Emerging fluorescence sensing technologies: from photophysical principles to cellular applications

Turn-On Fluorescence Chemical Sensing through Transformation of Self-Trapped Exciton States at Room Temperature

Most of the current fluorescence sensing materials belong to the turn-off type, which make it hard to detect toxic substances such as benzene, toluene, and xylene (BTX) due to the lack of active chemical sites, thereby limiting their development and practical use. Herein, we show a guest-host mechanism stemming from the confined emitter's self-trapped exciton (STE) states or electron-phonon coupling to achieve turn-on fluorescence. We designed a luminescent guest@metal-organic framework (LG@MOF) composite material, termed perylene@MIL-68(In), and established its E-type excimeric emission properties in the solid state. Upon exposure to BTX, especially xylene, we show that the E-excimer readily converts into the Y-excimer due to nanoconfinement of the MOF structure. Such a transformation elevates the fluorescence intensity, thus realizing a turn-on type fluorescent sensor for detecting BTX solvents. Our results further demonstrate that controlling the STE states of perylene at room temperature (vs the previous report of <50 K) is possible via nanoscale confinement, paving the way to enabling turn-on type luminescent sensors for engineering practical applications.

Synthesis, spectral characteristics and sensor ability of new polyamidoamine dendrimers, modified with curcumin

To prepare a novel highly photo-stable fluorescent chemosensor, curcumin was successfully immobilized to polyamidoamine dendrimer of zero (S1), first (S2) and second (S3) generations conjugated-UV absorber moieties. Chemical structure of synthesized chemosensors were well-analysed by FTIR, ¹H-NMR, ¹³CNMR, elemental analysis, DSC and UV-vis techniques. Photo-physical characteristics and solvatochromism effect of three novel chemosensors in organic solvents with different dielectric constants ranged 2.21-37.78 were studied. The pH determination ability of S1, S2 and S3 in the range of 2-12 were also examined. Newly synthesized materials were employed for detection of different metal cations including Ag⁺, Ba²⁺, Cu²⁺, Ca²⁺, Cd²⁺, Fe³⁺, Hg²⁺, Ni²⁺, Pb²⁺ and Zn²⁺ and their possibility to apply as a cation chemosensor were evaluated. The results showed significant changes in their fluorescence intensity upon the different pHs and cations indicating their possibility to apply as a pH and metal cation chemosensor. Among the new chemosensors under study, S1 represented high sensitivity to pH in the range of 4-8 and high selectivity for Cu²⁺ over the other cations.

Rhodols - synthesis, photophysical properties and applications as fluorescent probes

The formal replacement of one dialkylamino group in rhodamines with a hydroxyl group transforms them into rhodols. This apparently minor difference is not as small as one may think; rhodamines belong to the cyanine family whereas rhodols belong to merocyanines. Discovered in the late 19th century, rhodols have only very recently begun to gain momentum in the field of advanced fluorescence imaging. This is in part due to the increased understanding of their photophysical properties, and new methods of synthesis. Rationalization of how the nature and arrangement of polar substituents around the core affect the photophysical properties of rhodols is now possible. The emergence of so-called π-expanded and heteroatom-modified rhodols has also allowed their fluorescence to be bathochromically shifted into regions applicable for biological imaging. This review serves to outline applicable synthetic strategies for the synthesis of rhodols, and to highlight important structure-property relationships. In the first part of this Review, various synthetic methods leading to rhodols are presented, followed by structural considerations and an overview of photophysical properties. The second part of this review is entirely devoted to the applications of rhodols as fluorescent reporters in biological imaging.

An Amphiphilic Pyridinoyl-hydrazone Probe for Colorimetric and Fluorescence pH Sensing

A new pH sensor based on a substituted aroylhydrazide with a flexible side chain and a terminal trimethyl ammonium group (PHA⁺) was designed and synthesized. The terminal quaternary ammonium guarantees excellent solubility in water. At the same time, the probe is very soluble in hydrophobic envirornments. The pyridinoyl-hydrazone moiety acts as the pH-sensitive fluorophore/chromophore probe. Extensive physicochemical characterization has been performed on the bromide salt PHABr. DFT calculations, based on single-crystal X-ray data, permitted to rationalize the optical behavior. Molecular dynamics simulations permitted to clarify the mode of interaction with lipid membrane. The ability of the probe to change color and fluorescence in response to different pH and media of different polarity has been investigated. PHABr shows a remarkable pH-dependent behavior in both absorption and fluorescence spectra with high sensitivity and strong on-off switch effect at neutral pH, perceptible even to the naked eye.

