Rapid visible and solar photocatalytic Cr(VI) reduction and electrochemical sensing of dopamine using solution combustion synthesized ZnO–Fe2O3 nano heterojunctions: Mechanism Elucidation

In recent times cost-effective advanced materials with dual applications in photocatalytic water treatment and electrochemical sensing have been explored and developed. Herein we report facile solution combustion synthesis of ZnO/Fe₂O₃ (ZF) type-II heterojunction for electrochemical sensing of dopamine and visible assisted photocatalytic reduction of carcinogenic Cr(VI) into Cr(III). Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used to evaluate the compatibility of the electrodes for DA sensing. Furthermore, DPV curves revealed that ZF2 (ZnO: Fe₂O₃- 3:0.5) modified GCEs possessed 0.27 μM and 0.7 μM of LOD and LOQ with good linear range from 3 μM to 30 μM. Overall, reproducibility and interference studies confirm the efficient use of material for DA sensing. The junction ZF2 shows best performance with 88% Cr(VI) photo-reduction under visible light in 90 min and 100% reduction with tartaric acid as sacrificial agent (utilizing holes) in just 45 min. The effect of sacrificial agents and scavengers suggest the photogenerated electrons were major active species followed by ^●O₂⁻. The CB edges have enough potential for rapid reduction of hexavalent chromium under visible light and solar light. The photoluminescence and electrochemical impedance suggest lowered recombination, high charge separation and charge transfer capacity in the heterojunction. The transfer of electrons from conduction band of Fe₂O₃ to that of ZnO having high enough negative potential to reduce Cr(VI), thus utilizing the CB potential of wide band gap ZnO for rapid visible photocatalysis. In addition shortcomings as low conductivity Fe₂O₃ and high band gap of ZnO are both minimized in the junction. This study confirmed that popular semiconductors as ZnO and Fe₂O₃ have enough potential in dual role as electrochemical sensors and visible photocatalysts with best performance by optimizing the conditions and ratios.     

Fluorescent spongy carbon nanoglobules derived from pineapple juice: A potential sensing probe for specific and selective detection of chromium (VI) ions

Herein, we report, for the first time, the facile, green and one step hydrothermal synthesis of fluorescent spongy carbon nanoglobules (CNG) derived from pineapple juice as a sole carbon source without involving any strong acid treatment. The synthesized CNG not only exhibits hydrophilicity and can remain stable for several weeks but also demonstrates prominent blue color fluorescence under UV light (λ=365 nm). These CNG showed wide range of emission spectra in the visible region depending upon the different excitation wavelengths. The synthesized CNG were characterized in detail in terms of their morphological, structural, optical, compositional and thermal properties using numerous analytical and spectroscopic techniques. For application point of view, the perspective to utilize CNG as fluorescent sensing probe has also been explored with the introduction of various highly toxic metal ions and interestingly, it was observed that the fluorescence intensity of CNG was drastically quenched by chromium (VI) ions with high selectivity over the linear range of concentrations 0–18 µM with stern volmer constant 2.29×10⁴ M⁻¹. Such encouraging results revealed that simply prepared CNG are potential scaffold to fabricate highly sensitive and selective fluorescent sensors for the detection of highly hazardous and life threatening metal ions.     

Photo-enhancement of Cr(VI) reduction by fungal biomass of Neurospora crassa

Various organisms such as fungus are capable of reducing Cr(VI) to less toxic Cr(III). However, light-induced Cr(VI) reduction by fungus is less reported and needs to be explored since anthropogenic or natural activities may bring these two reactants into a sunlit environment. In this study, the interactions and reaction mechanisms of Cr(VI) on a model fungus, Neurospora crassa, were evaluated in the presence or absence of light. The influence of ferric ion, a widely distributed metal, on Cr(VI) reduction by the fungus was also investigated under illumination. The results show that 20–54% of added Cr(VI) (96.2 μM) was removed by 1 g of dead fungal biomass (i.e., 1–2.7 mg Cr(VI) reduction by 1 g biomass) at pH 1–3, after 6 h reaction in the dark. However, 96.2 μM Cr(VI) disappeared completely (i.e., 5 mg Cr(VI) reduction by 1 g biomass) under the same reaction time and experimental conditions when light was present. The rapid disappearance of Cr(VI) in solution was due to the reduction of Cr(VI) by the excited biomass upon light absorption, and the rates of redox reactions increased with a decrease at pH. Cr(VI) reduction could be further increased with the addition of 89.5 μM Fe(III) because the formation of Fe(II) from the photolysis of Fe–organic complexes enhanced Cr(VI) reduction. Spectroscopic studies indicated that the amide, NH, and carboxyl groups of N. c.-biomass may be responsible for initiating Cr(VI) reduction; comparatively, the cyclo-carbons of chitin, glucan, and their derivatives were more persistent to the oxidation by Cr(VI). Accordingly, fungi containing high amount of carboxyl, amide, and NH groups may be preferable as efficient reductants for scavenging Cr(VI) from environment. Upon the absorption of a renewable light source, Cr(VI) could be converted rapidly by the biomaterials to the less toxic Cr(III).     

