Dispersion enhancing effect of sonochemically functionalized graphene oxide for catalysing antioxidant efficacy of curcumin
Authors: Shrikant S Maktedar
Gopal Avashthi
Parth Malik
Man Singh
Journal: Ultrasonics sonochemistry (2017)
Discipline: CHEMISTRY
Keywords: Antioxidant activity; Curcumin; Emulsions; Functionalized graphene oxide.
Affiliations: School of Chemical Sciences, Central University of Gujarat, Gandhinagar 382030, India. Electronic address: ., School of Nano Scences, Central University of Gujarat, Gandhinagar 382030, India. Electronic address: .

Abstract:  The non-hazardous sonochemical approach has been developed for the functionalization of graphene oxide (GrO) with 5-Aminoindazole (5-AIND). The formation of f-(5-AIND) GrO is confirmed with 13C solid state NMR, HRXPS, XRD, Raman, TGA, DSC, FTIR etc. The &gt;80% cell viabilities on MCF-7 and Vero cell lines have confirmed the high cytocompatibility of f-(5-AIND) GrO. Additionally, the morphological impact on Vero cell line has evidently confirmed the biocompatibility of f-(5-AIND) GrO. As compared to GrO, the f-(5-AIND) GrO has demonstrated an enhanced antioxidant efficacy of 69.4-72%, evaluated with 2, 2-diphenyl-1-picrylhydrazyl (DPPH) free radical assay. With a similar objective (0.01-0.05)mL peanut oil based curcumin micro and nanoemulsions have been prepared using ethanol and glycerol as co-solvent and co-surfactant respectively. The prepared emulsions are subsequently characterised with respect to morphological and physicochemical parameters via density, surface tension, viscosity, friccohesity measurement and DLS analysis. Henceforth, with an expectation to achieve higher dispersion, an ethanolic dispersion of f-(5-AIND) GrO has been mixed with curcumin carrying emulsions in 1:1. Notably, the radical scavenging activities (RSA) of the combined formulations are significantly enhanced to an extent of 26.6%. 

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PubPeer comments:
 On page 216, author wrote following sentences u2013 u201cThe increased RSAs of curcumin in the emulsified environment show the catalysing impact of f-(5-AIND) GrO towards facilitating the proton release from curc. Such synergistic molecular activities open up the significant potential to further enhance a proton release mechanism from curc, through an indirect means. Therefore, the enhanced RSAs for combined samples generate a significant potential to still improve the free radical trapping mechanism using some other potent biocompatible proton releasing agents.u201d The provided explanation for the antioxidant activity mechanism is wrong. **Explanation** = It is found that while using MeOH or ethanol as solvent in DPPH study, acidic phenols of substrate react much faster than expected with DPPH radical, which would subvert the order of reactivity with respect to their actual antioxidant activity of substrate. [This sentence is taken from Litwinienko G, Ingold KU. J Org Chem. 2003, 68, 3433-3438.] There is possibility of five different mechanisms for different radicals from the curcumin under different polar conditions: (1) single electron transfer (SET), (2) radical adduct formation (RAF), (3) H atom transfer from neutral curcumin (HAT), (4) H atom transfer from deprotonated curcumin (HAT-D), and (5) sequential proton loss electron transfer (SPLET). Out of all these five it is proposed that the curcumin + DPPH reaction actually takes place mainly through the SPLET mechanism, while the reaction with u2022OCH3 (radical), and likely with other alkoxyl radicals, is governed by the HAT mechanism. [taken from = A. Galano, R. lvarez-Diduk and M. T. Ramu00edrez-Silva: Chemical Physics: Vol. 363 (2009), p. 13 ] **Question** = In this research article the authors are using the ethanol as a solvent for the Antioxidant activity assay by using the DPPH radical. So there is very strong possibility of involvement of the SPLET mechanism by considering the above argument. So the explanation provided by the authors is wrong. Why author is not mentioning anything about the SPLET mechanism? ,