4-1-1 Adsorption
The adsorption capacity of the MIP for BPA (50 mg/L) at the different time intervals is shown in Figure 4-1. It can be seen that the rebinding of MIP toward BPA reached the saturation after 3 hr. The long equilibrium period is due to the slow diffusion of BPA molecules from surface into the cavities of the rigid structural framework of MIP. Wang et al.[65] constructed BPA-imprinted polyethersulfone (PES) particles with a diameter of about 2.3 mm. The saturated binding for the imprinted particles was observed after about 60 hr.
The particle size controls the adsorption and reactive kinetics.[66] Thus, reducing diffusion length is the key point when a MIP is used for sensing applications.
0 1 2 3 4 5
Figure 4-1 Equilibrium adsorptions of the MIP toward BPA (50 mg/L).
47
Figure 4-2 shows the adsorption isotherm of the MIP toward BPA. The MIP had two adsorption ranges: one was in 1-18 mg/L, and the other one was in 20-100 mg/L. This phenomenon indicates that the adsorption strength of the MIP was dependent on the concentrations of BPA. The diffusion of BPA molecules in the imprinted polymer is in response to a concentration gradient. Concentration gradients are often regarded as the driving force for diffusion and the sorption of species onto the imprinted polymer.[67] The binding strength is larger when the MIP worked at high BPA concentrations (> 20 mg/L) because of higher driving force resulting from large concentration gradient in the MIP. The adequate driving force not only assists the sorption of BPA molecules onto the surface of MIP but also promotes the pore diffusion from surface to the buck of the imprinted polymer. In contrast, when the MIP worked at low BPA concentrations (< 18 mg/L), BPA molecules were difficult to diffuse into the rigid framework due to the insufficient driving force. Thus, BPA was only adsorbed on the surface instead of bulk cavities inside the imprinted polymer. That is why a saturated rebinding and Langmuir adsorption was observed under the BPA concentrations lower than 18 mg/L.
48
Figure 4-2 Adsorption isotherm of the MIP toward BPA.
In order to evaluate the specific affinity of MIP toward to the BPA, adsorptions in analogues solutions were carried out. Phenol, 1-naphthol and BP were chosen as the analogues for the selective recognition capacity due to their log Kow and chemical structure similar to those of BPA. Figure 4-3 shows the adsorption capacity and the selectivity factor of the MIP for 50 mg/L of BPA and its analogues. The adsorption capacities of the MIP and NIP toward to BPA were 4.62 and 0.22 mg/g, respectively. Correspondingly, the imprinted
49
factor was 20.8. The high imprinting factor was due to that the high polarity of EtOH promotes BPA molecules, which has low dielectric constant, bound to PTMOS via π-π interaction, thus forming well imprinted cavities.[68] The adsorption capacities of MIP were 2.99 mg/g for phenol, 4.09 mg/g for 1-naphtnol and 6.44 mg/g for BP. The MIP exhibited the selectivity factor of 0.6 for phenol, 0.9 for 1-naphthol and 1.4 for BP. It is obvious that the MIP was hard to differentiate BP and 1-naphthol from BPA. Only phenol was repelled from the MIP. The results demonstrated that the imprinted cavities show poor selectivity for the BPA from structural similarity.
These results reveal that the imprinted cavities have low capability to recognize the analogue that either has a similar shape with or a smaller size than the target compound. In addition, the low selectivity for 1-naphthol, which has two phenyl groups, also indicates that the strong π-π stacking between the functional monomer in the cavities and the analogues reduces the recognition ability of the IPC. Meng et al.[69] reported that a poor selectivity was resulted in an imprinted polymer for estrogenic compounds due to the hydrophobic interaction.
Although the interaction between fictional groups causes unselective adsorption for the analogues irrespective to molecular structures, this feature could be an advantage for water treatment because different kinds of EDCs can be removed efficiently using the IPC.
50
Figure 4-3 (a) Adsorption capacity and (b) selectivity factor of the MIP and its corresponding NIP for 50 mg/L of BPA and its analogues.
