Rhodamine-Based Cu2+ -Selective Fluorosensor: Synthesis, Mechanism, and Application in Living Cells

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.

Solvent-directed multiple correspondence fluorescent probe for highly selective and sensitive detection of Cu2+ and Mg2

A solvent-directed, new Schiff base multiple correspondence fluorescent probe, (E)-2-(2-hydroxybenzylidene) hydrazine-1-carboxamid (L), was synthesized for selective sensing of Cu2+ and Mg2+ ions. L showed excellent selectivity and high sensitivity toward Cu2+ in "turn off" mode with a detection limit of 40.5 nM in 10 mM, pH = 7.0 PBS buffer. Contrary to that, when acetonitrile was used as the solvent, L exhibited highly selective and sensitive fluorescence sensing ability for Mg2+ in "turn on" mode with a detection limit of 9.5 nM. L can coordinate to Cu2+ and Mg2+ in a 1:1 molar ratio, respectively, evidenced by Job's plot analysis. Their binding modes were investigated by NMR, IR and XPS spectroscopies. Moreover, the satisfied results were obtained when L was used to detect Cu2+ and Mg2+ in real water samples.

Influence of ortho group in rhodamine B hydrazide based Schiff base for selective recognition of Cu2+ and Fe3+ ions: A mechanistic approach by DFT and colorimetric studies

Herein we have implemented a computational approach in designing sensor molecules for the selective recognition of Cu²⁺ and Fe³⁺ ions. Seven rhodamine B hydrazide-based Schiff base derivatives were designed and analysed their chemosensing properties against Cu²⁺ and Fe³⁺ ions in ethanol solution theoretically. The theoretical calculations revealed that the selective recognition of Cu²⁺ and Fe³⁺ ions takes place via spirolactam ring-opening and there is a pivotal role of ortho substituents and N-heteroatoms. The two best chemosensors were synthesised and used for the detection of Cu²⁺ and Fe³⁺ ions by colorimetric methods.

N-doped carbon nanospheres as selective fluorescent probes for mercury detection in contaminated aqueous media: chemistry, fluorescence probing, cell line patterning, and liver tissue interaction

A precise nano-scale biosensor was developed here to detect Hg²⁺ in aqueous media. Nitrogen-doped carbon nanospheres (NCS) created from the pyrolysis of melamine-formaldehyde resin were characterized by FESEM, XRD, Raman spectra, EDS, PL, UV-vis spectra, and N₂ adsorption-desorption, and were used as a highly selective and sensitive probe for detecting Hg²⁺ in aqueous media. The sensitivity of NCS to Hg²⁺ was evaluated by photoluminescence intensity fluctuations under fluorescence emission in the vicinity of 390 nm with a λ_exc of 350 nm. The fluorescence intensity of the NCS probe weakened in the presence of Hg²⁺ owing to the effective fluorescence quenching by that, which is not corresponding to the special covalent liking between the ligand and the metal. The effects of the fluorescence nanoprobe concentration, pH, and sensing time were monitored to acquire the best conditions for determining Hg²⁺. Surprisingly, NCS revealed excellent selectivity and sensitivity towards Hg²⁺ in the samples containing Co²⁺, Na⁺, K⁺, Fe²⁺, Mn²⁺, Al³⁺, Pb²⁺, Ni²⁺, Ca²⁺, Cu²⁺, Mg²⁺, Cd²⁺, Cr³⁺, Li⁺, Cs⁺, and Ba²⁺. The fluorescence response was linearly proportional to Hg²⁺ concentration in 0.013-0.046 µM with a limit of detection of 9.58 nM. The in vitro and in vivo toxicological analyses confirmed the completely safe and biocompatible features of NCS, which provides promise for use for water, fruit, vegetable, and/or other forms of natural-connected materials exposed to Hg²⁺, with no significant toxicity noticed toward different cells/organs/tissues.

Development of a reusable fluorescent nanosensor based on rhodamine B immobilized in Stöber silica for copper ion detection

This work has the goal of developing and evaluating a reusable fluorescent nanosensor for detection of Cu(ii) ion in aqueous solution, based on the immobilization of rhodamine B in silica nanoparticles prepared according to a modified Stöber method. In order to do this, a standard ammonium hydroxide ethanolic solution was mixed to ethanol under constant stirring, followed by the addition of tetraethoxysilane (TEOS). To immobilize the fluorescent reagent in the silica nanoparticles, rhodamine B ethanolic solution was added to the reacting mixture at different times (2; 3; 4 and 5 h) after starting the synthesis (which always lasts 7 h). The nanosensor obtained with the addition of rhodamine B after 5 h of synthesis showed the best sensitivity, measured as the fluorescence quenching, which was proportional to Cu(ii) ion. The nanosensor was selective to Cu(ii) ions and showed a linear range from 2.0 to 12.0 μmol L^-1, detection limit of 0.40 μmol L^-1, quantification limit of 1.3 μmol L^-1, response time of 50 s, being possible to be reused 3 times. The nanosensor was applied to the determination of Cu(ii) in sugar cane spirit and the results obtained did not show significant differences from those obtained by atomic absorption spectrometry at a confidence level of 95%.

