Metal Directed Assembly of Terpyridine-Functionalized Gold

NANO LETTERS

Metal Directed Assembly of Terpyridine-Functionalized Gold Nanoparticles

2002 Vol. 2, No. 12 1345-1348

Tyler B. Norsten, Benjamin L. Frankamp, and Vincent M. Rotello* Department of Chemistry, UniVersity of Massachusetts, Amherst, Massachusetts 01003 Received July 29, 2002; Revised Manuscript Received September 27, 2002

ABSTRACT Terpyridine capped gold nanoparticles (ca. 2.0 nm diameter) form large aggregates in the presence of metal ions [Fe(II), Zn(II), Cu(I), Ag(I)]. The assembly process is a result of metal coordination between two terpyridines that are attached to separate nanoparticles. The stability of the aggregates in various solvents and in the presence of excess terpyridine can be controlled through choice of bridging metal. Small angle X-ray scattering experiments indicate regular interparticle distances that increase as the length of the supporting monolayer is extended.

The assembly of nanoparticles into defined macromolecular structures provides access to nanocomposites featuring useful chemical,1 electronic,2 and physical properties.3 Common approaches to noncovalent assembly strategies employ van der Waals/packing interactions,4 hydrogen bonding,5 ion pairing,6 and host-guest inclusion chemistry.7 These selfassembly methods provide direct access to extended structures from appropriately designed nanoparticle building blocks. Metal-ligand systems provide a means of expanding the structural diversity of self-assembly processes,8 as well as imparting functional attributes such as redox9 and photochemical properties10 to the resulting constructs. To explore the application of this methodology to nanocomposite fabrication, we have synthesized nanoparticles bearing terpyridine (terpy) ligands and studied their self-assembly using a variety of transition metals. We demonstrate here the application of this approach to the formation of stable inorganic nanocomposites featuring systematic control of interparticle spacing. Metal-mediated nanoparticle assembly was studied using mixed monolayer protected gold clusters (MMPCs) 4-6 (Scheme 1). In these nanoparticles, the distance of the terpyridine functionality to the metal core was systematically varied by altering the chain lengths of the terpy ligand and the supporting monolayers. The comparison of interparticle spacing as a function of monolayer thickness required uniform cluster core size for MMPCs 4-6. The control of core size was achieved by using the pentane thiol stabilized monolayer protected cluster (MPC 1) as the nanoparticle source for the fabrication of all MMPCs. MPC 1 was prepared following a previously reported protocol11 and * Corresponding author. E-mail: [email protected] 10.1021/nl020217m CCC: $22.00 Published on Web 11/09/2002

© 2002 American Chemical Society

Scheme 1

featured an average core diameter of ca. 2.0 nm.12 The octane (MPC 2) and undecane (MPC 3) clusters were synthesized from MPC 1 employing typical place exchange conditions13 using a large excess of the respective alkane thiol. Terpy-functionalized thiols 10-12 were prepared through similar synthetic routes (Scheme 2). In a typical procedure, the bromothioacetates14 were converted to the corresponding terpy thioacetates 7-9 by treatment with 4-hydroxyterpyridine15 under basic conditions. The acyl groups were then

Scheme 2

Figure 2. Representative PCM micrograph (a) and TEM micrographs (b,c) showing metal induced aggregation with MMPC 5 + Fe(H2O)6(BF4)2, and (d) TEM micrograph showing no aggregation with MPC 2 + Fe(H2O)6(BF4)2.

Figure 1. Changes in the UV-vis spectrum of a CHCl3 solution of MMPC 5 (5 × 10-7 M) as 2.5 µL aliquots of a (1:3) MeOH/ CHCl3 solution of Fe(H2O)6(BF4)2 (2.5 × 10-3 M) are added. Inset: 2.5 µL additions of the same Fe(H2O)6(BF4)2 solution to a CHCl3 solution terpyridine thioacetate 8.

