New Fluorogenic Sensors for Hg2+ Ions: Through-Bond Energy Transfer from Pentaquinone to Rhodamine

Architecture of multi-channel and easy-to-make sensors for selective and sensitive Hg2+ ion recognition through Hg‒C and Hg‒N bonds of naphthoquinone-aniline/pyrene union

The aim of this work is, for the first time, to develop new inexpensive, easy-to-make and multi-channel receptors, naphthoquinone-aniline/pyrene union ((Nq-An) and (Nq-Pyr)) and their Hg²⁺ complexes [Hg-(Nq-An)₂] and [Hg-(Nq-Pyr)₂] to supply an efficient solution to critical deficiencies to be encountered for Hg²⁺ recognition. This study is based on colorimetric, fluorometric, and voltammetric methods for determination of Hg²⁺ ions through Hg-C and Hg-N binding mode of the naphthoquinone-aniline/pyrene union in aqueous media. The binding mode of the receptors with Hg²⁺ cation was confirmed by usual characterization techniques for the synthesized Hg²⁺-complexes [Hg-(Nq-An)₂] / [Hg-(Nq-Pyr)₂] and voltammetric, ¹H NMR titration experiments as well as Job's method, indicating a 2:1 complex between the receptors and Hg²⁺ cation. The receptors showed a considerable color switching from orange to pink along with a red-shift of absorption wavelength, and fluorescence enhancement via the Chelation Enhanced Fluorescence effect (CHEF), and distinctive changes on the voltammogram of the electroactive naphthoquinone unit with Hg²⁺ cation. The experiments indicate that the sensors are highly selective and sensitive toward Hg²⁺ among the studied metal ions in aqueous media compared with other reported Hg²⁺ sensors.

Fluorescent, colourimetric, and ratiometric probes based on diverse fluorophore motifs for mercuric(II) ion (Hg2+) sensing: highlights from 2011 to 2019

Though it has not been shown to deliver any biological importance, mercuric(II) ion (Hg²⁺) is a deleterious cation which poses grievous effects to the human body and/or the ecosystem, hence, the need for its sensitive and selective monitoring in both environmental and biological systems. Over the years, there has been a great deal of work in the use of fluorescent, colourimetric, and/or ratiometric probes for Hg²⁺ recognition. Essentially, the purpose of this review article is to give an overview of the advances made in the constructions of such probes based on the works reported in the period from 2011 to 2019. Discussion in this review work has been tailored to the kinds of fluorophore scaffolds used for the constructions of the probes reported. Selected examples of probes under each fluorophore subcategory were discussed with mentions of the typically determined parameters in an analytical sensing operation, including modulation in fluorescence intensity, optimal pH, detection limit, and association constant. The environmental and biological application ends of the probes were also touched where necessary. Important generalisations and conclusions were given at the end of the review. This review article highlights 196 references.

A mitochondria-targeting ratiometric fluorescent probe for the detection of sulfur dioxide in living cells

A mitochondria-targeting ratiometric fluorescent probe was developed for the detection of sulfur dioxide in living cells.

Colorimetric Detection of Multiple Metal Ions Using Schiff Base 1-(2-Thiophenylimino)-4-(N-dimethyl)benzene

In this paper, a Schiff base ligand 1-(2-thiophenylimino)-4-(N-dimethyl)benzene (SL1) bearing azomethine (>C=N-) and thiol (-SH) moieties capable of coordinating to metals and forming colored metal complexes was synthesized and examined as a colorimetric chemosensor. The sensing ability toward the metal ions of Cu2+, Cr3+, Fe2+ Ni2+, Co2+, Mg2+, Zn2+, Fe2+, Fe3+, NH4VO3 (V5+), Mn2+, Hg2+, Pb2+, and Al3+ was investigated in a mixture of H2O and dimethylformamide (DMF) solvent using the UV–Visible spectra monitoring method. The synthesized Schiff base ligand showed colorimetric properties with Cr3+, Fe2+, Fe3+, and Hg2+ ions, resulting in a different color change for each metal that could be identified easily with the naked eye. The UV–Vis spectra indicated a significant red shift (~69–288 nm) from the origin after the addition of the ligand to these metal ions, which may be due to ligand-to-metal charge-transfer (LMCT). On applying Job’s plot, it was indicated that the ligand binds to the metal ions in a 2:1 ligand-to-metal molar ratio. SL1 behaves as a bidentate ligand and binds through the N atom of the imine group and the S atom of the thiol group. The results indicate that the SL1 ligand is an appropriate coordination entity and can be developed for use as a chemosensor for the detection of Cr3+, Fe2+, Fe3+, and Hg2+ ions.

