Preparation of Nanoparticle Coatings on Surfaces of Complex Geometry

Unlike LBL on colloids and flat surfaces, such procedure opens the door for the preparation of conformal nanostructured coatings for a variety of phot...
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NANO LETTERS

Preparation of Nanoparticle Coatings on Surfaces of Complex Geometry

2003 Vol. 3, No. 2 173-177

M. Todd Crisp and Nicholas A. Kotov* Chemistry Department, Oklahoma State UniVersity, Stillwater, Oklahoma 74078 Received November 14, 2002; Revised Manuscript Received December 16, 2002

ABSTRACT Layer-by-layer sequential adsorption technique (LBL) can be utilized as a deposition method for nanostructured coatings on highly curved micrometer to millimeter scale surfaces, which is demonstrated in this work on a model films of strongly luminescent CdTe nanoparticles. Unlike LBL on colloids and flat surfaces, such procedure opens the door for the preparation of conformal nanostructured coatings for a variety of photonics and lab-on-the chip devices with predictable technological importance. Confocal microscopy data showed that prepared CdTe/PDDA coatings were found to be uniform and continuous. Strong coupling of the nanoparticle luminescence into the optical fiber attributed to high refractive index of CdTe/PDDA composite was obtained. As applied to nanoparticle coatings, two unanticipated effects were observed: (1) gradual red shift of the luminescence spectrum as the layer thickness increases; (2) two-stage luminescence transient including the significant enhancement of the luminescence during first 100 min of UV illumination. Similar results were obtained for the coatings made in the glass tube interior, which can be considered as simple models of microfluidic devices with channels of complex geometry.

I. Introduction. Semiconductor nanoparticles (NPs) and other size-confined species offer a broad spectrum of optical properties for the design of photonic materials and devices. High luminescence efficiency, optical nonlinearity, polarization effects, and new photophysical and energy transfer phenomena are but a few of the properties attracting the interest of numerous research groups. Prototypes of several NP-based photonic and electronic devices have been already demonstrated.1 Similarly to classical semiconductor devices, the most likely implementation of NPs will be in the form of thin films. Layer-by-layer assembly (LBL) with polyelectrolytes2 was shown to be one of the most efficient ways of preparing NP films on flat substrates. It is quite universal and provides some degree of control over 2D and 3D film structure.3 The method of LBL assembly is ideally suited for growing thin NP films across a contoured substrate with variable curvature. To some extent, this idea was employed for building LBL films on spherical colloids.4,5 However, the coatings obtained on mesoscale surfaces of complex geometry have not received sufficient attention and remain to be utilized in various, possibly unique devices. For instance, the conformal films on curved transparent or reflective substrates are of great importance for photonic applications. The use of traditional deposition methods of optically active films, such as spin coating and vacuum evaporation, is often difficult for them because it leads to heterogeneous distribution across the substrate and shadowing. Standard sol-gel dipping techniques do not allow for the nanometer scale control of the film thickness. Since LBL is essentially a wet-dip method that utilizes the ability of LBL partners to adsorb on each other, and is governed only 10.1021/nl025896f CCC: $25.00 Published on Web 01/08/2003

© 2003 American Chemical Society

by chemical conditions in the thin layer of solvent in the vicinity of the surface, the uniform optically active films can be made by LBL equally efficiently both on flat and on high curvature and/or cavernous substrates. In this study, we demonstrate the preparation of uniform optical quality coatings made of highly luminescent NPs on optical fibers and tube interior. For 600 micron glass fibers, the successful coupling of the NP luminescence into the fiber was observed. The high refractive index of the hybrid semiconductor/polyelectrolyte layer deposited increases the efficiency of optical coupling acting as partial cladding layer. NP luminescence transmitted by the fiber was seen even for 1-2 LBL layers deposited. The NP coatings made in the interior of glass tubing can be considered as a simple model for making functional coating for microfluidics and sensing devices. II. Experimental Section. II.1. Sample Preparation. II.1.1. Coating of Fiber Optic Substrates. Colloidal solutions of CdTe nanoparticles stabilized by thioglycolic acid were prepared by the synthesis devised by Rogach et al.6 These particles were synthesized, allowed to reflux until their emissions peak was in the red region (∼630 nm), diluted 3 times, and pH adjusted to 8.0-8.5. The same synthesis was generally utilized for the production of all NPs used during the course of these experiments. Two different variations of substrate were used for the growth of luminescent thin films during the course of these experiments. The first to be investigated was a silica optical fiber purchased from Thor Labs Inc. It has a 600 µm diameter silica core cladded by silicone resin and overcoated with a plastic buffer layer. A section of the fiber was stripped of

