Study of Chitosan-Stabilized Ti3C2Tx MXene for Ultrasensitive and Interference-Free Detection of Gaseous H2O2.

The development of sensitive, selective, and reliable gaseous hydrogen peroxide (H2O2) sensors operating at room temperature still represents a remaining challenge. In this work, we have investigated and combined the advantageous properties of a two-dimensional Ti3C2Tx MXene material that exhibits a large specific surface area and high surface activity, with favorable conducting and stabilizing properties of chitosan. The MXene-chitosan membrane was deposited on the ferrocyanide-modified screen-printed working carbon electrode, followed by applying poly(acrylic acid) as an electrolyte and accumulation medium for gaseous H2O2. The sensor showed highly sensitive and selective electroanalytical performance for detecting trace concentrations of gaseous H2O2 with a very low detection limit of 4 µg m-3 (4 ppbv), linear response in the studied concentration range of 0.5-30.0 mg m-3, and good reproducibility with an RSD of 1.3%. The applicability of the sensor was demonstrated by point-of-interest detection of gaseous H2O2 during the real hair bleaching process with a 9 and 12% H2O2 solution.

2D Layered Bimetallic Phosphorous Trisulfides MI MIII P2 S6 (MI = Cu, Ag; MIII = Sc, V, Cr, In) for Electrochemical Energy Conversion.

Considerable improvements in the electrocatalytic activity of 2D metal phosphorous trichalcogenides (M2 P2 X6 ) have been achieved for water electrolysis, mostly with MII 2 [P2 X6 ]4- as catalysts for hydrogen evolution reaction (HER). Herein, MI MIII P2 S6 (MI  = Cu, Ag; MIII  = Sc, V, Cr, In) are synthesized and tested for the first time as electrocatalysts in alkaline media, towards oxygen reduction reaction (ORR) and HER. AgScP2 S6 follows a 4 e- pathway for the ORR at 0.74 V versus reversible hydrogen electrode; CuScP2 S6 is active for HER, exhibiting an overpotential of 407 mV and a Tafel slope of 90 mV dec-1 . Density functional theory models reveal that bulk AgScP2 S6 and CuScP2 S6 are both semiconductors with computed bandgaps of 2.42 and 2.23 eV, respectively and overall similar electronic properties. Besides composition, the largest difference in both materials is in their molecular structure, as Ag atoms sit at the midpoint of each layer alongside Sc atoms, while Cu atoms are raised to a similar height to S atoms, in the external segment of the 2D layers. This structural difference probably plays a fundamental role in the different catalytic performances of these materials. These findings show that MI (Cu, Ag) together with Sc(MIII ) leads to promising achievements in MI MIII P2 S6 materials as electrocatalysts.

Synthesis of Magnesium Phosphorous Trichalcogenides and Applications in Photoelectrochemical Water Splitting.

Promising applications of metal phosphorous trichalcogenides (M2 P2 X6 or MPX3 ) have been predicted in optoelectronics, photoelectrocatalysis, and water-splitting reactions, mainly due to its wide bandgap. Transition metals are widely used in the synthesis of MPX3 , however, divalent cations of alkaline earth metals can also be constituents in MPX3 2D layered structures. Herein, MgPX3 (X = S, Se) are synthesized and their photoelectrochemical (PEC) activity is tested in the hydrogen evolution and oxygen evolution reaction (OER) regions under a wide range of wavelengths. MgPSe3 photoelectrode shows the best PEC performance with a response of 1.6 ± 0.1 mA cm-2 under 420 nm. In the light-assisted OER, a 200 mV improvement is obtained in the overpotential at 10 mA cm-2 for MgPSe3 . The better performance of MgPSe3 is consistent with its lower optical bandgap (Eg  = 3.15 eV), as a result of the variation of electronegativity between selenide and sulfide.

Enhanced voltammetric performance of sensors based on oxidized 2D layered black phosphorus.

