IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 22 (2011) 325604 (8pp) doi:10.1088/0957-4484/22/32/325604

Upconversion in NaYF4:Yb, Ernanoparticles amplified by metalnanostructuresWei Deng1, L Sudheendra2, Jiangbo Zhao1, Junxiang Fu3,Dayong Jin1, Ian M Kennedy2 and Ewa M Goldys1,4

1 Department of Physics and Astronomy, Macquarie University, North Ryde 2112 NSW,Australia2 Mechanical and Aeronautical Engineering, University of California, Davis, CA 95616-5294,USA3 Department of Chemistry, University of California, Riverside, CA 92521, USA

E-mail: ewa.goldys@mq.edu.au

Received 16 February 2011, in final form 20 May 2011Published 19 July 2011Online at stacks.iop.org/Nano/22/325604

AbstractUpconversion (UC) fluorescence in NaYF4:Yb, Er nanoparticles amplified by metalnanostructures was compared in two nanostructure geometries: gold nanoshells surroundingnanoparticles and silver nanostructures adjacent to the nanoparticles, both placed on a dielectricsilica surface. Enhanced UC luminescence signals and modified lifetimes induced by these twometals were observed in our study. The UC luminescence intensities of green and red emissionswere enhanced by Ag nanostructures by a factor of approximately 4.4 and 3.5, respectively. Thecorresponding UC lifetimes were reduced ∼1.7-fold and ∼2.4-fold. In NaYF4:Yb, Ernanoparticles encapsulated in gold nanoshells, higher luminescence enhancement factors wereobtained (∼9.1-fold for the green emission and ∼6.7-fold for the red emission). However, theAu shell coating extended the red emission by a factor of 1.5 and did not obviously change thelifetime of green emission. The responsible mechanisms such as plasmonic enhancement andsurface effects are discussed.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Lanthanide-doped upconverting nanocrystals have recentlyemerged as an area of intense research activity, due totheir potential applications in life sciences research andsecurity [1–4]. In contrast to the conventional inorganicfluorophores [5, 6], organic dyes [7, 8] and semiconductorquantum dots [9, 10], these nanoparticles use inexpensive andhigh-power near-infrared (NIR) diode lasers as the excitationsource to produce visible luminescence. Under continuous-wave excitation at 980 nm, the particles exhibit superiorphotostability and unique upconversion (UC) luminescencethat has many attractive features, such as sharp absorptionand emission lines with full width at half maximum of�15 nm [11]. Furthermore, upconversion is excited in NIR

4 Author to whom any correspondence should be addressed.

which minimizes possible photodamage in biological systems,it limits the excitation of autofluorescence and it permits deepertissue penetration for bio-imaging [12, 13]. Despite theseadvantages, the nanoparticles typically have low emissionefficiency due to surface quenching, particularly strong insmall nanoparticles with large surface area/volume ratio anda relatively small photon absorption cross section. Intenseresearch efforts are currently directed toward maximizingUC quantum yield [14, 15] often by searching for optimumcombinations of different host matrices and rare-earth (RE)atoms.

In this work we explored two alternative strategies toachieve enhanced luminescence, by the addition of differentnoble metals in proximity to UC lanthanide nanophosphors.The comparison serves to elucidate some of the underlyingmechanisms that underpin metal enhancement of emissionfrom this class of materials. The nanoscale fluorophore–metal

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interactions give rise to the process known as metal-enhancedfluorescence (MEF) [16–19]. The MEF effect is dueto excited fluorophores interacting with surface plasmonresonances in metals; this effect is particularly strong in metalnanostructures. MEF has been shown to produce desirableeffects such as increased quantum yield of fluorophores, theirdecreased lifetimes, increased photostability and potential forimproved energy transfer [16]. Due to the above advantages,MEF has been widely used for fluorescence enhancementfrom organic dyes [20, 21] rare-earth complexes [22, 23] andquantum dots [24, 25]. Among different MEF substrates, silverand gold are the most widely investigated materials due totheir unique plasmonic properties, such as surface plasmonresonances in the visible wavelength range [26]. Recently,the coupling of UC materials with metal nanomaterials (Agand Au) has been reported, mainly focusing on Er in glasscomposites and film [27–29]. Such films are, however,incompatible with biological applications. To the best ofour knowledge, there have been no reports that demonstrateand compare the metal-enhanced UC luminescence achievedthrough the use of both silver and gold nanostructures withdifferent morphologies.

