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We experimentally demonstrate a novel refractive index (RI) sensor with the measurand infiltrated into the air channels of a single mode photonic crystal fiber (PCF). A tunable long-period grating is formed in a short length of PCF by use of the periodic mechanical pressure. Resonance shift with the change of the infiltrated liquid is observed with a sensitivity of 4.1times 10-6 RI unit in a refractive index range from 1.32 to 1.39. The thermal stability of the pure-silica-based PCF device shows a three times better performance compared with conventional single mode fiber.