pH-Responsive nano sensing valve with self-monitoring state property based on hydrophobicity switching

Abstract

Nano valves have been used in functional porous materials to control molecular transport by changing their properties in response to external stimuli. But most of them are limited by the blocking units and cannot show their state by themselves. Herein, pH switchable nano valves were constructed using mesoporous inverse opal photonic crystal, which realized free-blockage nano valves and achieved the monitoring of the state of the valve by the naked eye without an external indicator. The nano valves were modified by phenylamine groups, which has a convertible hydrophobic/hydrophilic property between deprotonation and protonation. The valves were hydrophobic enough to prevent solution passing through at pH 7.0, and meanwhile a green color was presented. With the decrease of the pH value of the solution, the valves became open and presented a yellow to red color because of the protonation of phenylamine groups followed by the invasion of solution. Thus, in this study not only a free-blockage valve but also nano sensing valve was constructed. We believe that our studies provide new insights into photonic crystal sensors and nano sensing valve.

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Introduction

Nano valves were often used in functional porous materials to control molecular transport by changing their properties in response to external stimuli, which was valuable in both theoretical research and practical applications. Many of the mechanisms of nano valves have been previously discussed. The most obvious one is based on a physical blockage such as polymers or nanoplugs (cyclodextrins, nanoparticles, DNA, etc.).^1–3 It is difficult for such valves to stem the nanopores completely especially for “large” pores, which can lead to some decline of flow rate. What's more, most of the blockage unit flows into solution, which results in the contamination of solution. Wettability is one of the most important properties of solid surfaces.⁴ There is a growing interest for some “smart” materials which can change reversibly between hydrophobicity and hydrophilicity under external stimuli including light,⁵ thermal treatment,^6,7 electric field,⁸ biochemical analytes,^9,10 pressure, and pH.^11,12 In recent years, our research group and others have applied smart hydrophobic materials to nano valves, realizing a free-blockage nano valve.^13–17 Such free-blockage nano valve can avoid the side effects caused by physical blocking units efficiently.

Photonic crystals (PCs) possess a periodic modulation of refractive index which creates a photonic bandgap. Light with a specific wavelength whose energy is in the bandgap is prohibited in the structure, thereby being reflected.¹⁸ The bandgap is determined by the periodicity and effective refractive index, so the shift of bandgap can be caused by the change of periodicity and effective refractive index.¹⁹ The responsive photonic crystals, a kind of PCs with tunable bandgaps by changing the periodicity and effective refractive index, are highly desired for some practical applications, such as biological and chemical sensors,^20–22 color displays,²³ inks,²⁴ and paints,²⁵ and many optically active components.²⁶ However, the tuning of structural colors (or photonic bandgaps shift) is limited by materials and structures.²⁷ To enlarge the bandgap shift, many researches have been done by using different structural materials such as gold, titania²⁸ or responsive polymer.^29,30 But not all materials can enlarge the shift, which limits the application of responsive photonic crystals. Inverse opal photonic crystal (IOPC) with porous structure is often formed by backfilling a sacrificial template of opal photonic crystal. Liquids can invade in the large portion of air cavities of IOPC, leading to a red shift of the bandgap.¹⁹ Because the inverse opals possess more void percentage (about 74%) than the opal structure (26%), their average refractive index can vary in wider range after liquids invasion.³¹ Despite broadly based and encouraging recent progress, it is still a great challenge to find a more efficient method to increase the bandgap shift and improve the sensitivity of responsive photonic crystals.

