| Deoxyribonucleic acid(DNA)is an important biological macromolecule to guide the development of biological growth and life function,and it is the genetic material of most organisms.The hybridization detection of single-stranded DNA(ssDNA)molecule shows significance to disease diagnosis,genetic testing in clinical medicine,and it is also important to the detection of biological reagents in the environment.At present,label-free detection of DNA molecules,such as optical detection,electrical detection and electrochemical detection,has been extensively studied becasue it is more convenient and faster than the method based on labeled detection.Among them,the method of electrical detection is simple,low cost,and becomes a common test method.As famous nanomaterials,carbon nanotube(CNT)and graphene show excellent electrical and mechanical properties,are always used to make micro-resonator.Their unique atomic structure can be combined with biomolecules,which indicates that CNT and graphene are very suitable for biosensing.The field-effect transistors produced on this basis are used to detect the hybridization of DNA.However,the testing instruments are complex and the experiment can not be effectivly repeated by using this kind of method.Based on this,the micro-resonator and biosensor are combined to explore the production of carbon nanotubes and graphene micro-resonator.And then the deviced was used to detect the concentration of single-stranded DNA solution.Traditional micro-resonators are silicon-based devices,and large parasitic capacitance exists between the electrodes of the device because of its own electrical property as semiconductor material.As a result,great signal loss could interference test accuracy when the device is applied high frequency(?MHz).Here we propose to fabricate a micro-resonator with a glass substrate to replace the traditional silicon-based device through self-aligned lithography process.Then the parasitic capacitance of the new device was measured.For comparison,we also test the parasitic capacitance of the device with the same structure based on silicon wafer.The results show that the parasitic capacitance of the micro-resonator fabricated by self-aligned process and based on glass substrate is 50 fF,while the parasitic capacitance of the silicon-based device is 2 pF.The carbon nanotubes were assembled onto the device by dielectrophoresis in order to form a suspending carbon nanotube film with two fixed ends.To verify its electrical properties,the device was tested with a semiconductor parametric analyzer as a transistor.The output and transfer characteristic curves were obtained.Then we built a test system based on our fabricated micro-resonator,vector network analyzer and preamplifier to detect the concentration of ssDNA.The transmission coefficient S21 was recorded after the ssDNA solution dropping on the CNT film of the micro-resonator and the corresponding frequency was seen as resonant frequency when the transmission coefficient S21 achieved peak value.The result shows that the resonant frequency is affected by the concentration of probe ssDNA solution when the probe ssDNA molecules bind with suspending CNT film.The resonant frequency decreases,that is,the so-called red-shit occurred.The value of resonant frequency shift increases with the increase of concentration of the probe ssDNA solution.The complementary ssDNA was measured after the micro-resonator with suspending CNT film saturating by probe ssDNA solution,while this time,the result is just opposite.The resonant frequency increases,that is,the so-called blue-shit occurred.The value of resonant frequency shift also increases with the increase of the concentration of complementary ssDNA solution.Real-time detection is finished in 60 minutes and the available testing limit of ssDNA concentration is 5 nMol/L.However,carbon nanotubes are classified into semiconducting and metal types by their chiral angles,which affect the electrical properties of carbon nanotubes themselves.And it is difficult to obtain one single type of carbon nanotubes with very high purity.In addition,the excellent electrical conductivity of carbon nanotubes is just limited along the tube,and the high degree of directional uniformity is still very difficult to achieve in the real life even though many methods have been proposed.These factors limit the performance of the micro-device based on carbon nanotube.Graphene,a new nanomaterial composed of single layer of carbon atoms with a honeycomb hexagonal structure,also shows high electrical conductivity and good mechanical properties just like carbon nanotube.Graphene tends to reunion in water because of Van der Waals force between single-layer carbon atoms,so the graphene dispersed in water is not easy to stabilize.Besides,the graphene cannot be easily assembled on the micro-device by dielectrophoresis.Therefore,the alternative plan is to assemble suspending graphene oxide film by using dielectrophoresis.The suspending graphene oxide film of the micro-resonator was then reduced in situ by hydrogen in a tubular furnace with low temperature(?200 ?).The morphology of reduced graphene oxide(RGO)was characterized by scanning electron microscopy(SEM)and atomic force microscopy(AFM).Then the electrical properties of RGO film were characterized by X-ray dif&actometer and Raman spectroscopy.In addition,the electrical properties of graphene oxide film after reduction were also tested.It shows that the best reduction effect of graphene oxide film on the micro-device occurs at 200 ?.The same testing system was used to detect the concentration of ssDNA solution based on suspending graphene film micro-resonator.The combination between probe single-stranded DNA and graphene causes the curve plotted by transmission coefficient S21 and the sweeping frequency to appear a whole left-shift.The resonant frequency corresponding to the center peak of the transmission coefficient S2,changes from 120 kHz to 0 when the concentration of probe ssDNA decreases from 5?Mol/L to 5 nMol/L.The curve plotted by transmission coefficient S21 and the sweeping frequency shows whole right-shift when the micro-resonator was used to measure the complementary ssDNA after saturation treatment with probe ssDNA.The blue-shift of the resonant frequency is 10 kHz when the concentration of complementary ssDNA solution is only 50 nMol/L.Finally,the test results between the carbon nanotube and graphene thin film devices were compared,it shows that the test accuracy of the carbon nanotube thin film device is higher,but the reproducibility is poor,while it is just opposite for the graphene resonator.This is probably related to the device's preparation method and the material's own performance.In general,we explored a practical method for rapidly detecting the concentration of single stranded DNA by using a micro nano-film resonator in real-time,which will enable real-time detection for other biomolecules in the future. |
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