Color-tunable luminescent hydrogels with tough mechanical strength and self-healing ability
颜色可调的发光水凝胶，具有坚韧的机械强度和自愈能力

Luminescent hydrogels are of immense importance for various applications nowadays, while it still remains a significant challenge to achieve excellent optical functional hydrogels combining robust mechanical properties and high self-healing efficiency under mild conditions. Herein, we describe the preparation of a series of color-tunable and robust luminescent hydrogels through the electrostatic interactions between positive-charged luminescent micelles and negatively-charged clay nanosheets. Further addition of poly (vinyl alcohol) (PVA) results in the obvious enhancement of the mechanical strength of hydrogels via physical cross-linking, leading to a more stable luminescent hydrogel with fast self-healing performances. The obtained luminescent hydrogels can also remain shape persistent even when immersed in organic solvents, affording novel luminescent organogels with outstanding optical performance.
发光水凝胶对于当今的各种应用具有极其重要的意义，而在温和条件下实现优异的光学功能水凝胶仍然是一个重大挑战，其结合了强大的机械性能和高自愈合效率。 在本文中，我们描述了通过正电荷发光胶束和带负电荷的粘土纳米片之间的静电相互作用制备一系列颜色可调且稳健的发光水凝胶。 进一步添加聚（乙烯醇）（PVA）通过物理交联导致水凝胶的机械强度明显增强，从而产生具有快速自愈合性能的更稳定的发光水凝胶。 所获得的发光水凝胶即使浸入有机溶剂中也可保持形状持久性，从而提供具有优异光学性能的新型发光有机凝胶。

Introduction

The design and synthesis of self-healing hydrogels remain a top concern for scientist worldwide for they show great promise in various application fields like sensors, electronic devices, regenerative medicine, tissue engineering scaffolds and drug delivery. They typically have the ability to regenerate and heal after the infliction of physical damage. Non-covalent interactions, including ionic interactions, hydrogen bonding, electrostatic interactions and π-π stacking, have emerged as powerful methods to construct healable hydrogels. However, most of the self-healing hydrogels reported so far are either mechanically robust or display rapid self-healing performance but not both because the self-healing efficiency and the robustness are opposing properties.Therefore, the development of hydrogels possessing both mechanical robustness and rapid self-healing properties is of high importance and remain a challenging task. Recently, a new class of supramolecular hydrogels consisting of a high portion of water (95-98 wt%) has been prepared by mixing a very small proportion of organic component with inorganic clay in water, which has been reported to have the properties of exceptionally great mechanical strength and self-heals rapidly upon damage. As consequence of this, the hydrogels can be easily modulated into free-standing objects of shape-persistence despite a high content of water. Another important aspect is adding extra functionalization such as fluorescence emission to the self-healing hydrogels to develop smart and adaptive functional materials. In terms of emission properties, lanthanide complexes (LnCs) are widely believed to be highly attractive because of their intriguing optical properties, such as sharp emission bands, large Stokes shift, high luminescent efficiency and long-lived excited states. Nevertheless, preparing highly luminescent hydrogels consisting of LnCs is not an easy task because (i) most of the LnCs are poor soluble in water and (ii) the luminescence of which can be effectively quenched by water molecules. A luminescent hydrogel with bright red luminescence characteristic of Eu3+ ions, mechanical robustness and rapid self-healing has been reported by us recently, which was based on the co-assembly of the exfoliated clay with ionic luminescent micelle consisted of luminescent europium (III) complexes within the hydrophobic part of the micelles driven by electrostatic interactions encapsulated, guaranteeing the high luminescence of the complex in the water-abundant conditions. Although this hydrogel has numerous advantages, (i) it is only based on the luminescence intensity of single emissive transition; (ii) the mechanical strength of hydrogel needs to be further improved to meet the practical needs.
自愈式水凝胶的设计和合成仍然是全球科学家最关心的问题，因为它们在传感器，电子设备，再生医学，组织工程支架和药物输送等各种应用领域中显示出巨大的潜力。 它们通常具有在造成物理伤害后再生和愈合的能力。 非共价相互作用，包括离子相互作用，氢键，静电相互作用和π-π堆积，已经成为构建可修复水凝胶的有效方法。 然而，到目前为止报道的大多数自愈合水凝胶要么具有机械强度，要么显示出快速的自愈性能，但不是两者都是因为自愈效率和稳健性是相反的性质。因此，具有机械强度和快速自愈性能的水凝胶的开发具有高度重要性并且仍然是具有挑战性的任务。最近，通过将极少量的有机组分与无机粘土混合在水中制备了一种由大部分水（95-98％（重量））组成的新型超分子水凝胶，据报道，这种水溶胶具有特殊的性质。强大的机械强度，并在损坏后自动愈合。因此，尽管水含量高，但水凝胶可以很容易地调制成形状持久的独立物体。另一个重要方面是为自愈合水凝胶添加额外的功能化，例如荧光发射，以开发智能和适应性功能材料。就发射性质而言，镧系元素络合物（LnCs）因其有趣的光学性质而被广泛认为是极具吸引力的，例如尖锐的发射带，大的斯托克斯位移，高发光效率和长寿命激发态。然而，制备由LnCs组成的高度发光水凝胶不是一件容易的事，因为（i）大多数LnCs在水中溶解性差，（ii）其发光可以被水分子有效地猝灭。 我们最近报道了一种具有Eu3 +离子的鲜红色发光特性，机械强度和快速自愈的发光水凝胶，其基于剥离粘土与由发光铕（III）络合物组成的离子发光胶束的共组装。 在胶束的疏水部分内，由封装的静电相互作用驱动，确保在水丰富的条件下复合物的高发光。 虽然这种水凝胶具有许多优点，但（i）它仅基于单个发射转变的发光强度; （ii）需要进一步改善水凝胶的机械强度以满足实际需要。

