1,8-naphthalimide modified [12]aneN₃ compounds as selective and sensitive probes for Cu²⁺ ions and ATP in aqueous solution and living cells

Fluorescent Biomass-Based Platform for Detection of ClO- in Cells and Water-Soluble Systems

The generation and presence of excessive hypochlorous acid derivative ionic form (ClO-) could cause various diseases, such as arteriosclerosis, DNA damage, and cardiovascular illness. It is a critical need to develop a highly sensitive sensor for reliable detection of ClO- in cells and water-soluble systems. In this work, a hydroxyl group has been introduced into the compound 2-amino-3-(((E)-4-(2-(2-(2-hydroxyethoxy)ethyl)-1,3-dioxo-2,3-dihydro-1H-benzo[de]isoquinolin-6-yl)benzylidene)amino)maleonitrile (NDC) to increase its solubility in water, at the same time, the hydrazone unit was designed as a specific recognition group for the "off-on" fluorescence probe of ClO-. The probe NDC presents high selectivity, sensitivity, anti-interference, and low detection limit (67 nM) for ClO-. The recognition mechanism that ClO- breaks the C=N bond and forms the fluorescent compound 4-(2-(2-(2-hydroxyethoxy)ethyl)-1,3-dioxo-2,3-dihydro-1H-benzo[de]isoquinolin-6-yl)benzaldehyde (ND-3) has been confirmed by time-of-flight mass spectrometry. The probe NDC presents a good performance in the actual test of water samples and can be designed as the test papers for the quick and convenient detection of ClO- range from 0 to 1 μM. Moreover, the practical application was demonstrated by the successful imaging of endogenous and exogenous ClO- in HeLa cells. Our fluorescent biomass-based platform opens vast possibilities for repeatability, sensitivity, and selectivity detection of ClO- in cells and water-soluble systems.

Research Progress on Structure-Activity Relationship of 1,8-Naphthalimide DNA Chimeras Against Tumor

The development of 1,8-naphthalimide derivatives as cell probes, DNA targeting agents, and anti-tumor drugs is one of the research hotspots in the field of medicine. Naphthalimide compounds are a kind of DNA embedder, which can change the topological structure of DNA by embedding in the middle of DNA base pairs, and then affect the recognition and action of topoisomerase on DNA. Aminofide and mitonafide are the first 2 drugs to undergo clinical trials. They have good DNA insertion ability, can embed DNA double-stranded structure, and induce topoisomerase II to cut part of pBR322DNA, but not yet entered the market due to their toxicity. In this paper, the design and structure-activity relationship of mononaphthalimide and bisaphthalimide compounds were studied, and the relationship between the structure of naphthalimide and anti-tumor activity was analyzed and discussed. It was found that a variety of structural modifications were significant in improving anti-tumor activity and reducing toxicity.

A novel fluorescent probe based on naphthimide for H2S identification and application

In view of the superior chemical activity of selenoether bond (-Se-) and the excellent optical properties of naphthimide, a novel fluorescent probe (NapSe) with near-rectangular structure, which contains double naphthimide fluorophores linked by selenoether bond, is designed for specific fluorescence detection of hydrogen sulfide (H2S). NapSe has excellent optical properties: super large Stokes Shift (190 nm) and good stability in a wide pH range. The selectivity of NapSe fluorescence detection of H2S is high, and displays excellent "turn-on" phenomenon and strong anti-interference. And the fluorescence intensity increased obviously, reaching 42 times. The time response of probe NapSe is very rapid (3 min) compared with other fluorescence probes that respond to H2S. It shows high sensitivity by calculating the detection limit (LOD) as low as 5.4 μM. Notably, the identification of H2S by probe NapSe has been successfully applied to the detection of test paper and the detection of exogenous and endogenous fluorescence imaging of MCF-7 breast cancer cells.

