Hydrophilization of Magnetic Nanoparticles with Modified Alternating Copolymers. Part 2: Behavior in solution.

Abstract

Aqueous solutions of iron oxide nanoparticles (NPs) stabilized by poly(maleic acid-alt-1-octadecene) (PMAcOD) modified with the 5,000 Da poly(ethylene glycol) (PEG) or the short ethylene glycol (EG) tails were analyzed by small-angle X-ray scattering (SAXS). Advanced SAXS data analysis methods were employed to systematically characterize the structure and interactions between the NPs. Depending on the type of the grafted tail and the grafting density all NPs can be separated into three groups. All the samples contain mixtures of individual nanoparticles, their dynamic clusters and aggregates, and the fractions of these species are different in the different groups. The first group consists of NPs coated with PMAcOD modified with the long PEG tails with the maximal grafting density, and the content of dynamic clusters and aggregates in the samples of this group does not exceed 4%. The samples from the second group with less dense coatings demonstrate a larger amount (5-7%) of the aggregates and dynamic clusters. The samples from the third group consisting of the NPs protected by EG modified PMAcOD contain mostly individual NPs and some amount of dumbbell dimers without noticeable aggregation. Importantly, the solution behavior of the NPs is independent on the iron oxide core size. Our results therefore provide means of predicting stabilization and avoiding aggregation of NPs based on the type of a protective shell.

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Hydrophilization of Magnetic Nanoparticles with Modified Alternating Copolymers. Part 2:
Behavior in Solution
Eleonora V. Shtykova,†Andrey Malyutin,‡Jason Dyke,‡Barry Stein,§Peter V. Konarev,⊥
Bogdan Dragnea,‡Dmitri I. Svergun,*,⊥and Lyudmila M. Bronstein*,‡
Institute of Crystallography, Russian Academy of Sciences, Leninsky pr. 59, 117333 Moscow, Russia,
Department of Chemistry, Indiana UniVersity, 800 East Kirkwood AVenue, Bloomington,
Indiana 47405, United States, Department of Biology, Indiana UniVersity, 1001 East Third Street, Bloomington,
Indiana 47405, United States, and EMBL, Hamburg Outstation, Notkestrasse 85, D-22603 Hamburg, Germany
ReceiVed: August 3, 2010; ReVised Manuscript ReceiVed: October 11, 2010
Aqueous solutions of iron oxide nanoparticles (NPs) stabilized by poly(maleic acid-alt-1-octadecene)
(PMAcOD) modified with the 5000 Da poly(ethylene glycol) (PEG) or the short ethylene glycol (EG) tails
were analyzed by small-angle X-ray scattering (SAXS). Advanced SAXS data analysis methods were employed
to systematically characterize the structure and interactions between the NPs. Depending on the type of the
grafted tail and the grafting density, all NPs can be separated into three groups. All the samples contain
mixtures of individual nanoparticles, their dynamic clusters, and aggregates, and the fractions of these species
are different in the different groups. The first group consists of NPs coated with PMAcOD modified with the
long PEG tails with the maximal grafting density, and the content of dynamic clusters and aggregates in the
samples of this group does not exceed 4%. The samples from the second group with less dense coatings
demonstrate a larger amount (5-7%) of the aggregates and dynamic clusters. The samples from the third
group, consisting of the NPs protected by EG-modified PMAcOD, contain mostly individual NPs and some
amount of dumbbell dimers without noticeable aggregation. Importantly, the solution behavior of the NPs is
independent of the iron oxide core size. Our results, therefore, provide means of predicting the stabilization
and avoiding aggregation of NPs based on the type of protective shell.
