Spatiotemporal Photopatterning on Polycarbonate Surface through

Letter pubs.acs.org/macroletters

Spatiotemporal Photopatterning on Polycarbonate Surface through Visible Light Responsive Polymer Bound DASA Compounds Sukhdeep Singh,*,† Karin Friedel,† Marcel Himmerlich,‡ Yong Lei,§ Gregor Schlingloff,† and Andreas Schober† †

Department of Nanobiosystem Technology, Institute of Micro- and Nanotechnologies MacroNano, Institute of Chemistry and Biotechnology, Technische Universität Ilmenau, Prof.-Schmidt-Str. 26/Heliosbau, 98693 Ilmenau, Germany ‡ Institute of Physics and Institute of Micro- und Nanotechnologies, Technische Universität Ilmenau, PF 100565, 98684 Ilmenau, Germany § Department of 3D Nanostructuring, Institute for Physics and IMN MacroNano (ZIK), Technische Universität Ilmenau, Prof. Schmidt Str. 26, 98693 Ilmenau, Germany S Supporting Information *

ABSTRACT: Besides interesting applications in drug delivery, photoresponsive molecules have great potential to serve as an efficient basis for postfunctionalization photopatterning of polymer surfaces. To the best of our knowledge, only UV light sources have been exploited as a photoinducer for creating patterned templates with or without hydrogels. In this work, we present a practically facile method for grafting visible light responsive donor−acceptor stenhouse adducts (DASAs) on aminofunctionalized polycarbonate surfaces. DASA grafted surfaces have shown excellent lithographic performance using visible light. The functionalized surfaces exhibit significant changes of their physical properties after being illuminated with visible light. By using suitable masks, well-defined patterns can be replicated with high precision and resolution. Since the DASA ligand synthesis and surface functionalization is not cumbersome, this method may serve as a facile protocol for obtaining photopatterned polymer surfaces for various applications.

A

UV light. Most of these photochromic molecules require the use of UV light for photoisomerization, which limits their use in biological systems due to damaging effects of UV radiation,9 especially for cells. However, by modifying the scaffold of a chromophore, a red-shift in azobenzene-based dyes has been reported.12 Very recently, the group of J. Read de Alaniz has introduced donor−acceptor stenhouse adducts (DASA) as efficient organic photochromic compounds that have the ability to change conformation during exposure to visible light.13 In this case, the open chain colored conformer 1 is hydrophobic, and after ring close under visible light, it turns to a colorless cyclic hydrophilic form 2 (see Figure 1). This transformation makes them ideal candidates for further investigating their ability to change wetting properties and lithographic performance when they are attached to a polymer surface.

mong the different classes of intelligent or smart materials, bioresponsive materials are considered as most challenging.1 Because apart from the tunable nature of the material, they require to possess biological and chemical cues that are responsible for bioactivity.2 During the last decades, a large number of research activities were documented in literatures that reflect the trust of the scientific community to develop such materials.3 The majority of stimuli for bioresponsive materials can be classified in two categories: (i) Invasive stimuli, for example, pH,4 temperature,5 ionic strength,6 and shear stress;7 (ii) Noninvasive stimuli, for example, magnetic or electrical6,8 light irradiation.9 A stimulation by light irradiation is ideal where the purpose is to achieve spatial and temporal resolutions in a noninvasive manner.10 The chemistry of small molecules, where the conformational changes upon photostimulation are occurring at a the molecular level, shall play a significant role in tuning the material properties.11 Azobenzenes, spiropyranes, and diarylethenes are among the most popular classes of organic photosensitive molecules that show excellent performance when exposed to © XXXX American Chemical Society

Received: September 7, 2015 Accepted: November 1, 2015

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DOI: 10.1021/acsmacrolett.5b00653 ACS Macro Lett. 2015, 4, 1273−1277

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ACS Macro Letters

Figure 1. Photoswitching of the triene derivative of Meldrum’s acid to its cyclopentenone isomer in solution.

