Paula Caregnato

Theoretical and Experimental Investigation on the Oxidation of Gallic Acid by Sulfate Radical Anions

By monitoring the decay of SO4*- after flash photolysis of aqueous solutions of S2O82- at different pH values, the kinetics of the reaction of SO4*- radicals with gallic acid and the gallate ion was investigated. The bimolecular rate constants for the reactions of the sulfate radicals with gallic acid and the gallate ion were found to be (6.3 +/- 0.7) x 10(8) and (2.9 +/- 0.2) x 10(9) M(-1) s(-1), respectively. On the basis of the oxygen-independent second-order decay kinetics and on their absorption spectra, the organic radicals formed as intermediates of these reactions were assigned to the corresponding phenoxyl radicals. DFT calculations in the gas phase and aqueous solution support formation of the phenoxyl radicals by H abstraction from the phenols to the sulfate radical anion. The observed recombination of the phenoxyl radicals of gallic acid to yield substituted biphenyls and quinones is also supported by the calculations. HPLC/MS product analysis showed formation of one of the predicted quinones.

Chloride anion effect on the advanced oxidation processes of methidathion and dimethoate: role of Cl2(·-) radical.

The reaction of phosphor-containing pesticides such as methidathion (MT) and dimethoate (DM) with dichloride radical anions (Cl(2)(·-)) was investigated. The second order rate constants (1.3 ± 0.4) × 10(8) and (1.1 ± 0.4) × 10(8) M(-1) s(-1) were determined for the reaction of Cl(2)(·-) with MT and DM, respectively. A reaction mechanism involving an initial charge transfer from the sulfide groups of the insecticides to Cl(2)(·-) is proposed and supported by the identified transient intermediates and reaction products. The formation of chlorinated byproducts was determined. The unexpected consequences of an efficient Cl(2)(·-) reactivity towards MT and DM on the degradation capacity by Advanced Oxidation Procedures applied to polluted waters containing the insecticides and Cl(-) anions is discussed.

Organic coating of 1-2-nm-size silicon nanoparticles： Effect on particle properties

Photoluminescent silicon nanoparticles 1–2 nm in size were synthesized by a wet chemical procedure and derivatized with propylamine (NH2SiNP). Surface NH2 groups were used as linkers for additional poly(ethylene glycol) (PEG) and folic acid (Fo) attachment (PEG-NHSiNP and Fo-NHSiNP, respectively) to enable efficient targeting of the particles to tumors and inflammatory sites. The particles were characterized by transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, ζ potential, dynamic light scattering, and time-resolved anisotropy.The photophysical properties and photosensitizing capacity of the particles and their interaction with proteins was dependent on the nature of the attached molecules. While PEG attachment did not alter the photophysical behavior of NH2SiNP, the attachment of Fo diminished particle photoluminescence. Particles retained the capacity for 1O2 generation; however, efficient 1O2 quenching by the attached surface groups may be a drawback when using these particles as 1O2 photosensitizers. In addition, Fo attachment provided particles with the capacity to generate the superoxide anion radical (O2−).The particles were able to bind tryptophan residues of bovine serum albumin (BSA) within quenching distances. NH2SiNP and PEG–NHSiNP ground state complexes with BSA showed binding constants of (3.1 ± 0.3) × 104 and (1.3 ± 0.4) × 103 M−1, respectively. The lower value observed for PEG-NHSiNP complexes indicates that surface PEGylation leads to a reduction in protein adsorption, which is required to prevent opsonization. An increase in particle luminescence upon BSA binding was attributed to the hydrophobic environment generated by the protein. NH2SiNP-BSA complexes were also capable of resonance energy transfer.

Electrostatic control of spin exchange between mobile spin-correlated radical pairs created in micellar solutions.

A series of photoinduced H-atom abstraction reactions between anthraquinone-2,6,-disulfonate, disodium salt (AQDS) and differently charged micellar substrates is presented. After a 248 nm excimer laser flash, the first excited triplet state of AQDS is rapidly formed and then quenched by abstraction of a hydrogen atom from the alkyl chain of the micelle surfactant, leading to a spin-correlated radical pair (SCRP). The SCRP is detected 500 ns after the laser flash using time-resolved (direct detection) electron paramagnetic resonance (TREPR) spectroscopy at X-band (9.5 GHz). By changing the charge on the surfactant headgroup from negative (sodium dodecyl sulfate, SDS) to positive (dodecyltrimethylammonium chloride, DTAC), TREPR spectra with different degrees of antiphase structure (APS) in their line shape were observed. The first derivative-like APS line shape is the signature of an SCRP experiencing an electron spin exchange interaction between the radical centers, which was clearly observable in DTAC micelles and absent in SDS micellar solutions. Solutions with surfactant concentrations well below the critical micelle concentration (cmc) or solutions where micellar formation had been disrupted (1:1 v/v CH(3)CN/H(2)O) also showed no APS line shapes in their TREPR spectra. These results support the conclusion that electrostatic forces between the sensitizer (AQDS) charge and the substrate (surfactant) headgroup charge are responsible for the observed effects. The results represent a new example of electrostatic control of a spin exchange interaction in mobile radical pairs.

