Pulsatile Releasing Platform of Nanocontainers Equipped with

Jan 16, 2013 - These platforms are characterized by a high loading capacity in ... in many systems where small controlled amounts of substances have t...
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Pulsatile Releasing Platform of Nanocontainers Equipped with Thermally Responsive Polymeric Nanovalves Michał Szuwarzyński, Leszek Zaraska, Grzegorz D. Sulka, and Szczepan Zapotoczny* Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krakow, Poland S Supporting Information *

ABSTRACT: Polymer-gated reservoirs built of an ordered porous anodic aluminum oxide (AAO) platform equipped with poly(N-isopropylacrylamide) (PNIPAM) brushes grafted from the surface using atom transfer radical polymerization were fabricated and studied. The brushes of different lengths were grafted from the AAO surface with pores having diameters of 30, 50, and 80 nm, respectively. Polymer brushes served as thermally responsive valves that can immediately open and close the pores just by crossing the PNIPAM lower critical solution temperature. Calcein was used as a model fluorescent molecule to follow a release process from the platform of the gated nanocontainers. The studied systems enable the burst release of the loaded substance, followed by diffusion-controlled release that depends on the pores’ diameter. These platforms are characterized by a high loading capacity in comparison to polymer films and can be loaded repeatedly. Importantly, the opening/closing of the nanocontainers is reversible, and a pulsatile release can be easily realized. Such platforms combined with highly localized heating devices can find potential applications in many systems where small controlled amounts of substances have to be released on demand, such as lab-on-a-chip, total analysis, and nano/microfluidic systems. KEYWORDS: anodic aluminum oxide (AAO), poly(N-isopropylacrylamide) (PNIPAM), pulsatile release, nanocontainers, polymer brushes, atom transfer radical polymerization (ATRP) for PNIPAM).13 Stimuli-responsive polymers have been commonly used to build carriers and releasing systems in the forms of film14,15 or brush16,17 platforms. Their thermal,18 pH,19,20 photo-, and ionic strength21 responsive behaviors have been applied as triggers for releasing substances entrapped among polymeric chains. Surface-grafted polymer brushes22,23 can be synthesized using surface-initiated controlled radical polymerizations24,25 that enable easy control over the brush length. It has been shown previously that polymer brushes may be grafted from the surface of nanoporous AAO.26 However, the systems studied so far focused on the tunable permeability of the decorated membranes27 or just the examination of stimuliresponsive behavior of such nanostructured surfaces.28−30 We propose here to apply such systems as a matrix of nanogated reservoirs. These new releasing platforms can address several problems of the existing loading/releasing systems on the nanoscale. Our platforms are characterized by a high loading capacity in comparison to thermally responsive polymer films, can be loaded and release a substance many times, and the release process may proceed in a controlled pulsatile manner (partial cargo delivery system). Thanks to such features, the polymer-gated platforms may be applied in, for example, the rapidly developing lab-on-a-chip systems31 and total analysis systems (TAS),32 and may find a general use in nano-33 and

1. INTRODUCTION Over the past decade, there has been an increasing interest in fabrication of micro(nano)carriers for controlled release of nanoparticles or molecules loaded in them. Most of them are based on dispersions in liquids,1,2 whereas the platforms designed on macroscopic supports are rather rare3 and mostly focused on degradable surfaces4 or a microfluidic approach.5 Such carrier types have usually unique properties and are designed for specific medical,6,7 biotechnological,8,9 or environmental protection10 devices. They are commonly used for defined substances only. Despite many practical applications, the systems based on surface-eroded carriers working at nanoscale have many disadvantages limiting their usage. They are characterized by disposable use and a rather high carrier-topayload weight ratio (limited loading capacity). It is also difficult to stop the ongoing release on demand. This work presents a new platform of nanocontainers that enables loading and controlled pulsatile release of picoliter quantities of cargo solution from square millimeters surface area. Advantageously, the containers may be easily reloaded in contrast to typical eroding polymeric carriers. Such polymergated smart nanocontainers were built from a porous anodic aluminum oxide (AAO) 11,12 platform that consists of hexagonally ordered nanopores, arranged practically parallel to each other. From their surface, thermally responsive poly(Nisopropylacrylamide) (PNIPAM) polymer brushes were grafted. Polymer brushes serve as thermally responsive valves that can immediately open and close just by crossing the PNIPAM lower critical solution temperature (LCST = 32 °C © 2013 American Chemical Society

