The construction of dual-emissive ratiometric fluorescent probes based on fluorescent nanoparticles for the detection of metal ions and small molecules

With the rapid development of fluorescent nanoparticles (FNPs), such as CDs, QDs, and MOFs, the construction of FNP-based probes has played a key role in improving chemical sensors. Ratiometric fluorescent probes exhibit distinct advantages, such as resistance to environmental interference and achieving visualization. Thus, FNP-based dual-emission ratiometric fluorescent probes (DRFPs) have rapidly developed in the field of metal ion and small molecule detection in the past few years. In this review, firstly we introduce the fluorescence sensing mechanisms; then, we focus on the strategies for the fabrication of DRFPs, including hybrid FNPs, single FNPs with intrinsic dual emission and target-induced new emission, and DRFPs based on auxiliary nanoparticles. In the section on hybrid FNPs, methods to assemble two types of FNPs, such as chemical bonding, electrostatic interaction, core satellite or core-shell structures, coordination, and encapsulation, are introduced. In the section on single FNPs with intrinsic dual emission, methods for the design of dual-emission CDs, QDs, and MOFs are discussed. Regarding target-induced new emission, sensitization, coordination, hydrogen bonding, and chemical reaction induced new emissions are discussed. Furthermore, in the section on DRFPs based on auxiliary nanoparticles, auxiliary nanomaterials with the inner filter effect and enzyme mimicking activity are discussed. Finally, the existing challenges and an outlook on the future of DRFP are presented. We sincerely hope that this review will contribute to the quick understanding and exploration of DRFPs by researchers.

A ratiometric fluorescent paper sensor based on dye-embedded MOF for high-sensitive detection of arginine

Ratiometric fluorescent sensors can suppress the interference of factors unrelated to analysis due to their built-in self-calibration characteristics, which exhibit higher sensitivity and more obvious visual detection in the process of qualitative and quantitative analysis. Herein, we constructed a ratiometric fluorescence probe based on fluorescent/colorimetric dual-mode method for the determination of arginine by encapsulating rhodamine B in-situ into UiO-66-NH2 MOFs (UiO-66-NH2@RhB). The as-prepared probe showed dual-emission characteristics under a single excitation wavelength. The fluorescence intensity of UiO-66-NH2 was increased significantly by arginine, while the emission peak intensity of rhodamine B remained stable, resulting in a single-signal response with fixed reference. Furthermore, the practicality of the presented sensor was successfully validated by quantitative detection of arginine in human serum. More significantly, paper-based sensors for arginine detection were devised by using carboxymethyl cellulose modified filter papers. Under the irradiation of ultraviolet light, the paper-based sensors would produce obvious color variation from lightpink to bluish violet. This work provided a convenient and efficient method for on-site detection of arginine.

Visible light irradiated photocatalytic C(sp3)-H phosphorylation of xanthenes and 9,10-dihydroacridines with P(O)-H compounds

Photocatalytic C(sp3)-H phosphorylation of xanthenes and 9,10-dihydroacridines with P(O)-H compounds under the irradiation of 18 W blue LEDs at room temperature using fluorescein as the photocatalyst and molecular oxygen (O2) as the sole oxidant has been achieved. The newly developed reaction provides direct access to 9-phosphorylated xanthene derivatives with good functional group compatibility.

A Piperazine Linked Rhodamine-BODIPY FRET-based Fluorescent Sensor for Highly Selective Pd2+ and Biothiol Detection

A class of rhodamine-based fluorescent sensors for the selective and sensitive detection of Pd2+ metal ions in aqueous media has been developed. A rhodamine-based sensor PMS and a rhodamine-BODIPY Förster resonance energy transfer (FRET)-pair sensor PRS have been incorporated with a piperazine linker and an O-N-S-N podand ligand for specific recognition of Pd2+ ion. Both probes displayed colorimetric and fluorescent ratiometric changes when exposed to Pd2+ , due to their spirolactam rings opening and restoring rhodamine conjugation. PRS is highly selective to Pd2+ over 22 other metal ions, showing a 0.6-fold ratiometric difference at I600nm /I515nm . Additionally, the lactam ring in Pd2+ coordinated PRS-Pd could be switched back to the closed form in the presence of various thiols, providing a "red-green traffic light" detection mechanism between red and green emission. Furthermore, PRS showed excellent cell viability and was successfully employed to image Pd2+ and the PRS-Pd complex ensemble could interchangeably detect biothiols including glutathione (GSH) in A549 human lung cancer cells.