1. Introduction
Small-angle X-ray scattering (SAXS) is a universal method
to analyze the structure of various disperse systems at a
resolution from about 1 to 100 nm.1SAXS is useful due to its
capability to comprehensively characterize complex systems,
including, for example, polymer matrices and nanocomposites
containing nanoparticles (NPs). The SAXS patterns provide
structural information both about nanoscale inhomogeneities
(particles or clusters) and about the internal ordering in the
sample. One of the important benefits of SAXS is that specimens
are studied in their natural media and aggregate states, allowing
nondestructive quantitative analysis of structural characteristics
of the materials in a native environment. Application of SAXS
provides sizes and shapes of various NPs, their distributions
and locations in NP-containing polymer matrices,2-8and
structural information about internal organization of the entire
system.9-14Development of novel SAXS data analysis methods,
including a new approach to model simulations, allows one to
build low-resolution three-dimensional structural models of
different kinds of polymer organizations. In particular, ab initio
methods for low-resolution shape reconstruction (e.g., program
DAMMIN)15along with rigid body modeling methods,16
although originally developed for biological systems, can be
successfully employed to analyze various polymer systems,
including those containing metals, and to build structural models
of complex natural or artificial materials.9,17-19
Various types of nanocomposites were actively studied using
SAXS. This method was crucial for determining the morphology
of NPs in thin films,20polymer networks,21clays in polymers,22,23
or NPs themselves.24,25SAXS has been used for characterizing
arrays of NPs;26-28the self-assembly in nanocomposites, includ-
ing mesoscopic materials;29-35the deformation behavior of
nanocomposites;36and for proving the existence of oxo-metal
domains in a nanocomposite.37
It is noteworthy that, for better characterization of material
structures, the SAXS studies should be complemented by other
methods, such as atomic force microscopy, X-ray diffraction,
transmission electron microscopy (TEM), dynamic light scat-
tering, ?-potential measurements, etc. The combination of the
techniques allows for comprehensive structural and functional
analyses of sophisticated nanocomposites, revealing fine changes
occurring during NP formation or other processes.2,6,7,10,14,38,39
In our recent work, monodisperse iron oxide nanoparticles
coated with phospholipids containing poly(ethylene glycol)
(PEG) tails39and those protected by poly(maleic acid-alt-1-
octadecene) (PMAcOD)38were studied by SAXS. It was shown
that, in both cases, mainly individual functionalized magnetic
NPs were formed, but these specimens also demonstrated
different kinds of interactions in solution. The iron oxide NPs
coated with PEGylated phospholipids formed dynamic nano-
clusters exhibiting regular interparticle distances, whereas those
coated with the alternating copolymer had a tendency to form
aggregates. For various applications of NPs, it is of paramount
importance to understand the influence of such properties of
* To whom correspondence should be addressed. E-mail: lybronst@
indiana.edu (L.M.B.), svergun@embl-hamburg.de (D.I.S.). Tel: +1-812-
855-3727 (L.M.B.), +49 40 89902 125 (D.I.S.). Fax: +1-812-855-8300
(L.M.B.), +49 40 89902 149) (D.I.S.).
†Russian Academy of Sciences.
‡Department of Chemistry, Indiana University.
§Department of Biology, Indiana University.
⊥EMBL.
J. Phys. Chem. C XXXX, xxx, 000
A
10.1021/jp1072846
 XXXX American Chemical Society

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NP protective shells as their thickness, charge, and density on
the behavior of the NPs in solution to prevent them from
aggregating or to control the aggregation behavior. In the present
work, using synchrotron SAXS and modern tools of SAXS data
interpretation, we studied solution interactions of iron oxide NPs
coated with PMAcOD modified with grafted 5000 Da poly-
(ethylene glycol) (PEG) or short ethylene glycol (EG) tails of
different grafting densities. The synthesis and characterization
of these NPs is described in Part 1 of this work.40Comparing
NPs of different sizes with the same shell, and different shells
on the NPs of the same size, we were able to identify the most
important factors influencing the behavior of NPs in solution.
In particular, the NP aggregation (which is an undesirable
phenomenon and needs to be minimized when magnetic NPs
are hydrophilized) appeared to be linked to the properties of
the shell, allowing one to control the aggregation by the
appropriate selection of the coating.
2. Experimental Section
2.1. Materials. Hexanes (85%), ethanol (95%), and acetone
(99.78%) were purchased from EMD and used as received.
