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Synthesis of Hybrid Gold-Gold Sulfide Colloidal Particles Jeroˆme Majimel,† Daniel Bacinello,† Etienne Durand,† Fabrice Valle´e,‡ and Mona Tre´guer-Delapierre*,† ICMCB-CNRS, UniVersity of Bordeaux I, 87 aV. Dr. Schweitzer, 33608 Pessac Cedex, France, and LASIM-CNRS, UniVersity of Lyon 1, 43 Bd du 11 NoVembre, 69622 Villeurbanne, France ReceiVed September 13, 2007. In Final Form: January 29, 2008 The nucleation and growth mechanism of nanometer size gold onto gold sulfide colloidal particles by irradiationinduced reduction is reported. The process is characterized by ultraviolet-visible spectroscopy, electronic diffraction, and high-resolution transmission electron microscopy, allowing for observation of several key intermediates and characteristics of the growth mechanism. The formation mechanism of gold on the surface of the gold sulfide particles is shown to depend strongly on the deposition rate. At low dose rate, gold nucleates preferentially onto specific gold-rich Au2S facets {110}, resulting in epitaxial growth. The gold crystal lattice plastically deforms near the interface to accommodate a substantial lattice mismatch. Upon increasing gold precursor concentration, this low dose rate results in growth of elongated gold island on the gold sulfide surface. At a high dose rate, several randomly oriented gold particles are simultaneously produced on gold sulfide, resulting in a layered structure. The absorption spectra of these particles show a dominant surface plasmon band, whose peak wavelength shifts markedly to the red as layered structure is formed.
Introduction Nanostructures composed of multiple materials have recently become the focus of intensive study with particular attention being paid to their optical properties and the enhanced role of the interface between materials. In this context, dielectric- or semiconductor-metal hybrid nanoparticles are particularly interesting, since they offer the possibility to combine different type of linear and nonlinear optical responses or to design surface plasmon resonance effects in new frequency ranges. This is in particular the case for nanoshell materials, whose surface plasmon resonance can be tuned over the infrared and visible spectrum offering many applications for biolabeling or plasmon mediated optical response enhancement (Raman scattering or luminescence of adsorbed molecule for instance). In order to manipulate the diverse properties of these materials or to create novel composite materials to meet technological requirements or to perform desired functions, it is necessary to establish control over the interface between the various components. In addition, a thorough understanding of the nature and character of this interface is required. In this work, we discuss the nucleation and growth process of gold nanoparticles onto gold sulfide semiconductor nanocores. Similar metal-semiconductor objects have already found widespread use in applications such as optics, sensors, photography, biomedical contrast imaging, and therapeutics.1-7 In this study, AuI is reduced onto the surface of the gold sulfide core by a radiolytic method, the mechanism of which is understood and elucidated. The factors that affect the gold deposition, such as diffusion of gold atoms across the semiconductor surface as well as the rate of reduction, are discussed. Special attention is * Corresponding author: E-mail:
[email protected]. † University of Bordeaux I. ‡ University of Lyon 1. (1) Jain, P. K.; Lee, K. S.; El-Sayed, I. H.; El-Sayed, M. A. J. Phys. Chem. B 2006, 110, 7238-7248. (2) West, J. L; Halas, N. Annu. ReV. Biomed. Eng 2003, 5, 285-292. (3) Wei, A. J. Surf. Sci. Nanotech. 2006, 4, 9-18. (4) Pastoriza-Santos, I.; Gomez, D.; Perez-Juste, J.; Liz-Marzan, L.; Mulvaney, P. Phys. Chem. Chem. Phys. 2004, 6, 5056-5060. (5) Averitt, R. D.; Sarkar, D.; Halas, N. Phys. ReV. Lett. 1997, 78, 4217-4220. (6) Averitt, R. D; Westcott, S. L; Halas N. JOSA B 1999, 16, 1824-1832. (7) Graf, C.; Van Blaaderen, A. Langmuir 2002, 18, 524-534.