Nanostructured Cerium-doped ZnO thin film – A breath sensor

Breath analysis has emerged as one of the accurate diagnostic techniques, which can be used to correlate disease conditions with the abnormal concentrations of certain biomarkers present in the exhaled breath. In this background, thin film based chemi-resistive Cerium-doped ZnO sensors were developed for detecting acetone and ethanolamine, which are the notable biomarkers of diabetes mellitus and carcinoma respectively. Ce-doped ZnO thin films were deposited on glass substrates using spray pyrolysis technique. X-ray diffraction patterns confirmed the formation of Ce-doped ZnO thin films with polycrystalline hexagonal wurtzite structure. Scanning electron micrographs and optical absorption spectra of Ce-doped ZnO thin films revealed the decrease in average grain size and increase in band gap respectively with an increase in Ce-dopant concentration. ZnO films with 0.004 and 0.008 M of Ce-dopant concentrations showed a better room temperature sensing response towards acetone and ethanolamine respectively. Highly selective nature of the developed sensing elements towards acetone and ethanolamine with swift response and recovery times can be considered as a non-invasive and cost effective method for the detection of diseases like diabetes mellitus and carcinoma.     

聚季铵盐强化超滤处理铬(VI)

以聚季铵盐-22(PQ22)为络合剂,研究Cr(VI)的强化超滤行为,考察聚电解质/金属质量比、pH值及外加盐对PQ22-Cr(VI)络合体系截留系数和膜通量的影响,并研究了络合体系的浓缩、解络合和洗涤过程. 结果表明,当聚电解质/金属质量比为80及pH=9时,Cr(VI)截留系数大于0.9;外加Cl-, NO3-和SO42-使Cr(VI)截留系数降低,且SO42-比NO3-和Cl-的影响更大;控制聚电解质/金属质量比为80及pH=9,当浓缩因子为20时,Cr(VI)浓度从初始的5 mg/L浓缩至82.6 mg/L;对浓缩液解络合,控制Cl-浓度为0.15 mol/L,解络合率为71.1%,以Cl-溶液对解络合液进行洗涤,Cr(VI)洗脱率可达95.9%. 聚季铵盐-22可循环使用.     

Efficient nitric oxide sensing on nanostructured La2MMnO6 (M: Co,Cu, Zn) electrodes

Selective detection of nitric oxide (NO) is a challenge for automotive exhaust monitoring systems due to the instability of sensing architectures operating under extreme environments. Herein, yttria-stabilized zirconia (YSZ) solid-electrolyte-based electrochemical gas sensor was developed using double perovskite electrodes (DPO) for selective detection of NO. The La₂MMnO₆ (M: Co, Cu, Zn) phases were synthesized by sol-gel processing of constituent salts and characterized for physicochemical and sensing properties to investigate the impact of transition metal cations present in octahedral environments on charge transport properties. The La₂ZnMnO₆ with a predominant Zn²⁺–Mn⁴⁺ charge ordering excelled in the sensing characteristics with high sensitivity (33 mV/decade for 3–80 ppm NO concentration), fast response/recovery time (52/42 s) and significant NO selectivity at 500 °C. The sensing behavior of double perovskites was comprehensively explored and found to abide by the mixed-potential model. Moreover, stable sensing properties over a period of three weeks indicate the here-described sensors to be potentially competitive for onboard exhaust monitoring in automobiles.     

