For SERS analysis, TNT aqueous solution was prepared with concentration of 106 mol L1. Silver colloids, TNT solution (pH
value was adjusted with NaOH solution), NaCl aqueous solution(0.6 mol L1) were mixed at the volume ratio of 8:1:1 in a vial. Then a drop of the mixture was dripped directly on a glass slide and dried naturally for the SERS experiments. Compared to the usual way of keeping the mixture in a small cuvette for measuring SERS spectra, this method can prevent suffering from the effect of Brownian motion and time-dependent coagulation [20], and the colloidal clusters for extremely large SERS enhancement factors can be distinguished easily as small bright flakes on the glass slide with the micro-Raman instrument