In this study we demonstrate metal-induced enhancementof the UC luminescence of NaYF4:Yb, Er nanoparticles intwo different experimental systems. In the first system thenanoparticles were made water-soluble by deposition of athin silica coating. Further, they were conjugated to silvernanostructures deposited on the surface of dielectric silicabeads. The silica coating additionally ensured nanoscaleseparation between nanoparticles and metal nanostructures,which is required to prevent metal quenching. The conjugationwas achieved by using a simple bioassay, where streptavidin(SA)-labeled UC nanoparticles were bound to the biotinylatedanti-mouse IgG antibody. This assay was carried out onsilica beads with and without Ag nanostructures under thesame experimental conditions to permit a valid comparisonof effectiveness. The Ag nanostructures on silica beads hada broad absorption band around 400 nm. In the second system,the NaYF4:Yb, Er nanoparticles were coated with ∼5–8 nmthick gold shells [30]. The gold nanoshells had a strongabsorbance in the green region of the visible spectrum andthey did not inhibit emission from Er3+ centered around 525and 545 nm. This is in direct contrast to the case of quantumdots, where the fluorescence emission from the quantum dots isquenched by the gold shell in the core–shell architecture [31].

2. Experimental details

2.1. Materials

The following materials were purchased from Sigma-Aldrichand used as received: YCl3 (99.9%), YbCl3 (99.9%), ErCl3(99.9%), oleic acid (OA, 90%), octadecene (ODE, 90%),methanol, NaOH, NH4F, ethanol, acetone, chloroform, CO-520, cyclohexane, hexane, tetraethyl orthosilicate (TEOS), am-monia (wt 30%), NaF (�99%), HCl (36.5–38.0%), HAuCl4,trisodium citrate, NaBH4, (3-aminopropyl)trimethoxysilane(APTMS), concentrated H2SO4, silver enhancing solution A,

silver enhancing solution B (enhancing kit), biotinylated rabbitanti-mouse IgG antibody, streptavidin (SA), glutaraldehyde,bovine serum albumin (BSA), phosphate buffered saline (PBS,pH 7.4), Tween 20 and poly-L-lysine. 400 nm silica beads werepurchased from Bangs Laboratories, Inc. Glass microscopeslides were obtained from Fisher Scientific. Nanopure water(>18.0 M�), purified using the Millipore Milli-Q gradientsystem, was used in all the experiments.

2.2. Preparation of SiO2-coated NaYF4:Yb, Er nanoparticles(first experimental system)

The SiO2-coated NaYF4:Yb, Er nanoparticles were firstsynthesized by thermal decomposition of rare-earth/sodiumchloride precursors and the fluoride reagent in oleic acid andoctadecene, then coated with the SiO2 shell via the hydrolysisof TEOS in a basic solution [32]. First, YCl3, YbCl3 andErCl3 were mixed with OA and ODE, and this solution washeated to 160 ◦C to form a homogeneous solution. Aftercooling down to room temperature, 10 ml methanol solutioncontaining NaOH and NH4F was slowly added with stirring for30 min. The solution was then heated to 300 ◦C for 1 h underargon protection and cooled down to room temperature. TheNaYF4:Yb, Er nanoparticles were precipitated with ethanoland washed with ethanol/water (1:1 v/v) three times. Theseas-prepared nanoparticles were dispersed in cyclohexane. CO-520 and ammonia were then added to form a water-in-oilmicroemulsion. After sonicating for 20 min, TEOS was addedinto the microemulsion with stirring for two days, resulting inSiO2-coated nanoparticles. They were precipitated by addingacetone and were washed with ethanol/water (1:1 v/v) twice.

