Electrochemical methods for ascorbic acid determination

Single laser synthesis of gold nanoparticles-polypyrrole-chitosan on laser-induced graphene for ascorbic acid detection

The simultaneous synthesis of gold nanoparticles (AuNPs) and graphene by laser ablation was demonstrated. The in-situ synthesis was performed by laser ablation of a polymer substrate covered with a gold precursor dispersion. The gold precursor was prepared in a copolymer solution of pyrrole (Py) and chitosan (Chi) to improve the nucleation of gold embedded on the laser-induced graphene electrode (LIGE). The morphology of AuNPs-pPy-Chi/LIGE was studied by scanning electron microscopy and characterized electrochemically by cyclic voltammetry. A comprehensive investigation of the electrochemical and physical features of the AuNPs-pPy-Chi/LIGE was carried out. The parameters of differential pulse voltammetry were adjusted to enhance the response to ascorbic acid (AA). The AuNPs-pPy-Chi/LIGE produced two linear ranges: from 0.25 to 5.00 and 5.00-25.00 mmol L-1. The limit of detection was 0.22 mmol L-1. Hundreds of electrodes were tested to demonstrate the excellent reproducibility of the AuNPs-pPy-Chi/LIGE fabrication. Overall, the proposed electrode allows the successful detection of AA in orange juice products with acceptable accuracy (recoveries = 97 ± 2 to 109.1 ± 0.7). The preparation strategy of the proposed AuNPs-pPy-Chi/LIGE could be adapted to detect other compounds or biomarkers.

Investigating metal (M = Mn, Fe, and Ni)-doped Co(OH)2 nanofibers for electrocatalytic oxygen evolution and electrochemical biosensing performance

To achieve efficient and cost-effective electrochemical water splitting, highly active and affordable nanostructured catalysts are the key requirement. The current study presents the investigations of the efficacy of metal (Mn, Fe and Ni)-doped Co(OH)2 nanofibers towards oxygen evolution via water splitting. Notably, Ni-doped Co(OH)2 demonstrates superior OER performance in KOH electrolyte, surpassing standard IrO2 with a modest potential of 1.62 V at 10 mA cm-2. The remarkable activity is attributed to the nanofiber structure, facilitating faster conduction and offering readily available active sites. Ni-doped Co(OH)2 nanofibers displayed enduring stability even after 1000 cycles. This work underscores the importance of transition-metal based catalysts as effective electrocatalysts, providing the groundwork for the development of cutting-edge catalysts. Additionally, the electrochemical sensing capability towards ascorbic acid is evaluated, with Ni-doped Co(OH)2 showing the most promising response, characterized by the lowest LOD and LOQ values. These findings highlight the potential of Ni-doped Co(OH)2 nanofibers for upcoming diagnostic detection devices.

Electrifying Fruit Juice: Integrating Applied Electroanalytical Chemistry into the Undergraduate Curriculum

Electroanalytical chemistry has been advanced through portable devices, providing methods and sensors for the detection of analytes with high sensitivity and accuracy. This subfield of electrochemistry has the potential to be utilized in industry and analytical quality control, in general. This results in an increasing demand for trained personnel, capable of operating benchtop and portable electroanalytical equipment. Although electrochemical techniques are routinely taught in theoretical undergraduate courses, they need to be more often incorporated into experimental didactic activities. Herein, we describe the application of an effective, hands-on, and low-maintenance experiment that can enhance the learning experience of electroanalytical methods and their use in industrial quality control settings. This activity is based on the detection of ascorbic acid (vitamin C) by employing cyclic voltammetry at unmodified glassy carbon electrodes (GCE) in real juice samples. This didactic experiment teaches students about the theoretical concepts of cyclic voltammetry, three-electrode cell setup, chemical reversibility, data treatment, and quantitative analysis. This teaching approach was conducted in a second-year analytical chemistry course and was easily adapted to social distancing measures imposed by the COVID-19 pandemic.

First-Order Derivative Spectrophotometry for Simultaneous Determination of Vitamin C and Nicotinamide: Application in Quantitative Analysis of Cocrystals

Vitamin C (l-ascorbic acid, ASC) and the amide form of vitamin B3 nicotinamide (NIC) can form cocrystals through hydrogen bonding. Currently, there is a lack of fast and reliable alternatives for precisely quantifying cocrystal components and their purity. Spectrophotometric analysis for quantifying such vitamin preparations is challenging due to overlapping absorbance bands in a narrow wavelength range in the ultraviolet (UV) region. Moreover, ASC undergoes progressive degradation if not diluted in a proper medium, requiring stability during quantitative analysis. This study adopted a fast, simple, and reliable two-component spectrophotometric assay for simultaneously determining ASC and NIC based on the first-order derivative spectrophotometry (FODS) method using sodium oxalate as a stabilizer for vitamin C. The FODS method showed linearity between 2 and 24 μg·mL-1 and good precision. The standard addition method was employed to validate FODS, with high recovery percentages (96.5 to 102.4% for ASC and 95.3 to 101.9% for NIC). The FODS method was successfully applied to quantify ASC and NIC in bulk powder produced by the gas antisolvent method. The proposed method could estimate cocrystal purity through mass balance regarding the expected 1:1 stoichiometry, confirmed by PXRD and DSC. Cocrystal purity determined by the FODS method (58-100%) aligned well with results from LC-MS (62-100%), with an accuracy exceeding 97%. The FODS method is as sensitive and accurate as high-performance liquid chromatography for simultaneously determining vitamin concentrations deriving from cocrystals. However, it is less costly, more efficient, and a suitable alternative to classical solid-state methods for estimating cocrystal purity.

