Quinoxaline sensitised lanthanide ion luminescence: Syntheses,spectroscopy and X-ray crystal structure of Na{1,4,7-tris[(N-diethyl)carbamoylmethyl]-1,4,7,10-tetraazacyclododecane-10-(2-methylquinoxaline)}I3 C7H8

The synthesis of a quinoxaline-appended aza-macrocyclic ligand together with corresponding Ln^III complexes are described. The luminescence properties of the complexes show that the quinoxaline unit sensitises both visible (Eu^III) and near-IR (Nd^III and Yb^III) emitting lanthanide ions. UV–Vis absorption and time-resolved photophysical studies together with X-ray structural data suggest that as well as contributing chromophorically, the quinoxaline moiety generally participates in the first coordination sphere of Ln^III. The luminescent pH response of the Eu^III complex is also reported. The form of the steady state spectrum changes profoundly in the pH range 2–12, implying a change in the coordination environment, which was confirmed with time-resolved lifetime measurements that suggest an increase in europium inner sphere hydration at acidic pH.     

Novel pH responsive luminescent poly(oxyethylene phosphate) tris(β-diketonate) europium (III) complexes

The paper reports on the dependence of the absorbance and luminescent intensity from pH of novel poly(oxyethylene phosphate) tris(β-diketonate) europium (III) complexes. The photophysical data obtained allow some preliminary assumptions about the nature of this phenomenon. Increase in luminescent efficiency is a consequence of enhanced efficiency of energy transfer caused by structural changes in complexes after water deprotonation. Remarkable change has been observed in photophysical properties of the polymer complexes by studying the fluorescent emission and excitation spectra and absorption recorded at various pH both in solution and in the solid state. Some of the complexes derivative of the dibenzoylmethane (DBM) show more than hundred times increase in the luminescence after alkalization. The pH value, at which the maximum luminescent efficiency appears, depends on the type of the fourth ligand. The difference between luminescent efficiency of the complexes in alkaline and neutral environment depends on the β-diketonate ligands and on polymer type as well.      

Environmental sensitivity of Ru(II) complexes: the role of the accessory ligands

A suite of Ru(II) complexes in which one ligand is pH responsive and the other two are varied in an effort to achieve improved photophysics has been synthesized and their potential as pH reporters assessed. The more general purpose of the study was to examine the role of the accessory ligands in heteroleptic reporter complexes and the degree to which such ligands can affect the performance of luminescent reporters. For this suite of complexes, judicious choice of the accessory ligand can alter both the pK(a)* and the dynamic range of response. It was found that the emission color and brightness were influenced by pH, but the lifetimes were only weakly affected. Surprisingly, some accessory ligands which should have improved luminescent properties essentially turned off the pH response. Several possible reasons for this observation are explored. It is suggested, and density functional theory (DFT) calculations support, that the relative π* levels of the pH sensitive and the accessory ligands are critical.     

Synthesis and Evaluation of Europium Complexes that Switch on Luminescence in Lysosomes of Living Cells

A set of four luminescent Eu^III complexes bearing an extended aryl-alkynylpyridine chromophore has been studied, showing very different pH-dependent behaviour in their absorption and emission spectral response. For two complexes with pK_a values of 6.45 and 6.20 in protein-containing solution, the emission lifetime increases very significantly following protonation. By varying the gate time during signal acquisition, the ‘switch-on’ intensity ratio could be optimised, and enhancement factors of between 250 to 1330 were measured between pH 8 and 4. The best-behaved probe showed no significant emission dependence on the concentration of endogenous cations, reductants, and serum albumin. It was examined in live-cell imaging studies to monitor time-dependent lysosomal acidification, for which the increase in observed image brightness due to acidification was a factor of 50 in NIH-3T3 cells.     

Dependence on pH of the Luminescent Properties of Metal Ion Complexes of 5-Chloro-8-hydroxyquinoline Appended Diaza-18-Crown-6

Luminescent properties of 5-chloro-8-hydroxyquinoline (CHQ) free and appended tothe amines in diaza-18-crown-6 (A₂18C6) were determined. These propertieswere compared to those of bivalent alkaline earth and post-transition metal ioncomplexes of the appended macrocycle (CHQ-A218C6). The luminescent properties were foundto be pH dependent. In the pH range 3 to 7, CHQ-A₂18C6 forms luminescent complexes withonly Zn²⁺ and Cd²⁺. At higher pH values, luminescent complexes wereformed with Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺. No luminescent complex was formed by Hg²⁺ over the pH range studied. This lariat macrocycle could findapplication as a chemosensor for several of the metal ions studied.     

