Ahiabu Andrews, Serpe Michael J
Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2.
ACS Omega. 2017 May 3;2(5):1769-1777. doi: 10.1021/acsomega.7b00103. eCollection 2017 May 31.
Rapidly responding stimuli-responsive materials can have a benefit in a myriad of applications, for example, sensing and biosensing, actuation, and in drug delivery systems. Thermo- and pH-responsive materials have been among the most widely studied, and can be triggered at physiologically relevant temperatures and pH. Here, we have used a "homologous series" of acids based on the acrylic acid (AAc) backbone and incorporated them into -isopropylacrylamide (NIPAm)-based microgels. Specifically, the acids used were AAc, methacrylic acid (MAAc), ethylacrylic acid (EAAc), and butylacrylic acid (BAAc), which have p 's in the range of 4.25-7.4. The resultant microgels were characterized by optical microscopy, and their responsivity to temperature and pH studied by dynamic light scattering. The microgels were subsequently used to generate optical devices (etalons) and their pH and temperature response was also investigated. We found that the devices composed of BAAc-modified microgels exhibit unusually fast response kinetics relative to those of the rest of the devices. We also found that the speed of the response decreased as the length of the acid pendant group decreased, with AAc-modified microgel-based devices exhibiting the slowest response kinetics. Finally, we showed that the kinetics of the device's temperature response also decreased as the length of the acid pendant group decreased, which we hypothesize is a consequence of the hydrophobicity of the acid groups, that is, increased hydrophobicity leads to faster responses. Understanding this behavior can lead to the rational design of fast responding materials for the applications mentioned above.
快速响应的刺激响应材料在众多应用中具有优势，例如传感与生物传感、驱动以及药物递送系统。热响应和pH响应材料是研究最为广泛的材料之一，并且可以在生理相关的温度和pH条件下被触发。在此，我们使用了基于丙烯酸（AAc）主链的一系列“同系物”酸，并将它们引入到基于N-异丙基丙烯酰胺（NIPAm）的微凝胶中。具体而言，所使用的酸为AAc、甲基丙烯酸（MAAc）、乙基丙烯酸（EAAc）和丁基丙烯酸（BAAc），它们的pKa值在4.25 - 7.4范围内。通过光学显微镜对所得微凝胶进行了表征，并通过动态光散射研究了它们对温度和pH的响应性。随后，这些微凝胶被用于制造光学器件（标准具），并对其pH和温度响应进行了研究。我们发现，由BAAc修饰的微凝胶组成的器件相对于其他器件表现出异常快速的响应动力学。我们还发现，响应速度随着酸侧基长度的减小而降低，基于AAc修饰的微凝胶的器件表现出最慢的响应动力学。最后，我们表明，器件的温度响应动力学也随着酸侧基长度的减小而降低，我们推测这是酸基团疏水性的结果，即疏水性增加导致响应更快。理解这种行为有助于合理设计用于上述应用的快速响应材料。
