Instrumentation & Measurement Magazine 24-4 - 7

Fig. 3. Multiwavelength LED-based light source developed for laboratory use. (a) Overview of the custom-made setup. The sample can be positioned with
micrometric screws. Trigger input/output lines are connected to the infrared camera, whereas USB connection to the computer enables the selection of
wavelength and intensity. (b) A custom-made electronic driver is connected to an octagonal-shaped LED panel. To ensure homogeneous illumination, a light mixing
rod is inserted between the LEDs and the sample. The LED driver is alimented by 3 power supplies and connected to a DAQ for communication with the computer.
To avoid overheating, the LED panel is directly mounted on a heat sink using heat conductive paste.
the results. The setup we developed to investigate magnetic
NPs (like SPIONs) is based on a commercial AMF generator.
AMFs are homogeneous over small spatial regions and as a
result, the amplitude will depend on the sample position. On
the contrary the phase remains unaffected as the ratio of the
in-quadrature and in-phase signals. In previous studies we
demonstrated that spatial variations can account for up to 18%
in signal variations [9]-[14].
To investigate photothermal NPs, we developed a light
source comprising custom-made octagonal-shaped LED
panels that emit stimulation light at various wavelengths
(ADOM GmbH) (Fig. 2b and Fig. 3). The panel can be designed
with a single wavelength for high intensity up to 1
W/cm2
or incorporate different wavelengths: 420 nm, 525
nm, 660 nm, 730 nm, 855 nm and 940 nm. To ensure homogeneous
illumination at the different wavelengths, a light
mixing rod (N-BK7, Edmond Optics) is positioned between
the sample and the LED array. A custom electronic driver
board was designed to modulate the LED intensities at the
stimulation frequency. This board is connected to a computer
via a DAQ card (USB 6008, National Instruments)
where a simple software (LabVIEW, National Instruments)
allows setting the LED intensity as well as the wavelengths
to be used. LED performances are dependent on the temperature
and in particular, the peak wavelength will shift
until the temperature stabilizes. Therefore, the LED panel is
mounted on a heat sink connected to a ventilator. A temperature
sensor is mounted directly on the LED board to monitor
the temperature during use.
June 2021
Dual Wavelength Mode for Measuring Plasmonic
NPs in Strong Absorbing Medium
Lock-in detection is a well-known approach in engineering
that demonstrates the ability to suppress constant, slowly
varying background signals and to enhance the signal-tonoise-ratio
(SNR). Nonetheless, the technique cannot cancel
out artifact signals at the stimulation frequency (or lock-in frequency).
In the case of photothermal NPs, this issue can arise
if the particles are embedded into a strong absorbing medium
that is generating heat at the same wavelength as the NPs
themselves (Fig. 4a). Following previous solutions developed
for similar problems in the field of photovoltaics, we propose
a dual wavelength approach (DWLIT) where two separate
wavelengths are used as stimulation. Both wavelengths are
amplitude-modulated at the stimulation frequency but out-ofphase
(Fig. 4c).
Advantages of LIT for the Investigation
of NPs
In recent review articles, common analytical methods are evaluated
in detail and classified with respect to their applicability
and flexibility for NP characterization. They generally share the
opinion that: the current measures for the detection and quantification
of NPs in complex matrices are not yet sufficiently
developed; depending on the given matrix, potential interference
signals and/or artifacts have to be reliably filtered; the
majority of the currently available tools are useful but often
very time-consuming, complex and demand great expertise
in terms of operation and data treatment; no single method
IEEE Instrumentation & Measurement Magazine
7

Instrumentation & Measurement Magazine 24-4

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