Copper and liquid crystal polymer bonding towards lead sensing

Lead (Pb) is a highly toxic and carcinogenic heavy metal causing adverse impacts on environment and human health, thus requiring its careful monitoring. In this work, we demonstrate the integration of copper (Cu) film-based electrodes toward Pb sensing. For this, we developed a direct bonding method for Cu thin film and liquid crystal polymer (LCP) substrate using oxygen plasma treatment followed by contact and heat at 230 °C. The oxygen plasma activation forms hydroxyl groups (OH−) on Cu and LCP. The activated surfaces further adsorb water molecules when exposed to clean room air during contact. After contact, hydrogen bonds are formed between the OH− groups. The interfacial water is removed when the contacted films are heated, leading to shrinkage of OH− chain. This results in an intermediate oxide layer linking the Cu and C sites of Cu and LCP respectively. A strong adhesion (670 N·m−1) is obtained between Cu/LCP that may offer prolonged use of the electrode without delamination in wet sensing applications. Anodic stripping voltammetry of Pb using Cu thin film electrode shows a stronger current peak than sputtered Cu electrode, which implies the significance of the direct bonding approach to integrate thin films. We also studied the electrochemical impedance that will enable modeling of integrated environmental sensors for on-site monitoring of heavy metals.