Voltammetric determination of folic acid at physiological pH values by using a glassy carbon electrode modified with a multilayer composite consisting of polyoxometalate (H8P2Mo16V2O62) and reduced graphene oxide and prepared via layer-by-layer self-assembly and in-situ photoreduction

In situ electrodeposition of bismuth oxide nanowires @MWNT on the carbon fiber microelectrode for the sensitively electrochemical detection of folic acid

A microminiaturized electrochemical device, BiO@CNW/CFE was fabricated based on the in situ co-electrodeposition of bismuth oxide nanowires (BiNWs) and multi-walled carbon nanotubes (MWNTs) on the surface of carbon fiber electrode (CFE). The nanostructure of BiNWs could bind MWNTs on the surface of CFE during the precipitation of bismuth at the potential of -1.1 V. The vimineous nanostructure of BiO@CNW improved the surface area and electrochemical activity of the microelectrode. With the low background noise, folic acid (FA) can be detected sensitively by BiO@CNW/CFE based on the electrochemical reduction via the method of square wave voltammetry. The linear range of FA in sodium acetate-acetic acid buffer was achieved in the range of 5.00 nM-200 nM, the detection limit was estimated to be 0.63 nM. The recoveries of FA in human serum and artificial cerebral spinal fluid were between 99% and 103%, which indicates BiO@CNW/CFE was a reliable sensor for the detection of FA in biological samples.

Bioresource-derived colloidal nitrogen-doped graphene quantum dots as ultrasensitive and stable nanosensors for detection of cancer and neurotransmitter biomarkers

Rapid and accurate detection of cancer and neurological diseases is a major issue that has received great attention recently to enable early therapy treatment. In this report, we utilize an atmospheric pressure microplasma system to convert a natural bioresource chitosan into nitrogen-doped graphene quantum dots (NGQDs) for photoluminescence (PL) based selective detection of cancer and neurotransmitter biomarkers. By adjusting the pH conditions during the detection, multiple biomolecules including uric acid (UA), folic acid (FA), epinephrine (EP), and dopamine (DA) can be simultaneously detected with high selectivity and sensitivity using a single material only. Linear relationships between the biomarker concentration and the PL intensity ratio are obtained starting from 0.8 to 100 μM with low limits of detection (LoDs) of 123.1, 157.9, 80.5, and 91.3 nM for UA, EP, FA, and DA, respectively. Our work provides an insight into the multiple biomarker detection using a single material only, which is beneficial for the early detection and diagnosis of cancer and neurological diseases, as well as the development of new drugs.

Folic Acid Determination in Food Samples Using Green Synthesized Copper Oxide Nanoparticles and Electro-Poly (Methyl Orange) Sensor

Copper (II) oxide nanoparticles (CuONPs) were green synthesized using Ocimum basilicum leaves aqueous extract in which polyphenols act as reducing and stabilizing agents. The synthesized CuONPs were characterized using X-ray diffraction spectroscopy, Fourier transform infrared spectroscopy, transmission electron microscopy, selected area electron diffraction, and Brunauer–Emmett–Teller (BET) surface area analysis. The analyses indicated the formation of crystalline rod-like monoclinic pure CuONPs with a mean grain size of 15 nm, a surface area of 396 m2 g−1, and a total pore volume of 0.71 cm3 g−1. A glassy carbon electrode (GCE) was modified using the synthesized CuONPs and electropolymerized poly(methyl orange) (PMO). The modified PMO/CuONPs/GCE electrode was electrochemically characterized and applied for the estimation of folic acid (FA) by cyclic voltammetry, chronoamperometry, linear sweep voltammetry, and differential pulse voltammetry techniques. The influence of pH (7), scan rate (50 mV/s), supporting electrolyte (0.1 M KCl) and FA concentration has been optimized. FA is precisely determined in the range from 0.01 to 1.5 µΜ with a low detection limit (0.002 µΜ), a low quantitation limit (0.068 µΜ), high reproducibility (RSD 0.37, 10 measurements), and high stability (98% activity after 50 days). FA in food samples was determined by the new sensor with high recoveries from 93 to 108.8%.

