Bright-yellow-emissive nitrogen-doped carbon nanodots as a fluorescent nanoprobe for the straightforward detection of glutathione in food samples

Development of a smartphone-assisted multiple colorimetric detection assay for GSH in food based on the degradation of gold nanorods

Glutathione (GSH) is a tripeptide and natural reducing agent composed of glutamic acid, glycine, and cysteine. Its level in the human body is closely linked to human health, such as diabetes, Alzheimer's disease, and cancer. The supplementation of exogenous GSH could bring health benefits and GSH detection in food is of considerable importance. However, the existing assays for GSH detection such as high-performance liquid chromatography/mass spectrometry, electrochemiluminescence and fluorescent nanoprobe were not satisfactory because of the disadvantages of equipment and site requirements. In this study, a multiple-colorimetric detection assay for GSH was developed based on GSH's reaction with gold nanorods. During the reaction with varying concentrations of GSH, the gold nanorods degraded into spherical nanoparticles with multiple color changes, which could be used to determine GSH concentrations. The transverse surface plasmon resonance absorption peak of gold nanorods (AuNRs) significantly shifted, indicating a novel mechanism distinct from etching or surface coating, which typically altered the longitudinal surface plasmon absorption peak. Under optimized conditions, the assay exhibited commendable specificity and reliability in actual samples. The assay accurately quantified GSH ranging from 1 to 10 µM, with detection limits of 439 nM and 260 nM for spectrophotometry and visual analysis, respectively. It was firstly to use GSH as a reducing agent to react with AuNRs in the presence of AgNO3 and the mechanism was different from etching or surface coating. The study's assay shows potential for detecting GSH in food samples and provides an alternative approach for the development of colorimetric detection assays based on AuNRs.

Construction of a reaction-based fluorescent sensor for tandem detection of Cu2+ and glutathione in wine

The purpose of this study was to develop a novel reaction-based fluorescent sensor for the detection of Cu2+ and glutathione in real wine samples. The sensor, tris-(2-pyridyl)-methylamine rhodol derivative, was synthesized and validated for the tandem and selective detection of both Cu2+ and glutathione. The sensor exhibited a strong linear correlation between fluorescence intensity and Cu2+ concentration ranging from 100 to 900 nM, while the in situ generated Cu2+ ensemble selectively detected glutathione with a robust linear response from 3 to 30 μM. The detection limits for Cu2+ and glutathione were as low as 28 nM and 0.60 μM, respectively. Additionally, the sensor enabled quantitative detection of Cu2+ and glutathione in real wine samples. This work provides the first reaction-based fluorescence sensor with an "on-off-on" fluorescence response for the tandem detection of Cu2+ and glutathione in wine, offering potential applications in food and beverage quality control.

Picric Acid Incorporated Fluorescent Nitrogen Doped Carbon Dot for "Turn-On" Detection of Glutathione

Glutathione (GSH), a non-protein thiol in living cells whose abnormal level indicates the onset of diseases like Alzheimer's, HIV, diabetes, cancer, Parkinson's, Dementia, etc. Herein, we have synthesized a low-cost, selective, and sensitive detection platform by using citric acid and urea-derived fluorescent carbon dots (NCDs) via the microwave-assisted method, showing fluorescence at 444 nm. This fluorescence was quenched with picric acid (PA), and this probe, picric acid incorporated nitrogen doped carbon dot (NCDs@PA) was further used for the detection of GSH. The characterization of the probe was done by photoluminescence study, UV-Visible absorption studies, ATR-FTIR, SEM, and DLS analysis. GSH induced fluorescence recovery due to the competitive binding of GSH to PA. GSH was detected within a linear range of 0.31 mM- 2.43 mM with a Limit of Detection (LoD) and Limit of Quantification (LoQ) of 32.10 µM and 107.32 µM, respectively. The sensor exhibited good selectivity and sensitivity towards GSH among various co-existing ions and biomolecules. The paper-strip-based sensing of glutathione was conducted to check practical applicability of the probe, and a real sample analysis was also conducted from spiked human samples.

Phenylboronic acid-functionalized copper nanoclusters with sensitivity and selectivity for the ratiometric detection of luteolin

A desirable ratiometric fluorescent probe was designed by using 3-carboxyphenylboronic acid functionalized polyethyleneimine ethoxylated modified copper nanoclusters (CPBA@PEI-CuNCs) for detecting luteolin (LTL, cis-diols structure) with sensitivity and selectivity. Characterization techniques were carried out with the TEM, PSD, FT-IR, XPS and XRD, confirming its successful formation with a quantum yield of 40.49 %. As the optimal excitation wavelength at 386 nm, the fluorescence intensity was detected by fluorophotometer. Meanwhile, the nanoprobe has two emission wavelengths 396 and 473 nm. According to the fluorescence quenching intensity ratio (F473/F396), the LOD reached as low as 1.22 nM within the range from 0.11 to 600 μM. The CPBA@PEI-CuNCs exhibited pH-controlled, covalent, and reversible binding properties. Using PEI capped by CuNCs, cytotoxicity is reduced for potential treatment purposes. Additionally, the developed probe was used for LTL detection, HepG2 cell imaging and pH-responsive drug delivery in real samples with carrot leaves, peanut shells, and perilla leaves samples. CPBA@PEI-CuNCs demonstrated repeatability and reproducibility, making it a cost-effective and practical tool for fluorescence analysis in detection.

