, this work demonstrates the feasibility of hydro-phobic nanorod fractionation in GXLs where nanorod sizefractionation is easily controlled by the system pressure, CO2antisolvent is recovered by simple depressurization, and theGXL media is minimally invasive, enabling facile GNRredispersion. This research also provides the groundwork fortheoretical modeling of nonspherical nanoparticle dispersibilityin GXLs and other solution, which will be instrumental inenhancing our fundamental understanding of nanorod behaviorin these and other systems, as well as post-synthesis processingoptimization at laboratory or commercial scales.

■ ASSOCIATED CONTENT*S Supporting InformationSupporting Information is available free of charge via theInternet at http://pubs.acs.org/.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected].



http://pubs.acs.org/

mailto:[email protected]

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This research was sponsored by the NSF BRIGE Award (GrantNo. EEC-082443). Fellowship funding at the time of this workwas provided by the South East Alliance for GraduateEducation and the Professoriate (SEAGEP) on the NationalScience Foundation (Award No. HRD-0450279).

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