Supporting Information to Highly Ordered Binary Assembly of Silica Mesochannels and Surfactant Micelles for Extraction and Electrochemical Analysis of Trace Nitroaromatic Explosives and Pesticides Fei Yan, Yayun He, Longhua Ding and Bin Su* Institute of Microanalytical Systems, Department of Chemistry, Zhejiang University, Hangzhou, 310058, China * Corresponding author. E-mail: subin@zju.edu.cn Table of Content S1. Molecular structure of nitroaromatic compounds S2. Characterizations of the BASMM/ITO electrode S2.1 TEM and SEM measurements S2.2. FTIR measurements S2.3. Contact angle measurements S2.4. Mass spectrometry measurements S3. CVs of TNT at the BASMM/ITO electrode S4. Optimized conditions for nitroaromatic explosives detection S4.1. Electrolyte solution S4.2. Preconcentration method S5. DPV results of other nitroaromatic explosives S6. Optimized conditions for nitroaromatic OPs detection S6.1. Electrolyte solution S6.2. Preconcentration method S7. DPV results of other nitroaromatic OPs S-1
S1. Molecular structure of nitroaromatic compounds Scheme S1. Molecular structures of studied nitroaromatic explosives Scheme S2. Molecular structures of studied nitroaromatic OPs S-2
S2. Characterizations of the BASMM/ITO electrode S2.1. TEM and SEM measurements Scanning electron microscopy (SEM) image was obtained on a SU-70 field-emission scanning electron microscope (Hitachi, Japan) at an accelerating voltage of 3.0 kv. Transmission electron microscopy (TEM) measurement was performed on a HT7700 microscope (Hitachi, Japan) operated at 120 kv. The TEM specimens were prepared by mechanically scraping mesoporous silica films from the ITO surface, dispersing them in ethanol before dropping onto the copper grids. The results were shown in Figure S1. Figure S1. (a) TEM image of BASMM film showing the mesopores as the bright spots. (b) Cross-sectional SEM image illustrating the BASMM layer on the ITO electrode surface with a thickness of ca. 97.5 nm. S-3
S2.2. FTIR measurements Fourier transform infrared (FTIR) measurements were recorded by using Thermo Fisher Scientific (Nicolet Nexus 470) spectrometer operated in the ATR mode. The formation of BASMM was confirmed by the CTAB absorption signature. CTAB shows characteristic peaks at 2925 and 2840 cm 1, which are attributed to the various C-H stretching vibrations in the FTIR spectra. 1 It can be seen from Figure S2 that these peaks are at 2920 and 2850 cm 1 for both pure CTAB and BASMM modified ITO electrode, which were not detected for SMs modified ITO electrode because CTAB micelles were excluded from the mesochannels. Meanwhile, the characteristic Si-O-Si peak of silica at 1090 cm 1 also appeared in the BASMM spectra. Above data proved the existence of hybrid structure of CTAB micelles and mesoporous silica. Figure S2. FTIR spectra of a bare ITO electrode (a), pure CTAB molecules (b), a BASMM/ITO electrode (c) and a SM/ITO electrode (d). S-4
S2.3. Mass spectrometry measurements Mass spectrometry (MS) measurements were performed on the matrix assisted laser desorption ionization time of flight (MALDI-TOF) mass spectrometer (Applied ultraflextreme, Bruker Daltonics). The instrument was equipped with a 337 nm nitrogen laser under the linear positive-ion mode and 2,5-dihydroxybenzoic acid (DHB) was used as the matrix. Prior to the MS measurement, the BASMM/ITO electrode was spotted with 1 μl DHB, followed by drying for 10 min. Figure S3 displays the mass spectra of a freshly prepared BASMM/ITO electrode. The peaks at m/z = 285.143 and 286.122 can be assigned to CTAB. 2 Figure S3. Mass spectrum obtained with the freshly prepared BASMM/ITO electrode. S-5
S2.4. Contact angle measurements Static contact angles were measured on a JC2000D2 contact angle system. As shown in Figure S4, the contact angles of the bare ITO, the BASMM/ITO and the SM/ITO electrodes were 42, 61 and 40, respectively. Obviously, the surface of the BASMM/ITO electrode exhibits a higher contact angle and better hydrophobicity. Although the polar head of CTAB, namely the ammonium group, is located on the exterior surface of the micelles, the partial exposure of the hydrocarbon chain of CTAB molecules at the micelle/water interface (as shown in Scheme 1) can increase the surface hydrophobicity. It can be also expected that the ordered aggregation of the hydrocarbon chain of CTAB molecules inside micellar structures could form more hydrophobic cores. Figure S4. Contact angle images of the bare ITO (a), the BASMM/ITO (b) and the SM/ITO electrode surfaces (c). S-6
S3. CVs of TNT at the BASMM/ITO electrode In order to examine the mass transport of TNT molecules inside the CTAB micelles, CVs of the BASMM/ITO electrode at different scan rates were recorded (see Figure S5a). As plotted in Figure S5b, both the first and second cathodic peak currents linearly increase with the square root of scan rate, suggesting a diffusion-controlled electrochemical process for the reduction of TNT at the ITO electrode. It also indicates that the extracted TNT molecules are well solubilized in the hydrophobic CTAB micelles and can diffuse freely to the underlying ITO electrode surface. Figure S5. (a) CVs of the BASMM/ITO electrode in 0.5 M NaCl containing 1 ppm TNT at different scan rates (from inner to outer: 10, 50, 100, 150, 200 mv s 1); (b) The dependence of the first (black) and second (red) reduction peak currents on the square root of the scan rate. S-7
