Effects of the Polymer Amount and pH on Proton Transport in Mesopores

Proton exchange membranes (PEMs) have various applications, such as in electrolysis technology for hydrogen generation, vanadium flow batteries for energy storage, and fuel cells for energy conversion. To increase PEM performance and expand the range of PEM applications, the underlying transport mechanisms of PEMs need to be understood. Mesoporous silica thin films are versatile model materials for proton transport investigation and are prepared with a pore size of ≈12 nm and film thickness of ≈565 nm by evaporation‐induced self‐assembly, providing an ordered, mesoporous, rigid matrix that allows us to deduce the structure‐property relationship with respect to proton conductivity. Different amounts of sulfonic acid‐bearing groups are introduced into the mesopores using the grafting‐through polymerization of sulfopropylmethacrylate. The relationship between proton transport and the pH of the surrounding solution in poly‐sulfopropylmethacrylate‐functionalized mesopores is investigated using electrochemical impedance spectroscopy. The proton conductivity is found to depend on both the proton concentration in solution and the number of proton transporting groups inside the pore, indicating the major role of charge regulation and the confinement effect on proton transport

Mesoporous Coatings with Simultaneous Light‐Triggered Transition of Water Imbibition and Droplet Coalescence

A systematic study of gating water infiltration and condensation into ceramic nanopores by carefully adjusting the wetting properties of mesoporous films using photoactive spiropyran is presented. Contact angle measurements from the side reveal significant changes in wettability after irradiation due to spiropyran/merocyanine-isomerization, which induce a wetting transition from dry to wet pores. The change in wettability allows the control of water imbibition in the nanopores and is reflected by the formation of an imbibition ring around a droplet. Furthermore, the photoresponsive wettability is able to overcome pinning effects and cause a movement of a droplet contact line, facilitating droplet coalescence, as recorded by high-speed imaging. The absorbed light not only effectuates droplet merging but also causes flows inside the drop due to heat absorption by the spiropyran, which results in gradients in the surface tension. IR imaging and particle tracking is used to investigate the heat absorption and temperature-induced flows, respectively. These flows can be used to manipulate, for example, molecular movement inside water and deposition inside solid mesoporous materials and are therefore of great importance for nanofluidic devices as well as for future water management concepts, which include filtering by imbibition and collection by droplet coalescence. © 2021 The Authors. Advanced Materials Interfaces published by Wiley-VCH Gmb

Mechanistic Understanding and Three‐Dimensional Tuning of Fluid Imbibition in Silica‐Coated Cotton Linter Paper Sheets

Paper‐based microfluidic devices are used in point of care diagnostic, sensor technology or lab‐on‐a‐chip devices. Although a number of studies has been reported, only relatively few paper‐based diagnostic tools are available on the market. A remaining challenge is the mechanistic understanding and precise design of capillary flow in paper. Here, silica coatings are applied to control paper wettability, fiber swelling, and thus fluid transport in all three dimensions of a paper sheet via a simple dip‐coating and post‐treatment process. By adjusting the three‐dimensional silica coating distribution, a three‐dimensional asymmetric wettability gradient within the paper sheet is obtained which controls the fluid distribution and imbibition. The correlation between silica coating amount and silica distribution with the resulting fluid behavior is systematically elaborated by analyzing the interaction between fiber and fluid as well as the fiber swelling by applying confocal microscopy. Three different silica‐amount dependent fluid distribution states are demonstrated. These new insights into the mechanism of fluid imbibition using simple silica coatings enable the specific design of different imbibition mechanisms and thus the adjustment of the microfluidic properties in paper‐based microfluidic devices with control over all three spatial dimensions of a paper sheet in one fabrication step

Proton and Calcium-Gated Ionic Mesochannels: Phosphate-Bearing Polymer Brushes Hosted in Mesoporous Thin Films As Biomimetic Interfacial Architectures

