Enhancement of Thermosensitivity of Gel-Immobilized Tunable

Oct 13, 2017 - This led to the manipulation of the optical stop-band wavelength of the colloidal photonic crystals with a higher thermosensitivity and...
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Letter Cite This: ACS Macro Lett. 2017, 6, 1196-1200

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Enhancement of Thermosensitivity of Gel-Immobilized Tunable Colloidal Photonic Crystals with Anisotropic Contraction Toshimitsu Kanai,*,† Hiroki Yano,† Naoto Kobayashi,† and Tsutomu Sawada‡ †

Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama, Kanagawa 240-8501, Japan National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

‡

S Supporting Information *

ABSTRACT: A thermosensitive poly(N-isopropylacrylamide) (PNIPAM) circular gel film containing single-crystalline colloidal crystals was prepared, and the circular edge was pinched with washers to restrain the in-plane shrinkage of the gel film. Upon heating, the film shrunk in the thickness direction selectively, resulting in a change in the crystal structure of colloids embedded in the anisotropic shrunken gel. This led to the manipulation of the optical stop-band wavelength of the colloidal photonic crystals with a higher thermosensitivity and over a wider range of wavelengths covering a wide margin on either side of the visible-light region. The present method for enhancement of the sensitivity is very simple, and the principle should also be applicable to the gel-immobilized colloidal crystals that respond to stimuli other than temperature changes. The present findings contribute to the progress of practical applications of gel-immobilized colloidal photonic crystals as tunable photonic crystals and biological and chemical sensors.

C

changes isotropically in three dimensions. Thus, in previous reports, the sensitivity of the gel-immobilized tunable colloidal photonic crystals to stimuli was quantified based on the change in the lattice constant of the crystal structures maintained in the isotropic shrunken gel. In this study, we demonstrated that the sensitivity of gelimmobilized colloidal photonic crystals can be enhanced by changing the crystal structure of colloids through the anisotropic shrinkage of the gel. We prepared single-crystalline colloidal crystals with the FCC structure immobilized in a circular gel film composed of thermosensitive poly(Nisopropylacrylamide) (PNIPAM), and then, we pinched the circular edge of the film with washers to restrain the in-plane shrinkage of the film. Upon heating, the gel film shrunk in the thickness direction selectively, resulting in a change in the crystal structure of colloids embedded in the anisotropic shrunken gel. We found that this led to the marked contraction of the lattice spacing in the thickness direction and suppression of the increase in the refractive index of colloidal crystals compared to the case of crystals in the isotropically deformed film. As a result, the optical stop-band wavelength could be manipulated with a higher thermosensitivity and over a wider range of wavelengths covering a significant margin on either side of the visible-light region. The circular gel film containing single-crystalline colloidal crystals was prepared by the following procedure (the details

olloidal crystals are three-dimensional periodic arrays of monodisperse colloidal particles.1−10 They form optical stop bands due to the spatial periodicity of the refractive index between the particle arrays and the surroundings and, hence, are expected to be used for intriguing applications as photonic crystals11−17 and reflective-type color pigments that do not undergo degradation under UV light.18,19 In particular, colloidal crystals immobilized in soft polymer gels have been developed and have received increasing attention due to their unique tunable properties.20,21 Since the gels change their volume in response to changes in environmental conditions, the lattice constant or the optical stop-band wavelength of the colloidal crystals embedded in the gel can be altered on demand by external stimuli. Therefore, they are potentially useful for applications such as in tunable lasers, tunable optical filters, optical switches, and biological and chemical sensors for monitoring chemical reactions or the changes in the environments through the stop-band wavelength or Bragg reflection color.22,23 To date, tunable colloidal photonic crystals that respond to a variety of external stimuli such as the temperature, pH, and magnetic field have been developed,24−26 and enhancement of the sensitivity is one of the most important issues for the practical applications. Although the sensitivity can be controlled with the concentrations of the monomer and cross-linker in the gel network,27,28 a facile and versatile method for the enhancement has not been achieved yet. Colloidal crystals are generally prepared through a self-assembly process, where the crystal structure is confined to face-centered cubic (FCC), hexagonal close-packed (HCP), or body-centered cubic (BCC) structures.29,30 In addition, the volume of common gels © XXXX American Chemical Society