Fluorescence sensor array for highly sensitive pattern recognition of biothiols in food based on tricolor upconversion luminescence metal-organic frameworks

Fluorescence nanomaterial sensors have exhibited excellent application potential in biothiols analyses. The fluorescence sensor arrays constructed from upconversion luminescence metal-organic frameworks nanocomposites (LMOFs) can provide impressive discrimination and exquisite fingerprinting capabilities for extremely similar analytes. Herein, an upconversion fluorescence sensor array based on LMOFs featuring UiO-type metal-organic frameworks-functionalized lanthanide-doped upconversion nanoparticles was proposed, wherein Cu2+ can make the fluorescence quenching of LMOFs and preferentially bind biothiols to recover fluorescence in different degrees forming unique fingerprinting. The fluorescence sensor array displayed an excellent pattern recognition for five biothiols (glutathione, homocysteine, N-acetylcysteine, and L/D-cysteine) even at 50 µM by linear discriminant analysis, and the discernment for the enantiomers of L/D-cysteine, as well as the accurate identification (90.0% accuracy) of biothiols in food samples (tea beverage and white wine). Such fluorescence sensor array might provide a simple and efficient detection method for biothiols.

Fruit waste-derived carbon dots with rhodamine B for the ratiometric detection of Fe3+ and Cu2

A green and eco-friendly solvothermal approach is proposed for the synthesis of carbon quantum dots (CQDs) from watermelon rind. The as-prepared CQDs exhibited superior teal fluorescence in aqueous solutions, with a quantum yield of 13.9%. The CQDs and rhodamine B (RhB) were demonstrated to selectively react with Fe3+ and Cu2+, leading to a fluorescence (FL) quenching effect, which was successfully used for constructing "double-response-off" type ratiometric FL probes. A comparative study was conducted to assess the sensitivity and accuracy of ratiometric fluorescent probes, specifically those based on CQDs alone and in combination with RhB, for the selective detection of Fe3+ and Cu2+. By plotting the ratio of the differential fluorescence (ΔF) signals of CQDs to that of RhB against the practical application analyte concentration, the detection limits for Fe3+ (1.75 μM) and Cu2+ (0.43 μM) were markedly improved. The quenching mechanism was further explored, and the detection of Fe3+ and Cu2+ in surface water was demonstrated, showcasing the potential of efficient and effective nanosensors based on a static quenching effect. Futhermore, the addition of ascorbic acid can restore the fluorescence quenched by Fe3+. Therefore, in the presence of copper and iron, the ratiometric probe can demonstrate the ability to identify two different metals.

Single-atom Fe nanozymes with excellent oxidase-like and laccase-like activity for colorimetric detection of ascorbic acid and hydroquinone

Although traditional Fe-based nanozymes have shown great potential, generally only a small proportion of the Fe atoms on the catalyst's surface are used. Herein, we synthesized single-atom Fe on N-doped graphene nanosheets (Fe-CNG) with high atom utilization efficiency and a unique coordination structure. Active oxygen species including superoxide radicals (O2•-) and singlet oxygen (1O2) were efficiently generated from the interaction of the Fe-CNG with dissolved oxygen in acidic conditions. The Fe-CNG nanozymes were found to display enhanced oxidase-like and laccase-like activity, with Vmax of 2.07 × 10-7 M∙S-1 and 4.54 × 10-8 M∙S-1 and Km of 0.324 mM and 0.082 mM, respectively, which is mainly due to Fe active centers coordinating with O and N atoms simultaneously. The oxidase-like performance of the Fe-CNG can be effectively inhibited by ascorbic acid (AA) or hydroquinone (HQ), which can directly obstruct the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB). Therefore, a direct and sensitive colorimetric method for the detection of AA and HQ activity was established, which exhibited good linear detection and limit of detection (LOD) of 0.048 μM and 0.025 μM, respectively. Moreover, a colorimetric method based on the Fe-CNG catalyst was fabricated for detecting the concentration of AA in vitamin C. Therefore, this work offers a new method for preparing a single-atom catalyst (SAC) nanozyme and a promising strategy for detecting AA and HQ.

