A Facile Method to Assemble PNIPAM-Containing Microgel Photonic Crystals一种装配含PNIPAM的微凝胶光子晶体的简便方法
Hydrogels have attracted great attention for potential applications in numerous fields, such as controlled drug delivery,chemical separation, sensors, catalysis, and enzyme immobilization, because of their environmental sensitiveness. Their swelling/deswelling behavior is particularly interesting for various applications. Based on this property, tunable photonic crystals can be fabricated by integrating hydrogels into the structure of the photonic crystals. The thus obtained crystals have potential application as visible “intelligent” chemical sensors. In the meantime, hydrogel inverse opals have also been developed which can be used as pH, glucose, temperature, humidity, solvent or UV-light sensors.Up to now, there are only a few reports on poly(N-isopropyl acrylamide) (PNIPAM) based microgel photonic crystals, most of them being produced from PNIPAM hydrogel colloids by means of centrifugation—a technology that is more or less difficult to apply.Herein, we report a new, facile, and time-saving method, called high-temperature-induced hydrophobic assembly (HTIHS), to prepare PNIPAM-based microgel photonic crystals from PNIPAM-containing hydrogel colloids. PNIPAM contains-CONH- and -CH-(CH3)2 groups.Below the lower critical solution temperature (LCST=33�18C), PNIPAM is hydrophilic because of the formation of a hydrogen bond between the -CONH-groups and H2O.Above this temperature, however, PNIPAM becomes hydrophobic because of the association between -CH-(CH3)2-groups. By simply taking advantage of this property, we have been able to establish the HTIHS method to fabricate PNIPAM microgel photonic crystals. When the PNIPAM hydrogel colloids remain at high temperatures, the water contained in the microgel is gradually squeezed out from the colloids, and the shrunk colloids become harder and harder. This offers a facile way for the colloids to gradually deposit and assemble into an ordered structure.
由于其对环境的敏感性，水凝胶因其在许多领域中的潜在应用而受到极大关注，例如受控药物递送，化学分离，传感器，催化和酶固定。
它们的膨胀/消溶胀行为对于各种应用特别有意义。 基于这种性质，可以通过将水凝胶整合到光子晶体的结构中来制造可调谐光子晶体。
由此获得的晶体具有作为可见“智能”化学传感器的潜在应用。 与此同时，还开发了水凝胶反蛋白石，可用作pH，葡萄糖，温度，湿度，溶剂或紫外线光传感器。
到目前为止，关于基于聚（N-异丙基丙烯酰胺）（PNIPAM）的微凝胶光子晶体的报道很少，其中大部分都是通过离心技术从PNIPAM水凝胶胶体中生产出来的 - 这种技术或多或少难以应用。
在这里，我们报告了一种新的，简便的，省时的方法，称为高温诱导疏水组装（HTIHS），用于从含PNIPAM的水凝胶胶体制备基于PNIPAM的微凝胶光子晶体。
PNIPAM含有-CONH-和-CH-（CH3）2基团。
低于临界溶解温度（LCST = 33~18℃），PNIPAM是亲水的，因为在-CONH-基团和H2O之间形成氢键。
然而，高于该温度，由于-CH-（CH 3）2 - 基团之间的缔合，PNIPAM变得疏水。 通过简单地利用这一特性，我们已经能够建立HTIHS方法来制造PNIPAM微凝胶光子晶体。
当PNIPAM水凝胶胶体保持在高温下时，微凝胶中含有的水逐渐从胶体中挤出，收缩的胶体变得越来越硬。 这为胶体逐渐沉积和组装成有序结构提供了一种简便的方法。