Received: October 1, 2017 Accepted: October 11, 2017

1196

DOI: 10.1021/acsmacrolett.7b00780 ACS Macro Lett. 2017, 6, 1196−1200

Letter

ACS Macro Letters are described in the Supporting Information). A suspension of monodisperse silica particles with a diameter of 210 nm was deionized in vials using mixed-bed ion-exchange resin and mixed with gelation reagents. The solution was then injected into a flat capillary cell (internal dimensions: 0.1 mm thick, 9 mm wide, 50 mm long) and processed with a momentary strong shear flow to form a single-domain crystal in the entire cell.31,32 The formed crystal was subsequently immobilized in the gel network by the photopolymerization of the gelation reagents in a UV-light exposure chamber.33 From the optical characterization such as transmission spectroscopy and Kossel line analysis,34,35 the colloidal crystal was identified as the FCC structure with a fixed crystallographic orientation determined by the cell geometry; the (111) lattice plane was parallel to the cell face, and one of the closest-packed directions in the (111) plane was parallel to the cell axis.32 When an external stimulus, i.e., heat in this case, was applied to the PNIPAM-immobilized colloidal crystal film, the gel shrunk isotropically in three dimensions, resulting in a decrease in the lattice constant of the crystal or the optical stop-band wavelength.28 However, the stop-band wavelength, which is measured from the Bragg reflection, is derived from the lattice spacing of the (111) lattice planes perpendicular to the thickness direction. Therefore, if the contraction of the lattice spacing in the thickness direction is increased selectively, the sensitivity can be enhanced. We considered that if the in-plane shrinkage of the gel film is restrained, the gel shrinkage in the thickness direction must increase complementarily. In addition, the crystal structure of colloids embedded in such an anisotropic shrunken gel changes, resulting in the lattice spacing in the thickness direction at the maximum-shrinkage state being smaller than that in the isotropic shrunken gel (Figure 1). This affords a wider tunable range of the Bragg wavelength, leading to the enhancement of the sensitivity to external stimuli.

Figure 2. Schematic diagram of the apparatus for pinching the circular edge of the gel film tightly and uniformly. Photographs show the film before and after pinching.

area visible through the transparent washer. When the degree of pinching was insufficient or nonuniform, the film slipped from between the washers during the shrinkage upon heating. When the circular edge was uniformly compressed to less than 56% of the film thickness at 4 °C, the film remained fixed with the washers during the experiment. The unpinched and pinched films were put into a water bath, and the temperature of water was increased from about 4 to 40 °C using a temperature controller. We confirmed using an optical microscope that the film did not shrink in-plane direction during heating. The reflection spectrum at normal incidence and the photograph of the films were recorded at various temperatures. The PNIPAM gel is a thermosensitive polymer that undergoes a volume transition at about 32 °C.36,37 Therefore, as the temperature of the unpinched film was increased, it began to shrink rapidly at the transition temperature, resulting in the color change as shown in Figure 3a. In the spectra, a reflection peak, which was due to the Bragg reflection derived from FCC (111) lattice planes perpendicular to the thickness direction, greatly shifted to a shorter wavelength at about 32 °C because of the contraction of the lattice spacing. Above the transition temperature, the peak remained constant at about 590 nm. In contrast, the pinched film exhibited various reflection colors, and the Bragg reflection peak shifted to a greater extent than that of the unpinched film did with increasing temperature (Figure 3b). In order to clarify the difference in the temperature-dependent changes between the unpinched and pinched films, the Bragg wavelengths and the difference, Δλ, are plotted as a function of temperature in Figure 3c and d, respectively. The Bragg wavelengths of the pinched film were always smaller than those of the unpinched film, and the ultimate Bragg wavelength in the maximumshrinkage state reached about 310 nm, which was approximately half of that of the unpinched film. Δλ gradually increased with the temperature until 34 °C and increased drastically thereafter. The small Δλ in the low-temperature range is mainly caused by the increase in the shrinkage in the thickness direction owing to the restraint of the in-plane shrinkage by pinching of the film. The large Δλ in the hightemperature range is mainly caused by the smaller lattice spacing of colloidal crystals at the maximum-shrinkage state achieved in the anisotropic shrunken gel film. These results indicate that the thermosensitivity is enhanced just by pinching the circular edge of the gel film.

