A dual-mode nanosensor based on carbon quantum dots and gold nanoparticles for discriminative detection of glutathione in human plasma

Synthesis of a metal-organic framework Cu-Mi-UiO-66-based fluorescent nanoprobe for the simultaneous sensing and intracellular imaging of GSH and ATP

This study reports a fluorescent nanoprobe operated in fluorescence turn-on mode for simultaneously sensing and imaging intracellular GSH and ATP. By using maleimide-derivatives as the ligand, the bimetallic nanoscale metal-organic framework (NMOF) Cu-Mi-UiO-66 has been synthesized for the first time using a straightforward one-step solvothermal approach, serving as a GSH recognition moiety. Subsequently, a Cy5-labeled ATP aptamer was assembled onto Cu-Mi-UiO-66 via strong coordination between phosphate and zirconium, π-π stacking and electrostatic adsorption to develop the dual-responsive fluorescence nanoprobe Cu-Mi-UiO-66/aptamer. Due to the photoinduced electron transfer (PET) effect between maleimide groups and the benzene ring of the ligand and the charge transfer between Cy5 and the Zr(IV)/Cu(II) bimetal center of the NMOF, the Cu-Mi-UiO-66/aptamer exhibits a fluorescence turn-off status. The Michael addition reaction between the thiol group of GSH and the maleimide on the NMOF skeleton results in turning on of the blue fluorescence of Cu-Mi-UiO-66. Meanwhile, upon specific interaction with ATP, the aptamer changes into internal loop structures and detaches from Cu-Mi-UiO-66, resulting in turning on of the red fluorescence of Cy5. The nanoprobe demonstrated an excellent sensing performance with a good linear range (GSH, 5.0-450.0 μM; ATP, 1.0-50.0 μM) and a low detection limit (GSH, 2.17 μM; ATP, 0.635 μM). More importantly, the Cu-Mi-UiO-66/aptamer exhibits good performance for tracing intracellular concentration variations of GSH and ATP in living HepG2 cells under different stimulations. This study highlights the potential of NMOFs for multiplexed analysis and provides a valuable tool for tumor microenvironment research and early cancer diagnosis.

Fluorescent carbon dots for sensing applications: a review

Luminescent carbon dots (CDs) are important class of nanomaterials with fantastic photoluminescence (PL) properties, great biocompatibility, extraordinary solubility in water, minimal expense, and so on. There are many methods for their preparation and they are mainly classed into two groups, top-down and bottom-up approaches. In order to understand the origin of fluorescence in quantum CDs, three mechanisms have been proposed namely molecular state, surface state, and quantum confinement phenomenon. Fluorescent CDs have significant application in the fields of biochemical sensing, photocatalysis, bioimaging, delivery of drugs, and other related fields. In this review article the application of quantum dots as detecting component, for the sensing of different targets, has been summed up. In fact, the detection of several analytes including, anions, cations, small molecules, polymers, cells, and microscopic organisms has been discoursed. Moreover, the future aspects of CDs as detecting resources have been explored.

Carbon nanospheres dual spectral-overlapped fluorescence quenching lateral flow immunoassay for rapid diagnosis of toxoplasmosis in humans

Toxoplasmosis is a common zoonotic disease caused by a protozoan parasite Toxoplasma gondii (Tox), approximately infecting one-third of human populations worldwide. This study developed the carbon nanospheres (CNPs) based dual spectral-overlapped fluorescence quenching lateral flow immunoassay (CNPs-FQLFIA) for detection of Tox antibodies (ToxAbs). The CNPs have been effectively coupled with Tox antigen (ToxAg), which can completely overlap the excitation and emission spectra of europium nanospheres (EuNPs) and CdSe/ZnS quantum dots (QDs) in testing strips of CNPs-QDs-FQLFIA or CNPs-EuNPs-FQLFIA. The sensitivity of CNPs-EuNPs-FQLFIA or CNPs-QDs-FQLFIA was 4 or 8 IU/mL under natural light readout, or both 4 IU/mL ToxAbs under ultraviolet (UV) light readout by the naked eyes, respectively. The limit of detection (LOD) of two types of CNPs-FQLFIA was both 1 IU/mL ToxAbs under UV light by a dry fluorescence analyzer, but no cross-reaction was found with other antibodies. The intra-assay coefficient variation (CV) of both CNPs-EuNPs-FQLFIA and CNPs-QDs-FQLFIA was less than 8%, while the inter-assay CV was less than 14%, respectively. The correlation coefficient (R2) of CNPs-EuNPs-FQLFIA or CNPs-QDs-FQLFIA to measure the different concentrations of ToxAbs spiked serum samples was 0.99712 and 0.99896, respectively. The CNPs-FQLFIA presented a characteristics of 94.3% sensitivity, 100% specificity and 98% accuracy for detection of ToxAbs in clinical serum samples. In conclusion, CNPs-FQLFIA with EuNPs or QDs fluorescence reporter was an easy, rapid, sensitive, precise and quantitative assay for detecting Tox antibodies in human blood samples.

