Water-soluble MoS<Subscript>2</Subscript> quantum dots are a viable fluorescent probe for hypochlorite

A method is described for the fluorometric determination of hypochlorite. It is making use of molybdenum disulfide quantum dots (MoS₂ QDs) as a fluorescent probe. The QDs are prepared by hydrothermal reaction of sodium molybdate with glutathione. They possess diameters typically ranging from 1.4 to 3.8 nm, excellent stability in water, and blue photoluminescence (with excitation/emission peaks located at 315/412 nm and a quantum yield of 3.7%). The fluorescence of the QDs is statically quenched by hypochlorite, and the Stern-Volmer plot is linear. Hypochlorite can be detected in the 5–500 μM concentration range with a 0.5 μM detection limit. The method has been successfully applied to the determination of hypochlorite in spiked samples of tap water, lake water, and commercial disinfectants.

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Graphical abstract Schematic of a method for the fluorometric determination of hypochlorite using MoS₂ quantum dots as a fluorescent probe. It has been applied to hypochlorite assay in spiked samples of tap water, lake water, and commercial disinfectants.

     

Method for determination of radium in seawater using MnO<Subscript>2</Subscript> co-precipitation technique

Radium is considered to be a useful tracer for studying various physical processes of seawater. There are several methods for analysis of radium; however, analysis of radium in seawater by those analytical techniques is a tedious job. Thus a new methodology was optimized for analysis of radium in sea water using MnO₂ co-precipitation followed by gamma spectrometry. The method produced good yield which ranged from 85–98%. The method is simple and requires less amounts of chemicals and no use of acids. Seawater from different western Indian coastal environments viz. Tarapur, Mumbai and Goa were collected and analysed for ²²⁶Ra and ²²⁸Ra using this method.     

A fluorescent glucose bioassay based on the hydrogen peroxide-induced decomposition of a quencher system composed of MnO<Subscript>2</Subscript> nanosheets and copper nanoclusters

The authors describe a fluorometric glucose assay that is based on the use of MnO₂ nanosheets and copper nanoclusters (CuNCs) acting as nanoprobes. The CuNCs were synthesized by using bovine serum albumin as a template by chemical reduction of copper(II) sulfate. On addition of MnO₂ nanosheets to a colloidal solution of CuNCs, the fluorescence of CuNCs (measured at excitation/emission wavelengths of 335/410 nm) is quenched. However, in the presence of enzymatically generated H₂O₂, the MnO₂ nanosheets are reduced to form Mn(II) ions. As a result, fluorescence intensity recovers. The glucose assay is based on the enzymatic conversion of glucose by glucose oxidase to generate H₂O₂ and glucuronic acid. The calibration plot is linear in the 1 μM to 200 μM glucose concentration range, and the detection limit is 100 nM. The method was successfully applied to the determination of glucose in spiked human serum samples.

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Graphical abstract A sensitive fluorescent bioassay is reported for the detection of glucose based on the hydrogen peroxide-induced decomposition of a quencher system composed of MnO₂ nanosheets and copper nanoclusters (CuNCs).

     

Electrochemical determination of 5-hydroxytryptamine using mesoporous SiO<Subscript>2</Subscript> modified carbon paste electrode

A mesoporous SiO₂ was synthesized according to the published work, and then used to modify the carbon paste electrode (CPE). The electrochemical behaviors of 5-hydroxytryptamine (5-HT) at the bare CPE and the mesoporous SiO₂ modified CPE were compared. Owing to the huge surface area, unique mesopores and strong adsorptive ability, the oxidation signal of 5-HT at the mesoporous SiO₂ modified CPE greatly increased, compared with that at the bare CPE. This clearly suggests that the mesoporous SiO₂ modified electrode shows efficient and remarkable enhancement effect towards 5-HT. Based on this, a sensitive, rapid and convenient electrochemical method was developed for the determination of 5-HT after optimizing the experimental parameters such as supporting electrolyte, content of mesoporous SiO₂ as well as accumulation time. The linear range is from 2.0 × 10⁻⁷ to 1.5 × 10⁻⁵ mol/l, and the limit of detection is as low as 8.0 × 10⁻⁸ mol/l after 2-min accumulation. The relative standard deviation (RSD) for 10 mesoporous SiO₂ modified CPEs is evaluated to be 6.7%. Finally, this novel method was successfully used to determine 5-HT in human blood serums.     

