A novel pH-mediated method was applied in this experiment
to completely replace CTAB from the surface of GNRs synthesized
above. The detailed procedure used was as follows: prior to the surface
modification of the GNRs, excess CTAB in the GNRs solutions
with different aspect ratios synthesized as above was removed by
two rounds of centrifugation at 8000 rpm for 20 min for each round.
The supernatant was carefully decanted and the GNRs were resuspended
in an equal amount of DI water. 20 mM MUA aqueous
solution was prepared by adding 44 mg MUA to 10 mL DI water.
In order to dissolve MUA, the mixture was first sonicated for several
minutes until the majority of the MUA was dispersed in water
which results in a white cloudy suspension. 0.2 M NaOH solution
was then added to the MUA solution (dropwise) with stirring and
periodic sonication, until the MUA dissolves. To replace the CTAB
on the surface of the GNRs, 4 mL of 20 mM MUA aqueous solution
was added to each 10 mL of purified GNRs solutions with different
aspect ratios prepared as above with vigorously stirring for
24 h. Excess MUA was removed by centrifugation twice at 8000 rpm
for 20 min each time, and the MUA-modified GNRs were then redispersed
in 0.01 M borate buffer at pH 9.