Real-time visualization the pH fluctuations of living cells with a ratiometric near-infrared fluorescent probe

In situ real-time quantitative monitoring pH fluctuation in complex living systems is vitally significant. In the current work, a ratiometric near-infrared (NIR) probe (MCyOH) was developed to confront this challenge. MCyOH exhibited good sensitivity, photostability, reversibility, and an ideal pKa (pKa = 6.65). Ratiometric character of MCyOH is beneficial to accuracy detect the pH fluctuations in living cells under different stimulation. The observations showed that intracellular pH was decreased when HepG2 cells under oxidative stress or starvation conditions. In particular, HepG2 cells was acidulated after addition of ethanol, however, the acidification phenomenon was attenuated or disappeared when HepG2 cells preincubated with disulfiram or fomepizole. Finally, MCyOH was successfully applied to observe the increasement of intracellular pH when HepG2 cells treated with fomepizole individually. Overall, MCyOH would be a practical candidate to explore pH-associated physiological and pathological varieties.

Terbium doping and energy level modification of zirconium organic frameworks as probes for the improved determination of histamine and visual inspection of food freshness

Food safety is a common concern among people, and the development of high-performance food freshness detection technology is crucial, but is still highly challenging. Fluorescent sensing based on metal organic frameworks is a promising technology to tackle this issue. In this work, three UiO-66 type Zirconium organic frameworks (ZrOFs) which are functionalized with varying numbers of hydroxyl groups to alter the energy levels, and partial replacement of Zirconium(IV) by Terbium(III) ions to introduce additional emitting centers, were explored as probes for the sensing of Histamine (His). With one hydroxyl group introduced, UiO-66-OH@Tb can be developed as ratiometric fluorescent probe with improved sensing performance, showing a wide detection range of 0 to 120 mg/L, and a low detection limit of 0.13 mg/L. UiO-66-OH@Tb can also be fabricated into composite film to function as visual sensing material of His. This work can provide instructions for the development of other fluorescent sensors.

A coumarin based ratiometric fluorescent probe for the detection of Cu2+ and mechanochromism as well as application in living cells and vegetables

In this paper, a novel coumarin-derived fluorescent probe NY was designed and synthesized. NY displayed a significant ratiometric fluorescence response towards Cu2+ in PBS buffer (10 mM, pH = 7.4), with the emission wavelength blue-shifted from 580 to 495 nm, and a fluorescence change from orange to green was evident under a 365 nm UV light. Meanwhile, NY had the advantages of high selectivity, short response time (5 min), low detection limit (1.3 × 10-8 M) and large binding constant (1.45 × 105 M-1) towards Cu2+. The binding mechanism between NY and Cu2+ was elucidated by FT-IR, 1H NMR titration, TOF-MS and Job's plot analysis. In addition, NY was successfully employed in the detection of Cu2+ within environmental water and vegetable samples with satisfactory results. Laser confocal microscopy imaging results showed that NY could easily penetrate HeLa cells membrane to target mitochondria and image Cu2+ in living cells. Furthermore, NY demonstrated mechanochromic properties by exhibiting orange-red fluorescence when subjected to mechanical grinding.

Diketopyrrolopyrrole-based fluorescent probe for visualizing over-expressed carboxylesterase in fever via ratiometric imaging

