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Photoionization (PI) and photoinduced electron transfer (PET) dynamics of coumarin 450 (C450) in micelles were investigated in the time domains of micro to femtoseconds using steady-state and time-resolved absorption and fluorescence spectroscopy. The PI of C450 occurs inside the micelles leads to the formation of C450 cation radical (CR) and hydrated electron, which is characterized by the respective transient absorption. The PI of C450 is monophotonic in nature and the yield is dependent on the charge of the micelles. The observation of amine CR in the transient absorption confirms the PET from amine to the excited state of C450 in micelles, which results in the quenching of both fluorescence intensity and lifetime. The decrease in femtosecond fluorescent decay of C450 and the absence of transient C450 radical anion in the presence of amine implies that the concerted ultrafast PET promoted PI and PET to the C450 CR–electron pair. The decrease in the time constant for the formation of relaxed state in the presence of amines is due to the ultrafast PET to the C450 CR–electron pair, which prevents the formation of a relaxed state through recombination at a longer time scale. In the present investigation, the recombination dynamics of the CR–electron pair is justified as one of the origins of the slow solvation in micelles. The influence of amine concentration on the decay of C450 CR indicates ET reaction between C450 CR and amine, which is further confirmed by the bleach recovery of C450 ground state in the presence of amine. [ABSTRACT FROM AUTHOR]

Highlights • A fluorogenic detection of Co2+, Cd2+ and Pb2+ metal ions was investigated using surface functionalized MoS 2 quantum dots. • The MoS 2 /COOH, MoS 2 /NH 2 and MoS 2 /SH QDs were prepared by facile hydrothermal method. • A lower detection limit was achieved. • Practicality of the sensors were evaluated in real water samples. Abstract The multiple sensing capabilities of molybdenum disulfide quantum dots (MoS 2 QDs) towards metal ions were scrutinized by tuning their surface functional groups. The MoS 2 QDs surface was individually modified with thiol-containing capping agents to form carboxylic-, amine- and thiol-functionalized MoS 2 QDs (MoS 2 /COOH, MoS 2 /NH 2 and MoS 2 /SH) by the facile hydrothermal method. Each as-prepared QDs exhibits strong excitation wavelength dependent fluorescence behavior. The design of MoS 2 QDs based metal ion sensor was implemented based on the fluorescence turn-on mechanism. These MoS 2 /COOH, MoS 2 /NH 2 and MoS 2 /SH QDs sensors exhibit superior performance towards the highly selective detection of Co2+, Cd2+ and Pb2+ ions, respectively, due to the varied association of each functional group towards metal ions. The resultant detection limit of Co2+, Cd2+ and Pb2+ was evaluated to be 54.5, 99.6 and 0.84 nM, respectively, and the related fluorescence turn-on mechanism is verified unambiguously. The binding energies were calculated for QDs with metal ions pairs and the results lent support to the determined sensitivity. The as-prepared QDs were also successfully demonstrated to detect the above metal ions in real water samples. While becoming potential candidates in the chemosensors based on the fluorescence probe, these surface modified MoS 2 QDs can offer an excellent sensing capability for specific metal ions with extremely high selectivity. [ABSTRACT FROM AUTHOR]

Gum Arabic (GA), a food hydrocolloid is a natural composite obtained from the stems and branches of Acacia Senegal and Acacia Seyal trees. GA structure is made up of highly branched arabinogalactan polysaccharides. Steady-state absorption, fluorescence, and time-resolved fluorescence spectral studies of acid hydrolyzed GA solutions were carried out at various pH conditions. The fluorescence in GA is predominantly attributed to the presence of tyrosine and phenylalanine amino acids. The presence of multi-emissive peaks at different pH condition is attributed to the exposure of the fluorescing amino acids to the aqueous phase, which contains several sugar units, hydrophilic and hydrophobic moieties. Time-resolved fluorescence studies of GA exhibits a multi-exponential decay with different fluorescence lifetime of varying amplitude which confirms that tyrosine is confined to a heterogeneous microenvironment. The existence of multi-emissive peaks with large variation in the fluorescence intensities were established by 3D emission contour spectral studies. The probable location of the fluorophore in a heterogeneous environment was further ascertained by constructing a time-resolved emission spectrum (TRES) and time-resolved area normalized emission spectrum (TRANES) plots. Fluorescence spectral technique is used as an analytical tool in understanding the photophysical properties of a water soluble complex food hydrocolloid containing an intrinsic fluorophore located in a multiple environment is illustrated. [ABSTRACT FROM AUTHOR]