The exceptional properties of 2D layered black phosphorus (BP) make it a promising candidate for electrochemical sensing applications and, even though BP is considered unstable and tends to degrade by the presence of oxygen and moisture, its oxidation can be beneficial in some situations. In this work, we present an unequivocal demonstration that the exposition of BP-based working electrodes to normal ambient conditions can indeed be advantageous, leading to an enhancement of voltammetric sensing applications. This point was proved using a BP modified screen-printed carbon electrode (BP-SPCE) for the voltammetric determination of dopamine (DA) as a model target analyte. Oxidized BP-SPCE (up to 35% of PxOy at the surface) presented an enhanced analytical performance with a 5-fold and 2-fold increase in sensitivity, as compared to bare-SPCE and non-oxidized BP-SPCE stored in anhydrous atmosphere, respectively. Good detection limit, repeatability, reproducibility, stability, selectivity, and accuracy were also achieved. Overall, the results presented herein display the prominent possibilities of preparing and working with BP based-sensors in normal ambient settings and showcase their implementation under physiological conditions.

Cobalt Phosphorous Trisulfide as a High-Performance Electrocatalyst for the Oxygen Evolution Reaction.

Two-dimensional (2D) layered materials are currently one of the most explored materials for developing efficient and stable electrocatalysts in energy conversion applications. Some of the 2D metal phosphorous trichalcogenides (M2P2X6 or MPX3 in its simplified form) have been reported to be useful catalysts for water splitting, although results have been less promising for the sluggish oxygen evolution reaction (OER) due to insufficient activity or compromised stability. Herein, we report the OER catalysis of a series of M2P2X6 (M2+ = Mn, Fe, Co, Zn, Cd; X = S, Se). From the series of MPX3, CoPS3 yields the best results with an overpotential within the range of values usually obtained for IrO2 or RuO2 catalysts. The liquid-phase exfoliation of CoPS3 even improves the OER activity due to abundant active edges of the downsized sheets, accompanied by the presence of surface oxides. The influence of the OER medium and underlying substrate electrode is studied, with the exfoliated CoPS3 reaching the lowest overpotential at 234 mV at a current density of 10 mA/cm2, also able to sustain high current densities, with an overpotential of 388 mV at a current density of 100 mA/cm2, and excellent stability after multiple cycles or long-term operation. Quantum chemical models reveal that these observations are likely tied to moieties on CoPS3 edges, which are responsible for low overpotentials through a two-site mechanism. The OER performance of exfoliated CoPS3 reported herein yields competitive values compared to those reported for other Co-based and MPX3 in the literature, thus holding substantial promise for use as an efficient material for the anodic water-splitting reaction.

A highly sensitive enzyme-less glucose sensor based on pnictogens and silver shell-gold core nanorod composites.

Herein, we successfully incorporated pnictogen-Au@AgNR composites, produced by mixing shear exfoliated pnictogen nanosheets with silver shell, gold core nanorods (Au@AgNRs), as novel electrode materials towards the development of a non-enzymatic electrochemical glucose sensor. The findings of this study conceptually prove the feasibility of incorporating pnictogen-based composites for future development of electrochemical sensors.

Towards Antimonene and 2D Antimony Telluride through Electrochemical Exfoliation.

Two-dimensional (2D) layered antimony (Sb) and antimony telluride (Sb2 Te3 ) are two valuable materials for optoelectronic devices and thermoelectric applications. Preparing high-quality sheets of these materials is the initial phase to promote their expected issues. Herein, micrometer-sized few-to-multilayered sheets of Sb and Sb2 Te3 have been obtained by electrochemical exfoliation. The layered rhombohedral Sb was exfoliated in Na2 SO4 and Li2 SO4 electrolytes by anodic-cationic intercalation, and Sb2 Te3 was exfoliated in Na2 SO4 . These findings are important contributions for the solution-based room-temperature electrochemical exfoliation, which is stable under glove-box-free conditions, to further improve the production of high-quality exfoliated sheets.

MoS2 versatile spray-coating of 3D electrodes for the hydrogen evolution reaction.

