4-Amino-1,8-naphthalimide based fluorescent photoinduced electron transfer (PET) pH sensors as liposomal cellular imaging agents: The effect of substituent patterns on PET directional quenching

Emissive triphenylamine functionalised 1,8-naphthalimide and naphthalene diimide fluorophores: aggregation, computation and biological studies

Four new aromatic imides bearing triphenylamino (TPA) moieties are reported each of which differ by the number and/or positional arrangements of the TPA units. Compounds 1-3 are 1,8-naphthalimides (naps) that contain N,N'-diphenyl-[1,1'-biphenyl]-4-amino (TPA-Ph) groups appended to the N-termini of the respective imides. Each differs by their functionalisation of the 4-position of the nap: nitro (1), amino (2), or an additional TPA group (3). By contrast, compound 4 is a naphthalene diimide (NDI) functionalised with TPA-Ph moieties on each N-terminus. These simple modifications produce molecules with vastly different optoelectronic and aggregation properties. This article studies these characteristics with particular focus directed toward the contrast in aggregation-caused quenching (ACQ) properties of 2 compared with the aggregation-induced emission (AIE) properties of 3. The distinct aggregation and photophysical properties of 2 and 3 are delicately exploited using self-assembly with an amphiphilic poloxamer to generate nanoparticles capable of delivering 2 and 3 into cells for biological imaging.

Novel 1,8-Naphthalimide Derivatives Inhibit Growth and Induce Apoptosis in Human Glioblastoma

Given the rapid advancement of functional 1,8-Naphthalimide derivatives in anticancer research, we synthesized these two novel naphthalimide derivatives with diverse substituents and investigated the effect on glioblastoma multiforme (GBM) cells. Cytotoxicity, apoptosis, cell cycle, topoisomerase II and Western blotting assays were evaluated for these compounds against GBM in vitro. A human GBM xenograft mouse model established by subcutaneously injecting U87-MG cells and the treatment responses were assessed. Both compounds 3 and 4 exhibited significant antiproliferative activities, inducing apoptosis and cell death. Only compound 3 notably induced G2/M phase cell cycle arrest in the U87-MG GBM cells. Both compounds inhibited DNA topoisomerase II activity, resulting in DNA damage. The in vivo antiproliferative potential of compound 3 was further validated in a U87-MG GBM xenograft mouse model, without any discernible loss of body weight or kidney toxicity noted. This study presents novel findings demonstrating that 1,8-Naphthalimide derivatives exhibited significant GBM cell suppression in vitro and in vivo without causing adverse effects on body weight or kidney function. Further experiments, including investigations into mechanisms and pathways, as well as preclinical studies on the pharmacokinetics and pharmacodynamics, may be instrumental to the development of a new anti-GBM compound.

Utilising a 1,8-naphthalimide probe for the ratiometric fluorescent visualisation of caspase-3

The development of selective and sensitive probes for monitoring caspase-3 activity-a critical enzyme involved in apoptosis-remains an area of significant interest in biomedical research. Herein, we report the synthesis and characterisation of a novel ratiometric fluorescent probe, Ac-DEVD-PABC-Naph, designed to detect caspase-3 activity. The probe utilises a 1,8-naphthalimide fluorophore covalently linked to a peptide sequence via a self-immolative p-aminobenzyl alcohol (PABA) linker. Upon enzymatic cleavage by caspase-3, the probe undergoes spontaneous degradation, releasing the free naphthalimide fluorophore, resulting in a ratiometric change in fluorescence emission. Spectroscopic studies revealed a time-dependent ratiometric fluorescent response, demonstrating the probe's ability to visualise caspase-3 activity with high sensitivity. Enzyme kinetics such as K m (Michaelis constant), k cat (turnover number), and LOD (Limit of Detection) were obtained, suggesting that the probe possesses comparable kinetic data to other probes in literature, but with the added benefits of ratiometric detection. Selectivity studies also demonstrated the probe's specificity for caspase-3 over other endogenous species and enzymes. Ac-DEVD-PABC-Naph may be a promising tool for the quantitative detection and fluorescent visualisation of caspase-3 activity in biological systems, with potential applications in apoptosis research and drug development.