S4. Optimized conditions for nitroaromatic explosives detection The process for nitroaromatic explosives detection was determined by optimizing the buffer solution and the preconcentration method, taking TNT as the example compound. Its second reduction peak current intensity occurred at 0.68 V in the DPV was optimized for the quantitative trace analysis. S4.1. Electrolyte solution Figure S6 shows the calibration curves of the BASMM/ITO electrode corresponding to current response for TNT in different aqueous supporting electrolytes. It can be seen that electrolyte does not significantly affect the current magnitude. But NaCl is slightly better than PBS and HAc/NaAc in terms of lowest detected concentration and sensitivity under the same preconcentration method (Table S1). Therefore, the NaCl was chosen as supporting electrolyte for the following experiments. Figure S6. Calibration curves for TNT detection at the BASMM/ITO electrode by DPV in different aqueous supporting electrolytes: 0.5 M NaCl, 0.5 M PBS (ph 6.0) and 0.5 M HAc/NaAc (ph 6.0). Before measurement, the preconcentration of TNT was performed by immersing the electrode in solutions under stirring for 3min. S-8
Table S1. Effect of the buffer solution on the detection of TNT Buffer (0.5 M) Lowest Detected Concentration (ppb) Sensitivity (μa/ppm) R NaCl 10 7.37 0.9968 PBS (ph=6) 20 6.60 0.9993 NaAc/HAc (ph=6) 10 5.98 0.9995 S4.2. Preconcentration method The preconcentration methods, including mechanical stirring and potentiostatic biasing, have been compared. For each method, the preconcentration time or the preconcentration potential was optimized. (1) Mechanical Stirring Figure S7 shows the effect of the preconcentration time on the current magnitude of 500 ppb TNT in NaCl solution under mechanical stirring. The current maximum was reached at 10 s, indicating a very rapid extraction/ preconcentration of TNT by the surfactant micelles. Figure S7. Influence of the stirring time on the current response of 500 ppb TNT in 0.5 M NaCl solution. S-9
Figure S8. Effect of the preconcentration potential (a) and time (b) on the current response of 500 ppb TNT in 0.5 M NaCl solution. In (a): preconcentration time 120 s; in (b): preconcentration potential 0.4 V. (2) Potentiostatic preconcentration Figure S8 shows the effects of the preconcentration potential and time on the current response of 500 ppb TNT in 0.5 M NaCl solution by using potentiostatic method. The preconcentration potential was investigated over a range of 0.6 V to 0.2 V (Figure S8a). The current response reached the maximum at the potential of 0.4 V and declined with increasing the potential from 0.4 V to 0.2 V. Hence, 0.4 V was selected in this study. As shown in Figure S8b, the current increased obviously within 90 s and nearly did not vary at longer preconcentration times, if applying 0.4 V as the preconcentration potential. Therefore, 90 s was chosen as the optimal preconcentration time. (3) Comparison of two preconcentration methods Figure S9 compares the calibration curves for various concentration of TNT at the BASMM/ITO electrode obtained with two preconcentration methods. Apparently, the lowest detected concentration for both two methods can be as low as 10 ppb and the linear correlation coefficient is comparable. However, S-10
considering the sensitivity for TNT detection (Table S2), the potentiostatic method was chosen eventually for further investigated. Figure S9. Calibration curves for TNT detection in 0.5 M NaCl solution after different preconcentration methods at the BASMM/ITO electrode by DPV. Table S2. Effect of the preconcentration method on the detection of TNT Method Lowest Detected Concentration (ppb) Sensitivity (μa/ppm) R Stirring 10 8.14 0.9994 Potentiostatic 10 11.85 0.9982 S-11
S5. DPV detection of other nitroaromatic explosives As shown in Figures S10-13, the nitroaromatic explosives have various characteristic current peaks, which are associated with the number of nitro group and the substitution group. When two or more reduction peak has been observed, the signal of the most prominent reduction peak is selected for the quantitative analysis. Figure S10. (a) DPV responses of the BASMM/ITO electrode in 0.5 M NaCl solution containing various concentrations of DNT. (b) The calibration curve. Error bars represent the standard deviations of three measurements. Figure S11. (a) DPV responses of the BASMM/ITO electrode in 0.5 M NaCl solution containing various concentrations of TNP. (b) The calibration curve. Error bars represent the standard deviations of three measurements. S-12
