A sensor platform (300) with a sensor surface (350) receives particles (400). A material extending from the sensor surface (350) having a refractive index higher than the one of the fluid (200), such that an electromagnetic wave (10) propagating in this platform material (310) and incident to the sensor surface (350) at an angle greater than the critical optical angle is totally reflected onto the sensor surface (350). The particles (400) suspended in the fluid (200) include a metallic material (410) enabling a localized surface plasmon resonance at resonant wavelength(s). An optical detector (102) detects a portion of a spectrum of the totally reflected wave (20), including the resonant wavelength(s). A processor determines a presence of the particles (400) on or close to the sensor surface (350) from a frustrated totally internal reflection (“FTIR”) signal retrieved from the detected wavelengths. The retrieving takes into account the detected resonant component(s) present in the FTIR signal.