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Fabricating Planar Nanoparticle Assemblies with Number Density Gradients Rajendra R. Bhat,† Daniel A. Fischer,‡ and Jan Genzer*,† Department of Chemical Engineering, North Carolina State University, Raleigh, North Carolina 27695-7905, and Material Science & Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Received January 11, 2002. In Final Form: April 5, 2002 We report on preparing assemblies of gold nanoparticles with continuous gradients in number density on flat silica-covered substrates. The methodology consists of (i) first forming a one-dimensional molecular gradient of amino groups (-NH2) on the substrate by vapor deposition of amine-terminated silane molecules, followed by (ii) attachment of gold nanoparticles to -NH2 functional groups by immersing the substrate in a colloidal gold solution. Experiments using atomic force microscopy reveal that the number density of nanoparticles on the substrate varies continuously as a function of the position on the substrate. Nearedge X-ray absorption fine structure studies confirm that the nanoparticle number density gradient is closely correlated with the concentration gradient of -NH2 groups anchored to the substrate. We demonstrate that the number density of nanoparticles within the gradient and the length of the gradient can be tuned by controlling the vapor diffusion of silane molecules. In addition we show that this simple methodology can be further extended to create double gradients, thus producing “a valley in nanoparticle concentration”.
Introduction Although the concept of nanotechnology was put forth more than 40 years ago by Richard Feynman in his visionary lecture at Caltech,1 it was not until the past decade that key advances were made in manipulation and control of materials on a “small” scale.2 This sudden outburst has been fueled, in part, by the potential for exploiting unique properties of nanostructures in commercially important applications, such as sensors,3 highefficiency solar cells,4 single molecule detectors,5 and electroluminescent devices.6 Many of these functional devices utilize nanoparticles, whose typical dimensions are intermediate between mesoscopic and molecular systems.7 Nanoparticle-based structures are also envisaged to play an important role in futuristic devices such as single-electron tunneling based computer chips8 and high-density information storage devices.9 To harness the useful properties of nanoparticles, it is imperative, in many instances, that they be bound to a solid, tangible substrate. Previous studies have shown that nanoparticles can be attached to a substrate through covalent or electrostatic interaction of particles with a surface-bound molecular or polymer film.7,10-12 For example, Natan and co-workers prepared two-dimensional arrays of gold nanoparticles * To whom correspondence should be addressed: phone, +1919-515-2069; e-mail,
[email protected]. † North Carolina State University. ‡ National Institute for Standards and Technology. (1) Feynman, R. P. Eng. Sci. 1960, 23, 22. (A copy of this famous talk is also available on the Internet at http://www.zyvex.com/nanotech/ feynman.html.) (2) Engineering a smaller world: From atomic manipulation to microfabrication. A special issue in Science 1991, 254, 1277. Rao, C. N. R.; Cheetham, A. K. J. Mater. Chem. 2001, 11, 2887. (3) Wohltjen, H.; Snow, A. W. Anal. Chem. 1998, 70, 2856. (4) Oregan, B.; Gratzel, M. Nature 1991, 353, 737. (5) Nie, S.; Emory, S. R. Science 1997, 275, 1102. (6) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (7) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18. (8) Service, R. F. Science 1997, 275, 303. (9) Ross, C. Annu. Rev. Mater. Res. 2001, 31, 203. (10) Kotov, N. A.; Dekany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065. (11) Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Anal. Chem. 1995, 67, 735.
