Analytical Chemistry Chemical Cytometry Quantitates Superoxide
Analytical Chemistry Chemical Cytometry Quantitates Superoxide
Analytical Chemistry Chemical Cytometry Quantitates Superoxide
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Figure 1. Extinction spectra of solutions of (A) 0.48 nM Tween 20-<br />
AuNPs and (B) 0.37 nM Tween 20-modified bare AuNPs (a) before<br />
and (b, c) after the addition of (b) 1 µMHg 2+ and (c) 1 µMAg + . Tween<br />
20-AuNPs are prepared in 20 mM phosphate at pH 12.0. The<br />
incubation time is 5 min.<br />
to reduce Ag + onto the Au surface. 38,39 The formation of Agcoated<br />
AuNPs enables Tween 20 to be removed from the NP<br />
surface, thereby inducing aggregation of the AuNPs in a highionic-strength<br />
solution.<br />
Evidence for the Formation of Hg-Au Alloys and Ag<br />
Shells. To test the hypothesis mentioned above, we monitored<br />
the extinction spectra of Tween 20-AuNPs in the absence and<br />
presence of Hg 2+ and Ag + . Curve a in Figure 1A shows that the<br />
SPR wavelength of Tween 20-AuNPs appears at 520 nm, indicating<br />
that they are well dispersed in 20 mM phosphate solution at pH<br />
12.0. In other words, Tween 20 molecules can indeed protect<br />
citrate-capped AuNPs against a high-ionic-strength solution. 40 After<br />
separately adding 1.0 µMHg 2+ (curve b) and 1.0 µMAg + (curve<br />
c), we observed a decrease in the strength of SPR band at 520<br />
nm and the formation of a new red-shift band. These changes<br />
were characteristic of AuNP aggregation. The Hg 2+ - and Ag + -<br />
(38) Xie, W.; Su, L.; Donfack, P.; Shen, A.; Zhou, X.; Sackmann, M.; Materny,<br />
A.; Hu, J. Chem. Commun. 2009, 5263–5265.<br />
(39) Xia, H.; Bai, S.; Hartmann, J.; Wang, D. Langmuir 2009.<br />
(40) Shen, C.-C.; Tseng, W.-L.; Hsieh, M.-M. J. Chromatogr., A 2009, 1216,<br />
288–293.<br />
induced NP aggregation was nearly complete after 5 min (SI<br />
Figure S2). Obviously, the deposition of Hg and Ag onto the Au<br />
surface enables Tween 20 to be removed, thereby driving NP<br />
aggregation. We ruled out the possibility that coordination<br />
between Tween 20 and metal ions induces the NP aggregation<br />
because Tween 20 does not contain any functional group to<br />
interact with Hg 2+ and Ag + . To ensure the role of citrate ions<br />
in the reduction of Hg 2+ and Ag + , the extinction spectra of bare<br />
AuNPs modified with Tween 20 were examined under the same<br />
conditions. The addition of Hg 2+ and Ag + to this type of AuNP<br />
resulted in a rare shift in the SPR wavelength (Figure 1B),<br />
clearly indicating that citrate ions are indispensable for detecting<br />
Hg 2+ and Ag + using Tween 20-AuNPs. On the other hand, we<br />
prepared Hg-Au alloy- and Ag-coated AuNPs and modified<br />
them with Tween 20. Compared to Tween 20-AuNPs, a blue<br />
shift in SPR and a decrease in SPR intensity were observed<br />
for Tween 20-modified Hg-Au alloy-coated AuNPs (SI Figure<br />
S3). This is attributed to the formation of Hg-Au alloy on the<br />
surface of the AuNPs. A similar phenomenon was reported when<br />
citrate-capped AuNPs were exposed in the presence of Hg vapor. 41<br />
Moreover, this kind of NPs was found to be unstable in a highionic-strength<br />
solution. This result reflects that Tween 20 molecules<br />
were not attached to the surface of Hg-Au alloy, thereby<br />
incapable of protecting NPs against a high-ionic strength solution.<br />
SI Figure S4 shows that the deposition of Ag on the surface of<br />
the AuNPs resulted in an increase SPR intensity relative to Tween<br />
20-AuNPs. 42,43 Similarly, Tween 20-modified Ag-coated AuNPs<br />
were aggregated in a high-ionic-strength solution because Tween<br />
20 molecules were not adsorbed on the surface of Ag shell.<br />
To provide further evidence for the formation of Hg-Au alloys<br />
and Ag shells, we used ICP-MS to quantitatively determine the<br />
composition of the precipitates, which were obtained by five cycles<br />
of centrifugation of a solution of Tween 20-AuNPs and metal ions.<br />
When a series of concentrations (0-1 µM) of Hg 2+ were present<br />
in a solution of 0.48 nM Tween 20-AuNPs, the molar ratio of<br />
Hg to Au in the precipitates gradually increased with increasing<br />
Hg 2+ concentration (Figure 2A). A similar phenomenon was seen<br />
in the case of Ag + (Figure 2B). If Hg-Au alloys and Ag shells<br />
did not form on the Au surface, the molar ratios of Hg to Au and<br />
Ag to Au should remain constant under these conditions. Under<br />
identical treatment conditions (five centrifugation/washing cycles),<br />
the composition of precipitates was also determined by EDX<br />
analysis. The Hg content in the precipitates increased with<br />
increasing Hg 2+ concentration (Figure 2C). We observed a<br />
similar phenomenon when different concentrations of Ag + were<br />
present in a solution of Tween 20-AuNPs (Figure 2D). These<br />
results are in agreement with those obtained by ICP-MS. These<br />
findings strongly support the idea that Hg 2+ - and Ag + -induced<br />
aggregation of Tween 20-AuNPs is indeed the result of the<br />
formation of Hg-Au alloys and Ag on the Au surface.<br />
Effect of Surfactant Chain Length, NP Concentration, and<br />
Ionic Strength. We next explored the effect of surfactant chain<br />
length on the metal-ion-induced aggregation of the AuNPs. The<br />
(41) Morris, T.; Copeland, H.; McLinden, E.; Wilson, S.; Szulczewski, G.<br />
Langmuir 2002, 18, 7261–7264.<br />
(42) Anandan, S.; Grieser, F.; Ashokkumar, M. J. Phys. Chem. C 2008, 112,<br />
15102–15105.<br />
(43) Guerrini, L.; Garcia-Ramos, J. V.; Domingo, C.; Sanchez-Cortes, S. J. Raman<br />
Spectrosc. 2010, 41, 508–515.<br />
<strong>Analytical</strong> <strong>Chemistry</strong>, Vol. 82, No. 16, August 15, 2010<br />
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