2.3. Preparation of Ag nanostructures on silica beads

The deposition of Ag nanostructures onto silica beads wasperformed using a protocol previously reported by ourgroup [33]. Firstly, APTMS was added to the silica beadssolution with stirring overnight. Secondly, the ∼10 nmas-prepared Au colloids were added to the above solutionwith stirring for 30–90 min to allow them to bind to thesilica surface, followed by washing with nanopure water threetimes. These Au colloids act as seeds for further depositionof Ag which surrounds and encapsulates each of the Aunanoparticles. The deposition of Ag nanostructures on thesilica beads was carried out as follows: equal amounts of thesilver enhancer solutions A and B were mixed using a vortex,followed by addition to the Au–silica solution with reactionfor 3 min. This process was carried out in the dark as the silverenhancer kit is light-sensitive. The resulting Ag nanostructure-coated silica beads were centrifuged and washed with nanopurewater three times to terminate the reaction between silica beadsand the enhancer solution.

2.4. Conjugation of SA with SiO2-coated NaYF4:Yb, Er

5 μl of APTMS and 400 μl of ammonia (wt 30%) were addedto 20 ml ethanol solution containing 1 mg of SiO2-coatedNaYF4:Yb, Er nanoparticles. After stirring for 24 h at roomtemperature (RT), nanoparticles were centrifuged and washed

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Figure 1. Schematic representation of conjugation SiO2-coated nanoparticles with streptavidin (SA) (a) and an SA–biotin-based assay (b).

twice with ethanol and three times with nanopure water tocompletely remove unreacted materials. These nanoparticleswere then added to 2 ml of PBS buffer containing 0.5 mgSA and 0.3 ml of 1% glutaraldehyde (see figure 1(a)). Afterstirring again for 22 h at 4 ◦C, 1.0 mg of NaBH4 was addedand the solution was incubated for 2 h at RT. The nanoparticleswere centrifuged and washed with PBS buffer three times. TheSA–UC nanoparticle conjugates were stored at 4 ◦C before use.

2.5. SA–biotin-based assay

The glass slides were cleaned by soaking in the piranhasolution overnight and then rinsed with nanopure water anddried in air before use. The slides were then dip coatedwith poly-L-lysine solution (freshly prepared solution: 8 mlof water + 1.0 ml of 1% poly-L-lysine solution + 1.0 mlof PBS buffer, pH 7.4). After drying in air, each slide wascovered with tape containing punched holes to form wells onthe surface. Each well was filled with a 20 μl suspension ofsilica beads: the first slide with Ag-coated silica beads andthe second with uncoated silica beads. After drying in air,each well was filled with 20 μl of 40 μg ml−1 biotinylatedrabbit anti-mouse IgG antibody solution. The slides wereincubated overnight in a humid chamber and rinsed with PBST(PBS with 0.05 % Tween 20) and PBS, which were used inall washing procedures. Blocking was performed by addingthe BSA solution and then incubating for 1 h in a humidchamber to minimize non-specific binding. After rinsing, eachwell was incubated for 1 h with 20 μl per well of SA–UCnanoparticle conjugates, followed by a rinse and stored at 4 ◦Cbefore measurement (see figure 1(b)).

2.6. Preparation of gold-shell-coated NaYF4:Yb, Ernanoparticles (second experimental system)

Synthesis of the gold-shell-coated nanoparticles was imple-mented by employing a facile one-pot technique [30]. Briefly,

0.2 M solution of YCl3, YbCl3 and ErCl3 comprising 78%YCl3, 20% YbCl3 and 2% ErCl3 was mixed with 0.2 M sodiumcitrate and 1 M NaF solution in a 1:2:4 volume ratio and heatedto 90 ◦C. Then 380 nanomoles of 0.1% HAuCl4 were added tothe formed solution, with heating continued for two and halfhours. The pink colored nanoparticles were centrifuged, whichcauses them to precipitate as solids, and dried at 80–100 ◦C.The resulting product in the form of a solid crust several mmthick was crushed to form a mixture of micro- and originalnanoparticles and heated to 450 ◦C for 12 h in a N2 flowfurnace, followed by suspension in water.

