A method for detection of presence or absence of analytes in fluids, in an optical system, comprising the steps of: a) contacting in a mixing container, a fluid sample and a plurality of silver nanoparticles, wherein each of the silver nanoparticles comprises, a core unprotected which is void of any coating that prevents plasmotic coupling between a pair of nanoparticles or among several nanoparticles and the silver nanoparticles are tagged with at least a molecular species with a defined SERS spectrum, the nanoparticles being functionalized with at least a selective ligand for a given receptor of a target analyte, the contact of the fluid sample with the nanoparticles being under conditions such that, when the fluid sample comprises at least an analyte, 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 and thus a plasmon coupling in between the nanoparticles through the generation of narrow inter-nanoparticle gaps; b) circulating the fluid mixture of the fluid sample and the plurality of nanoparticles through at least a passage (102A, 102B, 102C) driven by a pump, towards the detection section; c) subjecting a fluid mixture of the fluid sample and the plurality of nanoparticles to SERS passing said fluid mixture of fluid sample and nanoparticles through the detection section where an- emitter issues an incident monochromatic light to strike the fluid mixture of the fluid sample and the plurality of nanoparticles and wherein the incident monochromatic light is allowed to strike the fluid mixture passing through a detection section associated with the passage (102A, 102B, 102C), and wherein the passage is a microchannel and it is divided in a sections (102A, 102B and 102C): a narrow initial section of the passage that increases pressure drop of the flow necessary to finely control the flow discharge with the pump, the second section of the passage 102B expands the volume of the microchannel until the third section of the passage, 102C, wherein the dimensions of the third section are 0.4 mm width and 2 mm depths; d) measuring at least a SERS signal associated with the fluid mixture by a reader; e) spectrally analysing the at least one SERS signal of step c) by control means; f) 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, when the fluid mixture comprises at least a nanoparticle-analyte complex with the narrow inter-nanoparticle gap. a) contacting in a mixing container, a fluid sample and a plurality of silver nanoparticles, wherein each of the silver nanoparticles comprises, a core unprotected which is void of any coating that prevents plasmotic coupling between a pair of nanoparticles or among several nanoparticles and the silver nanoparticles are tagged with at least a molecular species with a defined SERS spectrum, the nanoparticles being functionalized with at least a selective ligand for a given receptor of a target analyte, the contact of the fluid sample with the nanoparticles being under conditions such that, when the fluid sample comprises at least an analyte, 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 and thus a plasmon coupling in between the nanoparticles through the generation of narrow inter-nanoparticle gaps; b) circulating the fluid mixture of the fluid sample and the plurality of nanoparticles through at least a passage (102A, 102B, 102C) driven by a pump, towards the detection section; c) subjecting a fluid mixture of the fluid sample and the plurality of nanoparticles to SERS passing said fluid mixture of fluid sample and nanoparticles through the detection section where an- emitter issues an incident monochromatic light to strike the fluid mixture of the fluid sample and the plurality of nanoparticles and wherein the incident monochromatic light is allowed to strike the fluid mixture passing through a detection section associated with the passage (102A, 102B, 102C), and wherein the passage is a microchannel and it is divided in a sections (102A, 102B and 102C): a narrow initial section of the passage that increases pressure drop of the flow necessary to finely control the flow discharge with the pump, the second section of the passage 102B expands the volume of the microchannel until the third section of the passage, 102C, wherein the dimensions of the third section are 0.4 mm width and 2 mm depths; d) measuring at least a SERS signal associated with the fluid mixture by a reader; e) spectrally analysing the at least one SERS signal of step c) by control means; f) 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, when the fluid mixture comprises at least a nanoparticle-analyte complex with the narrow inter-nanoparticle gap.