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

Abstract

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.

1. Introduction

Chemosensors, since their first exploitation in 1864, have since then been playing a vital role in analytical chemistry due to their high sensitivity, time efficiency, simple operation, and less complicated instruments [1,2,3,4,5]. These sensors have been useful in identifying targets such as cations (H⁺ and metal ions), anions (CN⁻, F⁻, etc.), and biomolecules (NADH, ATP, etc.) [6,7,8]. The detection of those targets is of great significance for monitoring food safety and human health. According to the Chinese standard, the threshold limit values in drinking water of Hg²⁺, Pb²⁺, As³⁺, CN⁻, F⁻ are 0.001, 0.01, 0.01, 1.5, 0.01 mg/L (5.0 × 10⁻⁹, 4.8 × 10⁻⁸, 4.8 × 10⁻⁷, 7.9 × 10⁻⁵, 3.8 × 10⁻⁷ M), respectively [9]. Traditional organic dyes contain a fluorescent core skeleton with a large conjugate system and auxochrome group altering wavelengths and enhancing fluorescence (such as amino, amide, etc.). In addition, many kinds of fluorescent materials have been developed, such as gold nanoparticles [10], quantum dots [11], metal-organic frameworks [12] covalent organic frameworks [13], and so on, as the development in fluorescent detection technology continues [14]. Although these emerging materials are widely used as fluorescent probes, cheap and easily synthetic organic dyes, such as rhodamine and its derivatives, are not replaced because of their great characteristics of convenience for structural modification, excellent molecular stability, and gratifying environmental compatibility [6].

Rhodamine, a group of xanthene-derivative dyes (Figure 1A) [15], was first prepared in 1905 by Noelting and Dziewonsky [16] and used for analytical research purposes (colorimetric detection of zinc, silver, etc.) from the middle of the 20th century [17]. Since Czarnik et al. [18] and Tae et al. [19], who pioneered works on rhodamine B and rhodamine 6G derivatives, respectively, as the fluorescent chemosensors for metal ions (Cu²⁺ and Hg²⁺), a lot of sophisticated researches on rhodamine have been published, among which rhodamine B-based and rhodamine 6G-based chemosensors were those most often referred. As displayed in Figure 1B, the unique and convenient close-open ring structure of rhodamine derivatives before and after the addition target would induce color and fluorescent changes, helping greatly to detect target analyte. The coordination between the sensor and target ion induces spirolactam ring opening and pronounces the colorimetric and/or fluorescent responses, which are usually non-fluorescent and colorless before the coordination (Figure 1B). Additionally, the rhodamine derivatives’ properties of fine biocompatibility and near-infrared fluorescence make them an excellent choice for bio-sensor construction.

Many reviews on chemosensors research have been reported in the last two decades. Chen et al. described and discussed mercury-ion sensors from 2008 to 2015 [20]. Kaur et al. comprehensively summarized the developments of colorimetric sensors for metal ions from 2011–2016 [1]. Roy, P. discussed recently reported sensors for aluminum detection [21]. These reviews mainly focused on chemosensor targets. Chen et al. described the progress of benzazole-based fluorescent probes, reported in 2017–2019, combining the design strategy, synthetic route, sensing mechanism, and applications [6]. Jain, N. and Kaur, N. published a review on chemosensors based on 1,8-naphthalimide reported during 2017–2021 [22]. Zhu, H., et al. reviewed the advances of 4-hydroxy-1,8-naphthalimide-based chemosensors in the past decade [23]. These reviews elaborated on the chromophores of chemosensors.

However, as a classic chromophore for chemosensor construction, there are no reviews on the developments of rhodamine-based chemosensors in recent years. The sensing mechanism of rhodamine-based sensors has been proposed to guide the sensor’s design [24]. The most common sensing mechanisms are photoinduced electron transfer (PET) [25,26], intramolecular charge transfer (ICT) [27,28], and fluorescence-resonance energy transfer (FRET) [24,29]. In this review, the endeavors of chemosensors based on rhodamine and its derivative from 2018 to 2022 are summarized and discussed. Generally, the colorless and non-fluorescent rhodamine turns red and shows a strong fluorescence when its closed spiro-ring is opened by metal complexation, analyte binding, or exposure to acidic conditions [17,30]. The rhodamine-based chemosensors are divided into three main categories according to the chemo sensor’s targets: cations (metal ions, H⁺), anions (CN⁻, F⁻, etc.), and small bio-functional molecules (H₂O₂, ATP, NADH). The concrete progress in this research area is elaborated in following sections. This review could bring new clues or bright ideas for further advances in rhodamine-based chemosensors in the future.

2. Rhodamine-Based Sensors for Metal Ions

Metal ions are partly beneficial to the body up to a certain concentration level. For example, iron (Ⅲ), zinc (Ⅱ), and copper (Ⅱ) play vital roles in neurophysiology and neuropathology [31], however, their alleviated levels are harmful and may cause physiological disorders [32]. Other metals, such as mercury (Ⅱ), lead (Ⅱ), cadmium (Ⅲ), and chromium (Ⅲ) are potentially toxic to both humans and the environment [1,5]. Hence, monitoring the levels of these metal ions in living bodies and the environments (groundwater, industrial sewage, and so on) is quite significant. Table 1 shows the reported fluorescent probes based on rhodamine derivatives, generally designed for divalent (Hg²⁺, Cu²⁺, Pb²⁺, Zn²⁺, Cd²⁺, Co²⁺) and trivalent (Fes³⁺, Cr³⁺, Al³⁺, As³⁺) metal ions.

2.1. Rhodamine-Based Sensors for Hg²⁺

A rhodamine-fluorene-based dual channel sensor was synthesized for selective detection of Hg²⁺ via both colorimetry and fluorometry (Table 1, No. 1), and the sensor’s solubility was greatly improved by the alkyl chain in the fluorene moiety, enabling its utility in 100% H₂O [33]. The prepared sensor selectively detected Hg²⁺ in 100% water using a strip with a good response and was then used as a digital printing material.

Hong M., et al. synthesized a spectral sensor through a one-step reaction between rhodamine B and thiobisethylamine, named 2-(2-((2-aminoethyl)thio)ethyl)-3′,6′-bis(diethylamino)spiro [isoindoline-1,9′-xanthen]-3-one (RMTE) (as shown in Table 1, No. 2) [34]. RMTE emitted a new fluorescence peak at 578 nm in the presence of Hg²⁺ with a remarkable visual change in fluorescence color (Figure 2).

Cicekbilek, F. et al., reported a Hg²⁺ sensor synthesized a facile Schiff base-type reaction between 9-anthraldehyde and aminated rhodamine [35] and used to detect Hg²⁺ with observable sensitivity and selectivity via colorimetric and fluorescence methods (Table 1, No. 3). The detection limit of the proposed sensor was 0.87 μM. The sensing mechanism of the as-prepared sensor to Hg²⁺ was based on a spirolactam ring-opening reaction, verified through UV-Vis, FTIR, and NMR titration analyses.

A promising colorimetric and fluorometric chemosensor named RDV was designed by Patil S. K., et al. for specific detection of Hg²⁺ (Table 1, No. 4) [36]. The color of the “turn-on” fluorescence sensor changed from colorless to pink before and after the addition of Hg²⁺ in an aqueous acetonitrile medium (Figure 3). The sensing limit of RDV for Hg²⁺ was 136 nM, revealing a high sensitivity of the sensor.

Kan, C. et al. constructed a novel reversible fluorescent probe named RBST (Table 1, No. 5), by the reaction of 2-thiopheneacetyl chloride and rhodamine B, [37]. RBST showed a sensitive and selective response to Hg²⁺ in EtOH/H₂O (2,1, v/v) with the detection limit of 0.11 μM (Figure 4). The reversibility of RSBT was attributed to the strong affinity of S²⁻ and Hg²⁺. Besides, the RSBT was used to detect Hg²⁺ in real water samples, suggesting its promising practicality.

Dewangan S., et al. synthesized a significant molecule through the functioning of the cymantrenyl hydrazone system by fluorescent tagging of rhodamine (Table 1, No. 6) [38]. The synthesized molecule displayed high selectivity and responsiveness to Hg²⁺ (Figure 5). The prepared sensor was applied to recognize Hg²⁺ in bacterial strains and THP-1 cancer cells, indicating its potential in bioimaging applications.

Wang Q., et al. synthesized three rhodamine based probes 1, 2, and 3 (Table 1, No. 7) and applied them for Hg²⁺ detection in Tris-HCl/C₂H₅OH (v:v = 3:7, 10 mM, pH = 7.2). All three probes were colorless at first but turned to visible pink after the addition of Hg²⁺ with the fluorescence increment. The detection limits of probes 1, 2, 3 to Hg²⁺ were 18 nM, 16 nM, 56 nM, respectively. These probes were successfully applied to both environmental samples and bioimaging applications, as shown in Figure 6.

A ferrocene-based rhodamine-hydrazone molecular probe (Table 1, No. 2) was synthesized using the solid-state synthetic method by Dewangan S., et al. [40]. The probe showed selective recognition of Hg²⁺. It was therefore applied to intracellular metal detection and bioaccumulation analysis, showing its promising application prospects. The proposed sensing mechanism of the prepared probe to Hg²⁺ was shown in Figure 7.

