A microfluidic device for detecting target molecules in a fluid, the microfluidic device having an array of chambers, each containing electrochemiluminescent (ECL) probes for reaction with the target molecules to form a probe-target complex, and electrodes positioned in each of the chambers for receiving an electrical pulse, the probe-target complexes being configured to emit a photon of light when excited by current between the electrodes, a flow-path for the fluid containing the targets, wherein, each of the chambers have a chamber inlet for fluid communication between the sample flow-path and the probes within the chamber, the chamber inlet being configured as a diffusion barrier to prevent diffusion of the probe-target complexes between the chambers to an extent that causes erroneous detection results.