9-Aryl-1,2-dihydropyrrolo[3,4-b]indolizin-3-one (Seoul-Fluor) as a smart platform for colorful ratiometric fluorescent pH sensors

In this communication, we report that 9-aryl-1,2-dihydropyrrolo[3,4-b]indolizin-3-one (Seoul-Fluor) can serve as a potential platform for colorful ratiometric fluorescent pH sensors by simple incorporation of pH responsive elements on Seoul-Fluor. Seoul-Fluor-based fluorescent pH sensors allow the emission- and pH-tuning ability upon protonation by varying their pK(a) values and electronic characteristics of substituents by a rational design.     

Ratiometric and water-soluble fluorescent zinc sensor of carboxamidoquinoline with an alkoxyethylamino chain as receptor

A novel "naked-eye" and ratiometric fluorescent zinc sensor (AQZ) of carboxamidoquinoline with an alkoxyethylamino chain as receptor was designed and synthesized. AQZ shows good water solubility and high selectivity for sensing; about an 8-fold increase in fluorescence quantum yield and a 75 nm red-shift of fluorescence emission upon binding Zn2+ in buffer aqueous solution are observed. Moreover, AQZ can enter yeast cells and signal the presence of Zn2+.     

Zinc ion-tetraphenylporphyrin interactions in the ground and excited states

In the present article, tetraphenylporphyrin a new ratiometric fluorescence sensitizer for zinc ion has been proposed. Electronic absorption, emission and (1)H NMR spectral characteristics of meso-tetraphenylporphyrin (TPP) have been studied in acetonitrile medium in the presence of zinc perchlorate. Absorption spectral studies indicate the formation of a new complex between zinc ion and the porphyrin moiety in the ground state as distinguished from the characteristics of metalo(zinc) porphyrin compound. The energy of maximum fluorescence of porphyrin shifts towards blue with the addition of Zn(ClO(4))(2). Steady state emission studies point to the existence of two emitting species viz, the solvated and the complexed porphyrin in equilibrium. The fluorescence emission of tetraphenylporphyrin at 651-nm bands decreases while that at 605 nm increases upon zinc ion interaction in acetonitrile. Thus, the TPP can behave as a ratiometric fluorescent sensor. This fluorescence modulation of TPP should be applicable to dual-wavelength measurement of various biomolecules or enzyme activities. (1)H NMR spectra of the porphyrin suffered a radical change with the addition of zinc perchlorate which points to the formation of a new porphyrin complex. This change is due to the difference in the electron-donating ability of the pyrrolic nitrogens before and after complexation with Zn(2+). The values of equilibrium constant for the binding process have been determined in acetone and acetonitrile, in both ground and excited states.     

Photophysics of a series of efficient fluorescent pH probes for dual-emission-wavelength measurements in aqueous solutions

This paper evaluates the 5-aryl-2-pyridyloxazole backbone to engineer donor-acceptor fluorescent pH probes after one- or two-photon absorption. Parent fluorophores, as well as derivatives that can be used to label biomolecules, can be easily obtained in good yields. These molecules exhibit a large one-photon absorption in the near-UV range, and a strong fluorescence emission that covers the whole visible domain. The 5-aryl-2-pyridyloxazole derivatives also possess significant cross sections for two-photon absorption. Upon pyridine protonation, large shifts were observed in the absorption spectra after one- and two-photon excitation, as well as in the emission spectra. This feature was used to measure the pK(a) of the investigated compounds that range between 2 and 8. In most of the investigated derivatives, the pK(a) increased upon light excitation and protonation exchanges took place during the lifetime of the excited state, as shown by phase-modulation fluorometry analysis. Several 5-aryl-2-pyridyloxazole derivatives are suggested as efficient probes to reliably measure the pH of aqueous solutions by means of ratiometric methods that are dependent on fluorescence emission.     

Water-soluble fluorescent boronic acid compounds for saccharide sensing: substituent effects on their fluorescence properties

Four new naphthalene-based boronic acid compounds (1-4) were synthesized. The effect of various carbohydrates on their fluorescence properties has been studied in aqueous phosphate buffer at pH 7.4. Different substitutions on the aniline group of the naphthalene ring resulted in significant differences in fluorescence properties for these four compounds. Compound 1 shows ratiometric fluorescence changes upon addition of a sugar. Compounds 2 and 3 do not show ratiometric fluorescence changes but show very large fluorescence intensity changes (about 70-fold fluorescence intensity increase). In addition to the quantifiable fluorescence property changes upon sugar addition, the fluorescence color changes of 1-3 are also visible to the naked eye. However, amidation of the aniline nitrogen atom significantly diminishes the fluorescence intensity of compound 4. The crystal structure of one boronic acid provided some insight into the structural features that are important for the fluorescence properties of these compounds.     

