Preparation of N/S doped carbon dots and their application in nitrite detection

Detection of carcinogens is generally recognized to be meaningful, especially for nitrites (NO₂⁻). Here blue-green fluorescent carbon dots (CDs) were successfully synthesized by using p-aminobenzenesulfonic acid, and their surfaces were identified to be abundant in the functional groups of amino, hydroxyl, and sulfuric acid. Importantly, the sulfuric acid group and aromatic primary ammonia groups on the surfaces of CDs showed the interactions with the nitrites to cause fluorescence quenching. The novel CDs showed high sensitivity and selectivity for NO₂⁻ detection with a low detection limit of 0.03 mM in water due to the fluorescence quenching effect of the CDs. Consequently, the proposed CDs here may provide a new way of monitoring NO₂⁻ in the target samples.

Detection of carcinogens is generally recognized to be meaningful, especially for nitrites (NO₂⁻).     

Ratiometric fluorescent sensors for nitrite detection in the environment based on carbon dot/Rhodamine B systems

A novel carbon dot/Rhodamine B-based ratiometric fluorescent probe was developed for a highly sensitivity and selective detection of nitrite (NO₂⁻). The probe showed colour changes from blue to orange under ultraviolet light in response to NO₂⁻ with a detection limit as low as 67 nM in the range of 0 to 40 μM. A ratiometric fluorescent test paper was successfully prepared using the probe solution, which demonstrated its feasibility towards a rapid and semi-quantitative detection of NO₂⁻ in real samples.

A visual ratiometric fluorescent sensor based on blue carbon dot/Rhodamine B is used to selectively detect NO₂⁻ in the environment.     

Carbon quantum dots as a fluorophore for “inner filter effect” detection of metronidazole in pharmaceutical preparations

With houttuynia cordata as carbon source, photoluminescent carbon quantum dots (CDs) were obtained via a one-step hydrothermal procedure. The absorption band of metronidazole (MNZ, maximum absorption wavelength at 319 nm) can well overlap with the excitation bands of CDs (maximum excitation wavelength at 320 nm). A fluorescent approach has been developed for detection of MNZ based on the inner filter effect (IFE), in which as-prepared CDs act as an IFE fluorophore and the MNZ as an IFE absorber. We have investigated the mechanism of quenching the fluorescence of CDs and found that the IFE leads to an exponential decay in fluorescence intensity of CDs with increasing concentration of MNZ, but showed a good linear relationship (R² = 0.9930) between ln(F₀/F) with the concentration of MNZ in the range of 3.3 × 10⁻⁶ to 2.4 × 10⁻⁴ mol L⁻¹. Due to the absence of surface modification of the CDs or establishing any covalent linking between the absorber (MNZ) and the fluorophore (CDs), the developed method is simple, rapid, low-cost and less time-consuming. Meanwhile, it possesses a higher sensitivity, wider linear range, and satisfactory selectivity and has potential application for detection of MNZ in pharmaceutical preparations.

CDs were prepared using Houttuynia cordata via hydrothermal process, the absorption band of MNZ can well overlap the excitation bands of CDs, a simple, rapid approach for detection of MNZ was established on the basis of IFE.     

Poly(quercetin)-bismuth nanowires as a new modifier for simultaneous voltammetric determination of dihydroxybenzene isomers and nitrite

Dihydroxybenzene isomers and nitrite, NO₂⁻, are present in the environment as highly toxic compounds and cause human cancer. In this study, for the first time poly(quercetin) (PQ) was synthesized from the reaction between quercetin (Q) and hydroquinone (HQ) as a linker. Bismuth nanowires (BNWs) were synthesized using a solvothermal technique and then the BNWs and PQ were used for preparation of a novel modified graphite paste electrode (GPE/PQ–BNWs) for simultaneous determination of dihydroxybenzene isomers; HQ, catechol (CC), resorcinol (RS) in the presence of NO₂⁻. The product was characterized using X-ray diffraction, field emission scanning electron microscopy and Fourier transform infrared spectroscopy. The electrochemical response characteristics of the modified GPE toward mix HQ, CC, RS and NO₂⁻ were investigated by cyclic voltammetry, differential pulse voltammetry and electrochemical impedance spectroscopy. Under the optimum conditions, detection limits of 0.12, 0.2, 0.82 and 4.5 μM were obtained for HQ, CC, RS and NO₂⁻, respectively. Moreover, the GPE/PQ–BNWs were applied to determine HQ, CC, RS and NO₂⁻ in water samples with satisfactory results.

