This review summarizes recent progress in the development and applications of metal–organic gels (MOGs) and their hybrids and derivatives dividing them into subclasses and discussing their synthesis, design and structure–property relationship.

Despite intensive research on the application of two-dimensional (2D) materials, including MXenes, in nanomedicine, the knowledge concerning the mechanisms responsible for their observed bio-effects is far from being understood. Here we present insight into the mechanism of toxicity in vitro of the 2D Ti₃C₂ MXene. The most important results of this work are that using simple, inexpensive, post-delamination treatments, such as ultrasonication or mild thermal oxidation it is possible to ‘tune’ the cytotoxicity of the Ti₃C₂T _z flakes. Sonication of Ti₃C₂T _z flakes, or sonication followed by mild oxidation in the water at 60 °C, renders them selectively toxic to cancer cells as compared to non-malignant ones. It relates to the appearance of superficial titanium (III) oxide (Ti₂O₃) layer corresponding to the type of post-treatment. The presence of surface-Ti₂O₃ results in a noticeably higher generation of oxidative stress compared to pristine 2D Ti₃C₂. Our findings give evidence that the sonication and thermal treatments were successful in changing the nature of the surface terminations on the Ti₃C₂T _z surfaces. This study makes a significant contribution to the future rationalized surface-management of 2D Ti₃C₂ MXene as well as encourages new rationalized applications in biotechnology and nanomedicine. Bullet points: 1. First study on 2D Ti₃C₂ MXene superficially oxidized to titanium (III) oxide i.e. Ti₂O₃. 2. By sonication Ti₃C₂Tz MXene flakes followed by mild thermal oxidation in the water at 60 °C for 24 h, it is possible to ‘tune’ the toxicity of the flakes to cancerous cell lines. 3. Decreases in cell viabilities were dose-dependent. 4. Highest cytotoxic effect was observed for thermally oxidized samples. 5. The thermally oxidized samples were also selectively toxic towards all cancerous cell lines up to 375 mg l⁻¹. 6. Reactive oxygen species generation was identified as a mechanism of toxicity.

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 Ti₃C₂T_x 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 (Ti₃C₂T_x). 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 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 Ti₃C₂T_x 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.