the buffer and cladding layers and cut into several sections 6 cm long. To ensure the ability to produce a number of samples with uniform characteristics, a multisample holder was fashioned from a small, 1.5 cm diameter Teflon wheel with eight holes (∼600 µm in diameter) arranged in a radial pattern around the center. The center was drilled and fitted onto a 3 mm glass rod used for manipulating this tool in the dipping process. Sections of optical fiber were then fitted into the Teflon wheel, and after that the entire assembly was placed into Piranha solution (30% H2O2 + 98% H2SO4) for 30 min for cleaning (DANGEROUS! Violently reacts with organics!). With the substrate prepared, the LBL assembly was initiated by immersion of the entire assembly into 50 mL of a 1 wt % solution of high molecular weight poly(diallyldimethylammonium chloride) (PDDA) with the pH adjusted to 8.0 and allowed to stand for 15 min. Next, the substrate was removed and inserted into an equal volume of deionized rinse water for 2 min. Upon removal from the rinse water, the substrate was inserted for 10 min into 50 mL of a 1 wt % solution of poly(acrylic acid) (PAA, average molecular weight ∼450 000). The pH of this solution was adjusted to 6.0. In 10 min of immersion, the fibers were removed from the PAA solution and inserted into DI rinse water for 2 min. With the completion of the rinse step, the fibers were again inserted into the PDDA solution and allowed to stand for 10 min. This top PDDA layer enables subsequent adhesion of the negatively charged NPs. The fibers were removed and rinsed for 2 min to complete the production of the organic foundation layer over the surface of the fibers. Notably, the growth of the NP film can be accomplished even without the organic foundation layer by simple alternation with PDDA, which was done for some results reported here. However, it was noted that the density of the adsorbed NP as well as film uniformity always improved when the polycation/polyanion layers were applied first. With the foundation layer in place, the optical fibers were immersed into a solution of CdTe NPs for 10 min. After the formation of the first NP bilayer was complete, the fibers were removed from solution and inserted into pH-adjusted deionized water for 2 min. From this point of the film growth process on, all solutions were kept basic (pH ∼8.0) to prevent slow degradation of CdTe luminescence. Following the rinse step, the fibers were removed and placed into the PDDA solution for 10 min followed by another 2 min deionized water rinse. This cycle of CdTe dip/rinse followed by PDDA dip/rinse was repeated until the desired number of bilayers, n, was achieved, producing a film denoted as (PDDA/CdTe)n with n usually being from 2 to 8. The foundation PDDA/ PAA bilayer is not included in the notations for brevity. In another experimental setting, the same optical fiber was cut in long sections (> 70 cm) and fitted with an SMA type connector at one end. The tip of each connector was polished using polishing kit obtained from Thor Labs. Following this procedure, a 20 cm length of the cladding layers was removed from the end opposite the SMA connector. To clean the exposed silica core, one end of the fiber to be modified with NPs was immersed for 30 min into a mixture of 174

