Functionalized Long Period Grating—Plasmonic Fiber Sensor Applied to the Detection of Glyphosate in Water

Selectivity was added to a refractometric fiber sensor based on a long-period grating coated with a film of gold nanoparticles by the functionalization with cysteamine. The sensor was applied to the detection of glyphosate in water. Sensor operation is based on the reaction between glyphosate molecules and cysteamine that modifies the effective refractive index of the long-period grating cladding modes. In the presence of water containing glyphosate, spectral changes occur in the long-period grating attenuation bands, allowing the sensor interrogation both in wavelength and intensity. The sensor relies on the high sensitivity of long-period gratings operating at the visible spectral range near the turning point, as well as on the resonance between the cladding modes and the plasmon resonance band of the nanoparticles. It was shown that the resonance between the LPG and LSPR bands increases the sensitivity of the LPG attenuation bands to the glyphosate in the samples. The sensor configuration proposed in this paper was tested with water samples deliberately contaminated with glyphosate showing a limit of detection about 0.02 μM.