pH-responsive Probe Research Articles

Tuning the p Ka of a pH Responsive Fluorophore and the Consequences for Calibration of Optical Sensors Based on a Single Fluorophore but Multiple Receptors.

Since Sørensen and Bjerrum defined the pH scale, we have relied on two methods for determining pH, the colorimetric or the electrochemical. For pH electrodes, calibration is easy as a linear response is observed in the interesting pH range from 1 to ∼12. For colorimetric sensors, the response follows the sigmoidal Bjerrum diagram of an acid-base equilibrium. Thus, calibration of colorimetric sensors is more complex. Here, seven pH responsive fluorescent dyes based on the same diazaoxatriangulenium (DAOTA) fluorophore linked to varying receptor groups were prepared. Photoinduced electron transfer (PeT) quenching from appended aniline or phenol receptors generated the pH response of the DAOTA dyes, and the position of the p Ka value of the dye was tuned using the Hammett relationship as a guideline. The fluorescence intensity of the dyes in a sol-gel matrix environment was measured as a function of pH in universal buffer, and it was found that the dyes behave as perfect pH responsive probes under these conditions. The response of optical pH sensors is nonlinear and was found to be limited to 2-3 pH units for a precision of 0.01 pH unit. As sensors with a broader sensitivity range can be achieved by mixing multiple dyes with different p Ka values, mixtures of dyes in solution were investigated, and a broad range pH sensor with a precision of 0.006 pH units over a range of 3.6 pH units was demonstrated. Further, approximating the sensor response as linear was considered, and a limiting precision for this approach was determined. As the responses of the pH responsive DAOTA dyes were found to be ideally sigmoidal and as the six dyes were shown to have p Ka values scattered over a range from ∼2 to ∼9, this allows for design of a broad range optical pH sensor in the pH range from 1 to 10. This hypothesis was tested using quaternary mixtures of the different DAOTA dyes, and these were found to behave as a direct sum of the individual components. Thus, while linear calibration is limited to a precision of 0.02 in a range of 2-3 pH units, calibration using ideal sigmoidal functions is possible in the range of 1-10 with a precision better than 0.01, and as good as 0.002 pH units.

Development of near-infrared sensitized core-shell-shell upconverting nanoparticles as pH-responsive probes.

Recently, the functionalization of nanoparticles, either within themselves or on the outer surface and its application in medicine, turned out to be the ultimate goal of nanotechnology. By providing these nanoparticles with chemical functional groups, one can force the nanoparticles to target the markers of the particular diseases or to measure the quantity and distribution of various intracellular species. In this paper, we report our development of a pH-responsive nanocomposite based on lanthanide-doped upconverting nanoparticles (UCNPs). Through multiphoton absorption and energy migration between spatially separated Nd3+, Yb3+, and Tm3+ in a three-layered NaYF4 host coated with FITC (fluorescein-5-isothiocyanate), this nanocomposite can measure the pH with high sensitivity. The fundamental acidity measurement is based on the pH-dependent equilibrium of the bright and dark states of FITC. The tremendous advantages of this system, regarding the pH measurement, come from the fact that the versatility of UCNP-imaging can fully be exploited. This includes the fact that (a) the optical wavelengths for the sensitization (980 nm and/or 808 nm) and the emission bands (UV, visible) are well separated, (b) the spectral overlap between FITC (absorption) and Tm3+ (emission) is substantially high, (c) there is no background signal due to the near-infrared laser, and (d) the signals are consistent regardless of the fluctuations by monitoring the ratio of blue band with respect to the unaffected self-reference (red and near-infrared bands). Moreover, the double shell structure is obviously superior to the core–shell structure in that it enhances the spectral separation between the sensitizer and the emitter in the upconversion process, inhibiting any unnecessary contamination in the spectra. Finally, it is noteworthy that Yb3+ plays crucial roles as a sensitizer at 980 nm excitation and a bridge above which 808 nm excitation migrates from Nd3+ to Tm3+via the Yb3+ excited state.

Design, synthesis and evaluation of HepDirect fluorescence probes mediated by asialoglycoprotein receptor

A series of HepDirect pH responsive fluorescence probes of glycosyl-rhodamine are synthesized. The recognition trend of glucoside < lactoside < galactoside illuminates that the galactose is a targeting group toward HepG2 cells and its recognition is affected by the bulk of targeting group. The HepG2 cells binding trend of RDGal1< RDGal2< RDGal3 supports the cluster effect of galactose. RDGal3 with the optimum targeting potential is verified to selectively recognize HepG2 cells via a galactose dependent ASGPR binding mechanism. These nontoxic probes could be controllably activated by the acidic microenvironment of cancer cells, which is very significant to achieve diagnosis of hepatocellular carcinoma and provide a novel HepDirect carrier for precision treatment of hepatocarcinoma.

Correction: A pH responsive AIE probe for enzyme assays.

Correction for 'A pH responsive AIE probe for enzyme assays' by Leilei Shi et al., Analyst, 2018, DOI: 10.1039/c7an01710c.

A pH responsive AIE probe for enzyme assays.