Supramolecular Recognition of Cytidine Phosphate in Nucleotides and RNA Sequences

Supramolecular recognition of nucleotides would enable manipulating crucial biochemical pathways like transcription and translation directly and with high precision. Therefore, it offers great promise in medicinal applications, not least in treating cancer or viral infections. This work presents a universal supramolecular approach to target nucleoside phosphates in nucleotides and RNA. The artificial active site in new receptors simultaneously realizes several binding and sensing mechanisms: encapsulation of a nucleobase via dispersion and hydrogen bonding interactions, recognition of the phosphate residue, and a self-reporting feature-"turn-on" fluorescence. Key to the high selectivity is the conscious separation of phosphate- and nucleobase-binding sites by introducing specific spacers in the receptor structure. We have tuned the spacers to achieve high binding affinity and selectivity for cytidine 5' triphosphate coupled to a record 60-fold fluorescence enhancement. The resulting structures are also the first functional models of poly(rC)-binding protein coordinating specifically to C-rich RNA oligomers, e.g., the 5'-AUCCC(C/U) sequence present in poliovirus type 1 and the human transcriptome. The receptors bind to RNA in human ovarian cells A2780, causing strong cytotoxicity at 800 nM. The performance, self-reporting property, and tunability of our approach open up a promising and unique avenue for sequence-specific RNA binding in cells by using low-molecular-weight artificial receptors.

A naphthalimide-tyrosine-based dicationic amphiphile for intracellular 'turn-on' simultaneous detection of ATP and CTP

We have developed a new naphthalimide-based amphiphile (YN-1) for the simultaneous detection of ATP and CTP. In YN-1, the cationic tyrosine-linked polyamine (+2 charge, hydrophilic unit) is appended at the -peri position of naphthalimide (hydrophobic unit). YN-1 and its Boc-protected compound 4 were characterized using state-of-the-art spectroscopic and optical techniques such as NMR, IR, UV-vis and fluorescence. The fluorescence data revealed that YN-1 showed a 'turn-on' (λ_em = 440 nm) fluorescence response for nanomolar detection of nucleoside triphosphates such as ATP and CTP in 20% HEPES buffer-DMSO solution. YN-1 also showed a concentration-based discrimination between ATP and CTP. YN-1 has been successfully applied for bioimaging of nucleoside triphosphates in MCF-7 live cancer cells with good compatibility. Therefore, the important findings from the present work will provide insight for future development of fluorescent probes to detect various kinds of essential nucleoside triphosphates.

A novel 1,8-naphthalimide-based Cu2+ ion fluorescent probe and its bioimaging application

A new 1,8-naphthalimide-based Schiff base compound, named as (Z)-2-butyl-6-(((2-hydroxyphenyl)imino)methyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (BHBD), has been simply synthesized with high yields. BHBD can be employed as a "turn-on" fluorescent probe for Cu²⁺ ion with high sensitivity, high selectivity and relatively low detection limit (0.48 × 10^-6 M). The fluorescence emission of BHBD is very weak in H₂O/THF (v/v: 7/3) mixture, which is significantly enhanced after addition of Cu²⁺ ion. The proposed mechanism is verified by ¹H NMR, Job's plot and TOF-MS experiments. Anti-interference experiment, cytotoxicity assay and pH influence results indicated that BHBD meets the requirements of bioimaging. Therefore, BHBD has been successfully applied in detecting Cu²⁺ ion in HeLa cells.

A mitochondria-targeting fluorescent sensor for on-off-on response to Cu2+ and ATP in cells and zebrafish

Cupric ion (Cu²⁺) and adenosine triphosphate (ATP) are functionally important in mitochondria and play essential roles in many important biological processes. In this work, a mitochondria-targeting fluorescent molecule Mito-A was used as a probe to detect Cu²⁺ and ATP. The results showed remarkable fluorescence quenching of Mito-A in the presence of Cu²⁺, and then the quenched fluorescence solution gradually recovered due to the ATP binding to Cu²⁺ from the structure of the molecule. Mito-A has high sensitivity to Cu²⁺ and ATP, with limits of detection (LOD) close to 40 nM and 0.43 μM, respectively. Cell imaging experiments showed that Mito-A has good mitochondria-targeting capabilities, and can be successfully employed for imaging Cu²⁺ and ATP in living cells and zebrafish.