Facile synthesis of Gd/Ru-doped fluorescent carbon dots for fluorescent/MR bimodal imaging and tumor therapy

Functional metal doping endows fluorescent carbon dots with richer physical and chemical properties, greatly expanding their potential in the biomedical field. Nonetheless, fabricating carbon dots with integrated functionality for diagnostic and therapeutic modalities remains challenging. Herein, we develop a simple strategy to prepare Gd/Ru bimetallic doped fluorescent carbon dots (Gd/Ru-CDs) via a one-step microwave-assisted method with Ru(dcbpy)3Cl2, citric acid, polyethyleneimine, and GdCl3 as precursors. Multiple techniques were employed to characterize the morphology and properties of the obtained carbon dots. The Gd/Ru-CDs are high mono-dispersity, uniform spherical nanoparticles with an average diameter of 4.2 nm. Moreover, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) confirmed the composition and surface properties of the carbon dots. In particular, the successful doping of Gd/Ru enables the carbon dots not only show considerable magnetic resonance imaging (MRI) performance but also obtain better fluorescence (FL) properties, especially in the red emission area. More impressively, it has low cytotoxicity, excellent biocompatibility, and efficient reactive oxygen species (ROS) generation ability, making it an effective imaging-guided tumor treatment reagent. In vivo experiments have revealed that Gd/Ru-CDs can achieve light-induced tumor suppression and non-invasive fluorescence/magnetic resonance bimodal imaging reagents to monitor the treatment process of mouse tumor models. Thus, this simple and efficient carbon dot manufacturing strategy by doping functional metals has expanded avenues for the development and application of multifunctional all-in-one theranostics.

Aggregation-Resistant, Turn-On-Off Fluorometric Sensing of Glutathione and Nickel (II) Using Vancomycin-Conjugated Gold Nanoparticles

Glutathione (GSH) and nickel (II) cation have an indispensable role in various physiological processes, including preventing the oxidative damage of cells and acting as a cofactor for lipid metabolic enzymes. An imbalance in the physiological level of these species may cause serious health complications. Therefore, sensitive and selective fluorescent probes for the detection of GSH and nickel (II) are of great interest for clinical as well as environmental monitoring. Herein, vancomycin-conjugated gold nanoparticles (PEI-AuNP@Van) were prepared and employed for the detection of GSH and nickel (II) based on a turn-on-off mechanism. The as-synthesized PEI-AuNP@Van was ~7.5 nm in size; it exhibited a spherical shape with face-centered cubic lattice symmetry. As compared to vancomycin unconjugated gold nanoparticles, GSH led to the turn-on state of PEI-AuNP@Van, while Ni2+ acted as a fluorescence quencher (turn-off) without the aggregation of nanoparticles. These phenomena strongly justify the active role of vancomycin conjugation for the detection of GSH and Ni2+. The turn-on-off kinetics was linearly proportional over the concentration range between 0.05-0.8 µM and 0.05-6.4 μM. The detection limits were 205.9 and 90.5 nM for GSH and Ni2+, respectively; these results are excellent in comparison to previous reports. This study demonstrates the active role of vancomycin conjugation for sensing of GSH and Ni2+ along with PEI-AuNP@Van as a promising nanoprobe.

On-site quantitation of xanthine in fish and serum using a smartphone-based spectrophotometer integrated with a dual-readout nanosensing assay

Rapid and quantitative biochemical analysis at points-of-need is imperative for food safety inspection. This work reports on: 1) a stand-alone smartphone-based "two-in-one" spectrophotometer (the SAFS) installed with a self-developed application (the SAFS-App) which can precisely collect both absorption spectra and fluorescence spectra in a reproducible manner within 5 s; and 2) a straightforward protocol for xanthine detection using fluorescent carbon nanodots and silver nanoparticles. The assay performed with the SAFS demonstrates high specificity towards xanthine, and a linear range of 1-60 μM with LODs of 0.38 and 0.58 μM for colorimetric and fluorometric readouts, respectively. The reliability and robustness of the SAFS are validated by on-site quantitation of xanthine in fish and serum samples, with comparable accuracy to HPLC method. More importantly, the SAFS presents itself as an appealing device which is accessible to everyone through the Internet of Things and can be tailored for diverse point-of-care testing applications.

Bacterial cellulose composite hydrogel for pre-concentration and mass spectrometric detection of thiol-containing biomarker

Simple soaking of bacterial cellulose (BC) membrane in carboxymethyl cellulose (CMC) solution yielded BC/CMC hydrogel having re-swellable property. Then, gold nanoparticles (AuNPs) were embedded in the BC/CMC hydrogel via in situ chemical reduction to form BC/CMC/AuNPs composite hydrogel. It was found that the composite hydrogel exhibited physical/chemical characteristics similar to those of BC. The AuNPs with an average diameter of 13 nm distributed uniformly within the BC/CMC matrix as verified by transmission electron microscopy. The novelty of this work is the application of the BC/CMC/AuNPs composite hydrogel for selective adsorption of an important thiol-containing biomarker of Alzheimer's disease, glutathione (GSH), prior to direct laser desorption/ionization mass spectrometric (LDI-MS) detection. GSH adsorbed in the BC/CMC/AuNPs composite hydrogel showed the high ionization signal in LDI-MS providing a linear range of 50-10,000 nM with a limit of detection as low as 54.1 nM, which is a cut-off level for distinguishing between normal individuals and Alzheimer's patients. It should be emphasized that an additional matrix was not necessary as AuNPs can act as self-matrix for LDI-MS analysis. Furthermore, the BC/CMC/AuNPs composite hydrogel can effectively preconcentrate GSH approximately 10 times upon adsorption allowing for ultrasensitive detection of GSH required for disease diagnosis.

