Scavenging performance and antioxidant activity of γ-alumina nanoparticles towards DPPH free radical: Spectroscopic and DFT-D studies

Accepted Manuscript Scavenging performance and antioxidant activity of γ-alumina nanoparticles towards DPPH free radical: Spectroscopic and DFTD studies

Mehdi Zamani, Ali Moradi Delfani, Morteza Jabbari PII: DOI: Reference:

S1386-1425(18)30394-9 doi:10.1016/j.saa.2018.05.004 SAA 16027

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received date: Revised date: Accepted date:

22 November 2017 7 April 2018 1 May 2018

Please cite this article as: Mehdi Zamani, Ali Moradi Delfani, Morteza Jabbari , Scavenging performance and antioxidant activity of γ-alumina nanoparticles towards DPPH free radical: Spectroscopic and DFT-D studies. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Saa(2017), doi:10.1016/j.saa.2018.05.004

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ACCEPTED MANUSCRIPT Scavenging performance and antioxidant activity of γ-alumina nanoparticles towards DPPH free radical: Spectroscopic and DFT-D studies Mehdi Zamani,* Ali Moradi Delfani, Morteza Jabbari School of Chemistry, Damghan University, Damghan 36716-41167, Iran

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ABSTRACT The radical scavenging performance and antioxidant activity of γ-alumina nanoparticles towards 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical were investigated by spectroscopic and computational methods. The radical scavenging ability of γ-alumina nanoparticles in the media with different polarity (i.e. i-propanol and n-hexane) was evaluated by measuring the DPPH absorbance

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in UV-Vis absorption spectra. The structure and morphology of -alumina nanoparticles before and

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after adsorption of DPPH were studied using XRD, FT-IR and UV-Vis spectroscopic techniques. The adsorption of DPPH free radical on the clean and hydrated γ-alumina (1 1 0) surface was

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examined by dispersion corrected density functional theory (DFT-D) and natural bond orbital (NBO) calculations. Also, time-dependent density functional theory (TD-DFT) was used to predict

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the absorption spectra. The adsorption was occurred through the interaction of radical nitrogen N•

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and NO2 groups of DPPH with the acidic and basic sites of -alumina surface. The high potential for the adsorption of DPPH radical on γ-alumina nanoparticles was investigated. Interaction of

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DPPH with Brønsted and Lewis acidic sites of γ-alumina was more favored than Brønsted basic sites. The following order for the adsorption of DPPH over the different active sites of γ-alumina was predicted: Brønsted base < Lewis acid < Brønsted acid. These results are of great significance for the environmental application of γ-alumina nanoparticles in order to remove free radicals. Keywords: γ-Alumina, DPPH, Antioxidant, Radical scavenger, Adsorption, DFT

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Correspondence should be addressed to M. Zamani (Fax: +98 233 522 0095; Tel: +98 233 522 0095; E-mail: [email protected])

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ACCEPTED MANUSCRIPT 1. Introduction Antioxidants are compounds that are added to oxidizable organic materials to delay autooxidation process and to extend the useful life of the substrates [1]. Relatively few chemical classes such as hindered phenols, hindered amines, trivalent phosphorus compounds, sulfide esters, metal dithiocarbamates and metal dithiophosphates are effective as antioxidants [1]. It is therefore

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necessary to develop the production and the characterization of the new classes of antioxidants. In recent years, nanoparticles with a practical application as antioxidant have received increasing attention in research and technology [2]. Some kinds of metal and metal oxide nanoparticles have been tested for their ability to serve as free radical scavengers to provide protection against chemical, biological, and radiological insults that promote the production of free radicals [3-25].

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The silver nanoparticles have antioxidant activity due to capped phenolic and flavonoid compounds

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and can be used against deleterious effects of free radicals [3-8]. Also, the gold [8-11], selenium [12, 13], platinum [14], cerium oxide [15-18], zinc oxide [19, 20], nickel oxide [21], copper oxide

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[22], tin oxide [23] and titanium dioxide [24] nanoparticles exhibited antioxidant and free radical scavenging activities. Using the metallic nanoparticles as novel tool for reliable assessment of

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antioxidant activity in food and biological samples is recently reviewed [25]. Alumina materials in various forms (χ, κ, γ, δ, θ, η and α) are obtained by thermal dehydration of

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different aluminum hydroxides (gibbsite, bayerite, nordstrandite, diaspore and boehmite) in the 250-

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1200 °C temperature [26]. These compounds are one of the important inorganic chemicals produced in greatest volume today for industrial applications as fillers, adsorbents, drying agents, catalysts, ceramics, abrasives and refractories [27]. γ-Alumina is generally produced by calcination of boehmite at 350-700 °C [28]. Experimental data and theoretical calculations have predicted two important models for this compound, i.e. spinel-like [29-31] and nonspinel [32-35] models. The spinel-like model is thermodynamically more stable [36]; and the nonspinel model has good agreement with experiments in terms of structural parameters and OH vibrational frequencies [32]. 2

ACCEPTED MANUSCRIPT γ-Alumina is well-known catalyst with both acidic and basic properties (Scheme 1). Recently, the acid-basic properties of the three relevant γ-alumina (1 0 0), (1 1 0), and (1 1 1) crystal planes have been investigated by DFT calculations [32]. The (1 1 0) surface, which contains the tri- and tetracoordinated aluminum atoms, is the most active plane of γ-alumina [32]. The Lewis acidity of γalumina is due to the electron deficiency of trivalent aluminum atoms which behave as Lewis acid

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sites. Also, the oxygen atoms on the surface may act as proton donor (Brønsted acid) or proton acceptor (Brønsted base). These acidic and basic sites have been looked upon as the active catalytic centers of γ-alumina. Many investigations have been attributed on the catalytic activity of γ-alumina and the adsorption of various molecules on the surface [37-62]. Brønsted acid

Brønsted base

H

H

O

O O

O

H

H

Al

O Al

Al

O

Al

Al

H H

O O

H O

Al

O Al

O H

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H

O

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H

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Lewis acid

Scheme 1. Schematic representation of the acidic and the basic sites on γ-alumina surface.

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DPPH assay [63] is routinely practiced for assessment of free radical scavenging potential of an

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antioxidant molecule and considered as one of the standard and easy colorimetric methods for the evaluation of antioxidant properties of pure compounds. DPPH molecule is a stable synthetic free

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radical which is widely used to evaluate the ability of compounds to act as free radical scavengers or radical hydrogen donors and to measure the antioxidant activity [63-69]. In the presence of antioxidants, DPPH turns from purple to pale yellow color corresponding to the reduced form of DPPH, i.e. 2,2-diphenyl-1-picrylhydrazine (DPPH-H) [70] (Scheme 2). The DPPH can also contribute in homolytic additions with other radicals species R•, yielding in many instances RDPPH-H-substituted derivatives [70].

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N

N +

N O2N

NO2

RH

+

HN O2N

Antioxidant

R

NO2

NO 2

NO2 DPPH

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DPPH-H

Scheme 2. Conversion of DPPH free radical to DPPH-H molecule in the presence of antioxidant. Due to the high surface area and high capacity for absorption of various molecules, γ-alumina nanoparticles were selected as a radical scavenger and antioxidant candidate in the present study.

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Moreover, the hydrated surface of γ-alumina nanoparticles is useful in promoting the DPPH adsorption and acting as a radical hydrogen donor. The free radical scavenging ability of γ-alumina

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nanoparticles was tested as bleaching of the DPPH in the media with different polarity (i.e. ipropanol and n-hexane). There is no computational report in the literature about the adsorption of

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DPPH free radical on the clean and hydrated γ-alumina surfaces so far. The new mechanism for radical scavenging and antioxidant activities of γ-alumina nanoparticles is proposed using UV-Vis,

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XRD and FT-IR spectroscopic data as well as DFT-D calculations. Also, the activity of γ-alumina

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for the first time.

