Fluid Flow Programming in Paper-Derived Silica-Polymer Hybrids

Single-fibre coating and additive manufacturing of multifunctional papers

Paper-based materials with precisely designed wettabilities show great potential for fluid transport control, separation, and sensing. To tune the wettability of paper, paper sheets are usually modified after the paper manufacturing process. This limits the complexity of the local wettability design. We combined the wettability design of the individual fibres with subsequent paper sheet fabrication through either fibre deposition or fibre printing. Using silica-based cellulose fibre functionalization, the wettability of the paper sheets, containing only one specific fibre type, could be gradually tuned from highly hydrophilic to highly hydrophobic, resulting in water exclusion. The development of a silica-functionalized fibre library containing mesoporous or dense silica coatings, as well as silica with varying precursor compositions, further enabled the variation of the paper wettability and fluid flow. By combining this fibre library with the paper fabrication process by (i) fibre deposition or (ii) fibre printing, the paper wettability architecture and thus the local fibre composition were adjusted without any further processing steps. This enabled the fabrication of papers with wettability integration, such as a wettability pattern or a Janus paper design, containing wettability gradients along the paper sheet cross section. This asymmetric wettability along all three spatial dimensions enabled side-selective oil-water separation.

Nanoscale pores introduced into paper via mesoporous silica coatings using sol-gel chemistry

Mesopores, with diameters between 2 and 50 nm, not only increase the specific surface area, but also generate hierarchically porous materials with specific properties such as capillary fluid transport, ion specific pore accessibility, or size exclusion. Paper is a strongly hierarchical, porous material with specific properties, such as capillary force-driven fluid transport. However, paper fibers change their morphology during the initial step of wood disintegration. This results in changes of the porous fiber structure. In particular paper fibers loose their mesopores during the final drying step in the fabrication process. Here, we investigate silica mesopore formation in paper by sol-gel chemistry and evaporation induced self-assembly to specifically introduce and rationally design mesopore formation and distribution in cotton linter and eucalyptus sulfate paper sheets. We demonstrate the importance of synchronizing the solvent evaporation rate and capillary fluid velocity to ensure mesopore formation as well as the influence of the fiber type and sol-gel solution composition. The combination of argon and krypton sorption, SAXS, TEM and CLSM provides systematic analysis of the porous structure and the silica distribution along the cellulose paper fiber length and cross-section. These results provide a deeper understanding of mesopore formation in paper and how the latter is influenced by paper fluidic properties.

Capillary Flow-Driven Microfluidics Combined with a Paper Device for Fast User-Friendly Detection of Heavy Metals in Water

Human exposure to heavy metals is a concerning global problem because of its detrimental effect on our health and ecosystem. Assessing the levels of these metals is cost- and labor-intensive and nonuser friendly because current analysis approaches typically rely on heavy instrumentations like inductively coupled plasma-mass spectrometry, which is only possible in centralized labs. Hence, simple economical detection methods are in high demand in developing countries and areas with insufficient infrastructure, professional experts, and appropriate environmental treatment. Several microfluidic paper-based analytical devices have been reported as promising alternatives to conventional testing methods for on-site heavy metal detection. Paper-based microfluidics are advantageous because of their simple fabrication, biodegradability, low cost, and ability to operate without pumps. However, typical assay times for current platforms are slow, and they typically rely on pipetting a fixed volume into the assay cards. This adds complexity in actual field scenarios. Here, we report a novel, inexpensive, and straightforward capillary-driven microfluidic device combined with paper for rapid and user-friendly detection of Ni(II), Cu(II), and Fe(III) in water. A colorimetric approach was adopted to quantify these metals. The device was able to produce a homogeneous color signal within 8 s of sample insertion. The limit of detection and limit of quantification were calculated to be 2 and 6.67 ppm for nickel, 0.3 and 1 ppm for Cu, and 1.1 and 3.67 ppm for Fe, respectively. The majority (>90%) of the collected samples showed recovery in the 80-110% range with acceptable accuracy and precision (<15% RSD) for a colorimetric device. This technique can be beneficial for rapidly assessing heavy metal exposure in drinking and surface water at drastically reduced assay time and is the first example of capillary flow-driven microfluidic devices as a transport medium for heavy metal detection.

