One-step synthesis of rhodamine-based Fe3+ fluorescent probes via Mannich reaction and its application in living cell imaging

Small-Molecule Fluorescent Probes for Binding- and Activity-Based Sensing of Redox-Active Biological Metals

Although transition metals constitute less than 0.1% of the total mass within a human body, they have a substantial impact on fundamental biological processes across all kingdoms of life. Indeed, these nutrients play crucial roles in the physiological functions of enzymes, with the redox properties of many of these metals being essential to their activity. At the same time, imbalances in transition metal pools can be detrimental to health. Modern analytical techniques are helping to illuminate the workings of metal homeostasis at a molecular and atomic level, their spatial localization in real time, and the implications of metal dysregulation in disease pathogenesis. Fluorescence microscopy has proven to be one of the most promising non-invasive methods for studying metal pools in biological samples. The accuracy and sensitivity of bioimaging experiments are predominantly determined by the fluorescent metal-responsive sensor, highlighting the importance of rational probe design for such measurements. This review covers activity- and binding-based fluorescent metal sensors that have been applied to cellular studies. We focus on the essential redox-active metals: iron, copper, manganese, cobalt, chromium, and nickel. We aim to encourage further targeted efforts in developing innovative approaches to understanding the biological chemistry of redox-active metals.

A Phenalenone-based Fluorescent Probe for the Detection of Fe3+ ions

A phenalenone based "turn on" probe was developed for selective and sensitive detection of Fe³⁺ ions in aqueous solutions. The thiophene-2-carboxaldehyde (receptor unit) was integrated into the 6-amino-1-phenalenone (6-AP) (signal reporter unit) through the C = N bond formation. The probe, 6-APT, operated through subsequent hydrolysis of the C = N bond induced by the coordination of Fe³⁺ ions to the heteroatoms to form highly fluorescent 6-AP. The probe displayed remarkable characteristics such as rapid response time (< 1 min), high analyte selectivity, and low limit of detection (1.3 µM). The sensing approach offered an accurate method for the detection of Fe³⁺ ions in real water samples (tap water and drinking water). In addition to the fluorometric response, the presence of Fe³⁺ ions can be monitored under daylight by the change in the color of the solution. Importantly, this study is the first example of a phenalenone-based sensor developed for metal ion sensing in literature.

A Highly Sensitive and Selective Fluorescein-Based Cu2+ Probe and Its Bioimaging in Cell

Copper is a vital trace metal in human body, which plays the significant roles in amounts of physiological and pathological processes. The application of copper-selective probe has attracted great interests from environmental tests to life process research, yet a few of sensitive Cu²⁺ tests based on on-site analysis have been reported. In this paper, a novel fluorescein-based fluorescent probe N4 was designed, synthesized, and characterized, which exhibited high selectivity and sensitivity to Cu²⁺ comparing with other metal ions in ethanol–water (1/1, v/v) solution. The probe N4 bonded with Cu²⁺ to facilitate the ring-opening, and an obvious new band at 525 nm in the fluorescence spectroscopy appeared, which could be used for naked-eye detection of Cu²⁺ within a broad pH range of 6–9. Meanwhile, a good linearity between the fluorescence intensity and the concentrations of Cu²⁺ ranged 0.1–1.5 eq. was observed, and the limit of detection of N4 to Cu²⁺ was calculated to be as low as 1.20 μm. In addition, the interaction mode between N4 and Cu²⁺ was found to be 1:1 by the Job's plot and mass experiment. Biological experiments showed that the probe N4 exhibited low biological toxicity and could be applied for Cu²⁺ imaging in living cells. The significant color shift associated with the production of the N4-Cu²⁺ complex at low micromolar concentrations under UV light endows N4 with a promising probe for field testing of trace Cu²⁺ ions.

Facile Synthesis of Water-Soluble Rhodamine-Based Polymeric Chemosensors via Schiff Base Reaction for Fe3+ Detection and Living Cell Imaging