A Dual-mode Ratiometric Fluorometric and Colorimetric Platform Based on Nitrogen-doped Carbon Dots and o-phenylenediamine for the Detection of Nitrite

In this study, a dual-mode ratiometric fluorometric and colorimetric platform for the determination of nitrite in pickles was proposed by exquisitely employing the fact that non-fluorescent o-Phenylenediamine (OPD) was oxidized by nitrite under acidic conditions to form fluorescent 2,3-diaminophenazine (DAP) (Em = 575), which meanwhile quench the fluorescent nitrogen-doped carbon dots (N-CDs) at 455 nm, the ratio of fluorescence intensity of DAP to N-CDs (F575/F455) changed with the increase of nitrite accompanied by visible color changes. Thus, nitrite can be quantitatively detected within a wide linear range (10-500 µM) with a low detection limit of 0.45 µM due to the high quantum yield of 39.7% of N-CDs. In addition, the colour of the N-CDs/OPD system changed from transparent to yellow when the nitrite was introduced, enabling colorimetric and on-site visual detection. The detection limit of the colorimetric method was 3.03 µM with a linear range of 10-500 µM. The proposed ratiometric fluorometric method has pleasant selectivity and good immunity to interference.

Using highly water-stable wool keratin/CsPbBr3 nanocrystals as a portable amine-responsive fluorescent test strip for onsite visual detection of food freshness

Perovskite nanocrystals (PNCs) have emerged as promising candidates for fluorescent probes owing to their outstanding photoelectric properties. However, the conventional CsPbBr3 (CPB) NCs are extremely unstable in water, which has seriously limited their sensing applications in water environment. Herein, we present a powerful ligand engineering strategy for fabricating highly water-stable CPB NCs by using a biopolymer of wool keratin (WK) as the passivator and the polyaryl polymethylene isocyanate (PAPI) as the cross-linking agent. In particular, WK with multi-functional groups can serve as a polydentate ligand to firmly passivate CPB NCs by the ligand exchange process in hot toluene; and then the addition of PAPI can further encapsulate CPB NCs by the crosslinking reaction between PAPI and WK. Consequently, the as-prepared CPB/WK-PAPI NCs can maintain ∼ 80 % of their relative photoluminescence (PL) intensity after 60 days in water, and they still maintain ∼ 40 % of their relative PL intensity even after 512 days in the same environment, which is one of the best water stabilities compared previously reported polymer passivation methods. As a proof-of their application, the portable CPB/WK-PAPI NCs-based test strips are further developed as a fluorescent nanoprobe for real-time and visual monitoring amines and food freshness. Among various amine analytes, the as-prepared test strips exhibit higher sensitivity towards conjugated amines, achieving a remarkable detection limit of 18.3 nM for pyrrole. Our research not only introduces an innovative strategy involving natural biopolymers to enhance the water stability of PNCs, but also highlights the promising potential of PNCs for visually and portably detecting amines and assessing food freshness.

Glutathione detection in water and milk using a new probe DCYP based on benzopyranonitrile

Glutathione (GSH) is a potent antioxidant, fragrance, and anti-browning agent in the field of food chemistry. The accurate GSH evaluation in food and vegetables is critical for instructing the right supplementation of GSH in body. However, most reported GSH fluorescent probes were utilized for the biological imaging. In this study, a new probe DCYP-GSH was developed by coupling of benzopyranonitrile as signal reporter to N-methylpyridine through C = C bond as binding site. Notably, a significant increase in fluorescence intensity and a λmax red-shift of DCYP-GSH in electron spectra were found as a result of the response to GSH. Quantitative detection of GSH in water and milk samples were achieved using probe DCYP-GSH. The development of DCYP-GSH was anticipated to provide an effective toolkit for food safe evaluation.

Aggregation-induced emission carbon dots as Al3+-mediated nanoaggregate probe for rapid and selective detection of tetracycline

Worldwide abuse of tetracycline (TC) seriously threatens environmental safety and human health. Metal-TC complexes formed by residual TC in the environment can also contribute to the spread of antibiotic resistance. Therefore, monitoring of TC residues is still required. Here, we report novel aggregation-induced emission carbon dots (AIE-Cdots) as nanoaggregate probes for the rapid and selective detection of TC residue. Riboflavin precursors with rotational functional groups led to the development of AIE-Cdots. The aggregation of AIE-Cdots was induced selectively for Al3+, amplifying the fluorescence signals owing to the restricted rotation of the side chains on the AIE-Cdot surface. The fluorescence signal of such Al3+-mediated nanoaggregates (Al3+-NAs) was further triggered by the structural fixation of TC at the Al3+ active sites, suggesting the formation of TC-coordinated Al3+-NAs. A linear correlation was observed in the TC concentration range of 0-10 μM with a detection limit of 42 nM. In addition, the strong Al3+ binding affinity of AIE-Cdots produced similar NAs and enhanced fluorescence signals in Al3+-TC mixtures. These AIE-Cdots-based nanoplatforms have a rapid response, good selectivity, and reliable accuracy for detecting TC or aluminum complexes, meeting the requirements for hazardous substance monitoring and removal in environmental applications.