In this paper, mesoporous structures were introduced into the IOPC to improve the efficiency of the bandgap shift. The mesoporous structures made the IOPC had more void space for solution to invade in, which could increase the bandgap shift efficiently. Besides, owing to the interconnecting hierarchically porous of IOPC provided fast mass transport channels, the IOPC could also be used as free-blockage nano valve. Thus, pH sensitive nano valves were created based on hydrophobicity switching materials and mesoporous IOPC, which realized free-blockage switching and achieved the monitor of the state of the valve by naked eyes without external indicator (Scheme 1a). We achieved it by modifying the mesoporous IOPC with phenylamine (Ph) groups which have a convertible hydrophobic/hydrophilic property between deprotonation and protonation. The Ph-functionalized valve was hydrophobic enough to prevent solution passing through at pH 7.0. With a decrease of the pH value of the solution, the valve changed from hydrophobic to hydrophilic, leading to the traverse of solution due to the protonation of Ph groups. The traverse of solution caused the change of effective refractive index, resulting in the red shift of bandgap. The degree of red shift was determined by the degree of the protonation and the corresponding volume of solution entering into macropores and mesopores in mesoporous IOPC (Scheme 1b). This smart mesoporous IOPC valve with self-monitoring state property is expected to find many applications, such as, but not limited to, sensing platforms and controlled release film.³² The drug release degree can be estimated by the valve state. The indicator-free detection greatly simplifies the sensing protocol, and more kinds of drugs besides that with fluorescence or color can be used. What's more, the films with high surface-to-volume ratio, stable structure, large pore diameter, and good biocompatible even can load biomacromolecules, which offers broad applications in biotechnology.³³

Scheme 1 (a) Schematic diagram illustrating the formation of the process for the preparation of pH sensitive mesoporous IOPC valve and its valve characteristic. (b) Schematic illustration of the diverse color results from the invasion of solutions in mesopores of IOPC vary with different pH values (pH 1.0, pH 3.0, pH 5.0, and pH 7.0), solutions with lower pH value invade in mesopores faster. Inset pictures for each pH situation are optical microscopy images of mesoporous IOPC at which one of different pH solution is deposited.

Experimental

Materials

TEOS, F127, PhAPTMS, styrene, sodium dodecyl benzene sulfonate (SDBS) and methyl methacrylate (MMA) were purchased from Sigma Company. Ethanol, sodium hydroxide, ammonium peroxodisulfate (APS), disodium hydrogen phosphate dodecahydrate, acrylic acid, hydrochloric acid, citric acid, sulfuric acid, hydrogen peroxide, and tetrahydrofuran were purchased from Sinopharm Chemical Reagent Co. Ltd. All buffers were prepared with ultra-pure MilliQ water (resistance > 18 MΩ cm⁻¹).

Instruments

Scanning electron microscopy (SEM) images were obtained with a field emission SEM (JSM-6700F). Transmission electron microscope (TEM) images were obtained by JEM-2010 microscopy. Reflectance spectrum was obtained with an R1 series spectroscopy meter (Shanghai Ideaoptics Corporation) attached to the Ideaoptics PG 2000 fiber optic spectrometer. The contact angles were measured using Dataphysics OCA20. Optical photos of IOPC were taken by ECLIPSE LV100ND (Nikon).

Preparation of monodispersed latex spheres

Monodispersed poly(styrenemethyl methacrylate-acrylic acid) (poly(St-MMA-AA)) latex spheres were prepared via improved batch emulsion polymerization.³⁴ MMA (10.00 mmol), acrylic acid (13.89 mmol), and styrene (182.60 mmol) dissolved in 100 ml water with sodium dodecyl benzene sulfonate (SDBS, 0.005 mmol) under stirring for 20 min at 60 °C. Following dropwise adding APS (1.06 mmol) with stirring for 12 h at 80 °C.

Fabrication of colloidal crystal films

The colloidal crystal films were fabricated by vertical deposition method on glass slides treated with piranha solution.³⁵ The glass slides inserted vertically into poly(St-MMA-AA) colloidal suspensions 5 wt% at 60 °C with a relative humidity of 60% for 3 days, and the liquid surface dropping velocity was 0.28 mm h⁻¹.

