Clickable Colloidal Photonic Crystals for Structural Color Pattern

Article Cite This: Langmuir 2018, 34, 13219−13224

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Clickable Colloidal Photonic Crystals for Structural Color Pattern Jialun Chen,†,‡ Panmiao Liu,† Xin Du,† and Zhuoying Xie*,†,‡ †

State Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering, and ‡National Demonstration Center for Experimental Biomedical Engineering Education, Southeast University, Si Pai Lou 2, Nanjing, Jiangsu, China 210096

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S Supporting Information *

ABSTRACT: Patterning colloidal photonic crystals have broad important applications in optical devices, functional coatings, full color displays, and colorimetric sensors. In this paper, a clickable colloidal photonic crystal using vinyl-modified sub-micrometer silica particles as building blocks was proposed to pattern photonic crystals. By click chemistry, different chemical groups were simply grafted to the clickable photonic crystals film and obtained wettability-encoded structure color patterns. The clickable photonic crystals provide a simple, controllable, and rapid path to pattern photonic crystals.

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INTRODUCTION Photonic crystals are an artificial microstructure arranged periodically by two or more media of different refractive indices.1,2 Because of their various unique optical properties such as photonic band gap, “low-photon” effect, and fluorescence enhancement effect,3−7 photonic crystals have wide application in reflector, laser device, anticounterfeiting, color display, and sensors.8−16 Among them, patterning colloidal photonic crystals greatly expands its application in multiaspects by integrating different functionalized photonic crystals into one substrate. The patterning colloidal photonic crystals is generally achieved through multiple approaches, such as patterned substrate-induced assembly,17−21 inkjet printing,22−26 or selective immobilization and modification.27−29 Patterned substrate-induced assembly is an approach; Gu and co-workers used superhydrophilic− superhydrophobic patterned substrate to achieve patterned photonic crystals. However, only one kind of photonic crystals could be formed. Through inkjet printing, photonic crystals with different structural colors could be formed, which improved the performance of patterned photonic crystals. To further enhance its practicality in fields like sensors, selective immobilization and modification was applied to achieve responsive photonic crystals patterns. Aizenberg and co-workers reported a colorimetric sensor for solvents determination. However, the requirement of vapor environment makes the process complex. Hence, we proposed a remarkably simple, rapid, and controllable postpatterning approach by selective modification on clickable colloidal photonic crystal. The clickable colloidal photonic crystal is built by monodispersed silica (SiO2) particles modified with vinyl group, which served as the reaction site with thioalcohol in grafted groups by click chemistry. The click reaction makes the various surface functionalization of colloidal photonic crystal, which usually requires harsh conditions, much easier to spark such as the chemical modification of superhydrophobic surfaces in situ,30 and has the advantages of quick chemical © 2018 American Chemical Society

reaction, simple reaction conditions, and high reaction rate. On the basis of the unique feature of the clickable colloidal photonic crystal, we further demonstrated a wettability-encoded structure color pattern with a resolution of 8 μm (Figure S1, Supporting Information) and stable chemical properties (Figure S2) by selective clicking modification of different hydrophobic or hydrophilic groups. Because of the difference of surface chemical properties, diverse structure color patterns were revealed under the soaking of the gradient concentration of ethanol aqueous solution.

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EXPERIMENTAL SECTION

Chemicals. The tetraethyl orthosilicate (TEOS), triethoxyvinylsilane, ammonia, ethanol, and N,N-dimethylformamide (DMF) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). The benzoin dimethyl ether (DMPA), β-mercaptoethanol, pentanethiol, octanethiol, N-dodecyl mercaptan, 1H,1H,2H,2H-perfluorodecanethiol were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). Ultrapure water was prepared by using a Milli-Q water purification system (Millipore, Bedford, MA, USA). Instruments. Transmission electron microscope (TEM) studies were carried out with the JEM-2100 (JEOL) operating at 80 kV accelerated voltage. Scanning electron microscope (SEM) studies were carried out with the Ultra Plus (Zeiss) operating at 2 kV accelerated voltage. Attenuated total reflection-infrared (ATR-IR) spectra were conducted with a NICOLET 5700 Infrared Fourier Transform Spectrometer (Thermo). Reflectance spectra measurements were performed with a QE6500 spectrometer (Ocean Optics) with DH-2000 Deuterium-Halogen Light Source (Ocean Optics). Contact angles were measured with a contact angle meter (Shanghai Zhongchen Digital Technology Apparatus Co., Ltd.). Fabrication of Clickable Photonic Crystals. First, SiO2 nanoparticles (NPs) were synthesized by the Stöber method. TEOS was Received: March 27, 2018 Revised: October 3, 2018 Published: October 23, 2018 13219