on immobilized monolayers of silanes with terminal groups having affinity for gold.11,12 The protocol involved in that process required formation of self-assembled monolayer (SAM) of silane molecules on the substrate followed by immersion of the substrate in a colloidal gold solution. This resulted in adsorption of particles on the substrate with a sub-monolayer coverage. To utilize nanoparticles as components of functional devices, it is vital to develop methods for placing particles into chemically and structurally well-defined environments. In this regard, patterning of nanoparticles represents an area of great significance. Several techniques such as micro-contact printing13,14 and lithography15,16 have been utilized to accomplish this goal. Although these methods of organizing nanoparticles into various architectures are potentially useful in some applications, they always result in sharp boundaries between the regions on the substrate which are covered with particles and those which are not. For some functions (such as property optimization), it is useful to vary the number of immobilized particles per unit area of the substrate (i.e., number density) continuously along the substrate. This kind of surface having a “nanoparticle gradient” provides a combinatorial surface for further manipulation. In addition, surfaces having gradients of nanoparticles can be envisaged to have tremendous practical utility in optoelectronic devices, sensors, etc. Yet, except for an early report,17 very little effort has been expended to fabricate and explore properties of this interesting architecture of nanoparticles. In this Letter, we show that a substrate having a number density gradient of adsorbed nanopar(12) Grabar, K. C.; Smith, P. C.; Musick, M. D.; Davis, J. A.; Walter, D. G.; Jackson, M. A.; Guthrie, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 1148. (13) Xia, Y.; Whitesides, G. M. Annu. Rev. Mater. Sci. 1998, 28, 153; Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 550. Xia, Y.; et al. Chem. Rev. 1999, 99, 1823. (14) He, H. X.; Zhang, H.; Li, Q. G.; Li, S. F. Y.; Liu, Z. F. Langmuir 2000, 16, 3846. (15) Vossmeyer, T.; DeIonno, E.; Heath, J. R. Angew. Chem., Int. Ed. 1997, 36, 1080. (16) Sato, T.; Hasko, D. G.; Ahmed, H. J. Vac. Sci. Technol., B 1997, 15, 45. (17) Baker, B. E.; Kline, N. J.; Trendo, P. J.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 8721.
10.1021/la025524m CCC: $22.00 © 2002 American Chemical Society Published on Web 05/02/2002
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ticles can be prepared by first forming a one-dimensional molecular gradient of amino groups (-NH2) on the substrate by vapor deposition of amine-terminated silane molecules, followed by attachment of gold nanoparticles to -NH2 functional groups by immersing the prepared molecular gradient in a colloidal gold solution. We identify a number of experimental parameters that can be varied to control the number density of particles along the gradient. Finally, we mention our ongoing research to utilize the gradient structure to prepare complex architectures. Materials and Methods Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4‚3H2O), trisodium citrate dihydrate, and (3-aminopropyl)triethoxysilane (APTES) were obtained from Aldrich and used as received. Concentrated HNO3 and HCl were purchased from Fisher Chemicals. Deionized (DI) water (resistivity >16 MΩ‚cm) was obtained using the Millipore water purification system. Silicon substrates with a ≈2 nm thick layer of native SiOx were purchased from Virginia Semiconductors. All glassware was washed with freshly prepared aqua regia solution (3:1 HCl:HNO3) and rinsed thoroughly with DI water. An aqueous solution of gold nanoparticles was prepared by citrate reduction of HAuCl4 following the method of Frens.18 The resulting particles have a diameter of ≈16.7 ( 1.8 nm, as determined from transmission electron microscopy (TEM) images of nanoparticles. The silicon wafer was cut into rectangular pieces (4.5 cm × 1.2 cm) that were exposed to ultraviolet/ozone (UVO) treatment for 30 min (Jelight Company, Inc., model 42)19 in order to generate a large number of hydroxyl groups, which are required for coupling silane molecules. A mixture of APTES and paraffin oil (PO) (1:1 w/w) was prepared and kept in a small rectangular container. This container was placed near the shorter edge of the UVO-treated silicon wafer, and the whole system was enclosed in a Petri dish at ambient conditions. After a predetermined period of time, the wafer was taken out of the container, washed with ethanol to remove physisorbed silane molecules, and immersed in an aqueous gold nanoparticle solution for 24 h. The substrate was then taken out of colloidal solution, washed thoroughly with DI water, and dried with nitrogen. To determine the number density of adsorbed gold nanoparticles, atomic force microscopy (AFM, Nanoscope III, Digital Instruments)19 tapping mode scans were taken at various positions along the longer side of the substrate (x direction). For each x position, three scans were taken along the transverse direction (y direction). The particle densities were determined from the AFM micrographs (1 µm × 1 µm) by manual counting. An average of three transverse measurements was taken for each x position. To determine the concentration profile of grafted APTES silane, we used near-edge X-ray absorption fine structure (NEXAFS) spectroscopy.20 The NEXAFS experiments were carried out on the NIST/Dow materials characterization endstation at the National Synchrotron Light Source at Brookhaven National Laboratory (NSLS BNL).21
Results and Discussion A number of approaches to synthesize surface-bound chemical gradients on millimeter to nanometer scale have been reported.22-24 A gradient preparation method must (18) Frens, G. Nat. Phys. Sci. 1973, 241, 20. (19) Certain commercial equipment is identified in this article in order to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the items identified are necessarily the best available for the purpose. (20) Sto¨hr, J. NEXAFS Spectroscopy; Springer-Verlag: Berlin, 1992. (21) For detailed information about the NIST/Dow Soft X-ray Materials Characterization Facility at NSLS BNL, see: http:// nslsweb.nsls.bnl.gov/nsls/pubs/newsletters/96-nov.pdf and Genzer, J. et al. Langmuir 2000, 16, 1993; Macromolecules 2000, 33, 1882. (22) Ruardy, T. G.; Schakenraad, J. M.; VanderMei, V. C.; Busscher, H. J. Surf. Sci. Rep. 1997, 29, 3. (23) Chaudhury, M. K.; Whitesides, G. M. Science 1992, 256, 1539.
