Dyer Group Research Projects
Polymer Brush Nanosponges Polymer Composite LC Thin Films Hydrogen Bonded LCs Fluorescent Biosensors Dyer Group Faculty Chemistry Department		Polymer Brush Nanosponges The control of interfacial properties is extremely important for the proper functioning of many devices. Take for instance a piece of tape; the adhesive must interact with the substrate in order to stick. Some tape may stick well to paper, but not at all to glass. The difference lies in the interaction energy between the adhesive and the substrate. The same is true for biological materials. Cell cultures and tissue may grow rapidly on one medium and not at all on another. Thus, the study and manipulation of interfacial phenomena is extremely important. Our group is developing the synthetic tools that will allow us to modify substrates, such as gold, glass, or silicon, with organic polymers. Organic polymers are easily deposited onto these substrates by the process of spin casting or dip coating. However, these polymer films are merely adsorbed to the surface and may become detached under certain conditions. For instance, a water soluble polymer may readily stick to glass, but it might dissolve if the substrate were immersed into an aqueous solution. If, however, the polymer were covalently linked to the substrate, then it would remain tethered to the substrate even when it was immersed into a solvent that might otherwise cause the polymer to dissolve and de-adsorb. Polymers that are bonded to the surface at one end are called “polymer brushes” because the chains may stretch out away from the surface, much like the bristles on a hairbrush. However, the orientation of the chains depends largely on the grafting density (i.e. the number of chains per unit area). Low density brushes tend to wet the surface and yield polymer chains that are randomly oriented, much like a spin cast film, whereas high density brushes may yield polymer chains that stretch out away from the surface. We are targeting high density brushes for use in sensor applications based on the electrical and optical response of brush polymers. We are examining the adsorption of peptides and fluorescent dye molecules onto polymer brushes in order to rapidly fractionate mixtures of compounds and analyze the components by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry. As Figure 1 illustrates, these films can swell by more than an order of magnitude in aqueous solutions, depending on the pH. More specifically, the anionic brushes expand at high pH (>6) and collapse at low pH(<6) due to protonation of the anionic carboxylates; we refer to these films as nanosponges since the collapse triggers a release of the solvent (i.e. the sponge is squeezed). And this release of solvent offers a method to deliver an analyte that had previously been fractionated by selective adsorption from a mixture. In this two-year project we are primarily interested in examining the loading capacity of the brush films (i.e. the amount of adsorbed peptide per unit volume), along with the various structural parameters that control the loading and release of compounds from the brush film. Furthermore, we are examining the temporal response of uptake and release of analytes, along with the collapse and expansion of the brush films. Since these are nanofilms we anticipate that their response will be faster than traditional bulk hydrogel films. Figure 1. AFM images of a random copolymer brush patterned by e-beam lithography and composed of N-isopropyl acrylamide (NIPAAM) and methacrylic acid monomers. (a) Dry brush height is 9 nm; (b) at pH 9 the brush swells to 112 nm; and (c) at pH 4 the brush contracts back to the dry height. Back to Top Dyer Home Page Faculty Department of Chemistry
Polymer Brush Nanosponges Polymer Composite LC Thin Films Hydrogen Bonded LCs Fluorescent Biosensors Dyer Group Faculty Chemistry Department		Polymer Composite Liquid Crystalline Thin Films Chiral Nematic LCs. The most common thermotropic liquid crystalline phase is the nematic (N), where the rod-shaped molecules align along a director axis (n). In the chiral nematic (N*), the director axis precesses about a helical axis (z) that is orthogonal to n (Figure 1). In addition, the helix pitch may be adjusted by adding chiral dopants or twist agents. Furthermore, we had previously synthesized N* LC-polymer composite films by a photoinitiated radical polymerization process involving three steps: First, a nematic liquid crystal mixture is doped with a chiral twist agent to adjust the helix pitch; Second, a rod-shaped monomer is dissolved in a solvent mixture consisting of the N* LC and a catalytic amount of polymer initiator (Figure 1); and Third, polymerization is initiated resulting in polymer that is partially phase-separated from the LC solvent. Ultraviolet-visible (UV-Vis) spectroscopy suggests that the phase-separated polymers exhibit selective Bragg reflection and therefore are helical. In addition, scanning electron micrographs have demonstrated helical polymer fibrils in these composites that are consistent with the N* solvent Figure 2. Rod-shaped monomers can align with the N* phase and the resulting polymer possesses a helical morphology. The helix pitch of the polymer is similar to the N* solvent and maintains the same twist direction (clockwise or counterclockwise). Molecular Sensors. For biosensor applications it is desirable to have polymers grafted to a surface via covalent bonds in order to increase the durability of the films. Our modified approach utilizes surface grafting polymerization techniques and begins with the deposition of a self-assembled monolayer (SAM) onto a surface such as gold, via a thiol precursor, or onto glass, via a chlorosilane or alkoxysilane precursor (Figure 2). The terminal group on the SAM precursor is either a monomeric unit, such as an acrylate or styryl group, or functionality capable of polymer initiation. Second, the SAM is immersed into a solvent mixture that consists of monomer and a chiral nematic liquid crystal. Third, the