Polymerized Microgel Colloidal Crystals: Photonic Hydrogels with Tunable Band Gaps and Fast Response Rates
聚合微凝胶胶体晶体：具有可调谐带隙和快速响应速率的光子水凝胶
Colloidal crystals, three-dimensional periodic arrays of submicron particles, have attracted considerable attention because of their novel applications in a wide range of disciplines, especially as photonic band-gap materials. Usually monodisperse hard microspheres, such as polystyrene (PS), poly(methyl methacrylate) (PMMA), and SiO2 spheres, are used to assemble colloidal crystals. However, soft hydrogel microspheres, especially poly(N-isopropylacrylamide) (PNIPAM) microgels, can also self-assemble into highly ordered colloidal crystals. In contrast to the hard spheres, PNIPAM microgels are stimuli-responsive. In addition, microgel colloidal crystals are intrinsically defect-tolerant, thanks to the soft nature of the particles. Therefore it is believed that PNIPAM microgels are promising candidates for the fabrication of large arrays of colloidal crystals. Unfortunately the ordered structure of microgel colloidal crystals is fragile. It remains an unresolved problem for their use in the large-scale production of photonic crystals.
胶体晶体，亚微米颗粒的三维周期性阵列，由于其在广泛的学科中的新应用，特别是作为光子带隙材料，因此引起了相当大的关注。通常使用单分散硬微球，例如聚苯乙烯（PS），聚（甲基丙烯酸甲酯）（PMMA）和SiO 2球，来组装胶体晶体。 然而，软水凝胶微球，尤其是聚（N-异丙基丙烯酰胺）（PNIPAM）微凝胶，也可以自组装成高度有序的胶体晶体。 与硬球相比，PNIPAM微凝胶具有刺激响应性。此外，由于颗粒的柔软性质，微凝胶胶体晶体本质上是耐缺陷的。 因此，人们认为PNIPAM微凝胶是制备大量胶体晶体的有希望的候选者。不幸的是，微凝胶胶体晶体的有序结构是脆弱的。它仍然是用于大规模生产光子晶体的一个尚未解决的问题。
Some efforts have been made to solve this problem. One way is to embed the colloidal crystals in a hydrogel matrix. Unfortunately the matrix may significantly reduce the extent of the volume phase transition of the microgel particles. Alternatively the microgel particles in colloidal crystals can be crosslinked directly, using a suitable crosslinker. However addition of the crosslinker solution may disturb the preformed ordered structure. Cross-linker diffusion is also time-consuming and always results in a heterogeneous distribution of the crosslinker. The cross-linking reactions are not efficient enough and may require harsh conditions. A new method was developed later in which P(NIPAM-NMA) microgels (NMA: N-hydroxymethylacrylamide) cross-link simultaneously when the microgels assemble into ordered structures at the air–water surface, taking advantage of the self-cross-linking property of NMA. This method is also time-consuming and difficult to control. Herein we propose a facile method to cross-link microgel colloidal crystals. The PNIPAM microgel particles, with polymerizable vinyl groups on their surface, first selfassemble into highly ordered colloidal crystals. Then the ordered structure is locked by light-initiated free-radical polymerization of the surface-bonded vinyl groups (see Scheme 1). The resulting polymerized microgel colloidal crystals (PMCC) can respond to both temperature and salt, and the band gap can be tuned in a wide range. In contrast to colloidal crystals embedded in a hydrogel matrix, the swelling of the microgel particles is almost unaltered. In addition the response of PMCC is quite fast.