Gauthama B. U., et al. designed a rhodamine-based sensor named RhB, with a selective and sensitive response for Hg²⁺ (Table 1, No. 9) [41]. The UV-Vis and fluorescence spectral profiles changed in the presence of Hg²⁺ due to the formation of the coordinate bond between RhB and Hg²⁺ (Figure 8). The detection limit was around 2.57 × 10⁻⁶ M and the prepared sensor detected Hg²⁺ in 100% aqueous solution by the naked eye.

Li, X., et al. synthesized a fluorometric and colorimetric chemosensor, named 1O and studied its responses for Hg²⁺ (Table 1, No. 10). The results suggested that Hg²⁺ induced remarkable colorimetric and fluorescent changes, due to the reaction between 1O and Hg²⁺ (Figure 9). Singh S., et al. described a rhodamine-based fluorescence sensor for Hg²⁺ detection in water samples (Table 1, No. 11) [43]. The influences of different analytical parameters on the analytical performance of the sensors were discussed. Results showed that replacing the fluorophore of rhodamine B with rhodamine 6G on the prepared sensor improved the limit of detection. In addition, the buffer optimization improved detection limits and selectivity. The prepared sensor was successfully applied for Hg²⁺ detection in tap and river samples.

Yilmaz, B., et al. [44] reported a novel rhodamine-based sensor, using the calixarene as the skeleton and appended with four rhodamine units at the upper rim (Table 1, No. 12). The prepared sensor showed high selective and sensitive response to Hg²⁺ with immediately turning on fluorescence output. Through in vitro and bio-imaging studies, the prepared sensor was cell permeable and suitable for real-time imaging of Hg²⁺ in living cells. Aduroja O., et al. synthesized a rhodamine-based sensor, RD6 (Table 1, No. 13) [45], with excellent selectivity sensitivity to Hg²⁺ in an aqueous buffer solution by colorimetric and fluorescent methods. The detection limit of RD6 for Hg²⁺ was 1.2 × 10⁻⁸ M. With the CN⁻ assistance, the prepared sensor realized reversibility due to the strong stability of Hg(CN)₂ (Figure 10).

Li M., et al. constructed a novel rhodamine-based material, FFC, for the detection and adsorption of Hg²⁺ (Table 1, No. 14) [46]. The cellulose was chosen as the skeleton and the rhodamine moiety was conjugated with cellulose as the recognition group, enabling excellent optical signals in the presence of Hg²⁺. After adding Hg²⁺, the intensity of fluorescence enhancement of FFC showed a 7-fold enhancement by triggering the spirolactam ring-opening reaction. The adsorption process of FFC to Hg²⁺ was fitted with the Langmuir adsorption model and pseudo-second-order kinetics. The prepared FFC was successfully applied for the detection and adsorption of Hg²⁺ in tap water. Liang F. et al. introduced an anthocyanin functional group in the rhodamine spiro ring and constructed a sensitive and selective near-infrared (NIR) fluorescent sensor, CCS-Hg for Hg²⁺ (Table 1, No. 15) [47]. The prepared CCS-Hg was applied to determine Hg²⁺ distribution in living cells. The sensing mechanism was verified through theoretical calculations (Figure 11).

Zhang Q., et al. designed a rhodamine-based sensor for selective detection of Hg²⁺ via the FRET mechanism (Table 1, No. 16) [48]. The synthesized material possessed great pseudo-Stokes (174 nm) in the presence of Hg²⁺ and showed fluorescent and ratiometric response with a fast reaction time (less than 30 s) (Figure 12). The limit of detection for Hg²⁺ was found to be 0.38 × 10⁻⁶ M.

2.2. Rhodamine-Based Sensors for Cu²⁺

Zhang J., et al. constructed a FRET-based sensor for the detection of Cu²⁺ (Table 1, No. 17) [24], by conjugating rhodamine with a 2-(1,3,3-trimethylindolin-2-ylidene) acetaldehyde moiety. The designed sensor demonstrated colorimetric and ratiometric responses to Cu²⁺ with a detection limit of 1.168 × 10⁻⁸ M (Figure 13). The prepared sensor was successfully applied to detect Cu²⁺ by test paper strips and in real water samples.

During grape wine production, copper could disorientate volatile sulfur compounds and cause turbidity formation, thus lowering the wine quality [71]. By decorating rhodamine B with a tetra azamacrocyclic ring, a probe was created to monitor the Cu²⁺ in wine with a detection limit of 4.38 × 10⁻⁸ M (Table 1, No.18) (Figure 14) [49]. The prepared sensor responded to Cu²⁺ only in acidic conditions (< pH 5.5).

Deepa A., et al. synthesized a fluorescent and colorimetric sensor, named 6GS2, exhibiting a specific response after binding with Cu²⁺ (Table 1, No.19) [50]. The spirocyclic structure of 6GS2 transformed into a non-cyclic form in the presence of Cu²⁺ and the fluorescent peak intensity increased in the H₂O/DMSO (9/1, v/v) medium. The fluorescence intensity increments of 6GS2 induced by Cu²⁺ is attributed to the PET mechanism (Figure 15).

Hosseinjani-Pirdehi H., et al., prepared a biocompatible fluorogenic chemosensor named RHA (Table 1, No. 20) [51]. Furan-2,5-dione and rhodamine combined with an iminohydrazine crosslinker were the receptor and fluorophore of RHA, respectively. The RHA served as a pH indicator in acidic media and a sensitive sensor with high selectivity to Cu²⁺ RHA was used for bio-imaging applications in acidic and neutral pH. A novel sensor, named RhP, was synthesized by integrating the rhodamine phosphonate group (Table 1, No. 21) [52]. RhP demonstrated highly sensitive and selective recognition for Cu²⁺ and the detection limit of RhP was 15 × 10⁻⁶ M. The sensing of Cu²⁺ by RhP was achieved in less than 15 s (Figure 16).

Wang Y., et al. synthesized a naked-eye and fluorescent chemosensor, by introducing pyrazinyl hydrazine in rhodamine B, for Cu²⁺ detection (Table 1, No. 22) [53]. The sensor color changed from colorless to amaranth after Cu²⁺ addition. The proposed response mechanism of the prepared sensor (Figure 17) was extrapolated through the results of mass spectra, fluorescence spectra, and theoretical calculations.

Yang L., et al. reported a novel rhodamine-based sensor, poly-NRG, modified by picolyl groups serving as the recognizing ligand for Cu²⁺ (Table 1, No. 23) [54]. The prepared poly-NRG was further modified into gel balls for the Cu²⁺ adsorption which showed a specific reaction in the presence of Cu²⁺ with obvious color changes and achieved recycling after treatment with EDTA solution (Figure 18). Hu L., et al. reported a NIR fluorescent sensor EtRh-N-NH₂ employing rhodamine as a fluorescent core and a hydrazine group as a responsive site for Cu²⁺ (Table 1, No. 24) [27]. The sensitivity and selectivity of the prepared sensor turned out to be satisfactory with the low detection limit (6 × 10⁻⁶ M). The proposed sensing mechanism was shown in Figure 19. Furthermore, EtRh-N-NH₂ was used in the bio-imaging of Cu²⁺ in living cells.

2.3. Rhodamine-Based Sensors for Fe³⁺

Cheng Z., et al. synthesized a rhodamine-based sensor, named J1, for Fe³⁺ detection (Table 1, No. 25) [32]. The colorless sensor solution visibly turned pink in the presence of Fe³⁺, realizing the detection of Fe³⁺ in an aqueous environment. The prepared sensor was successfully used in the bio-imaging of Fe³⁺ in living cells, indicating its promising application prospects. Liu Y., et al. synthesized an N, N-dithenoyl-rhodamine-based sensor for fluorescent and colorimetric Fe³⁺ detection (Table 1, No. 26) [55]. The excellent selectivity of the prepared sensor was attributed to the coordinating functional recognition group of two thiophene carbonyl groups and the detection limit was around 3.76 × 10⁻⁹ M (Figure 20). The reported sensor also exhibited satisfactory membrane permeability and was also used for bio-imaging in living cells.

Wang K. P., et al. (Table 1, No. 27) reported a Fe³⁺-detecting fluorescence sensor, BYY, synthesized by the reaction of 2-thiopheneacetic acid and N-(rhodamine-B)lactam-ethylenediamine [56]. The BYY sensor exhibited high sensitivity toward Fe³⁺ in EtOH/H₂O (2/3, v/v, pH = 7.2) with a fast response rate. Owing to a strong affinity between Na₄P₂O₇ and Fe³⁺, BYY realized the reversible binding of Fe³⁺ (Figure 21). BYY was successfully applied for the bio-imaging of Fe³⁺ in living cells.

Wang, Y., et al. designed a simple rhodamine-based fluorescent and colorimetric sensor, L1, exhibiting a good selectivity and sensitivity to Fe³⁺ (Table 1., No. 28) [57]. The detection limit of the L1 sensor was 0.29 × 10⁻⁶ M. The sensing process, which was shown in Figure 22, was chemically reversible at least three times on the alternate addition of AcO⁻ and Fe³⁺.