Fever is the result of inflammation and the innate self-defense response of organisms, can cause abnormal changes in the activity of many enzymes in organisms, including the important carboxylesterase (CE). Monitoring the activity changes of CE in vivo during a fever will help to understand heat-related pathological mechanisms. In this paper, we designed diketopyrrolopyrrole-based ratiometric fluorescent probes DPP-FBC-P and DPP-FBO-P containing alkyl chain and diethylene glycol monomethyl ether chain respective for detection of CE. Both probes could realized fast response to CE and displayed good selectivity and high sensitivity. Compared with DPP-FBO-P, DPP-FBC-P had better biocompatibility, larger signal to noise ratio (225-fold vs 125-fold) and lower detection limit (1.6 × 10-5 U/mL vs 4.2 × 10-5 U/mL). Moreover, the probe DPP-FBC-P had been successfully applied to image the endogenous CE in HepG2 cells and solid tumors, and also visualized the over expressed CE in fever cells. Most importantly, the changes of CE level in the liver of fever mice model induced by LPS were monitored with the assistance of DPP-FBC-Pvia dual channel ratio imaging for the first time. In addition, fluorescence color signal in solution was captured by smart phone, and the linear relationship between RGB ratio (G/R) and CE concentration was established. This work will provide a potential approach for investigating the physiological and pathological processes of heat related diseases.

A reversible and ratiometric fluorescent probe based on rhodol derivative with an ESIPT unit for monitoring copper ion content and in situ evaluation of related drugs in cells

The amount of copper ions in the environment has an immediate effect on ecology and food safety, Menkes syndrome and Wilson's disease cause accumulation and deficiency of copper ions in the body, respectively, and neurodegenerative diseases are also closely related to copper ion levels. However, the current copper ion detection technology has a high cost, complex operation, and other disadvantages. In this study, a ratiometric fluorescent probe (RB-DH) was rationally constructed to detect copper ions by coupling benzothiazole to rhodol derivatives. It can be used to determine copper ion concentrations in water samples, agricultural products, cells, and zebrafish. Importantly, due to the reversible response of RB-DH to copper ions, the fluctuation of intracellular copper ion content during the release of copper ion-related drugs (Copper gluconate and D-penicillamine) was successfully monitored with RB-DH for the first time. This study demonstrates RB-DH's potential application in the evaluation of related drug release effects and serves as a guide for the establishment of portable detection techniques for other important substances.

A ratiometric luminescence probe for selective detection of Ag+ based on thiolactic acid-capped gold nanoclusters with near-infrared emission and employing bovine serum albumin as a signal amplifier

When thiolactic acid-capped gold nanoclusters (AuNCs@TLA) with strong near-infrared (NIR, 800 nm) emission were applied to detect metal ions, only Ag+ induced the generation of two new emission peaks at 610 and 670 nm in sequence and quenching the original NIR emission. The new peak at 670 nm generated after the 800-nm emission disappeared utterly. The ratiometric and turn-on responses showed different linear concentration ranges (0.10-4.0 μmol·L-1 and 10-50 μmol·L-1) toward Ag+, and the limit of detection (LOD) was 40 nmol·L-1. Especially, the probe exhibited extremely high selectivity and strong anti-interference from other metal ions. Mechanism studies showed that the novel responses were attributed to the anti-galvanic reaction of AuNCs to Ag+ and formation of bimetallic nanoclusters. The two new emission peaks were due to the composition change and size growth of the metal core. Besides, bovine serum albumin (BSA) has been employed as a signal amplifier based on the assembly-induced emission enhancement properties of AuNCs, which improved the LOD to 10 nmol·L-1. Moreover, the ratiometric method is feasible for Ag+ detection in diluted serum with high recovery rates, showing large application potential in the biological system. The present study supplies a novel ratiometric probe for Ag+ with a two-stage response and provides a novel signal amplifier of BSA, which will facilitate and promote the application of NIR-emitted metal nanoclusters in biological system.

Ratiometric and visual determination of copper ions with fluorescent nanohybrids of semiconducting polymer nanoparticles and carbon dots

Developing nanohybrid composition based fluorescent carbon dots (CDs) for ratiometric detection of copper ions is highly appealing. Herein, a ratiometric sensing platform (GCDs@RSPN) for copper ions detection has been developed by loaded green fluorescence carbon dots (GCDs) on the surface of red emission semiconducting polymer nanoparticles (RSPN) through electrostatic adsorption. The GCDs, featuring abundant amino groups, can selectively bind copper ions to induce the photoinduced electron transfer, leading to fluorescence quenching. A good linearity within the range of 0-100 μM is obtained, and the limit of detection (LOD) is 0.577 μM by using obtained GCDs@RSPN as ratiometric probe to detect copper ion. Moreover, the paper-based sensor derived from GCDs@RSPN was successfully applied for the visual detection of Cu²⁺.

