Synthesis of Stable, Low-Dispersity Copper Nanoparticles and Nanorods and TheirAntifungal and Catalytic Properties

Yanhu Wei, Steven Chen, Bartlomiej Kowalczyk, Sabil Huda, Timothy P. Gray, andBartosz A. Grzybowski*Department of Chemical and Biological Engineering, Department of Chemistry, Northwestern UniVersity,2145 Sheridan Road, EVanston, Illinois 60208

ReceiVed: June 17, 2010; ReVised Manuscript ReceiVed: July 31, 2010

Low-polydispersity copper nanoparticles (NPs) and nanorods (NRs) were synthesized by thermal decompositionof copper(II) acetylacetonate precursors in the presence of surfactants. Exchange of weakly bound alkylamineligands for alkanethiols increased the stability of the NPs and, depending on the thiols’ terminal functionality,rendered them soluble in organic solvents or in water. The water-soluble nanoparticles stabilized with positivelycharged thiols exhibited long-term (months) stability and antifungal properties. The NPs and NRs stabilizedwith weakly bound alkylamine ligands are catalytically active in alkyne coupling reactions.

Introduction

The ability to synthesize nanostructures of controlled sizesand shapes is at the heart of modern nanotechnology and isimportant for their applications in optics, biodetection, andcatalysis.1-11 While for most noble metals (Au,12 Ag,12a,13 Pt,14

Pd15) various methods are known that yield monodispersenanoparticles and other types of nanostructures (rods,16 plates,17

cubes,18 wires,19 multibranches20), this nanosynthetic repertoireis not as broad for copper particles. Interest in copper nano-structures is spurred by their useful electrical, catalytic, optical,and antifungal/antibacterial properties.21-26 However, synthesisof stable and low-polydispersity Cu nanoparticles (CuNPs) hasproven challenging mainly because of the rapid air oxidationof metallic copper to Cu2+ ions or CuxO oxides (x ) 1 or 2).27

Consequently, most of the existing methods of CuNP synthesisproduce particles of large polydispersity and/or limited stability(see refs 27a and 28 and also Supporting Information, section1, for comparison with several recent methods). For coppernanorods (NRs), various methods based on template-basedreduction/electrochemical deposition,27b,29 vacuum vapor deposi-tion,30 structure-directing surfactants,31 and hydrothermalreduction31b,32 were reported, but many of these procedures sufferfrom the same limitations as those for CuNP synthesis (poorsize and shape control, low stability). Here, we report astraightforward procedure based on the thermal decompositionof precursor Cu salts27b in the presence of oleylamine (OAM)that yields low-polydispersity CuNPs or CuNRs of various sizes.The Cu nanostructures prepared by this route can be furtherfunctionalized by exchanging weakly bound surfactants to tighterbinding alkanethiols. This functionalization allows for thecontrol of surface chemistry and solubility of CuNPs/CuNRsin various solvents and yields nanostructures that are stable insolution for prolonged periods of time. Notably, copper nano-particles functionalized with positively charged thiols are stablein water for several months, during which they exhibit excellentantifungal properties. On the other hand, Cu NPs and NRsstabilized with weaker binding oleylamine ligandssthough less

stable than their thiol-protected counterpartssare catalyticallyactive in alkyne coupling reactions.

Experimental Section

Materials. Copper acetylacetonate (Cu(acac)2), iron(0) pen-tacarbonyl (Fe(CO)5), oleylamine (OAM), oleic acid (OA), 1,2-tetradecanediol, 1-hexadecanethiol, 3,4-dihydroxyhydrocinnamicacid, ascorbic acid, phenyl ether, and 1-octadecene were pur-chased from Sigma-Aldrich. Hydrogen tetrachloroaurate trihy-drate (HAuCl4 ·3H2O) was purchased from DF Goldsmith.N,N,N-Trimethyl(11-mercaptoundecyl)ammonium chloride (TMA,SH(CH2)11N(CH3)3Cl) was a generous gift of ProChimia Sur-faces (Gdansk, Poland). All solvents and reagents were usedwithout further purification.

