Study Of Liquid Crystal Biosensor Method For Protein And Metal Ions Assay

Liquid crystals (LCs) are materials which possess long-range orientationalorder and optical anisotropy, and their unique fast response to an external stimulusmakes them well-sutied as “sensing element”. The LC-based biosensors areusually based on the disruption from the binding event of ligands and receptors tothe alignment of LC molecules at the biosensor surfaces, and their utility in thetransduction of binding events into the optical signals has been demonstrated. TheLC-based biosensors usually need no extraneous markers and their optical signalscan be easily observed even under the natural light, making them sufficientlysimple and particularly useful for low-cost screening bioassay performed awayfrom central laboratories. To further improve the sensitivity and application fieldthe signal enhanced LC-based biosensor methods and the extending application toheavy metal ions have been investigated in the paper and described as follows:1. A novel liquid crystal（LC） biosensor was developed for the detection ofplatelet-derived growth factor BB (PDGF-BB) based on the orientation changes ofliquid crystals. One glass slide of the LC cell was first modified with theAPTES/DMOAP [(3-aminopropyl) trimethoxysilane/N, N-dimethyl-N-octadecyl(3-aminopropyl) trimethoxysilyl chloride] self-assembled monolayer (SAM) totrigger the homeotropic alignment of LC molecules and thereby produced a blackbackground optical image under the crossed polarized light, and then the specificaptamer of PDGF-BB was immobilized on SAM through glutaraldehydecrosslinking to construct a LC sensing substrate. In the presence of PDGF-BB, thestable triple helix structure of PDGF-BB aptamer was formed by the specificbinding event and thus led to the target PDGF-BB captured to the sensingsubstrate, making a visible optical change observed under the crossed polarizedlight via the huge steric effect of the PDGF-BB on disrupting the LC alignment.There was an obvious optical change when the concentration of target PDGF-BBwas5nM. The proposed LC biosensing method permits the label-free detection ofPDGF-BB with good selectivity and high sensitivity, making them sufficientlysimple and particularly useful for low-cost screening bioassay performed awayfrom central laboratories.2. Using the strong binding of Hg2+by two DNA thymine bases (T-Hg2+-T),we developed a new LC-based sensor for the detection of target Hg2+at a surfactant-laden aqueous LC interface. The adsorption of ionic surfactant (SDS) tothe LC-aqueous interface can lead to homeotropic LC alignment. In the absence ofHg2+, the adsorption of two signal-stranded DNA probes (DNA1and T9、DNA1andT6) to the SDS-laden aqueous LC interface modified the interfacial structureresult in a reorientation of the LC from homeotropic alignment to planarconfiguration, because the SDS and ssDNA can combine through hydrophobicinteraction at the LC-aqueous interface, which will reduce the concentration ofSDS in the LC-aqueous interface and have a disrupting effect on the alignment ofLC. Upon subsequent T-Hg2+-T hybridization, the ssDNA can form stable DNAduplexes and the DNA become less hydrophobic, allowing for a return to theinterfacial structure associated with homeotropic LC alignment. The shift ofoptical images from bright-to-dark has been observed under the crossed polarizedlight. The optical signal of LC-based sensor has an obvious change at the Hg2+concentration of low to1nM, showing good detection sensitivity and simpleoperation.3. A novel label-free LC-based sensing for the detection of Ag+is developedbased on the specific and strong binding of C-Ag+-C at a surfactant-laden aqueousLC interface. The long-chain alkyl groups of surfactant SDS interaction with theliquid crystal (5CB), and induce LC from planar to homeotropic orientation, and auniformly dark background optical image appeares. With the absence of Ag+, theSDS and ssDNA can combine through hydrophobic interaction at the LC-aqueousinterface, which can well disrupts the alignment of LC, generating an opticalsignal. This LC biosensor is simple and sensitive, and the detection limit is as lowas50nM.