Near-infrared dual-emission quantum dots-gold nanoclusters nanohybrid via co-template synthesis for ratiometric fluorescent detection and bioimaging of ascorbic acid in vitro and in vivo

The construction of dual-emissive ratiometric fluorescent probes based on fluorescent nanoparticles for the detection of metal ions and small molecules

With the rapid development of fluorescent nanoparticles (FNPs), such as CDs, QDs, and MOFs, the construction of FNP-based probes has played a key role in improving chemical sensors. Ratiometric fluorescent probes exhibit distinct advantages, such as resistance to environmental interference and achieving visualization. Thus, FNP-based dual-emission ratiometric fluorescent probes (DRFPs) have rapidly developed in the field of metal ion and small molecule detection in the past few years. In this review, firstly we introduce the fluorescence sensing mechanisms; then, we focus on the strategies for the fabrication of DRFPs, including hybrid FNPs, single FNPs with intrinsic dual emission and target-induced new emission, and DRFPs based on auxiliary nanoparticles. In the section on hybrid FNPs, methods to assemble two types of FNPs, such as chemical bonding, electrostatic interaction, core satellite or core-shell structures, coordination, and encapsulation, are introduced. In the section on single FNPs with intrinsic dual emission, methods for the design of dual-emission CDs, QDs, and MOFs are discussed. Regarding target-induced new emission, sensitization, coordination, hydrogen bonding, and chemical reaction induced new emissions are discussed. Furthermore, in the section on DRFPs based on auxiliary nanoparticles, auxiliary nanomaterials with the inner filter effect and enzyme mimicking activity are discussed. Finally, the existing challenges and an outlook on the future of DRFP are presented. We sincerely hope that this review will contribute to the quick understanding and exploration of DRFPs by researchers.

A pre-oxidized Eu-probe doped into bio-MOF-1 for ascorbic acid emission "off-on" detection in human serum

Ascorbic acid (or widely known as vitamin C, VC) is an essential antioxidant and a free radical scavenger in human body. The appeal for reliable fluorescent nanosensors always promotes the development of VC probe. In this work, a pre-oxidized Eu-probe (denoted as Eu-NO9) was synthesized. Without VC, Eu-NO9 was nearly non-emissive owing to the inefficient ligand energy transfer (ET) to Eu ion (caused by mismatched ligand level and long distance to Eu, as revealed by single crystal analysis and emissive parameters). By adding VC, the pre-oxidized ligand was deoxidized and its ET to Eu ion became efficient (confirmed by electrochemical analysis), with Eu(III) red emission intensity obviously increased. Then Eu-NO9 was doped into a porous host bio-MOF-1 for ascorbic acid detection (denoted as Eu-NO9@MOF). The molecular sieving effect of bio-MOF-1 improved sensing selectivity, and bio-MOF-1 blue emission (421 nm) was applied as a reference for Eu(III) red emission. Linear working curves were obtained within a wide working region of 0-100 μM, with LOD of 1.7 μM. A short response time of 192 s at 25 °C was confirmed. Practical sensing plates were prepared and found applicable for VC detection in fresh human serum. The advantage of this work was the combination of a pre-oxidized probe and a porous host which gave emission "turn on" fluorescence sensing for VC with good selectivity, linear calibration curve and wide working region.

Anisotropic nanocluster arrays to a diminished zone: different regimes of surface deposition of gold nanocolloids

Evaporation-induced assembly of nanoparticles has emerged as a versatile technique for the production of large-scale ordered structures and materials with complex features. In this study, we show that a dried particulate of an anisotropic nanocolloid undergoes non-ubiquitous surface morphological transitions at varying particle concentrations. Below 5 nM, deposits reveal the formation of linear arrays of AuNR clusters outside of the coffee ring and an annular CTAB-rich depletion zone in the inner vicinity of the coffee ring. For nanoparticle concentrations ≥5 nM, the outer cluster deposits disappear and a region of reduced AuNR density, sandwiched between the coffee ring and the depletion zone, analogous to the diminished zone, is observed. Within the coffee-ring deposits, nanoscale smectic AuNR assembly occurs via the expulsion of the cetyltrimethyl ammonium bromide (CTAB) bilayer, which contributes to the inward solutal Marangoni flow. An enhanced inward solutal Marangoni flow at high particle concentrations assists in the formation of a wider depletion zone, the emergence of the diminished zone and suppression of the width of the coffee-ring deposits. Through detailed analysis of data from ex situ (scanning electron microscopy, SEM) and in situ (contact angle and confocal imaging) measurements, we establish a direct correlation between the different evaporation modes and the various deposition regimes. A detailed mechanism for the surface morphology modulation of AuNR deposits by tuning the nanoparticle concentration in the drying sessile drop is discussed.

On the construction and performance of pre-oxidized probe inserted into charged MOF for the in vitro vitamin C detection: A highly selective emission turn-on sensing

Owing to the critical role of ascorbic acid (vitamin C, VC) in human bio-activities as an antioxidant and free radical scavenger, the development for an easy and fast VC detection has always been pushed. In this work, a traditional ligand bipyridine (bpy) was firstly oxidized (denoted as bpy-O) and then coordinated to Eu(III) (denoted as Eu-bpy-O). Due to the oxidization of bpy N atoms, which led to improper triplet ligand energy level and increased ligand-to-Eu distance, the efficient ligand energy transfer (ET) to Eu(III) was suspended, resulting in a low emission quantum yield of 1.6%. Upon being deoxidized by VC, the ligand ET to Eu(III) became efficient, with emission quantum yield recovered to 22.4%, showing emission turn-on effect for VC. To allow smooth analyte dispersion and uniform probe distribution, Eu-bpy-O was loaded into the micropores of bio-MOF-1 (Eu-bpy-O@MOF) with various doping levels. Good sensing selectivity was observed owing to the well-designed probe and bio-MOF-1 sieving effect. Linear fitting equations were obtained for the optimal sample named 5Eu-bpy-O@MOF within VC concentration region of 0-100 μM, showing LOD of 1.7 μM, sensitivity of 0.191 μM^-1, and response time of ∼240 s at 35 °C. The practical sensing performance of 5Eu-bpy-O@MOF was confirmed by its sensing plates upon human serum samples. The novelty of this work was the application of a pre-oxidized probe and a porous host, which offered emission "turn-on" effect for VC with promising performance.