已经做出一些努力来解决这个问题。 一种方法是将胶体晶体嵌入水凝胶基质中。 不幸的是，基质可以显着降低微凝胶颗粒的体积相变的程度。或者，胶体晶体中的微凝胶颗粒可以使用合适的交联剂直接交联。然而，添加交联剂溶液可能会干扰预先形成的有序结构。 交联剂扩散也是耗时的并且总是导致交联剂的不均匀分布。交联反应不够有效，可能需要苛刻的条件。 后来开发了一种新方法，其中当微凝胶在空气 - 水表面组装成有序结构时，P（NIPAM-NMA）微凝胶（NMA：N-羟甲基丙烯酰胺）同时交联，利用NMA的自交联性质 。该方法也很耗时且难以控制。 在这里，我们提出了一种简便的方法来交联微凝胶胶体晶体。PNIPAM微凝胶颗粒表面具有可聚合的乙烯基，首先自组装成高度有序的胶体晶体。然后通过表面键合的乙烯基的光引发的自由基聚合来锁定有序结构（参见方案1）。 所得聚合的微凝胶胶体晶体（PMCC）可以对温度和盐都起反应，并且带隙可以在很宽的范围内调节。 与嵌入水凝胶基质的胶体晶体相反，微凝胶颗粒的溶胀几乎不变。 此外，PMCC的反应非常快。
Polymerizable vinyl groups were introduced onto the surface of the microgel particles by modification of a P(NIPAM-AAc) microgel (AAc: acrylic acid) with 2hydroxyethyl methacrylate (HEMA) using N-(3-dimethylaminopropyl)-N’-ethylcarbodiimide hydrochloride (EDC) as catalyst (Scheme 1A, Supporting Information, Figure S1 and S2). An approximately 4.3 wt% solution of the HEMA-modified microgels, containing photo-initiator DEAP, were then prepared and allowed to self-assemble into highly ordered colloidal crystals (Scheme 1B).[3–5] With the formation of crystalline structure, the dispersion becomes iridescent, however, the ordered structure can be easily destroyed by heating or shearing.[18] To stabilize it, the dispersion was exposed to UV irradiation. With the polymerization of the surface vinyl groups, a cross-linked hydrogel film was obtained. Direct visual inspection reveals that the crystalline structure is intact after polymerization. The sample is still iridescent (Figure 1A), with crystallites ranging from hundreds of micrometers to millimeters in diameter. More importantly, a sharp diffraction peak was observed from the reflection spectra of the polymerized sample, confirming again its periodical ordered structure (Figure 1B).
通过使用N-（3-二甲基氨基丙基）-N'-乙基碳二亚胺盐酸盐改性P（NIPAM-AAc）微凝胶（AAc：丙烯酸）和甲基丙烯酸2-羟乙酯（HEMA），将可聚合的乙烯基团引入微凝胶颗粒的表面上（ EDC）作为催化剂（方案1A，支持信息，图S1和S2）。 然后制备含有光引发剂DEAP的约4.3wt％HEMA方案改性微凝胶溶液，并使其自组装成高度有序的胶体晶体（方案1B）。随着晶体结构的形成，分散体变成虹彩，然而，有序结构可以通过加热或剪切容易地破坏。 为了稳定它，将分散体暴露于UV照射。 通过表面乙烯基的聚合，获得交联的水凝胶膜。 直接目视检查显示聚合后晶体结构完整。 样品仍为彩虹色（图1A），微晶直径为数百微米至毫米。更重要的是，从聚合样品的反射光谱中观察到尖锐的衍射峰，再次证实了其周期性有序结构（图1B）。
The PMCC films remain thermosensitive. As shown in Figure 1A, the film is highly swollen and red in color at 18C. Upon heating, it shrinks gradually. Simultaneously, its color turns gradually to green (228C), blue (248C), and finally white(278C).Figure 1Bshowsthatthediffractionpeakofthe film gradually shifts to a shorter wavelength with increasing temperature. At 18C, the peak centers at 703 nm. It moves to 549, 482, and 405 nm when heated to 22, 24, and 258C, respectively.Note that the diffraction peak of the PMCC film, or the band gap, can be finely tuned in the whole visible spectrum, simply by changing temperature. The color change observedinFigure 1Aisinexcellentagreementwiththeshift of the diffraction peak. When heated to 278C, the peak shifts into the UV range, therefore the film becomes white (Figure 1A).