Herein, based on the same strategy reported in ref17, we prepared a series of robust, color-tunable, and self-healing luminescent hydrogels by the electrostatic interactions between the positive-charged luminescent micelles consisting of Ln(dpa)3-tpy-mim2 (dpa=2,6-pyridinedicarboxylic acid, Ln=Eu3+, Tb3+) (Scheme 1) and clay nanosheets under aqueous conditions. The advantageous features shown by the hydrogels reported herein compared with that reported in Ref17 include 1) the tunable and highly bright luminescence is achieved; 2) a much stronger mechanical strength achieves upon further addition of an appropriate amount of PVA. It is worth noting that newly obtained hydrogels constructed only by noncovalent forces, yields robust mechanical properties, exhibiting remarkable self-healing capability at room temperature. In addition, the hydrogels networks are strong enough for the replacement of incorporated water by organic solvents to prepare organogels. This work provides a convenient route to construct self-healing luminescent hydrogels with tough polymer networks, hopefully promoting current hydrogel research in self-healable devices, luminescent sensors.
在此，基于ref17中报道的相同策略，我们通过由Ln（dpa）3-tpy-mim2组成的带正电的发光胶束之间的静电相互作用制备了一系列稳健的，颜色可调的和自我修复的发光水凝胶。 （dpa = 2,6-吡啶二羧酸，Ln = Eu 3 +，Tb 3+）（方案1）和在水性条件下的粘土纳米片。本文报道的水凝胶显示的有利特征与Ref17中报道的相比包括1）实现可调谐和高亮度发光; 2）在进一步加入适量的PVA后，可以获得更强的机械强度。值得注意的是，新获得的水凝胶仅由非共价力构成，产生稳健的机械性能，在室温下表现出显着的自愈合能力。此外，水凝胶网络足够强，可以用有机溶剂代替掺入的水来制备有机凝胶。这项工作提供了一种方便的途径来构建具有坚韧聚合物网络的自我修复发光水凝胶，希望促进当前水凝胶研究自愈性设备，发光传感器。

Results and discussion

总之，我们通过自组装策略展示了一系列稳健的，颜色可调和自愈合的发光有机 - 无机混合水凝胶，其由发光胶束的静电相互作用驱动，所述发光胶束由镧系元素络合物和水溶液中的纳米片粘土组成。 在水凝胶中进一步加入极少量的PVA可以显着提高机械强度，并具有快速的自愈性能。 获得的自愈合水凝胶显示出明亮的发光，Q高达47.6，尽管最终产品中含有非常小部分的发光LC（_{0.08％）和非常大部分的水（}97％）。 通过改变Eu3 + / Tb3 +离子的摩尔比，可以容易地调节水凝胶的发光。 与相应水凝胶形状相同的发光有机凝胶可以通过用有机溶剂如THF和甘油直接置换水来获得，尽管它们的自愈性能消失。