Zhang M., et al. synthesized four rhodamine-based sensors through simple, high yield, and one-step Mannich reactions, and the acquired product M3 (Table 1, No. 29) exhibited a specific and sensitive response to Fe³⁺ in acetonitrile/Tris-HCl buffer solution (3:7, v/v; pH = 7.4) (Figure 23) [58]. The prepared Fe³⁺ sensor was successfully applied in bio-imaging by confocal laser scanning microscopy.

A rhodamine salicylaldehyde sensor, named RS, was constructed to modify a fluorescent PU foam named PU-RS, as a sensitive fluorescence sensor for Fe³⁺ (Table 1, No. 30) [59]. PU-RS showed ring-opened amide form and higher fluorescence intensity, due to the coordination between RS and Fe³⁺ in pure water (Figure 24). Liu X., et al. synthesized a rhodamine-based sensor (RhBUEA) by functionalizing the polyacrylamide hydrogel through free radical polymerization of RhBUEA and then constructed a fast, visual detection device for Fe³⁺ (Table 1, No. 31) [60]. The fabricated hydrogel showed good selectivity and sensitivity to Fe³⁺ in the pH range of 4.95 to 8.02 with a fast response time (less than 20 min). The prepared device was successfully used for Fe³⁺ detection in real environmental water samples.

A novel rhodamine-based sensor (RhB-EDA) was constructed by functionalizing rhodamine with the Fe³⁺ recognition group ethylenediamine (EDA) (Table 1, No. 32) [61]. The colorless and non-fluorescent RhB-EDA turned pink and showed high fluorescence in the presence of Fe³⁺ through the coordination-induced lactam ring opening mechanism (Figure 25).

Zhang J., et al. reported a rhodamine-based sensor named LXY (Table 1, No. 33) for quantitative and qualitative Fe³⁺ detection in HEPES buffer (10 mM, pH = 7.4)/CH₃CN (2:3, v/v) [62]. The detection limit of the LXY sensor for Fe³⁺ was 3.47 × 10⁻⁹ M. The prepared sensor achieved a visual detection of Fe³⁺ through the color and fluorescence change before and after the addition of Fe³⁺ (Figure 26) and was successfully used in bio-imaging of Caenorhabditis elegans, adult mice, and plant tissue.

A filter paper grafted with rhodamine B (paper-g-RhBUEA) was reported to detect Fe³⁺ via fluorescent and colorimetric assay by Liu X., et al. (Table 1, No. 34) [63]. The sensor paper-g-RhBUEA realized the on-site detection of Fe³⁺ using a self-developed smartphone mini application named “Colorimetric detector”. In addition, the fast visual detecting device paper-g-RhBUEA was successfully applied to the on-site detection of Fe³⁺ in environmental water samples. Cuc T. T. K., et al. designed and synthesized a multi-stimuli responsive rhodamine-based sensor, pseudo [3] rotaxane NI-DBC/RB-DBA (Table 1, No. 35), for the detection of Fe³⁺ [29]. The prepared mono-fluorophoric pseudo [3] rotaxane NI-DBC/RB-DBA transformed into bi-fluorophoric pseudo [3] rotaxane NI-DBC/RB-DBA-Fe³⁺ in the presence of Fe³⁺ by triggering the FRET process (Figure 27). The sensor exhibited high sensitivity to Fe³⁺ with a very low detection limit (3.90 × 10⁻⁸ M) and was applied to detect Fe³⁺ in pristine water samples.

Qiu X., et al. reported a water-soluble rhodamine-poly ethylene glycol (DRF-PEG) conjugated sensor as a dual-responsive sensor for Fe³⁺ detection (Table 1, No. 36) [28]. DRF-PEG was comprised of rhodamine 6G hydrazide and benzaldehyde-functionalized polyethylene glycol and synthesized by Schiff base reaction. The water solubility of this prepared sensor was significantly improved by introducing the PEG segment. The designed sensor was successfully used in ratiometric imaging of Fe³⁺ in living cells and Fe³⁺ detection in fetal bovine serum. The possible sensing mechanism of DRT-PEG is shown in Figure 28.

2.4. Rhodamine-Based Sensors for Other Metal Ions

A novel rhodamine-based sensor, RBTPA was proposed for the detection of Pb²⁺ by Chen H., et al. (Table 1, No. 37) [25]. This prepared sensor displayed a specific response for Pb²⁺ driven by the PET process and the proposed sensing mechanism is shown in Figure 29. Furthermore, RBPTA was successfully applied to detect Pd²⁺ in actual water samples and living cells. By test paper, RBPTA realizes the on-site and real-time quantitation of Pd²⁺.

Luo M., et al. synthesized a rhodamine-based sensor (R1) by appending indole to rhodamine B (Table 1, No. 38) [26]. The prepared sensor showed a fluorescent and colorimetric response in the addition of Pb²⁺. The detection limit of R1 for Pb²⁺ was 0.17 × 10⁻⁹ M which was suitable for the intra-cellular imaging of Pb²⁺. Xie X. et al. synthesized a rhodamine 6G hydrazide complex, (R6GH), synthesized for fluorescent and visual detection of Pb²⁺ (Table 1, No. 39) [64] which turned out to be an “off-on” output platform (Figure 30). A paper-based array was also developed by immobilizing R6GH on ordinary filter paper.

Karmegam M. V., et al. synthesized a phenothiazine–rhodamine (PTRH) sensor for Zn²⁺ sensing in solution and living cells (Table 1, No. 40) [65]. Several factors contributed to the FRET-ON mechanism for Zn²⁺ detection including the spectral overlap of the donor emission band, the phenothiazine groups in PTRH, and the absorption band, the ring-opened of the rhodamine fluorophore group. The possible binding model of PTRH with Zn²⁺ was shown in Figure 31 and the limit of detection of the proposed sensor was 2.89 × 10⁻⁸ M.

Zhang X., et al. designed and synthesized both rhodamine-B and rhodamine-based sensors, RBS and R6S (Table 1, No. 41), for Zn²⁺ detection [66] and both sensors showed good selectivity to Zn²⁺. The pink sensor solution changed to colorless after Zn²⁺ addition with “turn-on” fluorescence enhancement. RBS and R6S were all successfully applied for Zn²⁺ quantification inside mitochondria via fluorescence imaging. Paul S., et al. reported an electronically enriched rhodamine-based sensor, PBCMERI-23, via spirolactam ring opening by As³⁺ (Table 1, No. 42) [67]. The colorless and non-fluorescent PBCMERI-23 turned pink and showed bright yellow fluorescence after the addition of As³⁺ (Figure 32). PBCMERI-23 also exhibited nice cell permeability and was successfully applied to As³⁺ bio-imaging in living cells.

Kan C., et al. reported a supramolecular fluorescent sensor (RBGP) for Al³⁺ detection (Table 1, No. 43) [68]. The prepared sensor showed a fast and stable response to Al³⁺. Additionally, the response of RBGP was reversed by the addition of F⁻ (Figure 33). The prepared sensor was used in Al³⁺ bio-imaging in living cells, plant cells, and animal bodies.

RB-CR (Table 1, No. 44), conjugation between rhodamine B and phenylthiourea, was reported by Jiang T. et al. [69]. Spirolactam ring of RB-CR would open when triggered by Cr³⁺ (Figure 34). The UV-Vis absorption and fluorescence signal of RB-CR displayed pronounced enhancement.

A rhodamine-based sensor (Rh-Hyd) was reported by Rathinam B., et al. (Table 1, No. 45) [70]. The carbonyl and terminal amino groups of Rh-Hyd showed high affinity to Sn²⁺. The presence of Sn²⁺ induced spirolactam ring-opening reaction of Rh-Hyd and then changed the color and fluorescence of the prepared sensor. The proposed sensing mechanism of Rh-Hyd to Sn²⁺ was shown in Figure 35.

3. Rhodamine-Based Sensors for H⁺

It is significant to investigate dynamic pH in living cells since pH is an indicator of the body’s health and also plays a critical role in cellular processes, such as cell adhesion, cell growth, and ion transport [72,73,74]. The design inspiration for the H⁺ sensor, named Lyso-NIR-pH, was drawn from the certain correlation between heat stroke and lysosome acidity (Table 2, No. 1) [75]. Thus, the probe structured with an openable deoxylactam served as a response group to pH and the morpholine moiety acted as a recognizing unit for lysosome (Figure 36). This sensor acquired photo-bleaching and auto-fluorescence in vivo, revealed by previous similar sensors as a result of the fluorophore, Si-rhodamine with excellent photostable and NIR fluorescence.

Sensor No. 2 (Table 2) was developed for pH changes in lysosomes during induced abnormal cells, thus promoting cellular signaling responses [76]. This pH sensor exhibited excellent selectivity, sensitivity, and water solubility with a pKa value of 4.10. Remarkably, it was reused for five cycles, demonstrating strong and reproducible performances. pH probes with pKa around six were appropriate for monitoring intracellular pH changes and studying cell processes [77].

