Chemosensors based on traditional fluorescent dyes have always contributed to the development of chemical sensor areas. In this review, the rhodamine-based chemosensors’ improvements and applications from 2018 to 2022 are discussed, mainly focusing on cations (metal ions and H+), anions (CN−, F−, etc.), and small bio-functional molecules’ (thiols, amino acids, etc.) detection. Specifically, this review highlights the detection target, detection limit, detection solution system, detection mechanism, and performance of the rhodamine-based sensors. Although these rhodamine-based sensors are well developed, their repeatability and sensitivity still need significant improvement. This review is expected to bring new clues and bright ideas to researchers for further advances in rhodamine-based chemosensors in the future.

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Recent Developments in Rhodamine-Based Chemosensors: A Review of the Years 2018–2022

A first tricolor fluorescent pH nanosensor is presented, which was rationally designed from biocompatible carboxylated polystyrene nanoparticles and two analyte-responsive molecular fluorophores. Its fabrication involved particle staining with a blue-red-emissive dyad, consisting of a rhodamine moiety responsive to acidic pH values and a pH-inert quinoline fluorophore, followed by the covalent attachment of a fluorescein dye to the particle surface that signals neutral and basic pH values with a green fluorescence. These sensor particles change their fluorescence from blue to red and green, depending on the pH and excitation wavelength, and enable ratiometric pH measurements in the pH range of 3.0-9.0. The localization of the different sensor dyes in the particle core and at the particle surface was confirmed with fluorescence microscopy utilizing analogously prepared polystyrene microparticles. To show the application potential of these polystyrene-based multicolor sensor particles, fluorescence microscopy studies with a human A549 cell line were performed, which revealed the cellular uptake of the pH nanosensor and the differently colored emissions in different cell organelles, that is, compartments of the endosomal-lysosomal pathway. Our results demonstrate the underexplored potential of biocompatible polystyrene particles for multicolor and multianalyte sensing and bioimaging utilizing hydrophobic and/or hydrophilic stimuli-responsive luminophores.

Multicolor Polystyrene Nanosensors for the Monitoring of Acidic, Neutral, and Basic pH Values and Cellular Uptake Studies.

For the immediate detection of strong acids, it is advantageous to employ colorimetric textile sensors based on halochromic dyes, as such systems permit the detection of gas leaks with the naked eye. Here, we introduce a UV-induced photografting method to dye cotton fabrics in an eco-friendly manner and to improve the dyeability and wash fastness of halochromic dyes. In this UV photografting method, the radical-sensitive groups of the dye and the fiber were covalently bonded through radicals formed under UV irradiation to greatly reduce dye leaching. Two types of graftable-rhodamine dyes were synthesized by introducing a radical-sensitive group to a rhodamine derivative with a superior pH sensitivity, and textile sensors were fabricated via UV photografting. Subsequently, the eco-friendliness of the UV-induced photografting method and the gas detection performances, durabilities, and reusabilities of the fabricated textile sensors were investigated. All fabricated sensors exhibited distinctive color changes (ΔE >15) under acidic conditions, in addition to an outstanding durability (washfastness level: 4–5, performance retention rate after washing: 91%) and reusability after ten washing/drying cycles. Furthermore, the low consumption of solvent (∼91%), the salt/alkali-free nature of this method, its low energy consumption (∼85%), and the relatively short process time (∼41%) render this UV-induced photografting technique more environmentally friendly than conventional dyeing methods. Our results therefore indicate that textile sensors fabricated using the UV-induced photografting method are promising candidates for acid gas sensors in a wide range of applications. 

Facile and Eco-friendly Fabrication of a Colorimetric Textile Sensor by UV-induced Photografting for Acidic Gas Detection

Spectral characterization of the main pigments in the plant photosynthetic apparatus by theory and experiment

A multi-channel rhodamine-pyrazole-based chemosensor (RhPzNH2) was prepared by a simple synthetic method. The photophysical characteristics of the compound were determined in various organic solvents. The new probe responded discriminately to acidic and basic pHs; in acidic media (pH < 3) it produced a yellow emission centered at 555 nm, while in basic media (pH > 10) its emission was blue with a maximum at 460 nm. Moreover, the new RhPzNH2 sensor responded significantly to Cu2+ by increasing the absorbance at 530 nm (limit of detection (LOD) 7×10−8 M). The stoichiometric ratio of Cu2+ ions and the probe in the complex formed was found to be 1:2, respectively. Possible structural arrangements of the ligand molecule and Cu2+ in the complex were studied by DFT computations and the most likely structural organization was proposed. Furthermore, the responses of the [(RhPzNH2)2-Cu]2+ complex were significant towards Ba2+ and CN–. The substitution of Cu2+ in the complex decreased remarkably the absorbance at 530 nm by 72 %. On the other hand, CN– increased both the absorbance at 530 nm and the fluorescence emission at 550 nm by sharing it in the coordination within the complex increased solubility of the complex, and blocked the energy transfer from the compound to Cu2+. Moreover, the RhPzNH2 mimicked three different INHIBIT logic gates that can be integrated to work as a molecular digital comparator. 