DOI: 10.1021/acs.langmuir.8b00996 Langmuir 2018, 34, 13219−13224

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Langmuir added dropwise into the mixture of 300 mL of ethanol and 10 mL of ammonia. Through the hydrolysis and condensation reactions of TEOS, SiO2 NPs grow to the size we need. Next, 1 mL of triethoxyvinylsilane was diluted 10 times by ethanol and then added dropwise into the reaction system by an injection pump within 10 min at room temperature. The solution was stirred at 500 rpm to spread triethoxyvinylsilane rapidly in the system uniformly. Resulting from the low content of triethoxyvinylsilane, it will be exhausted quickly and connect to SiO2 NPs. The achieved vinyl-modified SiO2 NPs were washed twice with ethanol and water and redispersed in ethanol at the concentration at 2 wt %. Insert glass slides vertically into 2 wt % vinyl-modified SiO2 particles ethanol solution in a dry environment of 40 °C for 48 h. Modification of the Clickable Photonic Crystals. Cover vinylmodified photonic crystals with a quartz glass. The mask can be placed on the quartz glass if there is a need. Inject the reaction solution (DMF as solvent, 0.5 wt % DMPA and 10 wt % thioalcohol) from the interspace between glass and quartz. Place it under UV radiation for 60 s. Wash away the solution with ethanol.

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RESULTS AND DISCUSSION The clickable colloidal photonic crystals were fabricated by using vinyl-modified sub-micrometer silica (SiO2) particles as building blocks. The vinyl modification of SiO2 particles was performed by using triethoxyvinylsilane as substitutive silicon source after the formation of SiO2 particles in the modified Stöber system (Figure 1a). The Si−O−Et of triethoxyvinylsilane

Figure 2. (a) TEM of silica particles (left) before and (right) after vinyl modification. (b) ATR-IR spectra of silica particles (black line) before and (red line) after vinyl modification.

photonic crystals film was covered by a quartz glass, which eliminated the unevenly spreading light results from the curved liquid surface and separated most air from reaction. Reaction solution was injected into the interspace between substrate and quartz glass. The reaction solution was composed of DMF (N,N-dimethylformamide) as solvent, 0.5 wt % DMPA (benzoin dimethyl ether) as photoinitiator and 10 wt % thioalcohol as reactive monomer. Under UV radiation, DMPA cracked into abundant free radicals. The free radicals combined with hydrogen from thioalcohol, producing sulfhydryl radical. The sulfhydryl radical attacked CC double bond and captured a hydrogen from thioalcohol, resulting in new sulfhydryl radical.31 Figure 4c,d showed the SEM images of photonic crystals after modification by β-mercaptoethanol, which revealed that the particles as well as the structure of photonic crystals remained unchanged after click reaction. The variation of contact angle from 120.8° to 27.9° confirmed that the hydroxy group was successfully grafted to the surface of photonic crystals film. For further proof, we performed 13C solid nuclear magnetic resonance (SNMR) (Figure 5b). Peaks assigned to −CH2− CH2−OH at 26.0 and 61.2 ppm confirmed the successful modification of β-mercaptoethanol. A previous report has already confirmed that vinyl could react with most sulfhydryl groups regardless of chemical activities.32 In our research, we respectively introduced β-mercaptoethanol, pentanethiol, octanethiol, N-dodecylmercaptan, 1H,1H,2H,2Hperfluorodecanethiol as reactive monomer to modify the vinylmodified photonic crystals. To confirm the successful modification, we performed 13C solid nuclear magnetic resonance (SNMR) (Figure 5). All the samples showed the same peaks at about 17.0 and 58.2 ppm, which were assigned to the unreacted −O−CH2−CH3 groups of TEOS. Peaks attributable to vinyl at 129.9 and 133.2 ppm could be found in the spectra of vinylmodified SiO2 (Figure 5a), but decreased obviously in the spectra of β-mercaptoethanol (Figure 5b), pentanethiol (Figure 5c), octanethiol (Figure 5d), N-dodecylmercaptan (Figure 5e), and 1H,1H,2H,2H-perfluorodecanethiol (Figure 5f)-modified SiO2. Furthermore, peaks caused by different chemical groups could

