Excimer Emission Intensity Research Articles

Investigation of silica particle growth by incorporation of a pyrene fluorescent probe

The growth of silica particles synthesized by the Stöber method was investigated by the co-condensation of pyrene sulfonyl chloride (PySCl) coupled to (3-aminopropyl)triethoxysilane (APTES). Growth by monomer addition could be distinguished from growth by aggregation based on the intensity of the pyrene excimer emission of the resulting particles. The influence of the base concentration, water/ethanol ratio, solvent, temperature, and type of silica precursor was studied. At high hydrolysis rates, the growth occurred primarily by aggregation, whereas growth by monomer addition was predominant at low hydrolysis rates. The results were confirmed by evaluating the accessibility of the co-condensed pyrene moieties and by determining the porosity of the particles.

Nanothermometer Based on Polychlorinated Trityl Radicals Showing Two-Photon Excitation and Emission in the Biological Transparency Window: Temperature Monitoring of Biological Tissues.

Nanothermometers are emerging probes as biomedical diagnostic tools. Especially appealing are nanoprobes using NIR light in the range of biological transparency window (BTW) since they have the advantages of a deeper penetration into biological tissues, better contrast, reduced phototoxicity and photobleaching. This article reports the preparation and characterization of organic nanoparticles (ONPs) doped with two polychlorinated trityl radicals (TTM and PTM), as well as studies of their electronic and optical properties. Such ONPs having inside isolated radical molecules and dimeric excimers, can be two-photon excited showing optimal properties for temperature sensing. Remarkably, in TTM-based ONPs the emission intensity of the isolated radical species is unaltered increasing temperature, while the excimer emission intensity decreases strongly being thereby able to monitor temperature changes with an excellent thermal absolute sensitivity of 0.6-3.7%K-1 in the temperature range of 278-328K. The temperature dependence of the excimeric bands of ONPs are theoretically simulated by using electronic structure calculations and a vibronic Hamiltonian model. Finally, TTM-doped ONPs as ratiometric NIR-nanothermometers are tested with two-photon excitationwith enucleated pig eye sclera, as a real tissue model, obtaining a similar temperature sensitivity as in aqueous suspensions, demonstrating their potential as NIR nanothermometers for bio applications.

Ratiometric Nanothermometer Based on a Radical Excimer for In Vivo Sensing.

Ratiometric fluorescent nanothermometers with near-infrared emission play an important role in in vivo sensing since they can be used as intracellular thermal sensing probes with high spatial resolution and high sensitivity, to investigate cellular functions of interest in diagnosis and therapy, where current approaches are not effective. Herein, the temperature-dependent fluorescence of organic nanoparticles is designed, synthesized, and studied based on the dual emission, generated by monomer and excimer species, of the tris(2,4,6-trichlorophenyl)methyl radical (TTM) doping organic nanoparticles (TTMd-ONPs), made of optically neutral tris(2,4,6-trichlorophenyl)methane (TTM-αH), acting as a matrix. The excimer emission intensity of TTMd-ONPs decreases with increasing temperatures whereas the monomer emission is almost independent and can be used as an internal reference. TTMd-ONPs show a great temperature sensitivity (3.4% K-1 at 328K) and a wide temperature response at ambient conditions with excellent reversibility and high colloidal stability. In addition, TTMd-ONPs are not cytotoxic and their ratiometric outputs are unaffected by changes in the environment. Individual TTMd-ONPs are able to sense temperature changes at the nano-microscale. In vivo thermometry experiments in Caenorhabditis elegans (C. elegans) worms show that TTMd-ONPs can locally monitor internal body temperature changes with spatio-temporal resolution and high sensitivity, offering multiple applications in the biological nanothermometry field.

