Browsing by Author "Jockusch, Steffen"
Now showing 1 - 10 of 12
Results Per Page
Sort Options
- Aggregates of cucurbituril complexes in the gas phasePublication . Silva, José P. da; Jayaraj, Nithyanandhan; Jockusch, Steffen; Turro, Nicholas J.; Ramamurthy, V.The self-organization of cucurbit[n]uril (n = 7 and 8, CBs) complexes was probed by electrospray mass spectrometry. The self-association of CB complexes is a general phenomenon but shows some dependence on the absence, presence, and type of included guest molecules.
- Benzoin type photoinitiator for free radical polymerizationPublication . Esen, Duygu Sevinc; Arsu, Nergis; Silva, José P. da; Jockusch, Steffen; Turro, Nicholas J.Benzoin, a popular photoinitiator for free radical polymerization of vinyl monomers, was improved by introduction of two methyl thioether substituents. This new benzoin derivative showed an about 50 times higher light absorption in the near-UV spectral region and performed better than the unsubstituted benzoin in polymerization experiments in bulk solutions or films of acrylate monomers when low initiator concentrations are used. Laser flash photolysis, low temperature luminescence experiments and photoproduct studies by mass spectrometry suggest that a slow -cleavage mechanism (k = 2.2 x 105 s1) from the electronic triplet state with a quantum yield of 0.1 is the primary photoreaction to generate the initiating free radicals. (c) 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013, 51, 1865-1871
- Capsular complexes of nonpolar guests with octa amine host detected in the gas phasePublication . Silva, José P. da; Kulasekharan, Revathy; Cordeiro, Carlos; Jockusch, Steffen; Turro, Nicholas J.; Ramamurthy, V.Nanocapsules, made up of the deep cavitand octa amine and several guests, were prepared in aqueous acidic solution and were found to be stable in the gas phase as detected by electrospray ionization mass spectrometry (ESI-MS). The observed gas phase host-guest complexes contained five positive charges and were associated with several acid molecules (HCI or HBr).
- Fluorescence sensing of microplastics on surfacesPublication . Costa, Camila Q. V.; Cruz, Joana; Martins, Jorge; Teodosio, Maria; Jockusch, Steffen; Ramamurthy, V.; Da Silva, José PauloMicroplastics, nanoplastics and related products have been recently found in marine ecosystems worldwide, calling for new analytical methods for rapid detection and risk assessment. Fluorescence is a sensitive technique that when used with polarity probes can potentially detect low-polarity plastic particles in environments. Here, we evaluated the fuorescence technique to sense polystyrene microparticles directly on salt, silica and sand surfaces, using Nile Red and pyrene as polarity probes. Results show that all probes displayed fuorescence on silica and sand, whereas strong fuorescence quenching was observed on NaCl. Polystyrene particles increase the fuorescence intensity due to probe migration into their nonpolar microenvironment. In the presence of polystyrene, the spectra of Nile Red are shifted to shorter wavelengths, while the ratio of vibronic bands I1/I3 of fuorescence of pyrene decreases to about 1. Pyrene showed similar sensitivity toward surface-modifed carboxyl polystyrene particles. On NaCl, the emission of pyrene increases linearly with polystyrene content for concentrations from 0.5 to 20 µg/g. The detection limit of polystyrene microparticles on natural sea salt using pyrene as probe is about 0.2 µg/g, while on sand, the sensitivity is about one order of magnitude lower. Overall, although being of relatively low selectivity, the fuorescence technique can be used to determine a maximum content of plastic particles of few micrometers size with little sample preparation. Fluorescence, when used in conjunction with pyrene probe, allows for detection and quantifcation of microplastic particles in the sub-ppm range.