51
4-1-2 Characterizations
Figure 4-4 shows the SEM images of the MIP and NIP. The MIP was in regularly spherical shape with an average particle size of 55.63 ± 0.01 nm. On the other hand, the NIP shows square shape, like cube sugar, with an average particle size of around 77.50 ± 0.02 nm.
The steric effect has been confirmed to influence the rate of gelation and powder agglomerate state in the sol-gel process.[68] The presence of template molecules caused steric effect in sol-gel process, consequently hindering polymerization to cause smaller particles.
Corresponding, the NIP has square shape that resulted largely from without the BPA molecules existence and less steric effect.
Figure 4-4 SEM images of the MIP and the NIP.
Table 4-1 lists the surface properties of the MIP and NIP. It shows that the specific surface areas of the MIP and NIP were 5.9 and 1.4 m2/g, respectively. The pore sizes of the MIP and the NIP were 80.0 and 232.0 nm, respectively. The comparatively high surface area of the MIP was due to the small particle sizes. In addition, the enhanced ratio in the surface area of the MIP to the NIP about 4.2 was not consistent to the imprinting effect. This result
52
clearly demonstrates that BPA molecules was not only adsorbed on the surface but indeed diffused into cavities in the bulk of MIP.
Table 4-1 The surface properties of the MIP and its corresponding NIP.
Samples Specific surface areas (m2/g) Pore volume (cm3/g) Pore size (nm)
MIP 5.9 0.16 80.0
NIP 1.4 0.17 232.0
Figure 4-5 shows the FTIR spectra of BPA, NIP, and MIP before and after rebinding.
The MIP and NIP showed the Zr-O and Si-O-Si stretching at 490 and 1133 cm-1, respectively.[70,71] In addition, O-H stretching coming from physisorbed water was observed in the 3200-3400 cm-1 region.[72] The benzene ring adsorption at 696 and 741 cm-1 and the phenyl group at 1430 cm-1 were found in the MIP.[73] These phenomena confirm that the functional monomer was successfully incorporated in the framework of the MIP. The formation of hydrophobic interaction between PTMOS and BPA during sol-gel reaction maintains a regular organization of the template molecules inside the cross-linked matrix.
Further slow evaporation of EtOH resulted in homogeneous monoliths consisting of templates held by the π-π stacking interaction. After removal of all the template molecules by methanol extraction, the well imprinted cavities left in the polymer matrix.
53
4000 3500 3000 2500 2000 1500 1000 500 0
1500 1400 1300 1200 1100 1000 900 800 700 600 1.8 400-4000 cm-1 and (b) the magnification of the IR spectra of the MIP at 600-1500 cm-1.
54
4-2 Photonic crystal
4-2-1 Opal structure
The PS nanoparticles were prepared by an emulsion polymerization in this study.
Figure 4-6 shows the photo-image of the as-made PS suspensions. The PS suspensions exhibited polychrome feature in any directions because of the formation of stabilized microspheres colloidal crystals in the aqueous solution in the presence of the stabilizer (SDS).
Figure 4-7 shows the particle distribution of the PS microspheres. The average particle size was 163.0 nm, which is in good agreement with the particle size (163.5 nm) obtained from SEM images.
Figure 4-6 Photo image of the home-made PS suspensions.
Self-assembly of colloidal microspheres is an inexpensive and simple approach to fabricate three-dimensional photonic crystal.[74] In this study, gravitational sedimentation was used to fabricate three-dimensional colloidal crystals. The latex solution was dropped on a glass slide and the PS microspheres were self-arranged into a crystalline structure by the attractive capillary forces generated during water evaporation. The growth process of the photonic crystal can be controlled in part by adjusting the evaporation temperature.[75] The
55
effect of the evaporation temperature on the crystalline quality of PS colloidal crystals was examined. Figure 4-8 displays the SEM images of the PS opal structures dried at different temperatures (40-70oC). Loose structure was obtained at 40oC and a long-range compact ordered array was formed at 50oC. However, it turned to a little disorder as the temperature was greater than 50oC. This finding indicates that adequate heat is required to induce ordered crystalline structure during self-assembling process. Similar result was reported by Ye et.al [76] who fabricated a high quality PS colloidal crystals at an evaporation temperature of 55oC.
