Interaction and UV-A photodegradation study of graphene oxide with natural alkaloids

FEDERICO A. O. RASSE-SURIANI; VITTORIO BOFFA; FERNANDO S. GARCÍA EINSCHLAG

Congreso; XIV ELAFOT; 2019

Nowadays graphene oxide (GO) is among the most promising and functionalizable two-dimensional materials. Due to its high dispersability in water several researchers have explored the potential of GO for different biomedical and biotechnical applications. Chemically, GO has a fraction of carbon atoms with sp3 hybridization and covalently bounded to oxygen functional groups (e.g., hydroxyl, epoxide and carbonyl groups) attached on the basal planes and edges of GO sheets. The remaining carbon atoms, with sp2 hibrydization, form clusters of various sizes, which mainly determine the optical and electronic properties of GO. It is a non-stoichiometric compound with a non-homogeneous structure and a very large specific area. It was recently reported that the planar structure and the π-conjugation of GO endowed it with excellent ability to immobilize a large number of substances, including metals, drugs, biomolecules and fluorescent molecules. Since the analysis of the interaction between graphene oxide and small fluorescent molecules becomes important from a biotechnological viewpoint, we have investigated the behavior of GO suspensions in the presence of β-Carbolines (βCs). βCs are photoactive natural alkaloids present in a wide range of species, they derive from the structural unit 9H-pyrido[3,4-b]indole or norharmane. Therefore, βCs are biocompatible compounds that also show several unique properties such as high chemical and photochemical stabilities. It is well known that the physicochemical behavior of βCs in aqueous environments is very varied since the presence of different substituents in a tricyclic system bearing two nitrogen atoms, one pyridine and one indolic, strongly affects their properties. In addition, upon UV-A excitation, βCs alkaloids are efficient photosensitizers, that can produce non-negligible quantities of singlet molecular oxygen (1O2), as well as other reactive oxygen species (O2?− and H2O2). Moreover, most of them also show high fluorescence quantum yields. In this work we present a comparative study of the photodegradation of GO dispersions both in the presence and in the absence of βCs. Two GO samples, with different degree of natural oxidation, were used. In a first stage, interaction tests were carried out to characterize the molecular associations present in the mixtures. Subsequently, kinetic tests were carried out at pH 5.0, with stirring and under UV-A irradiation (λirrad = 368 ± 20 nm), both for the individual components, and for the mixtures of GO and βCs. The systems studied were monitored by UV-vis absorption and fluorescent excitation-emission spectroscopy. For the interpretation of spectroscopic results, multivariate analysis methods (MCR-ALS and PARAFAC) were applied in order to clarify the nature both of the interactions and of the photoinduced processes.The results show that GO have a strong electrostatic interaction with the βCs studied. Moreover, the presence of βCs produces a considerable decrease in the stability of the GO and the type of photoproducts obtained depends on the age of the starting GO.