We propose, in this paper, a sensor dedicated to lab-on-chip. This sensor is a passive microfluidic resonator based on the resonance of capillary waves. These waves are excited by natural thermal fluctuations and propagate at the interface between immiscible fluids. When the characteristic size of the interface is lower than the propagation length, resonance occurs. The power spectrum of the propagating waves is measured by an optical spectroscopic method: single beam dynamic light scattering. In this work, the concept of microfluidic resonator is theoretically and experimentally demonstrated. The overall system is robust, simple to use and has a potential resolution down to few molecules.