Daniel Bouša

Carboxylic Carbon Quantum Dots as a Fluorescent Sensing Platform for DNA Detection

The demand for simple, sensitive, affordable, and selective DNA biosensors is ubiquitous, due to the important role that DNA detection performs in the areas of disease diagnostics, environment monitoring, and food safety. A novel application of carboxylic carbon quantum dots (cCQD) is highlighted in this study. Herein, cCQD function as a nanoquencher in the detection of nucleic acid based on a homogeneous fluorescent assay. To that purpose, the performance of two types of cCQD, namely, citric acid QD and malic acid QD, is evaluated. The principle behind the sensing of nucleic acid lies in the different propensity of single-stranded DNA and double-stranded DNA to adsorb onto the surface of cCQD. For both types of cCQD, a superior range of detection of at least 3 orders of magnitude is achieved, and the potential to distinguish single-base mismatch is also exhibited. These findings are anticipated to provide valuable insights on the employment of cCQD for the fabrication of future DNA biosensors.

Layered Black Phosphorus: Strongly Anisotropic Magnetic, Electronic, and Electron‐Transfer Properties

Layered elemental materials, such as black phosphorus, exhibit unique properties originating from their highly anisotropic layered structure. The results presented herein demonstrate an anomalous anisotropy for the electrical, magnetic, and electrochemical properties of black phosphorus. It is shown that heterogeneous electron transfer from black phosphorus to outer- and inner-sphere molecular probes is highly anisotropic. The electron-transfer rates differ at the basal and edge planes. These unusual properties were interpreted by means of calculations, manifesting the metallic character of the edge planes as compared to the semiconducting properties of the basal plane. This indicates that black phosphorus belongs to a group of materials known as topological insulators. Consequently, these effects render the magnetic properties highly anisotropic, as both diamagnetic and paramagnetic behavior can be observed depending on the orientation in the magnetic field.

Air-stable superparamagnetic metal nanoparticles entrapped in graphene oxide matrix

Superparamagnetism is a phenomenon caused by quantum effects in magnetic nanomaterials. Zero-valent metals with diameters below 5 nm have been suggested as superior alternatives to superparamagnetic metal oxides, having greater superspin magnitudes and lower levels of magnetic disorder. However, synthesis of such nanometals has been hindered by their chemical instability. Here we present a method for preparing air-stable superparamagnetic iron nanoparticles trapped between thermally reduced graphene oxide nanosheets and exhibiting ring-like or core-shell morphologies depending on iron concentration. Importantly, these hybrids show superparamagnetism at room temperature and retain it even at 5 K. The corrected saturation magnetization of 185 Am2 kg–1 is among the highest values reported for iron-based superparamagnets. The synthetic concept is generalized exploiting functional groups of graphene oxide to stabilize and entrap cobalt, nickel and gold nanoparticles, potentially opening doors for targeted delivery, magnetic separation and imaging applications.

Pnictogen (As, Sb, Bi) Nanosheets for Electrochemical Applications Are Produced by Shear Exfoliation Using Kitchen Blenders

Layered materials are of high importance because of their anisotropy and as a source of 2D materials. Whilst there is a plethora of multi-elemental 2D materials, the number mono-elemental 2D materials is rather limited. Herein, we demonstrate that aqueous shear exfoliation can be used to obtain As, Sb, and Bi exfoliated nanosheets. Morphological and chemical characterization of the exfoliated materials shows a decrease in thickness, sheet-to-nanosheet scale, and partial oxidation owing to a higher surface area. The electrochemical performance is tested in terms of inherent electrochemistry, electron transfer, and sensing applications as demonstrated with ascorbic acid. Potential energy-related applications are evaluated in the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR), with shear-exfoliated Sb having the best electrochemical performance overall. These findings will have a profound impact on the preparation and application of 2D mono-elemental materials.

The Covalent Functionalization of Layered Black Phosphorus by Nucleophilic Reagents

Layered black phosphorus has been attracting great attention due to its interesting material properties which lead to a plethora of proposed applications. Several approaches are demonstrated here for covalent chemical modifications of layered black phosphorus in order to form P-C and P-O-C bonds. Nucleophilic reagents are highly effective for chemical modification of black phosphorus. Further derivatization approaches investigated were based on radical reactions. These reagents are not as effective as nucleophilic reagents for the surface covalent modification of black phosphorus. The influence of covalent modification on the electronic structure of black phosphorus was investigated using ab initio calculations. Covalent modification exerts a strong effect on the electronic structure including the change of band-gap width and spin polarization.

