A new type of phosgene probe with a limit of detection down to 0.12 nM, response time of less than 1.5 s, and high selectivity over other similarly reactive toxic chemicals was developed using ethylenediamine as the recognition moiety and 8-substituted BODIPY unit as the fluorescence signaling component. The probe undergoes sequential phosgene-mediated nucleophilic substitution reaction and intramolecular cyclization reaction with high rate, yielding a product with the intramolecular charge transfer (ICT) process from amine to the BODIPY core significantly inhibited. Owing to the emission feature of 8-substituted BODIPY that is highly sensitive to the substituent′s electronic nature, such inhibition on the ICT process strikingly generates strong fluorescence contrast by a factor of more than 23 300, and therefore creates the superhigh sensitivity of the probe for phosgene. Owing to the high reactivity of ethylenediamine of the probe in nucleophilic substitution reactions, the probe displays a very fast response rate to phosgene.Keywords: detection limit; ethylenediamine group; ICT; intramolecular cyclization; phosgene;

A colorimetric fluorescent probe with fluorescence emission feature sensitive to SO2 derivatives, i.e. bisulfite (HSO3−) and sulfite (SO32−), was developed based on the HSO3−/SO32−-mediated nucleophilic addition reaction of the probe that. This probe exhibited SO32− sensing ability with detection limit down to 46 nM and desired selectivity over other reference anions and redox species. The preliminary fluorescence bioimaging experiments have validated the practicability of the as-prepared probe for SO2 derivatives sensing in living cells.

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As a very sensitive technique, photoswitchable fluorescence not only gains ultrasensitivity but also imparts many novel and unexpected applications. Applications of near-infrared (NIR) fluorescence have demonstrated low background noises, high tissue-penetrating ability, and an ability to reduce photodamage to live cells. Because of these desired features, NIR-fluorescent dyes have been the premium among fluorescent dyes, and probes with photoswitchable NIR fluorescence are even more desirable for enhanced signal quality in the emerging optical imaging modalities but rarely used because they are extremely challenging to design and construct. Using a spiropyran derivative functioning as both a photoswitch and a fluorophore to launch its periodically modulated red fluorescence excitation energy into a NIR acceptor, we fabricated core–shell polymer nanoparticles exhibiting a photoswitchable fluorescence signal within the biological window (∼700–1000 nm) with a peak maximum of 776 nm. Live cells constantly synthesize new molecules, including fluorescent molecules, and also endocytose exogenous particles, including fluorescent particles. Upon excitation at different wavelengths, these fluorescent species bring about background noises and interferences covering nearly the whole visible region and therefore render many intracellular targets unaddressable. The oscillating NIR fluorescence signal with an on/off ratio of up to 67 that the polymer nanoparticles display is beyond the typical background noises and interferences, thus producing superior sharpness, reliability, and signal-to-noise ratios in cellular imaging. Taking these salient features, we anticipate that these types of nanoparticles will be useful for in vivo imaging of biological tissue and other complex specimens, where two-photon activation and excitation are used in combination with NIR-fluorescence photoswitching.Keywords: cellular imaging; Förster resonance energy transfer; near-infrared fluorescence; photoswitching; polymer nanoparticles

•Flavone-based highly selective fluorescent probe for Cys/Hcy sensing was developed.•The ability of the probe for micromolar level Cys/Hcy sensing was confirmed.•The practicability of the probe for intracellular Cys/Hcy sensing was validated.A new type of flavone-based fluorescent probe (DMAF) capable of cysteine (Cys)/homocysteine (Hcy) sensing with high selectivity over other amino acids was developed. Such type of probe undergoes Cys/Hcy-mediated cyclization reaction with the involvement of its aldehyde group, which suppresses of the photoinduced electron transfer (PET) process of the probe molecule and consequently leads to the enhancement of fluorescence emission upon excitation using visible light. The formation of product of the Cys/Hcy-mediated cyclization reaction was confirmed and the preliminary fluorescence imaging experiments revealed the biocompatibility of the as-prepared probe and validated its practicability for intracellular Cys/Hcy sensing.A new type of flavone-based fluorescent probe (DMAF) capable of cysteine (Cys)/homocysteine (Hcy) sensing with high selectivity over other amino acids was developed. Such type of probe undergoes Cys/Hcy-mediated cyclization reaction with the involvement of its aldehyde group, which suppresses of the photoinduced electron transfer (PET) process of the probe molecule and consequently leads to the enhancement of fluorescence emission upon excitation using visible light. The formation of product of the Cys/Hcy-mediated cyclization reaction was confirmed and the preliminary fluorescence imaging experiments revealed the biocompatibility of the as-prepared probe and validated its practicability for intracellular Cys/Hcy sensing.