A New Boron-Rhodamine-Containing Carboxylic Acid as a Sugar Chemosensor

We propose a boron-rhodamine-containing carboxylic acid (BRhoC) substance as a new sugar chemosensor. BRhoC was obtained by the Friedel-Crafts reaction of 4-formylbenzoic acid and N,N-dimethylphenylboronic acid, followed by chloranil oxidation. In an aqueous buffer solution at pH 7.4, BRhoC exhibited an absorption maximum (Abs_max) at 621 nm. Its molar absorption coefficient at Abs_max was calculated to be 1.4 × 10⁵ M^-1 cm^-1, and it exhibited an emission maximum (Em_max) at 644 nm for the excitation at 621 nm. The quantum yield of BRhoC in CH₃OH was calculated to be 0.16. The borinate group of BRhoC reacted with a diol moiety of sugar to form a cyclic ester, which induced a change in the absorbance and fluorescence spectra. An increase in the D-fructose (Fru) concentration resulted in the red shift of the Abs_max (621 nm without sugar and 637 nm with 100 mM Fru) and Em_max (644 nm without sugar and 658 nm with 100 mM Fru) peaks. From the curve fitting of the plots of the fluorescence intensity ratio at 644 nm and 658 nm, the binding constants (K) were determined to be 2.3 × 10² M^-1 and 3.1 M^-1 for Fru and D-glucose, respectively. The sugar-binding ability and presence of a carboxyl group render BRhoC a suitable building block for the fabrication of highly advanced chemosensors.

A novel fluorescent "OFF-ON" sensing strategy for Hg (II) in water based on functionalized gold nanoparticles

Mercury ion (Hg²⁺) is a heavy metal pollutant that can affect the safety of water environment and endanger human health. A novel detection strategy (GNPs-_L-Cys-Rh6G2) for Hg²⁺ based on a fluorescence "OFF-ON" was proposed. Gold nanoparticles (GNPs) were assembled with _l-cysteine (_L-Cys), which was used as a "bridge" to link with rhodamine 6G derivatives (Rh6G2). The fluorescence state transition of GNPs-_L-Cys-Rh6G2 switching from "OFF"-"ON" was observed because Hg²⁺ opened the spirolactam ring through a catalytic hydrolysis mechanism. The fluorescence signal of the GNPs-_L-Cys-Rh6G2 system mixed with Hg²⁺ in the concentration range of 10-100 μM was analyzed and determined with a limit of detection (LOD) of 2 μM (S/N = 3). Moreover, the spiked Hg²⁺ concentration in real water samples were successfully quantified by GNPs-_L-Cys-Rh6G2, which was in line with the ideal average recovery rate and relative standard deviation. The proposed strategy exhibited high specificity, sensitivity and stability, providing a novel sensing platform for heavy metal ions detection in water environment.

A Review on Recent Progress in Organic Fluorimetric and Colorimetric Chemosensors for the Detection of Cr3+/6+Ions

Chromium occurs in the environment primarily in two valence states, trivalent Cr3+ and hexavalent Cr6+, which have different physicochemical and biochemical properties. However, the higher concentration of Cr3+/6+ can cause various adverse effects on human health. Therefore, detecting Cr3+/6+ ions is important in various samples. Colorimetric and fluorescent chemosensors are the most powerful tools for the detection of Cr3+/6+ ions. These chemosensors have excellent bioimaging capability and significant sensitivity and selectivity. In this article, different colorimetric and fluorescent chemosensors based on organic compounds, including Schiff base, antipyrine, diarylethene, pyrene, crown ether, dansyl, pyridine, thiazole, coumarin, boradiazaindacene, rhodamine, imidazole, hydrazone, and other functional groups for detection of Cr3+/6+ ions have been reviewed, classified them according to different fluorophore and recognition mode. I hope this article will help the readers for the future design of highly effective, sensitive, and selective chemosensors for the detection and determination of Cr3+/6+ ions.