By combining leucine (Leu) and tetraphenylethene (TPE), a pH-sensitive aggregation induced emission (AIE) probe TPE-Leu was developed. The aliphatic amine in TPE-Leu was more easily protonated under acidic conditions, which made TPE-Leu more water soluble. Therefore, the protonated AIE probe showed weak fluorescence under acidic conditions. When the pH was changed to basic conditions, it showed strong fluorescence due to the hydrophobic nature of TPE-Leu. We demonstrated that the probe showed high selectivity toward pH changes with the coexistence of other potential species such as metal ions, redox agents, and biomolecules. In contrast, TPE-NH2 did not exhibit obvious pH-sensitive properties. Moreover, TPE-Leu was further utilized to develop a sensitive and selective sensing platform for urease and acetylcholinesterase (AChE) detection. The current study not only provides a new strategy for designing pH-sensitive fluorescent probes for bioassays but also broadens the applications of AIE probes.

PH‐Responsive Aminoproline‐Containing Collagen Triple Helices

(4S)- and (4R)-configured aminoproline (Amp) residues were used as pH-responsive probes to tune the thermal stability of collagen triple helices in acidic and basic environments. The different steric and stereoelectronic properties of amino versus ammonium groups lead to a switch of the ring pucker of Amp upon changing the pH. The choice of the position of Amp within collagen model peptides (CMPs) as well as the absolute configuration at C(4) of the pH-responsive probe allows for tuning of the stability of Amp-containing collagen triple helices over a broad range. Comparative quantum chemical calculations on the steric and stereoelectronic effects of amino and ammonium groups versus fluorine, hydroxy, chlorine, and methyl substituents support the experimental findings. The research also shows that substitution of the naturally occurring hydroxy group in collagen by electron-withdrawing groups with a larger hydration shell than that of the hydroxy group is not tolerated.

Porphyrin-cored dendrimers consisting of novel siloxane-poly (amido amine) dendron-like arms: Synthesis, characterization, and photophysical properties

Different generations of porphyrin-cored dendrimers consisting of novel siloxane- poly(amido amine) dendron-like arms have been synthesized via copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction of azide-modified 5,10,15,20-tetrakis(4-hydroxyphenyl)-porphyrin with different generations of propargyl focal point novel siloxane-poly(amido amine) typed dendron (Si-PAMAM). The used different generations of Si-PAMAM were obtained by the “arm-first” strategy based on the Michael addition reaction and Amidation reaction, orderly. The structure of porphyrin dendrimers is established by 1H NMR, 13C NMR, MALDI-TOF-MS and FT-IR analysis. PP-Si-PAMAM-Dm (m=1, 2, and 3), not only gave strong fluorescence emission and the intensity increased significantly as the generation growing, but also showed a remarkable pH-responsive in fluorescence emission. These interesting properties indicate that synthesized porphyrin dendrimers may be able to act as pH-responsive probes suitable for biomedical imaging, diagnosis and treatment.

Lanthanide coordination polymer probe for time-gated luminescence sensing of pH in undiluted human serum

Lanthanide coordination polymers (LCPs) have emerged as fascinating materials because of their specific structure and properties. In this work, utilizing hydrosoluble biomolecule of guanosine-5′-monophosphate (GMP) as bridging linker, lanthanide terbium ions (Tb3+) as metal nodes, and silver ions (Ag+) as sensitizers, we synthesized a pH responsive luminescent lanthanide CP probe of Tb/GMP/Ag. The probe possesses high luminescence due to the sensitization of Ag+; While in alkaline solutions, Ag+ in Tb/GMP/Ag immediately binds to OH−, forming Ag2O precipitation and resulting in a distinct fluorescence quenching of Tb/GMP/Ag. This probe displays high selectivity for OH− and a broader pH detection range of 7.5–13.0. In addition, based on the high anti-interference ability in serum, we applied Tb/GMP/Ag to measure pH in undiluted human serum samples, yielding satisfactory results.

Combinatorial Dansyl Library and its Applications to pH-Responsive Probes.

Herein, we report the first 48-membered, dansyl-based, combinatorial fluorescent library. From the electronic and structural properties of the probes, we analyzed their optical properties and chemical yields, with an average of 49 %. The molecules were examined for their pH responses, and DS-2 and DS-45 showed blue-shifts, whereas DS-7 and DS-40 showed red-shifts in wavelength with increasing pH. Finally, cell permeability was investigated by treating SNU-2292 cells. Our results demonstrate the potential application of this library in biosensors, bio-imaging and pH indicators.

From visible to far-red excitable chromophores derivatives of vitamin B6. Evaluation as pH-responsive probes and solvatochromic study

Owing to the great potential and application of pH probes in biological and environmental sciences, four aqueous soluble vitamin B6 derivatives 1–4 bearing visible excitable chromophores as coumarin, fluorescein, cresyl violet and nile blue were successfully synthetized and characterized. Compounds 2–4 are highly pH dependent revealed significant spectral changes in the ground and excited states, with exception of 1 whereas no changes in the excited state were visualized. In the same way, at pH 6–7 probes 1 and 3 exhibited the highest fluorescent quantum yield, with values of 0.65 and 0.51. Concerning the solvatochromic behaviour, a positive effect was witnessed for compound 4. Moreover, in the excited state were observed different colours ranging from red (CH2Cl2) to orange (CHCl3, CH3CN), to pink (EtOH), to green (DMSO) and to blue (H2O). The dipole moments in the ground and excited states were determined for compound 4, whereas values of μg.imineE = 3.62 D, μe.imineE = 12.35 D (Δμ = 8.73 D), μg.iminez = 3.38 D and μe.iminez = 11.54 D, (Δμ = 8.16 D), revealing that 4 is more polar in the excited state.

