Article pubs.acs.org/ac
Mass Quantification of Nanoparticles by Single Droplet Calibration Using Inductively Coupled Plasma Mass Spectrometry Sabrina Gschwind,† Harald Hagendorfer,‡ Daniel A. Frick,† and Detlef Günther*,† †
ETH Zurich, Department of Chemistry and Applied Biosciences, Laboratory of Inorganic Chemistry, Wolfgang-Pauli-Strasse 10, CH-8093 Zurich, Switzerland ‡ EMPA − Swiss Federal Laboratories of Material Science and Technology, Laboratory for Thin Films and Photovoltaics, Ueberlandstrasse 129, CH-8600 Duebendorf, Switzerland S Supporting Information *
ABSTRACT: Utilization of metallic engineered nanoparticles (ENP) is progressing rapidly; therefore, characterization of their most important properties, e.g., size/mass, elemental composition, and number concentration, is inevitable and currently uses a set of different techniques. In this work, a new setup is proposed for the quantitative size and mass determination of ENPs employing a monodisperse microdroplet generator (MDG) with transport efficiencies >95% coupled to an ICPMS. Two different MDG sample introduction configurations (vertical and horizontal) were tested, and their performance characteristics were evaluated. Due to a 5-fold reduced temporal jitter resulting in a shorter measurement time, the horizontal droplet introduction approach was used for the analysis of ENPs. With this setup, the quantification of Au, Ag, and CeO2 nanoparticles of different sizes and polydispersities was achieved. Results are compared to complementary techniques such as transmission electron microscopy (TEM) and asymmetric flow field flow fractionation (AF4), and advantages as well as limitations of this newly proposed technique are discussed.
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efficiency of the nebulized aerosol through the spray chamber entering the ICP must be known. This information can only be obtained using reference materials which are often not available for certain types of particles. Additionally, due to the unspecific and random droplet introduction, an integration time larger than 5 ms needs to be chosen (problem with split events into two integration times) and large memory effects will influence the accuracy.28 Microdroplet generation as a sample introduction system for e.g. flames or ICP in combination with various detectors has been used for a variety of fundamental and applied studies.29−37 A detailed description of how microdroplets are being used for analytical purposes can be found in Shigeta et al.38 and references therein. Garcia et al.39 reported on quantitative mass (size) determinations for small microparticles (SiO2: 0.53−2.06 μm; Au: 250 nm) by end-on ICP-OES using droplets of welldefined size for calibration. Their setup was based on a vertically arranged droplet introduction system and ICP source providing detection capabilities for particles significantly above the size range of nanoparticles. However, the general proof of principle of the quantification using internal standardization has been demonstrated for SiO2 particles (830 nm, 257 fg) with a 26% underestimation of their mass (189 fg). Injecting
anoparticles (NPs) are defined as objects less than 100 nm in all three dimensions1 and are produced and used in various fields. Due to their high surface-to-volume ratio,2 NPs possess unique features which make them useful in a wide range of applications, including medicine,3 consumer goods,4−6 energy production and storage,7 biomaterials,8 etc. The metal/ metal oxide engineered nanoparticles (ENPs) such as silver (Ag), titanium dioxide (TiO2), zinc oxide (ZnO), silicon dioxide (SiO2), and gold (Au) are most widely used.9 Toxicological and health related issues are discussed already substantially,10−16 but a detailed understanding of NP release and fate is not or only partially given for specific NP systems (e.g., Ag, CeO2).17−22 As a consequence, the detailed characterization of the physicochemical properties of metal/ metal-oxide and/or persistent ENPs is of increasing importance to gain a better understanding and tailor their performance for more specific, cost-effective, and safe applications. Currently, there is no method which can determine the most important properties of NPs including mass (size), particle number concentration, and elemental composition simultaneously on a routine basis. A method which possibly could fill this gap and has recently gained interest is single particle inductively coupled plasma mass spectrometry (sp-ICPMS). This technique is based on introducing a diluted NP suspension using conventional liquid nebulization directly into an ICP23−27 and enables one to measure the particle number concentrations and further to determine the particle masses.26 Thus, the transport © 2013 American Chemical Society
