Article pubs.acs.org/Langmuir
Using Light to Covalently Immobilize and Pattern Nanoparticles onto Surfaces Ellane J. Park,† Tina Wagenaar,† Siyan Zhang,‡ A. James Link,‡ Robert K. Prud’homme,‡ Jeffrey T. Koberstein,§ and Nicholas J. Turro*,† †
Department of Chemistry and §Department of Chemical Engineering, Columbia University, 3000 Broadway Avenue, New York, New York 10027, United States ‡ Department of Chemical and Biological Engineering, Princeton University, Princeton, New Jersey 80544, United States ABSTRACT: There is considerable current interest in developing methods to integrate nanoparticles into optical, electronic, and biological systems due to their unique sizedependent properties and controllable shape. We report herein a versatile new approach for covalent immobilization of nanoparticles onto substrates modified with photoactive, phthalimide-functional, self-assembled monolayers. Upon illumination with UV radiation, the phthalimide group abstracts a hydrogen atom from a neighboring organic molecule, leading to radical-based photografting reactions. The approach is potentially “universal” since virtually any polymeric or organic−inorganic hybrid nanoparticle can be covalently immobilized in this fashion. Because grafting is confined to illuminated regions that undergo photoexcitation, masking provides a simple and direct method for nanoparticle patterning. To illustrate the technique, nanoparticles formed from diblock copolymers of poly(styrene-b-polyethylene oxide) and laden with Hostasol Red dye are photografted and patterned onto glass and silicon substrates modified with photoactive phthalimide-silane self-assembled monolayers. Atomic force microscopy and Xray photoelectron spectroscopy are applied to characterize the grafted nanoparticle films while confocal fluorescence microscopy is used to image patterned nanoparticle deposition.
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INTRODUCTION Extensive research has been devoted to the development of methods to impart specific chemical functionality to the surface of both soft and hard substrates. More recently, this interest has expanded to include techniques for surface immobilization of various nanoparticles (NPs) because such systems have high potential for applications in optical, electronic, and biological devices due to their unique size-dependent properties and high surface to volume ratio.1−4 NPs have been grafted to surfaces by chemical reactions5−8 and lithography;9,10 however, these methods can be costly, can have limitations, and sometimes yield unstable, noncovalent immobilization of NPs. The use of UV radiation to photograft NPs to surfaces is a method that holds great potential and scope, but has been scarcely explored.11 We present a new “universal” approach that has the capability to covalently bond organic or organic−inorganic hybrid NPs to any substrate that can be functionalized with a photoactive selfassembled monolayer (SAM). The only true restriction of the method is that the NP surface must comprise hydrogen atoms that can be abstracted upon photoexcitation of the SAM. Hydrogen abstraction leads to radical formation which in turn initiates interfacial grafting reactions. To illustrate the method, SAMs presenting photoactive phthalimide (PI) groups are employed to photograft polymeric nanoparticles (PNPs) to © 2012 American Chemical Society
silicon wafers and glass substrates. Carbohydrate microarrays were constructed previously using a similar approach.12 The NP photografting process is shown schematically in Figure 1. PNPs formed by self-assembly of diblock copolymers, comprising a polystyrene (PS) core (loaded with Hostasol red dye) and a polyethylene oxide (PEO) shell, are covalently bound and immobilized onto a surface by photoexcitation of a PI-silane SAM formed on the substrate. PNPs can be patterned by a simple masking process, because a covalent bond forms between the photoactive SAM and the PNPs only in regions that are illuminated and undergo photoexcitation. X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and confocal fluorescence microscopy (CFM) are used to characterize the photoimmobilization of PNPs.