Figure 1. Schematic diagram of the change in the lattice spacing of colloidal crystals embedded in the isotropic and anisotropic shrunken gel.

In order to test this hypothesis, the gel film containing singlecrystalline colloidal crystals was cut into a circular fragment of 4 mm diameter and sandwiched between transparent plastic washers with an inner diameter of 2 mm fixed on a substrate with a hole (Figure 2). The circular edge of the film was tightly and uniformly pinched with the washers by turning three screws. The degree of pinching can be measured from the Bragg reflection wavelength and reflection color at the pinched 1197

DOI: 10.1021/acsmacrolett.7b00780 ACS Macro Lett. 2017, 6, 1196−1200

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ACS Macro Letters

where λhkl is the Bragg wavelength; nc is the refractive index of the colloidal crystals; dhkl is the lattice spacing of (hkl) planes, and θ is the incident light angle (θ = 90° for normal incidence). The value of nc can be approximated by volume-weighted average of the refractive indices of the components38 nc = n pϕp + ngelϕgel

(2)

where np and ngel are the refractive indices of the silica particle (np = 1.45) and hydrogel, respectively, and ϕp and ϕgel are the volume fraction of the particle and hydrogel, respectively. Since the hydrogel is composed of a polymer and water, the refractive index of the hydrogel can be approximated as ngel = npolϕpol/ϕgel + nwϕw/ϕgel, where npol and nw are the refractive indices of the polymer (npol = 1.45, PNIPAM) and water (nw = 1.33), respectively, and ϕpol and ϕw are the volume fraction of the polymer and water, respectively (ϕpol = 0.56ϕp, which is determined from the masses of the gelation reagent and particles added to the suspension). The colloidal crystals in the unpinched film maintain the FCC structure with the (111) lattice planes perpendicular to the thickness direction during the shrinkage. Therefore, the lattice spacing perpendicular to the incident light is d111 and is given, using the particle volume fraction ϕp and particle diameter d, by ⎛ ⎞1/3 2π 1 ⎟ ⎜ d111 = ⎜ · ⎟ ·d ⎝ 9 3 ϕp ⎠

(3)

By substituting the observed Bragg wavelength of the unpinched film at different temperatures into eq 1 and using eqs 2 and 3, the lattice spacing, particle volume fraction, and refractive index are estimated and are plotted as the open circles in Figure S1a, S1b, and 3e, respectively. The ultimate lattice spacing for the unpinched film in the maximum-shrinkage state is estimated to be d111 = 208 nm at ϕp = 0.42 and nc = 1.41. On the other hand, the colloidal crystals in the pinched film do not maintain the FCC structure during the shrinkage as shown schematically in Figure 1. In this case, the lattice spacing perpendicular to the incident light is determined as follows from geometrical considerations using the initial lattice spacing di111 and initial particle volume fraction ϕip at 4 °C before pinching: d′111 =

Figure 3. Photographs and the reflection spectra of the (a) unpinched and (b) pinched films at various temperatures. An unknown broad peak appeared at around 550 nm in the pinched film. It might be caused by the diffraction from silica particles. (c) Bragg wavelength of the unpinched and pinched films determined from the reflection spectra as a function of temperature (○: unpinched film, ●: pinched film). (d) Δλ as a function of temperature. (e) Refractive index of the unpinched and pinched films estimated from the Bragg wavelength as a function of temperature (○: unpinched film, ●: pinched film). Solid lines are calculation curves when the linear shrinkage rate in the thickness direction of the pinched film is 1.65-fold that of the unpinched film.

