18715
2005, 109, 18715-18718 Published on Web 09/17/2005
Growth of Uncapped, Subnanometer Size Gold Clusters Prepared via Electroporation of Vesicles Sixin Wu, Hongxia Zeng, and Zoltan A. Schelly* Center for Colloidal and Interfacial Dynamics, Department of Chemistry and Biochemistry, UniVersity of Texas at Arlington, Arlington, Texas 76019-0065 ReceiVed: August 4, 2005; In Final Form: August 31, 2005
Electric-field-induced transient pore formation (electroporation) in synthetic unilamellar vesicles is utilized for the preparation of subnanometer size uncapped gold quantum dots. With the precursor AuCl4- placed in the aqueous bulk solution and the reducing agent BH4- originally entrapped in the vesicles’ compartments, the redox reactionsthat occurs in the bulksis initiated by the opening of transient pores in the vesicles’ bilayers. The absence of caps permits (i) continued growth of the Au clusters formed, (ii) the assessment of their true absorption spectra unaltered by stabilizing ligands, and (iii) the previously inaccessible live observation of the growth of the clusters in the molecular size regime. The normally rapid self-aggregation of Au atoms is slowed to the time scales of hour and week by their adsorption at the exterior surface of the vesicles. The UV spectra exhibit novel, time-dependent, oscillating red and blue shifts of the characteristic absorption band, which can be attributed to the evolution of cluster size transiently halting at magic aggregation numbers corresponding to Au2, Au8, Au20, and Au34. Subsequent growth is associated with a monotonic red shift of the absorption band up to the characteristic surface plasmon absorption at 520 nm.
Introduction The interest in colloidal gold is known to span over several millennia: from the use of gold elixirs in ancient Egypt to the exploitation of the size-dependent properties of gold nanoparticles in the present days. With metal clusters exhibiting the most significant quantum size effects in the 1-10 nm diameter range, recent efforts have focused on the production of such tiny particles. Since these metal clusters play a central role in catalysis and nanotechnology, extensive studies involving metallic, especially gold, nanoclusters for nanoscale materials and devices have been in progress.1-3 The findings demonstrate clearly the complementary roles played by theoretical, gas, and condensed phase studies of clusters. Although gold is one of the least reactive metals, its nanoclusters have revealed surprising new catalytic properties.4,5 Consequently, much work has been invested in the synthesis and characterization of gold nanoclusters of uniform size. Numerous synthetic methods have been developed for this purpose, including the reduction of a gold salt by citrate to produce 12-20 nm size gold particles with a relatively narrow size distribution6,7 or reduction by borohydride in the presence of an alkanethiol capping agent to produce 1-3 nm particles.8,9 Variation of the thiol concentration leads to size control between 2 and 5 nm.10 Phosphine-stabilized gold clusters (1.4 ( 0.4 nm) have also been prepared and further converted to thiol-capped clusters by ligand exchange in order to improve their stability.11-15 Gold clusters of 1.3-8 nm in diameter were obtained in reverse micelles.16-18 Most methods to create small gold clusters (1-5 nm) take advantage of the * To whom
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strong capping action of thiols.19-23 Other capping agents used include disulfides,19 polymers with mercapto and cyano functional groups,20 and dendrimers.22,23 Reports on subnanometer size colloidal Au clusters are rather limited due to the difficulty of controlling the nucleation and growth steps occurring at intermediate stages of self-aggregation, owing to the lower stability of the smaller clusters.24-27 Predominantly, Au6 (some of them ligand free) were obtained on MgO powder support,24 dendrimer-encapsulated Au8 nanodots were synthesized in aqueous solution,25 functionalized, thiol-stabilized gold particles with a 0.8 ( 0.2 nm Au11 core were obtained by a ligand exchange reaction,26 and gold clusters composed of ∼10-13 atoms were prepared via reduction and stabilization by meso-2,3-dimercaptosuccinic acid.27 The very function capping agents are intended to serve (namely, to stabilize clusters at a certain size and to block their agglomeration), however, effectively also prevents cluster growth and potentially alters the surface energy of the particles. Because of these limitations, especially in the subnanometer size regime where the clusters exhibit molecular behavior and structures, the intrinsic properties of energetically preferred different cluster sizes have escaped observation. Due to the complexity of the electron configuration of gold atoms (5d106s1), theoretical investigation of gold clusters represents a computational challenge. Nonetheless, numerous studies attacked the problem,28 yielding somewhat deviating results depending on the type and level of approximations applied. Recent density functional theory (DFT) calculations on Aun (2 e n e 20) clusters, however, agree in predicting the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap to oscillate in magnitude with © 2005 American Chemical Society
18716 J. Phys. Chem. B, Vol. 109, No. 40, 2005
Figure 1. Formation of Au quantum dots (QDs) via electroporation (EP) of vesicles. No reaction occurs without application of the pulse.