We first studied the assembly conditions of the HTIHS method: We prepared PNIPAM microgel photonic crystals (MPCPNIPAM) at different sedimentation times (STs) (at 80 8C) from PNIPAM colloids using 5% methylenebisacrylamide (BIS) as cross-linking agent (see Experimental Section).Digital photos of the photonic crystals are shown in Figure 1a. Obviously, the size of the MPCPNIPAM films decreases and the structural colors of the crystals exhibit a blue shift when the ST is increased.All the photonic crystals obtained in this way exhibit more brilliant colors than those prepared by centrifugation. Figure 2 gives the relationships between the ST and the wavelength (lmax) of the reflection peak.As can be seen, lmax decreases gradually with increasing ST. The lmax from a single crystal can be evaluated by using a modified version of Bragg’s law, combined with Snell’s law, considering that the lattice constant is related to the (111) plane of a face-centered cubic
我们首先研究了HTIHS方法的装配条件：我们使用5％亚甲基双丙烯酰胺（BIS）作为交联剂，在PNIPAM胶体的不同沉降时间（STs）（80°C）下制备PNIPAM微凝胶光子晶体（MPCPNIPAM）（参见实验） 部分）。
光子晶体的数码照片如图1a所示。 显然，当ST增加时，MPCPNIPAM膜的尺寸减小并且晶体的结构颜色呈现蓝移。
以这种方式获得的所有光子晶体显示出比通过离心制备的那些更亮的颜色。 图2给出了ST与反射峰的波长（lmax）之间的关系。
可以看出，随着ST的增加，lmax逐渐减小。 考虑到晶格常数与面心立方的（111）面相关，可以使用布拉格定律的修正版本结合Snell定律来评估单晶的lmax。
At high temperatures, the diameter d of the microgels is reduced with increasing ST because of the loss of water, which causes a decrease in lmax, according to Equation (1). The microgel photonic crystals even become a transparent thin film at long STs (60 min).
在高温下，根据等式（1），由于水的损失，微凝胶的直径d随着ST的增加而减小，这导致lmax降低。 微凝胶光子晶体甚至在长ST（60分钟）时变成透明薄膜。
We also prepared PNIPAM-co-AA (AA=acrylic acid) microgel photonic crystals (MPCPNIPAM-co-AA) by using the HTIHS method at 80 8C.The obtained crystals also showed similar, brilliant structural colors. The structural colors of the MPCPNIPAM-co-AA change from red to green (and blue) when the ST is increased from 40 to 70 (and 100 min) at 80 8C, see Figure 1b. The lmax (shown in Figure 3) also exhibits a blue shift, which means that the increasing ST causes a deswelling of the PNIPAM-co-AA microgel and a decrease in its diameter.In both cases (MPCPNIPAM-co-AA and MPCPNIPAM), we can obtain very big crystals if we use large-enough containers—this is impossible with the centrifugation method. A comparison of the structural colors of these two kinds of MPCs indicates that a much longer ST is required for MPCPNIPAM-co-AA (as compared to MPCPNIPAM) to obtain the same green or blue color.Additionally, we were not able to obtain a red color in the case of MPCPNIPAM, which is due to the absence of the co-monomer AA. The PAA hydrogel is more hydrophilic and can absorb much water. It not only exhibits a high strength, but also shows a high water-keeping ability at high temperatures. When AA is copolymerized into the PNIPAM colloids, the copolymer hydrogel exhibits some properties of PAA.Therefore, this copolymer requires a longer time—as compared to pure PNIPAM—to reduce its water content. On the contrary, since PNIPAM colloids easily lose water at high temperatures, it is possible that the water content required to form red photonic crystals might have already past without the colloids having had enough time to arrange into an order structure. So, we were not able to obtain MPCs with a red structural color from pure PNIPAM colloids. Similarly, by using the centrifugation method, we could only prepare palegreen MPCPNIPAM. However, we could obtain red MPCPNIPAM with the HTIHS method if we used larger PNIPAM colloids.
