Schiff Bases: A Versatile Fluorescence Probe in Sensing Cations

Metal cations such as Zn²⁺, Al³⁺, Hg²⁺, Cd²⁺, Sn²⁺, Fe²⁺, Fe³⁺ and Cu²⁺ play important roles in biology, medicine, and the environment. However, when these are not maintained in proper concentration, they can be lethal to life. Therefore, selective sensing of metal cations is of great importance in understanding various metabolic processes, disease diagnosis, checking the purity of environmental samples, and detecting toxic analytes. Schiff base probes have been largely used in designing fluorescent sensors for sensing metal ions because of their easy processing, availability, fast response time, and low detection limit. Herein, an in-depth report on metal ions recognition by some Schiff base fluorescent sensors, their sensing mechanism, their practical applicability in cell imaging, building logic gates, and analysis of real-life samples has been presented. The metal ions having biological, industrial, and environmental significance are targeted. The compiled information is expected to prove beneficial in designing and synthesis of the related Schiff base fluorescent sensors.

Selenium-Based Fluorescence Probes for the Detection of Bioactive Molecules

Chemistry of organoselenium reagents have now become an important tool of synthetic organic and medicinal chemistry. These reagents activate the olefinic double bonds and used to archive the number of organic transformations under mild reaction conditions. A number of organoselenium compounds have been identified as potent oxidants. Recently, various organoselenium species have been employed as chemical sensors for detecting toxic metals. Moreover, a number of selenium-based fluorescent probes have been developed for detecting harmful peroxides and ROS. In this review article, the synthesis of selenium-based fluorescent probes will be covered including their application in the detection of toxic metals and harmful peroxides including ROS.

The development of an endoplasmic reticulum-targeting fluorescent probe for the imaging of 1,4-dithiothreitol (DTT) in living cells

1,4-Dithiothreitol (DTT) is a robust reducing agent that contributes significantly to the folding process of proteins and maintaining endoplasmic reticulum (ER) homeostasis. Abnormally high levels of DTT can lead to severe endoplasmic reticulum stress (ERS), which induces cell death. In addition, DTT can also hinder cell growth and enhance reactive oxygen species (ROS) production in the ER. Herein, an effective turn-on ER-targeting fluorescent probe, ER-DTT, was designed to image DTT for the first time. The probe ER-DTT was based upon naphthalimide as a fluorophore, p-toluenesulfonamide as an exceptional unit for ER-targeting, and sulfoxide as a response site for imaging DTT based on an intramolecular charge transfer (ICT) mechanism. Optical-response experiments showed that the probe ER-DTT had good selectivity and sensitivity for DTT. Furthermore, confocal microscopy indicated that ER-DTT was suitable for selectively targeting ER in living cells and could be implemented to recognize cellular DTT.

Two-photon imaging of 1,4-dithiothreitol (DTT) by a red-emissive fluorescent probe in living cells, tissues and animals

1,4-Dithiothreitol (DTT) is an important small-molecular reducing agent and has extensive applications in biochemistry, peptide/protein chemistry and clinical medicine. The development of effective methods for monitoring DTT is of great importance for its safe use and studying its toxicity to human. In this work, we present a two-photon red-emissive probe for the imaging of DTT in living cells, tissues and animals. The probe employed a two-photon red-emissive xanthene dye as the fluorophore and selected 2,4-dinitrophenylate as the novel recognition site for DTT. In response to DTT, the probe displayed excellent sensitivity and selectivity. The probe was successfully applied to the two-photon imaging of DTT in living cells, and the imaging of DTT in living tissues and animals.

Theoretical design and investigation of 1,8-naphthalimide-based two-photon fluorescent probes for detecting cytochrome P450 1A with separated fluorescence signal

Two-photon fluorescent probe for detecting CYP1A enzyme with separated fluorescence signal.

A highly selective ratiometric fluorescent probe for the sensitive detection of hypochlorous acid and its bioimaging applications

A rapid-response, highly selective and ratiometric fluorescent probe was synthesized to sensitively detect HOCl in living cells.