Gold nanocluster-confined covalent organic frameworks as bifunctional probes for electrochemiluminescence and colorimetric dual-response sensing of Pb2

The development of bifunctional signal probes based on a single component is highly desirable for sensitive and simple dual-mode detection of Pb²⁺. Here, novel gold nanocluster-confined covalent organic frameworks (AuNCs@COFs) were fabricated as a bisignal generator to enable electrochemiluminescence (ECL) and colorimetric dual-response sensing. AuNCs with both intrinsic ECL and peroxidase-like activity were confined into the ultrasmall pores of the COFs via an in situ growth method. On the one hand, the space-confinement effect of the COFs closed the ligand motion-induced nonradiative transition channels of the AuNCs. As a result, the AuNCs@COFs exhibited a 3.3-fold enhancement in anodic ECL efficiency compared to the solid-state aggregated AuNCs using triethylamine as the coreactant. On the other hand, due to the outstanding spatial dispersibility of the AuNCs in the structurally ordered COFs, a high density of active catalytic sites and accelerated electron transfer were obtained, leading to the promotion of the enzyme-like catalytic capacity of the composite. To validate its practical applicability, a Pb²⁺-triggered dual-response sensing system was proposed based on the aptamer-regulated ECL and peroxidase-like activity of the AuNCs@COFs. Sensitive determinations down to 7.9 pM for the ECL mode and 0.56 nM for the colorimetric mode were obtained. This work provides an approach for designing single element-based bifunctional signal probes for dual-mode detection of Pb²⁺.

Metal-Enhanced Fluorescent Carbon Quantum Dots via One-Pot Solid State Synthesis for Cell Imaging

In this study, a facile one-pot solid-state synthesis method is developed to shed light on the metal-enhanced fluorescence (MEF) effect in carbon quantum dots (CQDs) and gold nanoparticles (AuNPs) hybrid materials. This is one of the few studies on the solid-state synthesis of N-doped CQDs/gold hybrid nanomaterials. We have conducted various sets of experiments to reveal the role of individual reagents during the nucleation and growth of nanoparticles. We have demonstrated that the addition of a small amount of gold salt illustrates a paramount effect (10³-fold) in photoluminescence intensity. This effect is ascribed to MEF, which is caused due to interactions between the excited-state fluorophores and the free surface electrons of metal nanoparticles. It is interesting to note that a further increase of gold yields fluorescence quenching due to a large number of formed AuNPs causing fluorescence resonance energy transfer. By adjusting the volume ratio of gold salt and CD precursors, it is possible to obtain the CQDs-AuNPs hybrid with the highest fluorescence, which produces extensive visible light under 460 nm excitation. Synthesized materials have been successfully used for imaging human dermal fibroblasts and A549 lung epithelial cells. The dose-dependent cytotoxicity studies reveal that the hybrid structures do not have cytotoxicity.

In Situ Synthesis of Horseradish Peroxidase Nanoflower@Carbon Nanotube Hybrid Nanobiocatalysts with Greatly Enhanced Catalytic Activity

Organic-inorganic hybrid nanoflowers (NFs) consisting of horseradish peroxidase (HRP) and copper II (Cu²⁺) are successfully synthesized with the involvement of carbon nanotubes (CNTs) by in situ and post-modification methods. Catalytic activities of in situ synthesized HRP-NF@CNT (HRP-NF@CNT-Is) and post-modification-synthesized HRP-NF@CNTs (HRP-NF@CNT-Pm) are systematically examined. The 30 mg CNTs incorporated HRP-NF@CNT-Is (HRP-NF@CNT-30Is) exhibits greatly increased catalytic activity and stability toward 3,3',5,5'-tetramethylbenzidine (TMB), thanks to the synergistic effect between HRP-NF and CNTs and the peroxidase-like activity of CNTs in the presence of hydrogen peroxide (H₂O₂). While HRP-NF@CNT-30Is retains almost 85% of its initial activity even after 10 cycles, HRP-NF (without CNTs) loses half of its initial activity at the same experimental conditions. We study how two experimental parameters, the pH values and temperatures, influence the catalytic activity of HRP-NF@CNT-30Is, in addition to the fact that HRP-NF@CNT-30Is is employed to detect the presence of H₂O₂ and glutathione (GSH) with colorimetric and spectrophotometric readouts. For instance, HRP-NF@CNT-30Is is used to sensitively detect H₂O₂ in the range of 20 to 300 μM with an LOD of 2.26 μM. The catalytic activity of HRP-NF@CNT-30Is is suppressed in the presence of GSH, and then an obvious color change from blue to nearly colorless is observed. Using this strategy, GSH is also sensitively determined in the range of 20-200 μM with an LOD of 11.2 μM. We expect that HRP-NF@CNTs can be used as a promising and novel nanobiocatalyst for various biomedical and industrial applications in the near future.

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 sensitive dual mode turn-on fluorescence and colorimetric nanosensor for ultrasensitive detection of trace amount of gluten proteins in bread products based on crystalline nano cellulose and gold nanoparticles

In this work, Gold nanoparticles (AuNPs) encapsulated in the surface of crystalline nano cellulose grafted poly citric acid (CNC-g-PCA) and CNC-g-PCA/Au nanocomposite were synthesized successfully that exhibited stable and intense fluorescence property in aqueous buffer. A dual-mode nanosensor is reported with both colorimetric and fluorimetric readout based on citrate-protected AuNPs for discriminative detection of gluten proteins. The proposed sensing system consists of AuNPs and fluorescent CNCs, where CNCs function as a fluorimetric reporter and AuNPs serve a dual function as a colorimetric reporter and fluorescence quencher. The mechanism of the reported dual-mode nanosensor is based on two distance-dependent phenomena, the color change of AuNPs and FRET. The presence of gluten proteins can reverse the process by enlarging the inter-particle distance between AuNPs and CNCs and recovering the fluorescence emission of CNC. The linear range was 0.05 to 0.40 μgmL^-1 for UV-vis spectroscopy and 0.017 to 0.298 μgmL^-1 for fluorescence spectroscopy, The limit of detection was 4.43 ± 0.019 ngmL^-1 for UV-vis spectroscopy and 3.13 ± 0.033 ngmL^-1 for fluorescence spectroscopy (n = 6). The fabricated nanosensor was applied to the gluten analysis in gluten-free bread successfully.