PMCC薄膜保持热敏性。 如图1A所示，该膜在18℃下高度溶胀并呈红色。 在加热时，它逐渐收缩。 同时，其颜色逐渐变为绿色（228℃），蓝色（248℃），最后变为白色（278℃）。图1B显示随着温度升高，膜的衍射峰逐渐变为更短的波长。 在18℃时，峰值中心在703nm处。 当加热到22,24和258℃时，它分别移动到549,482和405nm。注意PMCC膜的衍射峰或带隙可以在整个可见光谱中微调，只需改变 温度。 图1A中观察到的颜色变化与衍射峰的偏移非常一致。 当加热到278℃时，峰值会转移到UV范围内，因此膜变白（图1A）。
The heat-induced shift of diffraction peak is attributed to the shrinkageof the microgel particles. The size of thefree particles dispersed in the same aqueous solution was measured by dynamic light scattering.As shown in Figure 1C, the particles shrink gradually with increasing temperature. Fast shrinkage was observed at 238C, which can be identified as the onset of phase transition. The particles in PMCC films should experience a similar heat-induced shrinkage. As a result the lattice constant of the crystal decreases (Scheme 1C) and the diffraction peak shifts to a shorter wavelength. The shrinkage of the microgel particles also results in an increased refractive index. As the dielectric constant modulation increases, light is diffracted more efficiently, therefore the intensity of the diffraction peak increases with increasing temperature. (Figure 1B)
衍射峰的热诱导偏移归因于微凝胶颗粒的收缩。 通过动态光散射测量分散在相同水溶液中的游离颗粒的尺寸。如图1C所示，颗粒随着温度的升高而逐渐收缩。 在238℃观察到快速收缩，这可以确定为相变的开始。 PMCC薄膜中的颗粒应经历类似的热诱导收缩。 结果，晶体的晶格常数减小（方案1C），衍射峰移向较短的波长。 微凝胶颗粒的收缩也导致折射率增加。 随着介电常数调制的增加，光被更有效地衍射，因此衍射峰的强度随着温度的升高而增加。 （图1B）
Immobilization of a PNIPAM microgel usually results in a reduced thermo-responsivity. When adsorbed onto a solid substrate, the swelling capacity can be reduced by one order of magnitude. When embedded in hydrogel matrixes, the swelling of the particles can also be reduced to a large degree. To study the effect of immobilization, the size of the microgel particles in the PMCC films was determined from the diffraction wavelength, l, which is described by Bragg�s law [Eq. (1)]:
PNIPAM微凝胶的固定通常导致热响应性降低。 当吸附在固体基质上时，溶胀能力可以降低一个数量级。当嵌入水凝胶基质中时，颗粒的溶胀也可以在很大程度上降低。为了研究固定化的效果，大小 PMCC薄膜中的微凝胶颗粒由衍射波长l确定，其由布拉格定律[方程式1]描述。（1）]：
where h, k, and l are the Miller indices, q the diffraction angle (908 for normal incidence), a the lattice constant, and n the refractive index of the film. PNIPAM microgel colloidal crystals are reported to have a bcc or fcc structure. Assuming a bcc lattice and identifying the observed diffraction peak as the (1,1,0) reflection, or assuming an fcc lattice and identifying the diffraction peak as the (1,1,1) reflection, the shortest distance between two microgel particles in the film, or the diameter of the particles in the film, Df, can be calculated from the diffraction wavelength lm, using the same equation [Eq. (2)]:
其中h，k和l是米勒指数，q是衍射角（垂直入射908），晶格常数，和n是膜的折射率。 据报道，PNIPAM微凝胶胶体晶体具有bcc 或fcc结构假设bcc晶格并将观察到的衍射峰识别为（1,1,0）反射，或假设为fcc晶格 并且将衍射峰识别为（1,1,1）反射，膜中两个微凝胶颗粒之间的最短距离，或膜中颗粒的直径Df，可以从衍射波长lm计算，使用 相同的等式[Eq。（2）]：
Using Equation (2), and taking n of the film as that of water (ca. 1.335) because of the high water content of the film, Df values at various temperatures were determined from the corresponding lm. As expected, Df decreases with increasing temperature, following a same trend as Dh.(Figure 1C) In addition, Df is comparable to Dh measured at the same temperature. In other words, immobilization of the microgel particles does not affect their swelling. Although cross-linked with neighboring particles in the film, they can still swell freely as those free particles. This behavior is very different from other immobilized microgel particles.
