Patterning of Nanoclays on Positively Charged Self ... - ACS Publications

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Patterning of Nanoclays on Positively Charged Self-Assembled Monolayers via Micromolding in Capillaries Moritz Buhl,† Mark Staniford,‡ Sebastian Lamping,† Martin Körsgen,§ Heinrich F. Arlinghaus,§ Ulrich Kynast,*,‡ and Bart Jan Ravoo*,† †

Organic Chemistry Institute and Center for Soft Nanoscience, Westfälische Wilhelms-Universität Münster, Corrensstrasse 40, 48149 Münster, Germany ‡ Institute for Optical Technologies, Münster University of Applied Sciences, Stegerwaldsstrasse 39, 48565 Steinfurt, Germany § Physics Institute, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany S Supporting Information *

ABSTRACT: Nanoclays are nanomaterials with versatile adsorptive properties. This contribution describes the generation of micropatterns of a nanoclay (“laponite”) on ammonium-terminated, self-assembled monolayers (SAMs) on glass and silicon. Microstructured immobilization of the laponite was performed using micromolding in capillaries (MIMIC). The immobilization was verified using contact angle goniometry, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), scanning electron microscopy (SEM), time-offlight secondary ion mass spectrometry (ToF-SIMS), and fluorescence microscopy. Furthermore, laponite was modified with Nile red to generate a fluorescence enhancement-based surface sensor for the vitamin choline.

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INTRODUCTION The properties of a surface have a large impact on the characteristics of the underlying bulk material or device and can either severely limit or significantly broaden their field of application.1 The generation of self-assembled monolayers (SAMs) on flat and curved surfaces is a versatile method to open a wide window of possible properties and their modification to different materials such as wettability, reactivity, electrochemical behavior and biocompatibility.2,3 In ongoing research, the modification of surface properties is of interest, for example, in materials and medical research.4−6 In many applications (for example, in microelectronic devices) a spatial differentiation of varying functionalities on substrates is required.7 One method for the generation of surface patterns is micromolding in capillaries (MIMIC). MIMIC is a soft lithography method which is widely applicable due to its tolerance toward many different compounds and surface materials.8 It can be used to form patterns with a high resolution on a wide variety of surfaces like glasses, metals, as well as substrates functionalized with SAMs.9 In MIMIC, a microstructured template mold (or “stamp”) made of polydimethylsiloxane (PDMS) is brought in contact with a surface. The substrate surface and the interstitials of the stamp form capillaries in which a solution can be drawn by simply drop casting a diluted solution of a compound of interest at the edge of the stamp on the surface. This method is cost-effective because it merely requires small amounts of substance and is easy and fast to perform without the need to operate cost © XXXX American Chemical Society

intensive devices. Among others, MIMIC has been used for the modification of surfaces with polymer brushes and for the generation of microelectronic devices.10−12 In this paper we report the micropatterning of a nanoclay (“laponite”, Laponite RD)13 using MIMIC. Laponite is a smectite clay that forms nanoscale platelets.13 It consists of exterior [SiO4] tetrahedra, sandwiching octahedral [Mg, Li (O, OH)] units organized in a so-called TOT (tetrahedral-octahedral-tetrahedral) structure. Due to the ion distribution, the 1-nm-thick and 25-nm-wide disc shaped laponite particles are negatively charged, leading to highly adsorptive behavior for a wide variety of molecules such as phthalocyanine (Pc)14,15 or rhodamine.16 In addition, the negative surface charges are compensated by intercalated sodium atoms, which can be released upon dispersion in water, resulting in individual platelets and hence transparent dispersions. Laponite has been extensively used as a rheological modifier in paints, inks, and surface coatings.13 Recently, broader applications in various fields have emerged. Besides using laponite as a filler material in organic composites to regulate mechanical properties,17,18 also a plethora of biomedical applications has been proposed in the past decade, Special Issue: Surfaces and Interfaces for Molecular Monitoring Received: December 23, 2016 Revised: March 24, 2017 Published: March 28, 2017 A

