Improving magnetic properties of ultrasmall magnetic
nanoparticles by biocompatible coatings
Rocio Costo*, M. Puerto Morales, Sabino Veintemillas-Verdaguer.
Supplementary data:
1. Characterization techniques:
The crystal structure of the samples was identified by X-ray powder diffraction
performed in a Bruker D8 Advance powder diffractometer by using the CuKα
radiation with an energy-discriminator (Sol-X) detector. The patterns were collected
between 5˚ and 90˚ in 2θ. The XRD spectra were indexed to an inverse spinel
structure. The average crystallite size was calculated by Scherrer’s formula using the
half width of the (311) X-ray diffraction peak.
Particle size was determined from TEM micrographs using a 200 keV JEOL-2000
FXII microscope. For observation of the sample in the microscope, the particles were
dispersed in isopropanol and a drop of the suspension was placed onto a copper grid
covered by a perforated carbon film. The mean particle size and distribution were
evaluated by measuring the largest internal dimension of at least 100 particles.
Exposure of the region of interest to the electron beam was minimized by obtaining
best focus, astigmatism and brightness on a region adjacent to the region of interest
before exposing the region of interest to the electron beam in order to minimize phase
changes and sample damage induced by the microscope electron beam.
Subnanometric structural characterization (subnanometric changes in particle size or
structure as stacking faults, dislocations or twin boundaries) was not performed by
HR-TEM for the same reason.
Infrared spectra of the samples diluted in 2% KBr were recorded between 4000 and
400 cm-1 in a Bruker IFS 66V-S spectrometer and a Nicolet FT-IR 20SXC.
The magnetic properties of the samples were recorded in a MagLabVSM vibrating
sample magnetometer from Oxford Instrument. For the measurement of powders, the
samples were dried in an inox-coated oven at 50˚C 24h. Afterwards, the samples were
accurately weighed and fitted into the sample holder. To minimize interactions in the
liquid samples, the dispersions were prepared at low concentrations (0.25 wt%). To
avoid nanoparticle aggregation due to the VSM magnetic field, liquid samples were
always measured in the frozen state. One hundred microliters of the sample were
placed in the sample holder and immersed in liquid nitrogen before being placed in
the VSM. The temperature was always kept under 250 K. Temperature-dependent
zero field cooling (ZFC) and field cooling (FC) magnetization measurements were
taken by initially cooling the samples to 5 K in the zero and 200 Oe fields,
respectively. Then, magnetization was measured during the heating cycle (3 K · min1
) from 5 to 250 K under a 200 Oe field. Hysteresis loops of the powder samples were
measured at RT and at 5 K at a rate of 5 kOe·min-1. Saturation magnetization was
evaluated by extrapolating to an infinite field the experimental data obtained in the
high field range where magnetization increases linearly with H which can be
approximated to a 1/H law. Exchange anisotropy studies were undertaken by freezing
the sample to 5 K in a 50 kOe field. Afterwards, hysteresis loops were carried out at 5
kOe·min-1. The magnitude of the loop shift (HS) and coercivity (Hc) were calculated
by HS= - (H0+ + H0-)/2 and Hc= (H0+ - H0-)/2, respectively.
Simultaneous thermogravimetric (TG) and differential thermal analysis (DTA)
analysis were performed on a Seiko TG/DTA 320U thermobalance. Samples were
heated from room temperature to 900˚C at 10˚C/min under an air flow of 100 ml/min.
Iron determination was carried out in an Inductively Coupled Plasma – Optical
Emission Spectrometry (ICP-OES) apparatus, OPTIME 2100DV from Perkin Elmer.
Determination of the carbon content was carried out in a Perkin Elmer 2400 CHNS/O
Series II analyzer.
A Zetasizer nano ZS by Malvern Instruments was used to determine both the
hydrodynamic size and Z-potential.
This device is equipped with a He‐Ne laser (4 mW, 633 nm), an automatic laser
attenuator (transmission from 100% to 0.0003%) and an avalanche photodiode
detector.
The mean hydrodynamic diameter of the aggregates was measured thanks to non
invasive back scatter (NIBS) technology and dynamic light scattering (DLS) which
allow measure sizes from 0.6 nm to 6 μm. To minimize the effect of multiple
scattering, measurements at three different sample concentrations were performed.
The result with the higher count number and the best position was selected as correct.
A log‐normal distribution function was used to fit the size data obtained.
It is worthy to mention that although the fundamental size distribution generated by
DLS is an intensity distribution, this can be converted to a volume distribution using
the refraction index (2.42 for maghemite) and the Mie theory. This volume
distribution can be further converted to a number distribution; however, number
distributions are of limited use as small errors in gathering data could lead to huge
errors in distribution by number.
In most of the experiments of this work, the hydrodynamic size distribution is
measured in terms of volume instead of intensity. The use of the volume mode is
specially appropriated in case of two or more different size populations in the sample.
In these cases when we measure in intensity mode, we are overestimating the
contribution of the large particles. This is because large particles scatter much more
light than small particles (the intensity of scattering of a particle is proportional to the
sixth power of its diameter ‐ from Rayleigh’s approximation).
2. X-ray diffraction analysis:
Intensity (a.u.)
(311)
UNCOATED SAMPLE
(440)
(400) (511)
10
20
30
40
50
60
70
80
90
2CuK
Figure S1: XRD diffractogram of the iron oxide nanoparticles synthesized by laser
pyrolysis before the coating process. The peaks have been indexed to a spinel
structure, -Fe2O3 JPCDS file Nº 39-1346
3. FTIR Spectra:
PAA
2,5
Absorbance Units
Absorbance Units
2,5
2,0
1,5
1,0
0,5
3500
3000
2500
2000
1500
1000
PAA
2,0
1,5
1,0
0,5
2000
500
-1
PAMIDRONIC
1,4
Absorbance Units
Absorbance Units
500
Wavenumber (cm )
1,2
1,0
0,8
0,6
0,4
PAMIDRONIC
1,2
1,0
0,8
0,6
0,4
0,2
2000
0,2
3500
3000
2500
2000
1500
1000
500
1500
CM-DEXTRAN
CM-DEXTRAN
Absorbance Units
1,0
0,8
0,6
0,4
0,2
3000
2500
500
Wavenumber (cm )
Wavenumber (cm )
3500
1000
-1
-1
Absorbance Units
1000
-1
Wavenumber (cm )
1,4
1500
2000
1500
1000
500
1,0
0,8
0,6
0,4
0,2
2000
-1
Wavenumber (cm )
1500
1000
500
-1
Wavenumber (cm )
Figure S2: FTIR spectra of the coating molecules recorded between 3600 and 300
cm-1 (left) and between 2000 and 300 cm-1 (right).