A novel ratiometric fluorescent probe for sensitive detection of jasmonic acid in crops

Herein, a novel ratiometric fluorescent probe was proposed for sensitive detection of jasmonic acid (JA) based on NCQDs@Co-MOFs@MIPs. The prepared NCQDs, with uniquely dual-emissive performance, are insensitive to JA due to electrostatic repulsion. Interestingly, the introduction of Co-MOFs not only avoided the self-aggregation of NCQDs, but changed the surface charge of NCQDs and triggered the response of NCQDs to JA. More importantly, the imprinted recognition sites from MIPs provided "key-lock" structures to specifically capture JA molecules, greatly improving the selectivity of the probe to JA. Under the synergistic actions of Co-MOFs and MIPs, JA can interact with NCQDs through photo-induced electron transfer (PET), resulting in the changes on emission intensity of the probe at E_m = 367 nm and 442 nm. Based on the observations, the quantification of JA was realized in the range of 1-800 ng/mL with the limit of detection (LOD) of 0.35 ng/mL. In addition, the probe was used for detecting JA in rice with satisfactory analysis results, indicating the probe holds great potential for monitoring JA levels in crops. Overall, this strategy provides new insights into the construction of practical probes for sensitive detection of plant hormones in crops.

A smartphone-based platform for ratiometric visualization of SARS-CoV-2 via an oligonucleotide probe

COVID-19 necessitates the development of reliable and convenient diagnostic tools. In this work, a facile 3D-printed smartphone platform was constructed that achieved reliable visual detection of SARS-CoV-2 by eliminating the effect of ambient light and fixing the camera position relative to the sample. The oligonucleotide probe is modified with orange-red-emitting TAMRA working as an internal standard and green-emitting FAM serving as a sensitive sensing agent. Under 365-nm UV excitation, the emission wavelengths of TAMRA and FAM are 580 nm and 518 nm, respectively. When the probes interact with the targets, the green fluorescence gradually restores while the orange-red fluorescence remains stable. Thus, a striking color transition from orange-red to green could be observed by the naked eye. The detection limit of SARS-CoV-2 nucleic acid is 0.23 nM, and the entire process of color change could be completed in 25 min. Furthermore, the RGB value analysis of the sample solution was conducted using a smartphone for reliable and reproducible discrimination of SARS-CoV-2. The proposed smartphone platform might establish a general method for visual detection of SARS-CoV-2 nucleic acid as well as other virus-related diseases.

Extending optical chemical tools and technologies to mice by shifting to the shortwave infrared region

Fluorescence imaging is an indispensable method for studying biological processes non-invasively in cells and transparent organisms. Extension into the shortwave infrared (SWIR, 1000-2000 nm) region of the electromagnetic spectrum has allowed for imaging in mammals with unprecedented depth and resolution for optical imaging. In this review, we summarize recent advances in imaging technologies, dye scaffold modifications, and incorporation of these dyes into probes for SWIR imaging in mice. Finally, we offer an outlook on the future of SWIR detection in the field of chemical biology.

Ratiometric fluorescence sensor for sensitive detection of inorganic phosphate in environmental samples

Fast, simple, and low-cost on-site visualized detection of inorganic phosphate (Pi) is in great demand since phosphate is the major reason of eutrophication. In this work, a ratiometric fluorescent probe composed by green carbon dots (GCDs) and red carbon dots (RCDs) has been established for high-sensitivity and selective sensing of Pi. A trend of color change from red to green is observed for the detection of Pi under ultraviolet light and the detection limit is 0.09 μM in the range of 0 to 55 μM. Fluorescent test paper prepared from the probe solution was successfully applied to semi-quantitative visual detection of Pi in real-world water and soil samples, which shows great real-world application potentials.

