A microfluidic sensor for sensing biological particles including a particle concentration device for performing concentration of particles in three dimensions, the sensor device comprising: a substrate which defines a lateral plane of said sensor, said substrate bearing a microfluidic channel or chamber for carrying a conductive fluid bearing said biological particles for concentration, wherein said channel has: upper and lower surfaces spaced apart in said direction perpendicular to said lateral plane, said upper surface being further from said substrate than said lower surface, and first and second electrodes spaced apart on said channel or chamber for defining an electric field therebetween; and a sensing surface on an inner surface of said channel or chamber for contact with said fluid to selectively sense said biological particles; wherein said particle concentration device comprises means for applying an ac voltage across said electrodes to perform simultaneously: i) electrohydrodynamic generation of a convection current flow in said fluid; and ii) 3D concentration of said particles in said fluid by dielectrophoretic attraction or repulsion of said biological particles to or from a region of increased electric field generated by said ac voltage across said electrodes; whereby said convection current flow and said 3D concentration increase a concentration of said biological particles towards said sensing surface of said sensor, characterised wherein: - one or both of said electrodes is disposed on said upper surface and - said sensing surface is opposite said electrode, and wherein in use, said biological particles circulate past said sensing surface and concentrate in three dimensions in a direction perpendicular to said lateral plane adjacent to said sensing surface. a substrate which defines a lateral plane of said sensor, said substrate bearing a microfluidic channel or chamber for carrying a conductive fluid bearing said biological particles for concentration, wherein said channel has: upper and lower surfaces spaced apart in said direction perpendicular to said lateral plane, said upper surface being further from said substrate than said lower surface, and first and second electrodes spaced apart on said channel or chamber for defining an electric field therebetween; and a sensing surface on an inner surface of said channel or chamber for contact with said fluid to selectively sense said biological particles; upper and lower surfaces spaced apart in said direction perpendicular to said lateral plane, said upper surface being further from said substrate than said lower surface, and first and second electrodes spaced apart on said channel or chamber for defining an electric field therebetween; and a sensing surface on an inner surface of said channel or chamber for contact with said fluid to selectively sense said biological particles; wherein said particle concentration device comprises means for applying an ac voltage across said electrodes to perform simultaneously: i) electrohydrodynamic generation of a convection current flow in said fluid; and ii) 3D concentration of said particles in said fluid by dielectrophoretic attraction or repulsion of said biological particles to or from a region of increased electric field generated by said ac voltage across said electrodes; i) electrohydrodynamic generation of a convection current flow in said fluid; and ii) 3D concentration of said particles in said fluid by dielectrophoretic attraction or repulsion of said biological particles to or from a region of increased electric field generated by said ac voltage across said electrodes; whereby said convection current flow and said 3D concentration increase a concentration of said biological particles towards said sensing surface of said sensor, characterised wherein: - one or both of said electrodes is disposed on said upper surface and - said sensing surface is opposite said electrode, - one or both of said electrodes is disposed on said upper surface and - said sensing surface is opposite said electrode, and wherein in use, said biological particles circulate past said sensing surface and concentrate in three dimensions in a direction perpendicular to said lateral plane adjacent to said sensing surface.