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BACKGROUND: Assembling framework nucleic acid (FNA) nanoarchitectures and tuning luminescent quantum dots (QDs) for fluorescence assays represent a versatile strategy in analytical territory. Rationally, FNA constructs could offer a preferential orientation to efficiently recognize the target and improve detection sensitivity, meanwhile, regulating size-dependent multicolor emissions of QDs in one analytical setting for ratiometric fluorescence assay would greatly simplify operation procedures. Nonetheless, such FNA/QDs-based ratiometric fluorescence nanoprobes remain rarely explored. RESULTS: We designed a sensitive and signal amplification-free fluorescence aptasensor for lead ions (Pb2+) that potentially cause extensive contamination to environment, cosmetic, food and pharmaceuticals. Red and green emission CdTe quantum dots (rQDs and gQDs) were facilely prepared. Moreover, silica nanosphere encapsulating rQDs served as quantitative internal reference and scaffold to anchor a predesigned FNA and DNA sandwich containing Pb2+ binding aptamer and gQD modified DNA signal reporter. On binding of Pb2+, the gQD-DNA signal reporter was set free, resulting in fluorescence quenching at graphene oxide (GO) interface. Owing to the rigid structure of FNA, the fluorescence signal reporter orderly arranged at the silica nanosphere could sensitively respond to Pb2+ stimulation. The dose-dependent fluorescence signal-off mode enabled ratiometric analysis of Pb2+ without cumbersome signal amplification. Linear relationship was established between fluorescence intensity ratio (I555/I720) and Pb2+ concentration from 10 nM to 2 µM, with detection limit of 1.7 nM (0.43 ppb), well addressing the need for Pb2+ routine monitoring. The designed nanoprobe was applied to detection of Pb2+ in soil, cosmetic, milk, drug, and serum samples, with the sensitivity comparable to conventional ICP-MS technique. SIGNIFICANCE: Given the programmable design of FNA and efficient recognition of target, flexible tuning of QDs emission, and signal amplification-free strategy, the present fluorescence nanoprobe could be a technical criterion for other heavy metal ions detection in a straightforward manner.

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A novel time-resolved fluorescence nanoprobe (PBMO, PLNR-BSA-Mn2+-OPD) is fabricated for the label-free determination of acetylcholinesterase (AChE). The ZnGeO:Mn persistent luminescence nanorod (PLNR) and Mn(II) are, respectively, exploited as the signal molecule and quencher to construct the PBMO nanopobe using bovine serum albumin (BSA) as the surface-modified shell and o-phenylenediamine (OPD) as the reducing agent. In the presence of H2O2, the persistent luminescence of PBMO at 530 nm is enhanced remarkably within 30 s due to the oxidation of Mn(II). H2O2 can react with thiocholine (TCh), which is produced through the enzymatic degradation of acetylcholine (ATCh) by AChE. The PBMO nanoprobe is successfully applied to the determination of AChE in the linear range of 0.08-10 U L-1, with a detection limit of 0.03 U L-1 (3σ/s). The practicability of this PBMO nanoprobe is confirmed by accurately monitoring AChE contents in human serum samples, giving rise to satisfactory spiking recoveries of 96.2-103.6%.

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In this work, a facile and fast aqueous-phase synthetic method is proposed to prepare water-soluble ZnS quantum dots stabilized simultaneously with glutathione and L-cysteine (ZnS QDs-GSH/L-Cys). As-synthesized ZnS QDs-GSH/L-Cys were monodispersed spherical nanocrystals with a mean diameter of 5.0 ± 0.7 nm. Besides, the obtained ZnS QDs-GSH/L-Cys emitted more intensive blue fluorescence and exhibited an improved stability in aqueous solution compared with ZnS quantum dots merely stabilized with GSH (ZnS QDs-GSH). Interestingly, Adriamycin, a representative anticancer drug, was added into the solution of ZnS QDs-GSH/L-Cys, the blue fluorescence of ZnS QDs-GSH/L-Cys was greatly enhanced instead of being quenched, which indicated that ZnS QDs-GSH/L-Cys can be used as an enhanced-fluorescence nanoprobe for determining Adriamycin. The observed fluorescent enhancement could be attributed to the blocking of photoinduced electron transfer (PET) in ZnS QDs-GSH/L-Cys due to the electrostatic interaction between the -COO- groups on the surface of quantum dots and the -NH3+ groups in Adriamycin, followed by the coordination interaction among ZnS QDs-GSH/L-Cys and Adriamycin. The fluorescence intensity of ZnS QDs-GSH/L-Cys presented a good linear response with the concentration of Adriamycin ranging from 2.0 to 20 µg•mL-1. The proposed fluorescent nanoprobe exhibited an excellent sensitivity with the LOD of 0.1 µg•mL-1 and a good accuracy for detecting Adriamycin.

