Communications

9-BBN Induced Synthesis of Nearly Monodisperseω-Functionalized Alkylthiol Stabilized GoldNanoparticles

Rajesh Sardar† and Jennifer S. Shumaker-Parry*

Department of Chemistry, UniVersity of Utah, 315 South1400 East, RM 2020, Salt Lake City, Utah 84112

ReceiVed October 28, 2008

Different wet-chemical synthetic methods have emergedfor the generation of nanometer-scale metal particles whichdisplay size, shape, and organization dependent opticalproperties that are exploited in a wide range of fields.1,2 Thesynthesis of monodisperse gold nanoparticles (AuNPs) withaverage diameters of <5.0 nm is one of the major challengesin nanotechnology research. The narrow particle size disper-sion is essential for controlling the properties of the materialsfor applications, specifically in nanoscale device develop-ment.3 The Brust two-phase synthetic approach typically is

used to generate monodisperse gold nanoparticles.4 However,this approach requires phase transfer agents, which introducesimpurities in the synthesized particles and requires extrapurification steps.5 In addition, the types of ω-functionalizedalkythiols that may be used as stabilizing ligands in thesynthesis are limited. For example, the formation of AuNPswas not observed using the Brust method of NaBH4 reductionof gold salt when the reaction was done in the presence ofalcohol- or acid-terminated thiol or disulfide ligands.6

Moreover, in the NaBH4 method, reduction of functionalgroups could take place; however, a detailed investigationof the nature of the functional groups after the metalnanoparticle synthesis using NaBH4 in the presence of thepreviously mentioned ligands has not to our knowledge beenreported.

To simplify the synthetic process, single-step approacheshave been developed for the generation of monodisperseAuNPs.7 Although these approaches eliminate the need forphase transfer, the reduction processes are only compatiblewith non-functionalized alkylamine or alkylthiol cappingligands used for stabilization of the nanoparticles. One ofthe alternative approaches in generation of low size disper-sion nanoparticles could be reduction of metal salts by a mildreducing agent. In the presence of stabilizing agents, the slowreduction could lead to better control over the particle growthprocess and generate nearly monodisperse nanoparticles.8 Inaddition, by using a mild reducing agent, a wide variety of

* Corresponding author. E-mail: shumaker-parry@chem.utah.edu.† Current address: Department of Chemistry, University of North Carolina at

Chapel Hill, Chapel Hill, NC 27599.(1) (a) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M. N.;

Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706–3712.(b) Tao, A.; Sinsermsuksakul, P.; Yang, P. Nat. Nanotech. 2007, 2,435–440. (c) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257–264.(d) Sau, T. K.; Murphy, C. J. J. Am. Chem. Soc. 2004, 126, 8648–8649. (e) Sardar, R.; Shumaker-Parry, J. S. Nano Lett. 2008, 8, 731–736.

(2) (a) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin,C. A. Science 1997, 277, 1078–1081. (b) Sonnichsen, C.; Reinhard,B. M.; Liphardt, J.; Alivisatos, A. P. Nat. Biotechnol. 2005, 23, 741–745. (c) Yu, A.; Liang, Z.; Cho, J.; Caruso, F. Nano Lett. 2003, 3,1203–1207. (d) He, L.; Musick, M. D.; Nicewarner, S. R.; Salinas,F. G.; Benkovic, S. J.; Natan, M. J.; Keating, C. D. J. Am. Chem.Soc. 2000, 122, 9071–9077.

(3) (a) Chen, S. W.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray,R. W.; Schaaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L.Science 1998, 280, 2098–2101. (b) Boyer, D.; Tamarat, P.; Maali,A.; Lounis, B.; Orrit, M. Science 2002, 297, 1160–1163.

(4) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R.J. Chem. Soc., Chem. Commun. 1994, 801–802.

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6550–6551. (b) Jana, N. R.; Peng, X. J. Am. Chem. Soc. 2003, 125,14280–14281.

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10.1021/cm802942x CCC: $40.75 2009 American Chemical SocietyPublished on Web 03/16/2009

functional groups could be incorporated as stabilizing ligandsduring synthesis.

