A method for capturing and detecting a molecule in a sample, comprising the following steps: a) mixing said sample with magnetic particles, each of said particles being coupled with an element capable of binding selectively to said molecule to be captured, so as to form at least one complex comprising a magnetic particle, said element and said molecule bound to said element, these magnetic particles having a diameter comprised between 5 nm and 500 nm, b) immobilizing said at least one complex on a support comprising ordered magnetic field micro-sources, and c) detecting the immobilised molecule by fluorescence, characterised in that spatial organization of the ordered magnetic field micro-sources defines strong magnetic field gradient areas and low magnetic field gradient areas, and in that the detection step is implemented directly after the immobilisation step, without any intermediate step of washing the support, by determining the fluorescence density at the surface of the ordered magnetic field micro-sources in the high magnetic field gradient areas, defining a specific signal, and in the low magnetic field gradient areas, defining a non-specific signal, in that said support is a micro-channel comprising in at least one of its walls a network of magnetic micro-sources arranged in an ordered manner, and in that the concentration of magnetic particles in the mixture obtained upon completion of step a) is higher than 10 6 particles/ml. a) mixing said sample with magnetic particles, each of said particles being coupled with an element capable of binding selectively to said molecule to be captured, so as to form at least one complex comprising a magnetic particle, said element and said molecule bound to said element, these magnetic particles having a diameter comprised between 5 nm and 500 nm, b) immobilizing said at least one complex on a support comprising ordered magnetic field micro-sources, and c) detecting the immobilised molecule by fluorescence, characterised in that spatial organization of the ordered magnetic field micro-sources defines strong magnetic field gradient areas and low magnetic field gradient areas, and in that the detection step is implemented directly after the immobilisation step, without any intermediate step of washing the support, by determining the fluorescence density at the surface of the ordered magnetic field micro-sources in the high magnetic field gradient areas, defining a specific signal, and in the low magnetic field gradient areas, defining a non-specific signal, in that said support is a micro-channel comprising in at least one of its walls a network of magnetic micro-sources arranged in an ordered manner, and in that the concentration of magnetic particles in the mixture obtained upon completion of step a) is higher than 10 6 particles/ml. characterised in that spatial organization of the ordered magnetic field micro-sources defines strong magnetic field gradient areas and low magnetic field gradient areas, and in that the detection step is implemented directly after the immobilisation step, without any intermediate step of washing the support, by determining the fluorescence density at the surface of the ordered magnetic field micro-sources in the high magnetic field gradient areas, defining a specific signal, and in the low magnetic field gradient areas, defining a non-specific signal, in that said support is a micro-channel comprising in at least one of its walls a network of magnetic micro-sources arranged in an ordered manner, and in that the concentration of magnetic particles in the mixture obtained upon completion of step a) is higher than 10 6 particles/ml.