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nanoparticles for adsorption of DPPH-H molecule was examined experimentally and theoretically

2. Experimental

2.1. Materials and instruments The powder of γ-alumina nanoparticles with high purity (>99%) (average size 20 nm and specific surface area 138 m2 g-1) was purchased from US Research Nanomaterials, Inc. DPPH free radical (>95%) was purchased from Sigma-Aldrich and the organic solvents used (i-propanol, n-hexane and cyclohexane) were prepared from Merck Chemical Co. The X-Ray diffraction (XRD) patterns were recorded in the range of 10-120° (2θ) by Bruker D8-Advance diffractometer with CuKα 4

ACCEPTED MANUSCRIPT radiation. The Fourier transform infrared (FT-IR) spectra in the range 400-3500 cm-1 were obtained using KBr pellets on a Perkin-Elmer RXI Fourier transform infrared spectrophotometer. The ultraviolet-visible (UV-Vis) absorption spectra in the range 200-800 nm were monitored by Analytik Jena SPECORD-205 UV-Vis spectrophotometer. In order to remove nanoparticles from

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solution system, HERMLE Z300 centrifuge was used.

2.2. DPPH adsorption test and radical scavenging activity measurements

Firstly, 0.5 grams of γ-alumina nanoparticles powder was dispersed in 10 mL of a 5.0e-5 mol L-1 solution of DPPH in solvent (i-propanol or n-hexane). The reaction tube was wrapped in aluminum foil and the mixture had been stirred for 3 h at 3-5 °C in a dark place. As a reference, 10 mL of a

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5.0e-5 mol L-1 DPPH solution was stirred in the absence of γ-alumina nanoparticles under the same

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conditions. After separation of the γ-alumina nanoparticles by centrifugation at 3000 rpm, the absorption spectrum of the resulting solution was recorded by UV-Vis spectrophotometer. The result was then compared with the absorption spectrum of the reference solution.

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The radical scavenging activity of γ-alumina nanoparticles in i-propanol and n-hexane was

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evaluated by measuring the decrease of DPPH absorbance around 520 nm (according to DPPH maximum absorption [70]) in UV-Vis absorption spectra. The DPPH scavenging percentage was

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estimated using Eq. 1, where ARef and AS are the absorption of DPPH in solutions without and

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including nanoparticles, respectively. DPPH scavenging (%) = (

ARef - AS )  100 ARef

(1)

The concentration of DPPH in each solution was estimated using Beer–Lambert law (Eq. 2) [71], where A is the measured absorbance, c is the concentration, ε is the molar absorptivity and b is the cell length. A = εbc

(2)

The concentration of adsorbed DPPH free radical on γ-alumina nanoparticles was calculated via the difference in concentration of DPPH solution in the absence and the presence of nanoparticles. 5

ACCEPTED MANUSCRIPT The DPPH adsorption percentage was calculated using Eq. 3, where CRef and CS are the concentration of DPPH in solutions without and including nanoparticles, respectively. Here, the DPPH scavenging and DPPH adsorption percentages are numerically equal. DPPH adsorption (%) = (

CRef - CS )  100 CRef

(3)

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2.3. DPPH-H adsorption test

The DPPH-H molecule is produced in-situ by dissolving the DPPH free radical in cyclohexane solvent [72]. The experiments were performed under the same conditions as described in section 2.2. The activity of γ-alumina nanoparticles for adsorption of DPPH-H molecule was evaluated by

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measuring the decrease of DPPH-H absorbance around 330 nm (according to DPPH-H maximum absorption [70]) in UV-Vis absorption spectra. The concentration of adsorbed DPPH-H molecule

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on γ-alumina nanoparticles was calculated by the difference in concentration of DPPH-H solution in the absence and the presence of nanoparticles. The DPPH-H adsorption percentage was calculated

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using Eq. 4, where CRef and CS are the concentration of DPPH-H in solutions without and including

CRef - CS )  100 CRef

(4)

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DPPH-H adsorption (%) = (

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nanoparticles, respectively.

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2.4. Computational details

The geometry of DPPH and DPPH-H molecules as well as nonspinel model of γ-alumina were optimized through dispersion corrected density functional theory (DFT-D) using DMOL3 [73, 74] program. The periodic boundary conditions (PBC) were used to describe the periodic structure of γalumina. The calculations were performed using the generalized gradient approximation (GGA) employing the Perdew−Burke−Ernzerhof PBE-D density functional [75] plus Grimme procedure [76] for dispersion correction and numerical basis set of double-zeta plus polarization quality (DNP) [73]. Effective core potentials (ECP) were used to treat the core electrons. Each basis 6

ACCEPTED MANUSCRIPT function is restricted to a cutoff radius of 4.5 Å. The optimization convergence thresholds for energy change, maximum force and maximum displacement between the optimization cycles were 1.0e-5 Ha, 0.002 Ha Å-1 and 0.005 Å, respectively. The fine integration accuracy and self-consistent field (SCF) tolerance (1.0e-6 Ha) as well as the k-point set of (1×1×1) were used for all calculations.

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The (1 1 0) crystal plane of γ-alumina was cleaved. A vacuum of 15 Å between slabs in the direction of the crystal lattice, perpendicular to the surface plane, and periodically repeated unit cell through space was imposed. The top layers of this surface were rigid and the bottom layers were constraint. The optimization of clean and hydrated super cells of γ-alumina (1 1 0) surface was performed using periodic DFT-D calculations. All of the possible orientations for the adsorption of

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DPPH and DPPH-H molecules over these surfaces were examined using condensed-phase

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optimized molecular potentials for atomistic simulation studies (COMPASS) force field [77]. The more stable structures with proper orientation of the adsorbed molecules on the surface were selected for further optimization through PBE-D/DNP level of theory.

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The adsorption (interaction) energy (ΔEads) of DPPH and DPPH-H molecules over γ-alumina (1 1

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0) surface was calculated by Eq. 5, where E(Adsorbed molecule on surface), E(Molecule) and E(Surface) refer to energy of the system after adsorption, energy of isolated molecule and energy of γ-alumina surface,

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respectively. The bonding nature of intramolecular interactions of DPPH and DPPH-H over clean

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and hydrated surfaces of γ-alumina was studied using energy decomposition analysis (EDA) of ΔEads to sum of atomic, sum of kinetic and electrostatic, exchange-correlation, spin polarization and dispersion energies.

ΔEads = E(Adsorbed molecule on surface) – (E(Molecule) + E(Surface))

(5)

The UV-Vis absorption spectra of DPPH and DPPH-H molecules before and after adsorption on γ-alumina active sites were calculated by time-dependent density functional theory (TD-DFT) [78] using Becke’s three-parameter hybrid functional [79] with the non-local Lee-Yang-Parr correlation [80] (B3LYP) and 6-311G(2d,p) or 6-311++G(2d,p) basis functions for the optimized geometries of 7

ACCEPTED MANUSCRIPT B3LYP-D/6-311G(2d,p) method, in the gas phase and in solvent using the polarized continuum model (PCM) [81]. These calculations were performed using Gaussian-09 software package [82]. The NBO version 5.0 [83] at B3LYP-D/6-311G(2d,p) level of theory was used for visualization of natural bond orbitals (NBOs) and estimation of the stabilization associated with delocalization of

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donor-acceptor NBOs.

3. Results and Discussion

3.1. DPPH adsorption test and measurement of radical scavenging activity

The radical scavenging activity of γ-alumina nanoparticles was evaluated by UV-Vis

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spectrophotometric method. The DPPH free radical has a maximum absorbance at about 520 nm, due to the delocalization of the unpaired electron over the aromatic molecule [70]. In order to

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evaluate the adsorption of DPPH on the surface of γ-alumina, the nanoparticles were dispersed in DPPH solution containing i-propanol or n-hexane. The solution of DPPH in these solvents without

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nanoparticles has a strong absorption band about 520 nm (Figs. 1 and 2). The observed decrease of peak intensity in the presence of nanoparticles can be related to the decrease of DPPH concentration

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in solution due to the adsorption of DPPH on γ-alumina surface. To ensure that the adsorption of

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DPPH over nanoparticles is responsible for the decrease of DPPH concentration (not related to the reduction of DPPH to DPPH-H), the purity of the extracted solvents was checked.

