Environmental Engineering Reference
In-Depth Information
FCS [24] can be used to measure the
D
of fluorescent macromolecules or particles under a wide range of solution conditions,
including low concentrations (≈ 0.5 mg l
−1
). In this technique, a laser light is focused into the sample of interest using confocal
optics, creating an illuminated confocal volume with approximately 0.5-1.0 µm
3
. Temporal fluctuations in the measured
fluorescence intensity are used to derive an autocorrelation curve, which, in the absence of other processes that affect sample
fluorescence (e.g., chemical reactions), is related to the translational diffusion of the fluorescent molecule or particle. The
advantage of this technique is that it works at or near single-molecule detection, providing a number average
D
for singly
labeled particles or a weight average
D
for conditions in which several fluorescent labels are bound to each particle. Two impor-
tant limitations of FCS are that the particles must be fluorescent, which can be achieved by using labeling techniques [25], and
the upper size limit of the aggregates is restricted by the size of the confocal volume (~1 µm
3
).
NTA [15, 26], based on a laser-illuminated microscopical technique, also relies, like DlS, on the scattering generated from
particles undergoing Brownian motion. However, the Brownian motion of the particles is analyzed in real time by a charged
coupled device (CCD) camera, and the mean squared distances that are traveled by each particle in two dimensions are simul-
taneously visualized and tracked by a particle-tracking image analysis program, in order to determine the number-based
D
.
Since the scattering of small particles varies significantly with particle radius, larger particles can mask the signal of the
smaller ones.
SP-ICP-MS [19, 27] requires an adequate time resolution and low particle number concentrations in order to ensure that each
pulse corresponds to one particle only. Thousands of individual intensity readings are acquired, each with a very short dwell
time (~10 ms), as a function of time, where the pulses above the background represent the measurement of an individual particle.
The number of counts of the pulse is related to the number of analyte atoms in the particle, and the frequency of the pulses is
proportional to the number concentration of particles. This technique requires little sample preparation and uncomplicated addi-
tional method development for a given matrix and/or analyte. Moreover, beside the determination of the particle concentration
and size (as low as 20 nm), it also allows the quantification of dissolved metal concentrations (on the ng l
−1
range). However,
this technique is highly dependent on the signal-to-noise ratio of the ICP-MS in use, which may significantly hinder analysis of
smaller-sized NPs.
FFF [28] is a chromatography-like elution technique based on the physical interactions of the sample with a field applied at
right angles to a thin, open channel. This field allows the separation of particles based on their
D
. The separation occurs because
the flow through the channel is laminar with a parabolic cross-sectional velocity profile. Mass-based
D
values are determined
when the FFF is coupled with an absorbance detector. This is a promising technique when combined with a more sensitive
element-specific detector such as ICP-MS.
In HDC [29], particle separation is solely based on particle size independently of particle type and density. It can separate
particles in the size range from 5 to 100 nm, but can go well above this limit, depending on the size column used. The column
is packed with nonporous beads building up flow channels or capillaries, resulting in the separation of particles by flow velocity
and the velocity gradient across the particle. In the narrow conduits, the larger particles cannot fully access slow-flow regions,
resulting in faster elution of these larger particles from the column, and higher retention times for smaller particles. HDC is
sufficiently robust to require no sample pretreatment, even for samples as complex as lake water and sewage sludge supernatant
[29]. When coupled with ICP-MS it allows the simultaneous analysis of most of the commonly used NMs in a single run, hav-
ing the great advantage over FFF of a fast sample analysis time, which is less than 10 min per sample. A drawback is the
quantitative aspect, which is still to be fully addressed, as quantitative ionic standards are not suitable for use in HDC columns.
On-column and post-column calibration using a range of standards with different sizes and concentrations for the NM of
interest could be used in order to surpass this limitation, but no such standards are still available for NMs.
In addition to size and size distribution, surface chemistry and functionalization should also be taken into account when
considering the transport of NMs. An overview of the numerous spectroscopic techniques available for characterizing the
surfaces of particles has been provided by Handy et al. [30].
Once NMs are released, their environmental fate determines how they will be biogeochemically processed, or weathered,
and which receptors will be exposed (organisms and exposure route). These transformations add up on other modifications of
the NMs that take place during their life cycle in an NM-enabled product, ultimately generating the NM form to which ecosys-
tems and the general population will be exposed to. Despite the critical impact that such transformations may have on the
chemical and biological activity of NMs, the knowledge available is scarce.
32.2.2
matrix Interactions
When considering the release of a stabilized NM into the environment, two key aspects must be analyzed: (1) the input of metal
ions into the matrix by dissolution of the NM, and (2) the interaction of the stabilizer soft shell with the released metal ions, as
well as with other metal ions and/or other macromolecules present in the matrix. To achieve this aim, it is not only necessary to
