Studies on fluorescein-V The absorbance of fluorescein in the ultraviolet, as a function of pH

The ultraviolet absorption spectum of an aqueous solution of highly-purified yellow fluorescein at ionic strength 0.10 has been measured at various pH values in the range from 0.15 to 8.70. The maxima at 227, 249 and 295 nm change little with pH, but the maximum found at 437 nm in acid medium changes greatly in absorbance and position on addition of alkali, resolving first into two maxima, at 455 and 475 nm, and finally becoming a single large maximum (at pH 8) at 490 nm. A unique feature of the absorption at 437 nm is that all four prototropic forms of fluorescein, H(3)Fl(+), H(2)Fl, HFl(-) and Fl(2-), absorb at this wavelength. The total absorbance at this wavelength first falls rapidly as the pH rises from 0.15, reaching a minimum at pH 3.63, then increases to a maximum at pH 5.3, and finally falls to steady value at pH > 8.0. The absorbance as a function of pH is defined by seven constants: three dissociation constants (K(H(3)Fl) = 6.61 x 10(-3), K(H(2)Fl) = 3.98 x 10(-5), K(HFl) = 4.36 x 10(-10)) and four molar absorptivities ((H(3)Fl) = 4.94 x 10(-4), (H(2)Fl) = 1.20(5) x 10(4), (HFl) = 2.16 x 10(4) and (Fl) = 7.61 x 10(3) 1.mole(-1).cm(-1)). Solutions of yellow fluorescein in water undergo rapid deterioration on exposure to daylight or fluorescent lighting but are stable in the dark.     

Study on various fluorescein derivatives as pH sensors

Various fluorescein derivatives were examined as fluorescent pH sensors. Fluorescein derivatives bearing benzylic amine moieties displayed opposite fluorescent changes compared to those of simple fluorescein derivatives upon pH changes. Photo-induced electron transfer (PET) mechanism controls the fluorescent changes of these derivatives. In this Letter, modulations of pK_a values were successfully demonstrated through variations of substituent groups.     

Evolution of fluorescein as a platform for finely tunable fluorescence probes

Fluorescence imaging is the most powerful technique currently available for continuous observation of dynamic intracellular processes in living cells. Suitable fluorescence probes are naturally of critical importance for fluorescence imaging, but only a very limited range of biomolecules can currently be visualized because of the lack of flexible design strategies for fluorescence probes. At present, design is largely empirical. Here we show that the carboxylic group of traditional fluorescein dyes, formerly considered indispensable, has been replaced with other substituents, affording various kinds of new fluoresceins. Further, by breaking out of the traditional structure of fluorescein, we developed the first and totally rational design strategy for novel fluorescence probes based on a strict photochemical basis. The value of this approach is exemplified by its application to develop a novel, highly sensitive, and membrane-permeable fluorescence probe for beta-galactosidase, which is the most widely used reporter enzyme.     

Studies on fluorescein-III The acid strengths of fluorescein as shown by potentiometric titration

Potentiometric back-titration of yellow solid fluorescein (H(2)Fl) and of red solid fluorescein in alkali with acid yielded titration curves that were practically identical in shape and position. The end-points at pH 8.5, 5.40 and 3.3 corresponded, respectively, to titration of the excess of standard alkali, and the successive protonations Fl(2-) + H(+) = HFl(-) and HFl(-) + H(+) = H(2)Fl. The pH at the mid-point of the first protonation yielded a value of 6.36 for pK(HFl) (ionic strength 0.10). Because of precipitation of yellow fluorescein during the second protonation step, a value for pK(H(2)Fl) could not be obtained. The total concentration of fluorescein at the first appearance of the precipitate fell on the curve for the solubility of yellow fluorescein as a function of pH. The titrations and the pK values found for the three acid groups of protonated fluorescein (H(3)Fl(+)) have been interpreted on the basis that in water fluorescein exists in only one structural form the yellow zwitterion. Similar back-titrations of alkalinized solutions of yellow or red fluorescein in 50% aqueous ethanol showed that in this medium fluorescein is present in only one form, presumably the quinonoid structure, with much weaker apparent acid functions, pK'(1) = 6.38 and PK'(2) = 7.16 (ionic strength 0.10).     

Lanthanide doped carbon dots as a fluorescence chromaticity-based pH probe

A colorimetric and fluorescent pH probe was designed by doping carbon dots (C-dots) with Eu(III), Tb(III) and 2,6-pyridinedicarboxylic acid (DPA). The resulting nanoparticles were applied as fluorescent indicators for pH values (best detected at excitation/emission wavelengths of 272/545, 614 nm). The pH induced optical effects are due to pH induced variations in energy transfer. The fluorescence of the probe shows a continuous color variation, and a linear change with pH values in the range from 3.0 to 10.0 can be established by using a Commission Internationale de L’Eclairage (CIE) chromaticity diagram. This new kind of pH nanoprobe is more accurate than previously reported pH indicator probes because the pH value can be calculated by using chromaticity coordinates that only depend on the chromaticity. The pH nanoprobe was applied to visualize pH values in human breast adenocarcinoma cells (MCF-7).