氨基苯基类中氮茚化合物的合成及作为质子探针的研究

设计合成了3个氨基苯基类中氮茚化合物, 研究了其在不同pH值的缓冲溶液中的荧光强度变化. 结果表明, 在pH=2.1～4.2之间, 该类化合物具有荧光敏感性. 化合物3a和3b可以作为良好的质子控制的荧光开关器件信号分子.     

Ratiometric Fluorescent Determination of the Zinc Ion Using a Terpyridine Derivative

Zinc is essential for normal growth and development, and hence selective recognition and detection for zinc has been a significant area of research. Here 4′-(4-methoxyphenyl)-2,2′:6′,2″-terpyridine is described for the ratiometric fluorescence quantification of zinc ion with high selectivity. The fluorescence of 4′-(4-methoxyphenyl)-2,2′:6′,2″-terpyridine at 406?nm was quenched in the presence of zinc, and a new emission band appeared at 452?nm. The ratiometric method for the determination of zinc ion was based on the dual fluorescence measurements at 406 and 452?nm. This fluorescence response is caused by the formation of a 1:1 complex between 4′-(4-methoxyphenyl)-2,2′:6′,2″-terpyridine and the zinc(II) ion. The analytical figures of merit for the protocol were obtained. The linear dynamic range extended for zinc concentrations from 3.0 to 40.0?µmol/L with a limit of detection of 0.28?µmol/L. Zinc was determined in water with satisfactory results.     

Solvent dependent fluorescent properties of a 1,2,3-triazole linked 8-hydroxyquinoline chemosensor: tunable detection from zinc(II) to iron(III) in the CH3CN/H2O system

A triazole-containing 8-hydroxyquinoline (8-HQ) ether 2 was efficiently synthesized in two steps from the "click" strategy. Compound 2 gave a strong fluorescence (Φ = 0.21) in nonprotic solvent like CH(3)CN, and a weak fluorescence (Φ = 0.06) in protic solvent like water. In water, a more than 100 nm red shift of the fluorescence maximum was observed for compound 2 in comparison with that in CH(3)CN. This fluorescence difference may be attributed to the intermolecular photoinduced proton transfer (PPT) process involving the protic solvent water molecules. Similarly, this intermolecular PPT process was also observed in the high-water-content CH(3)CN aqueous solution (e.g., CH(3)CN/H(2)O = 5/95, v/v). The water content in the CH(3)CN/H(2)O binary solvent mixture greatly affected the fluorescence intensity (e.g., Φ = 0.06 and 0.25 when CH(3)CN/H(2)O = 5/95 and 95/5, v/v, respectively) and emission wavelength. Using this interesting property, by simple variation of the water content in the CH(3)CN aqueous solution, compound 2 was tuned from a selective "turn-on" fluorescent sensor for Zn(2+) (CH(3)CN/H(2)O = 5/95, v/v) to a ratiometric one for Zn(2+) and a selective "turn-off" one for Fe(3+) (CH(3)CN/H(2)O = 95/5, v/v) over a wide range of pH value. In high-water-content (CH(3)CN/H(2)O = 5/95, v/v) aqueous solution compound 2 shows a selective "turn-on" response toward Zn(2+), with a 10-fold enhancement in the fluorescence intensity at 428 nm and a 62 nm blue shift of the emission maximum (490 to 428 nm) due to the inhibition of intermolecular PPT process upon chelating with Zn(2+). However, in a less polar solvent (CH(3)CN/H(2)O = 95/5, v/v) in which compound 2 has high fluorescence (quantum yield =0.25), it shows a ratiometric response toward Zn(2+), with a continuous decrease of the fluorescence intensity at 399 nm and an increase at 423 nm. More interestingly, in this case, it also exhibits a very sensitive, selective, and ratiometric fluorescence quenching in the presence of Fe(3+), with an 81 nm red shift of the emission maximum (399 to 480 nm) in a wide range of pH through a metal ligand charge transfer (MLCT) effect.     