possess the following virtues: (i) the gradient should be easy to prepare; (ii) the method should provide a good control over the wettability as well as steepness of the gradient; and (iii) it should offer the freedom of choosing a specific surface chemistry that can be harnessed for a particular application. On the basis of these criteria, the silane vapor diffusion technique described by Chaudhury and Whitesides23 qualifies as a preferred method. This methodology involves evaporating silane (having a general formula ω-(CH2)n-SiCl3, or ω-(CH2)n-Si(OR)3, where ω is a terminal functional group and R is a short alkyl) over a hydroxyl-terminated substrate. Small silane molecules have sufficiently high vapor pressure at room temperature and can thus evaporate readily. As silane molecules (APTES in our case) evaporate, they diffuse in the vapor phase and generate a concentration gradient in vapor phase. Upon impinging on the substrate, these molecules react with the hydrophilic moieties on the substrate and get immobilized. According to Chaudhury and Whitesides, kinetics of the whole process is determined primarily by the vapor phase diffusion of silane molecules, so that the gradient in vapor phase is transferred onto the substrate.25 This method offers several parameters that can be varied, albeit to a limited extent, to change the two basic characteristics of the gradient, viz., wettabilities of the opposite ends of the gradient and steepness of the region between them. Indeed, our group has recently demonstrated the ability to fine-tune these characteristics of the gradient prepared on a flexible substrate by using a combination of mechanical manipulation of the substrate and self-assembling nature of the silane molecules.26 The system that we chose to form a planar gradient of nanoparticles consists of a silicon substrate with its thin native SiOx layer, aqueous colloidal gold solution, and a silane with a headgroup having affinity for gold particles. The choice of the silane molecule used in this process is crucial as it determines the optimal conditions required for the binding between nanoparticles and the substrate. A number of chemical functional groups such as mercapto (-SH), amine (-NH2), cyano (-CtN), and phosphine (-Pt) show a great affinity to bind to a gold particle by virtue of their available lone pair of electrons.27 We decided to use -NH2 terminated silane (APTES) in our study. APTES is amenable to the vapor diffusion technique on account of its adequate vapor pressure at room temperature. The kinetics of gold particle adsorption on -NH2 silane has been well studied.12 Additionally, the -NH2/ gold system offers the possibility of controlling the number of the adsorbed gold particles on the -NH2 surface by adjusting pH of the colloidal solution.28 While using the vapor diffusion technique, we chose to employ a mixture of silane and low vapor pressure PO as the diffusing source.23 This choice allows us to vary the flux of evaporating silane molecules by simply varying the composition of the mixture. Gold particles prepared by the citrate reduction method have an inherent negative charge as a consequence of a weakly bound anion (such (24) Fuierer, R. R.; Carroll, R. L.; Feldheim, D. L.; Gorman, C. B. Adv. Mater. 2002, 14, 154. (25) We note, however, that recent work in our group (Efimenko, K.; Genzer, J. Unpublished results.) indicates that the process is more complicated. In a simple picture, one has to realize that the gradient growth is governed by at least three processes: diffusion in the vapor, diffusion along the substrate, and reaction with the surface anchor. (26) Efimenko, K.; Genzer, J. Adv. Mater. 2001, 13, 1560. (27) Grabar, K. C.; Allison, K. J.; Baker, B. E.; Bright, R. M.; Brown, K. R.; Freeman, R. G.; Keating, C. D.; Musick, M. D.; Natan, M. J. Langmuir 1996, 12, 2353. (28) Zhu, T.; Fu, X. Y.; Mu, T.; Wang, J.; Liu, Z. F. Langmuir 1999, 15, 5197.
