Methods to Determine Chain-Breaking Antioxidant Activity of Nanomaterials beyond DPPH•. A Review

Abstract

This review highlights the progress made in recent years in understanding the mechanism of action of nanomaterials with antioxidant activity and in the chemical methods used to evaluate their activity. Nanomaterials represent one of the most recent frontiers in the research for improved antioxidants, but further development is hampered by a poor characterization of the ''antioxidant activity'' property and by using oversimplified chemical methods. Inhibited autoxidation experiments provide valuable information about the interaction with the most important radicals involved in the lipid oxidation, namely alkylperoxyl and hydroperoxyl radicals, and demonstrate unambiguously the ability to stop the oxidation of organic materials. It is proposed that autoxidation methods should always complement (and possibly replace) the use of assays based on the quenching of stable radicals (such as DPPH^• and ABTS^•+). The mechanisms leading to the inhibition of the autoxidation (sacrificial and catalytic radical trapping antioxidant activity) are described in the context of nanoantioxidants. Guidelines for the selection of the appropriate testing conditions and of meaningful kinetic analysis are also given.

Keywords: ROS; antioxidant; assays; autoxidation; catalysis; nanoantioxidants; nanomaterial; oxygen; radicals; reactive oxygen species.

Figure 1

Nanoparticles with antioxidant activity: “nanoantioxidants”.

Figure 2

Nanoantioxidants 3 and 4 suspended in THF (inset A) and recovered with a neodymium magnet (inset B), peroxide test strips (QUANTOFIX Peroxide 100, inset C) reveal the absence of peroxide formation in samples containing the nanoantioxidants and the reference BHT. Adapted from Ref [5].

Figure 3

Structures of radical compound DPPH^• and ABTS^•+.

Scheme 1

General mechanism of inhibited autoxidation. RTA = radical trapping antioxidants; HAT = H-atom transfer; PRA = peroxyl radical addition.

Scheme 2

Typical mechanism of action of RTA. (A) Reaction of α-tocopherol (R = C₁₆H₃₃) with peroxyl radicals leading to a stoichiometry of 2 radicals trapped by each antioxidant. The mechanism of regeneration by ascorbate (AscH⁻) or by HOO^•, leading to radical trapping stoichiometries larger than 2, is also shown. (B) Reaction of catechols with peroxyl radicals, and effect of the deprotonation of one OH group on the antioxidant/prooxidant behaviour of catechols.

Figure 4

Similarities and differences between sacrificial (A), catalytic (B) and SOD-like nanoantioxidants (C). The reactions are exemplified for the case of nanoparticles that are only able to donate or accept electron, with protons being released by the solvent. However in many case nanoparticles can host protons on the surface thus displaying a behaviour identical to that of phenols or of quinones.

Figure 5

Different class of azoinitiators currently used in antioxidant research.

Figure 6

Oxidizable substrates for inhibited autoxidation studies (PC = phosphatidylcholine).

Figure 7

Methods (in orange boxes) used to study the rate of autoxidation. The methods that are not based on optical spectroscopy and thus are best suited to study nanoantioxidants are evidenced.

Scheme 3

Probes used to study autoxidation reactions.

Figure 8

O₂ consumption traces observed during the autoxidation of styrene 4.3 M in chlorobenzene initiated by 50 mM of AIBN in a sample volume 4 mL at 30 °C, inhibited by nanoparticles bearing an analogue of α-tocopherol on the surface (1, 10.3 mg/mL) and by the antioxidant dimer (2, 5 μM). The O₂ consumption was measured by a pressure transducer. Adapted from reference [98].

Figure 9

Catalytic antioxidant activity of polydopamine (PDA). Panel (A): O₂ consumption during the autoxidation of styrene (initiator AIBN, 30 °C) without any inhibitor (a), with 1,4-cyclohexadiene (CHD) as source of HOO^• radicals (b), with PDA (c), in the presence of both PDA and CHD (d). Panel (B): mechanism explaining the radical trapping activity of PDA showing the reduction of quinone moieties to semiquinone able to trap new radicals by cycling between the quinone/semiquinone/hydroquinone forms. The O₂ consumption was measured by a pressure transducer. Adapted from reference. [36].