Self-immolative spacers: kinetic aspects, structure-property relationships, and applications

Self-immolative spacers are covalent assemblies tailored to correlate the cleavage of two chemical bonds after activation of a protective part in a precursor: Upon stimulation, the protective moiety is removed, which generates a cascade of disassembling reactions leading to the temporally sequential release of smaller molecules. Originally introduced to overcome limitations for drug delivery, self-immolative spacers have gained wide interest in medicinal chemistry, analytical chemistry, and material science. For most applications, the kinetics of the disassembly of the activated self-immolative spacer governs functional properties. This Review addresses kinetic aspects of self-immolation. It provides information for selecting a particular self-immolative motif for a specific demand. Moreover, it should help researchers design kinetic experiments and fully exploit the rich perspectives of self-immolative spacers.

Visible-light-induced cleavage of 4-α-amino acid substituted naphthalimides and its application in DNA photocleavage

4-α-Amino acid substituted naphthalimides can be photocleaved at the C–N bond between the 4-amino and the amino acid residue under visible light irradiation, releasing a fluorophore, 4-aminonaphthalimide.

Amplified release through the stimulus triggered degradation of self-immolative oligomers, dendrimers, and linear polymers

In recent years, numerous delivery systems based on polymers, dendrimers, and nano-scale assemblies have been developed to improve the properties of drug molecules. In general, for the drug molecules to be active, they must be released from these delivery systems, ideally in a selective manner at the therapeutic target. As the changes in physiological conditions are relatively subtle from one tissue to another and the concentrations of specific enzymes are often quite low, a release strategy involving the amplification of a biological signal is particularly attractive. This article describes the development of oligomers, dendrimers, and linear polymers based on self-immolative spacers. This new class of molecules is designed to undergo a cascade of intramolecular reactions in response to the cleavage of a trigger moiety, resulting in molecular fragmentation and the release of multiple reporter or drug molecules. Progress in the development of these materials as drug delivery vehicles and sensors will be highlighted.

A new N-imidazolyl-1,8-naphthalimide based fluorescence sensor for fluoride detection

A chemosensor is reported with high sensitivity and selectivity for detection of fluoride anion. The recognition mechanism is attributed to a fluoride-triggered disruption of the hydrogen bond between imidazole and naphthalimide moieties, resulting in a noncoplanar geometry with low fluorescence.

A disulfide-linked naphthalimide dimer for Hg(II) detection in aqueous solution

A disulfide linked naphthalimide dimer probe was designed for mercury ion (Hg(2+)) recognition in this work. The recognition was based on the strong affinity of mercury for sulfur. The experimental results revealed that the probe exhibited high selectivity and sensitivity toward Hg(2+) in comparison to other metal ions via a turn-on and reversible response to Hg(2+) in neutral buffer solution. More importantly, the probe demonstrated a linear response for Hg(2+) over a concentration range from 0 to 150 μM with a detection limit of 0.38 μM, which is just the limit of the safe concentration for humans. Upon addition of 150 μM Hg(2+), the enhancement of fluorescence reached a maximum (∼7-fold). The performances of the probe indicated that it could meet the selectivity requirements for biomedical and environmental application and also was sensitive enough to detect Hg(2+) in environmental and biological samples.

Design, synthesis, and cyclization of 4-aminobutyric acid derivatives: potential candidates as self-immolative spacers

Self-immolative spacers have gained significant interest in recent years due to their utility in numerous prodrug, sensor and drug delivery systems. However, there are a very limited number of spacers that are capable of undergoing spontaneous and rapid reactions under mild conditions. To address this need, 4-aminobutyric acid derivatives were explored as a potential class of self-immolative spacers. Using a modular approach, eleven N- and α-substituted derivatives of 4-aminobutyric acid were synthesized, and their intramolecular cyclizations to γ-lactams were studied. Kinetics experiments were carried out at physiological pH and temperature, and the observed half-lives for the spacers ranged from 2 to 39 s, depending on the molecular structure. In addition, the pH dependence of the cyclization rate was also explored and it was found that cyclization still occurred rapidly at mildly acidic pH. Therefore, this class of compounds exhibits promise for incorporation into a variety of self-immolative systems where rapid cyclization reactions are desired.