Colloidal crystal film

The method for selective chemical sensing using colloidal crystal films is depicted in Fig. 4.1. The steps for the fabrication of the colloidal crystal films and determination of selective chemical sensing response are summarized in Table 4.2. 

Fig. 4.1 Diagram of the method for selective vapor detection that includes fabrication of core (1) and core shell (2) materials, their assembly into a colloidal crystal film (3), exposure of the film to different vapors (4), measurements of the spectral response of the film (5), and multivariate analysis of the spectra (6) to obtain a vapor selective response of the colloidal crystal film...

Assemble colloidal crystal film with core/shell X/Y submicron spheres responsive to both... 

Expose colloidal crystal film to both classes of vapors... 

Measure differential reflectivity spectral response of the colloidal crystal film... 

As an example of composite core/shell submicron particles, we made colloidal spheres with a polystyrene core and a silica shell. The polar vapors preferentially affect the silica shell of the composite nanospheres by sorbing into the mesoscale pores of the shell surface. This vapor sorption follows two mechanisms physical adsorption and capillary condensation of condensable vapors17. Similar vapor adsorption mechanisms have been observed in porous silicon20 and colloidal crystal films fabricated from silica submicron particles32, however, with lack of selectivity in vapor response. The nonpolar vapors preferentially affect the properties of the polystyrene core. Sorption of vapors of good solvents for a glassy polymer leads to the increase in polymer free volume and polymer plasticization32. 

Differentiation of vapor responses of the colloidal crystal film was accomplished with spectral measurements of the shape changes of the diffraction peak. Selectivity of response was obtained by applying multivariate data analysis to correlate these spectral changes to the effects of species of different chemical nature and to establish the identity and concentration of species. 

To overcome the limitation of detecting only a color change in the sensing colloidal crystal films, we apply a differential spectroscopy measurement approach coupled with the multivariate analysis of differential reflectance spectra. In differential spectroscopy, the differential spectrum accentuates the subtle differences between two spectra. Thus, in optical sensing, when the spectral shifts are relatively small, it is well accepted to perform measurements of the differential spectral response of sensing films before and after analyte exposure6 19. Therefore, the common features in two spectra of a sensing film before and after analyte exposure cancel and the differential spectrum accentuates the subtle differences due to analyte response. 

To evaluate the vapor responses of the colloidal crystal film, we measured differential reflectance spectra AR defined as ... 

Fabrication of Core-Shell Colloidal Crystal Films for Selective Chemical Sensing... 

Core-shell colloidal crystal films were prepared in three steps as outlined in Table 4.2. First, spherical submicron polystyrene particles were prepared by known methods38 39. The size of isolated polystyrene beads was 326 5 nm as determined by analysis of scanning electron microscopy (SEM) images using standard techniques. 

Fig. 4.2 TEM images of fabricated nanoparticles, (a) Isolated composite core/shell submicron particles, (b) Hollow silica submicron particles prepared by removing the polystyrene core to demonstrate the high quality of the formed sol gel shell of the composite nanospheres employed to prepare sensing colloidal crystal films...

Fig. 4.4 SEM image of a cross section of the beginning of the assembled colloidal crystal film to illustrate the growth of multiple layers. Reprinted from Ref. 15 with permission. 2008 Institute of Electrical and Electronics Engineers...

For the evaluation of the response of the sensor, we selected several vapors of different polarity. The vapors included water (H20), acetonitrile (ACN), toluene, and dichloromethane (DCM). Solvent polarity and refractive index of tested vapors are listed in Table 4.346 47. The spectral range for the evaluation of the vapor responses of the colloidal crystal film was selected as 700 995 nm, which covered only the fundamental Bragg diffraction peak on the (111) planes of the colloidal crystal film to further reduce effects from possible stacking defects in the film as suggested in the literature44. 

Changes in the differential reflectance spectra AR of the sensing film upon exposure to different vapors at various concentrations are presented in Fig. 4.9. These spectra illustrate several important findings. For polar vapors such as water and ACN (see Fig. 4.9a, b respectively), the differential reflectance spectra have a stable baseline and consistent well-behaved changes in the reflectivity as a function of analyte concentration. The response of the colloidal crystal film to nonpolar vapors such as DCM and toluene (see Fig. 4.9c, d respectively) is quite different compared with the response to polar vapors. There are pronounced analyte concentration-dependent baseline offsets that are likely due to... 

