Surface enhanced Raman spectroscopy (SERS) microfluidics biosensor for detecting single and/or multiple analytes

A biosensor for the detection of an analyte using surface-enhanced Raman spectroscopy (SERS), the biosensor comprising a SERS-active substrate and a microfluidic circuit device arranged to be in fluid communication with the SERS-active substrate, wherein the microfluidic circuit device comprises a concentration gradient generator having two or more mixing structures and (i) at least two inlets and (ii) at least two outlets, wherein a plurality of respective microchannels extend from each of the at least two outlets of the concentration gradient generator, and wherein the microfluidic circuit device further comprises at least one cross-flow line intersecting with and in fluid communication with each of the microchannels, wherein each of the at least one cross-flow line has an inlet for introducing an analyte binding molecule into the microfluidic circuit device, wherein each of the at least one cross-flow line which intersects with each of the microchannels defines respective reaction chambers in which a reaction between said analyte binding molecule and a sample from one of the at least two inlets occurs, and wherein the SERS-active substrate is comprised in or forms at least a portion of each of the reaction chambers.