Rh(III)-Catalyzed spiroannulation of ketimines with cyclopropenones via sequential C-H/C-C bond activation

An unprecedented Rh(III)-catalyzed [3+3]-spiroannulation of ketimines with cyclopropenones to access spiro[4,5]dienones has been developed. Sequential C-H/C-C bond activation and subsequent nucleophilic addition are disclosed in this process. This procedure represents the first example of the construction of spirolactams utilising cyclopropenones as 3C synthons. The remarkable advantages of this protocol are excellent chemo- and regio-selectivity, wide functional group tolerance, high reaction yields, and tolerance towards H₂O.

Electrochemical benzylic C-H arylation of xanthenes and thioxanthenes without catalyst and oxidant

A catalyst-free and oxidant-free C-H arylation of xanthenes and thioxanthenes using electrochemistry has been developed, which affords a number of cross-coupling products in moderate to good yields. This method is...

A tracer-type fluorescent probe for imaging adenosine triphosphate under the stresses of hydrogen sulfide and hydrogen peroxide in living cells

Adenosine triphosphate (ATP) is a direct energy source in cells and the core of the biochemical system, and is closely related to various metabolic activities in living organisms.

Novel Benzothiazole-Based Highly Selective Ratiometric Fluorescent Turn-On Sensors for Zn2+ and Colorimetric Chemosensors for Zn2+, Cu2+, and Ni2+ Ions

Metal ions play a very important role in environmental as well as biological fields. The detection of specific metal ions at a minute level caught much attention, and hence, several probes are available in the literature. Even though benzothiazole-based molecules have a special place in the medicinal field, only very few chemosensors are reported based on this moiety. The current work describes the design and synthesis of the benzothiazole-based chemosensor for a highly selective and sensitive detection of biologically important metal ions such as Zn²⁺, Cu²⁺, and Ni²⁺. The sensing studies of compound-1 showed a ratiometric as well as colorimetric response toward Zn²⁺, Cu²⁺, and Ni²⁺ ions and color changes from colorless to yellow and is found to be insensitive toward various metal ions (Cd²⁺, Cr³⁺, Mn²⁺, Pb²⁺, Ba²⁺, Al³⁺, Ca²⁺, Fe²⁺, Fe³⁺, Mg²⁺, K⁺, and Na⁺). Further, compound-1 exhibited ratiometric as well as turn-on-enhanced fluorescence response toward Zn²⁺ ions and turn off response for Cu²⁺ and Ni²⁺ ions. The Job plots revealed that the binding stoichiometry of compound-1 and metal ions is 2:1. The detection limits were found to be 0.25 ppm for Zn²⁺, while it was 0.30 ppm and 0.34 ppm for Ni²⁺ and Cu²⁺, respectively. In addition, density functional theory results strongly support the colorimetric response of metals, and the reversibility studies suggested that compound-1 can be used as a powerful chemosensor for the detection of Zn²⁺, Cu²⁺, and Ni²⁺ ions. The bioimaging data illustrated that compound-1 is a very effective ratiometric sensor for Zn²⁺ ions in live cells.

Bicolour fluorescent molecular sensors for cations: design and experimental validation

Molecular entities whose fluorescence spectra are different when they bind metal cations are termed bicolour fluorescent molecular sensors. The basic design criteria of this kind of compound are presented and the different fluorescent responses are discussed in terms of their chemical behaviour and electronic features. These latter elements include intramolecular charge transfer (ICT), formation of intramolecular and intermolecular excimer/exciplex complexes and Förster resonance energy transfer (FRET). Changes in the electronic properties of the fluorophore based on the decoupling between its constitutive units upon metal binding are also discussed. The possibility of generating fluorescent bicolour indicators that can capture metal cations in the gas phase and at solid-gas interfaces is also discussed.