Novel optical trace oxygen sensors based on platinum(II) and palladium(II) complexes with 5,10,15,20-meso-tetrakis-(2,3,4,5,6-pentafluorphenyl)-porphyrin covalently immobilized on silica-gel particles

New optical sensors for trace amounts of oxygen are based on platinum(II) and palladium(II) complexes of 5,10,15,20-meso-tetrakis-(2,3,4,5,6-pentafluorphenyl)-porphyrin covalently attached to the surface of amino-modified silica-gel particles. The dye-doped silica-gel particles are dispersed in silicone rubber. The Stern–Volmer plots show linear response and are virtually identical for either luminescence intensity and decay time. Other features include high photostability and rapid response times (∼150 ms in gas phase). The sensors based on the palladium(II) complex show significantly higher sensitivity (K_SV about 67 kPa⁻¹ at 25 °C) with the dynamic range from 0.02 to 100 Pa. The sensitivity of the platinum(II) complexes is significantly lower (K_SV = 3.7–4.2 kPa⁻¹, dynamic range 0.3–1000 Pa). The sensors can be suitable for application in breweries, water boilers and for marine research (monitoring of oxygen minimum zones).     

Studies on the photophysical characteristics of poly(carboxylic acid)s bound protoporphyrin IX and metal complexes of protoporphyrin IX

The copolymers of methacrylic acid with protoporphyrin IX (PPIX) and the metal complexes, zinc protoporphyrin IX and magnesium protoporphyrin IX were synthesised and characterised. Corresponding acrylic acid copolymers were also synthesised. The steady state absorption and fluorescence spectral properties of the macromolecular bound fluorophores PPIX, Zn-PPIX and Mg-PPIX were investigated. Poly(methacrylic acid) bound protoporphyrin IX, zinc protoporphyrin IX and magnesium protoporphyrin IX show an increase in the fluorescence intensity and lifetime with increase in the pH in the range 2-8 with a marked transition around pH 6.0-7.0. The fluorophore concentration in the dilute solution of the copolymers is micromolar and the fluorophore to the carboxylic acid monomer ratios in the copolymer is around 10⁻³. The molecular weight of the copolymers is 100 ± 10 kD. The fluorescence decay curves of all the fluorophore bound polymers follow biexponential decay fit independent of pH. Poly(MAA-co-PPIX) and poly(MAA-co-MgPPIX) undergo well marked pH induced structural transitions in the pH range of 6.0-7.0 whereas poly(MAA-co-ZnPPIX) undergoes pH induced structural transitions in the pH range of 4.0. In the case of polyacrylic acid copolymers the changes observed in the steady state and time resolved fluorescence studies are less marked. The distinct hydrophobic and hydrophilic environments experienced by the fluorophore bound to PMMA are attributed to the dynamics of the macromolecules in dilute aqueous solutions manifested by the α-methyl group present in the copolymer. The studies carried out using the fluorophores in the time windows from 2 ns to 12 ns indicate evolving trends in the dynamic coiling and reverse coiling of poly methacrylic acid chain.     

Development of Optical Sensors Based on Luminescent Transition-Metal Complexes

Luminescence-based optical sensors are becoming increasingly important particularly in the area of fibber optic sensors. Most luminescence sensors detect analyses based on the change in luminescent intensity or excited-state lifetime of the sensing material as a function of analyst concentration. Luminescent materials with long excited-state lifetime are essential for the development of inexpensive sensors because it is much simpler and less expensive to measure lifetime in microseconds than in nanoseconds. Luminescent dyes with absorption in the visible region would allow the use of inexpensive light sources such as light emitting diodes (LEDs). Recent studies indicate that luminescent transition metal complexes, especially those with platinum group metals, have desirable spectral characteristics and features including long excited state lifetimes, high luminescence quantum yields and intense visible absorptions. They also tend to be thermally, chemically and photochemically robust. We have been investigating new luminescent sensor materials with the aims to understand the behaviour of luminescent materials in polymer and gel support and to develop new sensor materials with desirable properties.     