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One-pot means was performed for the rapid preparation of copper nanoclusters (Cu NCs), which were employed as a fluorescence system for the sensitive apigenin measurement in pharmaceutical samples. Herein, CuCl2 aqueous solution was reduced to Cu NCs through ascorbic acid and the Cu NCs were protected through trypsin under 65 ℃ for 4 h. The entire preparation process was rapid, facile and environmentally friendly. The trypsin-capped Cu NCs were demonstrated through ultraviolet-visible spectroscopy, fluorescence spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and fluorescence lifetime, respectively. The Cu NCs revealed blue fluorescence with emission wavelength around 465 nm under the excitation wavelength of 380 nm. The fluorescence weakening feature of Cu NCs with apigenin was observed. On this basis, a facile and sensitive turn-off fluorescent nanoprobe for the sensing of apigenin in real samples was developed. The logarithm of relative fluorescence intensity revealed a good linear relationship with apigenin contents from 0.5 µM to 300 µM with the detection limit of 0.079 µM. The Cu NCs-based fluorescent nanosensor have been employed to measure the apigenin amounts in real samples such as medical saline, bovine and human serum. The results revealed excellent potential of this Cu NCs-based fluorescent nanoprobe for the convention computation of apigenin amounts in real samples.

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Tricresyl phosphate (TCP), a notable emerging pollutant with a high bioconcentration factor and biotoxicity, is a typical representative of aryl-organophosphorus flame retardants. The electrochemical and chromatographic technologies used in conventional TCP detection have a variety of drawbacks. Hence, it is crucial to suggest an easy, accurate, and selective method for detecting TCP. In this study, we presented a brand-new method based on NH2-MIL-53(Al) nanoprobe for the direct luminescence assay of TCP. NH2-MIL-53(Al) possessed an excellent crystal structure and superior optical qualities. Notably, the introduction of TCP caused a considerable dampening of the photoluminescence signal of the nanoprobe. The fluorescence response based on static quenching was verified by fluorescence lifetime decay curves. The thermodynamic analysis further concluded that TCP and nanoprobe spontaneously produced non-fluorescent complexes due to hydrophobic interaction. The quenching efficiency (F0-F)/F0 of the nanoprobe and the TCP concentration displayed good linearity in the scope of 0.3-3.0 µM (R2 = 0.996), and the LOD was 0.058 µM under the ideal detection conditions. More significantly, the technique was effectively used to identify TCP in lake and tap water (RSD ≤5.79%), which provided a fresh perspective on how to recognize OPFRs in environmental water.

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A simple, sensitive, and selective fluorometric method based on graphene quantum dots and Hg2+ is presented for the determination of tetracycline. The fluorescence emission of graphene quantum dots at 463 nm decreased in the presence of Hg2+ ions due to its electrostatic interaction with the negatively charged surface of quantum dots at pH = 8.0. The addition of tetracycline to this system resulted in the retrieval of the fluorescence emission of the graphene quantum dots proportional to the tetracycline concentration. This is because of the interaction between tetracycline and Hg2+ that results in the release of the quantum dots' surface. Under the optimized conditions, the calibration curve indicated good linearity in the range of 2.0-44.0 nmol L-1 with a detection limit of 0.52 nmol L-1 for tetracycline. The designed nanoprobe was capable of the determination of tetracycline in serum and urine samples.