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Graphical abstract Carbon dots modified with Eu(III) and Tb(III) complexes of 2,6-pyridinedicarboxylic acid (DPA) were prepared. The doped carbon dots were used as a pH-sensitive nanosensor. The fluorescence chromaticity of the nanoparticles changes with the variation of pH value.

     

Degradation of floor adhesives as a function of pH

Floor adhesives on cement-based substrates may degrade if the pH is high enough and this has in many cases led to emissions of odorous substances and deteriorated indoor air quality. We have used isothermal calorimetry to assess the degradation rate of two floor adhesives as a function of pH. The rate of heat production measured by the calorimeter is proportional to the reaction rate. The degradation rate was similar for a “standard” and a “low emitting” adhesive, but the low emitting adhesive did not release volatile reaction products. The results show that adhesive degradation is strongly pH dependent. A model of alkaline hydrolysis based on two reaction sites is discussed.     

Multiway chemometric decomposition of EEM of fluorescence of CdTe quantum dots obtained as function of pH

The effect of the pH (from 3 to 10) on the excitation emission matrices (EEMs) of fluorescence of CdTe quantum dots (QDs), capped with mercaptopropionic acid (MPA), were analyzed by multiway decomposition methods of parallel factor analysis (PARAFAC), a variant of the parallel factor analysis method (PARAFAC2) and multivariate curve resolution alternating least squares (MCR-ALS). Three different sized CdTe QDs with emission maximum at 555 nm (QDa), 594 nm (QDb) and 628 nm (QDc) were selected for analysis. The three-way data structures composed of sets of EEMs obtained as function of the pH (EEMs, pH) do not have a trilinear structure. A marked deviation to the trilinearity is observed in the emission wavelength order—the emission spectra suffers wavelength shift as the pH is varied. The pH-induced variation of the fluorescence properties of QDs is described with only one-component PARAFAC2 or MCR-ALS models—other components are necessary to model scattering and/or other background signals in (EEMs, pH) data structures. Bigger sized QDs are more suitable tools for analytical methodologies because they show higher Stokes shifts (resulting in simpler models) and higher pH range sensitivity. The pH dependence of the maximum wavelength of the emission spectra is particularly suitable for the development of QDs/EEMs wavelength-encoded pH sensor bioimaging or biological label methodologies when coupled to multiway chemometric decomposition.     

A long lifetime chemical sensor: study on fluorescence property of fluorescein isothiocyanate and preparation of pH chemical sensor

The fluorescence property of fluorescein isothiocyanate (FITC) in acid-alkaline medium was studied by spectrofluorimetry. The characteristic of FITC response to hydrogen ion has been examined in acid-alkaline solution. A novel pH chemical sensor was prepared based on the relationship between the relative fluorescence intensity of FITC and pH. The measurement of relative fluorescence intensity was carried out at 362 nm with excitation at 250 nm. The excellent linear relationship was obtained between relative fluorescence intensity and pH in the range of pH 1-5. The linear regression equation of the calibration graph is F = 66.871 + 6.605 pH (F is relative fluorescence intensity), with a correlation coefficient of linear regression of 0.9995. Effects of temperature, concentration of FITC on the response to hydrogen ion had been examined. It was important that this chemical sensor was long lifetime, and the property of response to hydrogen ion was stable for at least 70 days. This pH sensor can be used for measuring pH value in water solution. The accuracy is 0.01 pH unit. The results obtained by the pH sensor agreed with those by the pH meter. Obviously, this pH sensor is potential for determining pH change real time in biological system.     

Development of a fluorescein analogue, TokyoMagenta, as a novel scaffold for fluorescence probes in red region

We present a design strategy for fluorescence probes with a high off/on activation ratio in the red wavelength region, based on a novel fluorescein analogue in which the O atom at the 10 position of the xanthene chromophore is replaced with a Si atom. To demonstrate the usefulness of this strategy, we designed and synthesized a red-fluorescent probe for β-galactosidase, and showed that it works in live HEK293 cells.     

Rational principles for modulating fluorescence properties of fluorescein

Rational design strategies based on practical fluorescence modulation mechanisms would enable us to rapidly develop novel fluorescence probes for target molecules. Here, we present a practical and general principle for modulating the fluorescence properties of fluorescein. We hypothesized that (a) the fluorescein molecule can be divided into two moieties, i.e., the xanthene moiety as a fluorophore and the benzene moiety as a fluorescence-controlling moiety, even though there is no obvious linker structure between them, and (b) the fluorescence properties can be modulated via a photoinduced electron transfer (PeT) process from the excited fluorophore to a reducible benzene moiety (donor-excited PeT; d-PeT). To evaluate the relationship between the reduction potential of the benzene moiety and the fluorescence properties, we designed and synthesized various derivatives in which the reduction potential of the benzene moiety was fine tuned by introducing electron-withdrawing groups onto the benzene moiety. Our results clearly show that the fluorescence properties of fluorescein derivatives were indeed finely modulated depending upon the reduction potential of the benzene moiety. This information provides a basis for a practical strategy for rational design of novel functional fluorescence probes.     