The NIR fluorescence sensor of pH, based on hydroxymethyl germanium-rhodamine, showed an optimal pKa for the successful visualization of tumors in a mouse model. The proposed sensor was ranked at the top list among similar probes (Table 2, No.3) [78]. In comparison, the three pH-triggered NIR fluorescent probes (Table 2, No.4) with different pKa values (4.4 for Probe No. 4.1, 4.6 for Probe No. 4.2, and 4.8 for Probe No. 4.3) were synthesized with rhodamine dyes and tetraphenylethene used as FRET or through-bond energy transfer (TBET) donors [79]. The ratiometric sensors caused the aggregation-quenching effect, inducing 98.9% energy transfer from TPE to rhodamine and consequently showing excellent photostability and good selectivity for pH.

Yan Y., et al. reported a BODIPY-rhodamine-based sensor, BDP-RhB to measure the variation in pH of lysosomes in macrophages during inflammation (Table 2, No. 5) [80]. Based on FRET, BDP-RhB responded to the pH range of 8.0 to 4.0 (Figure 37). Importantly, the prepared sensor exhibited good lysosome targeting ability and demonstrated a decrease in pH value during inflammation in macrophages.

Table 2

Rhodamine-based sensors for H⁺.

No.	Molecular Structure	Target	Sensing Mechanism	SensorType	Sensitive pH Range	pKa	Solution Systems	Application	Ref.
1		H+	/	Fluorescent sensor	/	4.63	/	Living cells	[75]
2		H+	/	Fluorescent sensor	pH = 4.0–7.0	4.10	/	Living cells	[76]
3		H+	/	Fluorescent sensor	/	6.24	NaPi buffer	Living cells	[78]
4		H+	TBET andFRET	Fluorescent sensor	pH = 3.0–7.4	4.4 (1)4.6 (2)4.8 (3)	/	Living cells	[79]
5		H+	FRET	Ratiometric sensor	pH = 4.0–8.0	7.1	/	Living cells	[80]
6		H+	FRET	Fluorescent andcolorimetric sensor	/	/	DMF/H2O (v/v, 1:4) (pH = 7.2)	Living cells	[81]
7		H+	PET	Fluorescent andcolorimetric sensor	pH = 1.0–7.5	2.59	THF/H2O (v/v, 1: 1)	/	[82]

A novel aggregation-induced emission target molecule was proposed by Liu Z., et al. (Table 2, No. 6) [81], acquired from rhodamine 6G hydrazine and tetrastyrene carbaldehyde through a high boiling solvent (DMF/H₂O) volatilization method. The change in pH induced spiro ring opening structure and the color change (colorless to pink) of the prepared sensor (Figure 38).

Srivastava P., et al. reported a novel pH-responsive fluorescent sensor that consisted of three units (two phenanthrene and one rhodamine B), linked by click chemistry and possessed dual excitation and three color-emitting characteristics (Table 2, No. 7) [82]. Two phenanthrene units in the prepared structure provided more choices for signal response due to the abundant optical-spectral characteristics (Figure 39). The prepared sensor (pKa = 2.59 ± 0.04) showed a specific response in the pH range of 1.0 to 7.5 in THF-water mixtures in Britton-Robinson (B-R) buffer.

4. Rhodamine-Based Sensors for Anions and Others

Many small molecule indicators for cells or body state, such as H₂S, NO, and NADH, are playing a critical role in physiological and pathological processes including neuromodulation [83,84], apoptosis [85,86], and inflammation regulation [87,88,89]. There rhodamine derivatives sensors are also used for these indicator molecules, extending the application to biological systems.

Sensor No.1 [90] and No.2 (Table 3) [91] were used to detect NO over a wide pH range (4.0–12.0), designed with a characteristic cell membrane permeability for visualizing in vivo NO. Moreover, NO sensors are characterized by a high reaction rate due to their high chemical activity in short existence [90]. A selective and sensitive cationic fluorescent probe, named ROPD (Table 3, No. 1), was prepared by Wang Q., et al. [90] by grafting a NO-trapper o-phenylenediamine on a rhodamine fluorophore at the C-3 position through the Schiff base reaction. The presence of NO would block the PET of ROPD and induce fluorescence change (Figure 40). Sensor No. 2 (Table 3) was constructed from Rhodamine-B-en and 2-(pyridin-2-ylmethoxy) benzaldehyde by Alam R., et al. [91]. The presence of NO improved the formation of nitroso-hydroxylamine via the spirolactam ring opening mechanism (Figure 41). Resultantly, an obvious intensity enhancement in absorption and emission peaks was observed.

Rhodamine-based sensors designed for the detection of 4-dihydronicotinamide adenine dinucleotide (NADH), H₂O₂ (a kind of reactive oxygen species), and adenosine triphosphate (ATP) have gained significant attention in the bioimaging area. Sensor No.3 (Table 3) was synthesized via a redox-responsive 3-aminoquinoline appended rhodamine by Podder A., et al. This sensor was a sort of reaction type fluorescent sensor and showed a remarkably increasing fluorescent intensity in the presence of NADH [92]. Apart from the common detection function, the sensor (MQR) figured out the connection between NADH levels and metabolic abnormalities of high clinical significance (Figure 42).

Gu T., et al. reported another reaction type sensor, named RhB-NIR (Table 3, No.4), to detect the H₂O₂ in living cells [93]. This sensor was triggered by a classical Baeyer-Villiger reaction. Specifically, H₂O₂ attacked the carbonyl carbon of the amide bond in the sensor structure, causing the rearrangements of α-ketoamide groups (Figure 43). The formed intermediates were successively hydrolyzed, decarboxylated, and released into the fluorophore.

Sensor No.5 (Table 3) was also a kind of reaction-type sensor used for ATP detection [94] and Sensor No. 6 (Table 3) discriminated the ATP as hydrogen bonds formed between diethylenetriamine moiety of the sensor and phosphate groups of the target [95]. Three rhodamine derivatives (probes 1, 2, 3) were constructed by Liu Y. et al. (Table 3, No. 5) [94] and probe 2 demonstrated a specific target at mitochondrial and the real-time detection of ATP in living cells. Tikum A. F., et al. synthesized a NIR-fluorescent sensor (Table 3, No. 6), revealing a turn-on fluorescent response by changing the solution color (from colorless to purple) through the interaction with ATP (Figure 44) [95].

Apart from ATP, the rhodamine-based acid phosphatase (ACP)-detection sensor was reported based on the IFE between oxidized 3,3′,5,5′-tetramethylbenzidine (oxTMB), and rhodamine B [96]. Specifically, the fluorescence of rhodamine B was quenched by oxTMB, owing to spectral overlap between the emission of rhodamine B and the absorption of oxTMB. oxTMB was reduced by ascorbic acid, generated from ACP-inducing hydrolysis of 2-phospho-L-ascorbic acid trisodium salt. Thus, the internal filtering effect between rhodamine B and oxTMB was inhibited, realizing the determination of ACP.

Sensor Nos. 8–12 (Table 3) [97,98,99,100,101] were applied for anion detections. Among these, sensor No.8 (Table 3) was a Si-rhodamine-based sensor (SiNH) that showed a nanoscale response after capturing amphiphilic copolymers named mPEG-DSPE [97]. The amphiphilic mPEG-DSPE improved the water solubility of the prepared sensor which was applied to real-time analysis of endogenous ONOO⁻ in living cells (Figure 45).

Sensors for SH⁻ and ClO⁻ are useful in the environment and biosafety. SH⁻ is a highly toxic material and harmful to the environment, humans, and animals [102,103], and ClO− is a toxic disinfection by-product of hypochlorite salts [104], often used as a disinfectant in swimming pools and other water environments [105]. SH− sensor (Table 3 No. 9) [98] and ClO− sensors (Table 3 No. 10–11) [99,100] were applied to detect environment water samples and living cells, and both were convenient for monitoring human and animal health. Wu J., et al. chose the DNBS group as a cleavable quencher of rhodamine-based fluorescence since thiophenols removed the group and recovered the rhodamine fluorescence [98]. The sensing mechanism was attributed to the nucleophilic substitution reaction of thiophenols (Figure 46) and confirmed by HPLC.

Wang Z., et al. reported the rhodamine-based fluorescent sensor, named 6G-ClO (Table 3, No. 10), developed from 2-formyl rhodamine and used for ClO⁻ detection in water and HUVEC cells [99]. Apart from the application in ClO⁻-detection in water and living cells, a test strip was prepared with 6G-ClO and successfully used for semi-quantitative detection of the same molecule (Figure 47).

Zhang D., et al. reported a ClO⁻ sensor (SRh) (Table 3, No. 11) synthesized by the conjugation of dual-binding benzene and rhodamine B [100]. SRh showed an “off-on” response to ClO⁻, visible to the naked eye (Figure 48). The detection limit was calculated to be 2.43 × 10⁻⁹ M. Wang K., et al. synthesized a turn-on NIR fluorescent sensor with rhodamine derivatives for SO₃²⁻ detection (Table 3, No. 12) [101]. The addition of SO₂³⁻ attacked the 4-position carbonyl group in levulinic acid and induced the ester bond cleavage, resulting in the formation of the fluorophore. The ICT process was then restored, and the sensor showed the corresponding NIR fluorescence (Figure 49).

5. Rhodamine-Based Sensors for Multi-Targets

Sensors No. 1, 2, and 3 (Table 4) were developed for the detection of both Hg²⁺ and Cu²⁺. Chang L. L., et al. reported a Hg²⁺ and Cu²⁺-responsive sensor, RhBH-HB-Ac, based on a completely different sensing mechanism in the absence and presence of UV irradiation (Table 4, No. 1) [106]. Without UV irradiation, this probe selectively recognized Hg²⁺ through colorimetric and fluorescent responses. Under UV irradiation, Cu²⁺ induced a significant color change in the sensor (Figure 50).