Chloroform (Mallinckrodt, 100%), FeCl3·6H2O (98%, Aldrich),
docosane (99%, Aldrich), oleic acid (OA, 90%, Aldrich), oleic
acid sodium salt (97%, TCI), and TBE buffer (1.3 M Tris, 450
mM boric acid, 25 mM EDTA·Na2in H2O, Fluka) were used
without purification. PMAOD (30 000-50 000 Da, Aldrich) was
used as received. O-(2-aminoethyl)-o′-methyl polyethylene
glycols (PEG) with MW ) 5000 and (()-3-amino-1,2-pro-
panediol (98.0%) (EG) were purchased from Fluka and used as
received. Water was purified with a Milli-Q (Millipore) water
purification system (18 µS).
2.2. Synthetic Procedures. 2.2.1. Synthesis of Iron Oxide
Nanoparticles. For the detailed procedure, see our preceding
paper.41The synthesis of iron oleate was carried out according
to the published procedure.42Iron oleate was then purified and
dried for 24 h at room temperature and for 20 h at 70 °C in a
vacuum oven. Spherical iron oxide nanoparticles of various sizes
were then prepared through thermal decomposition of iron oleate
in the presence of oleic acid in docosane as a solvent. The details
of NP precipitation and purification are described in Part 1 of
this work. Three NP samples have been used with the sizes of
21.6 (NP1), 26.5 (NP2), and 19.3 nm (NP3), as measured using
the TEM images (Figure 1).
2.3. Encapsulation of Iron Oxide Nanoparticles with
PMAOD-PEG(EG). To encapsulate the iron oxide NPs in
PMAOD-PEG(EG), first, the modified PMAcOD polymers
were synthesized. Notations “P” and “G” are reserved for
PMAOD-PEG(5000) and PMAOD-EG, respectively. Mole
ratios of 1:20 (P1, G1), 1:30 (P2, G2), 1:60 (P3, G3), and 1:80
(P4) of PMAOD to PEG or EG were used. The details of the
modified PMAcOD synthesis and NP coating are described in
Part 1 of the present work.40
2.4. Characterization. Electron-transparent specimens for
TEM were prepared by placing a drop of a dilute solution onto
a carbon-coated Cu grid. Images were acquired at an accelerating
voltage of 80 kV on a JEOL JEM1010 transmission electron
microscope. Images were analyzed with the ImageJ software
package to estimate NP diameters. Normally, 150-300 NPs
were used for analysis.
The synchrotron radiation X-ray scattering measurements
were carried out in the European Molecular Biology Laboratory
(EMBL) on the storage ring DORIS III of the Deutsches
Elektronen Synchrotron (DESY, Hamburg) on the X33 camera
with a MAR Image plate detector.43The scattering was recorded
in the range of the momentum transfer, 0.07 < s < 5.0 nm-1,
where s ) (4π sin θ)/λ, 2θ is the scattering angle, and λ )
0.15 nm is the X-ray wavelength. All measurements were carried
out in a vacuum cuvette when exposure times of 2 min were
used to diminish the parasitic scattering. The scattering profiles
were corrected for the background scattering from distilled water
and processed using standard procedures.44The distance dis-
tribution functions, p(r), and radii of gyration, Rg, of the iron
oxide cores of the nanoparticles were calculated using an indirect
transform program GNOM.45
The interaction of the iron oxide cores in solution was
analyzed using several approaches. The low-resolution shapes
and internal structures of the NP cores were reconstructed ab
initio from the scattering patterns using the program DAM-
MIN.15Higher-angle portions of the scattering data correspond-
ing to the scattering from the individual NPs were employed
for the shape reconstruction. The scattering amplitudes from
DAMMIN models of the individual cores were computed using
CRYSOL.46A simulated annealing protocol was employed to
build the models of the NP clusters by SASREF,16minimizing
discrepancy between the experimental scattering data and the
curves calculated from the model assemblies at smaller scat-
tering angles responsible for interparticle interactions. Multiple
SASREF runs starting from different initial configurations were
performed by varying the number of individual particles in the
cluster.