Figure 4-7 Particle distribution of the PS microspheres.
56
Figure 4-8 SEM images of the PS opal structures dried at different temperatures. (a) 40oC, (b) and (c)50oC, (d) 60oC and (e) 70oC.
The crystal growth process consists of nucleus formation, transport of particles toward the ordered nucleus and crystallization.[77] Capillary force formed by water evaporation leads the PS microspheres moving to the lattice sites. When the thickness of water layer in the centre of the substrate becomes equal to the particle diameter, a nucleus of two-dimensional crystal suddenly forms. The particles in the thicker layer encircling the nucleus begin to move towards the ordered zone and upon reaching the boundary.[77] As a result, when the temperature was low (< 50oC), slow water evaporation resulted in small capillary force and caused poor colloidal crystalline structures. In contrast, rapid drying at
57
high temperature (> 50oC) led to a quick transport of PS nanoparticles and disordered structure because the time was insufficient for the PS nanoparticles to move and deposit at the proper lattice sites. Moreover, high drying tension caused cracks of the colloidal crystals.
Accordingly, it can be observed that the 52 layers piled up orderly and neatly along the perpendicular direction at drying temperature of 50oC (shown in Figure 4-8 (c)).
Figure 4-9 shows the UV-visible reflection spectra of the PS opal structures dried at various temperatures. The Bragg diffraction peak at 385 nm was observed in all the samples.
Moreover, the PS opal structure dried at 50oC showed the strongest reflective intensity, referring the highest crystalline quality of the sample than the other PS opal structures dried at lower or higher temperatures.
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4-2-2 Inverse opal structure
To optimize the inverse opal structure of ZrO2, the added volumes of the sol solution were adjusted. Figure 4-11 shows the SEM images of the inverse opal ZrO2 prepared with different volumes of the sol solution. Thick pore walls and irregular pore sizes were resulted when 10 L sol solution was added into the interstitial space of the PS colloidal crystals. This excess of the solution floated well-organized PS, consequently destroying the crystalline structure. The thicknesses of the pore walls decreased with decreasing addition amounts of the sol solutions. In addition, regular pore size of inverse opal structure was formed at 2.5 μL. The order porous structure can extend over 2.5 μm2 (shown in Figure 4-10 (e)).
Herein, 2.5 μL sol solution was set to prepare inverse opal ZrO2 photonic crystal.
59
Figure 4-10 SEM images of inverse opal ZrO2 prepared with different volumes of the sol solution (a) 10 μL, (b) 5 μL, (c) 3 μL, (d) 2.5 μL and (e) the low magnification of (d).
Figure 4-11 displays the TEM images of the inverse opal ZrO2 prepared with 25 μL sol solution. Highly ordered hexagonally packed pores were arranged in three dimensions, indicating the fcc lattice. The pore wall thickness of the inverse opal ZrO2 was about 33.3 nm. The average pore diameter of the ZrO2 inverse opal structure was about 100.0 nm, which was smaller than the particle size of the PS microspheres (163.0 nm). The decrease of pore size is due to the bulk shrinkage of ZrO2 inverse opal structure occurred after dissolving the PS templates. The shrinkage ratio was around 38.7%. As the ZrO2-PS composites soaking in toluene, PS templates continued to be removed. At same time, the film was
60
sharply contracted when the organic solvent was applied as extractant. In general, the evaporation of the extractant would result in contraction of the ZrO2-PS composites from surface tension. Surface tension at liquid-gas interfaces is responsible for the pressure decrease at the pore wall of the film during the evaporation.[78] Consequently, the shrinkage occurred after dissolving the PS templates with solvent extraction.
Figure 4-11 TEM image of the inverse opal ZrO2 prepared with 2.5 μL sol solution.
61
200 220 240 260 280 300 320
-10
200 250 300 350 400 450 500 550 600 0 figure was the UV-visible spectra of non-porous ZrO2.
