School for Radiological and Interdisciplinary Sciences (RAD-X), Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions and Jiangsu Provincial Key Laboratory of Radiation Medicine and Protection, Soochow University , Suzhou, Jiangsu 215123, China.
Department of Chemistry, University of Alberta , Edmonton, AB T6G 2G2, Canada.
ACS Appl Mater Interfaces. 2017 Aug 9;9(31):26539-26548. doi: 10.1021/acsami.7b05558. Epub 2017 Jul 26.
In this submission, the phase transition behavior for poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-co-AAc) microgels and their assemblies was investigated as a function of temperature and pH using UV-vis spectroscopy (to probe light scattering behavior) and quartz crystal microbalance with dissipation (QCM-D) measurements. PNIPAm-co-AAc microgels were "painted" onto Au-coated glass substrates (for UV-vis) and the Au electrode of a QCM crystal to generate monolayers. The subsequent deposition of another Au layer on top of the pNIPAm-co-AAc microgel layer yields what is known as an etalon. UV-vis/QCM-D measurements revealed that the temperature and pH responsivities for the microgel assemblies match well with their solution behavior. UV-vis spectroscopy shows that the transmittance of the microgel monolayers decreased with increasing solution temperature at pH 3.0. At pH 6.5, the AAc groups in the microgels were deprotonated, leading to strong Coulombic repulsive forces inside the microgels that prevented their collapse and lead to minimal change in the transmitted light intensity. However, QCM-D analysis reveals more complex behavior as it is sensitive to the viscosity/viscoelasticity and thickness changes of the microgel layer, which ultimately depends on the microgel chemical composition and the interaction of the etalon's Au layer with the crystal. The maximum sensitivity to temperature is 0.8 × 10 °C·Hz, which is the most sensitive pNIPAm microgel-based QCM temperature sensor thus far reported in the literature. Finally, we exploit this new understanding to characterize the pH and ionic strength of a solution using pNIPAm-co-XAAc microgel-based etalon coated crystals. The research results and the sensing demonstration can inspire new and improved sensor designs for a variety of analytes.
在本研究中，通过紫外可见分光光度法（用于探测光散射行为）和石英晶体微天平（QCM-D）测量，研究了聚（N-异丙基丙烯酰胺-共-丙烯酸）（pNIPAm-co-AAc）微凝胶及其组装体的相转变行为，作为温度和 pH 的函数。pNIPAm-co-AAc 微凝胶被“涂”在镀金玻璃基底（用于紫外可见）和 QCM 晶体的金电极上，以生成单层。随后，在 pNIPAm-co-AAc 微凝胶层的顶部沉积另一层 Au，得到所谓的标准具。紫外可见/QCM-D 测量表明，微凝胶组装体的温度和 pH 响应与它们的溶液行为非常吻合。紫外可见光谱显示，在 pH 3.0 时，随着溶液温度的升高，微凝胶单层的透过率降低。在 pH 6.5 时，微凝胶中的 AAc 基团去质子化，导致微凝胶内部产生强烈的库仑斥力，阻止其坍塌，导致透过光强度几乎没有变化。然而，QCM-D 分析揭示了更复杂的行为，因为它对微凝胶层的粘度/弹性和厚度变化敏感，这最终取决于微凝胶的化学成分以及标准具的 Au 层与晶体的相互作用。对温度的最大灵敏度为 0.8×10 °C·Hz，这是迄今为止文献中报道的最灵敏的基于 pNIPAm 微凝胶的 QCM 温度传感器。最后，我们利用这种新的认识，利用基于 pNIPAm-co-XAAc 微凝胶的标准具涂层晶体来表征溶液的 pH 值和离子强度。研究结果和传感演示为各种分析物的新型和改进型传感器设计提供了灵感。
