Electronic Supplementary Material
Electrochemically modified carbon fiber bundles as selective sorbent for online solid-phase microextraction of sulfonamides
XuLing1,2, Wenpeng Zhang1,2,Zilin Chen *1,2
1Key Laboratory of Combinatorial Biosynthesis and Drug Discovery (WuhanUniversity), Ministry of Education, and WuhanUniversitySchool of Pharmaceutical Science, Wuhan 430071, China
2State Key Laboratory of Transducer Technology, Chinese Academy of Sciences, Beijing10080, China
*Corresponding author, Phone: 86-27-68759893, Fax: 86-27-68759850, Email:
Optimization of polymerization conditions
The polymerization process of EDOT on carbon fiber may be strongly influenced by many factors, such as monomer and supporting analyte concentration, scan rates. To obtain higher extraction efficiency, several main affecting factors were investigated as discussed as follows.
Effect of monomer concentration. Sufficient monomer in the supporting electrolyte solution is the key of polymerization. Therefore, it is necessary to investigate the effect of monomer concentration on extraction performance. In general, the amount of monomer should be large enough to produce sufficient radical cation on surface of the electrode for polymerization, while rapid aggregation reaction should be taken into account and be prevented simultaneously, which may lead to heterogeneity of the obtained membrane. A suitable range of monomer concentration is between 0.5~5 mM. In our experiment we investigated five different concentrations gradient and triple respectively for each concentration. From the result shown in Fig. S3A, we can see that when the concentration is blew 5 mmol⋅L⁻¹, the extraction efficiency kept increasing gradually and a sharp increasing appeared when the concentration changed from 2.5 mmol⋅L⁻¹ to 5 mmol⋅L⁻¹. However, a doubled of concentration from 5 to 10 mmol⋅L⁻¹ did not bring doubled extraction efficiency, indicating that rapid aggregation reaction might occur among the monomers. As a result, we chose 5 mmol⋅L⁻¹ for our experiment.
Effect of scan rates. Scanning rates is an important kinetic parameter, which has a role in increasing the homogeneity of the coated membrane obviously, thus enhancing extraction efficiency and reducing extraction time. The effect of the scanning rate (0.025, 0.05, 0.075, 0.1, 0.2 V⋅s⁻¹) on the extraction efficiency of sulfamethoxazole was investigated. Results in Fig. S3B shows that the peak areas reached its maximum amounts at a scan rate of 0.025 V⋅s⁻¹. The results revealed that, the more slowly the scan rate, the better the extraction efficiency. However, it would take longer time to modify the fiber. Therefore, slower scan rates less than 0.025 V⋅s⁻¹ were not tested and it was chosen as an optimal option.
Effect of scan segments. The thickness of the electro-polymerized membrane may be influenced by scan segments in cyclic voltammetry. When it was varied between 30 and 150, the extraction efficiency varied sharply as Fig. S3C showed. It was interesting that the efficiency increased up to 90 scan segments and decreased afterwards. We can deduce that an increase of cyclic voltammetry segments up to 90 gave rise to growing rough surface of the film on the fiber, which generated a good porosity, and resulted in larger surface areas and availed the extractive potential of the membrane. The surface of the modified fiber by 120 scan segments may be smoother than that of 90 scans and therefore the effective surface for extraction decreased.
