Effects of size and concentration of zinc oxide on colorimetric response of polydiacetylene/zinc oxide nanocomposites to temperature and pH

Abstract

This research studies effects of ZnO concentration and particle size on properties of PDA/ZnO nanocomposite for sensing technology. To investigate the effect of ZnO concentration, ZnO with particle size of 65 nm is used and the concentrations are varied from 5, 10, 15, 20, 30 and 50wt% based on DA monomer. For studying the effect of particle size, as-received ZnO with sizes of 65, 174 and 224 nm are used along with their calcined particles with size in the range of 4-7µm. Monomers used in this research are 10,12-pentacosadiaynoic acid (PCDA) and 5,7-hexadecadiynoic acid (HDDA). PDA/ZnO nanocomposites prepared with 5 wt% ZnO are unstable. At 10-15 wt% ZnO, the suspensions are deep blue, showing high amount of polymerized materials. At higher ZnO concentration, less polymerized materials occur. This could be due to fewer amounts of monomers on ZnO surface with separate distance too long to be polymerized. All poly(PCDA)/ZnO nanocomposite changes from blue to red at ~75°C and exhibits completely reversible behavior upon cooling. Poly(HDDA)/ZnO nanocomposite changes from blue to purple ~60°C and exhibits partially reversible thermochromism. Color transition temperatures are similar for PDA/ZnO nanocomposites prepared with various ZnO sizes. However, the partially reversible thermochromism is more obvious when ZnO size increases. This could result from decreasing in surface area when size increases. For calcined 174 and 224 nm-ZnO, the particles could have insufficient surface area for all DA monomers to arrange on. Some DA monomers could remain in the system and form pure vesicles. Therefore, the nanocomposites exhibit partially reversible thermochromic behavior. For pH response, it is found that poly(HDDA)/ZnO is more sensitive than poly(PCDA)/ZnO nanocomposites. In addition, the nanocomposites prepared with as-received ZnO exhibit higher degree of colorimetric response to pH than the ones prepared with calcined particles.