Total Emission Time Resolved Decay: a Method for Measurement and Resolution of Broad-Band Emission.

This article reports a time-resolved fluorescence data acquisition technique termed as "Total Emission Time Resolved Decay" (TETRD). TETRD is recorded by using zero-order diffraction of emission grating in TCSPC instrument. TETRD decay curve has entire wavelength dependent decay information buried in it. Cut-off filters are used to avoid scattering contamination. Two existing approaches are used for analysing the interconnected TETRD data. (i) First, global analysis: for discretely decaying multiple components, TETRD dataset is analyzed using global analysis. The normalized pre-exponentials (αi) and relative amplitudes (fi) recovered from global analysis reflect the individual component emission more faithfully and resembles with steady-state spectral data as well. (ii) Second, stretched exponential fitting (StrEF): for continuous lifetime distribution systems, StrEF (I(t) = I0 exp[-(t/τ)1/h]) has been used to analyse TETRD data. The average lifetime (τ) of StrEF matches well with the average lifetime of multi-exponential fitting, the heterogeneity factor (h) of StrEF is an additional parameter, which informs about local heterogeneity in the system. It is shown that the lifetimes obtained with TETRD matches well with the lifetimes obtained using conventional time resolved emission spectra (TRES). TETRD holds advantage in rapid data acquisition and facilitates inclusion of another variable (like concentration, solvent composition, pH, excitation wavelength etc.) into experimental design. Further, with the use of an appropriate data analysis tool, the multi-component decay profiles can be resolved conveniently.

Study on the dependence of fluorescence intensity on optical density of solutions: the use of fluorescence observation field for inner filter effect corrections.

In this study, we report the identification of absorbance value of an analyte at the excitation wavelength that corresponds to the maximum of the observed fluorescence intensity obtainable for a certain instrument operating with right-angle fluorescence measurement (). The value depends on the fluorescence observation field (FOF) dimensions of the concerned spectrofluorometer. As the FOF varies from instrument to instrument, this study presents a simple method for obtaining FOF dimensions. Using the knowledge of FOF, absorbance of analyte at the excitation wavelength (Aλex) and emission wavelength (Aλem), we deduced a derived absorbance spectral parameter (Dabs). The observed fluorescence intensity of an analyte is proportional to the Dabs. While differentiating Dabs w.r.t. Aλex, the value of for the concentred spectrofluorometer was obtained and subsequently could be used for maximizing fluorescence sensitivity. It was observed that when the FOF was a point at the centre of a 1 cm path-length cuvette, the value was 0.87 with a progressive widening of FOF, the value increased gradually till ∼1.0. The proposed methodology was established using two well-known inner filter effect (IFE) correction models (Parker and Lakowicz model). The Dabs obtained from the Parker model corresponded well with the observed fluorescence data; however, the Dabs obtained using the Lakowicz model overestimated the loss of fluorescence because of IFE. Using equations derived from the Parker model, the correction of observed fluorescence intensity for IFE could be achieved. Furthermore, it is demonstrated that the commonly used the Lakowicz model loses its correction efficiency at absorbance values of ≥0.7.

Derived Absorbance Spectral Parameter as a Tool for Sensitive Fluorescence Measurements of Optically Dense Systems.

The present study proposes an approach to use the derived absorbance spectral parameter (Dabs) as a tool for sensitive fluorescence measurement of optically dense multifluorophoric systems. Mishra and group have demonstrated the use of Dabs in obtaining optimum offset wavelength for sensitive fluorescence measurement in synchronous fluorescence spectra (SFS) (Anal. Chim. Acta 2016, 940, 113-119; Anal. Chim. Acta 2008, 630 (1), 47-56). The Dabs is obtained with the knowledge of UV-vis absorbance spectral data of the system under study and fluorescence observation filed (FOF) parameters of the instrument. Dabs is proportional to the observed fluorescence intensity of the system. Excitation of an optically dense sample at Dabs maximum leads to maximum fluorescence intensity obtainable for that system even in the presence of considerable inner filter effect (IFE). The earlier studies were applicable to SFS at right-angle fluorescence measurement geometry and for samples showing only primary IFE. In the present study, the work has been expanded to general fluorescence measurements, even when both primary and secondary IFE is present and for different fluorescence measurement geometries (right-angle, 45° and front-face). The procedure for obtaining Dabs is simple and can be applied to other analyte-specific/unconventional fluorescence measurement strategies reported in the literature.

