Synthesis and photoluminescence properties of π-extended fluorene derivatives: the first example of a fluorescent solvatochromic nitro-group-containing dye with a high fluorescence quantum yield

Functional π-extended fluorene derivatives, 2,7-di(p-substituted-phenyl)fluorenes containing different functional groups such as hydrogen, trimethylsilyl (TMS), methoxycarbonyl, cyano, and nitro groups, were synthesized. Except for the nitro group, the resulting compounds exhibited extremely high fluorescence quantum yields (Φ_F >0.85 in chloroform). The diphenylfluorene containing nitro groups have higher fluorescence quantum yield (Φ_F = 0.31 in N,N’-dimethyl-formamide) than other nitro-group-containing fluorophores which were previously reported (Φ_F <0.1). Furthermore, this compound exhibited large Stokes’ shift with green to orange emission and unique on-off behavior of the emission by solvents.     

A light scattering and fluorescence emission coupled ratiometry using the interaction of functional CdS quantum dots with aminoglycoside antibiotics as a model system

Any signals, if their intensities have simple functional relationship with analyte concentration, can be applied to analytical purposes. Rayleigh light scattering signals and fluorescence signals are twins in flurospectroscopy, so the light scattering signals are the major interference when the Stokes shift is small. Herein, we propose a light scattering and fluorescence emission (LS-FL) coupled ratiometry using CdS quantum dots (QDs) as a fluorescence probe to detect aminoglycoside antibiotics (AGs). As model analytes, AGs, when attached to the surface of CdS-QDs via electrostatic interaction in aqueous medium, result in strong enhanced light scattering (LS) emission characterized at 376 nm and fluorescence quenching of CdS-QDs at 500 nm. Thus, a ratiometry using the coexistent light scattering and fluorescent emission signals has been proposed. Based on the linear relationship between logarithm of light scattering and fluorescence emission ratio (R) and logarithm of AGs concentration, a novel assay of AGs is established with the limits of detection (3σ) being 58-190 nmol l⁻¹, and applied successfully to detect AGs injection and serum samples.     

Absorption and emission behaviour of trans-p-coumaric acid in aqueous solutions and some organic solvents

The absorption and fluorescence behaviour of trans-p-coumaric acid (trans-4-hydroxycinnamic acid) is investigated in buffered aqueous solution over a wide range from pH 1 to pH 12, in un-buffered water, and in some organic solvents. Absorption cross-section spectra, fluorescence quantum distributions, fluorescence quantum yields, and degrees of fluorescence polarisation are measured. p-Coumaric acid exists in different ionic forms in aqueous solution depending on the pH. There is an equilibrium between the neutral form (p-CAH₂) and the single anionic form (p-CAH⁻) at low pH (pK_na ≈ 4.9), and between the single anionic and the double anionic form (p-CA²⁻) at high pH (pK_aa ≈ 9.35). In the organic solvents studied trans-p-coumaric acid is dissolved in its neutral form. The fluorescence quantum yield of trans-p-coumaric acid in aqueous solution is ?_F ≈ 1.4 × 10⁻⁴ for the neutral and the single anionic form, while it is ?_F ≈ 1.3 × 10⁻³ for the double anionic form. For trans-p-coumaric acid in organic solvents fluorescence quantum yields in the range from 4.8 × 10⁻⁵ (acetonitrile) to 1.5 × 10⁻⁴ (glycerol) were measured. The fluorescence spectra are 7700–10,000 cm⁻¹ Stokes shifted in aqueous solution, and 5400–8200 cm⁻¹ Stokes shifted in the studied organic solvents. Decay paths responsible for the low fluorescence quantum yields are discussed (photo-isomerisation and internal conversion for p-CA²⁻, solvent-assisted intra-molecular charge-transfer or ππ^∗ to nπ^∗ transfer and internal conversion for p-CAH₂ and p-CAH⁻). The solvent dependence of the first ππ^∗ electronic transition frequency and of the fluorescence Stokes shift of p-CAH₂ is discussed in terms of polar solute–solvent interaction effects. Thereby the ground-state and excite-state molecular dipole moments are extracted.     

