Electronic Supplementary Material
Electrochemically modified carbon fiber bundles as selective sorbent for online
solid-phase microextraction of sulfonamides
Xu Ling1,2, Wenpeng Zhang1,2, Zilin Chen *1,2
1
Key Laboratory of Combinatorial Biosynthesis and Drug Discovery (Wuhan University),
Ministry of Education, and Wuhan University School of Pharmaceutical Science, Wuhan
430071, China
2
State Key Laboratory of Transducer Technology, Chinese Academy of Sciences, Beijing
10080, China
*Corresponding
author,
Phone:
86-27-68759893,
Fax:
86-27-68759850,
Email:
chenzl@whu.edu.cn
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 1.5 ng⋅mL 5 ng⋅mL
0.5 ng⋅mL 1.5 ng⋅mL 5 ng⋅mL
⁻¹
⁻¹
⁻¹
⁻¹
⁻¹
⁻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
1.9
1.2
1.8
2.2
1.9
2.8
Sulfamethoxaz
ole
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