Wang K., et al. designed a novel bifunctional rhodamine-based sensor, comprising hydrazide and formylformic acid (Table 4, No. 2) [107]. The prepared sensor was used for differential detection of Cu²⁺ and Hg²⁺ by different sensing assays in CH₃CN-HEPES buffer solution (20 mM, pH 7.4) (1:9, v/v). A colorimetric change in the presence of Cu²⁺ was observed along with the fluorescent enhancement due to the coordination-induced spirolatam-ring opening process (Figure 51).

RHB (Table 4, No. 3) was a reusable spherical solid supported sensor prepared by a simple two-step route. (1) synthesis of aldehyde functional spherical crosslinked poly(vinylbenzaldehyde-co-divinylbenzene) microbeads (PVBA-DVB) via precipitation polymerization and (2) attaching the rhodamine hydrazide to aldehyde groups via imine linkages [108]. The prepared RHB sensor displayed selective turn-on fluorescence and colorimetric response to Cu²⁺ and Hg²⁺ (Figure 52).

Sensor No. 4, 5, and 6 (Table 4) were developed for the co-detection of Al³⁺ and Hg²⁺. Among these, sensor No. 4 was a chromenone-based rhodamine, synthesized by Mondal S., et al. [109], and was used for fluorometric and colorimetric recognition of Al³⁺ and Hg²⁺ in CH₃CN:H₂O (3:1, v/v, 10 mM HEPES buffer, pH = 6.85). In addition, the recognition of Al³⁺ and Hg²⁺ was discriminated by adding the tetrabutylammonium iodide salts. In the presence of I⁻, the color change was completely discharged, induced by Hg²⁺, and color was retained by Al³⁺ (Figure 53). Sensor TR (Table 4, No.5), was a rhodamine-based sensor appended with triphenylamine and developed for Al³⁺ and Hg²⁺ detection with different sensing mechanisms (Figure 54) [110]. Furthermore, TR-coated test papers were successively prepared for the rapid detection of Al³⁺ and Hg²⁺.

Hazra A., et al. synthesized a rhodamine-based sensor, named RBO (Table 4, No. 6) through the reaction of N-(rhodamine-6G)lactam-ethylenediamine and 2,1,3-benzoxadiazole-4-carbaldehyde [111] and used for selective detection of Al³⁺ and Hg²⁺ in 10 mM HEPES buffer in water: ethanol (1:9, pH = 7.4) (Figure 55). Furthermore, the induced response by Al³⁺ and Hg²⁺ was recovered with the assistance of fluoride or sulfide ions, thus opening a path for the construction of logic gates.

Chan W. C., et al. reported a novel rhodamine-based sensor, R34 (Table 4, No. 7), synthesized by the reaction of rhodamine B hydrazide and 3,4- dihydroxybenzaldehyde [112]. R34 exhibited a sensitive and selective response to Al³⁺ and Cu²⁺ and displayed significant color change from colorless to pink and colorless to orange for Al³⁺ and Cu²⁺, respectively (Figure 56). The prepared sensor was successfully used in actual water samples and living cells bio-imaging.

Zhang Z., et al. reported a rhodamine-based sensor with solvent-dependent binding properties for differential detection of Cr³⁺ and Cu²⁺ (Table 4, No. 8) [113]. The sensor depicted a turn-on fluorescence response with high selectivity and sensitivity to Cr³⁺ and Cu²⁺ in acetonitrile and ethanol solution, respectively (Figure 57). In acetonitrile medium, the acquired sensor showed different colorimetric, providing easy and differential analysis of Cr³⁺ and Cu²⁺ through naked eyes.

A sensor, with a dual function of distinguishing Al³⁺ and CN⁻, named sensor No.9 in Table 4 [114], showed strong fluorescence and pink color for Al³⁺, specifically recognizing CN⁻ with a detection limit of 0.815 × 10⁻⁸ M accompanied by reduced fluorescence intensity. The possible binding mechanism of the prepared sensor with Al³⁺ and CN⁻ is shown in Figure 58.

A rhodamine-based sensor, MF-N₃ (Table 4, No. 10), was reported by Zhu L., et al. to selectively accumulate in lysosomes and show turn-on fluorescence for both H₂S and H⁺ [115]. Owing to ingenious molecular design, MF-N₃ exhibited strong fluorescence with H₂S only in acid conditions, since endogenous H₂S is mainly produced in cells with pH in the acidic range. This sensor containing the spirolactam group was modified with 4-azidobenzyl carbamate that reacted with H₂S and then transformed to amine, enabling strong fluorescence (Figure 59).

Five rhodamine-based sensors (Table 4, No. 11–15) were designed with distinctive characters for the detection of trivalent metal ions, such as Fe³⁺, Cr³⁺, and Al³⁺. Among these, sensor No.11 (RDP), reported by Kilic H. and Bozkurt E., possessed the best selectivity and the lowest detection limit (2.50 × 10⁻⁹ M, 3.16 × 10⁻⁹ M, and 1.17 × 10⁻⁹ M for Fe³⁺, Cr³⁺ and Al³⁺, respectively). The interaction ratio between the prepared RDP and target ions was 1:2. Similarly, sensor No. 12 (Table 4) was a rhodamine 6G-benzylamine, synthesized by Das D., et al. [117]. The prepared sensor exhibited crystal structure due to only hydrocarbon skeletons in the extended part. In the presence of Fe³⁺, Cr³⁺, and Al³⁺, the sensor showed large enhancements in fluorescence intensity, thus, realizing bio-imaging in living cells.

Sensor No. 13 (Table 4), named RFMS, was constructed by rhodamine 6G functionalized mesoporous silica and served to recognize the target ions and remove them from the matrix [118]. The prepared RFMS had no absorption band in the visible region and possessed low fluorescence at 550 nm. In the presence of Fe³⁺, Cr³⁺, and Al³⁺, RFMS exhibited an absorption band at around 528 nm and the intensity was also enhanced (Figure 60). The opening of the spirolactam ring, induced by the interactions with Fe³⁺, Cr³⁺, and Al³⁺, resulted in a visible color change of RFMS solution from colorless to pink. The removal efficiency of RFMS for Fe³⁺, Cr³⁺, and Al³⁺ was up to 90%.

Roy A., et al. reported a rhodamine-based chemosensor, HL-CHO (Table 4, No. 14) for Fe³⁺, Cr³⁺, and Al³⁺ ions [119]. The fluorescence intensity of HL-CHO in HEPES buffer in methanol: water (9: 1, v/v) (pH 7.4) at 550 nm increased by 800, 588, and 1466 for Fe³⁺, Cr³⁺, and Al³⁺, respectively (Figure 61). Sensor No. 15 (Table 4), TAT-ROD was reported by Sadak A. E. and Karakuş E., showing high sensitivity and selectivity to Fe³⁺, Cr³⁺, and Al³⁺ via TBET pathway. The TAT-ROD enabled Fe³⁺, Cr³⁺, and Al³⁺ recovery in the presence of ethylenediaminetetraacetic acid (EDTA), as shown in Figure 62. Banerjee M., et al. reported a multi-signaling rhodamine-based sensor for rapid recognition of Zn²⁺, Cd²⁺, and Hg²⁺ (Table 4, No. 16) [121]. The prepared sensor displayed a specific response to Zn²⁺, Cd²⁺, and Hg²⁺ via PET-CHEF-FRET processes and was applied to detect Zn²⁺, Cd²⁺, and Hg²⁺ in sea fish and water samples.

Wang Y., et al. designed a rhodamine hydrazone (Table 4, No. 17), displaying exceptional selectivity to Cu²⁺, Co²⁺, and Al³⁺ (Figure 63) [122]. The prepared sensor exhibited fluorescence enhancement in methanol solution when Co²⁺ was added to the solution and showed a specific response to Al³⁺ in nearly pure water media. For Cu²⁺, the acquired sensor demonstrated a color change response in methanol or water solutions.

Pungut N. A. S., et al. reported a colorimetric sensor (RBOV) that displayed a sensitive and selective response to Cu²⁺, Ni²⁺, and Co²⁺ in an aqueous medium (Table 4, No. 18) [123]. The detection limits for three metal ions were 4.0 × 10⁻⁸ M, 3.2 × 10⁻⁷ M, and 4.8 × 10⁻⁷ M, respectively. The sensing mechanism of RBOV for Cu²⁺, Ni²⁺, and Co²⁺ was analyzed, demonstrating a 1:1 stoichiometry of sensor-metal complex for all target ions. Hazra A. and Roy P. synthesized a new rhodamine-based sensor (Table 4, No. 19) for the colorimetric and fluorescence detection of group 13 cations (Al³⁺, Ga³⁺, In³⁺, and Tl³⁺) in 10 mM HEPES buffer (pH = 7.4) in water/ethanol (1:9, v/v) [124]. The prepared sensor demonstrated fluorescence enhancement and color change because of spirocyclic ring opening induced by the target ions (Figure 64).