Received: March 1, 2013 Accepted: April 30, 2013 Published: April 30, 2013 5875
dx.doi.org/10.1021/ac400608c | Anal. Chem. 2013, 85, 5875−5883
Analytical Chemistry
Article
Figure 1. Two He/Ar based droplet delivery systems for the transport of droplet sizes below 95 μm ((A) 30−95 μm, (B) 95% transport efficiencies were achieved due to the faster desolvation of the droplets in helium compared to argon (90% reduced volume within 7.75 cm).41 For dissolved elements, present in every droplet, the ion signal peak areas could be determined with a good reproducibility (RSD < 10%). This system has been adapted and modified by Franze et al.42 who also reached a transport efficiency of approximately 100%. Shigeta et al.38 applied a similar system and achieved transport efficiencies of 20% for droplets of approximately 70 μm size (nozzle diameter 50 μm). With the application of a triple pulse sequence to the microdroplet dispenser, the size was reduced to 23 μm while obtaining a close to 100% transport efficiency. However, most of the studies using MDG in combination with ICPMS have been focused on the detection and counting of nanoparticles. The quantification capabilities of nanoparticles without welldefined NP reference materials, as demonstrated for ICPOES,39 have yet to be shown for MDG-ICPMS.
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EXPERIMENTAL SECTION Experimental Setup. The setup described by Gschwind et al.40 was modified to obtain a more stable transport. The initially two symmetrically arranged gas inlets of the adapter were replaced by four gas inlets (inner diameter 4 mm), and the inner diameter of the falling tube was increased to 10 mm to reduce optimization time for vertical positioning (Figure 1A). To facilitate the adaption of the MDG to commercial ICPMS instruments and to reduce the temporal jitter in arrival of the droplets at the ICP, the modified vertical setup described was transferred into a horizontal system. This new configuration consists of the adapter only (four gas inlets, total length 8 cm) and can be placed directly onto the torch of any commercial ICP. A schematic of the newly designed setup is shown in Figure 1B. Details of the droplet generator used have been described elsewhere.40,43 5876
dx.doi.org/10.1021/ac400608c | Anal. Chem. 2013, 85, 5875−5883
Analytical Chemistry
Article
this study were acquired on the basis of the directly connected horizontal setup using a nozzle diameter of 30 μm. Quantification of Au NP. All nanoparticles described in the Experimental Section were analyzed using AF4-DLS, TEM, and MDG-ICPMS. In Table 2, an overview of all measured Au NPs, stating the certified values from the producer and results obtained with different techniques, is presented. Figures 2−4 illustrate a selection of results for different sizes, monomodal and polymodal Au NPs in more detail, to better demonstrate the pros and cons of MDG-ICPMS in comparison to the more established techniques. Figure 2B presents the Au particle size distribution (PSD) of the NanoComposix suspension (88.7 ± 11.1 nm, NC 90) determined from TEM images (Figure 2A) and subsequent particle counting and analysis. The histogram contains the PSD measured for the longest and shortest distance of mostly nonspherical particles. The measured values of 85 ± 20 nm (measurement of the longest particle dimensions = dL) and 80 ± 25 nm (measurement of the shortest particle dimensions = dS) are in agreement with the values determined using AF4DLS (Figure 2C). Figure 2D (MDG-ICPMS) contains the internal standardization histogram (n = 4 × 105) and the inset shows a bimodal NP distribution. Quantifying the distribution with the low number of events provides a size value of 86 ± 8 nm. The higher frequency led to a size of 106 ± 7 nm. Considering the bimodal distribution as monomodal, the size yields 103 ± 13 nm. Figure 3A−D shows the