A microfluidic separation device is provided that includes a first sample channel region and a second sample channel region, where the first sample channel region has an array of channels that are smaller than the second channel region, a first detection region and a second detection region located at the interface of the first sample channel region, a detection channel, an illuminating electric field, Raman-scattering nanoparticles having surface plasmon resonances for detection when illuminated by the electric field, where the resonances create an enhanced local electric field along specific directions resulting in an enhanced Raman response, and a nanoparticle input channel disposed to input the nanoparticles into the second sample channel region, where the nanoparticles are larger than the cross-section of the first sample channel region and the cross-section of the second detection region, where the nanoparticles collect in the first detection region to form region of densely packed nanoparticles.