Biomass carbon and Ti2C3MXene quantum dots as ratiometric fluorescent probes for sensitive detecting malachite green in fish sample.

A visual detection method for malachite green (MG) in food was established based on 'double-response-OFF' ratiometric fluorescent paper-based sensor. Biomass carbon quantum dots (BCQDs) using broad bean shell, and Ti3C2MXene quantum (MQDs) dots modified by ethylenediamine were synthesized by solvothermal method. The MG and two kinds of quantum dots could undergo static quenching, and the fluorescence color of two kinds of quantum dots gradually changed from red to blue, eventually the fluorescence was quenched, and the pattern had a two-stage linear relationship using fluorescent spectrofluorometer in the range of 0.1-140.0µM and the detection limit of 0.07µM. On this basis, a BCQDs/MQDs ratiometric fluorescence paper-based sensor was constructed and applied to fish sample. Through mobile phone software-Color recognizer, RGB values of fluorescent paper-based sensor at various concentrations of MG were extracted. The results showed that MG concentration was linearly correlated withR' value of RGB in the range of 20.0-140.0µM with 16.5µM detection limit. The method had been applied to the determination of canned fish and fresh basa fish samples, and the recovery rates were 97.33%-108.93% and 96.04%-117.97%, respectively. It proved that the ratiometric fluorescent paper-based sensor could be used for the rapid visual quantitative detecting MG in real samples.

Electrochemical immuno-biosensors for the detection of the tumor marker alpha-fetoprotein: A review.

Alpha-fetoprotein (AFP) is a glycoprotein that has many important physiological functions, including transportation, immunosuppression, and induction of apoptosis by T lymphocytes. AFP is closely related to the development of hepatocellular carcinoma and many kinds of tumors, all of which can show high concentrations, so it is used as a positive test indicator for many kinds of tumors. This paper reviews recent advances in the detection of the tumor marker AFP based on three immuno-biosensors: electrochemical (EC), photoelectrochemical (PEC), and electrochemical luminescence (ECL). The electrodes are modified by different materials or homemade composites, different signaling molecules are selected as single probes or dual probes for the detection of AFP. The detection limit was as low as 3 fg/mL, which indicated that the AFP immunosensor had achieved highly sensitive detection. In addition, we also reviewed and summarized the current development status and application prospect of AFP immunoelectrochemical sensors. There are not too many researches on immunosensors based on dual-signal ratios, and the commonly used probes are methylene blue (MB) and ferrocene (Fc). It would be more innovative to have more novel signaling molecules as probes to prepare dual-signal ratio sensors.

Electrochemical sensor for ultrasensitive sensing of biotin based on heme conjugated with gold nanoparticles and its electrooxidation mechanism.

We report the fabrication of a facile sensor using heme conjugated with gold nanoparticles (AuNPs) in situ on a glass carbon electrode (GCE) for the ultrasensitive determination of biotin without antibody or streptavidin. The use of heme and AuNPs as dual amplifiers allows a very broad detection range from 0.0050 to 50.0000 µmol·L-1 and a very low detection limit of 0.0016 µmol·L-1. The mechanistic aspects were elucidated using electrochemical analyses and frontier orbital calculations showing that the electrooxidation of biotin involves a one-electron and a one-proton transfer, generating biotin sulfoxide. The heme/AuNPs/GCE sensor exhibited excellent selectivity, reproducibility and stability, indicating high robustness. The recovery was between 97.20 and 105.70% with RSD less than 8.71%, suggesting good practicability. Our studies demonstrate that this approach can be used to detect and quantify biotin in a range of foods, including milk, infant formula, flour, orange juice, mango juice, egg white and egg yolk. Furthermore, all measurements do not require any intricate preparation or pre-treatment of the foods, thus representing a great potential for point-of-care testing.

A facile indole probe for ultrasensitive immunosensor fabrication toward C-reactive protein sensing.

