Hierarchical porous metal-organic gels and derived materials: from fundamentals to potential applications.

Metal-organic gels (MOGs) emerged as a novel class of functional soft materials in which the scaffolding framework is fabricated by metal-ligand coordination in combination with other supramolecular interactions (for example, hydrogen bonding or π-π stacking). Through the combination of organic and inorganic (metal/metal-oxo clusters) building blocks, significant steps forward have been made in the development of new electrochemical sensors, superhydrophobic materials and ion storage devices, among others. These leaps forward are to some extend induced by the intrinsic hierarchical microporous/mesoporous pore structure of these metal-organic materials. Within this review we give an overview of recent developments of this growing field. First, we shed light onto the parallels to the well-established field of conventional gels and outline similarities and differences. Afterwards, we classify different types of MOGs according to their architectural/structural nature: (1) pristine MOGs, (2) hybrid MOGs, (3) crosslinking-based MOGs and (4) MOG-derived materials. Furthermore, we look at the different properties of MOGs and the requirements for the preparation of spatially patterned macro-structured MOGs by emerging additive manufacturing technologies. Moreover, different potential fields of application for MOGs and MOG derived materials are critically evaluated and potential improvements and pitfalls in comparison to traditional gel-based materials are given. Finally, a comprehensive outlook into future directions for the development of MOGs is provided.

Covalent Graphene-MOF Hybrids for High-Performance Asymmetric Supercapacitors.

In this work, the covalent attachment of an amine functionalized metal-organic framework (UiO-66-NH2  = Zr6 O4 (OH)4 (bdc-NH2 )6 ; bdc-NH2  = 2-amino-1,4-benzenedicarboxylate) (UiO-Universitetet i Oslo) to the basal-plane of carboxylate functionalized graphene (graphene acid = GA) via amide bonds is reported. The resultant GA@UiO-66-NH2 hybrid displayed a large specific surface area, hierarchical pores and an interconnected conductive network. The electrochemical characterizations demonstrated that the hybrid GA@UiO-66-NH2 acts as an effective charge storing material with a capacitance of up to 651 F g-1 , significantly higher than traditional graphene-based materials. The results suggest that the amide linkage plays a key role in the formation of a π-conjugated structure, which facilitates charge transfer and consequently offers good capacitance and cycling stability. Furthermore, to realize the practical feasibility, an asymmetric supercapacitor using a GA@UiO-66-NH2 positive electrode with Ti3 C2 TX MXene as the opposing electrode has been constructed. The cell is able to deliver a power density of up to 16 kW kg-1 and an energy density of up to 73 Wh kg-1 , which are comparable to several commercial devices such as Pb-acid and Ni/MH batteries. Under an intermediate level of loading, the device retained 88% of its initial capacitance after 10 000 cycles.

Unique cellular network formation guided by heterostructures based on reduced graphene oxide - Ti3C2Tx MXene hydrogels.

Two-dimensional (2D) materials remain highly interesting for assembling three-dimensional (3D) structures, amongst others, in the form of macroscopic hydrogels. Herein, we present a novel approach for inducing chemical inter-sheet crosslinks via an ethylenediamine mediated reaction between Ti3C2Tx and graphene oxide in order to obtain a reduced graphene oxide-MXene (rGO-MXene) hydrogel. The composite hydrogels are hydrophilic with a stiffness of ~20 kPa. They also possess a unique inter-connected porous architecture, which led to a hitherto unprecedented ability of human cells across three different types, epithelial adenocarcinoma, neuroblastoma and fibroblasts, to form inter-connected three-dimensional networks. The attachments of the cells to the rGO-MXene hydrogels were superior to those of the sole rGO-control gels. This phenomenon stems from the strong affinity of cellular protrusions (neurites, lamellipodia and filopodia) to grow and connect along architectural network paths within the rGO-MXene hydrogel, which could lead to advanced control over macroscopic formations of cellular networks for technologically relevant bioengineering applications, including tissue engineering and personalized diagnostic networks-on-chip. STATEMENT OF SIGNIFICANCE: Conventional hydrogels are made of interconnected polymeric fibres. Unlike conventional case, we used hydrothermal and chemical approach to form interconnected porous hydrogels made of two-dimensional flakes from graphene oxide and metal carbide from a new family of MXenes (Ti3C2Tx). This way, we formed three-dimensional porous hydrogels with unique porous architecture of well-suited chemical surfaces and stiffness. Cells from three different types cultured on these scaffolds formed extended three-dimensional networks - a feature of extended cellular proliferation and pre-requisite for formation of organoids. Considering the studied 2D materials typically constitute materials exhibiting enhanced supercapacitor performances, our study points towards better understanding of design of tissue engineering materials for the future bioengineering fields including personalized diagnostic networks-on-chip, such as artificial heart actuators.

The Influence of Oxygen Concentration during MAX Phases (Ti3AlC2) Preparation on the α-Al2O3 Microparticles Content and Specific Surface Area of Multilayered MXenes (Ti3C2Tx).