Fluorometric determination of the antibiotic kanamycin by aptamer-induced FRET quenching and recovery between MoS<Subscript>2</Subscript> nanosheets and carbon dots

A rapid and sensitive aptamer-based assay is described for kanamycin, a veterinary antibiotic with neurotoxic side effects. It is based on a novel FRET pair consisting of fluorescent carbon dots and layered MoS₂. This donor-acceptor pair (operated at excitation/emission wavelengths of 380/440 nm) shows fluorescence recovery efficiencies reaching 93 %. By taking advantages of aptamer-induced fluorescence quenching and recovery, kanamycin can be quantified in the of 4–25 μM concentration range, with a detection limit of 1.1 μM. The method displays good specificity and was applied to the determination of kanamycin in spiked milk where it gave recoveries ranging from 85 % to 102 %, demonstrating that the method serves as a promising tool for the rapid detection of kanamycin in milk and other animal-derived foodstuff.

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Graphical Abstract A fluorometric aptasensor was developed for the determination of kanamycin. It is based on a novel FRET pair of carbon dots and layered MoS₂. The fluorescence recovery efficiency reached 93 % with a good sensitivity, specificity and recoveries in spiked milk.

     

Voltammetric determination of chloramphenicol using a carbon fiber microelectrode modified with Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanoparticles

An electrochemical microsensor for chloramphenicol (CAP) was fabricated by introducing magnetic Fe₃O₄ nanoparticles (NPs) onto the surface of activated carbon fibers. This microsensor exhibited increased electrochemical response toward CAP because of the synergetic effect of the Fe₃O₄ NPs and the carbon fibers. Cyclic voltammograms were acquired and displayed three stable and irreversible redox peaks in pH 7.0 solution. Under optimized conditions, the cathodic current peaks at ?0.67 V (vs. Ag/AgCl). The calibration plot is linear in the 40 pM to 1 μM CAP concentration range, with a 17 pM detection limit (at a signal-to-noise ratio of 3). The sensor was applied to the determination of CAP in spiked sediment samples. In our perception, this electrocatalytic platform provided a useful tool for fast, portable, and sensitive analysis of chloramphenicol.

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Graphical abstract A sensitive carbon fiber microsensor modified with Fe₃O₄ nanoparticles is found to display two cathodic peaks when detecting chloramphenicol at 100 mV·s^?1 and at pH 7.0. The sensor was applied to the determination of chloramphenicol in sediment samples.

     

Carbon ceramic electrodes modified with mixed oxides SiO<Subscript>2</Subscript>/SnO<Subscript>2</Subscript> for determination of levofloxacin

The preparation of a carbon ceramic electrode modified with SnO₂ (CCE/SnO₂) using tin dibutyl diacetate as precursor was optimized by a 2³ factorial design. The factors analyzed were catalyst (HCl), graphite/organic precursor ratio, and inorganic precursor (dibutyltin diacetate). The statistical treatment of the data showed that only the second-order interaction effect, catalyst × inorganic precursor, was significant at 95% confidence level, for the electrochemical response of the system. The obtained material was characterized by scanning electron microscopy (MEV), X-ray diffraction (XRD), RAMAN spectroscopy, XPS spectra, and voltammetric techniques. From the XPS spectra, it was confirmed the formation of the Si–O–Sn bond by the shift in the binding energy values referred to Sn 3d3/2 due to the interaction of Sn with SiOH species. The incorporation of SnO₂ provided an increment of the electrode response for levofloxacin, with Ipa = 147.0 μA for the ECC and Ipa = 228.8 μA for ECC/SnO₂, indicating that SnO₂ when incorporated into the silica network enhances the electron transfer process. Under the optimized working conditions, the peak current increased linearly with the levofloxacin concentration in the range from 6.21×10^?5 to 6.97×10^?4 mol L^?1 with quantification and detection limits of 3.80×10^?5 mol L^?1 (14.07 mg L^?1) and 1.13×10^?5 mol L^?1 (4.18 mg L^?1), respectively.     