Fluorescent push-pull pH-responsive probes for ratiometric detection of intracellular pH.

A family of fluorescent push-pull pH-responsive probes based on 2-dicyanomethylidene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran as a strong electron acceptor group is described. Small structural variations allow obtaining pK(a) ranging from 4.8 to 8.6, underlining the role of the substituent in modulating the acidic properties. Remarkable changes in the optical properties (in particular the fluorescence intensity ratios) were observed as a function of pH. The most interesting probes with pK(a) close to neutrality were used for ratiometric imaging of intracellular pH.

Ratiometric pH responsive fluorescent probes operative on ESIPT

A series of C-8′-oxime-appended spirobenzopyrans 4a–4d were synthesized and developed as fluorescent pH ratiometric probes operative in HEPES/ACN (8:2, v/v) buffer solutions. Acidochromic conversion of spirobenzopyran to merocyanine open form can facilitate the subsequent enol–keto tautomerization giving dual emissive peaks via the excited state intramolecular proton transfer (ESIPT) mechanism. The pH titrations of 4a show a 68-fold increase in ratiometric intensities ratio (I645 nm/I522 nm) when the pH was switched from 8.0 to 4.0 with a pKa value of 5.90.

Azide Conjugatable and pH Responsive Near-Infrared Fluorescent Imaging Probes

The synthesis and photophysical characteristics of a pH responsive near-infrared fluorescence imaging probe is described. A key feature is the ability to conjugate the probe by an alkyne-azide cycloaddition reaction and its reversible response of fluorescence intensity across the physiological pH range.

Near Infrared Fluorescence-Based Bacteriophage Particles for Ratiometric pH Imaging

Fluorogenic imaging agents emitting in the near-infrared are becoming important research tools for disease investigation in vivo. Often pathophysiological states such as cancer and cystic fibrosis are associated with disruptions in acid/base homeostasis. The development of optical sensors for pH imaging would facilitate the investigation of these diseased conditions. In this report, the design and synthesis of a ratiometric near-infrared emitting probe for pH quantification is detailed. The pH-responsive probe is prepared by covalent attachment of pH-sensitive and pH-insensitive fluorophores to a bacteriophage particle scaffold. The pH-responsive cyanine dye, HCyC-646, used to construct the probe, has a fluorogenic pKa of 6.2, which is optimized for visualization of acidic pH often associated with tumor hypoxia and other diseased states. Incorporation of pH-insensitive reference dyes enables the ratiometric determination of pH independent of the probe concentration. With the pH-responsive construct, measurement of intracellular pH and accurate determination of pH through optically diffuse biological tissue is demonstrated.

Synthesis and luminescence studies of aryl substituted tetraamide complexes of europium(III): a new approach to pH responsive luminescent europium probes.

DOTA-tetraamide ligands having extended phenol or pyridine substituents have been synthesized. The (5)D(0) --> (7)F(J) deltaJ = 1 and 2 emission bands in the corresponding europium(III) complexes differ in their sensitivity to solution pH. This offers the potential for developing pH responsive probes for in vivo imaging that are independent of probe concentration.

Paramagnetic lanthanide(III) complexes as pH-sensitive chemical exchange saturation transfer (CEST) contrast agents for MRI applications.

The recently introduced new class of contrast agents (CAs) based on chemical exchange saturation transfer (CEST) may have a huge potential for the development of novel applications in the field of MRI. In this work we explored the CEST properties of a series of Lanthanide(III) complexes (Ln = Eu, Dy, Ho, Er, Tm, Yb) with the macrocyclic DOTAM-Gly ligand, which is the tetraglycineamide derivative of DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid). These complexes possess two pools of exchangeable protons represented by the coordinated water and the amide protons. Yb-DOTAM-Gly displays the most interesting CEST properties when its amide N-H resonance (16 ppm upfield H2O signal) is irradiated. Up to 70% suppression of the water signal is obtained at pH 8. As the exchange rate of amide protons is base-catalyzed, Yb-DOTAM-Gly results to be an efficient pH-responsive probe in the 5.5-8.1 pH range. Moreover, a ratiometric method has been set up in order to remove the dependence of the observed pH responsiveness from the absolute concentration of the paramagnetic agent. In fact, the use of a mixture of Eu-DOTAM-Gly and Yb-DOTAM-Gly, whose exchangeable proton pools are represented by the coordinated water (ca. 40 ppm downfield H2O signal at 312K) and amide protons, respectively, produces a pH-dependent CEST effect which is the function of the concentration ratio of the two complexes.