In this contribution, we report a general method for preparing photoresponsive polycarbonate surfaces and their behavior upon exposure to visible light. A simple protocol has been developed to functionalize the polycarbonate surface with the photoresponsive triene derivative of Maldrum’s acid. Spatially resolved patterns up to submicron scales were generated via photolithography. Easy functionalization steps by using intrinsic functional groups of polycarbonate make this method versatile. We believe that, due to the simplicity of the material and chemistry, this method can serve as an efficient protocol for realizing photopatterned surfaces. The conjugated furfural derivative of Meldrum’s acid 6 (see Scheme 1) is known to react with secondary amines in solution and to form an intensely colored triene derivative 1.14 However, the behavior of such ligands when react with amines on largely functionalized polymer surfaces is not well-known. The chemical strategy includes (i) the surface activation of the substrate, (ii) ligand design for conjugation, and (iii) photoactivation of ligands on the polymer surface. We have chosen polycarbonate as a substrate because it is recognized as an ideal material for disposable biomedical microdevices due to its good biocompatibility, low manufacturing cost, facile membrane formation, and thermoformability. Recently, we have reported the reactivity of primary amines with polycarbonate bulk material and introduced a method for amino surface functionalization using terminal diamines.15 Despite the above-mentioned advantages of polycarbonate, its surface modification is challenging due to a facile solubility of the material in most organic solvents.

Figure 2. (a) FT-IR spectra of polycarbonate at different stages of functionalization. Irradiation time was 18 h. Bottom: Kinetics of the light-induced photoswitching at the polymer surface in timedependent measurements of the UV−vis absorption (b) and the surface contact angle of water (c).

One possibility is to perform a surface plasma treatment of the polycarbonate and a subsequent reaction with APTES.16 Another possibility is the chemical functionalization of polycarbonate using reactive species in limited choice of solvents. To develop a simple and widely applicable method, we have chosen the second approach due to its higher practicability in standard chemical laboratories. Initially, we have tested various kinds of terminal diamines and concluded that under mild conditions, such as in 60 °C warm ethanol, it is possible to

Scheme 1. Chemical Strategy for DASA Functionalization of Polycarbonate Foils Using the Furfural Conjugate of Meldrum’s Acid

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DOI: 10.1021/acsmacrolett.5b00653 ACS Macro Lett. 2015, 4, 1273−1277

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Figure 3. O 1s, N 1s, and C 1s XPS core level spectra of functionalized polycarbonate surfaces before (blue) and after illumination for 18 h (red), including a deconvolution of the spectra into their main chemical components.

ions NH4+ (18 0.0 u), CH2N+ (28.0 u), CH4N+ (30.0 u), C2H4N+ (42.0 u), C2H6N+ (44.0 u), and C3H6N+ (56.0 u) that indicate the presence of amino functionalization. However, for sample 7, the attachment of the photoswitchable ligand is evident by appearance of new signals around 325 u in positive ion spectra as well as 71, 89, 135, 149, 191.1, and 223 u in negative ion spectra. Similarly, ATR-FTIR measurements of 5 reveal the existence of N−H stretching vibrations at 3275 cm−1 (see Figure 2a). Sample 7 exhibits the appearance of a new peak at 1603 cm−1 corresponding to conjugated olefinic functional groups due to the triene derivative. Moreover, due to the change in chemical nature of the surface, the wettability altered as the contact angle of water droplets changed from 83° (polycarbonate 3) to 45° after amine functionalization 5. An obvious increase in hydrophobicity was noticed after substrate 5 had been functionalized with the aliphatic triene derivative 7 (water contact angle 74°). In order to test the possibility of photoisomerization of the covalently bound hydrophobic triene 7 to its most probable zwitter ionic hydrophilic pentenone derivative 8, we exposed the samples to visible light (Osram crystal clear 100 light bulb, 100 W, 900 lm). For monitoring the changes in physiochemical properties, time-dependent UV−vis transmission spectra (Figure 2b) and changes in water contact angle (Figure 2c) were measured. A gradual decrease of the absorption feature around 550 nm is observed during ongoing illumination. In parallel, the water contact angle measurements have also shown a continuous increase of hydrophilicity as expected due to the formation of the zwitter ionic pentanone derivative 8 of Meldrum’s acid. Unlike the reversible behavior of DASA compounds in solution, we did not observe any reversibility of this process on polymer bound analogues. The irreversibility of the polymer bound DASA molecules under ambient conditions could be related to hydrophilic interactions between the polar cyclopentenone and the polyamine on the surface. In order to characterize the stability of the DASA compounds under these experimental conditions, we have investigated the 1H NMR signal of ligand 1 after 56 h of illumination. The resultant solution has shown the formation of polar cyclopentenone derivative 2, but no additional byproducts or decomposition fragments have been observed (see Supporting Information).