The on-off-on Fluorescence Sensor of Hollow Carbon Dots for Detecting Hg2+ and Ascorbic Acid

Carbon dots (CDs) have excellent fluorescence properties and can be used in many research fields. In this paper, carbon dots were prepared by microwave-assisted pyrolysis of citric acid and urea, characterized by transmission electron microscope (TEM), X-ray diffractometer (XRD), ¹³C-NMR spectrum, zeta potential, Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible (UV-vis) absorption and fluorescence spectra, and detected the Hg²⁺ and ascorbic acid (AA) sequentially. It showed that carbon dots were hollow, spherical particles and less than 10 nm, photoluminescence quantum yield of carbon dots was about 15%. The CDs were selective and sensitive to Hg²⁺ and AA based on the "on-off-on" fluorescence behavior. The detection limits of CDs for Hg²⁺ and AA were 0.138 μM and 0.212 μM, respectively. Fluorescence response mechanism of CDs was also discussed.

An inner filter effect-based nitrogen-doped carbon dots-CoOOH nanoflakes fluorescence probe for detection of ascorbic acid by chemical redox modulation

BACKGROUND

Ascorbic acid (AA) is an essential nutrient for humans, which must be obtained from vegetables, fruits, and other foods. The content of AA has become an important standard to evaluate the quality and nutritional value of food. The fluorescence sensing method based on nanomaterials is a good alternative for the rapid detection of AA. In this study, we developed an inner filter effect-based fluorescent probe that hybridized nitrogen-doped carbon dots (NCDs) with cobalt oxyhydroxide nanoflakes (CoOOH NFs).

RESULTS

An optimal NCDs was successfully selected because it has a strong fluorescence at 430 nm and the most significant quenching phenomenon with CoOOH NFs due to the inner filter effect. When adding AA into the NCDs-CoOOH NFs probe solution, a specific redox reaction will occur between the enediol group of AA and the CoOOH NFs to interfere with the quenching ability of CoOOH NFs and recover the fluorescence of NCDs. The recovered fluorescence intensities demonstrated a linear relationship with the concentrations of AA. The assay based on the NCDs-CoOOH NFs probe allows AA to be tested in a wide range of 5-200 μmol L^-1 with a detection limit of 2.31 nmol L^-1 . Furthermore, to evaluate its practical application, the NCDs-CoOOH NFs fluorescence probe was utilized to analyze AA in vegetable, fruit, and serum matrixes with satisfactory results.

CONCLUSION

An inner filter effect-based fluorescence probe for the rapid detection of AA was developed, and it has a good potential to be applied in both food and clinical testing. © 2022 Society of Chemical Industry.

Ce3+ and Fe2+ co-enhanced ratiometric fluorescence probe utilizing copper nanoclusters and coumarin for sensitive assay of hydrogen peroxide and glucose

A novel ratiometric fluorescent probe was constructed for sensitive assay of hydrogen peroxide (H₂O₂) and glucose, which utilized the synergistically enhanced effects of Ce³⁺ and Fe²⁺ on copper nanoclusters (CuNCs) and coumarin. In the CuNCs-Ce³⁺/Fe²⁺-coumarin system, Ce³⁺ triggered the aggregation-induced emission phenomenon of CuNCs, and Fe²⁺ catalyzed the Fenton reaction to efficiently yield hydroxyl radical (•OH). In the presence of H₂O₂, the 625-nm red fluorescence of CuNCs was sharply quenched owing to the oxidation of CuNCs to Cu(II) by •OH, but the 460-nm blue fluorescence of 7-hydroxycoumarin from the oxidation of coumarin by •OH dramatically increased. Based on the reversible changes in two fluorescence signals, a satisfactorily ratiometric probe was constructed for H₂O₂ assay with a detection limit (LOD) of 0.6 μM accompanied by a visual color variation from red to blue. For glucose assay, this ratiometric probe gave a linear range of 3.2-160 μM and LOD of 0.96 μM owing to the oxidization of glucose to yield H₂O₂ in the presence of glucose oxidase and O₂. Overall, the newly developed ratiometric probe shows a great prospect in real applications for visual assay of H₂O₂ and glucose by our naked eyes.