Binder-free and efficient voltammetric sensor based on Zn-Ca2CuO3 nanoparticles for simultaneous determination of amlodipine, acetaminophen, and ascorbic acid in hypertension patients

Amlodipine (AM) is a long active calcium channel blocker used to relax blood vessels by preventing calcium ion transport into the vascular walls and its supporting molecules acetaminophen (AP) and ascorbic acid (AA) are recommended for hypertension control and prevention. Considering their therapeutic importance and potential side effects due to over dosage, we have fabricated a sensor for individual and simultaneous determination of AA, AP, and AM in pharmaceuticals and human urine using novel Zn-doped Ca2CuO3 nanoparticles modified glassy carbon electrode (GCE). Optimally doped Ca2CuO3 (2.5 wt% Zn at Cu site) enhanced the detection of target molecules over much wider concentration ranges of 50 to 3130 µM for AA, 0.25 to 417 µM for AP, and 0.8 to 354 µM for AM with the corresponding lowest detection limits of 14 µM, 0.05 µM, and 0.07 µM, respectively. Furthermore, the Zn-Ca2CuO3/GCE exhibited excellent selectivity and high sensitivity even in the presence of several potential interfering agents. The usefulness of the developed electrode was tested using an amlodipine besylate tablet and urine samples of seven hypertension patients under medication. The results confirmed the presence of a significant amount of AP and AM in six patients' urine samples indicating that the personalized medication is essential and the quantum of medication need to be fixed by knowing the excess medicines excreted through urine. Thus, the Zn-Ca2CuO3/GCE with a high recovery percentage and good sensitivity shall be useful in the pharmaceutical and biomedical sectors.

High performance Pt-anchored MoS2based chemiresistive ascorbic acid sensor

Ascorbic acid (AA), known as vitamin C, is a vital bioactive compound that plays a crucial role in several metabolic processes, including the synthesis of collagen and neurotransmitters, the removal of harmful free radicals, and the uptake of iron by cells in the human intestines. As a result, there is an absolute need for a highly selective, sensitive, and economically viable sensing platform for AA detection. Herein, we demonstrate a Pt-decorated MoS2for efficient detection of an AA biosensor. MoS2hollow rectangular structures were synthesized using an easy and inexpensive chemical vapor deposition approach to meet the increasing need for a reliable detection platform. The synthesized MoS2hollow rectangular structures are characterized through field effect scanning electron microscopy (FESEM), energy-dispersive spectroscopy elemental mapping, Raman spectroscopy, and x-ray photoelectron spectroscopy. We fabricate a chemiresistive biosensor based on Pt-decorated MoS2that measures AA with great precision and high sensitivity. The experiments were designed to evaluate the response of the Pt-decorated MoS2biosensor in the presence and absence of AA, and selectivity was evaluated for a variety of biomolecules, and it was observed to be very selective towards AA. The Pt-MoS2device had a higher response of 125% against 1 mM concentration of AA biomolecules, when compared to that of all other devices and 2.2 times higher than that of the pristine MoS2device. The outcomes of this study demonstrate the efficacy of Pt-decorated MoS2as a promising material for AA detection. This research contributes to the ongoing efforts to enhance our capabilities in monitoring and detecting AA, fostering advancements in environmental, biomedical, and industrial applications.

Detection of ascorbic acid by persistent luminescent nanoparticles based on CoOOH nanosheets modification

Persistent luminescent nanomaterials (PLNPs) Zn0.8Ga2O4: Cr3+, Zr3+ with high brightness and good dispersion were prepared by hydrothermal method. The PLNPs were used as luminescent units, and CoOOH nanosheets were used as quenching agents. Based on the fluorescence internal filtering effect, the luminescence of PLNPs were effectively quenched by CoOOH modification on the surface of PLNPs. However, the introduction of ascorbic acid (AA) restored the luminescence of PLNPs and successfully achieved highly sensitive and selective detection of AA. This was due to a selective redox reaction between CoOOH and AA, in which CoOOH was reduced to Co2+. The degree of luminescence recovery of PLNPs showed a good linear relationship with AA concentration in the range 5-250 µM, with a detection limit of 0.72 µM. The recovery of actual spiked samples were 97.9-102.2%. This method is expected to provide reference for the study of other redox substances in biological systems.

Electroanalytical analysis of phenol oxidation using bacteria immobilized by a polycaprolactone coating on the copper electrode surface

The copper electrode modified by bacteria immobilised by a polycaprolactone film was successfully developed by electropolymerisation for the purpose of determining the presence of phenol. Electrochemical techniques such as square-wave voltammetry (SWV), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to characterize the electrochemical properties of the Cu-polymer/bacteria electrode. The results show that the intensity of the phenol oxidation peak increases with concentration, allowing us to obtain good analytical results with DL of 2.156 × 10-7 M and QL which is 7.2 × 10-7 M , confirming that the biosensor has excellent electroanalytical activity for phenol oxidation, with good stability and a wide linear range. Our electrode is based on a easily available and inexpensive material, as well as on its simple preparation, which has demonstrated high performance for phenol.

