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Bisulfite (HSO3−) and sulfite (SO32−) are commonly employed in food preservatives and are also significant environmental pollutants. Thus, developing an effective method for detecting HSO3−/SO32− is crucial for food safety and environment monitoring. In this work, based on carbon dots (CDs) and zeolitic imidazolate framework‐90 (ZIF‐90), a composite probe (named CDs@ZIF-90) is constructed. The fluorescence signal and the second-order scattering signal of CDs@ZIF-90 are employed to ratiometricly detect HSO3−/SO32−. This proposed strategy exhibits a broad linear range for HSO3−/SO32− determination (10 µM to 8.5 mM) with a limit of detection of 2.74 μM. This strategy is successfully applied for evaluating HSO3−/SO32− in sugar with satisfactory recoveries. Therefore, this work has uniquely combined the fluorescence and second-order scattering signals to establish a novel sensing system with a wide linear range, which is applicable for ratiometric sensing of HSO3−/SO32− in actual samples. [ABSTRACT FROM AUTHOR]

Layered nanostructures (LNs), including two-dimensional nanosheets, nanoflakes, and planar nanodots, show large surface-to-volume ratios, unique optical properties, and desired interfacial activities. LNs are highly promising as alternative probes and platforms due to numerous merits, e.g. signal amplification, improved recognition ability, and anti-interference capacity, for emerging sensing applications. Significantly, when stimuli-responsive aggregation occurs, the modified LNs show engineered morphologies, attractive optical absorption and fluorescence characteristics, which are remarkably programmable. On the basis of the altered aggregation behaviours of LNs, as well as their modulated physical and chemical characteristics, a series of novel sensing assays exhibiting enhanced sensitivity, simple operation, multiple functions, and improved anti-interference capacity are reported, contributing to both point-of-care testing and high-throughput measurements. Herein, the aggregation-induced response sensing strategies of LNs are comprehensively summarized with the classification of materials and variation of aggregated routes aiming at understanding dimension-dependent features, expanding nanoscale biosensor applications, and addressing key issues in disease diagnosis and environmental analysis. [ABSTRACT FROM AUTHOR]

Ratiometric assays, which can effectively surmount external interference, have attracted extensive research interests. Herein, a novel ratiometric sensing platform for Hg2+ is designed based on nitrogen-doped carbon dots (N-CDs) with two different optical signals. Under a single excitation, N-CDs have two emission peaks around 668 nm and 412 nm, which are second-order scattering and fluorescence, respectively. Upon the addition of Hg2+, the weak scattering emission at 668 nm can be increased apparently, while the strong fluorescence intensity at 412 nm is weakened. Moreover, the ratio of scattering intensity to fluorescence intensity is linearly dependent on Hg2+ concentration (0.1–10 μM and 10–30 μM, respectively), and the detection limit is 66 nM. In addition, the ratiometric sensing mechanism is investigated in detail, which is due to the combined effect of aggregation-induced fluorescence quenching and scattering enhancement. Furthermore, the developed sensing approach holds a promising application for Hg2+ detection in actual samples. [ABSTRACT FROM AUTHOR]

In this work, a facile and label-free ratiometric sensor is constructed for selective determination of norepinephrine (NE) by coupling second-order scattering (SOS) and fluorescence, two different and independent optical signals. Herein, polyethyleneimine (PEI) dilute solution medium shows an intensive SOS signal without any fluorescence response. Interestingly, NE can be selectively induced by PEI to emit bright fluorescence, and meanwhile causes an observable decrease in the SOS signal due to the interactions between NE and PEI. The simultaneous variation of the two independent signals can be used for ratiometric sensing of NE. Under the optimal conditions, the resultant ratiometric sensor displays high sensitivity and selectivity toward NE by simultaneously monitoring fluorescence and SOS signals with the same excitation wavelength. The proposed sensor exhibits a good linear relationship versus NE concentration in the range of 10.0 nM–45.0 μM with a detection limit of 2.0 nM (S/N = 3) and has been successfully applied to the determination of NE in real samples without the use of any extra reagent. The combination of fluorescence and SOS signals provides a new scheme for ratiometric sensor design, greatly simplifying experimental procedure and effectively enhancing detection accuracy. Moreover, the proposed analytical strategy further broadens the application of dilute solutions of polymers in research into optical sensor and green analytical chemistry. [ABSTRACT FROM AUTHOR]

This work proposed a rapid and novel fluorescence-sensing system using a complex of acridine orange (AO) and polystyrene sulfonate (PSS) to sensitively recognize and monitor cetyltrimethylammonium bromide (CTAB) in an aqueous medium. AO can interact with PSS and a complex is formed via electrostatic attraction and hydrophobic interaction. The fluorescence of AO is greatly quenched after the introduction of PSS. Upon its subsequent addition, CTAB can interact and form a complex with PSS because the electrostatic attraction between CTAB and PSS is much stronger than that between AO and PSS, which results in significant fluorescence recovery. Interestingly, the proposed method can be applied for the discrimination and detection of surfactants with different hydrocarbon chain lengths due to their different binding affinity toward PSS. The detection limit for CTAB is as low as 0.2 µg/mL and the linear range is from 0.5 to 3.5 µg/mL. Moreover, we applied the sensor to the successful detection of CTAB in water samples. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Highlights: [•] A simple, practical, and reliable method for detection of Cu2+ is developed. [•] The system shows high sensitivity, low detection limit, and good selectivity to Cu2+. [•] The entire measurement process for Cu2+ is very rapid and simple. [•] Detection of Cu2+ in the real sample has satisfied recovery and accuracy. [•] The hydrophilic, low-cost, and biocompatible PEI plays a unique role. [ABSTRACT FROM AUTHOR]

In order to detect ascorbic acid (AA) sensitively, a dual-signal optical sensor of a nanosystem with carbon dots (CDs)/MnO2 nanosheets based on fluorescence and second-order scattering (SOS) has been constructed. Here, MnO2 nanosheets, both as a fluorescence quencher and signal transducer of SOS, quench the blue fluorescence of CDs by an inner filter effect. Under the excitation of 300 nm, the nanosystem shows a fluorescence emission peak at 405 nm and a SOS peak at 610 nm, respectively. With the increase of AA , the lamellar structure of MnO2 nanosheets is etched into a smaller nanostructure, causing a decrease of the fluorescence recovery of CDs (405 nm) and decrease of the SOS signal of the MnO2 nanosheets (610 nm). According to the simultaneous changes of fluorescence and SOS signals, a dual-signal optical sensor toward AA is successfully constructed. Satisfactorily, the optical sensor for AA detection shows a detection limit of 88 and 105 nM for fluorescence and SOS, respectively. The practical application of the designed sensor is verified through the detection of AA content in vitamin C tablets, and satisfactory results are obtained [ABSTRACT FROM AUTHOR]

