i) providing a microfluidic channel comprising a surface plasmon resonance (SPR) active surface;

ii) immobilizing an analyte capture agent on the surface;

iii) contacting the analyte capture agent with the test solution and an enzyme labeled reagent under conditions for the analyte capture agent to form a complex with an analyte or the enzyme labeled reagent, wherein the enzyme is capable of forming a precipitate when reacted with a precipitatable substrate;

iv) flowing solution containing the precipitatable substrate of the enzyme through the microfluidic channel so as to contact the complex with the solution comprising the precipitatable substrate of the enzyme under conditions conducive to the formation of the precipitate product for a time period sufficient for forming the precipitate; and

v) detecting a change in refractive index near the surface as a function of spatial position on the surface due to precipitate formation using a SPR signal, wherein the change in the refractive index at a selected location is indicative of amount of the analyte on the surface at the selected location.