使用等式（2），并且由于膜的高含水量，将n的膜取为水的膜（约1.335），从相应的lm确定在各种温度下的Df值。 正如预期的那样，Df随着温度的升高而降低，遵循与Dh相同的趋势（图1C）。此外，Df与在相同温度下测量的Dh相当。 换句话说，微凝胶颗粒的固定不会影响它们的溶胀。 尽管与薄膜中的相邻颗粒交联，但它们仍然可以像那些自由颗粒一样自由膨胀。 这种行为与其他固定的微凝胶颗粒非常不同。
PNIPAM microgels are not only sensitive to temperature, but also ionic strength. Therefore ionic strength can also beusedasanexternalstimulustotunethecolorandbandgap of PMCC films. Figure 2A shows a PMCC film is almost transparent and colorless at [NaCl]=10 mm. It turns to red, green, and blue, when [NaCl] increases to 40, 256, and 562 mm, respectively. The corresponding reflection spectra of the film are shown in Figure 2B. With increasing [NaCl], the diffraction peak shifts gradually towards short wavelength, from 760 nm at [NaCl]=10 mm, to 403 nm at [NaCl]= 667 mm. Thelm is 760, 663, 561 and 452 nm, when [NaCl] is 10, 40, 256, and 562 mm, respectively, which corresponds well withthecolor.Inadditiontheintensityofthediffractionpeak increaseswithincreasing[NaCl].Allthesephenomenacanbe explained by the salt-induced deswelling of the microgel particles in the film, which results in a smaller lattice constant of the film and an increased refractive index of the microgel particles. Diameter of free particles, Dh, and diameter of particles in the film, Df, were also determined. As shown in Figure 2C, both Dh and Df decrease with increasing [NaCl]. At the same [NaCl], Df is comparable to Dh, suggesting again that the swelling of the microgel particles is not affected by immobilization.RelativelylargedeviationbetweenDh andDf at high [NaCl] may be explained by the partial aggregation of PNIPAM particles at high [NaCl]. As a result, Dh of the free particlesmeasuredbylightscatteringmaybelargerthantheir real size.
PNIPAM微凝胶不仅对温度敏感，而且对离子强度也敏感。因此，离子强度也可用作外部刺激以调节PMCC膜的颜色和带隙。图2A显示PMCC膜在[NaCl] = 10mm时几乎是透明和无色的。当[NaCl]分别增加到40,256和562 mm时，它变为红色，绿色和蓝色。薄膜的相应反射光谱如图2B所示。随着[NaCl]的增加，衍射峰逐渐向短波长移动，从[NaCl] = 10mm处的760nm，到[NaCl] = 667mm处的403nm。当[NaCl]分别为10,40,256和562 mm时，lm为760,663,561和452 nm，与颜色对应良好。此外，衍射峰的强度随着[NaCl]的增加而增加。所有这些现象可以通过盐诱导的膜中微凝胶颗粒的消溶胀来解释，这导致膜的晶格常数更小并且微凝胶颗粒的折射率增加。还测定了自由颗粒的直径Dh和膜中颗粒的直径Df。如图2C所示，Dh和Df都随着[NaCl]的增加而减少。在相同的[NaCl]下，Df与Dh相当，再次表明微凝胶颗粒的膨胀不受固定的影响。在高[NaCl]下Dh和Df之间的相对大的偏差可以通过PNIPAM颗粒的部分聚集来解释。高[NaCl]。结果，通过光散射测量的自由粒子的Dh可能大于它们的实际尺寸。
Because the volume phase transition of PNIPAM microgels is reversible, the response of PMCC films to external stimuli, in terms of both temperature and ionic strength, is also reversible. As an example, Figure 1A shows that, after a heating/cooling cycle, the color of the film is largely recovered when temperature is restored to be 208C. Similar recovery of the film color was found when [NaCl] is adjusted back to be 250 mm (Figure 2A). Figure S3 further shows that when temperature cycles between 19.0 and 23.88C, the diffraction peak shifts from approximately 587 to approximately 489 nm and then moves back. Only a small variation was observed in 5 heating/cooling cycles.