The reversibility of interactions of the composite colloidal crystal film with different vapors was also evaluated. Figure 4.10 illustrates the dynamic response of the sensing film at several wavelengths (770, 835, and 870 nm) upon triplicate exposures to water vapor at four concentrations (0.02, 0.04, 0.07, and 0.1 PIPo). The response and recovery kinetics upon exposure to water vapor were fully reversible and rapid (under 5 s). A comparison of dynamic response of the sensing film at five different wavelengths upon exposures to water and toluene vapors at four concentrations is presented in Fig. 4.11. These data illustrate that the direction, magnitude, and kinetics of the responses to these vapors were quite different. 

P/P0), the colloidal crystal film exhibited an additional increase in the response sensitivity likely due to the swelling of the core of the core/shell submicron particles. This effect was more pronounced upon exposure to toluene. 

We have shown a new concept for selective chemical sensing based on composite core/shell polymer/silica colloidal crystal films. The vapor response selectivity is provided via the multivariate spectral analysis of the fundamental diffraction peak from the colloidal crystal film. Of course, as with any other analytical device, care should be taken not to irreversibly poison this sensor. For example, a prolonged exposure to high concentrations of nonpolar vapors will likely to irreversibly destroy the composite colloidal crystal film. Nevertheless, sensor materials based on the colloidal crystal films promise to have an improved long-term stability over the sensor materials based on organic colorimetric reagents incorporated into polymer films due to the elimination of photobleaching effects. In the experiments... 

Potyrailo, R. A. Ding, Z. Butts, M. D. Genovese, S. E. Deng, T., Selective chemical sensing using structurally colored core shell colloidal crystal films, IEEE Sensors J. 2008, 8,815 822... 

Yamada, Y. Nakamura, T. Ishi, M. Yano, K., Reversible control of light reflection of a colloidal crystal film fabricated from monodisperse mesoporous silica spheres, Langmuir. 2006, 22, 2444 2446... 

O.D. (2006) Controlled assembly of SERS substrates templated by colloidal crystal films. Journal of Materials Chemistry, 16, 1207-1211. 

Y., Sato, O., and Fujishima, A. (2002) Metal-coated colloidal crystal film as surface-enhanced Raman scattering substrate. Langrnuir, 18, 5043-5046. 

The application of widespread standard polymer processing techniques to the formation of colloidal crystals was introduced by Ruhl et al. [60]. They demonstrated the fabrication of a large colloidal crystal film, which comprises core-shell latex particles. Upon coagulation the soft shell of the particles causes the formation of a rubbery mass, which can be uniaxially compressed. The radial horizontal flow during the compression induces crystallization of the particles from the surface of the plates inward. The soft shell constitutes the matrix in which the hard spheres are embedded. This technique is promising for efficient application to other polymer processing methods like extrusion or injection molding. 

FIGURE 8.1 SEM images of the cross sections of the silica colloidal crystal films produced using (a) and (c) 635 nm, (b) 850 nm, and (d) 1.0 xm spheres. Reproduced with permission from Reference 15. Copyright 2003 American Chemical Society. 

Q. Yan, Z. Zhou, and X S. Zhao, Inward-growing self-assembly of colloidal crystal films on horizontal substrates, Langmuir ACS J. Surf. Colloids, 21,... 

M. Ishii, H. Nakamura, H. Nakano, A. Tsukigase, and M. Harada, Lai e-domain colloidal crystal films fabricated using a fluidic cell, Langmuir, 21,5367-5371 (2005). 

Gu, Z.-Z., Wang, D., and Moehwald, H. (2007) Self-assembly of microspheres at the air/water/air interface into freestanding colloidal crystal films. Soji Matter, 3 (1), 68-70. 

Lozano G, Miguez H (2008) Relation between growth dynamics and the spatial distribution of intrinsic defects in self-assanbled colloidal crystal films. Appl Phys Lett 92 091904— 091906... 

Scheid D, Lederle C, Vowinkel S, Schafer CG, Stuhn B, Gallei M (2014) Redox- and mechano-chromic response of metallopolymer-based elastomeric colloidal crystal films. J Mater Chem C 2 2583-2590... 

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