Received: December 7, 2012 Revised: January 16, 2013 Published: January 16, 2013 514

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Figure 1. General synthetic route for grafting of PNIPAM brushes from the AAO surface. carried out in 0.3 M sulfuric acid at 1 °C and 0.3 M oxalic acid at 20 °C under a constant potential of 25 and 45 V, respectively. The duration of first anodization was 8 h for anodizing performed in sulfuric acid and 1 h when oxalic acid was used as an electrolyte. Asprepared anodized samples were submerged into a mixture of 6 wt % H3PO4 and 1.8 wt % H2CrO4 at 45 °C for 12 h to remove oxide layers. Subsequently, the second anodization was carried out under the same experimental conditions as were used in the first step. The duration of the second anodizing step was 10 h for H2SO4 and 4 h for H2C2O4. All anodization experiments were carried out in a simple electrochemical cell, and the temperature of the electrolyte was maintained by a powerful circulation system (Thermo Haake, DC10-K14). A Pb plate was used as the cathode. The Al area exposed to the electrolyte solution was 0.5 cm2. The distance between the cathode and the aluminum anode was 3 cm. After anodization, the chemical etching in 5 wt % H3PO4 was performed in order to increase the diameter of the nanopores. For samples anodized in sulfuric acid, the pore widening was performed at 30 °C for 30 min. The resulting pore diameter was about 30 nm. The pore widening treatment of samples anodized in oxalic acid was carried out at 25 °C for 80 min, and the size of pores increased up to 80 nm (see the Supporting Information for SEM images, Figure S1). 2.4. Synthesis of Polymer PNIPAM Brushes. Plates with nanoporous AAO were cleaned using ethanol and dichloromethane. Silicon wafers were purified in “piranha” solution (a mixture of H2SO4 and H2O2 at a 1:3 ratio) (Caution! “Piranha solution” should be handled with extreme care!) and cleaned in water. Polymer PNIPAM brushes were obtained on a smooth silicon surface and an Al2O3 nanoporous surface in three steps (see Figure 1). In the first step, an amide-silane initiator (APTS, 0.2 mL) was grafted from the surfaces by immersing the substrates overnight in APTS at room temperature under an argon atmosphere. The samples were cleaned using dichloromethane. In the next step, a solution of 2-isobromobutyryl bromide (BIB; 0.37 mL), triethylamine (Et3N; 0.41 mL), and dichloromethane (CH2Cl2; 60 mL) was added over silicon and AAO samples under an argon atmosphere, at room temperature, and left for 1 h. In the third step, PNIPAM brushes were obtained using the ATRP method. NIPAM monomer was added to the solution of water and methanol (at 1:1 v/v ratio) under an argon atmosphere at room temperature. To the resultant solution was injected a mixture of PMDETA and CuBr, and the solution was left for the proper time of synthesis. The molar ratio of the reactants was 1:0.01:0.035 (NIPAM/CuBr/PMDETA). The brushes were grafted for 1, 6, and 12 h from AAO substrates with 30 nm wide pores (AAO30). For the samples having 50 and 80 nm wide pores (AAO50 and AAO80, respectively), the polymerization time was set to be 4 h. 2.5. Loading/Releasing Procedure. AAO30, AAO50, AAO80 samples of similar size (60 mm2) were loaded with calcein by immersing in warm (T ≈ 40 °C) calcein solution (30 mM) and left for a few hours in order to let calcein penetrate the pores and the system

microfluidics34−36 where controlled and spatially confined delivery of substances is crucial. It is important that the platforms of reservoirs presented here have no moving parts (no mechanical caps) and could be easily implemented into devices. Using highly localized heating devices that utilize, for example, microwaves37,38 or infrared light,3 it should be possible to quickly39 open and close the nanopores and thus release molecules stored inside the devices, also with high spatial resolution.

