An Overview of Naphthylimide as Specific Scaffold for New Drug Discovery

Naphthylimides play a pivotal role as aromatic heterocyclic compounds, serving as the foundational structures for numerous pharmacologically significant drugs. These drugs encompass antibacterial, antifungal, anticancer, antimalarial, antiviral, anti-inflammatory, antithrombotic, and antiprotozoal agents. The planar and heteroaromatic characteristics of naphthylimides grant them a strong ability to intercalate into DNA. This intercalation property renders naphthylimide derivatives highly valuable for various biological activities. The advantageous pharmacological activity and ease of synthesis associated with naphthylimides and their derivatives provide significant benefits in the design and development of new compounds within this class. Currently, only a few such molecules are undergoing preclinical and clinical evaluations. In this paper, we have compiled the literature on naphthylimides reported by researchers from 2006 to 2024. Our focus lies on exploring the pharmacological activities of their analogues from a drug development and discovery perspective, while examining their structure-activity relationship and mechanisms of action.

Application of RET Approach for Ratiometric Response Enhancement of ICT Fluorescent Hg2+ Probe based on Crown-containing Styrylpyridinium Dye

Styrylpyridinium dye bearing azadithia-15-crown-5 ether receptor group SP and 4-alkoxy-1,8-naphthalimide fluorophore were linked using copper-catalyzed azide-alkyne cycloaddition click reaction to afford dyad compound NI-SP. Chemosensor NI-SP exhibited selective ratiometric fluorescent response to the presence of Hg2+ in aqueous solution due to the interplay between resonance energy transfer (RET) and intramolecular charge transfer (ICT) processes occurred upon excitation. The observed switching of the ratio of emission intensities in the blue and red channels R was higher than in the case of monochromophoric styrylpyridine derivative SP showing ratiometric response based on ICT mechanism only. Biological studies revealed that NI-SP penetrates into human lung adenocarcinoma A549 cells and accumulates in cytoplasm and lysosomes. When cells were pre-incubated with mercury (II) perchlorate, the ratio R was increased 2.6 times, which enables detection of intracellular Hg2+ ions and their quantitative analysis in the 0.7-6.0 μM concentration range.

Synthesis and Characterization of Pt(II) and Pd(II) Complexes with Planar Aromatic Oximes

A series of four Werner-type complexes of Pd(II) and Pt(II) with planar, isomeric conjugated aromatic naphtoquinone oximes were synthesized for the first time. These ligands were 1-oxime-2-naphtoquinone (HL1) and 2-oxime-1-napthoquinone (HL2). Compounds were characterized using thermal analysis, spectroscopic methods, and X-ray analysis. TG/DSC data were collected for pure starting organic ligands, their complexes, and indicated vigorous exothermic decomposition with at ~155 °C for starting HL and ~350 °C for transition metal complexes. Crystal structures for two Pt compounds with 2-oxime-1-quinone were determined and revealed the formation of the cis-geometry complexes and incorporation of molecules of stoichiometric solvents in the lattice: acetonitrile and nitrobenzene. Both solvents of crystallization displayed attractive interactions between their C-H groups and the oxygen atoms of the nitroso groups in complexes, leading to short distances in those fragments. Despite the presence of solvents of inclusion, the overall structure motifs in both compounds represent 1D columnar coordination polymer, in which the PtL2 units are held together via metallophilic interactions, thereby forming ‘Pt-wires’. The Hirshfield surface analysis was performed for both crystallographically characterized complexes. The results showed intermolecular π–π stacking and Pt–Pt interactions among the planar units of both complexes. In addition, the analysis also verified the presence of hydrogen bonding interactions between the platinum unit and solvent molecules. Solid bulk powdery samples of both PtL12 and PtL22 demonstrated pronounced photoluminescence in the near infrared region of spectrum at ~980 nm, being excited in the range of 750–800 nm. The NIR emission was observed only for Pt-complexes and not for pure starting organic ligands or Pd-complexes. Additionally, synthesized Pt-naphtoquinone oximes do not show luminescence in solutions, which suggests the importance of a 1D ‘metal wire’ structure for this process.