Paper-based visualization of auramine O in food and drug samples with carbon dots-incorporated fluorescent microspheres as sensing element

A paper-based assay for visualization of auramine O (AO) was for the first time established by using CFMs as a ratiometric fluorescent probe (RFP). The CFMs were melamine formaldehyde microspheres (MFMs) incorporated with carbon dots (CDs), where the CDs species as sensing units and MFMs as a signal amplification carrier. The proposed RFP can quantitatively measure AO content from 0.0 to 10.0 μM and exhibited an ultralow limit of detection (LOD, 15.7 nM). In particular, obvious luminescence color change of CFMs from blue to green was perceived with naked-eyes and therefore, a solution-based and a paper-based visualization platform were respectively proposed for on-site visual detection of AO with LODs of 1.15 μM and 0.83 μM, separately. Finally, those fluorescence methods were adopted in sensitively quantitative measurement of AO within various food and drug samples, providing new prospects for analysts and technical support in food quality monitoring.

Ultrahighly Sensitive and Selective Glutathione Sensor Based on Carbon Dot-Functionalized Solution-Gate Graphene Transistor

A new type of carbon dot (CD)-functionalized solution-gated graphene transistor (SGGT) sensor was designed and fabricated for the highly sensitive and highly selective detection of glutathione (GSH). The CDs were synthesized via a one-step hydrothermal method using DL-thioctic acid and triethylenetetramine (TETA) as sources of S, N, and C. The CDs have abundant amino and carboxyl groups and were used to modify the surface of the gate electrode of SGGT as probes for detecting GSH. Remarkably, the CDs-SGGT sensor exhibited excellent selectivity and ultrahigh sensitivity to GSH, with an ultralow limit of detection (LOD) of up to 10-19 M. To the best of our knowledge, the sensor outperforms previously reported systems. Moreover, the CDs-SGGT sensor shows rapid detection and good stability. More importantly, the detection of GSH in artificial serum samples was successfully demonstrated.

MnO2 nanoparticles decorated with Ag/Au nanotags for label-based SERS determination of cellular glutathione

A novel stimulus-responsive surface-enhanced Raman scattering (SERS) nanoprobe has been developed for sensitive glutathione (GSH) detection based on manganese dioxide (MnO₂) core and silver/gold nanoparticles (Ag/Au NPs). The MnO₂ core is not only capable to act as a scaffold to amplify the SERS signal via producing "hot spots", but also can be degraded in the presence of the target and thus greatly enhance the nanoprobe sensitivity for sensing of GSH. This approach enables a wide linear range from 1 to 100 µM with a 2.95 µM (3σ/m) detection limit. Moreover, the developed SERS nanoprobe represents great possibility in both sensitive detection of intracellular GSH and even can monitor the change of intracellular GSH level when the stimulant occurs. This sensing system not merely offers a novel strategy for sensitive sensing of GSH, but also provides a new avenue for other biomolecules detection.

Light-responsive benzobisthiazole as oxidase mimic for rapid determination of glutathione in food and vegetable

Accurate determination of glutathione (GSH) in food and vegetable is significant to instruct the appropriate supplementation of GSH in the human body. Light-responsive enzyme mimics have been widely used in detecting GSH due to controllable temporal and spatial accuracy. However, exploring a potential organic mimic enzyme with excellent catalytic efficiency keeps challenging. Herein, a benzobisthiazole organic oxidase mimic was successfully prepared by a simple and low-cost method. Based on its high light-responsive oxidase-like activity, it was used for high reliable colorimetric determination of GSH in food and vegetable for only 1 min with a large linear range of 0.02-30 μM and a low detection limit of 5.3 nM. This study provides a novel strategy to obtain powerful light-responsive oxidase mimics and holds great potential for rapid and accurate detection of GSH in food and vegetables.

Plasmonic Cu2-xSe Mediated Colorimetric/Photothermal Dual-Readout Detection of Glutathione

Plasmonic nanomaterials have attracted great attention in the field of catalysis and sensing for their outstanding electrical and optical properties. Here, a representative type of nonstoichiometric Cu_2-xSe nanoparticles with typical near-infrared (NIR) localized surface plasma resonance (LSPR) properties originating from their copper deficiency was applied to catalyze the oxidation of colorless TMB into their blue product in the presence of H₂O₂, indicating they had good peroxidase-like activity. However, glutathione (GSH) inhibited the catalytic oxidation of TMB, as it can consume the reactive oxygen species. Meanwhile, it can induce the reduction of Cu(II) in Cu_2-xSe, resulting in a decrease in the degree of copper deficiency, which can lead to a reduction in the LSPR. Therefore, the catalytic ability and photothermal responses of Cu_2-xSe were decreased. Thus, in our work, a colorimetric/photothermal dual-readout array was developed for the detection of GSH. The linear calibration for GSH concentration was in the range of 1-50 μM with the LOD as 0.13 μM and 50-800 μM with the LOD as 39.27 μM. To evaluate the practicability of the assay, tomatoes and cucumbers were selected as real samples, and good recoveries indicated that the developed assay had great potential in real applications.

