Whispering gallery mode (WGM) optical microcavities trap light in micro-scale volumes by continuous total internal reflection which leads to enhancement of light intensity within a confined region and longer photon lifetime. Consequently, light-matter interaction is enhanced making the WGM resonator an extremely sensitive platform for the detection of perturbations in and around the resonator. Here, we report mode-splitting in monolithic ultra-high-Q WGM microcavities for real-time and in-situ detection of single nanoparticles. We investigate experimentally and theoretically particle detection and sizing at single nanoparticle resolution using the mode-splitting technique. Theoretical calculations are in good agreement with the experimental results. The mode-splitting effect provides a 'self-reference sensing' technique that can overcome the limitations of current resonator-based sensors and in the meantime keep the advantages offered by resonant structures for high-performance sensing.