A simple paper-based ratiometric luminescent sensor for tetracyclines using copper nanocluster-europium hybrid nanoprobes

Tetracyclines (TCs), as one of the broad-spectrum antibiotics, are widely used to treat bacterial infections. The residues of TCs in animal-origin foods and drinking water have raised safety concerns and affected the public health. Thus, there is a high demand to develop a simple and rapid method for the detection of TCs. In this work, we developed a ratiometric luminescence probe for the sensitive and visualized detection of TCs. Specifically, tannic acid-stabilized copper nanoclusters (TA-CuNCs) with blue emission at 433 nm were synthesized. The luminescence of TA-CuNCs attenuated partially by the europium ions (Eu³⁺) due to the aggregation-induced quenching. When TCs were added to the TA-CuNCs-Eu³⁺ system, the luminescence of TA-CuNCs at 433 nm can be further quenched by the inner-filter effect, and the characteristic luminescence of Eu³⁺ at 617 nm emerged due to the formation of Eu³⁺-TCs complex. The ratio of the luminescence at 617 nm-433 nm increased linearly to the concentration of TCs. Additionally, we demonstrated the detection of oxytetracycline in real samples such as tap and lake water, milk, pharmaceutical industry wastewater, honey and soil extract with high recovery rate (97.25%-103.44%). Furthermore, a portable paper device is fabricated by the luminescent probe to conduct the on-site analysis of TCs.

Ratiometric fluorescent probe for the on-site monitoring of coexisted Hg2+ and F- in sequence

The monitoring of mercury and fluoride ions (Hg²⁺ and F^-) has aroused wide concerns owing to the high toxicity of Hg²⁺ and the duplicitous nature of F^- to human health. As far as we known, more than 100 million people in poverty-stricken areas are still at high risk of being over-exposed to Hg²⁺ and F^- via drinking water. Simple and cost-effective luminescent methods are highly promising for on-site water monitoring in rural areas. However, the development of multipurpose luminescent probes that are accurate and sensitive remains challenging. Herein, a new strategy for rationally designing a multipurpose ratiometric probe is present. The obtained probe is consisted of two emission units with energy transfer between them, which exhibit high coordination affinities to the two coexisted toxic targets (Hg²⁺ and F^-), respectively. Thus, two distinct routes for efficiently modulating the energy transfer in the probe are present to trigger the responses to the two targets in sequence. By detecting the shift of the emission color with a smartphone, an on-site water monitoring method is successfully established with the detection limits as low as 2.7 nM for Hg²⁺ and 1.9 μM for F^-. The present study can expend the toolbox for water monitoring in rural regions.

Ratiometric Probe for Rapid Naked Eye Detection of Toxic Hydrazine: Real Time Application in Strip Test, Spray Test and Soil Analysis

Striking colorimetric probe (CynH) for abrupt detection of hydrazine under complete aqueous solution was achieved. The water soluble probe was designed with electron "push-pull" strategy by coupling of 4-hydroxy benzaldehyde and 2, 3, 3-trimethylindolinine. The positively charged N-propylated indolinine make the probe completely soluble in water. The probe yields eye catching selective detection of hydrazine over other competing analytes with high sensitivity. Obvious colour change was observed from colourless to appearance of bright pink colour with hydrazine. It reacts quickly with hydrazine within 2 min and makes the probe an effective candidate for practical application. The real time application was demonstrated using paper strip to detect hydrazine vapour. This probe is superior to earlier reported probes because of its effective sensing of hydrazine displayed with various applications including real-time strip based sensing, spray test and soil analysis. In all the examinations, the probe yields distinct response with rapid naked eye colour change which overcomes the drawbacks of previous reports.