2. EXPERIMENTAL SECTION 2.1. Materials. N-isopropylacrylamide (NIPAM; 97%), diiodomethane (CH2I2; puriss), 2-isobromobutyryl bromide (BIB, 98%), and N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA; 99%) were obtained from Sigma Aldrich (St. Louis, MO). Ethanol (96%), hydrogen peroxide (30%), and sulfuric acid (95%, pure) were obtained from POCH S. A. (Gliwice) and triethylamine (Et3N; ultra, ≥99,5% GC), copper(I) bromide (CuBr, purum >98%), and aminopropyltrimethoxysilane (APTS) from Fluka Analytical. They were all used as received. To fabricate AAO layers, we used oxalic acid (pure) and perchloric acid (60%) (both from Chempur), phosphoric acid (85%), and chromic anhydride (99.9%) (both from POCH S.A, Gliwice). 2.2. Apparatus. The contact angle measurements were recorded on an OEG Surftens Universal apparatus (Frankfurt, Germany). Drops of CH2I2 on smooth silicon with polymer brushes and a nanoporous membrane with the same brushes were measured under water at different temperatures both below and above LCST. Atomic force microscope (AFM) images were obtained with a NanoScope IV Multimode atomic force microscope (Bruker, Santa Barbara, CA) working in the tapping mode with standard silicon cantilevers for measurements in the air (nominal spring constant of 40 N/m) and gold-covered Si3N4 cantilevers for water measurements (nominal spring constant of 0.12 N/m). Time-trace fluorescence spectra were measured, below and above LCST, using an SLM Aminco 8100 spectrofluorometer. Ellipsometry measurements were done using a J.A. Woollam ellipsometer, model 2000U. Data were recorded at 60, 65, and 70° for dry samples and at 70° for samples measured in water. 2.3. Fabrication of the Nanoporous AAO Substrates. The anodic aluminum oxide (AAO) membranes were prepared by a twostep anodization process, as described previously.40,41 Briefly, a highpurity aluminum foil (99.999%, Goodfellow, 0.5 mm in thickness) was used as a starting material. Before anodization, Al substrates with dimensions of 25 mm × 5 mm × 0.5 mm were degreased in ethanol and acetone, followed by the electrochemical polishing carried out in a mixture of perchloric acid (60 wt %) and ethanol (1:4 v/v) at 10 °C and a constant current density of 500 mA·cm−2 for 1 min. Subsequently, the samples were rinsed with water, ethanol, and dried. The porous AAO layers were fabricated by a two-step anodizing 515

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to reach the equilibrium state. The platforms were then cleaned by alternating immersions of the sample in ice-cold and warm (T ≈ 40 °C) water to eliminate potential traces of calcein adsorbed on the surface and subsequently dried in a stream of nitrogen. The releasing process of calcein from the loaded sample was realized by placing it in a quartz cuvette filled with water. It was followed using spectrofluorimetric measurements (λexc = 385 nm and λem = 520 nm, typical values for calcein) using an environmental holder to control the temperature and under magnetic stirring. The temperature was varied accordingly, and the fluorescence signal was measured simultaneously. Controlled experiments were also performed using silicon substrates with grafted PNIPAM brushes (6 h polymerization) and native AAO samples (without brushes). They were treated exactly the same way as the AAO samples.

3. RESULTS AND DISSCUSSION 3.1. Preparation and Characterization of PolymerGated Platforms of Nanocontainers. Nanoporous anodic aluminum oxide samples with hexagonally arranged pores closed at one side and having diameters of ca. 30 nm (AAO30) and a height equal to ca. 40 μm were fabricated and used as 2D platforms of nanocontainers for further studies. The volume of each individual container could be estimated to be ca. 30 aL. The pore outlets were capped with thermally responsive PNIPAM brushes that were grafted from the AAO surface using ATRP after various polymerization times (1, 6, and 12 h), as depicted in Figure 1. The topographies of the native AAO30 and the coated platforms were visualized using atomic force microscopy (AFM) in an aqueous medium at room temperature (see Figure 2). Polymer chains in PNIPAM brushes below LCST (T = 32 °C) are extended because the hydrophilic parts of the chains form hydrogen bonds with the surrounding water molecules and build a stable “scaffold”. Above LCST, the hydrogen bonds break and the chains collapse and aggregate. This behavior has been previously shown for longer and densely packed PNIPAM brushes on a smooth support,42 but it is hard to be visualized on the nanostructured AAO surface. As can be deduced from the AFM images, after 1 h of polymerization, the pores seem to be partially closed (reduced pore opening) (Figure 2B) and fully closed after 6 h (Figure 2C). The two-dimensional fast Fourier transform (2D-FFT) indicated that, after 6 h of polymerization, the hexagonal order of the pores was totally lost, whereas it was still preserved after 1 h. From the topological considerations (concave substrate geometry), it is likely that, after prolonged polymerization, cross-linking of the chains growing from the opposite sides of the pore’s mouth may occur, leading ultimately to permanent closure of the pores.43 Thus, the polymerization time should be limited to avoid such a possibility. The polymerization performed at the same conditions using a model smooth silicon surface yields PNIPAM brushes with the height equal to 21.3 ± 0.6 nm already after 1 h, as measured by spectroscopic ellipsometry in an aqueous medium. This value is larger than the average radius of the pores (ca. 15 nm) in the AAO30 samples and would suggest complete closing of the pores by PNIPAM brushes after 1 h of polymerization. Thus, it is clear that, for example, due to differences in the surface grafting density of the initiator between silicone and AAO (especially at the pore mouth), the obtained height value cannot be directly compared to those of the brushes that are grafted from AAO. Apparently, also the lengths of the chains that are grown on the top surface, at the pore edge and inside the pore in the AAO sample, should be different due to the confinement effect.