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EXPERIMENTAL SECTION
Synthesis of Alkyne-Decorated Polymeric Particles..13−16 Fluorescent PNPs were prepared via Flash NanoPrecipitation17 from poly(styrene-b-polyethylene oxide) diblock copolymers (PS-b-PEO). Hostasol red (structure given in Figure 1), a hydrophobic fluorescent dye, was encapsulated within the PNP cores. The surface functionality Received: October 4, 2011 Revised: June 29, 2012 Published: July 1, 2012 10934
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Figure 1. Schematic illustration of the photoimmobilization of PNPs onto a PI-SAM functionalized substrate. On each PNP, 20% of the PEG (i.e., PEO) chains are terminated with alkyne groups and the core is labeled with Hostasol red dye (note: figure not drawn to scale). of the PNPs was controlled by using a mixture of 80% OH-terminated and 20% alkyne-terminated block copolymers. Alkyne groups provide sites for subsequent alkyne−azide cycloaddition reactions (i.e., click reactions) that could be used to conjugate azide-functionalized biomolecules such as targeting ligands.16,18,19 Hydroxyl terminated diblock copolymer PS-b-PEO−OH (8 mg/mL, 1.8 mM, PS Mw ∼ 1500, PEO Mw ∼ 3000), alkyne terminated diblock copolymer PS-bPEO-alkyne (2 mg/mL, 0.44 mM, PS Mw ∼1500, PEO Mw∼3000), homopolymer PS−OH (10 mg/mL, 6.7 mM, Mw∼1500), and hostasol red (0.01 mg/mL, 30 μM) were dissolved in THF (12 mL/min), and rapidly mixed with three water streams (each at 40 mL/min). Immediately after PNP formation, THF was removed by dialysis against ultrapure water using Spectra/Por dialysis tubing with a molecular weight cutoff of 6000−8000. The PNP diameter in aqueous solution was ca. 100 nm as determined by dynamic light scattering. Synthesis of Phthalimide-undecyl-trimethoxysilane. A 3.3 mmol portion of 11- bromoundecanetrimethoxysilane (Gelest) was added to a solution containing an equimolar amount of potassium phthalimide (Aldrich) in 60 mL of anhydrous dimethylformamide (DMF, Aldrich). The solution was stirred overnight at room temperature (RT) under argon, and chloroform (50 mL) was added. The solution was transferred to a separatory flask containing 50 mL of H2O. The aqueous layer was separated and then extracted with two 20 mL portions of chloroform. The combined chloroform extract was washed several times with 20 mL of H2O. The chloroform was removed by rotovaporation, and residual DMF was removed on a high vacuum line to give a pale yellow liquid. Self-Assembly of Phthalimide-undecyl-trimethoxysilane onto Glass or Silicon Wafer. Glass (ArrayIt) or silicon (Wafer World) substrates were cleaned in piranha solution (2:1 H2SO4:30% H2O2) for 2 h followed by extensive rinsing with water and ethanol. Substrates were dried with a stream of argon and immersed in a 1 mmol solution of phthalimide-undecyl-trimethoxysilane in anhydrous toluene (Aldrich) in the dark for 12 h to form SAMs. Substrates were subsequently rinsed with toluene; sonicated three times for 2 min each in toluene, toluene/methanol (1:1), and methanol; and then were stored under argon until used. Photochemical Immobilization and Patterning of Polymeric Nanoparticles. PNPs were deposited onto the SAM modified substrates by spin-coating from aqueous solution. Most specimens were prepared by spin coating at 3000 rpm for 1 min after which they were stored under argon. Samples prepared for XPS analysis were spin coated at 250 rpm for 1 min to increase the film thickness (i.e., PNP surface coverage). The spin coating conditions employed produced