paid to structural characterization of the interface and the plastic deformation of the crystal lattice at the heterointerface. Materials and Methods Materials. Methanol and hydrochloric acid (HCl) were both purchased from J. T. Baker; nitric acid (HNO3) was purchased from Carlo Erba Reagenti, and all materials were used as received. For the synthesis of Au2S particles and the Au2S/Au composite particles, potassium cyanoaurate (KAu(CN)2, 99.98%, Aldrich) was purchased and used as received. Prior to experiments, all glassware was washed thoroughly with aqua regia and rinsed with deionized water. Synthesis of Au2S Particles.8,9 A solution (100 mL) of KAu(CN)2 of 0.5 mM is prepared. This solution is diluted to 0.2 mM in water (100 mL) and introduced into a three-neck flask. The solution is degassed by bubbling with Ar(g) for 30 min at 80 °C. The mixture is then bubbled with H2S(g) for approximately 80 min at 80 °C with stirring, at which point it is left under Ar(g) (without heating) for approximately 5 h to eliminate any excess H2S(g). This procedure yields a pale yellow solution consisting of Au2S nanoparticles. The linear absorption spectra of the solution can be used to confirm the elimination of the gold ions and the formation of the Au2S particles. The mechanism involved in the formation of the gold sulfide includes the generation of various gold sulfide complexes, such as Au0HSaq and Au(HS)2-, identified by Renders et al.10 AuCN-1AI + H2S T AuHSaq0, Au(HS)2-..., T Au2S
(1)
Synthesis of Au2S/Au Composite Particles. The deposition of colloidal gold onto the gold sulfide core is accomplished by radiolytic reduction of Au(CN)2- ions. The advantage of using ionizing radiation lies in the fact that the rate of the formation of the reducing radicals is well-known and can easily be regulated by changing the absorbed dose rate. The principle of the method has previously been described by Henglein and Meisel.11 The desired amount of KAu(CN)2 is added to the Au2S colloid solution along with 0.5 M methanol, and the solution is flushed with nitrous oxide. It is then irradiated either (8) Morris, T.; Copeland, H.; Szulczewski, G. Langmuir 2002, 18, 535-539. (9) Hirsh, H.; De Cugnac, A.; Gadet, M.; Pouradier, J. C. R. Acad. Sci. Paris 1966, 263, 1328-1330. (10) Renders, P. J.; Seward, T. M. Geochim. Cosmochim. Acta 1989, 53, 245-253. (11) Henglein, A.; Meisel, D. Langmuir 1998, 14, 7392-7396.
10.1021/la702829w CCC: $40.75 © 2008 American Chemical Society Published on Web 03/14/2008
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Figure 1. (a) Absorption spectra before and after addition of H2S to gold cyanide solution and after bubbling H2S for differnt periods of time (optical pass 2 mm). (b) High-resolution electron micrograph of a colloidal gold sulfide particle and its associated digital diffractogram. in a Cs137 source (2.7 kGy h-1) or with a 20 kW and 10 MeV accelerator delivering 15 µs electron pulses through a scanning beam (1-10 Hz) at a mean dose rate of 7 MGy h-1. The irradiation is performed until all AuI from Au(CN)2- was reduced; this end point is readily recognized by following the disappearance of the strong UV absorption bands of Au(CN)2-. In the presence of N2O, the reduced gold does not form pure gold particles but instead settles onto the gold sulfide particles. It is well-established that the primary radicals of water radiolysis, that is, hydrated electrons, hydroxymethyl radicals, and H atoms, are efficiently scavenged by N2O and methanol. The latter reaction yields hydroxymethyl radicals. H2O s∧∧∧f e-aq, OH, H (H2, H2O2)
(2)
e-aq + N2O + H2O f N2+ OH- + OH
(3)
CH3OH + OH (H) f •CH2OH + H2O (+H2)
(4)
These radicals then initiate the process of reducing the dissolved Au(CN)2-. Instrumentation. Transmission Electron Microscopy. The quality and totality of the deposition of gold onto the gold sulfide surface were visually evaluated and analyzed using transmission electron microscopy. Samples for high-resolution transmission electron microscopy (HRTEM) were prepared by depositing a drop of Au/ Au2S colloidal suspension onto a copper grid covered with a carbon film. The grid is then air-dried for 15 min. HRTEM observations were performed using a JEOL 2200 FS TEM equipped with a field emission gun operating at 200 kV and with a point resolution of 0.23 nm. High-resolution transmission electron micrographs were acquired with a Gatan Ultrascan CCD 2 k × 2 k camera and digital diffractograms were calculated using the Gatan Digital Micrograph software. Moreover, in order to be representative and statistically meaningful, many images from several regions of various samples were recorded and the most characteristic results are presented here. Absorption. All UV-vis absorption spectra were obtained at room temperature using a Cary 5E UV-vis-NIR spectrophotometer. For all measurements, samples were transferred into a Suprasil quartz cuvette with an optical path length of 10 mm. Baseline correction was performed prior to all measurements.
