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Thermoresponsive Coatings Strongly Adhering to (Semi)conducting Surfaces† Sabine Gabriel,‡ Anne-Sophie Duwez,§ Robert Je´roˆme,‡ and Christine Je´roˆme*,‡ Center for Education and Research on Macromolecules, UniVersity of Liege, B6 Sart-Tilman, B-4000 Liege, Belgium, and Unite´ de Chimie et Physique des Hauts Polyme` res and Research Center in Micro- and Nanoscopic Materials and Electronic DeVices, UniVersite´ catholique de LouVain, Place Croix du Sud 1, 1348 LouVain-la-NeuVe, Belgium ReceiVed May 30, 2006. In Final Form: July 20, 2006 Thermoresponsive brushes have been efficiently grafted onto (semi)conductive surfaces by a two-step process. In the first step, poly(N-succinimidyl acrylate) chains have been chemisorbed onto silicon or stainless steel by the electrografting method. Then, these modified electrodes were immersed in isopropylamine in order to transform the grafted chains to the thermoresponsive poly(N-isopropyl acrylamide). The thermal response of these brushes has been shown by AFM. This straightforward grafting process is quite attractive for surface modification in confined media and is thus promising for microfludics application.
Steadily increasing attention is paid nowadays to the preparation of smart surfaces, such as, among others, surfaces exhibiting thermoresponsive behavior. As an example, the coating of silicon or metallic channels of microfluidic systems with a thermoresponsive thin film is quite attractive, as far as the wetting ability or film thickness can be controlled by an external and easy to handle stimulus such as temperature. Indeed, variation in the swelling of the coating with temperature is expected to modify the rate of a flowing liquid within the coated channel and thus to control a localized reaction on a lab-on-a-chip device. Moreover, the coating of semiconducting material that exhibits an efficient thermal response upon applying a voltage by the so-called Joule effect is particularly interesting for local and external control of the coating swelling. A general approach commonly followed for the elaboration of such surfaces is the formation of a thin coating of a responsive polymer on top of the device surface. Obviously, this strategy implies the existence of a strong adhesion of the responsive coating to the substrate and particularly under the stimulation conditions, for example, upon heating.1 In this context, poly(N-isopropyl acrylamide) (PNIPAM) is probably one of the most popular thermoresponsive polymers, exhibiting a clear collapse of the chains above an easily reachable low critical solubility temperature (LCST) of around 32 °C. Whenever the surface of electrically (semi)conducting materials is concerned, electrochemistry is an appropriate technique to improve the adhesion to organic coatings.2,5 In this respect, electrografting of (meth)acrylates3 is a powerful technique for † Part of the Stimuli-Responsive Materials: Polymers, Colloids, and Multicomponent Systems special issue. * Corresponding author. E-mail:
[email protected]. Fax: 32 4366 3497. Tel: 32 4366 3491. ‡ University of Liege. § Universite ´ catholique de Louvain.
(1) Jones, D. M.; Smith, J. R.; Huck, W. T. S.; Alexander, C. AdV. Mater. 2002, 14, 1130. (2) (a) Stratmann, M. AdV. Mater. 1990, 2, 191. (b) Coulon, E.; Pinson, J.; Bourzat, J. D.; Commercon, A.; Pulicani, J. P. Langmuir 2001, 17, 7102. (c) Deniau, G.; Azoulay, L.; Je´gou, P.; Le Chevalier, G.; Palacin, S. Surf. Sci. 2006, 600, 675. (3) Je´roˆme, C.; Je´roˆme, R. In Stimuli-ResponsiVe Polymeric Films and Coatings; Urban, M. W., Ed.; ACS Symposium Series No. 912; American Chemical Society: Washington, DC, 2005; Chapter 6, p 84. (4) Le´cayon, G.; Bouizem, Y.; Le Gressus, C.; Reynaud, C.; Boiziau, C.; Juret, C. Chem. Phys. Lett. 1982, 91, 506.