Nocromix and 98% H2SO4 (DANGEROUS! Violently reacts with organics!). This was followed by thorough rinsing with deionized water and immersion into 150 mL of a 1 wt % solution of high molecular weight PDDA (pH 8.0) for 15 min. Upon completion of the PDDA dip, the fibers were carefully removed from the cylinder of PDDA and placed into a similar cylinder of deionized rinse water with the pH adjusted to 8.0 and allowed to stand for 2 min. Next, the fiber was removed from the rinse water and placed into 150 mL of a 1 wt % solution of PAA for 10 minutes, followed by removal and rinsing for 2 min. Then the fiber was placed into the PDDA solution, allowed to sit for 10 min, and rinsed in basic rinse water for 2 min. This completed the generation of the organic foundation layer. At this point, the building up of the NP layers was begun by immersion of the substrates into a solution of red CdTe NPs with a pH of 8.5 for 10 min. This step was followed by a two-minute dip into a cylinder of basic rinse water and then placing the fibers into the PDDA solution for 10 min followed by rinsing for 2 min. These steps were cycled until an appropriate number of bilayers, n, was generated, making a film with a cumulative notation (PDDA/CdTe)n. II.1.2. Coating of Glass Tubing. The growth of films inside glass tubing was achieved using methods nearly identical to those previously discussed. All chemicals, pH values, and times were kept the same. A 1 m piece of glass tubing was wrapped in aluminum foil and connected to a piece of Tygon hose with a small syringe attached to one end. The glass tubing was then clamped into a vertical position with the open end pointing down. Using the syringe to draw a mixture of H2SO4 and Nocromix up into the entire length of the glass tube and allowing the mixture to remain inside the tube for 15 min achieved cleaning of the tubing. Cleaning was followed by a rinse step. It was the same for rinsing the cleaning solution, polyelectrolyte, and NPs from the inside of the tubing during all stages of film growth. The Tygon tubing was connected to a rinse bottle with basic deionized water and the bottle used to pump 50 mL of rinse water through the tubing. Following this, the Tygon tubing was connected to a nitrogen bottle so that a stream of nitrogen could be passed through the tubing to dry out the inside. After completion of the cleaning, the inside wall of the glass tube was coated with a foundation of PDDA/PAA/PDDA by drawing the precursors into the tubing for 10 min each and then rinsing between layers. Once this step was finished, 10 bilayers of CdTe and PDDA were coated over the inside using a 10-min deposition time for each layer. To dry the films, nitrogen was passed through the tube for 30 min, after which the sample was placed under a nitrogen blanket and kept in the dark until the luminescence measurements were done. For spectroscopic measurements, a 7 cm long section of the sample tube was scored and broken off. These pieces were then placed vertically inside the cell compartment of the fluorometer for emission measurements. II. 2. Measurements and Instrumentation. To characterize the films grown on the optical fibers by fluorescence spectroscopy, a fluorescence cuvette was fitted with Teflon plugs in the bottom and top, which were made to hold the Nano Lett., Vol. 3, No. 2, 2003

fiber in precise alignment with the centerline of the cuvette. Samples were placed into the cuvette for measurement in a Spex Fluorolog 3 Fluorometer. Some variation of the in signal intensity upon rotation of the sample inside the cuvette (typical for samples of small physical dimensions) was mediated by accumulating several spectral scans (>10) and subsequent signal averaging. To avoid the variability problem, a fiber with male SMA connector was coupled to the spectrometer so that thin film emission was transmitted directly to the detector. A female SMA connection was fitted to the fluorometer so that the polished end of the fiber was in alignment with the photo multiplier tube of the fluorometer. To facilitate the excitation of the fiber, a BONDwand UV hand-held lamp was fitted to a delrin holding/excitation block, which holds a quartz cylinder in alignment with the UV lamp while preventing exposure to outside light sources. The quartz cylinder retains the exposed end of the fiber in place about the centerline of the cylinder for exposure to UV light. Confocal laser scanning microscopy images were performed on a Leica TCS SP2 spectral confocal microscopy system. III. Results and Discussion. III.1 Optical Fiber. The objective of this investigation was to produce uniform thin films of CdTe NPs on curved macroscopic surfaces, with obvious practical importance as coating for telecommunication, sensor, and other photonic devices. The observation of remarkably strong luminescence in the near-IR region at 1550 nm,7 LBL films of HgTe quantum dots which is the principal spectral region for fiberoptic telecommunication, makes this objective particularly relevant. Since the use of UV-vis absorption spectroscopy utilized for monitoring of LBL film growth on flat substrates is complicated for highly curved surfaces because of the complex interplay of refraction and reflection, this task was performed by fluorescence spectroscopy. The measurements were taken from the films assembled upon small sections of the 600 micron diameter silica optical fiber in fixed sample/ beam geometry and excitation/detection conditions. To ensure applicability of the film growth models developed for flat substrates, it is important to verify the linearity of the deposition.8 The linear dependence of the optical density of the flat substrates on the number of the deposition cycles was obtained for many NP/polyelectrolyte pairs.2,3,8,9 A similar linear relationship was observed in the LBL deposition on the optical fiber as well (Figure 1), although the increment of the fluorescence intensity growth had greater dispersion than for UV-vis absorption spectra. To assess uniformity of the NP coating over the substrate we utilized confocal microscopy that demonstrated its effectiveness for LBL-deposited films on mesoscale colloids.4,5,10 It provides better representation of film coverage over fairly large areas as compared to atomic force microscopy. Additionally, it also affords depth profiling of the particle distribution over the cylindrical surfaces. Figure 2a presents a series of NP-coated optical fiber images subsequently acquired for different focusing depths on a confocal microscope. As one can see, the thickness of prepared Nano Lett., Vol. 3, No. 2, 2003