Rational construction of interfaces based on multicomponent responsive systems in which molecular transport is mediated by structures of nanoscale dimensions has become a very fertile research area in biomimetic supramolecular chemistry. Herein, we describe the creation of hybrid mesostructured interfaces with reversible gate-like transport properties that can be controlled by chemical inputs, such as protons or calcium ions. This was accomplished by taking advantage of the surface-initiated polymerization of 2-(methacryloyloxy)ethyl phosphate (MEP) monomer units into and onto mesoporous silica thin films. In this way, phosphate-bearing polymer brushes were used as “gatekeepers” located not only on the outer surface of mesoporous thin films but also in the inner environment of the porous scaffold. Pore-confined PMEP brushes respond to the external triggering chemical signals not only by altering their physicochemical properties but also by switching the transport properties of the mesoporous film. The ion-gate response/operation was based on the protonation and/or chelation of phosphate monomer units in which the polymer brush works as an off-on switch in response to the presence of protons or Ca2+ ions. The hybrid meso-architectured interface and their functional features were studied by a combination of experimental techniques including ellipso-porosimetry, cyclic voltammetry, X-ray reflectivity, grazing incidence small-angle X-ray scattering, X-ray photoelectron spectroscopy, and in situ atomic force microscopy. In this context, we believe that the integration of stimuli-responsive polymer brushes into nanoscopic supramolecular architectures would provide new routes toward multifunctional biomimetic nanosystems displaying transport properties similar to those encountered in biological ligand-gated ion channels.Instituto de Investigaciones Fisicoquímicas Teóricas y AplicadasConsejo Nacional de Investigaciones Científicas y Técnica

Fluid Flow Programming in Paper-Derived Silica–Polymer Hybrids

In paper-based devices, capillary fluid flow is based on length-scale selective functional control within a hierarchical porous system. The fluid flow can be tuned by altering the paper preparation process, which controls parameters such as the paper grammage. Interestingly, the fiber morphology and nanoporosity are often neglected. In this work, porous voids are incorporated into paper by the combination of dense or mesoporous ceramic silica coatings with hierarchically porous cotton linter paper. Varying the silica coating leads to significant changes in the fluid flow characteristics, up to the complete water exclusion without any further fiber surface hydrophobization, providing new approaches to control fluid flow. Additionally, functionalization with redox-responsive polymers leads to reversible, dynamic gating of fluid flow in these hybrid paper materials, demonstrating the potential of length scale specific, dynamic, and external transport control