Scheme 1
synthetic polymers to be chemisorbed onto various materials, such as metals, alloys, and carbon.3 Originally reported for acrylonitrile in acetonitrile (ACN),4 this technique was extended to various (meth)acrylates by using dimethylformamide (DMF) as the solvent.5 Indeed, it has been shown that there is strong competition between the monomer and the solvent for adsorption on the electrode surface, which has to be in favor of the monomer for electrografting to occur. This can be achieved only for solvents having a high donor number, such as DMF, when a (meth)acrylate monomer is used. In this article, the electrografting technique has been extended for the first time to the formation of thermoresponsive PNIPAM coatings onto conducting and semiconducting materials. It is worth noting that the one-step process consisting of the electroreduction of a primary or secondary acrylamide monomer in DMF, or other high donor number solvents, is unsuccessful because as already reported the cathodically initiated chain propagation of such acrylamide derivatives does not proceed through the conventional direct addition of the activated double bond on one of the next monomer units but a rearrangement of the activated chain-end occurs, leading to addition via the nitrogen atom. The structure of the so-formed coating is thus deeply affected, avoiding any responsive behavior. Only the anodic electropolymerization of such monomers follows the expected polyaddition reaction, but in this case, the polymer is precipitated on the surface and no strong adhesion is observed.6 A two-step but still rather straightforward alternative to get electrografted PNIPAM is thus here described. In the first step, electrografting of the N-succinimidyl acrylate (NSA) (Scheme 1), which is of the acrylic type, was performed. This monomer bears an activated ester group that is highly reactive (5) Baute, N.; Je´roˆme, C.; Martinot, L.; Mertens, M.; Geskin, V. M.; Lazzaroni, R.; Bredas, J. L.; Je´roˆme, R. Eur. J. Inorg. Chem. 2001, 1097. (6) Akbulut, U.; Fernandez, J. E.; Birke, R. L. J. Polym. Sci., Polym. Chem. Ed. 1975, 13, 133.
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Figure 1. Voltammograms for the reduction of NSA in DMF/5 × 10-2 M TEAP at 20 mV/s on a Si substrate with [NSA] ) 0.1 M (1), 0.5 M (2), 1.0 M (3), and 1.5 M (4). (Inset) Reduction of NSA (1 M) on Si: (1) first scan, (2) second scan, and (3) third scan.
toward nucleophiles, thus making the electrografted coating particularly appropriate for the anchoring of amino derivatives. The second step consists of the reaction of the coating with isopropylamine in order to obtain the targeted responsive polymer. NSA has been electrografted by voltammetry in DMF added with tetraethylammonium perchlorate (TEAP 0.05 M) as a conducting salt onto a semiconducting silicon surface. Figure 1 shows voltammograms recorded for NSA solutions of various concentrations. As already reported for other materials, the first scanning of cathodic potentials (curves 1 to 4) shows a typical reduction peak below -2 V corresponding to the grafting peak7,8 where a film is deposited even in a good solvent (DMF) for the polymer (PNSA). When the potential is held at the top of this peak (Figure 1 inset, curve 1), the current drops considerably, which confirms the cathode passivation and thus the saturation of the grafting sites. Recording a second or a third time, we see that the potential (Figure 1 inset, curves 2 and 3) shows a deep decrease in the intensity of the grafting peak and completes the grafting efficiency. Moreover, a clear decrease in the intensity of the grafting peak while increasing the monomer concentration in the electrochemical bath (Figure 1, curves 1 to 4) indicates the faster passivation of the substrate due to the higher propagation speed of the electroinitiated chains at higher concentrations. All of these electrochemical observations are typical indications that chains are chemisorbed (grafted) whenever the cathodic potential reaches the maximum of this reduction peak. It is noteworthy that the electrografting process is applicable to silicon wafers in addition to a large variety of conducting substrates (noble metals, transition metals, metal alloys, etc). Indeed, similar curves have been recorded on steel and stainless steel. This step provides PNSA brushes with a grafting density of about 1 chain/100 nm2 as determined by AFM-based force spectroscopy9 and so would be the density of the PNIPAM chains because they are derived from the side-chain modification of the PNSA brushes. ATR-FTIR analysis (Figure 2A) of the stainless steel surface onto which a 1 M NSA solution has been reduced (at the potential at the top of the grafting peak) confirms the expected structure of PNSA film with the characteristic CdO vibrations at 17381778-1814 cm-1 and the C-N vibration at 1215 cm-1. For such a characterization technique, stainless steel substrates were preferred to silicon because no competing signal from the substrate is observed. However, the characteristic vibrations of the electrografted PNSA have also been observed by using silicon as a substrate. (7) Je´roˆme, C.; Gabriel, S.; Voccia, S.; Detrembleur, C.; Ignatova, M.; Gouttebaron, R.; Je´roˆme, R. Chem. Commun. 2003, 19, 2500. (8) Bureau, C. J. Electroanal. Chem. 1999, 479, 43. (9) Cuenot, S.; Gabriel, S.; Je´roˆme, R.; Je´roˆme, C.; Fustin, C. A.; Jonas A. M.; Duwez, A. S. Macromolecules, submitted for publication.
