Degradable and Thermosensitive Microgels with Tannic Acid as the Sole Cross-Linker.

Poly(N-isopropylacrylamide) (PNIPAM)-tannic acid (TA) microgels were successfully prepared via surfactant-free emulsion polymerization (SFEP) at 70 °C in aqueous solution using N-isopropylacrylamide (NIPAM) as the monomer and a natural polyphenol macromolecule, TA, as the sole cross-linker. The cross-linking network of the PNIPAM-TA microgels was confirmed to contain both physical cross-linking structures formed via hydrogen-bonding interactions between TA and PNIPAM chains and chemical cross-linking structures formed via capturing the radicals of propagating polymer chains by catechol and pyrogallol groups of TA. Furthermore, TA was applied to modify the surface of hydrophobic Fe3O4 nanoparticles, leading to hydrophilic Fe3O4@TA composite nanoparticles, which were successfully used as the cross-linker to fabricate PNIPAM-Fe3O4@TA organic-inorganic hybrid microgels. The obtained PNIPAM-TA and PNIPAM-Fe3O4@TA organic-inorganic hybrid microgels had a uniform spherical shape with a relatively narrow size distribution and exhibited thermosensitive behavior and pH-tunable degradation. The PNIPAM-TA microgels were stable in the pH range of 1.3-11.1 but underwent complete degradation with pH above 11.4. The PNIPAM-Fe3O4@TA hybrid microgels were partially degraded at pH values of 1.3 and 2.1, stable in the pH range of 3.1-11.1, and underwent complete degradation at pH above 11.4. The partial degradation of PNIPAM-Fe3O4@TA organic-inorganic hybrid microgels under strong acidic conditions was attributed to the disintegration of Fe3O4 nanoparticles. The complete degradation of both microgels at pH above 11.4 was attributed to the hydrolysis of ester groups of TA under strong alkali conditions.

Ultrastiff, Tough, and Healable Ionic-Hydrogen Bond Cross-Linked Hydrogels and Their Uses as Building Blocks To Construct Complex Hydrogel Structures.

We report the ultrastiff and tough poly(acrylamide- co-acrylic acid)/Na-alginate/Fe3+ (P(AM- co-AA)/Na-alginate/Fe3+) hydrogel via the formation of hybrid ionic-hydrogen bond cross-linking networks. The optimal P(AM- co-AA)/Na-alginate/Fe3+ hydrogel possessed super high elastic modulus (∼24.6 MPa), tensile strength (∼10.4 MPa), compression strength (∼44 MPa), and toughness (∼4800 J/m2). The P(AM- co-AA)/Na-alginate/Fe3+ hydrogel was highly stable and maintained its superior mechanical properties in 0.5-2 M NaCl solution, aqueous solution with pH ranging from 4 to 10. The ionic cross-linking networks of the P(AM- co-AA)/Na-alginate/Fe3+ hydrogels can be locally and selectively dissociated by treating with aqueous NaOH solution with pH of 13 for 1 min and reformed by locally adding the additional Fe3+ solutions, making the hydrogels healable and cohesive. The healed hydrogels from the cutting surfaces can bear a tensile strength of up to 7.1 MPa. Various complex hydrogel structures can be constructed by using the P(AM- co-AA)/Na-alginate/Fe3+ hydrogels as building blocks via the adhesion of as-prepared hydrogels.

Electrokinetics in Micro-channeled Cantilevers: Extending the Toolbox for Reversible Colloidal Probes and AFM-Based Nanofluidics.

The combination of atomic force microscopy (AFM) with nanofluidics, also referred to as FluidFM, has facilitated new applications in scanning ion conductance microscopy, direct force measurements, lithography, or controlled nanoparticle deposition. An essential element of this new type of AFMs is its cantilever, which bears an internal micro-channel with a defined aperture at the end. Here, we present a new approach for in-situ characterization of the internal micro-channels, which is non-destructive and based on electrochemical methods. It allows for probing the internal environment of a micro-channeled cantilever and the corresponding aperture, respectively. Acquiring the streaming current in the micro-channel allows to determine not only the state of the aperture over a wide range of ionic strengths but also the surface chemistry of the cantilever's internal channel. The high practical applicability of this method is demonstrated by detecting the aspiration of polymeric, inorganic and hydrogel particles with diameters ranging from several µm down to 300 nm. By verifying in-situ the state of the aperture, i.e. open versus closed, electrophysiological or nano-deposition experiments will be significantly facilitated. Moreover, our approach is of high significance for direct force measurements by the FluidFM-technique and sub-micron colloidal probes.

Highly Selective and Sensitive Detection of Pb2+ in Aqueous Solution Using Tetra(4-pyridyl)porphyrin-Functionalized Thermosensitive Ionic Microgels.

Tetra(4-pyridyl)porphyrin (TPyP)-functionalized thermosensitive ionic microgels (TPyP5-MGs) were synthesized by a two-step quaternization method. The obtained TPyP5-MGs have a hydrodynamic radius of about 189 nm with uniform size distribution and exhibit thermosensitive character. The TPyP5-MG microgel suspensions can optically respond to trace Pb2+ ions in aqueous solution with high sensitivity and selectivity over the interference of other 19 species of metal ions (Yb3+, Gd3+, Ce3+, La3+, Bi3+, Ba2+, Zn2+, Ni2+, Co2+, Mn2+, Cr3+, K+, Na+, Li+, Al3+, Cu2+, Ag+, Cd2+, and Fe3+) by using UV-visible spectroscopy. The sensitivity of TPyP5-MGs toward Pb2+ can be further improved by increasing the solution temperature. The limit of detection for TPyP5-MG microgel suspensions in the detection of Pb2+ in aqueous solution at 50 °C is about 25.2 nM, which can be further improved to be 5.9 nM by using the method of higher order derivative spectrophotometry and is much lower than the U. S. EPA standard for the safety limit of Pb2+ ions in drinking water. It is further demonstrated that the TPyP5-MG microgel suspensions have a potential application in the detection of Pb2+ in real world samples, which give consistent results with those obtained by elemental analysis.