Fabrication of Ph-functionalized mesoporous IOPC films

F127 (1.0 g) and concentrated HCl (1.5 ml) were dissolved in the mixture of EtOH (50 ml). After addition of the mix of different molar ratios of PhAPTMS to TEOS (1 : 2, 1 : 9, 0 : 1) the solution was stirred for 8 h at room temperature to get the Ph-functionalized mesoporous precursor. Then the P(St-MMA-AA) photonic crystal films was dipped in the mesoporous precursor and drawn out from the emulsion at a speed of 5 mm min⁻¹. After dried for 8 h at room temperature, the film was cured in an oven at 150 °C for 30 min in order to increase the mechanical strength and connectivity of the mesoporous framework. In the end, the film was immersed in tetrahydrofuran for 24 h to remove the P(St-MMA-AA) template. F127 in the mesostructured film were removed by refluxing in hydrochloric acid–ethanol solution.

Measurement of the optical properties

The bandgaps of PCs were measured by R1 series spectroscopy meter and characterized by reflectance spectrum. To measure the response of the mesoporous IOPC to pH, the IOPC was immersed into solutions with different pH (1.0, 3.0, 5.0 and 7.0) until solution just submerged the IOPC, at which point the reflectance spectrum was recorded. Optical photos of IOPC were taken with Nikon LV100ND which has a reflection mode to provide an image formed by light reflected from surface of IOPC.

Testing the contact angle

Contact angles were measured with Dataphysics OCA20 using the sessile drop method. Droplets of 3.0 μl volume were deposited on the surface enclosed in a 20 °C chamber to minimize evaporation.

Results and discussions

Fabrication and characterization of mesoporous IOPC films

Scheme 1a illustrates the preparation diagram of mesoporous IOPC film. Photonic crystals self-assembled from monodispersed latex suspension of P(St-MMA-AA) with a diameter of 310 nm via vertical deposition. The specific diameter of spheres ensured that the bandgap of the IOPC located in the visible range under different infiltrating levels. As shown in Fig. 1a, the spheres were in a face-centered cubic (fcc) arrangement with the close-packed plane (111) oriented parallel to the substrate, with each sphere touching six others in one layer.

Fig. 1 (a) SEM images of top surfaces of P(St-MMA-AA) opal. (b) SEM images of top surfaces of mesoporous IOPC, where P(St-MMA-AA) spheres with diameter of 310 nm are used. (c) SEM images of cross view of mesoporous IOPC. (d) TEM image of mesoporous IOPC.

The mesoporous IOPC films were obtained by removing opal P(St-MMA-AA) spheres, after being backfilled with Ph-functionalized SiO₂ precursor by dip-drawing method.^36,37 Different concentrations of N-phenylamino-propyltrimethoxysilane (PhAPTMS) were chosen to synthesize the precursor. The molar ratio of PhAPTMS to tetraethyl orthosilicate (TEOS) was chosen as 0 : 1, 1 : 9, and 1 : 2 under the conditions of a constant surfactant concentration, and the modified mesoporous IOPCs were accordingly named as mIOPC, mIOPC–Ph (1 : 9), mIOPC–Ph (1 : 2). An inverse opal structure was observed with SEM as shown in Fig. 1b (inset), where the air spheres were arranged in a hexagonally ordered array, consistent with a (111) plane arrangement of fcc structure as with the original latex templates. The center to center distance between the air spheres was in accord with original PS opals. Besides, there was just a little defect observed at nanoscale as shown in Fig. 1b, which was beneficial to the closure of solution. As the red arrows shown in Fig. 1b, the defects were point defects, which caused by some (but not much) big diameter spheres in opal film (Fig. S1†). The cross-view of SEM in Fig. 1c shows the thickness of the film was 2.3 μm.

TEM image of the mesoporous IOPC film showed that the macropores formed by P(St-MMA-AA) and mesopores formed by F127 were constructed (Fig. 1d). The interconnecting hierarchically porous structure provide fast mass transport channels from the macropores. Once the valve open, the solution could invade in both horizontal and vertical directions. Thus the film has ideal structure as nano valves. It can be observed the wall of the macropore system is composed of mesopores, which provide more space for solution to invade in.

