Electronic Supplementary Material
Electrochemical tyrosine sensor based on a glassy carbon electrode modified with a nanohybrid made from graphene oxide and multiwalled carbon nanotubes
Junhua Li · Daizhi Kuang · Yonglan Feng · Fuxing Zhang · Zhifeng Xu·Mengqin Liu· Deping Wang
1. Electrochemical impedance spectroscopy (EIS) result
Fig. S1.Nyquist diagrams of different electrodes in 5 mM Fe(CN)63−/4− (1:1) solutioncontaining 0.1 M KCl at open circuit potential with amplitude of 5 mV and frequency range from 105 Hz to 0.1 Hz; the inset is the equivalent circuit
In general, EIS was employed to get more information about the electrochemical properties of the modified electrodes. Fig. S1 shows the impedance changes of the different electrodes associated with the stepwise modification process. The value of the charge transfer resistance, Ret depends on the dielectric and insulating features at the electrode/electrolyte interface. In order to obtain the detailed information from the impedance spectrum, a simple equivalent circuit model (inset in the Figure) was used to fit the results. In the Nyquist plot of impedance spectra, the semicircle section at higher frequencies can characterize the electron transfer limited process and the linear section seen at lower frequencies may be attributed to the diffusion step. The Ret value which obtained at the bare GCE was about 80 Ω. After the modification of the GCE surface with GO or MWCNT, the Ret values were increased to approximate 110 Ω and 200 Ω respectively. This is because that carboxyl and hydroxy groups have been grafted into the GO and MWCNT surface during their preparation processes, and then negative charges of these groups can arouse electrostatic repulsion with Fe(CN)63−/4− at electrode surface, resulting in the formation of larger interface impedance. With further analysis comparing, Ret of GO is lower than that of MWCNT, indicating the electron transfer ability of GO is relatively greater than that of MWCNT. Furthermore, the Retof GO/MWCNT/GCE was about 160 Ω which was between the Ret values of GO/GCE and MWCNT/GCE, and this phenomenon confirmed that nanohybrid of GO/MWCNT was successfully immobilized on the GCE surface just as designed.
2. The calculation process of charge transfer coefficient (α)
According to Bard and Faulkner, the charge transfer coefficient of α can be given from Tafel equation:
Where i0 is exchange current density. From this equation, we know that a polt of logi vs. η, regarded as a Tafel plot, is a useful device for evaluating kinetic parameters. In general, there is an anodic branch with slope (1−α)F/2.3RT and a cathodic branch with slope −αF/2.3RT in Tafel plot. In this work, the average value of anodicslope for five Tafel measurementsis 9.82 V−1. So, the value of α was got to be 0.42.
3. The mechanism of L-tyrosine electro-oxidation
Fig. S2.Electrochemical oxidation mechanism of L-tyrosine at GO/MWCNT/GCE
4. Thelinear relationship between Q and t1/2
Fig. S3. The plot of Q versus t1/2 for GCE (a), MWCNT/GCE (b), GO/GCE (c) and GO/MWCNT/GCE (d) in 0.1 mM K3[Fe(CN)6]
5. The calculation process of adsorption capacity (Γ)
The surface adsorption concentration(Γ)of L-tyrosineon GO/MWCNT film can be estimated according to the followingequation:
where Γ is the surface coverage concentration, Ais the electrode surface area,n is the number of transferred electron andν is the potential scan rate. The symbolsF, R and T have their usual meanings.The linear relationship betweenip and v has been obtained above with slope of 0.01775. Here, as n = 2, A = 0.0617 cm2, the adsorption capacity (Γ) of the prepared sensoris calculated to be 7.654 × 10−8 mol cm−2 which is much greater than that of MWCNT/Au/cyclodextrin modified electrode (7.16 × 10−11 mol cm−2) and L-serine polymer modified electrode (4.68 × 10−10 mol cm−2).