Figure 4-12 shows the UV-visible reflection spectra of the porous and non-porous ZrO2. According the absorption edge (237 nm) of the non-porous ZrO2, its band gap was located about 5.2 eV. On the other hand, a distinguishable reflection peak at 257 nm was observed for the inverse opal ZrO2, revealing the crystalline feature and a photonic bandgap of 4.8 eV.
The photonic band position is determined by the d-spacing and the refractive index, and the corresponding wavelength can be calculated using Bragg’s law, 2d111(n2ef sin2)1/2.[51]
Sin θ is unity because the photonic band gap was determined from back-diffraction (reflectance) with a normal incidence to the (111) plane of the array. The refractive index of zirconia and air was 2.18 and 1.00, respectively.[79,80] Assume there are 6 hollow microspheres in a unit cell in the fcc close-packed structure, and the pore size and pore wall thickness of the inverse opal ZrO2 was d and w, respectively. Therefore, the volume fraction
62
of air (fair) isfair 5.66d3/(8d312d2w6dw2 w3); and the volume fraction of ZrO2 (fZr) is fZ fair
r 1 . Therefore, the volume fractions of ZrO2 and air are 31.0 % and 69.0 %, respectively. Accordingly, the theoretical phonic band gap was obtained about 261 nm after taking the above factors into the Bragg’s equation. This value is close to the experimental value (257 nm). Hence, the difference between the theoretical and experiment results could be due to the increase in the lattice spacing resulting from irregularity.
63
4-3 Optimization of imprinted photonic crystal (IPC)
Photonic band gap which can resist light pass through is the important feature of photonic crystal. Regularity within photonic crystal determines the formation of photonic band gap. Because the relative concentrations of the constituents in the sol solution would affect the microstructure of MIP and regularity of photonic crystal, the molar ratios among the cross-linker, solvent and functional monomer were optimized to obtain the highest detection ability of IPC.
4-3-1 Cross-linker
Figure 4-13 shows the UV-visible reflection spectra of the IPC prepared using various amounts of ZPO when the PTMOS/BPA/EtOH molar ratio was controlled at 1/1/100. The typical diffractive peak of a photonic band gap at 237 nm was observed when the ZPO content was 30-40 mM. However, the diffractive peak disappeared as the ZPO content was lower than 30 mM, indicating irregular porous structures. The lack of ordered porous structure was due primarily to that the concentration of the precursor was too low to construct a solid back-bone. Therefore, the molar ratio of cross-linker was controlled at 30 mM for the following preparation of sol solution.
64
Figure 4-13 UV-visible reflection spectra of the IPC prepared using various amounts of ZPO when the PTMOS/BPA/EtOH molar ratio was controlled at 1/1/100.
Figure 4-14 shows the UV-visible reflection spectra of the IPC before and after rebinding of BPA. The IPC was prepared using 30 and 40 mM ZPO when the PTMOS/BPA/EtOH molar ratio was controlled at 1/1/100. After rebinding of BPA, the reflection peak of the IPC blue shifted from 237.0 nm to 236.0 and 236.2 nm, respectively, when 30 and 40 mM of ZPO was used. The photonic band position is determined by using 2d111(n2ef sin2)1/2.[51]
Because the lattice spacing in the IPC was controlled, the blue-shift after rebinding of BPA was resulted from the decrease of the mean refractive index of the IPC. The imprinted IPC
65
∆λ = -1.0 nm ∆λ = -0.8 nm
involves a BPA/ZPO molar ratio of 1/30. The refractive index of BPA is 1.58. Thus, the mean refractive index of the solid backbone in the IPC theoretically decreased from 1.47 to 1.45 in case all the BPA molecules were imprinted into the cavities in the ZrO2. Therefore, the mean refractive index of the IPC decreased 0.02 and caused a blue-shift wavelength after rebinding of BPA. Correspondingly, the wavelength shift was about 3.3 nm. The smaller before and after rebinding of BPA.
4-3-2 Solvent
Figure 4-15 shows the UV-visible reflection spectra of the IPC, which were prepared using various amounts of EtOH, before and after binding of BPA. The amounts of EtOH were varied from 50 to 100 mM, and the ZPO/PTMOS/BPA molar ratio was set at 30/1/1.
