Colorimetric determination of ascorbic acid based on carbon quantum dots as peroxidase mimetic enzyme

Ultra-trace Ag doped carbon quantum dots with peroxidase-like activity for the colorimetric detection of glucose

Carbon quantum dots (CQDs) have significant applications in nanozymes. However, previous studies have not elucidated the structure-activity relationship and enzyme mechanism. In this study, we employed a one-step microwave method to synthesize ultra-trace Ag-doped carbon quantum dots (Ag-CQDs). In the presence of hydrogen peroxide (H2O2), we used the oxidative coupling reaction of 3,3',5,5'-tetramethylbenzidine (TMB) to evaluate the intrinsic peroxidase-like activity, kinetics, and mechanism of Ag-CQDs. The trace amount of doped Ag (1.64 %) facilitated electron transfer from the CQDs interior to the surface. The electron transfer triggered the peroxide activity of CQDs, producing hydroxyl radical (·OH), which oxidized the colorless TMB to blue-colored TMB (oxTMB). By coupling with glucose oxidase (GOx), the Ag-CQDs/H2O2/TMB system has been used for colorimetric glucose determination. The system demonstrated a low detection limit (0.17 µM), wide linear range (0.5-5.5 µM), and satisfactory results when fruit juice was analyzed. This study reports a feasible method for the colorimetric detection of glucose by synthesizing ultra-trace Ag-doped carbon quantum dots with peroxidase-mimicking activity.

Selective fluorescence detection of acetylsalicylic acid, succinic acid and ascorbic acid based on a responsive lanthanide metal fluorescent coordination polymer

A fluorescent sensor for highly selective and ultrasensitive detection of acetylsalicylic acid (ASA), succinic acid (SA), and ascorbic acid (AA) was reported. The water-soluble fluorescent ligand salicylic acid (Sal) was generated through catalyzing ASA by the hydrolase activity of zeolitic-imidazolate framework-8 (ZIF-8) or natural esterase (Est). The Sal can coordinate with 2-methylimidazole (2-MIm) and Ln(III) to form a fluorescent lanthanide coordination polymer (LCP), which has a fluorescence emission peak with the maximum wavelength at 412 nm (the excitation wavelength at 300 nm). Therefore, the detection of ASA can be achieved through the fluorescence intensity changes of LCPs in the system, which has comparable sensitivity and good selectivity (linear range of 0.031-1.00 mM and LODs of 11.72 and 3.22 μM) as compared to a direct reaction between Est/ZIF-8 and ASA for detecting ASA (linear range of 0.05-1.20 mM and limits of detection (LODs) of 4.43 and 4.58 μM). Furthermore, upon the addition of SA and AA, the fluorescence intensity of the reaction system can be enhanced and weakened through changing the energy resonance transfer pathways and affecting the enzymatic reaction process, respectively, realizing their sensitive and selective fluorescence detection. The established fluorescent sensors can work well in a wide linear range of SA concentrations from 0 to 2.50 mM (Est-based reaction system) and 0 to 1.50 mM (ZIF-8-based reaction system) with the LODs of 0.032 and 0.028 mM, respectively. The linear ranges of AA concentrations are from 0.0078 to 0.25 mM (Est-based reaction system) and 0.0078 to 0.13 mM (ZIF-8-based reaction system) with the LODs of 2.54 and 3.80 μM, respectively. The established sensors were successfully used in the detection of SA in rabbit plasma, with a recovery of 84.0%-98.7%. Additionally, the contents of ASA in Aspirin Enteric-Coated tablets and AA in vitamin C tablets were also determined by the developed methods.