Dihydroxybenzene isomers and nitrite, NO₂⁻, are present in environment as the high toxic compounds and cause human cancer. A novel GPE/PQ–BNWs for simultaneous determination dihydroxybenzene isomers; HQ, CC, RS in presence of NO₂⁻.     

A dual-emission nano-rod MOF equipped with carbon dots for visual detection of doxycycline and sensitive sensing of MnO4−

Herein, ethanediamine-modified carbon dots (CDs) were encapsulated into luminescent MOF(Eu), which was designed for a dual-emission hybrid material (CDs@MOF(Eu)) with diverse fluorescence applications. This material exhibited high selectivity and sensitivity towards doxycycline. With an increasing concentration of doxycycline, the blue light emission of CDs could be quenched, whereas the red light emission of MOF(Eu) was enhanced. In view of this result, more convenient “test paper” was used first as a new tool for doxycycline detection, the colour of which turned from blue-purple to red as observed by the naked eyes under 365 nm UV-irradiation. This hybrid material also was a probe for sensing MnO₄⁻ with a low limit of detection and good anti-interference performance. We propose that CDs can improve detection sensitivity compared with the original MOF(Eu). The possible sensing mechanism was discussed in detail. Importantly, the feasibility of this composite for sensing doxycycline in a simulated biological system and sensing MnO₄⁻ in tap water was investigated.

A dual-emission hybrid material could detect doxycycline and MnO₄⁻ sensitively. Test paper was regarded initially as a tool for doxycycline visual detection. A lower LOD of MnO₄⁻ showed that carbon dots can accelerate quenching speed of MOF(Eu).     

A new ratiometric fluorescence assay based on resonance energy transfer between biomass quantum dots and organic dye for the detection of sulfur dioxide derivatives

Sulfur dioxide (SO₂) is considered as the fourth gas signal molecule after nitric oxide (NO), carbon monoxide (CO) and hydrogen sulfide (H₂S). It plays important roles in several physiological processes. Therefore, the design and synthesis of nanoprobes for the detection of SO₂ derivatives in cells is of great significance. Herein, we report a new ratiometric fluorescence nanoprobe based on resonance energy transfer (RET) between biomass quantum dots (BQDs) and organic dye (DMI) for the detection of SO₂ derivatives. The proposed ratiometric fluorescence assay allows the determination of HSO₃⁻ in the range of 1.0 to 225 μM with a detection limit of 0.5 μM. Importantly, the proposed ratiometric fluorescence nanoprobe exhibits a high photostability and good selectivity for HSO₃⁻ over other chemical species including H₂S and biological mercaptans. Quantitation of HSO₃⁻ in cell lysates by using the nanoprobe is demonstrated.

A new ratiometric fluorescence assay has been developed for the detection of sulfur dioxide derivatives with repeatability and selectivity. The assay was applied to quantitate HSO₃⁻ in cell lysates with accurate results.     

Enhanced nitrite accumulation under mainstream conditions by a combination of free ammonia-based sludge treatment and low dissolved oxygen: reactor performance and microbiome analysis

