Wavelength-Selective Disruption and Triggered Release with

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Wavelength-Selective Disruption and Triggered Release with Photolabile Polyelectrolyte Multilayers Patricia Gumbley,† Damla Koylu,† Robert H. Pawle,† Bond Umezuruike,† Elise Spedden,‡ Cristian Staii,‡ and Samuel W. Thomas III*,† †

Department of Chemistry and ‡Department of Physics and Astronomy and Center for Nanoscopic Physics, Tufts University, Medford, Massachusetts 02155, United States S Supporting Information *

ABSTRACT: This paper describes photosensitive polyelectrolyte multilayers (PEMs), constructed using layer-by-layer (LbL) self-assembly, which contain polymers that comprise combinations of photocleavable methacrylate esters and photoinert cationic methacrylate esters. The solubility of these polymers in aqueous base depends upon irradiation with either ultraviolet or visible light. PEMs with dialkylaminocoumarin groups dissolve upon irradiation with visible light, while analogous films with nitrobenzyl ester-substituted polymers dissolve upon irradiation with UV light. Wavelength-selective release of fluorescent polymers as guests is demonstrated using this approach.

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INTRODUCTION Polyelectrolyte multilayers (PEMs) are multilayer films comprising alternating layers of polycations and polyanions; this electrostatic attraction between oppositely charged macromolecules provides the enthalpic driving force for assembly. Fabrication of PEMs is the most common application of the layer-by-layer (LbL) technique, which involves alternating exposure of a substrate to solutions of a polycation and a polyanion by deposition techniques such as submersion, spincasting, or spray-casting.1,2 Advantages of the LbL process to prepare multilayer films include (i) a simple protocol that does not require expensive equipment; (ii) nanoscale control of thickness of the assembled film through the number of deposition steps, (iii) the capability to coat irregularly shaped and small substrates,3,4 and (iv) amenability of incorporating biomolecules such as proteins or nucleic acids into the multilayer film.5,6 Experimental parameters such as pH,7,8 temperature,8,9 hydrolysis,3,10,11 light,12−15 magnetic fields,4 mechanical force,16 or electrical potential17,18 can perturb the properties of PEMs designed to respond to these stimuli, which affect the attractive interactions between layers by changes in the charge,5 amphiphilicity,13 swelling,8 and solubility of films.7,19 Stimuliresponsive LbL films have applications in drug delivery,3,20 selfhealing materials,21 ion selective transport membranes,22 antibacterial surfaces,23 photonic crystal structures,24 and actuators.8 Light has a number of unique features that make it attractive for stimuli-responsive materials: it can be applied remotely, allows for control over intensity and wavelength, and offers spatiotemporal resolution. Photoresponsive polymers have applications in tissue engineering, drug delivery, permselective membranes, and control over the wettability of surfaces.5,25−35 Photoreactive cross-linkers have been incorporated into LbL films to change both the solubility and permeability of the layers.19,36,37 Our group has previously © 2014 American Chemical Society

demonstrated photoresponsive PEMs by incorporating orthonitrobenzyl esters (NBEs) into cationic polymers that change their sign of charge upon photolysis.12,38 Photocleavable groups are useful tools in both synthetic chemistry and biological applications to “cage” functional groups, which irradiation and cleavage of the photolabile moiety deprotect. These groups are becoming increasingly popular in the design of functional polymeric materials because they enable convenient design of polymers with rationally designed light-responsive properties. NBE groups, which reveal carboxylic acids upon photolysis, are among the most commonly used photocleavable groups in design of such materials.39−41 The cleavage of NBE groups is therefore a facile way to impart negative charge onto a material using light.42,43 Coumarin derivatives, which can undergo photosolvolysis reactions or [2 + 2] photochemical cycloadditions, comprise another useful family of photoreactive groups.44 The photodimerization of coumarins has been used to change the solubility of films and micelles reversibly.19,45 In particular, N,N-dialkylaminocoumarins, although less frequently used than nitrobenzyl groups, are important photocleavable protecting groups, especially in biological applications, because visible light can cause their photolysis without the need for multiphoton processes.46−48 One of the unique features of light-responsive materials is wavelength selectivity: based on the chemical structure of photosensitive groups, different wavelengths of light can yield different responses.49 This is often the case with photochromic materials, where two wavelengths of light induce interconversion between two structures. In a conceptually different approach, a combination of photocleavable groups that respond Received: December 3, 2013 Revised: January 9, 2014 Published: January 24, 2014 1450