From waste to wealth: iron oxide doped hydroxyapatite-based biosensor for the colorimetric detection of ascorbic acid

Ascorbic acid plays a pivotal role in the human body. It maintains the robustness, enlargement, and elasticity of the collagen triple helix. However, the abnormal concentration of ascorbic acid causes various diseases, such as scurvy, cardiovascular diseases, gingival bleeding, urinary stones, diarrhea, stomach convulsions, etc. In the present work, an iron-doped hydroxyapatite (HAp@Fe2O3)-based biosensor was developed for the colorimetric detection of ascorbic acid based on a low-cost, biocompatible, and ubiquitous material. Due to the catalytic nature of HAp owing to the acidic and basic moieties within the structure, it was used as a template for HAp@Fe2O3 synthesis. This approach provides an active as well as large surface area for the sensing of ascorbic acid. The synthesized platform was characterized by various techniques, such as UV-Vis, FTIR, SEM, XRD, TGA, EDX, etc. The HAp@Fe2O3 demonstrated inherent peroxidase-like activity in the presence of 3,3',5,5'-tetramethylbenzidine (TMB) oxidized with the assistance of H2O2. It resulted in the color changing to blue-green, and after the addition of ascorbic acid, the color changed to colorless, resulting in the reduction of TMB. To achieve optimal sensing parameters, experimental conditions were optimized. The quantity of HAp@Fe2O3, H2O2, pH, TMB, time, and the concentration of ascorbic acid were fine-tuned. The linear range for the proposed sensor was 0.6-56 μM, along with a limit of detection of 0.16 μM and a limit of quantification of 0.53 μM. The proposed sensor detects ascorbic acid within 75 seconds at room temperature. The proposed platform was also applied to quantitatively check the concentration of ascorbic acid in a physiological solution.

Deinococcus wulumuqiensis R12 synthesized silver nanoparticles with peroxidase-like activity for synergistic antibacterial application

The use of a combination of several antibacterial agents for therapy holds great promise in reducing the dosage and side effects of these agents, improving their efficiency, and inducing potential synergistic therapeutic effects. Herein, this study provides an innovative antibacterial treatment strategy by synergistically combining R12-AgNPs with H2O2 therapy. R12-AgNPs were simply produced with the supernatant of an ionizing radiation-tolerant bacterium Deinococcus wulumuqiensis R12 by one-step under room temperature. In comparison with chemically synthesized AgNPs, the biosynthesized AgNPs presented fascinating antibacterial activity and peroxidase-like properties, which endowed it with the capability to catalyze the decomposition of H2O2 to generate hydroxyl radical. After the combination of R12-AgNPs and H2O2, an excellent synergistic bacteriostatic activity was observed for both Escherichia coli and Staphylococcus aureus, especially at low concentrations. In addition, in vitro cytotoxicity tests showed R12-AgNPs had good biocompatibility. Thus, this work presents a novel antibacterial agent that exhibits favorable synergistic antibacterial activity and low toxicity, without the use of antibiotics or a complicated synthesis process.

Recent advances in the colorimetric and fluorescence analysis of bioactive small-molecule compounds based on the enzyme-like activity of nanomaterials

Nanomaterials with enzyme-like activity have been widely used in the construction of colorimetric and fluorescence sensors due to their advantages of cost-effectiveness, high stability, good biocompatibility, and ease of modification. Furthermore, the colorimetric and fluorescence sensors, which are effective approaches for detecting bioactive small-molecule compounds, have been extensively explored due to their simple operation and high sensitivity. Recent significant researches have focused on designing various sensors based on nanozymes with peroxidase- and oxidase-like activity for the colorimetric and fluorescence analysis of different analytes. In this review, recent developments (from 2018 to present) in the colorimetric and fluorescent analysis of bioactive small-molecule compounds based on the enzyme-like activity of nanomaterials were summarized. In addition, the challenges and design strategies in developing colorimetric and fluorescent assays with high performance and specific sensing were discussed.