Functional dextran-based hydrogels

Dextran-based polymers are versatile hydrophilic materials, which can provide functionalized surfaces in various areas including biological and medical applications. Functional, responsive, dextran based hydrogels are crosslinked, dextran based polymers allowing the modulation of response towards external stimuli. The controlled modulation of hydrogel properties towards specific applications and the detailed characterization of the optical, mechanical, and chemical properties are of strong interest in science and further applications. Especially, the structural characteristics of swollen hydrogel matrices and the characterization of their variations upon environmental changes are challenging. Depending on their properties hydrogels are applied as actuators, biosensors, in drug delivery, tissue engineering, or for medical coatings. However, the field of possible applications still shows potential to be expanded. rnSurface attached hydrogel films with a thickness of several micrometers can serve as waveguiding matrix for leaky optical waveguide modes. On the basis of highly swelling and waveguiding dextran based hydrogel films an optical biosensor concept was developed. The synthesis of a dextran based hydrogel matrix, its functionalization to modulate its response towards external stimuli, and the characterization of the swollen hydrogel films were main interests within this biosensor project. A second focus was the optimization of the hydrogel characteristics for cell growth with the aim of creating scaffolds for bone regeneration. Matrix modification towards successful cell growth experiments with endothelial cells and osteoblasts was achieved.rnA photo crosslinkable, carboxymethylated dextran based hydrogel (PCMD) was synthesized and characterized in terms of swelling behaviour and structural properties. Further functionalization was carried out before and after crosslinking. This functionalization aimed towards external manipulation of the swelling degree and the charge of the hydrogel matrix important for biosensor experiments as well as for cell adhesion. The modulation of functionalized PCMD hydrogel responses to pH, ion concentration, electrochemical switching, or a magnetic force was investigated. rnThe PCMD hydrogel films were optically characterized by combining surface plasmon resonance (SPR) and optical waveguide mode spectroscopy (OWS). This technique allows a detailed analysis of the refractive index profile perpendicular to the substrate surface by applying the Wentzel Kramers Brillouin (WKB) approximation. rnIn order to perform biosensor experiments, analyte capturing units such as proteins or antibodies were covalently coupled to the crosslinked hydrogel backbone by applying active ester chemistry. Consequently, target analytes could be located inside the waveguiding matrix. By using labeled analytes, fluorescence enhancement was achieved by fluorescence excitation with the electromagnetic field in the center of the optical waveguide modes. The fluorescence excited by the evanescent electromagnetic field of the surface plasmon was 2 3 orders of magnitude lower. Furthermore, the signal to noise ratio was improved by the fluorescence excitation with leaky optical waveguide modes.rnThe applicability of the PCMD hydrogel sensor matrix for clinically relevant samples was proofed in a cooperation project for the detection of PSA in serum with long range surface plasmon spectroscopy (LRSP) and fluorescence excitation by LRSP (LR SPFS). rnDextran-basierte Polymere sind hydrophile Materialien, die zur Funktionalisierung von Oberflächen eingesetzt werden können. Funktionelle, responsive Hydrogele sind vernetzte, Polymere, die ihren Quellgrad bei Variation der externen Bedingungen ändern. Eine kontrollierte Anpassung der Hydrogeleigenschaften für spezifische Anwendungen sowie eine detaillierte Charakterisierung der optischen, mechanischen und chemischen Eigenschaften sind aus wissenschaftlichen und anwendungsorientierten Gesichtspunkten von großem Interesse. Dabei ist vor Allem die strukturelle Charakterisierung gequollener Hydrogele und die Charakterisierung der responsiven Änderung des Quellgrades bei Variation der externen Bedingungen eine Herausforderung. Entsprechend ihrer Eigenschaften finden Hydrogele Anwendung als Aktuatoren, in der Biosensorik, in der Pharmakotherapie, in medizinischen Beschichtungen oder als Matrices für Zellwachstum.rnOberflächengebundene Hydrogelfilme mit einer Filmdicke im Bereich von Mikrometern können als Wellenleiter für optische Leckwellenleitermoden dienen. Auf Basis stark quellender und wellenleitender Dextran basierter Hydrogelfilme wurde ein Biosensorkonzept entwickelt. Die Synthese einer Dextran basierten Hydrogelmatrix, ihre Funktionalisierung und die Charakterisierung der gequollenen Hydrogelfilme waren die Schwerpunkte innerhalb dieses Biosensorprojektes. Ein zweiter Schwerpunkt war die Optimierung der Hydrogele als Matrix für Zellwachstum zur Knochenregeneration. Dabei wurde Anhaftung und Wachstum von Endothelzellen und Osteoblasten beobachtet.rnEs wurde ein photovernetzbares, carboxymethyliertes, Dextran basiertes Hydrogel (PCMD) hergestellt und auf seine Quelleigenschaften und seine Struktur im gequollenen Zustand untersucht. Eine weitere Funktionalisierung wurde sowohl am löslichen Polymer, als auch nach Vernetzen am Hydrogelfilm durchgeführt. Das Ziel der Funktionalisierung war die externe Manipulation des Quellgrades und der Ladung der Hydrogelmatrix. Beide Eigenschaften sind einerseits für die Biosensorik als auch für Zellwachstum von Bedeutung. Die Modulation des Quellgrades durch pHIonenkonzentrationsänderungen oder durch Anlegen eines elektrischen oder magnetischen Feldes wurde dabei untersucht.rnDie PCMD Hydrogelfilme wurden optisch mittels Oberflächenplasmonenresonanz (SPR) und optischer Wellenleitermodenspektroskopie (OWS) untersucht. Mit Hilfe dieser Technik und unter Anwendung der Wentzel Kramers Brillouin Näherung (WKB) ist es möglich, das Brechungsindexprofil des Hydrogelfilms senkrecht zur Substratoberfläche zu bestimmen. rnUm Biosensorexperimente durchzuführen wurden selektive Analyterkennungseinheiten, wie z.B. Proteine oder Antikörper, mittels Aktivesterchemie kovalent an das vernetzte Polymer angebunden. Anschließend wurden Analyten innerhalb des Leckwellenleitermoden leitenden Hydrogelfilms angebunden und das elektromagnetische Feld der Wellenleitermoden konnte zur Detektion genutzt werden. Fluoreszenzmarkierte Analyten führen dabei zu einer Signalverstärkung. Das Fluoreszenzsignal, angeregt durch die optischen Wellenleitermoden, lag dabei um 2-3 Größenordnungen höher als das durch das evaneszente Feld des Oberflächenplasmons angeregte Signal. Zudem resultiert die Fluoreszenzdetektion in optischen Leckwellenleitermoden in einem geringeren Signal Rausch Verhältnis. rnDie Anwendbarkeit der PCMD Hydrogelsensormatrix für klinisch relevante Proben wurde in einem Kooperationsprojekt durch die Detektion des Tumormarkers PSA in Serum mittles langreichweitiger Oberlflächenplasmonenspektroskopie (LRSP) und Fluoreszenzdetektion mittles LRSP (LR SPFS) nachgewiesen.r