Nanolevel of detection of ascorbic acid using horse-radish peroxidase inhibition assay

Ascorbic acid plays a significant role in regulation of various bodily functions with high concentrations in immune cells and being involved in connective tissue maintenance. Commonly it is detected through various colorimetric methods. In this study, we propose a one-step simple method based on the inhibitory activity of ascorbic acid on horseradish peroxidase and hydrogen peroxide. The detection is observed by colorimetric changes to TMB (3,3',5,5' tetramethylbenzidine). The enzyme inhibition unit was optimized with a high level of linearity (r2 = 0.9999) and the level of detection and level of quantification were found to be 1.35 nM and 4.08 nM, respectively with higher sensitive compared to the HPLC method (11 μM). Both intra and inter-assays showed high correlations at different AA concentrations. (r2 > 0.9999). Similar results were also observed for vitamin C tablets, ascorbate salts, fruits, and market products (r2 = 0.999). There was negligible effect of interference by citric acid, lactic acid, tartaric acids, and glucose with high recoveries (>98%) at 1 mg/mL to 0.0078 mg/mL concentration ranges. The recovery error (RE%) was found to be less than 10%. Our detection method is distinguished by its simplicity, nano-level of detection, reproducibility, and potential application and adaptability as a point-of-use test.

Microfluidic chemistry assisted synthesis of cobalt quantum dot embedded nitrogen doped carbon with oxidase-like properties toward ascorbic acid detection

Ascorbic acid (AA) is a powerful antioxidant in food safety and disease treatment. It is of great significance to develop a low-cost, high-stability, and easy-to-operate colorimetric method for quantitative detection of AA in food or human body. Although various nanozymes have been developed for the colorimetric detection of AA, the size regulation of the catalytic center of nanozymes remains a challenge. In this work, we propose a combined strategy of flow chemistry synthesis and pyrolysis to realize the controllable adjustment of the catalytic center size of nanozymes. Zinc-cobalt zeolitic imidazole frameworks (ZnCo-ZIFs) with different sizes are synthesized by flow chemistry. Nitrogen-doped carbon materials with different Co catalytic centers (80 nm-10 nm) are then obtained by pyrolysis of ZnCo-ZIFs precursors. Among them, cobalt quantum dot embedded nitrogen-doped carbon (Co QDs/N-C) exhibits excellent oxidase activity, with Vmax and Km of 4.19 × 10-7 M s-1 and 0.12 mM. Therefore, a simple, low-cost, and stable colorimetric method for the detection of AA is established with a good linear relationship (3-500 μM) and low detection limit (0.40 μM). This work has certain guiding significance for the size regulation of catalytic center of nanozyme, and the detection method has broad application prospects in biochemical sensing field.

Biomedical Applications of CNT-Based Fibers

Carbon nanotubes (CNTs) have been regarded as emerging materials in various applications. However, the range of biomedical applications is limited due to the aggregation and potential toxicity of powder-type CNTs. To overcome these issues, techniques to assemble them into various macroscopic structures, such as one-dimensional fibers, two-dimensional films, and three-dimensional aerogels, have been developed. Among them, carbon nanotube fiber (CNTF) is a one-dimensional aggregate of CNTs, which can be used to solve the potential toxicity problem of individual CNTs. Furthermore, since it has unique properties due to the one-dimensional nature of CNTs, CNTF has beneficial potential for biomedical applications. This review summarizes the biomedical applications using CNTF, such as the detection of biomolecules or signals for biosensors, strain sensors for wearable healthcare devices, and tissue engineering for regenerating human tissues. In addition, by considering the challenges and perspectives of CNTF for biomedical applications, the feasibility of CNTF in biomedical applications is discussed.

Nonenzymatic Potentiometric Detection of Ascorbic Acid with Porphyrin/ZnO-Functionalized Laser-Induced Graphene as an Electrode of EGFET Sensors

Laser-induced graphene (LIG) has emerged as a highly versatile material with significant potential in the development of electrochemical sensors. In this paper, we investigate the use of LIG and LIG functionalized with ZnO and porphyrins-ZnO as the gate electrodes of the extended gate field effect transistors (EGFETs). The resultant sensors exhibit remarkable sensitivity and selectivity, particularly toward ascorbic acid. The intrinsic sensitivity of LIG undergoes a notable enhancement through the incorporation of hybrid organic-inorganic materials. Among the variations tested, the LIG electrode coated with zinc tetraphenylporphyrin-capped ZnO nanoparticles demonstrates superior performance, reaching a limit of detection of approximately 3 nM. Furthermore, the signal ratio for 5 μM ascorbic acid relative to the same concentration of dopamine exceeds 250. The practical applicability of these sensors is demonstrated through the detection of ascorbic acid in real-world samples, specifically in a commercially available food supplement containing l-arginine. Notably, formulations with added vitamin C exhibit signals at least 25 times larger than those without, underscoring the sensors' capability to discern and quantify the presence of ascorbic acid in complex matrices. This research not only highlights the enhanced performance of LIG-based sensors through functionalization but also underscores their potential for practical applications in the analysis of vitamin-rich supplements.

Peroxidase activity of a Cu-Fe bimetallic hydrogel and applications for colorimetric detection of ascorbic acid

A Cu-Fe bimetallic hydrogel (2-QF-CuFe-G) was constructed through a simple method. The 2-QF-CuFe-G metallohydrogel possesses excellent peroxidase-like activity to catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H2O2. The catalytic mechanism was confirmed by the addition of •OH radical scavenger isopropyl alcohol (IPA), tert-butyl alcohol (TBA) and ˙OH trapping agent terephthalic acid (TA). Remarkably, the resultant blue ox-TMB system can be used to selectively and sensitively detect ascorbic acid (AA) with an LOD of 0.93 μM in the range of 4-36 μM through the colorimetric method. Moreover, the assay based on the 2-QF-CuFe-G metallohydrogel can be successfully applied to detect AA in fresh fruits.

