Analytical Chemistry Chemical Cytometry Quantitates Superoxide

containing T-T mismatches. 23 Based on this concept, two
strands of DNA, which are designed to be complementary
except for a single T-T mismatch, are used to modify the
surface of the AuNPs. The resulting two types of DNAfunctionalized
AuNPs are selectively aggregated in the presence
of Hg 2+ based on T-Hg 2+ -T coordination and temperature
control. 24 Similarly, Hg 2+ was selectively detected using two
types of T-rich DNA-modified AuNPs and a T-rich DNA linker
at room temperature. 25 Citrate-capped AuNPs interacting with
single strands of T-rich oligonucleotide were found to be stable
in a high-salt solution. 26-28 When Hg 2+ causes the conformation
of T-rich DNA into a folded structure, this folded DNA cannot
be adsorbed onto the AuNP surface. Salt-induced NP aggregation
occurs because T-rich DNA is removed. In addition, it is
well-known that a Au surface exhibits a strong affinity for
Hg 2+ . 29-32 Thus, after the reduction of Hg 2+ with NaBH4, the
Hg(0) thus generated is strongly bonded onto the surface of
Au-based nanomaterials to form a solid amalgam-like structure.
The surface plasmon resonance (SPR) band of Au nanorods
and NPs in an excess of NaBH4 has been found to undergo a
blue shift and a decrease in intensity after Hg 2+ was added. 29,31
The only work on the detection of Ag + reported that AuNPs
functionalized with cytosine-(C)-rich oligonucleotide selectively
aggregated in the presence of Ag + based on the formation of
C-Ag + -C complexes. 33 Although these methods all show good
sensitivity and selectivity to Hg 2+ or Ag + , analysis of these two
metal ions using a single type of AuNPs remains a challenge.
In this study, we present a label-free, rapid, and homogeneous
method for sensing both Hg 2+ and Ag + using Tween 20stabilized
AuNPs (Tween 20-AuNPs). Because the surfaces of
Tween 20-AuNPs still had citrate ions, the reduction of Hg 2+
or Ag + with citrate resulted in the formation of Hg-Au alloy
and Ag on the Au surface. When the Tween 20 was removed,
NP aggregation occurred. We also investigated the effect of
masking agents on the selectivity of this probe. To demonstrate
its practicality, the present method was further applied to the
determination of Hg 2+ ,Ag + , and AgNPs in complex matrices.
EXPERIMENTAL SECTION
Chemicals. Hydrogen tetrachloroaurate (III) dehydrate,
Na2HPO4, and Na3PO4 were purchased from Alfa Aesar (Ward
Hill, MA). Trisodium citrate, ethylenediaminetetraacetic acid
(EDTA), Tween 20, Tween 40, Tween 60, Tween 80, NaBH4,
(23) Tanaka, Y.; Oda, S.; Yamaguchi, H.; Kondo, Y.; Kojima, C.; Ono, A. J. Am.
Chem. Soc. 2007, 129, 244–245.
(24) Lee, J.-S.; Han, M.-S.; Mirkin, C. A. Angew. Chem., Int. Ed. 2007, 46, 4093–
4096.
(25) Xue, X.; Wang, F.; Liu, X. J. Am. Chem. Soc. 2008, 130, 3244–3245.
(26) Yu, C.-J.; Cheng, T.-L.; Tseng, W.-L. Biosens. Bioelectron. 2009, 25, 204–
210.
(27) Li, D.; Wieckowska, A.; Willner, I. Angew. Chem., Int. Ed. 2008, 47, 3927–
3931.
(28) Liu, C.-W.; Hsieh, Y.-T.; Huang, C.-C.; Lin, Z.-H.; Chang, H.-T. Chem.
Commun. 2008, 2242–2244.
(29) Rex, M.; Hernandez, F. E.; Campiglia, A. D. Anal. Chem. 2006, 78, 445–
451.
(30) Leopold, K.; Foulkes, M.; Worsfold, P. J. Anal. Chem. 2009, 81, 3421–
3428.
(31) Lisha, K. P.; Anshup; Pradeep, T. Gold Bull. 2009, 42, 144–152.