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The UV-Vis absorption spectrum and the color of DPPH in i-propanol without and including nanoparticles are shown in Fig. 1. As seen from this figure, the purple color of solution was getting slightly lighter during the adsorption of DPPH free radical on γ-alumina in i-propanol. The value of 0.9e-5 mol L-1 was calculated as concentration of adsorbed DPPH on the surface (percentage of DPPH adsorption = 24%). Therefore, 24% of DPPH has been scavenged by γ-alumina nanoparticles. The UV-Vis absorption spectrum and the color of DPPH in n-hexane without and including nanoparticles are shown in Fig. 2. The elimination of the characteristic absorption peak at about 520 8

ACCEPTED MANUSCRIPT nm indicates the complete adsorption of DPPH free radical on γ-alumina nanoparticles (percentage of DPPH adsorption ≈ 100%). According to the results, the 100% of DPPH is scavenged by γalumina nanoparticles. During the adsorption of DPPH free radical on γ-alumina in n-hexane, the purple color of solution turns to white and the color of γ-alumina nanoparticles changes from white to purple (Fig. 2).

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The change of absorbance in UV-Vis absorption spectrum of DPPH in n-hexane in comparison to i-propanol is large. Due to the polar nature of γ-alumina (see Scheme 1), the polar molecules of solvent adsorb on the surface of nanoparticles better than DPPH free radical. For example, ipropanol is bonded through the charge transfer interaction and hydrogen bonding with the Lewis acid sites and the hydroxyl groups located on the surface of γ-alumina, respectively. As the non-

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polar solvents such as n-hexane are not capable for hydrogen bonding with the surface, it can be

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concluded that the γ-alumina nanoparticles are powerful DPPH radical scavenger in non-polar

3.2. DPPH-H adsorption test

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media.

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In order to evaluate the adsorption of DPPH-H on the γ-alumina surface, the nanoparticles were dispersed in DPPH solution plus cyclohexane. The DPPH-H molecule has a maximum absorbance

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at about 330 nm [70]. The solution of DPPH in cyclohexane without nanoparticles has a strong

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absorption band about 350 nm (Fig. 3), due to complete reduction of the DPPH free radical to DPPH-H. A yellow color is observed for this solution, because the conversion of DPPH to DPPH-H is done. The value of 1.2e-5 mol L-1 was obtained as concentration of adsorbed DPPH-H on the surface of nanoparticles (equal with 14% adsorption).

3.3. Characterization of nanoparticles The color changes of γ-alumina nanoparticles before and after adsorption of DPPH as well as the spontaneous conversion of DPPH to DPPH-H are shown in Fig. 4. When DPPH was adsorbed on 9

ACCEPTED MANUSCRIPT the γ-alumina, the color of nanoparticles changed from white to purple (DPPH/γ-alumina sample). When the as-prepared DPPH/γ-alumina nanoparticles were exposed to the air for 48 h at room temperature, the color of the nanoparticles changed from violet to yellow (DPPH-H/γ-alumina sample). It is expected that the adsorbed DPPH radicals on the γ-alumina are capable for abstracting the radical hydrogen atoms from the hydrated surface of γ-alumina to convert to DPPH-H

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molecules. Therefore, in addition to radical scavenging activity, the γ-alumina nanoparticles have antioxidant and radical neutralization properties. This prediction is further supported by UV-Vis, XRD and FT-IR spectroscopic analysis of nanoparticles which describe in the following.

3.3.1. UV-Vis spectra

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The solid state UV-Vis spectra of γ-alumina nanoparticles without and containing DPPH and

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DPPH-H molecules are shown in Fig. 4. The UV-Vis spectrum of γ-alumina nanoparticles displays a band in 210 nm. This band is preserved at the region from 210 to 220 nm for γ-alumina

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nanoparticles in the presence of DPPH and DPPH-H. The UV-Vis spectrum of as-prepared DPPH/γ-alumina exhibits two absorption bands at 240 and 330 nm (corresponds to the resonance of

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NO2 groups and the phenyl ring) and a band at 520 nm (corresponds to the resonance of N• radical and the phenyl ring). Since the later peak is the characteristic for DPPH free radical, the results

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confirm the adsorption of DPPH on γ-alumina nanoparticles. This characteristic absorption peak

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cannot be observed in the UV-Vis spectrum of DPPH/γ-alumina sample exposed to air meaning that the DPPH is under reduced form present on the surface, i.e. DPPH-H/γ-alumina.

3.3.2. XRD measurements The XRD patterns of γ-alumina nanoparticles before and after adsorption of DPPH are shown in Fig. 5. The typical XRD pattern of nanoparticles show characteristic reflections 2θ at 38, 46 and 67° correspond to the alumina γ phase [52]. The XRD pattern of DPPH free radical revealed multiple peaks in region between 2θ = 20-30° (Fig. 5). The observed change in the XRD pattern of γ10

ACCEPTED MANUSCRIPT alumina nanoparticles containing DPPH in the region of 20 to 30° is related to the adsorption of DPPH on the surface of nanoparticles. The XRD pattern of as-prepared DPPH/γ-alumina sample in this region is intense and broader. The XRD pattern of the DPPH/γ-alumina nanoparticles exposed to air indicates the transformation of DPPH to DPPH-H. The change in broadness and intensity observed in the region between 20-30° is attributed to distribution DPPH-H molecules on γ-alumina

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nanoparticles.

3.3.3. FT-IR spectra

The FT-IR spectra of γ-alumina nanoparticles containing DPPH and DPPH-H molecules together with pure nanoparticles are shown in Fig. 6. The main absorption bands for these compounds are

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labeled in this figure. The FT-IR spectrum of γ-alumina nanoparticles shows some absorption at

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400-1100 cm-1 which is related to Al-O stretching and O-Al-O bending modes [82]. The peak at 1550 cm-1 is attributed to H-O-H bending mode of adsorbed water on the surface [54]; and the peak at 2700-3800 is assigned to O-H stretch of the surface hydroxyl groups [54]. The adsorption of

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DPPH free radical on γ-alumina nanoparticles in DPPH/γ-alumina sample was confirmed by the

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observation of N-O (1520 and 1550 cm-1) and C-H (3100 cm-1) stretching absorbance peaks. The FT-IR spectrum of the DPPH/γ-alumina sample exposed to air containing DPPH-H molecules is

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completely different from as-prepared sample, especially at 2700-3800 cm-1 as a result of the

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appearance of N-H stretching absorption at 3431 cm-1. The results illustrate that all adsorbed DPPH free radicals on the surface of γ-alumina nanoparticles are completely converted to DPPH-H molecules. This conversion was accompanied by a change in the color of the nanoparticles from purple to yellow (Fig. 4).

3.4. Theoretical models and proposed mechanism A two step mechanism for the radical scavenging and antioxidant activities of γ-alumina nanoparticles was proposed. The first step is the adsorption of DPPH free radical over all active 11

ACCEPTED MANUSCRIPT sites of γ-alumina nanoparticles, including Lewis acidic, Brønsted acidic and basic sites (Scheme 3). During this step, the color of γ-alumina nanoparticles turns from white to purple. The second step is the neutralization of adsorbed DPPH radical which occurs by radical H transfer from the hydrated surface of γ-alumina to radical nitrogen of DPPH (Scheme 4). During this step, the color of γalumina nanoparticles turns from purple to yellow corresponds to the conversion of DPPH to

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DPPH-H. The ability of γ-alumina nanoparticles to scavenge and neutralize DPPH free radical was further analyzed by DFT-D calculations. The optimized structures of clean and hydrated (1 1 0) surfaces of γ-alumina super cells at PBE-D/DNP level of theory are shown in Fig. 7. The surface of nanoparticles is composed from tri- and tetra-coordinated aluminum atoms and di- and three-

O2N

O

N

N

Al

Al

O

O

NO2

N

N

O H O

Al

Al

O2N

O

O

O

O

Al

H

H

O

O

Al

NO2

H

H

O

Al

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O

O

N O

O2N

N

N O

O

Al

CE

NO2

AC

O

Al

ED

O

N

N

O

O

Al

O2N

N

N

N

NO2

O

NO2 N

O

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coordinated oxygen atoms which have important role in the catalytic activity of γ-alumina.

Al

N

N O

H H

O O

Al

N

O

H O

Al

O Al

O H

H

Scheme 3. Schematic representation of DPPH adsorption on the clean (top) and hydrated (bottom) surfaces of γ-alumina.

