Extended Data Fig. 3: Characterization of the SERS colloids.
From: Digital colloid-enhanced Raman spectroscopy by single-molecule counting
(a)-(c) Hya-Ag colloids, (d)-(f) citrate-Ag colloids and (g)-(i) citrate-Au colloids. (a), (d), (g) Shown are the colloidal suspensions contained in the vials (left) and transmission electron microscopic images of the colloids (right). (b), (e), (h) Extinction spectra. (c), (f), (i) Histograms of the hydrodynamic diameter obtained by dynamic light scattering. (j) Histogram of the intensity distribution among voxels in 10−7 M crystal violet detected by Hya-Ag colloids. The relative standard deviation is 2.8% among the 200 voxels, indicating that the hotspots present in each probed voxel were statistically comparable across voxels. To improve statistics, a relatively longer acquisition time was used for each voxel (i.e., 5 s) to accumulate more hotspots. (k) 10−10 M crystal violet (CV) and (l) 10−7 M L-cysteine were detected by the Hya-Ag colloids at 0.17, 0.25, 0.5, 1 and 1.5 nM. The detection efficiency is improved when the colloidal concentration increases, because of the increased hotspot density. With greater colloid concentration, the detection efficiency then decreases due to the increased background scattering. The histograms show the RPVs with error bars indicating the standard deviations (n = 3). The optimal concentration for Hya-Ag colloids was found around 0.5 nM.