less than monolayer coverage of PNPs. No attempts were made to optimize the spin coating conditions. Photoimmobilization was accomplished by irradiation at 254 nm for 15 min in a Rayonet photochemical reactor, after which samples were rinsed with water, dried with argon, and immediately characterized. A 400-mesh transmission electron microscopy (TEM) grid (Electron Microscopy Sciences) was used as a contact mask for all patterning experiments. After patterning, samples were rinsed with water and blown dry with argon. For both cases (with and without patterning), two sample groups were prepared: (1) irradiated surfaces as described above and (2) nonirradiated (i.e., “dark”) controls. The two treatments were identical except for elimination of the irradiation step for the control. The dark controls indicate the extent to which the PNPs physisorb or chemisorb in the absence of irradiation. Instrumentation. XPS spectra were recorded with a PHI 5500 spectrometer equipped with a hemispherical electron energy analyzer, a multichannel detector, and an Al Kα monochromator X-ray source run at 15 kV and 23.3 mA. The test chamber pressure was maintained below 2 × 10−9 Torr during spectral acquisition. A low-energy electron flood gun was used as required to neutralize surface charging. The binding energy (BE) was internally referenced to the aliphatic C 1s peak at 284.6 eV. Survey spectra were acquired using an analyzer pass energy of 93.9 eV and a BE resolution of 0.8 eV, while high-resolution spectra were acquired with a pass energy of 23.5 eV and a BE resolution of 0.05 eV. The takeoff angle is defined as the angle between the surface and the photoelectron detector. Spectra were deconvoluted using RBD software that fits a series of Gaussian− Lorentzian functions to each chemically shifted photoelectron peak, after subtracting an appropriate background. Atomic concentrations were calculated by normalizing peak areas with the elemental sensitivity factor data in the PHI database. AFM images (Thermo Probes Autoscope CP) were obtained in nontapping mode. The image dimensions ranged from 5 × 5 μm2 to 10 × 10 μm2. Images of patterned PNPs were collected with a 488 nm excitation wavelength using an inverted CFM (Olympus Fluoview; 20× objective lens).
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RESULTS The NP photografting technique is based on a widely known photochemical mechanism: irradiated aromatic carbonyl groups abstract a hydrogen atom from neighboring C−H groups to generate radicals that initiate radical-based grafting mechanisms. When incorporated within a SAM, this mechanism leads 10935
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Figure 2. Hydrogen abstraction mechanism for photografting PNPs to substrates modified with PI-SAMs. Photoexcitation of the carbonyl group leads to abstraction of a hydrogen atom from a PEO chain on a PNP and the promotion of covalent bond formation by radical recombination.
Figure 3. XPS survey spectra of (a) PI-SAM and (b) PNPs photografted onto PI-SAM-coated glass substrate at a takeoff angle of 45°.
to interfacial grafting via covalent bond formation as pictured in Figure 2.20 Benzophenone (BP) derivatives are among the most commonly used photoactive groups to fabricate thin polymer films and develop biological membranes.21−25 We previously employed similar phthalimide (PI) functional SAMs to successfully immobilize a variety of polymers and biopolymers,12 because the wetting properties of PI-functional surfaces are superior to those of BP-SAMs. Upon absorption
of a photon, a PI group is excited to the n−π* state; the electronically excited state abstracts a hydrogen atom from a nearby molecule, in the present case a PNP, to form a radical pair with one radical center attached to the SAM surface via the PI group and the other attached to the PNP. Recombination of these two radicals produces a covalent bond joining the SAM and the PNP. While other nongrafting processes such as disproportionation and back-transfer are possible, we have 10936
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confirmed by comparing the C/Si ratios for the two samples (i.e., the bottom line of Table 1). The C/Si ratio increases from 1.46 (PI-SAM) to 5.67 (PNP-grafted onto SAM). In contrast, negligible change is seen between the PI-SAM and the dark control. The increased C/Si ratio clearly indicates that PNPs have been photografted to the SAM surface. The fractional surface coverage of PNPs grafted to the SAM can be estimated by analysis of the Si 2p signals emanating from the substrate because these signals are attenuated when PNPs graft to the surface. The fractional surface coverage of PNPs, f, can be determined by application of an XPS patchy coverage model. The ratio of the Si 2p signal from the PNP grafted film, IPNP(θ), to that of the nonirradiated PI-monolayer, IM(θ), is given by