Results and Discussion Gold Sulfide Colloidal Particles. Prior to the deposition of gold, synthesis, analysis, and characterization of isolated gold sulfide core particles were carried out. The Au2S particles were prepared with a uniform size distribution in a surfactant free aqueous solution. The particles exhibit an average radius and
standard deviation of 22.5 nm and 3.5, respectively. Figure 1a shows the absorption spectrum of the synthesized colloidal particles, which is in good agreement with that found in the literature.8 The particles are regular crystals with a near perfect lattice structure. This is illustrated in Figure 1b, which shows a typical high-resolution electron micrograph of a gold sulfide core particle; the good lattice structure is further confirmed by the digital diffractogram given in the inset. Particles crystallize in a Pn3m cubic system with a lattice parameter of 5.02057(86) (ICDS file number 078718).12 For colloidal gold sulfide particles, such high quality structures have not been previously observed. No trace of Au2S3 was detected. Gold Deposition onto Gold Sulfide. Figure 2 shows highresolution transmission electron micrographs of several typical examples of the gold deposition onto the semiconductor core obtained under various experimental conditions. The gold was produced using radiolytic reduction of Au(CN)2- in solution. At a low rate of reduction, a few nanoscale gold monocrystallite, below 5 nm in size, are observed on the metal sulfide particle surface (dark contrast). The interreticular distance of the structure and the obtained electronic diffraction pattern confirm the formation of the metallic structure. The formation of gold by reduction of Au(CN)2- could principally occur in three ways: (1) reduction of AuI in solution followed by the migration of the gold nuclei to the surface of the gold sulfide particles, (2) reduction of AuI adsorbed onto the gold sulfide nanoparticles, and (3) donation of an electron to the Au2S particles by the methanol radicals produced upon irradiation and then reduction of AuI at the particle surface by the stored few electrons. The photochemical reduction on Au2S surface has been excluded, since no reduction of Au(CN)2- could be observed in the experiment where an aqueous solution containing AuI and Au2S was exposed to UV light. In the absence of Au2S seeds in the medium, no reduction of the cyanoaurate is observed over 1 h. In contrast, in the presence of Au2S seeds, the synthesis exhibits no induction period. This suggests that pathway 1 is exceedingly inefficient. Au(CN)2- is not directly reduced by the alcohol radicals, although the standard redox potential of the Au(CN)2-/Aubulk is more positive (-0.6 V) than that of CH2O/ ‚CH2OH (-1.1 V).13,14 The reason for this is that the gold atoms (12) Ishikawa, K.; Isonaga, T.; Watika, S.; Suzuki, Y. Solid State Ionics 1995, 79, 60-66. (13) Mosseri, S.; Henglein, A.; Janata, E. J. Phys. Chem. 1989, 93, 67916795.
Gold-Gold Sulfide Colloidal Particles
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Figure 2. High-resolution transmission electron micrographs showing typical morphologies of gold on Au2S colloidal particles: (a) at low dose rate, ratio ) 0.3; (b) same as (a) except that ratio ) 0.5; (c) high dose rate, ratio ) 0.5, dose ) 5 kGy.
cannot be formed free in solution. Taking into account the fact that the free enthalpy of sublimation of gold is 3.2 eV, the standard potential of the system Au(CN)2-/Au0 (atom) is -3.8 V.13,15 Thus, the reduction by organic radicals will be endoergic by 2.7 eV. In the presence of the Au2S colloidal particles, the redox potential of adsorbed ions Au(CN)2- on support are known to be more positive than for free ions in solutions. Therefore, alcohols radicals, not strong enough to reduce free solvated gold ions Au(CN)2- into zerovalent state, become able to reduce adsorbed ions and to achieve the surface growth of the metal clusters. The binding of adsorbed gold ions to the gold sulfide surface makes the reduction energetically more favorable than in solution (pathway 2).
