Fluorescent Compounds for Calibration of Excitation and Emission Units of Spectrofluorometer

Characterization of photoluminescence measuring systems (IUPAC Technical Report)

Procedures for the characterization of photoluminescence measuring systems are discussed, focusing on spectrofluorometers and fit-for-purpose methods including suitable standards. The aim here is to increase the awareness for the importance of a reliable instrument characterization and to improve the reliability and comparability of measurements of photoluminescence.

State-of-the art comparability of corrected emission spectra. 1. Spectral correction with physical transfer standards and spectral fluorescence standards by expert laboratories

The development of fluorescence applications in the life and material sciences has proceeded largely without sufficient concern for the measurement uncertainties related to the characterization of fluorescence instruments. In this first part of a two-part series on the state-of-the-art comparability of corrected emission spectra, four National Metrology Institutes active in high-precision steady-state fluorometry performed a first comparison of fluorescence measurement capabilities by evaluating physical transfer standard (PTS)-based and reference material (RM)-based calibration methods. To identify achievable comparability and sources of error in instrument calibration, the emission spectra of three test dyes in the wavelength region from 300 to 770 nm were corrected and compared using both calibration methods. The results, obtained for typical spectrofluorometric (0°/90° transmitting) and colorimetric (45°/0° front-face) measurement geometries, demonstrated a comparability of corrected emission spectra within a relative standard uncertainty of 4.2% for PTS- and 2.4% for RM-based spectral correction when measurements and calibrations were performed under identical conditions. Moreover, the emission spectra of RMs F001 to F005, certified by BAM, Federal Institute for Materials Research and Testing, were confirmed. These RMs were subsequently used for the assessment of the comparability of RM-based corrected emission spectra of field laboratories using common commercial spectrofluorometers and routine measurement conditions in part 2 of this series (subsequent paper in this issue).

Tunable phosphorescent NIR oxygen indicators based on mixed benzo- and naphthoporphyrin complexes

A series of π-extended phosphorescent palladium(II) and platinum(II) porphyrin complexes were synthesized, in which additional benzene rings are fused radially onto at least one of the four peripheral benzo groups. The photophysical properties of the metalloporphyrins palladium(II)-meso-tetra-(4-fluorophenyl)mononaphthotribenzoporphyrin (Pd1NF), cis-palladium(II)-meso-tetra-(4-fluorophenyl)dibenzodinaphthoporphyrin (Pd2NF), and palladium(II)-meso-tetra-(4-fluorophenyl)monobenzotrinaphthoporphyrin (Pd3NF) and the corresponding platinum(II) compounds (Pt1NF, cis-Pt2NF, Pt3NF) were investigated. The compounds under investigation absorb intensively in the near-infrared region (628-691 nm) and emit at room temperature at 815-882 nm. Phosphorescence quantum yields of the platinum(II) porphyrins range from 25 to 53% with luminescence decay times of 21 to 44 μs in deoxygenated toluene solutions at room temperature. The corresponding palladium(II) complexes exhibit quantum yields in the range of 7 to 18% with lifetimes of 106 to 206 μs. Density functional theory (DFT) calculations revealed nonplanar geometries for all complexes and corroborate the absorption characteristics. The subsequent π extension of the porphyrin system leads to near-infrared absorbing oxygen indicators with tailor-made luminescence properties as well as tunable oxygen sensitivity.

Fluorescence standards: Classification, terminology, and recommendations on their selection, use, and production (IUPAC Technical Report)

Chromophore-based fluorescence standards for the characterization of photo-luminescence measuring systems and the determination of relevant fluorometric quantities are classified according to their scope and area of application. General and type-specific requirements for suitable standards are derived for each class of standards. Metrological requirements linked to the realization of comparable measurements are addressed and recommendations on selecting, using, and developing fluorescence standards are given.

FLUORESCENCE POLARIZATION: MEASUREMENT WITH ULTRAVIOLET-POLARIZING FILTERS IN A SPECTROPHOTOFLUOROMETER

Ultraviolet-polarizing filters with large apertures facilitate the use of spectrophotofluorometers for measuring fluorescence polarization. Such filters (i) allow the use of polarized exciting light even in the ultraviolet region, (ii) make it possible to correct for grating anomalies which influence the observed polarization, and (iii) because of good transmission properties allow the entrance of sufficient light so that small slits and dilute solutions can be used. Under optimum conditions, polarization can be measured with a precision of +/-0.002. Results obtained are in excellent agreement with values cited elsewhere and indicate that standard spectrophotofluorometers can easily be adapted for accurate measurement of polarization.

Photochemistry of merocyanine 540: the mechanism of chemotherapeutic activity with cyanine dyes

The photophysical properties of merocyanine 540 have been determined in methanol solution over a modest temperature range. Triplet state population is inefficient (the limiting triplet quantum yield being 0.25) due to rapid isomerization of the central double bond from the first excited singlet state. Activation energies have been measured for isomerization from the excited singlet state (20 kJ mol-1) and for conversion of the resultant cis-isomer back to the original trans-form (63 kJ mol-1), both processes involving formation of a twisted species. The dye is easily oxidized to give an unstable adduct which decomposes on the sub-ms timescale. Reversible redox chemistry occurs upon excitation in the presence of electron acceptors. These various observations are discussed in terms of the known chemotherapeutic activity of MC540 and it is concluded that the most probable mechanisms for cytotoxicity involve either local thermal disruption of cell membranes or in situ photogeneration of toxins derived from breakdown of the dye.

