System for optically detecting at least one particle in a fluid, this fluid having a determined refractive index, comprising: - said at least one particle, stored before fluid is supplied and arranged to be suspended in the fluid thereafter, comprising a metallic material enabling a localized plasmon resonance at at least one resonant wavelength if excited at at least one excitation wavelength, a sensor platform comprising: o a sensor surface; o a platform material extending from the sensor surface, transparent to said at least one excitation wavelength and said at least one resonant wavelength, and having a refractive index higher than said determined refractive index and defining accordingly an optical critical angle from the normal of the sensor surface such that an electromagnetic wave propagating in this platform material and incident to the sensor surface at an angle greater than the critical optical angle is totally reflected onto the sensor surface; - an optical detector adapted to detect at least a portion of the spectrum of the optical wave which has been totally reflected onto the sensor surface, said portion including said at least one resonant wavelength; and - a processor arranged to determine a presence of said at least one particle on or close to the sensor surface from the frustrated totally internal reflection, "FTIR", signal retrieved from the detected wavelengths, wherein this retrieving takes into account the detected resonant components present in the FTIR signal, characterized in that said at least one particle is functionalized with a specific bond or chemistry reactive to a target analyte in the fluid, and in that the sensor surface is functionalized with specific bonds to a target analyte or to an analogue of the target analyte in the fluid. - said at least one particle, stored before fluid is supplied and arranged to be suspended in the fluid thereafter, comprising a metallic material enabling a localized plasmon resonance at at least one resonant wavelength if excited at at least one excitation wavelength, a sensor platform comprising: o a sensor surface; o a platform material extending from the sensor surface, transparent to said at least one excitation wavelength and said at least one resonant wavelength, and having a refractive index higher than said determined refractive index and defining accordingly an optical critical angle from the normal of the sensor surface such that an electromagnetic wave propagating in this platform material and incident to the sensor surface at an angle greater than the critical optical angle is totally reflected onto the sensor surface; o a sensor surface; o a platform material extending from the sensor surface, transparent to said at least one excitation wavelength and said at least one resonant wavelength, and having a refractive index higher than said determined refractive index and defining accordingly an optical critical angle from the normal of the sensor surface such that an electromagnetic wave propagating in this platform material and incident to the sensor surface at an angle greater than the critical optical angle is totally reflected onto the sensor surface; - an optical detector adapted to detect at least a portion of the spectrum of the optical wave which has been totally reflected onto the sensor surface, said portion including said at least one resonant wavelength; and - a processor arranged to determine a presence of said at least one particle on or close to the sensor surface from the frustrated totally internal reflection, "FTIR", signal retrieved from the detected wavelengths, wherein this retrieving takes into account the detected resonant components present in the FTIR signal, said at least one particle is functionalized with a specific bond or chemistry reactive to a target analyte in the fluid, and in that the sensor surface is functionalized with specific bonds to a target analyte or to an analogue of the target analyte in the fluid.