A new fluorescent pH probe for extremely acidic conditions

Coumarin-Synthetic Methodologies, Pharmacology, and Application as Natural Fluorophore

Coumarins are secondary metabolites made up of benzene and α-pyrone rings fused together that can potentially treat various ailments, including cancer, metabolic, and degenerative disorders. Coumarins are a diverse category of both naturally occurring as well as synthesized compounds with numerous biological and therapeutic properties. Coumarins as fluorophores play a key role in fluorescent labeling of biomolecules, metal ion detection, microenvironment polarity detection, and pH detection. This review provides a detailed insight into the characteristics of coumarins as well as their biosynthesis in plants and metabolic pathways. Various synthetic strategies for coumarin core involving both conventional and green methods have been discussed comparing advantages and disadvantages of each method. Conventional methods discussed are Pechmann, Knoevenagel, Perkin, Wittig, Kostanecki, Buchwald-Hartwig, and metal-induced coupling reactions such as Heck and Suzuki, as well as green approaches involving microwave or ultrasound energy. Various pharmacological applications of coumarin derivatives are discussed in detail. The structural features and conditions responsible for influencing the fluorescence of coumarin core are also elaborated.

A rhodamine coumarin-derived fluorescence probe that selectively detects Fe3+ and measures radiation doses

We synthesized a fluorescence ratiometric probe by combining coumarin and rhodamine B with ethylenediamine to sense Fe3+ and measure ionizing radiation doses. The presence of Fe3+ caused rhodamine to transition from a closed helical structure to an open-ring structure. Additionally, fluorescence resonance energy transfer (FRET) occurred between coumarin and rhodamine B. As a result, the fluorescence intensity at 405 nm (I405) due to coumarin was decreased, whereas that at 585 nm (I585) derived from open-ring structure rhodamine B was increased. The ratio of I585 and I405 (I585/I405) linearly increased as the Fe3+ concentration increased. The probe sensed Fe3+ in a 0-110 μM range, with a lower limit of detection (LOD) of 0.226 μM. Inspired by Fricke dosimeters, we extended the probe to measure X-ray doses using the fluorescence methodology. The probe measured X-ray doses in a 0-30 Gy range with a lower LOD of 0.5 Gy. Additionally, the dosing capability was independent of the dosing rates. Our probe showed potential for detecting Fe3+ and measuring ionizing radiation doses.

Dual-Responsive and Reusable Optical Sensors Based on 2,3-Diaminoquinoxalines for Acidity Measurements in Low-pH Aqueous Solutions

This work is focused on the age-old challenge of developing optical sensors for acidity measurements in low-pH aqueous solutions (pH < 5). We prepared halochromic (3-aminopropyl)amino-substituted quinoxalines QC1 and QC8 possessing different hydrophilic-lipophilic balance (HLB) and investigated them as molecular components of pH sensors. Embedding the hydrophilic quinoxaline QC1 into the agarose matrix by sol-gel process allows for fabrication of pH responsive polymers and paper test strips. The emissive films thus obtained can be used for a semi-quantitative dual-color visualization of pH in aqueous solution. Being exposed to acidic solutions with pH in the range of 1-5, they rapidly give different color changes when the analysis is performed in daylight or under irradiation at 365 nm. Compared with classical non-emissive pH indicators, these dual-responsive pH sensors allow for an increase in the accuracy of pH measurements, particularly in complex environmental samples. pH indicators for quantitative analysis can be prepared by the immobilization of amphiphilic quinoxaline QC8 using Langmuir-Blodgett (LB) and Langmuir-Schäfer (LS) techniques. Compound QC8 possessing two long alkyl chains (n-C₈H₁₇) forms stable Langmuir monolayers at the air-water interface, and these monolayers can be successfully transferred onto hydrophilic quartz and hydrophobic polyvinylchlorid (PVC) substrates using LB and LS techniques, respectively. The 30-layer films thus obtained are emissive, reveal excellent stability, and can be used as dual-responsive pH indicators for quantitative measurements in real-world samples with pH in the range of 1-3. The films can be regenerated by immersing them in basic aqueous solution (pH = 11) and can be reused at least five times.