shown that PI-SAMS can be used to successfully fabricate carbohydrate microarrays with high signal throughputs.12 In this previous work, there was no indication that photografted carbohydrates were degraded to any significant extent by the use of UV radiation, as they were found to retain their function. Contact angle goniometry was used to detect changes in the surface composition before and after photografting. The water contact angle of the PI-SAM prior to PNP photografting was approximately 65 ± 1° in agreement with previous data for a PI-SAM.12 After PNP photografting, the left and right side readings of the water contact angles were found to be inconsistent, an indication of significant surface roughness, and therefore not reported. Ellipsometry was not applied to measure the thickness of the photografted PNP layers due to their inherent roughness as well as the difficulty in calculating a refractive index for the composite PNP-SAM film. PNP photografting was characterized by XPS, AFM, and CFM. XPS reveals differences in the surface composition of samples before and after NP-grafting for a sampling depth of about 7 nm. XPS survey spectra of the PI-SAM functionalized glass substrates before and after PNP photografting (Figure 3) exhibit peaks that can be assigned to Si 2p (103.5 eV), Si 2s (155.0 eV), C 1s (284.6 eV), and O 1s (532.0 eV) photoelectrons.26 These photoelectrons that originate arise from carbon and oxygen atoms in the SAM and PNPs as well as from silicon and oxygen atoms in the silicon wafer and glass substrates.27 PNP photografting leads to an increase in the C 1s signal and a decrease in the Si 2p signal, indicating that the substrate is being covered by carbonaceous matter, that is, by the organic PNPs. The XPS atomic concentrations for the two specimens are shown in Table 1. The success of photografting is
IPNP(θ ) = (1 − f ) + f e−t / λsin θ IM(θ )
where t is the PNP film thickness, θ is the photoelectron takeoff angle, and λ is the electron mean free path. Because the thickness of a grafted PNP layer is of the order of 10 nm (see AFM data to follow), the second term in (eq 1) is negligible and the ratio of silicon signals after and before photografting becomes simply (1 − f). Application of this analysis to the XPS data yields a fractional PNP surface coverage of about 67%. The surface compositions of the samples may be examined by analysis of the high-resolution C 1s spectra, because different carbon chemical environments are manifest as chemical shifts. A bulk film of N-methyl phthalimide deposited onto a gold coated glass substrate was characterized as a model compound for the PI-SAM. The C 1s spectrum for this control is shown in Figure 4, left. Deconvolution of the spectrum provides evidence for the existence of three different carbon environments: the 288 eV peak comes from the two carbonyl carbons, the 286 eV peak is associated with the two phenyl carbons adjacent to the carbonyl carbons as well as the methyl carbon, and the 284.6 eV peak originates from the four remaining unshifted phenyl carbons. The spectrum for PI-SAM, Figure 4, right, can be represented by contributions from these same three carbon environments at the same binding energies found for the model compound. The peak heights differ, however, because the PISAM has additional unshifted carbons coming from the undecyl spacer, and because molecules in the SAM are preferentially
Table 1. Atomic Compositions (%) of PI-SAM, Dark Control, and PNP Photografted onto PI-SAM Determined by XPS peak
PI-SAM
dark control
PNP grafted onto PI-SAM
C 1s Si 2p3 C/Si
33.2 22.7 1.46
