Electron Spin Relaxation Studies of Polydopamine Radicals

We present a thoroughgoing electron paramagnetic resonance investigation of polydopamine (PDA) radicals using multiple electron paramagnetic resonance techniques at the W-band (94 GHz), electron nuclear double resonance at the Q-band (34 GHz), spin relaxation, and continuous wave measurements at the X-band (9 GHz). The analysis proves the existence of two distinct paramagnetic species in the PDA structure. One of the two radical species is characterized by a long spin-lattice T-1 relaxation time equal to 46.9 ms at 5 K and is assigned to the radical center on oxygen. The obtained data revealed that the paramagnetic species exhibit different electron spin relaxation behaviors due to different couplings to local phonons, which confirm spatial distancing between two radical types. Our results shed new light on the radical structure of PDA, which is of great importance in the application of PDA in materials science and biomedicine and allows us to better understand the properties of these materials and predict their future applications

Unraveling Origins of EPR Spectrum in Graphene Oxide Quantum Dots

Carbon nanostructures are utilized in a plethora of applications ranging from biomedicine to electronics. Particularly interesting are carbon nanostructured quantum dots that can be simultaneously used for bimodal therapies with both targeting and imaging capabilities. Here, magnetic and optical properties of graphene oxide quantum dots (GOQDs) prepared by the top-down technique from graphene oxide and obtained using the Hummers’ method were studied. Graphene oxide was ultra-sonicated, boiled in HNO3, ultra-centrifuged, and finally filtrated, reaching a mean flake size of ~30 nm with quantum dot properties. Flake size distributions were obtained from scanning electron microscopy (SEM) images after consecutive preparation steps. Energy-dispersive X-ray (EDX) confirmed that GOQDs were still oxidized after the fabrication procedure. Magnetic and photoluminescence measurements performed on the obtained GOQDs revealed their paramagnetic behavior and broad range optical photoluminescence around 500 nm, with magnetic moments of 2.41 µB. Finally, electron paramagnetic resonance (EPR) was used to separate the unforeseen contributions and typically not taken into account metal contaminations, and radicals from carbon defects. This study contributes to a better understanding of magnetic properties of carbon nanostructures, which could in the future be used for the design of multimodal imaging agents

Electron Spin Relaxation in Carbon Materials

This article focuses on EPR relaxation measurements in various carbon samples, e.g., natural carbons—anthracite, coal, higher anthraxolites, graphite; synthetically obtained carbons—glassy carbons, fullerenes, graphene, graphene oxide, reduced graphene oxide, graphite monocrystals, HOPG, nanoribbons, diamonds. The short introduction presents the basics of resonant electron spin relaxation techniques, briefly describing the obtained parameters. This review presents gathered results showing the processes leading to electron spin relaxation and typical ranges of electron spin relaxation rates for many different carbon types

Biomedical Applications of Graphene-Based Structures

Graphene and graphene oxide (GO) structures and their reduced forms, e.g., GO paper and partially or fully reduced three-dimensional (3D) aerogels, are at the forefront of materials design for extensive biomedical applications that allow for the proliferation and differentiation/maturation of cells, drug delivery, and anticancer therapies. Various viability tests that have been conducted in vitro on human cells and in vivo on mice reveal very promising results, which make graphene-based materials suitable for real-life applications. In this review, we will give an overview of the latest studies that utilize graphene-based structures and their composites in biological applications and show how the biomimetic behavior of these materials can be a step forward in bridging the gap between nature and synthetically designed graphene-based nanomaterials

CT, MR and EPR imaging of graphene oxide aerogels

The shape and intrinsic pore structure of pristine and partially reduced graphene oxide (prGO) aerogels/hydrogels was imaged using three complementary techniques: Magnetic Resonance Imaging (MRI), Computer Tomography (CT) and Electron Paramagnetic Resonance Imaging (EPRI). Each method is sensitive to different features of the sample: CT images are contrasted with mass density changes across sample, MRI with hydrogen density, nuclear relaxation times, diffusion, and finally EPRI with radical density. The complementarity of the techniques is presented on exemplary chosen prGO aerogels, which are currently researched for variety of applications, such as biomaterials in tissue engineering. All techniques have their strengths and weaknesses which are thoroughly pointed out and discussed. EPRI technique, which has been perfected only during last years, is used here for the first time to image carbon-based foams. Finally scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) is used to characterize surface and elemental content of fabricated prGO aerogels/hydrogels

The Influence of Oxygen Concentration during MAX Phases (Ti₃AlC₂) Preparation on the α-Al₂O₃ Microparticles Content and Specific Surface Area of Multilayered MXenes (Ti₃C₂T_x)

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

Tuning Properties of Partially Reduced Graphene Oxide Fibers upon Calcium Doping

The arrangement of two-dimensional graphene oxide sheets has been shown to influence physico-chemical properties of the final bulk structures. In particular, various graphene oxide microfibers remain of high interest in electronic applications due to their wire-like thin shapes and the ease of hydrothermal fabrication. In this research, we induced the internal ordering of graphene oxide flakes during typical hydrothermal fabrication via doping with Calcium ions (~6 wt.%) from the capillaries. The Ca2+ ions allowed for better graphene oxide flake connections formation during the hydrogelation and further modified the magnetic and electric properties of structures compared to previously studied aerogels. Moreover, we observed the unique pseudo-porous fiber structure and flakes connections perpendicular to the long fiber axis. Pulsed electron paramagnetic resonance (EPR) and conductivity measurements confirmed the denser flake ordering compared to previously studied aerogels. These studies ultimately suggest that doping graphene oxide with Ca2+ (or other) ions during hydrothermal methods could be used to better control the internal architecture and thus tune the properties of the formed structures