Recent progress and outlooks in rhodamine-based fluorescent probes for detection and imaging of reactive oxygen, nitrogen, and sulfur species

Reactive oxygen species (ROS), reactive nitrogen species (RNS), and reactive sulfur species (RSS) serve as vital mediators essential for preserving intracellular redox homeostasis within the human body, thereby possessing significant implications across physiological and pathological domains. Nevertheless, deviations from normal levels of ROS, RNS, and RSS disturb redox homeostasis, leading to detrimental consequences that compromise bodily integrity. This disruption is closely linked to the onset of various human diseases, thereby posing a substantial threat to human health and survival. Small-molecule fluorescent probes exhibit considerable potential as analytical instruments for the monitoring of ROS, RNS, and RSS due to their exceptional sensitivity and selectivity, operational simplicity, non-invasiveness, localization capabilities, and ability to facilitate in situ optical signal generation for real-time dynamic analyte monitoring. Due to their distinctive transition from their spirocyclic form (non-fluorescent) to their ring-opened form (fluorescent), along with their exceptional light stability, broad wavelength range, high fluorescence quantum yield, and high extinction coefficient, rhodamine fluorophores have been extensively employed in the development of fluorescent probes. This review primarily concentrates on the investigation of fluorescent probes utilizing rhodamine dyes for ROS, RNS, and RSS detection from the perspective of different response groups since 2016. The scope of this review encompasses the design of probe structures, elucidation of response mechanisms, and exploration of biological applications.

Targeted theranostics: Near-infrared triterpenoic acid-rhodamine conjugates as prerequisites for precise cancer diagnosis and therapy

Pentacyclic triterpenoic acids have shown excellent potential as starting materials for the synthesis of highly cytotoxic agents with significantly reduced toxicity for non-malignant cells. This study focuses on the development of triterpenoic acid-rhodamine conjugates with fluorescence shifted to the near-infrared (NIR) region for theranostic applications in cancer research. Spectral analysis revealed emission wavelengths around λ = 760 nm, enabling stronger signals and deeper tissue penetration. The conjugates were evaluated using SRB assays on tumor cell lines and non-malignant fibroblasts, demonstrating low nanomolar activity and high selectivity, similarly to their known rhodamine B counterparts. Additional staining experiments proved their mode of action as mitocans.

Near-Infrared Probes for Biothiols (Cysteine, Homocysteine, and Glutathione): A Comprehensive Review

Biothiols (cysteine, homocysteine, and glutathione) are an important class of compounds with a free thiol group. These biothiols plays an important role in several metabolic processes in living bodies when present in optimum concentration. Researchers have developed several probes for the detection and quantification of biothiols that can absorb in UV, visible, and near-infrared (NIR) regions of the electromagnetic spectrum. Among them, NIR organic probes have attracted significant attention due to their application in in vivo and in vitro imaging. In this review, we have summarized probes for these biothiols, which could work in the NIR region, and discussed their sensing mechanism and potential applications. Along with focusing on the pros and cons of the reported probes we have classified them according to the fluorophore used and summarized their photophysical and sensing properties (emission, response time, limit of detection).

Quantitative Hg2+ detection via forming three coordination complexes using a lysosome targeting quinoline - Fisher aldehyde fluorophore

This work describes (Z)-N-((Z)-2-(1,3,3-trimethylindolin-2ylidene)ethylidene)quinoline-8-amine (LYSO-QF), a high-performing and biocompatible dye comprised of quinoline and Fisher aldehyde moieties linked via an imine vinyl backbone with lysosome targeting ability that can be used to quantitatively detect the mercury ion (Hg²⁺) in biosystems and the natural environment. This is achieved by forming three different tetrameric, trimeric and dimeric complexes between Hg²⁺ and LYSO-QF with the limit of detection (LOD) of 11 nm. The complexes formed were analyzed with the aid of time-dependent density functional theory (TD-DFT) calculations. The concentration dependence of the Hg²⁺ complex fluorescence emission changes from grey-green to jade green and then to red as the different types of complex are formed. The favorable sensor properties of the LYSO-QF probe are demonstrated by monitoring different Hg²⁺ concentrations in buffer solutions, HeLa cells, zebrafish model samples and several different types of water sample. Experiments with Whatman paper strips demonstrate that the cost-effective LYSO-QF also has considerable potential for use in on-site Hg²⁺ detection with the naked eye.

