A method and system for detecting mercury in a hydrocarbon-containing fluid stores a sample of the hydrocarbon-containing fluid in a first reservoir. A liquid phase reagent solution is stored in a second reservoir. The liquid phase reagent solution includes nanoparticles with an affinity to mercury, wherein the nanoparticles are suspended as a colloid in the liquid phase reagent solution. The sample of the hydrocarbon-containing fluid is delivered from the first reservoir into a first port of a fluidic device while the liquid phase reagent solution is delivered from the second reservoir into a second port of the fluidic device such that the fluidic device produces slug flow. The slug flow is subject to optical analysis that determines concentration of mercury in the sample of the hydrocarbon-containing fluid.