Assessment of Progress - Research Progress on Environmental Health and Safety Aspects of Engineered Nanomaterials - page 30

 Characterizing released materials in complex environments 5
Characterization of materials that are released into the environment remains a challenge because
released materials are present at low concentrations, are often transformed during release, and must be
analyzed within structurally and compositionally heterogeneous matrices (von der Kammer et al. 2012).
For example, a nanoparticle released into a waterway may undergo a wide array of transformations that
would render it difficult to detect and characterize:
 The nanoparticle becomes highly diluted, and this makes detection and characterization
difficult even if it is not transformed.
 The nanoparticle surface coating or core may be fully or partially degraded, and this can result
in a complex mixture of unknown chemicals that are more difficult to detect and characterize. In addition,
it may not be possible to relate whatever is detected to the primary nanoparticles released and to
distinguish between degradation products and naturally occurring nanoscale species.
 The nanoparticle surface may be coated rapidly by natural organic matter or proteins, and this
can complicate detection and isolation and make it difficult to characterize the surface chemistry.
Releases have been tracked by monitoring the elemental compositions of macroscopic products or the
bulk environments into which nanomaterials are released. Such techniques as inductively coupled plasma
mass spectroscopy (ICP-MS) are now widely used to gain information about elemental composition of
ENMs in aqueous samples (for example, surface waters) (Heithmar 2011; Mitrano et al. 2012), but the
measurements produced provide no information on the speciation of the released material. The detection
of particle releases has also advanced, but gathering information on the compositions of those particles
within the matrix into which they have been released remains challenging. Both the strong elemental
signals from the matrix and the presence of naturally occurring nanoparticles complicate the analyses.
Some progress has been made in assessing both particle release and composition. For example,
single-walled CNTs can be separated from soil and sediment and quantified with near-infrared
fluorescence spectroscopy (Schierz et al. 2012). C 60 and C 70 fullerenes have been extracted from soils
using ultrasound and quantified by HPLC-MS (Perez et al. 2013), and from urine (Benn et al. 2011).
Single-particle ICP-MS methods have been developed that have proved useful for metal nanoparticles,
such as gold and silver (Heithmar 2011; Mitrano et al. 2012) in pore water extracted from soil. Those are
examples of methods that rely on separation of nanomaterials from or within a matrix followed by
analysis, but concern that the separation process itself can transform the materials remains (von der
Kammer et al. 2012). Separation methods that lack stationary phases, such as field-flow fractionation,
show the most promise for separating nanomaterials without altering them (Mitrano et al. 2012). Other
approaches to monitoring environmental transformations that obviate separation include monitoring of the
transformation of tethered nanoparticles (Glover et al. 2011) and use of x-ray-based spectroscopic
methods that provide speciation information in media without the need to desiccate samples (Lawrence et
al. 2012; Lombi et al. 2012; Lowry et al. 2012b). The generality of the new approaches and their
relevance to real-world samples remain to be determined.
There has been some progress toward this objective, but proper characterization of nanomaterial
releases will require additional progress in developing methods that can simultaneously determine the
particulate characteristics and the nanomaterial composition (including surface chemistry) of the released
material. Thus, the committee's assessment is that progress toward this goal should be graded as yellow.
 Modeling nanomaterial releases along the value chain
5 This indicator originally was phrased as “Characterizing released materials and associated receptor
environments” (NRC 2012, p. 181).