Gut Microbiome Proteomics in Food Allergies

Food allergies (FA) have dramatically increased in recent years, particularly in developed countries. It is currently well-established that food tolerance requires the strict maintenance of a specific microbial consortium in the gastrointestinal (GI) tract microbiome as alterations in the gut microbiota can lead to dysbiosis, causing inflammation and pathogenic intestinal conditions that result in the development of FA. Although there is currently not enough knowledge to fully understand how the interactions between gut microbiota, host responses and the environment cause food allergies, recent advances in '-omics' technologies (i.e., proteomics, genomics, metabolomics) and in approaches involving systems biology suggest future headways that would finally allow the scientific understanding of the relationship between gut microbiome and FA. This review summarizes the current knowledge in the field of FA and insights into the future advances that will be achieved by applying proteomic techniques to study the GI tract microbiome in the field of FA and their medical treatment. Metaproteomics, a proteomics experimental approach of great interest in the study of GI tract microbiota, aims to analyze and identify all the proteins in complex environmental microbial communities; with shotgun proteomics, which uses liquid chromatography (LC) for separation and tandem mass spectrometry (MS/MS) for analysis, as it is the most promising technique in this field.

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.

A Rapid, Fluorescence Switch-On Biosensor for Early Diagnosis of Sorghum Mosaic Virus

For the first time, a nanobiosensor was established for Sorghum mosaic virus (SrMV) detection. The biosensor consists of cadmium telluride quantum dots (CdTe QDs) conjugated to the specific antibody (Ab) against SrMV coat protein (CP) and carbon quantum dots (C QDs) labeled with SrMV coat protein. The formation of the fluorophore-quencher immunocomplex CdTe QDs-Ab+C QDs-CP led to a distinct decrease in the fluorescence intensity of CdTe QDs. Conversely, the emission intensity of CdTe QDs recovered upon the introduction of unlabeled CP. The developed biosensor showed a limit of detection of 44 nM in a linear range of 0.10-0.54 μM and exhibited the strongest fluorescence intensity (about 47,000 a.u.) at 552 nm. This strategy was applied to detect purified CP in plant sap successfully with a recovery rate between 93-103%. Moreover, the feasibility of the proposed method was further verified by the detection of field samples, and the results were consistent with an enzyme-linked immunosorbent assay (ELISA). Contrarily to ELISA, the proposed biosensor did not require excessive washing and incubation steps, thus the detection could be rapidly accomplished in a few minutes. The high sensitivity and short assay time of this designed biosensor demonstrated its potential application in situ and rapid detection. In addition, the fluorescence quenching of CdTe QDs was attributed to dynamic quenching according to the Stern-Volmer equation.

Bifunctional Mn-Doped N-Rich Carbon Dots with Tunable Photoluminescence and Oxidase-Mimetic Activity Enabling Bimodal Ratiometric Colorimetric/Fluorometric Detection of Nitrite

Multimodal detection is a promising paradigm because of its advantages of expanding usage scenarios and improving reliability. However, it is very challenging to design reasonable strategies to achieve the multimodal sensing of targets. Herein, we developed an unprecedented bimodal ratiometric colorimetric/fluorometric method by exploring a novel bifunctional artificial oxidase mimic, Mn-doped N-rich carbon dots (Mn-CDs), to achieve the high-performance determination of nitrite in complicated matrices. The Mn-CDs exhibited both tunable photoluminescence and high oxidase-like activity, effectively catalyzing the colorless 3,3',5,5'-tetramethylbenzidine (TMB) oxidation to generate blue TMB⁺. When nitrite was introduced, the TMB⁺ species generated would specifically react with nitrite to produce diazotized TMB⁺, resulting in a color change from blue to green and finally to yellow. Simultaneously, the fluorescence of Mn-CDs was quenched by the diazotized TMB⁺ product via the inner filter effect. Hence, the existence of nitrite could lead to the simultaneous variations of visual color and photoluminescence, providing the principal basis for the bimodal ratiometric colorimetric/fluorometric quantification of the target. With the method, excellent sensitivity, selectivity, reliability, and practicability for nitrite detection were verified. Our work proposes a new bimodal strategy for nitrite measurement using bifunctional CDs-based enzyme mimics, which will inspire future effort on the exploration of promising multifunctional nanozymes and their advanced applications in biochemical sensing.

Where Nanosensors Meet Machine Learning: Prospects and Challenges in Detecting Disease X

Disease X is a hypothetical unknown disease that has the potential to cause an epidemic or pandemic outbreak in the future. Nanosensors are attractive portable devices that can swiftly screen disease biomarkers on site, reducing the reliance on laboratory-based analyses. However, conventional data analytics limit the progress of nanosensor research. In this Perspective, we highlight the integral role of machine learning (ML) algorithms in advancing nanosensing strategies toward Disease X detection. We first summarize recent progress in utilizing ML algorithms for the smart design and fabrication of custom nanosensor platforms as well as realizing rapid on-site prediction of infection statuses. Subsequently, we discuss promising prospects in further harnessing the potential of ML algorithms in other aspects of nanosensor development and biomarker detection.

