Surface plasmon resonance based fiber optic potassium ion disposable sensing probe for soil testing

Abstract

Fabrication and characterization of a fiber optic surface plasmon resonance (SPR) based potassium ion (K+) sensor utilizing the coating of dicyclohexeno-18-crown-6 ionophore incorporated in plasticized PVC membrane along with lipophilic pH sensitive dye chromoionophore-I over the silver coated unclad core of the fiber have been carried out. The interaction between ion selective cocktail layer and incoming K+ changes the effective refractive index of the sensing layer which affects the resonance wavelength in the SPR spectrum. To achieve best sensor performance, the thickness of the sensing layer has been optimized. The sensor probe has been found to be irreversible due to the very strong interaction between ionophore and target K+. The ring type structure of ionophore used provides a close fit to the incoming K+ and hence interacts strongly with it which makes K+ incapable of leaving the cavity of the ionophore. The washing of sensor probe with DI water could not break this bonding. This results in one time use of the probe or disposable type sensor probe. However, the probe works for 0 to 180 µg/mL concentration range of K+. The sensor is highly selective to K+ and is not affected by nitrate, phosphate, calcium and sodium ions and can be used to check the concentration of K+ in soil. In addition, the sensor possesses a response time of around 50 sec. Since the probe has been fabricated on optical fiber it has various advantages such as miniaturization, low cost, fast response, capability of real time monitoring and remote sensing.