Water-soluble fluorescent polymer dots (PDs) were prepared from polyethylenimine and glutathione and are shown to be viable fluorescent probes for selective and sensitive determination of Hg(II). The PDs possess bright blue fluorescence (with excitation/emission peaks at 340/462 nm) which is quenched on addition of Hg(II). Based on these findings, a fluorometric assay was worked out. Fluorescence linearly drops in the 0.1 to 100 μM Hg(II) concentration range, and the limit of detection is 32 nM.The fluorescence of polymer dots prepared from glutathione and polyethyleimine (G-PEI PDs) is selectively quenched by Hg2+, and this finding was applied to the determination of Hg2+ in environmental water samples. [ABSTRACT FROM AUTHOR]

A convenient potassium tert -butoxide catalyzed addition–elimination reaction has been achieved using exo -cyclic enol ethers and aryl aldehydes as the starting materials. The transition-metal free reaction proceeded smoothly to afford 1,3-dihydroisobenzofuran derivatives with good to excellent yields. More importantly, the resulting products were discovered as novel fluorophores with good fluorescence properties and remarkable Stokes shifts. Changing the nature of the substituents in 1,3-dihydroisobenzofurans derivatives allowed the maximum emission wavelengths to be tuned between 438 and 597 nm and the Stokes shifts varied between 63 and 166 nm. In particular, derivative C27 containing a piperidyl and a cyano group showed the maximum emission wavelength of 597 nm and a Stokes shift of 166 nm. [ABSTRACT FROM AUTHOR]

In this contribution, a luminescent gold nanoclusters which were synthesized by bovine serum albumin as novel fluorescent probes were successfully utilized for the determination of d -penicillamine for the first time. Cupric ion was employed to quench the strong fluorescence of the gold nanoclusters, whereas the addition of d -penicillamine caused obvious restoration of fluorescence intensity of the Cu 2+ -gold nanoclusters system. Under optimum conditions, the increment in fluorescence intensity of Cu 2+ -gold nanoclusters system caused by d -penicillamine was linearly proportional to the concentration of d -penicillamine in the range of 2.0 × 10 −5 –2.39 × 10 −4 M. The detection limit for d -penicillamine was 5.4 × 10 −6 M. With the off–on fluorescence signal at 650 nm approaching the near-infrared region, the present sensor for d -penicillamine detection had high sensitivity and low spectral interference. Furthermore, the novel gold nanoclusters-based fluorescent sensor has been applied to the determination of d -penicillamine in real biological samples with satisfactory results. [ABSTRACT FROM AUTHOR]

[Display omitted] • OQDs with great sensing potency are synthesized by one-step method. • OQDs realize the distinguishing and sequential detection of Ag+ and Hg2+. • A unique Ag+-triggered AIE enhancement mechanism of OQDs is clarified. • A smartphone-based sensing platform is established based on chromatic change. Realizing the accurate recognition and quantification of heavy metal ions is pivotal but challenging in the environmental, biological, and physiological science fields. In this work, orange fluorescence emitting quantum dots (OQDs) have been facilely synthesized by one-step method. The participation of silver ion (Ag+) can evoke the unique aggregation-induced emission (AIE) of OQDs, resulting in prominent fluorescence enhancement, which is scarcely reported previously. Moreover, the Ag+-triggered turn-on fluorescence can be continuously shut down by mercury ion (Hg2+). This intriguing sequential fluorescence variation exhibits great sensing potency for discrimination and quantification of Ag+ and Hg2+. Meanwhile, our OQDs also exhibit good selectivity, sensitivity, and rapid response toward Ag+ and Hg2+ detection. Due to their high performance, OQDs have been applied to the determination of Ag+ and Hg2+ levels in daily necessities and water samples with satisfactory results. Moreover, a portable smartphone-assisted sensing platform based on chromatic change has been constructed, facilitating the real-time and naked-eye visualization in the resource-confined scene. We anticipate that the discovery of these OQDs would be advantageous for exploring novel QDs materials for fluorescence detection. [ABSTRACT FROM AUTHOR]

On-site monitoring of hazardous nitroaromatic explosive 2,4,6-trinitrophenol (TNP) is of great significance. In this work, we develop a bionic electronic eye-integrated dual-channel nanosensor to rapid and accurate quantification of TNP just via a smartphone. The sensing platform is composed of g-CQDs as the TNP-response recognition element and a smartphone as the optical signal detector, possessing simplicity, rapidness, and portability for TNP tracing. The transducer is based on novel green-emitting fluorescent carbon quantum dots (g-CQDs), which display color and fluorescence dual-signal response towards TNP. Under the optimum conditions, the color and fluorescence intensity change linearly with the increase of the TNP concentration in the range of 1.1–19.8 μM and 0.1–15.8 μM with a low detection limit of 0.4 μM and 27 nM, respectively. It is the inner filter effect that results in the sensitive fluorescence and color switch of CQDs by TNP. Also, the proposed sensor shows excellent selectivity towards TNP, because only TNP can compete for the excitation light with CQDs. Furthermore, the sensing platform is successfully employed to rapid, sensitive, and visual pH monitoring, fulfilling its bifunctional applications. Therefore, the smartphone-integrated dual-mode detection system is promising to be a new portable detector for on-site TNP and pH analysis. Unlabelled Image • A smartphone-driven bi-mode nanosensor based on CQDs for TNP monitoring is built. • The visual colorimetric mode fulfills real-time TNP quantization via a smartphone. • The fluorescent mode shows rapidness, sensitivity, and high specificity to TNP. • The inner filter effect results in the sensitive fluorescence and color switch. • The design is expanded to rapid, sensitive, and visual dual-mode sensing of pH. [ABSTRACT FROM AUTHOR]

Ammonia concentration together with pH values are important and closely linked indexes for aqueous systems. Rapid on-site determination of ammonia or pH is of great significance to environmental monitoring. In this work, a pH-switchable nanoprobe based on biomass carbon dots (CDs) is developed using a smartphone as a simple and handy instrument. The CDs demonstrate sensitive pH response in wide linear ranges of 6.1–13.6, and 2.0–13.6 with colorimetric and fluorescent channels, respectively. It is the pH-induced aggregation that governs the color and fluorescence switch. With the pH evolution caused by the dissolution of ammonia, the smartphone-integrated nanoprobe is applied to ammonia detection with a broad range of 0.5–300 mM. Moreover, the headspace single drop microextraction strategy can concentrate ammonia from matrix, offering a remarkably high selectivity for ammonia determination. Finally, the practical applications of this method for ammonia analysis obtained satisfactory results. Unlabelled Image • A smartphone-coalesced bi-mode CDs nanoprobe for pH monitoring is developed. • The colorimetric mode fulfills pH quantization via the smartphone conveniently. • The fluorescent mode shows high sensitivity to pH in the range of 2.0–13.6. • The pH-induced aggregation governs the color and fluorescence switch of CDs. • Selective ammonia sensing is fulfilled via extracting NH 3 from matrix. [ABSTRACT FROM AUTHOR]