On the causes of altered photophysics of luminescent metal complexes embedded in polymer hosts

A suite of luminescent Re(I) complexes has been prepared whose emissive properties are responsive to the probe's local environment. These complexes were embedded in a series of chemically similar polymers whose room temperature rigidity varied over a significant range. It is shown that the degree of local rigidity experienced by the embedded complexes significantly alters the observed emission in terms of both spectra and lifetime. Time resolved emission measurements show that the spectral shifts and lifetime complexity are correlated and track well the polymers' T(g) within the series. Fluorescence confocal microscopy did not show the presence of discrete domains, and thus, the environmental features responsible for the altered photophysics must be submicrometer in size.     

Buffer and sugar concentration dependent fluorescence response of a BODIPY-based aryl monoboronic acid sensor

The emission intensity response from a BODIPY-based aryl monoboronic acid was found to depend on the employed buffer at physiological pH (7.4). The structural prerequisites of the bound monosaccharides showed a clear discrimination between d-glucose and d-fructose. The different response behavior was ascribed to a major difference in the structural features of the arylboronate-monosaccharide complexes, under the employed buffered conditions. Supporting excitation measurements were found to correlate nicely with the emission experiments.     

Luminescent nanobeads for optical sensing and imaging of dissolved oxygen

A variety of luminescent oxygen nanosensors were prepared by addressable staining of poly(styrene-block-vinylpyrrolidone) nanobeads with metal–ligand complexes whose luminescence is quenched by oxygen. They display optimal sensitivity in responding to dissolved oxygen in concentrations from 0 to 100% air saturation. The nanobeads based on cyclometallated iridium(III) complexes with coumarins are especially promising due to excellent brightnesses. The nanosensors respond virtually in real time to altering oxygen concentration and are capable of recording even very rapid changes in oxygen partial pressure. Signals are obtained by determination of luminescence lifetime in the frequency domain and in the time domain, and by ratiometric measurement of luminescence intensity. The nanosensors have been applied to sensing and imaging of dissolved oxygen, to monitor the consumption of oxygen during enzymatic oxidation of glucose, and to monitoring dissolved oxygen in a growing culture of E. coli.     

A ratiometric approach for pH optosensing with a single fluorophore indicator

A new fiber-optic prototype of luminometer has been designed in order to perform ratiometric-based measurements for optical sensing purposes. The coupling of a pH-selective sensing phase to the fiber-optic prototype has been evaluated for robust pH optosensing in drinking water. The pH-sensitive material has been synthesized by entrapping a pH-sensitive luminescent indicator (mercurochrome) in a sol-gel inorganic matrix. The pH optosensing is based on the detection of pH-induced reversible changes in the mercurochrome fluorescent emission and in the light reflected by the sensing phase.The instrument has been constructed using low-cost and simple optoelectronic components. The active phase was excited by means of a visible 470 nm high intensity light emitting diode (LED). The radiant power of the LED was modulated using a sinusoidal function so that scattered light due to light sources of different frequency than the modulating signal (e.g. sunlight) can be easily removed by adequate electronic filtering of the emission signal. Both the fluorescence emission from the dye and the sensing phase reflected light were collected in a bifurcated fiber-optic to allow the ratiometric measurement.Two different ratiometric approaches have been evaluated. The analytical performance of the pH optrode using both measurement methods have been compared, between them and with simple fluorescence intensity measurements, in terms of sensitivity, measurement range, response time, repeatability and insensitivity to changes in excitation light intensity.The applicability of the developed pH optrode and methods has been tested for pH analysis in tap and bottled still mineral water samples. The results obtained showed good agreement with the corresponding pH values provided by a commercial glass electrode.In this work, pH was selected as a model analyte to evaluate the performance of the proposed methodology, although other optical sensors for different applications/analytes could benefit of this approach.     

Inside/outside Pt nanoparticles decoration of functionalised carbon nanofibers (Pt(19.2)/f-CNF(80.8)) for sensitive non-enzymatic electrochemical glucose detection