xCH2OH + AuIads f xCH2O + xH+ + Au0ads +
(5)
The pathway 3 could also occur although only a few electrons donated by the organic radicals could be kept on semiconductor nanoparticles. The organic radicals may transfer a few electrons (14) Schwarz, H.; Dodson, R. J. Phys. Chem 1989, 93, 409-414. (15) Henglein A. Ber. Bunsen-ges. 1977, 81, 556
to the gold sulfide particles, and the stored electrons then reduce Au(CN)2- yielding gold atoms.
xCH2OH + (Au2S)n f (Au2S)n- + xCH2O + xH+ (6) (Au2S)n- + Au(CN)2- f (Au2S)n Au + 2CN-
(7)
As the metal islands are formed on the gold sulfide surface from pathways 2 and 3, they can grow either via reduction of AuI adsorbed on preformed nuclei by organic radicals or by excess electrons stored in metal nuclei. Migration of gold atoms onto gold sulfide surface could also be considered. The atoms could migrate to gold nuclei at the surface of gold sulfide and accumulate at these sites. The accumulation of high concentrations of gold atoms permits the growth of nanometer size gold particles on the Au2S particles. Even if a small fraction of the radicals reduces gold(I) directly in solution, the formed Au0 atoms (or even small Au oligomers) may reach the surface of the gold sulfide and contribute to the deposited metal dot. A yield of 3.6 Au atoms/100 eV of absorbed radiation energy is calculated. This is somewhat less than the
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Figure 3. HRTEM micrograph illustrating the epitaxial growth of gold islands on the gold sulfide core. The interface dislocation network is presented in the inset, and the deviation to the expitaxial relationships is 4° for this particular example.
theoretical value of 4.4 atoms/100 eV, which is most likely due to the loss of radicals as a result of radical recombination prior to reaching the gold sulfide particles. Particularly, remarkable is the accumulation of the gold onto preferential gold sulfide facets. Under the experimental conditions described above, all the gold sulfide particles are systematically affected in the same manner and the growth of gold islands always occurs according to the following epitaxial relationships (Figure 3):
(110)Au//(110)Au2S; [11h1]Au//[11h1]Au2S The (110) plane of the fcc gold sulfide structure only contains gold atoms leading to gold-gold chemical bonds at the interface with the gold island. The strong Au-Au bonding energy may play a major role in the oriented growth process. Note that the (110) plane is the one exhibiting the highest surface energy on the gold structure. One can also observe the presence of a dislocation network perpendicular to the interface plane between the core and the island. It seems that the softer material, gold in this system, favors the creation of dislocations to match the lattice misfit between the two materials rather than elastically deforming. The ∆a/a lattice misfit value of about 18% is in good agreement with the presence of one dislocation for every five or six lattice planes as we can see in Figure 3. Finally, note that there is a small angular deviation to the epitaxial relationships mentioned above. This deviation changes as a function of the interface plane length (Figure 4). On increasing the concentration of the gold ionic precursor, the average size of the gold particle attached to the core surface increases. The deposited metal particle is not exactly spherical and becomes elongated. Figure 2b shows a HRTEM image of
a composite particle created with [AuI]/[Au2S] ) 0.5 and possessing a gold island of hemispherical shape with a measured radius of 10 nm. The produced additional gold atoms are deposited on the already formed gold nuclei, increasing its size but not the number of gold islands. This suggests that the rate of association of AuI with gold nuclei on gold sulfide is higher than the production rate of the reducing radicals and the rate at which the particles pick up electrons from the alcohol radicals. As a consequence, the new particles formed are not isolated or do not produce new islands onto gold sulfide but contribute to the growth of the preformed nuclei. The final radius of the islands is thus found to be systematically larger upon increasing the AuI/Au2S ratio. Moreover, the reduction of the adsorbed gold ions is thermodynamically favored over that of the free ions. As previously mentioned, the redox potential of ions adsorbed on clusters of the same metal is more positive than that of the free ions in solution. As a consequence, the reduction is achieved exclusively on the gold nuclei on the preferential gold sulfide sites. The rate of reduction of the gold ions, as compared to that of gold atom diffusion on the Au2S surface, is a key parameter in the deposition of the gold. Figure 2c shows a HRTEM image of resulting composite particles when a higher dose rate is used for initiating the reduction. It demonstrates that a composite particle with a random distribution of gold nanoparticles can be obtained by increasing the dose rate limits. This increase prevents the diffusion of gold atoms on the gold sulfide surface and results in growth of many randomly oriented gold particles. Higher dose rate leads to a faster electron storage and subsequent gold reduction. A large number of nuclei merge simultaneously over the surface of the gold sulfide particles in the early stage of the
Gold-Gold Sulfide Colloidal Particles
Figure 4. Evolution of the deviation to the gold island on the gold sulfide core epitaxial relationships as a function of the interface length.