Quantitative and accurate determination of iron ions play a vital role in maintaining environment and human health, but very few polymeric chemosensors were available for the detection of Fe³⁺ in aqueous solutions. Herein, a water-soluble rhodamine-poly (ethylene glycol) conjugate (DRF-PEG), as a dual responsive colorimetric and fluorescent polymeric sensor for Fe³⁺ detection with high biocompatibility, was first synthesized through Schiff base reaction between rhodamine 6G hydrazide and benzaldehyde-functionalized polyethylene glycol. As expected, the introduction of PEG segment in DRF-PEG significantly improved the water solubility of rhodamine derivatives and resulted in a good biosensing performance. The detection limit of DRF-PEG for Fe³⁺ in pure water is 1.00 μM as a fluorescent sensor and 3.16 μM as a colorimetric sensor at pH 6.5. The specific sensing mechanism of DRF-PEG toward Fe³⁺ is proposed based on the intramolecular charge transfer (ICT) mechanism, in which the O and N atoms in rhodamine moiety, together with the benzene groups from benzaldehyde-modified PEG segment, participate in coordination with Fe³⁺. Furthermore, DRF-PEG was applied for the ratiometric imaging of Fe³⁺ in HeLa cells and showed the potential for quantitative determination of Fe³⁺ in fetal bovine serum samples. This work provides insights for the design of water-soluble chemosensors, which can be implemented in iron-related biological sensing and clinical diagnosis.

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.

Novel Fluorescent Probe toward Fe3+ Based on Rhodamine 6G Derivatives and Its Bioimaging in Adult Mice, Caenorhabditis elegans, and Plant Tissues

A new fluorescent probe LXY based on the rhodamine 6G platforms has been designed, synthesized, and characterized, which could recognize Fe³⁺ effectively in HEPES buffer (10 mM, pH = 7.4)/CH₃CN (2:3, v/v). And the distinct color change and the rapid emergence of fluorescence emission at 550 nm achieved "naked eye" detection of Fe³⁺. The interaction mode between them was achieved by Job's plot, MS, SEM, and X-ray single-crystal diffraction. Importantly, the crystal structures proved that Fe³⁺ could induce the rhodamine moiety transform the closed-cycle form to the open-cycle form. But it is interesting that Fe³⁺ did not appear in the crystal structures. Meanwhile, the limit of detection (LOD) of LXY to Fe³⁺ was calculated to be 3.47 × 10^-9. In addition, the RGB experiment, test papers, and silica gel plates all indicated that the probe LXY could be used to distinguish Fe³⁺ quantitatively and qualitatively on-site. Moreover, the probe LXY has also been successfully applied to Fe³⁺ image in Caenorhabditis elegans, adult mice, and plant tissues. Thus, LXY was considered to have some potential for application in bioimaging.

Sulfonate substituted rhodamine hydrophilic fluorescent probes: Application to specific detection of Fe3+ and imaging in living fish

Two Sulfonate substituted rhodamine hydrophilic fluorescent probes RbS1 and RbS2 were designed and synthesized for specific detection of Fe³⁺. It was found that the probe RbS2 was stronger than RbS1 in the water solubility test. Both of them displayed responses to Fe³⁺ with a apparent fluorescence enhancement at 585 nm, accompanied with a distinct fluorescence change to pink. Upon addition of Fe³⁺ ions (0-16 μM), the emission intensity of RbS1 and RbS2 increased to 40 and 70 fold, which exhibited a good linear relationship with the concentration of Fe³⁺. The detection limits of RbS1 and RbS2 for sensing Fe³⁺ were 0.64 μM and 0.56 μM, respectively. The binding ratios of the RbS1 and RbS2 to Fe³⁺ were 1:1 and the recycling ability for Fe³⁺ was reasonable. RbS1 and RbS2 have been successfully applied to the determination of Fe³⁺ in real water samples with satisfactory recovery and accuracy. In further living fish imaging test, the probe RbS2 was distributed into abdomen, which exhibited better fluorescence imaging ability than that of RbS1.

"Turn-on" fluorescent probes based on Rhodamine B/amino acid derivatives for detection of Fe3+ in water

Five kinds of Fe³⁺ fluorescent probes (RhB-Gly, RhB-Ala, RhB-Try, RhB-Cys, and RhB-His) were synthesized and characterized by NMR and mass spectrometry, based on the "OFF-ON" mechanism of Rhodamine B derivatives. The RhB-His based probe showed remarkable sensing performance toward the detection for Fe³⁺ and showed high selectivity for Fe³⁺ in the presence of other metal ions (such as Fe²⁺, Hg²⁺, Zn²⁺, Ba²⁺, Al³⁺, Co²⁺, Cd²⁺, K⁺, Na⁺, Mn²⁺, Pd²⁺, Pb²⁺, Ca²⁺, Ni²⁺, Cu²⁺, and Ag⁺), in PBS buffer solution (containing 2% of EtOH, pH 7.4, 1.0 mmol/L). The enhancement of the fluorescence was linearly proportional with the concentration Fe³⁺ (from 0 to 20 μmol/L), while the detection limit reached 0.88 μmol/L with a response time of 15 s. The RhB-His probe was successfully applied to investigate real samples and living cell imaging.