Figure 2. AFM images obtained at room temperature in water of (A) the native nanoporous AAO30 platform and the same surface covered with PNIPAM brushes after (B) 1 h, and (C) 6 h of ATRP together with the representative cross sections and 2D-FFT pictures.

Nevertheless, even the molecular weights (brush length) are generally much smaller for the brushes grafted from AAO, they follow a similar trend with polymerization time as for the bulk reactions or grafting from flat surfaces.44 The relationship between the polymerization rate and the curvature of the 516

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Figure 3. Schematic drawing of loading and release processes using a nanoporous smart reservoir equipped with thermally responsive nanovalves built of PNIPAM brushes.

Figure 4. Cumulative release profiles of calcein at temperatures below and above LCST from AAO30 platforms gated with PNIPAM brushes obtained after 1 and 6 h of polymerization. The release is shown also for two control samples: native AAO30 and a smooth silicon surface covered with brushes after 6 h of polymerization. The arrow indicates the moment of increasing the temperature above LCST (all samples had a 60 mm2 surface area).

First, the brushes with a length sufficient to cap the 30 nm wide pores were selected. Apparently, the brushes obtained after 12 h of polymerization are too long and would permanently close the pore mouths, even above LCST when the polymer chains shrink (see contact angle measurements). Thus, for the loading/release studies, AAO30 after 1 and 6 h of polymerization were chosen. Polymer brushes work as thermally responsive valves that can immediately open and close the pores just by crossing the LCST. The schematic picture of loading and release processes to/from such nanoreservoirs is shown in Figure 3. Those processes were studied using calcein as a model fluorescent compound. Calcein was chosen due to its good water solubility and high fluorescence yield, which is necessary to follow very small amounts of the released compounds. AAO30 samples were loaded with calcein, and the release was followed by fluorimetry. The AAO platforms were immersed into warm calcein solution (30 mM) and left for a few hours in order to let calcein penetrate the pores and reach the equilibrium state. The platforms were then cleaned using cold and hot water. The time-dependent release of calcein from AAO30 platforms is shown in Figure 4. The fluorescence intensity of the solution above the immersed AAO30 gated platforms was followed at room temperature, and no significant increase of the signal was observed, confirming practically no leakage of calcein from the pores. After ca. 600 s, the temperature of the solution was raised to ca. 37 °C, which induced opening of the pores and release of the loaded calcein. Neither the native AAO30 platform (without gating brushes) nor the silicon substrate grafted with PNIPAM brushes (6 h polymerization) showed

grafting surface or limited diffusion of monomer into the pores has been recently the subject of experimental studies and modeling45 and is beyond the scope of this report. Contact angle measurements were subsequently performed to study the thermoresponsive behavior of the PNIPAM polymer brushes on the nanostructured AAO. For the AAO30 sample, after 1 h of polymerization, the contact angle values were found to be 43.8 ± 0.6° at room temperature and 50.1 ± 1.9° at ca. 40 °C, indicating the changes of wetting due to surface reorganization upon increasing the temperature above LCST. Further measurements were performed under water using diiodomethane as a probing solvent in order to ensure stable and uniform thermal conditions during the measurements. Diiodomethane was used since it is a hydrophobic liquid that does not mix with water and is much denser than it. It was shown that, for AAO30 after 1, 6, and 12 h of ATRP, the contact angle values for diiodomethane decreased upon increasing temperature above LCST (in contrast to water, which is polar) from 89.4° ± 1.6° to 88.6° ± 3.9°, from 100.9° ± 1.0° to 95.7° ± 1.5°, and from 131.2° ± 2.6° to 118.6° ± 1.3°, respectively. It is clear that the thicker the brushes were, the more distinct changes of the contact angle were observed. This observation is consistent with a lower contribution of the underlying AAO substrate for wetting of the thicker brushes. It has been also reported that the thicker PNIPAM layer with a higher molecular weight induces a larger conformational change and consequently a more pronounced wettability transition.46,47 3.2. The Release of Calcein from the Platform of Nanocontainers. AAO30 platforms covered with PNIPAM brushes were investigated further as a gated delivery system. 517