The fractions of aggregates in the samples were estimated
using the scattering intensity at zero scattering angle, I0, as is
shown below
where I0 mono, I0 whole, and I0 clustare the intensities at zero angle
for the monodisperse fraction, entire sample, and clusters
(aggregates), respectively, N denotes the number of the objects,
and V their volumes. The number of monomers in the cluster,
Figure 1. TEM images of 21.6 (a), 26.5 (b), and 19.3 (c) nm NPs.
I0mono∼ Nmono(Vmono)2
(∆) ) I0whole- I0mono) I0clust) Nclust(k*Vmono)2
B
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k, can be estimated as k ) (Rg/Rsingle)3, where Rgand Rsingleare
the radii of gyration of the cluster and of the individual NP,
respectively. Given that
one obtains the estimate of the volume fraction of monomers
or their number fraction
3. Results and Discussion
The samples characterized by SAXS are listed in Table 1
(see also notations in the Experimental Section). The charac-
teristics of most of the NPs coated with modified PMAcOD
are described in Part 1 of the present work.40The sample NP2-
P4 has a larger diameter of the iron oxide core (26.5 nm) than
that of NP3-P4 (19.3 nm) but the same stabilizing polymer to
detect the influence of the NP size on the interparticle interac-
tions. The other samples based on NP1 have the same iron oxide
core size (21.6 nm) but are coated with different shells to reveal
the effect of the properties of the protective coatings on the NP
aggregation. The concentrations of the samples were kept low
(in the range of 0.05-0.15 mg/mL) to minimize NP interactions
caused by the excluded volume effect.
Theexperimentalscatteringprofilesfromthesamplesinsolution,
shown in Figure 2, display distinctive maxima characteristic for
largely monodisperse systems of spherical particles.
The scattering patterns from the group of samples NP1-
PMAcOD, NP1-P1, NP1-P2, NP1-P3, NP1-G2, and NP1-G3
practically coincide with each other in the range of the maxima
due to the spherical form factor. Only two out of six of these
scattering patterns are shown in Figure 2 as the most different.
Experimental SAXS profiles from NP1-P1, NP1-P2, NP1-G2,
and NP1-G3 are not shown, being graphically indistinguishable
in this region. However, all curves of the group slightly differ
in the initial part of the experimental SAXS profile in the range
of the momentum transfer, 0.07 < s < 0.3 nm-1, responsible for
interparticle interactions. This observation indicates that the
structures of the NPs are similar, but their interactions in solution
are different. The SAXS curves from NP2-P4 and NP3-P4 are
significantly different from the others due to the difference of
the iron oxide core sizes. The average particle radii R for all
NPs were estimated from the position of the first minima, s1,
as R ) 4.49/s1. Moreover, the distance distribution functions
p(r) of the single NPs were evaluated using GNOM (Figure 3)
from the higher-angle portions of the data s > 0.3 nm-1to yield
the maximum sizes of the NPs, Dmax, and their radii of gyration,
Rsingle. The latter were employed to obtain an alternative estimate
of the radii of the single NPs, assuming that (Rsingle)2) (3/5)R2
(Table 1).
The obtained values correlate well with the TEM data. For
the same size (samples based on NP1), they also agree well
with each other within the experimental error. It should be noted
that the SAXS-derived sizes refer to the iron oxide cores only.
These cores have a much higher electron density contrast with
respect to water than the protective shells so that the latter
remain practically invisible for the X-rays. Some differences
in the radii of the iron oxide cores obtained from the first minima
and those calculated from the radii of gyration occur due to the
possible presence of small amounts of some larger scattering
objects because Rgis very sensitive to the presence of such
objects.
Further analysis of the SAXS data was divided into two parts:
(i) the determination of the structural characteristics of the
individual NPs from the higher-angle parts of the scattering data
(s > 0.3 nm-1) and (ii) the investigation of the particle
interactions reflected by the initial portion of the scattering
patterns. The structural models built at the first step were used
to evaluate the NP behavior in solution at the second phase.