Synthesis of CuNPs and NRs. In a typical experiment, 0.125mmol of copper acetylactonate, Cu(acac)2, was dissolved in 5mL of phenyl ether and 6 mL of OAM. To this so-called royalblue solution 2 mmol of 1,2-tetradecanediol was added. Thesolution was stirred and degassed by three vacuum pump/backfillcycles under Ar. The temperature of the solution was slowlyincreased (at ∼2 °C/min) over 1 h to 155 °C and held there foranother hour. Afterward, the solution was allowed to cool toroom temperature, and the particles were precipitated fromphenyl ether with ethanol and redispersed in toluene or hexanefor further use. Cu nanorods were obtained by the same routeas CuNPs, except that 0.1 equiv of dodecylammonium bromide,DDAB, was added as structure-directing surfactant.

Culturing of Stachybotrys Chartarum Fungus. A strainof S. chartarum (isolated from a house not associated withidiopathic pulmonary hemorrhage, Cleveland, OH) was obtainedfrom ATCC and cultured on a solid media composed ofcornmeal extract, 5% V8 juice, 3 g/L CaCO3, and 7.5 g/Lbacteriological agar as solidifying agent with pH adjusted to5.6-6.0 at 25 °C (ATCC medium 309).

Minimum Inhibitory Concentration Assay. After thegrowth medium (20 mL per plate) was autoclaved for steriliza-tion, it was mixed with CuNPs of concentrations 0.01-2 mM(in terms of metal atoms) and allowed to solidify. Colonies ofS. chartarum were picked using a cotton swab and were addedto sterile water and then brought to a concentration of roughly107 CFU. Next, 100 µL of this suspension was spread uniformly

* Corresponding author: e-mail: grzybor@northwestern.edu, Tel: (+1)847-491-3024.

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10.1021/jp1055683 2010 American Chemical SocietyPublished on Web 08/25/2010

onto each of the medium/CuNPs plates and left in an incubatorat 27 °C for 72 h to grow. The lowest concentration of coppernanoparticles which exhibited no mold growth was determinedas the minimum inhibitory concentration (MIC). Upon deter-mining the MIC of copper nanoparticles, different ratios of latexpaint to the MIC (1:1, 1:2, 1:5, 1:10, 1:20) were mixed withthe MIC of copper nanoparticles and the growth medium, andthe MIC assay was repeated. This was done to test whether theaddition of paint would affect the MIC.

Disk Diffusion Assay. Suspension of S. chartarum (100 µLof ∼107 CFU/mL) in sterile water was applied uniformly onthe surface of the growth medium plate. Wells (1 cm indiameter) were hollowed out in each of the growth-medium agarplates tested and served as reservoirs for loading coppernanoparticle suspensions (concentrations in terms of Cu atoms0.06, 0.08, 0.1, 0.2, 0.4, 0.6, 0.8, 0.9, and 1 mM) mixed with acommercial latex paint. The plates were then left in an incubatorat 27 °C for 48 h to grow. The zone of inhibition (ZoI)surrounding the hole was determined on the basis of fivemeasurements for each CuNP concentration studied.

Cu NP-Catalyzed Alkyne Coupling Reaction. For experi-mental details see section 3 of the Supporting Information.

Results and Discussion

Copper NPs were synthesized in phenyl ether using Cu(acac)2

as a salt precursor, oleylamine (OAM) as a surfactant, and 1,2-tetradecanediol as a reducing agent. The solution was degassedand stirred while its temperature was raised at a rate of ∼2 °C/

min to 155 °C and was held there for an additional 1 h. Duringheating, the color of the reaction mixture changed from blue todark red, signifying the formation of CuNPs.

The diameters, d, and the polydispersities, σ, of the CuNPsdepended on the ratio, �, of OAM to Cu(acac)2. TEM imagesof particles prepared at four different values of � are shown inFigure 1a-d. The surface plasmon resonance (SPR) band offreshly prepared CuNPs was at ∼590 nm (Figure 1e) and didnot shift much upon the size change from 34 to 9 nm (Figure1a-d). Both d and σ decreased with increasing �, and thepolydispersity was as low as 15% (for d ) 9 nm NPs at � )100). The OAM surfactant can play two roles in the synthesis:(i) as complexing agent with copper ion and (ii) as cappingagent for stabilizing formed particles. Here, the capping actionlikely dominates and causes the decrease of the particle sizewith an increase of the OAM concentration. However, thesesurfactant-coated NPs were stable for only short time. Forinstance, the color of toluene/hexane CuNP solution (degassedand under Ar) changed gradually from dark brown-red to bluewithin 2 weeks. This change was accompanied by a decay ofthe intensity of the SPR band at around 590 nm (curves 1 and2 in Figure 1e) and by increasing polydispersity of the sample,as verified by TEM. Decomposition of the NPs was even morerapid (1-2 days) if the solution was exposed to atmosphericoxygen that caused Cu oxidation.