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ACCEPTED MANUSCRIPT O2N

O2N

NO2 N

O

N

N

N

O

O

NO2

N

O H

O O

O

N

N O

O O

Al

NO2 N

N O

H

H

H

O2N

O

Al

Al

O

Al

H Al

O

H O

Al

O

O

Al

H

O2N

O2N

NO2 N

N

O2N

NO2

O2N

H

O

H

O O Al

Al

O

Al

O

Al

O

Al

H O

O

Al

O H

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H

N

N

H

O O

NO2

O2N

H

Al

O2N

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H

N

N

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H

Scheme 4. Schematic representation of DPPH neutralization via radical H transfer from hydrated

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surface of γ-alumina (top) and formation of DPPH-H molecule on the surface (bottom). The calculated adsorption energies and their components (sum of atomic, sum of kinetic and

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electrostatic, exchange-correlation, spin polarization and dispersion energies) for DPPH and DPPHH molecules are listed in Table 1. Theoretical calculations showed that the DPPH free radical

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adsorbs on the nanoparticles by interaction of radical nitrogen (N•) and NO2 of DPPH with the

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Lewis acid (>Al−), Brønsted acid (−OH2) and Brønsted base (−OH) of γ-alumina surface with highly negative interaction energy (Scheme 3 and Fig. 8); While, DPPH-H adsorbs on the nanoparticles by hydrogen bonding between −NH of DPPH-H with the Brønsted basic site of γalumina (−OH) with less negative interaction energy (Scheme 4 and Fig. 9). Adsorption of DPPH over Lewis acidic sites of γ-alumina (>Al−) by incorporation of both N• and NO2 with more negative adsorption energy (-49.8 kcal mol-1, Fig. 8a) is more favored than the adsorption via NO2 groups (-36.6 kcal mol-1, Fig. 8b). Based on the results of EDA analysis, the

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ACCEPTED MANUSCRIPT higher stability predicted for the former configuration (Fig. 8a) is due to the stronger dispersion and the sum of kinetic and electrostatic energies (Table 1). The best configuration for the interaction of DPPH radical with Brønsted acidic sites of γalumina (−OH2) is obtained where both N• and NO2 interact with one acidic proton (ΔEads = -57.4 kcal mol-1, Fig. 8c) versus interaction of N• and NO2 with different protons (ΔEads = -52.5 kcal mol, Fig. 8d). The more negative dispersion and the sum of kinetic and electrostatic energies are the

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1

result of stronger adsorption of the former configuration (Fig. 8c, Table 1).

Adsorption of DPPH over Brønsted basic sites of γ-alumina is favored only where the −OH is supported by hydrogen bonding with the neighboring Brønsted acidic sites of γ-alumina (>O−H), i.e. ΔEads = -46.7 kcal mol-1, Fig. 8e compared to the configuration without hydrogen bonding

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due to the greater dispersion energy (Table 1).

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(ΔEads = -21.4 kcal mol-1, Fig. 8f). The stronger interaction of the former configuration (Fig. 8e) is

According to the results of DFT-D calculations at PBE-D/DNP level of theory, the high capacity for the adsorption of DPPH free radical on γ-alumina nanoparticles was predicted. The interaction

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of DPPH with the Brønsted acidic sites of γ-alumina (Fig. 8c, d) is more favored than the Lewis

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acidic (Fig. 8a, b) and Brønsted basic (Fig. 8e, f) sites. The following order for the adsorption of

Brønsted acid.

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DPPH over the different active sites of γ-alumina was investigated: Brønsted base < Lewis acid <

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The abstraction of radical hydrogen from all of the Brønsted acidic (Fig. 8c, d) and some of the basic (Fig. 8e) sites is possible. The consequence of this process is the conversion of DPPH to its reduced form, i.e. DPPH-H; and the formation of radical hydroxyl groups on the surface of γalumina (Scheme 4). The calculated interaction energies for adsorption of DPPH-H molecule on the Brønsted basic sites of γ-alumina hydrated surface (−OH) are -28.9 and -28.5 kcal mol-1 at PBED/DNP level of theory (Fig. 9). In agreement with the experimental results of DPPH and DPPH-H adsorption tests, the lower adsorption energy for DPPH-H compared to DPPH was predicted. The

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ACCEPTED MANUSCRIPT EDA analysis of ΔEads values indicates that the difference in dispersion and the sum of kinetic and electrostatic energies is responsible for the difference in stability of these compounds (Table 1). NBO analysis is an efficient method for investigating charge transfer interactions. The surface plot and stabilization energy for more important interactions between electron donor and electron acceptor orbitals of DPPH and DPPH-H molecules with the main adsorption sites of γ-alumina (i.e.

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Lewis acid, Brønsted acid and Brønsted base) are shown in Fig. 10. The larger stabilization energy values indicate the more intensive interaction between electron donor and acceptor natural bond orbitals. In accordance with the previous results, NBO calculations at B3LYP-D/6-311G(2d,p) level of theory predict the stronger interaction for DPPH than DPPH-H with γ-alumina surface. Also, interaction of DPPH with Brønsted and Lewis acidic sites of γ-alumina is more favored than

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Brønsted basic sites. The nonbonding orbital of DPPH radical (nN•) can interact as a donor with the

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acceptor vacant nonbonding orbitals of Al Lewis acid sites (n*Al) with great stabilization energy, i.e. 19.8 and 27.3 kcal mol-1 (Fig. 10a, b). The stabilization energy due to the interaction of nN• orbital of DPPH with the virtual n*H and σ*O−H orbitals in γ-alumina Brønsted acidic sites is also large, i.e.

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23.7 and 17.2 kcal mol-1, respectively (Fig. 10c, d). While, the stabilization energy associated with

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the interaction of nN• orbital with antibonding orbital σ*O−H in γ-alumina Brønsted basic sites is small, i.e. 3.3 kcal mol-1 (Fig. 10e). Also, the hydrogen bonding interaction between nonbonding

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orbital nO of γ-alumina and antibonding orbital σ*N−H of DPPH-H molecule is 11.3 kcal (Fig. 10f),

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which is smaller than the values calculated for DPPH molecule. The color-filled maps of the electron density for interaction of DPPH (from N•) and DPPH-H (from −NH bond) with the main adsorption sites of γ-alumina (i.e. Lewis acid, Brønsted acid and base) calculated by B3LYP-D/6-311G(2d,p) level of theory are shown in Fig. 11. The electron cloud around the nitrogen radical of DPPH for interaction with acidic sites (Fig. 11a, b) is more polarized than that for basic sites (Fig. 11c). Therefore, interaction of DPPH with Brønsted and Lewis acidic sites (−OH2 and >Al−, respectively) is stronger than the interaction of DPPH with Brønsted basic site (−HO). 15

ACCEPTED MANUSCRIPT The TD-DFT calculated UV-Vis absorption spectra of DPPH and DPPH-H molecules in the gas phase and in solvents are shown in Fig. 12. The gas phase absorption spectrum of DPPH at TDB3LYP/6-311G(2d,p) and TD-B3LYP/6-311++G(2d,p) levels of theory shows the absorption maximum at 507 and 517 nm, respectively. The absorption spectrum of DPPH in n-hexane and ipropanol solvents indicates the absorption maximum at 525 and 530 nm, respectively. In the case of

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DPPH-H gas phase spectrum, the absorption maximum appears at 335 and 340 nm for TDB3LYP/6-311G(2d,p) and TD-B3LYP/6-311++G(2d,p) levels of theory, respectively. The absorption spectrum of DPPH-H in cyclohexane displays the absorption maximum at 338 nm. The calculated results for both of DPPH and DPPH-H molecules show a close agreement with the experimental values, i.e. 520 and 330 nm [70], respectively.

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The UV-Vis absorption spectra of free and adsorbed DPPH and DPPH-H molecules on the main

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adsorption sites of γ-alumina calculated by TD-B3LYP/6-311G(2d,p) level of theory is shown in Fig. 13. Adsorption over different active sites of γ-alumina leads to shifting of the absorption maximum. The DPPH/γ-alumina Lewis acid (>Al−) complex shows the absorption maximum at

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493 nm; while the DPPH/γ-alumina Brønsted acid (−OH2) and Brønsted base (−OH) complexes

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display the absorption maximum at 525 nm. Therefore, adsorption of DPPH on Lewis acidic sites of γ-alumina shows blue shift, whereas adsorption of DPPH on Brønsted acidic and basic sites of γ-

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alumina exhibits red shift. The absorption spectrum DPPH-H/γ-alumina Brønsted base (−OH)

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complex shows the absorption maximum at 357 nm, which is also red-shifted compared to free DPPH-H. These results suggest that DPPH adsorbs over all active sites of γ-alumina, especially acidic sites, while DPPH-H interacts with the basic positions.