Water/Silica Nanoparticle Interfacial Kinetics of Sulfate, Hydrogen Phosphate, and Dithiocyanate Radicals

Abstract The values of the rate constants for the reactions of the sulfate (2.5 × 109 M−1 s−1) and hydrogen phosphate (2.2 × 108 M−1 s−1) radicals with silica nanoparticles are obtained by flash photolysis experiments with silica suspensions containing S2O82− or P2O84−, respectively. The interaction of these radicals with the silica nanoparticles leads to formation of transients, probably adsorbed sulfate and hydrogen phosphate radicals, with absorption maxima at around 320 and 350 nm, respectively. A different mechanism takes place for the interaction of the less oxidizing dithiocyanate radicals with the silica nanoparticles. These radicals selectively react with the dissociated silanol groups of the nanoparticles with a rate constant at 298.2K of 7 × 107 M−1 s−1 (per mol of SiO− groups), and there is no evidence for their adsorption at the surface. All the results are discussed in terms of the Smoluchowski equation and redox potential of the inorganic radicals.

Reactions of CL˙/CL2˙- radicals with the nanoparticle silica surface and with humic acids : Model reactions for the aqueous phase chemistry of the atmosphere

Reactions of chlorine radicals might play a role in aqueous aerosols where a core of inorganic components containing insulators such as SiO2 and dissolved HUmic‐LIke Substances (HULIS) are present. Herein, we report conventional flash photolysis experiments performed to investigate the aqueous phase reactions of silica nanoparticles (NP) and humic acid (HA) with chlorine atoms, Cl•, and dichloride radical anions, Cl2•−. Silica NP and HA may be taken as rough models for the inorganic core and HULIS contained in atmospheric particles, respectively. Both Cl• and Cl2•− were observed to react with the deprotonated silanols on the NP surface with reaction rate constants, k ± σ, of (9 ± 6) × 107 M−1 s−1 and (7 ± 4) × 105 M−1 s−1, respectively. The reaction of Cl• with the surface deprotonated silanols leads to the formation of SiO• defects. HA are also observed to react with Cl• and Cl2•− radicals, with reaction rate constants at pH 4 of (3 ± 2) × 1010 M−1 s−1 and (1.2 ± 0.3) × 109 M−1 s−1, respectively. The high values observed for these constants were discussed in terms of the multifunctional heterogeneous mixture of organic molecules conforming HA.

Properties of singlet- and triplet-excited states of hemicyanine dyes

The fluorescence emission spectra and fluorescence quantum yields of hemicyanine dyes LDS 698, LDS 722, and LDS 730 were measured in different media. No transient species was detected in the laser flash-photolysis experiments performed with Ar-saturated solutions of the dyes in methanol. However, in the presence of 0.08 M potassium iodide, the absorption of the triplet states was clearly observed. Oxygen consumption measurements in the absence and presence of a chemical trap (furfuryl alcohol) in MeOH: H2O (φr = 1: 1) solutions of the dyes containing KI confirmed the generation of singlet molecular oxygen.

Versatile silicon nanoparticles with potential uses as photoluminiscent sensors and photosensitizers

Silicon nanoparticles (including silicon clusters and 1–4 nm size silicon nanocrystals, SiCs) combine size – dependent photoluminescence, the capacity for singlet oxygen and superoxide radical anion generation, and the richness of silicon surface derivatization. Surface modifications as coating/linking with folate, antibodies, adjuvants, and a plethora of other substances may lead to an increased aqueous solubility, stability, biocompatibility, targeting potential, and circulation time in biological systems. Size, synthetic procedures and surface derivatization/oxidation may strongly affect the particles efficiency for reactive oxygen species photosensitization and the interaction with small molecules and biological entities. Thus, through intelligent design it is possible to develop multifunctional nanoparticles with potential applications in imaging, diagnosis, and therapy. Herein, we present and discuss the properties that make SiCs potential photosensitizers for biological uses and describe the most widely used synthesis and surface functionalization procedures in order to help understanding the basics of photoluminescent SiCs and as a guide for researchers aiming to find new applications based on these particles.

Silanization effect on the photoluminescence characteristics of crystalline and amorphous silicon nanoparticles

Silicon nanoparticles synthesized by two different methods were surface modified with 3-mercaptopropyltrimethoxysilane. The particles of ~2 nm size exhibit photoluminescence (PL) in the UV-Vis range of the spectrum. The most intense PL band at 430 nm with an emission lifetime of 1-2 ns is attributed to the presence of the surface defects Si-O-Si, generated after anchoring the organic molecule onto the interface. The excitation-emission matrix of this band is essentially independent of the technique of synthesis, crystalline structure, and size of the silicon nanoparticles.