2.7. Characterization

The extinction spectra of silica beads with and without Agnanostructures were measured using a Cary spectrophotometer(Cary 5000 UV–vis–NIR, Varian Inc.). Transmission electronmicroscopy (TEM) images were taken on a PHILIPS CM10system at an accelerating voltage of 100 kV. The sampleswere prepared by placing a drop of dilute ethanol dispersioncontaining samples on the surface of a copper grid. UCluminescence spectra were obtained using a Fluorolog-Tau-3 system (JY Horiba). In this measurement, the glass slideswith dry samples were maintained at a 45◦ illumination inan upright position in the solid sample holder. The spectralwidth was set to 8 nm. The emission spectrum was recordedover a range of 500–750 nm. UC luminescence lifetime wasmeasured using a purpose-built epi-fluorescence microscopysystem (10× objective; dichroic beam splitter (Zeiss, FT395)).In the measurements, a pair of emission filters (FF01-543/22-25 BP filter from Semrock (543 nm, 25 nm bandpass) forgreen emission and a 5915-A clarity BP filter (640 nm, 30 nmbandpass) from Newfocus for red emission) were employedto extract green and red emission, respectively. A diodelaser (980 nm, maximum 1 W in CW mode) (Beijing ViashoTechnology Co., Ltd), with a 1 m fiber (N.A: 0.22 and� 200 μm core) was used as the excitation source in both

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Figure 2. TEM image of SiO2-coated NaYF4:Yb, Er (a), the UC luminescence spectra of SiO2-coated NaYF4:Yb, Er nanoparticles (b),histograms of shell thickness (c) and core diameter (d).

UC spectrum and lifetime measurements. We used 500 μspulses at 1 kHz triggered by TTL signals. The UC signal wascollected by a cooled (−20 ◦C) SPMMini3035X08A1 solid-state photomultiplier (SPMT) from SensL. The preamplifierelectronics were configured to have a bandwidth of 2 MHz.The system was able to detect signals with decay times longerthan 150 ns. The samples were measured dry. The valuesof lifetime were derived by a single exponential curve fitting(Origin 8.0 software) with the residual error function χ2 valueof less than 1.0.

3. Results and discussion

The surface modification of these nanoparticles in the firstexperimental system was achieved using a silica coating whichprotects the core from the environment and can be modifiedwith amines, thiols and carboxyl groups. The uniformityof pure and silica-coated nanoparticles was confirmed byTEM images (figures 2(a), (c), (d)). The average size ofnanoparticles was ∼28 nm and the thickness of the silica shellwas ∼11 nm. The corresponding UC luminescence spectrumexhibits three distinct Er3+ emission bands around 525 nm,545 nm and 660 nm which correspond to 2H11/2 → 4I15/2,4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions, respectively(figure 2(b)) [34]. In the second system, the presence ofgold shells was confirmed by the TEM image, as shown infigure 3(a). The TEM studies also confirmed that Au coatinghad good uniformity and reproducibility. Figure 3(b) shows theabsorption spectra of Au coated and uncoated samples, wherecoated nanoparticles display a green plasmon band around510–560 nm. The respective size histograms are presented infigure 3(c).

The TEM images and extinction spectra of Ag nano-structure-coated silica beads were obtained to verify the sizeand surface plasmon energy (figure 4). Figure 4(a) showsthe TEM image of Ag nanostructure-coated silica beads. Asexpected, Au nanoparticles on silica beads served as nucleationsites for the growth of silver nanostructures, producingcontrolled, high and relatively even silver coverage. Theformation of silver nanostructure clusters was also observed.In such clusters, where particles are in close proximity, thecoupling of the neighboring particles plays an important roleand modifies the dipole–dipole interaction between the dipolemoments of the coupled particles. This dipole couplingconcept has been successfully applied to the modeling of theoptical properties of the silver fractal clusters [35]. Figure 4(b)shows the corresponding extinction spectra of silica beads withand without Ag nanostructures. The obvious Ag plasmonpeak at around ∼400 nm was observed with Ag nanostructuresadhering to the silica surface compared to that from pure silicabeads. Moreover, the shape of such extinction spectra suggeststhat light incident on the silica beads is scattered by silvernanostructures.

We first assessed the effect of silver nanostructures onUC luminescence amplification using an SA–biotin bindingsystem. The glass slide was coated with Ag–silica beadsand biotinylated IgG antibody molecules; SA–UC nanoparticleconjugates were then bound to silica beads via the avidin–biotin recognition mechanism. The impact of the nearbyAg nanostructures on luminescence was measured. Wechecked the effect of these nanostructures on the spectralshape of the UC emission. In the samples with andwithout Ag nanostructures, as shown in figure 5(a), we wereable to observe very similar spectral signatures, with theemission bands typical of Er3+ around 525, 545 and 660 nm.

