Carbon quantum dots originated from chicken blood as peroxidase mimics for colorimetric detection of biothiols

Carbon-based materials from waste PVC/iron chips dechlorination as peroxidase mimics for total antioxidant capacity biosensing

The monitoring of antioxidant capacity is very important to evaluate the quality of antioxidant foods or drugs for market regulation. Herein, dechlorination treatment of waste PVC/scrap irons were conducted in subcritical water to obtain carbon-based Fe composites (CM-Fe-dPVC) with peroxidase-like activity. The electron bonding of C 2p and Fe 3d orbital led to strong electron migration ability. CM-Fe-dPVC exhibited excellent activity of simulated peroxidase. Vitamin C (VC) and CM-Fe-dPVC had competitive behaviors on •OH generation in TMB oxidation reaction. A portable paper based colorimetric test kit was developed for monitoring total antioxidant capacity of beverages and pharmaceuticals on the market (with the detection limit of 0.1 μM for Vc). The results of life cycle assessment (LCA) revealed that the proposed strategy had low global warming potential. This research could provide important reference for high value recycling of organic solid wastes.

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.

A nanozyme with switchable enzyme-like activity for the logic gates detection of thymol and hydrogen peroxide in honey

A new nanozyme (CuGaa) with switchable enzyme-like activity of peroxidase and polyphenol oxidase was successfully prepared based on guanidinoacetic acid and copper. The two enzyme-like activities can be easily switched by changing temperature or adding MnCl2. At 4 °C, polyphenol oxidase-like activity decreased to nearly 1%, and the material is mainly characterized by peroxidase-like activity at this point. However, at 60 °C in the presence of 20 mM MnCl2, the peroxidase-like activity decreased to nearly 10%, and the polyphenol oxidase-like activity of the materials increased to 140%. Based on the switchable enzyme-like activity of CuGaa, detection methods for thymol and hydrogen peroxide were developed. In addition, a rapid combination strategy was further established combined with logic gate technology for the facile identification of complex contamination in honey, which provided new ideas for low-cost and rapid honey identification.

Integration of Myrica rubra-based N-doped carbon dots with Fe3S4 as excellent peroxidase mimics for colorimetric assay and smartphone-based intelligent sensing of p-aminophenol in waters

To realize the reutilization of waste Myrica rubra in the analytical field, we synthesized Myrica rubra-based N-doped carbon dots (MN-CDs) and further anchored them onto the surface of Fe3S4 to fabricate Fe3S4@MN-CD nanocomposites. The as-fabricated nanocomposites possessed higher peroxidase-mimetic activity than its two precursors, resulting from the synergistic effect between them, and could catalyze colorless 3,3',5,5'-tetramethylbenzidine (TMB) into deep blue oxTMB with a strong 652-nm absorption. Under optimized conditions (initial solution pH, 3.5; incubation temperature, 35 ℃; Fe3S4@MN-CD concentration, 50 µg mL-1, and 652-nm absorption), Fe3S4@MN-CDs were employed for colorimetric assay of p-aminophenol (p-AP) with wide linear range (LR, 2.9-100 µM), low detection limit (LOD, 0.87 µM), and satisfactory recoveries (86.3-105%) in environmental waters. Encouragingly, this colorimetric assay provided the relative accuracy of 97.0-99.4% as compared with  conventional HPLC-UV detection. A portable smartphone-based colorimetric application was developed by combining the Fe3S4@MN-CD-based visually chromogenic reaction with a "Thing Identify" APP software. Besides, we engineered an image-capturing device feasible for field use, in which the internal-compact sealing prevented external light source from entering photography chamber, thereby reducing light interference, and also the bottom light source enhanced the intensity of blue imaging. This colorimetric platform exhibited satisfactory LR (1-500 µM), low LOD (0.3 µM), and fortification recoveries (86.6-99.6%). In the chromogenic reaction catalyzed by Fe3S4@MN-CDs, ·O2- played a key role in concomitant with the participation of •OH and h+. Both the colorimetric assay and smartphone-based intelligent sensing show great promising in on-site monitoring of p-AP under field conditions.

