Analytical Chemistry Chemical Cytometry Quantitates Superoxide
Analytical Chemistry Chemical Cytometry Quantitates Superoxide
Analytical Chemistry Chemical Cytometry Quantitates Superoxide
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Figure 2. (A, B) Effect of the concentration of (A) Hg 2+ and (B) Ag + on the composition ratio of (A) Au to Ag and (B) Au to Hg of the precipitates.<br />
A series of concentration of (A) 0-1000 nM Hg 2+ and (B) 0-1000 nM Ag + was added to 1.0 mL of 0.48 nM Tween 20-AuNPs. The precipitates<br />
were obtained by five cycles of centrifugation of the resulting solutions. (C, D) EDX spectra of the precipitates obtained after the addition of (a)<br />
0.1, (b) 1, and (c) 10 µM (C) Hg 2+ and (D) Ag + to a solution of 0.48 nM Tween 20-AuNPs. Tween 20-AuNPs are prepared in 20 mM phosphate<br />
at pH 12.0. The incubation time is 5 min.<br />
extinction values of the solution at 650 and 520 nm corresponded<br />
to the quantities of dispersed and aggregated AuNPs, respectively.<br />
Thus, the molar ratio of dispersed to aggregated AuNPs can be<br />
expressed by the ratio of the extinction value Ex at 650 nm to<br />
that at 520 nm (Ex650 nm/Ex520 nm). As shown in Figure 3A, the<br />
addition of both Hg2+ and Ag + to a solution of Tween 20-AuNPs<br />
resulted in a high value of Ex650 nm/Ex520 nm. However, when<br />
we replaced Tween 20 with Tween 40, the value of Ex650 nm/<br />
Ex520 nm became small. This suggests a relatively small amount<br />
of Hg-Au alloys or Ag on the surface of Tween 40-modified<br />
AuNPs, resulting in a small degree of NP aggregation. Similar<br />
phenomena were observed in the case of Tween 60- and Tween<br />
80-modifed AuNPs. To further confirm our hypothesis, ICP-<br />
MS was used to determine the composition of the NPs. After<br />
adding 1 µM Hg2+ to different kinds of AuNPs, the concentrations<br />
of Hg in Tween 20-, 40-, 60-, and 80-modified AuNPs were<br />
148, 64, 58, and 51 ppb, respectively. Similarly, upon the<br />
addition of 1 µM Ag + , the concentrations of Ag in Tween 20-,<br />
40-, 60-, and 80-modified AuNPs were 65, 19, 24, and 25 ppb,<br />
respectively. On the basis of these results, Tween 20 was<br />
selected for the following studies.<br />
Previous studies have shown that the sensitivity of a AuNPbased<br />
sensor is highly dependent on the concentration of<br />
6834 <strong>Analytical</strong> <strong>Chemistry</strong>, Vol. 82, No. 16, August 15, 2010<br />
AuNPs. 44 At relatively high concentrations of Tween 20-AuNPs,<br />
the deposition of Hg2+ or Ag + on the surface of a single particle<br />
decreased, reducing the degree of NP aggregation. Moreover,<br />
the aggregation rate of NPs increased with increasing NP<br />
concentration. It can be seen that the optimum concentrations<br />
of Tween 20-AuNPs for sensing Hg2+ and Ag + were 0.24 and<br />
0.48 nM, respectively, when the incubation time was fixed at 5<br />
min (Figure 3B). Moreover, we investigated the effect of Na3PO4<br />
concentration on the colorimetric sensitivity of Tween 20-<br />
AuNPs to Hg2+ and Ag + .SI Figure S5 shows that the zeta<br />
potential of Tween 20-AuNPs reduced with increasing Na3PO4<br />
concentration, implying that electrostatic repulsion between<br />
Tween 20-AuNPs decreased with increasing ionic strength of<br />
the solution. Thus, under conditions of high ionic strength, the<br />
slight electrostatic repulsion between Tween 20-AuNPs provided<br />
a low barrier for metal-ion-induced NP aggregation. As<br />
expected, the difference in Ex650 nm/Ex520 nm for the cases with<br />
and without 1.0 µM Hg2+ gradually increased with increasing<br />
Na3PO4 concentration and reached a plateau at 80 mM Na3PO4<br />
(Figure 3C). A similar effect was found in the analysis of Ag +<br />
(Figure 3D). Consequently, 80 mM Na3PO4 was chosen for the<br />
following studies.<br />
(44) Huang, C.-C.; Tseng, W.-L. Anal. Chem. 2008, 80, 6345–6350.