Single-Atomic Iron Doped Carbon Dots with Both Photoluminescence and Oxidase-Like Activity

Multifunctional nanozymes can benefit biochemical analysis via expanding sensing modes and enhancing analytical performance, but designing multifunctional nanozymes to realize the desired sensing of targets is challenging. In this work, single-atomic iron doped carbon dots (SA Fe-CDs) are designed and synthesized via a facile in situ pyrolysis process. The small-sized CDs not only maintain their tunable fluorescence, but also serve as a support for loading dispersed active sites. Monoatomic Fe offers SA Fe-CDs exceptional oxidase-mimetic activity to catalyze 3,3',5,5'-tetramethylbenzidine (TMB) oxidation with fast response (V_max  = 10.4 nM s^-1 ) and strong affinity (K_m  = 168 µM). Meanwhile, their photoluminescence is quenched by the oxidation product of TMB due to inner filter effect. Phosphate ions (Pi) can suppress the oxidase-mimicking activity and restore the photoluminescence of SA Fe-CDs by interacting with Fe active sites. Based on this principle, a dual-mode colorimetric and fluorescence assay of Pi with high sensitivity, selectivity, and rapid response is established. This work paves a path to develop multifunctional enzyme-like catalysts, and offers a simple but efficient dual-mode method for phosphate monitoring, which will inspire the exploration of multi-mode sensing strategies based on nanozyme catalysis.

A Review on Graphene Quantum Dots for Electrochemical Detection of Emerging Pollutants

Graphene quantum dots which are known as zero-dimensional materials are gaining increasing attention from researchers all over the world. This is predicated upon their relatively unique chemiluminescent, fluorescent, electrochemiluminescent, and electronic properties. The precise mechanism of electrochemiluminescence continues to be a subject of debate in the research world, and this is important in identifying synthetic pathways for graphene quantum dots. Heavy metals and other emerging pollutants are global health and environmental concerns. Several studies have reported the sensitivity and limit of detection of graphene quantum dots up to the nano-, pico-, and femto- levels when used as sensors. This review seeks to bridge information gaps on the reported electrochemiluminescence chemosensors for emerging pollutants using graphene quantum dots under the sub-headings, synthesis, characterization, electrochemiluminescence chemosensor detection, and comparison with other detection methods.

Colorimetric sensing of glucose and GSH using core-shell Cu/Au nanoparticles with peroxidase mimicking activity

The catalytic properties of bimetallic nanoparticles have been widely studied by researchers in many fields. In this paper, core-shell Cu/Au nanoparticles (Cu/Au NPs) were synthesized by a simple and mild one-pot method, and their peroxidase activity was proved by catalyzing the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) with color change to blue. The change of solution color and absorbance strongly depends on the concentration of H₂O₂, so it can be used for direct detection of H₂O₂ and indirect detection of glucose. What's more, GSH can efficiently react with the hydroxyl radicals from H₂O₂ catalyzed by core-shell Cu/Au NPs to inhibit the production of ox-TMB. Thus, the concentration of GSH can be determined by the decrease in the absorbance of the solution at 652 nm. The results showed that our proposed strategy had good detection range and detection limit for the detection of glucose and GSH. This method has been used in the detection of practical samples and has great application potential in environmental monitoring and clinical diagnosis.

Polymer Carbon Nanodots: A Novel Electrochemiluminophore for Dual Mode Detection of Ferric Ions

The development of simple and effective dual-mode analytical methods plays crucial regulatory roles in the discrimination of relevant target species, because of their built-in cross reference correction and high accuracy. In this work, a novel polymer carbon nanodots (PCNDs) prepared from a facile one-pot hydrothermal procedure using readily available l-tryptophan and l-phenylalanine as precursors, showed excellent aqueous solubility and blue fluorescence property with a high quantum yield of 29%. Moreover, the PCNDs was demonstrated to be a robust luminophore with electrochemiluminescence (ECL) efficiency of 43% was achieved by using K₂S₂O₈ as a coreactant. The spooling ECL spectroscopy was employed to take insight into excited states responsible for the potential-dependent ECL emissions. Most importantly, when introduced into construction of the fluorescence and ECL dual mode sensing platform, for the first time, the PCNDs displayed prominent performance for the detection of ferric ions (Fe³⁺). The ferric ions could be quantified ranging from micromolar to millimolar with a detection limit of 0.22 and 5.3 μM, respectively. Such a dual-functional sensing platform also exhibits excellent selectivity, reproducibility and stability. Results from this work indicate that PCNDs showing great promise as a bright luminophore for the fabrication of low-cost, high-performance dual-signal readout platforms for ferric ions determination.

A Yellow Fluorescence Probe for the Detection of Oxidized Glutathione and Biological Imaging

It is well-known that the ratio of reduced l-glutathione (GSH) to oxidized l-glutathione (GSSG) is a vital biomarker for monitoring overall cellular health, thus detecting the intracellular concentration of glutathione is of great significance. Recently, an increasing number of reports have published various methods for GSH detection, but studies on the detection of GSSG are still rare. Here, we report a kind of new yellow fluorescent carbon dots (CDs) for the detection of GSSG through a fluorescence "off-on" process. Because the surface is rich in amino groups, the CDs show a positive potential. When the concentration of GSSG was continuously increased, the CDs' fluorescence dropped sharply, while the fluorescence gradually recovered after the addition of sodium sulfide. The phenomenon of fluorescence quenching is linear with the concentration of the quencher (GSSG)(0-200 μM), and 0.18 μM is calculated as the detection limit. More interestingly, as a fluorescent probe, the CDs can be further used for fluorescence imaging in living cells and zebrafish.

