Surface-Enhanced Raman Spectroscopy Using Commercially Available Au Colloids

Surface-enhanced Raman spectroscopy (SERS) has received considerable attention in recent years as an analytical technique and was the subject of a recent review article. The principal reasons for this attention are the sensitivity of the technique and the considerable information inherent in a Raman spectrum. Many different types of SER substrates have been described for analytical purposes including metal electrodes, metal colloids, and metal-coated substrates, such as Ag-coated microspheres and microspheres deposited on filter paper, as well as Ag-coated paper and fumed silica. Of these substrates, metal-coated fumed silica, metal colloids, and filter paper are particularly attractive because of the ease of sample preparation. The use of metal-colloid solutions has the added advantage that conventional liquid sample cells, may be used. Although sample preparation is simple with metal colloids after they are made, it is time consuming to prepare the colloids themselves. To obtain reproducible and optimal results, it is necessary to sort the colloids according to size before they are used. Although simple sorting methods have been described, they are time consuming, requiring many days for the colloids to settle and separate. On the other hand, some colloids can be stable for months once they are prepared and sorted.