Partial nitritation under mainstream conditions is one of the major bottlenecks for the application of deammonification processes to municipal wastewater treatment plants. This study aimed at evaluating the combination effect of a side-stream free ammonia (FA) treatment and low dissolved oxygen (0.2 ± 0.1 mg L⁻¹) on inhibiting nitrite oxidizing bacteria (NOB) from enhancing nitrite accumulation in long-term lab-scale experiments. Two continuous floccular sludge reactors treating low-strength synthetic wastewater (60 mg N–NH₄⁺ L⁻¹ without COD) with a fixed nitrogen loading rate of 0.22 ± 0.03 g N per L per day were operated in a varied temperature range of 7–31 °C, with one acting as the experimental reactor and the other as the control. Side-stream sludge treatment with a stepwise elevation of FA concentration (65.2–261.1 mg NH₃ L⁻¹) was carried out every day in the experimental reactor; the nitrite accumulation ratio (NAR, (NO₂–N/(NO₂⁻–N + NO₃⁻–N) × 100%)) in the experimental reactor was always about twice that in the control one. Quantitative PCR (q-PCR) and high-throughput sequencing analyses showed the dominant NOB was mostly Nitrobacter, while there was an alternating trend between Nitrobacter and Nitrospira. Even though the whole microbial communities of each experimental stage between the two reactors were relatively clustered due to an incomplete NOB washout, three abundant metabolisms (amino acid metabolism, pyruvate metabolism and nitrogen metabolism) and key functional genes of nitrification predicted by PICRUSt in the experimental reactor were enriched, providing a better understanding of nitrite accumulation. These results have demonstrated that the positive hybrid effects of FA side-stream sludge treatment and a low DO could enhance nitrite accumulation. It is expected that a complete washout of NOB would be achieved after further process optimization.

An introduction of the combination of side-stream sludge treatment using FA and low DO could more effectively enhance nitrite accumulation than single low DO.     

Reed-derived fluorescent carbon dots as highly selective probes for detecting Fe3+ and excellent cell-imaging agents

A kind of highly selective and sensitive fluorescent probe for detecting Fe³⁺, carbon dots (CDs), was prepared with renewable reed naturally containing C, N, O, and S elements as a green and eco-friendly carbon source by a simple hydrothermal process. The fluorescence of CDs without purification and surface modification can be quenched by Fe³⁺ in a wide concentration range of 0 to 362 μmol L⁻¹ (concentration of Fe³⁺), with detection limits as low as 0.014 μmol L⁻¹ in 0–50 μmol L⁻¹. Characterizations, such as TEM, XPS, Raman and FTIR, confirmed that the static quenching mechanism involved the generation of non-luminescent complexes between Fe³⁺ and functional groups (carboxyl group, sulfur-oxyl group and hydroxyl group) on the surface of CDs and with the aggregation of CDs. More importantly, CDs had good biocompatibility and nontoxicity according to an MTT cell-viability assay, and cells labeled with CDs emitted blue, green and red color fluorescence. Thus, the static quenching mechanism was confirmed. So, this reed-derived natural CD solution can be utilized in detecting Fe³⁺, culture cells, and cell imaging.

A highly selective and sensitive fluorescent probe for detecting Fe³⁺, carbon dots (CDs), was prepared with renewable reed naturally containing C, N, O, and S elements as a green and eco-friendly carbon source by a simple hydrothermal process.     

Wild jujube-based fluorescent carbon dots for highly sensitive determination of oxalic acid

Fluorescent carbon dots (CDs) were synthesized by a one-step hydrothermal treatment of wild jujube and dl-tryptophan. The structure and properties of the CDs were confirmed by transmission electron microscopy, X-ray photoelectron spectroscopy, ultraviolet visible absorption spectroscopy, fluorescence spectroscopy and so on. The as-prepared CDs exhibit excellent excitation-independent but pH-dependent (4.0–12.0) fluorescent features and emit blue strong fluorescence under 365 nm light. Hg²⁺ can decrease the fluorescence intensity of the CDs through static quenching, while the addition of oxalic acid (OA) recovers it owing to the coordination binding between oxalic acid and Hg²⁺. Based on this, the as-prepared CDs were used as a new “off–on” fluorescent probe for highly sensitive determination of oxalic acid with a wide linear detection range of 0.1–20 mg L⁻¹ and a low detection limit of 0.057 mg L⁻¹. Moreover, the fluorescent probe was successfully applied to detect oxalic acid in tomato and cherry tomato samples with satisfactory results.