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We demonstrate a novel ratio fluorescence nanoprobe for dipicolinic acid (DPA) as an anthrax biomarker based on layered rare-earth hydroxide (LRH). 3-Amino-benzenesulfonic acid (AS) was intercalated into layered terbium hydroxide to form composite and then delaminated into nanosheets in formamide. The monolayer nanosheets were beneficial to expose the Ln3+ luminescence centers to the environment more completely, contributing a high sensitive detection to the environment. With the increase of DPA concentration, the emission intensity of AS kept constant which worked as a stable internal reference, while the fluorescence of Tb3+ was enhanced obviously due to the antenna effect. In the 0.05-5.0 µM concentration range, the I544/I360 fluorescence ratio changed with the DPA concentration, which exhibited a good linear relationship (R2 = 0.999) and an ultralow detection limit of 3.8 nM. In addition, the probe showed high selectivity and sensitivity to the DPA detection as an anthrax biomarker, which can be applied in real tap water with good performances. This work could extend the applications of LRH nanosheets in detection and offer an extremely effective and easy technique for detecting DPA.

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A near-infrared (NIR) fluorescence nanoprobe named RhI-DOX@ZIF-90 has been synthesized by wrapping the guest molecule (RhI and DOX) into ZIF-90 framework. The nanoprobe itself is non-fluorescent and the drug (DOX) is inactive. Upon the addition of ATP, the structure of RhI-DOX@ZIF-90 is degraded. The fluorescence of RhI is recovered and DOX is released. The nanoprobe can detect ATP with high sensitivity and selectivity. There is good linear relationship between the nanoprobe and ATP concentration from 0.25 to 10 mM and the detection limit is 0.10 mM. The nanoprobe has the ability to monitor the change of ATP level in living cells and DOX is released inducing apoptosis of cancer cells. RhI-DOX@ZIF-90 is capable of targeting mitochondria, which provides a basis for improving the efficiency of drug delivery by mitochondrial administration. In particular, the nanoprobe is preferentially accumulated in the tumor sites and detect ATP in tumor mice by fluorescence imaging using near-infrared fluorescence. At the same time, DOX can be released accurately in tumor sites and have good anti-tumor efficiency. So, this nanoprobe is a reliable tool to realize early diagnosis of cancer and improve effect of anticancer drug.

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Nucleic acids as the important tumor markers play a crucial role in the identification of cancer. Various kinds of probes such as gold nanoparticles and graphene oxide have been explored to detect different nucleic acid markers. However, the existing probes are mostly used to detect a single tumor marker and susceptible to harsh conditions in the complex and dynamic physiological environment, which may lead to false positive results and greatly limit the sensing performance of the probe. Herein, a powerful and reliable Au-Se probe was developed for high-fidelity imaging of two cancer markers simultaneously in living cells. Compared with the traditional nucleic acid probe based on the Au-S bond, this probe was more stable against biological thiols and could effectively distinguish normal cells and cancer cells to avoid false positive results, which is more suitable for imaging in a complex physiological environment. This strategy will provide more valuable insights into designing and exploring novel biosensors in the future.

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A novel dual-functional nanoprobe was designed and synthesized by facile assembly of quinoline derivative (PEIQ) and meso-tetra (4-carboxyphenyl) porphine (TCPP) via electrostatic interaction for simultaneous sensing of fluorescence of Zn2+ and pH. Under the single-wavelength excitation at 400 nm, this nanoprobe not only exhibits "OFF-ON" green fluorescence at 512 nm by specific PEIQ-Zn2+ chelation, but also presents red fluorescence enhancement at 654 nm by H+-triggered TCPP release. The nanoprobe demonstrated excellent sensing performance with a good linear range (Zn2+, 1-40 µM; pH, 5.0-8.0), low detection limit (Zn2+, 0.88 µM), and simultaneous response towards Zn2+ and pH in pure aqueous solution within 2 min. More importantly, this dual-functional nanoprobe demonstrates the capability of discerning cancerous cells from normal cells, as evidenced by the fact that cancerous HepG2 cells in tumor microenvironment exhibit substantially higher red fluorescence and significantly lower green fluorescence than normal HL-7702 cells. The simultaneous, real-time fluorescence imaging of multiple analytes in a living system could be significant for cell analysis and tracking, cancer diagnosis, and even fluorescence-guided surgery of tumors.