conjugate the primary aliphatic amines with the carbonate moiety of the polycarbonate via urethane linkage. This treatment leads to the formation of diamine-based dangling amino groups as free handle for further attachment of specific ligands. A performed fluorescent labeling with dansyl chloride has confirmed the attachment of amine groups at the polymer surface. After testing different reaction conditions for terminal and polymeric amines, it is found that the branched polyethylenimine has served the purpose in a very good way because of the observed high density and uniformity of the amino functionalization. Further, this procedure is suitable with respect to the negligible damage caused to the polymer surface as investigated by scanning electron microscopy (SEM). Additionally, we have tested the nitration/reduction method as an alternative procedure and found its unsuitability as it deteriorates the quality of the thin film samples.17 After standardizing the amino-functionalization of the polycarbonate surface, the next step is conjugation of the triene derivative of the Meldrum’s acid 1 on the surface. The intense color of the triene derivative 1 is an added advantage for observing the reaction performance just by noticing the appearance of a purple color on the polymer surface (Scheme 1). For this purpose, we have prepared a precursor ligand furylidene-Meldrum acid 6 by reacting 2-furfural with Meldrum’s acid, as reported by Read de Alaniz et al.14 For synthesizing the DASA ligand on the surface of amine functionalized polycarbonate, various reaction conditions were tested. Treating amino-functionalized 1 cm2 sample with 4 mL of 2% ethanolic solution of furylidene-Meldrum’s acid 6 at room temperature for 15 min generated DASA triene 7 on the surface, as proven by nuclear magnetic resonance (NMR) measurements, which are included in the Supporting Information. The surface reaction can be very easily observed by noticing the appearance of the purple color on the amino functionalized polymer film 5. In addition to the optical inspection various physical and chemical properties of the surface have changed after incorporation of the ligand. A comparison of time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis of 3, 5, and 7 clearly indicates the appearance of different fragments under similar bombardment conditions (for detail fragmentation spectra please see Supporting Information). Sample 5 has shown the secondary 1275

DOI: 10.1021/acsmacrolett.5b00653 ACS Macro Lett. 2015, 4, 1273−1277

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linker group. A comparison of time-of-flight secondary ion mass spectrometry (ToF-SIMS) measurements of samples before and after light exposure also indicates changes in the fragmentation patterns. SIMS scans with negative ions have shown the appearance of new secondary ions at 55 (C3H4O), 57, 65, 69, and 97 u for the surface that had been illuminated for 18 h. Similarly, the positive ion mode measurements of the illuminated surface has shown the appearance of secondary ion fragments at 86, 91, 148, 182, and 233 u (see Supporting Information for detailed spectra). The surface functionalization of the polycarbonate as well as the light-assisted changes in chemical composition have additionally been investigated by X-ray photoelectron spectroscopy (XPS) in grazing emission (takeoff angle 60° with respect to the surface normal) using MgKα radiation. A comparison of the N 1s, C 1s, and O 1s core level spectra of the functional layer prior and after illumination is shown in Figure 3. Distinct differences in the spectra can be identified, which are indicative for structural changes of the functional molecules at the surface upon illumination. The relative intensity of the N 1s signal is reduced by ∼30%, while the oxygen and carbon total intensities based on analysis of peak area are not noticeably changed. However, the peak shape of the O 1s and C 1s spectra indicate important structural changes. For simplicity of the data analysis, the C 1s spectra were fitted by three chemical states accounting for C−C(H), C−O(H), and CO bonds with increasing binding energy (BE). Two states were used for the O 1s spectra where the component with lower BE is attributed to OC bonds and the other one to O(H)−C groups at the surface. We want to note that the actual structures of the attached functional molecules (7 and 8 in Scheme 1) include several carbon and oxygen bonds with slightly varying BE, which cannot be resolved in this experiment. Therefore, we use this simple approach for fitting the data in order to document the main changes in core level spectral shape. The O 1s spectrum is found to be narrower after illumination, reducing the width at the high BE range. We attribute this change to the variation of the molecule by transformation of the conjugated triene backside chain into a cyclopentenone ring structure, while the initial oxygen of the −OH group is transferred to a different molecule site in the cyclopentenone ring structure, which exhibits lower binding energy. At the same time, the relative amount of C atoms with lower BE is enhanced and C− O(H) contributions are reduced. We account this change to the structural rearrangement of the molecule, including hydrogen atom and charge transfer taking place at different molecule sites during transformation. These effects also influence the configuration of the nitrogen linker atom, documented by the increase of 0.2 eV in the N 1s binding energy as well as the more effective attenuation of electrons from the N 1s state resulting in lower total N 1s intensity after illumination. Consequently, the observed synchronous trends in chemical states support the model of light-induced structural modification of the functional surface molecules, as shown in Scheme 1. Due to significant physio-chemical changes and the distinct difference in color comparing illuminated and nonilluminated regions, the substrate 7 becomes a good candidate to test patterning by photolithography. Unlike photocleavable lithographic methods, the visible light-induced rearrangement of the molecules on the surface could generate desired patterns without releasing additional chemical species. Several tests of various foil and chromium masks revealed that a complete