dx.doi.org/10.1021/cm403979p | Chem. Mater. 2014, 26, 1450−1456

Chemistry of Materials

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charge−charge interactions with anionic PSS. A similar design was recently reported to yield multiresponsive micelles.56 As shown in Scheme 2, esterification of methacryloyl chloride with the appropriate alcohol53 yielded either nitrobenzyl

to different wavelengths of light can yield multifunctional materials that show wavelength-selective responses. Structurally different nitrobenzyl groups have been used to alter the swelling and degradation of gels with irradiation at different wavelengths based on their different reactivities.50−52 Multifunctional surfaces consisting of photocaged organosilanes with up to four different wavelength-selective, structurally different photolabile groups have previously been shown.53 Additionally, the cleavage of coumarins induced by visible light has been used for selective removal in the presence of NBE groups for peptide uncaging,54 surface patterning,46 and directed particle deposition.55 Herein we describe a combination of this spectral differentiation with the spatial segregation that the LbL technique enables to yield wavelength-selective dissolution of PEMs, as well as wavelength-selective release of fluorescent guest materials.

Scheme 2. Synthesis of Photolabile Polymers P1 (NBE Side Chains) and P2 (Dialkylaminocoumarin Side Chains)a

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RESULTS AND DISCUSSION Previously, our group reported a polycation, prepared by ringopening metathesis polymerization (ROMP), with photocleavable cationic moieties that became polyanionic upon irradiation. This photoinduced charge-shifting behavior resulted in dissolution of UV-irradiated PEMs containing this polycation.12,38 To facilitate straightforward incorporation of different photocleavable groups, scale-up of material, and tuning of polymer properties through copolymerization, we designed a different class of polymerspoly(methacrylate)s that comprise cationic side chains and photocleavable esters on different monomers. Scheme 1 shows the design of the materials in this Scheme 1. Design of Photosensitive Polycations That Have Reduced Net Charge upon Irradiation with Either Visible (Coumarinyl Groups) or UV (Coumarinyl or Nitrobenzyl Groups) Lighta

a

In both polymers, the molar ratio of the two comonomers was ∼1:1. P1 and P2 are random copolymers.

a

methacrylate 1 or diethylaminocoumarinyl (DEACM) methacrylate 2.37,57 AIBN-initiated free radical polymerization of either 1 or 2 with DMAEMA in toluene followed by precipitation into hexanes, and collection by filtration gave random copolymers with the general structures of P1 and P2. We used molar feed ratios of DMAEMA to photolabile monomer (x:y) of 1:1; decreasing the relative amount of photolabile monomer resulted in polymer films that resisted complete dissolution upon photolysis (vide infra). As illustrated in Figure 1, 1H NMR spectra of the resultant polymers revealed their ratio of monomer incorporation through the ratio of integrals for the resonances due to benzylic protons (labeled either “b” or “c”) in the photoreactive side chains to integrals for the resonances labeled “a” for DMAEMA side chains as shown in Figure 1; the maximum observed deviation from the expected 1:1 ratio of incorporation was 1:1.1 (x:y). Figure 1c shows solution phase UV/vis spectra of P1 and P2: consistent with our experimental design, the absorbance of dialkylaminocoumarin-containing P2 stretches into the visible, while the absorbance of nitrobenzyl-containing P1 is confined to the UV. Analysis of these polymers by gel permeation chromatography (GPC), using THF with 2% (v/v) triethylamine as the mobile phase, gave distributions of molecular weights of Mn = 15 kDa and Mw = 45 kDa for P1 and Mn = 7.1 kDa and Mw = 11 kDa for P2, relative to polystyrene standards with narrow distributions of molecular weight. Under the free-radical polymerization conditions used here, the presence of the

All polymers are random copolymers.