same PSD measurements, as carried out for NC 90, for the bimodal Au NP suspension obtained from BAM (114.0 ± 11.0, BAM 12). With TEM analyses, it was possible to discriminate the small (20 ± 5 nm) from the large (97 ± 20 nm) particle fraction, whereas AF4-DLS and MDGICPMS provided sizes for the larger fraction only (AF4-DLS: 100 ± 14 nm; MDG-ICPMS: 120 ± 11 nm). The results in Figure 4A−D represent the smallest Au particle size purchased from NanoComposix (28.8 ± 3.3 nm, NC 30). The presence of low amounts of larger particles was detected by AF4-DLS measurements, and the TEM images supported the existence of agglomerates. Also, with MDGICPMS measurements, the evidence for the presence of larger particles (e.g., I > 500 counts) was confirmed. The signal of the primary particle fraction, partially overlapping with the internal standard, yielded a size of 30 ± 10 nm. The results obtained are in agreement within the uncertainty of the certified, TEM, and AF4-DLS data. Quantification of Ag and CeO2 NP. Next to Au NP, which were exhibited to be relatively inert in the environment, the system was assessed for other more commonly used and/or “problematic” types of NPs. Ag NPs are already employed for commercial applications and have been extensively studied due to the known toxic effects of Ag ions in the aquatic environment.44−48 The size quantification for Ag using external calibration (Figure 5A) yielded sizes of 56 ± 21 nm and 108 ± 29 nm, which are in agreement with the data provided by the supplier (53.1 ± 4.1 nm and 107.0 ± 7.6 nm) and our SEM image evaluation (data not shown), even though the uncertainty was high (approximately 30%). To provide a proof of principle for oxide NPs, which belong to the most applied inorganic nanoparticles, flame-spray synthesized polydisperse CeO2 NPs were quantified using external calibration (Figure 5C). The sizes determined assuming ideal stoichiometry were in the range of 15−80 nm using MDGICPMS (Figure 5D) and again were in agreement to the
The measurements reported in this study were carried out using a sector field ICPMS (Element2, ThermoFisher, Bremen, Germany). A more detailed description about operation conditions, data acquisition, processing, and evaluation can be found in the Supporting Information SI1. This instrument allows one to achieve detection efficiencies of approximately 10−4 counts/atom which is an order of magnitude lower than recently reported values.38 In addition to MDG-ICPMS measurements, the same NP suspensions were analyzed by AF4-DLS and TEM (information on the experimental setup: Supporting Information SI2 and SI3). Sample Preparation. Diluted suspensions of commercially available monodisperse Ag (0.02 mg mL−1, stabilized in 2 mM phosphate buffer) and Au NPs (0.05 mg mL−1, stabilized with tannic acid) with sizes of 53.1 ± 4.1 nm, 107.0 ± 7.6 nm and 28.8 ± 3.3 nm (NC 30), 48.1 ± 4.2 nm (NC 50), and 88.7 ± 11.1 nm (NC 90), respectively (NanoComposix, San Diego, USA), monodisperse Au NPs in water with sizes of 65.5 ± 6.2 nm (BAM 8), 88.6 ± 11.1 nm (BAM 10), and 114.0 ± 11.0 nm (BAM 12) (5 × 1010 particles mL−1, BAM, Berlin, Germany) and polydisperse CeO2 NP suspension stabilized in Dehscofix 108 (Huntsman International LLC, Salt Lake City, USA) (10− 100 nm, Nanograde Ltd., Staefa, Switzerland) were analyzed in this study. The quantification of the NP mass was based on two calibration strategies. For the internal standardization approach, the diluted NP suspension was mixed with an appropriate amount of diluted standard solution. For external standardization, standard solutions and NP suspensions were used sequentially. A more detailed description on sample preparation can be found in the Supporting Information SI4.
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RESULTS Vertical versus Horizontal MDG. The operating conditions and characteristics of the modified vertical and the newly designed horizontal setup are summarized in Table 1. Table 1. Typical Operation Conditions and Characteristics of the Two Different Setups Coupling the Dispenser to the ICP Torch
gas at dispenser gas at adapter 2nd gas supply in front of the ICP total helium gas flow, L min−1 total argon gas flow, L min−1 transportable droplet size range, μmb temporal jitter, msa
vertical arrangement
horizontal arrangement
helium argon argon 0.35 0.9
