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Breast cancer is the most common cancer in women and second most deadly. The onventional treatment involves a non-specific systemic delivery of therapeutic agent and this often results in unwarranted side effects which greatly decrease the patient’s quality of life. This work aims to identify aptamers capable of specifically targeting the breast cancer cell surface protein to develop: 1) multifunctional magnetic nanoparticles for in vivo breast cancer detection and targeted therapy; 2) biosensor for the detection of tumor biomarker. Human epidermal growth factor receptor 2 (HER2) is a transmembrane protein overexpressed in 20-30% of breast cancers. HER2 positive cancer is notorious for higher fatality and recurrence rates primarily due to strong drug resistance of HER2 positive cancer cells. Herceptin, a monoclonal antibody cognitive of HER2 extracellular domain (ECD), is currently used in the tr atment of HER2 positive cancer. Aptamer has advantages over monoclonal antibody in targeting HER2 in terms of lower cost, equal or greater targeting specificity, lower immunogenicity, more hydrophilic (hence less likely to cause particles aggregation in an aqueous environment once it is immobilized onto the particles), greater stability, and reproducible synthesis. We characterized the binding affinity and specificity of single-stranded DNAs, screened via 10 rounds of CE-based SELEX method, against HER2 ECD. The percentage of ssDNA eluted from the nitrocellulose membrane filter binding assay, measured for the fractionated DNA pool following each round of CE-based SELEX screening increased with selection rounds, indicating successful enrichment of aptamers with enhanced affinity to the target protein. The affinity of the selected aptamers to HER2/Fc protein after the 10th round was also tested by capillary electrophoresis. After incubation of HER2/Fc protein with the selected aptamers at 1:1 ratio, a significant reduction (ca. 46%) of free aptamer peak during voltage separation (as compared to the aptamer peak in the absence of protein) was found, further confirming that 10 rounds of CE SELEX screening led to a successful enrichment of affinity aptamers to HER2/Fc. The specificity of the screened aptamers toward HER2 positive breast cancer cell line (SkBr3) is under investigation with the use of breast cancer cell line (MDA-MB-231) lacking HER2 ECD as a control. Following the confirmation of specificity of the screened aptamers, their targeting efficacy (through fluorescence microscopy observation), cell ytotoxicity, and cell growth inhibition (via MTT assay) will be further tested to judge the suitability of harnessing the screened aptamers for targeted aptamer-based imaging, therapy and diagnostics of breast cancer.