Figure 1. Fluorescence spectra obtained for the sequentially adsorbed (PDDA/CdTe)n multilayers, n ) 2-7. In the inset: dependence of the fluorescence intensity at 636 nm on the number of deposition cycles, n. This wavelength was selected as the maximum luminescence spectrum for the second PDDA/CdTe bilayer.

Figure 2. Confocal microscopy examination of the optical fiber coated with (PDDA/CdTe)8. (a) Depth profile image sequence. Numbers 1-6 denote gradual change of imaging depth from the middle of the fiber to the top surface. (b) Composite image of a piece of fiber surface. (c) Axial image of the fiber from the polished coated end.

coatings is uniform; they are also homogeneous and virtually defect-free over the entire surface. This can also be confirmed by the side-view and axial image demonstrating the deposition of the uniform and continuous luminescent layer around the entire fiber circumference (Figure 2b,c). A noticeable red shift in the luminescence peak was observed with each additional layer (Figure 1) during the first few deposition cycles. The emission spectrum stabilizes at the 5th deposition cycle, producing a total shift of 4-6 nm. This effect was attributed to the energy transfer phenomena NP solids.11 In the dispersion used for NP LBL assembly, the distribution of the NP diameters is about 15%. 175

The exciton hopping can occur between the closely packed NPs throughout the film. The larger particles with narrower band gap act as acceptors of the excitonic state energy of smaller NP with wider band gap, similarly to the energy transfer in the stratified layers of CdTe of different diameters (nanorainbows).12 Naturally, extensive hopping leads to the preferential emission by bigger NPs and progressive red shift of the film emission, which happens as the film thickness increases. Note that the excitation beam spot is large enough so as to completely surround the sample and strong enough to provide virtually constant intensity throughout the film, regardless of the deposition cycle number, n. It is necessary to point out that in addition to the energy transfer, the luminescence red-shift can perhaps be ascribed to partial size-selection process during the LBL assembly procedure when smaller NPs are partially removed from the freshly adsorbed film by the rinsing step. Although possible, we consider this process less significant in the present case, because a concomitant shift of UV-vis absorption characteristics with n was not observed for the LBL assembly of CdTe on flat surfaces under similar conditions.12,13 III.2 Fiber Optic Coupled Luminescence. The demonstration of successful coupling of the NP emission in the optical fiber is critical for all potential applications of this system. A piece of optical fiber ca. 2 m in total length with a standard SMA connector on one end was coated with (PDDA/CdTe)3 multilayers on the other end over a distance of 0.2 m. Its luminescent end was placed into the glass holding cylinder inserted into the enclosed excitation block shielded from the ambient light with an internal black light lamp (primary emission at 365 nm, BONDwand). The SMA connector was then coupled to its counterpart attached to the photomultiplier entrance slit of the fluorometer with the excitation slit of the spectrofluometer completely closed. After that, the luminescent end was excited with the UV lamp. The red light emitted by NPs was guided by the optical fiber into the monochromator. Strong emission signals were measured and could easily be seen by the naked eye under ambient room lighting, which indicated successful coupling of the NP emission in the device. Aside from the small luminescence red shift mentioned above, the measured spectra of fiber optic coupled emission followed those of the original dispersion of mixed NPs (Figure 3). Monitoring of the time dependence of the signal intensity (Figure 4) revealed an unexpected two-step kinetic process, which needs to be taken into account for various applications. Initially the signal increased approximately three-five times of the starting intensity (Figure 4, trace 1), while after about an hour of illumination the reverse process set it (Figure 4, trace 2), leading to a slowly decreasing plateau. The initial signal rise should be attributed to the photoinduced luminescence activation observed previously for CdTe as well as for CdSe nanoparticles.14,15 For the CdTe NPs used here, Rogach et al. associated this luminescence enhancement with the formation of the short band gap layer of CdS on NP surface from thiols used as stabilizers, while other processes taking place in the presence of oxygen were shown to occur 176

