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Electrochemically Controlled Swelling and Mechanical Properties of a Polymer Nanocomposite

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Electrochemically Controlled Swelling and Mechanical Properties of a Polymer Nanocomposite ARTICLE Electrochemically Controlled Swelling and Mechanical Properties of a Polymer Nanocomposite Daniel J. Schmidt,† Fevzi C¸ . Cebeci,†,‡ Z. Ilke Kalcioglu,‡ Samantha G. Wyman,† Christine Ortiz,‡ Krystyn J. Van Vliet,‡,§,* and Paula T. Hammond†,* †Department of Chemical Engineering, ‡Department of Materials Science and Engineering, and §Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 timuli-responsive polymeric materials S have attracted much interest for a va- ABSTRACT We present the layer-by-layer assembly of an electroactive polymer nanocomposite thin film riety of applications including drug containing cationic linear poly(ethyleneimine) (LPEI) and 68 vol % anionic Prussian Blue (PB) nanoparticles, which delivery, biological and chemical sensing, allow for electrochemical control over film thickness and mechanical properties. Electrochemical reduction of the separations, and mechanical actuation, PB doubles the negative charge on the particles, causing an influx of water and ions from solution to maintain among others.1Ϫ5 Control over the mechan- ical properties of a surface is of particular in- electroneutrality in the film; concomitant swelling and increased elastic compliance of the film result. Reversible terest for the modulation of cellular behav- swelling upon reduction is on the order of 2؊10%, as measured via spectroscopic ellipsometry and electrochemical ior, as it is well-known that adhesion of cells atomic force microscopy. Reversible changes in the Young’s elastic modulus of the hydrated composite film upon to surfaces can be modulated via the stiff- reduction are on the order of 50% (from 3.40 to 1.75 GPa) as measured with in situ nanoindentation, and a 6,7 ness of those surfaces. In the area of bio- qualitative increase in viscous contributions to energy dissipation upon redox is indicated by electrochemical mimicry, researchers have recently de- quartz crystal microbalance. Electrochemical stimuli maintain a mild operating environment and can be applied signed a chemoresponsive polymer nanocomposite to mimic the dermis of the rapidly, reversibly, and locally. We maintain that electrochemical control over the swelling and mechanical sea cucumber, a marine organism that can behavior of polymer nanocomposites could have important implications for responsive coatings of nanoscale reversibly alter its stiffness.8 Stimuli- devices, including mechanically tunable surfaces to modulate behavior of adherent cells. responsive composite materials open up the possibility of tuning percolative behav- KEYWORDS: polymer nanocomposite · electrochemistry · Prussian Blue · responsive Downloaded by MIT on July 22, 2009 ior, which can dramatically alter mechanical, materials · layer-by-layer thin film · swelling · nanoindentation electrical, optical, and other composite properties.9,10 In this work, we have de- charged materials onto a charged sub- signed and characterized an electrochemi- strate. While the stiffness of LbL films can cally responsive polymer nanocomposite be controlled by post-assembly Published on July 22, 2009 http://pubs.acs.org | doi: 10.1021/nn900526c thin film with actively tunable mechanical cross-linking,12,13 as well as assembly pH properties. Electrochemical stimuli are ad- and choice of polymer,7 these films can also vantageous, in comparison with tempera- be stimuli-responsive.14 It is known that ture or pH changes, in that they can be ap- changes in humidity15 and ionic strength16 plied reversibly, rapidly, and locally (i.e.,at can reversibly alter the stiffness of LbL films an electrode instead of throughout the by changing the degree of film hydration bulk). Moreover, the structure and function and the degree of ionic cross-linking, re- of biological molecules, cells, and organ- spectively. There are only a few examples isms, for example, may be inadvertently al- in the literature of reductionϪoxidation tered by large changes in temperature or (redox)-driven swelling of LbL films. The pH, whereas application of a small voltage Calvo group has studied multilayer films maintains a comparatively mild containing a ferrocene-derivatized poly- *Address correspondence to environment. allylamine (PAH-Fc)17 as well as an osmium [email protected], [email protected]. Layer-by-layer (LbL) assembly, intro- complex-derivatized polyallylamine (PAH- Received for review May 21, 2009 duced by Decher in the 1990s,11 was em- Os),18 which can swell by 10% of initial film and accepted July 15, 2009. ployed here for the fabrication of an electro- thickness upon oxidation of the Os(II). The active thin film composite. LbL assembly is Vancso group has layered anionic