increasing cluster size (n).28,29 To our knowledge, the corresponding experimental observation in the absorption spectra of growing gold clusters has not been reported previously. In the preset communication, we confirm the theoretical prediction by reporting the novel observation of oscillating red and blue shifts of the UV absorption band of slowly growing subnanometer size uncapped gold clusters. Access to these materials is a prerequisite to the detailed study of the electronic and optical properties of subnanometer particles and the assessment of their utility as building blocks in nanoscale devices. Experimental Section Gold clusters are produced through the reduction of the precursor AuCl4- by BH4- in aqueous solution under the controlled conditions offered by the electroporation of synthetic vesicles. Electroporation is the opening of fully reversible, temporary pores in the bilayer of synthetic phospholipid vesicles and cells, induced by the application of a high-voltage rectangular electric pulse of suitable strength and duration to the suspension.30 Utilization of this method for creating quantum dots, developed in our laboratory,31 was demonstrated previously upon obtaining ultra small, uncapped AgBr,31 CdS,32 and PbS33 nanoclusters. The essence of the method can be summarized as follows.34 Under the influence of the applied electric field (E), the time-average spherical vesicles become polarized and slightly elongated to prolate ellipsoids with their major axes parallel to E. The resulting structural anisotropy of the solution entails optical anisotropy that manifests itself in transient electric birefringence.35,36 Since the membrane is not stretchable by moderate forces, deformation of the spherical shell leads to an elevation of pressure in the vesicle’s interior and an increase of membrane curvature (destabilization of membrane structure) at the two polar cap regions of the vesicle which face the electrodes. The polar cap regions are also the sites of the maximum induced transmembrane potential. The result of these combined effects is the opening of temporary holes in the polar cap regions, provided the strength and length of the electric pulse exceed certain critical values. During opening of the pores, a fraction of the vesicle’s entrapped content is ejected into the bulk solution. This circumstance allows for the metered admission of a small amount of a reactant (BH4-) throughout into the bulk solution of the reaction partner (AuCl4-), without causing high local concentrations that would be inevitable in macroscopic scale mixing. (During the limited lifetime of the pores, the outward flux of BH4- solution and the large size of hydrated AuCl4- ions prevent the latter from entering the vesicle. Upon termination of the pulse, the vesicle resumes its spherical shape in a few microseconds and the pores start to shrink and reseal within ∼1 ms.) The avoidance of high local concentrations is crucial for the production of only a small amount of homogeneously distributed Au atoms. Similarly important is their adsorption on the exterior surface of the vesicles which contributes greatly to the observed sluggishness of selfaggregation and cluster growth. The overall process is depicted schematically in Figure 1.
Letters Preparation, Characterization, and Loading of Vesicles. All chemicals were used as received, without further purification. Unilamellar bilayer vesicles were prepared from the synthetic lipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC; Avanti Polar Lipids). After evaporating the chloroform solvent and drying under reduced pressure, large multilamellar vesicles (MLVs) were formed by hydrating the lipid film with 5 × 10-3 M NaBH4 (Aldrich, 99%) aqueous solution. The resulting solution of MLVs with BH4- both entrapped inside the vesicles and present in the bulk medium had a lipid concentration of 6 mg/mL. To obtain unilamellar vesicles, the MLV suspension was extruded five times (Extruder, Lipex Biomembranes) through two stacked polycarbonate filters with a pore size of 200 nm under a nitrogen pressure of up to 3.4 atm. To replace the BH4- ions in the bulk medium with the precursor AuCl4-, equal volumes of the vesicle solution and a 7.5 × 10-3 M NaAuCl4 (Aldrich, 99%) aqueous solution were mixed slowly (over a period of 10 min). In the resulting dark brown solution of colloidal gold, the BH4- ions were used up from the bulk by the reaction and an excess amount of AuCl4- remained. The gold particles formed outside the vesicles were separated by centrifugation (Hettich EBA 12R) at 29 515g for 99 min. A significant fraction of the loaded vesicles were also dragged by the precipitate to the sediment. Only the clear supernatants (comprising the left side of the reaction in Figure 1) were used for electroporation and for dynamic light scattering measurements. The latter revealed a mean hydrodynamic diameter of 〈Dh〉 ) 140 nm, with a polydispersity of 0.1603, for the loaded DMPC vesicles. Electroporation. Electroporation of the loaded vesicles was carried out in an instrument designed for studying the transient electric birefringence of colloidal solutions. Since the instrument and its operation were described previously,33 only the details relevant for the present purpose are summarized. The sample solution is placed between a pair of gold-plated stainless steel electrodes (2.5 mm apart) separated by Teflon spacers. A single, 500 µs long, high-voltage (1.5 kV) rectangular pulse of a pulse generator (Cober 605P, rise and fall times