因为PNIPAM微凝胶的体积相变是可逆的，所以PMCC膜在温度和离子强度方面对外部刺激的响应也是可逆的。 作为一个例子，图1A表明，在加热/冷却循环之后，当温度恢复到208℃时，膜的颜色很大程度上恢复。 当将[NaCl]调节回250mm时，发现膜颜色的类似恢复（图2A）。 图S3进一步显示当温度在19.0和23.88℃之间循环时，衍射峰从大约587移动到大约489nm然后向后移动。 在5个加热/冷却循环中仅观察到小的变化。
A big problem for macroscopic hydrogels is that their response to external stimuli is extremely slow. To study the responserateofthenewphotonichydrogel,theshiftcourseof the diffraction peak of a PMCC film, as a result of a jump in ionic strength, was monitored.(Figure 3A) Similar to many hydrogels, the kinetic curves of PMCC can be well-fitted with a single-exponential function (Figure 3B), suggesting its swelling/deswelling can be described by Tanaka–Fillmore theory. According to this theory, the swelling/deswelling kinetics of a hydrogel are governed by the cooperative diffusion of the gel network. It is noteworthy that ordinary hydrogels are networks of linear polymer chains, while PMCCs are networks of spherical microgel particles. From single-exponential fitting, the characteristic response time, t, was determined to be 150.3, 131.8, and 114.5 s, when [NaCl] increases stepwise from 200 to 250, 300, and 350 mm (Fig-ure 3C). Considering the size of the gel (side length is approximately 0.7 cm at [NaCl]=350 mm), the response rate is extremely fast. The collective diffusion coefficient of the PMCC film was estimated to be around 4�10�4 cm2s�1,which is about three orders of magnitude larger than that of ordinary PNIPAM hydrogels (on the order of 10�7 cm2s�1). ThefastresponseofPMCCmaybeattributed to its structural unit, the small microgel particles. It is well established that microgels respond much fast than macroscopic hydrogels. The interconnected porous structure ofthePMCCfilm,whichallowsthesolventtodiffuseintoand out of the gel smoothly, should also play an important role.[33] Previously it was reported that an inverse opal hydrogel, with a similar mesoporous structure, responds more than 1000 times faster than ordinary homogeneous gels.[34] Usually porous structures were introduced into hydrogels by polymerization in the presence of pore-forming agents followed by removal of these agents.[27] In contrast, the interconnected porous structure in PMCC, which is actually the interstitial space among the microgel particles, is inherent. (Scheme 1C)
宏观水凝胶的一个大问题是它们对外部刺激的反应非常缓慢。为了研究新光子水凝胶的响应速率，监测PMCC薄膜衍射峰的移动过程，这是由于离子强度的跳跃所致。（图3A）与许多水凝胶相似，PMCC的动力学曲线可以很好地拟合单指数函数（图3B），表明它的膨胀/消溶胀可以用Tanaka-Fillmore理论来描述。根据该理论，水凝胶的溶胀/消溶胀动力学由凝胶网络的协同扩散控制。值得注意的是，普通水凝胶是线性聚合物链的网络，而PMCC是球形微凝胶颗粒的网络。从单指数拟合，当[NaCl]从200逐步增加到250,300和350mm时，特征响应时间t被确定为150.3,131.8和114.5s（图3C）。考虑到凝胶的大小（在[NaCl] = 350mm时边长约为0.7cm），响应速度非常快。 PMCC薄膜的总扩散系数估计约为4-10±4cm2s-1，比普通PNIPAM水凝胶大约三个数量级（大约10-7cm2s-1）。 PMCC的快速响应可归因于其结构单元，即小的微凝胶颗粒。众所周知，微凝胶比宏观水凝胶反应快得多。 PMCC薄膜的互连多孔结构，其允许溶剂平稳地扩散进出凝胶，也应该起重要作用。此前据报道，具有类似中孔结构的反蛋白石水凝胶响应速度比普通均质凝胶快1000倍。通常在成孔剂存在下通过聚合将多孔结构引入水凝胶中，然后除去这些试剂。相反，PMCC中的互连多孔结构（其实际上是微凝胶颗粒之间的间隙空间）是固有的。 （方案1C）
In conclusion, we developed a facile method to stabilize the highly ordered structure of a PNIPAM microgel colloidal crystal. The cross-linking reaction is highly efficient and does not require the addition of a cross-linker, therefore will not disturb the highly ordered structure. The resulting polymerized microgel colloidal crystals can respond to external stimuli, including temperature and ionic strength. Using these stimuli the color and the band gap of the hydrogel can be finely tuned in the whole visible range. The response is reversible and fast. The swelling of the immobilized microgel particles is almost unaffected, which is quite different from those stabilized by embedding in a hydrogel matrix. The inherent mesoporous structure not only provides a quick response, but also allows big biomolecules to diffuse into the interiorofthegel.AsthelowrefractiveindexofthePNIPAM microgel can be improved by the incorporation of inorganic particles as shown by Karg et al.,[35] it is expected this new photonic hydrogel will find applications in areas such as sensing and displays.