A novel ratiometric nanoprobe based on copper nanoclusters and graphitic carbon nitride nanosheets using Ce(III) as crosslinking agent and aggregation-induced effect initiator for sensitive detection of hydrogen peroxide and glucose

Herein, glutathione-capped copper nanoclusters (CuNCs) and graphitic carbon nitride nanosheets (g-C₃N₄ NSs) were synthesized by a facile one-pot chemical reduction and directly thermal pyrolysis following ultrasonic exfoliation approaches, respectively. The introduction of Ce(III) (Ce³⁺) played dual functions in constructing a fluorescence-enhanced ratiometric nanoprobe (g-C₃N₄ NSs-Ce³⁺-CuNCs), i.e., triggering aggregation-induced emission of CuNCs and conjugating g-C₃N₄ NSs with CuNCs by virtue of electrostatic and coordination interactions. The as-fabricated nanohybrid displayed 460 and 625 nm dual-emitting peaks, attributing to the emission of g-C₃N₄ NSs and CuNCs, respectively. Upon addition of H₂O₂, the 625 nm emission was dramatically quenched, whereas the 460 nm emission remained nearly unchanged, thereby causing obvious color changes from purple to blue under a 365-nm UV lamp. A ratiometric fluorescent assay, based on g-C₃N₄ NSs-Ce³⁺-CuNCs, was devised for sensitive and visual detection of H₂O₂, which spanned the linear range of 2-100 μM with a detection limit of 0.6 μM. In the presence of glucose oxidase, the ratiometric nanoprobe could be simultaneously employed to detect glucose across the linear range of 1.6-320 μM with a detection limit of 0.48 μM. In milk and human serum samples, the fortified recoveries for H₂O₂ and glucose by the nanoprobe were in the range of 95.5-103.6% with RSDs <3.8%. The real detection levels for glucose are consistent with those by a standard glucometer. As such, the ratiometric nanoprobe offers a promising methodology for several practical applications, such as point-of-care diagnosis and workplace health evaluations.

Visible-light and near-infrared fluorescence and surface-enhanced Raman scattering point-of-care sensing and bio-imaging: a review

This review article deals with the concepts, principles and applications of visible-light and near-infrared (NIR) fluorescence and surface-enhanced Raman scattering (SERS) in in vitro point-of-care testing (POCT) and in vivo bio-imaging. It has discussed how to utilize the biological transparency windows to improve the penetration depth and signal-to-noise ratio, and how to use surface plasmon resonance (SPR) to amplify fluorescence and SERS signals. This article has highlighted some plasmonic fluorescence and SERS probes. It has also reviewed the design strategies of fluorescent and SERS sensors in the detection of metal ions, small molecules, proteins and nucleic acids. Particularly, it has provided perspectives on the integration of fluorescent and SERS sensors into microfluidic chips as lab-on-chips to realize point-of-care testing. It has also discussed the design of active microfluidic devices and non-paper- or paper-based lateral flow assays for in vitro diagnostics. In addition, this article has discussed the strategies to design in vivo NIR fluorescence and SERS bio-imaging platforms for monitoring physiological processes and disease progression in live cells and tissues. Moreover, it has highlighted the applications of POCT and bio-imaging in testing toxins, heavy metals, illicit drugs, cancers, traumatic brain injuries, and infectious diseases such as COVID-19, influenza, HIV and sepsis.

Photochemistry of phthalocyanine based on spin angular momenta: a kinetic study of fluorescent probes for ascorbic acid

Ascorbic acid, i.e., vitamin C, is a well-known essential nutrient, and has attracted considerable attention as a new candidate for cancer therapy. Previously, R2c consisting of silicon tetra-tert-butylphthalocyanine (SiPc) and two 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) radicals has been encapsulated into the hydrophobic cavity of dimeric bovine serum albumin (BSA), i.e., R2c@(BSA)2, and the system was found to behave as a highly sensitive and selective fluorescent probe for detecting ascorbic acid not only in aqueous solutions but also in vivo. In this study, kinetics of the reaction of R2c@(BSA)2 with ascorbic acid have been studied based on the temporal evolution of fluorescence. Global fitting of the concentration dependence using the Runge-Kutta method revealed the existence of stepwise two proton-coupled electron transfer processes. The rate constants for the reactions with ascorbic acid (k AA ) and ascorbate radical (k AR ) were 3 × 10 and 1 × 105 min−1 M−1, respectively, suggesting that the reaction with ascorbate radical was much faster than that with ascorbic acid. These results were further corroborated by theoretical calculations of the Gibbs free energy differences and by spin statistical factors. The analysis presented herein will aid in understanding the two proton-coupled electron transfer processes in the reaction with ascorbic acid.

Inner filter effect between upconversion nanoparticles and Fe(ii)-1,10-phenanthroline complex for the detection of Sn(ii) and ascorbic acid (AA)

Dual-function and multi-function sensors can use the same material or detection system to achieve the purpose of detection of two or more substances. Due to their high sensitivity and specificity, dual-function and multi-function sensors have potential applications in many fields. In this article, we designed a dual-function sensor to detect Sn(ii) and ascorbic acid (AA) based on the inner filter effect (IFE) between NaYF₄:Yb,Er@NaYF₄@PAA (UCNPs@PAA) and Fe(ii)–1,10-phenanthroline complex. Fe(ii)–1,10-phenanthroline complex has strong absorption in most of the ultraviolet-visible light range (350 nm–600 nm), and this absorption band overlaps with the green emission peak of UCNPs@PAA at 540 nm; Fe(ii)–1,10-phenanthroline complex can significantly quench the green light emission of UCNPs@PAA. When Sn(ii) or AA is added to the UCNPs@PAA/Fe(iii)/1,10-phenanthroline, they can reduce Fe(iii) to Fe(ii). Fe(ii) can react with 1,10-phenanthroline to form an orange complex, thereby quenching the green light emission of UCNPs@PAA. And the quenching efficiency is related to the concentration of Sn(ii) and AA; there is a linear relationship between quenching efficiency and the concentration of Sn(ii) and AA, within a certain concentration range the detection limits of this dual-function sensor for Sn(ii) and AA are 1.08 μM and 0.97 μM, respectively. In addition, the dual-function sensor can also detect Sn(ii) and AA in tap and spring water.

Based on the inner filter effect (IFE), we use UCNPs to develop a dual-function sensors, which can realize sensitive and selective detection for the Sn(ii) and ascorbic acid (AA).