Naphthalimide-rhodamine based fluorescent probe for ratiometric sensing of cellular pH

The pH values of lysosomes in cancer cells is slightly lower than that in normal cells, which can be used to distinguish cancer cells from normal cells. According to this, a naphthalimide-rhodamine based fluorescent probe(hereafter referred to as RBN) with a pK_a of 4.20 was designed and synthesized for ratiometric sensing of cellular pH via fluorescence resonance energy transfer(FRET), which can respond to different pH precisely through ratiometric fluorescence intensity(Ⅰ_(577)/Ⅰ_(540)). RBN can be employed to distinguish cancer cells from normal cells on the basis of different fluorescent response, in particular, RBN showed excellent water solubility and low cell toxicity, all these are quite significant for potential application in cancer diagnose and therapy.     

Enhanced oxidase-like activity of g-C3N4 nanosheets supported Pd nanosheets for ratiometric fluorescence detection of acetylcholinesterase activity and its inhibitor

The undesirable enzymatic activity of nanozymes under near neutral p H condition and the traditional single signal output always restrict the analytical application of nanozyme-based biosensors.Herein,graphitic carbon nitride nanosheets supported palladium nanosheets composite （Pd/g-C₃N₄） with both oxidase-like activity and fluorescent property is synthesized.Notably,Pd/g-C₃N₄exhibits enhanced oxidase-like activity compared to Pd NSs under p H 7.4.By c...     

Ratiometric chemosensor for fluorescent determination of Zn2+ in aqueous ethanol

A new ratiometric and selective fluorescent chemosensor (1) for quantification of zinc ions in aqueous ethanol has been synthesized and investigated in this work. In an environmentally friendly media of 30% (v/v) water/ethanol and 10 mM Tris-HCl neutral buffer (pH 7.03), 1 displayed selective Zn²⁺ ratiometric fluorescence response, with a dynamic working range of 1.0-8.0 μM and a detection limit of 0.5 μM Zn²⁺. The determination of Zn²⁺ in synthesized water sample was also successful.     

Developing a novel ratiometric fluorescent probe based on ESIPT for the detection of pH changes in living cells

As an important parameter of intracellular metabolism, pH plays important roles in maintaining normal physiological processes. The abnormal pH could cause disorder of cell function which may cause neurological diseases. Herein, we present two novel ratiometric fluorescent probes to detect pH changes. The probes employed 2-(2′-hydroxyphenyl)benzothiazole as fluorescent platform, and displayed desirable fluorescence response to pH on the basis of excited state intramolecular proton transfer (ESIPT) process. The probe BtyC-1 showed green fluorescence at 546 nm under acidic conditions, while it displayed strong blue fluorescence at 473 nm and weak green fluorescence at 546 nm under alkaline conditions. Biological experiments demonstrated that the probe BtyC-1 could be successfully applied for the ratiometric imaging of cellular pH and the NH₄Cl-induced pH changes in living cells.     

Ratiometric, fluorescent BODIPY dye with aza crown ether functionality: synthesis, solvatochromism, and metal ion complex formation

A new pH and metal ion-responsive BODIPY-based fluorescent probe with an aza crown ether subunit has been synthesized via condensation of 4-(1,4,7,10-tetraoxa-13-aza-cyclopentadec-13-yl)-benzaldehyde with the appropriate 1,3,5,7-tetramethyl substituted boron dipyrromethene moiety. Steady-state and time-resolved fluorometries have been used to study the spectroscopic and photophysical characteristics of this probe in various solvents. The fluorescence properties of the dye are strongly solvent dependent: increasing the solvent polarity leads to lower fluorescence quantum yields and lifetimes, and the wavelength of maximum fluorescence emission shifts to the red. The Catalan solvent scales are found to be the most suitable for describing the solvatochromic shifts of the fluorescence emission. Fluorescence decay profiles of the dye can be described by a single-exponential fit in nonprotic solvents, whereas two decay times are found in alcohols. Protonation as well as complex formation with several metal ions are investigated in acetonitrile as solvent via fluorometric titrations. The aza crown ether dye undergoes a reversible (de)protonation reaction (pKa = 0.09) and shows a approximately 50 nm blue shift in the excitation spectra and a 10-fold fluorescence increase upon protonation. The compound also forms 1:1 complexes with several metal ions (Li(+), Na(+), Mg(2+), Ca(2+), Ba(2+), Zn(2+)), producing large blue shifts in the excitation spectra and significant cation-induced fluorescence amplifications.     