(32) Barrosse-Antle, L. E.; Xiao, L.; Wildgoose, G. G.; Baron, R.; Salter, C. J.;
Crossley, A.; Compton, R. G. New J. Chem. 2007, 31, 2071–2075.
(33) Li, B.; Du, Y.; Dong, S. Anal. Chim. Acta 2009, 644, 78–82.
ascorbic acid, and NaCl were ordered from Sigma-Aldrich
(Louis, MO). LiCl, KCl, MgCl2, CaCl2, SrCl2, BaCl2, CrCl3,
MnCl2, FeCl2, FeCl3, CoCl2, NiCl2, CuCl2, ZnCl2, Cd(ClO4)2,
AlCl3, Pb(NO3)2, HgCl2, and AgNO3 were purchased from Acros
(Geel, Belgium). Water used in all experiments was doubly
distilled and purified by a Milli-Q system (Millipore, Milford,
MA).
Characterization of the AuNPs. Extinction spectra of the
AuNPs were measured using a double-beam UV-visible spectrophotometer
(Cintra 10e; GBC, Victoria, Australia). High-resolution
transmission electron microscopy (HRTEM, FEI Tecnai G2 F20
S-Twin working at 200 kV) was used to collect HRTEM images
of dispersed and aggregated AuNPs. Energy-dispersive X-ray
(EDX) spectra were obtained using a HRTEM microscope. The
zeta potential and size distribution of the AuNPs were measured
using Delsa nano zeta potential and submicrometer particle size
analyzer (Beckman Coulter Inc., U.S.). The hydrodynamic size
of the AuNPs was measured using dynamic light scattering (DLS)
(N5 Submicrometer Particle Size Analyzer, Beckman Coulter Inc.,
U.S.).
To understand the sensing mechanism, we equilibrated aliquots
(1.0 mL) of 0.48 nM Tween 20-AuNPs in the presence of
Hg 2+ (0-10 µM) or Ag + (0-10 µM) for 5 min at ambient
temperature. The resulting mixture was subjected to centrifugation
at 17 000 rpm for 10 min. Following removal of the
supernatants, the precipitates were washed with water. After
five centrifugation/washing cycles, the pellets were resuspended
in water. A portion of the samples (∼200 µL) was
diluted to 50-fold and then measured by ICP-MS (Perkin-Elmer-
SCIEX, Thornhill, ON, Canada). Additionally, the composition
of the obtained pellets was analyzed by EDX spectroscopy.
For surface-assisted laser desorption/ionization time-of-flight
ionization mass spectrometry (SALDI-TOF MS) (Autoflex, Bruker)
measurements, citrate-capped AuNPs and Tween 20-AuNPs were
separately pipetted into a stainless steel 384-well target (Bruker
Daltonics) and dried under ambient temperature. Desorption/
ionization was obtained by using a 337-nm-diameter nitrogen laser
witha3nspulse width. MS experiments were performed in the
positive-ion mode on a reflectron-type TOF MS equipped with a
3 m flight tube. To obtain good resolution and signal-to-noise
ratios, the laser power was adjusted to slightly above the threshold,
and each mass spectrum was generated by averaging 500 laser
pulses.
Nanoparticle Synthesis. We prepared citrate-capped AuNPs
by means of the chemical reduction of a metal salt precursor
(hydrogen tetrachloroaurate, HAuCl4) in the liquid phase. To
achieve this, we rapidly added HAuCl4 (0.35 M, 54 µL) to a
solution of sodium citrate (2.55 mM, 60 mL) that was heated
under reflux. This heating continued for an additional 15 min,
during which time the color of the solution changed to a deep
red. The size of citrate-capped AuNPs determined by TEM
images was 13 ± 1 nm. The SPR wavelength of citrate-capped
AuNPs located at 520 nm. The particle concentration of the
AuNP solution was estimated to be 4.8 nM by Beer’s law; the
extinction coefficient of 13 nm AuNPs at 520 nm is 2.7 × 10 8
M -1 cm -1 . Tween 20-AuNPs were synthesized by adding Tween
20 (10% v/v, 240 µL) to a solution of citrate-capped AuNPs (4.8
Analytical Chemistry, Vol. 82, No. 16, August 15, 2010
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