A facile one-step MoS2 spray-coating method was applied to a range of rigid, flexible, porous and 3D printed carbon-based surfaces, yielding high loadings in MoS2 flakes. The characterization of MoS2 flakes from a commercial lubricant spray reveals up to micron-scale bulk sheets of the layered material, constituted in its majority by the semiconducting 2H polymorph, in the presence of the metallic 1T phase. Consequently, the process generates MoS2 spray-coated surfaces with improved hydrogen evolution reaction (HER) catalytic performance. In the case of carbon-based screen printed electrodes (SPE), a short-term thermal post-treatment of the MoS2 spray-coated SPE had a further beneficial effect in the HER overpotential. The MoS2 spray-coated 3D metallic meshes held the lowest HER overpotential of the series. Finally, MoS2 spray-coated 3D printed electrodes yielded improved heterogeneous charge transfer and a 500 mV shift in the required overpotential at a current density of -10 mA cm-2. The MoS2 spray-coated 3D printed electrode displayed an abundant coverage at the inner, external and planar zones of electrodes by the MoS2 sheets, even after long-term operation conditions. These outcomes can be beneficial for future tailoring of MoS2 spray-coated surfaces and their implementation in energy conversion technologies.

Cytotoxicity of Shear Exfoliated Pnictogen (As, Sb, Bi) Nanosheets.

The experimental achievement of phosphorene, which exhibits superior electronic, physical, and optical properties has spurred recent interest in other Group 15 elemental 2D nanomaterials such as arsenene, antimonene, and bismuthene. These unique and superior properties of the pnictogen nanosheets have spurred intensive research efforts and led to the discovery of their diversified potential applications; for instance, optical Kerr material, photonic devices, pnictogen-decorated microfibers, high-speed transistors, and flexible 2D electronics. Previous studies have mainly been dedicated to study the synthesis, properties, and applications of the heavy pnictogens nanosheets; however, the toxicological behaviour of these nanosheets has yet to be established. Herein, the cytotoxicity study of pnictogen nanosheets (As, Sb, and Bi) was conducted over 24 h of incubation with various concentrations of test materials and adenocarcinoma human lung epithelial A549 cells. After the treatment period, the remaining cell viabilities were obtained through absorbance measurements with WST-8 and MTT assays. These findings demonstrate that the toxicity of pnictogen nanosheets decreases down Group 15, whereby arsenic nanosheets are considered to be the most toxic, whereas bismuth nanosheets induce low cytotoxicity. The findings of this study constitute an important initial step towards enhancing our understanding of the toxicological effects of pnictogen nanosheets in light of their prospective commercial applications.

Pnictogen-Based Enzymatic Phenol Biosensors: Phosphorene, Arsenene, Antimonene, and Bismuthene.

Two-dimensional materials have allowed for great advances in the biosensors field and to obtain sophisticated, smart, and miniaturized devices. In this work, we optimized a highly sensitive and selective phenol biosensor using 2D pnictogens (phosphorene, arsenene, antimonene, and bismuthene) as sensing platforms. Exfoliated pnictogen were obtained by the shear-force method, undergoing delamination and downsizing to thin nanosheets. Interestingly, compared with the other tested elements, antimonene exhibited the highest degree of exfoliation and the lowest oxidation-to-bulk ratio, to which we attribute its enhanced performance in the phenol biosensor system reported here. The proposed design represents the first biosensor approach developed using exfoliated pnictogens beyond phosphorene.

Metal Phosphorous Trichalcogenides (MPCh3 ): From Synthesis to Contemporary Energy Challenges.

Owing to their unique physical and chemical properties, layered two-dimensional (2D) materials have been established as the most significant topic in materials science for the current decade. This includes layers comprising mono-element (graphene, phosphorene), di-element (metal dichalcogenides), and even multi-element. A distinctive class of 2D layered materials is the metal phosphorous trichalcogenides (MPCh3 , Ch=S, Se), first synthesized in the late 1800s. Having an unusual intercalation behavior, MPCh3 were intensively studied in the 1970s for their magnetic properties and as secondary electrodes in lithium batteries, but fell from scrutiny until very recently, being 2D nanomaterials. Based on their synthesis and most significant properties, the present surge of reports related to water-splitting catalysis and energy storage are discussed in detail. This Minireview is intended as a baseline for the anticipated new wave of researchers who aim to explore these 2D layered materials for their electrochemical energy applications.