The synthesis of carbon dots by folic acid and utilized as sustainable probe and paper sensor for Hg2+ sensing and cellular imaging

In this work, the blue emission carbon dots (FA-CDs) are synthesized by one-pot solvothermal method by using folic acid as precursor. The FA-CDs emitted bright emission at 445 nm when excited at 360 nm with the QY of 31.2 %. The FA-CDs exhibit sensitive quenching response to Hg²⁺ with variable concentrations systematically, which determined FA-CDs can be employed as fluorescent probe, with a reliable linear relationship between fluorescence intensity and Hg²⁺ concentration, and a limit of detection (LOD) of 1.29 nM. Notably, the quenched FA-CDs can be recovered by using EDTA saturated solution with the emission comparable to initial in succession. The FA-CDs based paper sensor can be explored with similar detection performance, and it can also be restored by EDTA saturated solution. Both the restored CDs and paper sensor can be reused in the next turn for detecting Hg²⁺, which allowed the FA-CDs and their paper sensor can be serviced as sustainable probe for Hg²⁺ detection. The visual LOD of paper sensor can be determined at 0.1 μM, notably, the paper sensor can be reused at least 3 times with good performance, which is beneficial to environmental protection and saving resources. Possess excellent water solubility and non-toxic properties, the cellular imaging of FA-CDs was evaluated with excellent quality fluorescent image results. The FA-CDs provide a promising convenient fluorescent probe for multi-application in detection and imaging.

Fluorescent Sensing of Glutathione and Related Bio-Applications

Glutathione (GSH), as the most abundant low-molecular-weight biological thiol, plays significant roles in vivo. Abnormal GSH levels have been demonstrated to be related to the dysfunction of specific physiological activities and certain kinds of diseases. Therefore, the sensing of GSH is emerging as a critical issue. Cancer, with typical high morbidity and mortality, remains one of the most serious diseases to threaten public health. As it is clear that much more concentrated GSH is present at tumor sites than at normal sites, the in vivo sensing of GSH offers an option for the early diagnosis of cancer. Moreover, by monitoring the amounts of GSH in specific microenvironments, effective diagnosis of ROS levels, neurological diseases, or even stroke has been developed as well. In this review, we focus on the fluorescent methodologies for GSH detection, since they can be conveniently applied in living systems. First, the fluorescent sensing methods are introduced. Then, the principles for fluorescent sensing of GSH are discussed. In addition, the GSH-sensing-related biological applications are reviewed. Finally, the future opportunities in in the areas of fluorescent GSH sensing-in particular, fluorescent GSH-sensing-prompted disease diagnosis-are addressed.

A facile fluorescence platform for chromium and ascorbic acid detection based on "on-off-on" strategy

In this paper, a facile and rapid fluorescence "on-off-on" strategy for the detection of chromium (Cr(VI)) and ascorbic acid (AA) was developed, which was based on the water-soluble carbon dots (CDs). The CDs was synthesized by a microwave-assisted treatment of L-tartaric acid, citric acid, and urea. The CDs have many advantages, such as high fluorescence quantum yield (20.5%) and good fluorescence stability. Based on inner filter effect (IFE) and static quenching, the fluorescence of the CDs can be quenched by Cr(VI) quickly; while the reduction of IFE and reducing action can make the fluorescence of the CDs recover by AA efficiently. Moreover, under the optimal experimental conditions, the CDs had a good detection performance for Cr(VI) in the range of 0.8 ∼ 189 µM with the limit of detection (LOD) of 0.16 µM. The linear detection for AA was ranged from 0.43 to 25.7 µM with a LOD of 0.1 µM. More importantly, the as-constructed fluorescence detecting platform was successfully applied for Cr(VI) and AA detection in the environmental samples and fruit samples, respectively. In addition, the application potential of the CDs in fluorescent films and anti-counterfeiting materials was further discussed in detail. This work will provide a novel idea for designing a portable sensor based on the CDs to quickly and sensitively detect Cr(VI) and AA.

Iron porphyrin-based porous organic polymer with high peroxidase-like activity as colorimetric sensor for glutathione and ascorbic acid assay

A new iron porphyrin-based organic polymer (Fe-POP) was synthesized through the William ether reaction. The as-prepared Fe-POP presented high chemical stability, wide pore distribution, high iron content, and strong affinity with 3,3',5,5'-tetramethylbenzidine (TMB) and hydrogen peroxide (H₂O₂), which contributed to its excellent peroxidase-mimicking performance. In the presence of H₂O₂, Fe-POP could catalyze the transparent TMB into blue ox-TMB, which could be easily distinguished by the naked eyes. Moreover, glutathione (GSH) and ascorbic acid (AA) could convert blue ox-TMB into colorless TMB due to the inhibitory effect of GSH/AA to the catalytic oxidation of TMB. Based on this phenomenon, a rapid and sensitive colorimetric method for the assay of H₂O₂, GSH, and AA was developed using Fe-POP as sensor. The detection limits of H₂O₂, GSH, and AA  were 1.37, 0.44, and 0.33 μM, respectively. Finally, the colorimetric method based on Fe-POP was used to evaluate the GSH and AA content in real samples, which provided the guidance for GSH and AA supplements in our daily diet, suggesting the significant potential of Fe-POP in practical applications.