Crumpled MXene nanosheets for sensing of ascorbic acid in food, biological fluids, and erythrocytes in-vitro microenvironment

In this work, a simple and facile method was developed to achieve controlled oxidation and enhance the surface area of MXene nanosheets and their utilization in the efficient sensing of ascorbic acid (AA or vitamin C). After etching of MAX phase to MXene via the MILD technique, controlled flash oxidation was carried out in the open air environment for 1.5 h, followed by flocculation of oxidized MXene nanosheets by using H2SO4, consequently achieving crumpled MXene possessing anatase phase, porosity, and improved surface area as revealed and confirmed by SEM, TEM, Raman, and BET analysis results. The as-prepared crumpled MXene was coated over a glassy carbon electrode (GCE) and used to determine AA successfully via cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and differential pulse voltammetry (DPV) with a linear concentration range of 300 μM to 0.005 μM with a detection limit (LOD) of 2 nM (2.8 % RSD and S/N = 3). The developed electrochemical sensor was used to determine the AA in various actual samples such as juice, urine, serum, and erythrocytes spiked with AA with excellent recoveries in the 94-103 % range. The sensor also demonstrated excellent reproducibility (~1 % RSD for five repetitive assays) and a shelf life of nearly one month with a negligible decrease in response. Furthermore, it lost only 10 % of its response for the next ten days. It also showed satisfactory selectivity toward AA in the presence of other similar compounds, including uric acid (UA), dopamine (DA), and glucose.

Electrochemically Activatable Liquid Organic Hydrogen Carriers and Their Applications

Hydrogen has been chosen as an environmentally benign energy source to replace fossil-fuel-based energy systems. Since hydrogen is difficult to store and transport in its gaseous phase, thermochemical liquid organic hydrogen carriers (LOHCs) have been developed as one of the alternative technologies. However, the high temperature and pressure requirements of thermochemical LOHC systems result in huge energy waste and impracticality. This Perspective proposes electrochemical (EC)-LOHCs capable of more efficient, safer, and lower temperature and pressure hydrogen storage/utilization. To enable this technology, several EC-LOHC candidates such as isopropanol, phenolic compounds, and organic acids are described, and the latest research trends and design concepts of related homo/hetero-based electrocatalysts are discussed. In addition, we propose efficient fuel-cell-based systems that implement electrochemical (de)hydrogenation of EC-LOHCs and present prospects for relevant technologies.

Impedimetric immunosensor for microalbuminuria based on a WS2/Au water-phase assembled nanocomposite

An electrochemical impedimetric biosensor for human serum albumin (HSA) determination is proposed. The biosensor is based on water-phase assembled nanocomposites made of 2D WS₂ nanoflakes and Au nanoparticles (AuNPs). The WS₂ has been produced using a liquid-phase exfoliation strategy assisted by sodium cholate, obtaining a water-stable suspension that allowed the straightforward decoration with AuNPs directly in the aqueous phase. The resulting WS₂/Au nanocomposite has been characterized by atomic force microscopy and Raman spectroscopy and, then, employed to modify screen-printed electrodes. Good electron-transfer features have been achieved. An electrochemical immunosensing platform has been assembled exploiting cysteamine-glutaraldehyde covalent chemistry for antibody (Ab) immobilization. The resulting immunosensor exhibited good sensitivity for HSA detection (LOD = 2 ng mL^-1), with extended linear range (0.005 - 100 µg mL^-1), providing a useful analytical tool for HSA determination in urine at relevant clinical ranges for microalbuminuria screening. The HSA quantification in human urine samples resulted in recoveries from 91.8 to 112.4% and was also reproducible (RSD < 7.5%, n = 3), with marked selectivity. This nanocomposite, thanks to the reliable performance and the ease of the assembling strategy, is a promising alternative for electrochemical immunosensing of health relevant markers.

Different Aspects of the Voltammetric Detection of Vitamins: A Review

Vitamins comprise a group of organic chemical compounds that contribute significantly to the normal functioning of living organisms. Although they are biosynthesized in living organisms, some are also obtained from the diet to meet the needs of organisms, which is why they are characterized as essential chemical compounds. The lack, or low concentrations, of vitamins in the human body causes the development of metabolic dysfunctions, and for this reason their daily intake with food or as supplements, as well as the control of their levels, are necessary. The determination of vitamins is mainly accomplished by using analytical methods, such as chromatographic, spectroscopic, and spectrometric methods, while studies are carried out to develop new and faster methodologies and techniques for their analysis such as electroanalytical methods, the most common of which are voltammetry methods. In this work, a study is reported that was carried out on the determination of vitamins using both electroanalytical techniques, the common significant of which is the voltammetry technique that has been developed in recent years. Specifically, the present review presents a detailed bibliographic survey including, but not limited to, both electrode surfaces that have been modified with nanomaterials and serve as (bio)sensors as well as electrochemical detectors applied in the determination of vitamins.

Multiwalled Carbon Nanotubes-Modified Metallic Electrode Prepared Using Chemical Vapor Deposition as Sequential Injection Analysis Detector for Determination of Ascorbic Acid

A carbon nanotubes modified silver electrode (CNTs-Ag) was prepared via catalytic chemical vapor deposition and characterized. The morphology, crystallinity, elemental composition, and other quality parameters of the prepared electrode were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman techniques. The characterization results revealed the modification of the silver metal surface with CNTs of good characteristics. A sequential injection analysis (SIA) system was developed for studying the reaction of ascorbic acid with KIO₃ using the prepared CNTs-Ag electrode. Electrodes were polarized with both direct current (DC) and periodic square wave (SW). Various experimental conditions affecting the differential electrolytic potentiometric (DEP) peak such as current density, SW bias value, and flow rate were appraised. Under the optimum conditions, good linear responses for ascorbic acid were obtained in the range of 60.0-850.0 µM for both types of polarization with detection limits of 14.0-19.0 µM. The results obtained showed that the periodic polarization method was more sensitive than DC polarization and the electrode response was faster. Ascorbic acid in pharmaceutical tablets was determined with satisfactory results using this method. The prepared CNTs-based electrode exhibited good performance for a long period of use. The method is simple, rapid, and inexpensive for routine analysis.