Figure 4. Different patterns generated by a photolithographic process using visible light exposure for 18 h: (A) line 19.44 μm, space 20.27 μm (original mask: line 20 μm; space 20 μm); (B) line 39.69 μm, space 40.53 μm (original mask: line 40 μm; space 40 μm); (C) line 59.96 μm, space 59.96 μm (original mask: line 60 μm; space 60 μm); (D) line 99.65 μm, space 100.49 μm (original mask: line 100 μm; space 100 μm). (E) Structure obtained using a graded photolithograpic test mask showing performance from 3 μm to submicron length (inserted numbers represent the line width in μm). (F) Color brightness line profiles of the 20 μm pattern in A. (G) Cross-sectional micrograph of a functional foil showing a homogeneous surface functionalization and no penetration into the bulk.

In addition, ATR-IR spectra of the sample before and after light exposure points toward formation of the expected isomer 8 on the surface. The appearance of a medium peak at 1663 cm−1 revealed the formation of new CC bonds in the cyclopentenone conformation for the illuminated sample. Another noticeable change is the disappearance of a shoulder at 1347 cm−1 that occurs due to the change of hybridization from sp2 to sp3 of the carbon atom neighboring the nitrogen 1276

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transfer of the patterns to substrate 7 is feasible using a visible light exposure of 18 h. In order to check the spatial resolution, we have used different masks with line widths of a few micrometers (Figure 4). The width of the resulting colored stripes on the polycarbonate foil is consistent with the dimensions of the photomask structures (Figure 4A−D). The line profile measured across the stripe pattern with 20 μm periodicity exhibits sharp edges below 2 μm (Figure 4F). It is worth mentioning that patterns finer than 100 μm are better transferable using chromium masks as compared to foil masks. By using a photolithography test mask we have furthermore demonstrated the possibility of developing patterns with structures smaller than 1 μm (Figure 4E). In addition, we have tested the penetration of the photoswitchable ligand into the polymer bulk. The cross-sectional micrograph (Figure 4G) of the thin functionalized polymer foil 7 indicates that the triene ligand is only attached at the surface of the polymer. In conclusion, we have introduced a new method for preparing a photosensitive polymer surface that has shown significant change in their physical and chemical properties when exposed to visible light. For biomedical applications, we consider this photoisomerization as a more suitable approach compared to photocleavage where released species can interfere with the system. Since simple chemical procedures are involved in this protocol, we consider this process chain to be more general and applicable. Moreover, the efficiency of the patterning process in terms of exposure time can be improved by preforming the experiments in ethanol. Further applications of these surfaces for BioLithoMorphie applications to mimic tissue18 and ultimately body on a chip systems19 are under investigation.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.5b00653. Experimental details and additional supporting figures (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +49 3677 693379. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully thank Federal Ministry of Education and Research (BMBF) for providing financial support within the Initiative “Centre for Innovation Competence” Meta-ZIK (BioLithoMorphie: FKZ 03Z1M511). Also, the financial support from Optimi (FKZ: 16SV3701, FKZ 16SV5473), Carl Zeiss FKZ 0563-2.8/399/1, and by the Thuringian Ministry of Culture (Nanozellkulturen, FKZ: B714-09064) is gratefully acknowledged. Additionally, authors thank Prof. Uwe Ritter, Prof. Stefan Krischok, Dr. Yixin Zhang, and Dr. Arne Albrecht for helpful scientific discussions. We thank Jörg Hampl and Martin Baca for providing masks and Katrin Risch and Susann Günther for technical support and ION-TOF GmbH for ToF-SIMS measurements. 1277

DOI: 10.1021/acsmacrolett.5b00653 ACS Macro Lett. 2015, 4, 1273−1277