Figure 5. Absorption spectra of gold produced onto gold sulfide colloidal particles (a) at low dose rate and (b) at high dose rate. Numbers refer to the [AuI]/[Au2S] ratio. [Au2S] ) 0.06 mmolL-1; dose ) 5 kGy.
reaction. The critical size is rapidly reached, limiting the diffusion of atoms or oligomers toward preferential facets containing only gold atoms. Absorption Spectral Feature of the Composites Particles. Figure 5 shows the absorption spectra of gold nanoparticles on gold sulfide. In the absence of gold, the absorption spectrum of colloidal gold sulfide particles is produced by well separated particles. It exhibits a rise for wavelength smaller than about 600 nm, consistent with the estimated Au2S energy gap,6 with no features (the large absorption increase in the UV part of the spectrum is dominated by the solvent). The evolution of the absorption band as a function of the concentration of deposited gold at low dose rate is shown in Figure 5a and is ascribed to
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gold cluster formation and growth. For very low gold concentrations reduced at low dose rate, only the absorption of Au2S is observed (not shown). No absorption in the 520 nm region due to the development of surface plasmon band of pure gold nanoparticles can be observed. This is consistent with many literature reports of the absence of the surface plasmon band for particles of diameter less than 2 nm. At [AuI]/[Au2S] ) 0.3, this surface plasmon band is first seen at 515 nm. With increasing gold concentration, the intensity of this peak increases and slightly red shifts (ratio ) 0.3, 515 nm; 0.5, 516 nm; 1, 518 nm; 2, 520 nm) toward that of pure spherical gold particles (about 520 nm for 10 nm diameter spheres). The main factor responsible for the increase in intensity and the continuous shift to longer wavelengths is the increase in gold content concomitant with an increase in the size of the metallic island, as observed in TEM measurements (Figure 2). With gold content, the intensity of the gold-related absorption features increases, and with increasing particle size, the surface plasmon band also becomes more pronounced, as it spectrally narrows due to reduction of the electron surface contribution to the electron scattering rate.16 The frequency shift with size increase is due to reduction of the quantum size and environment effects and has already been investigated in a number of experimental and theoretical studies of pure gold colloidal solutions as well as other metals.17-19 In our experiments, the observed band is attributed to the formation of gold onto gold sulfide particles. One could argue that it is produced by individual gold particles formed in the solution. However, as previously mentioned, the direct reduction is energetically not favorable in solution and no significant fraction of isolated gold particles were observed in TEM data. The fact that the spectra resemble those of pure spherical gold particles suggests that the gold sulfide substrate has little influence on the resonance modes of the metal dot, despite its higher refractive index (2.33)20 compared to that of water (1.33). A more detailed description would require numerical computation of the optical response of a nano-object in an asymmetric environment,21 a weak effect being anticipated in the case of gold because of the blocking of the surface plasmon resonance frequency by the interband transitions. The surface plasmon band does not change when the solution is exposed to air. The gold/gold sulfide nanocomposites were stable over several weeks in air. When the gold deposition was carried out at a higher dose rate (Figure 5b), strong changes were observed in the measured spectra. For a low gold ratio, the shape of the UV-vis spectra becomes broader and the plasmon band becomes asymmetrical with a long wavelength wing. For higher gold ratio (g0.5), a new optical feature develops at longer wavelength above 600 nm. Under these conditions, smaller multiple particles instead of a larger island are expected. Furthermore, imperfections on the surface of gold particles surface and lattice defects may exist when the gold particles are formed very rapidly. Both effects are expected to increase electron scattering and, consequently, the damping on the plasmon oscillations, leading to a broadening of the absorption band.18 Furthermore, the different gold particles formed on the same Au2S core are strongly coupled by dipoledipole interaction, if their distance is typically smaller than their size. This also induces a broadening of the plasmon band and (16) Kreibig, U.; Fragstein, C. Z. Phys. 1969, 224, 307-323. (17) Kreibig U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995; p 532. (18) Mulvaney, P. Langmuir 1996, 12, 788-800. (19) Link, S.; El-Sayed, M. A. J. Phys. Chem 1999, 103, 8410-8426. (20) Pankove, J. Optical Processes in Semiconductors; Dover: New York, 1975; p 422. (21) Kelly, K. L.; Coronado, E.; Lin Zhao, L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668-677.