Solvent-polarity reconfigurable fluorescent 4-piperazino-N-aryl-1,8-naphthalimide crown ether logic gates

Four compounds 1-4 were designed and synthesised, comprising a 4-amino-N-aryl-1,8-naphthalimide fluorophore, a piperazine receptor, and an aryl group, as fluorescent logic gates. At the imide position, the substituent is phenyl (1), 1,2-dimethoxyphenyl (2), benzo-15-crown-5 (3), or benzo-18-crown-6 (4). Molecules 1 and 2 are constructed according to a fluorophore-spacer-receptor format, while 3 and 4 are engineered according to a receptor1-spacer1-fluorophore-spacer2-receptor2 format based on photoinduced electron transfer and internal charge transfer mechanisms. The compounds were studied in water, water/methanol mixtures of different ratios, and methanol by UV-visible absorption and steady-state fluorescence spectroscopy, as a function of pH, metal ions and solvent polarity. The excited state of 1-4 is 8.4 ± 0.2 in water, 7.6 ± 0.1 in 1 : 1 (v/v) water/methanol, and 7.1 ± 0.3 in methanol. The of 3 in water is 0.92 and the and of 4 in water are 2.3 and 2.9. 1H NMR data in D2O and CD3OD confirm H+ interaction at the piperazine moiety, and Na+ and Ba2+ binding at the benzo-15-crown-5 and benzo-18-crown-6 moieties of 3 and 4. By altering the solvent polarity, the fluorescent logic gates can be reconfigured between TRANSFER logic and AND logic. Molecules with polarity reconfigurable logic could be useful tools for probing the microenvironment of cellular membranes and protein interfaces.

Fluorescent RET-Based Chemosensor Bearing 1,8-Naphthalimide and Styrylpyridine Chromophores for Ratiometric Detection of Hg2+ and Its Bio-Application

Dyad compound NI-SP bearing 1,8-naphthalimide (NI) and styrylpyridine (SP) photoactive units, in which the N-phenylazadithia-15-crown-5 ether receptor is linked with the energy donor naphthalimide chromophore, has been evaluated as a ratiometric fluorescent chemosensor for mercury (II) ions in living cells. In an aqueous solution, NI-SP selectively responds to the presence of Hg²⁺ via the enhancement in the emission intensity of NI due to the inhibition of the photoinduced electron transfer from the receptor to the NI fragment. At the same time, the long wavelength fluorescence band of SP, arising as a result of resonance energy transfer from the excited NI unit, appears to be virtually unchanged upon Hg²⁺ binding. This allows self-calibration of the optical response. The observed spectral behavior is consistent with the formation of the (NI-SP)·Hg²⁺ complex (dissociation constant 0.13 ± 0.04 µM). Bio-imaging studies showed that the ratio of fluorescence intensity in the 440–510 nm spectral region to that in the 590–650 nm region increases from 1.1 to 2.8 when cells are exposed to an increasing concentration of mercury (II) ions, thus enabling the detection of intracellular Hg²⁺ ions and their quantitative analysis in the 0.04–1.65 μM concentration range.

Enhanced Photodynamic Efficacy Using 1,8-Naphthalimides: Potential Application in Antibacterial Photodynamic Therapy

This study addresses the need for antibacterial medication that can overcome the current problems of antibiotics. It does so by suggesting two 1,8-naphthalimides (NI1 and NI2) containing a pyridinium nucleus become attached to the imide-nitrogen atom via a methylene spacer. Those fluorescent derivatives are covalently bonded to the surface of a chloroacetyl-chloride-modified cotton fabric. The iodometric method was used to study the generation of singlet oxygen (¹O₂) by irradiation of KI in the presence of monomeric 1,8-naphthalimides and the dyed textile material. Both compounds generated reactive singlet oxygen, and their activity was preserved even after they were deposited onto the cotton fabric. The antibacterial activity of NI1 and NI2 in solution and after their covalent bonding to the cotton fabric was investigated. In vitro tests were performed against the model gram-positive bacteria B. cereus and gram-negative P. aeruginosa bacteria in dark and under light iradiation. Compound NI2 showed higher antibacterial activity than compound NI1. The light irradiation enhanced the antimicrobial activity of the compounds, with a better effect achieved against B. cereus.