Highly improved sensing of dopamine by using glassy carbon electrode modified with MnO<Subscript>2</Subscript>, graphene oxide,carbon nanotubes and gold nanoparticles

A composite material obtained by ultrasonication of graphene oxide (GO) and multi-walled carbon nanotubes (MWCNTs) was loaded with manganese dioxide (MnO₂), poly(diallyldimethylammonium chloride) and gold nanoparticles (AuNPs), and the resulting multilayer hybrid films were deposited on a glassy carbon electrode (GCE). The microstructure, composition and electrochemical behavior of the composite and the modified GCE were characterized by transmission electron microscopy, Raman spectra, energy-dispersive X-ray spectroscopy, electrochemical impedance spectroscopy and cyclic voltammetry. The electrode induces efficient electrocatalytic oxidation of dopamine at a rather low working voltage of 0.22 V (vs. SCE) at neutral pH values. The response is linear in the 0.5 μM to 2.5 mM concentration range, the sensitivity is 233.4 μA·mM￣¹·cm￣², and the detection limit is 0.17 μM at an SNR of 3. The sensor is well reproducible and stable. It displays high selectivity over ascorbic acid, uric acid and glucose even if these are present in comparable concentrations.

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Graphical abstract Gold nanoparticles were self-assembled onto the surface of the MnO₂ decorated graphene oxide-carbon nanotubes composites with poly(diallyldimethylammonium chloride) (PDDA) as a coupling agent. Further, a sensitive electrochemical sensor of dopamine was developed via immobilizing this nanocomposite on a glassy carbon electrode (GCE).

     

Voltammetric determination of 4-nitrophenol using a glassy carbon electrode modified with a gold-ZnO-SiO<Subscript>2</Subscript> nanostructure

A nanostructured material of the type Au-ZnO-SiO₂ is described that consists of ZnO and gold nanoparticles (NPs) dispersed into a silica matrix and used to construct a voltammetric sensor for 4-nitrophenol. The AuNPs and ZnO NPs are anchored onto the silica network which warrants the nanostructures to be stable in various environments. It also facilitates the electron transfer between the electrolyte and the glassy carbon electrode (GCE). The properties of the nanostructure as a modifier for the GCE were investigated by energy dispersive spectrometry, X-ray diffraction spectroscopy, and transmission electron microscopy. It is shown that the nanostructure increases the surface area. Hence, the cathodic and anodic current in differential pulse voltammetry of 4-nitrophenol are considerably enhanced in comparison to a bare GCE. Under optimum conditions, the currents for oxidation and reduction are proportional to the concentration of 4-nitrophenol in the 0.05–3.5 μM and 0.01–1.2 μM concentration ranges, with 13.7 and 2.8 nM detection limits, respectively. The sensor has excellent sensitivity, fast response, long-term stability, and good reproducibility. It is perceived to be a valuable tool for monitoring 4-nitrophenol in real water samples.

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Graphical abstract Schematic of voltammetric sensor for 4-nitrophenol. It is based on GCE modified with gold-ZnO-SiO₂ nanostructure. It exhibited the improvement in performance for both oxidation and reduction peaks in terms of linearity, concentration range, detection limit, and sensitivity.

     

Liquid-phase MnO<Subscript>2</Subscript>-modification of clinoptilolite

Formation mechanism of the MnO₂ phase in the reaction of heterogeneous synthesis between Mn²⁺ and MnO ₄ ^- ions on a solid aluminosilicate surface in aqueous solutions was studied. It was shown that, for lowsilica forms, the Mn²⁺ ion is oxidized by the MnO ₄ ^- ion uniformly across the grain depth to give the MnO₂ phase and manganese manganites. For high-silica materials, the MnO₂ phase is formed on the outer surface of grains, with the decomposition of the MnO ₄ ^- ion and formation of the MnO₂ phase and molecular oxygen. It was found that, for the clinoptilolite rock used as a solid support, the yield of the MnO₂ phase and its distribution over the particle volume depend on the penetration capacity of the MnO ₄ ^- ion into the porous structure of this rock, determined by its composition. It is shown that the amount of the MnO₂ phase grows with increasing concentration of the MnO ₄ ^- ion and treatment duration, with the phase thickness being 15–20 and 350–1050 μm for, respectively, high- and low-silica samples.     