我们还在80℃下使用HTIHS方法制备了PNIPAM-co-AA（AA =丙烯酸）微凝胶光子晶体（MPCPNIPAM-co-AA）。
所获得的晶体也显示出类似的明亮结构颜色。 当ST在80℃下从40增加到70（和100分钟）时，MPCPNIPAM-co-AA的结构颜色从红色变为绿色（和蓝色），见图1b。 lmax（图3中所示）也呈现蓝移，这意味着增加的ST导致PNIPAM-co-AA微凝胶的消溶胀和其直径减小。
在这两种情况下（MPCPNIPAM-co-AA和MPCPNIPAM），如果我们使用足够大的容器，我们可以获得非常大的晶体 - 离心方法是不可能的。 这两种MPC的结构颜色的比较表明，MPCPNIPAM-co-AA（与MPCPNIPAM相比）需要更长的ST以获得相同的绿色或蓝色。
另外，在MPCPNIPAM的情况下，我们无法获得红色，这是由于不存在共聚单体AA。 PAA水凝胶更亲水，可吸收大量水分。 它不仅具有高强度，而且在高温下也具有高保水性。 当AA共聚到PNIPAM胶体中时，共聚物水凝胶表现出PAA的一些性质。
因此，与纯PNIPAM相比，该共聚物需要更长的时间来降低其含水量。 相反，由于PNIPAM胶体在高温下容易失水，因此形成红色光子晶体所需的水含量可能已经过去，而胶体没有足够的时间排列成有序结构。 因此，我们无法从纯PNIPAM胶体中获得具有红色结构色的MPC。 同样，通过使用离心方法，我们只能制备淡绿色的MPCPNIPAM。 但是，如果我们使用更大的PNIPAM胶体，我们可以使用HTIHS方法获得红色MPCPNIPAM。
The structural color of MPCPNIPAM-co-AA is mainly dependent on the water content for a specific polymeric composition if the colloidal size is fixed. Figure 3 gives the relationships between lmax, the water content, and the ST. The ST actually controls the water content, which subsequently determines lmax or the structural color of the MPCPNIPAM-co-AA. The curves in Figure 3 indicate that the water content decreases almost linearly when the color of the photonic crystal changes from red to blue. The variation of the water content between green and blue is smaller than that between red and green; lmax shows a similar trend.
如果胶体尺寸是固定的，MPCPNIPAM-co-AA的结构颜色主要取决于特定聚合物组合物的水含量。 图3给出了lmax，含水量和ST之间的关系。 ST实际上控制水含量，其随后确定lmax或MPDNIPAM-co-AA的结构颜色。 图3中的曲线表明，当光子晶体的颜色从红色变为蓝色时，水含量几乎呈线性下降。 绿色和蓝色之间的含水量变化小于红色和绿色之间的含水量; lmax显示出类似的趋势。
As discussed above, a high temperature makes the PNIPAM colloids hydrophobic and induces their ordered arrangement. Thus, the temperature is an important factor for PNIPAM-colloid assembly. We investigated the assembly at various temperatures, namely, 70, 60, and 50C, and compared the obtained MPCPNIPAM-co-AA (see Figure 1 b). At 70 8C, we still obtain MPCPNIPAM-co-AA with structural colors from red to blue by changing the ST, even though the needed time is longer for getting a specific color than that at 80C. As the assembling temperature is further decreased (for example, to 60 or 50 C), the time needed to get red MPCPNIPAM-co-AA becomes even longer. The red color is clear, but it is inhomogeneous. Actually, it is even difficult to get homogeneous green or blue MPCPNIPAM-co-AA in a reasonable time.
如上所述，高温使PNIPAM胶体疏水并诱导它们的有序排列。 因此，温度是PNIPAM-胶体组装的重要因素。 我们在各种温度即70,60和50℃下研究了组件，并比较了获得的MPCPNIPAM-co-AA（参见图1b）。 在70℃，我们仍然通过改变ST获得具有从红色到蓝色的结构颜色的MPCPNIPAM-co-AA，即使获得特定颜色所需的时间比在80℃时更长。 随着组装温度进一步降低（例如，降至60或50℃），获得红色MPCPNIPAM-co-AA所需的时间变得更长。 红色是清晰的，但它是不均匀的。 实际上，甚至很难在合理的时间内获得均质的绿色或蓝色MPCPNIPAM-co-AA。
The above discussion confirms that the sedimentation temperature plays an important role in the assembly of microgel photonic crystals. High temperatures are favorable for the formation of MPCPNIPAM-co-AA at short times, but some bubbles are found on the surface of the MPCPNIPAM-co-AA film when the temperature increases over 908C. Therefore, the temperature should be appropriately chosen. At a given temperature, the ST has a direct effect on the water content. Longer times lead to less water content in the microgel, which brings about a blue shift of both the structural color and lmax. By using the HTIHS method, we can easily obtain any size and color (between red, green or blue) of MPCPNIPAM-co-AA.