Solid Fluorescence pH Sensors Based on 1,8-Naphthalimide Copolymers Synthesized by UV Curing

Novel water-swollen photo-crosslinked membranes were obtained by copolymerization of the N-vinylpyrrolidone, butyl acrylate and ethyl methacrylate monomers functionalized with naphthalimide groups, as pH sensitive fluorescence probes. For that purpose, two monomers with pending naphthalimide groups anchored to ethyl methacrylate through alkyl chains with different length, were previously synthesized. The membranes were characterized using different techniques. The pH dependence of absorbance and the corresponding quenching of fluorescence were investigated and related to the structure of naphthalimide substituents linked to the membrane. The new solid sensors exhibited sensitive fluorescence changes at pH < 3, and lower time response was determined for membranes where the sensing group was linked through longer alkyl chain to the polymer matrix. The membranes were solid, thermally stable and easily handled to be applied as sensor materials, and showed a reversible behavior, which is an important feature for further fabrication of an economical on-site tool for the acidity detection in aqueous environments.

Unique Photophysical Properties of 1,8-Naphthalimide Derivatives: Generation of Semi-stable Radical Anion Species by Photo-Induced Electron Transfer from a Carboxy Group

The development of anion sensors for selective detection of a specific anion is a crucial research topic. We previously reported a selective photo-induced colorimetric reaction of 1-methyl-3-(N-(1,8-naphthalimidyl)ethyl)imidazolium (MNEI) having a cationic receptor in the presence of molecules having multiple carboxy groups, such as succinate, citrate, and polyacrylate. However, the mechanism underlying this reaction was not clarified. Here, we investigate the photo-induced colorimetric reaction of N-[2-(trimethylammonium)ethyl]-1,8-naphthalimide (TENI), which has a different cationic receptor from MNEI and undergoes the photo-induced colorimetric reaction, and its analogues to clarify the reaction mechanism. The TENI analogues having substituents on the naphthalene ring provide important evidence, suggesting that the colorimetric chemical species were radical anions generated via photo-induced electron transfer from carboxylate to the naphthalimide derivative. The generation of the naphthalimide-based radical anion is verified by ¹H NMR and cyclic voltammetry analyses, and photo-reduction of methylene blue is mediated by TENI. In addition, the role of the cationic receptor for the photo-induced colorimetric reaction is investigated with TENI analogues having different hydrophilic groups instead of the trimethylammonium group. Interestingly, the photo-induced colorimetric reaction is observed in a nonionic analogue having a polyethylene glycol group, indicating that the colorimetric reaction does not require a cationic receptor. On the other hand, we reveal that the trimethylammonium group stabilizes the radical anion species. These generation and stabilization phenomena of naphthalimide-based radical anion species will contribute to the development of sophisticated detection systems specific for carboxylate.

Nature of Linear Spectral Properties and Fast Electronic Relaxations in Green Fluorescent Pyrrolo[3,4-c]Pyridine Derivative

The electronic nature of 4-hydroxy-1H-pyrrolo[3,4-c]pyridine-1,3,6(2H,5H)-trione (HPPT) was comprehensively investigated in liquid media at room temperature using steady-state and time-resolved femtosecond transient absorption spectroscopic techniques. The analysis of the linear photophysical and photochemical parameters of HPPT, including steady-state absorption, fluorescence and excitation anisotropy spectra, along with the lifetimes of fluorescence emission and photodecomposition quantum yields, revealed the nature of its large Stokes shift, specific changes in the permanent dipole moments under electronic excitation, weak dipole transitions with partially anisotropic character, and high photostability. Transient absorption spectra of HPPT were obtained with femtosecond resolution and no characteristic solvate relaxation processes in protic (methanol) solvent were revealed. Efficient light amplification (gain) was observed in the fluorescence spectral range of HPPT, but no super-luminescence and lasing phenomena were detected. The electronic structure of HPPT was also analyzed with quantum-chemical calculations using a DFT/B3LYP method and good agreement with experimental data was shown. The development and investigation of new pyrrolo[3,4-c]pyridine derivatives are important due to their promising fluorescent properties and potential for use in physiological applications.