Functionalized cadmium sulfide quantum dots as fluorescence probe for silver ion determination

CdS quantum dots (QDs) modified with l-cysteine has been prepared by one step. They are water-soluble and biocompatible. To improve CdS QDs stability and interaction between silver ion and functionalized CdS QDs in aqueous solution, some amounts of fresh l-cysteine were added to functionalized CdS solution. Based on the characteristic fluorescence enhancement of CdS QDs at 545 nm by silver ions in the presence of some amounts of fresh l-cysteine, simultaneously, a gradual red shift of fluorescence emission bands of CdS QDs from 545 to 558 nm was observed. A simple, rapid, sensitive and specific detection method for silver ion was proposed. Under optimum conditions, the fluorescence intensity of CdS QDs is linearly proportional to silver concentration from 2.0 × 10⁻⁸ to 1.0 × 10⁻⁶ mol/L with a detection limit of 5.0 × 10⁻⁹ mol/L. In comparison with single organic fluorophores, functionalized CdS quantum dots are brighter, more stable against photobleaching, and don’t suffer from blinking. Furthermore, owing to the fluorescence enhancement effect of CdS QDs by silver ion, the proposed method showed lower detection blank and higher sensitivity. Possible fluorescence enhancement mechanism was also studied.     

Molecular fluorescence analysis of rainwater: Effects of sample preservation

Very different filtration and preservation procedures may be found in the literature on the study of the rainwater dissolved organic fraction. Thus, the influence of sample filtration and preservation procedures on the fluorescence of rainwater dissolved organic matter (DOM) was studied in this work. Rainwater was filtered through different filters (quartz 0.22 μm or PVDF 0.45 μm) and excitation (λ_em = 415 nm) and synchronous (Δλ = 70 nm) fluorescence spectra were obtained at the same day of collection, or after preservation by refrigeration (1-7 days) or by freezing (1-4 weeks). The excitation-emission matrix (EEM) spectra of rainwater showed six types of fluorescent bands: two corresponding to humic-like bands, and four resembling proteins. Then, the excitation and synchronous spectra were chosen in order to monitor changes in the humic-like and protein-like bands, respectively. The filtration procedures adopted in this work did not affect the fluorescence properties of the rainwater samples. However, these properties were differently preserved by refrigeration or freezing: after refrigeration, filtered rainwater maintained the original fluorescent properties for at least 4 days, while after freezing fluorescent properties were not always preserved since it occurred a decrease of protein-like fluorescence intensity.     

Synthesis, luminescence properties, and theoretical insights of N-alkyl- or N,N-dialkyl-pyrene-1-carboxamide

We report the synthesis and photophysical properties of N-alkyl- or N,N-dialkyl-pyrene-1-carboxamide. These derivatives, as well as pyrene, exhibited blue emission. N-Alkyl-type derivatives exhibited strong fluorescence emission (Φ_fl = 0.61 in EtOH) in both nonpolar and polar solvents. On the other hand, N,N-dialkyl-type derivatives showed weak fluorescence emission (Φ_fl <0.01) due to vibrational deactivation. However, in highly viscous solvents such as glycerin, the quantum efficiencies of N-alkyl-type (Φ_fl = 0.91) and N,N-dialkyl-type (Φ_fl = 0.082) derivatives were increased. We also investigated the fluorescence mechanism of these compounds using time-dependent density-functional theory (TD-DFT). From these results, we find that highly fluorescent pyrene-1-carboxamide derivatives can be designed by introducing an appropriate functional group at the nitrogen atom of the amide. Thus, N,N-dialkyl-type pyrene-1-carboxamide has considerable potential for use in applications such as environmental response sensors and probes.     

The inner filter effects and their correction in fluorescence spectra of salt marsh humic matter

The inner filter effects in synchronous fluorescence spectra (Δλ = 60 nm) of sedimentary humic substances from a salt marsh were studied. Accordingly to their type and the influence of plant colonization, these humic substances have different spectral features and the inner filter effects act in a different manner. The fluorescence spectra of the humic substances from sediments with colonizing plants have a protein like band (λ_exc = 280 nm) which is strongly affected by primary and secondary inner filter effects. These effects were also observed for the bands situated at longer wavelengths, i.e., at λ_exc = 350 nm and λ_ex = 454 nm for the fulvic acids (FA) and humic acids (HA), respectively. However, they are more important for the band at 280 nm, causing spectral distortions which can be clearly seen when the spectra of solutions 40 mg L⁻¹ of different samples (Dissolved Organic Carbon – DOC ∼ 20 mg L⁻¹) are compared with and without correction of the inner filter effects. The importance of the spectral distortions caused by inner filter effects has been demonstrated in solutions containing a mixture of model compounds which represent the fluorophores detected in the spectra of sedimentary humic samples. The effectiveness of the mathematical correction of the inner filter effects in the spectra of those solutions and of solutions of sedimentary humic substances was studied. It was observed that inner filter effects in the sedimentary humic substances spectra can be mathematically corrected, allowing to obtain a linear relationship between the fluorescence intensity and humic substances concentration and preventing distortions at concentrations as high as 50 mg L⁻¹ which otherwise would obscure the protein like band.     

