Hierarchically Structured Nanoporous Poly(Ionic Liquid) Membranes: Facile Preparation and Application in Fiber-optic ph Sensing

Transcription

1 Supporting Information Hierarchically Structured Nanoporous Poly(Ionic Liquid) Membranes: Facile Preparation and Application in Fiber-optic ph Sensing Qiang Zhao, a Mingjie Yin, b A. Ping Zhang, b Simon Prescher, a Markus Antonietti, a Jiayin Yuan* a a Max-Planck-Institute of Colloids and Interfaces, Department of Colloid Chemistry, D Potsdam, Germany b Photonics Research Centre, Department of Electrical Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong SAR, China. 1. Experimental 1.1 Chemicals Lithium bis(trifluoro methanesulfonyl)imide (LiTf 2 N, 99.95%), sodium tetraborate (NaBF 4, 98%), and potassium hexafluorophosphate (KPF 6, 97%) were purchased from Sigma-Aldrich and used without further purification. Poly(acrylic acid) (PAA, solid powder, MW=2000) was purchased from Sigma-Aldrich. Poly(1-cyanomethyl-3-vinylimidazolium bromide) (PCMVImBr) was synthesized according to our previous method (Chem. Mater. 2010, 22, ) Its proton nuclear magnetic resonance ( 1 H-NMR) spectrum was shown in Figure S1. d e a b H 2 O DMSO c c a b d e ppm Figure S1 1 H-NMR spectrum of PCMVImBr. S1

2 The apparent molecular weight and PDI of PCMVImBr was g/mol and 2.95 (measured by GPC, eluent: water with 0.2 M Na 2 SO 4 + 1% acetic acid at room temperature). Poly(3-cyanomethyl-1-vinylimidazolium bis(trifluoro methanesulfonyl)imide) (PCMVImTf 2 N) was prepared by anion exchange of PCMVImBr with LiTf 2 N salts. Hence, the apparent molecular weight of the PCMVImTf 2 N is calculated to be g/mol. All the solvents in this study were of analytic grade. 1.2 Preparation of porous PCMVImTf 2 N-PAA complex membrane (PPM) 1.0 g of PCMVImTf 2 N PIL and 0.18 g of poly(acrylic acid) (MW: 2000 g/mol) were dissolved in 10 ml of dimethylformamide (DMF) solvent to form an homogeneous solution, which was then cast onto a clean glass plate. The liquid film on the glass place was dried at 80 o C for 1h, and soaked in 0.2 wt% aqueous ammonia (ph=10.8, 20 o C, 2 h). After the soaking step, a yellowish and flexible freestanding membrane can be easily detached from the glass substrate. For the water permeation test, a freshly prepared membrane was supported onto a regenerated cellulose porous membrane filter (Whatman, average pore: 0.2 µm) and fixed into a home-made, dead end filtration apparatus. The external pressure 3 bar was generated by compressed nitrogen. 1.3 preparation of fiber optic ph sensor Structure of the thin-core fiber interferometer (TCFMI) sensor was shown in Scheme S1. The TCFMI sensor was cleaned with a piranha solution (8 : 2 v/v mixture of concentrated H 2 SO 4 (98 %) and H 2 O 2 (50%)) for 20 min followed by thorough rinsing with deionized water and drying with nitrogen. (Caution! Piranha solution is a strong oxidizing agent. Handle with extreme care!). The fiber sensor was soaked in PCMVImTf 2 N+PAA blend solution for 1.5 h to coat a polymer layer onto the fiber surface. Afterwards, the coated fiber sensor was dried at 80 o C for 1 h, and was soaked in 0.2 wt% aqueous ammonia for 2 h. Then it was immersed into DI water for 10 min. Sensing mechanism: Due to the mode mismatch at the splicing point between the standard sing-mode fiber (SMF) and thin-core fiber (TCF), optical fiber cladding modes will be excited and interact with PPM coating through evanescent wave. The light of cladding modes coupled back to the SMF at the second splicing point will interfere with the core mode and form an S2

3 interference fringe in the transmission spectrum. This spectrum can be used for precisely analyzing the change of refractive index of PPM coating for biochemical sensing applications. Scheme S1 A schematic thin-core fiber interferometer (TCFMI) fiber optics coated with PPM. 1.4 Characterization methods FT-IR spectra were performed on a BioRad 6000 FT-IR spectrometer; samples were measured in solid state using a Single Reflection Diamond ATR. Scanning electron microscopy (SEM) was performed on a GEMINI LEO 1550 microscope at 3 kv. Samples were coated with gold before examination. Confocal laser scanning microscopy (Leica TCS SP, Germany) equipped with a 100X/ oil immersion objective was used to record the cross sectional images of the film in inverted microscope mode. 2. Supplementary data PAA Complex membrane COOH COO Wavenumbers (cm -1 ) Figure S2 FT-IR spectra of PCMVImTf 2 N-PAA complex membrane (red) and pristine PAA polymer (black). The absorption band at 1700 cm -1 and 1550 cm -1 are ascribed to C=O stretching in COOH and COO - NH + 4 groups, respectively. S3

