In this work, we developed a microwave stripline sensor based on the optimized double quadratic-shape design and the concentration of Ag nanoparticles is measured in aqueous solutions by using a real-time microwave near-field electromagnetic interaction. The {tiS}₁₁ microwave reflection parameter of the sensor at the resonant frequency was changed depending on the Ag nanoparticles concentration in the aqueous solution. It is possible to determine the Ag nanoparticles concentration in the range of 0–50 ώg/l at the operating frequency of about 2 GHz. The measured minimum registered signal and concentration were equal to 0.03 dB/(ώg/l) and 1 ώg/l, respectively. The electromagnetic response of these type of systems is explained by the structural changes of water due to the Ag nanoparticles ablation. Because of the low cost and easy fabrication process, this system enables one to develop a chemical sensor for non-invasive monitoring of metal nanoparticles in liquid media.