Graphical Abstract

Polyoxometalate-based materials in extraction, and electrochemical and optical detection methods: A review

Polyoxometalates (POMs) as metal-oxide anions have exceptional properties like high negative charges, remarkable redox abilities, unique ligand properties and availability of organic grafting. Moreover, the amenability of POMs to modification with different materials makes them suitable as precursors to further obtain new composites. Due to their unique attributes, POMs and their composites have been utilized as adsorbents, electrodes and catalysts in extraction, and electrochemical and optical detection methods, respectively. A survey of the recent progress and developments of POM-based materials in these methods is therefore desirable, and should be of great interest. In this review article, POM-based materials, their properties as well as their identification methods, and analytical applications as adsorbents, electrodes and catalysts, and corresponding mechanisms of action, where relevant, are reviewed. Some current issues of the utilization of these materials and their future prospects in analytical chemistry are discussed.

Polyoxometalate Functionalized Sensors: A Review

Polyoxometalates (POMs) are a class of metal oxide complexes with a large structural diversity. Effective control of the final chemical and physical properties of POMs could be provided by fine-tuning chemical modifications, such as the inclusion of other metals or non-metal ions. In addition, the nature and type of the counterion can also impact POM properties, like solubility. Besides, POMs may combine with carbon materials as graphene oxide, reduced graphene oxide or carbon nanotubes to enhance electronic conductivity, with noble metal nanoparticles to increase catalytic and functional sites, be introduced into metal-organic frameworks to increase surface area and expose more active sites, and embedded into conducting polymers. The possibility to design POMs to match properties adequate for specific sensing applications turns them into highly desirable chemicals for sensor sensitive layers. This review intends to provide an overview of POM structures used in sensors (electrochemical, optical, and piezoelectric), highlighting their main functional features. Furthermore, this review aims to summarize the reported applications of POMs in sensors for detecting and determining analytes in different matrices, many of them with biochemical and clinical relevance, along with analytical figures of merit and main virtues and problems of such devices. Special emphasis is given to the stability of POMs sensitive layers, detection limits, selectivity, the pH working range and throughput.

Ferrocene-reduced graphene oxide-polyoxometalates based ternary nanocomposites as electrochemical detection for acetaminophen

Developing a convenient and accurate method for the determination of acetaminophen (APAP) content is very vital, and ferrocene (Fc) based nanocomposites coupled with polyoxometalates (POMs) as electrochemical sensor is a promising approach to address the issues. Herein, a new ternary nanocomposite of Fc based carbon nanomaterials (Fc-rGO) with PMo₁₂ (Fc-rGO/PMo₁₂, rGFP-n) was successfully fabricated, and the electrochemical activities and APAP detection of rGFP-n as electro-active materials were systematically investigated, and results of the differential pulse voltammetry (DPV) and electro-active surface area (0.0332 cm²) show that rGFP-1 is an excellent electrochemical sensor for APAP, and the proportion of Fc in rGFP-n can affect the charge transfer between APAP and rGFP. Under the optimal experimental conditions, rGFP-1 can be used to detect APAP with the limit of detection (LOD) of 13.27 nM (S/N = 3), the sensitivity of 36.81μA⋅μM^-1cm^-2, and the detection range from 1×10^-6 to 1×10^-3M, meeting the lowest plasma concentration of APAP (1.3 mM).

Microplasma-Tunable Graphene Quantum Dots for Ultrasensitive and Selective Detection of Cancer and Neurotransmitter Biomarkers

The effective and precise detection of cancer and neurotransmitter biomarkers including folic acid (FA), dopamine (DA), and epinephrine (EP) are essential for early detection and diagnosis of cancer and neurological disorders and for the development of new drugs. However, it remains challenging to detect FA, DA, and EP with high selectivity and sensitivity with a single material. Herein, we report a photoluminescence (PL)-based selective sensing of FA, DA, and EP with nitrogen-doped graphene quantum dots (NGQDs) synthesized from biocompatible chitosan under ambient conditions using atmospheric pressure microplasmas. By regulating the pH, the selective detection is achieved in broad ranges from 0.8 to 80 μM for FA and 0.4 to 100 μM for both DA and EP with the very low limits of detections of 81.7, 57.8, and 16.7 nM for FA, DA, and EP, respectively. The developed PL sensing method shows the high throughput of 5000 detections per hour. Moreover, highly stable colloidal NGQD dispersion with 100 μg/mL concentration for at least 100 PL detections is produced in 1 h by a single microplasma, and the process is scalable. The mechanisms of the outstanding performance are related to the enhanced, size-dependent π-π stacking attraction between the NGQDs and the pH-regulated chemical states of the analytes and the associated pH-specific photo-induced electron transfer and PL.