Tumor Microenvironment-Induced Drug Depository for Persistent Antitumor Chemotherapy and Immune Activation

Herein, a drug-loading nanosystem that can in situ form drug depository for persistent antitumor chemotherapy and immune regulation is designed and built. The system (DOX@MIL-LOX@AL) is fabricated by packaging alginate on the surface of Doxorubicin (DOX) and lactate oxidase (LOX) loaded MIL-101(Fe)-NH2 nanoparticle, which can easily aggregate in the tumor microenvironment through the cross-linking with intratumoral Ca2+. Benefiting from the tumor retention ability, the fast-formed drug depository will continuously release DOX and Fe ions through the ATP-triggered slow degradation, thus realizing persistent antitumor chemotherapy and immune regulation. Meanwhile, LOX in the non-aggregated nanoparticles is able to convert the lactic acid to H2O2, which will be subsequently decomposed into ·OH by Fe ions to further enhance the DOX-induced immunogenic death effect of tumor cells. Together, with the effective consumption of immunosuppressive lactic acid, long-term chemotherapy, and oxidation therapy, DOX@MIL-LOX@AL can execute high-performance antitumor chemotherapy and immune activation with only one subcutaneous administration.

Turning food waste into value-added carbon dots for sustainable food packaging application: A review

Carbon dots (CDs) are a recent addition to the nanocarbon family, encompassing both crystalline and amorphous phases. They have sparked significant research interest due to their unique electrical and optical properties, remarkable biocompatibility, outstanding mechanical characteristics, customizable surface chemistry, and negligible cytotoxicity. Their current applications are mainly limited to flexible photonic and biomedical devices, but they have also garnered attention for their potential use in intelligent packaging. The conversion of food waste into CDs further contributes to the concept of the circular economy. It provides a comprehensive overview of emerging green technologies, energy-saving reactions, and cost-effective starting materials involved in the synthesis of CDs. It also highlights the unique properties of biomass-derived CDs, focusing on their structural performance, cellular toxicity, and functional characteristics. The application of CDs in the food industry, including food packaging, is summarized in a concise manner. This paper sheds light on the current challenges and prospects of utilizing CDs in the packaging industry. It aims to provide researchers with a roadmap to tailor the properties of CDs to suit specific applications in the food industry, particularly in food packaging.

Development and application of naturally derived, cost-effective CQDs with cancer targeting potential

Carbon quantum dots (CQDs) derived from natural sources have obtained potential interest in biomedical imaging and therapy because of their excellent biocompatibility properties, which include water solubility, simple synthesis and low cytotoxicity. Here the cytotoxicity of ethylene-diamine doped carbon quantum dots (N-CQDs) delivered to breast cancer MCF-7 cells was investigated. Folic acid was used to raise folate recognition and increase FA-NCQD accumulation in the cells, then apoptosis was assayed using nuclear fragmentation, acridine orange labeling, fluorescence imaging, flow cytometry, and caspase 3 expression. The data show that functionalization of these CQDs, derived from a natural source, have potential application in eliminating cancer cells, as shown here for the invasive breast cancer cells, MCF-7. This nano-delivery system provides a novel target therapy possibility therapeutic approach for cancer cells.

