Biomedical Engineering Reference
In-Depth Information
1.7.1 p
hysIcocheMIcal
c
haracterIzatIoN
As discussed earlier, the physical and chemical properties of NMs play a very important role in their
interaction with biological systems. The physicochemical properties of NPs must be scrutinized
in detail to interpret any results of NP-induced toxicity by optimizing the NP design. Specific NP
properties that influence cellular toxicity are still not fully understood. Hence, a thorough charac-
terization of the NP is essential.
There are diverse approaches that are commonly used to characterize these properties, several of
which are described in Table 1.6. Size (distribution) and shape determinations are typically evalu-
ated with one or more of the following: dynamic light scattering (DLS) (Murdock et al. 2008),
transmission electron microscopy (TEM), scanning electron microscopy (SEM) (Love et al. 2012,
Marquis et al. 2009), and AFM. Field flow fractionation (FFF) is a chromatography-like technique
in which the partition of sample species is achieved in a thin, open channel, and the particle size
distribution can be calculated directly from the first principles (Bohnsack et al. 2012). Crystal struc-
ture is generally elucidated by x-ray diffraction and the surface area is determined by the Brunauer-
Emmett-Teller (BET) method by nitrogen adsorption and desorption isotherms (Maurer-Jones et al.
2010).
The surface chemical composition can be examined through various techniques, such as induc-
tively coupled-mass spectrometry (ICP-MS) and inductively coupled-optic emission spectrometry
(ICP-OES), which utilize an inductively coupled plasma as the ion source and can detect metals
(and some nonmetals) at concentrations below one part per trillion. ICP-MS and ICP-OES are also
capable of monitoring isotopic speciations for the ions of choice (Bohnsack et al. 2012). Other tech-
niques are secondary ion mass spectroscopy (SIMS), liquid chromatography-mass spectrometry
(LC-MS), matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF), and are used
specifically for an elemental analysis along with mass determinations. The elemental composition
of NP surfaces can be determined by Auger electron spectroscopy (AES), electron energy loss spec-
troscopy (EELS), and x-ray photoelectron spectroscopy (XPS). These are all high-vacuum tech-
niques and vary in their capabilities and surface sensitivities (Love et al. 2012, Powers et al. 2012).
NP uptake by cells is a vital factor to the assessment nanotoxicity. Flow cytometry (FCM) has
been used in the field of biochemistry to analyze thousands of cells in a second, which is advanta-
geous over TEM and ICP-MS techniques (Ibuki and Toyooka 2012). Electron paramagnetic reso-
nance (EPR)-electron spin resonance (ESR) are specialized methods to measure free radicals either
directly or by “spin trapping” them with a reference molecule. They are powerful techniques to
quantify oxygen ROS in toxicological evaluations of NMs. The ion abrasion SEM (IA-SEM) and
focused ion beam SEM (FIB-SEM), in addition to soft x-ray tomography, are being used to elucidate
three-dimensional displays of cells and tissues (Powers et al. 2012). Synchrotron radiation-induced
x-ray fluorescence (SRXRF) can provide the local, electronic, and molecular structures around the
atom of interest with sub-Angstrom spatial resolutions. It has also been utilized to investigate the
uptake of organic mercury in zebra fish larvae (Bohnsack et al. 2012).
Along with the physical characteristics of NPs, dose characterizations are also critical for the
interpretation of results. The calculation of a dosing metric is complicated because little is known
about appropriate doses and how aggregation or stability influences effective dosings (Love et al.
2012). These characterization challenges are ripe for the study and application of the collective
expertise of analytical chemists.
Various combinations of methods may be utilized to validate NM properties, as each method
has its own disadvantages and advantages. However, chemical characterization should accompany
physical characterization to assess the presence of contaminants in test samples, although exhaus-
tive characterization is time consuming as well as very expensive. There is a need to devise a battery
of standardized tests to adequately assess these properties, so that the data that are obtained are
comparative and reproducible. There is also a need for a standardized reference material for NMs
that can be used by toxicologists, so that data can be compared with different studies.