A fluorescence ratiometric nano-pH sensor based on dual-fluorophore-doped silica nanoparticles

We have synthesized dual-fluorophore-doped core-shell silica nanoparticles used as ratiometric pH sensor. The nanoparticles were prepared with a reverse microemulsion technique by simultaneously encapsulating two different fluorophores, the pH-sensitive dye fluorescein as a pH indicator and the pH-insensitive dye phenosafranine as an internal reference for fluorescence ratiometric measurement, into silica shell. The nanoparticles prevent the fluorescence dyes leaching from the silica matrix when immersed inside water. The hydrophilic silica shells were made by hydrolysing and polymerizing tetraethoxysilane (TEOS) in water-in-oil microemulsion. The fluorescence intensity ratio of the two dyes varied linearly as a function of pH in the range from 4.0 to 8.0. The sensor was also applied to measure pH of real water samples. The results are in good agreements with that using the conventional glass electrode method. The as-prepared fluorescent nanoparticles showed rapid response, excellent stability and high reproducibility as pH sensors.     

Solvent and pH dependent fluorescent properties of a dimethylaminostyryl borondipyrromethene dye in solution

Steady-state and time-resolved fluorescence techniques have been used to study the photophysical properties of the fluorescent BODIPY-derived dye 3-{2-[4-(dimethylamino)phenyl]ethenyl}-4,4-difluoro-8-(4-methoxyphenyl)-1,5,7-trimethyl-3a,4a-diaza-4-bora-s-indacene. This compound has been synthesized via a microwave-assisted condensation of p-N,N-dimethylaminobenzaldehyde with the appropriate 1,3,5,7-tetramethyl substituted borondipyrromethene unit. The fluorescence properties of the dye are strongly solvent dependent: increasing the solvent polarity leads to lower fluorescence quantum yields and lifetimes, and the wavelength of maximum fluorescence emission shifts to the red. The Catalán solvent scales are found to be the most suitable for describing the solvatochromic shifts of the fluorescence emission. These are dominated by polarity/polarizability effects, as confirmed by quantum-chemical calculations performed in the dielectric continuum approximation. Fluorescence decay profiles of the dye can be described by a single-exponential fit in most solvents investigated, while two decay times are found in alcohols. The dye undergoes a reversible protonation-deprotonation reaction in the acidic pH range with a pK(a) of 2.25 in acetonitrile solution. Fluorimetric titrations as a function of pH produce fluorescence emission enhancements at lower pH. The fluorescence excitation spectra show a hypsochromic shift from 600 nm for the neutral amine to 553 nm for the ammonium form, so that ratiometric measurements can be used to determine pK(a).     

Ratiometric sensing of Hg2+ based on the calix[4]arene of partial cone conformation possessing a dansyl moiety

A new fluorescent chemosensor based on the calix[4]arene of partial cone conformation possessing a dansyl moiety has been synthesized. The chemosensor demonstrates selective optical recognition of Hg(2+) and Cu(2+) in two contrasting modes. The receptor exhibited ratiometric sensing of Hg(2+) and "ON-OFF" type of fluorescence behavior in the presence of Cu(2+). The compound behaves as a fluorescent molecular switch upon chemical inputs of Hg(2+) and Cu(2+) ions.     

Bis-azamacrocyclic anthracene as a fluorescent chemosensor for cations in aqueous solution

A bis-azamacrocyclic anthracene (L2), which has two a 12-membered cyclic tetraamine (cyclen) connected through a 9,10-dimethylanthracene spacer, has been synthesized as a new fluorescent chemosensor for detection of pH and metal cations in aqueous solution (cyclen = 1,4,7,10-tetraazacyclododecane, L2 = 9,10-bis(1,4,7,10-tetraazacyclododecane-1-ylmethyl)anthracene). The fluorescence response of L2 has been studied in comparison to that of the previously reported monoazamacrocyclic anthracene (L1 = 1-(9-anthrylmethyl)-1,4,7,10-tetraazacyclododecane). Plots of the fluorescence intensity of L2 against pH demonstrate a sigmoidal curve with pKa 7.4, which is lower than that of L1 (8.3). Potentiometric titration reveals that the increase in the L2 fluorescence requires protonation of both cyclen rings, thus resulting in the lower pKa value. L2 demonstrates impressive fluorescence response against metal cations. At basic pH, upon addition of Zn2+ or Cd2+, L1 leads to an increase in the fluorescence intensity with a 1:1 metal-intensity response. L2, however, shows a 2:1 response to Zn2+, while showing a 1:1 response to Cd2+. At neutral pH, L1 fluorescence decreases upon addition of Zn2+ or Cd2+ because of a formation of metal-anthracene pi complex. L2, however, still demonstrates a Zn2+-induced increase in intensity with a 2:1 response, while no change in intensity is observed upon Cd2+ addition. The obtained findings suggest potential utilities of L2 as a new type fluorescent chemosensor for the detection of cations in aqueous solution.     