Synthesis of POMOFs with 8-fold helix and its composite with carboxyl functionalized SWCNTs for the voltammetric determination of dopamine

Although many satisfactory studies have been developed for biomolecule detection, the complexity of biofluids still poses a major challenge to improve the performance of nanomaterials as electrochemical sensors. Herein, unprecedented polyoxometalate-based metal-organic frameworks (POMOFs) with 8-fold meso-helical feature, [Ag₅(trz)₄]₂[PMo₁₂O₄₀] (PAZ), were synthesized and explored as electrochemical sensors to detect dopamine (DA). To improve the conductivity of PAZ and the binding ability with single-walled carbon nanotubes (SWCNTs), the nanocomposite of carboxyl functionalized SWCNTs (SWCNTs-COOH) with nano-PAZ (NPAZ), NPAZ@SWCNTs-COOH, was fabricated, and transmission electron microscopy (TEM) shows that NPAZ can interact stably and uniformly with SWCNTs-COOH, owing to more defect sites on the surface of SWCNTs-COOH. The electrochemical result of NPAZ@SWCNTs-COOH/GCE towards detecting DA shows that the linear range was from 0.05 to 100 μM with a detection limit (LOD) of 8.6 nM (S/N = 3). A new electrochemical biosensing platform by combining 8-fold helical POMOFs with SWCNTs-COOH was developed for enhancing detection of dopamine for the first time, exhibiting the lowest detection limit to date.

Carbon quantum dots as fluorescence sensors for label-free detection of folic acid in biological samples

Carbon quantum dots (CQDs) have been fabricated by a facile single-step pyrolysis method from citric acid and ethylene imine polymer. When excited at 359 nm, CQDs show intense blue fluorescence at 434 nm. The fluorescence can be effectively quenched by folic acid (FA), which is attributed to the combination of static quenching and inner filter effect. Thus, the CQDs are developed as an efficient fluorescent sensing platform for label-free sensitive and selective detection of FA. Key parameters influencing the detection were investigated, such as incubation time, salt concentration, selectivity and potential interferences. Under the optimal conditions, a good linearity was observed for the emission intensity against 1.14-47.57 μM with a correlation coefficient of 0.99. The limit of detection was found to be 0.38 μM. The practical application of the sensing system was demonstrated by analyzing FA in human urine samples. The sample recoveries fell in the range of 82.0%-113.1% with RSDs ≤ 10.9%, which presented its reliable and feasible application in real samples.

A multilayer assembly of two mixed-valence Mn16-containing polyanions and study of their electrocatalytic activities towards water oxidation

Herein, two mixed-valence Mn16-containing polyanions, (Mn16) [MnIII10MnII6O6(OH)6(PO4)4(A-a-SiW9O34)4]28- (Mn16-Cs) and [MnIII4MnII12(OH)12(PO4)4(A-a-SiW9O34)4]28- (Mn16-Rb), were successfully fabricated on an indium tin oxide (ITO)-coated glass electrode and a glass carbon electrode (GCE) by a layer-by-layer assembly method. Moreover, four composite films, i.e. [PDDA/Mn16-Cs]n, [PDDA/Mn16-Rb]n, [Mn16-Cs/Rubpy]n, and [Mn16-Rb/Rubpy]n (PDDA: poly(diallyldimethylammonium chloride); Rubpy: tris(2,2'-bipyridyl)ruthenium(ii) chloride; n = 1-10), were constructed for comparison and characterized by UV-visible spectroscopy, cyclic voltammetry (CV), and X-ray photoelectron spectroscopy (XPS). Their electrocatalytic activities towards water oxidation were studied under the same experimental conditions. The results of the controlled experiments indicate that (1) all the four films exhibit expected electrocatalytic activities towards water oxidation; (2) the electrocatalytic activity of Mn16-Cs is better than that of Mn16-Rb in solution and composite films; and (3) the electrocatalytic activities of the composite film [Mn16/Rubpy]n are better than those of the composite film [PDDA/Mn16]n.