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Achieving highly sensitive and selective detection of trace-level As(III) and clarifying the underlying mechanism is still a intractable problem. The electroanalysis of As(III) relies on the electrocatalytic ability of the sensing interface. Herein, we first adopt single-atom catalysts as the electrocatalyst in As(III) detection. Cobalt single-atoms anchored on nitrogen-doped carbon material (Co SAC) were found to have an extraordinary sensitivity of 11.44 µA ppb-1 with excellent stability and repeatability, which so far is the highest among non-noble metal nanomaterials. Co SAC also exhibited a superior selectivity toward As(III) compared with some bivalent heavy metal ions (HMIs). Combining X-ray absorption spectroscopy (XAFS), density functional theory (DFT) calculation, and reaction kinetics simulation, we demonstrated that Co single atoms stabilized in N2C2 support serve as active sites to catalyze H3AsO3 reduction via the formation of Co-O hybridization bond, leading to a lower energy barrier, promoting the breakage of As-O bonds. Importantly, the first electron transfer is the rate-limiting step of arsenic reduction and is found to be more favorable on Co-SAC both thermodynamically and kinetically. This work not only expands the potential applicaiton of single-atom catalysts in the detection and treatment of As(III), but also provides atomic-level catalytic insights into HMIs sensing interfaces.

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Metal hydroxide nanomaterials are widely applied in the energy and environment fields. The electrochemical performance of such materials is strongly dependent on their crystal phases. However, as there are always multiple factors relating to the phase-dependent electrochemistry, it is still difficult to identify the determining one. The well-defined crystal phases of α- and ß-FeOOH nanorods are characterized through the transmission electron microscopy by a series of rotation toward one rod, where the cross-section shape and the growth direction along the [001] crystalline are first verified for 1D FeOOH nanostructures. The electrosensitivity of the two materials toward Pb(II) is tested, where α-FeOOH performs an outstanding sensitivity whilst it is only modest for ß-FeOOH. Experiments via Fourier transform infrared spectroscopy, X-ray absorption fine structure (XAFS), etc., show that α-FeOOH presents a larger Pb(II) adsorption capacity due to more surficial hydroxyl groups and weaker PbO bond strength. The reaction kinetics are simulated and the adsorption capacity is found to be the determining factor for the distinct Pb(II) sensitivities. Combining experiment with simulation, this work reveals the physical insights of the phase-dependent electrochemistry for FeOOH and provides guidelines for the functional application of metal hydroxide nanomaterials.

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The presence of antibiotics in aquatic environments has attracted global concern. The Fenton system is one of the most popular methods for eliminating antibiotics in aquatic environments, but the existing Fenton system is limited due to the potential for secondary pollution, and the narrow pH range (∼3-5). In this study, we report that the bottlenecks for high-strength tetracycline (TC) wastewater treatment under neutral conditions can be tackled well by a class of mixed-valence W/Mo containing oxides (WMoO-x) with tunable morphologies. Triethanolamine was selected as a structure-directing agent to control the morphologies of the catalysts going from ultrathin nanowires (UTNWs) to wire-tangled nanoballs (WTNBs). As a proof of concept, the most efficient catalyst in the batch samples, WMoO-1 ultrathin nanowires, was employed as a model material for TC degradation, in which the coordinatively unsaturated metal atoms with oxygen defects serve as the sites for TC chemisorption and electron transfer. As a result, 91.75% of TC was degraded in 60 min for the initial TC concentration of 400 µM. Furthermore, LC-MS analysis confirmed that the TC could be degraded to nontoxic by-products without benzene rings, and finally mineralized to CO2 and H2O. ICP-MS and cycle experiments showed the good stability and reusability of WMoO-1 UTNWs in the Fenton-like system. The findings of this work provide fresh insights into the design of nanoscale catalyst morphology and reaffirm the versatility of doping in tuning catalyst activity, extending the range of the optimal pH values to neutral conditions. This is significant for the expansion of the heterogeneous Fenton-like family and its application in the field of water treatment.

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An ultrasensitive electrochemical detection of heavy metal ions is achieved via defect- and phase-engineering of Mn-mediated MoS2 nanosheets. We find for the first time that chemical interactions between Pb(ii) and active S atoms in Mn-MoS2 facilitate the electron transfer and in situ catalytic redox reactions.