Processing of highly porous bioglass monoliths by hydrothermal hot pressing

Porous bioglass monoliths have been processed by hydrothermal hot pressing (HyHP) from sol-gel and melt-derived bioglass powders of composition (in mol %): SiO₂–CaO–P₂O₅ (55.0-40.0-5.0) and SiO₂–CaO–Na₂O–P₂O₅ (47.2-26.4-23.8-2.6), respectively. An open porosity of >70% ever reached in 3D structures is reported for monoliths issued from sol-gel powders. Dissolution studies were performed in simulated body fluid (SBF) for 1–30 days. The monoliths were analysed using X-Ray Diffraction (XRD), Fourier Transform Infra-Red (FTIR) spectroscopy and Scanning Electron Microscopy (SEM) to observe the formation of an apatite-like layer and elemental concentration of SBF was evaluated using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). A higher kinetics in the development of apatite layer was observed for sol-gel derived monoliths. This result is explained by the high surface areas of the nanosized sol-gel powders and the possibility of HyHP to create large porosity (mesoporous monoliths) and retain large surface areas. HyHP is also effective in processing 3D-bioglass structures with porosity gradient by co-sintering powders of different size.     

Surface modification of monolithic PolyHIPE Polymers for anionic functionality and their ion exchange behavior

Monolithic PolyHIPE Polymer (PHP), being a highly porous, low density, open cellular material was produced by polymerization of a high internal phase emulsion (HIPE) in which the polymerizable continuous phase consisted of monomers, styrene (STY), and divinyl benzene (DVB). The inner dispersed phase (90 vol %) was an aqueous solution containing 0.4 wt % potassium persulphate as initiator. The resulting porous structure had 12% crosslinking density. Surface chemistry of the monoliths was modified by chloromethylation and amination to impart anionic functionality. Surface modified monoliths had ion exchange capacity of 3.01 meq/g, and had the ability to uptake water about 10 times of its mass. It was used Cr (VI) ion removal from aqueous solution. The experimental results investigated for both the Langmuir and the Dubinin–Radushkevich adsorption models. The maximum Cr (VI) adsorptions are 126.6 mg Cr (VI)/g and 129.3 mg Cr (VI)/g, respectively. The mean free energy E of adsorption is 11.18 kJ/mol according to the Dubinin–Radushkevich adsorption model, indicating that the adsorption occurs through a chemical ion‐exchange process and it is not diffusion limited. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42286.     

Electrocatalytic reduction of chromium by poly(aniline-co-o-aminophenol): An efficient and recyclable way to remove Cr(VI) in wastewater

Poly(aniline-co-o-aminophenol) (PANOA)-modified glassy carbon electrode (GCE) was first used to investigate the electrocatalytic reduction of dichromate in a NaCl solution of pH 5.0. The results of cyclic voltammograms and UV–vis spectra demonstrated that the reduction of Cr(VI) occurred at PANOA-modified GCE. The FT-IR, ESR and XPS results showed that the Cr(VI) can be doped in the PANOA films and can convert to less-toxic Cr(III). The doping level Cr/N was 78.2% and trace amount of Cl (0.42%) was detected in the doped PANOA, which indicated that the doping process is effective and PANOA had rather good ion selectivity in the 0.10 M NaCl supporting electrolyte. The factors influenced the reduction were also considered. Various initial concentrations of Cr(VI) had different removal rates. The maximum removal rate of Cr(VI) at 20 mg L⁻¹ (32.3%) was better than that of at 5 mg L⁻¹ (22.9%). The solution pH had little effect on Cr(VI) reduction and doping process of the PANOA because PANOA had good electrochemical activity and stability in a wide range of pHs (from pH 4 to pH 8).     

聚丙烯酸复合铝改性膨润土制备及其对Cr(VI)的吸附

为改善膨润土对Cr(VI)的吸附性能,用铝、丙烯酸聚合及十六烷基三甲基溴化铵改性合成聚丙烯酸复合铝改性膨润土。采用X射线衍射、红外光谱和扫描电子显微镜对天然膨润土和改性膨润土进行表征。结果表明,在25℃、溶液pH=5~6时,0.5 g吸附剂对200 mL浓度为20 mg/L Cr(VI)的平衡吸附量为1.996 mg/g,平衡吸附时间为130 min,且固液分离容易。吸附过程较好地符合Lagergren二级吸附动力学方程和Freundlich等温吸附方程,颗粒状吸附剂对Cr(VI)的吸附以络合吸附为主。     

Sol-gel methods for the assembly of metal chalcogenide quantum dots

Sol-gel chemistry represents a powerful method for assembling metal chalcogenide quantum dots into 3D connected architectures without the presence of intervening ligands to moderate particle-particle interactions. Wet gels prepared by the oxidative loss of thiolate surface groups from chalcogenide nanoparticles can be converted to xerogels (low porosity) or aerogels (high porosity), and the quantum-confinement effects in these low-dimensional networks decrease with increasing density of the network. In this Account, we describe the application of sol-gel chemistry to the formation of CdSe architectures and discuss how surface modification can lead to highly luminous monoliths, concluding with the prospects of these unique materials for applications in sensing and photovoltaics.     