上述讨论证实沉降温度在微凝胶光子晶体的组装中起重要作用。 高温有利于在短时间内形成MPCPNIPAM-co-AA，但当温度升高超过908℃时，在MPCPNIPAM-co-AA膜的表面上发现一些气泡。 因此，应适当选择温度。 在给定温度下，ST对水含量具有直接影响。 更长的时间导致微凝胶中的水含量更少，这导致结构颜色和lmax的蓝移。 通过使用HTIHS方法，我们可以轻松获得MPCPNIPAM-co-AA的任何大小和颜色（红色，绿色或蓝色之间）。
The PNIPAM hydrogel is hydrophilic at room temperature— and so are the obtained MPCPNIPAM-co-AA, which easily absorb moisture upon contact with water. According to Bragg’s law, water content directly affects the diameter of PNIPAM-co-AA microgels and consequently causes changes in lmax or structural color of the MPCPNIPAM-co-AA. Figure 1c shows the reversible changes in the structural color of MPCPNIPAM-co-AA when in contact with water and dry air. The color changes from blue to red when the crystals absorb some water because of gradual swelling. Reversely, the structural color shifts to blue when water is evaporated from the MPCPNIPAM-co-AA into the air; finally, both the structural color and the volume return to their original state. This distinct change in color demonstrates that the response of the MPCPNIPAM-co-AA to water is reversible and perceptible with the naked eye. Figure 4 displays the blue shift of lmax from 662 to 464 nm during the air-drying process. The relationship between water content and lmax (structure color) is almost linear. Generally, MPCPNIPAM-co-AA with 71–80% water are red whereas the crystals with 55–70% water are green and those with 28–54% water are blue or purple.
PNIPAM水凝胶在室温下是亲水的 - 所获得的MPCPNIPAM-co-AA也是亲水的，其在与水接触时容易吸收水分。根据布拉格定律，水含量直接影响PNIPAM-co-AA微凝胶的直径，从而引起MPCPNIPAM-co-AA的lmax或结构颜色的变化。图1c显示了MPCPNIPAM-co-AA与水和干燥空气接触时结构颜色的可逆变化。当晶体因逐渐膨胀而吸收一些水时，颜色从蓝色变为红色。相反，当水从MPCPNIPAM-co-AA蒸发到空气中时，结构颜色变为蓝色;最后，结构颜色和体积都恢复到原始状态。这种明显的颜色变化表明MPCPNIPAM-co-AA对水的响应是可逆的并且用肉眼可察觉。图4显示了在空气干燥过程中lmax从662nm到464nm的蓝移。水含量与lmax（结构颜色）之间的关系几乎是线性的。通常，具有71-80％水的MPCPNIPAM-co-AA是红色而具有55-70％水的晶体是绿色而具有28-54％水的晶体是蓝色或紫色。
MPCPNIPAM-co-AA are also sensitive to acetone, with their structural colors changing gradually from blue—through green—to pink when acetone is continually absorbed (see Figure 1d, top). A blue shift is observed with the evaporation of this solvent, but finally, the original structural color can almost be recovered (see Figure 1d, bottom). According to Bragg’s law, the structural color of a photonic crystal is mainly dependent on the average refractive index na and the diameter d of the colloids (na depending on the solvent). The refractive indexes of pure water and acetone are 1.3327 and 1.3588, respectively. The difference is so small that the na cannot be the reason for the change in the structural color when acetone is absorbed into or desorbed from the MPCPNIPAM-co-AA. This suggests that the PNIPAM-co-AA hydrogel colloids swell at higher acetone contents, which causes a red shift in the structural color. Since acetone easily evaporates in air, the MPCPNIPAM-co-AA show a reverse change in their structural color during evaporation.
