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J. Phys. Chem. B 2008, 112, 14422–14426
Cluster Precursors of Uncapped CdS Quantum Dots via Electroporation of Synthetic Liposomes. Experiments and Theory† Hongxia Zeng, Raji Reddy Vanga, Dennis S. Marynick,* and Zoltan A. Schelly* Department of Chemistry and Biochemistry, UniVersity of Texas at Arlington, Arlington, Texas 76019-0065 ReceiVed: March 28, 2008; ReVised Manuscript ReceiVed: September 29, 2008
Subnanometer size cluster precursors of uncapped CdS quantum dots were produced via the electroporation of synthetic dioleoylphosphatidylcholine (DOPC) unilamellar bilayer vesicles of mean hydrodynamic diameter 〈Dh〉 ) 175 nm. During electroporation, Cd2+ ions are ejected from the interior compartments of the vesicles into the bulk solution where they react with S2- ions to form CdS monomers. The monomers adsorb on the exterior surface of the vesicles, where their spontaneous self-aggregation to (CdS)n clusters occurs on the hour and day time scale. The stepwise growth of the clusters was monitored through the time evolution of the UV absorption spectrum of the solution. The process is characterized by initial stepwise blue shifts of the absorption maxima: 285 nm f 269 nm f 245/275 nm f 240 nm f 236 nm, followed by a red shift to 494 nm. Nonlocal density functional theory (DFT) calculations of the optimized geometry and HOMO-LUMO gap of (CdS)n particles with n ) 1-6 were carried out. The optimized structures are characterized by strong Cd-Cd bonds, with the S atoms bridging those bonds or capping the faces of the Cd polyhedra. The structure of such clusters bears no resemblance to fragments of the bulk crystal. The trend of the calculated HOMO-LUMO gaps facilitates the attribution of aggregation numbers (n) to particular clusters responsible for the observed absorption bands: n ) 1 (285 nm), n ) 2 (269 nm), n ) 4 (245/275 nm f 240 nm), n ) 5 (236 nm), and larger quantum dots absorbing around 494 nm. The multiple bands assigned to the tetramer reflect the existence of its two distinct structures with similar stability. Introduction Besides the fundamental importance of exhibiting quantum size effects (QSE), metal (Me) and semiconductor (MeX) nanoparticles have attracted considerable interest because of their potential practical applications including labeling, drug delivery, catalysis, photocatalysis, microelectronics, nonlinear optics, and solar energy conversion.1 In the synthesis of such quantum dots, a major experimental challenge is arresting the spontaneous, rapid self-aggregation of the nascent Me atoms or MeX molecules at a desired nanoscopic size. For obtaining CdS nanoparticles, numerous strategies have been exploited to accomplish size control including the use of zeolite cavity,2-4 reverse microemulsion,5 polymer stabilization,6 gel electrophoresis,7 vesicle,8 and size-selective precipitation.9 Particles created this way are large enough for the existence of an electronic band structure; hence, they exhibit the archetypal quantum size effect of decreasing excitation energy, that is, a monotonic red shift of the absorption band, with increasing particle size. The most often used general strategy, however, is capping the particles during synthesis, which allows the creation of molecular size as well as larger clusters. In the case of cadmium sulfide, the UV-vis absorption spectra of thiophenolate ligandcapped species were studied for the (Cd1) monomer10 [Cd(SPh)4]2- and the cadmium polynuclear clusters such as [Cd2(SPh)6]2-,(Cd2),11 [Cd4(SPh)10]2-,(Cd4),10 [Cd10S4(SPh)16]4-, (Cd10),10,12 and Cd32S14(SPh)36, (Cd32).13 Blue shift was found for proceeding from (Cd1) to (Cd4), and red shift appears from † Part of the “Janos H. Fendler Memorial Issue”. * To whom correspondence should be addressed. E-mail: schelly@ uta.edu.