Ascorbic Acid-Caused Quenching Effect of Protein Clusteroluminescence Probe: The Fast Fluorescent Detection of Ascorbic Acid in Vegetables

It is interesting and meaningful to explore fluorescent probes for novel rapid detection methods. In this study, we discovered a natural fluorescence probe, bovine serum albumin (BSA), for the assay of ascorbic acid (AA). Due to clusterization-triggered emission (CTE), BSA has the character of clusteroluminescence. AA shows an obvious fluorescence quenching effect on BSA, and the quenching effect increases with increasing concentrations of AA. After optimization, a method for the rapid detection of AA is established by the AA-caused fluorescence quenching effect. The fluorescence quenching effect reaches saturation after 5 min of incubation time and the fluorescence is stable within more than one hour, suggesting a rapid and stable fluorescence response. Moreover, the proposed assay method shows good selectivity and a wide linear range. To further study the mechanisms of AA-caused fluorescence quenching effect, some thermodynamic parameters are calculated. The main intermolecular force between BSA and AA is electrostatic, presumably leading to the inhibiting CTE process of BSA. This method also shows acceptable reliability for the real vegetable sample assay. In summary, this work will not only provide an assay strategy for AA, but also open an avenue for the application expansion of CTE effect of natural biomacromolecules.

Toward Achieving Rapid Estimation of Vitamin C in Citrus Peels by NIR Spectra Coupled with a Linear Algorithm

Citrus peels are rich in bioactive compounds such as vitamin C and extraction of vitamin C is a good strategy for citrus peel recycling. It is essential to evaluate the levels of vitamin C in citrus peels before reuse. In this study, a near-infrared (NIR)-based method was proposed to quantify the vitamin C content of citrus peels in a rapid way. The spectra of 249 citrus peels in the 912-1667 nm range were acquired, preprocessed, and then related to measured vitamin C values using the linear partial least squares (PLS) algorithm, indicating that normalization correction (NC) was more suitable for spectral preprocessing and NC-PLS model built with full NC spectra (375 wavelengths) showed a better performance in predicting vitamin C. To accelerate the predictive process, wavelength selection was conducted, and 15 optimal wavelengths were finally selected from NC spectra using the stepwise regression (SR) method, to predict vitamin C using the multiple linear regression (MLR) algorithm. The results showed that SR-NC-MLR model had the best predictive ability with correlation coefficients (r_P) of 0.949 and root mean square error (RMSE_P) of 14.814 mg/100 mg in prediction set, comparable to the NC-PLS model in predicting vitamin C. External validation was implemented using 40 independent citrus peels samples to validate the suitability of the SR-NC-MLR model, obtaining a good correlation (R² = 0.9558) between predicted and measured vitamin C contents. In conclusion, it was reasonable and feasible to achieve the rapid estimation of vitamin C in citrus peels using NIR spectra coupled with MLR algorithm.

Environment-Friendly Ascorbic Acid Fuel Cell

Recently, ascorbic acid (AA) has been studied as an environment-friendly fuel for energy conversion devices. This review article has deliberated an overview of ascorbic acid electrooxidation and diverse ion exchange types of AA-based fuel cells for the first time. Metal and carbon-based catalysts generated remarkable energy from environment-friendly AA fuel. The possibility of using AA in a direct liquid fuel cell (DLFC) without emitting any hazardous pollutants is discussed. AA fuel cells have been reviewed based on carbon nanomaterials, alloys/bimetallic nanoparticles, and precious and nonprecious metal nanoparticles. Finally, the obstacles and opportunities for using AA-based fuel cells in practical applications have also been incorporated.

A New Method for the Determination of Total Content of Vitamin C, Ascorbic and Dehydroascorbic Acid, in Food Products with the Voltammetric Technique with the Use of Tris(2-carboxyethyl)phosphine as a Reducing Reagent

The objective of the study was to develop a new method for the determination of the total content of vitamin C and dehydroascorbic acid in food, based on the technique of differential pulse voltammetry with the use of a boron-doped diamond electrode modified with mercury film. A comparison was made between the results obtained with the developed method and a proposed reference method based on high-performance liquid chromatography with spectrophotometric detection. The reduction of dehydroascorbic acid was performed with the use of tris(2-carboxyethyl)phosphine. The interference caused by the presence of tris(2-carboxyethyl)phosphine during the voltammetric determination of ascorbic acid was effectively eliminated through a reaction with N-ethylmaleimide. The conducted validation of the voltammetric method indicated that correct results of analysis of the total content of vitamin C and ascorbic acid were obtained. Analysis of the content of dehydroascorbic acid was imprecise due to the application of the differential method. The results of the analyses and the determined validation parameters of the developed method are characterised by a high degree of conformance with the results obtained with the chromatographic reference method, which indicates the equivalence of the two methods.

