A novel fluorescent probe for ascorbic acid based on seed-mediated growth of silver nanoparticles quenching of carbon dots fluorescence

Recent advances in detection techniques for vitamin analysis: A comprehensive review

Vitamins are vital micronutrients that play critical roles in human growth and development. However, vitamins are highly susceptible to degradation by light, heat, oxygen, and interactions with other food components during processing and storage. Additionally, insufficient intake or malabsorption can lead to vitamin deficiencies, resulting in various diseases. Since the human body cannot synthesize most vitamins, they must be sourced through diet or supplementation. Therefore, vitamin analysis is critical for meeting human nutritional needs and ensuring quality control. In recent years, significant advancements have been made in vitamin analysis. Here, we propose a comprehensive and critical evaluation of detection methods for water- and fat-soluble vitamins that have been studied over the past five years, including microbiology-, spectroscopy-, liquid chromatography-mass spectrometry-, electrochemistry-, sensor-, and immunoassay-based analysis techniques. Notably, immunoassays are highlighted for their simplicity, affordability, and high sensitivity. Finally, the current challenges and prospects of vitamin analysis are discussed.

Highly efficient determination of trace ascorbic acid in vitamin C tablets by boronate affinity-modified magnetic metal-organic frameworks

Ascorbic acid (AA) plays an important role in many life processes. The chronic nutritional deficiency of AA will lead to the symptoms of scurvy. Therefore, the sensitive quantitative detection of AA is most important in the pharmaceutical analysis, food industry and diagnostic application. In this study, a dual-functional magnetic metal-organic frameworks (Fe3O4@SiO2@UiO-PBA) nanoparticles was synthesized by modifying phenylboronic acid to the surface of magnetic UiO-66-NH2 via postsynthetic modification for selectively and sensitively florescent detection of AA. Due to the abundant amino groups and grafted phenylboronic acid, the proposed nanoparticles have the dual properties of hydrophilicity and boronate affinity. Under optimum conditions, the obtained Fe3O4@SiO2@UiO-PBA nanoparticles can detect AA within 30 s, and has a good linear relationship with the concentration of AA in the range of 5.0-60 μM with a detection limit of 2.5 μM (S/N = 3). In addition, the prepared Fe3O4@SiO2@UiO-PBA nanoparticles showed excellent selectivity and great potential application in the highly efficient determination of trace AA in vitamin C tablets. These results indicated that a convenient method was proposed to develop fluorescent probes for rapid and sensitive detection of trace AA in real samples.

Emission enhanced fluorometric biosensor by functionalized carbon polymer dots for glutathione detection in human real samples and molecular logic gate operation

Glutathione (GSH), an active peptide, plays pivotal roles in many physiological processes and detection of GSH inside of human body is of great importance for the playing of its biological effects. Here silver-phosphorus co-doped carbonized polymer dots (Ag@PCPDs) were prepared via solvothermal treatment of citric acid and phytic acid in the presence of Ag+ for GSH determination. The physicochemical and optical performance of the Ag@PCPDs were characterized by X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared (FT-IR), X-ray powder diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), fluorescence spectroscopy and ultraviolet-visible (UV-Vis) spectroscopy analyses. The prepared Ag@PCPDs have outstanding water solubility with high monodispersity (7.81 ± 0.31 nm) and exhibited excellent optical properties with excitation-dependent emission, high photostability, pH, and ionic strength tolerance. An optimized excitation at 358 nm, the Ag@PCPDs showed strong photoluminescent (PL) emission at 456 nm with a PL quantum yield (QYs) of 15.6%. Furthermore, the Ag@PCPDs were used as a PL sensing platform for detection GSH in a linear range of 0-200 μM with a low limit of detection at 0.68 μM. In addition, the proposed system can construct molecular logic gates with GSH and Fe3+ ions as the chemical inputs and PL emissions as the output. And the Ag@PCPDs were successfully used for GSH determination in real samples resulting in high sensitivity and satisfactory recoveries (92.81--107.45%). More importantly, the Ag@PCPDs showed low cytotoxicity at 500 μg/mL and superior cell imaging capability in HeLa cells, which offer a new path for detection and categorization of GSH in biomedical applications.