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Hypoxia is an obvious characteristic of cancer, especially solid tumors. which may give rise to the expansion of invasion and metastasis. Exploring near-infrared (NIR) nanoprobes that could accurately evaluate the degree of hypoxia will contribute to the assessment of the degree of malignant neoplasms, so as to adopt more accurate and individualized treatment options Here, we have developed a self-assembled NIR organic nanoprobe to specifically and authoritatively detect the oxygen concentration in vivo and in vitro to evaluate the level of hypoxia. The organic nanoprobe mainly contains two motifs: a fluorophore moiety NRh-NH2 for NIR fluorescence imaging and hypoxia-sensitive moiety Azonaphthalene derivatives for quenching NIR emissions, detecting oxygen in hypoxic regions and improving the hydrophilicity. The nanoprobes were used for detection of oxygen in a variety of living cells under different conditions and real-time bioimaging of neoplasms in live mice. This design strategy provides ideas for the development of nanoprobes for the diagnosis of tumors and other hypoxia-related diseases.

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A sensitive and robust fluorescent assay of 6-MP is described which relies on the facile assembly of a fluorescence nanoprobe by design of silica nanosphere encapsulated CdTe quantum dots (CdTe QDs) as scaffold, coupling with chemically tethered folic acid (FA)-protected silver nanoparticles (AgNPs) that function as responsive element. In this way a stable ternary core-shell-satellite nanostructure with dual-emission signals can be established. On binding to the target molecules, 6-MP, FA molecules initially occupied by AgNPs are liberated to give dose-dependent fluorescence emission, which can further form a self-calibration ratiometric fluorescence assay using CdTe QDs as an internal reference. The nanoprobe color vividly changes from red to blue, enabling the direct visual detection. The linear concentration range is 0.15~50 µM with the detection limit of 67 nM. By virtue of the favorable selectivity and robust assays, the nanoprobe was applied to 6-MP detection in urine samples, with recoveries from 97.3 to 106% and relative standard deviations (RSD) less than 5%. Graphical abstract.

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Adenosine triphosphate (ATP) is mainly produced in mitochondria and plays an important role in lots of pathological processes such as colitis. Unfortunately, to date, few suitable fluorescence probes have been developed for monitoring the ATP level in colitis. Herein, a fluorescence nanoprobe named NIR@ZIF-90 is proposed and prepared by encapsulating a rhodamine-based near-infrared (NIR) dye into zeolitic imidazolate frameworks (ZIF-90). The nanoprobe is nonfluorescent because the emission of NIR is suppressed by the encapsulation, while in the presence of ATP, the framework of ZIF-90 is dissembled to release NIR and thus NIR fluorescence at 750 nm is observed. The nanoprobe shows high sensitivity to ATP with a 72-fold increase and excellent selectivity to ATP over other nucleotides. Moreover, with low cytotoxicity and good mitochondria-targeted ability, NIR@ZIF-90 is used to image ATP in colorectal cancer cells (HCT116). In addition, due to the NIR emission, the nanoprobe is further employed to successfully monitor the ATP level in a colitis mouse model. To the best of our knowledge, the nanoprobe is the first example to study colitis in vivo with the guidance of ATP, which will provide an efficient tool for understanding colitis.

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Carbon dots and gold nanoclusters co-encapsulated by zeolitic imidazolate framework-8 (CDs/AuNCs@ZIF-8) have been obtained at room temperature. The composite has been applied to the ratiometric fluorescence determination of mercury(II). The composite shows fluorescence emission maxima at 440 and 640 nm under 360 nm excitation, due to the CDs and AuNCs, respectively (associated quantum yields were 18% and 17%, respectively). In the presence of Hg2+, the fluorescence at about 640 nm is quenched, while the fluorescence at about 440 nm is unaffected. The CDs/AuNCs@ZIF-8 composite allows the sensitive detection of Hg2+, with the fluorescence intensity ratio (I640/I440) decreasing linearly with Hg2+ concentration over the range 3-30 nM. The fluorescence emission of the composite changes color from red to blue with increasing Hg2+ under UV excitation, which can easily be discerned visually. This visual detection of Hg2+ is due to the high fluorescence quantum yields of the CDs and AuNCs and the ~ 200 nm separation between the two emission maxima. Graphical abstract (A) Schematic diagram showing the operating principle of the determination for Hg(II). (B) Digital graph of the solutions in absence and presence of 30 nM Hg(II) under a portable UV lamp.

