Monolayer-Protected Gold Nanoparticles Prepared

Monolayer-Protected Gold Nanoparticles Prepared

(Parte 1 de 3)

DOI: 10.1021/la901847s 13855Langmuir 2009, 25(24), 13855–13860 Published on Web 07/28/2009 pubs.acs.org/Langmuir ©2009 American Chemical Society

Monolayer-Protected Gold Nanoparticles Prepared Using Long-Chain Alkanethioacetates†

Shishan Zhang, Gyu Leem, and T. Randall Lee*

Departments of Chemistry and Chemical Engineering, University of Houston, 4800 Calhoun Road, Houston, Texas 77204-5003

Received May 23, 2009. Revised Manuscript Received June 25, 2009

This letter describes the preparation of monolayer-protected nanoparticle clusters (MPCs) from the adsorption of n-tetradecanethioacetate onto colloidal gold nanoparticles using the Brust-Schiffrin two-phase synthesis method. The MPCs were characterized by transmission electron microscopy (TEM), ultraviolet-visible (UV-vis) spectroscopy, 1H nuclear magnetic resonance (NMR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy. These studies found that the monolayer coatings on the gold nanoparticles were nearly indistinct with regard to chemical composition, monolayer structure, and Au-S ligation when compared to those prepared from the analogous adsorption of n-tetradecanethiol (i.e., the thioacetate headgroup adsorbs to gold as a thiolate, with concurrent loss of the acetyl group). Under equivalent conditions of formation, however, the size of the gold nanoparticles formed was larger when using the alkanethioacetate adsorbate (e.g., 4.9 ( 1.2 nm) compared to the alkanethiol adsorbate (e.g., 1.6 ( 0.3 nm). The observed difference in size is rationalized on the basis of the stronger ligating ability of the thiol compared to that of the thioacetate during gold nanoparticle nucleation and/or growth. The use of alkanethioacetates affords significant control of particle size and allows the formation of MPCs with thiol-sensitive ω-functional groups.

Introduction

Self-assembled monolayers (SAMs) generated by the spontaneous assembly of organic molecules on two-dimensional (2-D) substrates are widely used in a variety of technological applications. 1,2 The process of self-assembly is associated with the spontaneous adsorption and organization of an active surfactant-like species on a solid surface and therefore includes a variety of adsorbates and substrates, such as phosphines on platinum or palladium,3-5 silanes on silica,6,7 and thiols on gold, silver, and copper.8 Research involving SAMs on flat substrates has been adapted to form three-dimensional (3-D) SAM-coated structures, specifically, monolayer-protected nanoparticle clusters (MPCs), which can be handled as isolable species and further functionalized with a variety of reagents.9 Since the initial description of Au55(PPh3)12Cl6 by Schmid,10 researchers have prepared MPCs using a var- iety of ligands, including thiols, disulfides, dialkyl sulfides, thiosulfates, xanthates, carbamates, phosphines, phosphine oxides, amines, carboxylates, selenides, and isocyanides. 1

Furthermore, the use of the Brust-Schiffrin12 two-phase synthesis method now allows facile tailoring of the surface properties of thiolate-functionalized nanoparticles by selecting from structurally diverse alkanethiols or alkyl disulfides having various chain lengths and/or ω-functional groups.9,13

Given that the interfacial properties of SAMs are critical in technologicalapplications,1,2,11researcherswhoutilizeSAMshave explored various protecting-groupstrategies14 in cases where the targeted ω-terminal groups are reactive toward thiols15 or some intermolecularreactionsmayoccur(e.g.,aromaticthiolscaneasily undergo oxidation to form disulfide in the presence of a small amountof oxygen).16 Notably, severalresearchgroups17-19 have successfullyutilizedtheS-acetyl-protectedspeciesintheformation of 2-D SAMs on flat gold substrates. In these studies, the direct attachmentof adsorbatesvia their terminalthioacetate group was alsoobserved; likeSAMsderived from alkanethiols,the adsorbed sulfur species were thiolates. For the alkanethioacetate system, however, SAM formation occurred less readily than with analogousthiol-terminatedadsorbates.In lightofthese studies,we were surprised to find no reports of the use of thioacetate-terminated adsorbatesto prepare3-D SAMs on colloidalsubstrates.