4 Figure S3 SEM image on the surface morphologies of PCMVImTf 2 N-PAA complex membrane. (a) top surface (in zone I), (b) bottom surface (in zone II). The pores in zone II are in the range of nm (Figure 1d). The pores on the bottom surface (here in b) were deformed into submicron sizes owing to its direct contact with the glass plate substrate. Figure S4 SEM morphologies of PCMVImTf 2 N-PAA blend film after being dried at 80 o C for 1h (a,b), followed by soaking in neutral water without ammonia for 2 h (c,d). Scale bars are 500 nm. It can be seen that no pores were created before/without the ammonia activation. S4

5 100 Residual DMF (wt%) Drying time (min) Figure S5 Effect of the drying time at 80 o C on the residual DMF solvent in the thin film of the PCMVImTf 2 N-PAA polymer blend film system. Figure S6 Cross-sectional view of a PCMVImTf 2 N-PAA PPM. The sulfur content (measured by EDX) in Zone I and Zone II were 0.95 mol% and 5.1 mol%, respectively. Note: sulfur only exists in Tf 2 N - counter anions. Higher sulfur content stands for lower ionic comlexation degree because ionic complexation releases Tf 2 N counter anions. Thus, zone I has a higher complexation degree than zone II. S5

6 Figure S7 SEM cross-section view of (a,b) the PCMVImBF 4 -PAA complex membrane and (c,d) the PCMVImPF 6 -PAA complex membranes prepared by the processing method shown in Figure 1a in the main text. Note that no membrane was formed from the PCMVImBr-PAA system because both polymer components are water soluble. Figure S8 SEM cross-section view of porous polyelectrolyte membranes made from another two water insoluble cationic polyelectrolytes. Membranes were prepared by the same method shown in Figure 1a. Note that no porous membranes can be obtained if their water-soluble structure forms with Cl - anion were used. It indicates the hydrophobicity of the PIL component is necessarily needed for the pore formation. S6

7 Figure S9 (a) transmission spectrum of a optic fiber before (red line) and after (blue) being coated with a PCMVImTf 2 N-PAA PPM, (b) cross-sectional and (c) top surface morphologies of a PPM coated on the fiber. Note: the PPM coated on the fiber was cracked by liquid nitrogen to examine the cross-sectional morphology. It is seen that the cross-sectional pores have smaller sizes compared to that of flat PPM. Both the surface pore and corssectional pore facilitated the mass transport to reach a fast sensing response Loss (db) a Wavelength (nm) Loss (db) Wavelength (nm) b Figure S10. Original spectra of PCMVImTf 2 N-PAA membrane-coated TCFMI ph sensor recorded at (a) ph: 2.24~6.68, (b) ph: 7.15~10. S7

8 Wavelength (nm) ph=6.68 t r =45 s ph=3.96 t f =120 s Time(s) Figure S11. Reference experiment: Dynamic responses of the PCMVImTf 2 N-PAA blend TCFMI ph sensor in solutions of ph 6.68 and 3.96 alternatively. Note: the sensor is made by coating TCFMI fiber optics with the PCMVImTf 2 N-PAA mixture solution in DMF, dried at 80 o C for 1 h, followed by soaking it in neutral water instead of 0.2 wt% aqueous NH 3. This coating is nonporous (Figure S4 c,d). We can see that the ph sensor prepared from this nonporous coating shows much slower responsive rate: t r = 45 s, t f = 120 s. Table S1 Sensor performance (sensitivity and response time) of the present system and other ph fiber-optics sensors. It is seen that PPM sensor in this study is superior in terms of both the sensitivity and responsive rate. Polymer coating Sensitivity a Responsive time Ref (PAH/PAA) nm/ph, 0.45 nm/ph T r =120s; T f =200 s (PDDA/PAA) nm/ph, T r =240 s; T f =160 s (PEC/PDDA) nm/ph, 0.85 nm/ph T r =30 s; T f =50 s (PDDA/PAA) nm/ph, T r =20 s; T f =15 s PCMVImTf 2 N-PAA 2.04 nm/ph, 2.48 nm/ph T r = 5 s; T f =12 s This work Note: sensitivity in acid and base region, respectively 1. B. Gu, M.-J. Yin, A. P. Zhang, J.-W. Qian, S. He, Opt. Express 2009, 17, Z. Gui, J. Qian, M. Yin, Q. An, B. Gu, A. Zhang, J. Mater. Chem. 2010, 20, M. Yin, B. Gu, Q. Zhao, J. Qian, A. Zhang, Q. An, S. He, Anal. Bioanal. Chem. 2011, 399, S8