Fiber optic surface plasmon resonance based disposable probe for the detection of phosphate ion in soil

Fabrication and characterization of a fiber optic surface plasmon resonance (SPR) based phosphate ion sensor utilizing the coating of cobalt oxide (CoO) over the gold coated unclad core of the fiber have been carried out. The interaction between CoO layer and incoming phosphate ion changes the effective refractive index of the CoO layer which affects the resonance wavelength in the SPR spectrum. The performance of the sensor has been evaluated in acidic solution (pH 4) of inorganic phosphate salt (KH2PO4). The sensor probe has been found to be irreversible implying one time use of the probe or disposable probe. However, the probe works for 0-100 ppm (0-1.03 mM) concentration range of the phosphate ion (H2PO4- ). The sensor possesses maximum sensitivity of 0.3155 nm/ppm with 4.96 ppm as the limit of detection. The sensor is highly selective, fast in response and is suitable to determine the phosphate ion concentration in soil. Since the probe has been fabricated on optical fiber it has the advantages of simplicity, miniaturization and low cost.