Dual-channel ratiometric recognition of Al3+ and F- ions through an ESIPT-ESICT signalling mechanism

An optical probe 1 has been synthesized comprising naphthalimide unit conjugated with Schiff base, exhibiting excited state intramolecular proton transfer and intramolecular charge transfer as a potential sensor for Al³⁺ and F^- ions using standard spectroscopic techniques. The probe 1 exhibited local and charge-transfer excitation at 340 nm and 460 nm, respectively. On excitation at 460 nm, probe 1 displayed two emission bands at 510 nm and 610 nm, accompanied by Stokes' shift of 50 nm and 150 nm, respectively. The solvatochromic effect and theoretical calculation depicted that the representative emissions resulted from the ESICT/ESIPT phenomenon. Upon addition of Al³⁺ ions, the charge transfer excitation at 460 nm was enhanced ratiometrically to local excitation at 340 nm and showed a color change from orange to yellow. Similarily, probe 1.Al³⁺ displayed emission enhancement at 540 nm in H₂O/CH₃CN (1:9; v/v) and showed a color change from yellow to blue-green emission. Following the detection of Al³⁺ ions, hydrolysis of probe 1 to its reacting precursors was observed. The detection of Al³⁺ ions was also demonstrated in surfactant-containing water. The limit of detection (LOD) of probe 1 (H₂O/CH₃CN (1:9; v/v)) towards Al³⁺ ions was measured to be 3.2 × 10^-8 M. The probe 1 displayed a ratiometric absorption response towards F^- ions with a new peak at 570 nm and showed a color change from orange to purple. The probe 1.F^- displayed a decrease in emission at 635 nm. The LOD of probe 1 (CH₃CN) towards F^- ions was measured to be 7.5 × 10^-7 M.

Demonstration of Selective Single-Barium Ion Detection with Dry Diazacrown Ether Naphthalimide Turn-on Chemosensors

Single-molecule fluorescence imaging (SMFI) of gas-phase ions has been proposed for "barium tagging," a burgeoning area of research in particle physics to detect individual barium daughter ions. This has potential to significantly enhance the sensitivity of searches for neutrinoless double-beta decay (0νββ) that is obscured by background radiation events. The chemistry required to make such sensitive detection of Ba²⁺ by SMFI in dry Xe gas at solid interfaces has implications for solid-phase detection methods but has not been demonstrated. Here, we synthesized simple, robust, and effective Ba²⁺-selective chemosensors capable of function within ultrapure high-pressure ¹³⁶Xe gas. Turn-on fluorescent naphthalimide-(di)azacrown ether chemosensors were Ba²⁺-selective and achieved SMFI in a polyacrylamide matrix. Fluorescence and NMR experiments supported a photoinduced electron transfer mechanism for turn-on sensing. Ba²⁺ selectivity was achieved with computational calculations correctly predicting the fluorescence responses of sensors to barium, mercury, and potassium ions. With these molecules, dry-phase single-Ba²⁺ ion imaging with turn-on fluorescence was realized using an oil-free microscopy technique for the first time-a significant advance toward single-Ba²⁺ ion detection within large volumes of ¹³⁶Xe, plausibly enabling a background-independent technique to search for the hypothetical process of 0νββ.

