We demonstrate a simple refractive index sensor (RI) sensing system based on a biconical tapered optical fiber (BTOF),
which is fabricated by heat pulling method, utilizing a CO<sub>2</sub> laser. In this work we explore the application of these sensors
for the detection of label free single stranded DNA (ssDNA) in real time. During the experiment, the target ssDNA did
not need to be labeled with a fluorescent tag, which is expensive and complicated. The change in output optical
transmission of the tapered fiber was recorded for Poly-L-Lysine (PLL) coating, ssDNA probe immobilization and
hybridization. The result indicated that due to the hybridization with the complementary target ssDNA on the tapered
surface, the RI of surrounding medium changes which leads to changes in the characteristics of the tapered region and
change in the output power of the sensor.