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Porous ZnO nanobelts sensitized with Ag nanoparticles have been prepared via a partial cation-exchange reaction assisted by a thermal oxidation treatment, employing ZnSe·0.5N2H4 nanobelts as precursors. After partially exchanged with Ag+ cations, the belt-like morphology of the precursors is still preserved. Continuously calcined in air, they are in situ transformed into Ag nanoparticles sensitized porous ZnO nanobelts. The size of the Ag nanoparticles can be tuned through manipulating the amount of exchanging Ag+ cations. Considering the porous and belt-like nanostructure, sensing characteristics of ZnO and the catalytic activity of Ag nanoparticles, the gas sensing performances of the as-prepared Ag nanoparticles sensitized porous ZnO nanobelts have been carefully investigated. The results indicate that Ag nanoparticles significantly enhance the sensing performances of porous ZnO nanobelts toward typical volatile organic compounds. Especially, a good selectivity has been demonstrated toward acetic acid gas with a low detection limit less than 1 ppm. Furthermore, they also display a good reproducibility with a short response/recovery time due to the thin, uniform and porous sensing film, which is fabricated with the assembled technique and in situ calcined approach. Finally, their sensing mechanism has been further discussed.

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Mutual interference is a severe issue that occurs during the electrochemical detection of heavy metal ions. This limitation presents a notable drawback for its high sensitivity to specific targets. Here, we present a high electrochemical sensitivity of ∼237.1 µA cm-2 µM-1 toward copper(II) [Cu(II)] based on oxygen-deficient titanium dioxide (TiO2- x) nanosheets. We fully demonstrated an atomic-level relationship between electrochemical behaviors and the key factors, including the high-energy (001) facet percentage, oxygen vacancy concentration, surface -OH content, and charge carrier density, is fully demonstrated. These four factors were quantified using Raman, electron spin resonance, X-ray photoelectron spectroscopy spectra, and Mott-Schottky plots. In the mutual interference investigation, we selected cadmium(II) [Cd(II)] as the target ion because of the significant difference in its stripping potential (∼700 mV). The results show that the Cd(II) can enhance the sensitivity of TiO2- x nanosheets toward Cu(II), exhibiting an electron-induced mutual interference effect, as demonstrated by X-ray absorption fine structure spectra.

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Developing a new ultrasensitive interface to detect As(III) is highly desirable because of its seriously toxic and low concentration in drinking water. Recently, Fe3O4 nanoparticles of high adsorption toward As(III) become very promising to be such an interface, which is still limited by the poor understanding of their surface physicochemical properties. Herein, we report that dumbbell-like Au/Fe3O4 nanoparticles, when being modified the screen-printed carbon electrode, can serve as an efficient sensing interface for As(III) detection with an excellent sensitivity of 9.43 µA ppb-1 and a low detection limit of 0.0215 ppb. These outstanding records were attributed to the participation of Fe(II)/Fe(III) cycle on Fe3O4 surface in the electrochemical reaction of As(III) redox, as revealed by X-ray photoelectron spectroscopy, X-ray absorption near edge structure, and extended X-ray absorption fine structure. This work provides new insight into the mechanism of electroanalysis from the viewpoint of surface active atoms, and also helps to predict the construction of ultrahighly sensitive electrochemical sensors for other heavy metal ions with nonprecious redox active materials.

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Nanocrystals generally suffer from agglomeration because of the spontaneous reduction of the system surface energy, resulting in blocking the active sites from reacting with target ions, and then severely reducing the electrochemical sensitivity. In this article, a highly ordered self-assembled monolayer array is successfully constructed using ∼14 nm Co0.6Fe2.4O4 nanocubes uniformly and controllably distributed on the surface of a working electrode (glass carbon plate). The large area and high exposure of the surface defects on Co0.6Fe2.4O4 nanocubes are clearly characterized by high-resolution transmission electron microscopy (HRTEM) and atomic-resolution high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Expectedly, a considerable sensitivity of 2.12 µA ppb-1 and a low limit of detection of 0.093 ppb are achieved for As(III) detection on this highly homogeneous sensing interface; this excellent electroanalysis performance is even better than that of noble metals electrodes. Most importantly, this approach of uniformly distributing the small-sized defective nanoparticles on the electrode surface provides a new opportunity for modifying the electrodes, as well as the realization of their applications in the field of environmental electroanalysis for heavy metal ions.