removed by treatment with NaOMe to provide the corresponding terpy thiols 10-12. Place exchange13 of the appropriate thiol functionalized terpy into the corresponding MPC yielded MMPCs 4-6; 1H NMR end group analysis demonstrated that the ratio of ligand to packing thiol on MMPCs 4-6 was ca. 1:3. Preparation of MMPCs 4-6 set the stage to investigate the metal-mediated assembly of the nanoparticles. We initially chose to assemble the MMPCs using iron(II) because of its known ability to form stable hexacoordinate bisterpy complexes under mild conditions.9b,10a Coordination of the MMPCs 4-6 to the iron was monitored by UV-vis spectroscopy following the formation and subsequent increase of the distinctive Fe(terpy)2 MLCT absorption band (450-600 nm) located beneath the surface plasmon resonance band of the particle, as Fe(H2O)6(BF4)2 was added to a solution of the MMPC. No visible aggregation was observed at the low concentrations necessary to study this association by UV-vis spectroscopy (5 × 10-7 M), presumably because at these concentrations the MMPCs are able to form discrete soluble aggregates. However, extended aggregates of MMPCs 4-6 could be readily obtained through the addition of a more concentrated MeOH solution of Fe(H2O)6(BF4)2 (50 µL, 4 mg/mL) to a CHCl3 solution of the nanoparticle (2 mL, 1 mg/mL). Agitation of the dark mixture resulted in the immediate formation of microscopic particulates, and subsequent standing yielded clear solutions indicating complete association of the nanoparticles. 1346

The overall morphology of the Fe nanocomposites was then investigated by phase contrast microscopy (PCM), (Figure 2a). Microscopically, the Fe-bridged composites appeared as porous extended structures containing numerous internal voids and cavities. Transmission electron microscopy (TEM) was used to determine the nanoscale structure of these aggregates by drop casting aggregate suspensions in CHCl3 onto carbon-coated copper grids. Figure 2a and 2b show representative TEM examples of typical MMPC-Fe aggregates. Little free nanoparticle was observed on the TEM grid, verifying complete uptake of nanoparticle into the aggregates. No aggregation was observed in the control systems by TEM when (a) MMPCs were drop-cast without added metal or (b) nonfunctionalized MPCs were drop-cast with added metal (Figure 2d). It is evident from the TEM micrographs that the nanoparticles within the aggregates are tightly assembled (Figure 2c), exhibiting substantially more structure than the controls mentioned above. Aggregates could also be obtained when other metals such as Ag(CH3CN)4BF4, Zn(H2O)4(BF4)2, and Cu(CH3CN)4PF6 were added to MMPCs 4-6 under identical conditions. The aggregates formed by the addition of the Ag (Figure 3a) and Cu salts to MMPC 5 appeared much denser by PCM, exhibiting fewer internal voids and cavities as compared to the Fe and Zn aggregates.16 This may result from a kinetic vs a thermodynamic assembly process whereby the more strongly coordinating hexacoordinate terpy complexes (Fe and Zn)17 form very rapidly and essentially irreversibly on the molecular time scale, generating kinetic aggregates filled with many voids and cavities. In the case of the weaker tetracoordinate complexes (Ag and Cu),18 more densely packed aggregates are formed as result of an “on-off”, thermodynamically controlled assembly process, which allows for reorganization and maximization of coordination contacts within the assembly. The nanoparticles within the Ag (Figure 3d), Cu, and to a lesser extent the Zn aggregates could be redispersed by Nano Lett., Vol. 2, No. 12, 2002

Figure 3. Representative PCM micrograph (a) and TEM micrographs (b,c) showing metal induced aggregation with MMPC 5 + Ag(CH3CN)4BF4, and (d) disaggregation of Ag aggregates upon addition of excess 2,2′:6′, 2′′-terpyridine.