A pyrene-derived ratiometric fluorescent probe for pH monitoring in cells and zebrafish based on monomer-excimer emission

Acidity in biological microenvironments plays a crucial role in physiology and pathology, so that pH monitoring can provide insights into the mechanisms underlying pH-related processes. Many pH ratiometric fluorescent probes have been developed based on intramolecular charge transfer (ICT) and fluorescence resonance energy transfer (FRET) principles. In contrast, those based on intermolecular excimer are scarce. The present work rationally designed and readily synthesized a pyrene-derived ratiometric fluorescent probe (Pyrene-PA) for pH detection using the conversion of monomer-excimer emission. In an acidic aqueous medium, Pyrene-PA exists in a cationic monomer with an emission centered at 463 nm. As pH increases, Pyrene-PA converts to neutral molecules and exhibits reduced dispersibility, generating an intense excimer emission around 630 nm. Based on the above mechanism, Pyrene-PA displays a sensitive, selective, photostable, and reversible response to pH changes without interference from coexisting species such as anions, cations, thiols, and reactive oxygen species. The ratio of fluorescence intensity at 630 nm and 463 nm increases linearly with pH values from 4.0 to 7.0, and the pKa value is 5.6, suitable for pH quantification in acidic to neutral environments. Moreover, Pyrene-PA with low cytotoxicity and good cell penetrability enables monitoring pH changes in cells and zebrafish.

Measurements of viscosity profile in elastohydrodynamic lubrication film using fluorescence method under various contact pressure conditions

This paper describes rheological properties of hydrodynamic and elastohydrodynamic lubrication (EHL) film under various contact pressure conditions using the fluorescence measurement method. In this measurement method, the viscosity and the information about the state of the lubricant (liquid or solid) can be obtained via the analysis of fluorescence spectrum of a pyrene excimer emission. Previous research revealed that the intensity of excimer emission decreased with increasing of viscosity. In addition, the intensity ratio between the excimer peak and lubricant 5P4E fluorescence peak was introduced for the analysis, and the ratio decreased with increasing viscosity of lubricant. In this study, the viscosity profile obtained from the relationship between the intensity ratio and viscosity showed that the state of lubricant in the lubrication film corresponded almost exactly to the lubrication regime proposed in previous researches. It is concluded that the viscosity measurement technique proposed in this study is useful method for practical uses in hydrodynamic and elastohydrodynamic conditions, while in the high viscosity range, the measured viscosity varies widely due to the variation of intensity ratio.

Mechanically Responsive Luminescent Polymers Based on Supramolecular Cyclophane Mechanophores.

A new approach to cyclophane-based supramolecular mechanophores is presented. We report a mechanically responsive cyclic motif that contains two fluorescent 1,6-bis(phenylethynyl)pyrene moieties that are capable of forming intramolecular excimers. The emission spectra of dilute solutions of this cyclophane and a polyurethane elastomer into which a small amount of the mechanophore (0.08 wt %) had been covalently integrated are dominated by excimer emission. Films of the cyclophane-containing polyurethane also display a considerable portion of excimer emission, but upon deformation, the fluorescence becomes monomer-dominated and a perceptible change from cyan to blue is observed. The response is instant, reversible, and consistent with a mechanically induced change of the molecular conformation of the mechanophore so that the excimer-promoting interactions between the luminophores are suppressed. In-depth investigations show a correlation between the applied strain and the emission color, which can conveniently be expressed by the ratio of monomer to excimer emission intensity. The current study suggests that cyclophanes can be utilized to develop various supramolecular mechanophores that detect and visualize weak forces occurring in polymeric materials or generated by living tissues.

Dynamics of Anthracene Excimer Formation within a Water-Soluble Nanocavity at Room Temperature.

