title: Dynamic Gradient Directed Molecular Transport and Concentration in Hydrogel Films.
abstract: Materials which selectively transport molecules along defined paths offer new opportunities for concentrating, processing and sensing chemical and biological agents. Here, we present the use of traveling ionic waves to drive molecular transport and concentration of hydrophilic molecules entrained within a hydrogel. The traveling ionic wave is triggered by the spatially localized introduction of ions, which through a dissipative ion exchange process, converts quaternary ammonium groups in the hydrogel from hydrophilic to hydrophobic. Through a reaction-diffusion process, the hydrophobic region expands with a sharp transition at the leading edge; it is this sharp gradient in hydrophilicity that drives the transport of hydrophilic molecules dispersed within the film. The traveling wave moved up to 450 μm within 30 min, while the gradient length remained 20 μm over this time. As an example of the potential of molecular concentration using this approach, a 70-fold concentration of a hydrophilic dye was demonstrated.
authors.affiliation: Department of Materials Science and Engineering, Department of Chemistry, Beckman Institute for Advanced Science and Technology, Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana Champaign, Urbana, IL, 61801, USA.
authors.forename: Tsung-Han
authors.initials: TH
authors.lastname: Tsai
authors.affiliation: Department of Materials Science and Engineering, Department of Chemistry, Beckman Institute for Advanced Science and Technology, Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana Champaign, Urbana, IL, 61801, USA.
authors.forename: Mohammad A
authors.initials: MA
authors.lastname: Ali
authors.affiliation: Department of Materials Science and Engineering, Department of Chemistry, Beckman Institute for Advanced Science and Technology, Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana Champaign, Urbana, IL, 61801, USA.
authors.forename: Zhelong
authors.initials: Z
authors.lastname: Jiang
authors.affiliation: Department of Materials Science and Engineering, Department of Chemistry, Beckman Institute for Advanced Science and Technology, Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana Champaign, Urbana, IL, 61801, USA.
authors.forename: Paul V
authors.initials: PV
authors.lastname: Braun
keywords: chemical sensors; dynamic gradient; molecular transport; reaction-diffusion; signal amplification; traveling waves