A smartphone-integrated dual-mode nanosensor based on Fe3O4@Au for rapid and highly selective detection of glutathione

A simple, rapid and straightforward method for detecting reduced glutathione (GSH) was developed supported on smartphone analysis software package and a peroxide simulated catalyst nanoparticles (Fe₃O₄@Au) system. The nanocomposite was prepared by self-assembling technique, and the characterization was carried out using transmission electron microscopy, Fourier transforms infrared, and X-ray diffractometer. Fe₃O₄@Au materials have catalyzed the oxidation of a typical colorimetric substrate in the presence of H₂O₂, with the color changes from colorless to green oxidized. A smartphone with a free self-developed app referred to as "Color Capture" was accustomed live the RGB (red-greenblue) values of color intensity within the Fe₃O₄@Au system and computationally convert them GSH concentrations. The smartphone detection system showed high property and sensitivity of GSH detection. It gave a constant correlation (R² = 0.9973) between the colour intensity of I and the GSH concentration, with a linear vary of 0-0.25 mmol/L, and a detection limit of 0.013 μmol/L. The results obtained were most consistent with the results obtained in ultraviolet spectrophotometry. The colorimetric system is based on smartphone analysis software developed to detect GSH in actual samples with potential application values.

Synthesis of a cerium-based nanomaterial with superior oxidase-like activity for colorimetric determination of glutathione in food samples

Enzyme-like nanomaterials have received significant attention for their high stability and low cost. However, most nanomaterials require complicated synthesis processes, limiting the range of their potential applications. In this study, a novel cerium-based nanomaterial was fabricated in a facile manner from a mixture of dipicolinic acid (DPA), guanosine 5'-monophosphate (GMP), and cerium acetate under ambient conditions. The obtained nanomaterial, designated as DPA-Ce-GMP, exhibited superior oxidase-like activity owing to the mixed valence (Ce³⁺/Ce⁴⁺) of cerium ions. DPA-Ce-GMP efficiently catalyzed the oxidation of 3,3,5,5-tetramethylbenzidine (TMB), achieving a color reaction without requiring hydrogen peroxide. Thus, DPA-Ce-GMP was incorporated into a simple, rapid, and sensitive colorimetric sensor for glutathione (GSH) detection. Within this sensor, TMB oxidation is inhibited by the reducibility of GSH. The sensor exhibits a linear response over two concentration ranges (0.05-10 and 10-40 μM), and its detection limit is 17.1 nM (3σ/slope). The proposed sensor was successfully applied to GSH quantification in food samples. The developed sensor provides an efficient biomimic oxidase for GSH detection in real samples. Facile approach to prepare cerium-based nanomaterial with superior oxidase-like activity for colorimetric detection of glutathione in food samples.

Chitosan and κ-carrageenan-derived nitrogen and sulfur co-doped carbon dots "on-off-on" fluorescent probe for sequential detection of Fe3+ and ascorbic acid

This study develops a high sensitive and selective "on-off-on" fluorescent probe for sequential detection of iron ion (Fe³⁺) and ascorbic acid (AA) based on nitrogen and sulfur co-doped carbon dots (N, S-CDs), which were synthesized by using chitosan and κ-carrageenan as raw materials through one-step hydrothermal protocol. The synthesized N,S-CDs possess particularly high quantum yield (QY = 59.31%), excellent stability and excitation dependent behavior, showing great potential for practical applications. Furthermore, N,S-CDs provided high selectivity and strong anti-interference to Fe³⁺ due to its fluorescence quenching performance, revealing a wide linear concentration range from 1 to 100 μM for the detection of Fe³⁺ ion with an extremely low limit of detection of 57 nM, and presented reliable and accurate results in actual sample detection of Fe³⁺. The overall fluorescence quenching mechanism of N,S-CDs with Fe³⁺ was due to the formation of N,S-CDs/Fe³⁺ initiated to the aggregation and electron transfer of N,S-CDs, resulting in the static quenching of fluorescence. More interestingly, AA could reduce Fe³⁺ to Fe²⁺ and efficaciously recover the quenched fluorescence of N,S-CDs/Fe³⁺. N,S-CDs/Fe³⁺ as "turn-on" fluorescent probe was further applied for detecting AA in a linear range of 0.5-90 μM with a detection limit of 38 nM.