Carbon dots synthesized from wild jujube and dl-tryptophan can serve as sensitive off–on fluorescent sensors for the detection of oxalic acid.     

Ultrasensitive and bifunctional ZnO nanoplates for an oxidative electrochemical and chemical sensor of NO2: implications towards environmental monitoring of the nitrite reaction

Herein, we focused on the one pot synthesis of ZnO nanoplates (NP edge thickness of ∼100 nm) using a chemical emulsion approach for chemical (direct) and electrochemical (indirect) determination of NO₂. The structural and morphological elucidation of the as-synthesized ZnO NPs was carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive analysis of X-ray (EDAX), thermogravimetric analysis (TGA) and BET-surface area measurements. The XRD studies of the as-synthesised NPs reveal that ZnO NPs have a Wurtzite type crystal structure with a crystallite size of ∼100 nm. Such ZnO NPs were found to be highly sensitive to NO₂ gas at an operating temperature of 200 °C. Electrocatalytic abilities of these ZnO NPs towards NO₂/NO₂⁻ were verified through cyclic voltammetry (CV) and linear sweep voltammetry (LSV) using aqueous 1 mM NO₂⁻ (nitrite) in phosphate buffer (pH 7) solution. The results revealed enhanced activity at an onset potential of 0.60 V vs. RCE, achieved at a current density of 0.14 mA cm⁻². These ZnO NPs show selective NO₂ detection in the presence of other reactive species including CO, SO₂, CH₃OH and Cl₂. These obtained results show that this chemical route is a low cost and promising method for ZnO NPs synthesis and recommend further exploration into its applicability towards tunable electrochemical as well as solid state gas sensing of other toxic gases.

Herein, we focused on the one pot synthesis of ZnO nanoplates (NP edge thickness of ∼100 nm) using a chemical emulsion approach for chemical (direct) and electrochemical (indirect) determination of NO₂.      

Shock response of condensed-phase RDX: molecular dynamics simulations in conjunction with the MSST method

We have performed molecular dynamics simulations in conjunction with the multiscale shock technique (MSST) to study the initial chemical processes of condensed-phase RDX under various shock velocities (8 km s⁻¹, 10 km s⁻¹ and 11 km s⁻¹). A self-consistent charge density functional tight-binding (SCC-DFTB) method was used. We find that the N–NO₂ bond dissociation is the primary pathway for RDX with the NO₂ groups facing (group 1) the shock, whereas the C–N bond scission is the dominant primary channel for RDX with the NO₂ groups facing away from (group 2) the shock. In addition, our results present that the NO₂ groups facing away from the shock are rather inert to shock loading. Moreover, the reaction pathways of a single RDX molecule under the 11 km s⁻¹ shock velocity have been mapped out in detail, NO₂, NO, N₂O, CO and N₂ were the main products.

We have performed molecular dynamics simulations in conjunction with the multiscale shock technique (MSST) to study the initial chemical processes of condensed-phase RDX under various shock velocities (8 km s⁻¹, 10 km s⁻¹ and 11 km s⁻¹).     

Reversible ammonia uptake at room temperature in a robust and tunable metal–organic framework

Ammonia is useful for the production of fertilizers and chemicals for modern technology, but its high toxicity and corrosiveness are harmful to the environment and human health. Here, we report the recyclable and tunable ammonia adsorption using a robust imidazolium-based MOF (JCM-1) that uptakes 5.7 mmol g⁻¹ of NH₃ at 298 K reversibly without structural deformation. Furthermore, a simple substitution of NO₃⁻ with Cl⁻ in a post-synthetic manner leads to an increase in the NH₃ uptake capacity of JCM-1(Cl⁻) up to 7.2 mmol g⁻¹.

Recyclable and tunable ammonia adsorption with JCM-1 and JCM-1(Cl⁻) at room temperature occurs reversibly without structural decomposition.     

