An optical sensor and corresponding method of operation can detect a phase transition and/or related property of a hydrocarbon-based analyte. The optical sensor includes an optical element with a metallic film coupled or integral thereto, with a sample chamber holds the hydrocarbon-based analyte such that the hydrocarbon-based analyte is disposed adjacent the metallic layer. The optical sensor further includes a light source configured to direct light through the optical element such that the light is reflected by the metallic layer under conditions of surface plasmon resonance. The optical sensor analyzes the reflected light to detect a phase transition and/or related property of a hydrocarbon-based analyte.