TABLE 1: NP Radii, Their Maximum Sizes from SAXS,
and the Diameters from TEM
sample
R ) 4.49/s1(nm)
10.8 ( 0.2
10.7 ( 0.2
10.7 ( 0.2
10.8 ( 0.2
10.7 ( 0.2
10.7 ( 0.2
12.2 ( 0.3
9.7 ( 0.1
R ) 1.29Rsingle(nm)
11.1 ( 0.2
11.1 ( 0.2
11.1 ( 0.2
11.1 ( 0.2
11.0 ( 0.2
11.1 ( 0.2
12.6 ( 0.3
10.0 ( 0.2
Dmax(nm)
22.0 ( 1.0
22.0 ( 1.0
22.0 ( 1.0
22.0 ( 1.0
22.0 ( 1.0
22.0 ( 1.0
26.0 ( 1.0
19.5 ( 1.0
DTEM
(nm)
NP1-P1
NP1-P2
NP1-P3
NP1-G2
NP1-G3
NP1-PMAcOD
NP2-P4
NP3-P4
21.6
21.6
21.6
21.6
21.6
21.6
26.5
19.3
Nmono/Nclust) k2(I0mono/I0clust)
Vm) k(I0mono/I0clust)/(1 + k(I0mono/I0clust))
nm) k2(I0mono/I0clust)/(1 + k2(I0mono/I0clust))
Figure 2. Experimental SAXS profiles from iron oxide NPs in solution
for NP1-PMAcOD (1), NP1-P3 (2), NP2-P4 (3), and NP3-P4 (4). The
scattering curves from samples with different core sizes are displaced
down by two logarithmic units for better visualization.
Figure 3. Distance distribution functions calculated by the program
GNOM from the experimental SAXS curves for NP1-PMAcOD (1),
NP1-P3 (2), NP2-P4 (3), and NP3-P4 (4). Distance distribution
functions for NP1-P1, NP1-P2, NP1-G2, and PN1-G3 are not shown
because they coincide with the functions for NP1-PMAcOD (1) and
NP1-P3 (2).
Hydrophilization of Magnetic NPs with Copolymers
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For this, the higher-angle portions of the scattering data
corresponding to the particle form factor reflect the shape and
the inner structure of the individual iron oxide cores, which
can be reconstructed ab initio at low resolution. The distance
distribution functions in Figure 3 were back transformed to
extrapolate the form factor of the individual particle to zero
angle scattering, and the shape was further reconstructed by the
program DAMMIN. The typical shapes of the iron oxide cores
with different diameters represented by densely packed beads
are shown in the insets of Figure 4.
The scattering patterns computed from the bead models yield
very good fits to the experimental data with a discrepancy of
about 2.0-3.0 for all the samples. All NPs display a multilay-
ered structure, reflecting the process of the formation of the iron
oxide core from smaller nuclei, as discussed in our preceding
papers.38,39The ab initio models of the individual particles were
further employed to determine the influence of the core sizes
and the protective shell on the behavior of the magnetic NPs in
solution. For this, the initial portions of the scattering patterns
displayed in Figure 5a were analyzed, revealing information
about the interparticle interactions.
As one can see from Figure 5, the initial portions of the
experimental scattering profiles in the range of the momentum
transfer, 0.07 < s < 0.3 nm-1, can be divided into three
groups. The first group (I), containing samples NP1-P3, NP2-
P4, and NP3-P4, demonstrates distinct shoulders at about
0.10-0.20 nm-1, reflecting interference between the particles
and regularly spaced NPs (dynamic clusters) similar to those
observed in our preceding papers.38,39The scattering curves
from NP1-PMAcOD, NP1-P1, and NP1-P2 of the second group
(II) display weak shoulders, whereas the scattering from the
last group (III) of the samples NP1-G2 and NP1-G3 displays
small interference effects. All these samples contain mixtures
of individual nanoparticles, dynamic clusters, and aggregates,
but the fractions of these kinds of scattering objects are different
in the different groups. To quantitatively assess the content of
the three fractions, we analyzed the behavior of the distance
distribution functions p(r) calculated using the entire scattering
curves (Figure 5b). These p(r) functions can be divided into
the same three groups observed for the initial portion of the
scattering patterns (Figure 5a). Considering the profiles of the
p(r) functions, the largest contributions from the clusters or
aggregates are present in the samples of the first group, whereas
the samples of the third group contain mainly individual NPs
with a small fraction of relatively small (presumably dimeric)
aggregates. The amount of aggregates in each sample was
quantitatively estimated using the scattering intensity at zero
scattering angle, I0, as described in Experimental Section, and
the results are presented in Table 2.