Novel fluorescence sensor for the selective recognition of tetracycline based on molecularly imprinted polymer-capped N-doped carbon dots

A novel fluorescent probe based on molecularly imprinted polymers (MIPs) coupled with N-doped carbon dots (CDs) was prepared and used for specific recognition and sensitive determination of tetracycline (TC). N-doped CDs were synthesized using citric acid as a carbon source and ethylenediamine as a nitrogen source by a microwave assisted pyrolysis method. The determination conditions such as the solvents, material amount, pH value, and temperature were optimized. The CDs-MIPs have the best quenching on TC in water. The proposed method used for TC determination in milk powder samples had a detection limit of 0.054 μg mL⁻¹ and a wide range of 0.5–30 μg mL⁻¹. Meanwhile, satisfactory recoveries were obtained ranging from 95 to 108%. Oxytetracycline, chlorotetracycline and most of the coexisting substances showed no obvious interference indicating that the CDs-MIP probe exhibited high selectivity due to the presence of imprinted sites. Charge transfer from CDs-MIPs to TC may be through the mechanism of fluorescence quenching. This work gives a feasible strategy for the synthesis of N-doped carbon dot based molecularly imprinted polymers used as a fluorescent sensor in the food analysis field.

A novel fluorescent probe based on MIP coupled with N-doped CDs was prepared and used for sensitive recognition of tetracycline.     

Microwave assisted synthesis of negative-charge carbon dots with potential antibacterial activity against multi-drug resistant bacteria

In this research, negative-charge carbon dots (CDs) were synthesized in one-step using a microwave and found to have potential antibacterial ability against multi-drug resistant bacteria. The CDs were synthesized by using citric acid and urea as precursors, and characterized by FT-IR, TEM and fluorescence spectrophotometry. The average size of CDs was about 2.5 nm, and the ζ potential was −11.06 mV. In the following antibacterial activity test, time-killing curve experiments and colony-forming assay were carried out to determine the minimum bactericidal concentration (MBC) and minimum inhibitory concentration (MIC) of the CDs against methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-intermediate Staphylococcus aureus (VISA). The data showed the MBC of the CDs against MRSA is 2.5 mg mL⁻¹, and the MIC of the CDs against MRSA is 0.63 mg mL⁻¹; the MBC of the CDs against VISA is 1.25 mg mL⁻¹, and the MIC of the CDs against VISA is 0.63 mg mL⁻¹. The results demonstrated that the negative-charge CDs have potential against multi-drug resistant Staphylococcus aureus (S. aureus), and may serve as alternatives for therapy in the future.

In this research, negative-charge carbon dots (CDs) were synthesized in one-step using a microwave and found to have potential antibacterial ability against multi-drug resistant bacteria.     

Cancer antigen 125 assessment using carbon quantum dots for optical biosensing for the early diagnosis of ovarian cancer

Fluorometric quantification of biological molecules is a key feature used in many biosensing studies. Fluorescence resonance energy transfer (FRET) using highly fluorescent quantum dots offers highly sensitive detection of the in-proximity wide variety of analyst molecules. In this contribution, we report the use of carbon quantum dots (CDs) for the ultrasensitive optical biosensing of cancer antigen 125 (CA-125) in the early malignant stage. This approach is based on monitoring the quenching of CDs luminescence at 535 nm by CA-125 after excitation at 425 nm and pH 10. The calibration of this method was performed in the concentration range of CA-125 from 0.01 to 129 U ml⁻¹ (R² = 0.99) with a detection limit of 0.66 U ml⁻¹, which matches remarkably with the standard chemiluminometric method in control and real patient samples. The sensing mechanism for cancer antigen 125 assessment was discussed on the basis of fluorescence quenching of CDs and time-resolved photoluminescence spectroscopy. The current method is easy, sensitive, cost-effective and provides a wide range of validity, which helps in overcoming the limitations of high cost and time consumption exhibited by many other traditional clinical assays for CA-125 quantification.

Fluorometric quantification of biological molecules is a key feature used in many biosensing studies.     