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Figure 1. (upper panel) AFM images of gold particles adsorbed along a substrate prepared by evaporating an APTES/PO mixture for 5 min followed by immersion in colloidal gold solution for 24 h (edge of each image ) 1 µm). (lower panel) Particle number density profile (left) for two gradients prepared by evaporating APTES/PO mixtures for 3 (b) and 5 (9) min. The data points represent an average from three transverse scans along the gradient taken at the center of the sample (y ) 0 mm) and y ) -3 mm and y ) +3 mm. The line represents the PEY NEXAFS profile (right) of N-H bonds from an ATEPS gradient prepared by evaporating APTES/PO mixture for 5 min. The area around the PEY NEXAFS line denotes the measurement uncertainty (based on nine line scans along the gradient taken between -3 mm and +3 mm from the center of the sample).
as citrate, chloride ions) coating.7,12,29 Repulsion between ionic coatings on the nanoparticles keeps them from aggregating in solution. The gold sol prepared by citrate reduction is slightly acidic in nature (pH ≈ 6.5). When APTES-coated substrate is immersed in such an acidic gold colloidal solution, -NH2 groups get protonated, forming -NH3+, and bind electrostatically to negatively charged gold particles, thus immobilizing them on the substrate.12,28 As will be demonstrated later, gold particles bind only to those parts of the substrate to which silane molecules are attached, thus resulting in the formation of a number density gradient of gold particles. The upper panel in Figure 1 shows AFM images taken at various positions along the longer side of the specimen (x direction) prepared by first allowing silane molecules to diffuse for 5 min and then immersing the silanized substrate in gold colloid for 24 h. A gradient in number density of gold nanoparticles is clearly evident as one scans farther away from the end of the substrate that was closer to the silane diffusion source. The particle number density changes continuously from ≈500 particles/µm2 near the end of the substrate closer to the diffusing source to zero at the other end. As suggested by Chaudhury and Whitesides,23 the molecular density and the gradient (29) Handley, D. A. In Colloidal Gold: Principles, Methods and Applications; Hayat, M. A., Ed.; Academic Press: San Diego, CA, 1989; Vol. 1, Chapter 1.
length can be tailored by adjusting the silane concentration in the silane/PO diffusing source and the diffusion time. In this work, we kept the APTES/PO ratio in the diffusing source constant and increased the length of the gradient by increasing the APTES diffusion time. The lower panel in Figure 1 shows the number density profile of nanoparticles along the gradient (x direction) for two samples with silane diffusion times of 3 (b) and 5 (9) min. It can be clearly seen that as the silane diffusion time increases, the number of the adsorbed gold particles increases and the nanoparticles adsorb over longer distances on the substrate. We utilized NEXAFS to measure the concentration of the grafted APTES molecules after nanoparticle deposition. By comparison of the results of the NEXAFS and AFM measurements, the relationship between the spatial variation of the particle density and the APTES concentration along the gradient was established. NEXAFS involves the resonant soft X-ray excitation of a K or L shell electron to an unoccupied low-lying antibonding molecular orbital of σ symmetry, σ*, or π symmetry, π*.20 The initial-state K shell excitation gives NEXAFS its element specificity, while the final-state unoccupied molecular orbitals provide NEXAFS with its bonding or chemical selectivity. A measurement of the intensity of NEXAFS spectral features thus allows for the identification of chemical bonds and determination of their relative
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population density within the sample. In a typical NEXAFS experiment, one measures the intensity of the Auger electrons that escape from the sample in the form of the partial electron yield (PEY) signal, whose probing depth is ≈2 nm. If the particles formed a closed packed structure, no (or only a limited) signal originating from the APTES monolayer can penetrate through the particles and be detected. However, adsorption of gold nanoparticles on a silanized substrate does not result in a complete monolayer coverage due to the interparticle repulsion.11,12 This facilitates the determination of the APTES concentration profile even after particle adsorption by collecting PEY NEXAFS line scans at a fixed photon excitation energy corresponding to the N-H bond (≈406.0 eV) along the x direction of the sample.30 An average from several PEY NEXAFS scans (measured between -3 and +3 mm from the center of the sample) for the sample prepared by first evaporating APTES for 5 min and then immersing the substrate into a colloidal gold solution for 24 h is depicted in Figure 1. There