34.7 20.1 1.73
61.8 10.9 5.67
(1)
Figure 4. High-resolution C 1s spectra: (left) bulk sample of N-methyl phthalimide model compound; (right) PI-SAM on silicon wafer. The noisy black lines are the experimental data, the three sets of smooth solid black lines are the results of deconvolution assuming three different carbon environments, and the red lines are the total best fit from the convolution process. Data is taken at a 45° takeoff angle. 10937
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PNPs have been successfully photografted. A total of four different carbon environments are necessary to represent the sample with photografted PNPs. The peak at 284.6 eV peak is associated with unshifted carbons bonded only to other carbons as found in the SAM, the dye, and the PS portion of the PNP. The 287.6 eV peak can be ascribed to a variety of carbon species including carbons adjacent to carbonyls and sulfur in the dye as well as carbons adjacent to the nitrogen and carbonyls of the PI groups in the SAM. The peak at 285.6 eV is assigned to the ether carbons of PEO. The peak at 288.0 eV, associated with carbonyl carbons, is attenuated after photografting, as these carbons are covered up by the PNPs. The intensity of the π to π* shakeup satellite for the PS core, observed at 291.9 eV, accounts for about 2.5% of the total C 1s signal, while a typical shakeup satellite contributes 5−10% of the total signal for PS.27 Attenuation of the π to π* transition signal indicates that PS chains in the grafted PNP cores are covered by a layer of PEO chains from the PNP coronae. AFM provides a method to directly image the surface morphology of photografted PNPs. The AFM image for the SAM control, shown in Figure 6a, is essentially featureless and leads to an rms surface roughness of 0.897 nm. The rms roughness increases to 3.52 nm after PNP photografting, consistent with the appearance of island-like features on the surface as shown in Figure 6b. This result is consistent with a previous model suggesting that lateral capillary forces may lead to PNP aggregation during the spincoating/drying process.28 A line scan across Figure 6b reveals that the surface features are approximately 10 nm in height and 750 nm in width, as
oriented somewhat normal to the surface, as suggested by Figure 1. Upon PNP photografting to the PI-SAM, the high resolution C 1s region of the XPS spectrum changes markedly, as shown in Figure 5. The unshifted carbon peak, the most
Figure 5. High-resolution C 1s spectrum for PNPs photografted onto a PI-SAM. The noisy black line is the experimental data; the four sets of smooth solid black lines are the results of deconvolution assuming four different carbon environments, and the red line is the total best fit from the convolution process. Data is taken at a 45° takeoff angle.
prominent peak found for the model compound and the PISAM, is now overshadowed by a new peak at 285.6 eV that can be assigned to the ether carbons of PEO, a clear indication that
Figure 6. Atomic force microscopy (AFM) images of (a) the PI-SAM on a silicon wafer and (b) PNPs photografted onto the PI-SAM, and (c) the line scan crossing the AFM image in part b. 10938
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Figure 7. Schematic illustration of photopatterning PNPs on a PI-SAM-functionalized surface by use of a contact photomask with a grid-pattern. (a) Only PNPs that are exposed to radiation in unmasked areas are covalently bound to the surface. (b) The pattern of covalently bound PNPs (i.e., the pattern of fluorescence) is produced by washing away unexposed, unbound PNPs.
Figure 8. CFM images of PNPs photopatterned with a 50 μm TEM grid used as a contact mask: (a) before and (b) after rinsing away nongrafted PNPs (the dye was excited at 488 nm). The contrast and brightness of CFM images were modified to distinguish pattern features. ImageJ software was used to produce the line scans presented in the lower two graphs, following the white line trajectories traced on the CFM images.