Lanthanide-functionalized metal-organic frameworks based ratiometric fluorescent sensor array for identification and determination of antibiotics

Antibiotics have made great contributions to the improvement of human health and life quality. However, the current abuse of antibiotics not only has a serious impact on the environment, but also endangers people's health. For this reason, the simultaneous identification and accurate determination of as many antibiotics in the environment, food and organisms as possible is critical. Herein, a ratiometric fluorescent sensor array based on Eu³⁺ and Tb³⁺ co-doped metal-organic frameworks (MOFs) was fabricated. Benefiting from the sensitization of the organic ligands to Eu³⁺ and Tb³⁺, the reaction of MOFs with various antibiotics resulted in different responses to the ratio of fluorescent intensity at 545 nm and 616 nm (F₅₄₅/F₆₁₆). After these responses were differentiated by principal component analysis (PCA), totally eight kinds of 25 antibiotics were well distinguished with the existence of interfering substances. The proposed sensor array exhibited high accuracy (98%) for the identification of 48 unknown samples in water and outstanding quantitative ability for the mixture of antibiotics. Finally, the practicability of the sensor array for the analysis of real samples was proved. In this strategy, we have not only provided an efficient way for the comprehensive identification and determination of antibiotics, but also promised new opportunities for the development of ratiometric signal based sensor array.

A "switch-on" fluorescence assay based on silicon quantum dots for determination of ascorbic acid

Water dispersible silicon quantum dots (SiQDs) showing blue fluorescence were synthesized with 3-aminopropyltriethoxysilane (APTES) as silicon source. Based on the synthesized SiQDs as the photoluminescence unit, MnO₂ nanosheets (NS) as the quencher, a "switch-on" fluorescence assay for the determination of ascorbic acid (AA) was designed. The fluorescence of SiQDs can be effectively quenched by MnO₂ NS because of the internal filtration effect. In the presence of AA, MnO₂ is reduced to Mn²⁺, so that the fluorescence of SiQDs is partially recovered. The recovered fluorescence intensity was related to the concentration of AA. Under the optimal experimental conditions, the linear response range of the assay to AA is 1-80 µM, and the detection limit is 0.48 µM. The method for the determination of AA has the advantages of simple, low cost, good selectivity and sensitivity. The assay has been successfully applied to the quantification of AA in beverage (mizone) samples, which proves the practicability of the assay.

A dual luminescent sensor coordination polymer for simultaneous determination of ascorbic acid and tryptophan

Simultaneous high sensitivity detection of biomolecules is important for research in medicine, living cells and environmental samples. In this work, a water stable coordination polymer, [Cd₂(bptc)(4,4'-bpy)(H₂O)₃]ˑH₂O 1 (H₄bptc = 2,3,3',4'-biphenyl tetracarboxylic acid, 4,4'-bpy = 4,4'-bipyridine), was designed and successfully synthesized as a luminescent sensor for simultaneous recognition of Ascorbic Acid (AA) and L-Tryptophan (L-Trp) based on luminescent -OFF and -ON, respectively. Importantly, the proposed sensing system showed an excellent performance with high K_SV values of 4.85 × 10⁴ M^-1, 9.60 × 10⁷ M^-1 and low limit of detection (LOD) of 0.28 nM, 63 nM, respectively. In addition, the probable mechanisms are also discussed. The luminescent quenching behavior by AA can be mainly attributed to the static resonance energy transfer between complex 1 and the analytes. Whereas the enhancing effect of L-Trp comes from the intrinsic strong luminescence for L-Trp itself and photo-competitive mechanism between CP 1 sensor and L-Trp, supposedly. In addition, the repeatability of both systems were also investigated.

Nanoscale Dynamic Readout of a Chemical Redox Process Using Radicals Coupled with Nitrogen-Vacancy Centers in Nanodiamonds

Biocompatible nanoscale probes for sensitive detection of paramagnetic species and molecules associated with their (bio)chemical transformations would provide a desirable tool for a better understanding of cellular redox processes. Here, we describe an analytical tool based on quantum sensing techniques. We magnetically coupled negatively charged nitrogen-vacancy (NV) centers in nanodiamonds (NDs) with nitroxide radicals present in a bioinert polymer coating of the NDs. We demonstrated that the T₁ spin relaxation time of the NV centers is very sensitive to the number of nitroxide radicals, with a resolution down to ∼10 spins per ND (detection of approximately 10^-23 mol in a localized volume). The detection is based on T₁ shortening upon the radical attachment, and we propose a theoretical model describing this phenomenon. We further show that this colloidally stable, water-soluble system can be used dynamically for spatiotemporal readout of a redox chemical process (oxidation of ascorbic acid) occurring near the ND surface in an aqueous environment under ambient conditions.

Facile synthesis of carboxymethyl cellulose sulfur quantum dots for live cell imaging and sensitive detection of Cr(VI) and ascorbic acid

Nowadays the synthesis of stable fluorescent sulfur quantum dots (SQDs) remains a big challenge. Herein, the utilization of carboxymethyl cellulose (CMC) to synthesis of SQDs is reported. Benefiting from the unique composition and structure of CMC macromolecule, the resulted CMC-SQDs simultaneously show high aqueous dispersibility and stability, tunable emission, stable fluorescence and low cytotoxicity, which make them promising for working as a fluorescent probe. Fluorescence detection experiments suggested that the CMC-SQDs could serve as a fluorescence on-off-on switch to sensitive and selective detection of Cr(VI) and ascorbic acid (AA) based on the inner filter effect (IFE). The limit of detection towards Cr(VI) and AA can reach 0.024 and 0.18 μM with linear range of 0.5-225 and 1-300 μM, respectively, which compares favorably to other reported fluorescent probes. In addition, the employment of fluorescent CMC-SQDs for practical detection of Cr(VI) and AA was also studied.