Excited anthracene is well-known to photodimerize and not to exhibit excimer emission in isotropic organic solvents. Anthracene (AN) forms two types of supramolecular host-guest complexes (2:1 and 2:2, H:G) with the synthetic host octa acid in aqueous medium. Excitation of the 2:2 complex results in intense excimer emission, as reported previously, while the 2:1 complex, as expected, yields only monomer emission. This study includes confirming of host-guest complexation by NMR, probing the host-guest structure by molecular dynamics simulation, following the dynamics AN molecules in the excited state by ultrafast time-resolved experiments, and mapping of the excited surface through quantum chemical calculations (QM/MM-TDDFT method). Importantly, time-resolved emission experiments revealed the excimer emission maximum to be time dependent. This observation is unique and is not in line with the textbook examples of time-independent monomer-excimer emission maxima of aromatics in solution. The presence of at least one intermediate between the monomer and the excimer is inferred from time-resolved area normalized emission spectra. Potential energy curves calculated for the ground and excited states of two adjacent anthracene molecules via the QM/MM-TDDFT method support the model proposed on the basis of time-resolved experiments. The results presented here on the excited-state behavior of a well-investigated aromatic molecule, namely the parent anthracene, establish that the behavior of a molecule drastically changes under confinement. The results presented here have implications on the behavior of molecules in biological systems.

Steric Engineering of Cyclometalated Pt(II) Complexes toward High-Contrast Monomer-Excimer-Based Mechanochromic and Vapochromic Luminescence.

Ligands play a crucial role in the supramolecular photoluminescence properties of Pt(II) square-planar complexes. To improve the luminescence color responses of N∧C∧N cyclometalated Pt(II) complexes to external stimuli such as mechanical stress and chemical vapors, we have conducted a steric engineering of the previous systems 1a-1d [Inorg. Chem. 2017, 56, 4978-4989] by introducing two tert-butyl groups to the tridentate ligand to form complexes 2a-2c. Unlike the "too low" or "too high" steric hindrance of the NCNPt core in 1a-1d, the combined steric effects of the tert-butyl groups at one side and the pentiptycene group at the other side of the NCNPt core in 2b are "just right" for generating as-prepared powders with pure monomer (green) emission or pure excimer (red) emission, depending on the rate of precipitation from solutions. The synergistic steric effects are also beneficial to the solid-state luminescence quantum efficiency (30-36%). As a result of the differences in steric interactions and thus in the relative monomer vs excimer emission intensity, each complex of 2a-2c performs a two-step luminescence mechanochromism and vapochromism with different color patterns. This work provides an intriguing example of steric engineering of Pt(II) complexes toward highly emissive molecular solids with high-contrast mechanochromic and vapochromic luminescence.

Positive Identification of DNA Cleavage by CRISPR-Cas9 Using Pyrene Excimer Fluorescence to Detect a Subnanometer Structural Change.

The Cas9 nuclease binds and cleaves DNA through its large-scale structural rearrangements. However, its unique property of not releasing the cleaved DNA has forbidden spectroscopic detection of the cleavage event. Here, we employ a novel fluorescence probe based on pyrene excimer emission to detect a minute structural change not detectable by other methods and demonstrate its applicability to spectroscopic tracking of the Cas9 nuclease activity in time. We show that the intensity of excimer emission depends sensitively on a subtle change in the structural environment of the target nucleic acid, which enables discrimination between cleaved and uncleaved nucleic acids within the DNA/Cas9/gRNA ternary complex. Kinetic parameters were obtained from the temporal evolution of the excimer emission, which revealed that DNA binding is hardly affected by PAM-distal mismatches, whereas the rate of cleavage by Cas9 decreases dramatically even with a 1-bp mismatch. Spectroscopic studies using the pyrene-based probe should be promising for biomolecular systems affected by subnm structural changes.

Highly Sensitive Ratiometric Fluorescent Detection of Indium(III) Using Fluorescent Probe Based on Phosphoserine as a Receptor.

