Ti3C2Tx MXene has gained significant attention for biosensor and supercapacitor applications because of 1) its metallic conductivity, large surface area, and reversible surface redox reactions led to high pseudocapacitance and high-rate performance; 2) the unique 2D morphology and high biocompatibility drive great motivation to design advanced nanohybrid systems with bio-receptors; 3) the high density of surface functional groups offers improved biomolecule loading and flexibility for further functionalization. In this thesis, biosensors and electrochemical energy storage devices based on Ti3C2Tx MXene are proposed. Specifically, Ti3C2Tx nanosheets were uniformly functionalized with aminosilane to provide a covalent binding for the immobilized bio-receptor (anti-CEA) for label-free ultrasensitive detection of cancer biomarker (CEA). [Ru(NH3)6]3+ is discovered as the preferable redox probe for biosensing. The fabricated MXene-based sensor exhibits a more comprehensive linear detection range and high sensitivity. Further, Ti3C2Tx nanosheets were introduced as the transducer, and Ti3C2Tx /Prussian blue (Ti3C2Tx/PB) composite was synthesized for sensitive detection of hydrogen peroxide. Meanwhile, a one-step patterning process for highly conductive nitrogen-doped laser-scribed graphene (N-LSG) has been developed. Working electrodes (Ti3C2Tx/PB/N-LSG) were extended by using different enzymes for corresponding biomarker detection, namely glucose, lactate, and alcohol. The enzyme/Ti3C2Tx/PB/N-LSG electrodes exhibit significantly improved electrocatalytic activity and outperform previously reported on-chip graphene-based biosensors. Further, a stretchable, wearable, and multifunctional Ti3C2Tx-based biosensor were designed for durable and sensitive detection of biomarkers in sweat. A unique modular design enabled a simple exchange of the specific sensing electrode to target the desired analytes, while an implemented three-phase interface design for the constant supply of oxygen led to superior sensor performance and stability. As expected, during in-vitro perspiration monitoring of human subjects, the physiochemistry signals (glucose and lactate level) could be measured simultaneously with high sensitivity and good repeatability, outperforming traditional reported graphene/PB- and CNTs/PB-based biosensors. Finally, we developed an in-plane hybrid microsupercapacitor, employing battery-type CuFe-Prussian blue analog (CuFe-PBA) as the positive electrode and pseudocapacitive Ti3C2Tx as the negative electrode. Due to the excellent match of the two types of high-rate performance materials in proton-based electrolyte, the designed on-chip device achieved excellent electrochemical performance.

During recent years, new types of materials have been discovered with unique properties. One family of such materials are two-dimensional materials, which include graphene and MXene. These materials are stronger, more flexible, and have higher conductivity than other materials. As such they are highly interesting for new applications, e.g. specialized in vivo drug delivery systems, hydrogen storage, or as replacements of common materials in e.g. batteries, bulletproof clothing, and sensors. The list of potential applications is long for these new materials. As these materials are almost entirely made up of surfaces, their properties are strongly influenced by interaction between their surfaces, as well as with molecules or adatoms attached to the surfaces (surface groups). This interaction can change the materials and their properties, and it is therefore imperative to understand the underlying mechanisms. Surface groups on two-dimensional materials can be studied by Transmission Electron Microscopy (TEM), where high energy electrons are transmitted through a sample and the resulting image is recorded. However, the high energy needed to get enough resolution to observe single atoms damages the sample and limits the type of materials which can be analyzed. Lowering the electron energy decreases the damage, but the image resolution at such conditions is severely limited by inherent imperfections (aberrations) in the TEM. During the last years, new TEM models have been developed which employ a low acceleration voltage together with aberration correction, enabling imaging at the atomic scale without damaging the samples. These aberration-corrected TEMs are important tools in understanding the structure and chemistry of two-dimensional materials. In this thesis the two-dimensional materials graphene and Ti3C2Tx MXene have been investigated by low-voltage, aberration-corrected (scanning) TEM. High temperature annealing of graphene covered by residues from the synthesis is studied, as well as the structure and surface groups on single and double Ti3C2Tx MXene. These results are important contributions to the understanding of this class of materials and how their properties can be controlled.

The demand for today's material resources for energy storage is rapidly increasing and can result in both environmental and political conflicts that can affect the development of electronic devices due to high prices and limitations of raw materials for batteries. In this study, potential future composite electrodes were synthesised with an ex-situ approach by compositing redox-active 2,6-Diaminoanthraquinone and 1,3,5-Triformylphloroglucinol covalent organic framework (DAAQ-TFP COF) with conductive delaminated Ti3C2Tx MXene to maximise the number of redox-active moieties during cycling. In addition, solvothermal synthesis with the implementation of mechanical grinding as an exfoliation method was used to try to obtain DAAQ-TFP nanosheets to increase both the contact area between the two materials and the number of charge carriers. The sample was analysed with PXRD and BET surface analysis to characterise the crystallinity meanwhile SEM was utilised to study the morphology of the COF and the composite material. The specific capacitance of each electrode was estimated by cyclic voltammetry. The study showed a decrease in reduced specific capacitance with lower MXene content. Hence, this concludes pure Ti3C2Tx sheets have the highest capacitance contribution with a value of 48.79 Fg-1 meanwhile the composite electrode with a ratio of 1:1 was estimated to 32.26 Fg-1 with 0.0928 % of its moieties undergoing a redox reaction. A reduced capacitance with an increased COF-MXene ratio indicates that MXene contributes with more capacity relative to the COF, in combination with a non-successful exfoliation of DAAQ-TFP to single-layered nanosheets, reducing the interactions between the two materials.