The optical properties of opal PC, IOPC, and mesoporous IOPC films were evaluated using reflection spectra. The bandgap peak position of photonic crystal with FCC lattice can be estimated by Bragg's law

where d is the (111) plane spacing, D is the sphere diameter, n_eff is the effective refractive index, respectively. We use the Maxwell–Garnett approximation to determine n_eff

n_eff = (ϕ_sn_s² + (1 − ϕ_s)n_m²)^1/2 (2)

where ϕ is the volume fraction of sphere, n_s and n_m are refractive indexes of sphere and matrix respectively. So band gap is governed by lattice parameter and effective refractive index. As shown in Fig. 2a the bandgap of the IOPC (530 nm) had obvious blue shift compared with that of the opal P(St-MMA-AA) (788 nm), which was attributable to the decrease of refractive index n_s (eqn (2)) caused by air spheres instead of polymer spheres. And it can be seen that the bandgap of the mIOPC (446 nm) had a blue shift compared with normal IOPC (526 nm), ascribed to the mesoporous structure which brought about the decrease of average refractive index n_m (eqn (2)). In other words, the mesopores decreased the volume fraction of silica matrix with high relative refraction index, while increased the volume fraction of air sphere with low relative refraction index. The reflection spectra for mIOPC, mIOPC–Ph (1 : 9), mIOPC–Ph (1 : 2) shown in Fig. 2b presents the bandgap positions at 448 nm, 498 nm, 530 nm respectively. The diversity in bandgap position of these mesoporous IOPC was caused by the difference in effective refractive index. Briefly speaking, the introduction of the Ph groups led to the increase of the effective refractive index n_m (eqn (2)), which resulted in the red shift of the bandgap of the mesoporous IOPC.

Fig. 2 (a) The reflection spectra of opal PC, IOPC and mIOPC. (b) The reflection spectra of mIOPC, mIOPC–Ph (1 : 9), mIOPC–Ph (1 : 2).

pH sensitive optical change of mesoporous IOPC films

To investigate the influence of mesoporous structure on shift of bandgap caused by invading of liquid, we prepared IOPC without surfactant named IOPC–Ph (1 : 2) which also modified Ph groups (molar ratio of PhAPTMS to TEOS was 1 : 2). Both IOPC–Ph (1 : 2) and mIOPC–Ph (1 : 2) were immersed in pH 1.0 solution, and the bandgaps changing with time were recorded in Fig. 3. It can be seen that, the average bandgap of IOPC–Ph (1 : 2) in air was 561 nm, while that of mIOPC–Ph (1 : 2) was 525 nm. And the bandgap of mIOPC–Ph (1 : 2) in solution didn't exceed that of IOPC–Ph (1 : 2), because the refractive index of water (1.33) is less than index of silica (1.45), which made the average refractive index of mIOPC–Ph (1 : 2) in solution be less than that of IOPC–Ph (1 : 2). The inset of Fig. 3 shows the cumulative shift of bandgaps (Δ bandgap) for IOPC–Ph (1 : 2) and mIOPC–Ph (1 : 2) respectively. In the first 1 min, solution invaded in both IOPC–Ph (1 : 2) and mIOPC–Ph (1 : 2) quickly caused by the surface switching from hydrophobicity to hydrophilicity. The invasion of solution gave rise to air spheres with low relative refraction index replaced by water spheres with high relative refraction index. The increase of refractive index n_s (eqn (2)) led to the red shift of bandgap for both IOPC–Ph (1 : 2) and mIOPC–Ph (1 : 2). But the mIOPC–Ph (1 : 2) had more shift degree than IOPC–Ph (1 : 2), it was because that there was a little liquid invading in the mesoporous structure of mIOPC–Ph (1 : 2). Besides, the refractive index n_m (eqn (2)) of mIOPC–Ph (1 : 2) was less than that of IOPC–Ph (1 : 2), so the increase of refractive index n_s (eqn (2)) had more influence on effective refractive index n_eff (eqn (2)) to the mIOPC–Ph (1 : 2). In the next time, with more and more Ph groups in the mesoporous structure were protonized, more liquid invaded in the mesopores, which resulted in the increase of the average refractive index n_m (eqn (2)). So there was more red shift of mIOPC–Ph (1 : 2) than that of IOPC–Ph (1 : 2) without mesoporous structure under the same condition.