C-reactive protein (CRP) is a protein biomarker for acute phase response. Herein, we fabricate a highly sensitive electrochemical immunosensor for CRP on a screen-printed carbon electrode (SPCE) with indole as a novel electrochemical probe and Au nanoparticles for signal amplification. Amongst, indole appeared as transparent nanofilms on the electrode surface, and underwent a one-electron and one-proton transfer to form oxindole during the oxidation process. Upon optimization of experimental conditions, a logarithmic correlation between CRP concentration (0.0001-100 µg∙mL-1) and response current was revealed with a detection limit of 0.03 ng∙mL-1 and a sensitivity of 5.7055 µA∙µg-1∙mL∙cm-2. The sensor exhibited exceptionally distinction selectivity, reproducibility and stability of the electrochemical immunosensor studied. The recovery rate of CRP in human serum samples determined by the standard addition method, ranged between 98.2-102.2%. Overall, the developed immunosensor is promising, and has the potential for CRP detection in real human serum samples.

Facile bimetallic co-amplified electrochemical sensor for folic acid sensing based on CoNPs and CuNPs.

Folic acid (FA) is essential for human health, particularly for pregnant women and infants. In this work, a glassy carbon electrode (GCE) was modified by a bimetallic layer of Cu/Co nanoparticles (CuNPs/CoNPs) as a synergistic amplification element by simple step-by-step electrodeposition, and was used for sensitive detection of FA. The proposed CuNPs/CoNPs/GCE sensor was characterized by differential pulse voltammetry (DPV), electrochemical impedance spectroscopy (EIS) and field emission scanning electron microscopy (FESEM). Then, under optimal conditions, a linear relationship was obtained in the wide range of 110.00-1750.00 µM for the detection of FA with a limit of detection (LOD) of 34.79 µM (S/N = 3). The sensitivity was calculated as 0.096 µA µM-1 cm-2. Some interfering compounds including glucose (Glc), biotin, dopamine (DA), and glutamic acid (Glu) showed little effect on the detection of FA by amperometry (i-t). Finally, the average recovery obtained was in a range of 91.77-110.06%, with a relative standard deviation (RSD) less than 8.00% in FA tablets, indicating that the proposed sensor can accurately and effectively detect the FA content in FA tablets.

Enzyme-free glucose sensors with efficient synergistic electro-catalysis based on a ferrocene derivative and two metal nanoparticles.

Gold electrodes (GE) were modified by the deposition of copper nanoparticles (CuNPs) and cobalt nanoparticles (CoNPs), followed by drop-casting of the ferrocene derivative FcCO-Glu-Cys-Gly-OH (Fc-ECG), resulting in two enzyme-free electrochemical sensors Fc-ECG/CuNPs/GE and Fc-ECG/CuNPs/GE. The ferrocene-peptide conjugate acts as an effective redox mediator for glucose oxidation, while metal nanoparticles acted as non-biological sites for glucose oxidation. Field emission scanning electron microscopy (FESEM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were carried out for characterization, while differential pulse voltammetry (DPV) was used for glucose quantification. Under optimized conditions, DPV shows a linear relationship between glucose concentration and the peak current. Both sensors showed a surprisingly high sensitivity of 217.27 and 378.70 µA mM-1 cm-2, respectively. A comparison to other glucose sensors shows a sensitivity that is 25 times higher. The sensors exhibit good reproducibility, stability, and repeatability. In injection experiments, recovery rates were 87.39-107.65% and 100.00-106.88%, respectively.

One step construction of an electrochemical sensor for melamine detection in milk towards an integrated portable system.

Excessive intake of melamine (MEL) can be harmful to human health, and it is important to establish a rapid and accurate MEL detection method. As the electrochemical activity of MEL is very low, ferrocenylglutathione (Fc-ECG) was used as an electron transfer mediator to assist with the detection of MEL using screen-printed carbon electrode (SPCE). This modified electrode (MEL/Fc-ECG/SPCE) was prepared by stepwise drop-casting and was fully characterized. Results showed that MEL significantly enhanced signal of Fc-ECG/SPCE sensor due to the three p-π conjugated double bonds that facilitated electron transfer. Under optimal conditions, the sensor exhibits two linearities in the range of 0.20-2.00 µM and 8.00-800 µM, with a sensitivity of 15.03 µA·µM-1·cm-2. The selectivity, stability, and reproducibility were investigated and successfully used to detect MEL in raw milk and confirms safety verification of foods. Moreover, a portable testing platform was designed for MEL detection based on a CH32 chip.

Na3V2(PO4)2F3@bagasse carbon as cathode material for lithium/sodium hybrid ion battery.