Advanced fluorescence microscopy including single-molecule localization-based super-resolution imaging techniques requires bright and photostable dyes or proteins as fluorophores. The photophysical properties of fluorophores have been proven to be crucial for super-resolution microscopy’s localization precision and imaging resolution. Fluorophores TAMRA and Atto Rho6G, which can interact with macrocyclic host cucurbit[7]uril (CB7) to form host-guest compounds, were found to improve the fluorescence intensity and lifetimes of these dyes. We enhanced the localization precision of direct stochastic optical reconstruction microscopy (dSTORM) by introducing CB7 into the imaging buffer, and showed that the number of photons as well as localizations of both TAMRA and Atto Rho6G increase over 2 times.

A pair of reversible photochemical reactions correlates their reactant and product specifically, and such a correlation uniquely distinguishes their correlated signal from others that are not linked by this reversible reaction. Here a nanoparticle-shielded fluorophore is photodriven to undergo structural dynamics, alternating between a green-fluorescence state and a red-fluorescence state. As time elapses, the fluorophore can be in either state but not both at the same time. Thus, the red fluorescence is maximized while the green fluorescence is minimized and vice versa. Such an antiphase dual-color (AD) corelationship between the red and green fluorescence maxima as well as between their minima can be exploited to greatly improve the signal-to-noise ratio, thus enhancing the ultimate detection limit. Potential benefits of this correlation include elimination of all interferences originating from single-color dyes and signal amplification of AD photoswitching molecules by orders of magnitude.

Hybrid fluorescent nanoparticles (NPs) capable of fluorescing near-infrared (NIR) light (centered ∼730 nm) upon excitation of 800 nm laser light were constructed. A new type of conjugated polymer with two-photon excited fluorescence (TPEF) feature, P-F8-DPSB, was used as the NIR-light harvesting component and the energy donor while a NIR fluorescent dye, DPA-PR-PDI, was used as the energy acceptor and the NIR-light emitting component for the construction of the fluorescent NPs. The hybrid NPs possess δ value up to 2.3 × 106 GM per particle upon excitation of 800 nm pulse laser. The excellent two-photon absorption (TPA) property of the conjugated polymer component, together with its high fluorescence quantum yield (ϕ) up to 45% and the efficient energy transfer from the conjugated polymer to NIR-emitting fluorophore with efficiency up to 90%, imparted the hybrid NPs with TPEF-based NIR-input-NIR-output fluorescence imaging ability with penetration depth up to 1200 μm. The practicability of the hybrid NPs for fluorescence imaging in Hela cells was validated.Keywords: conjugated polymer; energy transfer; multiphoton fluorescence bioimaging; near-infrared (NIR) fluorescence; two-photon absorption

A new type of resorufin-based dual-functional fluorescent probe whose fluorescence emission features are sensitive to thiol compounds and redox homeostasis was developed. Thiols-triggered nucleophilic substitution of the probes converts the nonfluorescent probe to the highly fluorescent resorufin moiety; the released resorufin not only enables fluorescence signaling specific for thiol compounds but functions as a redox indicator with sensitive colorimetric and fluorescence emission change upon redox variation. Preliminary fluorescence imaging experiments have revealed the biocompatibility of the as-prepared probes and validated their practicability for thiol sensing and redox homeostasis mapping in living cells.

A novel strategy for modulating the photophysics of organic dyes in super-resolution fluorescence imaging using an external magnetic field was reported. The magnetic field induced increase in fluorescence intensity, localization number of probe molecules, and the number of photons emitted per molecule as compared to those acquired without a magnetic field were experimentally confirmed. Improved dSTORM localization precision and imaging resolution were consequently achieved.

An anthranol derivative (DTDDP) and an anthraquinone derivative diastereomer (DDDP) without typical donor–acceptor (D–A) structural features were synthesized from 1,4-diaminoanthraquinone via a one-step reaction. The absorption and fluorescence emission properties of DTDDP and DDDP were investigated in common organic solvents with different polarity. In spite of a lack of apparent D–A structure, DTDDP exhibited apparent solvatochromic fluorescence emission with shift >80 nm in aprotic media. The linear correlation of the Stokes shift of DTDDP in a Lippert–Mataga plot was observed, and the change in the dipole moment of DTDDP upon excitation, 10.7 D, was determined. Time-dependent density functional theory (TDDFT) calculation confirmed the marked hole–electron separation of the excited DTDDP molecule as compared to that of the excited DDDP molecule. The experimental results and the theoretical calculation revealed the excited-state intramolecular charge-transfer nature of the emitting state of DTDDP molecules, which is responsible for the observed solvatochromic fluorescence emission features. The photooxidation-mediated facile conversion of DTDDP to its anthraquinone form counterpart DDDP was also confirmed.

A new type of ratiometric fluorescent probe capable of detecting Hg2+ ions at nanomolar-concentration level with high selectivity was developed based on an indole-trizole-rhodamine triad and its practicability for intracellular Hg2+ sensing was verified. The as-prepared fluorescent probe is capable of detecting Hg2+ over other competing metal ions including Ag+ with high selectivity. The synergistic effect of Hg2+-assisted conversion of the nonfluorescent ring-closed rhodamine moiety to the highly fluorescent ring-open form as well as the fluorescence signal amplification originating from the Förster resonance energy transfer (FRET) from indole-trizole conjugate to rhodamine moiety contributed to a detection limit of 11 nM of the probe for Hg2+ sensing.

