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A huge quantities of parbens, esters of p-hydroxybenzoic acid, are
used in cosmetic and personal care products as a preservative substance. Due to
their detrimental effects on ecosystem and human health, taking precautionary
measures to remove these compounds is a must. In this study, a silica nanotube
is selected as a novel sensor to adsorb of the most common parabens which are
methyparaben and propylparaben molecules. To do so, Density Functional Theory calculation
is investigated. The calculated adsorption energies of the most stable
configurations for Methyl [email protected] and Propyl [email protected]
complexes are about -0.238, -0.242 eV, respectively. It was observed in the
course of experiments that the electronic properties of nanotube experience
dramatic changes in case of interaction between the parabens and nanotube.
These changes lead to declining the HOMO/LUMO gap of the tube to its original
value. Experimental results demonstrate that this adsorption can also increase
the electrical conductivity of the nanotube. It means that the utilized silica
nanotube can detect the existence of  methyl
and propyl parabens molecules in environment.  

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Methyl paraben; Propyl paraben; Computational calculation;
Electrical conductivity; SiO2 NT.







are esters of parahydroxybenzoic acid which mainly include methylparaben (MP),
ethylparaben (EP), propylparaben (PP), butylparaben (BuP) and benzylparaben
(BP). It is nearly a century, parabens, alone or in combination with other
compounds, have been routinely utilized as preservatives in formulation of
approximately all types of cosmetics, pharmaceuticals and food products because
of having antibacterial and antifungal effects1-2 . Furthermore, the widespread use of parpens as the most common
preserving agents is attributed to their extraordinary physical and chemical
properties such as having imperceptible odor or taste, chemical stability at
high temperature or wide pH range, low toxicity and low production cost3. Researches have shown a direct correlation exists between
increasing the anti-microbial activity of parabens and the chain length of
their aster group; however, the shorter esters including methylparaben and
propylparaben are frequently used since they are more soluble in water4567.

the past few years, the vast use of 
paraben compounds  has raised  worldwide concerns about their unpredictable
influence over human life and environment systems .Although paraben’s safety
have been proven by various studies, currently, some vitro studies have
identified parabens as Endocrine-Active Chemicals (EACs) which might make an
adverse impact on human health8. Since, parabens are able to be absorbed by skin, their estrogenic
activity may increase the risk of breast cancer or the male reproductive tract910

order to remove these compounds, a number of technics have been applied. For
example, extraction of parabens from cosmetic products and environmental
samples using Solid Phase Extraction (SPE) 11, detection of parabens by Gas Chromatography–Mass Spectrometry
(GC–MS) 12 and  adsorption13. Among them, the adsorption technics has been considered as a
cost-effective and practical way to remove and detection of specific compounds14.

nanotubes (SiO2NT), discovered by Nakamura and coworkers 15 in1995, have drawn a great deal of attention because of their
exceptional properties such as hydrophilic nature, easy colloidal suspension
formation, and distinctive inner and outer surface. In addition, their
widespread applications in many fields including separation, adsorption,
catalysis, optics, electronics, drug delivery and chemical sensing lead to
remarkable advances in the use of silica nanotubes in medicine and industry161718. Since the discovery of SiO2NTs, a wide range of
research on their preparation and the applications have been carried out.
Utilizing the SiO2NTs as the catalyst carriers by Zhang and his
coworkers is example of finding research 19. Additionally, For the first time, SiO2NTs were
introduced as adsorbents for heavy metals due to their intriguing
characterizations by Wang.20.

in an experimental investigation the adsorption MP, EP and PP on PS/Fe3O4
was studied by Chen group13. Afterward, we decided to investigate the interaction of SiO2NT
with paraben molecules, such as the geometrical structures, energetic and
electronic properties based on the density functional theory and explore the feasibility
of applying SiO2NT as a novel detection for the removing methyl and
propyl paraben molecules.



All Density
functional Theory (DFT )calculations were performed using the GAMMES suite of
program21. In this work, a single-walled silica nanotube, (SiO2NT)
consisting of 24 Si and 52 O atoms that the end atoms were saturated by
hydrogen atoms to decrease the boundary effects, was selected (Fig. 1). The
geometric optimizations were carried out on the pristine SiO2NT and
its different MP-SiO2NT and PP- SiO2NT complexes at B3LYP
 level of theory and 6-31G(d) standard
basis set with no symmetry constrain22 23. B3LYP is considered as a common used functional in the
investigation of these structures. Also, Molecular Electrostatic Potential
(MEP), Natural Bonding Orbital (NBO) analysis 24, Density of States (DOS) and all energy calculation were performed
at the same level of theory. DOS results
were obtained from Gauss Sum program25.

Furthermore, Vibrational frequencies were also
calculated at the same level to confirm that all the stationary points
correspond to true minima on the potential energy surface. The interaction of
MP and PP with SiO2NT was investigated through computing energy
adsorption (Eads ) by using the following Eq:



 E([email protected])


E(nanotube)                                                                                (1)


E([email protected] nanotube) is the total energy of MP and PP molecules adsorbed on
the surface of SiO2NT, E(tube) and E(adsorbate) are the total
energies of the pristine SiO2NT and free paraben molecules, respectively.
By the definition, a negative value of Eads indicates exothermic adsorption.

and discussion:

optimized structure of bare SiO2NT is shown in Fig. 1. After
optmization, the length and diameter of tube were calculated to be about 16.59 and 3.98 Å,
respectively. Moreover, the length of Si-Oxygen bonds of the SiO2 nanotube
were calculated. Two types of Si-oxygen bond could be found: One with the bond

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