The nano-scale spherical Na3V2(PO4)2F3 with a NASICON structure phase was prepared with a spray drying technique, and the bagasse in Guangxi, China was selected as the carbon source to prepare Na3V2(PO4)2F3/C. The optimal preparation conditions of the composite determined using thermogravimetry, X-ray diffraction, scanning electron microscopy and electrochemical testing were: a calcination temperature of 650 °C and a 20% carbon source. The Na3V2(PO4)2F3/C has obvious redox peaks, determined by cyclic voltammetry (CV), at 3.90 V and 3.75 V, 4.32 V and 4.15 V. These two pairs of redox peaks correspond to the escape/intercalation of the two pairs of Li+/Na+. Notably, compared with pure Na3V2(PO4)2F3, the specific discharge capacity of Na3V2(PO4)2F3/C-20%, which were used as a cathode material for lithium-sodium hybrid ion batteries, increased from 55 mA h g-1 to 125 mA h g-1, which was an improvement of twofold.

Convenient Heme Nanorod Modified Electrode for Quercetin Sensing by Two Common Electrochemical Methods.

Quercetin (Qu) is one of the most abundant flavonoids in the human diet. High concentrations of Qu can easily cause adverse effects and induce inflammation, joint pain and stiffness. In this study, Heme was used as a sensitive element and deposited and formed nanorods on a glassy carbon electrode (GCE) for the detection of Qu. The Heme/GCE sensor was characterized using scanning electron microscopy (SEM), cyclic voltammetry (CV), differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS) techniques. Under optimized conditions, the developed sensor presented a linear concentration ranging from 0.1 to 700 µmol·L-1 according to the CV and DPV methods. The detection limit for the sensor was 0.134 µmol·L-1 and its sensitivity was 0.12 µA·µM-1·cm-2, which were obtained from CV analysis. Through DPV analysis we obtained a detection limit of 0.063 µmol·L-1 and a sensitivity of 0.09 µA·µM-1·cm-2. Finally, this sensor was used to detect the Qu concentration in loquat leaf powder extract, with recovery between 98.55-102.89% and total R.S.D. lower than 3.70%. The constructed electrochemical sensor showed good anti-interference, repeatability and stability, indicating that it is also usable for the rapid detection of Qu in actual samples.

Synthesis of Mn-MOFs loaded zinc phosphate composite for water-based acrylic coatings with durable anticorrosion performance on mild steel.

A new metal-organic framework (MOF) compound [Mn2(2,2'-bca)2(H2O)2] n (Mn-MOFs) was successfully synthesized by solvothermal method, and Mn-MOFs@Zn material was prepared by loading zinc phosphate onto Mn-MOFs by ball milling, then Mn-MOFs@Zn was added to the water-based acrylic paint to prepare Mn-MOFs@Zn@acrylic coating. The AC impedance test results showed Mn-MOFs@Zn@acrylic coating has higher corrosion inhibition performance and stability to mild steel when compared with blank coating. The impedance modulus of the blank coating in the low frequency region decreased by 90%, and the R ct showed a trend of first increasing and then decreasing over time, the maximum R ct was only 303.8 Ω, which was only Mn-MOFs@Zn@acrylic coating one-seventh of the R ct value. The artificial scratch experiment showed that the Mn-MOFs@Zn@acrylic coating only slightly corrodes at the scratches, because Mn-MOFs@Zn material made the coating a self-repairing function and improved the durable anticorrosion performance of the acrylic coating.

Real-Time Electrochemical Detection of Uric Acid, Dopamine and Ascorbic Acid by Heme Directly Modified Carbon Electrode.

Several simple electrochemical sensors were designed for three analytes, uric acid (UA), dopamine (DA) and ascorbic acid (AA) detection via electrolytic deposition of three functional materials heme, Fc(Cys)2 and Fc-ECG onto glassy carbon electrodes (GCEs) without other medium. Characterization of the modified GCEs was conducted using SEM, TEM and DPV methods. Results showed that the heme/GCE demonstrated notable electro-catalytic capabilities, towards DA, AA, and UA oxidation in a solution of phosphate buffer. The current signals of DPV technique for three analytes shown up three defined oxidated peaks, the peak potential differences was 192 mV for AA and DA, and 142 mV for DA and UA. Under optimal conditions, it can be obtained linear responses for AA, DA and UA in the following concentration ranges: 10 to 50, 5 to 20, and 2.5 to 20 µmol·L-1, and the limits of detection was calculated as 0.76, 0.50 and 0.63 µmol·L-1, respectively. The results demonstrated that the heme directly modified GCE was a well-suited electrochemical sensor for determining UA, DA and AA in actual samples.

