Improved surface enhanced Raman spectroscopy (SERS) of biological targets in liquids is provided. Nanoparticles are treated with a surfactant to provide an electrostatic attraction between the nanoparticles and the biological targets. The resulting clustering of the nanoparticles at the biological targets improves the SERS signal. Such SERS spectroscopy enables real time monitoring of the biological targets, thereby enabling a wide variety of assays etc.