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Tailoring Molecular Permeability of NanochannelMicelle Membranes for Electrochemical Analysis of Antioxidants in Fruit Juices without Sample Treatment Fei Yan, and Bin Su Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b02823 • Publication Date (Web): 24 Oct 2016 Downloaded from http://pubs.acs.org on October 28, 2016
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Analytical Chemistry
Tailoring Molecular Permeability of Nanochannel-Micelle Membranes for Electrochemical Analysis of Antioxidants in Fruit Juices without Sample Treatment Fei Yan and Bin Su* Institute of Analytical Chemistry, Department of Chemistry, Zhejiang University, Hangzhou, 310058, China Corresponding Author *E-mail:
[email protected]; Homepage: http://mypage.zju.edu.cn/binsu.
ABSTRACT: Antioxidants are widely found or used in food, pharmaceutical and cosmetics industries, thus rapid and sensitive detection of antioxidants is of great interest. The present work reports a simple and fast electrochemical method for direct analysis of antioxidants in fruit juices by modulating the permeability of mesochannels on the electrode surface. This goal was achieved by growing vertical silica mesochannel array (SMA) with a channel diameter of 2 3 nm on the indium tin oxide (ITO) electrode surface using the cylindrical micelles (CMs) as the template. As-prepared electrodes, designed as CM@SMA/ITO, are only permeable to lipophilic antioxidants, e.g. retinol, with the hydrophobic hydrocarbon cores of CMs. After excluding CMs from silica mesochannels, the ITO electrode modified with bare SMA, namely SMA/ITO, possesses a high density of silanol groups on the channel wall and thus is only permeable to hydrophilic antioxidants, such as ascorbic acid (AA). Two types of sensors allowed the selective analyses of retinol and AA in buffer solutions and demonstrated a wide linear range for retinol
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(1 60 M) and AA (10 2000 M), respectively, and a low detection limit (0.65 M for retinol and 0.52 M for AA). Moreover, the SMA/ITO electrode can selectively determine the concentration of AA in orange juice. The CM@SMA/ITO electrode can measure the sum activity of lipophilic antioxidants, such as retinol, α-tocopherol and others possibly co-existed, in carrot juice. In addition, the ultrasmall mesochannels and CMs could effectively exclude the access of large substances, rendering an excellent anti-fouling and anti-interference ability for direct analysis of antioxidants in fruit juices without sample pre-treatment.
INTRODUCTION Antioxidants are capable of scavenging excess radical and reactive oxygen species related to cardiovascular diseases, inflammatory disorder and cancer, thus protecting the living organisms from harmful effects of oxidative stress.1,2 Moreover, antioxidants have been extensively found or used in food, pharmaceutical and cosmetics industries. Therefore, developing a rapid, simple and sensitive approach for antioxidants quantification is of great significance. Common antioxidants, including vitamins, flavonoids, carotenoid and phenolic compounds, are often reducing agents and take into function by oxidation of themselves. Among them, retinol (namely vitamin A) is a kind of liposoluble antioxidants and is essential for vision.3 Water-soluble ascorbic acid (AA) is a powerful antioxidant that participates in many biological processes.4 The structures of antioxidants including hydrophobic retinol, hydrophilic AA, and amphiphilic trolox are shown in Scheme 1a.
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Analytical Chemistry
Scheme 1. (a) Molecular structures of studied antioxidants. (b) Experimental setup for electrochemical measurements.
So far, conventional methods for detection of antioxidants are usually based on spectrophotometry or fluorescence, which evaluates the antioxidants by a certain standard, such as trolox equivalent antioxidant capacity (trolox is 6-hydroxy-2,5,7,8-tetramethylchroman2-carboxylic acid, a amphiphilic antioxidant),5,6 2,2-diphenyl-picrylhydrazyl radicals scavenging capacity,7 the ferric reducing antioxidant power,8 and the oxygen radical absorbance capacity.9 Compared with aforementioned methods, electrochemical sensors have received a great deal of attention because of their simplicity, low-cost and high sensitivity.10-16 Recently, various materials, such as carbon nanotubes,17-19 surfactants,20-22 graphene,23-25 and DNA,26-29 have been extensively used to modify the sensor surface to improve analytical performance for antioxidants. In most cases, the total antioxidant capacity of antioxidants could be evaluated. However, qualitative and 3
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quantitative simultaneously detection of antioxidants is yet highly desired. More recently, Kunitake et al. have reported that individual antioxidant activity of hydrophilic, lipophilic or amphiphilic compounds can be simultaneously measured by modulating the electrode surface lipophilicity and using a bicontinuous microemulsion.30,31 Mesoporous silica materials have aroused enormous interest because of their regular structure with a high surface area, uniform pore distribution and tunable pore size.32-36 Using organic templates, versatile silica mesostructures can be prepared in the form of either particles or thin films.36-38 Among these, perpendicularly oriented mesochannels are in favor of the mass transport to the underlying substrate and have been extensively applied in catalysis, sensing, molecular separation and many other areas.36,39-42 Bare silica mesochannels have silanol groups on the surface and are thus preferentially permeable to hydrophilic substance. If retaining the template structures, such as cylinder micelles (CMs), in mesochannels, the hydrophobic cores of micelles will selectively extract or concentrate nonpolar organic compounds via the hydrophobic effect.43-45 Furthermore, the combination of size exclusion effect and hydrophobic extraction capacity, the extraction and concentration can be applied for the direct analysis in real or complex samples (e.g. soil dispersions, human blood serum, milk and whole blood) without sample pre-treatment.46,47 In this work, we demonstrated that hydrophilic and lipophilic antioxidants, such as AA and retinol, can be selectively determined by the control of lipophilicity of silica mesochannels attached to the ITO electrodes. Bare silica mesochannel array (SMA) modified ITO electrodes, namely SMA/ITO, can accurately measure hydrophilic AA. If retaining CMs inside SMA, the electrode, designated as CM@SMA/ITO, can quantitatively detect hydrophobic retinol. 4
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Analytical Chemistry
EXPERIMENTAL SECTION Chemicals and Materials. All chemicals and reagents were of analytical grade or higher and used as received without further purication. Ultrapure water (18.2 MΩ·cm) was used to prepare all aqueous solutions throughout the work. Tetraethoxysilane (TEOS, ≥99.0%), hexaammineruthenium(III) chloride (Ru(NH3)6Cl3, 98%), trolox (97%), -tocopherol and cetyltrimethylammonium bromide (CTAB, ≥99%) were ordered from Sigma. Concentrated ammonia aqueous solution (25 wt %), retinol (95%), ascorbic acid (AA, ≥99.0%), -carotene (96%), potassium ferricyanide (K3[Fe(CN)6]), and potassium hydrogen phthalate (KHP) were purchased from Aladdin. Hydroxymethylferrocene (FcMeOH, 97%) was obtained from Alfa Aesar. The stock solutions of retinol and trolox were prepared with methanol and ethanol, respectively, and stored at 4 °C for use. ITO coated glass (surface resistivity