Highly selective and sensitive detection of glutathione over cysteine and homocysteine with a turn-on fluorescent biosensor based on cysteamine-stabilized CdTe quantum dots

In this work, cysteamine (CA) stabilized CdTe quantum dots (QDs) (CA-CdTe QDs) and sodium citrate stabilized gold nanoparticles (AuNPs) were prepared. Because of the strong electrostatic interaction and spectral overlap of emission spectrum of CA-CdTe QDs and absorption spectrum of AuNPs, a highly effective fluorescence resonance energy transfer (FRET) system was formed and the fluorescence of CA-CdTe QDs was strongly quenched. The synthesized CA-CdTe and AuNPs were self-assembled to large clusters due to the electrostatic attraction and the fluorescence of CA-CdTe was sharply quenched as a result of FRET. Under the optimum pH of 5.5, the positive GSH could assemble with negative AuNPs through electrostatic interaction and destroy the FRET system of CA-CdTe and AuNPs, due to the red shift of absorption wavelength of AuNPs caused by aggregation. The fluorescence of CA-CdTe recovered, and the recovered fluorescence efficiency shows a linear function against the GSH concentrations from 6.7 nM to 0.40 μM, with a detecting limit of 3.3 nM. The quenched emission of CA-CdTe could be recovered attributed to the aggregation of AuNPs by GSH. Under optimal conditions, the sensing system was successfully applied in the detection of GSH in real human blood plasma samples with a recovery of 99.5-102.3%, showing a promising future for the highly sensitive and selective GSH detection in the human blood plasma samples.

Tyndall-Effect-inspired assay with gold nanoparticles for the colorimetric discrimination and quantification of mercury ions and glutathione

This work initially reports a new nanosening method for simple, sensitive, specific, visual detection of mercury (II) (Hg²⁺) and glutathione (GSH) using the Tyndall Effect (TE) of the same colloidal gold nanoparticle (GNP) probes for efficient colorimetric signaling amplification. For the TE-inspired assay (TEA) method, arginine (Arg) molecules are pre-modified on the GNPs' surfaces (Arg-GNPs). Upon the Hg²⁺ introduction, it can be specifically coordinated with the terminal -NH₂ and -COOH groups of the Arg molecules to make the Arg-GNPs aggregate, producing a significantly-enhanced TE signal in the reaction solution after its irradiation by a 635-nm red laser pointer pen. On the other hand, the introduction of the GSH results in the production of the original Arg-GNPs' weak TE response, as it is able to bind such metal ion via mercury-thiol reactions to inhibit the above aggregation. Under the optimal conditions, the utility of the new TEA method is well demonstrated to quantitatively detect the Hg²⁺ and GSH with the aid of a smartphone as a portable TE reader during the linear concentration ranges of 50-3000 and 10-3000 nM, respectively. The detection limits for the Hg²⁺ and GSH are estimated to be as low as ∼3.5 and ∼0.3 nM, respectively. The recovery results obtained from the detection of Hg²⁺ in the complex tap and pond water samples and the assay of GSH in real human serum and urine samples are also satisfactory.

BSA-stabilized manganese phosphate nanoflower with enhanced nanozyme activity for highly sensitive and rapid detection of glutathione

The development of an efficient protein-inorganic nanohybrid with superior nanozyme activity for highly sensitive detection of glutathione (GSH) is essential for early diagnosis of human diseases. Herein, a rapid and highly sensitive colorimetric assay using self-assembled bovine serum albumin-hydrated manganese phosphate nanoflowers (MnPNF) as a biomimic oxidase is developed for GSH detection in human serum. The BSA can complex with Mn²⁺ to serve the nucleation center to produce MnPNF in the presence of phosphate-buffered saline (PBS). The morphology and surface characterization results show that the MnPNF is assembled with hierarchical nanoplates to form 500 nm nanoflowers. The oxidase-like activity of MnPNF is based on the redox reaction with 3,3',5,5'-tetramethylbenzidine. However, the addition of GSH can reduce MnPNF to Mn²⁺, and subsequently supresses the oxidase-like activity and a yellow color at 450 nm is observed in the presence of H₂SO₄. The MnPNF-based nanozyme exhibits excellent sensing ability toward GSH detection, and a good linear relationship between the change in absorbance at 450 nm and the added amounts of GSH at 50 nM-10 μM with low limits of detection of 20 and 26.6 nM in the PBS and diluted human serum, respectively, is observed. Moreover, the sensing probe shows a superior selectivity over the other 16 interferences, which drive the determination of GSH feasible in real human serum. Since the MnPNF can be simply prepared at room temperature and no functionalization is required, this assay can be used to design the highly efficient biomimic oxidase for effective sensing of GSH and other disease-related biomolecules in biological fluid samples.

Turn-on signal fluorescence sensor based on DNA derived bio-dots/polydopamine nanoparticles for the detection of glutathione

A convenient, fast, sensitive and highly selective fluorescence sensor for the detection of glutathione (GSH) based on DNA derived bio-dots (DNA bio-dots)/polydopamine (PDA) nanoparticles was constructed. The fluorescent switch of DNA bio-dots was induced to turn off because of fluorescence resonance energy transfer (FRET) reactions between DNA bio-dots and PDA. The presence of GSH blocked the spontaneous oxidative polymerization of dopamine (DA) to PDA, leading the fluorescent switch of DNA bio-dots to be "turned on". The degree of fluorescence recovery of DNA bio-dots is linearly correlated with the concentration of GSH within the range of 1.00-100 μmol L-1, and the limit of detection (LOD) is 0.31 μmol L-1 (S/N = 3, n = 9). Furthermore, the fluorescence sensor was successfully used to quantify GSH in human urine and glutathione whitening power, indicating the fluorescence sensor has potential in the detection of human body fluids and pharmaceutical preparations.

Ratiometric fluorescence detection of pathogenic bacteria based on dual-recognition nanoprobes with controllable G-quadruplex release

A dual-recognition carbon dot-based nanoprobe with controllable G-quadruplex release is developed for ratiometric fluorescence detection of pathogenic bacteria in a fast and precise way, which opens a promising avenue for efficient detection and early warning of pathogenic bacteria in food matrices.