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Based on the inner filter effect mechanism of quantum dots, a ratiometric fluorescence nanoprobe was constructed for the determination of Pb(II) ion. Green emitting quantum dots conjugated with DNA substrate (DNA2) acted as donors providing green fluorescence, while gold nanoparticles coupled with DNA enzyme (DNA1) as acceptors quench the green fluorescence. Meanwhile, Fe3O4 nanosphere served as magnetic substrates to facilitate separation process and red fluorescence as an "inner rule" to eliminate the background signal. In the presence of Pb(II) ion, the DNA1 specifically recognize and capture Pb(II) ion with enhanced catalytic activity, which can cleave DNA2 and "turn on" the green fluorescence (I540), while the red fluorescence (I630) remained unchanged. In this way, the ratio of I540/I630 reflects the Pb(II) ion in the system, enabling the quantitative and selective determination of Pb(II) ion over nine different metal ions. Under optimal conditions, the ratiometric fluorescence assay showed good linearity (R2 = 0.98) within the range 10 to 100 ng mL-1. The limit of detection (LOD) was calculated to be 1.79 pg mL-1 (S/N = 3, n = 3, ±3.8%). The proposed fluorescence nanoprobe provides better sensitivity and accuracy than non-ratiometric signal evaluation for Pb(II) ion determination. Schematic representation of ratiometric fluorescence nanoprobe for Pb(II) ion detection using green fluorescence of I540 as "signal switch" and red fluorescence of I630 as "inner rule." Graphical abstract.

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Herein, a dual-emission metal-organic framework based ratiometric fluorescence nanoprobe was reported for detecting copper(II) ions. In particular, carbon dots (CDs) and gold nanoclusters (AuNCs) were embedded into ZIF-8 (one of the classical metal-organic frameworks) to form CDs/AuNCs@ZIF-8 nanocomposites, which exhibited dual-emission peaks at UV excitation. In the presence of Cu2+, the fluorescence attributed to AuNCs can be rapidly quenched, while the fluorescence of CDs serves as reference with undetectable changes. Therefore, the CDs/AuNCs@ZIF-8 nanocomposites were utilized as a ratiometric fluorescence nanoprobe for sensitive and selective detection of Cu2+. A good linear relationship between the ratiometric fluorescence signal of CDs/AuNCs@ZIF-8 and Cu2+ concentration was obtained in the range of 10-3-103 µM, and the detection limit was as low as 0.3324 nM. The current ratiometric fluorescence nanoprobe showed promising prospects in cost-effective and rapid determination of Cu2+ ions with good sensitivity and selectivity. Furthermore, this nanoprobe has been successfully applied for the quantitative detection of Cu2+ in serum samples, indicating its value of practical application. Graphical abstract Carbon dots (CDs) and gold nanoclusters (AuNCs) were embedded into metal-organic frameworks (ZIF-8) to form CDs/AuNCs@ZIF-8 nanocomposites, which exhibited dual-emission peaks at 365 nm excitation. In the presence of Cu2+, the fluorescence emission peak at 574 nm can rapidly respond by quenching, while the fluorescence at 462 nm serves as reference with undetectable changes.

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Intracellular pH level plays an important role in physiological and pathological processes. The development of nanoprobes for detecting in vivo pH levels is especially important for early diagnosis of disease. Therefore, we develop a hydrophilic carbon points (CDs) using quercetin and ethylenediamine as precursors to monitor intracellular pH. Under optimized conditions, the prepared CDs not only have uniform particle size and morphology, but also possess strong green fluorescence, photostability, and photoreversibility in water medium. The CDs exhibit pH-sensitive fluorescence effect under acidic and alkaline conditions, which is used to achieve "off-on-off" detection pH (from 3.5 to 13.5). Meanwhile, the pH-dependent mechanism is further investigated and explained, which is the fluorescence quenching caused by the pH-induced aggregation. Based on the pH-sensitive characteristics of CDs, it has been applied to the detection of aspartic acid and glutamic acid. More importantly, when applied to live cells, the pH-probe exhibits low cytotoxicity and high sensitivity, and is successfully used in intracellular pH fluorescence imaging. Consequently, this nanoprobe is expected to be used for real-time monitoring of intracellular pH level.