Functional Protection of Exfoliated Black Phosphorus by Noncovalent Modification with Anthraquinone.

Few and monolayer black phosphorus (phosphorene) is currently an intensively researched material. Shear exfoliated black phosphorus (BPSE) nanosheets were functionalized with the redox active antraquinone (AQ) that can provide additional charge storage capacity. The noncovalent interaction of BP with AQ occurs due to van der Waals interactions. X-ray photoelectron spectroscopy results show that AQ coverage of BPSE nanosheets led to a stabilization against BPSE degradation. Electrochemistry of the BPSE-AQ shows that AQ is stably anchored onto BPSE and exhibits redox peaks stable for more than 100 cycles. The surface coverage by AQ on BPSE is estimated to be 1.25 nmol AQ/mg BP and electron-transfer rate constant ( kET) of 33 s-1. Furthermore, the proposed modification greatly increases the gravimetric capacitance of BPSE-AQ with respect to the starting BPbulk. Such coating of BP not only protects BP from degradation but also brings electroactive functionality to this two-dimensionally layered material.

Synergetic Metals on Carbocatalyst Shungite.

The naturally occurring Palaeoproterozoic carbon mineral shungite is a complex raw carbon microporous matrix, loaded with a wide range of elements. Shungite exhibits a disordered and amorphous structure with highly irregular building blocks. Shungite incorporates metals in its structure; typically catalytic elements such Fe and Ni are present, as well as the toxic elements Pb and As at mg g-1 levels. We show here that incorporation of the metals in the carbon matrix of shungite leads into synergistic catalytic effect. We investigate the application of shungite in energy related electrochemical catalytic reactions, such as the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). All elements have a synergetic effect, thus contributing for shungite's interesting catalytic performance towards a different range of electrochemical reactions, outperforming other tested carbon allotropes, such as carbon black, metal loaded carbon nanotubes, fullerene, and glassy carbon. These findings have profound impact on the application of the natural carbon materials for catalysis.

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.

Black Phosphorus Rediscovered: From Bulk Material to Monolayers.

Phosphorus is a nonmetal with several allotropes, from the highly reactive white phosphorus to the thermodynamically stable black phosphorus (BP) with a puckered orthorhombic layered structure. The bulk form of BP was first synthesized in 1914, but received little attention until it was rediscovered in 2014 as a member of the new wave of 2D layered nanomaterials. BP can be exfoliated to a single sheet that acts as a semiconductor with a tunable direct band gap, a high carrier mobility at room temperature, and an in-plane anisotropy. The development of BP applications is hampered by surface degradation, thus efforts to achieve effective BP passivation are ongoing, such as its integration in van der Waals heterostructures. BP has been tested as a novel nanomaterial in batteries, transistors, sensors, and photonics. This Review begins with the origin of the BP story, following the path from a bulk material to modern few/single layers. The physical and chemical properties are summarized, and the state-of-the-art of BP applications highlighted.

Multifunctional electrocatalytic hybrid carbon nanocables with highly active edges on their walls.

Graphene sheets exhibit fast heterogeneous electron transfer at the edges while at the basal plane the electron transfer is much slower. Carbon nanotubes (CNTs) represent fascinating quasi-1-dimensional materials due to their electronic and mechanical properties which enable the formation of (unlike graphene) robust, flexible and well defined three dimensional structures. Because CNTs are created from "rolled up" graphene sheets, exposing mostly inactive walls, they generally exhibit poor electrochemical properties. In contrast, graphene sheets can exhibit fast electron transfer rates but are often prone to "restacking" which hinders their true electrochemical potential. Here we obviate this problem by partial unzipping of CNTs, where their inner core creates nanocables with high electrical conductivity while the outer unzipped graphene layers full of edges and defects act as highly electroactive materials. Metallic nanoparticles are introduced into graphene oxide/CNT hybrid structures (GOCNT), so they do catalyze reactions which are not catalyzed by carbon. We show that in combination with trace metal doping, these nanocables act as efficient electrocatalysts towards oxidation of biomarkers and energy related applications, such as hydrogen evolution reaction. Such hybrid graphene/CNT/metallic nanoparticles present universal well-structured catalysts which should find wide applications in electrochemical devices. GOCNTs rich in oxygenated groups show much promise in pollution management, thus their adsorption behaviour was investigated to establish their ability to remove harmful heavy-metal pollutants. The results show an increasing trend in the concentration of oxygen functional groups, directly correlated with the GOCNT adsorption capacity.