Target-Activating and Toehold Displacement Ag NCs/GO Biosensor-Mediating Signal Shift and Enhancement for Simultaneous Multiple Detection

Herein, we demonstrate that a new multicolor silver nanoclusters/graphene oxide (Ag NCs/GO) hybrid material, upon target response, undergoes a configuration transformation, based on entropy-driven enzyme-free toehold-mediated strand displacement reaction, achieving emission shift and enhancement. To realize the aim above, two different synthesis routes (route I and II) of synthesizing fluorescent Ag NCs for constructing toehold displacement Ag NCs/GO biosensor is designed and performed. Influenza A virus subtype genes (H1N1 and H5N1) as a model can efficiently initiate the operation of entropy-driven displacement reaction, resulting in activatable fluorescence. Red-emitting and green-emitting Ag NCs tethering the complementary sequence of H1N1 (pDNA1) and H5N1 (pDNA2) are indirectly immobilized on GO surface through binding with capture DNA (cDNA1 and cDNA2), respectively, forming multicolor pDNA-Ag NCs/GO nanohybrid materials. However, they do not exhibit nearly fluorescence signals attributed to energy transfer from donor Ag NCs to acceptor GO. Upon adding targets H1N1 and H5N1 (tDNA1 and tDNA2), pDNA1-Ag NCs and pDNA2-Ag NCs detach from GO, based on toehold-mediated strand displacement reaction, which interferes the energy transfer and leads to significant fluorescence enhancement. More interestingly, the activatable process is accompanied by remarkable hypsochromic shift (19 nm) or bathochromic shift (21 nm) emission with quite high fluorescence recovery rates (823.35% and 693.62%). Therefore, based on these phenomena, a novel multiple approach has been developed with the assistance of toehold displacement and Ag NCs/GO nanohybrid materials. As for the remarkable emission recovery and multichannel signal, the proposed approach displays the promising application prospect in accurate diagnosis and treatment.

Aptamer-Based Fluorescent Biosensor for the Rapid and Sensitive Detection of Allergens in Food Matrices

Food allergies have seriously affected the life quality of some people and even endangered their lives. At present, there is still no effective cure for food allergies. Avoiding the intake of allergenic food is still the most effective way to prevent allergic diseases. Therefore, it is necessary to develop rapid, accurate, sensitive, and reliable analysis methods to detect food allergens from different sources. Aptamers are oligonucleotide sequences that can bind to a variety of targets with high specificity and selectivity, and they are often combined with different transduction technologies, thereby constructing various types of aptamer sensors. In recent years, with the development of technology and the application of new materials, the sensitivity, portability, and cost of fluorescence sensing technology have been greatly improved. Therefore, aptamer-based fluorescence sensing technology has been widely developed and applied in the specific recognition of food allergens. In this paper, the classification of major allergens and their characteristics in animal and plant foods were comprehensively reviewed, and the preparation principles and practical applications of aptamer-based fluorescence biosensors are summarized. In addition, we hope that this article can provide some strategies for the rapid and sensitive detection of allergens in food matrices.

Development of gold nanoparticles-aptamer nanocomposite for multiplexed analysis of antibiotics and design of molecular logic gates

The widespread use of antibiotics caused severe problems of antibiotic residues in foodstuffs and water, posing a serious threat to public health and thus urging the development of sensitive, selective, and rapid detection methods for antibiotics. In this study, a fluorescence resonance energy transfer (FRET)-based system is developed for the multiplexed analysis of chloramphenicol (CAP) and streptomycin (Strep) with detection limits of 2.51 and 8.69μg l^-1, respectively. The FRET-based system consists of Cy3-tagged anti-CAP aptamer-conjugated gold nanoparticles (AuNPs) (referred to as AuNPs-AptCAP) and Cy5-tagged anti-Strep aptamer-conjugated AuNPs (referred to as AuNPs-AptStrep). In addition, AuNPs-AptCAP and AuNPs-AptStrep have been demonstrated to serve as signal transducers for implementing a series of logic operations such as YES, NOT, INH, OR, (2-4)-Decoder and even more complicated multi-level logic gates (OR-INH). Based on the outputs of logic operations, it could be figured out whether targeted analytes were present or not, thus enabling multiplex sensing and evaluation of pollution status. This proof of concept study might provide a new route for the enhanced sensing performance to distinguish different pollution status as well as the design of molecular mimics of logic elements to demonstrate better applicability.

Gadolinium-porphyrin based polymer nanotheranostics for fluorescence/magnetic resonance imaging guided photodynamic therapy

Nanotheranostics for fluorescence/magnetic resonance (FL/MR) dual-modal imaging guided photodynamic therapy (PDT) are highly desirable in precision and personalized medicine. In this study, a facile non-covalent electrostatic interaction induced self-assembly strategy is developed to effectively encapsulate gadolinium porphyrin (Gd-TCPP) into homogeneous supramolecular nanoparticles (referred to as Gd-PNPs). Gd-PNPs exhibit the following advantages: (1) excellent FL imaging property, high longitudinal relaxivity (16.157 mM^-1 s^-1), and good singlet oxygen (¹O₂) production property; (2) excellent long-term colloidal stability, dispersity and biocompatibility; and (3) enhanced in vivo FL/MR imaging guided tumor growth inhibition efficiency for CT 26 tumor-bearing mice. This study provides a new strategy to design and synthesize metalloporphyrin-based nanotheranostics for imaging-guided cancer therapy with enhanced theranostic properties.