Two-photon ratiometric fluorescent probe based on NBD-amine functionalized semiconducting polymer nanoparticles for real-time imaging of hydrogen sulfide in living cells and zebrafish

The thiolysis of 7-nitro-1,2,3-benzoxadiazole amine (NBD-A) paves the way for specific sensing of H₂S over biothiols and real-time imaging in living organisms. Rational fabrication of NBD-A-based probe with ratiometric mode and two-photon excitation is highly appealing to achieve high quality bioimaging. In this work, the NBD-A molecules are assembled with poly(9,9-dioctylfluorenyl-2,7-diyl) polymer nanoparticles, defined as NBD@PFO, to construct two-photon ratiometric probes for H₂S detection through the fluorescence resonance energy transfer (FRET). For the construction of NBD@PFO nanohybrids, polymer nanoparticles are employed as the NBD-A molecular vehicle, energy donor and two-photon absorber, while NBD-A is served as the response unit and energy acceptor. Taking advantages of NBD-A and polymer nanoparticles, the obtained NBD@PFO probes exhibit high selectivity, fast response (<5 s), ratiometric detection and two-photon excitation. Our results indicate that NBD@PFO nanohybrids are successfully applied for monitoring of H₂S concentration in living cells and zebrafish, exhibiting great potential of polymer nanoparticles to improve the imaging capability of an organic small molecular probe.

Gold nanoparticles-mediated ratiometric fluorescence aptasensor for ultra-sensitive detection of Abscisic Acid

Herein, a novel ratiometric aptasensor based on carbon quantum dots@2-Methylimidazole zinc salt (CQDs@ZIF-8) and aptamer-functionalized gold nanoparticles (Apt-AuNPs) was developed for highly sensitive detection of ABA by fluorescence spectrometry. The CQDs@ZIF-8 nanomaterials displayed dual-emission properties at 490 nm and 657 nm with excitation at 420 nm were synthesized for the first time. ZIF-8 not only served as an anchor point for CQDs but also acted as a modulator to regulate fluorescence signals of CQDs. Interestingly, introduction of ZIF-8 changed the quenching properties of the AuNPs on CQDs. The AuNPs quenched the fluorescence of CQDs@ZIF-8 at 490 nm but not at the second peak of 657 nm. Few studies have been reported on the ineffectiveness of AuNPs in fluorescence quenching as far as we know. In this study, we found that incorporation of ABA triggered the aggregation of AuNPs due to the specific ABA-aptamer recognition and this changed the fluorescence intensity of the ratiometric probe (CQDs@ZIF-8@Apt-AuNPs). The proposed probe increased the sensitivity and selectivity of determining ABA levels in rice seeds in the range of 0.100-150 ng/mL with an LOD of 30.0 ng/L. Importantly, the method proposed here offers a new unique strategy for the construction of ratiometric probes and ultra-sensitive measurement of biomolecules.

Colorimetric recognition of hydrazine in aqueous solution by a bromophenol blue-tethered ion-pair-like ratiometric probe

Hydrazine or hydrazine hydrate (N₂H₄·H₂O) is a potential neurotoxin and has several mutagenic effects in physiological systems. Therefore, the development of synthetic organic probes that are sensitive and selective to hydrazine is of tremendous importance. Unfortunately, however, the hydrazine-selective sensing probes that rely upon minimum usage of the organic solvents (≤5%, v/v) are still rarer. In this work, an ion-pair-like mono acetate derivative of bromophenol blue has been developed as a fairly selective ratiometric probe for the naked-eye recognition of hydrazine in a solution of tris buffer and EtOH (19:1, v/v) at physiological pH. The chromogenic signalling relies upon hydrazine-induced cleavage of an ester moiety of the probe to its resonance stabilized quinonoid form, resulting in momentous variations in its spectrophotometric profile. Meanwhile, the colour of the probe solution changed from mustard yellow to blue within few minutes. This sensing assay could be successfully applied in the recognition of hydrazine in real environmental and pharmaceutical samples with satisfactory recoveries. Given the cost-effectiveness, simplicity and versatility, for instance, direct analysis of colorimetric probes, it is reasonable to propose that the present method can serve as a complementary method for prompt inspection of hydrazine in boiler feed water.