Functional Metalloblock Copolymers for the Preparation and In Situ Functionalization of Porous Silica Films

Stimuli-responsive mesoporous silica films were prepared by evaporation-induced self-assembly through the physical entrapment of a functional metalloblock copolymer structuring agent, which simultaneously served to functionalize the mesopore. After end-functionalization with a silane group, the applied functional metalloblock copolymers were covalently integrated into the silica mesopore wall. In addition, they were partly degraded after the formation of the mesoporous film, which enabled the precise design of accessible mesopores. These polymer-silica hybrid materials exhibited remarkable and gating ionic permselectivity and offer the potential for highly precise pore filling design and combination with high-throughput printing techniques. This in situ functionalization strategy of mesoporous silica using responsive metalloblock copolymers has the potential to improve how we approach the design of complex architectures at the nanoscale for tailored transport. This functionalization strategy paves the way for a variety of technologies based on molecular transport in nanoscale pores, including separation, sensing, catalysis, and energy conversion.

Metallopolymer-Based Block Copolymers for the Preparation of Porous and Redox-Responsive Materials

Metallopolymers are a unique class of functional materials because of their redox-mediated optoelectronic and catalytic switching capabilities and, as recently shown, their outstanding structure formation and separation capabilities. Within the present study, (tri)block copolymers of poly(isoprene) (PI) and poly(ferrocenylmethyl methacrylate) having different block compositions and overall molar masses up to 328 kg mol^-1 are synthesized by anionic polymerization. The composition and thermal properties of the metallopolymers are investigated by state-of-the-art polymer analytical methods comprising size exclusion chromatography, ¹H NMR spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. As a focus of this work, excellent microphase separation of the synthesized (tri)block copolymers is proven by transmission electron microscopy, scanning electron microcopy, energy-dispersive X-ray spectroscopy, small-angle X-ray scattering measurements showing spherical, cylindrical, and lamellae morphologies. As a highlight, the PI domains are subjected to ozonolysis for selective domain removal while maintaining the block copolymer morphology. In addition, the novel metalloblock copolymers can undergo microphase separation on cellulose-based substrates, again preserving the domain order after ozonolysis. The resulting nanoporous structures reveal an intriguing switching capability after oxidation, which is of interest for controlling the size and polarity of the nanoporous architecture.

Free-Standing and Self-Crosslinkable Hybrid Films by Core-Shell Particle Design and Processing

The utilization and preparation of functional hybrid films for optical sensing applications and membranes is of utmost importance. In this work, we report the convenient and scalable preparation of self-crosslinking particle-based films derived by directed self-assembly of alkoxysilane-based cross-linkers as part of a core-shell particle architecture. The synthesis of well-designed monodisperse core-shell particles by emulsion polymerization is the basic prerequisite for subsequent particle processing via the melt-shear organization technique. In more detail, the core particles consist of polystyrene (PS) or poly(methyl methacrylate) (PMMA), while the comparably soft particle shell consists of poly(ethyl acrylate) (PEA) and different alkoxysilane-based poly(methacrylate)s. For hybrid film formation and convenient self-cross-linking, different alkyl groups at the siloxane moieties were investigated in detail by solid-state Magic-Angle Spinning Nuclear Magnetic Resonance (MAS, NMR) spectroscopy revealing different crosslinking capabilities, which strongly influence the properties of the core or shell particle films with respect to transparency and iridescent reflection colors. Furthermore, solid-state NMR spectroscopy and investigation of the thermal properties by differential scanning calorimetry (DSC) measurements allow for insights into the cross-linking capabilities prior to and after synthesis, as well as after the thermally and pressure-induced processing steps. Subsequently, free-standing and self-crosslinked particle-based films featuring excellent particle order are obtained by application of the melt-shear organization technique, as shown by microscopy (TEM, SEM).