Because of its mild reducing character, 9-borabicyclo-[3.3.1]nonane (9-BBN) has been widely used in organicsynthesis specifically in various hydroboration reactions.9 Forthe first time, we report the application of 9-BBN as areducing agent in an efficient, single step synthesis of AuNPs.The synthesized AuNPs are uniform in size and have lowsize dispersion (<10%) without any post-synthesis processing.We exploit this mild reduction process for in situ synthesisof various ω-functionalized (acid, amide, or alcohol) alkyl-thiols or azide terminated disulfide-protected AuNPs; seeScheme 1. We also demonstrate that the methodology isapplicable in the synthesis of silver (Ag), palladium (Pd),and platinum (Pt) nanoparticles. The alkylthiol stabilized goldnanoparticles were synthesized at room temperature or at60 °C via a single step reduction process.

In the synthetic approach, 0.017 g (0.05 mmol) ofEt3PAuCl was dissolved in 100 mL of toluene. The solutionwas stirred for 5 min, then 0.17 mL (0.5 mmol) of1-octadecanethiol (ODT) was injected, and stirring wascontinued for another 30 min. Next, 0.2 mL of 0.5 M 9-BBNin THF was added followed by immediate injection of 0.005mL (0.01 mmol) of trioctylamine (TOA). The color of thesolution gradually changed from orange to light purple. At65 min, the solution color was reddish-purple and exhibiteda stable λmax at 520 nm. No further absorption changes wereobserved (Supporting Information, Figure 1). The finalsolution collected 65 min after addition of 9-BBN wasanalyzed by transmission electron microscopy (TEM) todetermine the particle size, size distribution, and organization.The TEM analysis shows nearly monodisperse particles withan average diameter of 3.3 ( 0.3 nm (see Figure 1). Inaddition, the particles formed highly ordered two-dimensional(2-D) arrays on the TEM grid as shown in Figure 1A.

Substantial efforts have been dedicated to functionalizeAuNPs using different ω-functionalized alkylthiols to im-prove their versatility for applications such as in organicmolecule and biomolecule sensing and implementation inelectronic and photonic devices.10 For example, Murray andco-workers have extensively studied the thiol functionaliza-

tion of AuNPs and showed that chemical properties such asrate of electron transfer of the monolayer protected clusters(MPCs) is dependent on the AuNP surface ligands.11 Dif-ferent approaches have been used to obtain thiol protectedAuNPs through ligand exchange reactions.12 In most cases,the AuNPs are synthesized using the Brust method or avariation and the particles are subsequently functionalizedwith different thiolated surfactants than used during synthesisusing an exchange process. Typically, in the place-exchangeprocess some foreign ligand is added to the as-synthesizedAuNP solution and the mixture is allowed to react. Eventhough the ligand exchange reactions have shown greatsuccess in producing functional nanoparticles, the methodrequires a long time13 and complete exchange of the originalligand has not been demonstrated.

We exploit the mild reduction process in the 9-BBN-basedsynthesis to expand the types of ω-functionalized thiols thatmay be incorporated in situ, eliminating the need foradditional place-exchange reactions. We used 9-BBN in asimple and single step synthesis to generate various ω-func-tionalized thiols such as 11-mercaptoundecanoic acid (MUA),11-mercapto-1-undecanoic acid (MUOH), 11-mercaptoun-decanamide (MUDA), and azide-terminated undecyl disulfide(AUDDS) capped AuNPs. The choice of ligands was madein such a way that further chemistry such as attachment ofvarious redox functionalities to nanoparticle surfaces byamide or ester formation or azide “click chemistry” couldbe possible.14

In the synthetic procedure, 0.017 g (0.05 mmol) ofEt3PAuCl was dissolved in 100 mL of toluene in air at roomtemperature. The solution was stirred for 5 min, and at thispoint 0.066 g (0.3 mmol) of 11-mercaptoundecanoic acid

(9) (a) Knights, E. F.; Brown, H. C. J. Am. Chem. Soc. 1968, 90, 5281–5283. (b) Wang, K. K.; Scouten, C, G.; Brown, H. C. J. Am. Chem.Soc. 1982, 104, 531–536. (c) Burkhardt, E. R.; Matos, K. Chem. ReV.2006, 106, 2617–2650, and references therein.