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In cell signal transduction pathways, a series of biochemical reactions and interactions between proteins guarantee physiological responses indicating cell functionality. However, there are a variety of upstream and downstream signal molecules in these pathways with multiple levels of cross-regulation, making it difficult to sequential visualize their relationships in living organisms in complex environments. To investigate the interrelationships among intracellular signaling pathways, a Au-Se bonded nanoprobe with extraordinary stability and strong anti-interference ability was designed and prepared to monitor the evolution of two kinds of apoptosis biomarkers in real time. Two different peptide chains decorated with two dyes were functionalized on the surface of Au nanoparticles (NPs) via Au-Se bonds. These peptide chains can be respectively cleaved by upstream cathepsin B proteins and downstream caspase-3 proteins to trigger fluorescence recovery. Moreover, when the living cells were stimulated to induce the apoptotic pathway, cathepsin B and caspase-3 were activated in turn with signals sequentially recovered at 2 and 4 h, respectively. This fluorescent nanoprobe can be used in complicated biological systems to achieve real-time in situ monitoring of the sequential activation of signal molecules in intracellular pathways and provides a novel approach for the future investigation of protein interactions in vivo.

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Efficient and instant detection of biological threat-agent anthrax is highly desired in the fields of medical care and anti-terrorism. Herein, a new ratiometric fluorescence (FL) nanoprobe was elaborately tailored for the determination of 2,6-dipicolinic acid (DPA), a biomarker of anthrax spores, by grafting terbium ions (Tb3+) to the surface of carbon dots (CDs). CDs with blue FL were fabricated by a simple and green method using schizochytrium as precursor and served as an FL reference and a supporting substrate for coordination with Tb3+. On account of the absorbance energy transfer emission effect (AETE), green emission peaks of Tb3+ in CDs-Tb nanoprobe appeared at 545 nm upon the addition of DPA. Under optimal conditions, good linearity between the ratio FL intensity of F545/F445 and the concentrations of DPA was observed within the experimental concentration range of 0.5-6 µM with the detection limit of 35.9 nM, which is superior to several literature studies and significantly lower than the infectious dosage of the Bacillus anthracis spores. Moreover, the CDs-Tb nanoprobe could sensitively detect DPA in the lake water sample. This work offers an efficient self-calibrating and background-free method for the determination of DPA.

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The employment of aggregation induced emission (AIE) species for detecting analytes has become ubiquitous in many applications ranging from environmental monitoring to novel chemical sensing processes. Herein, a new organic building block (4,4',4″,4″'-(ethene-1,1,2,2-trayltetrakis (benzene-4,1-diyl))tetrakis(1-methylpyridin-1-ium) boric acid (TPE-B)) has been synthesized and such chromophore exhibits very weak emission in aqueous solution. The molecule-surfactant interaction can lead to distinguished yellow emissions and the incorporation of sodium dodecyl sulfonate (SDS) will generate morphological changes from irregular organic clusters to aggregated nanoparticles with the size of 45 nm. A six-fold intensity enhancement has been observed and the electrostatic forces are believed to act as the primary role for the selective response to SDS. Based on the in situ established TPE-B-SDS framework, a switched-off effect has been observed in the presence of ClO- and this signal change will allow us to accurately determine the concentration of such reactive oxygen species (ClO-). The limits of detection for SDS and ClO- are calculated to be 54.2 nM and 14.2 nM, respectively. These excellent optical properties have been extended into practical range and the results for the detection of SDS and ClO- in tap water samples are satisfactory. It is anticipated that the responsive probe will provide deeper insights into multi-targets sensing in extensive systems.

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