Instrumentation & Measurement Magazine 24-4 - 8

Fig. 4. Dual wavelength mode lock-in thermal imaging (DWLIT). (a) Pictorial absorption spectrum of spherical gold NPs embedded in an absorbing medium. The
NPs exhibit plasmonic-based enhanced absorption at 525 nm where the medium is also strongly absorbing. Performing standard amplitude-modulated LIT (see
(b)) illuminating the sample at around 525 nm would lead to the addition of the heat generated by the medium and the NPs. Performing DWLIT using an additional
wavelength (420 nm for example) where the medium absorption is similar to 525 nm allows the lock-in principle to reject the heat generated by the medium
(see (c)). (b) Standard LIT where the stimulation signal amplitude is modulated at a defined stimulation frequency (1 Hz in this example). (c) DWLIT where the
stimulation wavelength is switched between 420 nm and 525 nm allowing a better rejection of the heat generated by the medium.
is capable of resolving all relevant material-related parameters
and NP types; and novel, affordable and straight-forward
analytical methods should be developed. In general, main analytical
challenges are related to several distinct parameters
including e.g., the low concentration of NPs in products, the
complexity and variability of the matrix (including solids, gels,
cells, etc.), the presence of potentially high concentrations of
proteins, electrolytes, synthetic polymers, binder, or pigments,
or the change of NP composition (e.g., dissolution), dispersion
state (i.e., agglomeration), or surface chemistry.
Existing techniques can be very powerful for specific NPs
embedded or suspended in particular matrices/liquids but
frequently face detrimental limitations when the environmental
conditions are altered. As a consequence, plenty of
complementary analytical methods have emerged over the
last decades, all with certain advantages but also constraints.
Especially heterogenous and polydisperse samples have
proven to be consistently demanding when it comes to their
analysis. Many techniques are highly complex in terms of
sample preparation, operation, and data analysis, which renders
them time- and cost-intensive. A common fact is that they
are also rarely available and therefore restricted in access. Table
1 presents the different standard methods currently used
to investigate NPs in complex environments, depending on
the objectives.
The extraction of NPs from their solid matrix is often inevitable
to perform proper NP investigations (e.g., Inductively
Coupled Plasma (ICP)). Nonetheless, such measurements are
destructive and besides, the differentiation with charged NPs
can be cumbersome. LIT offers an in-situ alternative that is
nondestructive and NP specific.
To investigate NPs in suspension, dynamic light scattering
(DLS) is arguably the most common method that offers
8
high sensitivity and resolves even small NPs. However, measuring
heterogenous and non-purified samples becomes
extremely challenging. On the contrary, LIT can measure both
samples in liquid and solid states, and thanks to the imaging
nature of the techniques, multiple samples can be investigated
simultaneously in a time efficient manner. In biological media,
investigations are often achieved using imaging methods
based on Electron Microscopy (EM), Fluorescence Microscopy
(FM) or Dark-field Microscopy (DM). For high resolution
NP visualization, EM is the number one option, but is usually
fairly time-intensive, expensive and the obtained images
are not necessarily representative for the entire sample. In biological
settings, FM is often applied to evaluate NP uptake.
Nonetheless, the technique requires dye-labelling of NPs, and
recent studies have demonstrated that the presence of dyes
can significantly alter NP-cell interactions and lead to potential
misinterpretation of the data. On the other hand, DM is
an expensive method and requires expertise for the interpretation
of the data. Even if LIT cannot provide similar spatial
resolution as EM or FM, it provides an overview of the overall
sample, does not require any sample preparation or labelling
and is cost effective. Finally, a reliable method is lacking for the
investigation of carbon-containing NPs due to their complexity.
Preliminary investigations with LIT are very promising
and highlight that LIT can be used for the precise estimation of
carbon nanotube content in biological environments in a timeefficient
and label-free fashion.
Summary
The increased use of NPs in a large variety of products triggers
the necessity to develop analytical tools that can detect
and characterize NPs in complex but rather common environments
like cell cultures or gels. Among the current methods
IEEE Instrumentation & Measurement Magazine
June 2021

Instrumentation & Measurement Magazine 24-4

Table of Contents for the Digital Edition of Instrumentation & Measurement Magazine 24-4