Lysosome targeting, Cr(vi) and l-AA sensing, and cell imaging based on N-doped blue-fluorescence carbon dots

N-doped blue-fluorescence carbon dots (N-CDs) were fabricated via a one-pot hydrothermal method using folic acid and p-phenylenediamine. The obtained N-CDs exhibited strong fluorescence (FL) with a considerable quantum yield (QY) of 21.8% and exceptional optical stability under different conditions. Upon introducing Cr(vi), blue FL of N-CDs was distinctly quenched. On subsequent addition of l-AA, the FL of N-CDs could be partially recovered. The fluorescence changes of N-CDs have been utilized to detect Cr(vi) and l-AA in aqueous solutions with linear ranges of 0.10-150 μM and 0.75-2.25 mM, respectively, as well as limit of detection values of 9.4 nM and 25 μM, respectively. Furthermore, as-obtained N-CDs can be extended to monitor the fluctuation of intracellular Cr(vi) and l-AA. More intriguingly, N-CDs can target lysosomes with a satisfactory Pearson correction coefficient of 0.87, which indicates a promising application prospect in the biomedical field.

A novel electrochemiluminescence sensor based on resonance energy transfer from MoS2QDs@g-C3N4 to NH2-SiO2@PTCA for glutathione assay

In this work, a solid-state electrochemiluminescence (ECL) sensor based on resonance energy transfer (RET) was proposed using MoS2QDs@g-C3N4 as a donor and NH2-SiO2@PTCA as an acceptor. Herein, MoS2QDs could significantly facilitate the stability and efficiency of the ECL of g-C3N4. PTCA provided a large platform to anchor NH2-SiO2 nanoparticles. The prepared MoS2QDs@g-C3N4 exhibited good spectral overlap with the UV-vis absorption spectrum of NH2-SiO2@PTCA. Based on this, we designed an "off-on" ECL sensing strategy for sensitive and selective detection of glutathione (GSH). Under the best conditions, the linear range of the sensor for GSH detection was from 0.001 to 100 μM with a detection limit of 0.63 nM (S/N = 3). More importantly, GSH in commercial samples can be detected using the proposed sensor, which indicated its superior detection capabilities and potential application value in commercial medicines.

Highly Fluorescent N-Doped Carbon Quantum Dots Derived from Bamboo Stems for Selective Detection of Fe3+ Ions in Biological Systems

The establishment of sensing platform for trace analysis of Fe³⁺ in biological systems is meaningful for health monitoring. Herein, a Fe³⁺ sensitive fluorescent nanoprobe was constructed based on highly fluorescent N-doped carbon quantum dots (NCQDs) derived from bamboo stems through a hydrothermal method employing ethylenediamine as the nitrogen dopant. The prepared NCQDs had a uniformly distributed size and their mean size was around 2.43 nm. Abundant functional groups (C=N, N-H, C=O, and carboxyl) anchored on NCQDs demonstrated successful doping of N in CQDs. The obtained NCQDs possessed a high fluorescence quantum yield of 20.02% and outstanding fluorescence stability over a wide pH range and at high ionic strengths. Moreover, Fe³⁺ ions presented a specific fluorescent quenching effect to the as-prepared NCQDs. The calibration curve for fluorescence quenching degree corresponding to Fe³⁺ concentration showed a linear response in a range of 0.01-10 µM, and detection limit was 0.486 µM, which indicated that the NCQDs had high sensitivity to Fe³⁺ ions. Ascribed to these unique properties, the NCQDs were selected as luminescent probes for trace amount of Fe³⁺ ions in human serum. These results demonstrated their promising use in clinical diagnostics and other biologically relevant studies.

Development of an ultrasensitive spectrophotometric method for carmine determination based on fluorescent carbon dots

A high-efficiency spectrophotometric method based on nitrogen-doped fluorescent carbon dots (N-FCDs) was developed for the ultrasensitive determination of carmine (CRM) in foodstuffs. The N-FCDs were fabricated via a one-pot hydrothermal method with m-phenylenediamine as the starting material. The detection principle was based on the fluorescence quenching effect of N-FCDs by CRM, where their interaction was due to the inner filter effect (IFE) and static quenching. A good linear relationship was established for CRM detection in a concentration range of 0.1-10.0 μM with a detection limit as low as 11.2 nM. The proposed method achieved satisfactory results for CRM determination in commercial food products with recoveries better than 98.6% and relative standard deviations (RSDs) less than 4.07%. The method established in this study was simple, ultrasensitive and reliable for rapid detecting CRM in a food matrix, which could be potentially used as a useful sensing agent for the analysis of additive food colourants.