[12]aneN3-based multifunctional compounds as fluorescent probes and nucleic acids delivering agents

A series of multifunctional compounds (MFCs) 1a-1e based on 1,8-naphthalimide and [12]aneN3 building blocks were designed and synthesized. They were used as not only fluorescent probes for recognition of Cu2+ ions but also as non-viral gene vectors for DNA and RNA delivery. Furthermore, their complexes with Cu2+ (1-Cu) could also selectively stain lysosome in HeLa cells. In order to achieve high performance multifunctional materials, structure-performance relationship of MFCs 1a-1e was studied. It was found that MFCs 1a-1e exhibited highly selective fluorescence turn-off for Cu2+, without interference by other metal ions in aqueous solution. The fluorescence emission of 1a-1e was quenched by a factor of 10-fold, 47-fold, 6-fold, 64-fold, and 15-fold respectively in the presence of Cu2+ ions. Due to high sensitivity, good water solubility, and low cytotoxicity, MFCs 1a-1d were successfully applied in the recognition of Cu2+ and selectively staining lysosome in HeLa cells. Most importantly, MFCs 1a and 1b had excellent HeLa cell selectivity in RNA delivery, and their performances were far better than lipofectamine 2000 and 25 kDa PEI.

Alkyl Conformation and π-π Interaction Dependent on Polymorphism in the 1,8-Naphthalimide (NI) Derivative

The 1,8-naphthalimide (NI) derivative Lumogen F Violet 570 exhibits different photoluminescence (PL) and aggregation-caused quenching properties due to its crystal polymorphism, which depends on the solvent evaporation process in tetrahydrofuran solution. In the slow drying process, molecules aggregated into an energetically more stable form (time-dependent density functional theory calculation), of which the PL peak maximum was 453 nm, corresponding to blue emission at the 365 nm excitation. However, the fast evaporation process induces an energetically less stable form, with a PL peak maximum of 508 nm, corresponding to green emission. The main difference between the two crystal structures is the alkyl conformation, as confirmed by X-ray single-crystal analysis. Due to the different alkyl conformations, NI groups aggregated into more obliquely aligned structures that emit blue PL, which plays a role in weakening the π-π interactions between molecules relative to green PL crystals. We found that the conformational stable molecular stacking induced instability in the electronic energy levels of the blue crystal compared to the green crystal.

A new 1,8-naphthalimide-based fluorescent “turn-off” sensor for detecting Cu2+ and sensing mechanisms

In this article, a new “turn-off” fluorescent sensor N- n-butyl-4-{2-[(ethylimino)methyl]phenol}-1,8-naphthalimide (HL) for CuII ions is synthesized, which contains a 1,8-naphthalimide moiety as the fluorophore and a Schiff base as the recognition group. As expected, it exhibits high selectivity and sensitivity for detecting CuII ions over other common metal ions in acetonitrile–2-(4-(2-hydroxyethyl)-1-piperazinyl)-ethanesulfonic acid (HEPES) (1:1 v/v, pH = 7.4) solution. In addition, the fluorescence intensity for HL showed a good linearity with the concentration of CuII ions in the range of 0.5–5.0 μM. The 2:1 binding stoichiometry between HL and CuII ions was established on the basis of combined fluorescence titrations, a Job’s plot, single-crystal X-ray analysis and mass spectrometry. The quenching response of HL toward CuII ions is attributed to the reverse photoinduced electron transfer mechanism. The proposed sensor HL is preliminarily applied to quantify CuII ions in water samples from the Yellow River and tap water.

1,8-Naphthalimide-Based Multifunctional Compounds as Cu2+ Probes, Lysosome Staining Agents, and Non-viral Vectors

A series of multifunctional compounds (MFCs) 1a-1d based on 1,8-naphthalimide moiety were designed and synthesized. Due to the good fluorescence property and nucleic acid binding ability of 1,8-naphthalimide, these MFCs were applied in Cu2+ ion recognition, lysosome staining as well as RNA delivery. It was found that these MFCs exhibited highly selective fluorescence turn-off for Cu2+ in aqueous solution. The fluorescence emission of 1a-1d was quenched by a factor of 116-, 20-, 12-, and 14-fold in the presence of Cu2+ ions, respectively. Most importantly, 1a-Cu and 1b-Cu could be used as imaging reagents for detection of lysosome in live human cervical cancer cells (HeLa) using fluorescence microscopy. Furthermore, in order to evaluate the RNA delivery ability of 1a-1d, cellular uptake experiments were performed in HeLa, HepG2, U2Os, and MC3T3-E1 cell lines. The results showed that all the materials could deliver Cy5-labled RNA into the targeted cells. Among them, compound 1d modified with long hydrophobic chain exhibited the best RNA delivery efficiency in the four tested cell lines, and the performance was far better than lipofectamine 2000 and 25 kDa PEI, indicating the potential application in non-viral vectors.

