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A Sensitive and Quantitative pEPR
Detection System for SPIO Nanoparticles
X. Li, G. Torfs, J. Vandewege, J. Bauwelinck,
and J. R. Verbiest
This letter presents a new system for super paramagnetic iron oxide
nanoparticles (SPIONs) quantification. The proposed system design
relies on the particle electron paramagnetic resonance (pEPR) technique
and it utilizes the linear response of SPIONs to generate a pEPR signal
under a static field. The pEPR signal intensity is proportional to the
SPIONs concentration, by extracting the pEPR signals, the quantitative
information of SPIONs is derived. To evaluate the system performance,
sensitivity measurements have been conducted with two commercial
SPIONs, FeraSpin M and Resovist, and reveal a measurement
sensitivity of 300 ng..
Introduction: Super paramagnetic iron oxide nanoparticles (SPIONs)
have been recently implemented in medical and biomedical research
applications, such as disease diagnosis, hyperthermia and drug delivery
[1]. Accurate quantification of SPIONs is crucial in these applications
[2-3]. Several methods to quantification of iron ions in aqueous solution
have been described in the past, among these are inductively coupled
plasma spectrometry (ICP), Perls' Prussian blue reaction spectroscopy,
fluorometry and continuous wave electron paramagnetic resonance
(CW EPR) spectrometry [3]. ICP offers extremely high sensitivity,
although comes from high cost, and it is neither suitable for ex vivo
analysis [4]. In contrast to Perls' reaction-based colorimetry technology
and the fluorometry, the CW EPR technology had been shown to be a
more sensitive technique for SPION quantification [4]. CW EPR
spectrometer allows indirect EPR response measurements by recording
microwave power absorption during spin resonance, and a sweep of
static field is required .
In this letter, we present a new quantitative measurement system for
SPIONs based on the proposed particle electron paramagnetic response
(pEPR) technique [5]. Compared to a typical EPR spectrometric process,
pEPR technology directly detects EPR radiated from SPIONs, and the
pEPR signal intensity is proportional to the magnetization of the
nanoparticles. Due to the high magnetic susceptibility of SPIONs, a
high magnetization can be achieved at a relatively low magnetic field
(<50 mT) configuration, the low static magnetic field results in a low
resonant frequency, which is particularly interesting for biological and
medical applications, because the dielectrics losses in biological units
are significantly lower at low frequencies [5]. Moreover, pEPR
technology only requires fix static magnetic field configuration, hereby
the sweep of static field is no longer indispensable.
A b
Fig. 1 pEPR measurement fields configuration
a Positive B0 field configuration;
b Positive and negative B0 field configuration.
Measurement principle: In pEPR technology, the signal intensity of the
pERP wave is directly proportional to the concentration of the SPIONs.
By precisely detecting the active pEPR wave amplitude, the quantity of
SPIONs can be derived subsequently. To generate the pEPR signal, a
static magnetic field, B0, is required to orient the electron spins and an
alternating electromagnetic (EM) field, B1, at the corresponding Larmor
frequency is required to resonate the spins, as shown in Fig. 1 (a). M
represents the magnetization of spin, which precesses about B0 field
direction. The two fields, B0 and B1, are perpendicularly imposed on the
SPIONs. When resonating, the spin absorbs RF energy from the EM
field and emits a pEPR wave, My, at the Larmor frequency [5].
However, in pEPR measurements, the signal intensity of nanoparticle
sample is also determined by the static magnetic field strength, the EM
field strength and local temperature [6]. Hence, all these parameters are
expected to be highly stable during quantification measurement.
In the presented system, the relative orientation of the magnetic fields
is critical. The static field, B0, and the EM field, B1, are perpendicular
and the pEPR detection is set along y axis to avoid disturbance from the
static field and the EM field, as shown in Fig. 1. Nevertheless, in
practical, an EM field feed through signal is still presented together with
the pEPR signal because of strong coupling. Additionally, based on the
fact that the direction of SPIONs magnetization, M, will respond to a
direction change of B0 field, as shown in Fig. 1(b), a positive and
negative B0 field configuration is introduced in the proposed system. By
subtracting the received signals under positive and negative B0 field, the
feed through signal of EM field is eliminated as shown in (1) [7], SPositive
and SNegative represent the received signals under positive and negative
B0 field respectively, and SpEPR represents the pEPR signal, which is
proportional to the SPIONs quantity.
noiseSSS pEPRNegativePositive 2
(1)
Fig. 2 Conceptual block diagram of pEPR system
System architecture: The high level system architecture is shown in Fig.