Review on Carbon Dot-Based Fluorescent Detection of Biothiols

Biothiols, such as cysteine (Cys), homocysteine (Hcy), and glutathione (GSH), play a vital role in gene expression, maintaining redox homeostasis, reducing damages caused by free radicals/toxins, etc. Likewise, abnormal levels of biothiols can lead to severe diseases, such as Alzheimer's disease (AD), neurotoxicity, hair depigmentation, liver/skin damage, etc. To quantify the biothiols in a biological system, numerous low-toxic probes, such as fluorescent quantum dots, emissive organic probes, composited nanomaterials, etc., have been reported with real-time applications. Among these fluorescent probes, carbon-dots (CDs) have become attractive for biothiols quantification because of advantages of easy synthesis, nano-size, crystalline properties, low-toxicity, and real-time applicability. A CDs-based biothiols assay can be achieved by fluorescent "Turn-On" and "Turn-Off" responses via direct binding, metal complex-mediated detection, composite enhanced interaction, reaction-based reports, and so forth. To date, the availability of a review focused on fluorescent CDs-based biothiols detection with information on recent trends, mechanistic aspects, linear ranges, LODs, and real applications is lacking, which allows us to deliver this comprehensive review. This review delivers valuable information on reported carbon-dots-based biothiols assays, the underlying mechanism, their applications, probe/CDs selection, sensory requirement, merits, limitations, and future scopes.

A novel colorimetric detection of glutathione based on stable free radical TEMPO oxidation of 3,3',5,5'-tetramethylbenzizine (TMB) via Copper(II) acetylacetonate catalysis

In this work, a new colorimetric method for the determination of Glutathione (GSH) on the basis of stable free radical 2,2,6,6 - tetramethylpiperidine - 1 - oxyl (TEMPO) oxidation of 3,3',5,5'-tetramethylbenzizine (TMB) via copper(II) acetylacetonate (Cu(acac)₂) catalysis was proposed. TEMPO was catalyzed by Cu(acac)₂ to produce TEMPO⁺, then TEMPO⁺ oxidized TMB to produce oxidized TMB (ox - TMB). The resulting ox - TMB showed blue and possessed a distinct absorption peak about 650 nm. Whereas, GSH prohibited the generation of ox - TMB through inhibiting TMB oxidation. As compared to the case that GSH was absent, significantly enhanced absorption was determined, and was proportional to GSH amount. On this basis, a qualitative and quantitative detection method of GSH with the naked eye and the microplate reader was achieved. The developed TEMPO - based method achieved GSH biosensing with improved sensitivity in a good specificity - manner. The limit of detection (LOD) was 90 μM via naked eye, and the microplate reader was 4.71 μM. And the stable free radical TEMPO possessed higher stability and lower toxicity than traditional oxidant of H₂O₂. Moreover, this TEMPO - based method achieved good results in the detection of GSH in human serums.

Fighting Non-Small Lung Cancer Cells Using Optimal Functionalization of Targeted Carbon Quantum Dots Derived from Natural Sources Might Provide Potential Therapeutic and Cancer Bio Image Strategies

Non-small cell lung cancer (NSCLC) is an important sub-type of lung cancer associated with poor diagnosis and therapy. Innovative multi-functional systems are urgently needed to overcome the invasiveness of NSCLC. Carbon quantum dots (CQDs) derived from natural sources have received interest for their potential in medical bio-imaging due to their unique properties, which are characterized by their water solubility, biocompatibility, simple synthesis, and low cytotoxicity. In the current study, ethylene-diamine doped CQDs enhanced their cytotoxicity (98 ± 0.4%, 97 ± 0.38%, 95.8 ± 0.15%, 86 ± 0.15%, 12.5 ± 0.14%) compared to CQDs alone (99 ± 0.2%, 98 ± 1.7%, 96 ± 0.8%, 93 ± 0.38%, 91 ± 1.3%) at serial concentrations (0.1, 1, 10, 100, 1000 μg/mL). In order to increase their location in a specific tumor site, folic acid was used to raise their functional folate recognition. The apoptotic feature of A549 lung cells exposed to N-CQDs and FA-NCQDs was characterized by a light orange-red color under fluorescence microscopy. Additionally, much nuclear fragmentation and condensation were seen. Flow cytometry results showed that the percentage of cells in late apoptosis and necrosis increased significantly in treated cells to (19.7 ± 0.03%), (27.6 ± 0.06%) compared to untreated cells (4.6 ± 0.02%), (3.5 ± 0.02%), respectively. Additionally, cell cycle arrest showed a strong reduction in cell numbers in the S phase (14 ± 0.9%) compared to untreated cells (29 ± 0.5%). Caspase-3 levels were increased significantly in A549 exposed to N-CQDs (2.67 ± 0.2 ng/mL) and FA-NCQDs (3.43 ± 0.05 ng/mL) compared to untreated cells (0.34 ± 0.04 ng/mL). The functionalization of CQDs derived from natural sources has proven their potential application to fight off non-small lung cancer.