paper, which are random copolymers of 2-(N,Ndimethylamino)ethylmethacrylate (DMAEMA)which contains the cationic group and provides favorable charge−charge interactions for PEM formation with the polyanion, poly(styrene sulfonate) (PSS)and photolabile methacrylates with either NBE or dialkylaminocoumarin side chains. Photolysis of the photoresponsive groups deprotects carboxylic acid groups, which are anionic in weak aqueous base. The results from photolysis and rinsing in aqueous base using our structural design of photolabile polymers is therefore conversion of the cationic polymers into zwitterionic polymers that have weak 1451



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either DMF or water and (2) a 0.2% (w/v) solution of PSS in aqueous 0.1 M NaCl, with intermediate rinsing steps using deionized water, yielded PEMs that we monitored by UV/vis spectrophotometry. The shapes of the absorbance spectra of the PEMs showed that the layer-by-layer procedure yielded films that included the photoreactive groups (Figure 2): P1/

Figure 2. Light-induced disruption of photolabile PEMs: (a) UV light irradiation and rinsing of 39 bilayers of P1/PSS; (b) visible light irradiation and rinsing of 45 bilayers of P2/PSS. Figure 1. a and b: 1H NMR spectra of polymers P1 and P2 in CDCl3. Integrals of indicated protons yield the ratio of incorporation of DMAEMA and 1 or 2. The letter “S” indicates residual CHCl3, while * indicates CH2Cl2. c: Normalized electronic absorbance spectra of P1 and P2 in CH2Cl2.

PSS films had a local maximum at 262 nm (NBE chromophore) and a shoulder at ∼220 nm (arylsulfonate group), while P2/PSS films had maxima attributable to the DEACM chromophore at 382 and 250 nm. As shown in the Supporting Information, absorbance values of the films increased linearly with respect to the number of deposition steps. The linear relationship between absorbance and the number of layers indicates that there is not complete reorganization of the polyelectrolytes during film assembly.58 Profilometry and AFM measurements indicated that the thicknesses of these films were 1−2 nm/bilayer. Figure 3a shows an AFM image of a 15-bilayer P2/PSS film, which has an RMS roughness of 1.8 nm and a thickness of 14 nm. Our films often showed small areas of significantly thicker material (see Supporting Information Figure S7 for an example, which has an

nitro group in nitrobenzyl methacrylate had no obvious detrimental effect on the polymerization; increasing the relative loading of 1 to DMAEMA from 1:1 to 3:1 gave a similar distribution of molecular weights (Mn = 22 kDa, Mw = 41 kDa). Commercially available PSS was used together with either of the photoresponsive polymers P1 or P2 to fabricate photoreactive PEMs. One equivalent of p-toluenesulfonic acid (PTSA) was used to protonate the tertiary amines in the purified photoreactive copolymers providing the positive charge for LbL self-assembly. Alternating immersion of plasma cleaned quartz slides into (1) a 0.2% (w/v) solution of P1 or P2 in 1452



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Information). Films of P1/PSS either kept in the dark or heated to 60 °C and subsequently rinsed with sodium bicarbonate showed no change in absorbance, which indicates that irradiation with light is required for the films to be soluble in the aqueous bicarbonate. In addition, although polymers with a lower ratio of photoresponsive monomer to cationic monomer (incorporation of 1:DMAEMA of 1:3) assembled into PEMs, UV irradiation for extended periods followed by rinsing did not result in complete removal of these films from the substrate; this result demonstrates that disruption of the attractive interpolymer forces in the film requires a sufficient magnitude of photoinduced change in polymer structure. In analogous fashion, irradiation with wavelengths of light greater than 400 nm for 30 min disrupted films of P2/PSS. After washing the irradiated film for 10 min with aqueous base, absorbance spectrophotometry (Figure 2b) and AFM images (Figure 3b) showed that the PEM film was removed from the substrate. Films of P1/PSS did not react or become soluble in aqueous NaHCO3 upon irradiation with λ > 400 nm, suggesting that control of the wavelengths of light used for photolysis can yield selective disassembly of the films. We combined this difference in wavelength sensitivity of NBE and DEACM groups with the spatial differentiation that the layer-by-layer technique offers to sequentially disrupt the two classes of photolabile PEMs deposited on the same substrate (Figure 4). We performed three sets of LbL

Figure 3. Atomic force microscopy images of a 15-bilayer film of P2/ PSS before (a) and after (b) irradiation with visible light and rinsing with 0.1 M NaHCO3.