Fig. 3 Bandgaps of IOPC–Ph (1 : 2) (without surfactant) and mIOPC–Ph (1 : 2) (with surfactant) shift with time in pH 1.0 solution. The inset illustrates cumulative shift of bandgaps of IOPC–Ph (1 : 2) and mIOPC–Ph (1 : 2) in pH 1.0 solution.

To study the valve performance of the mesoporous IOPC films, we immersed the mIOPC–Ph (1 : 2) into solution with pH 1.0 and 7.0. It can be seen in Fig. 4a and b, the mesoporous IOPC film immersed in pH 1.0 solution had a red shift (from 530 nm to 605 nm) as time goes on, while the mesoporous IOPC film immersed in pH 7.0 solution had a stable bandgap positions. The red shift was caused by the increase of refractive index n_s (eqn (2)), as a result of the invading of solution. Furthermore, it was the protonation of the Ph groups that caused the mesoporous IOPC film to switch from hydrophobicity to hydrophilicity. Until surface tension could not hold the liquid on the mesoporous IOPC films anymore, liquid invaded into mesoporous IOPC, and the valves switched on. The mesoporous IOPC film immersed in pH 7.0 just kept hydrophobicity constant which prevented liquid invading in, and the valves stayed off.

Fig. 4 (a) Reflectance spectra of mIOPC–Ph (1 : 2) vary with time (0.5–60 min) in pH 1.0 solution. (b) Reflectance spectra of mIOPC–Ph (1 : 2) vary with time (0.5–60 min) in pH 7.0 solution.

To analyze the influence of modified ratio of PhAPTMS on valve performance, we immersed mIOPC, mIOPC–Ph (1 : 9) and mIOPC–Ph (1 : 2) into solution with pH 1.0, 3.0, 5.0, and 7.0 respectively. The cumulative shift of bandgaps of different mesoporous IOPC in diverse pH solution were recorded. In pH 7.0 solution, the pH value was higher than the pK_a of the Ph group (pK_a = 5.02), so the Ph groups were difficult to be protonized. As shown in Fig. 5a, the bandgap of mIOPC, mIOPC–Ph (1 : 9), and mIOPC–Ph (1 : 2) shifted about 72 nm, 14 nm and 7 nm respectively in the first one minute. The bandgap shift of mIOPC was caused by the invading of liquid obviously, while the bandgap shift of mIOPC–Ph (1 : 9) and mIOPC–Ph (1 : 2) may attributed to the partial sink-in of liquid layer as a result of the balance between surface tension and hydraulic pressure. There were more Ph groups on the surface of mIOPC–Ph (1 : 2) than that on mIOPC–Ph (1 : 9), so mIOPC–Ph (1 : 2) had more surface tension to keep more liquid away from the surface of mesoporous IOPC, which led to less change of refractive index and showed less bandgap shift. The bandgaps became invariant quickly for different reasons, in the case of mIOPC it was because the invasion of liquid reached the limit quickly, while for mIOPC–Ph (1 : 9) and mIOPC–Ph (1 : 2) it was the result of quick balance between the surface tension and hydraulic pressure. Fig. 5b shows the shift degree of mIOPC, mIOPC–Ph (1 : 9) and mIOPC–Ph (1 : 2) in pH 5.0 solution. Liquid also could invade into mIOPC quickly because of the hydrophilicity, so the bandgap shifted about 73 nm and became almost immobile after that. Because pH 5.0 was slightly less than the pK_a of the Ph group, the pH 5.0 solution could protonize the Ph groups on the surface and mesopores internal surface of mIOPC–Ph (1 : 9) step by step, which gave rise to the gradual red shift of bandgap. But there were more Ph groups on mIOPC–Ph (1 : 2) than that on mIOPC–Ph (1 : 9), which made it difficult for liquid invading in the structure. So the red shift of mIOPC–Ph (1 : 2) was less than mIOPC–Ph (1 : 9). In the case of pH 3.0 as shown in Fig. 5c, mIOPC had the same performance as in pH 5.0 and pH 7.0 solution. The pH 3.0 was much less than the pK_a of the Ph group, thus the protonation of Ph groups was more rapid. Accordingly, mIOPC–Ph (1 : 9) and mIOPC–Ph (1 : 2) in pH 3.0 solution had more red shift than that in pH 5.0 solution. But the degree of red shift of mIOPC–Ph (1 : 2) was less than that of mIOPC–Ph (1 : 9) because there were more Ph groups on mIOPC–Ph (1 : 2). With the increase of acidity, as shown in Fig. 5d, protonation occurred more rapidly with the increase of protons, because pH 1.0 was far less than the pK_a of the Ph group. The reaction was so fast that the performance of mIOPC–Ph (1 : 9) became closer to mIOPC. And the Ph groups on mIOPC–Ph (1 : 2) could be protonized thoroughly and gradually, so the red shift almost reached its limit, about 73 nm. All in all, with the increase of Ph groups, the capacity of solution to invade in the inverse opal and mesopores became negative, and showed less red shift. It is noteworthy that the shift degree of mIOPC–Ph (1 : 2) didn't exceed the mIOPC–Ph (1 : 9), and the same with mIOPC–Ph (1 : 9) as mIOPC. It was because that the more Ph groups on mesoporous IOPC, the bigger refractive index n_m (eqn (2)) it was. When the n_s (eqn (2)) changed, effective refractive index n_eff (eqn (2)) with small index n_m (eqn (2)) would change more than that with big n_m (eqn (2)).