Fluorescence detection of total count of Escherichia coli and Staphylococcus aureus on water-soluble CdSe quantum dots coupled with bacteria

The aim of this paper was to demonstrate a fluorescence measurement method for rapid detection of two bacterial count by using water-soluble quantum dots (QDs) as a fluorescence marker, and spectrofluorometer acted as detection apparatus, while Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) were as detection target bacteria. Highly luminescent water-soluble CdSe QDs were first prepared by using thioglycolic acid (TGA) as a ligand, and were then covalently coupled with target bacteria. The bacterial cell images were obtained using fluorescence microscopy. Our results showed that CdSe QDs prepared in water phase were highly luminescent, stable, and successfully conjugated with E. coli and S. aureus. The fluorescence method could detect 10²-10⁷ CFU/mL total count of E. coli and S. aureus in 1-2 h and the low detection limit is 10² CFU/mL. A linear relationship of the fluorescence peak intensity and log total count of E. coli and S. aureus have been established using the equation Y = 118.68X − 141.75 (r = 0.9907).     

Study on spectroscopic characterization of Pd porphyrin and its interaction with ctDNA

The fluorescence and solid substrate room temperature phosphorescence (SS-RTP) properties of Pd(II) meso-tetrakis (4-N-methyl-pyridiniumyl) porphyrin (Pd(II)TMPyP) were studied. The factors influencing the SS-RTP emission, such as filter type, inorganic salt sort, drying temperature, pre-drying time and drying time were investigated in detail. Strong SS-RTP signal can be induced on the slow speed filter paper in the presence of the external inorganic salt, Ca(NO₃)₂, with the maximum excitation and emission wavelengths at 421 nm and 675 nm, respectively. The interaction between calf thymus DNA (ctDNA) and Pd(II)TMPyP was investigated at pH 7.2 using SS-RTP, fluorescence and UV-vis spectroscopy. The SS-RTP intensity of Pd(II)TMPyP was enhanced efficiently with the increasing amount of ctDNA. This phenomenon demonstrates that the intercalated porphyrin is shielded by ctDNA to avoid collision quenching. This result was supported by SS-RTP lifetime measurement, SS-RTP anion quenching experiment and fluorescence polarization measurement. Furthermore, with the addition of ctDNA, the UV-vis spectra of Pd(II)TMPyP shows apparent hypochromicity (40%) at the Soret maximum of 417 nm and a red shift of Δλ = 15 nm, also indicating that Pd(II)TMPyP intercalates into ctDNA bases. The binding constant of Pd(II)TMPyP to ctDNA was calculated to be 4.41 × 10⁵ L/mol based on the derivative McGhee-von Hippel plots.     

Absorption and emission spectroscopic characterization of some azo dyes and a diamino-maleonitrile dye

A linear and nonlinear optical spectroscopic characterization is carried out on three azo dyes (Reactive orange 1, Reactive violet 8, and Acidproof purplish red), and on N-(p-hydroxybenzylidene)-diamino-maleonitrile. Fluorescence quantum distributions, fluorescence quantum yields, and fluorescence lifetimes are measured. The saturable absorption is studied by nonlinear transmission measurements with intense picosecond laser pulses. The ground-state absorption recovery is studied by picosecond time-resolved pump and probe measurements. Absolute ground-state absorption cross-sections, excited-state absorption cross-sections, and dye concentrations are extracted from saturable absorption studies. The azo dyes have fluorescence lifetimes and ground-state absorption recovery times of around 2 ps and their excited-state absorption cross-sections are small (measured at 527 nm) making them good mode-locking dyes for picosecond and femtosecond lasers. The investigated diamino-maleonitrile dye exhibits sub-picosecond fluorescence lifetime and slow ground-state absorption recovery (>1 ns).     