(Cd4) through (Cd10), and (Cd32) clusters. Capping, however, has the disadvantage of altering the intrinsic structure and electronic properties of the (CdS)n cluster, and precluding observation of the time evolution of the aggregation number (n). In the present paper, we report the preparation of naked (CdS)n clusters as well as larger quantum dots by utilizing the electroporation of synthetic liposomes (vesicles), a method developed in our laboratory14 and already used for creating uncapped clusters and quantum dots of AgBr,14 PbS,15 ZnS,16 and gold.17 Results of density functional theory (DFT) calculations on the structure and HOMO-LUMO gap of (CdS)n particles with n ) 1-6 are utilized for the assignment of aggregation numbers (n) to the particular species responsible for the time-dependent absorption spectra we observed. Experimental Methods Electroporation is the fully reversible opening of temporary pores across the bilayer membrane of cells and synthetic vesicles induced by the application of an external, high-voltage electric field E square pulse. The technique is often used in molecular biology for transfection.18 The sequence of physical events19 relevant for this communication are the (1) global polarization, (2) induction of a transmembrane potential ∆φ, and (3) a slight elongation of the original time-average spherical vesicle to a prolate ellipsoid of revolution, all in a direction parallel with E. Deformation of the spherical vesicle results in (4) an increase of pressure ∆p in its interior compartment relative to that in the bulk solution. These events are completed within about less than 10 µs. Above threshold values of the applied field strength E and pulse length ∆t, (5) a few pores open up at the two polar
10.1021/jp802676z CCC: $40.75 2008 American Chemical Society Published on Web 10/22/2008
Cluster Precursors of Uncapped CdS Quantum Dots
J. Phys. Chem. B, Vol. 112, No. 46, 2008 14423
Figure 1. Cartoon of the synthesis of (CdS)n clusters via electroporation (EP) of vesicles.
cap regions (facing the electrodes) of the ellipsoid through which (6) a portion of the vesicle’s compartment (e.g., an aqueous solution) is ejected into the bulk. While the field is maintained, (7) the diameter of the pores grows perpendicular to E; thus, their ultimate size can be controlled by the pulse length applied. After termination of the pulse, (8) the vesicles resume their spherical shape on the order of 10 µs, followed by (9) a much slower resealing of the pores on the millisecond time scale. (The slow closure of the pores is the period when transfection of cells can occur.) Under the conditions of electroporation, the compartmentalized nature of a vesicle suspension is eminently suitable for the creation of uncapped clusters and quantum dots. Namely, with Cd2+ ions (ionic radius 103 pm) initially entrapped in the interior compartment and the S2- ions (ionic radius 184 pm) placed in the bulk solution, their reaction
Cd2+(inside) + S2-(outside) f CdS(outside)
(1)
can only occur upon the ejection of a portion (an estimated total of 3.3 × 1015) of Cd2+ ions into the bulk. Hence, high local concentrations are avoided, which would inevitably lead to the very rapid self-aggregation of CdS to large clusters or quantum dots on the microsecond and millisecond time scale. An additional important factor for slowing down self-aggregation is the adsorption of the dipolar CdS monomer at the exterior surface of the vesicles where the growth steps
CdS + CdS f (CdS)2
(2)
(CdS)2 + (CdS) f (CdS)3
(3)
....... (CdS)n + (CdS)m f (CdS)n+m
(4)
occur on the minute and hour time scale. The overall process is depicted in Figure 1. Preparation and Characterization of Loaded Vesicles. Unilamellar bilayer vesicles of mean hydrodynamic diameter 〈Dh〉 )175 nm were prepared from the synthetic phospholipid dioleoylphosphatidylcholine (DOPC; >99%; Avanti Polar Lipids). The general procedure of characterization20 and loading of the vesicles with Cd2+ (inside) and S2- (outside) was analogous to that used for the preparation of PbS clusters described previously.15,19 During an intermediate stage of solution preparation, Cd2+ ions are present both inside and outside the vesicles. To replace the Cd2+ ions from the bulk, they are overtitrated with aqueous Na2S, leaving a specific desired excess of S2- ions outside the vesicles. The resulting CdS precipitate is removed by centrifugation at 23900g for up to 90 min, and the clear supernatant is used for electroporation and spectral studies. Although the supernatantsconsisting of 2 mg/mL DOPC, with a local concentration of 1 × 10-2 M Cd2+
ions inside the vesicles and a global concentration of 1 × 10-2 M S2- ions in the bulkswas free of spontaneous transmembrane reaction for 2 weeks tested, fresh solutions were used in all electroporation experiments. Theoretically, about a total of 3.3 × 1017 Cd2+ ions are entrapped in the vesicles in 3.5 mL of solution used for the experiments. All procedures were carried out at 23 °C under the exclusion of light, except for the unavoidable illumination during dynamic light scattering and spectral measurements. The water used was double-deionized and distilled. Electroporation Experiments. An instrument designed for transient electric birefringence and light scattering studies of organized assemblies was used for electroporation of the loaded vesicles. Details of the instrument and its operation have been described previously.15,21 The sample solution is placed between a pair of gold-plated stainless steel electrodes (2.5 mm apart) in a quartz cell. The high-voltage (up to 2.1 kV) square pulse with rise and fall times
