| Liquid crystals (LCs) are materials that can exhibit the mobility of liquids andthe anisotropy of solid crystals. The specific optical and dielectric anisotropies of LCsmake them widely applied in the fields of information communication technology,aerospace engineering, and microelectronic technology. Recently, LC-basedbiosensing technology has attracted particular attention in the study ofantibody-antigen, protein-ligand, enzyme, nucleic acid and microorganism, becausethe long-range orientational order and optical anisotropy of LCs can rapidly transformbiomolecular binding events into amplified optical signals, easily observed even withnaked eye. Unlike most current analytical approaches, which generally require labeledspecies such as fluorophores or radioactive isotopes, and sophisticated analysisprocedures with expensive laboratory-based equipments, LC-based biosensingmethods permits label-free detection with high sensitivity and can be carried outunder ambient lighting even without the need for electrical power, making themsufficiently simple and particularly useful for low-cost screening bioassay performedaway from central laboratories. However, the LC biosensing technique based on directbiomolecular binding events is difficult to meet the demand of ultrasensitive DNAassays, and the research of LC-based DNA biosensing technique at the LC-aqueousinterface is still unclear. There are thus significant interests in seeking the signalenhancement strategies and exploiting clearer sensing mechanisms for the LC-basedDNA biosensing techniques to circumvent the urgent problems.Part one: Studies of liquid crystal biosnensors at the solid-LC interfaceIn chapter2: A novel signal enhanced liquid crystal biosensing approach basedon the enzymatic silver deposition has been developed for the highly sensitivedetection of specific DNA sequences. Firstly, a chemically functionalized surface on aplane glass slide is obtained by self-assembling a (3-aminopropyl) trimethoxysilane(APS)/N, N-dimethyl-n-octadecyl (3-aminopropyl) trimethoxysilyl chloride (DMOAP)film. Then, the DNA immobilization is realized by binding a capture DNA probe tothe APS/DMOAP film through a cross-linker, followed by hybridizations of a targetDNA and a biotinylated detection DNA probe. Subsequently, the streptavidin-alkalinephosphatase (Sv-ALP) is bound to the biotin of the detection probe and then catalyzesthe hydrolysis of ascorbic acid2-phosphate (AA-p) to form ascorbic acid. The latter, in turn, reduces the silver ions in solution to form the deposition of metallic silver onthe substrate surface. Results showed that the mixing self-assembled film ofAPS/DMOAP was an effective biosensing substrate for LC biosensors, whichprovides functional amino groups for biomolecular immobilization by covalent bondformation and also the long alkyl chains for LC homeotropic alignment by short-rangeinteractions as well. The enzymatic silver nanoparticles deposited on the LC sensingsubstrate greatly changed the surface topology and further induced ahomeotropic-to-tiled transition of the LC molecules surrounding them, resulting in anobvious change of the optical appearances of LC biosensors from a dark backgroundto a birefringent texture before and after enzymatic silver deposition. In comparisonwith the existing LC biosensing methods established on the disruption of LCsorientation via direct biomolecular binding events, the sensitivity of the proposedprotocol depends on the condition of the enzymatic reaction instead of the size andamount of the biomolecules. The combination of enzymatic signal amplification andLCs-based imaging contributed a highly selective and ultrasensitive method for thedetection of DNA low to0.1pM. Therefore, the silver enhancing method wouldpossess a potential to offer highly sensitive detection for varieties of analytes andplay a crucial role in expanding the application scope of LC biosensing technique.In chapter3: A label-free LC biosensing method based on the aptamer and goldnanoparticles was developed for the highly sensitive detection of thrombin. Becausethe aptamer and its target possess high affinity and specificity, and the goldnanoparticles own good biocompatibility, high loading capacity and steady sizestability. Firstly, a hairpin DNA probe with-NH2group modified at the5′end wasimmobilized at the APES/DMOAP film by a cross-linker. Then, the formation ofG-quadruplex structure through the aptamer-thrombin binding events can lead to thehairpin DNA probe opened its loop and the-biotin group at its3′end stood far awayAPS/DMOAP film. Finally, the gold nanoparticles can be captured to the substratesthrough the binding events of the biotin and the streptavidin-gold nanoparticles. Thecombination of aptamer recognition and gold nanoparticles signal enhancement forthe LC-based biosensing contributed a highly selective and sensitive approach for thedetection of thrombin low to1pM. Results well revealed that the gold nanoparticle isan excellent ‘signal enhancement element’. This is a simple, label-free and sensitivedetection for LC-based imaging assay.Part two: Studies of liquid crystal biosnensors at the LC-aqueous interfaceand liquid crystal DNA logic gates In chapter4: A novel highly-sensitive LC biosensing approach based ontarget-triggering DNA dendrimers was developed for the detection of p53mutationgene segment at the LC-aqueous interface. In this chapter, the mutant-type p53genesegment was the target to trigger the formation of DNA dendrimers from hairpin DNAprobes by hybridization chain reaction, and the latter as a ‘signal enhancementelement’ further induced the LC reorientation from tilted to homeotropic alignment,resulting in a corresponding optical changes of LC biosensors from birefringent tohoneycombed textures or dark framework. The distinct optical reorientationalappearances can serve as a characteristic signal to distinguish target concentrationsranging from0.08nM to8nM. Moreover, these optical phenomena suggest that theLC reorientation is related to the electric-dipole coupling between the adsorbed DNAand LC molecules, the conformational constraints of DNA and the internal electricfield induction upon hybridization. This proposed biosensing method withoutbiological immobilizations is simple and safe to prepare, and it has a great potentialfor label-free and highly sensitive bioassays performed away from centrallaboratories.In chapter5: Utilizing the principles of metal ion-mediated base pairs (C-Ag+-C,T-Hg2+-T), the LC reorientation induced by both the conformational change of DNAand charge density changes of the LC-aqueous interface, two simple and intelligentLC logic devices with ‘AND’ and ‘INHIBIT’ functions were constructed for the rapid,selective, and multiplexed analysis of Ag+, Hg2+and H+. The ‘AND’ logic gate wasconstructed by introducing the Ag+and Hg2+as two inputs to control theconformational change of C-/T-rich DNA probe, and the optical appearances of LC asoutputs. This ‘AND’ gate generates the true signal of ‘1’ if and only if the inputs areboth true (‘1’), which can realize the simultaneous detection of Ag+and Hg2+low to6M. The ‘INHIBIT’ gate was constructed by introducing the H+as one input, Ag+andHg2+as another input, both the conformational change of DNA probe and the chargedensity change of the LC-aqueous interface as logic switch, and the opticalappearances of LC as outputs. This ‘INHIBIT’ gate generates the true signal of ‘1’ ifthe input state is (‘0’,‘1’), if not, it generates the true signal of ‘0’, which not onlycan realize the simultaneous detection of Ag+and Hg2+, but also can be used for thedetection of strong acid low to200M. These two simple, label-free, and intelligentLC logic gates can open up a new platform for the multiplexed detection of metal ionsand enrich the application scope of LC biosensing technique. |
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