Nanosized graphane (C1H1.14)n by hydrogenation of carbon nanofibers by Birch reduction method

Graphane, fully hydrogenated graphene with the composition (C1H1)n, has been theoretically predicted but never experimentally realized. Graphane stands out of the variety of heteroatom modified graphene for its well defined structure. Here we show that by employing Birch reduction on graphite nanofibers, one can reach hydrogenation levels close to 100%. We name this material graphane or graphane-like since its composition is relatively close to ideal theoretical stoichiometry C1H1. We systematically study the effect of the size and structure of the starting material and conditions of the synthesis. The morphology and properties of the synthesized graphane-like material are strongly dependent on the structure of the starting material. The extremely highly hydrogenated nanographanes should find applications ranging from nanoelectronics to electrochemistry such as in supercapacitors or electrocatalysts.

Fluorographene Modified by Grignard Reagents: A Broad Range of Functional Nanomaterials

Fluorographene is the youngest stoichiometric derivative of graphene; hence, its reactivity is only poorly explored. Compared to graphene, the significantly higher reactivity of C-F bonds makes this material a suitable platform for a large number of chemical modifications. Fluorographene is also the only member of the halographene family that can be prepared in the stoichiometric composition (C1 F1 ). Herein, the chemical modification of fluorographene with Grignard reagents, which are well known in organic synthesis for the formation of new C-C bonds, is presented. The reaction with alkyl magnesium bromides led to successful modification of fluorographene with ethyl, vinyl, ethynyl and propargyl groups. Chemical characterisation showed the presence of covalently bonded functional groups in a high concentration exceeding one functional group per C6 motif. The reactivity of Grignard reagents with fluorographene decreased from ethyl to ethynyl. The terminal carbon-carbon triple bonds were used for click reactions with organic azides leading to the formation of triazole rings. These findings open up a broad spectrum of opportunities for simple and robust modification of graphene by chemical reactions proceeding at room temperature under mild conditions. These results have major application potential in sensing, biomedical and energy-related applications.

Toward graphene chloride: chlorination of graphene and graphene oxide

Halogenated graphene derivatives are interesting due to their outstanding physical and chemical properties. In this paper, we present various methods for the synthesis of chlorinated graphene derivatives from graphene oxide and thermally reduced graphene. We performed exfoliation of graphene oxide in a chlorine atmosphere, plasma assisted exfoliation of graphite oxide using microwave radiation and finally direct chlorination of thermally reduced graphene by liquid chlorine under deep UV irradiation. The influence of the chlorination method on the resulting chlorinated graphenes was investigated by characterization of the graphenes, which was carried out using various techniques, including SEM, SEM-EDS, high-resolution XPS, FTIR, STA and Raman spectroscopy. Electrochemical properties were investigated by cyclic voltammetry. Although the graphenes were structurally similar, they involved remarkably different chlorine concentrations. The most highly chlorinated graphene exhibits a chlorine concentration of 11.7 at%. Chlorinated graphenes with such properties could be used for reversible chlorine storage or as a starting material for further chemical modifications.

Hydrogenated Graphenes by Birch Reduction: Influence of Electron and Proton Sources on Hydrogenation Efficiency, Magnetism, and Electrochemistry

Interest in chemical functionalisation of graphenes today is largely driven by associated changes to its physical and material properties. Functionalisation with hydrogen was employed to obtain hydrogenated graphenes (also termed graphane if fully hydrogenated), which exhibited properties including fluorescence, magnetism and a tuneable band gap. Although the classical Birch reduction has been employed for hydrogenation of graphite oxide, variation exists between the choice of alkali metals and alcohols/water as quenching agents. A systematic study of electron (Li, Na, K, Cs) and proton sources (tBuOH, iPrOH, MeOH, H2O) has been performed to identify optimal conditions. The proton source exerted a great influence on the resulting hydrogenation with water and out-performed alcohols, and the lowest carbon-to-hydrogen ratio was observed with sodium and water with composition of C1.4H1O0.3. Although ferromagnetism at room temperature correlates well with increasing hydrogen concentrations, small contributions from trace iron impurities cannot be completely eliminated. In contrast, hydrogenated graphenes exhibit a significant paramagnetic moment at low temperatures that has no correlation with impurities, and therefore, originates from the carbon system. This is in comparison to graphene, which is strongly diamagnetic, and concentrations of paramagnetic centres in hydrogenated graphenes are one order of magnitude larger than that in graphite. Nonetheless, hydrogenation over a particular level might also excessively disrupt intrinsic sp(2) conjugation, resulting in unintended reduction of electrochemical properties. This was observed with heterogeneous electron-transfer rates and it was postulated that hydrogenated graphenes should generally have high defect densities, but only moderately high hydrogenation, should they be employed as electrode materials.

Impact Electrochemistry: Detection of Graphene Nanosheets Labeled with Metal Nanoparticles through Oxygen Reduction Mediation

We have prepared Pt/Fe- and Fe-nanoparticle-labeled graphene sheets and demonstrate that these sheets can be detected by using impact electrochemistry through oxygen reduction mediation.