DOI: 10.1021/acs.langmuir.6b04618 Langmuir XXXX, XXX, XXX−XXX

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in water) were drop casted at the edge of the stamp on the surface. The dispersion thus spreads along the cavities of the stamp via capillary forces and generates the pattern (inverse dots or stripes). After 5 min, excess laponite dispersion was removed via blowing an argon stream over the structure, and the substrate was submerged in water and the stamps disengaged from the layer. Finally, the substrates were sonicated in water and dried carefully in an argon stream. Incubation of Laponites with Choline. A 5 mM solution of choline in water was prepared and diluted to result in concentrations of 1 mM, 100 μM, 10 μM, and 1 μM. The substrates were covered with 200 μL of the respective choline solution for 1 min. Afterward, the solution was removed in an argon stream, and the substrates were investigated by fluorescence microscopy. For the reversibility experiments, the substrates were rinsed with distilled water. Instrumentation. AFM was performed using a NanoWizard 3 (JPK Instruments). The measurements were operated in tapping mode with Veeco RTESP-Tapping Mode etched silicon Probes, a scan rate of 0.5 Hz, and a set point of 0.9 V, while images were recorded with a resolution of 1024 × 1024 pixels. Data evaluation was done using Gwyddion v2.41. For SEM imaging, an AURIGA Scanning Electron Microscope (Carl-Zeiss) was used. Detection was done using the in-lens detector with an acceleration voltage of 3 kV. The substrates were taped to the Al specimen holder using conductive carbon tape. Fluorescence microscopy was done with an Olympus BX53 fluorescence microscope (Olympus K. K.). Contact angle analysis was performed using a KRÜ SS DSA 100 (A.KRÜ SS Optronic GmbH, Germany) in combination with Drop Shape Analysis v1.90.0.14. Static contact angles were measured with 5 μL drops of ultrapure water. X-ray photoelectron spectroscopy was performed with a Kratos Axis Ultra (KRATOS) using monochromated Al Kα irradiation with an excitation energy of 1486.6 eV. For region scans a pass energy of 0.02 eV was employed. The obtained data was analyzed with CasaXPS Software Suite v2315. All spectra were calibrated to the binding energy of the C-1s-orbital in aliphatic carbon−carbon chains (285 eV). ToF-SIMS measurements were carried out using a TOF V (IONTOF GmbH, Münster) compatible instrument equipped with a 30 keV liquid metal ion gun (IONTOF GmbH, Münster). As primary ions, Bi3+ clusters with a pulsed current of 0.05 pA were used.

as a drug delivery system for the anticancer drug doxorubicin,19,20 for example, or as a functional additive in tissue engineering.21,22 Based on the outstanding film-forming capabilities of this artificial clay mineral, various studies on functional laponite films were also performed, elaborating on the bioactivity of the materials to improve packaging in food storage.23 The possibility to construct self-assembled layer-bylayer (LBL) films based on laponite has tremendously sparked the interest of numerous scientists.24,25 A manifold of systems were manufactured from covalent LBL films26 to obtain adsorptive multilayer systems for the study of electronic properties,27 physical processes, such as energy transfer between dyes,28 or even as a potential sensor.25 Hitherto, we are not aware of any example of microstructured laponite films. It is likely that by combining the enormous adsorptive character of laponite with a straightforward micropatterning method, versatile sensors can be developed. In this contribution we describe the preparation of surface patterns of laponite by using MIMIC on glass and silica substrates functionalized with ammonium-terminated SAMs. The immobilization was monitored using contact angle goniometry, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), scanning electron microscopy (SEM), and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Additionally, laponite was loaded with rhodamine B prior to patterning to enable the visualization of successful MIMIC with fluorescence microscopy. Finally, patterned laponite particles, modified with Nile Red, were exposed to choline (formerly erroneously denoted as vitamin B4), yielding a substantial fluorescence enhancement, which may serve as proof of concept for its potential in microsensing.