This table reveals the same separation of samples into three
groups. The volume fractions of aggregates or clusters in all the
samples are approximately the same, but their numerical fractions
are different because of the different number of individual
nanoparticles per cluster. Note that the employed estimate k )
(Rclust/Rsingle)3assumes the shape similarity of the clusters and the
individual NPs and gives only a rough (and usually somewhat
overestimated) value, which is still sufficiently accurate for
quantitative analysis. Although the value of k is the largest for the
first group, the content of clusters and aggregates in the samples
of the group still does not exceed 4%. This suggests that, in this
case, the protective shell successfully prevents NPs from aggregat-
ing,andtheshouldersatthesmallanglesreflectmostlyinterference
Figure 4. Examples of the shape restoration for the cores with different diameters: (a) shape restoration for the cores with a diameter of 22.0 nm
(NP1-PMAcOD, NP1-P1, NP1-P2, NP1-P3, NP1-G2, and NP1-G3), (b) shape restoration for the core with a diameter of 26.0 nm (NP3-P4), and (c) shape
restoration for the core with a diameter of 19.5 nm (NP3-P4). Curves: (1) the experimental data, (2) the GNOM curves extrapolated to zero angle, and (3)
the scattering patterns computed from the bead model (all for the cores). Insets: ab initio bead models reconstructed from the scattering data.
Figure 5. Comparison of the initial portion of the scattering profiles (a) and distance distribution functions calculated on the whole experimental
curves: (b) for NP1-P3 (1), NP2-P4 (2), and NP3-P4 (3) and (c) for NP1-PMAcOD (4), NP1-P1 (5), NP1-P2 (6), NP1-G2 (7), and NP1-G3 (8).
D
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between the particles and an existence of the dynamic clusters.
The samples of the second group demonstrate a larger amount (up
to5-7%)ofaggregatesanddynamicclusters.Thelastgroup(NP1-
G2 and NP1-G3) contains a significant amount (12%) of relatively
small structural formations consisting, on average, of two NPs
(presumably,NPdimers)inadditiontotheindividualnanoparticles.
We believe that the dimers can be considered as new structural
formations (rather than aggregates) that have emerged due to
specific interactions between their protective shells. Indeed, the
distance distribution functions for these samples (Figure 5b)
demonstrate a complete absence of larger aggregates, but the
presence of some amount of double-sized nanoparticles (see the
tails of the p(r) functions in the range of 25-35 nm) instead. It
should be noted that concentrations of the samples were kept in
the range of 0.05-0.15 mg/mL. Thus, the effect of the excluded
volume on the interparticle interactions can be completely ne-
glected, so the interactions in solution should be attributed solely
tomagneticinteractionsoftheironoxidecoresandtotheproperties
of the protective shells.
The structures formed by the NPs due to the interparticle
interactions were further characterized by rigid body modeling.
The calculations were carried out in the range of scattering
angles from 0.1 up to 0.4 nm-1, that is, at the very beginning
of the experimental scattering curves containing the major
information about the ensembles of individual NPs in solution.
Figure 6 demonstrates the typical spatial models calculated by
the program SASREF for the different groups of the specimens.
Group I containing scattering from NP1-P3, NP2-P4, and NP3-
P4 displays a regular structure of NPs with an equal distance
between the nearest neighbors (dynamic clusters). The samples of
the group have different core sizes, but practically equally dense
and thick protective shells. These shells are formed by a compara-
tivelydensebrushofthe5000DaPEGtailsonthePMAcODchain
(60-80% of all units are modified with PEG tails). Therefore, the
crucial factor determining the behavior of the NPs coated with the
modified PMAcOD is the properties of the protective shells and
not the size of the magnetic core, despite the fact that the larger
the particles from the same material, the stronger the magnetic
interactions. The thickness of the protective shell can be evaluated
as half the distance between the NPs, yielding about 1.4-1.5 nm.