A Highly Selective Fluorescent Probe for Hg2+ Based on a 1,8-Naphthalimide Derivative

Hg2+ has a significant hazardous impact on the environment and ecosystem. There is a great demand for new methods with high selectivity and sensitivity to determine mercury in life systems and environments. In this paper, a novel turn-on Hg2+ fluorescent probe has been reported with a naphthalimide group. The Hg2+ fluorescent probe was designed by the inspiration of the well-known specific Hg2+-triggered thioacetal deprotection reaction. A 1,2-dithioalkyl group was chosen as the specific recognition site of Hg2+. The probe showed weak fluorescence without Hg2+, and the color of the solution was light yellow. In the presence of Hg2+, the probe reacted specifically with the mercury ion to produce an aldehyde and emitted strong fluorescence, and the color of the solution also turned light green, thus realizing the monitoring of the mercury ion. The Hg2+ fluorescent probe showed outstanding sensitivity and selectivity toward Hg2+. Furthermore, the Hg2+ fluorescent probe could work in a wide pH range. The linear relationship between the fluorescence intensity at 510 nm and the concentration of Hg2+ was obtained in a range of Hg2+ concentration from 2.5 × 10-7 to 1.0 × 10-5 M. The detection limit was found to be 4.0 × 10-8 M for Hg2+. Furthermore, with little cell toxicity, the probe was successfully applied to the confocal image of Hg2+ in PC-12 cells.

Synthesis, Photophysical Characterization, and Sensor Activity of New 1,8-Naphthalimide Derivatives

Three new 1,8-naphthalimide derivatives M1-M3 with different substituents at the C-4 position have been synthesized and characterized. Their photophysical properties have been investigated in organic solvents of different polarity, and their fluorescence intensity was found to depend strongly on both the polarity of the solvents and the type of substituent at C-4. For compounds M1 and M2 having a tertiary amino group linked via an ethylene bridge to the chromophore system, high quantum yield was observed only in non-polar media, whereas for compound M3, the quantum efficiency did not depend on the medium polarity. The effect of different metal ions (Ag⁺, Ba²⁺, Cu²⁺, Co²⁺, Mg²⁺, Pb²⁺, Sr²⁺, Fe³⁺, and Sn²⁺) on the fluorescence emission of compounds M1 and M2 was investigated. A significant enhancement has been observed in the presence of Ag⁺, Pb²⁺, Sn²⁺, Co²⁺, Fe³⁺, as this effect is expressed more preferably in the case of M2. Both compounds have shown significant pH dependence, as the fluorescence intensity was low in alkaline medium and has been enhanced more than 20-fold in acidic medium. The metal ions and pH do not affect the fluorescence intensity of M3. Density-functional theory (DFT) and Time-dependent density-functional theory (TDDFT) quantum chemical calculations are employed in deciphering the intimate mechanism of sensor mechanism. The functional properties of M1 and M2 were compared with polyamidoamine (PAMAM) dendrimers of different generations modified with 1,8-naphthalimide.

Spectral Characteristics and Sensor Ability of a New 1,8-Naphthalimide and Its Copolymer with Styrene

In this study, a novel 6-(allylamino)-2-(2-(dimethylamino)ethyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (NI3) was synthesized and characterized. Its copolymer with styrene was also obtained. The photophysical characteristics of NI3 were investigated in organic solvents and the results were compared with those of its structural analogue, 2-allyl-6-((2-(dimethylamino)ethyl)amino)-1H-benzo[de]isoquinoline-1,3(2H)-dione (NI4). The influences of the pH in the medium and different metal ions on the fluorescent intensity of monomers and polymers were also investigated. Computational tools (DFT and TDDFT calculations) were employed when studying the structure and properties of the 1,8-naphthalimide-based chromophores. Although the position of the N,N-dimethylaminoethylamine receptor fragment did not significantly impact proton detection, it was still important for detecting metal ion sensor ability, especially for monomeric 1,8-naphthalimide structures and their copolymers with styrene.

Microwave-Assisted Synthesis and Fluorescent Properties of 4-Phenyl-1,8-naphthalimide

4-Phenyl-1,8-naphthalimide was synthesized by imidation of commercially available 4-bromo-1,8-naphthalic anhydride, followed by Suzuki coupling with phenyl boronic acid, both under microwave heating. The microwave-assisted reactions were found to be faster and more efficient than reactions carried out by heating in oil-baths. While this compound was quite fluorescent in solvents such as chloroform, methanol and ethanol, it is virtually non-fluorescent in DMSO; however, upon the addition of water to DMSO solutions of this dye, fluorescence was restored, suggesting a tendency for aggregation-induced emission. The fluorescent properties of 4-phenyl-1,8-naphthalimide were found to be influenced by salt concentrations, likely as a result of hydrophobic effects. While this dye does not show binding to DNA, presence of bovine serum albumin leads to effective fluorescence quenching.