Recent Advances in Activatable Organic Photosensitizers for Specific Photodynamic Therapy

Photodynamic therapy is an alternative modality for the therapy of diseases such as cancer in a minimally invasive manner. The essential photosensitizer, which acts as a catalyst when absorbing light, converts oxygen into cytotoxic reactive oxygen species that ablate malignant cells through apoptosis and/or necrosis, destroy tumor microvasculature, and stimulate immunity. An activatable photosensitizer whose photoactivity could be turned on by a specific disease biomarker is capable of distinguishing healthy cells from diseased cells, thereby reducing off-target photodamage. In this Minireview, we highlight progress in activatable organic photosensitizers over the past five years, including: (i) biorthogonal activatable BODIPYs; (ii) activatable Se-rhodamine with single-cell resolution; (iii) silicon phthalocyanine targeting oxygen tension; (iv) general D-π-A scaffolds; and (v) AIEgens. The potential challenges and opportunities for developing new types of activatable organic photosensitizers to overcome the hypoxia dilemmas of photodynamic therapy are discussed.

Novel PET-pperated rosamine pH-sensor dyes with substitution pattern-tunable pKa values and temperature sensitivity

We present the synthesis and characterization of a family of regioisomerically pure pH-sensitive rosamine fluorophores consisting of xanthene fluorophore cores and differently substituted phenol moieties.

FRET based ratiometric switch for selective sensing of Al3+ with bio-imaging in human peripheral blood mononuclear cells

FRET based ratiometric switch for selective sensing of Al³⁺ with bio-imaging in human peripheral blood mononuclear cells (PBMCs).

Through bond energy transfer (TBET)-operated fluoride ion sensing via spirolactam ring opening of a coumarin–fluorescein bichromophoric dyad

The detection of fluoride ions in a competitive environment often poses several challenges. In this work, we have designed and synthesized a coumarin functionalized fluorescein dyad (R3) which represents an ideal through bond energy transfer (TBET) fluorophore with the coumarin unit as donor and fluorescein unit as acceptor. The bichromophoric dyad demonstrates the detection of fluoride ions in the parts per billion (ppb) concentration level (22.8 ppb) with high selectivity via a TBET emission signal at 548 nm with a diagnostic bright yellow colour fluorescence output. Based on UV-visible, fluorescence, ¹H NMR and DFT studies, it is shown that the fluoride ion induces the opening of the spirolactam ring of the fluorescein moiety and provides a π-conjugation link between the donor and acceptor units enabling a TBET phenomenon with a larger pseudo-Stokes shift of 172 nm. To the best of our knowledge, this is the first report where the fluoride ion is detected via a TBET signal between the coumarin and fluorescein units in a bichromophoric dyad.

A coumarin–fluorescein based bichromophoric dyad detects fluoride ions in parts per billion concentration level via a TBET emission signal at 548 nm with a diagnostic bright yellow fluorescence.

Synthesis of Pyridyl and N-Methylpyridinium Analogues of Rosamines: Relevance of Solvent and Charge on Their Photophysical Properties

A series of pyridyl analogues of rosamines was prepared by employing two methodologies: (i) the conventional-heating condensation of a pyridinecarboxaldehyde with 3-(diethylamino)phenol in propionic acid, and (ii) the novel ohmic-heating assisted condensation under "on water" conditions, followed by oxidation. The 4-pyridyl substituted rosamine was further converted into the N-methylpyridinium derivative through N-alkylation using methyl iodide. The influence of the position and cationization of the nitrogen atom of the pyridyl ring in the physicochemical properties of fluorophores was investigated by ¹ H, ¹³ C, ¹⁵ N NMR spectral analysis, UV/Vis and fluorescence spectroscopy, single-crystal X-ray diffraction (4-pyridyl and N-methylpyridinium derivatives) and thermal-behavior analysis. Curiously, for ethanolic solutions of 4-pyridyl and N-methylpyridinium derivatives an extinction of color and fluorescence over time was observed. This phenomenon was further studied and the data revealed that it is the result of nucleophilic addition of ethoxide ion to the central 9-position of the xanthene. The kinetics of the process is slower for the 4-pyridyl rosamine, which emphasizes the importance of the charge in the N-methylpyridinium analogue in the reactivity of the molecule towards a nucleophile agent. This phenomenon is reversible, meaning that the compounds can be rapidly recovered by decreasing the pH, opening new avenues in the sensing applications of this class of rosamines.