Signaling strategies of silver nanoparticles in optical and electrochemical biosensors: considering their potential for the point-of-care

Silver nanoparticles (AgNPs) have long been overshadowed by gold NPs' success in sensor and point-of-care (POC) applications. However, their unique physical, (electro)chemical, and optical properties make them excellently suited for such use, as long as their inherent higher instability toward oxidation is controlled. Recent advances in this field provide novel strategies that demonstrate that the AgNPs' inherent capabilities improve sensor performance and enable the specific detection of analytes at low concentrations. We provide an overview of these advances by focusing on the nanosized Ag (in the range of 1-100 nm) properties with emphasis on optical and electrochemical biosensors. Furthermore, we critically assess their potential for point-of-care sensors discussing advantages as well as limitations for each detection technique. We can conclude that, indeed, strategies using AgNP are ready for sensitive POC applications; however, research focusing on the simplification of assay procedures is direly needed for AgNPs to make the successful jump into actual applications.

The role of positively charge poly-L-lysine in the formation of high yield gold nanoplates on the surface for plasmonic sensing application

An anisotropic structure, gold (Au) nanoplates was synthesized using a two-step wet chemical seed mediated growth method (SMGM) directly on the substrate surface. Prior to the synthesis process, poly-l-lysine (PLL) as a cation polymer was used to enhance the yield of grown Au nanoplates. The electrostatic interaction of positive charged by PLL with negative charges from citrate-capped gold nanoseeds contributes to the yield increment. The percentage of PLL was varied from 0% to 10% to study the morphology of Au nanoplates in term of shape, size and surface density. 5% PLL with single layer treatment produce a variety of plate shapes such as hexagonal, flat rod and triangular obtained over the whole substrate surface with the estimated maximum yield up to ca. 48%. The high yield of Au nanoplates exhibit dual plasmonic peaks response that are associated with transverse and longitudinal localized surface plasmon resonance (TSPR and LSPR). Then, the PLL treatment process was repeated twice resulting the increment of Au nanoplates products to ca. 60%. The thin film Au nanoplates was further used as sensing materials in plasmonic sensor for detection of boric acid. The anisotropic Au nanoplates have four sensing parameters being monitored when the medium changes, which are peak position (wavelength shift), intensity of TSPR and LSPR, and the changes on sensing responses. The sensor responses are based on the interaction of light with dielectric properties from surrounding medium. The resonance effect produces by a collection of electron vibration on the Au nanoparticles surface after hit by light are captured as the responses. As a conclusion, it was found that the PLL treatment is capable to promote high yield of Au nanoplates. Moreover, the high yield of the Au nanoplates is an indication as excellent candidate for sensing material in plasmonic sensor.

A sensitive photothermometric biosensor based on redox reaction-controlled nanoprobe conversion from Prussian blue to Prussian white

As a new low-cost photothermal nanoprobe, Prussian blue nanoparticles (PB NPs) have been demonstrated to have more potential in photothermometric-based point-of-care testing (POCT) application. However, most of the existing PB NP-based photothermometric sensors were constructed mainly relying on in situ generation of PB NPs or their combination with antigens and antibodies, therefore usually suffering from the inherent defects like complicated preparation and cumbersome surface process as well as high-cost modification. To break this limitation of PB NP-based photothermometric POCT, we proposed an ingenious redox reaction-controlled nanoprobe conversion strategy and successfully applied to photothermometric detection of ascorbate oxidase (AAO). In this design, the heat of PB NP photothermal system under 808-nm laser irradiation dramatically decreased with the addition of AA, due to a unique AA-induced Prussian blue to Prussian white (PB-to-PW) conversion. Upon AAO addition, the heat of reaction system increased because of the enzymatic catalytic reaction between AAO and AA, which led to a significant reduction of AA and resultantly inhibited PB-to-PW conversion. Such target-mediated nanoprobe conversion resulted in an obvious temperature change that could be easily detected by a common thermometer and exhibited good linear ranges from 0.25 to 14 mU/mL with a detection limit as low as 0.21 mU/mL for POCT analysis of AAO. This facile, convenient, and portable photothermometric sensing platform provides an innovative route for the design of PB NP nanoprobe-based photothermometric detection methods. A sensitive photothermometric AAO sensor based on a redox reaction-controlled nanoprobe conversion strategy from Prussian blue to Prussian white.