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EXPERIMENTAL SECTION

Preparation of SAMs on Glass and Silicon. Silicon and glass slides were cut into suitable sizes and were cleaned by treatment in an ultrasonic bath, first in acetone, then in ethanol, and afterward in distilled water for 10 min each. To activate the surface, the substrates were exposed to a mixture of concentrated sulfuric acid and 30% hydrogen peroxide (3:1 vol.) for 30 min. After cleaning the activated substrates by extensively rinsing them with distilled water, they were carefully dried in an argon stream. The activated, cleaned and dried glass or silicon slides were put in a solution of N-trimethoxysilylpropylN,N,N-trimethylammonium chloride (0.10 vol %) in methanol for 18 h. Afterward, the substrates were rinsed with acetone and ethanol, and the success of the SAM generation was verified by contact angle measurements. Loading of Laponites with Dyes. For the loading of the laponites with dyes, the required amounts of laponite and dye were calculated according to a published procedure.16,29 Typically, 300 mg of laponites were stirred in absolute ethanol and 480 μL of a dye stock solution (1 mmol/L) was added. After 1 h, the solution was filtered, and the laponites were dried in vacuum. Preparation of PDMS Stamps. PDMS stamps were prepared using Sylgard 184 Silicone Elastomer Kit (Dow Corning). PDMS was mixed with the curing agent in a 10:1 ratio and agitated with a glass bar for 5 min. The mixture was put on a silicon master, and remaining air in the mixture was removed in vacuum using a desiccator. The PDMS mixture was cured at 80 °C overnight. If not otherwise mentioned, the stamps were cut into suitable pieces and treated in a UV ozonizer (PSD-UV, Novascan Technologies, Inc.) for 55 min. The freshly oxidized stamps were immediately stored in distilled water. Prior to use, the stamps were carefully dried in an argon stream. Patterning of Laponites. Commercially available Laponite RD from BYK Additives and Instruments was used in this work.13 Freshly oxidized stamps were carefully placed on the ammonium terminated SAMs. Five microliters of the respective laponite dispersions (0.1 wt %

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RESULTS AND DISCUSSION Laponite was immobilized on SAMs of N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride prepared according to a literature procedure.30 For contact angle goniometry, a freshly oxidized flat stamp was carefully placed on a freshly prepared substrate. A drop of 5 μL 0.1 wt % solution of laponite in ultrapure water was carefully drop casted at the edge of the stamp. After verification that the solution entered the entire area between stamp and substrate via capillary forces, the surface was incubated for additional 5 min. The stamp was removed, and the surface was sonicated in ultrapure water followed by contact angle measurement (Figure 1). The contact angle of a freshly prepared ammonium-terminated SAM was determined to be 11.4° ± 1.9 (Figure 1A), as opposed to the nonfunctionalized surfaces, for which 25.1° ± 0.1 were found (Figure 1 A). After functionalization with laponite, the contact angle increased to 42.9° ± 0.8. This is a first indication for the successful immobilization of laponite on a major part of the targeted area. Furthermore, a silica substrate homogeneously modified with laponite was prepared and investigated using X-ray photoelectron spectroscopy (XPS) (Figure 1D, red) and compared with an unmodified ammonium-functionalized substrate (Figure 1D, black). The appearance of the element specific magnesium band is an additional proof for the presence of laponite on the modified substrate. B


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the patterned surface showed patterns that are an exact replica of the stamp. At this point it has to be stated that it appeared to be very difficult to obtain a suitable line scan due to irregularities within the microstructures and the thinness of the laponite layer. The irregulaties in the structures can also be observed in the phase mode AFM (Figure 2 B). Additionally, a laponite patterned surface was analyzed using SEM. The structure of the stamp was successfully reproduced in inverse patterns (Figure 2 C). These results show the successful generation of a laponite patterned surface in microscale patterns with high edge resolution.

