A Solid‐State‐Emissive 1,8‐Naphthalimide Probe Based on Photoinduced Electron Transfer and Aggregation‐Induced Emission

Fluorescent Probes as a Tool in Diagnostic and Drug Delivery Systems

Over the last few years, the development of fluorescent probes has received considerable attention. Fluorescence signaling allows noninvasive and harmless real-time imaging with great spectral resolution in living objects, which is extremely useful for modern biomedical applications. This review presents the basic photophysical principles and strategies for the rational design of fluorescent probes as visualization agents in medical diagnosis and drug delivery systems. Common photophysical phenomena, such as Intramolecular Charge Transfer (ICT), Twisted Intramolecular Charge Transfer (TICT), Photoinduced Electron Transfer (PET), Excited-State Intramolecular Proton Transfer (ESIPT), Fluorescent Resonance Energy Transfer (FRET), and Aggregation-Induced Emission (AIE), are described as platforms for fluorescence sensing and imaging in vivo and in vitro. The presented examples are focused on the visualization of pH, biologically important cations and anions, reactive oxygen species (ROS), viscosity, biomolecules, and enzymes that find application for diagnostic purposes. The general strategies regarding fluorescence probes as molecular logic devices and fluorescence–drug conjugates for theranostic and drug delivery systems are discussed. This work could be of help for researchers working in the field of fluorescence sensing compounds, molecular logic gates, and drug delivery.

A Highly Water-Soluble and Solid State Emissive 1,8-Naphthalimide as a Fluorescent PET Probe for Determination of pHs, Acid/Base Vapors, and Water Content in Organic Solvents

A new highly water-soluble 1,8-naphthalimide fluorophore designed on the “fluorophore-spacer-receptor1-receptor2” model has been synthesized. Due to the unusually high solubility in water, the novel compound proved to be a selective PET-based probe for the determination of pHs in aqueous solutions and rapid detection of water content in organic solvents. Based on the pH dependence of the probe and its high water solubility, the INH logic gate was achieved using NaOH and water as chemical inputs, where NaOH is the disabler and the water is an enabler. In addition, the probe showed effective fluorescence “off-on” reversibility on glass support after exposure to acid and base vapors, which defines it as a promising platform for rapid detection of acid/base vapors in the solid-state, thus extending the molecular sensing concept from solution to the solid support.

Enhanced photostability of aggregation induced emission by hydrophobic groups

Since the aggregation induced emission (AIE) phenomenon was reported, many research groups have used this unique AIE effect to develop chemo- or bio-sensors for detecting ions, gases, explosives, proteins, and enzymes. Most of these sensors work in their aggregate state, therefore, fluorescence stability has become one of the important problem, and unfortunately, as far as we know, there is no paper to discuss what factors can improve the fluorescence stability of AIE compounds in the aggregate state, if the fluorescence stability of the sensors are poor, there will seriously affect the detection result. In this article, we found that compounds with hydrophobic long alkyl substituents can maintain stable fluorescence intensity for a long time in the aggregate state. In addition, cyclohexane is introduced as a hydrophobic substituent. Therefore, the fluorescence stability of the aggregates also increased by 90% within 1800s. The aggregation solutions of CB-3 and CSB-2 were left for two weeks, and no significant changes were found in the fluorescence intensity. Molecular dynamics simulation (MDS) shows that the presence of hydrophobic substituents in compounds cause the molecules to be closely interspersed with each other, hence, making it difficult to change the optical properties, microstructure and stacking mode of the AIE aggregates by external stimulations. The introduction of hydrophobic substituents improves the fluorescence stability of AIE compounds, and makes the AIE phenomenon more valuable in the fields of biological/chemical sensing and imaging.

A chemosensoring molecular lab for various analytes and its ability to execute a molecular logical digital comparator

We reported here the unique ability of a Rhodamine 6G-based probe (3) to detect discriminately several targets, including H⁺, HO^-, Cu²⁺, Hg²⁺, Fe³⁺, Co²⁺, Cd²⁺, Zn²⁺, Sn²⁺, Ni²⁺, Al³⁺, Pb²⁺, Ce³⁺and Ag⁺, by unambiguously colorimetric and fluorimetric outcomes. In aqueous solutions, the presence of proton induced the ring-opening of rhodamine moiety but the presence of hydroxide induced the conversion of 2-hydroxyphenyl hydrazone moiety from the non-fluorescent benzenoid form into the fluorescent quinoid form. The probe could to distinguish between different cations in DMF and to work like an artificial tongue at molecular level. Several logic gates including OR, INHIBIT and TRANSFER, were performed by the probe. Moreover, the probe is able to execute three INHIBIT logic gates by two inputs, which was exploited to execute a digital molecular comparator.

Aminonaphthalimide hybrids of mitoxantrone and amonafide as anticancer and fluorescent cellular imaging agents

Novel water-soluble 4-aminonaphthalimides were synthesised and their cellular fluorescent imaging, cytotoxicity and ability to induced apoptosis evaluated. The lead compound 1 was designed from the cross-fertilisation of the basic hydrophilic amino pharmacophore of mitoxantrone, and an aminonaphthalimide scaffold of the drug candidate, amonafide. The compounds are also fluorescent pH probes based on photoinduced electron transfer (PET) and internal charge transfer (ICT). The compounds are sensitive to solvent polarity with large Stoke shifts (>90 nm) and provide emissive-coloured solutions (blue to yellow). Excited state pK_as of 9.0-9.3 and fluorescence quantum yields of 0.47-0.58 were determined in water. The cytotoxicity and cellular fluorescent imaging properties of the compounds were tested on human cancer cell lines K562 and MCF-7 by the MTT assay, phase contrast and fluorescence microscopy. Compounds 1 and 3 with flexible aminoalkyl chains exhibited GI₅₀ comparable to amonafide, while 2 and 4 with a rigid piperazine moiety and butyl chain are less cytotoxic. Fluorescence microscopy with 1 allowed for the visualization of the intracellular microenvironment exemplifying the potential utility of such hybrid molecules as anticancer and fluorescent cellular imaging agents.