Phenothiazinyl Boranes: A New Class of AIE Luminogens with Mega Stokes Shift, Mechanochromism, and Mechanoluminescence.

Phenothiazines with a dimesityl boron moiety, a new class of aminoboranes with B-N linkage, were synthesized. These aminoboranes exhibited interesting photophysical behavior including aggregation-induced emission (AIE), mechanochromism (MC), mechanoluminescence (ML), and a mega Stokes shift (up to 312 nm in hexane). The solid-state emission of the aminoboranes could be switched reversibly by grinding-fuming processes. Furthermore, the phenothiazine derivative with a bromo and an arylborane group at 3- and 7-positions exhibited bright mechanoluminescence.

Understanding the photophysics of stercobilin-Zn(II) and urobilin-Zn(II) complexes towards faecal pigment analysis.

A detailed photophysical study of two faecal pigments (FPs), Urobilin (UB) and Stercobilin (SB), and their zinc complexes [FP-Zn(II)] was carried out. The enhancement of UB and SB fluorescence resulting from the formation of their Zn(II) complexes was attributed to the complexation-induced rigidity of the chromophoric units, and the corresponding decrease of nonradiative decay rate constants of the excited singlet states (knr). The effect of various physicochemical environments was also studied in detail in order to understand the fluorescence behaviour of the Zn(II) complexes. FP-Zn(II) complexes have a lower solubility in water that results in the formation of molecular aggregates. The aggregation-induced loss of fluorescence of FP-Zn(II) complexes could be overcome by using the appropriate mixture of ethanol and water (70:30). Molecular orbital calculations on the FP-Zn(II) complexes provided a good idea of the geometry of the complexes and helped rationalise the enhancement of fluorescence after complexation. This study could pave the way towards developing a convenient non-extraction aqueous phase analytical procedure for detection of FPs using Zn(II) complexation method.

Use of zero order diffraction of a grating monochromator towards convenient and sensitive detection of fluorescent analytes in multi fluorophoric systems.

White light excitation fluorescence (WLEF) is known to possess analytical advantage in terms of enhanced sensitivity and facile capture of the entire fluorescence spectral signature of multi component fluorescence systems. Using the zero order diffraction of the grating monochromator on the excitation side of a commercial spectrofluorimeter, it has been shown that WLEF spectral measurements can be conveniently carried out. Taking analyte multi-fluorophoric systems like (i) drugs and vitamins spiked in urine sample, (ii) adulteration of extra virgin olive oil with olive pomace oil and (iii) mixture of fabric dyes, it was observed that there is a significant enhancement of measurement sensitivity. The total fluorescence spectral response could be conveniently analysed using PLS2 regression. This work brings out the ease of the use of a conventional fluorimeter for WLEF measurements.

Novel use of UV broad-band excitation and stretched exponential function in the analysis of fluorescent dissolved organic matter: study of interaction between protein and humic-like components.

A combination of broad-band UV radiation (UV A and UV B; 250-400 nm) and a stretched exponential function (StrEF) has been utilised in efforts towards convenient and sensitive detection of fluorescent dissolved organic matter (FDOM). This approach enables accessing the gross fluorescence spectral signature of both protein-like and humic-like components in a single measurement. Commercial FDOM components are excited with the broad-band UV excitation; the variation of spectral profile as a function of varying component ratio is analysed. The underlying fluorescence dynamics and non-linear quenching of amino acid moieties are studied with the StrEF (exp(-V[Q] ß )). The complex quenching pattern reflects the inner filter effect (IFE) as well as inter-component interactions. The inter-component interactions are essentially captured through the 'sphere of action' and 'dark complex' models. The broad-band UV excitation ascertains increased excitation energy, resulting in increased population density in the excited state and thereby resulting in enhanced sensitivity.