Figure 1. Contact angle measurement of water on (A) unmodified activated glass slides (25.0° ± 0.1), (B) ammonium-terminated SAM (11.4 ± 1.9), and (C) Laponite modified surface (42.9 ± 0.8). (D) XPS measurements of laponite-modified surfaces (red) in comparison with unmodified ammonium-terminated SAMs (black).

In the following experiments, laponite was immobilized by MIMIC employing structured stamps (either 5 μm stripes spaced by 3 or 5 μm dots spaced by 3 μm). To this end, a stamp was placed on the surface, and a droplet of the laponite solution was drop casted to the edge of the stamp (Figure 2A). In the case of line patterns, it was crucial to incubate the side orthogonal to the lines to enable the solution to penetrate the surface alongside the capillaries formed between surface and stamp. The incubation process could be monitored through the transparent stamp. The excess of the solution was removed in an argon stream 5 min after the complete wetting of the capillaries and the substrates were submerged in water. The stamps were removed under water to prevent residual laponite dispersion from spreading over the freshly prepared structured surfaces. The substrates were sonicated in ultrapure water prior to AFM measurements (Figure 2 B). The phase mode AFM of

Figure 3. Positive-mode ToF-SIMS measurements of patterned surfaces (5 μm dots spaced by 3 μm). (A) Laponite specific signals of Li and Mg secondary ions. (B) SAM and substrate specific secondary ions of silica and from silane.

To further clarify that the micropatterns are indeed composed of laponite, Tof-SIMS measurements were performed (Figure 3). The ToF-SIMS images display discrete boundaries of laponite-free dots and the laponite containing area in-between the dots. Here again, the dots display large

Figure 2. (A) Schematic representation of the immobilization of laponites via micromolding in capillaries (MIMIC) and the subsequent analysis of laponite patterns using (B) AFM (3 μm stripes spaced by 5 μm) and (C) SEM (5 μm dots spaced by 3 μm, scale bar 20 μm). C


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Langmuir amounts of Si and C3H8N+, presumably originating from the silane and the substrate, while the interstitial areas clearly show the presence of Mg and Li originating from the laponite. For fluorescence microscopy analyses, laponite was loaded with rhodamine B according to a previously published procedure.16 The rhodamine B-modified laponite was immobilized by MIMIC as described before. Stamps with lines (5 μm stripes spaced by 25 μm) and dots (10 μm dots spaced by 5 μm) were used. After the patterning and sonication, the surfaces were investigated via fluorescence microscopy (Figure 4). Again, the inverse image of the stamp structure is observed

water, resulting in a concentration of 0.1 wt %. The patterning was performed as described above using a stamp with 5 μm stripes spaced by 20 μm. After sonication the substrates were incubated with varying amounts of a choline solution (1 μM, 10 μM, 100 μM, and 1 mM in distilled water) for 1 min in order evaluate concentration-dependent enhancement effects. The surfaces were investigated before and after the incubation with choline (Figure 5 A, fluorescence micrographs are provided as

Figure 4. Fluorescence image of rhodamine B-modified laponite patterns on ammonium-terminated self-assembled monolayers. Inverse patterns of stamps with (A) 5 μm stripes spaced by 25 μm and (B) 10 μm dots spaced by 5 μm. (scale bar 50 μm).

Figure 5. (A) Nile Red fluorescence intensity enhancement as a function of choline concentration. (B) Reusability of the surface sensor by repetitive incubation with choline (100 μM, odd numbers) followed by rinsing of the substrate with water (even numbers).