Fluorescent Probes for Disease Diagnosis

The identification and detection of disease-related biomarkers is essential for early clinical diagnosis, evaluating disease progression, and for the development of therapeutics. Possessing the advantages of high sensitivity and selectivity, fluorescent probes have become effective tools for monitoring disease-related active molecules at the cellular level and in vivo. In this review, we describe current fluorescent probes designed for the detection and quantification of key bioactive molecules associated with common diseases, such as organ damage, inflammation, cancers, cardiovascular diseases, and brain disorders. We emphasize the strategies behind the design of fluorescent probes capable of disease biomarker detection and diagnosis and cover some aspects of combined diagnostic/therapeutic strategies based on regulating disease-related molecules. This review concludes with a discussion of the challenges and outlook for fluorescent probes, highlighting future avenues of research that should enable these probes to achieve accurate detection and identification of disease-related biomarkers for biomedical research and clinical applications.

A novel turn-on fluorescent probe for detection of pH in extremely acidic environment and its application

A water-soluble turn-on fluorescent probe PNAP for pH has been designed and synthesized. PNAP was consist of pyrene as fluorophore and morpholine as receptor. Owing to the photoinduced electron transfer (PET) effect, the fluorescence of PNAP was quenched, while PNAP exhibited a remarkable "turn-on" fluorescence with the increase of acidity. Notably for its pKa of 2.15, PNAP was one of the pH fluorescent probes used in extremely acidic environments. Furthermore, PNAP also displayed good repeatability, strong anti-ion interference ability, high sensitivity and selectivity toward pH. In addition, PNAP has been successfully applied to the test strips and monitor the pH of environment water samples and realistic samples, showing its good promising prospect.

Colorimetric and fluorescence dual-mode pH sensor based on nitrogen-doped carbon dots and its diverse applications

A dual-mode pH sensor based on nitrogen-doped carbon dots (N-CDs) with the source of o-phenylenediamine and tryptophan has been constructed. Under the stimulation of pH, the N-CDs exhibited prominent both color and fluorescence changes, leading to the rarely discovered colorimetric and fluorescent dual-readouts for the evaluation of pH. The mathematic relationship was established between pH and fluorescence intensity of N-CDs, and between pH and the UV-Vis absorbance ratio at 630 nm and 488 nm of N-CDs, respectively, over a quite broad pH range of 2.2 to 12.0. Multiple techniques are used to explore the dual-mode pH-responsive mechanism, and the preliminary explanation is put forward. The experimental results show that the N-CDs have visualized pH sensing applicability for actual samples, including various water samples and HeLa cell. Furthermore, the N-CD ink is developed for successful information encryption and anti-counterfeiting. This work might provide valuable insights into the sensing mechanism of CDs, and the application potential of CDs in broader fields.

Photoinduced electron transfer (PeT) based fluorescent probes for cellular imaging and disease therapy

Typical PeT-based fluorescent probes are multi-component systems where a fluorophore is connected to a recognition/activating group by an unconjugated linker. PeT-based fluorescent probes are powerful tools for cell imaging and disease diagnosis due to their low fluorescence background and significant fluorescence enhancement towards the target. This review provides research progress towards PeT-based fluorescent probes that target cell polarity, pH and biological species (reactive oxygen species, biothiols, biomacromolecules, etc.) over the last five years. In particular, we emphasise the molecular design strategies, mechanisms, and application of these probes. As such, this review aims to provide guidance and to enable researchers to develop new and improved PeT-based fluorescent probes, as well as promoting the use of PeT-based systems for sensing, imaging, and disease therapy.

4-Amino-1,8-naphthalimide-ferrocene conjugates as potential multi-targeted anticancer and fluorescent cellular imaging agents

Herein we present eight ferrocenyl 4-amino-1,8-naphthalimides. Designed as fluorescent logic gates for acidity and oxidisability, the molecules have been repurposed as anti-proliferation and cellular imaging agents. The compounds were studied in vitro against MCF-7 and K562 cancer cell lines by the MTT method. Compounds with protonable secondary amines tended to exhibit greater cytotoxicity than those with tertiary amines. Compounds with no measurable GI₅₀ values within a 24 hour time window, as well as at shorter exposure times, may be suitable as fluorescent cellular imaging probes.