ϕp

i d111

(4)

By substituting the observed Bragg wavelength of the pinched film at different temperatures into eq 1 and using eqs 2 and 4, the lattice spacing, particle volume fraction, and refractive index are estimated and are shown as the solid circles in Figure S1a, S1b, and 3e, respectively. The ultimate lattice spacing for the pinched film in the maximum-shrinkage state is estimated to be d′111 = 114 nm at ϕp = 0.25 and nc = 1.38. This lattice spacing is as small as about half of the particle diameter, which cannot be attained in the FCC structure even if the closest-packed structure is formed (d111 = 172 nm). The solid line in Figure S1a represents the lattice spacing d′111 calculated from that of the unpinched film, i.e., d111, when the linear shrinkage rate in the thickness direction of the pinched film, which is defined as

The temperature-dependent changes in the Bragg wavelength are quantitatively discussed by using the Bragg’s Law as follows λhkl = 2ncdhkl sin θ

ϕpi

SLpinched =

i d111 − d ′111 i d111

film, SLunpinched =

(1) 1198

× 100, is 1.65-fold that of the unpinched

i d111 − d111 i d111

× 100. In addition, the solid line in DOI: 10.1021/acsmacrolett.7b00780 ACS Macro Lett. 2017, 6, 1196−1200

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ACS Macro Letters

the Bragg wavelength derived from the (002) lattice planes is given by

Figures S1b, 3e, and 3c shows the particle volume fraction, refractive index, and Bragg wavelength of the pinched film, respectively, estimated by using the calculated lattice spacing and eqs 1, 2, and 4. These curves are in good agreement with the plots of the pinched film. This indicates that pinching the circular edge of the gel film causes the linear shrinkage rate in the thickness direction of the pinched film to increase and become 1.65-fold that of the unpinched film. In order to confirm the unilateral shrinkage of the colloidal crystals in the pinched film, angle-dependent transmission spectra were measured. Figure 4a and 4b shows the incident light-angle

λ 002 =

3 anc sin δ sin(90° − δ + θc)

(5)

where a is the distance between nearest-neighbor particles in the (111) lattice plane; δ is the angle between the (111) and (002) lattice planes; and θc is the incident light angle in the colloidal crystals, which was calculated based on the incident light angle in air, θa, using Snell’s Law, nc = sin θa/sin θc. Thus, the incident light-angle dependent change in λ002 could be calculated using nc and a, which were determined from the Bragg wavelength derived from the (111) lattice planes for normal incidence and δ. The plots of the (002) dip wavelength for the pinched film were best fitted at δ = 53.5°, which is shown as a solid line in Figure 4c. This value is in reasonable agreement with the angle determined from tan δ =

