Reivindicaciones

1. Sensor for the determination of solute concentrations in solutions, characterized in that said sensor comprises at least one porous, optically active, birefringent layer which is prepared by physical vapor deposition in a glancing geometry. 2. Sensor according to claim 1, characterized in that it further comprises two porous one-dimensional photonic crystals formed by birefringent layers with different refractive indexes separated by a porous central layer defining a Bragg microcavity, said crystal layers being prepared by physical vapor deposition in a glancing geometry. 3. Sensor according to claims 1 or 2, characterized in that the photonic crystals comprise alternant layers of SiO<2> and TiO<2> . 4. Sensor according to anyone of claims 1 to 3, characterized in that the inner part of the pores of the porous layers are functionalized. 5. Apparatus for the determination of solute concentrations in solutions which comprises a microfluidic device (1), wherein said apparatus comprises a sensor (2) as defined in anyone of claims 1 to 4, embedded into said microfluidic device. 6. Apparatus according to claim 5, characterized in that said microfluidic device (1) comprises two near IR and visible range transparent windows, said sensor (2) being located between said transparent windows in the microfluidic device (1). 7. Apparatus according to claim 5, characterized in that the sensor (2) is located on the tip of an optical fiber insertable in the microfluidic device (1). 8. Method for the determination of solute concentrations in solutions by means of an apparatus as defined in claim 5, said method characterized in that it comprises: i) contacting sensor (2) with the solution containing the solute whose concentration is to be determined, ii) illuminating linearly polarized light onto sensor (2), once it has been infiltrated with the solution whose solute concentration is to be determined. Iii) analyzing light transmitted through the microfluidic device by means of a polarization analyzer which splits the parallel (11) and perpendicular (12) components with respect to the polarization vector of the light beam illuminating the microfluidic device (1). iv) determining the respective intensities (11/12) of said light beams by means of photodiodes. v) calculating the 11/12 intensities ratio. vi) determining solute concentration from variations in the intensities 11/12 ratio through correlation with a calibration curve. 9. Method according to claim 8, characterized in that the sensor (2) is integrated in a microfluidic device (1) which comprises two near IR and visible range transparent windows, said sensor (2) being located between said transparent windows, data being thereby retrieved in reflection mode. 10. Method according to claim 8, characterized in that the sensor (2) is located on the tip of an optical fiber which is directly introduced in the solution, data being thereby retrieved in reflection mode, by means of reflection spectra registered from polarizers in aligned or crossed configuration, resulting in spectra as function of solute concentrations.