Synthesis, characterization and corrosion inhibition behavior of 2-aminofluorene bis-Schiff bases in circulating cooling water.

In this work, two new bis-Schiff bases, namely 2-bromoisophthalaldehyde-2-aminofluorene (M1) and glutaraldehyde 2-aminofluorene (M2), were synthesized, and their structures were characterized and confirmed by infrared spectroscopy, Fourier transform mass spectrometry and UV-visible spectroscopy. Their corrosion inhibition performance on carbon steel in simulated circulating cooling water was investigated by weight loss measurements and electrochemical measurements. The potentiodynamic polarization curves confirmed that two bis-Schiff bases are anode-type inhibitors; electrochemical impedance spectroscopy tests showed that M1 and M2 possess the best inhibition efficiencies of 96.25% and 99.15% at the optimal concentration of 2.50 mmol L-1, respectively. The weight loss results showed that M1 and M2 exhibit maximum η w values of 92.62% and 96.31%, respectively. Scanning electron microscopy showed that the inhibitors inhibited carbon steel corrosion. The adsorption isotherm measurements indicated that the two inhibitors exhibited physicochemisorption mechanisms and followed Langmuir adsorption isotherms. The relationships between the molecular structure and inhibition behavior of the inhibitors were explored by density functional theory, frontier molecular orbital studies, and Fukui index analysis, which affirmed that M2 possesses higher corrosion inhibition efficiency than M1.

The construction of a simple sensor for the simultaneous detection of nitrite and thiosulfate by heme catalysis.

Several simple sensors were fabricated through a one-step method. By depositing electro-active compounds, such as ß-cyclodextrins (ß-CD), heme, dopamine (DA), or Fc-ECG, onto a screen-printed electrode (SPE), the successful simultaneous detection of nitrite (NO2 -) and thiosulfate (S2O3 2-) ions was observed. Under optimal operating conditions, the notable electrocatalytic abilities of a Heme/SPE sensor were detected for the oxidation of NO2 - and S2O3 2-, with remarkable peak potential differences, after characterization via SEM, CV, and DPV. Linear relationships were obtained in the ranges of 5.0-200.0 µmol L-1 and 1.0-100.0 µmol L-1 for the current response versus concentration of NO2 - and S2O3 2-, respectively. The limits of detection were determined to be 1.67 and 0.33 µmol L-1 while the sensitivities of detection were noted to be 0.43 and 1.43 µA µM-1 cm-2, respectively. During the detection of NO2 - and S2O3 2-, no interfering common ions were observed. Furthermore, average recoveries from 96.0 to 104.3% and a total R.S.D. of less than 3.1% were found for the detection of NO2 - and S2O3 2- in pickled juice and tap water using the simple sensor. These results showed that rapid and precise measurements for actual application in NO2 - and S2O3 2- detection could be conducted in food samples, indicating a potential use in food safety.

Synthesis of MOFs/GO composite for corrosion resistance application on carbon steel.

Two unreported metal-organic frameworks [Cu(6-Me-2,3-pydc)(1,10-phen)·7H2O] n (namely Cu-MOF) and [Mn2(2,2'-bca)2(H2O)2] n (namely Mn-MOF) were synthesized by a solvothermal method and their structures were characterized and confirmed by elemental analysis, X-ray single crystal diffraction, Fourier infrared spectroscopy and thermogravimetric analysis. Cu-MOF/graphene (Cu-MOF/GR), Cu-MOF/graphene oxide (Cu-MOF/GO), Mn-MOF/graphene (Mn-MOF/GR) and Mn-MOF/graphene oxide (Mn-MOF/GO) composite materials were successfully synthesized by a solvothermal method and characterized and analyzed by PXRD, SEM and TEM. In order to study the corrosion inhibition properties of the Cu-MOF/GR, Cu-MOF/GO, Mn-MOF/GR and Mn-MOF/GO composite materials on carbon steel, they were mixed with waterborne acrylic varnish to prepare a series of composite coatings to explore in 3.5 wt% NaCl solution by electrochemical measurements and results showed that the total polarization resistance of the 3% Cu-MOF/GO and 3% Mn-MOF/GO composite coatings on the carbon steel surface were relatively large, and were 55 097 and 55 729 Ω cm2, respectively, which could effectively protect the carbon steel from corrosion. After immersion for 30 days, the 3% Mn-MOF/GO composite still maintained high corrosion resistance, the |Z| values were still as high as 23 804 Ω cm2. Therefore, MOFs compounded with GO can produce a synergistic corrosion inhibition effect and improve the corrosion resistance of the coating; this conclusion is well confirmed by the adhesion capability test.