Novel fluorescein- and pyridine-conjugated schiff base probes for the recyclable real-time determination of Ce3+ and F. Methods Appl Fluoresc 2019;8:015002. [PMID: 31622961 DOI: 10.1088/2050-6120/ab4ee7]

The effect of aminopyridines substituted at different positions on the fluorescence properties deserves to be studied. Since 2-aminopyridyl-based probes have been reported, the effects of 3-aminopyridine and 4-aminopyridine on the performance of fluorescein probes were discussed in here. Two Schiff base fluorescein probes FN-1, FN-2 were designed and synthesized. Among them, since the ligand shows a highly selective and sensitive response to metal charge transfer (LMCT), the fluorescence of FN-1 can be quenched by Ce³⁺ ions in PBS buffer. At the same time, a specific precipitation reaction between Ce³⁺ and F^- releases the free probe to restore the fluorescence of FN-1. Therefore, FN-1 can be used for the recyclable 'ON-OFF-ON' detection of Ce³⁺and F^-. The detection limits for Ce³⁺and F^- are 4.48 μM and 11.58 μM in concentration range of 0-50 μM and 0-150 μM. However, due to the para position of N and amino groups on 4-aminopyridine, the spatial structure of FN-2 cannot be complexed with ions, resulting in poor selectivity. Polyvinylidene fluoride (PVDF) membrane containing FN-1 were prepared for the real-time qualitative detection of Ce³⁺and F^- in real water samples. FN-1 exhibits high water solubility and biocompatibility and has been successfully applied to biological imaging in vascular smooth muscle cells (VSMCs).

A stimuli responsive triplet-triplet annihilation upconversion system and its application as a ratiometric sensor for Fe3+ ions

A ratiometric fluorescent sensor for the detection of Fe3+ ions is achieved based on triplet-triplet annihilation upconversion (TTA-UC) luminescence. A new anthracene derivative (named as DHTPA) is designed and synthesized and reveals similar optical properties to 9,10-diphenylanthracene (DPA) and is used as a stimuli responsive annihilator in a TTA-UC system due to its complexation ability. As a result, the UC emission can be significantly quenched by Fe3+ ions, while the phosphorescence (PL) emission of sensitizer palladium(ii) octaetylporphyrin (PdOEP) remains nearly constant, which makes the PL signal an appropriate internal reference for the UC signal. The UC and ratio signals (I UC/I PL) both reveal a good linear relationship with Fe3+ ion concentration, which for the first time makes the TTA-UC system a perfect ratiometric sensor for Fe3+ ion detection. This sensing method will open a novel avenue to achieve ratiometric sensors in chemical and biological fields.

Fluorescent probes with high pKa values based on traditional, near-infrared rhodamine, and hemicyanine fluorophores for sensitive detection of lysosomal pH variations

Sterically hindered fluorescent probes (A-C) have been developed by introducing 2-aminophenylboronic acid pinacol ester to a traditional, A, a near-infrared rhodamine dye, B, and a near-infrared hemicyanine dye, C, forming closed spirolactam ring structures. Probe A was non-fluorescent under basic pH conditions whereas probes B and C were moderately fluorescent with fluorescence quantum yields of 9% and 5% in pH 7.4 PBS buffer containing 1% ethanol, respectively. With all probes increasing acidity leads to significant increases in fluorescence at 580 nm, 644 and 744 nm for probes A, B and C with fluorescence quantum yields of 26%, 21% and 10% in pH 4.5 PBS buffer containing 1% ethanol, respectively. Probes A, B and C were calculated to have pKa values of 5.81, 5.45 and 6.97. The difference in fluorescence under basic conditions is ascribed to easier opening of the closed spirolactam ring configurations due to significant steric hindrance between the 2-aminophenylboronic acid pinacol ester residue and an adjacent H atom in the xanthene derivative moiety in probe B or C. The probes show fast, reversible, selective and sensitive fluorescence responses to pH changes, and are capable of sensing lysosomal pH variations in living cells.