Indium is one of the most widely used scarce metals for manufacturing various electronic devices including notebooks, mobile phones, and PC monitors. Recent studies revealed that indium and its compound could cause several toxicities to human beings and animals. However, there is no report about ratiometric fluorescent detection of In(III) in aqueous solutions. We synthesized a fluorescent probe (1) for In(III) based on a phosphoserine as a receptor with a pyrene fluorophore using solid phase synthesis. 1 showed highly sensitive ratiometric response to In(III) in purely aqueous solutions by increasing excimer emission intensity at 476 nm with a concomitant decrease in monomer emission intensity at 395 nm. 1 showed sensitive ratiometric responses to In(III) over a wide range of pH (2 < pH < 8) and exhibited a highly selective ratiometric response to In(III) among 18 tested metal including Al(III) and Ga(III). Job's plot analysis indicated that 1 preferred to form a 2:1 complex with In(III) and the binding affinity for In(III) was measured to be 2.3 × 1012 M-2 ( R2 = 0.989). 1 showed linear ratiometric responses to nanomolar concentrations (0-750 nM) of In(III) and the detection limit was calculated to be 64 nM ( R2 = 0.992) in aqueous solution. The binding mode study using NMR, IR, and CD spectroscopies revealed that the phosphate and the amide groups of the receptor of 1 played an important role for the binding with In(III). Moreover, 1 was suitable for the ratiometric detection of In(III) in tap water and groundwater. 1 showed much better detection properties than those of the colorimetric methods using EDTA with Eriochrome black T (EBT) and 4-(2-pyridylazo) resorcinol (PAR) for the detection of In(III) in tap water and groundwater.

J-type dimer of Auramine O dye upon encapsulation in cucurbit[8]uril host showing intense excimer emission

The host−guest interactions between Auramine O (AuO) dye and cucurbit[n]uril (CB[n], n = 7, 8) were investigated in aqueous solution by 1H NMR, UV/Vis, fluorescence spectroscopic techniques and single crystal diffraction. The results indicate the simultaneous formation of 1:1 and 2:1 inclusion complexes between CB[7] and AuO with two binding constants as 1.17 × 105 M−1 and 3.99 × 102 M−1 by UV/Vis spectroscopy titration (1.20 × 105 M−1 and 5.26 × 103 M−1 by fluorescence spectroscopy) in phosphate buffer (pH 7.4). And a 1:2 homoternary inclusion complex CB[8]·AuO2 with binding constant as 2.95 × 109 M−2 (by fluorescence spectroscopy titration) is detected. Interestingly, the appearance of intense new broad emission band with big red-shift (100 nm) with respect to that of AuO proves the formation of excimer upon encapsulation of AuO molecules in CB[8]. Determination of single crystal structure of CB[8]·AuO2 further discloses the existence of J-type dimer with intermolecular CH···π interactions between two included phenyl groups on AuO molecules. Upon host−guest complexation of AuO with CB[7] or CB[8], the pKa shifts of AuO were measured.

Potential of Pyridine Amphiphiles as Staphylococcal Nuclease Inhibitor.

The present study explores the potential of pyridine-based synthetic amphiphiles C1 and C2 having 4-carbon and 12-carbon hydrophobic tails, respectively, as staphylococcal nuclease inhibitors. UV-visible titration with calf-thymus DNA (CT-DNA) revealed a hypochromic shift in the absorbance bands of C1 and C2, whereas fluorescence titration indicated a reduction in the emission intensity of the monomer bands of the amphiphiles. Interaction of deoxyribonuclease I (DNase 1) and micrococcal nuclease (MNase) with C1 or C2 led to a decrease in the emission intensity of tryptophan at λ=345 nm along with an increase in the monomer emission intensity of C1 and C2 at λ=375 nm for DNase I and excimer emission intensity at λ=470 nm for both DNase I and MNase. Scatchard's analysis indicated superior interaction of C2 with DNase I. Circular dichroism spectroscopy revealed major changes in the secondary structures of both DNase I and MNase upon interaction with the amphiphiles. A solution-based nuclease assay in conjunction with gel electrophoresis indicated amphiphile-mediated protection against nuclease-directed DNA cleavage. Interestingly, C2 could render inhibition of nuclease present in the culture supernatant of Staphylococcus aureus MTCC 96, which highlights the therapeutic prospect of the amphiphile against S. aureus.