A hydrophilic naphthalimide-based fluorescence chemosensor forCu2+ ion: Sensing properties, cell imaging and molecular logic behavior

In this work, a hydrophilic naphthalimide-based fluorescence chemosensor (sensor 1) was synthesized for Cu²⁺ recognition, in which 2-(2-aminoethoxy)ethanol was introduced to improve the hydrophily and Schiff base acted as the multidentate ligand for Cu²⁺. The effect factors, sensing mechanism and regenerability of sensor 1 for Cu²⁺ sensing were systematically investigated. It was found that sensor 1 displayed a long emission wavelength of 532 nm upon excited in visible light region (436 nm), and the good water solubility made it utilized in aqueous media. It could selectively react with Cu²⁺ over other common metal ions to form a 2:1 complex within 1 min and result in significant fluorescence quench. The fluorescence change was linear to 0.5-10.0 μmol L^-1 of Cu²⁺ with a low detection limit of 3.74 × 10^-8 mol L^-1. Sensor 1 has been successfully utilized for analyzing Cu²⁺ in water samples as well as imaging cellular Cu²⁺. Moreover, in view of fluorescence "on-off-on" switches of sensor 1 induced by Cu²⁺ and EDTA, an IMPLICATION logic gate was constructed based on fluorescence mode with Cu²⁺ and EDTA as inputs.

Enhanced ion binding by the benzocrown receptor and a carbonyl of the aminonaphthalimide fluorophore in water-soluble logic gates

Fluorescent logic gates with benzocrown ethers attached at the imide naphthalimide exhibit synergistic binding of Na⁺ and K⁺ in aqueous methanol and water.

The Design and Synthesis of Fluorescent Coumarin Derivatives and Their Study for Cu2+ Sensing with an Application for Aqueous Soil Extracts

A series of fluorescent coumarin derivatives 2a–e were systematically designed, synthesized and studied for their Cu²⁺ sensing performance in aqueous media. The sensitivities and selectivities of the on-to-off fluorescent Cu²⁺ sensing signal were in direct correlation with the relative arrangements of the heteroatoms within the coordinating moieties of these coumarins. Probes 2b and 2d exhibited Cu²⁺ concentration dependent and selective fluorescence quenching, with linear ranges of 0–80 μM and 0–10 μM, and limits of detection of 0.14 μM and 0.38 μM, respectively. Structural changes of 2b upon Cu²⁺ coordination were followed by fluorescence titration, attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), mass spectrometry, and single crystal X-ray diffraction on the isolated Cu²⁺-coumarin complex. The results revealed a 1:1 stoichiometry between 2b and Cu²⁺, and that the essential structural features for Cu²⁺-selective coordination are the coumarin C=O and a three-bond distance between the amide NH and heterocyclic N. Probe 2b was also used to determine copper (II) levels in aqueous soil extracts, with recovery rates over 80% when compared to the standard soil analysis method: inductively coupled plasma-mass spectrometry (ICP-MS).

Surface Functionalization of Cotton Fabric with Fluorescent Dendrimers, Spectral Characterization, Cytotoxicity, Antimicrobial and Antitumor Activity

Poly(propylenimine) dendrimers from first and third generations modified with 1,8-naphthalimide units and their Zn(II) complexes have been investigated by absorption and fluorescence spectroscopy. These dendrimers have been deposited on a cotton cloth by the extraction method, producing yellow-colored textile materials. They have been characterized by defining their color coordinates L*a*b*, XYZ and xy. The antimicrobial activity of dendrimers has been investigated in vitro against model gram-positive and gram-negative bacteria and yeasts. Being deposited onto the surface of cotton fabric, the studied dendrimers reduced bacterial growth and prevented the formation of bacterial biofilm. Anticancer and cytotoxicity activities have also been performed against HeLa and Lep-3 human tumor cell lines as model systems.