Alkanethioacetates can be readily prepared from mesylateor halo-functionalized organic precursors. The preparation of

†Part of the “Langmuir 25th Year: Nanoparticles synthesis, properties, and assemblies” special issue.

*To whom correspondence should be addressed. E-mail: trlee@uh.edu (1) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M.

Chem. Rev. 2005, 105, 1103. (2) Ulman, A. Chem. Rev. 1996, 96, 1533. (3) Mallat, T.; Broennimann, C.; Baiker, A. Appl. Catal. 1997, 149, 103. (4) Mitchell, G. E.; Henderson, M. A.; White, J. M. J. Phys. Chem. 1987, 91, 3808. (5) Ugo, R. Coord. Chem. Rev. 1968, 3, 319. (6) Gun, J.; Iscovici, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 101, 201. (7) Wasserman, S. R.; Tao, Y. T.; Whitesides, G. M. Langmuir 1989, 5, 1074. (8) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9022. (9) Templeton, A. C.;Wuelfing, W.P.;Murray, R. W.Acc. Chem. Res. 2000, 3, 27. (10) Schmid, G.; Pfeil, R.; Boese, R.; Bandermann, F.; Meyer, S.; Calis,

G. H. M.; van der Velden, J. A. W. Chem. Ber. 1981, 114, 3634. (1) Daniel, M.-C.; Astruc, D. Chem. Rev. 2004, 104, 293. (12) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801.

(13) Ingram, R.S.;Hostetler, M.J.;Murray, R.W. J.Am. Chem.Soc.1997,119, 9175. (14) Greene, T. W. Protective Groups in Organic Synthesis, 3rd ed.; Wiley: New

York, 1999. (15) Witt, D.; Klajn, R.; Barski, P.; Grzybowski, B. A. Curr. Org. Chem. 2004, 8, 1763. (16) Tarbell, D. S. In Organic Sulfur Compounds; Kharasch, N., Ed.; Pergamon

Press: New York, 1961; Vol. 1, p 97. (17) Tour, J. M.; Jones, L. I.; Pearson, D. L.; Lamba, J. J. S.; Burgin, T. P.;

Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. J. Am. Chem. Soc. 1995, 117, 9529. (18) Kang, Y.; Won, D.-J.; Kim, S. R.; Seo, K.; Choi, H.-S.; Lee, G.; Noh, Z.;

Lee, T. S.; Lee, C. Mater. Sci. Eng., C 2004, C24, 43. (19) B ethencourt, M. I.; Srisombat, L.; Chinwangso, P.; Lee, T. R. Langmuir 2009, 25, 1265.

13856 DOI: 10.1021/la901847s Langmuir 2009, 25(24), 13855–13860

Letter Zhang et al.

thioacetate-terminated organic molecules is simpler than that of analogous thiol-terminated organic molecules because the former synthesis requires relatively mild reaction conditions (e.g., no strong reducing agents and/or acidic conditions). Furthermore, nanoparticle growth using the popular Brust- Schiffrin method is believed to proceed via a nucleationgrowth-passivation process9 where the average diameter of the resultant gold nanoparticles becomes smaller when a larger thiol/ gold molar ratio is used, the cooled reducing agent is added rapidly, or sterically bulky ligands are involved.13,20-23 We hypothesized that the use of a weaker binding adsorbate, such as an alkanethioacetate, might prolong the growth processes and thusofferlargernanoparticles thanthose affordedusingstandard alkanethiol-based adsorbates.

To this end, this letter describes the preparation and characterization of SAM-protected gold nanoparticles derived using n-tetradecanethioacetate as a passivating agent. The results are compared to those obtained via analogous passivation using n-tetradecanethiol. The product MPCs were characterized by transmission electron microscopy (TEM), ultraviolet-visible (UV-vis) spectroscopy, 1H nuclear magnetic resonance (1H NMR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy. The studies reveal that the MPCs prepared using n-tetradecanethioacetate are markedly larger than those prepared using n-tetradecanethiol.