2. Four main blocks can be distinguished: an RF transmitter (TX), an
RF receiver (RX), the coils and a control and data processing unit. The
coils are the central part of the pEPR system, which include a
Helmholtz coil, RX- and TX-coil. The Helmholtz coil builds the B0
field, and a low jitter synthesizer followed by a power amplifier (PA)
drives the TX coil which establishes the EM field, B1, that excites the
nanoparticles to resonance. The emitted pEPR signal is captured by the
RX coil, amplified by the low noise amplifier (LNA) and sampled with
an analog to digital converter (ADC). The digital processing unit
analyzes the digitalized pEPR signal and processes the collected data to
yield the quantification result of SPIONs. To achieve high measurement
sensitivity, a high quality factor RX coil is utilized, and a high isolation
between the TX coil and the RX coil is achieved by careful positioning
the two coils in the proposed system. Besides high isolation, precise
control of the measurement timing is also important, low jitter ADC
clock is highly applied. Furthermore, to further reduce the system noise
level, each measurement result would be repeated and averaged over a
million times.
Results and discussion: The proposed system was evaluated by
performing pEPR measurements on two different SPIONs, namely
FeraSpin M (Miltenyi Biotec, Leiden, Netherlands) and Resovist
(Schering AG, Berlin, Germany). Samples with different SPION
concentrations were prepared in 150 μL demineralized water. During
the measurements, a magnetic field of 10 mT and a RF frequency that
2
a b
Fig. 3 pEPR measurement results
a Five Resovist samples between 0.1 μg and 2 μg were quantified by
pEPR, an error bar of one SD was added on each sample measurement;
b Ten FeraSpin samples between 0.08 μg to 41 μg were quantified by
pEPR, an error bar of one SD was added on each measurement.
corresponds to the Larmor frequency were employed. Each
concentration was quantified 10 times and the mean value and the
standard deviation (SD) of the pEPR signal intensity were calculated.
The measurement results were plotted in Fig. 3. Both measurement
results exhibited the pERP signal intensity was proportional to the
SPION quantities of sample solutions. The pEPR signal intensity of
Resovist was 1.87 mV/μmol, in Fig. 3 (a), which is almost two times
smaller than 3.49 mV/μmol of FeraSpin M, in Fig. 3 (b), caused by the
different characteristics of the two SPIONs. By comparing the deviation
with the SD of each measurement result, the detective sensitivity of the
proposed system was estimated as 300 ng.
Fig. 4 FeraSpin M measurement with both pEPR and ICP-OES and an
error bar of one SD was added on each measurement. Two wavelengths
of iron ion, 238.204 nm and 259.940 nm, were chosen for FeraSpin M
under ICP-OES measurements.
Subsequently, a reference measurement was performed to validate
previous measurement results. 8 FeraSpin M samples ranging from 0.3
μg to 41 μg were quantified by inductively coupled plasma optical
emission spectrometry (ICP-OES). The iron ions quantity results
derived from ICP-OES measurement were plotted together with the
pEPR results in Fig. 4. The measurement results of the pEPR technique
showed agreement with ICP-OES results, proving the viability of the
presented pEPR technique, and verifying the previous measurement
sensitivity.
Conclusion: This letter demonstrates a sensitive quantification system
for SPIONs based on the pEPR technique, which allows direct
measurements of the emitted signal from SPIONs under specific
magnetic fields configurations. The designed system was evaluated by
quantifying two types of commercial nanoparticle, and the experiment
results were also validated by ICP-OES measurements. The
measurements prove that the presented system has a sensitivity down to
300 ng.
X. Li, G. Torfs, J. Vandewege, J. Bauwelinck (INTEC_Design, UGent,
Gent 9000, Belgium)
E-mail: xiao.li@intec.ugent.be
J. R. Verbiest (Pepric NV, Kapeldreef 75, Leuven 3001, Belgium)
E-mail: joeri.verbiest@pepric.com
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