RMS roughness of 4.7 nm for a 40 bilayer-thick film of P1/ PSS). Potential explanations for this observation could be due to the relatively hydrophobic nature of polycations, such that rinsing with water may not always remove them from the surface completely. This roughness was not significant enough to interfere with analysis of the films by UV/vis spectrophotometry. Irradiation of photosensitive PEMs containing P1 or P2 with wavelengths of light that the photolabile groups absorb leads to their cleavage. When immersed in aqueous base, the resulting negatively charged carboxylate groups on the polymer backbone lead to increased electrostatic repulsion between the photoproduct polymers and the negatively charged PSS, which results in removal of the film from the substrate. After irradiation of P1/PSS films with a 200 W Hg/Xe lamp at λ > 295 nm for 30 min, the UV/vis spectrum of the film shows a bathochromic shift of the absorbance due to the NBE group, which is indicative of NBE photolysis,28 while the position of the peak associated with PSS at 220 nm did not change. Immersion of the slides in 0.1 M aqueous NaHCO3 for 10 min resulted in removal of the films from the substrate as shown by the UV−vis spectrum (Figure 2a) and AFM (Supporting

Figure 4. Demonstration of wavelength-selective disruption using sequential deposition of (i) 31 bilayers P1/PSS, (ii) 2 bilayers of PDAC/PSS and (iii) 50 bilayers of P2/PSS followed by sequential irradiation and rinse steps with (iv) λ > 400 nm and v) λ > 295 nm.

sequentially on the same substrate: (i) 31 bilayers of P1/PSS, (ii) 2 photoinert bilayers of polydiallyldimethylammonium chloride (PDAC) and PSS, and (iii) 50 bilayers of P2/PSS. Layers containing P2/PSS were disrupted by irradiation with λ > 400 nm light while the layers of P1/PSS remained on the substrate. We designed the films for layers containing P2 to be removed first because the UV irradiation conditions necessary to cleave the NBE groups of P1 also cleave the DEACM groups. Therefore, a reverse configuration in which the layers of P1/PSS are deposited after layers of P2/PSS could yield disruption of all layers upon UV irradiation instead of only one set of layers. Figure 4 shows that the absorbance of a film with 1453



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Figure 5. Wavelength-selective release of dye-substituted polyethyleneimine (PEI) polymers from photolabile PEM films. Dye release was monitored by fluorescence measurements of the aqueous base used to rinse the films upon irradiation. Visible light (405 nm, 45 min duration) released rhodamine-substituted polymers selectively due to the photolysis of coumarin groups in P2/PSS films; subsequent treatment with UV light (10 min duration) released Alexa 488-substituted polymers from P1/PSS films. The procedure was more selective when the two types of films were on different substrates (a) than when they were on the same substrate (b). At least one bilayer of PDAC/PSS was present between each set of different chromophore-labeled polymers. For clarity, the schematics of film composition and dye release omit the presence of PEM film on the “bottom” side of the planar quartz substrate.