shown in Figure 6c. An individual PNP has a diameter of about 100 nm in aqueous solution. If we assume that each PNP retains constant volume when photografted, the diameter of a flattened PNP with a 10 nm height would be about 130 nm, much smaller than the 750 nm islands observed. PNPs therefore graft in clusters, consistent with the formation of aggregates on the surface prior to photografting. While we do not know how strongly the PNPs are bound to the substrates, the AFM experiments do confirm that the PNPs are covalently bound to the SAM as they are not simply pushed around by the AFM tip. Analysis of the AFM images provides an estimate of the PNP surface coverage of about 20−25%. For the purpose of this paper, no attempts were made to optimize the spin coating conditions so as to obtain complete monolayer PNP coverage. It is likely, however, that the coverage can be controlled up to a close-packed monolayer by adjusting the spin coating conditions. It is well-known that the thickness of polymer films can be controlled by adjusting either the rotational speed or solution concentration used for spin coating. While it is not
clear that a hybrid nanoparticle would follow the same relationship, we have found herein that a decrease in the rotational speed from 3000 to 250 rpm increases the surface coverage from about 20−25% to about 67%, a factor of 2.7− 3.4. It is interesting to note that if we assume that the PNP coverage scales inversely with the square root of the spin coating rpm (i.e., the thickness of polymer thin films scales with the square root of the spin coating rpm), we would predict an increase in thickness by a factor of 3.5, close to what is observed experimentally. The exact structure of the photografted PNPs is not obvious at this time. The thickness of photografted PNPs is substantially less than their size in solution, but the PEO coronae would be swollen in solution and would collapse after photografting and drying. The latter effect would lead to an expected thickness that is lower than the diameter in solution, but it would seem that some PNP deformation would be required to yield a final thickness of 10 nm. The lower surface tension polystyrene blocks that form the PNP core are 10939
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posure to UV radiation, the phthalimide group can extract a hydrogen atom from a nearby nanoparticle to form a pair of radicals that ultimately lead to covalent grafting between the nanoparticle and the self-assembled monolayer by virtue of a radical recombination process. XPS and AFM measurements confirm nanoparticle immobilization in the form of islands comprising multiple nanoparticles. The height of the islands is less than the nanoparticle diameter in solution, indicating that nanoparticles adopt a flattened, deformed morphology. The surface is found to be enriched in polyethylene oxide, the component that forms the shell of the nanoparticles, illustrating that the core−shell structure of the nanoparticles manifest in solution is preserved to some extent when photografted to the photoactive monolayer. Nanoparticle immobilization is patterned by exposure of the films though a contact mask. Successful patterning is confirmed by confocal fluorescence microscopy using nanoparticles loaded with a fluorescent dye. While there is no direct evidence of covalent bonding between the PNP and the photoactive monolayer, the experimental evidence in support of covalent bond formation is substantial: the mechanism of covalent bond formation by phthalimides is well-known and we have used it in previous studies,12 controls show no physisorption or chemisorption, the PNPs cannot be moved with an AFM tip, PNPs are successfully patterned by using a mask, and unirradiated PNPs are easily removed by simple washing. The new method of nanoparticle immobilization is essentially universal, because it is capable of photografting almost any organic or organic−inorganic hybrid nanoparticle to any surface that can be functionalized with a phthalimide functionalized self-assembled monolayer.