Labeling of Microthrix parvicella in situ: A novel FRET probe based on bisoctyl rhodamine B

Filamentous bacteria, particularly Microthrix parvicella, are mainly responsible for bulking or foaming of activated sludge. Based on the affinity of M. parvicella to the hydrophobic characteristics of long-chain fatty acids, a novel bisoctyl rhodamine B (BORB) and a novel fluorescence resonance energy transfer (FRET) complex probe were prepared herein to study their properties. When the FRET probe was used in in situ activated sludge, M. parvicella was clearly labeled at 20 nmol/L, which was a reduction of 50 times compared to that of the BORB (1 μmol/L) alone and 500 times compared to the carbazole-quinoline probe reported previously. Compared with fluorescence in situ hybridization, M. parvicella could be clearly labeled using BORB and the FRET probe in situ without requiring complicated pretreatments (i.e., shock and broken process, fixed sample, digestion, and lysozyme treatment). This study discusses the facile approach developed for labeling M. parvicella in early warning expansion, thereby inhibiting and controlling sludge bulking in situ.

Synthesis and spectral analysis of fluorescent probes for Ce4+ and OCl- ions based on fluorescein Schiff base with amino or hydrazine structure: Application in actual water samples and biological imaging

Two Schiff base fluorescein probes (FDA, FDH) based on fluorescein-aldehyde and nitroaniline derivatives were synthesized. The effects of amino and hydrazine substituents in fluorescein backbones were examined via fluorescence and absorbance spectra. In the presence of Ce⁴⁺, the fluorescence of FDA was quenched due to the ligand to metal charge transfer (LMCT). Hypochloric acid can react with the CN bond, and blocking the photo induced electron transfer (PET) of FDH leads to enhancement of the fluorescence. FDA showed detection limits for Ce⁴⁺ and OCl^- as low as 63 nM in concentration range of 0-4 μM. FDH showed detection limits for OCl^- as low as 0.8 μM in concentration rang 0-100 μM. Polyvinylidene fluoride (PVDF) membrane containing the probes was prepared for the real-time qualitative detection of Ce⁴⁺ and OCl^- in real water samples. The probes were successfully applied to biological imaging in vascular smooth muscle cells (VSMCs) and are expected to find applications in biosensing.

Ratiometric fluorescent probe based on pyrrole-modified rhodamine 6G hydrazone for the imaging of Cu2+ in lysosomes

A novel rhodamine-based Schiff base derivative was obtained via the simple condensation of substituted formyl-1H-pyrrole and rhodamine 6G hydrazone. Fluorescence resonance energy transfer enabled the subsequent use of the derivative as a naked-eye colorimetric and ratiometric fluorescent sensor for Cu²⁺ in semi-aqueous solution, and the existence of the morpholine group enabled the further application of the sensor in imaging Cu²⁺ in the lysosomes of HeLa cells.

Modular design for fluorophore homodimer probes using diethylentriamine as a common spacer

Cationic fluorophore homodimer probes 1 and 2 bearing 7-aminocoumarin and naphthalimide dyes, respectively, connected via diethylenetriamine (DETA) spacer, have been developed to demonstrate the validity of our modular probe design on the basis of the triamine-based spacer.

A novel ratiometric and reversible fluorescence probe with a large Stokes shift for Cu2+ based on a new clamp-on unit

A novel ratiometric and reversible chemosensor 4-((2-(Benzo[d]thiazol-2-yl)phenyl)ethynyl)-N,N-diethylaniline (BT-1) based on ortho-arylethynyl benzothiazole with large Stokes shift (Δλ≈190 nm) was designed and synthesized to recognize Cu²⁺. Copper ion induces a remarkable fluorescence enhancement and causes formation of a BT-1-Cu complex. The clamp-on coordination mode of BT-1 to Cu²⁺ was demonstrated using Job's plot, mass spectrum (MS) and DFT calculations. The calculations also indicate that Cu²⁺ was chelated to BT-1 through N and alkyne instead of S and alkyne. The probe could quantify Cu²⁺ with the detection limit of 3.2 × 10^-9 M. The in vitro imaging results indicated that the probe BT-1 was membrane-permeable and could be applied into the recognition of Cu²⁺ ions in living cells.