Chemometric-Assisted Litmus Test: One Single Sensing Platform Adapted from 1-13 to Narrow pH Ranges

A novel 3 × 4 colorimetric sensing platform, named the chemometric-assisted litmus test (CLT), has been developed by covalently anchoring commercial pH indicators to ethylene vinyl alcohol (EVOH). The proposed device can be exploited for pH determinations in a wide range from 1 to 13 and in specific narrow ranges, achieving sufficient accuracy and errors below 0.5 pH units. The experimental procedure is simple, quick and reliable; equilibration is reached in less than 2 h, CLT pictures are acquired by a camera, and data treatment is performed applying chemometric techniques such as principal component analysis (PCA) and partial least square regression (PLS) to RGB indices.

An ESIPT based versatile fluorescent probe for bioimaging live-cells and E. coli under strongly acidic conditions

A BTNN probe undergoes a 146 times increase in fluorescence intensity at 530 nm on lowering the pH from 7.0 to 2.0 and has been deployed for the bioimaging of MG-63 live cells and E. coli bacteria at different pH levels.

A sensitive pH fluorescent probe based on triethylenetetramine bearing double dansyl groups in aqueous solutions and its application in cells

pH fluorescent probes possess many advantages, including intracellular detection, rapid response time and nondestructive testing. In this paper, a highly selective and sensitive fluorescent pH probe based on triethylenetetramine bearing double dansyl groups (1) was synthesized. This probe offers fluorescent measurement of pH value in the range of 5.81-7.21 in aqueous solution, with an 8.64-fold enhancement of fluorescent emission intensity over the unmodified probe. Probe 1 shows a fluorescent color change from a pale yellow to bright green when the pH is increased from 5.81 to 7.21. In addition, probe 1 shows good potential as a fluorescent visualizing sensor for pH values in living GS cells of epinepheluscoioides. The mechanism of the fluorescent response of probe 1 to solution pHs was further clarified by NMR, fluorescent spectra, and UV-vis absorption spectra. The results indicate that the fluorescent emission will shift with an increase in solution pHs, due to increasing deprotonation of the nitrogen atom on the sulfonamides. Deprotonation of the sulfonamide group will inhibits the intramolecular charge transfer process between the imino group and the naphthalene ring, resulting in the recognition phenomenon of blue shift and enhancement of fluorescent emission intensity.

Synthesis and application of coumarin fluorescence probes

In recent years, the research on fluorescent probes has developed rapidly. Coumarin fluorescent probes have also been one of the hot topics in recent years. For the synthesis and application of coumarin fluorescent probes, great progress has been made. Coumarin fluorescent probes have become more and more widely used in biochemistry, environmental protection, and disease prevention, and have broad prospects. This review introduces the three main light emitting mechanisms (PET, ICT, FRET) of fluorescent probes, and enumerates some probes based on this light emitting mechanism. In terms of the synthesis of coumarin fluorescent probes, the existing substituents on the core of coumarin compounds were modified. Based on the positions of the modified substituents, some of the fluorescent probes reported in the past ten years are listed. Most of the fluorescent probes are formed by modifying the 3 and 7 position substituents on the mother nucleus, and the 4 and 8 position substituents are relatively less modified. In terms of probe applications, the detection and application of coumarin fluorescent probes for Cu²⁺, Hg²⁺, Mg²⁺, Zn²⁺, pH, environmental polarity, and active oxygen and sulfide in the past ten years are mainly introduced.

In recent years, the research on fluorescent probes has developed rapidly.

A red-emitting styrylnaphthalimide-based fluorescent probe providing a ratiometric signal change for the precise and quantitative detection of H2O2

A red-emitting and ratiometric fluorescence probe 1 for detecting H₂O₂, based on a styrylnaphthalimide-boronate ester was developed. Upon a H₂O₂-mediated hydrolysis of boronate ester, probe 1 was transformed to 2 with a ratiometric fluorescence change, decrease at 535 and increase at 640 nm. It was also found that the fluorescent reaction of 1 with H₂O₂ in solution could be completed within 10 min and the detection limit was estimated to be 0.30 μM. Moreover, this ratiometric change was highly selective for H₂O₂ over other redox species, metal ions, and anions. Also, this system was found to be capable of detecting H₂O₂ in the pH range of 6-9. Furthermore, probe 1 was preferentially accumulated into the endoplasmic reticulum (ER) in the live HeLa cells, and an increased H₂O₂ level in the presence of an ER stress inducer, thapsigargin was revealed.