Abstract Ratiometric sensing suffers from less interference and can obtain more accurate results than single-signal assay. Here, a new ratiometric optical sensing strategy for H 2 O 2 detection is developed by etching silver nanoparticles (AgNPs) to deactivate fluorescence resonance energy transfer (FRET) and reduce Rayleigh scattering based on a hyphenated technique of fluorescence and second-order Rayleigh scattering (SRS). The ratiometric detection of H 2 O 2 is achieved through exploiting a hybrid system fabricated by fluorescent carbon dots and silver nanoparticles (CDs/AgNPs). In the CDs/AgNPs system, the fluorescence of CDs is quenched because of FRET, and the scattering is strong due to the intrinsic high light-scattering power of AgNPs. With the introduction of H 2 O 2 , the AgNPs are etched and the CDs are released from the AgNP surface, resulting in the fluorescence enhancement and scattering decline. As a result, ratiometric sensing of H 2 O 2 can be achieved based on the CDs/AgNPs system by simultaneous collection of fluorescence and SRS signals. The sensing system is further used for H 2 O 2 -generation bioanalysis, and as a proof-of-concept, ratiometric assay of glucose and evaluation of glucose oxidase activity are performed successfully. This work provides a new perspective for sensing applications of plasmonic nanoparticles. Graphical abstract Image 1 Highlights • A ratiometric sensor of H 2 O 2 was fabricated by Ag nanoparticles (AgNPs) and carbon dots (CDs). • The sensor was developed by combining fluorescence and second-order Rayleigh scattering. • The CDs/AgNPs system was endowed with weak fluorescence and strong scattering. • Fluorescence increased and scattering declined after AgNPs were etched by H 2 O 2. • The sensor was extended to ratiometric assay of glucose and glucose oxidase activity. [ABSTRACT FROM AUTHOR]

Abstract A simple and label-free fluorescence turn-on method is proposed for the discrimination and detection of epinephrine (Ep) and dopamine (DA) via polyethyleneimine (PEI)-initiated in situ copolymerization and excitation wavelength switch. The PEI solution in the presence of Ep, DA and the mixture of Ep and DA are denoted as P Ep-PEI , P DA-PEI and M Ep+DA , respectively. In this study, PEI aqueous solution medium initiates the auto-oxidization of Ep and DA and the bioinspired copolymerization. These resultant copolymers emit yellow-green fluorescence color with a fluorescence emission maximum at 515 nm. Interestingly, these fluorescent copolymers exhibit distinct different excitation spectra, although Ep and DA are structurally very similar. P DA-PEI exhibits only one excitation peak at 385 nm, and P Ep-PEI shows dual-excitation mode with two significant excitation peaks at 328 nm and 405 nm, respectively. M Ep+DA also shows dual-excitation mode with two excitation peaks at 330 nm and 395 nm, respectively. Thus, individual Ep, DA, and their mixture can be discriminated based on the different excitation spectral shapes and peak locations of P Ep-PEI , P DA-PEI and M Ep+DA. Furthermore, the quantitative analysis of Ep and DA in mixture can also be achieved by switching excitation wavelength between 330 and 395 nm and monitoring the fluorescence emission intensity of M Ep+DA at 515 nm. The fluorescence intensity of M Ep+DA only related to the concentration of Ep when excited at 330 nm. Moreover, the concentration of DA can also be calculated by subtracting the fluorescence intensity of P Ep-PEI from the total fluorescence intensity when excited at 395 nm. The resultant method has been used to simultaneously detect Ep and DA in human urine samples. The proposed fluorescence system is facile, eco-friendly, low-cost, and time-saving, and also provides a new and simple path for discriminating analogues. Graphical abstract Image 1 Highlights • PEI played multiple roles as an alkaline buffer, fluorescence initiator and reactant. • PEI initiated fluorescence turn-on response of Ep and DA with different excitation spectra. • Fluorescence excitation spectra can distinguish between Ep and DA. • Ep and DA in PEI can be simultaneously detected by switching excitation wavelength. • The fluorescence strategy for sensing Ep and DA was simple, low-cost and eco-friendly. [ABSTRACT FROM AUTHOR]

Abstract In this work, novel photoluminescent Ag nanoclusters (Ag NCs) with red emission are synthesized and successfully used for detecting iodide ion (I−). The dihydrolipoic acid (DHLA) is used as the stabilizing agent and beta-cyclodextrin (β-CD) is used as the auxiliary stabilizing agent. DHLA and β-CD are combined with Ag atoms by the formation of Ag S bonds and hydrophobic interaction, respectively. Functionalization of β-CD endows good photoluminescent properties and solubility in water to the Ag NCs. The obtained DHLA and β-CD-protected Ag NCs (DHLA/β-CD-Ag NCs) are spherical and display a dispersed state. However, the DHLA/β-CD-Ag NCs are aggregated in the presence of I−, accompanied by the decrease in their fluorescence intensity. Because the integrity of β-CD cavities is retained on the surface of DHLA/β-CD-Ag NCs, which preserves their capability for I− host–guest recognition, the DHLA/β-CD-Ag NCs combine with I− through the formation of inclusion complexes. Based on this phenomenon, the prepared DHLA/β-CD-Ag NCs can be designed as a novel fluorescent probe for I− detection. The limit of detection (LOD) is calculated as 0.06 μM, indicating that it is an ideal probe for I− detection in practical applications. Graphical Abstract Unlabelled Image Highlights • The synthesis of DHLA/β-CD-Ag NCs is simple, rapid, and low-cost. • The DHLA/β-CD-Ag NCs show good optical properties, stability, and solubility. • I− triggers aggregation of DHLA/β-CD-Ag NCs, causing the quenching of fluorescence. • Analysis of I− in real samples with good sensitivity and selectivity is achieved. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • A ratiometric fluorescent strategy for alkaline phosphatase (ALP) activity assay was proposed. • g-C3N4 and CoOOH nanoflakes were used to develop the sensing platform. • The assay system based on the g-C3N4/CoOOH hybrid responded to ALP sensitively. • Competitive redox reaction was responsible for the ratiometric fluorescence response. • The ratiometric method was applied to quantitative analysis of ALP activity in serum samples. Abstract The development of ratiometric fluorescent strategy is very significant and challenging in bioanalysis. Herein, graphitic carbon nitride (g-C 3 N 4) nanosheets and cobalt oxyhydroxide (CoOOH) nanoflakes are exploited for ratiometric fluorescence assay of alkaline phosphatase (ALP) activity. In the g-C 3 N 4 /CoOOH nanohybrid, g-C 3 N 4 nanosheets serve as a signal unit and the CoOOH nanoflakes function as a recognition element, and initially, the fluorescence of g-C 3 N 4 is quenched by the CoOOH nanoflakes. In the absence of target, the CoOOH nanoflakes of the nanohybrid system are able to oxidize o-phenylenediamine (OPD), and the resultant oxidation product (OxOPD) quenches the blue emission of g-C 3 N 4 and meanwhile emits orange fluorescence which acts as another signal element. However, an efficient redox reaction between ascorbic acid (AA) and CoOOH can cause decomposition of the CoOOH nanoflakes, and additionally, ALP can catalytically hydrolyze L-ascorbic acid-2-phosphate (AAP) to generate AA. Thus, in the presence of target, the CoOOH nanoflakes are destroyed by AA preferentially and the OPD is rarely oxidized to OxOPD, accompanied with strong blue emission of g-C 3 N 4 and weak orange fluorescence from OxOPD. Target-dependent dual-signal change made the ratiometric assay possible, and also AA-induced signal variation was investigated and attributed to the stronger reducing capacity of AA than OPD. The ratiometric sensing platform for ALP activity assay provides a new perspective for the applications of two-dimensional nanomaterials to develop novel and sensitive biosensors. [ABSTRACT FROM AUTHOR]