Efficient photocatalytic removal of aqueous Cr(VI) by N-F-Al tri-doped TiO<Subscript>2</Subscript>

As chromium is a common heavy metal contaminant in water, we have prepared N-F-Al tri-doped TiO₂ catalyst for Cr(VI) removal under visible light. The sample was prepared via a sol-gel method and was characterized by XRD, BET, UV-vis DRS, XPS and SEM techniques. In the photocatalytic experiments, effects of Al/Ti ratio, F/Ti ratio, calcination temperature and different dopants were investigated. The optimum Al/Ti molar ratio, F/Ti ratio and calcination temperature proved to be 0.01, 0.1 and 500 °C, respectively, which is in accordance with the characterization analysis. Catalysts prepared under this condition showed a high photoactivity for Cr(VI) removal in water.     

Re‐use of Exhausted Ground Coffee Waste for Cr(VI) Sorption

Abstract

Exhausted ground coffee waste has been investigated as metal biosorbent for Cr(VI) from aqueous solution. Maximum metal sorption was found to occur at initial pH 3.0. Kinetic studies revealed that the initial uptake was quite rapid; nevertheless, it took five days to reach equilibrium. The value of the Langmuir maximum uptake was found to be 10.2 mg Cr(VI)/g waste. The sorbent is able to reduce hexavalent chromium to its trivalent form. A solution of 1 M NaOH was the most effective desorption agent and after 24 hours contact 42% of total chromium was desorbed in both hexavalent and trivalent oxidation states.     

Synthesis and optimization strategies of nanostructured metal oxides for chemiresistive methanol sensors

Thanks to the merits such as high specific surface areas, superior electronic conduction and unique gas diffusion path derived from the nanoscales, the demand for detecting methanol has contributed to the rapid expansion of gas sensors based on metal oxide nanostructures. In this review, the “process-structure-performance” correlations of metal oxide nanostructures utilized in the detection of methanol are analyzed. The sensing mechanisms of nanostructured metal oxides operated at different temperatures for methanol monitoring are first introduced. Subsequently, various synthesis processes (e.g. hydrothermal, sol-gel and electrospinning) utilized to modulate the structure and morphology of metal oxide nanostructures are discussed. Given the limitations that existed in methanol gas sensors, numerous optimization strategies including doping, surface modifications, newly designed structures and morphologies, the self-doping defects are enumerated to dramatically enhance the sensing properties represented by the improvement of sensitivity, the reduction of working temperature, the decrease of detection limit, etc. Additionally, the challenges and future research directions of advanced methanol sensors based on metal oxide nanostructures are proposed. It is ultimately expected that this review will help break the bottleneck of nanostructured metal oxides gas sensors in the field of methanol detection, and further promote the actual application of chemiresistive methanol sensors.     

Molecular Structure of Binary Chromium(III)–DNA Adducts

Chromium(VI) is a carcinogen and mutagen, and its mechanisms of action are proposed to involve binding of its reduction product, chromium(III), to DNA. The manner in which chromium(III) binds DNA has not been established, particularly at a molecular level. Analysis of oligonucleotide duplex DNAs by NMR, EPR, and IR spectroscopies in the presence of chromium(III) allows the elucidation of the Cr binding site. The metal centers were found to interact exclusively with guanine N7 positions. No evidence of chromium interactions with other bases or backbone phosphates nor of Cr forming intra-strand crosslinks between neighboring guanine residues was observed.     