Recent Progress in Small Spirocyclic, Xanthene-Based Fluorescent Probes

The use of fluorescent probes in a multitude of applications is still an expanding field. This review covers the recent progress made in small molecular, spirocyclic xanthene-based probes containing different heteroatoms (e.g., oxygen, silicon, carbon) in position 10'. After a short introduction, we will focus on applications like the interaction of probes with enzymes and targeted labeling of organelles and proteins, detection of small molecules, as well as their use in therapeutics or diagnostics and super-resolution microscopy. Furthermore, the last part will summarize recent advances in the synthesis and understanding of their structure-behavior relationship including novel computational approaches.

A Copper (II) Ensemble-Based Fluorescence Chemosensor and Its Application in the 'Naked-Eye' Detection of Biothiols in Human Urine

Quick and effective detection of biothiols in biological fluids has gained increasing attention due to its vital biological functions. In this paper, a novel reversible fluorescence chemosensor (L-Cu2+) based on a benzocoumarin-Cu2+ ensemble has been developed for the detection of biothiols (Cys, Hcy and GSH) in human urine. The chemosensing ensemble (L-Cu2+) contains a 2:1 stoichiometry structure between fluorescent ligand L and paramagnetic Cu2+. L was found to exclusively bond with Cu2+ ions accompanied with a dramatic fluorescence quenching maximum at 443 nm and an increase of an absorbance band centered at 378 nm. Then, the in situ generated fluorescence sluggish ensemble, L-Cu2+, was successfully used as a chemosensor for the detection of biothiols with a fluorescence "OFF-ON" response modality. Upon the addition of biothiols, the decomplexation of L-Cu2+ led to the liberation of the fluorescent ligand, L, resulting in the recovery of fluorescence and absorbance spectra. Studies revealed that L-Cu2+ possesses simple synthesis, excellent stability, high sensitivity, reliability at a broad pH range and desired renewability (at least 5 times). The practical application of L-Cu2+ was then demonstrated by the detection of biothiols in human urine sample.

A novel strategy for rhodamine B-based fluorescent probes with a selective glutathione response for bioimaging in living cells

The aim of this study was to overcome the reported shortcomings of the glutathione (GSH) detection of rhodamine-based fluorescent probes, such as poor selectivity to thiol groups and reversible unstable covalent binding with the thiol groups.

A water-soluble dual-site fluorescent probe for the rapid detection of cysteine with high sensitivity and specificity

A water-soluble turn-on fluorescent probe has been rationally designed and synthesized to distinguish Cys from Hcy and GSH in less than five seconds. It has high sensitivity and strong anti-interference ability to other amino acids and ions. Its ultra-rapid detection of Cys in aqueous systems could be attributed to dual response sites imparted by the probe.

A highly sensitive two-photon fluorescent probe for glutathione with near-infrared emission at 719 nm and intracellular glutathione imaging

