In the process of silver TNPs transformation to nanoparticles,the characteristic UV–vis absorption peak of 558 nm (attributed to silver TNPs) and 416 nm (attributed to silver nanoparticles)decreased and increased, respectively (shown in Fig. 1B). A relationship between A558/A416 and iodide concentration was shown in Fig. 4. The value of A558/A416 can be used for describing the transformation process of silver TNPs to nanoparticles. When silver TNPs are fused by iodide completely, A558/A416 is invariable with the increasing iodide. Thus, there is an iodide concentration that silver TNPs are fused by iodide exactly. By using Origin data analysis software to estimate the slope change (the two red lines inFig. 4), we calculated the iodide concentration which fused silver TNPs exactly, and defined as critical iodide concentration (cci). As silver TNPs show blue color and silver nanoparticles show yellow,we can observe obvious color changes of silver TNPs solution from blue to yellow during this transformation process (color pictures in Fig. 4). When silver TNPs were fused by iodide at the critical concentration (cci) exactly, the color of the solution shows as a critical Fig. 4. Estimation of critical iodine concentration and critical color for silver TNPs transformation. Experimental condition: silver TNPs concentration, 0.5394X M; pH,6.8; reaction time, 30 min. color. As it showed in the Fig. 4, the critical color is an intermediate color between blue and yellow, shows like orange.