Fluorescent polydopamine nanoparticles as a nanosensor for the sequential detection of mercury ions and l-ascorbic acid based on a coordination effect and redox reaction

Herein, a novel fluorescence nanosensor using intrinsic fluorescent polydopamine nanoparticles (PDA NPs) as an effective signal reporter has been constructed for the simple, rapid and sequential detection of mercury ions (Hg2+) and l-ascorbic acid (AA) based on a coordination effect and redox reaction. The fluorescence of the PDA NPs could be specifically quenched by Hg2+ through intense coordination effects between the Hg2+ and the groups (catechol, amine, ketone and imine) on the surface of the PDA NPs. However, when AA and Hg2+ coexisted in solution, the fluorescence of the PDA NPs pronouncedly recovered via the redox reaction of Hg2+, with it being reduced to Hg0 by AA. The fluorescence quenching mechanism of Hg2+ towards the PDA NPs and the redox reaction between Hg2+ and AA were also fully investigated. The nanosensor exhibited high sensitivity and desirable selectivity for Hg2+ and AA detection. Moreover, the strategy was successfully explored in real samples (tap water, lake water and human serum samples) with satisfactory recoveries. The developed nanosensor provides new sights and good inspiration for Hg2+ and AA detection under real conditions.

Activity-Based Sensing of Ascorbate by Using Copper-Mediated Oxidative Bond Cleavage

Ascorbate is an important biological reductant and enzyme cofactor. Although direct detection through ascorbate-mediated reduction is possible, this approach suffers from poor selectivity due to the wide range of cellular reducing agents. To overcome this limitation, we leverage reduction potential of ascorbate to mediate a copper-mediated oxidative bond cleavage of ether-caged fluorophores. The copper(II) complexes supported by a {bis(2-pyridylmethyl)}benzylamine or a {bis(2-pyridylmethyl)}(2-methoxybenzyl)amine ligand were identified as an ascorbate responsive unit and their reaction with ascorbate yields a copper-based oxidant that enables rapid benzylic oxidation and the release of an ether-caged dye (coumarin or fluorescein). The copper-mediated bond cleavage is specific to ascorbate and the trigger can be readily derivatized for tuning photophysical properties of the probes. The probes were successfully applied for the fluorometric detection of ascorbate in commercial food samples, human plasma, and serum, and within live cells by using confocal microscopy and flow cytometry.

Near-Infrared Dual-Emission Ratiometric Fluorescence Imaging Nanoprobe for Real-Time Tracing the Generation of Endogenous Peroxynitrite in Single Living Cells and In Vivo

Peroxynitrite (ONOO^-) is a highly reactive nitrogen species with potent oxidant and nitrating properties. Its excessive generation can cause DNA and protein damage, thereby contributing to cell injury, and it is closely related to the development of many diseases. Thus, there is an urgent need for a reliable method to determine changes in the steady-state levels of ONOO^- in vivo. Ratiometric imaging, due to its built-in self-calibration system, can reduce artifacts and enable reliable in vivo imaging. In this study, we designed and prepared near-infrared (NIR) biomass quantum dots (NI-BQDs) and covalently coupled them with the NIR dye Cyanine7 (Cy7) to construct an NIR dual-emission nanoprobe (NI-BQD-Cy7) for real-time tracing the generation of endogenous ONOO^- in single living cells and in vivo by ratiometric fluorescence imaging. NI-BQD-Cy7 exhibited high detection sensitivity and selectivity for ONOO^- in the mitochondria. Additionally, it can produce dual NIR fluorescence emission, thus allowing in situ ratiometric fluorescence imaging to real-time trace the generation and concentration changes of ONOO^- in vivo. The application of the proposed NIR dual-emission nanoprobe can provide accurate information for the study of the biological function of ONOO^- in single living cells and in vivo, and it is very useful to explain the mechanism of cell damage caused by ONOO^-.

Preparation of highly crystalline nitrogen-doped carbon dots and their application in sequential fluorescent detection of Fe3+ and ascorbic acid

Carbon dots (CDs) have been a new class of fascinating carbon-based fluorescent nanomaterials. In the present work, new N-doped CDs with highly crystalline graphite structures are prepared from renewable precursors, chitosan and tartaric acid, and are well characterized. The prepared CDs are applied as a biocompatible fluorescent sensor for the sequential detection of Fe³⁺ and AA. Among various transition metal ions, Fe³⁺ can selectively quench the fluorescence of CDs. Upon the further addition of AA, the quenched fluorescence of CDs is then restored as Fe³⁺ is reduced to Fe²⁺ by AA, which can be utilized for the fluorescent determination of AA. A good linear relationship in the range of 0-150 μM of AA concentration is established with a low detection limit of 0.02 µM. Moreover, the practical applications of this fluorescent sensing method in measurement of AA in food samples are successfully realized with satisfactory results.

