High Electrochemical Sensitivity of TiO2- x Nanosheets and an Electron-Induced Mutual Interference Effect toward Heavy Metal Ions Demonstrated Using X-ray Absorption Fine Structure Spectra.

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.

In Situ Underwater Laser-Induced Breakdown Spectroscopy Analysis for Trace Cr(VI) in Aqueous Solution Supported by Electrosorption Enrichment and a Gas-Assisted Localized Liquid Discharge Apparatus.

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.

Electrochemical Detection of Trace Arsenic(III) by Nanocomposite of Nanorod-Like α-MnO2 Decorated with ∼5 nm Au Nanoparticles: Considering the Change of Arsenic Speciation.

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.

Iron Oxide with Different Crystal Phases (α- and γ-Fe2O3) in Electroanalysis and Ultrasensitive and Selective Detection of Lead(II): An Advancing Approach Using XPS and EXAFS.

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.

Electroadsorption-assisted direct determination of trace arsenic without interference using transmission X-ray fluorescence spectroscopy.

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.

An ultra-sensitive electrochemical sensor of Ni/Fe-LDH toward nitrobenzene with the assistance of surface functionalization engineering.

Hypersensitive detection of organic pollutions with high toxicity in drinking water always keeps its challenge in electroanalysis due to their low concentration and electrochemical redox inert. In this work, a novel nanomaterial modified electrode for the sensitive detection of nitrobenzene (NB) is presented, based on environmental friendly and cost-effective Ni/Fe layered double hydroxides functionalized with sodium dodecyl sulfate (Ni/Fe(SDS)-LDH). Such 2D layered composites were prepared and used to improve the sensitivity for NB detection, due to its good catalytic activity for NB reduction. Besides, the proposed electrode shows a remarkably promoted sensitivity to NB compared to Ni/Fe-LDHs modified one. It is because that the surface modifier SDS can provide more adsorption sites to significantly improve the adsorption of NB, which has been confirmed by the adsorption experiment and the characterization of Fourier transform infrared spectroscopy (FTIR). As a result, an impressive sensing behaviour is achieved at the proposed Ni/Fe(SDS)-LDHs modified electrode with a sensitivity of 15.79 µA µM-1 cm-2. This work provides a promising way to build more advanced nanomaterials to electrochemical detection of organic pollution based on energetically synergizing of adsorption by surface functionalization engineering.

Competitive adsorption behavior toward metal ions on nano-Fe/Mg/Ni ternary layered double hydroxide proved by XPS: Evidence of selective and sensitive detection of Pb(II).

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.