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