A Review on Electrochemical Microsensors for Ascorbic Acid Detection: Clinical, Pharmaceutical, and Food Safety Applications

Nowadays, micro-sized sensors have become a hot topic in electroanalysis. Because of their excellent analytical features, microelectrodes are well-accepted tools for clinical, pharmaceutical, food safety, and environmental applications. In this brief review, we highlight the state-of-art electrochemical non-enzymatic microsensors for quantitative detection of ascorbic acid (also known as vitamin C). Ascorbic acid is a naturally occurring water-soluble organic compound with antioxidant properties and its quantitative determination in biological fluids, foods, cosmetics, etc., using electrochemical microsensors is of wide interest. Various electrochemical techniques have been applied to detect ascorbic acid with extremely high sensitivity, selectivity, reproducibility, and reliability, and apply to in vivo measurements. This review paper aims to give readers a clear view of advances in areas of electrode modification, successful strategies for signal amplification, and miniaturization techniques used in the electroanalytical devices for ascorbic acid. In conclusion, current challenges related to the microelectrodes design, and future perspectives are outlined.

Cation-Exchange in Metal-Organic Framework as a Strategy to Obtain New Material for Ascorbic Acid Detection

Ascorbic acid (AA) is an important biomolecule, the deficiency or maladjustment of which is associated with the symptoms of many diseases (e.g., cardiovascular disease or cancer). Therefore, there is a need to develop a fluorescent probe capable of detecting AA in aqueous media. Here, we report the synthesis, structural, and spectroscopic characterization (by means of, e.g., XRD, XPS, IR and Raman spectroscopy, TG, SEM, and EDS analyses), as well as the photoluminescent properties of a metal-organic framework (MOF) based on Cu²⁺ and Eu³⁺ ions. The ion-exchange process of the extraframework cation in anionic Cu-based MOF is proposed as an appropriate strategy to obtain a new material with a nondisturbed structure and a sensitivity to interaction with AA. Accordingly, a novel Eu[Cu₃(μ₃-OH)(μ₃-4-carboxypyrazolato)₃] compound for the selective optical detection of AA with a short detection time of 5 min is described.

Nano- and Microsensors for In Vivo Real-Time Electrochemical Analysis: Present and Future Perspectives

Electrochemical nano- and microsensors have been a useful tool for measuring different analytes because of their small size, sensitivity, and favorable electrochemical properties. Using such sensors, it is possible to study physiological mechanisms at the cellular, tissue, and organ levels and determine the state of health and diseases. In this review, we highlight recent advances in the application of electrochemical sensors for measuring neurotransmitters, oxygen, ascorbate, drugs, pH values, and other analytes in vivo. The evolution of electrochemical sensors is discussed, with a particular focus on the development of significant fabrication schemes. Finally, we highlight the extensive applications of electrochemical sensors in medicine and biological science.

Epitaxial Self-Assembly of Interfaces of 2D Metal-Organic Frameworks for Electroanalytical Detection of Neurotransmitters

This paper identifies the electrochemical properties of individual facets of anisotropic layered conductive metal-organic frameworks (MOFs) based on M3(2,3,6,7,10,11-hexahydroxytriphenylene)2 (M3(HHTP)2) (M = Co, Ni). The electroanalytical advantages of each facet are then applied toward the electrochemical detection of neurochemicals. By employing epitaxially controlled deposition of M3(HHTP)2 MOFs on electrodes, the contribution of the basal plane ({001} facets) and edge sites ({100} facets) of these MOFs can be individually determined using electrochemical characterization techniques. Despite having a lower observed heterogeneous electron transfer rate constant, the {001} facets of the M3(HHTP)2 systems prove more selective and sensitive for the detection of dopamine than the {100} facets of the same MOF, with the limit of detection (LOD) of 9.9 ± 2 nM in phosphate-buffered saline and 214 ± 48 nM in a simulated cerebrospinal fluid. Langmuir isotherm studies accompanied by all-atom MD simulations suggested that the observed improvement in performance and selectivity is related to the adsorption characteristics of analytes on the basal plane versus edge sites of the MOF interfaces. This work establishes that the distinct crystallographic facets of 2D MOFs can be used to control the fundamental interactions between analyte and electrode, leading to tunable electrochemical properties by controlling their preferential orientation through self-assembly.

Interfacial Characterization of Polypyrrole/AuNP Composites towards Electrocatalysis of Ascorbic Acid Oxidation

Polypyrrole (PPy) is an interesting conducting polymer due to its good environmental stability, high conductivity, and biocompatibility. The association between PPy and metallic nanoparticles has been widely studied since it enhances electrochemical properties. In this context, gold ions are reduced to gold nanoparticles (AuNPs) directly on the polymer surface as PPy can be oxidized to an overoxidized state. This work proposes the PPy electrochemical synthesis followed by the direct reduction of gold on its surface in a fast reaction. The modified electrodes were characterized by electronic microscopic and infrared spectroscopy. The effect of reduction time on the electrochemical properties was evaluated by the electrocatalytic properties of the obtained material from the oxidation of ascorbic acid (AA) and electrochemical impedance spectroscopy studies. The presence of AuNPs improved the AA electrocatalysis by reducing oxidation potential and lowering charge transfer resistance. EIS data were fitted using a transmission line model. The results indicated an increase in the electronic transport of the polymeric film in the presence of AuNPs. However, PPy overoxidation occurs when the AuNPs’ deposition is higher than 30 s. In PPy/AuNPs 15 s, smaller and less agglomerated particles were formed with fewer PPy overoxidized, confirming the observed electrocatalytic behavior.