Hydrazine (N 2 H 4) is a highly toxic and versatile chemical raw material, which poses a serious threat to the environment and human health when used in large quantities. However, the traditional methods for the detection of N 2 H 4 have the disadvantages of time-consuming, complicated operation and expensive instruments. In contrast, fluorescence probes have many advantages, such as simple operation, high sensitivity, good selectivity, and fast response time. Therefore, there is an urgent need for a fluorescence probe that can rapidly and accurately detect the presence of N 2 H 4 and monitor the changes in its concentration. For this purpose, we designed and synthesized a series of myricetin fluorescence probes 3-(substituent group)-5,7-dimethoxy-4-oxo-2-(3,4,5-trimethoxy. phenyl)-4 H -chromen-4-one (Myr-R) for N 2 H 4 detection. In the presence of N 2 H 4 , the probe 5,7-dimethoxy-3-(2,3,4,5,6-pentafluorobenzoate)-2-(3,4,5-trimethoxyphen-yl). -4 H -chr-omen-4-one (Myr-3) shows significant fluorescence changes, double emission properties and a large Stokes shift (183 nm), and exhibits high selectivity and sensitivity to N 2 H 4 (The detection limit is 93 nM). Importantly, the qualitative and quantitative analysis of N 2 H 4 in water, soil, and air can be accomplished using fluorescence, smartphone, and UV lamps coupled with Myr-3. In addition, Myr-3 can be used for monitoring and imaging intracellular N 2 H 4. Meanwhile, the fluorophore 3-hydroxy-5,7-dimethoxy-2-(3,4,5-trimethoxyphenyl)-4H-benzopyran-4-one (Myr-Me) was applied to fingerprinting of different substrate materials due to the fact that it exhibits strong yellow fluorescence emission in the solid state and shows excellent contrast and high resolution. The probe Myr-3 is not only able to rapidly detect N 2 H 4 in complex environments, but also can be used for imaging intracellular N 2 H 4. In addition, the fluorophore Myr-Me can be used as an effective imaging agent for visual fingerprinting. These properties enable the probe Myr-3 and the fluorophore Myr-Me for a wide range of potential applications in related fields. [Display omitted] • Myricetin-based fluorescence systems with AIE and ESIPT properties are constructed. • A novel fluorescent probe (Mry-3) for the detection of hydralazine was synthesized. • Mry-3 exhibits good selectivity, sensitivity and low detection limit for hydrazine. • Mry-3 can be used for real-time monitoring of hydrazine in the environment. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • A pH-mediated reversible fluorescence nanoswitch was designed for sensing 4-NP. • The fluorescence sensing mechanism was demonstrated deriving from IFE. • A portable polyamide film-based sensor was developed for visually detecting 4-NP. • The developed approach shows good sensitivity and selectivity for 4-NP. • The proposed method was successfully applied to detect 4-NP in real water samples. Abstract Being a common hazardous waste, 4-nitrophenol (4-NP) has caused a serious threat to humans and environment. Therefore, rapid and selective detection of 4-NP, especially using a simple and portable instrument, is highly desired for human health and environmental monitoring. Herein, we develop a novel pH-mediated reversible fluorescence nanoswitch for selectively detecting 4-NP by using water-soluble fluorescent polymer carbon dots (PCDs) as a probe. The fluorescence of PCDs can be quenched by 4-NP via inner filter effect (IFE) because its excitation spectrum well overlaps with the absorption spectrum of 4-NP under alkaline condition. However, an obvious blue shift of the absorption peak of 4-NP occurs under acidic condition, causing the fluorescence recovery of PCDs due to the disappearance of IFE. On the basis of this principle, a pH-mediated reversible fluorescence nanoswitch was constructed and a broad linear range was obtained from 0.5 to 60 μM with a detection limit of 0.26 μM for 4-NP. Furthermore, this approach was successfully applied to detect 4-NP in real water samples and a portable polyamide film-based sensor was developed for visual detection of 4-NP, which offers a promising platform for the detection of 4-NP in on-site and resource-poor settings. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The BSA-AuNCs with dual emissions were prepared by the hydrothermal method. • The BSA-AuNCs were used as a ratiometric probe for malachite green (MG) detection. • The ratiometric probe can be cleverly fabricated without further modification. • The probe was utilized to reliable and sensitive detection of MG in water samples. Abstract The ratiometric fluorescence measurement has drawn great attention due to its selective and sensitive sensing property for numerous analytes. In this sensing system, a ratiometric fluorescence method is described for the determination of malachite green (MG) based on bovine serum albumin-stabilized Au nanoclusters (BSA-AuNCs) with dual emission. The as-prepared BSA-AuNCs exhibit two emission peaks around 430 and 610 nm under a single excitation, which may be from the intrinsic structure of BSA and AuNCs, respectively. As MG was added, the fluorescence at 610 nm was obviously quenched, and the fluorescence intensity at 430 nm decreased slowly, which might be mainly ascribed to fluorescence resonance energy transfer and dynamic quenching. The fluorescence intensity ratio of F 430 / F 610 shows good linearity for detecting MG in the 0.3–20 μM range with a low detection limit of 0.19 μM. In addition, the ratiometric sensing strategy is successfully applied to real water samples, which reveals its potential practicability for detecting MG in environmental water samples. [ABSTRACT FROM AUTHOR]

Since uracil-DNA glycosylase (UDG) is closely related to some human diseases, monitoring and detection of UDG activity have great significance in its clinic diagnosis and functional study. Here we demonstrate a sensitive cascade multi-amplification fluorescence strategy for the active UDG assay with the help of exonuclease III (Exo III). Under the cleavage reaction of UDG, the double-stranded DNA containing peculiar uracil bases separated into two dissociative single-stranded DNA, which separately hybridized with two cyclic hairpin probes to launch the Exo III-assisted dual cycles. The hairpin DNA modified with fluorophore and quenching group was employed as the signal probe. After the dual cycles, countless same short DNA fragments were generated to hybridize with signal probes, initiating a new Exo III-assisted cyclic amplification and releasing numerous single-stranded DNA which carried only fluorophores. Thus, the fluorescence intensity of the detection system was enhanced. This study obtained the detection limit as low as 2.4 × 10 −4  U/mL for detecting the UDG activity. And it performed satisfactory selectivity and well practical applicability by analysis of the HeLa cell lysate, providing a potential method for clinic diagnosis and functional study of UDG activity. [ABSTRACT FROM AUTHOR]