A highly efficient and reproducible approach for effective Pt nanoparticles dispersion and excellent decoration (inside/outside) of functionalised carbon nanofibers (f-CNF) is presented. The surface morphological, compositional and structural characterisations of the synthesised Pt(19.2)/f-CNF(80.8) material were examined using transmission electron microscopy (TEM/STEM/DF-STEM), energy-dispersive X-ray spectrometry (EDS), thermogravimetric analysis (TGA/DTG), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Cyclic voltammetry (CV) was employed in order to confirm the typical electrochemical response for Pt. The aim of the work was to improve the utility of both the supporting matrix (via the use of both inner/outer surfaces of nanofibers) and precious Pt, together with the sensitive glucose determination. TEM data indicated successful nanoparticle decoration with average Pt particle size 2.4 nm. The studies demonstrated that utilisation of the inner surface of the nanofibers, together with the modified outer surface characteristics using chemical treatment, enables excellent decoration, effective dispersion and efficient impregnation of Pt nanoparticles on carbon nanofibers. Pt(19.2)/f-CNF(80.8) exhibited excellent amperometric response (sensitivity = 22.7 μAmM(-1)cm(-2) and LoD = 0.42 μM) towards direct glucose sensing, over the range 0-10 mM glucose, in neutral conditions (pH 7.4). The improved carbon surface area for nanoparticle decoration, inner surface structure and morphology of nanofibers together with the presence of functional groups provided strong interactions and stability. These features together with the effective nanoparticle dispersion and decoration resulted in excellent catalytic response. The decorated nanoscaled material (Pt(19.2)/f-CNF(80.8)) is capable of large scale production, providing sensing capability in neutral conditions, while eliminating the temperature sensitivity, pH and lifetime issues associated with glucose enzymatic sensors and holds great promise in the quantification of glucose in real clinical samples.     

Luminescent nanoparticles prepared by encapsulating lanthanide chelates to silica sphere

The luminescent nanoparticles were prepared by encapsulating the [LnL₄]^? (Ln = Eu, Tb; L = BTFA, HFAA, TTFA, TFAA) complexes anion into the silicon framework. We firstly synthesized a series of novel siloxy-bearing lanthanide complex precursor, and then encapsulated them into the silica sphere by a modified Stöber process. As a result, four europium and two terbium tetrakis β-diketonate complexes functionalized silica sphere nanoparticles were obtained and characterized in detail using Fourier transform infrared spectra, X-ray diffraction, scanning electronic microscope, thermogravimetric analysis, luminescence excitation and emission spectroscopy, luminescence lifetime measurements, and diffuse reflectance UV–Vis spectroscopy. The result shows that these luminescent nanoparticles maintain the distinctive luminescence character of lanthanide chelate including broad excitation spectra, line-like emission spectra, high quantum efficiency, and long luminescent lifetime, which makes them great potential application in the synthesis of luminescent nanoparticle.     

Solution Chemistry of Europium(III) Aqua Ion at Micromolar Concentrations as Probed by Direct Excitation Luminescence Spectroscopy

Direct excitation europium(III) luminescence spectroscopy is used to study the speciation of aqueous europium(III) ions at micromolar concentrations and near neutral pH. The pH and concentration dependence of the europium(III) ⁷F₀→⁵D₀ excitation peak is consistent with the formation of both mononuclear and dinuclear europium(III) hydroxide complexes at pH 6.5. Luminescence intensity and lifetime quenching studies in the presence of Nd^III at pH 5.0 and 6.5 support the formation of a dinuclear complex at pH 6.5. Steady state excitation and time‐resolved luminescence spectroscopy are consistent with the formation of innersphere nitrate and fluoride complexes, but outersphere perchlorate and chloride complexes at pH 6.5 and 5.0.     

Luminescence decay time encoding of magnetic micro spheres for multiplexed analysis

Magnetic microspheres are optically encoded by doping with three luminescent dyes. The combination of a fluorophore with a nanosecond decay profile and two phosphorescent Ruthenium metal ligand complexes with a microsecond decay profile generates a characteristic signature described by three features: bead brightness, luminescent decay time and dual lifetime referencing (DLR). The beads are identified by time resolved imaging in the microsecond range. A series of fluorophores is tested and the interference of the resulting luminescent code in the red and green label detection channels is investigated. A detailed staining procedure is worked out to increase the staining efficiency of the dyes with hydrophilic character into the lipophilic polystyrene microspheres. A mathematical model is established to calculate the dye amounts that are needed for staining a bead family with a specific feature set. Nineteen bead families were prepared representing the grid points in the three planes of a cube referring to the three features. The coefficient of variation over all bead families is 7%, 1.4% and 1.6% for bead brightness, luminescence decay time and DLR, respectively. The combination of these features and the bead size as additional feature enables the creation of 840 distinguishable bead families.     