in high-resolution patterns over a large surface area. These observations clearly demonstrate that MIMIC is a convenient method to form patterned laponite surfaces in a fast and easy manner and further proves that the dye-laponite hybrid remains luminescent upon deposition as a thin film. In further studies, the suitability of the micropatterns as a biosensitive recognition site for a surface sensor was established. To this end, laponite particles were loaded with Nile Red, which typically forms H-dimers on such surfaces, strongly diminishing the fluorescence quantum yield of the dye. However, upon incubation with choline, the interaction of Nile Red dimers on the laponite surface is weakened, and the Hdimers dissociate to a certain extent.16 In this way, a response of the surface indicating the presence of choline is generated, which is observable with fluorescence microscopy or spectroscopy. For this purpose, laponite was loaded with Nile Red by dispersing laponite and Nile Red in absolute ethanol, resulting in a ratio of 2 Nile Red molecules per laponite disc. The mixture was stirred for 2 h, and the solvent was removed in vacuum. The modified laponite particles were dispersed in

Supporting Information). For more sophisticated, future quantitative analyses, the reversibility of the system was tested by rinsing the substrates with water after the incubation with choline (100 μM). This experiment was done in a repetitive cycle (Figure 5 B). The fluorescence enhancement is clearly visible, indicating that choline induced a significant disaggregation of the Nile Red H-dimers, and conversely, that choline can be detected by the fluorescence increase from the laponite micropattern, even in a repetitive cycle. From our limited data set (Figure 5 A), we estimate that the limit of detection for choline is below 10 μM, while the laponite surface is saturated above 1 mM of choline.

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CONCLUSION We demonstrated the successful generation of laponite patterns on glass and silicon substrates using MIMIC. Additionally, as a proof of concept, Nile Red-modified laponites were patterned, and a strong choline induced fluorescence enhancement could readily be detected. It can be foreseen that based on this strategy, more complex bioactive interfaces can be obtained by D