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Hierarchical morphology-dependent gas-sensing performances have been demonstrated for three-dimensional SnO2 nanostructures. First, hierarchical SnO2 nanostructures assembled with ultrathin shuttle-shaped nanosheets have been synthesized via a facile and one-step hydrothermal approach. Due to thermal instability of hierarchical nanosheets, they are gradually shrunk into cone-shaped nanostructures and finally deduced into rod-shaped ones under a thermal treatment. Given the intrinsic advantages of three-dimensional hierarchical nanostructures, their gas-sensing properties have been further explored. The results indicate that their sensing behaviors are greatly related with their hierarchical morphologies. Among the achieved hierarchical morphologies, three-dimensional cone-shaped hierarchical SnO2 nanostructures display the highest relative response up to about 175 toward 100 ppm of acetone as an example. Furthermore, they also exhibit good sensing responses toward other typical volatile organic compounds (VOCs). Microstructured analyses suggest that these results are mainly ascribed to the formation of more active surface defects and mismatches for the cone-shaped hierarchical nanostructures during the process of thermal recrystallization. Promisingly, this surface-engineering strategy can be extended to prepare other three-dimensional metal oxide hierarchical nanostructures with good gas-sensing performances.

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Hypersensitive and highly selective nanomaterials for the measurement of heavy metal ions (HMIs) hold a key to electro-analysis. Various works improved the results of analysis but without scientific understanding. Herein, Fe/Mg/Ni-layered double hydroxide (LDH) has been successfully prepared and its electrochemical behavior for Pb(II) detection is also studied using square wave anodic stripping voltammetry (SWASV). The well performance of electrochemistry suggest that the modification with Fe/Mg/Ni-LDH significantly promotes the selectivity and sensitivity toward Pb(II). The sensitivity on Fe/Mg/Ni-LDH modified glassy carbon electrode (GCE) is 68.1µAµM-1 over the range from 0.03 to 1.0µM under the optimized conditions. Otherwise, the selectivity, anti-interference, stability measurements and practical implications of Fe/Mg/Ni-LDH modified GCE are also performed. What,s more, a reasonable mechanism of detection for Pb(II) including selectivity and sensitivity is proposed based on adsorption and characterized using XPS and XRD. These findings provide a potentially excellent material to improve the sensitivity and selectivity for toxic metal ions as well as a deep understanding of detection.

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Traditional laser-induced breakdown spectroscopy (LIBS) always fails to directly detect target in aqueous solution due to rapid quenching of emitted light and adsorption of pulse energy by surrounding water. A method is proposed for the in situ underwater LIBS analysis of Cr(VI) in aqueous solution freed from the common problems mentioned above by combining a gas-assisted localized liquid discharge apparatus with electrosorption for the first time. In this approach, the introduction of the gas-assisted localized liquid discharge apparatus provides an instantaneous gaseous environment for underwater LIBS measurement (that is, the transfer of sampling matrix is not needed from aqueous solution to dry state). The preconcentration of Cr(VI) is achieved by electrosorption with a positive potential applied around adsorbents, which can promote the adsorption of Cr(VI) and inhibit that of the coexisting cations leading to a good anti-interference. Amino groups functionalized chitosan-modified graphene oxide (CS-GO) is utilized for Cr(VI) enrichment, which can be protonated to form NH3+ in acidic condition promoting the adsorption toward Cr(VI) by electrostatic attraction. The highest detection sensitivity of 5.15 counts µg-1 L toward Cr(VI) is found for the optimized electrosorption potential (EES = 1.5 V) and electrosorption time (tES = 600 s) without interference from coexisting metal ions. A corresponding limit of detection (LOD) of 12.3 µg L-1 (3σ method) is achieved, which is amazingly improved by 2 or even 3 orders of magnitude compared to the previous reports of LIBS.

RESUMEN

Herein, we revealed that the electrochemical behaviors on the detection of heavy metal ions (HMIs) would largely rely on the exposed facets of SnO2 nanoparticles. Compared to the high-energy {221} facet, the low-energy {110} facet of SnO2 possessed better electrochemical performance. The adsorption/desorption tests, density-functional theory (DFT) calculations, and X-ray absorption fine structure (XAFS) studies showed that the lower barrier energy of surface diffusion on {110} facet was critical for the superior electrochemical property, which was favorable for the ions diffusion on the electrode, and further leading the enhanced electrochemical performance. Through the combination of experiments and theoretical calculations, a reliable interpretation of the mechanism for electroanalysis of HMIs with nanomaterials exposed by different crystal facets has been provided. Furthermore, it provides a deep insight into understanding the key factor to improve the electrochemical performance for HMIs detection, so as to design high-performance electrochemical sensors.