Paracetamol-Mediated Synthesis of Silver Nanoparticles and Their Functionalization with Ionic Liquid for the Colorimetric Biosensing of Ascorbic Acid

Ascorbic acid is a vital biomolecule for human beings. When the body's level of ascorbic acid is abnormal, it can lead to a number of illnesses. Its appropriate concentration is necessary for the oxidation of prostaglandins and cyclic adenosine monophosphate, the production of dopamine, norepinephrine, epinephrine, and carnitine, and the expansion and durability of the collagen triple helix in humans. In the present work, silver nanoparticle synthesis was performed through a paracetamol-mediated approach. Different characterization techniques, such as X-ray diffractometry (XRD), energy dispersive X-ray (EDX), Fourier transform infrared (FTIR), and scanning electron microscopy (SEM), were used to confirm the prepared nanoparticles. Subsequently, the prepared Ag NPs functionalized with an ionic liquid were used as a sensing platform for ascorbic acid in blood serum samples. To achieve the best possible results, the proposed biosensor was optimized with different parameters such as TMB concentration, time, amount of capped nanoparticles (NPs), and pH. The proposed biosensor offers a sensitive and straightforward method for ascorbic acid with a linear range from 2 × 10-9 to 3.22 × 10-7 M, an LOD of 1.3 × 10-8 M, an LOQ of 4.3 × 10-8 M, and an R2 of 0.9996, Moreover, applications of the proposed biosensor were successfully used for the detection of ascorbic acid in samples of human plasma, suggesting that Ag NPs with high peroxidase-like activity, high stability, and facile synthesis exhibited promising applications in biomedical fields.

Accuracy improvement via novel ratiometry design in distance-based microfluidic paper based analytical device: instrument-free point of care testing

Developing accurate, precise, instrument-free, and point-of-need microfluidic paper-based devices is highly significant in clinical diagnosis and biomedical analysis. In the present work, a ratiometric distance-based microfluidic paper-based analytical device (R-DB-μPAD), along with a three-dimensional (3D) multifunctional connector (spacer), was designed to improve the accuracy and detection resolution analyses. Specifically, the novel R-DB-μPAD was used for the accurate and precise detection of ascorbic acid (AA) as a model analyte. In this design, two channels were fabricated as detection zones, with a 3D spacer located between the sampling and detection zones to improve the detection resolution by preventing the reagents mixing from overspreading between these zones. Two probes for AA were used: Fe³⁺ and 1,10-phenanthroline were deposited in the first channel, and oxidized 3,3',5,5'-tetramethylbenzidine (oxTMB) was added to the second channel. Accuracy improvement of this ratiometry-based design was achieved by enhancing the linearity range and reducing the volume dependency of the output signal. Moreover, the 3D connector improved the detection resolution by eliminating the systematic errors. Under the optimal conditions, the ratio of the distances of the color bands in the two channels was used to construct an analytical calibration curve in the range from 0.05 to 1.2 mM, with a limit of detection of 16 μM. The proposed R-DB-μPAD combined with the connector was successfully used for the detection of AA in orange juice and vitamin C tablets with satisfactory accuracy and precision. This work opens the door for multiplex analysis of various analytes in different matrices.

Sensitive acid phosphatase assay based on light-activated specific oxidase mimic activity

Acid phosphatase (ACP) is a key marker in the diagnosis of many diseases. However, exploiting a simple and sensitive sensor for the real-time quantitative analysis of ACP is still challenging. Herein, we attempted to develop a sensitive colorimetric sensing strategy for the detection of ACP based on light-activated oxidase mimic property of carbon dots (CDs). The synthesized CDs were proved to be capable of intrinsic light-activated oxidase mimic activity, which could generate reactive oxygen species to oxidize chromogenic substrate under ultraviolet light stimulation. Interestingly, this light-activated oxidase mimic behavior would be effectively suppressed by the antioxidant ascorbic acid (AA), a product from the hydrolysis of 2-phospho-L-ascorbic acid trisodium (AAP) mediated by ACP. Based on the above property, a facile and sensitive colorimetric sensing method for ACP was developed. Under the optimal conditions, the linear range for ACP 0.1-5.5 U/L, and the detection limit was 0.056 U/L. Compared with conventional nanozyme based ACP assay systems, the catalytic activity of light-activated nanozyme could be conveniently regulated by switching the light on and off, which made it easier to precisely control the extent of the reaction and ensured the accuracy of the assay. In addition, the proposed sensing system would be readout directly by the naked eye or smartphone-based RGB analysis system, and have been successfully applied to analyze diluted in diluted fetal bovine serum and urine samples spiked with ACP. All these results indicated that this approach holds good promise for future applications in clinical analysis and point-of-care (POC) biosensor platforms.