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Figure 5. Cumulative release profiles of calcein at temperatures below and above LCST from AAO platforms gated with PNIPAM brushes and the brushes grafted from silicone (control). The arrow indicates the moment of increasing the temperature above LCST (all samples had a 60 mm2 surface area).

such a behavior. Practically no variations of the fluorescence intensity was observed for both controlled systems that underwent the same loading/release procedure as the gated platforms. It seems that calcein cannot be efficiently embedded in the PNIPAM brushes. Calcein adsorption on AAO and subsequent desorption should be also ruled out as a possible explanation of the release profile for the gated AAO. For both studied AAO30 platforms, the concentration of the released calcein after opening the valves saturates after a few minutes at the same level after some initial fluctuations. This similarity indicates that loading capacities of both systems are practically the same. Such an observation implies also that, in both cases, the pores are closed at lower temperature, preventing leakage of calcein. Thus, even partial capping of the pores with highly hydrated brushes seems to form a barrier for the diffusion of calcein out of the pores. However, it cannot be excluded that depletion in the gated pores observed in the AFM picture (see Figure 2B) is the result of penetration of the AFM tip into the unsupported brushes closing the whole pore. Nevertheless, there is one clear difference between both release profiles. In the case of longer brushes, immediately after raising the temperature, the burst release occurs that manifests in a sudden jump of the fluorescence intensity, indicating an increase of the local calcein concentration in the light path. Such a burst release of calcein may be explained by a significant change of the wetting at the pore outlets and to some extent also inside the pores (the farther from the pore top, the shorter PNIPAM chains should be present due to limited diffusion of the monomers during polymerization). According to the classical Washburn equation for capillaries,48 the liquid penetration depth depends on the wettability of the capillary surface. Thus, the transition toward worse wetting of the pores by water may induce removal of some solution from the pore. There is a smaller difference in wettability of the surface

covered by shorter brushes, so the burst release effect is negligible in this case. Such launching of the solution from the pores can be also rationalized by some residual air at the bottom of the pores, as it was shown recently.49 The release process was further investigated for the other AAO brush-gated platforms. In particular, the platforms with the same theoretical loading capacity (per projected surface area), but with a different pore diameter, were chosen. Because of the relatively good wettability of the pore inner walls (both native ones and covered with PNIPAM brushes) below LCST, the aqueous solutions can penetrate practically the whole pores. The total volume of all pores on the studied AAO30 platform (60 mm2 surface area) was calculated to be 5.6 nL. A similar capacity (5.9 nL) was calculated for the platform with the average pore diameter of 50 nm and 50 μm in thickness (AAO50). Despite the different pore sizes, the mentioned AAO platforms have similar capacities due to different pore-to-pore distances (see the Supporting Information). The PNIPAM brushes were grafted from the AAO50 sample for 4 h to make sure that the outlets are well-sealed (1 h polymerization was enough for 30 nm wide pores). Additionally, the sample with a pore diameter equal to 80 nm (AAO80) and a 3 times bigger capacity (15.1 nL) was investigated. Similar to AAO50, the brushes on AAO80 were grown for 4 h. For those platforms, loading and release processes were examined under the same conditions, and the results are presented in Figure 5. Similar to the previous observations for AAO30, there was no leakage observed for the other gated platforms stored at room temperature (below LCST). As can be judged from the cumulative profiles, the total released amount of calcein was several times larger for AAO50 than for AAO30 despite a very similar volume of the pores. Apparently, AAO50 exhibits a much higher loading capacity (for AAO30, the saturation value was achieved in the studied period) that can be explained by a 518

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Figure 6. Cumulative releasing profiles of calcein from AAO80 platforms gated with PNIPAM brushes. (A, C, E) Temperature below LCST. (B, D) Temperature above LCST.