Role of Synthetic Plant Extracts on the Production of Silver-Derived Nanoparticles

The main antioxidants present in plant extracts-quercetin, β-carotene, gallic acid, ascorbic acid, hydroxybenzoic acid, caffeic acid, catechin and scopoletin-are able to synthesize silver nanoparticles when reacting with a Ag NO3 solution. The UV-visible absorption spectrum recorded with most of the antioxidants shows the characteristic surface plasmon resonance band of silver nanoparticles. Nanoparticles synthesised with ascorbic, hydroxybenzoic, caffeic, and gallic acids and scopoletin are spherical. Nanoparticles synthesised with quercetin are grouped together to form micellar structures. Nanoparticles synthesised by β-carotene, were triangular and polyhedral forms with truncated corners. Pentagonal nanoparticles were synthesized with catechin. We used Fourier-transform infrared spectroscopy to check that the biomolecules coat the synthesised silver nanoparticles. X-ray powder diffractograms showed the presence of silver, AgO, Ag2O, Ag3O4 and Ag2O3. Rod-like structures were obtained with quercetin and gallic acid and cookie-like structures in the nanoparticles obtained with scopoletin, as a consequence of their reactivity with cyanide. This analysis explained the role played by the various agents responsible for the bio-reduction triggered by nanoparticle synthesis in their shape, size and activity. This will facilitate targeted synthesis and the application of biotechnological techniques to optimise the green synthesis of nanoparticles.

Sensitive detection of picric acid in an aqueous solution using fluorescent nonconjugated polymer dots as fluorescent probes

Nonconjugated polymer dots (NPDs) were successfully used as fluorescent probes to selectively and sensitively detect picric acid (PA). The NPDs were prepared from polyethylenimine and 1,4-phthalaldehyde under mild conditions and had excitation and emission maxima of 351 and 474 nm, respectively. Fluorescence of the NPDs was efficiently quenched by PA through the inner filter effect because of the overlapping PA absorption band and NPD excitation spectrum. The NPDs allowed PA to be determined with a high degree of sensitivity. The linear range was 0-140μM and the detection limit was 0.5μM. The work involved developing a novel method for synthesizing NPDs and a promising platform for determining PA in environmental media.

A sensitive turn-off-on fluorometric sensor based on S,N co-doped carbon dots for environmental analysis of Hg(II) ion

A simple and sensitive fluorescence turn-off-on sensor was established by means of S,N co-doped carbon dots (S,N-CDs) and Ag nanoparticles (AgNPs) for the determination of Hg²⁺ . For this purpose, blue emissive S,N-CDs were hydrothermally synthesized and characterized using transmission electron microscopy, Fourier transform infrared spectroscopy, and energy dispersive X-ray spectroscopy. We observed that the fluorescence intensity of the as-prepared S,N-CDs was impressively quenched by AgNPs. The quenching mechanism was studied and attributed to nanosurface energy transfer and the inner filter effect between S,N-CDs and AgNPs. Furthermore, by adding Hg²⁺ , the fluorescence intensity of S,N-CDs/AgNPs was restored as a result of aggregation of AgNPs in the presence of Hg²⁺ . Based on these facts, S,N-CDs and AgNPs were exploited to design a sensitive turn-off-on sensor for analysis of Hg²⁺ . The recovered fluorescence signal was proportional to the concentration of Hg²⁺ in the range 1.5-2000 nM with a detection limit of 0.51 nM. The established sensor was used with satisfactory results for measurement of Hg²⁺ in environmental water samples.

Recent advances in the construction and analytical applications of carbon dots-based optical nanoassembly

Recently, more and more attention has been focused on the construction and analytical applications of optical nanoassembly through combining carbon dots (CDs) with various other functional nanomaterials. The rational design and manufacture of CDs-based optical nanoassembly will be critical to meeting the needs of analytical science. The last decade has witnessed the immense potential of CDs-based optical nanoassembly in multiple sensing applications owing to their controlled optical properties, adjustable surface chemistry and microscopic morphology. This feature article collects the recent advances in the research and development of CDs-based optical nanoassembly and their applications in analytical sensors, aiming to provide vital insights and suggestions to inspire their broad sensing applications.