A dansyl-appended N-heterocycle for Cu2+ and S2- recognition via a displacement mode

A novel l-proline based heterocycle 3 of C₂ symmetry has been designed and synthesized for cation and anion recognition in aqueous solution. Ligand 3 shows a strong affinity to Cu²⁺ ion, and their interaction induces a remarkable fluorescence quenching in DMSO:H₂O=9:1 (HEPES buffer, 0.01M, pH7.4) among various metal ions. Both the in-situ generated and isolated 3-Cu²⁺ complex exhibit specific fluorescence recovery upon addition of S^2-, even in the presence of S₂O₃^2-, L-histidine, and thiol-containing amino acids. For this dual functional switch, a combination of ¹H NMR titration, ESI mass and FT-IR spectra suggest that its sensing behavior is via a displacement mode. Sequential "on-off-on" fluorescence bio-imaging of the heterocycle 3 to Cu²⁺ and S^2- was carried out in HeLa cells.

Molecular Receptors for Recognition and Sensing of Nucleotides

Nucleotides are constituents of nucleic acids and they have a variety of functions in cellular metabolism. Synthetic receptors and sensors are required to reveal the role of nucleotides in living organisms and mechanisms of signal transduction events. In recent years, a large number of nucleotide-selective synthetic receptors have been devised, which utilize different molecular designs and sensing mechanisms. This Minireview presents recent progress in the design of synthetic molecular receptors for selective recognition of nucleotides in aqueous solution. The binding properties of receptors and the origins of their selectivity for a particular nucleotide are discussed.

Rational design of an "on-off-on" fluorescent switch for Cu2+ and histidine based on chiral macrocyclic dioxopolyamine

A novel chemosensor 1 having a chiral macrocyclic dioxopolyamine of C₂ symmetry as a receptor and anthracene as a signal unit has been designed and synthesized for cations and α-amino acids recognition in DMSO-HEPES buffer (1:9, v/v, pH 7.2). The ligand exhibited selective response to Cu²⁺ even in the presence of other metal ions with a fluorescence "switch-off" behavior. Additionally, the in situ generated 1-Cu²⁺ ensemble displayed specific recognition to histidine by a "switch-on" fluorescence response. For this dual functional switch, its sensing behavior via a displacement mode was confirmed by ¹H NMR titration and ESI mass spectroscopy. Sequential "on-off-on" fluorescence responses of 1 to Cu²⁺ and histidine are successfully applied in HeLa cells.

A Highly Selective and Sensitive Fluorescent Turn-Off Probe for Cu2+ Based on a Guanidine Derivative

A new highly selective and sensitive fluorescent probe for Cu²⁺, N-n-butyl-4-(1′-cyclooctene-1′,3′,6′-triazole)-1,8-naphthalimide (L), was synthesized and evaluated. The structure of compound L was characterized via IR, ¹H-NMR, ¹³C-NMR and HRMS. The fluorescent probe was quenched by Cu²⁺ with a 1:1 binding ratio and behaved as a “turn-off” sensor. An efficient and sensitive spectrofluorometric method was developed for detecting and estimating trace levels of Cu²⁺ in EtOH/H₂O. The ligand exhibited excitation and emission maxima at 447 and 518 nm, respectively. The equilibrium binding constant of the ligand with Cu²⁺ was 1.57 × 10⁴ M⁻¹, as calculated using the Stern-Volmer equation. Ligand L is stable and can be used to detect Cu²⁺ in the range of pH from 7 to 12. The sensor responded to Cu²⁺ rapidly and a large number of coexisting ions showed almost no obvious interference with the detection.

A highly sensitive and fast responsive naphthalimide-based fluorescent probe for Cu2+ and its application

The response of the probe L to Cu²⁺ is reversible and very fast (20 s). L has a low detection limit of 49 nM and was used for imaging of Cu²⁺ in MCF-7 cells with satisfying results. The sensor L can be analyzed with a molecular logic gate.

A novel hexahydroquinazolin-2-amine-based fluorescence sensor for Cu2+ from isolongifolanone and its biological applications

The isolongifolanone derivative (2c) exhibit highly selective and sensitive fluorescence quenching towards copper ions, and this was used for real-time sensing of cooper ions in vivo.