Figure 3. Spectra of the fluorescence signal obtained from NP dispersion excited in the cell compartment of spectrofluometer (1) and the signal transmitted by optical fiber with (PDDA/CdTe)3 coated end (2).

Figure 4. Fluorescence transient of the signal transmitted by the optical fiber upon continuous illumination with UV light.

for citrate-stabilized CdSe.15 Subsequently, reduction of the luminescence intensity in the second part of the curve corresponds to the well-known photobleaching of CdTe (oxidation of Te2-). In addition to the high quantum yield of CdTe (20-30%), the strong signal output by the optical fiber should be attributed to the high index of refraction for nanoparticles, 2.6 for CdTe, which translates into a high refractive index of the film determined by ellipsometry to be 1.8. In part, the deposited LBL film can act as a thin cladding promoting the coupling of the NP luminescence into the optical fiber and helping it to channel the light down the length of the fiber. III.3 Glass Tubing Substrate. Another type of curved substrate investigated was glass tubing with an internal diameter of 3 mm. The preparation of optical and biological coatings in channels of complex geometry is paramount for microfluidics and lab-on-a-chip devices. The glass tube is treated here as a simple model system of such microfluidic systems. Additionally, the tube geometry is particularly suitable for sensing applications. Glass tubing was coated inside with LBL films of CdTe NPs under the same deposition conditions as optical fibers. Excitation of tube samples with coated interior by a UV hand lamp produced fluorescence strong enough to be seen with the naked eye (Figure 5) with characteristic red emission of the NP coating. As expected, the coating is uniform over Nano Lett., Vol. 3, No. 2, 2003

materials more resistant to photobleaching than CdTe, such as core-shell CdSe/CdS nanocolloids for UV-vis range and IR-emitting II-VI and III-V semiconductor NP more appropriate for the current signal processing technology. Acknowledgment. N.A.K. thanks the NSF-CAREER, NSF-Biophotonics, AFOSR, OCAST, and Nomadics Inc. for financial support of this project. M.T.C. and N.A.K. thank the Lou Wentz Foundation for the Undergraduate Research Scholarship award to M.T.C. References Figure 5. Photographs of a glass tubing with interior coated by (PDDA/CdTe)10 film in day light (a) and under UV illumination (b). Red luminescence of the NP is clearly visible. The blue reflections are the images of the UV lamp used for the excitation. The notches perpendicular to the tube axis were made to allow a clean break of individual test specimens.

Figure 6. The luminescence spectra of original NP dispersion (1) and the glass tubing with deposited (PDDA/CdTe)5 film (2).

the entire length and surface, similarly to optical fibers. In the emission spectrum, a slight red shift of the CdTe multilayer as compared to the parent NPs was also observed, being otherwise identical to the spectrum of original dispersion. (Figure 6).

Conclusion This study fills the gap in the application of LBL assembly as a method for production of nanostructured coatings on highly curved and complex surfaces. While anticipated, the realization of such process should be demonstrated. Importantly, it opens the door for numerous photonics, microfluidics,16 and possibly some biomedical devices.17 Strong coupling of the nanoparticle luminescence into the optical fiber observed here makes possible the utilization of LBL deposition scheme for a variety of optical devices taking advantage of quantum confinement effects. Further optimization of the coatings should possibly include different NP

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