The Locklin Group is interested in growing functional polymers from surfaces ("grafting from") using different Surface Initiated Polymerization (SIP) techniques. This is a technique based on the growth of polymer molecules at the surface of a substrate (such as glass, metal, or plastic) in situ from a surface bound initiator, which results in the covalent attachment of polymer molecules to this substrate. Polymer layers in which the polymer chains are irreversibly immobilized to the substrate are especially attractive for a wide variety of applications, as these layers have excellent long-term stability, even in rather adverse environments. In addition to improved stability, the number of functional groups present at a surface can be greatly enhanced by connecting large polymer molecules with functional groups (present in each monomer repeat unit) to the surface. Currently, we are using ring-opening metathesis polymerization (ROMP), Kumada transfer polycondensation (KCTP), atom-transfer radical polymerization (ATRP) and conventional free radical polymerization to grow functional coatings for the following applications: Stimuli responsive surfaces, photo-induced mechanical motion, sensors for biological arrays, antimicrobial coatings and enzymatic biofuel cells.

We are working with polymers that contain photochromic substitutents based on spiropyran derivatives. Spiropyrans are a group of light-switchable organic molecules whose photochromism is achieved through a photocleavage reaction of the spiro C-O bond. This allows reversible switching between a colorless closed form (SP) and a colored merocyanine open form (MC) which can be shown in the photo to the right. The photoinduced geometry change between the spiropyran and merocyanine forms are accompanied by a large change in dipole moment and the onset of absorbance in the visible region. The reaction is both thermally and photochemically reversible. If confined to an interface, the change in dipole moment influences the surface free energy, which in turn, gives rise to a switching of wettability. Our goal is to use photochromic polymer brushes to exploit these changes in dipole moment (and ultimately molecular conformation) to tune surface properties using light as the stimulus.

Fries, K.H.; Driskell, J.D.; Samanta, S.; Locklin, J. "Spectroscopic Analysis of Metal Ion Binding in Spiropyran Containing Copolymer Thin Films," Anal. Chem., 2010, 82, 3306-3314. DOI: 10.1021/ac1001004
Fries, K.; Samanta, S.; Orski, S.; Locklin, J. "Reversible Colorimetric Ion Sensors Based on Surface-Initiated Polymerization of Photochromic Polymers," Chem. Comm., 2008, 6288. DOI: 10.1039/b818042c

We have adapted the Kumada-type catalyst-transfer polycondensation to the formation of electroactive conjugated polymer brushes directly from surface-bound initiators. The use of arylmagnesium precursors are straight-forward to prepare, use inexpensive starting materials, and are amenable to thiophene-based monomers, which allow easy synthetic accessibility and versatility in terms of tuning the conjugated backbone with substitution of donating or withdrawing groups. As shown in the scheme below, we have fabricated gold surfaces functionalized with monolayers of an aryl bromide moiety that undergoes oxidative addition when placed in a solution of a reactive Ni(0) species to produce a surface-bound metal-mediated coupling catalyst. To date, we have successfully polymerized thiophene and phenylene A-B type monomers from the surface-bound catalyst in a surface-initiated Kumada catalyst-transfer polycondensation (SI-KCTP).

Marshall, N; Sontag, S.K.; Locklin, J. "Surface-initiated polymerization of conjugated polymers," Chem. Comm. 2011, ASAP. DOI: 10.1039/C1CC10483G

Surfaces containing reactive ester polymer brushes were functionalized with cyclopropenone-masked dibenzocyclooctynes for the light activated immobilization of azides using catalyst-free click chemistry. The photodecarbonylation reaction in the amorphous brush layer is first order for the first 45 s with a rate constant of 0.022 s−1. The catalyst-free cycloaddition of surface bound dibeznocyclooctynes proceeds rapidly in the presence of azides under ambient conditions. Photolithography using a shadow mask was used to demonstrate patterning with multiple azide containing molecules. This surface immobilization strategy provides a general and facile platform for the generation of multicomponent surfaces with spatially resolved chemical functionality.

Orski, S.V.; Poloukhtine, A.A.; Arumugam, S.; Mao, L.; Popik, V.V.; Locklin, J. "High Density Orthogonal Surface Immobilization via Photoactivated Copper-Free Click Chemistry," J. Am. Chem. Soc. 2010, 132, 11024-11026. DOI: 10.1021/ja105066t

Antimicrobial copolymers of hydrophobic N-alkyl and benzophenone containing polyethylenimines were synthesized from commercially available linear poly(2-ethyl- 2-oxazoline), and covalently attached to surfaces of synthetic polymers, cotton, and modified silicon oxide using mild photo-cross-linking. Specifically, these polymers were applied to polypropylene, poly(vinyl chloride), polyethylene, cotton, and alkyl-coated oxide surfaces using solution casting or spray coating and then covalently cross-linked rendering permanent, nonleaching antimicrobial surfaces. The photochemical grafting of pendant benzophenones allows immobilization to any surface that contains a C-H bond. Incubating the modified materials with either Staphylococcus aureus or Escherichia coli demonstrated that the modified surfaces had substantial antimicrobial capacity against both Gram-positive and Gram-negative bacteria (>98% microbial death).

Dhende, V.; Samanta, S.; Jones, D.M.; Hardin, I.R.; Locklin, J. "One-Step Synthesis of Permanent, Nonleaching, Ultrathin Antimicrobial Coatings for Textiles and Plastics," Applied Materials & Interfaces, 2011, ASAP. DOI:10.1021/am200324f

Thiolene-based microfluidic devices have been coupled with surface plasmon resonance imaging, SPRI, to provide an integrated platform to study interfacial interactions in both aqueous and organic solutions. In this work, we develop a photolithographic method that interfaces commercially available thiolene resin to gold and glass substrates to generate microfluidic channels with excellent adhesion that leave the underlying sensor surface free from contamination and readily available for surface modification through self-assembly. These devices can sustain high flow rates and have excellent solvent compatibility even with several organic solvents. To demonstrate the versatility of these devices, we have conducted nanomolar detection of streptavidin-biotin interactions using in situ SPRI.

Sheppard, G.; Oseki, T.; Baba, A.; Patton, D.; Kaneko, F.; Mao, L.; Locklin, J. "Thiolene-based microfluidic flow cells for surface plasmon resonance imaging," Biomicrofluidics, 2011, 2, 26501-26507. DOI: 10.1063/1.3596395