Experimental Section

Materials.Waterwaspurifiedtoaresistanceof18MΩusinga

Milli-Q academic system. All chemicals were used as received from the indicated companies without additional purification:

NaBH4, toluene, and hexane (EM Science), HAuCl4 (Strem), absolute ethanol (McKormick Distilling Co.), carbon tetra- chloride (CCl4, Acros), myristyl mercaptan (C14SH, TCI), and potassium thioacetate, 1-bromotetradecane, and tetraoctylam- monium bromide (TOAB, Aldrich).

Synthesis of the Adsorbates. The thioacetate adsorbate was synthesized following the procedure reported by Evans et al.24 Potassium thioacetate (1.78 g, 10 mmol)was dissolved in absolute ethanol (100mL), and the mixture was degassed by bubbling with argon for 30 min. An aliquot of 1-bromo-tetradecane (2.86 g, 10 mmol) was then added, and the solutionwas refluxed for 12 h. Thevolatileswereremovedbyevaporationundervacuum,andthe residue was dissolved in water (100 mL) and then extracted with diethylether(3 100mL).Thecombinedorganiclayersweredried overanhydrous magnesiumsulfate,filtered, andevaporated under vacuum. The yellowish residue was subjected to column chromatography on silica gel (5% diethyl ether in hexanes) to affordpure n-tetradecanethioacetate (C14SAc) as a liquid (2.21 g, 8.1 mmol, 81%yield).Thestructurewasconfirmedby1HNMRspectroscopy in CDCl3 usingaQ E-300s pectrometer.24 Preparation of SAM-Protected Gold Nanoparticles

Using C14SH/C14SAc. All glassware used in the preparation and storage of the MPCs was treated with aqua regia, rinsed with water, and cleaned with piranha solution (7:3 concentrated

H2SO4/30 wt % H2O2). (Caution! Piranha solutionreacts violently with organic materials and should be handled with care!)

Gold nanoparticles coated with C14SH and C14SAc were synthesized in toluene/H2O in a similar fashion to the work of Brust et al.12 except that the S/Au mole ratios were maintained at aqueous solution of HAuCl4 (0.09 mmol) was placed in a 50 mL round-bottomedflask.Tothissolution,8.0mLofa2.5 10-2M solution of TOAB (0.2 mmol) in toluene was added while stirring vigorously. To provide for complete phase transfer of the gold salt, stirring was continued for at least 15 min. The phase transfer was confirmed visually by observing the disappearance of the pale-yellow color of the aqueous phase and the appearance of the reddish orange color of the organic phase. The adsorbate (C14SH or C14SAc; 0.09 mmol) was then added dropwise to the organic phase while stirring. A 7.5 mL aliquot ofa freshlyprepared0.1 M aqueous solution of NaBH4 (0.75 mmol) was then added dropwise over 5 min. The mixture was allowed to stir for 24 h. The organic layer was separated using a micropipet, washed with water several times to remove excess TOAB, dried over MgSO4, andconcentratedtoca.1mLbyrotaryevaporation.Theresulting product was diluted with 50 mL of ethanol and stored at -50 C overnight. The functionalized gold nanoparticles were then collected as a brownish-black precipitate by centrifugation and were thoroughly rinsed with ethanol (3 50 mL).

Control Experiment to Probe the Possible Reduction of

Alkanethioacetate by NaBH4. The procedure in the preceding paragraph was performed without the addition of a metal salt

(HAuCl4). Separation of the organic and aqueous phases followed by the evaporation of each phase gave in each case a white residue. Each residue was dissolved in CDCl3 and characterized by 1H NMR spectroscopy. No thiol species were detected in the residue containing C14SAc (vide infra). Characterization of Nanoparticles. TEM images were col- lected on a JEOL JEM-2010 electron microscope operating at an accelerating bias voltage of 200 kV. Samples were prepared by depositing a film of nanoparticles onto a carbon-coated copper grid. UV-vis spectra were collected using a Cary 50 scan UV- visible extinction spectrometer. All samples were placed in a quartz cuvette having a 1 cm optical path length, and the baseline of each spectrum was corrected by subtracting a spectrum of the corresponding solvent. 1H NMR spectra were recorded on a General Electric QE-300 spectrometer operating at 300 MHz in