Near-infrared fluorescent probes based on TBET and FRET rhodamine acceptors with different pK a values for sensitive ratiometric visualization of pH changes in live cells

Three near-infrared ratiometric fluorescent probes (A-C) based on TBET and FRET near-infrared rhodamine acceptors with different pK a values were designed and synthesized to achieve sensitive ratiometric visualization of pH variations in lysosomes in visible and near-infrared channels. Tetraphenylethene (TPE) was bonded to near-infrared rhodamine dyes through short electrical π -conjugation linkers to prevent an aggregation-caused quenching (ACQ) effect and allow highly efficient energy transfer of up to 98.9% from TPE donors to rhodamine acceptors. Probes A-C respond to pH variation from 7.4 to 3.0 in both buffer solutions and live cells with significant decreases of donor fluorescence and concomitant extraordinary increases of rhodamine acceptor fluorescence because of highly efficient energy transfer. In addition, probe C is capable of determining pH fluctuations in live cells treated with chloroquine. The probes show good photostability, excellent cell membrane permeability, high selectivity to pH, and two well-resolved emission peaks to ensure accurately comparative and quantitative analyses of intracellular pH changes.

A colorimetric and “off–on” fluorescent Pd2+ chemosensor based on a rhodamine-ampyrone conjugate: synthesis, experimental and theoretical studies along with in vitro applications

We successfully designed and developed a rhodamine based “turn-on” chemosensor L for the detection of Pd²⁺ ions down to 1.19 × 10⁻⁵ M (11.9 μM).

A FRET-based colorimetric and ratiometric fluorescent probe for the detection of Cu2+ with a new trimethylindolin fluorophore

The possible interaction mechanism between probe RhF and Cu²⁺ ions.

A Ratiometric Fluorescent Probe for Cisplatin: Investigating the Intracellular Reduction of Platinum(IV) Prodrug Complexes

The Pt^IV prodrug strategy has emerged as an excellent alternative to tackle the problems associated with conventional Pt^II drug therapy. However, there is a lack of tools to study how this new class of Pt^IV drugs are processed at the cellular level. Herein, we report the first ratiometric probe for cisplatin detection and use it to investigate Pt^IV anticancer complexes in biological systems. The probe was able to distinguish between cisplatin and its Pt^IV derivatives, allowing us to probe the intracellular reduction of Pt^IV prodrug complexes. The correlation between the amount of active Pt^II species available after intracellular reduction of Pt^IV complexes and their cytotoxicity and the role glutathione plays in the reduction of Pt^IV complexes were investigated.

A novel fluorescent long-chain fatty acid-substituted dye: labeling and biodegrading of Microthrix parvicella

Microthrix parvicella (M. parvicella) is a filamentous bacterium that induces bulking in activated sludge. Here, we used the affinity of long-chain fatty acids (LCFA) for M. parvicella to create a novel fluorescent probe of carbazole modified by LCFA. The structure was characterized by ¹H NMR spectroscopy and mass spectrometry. The spectral properties, photostability, and hydrophobic properties of the probe were also characterized. Fluorescent-labeling results showed that it can label M. parvicella in situ and could be biodegraded via metabolism. The stable docking mode of carbazole probes with different fatty acid chains and lipases was also docked by the density functional tight-binding (DFTB) method.