Photoelectrochemical aptasensor for sulfadimethoxine using g-C<Subscript>3</Subscript>N<Subscript>4</Subscript> quantum dots modified with reduced graphene oxide

A novel photoelectrochemical (PEC) aptasensor with graphitic-phase carbon nitride quantum dots (g-C₃N₄; QDs) and reduced graphene oxide (rGO) was fabricated. The g-C₃N₄ QDs possess enhanced emission quantum yield (with an emission peak at 450 nm), improved charge separation ability and effective optical absorption, while rGO has excellent electron transfer capability. Altogether, this results in improved PEC performance. The method is making use of an aptamer against sulfadimethoxine (SDM) that was immobilized on electrode through π stacking interaction. Changes of the photocurrent occur because SDM as a photogenerated hole acceptor can further accelerate the separation of photoexcited carriers. Under optimized conditions and at an applied potential of +0.2 V, the aptasensor has a linear response in the 0.5 nM to 80 nM SDM concentration range, with a 0.1 nM detection limit (at S/N =?3). The method was successfully applied to the analysis of SDM in tap, lake and waste water samples.

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Graphical abstract Graphitic-phase carbon nitride (g-C₃N₄) quantum dots (QDs) and reduced graphene oxide (rGO) were used to modify fluorine-doped SnO₂ (FTO) electrodes for use in a photoelectrochemical (PEC) aptasensor. SDM oxidized by the hole on valance band (VB) of g-C₃N₄ QDs promote the separation of electron in the conductive band (CB), which made the changes of photocurrent signal.

     

Time-resolved enzymatic determination of glucose using a fluorescent europium probe for hydrogen peroxide

An enzymatic assay for glucose based on the use of the fluorescent probe for hydrogen peroxide, europium(III) tetracycline (EuTc), is described. The weakly fluorescent EuTc and enzymatically generated H₂O₂ form a strongly fluorescent complex (EuTc–H₂O₂) whose fluorescence decay profile is significantly different. Since the decay time of EuTc–H₂O₂ is in the microseconds time domain, fluorescence can be detected in the time-resolved mode, thus enabling substantial reduction of background fluorescence. The scheme represents the first H₂O₂-based time-resolved fluorescence assay for glucose not requiring the presence of a peroxidase. The time-resolved assay (with a delay time of 60 s and using endpoint detection) enables glucose to be determined at levels as low as 2.2 mol L^–1, with a dynamic range of 2.2–100 mol L^–1. The method also was adapted to a kinetic assay in order to cover higher glucose levels (mmol L^–1 range). The latter was validated by analyzing spiked serum samples and gave a good linear relationship for glucose levels from 2.5 to 55.5 mmol L^–1. Noteworthy features of the assay include easy accessibility of the probe, large Stokes shift, a line-like fluorescence peaking at 616 nm, stability towards oxygen, a working pH of approximately 7, and its suitability for both kinetic and endpoint determination.     

LiMO<Subscript>2</Subscript>@Li<Subscript>2</Subscript>MnO<Subscript>3</Subscript> positive-electrode material for high energy density lithium ion batteries

Li[Ni_1/3Co_1/3Mn_1/3]O₂ (NCM 111) is a promising alternative to LiCoO₂, as it is less expensive, more structurally stable, and has better safety characteristics. However, its capacity of 155 mAh g^?1 is quite low, and cycling at potentials above 4.5 V leads to rapid capacity deterioration. Here, we report a successful synthesis of lithium-rich layered oxides (LLOs) with a core of LiMO₂ (R-3m, M?=?Ni, Co) and a shell of Li₂MnO₃ (C2/m) (the molar ratio of Ni, Co to Mn is the same as that in NCM 111). The core–shell structure of these LLOs was confirmed by XRD, TEM, and XPS. The Rietveld refinement data showed that these LLOs possess less Li⁺/Ni²⁺ cation disorder and stronger M*–O (M*?=?Mn, Co, Ni) bonds than NCM 111. The core–shell material Li_1.15Na_0.5(Ni_1/3Co_1/3)_core(Mn_1/3)_shellO₂ can be cycled to a high upper cutoff potential of 4.7 V, delivers a high discharge capacity of 218 mAh g^?1 at 20 mA g^?1, and retains 90 % of its discharge capacity at 100 mA g^?1 after 90 cycles; thus, the use of this material in lithium ion batteries could substantially increase their energy density.