A multi-channel rhodamine-pyrazole based chemosensor for sensing pH, Cu2+, CN- and Ba2+ and its function as a digital comparator

We present the rational design, synthesis and spectroscopic characterization of a novel dual excitation, three color emitting, pH-responsive fluorescent probe consisting of two phenanthrene and one rhodamine B units linked by click chemistry. The rhodamine moiety, excitable at λEx = 315 nm and at λEx = 560 nm in its ring-opened form, provides the pH-responsive fluorophore, while the pH-insensitive phenanthrene, excited at λEx = 315 nm, serves as inert internal reference, The presence of two phenanthrene moieties enables a blue monomer and a blueish green excimer emission at 351 nm and 500 nm, respectively. Opening of the rhodamine B spirolactam ring at an acidic pH below 5.0 (pKa = 2.59 ± 0.04) switches on its emission at 580 nm. Simultaneously, the phenanthrene excimer emission decreases caused by a change in orientation of the phenanthrene units, while the monomer emission is barely affected. This sensor design enables ratiometric measurements in the low acidic pH range utilizing the intensity ratios of the rhodamine B and phenanthrene excimer emission at 580 nm and 500 nm. Alternatively, also the intensity ratios of the rhodamine B and the phenanthrene monomer emission could be exploited or the sum of the phenanthrene monomer and excimer fluorescence. To the best of our knowledge, this is the first report of ratiometric sensing utilizing such a versatile type of tricolor emissive dyad probe bearing phenanthrene moieties and showing phenanthrene monomer and excimer emission.

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Synthesis and spectroscopic characterization of a fluorescent phenanthrene-rhodamine dyad for ratiometric measurements of acid pH values

A user-friendly, Python-based quantum mechanics/Gromacs interface: gmx2qmmm

Abstract A 3‐amino‐functionalized phosphabenzene (phosphinine) has been synthesized and structurally characterized. The pyramidalized nitrogen atom of the dimethylamino substituent indicates only a weak interaction between the lone pair of the nitrogen atom and the aromatic phosphorus heterocycle, resulting in somewhat basic character. It turned out that the amino group can indeed be protonated by HCl. In contrast to pyridines, however, the phosphabenzene‐ammonium salt undergoes a selective ring contraction to form a hydroxylphospholene oxide in the presence of additional water. Based on deuterium labeling experiments and quantum chemical calculations, a rational mechanism for this hitherto unknown conversion is proposed.

Phospholenes from Phosphabenzenes by Selective Ring Contraction

Weakly coordinating anions (WCAs) have attracted much attention in recent years due to their ability to stabilise highly reactive cations. It may well be argued, however, that a profound understanding of what truly defines a WCA is still lacking, and systematic studies to unravel counterion effects are scarce. In this work, we investigate a supramolecular pseudorotaxane formation reaction, subject to a selection of anions, ranging from strongly to weakly coordinating, which not only aids in fostering our knowledge about anion coordination properties, but also provides valuable theoretical insight into the nature of the mechanical bond. We employ state-of-the-art DFT-based methods and tools, combined with isothermal calorimetry and 1H NMR experiments, to compute anion-dependent Gibbs free association energies ΔGa, as well as to evaluate intermolecular interactions. We find correlations between ΔGa and the anions’ solvation energies, which are exploited to calculate physico-chemical reaction parameters in the context of coordinating anions. Furthermore, we show that the binding situation within the (pseudo)rotaxanes can be mostly understood by straight-forward electrostatic considerations. However, quantum-chemical effects such as dispersion and charge-transfer interactions become more and more relevant when WCAs are employed.

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The Interplay of Weakly Coordinating Anions and the Mechanical Bond: A Systematic Study of the Explicit Influence of Counterions on the Properties of (Pseudo)rotaxanes

Jan Felix Witte

Papers

Synthesis and spectroscopic characterization of a fluorescent phenanthrene-rhodamine dyad for ratiometric measurements of acid pH values

Spectral characterization of the main pigments in the plant photosynthetic apparatus by theory and experiment

A user-friendly, Python-based quantum mechanics/Gromacs interface: gmx2qmmm

Phospholenes from Phosphabenzenes by Selective Ring Contraction

The Interplay of Weakly Coordinating Anions and the Mechanical Bond: A Systematic Study of the Explicit Influence of Counterions on the Properties of (Pseudo)rotaxanes