Photocontrolled Liquid Crystal Droplets and Solution Tuning for Sub-nanomolar Protein Detection

Understanding the interactions of native proteins with the liquid crystal (LC)-aqueous interface is fundamental to achieving ultimate performance from LC-based biosensing platforms. However, one of the most explored LCs for sensing of various analytes, 4-cyano-4'-pentyl biphenyl (5CB), is oleophilic and does not interact with globular and hydrophilic proteins. This impedes its utility in direct protein-sensing. Here, we show how controlled photoirradiation of 5CB coupled with tuning of several solution properties can induce interactions, impart responsiveness, and provide modulated nonspecific ultrasensitivity of the 5CB-aqueous interface toward proteins. UV-irradiation of 5CB forms several byproducts, including 4-cyano-4'-biphenyl carboxylate (CBA), an amphiphilic molecule. Microfluidics-based microemulsion droplets of UV-5CB exhibit a radial defect configuration and a highly negative charge, both attributed to the adsorption of CBA at the LC-aqueous interface. On exposure to model globular protein, myoglobin (Mb), and mediated through electrostatic interaction with amphiphilic CBA, these droplets transform to a bipolar defect configuration. Interestingly, an optimum UV-irradiation of 5CB is required for ultrasensitive protein detection; an extended irradiation time has deleterious effects. This is linked to the increase in the CBA concentration, which disrupts the packing and long-range order of the 5CB molecules, as shown by differential scanning calorimetry and small-angle X-ray scattering studies. Remarkably, a combination of tuned buffer concentration and a 4-times reduction in the number density of LC-droplets further advances the protein sensitivity to ∼0.95 nM, viz. ≥25 times better compared to previous reports. Overall, this work advances the field of LC-based systems by providing a fundamental understanding and precise control of interactions for optimizing protein sensitivities and paves the way for further studies to include specificity.