An optical-microcavity-based sensor is an important building block for an optofluidics system, because it allows us to fabricate small devices with high sensitivity. Here we describe the detection of NaCl and pH in water using a silica toroid microcavity. First we demonstrate the detection of NaCl particles, and show that a detection sensitivity of 0.38 mM is possible with a sample volume of 0.03 nl. Then, we report the detection of NaCl ions in liquid and demonstrate a sensitivity of 3.20 mM and also, in principle, the detection of pH with a sensitivity of 0.14 pH. Finally, we integrate a tapered optical fiber, a silica toroid microcavity, and a fluidic channel for future optofluidics applications.