A visible-near-infrared fluorescent probe for peroxynitrite with large pseudo-Stokes and emission shift via through-bond energy and charge transfers controlled by energy matching

We reported a visible-near-infrared fluorescent probe for peroxynitrite detection with large pseudo-Stokes and emission shifts, based on through-bond energy transfer (TBET) in combination with intramolecular charge transfer (ICT). Pyrene was chosen as a fluorophore (acceptor), which has monomer/excimer fluorescence characteristics. A conjugated 1,2-dimethylenehydrazine structure was a linker and phenyl boronate was selected as a reaction site (donor) to design the probe (Py-PhB) using the chemical transformation from boronate to phenol, which results in the increase of the energy of the donor to match the energy of the acceptor and simultaneously achieves TBET and ICT between the donor (phenolate) and the acceptor (pyrene), leading to a fluorescence 'OFF-ON' in a red-shifted region and a large emission shift. The results show that the probe exhibits high selectivity to ONOO^- with a detection limit of 3.54 μM. Favorable ICT from phenolate to pyrene makes the probe possess a large monomer emission shift (183 nm), red-shifted to organe-red light (598 nm). TBET ensures the probe with a large pseudo-Stokes shift of 244 nm. Furthermore, its excimer emits a near-infrared light (720 nm), which is extremely beneficial for bioimaging. In short, this probe offers a novel design strategy for designing the TBET fluorescent sensors emitting red or NIR light with large pseudo-Stokes and emission shifts.

A novel cationic iridium(iii) complex with a thiorhodamine-based auxiliary ligand: application for luminescent and colorimetric detection of Hg2+in an aqueous solution

Detecting Hg²⁺in an aqueous solution with detection limits of 16 ppb by luminescence detection and 80 ppb by absorption detection.

AIEgens for dark through-bond energy transfer: design, synthesis, theoretical study and application in ratiometric Hg2+ sensing

A novel dark through-bond energy transfer (DTBET) strategy is proposed and applied as the design strategy to develop ratiometric Hg²⁺ sensors with high performance. Tetraphenylethene (TPE) derivatives with aggregation-induced emission (AIE) characteristics are selected as dark donors to eliminate emission leakage from the donors. The TBET mechanism has been adopted since it experiences less influence from spectral overlapping than Förster resonance energy transfer (FRET), making it more flexible for developing cassettes with large pseudo-Stokes shifts. In this work, energy transfer from the TPE derivatives (dark donor) to a rhodamine moiety (acceptor) was illustrated through photophysical spectroscopic studies and the energy transfer efficiency (ETE) was found to be up to 99%. In the solution state, no emission from the donors was observed and large pseudo-Stokes shifts were achieved (>280 nm), which are beneficial for biological imaging. Theoretical calculations were performed to gain a deeper mechanistic insight into the DTBET process and the structure-property relationship of the DTBET cassettes. Ratiometric Hg²⁺ sensors were rationally constructed based on the DTBET mechanism by taking advantage of the intense emission of TPE aggregates. The Hg²⁺ sensors exhibited well resolved emission peaks. >6000-fold ratiometric fluorescent enhancement is also achieved and the detection limit was found to be as low as 0.3 ppb. This newly proposed DTBET mechanism could be used to develop novel ratiometric sensors for various analytes and AIEgens with DTBET characteristics will have great potential in various areas including light harvesting materials, environmental science, chemical sensing, biological imaging and diagnostics.

A through bond energy transfer based ratiometric probe for fluorescent imaging of Sn2+ ions in living cells

A 4-(naphthalen-1-ylethynyl) aniline appended rhodamine based fluorescent chemosensor ‘NAP-RD’ is synthesized which undergoes through bond energy transfer in the presence of Sn²⁺ ions in mixed aqueous media.

Novel triphenylamine based rhodamine derivatives: synthesis, characterization, photophysical properties and viscosity sensitivity

Five novel triphenylamine based deep red to NIR emitting rhodamine derivatives were synthesized and characterized by ¹H and ¹³C NMR spectroscopy and elemental analysis.

A ratiometric fluorescence sensor for HOCl based on a FRET platform and application in living cells

A ratiometric fluorescent probe CRSH based on a FRET platform for detecting HOCl. CRSH showed high selectivity, excellent sensitivity and a fast response toward HOCl.

A carbazole based “Turn on” fluorescent sensor for selective detection of Hg2+ in an aqueous medium

A colorimetric and fluorescent Hg²⁺ selective sensor based on carbazole-barbituric acid conjugate (CBA) is developed. It exhibited visual colour change and a turn-on emission at 593 nm and is selective and sensitive for Hg²⁺ in aqueous medium.

Optical sensor: a promising strategy for environmental and biomedical monitoring of ionic species

In this review, we cover the recent developments in fluorogenic and chromogenic sensors for Cu²⁺, Fe²⁺/Fe³⁺, Zn²⁺and Hg²⁺.

Rhodamine appended hexaphenylbenzene derivative: through bond energy transfer for sensing of picric acid

A rhodamine appended hexaphenylbenzene based derivative 5 has been designed and synthesized, which displays through bond energy transfer in the presence of picric acid in MeOH and serves as a "turn on" sensor for picric acid.

