Optical sensors based on liquid crystal droplets for heavy metal ion detection

Industrial and agricultural development has contributed to increased heavy metal pollution. Consequently, efficient heavy metal ion detection technologies are essential for comprehensive environmental monitoring and for safeguarding ecosystems and human health. We developed a rapid, user-friendly, and highly sensitive label-free sensing platform based on 4-cyano-4′-pentylbiphenyl (5CB) droplets for heavy metal ions detection. Five representative heavy metal ions and four light metal ions were chosen for feasibility testing. By varying solution pH and heavy metal ion concentration, we observed stearic-acid-mediated optical responses in the 5CB droplets. At 400 pM Cu(II), the sensor exhibited a rapid response, stabilizing within approximately 30 s, while at 30 pM the response time increased to around 330 s. Similarly, at 800 pM Fe(III) the response stabilized within about 40 s, whereas at 100 pM stabilization required approximately 420 s. The sensor's detection limits were 10–30 pM for Cu(II) and 80–100 pM for Fe(III). Compared to hemispherical designs, spherical 5CB microdroplets offer more uniform molecular alignment and improved optical clarity, thereby enhancing sensitivity and reducing detection time. Furthermore, the sensor can differentiate between heavy metal ions and light metal ions.