Podshibyakin V. A., et al. reported a solvent-dependent multifunctional rhodamine-based sensor for visible selective detection of Al³⁺, Cu²⁺, Hg²⁺, S²⁻ and CN⁻ (Table 4, No. 20) [125]. This prepared sensor showed selectivity to Al³⁺ in ethanol. The complexes of the sensor and Cu²⁺ and Hg²⁺ were used to detect S²⁻ and CN⁻, respectively (Figure 65).

6. Conclusions and Perspective

Rhodamine and its derivatives possess excellent optical properties: good photo-irradiation stability, high fluorescence quantum yield, and so on. Chemosensors based on rhodamine and its derivatives have attracted extensive attention in areas of chemistry material, biology, medical, and health care. These sensors play an important role as simple, fast, economic, and efficient tools for monitoring environmental quality, health, and mechanism of physiological activity disorder. The unique and convenient close-open ring structure of rhodamine derivatives before and after the addition target induce color and fluorescent changes, showing great benefit for obviously detecting target analyte. Moreover, the fine biocompatibility and near-infrared fluorescence of rhodamine derivatives make them an excellent choice for bio-sensor construction. This review has summarized the development and achievements of rhodamine-based sensors in the past four years and found that there are still spaces for improvements in sensor design and synthesis. The review is intended to bring new clues or bright ideas for further advances in rhodamine-based chemosensors.

The Schiff base reaction is the most common synthesis mechanism when designing rhodamine-based sensors. Other reactions, such as the Mannich reaction, and the Baeyer-Villiger reaction, could be considered in the future. As for the sensing mechanism, coordination between the sensor and target ions is the most common and the coordination atoms are mainly N, O, and S. Other sensing mechanisms, such as reaction-type sensing, especially reversible reaction-type sensing mechanisms, are also attractive choices.

For sensitivity and selectivity, the coordination groups containing electron-rich structures such as carbon-oxygen double bonds. Carbon-nitrogen double bonds are the primary choice when designing the sensors. Apart for sensitivity and selectivity, water solubility is the most significant property of rhodamine-based sensors, concerning the applications in environmental samples. Besides, cell membrane-permeability, cytotoxicity, and organelle localization are all necessary factors that should be taken into consideration in biological applications. In addition, probes based on fluorophore with NIR fluorescence are better due to their anti-interference capability.

As for the rhodamine-based sensor for multi-targets, there are a few strategies for selective recognition; for example, the “multi recognition group” strategy, and the “solvent-dependent sensing” strategy. The former is more reported whereas the latter is superior in sensing performance due to the advantage of interference elimination. Thus, the rhodamine-based sensors for multi-targets via the “solvent-dependent sensing” strategy need to be further developed.

Author Contributions

Writing–Review and Editing, Investigation, Y.W.; Investigation, X.W.; Investigation, W.M.; Supervision, R.L. and W.Z.; Funding Acquisition, Validation, H.G. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

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Figure 1. Chemical structures of rhodamine B and rhodamine 6G (A) and the general sensing principles of rhodamine-based sensors (B).

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Figure 2. The proposed sensing mechanism of RMTE (Table 1, No. 2) for Hg2+ [34].

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Figure 3. The possible mechanism of RDV (Table 1, No. 4) to Hg2+ [36].

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Figure 4. The proposed sensing mechanism of RBST (Table 1, No. 5) to Hg2+ [37].

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Figure 5. The possible sensing mechanism of the prepared sensor (Table 1, No. 6) to Hg2+ [38].

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Figure 6. The possible mechanism of Probe 2 (Table 1, No. 7) to Hg2+ [39].

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Figure 7. The proposed sensing mechanism of the prepared probe (Table 1, No. 8) to Hg2+ [40].

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Figure 8. The possible sensing mechanism of RhB (Table 1, No. 9) to Hg2+ [41].

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Figure 9. The proposed reaction mechanism between 1O (Table 1, No. 10) and Hg2+ [42].

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Figure 10. The proposed sensing mechanism of RD6 (Table 1, No.13) to Hg2+ [45].

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Figure 11. The proposed sensing mechanism of CCS-Hg (Table 1, No. 15) [47].

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Figure 12. The proposed sensing mechanism of the prepared sensor (Table 1, No. 16) for Hg2+ [48].

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Figure 13. The proposed sensing mechanism of the prepared sensor (Table 1, No. 17) [24].

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Figure 14. The proposed reaction mechanism of the prepared sensor (Table 1, No.18) [49].

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Figure 15. The proposed mechanism of 6GS2 (Table 1, No. 19) for sensing Cu2+ [50].

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Figure 16. The proposed detecting mechanism of RhP (Table 1, No. 21) [52].

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Figure 17. The proposed response mechanism of the prepared sensor (Table 1, No. 22) for Cu2+ [53].

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Figure 18. The proposed sensing process of NRG (Table 1, No. 23) [54].

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Figure 19. The proposed sensing mechanism of EtRh-N-NH2 (Table 1, No. 24) [27].

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Figure 20. The proposed sensing mechanism of the prepared sensor (Table 1, No. 26) to Fe3+ [55].

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Figure 21. The proposed sensing mechanism of BYY (Table 1, No. 27) for Fe3+ [56].

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Figure 22. The proposed sensing mechanism of the prepared sensor L1 (Table 1, No. 28) to Fe3+ [57].

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Figure 23. The proposed sensing mechanism of M3 (Table 1, No. 29) to Fe3+ [58].

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Figure 24. The proposed binding mechanism of RS (Table 1, No. 30) to Fe3+ [59].

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Figure 25. The proposed sensing mechanism of RhB-EDA (Table 1, No. 32) for Fe3+ [61].

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Figure 26. The proposed sensing mechanism of LXY (Table 1, No. 33) to Fe3+ [62].

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Figure 27. The possible coordination mechanism of pseudo [3]rotaxane NI-DBC/RB-DBA (Table 1, No. 35) with Fe3+ [29].

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Figure 28. The proposed coordination mechanism of DRF-PEG (Table 1, No. 35) towards Fe3+ [28].

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Figure 29. The proposed sensing mechanism of RBTPA (Table 1, No. 37) to Pb2+ [25].

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Figure 30. The possible sensing mechanism of R6GH (Table 1, No. 39) to Pb2+ [64].

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Figure 31. The proposed binding mode of PTRH (Table 1, No. 40) with Zn2+ [65].

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Figure 32. The proposed sensing mechanism of PBCMERI-23 (Table 1, No. 42) to As3+ [67].

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Figure 33. The proposed sensing mechanism of RBGP (Table 1, No. 43) to As3+ [68].

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Figure 34. The possible binding mechanism of RB-CR (Table 1, No. 44) to Cr3+ [69].

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Figure 35. The possible sensing mechanism of Rh-Hyd (Table 1, No. 45) to Sn2+ [70].

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Figure 36. The response mechanism of Lyso-NIR-pH (Table 2, No. 1) to pH in lysosome [75].

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Figure 37. The response mechanism of BDP-RhB (Table 2, No. 5) to pH [80].

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Figure 38. The concluded sensing mechanism of this prepared sensor (Table 2, No. 6) for pH [81].

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Figure 39. The possible sensing mechanism of the prepared sensor (Table 2, No. 7) for pH [82].

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Figure 40. The proposed sensing mechanism of ROPD (Table 3, No. 1) for NO [90].

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Figure 41. The proposed reaction mechanisms of the prepared sensor (Table 3, No. 2) with NO [91].

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Figure 42. The possible sensing mechanism of MQR (Table 3, No. 3) for NADH [92].

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Figure 43. The proposed sensing mechanism of RhB-NIR (Table 3, No. 4) for H2O2 [93].

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Figure 44. The proposed sensing mechanism of the prepared sensor (Table 3, No. 6) for ATP [95].

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Figure 45. The proposed sensing mechanism of SiNH (Table 3, No. 8) [97].

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Figure 46. The proposed sensing mechanism of the prepared sensor (Table 3, No. 9) for PhSH [98].

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Figure 47. The proposed mechanism of 6G-ClO (Table 3, No. 10) with ClO− [99].

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Figure 48. The proposed sensing mechanism of SRh (Table 3, No. 11) with ClO− [100].

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Figure 49. The possible reaction mechanism of the prepared sensor (Table 3, No. 12) for SO32− [101].

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Figure 50. The sensing mechanism of RhBH-HB-Ac (Table 4, No. 1) [106].

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Figure 51. Proposed binding modes of the prepared sensor (Table 4, No. 2) with Cu2+ and Hg2+ [107].

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Figure 52. The proposed sensing mechanism of RHB (Table 4, No. 3) to Cu2+ and Hg2+ [108].

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Figure 53. The sensing mechanism of the prepared sensor (Table 4, No. 4) for Al3+ and Hg2+ [109].

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Figure 54. Proposed sensing mechanism of TR (Table 4, No. 5) for Al3+ and Hg2+ [110].

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Figure 55. The possible sensing mechanism of RBO (Table 4, No. 6) for Al3+ and Hg2+ [111].

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Figure 56. The proposed mechanism for interaction between R34 (Table 4, No. 7) and Al3+/Cu2+ [112].

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Figure 57. The sensing mechanism of this prepared sensor (Table 4, No. 8) for Cr3+ and Cu2+ [113].

