NP solution was colorless after coloration and the assembling trend
of silver NPs was the same as that at pH = 4. The coloration did not
occur when the pH value was 8. The pH value played an important
role in the coloration process because the surface properties of
wool fiber are varied with different pH values. In an early study
[61], the wool fibers were milled into a fine powder (5 lm).
And the zeta potential of the wool powder at pH = 4 was measured
to be 51.7 mV. The zeta potentials of silver NPs at pH = 4 were obtained
to be 31.2 mV, 17.5 mV and 8.91 mV for nanoprism I,
nanodisks I and II, respectively. The zeta potential measurement
indicates that the assembling of silver NPs on wool fibers can be
realized by the interaction between positive charges on surface
of wool fibers and negative charges on the silver NPs.