Preparation and characterization of vacuum deposited semiconducting nanocrystalline polymeric thin film sensors for detection of HCl

Detection and monitoring of hazardous species like HCl, by fast, simple, cost effective, highly specific and selective devices is the main issue for environmental scientists. The currently available detection methods are either indirect or time consuming and expensive. These sensors are operated at high temperatures and response time is very slow. The nanocomposite polymeric combinations have come out as efficient sensors for detection of hazardous gases. Nanocomposite copolymers of aniline and formaldehyde synthesized by chemical polymerization with metal complex of Fe-Al have been found to act as high selective and efficient sensors for HCl. Thin films of Fe-Al doped nanocomposite copolymer evaporated under high vacuum on various substrates like mica and glass have been found to exhibit high stability, sensitivity, selectivity and a fast response towards the presence of HCl vapors in the air. High stability, sensitivity, selectivity and specificity are obtained by optimization of doping (Fe-Al) in nanocomposite copolymer during synthesis. These sensors operate at room temperature. The preparation, optical, electrical, and structural characterization and behavioral acceptance test on the gas sensing properties of these sensors are reported here.     

Photocatalytical reduction of Cr(VI) over ZnO powder

The photocatalytical reduction of Cr(VI) in aqueous suspensions of ZnO under uv-illumination is studied. The amount of Cr(VI) photoreduced at different irradiation times, initial Cr(VI) concentrations, pH and temperatures have been determined. Different ZnO—doped samples have been used in order to improve the yield of the Cr(VI) photoreduction. Finally, some experiments under sunlight have been performed. The results obtained in the present work show that ZnO can be used as efficient catalyst for the photoreduction of Cr (VI) in aqueous solutions even under sunlight exposure.     

Synthesis of strontium (Sr) doped hydroxyapatite (HAp) nanorods for enhanced adsorption of Cr (VI) ions from wastewater

The development of high-efficiency adsorbents for heavy metal ion removal from wastewater is highly desirable and challenging due to their synthesis complexity and low adsorption capacities. Herein, we reported the synthesis of strontium (Sr) doped hydroxyapatite (HAp) for the increased Cr (VI) adsorption. The effects of pH, temperature, and time on adsorption performances were studied. As a result, the Sr-HAp nanorods can achieve a Cr (VI) adsorption capacity of 443 mg/g, which is significantly higher than that of HAp nanorods (318 mg/g). To better understand the adsorption mechanism, the Langmuir isotherm model was established. The modeling results indicated that Langmuir monolayer chemical adsorption contributed to the efficient Cr (VI) ion removal for Sr-HAp nanorods adsorbents. The surface area and surface functional groups (O–H) contributed to the different Cr (VI) adsorption capacities between HAp and Sr-HAp.     

Adsorptive removal of hexavalent chromium using sawdust: Enhancement of biosorption and bioreduction

Simultaneous biosorption and bioreduction experienced by Cr(VI) species were initially validated by mixing 500 µM of Cr(VI) with 0.20 g of sawdust biosorbent. Accordingly, a complete disappearance of Cr(VI) species with emergence of 25% of Cr(III) ions was recorded; evidencing that both processes transpired simultaneously. An increase in the initial Cr(VI) concentration to 1500 µM could further induce more reduced Cr(VI) concentration. The enhancement of Cr(VI) biosorption and bioreduction was found to occur upon increasing the sawdust dosage to 4 g L^?1 and decreasing the solution pH to 2. However, the presence of a Zn(II) co-pollutant at 2500 µM had deteriorated the removal of Cr(VI) species.     

Low temperature TiO2 based gas sensors for CO2

Monitoring the level of CO₂, especially in closed spaces, is more and more required in technological applications, or in human activities. Since most of the literature data reveal CO₂ detection materials with high sensitivities over 300 °C, here we have concentrated on the gas sensing abilities of Cr doped TiO₂ thin films in front of CO₂, close to the room temperature and at atmospheric pressure. The films were obtained by RF reactive sputtering. The undoped films contain a mixture of anatase and rutile phases. With the increase of Cr content, the crystallites size decreases, and the films become pure rutile for a 4 at% Cr concentration. We found out that these material based sensors are more sensitive to CO₂ for higher Cr concentration, the optimum operating temperature approaching to the room temperature, determining in fact low energy consumption. The explanation is related to the observed increase of oxygen vacancies number (which we have evidenced and clarified), and also to the presence of the rutile phase, whose higher dielectric constant (compared to anatase), and its finer crystallites, determine a better gas sensing. More, the surface active area in front of CO₂ increases, as the films become rougher for higher Cr contents. The increase of Cr³⁺ percentage enhance the power of interaction with the adsorbed species (O₂ and/or CO₂). A grain boundary model was proposed for the thermal activation of the electrical conductivity. The energy barrier height at the grain boundary, the impurities concentration (characteristic parameters of this model) were calculated and found to agree well with the data in the literature.