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Intrinsically low conductivity and poor reactivity restrict many semiconductors from electrochemical detection. Usually, metal- and carbon-based modifications of semiconductors are necessary, making them complex, expensive, and unstable. Here, for the first time, we present a surface-electronic-state-modulation-based concept applied to semiconductors. This concept enables pure semiconductors to be directly available for ultrasensitive electrochemical detection of heavy-metal ions without any modifications. As an example, a defective single-crystalline (001) TiO2 nanosheet exhibits high electrochemical performance toward Hg(II), including a sensitivity of 270.83 µA µM-1 cm-2 and a detection limit of 0.017 µM, which is lower than the safety standard (0.03 µM) of drinking water established by the World Health Organization (WHO). It has been confirmed that the surface oxygen vacancy adsorbs an O2 molecule while the Ti3+ donates an electron, forming the O2•- species that facilitate adsorption of Hg(II) and serve as active sites for electron transfer. These findings not only extend the electrochemical sensing applications of pure semiconductors but also stimulate new opportunities for investigating atom-level electrochemical behaviors of semiconductors by surface electronic-state modulation.

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It has been reported that the majority of groundwater shows weak alkaline in which the As(III) species would be present as neutral H3AsO3 species and ionized H2AsO3- species. However, as most reported previously, electrochemical detection of As(III) has been operated under acidic conditions and the nonionic As(III) (H3AsO3) is the dominant species. Therefore, considering the change of As(III) speciation in different pH conditions, to develop a reliable method for the detection of As(III) in alkaline media might be more meaningful for practical applications. Here, combined the multilayer adsorption of nanorod-like α-MnO2 with the excellent electrocatalytic ability of ∼5 nm Au nanoparticles (AuNPs), an efficient and ultrahigh anti-interference electrochemical detection of As(III) with AuNPs/α-MnO2 nanocomposite in alkaline media (nearly real water environment) was developed. Notably, we have provided a thorough electrochemical analytical investigation to confirm the advantage of As(III) detection in alkaline media. The system was evaluated by a series of interference tests, and no obvious interference from commonly coexisting substances (referring to the groundwater, Togtoh region, Inner Mongolia, China) was observed in alkaline media. Furthermore, electrodes robust stability and excellent reproducibility were obtained. Under the optimized conditions, the limit of detection (3σ method) toward As(III) was 0.019 ppb, and the obtained sensitivity was 16.268 ± 0.242 µA ppb-1 cm-2. Finally, the proposed method has been successfully employed for detection of As(III) in a real water sample.

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Porous and single-crystalline ZnO nanobelts have been prepared through annealing precursors of ZnSe · 0.5N2H4 well-defined and smooth nanobelts, which have been synthesized via a simple hydrothermal method. The composition and morphology evolutions with the calcination temperatures have been investigated in detail for as-prepared precursor nanobelts, suggesting that they can be easily transformed into ZnO nanobelts by preserving their initial morphology via calcination in air. In contrast, the obtained ZnO nanobelts are densely porous, owing to the thermal decomposition and oxidization of the precursor nanobelts. More importantly, the achieved porous ZnO nanobelts are single-crystalline, different from previously reported ones. Motivated by the intrinsic properties of the porous structure and good electronic transporting ability of single crystals, their gas-sensing performance has been further explored. It is demonstrated that porous ZnO single-crystalline nanobelts exhibit high response and repeatability toward volatile organic compounds, such as ethanol and acetone, with a short response/recovery time. Furthermore, their optoelectronic behaviors indicate that they can be promisingly employed to fabricate photoelectrochemical sensors.

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Iron oxide with different crystal phases (α- and γ-Fe2O3) has been applied to electrode coatings and been demonstrated to ultrasensitive and selective electrochemical sensing toward heavy metal ions (e.g., Pb(II)). A range of Pb(II) contents in micromoles (0.1 to 1.0 µM) at α-Fe2O3 nanoflowers with a sensitivity of 137.23 µA µM(-1) cm(-2) and nanomoles (from 0.1 to 1.0 nM) at γ-Fe2O3 nanoflowers with a sensitivity of 197.82 µA nM(-1) cm(-2) have been investigated. Furthermore, an extended X-ray absorption fine structure (EXAFS) technique was applied to characterize the difference of local structural environment of the adsorbed Pb(II) on the surface of α- and γ-Fe2O3. The results first showed that α- and γ-Fe2O3 had diverse interaction between Pb(II) and iron (hydro)oxides, which were consistent with the difference of electrochemical performance. Determining the responses of Cu(II) and Hg(II) as the most appropriate choice for comparison, the stripping voltammetric quantification of Pb(II) with high sensitivity and selectivity at γ-Fe2O3 nanoflower has been demonstrated. This work reveals that the stripping performances of a nanomodifier have to be directly connected with its intrinsic surface atom arrangement.