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Figure 3. TEM image of gold-shell-coated NaYF4:Er, Ybnanocrystals (a), absorption spectra of Au-coated and uncoatednanoparticles (b) and size distribution of coated and uncoatednanoparticles measured by dynamic light scattering (c).

Ag nanostructures clearly enhanced UC luminescence withenhancement factors of ∼4.4 and ∼3.5 for green and redemission, respectively, compared to those from the controlsample without Ag nanostructures. These values wereobtained by dividing the integrated luminescence intensities forsamples with Ag nanostructures after background subtraction(integrated luminescence intensities around 525, 545 and660 nm wavelengths only, from Ag nanostructures without thebioassay) by those without Ag nanostructures. The intensityvalues were also corrected for the differences in the numberof nanoparticles bound in each sample, determined fromthe Er fluorescence of the top solution remaining after thecompletion of bioassays. For a more complete characterization

of the effect of Ag nanostructures on UC luminescence, wealso carried out lifetime measurements. As discussed inthe literature, MEF is characterized by an increase in thequantum yield and a decrease in the lifetime of a fluorophorelocated in the proximity of a metallic nanostructure [36]. Thefluorescence lifetime provides unambiguous confirmation ofthe metal-induced fluorescence enhancement effect in the firstsystem. The measured decay curves of UC luminescencefrom samples with and without Ag nanostructures indicate aclearly decreased lifetime observed in the presence of the silvernanostructures near these nanoparticles (figure 6), which wasreduced from 211 μs to 127 μs for green emission and from654 μs to 276 μs for red emission, respectively. Such reducedlifetime has two major implications, higher photostability and ahigher number of emitted photons per unit time under the sameexcitation conditions. Both these properties make it possibleto use shorter exposure time or obtain higher signal to noisefor comparable exposure times, thus increasing sensitivity anddetectivity in luminescence measurements.

In general, MEF arises from the enhancement of the localelectromagnetic field (referred to as excitation enhancement)and also from an increase in the radiative decay rate (referredto as emission enhancement). The distinction between thesetwo contributions is usually not simple but an estimate ofrelative contributions is possible from the data in figures 5(a)and 6. Excitation enhancement produces a higher excitationrate but it does not change the lifetime of the fluorophore.When the interparticle distances are reduced, the enhancementof the local electromagnetic fields becomes strong [36],which magnifies the excitation enhancement induced by thesenanostructures. In our case, the silver nanostructures on thesurface of the silica beads were very close to each other,resulting in a strong coupling effect between nanostructures.This coupling effect has a significant effect on the excitingelectromagnetic field [24, 25, 37]. We also found that theenhancement was not equal for both emission bands. Theluminescence enhancement for the green band was highercompared to the red band because the Ag plasmon hada better overlap. This wavelength-dependent enhancementcan be attributed to emission enhancement [27]. Therefore,both excitation and emission enhancement are likely to havecontributed to the strong UC luminescence enhancement inthis case. In addition, the luminescence decays of sampleswith Ag nanostructures were shorter than those of the controlsamples without Ag nanostructures, which is in accordancewith previously described MEF [24, 25, 27, 38]. This providesadditional evidence for the partial contribution from emissionenhancement, which can also reduce the lifetime of thefluorophores [36].

We extended a similar analysis to the second systemunder investigation, the gold-shell-coated nanoparticles. Asshown in figure 5(b), in samples with gold nanoshells bothgreen and red emissions were increased, by a factor of ∼9.1times and ∼6.7 times, respectively. Investigations on metalsurface enhanced luminescence of nanocrystals indicate thatthe degree of fluorescence enhancement is correlated with thespectral overlap between the absorption or emission band ofthe phosphors and the surface plasmon band of metals [39].

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Figure 4. TEM image of Ag nanostructure-coated silica beads (a) and absorption spectra of the pure and Ag nanostructure-coated silicabeads (b).

Figure 5. The UC luminescence spectra observed from NaYF4:Er, Yb nanoparticles with and without Ag nanostructures (a) and with andwithout gold nanoshells (b).

a b

Figure 6. Representative UC luminescence decay times of green (a) and red (b) emissions from NaYF4:Er, Yb nanoparticles with and withoutAg nanostructures.