A novel water-soluble fluorescent polydopamine derivative, polydopamine-glutathione nanoparticles (PDA-G (-S-)NPs), was synthesized by the Michael addition reaction between dopamine (DA) and reduced glutathione (GSH). Compared to a direct polymerization of dopamine, the as-prepared PDA-G (-S-)NPs have stronger fluorescence emission intensity. Also, the synthesis does not need any hazardous organic solvents and the process is simple. Additionally, the roles of GSH and hydrogen peroxide (H 2 O 2 ) in enhancing the fluorescence intensity are discussed in detail. The thioether in PDA-G (-S-)NPs is easily oxidized by hydrogen peroxide to sulfoxide and sulfone groups, accompanied by a decrease in fluorescence intensity. Therefore, the PDA-G (-S-)NPs can be applied to construct a fluorescent sensor for the sensitive detection of hydrogen peroxide. Based on the transformation of glucose into gluconic acid and H 2 O 2 in the presence of glucose oxidase, the PDA-G (-S-)NPs system was further utilized to sensing glucose. The linear ranges and detection limits of hydrogen peroxide and glucose are (0.5–6 μM, 2–130 μM) and (0.15 μM, 0.6 μM), respectively. [ABSTRACT FROM AUTHOR]

Ferric ions (Fe 3+ ) play an important role in various chemical and physiological functions. Thus, analytical methods for the detection of Fe 3+ in the environment are highly in demand. In this work, a simple chemical reduction method for synthesizing water soluble l -histidine capped copper nanoparticles ( l -His-CuNPs) was successfully established. l -Histidine and ascorbic acid were acted as the capping scaffold and reducing agent, respectively. The as-prepared l -His-CuNPs were characterized by fluorescence spectroscopy, ultraviolet-visible spectrophotometry, Fourier-transform infrared spectroscopy, and transmission electron microscopy. The l -His-CuNPs show maximum luminescence at 495 nm when excited at 385 nm. Based on an aggregation-induced fluorescence quenching (AIQ) mechanism, a sensitive and selective sensor for Fe 3+ was developed. The detection limit (3s/slope) is 82 nM and the linear range is from 0.10 to 20 μM. Moreover, the developed sensor was successfully employed in the detection of Fe 3+ in real water samples. [ABSTRACT FROM AUTHOR]

Using carbon dots as the reductant and stabilizer, a visible and fluorescent method was developed for melamine detection in raw milk with one-step synthesized silver nanoparticles. In this work, carbon dots (C-dots) were applied to reduce silver ions and stabilize the nanoparticles, resulting in the formation of silver nanoparticles (AgNPs). As a result, the inherent fluorescence emission of C-dots was significantly reduced after the formation of AgNPs. However, in the presence of melamine, silver ions could interact with the nitrogen atoms in amine and triazine groups of melamine. With 0–2 μM melamine, aggregated AgNPs were found after the reduction by C-dots, resulting in color and absorbance changes. With further increase of melamine (2–20 μM), both formation and aggregation of AgNPs were inhibited, and the fluorescence was gradually increased. This optical platform was optimized for melamine detection and then was applied to detecting melamine in raw milk samples. The results for melamine assay based on visible and fluorescent method showed the requisite sensitivity with a low detection limit of 30 nM, as well as high selectivity. [ABSTRACT FROM AUTHOR]

In this paper, a simple sensor platform is presented for highly selective and sensitive detection of dissolved ammonia in aqueous solutions without pretreatment based on temperature gradient headspace single drop microextraction (HS-SDME) technique, and fluorescence and UV–vis spectrophotometry are utilized with the Ag nanoclusters (Ag NCs) functioned by citrate and glutathione as the probe. The sensing mechanism is based on the volatility of ammonia gas and the active response of Ag NCs to pH change caused by the introduction of ammonia. High pH can make the Ag NCs agglomerate and lead to the obvious decrease of fluorescence intensity and absorbance of Ag NCs solution. Moreover, the presented method exhibits a remarkably high selectivity toward dissolved ammonia over most of inorganic ions and amino acid, and shows a good linear range of 10–350 μM (0.14–4.9 mg N L −1 ) with a low detection limit of 336 nM (4.70 μg N L −1 ) at a signal-to-noise ratio of 3. In addition, the practical applications of the sensor have been successfully demonstrated by detecting dissolved ammonia in real samples. [ABSTRACT FROM AUTHOR]

Lanthanide luminescent sensors for colorimetric and ratiometric fluorescence dual-readout detection of tyrosinase activity by introducing characteristic reaction followed by inhibition assay. [Display omitted] • Lanthanide sensors are used to detect TYR by introducing a characteristic reaction. • The sensitivity is enhanced by colorimetric and ratiometric fluorescence dual modes. • The selectivity is improved by introducing a characteristic fluorescence reaction. • Visualization is facilitated by changes in color and chromaticity. • Tyrosinase inhibition assay and inhibitor detection are realized. As the biomarker of dermatoses, tyrosinase (TYR) has various biological functions by regulating the production of melanin. Here, we proposed lanthanide luminescent sensors for colorimetric and ratiometric fluorescence dual-readout detection of TYR activity by introducing a characteristic blue fluorescence reaction. With the increase of TYR, the fluorescence signals of lanthanide luminescent probes (dipicolinic acid (DPA)-Eu, DPA-Tb, and DPA-Eu/Tb) are decreased, while the blue signal is turned "on", which improves the sensitivity and selectivity of ratiometric fluorescence sensing, with good linear range and low detection limit (0.05 U mL−1 for the DPA-Eu system or DPA-Tb system, 0.01 U mL−1 for the DPA-Eu/Tb system). In addition, colorimetric analysis of TYR is achieved by recording the absorbance value at 420 nm (The linear range of three system is 0.5–40 U mL−1). Moreover, the color changes under natural light and chromaticity changes under UV light facilitate the visual and qualitative analysis of TYR. Furthermore, the lanthanide sensors enable precise quantification of TYR in real samples. Subsequently, arbutin as an inhibitor sufficiently inhibited TYR activity, and arbutin was detected by "turn-off" fluorescence mode. More importantly, this study not only realizes dual-mode detection and inhibition of TYR activity, but also broadens the application of TYR detection in medical research and cosmetics industry, as well as provides a new idea for designing optical sensors. [ABSTRACT FROM AUTHOR]

[Display omitted] • Probe NOSA is used for fluorescent detection of ArSHs based on cascade reaction. • Probe NOSA enables the monitoring of ArSHs level in foods and condiments. • Composite probe NOSA@CMMT is constructed for chromatic sensing of ArSHs. Thiophenols (ArSHs) are widely used as popular flavoring ingredients for making daily dishes. Dissecting the ArSHs contents in common foodstuffs is meaningful in the field of food safety science. Herein, a novel small-molecule sensor 2-(1 H -benzo[ d ]imidazol-2-yl)-3-(2-(2,4-dinitrophenoxy)-4-morpholinophenyl)acrylonitrile (NOSA) has been tailored. The NOSA is able to respond to ArSHs, spontaneously yielding highly green-emissive fluorescent iminocoumarin (I 500). This cascade reaction-based strategy is sensitive (limit-of-detection = 2.8 nM), rapid (within 5 min), and selective toward ArSH flavors. Probe NOSA has been applied to the determination of ArSHs in real-life meat products and condiments. Moreover, a far-red fluorescent compound, 2-(7-(diethylamino)-4-(4-(methylthio)styryl)-2 H -chromen-2-ylidene)malononitrile (CMMT), has been first combined with NOSA to construct a composite probe NOSA@CMMT for the ratiometric detection of ArSHs (I 500 / I 630). System NOSA@CMMT exhibits a conspicuous fluorescence change from deep-red to light-green. Benefitted from the gorgeous chromatic fluctuation, a smartphone-integrated analysis platform is established for the real-time evaluation of ArSHs level. [ABSTRACT FROM AUTHOR]