is a remarkable agreement between the gold particle density profile determined from AFM measurements and the PEY NEXAFS N-H bond intensity profile for the APTES SAM on the substrate. This bolsters our assertion that gold particle gradient is indeed formed on the underlying -NH2 gradient. The potential of the vapor diffusion technique in forming planar molecular gradients can be utilized to prepare other interesting architectures of nanoparticles. For example, as can be seen from Figure 2, we can construct “double gradient” architecture of nanoparticles by diffusing silane molecules simultaneously from both ends of the substrate, thus producing a “valley in nanoparticle concentration”. This architecture also shows an excellent agreement between the particle gradient and the underlying silane gradient.31 As mentioned earlier, the silane diffusion technique of preparing molecular gradients allows one to have some control over the characteristics of the gradient. Effect of one such controlling factor, viz., the time of diffusion of silane molecules in vapor phase, was already discussed. Some other parameters, which may be varied to tune the concentration gradient of APTES on the substrate and hence the gold particle gradient, include (i) composition of the silane/PO mixture, (ii) temperature of the silane diffusion source, (iii) temperature of the substrate, (iv) concentration of the hydroxyl groups on the substrate, and (v) humidity. In addition to these factors, we expect pH of the gold colloid solution and the time of immersion of the substrate in the colloid solution to have a substantial effect on the gradient characteristics. The basis for such an expectation lies in the study conducted by Liu and co-workers28 and Grabar et al.12 Liu and co-workers studied the effect of pH of gold colloid solution on binding of gold nanoparticles to p-aminothiophenol SAM on gold substrate. They reported that in highly alkaline solution, where amino groups were not protonated, gold particles (30) During the NEXAFS experiments, the spot area of the X-ray beam on the sample was about 0.25 mm2 and the scan step size in the x direction was 0.5 mm. (31) A comparison of the number density profiles of gold nanoparticles in the double and single gradients reveals that while the lengths of the gradient are the same, the number density of the gold nanoparticles in the double gradient is about half the corresponding value in the single gradient case. As illustrated by the NEXAFS data in Figure 2, this behavior is a consequence of a lower concentration of the APTES molecules grafted on the substrate.
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Figure 2. Particle number density (left) and PEY NEXAFS (right) profile for a “double gradient” prepared by evaporating APTES/PO mixture for 3 min from both ends of the substrate followed by immersion in colloidal gold solution for 24 h. The data points represent an average from three transverse scans along the gradient taken at the center of the sample (y ) 0 mm), y ) -3 mm, and y ) +3 mm. The area around the PEY NEXAFS line denotes the measurement uncertainty (based on seven line scans along the gradient taken between -3 mm and +3 mm from the center of the sample).
could not be adsorbed on the substrate, whereas under acidic conditions, amino groups were protonated thus facilitating adsorption of negatively charged gold particles. Grabar et al.12 found that the kinetics of adsorption of gold particles on a complete APTES SAM is diffusionlimited and shows a time1/2 dependence before reaching an interparticle repulsion-controlled regime. Hence, if we deposit gold particles on silane gradients for varying amounts of time, different number of particles will be adsorbed for different deposition time, thus generating gradients of varying characteristics. Work is in progress to carefully study the effect of these parameters and to utilize the gradients in nanotechnology applications. Acknowledgment. This research was funded by the NSF CAREER award, Grant No. DMR98-75256. NEXAFS experiments were carried out at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences. We thank Professor Gregory N. Parsons for allowing us to use his AFM and Professor Richard J. Spontak for taking TEM micrographs of the gold nanoparticles. We also thank Dr. Kirill Efimenko and Mr. Kevin Bray for their assistance during the course of NEXAFS and AFM experiments, respectively. Note Added in Proof. While the manuscript was under review, we became aware of another study of particle density gradients on solid substrates. Plummer and Bohn [Langmuir 2002, 18, 4142-4149] reported on electrochemically generating a gradient of amino-terminated thiol-based self-assembled monolayer (SAM) on a goldcovered substrate. To produce particle gradients they attached carboxylic acid-modified, fluorescently doped polystyrene nanospheres (diameter of 200 nm) to the amino-termini of the gradient SAM. LA025524M