Effects of the Polymer Amount and pH on Proton Transport in Mesopores

Abstract Proton exchange membranes (PEMs) have various applications, such as in electrolysis technology for hydrogen generation, vanadium flow batteries for energy storage, and fuel cells for energy conversion. To increase PEM performance and expand the range of PEM applications, the underlying transport mechanisms of PEMs need to be understood. Mesoporous silica thin films are versatile model materials for proton transport investigation and are prepared with a pore size of ≈12 nm and film thickness of ≈565 nm by evaporation‐induced self‐assembly, providing an ordered, mesoporous, rigid matrix that allows us to deduce the structure‐property relationship with respect to proton conductivity. Different amounts of sulfonic acid‐bearing groups are introduced into the mesopores using the grafting‐through polymerization of sulfopropylmethacrylate. The relationship between proton transport and the pH of the surrounding solution in poly‐sulfopropylmethacrylate‐functionalized mesopores is investigated using electrochemical impedance spectroscopy. The proton conductivity is found to depend on both the proton concentration in solution and the number of proton transporting groups inside the pore, indicating the major role of charge regulation and the confinement effect on proton transport

Ultrashort Peptide Grafting on Mesoporous Films and Its Impact on Ionic Mesopore Accessibility

An approach for direct in-pore solid-phase ultrashort peptide synthesis on mesoporous films using the amino acids arginine, leucine, and glycine is presented. Although the number of grafted amino acids remains low, the ionic mesopore accessibility can be gradually adjusted. The addition of arginine in up to five reaction cycles leads to a progressive increase in positive mesopore charge density, which gradually increases the anionic mesopore accessibility at acidic pH. At basic pH, the remaining silanol groups at the pore wall still dominate counter-charged cation mesopore accessibility. Thus, specific peptide sequence design is demonstrated to be a sensitive tool for molecular transport control in nanoscale pores. Overall, the direct in-pore solid-phase ultrashort peptide synthesis on mesoporous films using the sequences of different amino acids opens up exciting opportunities for the development of innovative materials with precisely tailored properties and functions based on specific peptide sequence design

Pushing the limits of nanopore transport performance by polymer functionalization

Inspired by the design and performance of biological pores, polymer functionalization of nanopores has emerged as an evolving field to advance transport performance within the last few years. This feature article outlines developments in nanopore functionalization and the resulting transport performance including gating based on electrostatic interaction, wettability and ligand binding, gradual transport controlled by polymerization as well as functionalization-based asymmetric nanopore and nanoporous material design going towards the transport direction. Pushing the limits of nanopore transport performance and thus reducing the performance gap between biological and technological pores is strongly related to advances in polymerization chemistry and their translation into nanopore functionalization. Thereby, the effect of the spatial confinement has to be considered for polymer functionalization as well as for transport regulation, and mechanistic understanding is strongly increased by combining experiment and theory. A full mechanistic understanding together with highly precise nanopore structure design and polymer functionalization is not only expected to improve existing application of nanoporous materials but also opens the door to new technologies. The latter might include out of equilibrium devices, ionic circuits, or machine learning based sensors