Detection of Biomolecules Using Volume-Amplified Magnetic Nanobeads
Abstract: This thesis describes a new approach to biomolecular analysis, called the volume-amplified magnetic nanobead detection assay (VAM-DNA). It is a sensitive, specific magnetic bioassay that offers a potential platform for the development of low-cost, easy-to-use diagnostic devices. The VAM-NDA consists of three basic steps: biomolecular target recognition, enzymatic amplification of the probe-target complex using the rolling circle amplification (RCA) technique, and addition of target complementary probe-tagged magnetic nanobeads which exhibit Brownian relaxation behavior. Target detection is demonstrated by measuring the frequency-dependent complex magnetization of the magnetic beads. The binding of the RCA products (target DNA-sequence coils) to the bead surface causes a dramatic increase in the bead size, corresponding essentially to the size of the DNA coil (typically around one micrometer). This causes a decrease in the Brownian relaxation frequency, since it is inversely proportional to the hydrodynamic size of the beads. The concentration of the DNA coils is monitored by measuring the decrease in amplitude of the Brownian relaxation peaks of free beads.The parameters oligonucleotide surface coverage, bead concentration, bead size and RCA times were investigated in this thesis to characterize features of the assay. It was found that all of these parameters affect the outcome and efficiency of the assay.The possibility of implementing the assay on a portable, highly sensitive AC susceptometer platform was also investigated. The performance of the assay under these circumstances was compared with that using a superconducting quantum interference device (SQUID); the sensitivity of the assay was similar for both platforms. It is concluded that, the VAM-NDA opens up the possibility to perform biomolecular detection in point-of-care and outpatient settings on portable platforms similar to the one tested in this thesis.Finally, the VAM-NDA was used to detect Escherichia coli bacteria and the spores of Bacillus globigii, the non-pathogenic simulant of Bacillus anthracis. A limit of detection of at least 50 bacteria or spores was achieved. This shows that the assay has great potential for sensitive detection of biomolecules in both environmental and biomedical applications.
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