Tuning of the perylene probe excimer emission with silver nanoparticles

Silver nanoparticles (Ag NPs) enhanced perylene probe excimer emission is reported for the first time. It was observed that strong interactions between the perylene probe and the Ag NPs induced co-aggregation. As a result, a new in situ generated plasmonic absportion band of the Ag NPs at longer wavelength emerged. The monomer emission of the perylene probe was efficiently quenched, and dramatically enhanced probe excimer emission was observed. A remarkable emission enhancement of over 1000 fold was obtained compared to anionic polymers and other nanoparticles. The excimer emission intensity could be finely modulated by the size of the Ag NPs and the functionalities of the perylene probe. The observed Ag NPs enhanced perylene probe excimer emission shows good potential for the development of novel sensing techniques for various bioanalytical applications.

Study on Changes in the Rheologic Properties of EHL Film Using Fluorescence Measurements

This article describes a fundamental study using the fluorescence measurement method to understand the rheologic properties (such as viscosity and solidification) of elasto-hydrodynamic lubrication (EHL) film. The measurement principle is based upon the excimer emission of a pyrene fluorescence agent, which depends on the viscosity. To investigate the relationship between the excimer emission and viscosity, the measurements were taken under low temperature, with the results showing that the excimer emission decreased with decreasing temperature because of increasing of viscosity. No remarkable peak was observed below the temperature at which the state changed from liquid into a viscoelastic or elastic–plastic solid. The fluorescence was clearly observed in EHL film, and its spectrum revealed that the intensity of excimer emission decreases along the flow direction, with a contact area having the same behavior as that of the elastic–plastic solid at 243 K. These results suggest that the viscosity increased at the inlet of contact because of increasing hydrodynamic pressure under the wedge action and that the state in the contact region changes to solid because of high contact pressure. This study also proposes that this measurement method can be used to understand the viscosity change and solidification in the lubrication film.

Template-directed excimer formation via specific non-covalent interactions between pyrene guanidinium derivatives and nucleic acids

Structurally distinct guanidinium derivatives were evaluated for their ability to interact non-covalently with various nucleic acid sequences. Among the evaluated derivatives, 4-[(pyrene-1-ylmethyl)amino]butyl] guanidinium (pbg) was found to demonstrate strong excimer emission upon nucleic acid addition and high levels of discrimination between ds- and ss-DNA. The intensity of excimer emission proved to be dependent on the length of the linker probe as well as the oligonucleotide length and sequence. In particular, G-quadruplex prone structures were found to induce the highest excimer emissions among all nucleic acids tested.

Pyreneamide-based dipodal probes for ultra-sensitive and selective detection of 3,5-dinitrosalicylic acid in an aqueous solution

Pyrene-appended dipodal probes (probes 1 and 2) with differences in conformational rigidity were synthesized for sensitive detection of 3,5-dinitrosalicylic acid (DNSA) in an aqueous solution. Both dipodal probes had two pyrene units and exhibited quenching caused by aggregation (ACQ) at high water content (>60%) in DMSO. As the pyrene dimer (e.g., excimer) was a dominant conformation in 99% aqueous solution, both probes showed sufficient excimer emission intensity at 486 nm (probe 1) or 480 nm (probe 2) upon excitation. The excimer emission of the probes (probes 1 and 2) was sensitively quenched in the presence of DNSA. Probe 1 selectively responded to DNSA among various carboxylic acids (CAs), while probe 2 was responsive to both DNSA and 5-NSA. Density functional theory (DFT) calculations suggest that, in the presence of DNSA, the probes undergo structural changes, leading to formation of new π–π interactions between each pyrene unit of the probe and the benzene ring of the guest molecule. This new π–π interaction enabled energy transfer from pyrene to the non-radiative guest molecule (DNSA), resulting in fluorescence quenching in the probes. The limit of detection (LOD) of probe 1 with DNSA was observed to be 1 nM, the lowest value reported so far. This ultra-sensitivity toward DNSA detection was retained even in the case of a probe 1-coated TLC strip. Probe 3, which had only one pyrene unit, gave produced a monomeric emission spectrum with λmax = 406 nm, along with aggregation-induced emission enhancement (AIEE) behavior. The interactions of DNSA with individual probes were examined by UV-visible, fluorescence, and 1H NMR spectroscopies and were further supported by DFT calculations.

