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We present a label-free aptamer sensor based on an array of silicon micro-ring resonators. An array of sensors consists of three sensor rings and a reference ring was fabricated and its surface sensitivity and the limit of detection was obtained using polyelectrolyte multilayer. The mass sensitivity and the limit of detection of the fabricated sensor ring is 0.35 nm/ng mm-2 and 42.8 pg/mm2 in air and 0.24 nm/ngmm-2 and 62.5 pg/mm2 in water, respectively. We successfully demonstrated the binding assay over a broad range of protein concentrations using a model system with human thrombin as the analyte and DNA aptamers as immobilized ligands.