A Surface Plasmon Resonance Optical Fibre Sensor for Testing Detergent Cleaning Efficiency

The objective of this paper is to report a feasibility study on the use of optical fibre and surface plasmon resonance (SPR) for determining detergent efficiency. The concept presented comprises a miniaturised dip-sensor for enabling automated on-line testing. In this way, the effect of formulation, concentration and temperature on the cleaning behavior of various surfactants and commercial cleaners is investigated. For this purpose, the decladded core of an optical fibre was sputtered with gold and afterwards coated with defined layers of stearic acid or animal fat to obtain a kind of model soil. The deposition of stearic acid was performed using a Langmuir-Blodgett through, and the sensor followed on-line the deposition of the respective monolayers by UV/VIS spectroscopy, appearing as a distinct and constant shift in wavelength. Moreover, functional coatings were applied above the gold layer to achieve a variation of the hydrophilicity of the sensor surface. The SPR sensor proved to be easy to use, accurate and flexible. It offers a new solution that could replace the existing methods for detergency sensing and with a customised design it could be a useful industry tool since the small size of the dip sensor promises massive testing. The experiment also showed that functionalising the sensing zone could act as a way to mimic the potential substrates for cleaning.