′ d111 3 3

a

, i.e., δ

= 52.3°. Another important feature is that pinching the circular edge of the film decreases the volume shrinkage rate despite increasing the linear shrinkage rate in the thickness direction; this is shown in Figure S1b, where the particle volume fraction of the pinched film is smaller than that of the unpinched film at the same temperature. Since the refractive index of colloidal crystals is a function of the particle volume fraction as is given by eq 2, pinching the edge of the film suppresses the increase in the refractive index during the shrinkage upon heating (Figure 3e). Thus, according to eq 1, the combination of the significant contraction of the lattice spacing in the thickness direction and the suppression of the increase in the refractive index of the colloidal crystals in the anisotropic shrunken gel results in the Bragg wavelength being smaller than that in the isotropic shrunken gel. In conclusion, we have demonstrated that the thermosensitivity of the PNIPAM-immobilized tunable colloidal photonic crystals can be enhanced by restraining the in-plane shrinkage of the gel. We found that the colloidal crystals in the anisotropic shrunken gel film attained the marked contraction of the lattice spacing in the thickness direction and suppression of the increase in the refractive index of colloidal crystals. These enabled the manipulation of the optical stop-band wavelength with a higher sensitivity and over a wider range of wavelengths, which cannot be achieved in the isotropic shrunken gel. While we demonstrated the responsiveness of the colloidal crystals toward temperature changes, the principle should also be applicable to responsiveness to changes in other stimuli. The present method for enhancement of the sensitivity is very simple in that the edge of the circular gel film is pinched, and the present findings contribute to the progress of the practical applications of gel-immobilized colloidal photonic crystals as tunable photonic crystals and biological and chemical sensors for monitoring environmental changes.

Figure 4. Angle-dependent transmission spectra of the (a) unpinched and (b) pinched films at 20 °C. The incident angle of the light is swung in the plane perpendicular to the flow direction starting from the direction normal to the film surface to 30°. (c) Plots of the Bragg wavelength vs incident light angle obtained from a and b [○: (111) dip from unpinched film, ●: (111) dip from pinched film, △: (002) dip from unpinched film, ▲: (002) dip from pinched film]. The solid lines are the calculation curves obtained using the Bragg conditions. The curves were best fitted with an error of less than 3°. (d) Illustration of the (111) and (002) lattice planes.

dependence of the transmission spectra of the unpinched and pinched films, respectively, at 20 °C on the plane normal to the flow direction. For both films, the dips due to Bragg reflections derived from the (111) and (002) lattice planes were observed and shifted to shorter and longer wavelengths, respectively, when the incident light angle was increased. However, although the (111) dip wavelength for the pinched film was smaller than that for the unpinched film at the same incident light angle, the (002) dip wavelength for the pinched film was larger than that for the unpinched film (Figure 4c). This red-shift of the (002) dip indicates that the crystals in the pinched film selectively shrank in the thickness direction. The (002) lattice planes were inclined from the horizontal direction, different from the (111) lattice planes, as schematically shown in Figure 4d. When the crystals selectively shrank in the thickness direction, the (002) lattice planes faced toward the thickness direction while decreasing the lattice spacing. This led to the red-shift because the Bragg wavelength increased with increasing incident light angle to the lattice planes. From the geometrical considerations,

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00780. Experimental details and supplementary data (PDF) 1199

DOI: 10.1021/acsmacrolett.7b00780 ACS Macro Lett. 2017, 6, 1196−1200

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ACS Macro Letters

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(32) Kanai, T.; Sawada, T.; Toyotama, A.; Kitamura, K. Adv. Funct. Mater. 2005, 15, 25−29. (33) Toyotama, A.; Kanai, T.; Sawada, T.; Yamanaka, J.; Ito, K.; Kitamura, K. Langmuir 2005, 21, 10268−10270. (34) Kanai, T.; Sawada, T.; Maki, I.; Kitamura, K. Jpn. J. Appl. Phys. 2003, 42, L655−L657. (35) Comoretto, D., Ed.; Organic and Hybrid Photonic Crystals; Springer International Publishing: Switzerland, 2015. (36) Saunders, B. R.; Vincent, B. Adv. Colloid Interface Sci. 1999, 80, 1−25. (37) Shah, R. K.; Kim, J. W.; Agresti, J. J.; Weitz, D. A.; Chu, L. Y. Soft Matter 2008, 12, 2303−2309. (38) Kanai, T.; Yamamoto, S.; Sawada, T. Macromolecules 2011, 44, 5865−5867.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Toshimitsu Kanai: 0000-0002-3909-6324 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI (Grant Number 22686063). REFERENCES

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DOI: 10.1021/acsmacrolett.7b00780 ACS Macro Lett. 2017, 6, 1196−1200