MPCPNIPAM-co-AA对丙酮也很敏感，当丙酮不断被吸收时，它们的结构颜色从蓝到绿逐渐变为粉红色（见图1d，上图）。随着该溶剂的蒸发，观察到蓝移，但最终，原始结构颜色几乎可以恢复（参见图1d，底部）。根据布拉格定律，光子晶体的结构颜色主要取决于平均折射率na和胶体的直径d（na取决于溶剂）。纯水和丙酮的折射率分别为1.3327和1.3588。差异非常小，以至于当丙酮被MPCPNIPAM-co-AA吸收或解吸时，na不能成为结构颜色变化的原因。这表明PNIPAM-co-AA水凝胶胶体在较高的丙酮含量下膨胀，这导致结构颜色的红移。由于丙酮在空气中容易蒸发，因此MPCPNIPAM-co-AA在蒸发过程中显示出其结构颜色的反向变化。
The hydrogel is quite hydrophilic and has a tendency to absorb and retain water, a trend that depends on the degree of cross-linking (BIS content) of the polymer network. Therefore, we have prepared PNIPAM-co-AA colloids with three different cross-linking degrees and have studied the properties of the obtained MPCPNIPAM-co-AA, assembled by using the HTIHS method. The microgel photonic crystals in Figure 1b are all assembled from PNIPAM-co-AA colloids with 5% BIS. PNIPAM-coAA colloids with 3% BIS are difficult to assemble into microgel photonic crystals, whereas those with 7% BIS are more easy to produce. Figure 1e shows optical photos of such crystals, assembled at 80C. To obtain green or blue photonic crystals, much shorter STs are required for colloids with 7% BIS. For green photonic crystals, the difference in the ST is 50 minutes (i.e. 5% BIS: 70 min; 7% BIS: 20 min). This difference almost equals that between the two blue crystals (i.e. 5% BIS: 100 min; 7% BIS: 50 min). So, we can also obtain color-tunable photonic crystals by adjusting the degree of cross-linking of the hydrogel colloids.
水凝胶非常亲水并且具有吸收和保留水的趋势，这一趋势取决于聚合物网络的交联度（BIS含量）。因此，我们制备了具有三种不同交联度的PNIPAM-co-AA胶体，并研究了使用HTIHS方法组装的所得MPCPNIPAM-co-AA的性质。图1b中的微凝胶光子晶体全部由具有5％BIS的PNIPAM-co-AA胶体组装而成。具有3％BIS的PNIPAM-coAA胶体难以组装成微凝胶光子晶体，而具有7％BIS的那些胶体更容易生产。图1e显示了在80℃下组装的这种晶体的光学照片。为了获得绿色或蓝色光子晶体，具有7％BIS的胶体需要更短的ST。对于绿色光子晶体，ST的差异是50分钟（即5％BIS：70分钟; 7％BIS：20分钟）。该差异几乎等于两个蓝色晶体之间的差异（即5％BIS：100分钟; 7％BIS：50分钟）。因此，我们还可以通过调节水凝胶胶体的交联度来获得颜色可调的光子晶体。
In conclusion, high-temperature-induced hydrophobic assembly (HTIHS) has been successfully applied as a convenient method for creating brilliant PNIPAM and PNIPAM-co-AA microgel photonic crystals from hydrogel colloids. Both the sedimentation temperature and sedimentation time affect the structural colors (or lmax) of the microgel photonic crystals. High temperatures lead to an easier assembly of the colloids into ordered structures with a suitable water content that directly determines the structural color of the microgel photonic crystals. We can produce large MPCPNIPAM-co-AA, MPCPNIPAM or other PNIPAM containing MPCs by using the HTIHS method, which is impossible with the centrifugation method. The obvious changes in color demonstrate that the responses of the MPCPNIPAM-co-AA to water and acetone are reversible and perceptible with the naked eye. The degree of cross-linking of the colloids also affects the structural colors or the lmax value of the MPCPNIPAM-co-AA. We believe that this remarkably simple technique represents a facile route to fabricate PNIPAM-based microgel photonic crystals with promising applications in sensors, detectors or even chemical and surface modification.
