Article pubs.acs.org/ac
Capabilities of Single Particle Inductively Coupled Plasma Mass Spectrometry for the Size Measurement of Nanoparticles: A Case Study on Gold Nanoparticles Jingyu Liu,†,‡ Karen E. Murphy,† Robert I. MacCuspie,‡,§ and Michael R. Winchester*,† †
Chemical Sciences Division and ‡Materials Measurement Science Division, Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States S Supporting Information *
ABSTRACT: The increasing application of engineered nanomaterials (ENMs) in consumer and medical products has motivated the development of single-particle inductively coupled plasma mass spectrometry (spICP-MS) for characterizing nanoparticles under realistic environmental exposure conditions. Recent studies have established a set of metrological criteria and evaluated the feasibility of spICP-MS for sizing or quantifying various highly commercialized ENMs. However, less is known about the performance of spICP-MS for detecting nanoparticles with sizes greater than 80 nm. This paper presents a systematic study on spICP-MS for accurate size measurement of gold nanoparticles from 10 to 200 nm. We show that dwell time contributes significantly to the quality of data, with the optimal dwell time that limits split particle events, particle coincidences and false positives being 10 ms. A simple approach to correct for split particle events is demonstrated. We show that transient features of single particle events can be temporally resolved on a conventional quadrupole ICP-MS system using a sufficiently short dwell time (0.1 ms). We propose an intensity-size diagram for estimating the linear dynamic size range and guiding the selection of ICP-MS operating conditions. The linear dynamic size range of the ICP-MS system under standard (highest) sensitivity conditions is 10 to 70 nm but can be further extended to 200 nm by operating in less sensitive modes. Finally, the ability of spICP-MS to characterize heterogeneous forms of metal containing nanoparticles is evaluated in mixtures containing both dissolved and poly disperse nanoparticulate Au.
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In recent years, inductively coupled plasma mass spectrometry (ICP-MS) operated in time-resolved analysis (TRA) mode is emerging as a promising analytical tool capable of both sizing and counting metal-containing nanoparticles (NPs) in aqueous suspensions.12−15 The superior sensitivity and elemental specificity of ICP-MS makes TRA mode, more commonly called single particle ICP-MS (spICP-MS), an advanced technique for nanomaterial characterization with the potential to overcome the challenges of environmental sample measurements. Additionally, simultaneous detection of both dissolved and particulate species, as well as characterization of poly disperse systems can be achieved by spICP-MS.16,17 The theory of spICP-MS for characterizing colloids in aqueous suspensions was first proposed by Degueldre et al.12−15 The authors described the equations used in data analysis and demonstrated the use of spICP-MS for measuring inorganic colloids. The improvement of commercial ICP-MS systems and the demand of innovative nanomaterial analysis approaches have driven rapid development of spICP-MS in the last three years. A set of metrological criteria has been established, for example, number
he nanotechnology revolution has boosted increasing production and application of engineered nanomaterials (ENMs) in consumer and medical products. Examples include textiles, cosmetics, food packaging, pesticides, sporting goods, paint, optics, and medical devices.1 The nanotechnology market is estimated to reach $1 trillion by 2015,1 and $3.3 trillion by 2018.2 The rapidly growing production volume has raised concern about the increased emission of ENMs into the environment during manufacture, use, and disposal of ENMs and related products.3 There is general agreement that the fate, transport, stability, and potential risks of ENMs highly depend on their physicochemical properties, including chemical composition, crystal structure, particle size and shape, surface chemistry, and agglomeration/aggregation state.1,4,5 However, characterizing and quantifying ENMs under realistic environmental exposure conditions remains a substantial challenge.4,6−8 The in situ measurement is difficult for most routine nanoscale characterization tools because of the extremely low environmental concentrations of ENMs9,10 (on the order of ng L−1), compromised sensitivity in a complex environmental matrix, and interference from natural colloids.11 Thus, there is a need in the nanotechnology community to advance in situ nanomaterial characterization metrology. This article not subject to U.S. Copyright. Published XXXX by the American Chemical Society