Enzyme-free colorimetric detection of biothiols based on the photoinduced oxidation of 3,3',5,5'-tetramethylbenzidine

Realizing the rapid and on-site detection of biothiols in complex biological and food samples using simple assays and devices remains a major challenge. In this study, biothiols containing sulfhydryl groups were found to be able to inhibit the photo-triggered oxidation of 3,3',5,5'-tetramethylbenzidine (TMB). Based on the discovery, using the commercially available and low-cost TMB as the chromogenic substrate, an enzyme-free colorimetric approach was developed for the rapid determination of biothiols. The method does not involve the introduction of any natural enzymes, nanoenzymes, and external oxidants. The mechanisms of the photoinduced oxidation of TMB and the detection of biothiols were proposed. Furthermore, a smartphone-based portable device integrated with test strips was constructed by the 3D printing technique. This device can simultaneously meet the requirements of the photocatalytic oxidation reaction of TMB and the detection of biothiols. The entire process only takes less than 5 min. The successful detection of cysteine in urine and milk samples demonstrates the great potential of the device in the on-site assays.

Investigation on the Peroxidase-like Activity of Vitamin B6 and Its Applications in Colorimetric Detection of Hydrogen Peroxide and Total Antioxidant Capacity Evaluation

The peroxidase-like activity of vitamin B6 (VB6) was firstly demonstrated by catalyzing the peroxidase chromogenic substrate 3,3',5,5'-tetramethylbenzidine (TMB) at the existence of H2O2. The influence of different factors on the catalytic property of VB6, including pH, temperature, VB6 concentration, and incubation time, were investigated. The steady-state kinetic study results indicate that VB6 possesses higher affinity to H2O2 than natural horseradish peroxidase and some other peroxidase mimics. Besides, the radical quenching experiment results confirm that hydroxyl radical (•OH) accounts for the catalytic process. Based on the excellent peroxidase-like catalytic activity of VB6, the colorimetric methods for H2O2 and gallic acid (GA) detection were developed by measuring the absorbance variance of the catalytic system. Under the optimal conditions, the linear ranges of the methods for H2O2 and GA determination with good selectivity are 50.0-600.0 μM and 10.0-50.0 μM, respectively. In addition, the developed method was applied in the detection of H2O2 in milk samples and evaluation of total antioxidant capacity of different tea infusions. This study may broaden the application prospect of VB6 in environmental and biomedical analysis fields, contribute to profound insight of the physiological functions of VB6, as well as lay foundation for further excavation of small-molecule peroxidase mimics.

Determination of catechin and glutathione using copper aspartate nanofibers with multiple enzyme-like activities

Copper aspartate nanofibers were facilely prepared based on aspartic acid and copper (CuAsp nanofibers). It is found that the prepared CuAsp nanofibers have catalytic activities of five enzymes, including peroxidase, laccase, catalase, ascorbate oxidase, and superoxide dismutase mimetic activities. The kinetic and catalytic properties of CuAsp nanofibers were systematically investigated, showing their high catalytic activity, excellent stability, and reusability. The laccase mimetic activity of nanofibers could be used to detect catechin in the range 20-1200 µM with a detection limit of 5.88 µM. In addition, a sensing platform for glutathione with a detection limit of 0.25 µM and a detection range of 1-50 µM was established based on CuAsp nanofibers which have the peroxidase-mimicking activity. The sensor had good selectivity and could detect glutathione in actual samples of human serum. Therefore, CuAsp nanofibers with multi-enzyme activity have broad application prospects such as biosensing, environmental management, and disease diagnosis.