A novel fluoro-chromogenic Cu2+ probe for living-cell imaging based on rhodamine 6G-pyridine conjugation

A novel fluoro-chromogenic rhodamine spirolactam probe (RP) has been prepared through the condensation of rhodamine hydrazine and 2-acetylpyridine, which displayed the detection of Cu²⁺ with high selectivity over a large number of other common metal ions. It shows a "turn-on" response to paramagnetic Cu²⁺ with an about 12-fold enhancement, and a color change from colorless to red that is observable by the naked eye. These changes are ascribed to the ring-opening of the spirolactam in RP, and subsequent host-guest coordination. The 2:1 binding stoichiometry of RP to Cu²⁺ was confirmed by Job's and B-H plots. The resulting fluorescence enhancement can be used to detect Cu²⁺ at concentrations from 2.0 to 20.0 μM with a limit of detection of 0.21 μM, which was lower than the maximum allowable Cu²⁺ level set by the WHO. Finally, RP has been utilized to monitor Cu²⁺ in living cells and natural water. Graphical abstract.

A fluorescein-based chemosensor for “turn-on” detection of Hg2+ and the resultant complex as a fluorescent sensor for S2− in semi-aqueous medium with cell-imaging application: experimental and computational studies

A fluorescein hydrazone based probe selectively recognizes Hg²⁺ ion with live cell imaging application.

A Rhodamine Derivative Based Chemosensor with High Selectivity and Quick Respond to Cr3+ in Aqueous Solution

In this paper, a new kind of colorimetric chemsensor aiming at detecting Cr³⁺ has been synthesized, and it is based on the "Off-On" effect of a rhodamine derivative. Comparing with other metal irons (Na⁺, K⁺, Ni²⁺, Hg²⁺, Fe³⁺, Mn²⁺, Co²⁺, Cd²⁺, Cu²⁺, Pb²⁺, Zn²⁺, Mg²⁺, Ba²⁺, Ag⁺, Fe²⁺, Ce³⁺), the chemsensor has a quick and accurate response to Cr³⁺ in H₂O-EtOH solution (4/1, v/v). There is an obvious change in color, from colorless to bright pink when Cr³⁺ is detected. According to the fitting curve based on Benesi-Hildebrand equation and working curve of absorption strength in UV-vis spectrum, the binding pattern of Cr³⁺ and the rhodamine derivative follows a 1:1 stoichiometry. The chemsensor shows great potential in monitoring Cr³⁺ in the aqueous medium with high efficiency, which is supposed to complete the recognition in the minimum as 5.2 × 10^-7 mol/L within 5 min.

A multi-responsive thiosemicarbazone-based probe for detection and discrimination of group 12 metal ions and its application in logic gates

A new simple 3-in-1 multi-response thiosemicarbazone-based chemosensor has been synthesized and characterized.

A novel colorimetric and turn-on fluorescent chemosensor for iron(III) ion detection and its application to cellular imaging

A novel rhodamine-based dual probe Rh-2 for trivalent ferric ions (Fe(3+)) was successfully designed and synthesized, which exhibited a highly sensitive and selective recognition towards Fe(3+) with an enhanced fluorescence emission in methanol-water media (v/v=7/3, pH=7.2). The probe Rh-2 could be applied to the determination of Fe(3+) with a linear range covering from 3.0×10(-7) to 1.4×10(-5)M and a detection limit of 1.24×10(-8)M. Meanwhile, the binding ratio of Rh-2 and Fe(3+) was found to be 1:1. Most importantly, the fluorescence and color signal changes of the Rh-2 solution were specific to Fe(3+) over other commonly coexistent metal ions. Moreover, the probe Rh-2 has been used to image Fe(3+) in living cells with satisfying results.

An ultra sensitive fluorescent nanosensor for detection of ionic copper

A stable and ultra sensitive nano-scale fluorescent chemo-sensor for trace amounts of Cu(2+) was proposed. The Cu(2+) selective fluoroionophore 2-{[(2-aminophenyl)imino]methyl}-4,6-di-tert-butylphenol (DMK-7) was encapsulated in polymeric ethyl cellulose. The sensing membranes were fabricated in form of nanofibers and thin films. When embedded in polymers, the exploited DMK-7 dye exhibited enhanced photophysical characteristics in absorbance, Stoke's shift, fluorescence quantum yield, and short and long-term photostability with respect to the solution phase. Sensing abilities of the nanofibers and thin films were tested by steady state and time resolved fluorescence spectroscopy. To our knowledge, this is the first attempt using the DMK-7-doped electrospun nanofibrous materials for copper sensing. The offered sensor displayed a sensitive response with a detection limit of 3.3×10(-13) M for Cu(2+) ions over a wide concentration range of 5.0×10(-12)-5.0×10(-5). Additionally, exhibited high selectivity over convenient cations; Na(+), K(+), Ca(2+), Mg(2+), NH4(+) and Ag(+), Al(3+), Ba(2+), Co(2+), Cr(3+), Fe(3+), Fe(2+), Hg(2+), Li(+), Mn(2+), Ni(2+), Pb(2+), Sn(2+) and Zn(2+).