addition of a large excess of free 2,2′:6′, 2′′-terpyridine ligand, demonstrating the reversibility of these weaker coordinating systems. All of the resulting aggregates were insoluble in common organic solvents (DMF, MeOH, CH3CN, acetone, EtOAc) and were thermally stable, withstanding decomposition in boiling CHCl3, with the exception of the Ag aggregates which could be partially redissolved in DMF. The Fe aggregates showed no dissociation under any of the conditions tested. The internal structure of these aggregates was quantified using small angle X-ray scattering (SAXS). In these experiments, the precipitates formed from the addition of Fe(H2O)6(BF4)2 to CHCl3 solutions of MMPCs 4-6 were deposited onto Mylar film sheets. SAXS experiments revealed that the increase in monolayer chain length from MMPC 4 to 6 results in a gradual shift in qmax to lower values, indicating a greater average interparticle separation (Figure 4). Table 1 lists the q values and the corresponding interparticle spacings for all of the aggregates studied. The observed increases in spacing (∼ 0.9 Å per carbon atom) are in good agreement with other studies that address interparticle spacing as a function of alkyl chain length.4a The observed values for the core-to-core distances in all of the aggregates are approximately 65% of that expected from structures featuring Fe(terpy)2 moieties flanked by fully extended alkyl chains. Interdigitation between adjacent monolayers has been shown to cause an observed decrease in the core-to-core distance;4c however, this is unlikely in our system as the coordinated bulky terpy units would prevent interdigitation. It has also been speculated that an observed decrease in particle spacing results from the monolayer collapsing upon solvent evaporation.4a The latter hypothesis was verified by performing the SAXS experiments on the same Fe aggregates suspended in CHCl3 in a sealed capillary. In these cases, a similar trend was observed as the qmax shifted to even lower values, indicating a greater extension of the monolayer chains within the solvated aggregates. Similar values were observed for Ag aggregated MMPCs 4-6, corroborating the results Nano Lett., Vol. 2, No. 12, 2002

Figure 4. SAXS plots of (a) Fe aggregated MMPCs 4-6 drop cast onto Mylar film and (b) Fe aggregated MMPCs 4-6 in capillaries suspended in CHCl3. Table 1: Values of q (nm-1), 2π/q (nm), and Maximum Interparticle Distances for MMPCs 4-6 solida

solutionb

nanoparticle

q (nm-1)

2π/q (nm)

q (nm-1)

2π/q (nm)

max. interparticle spacingc (nm)

MMPC 4 MMPC 5 MMPC 6

1.97 1.83 1.67

3.18 3.43 3.77

1.76 1.52 1.40

3.57 4.13 4.49

4.7 5.5 6.2

a SAXS data obtained from aggregates drop-cast from a CHCl suspen3 sion onto Mylar film. b SAXS data obtained from CHCl3 suspension of c aggregates in a sealed 0.5 mm capillary tube. Maximum interparticle spacings were determined by molecular modeling as the core-to-core distances using fully extended terpy monolayers and 2.0 nm diameter nanoparticles.

obtained on the Fe nanocomposites and demonstrating the generality of the assembly strategy.19 In summary, we have shown that nanoparticle aggregation can be facilitated by transition metals coupled with terpyfunctionalized MMPCs. The terpy MMPCs have the capability to coordinate several different metals, demonstrating the versatility of this approach to both particle assembly and facile generation of various nanocomposites. This methodology is highly versatile: interparticle spacing can be controlled by the altering the length of the supporting monolayer while the overall strength of the assemblies can be controlled through choice of bridging metal atom. Expansion of this method to control the functional profile of electronic and magnetic nanoparticle aggregates is underway and will be reported in due course. 1347

Acknowledgment. This research was supported by the NSF (CHE 9905492 and MRSEC facilities). T.B.N. acknowledges support from NSERC (Canada) in the form of a postdoctoral fellowship. Supporting Information Available: The synthesis and characterization of terpys 7-12 and MMPCs 4-6, PCM and TEM micrographs of Zn and Cu aggregates, SAXS data for the MMPCs 4-6 with Ag. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) (a) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (b) Schmid, G.; Baumle, M.; Geerkens, M.; Heim, I.; Osemann, C.; Sawitowski, T. Chem. Soc. ReV. 1999, 28, 179. (2) Ung, T.; Liz-Marza´n, L. M.; Mulvaney, P. J. Phys. Chem. B 2001, 105, 3441. (3) (a) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (b) Sun, S.; Anders, S.; Hamann, H. F.; Thiele, J.-U.; Baglin, J. E. E.; Thomson, T.; Fullerton, E. E.; Murray, C. B.; Terris, B. D. J. Am. Chem. Soc. 2002, 124, 2884. (c) Pileni, M. P. J. Phys. Chem. B 2001, 105, 3328. (4) (a) Martin, J. E.; Wilcoxon, J. P.; Odinek, J.; Provencio, P. J. Phys. Chem. B 2000, 104, 9475. (b) Fink, J.; Kiely, J.; Bethell, D.; Schiffrin, D. J. Chem. Mater. 1998, 10, 922. (c) Wang, Z. L.; Harfenist, S. A.; Whetten, R. L.; Bentley, J.; Evans, N. D. J. Phys. Chem. B 1998, 102, 3068. (d) Beomseok, K.; Tripp, S. L.; Wei, A. J. Am. Chem. Soc. 2001, 123, 7955. (5) (a) Boal, A. K.; Gray, M.; IIhan, F.; Clavier, G. M.; Kapitzky, L.; Rotello, V. M. Tetrahedron 2002, 58, 765. (b) Boal, A. K.; Rotello, V. M. Langmuir 2000, 16, 9527. (c) Simard, J.; Briggs, C.; Boal, A. K.; Rotello, V. M. Chem. Commun. 2000, 1943. (d) Frankamp, B. L.; Uzan, O.; IIhan, F.; Boal, A. K.; Rotello, V. M. J. Am. Chem. Soc. 2002, 124, 892. (6) (a) Kolny, J.; Kornowski, A.; Weller, H. Nano Lett. 2002, 2, 361. (b) Boal, A. K.; Galow, T. H.; IIhan, F.; Rotello, V. M. AdV. Funct. Mater. 2001, 11, 1. (c) Hao, E.; Yang, B.; Zhang, J.; Zhang, X.; Sun, J.; Shen, J. Chem. Mater. 1998, 8, 1327.