Carbon dots synthesized from microorganisms and food by-products: active and smart food packaging applications

Nanotechnology is rapidly becoming a commercial reality for application in food packaging. In particular, the incorporation of nanoparticles into packaging materials is being used to increase the shelf life and safety of foods. Carbon dots (C-dots) have a diverse range of potential applications in food packaging. They can be synthesized from environmentally friendly sources such as microorganisms, food by-products, and waste streams, or they may be generated in foods during normal processing operations, such as cooking. These processes often produce nitrogen- and sulfur-rich heteroatom-doped C-dots, which are beneficial for certain applications. The incorporation of C-dots into food packaging materials can improve their mechanical, barrier, and preservative properties. Indeed, C-dots have been used as antioxidant, antimicrobial, photoluminescent, and UV-light blocker additives in food packaging materials to reduce the chemical deterioration and inhibit the growth of pathogenic and spoilage microorganisms in foods. This article reviews recent progress on the synthesis of C-dots from microorganisms and food by-products of animal origin. It then highlights their potential application for the development of active and intelligent food packaging materials. Finally, a discussion of current challenges and future trends is given.

Hollow POM@MOF-derived Porous NiMo6 @Co3 O4 for Biothiol Colorimetric Detection

Developing highly active and sensitive peroxidase mimics for _L -cysteine (_L -Cys) colorimetric detection is very important for biotechnology and medical diagnosis. Herein, polyoxometalate-doped porous Co₃ O₄ composite (NiMo₆ @Co₃ O₄ ) was designed and prepared for the first time. Compared with pure and commercial Co₃ O₄ , NiMo₆ @Co₃ O₄ (n) composites exhibit the enhanced peroxidase-mimicking activities and stabilities due to the strong synergistic effect between porous Co₃ O₄ and multi-electron NiMo₆ clusters. Moreover, the peroxidase-mimicking activities of NiMo₆ @Co₃ O₄ (n) composites are heavily dependent on the doping mass of NiMo₆ , and the optimized NiMo₆ @Co₃ O₄ (2) exhibits the superlative peroxidase-mimicking activity. More importantly, a sensitive _L -Cys colorimetric detection is developed with the sensitivity of 0.023 μM^-1 and the detection limit at least 0.018 μM in the linear range of 1-20 μM, which is by far the best enzyme-mimetic performances, to the best our knowledge.

Dual-signal uric acid sensing based on carbon quantum dots and o-phenylenediamine

Fluorescent carbon quantum dots (CQDs), which showed excitation-dependent emission characteristics, were prepared using a facile hydrothermal method. The structure and optical properties of CQDs were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, UV-Vis spectroscopy, and fluorescence spectroscopy. These CQDs also showed peroxidase-like activity and could catalyze the H₂O₂-mediated oxidation of o-phenylenediamine (OPD) to form 2,3-diaminophenazine (DAP) with an absorption peak at 420 nm. DAP exhibited an obvious fluorescence emission at 550 nm under the excitation of 360 nm. On the other hand, it decreased the fluorescence of CQDs at 450 nm via inner filter effect. The experimental results indicated that the H₂O₂ concentration affected the color of DAP and the fluorescence intensity of CQDs and DAP. Thus, a colorimetric and ratiometric fluorescence dual-signal method was established for measuring the concentrations of H₂O₂ and uric acid (UA). The effects of pH, incubation temperature, incubation time, and OPD concentration on the response were investigated. Under the conditions of pH 7.5, temperature 50 °C, incubation time 30 min, and OPD 1.5 mM, the absorbance and fluorescence intensity ratio responses were linearly dependent on UA concentration ranging from 5.0 μM to 100 μM. The limits of detection were 0.7 and 0.5 μM with a colorimetric method and ratiometric fluorescence method, respectively. More importantly, this dual responsive method has been applied to the determination of UA in urine samples with satisfactory results.