The role and effectiveness of monoculture and polyculture phytoremediation systems in fish farm wastewater

Phytoremediation offers a sustainable solution to aquaculture pollution, but studies with critical evaluations of the treatment performances of macrophyte systems are limited. This study intended to evaluate the roles and treatment profiles of Spirodela polyrhiza (L.) Schleid. and Lemna sp. systems in terms of ammonia, nitrate, nitrite, phosphate (NH₃–N, NO₃⁻–N, NO₂⁻–N, PO₄³⁻), chemical oxygen demand (COD), turbidity, and total suspended solids (TSS) on fish farm wastewater and to elucidate the rationale behind the removal of the pollutants and the changes in a raceway pond rig. The nitrogen and phosphorus removal in the Spirodela polyrhiza monoculture system outperformed the other configured systems. An 81% reduction in ammonia (to 3.90 mg of NH₃-N/L), and sharp declines of up to 75%, 88%, and 71% in TSS, turbidity, and COD levels were recorded within two days, while significant decreases in nitrate, nitrite, and phosphate levels were observed. This indicated that the system could inhibit nitrate and nitrite spikes in waters (nitrification) via reducing the available ammonia and limiting subsequent nitrite and nitrate conversion, while reducing TSS in algal-bloom wastewater via shading. High biomass productivity and superior protein content were observed in the macrophyte systems (S. polyrhiza + Lemna sp. polyculture system), with up to 112% and 12% increases, respectively. This study demonstrated that the S. polyrhiza monoculture system is effective at treating fish farm wastewater, lowering the levels of relevant inorganic and organic pollutants, and it could be used as a biofilter for natural waters, preserving the existing ecology.

Monoculture outperformed polyculture in terms of phytoremediation capabilities, indicating the importance of macrophyte selection. The species-wise pollutant removal abilities and their proportional densities dictate the treatment performance.     

Comparative study on synchronous adsorption of arsenate and fluoride in aqueous solution onto MgAlFe-LDHs with different intercalating anions

In this study, a series of MgAlFe-LDHs (Cl⁻, NO₃⁻, intercalation, and calcined products of a CO₃²⁻ interlayer) was synthesized and used for adsorption of arsenate and fluoride in individual contaminants and coexisting pollutant systems. Effects of various factors such as initial pH of solution, dosage of materials, coexisting ions, contact time, and initial pollutant concentrations were evaluated. Experimental results showed that different intercalating anions had a significant effect on adsorption performance of arsenate and fluoride in water. The adsorption of arsenate and fluoride on MgAlFe-CLDH, MgAlFe–Cl-LDH or MgAlFe–NO₃-LDH can be described by different adsorption isotherm equations. During the simultaneous removal process, arsenate and fluoride competed for adsorption sites of the adsorbent materials, and the fluoride ions had advantages in the competitive adsorption on MgAlFe–Cl-LDH and MgAlFe–NO₃-LDH. MgAlFe–NO₃-LDH was used to adsorb arsenate and fluoride in coexisting pollution systems (the concentration of each pollutant was 2 mg L⁻¹, the adsorbent dosage was 1.5 g L⁻¹). The remaining arsenic concentration was reduced to less than 10 μg L⁻¹ and the remaining fluoride ion concentration to below 20 μg L⁻¹ which meets the World Health Organization''s, EPA''s and China''s drinking water standards for arsenic and fluoride limits. A possible mechanism is discussed with support from further XRD, SEM, and XPS analysis of the materials after their adsorption.

During the simultaneous removal process, arsenate and fluoride competed for the adsorption sites of the adsorbent materials.     