Highly non-planar dendritic porphyrin for pH sensing: observation of porphyrin monocation

Metal-free porphyrin-dendrimers provide a convenient platform for the construction of membrane-impermeable ratiometric probes for pH measurements in compartmentalized biological systems. In all previously reported molecules, electrostatic stabilization (shielding) of the core porphyrin by peripheral negative charges (carboxylates) was required to shift the intrinsically low porphyrin protonation pK(a)'s into the physiological pH range (pH 6-8). However, binding of metal cations (e.g., K(+), Na(+), Ca(2+), Mg(2+)) by the carboxylate groups on the dendrimer could affect the protonation behavior of such probes in biological environments. Here we present a dendritic pH nanoprobe based on a highly non-planar tetraaryltetracyclohexenoporphyrin (Ar(4)TCHP), whose intrinsic protonation pK(a)'s are significantly higher than those of regular tetraarylporphyrins, thereby eliminating the need for electrostatic core shielding. The porphyrin was modified with eight Newkome-type dendrons and PEGylated at the periphery, rendering a neutral water-soluble probe (TCHpH), suitable for measurements in the physiological pH range. The protonation of TCHpH could be followed by absorption (e.g., ε(Soret)(dication)～270,000 M(-1) cm(-1)) or by fluorescence. Unlike most tetraarylporphyrins, TCHpH is protonated in two distinct steps (pK(a)'s 7.8 and 6.0). In the region between the pK(a)'s, an intermediate species with a well-defined spectroscopic signature, presumably a TCHpH monocation, could be observed in the mixture. The performance of TCHpH was evaluated by pH gradient measurements in large unilamellar vesicles. The probe was retained inside the vesicles and did not pass through and/or interact with vesicle membranes, proving useful for quantification of proton transport across phospholipid bilayers. To interpret the protonation behavior of TCHpH we developed a model relating structural changes on the porphyrin macrocycle upon protonation to its basicity. The model was validated by density functional theory (DFT) calculations performed on a planar and non-planar porphyrin, making it possible to rationalize higher protonation pK(a)'s of non-planar porphyrins as well as the easier observation of their monocations.     

A highly selective colorimetric and ratiometric two-photon fluorescent probe for fluoride ion detection

A naphthalimide-based highly selective colorimetric and ratiometric fluorescent probe for the fluoride ion displayed both one- and two-photon ratiometric changes. Upon reaction with the F(-) (TBA(+) and Na(+) salts) anion in CH(3)CN as well as in aqueous buffer solution, probe 1 shows dramatic color changes from colorless to jade-green and remarkable ratiometric fluorescence enhancements signals. These properties are mechanistically ascribed to a fluoride-triggered Si-O bond cleavage that resulted in a green fluorescent 4-amino-1,8-naphthalimide.     

Development of FRET-based dual-excitation ratiometric fluorescent pH probes and their photocaged derivatives

Dual-excitation ratiometric fluorescent probes allow the measurement of fluorescence intensities at two excitation wavelengths, which should provide a built-in correction for environmental effects. However, most of the small-molecule dual-excitation ratiometric probes that have been reported thus far have shown rather limited separation between the excitation wavelengths (20-70 nm) and/or a very small molar absorption coefficient at one of the excitation wavelengths. These shortcomings can lead to cross-excitation and thus to errors in the measurement of fluorescence intensities and ratios. Herein, we report a FRET-based molecular strategy for the construction of small-molecule dual-excitation ratiometric probes in which the donor and acceptor excitation bands exhibit large separations between the excitation wavelengths and comparable excitation intensities, which is highly desirable for determining the fluorescence intensities and signal ratios with high accuracy. Based on this strategy, we created a coumarin-rhodamine FRET platform that was then employed to develop the first class of FRET-based dual-excitation ratiometric pH probes that have two well-resolved excitation bands (excitation separations>160 nm) and comparable excitation intensities. In addition, these pH probes may be considered as in a kind of "secured ratioing mode". As a further application of these pH probes, the dual-excitation ratiometric pH probes were transformed into the first examples of photocaged dual-excitation ratiometric pH probes to improve the spatiotemporal resolution. It is expected that the modular nature of our FRET-based molecular strategy should render it applicable to other small-molecule dual-dye energy-transfer systems based on diverse fluorescent dyes for the development of a wide range of dual-excitation ratiometric probes with outstanding spectral features, including large separations between the excitation wavelengths and comparable excitation intensities.