Carbon dots with strong excitation-dependent fluorescence changes towards pH. Application as nanosensors for a broad range of pH

In this study, preparation of novel pH-sensitive N-doped carbon dots (NCDs) using glucose and urea is reported. The prepared NCDs present strong excitation-dependent fluorescence changes towards the pH that is a new behavior from these nanomaterials. By taking advantage of this unique behavior, two separated ratiometric pH sensors using emission spectra of the NCDs for both acidic (pH 2.0 to 8.0) and basic (pH 7.0 to 14.0) ranges of pH are constructed. Additionally, by considering the entire Excitation–Emission Matrix (EEM) of NCDs as analytical signal and using a suitable multivariate calibration method, a broad range of pH from 2.0 to 14.0 was well calibrated. The multivariate calibration method was independent from the concentration of NCDs and resulted in a very low average prediction error of 0.067 pH units. No changes in the predicted pH under UV irradiation (for 3 h) and at high ionic strength (up to 2 M NaCl) indicated the high stability of this pH nanosensor. The practicality of this pH nanosensor for pH determination in real water samples was validated with good accuracy and repeatability.     

Ultrasonic-assisted leaching of trace metals from sediments as a function of pH

The effects of ultrasonication on the leaching of trace metals form sediments as a function of pH have been investigated using atomic absorption spectrometry. Cu, Pb, Ni, Zn and Mn were leached from sediments by ultrasonic effect using phthalate buffers at pH values of 2.2-6.0. Parameters influencing leaching, such as leaching time, solution matrix, wet or dry sample and final pH were evaluated. Results from ultrasonic leaching experiments were comparable to those of conventional procedure. In addition, this ultrasonic-assisted leaching method reduces the time required for conventional method approximately from 12 h to 25 min. Depending on the metal and sample type, metal removal increased linearly or exponentially with decreasing pH. The accuracy of the method was tested by comparing obtained results with this of conventional method. The average relative standard deviation (R.S.D.) of ultrasonic-assisted leaching method (ULM) varied between 1.71 and 3.00% for N = 36, depending on the analyte. This technique shows promise for studying chemical and biological availability and uptake/release processes for metals in sediment and soil as a function of pH.     

Reaction rates and dissolution mechanisms of quartz as a function of pH

In this computational study, we present the dissolution rates for quartz as a function of pH at 298 K. At any given pH, the dissolution of the quartz surface depends on the distribution of protonated, deprotonated, or neutral species. The dissolution mechanism for each of these three species was investigated by ab initio electronic structure calculations to obtain the reaction profile. Using the barrier height along with the partition functions for the transition state and the reactants in the rate-limiting steps, we calculated the TST rate constants for the reactions for the temperature range of 200-500 K. At 298 K the rate constant (s-1) for the dissolution of neutral species was found to be several orders of magnitude smaller than the rate-limiting steps for the protonated and deprotonated species. The values of the rate constants were used in the rate law expression to calculate the overall dissolution rate (mol m-2 s-1) at a given pH. The calculated rates were compared to previously reported experimental and theoretical rates and were found to be in good agreement over 2-12 pH range.     

Cu triggered fluorescence sensor based on fluorescein derivative for Pd detection

This Letter describes the synthesis of a novel fluorescein-based derivative used as the fluorescence sensor for Pd²⁺ detection. The sensor can show highly selective and sensitive ‘off-on’ fluorescence response only in the presence of Cu²⁺ as a synergic trigger, which presents a new strategy for Pd²⁺ detection method.     

pH-Responsive amorphous room-temperature phosphorescence polymer featuring delayed fluorescence based on fluorescein

Numerous researchers have paid attention to achieve metal-free phosphorescence by exploring new structures or new mechanisms. Herein, a facile way is introduced to endow a common fluorescence dye, tetrabromofluorescein (4Br-Flu), some fabulous optical characteristics such as dual emission including thermally activated delayed fluorescence, room-temperature phosphorescence (RTP), and the excellent pH-sensitivity. Shortly, 4Br-Flu with good light-emitting properties is composed into the polymer system. The multiple bromine atoms promote the spin-orbit coupling effect and facilitate triplet excitation. Especially, the hydrogen bonding network of the polymer restricts the molecular motion of 4Br-Flu so that the system can emit long-wavelength RTP when 4Br-Flu is doped into polyvinyl alcohol or co-polymerized with acrylamide. Due to the reversible transformation of protonation and deprotonation, the 4Br-Flu based polymer responded to acid and alkali like a phosphorescent switch which makes it an excellent hydrogen chloride/ammonia gas leak detector in dry environment.