Figure 1. (a) Schematic of the synthesis of vinyl-modified SiO2. (b) Mechanism of vinyl modification.

was hydrolyzed to Si−OH and then condensed with the Si−OH on the surface of SiO2 nanoparticles and form CC−Si−O−Si bond at last, while the vinyl was inert during this process. The detail mechanism could be represented by the scheme in Figure 1b. As shown in Figure 2a, the obtained vinyl-modified SiO2 particles were standard spherical and highly monodispersed. Attenuated total reflection-infrared spectroscopy (ATR-IR) was used to investigate the modified vinyl on the surface of SiO2 particles. When the ATR-IR spectra of the silica particles are compared before and after vinyl modification (Figure 2b), the characteristic peaks at 953, 1400, and 1630 cm−1 could be found, which contributed to the CC twist and CH2 wagging,  CH2 scissors, and CC stretch, respectively. The peaks at 1100 cm−1 were caused by Si−O. This result demonstrated the existence of vinyl. The clickable photonic crystals film was fabricated by inserting glass slides vertically into 2 wt % vinyl-modified SiO2 particles ethanol solution in a dry environment of 40 °C. As ethanol solvent evaporated, these particles self-assembled on the slides forming opal structure. As shown in Figure 4a, the particles presented a hexagonal close-packed structure. Figure 3 showed the process schematic and theory of the alkyl-modified photonic crystals by the clickable reaction. Vinyl-modified 13220

DOI: 10.1021/acs.langmuir.8b00996 Langmuir 2018, 34, 13219−13224

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Figure 3. (a) Schematic of the alkyl-modified photonic crystals by the clickable reaction. (b) Reaction mechanism of clickable reaction with vinyl and sulfydryl.

Figure 4. SEM images of the front view and side view of clickable photonic crystals (a,b) before and (c,d) after hydroxy modification. The inserts were the contact angle images of photonic crystals before and after hydroxy modification, respectively.

be found in the spectra. For example, peaks assigned to −CH2− CH2−OH at 26.0 and 61.2 ppm confirmed the successful modification of β-mercaptoethanol. This evidence demonstrated successful modification. After clickable reaction described above, the obtained photonic crystals performed different wetting properties based on the modified functional groups. Contact angle measurement was a typical representative because different chemical groups changed the surface free energy, which determined the contact angle. As shown in Figure 6a, the contact angles between water and these photonic crystal surfaces were 27.9° for hydroxy modification, 122.3° for pentyl modification, 123.5° for octyl modification, 125.4° for dodecyl modification, 132.4° for perfluorinated decyl modification, respectively, which were all different from the vinyl surface of 120.8°. The different contact

angle was caused by the different surface energy. The lower the surface energy, the more hydrophobic it was and the bigger the contact angle. Furthermore, the wettability of modified photonic crystals responded to the ethanol concentration in water. Figure 6b showed the variation of contact angle with the ethanol concentration by using the perfluorinated decyl-modified photonic crystal as a typical example. When the ethanol concentration was below 80%, the contact angle linearly decreased as the increase of ethanol concentration. But when the ethanol concentration exceeded 80%, the contact angle declined sharply. The contact angle decrease was because the tension of the solution decreases with the increase of ethanol concentration. When the ethanol concentration was above 80%, the solution was able to wet the perfluorinated decyl surface and immerse into the photonic crystals, which caused the sharp decline. 13221

DOI: 10.1021/acs.langmuir.8b00996 Langmuir 2018, 34, 13219−13224

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Figure 5. 13C SNMR result of (a) vinyl, (b) hydroxy, (c) pentyl, (d) octyl, (e) dodecyl, and (f) perfluorinated decyl PCs.

Figure 6. (a) Contact angle of hydroxy-, vinyl-, pentyl-, octyl-, dodecyl-, perfluorinated decyl-modified photonic crystals. (b) Contact angle of different concentrations of ethanol on the perfluorinated decyl photonic crystals. (c) Corresponding relationship between the ethanol concentration and contact angles on the perfluorinated decyl photonic crystals.

index is decided by the compositions of photonic crystals as eq 2 shows. In this research, n1 and n2 refer to reflective indices of SiO2 NPs and air/ethanol−water solution, and f refers to volume ratio of each composition.