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Figure 58. The possible binding mechanism of the prepared sensor (Table 4, No. 9) for Al3+ and CN− [114].

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Figure 59. The sensing mechanism of MF-N3 (Table 4, No. 10) for dual-response to H2S and protons [115].

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Figure 60. The possible sensing mechanism of RFMS (Table 4, No. 13) to Fe3+, Cr3+ and Al3+ [118].

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Figure 61. The proposed sensing mechanism of HL-CHO to Fe3+, Cr3+ and Al3+ [119].

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Figure 62. The proposed binding mechanism of TAT-ROD (Table 4, No. 15) to Fe3+, Cr3+ and Al3+ [120].

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Figure 63. The proposed reaction mechanism of the prepared sensor (Table 4, No.17) with Cu2+, Co2+, and Al3+ [122].

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Figure 64. The possible sensing mechanism of the prepared sensor (Table 4, No.19) to Al3+, Ga3+, In3+ and Tl3+ [124].

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Figure 65. The proposed mechanism of coordination between the prepared sensor (Table 4, No. 20) and the target ions [125].

Table 1

Rhodamine-based sensors for metal ions.

No.	Molecular Structure	Target	Sensing Mechanism	Sensor Type	Detection Limit(M)	Bonding Ratio *	Solution Systems	Application	Ref.
1	[Image omitted. Please see PDF.]	Hg2+	/	Colorimetric and fluorescent probe	7.48 × 10−9	1:2	100% H2O	Test strip	[33]
2	[Image omitted. Please see PDF.]	Hg2+	/	Colorimetric sensor	/	1:1	CH3CN/H2O(1/99, v/v)HEPES buffer(pH = 7.05)	Actual water samples, living cells	[34]
3	[Image omitted. Please see PDF.]	Hg2+	/	Fluorescent andcolorimetric sensor	8.73 × 10−7	/	DMF	Living cells	[35]
4	[Image omitted. Please see PDF.]	Hg2+	/	Fluorescent andcolorimetric sensor	1.36 × 10−7	1:1	CH3CN/H2O (3/7, v/v)	Actual water samples and test paper	[36]
5	[Image omitted. Please see PDF.]	Hg2+	/	Fluorescent andcolorimetric sensor	0.11 × 10−9	1:1	EtOH/H2O (2/1, v/v)	Actual water samples, MCF-7 cells, zebrafish and soybean rhizome tissues	[37]
6	[Image omitted. Please see PDF.]	Hg2+	/	Fluorescent sensor	/	1:1	/	Living cells	[38]
7	[Image omitted. Please see PDF.]	Hg2+	/	Fluorescent andcolorimetric sensor	1.8 × 10−81.6 × 10−85.6 × 10−8	1:1	Tris-HCl/C2H5OH(3/7, v/v,pH = 7.2)	Living cells	[39]
8	[Image omitted. Please see PDF.]	Hg2+		Fluorescent sensor	/	1:1	C2H5OH/PBS buffer(2/8, v/v,pH = 7.4)	Living cells	[40]
9	[Image omitted. Please see PDF.]	Hg2+	/	Fluoric-metric sensor	2.57 × 10−6	1:1	EtOH/HEPES buffer(1:1, v/v)	Test paler and living cells	[41]
10	[Image omitted. Please see PDF.]	Hg2+	FRET	Fluorescent andcolorimetric sensor	2.46 × 10−7	1:1	/	Water sample	[42]
11	[Image omitted. Please see PDF.]	Hg2+	/	Fluorescent sensor	0.27 μg/L	1:1	/	Water sample	[43]
12	[Image omitted. Please see PDF.]	Hg2+	/	Fluorescent andcolorimetric sensor	3.4 × 10−9	1:1	HEPES/DMF (1/1, v/v)	Living cells	[44]
13	[Image omitted. Please see PDF.]	Hg2+	/	Fluorescent andcolorimetric sensor	1.2 × 10−8	1:1	CH3CN/H2O (9:1v/v)	Test strip	[45]
14	[Image omitted. Please see PDF.]	Hg2+	/	Fluorescent andcolorimetric sensor	9.18 × 10−7	1:1	/	Tap water,colorimetric card	[46]
15	[Image omitted. Please see PDF.]	Hg2+	/	Fluorescentsensor	/	2:1	H2O/EtOH (3/2, v/v,pH = 7.3);	Living cells	[47]
16	[Image omitted. Please see PDF.]	Hg2+	FRET	Colorimetric and ratiometric fluorescent sensor	0.81 × 10−6	1:1	CH3CN/H2O(1/1, v/v)	Living cells	[48]
17	[Image omitted. Please see PDF.]	Cu2+	FRET	Colorimetric and ratiometric probe	1.168 × 10−8	1:1	CH3CN/HEPES buffer(4/1, v/v)(pH = 7.3)	Actual water samples	[24]
18	[Image omitted. Please see PDF.]	Cu2+	/	Fluorescent sensor	4.38 × 10−8	1:1	H2O	White wine samples	[49]
19	[Image omitted. Please see PDF.]	Cu2+	PET	Fluorescent andcolorimetric sensor	7.4 × 10−8	1:1	H2O/DMSO(9/1, v/v)	Test paper	[50]
20	[Image omitted. Please see PDF.]	Cu2+	/	Fluorescent andcolorimetric sensor	0.69 × 10−8	1:2	Acetone/H2O(1/2, v/v)	Living cells	[51]
21	[Image omitted. Please see PDF.]	Cu2+	/	Fluorescent andcolorimetric sensor	/	1:1	HEPES/CH3CN(4/1, v/v)(pH = 7.0)	Living cells	[52]
22	[Image omitted. Please see PDF.]	Cu2+	/	Fluorescent andcolorimetric sensor	8.99 × 10−8	1:2	CH3CN/H2O(1/2, v/v)(pH = 7.2, 50 mM tris–HCl)	HepG2 living cells	[53]
23	[Image omitted. Please see PDF.]	Cu2+	/	Fluorescent andcolorimetric sensor	3.4 × 10−8	1:2	CH3OH/H2O (4:1, v/v)	Gel balls	[54]
24	[Image omitted. Please see PDF.]	Cu2+	ICT	Fluorescent sensor	6 × 10−6	2:1	PBS/EtOH(1/1, v/v)	/	[27]
25	[Image omitted. Please see PDF.]	Fe3+	/	Colorimetric sensor	2.54 × 10−8	1:1	EtOH/H2O(5/5, v/v)Tris–HCl(pH = 7.4)	Living cells	[32]
26	[Image omitted. Please see PDF.]	Fe3+	/	Fluorescent andcolorimetric sensor	3.76 × 10−9	1:1	MeCN/H2O(1/1, v/v)(pH = 7.2)	Living cells	[55]
27	[Image omitted. Please see PDF.]	Fe3+	/	Fluorescent andcolorimetric sensor	2.7 × 10−7	2:1	EtOH/H2O (2/3, v/v)(pH = 7.2)	Living cells	[56]
28	[Image omitted. Please see PDF.]	Fe3+	/	Fluorescent andcolorimetric sensor	7.4 × 10−6	2:1	CH3CN/Tris–HClbuffer(9/1, v/v;pH = 7.00)	Actual water samples	[57]
29	[Image omitted. Please see PDF.]	Fe3+	/	Fluorescentsensor	5.2 × 10−6	1:1	Acetonitrile/Tris-HCl buffer(3/7, v/v;pH = 7.4)	Living cells	[58]
30	[Image omitted. Please see PDF.]	Fe3+	/	Fluorescentsensor	1.64 × 10−6	1:1	Pure water	Water	[59]
31	[Image omitted. Please see PDF.]	Fe3+	/	Colorimetric sensor	0.88 × 10−7	/	Polyacrylamide	Water	[60]
32	[Image omitted. Please see PDF.]	Fe3+	/	Fluorescent andcolorimetric sensor	/	1:1	/	/	[61]
33	[Image omitted. Please see PDF.]	Fe3+	/	Fluorescentsensor	3.47 × 10−9	/	HEPES/CH3CN(2:3, v/v)(10 mM,pH = 7.4)	Caenorhabditis elegans, adult mice, plant tissue	[62]
34	[Image omitted. Please see PDF.]	Fe3+	/	Fluorescent andcolorimetric sensor	0.19 × 10−9	/	Rhodamine-grafted paper	Environmental water	[63]
35	[Image omitted. Please see PDF.]	Fe3+	FRET	Fluorescentsensor	3.90 × 10−8	/	Dioxane/H2O(4:6, v/v)	Paper test strips,water samples	[29]
36	[Image omitted. Please see PDF.]	Fe3+	ICT	Fluorescent andcolorimetric sensor	3.16 × 10−6	1:2	H2O(pH = 6.5)	Living cells,fetal bovine serum samples	[28]
37	[Image omitted. Please see PDF.]	Pb2+	PET	Colorimetric sensor	4.2 × 10−9	1:1	EtOH/H2O (3/2, v/v)HEPES Buffer(pH = 7.4)	Living cells,test paper	[25]
38	[Image omitted. Please see PDF.]	Pb2+	PET	Fluorescent andcolorimetric sensor	0.17 × 10−9	1:1	CH3OH/PBS buffer(1/1, v/v)pH = 7.40)	Living cells	[26]
39	[Image omitted. Please see PDF.]	Pb2+	/	Fluorescent andcolorimetric sensor	2.5 × 10−6	1:1	/	Tap water, soil, fish, shrimp	[64]
40	[Image omitted. Please see PDF.]	Zn2+	FRET	Fluorescentsensor	2.89 × 10−8	1:1	H2O/Acetonitrile(8/2, v/v)	Living cells	[65]
41	[Image omitted. Please see PDF.]	Zn2+	ICT	Fluorescentsensor	1.8–1.9 × 10−9	/	50 mM,HEPES buffer, pH = 7.54	Living cells,mitochondria	[66]
42	[Image omitted. Please see PDF.]	As3+	/	Fluorescent andcolorimetric sensor	0.164 μg/L	/	Acetonitrile/HEPES buffer(4/1, v/v,pH = 7.4)	Living cells	[67]
43	[Image omitted. Please see PDF.]	Al3+	/	Fluorescentsensor	0.146 × 10−6	1:1	/	Living cells, plant cells and living bodies	[68]
44	[Image omitted. Please see PDF.]	Cr3+	/	Fluorescentsensor	/	1:1	/	Living cells	[69]
45	[Image omitted. Please see PDF.]	Sn2+	ICT	Fluorescent andcolorimetric sensor	1.62 × 10−7	1:1	EtOH/H2O (8/2, v/v)	Living cells	[70]