Therefore one could attribute this larger enhancement (∼9.1-fold) for green emission to better spectral overlap of thegold plasmon band between 500 and 550 nm with thegreen luminescence, which can cause a better plasmoniccoupling. We also found that the enhancement value (∼6.7-fold) for red emission induced by gold nanoshells was higher,compared to that induced by silver nanostructures (∼3.5fold), and this again would be consistent with the sameargument. However, other effects can also produce similarrelationships between fluorescence intensities. The ratio ofred to green UC fluorescence depends on nanoparticle size,and therefore it may not be the same for the Ag-enhancedand Au-coated nanoparticles whose NaYF4 core sizes are

not identical. The fluorescence enhancement depends onthe proximity of the fluorophore to the metal and, for theAg nanoparticles, it depends on the length of the molecularlinker; this distance for Ag and Au structures is different.Hence the relationship between the red and green fluorescenceintensity in the case of each metal (Ag and Au) or between Agand Au nanostructures for selected (red or green) intensitiescannot provide unequivocal support that the fluorescenceenhancement in Au-coated samples is due to plasmonic effects.In contrast, the UC lifetimes in Au-coated nanoparticles veryclearly show the behavior opposite to that expected in the caseof metal enhancement acting in isolation. Figure 7 shows thatthe red emission in gold-shell-coated samples had a longer

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Figure 7. Representative UC luminescence decay times of green (a) and red (b) emissions from NaYF4:Er, Yb nanoparticles with and withoutgold nanoshells.

lifetime (∼1190 μs) than in uncoated ones (∼812 μs), whilethe lifetimes of the green emission in these two samples aresimilar (∼882 μs). These lifetime values can be interpretedwhen we consider the effect of surface recombination whichis known to affect the green/red upconversion in a majorway [40, 41]. This surface recombination may be differentin gold-shell-coated and uncoated nanoparticles. Its effect issuperimposed and may dominate over the plasmonic effects.The plasmonic effect of the gold-shell coating is expectedto reduce the lifetime in both green and red band, whilethe same coating may protect nanoparticles from surfacerecombination, producing the opposite effect of extending thelifetimes. Thus we interpret the similarity of lifetimes inAu-coated nanoparticles and uncoated nanoparticles as due tosurface recombination counterbalancing the plasmonic effects.

Finally we discuss how the presence of quite significantfluorescence enhancement in Au-coated samples can bereconciled with comparatively similar UC fluorescencelifetimes for Au-coated and uncoated samples. Thisfluorescence enhancement can be caused by the fact thatthe Au-coated nanoparticles experience different and higherelectric field under the same excitation conditions as theuncoated samples, which means excitation enhancement is inoperation. Such enhanced electric field due to the presence ofmetal nanoshells has been extensively discussed in [42].

4. Conclusion

We demonstrated the feasibility of using both silvernanostructures and gold nanoshells for UC luminescenceenhancement. These two types of noble metal nanomaterialsproduced clear luminescence enhancement (∼4.4-fold forgreen emission and ∼3.5-fold for red emission in the case ofsilver nanostructures; ∼9.1-fold and ∼6.7-fold in the case ofgold nanoshells). We also demonstrated reduced luminescencelifetimes induced by Ag nanostructures, confirming that theMEF effect applied to UC luminescence enhancement. Thelifetime results for gold nanoshells (extension of red lifetimeand no change of green lifetime upon Au coating) are attributedto a combination of plasmonic and surface recombinationeffects which cannot be untangled in this geometry. Althoughfurther study will be necessary to fully elucidate the exactunderlying mechanism, our results prove that UC emission can

be enhanced by noble metals such as silver and gold, whichpromise to yield a broader range of applications of metal–UC nanoparticle composites in bioassays, bio-imaging andenergy conversion that require ultrahigh sensitivity and lowbackground.

Acknowledgments

The project described was supported in part by AwardNumber P42ES004699 from the National Institute ofEnvironmental Health Sciences. The content is solely theresponsibility of the authors and does not necessarily representthe official views of the National Institute of EnvironmentalHealth Sciences or the National Institutes of Health. Thesupport of USDA Award number 2009-35603-05070 is alsogratefully acknowledged.

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