Imidazolyl-benzocoumarins as ratiometric fluorescence probes for biologically extreme acidity

A rational approach to develop a fluorescent probe for sensing biologically "extreme" acidity (pH <3) is disclosed. The probe, a push-full type 3-(imidazolyl)benzocoumarin dye, has the lowest pK_a = 1.3 among ratiometric probes known so far, which is ascribed due to a unique sensing mechanism. The probe has high quantum yields, high chemical stability and good aqueous solubility. The probe was successfully applied to ratiometric fluorescence imaging of intrabacterial acidity from pH 4.0-1.0, offering a practical means for studying biological systems under the extreme pH conditions.

Glutathione protected bimetallic gold-platinum nanoclusters with near-infrared emission for ratiometric determination of silver ions

A controlled method to prepare glutathione-protected bimetallic gold-platinum nanoclusters (Au-PtNCs) has been established. The Au-PtNCs show either strong red (625 nm) or near-infrared (NIR, 805 nm) emission. Further characterizations indicated that the average particle size grows from 1.42 to 1.78 nm, the larger particles being responsible for the redshift of emission. The NIR emitted Au-PtNCs are applied as a novel ratiometric probe of Ag(I), which induces a new emission peak at ~635 nm and quenches the initial emission gradually. The determination shows very high selectivity toward Ag(I) among other metal ions. A limit of determination (10 nM) and the linear range (0.10 to 15 μM) are achieved, which is much lower than the EPA mandate of 0.46 μM for Ag(I) in drinking water. The response mechanism is attributed to the fact that the added Ag(I) has been reduced by the core of Au-PtNCs and deposited on the surface, which induces new fluorescence emission around 635 nm. In addition, the ratiometric method is feasible for Ag(I) determination in serum serum with good recovery (between 98.3% and 102.0%, n = 3), showing very high application potential. The present study provides a controlled method to prepare Au-PtNCs with strong red and NIR emission and supplies a novel NIR ratiometric probe of Ag(I). Schematic presentation of the controlled preparation of glutathione-protected bimetallic gold-platinum nanoclusters (Au-PtNCs) with either red or near-infrared (NIR) emission, and application in ratiometric detection of Ag(I) with high selectivity and sensitivity.

Sensing of mercury ions in Porphyra by Copper @ Gold nanoclusters based ratiometric fluorescent aptasensor

A novel aptamer-modified Copper @ Gold nanoclusters (apt-Cu@Au NCs) based ratiometric fluorescent probe was developed for mercury ions (Hg²⁺) determination in Porphyra. The apt-Cu@Au NCs were well dispersed in solution without Hg²⁺ but combined together for the formation of thymidine-Hg-thymidine structure with the addition of Hg²⁺, which further caused the changes in their fluorescence intensities owing to fluorescence resonance energy transfer. Along with that, the changes in fluorescent colors are visible to the naked eye. Accordingly, Hg²⁺ were determined ranging from 0.1 to 9.0 μM by fluorescence analysis with the detection limit of 4.92 nM. Moreover, a homemade device utilizing smartphone and microfluidic chip was designed for colorimetric determination of Hg²⁺ ranging from 0.5 to 7.0 μM with good portability and usefulness. The proposed methods were used for Hg²⁺ detection in Porphyra with the recoveries of 101.83-114.00%, suggesting the considerable potential for evaluating Hg²⁺ levels in aquatic products.