Optical properties of bis(cyclopentadienyl)magnesium: excimer-type luminescence of the bis(cyclopentadienyl) ligand frame

The electronic spectra of solid MgCp₂ (Cp = cyclopentadienyl) show features which indicate the presence of intramolecular interligand interactions. The fluorescence of MgCp₂ (λ_max = 363 nm) undergoes a considerable Stokes shift which is apparently caused by a bonding attraction between both Cp rings in the excited state. An additional phosphorescence of the (Cp⁻)₂ fragment (λ_max = 535 nm) appears at 77 K.     

Multiway chemometric decomposition of EEM of fluorescence of CdTe quantum dots obtained as function of pH

The effect of the pH (from 3 to 10) on the excitation emission matrices (EEMs) of fluorescence of CdTe quantum dots (QDs), capped with mercaptopropionic acid (MPA), were analyzed by multiway decomposition methods of parallel factor analysis (PARAFAC), a variant of the parallel factor analysis method (PARAFAC2) and multivariate curve resolution alternating least squares (MCR-ALS). Three different sized CdTe QDs with emission maximum at 555 nm (QDa), 594 nm (QDb) and 628 nm (QDc) were selected for analysis. The three-way data structures composed of sets of EEMs obtained as function of the pH (EEMs, pH) do not have a trilinear structure. A marked deviation to the trilinearity is observed in the emission wavelength order—the emission spectra suffers wavelength shift as the pH is varied. The pH-induced variation of the fluorescence properties of QDs is described with only one-component PARAFAC2 or MCR-ALS models—other components are necessary to model scattering and/or other background signals in (EEMs, pH) data structures. Bigger sized QDs are more suitable tools for analytical methodologies because they show higher Stokes shifts (resulting in simpler models) and higher pH range sensitivity. The pH dependence of the maximum wavelength of the emission spectra is particularly suitable for the development of QDs/EEMs wavelength-encoded pH sensor bioimaging or biological label methodologies when coupled to multiway chemometric decomposition.     

Sensitive and selective fluorescence determination of trace hydrazine in aqueous solution utilizing 5-chlorosalicylaldehyde

A facile fluorescent method for the determination of hydrazine in aqueous solution with excellent sensitivity was developed. 5-Chlorosalicylaldehyde (CS), a readily commercially available compound, was applied as the derivatization reagent in this work. Under the addition of CS to hydrazine aqueous solution (ethanol/water/acetic acid = 30/66/4), an intense fluorescence enhancement was observed at 570 nm with a large stokes shift of ∼170 nm. Upon the optimal condition, the fluorescence intensity linearly increased with the concentration of hydrazine in the range of 0.2 and 9.3 μM with a correlation coefficient of R² = 0.9995 (n = 10) and a detection limit of 0.08 μM. The R.S.D. was 2.0% (n = 5). Determination of hydrazine in river and drinking water samples was successfully performed. Hydrazine vapor sensing by the proposed method was also reported.     

Chemical interactions and yellowing in chitosan-treated cellulose

The chemical interactions between cellulose and chitosan were studied in chitosan-treated celluloses using diffuse reflectance spectroscopic techniques (UV-Vis and FTIR) and fluorescence spectroscopy. The materials were obtained by treating pure cotton cellulose with chitosan in dilute acetic acid solutions of different concentration, and some samples were exposed to different thermal treatments in air. Other related materials, including cellulose treated with benzyl amine and chitosan-treated with acetaldehyde, were also prepared and studied as models for chitosan-treated celluloses. The spectra of the treated celluloses showed new absorption and emission bands that revealed the existence of chemical interactions. These bands were assigned to conjugated imines produced in the reaction of the chitosan amino groups with cellulose carbonyl groups. The reaction is strongly amplified at temperatures above 100 °C, causing the intense yellowing of chitosan-treated celluloses.     

A new fluorescent pH probe for extremely acidic conditions

A novel turn-off fluorescent probe based on coumarin and imidazole moiety for extremely acidic conditions was designed and developed. The probe with pK_a = 2.1 is able to respond to very low pH value (below 3.5) with high sensitivity relying on fluorescence quenching at 460 nm in fluorescence spectra or the ratios of absorbance maximum at 380 nm to that at 450 nm in UV–vis spectra. It can quantitatively detect pH value based on equilibrium equation, pH = pK_a − log[(I_x − I_b)/(I_a − I_x)]. It had very short response time that was less than 1 min, good reversibility and nearly no interference from common metal ions. Moreover, using ¹H NMR analysis and theoretical calculation of molecular orbital, we verified that a two-step protonation process of two N atoms of the probe leaded to photoinduced electron transfer (PET), which was actually the mechanism of the fluorescence quenching phenomenon under strongly acidic conditions. Furthermore, the probe was also applied to imaging strong acidity in bacteria, E.coli and had good effect. This work illustrates that the new probe could be a practical and ideal pH indicator for strongly acidic conditions with good biological significance.     