"Turn-on" fluorometric probe for α-glucosidase activity using red fluorescent carbon dots and 3,3',5,5'-tetramethylbenzidine

A turn-on method for determining α-glucosidase activity is described using a chemical redox strategy in which the fluorescence of red fluorescent carbon dots (CDs) is modulated. The red fluorescent CDs were prepared using a solvothermal method with p-phenylenediamine and sodium citrate. The excitation and emission maxima of the CDs were 490 and 618 nm, respectively. Ce⁴⁺ ions catalyze the oxidation of the colorless substrate 3,3',5,5'-tetramethylbenzidine (TMB) to give a blue oxidized TMB product (oxTMB). Absorption by oxTMB overlaps with the red light emitted by the CDs because of the fluorescence inner filter effect; therefore the presence of oxTMB decreases the intensity of fluorescence emission by the CDs. However, hydrolysis of L-ascorbic acid-2-O-α-D-glucopyranosyl by the enzyme α-glucosidase causes formation of ascorbic acid . Ascorbic acid reduces oxTMB to TMB, so that the inner filter effect disappeared and the fluorescence recovered. The strategy allows α-glucosidase activity to be successfully determined down to 0.02 U mL^-1 and gives a dynamic linear range of 0-5.5 U mL^-1. The strategy is very selective for α-glucosidase activity in the presence of potentially interfering substances. The method has been successfully applied to the determination of α-glucosidase activity in spiked human serum samples and gave satisfactory results. Graphical Abstract Schematic of the method used to prepare the carbon dots and the mechanisms involved in determining α-glucosidase activity.

Colorimetric strategy for ascorbic acid detection based on the oxidase-like activity of silver nanoparticle single-walled carbon nanotube composites

A colorimetric assay for the determination and quantification of ascorbic acid (AA) is presented using silver nanoparticle (AgNP) single-walled carbon nanotube (AgNP/SWCNT) nanocomposites prepared using a microwave-assisted method. The AgNP/SWCNT nanocomposites possessed oxidase-like properties toward 3,3',5,5'-tetramethylbenzidine (TMB) and could catalyze the oxidation of TMB to form a blue oxidation product (λ_max = 652 nm) in the absence of H₂ O₂ . AA can specifically inhibit the oxidation of TMB, resulting in a decline of the absorbance value and blue colour fading. As such, amounts of AA can be assessed easily by the unaided eye and quantitatively using an ultraviolet-visible light spectrophotometer. Under the optimal reaction conditions, this strategy showed a good linearity ranging from 0.4 μM to 5.0 μM for AA detection, and the limit of detection was 130 nM. This assay was also applied for AA measurement in vitamin C tablets and juice samples that yielded satisfactory results.

Optical sensors based on silver nanoparticles for determination of pharmaceuticals: An overview of advances in the last decade

This review focuses on optical nanosensors based on silver nanoparticles (Ag NPs) and demonstrates their applications in the determination of pharmaceutical compounds in the last decade. Such optical sensors have received high attention in the analytical field owing to their low cost and simplicity since they do not require any complex or expensive instrumentation. This article reviews Ag NP-based optical methods for the determination of pharmaceutical compounds from 2010 to 2020. The reported optical methods are classified into four types: spectrophotometry, spectrofluorimetry, scattering and chemiluminescence. Ag NPs play different roles in the different sensing platforms used by these methods, the details of which are carefully explained in this review. Moreover, the relevant analytical parameters of the developed methods are categorized by role and tabulated. It is hoped that this review will stimulate further research in this field with similar nanostructures.

Convenient synthesis of carbon nanodots for detecting Cr(vi) and ascorbic acid by fluorimetry

Carbon nanodots (CDs) were simply synthesized from Sophora flavescens Ait. “On–off–on” fluorescent probes for the sensitive and selective detections of Cr(iv) and ascorbic acid (AA) were founded and well applied in real samples.