Gabriel et al.
Figure 2. ATR-FTIR spectra for PNSA (A) and PNIPAM (B) electrografted onto stainless steel.
Figure 3. Force-distance curves (gray, approach profile; black, retraction profile) obtained in milliQ water below the LCST (at 22 °C) and above the LCST (at 42 °C) between a PNIPAM-modified silicon substrate and an AFM tip.
In the second step, the chemisorbed PNSA film (which cannot be dissolved by any good solvent) was converted to PNIPAM. For this purpose, the PNSA-modified substrate was immersed at room temperature for 48 h in a 0.5 M solution of commercially available isopropylamine (Sigma-Aldrich, g99.5%) in DMF containing a catalytic amount of 4-(dimethylamino)pyridine. ATR-FTIR analysis (Figure 2B) of the surface after intensive washing by DMF confirms the success of the derivatization that appears to be complete within the detection limit of the analysis method; the typical peaks for PNSA completely disappeared, and the characteristic N-H stretching at 3290 cm-1, the CdO vibration at 1651 cm-1, the NH bending at 1546 cm-1, the CH3 asymmetric deformation at 1444 cm-1, and CN and NH mixed vibrations at 1389 cm-1 were observed. Finally, the thermoresponsive behavior of the PNIPAM coating has been shown by recording force-distance curves by atomic force microscopy in water at room temperature (22 °C) (below the LCST) and at 42 °C (above LCST). Below the LCST, repulsive forces, characteristic of the compression of a swollen brush in a good solvent,10 were observed upon the approach of the tip to the surface. From this curve, the coating thickness at room temperature, thus in the swollen state, could be estimated to be 80 ( 10 nm. Upon retraction, a low adhesive interaction or a
Coatings Adhering to (Semi)conducting Surfaces
few bridging interactions are observed. This is the classical behavior of hydrophilic PNIPAM, previously reported for PNIPAM-modified substrates. 11 The temperature in the liquid cell was then raised to 42 °C to induce the change from hydrophilic to hydrophobic. In this case, no compression profile could be detected anymore, which means that the polymer layer is collapsed. Upon tip retraction, a huge adhesion force appears, characteristic of the hydrophobic behavior of PNIPAM.1 The thickness in the collapsed state is estimated to be 9 ( 3 nm. When the solution is cooled, hydrophilic behavior is recovered. The temperature dependence of the tip/surface interactions correlates well with the expected physicochemical properties of the polymer brush undergoing repeated hydrophilic/hydrophobic transitions as expected for PNIPAM coatings.1 In summary, the present study reports on an efficient strategy (10) Taunton, H. J.; Toprakcioglu, C.; Fetters, L. J.; Klein, J. Nature 1988, 332, 712. (11) Goodman, D.; Kizhakkedathu, J. N.; Brooks, D. E. Langmuir 2004, 20, 3297.
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to prepare chemisorbed thermoresponsive thin films of PNIPAM on silicon and metallic surfaces by a two-step process. First, the electrografting of the easily prepared N-succinimidyl acrylate was demonstrated on silicon. Then, the high reactivity of the surface toward isopropylamine easily leads to strongly adhering and thermoresponsive PNIPAM coatings. Deeper investigations of the coating response versus temperature are in progress. This two-step process is quite attractive for applications in microfluidics,12 with the small size of both the NSA monomer and the isopropylamine reactant being suitable for the modification of confined surfaces. Acknowledgment. We are grateful to the Belgian Science Policy in the framework of PAI V/03. C.J. is Research Associate of the FNRS. LA061541Q (12) Delvaux, M.; Demoustier-Champagne, S. Biosens. Bioelectron. 2003, 18, 943.