Visualization of mercury(ii) accumulation in vivo using bioluminescence imaging with a highly selective probe

Mercury is a highly toxic environmental pollutant that negatively affects human health. Thus, an in vivo method for noninvasive imaging of mercury(ii) and visualization of its accumulation within living systems would be advantageous. Herein, we describe a reaction-based bioluminescent probe for detection of mercury(ii) in vitro and accumulation in vivo. The application of this probe would help to shed light on the intricate contributions of mercury(ii) to various physiological and pathological processes.

Magnesium tetrapyrazinoporphyrazines: tuning of the pKa of red-fluorescent pH indicators

Fluorescence of Mg(ii) complexes of tetrapyrazinoporphyrazines can be switched off and on depending on pH.

A fluorescent pH probe for an aqueous solution composed of 7-hydroxycoumarin, Schiff base and phenanthro[9,10-d]imidazole moieties (PICO)

The pH fluorescent probe 7-hydroxy-4-methyl-8-(((2-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)imino)methyl)-2H-chromen-2-one (PICO) contains a donor–π–acceptor (D–π–A) conjugated system. The ‘off−on’ probe PICO has a pKa value of 8.01 and its fluorescence intensity is enhanced with increasing pH.

A reaction-based luminescent switch-on sensor for the detection of OH- ions in simulated wastewater

A series of luminescent iridium(iii) complexes were synthesized and evaluated for their ability to interact with hydroxide ions in semi-aqueous media at ambient temperature. Upon the addition of OH^-, a nucleophilic aromatic substitution reaction takes place at the bromine groups of the N^N ligand of complex 1, resulting in the generation of a yellow-green luminescence. Complex 1 showed a 35-fold enhanced emission at pH 14 when compared to neutral pH, and the detection limit for OH^- ions was 4.96 μM. Complex 1 exhibited high sensitivity and selectivity, long-lived luminescence and impressive stability. Additionally, we have demonstrated the practical application of complex 1 to detect OH^- ions in simulated wastewater.

A quinoline-based fluorometric and colorimetric dual-modal pH probe and its application in bioimaging

The compound (E)-8-hydroxyl-2-[(E)-2-(2, 4-dihydroxyphenyl)vinyl]-quinoline (1) has been developed as a fluorometric and colorimetric dual-modal probe for pH detection in solution and in vivo. Remarkable changes in the fluorescence intensity with large Stokes shifts and colorimetric responses were observed as a function of pH. The sensing mechanisms involving protonation and deprotonation processes over the acidic and alkaline pH ranges were confirmed by ¹H NMR and IR spectroscopic analysis. Furthermore, the application of probe 1 for the imaging of live PC3 cells was successfully achieved. Test strips based on probe 1 were fabricated, and were found to act as a convenient and efficient pH test kits.

Imaging of lysosomal pH changes with a novel quinoline/benzothiazole probe

A fluorescent pH probe BTVQ for imaging lysosomal pH fluctuations in live cells and extreme acidity in E. coli cells.

A novel pyrene-based dual multifunctional fluorescent probe for differential sensing of pH and HSO3− and their bioimaging in live cells

We designed and synthesized a new fluorescent probe for detecting pH and HSO₃⁻ using pyrene as an electron donor (D) and pyridine as an electron acceptor (A).

Ratiometric fluorescent pH nanoprobes based on in situ assembling of fluorescence resonance energy transfer between fluorescent proteins

pH-dependent protein adsorption on mesoporous silica nanoparticle (MSN) was examined as a unique means for pH monitoring. Assuming that the degree of protein adsorption determines the distance separating protein molecules, we examined the feasibility of nanoscale pH probes based on fluorescence resonance energy transfer (FRET) between two fluorescent proteins (mTurquoise2 and mNeonGreen, as donor and acceptor, respectively). Since protein adsorption on MSN is pH-sensitive, both fluorescent proteins were modified to make their isoelectric points (pIs) identical, thus achieving comparable adsorption between the proteins and enhancing FRET signals. The adsorption behaviors of such modified fluorescent proteins were examined along with ratiometric FRET signal generation. Results demonstrated that the pH probes could be manipulated to show feasible sensitivity and selectivity for pH changes in hosting solutions, with a good linearity observed in the pH range of 5.5-8.0. In a demonstration test, the pH probes were successfully applied to monitor progress of enzymatic reactions. Such an "in situ-assembling" pH sensor demonstrates a promising strategy in developing nanoscale fluorescent protein probes. Graphical abstract Working principle of the developed pH sensor TNS; and FRET Ratio (I₅₂₈/I₄₆₀) as a function of pH under different protein feed ratios (mNeonGreen to mTurquoise2).