Fluorescence Turn-On Detection of Ascorbic Acid Using a Self-Assembled Lanthanide Polymer Nanoparticle

Ascorbic acid (AA), or vitamin C, is an important reactive biological molecule in vivo, and an abnormal level of AA is associated with many diseases. Therefore, the rapid, sensitive, and selective detection of AA levels is of significance in cases of medical assay and diagnosis. Compared with other nanoparticles, lanthanide coordination polymer nanoparticles (Ln-CPs) have been demonstrated as the excellent biomolecule sensing platforms due to their unique optical properties and intrinsic porosities. In this work, the cerium coordination polymer nanoparticles ATP-Ce-Tris were synthesized in a simple and quick way. The synthesized ATP-Ce-Tris nanoparticle shows the characteristic peak of Ce³⁺ located at 365 nm, which is corresponding to the 4f→5d transition of Ce³⁺. In the presence of Fe³⁺, the fluorescence of ATP-Ce-Tris quenched, and the following added ascorbic acid (AA) makes it restoring effectively. Based on this, we constructed a fluorescence probe with excellent sensitivity for AA sensing in a wide linear relationship from 0.05 to 500 μM. The detection limit was as low as 18 nM (signal-to-noise ratio of three), which is one or two orders of magnitude lower than those of reported sensors. The proposed sensing systems also exhibits excellent sensitivity for AA detection in human serum sample, exploiting a valuable platform for AA analysis in clinic diagnostic and drug screening.

One-step rapid synthesis of Ni6(C12H25S)12 nanoclusters for electrochemical sensing of ascorbic acid

Metal nanoclusters (NCs) are highly desirable as active catalysts due to their highly active surface atoms. Among the reported metal clusters, nickel nanoclusters (Ni NCs) have been less well developed than others, such as gold, silver and copper. Herein a simple method is developed to synthesize atomically precise Ni clusters with the molecular formular of Ni₆(C₁₂H₂₅S)₁₂. Moreover, the single crystal of the Ni₆(C₁₂H₂₅S)₁₂ cluster is also obtained. The composition, morphology and optical properties of the prepared Ni₆ clusters are characterized by X-ray crystallography, XPS, XRD, SEM, HRTEM, FTIR and UV-Vis spectroscopy. The Ni cluster is composed of six nickel atoms that form a hexagonal ring with an exterior 1-dodecanethiol shell, resembling a double crown. Meanwhile, the Ni₆ NCs can be self-assembled into nanosheets due to their uniform size. It was found that the Ni₆(C₁₂H₂₅S)₁₂ clusters loaded on carbon black exhibit higher electrocatalytic activity than Ni nanoparticles towards ascorbic acid (AA) oxidation. The Ni₆ clusters show high sensing performance for AA with a wide linear range (1-3212 μM) and a low detection limit of 0.1 μM (S/N = 3). The significantly enhanced catalytic activity can be ascribed to the high fraction of surface Ni atoms with low coordination in the sub-nanometer clusters. The present work not only provides a straightforward method for synthesizing atomically precise metal clusters but also indicates that ultrasmall Ni clusters can be used as highly efficient catalysts for the electrochemical detection of AA.

o-Phenylenediamine/gold nanocluster-based nanoplatform for ratiometric fluorescence detection of alkaline phosphatase activity

This study demonstrates a novel and convenient ratiometric fluorescent method for the detection of alkaline phosphatase (ALP) activity. Amino-functionalized mesoporous silica nanoparticle-gold nanoclusters (MSN-AuNCs) nanocomposites were integrated with o-phenylenediamine (OPD) to form a ratiometric fluorescence nanoplatform. The presence of ALP induced the generation of quinoxaline (QX) derivative which called 3-(dihydroxyethyl)furo[3,4-b]quinoxaline-1-one (DFQ) with strong fluorescence emission at 450 nm, while the orange-red fluorescence of MSN-AuNCs at 580 nm was slightly quenched. Meanwhile, an obvious fluorescence color change from orange-red to purple and finally to blue can be observed by naked eyes with the increasing of ALP concentration. Therefore, employing the fluorescence emission of DFQ at 450 nm as the reporter signal and the fluorescence emission of MSN-AuNCs at 580 nm as a reference signal, a sensitive ratiometric detection method for ALP was developed. Quantitative detection of ALP activity in the linear range from 0.2 to 80 U/L with a detection limit of 0.1 U/L can be realized in this way, which endows the assay with high sensitivity enough for practical detection of ALP in human serum samples.

Biomineralization synthesis of a near-infrared fluorescent nanoprobe for direct glucose sensing in whole blood

A near-infrared (NIR) fluorescent nanoprobe that enables to circumvent the interference of background absorption and fluorescence in whole blood was developed for the direct sensing of blood glucose. Here, NIR fluorescent protein (iRFP) and glucose oxidase (GOx) were collectively deployed as the templates for the biomineralization of Mn2+ to prepare a NIR fluorescent nanoprobe (iRFP-GOx-MnO2 nanoparticles, iRGMs), in which the fluorescence of iRFP was effectively quenched by MnO2via energy transfer. When the iRGMs were mixed with whole blood samples, GOx can convert blood glucose into gluconic acid, as well as H2O2, which will reduce MnO2 and decompose the iRGMs. As a result, the NIR fluorescence of iRFPs was restored, providing a fluorometric assay for the direct detection of blood glucose. Owing to the high efficiency of the cascade reaction and the low background interference of the NIR fluorescence signal, accurate and rapid analysis of the glucose levels in whole blood samples was achieved using the iRGMs. Moreover, an iRGM-based paper device that only requires 5 microliters of samples was also demonstrated in the direct assay of blood glucose without any pretreatment, affording an alternative approach for the accurate monitoring of blood glucose levels.

A turn-on luminescent probe for Fe3+ and ascorbic acid with logic gate operation based on a zinc(ii)-based metal–organic framework

A zinc(ii)-based metal–organic framework exhibits fluorescence turn-on behaviour for Fe³⁺ and AA with high sensitivity and selectivity.

Nanostructured MnO2 nanosheets grown on nickel foam: an efficient and readily recyclable 3D artificial oxidase for the colorimetric detection of ascorbic acid

.An efficient and readily recyclable 3D artificial oxidase (NF@MnO₂) for the colorimetric detection of ascorbic acid was well constructed with nickel foam as the substrate.

