Abstract We developed a liquid crystal (LC)-based sensor system for detecting formaldehyde in aqueous solutions. In this system, nematic LCs were doped with 4′-pentyl-4-biphenylamine (PBA), which acts as the amphiphilic molecule to align at LC/aqueous interface. Such alignment induced the homeotropic orientation of LCs and a dark signal was observed under polarized light. Formaldehyde in the aqueous solutions would react with the amine group of PBA to form corresponding imine, which reduced the amount the PBA at LC/aqueous interface. As a result, the orientation of LCs was disrupted and a dark-to-bright transition of the LC images was observed. By using this mechanism, 0.5 mM of formaldehyde can be detected in 10 min with good selectivity. Moreover, we demonstrated that this system can be applied to detect formaldehyde in commercial facial toners with the limit of detection (LOD) of 30 mM, which is below the safety level of cosmetic products regulated by USA and the European Union. Because the LC-based sensor system is cost-effective and easy to use, while its signals can be simply differentiated through the naked-eye under ambient light, it shows great potential for the applications of formaldehyde detection in the daily necessities for general family.

中文翻译：

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用于检测水溶液中甲醛的基于液晶的传感器系统

摘要 我们开发了一种基于液晶 (LC) 的传感器系统，用于检测水溶液中的甲醛。在该系统中，向列 LC 掺杂了 4'-戊基-4-联苯胺 (PBA)，PBA 作为两亲分子在 LC/水界面对齐。这种排列导致 LC 的垂直取向，在偏振光下观察到暗信号。水溶液中的甲醛会与 PBA 的胺基反应生成相应的亚胺，从而降低了 LC/水界面处的 PBA 量。结果，LC 的方向被破坏，并且观察到 LC 图像从暗到亮的转变。通过使用这种机制，可以在 10 分钟内以良好的选择性检测到 0.5 mM 的甲醛。而且，我们证明该系统可用于检测商用面部爽肤水中的甲醛，检测限 (LOD) 为 30 mM，低于美国和欧盟规定的化妆品安全水平。由于基于LC的传感器系统具有成本效益和易于使用的特点，而其信号在环境光下可以通过肉眼简单区分，因此在甲醛检测在普通家庭的日常生活用品中的应用显示出巨大的潜力。