Phenotyping methods to assess heat stress resilience in grapevine

Global warming has become an issue in recent years in viticulture, as increasing temperatures have a negative impact on grapevine (Vitis vinifera) production and on wine quality. Phenotyping for grapevine response to heat stress is, therefore, important to understand thermotolerance mechanisms, with the aim of improving field management strategies or developing more resilient varieties. Nonetheless, the choice of the phenotypic traits to be investigated is not trivial and depends mainly on the objectives of the study, but also on the number of samples and on the availability of instrumentation. Moreover, the grapevine literature reports few studies related to thermotolerance, generally assessing physiological responses, which highlights the need for more holistic approaches. In this context, the present review offers an overview of target traits that are commonly investigated in plant thermotolerance studies, with a special focus on grapevine, and of methods that can be employed to evaluate those traits. With the final goal of providing useful tools and references for future studies on grapevine heat stress resilience, advantages and limitations of each method are highlighted, and the available or possible implementations are described. In this way, the reader is guided in the choice of the best approaches in terms of speed, complexity, range of application, sensitivity, and specificity.

Development and Optimization of Electrochemical Method for Determination of Vitamin C

The focus of this work was to develop a simple electrochemical method for the determination of vitamin C (VitC) by using a specially constructed microelectrode made from pyrolytic graphite sheet (PGS). A procedure for quantifying VitC in a real sample was established. VitC shows a single quasi-reversible reaction. The method was optimized, and analytical determination was performed by using cyclic voltammetry and square wave voltammetry for electroanalytical purposes. The obtained results show a linear response of the PGS electrode in a wide concentrations range. For the lower concentration range, 0.18–7.04 µg L−1, the sensitivity is 11.7 µAcm−2/mgL−1, while for the higher concentration range, 10.6–70.4 µg L−1, the sensitivity is 134 µAcm−2/mgL−1, preserving the linearity of 0.998 and 0.999. The second objective was to determine the effect of the addition of five different types of “green” biowaste on plant growth, VitC content, and antioxidant activity in arugula (Eruca sativa L.) using the developed method. After three weeks of cultivation, small differences in growth and large differences in certain nutritional characteristics were observed. The addition of black coffee makes the soil slightly alkaline and causes a significant increase in VitC content and antioxidant activity.

Voltammetric Determination of Ascorbic Acid Content in Cabbage Using Anthraquinone Modified Carbon Paste Electrode

In this study, a low-cost, sensitive, and efficient voltammetric method based on anthraquinone modified carbon paste electrode was developed for determination of ascorbic acid in cabbage samples. After cyclic voltammetry was used to investigate the electrochemical behavior of ascorbic acid and to study dependence of oxidative peak current on scan rate and pH, square wave voltammetric method was developed for direct determination of ascorbic acid in cabbage samples. In contrast to the unmodified carbon paste electrode, a remarkable enhancement in oxidative peak current at anthraquinone modified carbon paste electrode confirmed electrocatalytic property of the modifier towards oxidation of ascorbic acid. A better correlation coefficient for the dependence of peak current on the square root of scan rate ( $R^2=0.9963$ ) than on the scan rate ( $R^2=0.9781$ ) indicated that the oxidation of ascorbic acid at anthraquinone modified carbon paste electrode is predominantly governed by diffusion-controlled process. Square wave amplitude, square wave step potential, and square wave frequency are optimized for the investigation of AA in cabbage. The optimized values are 30 mV, 7 mV, and 35 Hz, respectively. Under the optimized method and solution parameters, an excellent linear response was observed between square wave voltammetric peak current of AQMCPE and concentration of ascorbic acid in the range $5\times {10}^{-5}$ to $4\times {10}^{-3}$  M with a better correlation coefficient ( $R^2=0.9993$ ) and detection limit ( $\text{LOD}=1.84\times {10}^{-6}$  M). The ascorbic acid content of the three cabbage samples from three different cabbage growing areas was found in the range $3.686\pm 0.026$ – $8.298\pm 0.011$  mg/g of powdered cabbage. Excellent recovery results between 95.042 and 96.139% for spiked ascorbic acid in cabbage samples confirmed the potential applicability of the developed method based on AQMCPE for the determination of ascorbic acid in real samples like cabbage.

Non-destructive assessment of vitamin C in foods: a review of the main findings and limitations of vibrational spectroscopic techniques

The constant increase in the demand for safe and high-quality food has generated the need to develop efficient methods to evaluate food composition, vitamin C being one of the main quality indicators. However, its heterogeneity and susceptibility to degradation makes the analysis of vitamin C difficult by conventional techniques, but as a result of technological advances, vibrational spectroscopy techniques have been developed that are more efficient, economical, fast, and non-destructive. This review focuses on main findings on the evaluation of vitamin C in foods by using vibrational spectroscopic techniques. First, the fundamentals of ultraviolet–visible, infrared and Raman spectroscopy are detailed. Also, chemometric methods, whose use is essential for a correct processing and evaluation of the spectral information, are described. The use and importance of vibrational spectroscopy in the evaluation of vitamin C through qualitative characterization and quantitative analysis is reported. Finally, some limitations of the techniques and potential solutions are described, as well as future trends related to the utilization of vibrational spectroscopic techniques.

Electrochemical Methods for the Analysis of Milk

The milk and dairy industries are some of the most profitable sectors in many countries. This business requires close control of product quality and continuous testing to ensure the safety of the consumers. The potential risk of contaminants or degradation products and undesirable chemicals necessitates the use of fast, reliable detection tools to make immediate production decisions. This review covers studies on the application of electrochemical methods to milk (i.e., voltammetric and amperometric) to quantify different analytes, as reported over the last 10 to 15 years. The review covers a wide range of analytes, including allergens, antioxidants, organic compounds, nitrogen- and aldehyde containing compounds, biochemicals, heavy metals, hydrogen peroxide, nitrite, and endocrine disruptors. The review also examines pretreatment procedures applied to milk samples and the use of novel sensor materials. Final perspectives are provided on the future of cost-effective and easy-to-use electrochemical sensors and their advantages over conventional methods.

