Analytical Chemistry Chemical Cytometry Quantitates Superoxide

Scheme 1. Illustration of the Mechanism of Tween 20-AuNPs for Sensing Hg 2+ and Ag +
nM, 60 mL). 34 To investigate the effect of the kind of the
surfactant on the sensing of Hg 2+ and Ag + , Tween 20 was
replaced by Tween 40, Tween 60, and Tween 80, once at a time.
Additionally, to test the effect of the type of the AuNPs on the
detection of Hg 2+ and Ag + , we replaced citrate-capped AuNPs
with bare AuNPs in the synthesis of Tween 20-AuNPs. The
preparation of bare AuNPs (3.7 nM; 12 nm) is described in
the Supporting Information (SI).
Sample Preparation. Tween 20-AuNPs were prepared in
100-1000 mM sodium phosphate solution at pH 12.0. For Hg 2+
sensing, metal ions (800 µL, 125-1250 nM) were added to a
solution containing Tween 20-AuNPs (100 µL, 0.96-14.4 nM)
and NaCl (100 µL, 1 M). For Ag + sensing, metal ions (800 µL,
125-1250 nM) were added to a solution containing Tween 20-
AuNPs (100 µL, 0.96-14.4 nM) and EDTA (100 µL, 0.1 M).
We equilibrated the resulting solutions at ambient temperature
for the optimum incubation time, and then recorded the
extinction spectra of the solutions.
Analysis of Real Samples. Samples of drinking water and
seawater (pH 7.9) were collected from National Sun Yat-sen
University campus. We then prepared a series of samples by
“spiking” them with standard solutions of Hg 2+ (125-1250 nM)
or Ag + (375-1250 nM). These spiked samples (800 µL) were
added either to a solution containing 100 µL of 2.4 nM Tween
20-AuNPs and 100 µL of 1 M NaCl or to a solution containing
100 of 4.8 nM Tween 20-AuNPs and 100 µL of 0.1 M EDTA.
We incubated the resulting solutions for 5 min before measuring
their extinction spectra.
On the other hand, this proposed method was utilized to detect
10 nm AgNPs in drinking water. The preparation of citrate-capped
AgNPs 35 is described in the SI. Different concentrations of AgNPs
(1-10 pM) present in drinking water were oxidized to Ag + with
(34) Huang, C.-C.; Tseng, W.-L. Analyst 2009, 134, 1699–1705.
(35) Wei, H.; Chen, C.; Han, B.; Wang, E. Anal. Chem. 2008, 80, 7051–7055.
6832 Analytical Chemistry, Vol. 82, No. 16, August 15, 2010
a solution of 1 µM H2O2 and 1 µM H3PO4. After 10 min, the
resulting solution (800 µL) was added to a solution containing
100 of 4.8 nM Tween 20-AuNPs and 100 µL of 0.1 M EDTA.
The extinction spectra of the resulting solutions were recorded
after 5 min incubation.
RESULTS AND DISCUSSION
Sensing Mechanism. SALDI-TOF MS technique is applied
to the detection of small molecules that are adsorbed on the
surface of the AuNPs when AuNPs are used as SALDI matrices. 36,37
Thus, we utilized this technique to determine whether citrate ions
were adsorbed on the surface of Tween 20-AuNPs. SI Figure S1A
shows the SALDI spectrum of citrate-capped AuNPs. The peak
detected at m/z 258.00 corresponded to [citrate +3Na] + . This
peak was also observed in the SALDI spectrum of Tween 20-
AuNPs (SI Figure S1B), indicating that citrate ions are capped
on the surface of Tween 20-AuNPs. Moreover, the zeta potential
of Tween 20-AuNPs was found to be -14.0 ± 0.8 mV in 10 mM
phosphate at pH 12.0. These results suggest that a neutral Tween
20 coating only shields the citrate ions (no displacement occurs).
Accordingly, we reasoned that citrate ions adsorbed onto the NP
surface can act as a reducing agent for Hg 2+ and Ag + when
citrate-capped AuNPs have been modified with Tween 20.
Scheme 1 shows the mechanism by which Tween 20-AuNPs
sense Hg 2+ and Ag + . Because of the high affinity between Au
and Hg, 29-32 the reduced Hg(0) is directly deposited onto the
Au surface through the formation of Hg-Au alloys; meanwhile,
Tween 20 molecules are desorbed from the AuNPs. The removal
of the stabilizer (Tween 20) causes the AuNPs to aggregate under
conditions of high ionic strength. Also, citrate ions can be used
(36) Su, C.-L.; Tseng, W.-L. Anal. Chem. 2007, 79, 1626–1633.
(37) Wu, H.-P.; Yu, C.-J.; Lin, C.-Y.; Lin, Y.-H.; Tseng, W.-L. J. Am. Soc. Mass
Spectrom. 2009, 20, 875–882.