Fluorometric sensing of Hg2+ ions in aqueous medium by nano-aggregates of a tripodal receptor

Two new tripodal receptors (1–2) have been synthesized and characterized by various spectroscopic techniques. The nano-aggregates of 1 and 2 (N1 and N2) have been prepared by a re-precipitation method in aqueous medium and have shown different photo-physical properties. Nano-aggregates of 1 (N1) can selectively recognize Hg(2+) in aqueous medium in the presence of other metal ions with enhancement in fluorescent intensity. The response was linearly proportional to the concentration of Hg(2+) in the range 0–10 μM with a detection limit of 2.4 nM. The mechanism of selective binding of Hg(2+) by N1 has also been supported by theoretical studies. To the best of our knowledge, this work represents the first report on substituted thiourea based nano-aggregates for nano-molar detection of mercury in aqueous medium.

A fluorescence ratiometric chemosensor for Fe³⁺ based on TBET and its application in living cells

Based on a through bond energy transfer (TBET) between rhodamine and naphthalimide fluorophores, a fluorescent ratiometric chemosensor L was designed and prepared for highly selective detection of Fe(3+) in aqueous solution and in living EC109 cells. These significant changes in the fluorescence color could be used for naked-eye detection. The reversibility established the potential of the probe as chemosensor for Fe(3+) detection.

A specific probe for Hg2+ to delineate even H+ in pure aqueous buffer/Hct116 colon cancer cells: Hg(ii)–η2-arene π-interaction and a TBET-based fluorescence response

Ligand design that allows a Hg(ii)–η²-arene π-interaction as well as a TBET-based luminescence response to distinguish between the cellular uptake of Hg²⁺ and H⁺ in live Hct116 colon cancer cells.

An overview of the recent developments on Hg2+ recognition

Adverse influences of mercury on living organisms are well known. Despite efforts from various regulatory agencies, the build-up of Hg²⁺ concentration in the environment is of serious concern. This necessitates the search for new and efficient reagents for recognition and detection of Hg²⁺ in environmental samples.

Selective sensing of Hg²⁺ using rhodamine-thiophene conjugate: red light emission and visual detection of intracellular Hg²⁺ at nanomolar level

Rhodamine-thiophene conjugate (L) has been synthesized and characterized by (1)H NMR, FTIR and mass spectra. L shows a large enhancement in emission intensity in presence of Hg(2+). Moreover, naked eye color of L becomes intense red in presence of Hg(2+). The lowest detection limit for Hg(2+) is 1 × 10(-9)M in HEPES buffer (0.1M in EtOH/water, 1/1, v/v, pH 7.4). Hg(2+) induced chelation enhanced fluorescence (CHEF) is associated with spirolacram ring opening of the rhodamine unit. Trace level intracellular Hg(2+) is visualized under fluorescence microscope.

Through bond energy transfer: a convenient and universal strategy toward efficient ratiometric fluorescent probe for bioimaging applications

Fluorescence resonance energy transfer (FRET) strategy has been widely applied in designing ratiometric probes for bioimaging applications. Unfortunately, for FRET systems, sufficiently large spectral overlap is necessary between the donor emission and the acceptor absorption, which would limit the resolution of double-channel images. The through-bond energy transfer (TBET) system does not need spectral overlap between donor and acceptor and could afford large wavelength difference between the two emissions with improved imaging resolution and higher energy transfer efficiency than that of the classical FRET system. It seems to be more favorable for designing ratiometric probes for bioimaging applications. In this paper, we have designed and synthesized a coumarin-rhodamine (CR) TBET system and demonstrated that TBET is a convenient strategy to design an efficient ratiometric fluorescent bioimaging probe for metal ions. Such TBET strategy is also universal, since no spectral overlap between the donor and the acceptor is necessary, and many more dye pairs than that of FRET could be chosen for probe design. As a proof-of-concept, Hg(2+) was chosen as a model metal ion. By combining TBET strategy with dual-switch design, the proposed sensing platform shows two well-separated emission peaks with a wavelength difference of 110 nm, high energy transfer efficiency, and a large signal-to-background ratio, which affords a high sensitivity for the probe with a detection limit of 7 nM for Hg(2+). Moreover, by employing an Hg(2+)-promoted desulfurization reaction as recognition unit, the probe also shows a high selectivity to Hg(2+). All these unique features make it particularly favorable for ratiometric Hg(2+) sensing and bioimaging applications. It has been preliminarily used for a ratiometric image of Hg(2+) in living cells and practical detection of Hg(2+) in river water samples with satisfying results.

Energy transfer cassettes based on organic fluorophores: construction and applications in ratiometric sensing

This tutorial review presents some recent developments in the construction and applications of cassettes based on resonance energy transfer between fluorescent dyes in the visible and infrared region. We focused on the contributions of different connections between the energy donor and acceptor according to the "through-space" and "through-bond" methods, and emphasised their applications in ratiometric sensing for the detection of ions and small molecules.