总之，高温诱导疏水组装（HTIHS）已成功应用于从水凝胶胶体中产生明亮的PNIPAM和PNIPAM-co-AA微凝胶光子晶体的方便方法。沉降温度和沉降时间都影响微凝胶光子晶体的结构颜色（或lmax）。高温导致胶体更容易组装成具有合适水含量的有序结构，其直接决定了微凝胶光子晶体的结构颜色。我们可以使用HTIHS方法生产大型MPCPNIPAM-co-AA，MPCPNIPAM或其他含有MPC的MPIP，离心法是不可能的。颜色的明显变化表明MPCPNIPAM-co-AA对水和丙酮的反应是可逆的并且用肉眼可察觉。胶体的交联度也影响MPCPNIPAM-co-AA的结构颜色或lmax值。我们相信这种非常简单的技术代表了制造基于PNIPAM的微凝胶光子晶体的简便途径，在传感器，探测器甚至化学和表面改性方面具有广阔的应用前景。
Experimental Section
Monodisperse PNIPAM-based colloids were prepared by means of precipitation polymerization at 70C. Briefly, NIPAM (2.0 g), a proper amount of AA (acrylic acid), and a cross-linking monomer BIS (N,N’-methylenebisacrylamide) were dissolved in deionized water (140 mL) in a 250 mL four-neck flask equipped with a condenser, a nitrogen inlet, a stirrer, and a thermometer. Nitrogen was bubbled into the solution, and the vessel was kept at 70C. After temperature equilibrium and oxygen removal, potassium persulfate (0.2 g), dissolved in 20 mL deionized water, was added into the flask to initiate polymerization. The reaction was continued for six hours under mild stirring.
通过在70℃下的沉淀聚合制备单分散的PNIPAM基胶体。 简而言之，将NIPAM（2.0g），适量的AA（丙烯酸）和交联单体BIS（N，N'-亚甲基双丙烯酰胺）溶解在250mL四颈烧瓶中的去离子水（140mL）中。 配有冷凝器，氮气入口，搅拌器和温度计。 将氮气鼓入溶液中，并将容器保持在70℃。 在温度平衡和除氧后，将溶解在20mL去离子水中的过硫酸钾（0.2g）加入烧瓶中以引发聚合。 在温和搅拌下继续反应6小时。
PNIPAM microgel photonic crystals were assembled from the above PNIPAM emulsion by using the high-temperature-induced hydrophobic assembly (HTIHS) method. A container with a PNIPAM emulsion (10 mL) was placed in an oven at a certain temperature (for example, 80C). Then, water was extracted from the colloids and moved out at high temperatures, owing to a transition from the hydrophilic to the hydrophobic state, and the PNIPAM colloids assembled into a milky mass. After a certain time, for example, 40 min, the container was taken out and placed horizontally on a table. After about 2 to 20 min of balance time, the PNIPAM microgel photonic crystals appeared exhibiting brilliant colors.
PNIPAM微凝胶光子晶体由上述PNIPAM乳液通过使用高温诱导的疏水组装（HTIHS）方法组装而成。 将具有PNIPAM乳液（10mL）的容器置于特定温度（例如，80℃）的烘箱中。 然后，由于从亲水状态到疏水状态的转变，从胶体中提取水并在高温下移出，并且PNIPAM胶体组装成乳状物质。 在一定时间后，例如40分钟，取出容器并水平放置在桌子上。 经过约2至20分钟的平衡时间后，PNIPAM微凝胶光子晶体呈现出鲜艳的色彩。
The structural colors of the microgel photonic crystals were recorded with a Nikon CoolPix4300 digital camera, and the reflection spectra were measured using an AvaSpec-2048-SPU fiber-optic spectrometer.
用Nikon CoolPix4300数码相机记录微凝胶光子晶体的结构颜色，并使用AvaSpec-2048-SPU光纤光谱仪测量反射光谱。
Keywords: colloids · photonic crystals · reflection spectra · solvent effects · structural colors
关键词：胶体·光子晶体·反射光谱·溶剂效应·结构色
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