[Display omitted] • DOPC and DPPC Si-SUVs with different contents of cholesterol were synthesized and characterized. • Silica nanoparticles were utilized to avoid the size effect on Si-SUVs membrane dynamics. • SM-FCS with auto- and cross-correlation analyses used to extract diffusion frequency and triplet-state lifetime of DiD dye. • 2D correlation plots were exploited to recognize Si-SUVs membrane dynamics as a function of cholesterol content. The effect of gradual cholesterol increase on the dynamics of 1,2-dioleoyl- sn -glycero-3-phosphocholine (DOPC) and 1,2-dipalmitoyl- sn -glycero-3-phosphocholine (DPPC) membranes of small unilamellar vesicles (SUVs) sized below 50 nm was explored using single-molecule fluorescence correlation spectroscopy (SM-FCS). The dye 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindodicarbocyanine (DiD) was entrenched into the membrane as a fluorescent probe, and silica nanoparticles were used to obtain SUVs of similar size, termed Si-supported small unilamellar vesicles (Si-SUVs). Si-SUVs of three different compositions for each DOPC and DPPC with cholesterol were synthesized. Auto-correlation and cross-correlation methods were implemented to analyze the fluorescence trajectories of single DiD molecules to determine their diffusion frequencies and triplet state lifetimes. The rigidity of the DOPC membrane increased with increasing cholesterol concentration, which led to an increase of the triplet state lifetime and a decrease in the diffusion frequency of DiD molecules. Thus, pure DOPC Si-SUVs, homogeneous in nature, showed a single block in the 2D correlation plot between triplet state lifetime and diffusion frequency, whereas multiple blocks appeared in the case of cholesterol-added DOPC Si-SUVs due to the inhomogeneity of the rigid membrane. On the contrary, DPPC Si-SUVs showed a reverse effect, where the membrane rigidity decreased with increasing cholesterol concentration. Thus, three blocks in the 2D plot of pure DPPC Si-SUVs were merged into one block when the cholesterol concentration in the membrane reached 40% of the total molar concentration as the membrane became homogeneous. [ABSTRACT FROM AUTHOR]

The dynamics and mechanism of the photoinduced electrontransfer(PET) reaction between coumarin 307 (C307) and aromatic amines inmicelles have been studied by using steady-state (S–S) andtime-resolved (T-R) absorption and fluorescence spectroscopy. Basedon the fluorescence quenching time scale, PET in micelles is groupedinto two types: (i) ultrafast electron transfer (ET) due to the closecontact of the donor and acceptor in micelles and (ii) diffusion averageddynamic electron transfer (DADET) which is controlled by the diffusionof the reactants in micellar Stern layer and diffusion of the micelles.The DADET does not affect the photoionization and solvation processeswhereas ultrafast ET competes with the photoionization and fasterthan the solvation process. Both ultrafast and DADET shows Marcusinversion in the ET rates at the similar exergonicity and indicatesthat the role of diffusion and solvent reorganization is negligibletoward the activation barrier for the ET reaction in micelles. Theactivation barrier for the ET reactions in micelles is mainly dueto intramolecular reorganization energy. The intramolecular reorganizationenergy must be higher in CTAB due to the photoionization and subsequentrecombination and also involvement of triplet state in the PET. TheET reaction between coumarin radical cation and amine is reportedfor the first time in the C307-amine systems in micelles which areconfirmed by the effect on amine concentration of the decay of coumarinradical cation and the dynamics of the ground-state recovery of C307.A mechanism for the PET reaction between C307–amine systemsis proposed in micelles including photoionization, ultrafast and dynamicET, and solvation dynamics. [ABSTRACT FROM AUTHOR]

Photophysical and electrochemical techniques were employed to hydrogen-bonding self assemblies forming solutes (Urea, Dimethylurea and Tetramethylurea) in the presence of 4-dicyanomethylene 2, 6-dimethyl-4H-pyran (DDP) dye. Addition of urea derivatives to DDP dye (Intramolecular Charge Transfer (ICT)) results in a fluorescence enhancement accompanied with a significant shift. Fluorescence lifetime behavior exhibits a tri-exponential decay with a large variation in the fluorescence lifetime and relative amplitude distribution. The coexistence of three different fluorescence lifetime components of DDP with urea derivatives signifies the existence of heterogeneous micro environment. The dye is surrounded by varying proportion of solute and water molecules are established from fluorescence lifetime studies. Urea derivatives govern the excited state characteristics of DDP dye resulting in the formation and promotion of different microenvironment which are clearly distinguishable. The existence of multi environment attributed to urea-water structural behaviour is authenticated by electrochemical impedance spectral studies (EIS). A large variation in the contour pattern, shape and intensity in 3D fluorescence contour spectra of dye with urea validate the existence of dye in a heterogeneous micro environment. The hydrophobicity of urea derivatives along with the hydrogen-bonding properties of urea-water and urea-urea influence the photophysical and electrochemical nature of dye is emphasized. Unlabelled Image • Photophysical and electrochemical properties of DDP dye with urea derivatives is entirely different from DCM type dyes. • Absorption, emission, and fluorescence lifetime of urea derivatives with DDP dye results in a fluorescence enhancement accompanied with shift in emission maxima and hydrogen-bonding interaction exists between DDP dye with urea derivatives. • Fluorescence spectral characteristics along with electrochemical impedance spectral behaviour of dye are confined to a heterogeneous environment. [ABSTRACT FROM AUTHOR]