A Personal Journey across Fluorescent Sensing and Logic Associated with Polymers of Various Kinds

Our experiences concerning fluorescent molecular sensing and logic devices and their intersections with polymer science are the foci of this brief review. Proton-, metal ion- and polarity-responsive cases of these devices are placed in polymeric micro- or nano-environments, some of which involve phase separation. This leads to mapping of chemical species on the nanoscale. These devices also take advantage of thermal properties of some polymers in water in order to reincarnate themselves as thermometers. When the phase separation leads to particles, the latter can be labelled with identification tags based on molecular logic. Such particles also give rise to reusable sensors, although molecular-scale resolution is sacrificed in the process. Polymeric nano-environments also help to organize rather complex molecular logic systems from their simple components. Overall, our little experiences suggest that researchers in sensing and logic would benefit if they assimilate polymer concepts.

Investigation of cation binding and sensing by new crown ether core substituted naphthalene diimide systems

Different modes of cation binding lead to very different optical readouts from two structurally similar sensors.

Bright molecules for sensing, computing and imaging: a tale of two once-troubled cities

The circumstances in Colombo, Sri Lanka, and in Belfast, Northern Ireland, which led to a) the generalization of luminescent PET (photoinduced electron transfer) sensing/switching as a design tool, b) the construction of a market-leading blood electrolyte analyzer and c) the invention of molecular logic-based computation as an experimental field, are delineated. Efforts to extend the philosophy of these approaches into issues of small object identification, nanometric mapping, animal visual perception and visual art are also outlined.

Sensitive and selective PET-based π-expanded phenanthrimidazole luminophore for Zn2+ ion

The novel photoinduced electron transfer (PET) chemosensor, 1-(1-(4-methoxyphenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)naphthalen-2-ol [MPPN] and its zinc complex were synthesised and characterized by electronic spectral and Frontier molecular orbital energy analysis. MPPN becomes efficient fluorescent chemosensor upon binding with metal ions and shows a strong preference toward Zn(2+) ion. Density Functional theory (DFT) calculations reveal that luminescence of free MPPN originates from its orbital structure in which two π-orbitals (HOMO and HOMO-1) of the imidazole ring are situated between two π-orbitals (HOMO-2 and LUMO) of the naphthyl fragment. Therefore the absorption and emission processes occur between the two π- orbitals (HOMO-2 and LUMO). The two higher energy imidazole orbitals (HOMO and HOMO-1) serve as quenchers for the excited state of the molecule through nonradiative processes. Upon binding with Zn(2+) ion, MPPN becomes a highly luminescent with λemi - 421 nm. The significant enhancement of luminescence upon binding with Zn(2+) ion is attributed to the stabilization of HOMO-2 and HOMO-1 π-orbitals of imidazole ring upon their engagement in new bonds with Zn(2+) ion. The affinity of MPPN to zinc ion is found to be very high [K = 6 × 10(6) M(-1)] when compared with other metals ions. The nonlinear absorption coefficient γ for MPPN is 1.9 × 10(-12) m/W and 3.9 × 10(-11) m/W for MPPN-Zn complex.

Selective fluorescence sensing of Mg2+ ions by Schiff base chemosensor: effect of diamine structural rigidity and solvent

Highly selective strong turn-on fluorescence for Mg²⁺ (Φ = 0.03 to 0.57) was realized with a simple Salen based Schiff base chemosensor (1a) using dimethylformamide (DMF) or dimethyl sulfoxide (DMSO) as solvent.

Synthesis, complexation, and fluorescence behavior of 3,4-dimethylthieno[2,3-b]thiophene carrying two monoaza-15-crown-5 ether groups

A novel fluoroionophore 1 based on 3,4-dimethylthieno[2,3-b]thiophene bearing two monoaza-15-crown-5 ethers at 3- and 4-positions was prepared. UV-vis and fluorescence responses of compound 1 upon the addition of alkali and alkaline earth metal cations were evaluated in acetonitrile solution. Receptor 1 showed unique response for Ba(2+) due to the formation of an intramolecular sandwich complex.

