Liquid crystal-based optical sensor for the detection of cysteine through competitive host-guest inclusion

A β-cyclodextrin (β-CD) inclusion complex containing L-cysteine (CYS) as a guest was prepared by freeze drying method. Formation of the inclusion complex was confirmed by powder X-ray diffraction pattern, Fourier-transform infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry and scanning electron microscopy. The interaction between CYS and β-CD was also analysed by conductivity measurement in aqueous solution. Thereafter, we developed a liquid crystal (LC)-based sensor system to detect CYS in aqueous solution following the principle of competitive host–guest inclusion. Here the guests sodium dodecyl sulphate (SDS) and CYS compete for the same hydrophobic cavity of host β-CD. The dark optical image observed by polarising optical microscope confirms that CYS forms inclusion complex with β-CD leaving SDS free which aligns LCs homeotropically. The constructed LC-based sensor could detect CYS up to 30 µM in aqueous solution. Moreover the practical approach of this system was validated by sensing CYS in human serum.