Luminescence and Electrochemical Activity of New Unsymmetrical 3-Imino-1,8-naphthalimide Derivatives

A new series of 1,8-naphtalimides containing an imine bond at the 3-position of the naphthalene ring was synthesized using 1H, 13C NMR, FTIR, and elementary analysis. The impact of the substituent in the imine linkage on the selected properties and bioimaging of the synthesized compounds was studied. They showed a melting temperature in the range of 120-164 °C and underwent thermal decomposition above 280 °C. Based on cyclic and differential pulse voltammetry, the electrochemical behavior of 1,8-naphtalimide derivatives was evaluated. The electrochemical reduction and oxidation processes were observed. The compounds were characterized by a low energy band gap (below 2.60 eV). Their photoluminescence activities were investigated in solution considering the solvent effect, in the aggregated and thin film, and a mixture of poly(N-vinylcarbazole) (PVK) and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PBD) (50:50 wt.%). They demonstrated low emissions due to photoinduced electron transport (PET) occurring in the solution and aggregation, which caused photoluminescence quenching. Some of them exhibited light emission as thin films. They emitted light in the range of 495 to 535 nm, with photoluminescence quantum yield at 4%. Despite the significant overlapping of its absorption range with emission of the PVK:PBD, incomplete Förster energy transfer from the matrix to the luminophore was found. Moreover, its luminescence ability induced by external voltage was tested in the diode with guest-host configuration. The possibility of compound hydrolysis due to the presence of the imine bond was also discussed, which could be of importance in biological studies that evaluate 3-imino-1,8-naphatalimides as imaging tools and fluorescent materials for diagnostic applications and molecular bioimaging.

Fluorescent 4-amino-1,8-naphthalimide Tröger's bases (TBNaps) possessing (orthogonal) 'α-amino acids', esters and di-peptides and their solvent dependent photophysical properties

The synthesis of fifteen luminescent bis-naphthalimide based Tröger's bases (TBNaps) derived from 4-amino-1,8-naphthalimide (4-Amino-Nap) precursors is described; these scaffolds possess α-amino acids, esters or di-peptides conjugated at the imide site and show minor fluorescence in aqueous solution while being highly emissive in organic solvents. The investigation shows that these TBNaps possessing ICT excited state properties are capable of generating either positive or negative solvatochromic effects in response to changes in polarity and/or the hydrogen bonding capabilities of the medium.

Preparation and properties of a novel polystyrene solid-phase fluorescence pH sensor based on a naphthalimide derivative

A novel naphthalimide-based solid-phase fluorescence pH sensor (PS-Acyl-II) is prepared by immobilization of a small molecule probe (II) on polystyrene microspheres through an ester bond and is characterized by Fourier-transform infrared spectroscopy, optical microscope, scanning electron microscope, and conductiometric titrations. The sensor can determine the pH of a solution within the pH 4.0–7.0, free from interference of common metal ions, and can be reused several times. The geometries of II, PS-Acyl-II, and its product with H+ are optimized at the B3LYP/6-31G** level by density functional theory. The charge distribution, orbital interactions, and bonding characteristics are analyzed and compared in detail to discuss the recognition mechanism and structure–fluorescence property relationships.

Synthesis, structure-fluorescence relationships and density functional theory studies of novel naphthalimide-piperazine-pyridine-based polystyrene sensors for Hg(ii) detection

Two novel naphthalimide-piperazine-pyridine-based polystyrene solid-phase fluorescent sensors PS-NA and PS-ND with different lengths of the linker were synthesized and shown to be able to detect Hg(ii) ions. Their structures were characterized by Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) analysis. Fluorescence properties, including response time, pH effects, fluorescence titration, metal ion selectivity and regeneration, were investigated and compared. Sensor PS-NA displayed a higher fluorescence response to Hg(ii) than PS-ND, with a lower detection limit of 1.01 μM. The detection mechanism involving the Hg(ii) chelation-induced photo-induced electron transfer (PET) was proposed with the aid of density functional theory (DFT) calculations. Sensors PS-NA and PS-ND with seven other similar sensors from our previous studies were collected together for thorough structure-fluorescence relationship (SFR) studies. Sensor PS-NA being recyclable and environmentally friendly was successfully employed in the fluorescence detection of Hg(ii) in real water samples, indicating its good potential in practical application.

Fluorescent supramolecular hierarchical self-assemblies from glycosylated 4-amino- and 4-bromo-1,8-naphthalimides

The investigation into the self-assembly formation of the glycan based 4-amino- and 4-bromo-1,8-naphthalimide (Nap) structures1–3is presented.