A sensitive and rapid detection of glutathione based on a fluorescence-enhanced "turn-on" strategy

Glutathione (GSH) plays important roles in the human body including protecting cells from oxidative damages and maintaining cellular redox homeostasis. Thus, developing a fast and sensitive method for detecting GSH levels in living bodies is of great importance. Many methods have been developed and used for GSH detection, such as high-performance liquid chromatography, capillary electrophoresis, and fluorescence resonance energy-based methods. However, these methods often lack sensitivity as well as efficiency. Herein, a rapid and sensitive method for glutathione detection was developed based on a fluorescence-enhanced "turn-on" strategy. In this study, a unique and versatile bifunctional linker 3-[(2-aminoethyl) dithio]propionic acid (AEDP)-modified gold nanoparticle (Au@PLL-AEDP-FITC) probe was designed for the simple, highly sensitive intracellular GSH detection, combined with the FRET technique. In the presence of GSH, the disulfide bonds of AEDP on Au@PLL-AEDP-FITC were broken through competition with GSH, and FITC was separated from gold nanoparticles, making the fluorescence signal switch to the "turn on" state. A change in the fluorescence signal intensity has a great linear positive correlation with GSH concentration, in the linear range from 10 nM to 180 nM (R2 = 0.9948), and the limit of detection (LOD) of 3.07 nM, which was lower than other reported optical nanosensor-based methods. Au@PLL-AEDP-FITC also has great selectivity for GSH, making it promising for application in complex biological systems. The Au@PLL-AEDP-FITC probe was also successfully applied in intracellular GSH imaging in HeLa cells with confocal microscopy. In short, the Au@PLL-AEDP-FITC probe-based fluorescence-enhanced "turn-on" strategy is a sensitive, fast, and effective method for GSH detection as compared with other methods. It can be applied in complex biological systems such as cell systems, with promising biological-medical applications in the future.

A Highly Sensitive and Selective Probe for the Colorimetric Detection of Mn(II) Based on the Antioxidative Selenium and Nitrogen Co-Doped Carbon Quantum Dots and ABTS•

Herein, selenium and nitrogen co-doped carbon quantum dots (Se/N-CQDs) were hydrothermally synthesized by using citric acid, histidine, and sodium selenite, which had sp³ and sp² hybridized carbon atoms and showed excitation-dependent fluorescence behavior. Furthermore, due to the redox reaction of ABTS^•+ and Se/N-CQDs, Se/N-CQDs had the excellent antioxidant capacity that it was demonstrated by scavenging ABTS^•+ with the fading of blue. Based on the synergistic effect of Se/N-CQDs and Mn(II) on ABTS^•+, Se/N-CQDs and ABTS^•+, as a stable, sensitive, selective, and reproducible colorimetric sensor, was applied to the detection of Mn(II) with a detection limit of 1.69 μM and a linear range of 0 to 142.90 μM. More importantly, the probe was successfully applied to detecting Mn(II) in tap water, illustrating that it could be a promising tool for Mn(II) detection in water environments.

Fabrication of multi-functional carbon dots based on "one stone, three birds" strategy and their applications for the dual-mode Fe3+ detection, effective promotion on cell proliferation and treatment on ferric toxicosis in vitro

The ingenious design of multi-functional materials to simultaneously achieve the accurate detection of targets and effective treatment of target-related diseases is of great significance for both practical and clinical applications. Accordingly, based on their advantages of facile synthesis and function designability, functional nanomaterials have become promising candidates for integrating multi-functionality into one platform, especially carbon dot (CD)-based materials. Herein, deferoxamine (DFO)-inspired CDs with integrated "sense and treatment" potential were elaborately designed and fabricated via a one-pot hydrothermal synthesis by employing l-aspartic acid (Asp) and 2,5-diaminobenzenesulfonic acid (DABSA) as the reactants. A series of characterization results distinctly confirmed that the synthesized CDs possessed a unique chemical composition, uniform spherical morphology (diameter of around 5 nm) and good dispersibility in aqueous solution, exhibiting excellent fluorescence stability under different conditions. Owing to the complexation interaction between Fe3+ and the functional groups of CDs, the selective and sensitive detection of Fe3+ could be successfully realized through fluorescent and colorimetric dual-mode detection based on the statistic quenching in the initial stage, and subsequently the FRET process. Furthermore, these CDs could be utilized for cellular imaging and effective Fe3+ detection due to their outstanding biocompatibility and cytoplasmatic distribution. More significantly, these DFO-inspired CDs could remarkably promote the proliferation of various mammalian cells. Particularly, the results in this work obviously indicated that this type of CDs could weaken the damage of Fe3+ towards the physiological behaviors of cells, helping the cells to regain their capability of differentiation after ferric toxicosis. Therefore, this work presents an original approach for the design and fabrication of multi-functional materials according to the "one stone, three birds" strategy, which may be an optional solution to develop various multi-functional platforms for disease diagnosis and corresponding clinical treatment.

A carbon-based nanocarrier for efficient gene delivery

Aim: Several types of nanocarriers, most of which show significant cytotoxicity, have been developed to overcome the problem of gene-delivery barriers. Biocompatibility, low toxicity and water solubility of carbon nanodots (CNDs) are major advantages that recommend them as delivery systems. Materials & methods: We present a simple method to produce positively charged CNDs. Ethanolamine, ethylenediamine and hydrogen peroxide were utilized to synthesize these CNDs. Results & conclusion: Our results indicated that delivery of the CND-siGFP complex led to significant switching-off of the fluorescence of the GFP-expressing A549 cell. Next, the A549 cells were transfected with siRNA against BiP, which is a pivotal protein in the chemotherapy resistance of cancer cells. The expression levels of BiP decreased remarkably.

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.

Carbon dots as naked eye sensors

Optical sensors are always fascinating for chemists due to their selectivity, sensitivity, robustness and cost-effective nature.

Label-free colorimetric detection of glutathione by autocatalytic oxidation of o-phenylenediamine based on Au3+ regulation and its application

A label-free, rapid, and highly sensitive colorimetric assay for the detection of glutathione (GSH) was developed.