The IPC all had clear reflection peak with different EtOH contents, indicating that the EtOH content in the sol solution has little influence on the formation of photonic crystal structure.
66
∆λ = -1.8 nm
∆λ = -1.6 nm
The wavelength shifts of the IPC for the EtOH content of 50, 60, 70, 80, 90 and 100 mM were 1.8, 1.6, 2.4, 3.2, 2.2 and 1.0 nm, respectively. When the amount of EtOH was 80 mM, the IPC had the largest wavelength shifts. The difference in wavelength shifts between the IPC were resulted from different adsorption amounts of BPA. Thus, the content of EtOH dominated different imprinting level in the IPC. Rapid solvent evaporation at high EtOH contents caused fast gelation, thus leading to poor imprinting process. On the other hand, relatively high precursor concentrations resulting from low contents of EtOH increased gelation rate. The quick solidification also provides insufficient time for well-organization between the template, functional monomer and cross-linker. The incomplete imprinted cavities prevent well recognition of BPA molecules due to dissimilarity of their shapes.
200 220 240 260 280
67 amounts of EtOH, before and after binding of BPA.
The amounts of EtOH control the regularity of the framework. Figure 4-16 shows the SEM images of the IPC prepared using different amounts of EtOH. When the EtOH molar ratio was ranged 50-60 mM, regular pore structures were not seen from the top-view images.
However, the interior structures still had a three-dimensional porous framework. Table 4-2 lists the wall thickness, pore size and shrinkage of the IPC. The wall thickness decreased from 50.0 to 34.8 nm when the EtOH molar ratios increased from 70 to 80 mM. The pore size decreased from 104.3 to 56.5 nm with decreasing the EtOH content from 80 to 50 mM;
relatively, the shrinkage was increased from 36.0% to 65.3% with EtOH decreased. The thicker wall and smaller pore diameter obtained from lower solvent contents was due to increased viscosity of the sol solution.[81] High sticky sol solutions hardly flow through the interstitial space between the PS colloidal, thus ether undergoing gelation before seeping or expanding the space between the colloidal templates.
On the other hand, a high level of irregular pore shapes appeared in the IPC when the amounts of EtOH were larger than 80 mM. When the amounts of EtOH were increased from
68
80 to 100 mM, the pore size decreased from 104.3 to 76.1 nm. The shrinkage was increased from 36.0% to 53.3% with EtOH increased. It is presumably due to that the original PS colloidal was disordered and the interstices were easier to be filled with sol solution at higher EtOH content. Consequently, a long-range ordered array was formed at 80 mM of EtOH content.
69
Figure 4-16 SEM images of the IPC prepared using different amounts of EtOH. (a) and (b) 50 mM, (c) 60 mM, (d) 70 mM, (e) 80 mM, (g) 90 mM and (g) 100 mM.
Table 4-2 Structural properties of the IPC.
EtOH volumes Wall thickness (nm) Pore diameter (nm) Shrinkage (%)
50 - 56.5 65.3
60 - 71.7 56.0
70 50.0 73.9 54.7
80 34.8 104.3 36.0
90 41.3 84.8 48.0
100 41.3 76.1 53.3
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4-3-3 Functional monomer
The role of functional monomers in the molecularly imprinting method is to create recognition sites by leaving interacting chemical functional groups in cavity for rebinding.
The adsorption ability of MIP is determined by the optimal amount of functional monomer relative to template used for the polymerization.[82] When this ratio of functional monomer is equal to the template, the imprinted polymer displayed the highest capacity than the polymers prepared with lesser or greater amounts of functional monomer.[83] To study the influence of the content of the functional monomer on the detection ability, the IPC were
The adsorption ability of MIP is determined by the optimal amount of functional monomer relative to template used for the polymerization.[82] When this ratio of functional monomer is equal to the template, the imprinted polymer displayed the highest capacity than the polymers prepared with lesser or greater amounts of functional monomer.[83] To study the influence of the content of the functional monomer on the detection ability, the IPC were