A novel dithiourea-appended naphthalimide "on-off" fluorescent probe for detecting Hg2+ and Ag+ and its application in cell imaging

An effective dithiourea-appended 1,8-naphthalimide fluorescent probe was designed and synthesized. This probe could recognize Hg²⁺ and Ag⁺ sensitively and selectively in neutral and alkaline conditions. Moreover, the probe detected Hg²⁺ alone at pH between 2 and 6. The sensing ability of the probe was explored by UV-vis, fluorescence, FTIR and ¹H NMR spectroscopy. The probe was quenched by Hg²⁺ and Ag⁺ with 1:1 binding ratios in MeCN/H₂O (4/1, v/v) mixed solution with binding constants of 3.76 × 10⁴ L mol^-1 and 2.47 × 10⁴ L mol^-1, respectively. The linear concentration ranges for Hg²⁺ and Ag⁺ were 0-17 μmol L^-1 and 0-24 μmol L^-1 with detection limits of 0.83 μmol L^-1 and 1.20 μmol L^-1, respectively, which allowed for the quantitative determination of Hg²⁺ and Ag⁺. The new probe, 3a, was successfully applied to the fluorescence imaging of Hg²⁺ and Ag⁺ in HepG2 cells, demonstrating its potential application in biological science. Moreover, 3a was used to measure Hg²⁺ and Ag⁺ in tap water, drinking water and ultrapure water samples. The recoveries of Hg²⁺ and Ag⁺ in water samples were 96-99% and 98-103%, respectively. Therefore, the proposed method showed promising perspectives for its application, aimed at detecting Hg²⁺ and Ag⁺ in fluorescence imaging and real water samples.

Development of a unique reversible fluorescent probe for tracking endogenous sulfur dioxide and formaldehyde fluctuation in vivo

A FRET-based reversible fluorescent probe for sensing SO₂ and FA was designed. The probe was first used for imaging endogenous SO₂ and FA in vitro and in vivo. Moreover, we first found that the interaction of SO₂ and FA can reduce the cytotoxicity.

A “weak acid and weak base” type fluorescent probe for sensing pH: mechanism and application in living cells

A simple pH fluorescent probe, N-(6-morpholino-1, 3-dioxo-1H-benzo[de]isoquinolin-2(3H)-yl) isonicotinamide (NDI), based on naphthalimide as the fluorophore and isonicotinic acid hydrazide as the reaction site was synthesized and characterized. It is useful for monitoring acidic and alkaline pH. The results of pH titration indicated that NDI exhibits obvious emission enhancement with a pK_a of 4.50 and linear response to small pH fluctuations within the acidic range of 3.00–6.50. Interestingly, NDI also displayed strong pH-dependent characteristics with pK_a 9.34 and linearly responded to an alkaline range of 8.30–10.50. The sensing response mechanism was confirmed by ¹H NMR and ESI-MS spectroscopy. The mechanism of the optical responses of NDI toward pH was also determined by density functional theory (DFT) calculations. In addition, NDI displayed a highly selective and sensitive response to hydrogen ions and hydroxyl ions. The probe was successfully applied to image acidic and alkaline pH value fluctuations in HeLa cells and has lysosomal targeting ability.

When the probe was in the protonation process, the fluorescence intensity gradually decreased, whereas when the probe was in the deprotonation process, the fluorescence intensity gradually increased.

Recent Progress on the Evolution of Pourbaix Sensors: Molecular Logic Gates for Protons and Oxidants

Recent progress in the area of molecular logic, in particular molecules capable of sensing for acidity and oxidizability, are gathered together in this short review. Originally proposed as AND logic gates that provide a high fluorescence output when simultaneously protonated and oxidized, the concept has been extended from two-input to three-input variants and to include molecules that function as INHIBIT logic gates. Photochemical concepts such as photoinduced electron transfer (PET) and internal charge transfer (ICT) are exploited as favorite design concepts. This review highlights the evolution of Pourbaix sensors with anthracene, pyrazoline, and naphthalimide fluorophores. Future applications abound in various disciplines from corrosion science, material science, geochemistry to cell imaging.