3.4.2. The lowest detection concentration and linear range
The anodic stripping voltammograms of Ag+ (Fig. 6 (a)), Cu2+
(Fig. 7(c)) and the corresponding calibration plots for Ag+ (Fig. 6(b))
and Cu2+ (Fig. 7(d)) are recorded at the optimum deposition
potential and time. During the detection of Ag+, anodic peaks corresponding
to Ag
→
Ag+ appear at around 0.2 V at low concentrations
and the position of this stripping peak slightly shifts to the more
positive potential with increasing ion concentration. This can be
explained by the Nernst equation and is usually observed during
in the heavy metal ions detection [20]. Noticeably, the shapes
of the anodic peaks are asymmetric showing a sluggish rise followed
by a sharp drop, caused by non-uniform electron-transfer
kinetics on the surface of the hybrid films where phase interfaces
exist. The peak currents show an excellent linear relationship
(r2 ≥
0.997) with the ion concentration over a wide range from 10
to 1000 ppb (Fig. 6(b)). The detection limit of Ag+ is calculated to be
5.8 ppb based on five individual measurements. The anodic stripping
voltammograms of Cu2+ detection exhibit similar trends to
those of Ag+. The position of anodic peaks is around
−0.1 V at low
Cu2+ concentration. The intensities of anodic peak also increase linearly
with metal ion concentrations with r2 ≥
0.998 over a wide
range. The detection limit for Cu2+ is calculated to be 5.6 ppb which
is lower than that on the B-doped diamond (10 ppb) [36]. The background
currents of the diamond/graphite electrode in Ag+ and Cu2+
detection are
∼1.5 A/cm2 and
∼4.7 A/cm2, which are of the same
magnitude with B-doped diamond electrodes [36]. The simultaneous
determination of Ag+ and Cu2+ is also possible on the hybrid