Analytical Chemistry Chemical Cytometry Quantitates Superoxide

Figure 4. The value of Ex650 nm/Ex520 nm of a solution of 80 mM Na3PO4 containing (A) 0.24 nM Tween 20-AuNPs and 0.1 M NaCl and (B) 0.48
nM Tween 20-AuNPs and 0.01 M EDTA upon the addition of (A) 1 µM Hg 2+ and 100 µM other metal ions and (B) 1 µM Ag + and 100 µM other
metal ions. The incubation time is 5 min.
ments. SI Figure S7 shows the concentration-dependent TEM
images of the aggregated AuNPs after adding different concentrations
(0, 0.1, 1, and 10 µM) of Hg 2+ or Ag + to a solution
of Tween 20-AuNPs. Moreover, the hydrodynamic size of
the aggregated AuNPs increased with an increase in the
concentration of Hg 2+ or Ag + (SI Figure S8). These results
provide clear evidence that the aggregation degree of Tween
20-AuNPs is highly dependent on the concentration of Hg 2+
or Ag + . We observed that the ratio Ex650 nm/Ex520 nm
increased linearly with increasing Hg 2+ and Ag + concentration
over the range of 200 to 800 nM (R 2 ) 0.9943) and 400
to 1000 nM (R 2 ) 0.9935), respectively (Figure 5C and D).
A difference in linearity between two metal ions could be due
to that the sensing mechanism of Tween 20-AuNPs for Hg 2+
is different from that for Ag + . This probe could detect Hg 2+
and Ag + at concentrations as low as 100 and 100 nM,
respectively. The result is useful for detecting Ag + in
drinking water, because the maximum level of silver in
drinking water permitted by the United States Environmental
Protection Agency (EPA) is 50 µg/L (∼460 nM).
To test the practicality of the present approach, a solution of
0.24 nM Tween 20-AuNPs was used to analyze Hg 2+ in drinking
water and seawater. As shown in SI Figures S9 and S10,
Ex650 nm/Ex520 nm increased linearly upon increasing the spiked
concentration of Hg 2+ in drinking water over the range of
200-600 nM (R 2 ) 0.9944) and in seawater over the range of
300-1000 nM (R 2 ) 0.9977). Evidence of the Hg 2+ -induced
aggregation of Tween 20-AuNPs in seawater can be seen in
the HRTEM images (SI Figure S11). The lowest detectable
concentrations of Hg 2+ in drinking water and seawater were
6836 Analytical Chemistry, Vol. 82, No. 16, August 15, 2010
estimated to be 200 and 100 nM, respectively. Although the
sensitivity of this probe is insufficient to detect the maximum
level of mercury (2 ppb) in drinking water permitted by the
U.S. EPA, we suggest that Tween 20-AuNPs can be used as
probes for solid-phase preconcentration of mercury in complex
matrices prior to ICP-MS analysis. 30
We also evaluated the feasibility of this approach for sensing
of Ag + and AgNPs in drinking water. We obtained a linear
correlation (R 2 ) 0.9963) between the ratio Ex650 nm/Ex520 nm
and the concentration of Ag + spiked into the drinking water
over the range of 400-1000 nM (SI Figure S12), which includes
the maximum permissible limit of silver in drinking water.
Moreover, since hazardous AgNPs may pose threats to human
health or the environment, 45 this approach was further used to
monitor AgNPs in drinking water. Under acidic conditions (1.0
µM H3PO4), AgNPs (10 ± 2 nm) are oxidized to Ag + ions with
1.0 mM H2O2. 12 The oxidation of AgNPs to Ag + was complete
after 10 min. The generated Ag + was directly detected by 0.48
nM Tween 20-AuNPs. The degree of aggregation of Tween 20-
AuNPs increased when the concentration of AgNPs was
increased from 1 to 10 pM (SI Figure S13). The correlation
coefficient (R 2 ) for the determination of AgNPs in the range
1-6 pM was 0.9988. These results suggest that this probe will
be suitable for routine assays of AgNPs in consumer products
such as cosmetics and fabrics. 46 Table 1 shows the quantitative
measurements of Hg 2+ ,Ag + , and AgNPs in different matrices
based on the use of Tween 20-AuNPs.
(45) Lubick, N. Environ. Sci. Technol. 2008, 42, 8617.
(46) Benn, T. M.; Westerhoff, P. Environ. Sci. Technol. 2008, 42, 4133–4139.