restarted. Such a pulsatile process of a substance release from AAO80 was followed and is presented in Figure 6. Calcein molecules loaded inside the pores (A) after crossing the LCST start to be released (B). The release process can be stopped on demand just by lowering the temperature down to below the LCST value (C). This process can be repeated many times ((D) release; (E) restrain). The release of substances from the AAO pores can be described by a well-known Higuchi model;52 however, for the first calcein release (after the period of a burst release and before (C)), a linear release profile is observed. This relationship can be assigned to the escape of calcein molecules from just the upper part of the pore. After a while (in Figure 6 after the opening of the pores again), the release profile reaches the classical square-root dependence on time. At this moment, the concentration of calcein around the pore outlets is lower than that during the previous release phase and the diffusion of the molecules from the deeper regions of the pores contribute to the release rate.

larger available volume (not occupied by PNIPAM brushes) in the larger pores compared to the smaller ones. Smaller wettable pores should be filled faster than larger ones, so the kinetic issue cannot explain the observed difference.50 In the initial period of release, the profiles for both studied platforms (AAO50 and AAO80) look very turbulent with a significant “jump” in the early phase of the release. Those fluctuations in the fluorescence signal are the result of a local increase of the calcein concentration in the light path due to quick release of the solution from the pores. Complex equilibria of different forms of calcein (with different fluorescence quantum yields) that stabilize upon a short time might partially contribute as well. That behavior was reproducible here and seemed to be previously observed.51 The mentioned burst release was also observed for AAO30 (after 6 h of polymerization) and can be explained as proposed above. It is clear that, for wider pores, the effect is more pronounced since the interior of the pores should be covered by PNIPAM brushes to a larger extent compared to that of AAO50 obtained after the same polymerization time. The brushes should be also longer. It is due to faster diffusion of the monomer molecules during polymerization into the confined space of the pores. Thus, the change in wettability, as induced by the transition at LCST, affects larger parts of the pores, resulting in an exceptional burst release in the case of AAO80. For the same brush length, but smaller pore diameter, the release was clearly slower after the burst release period. At this regime, the removal of calcein from the pores seems to be controlled by diffusion as the releasing profiles can be approximated by linear relationships with the slope, indicating release rates. Thus, releasing from larger pores (AAO80) is ca. 3 times faster than for smaller pores (AAO50). Such fitting could be applied only at the beginning of the release when the transport of the molecules from deeper regions of the pores may be neglected (see further for details). The release can be realized in both a continuous (Figures 4 and 5) and a pulsatile (Figure 6) manner. It is a very important feature of this system. The release process from AAO platforms can be stopped on demand very rapidly and subsequently

4. CONCLUSIONS A novel platform of nanocontainers built of AAO capped with PNIPAM thermally responsive brushes was prepared, and its performance was characterized. Several platforms with different pore sizes (30, 50, and 80 nm) and brush lengths were studied. It was shown that the PNIPAM brushes at temperatures below LCST effectively seal the outlets of the pores such that practically no leakage of the loaded molecules of a highly fluorescent dye, calcein, from the pores could be observed. However, raising the temperature just above LCST leads to opening of the polymeric valves, and a burst release of the loaded calcein can be observed. It seems that the effect is enhanced by the change of the wettability also inside the pores, which is related to the phase transition of the PNIPAM brushes. After the initial fluctuations, the release proceeds first in a linear fashion and finally reaches a square-root dependence on time that is typical for the Higuchi model. The release can be stopped and restarted again on demand just by applying small changes of temperature, resulting in crossing the LCST of the brushes. Both the loading capacity and the release rate may 519

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be tuned by varying the pore diameter, whereas the grafted brush length is not a very critical parameter, providing it is large enough to cap the pores. The platforms may be repeatedly loaded and unloaded in a controlled manner. What is more, the nanocontainers may be addressed with localized heating devices that should allow high spatial resolution of the release as required for future applications in nano- and microfluidic systems.

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ASSOCIATED CONTENT

* Supporting Information S

AAO platform details and SEM images (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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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 The authors would like to thank the Polish Ministry of Science and Higher Education for the financial support (“Ideas Plus” grant no. IdP2011 000561). The research was carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (contract no. POIG.02.01.00-12-023/08). The authors would like to thank Agnieszka Puciul-Malinowska for ellipsometric measurements.

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REFERENCES

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dx.doi.org/10.1021/cm303930y | Chem. Mater. 2013, 25, 514−520