both samples with NaHCO3. Reaction progress was monitored by absorbance spectra of the films. This procedure resulted in highly wavelength-selective release of dye-substituted PEI. On the basis of fluorescence spectroscopy of the soluble extracts from the films, after visible irradiation, ∼85% of the rhodamine in the sample was released, but less than 10% of the Alexa 488 dye in the sample was released. Subsequent irradiation with UV light resulted in release of all remaining material from the slides, which contained over 90% of the original amount of Alexa 488 and approximately 15% of the original amount of rhodamine, which was not removed in the initial visible irradiation step. Integrating the two sets of dyes and photolabile groups into the same sample, relying only upon the deposition sequence to separate them, also yielded wavelength-selective release (Figure 5b), albeit with less selectivity, presumably because of interlayer diffusion of polymer chains. On the basis of the low absorbances of the dyes in the films ( 295 nm and rinsing removed the remaining P1/PSS layers from the substrate. When we did not include PDAC/PSS to act as a barrier between the two groups of photolabile materials, irradiation with visible light and rinsing resulted in significant loss of the P1/PSS material from the substrate, highlighting that polymer interpenetration between layers does occur to a degree in these films, which affects the spatial separation of different groups of multilayer films deposited on the same substrate. As a proof-of-concept demonstration of wavelength-selective release of guests from these films, we prepared PEM films that included poly(ethyleneimine) (PEI) labeled with different dye pendants as cationic layers in the structures of films and monitored their light-induced release into aqueous solution by fluorescence spectroscopy. We used cationic PEI substituted with either Alexa 488 (Alexa-PEI) or rhodamine B (Rhodamine-PEI) as model guests for release into solution upon photolytic disruption of PEM films. In the first experiment (Figure 5a), we prepared two separate PEM film samples: (1) 9 bilayers of P1/PSS followed by 1 bilayer of PDAC/PSS and 3 bilayers of Alexa-PEI/PSS and (2) 32 bilayers of P2/PSS followed by 2 bilayers of PDAC/PSS and 2 bilayers of Rhodamine-PEI/PSS. We irradiated the two samples at the same time under identical conditions in two steps: (1) irradiation with visible light (405 nm, 45 min) followed by rinsing both samples with NaHCO3 and then (2) irradiation with UV (>295 nm, 10 min) light followed by again rinsing

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CONCLUSION We have developed a general design platform for photolabile polyelectrolyte multilayer films that are amenable to fabrication by the versatile layer-by-layer self-assembly approach. The combination of spatial segregation enabled by LbL and wavelength selectivity controlled by the chemical structures of photocleavable groups allows selectivity in the release of fluorescent guests from these materials. Although the current approach uses short wavelength light for release, recent developments using two-photon photochemistry of these 1454



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room temperature. The labeled polymer was purified by dialysis (3.5 K MWCO) in DI water for a minimum of three days. LbL Assembly. Quartz slides (Advalue Technologies) were cleaned with lint-free cloths to remove debris and then plasma cleaned with a Harrick Plasma Cleaner (PDC-32G) for 2 min. Films were assembled using a Midas III Automated Slide Stainer. Slides were first immersed in a positively charged polymer solution (0.2 wt % in 150 mL of DMF with 1 equiv of p-toluenesulfonic acid) and then rinsed in DI water (1000 mL/min) for 5 min. Next, the slides were immersed in the anionic polymer solution (poly(sodium 4-styrene sulfonate)) in 0.1 M NaCl (0.2 wt %)) for 10 min and then rinsed in DI water (1000 mL/min) for 5 min. This process was repeated until the desired number of layers was added. Alexa-PEI and RhodaminePEI in 0.1 M NaCl were added to the films in the same manner. Irradiation Experiments. A 200 W Hg/Xe (Newport-Oriel) lamp with a condensing lens and electronic shutter was used to irradiate the layer-by-layer self-assembled films. For P2 cleavage, a 400 nm longpass filter (11 mW/cm2 for light between 400 and 515 nm that the coumarin can absorb) or a 405 nm interference filter (1.9 mW/cm2) was used. P1 cleavage was accomplished using a 295 nm long-pass filter (26 mW/cm2 of wavelengths between 295 and 400 nm that the nitrobenzyl ester can absorb). After irradiation, films were rinsed in 0.1 M NaHCO3 for 10 min to remove the photocleaved layers and then dried with compressed air.

types of photocleavable groups gives this approach promise in the release of bioactive substances. Future work in our laboratory is focusing on the incorporation of additional photosensitive moieties for the wavelength-selective release of more than two guests by using other wavelengths of light, expanding the types of guest molecules that are released, and developing materials with new types of wavelength-selective responses by combining wavelength selectivity with other approaches to physical or chemical segregation.