Received: November 20, 2013 Accepted: February 28, 2014
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dx.doi.org/10.1021/ac403775a | Anal. Chem. XXXX, XXX, XXX−XXX
Analytical Chemistry
Article
concentration limit,18,19 intensity threshold for discriminating particles from background signal,18,20 and a protocol for counting and sizing NPs using existing particle standards.21 Various highly commercialized ENMs have been characterized by spICP-MS.16−22 However, some important limitations still remain. By the most recent EU definition, a nanomaterial is “a natural, incidental, or manufactured material containing particles, in an unbound state or as an aggregate or as an agglomerate and where, for 50% or more of the particles in the number size distribution, one or more external dimensions is in the size range 1−100 nm”.23 Upon exposure to environmental and biological systems, NPs tend to form agglomerates/ aggregates, whose dimensions may span a broad size range.24 The existing studies, however, focus mostly on particle sizes from 20 to 80 nm. Less is known about the performance of spICP-MS for larger particles. Degueldre et al.15 demonstrated the feasibility of spICP-MS measurement for gold colloids with diameters from 80 to 250 nm, but recent studies reported that spICP-MS yielded significantly lower particle size for large particles (∼100 nm) compared with other sizing techniques.16,17,19 The performance of spICP-MS for measuring particles of 80 nm or larger needs careful assessment. The objective of this study is to establish a protocol for guiding the selection of spICP-MS operating mode on a commercial ICP-MS system to achieve accurate size measurement of NPs over a wide size range. Gold NPs (AuNPs) from 10 to 200 nm were used as the model material. (We realize that particles with sizes greater than 100 nm are not normally considered to be in the nanoscale. Nevertheless, all of the particles under study will be called nanoparticles in this paper for ease of communication.) AuNPs were selected because of their chemical and colloidal stability, as well as the availability of three National Institute of Standards and Technology (NIST) reference materials for AuNPs with nominal diameters of 10, 30, and 60 nm. The effect of dwell time on the quality of spICP-MS intensity data was first examined, with consideration of split particle events, false positives, and particle coincidence. We report that the transient signal of a particle event can be captured using sufficiently short dwell time on a conventional quadrupole ICP-MS. The sizing capability of spICP-MS is compared to transmission electron microscopy (TEM), and we demonstrate the use of an ICP-MS intensity-size diagram for predicting the size linear dynamic range of AuNPs, which is 10−70 nm using standard (highest) sensitivity mode. Four low sensitivity operating modes were studied to extend the size measurement to larger size AuNPs (80, 100, and 200 nm). Finally, this paper presents the first measurement of the size distribution of a suspension containing AuNPs ranging in size from 20 to 200 nm using spICP-MS by combining size analysis under standard and reduced sensitivity conditions.
were stabilized with citrate and supplied as monodisperse suspensions in water. As given in the corresponding Reports of Investigation, the NIST reference materials were characterized by various sizing techniques. However, in the present study, we used only the diameters measured by TEM, because Ted Pella used TEM to characterize the AuNPs that we obtained from them. The TEM diameters are 8.9 ± 0.1, 27.6 ± 2.1, and 56.0 ± 0.5 nm, respectively. [The TEM diameters are reference values that are a best estimate of the true value, where all known or suspected sources of bias have not been fully investigated by NIST.] The Au mass fractions are 51.56 ± 0.23, 48.17 ± 0.33, and 51.86 ± 0.64 μg g−1, respectively. [The Au mass fractions are information values that are considered to be of interest to the reference material user, but insufficient information is available to assess adequately the uncertainty associated with the values or a limited number of analyses were performed.] Both the TEM and Au mass fraction uncertainties are expanded to approximately 95% confidence. As provided in the vendor’s product data sheets, the average TEM diameters of the Ted Pella AuNP suspensions are 20.0, 40.0, 80.0, 99.6, and 203.6 nm, respectively, with relative standard deviations