Development of a highly sensitive fluorescence method for tartrazine determination in food matrices based on carbon dots

In this work, an ultrasensitive sensing system based on fluorescent carbon dots (CDs) was developed for the tartrazine (Tar) determination. The CDs were prepared via a simple one-pot hydrothermal method with m-phenylenediamine as the only precursor. The physical and chemical properties were in detail characterized by transmission electron microscopy (TEM), MALDI-TOF MS, UV-vis absorption and photoluminescence (PL) spectroscopy, elemental analysis, and Fourier transform infrared spectroscopy (FTIR). Upon exposure to Tar, the fluorescence of CDs was efficiently quenched via the dynamic interaction between CDs and Tar as well as the inner filter effect (IFE). With this information, the CDs were proposed as a fluorescence probe for Tar detection. It was found that CDs had high sensitivity and selectivity for Tar sensing, and the linear relationship was observed in the range of 0.01-25.0 μM with the corresponding detection limit (3σ/k) of 12.4 nM, which is much more sensitive than any of the existed CD-based sensing platform. The investigated sensing system was finally utilized for Tar sensing in various food matrices with a high degree of accuracy. The spiked recoveries were in a range of 96.4-105.2%, and the relative standard deviations (RSDs) were lower than 4.13%. This work highlights the great application prospects of CDs for Tar sensing in a rapid, simple, and sensitive way.

Yellow emission N-doped fluorescent carbon dots as fluorescent nanoprobes for the detection of L-threonine in real samples

High quantum yield carbon dots and their applications in the detection of L-threonine.

Orange emissive carbon nanodots for fluorescent and colorimetric bimodal discrimination of Cu2+ and pH

We have facilely synthesized orange emissive carbon nanodots (O-CDs) via a hydrothermal method using citric acid and 5-aminosalicylic acid.

A one-pot synthesis of fluorescent N,P-codoped carbon dots for vitamin B12 determination and bioimaging application

N,P-codoped carbon dots were synthesised using l-arginine and phosphoric acid and explored for the detection of vitamin B₁₂ (VB₁₂) and bioimaging.

A water-stable zinc(ii)–organic framework as an “on–off–on” fluorescent sensor for detection of Fe3+ and reduced glutathione

A zinc(ii) metal–organic framework exhibits fluorescence on–off–on behaviour for Fe³⁺ and reduced glutathione in PBS solution and in real samples.

Deep eutectic solvent assisted synthesis of carbon dots using Sophora flavescens Aiton modified with polyethyleneimine: Application in myricetin sensing and cell imaging

Here, an efficient method for synthesizing carbon dots (CDs) using a deep eutectic solvent (DES) was developed. To investigate the influence of different DESs on the quantum yield of CDs, different hydrogen-bonding acceptors (HBAs) and hydrogen-bonding donors (HBDs) were used to synthesize the DES and prepare CDs. Using Sophora flavescens Aiton as precursor, CDs were prepared using choline chloride (ChCl)/urea based DES as reaction media and doping agent in the presence of water. The CDs showed strong blue fluorescence and were further modified with polyethyleneimine (CDs@PEI). The fluorescence intensity of CDs@PEI was selectively quenched by myricetin with a limit of detection (LOD) of 10 nM. Furthermore, CDs@PEI was used to analyze myricetin in the extracts that were fluorescent by DES with satisfactory performance of Abelmoschus manihot (Linn.) Medicus flowers, vine teas and blueberries. Finally, the bio-imaging application of CDs@PEI was tested and the results confirmed its potential application in bio-imaging.

Near-infrared emission Cu, N-doped carbon dots for human umbilical vein endothelial cell labeling and their biocompatibility in vitro

Quantum dots (QDs) are luminescent semiconductor nanomaterials (NMs) with various biomedical applications, but the high toxicity associated with traditional QDs, such as Cd-based QDs, limits their uses in biomedicine. As such, the development of biocompatible metal-free QDs has gained extensive research interests. In this study, we synthesized near-infrared emission Cu, N-doped carbon dots (CDs) with optimal emission at 640 nm and a fluorescence quantum yield of 27.1% (in N,N-dimethylformamide [DMF]) by solvothermal method using o-phenylenediamine and copper acetate monohydrate. We thoroughly characterized the CDs and showed that they were highly fluorescent and stable under different conditions, although in highly acidic (pH = 1-2) or alkaline (pH = 12-13) solutions, a redshift or blueshift of fluorescence emission peak of Cu, N-doped CDs was also observed. When exposed to human umbilical vein endothelial cells (HUVECs), Cu, N-doped CDs only significantly induced cytotoxicity at very high concentrations (100 or 200 μg/ml), but their cytotoxicity appeared to be comparable with carbon black (CB) nanoparticles (NPs) at the same mass concentrations. As the mechanisms, 200 μg/ml Cu, N-doped CDs and CB NPs promoted endoplasmic reticulum (ER) stress proteins IRE1α and chop, leading to increased cleaved caspase 3/pro-caspase 3 ratio, but CB NPs were more effective. At noncytotoxic concentration (50 μg/ml), Cu, N-doped CDs successfully labeled HUVECs. In summary, we successfully prepared highly fluorescent and relatively biocompatible CDs to label HUVECs in vitro.