Carbon dot-based fluorometric optical sensors: an overview

Fluorescent carbon dots (CDs) are a new class of carbon nanomaterials and have demonstrated excellent optical properties, good biocompatibility, great aqueous solubility, low cost, and simple synthesis. Since their discovery, various synthesis methods using different precursors were developed, which were mainly classified as top-down and bottom-up approaches. CDs have presented many applications, and this review article mainly focuses on the development of CD-based fluorescent sensors. The sensing mechanisms, sensor design, and sensing properties to various targets are summarized. Broad ranges of detection, including temperature, pH, DNA, antibiotics, cations, cancer cells, and antibiotics, have been discussed. In addition, the challenges and future directions for CDs as sensing materials are also presented.

Bovine serum albumin encapsulation of near infrared fluorescent nano-probe with low nonspecificity and cytotoxicity for imaging of HER2-positive breast cancer cells

Breast cancer with HER2 overexpressing type links to malignant tumor growth and poor clinical outcome. Successful development of sensitive and selective nano-probe for identification of HER2-positive breast cancer cells is of great importance for breast cancer early diagnosis, subtype classification, and treatment planning. Herein, we report a HER2 antibody conjugated near infrared (NIR) emitted MnCuInS/ZnS qumtun dots (QDs) encapsulated bovine serum albumin (BSA) nano-probe for accurately targeted imaging of HER2-positive breast cancer cells. This NIR nano-probe shows good biocompatibility, low nonspecificity and cytotoxicity, high colloidal stability, and allows HER2-positive breast cancer cell identification with good selectivity. The practicality of this targeted NIR fluorescent nano-probe was proved by successful identifying HER2-positive breast cancer cells from HER2-negative breast cancer cells, which indicates that it can be efficiently applied in selective screening of HER2 overexpressing cancer cells, and provide a platform for the strategy design on the distinction of different breast cancer subtypes.

Dual-emission ratiometric nanoprobe for visual detection of Cu(II) and intracellular fluorescence imaging

Copper is an essential mineral nutrient for the human body. However, excessive levels of copper accumulated in the body can cause some diseases. Therefore, it is great significant to establish a sensitive bioprobe to recognize copper ions (Cu²⁺) in vivo. In our work, nitrogen-doped carbon dots (N-CDs) and gold nanoclusters (Au NCs) are selected as luminescent nanomaterials and the Au NCs/N- CDs nanohybrids is successfully synthesized by coupling method. The Au NCs/N-CDs exhibited characteristic dual-emission peaks at 450 and 620 nm when excited by a single-wavelength of 380 nm. When different amounts of Cu²⁺ are introduced, the fluorescence intensity of the Au NCs is gradually weakened and fluorescence intensity of the N-CDs is almost unchanged, which can facilitate the visual detection of Cu²⁺. The Au NCs/N-CDs nanohybrid possesses good selectivity to Cu²⁺ with a limit of detection (LOD) is 3.5 μM and linear detection range of 10-150 μM. Visualization detection of Cu²⁺ is implemented by using nanoprobe in water samples. Furthermore, the ratiometric nanoprobe is utilized to the toxicity test of liver cancer cells, indicating excellent biocompatibility and low toxicity. This nanoprobe has been used to the intracellular fluorescence imaging. Moreover, this method is expected to be used to monitor the changes of Cu²⁺ concentration in hepatocytes.

Water-dispersed silicon quantum dots for on-off-on fluorometric determination of chromium(VI) and ascorbic acid

Water-dispersed silicon quantum dots (SiQDs) with the quantum yield of 25% was prepared using aminopropyltrimethoxysilane as the silicon source and ascorbic acid (AA) as the reduction reagent. The SiQDs display blue fluorescence with excitation/emission peaks at 350 nm/440 nm. The synthesized SiQDs are shown to be a viable "on-off-on" fluorescent probe for the detection of Cr(VI) and AA. Cr(VI) ions exert an inner filter effect on the fluorescence of the SiQDs which results in a reduction of fluorescence (off-state). On addition of AA, Cr(VI) is chemically reduced to Cr(III) which weakens the inner filter effect and restores fluorescence (on-state). The method has low detection limits for both Cr(VI) and AA (0.16 μM and 0.57 μM, respectively). It was applied to the analysis of spiked lotus seeds and human serum samples. Graphical abstract A simple and facile "on-off-on" fluorometric method for Cr(VI) and ascorbic acid (AA) was developed using water-soluble silicon quantum dots (SiQDs) as the fluorescent probe. The approach was also used to assay Cr(VI) and AA in the lotus seeds and human serum, respectively.

Quantum Dots in Ophthalmology: A Literature Review

Purpose: The aim of the current review was to summarize the current applications, the latest advances and importantly, highlight research gaps in the use of quantum dots in the eye. Quantum dots are nanoscale semiconductor crystals with characteristic size and tunable optical properties, which deliver bright and stable fluorescence suitable for bioimaging and labelling. Methods: A systematic search was conducted following the PRISMA guidelines. This review systematically searched published data to summarize the characteristics and applications of quantum dots in ophthalmology. Two hundred and eighty published articles were initially selected for this review following searches using the criteria quantum dots AND nanoparticles AND ophthalmology in the databases PubMed, MEDLINE, Scopus, Embase and Web of Science. Results: After duplicates were removed, a total of 22 eligible articles were included for the review. Quantum dots potentially provide a range of diagnostic and therapeutic applications in ophthalmology. Quantum dots offer visible and near-infrared emission, which is highly desirable for bioimaging, due to reduced light scattering and low tissue absorption. Their applications include in vivo bioimaging, labelling of cells and tissues, delivery of genes or drugs and as antimicrobial composites. Conclusion: Quantum dots have been used in ophthalmology for bioimaging, electrical stimulation and tracking of gene/stems cells, and ocular lymphatics. However, there is no detailed description of their desirable characteristics for use in ophthalmology, and there is limited information about their cytotoxicity to ocular cells and tissues.