A colorimetric and fluorescent pH probe for imaging in E. coli cells

MDAKexhibited turn-off fluorescence as the pH decreased. Simultaneously, the color of the solution changed from yellow to colorless.

A ratiometric fluorescent pH probe based on keto–enol tautomerization for imaging of living cells in extreme acidity

A ratiometric fluorescent pH probe (DDXC) has been developed for extreme acidity, the sensing mechanism of which is based on the reversible protonation of the carbonyl oxygen followed by keto–enol tautomerization.

Luminescence materials for pH and oxygen sensing in microbial cells - structures, optical properties, and biological applications

Luminescence including fluorescence and phosphorescence sensors have been demonstrated to be important for studying cell metabolism, and diagnosing diseases and cancer. Various design principles have been employed for the development of sensors in different formats, such as organic molecules, polymers, polymeric hydrogels, and nanoparticles. The integration of the sensing with fluorescence imaging provides valuable tools for biomedical research and applications at not only bulk-cell level but also at single-cell level. In this article, we critically reviewed recent progresses on pH, oxygen, and dual pH and oxygen sensors specifically for their application in microbial cells. In addition, we focused not only on sensor materials with different chemical structures, but also on design and applications of sensors for better understanding cellular metabolism of microbial cells. Finally, we also provided an outlook for future materials design and key challenges in reaching broad applications in microbial cells.

Two 1,8-naphthalimide-based proton-receptor fluorescent probes for pH determination

Two newly synthesised 1,8-naphthalimide-based proton-receptor fluorescent probes

A comparative study of lysosome-targetable pH probes based on phenoxazinium attached with aliphatic and aromatic amines

Phenoxazinium was used as a fluorophore for the design of pH probes by the photoinduced electron transfer (PET) mechanism.

A single fluorescent probe for multiple analyte sensing: efficient and selective detection of CN−, HSO3− and extremely alkaline pH

A water-soluble broadly emitting (500–650 nm) fluorescent probe (IECBT) for the selective and sensitive detection of multiple analytes, including CN⁻, HSO₃⁻ and extremely alkaline pH, is designed and synthesized.

A novel "off-on" colorimetric and fluorescent rhodamine-based pH chemosensor for extreme acidity

A novel "off-on" colorimetric and fluorescent rhodamine analogue was synthesized and characterized, and used to monitor extreme acidity (below pH 3.5) via the photophysical response to pH. The colorless spirocyclic structure at high pH (pH⩾7.0) opened to the colored and highly fluorescent form at very low pH (pH<3.0). This sensitive pH probe was characterized with short response time, good reversibility and no interaction with interfering metal ions, and the quantitative relationship between the fluorescence intensity and pH value was consistent with the equilibrium equation pH=pKa-log[(Imax-I)/(I-Imin)]. The fluorescent response to strong acidity was further verified by fluorescent imaging of bacteria, Escherichia coli, which contributed to the development of more useful colorimetric and fluorescent sensors based on the rhodamine platform for measuring intracellular pH in extremely acidic conditions.

A new probe based on rhodamine B and benzothiazole hydrazine for sensing hypochlorite in living cells and real water samples

We have developed a novel fluorescent probe (RBT) based on rhodamine B and benzothiazole hydrazine units for the detection of hypochlorite in living cells and real water samples with excellent selectivity and sensitivity.

A ratiometric lysosomal pH probe based on the naphthalimide–rhodamine system

The ratiometric pH probe RNL was constructed by integrating the naphthalimide moiety as an FRET donor into the rhodamine moiety as an FRET acceptor. The probe with a pK_a of 4.82 could detect pH in the range of 4.50–5.50, selectively stain lysosome and detect lysosomal pH changes.