(10) (a) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293–346, andreferences therein. (b) Plillips, R, L.; Miranda, O. R.; You, C,-C.;Rotello, V. M.; Bunz, U. H. F. Angew. Chem., Int. Ed. 2008, 47, 2590–2594. (9) Thomas, K. G.; Kamat, P. V. Acc. Chem. Res. 2003, 36,888–898.

(11) (a) Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27–36. (b) Stiles, R. L.; Balasubramanian, R.; Feldberg, S. W.; Murray,R. W. J. Am. Chem. Soc. 2008, 130, 1856–1865, and reference therein.

(12) (a) Brown, L. O.; Hutchison, J. E. J. Am. Chem. Soc. 1999, 119, 12384–12385. (b) Brown, L. O.; Hutchison, J. E. J. Am. Chem. Soc. 1999,121, 882–883. (c) Wang, G.; Huang, T.; Murray, R. W.; Menard, L.;Nuzzo, R. G. J. Am. Chem. Soc. 2005, 127, 812–813. (d) Hong, R.;Emrick, T.; Rotello, V. M. J. Am. Chem. Soc. 2004, 126, 13572–13573.

(13) (a) Song, Y.; Murray, R. W. J. Am. Chem. Soc. 2002, 124, 7096–7102. (b) Guo, R.; Song, Y.; Wang, G.; Murray, R. W. J. Am. Chem.Soc. 2005, 127, 2752–2757. (c) Kassam, A.; Bremner, G.; Clark, B.;Ulibarri, G.; Lennox, R. B. J. Am. Chem. Soc. 2006, 128, 3476–3477.

(14) (a) Ingram, R. S.; Murray, R. W. Langmuir 1998, 14, 4115. (b)Yamada, M.; Nishihara, H. Langmuir 2003, 19, 8050. (c) Brennan,J. L.; Hatzakis, N. S.; Tshikhudo, T. R.; Dirvianskyite, N.; Razumas,V.; Patkar, S.; Vind, J.; Svendsen, A.; Nolte, R. J. M.; Rowan, A. E.;Brust, M. Bioconjugate Chem. 2006, 17, 1373–1375.

Scheme 1. 9-BBN Induced Single Step Synthesis ofAlkylthiol Stabilized Gold Nanoparticles

Figure 1. (A) TEM image and (B) histogram of particle size analysis of9-BBN induced synthesis of ODT-stabilized AuNPs. Insert in (A) showshigher magnification TEM image with 40 nm scale bar.

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(MUA) was added and stirring was continued for another30 min. The solution was heated until the temperaturereached 60 °C. At this point, 0.6 mL of 0.5 M 9-BBN inTHF and 0.010 mL (0.02 mmol) of TOA were added to thereaction mixture. Immediately after addition of 9-BBN, thecolor of the solution changed to orange followed by lightpurple, and within 10 min it was a reddish-purple coloredsuspension. The solution was heated for another 10 min, thenremoved from heat, and stirred at room temperature for 30min. The solution was centrifuged resulting in a reddish-purple colored solid. The solid was dried under nitrogen forfurther analysis. Under similar reduction conditions andidentical molar ratio of reagents, the AuNPs also weresynthesized in the presence of MUOH, MUDA, and AUDDS.The ligand stabilized AuNPs were characterized by UV-visible spectroscopy. The MUA-capped particles display anabsorption peak (λmax) at 528 nm (Supporting Information,Figure 2). TEM analysis of the particles (Figure 2A) showsthat the MUA-capped nanoparticles are nearly monodispersewith an average size of 1.9 ( 0.3 nm. The AuNPssynthesized in the presence of other ligands also werecharacterized by TEM, and the particle sizes of MUDA,MUOH, and AUDDS were found to be 1.4 ( 0.2, 2.0 (0.3, and 2.2 ( 0.3 nm respectively; see Figure 2 andSupporting Information, Figure 3 and Table 1. The ligandstabilized AuNPs were analyzed by FTIR to determine thenature of the functional groups on the ligands attached tothe nanoparticle surface. The MUA- and MUDA-cappedAuNP conjugates displayed absorption peaks at 1710 and1667 cm-1 which are assigned to CdO stretching in the-COOH and -CONH2 of MUA and MUDA, respectively;