A Photoluminescent Colorimetric Probe of Bovine Serum Albumin-Stabilized Gold Nanoclusters for New Psychoactive Substances: Cathinone Drugs in Seized Street Samples

Screening of illicit drugs for new psychoactive substances-namely cathinone-at crime scenes is in high demand. A dual-emission bovine serum albumin-stabilized gold nanoclusters probe was synthesized and used for quantitation and screening of 4-chloromethcathinone and cathinone analogues in an aqueous solution. The photoluminescent (PL) color of the bovine serum albumin-stabilized Au nanoclusters (BSA-Au NCs) probe solution changed from red to dark blue during the identification of cathinone drugs when excited using a portable ultraviolet light-emitting diodes lamp (365 nm). This probe solution allows the PL color-changing point and limit of detection down to 10.0 and 0.14 mM, respectively, for 4-chloromethcathinone. The phenomenon of PL color-changing of BSA-Au NCs was attributed to its PL band at 650 nm, quenching through an electron transfer mechanism. The probe solution was highly specific to cathinone drugs, over other popular illicit drugs, including heroin, cocaine, ketamine, and methamphetamine. The practicality of this BSA-Au NCs probe was assessed by using it to screen illicit drugs seized by law enforcement officers. All 20 actual cases from street and smuggling samples were validated using this BSA-Au NCs probe solution and then confirmed using gas chromatography-mass spectrometry. The results reveal this BSA-Au NCs probe solution is practical for screening cathinone drugs at crime scenes.

Clearable Theranostic Platform with a pH-Independent Chemodynamic Therapy Enhancement Strategy for Synergetic Photothermal Tumor Therapy

Chemodynamic therapy (CDT) is an emerging field, which utilizes intratumoral iron-mediated Fenton chemistry for cancer therapy. However, the slightly acidic tumor environment is improper for the classical Fenton reaction, which is generally energetic in a narrow pH range (e.g., pH = 3-4). Herein, a kind of ultrasmall bovine serum albumin (BSA)-modified chalcopyrite nanoparticles (BSA-CuFeS₂ NPs) was synthesized via a facile aqueous biomineralization strategy, which shows high dispersity and biocompatibility. Interestingly, the obtained BSA-CuFeS₂ shows a pH-independent Fenton-like reaction, which could exert Fenton-like activity to efficiently generate ^•OH under a weak acidic tumor environment. Combined with the extraordinarily high photothermal conversion (38.8%), BSA-CuFeS₂ shows the synergistic function of high photothermal therapy (PTT) and enhanced CDT, that is, PTT/CDT. Importantly, such ultrasmall BSA-CuFeS₂ NPs measuring around 4.9 nm can be quickly cleared out of the body through kidneys and liver, thus effectively avoiding long-term toxicity and systemic toxicity. Moreover, BSA-CuFeS₂ NPs can act as an efficient T₂-weighted magnetic resonance imaging (MRI) contrast agent to guide tumor ablation in vivo. This work offers a universal approach to boost production ^•OH by a pH-independent Fenton-like reaction strategy and achieves MRI-guided synergistic enhanced photothermal-CDT for highly efficient tumor treatment.

Rapid and simple detection of ascorbic acid and alkaline phosphatase via controlled generation of silver nanoparticles and selective recognition

Ascorbic acid (AA) and alkaline phosphatase (ALP) serve as an important coenzyme and enzyme in multiple biological metabolism reactions, respectively, and abnormal levels of these substrates have been associated with several diseases. Herein, a new and simple fluorescence strategy has been developed for AA and ALP sensing by exploiting CdTe quantum dots (QDs) as an effective signal indicator. This method is mainly based on the selective fluorescence-quenching reaction between Ag+ and CdTe QDs, as opposed to silver nanoparticles (Ag NPs); Ag+ can be reduced to Ag NPs by AA. Furthermore, by taking advantage of AA as a mediator, this strategy is further exploited for ALP assay given that ALP can cause the hydrolysis of l-ascorbic acid-2-phosphate (AAP), which yields AA. Under optimal conditions, controlled generation of Ag NPs and the selective recognition-based sensing system exhibit high sensitivity toward AA and ALP with limits of detection (LODs) of 3 μM and 0.25 U L-1 and linear ranges of detection from 0 to 800 μM and 1 to 1000 U L-1, respectively. Moreover, the sensor was successfully used for assaying AA in fruit juice and ALP in human serum. The results demonstrate that the proposed fluorescence strategy has significant advantages, such as its simplicity, cost-effectiveness, and rapid runtime, and the operational convenience of this label-free method further demonstrates its potential for constructing effective sensors with biochemical and clinical applications.

Recent advances on fluorescent biomarkers of near-infrared quantum dots for in vitro and in vivo imaging

Luminescence probe has been broadly used for bio-imaging applications. Among them, near-infrared (NIR) quantum dots (QDs) are more attractive due to minimal tissue absorbance and larger penetration depth. Above said reasons allowed whole animal imaging without slice scan or dissection. This review describes in vitro and in vivo imaging of NIR QDs in the regions of 650-900 nm (NIR-I) and 1000-1450 nm (NIR-II). Also, we summarize the recent progress in bio-imaging and discuss the future trends of NIR QDs including group II-VI, IV-VI, I-VI, I-III-VI, III-V, and IV semiconductors.