A novel 3D paper-based microfluidic electrochemical glucose biosensor based on rGO-TEPA/PB sensitive film.

A novel 3D paper-based microfluidic screen-printed electrode (SPE) composed of two layers was constructed by photolithography and screen-printing technology. Aldehyde functionalized hydrophilic zone of the counter and reference electrodes layer was prepared for glucose oxidase immobilization. Highly conductive prussian blue deposited reduced graphene oxide-tetraethylene pentamine (rGO-TEPA/PB) modified paper working electrode layer can be used as an electrochemical sensitive membrane for quantitative detection of hydrogen peroxide (H2O2), which was the enzyme-catalyzed reaction product. Therefore, this 3D paper-based microfluidic electrochemical biosensor can be used for quantitative detection of glucose. Under optimum conditions, the proposed biosensor can be used for quantitative determination of glucose over a wide linear range of 0.1 mM-25 mM with detection limit of 25 µM. Finally, the 3D paper-based microfluidic electrochemical biosensor was applied to determine glucose in human sweat and blood, and the obtained results were in good consistency with values measured by Roche's blood glucose meter. In addition, the proposed 3D paper-based electrochemical device showed good repeatability, stability, and anti-interference, which would be of great potential to monitor glucose in complex biological fluids.

A Facile Electrochemical Sensor Labeled by Ferrocenoyl Cysteine Conjugate for the Detection of Nitrite in Pickle Juice.

Simple and facile electrochemical sensors for nitrite detection were fabricated by directly depositing ferrocenoyl cysteine conjugates Fc[CO-Cys(Trt)-OMe]2 [Fc(Cys)2] or Fc[CO-Glu-Cys-Gly-OH] [Fc-ECG] on screen-printed electrodes (SPEs). The modified carbon electrodes were characterized by scanning electron microscopy (SEM), cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Results indicated that Fc-ECG/SPE sensor showed enhanced current response and a lower overpotential than Fc(Cys)2/SPE sensor for nitrite detection. Optimal operating conditions were estimated for nitrite detection by DPV. The concentration of nitrite showed a good linear relationship with the current response in the range of 1.0⁻50 µmol·L-1 and with 0.3 µmol·L-1 as the concentration for limit of detection. There were no interferences from most common ions. The development of this electrochemical sensor was used for nitrite detection in pickled juice with a R.S.D. lower than 2.1% and average recovery lower than 101.5%, which indicated that disposable electrochemical sensor system can be applied for rapid and precise nitrite detection in foods.

A sensitive and selective electrochemical biosensor for the determination of beta-amyloid oligomer by inhibiting the peptide-triggered in situ assembly of silver nanoparticles.

Soluble beta-amyloid (Aß) oligomer is believed to be the most important toxic species in the brain of Alzheimer's disease (AD) patients. Thus, it is critical to develop a simple method for the selective detection of Aß oligomer with low cost and high sensitivity. In this paper, we report an electrochemical method for the detection of Aß oligomer with a peptide as the bioreceptor and silver nanoparticle (AgNP) aggregates as the redox reporters. This strategy is based on the conversion of AgNP-based colorimetric assay into electrochemical analysis. Specifically, the peptide immobilized on the electrode surface and presented in solution triggered together the in situ formation of AgNP aggregates, which produced a well-defined electrochemical signal. However, the specific binding of Aß oligomer to the immobilized peptide prevented the in situ assembly of AgNPs. As a result, a poor electrochemical signal was observed. The detection limit of the method was found to be 6 pM. Furthermore, the amenability of this method for the analysis of Aß oligomer in serum and artificial cerebrospinal fluid (aCSF) samples was demonstrated.