and cat- 10.1021/nn900526c CCC: $40.75 a simple and versatile process that involves ionic polyferrocenylsilanes to form multi- the sequential adsorption of oppositely layer capsules, which expand and increase © XXXX American Chemical Society www.acsnano.org VOL. XXX ▪ NO. XX ▪ 000–000 ▪ XXXX A their permeability upon chemical oxidation of the fer- ing.30 When PB is synthesized in its “soluble” form, rocene units.19 Most recently, Grieshaber et al. reported potassium ions dissociate in aqueous solution, re- on a poly(L-glutamic acid)/poly(allylamine hydrochlo- sulting in negatively charged surface and interior ride) multilayer film that takes up ferrocyanide ions sites; this dissociation results in an electrostatically from solution and can expand and contract by 5Ϫ10% stabilized colloidal suspension. In the native PB oxi- in response to electrochemical oxidation and reduction dation state, roughly half of the iron atoms are in the of the ferrocyanide species.20 None of the above re- Fe(II) redox state and the remaining are in the Fe(III) ports on redox-driven swelling quantify the concurrent redox state.31 PB can be electrochemically reduced mechanical changes occurring in the films. Further- to Prussian White (PW) or oxidized to Prussian Yel- more, in contrast to these three examples, the work pre- low (PX) (Figure 1a). The PW state has a charge of Ϫ2 ARTICLE sented here utilizes a nanoparticle-based assembly, as per unit cell, the PB state has a charge of Ϫ1 per redox-active Prussian Blue nanoparticles serve as an unit cell, and the PX state is neutral. This charge integral component of a polymer nanocomposite. shifting capability of PB makes it a very interesting (Srivastava and Kotov recently reviewed the field of LbL- material to study, particularly as a part of compos- assembled nanocomposites,21 though to our knowl- ites where altering PB charge can alter properties of edge previous LbL nanocomposite systems have not the composite as a whole. In previous work, we have been investigated for reversible swelling properties.) shown that oxidizing Prussian Blue nanoparticles The advantages in using a redox-active nanoparticle in- can lead to sufficient loss of charge to yield film dis- clude the potential to manipulate particle loading22 at solution and controlled release in LbL thin or near a percolation threshold, thus yielding more dra- films.23,30 matic shifts in electrochemical and mechanical proper- Here we characterize the electrochemically con- ties, the ability to achieve the multiple redox states ex- trolled swelling and mechanical behavior of electro- hibited by inorganic materials, and the ability to active LbL thin films containing Prussian Blue nano- manipulate surface charge and thus directly impact particles as the polyanion and linear polyethyleneimine electrostatics within the film with the accompanying re- (LPEI) as the polycation. Specifically, we apply Ϫ0.2 V dox behavior, independent of a polyelectrolyte conju- (vs Ag/AgCl) to reduce the particles from the PB ϩ gate. These properties make this system interesting as state to the PW state and 0.6 V to oxidize the par- a potential mechanomutable material for which stiff- ticles back to the PB state. This negative voltage ness can be altered significantly with a readily con- level is sufficient to fully reduce the PB nanoparti- trolled external stimulus. cles, and this positive voltage level is sufficient to Prussian Blue (PB), or iron(II) hexacyanoferrate(III), fully reoxidize the PB without inducing dissolution 23 is the first known, mixed valence transition metal that can occur at even higher voltages. Upon elec- complex. Researchers have used PB for a wide range trochemical reduction, the negative charge on the Downloaded by MIT on July 22, 2009 of applications including electrochromic devices,23,24 particle surface and interior is doubled as estimated glucose sensors,25,26 and molecular magnets.27Ϫ29 from the valence of the two redox states. In response Our group recently utilized Prussian Blue nanoparti- to the excess negative charge created in the film, cles (PB) for an electroactive controlled release coat- cations and water from the electrolyte solution en- ter the film to maintain electroneutrality (Fig- Published on July 22, 2009 http://pubs.acs.org | doi: 10.1021/nn900526c ure 1b). We have investigated the swelling phenomenon using spectroscopic ellipsome- try and electrochemical atomic force micros- copy (EC-AFM). Nanoindentation and an elec- trochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) were used to measure the mechanical properties of films in the oxidized and reduced states. To our knowledge, this is the first direct observa- tion of mechanical property manipulation carried out in an electrochemical cell with a redox-active thin film. We believe that elec- trochemical control over the swelling and mechanical
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