The high specific surface area of multilayered two-dimensional carbides called MXenes, is a critical feature for their use in energy storage systems, especially supercapacitors. Therefore, the possibility of controlling this parameter is highly desired. This work presents the results of the influence of oxygen concentration during Ti3AlC2 ternary carbide-MAX phase preparation on α-Al2O3 particles content, and thus the porosity and specific surface area of the Ti3C2Tx MXenes. In this research, three different Ti3AlC2 samples were prepared, based on TiC-Ti2AlC powder mixtures, which were conditioned and cold pressed in argon, air and oxygen filled glove-boxes. As-prepared pellets were sintered, ground, sieved and etched using hydrofluoric acid. The MAX phase and MXene samples were analyzed using scanning electron microscopy and X-ray diffraction. The influence of the oxygen concentration on the MXene structures was confirmed by Brunauer-Emmett-Teller surface area determination. It was found that oxygen concentration plays an important role in the formation of α-Al2O3 inclusions between MAX phase layers. The mortar grinding of the MAX phase powder and subsequent MXene fabrication process released the α-Al2O3 impurities, which led to the formation of the porous MXene structures. However, some non-porous α-Al2O3 particles remained inside the MXene structures. Those particles were found ingrown and irremovable, and thus decreased the MXene specific surface area.

Cytotoxicity Assessment of Ti-Al-C Based MAX Phases and Ti3C2Tx MXenes on Human Fibroblasts and Cervical Cancer Cells.

MXenes are a novel family of 2D materials, which are extensively investigated for common use in energy storage systems, nanoelectronics, and electromagnetic shielding. Although their unique physicochemical properties render their wide applicability, their cytotoxic response and safety use still remain a concern. From this perspective, it is imperative to perform an in vitro investigation of the influence of different forms of MXenes and their precursors on the human cell lines. Therefore, we prepared a selection of multi-, few-, and single-layered Ti3C2Tx, as well as TiC, Ti2AlC, and Ti3AlC2, and as recently indicated in nanomaterials safety field, we fully characterized their morphology and size (electron microscopies, atomic force microscopy and dynamic light scattering), purity (Raman spectroscopy and X-ray powder diffraction), as well as surface charge (zeta potential). Then, we investigated and compared several biological effects (cytotoxicity, membrane permeability, reactive oxygen stress, and mechanical stress) induced by MXenes, TiC, and parental MAX phases on the human fibroblasts (MSU1.1) and cervical cancer cells (HeLa), as model cells differing by their tumorigenicity. The analyses revealed that exposure to higher concentrations (≥400 µg/mL) of TiC, Ti2AlC, and Ti3AlC2 particles with the sizes <44 µm could be harmful, inducing a significant cytotoxic effect via oxidative and mechanical stress generation. All of the Ti3C2Tx forms remained safe to MSU1.1 cells with only slight cytotoxic behavior in the highest concentration regime. The cytotoxic behavior was also cell-type dependent, with higher cytotoxicities observed for cells of cancer origin. Finally, the cell response toward multilayered MXenes in an in vitro system, using scanning electron microscopy was depictured. Our work increases understanding of the safe use of MXene materials and points toward their possible use in fields spanning from energy storage systems to medical devices.

GQDs-MSNs nanocomposite nanoparticles for simultaneous intracellular drug delivery and fluorescent imaging.

Although number of stimuli-responsive drug delivery systems based on mesoporous silica nanoparticles (MSNs) have been developed, the simultaneous real-time monitoring of carrier in order to guarantee proper drug targeting still remains as a challenge. GQDs-MSNs nanocomposite nanoparticles composed of graphene quantum dots (GQDs) and MSNs are proposed as efficient doxorubicin delivery and fluorescent imaging agent, allowing to monitor intracellular localization of a carrier and drug diffusion route from the carrier. Graphene quantum dots (average diameter 3.65 ± 0.81 nm) as a fluorescent agent were chemically immobilized onto mesoporous silica nanoparticles (average diameter 44.08 ± 7.18 nm) and loaded with doxorubicin. The structure, morphology, chemical composition, and optical properties as well as drug release behavior of doxorubicin (DOX)-loaded GQDs-MSNs were investigated. Then, the in vitro cytotoxicity, cellular uptake, and intracellular localization studies were carried out. Prepared GQDs-MSNs form stable suspensions exhibiting excitation-dependent photoluminescence (PL) behavior. These nanocomposite nanoparticles can be easily DOX-loaded and show pH- and temperature-dependent release behavior. Cytotoxicity studies proved that GQDs-MSNs nanocomposite nanoparticles are nontoxic; however, when loaded with drug, they enable the therapeutic activity of DOX via its active delivery and release. GQDs-MSNs owing to their fluorescent properties and efficient in vitro cellular internalization via caveolae/lipid raft-dependent endocytosis show a high potential for the optical imaging, including the simultaneous real-time optical tracking of the loaded drug during its delivery and release. Graphical abstractᅟ.

Anchoring Fe3O4 nanoparticles in a reduced graphene oxide aerogel matrix via polydopamine coating.

Reduced graphene oxide-magnetite hybrid aerogels attract great interest thanks to their potential applications, e.g., as magnetic actuators. However, the tendency of magnetite particles to migrate within the matrix and, ultimately, escape from the aerogel structure, remains a technological challenge. In this article we show that coating magnetite particles with polydopamine anchors them on graphene oxide defects, immobilizing the particles in the matrix and, at the same time, improving the aerogel structure. Polydopamine coating does not affect the magnetic properties of magnetite particles, making the fabricated materials promising for industrial applications.

Electron Paramagnetic Resonance Imaging and Spectroscopy of Polydopamine Radicals.

A thorough investigation of biomimetic polydopamine (PDA) by Electron Paramagnetic Resonance (EPR) is shown. In addition, temperature dependent spectroscopic EPR data are presented in the range 3.8-300 K. Small discrepancies in magnetic susceptibility behavior are observed between previously reported melanin samples. These variations were attributed to thermally acitivated processes. More importantly, EPR spatial-spatial 2D imaging of polydopamine radicals on a phantom is presented for the first time. In consequence, a new possible application of polydopamine as EPR imagining marker is addressed.