4. Conclusions In this study the radical scavenging ability and antioxidant activity of γ-alumina nanoparticles towards DPPH free radical were investigated in the media with different polarity by measuring the DPPH absorbance in UV-Vis absorption spectra. The structure and morphology of -alumina 16

ACCEPTED MANUSCRIPT nanoparticles before and after adsorption of DPPH were studied using UV-Vis, XRD and FT-IR spectroscopic methods. Due to the high surface area of -alumina nanoparticles, the high potential for the adsorption of DPPH radical was investigated. DPPH adsorbs on the -alumina nanoparticles by interaction of radical nitrogen (N•) and NO2 with the acidic and basic sites of -alumina surface with highly negative interaction energy. Interaction of DPPH with Brønsted and Lewis acidic sites

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of γ-alumina is more favored than Brønsted basic sites. The following order for the adsorption of DPPH over the different active sites of γ-alumina was predicted: Brønsted base < Lewis acid < Brønsted acid. Adsorption over different active sites of γ-alumina leads to shifting of the absorption maximum. Adsorption of DPPH on Lewis acidic sites of γ-alumina shows blue shift, whereas adsorption of DPPH on Brønsted acidic and basic sites of γ-alumina exhibits red shift. DPPH-H

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adsorbs on the basic sites of γ-alumina with less negative interaction energy. The experimental

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adsorption tests and theoretical calculations predict stronger adsorption of DPPH than DPPH-H on the surface of -alumina nanoparticles. The γ-alumina nanoparticles are powerful DPPH radical

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scavenger in non-polar media. The new mechanism for radical scavenging and antioxidant activities of γ-alumina nanoparticles was proposed. When DPPH was adsorbed and neutralized on the

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surface, the color of γ-alumina nanoparticles changed from white to purple and yellow, respectively.

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These results are of great significance for the environmental application of γ-alumina nanoparticles

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in order to remove free radicals.

Acknowledgment

We would like to thank the research committee of Damghan University for supporting this work.

References [1] P.P. Klemchuk, Antioxidants, in: Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005.

17

ACCEPTED MANUSCRIPT [2] P. Kovacic, R. Somanathan, Nanoparticles: Toxicity, radicals, electron transfer, and antioxidants, in: D. Armstrong, D.J. Bharali, (Eds.), Oxidative Stress and Nanotechnology: Methods and Protocols, Springer, New York, 2013. [3] A.K. Mittal, A. Kaler, U.C. Banerjee, Free radical scavenging and antioxidant activity of silver nanoparticles synthesized from flower extract of rhododendron dauricum, Nano Biomed. Eng. 4

SC RI PT

(2012) 118-124. [4] H.M. El-Rafie, M.A. Hamed, Antioxidant and anti-inflammatory activities of silver nanoparticles biosynthesized from aqueous leaves extracts of four Terminalia species, Adv. Nat. Sci.: Nanosci. Nanotechnol. 5 (2014) 035008.

[5] A. Saravanakumar, M. Ganesh, J. Jayaprakash, H.T. Jang, Biosynthesis of silver nanoparticles

NU

using Cassia tora leaf extract and its antioxidant and antibacterial activities, J. Ind. Eng. Chem. 28

MA

(2015) 277-281.

[6] V. Ravichandran, S. Vasanthi, S. Shalini, S.A. Ali Shah, R. Haris, Green synthesis of silver

ED

nanoparticles using Atrocarpus altilis leaf extract and the study of their antimicrobial and antioxidant activity, Mate. Lett. 180 (2016) 264-267.

PT

[7] J. Seralathan, P. Stevenson, S. Subramaniam, R. Raghavan, B. Pemaiah, A. Sivasubramanian, A. Veerappan, Spectroscopy investigation on chemo-catalytic, free radical scavenging and

CE

bactericidal properties of biogenic silver nanoparticles synthesized using Salicornia brachiata

AC

aqueous extract, Spectrochim. Acta A 118 (2014) 349-355. [8] J.R. Nakkala, E. Bhagat, K. Suchiang, S.R. Sadras, Comparative study of antioxidant and catalytic activity of silver and gold nanoparticles synthesized from Costus Pictus leaf extract, J. Mate. Sci. Technol. 31 (2015) 986-994. [9] A. Muthuvel, K. Adavallan, K. Balamurugan, N. Krishnakumar, Biosynthesis of gold nanoparticles using Solanum nigrum leaf extractand screening their free radical scavenging and antibacterial properties, Biomed. Prev. Nutr. 4 (2014) 325-332.

18

ACCEPTED MANUSCRIPT [10]

H. Li, X. Ma, J. Dong, W. Qian, Development of methodology based on the formation

process of gold nanoshells for detecting hydrogen peroxide scavenging activity, Anal. Chem. 81 (2009) 8916-8922. [11]

R. Isono, T. Yoshimura, K. Esumi, Preparation of Au/TiO2 nanocomposites and their

catalytic activity for DPPH radical scavenging reaction, J. Colloid Interf. Sci. 288 (2005) 177-183. B.X. Gao, J. Zhang, L. Zhang, Hollow sphere selenium nanoparticles: Their in-vitro anti

hydroxyl radical effect, Adv. Mater. 14 (2002) 290-293. [13]

B. Huang, J. Zhang, J. Hou, C. Chen, Free radical scavenging efficiency of nano-se in vitro,

Free Radical Bio. Med. 35 (2003) 805-813. [14]

SC RI PT

[12]

A. Watanabe, M. Kajita, J. Kim, A. Kanayama, K. Takahashi, T. Mashino, Y. Miyamoto, In

Y. Xue, Q. Luan, D. Yang, X. Yao, K. Zhou, Direct evidence for hydroxyl radical

MA

[15]

NU

vitro free radical scavenging activity of platinum nanoparticles, Nanotechnology 20 (2009) 455105.

scavenging activity of cerium oxide nanoparticles, J. Phys. Chem. C 115 (2011) 4433-4438. Y.Y. Tsai, J. Oca-Cossio, K. Agering, N.E. Simpson, M.A. Atkinson, C.H. Wasserfall, I.

Constantinidis,

ED

[16]

W. Sigmund, Novel synthesis of cerium oxide nanoparticles for free radical

[17]

PT

scavenging, Nanomedicine 2 (2007) 325-332. E. Grulke, K. Reed, Ma. Beck, X. Huang, A. Cormack, S. Seal, Nanoceria: factors affecting

S. Das, J. M. Dowding, K.E. Klump, J.F. McGinnis, W. Self, S. Seal, Cerium oxide

AC

[18]

CE

its pro- and antioxidant properties, Environ. Sci-Nano 1 (2012) 429-444.

nanoparticles: applications and prospects in nanomedicine, Nanomedicine 8 (2013) 1483-1508. [19]

P.C. Nagajyothi, S.J. Cha, I.J. Yang, T.V.M. Sreekanth, K.J. Kim, H.M. Shin, Antioxidant

and anti-inflammatory activities of zinc oxide nanoparticles synthesized using Polygala tenuifolia root extract, J. Photoch. Photobio. B: Biology 146 (2015) 10-17. [20]

P.C. Nagajyothi, T.V.M. Sreekanth, C.O. Tettey, Y.I. Jun, S.H. Mook, Characterization,

antibacterial, antioxidant, and cytotoxic activities of ZnO nanoparticles using Coptidis Rhizoma, Bioorg. Med. Chem. Lett. 24 (2014) 4298-4303. 19

ACCEPTED MANUSCRIPT [21]

J.P. Saikia, S. Paul, B.K. Konwar, S.K. Samdarshi, Nickel oxide nanoparticles: A novel

antioxidant, Colloid Surf. B 78 (2010) 146-148. [22]

D. Dasa, B.C. Nathb, P. Phukonc, S.K. Dolui, Synthesis and evaluation of antioxidant and

antibacterial behavior of CuO nanoparticles, Colloid Surf. B 101 (2013) 430-433. [23]

V.K. Vidhu, D. Philip, Biogenic synthesis of SnO2 nanoparticles: Evaluation of antibacterial

[24]

SC RI PT

and antioxidant activities, Spectrochim. Acta A 134 (2015) 372-379. T. Santhoshkumar, A.A. Rahuman, C. Jayaseelan, G. Rajakumar, S. Marimuthu, A.V.

