To improve the colorimetric determination of Cu2+ for rapid
on-site screening applications, the developed approach using a
paper-based sensor was applied. The S2O3
2/CTAB/AgNPls was
pre-prepared with the test zone and control zone on the paperbased
sensor as fabricated by wax printing method (Fig. 2a). For
Cu2+ measurement, the sample solution at 20mL was applied at
the sample zone and then the solution flowed outward via
capillary forces to the eight detection zones. In the absence of Cu2
+, the color results at the detection zone were not changed
(Fig. 2b). However, in the presence of Cu2+, the color at the
detection zones changed from violet-red to colorless with
increasing Cu2+ concentration, which can be monitored by the
naked eye after 120 s (Fig. 2c and d). The SEM image of the paper
based-sensor at the detection zone is shown in Fig. 2 both (e)
without and (f) with AgNPls and (g) AgNPls in the presence of
Cu2+. The results indicated that the modified AgNPls were etched
by Cu2+. Additionally, the TEM image (Fig. 2h) clearly shows that
the S2O3
2/CTAB/AgNPls was well dispersed in the aqueous
solution. The average size of S2O3
2/CTAB/AgNPls was approximately
30 nm. After the addition of Cu2+ (Fig. 2i), the S2O3
2/
CTAB/AgNPls would be catalytically oxidized and etched into the
solution. The average size of S2O3
2/CTAB/AgNPls were decreased
after incubation for 5 min at room temperature.