SERS NPs (S440 and S420, Oxonica Materials Inc., Mountain View, California) are composed of a 60-nm-diameter Au core with a Raman active molecular monolayer adsorbed onto it. They are encapsulated with a 30-nm silica shell, with an overall diameter of 120 nm. Each type of SERS NP consists of a different Raman active molecular layer with its own unique Raman spectrum. To simplify, instead of nomenclature based on the Raman active molecular monolayer, a three-digit suffix (e.g., S440, S421) is used to name each type of SERS NP. Using a similar procedure as MNPs targeting, a solution of folate-PEG5k-maleimide (Nanocs Inc., PG2-FAML-5k, 0.8 mM) and methoxy-PEG2k-maleimide (Sigma Aldrich, 4 mM) was added to thiolated SERS NPs (1.3 nM, ) and mixed at room temperature on a rocker overnight, followed by three runs of centrifugation (14,000 rpm, 5 min) to eliminate excess folate-PEG-maleimide and resuspension in MES buffer. The diagrammatic representation of the conjugated SERS NP is shown in Fig. 1(b).