Recent Developments in Carbon Quantum Dots: Properties, Fabrication Techniques, and Bio-Applications

Carbon dots have gained tremendous interest attributable to their unique features. Two approaches are involved in the fabrication of quantum dots (Top-down and Bottom-up). Most of the synthesis methods are usually multistep, required harsh conditions, and costly carbon sources that may have a toxic effect, therefore green synthesis is more preferable. Herein, the current review presents the green synthesis of carbon quantum dots (CQDs) and graphene quantum dots (GQDs) that having a wide range of potential applications in bio-sensing, cellular imaging, and drug delivery. However, some drawbacks and limitations are still unclear. Other biomedical and biotechnological applications are also highlighted.

Nanozyme based on CoFe2O4 modified with MoS2 for colorimetric determination of cysteine and glutathione

A nanozyme based on CoFe₂O₄ modified with MoS₂ was constructed for colorimetric determination of cysteine (Cys) and glutathione (GSH). Firstly, ferrite CoFe₂O₄ is synthesized, and it is then modified by MoS₂ to form a flower-like polymer (MoS₂@CoFe₂O₄). In the presence of H₂O₂, a redox interaction takes place, and the resulting hydroxyl promoted a colorimetric conversion from colorless to blue in the presence of 3,3',5,5'-tetramethylbenzidine (TMB). However, once Cys or GSH is added, they are capable to compete with the interaction of the hydroxyl with TMB, resulting in an inhibition of the colorimetric conversion. The colorimetric distinction is sensitive to the amount of target. The results obtained proved that the catalytic efficiency of MoS₂@CoFe₂O₄ is 4.4-fold and 1.8-fold to that of MoS₂ and CoFe₂O₄. Meanwhile, the K_m values to TMB and H₂O₂ are 0.067 and 0.048 mM, respectively, which are 6.5-fold and 77-fold, respectively smaller than those of natural peroxidase such as HPR. This indicates that the MoS₂@CoFe₂O₄ possesses a favorable interaction affinity. Additionally, the colorimetric distinction caused by the competition between TMB and cysteine or glutathione is obvious. The signal responses to cysteine and glutathione are linear in the range 0.5~15 μM and 0.5~35 μM, and the LODs are 0.10 and 0.21 μM, respectively. In practical assay of Cys in serum, the RSD of the sample tests is 4.6%, and the recoveries for the spiked assays are 95.3% and 96.0% with the RSD of 2.1% and 4.2%, respectively.

A porous organic polymer nanosphere-based fluorescent biosensing platform for simultaneous detection of multiplexed DNA via electrostatic attraction and π–π stacking interactions

One key challenge in oligonucleotide sequence sensing is to achieve multiplexed DNA detection in one sensor. Herein, a simple and efficient fluorescent biosensing platform is constructed to simultaneously detect multiplexed DNA depending on porous organic polymer (POP) nanospheres. The developed sensor is based on the concept that the POP nanospheres can efficiently quench the fluorescence emission of dye-labeled single-stranded DNA (ssDNA). Fluorescence quenching is achieved by the non-covalent assembly of multiple probes on the surface of POP nanospheres through electrostatic attraction and π–π stacking interactions, in which the electrostatic attraction plays a more critical role than π–π stacking. The formed dsDNA could be released off the surface of POP via hybridizing with the target DNA. Consequently, the target DNA can be quickly detected by fluorescence recovery. The biosensor could sensitively and specifically identify three target DNAs in the range of 0.1 to 36 nM, and the lowest detection limits are 50 pM, 100 pM, and 50 pM, respectively. It is noteworthy that the proposed platform is successfully applied to detect DNA in human serum. We perceive that the proposed sensing system represents a simple and sensitive strategy towards simultaneous and multiplexed assays for DNA monitoring and early clinical diagnosis.