[Display omitted] • Aldehyde functional dual-emissive CDs are readily obtained by one-step synthesis. • This fluorescent probe is developed for ratiometric detection of HSO 3 −. • The dual-emissive CDs are applied for the detection of HSO 3 − in food samples. • A smartphone-assisted sensing platform is used to detect HSO 3 − on-site. Bisulfite (HSO 3 −) is used as a food additive for its antibacterial and antioxidant properties. However, excessive intake of HSO 3 − is harmful to humans. Here, for the first time, aldehyde-functionalized dual-emissive carbon dots (D-CDs) are synthesized in one-step for direct ratiometric sensing of HSO 3 −. Due to the nucleophilic addition reaction between HSO 3 − and aldehyde of D-CDs, the fluorescence transforms from green to deep-blue. The linear range of the probe is 0.1–30 μmol/L with a detection limit of 42 nmol/L. Moreover, D-CDs show good selectivity and a fast reaction time (<5 min) toward HSO 3 −. The probe has been applied to trace HSO 3 − detection in food samples. The recoveries range from 96.5 % to 107.0 % with relative standard deviations below 6.5 %. In addition, a smartphone sensing platform has been designed, which provides a wider application prospect for the real-time monitoring of HSO 3 − in food. [ABSTRACT FROM AUTHOR]

Ratiometric fluorescence sensors gain stronger anti-interference ability via self-calibration. Nevertheless, ratiometric analysis of phosphate (Pi) still faces problems such as complicated construction process of dual emission probes and possible interferences from outputting mono-category fluorescent signal. Herein, we propose a "kill two birds with one stone" strategy to address these challenges, by simply introducing a single-component probe, porphyrin paddlewheel framework-3 (PPF-3) nanosheets without modification, encapsulation or complex, to integrate fluorescence (FL)-second-order scattering (SOS) dual-signal for ratiometric detection of Pi. PPF-3 nanosheets are constructed by coordination of Co2+ with 5,10,15,20-tetrakis(4-carboxyl-phenyl)-porphyrin (TCPP) ligands, displaying weak FL and strong SOS, two different and independent signals. In the response system to Pi, Co2+ and TCPP serve as the recognition element and signal unit, respectively. After interacting with Pi, the high affinity for Co2+ makes Pi snatch Co2+ from the PPF-3 nanosheets, causing their structure disassembly (SOS decrease) and TCPP release (FL increase). Finally, the FL-SOS ratiometric platform is successfully employed to access Pi in real water samples. Synchronous collection of FL and SOS from the single-component probe provides a simpler and more efficient way on ratiometric sensor design as well as a new useful technique for monitoring target-induced aggregation and disaggregation behavior. [Display omitted] • PPF-3 nanosheets output FL-SOS dual-signal without encapsulation or complex. • Collecting FL-SOS dual-signal provides a new avenue for ratiometric sensor design. • PPF-3 nanosheets own larger surface area and more accessible active sites for Pi. • The ratiometric system is suitable for Pi detection in water examination. • A universal assay monitors target-induced aggregation and disaggregation behavior. [ABSTRACT FROM AUTHOR]

We synthesized polyethyleneimine capped ZnO quantum dots (PEI-ZnO QDs) which dispersed well in ethanol by using a two-step method. UV–visible absorption spectroscopy, photoluminescence, Fourier transform infrared spectroscopy, high resolution transmission electron microscope, and field emission scanning electron microscope were used to characterize the as-prepared PEI-ZnO QDs. It emitted green-yellow fluorescence (527 nm) for excitation at 355 nm. We used the PEI-ZnO QDs to determine the alcoholic strength because of its different property in water and ethanol which caused decrease in the PEI-ZnO QDs fluorescence intensity with increasing water content. With this method, we determined the alcoholic strength of different artificial samples with a linear range from 20% to 60% vol and detected commercial spirits with satisfactory results. [ABSTRACT FROM AUTHOR]

Silver ions have been widely applied to many fields and have harmful effects on environments and human health. Herein, a label-free optical sensor for Ag + detection is constructed based on thiazole orange (TO) as a fluorescent probe for the recognition of i-motif DNA structure change at neutral pH. Ag + can fold a C-rich single stranded DNA sequence into i-motif DNA structure at neutral pH and that folding is reversible by chelation with cysteine (Cys). The DNA folding process can be indicated by the fluorescence change of TO, which is non-fluorescent in free molecule state and emits strong fluorescence after the incorporation with i-motif DNA. Thus, a rapid, sensitive, and selective method for the detection of Ag + and Cys is developed with a detection limit of 17 and 280 nM, respectively. It is worth noting that the mechanism underlying the increase of the fluorescence of thiazole orange in the presence of i-motif structure is explained. Moreover, a fluorescent DNA logic gate is successfully designed based on the Ag + /Cys-mediated reversible fluorescence changes. The proposed detection strategy is label-free and economical. In addition, this system shows a great promise for i-motif/TO complex to analyze Ag + in the real samples. [ABSTRACT FROM AUTHOR]

Novel magnesium and nitrogen co-doped carbon quantum dots (Mg-N-CQDs) were developed as a fluorescent switch for sensitive and selective sensing of Hg(II) and cysteine (Cys). The Mg-N-CQDs synthesized by hydrothermal method possess high fluorescence intensity. However, after the addition of Hg(II), the fluorescence of Mg-N-CQDs aqueous solution was dramatically quenched, which may be attributed to the formation of non-fluorescent complex between Mg-N-CQDs and Hg(II). Interestingly, the fluorescence of the Mg-N-CQDs aqueous solution containing Hg(II) could be recovered gradually in the presence of Cys, due to the stronger binding affinity of Hg(II) toward Cys than toward Mg-N-CQDs. Based on these phenomena, a fluorescent probe for detection of Hg(II) and Cys was explored, and further applied to detect Hg(II) in real water samples with satisfactory results. Moreover, an IMPLICATION logic gate was constructed using Hg(II)/Cys as inputs and the fluorescence signal of Mg-N-CQDs as an output. [ABSTRACT FROM AUTHOR]