Figure S12. (a) DPV responses of the BASMM/ITO electrode in 0.5 M NaCl solution containing various concentrations of NP. (b) The calibration curve. Error bars represent the standard deviations of three measurements. Figure S13. (a) DPV responses of the BASMM/ITO electrode in 0.5 M NaCl solution containing various concentrations of NB. (b) The calibration curve. Error bars represent the standard deviations of three measurements. S-13
Figure S14. DPVs of 1 ppm nitroaromatic explosives (except TNP at 0.08 ppm) and the mixture of five nitroaromatic explosives at the BASMM/ITO electrode in 0.5 M NaCl. Table S3. Peak potentials obtained from DPV shown in Figure S14. Nitroaromatic Explosives Ep / V Mixture 0.44 0.66 0.81 TNT 0.45 0.68 0.82 DNT 0.59 0.8 TNP 0.63 0.76 NP 0.72 NB 0.76 S-14
S6. Optimized conditions for nitroaromatic OPs detection To investigate the effect of buffer and preconcentration method on the OPs detection, paraoxon was used as the model. Its reduction peak current intensity in the DPV was optimized for the quantitative trace analysis. S6.1. Electrolyte solution As illustrated in Figure S15, the effect of various supporting electrolyte on DPV response of paraoxon was investigated. It was found that NaCl was better than PBS and Na 2 SO 4 regarding to lowest detected concentration and sensitivity under the same preconcentration method (Table S4). As a result, the NaCl was chosen as supporting electrolyte for following experiments. Figure S15. Calibration curves for paraoxon detection in different aqueous supporting electrolyte at the BASMM/ITO electrode by DPV. Before measurement, the electrode was preconcentrated by stirring for 3 min. Table S4. Effect of supporting electrolyte on the detection of paraoxon Buffer (0.1 M) Lowest Detected Concentration (ppb) Sensitivity (μa/ppm) R NaCl 5 22.89 0.9991 PBS (ph = 6) 10 21.603 0.9997 Na 2 SO 4 5 14.58 0.9977 S-15
S6.2. Effect of preconcentration method (1) Preconcentration time in the mechanical stirring method Figure S16 shows the influence of the preconcentration time on the current response of 300 ppb TNT in 0.1 M NaCl solution under the mechanical stirring. The current maximum was reached at 10 s, indicating a very rapid extraction/ preconcentration of paraoxon by the surfactant micelles. Figure S16. Influence of the stirring time on the current response of 300 ppb paraoxon in 0.1 M NaCl solution. (2) Potentiostatic preconcentration Figure S17 shows the effects of preconcentration potential and time on the current response of 300 ppb paraoxon in 0.1 M NaCl solution by using potentiostatic method. It was found that the preconcentration potential of 0 V is appropriate. And using this potential as the preconcentration potential, the current plateau was obtained after 30 s. Hence, applying the pre-potential of 0 V for 30 s to the BASMM/ITO electrode represents the optimized option in this preconcentration method. S-16
Figure S17. Effect of the preconcentration potential (a) and time (b) on the current response of 300 ppb paraoxon in 0.1 M NaCl solution. In (a): the preconcentration time 120 s; in (b): the preconcentration potential 0 V. (3) Comparison of two preconcentration methods Figure S18 compares the calibration curves for paraoxon detection obtained with two preconcentration methods. And the results are collected in the Table S5. Considering a lower detected concentration, a higher sensitivity and a better correlation coefficient were achieved by the potentiostatic preconcentration, it was thus used eventually. S-17
Figure S18. Calibration curves for paraoxon detection in 0.1 M NaCl solution after different preconcentration methods at the BASMM/ITO electrode by DPV. Table S5. Effect of preconcentration method on the detection of paraoxon. Method Lowest detected concentration (ppb) Sensitivity (μa/ppm) R Stirring 10 4.428 0.9970 Potentiostatic 5 14.361 0.9974 S-18
S7. DPV responses of other nitroaromatic OPs Figure S19. (a) DPV responses of the BASMM/ITO electrode in 0.1 M NaCl solution containing various concentrations of methyl parathion. (b) The calibration curve for methyl parathion. Error bars represent the standard deviations of three measurements. Error bars represent the standard deviations of three measurements. Figure S20. (a) DPV responses of the BASMM/ITO electrode in 0.1 M NaCl solution containing various concentrations of fenitrothion. (b) The calibration curve for fenitrothion. Error bars represent the standard deviations of three measurements. S-19
Figure S21. DPVs of 0.2 ppm nitroaromatic OPs (except fenitrothion at 7 ppm) at BASMM/ITO electrode. Table S6. Peak potentials obtained from DPV shown in Figure S21 Nitroaromatic OPs Ep / V paraoxon 0.67 methyl parathion 0.69 fenitrothion 0.75 References 1. Samanta, S., Giri, S., Sastry, P., Mal, N., Manna, A., Bhaumik, A., Ind. Eng. Chem. Res. 2003, 42, 3012-3018. 2. Guo, Z., Zhang, Q., Zou, H., Guo, B., Ni, J., Anal. Chem. 2002, 74, 1637-1641. S-20