Asunto(s)

RESUMEN

Most gold nanoparticle-based electrodes have been utilized for the analysis of highly toxic As(III), while nano-Fe3O4 materials are currently attracting considerable interest as an adsorbent for the removal of As(III). However, the combination of gold nanoparticles with Fe3O4 nanoadsorbents for stripping voltammetry is, to the best of our knowledge, unexplored. Here, a sensing interface for ultrasensitive detection of As(III) is designed and constructed by abundantly dispersing Au nanoparticles (Au NPs) on the surface of the Fe3O4 nanosphere. The Au@Fe3O4 nanospheres are covered by the room temperature ionic liquid (RTIL) and then modified on the screen-printed carbon electrode (SPCE). By combining the excellent catalytic properties of the Au nanoparticles (∼3-9 nm in diameter) with the good adsorption capacity of Fe3O4 nanospheres toward As(III), as well as the good conductivity of RTIL, the Au@Fe3O4-RTIL shows excellent performance in the detection of arsenic under nearly neutral conditions without modifying the morphology of the sensing interface. Through optimization of the experimental conditions, an ultrahigh sensitivity of 458.66 µA ppb(-1) cm(-2) from 0.1 to 1 ppb with a detection limit (3σ method) of 0.0022 ppb was obtained. The reproducibility and reliability of the Au@Fe3O4-RTIL sensing interface was also evaluated with good results. Finally, we used this platform to analyze real samples.

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Ag2Se nanobelts are prepared through employing ZnSe nanobelts as templates via a facile cation exchange approach. The templates are derived from precursor ZnSe·0.5N2 H4 nanobelts, which are synthesized by a simple hydrothermal method. As-synthesized precursor nanobelts are with 200 nm in width and several hundreds of micrometers in length. Annealed in N2 , they are transformed into ZnSe nanobelts with preserving their initial morphology. Following with a complete replacement of Zn(2+) by Ag(+), Ag2Se nanobelts with single crystalline are obtained via a cation-exchange reaction. Combined with the Langmuir-Blodgett assembly technique, regular films of ZnSe nanobelts can be achieved on transparent glass substrates and Si wafers with interdigital Au electrode arrays. Further, the optical and electrical evolutions are investigated from ZnSe nanobelts to Ag2 Se nanobelts. Finally, the resistive switching characteristic are carefully explored for Ag2Se nanobelts regularly arranged on interdigital Au microelectrodes. The results indicate that it is analogous to complementary resistive switching behaviors, which is different from that of traditional two terminal devices about previously reported Ag2Se. In order to clarify this phenomenon, a possible mechanism has been proposed and indirectly demonstrated through in situ SEM (scanning electron microscropy) observation.

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An analytical technique based on electroadsorption and transmission X-ray fluorescence (XRF) for the quantitative determination of arsenic in aqueous solution with ppb-level limits of detection (LOD) is proposed. The approach uses electroadsorption to enhance the sensitivity and LOD of the arsenic XRF response. Amine-functionalized carbonaceous microspheres (NH2-CMSs) are found to be the ideal materials for both the quantitative adsorption of arsenic and XRF analysis due to the basic amine sites on the surface and their noninterference in the XRF spectrum. In electroadsorptive X-ray fluorescence (EA-XRF), arsenic is preconcentrated by a conventional three-electrode system with a positive electricity field around the adsorbents. Then, the quantification of arsenic on the adsorbents is achieved using XRF. The electroadsorption preconcentration can realize the fast transfer of arsenic from the solution to the adsorbents and improve the LOD of conventional XRF compared with directly determining arsenic solution by XRF alone. The sensitivity of 0.09 cnt ppb(-1) is obtained without the interferences from coexisted metal ions in the determination of arsenic, and the LOD is found to be 7 ppb, which is lower than the arsenic guideline value of 10 ppb given by the World Health Organization (WHO). These results demonstrated that XRF coupled with electroadsorption was able to determine trace arsenic in real water sample.

Asunto(s)

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Facet-dependent stripping behavior in the determination of Pb(II): Well-defined Cu2O microcrystals with different structures show facet-dependent electrochemical behaviors toward heavy metal ions. This provides an important insight into the understanding the efficiency of facet-dependent properties of microcrystals on electroanalytical performance for the rational design of electrochemical analytical techniques for efficient detection of heavy metal ions.