Mercury ion is one of the most hazardous metal pollutants that can cause deleterious effects on human health and the environment even at low concentrations. It is necessary to develop new mercury detection methods with high sensitivity, specificity and rapidity. In this study, a novel and green strategy for synthesizing D-penicillamine-capped copper nanoparticles (DPA-CuNPs) was successfully established by a chemical reduction method, in which D-penicillamine and ascorbic acid were used as stabilizing agent and reducing agent, respectively. The as-prepared DPA-CuNPs showed strong red fluorescence and had a large Stoke's shift (270 nm). Scanning electron microscopy, transmission electron microscopy, Fourier-transform infrared spectroscopy, fluorescence spectroscopy, and ultraviolet-visible spectrophotometry were utilized to elucidate the possible fluorescence mechanism, which could be aggregation-induced emission effect. Based on the phenomenon that trace mercury ion can disperse the aggregated DPA-CuNPs, resulting in great fluorescence quench of the system, a sensitive and selective assay for mercury ion in aqueous solution with the DPA-CuNPs was developed. Under optimum conditions, this assay can be applied to the quantification of Hg 2+ in the 1.0–30 μM concentration range and the detection limit (3σ/slope) is 32 nM. The method was successfully applied to determine Hg 2+ in real water samples. [ABSTRACT FROM AUTHOR]

We describe here a fluorometric assay for sensitive detection of oligonucleotides, based on a target recycling amplification strategy driven by toehold-mediated strand displacement reaction and on exonuclease III (Exo Ш)-assisted fluorescence background suppression strategy. The network consists of a pair of partially complementary DNA hairpins (HP 1 and HP 2 ) with 3′ overhang ends, between which the spontaneous hybridization is kinetically hindered by the stems. The target DNA is repeatedly used to trigger a recycling progress between the hairpins, generating numerous HP 1 –HP 2 duplex complexes. Exo III was then employed to digest the double strand parts of the residual hairpins and the intermediate products. The fluorescent dye, SYBR Green I, binds to the double-strand DNA products and emits strong fluorescence to achieve sensitive detection of the target DNA with the detection limit of 5.34 pM. Moreover, this proposed strategy showed high discrimination efficiency towards target DNA against mismatched DNA and was successfully applied in the analysis of human serum sample. [ABSTRACT FROM AUTHOR]

• A water-soluble fluorescent probe BIS with large Stokes shift was synthesized. • The BIS shows a dual-mode fluorescence sensing pattern toward HSO 3 − and HClO. • The BIS enables the monitoring of HSO 3 − and HClO levels in foods and beverages. • Two sensing platforms based on chromatic change are used for visual detection. • Probe BIS is applied to in vivo fluorescence imaging of Nile tilapias. Herein, a novel hemicyanine derivative (E)-3-(1,1-dimethyl-2-(4-(methylthio)styryl)-1 H -benzo[ e ]indol-3-ium-3-yl)propane-1-sulfonate (BIS) has been reasonably designed. Compound BIS is long-wavelength emissive and water-soluble with a large Stokes shift. Intriguingly, probe BIS provides a dual-mode fluorescence response pattern for the sensing of bisulfite (HSO 3 −) and hypochlorous acid (HClO) with great limit of detections (3.6 and 57.4 nM). First, the 1,4-Michael addition of HSO 3 − on the conjugated double bond triggers a ratiometric response (I 465 / I 575). Second, the rapid oxidation of HClO on the thioether moiety provides a turn-on response (I 575). Evaluation of HSO 3 − and HClO levels in dried fruit, beverage, and water samples has been carried out with satisfactory results. Moreover, motivated by an impressive chromatic variation (red to blue), smartphone-assisted signal readout system and thin-film sensing platform are facilely constructed for real-time and on-site measurement of HSO 3 − levels. Furthermore, probe BIS is used for the in vivo imaging of HSO 3 − in edible fish models. [ABSTRACT FROM AUTHOR]

Highlights: [•] Cu nanoclusters with size smaller than 2nm was utilized as a fluorescence probe. [•] The lable-free method had good selectivity, low toxicity, and low cost. [•] Cu nanoclusters using polyethyleneimine as a template possessed good stability. [•] Water-soluble Cu nanoclusters had been applied successfully in human serum sample. [Copyright &y& Elsevier]

Highlights: [•] Polyethyleneimine-capped silver nanoclusters were label-free, cost-effective. [•] This method for copper ions detection possesses a fluorescence signal-on response. [•] This sensor for copper ions was selective, simple, and highly sensitive. [Copyright &y& Elsevier]

A dual-mode sensing system of ratiometry and turn-off based on the carbon dots (CDs) and MnO 2 nanosheets nanocomposite for H 2 O 2 and glucose detection is constructed, in which the ratio of fluorescence to first order scattering (FOS) is used as the signal value of the ratiometric mode, and the intensity of second order scattering (SOS) is used as the signal value of the turn-off mode. [Display omitted] • An optical sensing system with ratiometric and turn-off dual modes was constructed. • Three signals (FOS, fluorescence, and SOS) can respond to the target simultaneously. • The three signals were applied to the quantitative detection of H 2 O 2 and glucose. • This work broadened the study of fluorescence combined with FOS and SOS. • The dual-mode detection method exhibited good sensitivity and high selectivity. The optical sensor with ratiometric and turn-off dual modes is constructed to detect H 2 O 2 and glucose based on blue fluorescent carbon dots (CDs) and MnO 2 nanosheets with great ability of fluorescence quenching and scattering. Employing CDs@MnO 2 nanosheets nanocomposite as the probe, H 2 O 2 is detected by simultaneously collecting first-order scattering (FOS, 353.5 nm), fluorescence (440 nm), and second-order scattering (SOS, 710 nm) under the excitation of 350 nm. H 2 O 2 with strong oxidation property can etch the lamellar structure of MnO 2 nanosheets into nano-fragments, which made the fluorescence of CDs in the system recover and the scattering intensity (FOS and SOS) of the system decrease significantly. Therefore, the optical sensor combined FOS and fluorescence signals in ratiometric mode, and SOS signal in turn-off mode to realize sensitive determination of H 2 O 2. The linear ranges of ratiometric mode and turn-off mode for H 2 O 2 detection were 0.2–40 and 0.2–15 μM, respectively. And the limits of detection (LODs) of two modes were 73 and 104 nM, respectively. Furthermore, the sensor was also successfully applied to the detection of glucose which can react to produce H 2 O 2. Satisfactorily, the LODs of this sensor for glucose detection were 95 and 113 nM for ratiometric mode and turn-off mode, respectively. This work not only provides a new method for the accurate detection of H 2 O 2 and glucose, but also extends a new idea for the study of the combination of scattering and fluorescence. [ABSTRACT FROM AUTHOR]

[Display omitted] • ZIF-8@GMP-Tb nanoparticles were constructed to detect ALP. • The system is simple, fast, sensitive, and selective. • The sensing mechanism of ALP was discussed. • This strategy can be applied to biological sample and ALP inhibitor investigation. Luminescent metal–organic frameworks (LMOFs) and their functional materials with unique characteristics can provide the basis for the construction of new analytical techniques, which can meet the continuous demand for various fields. In this work, guanosine monophosphate (GMP), terbium ion (Tb3+) and zeolitic imidazolate framework-8 (ZIF-8) are self-assembled to form a ZIF-8@GMP-Tb nanocomplex, which can be utilized as a ratiometric fluorescent probe to monitor alkaline phosphatase (ALP) activity. Specifically, with adding ALP, the fluorescence intensity at 547 nm (one of the characteristic emission peaks of Tb3+) obviously decreased. Meanwhile, the conjugated structure of GMP increased the fluorescence of ZIF-8 (located at 330 nm). The possible mechanism was proposed through the characterization of the materials. Based on the variation of the emission peaks at 330 and 547 nm, the ratiometric fluorescent sensor of ALP has a linear range of 0.25–20 U/L. Moreover, applying this sensing system to the detection of ALP in the human serum sample and ALP inhibitor investigation possesses satisfactory results. This work provides a new perspective for the utilization of ZIF-8 and lanthanide ions in manufacturing simple and sensitive sensors. [ABSTRACT FROM AUTHOR]