- Photochemical oxidation of polyethylene terephthalate microplastics adsorbed on sand and silica surfacesPublication . Q. V. da Costa, Camila; Nobahar, Amir; Egea-Corbacho, Agata; Jockusch, Steffen; Power, Deborah Mary; Ramamurthy, Vaidhyanathan; Silva, José Paulo daThe environmental contamination by plastics, microplastics, and related compounds is a major concern. While the detection and release of micro- and nanoparticles from these materials have been widely studied, the formation and release of molecules resulting from their degradation in the environment have been overlooked. This work presents a study of the products released from poly(ethylene terephthalate) (PET) irradiated as pure particles and adsorbed on silica and sand surfaces under different irradiation conditions. The role of oxygen was also evaluated. The products were identified by gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-high resolution mass spectrometry (LC-HRMS). The main released molecules can be accounted for by considering the cleavage of alpha- and beta-bonds next to the ester moiety of the polymer chain. Volatile products such as benzene as well as monomer units of the polymer and related products were identified. In the presence of oxygen, acetic acid and products resulting from hydroxylation at the benzenic ring or at the ethyl moiety were detected. Adsorption on silica and sand has little effect on the photoproduct distributions. The irradiation at 360 nm leads to distributions similar to the ones observed at 257 nm, but the reaction rate is lower. The identified product ethylene terephthalate is a marker of PET plastics and particles and can therefore be used to evaluate the environmental contamination by this polymer material.
- Photochemistry of 4-chlorophenol and 4-chloroanisole adsorbed on MFI zeolites: supramolecular control of chemoselectivity and reactive intermediate dynamicsPublication . Silva, José P. da; Jockusch, Steffen; Martinho, José M. G.; Ottaviani, M. Francesca; Turro, Nicholas J.The phototransformation of aryl chlorides adsorbed on MFI zeolites is markedly different from that observed in solution and other solid surfaces such as silica. The formation of radical cations and the constraints imposed by the channels shift the reactivity from the C-CI bond to the O-R bond. Irradiation generates kinetically stabilized intermediates that can be characterized using conventional steady-state spectroscopic techniques, and these intermediates can be used as ordinary chemical reagents.
- Polyphenol and volatile profiles of pomegranate (Punica granatum L.) fruit extracts and liquorsPublication . Galego, Ludovina; Jockusch, Steffen; Silva, José P. daThis work reports on the preparation of new aromatic pomegranate liquors by maceration of pomegranate juice and arils in Arbutus unedo L. distillate. The volatile, anthocyanin and ellagitannin profiles were monitored after each step. Aromatic compounds of pomegranates, such as limonene, 1-hexenol and trans-caryophyllene, were detected in trace levels and showed little contribution to the liquor aroma. The main volatiles of the liquors, namely isobutanol, isopentanols, vitispiranes and volatile esters, are given by the fruit distillate. The anthocyanin contents of the pomegranate juice were around 135mgL1 but decreased to about 5% of this value in the final liquors. Anthocyanin diglucoside compounds showed higher stability than their monoglucoside analogs. Punicalagins and punicalins showed contents between 80 and 135mgL1, depending on the used starting pomegranate material. These ellagitannins account for 60% of the total polyphenols. The procedure can be used to prepare aromatic pomegranate liquors with high polyphenol content.
- Probing the photoreactivity of aryl chlorides with oxygenPublication . Silva, José P. da; Jockusch, Steffen; Turro, Nicholas J.Molecular oxygen was used to probe the mechanism of the phototransformation of chlorobenzene and 4-chloroanisole in organic solvents. Laser. ash photolysis, electron paramagnetic resonance and product distribution studies clarified the reaction mechanisms of these compounds under a wide range of conditions. The main primary photochemical reaction step is the homolytic cleavage of the C-Cl bond to produce a triplet radical pair in the solvent cage. In non-polar solvents hydrogen abstraction, after radical diffusion, leads to reduction. In polar solvents, in addition to H-abstraction, electron transfer within the caged radical pair occurs and leads to an ion pair (phenyl cation and Cl(-)). In the presence of oxygen, phenyl radicals can form phenylperoxyl radicals which have a bathochromically shifted absorption, thus making the homolytic cleavage visible by. ash photolysis. The peroxyl radicals can couple, leading to more polar compounds, or undergo back reaction to the phenyl radical. For concentrations of the aryl chlorides of higher than 10(-3) M, dimerization becomes an important transformation process and occurs after reaction of the transients with ground state molecules. In addition, excimer formation is postulated to be involved in the dimerization process.