The role of particulate matter in reduced visibility and anionic composition of winter fog: a case study for Amritsar city

Severe fog events during winter months in India are a serious concern due to the higher incidence of road accidents, flight delays and increased occurrence of respiratory diseases. The present paper is an attempt to study the twenty fog samples collected from the rooftop of an academic building of Guru Nanak Dev University, Amritsar, India from November 2017 to January 2018. Fog samples were analysed for various parameters viz. pH, electrical conductivity (EC), chloride (Cl⁻), nitrate (NO₃⁻) and sulphate (SO₄²⁻) levels. The pH, EC, and Cl⁻, NO₃⁻ and SO₄²⁻ levels in the fog samples were estimated as 6.3–7.9, 240–790 μS cm⁻¹, 108–2025 μeq L⁻¹, 105–836 μeq L⁻¹ and 822–5642 μeq L⁻¹, respectively. It was noticed that sulphate was the dominant anion in fog samples. The SO₄²⁻ to NO₃⁻ molar ratio in the fog was estimated as 7.6 which suggests the burning of fossil fuel as the major pollutant from vehicular exhausts. Multiple regression analysis was performed to evaluate the effect of PM_2.5/PM₁₀ ratio and relative humidity (RH) on visibility. A box-cox plot of power transformation produced better model fitting, employing a square root transformation of the visibility which indicated that the PM_2.5/PM₁₀ and RH have an exponential effect on visibility.

Severe fog events during winter months in India are a serious concern due to the higher incidence of road accidents, flight delays and increased occurrence of respiratory diseases.     

In-plasma-catalysis for NOx degradation by Ti3+ self-doped TiO2−x/γ-Al2O3 catalyst and nonthermal plasma

In an attempt to realize the efficient treatment of NO_x, a mixed catalyst of Ti³⁺ self-doped TiO_2−x and γ-Al₂O₃ was constructed by reducing commercial TiO₂. The degradation effect on NO_x was evaluated by introducing the mixed catalyst into a coaxial dual-dielectric barrier reactor. It was found that the synthesized TiO_2−x could achieve considerable degradation effects (84.84%, SIE = 401.27 J L⁻¹) in a plasma catalytic system under oxygen-rich conditions, which were better than those of TiO₂ (73.99%) or a single plasma degradation process (26.00%). The presence of Ti³⁺ and oxygen vacancies in TiO_2−x resulted in a relatively narrow band gap, which contributed to catalyzing deeply the oxidation of NO_x to NO₂⁻ and NO₃⁻ during the plasma-induced “pseudo-photocatalysis” process. Meanwhile, the TiO_2−x showed an improved discharge current and promoted discharge efficiency, explaining its significant activation effect in the reaction. Reduced TiO_2−x could achieve an impressive degradation effect in a long-time plasma-catalysis process, and still maintained its intrinsic crystal structure and morphology. This work provides a facile synthesis procedure for preparing Ti³⁺ self-doped TiO_2−x with practical and scalable production potential; moreover, the novel combination with plasma also provides new insights into the low-temperature degradation of NO_x.

TiO_2−x has a smaller forbidden band width, more abundant Ti³⁺ and oxygen vacancies, so as to obtain a better and more stable degradation effect of NO_x in plasma-catalysis process.     

Crystallization of colourless hexanitratoneptunate(iv) with anhydrous H+ countercations trapped in a hydrogen bonded polymer with diamide linkers

Colourless crystalline compounds of centrosymmetric [Np(NO₃)₆]²⁻ were yielded from 3 M HNO₃ aq in the presence of double-headed 2-pyrrolidone derivatives (L). In the obtained crystal structures, H⁺ was also involved as a countercation to compensate for the negative charge of [Np(NO₃)₆]²⁻, where the initial hydration around H⁺ was fully removed during crystallization despite it having the strongest hydration enthalpy. Instead, this anhydrous H⁺ was captured by L to form a [H⁺⋯L]_n hydrogen bonded polymer. In [Np(NO₃)₆]²⁻, the Np⁴⁺ centre is twelve-coordinated with 6 bidentate NO₃⁻, and therefore, present in an icosahedral geometry bearing inversion centre. In such a centrosymmetric system, any f–f transitions stemming from the 5f³ electronic configuration of Np⁴⁺ are electric-dipole forbidden. This is the reason why the compounds currently obtained were colourless unlike ordinary Np(iv) species, which are olive-green.

Centrosymmetric hexanitratoneptunate(iv) crystallizes with anhydrous proton countercations trapped in a hydrogen bonded polymer with diamide linker molecules to give colourless crystals.