Kirthi, K. Velayutham, J. Thomas, J. Venkatesan, S.K. Kim, Green synthesis of titanium dioxide nanoparticles using Psidium guajava extract and its antibacterial and antioxidant properties, Asian Pac. J. Trop. Med. 7 (2014) 968-976.

D. Vilela, M.C. González, A. Escarpa, Nanoparticles as analytical tools for in-vitro

NU

[25]

[26]

MA

antioxidant-capacity assessment and beyond, Trac-Trend Anal. Chem. 64 (2015) 1-16. R.T. Yang, Adsorbents: Fandamentals and applications, John wiley & sons 1nc, New York,

[27]

ED

2003.

L.K. Hudson, C. Misra, A.J. Perrotta, K. Wefers, F.S. Williams, Aluminum Oxide in:

[28]

PT

Ullmann's Encyclopedia of Industrial Chemistry Wiley-VCH, Weinheim, 2005. P.S. Santos, H.S. Santos, S.P. Toledo, Standard transition aluminas. Electron microscopy

E.J.W. Verway, The crystal structure of y-Fe2O3 and y-Al2O3, Z. Krist-Krist. Mater. 91

(1935) 65–69. [30]

AC

[29]

CE

studies, Mat. Res. 3 (2000) 104-114.

E. Menendez-Proupin, G. Gutierrez, Electronic properties of bulk γ− Al2O3, Phys. Rev. B 72

(2005) 035116. [31]

L. Smrcok, V. Langer, J. Krestan, γ-Alumina: a single-crystal X-ray diffraction study, Acta

Cryst. C 62 (2006) i83–i84. [32]

M. Digne, P. Raybaud, H. Toulhoat, Use of DFT to achieve a rational understanding of

acid–basic properties of γ-alumina surfaces, J. Catal. 226 (2004) 54-68. 20

ACCEPTED MANUSCRIPT [33]

X. Krokidis, P. Raybaud, A.E. Gobichon, B. Rebours, P.Euzen, H. Toulhoat, Theoretical

study of the dehydration process of boehmite to γ-alumina, J. Phys. Chem. B 105 (2001) 51215130. [34]

G. Paglia, C.E. Buckley, A.L. Rohl, B.A. Hunter, R.D. Hart, J.V. Hanna, L.T. Byrne,

Tetragonal structure model for boehmite-derived γ-alumina, Phys. Rev. B 68 (2003) 144110. G. Paglia, A.L. Rohl, C.E. Buckley, G.D. Gale, Determination of the structure of γ-alumina

SC RI PT

[35]

from interatomic potential and first-principles calculations: The requirement of significant numbers of nonspinel positions to achieve an accurate structural model, Phys. Rev. B 71 (2005) 224115. [36]

A.R. Ferreira, M.J.F. Martins, E. Konstantinova, R.B. Capaz, W.F. Souza, S.S.X. Chiaro,

A.A. Leitao, Direct comparison between two γ‐alumina structural models by DFT calculations, J.

O. Maresca, A. Allouche, J.P. Aycard, M. Rajzmann, S. Clemendot, F. Hutschka, Quantum

MA

[37]

NU

Solid State Chem. 184 (2011) 1105-1111.

study of the active sites of the γ-alumina surface: chemisorption and adsorption of water, hydrogen

ED

sulfide and carbon monoxide on aluminum and oxygen sites, J. Mol. Struc-Theochem 505 (2000) 81-94.

P. Hirva, T.A. Pakkanen, The interaction of amine bases on the Lewis acid sites of

PT

[38]

[39]

CE

aluminum oxide—a theoretical study, Surf. Sci. 277 (1992) 389-394. M. Lindblad, T.A. Pakkanen, Cluster models for the interaction of HC1 with non-polar

[40]

AC

surfaces of γ-Al2O3, Surf. Sci. 286 (1993) 333-345. K.C. Hass, W.F. Schneider, A. Curioni, W. Andreoni, The chemistry of water on alumina

surfaces: Reaction dynamics from first principles, Science 282 (1998) 265-268. [41]

J. Fernandez Sanz, H. Rabaa, F.M. Poveda, A.M. Marquez, C. Calzado, Theoretical models

for γ‐Al2O3 (110) surface hydroxylation: An ab initio embedded cluster study, Int. J. Quantum Chem. 70 (1998) 359-365.

21

ACCEPTED MANUSCRIPT [42]

O. Maresca, A. Ionescu, A. Allouche, J.P. Aycard, M. Rajzmann, F. Hutschka, Quantum

study of the active sites of the γ alumina surface (II): QM/MM (LSCF) approach to water, hydrogen disulfide and carbon monoxide adsorption, J. Mol. Struct. Theochem 620 (2003) 119-128. [43]

H.A. Dabbagh, M. Zamani, B.H. Davis, Nanoscale surface study and reactions mechanism

of 2-butanol over the γ-alumina (100) surface and nanochannel: A DFT study, J. Mol. Catal. A:

[44]

SC RI PT

Chem. 333 (2010) 54-68. A. Ionescu, A. Allouche, J.P. Aycard, M. Rajzmann, F. Hutschka, Study of γ-alumina

surface reactivity: Adsorption of water and hydrogen sulfide on octahedral aluminum sites, J. Phys. Chem. B 106 (2002) 9359-9366.

L. Marina, T.A. Pakkanen, Cluster models for the interaction of HCl with non-polar surfaces

of γ-Al2O3, Surf. Sci. 286 (1993) 333-345.

M. Zamani, H.A. Dabbagh, Surface modification of γ-alumina by NaNO2, NaNO3, HNO2,

MA

[46]

NU

[45]

HNO3 and H2SO4: A DFT-D approach, Iran. J. Catal. 6 (2016) 345-353. [47]

S. Roy, G. Mpourmpakis, D.Y. Hong, D.G. Vlachos, A. Bhan, R.J. Gorte, Mechanistic

D.A. De Vito, F. Gilardoni, L. Kiwi-Minsker, P.Y. Morgantini, S. Porchet, A. Renken, J.

PT

[48]

ED

study of alcohol dehydration on γ-Al2O3, ACS Catal. 2 (2012) 1846-1853.

Weber, Theoretical investigation of the adsorption of methanol on the (110) surface of γ-alumina, J.

S. Cai, K. Sohlberg, Adsorption of alcohols on γ-alumina (110C), J. Mol. Catal. A: Chem.

AC

[49]

CE

Mol. Struct. Theochem 469 (1999) 7-14.

193 (2003) 157-164. [50]

G. Feng, C. Huo, C. Deng, L. Huang, Y. Li, J. Wang, H. Jiao, Isopropanol adsorption on γ-

Al2O3 surfaces: A computational study, J. Mol. Catal. A: Chem. 304 (2009) 58-64. [51]

M. Zamani, H.A. Dabbagh, Adsorption behavior of the primary, secondary and tertiary

alkyl, allyl and aryl alcohols over nanoscale (100) surface of γ-alumina, J. Nanoanal. 1 (2014) 2130.

22

ACCEPTED MANUSCRIPT [52]

H.A. Dabbagh, K. Taban, M. Zamani, Effects of vacuum and calcination temperature on the

structure, texture, reactivity, and selectivity of alumina: Experimental and DFT studies, J. Mol. Catal. A: Chem. 326 (2010) 55-68. [53]

M.A. Christiansen, G. Mpourmpakis, D.G. Vlachos, Density functional theory-computed

mechanisms of ethylene and diethyl ether formation from ethanol on γ-Al2O3 (100), ACS Catal. 3

[54]

SC RI PT

(2013) 1965-1975. Z. Fang, Y. Wang, D.A. Dixon, Computational study of ethanol conversion on Al8O12 as a

Model for γ-Al2O3, J. Phys. Chem. C 119 (2015) 23413–23421 [55]

K. Larmier, C. Chizallet, N. Cadran, S. Maury, J. Abboud, A.F. Lamic-Humblot, ́E.

Marceau, H. Lauron-Pernot, Mechanistic investigation of isopropanol conversion on alumina

Z. Zuo, P. Han, J. Hu, W. Huang, Effect of surface hydroxyls on DME and methanol

MA

[56]

NU

catalysts: location of active sites for alkene/ether production, ACS Catal. 5 (2015) 4423-4437.

adsorption over γ-Al2O3 (hkl) surfaces and solvent effects: a density functional theory study, J.