Electrochemical Detection of Ascorbic Acid in Oranges at MWCNT-AONP Nanocomposite Fabricated Electrode

Ascorbic acid (AA) is an essential vitamin in the body, influencing collagen formation, as well as norepinephrine, folic acids, tryptophan, tyrosine, lysine, and neuronal hormone metabolism. This work reports on electrochemical detection of ascorbic acid (AA) in oranges using screen-print carbon electrodes (SPCEs) fabricated with multi-walled carbon nanotube- antimony oxide nanoparticle (MWCNT-AONP) nanocomposite. The nanocomposite-modified electrode displayed enhanced electron transfer and a better electrocatalytic reaction towards AA compared to other fabricated electrodes. The current response at the nanocomposite-modified electrode was four times bigger than the bare electrode. The sensitivity and limit of detection (LOD) at the nanocomposite modified electrode was 0.3663 [AA]/µM and 140 nM, respectively, with linearity from 0.16–0.640 μM and regression value R² = 0.985, using square wave voltammetry (SWV) for AA detection. Two well-separated oxidation peaks were observed in a mixed system containing AA and serotonin (5-HT); and the sensitivity and LOD were 0.0224 [AA]/µA, and 5.85 µΜ, respectively, with a concentration range from 23 to 100 µM (R² = 0.9969) for AA detection. The proposed sensor outperformed other AA sensors reported in the literature. The fabricated electrode showed great applicability with excellent recoveries ranging from 99 to 107 %, with a mean relative standard deviation (RSD) value of 3.52 % (n = 3) towards detecting AA in fresh oranges.

Advances in fluorescence sensing enabled by lanthanide-doped upconversion nanophosphors

Lanthanide-doped upconversion nanoparticles (UCNPs), characterized by converting low-energy excitation to high-energy emission, have attracted considerable interest due to their inherent advantages of large anti-Stokes shifts, sharp and narrow multicolor emissions, negligible autofluorescence background interference, and excellent chemical- and photo-stability. These features make them promising luminophores for sensing applications. In this review, we give a comprehensive overview of lanthanide-doped upconversion nanophosphors including the fundamental principle for the construction of UCNPs with efficient upconversion luminescence (UCL), followed by state-of-the-art strategies for the synthesis and surface modification of UCNPs, and finally describing current advances in the sensing application of upconversion-based probes for the quantitative analysis of various analytes including pH, ions, molecules, bacteria, reactive species, temperature, and pressure. In addition, emerging sensing applications like photodetection, velocimetry, electromagnetic field, and voltage sensing are highlighted.

Different Electrochemical Behavior of Cationic Dopamine from Anionic Ascorbic Acid and DOPAC at CNT Yarn Microelectrodes

Carbon nanotube yarn microelectrodes (CNTYMEs) have micron-scale surface crevices that momentarily trap molecules. CNTYMEs improve selectivity among cationic catecholamines because secondary reactions are enhanced, but no anions have been studied. Here, we compared fast-scan cyclic voltammetry (FSCV) of dopamine and anionic interferents 3,4 dihydroxyphenylacetic acid (DOPAC) and L-ascorbic acid (AA) at CNTYMEs and carbon fiber microelectrodes (CFMEs). At CFMEs, dopamine current decreases with increasing FSCV repetition frequency at pH 7.4, whereas DOPAC and AA have increasing currents with increasing frequency, because of less repulsion at the negative holding potential. Both DOPAC and AA have side reactions after being oxidized, which are enhanced by trapping. At pH 4, the current increases for DOPAC and AA because they are not repelled. In addition, AA has a different oxidation pathway at pH 4, and an extra peak in the CV is enhanced by trapping effects at CNTYMEs. At pH 8.5, co-detection of dopamine in the presence of DOPAC and AA is enhanced at 100 Hz frequency because of differences in secondary peaks. Thus, the trapping effects at CNTYMEs affects anions differently than cations and secondary peaks can be used to identify dopamine in mixture of AA and DOPAC with FSCV.

High-Performance Piezo-Electrocatalytic Sensing of Ascorbic Acid with Nanostructured Wurtzite Zinc Oxide

Nanostructured piezoelectric semiconductors offer unprecedented opportunities for high-performance sensing in numerous catalytic processes of biomedical, pharmaceutical, and agricultural interests, leveraging piezocatalysis that enhances the catalytic efficiency with the strain-induced piezoelectric field. Here, a cost-efficient, high-performance piezo-electrocatalytic sensor for detecting l-ascorbic acid (AA), a critical chemical for many organisms, metabolic processes, and medical treatments, is designed and demonstrated. Zinc oxide (ZnO) nanorods and nanosheets are prepared to characterize and compare their efficacy for the piezo-electrocatalysis of AA. The electrocatalytic efficacy of AA is significantly boosted by the piezoelectric polarization induced in the nanostructured semiconducting ZnO catalysts. The charge transfer between the strained ZnO nanostructures and AA is elucidated to reveal the mechanism for the related piezo-electrocatalytic process. The low-temperature synthesis of high-quality ZnO nanostructures allows low-cost, scalable production, and integration directly into wearable electrocatalytic sensors whose performance can be boosted by otherwise wasted mechanical energy from the working environment, for example, human-generated mechanical signals.