Sensor, apparatus and method for determining a concentration of solute in a solution (PCT /ES2016/070764) The document describes a sensor, apparatus and method for the direct analysis of fluids in order to determine a concentration of solutes in solutions, such as the concentration of glucose in aqueous solutions. Transducers with porous birefringent monolayer or multilayer structures (such as a planar photonic structure in the form of a Bragg microcavity) may be used as sensors. The method of the invention can be performed in two possible implementations; a first one, wherein the sensor (2) is a photonic crystal in a microfluidic device (1) which comprises two transparent windows, data being retrieved in transmission mode and a second one, wherein the photonic crystal sensor is located on the tip of an optical fiber, data being retrieved in reflection mode.