Biomacromolecules 2005, 6, 1224-1225
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Communications Phospholipid-Stabilized Au-Nanoparticles Peng He and Marek W. Urban* School of Polymers and High Performance Materials, Shelby F. Thames Polymer Science Research Center, The University of Southern Mississippi, Hattiesburg, Mississippi 39406 Received March 13, 2005; Revised Manuscript Received March 29, 2005
This communication outlines a simple two-step approach of modification of 1 nm diameter Au nanoparticles using an aqueous solution of (1,2-dipalmitoyl-sn-glycero-3-phosphothio-ethanol) phospholipid (PL). Transmission electron microscopy as well as particle size analysis show that, as a result of PL reactions with Au particles, the initial Au nanoparticle size increases to 5 nm. Considering the size of the PL and their ability to form liposomes, 5 nm diameter spheres indicate that the PL bilayer was attached to the surface of Au particles and the PL-Au interactions are facilitated by the presence of thiol functionality. The change of surface electronic properties of PL-stabilized Au particles is manifested by the disappearance of the 217 and 290 nm absorbances due to 5d-6sp transitions in Au, which is likely attributed to the presence of S-H functionalities which increase the free electron density of the particle. As a consequence, two surface plasmons resulting from a collective oscillation of electrons in response to UV excitation disappear. Although synthesis of colloidal gold has been of interest for a long time,1 continuous interest in this area comes from many potential biological and nanotechnology applications. As a consequence, numerous recently published synthetic efforts have been made to prepare and/or stabilize Au nanoparticles in various environments ranging from DNAAu assemblies1,2 to thiol,3,4 carboxylate,4,5 or alkanethiol6 modifications and more complex Au porphyrin or fullerene clusterization,7,8 just to name a few. Although these approaches indeed provided valuable insights into the modification of Au nanoparticles, our recent studies on polymeric colloidal dispersions have shown,9,10 that the use of phospholipids (PL) in stabilizing colloidal dispersions open new opportunities for stabilizing solid particles using these biologically active species. This communication describes a simple procedure of how PL can be utilized to modify Au nanoparticles and form stable nanodispersions. Specific steps in this development are illustrated in Scheme 1. As seen, the first step is to prepare Au nanoparticles using a wellestablished protocol.11 Such gold nanoparticle aquous solutions at concentration levels of 0.001 M were stirred and (1,2-dipalmitoyl-sn-glycero-3-phosphothio-ethanol) PL was added. Its concentration was 0.01 M (10× the concentration of Au nanoparticles). The mixture was stirred for 24 h under ambient conditions. This is shown as step 2 in Scheme 1. In an effort to determine if indeed Au nanoparticles were “coated” with PL, Figure 1A illustrates a TEM image of Au nanoparticles prepared in step 1. Their average particle size diameter as determined from the image and the particle size analysis shown in Figure 2 is 1 nm. However, PL-modified Au nanoparticles images shown in Figure 1,B clearly illustrate that upon modification an average particle diameter * To whom correspondence should be addressed.
Scheme 1. Preparation of PL-Modified Au Nanoparticles
increases to 5 nm ((1), which is also confirmed by the particle size analysis data illustrated in Figure 2. It should be also pointed out that the data presented in Figure 2 represent two overlaid separate particle size measurements obtained in step 1 of Scheme 1 (Au only) and PL-modified Au nanoparticles in step 2. The particle size data indicate that, in order to form 5 nm diameter spheres, the actual thickness of the PL layer deposited is 2 nm. Considering the size of the PL molecules and the fact that these species form liposomes, the presence of a 5 nm spheres indicates that the PL bilayer was attached to the surface of Au particles and the PL-Au interactions are facilitated by the presence of thiol functionality. To further verify the presence of PL-Au modified nanoparticles, UV-vis spectra were collected. As shown in Figure 3, UV-vis spectra of HAuCl4 starting solution, Au nanoparticles, and PL-modified Au nanoparticles show the disappearance of the 217 and 290 nm absorbances due to 5d-6sp transitions in Au.12 Several theoretical models have been proposed regarding this behavior which is attributed to the presence of S-H functionalities. Apparently, thiolPL increases the free electron density of the particle, and therefore, two surface plasmons resulting from a collective oscillation of electrons in response to UV excitation detected at 217 and 290 nm will disappear.
10.1021/bm0501961 CCC: $30.25 © 2005 American Chemical Society Published on Web 04/12/2005
Communications
Biomacromolecules, Vol. 6, No. 3, 2005 1225
Figure 3. UV-vis spectra of HAuCl4 (H2O), Au nanoparticles (H2O), and phospholipid-modified Au nanoparticles (H2O).
Figure 1. TEM images of Au and phospholipid-modified Au nanoparticles.
for 1 h at 13 000 rpm. The resulting supernatants were removed and 10 mL of deionized water was added. The Au PL-stablized nanoparticles were redispersed and agitated for an additional 24 h, followed by centrifuging for 1 h at 13 000 rpm. Upon removal of the supernatants, the resulting particles were rinsed with deionized water (10 mL) with brief agitation. The supernatants were removed again, and the particles were dried in a vacuum for 24 h to prepare for characterization. Transmission electron micrographs were acquired on a Zeiss EM 109T microscope using an accelerating voltage of 50 kV. Liquid specimens were diluted in deioninzed water by a factor of 100, followed by casting onto Formvar-coated copper grids (T. Pella, Inc.). The specimens were not stained prior to the analysis and lack of the image contrast between the Au core and PL bilayers results from their high electron density.13 Acknowledgment. Major support for these studies from the National Science Foundation Materials Research Science and Engineering Center (DMR 0213883) is acknowledged. Supporting Information Available. IR analysis data are available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 2. Particle size analysis of Au and phospholipid-modified Au nanoparticles.
In the synthesis of Au nanoparticles stabilized by PLs, the following process was employed: first, Au nanoparticles were prepared by adding 3.94 mg of HAuCl4‚3H2O to 10 mL of deionized water (0.001 M solution) in the presence of (1,2-dipalmitoyl-sn-glycero-3-phosphothio-ethanol) PL. The concentration of PL was 10 times greater than HAuCl4‚ 3H2O (0.01 M). The mixture was stirred for 2 h to ensure homogeneity. Next, 5 mg of NaBH4 solution (0.001 M) was added dropwise, and the mixture was continuously stirred at room temperature for 48 h, resulting in a gold aqueous solution. The resulting solution was removed and centrifuged
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