Synthesis,characterization and fluorescence behavior of new fluorescent probe phthalocyanines bearing coumarin substituents

In this study, we report a new ligand, 6-hexyloxy-3-[p-(3′,4′-dicyanophenoxy)phenyl]coumarin, and its fluorescent tetrasubstituted phthalocyanines {M[Pc(OBzCou)₄], M = 2H, Zn(II), Co(II); Bz: Benzene}. The effect of the coumarin derivative on the intensity of the fluorescence spectra of the metal-free (H₂Pc) and zinc phthalocyanine (ZnPc) derivatives was investigated. The change of the emission properties of both the coumarin moieties and the phthalocyanine core in the presence of the metal ion and the ring-opening reaction of the coumarin were studied by means of steady-state fluorescence spectroscopy. The radiative decay of the Pcs and the treated coumarin substituents bound to the Pcs was examined. The novel chromogenic compounds were characterized by elemental analysis, ¹H NMR, ¹³C NMR, Maldi-TOF, IR and UV–Vis spectral data. The photophysical properties of the Pcs are extensively affected by their state of aggregation: in particular, dimerization and aggregation result in a remarkable modification of the absorption and emission bands and may induce significant quenching of the usually strong Pc fluorescence. The electronic spectra exhibit a band of coumarin identity together with characteristic Q and B bands of the phthalocyanine core.     

Sequential injection immunoassay for human bone morphogenic protein-7 using an immunoreactor immobilized with anti-human bone morphogenic protein-7 antibody–CdSe/ZnS quantum dot conjugates

The detection of human bone morphogenic protein-7 (BMP-7) was achieved using a sequential injection immunoassay (SIIA) system. The SIIA system is based on the binding between BMP-7 and anti-human BMP-7 (AbBMP7)–CdSe/ZnS quantum dot (QD) conjugates immobilized onto a glass disk or an optical fiber, using fluorescence detection at excitation and emission wavelengths of 470 nm and 580 nm, respectively. The AbBMP7–QD conjugates were prepared by conjugating anti-human BMP-7 antibody (AbBMP7) to hydrophilic CdSe/ZnS core/shell quantum dots (QDs). The SIIA system was fully automated using software written in the LabVIEW™ development environment. The analytical performance of the SIIA system was characterized with a number of variables such as carrier flow rate and elution buffer. Under partially optimized operating conditions, the SIIA system had a linear calibration graph at up to 10.0 ng mL⁻¹ BMP-7 (R² ≥ 0.975) and a sample frequency of two samples per hour. The SIIA system with an optical fiber immunosensor was used to detect and quantify BMP-7 in spiked real samples obtained from a biological process with recoveries in the range of 95–102%.     

Synthesis, structures, and photophysical properties of silicon and carbon-bridged ladder oligo(p-phenylenevinylene)s and related π-electron systems

A series of partially or fully fused ladder oligo(p-phenylenevinylene)s (LOPVs) and related π-electron systems has been synthesized. Thus, the intramolecular reductive cyclization of o-silyl-substituted bis(phenylethynyl)benzenes with lithium naphthalenide produces partially silicon-bridged bis(styryl)benzenes consisting of silaindene or disilaindacene skeletons. By combining this cyclization with the Friedel-Crafts type electrophilic cyclization, a homologous series of the fully fused LOPVs and related compounds, bearing silicon and carbon bridges, has been synthesized in fairly good yields. The longest example of the LOPVs is the 13-ring-fused system that has a nearly flat π-conjugated framework with a length of 2.9 nm, as proven by X-ray crystallography. All the produced ladder π-electron systems show intense fluorescence in the visible region with high quantum yields as well as relatively small Stokes shifts. As the silicon contents increase or the disilaindacene skeleton is incorporated, the emission maxima shift to the longer wavelengths and the fluorescent quantum yields slightly decrease. These trends can be rationalized as due to the σ* effect of silicon, wherein the silicon bridges contribute to the electronic structure through σ*-π* orbital interaction that cause the red-shifts in the emission maxima and suppress the radiative decay process from the singlet excited state.