Electronic Supplementary Material
An electrochemical sensor based on SiO2@TiO2 embedded molecularly imprinted polymers for selective and sensitive determination of theophylline
Journal of Solid State Electrochemistry
Tian Gan1,2,3· Aixia Zhao1· Zhikai Wang1· Pan Liu1· Junyong Sun1· Yanming Liu1,2
1 College of Chemistry and Chemical Engineering Institute for Conservation and Utilization of Agro−bioresources in Dabie Mountains, Xinyang Normal University, Xinyang 464000, People’s Republic of China
2 Henan Key Laboratory of Biomolecular Recognition and Sensing, Shangqiu Normal University, Shangqiu 476000, People’s Republic of China
3 Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), Wuhan University, Wuhan 430072, People’s Republic of China
Corresponding author: Tian Gan,
Yanming Liu,
Optimization of parameters for theophylline determination
Owing to its high sensitivity and resolution, the DPV technique was applied to investigate the effects of experimental parameters on the oxidation peak current of K3Fe(CN)6 probe at the SiO2@TiO2@MIP/CPE.
The template/monomer mole ratio in the polymerization solution is found to exert an influence on the amount of imprinted cavities available for the selective rebinding of theophylline template. Fig. S1A shows the effect of different mole ratio of theophylline/MAA during the polymerization process on the sensor response. It was observed that the largest peak current was obtained when the theophylline/MAA mole ratio was 1:4. When the amount of template was further decreased, the K3Fe(CN)6 response on the sensor was found to decrease, attributing to the lower number of recognition cavities and consequently lower sensitivity.
The effect of the template/cross−linker mole ratio on the DPV responses of the sensor was examined in Fig. S1B. With the molar ratio between theophylline and EGDMA decreasing from 1:10 to 1:35, a biggest peak current response was found at the ratio of 1:20 for the sensor. It is inferred that enough amount of EGDMA is crucial for the fabrication of polymer texture, and the structure of the polymer becomes more compact and less porous with too much EGDMA. Hence, the ratio of 1:20 was chosen as the best for theophylline/EGDMA mole ratio.
For comparison, 0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6 gSiO2@TiO2 was added in the solutions for imprinting polymerization, respectively. As Fig. S1C showed, the peak current varied with the amount of SiO2@TiO2 added, and a maximum was reached when 0.4 g of SiO2@TiO2 was embedded in the polymer. This was related to the change of imprinted sites density. Consequently, 0.4 g of SiO2@TiO2 was most suitable and adopted.
A pivotal step in the preparation of an imprinted sensor is the removal of the template molecules to form recognition sites for sensitive voltammetric analysis. All of the theophylline molecules must be completely removed from the polymer matrix for the cavities generation in order to ensure good sensitivity and reproducibility of the sensor. Deionized water, alcohol, methanol, acetonitrile, 0.1 M sodium hydroxide solution, methanol/acetic acid, and methanol/water were used respectively for the formation of cavities. The results indicated that methanol completely and quickly removed the template molecules resulting from the affinity of the solvent with the polymer [1]. Fig. S1D shows the variation of the current at different extraction time (3 to 18 min). As can be seen, the current value rapidly increased with a prolongation of the immersion time, and it was found to be unchangeable after the immersion time was above 12 min, owing to the complete exposure of imprinted cavities. Thus, the immersion in methanol for 12 min was chosen as the best condition for the elution of theophylline from the polymer particles.
The mass ratio between SiO2@TiO2@MIP and graphite in carbon paste has great effect on the sensitivity and selectivity of the sensor, as shown in Fig. S1E. The current signal varied significantly with the SiO2@TiO2@MIP/graphite mass ratio, and reached a maximum value with a mass ratio of 1:4. With an increase of the content of SiO2@TiO2@MIP, the imprinted binding sites for theophylline increased, whereas the non−conductive modified polymer became more. Therefore, an unsuitable amount of SiO2@TiO2@MIP modified on the sensor would decrease the response K3Fe(CN)6.
The pH of the K3Fe(CN)6 solution is another factor for evaluation the electrochemical behaviors of the sensor. As shown in Fig. S1F, a maximum electrochemical oxidation signal was observed at pH 6.5, which was adopted in the subsequent experiments.
Fig. S1Effects of the mole ratio of template to monomer (A), mole ratio of template to cross−linker (B), mass of SiO2@TiO2 (C), extraction time (D), mass ratio of SiO2@TiO2@MIP to graphite (E), and pH (F) on the response current of 5.0 mMK3Fe(CN)6 on SiO2@TiO2@MIP/CPE
The effect of incubation time on the current signal was investigated by DPV measurement to evaluate the optimum time for achieving theophylline maximum adsorption on the electrochemical sensor. After the theophylline molecules were extracted from the polymer matrix, the sensor was immersed in 0.1 M theophylline solution at pH 6.5 for different periods of time, followed by the careful rinsing with deionized water to remove the adsorbed physical residues. And then the sensor was immersed into the 0.1 M PBS (pH 6.5) buffer solutioncontaining 5.0 mM K3Fe(CN)6 as a probe to study the current response. The results in Fig. S2 demonstrated that the oxidation current decreased dramatically up to 10 min, and then it was leveled off steadily. With the increase of incubation time, the occupying cavities in SiO2@TiO2@MIP occurred, and the oxidation current of K3Fe(CN)6 decreased as a consequence. However, the oxidation current almost remained unchanged when the incubation time was longer than 10 min, which indicated the high affinity and rapid recognition ability of the imprinted sensor to the target theophylline molecules. So, 10 min was selected as the optimum incubation time for all further measurements.
Fig. S2 Effect of incubation time on the response current of 5.0 mMK3Fe(CN)6 on SiO2@TiO2@MIP−theophylline/CPE
Table S1Detection and recovery of theophylline in tea, human blood serum and urine samples (n = 6)
Samples / Detection / Recovery testBy this method (M) / RSD () / By HPLC−UV method (M) / Relative error () / Spiked (M) / Found (M) / Recovery ()
Black tea / 6.32 / 2.3 / 6.21 / 1.8 / 10.0 / 10.4 / 104
Green tea / 5.74 / 3.1 / 5.90 / −2.7 / 5.00 / 4.64 / 92.8
Human blood plasma / / / / / / / / / 2.00 / 1.89 / 94.5
Human urine / / / / / / / / / 5.00 / 5.06 / 101
References
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