polymerization is initiated thermally or photochemically and the resulting polymer is tethered to the surface via covalent bonds. Finally, the liquid crystalline solvent is rinsed away, thus yielding a well-defined chiral surface with helical periodicity. Figure 3. Our approach begins with the deposition of a self-assembled monolayer such that the terminal group (X) is a monomeric unit or functionality capable of initiating polymerization. The modified substrate is then immersed into a solvent mixture that consists of a chiral nematic LC and monomer. Polymerization is initiated thermally or photochemically resulting in a polymer-LC composite film. Removal of the LC solvent should yield a grafted polymer modified surface with a helical morphology that is consistent with the N* phase before polymerization. Selective Bragg reflection is a result of the helical structure of the N* phase. The wavelength (l) of Bragg reflected light is equal to the product of the average refractive index (n) and the helix pitch (p), (i.e. l=np). Light that satisfies this rule and is incident normal to a planar aligned N* LC will be split into two circularly polarized components, one of which is reflected and the other transmitted. Therefore, the reflected color of the N* LC is dependent on the pitch and is sensitive to environmental stress. For instance, a change in temperature may compress or expand the helix pitch and result in a color change. We will utilize reflective UV-Vis. spectroscopy to demonstrate that the Bragg reflection from the surface is sensitive to environmental stress, (e.g. temperature, pressure, pH, etc.). It should be possible to attach functionalities to these helical polymers that will selectively bind to a variety of analytes, thus causing a change in pitch and a detectable response. Figure 3 illustrates our strategy for creating a biosensor by utilizing proteins such as avidin which selectively bind to the small organic ligand, biotin. Recently, Abbott and coworkers used such a strategy to demonstrate an optical sensor based on the reorientation of LCs after a ligand-receptor-binding event. Based on this precedence, we propose that biotin could be tethered directly to the helical polymer fibrils by adding a small amount of biotin monomer to the polymerization mixture. Since the polymer will not be soluble in an aqueous environment, it may be necessary to incorporate a soluble tether between the polymer and ligand. This could be accomplished with an oligomer of polyethylene glycol (PEG), or other water-soluble linkers. Figure 4. Illustration of a bioactive surface capable of sensing a protein-binding interaction. Proteins such as avidin may bind to a biotin molecule that is tethered directly to the helical fibril. A water soluble tether may be added between the ligand and the insoluble polymer fibril to increase activity. The bound protein may interact with the grafted polymer and alter the helix pitch. Thus, a color change is effected that could be visible to the human eye. Back to Top Dyer Home Page Faculty Department of Chemistry
Polymer Brush Nanosponges Polymer Composite LC Thin Films Hydrogen Bonded LCs Fluorescent Biosensors Dyer Group Faculty Chemistry Department		Hydrogen Bonded Liquid Crystalline Polymers Introduction to polar order. One of our long-term goals is to create a stable noncentrosymmetric polar organic thin film. Noncentrosymmetric thin films possess three useful properties that are technologically important. First, pyroelectricity is the electrical response of a material due to a change in temperature; such materials may be used for infrared sensing (e.g. the detection of gasses and small molecules). Currently, pyroelectric devices are limited by weak response at ambient temperatures and manufacturing difficulties. Second, piezoelectricity is the electrical response of a material due to a change in pressure; these materials have many uses including vibration detectors, acoustic detectors, scanning probe microscopy, and biosensors. Third, second order nonlinear optics may be utilized to create new materials for electrooptic modulators in order to improve optical communications technology. Currently, inorganic crystals dominate the market for all three of these areas. However, organic materials are easier to process and modify than inorganics, thus they hold tremendous potential in the emerging field of nanotechnology. Figure 5. (a) Hypothetical polar crystal possesses axial order and polar order due to the orientation of the molecular dipoles. (b) Liquid crystals may possess large molecular dipoles and axial order. However, polar order along the LC director has never been observed because there is an equal number density of molecular dipoles pointing in both directions. Figure 5a illustrates a hypothetical acentric crystal where the dipole moments of the molecules have a preferred alignment along an axis, therefore the crystal exhibits polar order and possesses a macroscopic polarization density. Figure 5b illustrates a common liquid crystalline phase that does not possess polar order because the dipole moments of the molecules do not exhibit an orientational bias along the director axis (n). Nevertheless, LCs, or mesogens, do possess axial order and therefore could be useful if the dipoles may be oriented in a polar fashion. We are attempting to control the orientation of hydrogen bonded liquid crystalline aggregates by utilizing surface interactions. Hydrogen bonding LCs. LC polymers are known where the monomers aggregate via intermolecular hydrogen bonds. Typically, these polymers consist of two monomers terminated at both ends with identical units as illustrated in Figure 6a. Thus, each co-monomer is symmetric, exhibiting donor or acceptor properties but not both. These polymer networks form centrosymmetric chains yielding nonpolar macromolecules because the monomer dipoles oppose each other. However, positioning the H-bond acceptor and donor at opposite ends of a single molecule should result in a molecule capable of intermolecular H-bonding (Figure 6b). Figure 6. a) Centrosymmetric H-bonding monomers will form main chain copolymers where the dipoles cancel out along the chain axis. b) Noncentrosymmetric monomers can form homopolymers where the dipoles are pointing in the same direction along the chain axis. Such chains may be stretched out in a mesophase, thus aligning a chromophore or large dipolar unit. Clearly a polymer macromolecule as described in Figure 6b would possess polar order along the chain. However, this does not necessarily afford polar order in the bulk material because the polymer chains could form random coils, loops, or linear strands with isotropic orientations. Anisotropy may be introduced when the polymer chains self-assemble along a director axis forming nematic or smectic LCs. However, polar order along the LC director is generally not observed because an equal number density of molecular dipoles will be pointing in both directions (Figure 7). The self-assembled monolayer (SAM) technique suggests an approach for inducing polar order along the liquid crystal director. Figure 7b describes a hypothetical liquid crystal cell where the top and bottom surfaces are coated with SAMs terminated with complementary H-bonding pairs (e.g. pyridine and carboxylic acids). Growth of the liquid crystal polymer should initiate from the surface in a polar fashion. Furthermore, the LC supramolecular structure should orient the polymer strands perpendicular to the surface and along n. Alkoxysilanes and chlorosilanes provide ideal precursors for the deposition of SAMs onto glass, silicon, and similar surfaces. In addition, thiols may be deposited onto conductive surfaces such as gold, thus allowing us to pole the sample, if necessary via the application of an electric field. By utilizing the LC alignment in the bulk and at the surface, we hope to create materials that form crystalline films with polar order. Figure 7. Liquid crystalline polymer chains oriented perpendicular to surface. Polar order is induced when the two surfaces are coated with complementary hydrogen bonding self-assembled monolayers. The design strategy for the LCs is described in Figure 8; our first generation compounds possessed benzoic acid donors and stilbazole acceptors. These compounds had very high melting points and low solubility. Currently, we are exploring second generation systems with different linker units, lateral substitutions, and end groups. By changing the structure of this system, we hope to lower the melting temperatures and increase the processibility. Figure 8. Design strategy for H-bonding liquid crystalline monomers. The liquid crystalline properties are characterized by a variety of techniques. As Figure 9 illustrates, polarized optical microscopy is used to visualize the textures and determine the specific mesophase as well as the alignment. For instance, the nematic (N) phase is quite different from the layered smectic A (SA) phase. DSC is used to confirm the LC transition temperatures and melting point. We also use thermogravimetric analysis to determine the decomposition temperatures. Figure 9. Polarized optical micrographs of a H-bonded liquid crystal. Another critical issue that we explore is the nature of the H-bonding at the SAM surface. It is important that the terminal groups interact with adsorbed LCs from the bulk. To probe this we use grazing incidence FT-IR, also referred to as reflection-absorption IR spectroscopy (RAIRS), where the IR beam is reflected from the substrate at a very small angle. As Figure 10a illustrates, the SAM of mercaptohexadecanoic acid (4) exhibits a carbonyl stretch at 1720 cm^-1, suggesting little face-to-face H-bonding. However, after immersing the SAM into a solution of compound 5, we see a dramatic shift to 1705 cm^-1, suggesting face-to-face H-bonding between the acid moiety of the SAM and 5. Furthermore, the thickness, as measured by ellipsometry, increases by a factor of two. Figure 10. FT-RAIR spectra of (a) SAM of mercaptohexadecanoic acid (4) on gold; (b) SAM of 4 after immersion in solution of compound 5; and (c) FT-IR of compound 5. The thickness was measured by ellipsometry. Back to Top Dyer Home Page Faculty Department of Chemistry

Polymer Brush Nanosponges Polymer Composite LC Thin Films Hydrogen Bonded LCs Fluorescent Biosensors Dyer Group Faculty Chemistry Department

Fluorescent Biosensors The specific aim of this project is to evaluate the feasibility of developing novel fluorescent probes that utilize photoinduced electron transfer (PET) perturbation as a transduction mechanism. The primary challenge in developing effective fluorescent sensors/probes lies in the often mutually exclusive requirements of selectivity and sensitivity. Traditional approaches to sensor design have relied primarily on binding selectivity. Herein we propose the possibility of designing sensing molecules whose transduction mechanism is based on both binding and response selectivity. Importantly, the selective response will be obtained by the degree of perturbation caused by the binding event, not solely by the binding affinity. Thus, it may be possible to design sensors that bind to a series of analytes, but only one analyte elicits a fluorescence response, hence the term response selectivity. If successful the proposed sensors will undoubtedly fill a broad range of applications and we envision a significant impact in the development of new technologies for metabolomics research. Because our method is fundamentally based on fluorescence and molecular recognition, selectivity and sensitivity are inherent to the approach. Our approach utilizes the combined expertise of the investigative team, which includes theoretical/computational (Wang), synthetic organic (Dyer), and analytical (McCarroll) chemistries. Figure 11. The sensor consists of four segments, a fluorophore, spacer unit, electron receptor, and a recognition site. Binding of a guest may perturb the electronic state of the receptor and turn on or turn off fluorescence. Back to Top Dyer Home Page Faculty Department of Chemistry