总之，我们开发了一种简便的方法来稳定PNIPAM微凝胶胶体晶体的高度有序结构。交联反应是高效的并且不需要添加交联剂，因此不会干扰高度有序的结构。所得聚合的微凝胶胶体晶体可响应外部刺激，包括温度和离子强度。使用这些刺激，可以在整个可见范围内精细调节水凝胶的颜色和带隙。响应是可逆且快速的。固定化微凝胶颗粒的溶胀几乎不受影响，这与通过嵌入水凝胶基质中稳定的那些完全不同。固有的中孔结构不仅提供快速响应，而且还允许大的生物分子扩散到凝胶内部。由于可以通过掺入无机颗粒来改善PNIPAM微凝胶的低折射率，如Karg等人所示。预计这种新型光子水凝胶将在传感和显示等领域得到应用。
Figure 1. A) Photographs of a freestanding PMCC film taken when temperature rising from 1 to 278C. The last one (bottom right) was taken when the film was cooled back to 208C. The film was immersed in 0.5m NaCl solution. pH 3.0. Scale bar: 0.5 cm. B) Reflection spectra of the PMCC film measured at various temperatures. C) Hydrodynamic diameter (Dh) of microgel particles in diluted microgel dispersion measured by dynamic light scattering and the shortest distance between two microgel particles in PMCC films, or the diameter of the particles (Df), calculated from light diffraction, as a function of temperature.
图1. A）当温度从1升到278℃时拍摄的独立式PMCC薄膜的照片。 当薄膜冷却回到208℃时，取最后一个（右下）。 将膜浸入0.5m NaCl溶液中。 pH3.0。 比例尺：0.5厘米。 B）在各种温度下测量的PMCC膜的反射光谱。 C）通过动态光散射测量的稀释的微凝胶分散体中的微凝胶颗粒的流体动力学直径（Dh）和PMCC膜中两个微凝胶颗粒之间的最短距离，或者由光衍射计算的颗粒直径（Df），作为函数的函数。 温度。
Figure 2. A) Photographs of a freestanding PMCC film taken with [NaCl] increasing from 0 to 750 mm. The last one (bottom right) was taken when [NaCl] was decreased back to 250 mm. Scale bar: 0.5 cm. pH 3.0. T=238C. B) Reflection spectra of the PMCC film measured at various [NaCl] values. C) Hydrodynamic diameter (Dh) of microgel particles in diluted microgel dispersion measured by dynamic light scattering and the shortest distance between two microgel particles in the PMCC film, or the diameter of the particles (Df), calculated from light diffraction, as a function of [NaCl] in the media.
图2. A）用[NaCl]从0增加到750 mm的独立PMCC薄膜的照片。 当[NaCl]减少到250mm时，取最后一个（右下）。 比例尺：0.5厘米。 pH3.0。T =238℃。 B）在各种[NaCl]值下测量的PMCC膜的反射光谱。 C）通过动态光散射测量的稀释的微凝胶分散体中的微凝胶颗粒的流体动力学直径（Dh）和PMCC膜中两个微凝胶颗粒之间的最短距离，或者由光衍射计算的颗粒直径（Df）作为函数 [NaCl]在培养基中的含量。
Figure 3. A) Reflection spectra showing the response of a PMCC film with time upon increasing [NaCl] from 300 to 350 mm. The spectra were recorded every 25 s. pH 3.0. T=238C. B) Shift of the diffraction peak position with time in response to stepwise increase in [NaCl] as indicated. The solid line shows the best single exponential fit to the data.
图3. A）反射光谱显示PMCC薄膜随着[NaCl]从300毫米增加到350毫米时的响应。 每25秒记录光谱。 pH3.0。T =238℃。 B）如所示，响应于[NaCl]的逐步增加，衍射峰位置随时间的移动。 实线显示数据的最佳单指数拟合。