Electrochemical Fluorographane: Hybrid Electrocatalysis of Biomarkers, Hydrogen Evolution, and Oxygen Reduction.

Fluorographane (C1 Hx F1-x+δ )n is a new member of the graphene family that exhibits hydrophobicity and a large band gap that is tunable based on the level of fluorination. Herein, sensing and energy applications of fluorographane are reported. The results reveal that the carbon-to-fluoride ratio of fluorographane has a great impact on the electrochemical performance of the materials. Lowered oxidation potentials for ascorbic and uric acids, in addition to a catalytic effect for hydroquinone and dopamine redox processes, are obtained with a high fluoride content. Moreover, fluorographane, together with residual copper- and nickel-based doping, acted as a hybrid electrocatalyst to promote hydrogen evolution and oxygen reduction reactions with considerably lower onset potentials than those of graphane (starting material), which makes this a promising material for a broad range of applications.

Chemometric analysis of voltammetric data on metal ion binding by selenocystine.

The behavior of selenocystine (SeCyst) alone or in the presence of various metal ions (Bi(3+), Cd(2+), Co(2+), Cu(2+), Cr(3+), Ni(2+), Pb(2+), and Zn(2+)) was studied using differential pulse voltammetry (DPV) over a wide pH range. Voltammetric data matrices were analyzed using chemometric tools recently developed for nonlinear data: pHfit and Gaussian Peak Adjustment (GPA). Under the experimental conditions tested, no evidence was found for the formation of metal complexes with Bi(3+), Cu(2+), Cr(3+), and Pb(2+). In contrast, SeCyst formed electroinactive complexes with Co(2+) and Ni(2+) and kinetically inert but electroactive complexes with Cd(2+) and Zn(2+). Titrations with Cd(2+), Co(2+), Ni(2+), and Zn(2+) produced data that were reasonably consistent with the formation of stable 1:1 M(SeCyst) complexes.

Combination of chemometrically assisted voltammetry, calorimetry, and circular dichroism as a new method for the study of bioinorganic substances: application to selenocystine metal complexes.

Selenium-containing compounds play an important role in antioxidant defense systems, binding to toxic metals, preventing their uptake into cells, and thus protecting cells from metal-induced formation of reactive oxygen species. Here, we present a proposal for a relatively new method as a complement to the more usual methods used in selenium studies. A systematic study of the metal-binding properties of selenocystine (SeCyst) in the presence of divalent metal cations (Cd, Co, Hg, Ni, and Zn) is reported. Isothermal titration calorimetry provides thermodynamic parameters of the systems. Titrations produced curves that could be fit reasonably well to the one set of sites model. The data clearly demonstrate that one M(2+) binds one SeCyst molecule, and the stable M(SeCyst) complex is formed under these conditions. The order of the SeCyst binding constant for the metal ions is Hg(2+) > Cd(2+) ~ Zn(2+) > Ni(2+)> Co(2+). Cadmium ion was selected as a modulator for the behavior of SeCyst in the presence of a nonessential metal, and zinc was selected for the case of an essential element. These interactions of SeCyst with Cd(2+) and Zn(2+), either individually or combined, were studied in aqueous buffered solutions at physiological pH by differential pulse polarography and circular dichroism spectroscopy. Furthermore, recently developed chemometric tools were applied to differential pulse polarography data obtained in mixtures of SeCyst and glutathione in the presence of Cd(2+) at physiological pH.