[57]

ED

Mol. Model. 18 (2012) 5107-5111.

Z. Zuo, W. Huang, P. Han, Z. Gao, Z. Li, Theoretical studies on the reaction mechanisms of

Gen. 408 (2011) 130-136.

P. Hirunsit, K. Faungnawakij, S. Namuangruk, C. Luadthong, Catalytic behavior and

CE

[58]

PT

AlOOH-and γ-Al2O3-catalysed methanol dehydration in the gas and liquid phases, Appl. Catal. A:

AC

surface species investigation over γ-Al2O3 in dimethyl ether hydrolysis, Appl. Catal. A: Gen. 460 (2013) 99-105. [59]

S. Cai, V. Chihaia, K. Sohlberg, Interactions of methane, ethane and pentane with the

(110C) surface of γ-alumina, J. Mol. Catal. A: Chem. 275 (2007) 63-71. [60]

S. Cai, K. Sohlberg, Adsorption of 1-hexene on γ-alumina (110C), J. Mol. Catal. A: Chem.

248 (2006) 76-83. [61]

M.L. Ferreira, E.H. Rueda, Theoretical characterization of alumina and sulfated-alumina

catalysts for n-butene isomerization, J. Mol. Catal. A: Chem. 178 (2002) 147-160. 23

ACCEPTED MANUSCRIPT [62]

M. Mozaffari Majd, H.A. Dabbagh, H. Farrokhpour, A. Najafi Chermahini, Theoretical

study on the adsorption and relative stability of conformers of l-ascorbic acid on γ-alumina (100) surface, J. Mol. Struct. 1147 (2017) 185-191. [63]

O.P. Sharma, T. K. Bhat, DPPH antioxidant assay revisited, Food Chem. 113 (2009) 1202-

1205. A. Karioti, D. Hadjipavlou-Litina, M.L. Mensah, T.C. Fleischer, H. Skaltsa, Composition

SC RI PT

[64]

and antioxidant activity of the essential oils of Xylopia aethiopica (Dun) A. Rich. (Annonaceae) leaves, stem bark, root bark, and fresh and dried fruits, growing in Ghana, J. Agr. Food Chem. 52 (2004) 8094-8098. [65]

S. Kordali, A. Cakir, A. Mavi, H. Kilic, A. Yildirim, Screening of chemical composition and

NU

antifungal and antioxidant activities of the essential oils from three Turkish Artemisia species, J.

[66]

MA

Agr. Food Chem. 53 (2005) 1408-1416.

M.H. Alma, A. Mavi, A. Yildirim, M. Digrak, T. Hirata, Screening chemical composition

ED

and in vitro antioxidant and antimicrobial activities of the essential oils from Origanum syriacum L. growing in Turkey, Biol. Pharm. Bull. 26 (2003) 1725-1729. M. Jabbari, F. Gharib, Solvent dependence on antioxidant activity of some water-insoluble

PT

[67]

flavonoids and their cerium(IV) complexes, J. Mol. Liq. 168 (2012) 36-41. M. Jabbari, H. Mir, A. Kanaani, D. Ajloo, Kinetic solvent effects on the reaction between

CE

[68]

AC

flavonoid naringenin and 2,2-diphenyl-1-picrylhydrazyl radical in different aqueous solutions of ethanol: An experimental and theoretical study, J. Mol. Liq. 196 (2014) 381-391. [69]

M. Jabbari H.R. Moallem, Effect of solute–solvent interactions on DPPH radical scavenging

efficiency of some flavonoid antioxidants in various binary water–methanol mixtures, Can. J. Chem. 93 (2015) 558-563. [70]

P. Ionita, Is DPPH stable free radical a good scavenger for oxygen active species, Chem.

Pap. 59 (2005) 11-16.

24

ACCEPTED MANUSCRIPT [71]

D.L. Pavia, G.M. Lampman, G.S. Kriz, J.A. Vyvyan, Introduction to spectroscopy (4th ed.),

Brooks/Cole Cengage Learning, USA, 2008. [72]

L. Valgimigli, K.U. Ingold, J. Lusztyk, Solvent effects on the reactivity and free spin

distribution of 2,2-diphenyl-1-picrylhydrazyl radicals, J. Org. Chem. 61 (1996) 7947-7950. [73]

B. Delley, An all‐electron numerical method for solving the local density functional for

[74]

SC RI PT

polyatomic molecules, J. Chem. Phys. 92 (1990) 508-517. B. Delley, From molecules to solids with the DMol3 approach, J. Chem. Phys. 113 (2000)

7756-7764. [75]

J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple,

Phys. Rev. Lett. 77 (1996) 3865.

S. Grimme, Semiempirical GGA-Type density functional constructed with a long-range

NU

[76]

[77]

MA

dispersion correction, J. Comput. Chem. 27 (2006) 1787.

H. Sun, COMPASS: an ab initio force-field optimized for condensed-phase applications

[78]

ED

overview with details on alkane and benzene compounds, J. Phys. Chem. B 102 (1998) 7338-7364. R. Bauernschmitt, R. Ahlrichs, Treatment of electronic excitations within the adiabatic

A.D. Becke, Density-functional thermochemistry. III. The role of exact exchange, J. Chem.

CE

[79]

PT

approximation of time dependent density functional theory, Chem. Phys. Lett. 256 (1996) 454-464.

Phys. 98 (1993) 5648-5652.

C. Lee, W. Yang, R.G. Parr, Development of the Colle-Salvetti correlation-energy formula

AC

[80]

into a functional of the electron density, Phys. Rev. B 37 (1988) 785-789. [81]

J. Tomasi, B. Mennucci, R. Cammi, Quantum mechanical continuum solvation models,

Chem. Rev. 105 (2005) 2999-3093. [82]

M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G.

Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. 25

ACCEPTED MANUSCRIPT Montgomery, Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J.

SC RI PT

Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian 09, Revision D.01, Gaussian, Inc., Wallingford CT, 2013. [83]

E.D. Glendening, J.K. Badenhoop, A.E. Reed, J.E. Carpenter, J.A. Bohmann, C.M. Morales,

F. Weinhold, NBO 5.0, Theoretical Chemistry Institute and Department of Chemistry, University of Wisconsin, Madison, 2001.

J.M. Saniger, Al-O infrared vibrational frequencies of γ-alumina, Mater. Lett. 22 (1995)

NU

[84]

AC

CE

PT

ED

MA

109-l 13.

26

ACCEPTED MANUSCRIPT Table 1. Theoretical estimation of adsorption energy and its components for the adsorption of DPPH and DPPH-H molecules over γ-alumina (1 1 0) surface, calculated via PBE-D/DNP level of theory (kcal mol-1).

DPPH (N• and NO2) DPPH (N• and NO2) DPPH-H (NH and NO2) DPPH-H (NH and NO2)

Adsorption energy

Fig.

-16.7

-13.1

89.3

-78.8

-30.5

-49.8

8a

-16.7

-1.5

82.1

-16.7

-24.0

92.0

-16.7

-11.5

87.7

-16.7

-9.9

-16.7

32.2

-16.9

24.1

-16.9

26.0

SC RI PT

DPPH (N• and NO2)

Dispersion energy

-78.8

-21.7

-36.6

8b

-78.8

-29.9

-57.4

8c

-78.8

-33.3

-52.5

8d

87.3

-78.8

-28.7

-46.7

8e

62.4

-78.8

-20.5

-21.4

8f

66.7

-79.8

-23.1

-28.9

9a

68.2

-79.8

-26.1

-28.5

9b

CE

DPPH (N• and NO2)

Spin polarization energy

AC

DPPH (NO2)

Exchangecorrelation energy

NU

Lewis acid (>Al−) Lewis acid (>Al−) Brønsted acid (−OH2) Brønsted acid (−OH2) Brønsted base (−OH) Brønsted base (−OH) Brønsted base (−OH) Brønsted base (−OH)

Sum of kinetic and electrostatic energies

MA

DPPH (N• and NO2)

Sum of atomic energies

ED

Alumina position

PT

Interacting species

27

ACCEPTED MANUSCRIPT Figure captions Fig. 1. The color and UV-Vis absorption spectrum of DPPH in i-propanol without and containing γalumina nanoparticles. Fig. 2. The color and UV-Vis absorption spectrum of DPPH in n-hexane without and containing γalumina nanoparticles.