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EXPERIMENTAL SECTION

General Considerations. All chemicals were purchased and used as received from commercial sources except for methacryloyl chloride, which was distilled immediately before use. The following compounds were synthesized according to previously reported procedures: 7((diethylamino)coumarin-4-yl)methyl methacrylate (1)37 and 2-nitrobenzyl methacrylate (2).59 Dry solvent was obtained from Innovative Technologies PureSolv 400 solvent purifier. All reactions were run under standard air-free techniques with magnetic stirring. Silica gel (230−400 mesh) was used as the stationary phase for flash chromatography. NMR spectra were obtained on a Bruker DPX-300 or Bruker Avance III 500 spectrometer. Chemical shifts are reported relative to the solvent used (7.27 for CHCl3). Molecular weight distribution measurements of the polymers were conducted with a Shimadzu Gel Permeation Chromatography (GPC) system equipped with a TOSOH TSKgel GMHhr-M mixed-bed column and guard column (5 μm), equipped with both UV and refractive index detectors. THF with 2% triethylamine (v/v) was the GPC mobile phase and was eluted at 0.75 mL/min. The column was calibrated with low polydispersity poly(styrene) standards (TOSOH, PSt Quick Kit). Electronic absorbance spectra were obtained with a Varian Cary 100 UV−vis spectrometer in double beam mode using a quartz slide or solventcontaining quartz cuvette (NSG Precision Cells) for background subtraction spectra. Fluorescence emission spectra were obtained using a Cary Eclipse spectrometer. Film thicknesses were obtained with a Veeco Dektak 6 M stylus profilometer. Topographical data was acquired using an Asylum Research MFP-3D Atomic Force Microscope (Santa Barbara, CA). Scanning was performed in contact mode with a silicone cantilever, nominal spring constant k = 2 N/m. Scans were performed over 5 × 5 um regions at a scan rate of 1 Hz. RMS roughness values were calculated from the AFM scan data using Asylum Research MFP3D software running in Igor Pro. General Polymerization Procedure. 1 or 2, 2-(dimethylamino)ethyl methacrylate, and 1% (w/w) azobisisobutyronitrile (AIBN) were dissolved in toluene and sparged with argon for a minimum of 15 min. The reaction was then heated to 65 °C and stirred overnight. The toluene was removed in vacuo, and the polymer was dissolved in chloroform and precipitated into hexanes. P1: 1H NMR (300 MHz, CDCl3): δ 8.14−7.97 (1H), 7.81−7.58 (2H), 7.57−7.42 (1H), 5.50−5.20 (2H), 4.18−3.87 (2H), 2.64−2.39 (2H), 2.35−2.14 (6H), 2.11−1.67 (4H), 1.17−0.61 (6H). Mn = 15 kDa, Mw = 45 kDa (GPC). P2: 1H NMR (300 MHz, CDCl3): δ 7.41−7.29 (1H), 6.68−6.53 (1H), 6.52−6.36 (1H), 6.18−6.04 (1H), 5.18−4.91 (2H), 4.14−3.92 (2H), 3.51−3.26 (4H), 2.62−2.42 (2H), 2.33−2.12 (6H), 2.11−1.70 (4H), 1.30−0.69 (12H). Mn = 7.1 kDa, Mw = 11 kDa (GPC). Alexa-PEI Synthesis. Polyethylenimine (11 mg) was dissolved in 2 mL of 0.1 M NaHCO3. A total of 1 mg of Alexa Fluor 488 carboxylic acid succinimidyl ester was dissolved in 0.1 mL of DMSO and added to the polymer solution. The reaction was stirred for 1 h at room temperature. The labeled polymer was purified by dialysis (3.5 K MWCO) in DI water for a minimum of three days. Rhodamine-PEI Synthesis. Polyethylenimine (300 mg) was dissolved in 20 mL of 0.1 M NaHCO3 buffer (pH = 9). A total of 38 mg of rhodamine B isothiocyanate was dissolved in 5 mL of DMSO and added to the polymer solution. The reaction was stirred for 1 h at

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ASSOCIATED CONTENT

S Supporting Information *

Additional AFM images, results of rinsing unirradiated films with aqueous base, additional demonstrations of wavelength selectivity, and growth of films as a function of layers deposited. 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 are grateful to the National Science Foundation (DMR-1151385 and CBET-1067093) for financial support of this research.

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REFERENCES

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