In vitro photodynamic activity of kryptocyanine

The photophysical properties of 1,1'-diethyl-4,4'-carbocyanine chloride (kryptocyanine) have been measured in methanol solution and for the dye bound to human serum albumin, incorporated in neutral micelles and after incubation with leukemia cells. In all cases, it is found that formation of the triplet state of the dye occurs with low efficiency and that illumination of the dye under aerobic conditions does not produce significant yields of O2(1 delta g). Instead, the only efficient photoprocess involves rapid internal conversion from the first excited singlet state to the ground state, probably via isomerization of the polymethine sequence. These findings are discussed with respect to the demonstrated ability of kryptocyanine to photodestroy leukemic cells.

Photosensitized oxidation of biomaterials and related model compounds

Aluminium trisulfonatophthalocyanine (A1PCS), a dye being widely advocated for use in photodynamic therapy, produces singlet oxygen with a quantum yield of 0.34 in oxygenated water at pH 7. Triplet A1PCS abstracts an electron from a variety of amines and phenols, the rate of electron transfer depending upon the thermodynamic driving force, forming the A1PCS radical anion. This latter species reduces molecular oxygen to superoxide ions with high efficiency. The triplet state also abstracts an electron from biological components, including NADH, vitamin C, cysteine, methionine, tyrosine, tryptophan, uracil, and guanine, but not from DNA. These results suggest that photoinduced electron abstraction from appropriate biomaterials could compete with singlet oxygen production under in vivo conditions.

Nile red as a polarity-sensitive fluorescent probe of hydrophobic protein surfaces

Nile red is an uncharged hydrophobic molecule whose fluorescence is strongly influenced by the polarity of its environment. It interacts with many, but not all, native proteins, including beta-lactoglobulin, kappa-casein, and albumin, with a wide range of spectral changes for different proteins. It detects the exposure or formation of new hydrophobic surfaces induced by ligand binding to calmodulin, oligomerization of melittin, or unfolding of ovalbumin during early thermal denaturation. The dye is photostable, the working wavelength range is broad and removed from those at which many biomolecules absorb, the fluorescence is unaffected by pH between 4.5 and 8.5, the quantum yield is high, and hydrophobic sites on proteins may be investigated in dilute solutions.

Microspectrofluorometry in biogenic amine research

Microspectrofluorometry has played an important part in the development and understanding of fluorescence techniques for the demonstration of biogenic amines. This paper reviews briefly the techniques currently available for the characterization and localization of biogenic amines, and considers the basic design considerations applicable to the construction of a microspectrofluorometer, with special emphasis on the automated correction of spectral data. Spectral data for the commonly occurring biogenic amines dopamine, noradrenaline and 5-hydroxytryptamine have been summarized and the criteria applicable to the spectral analysis of the individual amines and their mixtures discussed. Discrepancies between spectral data derived from tissues and model systems are reviewed and emphasis is given to the importance of biochemical studies of amines prior to definitive microspectrofluorometric analysis. The conceptual basis of quantitative microspectrofluorometry is reviewed with a particular emphasis on the limitations of such studies. The analysis of terminal fields by scanning microspectrofluorometry and its application to the study of extracellular amine distribution is discussed.

Determination of oxytetracycline induced fluorescence and autofluorescence as an indicator of age and sex differences in rat femur

Simple and rapid methods for the determination and detection of autofluorescence and oxytetracycline (OTC) induced fluorescence in powdered and intact bone (without previous chemical extraction of the antibiotic) were used to estimate age and sex related differences in the rat femur. The autofluorescence always had collagen characteristics; it increased with age and was not sex dependent. The intensity of OTC induced fluorescence decreased with age, being higher in males than in females. In the oldest animals, however a slight increase in the fluorescence intensity was observed, and sex differences disappeared. The spectrofluorometric measurements of intact femora indicate a higher OTC retention on the bone surface of young animals and adult males but not in adult females. The results indicate that the determination of OTC retention in the bone using our quantitative fluorometric method and the detection of OTC deposition at bone surface using a spectrofluorometric technique can be a sensitive and inexpensive tool for experimental studies of bone changes in physiological and pathological conditions.

Standardization of fluorescence spectra and the calibration of spectrofluorimeters

A technique is proposed that can be used to calibrate the amplitude of fluorescence spectra so that spectra reported by different laboratories can be compared quantitatively. The technique is based upon the scattering of light by a solution of polystyrene. In order to compare quantitatively data obtained from different spectrofluorimeters, these terms are defined: the spectrofluorimetric sensitivity, the noise-equivalent concentration, and the fluorophotometric reproducibility. The author shows how these quantities can be measured with the polystyrene standard.