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Graphical Abstract Average voltage vs. number of cycles for the core–shell and pristine materials at 20 mA g^?1 for 10 cycles followed by 90 cycles at 100 mA g^?1

     

Improved peroxidase mimetic activity of a mixture of WS<Subscript>2</Subscript> nanosheets and silver nanoclusters for chemiluminescent quantification of H<Subscript>2</Subscript>O<Subscript>2</Subscript> and glucose

It is reported that a mixture of WS₂ nanosheets (WS₂ NS) and silver nanoclusters (AgNCs) displays strongly enhanced peroxidase-mimicking activity. The catalytic effect of the mixture was studied by colorimetry, fluorometry, chemiluminescence (CL) and electrochemistry. The effect is interpreted in terms of a difference between the Fermi energy level of the two nanomaterials. This leads to the formation of charge separation regions which act as active sites for enzyme mimetic interaction with the substrates. The mixture of WS₂ NS and AgNCs was exploited for the non-enzymatic determination of H₂O₂ and glucose. A stopped-flow method was applied as a sensitive CL detection system using the bicarbonate-H₂O₂ reaction. The mixture has a powerful peroxidase mimicking activity on the bicarbonate-H₂O₂ CL reaction, and this effect is much larger than that of any single constituent. In addition, the CL emission is improved several times by using the stopped-flow technique. Under optimum condition, H₂O₂ can be determined in the 2.5–1500 nM concentration range. Moreover, glucose levels in human serum can be quantified via glucose oxidase based oxidation which leads to the generation of H₂O₂. Using this CL assay, a linear relationship was obtained between the intensity of the CL emission and glucose concentration in the range of 0.03–20 μM, with a limit of detection (3S) of 13 nM.

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Graphical abstract An enhanced peroxidase-like catalytic activity for WS₂ nanosheets (WS₂ NS) was revealed in the presence of silver nanoclusters (AgNCs), and was exploited for the non-enzymatic determination of H₂O₂, and of glucose (via glucose oxidase; GOx) using a stopped-flow CL method.

     

Determination of nanomolar lead (II) using H<Subscript>2</Subscript>O<Subscript>2</Subscript>-oxidized activated carbon modified electrode

The virgin activated carbon (AC) was oxidized by 30% H₂O₂ under the ultrasonic condition for 6 h (denoted as AC-6). The electrochemical response of Pb²⁺ at the AC-6 modified paste electrode was investigated, suggesting that AC-6 shows much higher accumulation efficiency to trace levels of Pb²⁺. Based on this, a sensitive and convenient electrochemical method was developed for the determination of Pb²⁺ utilizing the excellent properties of AC-6. In pH 3.6 HAc-NaAc buffer, Pb²⁺ was easily accumulate at the surface of AC-6 modified paste electrode, then reduced to Pb at −1.20 V. During the following anodic sweep, the reduced Pb was oxidized and resulted in an oxidation stripping peak at −0.58 V. The stripping peak current is proportional to the concentration of Pb²⁺ over the range from the 8.0 × 10⁻⁹ to 2.0 × 10⁻⁶ mol l⁻¹, and the limit of detection is as low as 2.0 × 10⁻⁹ mol l⁻¹. Finally, this newly-developed method was successfully employed to determine Pb²⁺ in water samples.     