The particles became more stable after exchange of weakOAM ligands with tighter binding thiols. The improved NPstability was evidenced by TEM images of CuNPs stored for

Figure 1. TEM images of Cu nanoparticles prepared with (a) � ) 9 (d ) 34 nm, σ ) 32%), (b) � ) 25 (d ) 27 nm, σ ) 27%), (c) � ) 50 (d) 21 nm, σ ) 18%), and (d) � ) 100 (d ) 9 nm, σ ) 15%). (e) UV-vis spectra of CuNPs in toluene: 1, freshly prepared CuNPs coated withOAM; 2, Cu/OAM NPs stored under argon for 10 days; 3, freshly prepared CuNPs protected with 1-undecanethiol; 4 and 5, CuNPs coated with1-undecanethiol and stored under argon for 10 and 20 days, respectively. (f) A representative TEM image of CuNPs coated with 1-undecanethiolafter storing in toluene under argon for several weeks. (g) UV-vis spectra of freshly prepared CuNPs in H2O (solid curve) and the same NPs storedin water for 2 months (dotted curve). (h) A representative image of Cu/TMA NPs stored in water for two months. (i) The HRTEM image ofalkylthiol-coated CuNPs (synthesized at � ) 100) stored in toluene under argon for 2 weeks. The distance between two adjacent lattice planes is2.2 Å. Scale bars in (a) and (b) are 50 nm, those in (c), (d), (f), and (h) are 20 nm, and that in (i) is 2 nm.

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several weeks (Figure 1f) and by the corresponding UV-visspectra (curves 3, 4, and 5 in Figure 1e). The HRTEM imageof alkylthiol-capped CuNPs (after storing in toluene under argonfor 2 weeks) indicated that the distance between two adjacentlattice planes of CuNPs is 2.2 Å (Figure 1i), which is consistentwith the reported value of copper fcc {111}.30,31,33 The SPRband of thiolated CuNPs shifted with increased storage timefrom ∼590 to ∼690 nm (at 20 days) because of partialaggregation/merging of CuNPs. The NPs stabilized with 1-un-decanethiol were readily soluble in nonpolar solvents such astoluene or hexane; on the other hand, stabilization with positivelycharged thiols, TMA, gave particles soluble in water. In thelatter case, the particles remained stable for several weeks andeven months (especially in the presence of 0.1 M ascorbic acid,preventing Cu oxidation), which was confirmed by TEMimaging (Figure 1h) and was evidenced by only minute changesin the UV-vis spectra (Figure 1g). Interestingly, attempts toprepare negatively charged NPs covered with deprotonatedcarboxylic acid thiols (e.g., HS-(CH2)10-COO-) proved unsuc-cessful, likely because the coordination of copper to COO- headgroups shifts the equilibrium from Cu(0) to Cu(II).

The procedure for the synthesis of CuNRs was identical tothat of CuNPs with the exception that a small amount (∼0.1equiv) of didecyldimethylammonium bromide (DDAB) wasused as a structure-directing surfactant. The aspect ratio, R )length/width, of the Cu NRs depended on the ratio, �, of OAMto Cu(acac)2. Figures 2a,b show SEM and TEM images of CuNRs prepared at two different values of �. As � was decreasedfrom 100 to 60, the value of R increased from 13 to 64, withthe rods becoming both longer and thinner. In addition, the valueof R increased slightly with increasing the concentration ofDDAB. In this case, however, the yield and monodispersity ofNRs became markedly worse.