A new family of Ru(II) polypyridyl complexes containing open-chain crown ether for Mg2+ and Ca2+ probing

Six polypyridyl bridging ligands BL(1-6) containing open-chain crown ether, where BL(1-3) formed by the condensation of 4,5-diazafluoren-9-hydrazine with 1,7-bis-(4-formylphenyl)-1,4,7-trioxaheptane, 1,10-bis-(4-formylphenyl)-1,4,7,10-tetraoxadecane, and 1,13-bis-(4-formylphenyl)-1,4,7,10,13-pentaoxatridecane, respectively, BL(4-6) formed by the reaction of 9-(4-hydroxy)phenylimino-4,5-diazafluorene with diethylene glycol di-p-tosylate, triethylene glycol di-p-tosylate, and tetraethylene glycol di-p-tosylate, respectively, have been synthesized. Reaction of Ru(bpy)(2)Cl(2).2H(2)O with BL(1-6), respectively, afforded six bimetallic complexes [(bpy)(2)RuBL(1-6)Ru(bpy)(2)](4+) as PF(6)(-) salts. Cyclic voltammetry of these complexes is consistent with one Ru(II)-centered oxidation around 1.32V and three ligand-centered reductions. These complexes show metal-to-ligand charge transfer absorption at 413-444 nm and emission at 570 nm. Binding behavior of complexes with alkali and alkaline-earth metal ions are investigated by UV-vis absorption, fluorescence, and cyclic voltammetry. Addition of alkali and alkaline-earth metal ions to the solution of [(bpy)(2)RuBL(1-6)Ru(bpy)(2)](PF(6))(4) all result in a progressive quenching of fluorescence, a hyperchromic effect of UV-vis absorption, and a progressive cathodal shift of Ru(II)-centered E(1/2). Ru-BL(2) and Ru-BL(5) show the highest binding ability toward Mg(2+) among the five cations examined while Ru-BL(3) and Ru-BL(6) exhibit good selective recognition ability to Ca(2+).

A versatile strategy for the synthesis of crown ether-bearing heterocycles: Discovery of calcium-selective fluoroionophore

Heterocycles have been modified in various ways in the search for new functions, but few examples are known of crown ethers incorporating heterocycles in macro-ring systems. Here we report a simple and versatile synthesis of crown ether-bearing heterocycles. An acylurea moiety in the heterocycles is efficiently transformed to 'crown ether' of various ring sizes. The products included a Ca(2+)-selective fluoroionophore. Our simple methodology is expected to provide many novel functional heterocyclic compounds, including fluoroionophores and candidate pharmaceuticals.

Sensitive and Selective Photoinduced-Electron-Transfer-Based Sensing of Alkylating Agents

Photoinduced-electron-transfer (PET)-based chemosensing is a very elegant way of reporting the presence of a guest species in solution. This method was successfully applied for the detection of different ionic species, such as cations, anions, and protons. Herein, we report on the application of the PET chemosensing concept for the efficient and selective detection of different alkylating agents. 2-(2-Dimethylaminoethyl)benzo[de]isoquinoline-1,3-dione (1) was found to be a highly selective and effective PET chemosensor that turns luminescent upon reacting with different alkylating agents. This PET-based system detected even rather weak alkylating agents, such as dichloromethane. A PET-based sensor that consists of 1 as the active component could detect rather low concentrations of alkylating agents in solution and in the gas phase.

Selective binding and inverse fluorescent behavior of magnesium ion by podand possessing plural imidazo[4,5-f]-1,10-phenanthroline groups and its Ru(II) complex

[structure: see text] Two podands, 4,4'-[(ethylenedioxy)bis(ethyleneoxy)]bis[1-(2-imidazo[4,5-f]-1,10-phenanthroline)benzene] (1) and [Ru(phen)(2)](2)(1)(PF(6))(4) (2) complex, were synthesized from 1,10-phenanthroline. The photophysical behavior and the binding ability of 1 and 2 with some alkali metal and alkaline earth cations were investigated by UV-vis and fluorescence spectrometry and (1)H NMR experiments as well as fluorescence lifetime measurements. The complex stability constants (K(S)) and Gibbs free energy changes (DeltaG degrees ) for the stoichiometric 1:1 complexation of 1 and 2 with the cations were obtained by the fluorimetric titrations. The podands 1 and 2 exhibit different fluorescent behavior in the cations examined, i.e., fluorescence quenching for 1, and fluorescence enhancement for 2. In particular, 1 showed responses specific for Mg(2+), resulting in readily distinguishable by eye.