Selective and Sensitive Detection of Cyanide Based on the Displacement Strategy Using a Water-Soluble Fluorescent Probe.

A water-soluble fluorescent probe (C-GGH) was used for the highly sensitive and selective detection of cyanide (CN(-)) in aqueous media based on the displacement strategy. Due to the presence of the recognition unit GGH (Gly-Gly-His), the probe C-GGH can coordinate with Cu(2+) and consequently display ON-OFF type fluorescence response. Furthermore, the in situ formed nonfluorescent C-GGH-Cu(2+) complex can act as an effective OFF-ON type fluorescent probe for sensing CN(-) anion. Due to the strong binding affinity of CN(-) to Cu(2+), CN(-) can extract Cu(2+) from C-GGH-Cu(2+) complex, leading to the release of C-GGH and the recovery of fluorescent emission of the system. The probe C-GGH-Cu(2+) allowed detection of CN(-) in aqueous solution with a LOD (limit of detection) of 0.017 µmol/L which is much lower than the maximum contaminant level (1.9 µmol/L) for CN(-) in drinking water set by the WHO (World Health Organization). The probe also displayed excellent specificity for CN(-) towards other anions, including F(-), Cl(-), Br(-), I(-), SCN(-), PO4 (3-), N3 (-), NO3 (-), AcO(-), SO4 (2-), and CO3 (2-).

Interaction Study of Ferrocene Derivatives and Heme by UV-Vis Spectroscopy.

The interaction between ferrocene derivatives, such as Fc(COOH)2(λmax=286 nm), Fc(OBt)2(λmax=305 nm), Fc(Cys)(λmax=289 nm) and heme(λmax=386 nm) were studied by UV-Vis spectroscopy, respectively. The results show that, when the concentration of heme is fixed, the absorbance of heme increases with the increase of Fc(COOH)2 and Fc(Cys) concentration, the absorbance of heme almost keep the same when Fc(OBt)2 concentration increases; when the concentration of ferrocene derivatives are fixed, the absorbance of Fc(COOH)2 and Fc(Cys) also increases with the increase of heme concentration, the absorbance of Fc(OBt)2 almost keep the same when heme concentration increase. It is demonstrated that the hydrogen bonding interactions happen between Fc(COOH)2, Fc(Cys) and heme, none of Fc(OBt)2, the formation of hydrogen bonding lead to the growth of molecular chain, the bigger molecule can absorb more energy and increase the absorbance. Meanwhile, the stability of molecule is affected by the formation of hydrogen bonding, when the reaction time increases from 0.5 h to 18 h and 48 h, the absorbance at λmax=384 nm change from 2.64 to 2.53 and 2.51 with fixed concentration of Fc(COOH)2, the absorbance at λmax=384 nm change from 1.76 to 1.72 and 1.68 with fixed concentration of heme, the absorbance at λmax=397 nm change from 2.74 to 2.63 and 2.55 with fixed concentration of Fc(Cys), and the absorbance at λmax=397 nm change from 1.82 to 1.58 and 1.49 with fixed concentration of heme, respectively.

Determination of Residual Solvents in Linezolid by Static Headspace GC.

A headspace gas chromatographic method was developed for the determination of residual solvents in linezolid active substances. The solvents include petroleum ether (60-90°C), acetone, tetrahydrofuran, ethyl acetate, methanol, dichloromethane (DCM) and pyridine. The method showed the possibility to detect the tested solvents with a linear determination correlation coefficient (r) greater than 0.9995 except for petroleum ether (0.9980). The limits of detection ranged between 0.12 µg/mL (petroleum ether) and 3.56 µg/mL (DCM), and the limits of quantity ranged between 0.41 µg/mL (petroleum ether) and 11.86 µg/mL (DCM). The method achieved good accuracy (recoveries ranging from 92.8 to 102.5%) and precision for both run-to-run and day-to-day assay (relative standard deviation ranging from 0.4 to 1.3%) for all seven solvents concerned, which were applied in the quality control of three batches of linezolid successfully.