CeO2–Co3O4 nanocomposite with oxidase-like activity for colorimetric detection of ascorbic acid†

A CeO₂–Co₃O₄ nanocomposite (NC) was prepared and characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction. The obtained CeO₂–Co₃O₄ NC displayed biomimicking oxidase-like activity, which can catalytically oxidize the 3, 3′, 5, 5′-tetramethylbenzidine (TMB) substrate from colorless to the blue oxidized TMB (ox-TMB) product with a characteristic absorption peak at 652 nm. When ascorbic acid (AA) was present, ox-TMB would be reduced, resulting in a lighter blue and lower absorbance. On the basis of these facts, a simple colorimetric method for detection of AA was established with a linear relationship ranging from 1.0 to 500 μM and a detection limit of 0.25 μM. When this method was used to detect AA in human serum and commercially available vitamin C tablet samples, a good recovery of 92.0% to 109.0% was obtained. Besides, the catalytic oxidation mechanism was investigated, and the possible catalytic mechanism of CeO₂–Co₃O₄ NC can be described as follows. TMB is adsorbed on the CeO₂–Co₃O₄ NC surface and provides lone-pair electrons to the CeO₂–Co₃O₄ NC, leading to an increase in electron density of the CeO₂–Co₃O₄ NC. An increased electron density can improve the electron transfer rate between TMB and the oxygen absorbed on its surface to generate O₂˙⁻ and ˙O₂, which further oxidize TMB.

A CeO₂–Co₃O₄ nanocomposite (NC) was prepared and characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction.

Highly sensitive and convenient aptasensor based on Au NPs@Ce-TpBpy COF for quantitative determination of zearalenone

In this work, an aptasensor based on a portable U-disk electrochemical workstation in combination with a screen-printed electrode (SPE) is demonstrated for the quantitative determination of zearalenone (ZEN). The aptamer is immobilized on Au NPs@Ce-TpBpy COF (Covalent organic frameworks), which is modified on the surface of glassy carbon electrode. ZEN specifically binds to ZEN aptamer, which hinders the electron transfer and decreases the catalytic current of Au NPs@Ce-TpBpy COF for the reduction of hydrogen peroxide, measured by chronoamperometry (i-t). The quantitative detection of ZEN toxin is realized by a decrease of the catalytic current (ΔI). Under the optimal experimental conditions, the aptamer sensor exhibited excellent sensitivity, selectivity, reproducibility. A wide linear range of 1 pg mL-1-10.0 ng mL-1 with a detection limit of 0.389 pg mL-1 (at 3σ) was obtained. The linear equation is ΔI = 0.401 lg c + 1.948 with a correlation coefficient of 0.9906. The recovery is in the range of 93.0-104.7% for the cornflour samples. The proposed method offers a new strategy for the rapid, inexpensive, and real-time detection of ZEN.

Smart Materials Enabled with Artificial Intelligence for Healthcare Wearables

Contemporary medicine suffers from many shortcomings in terms of successful disease diagnosis and treatment, both of which rely on detection capacity and timing. The lack of effective, reliable, and affordable detection and real‐time monitoring limits the affordability of timely diagnosis and treatment. A new frontier that overcomes these challenges relies on smart health monitoring systems that combine wearable sensors and an analytical modulus. This review presents the latest advances in smart materials for the development of multifunctional wearable sensors while providing a bird's eye‐view of their characteristics, functions, and applications. The review also presents the state‐of‐the‐art on wearables fitted with artificial intelligence (AI) and support systems for clinical decision in early detection and accurate diagnosis of disorders. The ongoing challenges and future prospects for providing personal healthcare with AI‐assisted support systems relating to clinical decisions are presented and discussed.