Synthesis and application of a ratiometric probe for hydrogen peroxide

Molecular imaging of biological analytes provides detailed insights into signaling processes. Ratiometric probes are particularly attractive due to the ability to quantify analyte production. However, design strategies for ratiometric probes can be hindered by spectral overlap of the product and reactant species. In this chapter, we provide protocols for the synthesis and application of RF₆₂₀, a ratiometric probe for H₂O₂ displaying dramatic changes in both excitation and emission wavelengths, designed using an approach we term chemoselective alteration of fluorophore scaffolds. The probe contains a chemoselective functional group within a fluorescent xanthene scaffold, resulting in the in situ synthesis of a new fluorophore upon reaction with H₂O₂. Under physiological conditions, RF₆₂₀ exhibits far-red to near-infrared excitation and emission, and upon reaction with H₂O₂, RF₆₂₀ is chemically converted into tetramethylrhodamine, producing a significant (~66nm) blue-shift in excitation and emission. RF₆₂₀ can be used for ratiometric, molecular imaging of endogenous H₂O₂ production in living cells.

A new colorimetric and ratiometric probe for highly selective recognition and bioimaging of ClO- and Al3

In this study, a new fluorescence probe HMAQ based on quinazoline and diaminomaleonitrile was constructed for sensing ClO^- and Al³⁺. A fluorescence blue-shift with 102 nm together with a color change from golden-yellow to colorless was found by hypochlorite-induced hydrolysis of -CH=N- group to release the initial fluorophore. Besides, Al³⁺ could cause a 72-nm blue-shifted emission spectra and a color change from golden-yellow to brown. As expected, HMAQ exhibited a satisfactory selectivity and sensitivity to ClO^-/Al³⁺ with a quick response. Most notably, the reversibility of the [HMAQ+Al³⁺] complex could be used to detect ClO^- and Al³⁺ simultaneously without mutual interferences. The detection limits of HMAQ for ClO^- and Al³⁺ were turned out to be 10.2 nM and 1.56 nM, respectively. The high-performance results of real-time detections demonstrated the enormous potential of HMAQ in real-water samples and living cells.

Discrimination of Various Amine Vapors by a Triemissive Metal-Organic Framework Composite via the Combination of a Three-Dimensional Ratiometric Approach and a Confinement-Induced Enhancement Effect

Multiemissive sensors are being actively pursued, because of their ratiometric luminescent detection capabilities, which demonstrates better sensitivity and selectivity than conventional single-emission sensors. Herein, we present a trichromatic white-light-emitting metal-organic framework (MOF) composite (Z3) by simultaneously incorporating red/green-emitting Pt/Ru complex cations into porous blue-emitting bio-MOF-1 through post-synthetic modification. With the help of a three-dimensional (3-D) dual-ratiometric luminescence recognition method, and unique turn-on responses of the red emission toward amine compounds (ACs), including NH₃ and aliphatic amines, via confinement-induced luminescence enhancement effect, Z3 can work as a dual-ratiometric luminescent sensor for discrimination of 7 out of 11 AC vapors. This work not only provides a new AC sensing mechanism (confinement effect) that can induce a "turn-on" response but also proves that the accuracy and selectivity of composite sensor can be greatly improved through the combination of 3-D recognition method and the confinement effect. Thus, it open up fresh opportunities to develop composite sensors with excellent sensing and differentiating ability.

Aggregation-enhanced emission enables phenothiazine coumarin as a robust ratiometric fluorescent for rapid and selective detection of HClO

By taking advantage of phenothiazine moiety as an electron-donating group, a novel donor-acceptor (D-A) type coumarin dye, PTZ-Et, was developed. The introduction of phenothiazine moiety not only caused emission red-shifting and Stokes shift enlarging, but also endowed PTZ-Et with significant aggregation-enhanced emission (AEE) features, thereby enabled PTZ-Et as a robust ratiometric fluorescent probe for HClO detection. Upon oxidation of the sulfur atom on phenothiazine into sulfoxide, PTZ-Et displayed remarkable ratiometric fluorescence response (over 150 folds variations of F₅₃₄/F₆₂₆) toward HClO with rapid response time (<30 s) and ultra-sensitivity (LOD = 15 nM). Additionally, the corresponding sensing mechanism of PTZ-Et for HClO was fully elucidated through the successful purification and well characterization (¹H NMR, ¹³C NMR, HRMS, and single crystal data) of the corresponding reaction product between PTZ-Et and HClO. Significantly, PTZ-Et was capable of monitoring both exogenous and endogenous HClO in living RAW 264.7 cells by ratiometric fluorescence imaging.

