Abstract: A method and a system for detection of presence or absence of analytes in fluids, the method comprising the steps of: a) contacting a fluid sample and a plurality of nanoparticles, the nanoparticles being functionalized with a selective ligand, the contact of the fluid sample with the nanoparticles being under conditions such that, the nanoparticles aggregate selectively on surface of their target analyte, forming a nanoparticle-analyte complex, the aggregation promoting the concentration of the nanoparticles on the surface of the target analyte; b) subjecting a fluid mixture of the fluid sample and the plurality of nanoparticles to SERS; c) measuring at least a SERS signal associated with the fluid mixture; d) spectrally analyzing the at least one SERS signal of step c); e) recognizing at least one defined SERS spectrum of the nanoparticle-analyte complex, through a SERS signal enhanced by the at least one narrow inter-nanoparticle gap.