We have studied the changes in physical and chemical properties of cationic poly(2-(methacryloyloxy)ethyltrimethylammonium chloride) brushes after collapse driven by ion-pairing interactions in the presence of ClO 4anions. Results derived from the quartz crystal microbalance technique, atomic force microscopy, Fourier transform infrared spectroscopy, and contact angle goniometry indicate that ion-paired collapsed polyelectrolyte brushes suffer a dramatic loss of water accompanied by conformational changes leading to markedly different mechanical properties. This scenario is completely different from polyelectrolyte brushes whose collapse is simply driven by pure Coulombic screening, for example, in the presence of Clanions. In addition, wetting measurements indicated that ion-pairing interactions can be used to switch surface characteristics from hydrophilic to hydrophobic in a reversible manner. The immediate implications of these experimental results are related to the promising use of polyelectrolyte brushes as biolubricants and the design of "smart" surfaces exhibiting ion-sensitive reversible changes in interfacial properties.

Polymers are ideal building blocks for nanofabrication and the engineering of surfaces because of their response in shape and size to environmental changes in polymer concentration, ionic strength, pH, temperature, and solvent. Furthermore, since they are easy to synthesize, as well as chemically and mechanically robust, polymers are ideal materials to bridge the gap between biological nanomachines and silicon-based devices. Of particular interest herein is the exploration of the use of polymer brushes as nanoactuators. [1] Polyelectrolytes, that is, polymers containing charged monomers, change from an extended conformation in a solution of low ionic strength to a coiled conformation in solutions of high ionic strength.[2] When polyelectrolytes are covalently attached to a surface to form a brush, the polymer chains show a strongly extended (stretched) conformation in pure water, as a result of both the repulsion between neighboring chains and the repulsion between the monomers. [3][4][5][6] In contrast, when polymer brushes are placed in electrolyte solutions, the charges of the pendant groups in the polymer chains are screened, and the minimization of the electrostatic repulsions leads to entropically more favorable collapsed conformations. [4][5][6][7][8] This collapse is comparable to transitions from extended to coil-like conformations commonly observed for polyelectrolytes in solution. [3,4] In the case of polymer brushes, there are constraints on the degrees of freedom related to conformational changes, and as a result the height and the amount of water on the inner brush environment are reduced. [5,8] Our aim is to use the ionic-strength-sensitive behavior of polyelectrolyte brushes [9][10][11] in combination with specific binding interactions and ion-size-exclusion effects to design smart polymer surfaces that can be switched between a permanently collapsed and an extended state, where both states have different surface properties. In addition, we aim to find a route toward a "fast switch" that reversibly alternates between different polymer brush conformations, and as a consequence gain control over surface properties [12,13] such as wettability, hydration, and stiffness. Reversibly manipulating the polymer-brush conformation in a chemically selective way (not only by changing the ionic strength) will be a crucial step toward the use of surface-confined polyelectrolytes as nanoactuators.Cationic brushes of [2-(methacryloyloxy)ethyl]trimethylammonium chloride (METAC) [14] were grown from gold surfaces, patterned into regions containing either initiator or methyl-terminated monolayers (Figure 1 a, b), by atom-transfer radical polymerization (ATRP) in an oxygen-free methanol/water (4:1) mixture. [15,16] The responsive behavior of the METAC brushes in different electrolyte environments (NaCl and MgSO 4 ) was studied under liquids using atomic force microscopy (AFM) and a quartz crystal microbalance with dissipation (QCM-D). [17] The conformational changes in the METAC brush in the presence of 1m NaCl electr...

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Polymers are ideal building blocks for nanofabrication and the engineering of surfaces because of their response in shape and size to environmental changes in polymer concentration, ionic strength, pH, temperature, and solvent. Furthermore, since they are easy to synthesize, as well as chemically and mechanically robust, polymers are ideal materials to bridge the gap between biological nanomachines and silicon-based devices. Of particular interest herein is the exploration of the use of polymer brushes as nanoactuators. [1] Polyelectrolytes, that is, polymers containing charged monomers, change from an extended conformation in a solution of low ionic strength to a coiled conformation in solutions of high ionic strength.[2] When polyelectrolytes are covalently attached to a surface to form a brush, the polymer chains show a strongly extended (stretched) conformation in pure water, as a result of both the repulsion between neighboring chains and the repulsion between the monomers. [3][4][5][6] In contrast, when polymer brushes are placed in electrolyte solutions, the charges of the pendant groups in the polymer chains are screened, and the minimization of the electrostatic repulsions leads to entropically more favorable collapsed conformations. [4][5][6][7][8] This collapse is comparable to transitions from extended to coil-like conformations commonly observed for polyelectrolytes in solution. [3,4] In the case of polymer brushes, there are constraints on the degrees of freedom related to conformational changes, and as a result the height and the amount of water on the inner brush environment are reduced. [5,8] Our aim is to use the ionic-strength-sensitive behavior of polyelectrolyte brushes [9][10][11] in combination with specific binding interactions and ion-size-exclusion effects to design smart polymer surfaces that can be switched between a permanently collapsed and an extended state, where both states have different surface properties. In addition, we aim to find a route toward a "fast switch" that reversibly alternates between different polymer brush conformations, and as a consequence gain control over surface properties [12,13] such as wettability, hydration, and stiffness. Reversibly manipulating the polymer-brush conformation in a chemically selective way (not only by changing the ionic strength) will be a crucial step toward the use of surface-confined polyelectrolytes as nanoactuators.Cationic brushes of [2-(methacryloyloxy)ethyl]trimethylammonium chloride (METAC) [14] were grown from gold surfaces, patterned into regions containing either initiator or methyl-terminated monolayers (Figure 1 a, b), by atom-transfer radical polymerization (ATRP) in an oxygen-free methanol/water (4:1) mixture. [15,16] The responsive behavior of the METAC brushes in different electrolyte environments (NaCl and MgSO 4 ) was studied under liquids using atomic force microscopy (AFM) and a quartz crystal microbalance with dissipation (QCM-D). [17] The conformational changes in the METAC brush in the presence of 1m NaCl electr...

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The preparation of two new tripodal "pinwheel" type anion hosts based on a triethylbenzene core and bipyridinium or ethylnicotinium arms is reported. The new materials bind anions via CH...anion interactions. Complexes with Br(-) and PF(6)(-) have been characterised by X-ray crystallography as both solvates in a pure form. In the bipyridinium host CH...F interactions to PF(6)(-) induce a chiral C(3) symmetric conformation that is disrupted in the hydrate. The compound is also selective for ATP(2-) in aqueous acetonitrile.