Synthesis of profluorescent nitroxides as sensors

Abstract

Profluorescent nitroxides have become powerful tools for diagnostic and analytical applications of a variety of antioxidants and radical species, such as ascorbic acid (AA) as well as reactive oxygen species (ROS). Due to their interesting properties, they have been continuously designed and developed as sensors to qualitatively and quantitatively determine various analytes in both living organisms and natural resources for many past decades. Although, profluorescent nitroxides have been widely used as AA sensors, the accuracy of detection is still highly demanded. Furthermore, inhibition of collagen degradation in cartilage and detection of mass production of ROS using profluorescent nitroxides are also considerably required to prevent osteoarthritis usually found in elderly. Due to all of mentioned reasons, new profluorescent nitroxides for AA and ROS detection were synthesized. The former were ProN6 and ProN7 in which the precise detection relied on cascade reductive lactonization to prevent reoxidation of generated hydroxyamine. The synthetic approach consisted of michael addition, reductive cyclization, vinylation, click coupling and ester coupling. Kinetic study suggested that rate of AA detection by ProN6 was greater than that of ProN7 about 1.5 folds. Density functional theory (DFT) also demonstrated that Gibbs free energy of O-acylalkoxyamine from ProN6 was lower than that of ProN7. Electron paramagnetic resonance (EPR) spectra showed three characteristic peaks of nitroxides, which were dramatically reduced by AA overnight. Limit of AA detection was found to be 77.9 nM with a good linear relationship (R2 = 0.996) using ProN6 as probe in a solution. Moreover, AA was selectively detected over other competitive antioxidants. Quantity of AA in commercial supplements was investigated using paper-based ProN6 probe with a good linear relationship (R2 = 0.997) and moderate detection limit (195.9 ?M). The latter was profluorescent nitroxide for ROS detection, scavenging of forming carbon-centered radials and inhibition of collagen degradation. Probe PN1 has been developed using a similar protocol of AA sensor using michael addition, reductive cyclization and click coupling as main strategies. This probe was also converted into acrylate monomer PN2, which was subsequently incorporated with methyl methacrylate (MMA) by anionic polymerization to accomplish PN2-MMA copolymer, which might be applied as a artificial bone cement for clinical practice. Reassembly curve of acid soluble collagen (ASC) in the presence of ROS and profluorescent probe showed the efficiency of inhibition of collagen degradation by probe PN1. Furthermore, morphology of ASC observed by scanning electron microscope (SEM) confirmed the potential of the probe. PN1 probe and PN2-PMMA-based sensor satisfactorily exhibited high sensitivity of indirect detection of ROS in reassembled collagen solution with detection limits of 1.1 and 2.7 ?M, respectively.profluorescent nitroxide, ??????????? (AA), ?????????????????????, cascade reductive cyclization, reactive oxygen species (ROS), ?????????? ROS, ????????????????????????