CDCl3 and internally referenced to 7.26 ppm. XPS spectra were obtained using a PHI 5700 X-ray photoelectron spectrometer equipped with a monochromatic Al KR X-ray source (hν= 1486.7 eV). The spectrometer was operated at high resolution withapassenergyof23.5eV,aphotoelectrontakeoffangleof45 fromthesurface,andananalyzerspotdiameterof2mm.Thebase pressureinthechamberduringmeasurementswas3 10-9Torr, and the spectra were collected at room temperature. The binding energies of S were referenced to the Au4f7/2 peak at 84.0 eV. Infrared spectra were collected using a Nicolet Magna-IR 860

Fourier transform spectrometer. The IR spectra of nanoparticles in solution were collected by dispersing the nanoparticles in carbon tetrachloride and placing an aliquot of the solution between two KBr plates. The IR spectra of nanoparticles in the solid state were collected by depositing an aliquot of the solventdispersednanoparticlesonaKBrplateandallowingthesolventto evaporate prior to data collection.

Results and Discussion

Solubilities of C14SAc-Functionalized Gold Nanoparticles in Organic Solvents. Gold nanoparticles synthesized using n-tetradecanethioacetate as described above were precipitated by centrifugation in ethanol. These SAM-coated nanoparticles could be readily redissolved in a variety of organic solvents, including hexane, toluene, CCl4,C H2Cl2, and THF. Given that bare gold nanoparticles (i.e., those prepared with no SAM coat- ing) typically aggregate in organic solvents,25 the observed(20) Leff,D.V.;Ohara,P.C.;Heath,J.R.;Gelbart,W.M.J.Phys.Chem. 1995, 9, 7036.

(21) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604. (2) Chen, S.; Murray, R. W. Langmuir 1999, 15, 682. (23) Chen, S. Langmuir 1999, 15, 7551. (24) Evans, R. M.; Owen, L. N. J. Chem. Soc. 1949, 244. (25) Zhu, T.; Vasilev, K.; Kreiter, M.; Mittler, S.; Knoll, W. Langmuir 2003, 19, 9518.

Zhang et al. Letter

resistance to aggregation here strongly indicates that the alkanethioacetate moieties form at least a partial monolayer on the surface of the gold nanoparticles.

Sizes and Morphologies of C14SH- and C14SAc-Functionalized Gold Nanoparticles. Figure 1a-d shows TEM images of gold nanoparticles synthesized with S/Au molar ratios of 1:1 and 1:3 based on the stoichiometry of the starting reagents. The corresponding statistical analysis of the particle sizes is provided in Figure 2a-d, which demonstrates that the diameters of the

and 7.2 ( 0.8 nm (C14SAc) for a 1:3 S/Au molar ratio. Two important trends can be drawn from these data: (1) the alka- nethioacetate affords significantly larger particles than does the alkanethiol,and(2)adecreaseintheS/Aumolarratiogivesriseto anincreaseinthesizeofthegoldnanoparticles.Whereasthelatter phenomenon can be rationalized on the basis of the aforementioned nucleation-growth-passivation kinetics model in which the sulfur-containing agents inhibit the growth process,9,13,20,23,26 the former phenomenon must be attributed to a difference in behaviorforthetwotypesofadsorbates,wherethethiolsarebetter inhibitors (i.e., stronger ligating agents) than the thioacetates.17 We note that this trend is consistent with the preferred ligation of thiols versus thioacetates to the surface of gold during SAM formation (i.e., thiols were observed to bind more readily than thioacetates).17-19 An analogous trend has been observed for dialkyl sulfides27 and alkanethiosulfates28 on flat gold as well as dialkyl sulfides29 and alkanethiosulfates30 during gold nanoparticle growth. The use of alkanethioacetates affords more stable SAMs and thus more stable MPCs than does the use of dialkyl sulfides (covalent bonding vs dative bonding, respectively).27,29

(Parte 1 de 3)

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