Fig. 5 Cumulative shift of bandgaps of mIOPC, mIOPC–Ph (1 : 9) and mIOPC–Ph (1 : 2) in pH 7.0 (a), pH 5.0 (b), pH 3.0 (c), and pH 1.0 (d) solutions.

The valve performance of mesoporous IOPCs with different modification proportions of Ph groups in different pH solutions were also studied. The bandgaps of mIOPC, mIOPC–Ph (1 : 9) and mIOPC–Ph (1 : 2) changed along with time in different pH solutions were recorded as Fig. S2–S4.† It was obvious that, with the decrease of pH value, the solutions invade in the macropores and mesopores became more easily accessible, which causes more red shift of bandgap. And it can be seen from Fig. S4† that 1 : 2 was the right ratio of PhAPTMS to TEOS to distinguish different solution with pH value under pH 7.0. Thus, we chose mIOPC–Ph (1 : 2) as ideal pH sensitive mesoporous IOPC valve. Furthermore, the performance of mIOPC–Ph (1 : 2) under different pH solution in a relative long time has been studied as shown in Fig. 6. Protonation took place when pH lower than pK_a, and the lower pH value the more protonated Ph groups there were. As a result, the solution at pH 7.0 couldn't invade in mIOPC–Ph (1 : 2) at any time, while the solution at pH 1.0 invaded in the structure fast until it reached the limit at about 30 min, and then kept stable. The solution at pH 5.0 invaded in the structure gradually and showed a declining trend, and the shift degree was closer to the shift of pH 1.0 solution at longer times. The solution at pH 3.0 invaded the structure step by step too, but the degree of invasion was not as high as pH 5.0 solution, which formed a gentler red shift than pH 5.0 solution.

Fig. 6 Bandgaps of mIOPC–Ph (1 : 2) changed along with time in pH 7.0, pH 5.0, pH 3.0, and pH 1.0 solutions for a long time, 180 min.

Performance visual characterization

A small enough diameter hydrophobic pores remain dry in water and can withstand a high pressure difference, ΔP, defined by the Laplace equation:

ΔP > 4|Δγ|/D_pore = 4|γ cos θ|/D_pore (3)

where D_pore is the pore diameter. Δγ is the difference between the solid/vapor surface tension, γ_sv, and the solid/liquid surface tension, γ_sl. The value of Δγ is related to the contact angle on the pore solid, θ, through the Young equation:

Δγ = γ_sv − γ_sl = γ cos θ (4)

where γ is the liquid/vapor surface tension. The critical pressure exceeds 100 kPa even for the very modest contact angles, θ ∼ 94°, for D_pore = 300 nm, which could create a natural plug against water intrusion. However, when the CA becomes smaller than 93°, the plug disappears and the valve becomes open.