High-resolution TEM images of Cu NRs show that the latticespacing on the sides of CuNRs is 2.2 Å (Figure 2c, left),indicating that these planes are fcc {111}.30,31,33 While we donot know the details of the mechanism of CuNR formation,this crystallographic finding suggests that DDAB controls thegrowth of nanorods by stabilizing (111) facets of copper17asinthe presence of such a stabilization, crystal growth is favoredfrom higher surface energy “tips” of the rods.

The color of freshly prepared Cu nanorods was usuallybrown-pink, and the rods were more stable than the correspond-ing CuNPs stabilized with OAM; still, they gradually oxidizedafter several weeks. After ligand exchange, however, the rods

covered with thiolate SAMs remained stable in solution (evenopen to atmosphere and without antioxidants) for severalmonths. This was confirmed by TEM images in Figure 2d, byonly slight decrease of the SPR band intensity (see SupportingInformation, section 2), and by the unchanged color of Cu NRtoluene solution stored under atmosphere for several months.

One of the most promising applications of CuNPs and NRsis as antifungal agents. The water-soluble nanoparticles syn-thesized by our method are especially useful for this purposeowing to their long-term stability. Interestingly, while the NPsthemselves are not antifungal, they slowly oxidize and releasecupric ions (Cu2+), which have the ability to generate toxichydroxyl free radicals when near the lipid membrane. Thesefree radicals cause oxidative degeneration of lipids comprisingcell membrane,34,35 leakage of intracellular substances such asK+ ions,36 alteration of key biochemical processes inside of thecell,37-39 and, ultimately, cell death. We have verified byinductively coupled plasma mass spectrometry (ICP-AES)analysis of the aqueous NP solutions that Cu NPs and NRsstabilized by TMA thiols release ca. 2.3% of Cu atoms permonth at an approximately constant rate. We then tested theantifungal efficiency of the CuNPs/NRs against Stachybotryschartarum fungi (a common indoor house mold that has beenlinked to various health effects and the “sick building syn-drome”40) using the so-called disk diffusion assay (DDA) andthe minimum inhibitory concentration (MIC) assay describedin the Experimental Section. In the first assay, the radii, r, ofthe inhibition zones surrounding CuNP reservoirs depended onthe concentration of the nanoparticles, c (Figure 3a). Followingthe standard procedure,41 these radii were fitted (Figure 3b) tothe solution of the one-dimensional, radial diffusion equation,(r - r0)2 ) 4Dt ln(c/c0), where r0 ) 5 mm is the radius of theNP reservoir, D is diffusion constant of CuNPs/Cu2+, t standsfor time, and c0 is the critical NP concentration required toinhibit any growth of the fungus. Rearranging this equation andplotting ln c ) (r - r0)2/4Dt + ln c0 gives the critical

Figure 2. (a) SEM image of Cu nanorods, � ) 100, average aspectratio, R ≈ 13 (average length/width ) 841.6 nm/63.5 nm). (b) TEMimage of Cu nanorods, � ) 60, R ≈ 64 (average length/width ) 1338nm/21 nm). (c) HRTEM image of Cu nanorods, � ) 100; distancebetween two adjacent lattice planes is 2.2 Å. (d) TEM image of Cunanorods coated with 1-undecanethiol after storing under atmospherefor several months. Scale bars in (a) 400 nm, (b, d) 200 nm, and (c) 2nm for the left and 200 nm for the right images.

Figure 3. Antifungal and catalytic properties of CuNPs. (a) The radiiof the zones of inhibition surrounding CuNP reservoirs increase withnanoparticle concentration. Scale bar ) 3 cm. (b) Radii of the inhibitionzones plotted as a function of CuNP concentration. The solid curve isa theoretical, logarithmic fit. Red markers correspond to experimentaldata. Error bars are from five independent experiments for eachcondition. The inset gives a semilog plot of the data. Critical inhibitoryconcentration 55.46 µM is estimated from the y-intercept. (c) Variousw/w ratios of the fungus medium applied onto commercial paintswithout CuNPs (top row) and containing 40 µM CuNP (bottom row).In all cases, CuNPs inhibit the growth of the mold. Scale bar ) 3 cm.(d) The CuNP catalyzed coupling of phenylacetylene. (e) An opticalimage of PDMS cubes loaded with catalytic CuNPs. Scale bar ) 3mm.

