Reversible Loading and Unloading of Nanoparticles in “Exponentially” Growing Polyelectrolyte LBL Films Sudhanshu Srivastava, ² Vincent Ball, ‡ Paul Podsiadlo, ² Jungwoo Lee, ² Peter Ho, ² and Nicholas A. Kotov* ,² Departments of Chemical Engineering, Materials Science and Engineering, Biomedical Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109 and Institut National de la Sante ´ et de la Recherche Me ´ dicale, Unite ´ Mixte de Recherche 595, UniVersite ´ Louis Pasteur-Faculte ´ de Chirurgie Dentaire, 11 rue Humann, 67085 Strasbourg Ce ´ dex, France Received December 26, 2007; E-mail: kotov@umich.edu The layer-by-layer (LBL) assembly, based on sequential adsorp- tion of oppositely charged components, is one of the most established methods for the preparation of thin films with controlled properties. 1,2 The LBL technique is not limited to polyelectrolytes, but almost any type of macromolecular species (charged preferred) including inorganic molecular clusters, 3 nanoparticles (NPs), 4-5 nanowires, 6 organic dyes, 7 polypeptides, 8 DNA, 9 or viruses 10 can be used as the assembly components. Most of these LBL films have been loaded with active molecules only during the preparation by using the species of interest as active constituents in the film buildup. The significance of finding a method to reversibly load and unload NPs in LBL films is 4-fold. (1) Most obviously, it would enable fast and universal preparation of NP-based coatings with a variety of functionalities. (2) The mobility of NPs inside the polymeric matrix would enable new methods of control over self- assembly processes. 11 (3) A dynamic exchange process is essential in the development of fine separation tools for NPs. (4) Last but not the least, it would create important opportunities for biomedical applications using organic/inorganic nanocolloids, proteins, DNA, RNA, etc. in controlled-release devices. Recent advances in the fundamental studies of LBL films suggest that the highly hydrated exponentially growing films 12 can incor- porate multivalent ions, 13 dyes, 14 and small drugs. 15 These com- pounds can be released upon triggering by an external signal, e.g., an ion exchange process, 13a or a change in the pH. 14 Along the same lines, it was also observed that some LBL films are deformed when put in contact with colloidal particles. 16 Using these findings as a foundation one can hypothesize that LBL films can indeed have the ability to load/unload nanoscale species in a controlled fashion. The concept of loading and unloading an LBL film after its buildup would be of real interest and has not been validated very well yet. It has just been observed that certain combinations of polyelectrolytes allow for the completely irreversible loading of proteins or nanoparticles. 17 The fundamental possibility of reversible loading-unloading processes is the subject of this communication and is essential for enabling applications mentioned above. To demonstrate this functionality we used exponentially growing LBL films made from poly(diallyldimethylammonium chloride) (PDDA) and poly(acrylic acid) (PAA). The films were prepared by dipping alternatively a glass substrate in 0.5% w/v PDDA and 1% w/v PAA solutions. We note that exponential growth in the PDDA/PAA system is quite unusual and to our knowledge has not been reported yet (see Supporting Information). Exponential growth stipulates an increased mobility of the polymer chains in the films, 12 which, in turn, opens the possibility for fairly large colloids to penetrate inside them. In the initial experiments we used negatively charged thioglycolic acid-capped CdTe quantum dots (NP1, Figure 1a, 4 nm diameter, zeta potential )-50 mV). As prepared (PDDA/ PAA) n films, with n ) 45 (where n is the number of LBL deposition cycles), were exposed to a CdTe suspension at pH 9 (Figure 1a). After 7 h of exposure, the films appeared highly swollen and displayed characteristic adsorption and luminescence of NPs (Figure 1a, inset). Release of the incorporated CdTe and reproducibility were demonstrated by immersing the CdTe-loaded films in pure water at pH 9. The swollen films turned colorless in ∼30 h indicating release of NP1 from the films (Figure 1a). Clear evidence of loading and unloading of NP1 in (PDDA- PAA) 100 multilayer films was obtained by confocal microscopy of the cross sections (Figure 1b). After loading for 6 h, the films appeared to be evenly filled with the NPs. The fluorescence signal disappeared after the loaded films were placed in contact with pH 9 water for 24 h (Figure 1c) under identical conditions. Bright field optical images of cross sections show no microscale morphological changes of the films between the two loaded and empty states (Fig- ure 1b,c, right panels). Distribution of NPs in the film may not necessarily be uniform and can involve both lateral and vertical gradients. To better understand the loading/unloading mechanism, we measured the release kinetics of NP1 in water at pH 9 and 7 and in films covered with a capping layer of a linear polyelectrolytes LBL combination of (PDDA-PSS) 10 . 18 In the past, it was demon- strated that exponentially growing LBL films can be capped with impermeable capping layers made either from linearly growing LBL films 18 or from hydrolyzable polyesters. 19 We found that the pH 9 environment resulted in the release of the NPs in a characteristic ² University of Michigan. ‡ Universite ´ Louis Pasteur. Figure 1. (a) Loading and unloading of (PDDA-PAA)45 films (pH 9) with green fluorescence-emitting NP1 as followed by UV-vis absorbance at 530 nm (note the large change in absorbance). The insets are fluorescence photographs of the filled (top) and empty (bottom) films. (b) Confocal microscopy images of (PDDA-PAA) 100 films loaded by 6 h of exposure to NP1 solution and (c) empty films after 24 h exposure to pH 9 water. The right panels show white light images. Published on Web 03/06/2008 3748 9 J. AM. CHEM. SOC. 2008, 130, 3748-3749 10.1021/ja7110288 CCC: $40.75 © 2008 American Chemical Society