Highly selective ratiometric fluorescent probes for the detection of Fe3+ and its application in living cells

It's of vital importance to detect heavy metals in environment and living cells. In this work, four near-infrared regions boron dipyrromethene (BODIPY) probes (QBPH, PBPH, QBP and PBP) are constructed based on two BODIPY precursors (QB, PB) for sensing of Fe³⁺. As expected, these four probes exhibit obvious colorimetric and ratiometric response to Fe³⁺. In addition, QBP and PBP display highly sensitive and selective performance for detection of Fe³⁺. More importantly, QBP and PBP are successfully applied to near infrared imaging and detection of Fe³⁺ in living A549 cells; it indicates that these novel designed probes could be a useful tool for the studies of Fe³⁺ in living cells.

A BODIPY-based ratiometric probe for sensing and imaging hydrogen polysulfides in living cells

Hydrogen polysulfides (H₂S_n, n > 1) have attracted increasing attention in biological systems due to its redox signaling effect. To illustrate the process of the physiological and pathological roles played by H₂S_n, accurate detection is highly desired. In this work, we report a BODIPY-based fluorescent probe (BDP-PHS) for ratiometric H₂S_n sensing. BDP-PHS shows higher sensitivity and selectivity ratiometric response toward H₂S_n than various biological related species. Moreover, BDP-PHS has been successfully applied in imaging of H₂S_n in living cells.

A novel ratiometric fluorescence probe for highly sensitive and specific detection of chlorotetracycline among tetracycline antibiotics

It is of great importance to detect chlorotetracycline (CTC) in a highly sensitive and specific way because of its wide distribution in aquaculture and animal husbandry. Herein, we propose a novel ratiometric fluorescence strategy to assay CTC by using bovine serum albumin stabilized gold nanoclusters (BSA-AuNCs). The BSA-AuNCs consisting of 25 gold atoms (Au₂₅NCs) display a red emission at 640 nm (λ_ex = 370 nm). In the presence of CTC, a new blue emission at 425 nm is emerged and its intensity dramatically increases with the addition of more the analyte; meanwhile the red emission at 640 nm shows a linear decrease reversely. However, at identical conditions neither the analogues of CTC as tetracycline (TC), oxytetracycline (OTC) or doxycycline (DC) induces similar response of BSA-AuNCs. Such interesting phenomenon is proven related to the conversion from large Au₂₅NCs to smaller nanoclusters composing 8 gold atoms (Au₈NCs), which intrinsically originate from the interaction between CTC and the ligand BSA. Therefore, a ratiometric probe is established to sensitively detect CTC in the wide range (0.2-10 μM) with a low limit of detection (LOD) at 65 nM. In addition, this strategy can also be applied to assay CTC in human serum, showing great promise for practical applications in future.

Self-assembled fluorescent Ce(Ⅲ) coordination polymer as ratiometric probe for HIV antigen detection

The viral capsid protein p24 of human immunodeficiency virus is expressed at different level during viral invasion. Detection of p24 is of great importance in acquired immunodeficiency syndrome monitoring and therapy. A ratiometric probe that is easily-synthesized was constructed based on self-assembled fluorescent Ce(Ⅲ) and fluorescein. Fluorescein was used as reference. Hydrogen peroxide quenches the fluorescence of the Ce(III) easily but does not quench the fluorescence of fluorescein. The mechanism of reaction was discussed. Benefiting from the sensitive response to hydrogen peroxide, this probe was applied for p24 detection in enzyme linked immunoassay. The fluorescence ratio was in a good linear relationship with the concentration of p24, and the detection limit was 1.1 pg mL^-1. This proposed method has shown potential in virus detection with easy operation.