see Figure 3.15a,b In the case of MUDA-capped AuNPs, wealso observed a peak at 1608 cm-1 of sNsH stretchingdeformation.15c The azide terminated disulfide capped AuNPsdisplay a peak at 2094 cm-1 which is due to sNdNstretching of the sN3 group, and additionally a shoulder at1221 cm-1 also appeared because of CsN stretching ofCH2sN3 in Figure 3.15b The FTIR data shows that thefunctional groups attached to the nanoparticle surface arenot altered and do not undergo chemical reduction by 9-BBN.The 9-BBN reduction methodology provides a significantadvantage in extending the range of functional groups usedfor AuNP surface functionalization in comparison to theBrust two-phase synthetic approach where the reductionreaction is not compatible with acid or alcohol terminatedthiols.6 The AuNPs were synthesized in the presence ofvarious functionalized thiols and produced particles in thesize range of 1.4-3.3 nm without any post-synthesis treat-ment. The synthesized AuNPs can be be purified by washingwith cold hexane which allows removal of the soluble 9-BBNfrom the insoluble nanoparticles. Detailed purification pro-cesses and large scale synthesis are under investigation.

The versatility of the 9-BBN induced nanoparticle syn-thesis was investigated by applying this strategy to produceother noble metal nanoparticles. The surfactant stabilizednearly monodisperse Ag, Pd, and Pt nanoparticles also weresynthesized using 9-BBN as an efficient reducing agent. Thesizes of the synthesized particles were 2.0 ( 0.3, 3.4 ( 0.9,and 1.7 ( 0.4 nm for Ag, Pd, and Pt, respectively; see Figure4 and Supporting Information, Figure 4. Detailed descriptionof the synthetic protocols are included in the SupportingInformation.

In conclusion, we have reported a single step efficientmethod of producing thiol-stabilized, nearly monodispersegold nanoparticles using 9-BBN as a reducing agent. Thisis the first demonstration of 9-BBN as a reducing agent inmetal nanoparticle synthesis. The synthetic protocol basedon this mild reducing agent also provided the opportunityto directly functionalize AuNPs with a variety of thiolatedligands during the synthesis. The reducing agent also wasused to synthesize ligand stabilized Ag, Pd, and Pt nano-particles with low size dispersion.

Supporting Information Available: Detailed synthetic proce-dures, TEM image, and table of particle size analysis (PDF). Thismaterial isavailablefreeofchargevia theInternetathttp://pubs.acs.org.

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(15) (a) Ojha, U.; Rajkhowa, R.; Agnihotra, S. R.; Faust, R. Macromolecules2008, 41, 3832–3841. (b) Wood, B. R.; Langford, S. J.; Cooke, B. M.;Lim, J.; Glenister, F. K.; Duriska, M.; Unthank, J. K.; McNaoghton,D. J. Am. Chem. Soc. 2004, 126, 9233–9239. (c) Sardar, R.; Bjorge,N. S.; Shumaker-Parry, J. S. Macromolecules 2008, 41, 4347–4352.

Figure 2. TEM images of thiol capped AuNPs: (A) MUA, (B) MUDA,and (C) AUDDS, respectively. Scale bars are 40 nm.

Figure 3. FTIR spectra of (A) AuNPs-MUA, (B) AuNPs-MUDA, and (C)AuNPs-AUDDS conjugates.

Figure 4. TEM images of ligand stabilized (A) Ag, (B), Pd, and (C) Ptnanoparticles. Scale bars are 60 nm.

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