Submicrometric Lipobead-Based Fluorescence Sensors for Chloride Ion Measurements in Aqueous Solution

This paper describes the preparation and optimization of the analytical properties of fluorescence-based submicrometric chloride ion sensing lipobeads. Fluorescence sensing lipobeads are polystyrene nanoparticles that are coated with a phospholipid membrane that contains a fluorescent indicator for a targeted analyte. In this study, the halide-specific fluorescence dye, lucigenin, was immobilized into the phospholipid membrane of the lipobeads to enable chloride ion detection. The fluorescence intensity of lucigenin decreases with increasing chloride ion concentration due to dynamic quenching. Lipobeads that contained only lucigenin were ineffective as chloride ion sensors due to poor partition of the water-soluble lucigenin molecules into the phospholipid membrane and high leakage rate of immobilized lucigenin molecules to the aqueous solution. To stabilize the chloride ion sensing lipobeads we coimmobilized hexadecanesulfonate molecules into the phospholipid membrane. The formation of ion pairs between hexadecanesulfonate and lucigenin decreased the hydrophilicity of the dye, increased its partition rate into the membrane, increased the brightness of the particles, and significantly decreased the leakage rate of the hydrophobic ion pair from the membrane to the solution. To further improve their chloride ion sensitivity, we also immobilized the chloride ionophore [9] mercuracarborand-3 into the lipobead membrane. The study resulted in a unique submicrometric chloride ion sensor, which is suitable for chloride ion measurements in biological fluids.

Bis(isoquinoline N-oxide) pincers as a new type of metal cation dual channel fluorosensor

[structure: see text] A new type of donor-spacer-acceptor podand system has been synthesized and proved as an efficient dual channel fluorosensor for Li+, Mg2+, and Ca2+. The known ability for the N-oxide function to bind Lewis acids is the key step in the appearance of a new emitting charge-transfer (CT) excited state. The occurrence of this CT state for alkaline earth (Mg2+ and Ca2+) and not for alkaline metals (Li+) provided a new type of dual channel fluorosensors.

Dual-Signaling Fluorescent Chemosensors Based on Conformational Restriction and Induced Charge Transfer

Twist and shout! By restricting conformation, 2,6-biarylpyridines can be induced to "speak" to us through increased fluorescence emission. In combination with induced charge transfer, this allows these simple fluorophores to reveal otherwise silent binding events to alkali metal and alkaline earth cations through brightening and color change (see picture; from left to right: no metal, Li⁺ , Mg²⁺ , and Ca²⁺ ).

A combinatorial approach to discover new chelators for optical metal ion sensing

This paper reports the synthesis and characterization of metal-binding indicators with diverse optical responses on exposure to various heavy metal ions. A combinatorial approach, based on azo coupling of diazonium salts with either phenolic compounds or aromatic amines, generated a library of azo dyes. Each reaction mixture, containing the product(s) of azo coupling, was incubated with a series of solutions, each containing a different heavy metal ion. The absorbance and, in some cases, fluorescence spectra of the resulting complexes were recorded. The metal chelates showed extensive diversity in their UV-visible absorbance spectra upon binding to selected metal ions. Of the azo dyes prepared, the terdentate dyes were particularly useful, providing distinct spectral responses to three or more metal ions in a panel of seven.

Synthesis and application of submicrometer fluorescence sensing particles for lysosomal pH measurements in murine macrophages

Phagocytosis of bioparticles such as bacteria and viruses by macrophages is a critical component of the immune response against infections. In this paper we describe the synthesis of submicrometer fluorescent particles with pH sensing capability. The particles are used to measure the pH and to monitor the effect of chloroquine, an antimalarial drug, on the pH in the lysosome, the cellular organelle involved in the phagocytosis process. The synthesis of the pH sensing particles is realized by the covalent attachment of amine reactive forms of Oregon Green (pH sensitive dye) and Texas Red (pH insensitive dye) to the surface of amino-modified submicrometer polystyrene particles. The particles are absorbed by J774 Murine Macrophages through phagocytosis and directed to lysosomes. Despite the high lysosomal levels of digestive enzymes and acidity, the absorbed particles remain stable for 12 h in the cells when they are stored in a PBS buffer solution at pH 7.4. The pH dynamic range of the sensing particles is between pH 4.5 and 7 with a sensitivity of 0.1 pH units. Exposure of the cells to chloroquine increases the lysosomal pH from 4.8 to 6.5. The effect is concentration-dependent.