Mesoporous amorphous MnO<Subscript>2</Subscript> as electrode material for supercapacitor

A kind of novel mesoporous, electrochemical active material, amorphous MnO₂ has been synthesized by an improved reduction reaction and using supramolecular as template. The synthesized sample was characterized physically by thermogravimetric analysis, X-ray diffraction, transmission electron microscope (TEM), and Brunauer–Emmett–Teller (BET) surface area measurement, respectively. Electrochemical characterization was performed using cyclic voltammetry and chronopotentiometry in 2 mol/l KOH aqueous solution electrolyte. The results of BET and TEM analysis indicated that supramolecular template plays an important role in the process of big specific surface area mesoporous material forming. After sintering at 200 °C, the sample still remained an amorphous structure, and its specific capacitance reached 298.7 F/g and presented a very stable capacitance after 500 cycles. In addition, the electrochemical process, such as ion transfer and electrical condition, was also investigated with electrochemical impedance spectroscopy.     

The role of carbon nanotubes on the capacitance of MnO<Subscript>2</Subscript>/CNTs

The electrochemical behavior of MnO₂/carbon nanotubes (CNTs) has been studied by using cyclic voltammetry, galvanostatic charge discharge measurement and electrochemical impedance spectroscopy in 0.5 M Na₂SO₄ solution. The loading mass of CNTs, the potential sweep rate as well as the frequency have been investigated in detail to make clear of their influence on capacitance, resistance, and relaxation time constant. The dependence of the voltammetric surface charge q* on different loading mass of CNTs and potential scan rate has been investigated. With the addition of CNTs, resistance and relaxation time constant of the material are reduced and the rate capability increased. In particular, CNTs is beneficial for the outer surface capacitance contribution of MnO₂. The outer surface capacitance contribution of MnO₂/CNTs (1: 1) can reach 67% total capacitance contribution.     

Glassy carbon electrode modified with hemin and new melamine compounds for H<Subscript>2</Subscript>O<Subscript>2</Subscript> amperometric detection

In an attempt to increase the stability and efficiency of hemin-modified electrodes, the present work reports the preparation of a new modified glassy carbon electrode obtained by immobilization of hemin (Hm) on the electrode surface together with a new N-substituted melamine (2,4,6-triamino-1,3,5-triazine) based G-2 dendrimer comprising p-aminophenol as peripheral unit (Den) or with one of its analogues, a melamine G-0 dimer (Dim). Basic structural features, able to determine intimate relationships between Hm and Dim (or Den) at room temperature in solid state, were evidenced with the use of vibrational analysis carried out by FT-IR. This method revealed contacts between Hm and Dim or Den respectively as H-bond interactions, proton-interchange, and π-π stacking interactions. The new modified electrodes were characterized by cyclic voltammetry and electrochemical impedance spectroscopy and tested for amperometric detection of H₂O₂. In this purpose, GC/Hm-Dim electrode exhibited better catalytic properties than GC/Hm-Den electrode, but lower stability.     

Fluorometric turn-on determination of the activity of alkaline phosphatase by using WS<Subscript>2</Subscript> quantum dots and enzymatic cleavage of ascorbic acid 2-phosphate

A method is described for the determination of the activity of alkaline phosphatase (ALP). It is based on the reversible modulation of the fluorescence of WS₂ quantum dots (QDs). The fluorescence of the QDs is quenched by Cr(VI) but restored by free ascorbic acid (AA). The detection scheme relies on the fact that ALP hydrolyzes the substrate ascorbic acid 2-phosphate to produce AA, and that enzymatically generated AA can restore the fluorescence of the QDs. The signal (best measured at excitation/emission peak wavelengths of 365/440 nm) increases linearly in the 0.5 to 10 U·L^?1 ALP activity range, with a detection limit of 0.2 U·L^?1. The method was applied to the determination of ALP activity in human serum samples and demonstrated satisfactory results.

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Graphical abstract The fluorescence of chromate-loaded tungsten disulfide quantum dots (QDs) is quenched but restored after reaction with ascorbic acid that is formed by the catalytic action of alkaline phosphatase (ALP) on ascorbic acid 2-phosphate (AAP). The increase in fluorescence can be related to the activity of ALP.

     

Effect of the alloying component MnO<Subscript>2</Subscript> on the type of V<Subscript>2</Subscript>O<Subscript>5</Subscript> electrocrystallization

The methods of polarization curves E-log I _a and atomic-force microscopy were used to study the mechanisms by which the alloying component MnO₂ affects the electrocrystallization of V₂O₅ in electrolysis from mixed solutions of salts of these metals.