Electronic Supplementary Material (ESM)
Graphene-ceramic hybrid nanofibers for ultrasensitive electrochemical determination of ascorbic acid
Masoud Taleb1, Roman Ivanov1, Sergei Bereznev2, Sayed Habib Kazemi3,
Irina Hussainova1, 4, 5*
1 Tallinn University of Technology, Department of Materials Engineering, Ehitajate 5, Tallinn, Estonia;
2 Tallinn University of Technology, Department of Materials Science, Ehitajate 5, Tallinn, Estonia;
3Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan, Iran;
4ITMO University, Kronverksky 49, St. Petersburg, Russian Federation.
5University of Illinois at Urbana-Champaign, Department of Mechanical Science and Engineering, Urbana, IL 61801, USA
* Corresponding author: Tel. +372 6203371, e-mail: (Irina Hussainova)
Raman Spectra
The Raman scattering spectra obtained for ANF-C300 and ANF-C700 is shown in Fig. S1. The narrow bands suggest nanocrystalline structure of foliates. The materials exhibit the well-pronounced G band peak at 1590 cm-1 indicating the formation of a graphitized structure and a strong D peak at around 1350 cm-1 corresponding to the disorder-induced phonon mode [1]. The position and intensity of 2D band points to the presence of several layers of graphene sheets [1]. The ratio between intensities of the D and G peaks, or the value of area-averaged ID / IG, which can be used for estimation of the amount of structural defects/edges and the degree of graphitization in graphitic materials, is calculated to be 1.33 and 1.45 for the ANF-C300 and ANF-C700, respectively. Because of large number of the edge planes in the direction parallel to incident polarization as well as the twisted graphitic planes of multi-layer graphene foliates, the ID / IG ratio is expectedly high. The Raman spectra also demonstrate the weak second order 2D and 2D' peaks at around 2700 and 2962 cm−1, which can be explained by the crystallographic ordering of the graphitic planes [2,3].
Fig. S1. Raman spectra of the ANF-C300 and ANF-C700 samples measured with 534 nm excitation; inset demonstrates the direction of incident polarization.
Calculating electroactive surface area
The electroactive surface area of the electrode was calculated using the peak currents for a reversible reaction by applying the Randles–Sevcik equation (Eq. 1) [4]:
ip = (2.69 × 105)n3/2ACD1/2v1/2 (1)
where is the peak current (A), n is number of transferred electrons (n = 1), A is the surface area of the electrode (cm2), C is the concentration of Fe(CN)63−/4− in analyte (mol cm−3), D is the ferricyanide diffusion coefficient (6.67 × 10−6 cm2 s−1) and v is the potential scan rate (V s−1).
Fig. S2. CV profiles of a) ANF-C300; b) ANF-C700 electrodes recorded in 5 mM K3Fe(CN)6 in 0.1 M KCl solution at different scan rates.
Influence of scan rate on the oxidation peak current
The influence of scan rate on the oxidation peak current and potential both ANF-C300 and ANF-C700 samples is shown for in Fig. S3. Plot of log peak current versus log scan rate is linear and the slopes of the lines are 0.21 and 0.43, for ANF-C300 and ANF-C700 electrodes, respectively (Fig. S3c, 3d). The slope determined for AA oxidation at ANF-C700 electrode is very close to the theoretical value of 0.5 reported for the diffusion-controlled processes [5]. The relation, between the anodic peak current and the square root of scan rate is described as y = 8.009x + 76.297 (R2 = 0.994) and y = 24.294x + 14.046 (R2 = 0.994), supporting the diffusion controlled oxidation of ascorbic acid in prepared samples (Fig. S3e, 3f). Compared to ANF-C700 material, ANF-C300 has less carbon foliates in its structure and shows some non-diffusional contribution than the ANF-C700. Meanwhile, minor non-linearity can be seen for ANF-C300 between the anodic peak current and the square root of scan rate (Fig. S3e, 3f) which can be due to the possibility of adsorption of analyte and its redox product on the structure of carbonous electrode. Figs. S3g and 3h show a linear relationship between the peak potential (Ep) and ln ʋ confirming irreversible reactions on the surface of the modified electrodes during oxidation of AA.
Fig. S3. a, b) CVs of AA oxidation reaction at ANF-C300 and ANF-C700 electrodes, respectively, in 0.1 M phosphate buffer (pH = 7.0) containing 7 mM of AA at different potential scan rates; c, d) plots of Ip vs. (ʋ)1/2; e, f) plots of log Ip vs. log ʋ; and g,h) plots of Ep vs. ln ʋ.
Cottrell equation
For an electroactive material with diffusion coefficient of D, the current for the electrochemical reaction (at a mass transport restricted rate) is described by the Cottrell equation [5]:
I=nFACD12π12t12=Kt-1/2 (2)
where C is the bulk concentration (mol cm-3), D is the diffusion coefficient (cm2 s-1), n is stoichiometric number of electrons involved in the reaction, F is Faraday’s constant (96,485 C/equivalent) and A is electrode area (cm2). Under diffusion control, a plot of I vs. t-1/2 is linear (Fig. S4a-b insets) and the value of D can be calculated from the slope.
Fig. S4. Chronoamperometric responses of a) ANF-C300; and b) ANF-C700 at a potential of 0.1 V vs. SCE in 0.1M phosphate buffer (pH = 7.0) in different concentrations of AA noted on the curves. Left insets in both Figs. show the plot of I vs. t-1/2, and right insets show the slopes of the resulting straight lines versus the concentration for the corresponding modified electrodes.
Fig. S5. Amperometric responses to the successive addition of 30 µM AA, 20 µM UA, 20 µM DA, 1 mM Ammonium chloride, 1 mM FeCl3, 1 mM MgCl2, 1 mM KCl, 1 mM citric acid, 1 mM Na2SO4, 1 mM NaCl, 1 mM Urea, 1 mM H2O2 in 0.1 M phosphate buffer (pH 7.0) at 0.1 V.
Fig. S6. CV responses of ANF-C700 measured in the presence of 7 mM of AA and in phosphate buffer (pH = 7.0) during 10 days noted on the figure (scan rate = 50 mVs-1).
References (The numbering here is only for the Electronic Supplementary Material)
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