Optimization of parameters for online solid-phase microextraction-HPLC
The sorption of sulfonamides molecules onto the surface of modified fibers is mainly due to the hydrophobic interaction, hydrogen bonds and electrostatic force, which may be affected by pH of the solution to a great extent. Therefore, pH value of sample solution is an important parameter for SPME. The pH values among 4.0-9.0 were investigated, and the pH value was adjusted by 0.01 mol⋅L⁻¹ NaOH and HCl solution. Sample solutions of 10 mL were loaded onto the polymer sorbent at a constant flow rate (1 mL⋅min⁻¹). Total peak areas were calculated at each pH value and the results are shown in Fig. S5A. The extraction efficiency increased along with the increase of pH in the range of 4.0–6.0, a maximum was observed in pH 6, and higher pH value would result in lower extraction efficiency. This can be explained by the dissociation state of sulfonamides molecules. The molecules are sparingly soluble ampholyte because of the hydrophobic benzene ring, yet with hydrophilic functional group like amidogen and sulfonyl. Concerning its insolubility, partial acidic condition may be advantageous for the hydrophobic interaction. Dissociation of the molecule would increase its solubility and electrostatic force, but the hydrogen bonds may be destroyed in strong solution. In general, mild aqueous condition is advantageous for the formation of hydrogen bonds between sulfonamides and PEDOT. In our experiment we noticed that sulfonamides were better extracted in aqueous solution than in methanol. However, anions such as OH− are easy to bind with quaternary ammonium of the analytes to weaken the electrostatic force between sulfamethoxazole and PEDOT. The extraction efficiency is the result of equilibrium of these three kinds of interactions. With all factors taken into consideration, pH 6.0 is selected in the following studies.
The sample solution is loaded by pushing the pre-extraction solution through the packed PEEK tube, thus the sample flow rate has potential effect on the contact between sulfonamides and the adsorbent. In our experiment, the effect of sample flow rate was studied; the values were from 0.5 to 1.5 mL⋅min⁻¹ controlled by a syringe pump. As shown in Fig. S5B, there is a gradual peak areas fall-off in the examined sample flow rate, indicating that sample flow rate has big influence on extraction efficiency. Considering analysis time and pressure, which might increase along with the flow rate, 0.5 mL⋅min⁻¹ of flow rate was applied for further studies.
In-tube SPME based on packed sorbents is a nonequilibrium absorption process and the extraction efficiency of the PEDOT is closely related to sample volume. Volumes of pre-extraction solution in the range of 2.0–20 mL were loaded onto the PEEK tube separately, and the extraction efficiencies were investigated. As shown in Fig. S5C, the peak areas increase rapidly along with increase of the sample volume from 5 to 15 mL and increased slowly from 15 to 20 mL. Sample volume of 20 mL was selected for ultimate extraction.
Fig. S1. Cyclic voltammogram of modification.(monomer concentraction: 5 mmol⋅L⁻¹, scan rate: 0.025 V⋅s⁻¹, 90 scan segments).
Fig. S2. Mechanism of electrochemical polymerization of PEDOT.
Fig. S3. FTIR spectra of PEDOT membrane
Fig. S4. Optimization for polymerization: (A) monomer concentration, (B) scan rates, (C) scan segments. 20 ml sample solution (500 pg⋅mL⁻¹, pH 7) was loaded at 1 mL⋅min⁻¹ by syringe pump.
Fig. S5. Optimization for extraction: (A) Sample pH, (B) Sample flow rates, (C) Sample volume.
Table S1. RSD for determination of Sulfonamides in rat plasma samples
Compounds / Intra-day (RSD%, n=5) / Inter-day (RSD%, n=5)0.5 ng⋅mL⁻¹ / 1.5 ng⋅mL⁻¹ / 5 ng⋅mL⁻¹ / 0.5 ng⋅mL⁻¹ / 1.5 ng⋅mL⁻¹ / 5 ng⋅mL⁻¹
Sulfadiazine / 2.0 / 1.4 / 1.1 / 3.7 / 2.6 / 2.2
Sulfadimidine / 3.3 / 1.9 / 2.4 / 4.5 / 2.2 / 4.6
Sulfamethoxazole / 1.9 / 1.2 / 1.8 / 2.2 / 1.9 / 2.8
Table S2. Recoveries for determination of Sulfonamides in rat plasma sample
Compounds / Recovery (RSD%, n=3)0.5 ng⋅mL⁻¹ / 1.5 ng⋅mL⁻¹ / 5 ng⋅mL⁻¹
Sulfadiazine / 91.7±4.3 / 95.3±1.5 / 96.5±4.4
Sulfadimidine / 95.7±3.6 / 96.3±1.3 / 95.2±1.2
Sulfamethoxazole / 93.4 + 2.2 / 94.1 + 0.2 / 97.8 + 2.7