- Quantitative analysis of biogenic polyamines in distilled drinks by direct electrospray ionization tandem mass spectrometry using a nanocontainerPublication . Galego, Ludovina; Rodrigues, Maria A. A.; Mendes, Débora C.; Jockusch, Steffen; Silva, José P. daRATIONALE: Biogenic polyamines in drinks have been implicated in undesirable physiological effects. Methods for their detection and quantification usually involve derivatization, pre-concentration and clean-up. To assist the evaluation of the potential risk of distillates, it was important to develop a simple and fast analytical method, which is described in this study.METHODS: Biogenic polyamines were selectively encapsulated after addition of a nanocontainer, cucurbit[7] uril (50 mu M), to the distilled drink samples, which were acidified with HCl (pH 3) prior addition of the nanocontainer. The quantification of polyamines was achieved by direct infusion electrospray ionization quadrupole ion trap mass spectrometry in the multiple reaction monitoring mode, using encapsulated amantadine as internal standard, and by monitoring signals originating from their host-guest complexes.RESULTS: Six point calibration curves, ranging from 0.5 mu M to 20 mu M of polyamines in water and ethanol/water (50: 50), were used to establish instrument response. The method was validated by analysis of fortified Arbutus spirits. Samples of Arbutus and grape pomace spirits were also analyzed. Linear responses were observed for all polyamines and were similar in water, hydro-alcoholic solutions and fortified Arbutus spirits. Putrescine the simple polyamine was detected only in grape pomace distillate samples.CONCLUSIONS: A quantitative method was developed for rapid and simple analysis of biogenic polyamines in distilled drinks. The detection limits depend on the ionization properties of the samples. Encapsulated amantadine can be used to probe these properties and method application. Copyright (C) 2016 John Wiley & Sons, Ltd.
- Self aggregation of supramolecules of nitroxides@cucurbit[8]uril revealed by EPR spectraPublication . Jayaraj, Nithyanandhan; Porel, M; Ottaviani, MF; Maddipatla, MVSN; Modelli, A; Silva, José P. da; Bhogala, BR; Captain, B; Jockusch, Steffen; Turro, NJ; Ramamurthy, V.Supramolecular complexation behavior of cucurbiturils with paramagnetic nitroxide spin probes was examined by H-1 NMR, X-ray diffraction studies of crystals, computation, and EPR. Both cucurbit[7]uril (CB7) and cucurbit(S]uril (CB8) form a 1:1 complex with 4-(N,N,N-trimethylammonium)-2,2,6,6-tetramethylpiperidinyl-N-oxy bromide (CAT 1). The structure of the complex in the solid state was inferred by X-ray diffraction studies and in the gas phase by computation (B3LYP/6-31G(d)). Whereas ESI-MS data provided evidence for the existence of the complex in solution, indirect evidence was obtained through H-1 NMR studies with a structural diamagnetic analogue, 4-(N,N,N-trimethylammonium)-2,2,6,6- tetramethyl-N-methylpiperidine iodide (DCAT1). The EPR spectrum of the CAT1@CB7 complex consisting of three lines suggested that probe CATI is associated with host CB7 such that the nitroxide part is exposed to water. The spectral pattern was independent of the concentration of the complex and the presence of salt such as NaCl. The most interesting observation was made with CB8 as the host. In this case, in addition to the expected three-line spectrum, an additional spectrum consisting of seven lines was recorded. The contribution of the seven-line spectrum to the total spectrum was dependent on the concentration of the complex and added salt (NaCl) to the aqueous solution. The coupling constant for the seven-line spectrum for N-14-substituted CATI is 5 G, and that for the four-line spectrum for N-15-substituted CATI is 7.15 G. The only manner by which we could reproduce the observed spectra by simulation for both N-14- and N-15-substituted CAT1@CB8 was by assuming a spin exchange among three nitroxide radicals. To account for this observation, we hypothesize that three CAT1 molecules included within CB8 interact in such a way that there is an association of three supramolecules Of CAT1@CB8 (i.e., [CAT1@CB8](3)) in 11 triangular geometry that leads to spin exchange between the three radical centers. We have established, with the help of 13 additional examples, that this is it general phenomenon. We are in the process of understanding this unusual phenomenon.