On the basis of the data of bandgap shift, mIOPC–Ph (1 : 2) was chosen to further investigate the convertible hydrophilic/hydrophobic property induced by Ph group. We measured the apparent CAs for four different pH solution on the surface of mIOPC–Ph (1 : 2), as shown in Fig. 7b, where 2.0 μl solution of pH 1.0, pH 3.0, pH 5.0, pH 7.0 were deposited, which showed apparent CAs of 82°, 92°, 103°, and 112°, respectively. It was the different protonation degree that resulted in the variation of CAs. Besides, we recorded the change of CAs with time from 5 s to 650 s (video S1†) and fit the data with liner function (Fig. 7c). The slope of the CA changing curves were −0.036 (R² = 0.993), −0.026 (R² = 0.992), −0.023 (R² = 0.979), and −0.022 (R² = 0.980) corresponding to pH 1.0, pH 3.0, pH 5.0, and pH 7.0 solutions respectively. We can see clearly in the video S1† that the CAs of pH 7.0 droplet decreased with the water evaporation. What is meaningful is that the speed of decrease of CAs, namely the slope of the curves, demonstrated the ability to invade in the valves. With the increase of pH value, it was more and more difficult to invade in the valves, which was in accordance with the bandgap shift data we had provided.

Fig. 7 (a) Photograph of a mIOPC–Ph (1 : 2) with different pH drops on it. (b) Optical images showing contact angles (CAs) of liquid drops on the surface of mIOPC–Ph (1 : 2), where 2.0 μL of pH 1.0, pH 3.0, pH 5.0, and pH 7.0 are deposited. (c) Change of CAs of different pH droplets on mIOPC–Ph (1 : 2). (d) Optical microscopy images of mesoporous IOPC at which different pH solution is deposited.

The relationship between CAs and visual color of different pH solution was shown vividly as Fig. 7a. The pH 1.0 droplet on the surface of mIOPC–Ph (1 : 2) had the smallest CA meanwhile showed red color, while pH 7.0 droplet had the biggest CA and showed green color. Between the pH 1.0 and pH 7.0 droplets were pH 3.0 and pH 5.0 droplets with middle CAs and yellow color. To prominently show the color on mIOPC–Ph (1 : 2), we used optical microscope to record the images as shown in Fig. 7d, which was in accordance with the data of CAs. The pH 7.0 solution showed green color because the solution couldn't invade in the opal structure, while the diverse colors of pH 5.0, pH 3.0, and pH 1.0 were resulted from the different degree of invasion in mesopores as shown in Scheme 1b.

Conclusion

In this paper, a pH sensitive mesoporous IOPC valve which could monitor the valve state by naked eyes was achieved through the conversion of hydrophobic/hydrophilic on the surface and internal surfaces of mesopores. The natural porous structure of IOPC was modified with PhAPTMS to produce convertible hydrophobic valves. Furthermore, the mesoporous structure was introduced to increase the shift of bandgap. The membrane kept hydrophobicity at neutral conditions and presented green color, but it kept open at slightly acidic conditions and presented yellow to red color determined by pH value and infiltration time. The mesoporous IOPC film could be used for free-blockage nanovalves with stable structure to show the state real time in visual, which made them good candidates for applications requiring efficient gating, status indicator, and structure stable. Therefore, our approach to creating convertible hydrophobic mesoporous inverse opal photonic crystals provides new opportunity in a wide range of photonic crystal and nano valve applications.

Acknowledgements

The authors would like to thank the NSFC (21171019, 51373023), Beijing Natural Science Foundation (2122038), and the Fundamental Research Funds for the Central Universities and NCET-11-0584.

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Footnote

† Electronic supplementary information (ESI) available: The band gaps of different IOPC–Ph changed along with time in diverse pH solutions; video for change of CAs of different pH drops on mIOPC–Ph (1 : 2). See DOI: 10.1039/c6ra08948h

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