Development And Basic Application Study Of Multi-biodetection Based On AC Susceptibility

Magnetism is an important property for magnetic materials.Susceptibility is one important parameter to describe the magnetizationproperty of materials. When a certain kind of material is placed in an ACmagnetic field, we can use AC susceptibility to help us understand therelation between magnetic moment and the structure of the material.Nanoparticles are one kind of material that are widely used inbio-medicine such as bio-detection, drug delivery, magnetic hyperthermiaand MRI contrast agent. AC susceptibility measurement can be developedto be a promising method in bio-detection. But till now, the researchinterests are mostly focused on the susceptibility of bulk materials ratherthan magnetic fluid. Therefore, to establish methodology for measuringmagnetic fluid’s susceptibility becomes essential and it will be thecornerstone of further applications.The theoretical base of this article is the relationship betweenmagnetic nanoparticles’ Brownian relaxation characteristic frequency andthe hydrodynamic size of magnetic nanoparticles. When thebio-molecules are attached to magnetic nanoparticles, the hydrodynamicsize of the particle will rise, causing the characteristic frequency shift tolower range. It is very sensitive to use AC susceptibility to observe thevariation of the hydrodynamic size of nanoparticles because the exponentnumber is3. Small changes of hydrodynamic size will cause huge shift of characteristic frequency. The thesis is divided into4parts:Firstly, we established a home-made AC susceptibility measurementsystem. It is based on the fundamental principles of AC MutualInductance Bridge. The system can be divided into a coil assembly and alock-in amplifier. We wind coils with4different structures which areused for different frequency range. The results show the system canmeasure at frequency from1Hz to1MHz. then we use standard samplesto calibrate the system, making the measurement more reliable.Secondly, We studied the AC susceptibility of several typicalnanoparticles and microbeads using the system we have alreadyestablished. According to the theory we have proved before, we useMatlab to analyze the AC susceptibility spectrum by fitting data via LeastSquare Method and then study the relationship between AC susceptibilityand magnetic moment of specific samples. The results reveal that therelationship between magnetic moment and hydrodynamic size of theparticles relies much on the synthesize method of different samples. Also,by comparing the difference between calculated and measuredhydrodynamic size, we point out the lack of concern of the diversificationof the relations mentioned above in previous study.Finally, by mixing the nanoparticles with different hydrodynamicsize, we expected to make this kind of detection a multiplexedbio-detection and study the feasibility in material aspect. The resultsdepict that the AC susceptibility of microbeads with different sizessynchronize with the simple additive law. The mathematic ways toresolve the spectrum is still in progress.