New DTPA-derived bis-naphthalenophanes: fluorescence, protonation, and complexation with aromatic amines

Two fluorescent, water-soluble bis-naphthalenophane isomers with six carboxylate arms, abbreviated as (bis-dtpa14nap)H6 and (bis-dtpa15nap)H6, were synthesized, which consist of two 1,4- or 1,5-substituted naphthalene rings interlinked by two diethylenetriaminepentaacetic (DTPA) chains through amide-linkages. Both DTPA-based macrocycles exhibit intense excimer and monomer emission bands, which sensitively respond to pH in three protonation steps; more sensitive is the 1,4-naphthyl isomer. The full pH-emission profiles have confirmed that the mono-protonated anion (bis-dtpanap)H5 − is the major protonation species at the physiological pH. Fluorometric titrations at pH 7.2 have proven that the 1,4-naphthalenophane anion forms 1:1-complexes with cationic phenethylamine (formation constant, 5700 M−1) and histamine (3000 M−1), excluding tryptamine cation, whereas the 1,5-isomer does not react with any of the three amines. The primary binding forces are electrostatic interactions between the CH2CO2 − arms of 1,4-naphthalenophane and the CH2CH2NH3 + chain of an aromatic amine. The resulting ion-pair is stabilized by encapsulation of the guest molecule in 1,4-napthalenophane cavity, while the 1,5-isomer cannot encapsulate. NMR studies have demonstrated that 1,4-napthalenophane has a higher freedom in reorientation of naphthalene rings. Such geometrical properties controlled by selection of naphthalene units are the feature of the new naphthalenophanes, and are responsible for the pH‒emission profiles and the complexation.

Rational design and studies of excimer forming novel dual probes to target RNA

In this paper, we report structure-based rational design and physico-chemical and biological studies of novel pyrene excimer forming dual probes for visualization of intracellular RNAs. Herein, the probes based on 2′-O-methyl RNA with linkers of different structure and length between pyrene moiety and ribose are studied with respect to their hybridization and spectral properties. We found optimal linkers that provide more intense excimer emission (at ∼480nm) of RNA-bound probes; particularly, the length of the linker arm of the 3′-component of dual probes plays a key role in formation of pyrene excimer. Calculated molecular dynamics trajectories and probability distributions of pyrene-pyrene dimer formation upon hybridization of the dual probes with RNA target are in agreement with the obtained fluorescence spectroscopy data for the corresponding duplexes. Our study demonstrates the excellent binding properties of new dual probes to structured RNA and their feasibility for the visualization of intracellular RNA targets.

Ratiometric Sensing of Hydrogen Peroxide Utilizing Conformational Change in Fluorescent Boronic Acid Polymers.

We demonstrate that the copolymers containing boronic acid and pyrene units can be utilized for the fluorometric sensing of hydrogen peroxide (H2O2) in aqueous solutions. The copolymer exists in a relatively extended conformation in the absence of H2O2, whereas the polymer chain is contracted by the reaction of boronic acid moieties with H2O2 to form phenol groups. This conformational change induces aggregation of the originally isolated pyrene groups. As a result, relative intensity of excimer emission with respect to monomer emission increases with H2O2 concentration. Accordingly, the present methodology enables us to measure H2O2 by means of ratiometric fluorescence change in the range of 0–30 μM.

Color-tunable fluorescent nanoparticles encapsulating trialkylsilyl-substituted pyrene liquids

Fluorescent nanoparticles are prepared with fluid trialkylsilylpyrenes, which exhibit intense excimer emission and spontaneously absorb dopant dyes, enabling extensive color tuning without premixing.