*: The bonding ratio is between probe and target.

Table 3

Rhodamine-based sensors for anions and others.

No.	Molecular Structure	Target	Probe Type	Sensing Mechanism	Sensor Type	Solution Systems	Application	Ref.
1	[Image omitted. Please see PDF.]	NO	Turn-on	PET blocked	Fluorescent probe	PBS buffers(pH = 7.4)	Living cells	[90]
2	[Image omitted. Please see PDF.]	NO	Turn-on	/	Fluorescent probe	HEPES buffers(pH = 7.4)	Living cells and zebrafish	[91]
3	[Image omitted. Please see PDF.]	NADH	Turn-on	/	Fluorescent probe	PBS buffers(pH = 7.4)	Living cells	[92]
4	[Image omitted. Please see PDF.]	H2O2	Turn-on	PET blocked	Fluorescent sensor	PBS with 0.1% DMSO(pH = 7.4,)	Living cells	[93]
5	[Image omitted. Please see PDF.]	ATP	Turn-on	/	Fluorescent andcolorimetric sensor	PBS solution(containing 40% DMSO)	Living cells	[94]
6	[Image omitted. Please see PDF.]	ATP	Turn-on	/	Fluorescent andcolorimetric sensor	EtOH/PBS buffer(1/99, v/v;pH = 7.4)	Living lysosomes	[95]
7	[Image omitted. Please see PDF.]	ACP	Turn-on	IFE	Fluorescent andcolorimetric sensor	/	Human serum	[96]
8	[Image omitted. Please see PDF.]	Peroxynitrite (ONOO−)	Turn-on	PET blocked	Fluorescent probe	H2O/CH3CN (7:3, v/v)HEPES buffers(pH = 7.4)	Living cells	[97]
9	[Image omitted. Please see PDF.]	Thiophenols(-SH)	Turn-on	PET blocked	Fluorescent probe	DMSO/HEPES solution(1:1, v/v)(pH = 7.4)	Water andliving cells	[98]
10	[Image omitted. Please see PDF.]	ClO−	Turn-on	/	Fluorescent andcolorimetric sensor	C2H5OH/H2O(1:1, v/v)	Living lysosomes and real water samples	[99]
11	[Image omitted. Please see PDF.]	ClO−	Turn-on	/	Fluorescent andcolorimetric sensor	CH3CH2OH-Tris (7/3, v/v,pH = 7.4)	Living cells	[100]
12	[Image omitted. Please see PDF.]	SO32−	Turn-on	ICT	Fluorescent probe	PBS buffer(10 mM, pH 7.4, 1% DMSO)	Living cells and zebrafish	[101]

Table 4

Rhodamine-based sensors for multi-targets.

No.	Molecular Structure	Target	Sensing Mechanism	Sensor Type	LOD	Solution Systems	Application	Ref.
1	[Image omitted. Please see PDF.]	Hg2+Cu2+	/	Colorimetric and fluorescent probe	7.1 × 10−8 1.5 × 10−7	IPA/H2O (8/1, v/v)(Iso-Propyl Alcohol)	/	[106]
2	[Image omitted. Please see PDF.]	Cu2+Hg2+	/	Fluorescent andcolorimetric sensor	/	CH3CN/HEPES buffer (1/9, v/v;pH = 7.4)	/	[107]
3	[Image omitted. Please see PDF.]	Cu2+Hg2+	/	Fluorescent andcolorimetric sensor	5.06 × 10−73.90 × 10−7	CH3CN/H2O (8/2, v/v)(pH = 7.2)	/	[108]
4	[Image omitted. Please see PDF.]	Hg2+Al3+	FRET	Fluorescent andcolorimetric sensor	1.26 × 10−71.9 × 10−7	MeCN/H2O(3/1, v/v)HEPES buffer(pH = 6.85)	/	[109]
5	[Image omitted. Please see PDF.]	Hg2+Al3+	FRETPET	Fluorescent probe	0.48 × 10−67.18 × 10−8	MeCN/H2O (9/1, v/v)	Test paper	[110]
6	[Image omitted. Please see PDF.]	Hg2+Al3+	/	Fluorescentsensor	1.60 × 10−56.54 × 10−6	H2O/EtOH (1:9, v/v)(pH = 7.4)	River water,paper test	[111]
7	[Image omitted. Please see PDF.]	Cu2+Al3+	/	Fluorescent andcolorimetric sensor	9.90 × 10−95.63 × 10−9	MeCN/H2O(6:4, v/v)	Living cells,natural mineral water,tap water	[112]
8	[Image omitted. Please see PDF.]	Cr3+Cu2+	/	Colorimetric sensor	4.7 × 10−87.6 × 10−8	CH3CN solution Ethanol solution	/	[113]
9	[Image omitted. Please see PDF.]	Al3+CN−	PET blocked	Fluorescent andcolorimetric sensor	1.68 × 10−90.82 × 10−9	HEPES buffer (pH = 7.04)	Living cells	[114]
10	[Image omitted. Please see PDF.]	H+H2S	ICT (Intramolecular charge transfer)	Fluorescent probe		/	Living cells	[115]
11	[Image omitted. Please see PDF.]	Cr3+Al3+Fe3+	/	Fluorescent andcolorimetric sensor	3.16 × 10−91.17 × 10−92.50 × 10−9	H2O/EtOH(1/1, v/v)	/	[116]
12	[Image omitted. Please see PDF.]	Cr3+Al3+Fe3+	/	Colorimetric sensor	2.28 × 10−61.28 × 10−61.34 × 10−6	H2O/CH3CN (4/1, v/v, pH = 7.2)	Living cells	[117]
13	[Image omitted. Please see PDF.]	Cr3+Al3+Fe3+	/	Fluorescent andcolorimetric sensor	9.18 × 10−95.40 × 10−97.23 × 10−7	H2O/EtOH(14/1, v/v)	/	[118]
14	[Image omitted. Please see PDF.]	Cr3+Al3+Fe3+	/	Fluorescent andcolorimetric sensor	1.58 × 10−86.97 × 10−91.4 × 10−8	CH3OH/H2O(9/1, v/v)HEPES buffer(pH = 7.4)	Living cells	[119]
15	[Image omitted. Please see PDF.]	Cr3+Al3+Fe3+	TBET	Ratiometric fluorescent and colorimetric sensor	1.70 × 10−42.3 × 10−52.5 × 10−5	MeOH	Test strips	[120]
16	[Image omitted. Please see PDF.]	Zn2+Cd2+Hg2+	PET-CHEFPET-CHEFPET-FRET	Fluorescent andcolorimetric sensor	1.2 × 10−99.6 × 10−91.5 × 10−10	MeOH/H2O, (4/1, v/v)HEPES buffer(pH = 7.4)	Sea fish and water samples	[121]
17	[Image omitted. Please see PDF.]	Cu2+Co2+Al3+	/	Fluorescent sensor	8.68 × 10−8/1.06 × 10−9	CH3OHCH3OH/H2OH2O	Living cells	[122]
18	[Image omitted. Please see PDF.]	Cu2+Ni2+ Co2+	/	Colorimetric sensor	4.0 × 10−83.2 × 10−74.8 × 10−7	DMSO/TN(2:1, v/v) (pH = 7.5)	Test strips	[123]
19	[Image omitted. Please see PDF.]	Al3+Ga3+In3+Tl3+	/	Fluorescent andcolorimetric sensor	2.66 × 10−81.04 × 10−78.19 × 10−83.10 × 10−8	10 mM HEPES buffer (pH = 7.4) in water/ethanol (1:9, v/v)	River water	[124]
20	[Image omitted. Please see PDF.]	Al3+Cu2+ Hg2+ S2−CN−	ICT	Fluorescent andcolorimetric sensor	2.1 × 10−73.2 × 10−74.0 × 10−7//	CH3CN (5.0 × 10−5 mol/L)	Logic gate	[125]

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