thermodynamically favored to locate at the surface in the dry state, yet XPS experiments indicate that the PEO chains dominate the surface. The PS core and PEO shell structure of the PNPs found in solution therefore seem to be preserved to some extent when they are photografted, a result that might be expected because the PS cores are in the glassy state. Additional structural characterization, however, is needed to establish whether there is any truth to these speculations. PNP grafting can be patterned by exposure through a contact mask, in the present case a simple TEM grid, as illustrated by the schematic presented in Figure 7. PNPs covalently attach only to light-exposed regions while the PNPs in the dark unexposed regions do not photograft and are removed by rinsing. The top view in Figure 7 illustrates schematically how the PNP pattern can be imaged. Fluorescence should emanate only from within grid box regions that contain photografted PNPs loaded with dye, while fluorescence should not be observed in regions where gridlines mask the exciting radiation and dye-laden PNPs do not photograft. CFM images of photopatterned PNPs are shown in Figure 8 based upon a contact mask consisting of gridlines distributed on 50 μm centers. Figure 8a contains an image of the film after exposure to UV radiation through the mask, but without washing away nongrafted PNPs. Somewhat surprisingly, an image of the grid is visible, even though the PNP distribution is the same across the entire sample. This effect is caused by partial photobleaching of the encapsulated dye when exposed to the UV radiation. When the unwashed sample is excited in the CFM, unexposed nongrafted PNPs will appear brighter than light-exposed, photografted PNPs because the dye in the unexposed PNPs has not experienced photobleaching and will thus show a higher fluorescent signal. The brighter regions in Figure 8a therefore correspond to nongrafted PNPs that have not been photobleached, while the relatively darker regions correspond to the partially photobleached, photografted PNPs. Figure 8b shows the CFM image for the sample after washing out the nongrafted PNPs. In this case, regions where nongrafted PNPs have been removed by washing appear darker, while regions containing the partially photobleached, photografted PNPs appear relatively brighter. The edges of the image are not crisp because of the use of a TEM grid as a mask; crisper patterns would require the use of a mask projection system. While we have not examined the resolution of patterning, the maximum resolution is limited by the size of the nanoparticle, since this is the smallest grafting element. The image in Figure 8b nonetheless confirms that PNPs can be successfully patterned by photografting through a mask. The lower two graphs in Figure 8 show the result of line scans across the CFM images (shown as white lines in the fluorescence micrographs). The line scan for the photografted PNPs is noisy because much of the dye is photobleached by the radiation required for photografting leading to a low fluorescence signal; however, the pattern of PNPs is clearly visible. One can conclude that PNPs are successfully photografted to the PI-SAM in a periodic pattern with a 50 μm repeat distance, matching the dimensions of the TEM grid used as a contact mask.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research is supported by the National Science Foundation through Grants CHE 11-11398, IGERT-02-21589, and DMR0704054. T.W. thanks Columbia University for support of the Summer Research Program for Science Teachers. The authors thank Marissa Solomon for assistance in CFM imaging and Damien Maillard for AFM imaging.
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REFERENCES
(1) Taton, T. A.; Mirkin, C. A.; Letsinger, R. L. Scanometric DNA array detection with nanoparticle probes. Science 2000, 289, 1757. (2) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. A DNA-based method for rationally assembling nanoparticles into macroscopic materials. Nature 1996, 382, 607. (3) Lu, Y.; Fan, H.; Stump, A.; Ward, T. L.; Rieker, T.; Brinker, C. J. Aerosol-assisted self-assembly of mesostructured sphericalnanoparticles. Nature 1999, 398, 223. (4) Majetich, S. A.; Jin, Y. Magnetization directions of individual nanoparticles. Science 1999, 16, 284. (5) Ma, Y.; Qian, L.; Huang, H.; Yang, X. Buildup of gold nanoparticle multilayer thin films based on the covalent-bonding interaction between boronic acids and polyols. J. Colloid Interface Sci. 2006, 295, 583. (6) An, Y.; Chen, M.; Xue, Q.; Liu, W. Preparation and self-assembly of carboxylic acid-functionalized silica. J. Colloid Interface Sci. 2007, 311, 507.