[Display omitted] • The dsDNA nanobridge illuminates the fluorescence of the CV/G-quadruplex complex. • No molecular markers and expensive porphyrin fluorescent molecules were used. • Only three DNA single strands are used to realize the detection of two pollutants. • This platform has good selectivity and applicability, and builds a logical AND gate. DNAzyme and G-quadruplex are purposed as universal recognition elements to achieve multi-functional signal amplification and readout, showing great potential in the design of biosensors and nanodevices. Herein, the single-stranded DNA (ssDNA) opens the dumbbell-shaped hairpin probes, the generated double-stranded DNA (dsDNA) is constructed as a nanobridge, and the released G-rich sequence is folded into a G-quadruplex village. The fluorescence of crystal violet/G-quadruplex complex villages is enhanced by the dsDNA nanobridge connections. Under the excitation wavelength of 580 nm, the fluorescence emission intensity at 624 nm exhibits a good linearity with the concentration of crystal violet (CV) in the range of 10–250 nM, which can be used for sensitive detection of CV. Additionally, combined with Pb2+-dependent DNAzyme, a platform for quantitative detection of Pb2+ is established. In the concentration range of 5 nM – 25 μM, the fluorescence signal increases with the increase of Pb2+ concentration. The quantitative detection limit of Pb2+ by this method is 5 nM, and it possesses good selectivity and recovery for Pb2+ in a variety of water samples. More importantly, the platform can sensitively detect various metal ions by changing the DNAzyme sequence. It has potential application in the field of environmental monitoring. [ABSTRACT FROM AUTHOR]

Based on the 7-nitro-1,2,3-benzoxadiazole (NBD) unit, many fluorescent probes have been developed for the differential detection of biothiols. Among them, however, the use of fluorescent probes for rapid simultaneous identification of biothiols in vivo has been rarely reported. In this work, a simple fluorescent probe (NC-NBD) is rationally designed and developed according to the different reactivity of biothiols, which can simultaneously distinguish homocysteine (Hcy), cysteine (Cys), and glutathione (GSH) by adjusting two different fluorescence signals (green and yellow). The probe NC-NBD can discriminatively detect biothiols using two fluorescence channels with fast response (120–360 s), which is suitable for real-time monitoring of biothiols in biological systems. As expected, the probe NC-NBD with excellent performance is able to determine the levels of Cys/Hcy and GSH in human serum samples, and therefore has potential applications in biological specimens. The robust probe shows low toxicity and high sensitivity when applied to living cells, and successfully achieves rapid sequence distinguishing of biothiols and intracellular imaging by using two different fluorescence signals. The probe can provide an effective tool for the real-time monitoring of intracellular biothiols. Image 1 • A fluorescent (FL) probe based on 7-nitro-1,2,3-benzoxadiazole (NBD) was developed. • The simple probe was used for rapid and sensitive sequence detection of biothiols in vitro. • The probe can determine the levels of Cys/Hcy and GSH in human serum samples. • The probe senses biothiols by two FL signals and images them in living cells. [ABSTRACT FROM AUTHOR]

• A ratiometric fluorescence method was constructed to detect dopamine. • Mechanism of sensing for dopamine has been discussed. • The strategy owned good selectivity and sensitivity for detection of dopamine. • This strategy can be successfully applied to biological samples. For small size metal nanoclusters, it is expected that template ligand will play an essential role in regulating the properties and structure. Using histidine (His) templated Ag nanoclusters and glutathione (GSH) and dihydrolipoic acid (DHLA) protected silver nanoclusters as models, due to the different electron donating abilities of different ligands, two silver nanoclusters with similar sizes have fluorescence emission peaks at different wavelengths. Furthermore, dopamine (DA) is a good electron donor, but is easily oxidized to dopaquinone, which can act as an electron accepter. Therefore, with addition of DA, which is easily oxidized by dissolved oxygen, the fluorescence of His-Ag nanoclusters quenches. Meanwhile, the ligands of GSH-DHLA-Ag nanoclusters contain sulfhydryl group, which has stronger reducing ability and stronger electron donating ability. Therefore, even in the presence of dopaquinone, the electron-donating effect of the ligand of the GSH-DHLA-Ag nanoclusters on the metal cores does not decrease significantly. Moreover, the addition of DA causes a certain degree of aggregation of GSH-DHLA-Ag nanoclusters, and the fluorescence intensity is enhanced due to the aggregation-inducing emission enhancement effect. Based on two types of silver nanoclusters, a ratiometric fluorescent sensor for DA analysis has been established. The linear range is 0−800 nM, and the limit of detection based on 3δ/s is 10 nM. [ABSTRACT FROM AUTHOR]

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. Unlabelled Image • Hydrophilic CDs are fabricated by the hydrothermal method. • The CDs exhibit photostability, photoreversibility and pH-sensitive properties. • The CDs were developed for fluorescent "off-on-off" detection of pH. • The nanoprobe was used to detect acidic amino acids. • The nanoprobe was exploited to real-time monitoring of intracellular pH level. [ABSTRACT FROM AUTHOR]

• Trace NE and Ep are detected via simple and rapid chemically-modulated fluorescence. • A new fluorescence method is constructed for the sensitive determination of DβH. • The proposed fluorescence strategy is simple, highly efficient, and eco-friendly. Monitoring the concentration of norepinephrine (NE), epinephrine (Ep) and dopamine-β-hydroxylase (DβH) is of great importance in studying and diagnosing related diseases. In this study, a simple chemically-modulated turn-on fluorescence strategy is proposed for rapid, selective and visual discrimination of trace NE and Ep. Herein, Ep is differentiated from other catecholamines via its fast fluorescence response in 1.0 M NaOH medium. Furthermore, NE is alkalized with 0.01 M NaOH and is then immediately triggered by polyethyleneimine to emit bright cyan fluorescence, whereas Ep and DA show no any fluorescence response. The fluorescence intensity of resultant solution is linearly proportional to the concentration of Ep and NE in the range of 50.0–10000.0 nM and 10.0–10000.0 nM, respectively. Moreover, NE and Ep as low as 500.0 nM can be fluorescently identified with naked-eye, which is convenient for rapidly preliminary diagnosis and self-test of hypercatecholaminism. Inspired by the catalytic property of DβH transforming DA into NE, the proposed strategy enables a simple and high-efficient fluorescence method to monitor DβH by using trace DA as substrate and directly quantifying NE. The method is simple, smart, highly efficient and eco-friendly, and has been applied to the rapid assay of NE, Ep and DβH in real samples. [ABSTRACT FROM AUTHOR]