Progress in the Application of Carbon Dots-Based Nanozymes

As functional nanomaterials with simulating enzyme-like properties, nanozymes can not only overcome the inherent limitations of natural enzymes in terms of stability and preparation cost but also possess design, versatility, maneuverability, and applicability of nanomaterials. Therefore, they can be combined with other materials to form composite nanomaterials with superior performance, which has garnered considerable attention. Carbon dots (CDs) are an ideal choice for these composite materials due to their unique physical and chemical properties, such as excellent water dispersion, stable chemical inertness, high photobleaching resistance, and superior surface engineering. With the continuous emergence of various CDs-based nanozymes, it is vital to thoroughly understand their working principle, performance evaluation, and application scope. This review comprehensively discusses the recent advantages and disadvantages of CDs-based nanozymes in biomedicine, catalysis, sensing, detection aspects. It is expected to provide valuable insights into developing novel CDs-based nanozymes.

Continuous response fluorescence sensor for three small molecules based on nitrogen-doped carbon quantum dots from prunus lannesiana and their logic gate operation

In this study, an environmentally friendly and water-soluble nitrogen-doped carbon quantum dots (N-CQDs) with quantum yield (QY) of 8.59% were prepared by one-step hydrothermal synthesis without any chemical reagent using the leaves of prunus lannesiana as precursors. The properties and quality of N-CQDs were investigated by Ultraviolet-visible absorption spectroscopy, infrared spectroscopy, X-ray photoelectron spectroscopy, zeta potential, high-resolution transmission electron microscopy and fluorescence spectroscopy. The fluorescence of the prepared N-CQDs can be quenched by Fe³⁺ through the synergistic effect of the formation of non-fluorescent complex and internal filtration effect (IFE) between Fe³⁺ and N-CQDs. And the quenched fluorescence can be "turned on" after adding ascorbic acid (AA) because Fe³⁺ can be released from the surface of N-CQDs through the redox reaction between AA and Fe³⁺. While the restored fluorescence can be "turned off" again by hydrogen peroxide (H₂O₂) due to the re-oxidation of Fe²⁺ to Fe³⁺. So, the three inputs "logic gate" is achieved and the "on-off-on-off" continuous response fluorescence sensor is formed, which can be applied for the continuous detection of Fe³⁺, AA and H₂O₂ with the linear range of 40-260 μM, 10-200 μM and 40-140 μM, respectively. Finally, the sensor was successfully applied to determine Fe³⁺, AA and H₂O₂ in real samples with the satisfactory recoveries (95.35%-104.10%) and repeatability (relative standard deviation (RSD) ≤ 1.68%). The continuous response fluorescence sensor prepared by simple green synthesis route has the characteristics of fast response, acceptable sensitivity and good selectivity.

Sensor and Bioimaging Studies Based on Carbon Quantum Dots: The Green Chemistry Approach

Since carbon quantum dots have high photoluminescent efficiency, it has been a desired material in sensor and bioimaging applications. In recent years, the green chemistry approach has been preferred and the production of quantum dots has been reported in many studies using different precursors from natural, abundant, or waste sources. Hydrothermal, chemical oxidation, microwave supported, ultrasonic, solvothermal, pyrolysis, laser etching, solid-state, plasma, and electrochemical methods have been reported in the literature. In this review article, green chemistry strategies for carbon quantum dot synthesis is summarized and compared with conventional methods using methodologic and statistical data. Furthermore, a detailed discussion on sensor and bioimaging applications of carbon quantum dots produced with green synthesis approaches are presented with a special focus on the last decade.