This communication reports a simple and efficient fluorescent biosensing platform to simultaneously detect multiplexed DNA depending on porous organic polymer (POP) nanospheres by electrostatic attraction and π–π stacking interaction.

Nitrogen-Doped Carbon Quantum Dots from Poly(ethyleneimine) for Optical Dual-Mode Determination of Cu2+ and l-Cysteine and Their Logic Gate Operation

In this work, nitrogen-doped carbon quantum dots from poly(ethyleneimine) (PQDs) were synthesized by a low-cost and facile one-step hydrothermal method without other reagents. A quantum yield (QY) of up to 23.2% with maximum emission at 460 nm under an excitation wavelength of 340 nm was ascribed to the high nitrogen doping (20.59%). The PQDs selectively form a blue complex with Cu²⁺ accompanied by strong quenching of the fluorescence emission. Meanwhile, the PQD-Cu²⁺ complex exhibited selective fluorescence recovery and color disappearance on exposure to l-cysteine (Cys). The electron transfer from amino or oxygen groups on the PQDs to Cu²⁺ leads to fluorescence quenching, and a chromogenic reaction of the cuprammonium complex results in a color change. The strong affinity between Cys and Cu²⁺ causes the detachment of Cu²⁺ from the surface of PQDs, so the color of the solution disappears and the fluorescence of PQDs recovers. Under the optimized condition, the proposed sensor was applied to detect Cu²⁺ in the linear range of 0-280 μM. A detection limit of 4.75 μM is achieved using fluorescence spectroscopy and 4.74 μM by monitoring the absorbance variation at 272 nm. For Cys detection, the linear range of 0-800 μM with detection limits of 28.11 μM (fluorescence determination) and 19.74 μM (peak shift determination at 272 nm) was obtained. Meanwhile, the PQD-Cu²⁺ system exhibits distinguishable responses to other biothiols such as l-glutathione (GSH) and dl-homocysteine (Hcy). Based on the multimode signals, an "AND" logic gate was constructed successfully. Interestingly, besides Cu²⁺, Fe³⁺ can also quench the fluorescence of PQDs and the PQD-Fe³⁺ system exhibits superior selectivity for Cys detection. Most importantly, the proposed assay is not only simple, cheap, and stable but also suitable for detecting Cu²⁺ and Cys in some real samples.

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.

A fluorescein-gold nanoparticles probe based on inner filter effect and aggregation for sensing of biothiols

Cysteine (Cys), homocysteine (HCys) and glutathione (GSH) are sulfhydryl-containing amino acids known as biothiols being able to bind to gold nanoparticles (AuNPs) via sulfhydryl group, resulting in the aggregation of AuNPs. Owning to their inner filter effect, AuNPs can weaken or even quench the fluorescence of fluorescein. However, the introduction of biothiols to fluorescein-AuNPs leads to the recovery of fluorescein fluorescence. Thus, a simple and reliable turn on fluorescence method was developed for monitoring biothiols with fluorescein-AuNPs as a probe. Several factors, including AuNPs concentration, pH value and incubation time, which might influence the fluorescence reclamation of fluorescein-AuNPs probe, were optimized by taking Cys as an example at room temperature. Under the optimal conditions, sensitive sensing of Cys, HCys and GSH was achieved. The detection limits for Cys, GSH, and HCys were 0.027, 0.023, and 0.030μΜ, respectively. This method was used to the determination of Cys in human serum samples with high precision and accuracy, indicating the potential of the method in practical applications with simple operation, good accuracy and high sensitivity.

One-pot synthesis of luminol–gallium nanoassemblies and their peroxidase-mimetic activity for colorimetric detection of pyrophosphate

Luminol–Ga nanoassemblies exhibit peroxidase-mimetic activity. Colorimetric detection of PPi is developed owing to the formation of a complex between PPi and Ga³⁺.