Highly Selective Potentiometric Oxalate Ion Sensors Based on Ni(Ⅱ) Bis-(m-aminoacetophenone)ethylenediamine

A plasticized PVC (polyvinyl chloride) membrane based oxalate ion selective electrode has been developed by using the condensation product of m‐aminoacetophenone and ethylenediamine. The transition metal complexes of the ligand N,N′‐bis(m‐aminoacetophenene)ethylenediamine (L) were synthesized and incorporated as ionophore for the synthesis of oxalate ion selective electrodes. Most appropriate result in terms of dynamic range, detection limit and response behavior was determined for the Ni(II) bis‐(m‐aminoacetophenone)ethylenediamine complex. The electrode demonstrated higher selectivity for oxalate ion with improved performance as compared to other carriers reported in past. The electrode shows Nernstian slope of (?28.5±0.4) mV·decade^?1 with improved linear range of 1×10^?1?1×10^?7 mol·L^?1, with a comparatively lower detection limit in the pH range of 5–10.5, giving a relatively fast response within 10 s and reasonable reproducibility. The selectivity coefficient was calculated using matched potential method and fixed interference method. The lifetime of the electrode was found to be nearly 2 months. The response mechanism and the interaction of oxalate ion with the complexes have been discussed by UV‐visible spectroscopic technique. Further the electrode was also successfully applied to determine the oxalate content in water samples.     

Mixed Ligand Complexes of Am3+, Cm3+ and Eu3+ with HEDTA and HEDTA + NTA—Complexation Thermodynamics and Structural Aspects

The stability constants and the associated thermodynamic parameters of formation for the 1:1 binary complexes of Am³⁺, Cm³⁺ and Eu³⁺ with N-(2-hydroxyethyl) ethylenediaminetriacetate (HEDTA) and their 1:1:1 ternary complexes with HEDTA + NTA (nitrilotriacate) were determined by distribution ratio measurements using solvent extraction in aqueous solutions of I=0.10?mol?L^?1 (NaClO₄) at temperatures of 0?C45?°C. Formation of these complexes is favored by both the enthalpy (exothermic) and the entropy (endothermic) terms. Luminescence lifetime measurements with Cm and Eu were used to study the coordination environment of these complexes over a range of concentrations and pH values. In the binary complexes M(HEDTA), HEDTA is a hexadentate ligand with three waters of hydration, while in the ternary complexes M(HEDTA)(NTA)^3? we propose that the HEDTA retaines hexadentate coordination with NTA binding via three sites, depending on the pH of the solution, with the observation that the complex may contain a single water of hydration.     

X-ray structure and temperature dependent luminescent properties of two bimetallic europium complexes

The structural, luminescent and temperature dependent luminescent properties of two homodinuclear europium complexes bridged by 2,2′-bipyrimidine (bpm) are reported. β-Diketonate ligands 4,4,4-trifluoro-1-(2-furyl)-1,3-butanedione (tfa) and 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione (tta) are used as capping ligands resulting in complexes of the form [Eu(tfa)₃]₂bpm (1) and [Eu(tta)₃]₂bpm (2). All Eu^III ions are eight coordinate with six O atoms from the β-diketones and two N atoms from the polyazine bridging ligand. Excitation of the β-diketonate ligands tfa or tta at ca. 340 nm in toluene solutions results in the characteristic Eu^III emission in the visible region of the spectrum. The emission intensity and lifetime associated with the Eu^III centers decrease as the temperature of the solution is increased. Lifetime measurements are fit to a monoexponential while the temperature dependent lifetime data is fit to an Arrhenius-type equation. Evaluation of the data in comparison to data obtained from the monometallic Eu^III analogs reveal very similar photoluminescent properties. This suggests little electronic communication between Eu^III ions via the polyazine bpm bridging ligand.     

Pyrazole‐substituted Near‐infrared Cyanine Dyes Exhibit pH‐dependent Fluorescence Lifetime Properties

Near‐infrared heptamethine cyanine dye is functionalized with pyrazole derivatives at the meso‐position to induce pH‐dependent photophysical properties. The presence of pyrazole unsubstituted at ¹N‐position is essential to induce pH‐dependent fluorescence intensity and lifetime changes in these dyes. Replacement of meso‐chloro group of cyanine dye IR820 with ¹N‐unsubstituted pyrazole resulted in the pH‐dependent fluorescence lifetime changes from 0.93 ns in neutral media to 1.27 ns in acidic media in DMSO. Time‐resolved emission spectra (TRES) revealed that at lower pH, the pyrazole consists of fluorophores with two distinct lifetimes, which cor‐responds to pH‐sensitive and non‐pH‐sensitive species. In contrast, ¹N‐substituted pyrazoles do not exhibit pH response, suggesting excited state electron transfer as the mechanism of pH‐dependent fluorescence lifetime sensitivity for this class of compounds.