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a red-shift of its spectral position. This effect has been intensively studied theoretically and experimentally.17,22-24 The surface plasmon resonance band shape and frequency strongly depend on the degree of coupling of the particle as well as the orientation of the individual particles within the coupled system. Ruda et al. have modeled a similar red-shift and asymmetrical broadening for metallic particles coupled to a dielectric or semiconductor spherical core.25 For strongly interacting particles deposited on core and in the case of their percolation eventually, the spectrum acquires a long wavelength resonance, characteristic of the formation of metallic quasi-continuous nanoshell. Its peak wavelength decreases as shell thickness increases, eventually reaching that of the surface plasmon resonance of a nanosphere for very thick shell. The observed optical response and its evolution with gold concentration and dose rate suggest that the nanostructures grown by this approach at high dose rate are of a layered structure with formation of a quasi-continuous gold shell around the gold sulfide core. For low gold ratio (0.3), a discontinuous layer of small and weakly interacting gold nanoparticles is formed on the Au2S surface. The optical spectrum thus mainly reflects that of quasi-individual gold nanoparticles broadened by the presence of defects and by the weak nanoparticle interaction. For higher concentration, a layer of larger, closely packed nanoparticles is formed. Because of their small separation, they are strongly optically coupled, leading to the observation of a nanoshell type of spectrum with a red-shifted surface plasmon resonance (Figure 5b). Finally, we emphasize that the surface (22) Kreibig, U.; Althoff, A.; Pressman, H. Surf. Sci. 1981, 106, 308-317. (23) Kreibig, U.; Genzel, U. Surf. Sci. 1985, 156, 678-685. (24) Quinten, M.; Schonauer, D.; Kreibig, U. Z. Phys. D 1989, 26, 239-245. (25) Ruda, H. E.; Shik, A. Phys. ReV. B 2005, 71, 245328-1-8.
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plasmon resonance of gold nanoshell produced onto silica or iron oxide4,26-28 shows comparable absorption features as observed here for gold sulfide-gold-water nanoshell.
Conclusions In summary, small gold particles are directly produced onto gold sulfide colloidal particles by γ radiolysis without the use of stabilizers. Analysis has been done on the key parameters and conditions to practically synthesize metallic shell onto a semiconductor core. Evolution of the optical properties has been studied as a function of the metal concentration and irradiation dose. By controlling the reaction parameters, composite nanostructures of differing morphologies (nanohalter, nanoshell) have been obtained. This simple synthesis paves the way for the investigation of growth behavior of metallic nanostructures and provides a route to making promising materials for building functional nanodevices. This method can be extended to many different material systems and it provides a simple, convenient method for synthesizing metal based nanostructures. Acknowledgment. This work was supported by the FAME European network of excellence. The authors thank the CREMEM (University of Bordeaux 1) for the use of its transmission electron microscopy facilities. LA702829W (26) Wang L.; Luo J.; Fan Q.; Suzuki M.; Suzuki I.; Engelhard M.; Lin Y.; Kim N.; Wang J.; Zhong C. J. Phys. Chem. B 2005, 109, 21593-21601. (27) Westcott, S. L.; Jackson, J. B.; Radloff, C.; Halas, N. Phys. ReV. B 2002, 66, 155431-155435. (28) Prasad, V.; Mikhailovsky, A.; Zasadzinski, J. A. Langmuir 2005, 21, 7528-7532.