Bromine-functionalized polythiophene (P3BrHT) block copolymers with different block ratios were synthesized and their micro-phase separation features in the solid state that allow for retaining molecular packing identities of the unmodified polythiophene (P3HT) were experimentally confirmed.

•New type of fluorescent probe for Ag+ and Hg2+ sensing with high selectivity and sensitivity.•First demonstration of single-fluorophore-based Ag+-selective TURN-ON and Hg2+-selective TURN-OFF type dual-channel fluorescence signaling system.•Single-fluorophore-based probes for simultaneous determination of Ag+ and Hg2+ without crosstalk.A new type of fluorescent probe capable of detecting Ag+ and Hg2+ in two independent channels was developed in the present work. Specifically, in CH3CN–MOPS mixed solvents with CH3CN/MOPS ratio (v/v) of 15/85, this type of probe fluoresced weakly, and the addition of Ag+ remarkably induced fluorescence enhancement of the probe. In CH3CN–MOPS mixed solvents with the percentage of CH3CN increased up to 65%, the probe was highly fluorescent and addition of Hg2+ dramatically induced the fluorescence quenching. Thus, using such single-fluorophore-based probe and tuning the polarity of the mixed solvent, Ag+, and Hg2+ can be detected in independent channels with high selectivity and sensitivity. As a result, the mutual interference usually encountered in most cases of Ag+ and Hg2+ sensing owing to the similar fluorescence response that these two ions induced, can be effectively circumvented by using the probes developed herein.A new type of single-fluorophore-based fluorescent probes that displayed Ag+-selective TURN-ON and Hg2+-selective TURN-OFF type dual-channel antiphase-like fluorescence signaling features was developed, and their unique ability of enabling simultaneous determination of Ag+ and Hg2+ over other reference metal ions was confirmed.

Conjugated polymer nanoparticles (CPNs) were developed based on a polyfluorene-based conjugated polymer with thiophene units carrying pyridyl moieties incorporated in the backbone of polymer chains (PFPyT). Hybrid CPNs fabricated from PFPyT and an amphiphilic polymer (NP1) displayed pH-sensitive fluorescence emission features in the range from pH 4.8 to 13, which makes them an attractive nanomaterial for wide range optical sensing of pH values. The fluorescence of hybrid CPNs based on chemically close polyfluorene derivatives without pyridyl moieties (NP3), in contrast, remains virtually unperturbed by pH values in the same range. The fluorescence emission features of NP1 underwent fully reversible changes upon alternating acidification/basification of aqueous dispersions of the CPNs and also displayed excellent repeatability. The observed pH sensing properties of NP1 are attributed to protonation/deprotonation of the nitrogen atoms of the pyridine moieties. This, in turn, leads to the redistribution of electron density of pyridine moieties and their participation in the π-conjugation within the polymer main chains. The optically transparent amphiphilic polymers also exerted significant influence on the pH sensing features of the CPNs, likely by acting as proton sponge and/or acid chaperone.

Fluorescent nanoparticles (NPs) for Al3+ sensing with high selectivity were developed from a type of carbazole-based conjugated polymer with a two-dimensional donor–π bridge–acceptor (D–π–A) structure. These NPs are characterized by their small particle diameter (∼18 nm), far-red fluorescence emission (centered ∼710 nm), and Al3+-induced fluorescence enhancement with high selectivity owing to an Al3+-triggered inhibition on the intramolecular charge transfer (ICT) processes between the conjugated backbone and the pendant acceptors. This type of nanoparticle is easily suspended in aqueous solutions, indicating their practical applicability in physiological media, and their ability for intracellular Al3+ sensing was confirmed. As compared to other types of conjugated polymer based probes showing metal ion mediated fluorescence quenching, these as-prepared NPs possess analyte-enhanced fluorescence emission, which is analytically favored in terms of sensitivity and selectivity. Fluorescence emission with wavelengths in the biological window of maximum optical transparency (∼700 to 1000 nm) is expected to impart a salient advantage for biological detection applications to these as-prepared probes. The superior features of merit of this new type of fluorescent probe, together with the validation of practicability for intracellular Al3+ ion sensing, are indicative of their potential for application in fluorescence-based imaging and sensing, such as investigations on Al3+-related physiological and pathological processes.

A novel strategy for modulating the photophysics of organic dyes in super-resolution fluorescence imaging using an external magnetic field was reported. The magnetic field induced increase in fluorescence intensity, localization number of probe molecules, and the number of photons emitted per molecule as compared to those acquired without a magnetic field were experimentally confirmed. Improved dSTORM localization precision and imaging resolution were consequently achieved.

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