Sensitive and rapid detection of Cr3+ in live cells by a red turn-on fluorescent probe

A new probe RB-CR for the detection of Cr³⁺ was constructed based on the conjugation between rhodamine B and phenylthiourea. The Cr³⁺ could trigger opening of the rhodamine spirolactam of RB-CR upon complexation selectively, sensitively and rapidly, resulting in a pronounced enhancement of absorption and fluorescence signal. Further fluorescence imaging study has suggested that RB-CR could be applied as a probe for Cr³⁺ surveillance in living HepG2 cells with low cytotoxicity, which provide us the feasibility of exploring the Cr³⁺ recognition process by the turn-on fluorescence response.

Novel rhodamine probe for colorimetric and fluorescent detection of Fe3+ ions in aqueous media with cellular imaging

A novel rhodamine-pyridine conjugated spectroscopic probe RhP was synthesized and its X-ray single crystalline properties were revealed with tabulation. The RhP displayed a distinct pale-pink colorimetric and "turn-on" fluorescent response to Fe³⁺ in aqueous media [H₂O:DMSO (95:5, v/v)] than that of other interfering ions. During the Fe³⁺ recognition, the absorption (UV-Vis) and photoluminescence (PL) spectral studies revealed new peaks at 561 and 592 nm, respectively. The 1:1 stoichiometry and binding sites were verified by Job's plot, ESI-mass, and ¹H NMR titrations. Subsequently, LOD and binding constant for RhP + Fe³⁺ complex were estimated as 102.3 nM and 6.265 × 10⁴ M^-1 from linear fitting and Benesi-Hildebrand plots, correspondingly. Sensor reversibility of RhP + Fe³⁺ by EDTA was demonstrated by UV/PL and TRPL investigations. Moreover, the photoinduced energy transfer mechanism and band gap changes were established from the DFT interrogations. Lastly, cellular imaging studies were carried out to authenticate the real applicability of RhP in Fe³⁺ detection.

A rhodamine-based dual chemosensor for the naked-eye detection of Hg2+ and enhancement of the fluorescence emission for Fe3

A novel fluorescent chemosensor based on trimesoyl chloride-rhodamine (TR) was successfully synthesized. Rising chromogenic and fluorogenic spectral enhancements could be observed in trimesoyl chloride-rhodamine (TR) probes when Hg²⁺ and Fe³⁺ were added, respectively. TR has shown selectivity for Hg²⁺ and Fe³⁺ with high sensitivity due to metal ion complexation induced photophysical "turn-on" signaling responses. The detection limit towards Hg²⁺ was 2.46 × 10^-8 M as determined by the 3σ method. At the same time, fluorogenic spectral enhancements were observed in TR, which exhibits a superior sensitive and selective recognition towards Fe³⁺ with 4.11 × 10^-8 M of the detection limit. The test strips were used for colorimetric and simple detection towards Hg²⁺, which might finally enable the advancement of the Hg²⁺ sensor in the field of on-site detection.

A Coumarin-Based Fluorescent Probe for Ratiometric Detection of Cu2+ and Its Application in Bioimaging

The fluorescent probe L, based on naphthalimide-modified coumarin, was designed, synthesized, and characterized, which could recognize Cu2+ from other cations selectively and sensitively in HEPES buffer (10 mM, Ph = 7. 4)/CH3CN (1:4, V/V). When the probe L interacted with Cu2+, the color and the fluorescent intensity changed obviously and it provided the naked-eye detection for Cu2+. The recognition mode between them was achieved by Job's plot, IR, MS, SEM, and 1HNMR. In addition, test strips made from L could still interact with Cu2+ in tap water effectively. The limit of detection (LOD) of L was 3.5 × 10-6 M. Additionally, the density functional theory (DFT) calculation method was used to analyze the action mechanism of L toward Cu2+. Importantly, the fluorescent probe L could demonstrate favorable selectivity toward Cu2+ in Caenorhabditis elegans. Thus, L was considered to have some potential for application in bioimaging.

Ester and amide derivatives of rhodamine B exert cytotoxic effects on different human tumor cell lines

Three esters of rhodamine B (1–3) differing in their alkyl chain lengths as well as several rhodamine B amides (4–9) were synthesized in good yields and tested for their cytotoxicity in SRB assays employing several human tumor cell lines. The rhodamine B esters were unselective but showed cytotoxicity of as low as EC50 = 0.15 ± 0.02 µM. The rhodamine B amides were slightly less cytotoxic but showed good selectivity against MCF-7 and A2780 tumor cell lines. Especially a morpholinyl derivative 4 was ~20 time more cytotoxic for MCF-7 than for nonmalignant NIH 3T3 cells.