M. parvicella in situ could be biodegraded by a novel fluorescent probe of carbazole modified by LCFA(FP1).

New Near-infrared Fluorescent Probes with Single-photon Anti-Stokes-shift Fluorescence for Sensitive Determination of pH Variances in Lysosomes with a Double-Checked Capability

Two near-infrared luminescent probes with Stokes-shift and single-photon anti-Stokes-shift fluorescence properties for sensitive determination of pH variance in lysosomes have been synthesized. A morpholine residue in probe A which serves as a targeting group for lysosomes in viable cells was attached to the fluorophores via a spirolactam moiety while a mannose residue was ligated to probe B resulting in increased biocompatibility and solubility in water. Probes A and B contain closed spirolactam moieties, and show no Stokes-shift or anti-Stokes-shift fluorescence under neutral or alkali conditions. However, the probes incrementally react to pH variance from 7.22 to 2.76 with measurable increases in both Stokes-shift and anti-Stokes-shift fluorescence at 699 nm and 693 nm under 645 nm and 800 nm excitation, respectively. This acid-activated fluorescence is produced by the breaking of the probe spirolactam moiety, which greatly increased overall π-conjugation in the probes. These probes possess upconversion near-infrared fluorescence imaging advantages including minimum cellular photo-damage, tissue penetration, and minimum biological fluorescence background. They display excellent photostability with low dye photobleaching and show good biocompatibility. They are selective and capable of detecting pH variances in lysosomes at excitation with two different wavelengths, i.e., 645 and 800 nm.

Al(iii)-responsive "off-on" chemosensor based on rhodamine derivative and its application in cell imaging

In this work, a new rhodamine chemosensor (P) with excellent photochromic properties upon vis irradiation was designed and synthesized. The fabricated chemosensor P could detect Al3+ via the opening of the spirolactam ring of the rhodamine unit with high selectivity and sensitivity. The spirolactam ring opening was confirmed by NMR and infrared spectroscopy. Upon binding with Al3+, the generated 1 : 1 P-Al3+ complex, confirmed by Job's plot titrations and mass spectrometry analysis, could exhibit a remarkable fluorescence enhancement with a limit of detection (LOD) of 0.16 μM. Furthermore, the sensing of P to Al3+ in vivo was also studied quantitatively and qualitatively in detail, and the results showed that the coordination between P with Al3+ was reversible in living cells.

Fluorescent Probes Based on π-Conjugation Modulation between Hemicyanine and Coumarin Moieties for Ratiometric Detection of pH Changes in Live Cells with Visible and Near-infrared Channels

We report two ratiometric fluorescent probes based on π-conjugation modulation between coumarin and hemicyanine moieties for sensitive ratiometric detection of pH alterations in live cells by monitoring visible and near-infrared fluorescence changes. In a π-conjugation modulation strategy, a coumarin dye was conjugated to a near-infrared hemicyanine dye via a vinyl connection while lysosome-targeting morpholine ligand and o-phenylenediamine residue were introduced to the hemicyanine dye to form closed spirolactam ring structures in probes A and B, respectively. The probes show only visible fluorescence of the coumarin moiety under physiological and basic conditions because the hemicyanine moieties retain their closed spirolactam ring structures. However, decrease of pH to acidic condition causes spirolactam ring opening, and significantly enhances π-conjugation within the probes, thus generating new near-infrared fluorescence peaks of the hemicyanine at 755 nm and 740 nm for probes A and B, respectively. Moreover, the probes display ratiometric fluorescence response to pH with decreases of the coumarin fluorescence and increases of the hemicyanine fluorescence when pH changes from 7.4 to 2.5. The probes are fully capable of imaging pH changes in live cells with good ratiometric responses in visible and near-infrared channels, and effectively avoid fluorescence blind spots under neutral and basic pH conditions - an issue that typical intensity-based pH fluorescent probes run into. The probe design platform reported herein can be easily applied to prepare a variety of ratiometric fluorescent probes for detection of biological thiols, metal ions, reactive oxygen and nitrogen species by introducing appropriate functional groups to hemicyanine moiety.

New near-infrared rhodamine dyes with large Stokes shifts for sensitive sensing of intracellular pH changes and fluctuations

New near-infrared rhodamine dyes with large Stokes shifts were developed and applied for sensitive detection of cellular pH changes and fluctuations by incorporating an additional amine group with fused rings into the rhodamine dyes to enhance the electron donating ability of amine groups and improve the spectroscopic properties of the dyes.