By the Bragg−Snell equation, the relation between reflection peak and refractive index is shown in eq 1. λ, n, d, and θ refer to wavelength of reflection light, reflective index of photonic crystals, size of the cells, angle between photonic crystals, and viewer’s sight (90° in the measurement). And the reflective

λ = 2n × d sin θ 13222

(1) DOI: 10.1021/acs.langmuir.8b00996 Langmuir 2018, 34, 13219−13224

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Langmuir n=

n12 × f1 + n2 2 × f2

average refractive index. When the concentration changed from 30% to 40%, the solution completely infiltrated into the photonic crystals and totally replaced the air, which greatly changed the average refractive index. The blue shift of the reflection peak above 80% was caused by the decrease of refractive index of ethanol aqueous solution. It is remarkable that a different chemical group resulted in different ethanol concentration for full infiltration, which led to the drastic reflection peak red shift and visible color change. As mentioned above, UV exposure will trigger and control the click chemistry reaction. Such characters mean potential to encode different information in this single system, which improves the ability to be used in many fields such as data encryption and sensors. As shown in Figure 8a, the patterns of “S”, “E”, and “U” were respectively modified by β-mercaptoethanol, pentanethiol, N-dodecylmercaptan using templates, and the rest of the region was modified by 1H,1H,2H,2Hperfluorodecanethiol. Since different chemical groups presented different full infiltration concentrations of ethanol solution, the patterns were gradually revealed at 0%, 30%, and 40% concentration of ethanol solution, respectively (Figure 8b).

(2)

Therefore, the wetting of modified photonic crystal by various concentrations of ethanol−water solution resulted in the changes of reflection peak (as shown in Figure 7 and Figures S3 and S4)

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Figure 7. Reflection peaks of hydroxy, pentyl, octyl, dodecyl, and perfluorinated decyl photonic crystals substrates in different concentrations of ethanol aqueous solution, respectively.

CONCLUSIONS We proposed a simple path to pattern photonic crystals by the clickable photonic crystals built by vinyl-modified sub-micrometer silica particles. By click reaction under UV exposure with thioalcohol, different chemical groups were simply grafted to the clickable photonic crystals film and obtained wettability-encoded structure color patterns. Depending on the difference of surface chemical properties, different structure color patterns were revealed under the soaking of the gradient concentration of ethanol aqueous solution. The clickable photonic crystals provide a simple, controllable, and rapid path to pattern photonic crystals and have potential applications not only in photonic nose or in tongue functionalized with numerous chemical identification

because of the changes of average refractive index of compositions. For the example of octyl surface photonic crystals, the reflection peak was slowly red-shifted when the ethanol concentration was below 30%, rapidly red-shifted from 30% to 40%, slowly red-shifted again above 40%, and slowly blue-shifted above 80%. The phenomenon of reflection peak shift was consistent with the change of average refractive index of compositions, the corresponding numerical average refractive index is shown in Table S1. When the concentration was below 30%, the vapor of solution in the pores photonic crystal increased the

Figure 8. (a) Schematic of modifying different chemical groups in one piece of photonic crystal film with the differently patterned photomask. (b) Photographs of chemical-encoded photonic crystal patterns in different concentrations of ethanol aqueous solution. From left to right: air, 0%, 30%, and 40% ethanol aqueous solution. 13223

DOI: 10.1021/acs.langmuir.8b00996 Langmuir 2018, 34, 13219−13224

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probes, but also in information encryption with different encoded chemical information.

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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/acs.langmuir.8b00996. Resolution of the patterns, chemical stability of the patterns, reflectance spectra of different samples for different concentrations of ethanol aqueous solution, and reflect indices of photonic crystals (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jialun Chen: 0000-0003-0828-432X Zhuoying Xie: 0000-0003-3534-1924 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the research fund of the National Key Research and Development Program of China (No. 2017YFA0700404), the Natural Science Foundation of Jiangsu Province (Grant BK20150024), and the Fundamental Research Funds for the Central Universities.

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REFERENCES

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DOI: 10.1021/acs.langmuir.8b00996 Langmuir 2018, 34, 13219−13224