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(7) (a) Liu, J.; Mendoza, S.; Roma´n, E.; Lynn, M. J.; Xu, R.; Kaifer, A. E. J. Am. Chem. Soc. 1999, 121, 4304. (b) Liu, J.; Alvarez, J.; Ong, W.; Kaifer, A. E. Nano. Lett. 2001, 1, 57. (8) (a) Leininger, S.; Olenyuk, B.; Stang, P. J. Chem. ReV. 2000, 100, 853. (b) Fujita, M.; Ogura, K. Acc. Chem. Res. 1999, 32, 53. (c) Piguet, C.; Bernardinelli, G.; Hopfgartner, G. Chem. ReV. 1997, 97, 2005. (d) Lehn, J.-M. Supramolecular Chemistry: Concepts and PerspectiVes; VCH: Weinheim, 1995; p 144-160. (9) (a) Sauvage, J.-P.; Collin, J.-P.; Chambron, J.-C.; Guillerez, S.; Coudret, C.; Balzani, V.; Barigelletti, F.; De Cola, L.; Flamigni, L. Chem. ReV. 1994, 94, 993. (b) Constable, E. C. Cargill Thompson, A. M. W. J. Chem Soc. Dalton. Trans. 1994, 1409. (10) (a) Norsten, T. B., Chichak, K.; Branda, N. R. Tetrahedron 2002, 58, 639. (b) Ca´rdenas, D. J.; Collin, J.-P.; Gavin˜a, P.; Sauvage, J.P.; De Cian, A.; Fischer, J.; Armaroli, N.; Flamigni, L.; Vicinelli, V.; Balzani, V. J. Am. Chem. Soc. 1999, 121, 5481. (11) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (12) The average particle size was determined by TEM analysis. (13) (a) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir 1999, 15, 3782. (b) Hostetler, M. J.; Green, S. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212. (14) Pearson, A. J.; Hwang, J.-J. J. Org. Chem. 2000, 65, 3466. (15) Constable, E. C.; Ward, M. D. J. Chem. Soc., Dalton Trans. 1990, 1405. (16) See Supporting Information for PCM and TEM micrographs of Cu and Zn nanocomposites. (17) Terpy binds Zn(II) in a hexacoordinate geometry similar to Fe(II); (Schubert, U. S.; Eschbaumer, C.; Andres, P.; Hofmeier, H.; Weidl, C. H.; Herdtweck, E.; Dulkeith, E.; Morteani, A.; Heckner, N. E.; Feldmann, J. Synthetic Metals 2001, 121, 1249). (18) Terpy binds Cu(I) and Ag(I) in a four-coordinate near-tetrahedral geometry; (Baum, G.; Constable, E. C.; Fenske, D,; Housecroft, C. E.; Kulke, T.; Neuburger. M.; Zehnder, M. J. Chem. Soc., Dalton Trans. 2000, 945). (19) See Supporting Information for details.

NL020217M

Nano Lett., Vol. 2, No. 12, 2002