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CONCLUSION Nanoparticles formed from poly(styrene-b-polyethylene oxide) block copolymers are successfully photografted onto glass substrates and silicon wafers modified with photoactive, phthalimide-functional, self-assembled monolayers. Upon ex10940
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(7) Riboh, J. C.; Haes, A. J.; McFarland, A. D.; Yonzon, C. R.; Van Duyne, R. P. A nanoscale optical biosensor: Real-time immunoassay in physiological buffer enabled by improved nanoparticle adhesion. J. Phys. Chem. B 2003, 107, 1772. (8) Fu, Y.; Xu, H.; Bai, S.; Qiu, D.; Sun, J.; Wang, Z.; Zhang, X. Fabrication of a stable polyelectrolyte/Au nanoparticles multilayer film. Macromol. Rapid Commun. 2002, 23, 256. (9) Terekhin, V. V. V. Formation of ordered nanoparticle assemblies by block copolymer lithography methods. Russ. Chem. Rev. 2011, 80. (10) Beh, W. S.; Kim, I. T.; Qiu, D.; Xia, Y.; Whitesides, G. M. Fabrication of ordered two-dimensional arrays of micro- and nanoparticles using patterned self-assembled monolayers as templates. Adv. Mater. 1999, 11, 1038. (11) Wang, Y.; Zhong, W.; Jiang, N.; Yang, W. Directly fabricating monolayer nanoparticles on a polymer surface by UV-induced MMA/ DVB microemulsion graft polymerization. Macromol. Rapid Commun. 2005, 26, 87. (12) Carroll, G. T.; Wang, D.; Turro, N. T.; Koberstein, J. T. Photochemical micro-patterning of carbohydrates on a surface. Langmuir 2006, 22, 2899. (13) Akbulut, M.; Ginart, P.; Gindy, M.; Theriault, C.; Chin, K.; Soboyejo, W.; Prud’homme, R. K. Generic method of preparing multifunctional fluorescent nanoparticles using flash nanoprecipitation. Adv. Funct. Mater. 2009, 19, 1. (14) Kumar, V.; Adamson, D. H.; Prud’homme, R. K. Fluorescent polymeric nanoparticles: Aggregation and phase behavior of pyrene and amphotericin B molecules in nanoparticle cores. Small 2010, 6, 2907. (15) Zhang, S.; Adamson, D. H.; Prud’homme, R. K.; Link, A. J. Photocrosslinking the polystyrene core of block-copolymer nanoparticles. Polym. Chem. 2011, 2, 665. (16) Zhang, S. Y.; Prud’homme, R. K.; Link, A. J. J. Synthesis and Evaluation of Clickable Block Copolymers for Targeted Nanoparticle Drug Delivery. Mol. Pharmaceutics 2012, DOI: dx.doi.org/10.1021/ mp3000748. (17) Johnson, B. K.; Prud’homme, R. K. Flash nanoprecipitation of organic actives and block copolymers using a confined impinging jets mixer. Aust. J. Chem. 2003, 56, 1021. (18) Huisgen, R.; Szeimies, G.; Mobius, L. 1.3-Dipolare cycloadditionen, XXXII. Kinetik der additionen organischer azide an CCmehrfachbindungen. Chem. Ber./Recl. 1967, 100, 2494. (19) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Click chemistry: Diverse chemical function from a few good reactions. Angew. Chem., Int. Ed. 2001, 40, 2004. (20) Turro, N. T. Principles of Molecular Photochemistry: An Introduction; University Science Books: Sausalito, CA, 2009. (21) Hwang, L. Y.; Gotz, H.; Hawker, C. J.; Frank, C. W. Glycoacrylate copolymers for bilayer tethering on benzophenone-modified substrates. Colloids Surf., B 2007, 54, 127. (22) Dormán, G.; Prestwich, G. D. Benzophenone photophores in biochemistry. Biochemistry 1994, 33, 5661. (23) Dormán, G.; Prestwich, G. D. Using photolabile ligands in drug discovery and development. Trends Biotechnol. 2000, 18, 64. (24) Jeyaprakash, J. D.; Samuel, S.; Ruhe, J. A facile photochemical surface modification technique for the generation of microstructured fluorinated surfaces. Langmuir 2004, 20, 10080. (25) Prucker, O.; Naumann, C. A.; Ruhe, J.; Knoll, W.; Frank, C. W. Patterned attachment of ultrathin polymer layers to solid substrates. J. Am. Chem. Soc. 1999, 121, 8766. (26) Wagner, C. D.; Naumkin, A. V.; Kraut-Vass, A.; Allison, J.; Powell, C. J.; Rumble, J. R. NIST X-ray Photoelectron Spectroscopy Database; Measurement Services Division of the National Institute of Standards and Technology (NIST) Technology Services: Washington, DC, 2003. (27) Spells, S. J. Characterization of Solid Polymers; Chapman & Hall: London, U.K., 1994. (28) Thill, A.; Spalla, O. Aggregation due to capillary forces during drying of particle submonolayers. Colloids Surf., A 2003, 217, 143.
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