Aptamer Biosensors Based On Liquid Crystals For The Detection Of Sulfadimethoxine

Sulfadimethoxine（SDM）,one of the most common antibiotics,has been used as a feed additive agent for the treatment and prevention of animal diseases due to its high antibacterial activity.However,the inappropriate use of SDM could lead to severe side effects.Therefore much concern has been rised in recent years about SDM residues in animal foods.Early approaches have been applied to the detection of SDM,such as triphenyl tetrazolium chloride method,high performance liquid chromatography,enzyme-linked immunosorbent assay and fluorescence method.Though these conventional methods exhibited specificity and sensitivity for SDM analysis,drawbacks still existed among them,such as consuming longer time,higher cost,complex operation and being unsuitable for small laboratory.Hence,it is necessary to establish a low-cost and simple method for SDM detection.Liquid crystals（LCs）are eye-catching materials that possess various superior properties,especially sensitive orientation response and optical anisotropy.LCs act as both signal transducers and signal amplifiers in biosensors,and the LCs-based biosensor can visually detect target molecules without the help of other complex devices,therefore the LCs-based biosensors have been widely applied in the detection of proteins,DNA,pathogens and so on.The core step to design a LCs-based biosensors is to construct a functional sensing interface that can make the LCs vertically oriented,as for the LCs-based sensing solid interfaces,the frequently-used methods that past studies have mentioned for the immobilization of aptamer were as follws:streptavidin-modified aptamers were immobilized onto the biotin-modified sensing substrate by avidin-biotin interaction;amino-decorated aptamers were immobilized onto the sensing substrate containing 3-aminopropyltriethoxysilane（APTES）and glutaraldehyde（GA）with a two-step modification.Although the above methods have been proved to be effective and feasible for the immobilization of the aptamer,there were still some weaknesses,such as complex operation and expensive fabrication.Therefore,it is necessary to explore some new methods for immobilization of aptamer.In order to overcome both the shortcomings of the existing methods for SDM detection and the problems of past studies for aptamer immobilization in LCs sensing systems,two LCs-based biosensors with different functional substrates were constructed for the detection of SDM.The main contents of this paper include the following two parts:（1）A LCs-based biosensor with a sensing substrate decorated with the mixed self-assembled monolayers of triethoxybutyraldehyde silane（TEA）/N,N-dimethyl-N-octadecyl-3-aminopropyl trimethoxychloride（DMOAP）was constructed for the detection of SDM.We selected TEA and DMOAP to decorate the sensing substrate,DMOAP was used to induce the homeotropic alignments of LCs,while TEA was used to immobilize the aptamer of SDM that modified with amino-group.Firstly,we investigated the effect of the DMOAP/TEA ratio on the orientation behavior of liquid crystals.Next,we discussed the influence of the concentration of immobilized aptamers on the orientation of LCs.In the presence of SDM molecules,SDM-aptamer specific reaction could significantly affect the topography of the sensor and disturb the original perpendicular alignment of LCs,and finally cause the optical image of 5CB change from uniform black to bright color texture.Atomic force microscope（AFM）experiment and contact angle test were also carried out to measure the changes of surface topography and surface free energy after the surface of the sensor substrate was combined with SDM molecules,which theoretically proved that the detecting principle of the biosensor was practicable.In addition,we studied the detection limit and selectivity of the sensor,the results showed that the biosensor has very good selectivity and the detection limit of SDM reached 10μg L^-1.Compared with the two-step functional modification（by using APTES and GA）of sensor substrate in the early studies,TEA would directly provide aldehyde group to capture the aptamer,which simplified the process of fabricating the sensor and ensured the repeatability of the sensing system.（2）Another LCs-based biosensor was also design for the detection of SDM by using aptamer both as the agent to induce the homeotropic orientation of 5CB and the recognition molecule of SDM.In order to further simplify the construction of the sensing interface,the aptamers of suitable length and concentration were chose to induce the homeotropic alignment of liquid crystal（5CB）.We investigated the effects of aptamers on the orientation of LCs that may caused by both the length and the concentration of self-assembled aptamer,the result showed that the self-assembled aptamers with a certain length and a certain surface density（concentration）could make the LCs vertically oriented and produce a uniform dack polarized image.In the presence of SDM,the compounds formed by SDM molecules and its aptamers would destroy the initially homeotropic orientation of 5CB molecules and lead to great changes in the optical images.Furthermore,the performances of the biosensor were investigated,under the optimum conditions,the detection limit of SDM was as low as 0.9μg L^-1.Compared with the method for the modification of sensing substrate which was mentioned in（1）,this method has a simpler preparation process and a lower detection limit.