SC RI PT

Fig. 3. The color and UV-Vis absorption spectrum of DPPH-H in cyclohexane without and containing γ-alumina nanoparticles.

Fig. 4. The color and solid state UV-Vis spectrum of γ-alumina nanoparticles before and after adsorption of DPPH and DPPH-H.

Fig. 5. The XRD pattern of γ-alumina nanoparticles before and after adsorption of DPPH and

NU

DPPH-H.

MA

Fig. 6. The FT-IR spectrum of γ-alumina nanoparticles before and after adsorption of DPPH and DPPH-H.

BPE-D/DNP level of theory.

ED

Fig. 7. The optimized structures of clean and hydrated (110) surfaces of γ-alumina, calculated by

PT

Fig. 8. The optimized structures of γ-alumina (110) surface after adsorption of DPPH radical on the

level of theory.

CE

Lewis acidic (a, b), Brønsted acidic (c, d) and Brønsted basic (e, f) sites, calculated by BPE-D/DNP

AC

Fig. 9. The optimized structures of γ-alumina (1 1 0) surface after adsorption of DPPH-H molecule on Brønsted basic sites (a, b), calculated by BPE-D/DNP level of theory. Fig. 10. The surface plot and the stabilization energy of NBO donor-acceptor interactions for interaction of DPPH (a-e) and DPPH-H (f) molecules with the main adsorption sites of γ-alumina, i.e. Lewis acid (a, b), Brønsted acid (c, d) and Brønsted base (e, f), calculated by B3LYP-D/6311G(2d,p) level of theory.

28

ACCEPTED MANUSCRIPT Fig. 11. The color-filled map of the electron density for interaction of DPPH (a-c) and DPPH-H (d) molecules with the main adsorption sites of γ-alumina, i.e. Lewis acid (a), Brønsted acid (b) and Brønsted base (c, d), calculated by B3LYP-D/6-311G(2d,p) level of theory. Fig. 12. The UV-Vis spectra of DPPH (dashed line) and DPPH-H (solid line) molecules in the gas phase and in solvents, calculated by TD-B3LYP/6-311G(2d,p) and TD-B3LYP/6-311++G(2d,p)

SC RI PT

levels of theory. Fig. 13. The UV-Vis spectra of free and adsorbed DPPH and DPPH-H molecules on the main

AC

CE

PT

ED

MA

NU

adsorption sites of γ-alumina, calculated by TD-B3LYP/6-311G(2d,p) level of theory.

29

MA

NU

SC RI PT

ACCEPTED MANUSCRIPT

AC

CE

PT

ED

Fig. 1

30

MA

NU

SC RI PT

ACCEPTED MANUSCRIPT

AC

CE

PT

ED

Fig. 2

31

MA

NU

SC RI PT

ACCEPTED MANUSCRIPT

AC

CE

PT

ED

Fig. 3

32

MA

NU

SC RI PT

ACCEPTED MANUSCRIPT

AC

CE

PT

ED

Fig. 4

33

ACCEPTED MANUSCRIPT

SC RI PT

Intensity

DPPH-H/γ-Alumina

DPPH/γ-Alumina

MA

NU

γ-Alumina

30

40

50

ED

20

60

70

2θ Fig. 5

AC

CE

PT

10

DPPH

34

80

90

100

110

120

ACCEPTED MANUSCRIPT γ-Alumina

H-O-H bend O-H stretch

Al-O stretch and O-Al-O bend

SC RI PT

DPPH/γ-Alumina

C-H stretch

H-O-H bend O-H stretch

N-O stretch

MA

NU

DPPH-H/γ-Alumina

C-H stretch

O-H stretch 3500

3000

2500

H-O-H and N-H bend

2000

N-O stretch

1500

Wavenumber (cm-1)

Fig. 6

AC

CE

PT

4000

ED

N-H stretch

35

1000

500

0

ACCEPTED MANUSCRIPT

Al64O100H8

Al64O100H8

SC RI PT

Al64O96

Al64O100H8

AC

CE

PT

ED

MA

NU

Fig. 7

36

ACCEPTED MANUSCRIPT b

ΔEads = −49.8 kcal mol-1

SC RI PT

a

ΔEads = −36.6 kcal mol-1

d

ED

MA

NU

c

ΔEads = −57.4 kcal mol-1

f

AC

CE

PT

e

ΔEads = −52.5 kcal mol-1

ΔEads = −46.7 kcal mol-1

ΔEads = −21.4 kcal mol-1

Fig. 8

37

ACCEPTED MANUSCRIPT b

ΔEads = −28.9 kcal mol-1

SC RI PT

a

ΔEads = −28.5 kcal mol-1

AC

CE

PT

ED

MA

NU

Fig. 9

38

ACCEPTED MANUSCRIPT

a

b

nN•

nN•

n

Al

nN• → n*Al (27. 3 kcal mol-1)

nN• → n*Al (19. 8 kcal mol-1)

d

NU

c

ED

n*H

CE

PT

nN• → n*H (23.7 kcal mol-1)

AC

nN•

MA

nN•

e

SC RI PT

n*Al *

σ*O-H

nN• → σ*O-H (17. 2 kcal mol-1)

f

nN•

σ*N-H σ*O-H nO nN• → σ*O-H (3.3 kcal mol-1)

nO → σ*N-H (11. 3 kcal mol-1)

Fig. 10

39

ACCEPTED MANUSCRIPT a

b

H H

O

Al

SC RI PT

N

c

N

d

N

O H

N

Fig. 11

AC

CE

PT

ED

MA

NU

H

40

O H

200

300

400

SC RI PT

Intensity

ACCEPTED MANUSCRIPT

500 600 Wavelength (nm)

700

MA

NU

DPPH-H gas phase, TD-B3LYP/6-311++G(2d,p) DPPH-H gas phase, TD-B3LYP/6-311G(2d,p) DPPH-H in cyclohexane, TD-B3LYP/6-311G(2d,p) DPPH gas phase, TD-B3LYP/6-311++G(2d,p) DPPH gas phase, TD-B3LYP/6-311G(2d,p) DPPH in n-hexane, TD-B3LYP/6-311G(2d,p) DPPH in i-propanol, TD-B3LYP/6-311G(2d,p)

AC

CE

PT

ED

Fig. 12

41

800

200

300

400

SC RI PT

Intensity

ACCEPTED MANUSCRIPT

500 600 Wavelength (nm)

700

MA

NU

Free DPPH DPPH/Alumina Lewis acid (>Al−) DPPH/Alumina Brønsted acid (−OH2) DPPH/Alumina Brønsted base (−OH) Free DPPH-H DPPH-H/Alumina Brønsted base (−OH)

AC

CE

PT

ED

Fig. 13

42

800

ACCEPTED MANUSCRIPT

500 600 Wavelength (nm)

700

PT

ED

MA

Free DPPH DPPH/Alumina Lewis acid (>Al−) DPPH/Alumina Brønsted acid (−OH2) DPPH/Alumina Brønsted base (−OH) Free DPPH-H DPPH-H/Alumina Brønsted base (−OH)

800

NU

400

CE

300

AC

200

SC RI PT

Intensity

Graphical abstract

43

ACCEPTED MANUSCRIPT Highlights Radical scavenging activity of γ-alumina towards DPPH radical was investigated



The high potential for adsorption of DPPH radical on nanoparticles was investigated



The new mechanism for antioxidant activity of γ-alumina was proposed



γ-Alumina nanoparticles were powerful DPPH radical scavenger in non-polar media



Aadsorption order over different active sites of γ-alumina: Brønsted base < Lewis acid <

SC RI PT



AC

CE

PT

ED

MA

NU

Brønsted acid

44