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(11) Yan, J.; Du, Y.; Liu, J.; Cao, W.; Sun, X.; Zhou, W.; Yang, X.; Wang, E. Fabrication of Integrated Microelectrodes for Electrochemical Detection on Electrophoresis Microchip by Electroless Deposition and Micromolding in Capillary Technique. Anal. Chem. 2003, 75 (20), 5406−5412. (12) Vonhören, B.; Langer, M.; Abt, D.; Barner-Kowollik, C.; Ravoo, B. J. Fast and Simple Preparation of Patterned Surfaces with Hydrophilic Polymer Brushes by Micromolding in Capillaries. Langmuir 2015, 31 (50), 13625−13631. (13) BYK Additives & Instruments. Laponite: Performance additives https://www.byk.com/fileadmin/byk/additives/product_groups/ rheology/former_rockwood_additives/technical_brochures/BYK_BRI21_LAPONITE_EN.pdf. (14) Staniford, M. C.; Lezhnina, M. M.; Kynast, U. H. Phthalocyanine Blue in Aqueous Solutions. RSC Adv. 2015, 5 (6), 3974−3977. (15) Staniford, M. C.; Lezhnina, M. M.; Gruener, M.; Stegemann, L.; Kuczius, R.; Bleicher, V.; Strassert, C. A.; Kynast, U. H. Photophysical Efficiency-Boost of Aqueous Aluminium Phthalocyanine by Hybrid Formation with Nano-Clays. Chem. Commun. 2015, 51 (70), 13534− 13537. (16) Felbeck, T.; Mundinger, S.; Lezhnina, M. M.; Staniford, M.; Resch-Genger, U.; Kynast, U. H. Multifold Fluorescence Enhancement in Nanoscopic Fluorophore-Clay Hybrids in Transparent Aqueous Media. Chem. - Eur. J. 2015, 21 (20), 7582−7587. (17) Pavlidou, S.; Papaspyrides, C. D. A Review on Polymer-Layered Silicate Nanocomposites. Prog. Polym. Sci. 2008, 33 (12), 1119−1198. (18) Chiu, C. W.; Lin, J. J. Self-Assembly Behavior of PolymerAssisted Clays. Prog. Polym. Sci. 2012, 37 (3), 406−444. (19) Wu, Y.; Guo, R.; Wen, S.; Shen, M.; Zhu, M.; Wang, J.; Shi, X. Folic Acid-Modified Laponite Nanodisks for Targeted Anticancer Drug Delivery. J. Mater. Chem. B 2014, 2 (42), 7410−7418. (20) Chen, G.; Li, D.; Li, J.; Cao, X.; Wang, J.; Shi, X.; Guo, R. Targeted Doxorubicin Delivery to Hepatocarcinoma Cells by Lactobionic Acid-Modified Laponite Nanodisks. New J. Chem. 2015, 39 (4), 2847−2855. (21) Ordikhani, F.; Dehghani, M.; Simchi, A. Antibiotic-Loaded Chitosan Laponite Films for Local Drug Delivery by Titanium Implants: Cell Proliferation and Drug Release Studies. J. Mater. Sci.: Mater. Med. 2015, 26, 269. (22) Xavier, J. R.; Thakur, T.; Desai, P.; Jaiswal, M. K.; Sears, N.; Cosgriff-Hernandez, E.; Kaunas, R.; Gaharwar, A. K. Bioactive Nanoengineered Hydrogels for Bone Tissue Engineering: A GrowthFactor-Free Approach. ACS Nano 2015, 9 (3), 3109−3118. (23) Li, X.; Liu, A.; Ye, R.; Wang, Y.; Wang, W. Fabrication of Gelatin-Laponite Composite Films: Effect of the Concentration of Laponite on Physical Properties and the Freshness of Meat during Storage. Food Hydrocolloids 2015, 44, 390−398. (24) Lutkenhaus, J. L.; Olivetti, E. A.; Verploegen, E. A.; Cord, B. M.; Sadoway, D. R.; Hammond, P. T. Anisotropic Structure and Transport in Self-Assembled Layered Polymer-Clay Nanocomposites. Langmuir 2007, 23 (16), 8515−8521. (25) Patro, T. U.; Wagner, H. D. Layer-by-Layer Assembled PVA/ Laponite Multilayer Free-Standing Films and Their Mechanical and Thermal Properties. Nanotechnology 2011, 22 (45), 455706. (26) Ren, W.; Wu, R.; Guo, P.; Zhu, J.; Li, H.; Xu, S.; Wang, J. Preparation and Characterization of Covalently Bonded PVA/ Laponite/HAPI Nanocomposite Multilayer Freestanding Films by Layer-by-Layer Assembly. J. Polym. Sci., Part B: Polym. Phys. 2015, 53 (8), 545−551. (27) Shan, D.; Han, E.; Xue, H.; Cosnier, S. Self-Assembled Films of Hemoglobin/laponite/chitosan: Application for the Direct Electrochemistry and Catalysis to Hydrogen Peroxide. Biomacromolecules 2007, 8 (10), 3041−3046. (28) Dey, D.; Bhattacharjee, D.; Chakraborty, S.; Hussain, S. A. Effect of Nanoclay Laponite and pH on the Energy Transfer between Fluorescent Dyes. J. Photochem. Photobiol., A 2013, 252, 174−182. (29) Felbeck, T.; Behnke, T.; Hoffmann, K.; Grabolle, M.; Lezhnina, M. M.; Kynast, U. H.; Resch-Genger, U. Nile-Red-Nanoclay Hybrids:

micropatterning of laponites and absorption of reporter molecules and/or ligands, e.g., by designing multilayer structures. Eventually, other coadsorbates such a phthalocyanines may be useful as well, since these may be employed in studies on microbial adhesion and photocatalytic deactivation, or in electro-optical devices as structured electrode materials.

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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/acs.langmuir.6b04618. Fluorescence microscopy pictures of Nile Red-decorated laponites patterned on surfaces and incubated with different choline concentrations (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Bart Jan Ravoo: 0000-0003-2202-7485 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. M.B. and M.S. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Lukas Ibing at MEET for the SEM measurements. REFERENCES

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Patterning of Nanoclays on Positively Charged Self ... - ACS Publications

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