15614 J. Phys. Chem. C, Vol. 114, No. 37, 2010 Wei et al.

concentration (from the y-intercept) of 55.46 µM in terms ofCu atoms,42 which is close to the minimum inhibitory concen-tration of 40 µM determined by the MIC assay (see Experi-mental Section). This MIC value is lower than the Stachybotryschartarum MIC values reported for carneic acid (75 µM, 25µg/mL), penicillin G (299 µM, 100 µg/mL), or itraconazole (142µM-142 mM; 0.1-100 mg/mL)43,44 but higher than 2.5 µM(3.12 µg/mL)43 for actinomycin D (which, however, is highlytoxic). The efficiency of the CuNPs as antifungal agents is alsoevidenced by the results of experiments where these particleswere mixed with commercial acrylic paints (Figure 3c).

Another potential area of application of copper nanostructuresis in catalysis. In this case, however, stabilization of the particleswith tightly binding thiols is undesirable as it poisons thesurfacesindeed, we verified that thiol-stabilized CuNPs or NRsare not catalytically active in a model alkyne coupling reaction.This reaction, however, proceeds well in the presence of CuNPs/NRs stabilized with weak OAM ligands. Unfortunately,these NPs are unstable, and a significant fraction of themdecomposes during the reaction. To solve this problem, weoccluded the particles in a poly(dimethylsiloxane), PDMS,matrix which efficiently protects Cu NPs from oxidation in air.45

Figure 3d summarizes the results for the system in which 33 g,3 × 3 × 3 mm3 cubes of PDMS (Figure 3e) containing 0.1equiv (in terms of Cu atoms) of Cu NPs (or Cu NRs) wereadded to a mixture of 1.25 mmol of phenylacetylene in 5 mLof pyridine and 10 mL of freshly distilled tetrahydrofuran (THF,or dichloromethane, DCM) and were stirred at room temperatureand under argon for 36 h. Since THF swells PDMS matrix,46

the substrates were able to diffuse into the cubes47 where theyreacted on the NPs. The yield of the coupling product, 1,4-diphenyl-1,3-dibutyne, was as high as ∼90% (after columnpurification). We make the following comments about theseresults: (i) first, the relative bulkiness of the NPs prevented theiroutflow/leaking from the cubes upon solvent swelling. This wasin contrast to previously published systems,47 where Cu(OAc)2

salt was occluded in PDMS but leaked during the reactionreducing the reusability of the cubessin our case, where leakingwas minimal, the same cubes could be used to catalyze reactionsin several consecutive batches of reactants. (ii) Second, Cu NPsexhibited higher catalytic activities compared with Cu NRs(∼50% conversion after 36 h in THF). (iii) Third, we note thatthe obvious practical advantage of NP-loaded PDMS cubes overfree NPs is that the former can easily retrieved from the mixtureupon reaction completion.

Conclusions

In summary, we described a straightforward synthetic methodleading to stable and relatively low-dispersity copper nanopar-ticles and nanorods which, via ligand exchange, can be madesoluble in either hydrophobic solvents (e.g., toluene or hexane)or in water and can be used as antifungal agents or catalysts.The yields and monodispersities of the Cu NRs could beimproved further by adjusting the concentrations of DDAB and/or OAM or by using other structure-directing reagents (e.g.,CTAB or AgNO3).48 In the future, it would be interesting toquantify catalytic activities of copper nanostructures in otherreactions including “click,”49 Ullmann,50 or Sonogashira.51 Lastbut not least, the procedures described here can be, after minormodifications, extended to the synthesis of other types ofnanostructures (see Supporting Information, section 4).

Acknowledgment. This work was supported by the PewScholarship in the Biomedical Sciences, the Sloan Foundation

Research Fellowship, and the Camille & Henry DreyfusTeacher-Scholar Award (to B.A.G.).

Supporting Information Available: (1) Comparison withthe previously reported procedures of CuNP/NR syntheses, (2)UV-vis spectra of fresh and aged nanorods, (3) experimentaldetails of Cu NP catalyzed alkyne coupling, and (4) extensionof NP synthesis to other materials. This material is availablefree of charge via the Internet at http://pubs.acs.org.

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