A near-infrared turn-on two-photon fluorescent probe ST-BODIPY for glutathione-specific detection was designed and synthesized by attaching triphenylamine to BODIPY skeleton through the Knoevenagel condensation to prolong the maximum emission wavelength to the NIR region. And 2,4-dinitrobenzenesulfonyl group (DNBS), as the fluorescence quencher and thiol recognition moiety, was modified in 8 position of BODIPY. In the presence of GSH, the probe afforded an "off-on" signal response with a significant NIR fluorescence enhancement centered at 719 nm accompanying by quantum yield increased to 0.44, which was ascribed to the glutathione-induced S_NAr (aromatic substitution) reaction. Surprisingly, we found that the probe could discriminate GSH from other biothiols including Cys and Hcy upon the addition of intracellular concentrations of them. Time-dependence also demonstrated that the probe could distinguish GSH from Cys and Hcy under physiological environment. The limit of detection (LOD) for GSH was calculated as 25.46 nM from the titration experiments, which is lower than most previously reported GSH-selective probes. Under the Ti:sapphire pulsed laser's 800 nm irradiation, ST-BODIPY toward GSH generated an "off-on" signal response with a significant enhancement of fluorescence emission at 719 nm after treatment with GSH. Besides, the 2PA cross section value (σ₂) was calculated to be 410 GM, suggesting that it could not only function well as an excellent two-photon fluorescent probe for the detection of intracellular GSH, but also be applied for two-photon imaging with high sensitivity in living cells. Moreover, ST-BODIPY probe has been successfully employed for monitoring exogenous and endogenous GSH in MCF-7 cells with satisfying results, perhaps it was feasible for detecting abnormal contents of GSH in a biological system and accomplishing the goal of maintaining normal human activities.

A rhodol-hemicyanine based ratiometric fluorescent probe for real-time monitoring of glutathione dynamics in living cells

A new rhodol-hemicyanine based ratiometric and reversible fluorescent probe has been developed for real-time monitoring of glutathione dynamics in living cells.

A near-infrared fluorescent probe for the discrimination of cysteine in pure aqueous solution and imaging of cysteine in hepatocellular carcinoma cells with facile cell-compatible ability

A NIR fluorescence sensor for selectively detecting cysteine in aqueous solution with fast response and long emission wavelength was synthesized.

Synthesis of a BODIPY disulfonate near-infrared fluorescence-enhanced probe with high selectivity to endogenous glutathione and two-photon fluorescent turn-on through thiol-induced SNAr substitution

A BODIPY disulfonate BODIPY-diONs with two-photon fluorescent turn-on effect was developed as fluorescence probe for selective detection of glutathione over cysteine and homocysteine. BODIPY-diONs is weakly fluorescent due to the 2,4-dinitrobenzenesulfonyl quencher group. When GSH was added, a S_NAr substitution reaction was triggered. The red emission of the BODIPY fluorophore at 675 nm was switched on, with a 27-fold emission enhancement in fluorescence intensity. The color of the solution changed from blue to green together with fluorescence appeared within 5 s. The absorbance and emission maxima of the probe BODIPY-diONs were achieved at 650 nm and 675 nm, respectively (quantum yield: 0.11). Interestingly, under the sapphire pulsed laser's 800 nm irradiation, in presence of GSH, the two-photon excited fluorescence (TPEF) of probe BODIPY-diONs was turned on, affording an OFF-ON response signal and a strong emission band at 682 nm. Furthermore, for detection of GSH, the chemodosimeter BODIPY-diONs exhibits high sensitivity and excellent anti-interference with low detection limit of 0.17 μM, and it works effectively within a wide pH range. Furthermore, the imaging studies proved that the probe BODIPY-diONs is suitable for the detection of GSH in complete physiological media.

A NIR facile, cell-compatible fluorescent sensor for glutathione based on Michael addition induced cascade spirolactam opening and its application in hepatocellular carcinoma

A NIR fluorescence probe with NIR emission wavelength at 746 nm and high quantum yield of 0.36 was designed and synthesized to selectively detect GSH over Hcy and Cys in living systems.

Mercaptopyrimidine-directed gold nanoclusters: a suitable fluorescent probe for intracellular glutathione imaging and selective cancer cell identification

Herein, we demonstrate a novel, facile, and suitable strategy for imaging GSH based on mercaptopyrimidine-directed gold nanoclusters (Au NCs).

A near-infrared xanthene fluorescence probe for monitoring peroxynitrite in living cells and mouse inflammation model

A novel near-infrared xanthene fluorescence probe for monitoring peroxynitrite in vitro and in vivo.