Bionanosensor based on N-doped graphene quantum dots coupled with CoOOH nanosheets and their application for in vivo analysis of ascorbic acid

Herein, we employ 3D nitrogen-doped porous graphene frameworks (NPG) as raw material to prepare emissive nitrogen doped graphene quantum dots (r-NGQDs) via chemical oxidation method. The as-prepared fluorescent r-NGQDs was integrated with CoOOH nanosheets to construct a sensing platform for in vivo ascorbic acid (AA) analysis. Initially, the fluorescence emission intensity of r-NGQDs was quenched by CoOOH nanosheets based on the inner filter effect (IFE). Then the quenched intensity of r-NGQDs and CoOOH nanosheets system was enlightened by addition of AA, since AA could consume CoOOH nanosheets through redox reaction, leading to the release of r-NGQDs and fluorescence restoration. Moreover, the restored fluorescence intensity of r-NGQDs is highly dependent on the concentration of AA which endows them as a quantitative analysis of AA with a limit of detection (LOD) reach up to1.85 μM (n = 3) in aqueous solution. Finally, the as constructed bionanosensor was further employed for in vivo analysis of AA in living rat brain microdialysate with basal value up to 9.4 ± 1.4 μM (n = 3).

Green synthesis of carbon dots using the flowers of Osmanthus fragrans (Thunb.) Lour. as precursors: application in Fe3+ and ascorbic acid determination and cell imaging

In this work, dried flowers of Osmanthus fragrans Lour. were applied as green precursors to synthesize carbon dots (CDs) by a green hydrothermal method for the first time. The CDs showed strong blue fluorescence at 410 nm under 340-nm excitation with a quantum yield of approximately 18.53%. Furthermore, the CDs were applied for the sensitive detection of Fe³⁺. The linear response of Fe³⁺ ranged from 10 nM to 50 μM with a limit of detection as low as 5 nM. In addition, other ions were used as competitive substances to explore the selectivity of CDs for Fe³⁺. The fluorescence quenching effect of Fe³⁺ was much stronger, which demonstrated that the CDs had high selectivity for Fe³⁺ and they can be employed for the selective detection of Fe³⁺. The potential fluorescence quenching mechanism between CDs and Fe³⁺ was identified as the inner filter effect. The CDs were then used as a fluorescent sensor for the detection of Fe³⁺ in water samples and human serum; the recovery range was 93.76-113.80% (relative standard deviation less than 0.79%). These results indicate that the CDs can be applied for the sensitive and selective detection of Fe³⁺ in real samples. Moreover, on the basis of the redox reaction between Fe³⁺ and ascorbic acid (AA), the CD-Fe³⁺ system can be used as a fluorescent "off-on" sensor for the detection of AA with a limit of detection of 5 μM. What is more, because of their low toxicity and biocompatibility, the CDs can also be used for cell imaging and acted as a fluorescent probe for fluorescence imaging of Fe³⁺ and AA in living cells. These results demonstrate that the CDs have great potential for application in the fields of sensing, bioimaging, and even disease diagnosis.

Silver nanoparticles capped with carbon dots as a fluorescent probe for the highly sensitive "off-on" sensing of sulfide ions in water

The present study illustrates the synthesis of silver nanoparticles capped with carbon dots (AgNPs-CDs) and their application towards the sensitive and selective sensing of sulfide ions by colorimetry and spectrofluorimetry methods. The CDs were prepared from l-asparagine by pyrolysis at 234 °C. The as-synthesized CDs were then utilized as reducing and capping agents for the synthesis of AgNPs-CDs by the wet chemical method. The size of the AgNPs-CDs was found to be ~ 5.2 nm. They show a characteristic surface plasmon resonance band at 417 nm and emission maximum at 441 nm when excited at 348 nm. Since the AgNPs were formed on the surface of CDs, the emission intensity of AgNPs-CDs was drastically decreased in contrast to that of CDs. The as-synthesized AgNPs-CDs were then successfully used for the sensitive and selective determination of sulfide ions. The addition of 0.1 μM sulfide ions to AgNPs-CDs leads to a decrease in the absorbance intensity at 417 nm aside turning from yellow to colorless. In the contrary, the emission was "turned on" after the addition of sulfide ions. The decrease in the absorbance and increase in the emission were attributed to the rapid formation of Ag₂S. Finally, the practical application of the present method was demonstrated by determining dissolved H₂S in tap water samples.