Fluorescent N-oxides: applications in bioimaging and sensing

N-Oxides, due to their zwitterionic nature and ability to form hydrogen bonds through the oxide ion, are highly water-soluble and widely used in biological and pharmacological studies. The N-oxide structural scaffold is introduced into molecules, enabling "turn-on" fluorescence via an intramolecular charge transfer (ICT) process. This process occurs when the N-O bond is cleaved, either through an enzymatic reaction under hypoxic conditions or by using Fe(II), which allows rapid and selective detection of Fe(II) at nanomolar concentrations both in vitro and in vivo. This review focuses on the literature published between 2010 and 2024, particularly emphasising N-oxide fluorophores and their applications in hypoxic cell lines, Fe(II) detection, and bioimaging.

Detecting labile heme and ferroptosis through 'turn-on' fluorescence and lipid droplet localization post Fe2+ sensing

Iron, a crucial biologically active ion essential for metabolic processes in living organisms, plays a vital role in biological functions, and imbalances in iron levels can lead to various diseases. In this study, we have developed two simple "turn-on" fluorescent probes, NOPy and NOCN, for the quick and selective detection of Fe2+ at nanomolar levels (LOD of 35 nM), accompanied by significant absorption and emission shifts, along with colorimetric demarcation. Both fluorophores exhibit an excellent "turn-on" emission response upon encountering Fe2+ in the cells. Flow cytometry and confocal fluorescence imaging studies demonstrate enhanced fluorescence signals in response to labile iron, efficiently detecting heme during erastin-induced ferroptosis. Interestingly, we also observed that the product formed after Fe2+ sensing localizes within the lipid droplets. These water-soluble and highly sensitive reactive probes, NOPy and NOCN, enable investigations of iron-dependent physiological and pathological conditions. The development of these probes represents an advancement in the field, offering a rapid and selective means for detecting Fe2+ with minimal cytotoxicity.

Anion-assisted supramolecular polymerization of luminescent organic π-conjugated chromophores in a moderately polar solvent: tunable nanostructures and their corresponding effects on electronic properties

Supramolecular polymers of π-conjugated organic chromophores have emerged as promising candidates in organic electronics because of their dynamic and highly ordered molecular organization. Herein, we demonstrate the formation of luminescent, highly conducting supramolecular polymers of a functionalized naphthalimide π-chromophore-based organic semiconductor in a moderately polar organic solvent (tetrahydrofuran) by overcoming solute-solvent H-bonding via assistance from fluoride anions. The polymerization is exclusively guided by the synergistic effects of cascade H-bonding (F-⋯H-N- of primary amines, followed by -CO⋯H-N- of amides), π-π stacking and hydrophobic interactions. An increasing molar equivalent of anions leads to a morphology transition from 1D nanowires to 2D nanosheets via nanotubes and nanorings, but above a particular threshold of the same anion, depolymerization-mediated disruption of long-range order and formation of non-luminescent spherical particles was observed. Such significant impacts of anions in supramolecular polymerization-depolymerization were utilized in modulating the electronic properties of this naphthalimide-based organic semiconductor.

Monitoring of Fe (II) ions in living cells using a novel quinoline-derived fluorescent probe

Physiologically, Fe(III) and Fe(II) is the most important redox pairs in a variety of biological and environmental procedures with its capability of transition. The detection of physiological iron, especially Fe(II), has become the recent research focus of investigations on revealing the mechanism of iron-related metabolism. In this work, we exploited a novel quinoline-derived fluorescent probe, YTP, for the detection of Fe(II). It could monitor the level of Fe(II) with a linear range of 0-2.0 equivalent and the detection limit of 0.16 µM. High selectivity from other analytes including Fe(III) and steadiness for over 24 h confirmed the practicability of YTP. YTP was further applied in real buffer systems and in cellular imaging. The probe could achieve the semi-quantitative monitoring of Fe(II) in living cells. This work provided a potential implement for the detection of Fe(II), and raised important information for further researches on the redox pairs of iron, in mechanism and in practice.

Accelerated chromate reduction by tea waste: Comparison of chromate reduction properties between water and ice systems

The concentration of chromium (Cr) in natural water and soil environments has gradually increased in recent decades, owing to intensive use of Cr in industry and its subsequent disposal. In this study, we performed a comparison study on chromate (Cr⁶⁺) reduction by tea waste (green tea, black tea, red tea, and chamomile) in water (25 ^°C) and ice (-20 ^°C) to develop a new strategy for environmental-friendly stabilization of hazardous Cr⁶⁺ by freezing. This study shows that the freezing process can enhance the reduction of Cr⁶⁺ by tea waste. The residual Cr⁶⁺ concentration ratios (C/C₀, where C is the concentration of Cr⁶⁺ after the reaction (5 h) and C₀ is the initial concentration of Cr⁶⁺ (20 μM) in the system) by tea wastes in water were in the range of 0.71 (green tea) to 0.92 (chamomile); however, the ratios dramatically decreased under the freezing process (i.e., 0.06 by green tea, 0.13 by black tea, 0.18 by red tea, and 0.08 by chamomile). According to the results obtained from the fluorescent, chromatographic, and spectroscopic analyses, under the freezing process, the enhanced reduction of Cr⁶⁺ could be explained by the freeze concentration of Cr⁶⁺, phenolic components in tea extracts, and protons in small liquid pockets in liquid-like layers (LLLs). In addition, the proposed system can efficiently purify the real Cr⁶⁺-containing wastewater (i.e., electroplating wastewater), indicating that the system will be economically feasible in cold regions (i.e., polar regions).

Visualization of penicillin G acylase in bacteria and high-throughput screening of natural inhibitors using a ratiometric fluorescent probe

A ratiometric fluorescent probe (PNA) was developed to sense and image endogenous bacterial penicillin G acylase (PGA). Oleanolic acid was discovered as a potential natural inhibitor of PGA using high-throughput screening techniques based on PNA.