This value agrees with the minimal thickness of the NP shell
determined by us in the preceding paper,39where a PEGylated
phospholipid served as a stabilizing molecule. This thickness can
account for strongly interdigitated hydrophobic tails, but it does
not take into consideration the PEG tails. The latter are apparently
squeezed out of the interparticle space of the dynamic clusters.
Group II (NP1-P1, NP1-P2, and NP1-PMAcOD) demonstrates
a stronger interparticle attraction: in this case, the average distance
between closely located NPs is about 1.0-1.2 nm, which demon-
strates not only squeezing out of the PEG tails but also distortion
of the hydrophobic bilayer. Here, the PEG brush is significantly
less dense with 20% (NP1-P1) or 30% (NP1-P2) of maleic
anhydride units containing a PEG tail. Besides dynamic clusters,
there also exists apparently a significant amount of aggregates in
solution. This is easily explained by the way of stabilization of
these particles. Indeed, PMAcOD-coated NPs are stabilized purely
due to electrostatic interactions. This allows for a good stabilization
due to electrostatic repulsion, but the shell is so thin that permanent
aggregation is also possible (although the degree of the aggregation
is low). NP1-P1 (20% PEG tails) and NP1-P2 (30% PEG tails)
have a lower negative charge (see Part 1 of the present work,40
Table 2) than NP1-PMAcOD, but higher steric stabilization; thus,
they may form both dynamic clusters and aggregates.
The rigid body analysis of all the scattering profiles was
performed with the minimum amount of NPs sufficient to fit
the scattering data. For the two preceding groups, three particles
had to be taken into account. The data from the last group (NP1-
G2 and NP1-G3) could be well fitted by a dimeric dumbbell
structure (Figure 6III), suggesting the presence of permanent
dimeric structural formations, coexisting with individual nano-
particles. It is noteworthy that G2 and G3 copolymers have a
smaller charge than that of PMAcOD; thus, electrostatic
stabilization decreases. However, the grafted tails are very short,
so the steric stabilization is not realized. On the other hand, the
addition of two hydroxyl groups per each G tail allows for higher
hydrophilicity, but also additional hydrogen bonding between
the adjacent shells. Apparently, this combination leads to
dumbbell structures. At the same time, the solutions of NP1-
G2 and NP1-G3 demonstrate the lowest content of large
conglomerates to compare with groups I and II.
In general, the independent rigid body analysis fully con-
firmed the results obtained when considering the behavior of
the distance distribution functions and the initial portions of
the scattering profiles.
4. Conclusions
The structural analysis performed to understand the influence
of the type of the magnetic NP shell on the solution behavior
of NPs in water revealed a dependence of the NP aggregation
on the type of the grafted tails and the grafting density of the
modified PMAcOD. The negatively charged NPs protected by
the modified copolymers of the P and G series showed only a
small amount of real aggregates and high stability of individual
NPs in aqueous solutions, the solution behavior which is
important for practical applications of magnetic NPs. Neverthe-
less, there are differences between the NP specimens depending
on the protective shell. The detailed analysis carried out by
independent methods demonstrated that the NP coating by
modified PMAcOD with a high degree (60-80%) of grafted
5000 Da PEG tails leads to minimal aggregation in solution.
TABLE 2: Contents of Clusters (Aggregates) in the Samples
samples group
Rg
(nm)
Dmax 1
(nm)
k ) (Rg/Rsingle)3a
Vsingleb
Nsinglec
NP1-P3
NP2-P4
NP3-P4
NP1-P1
NP1-P2
NP1-PMAcOD
NP1-G2
NP1-G
I 16.5
13.7
17.3
14.1
13.4
12.3
9.9
39.9
50
40
55
45
45
40
35
35
7.1
5.6
5.2
4.2
3.5
2.8
1.5
1.5
0.83
0.80
0.82
0.83
0.84
0.83
0.83
0.83
0.97
0.96
0.96
0.95
0.95
0.93
0.88
0.88
II
III
aRg and Rsingle are the radii of the clusters and individual NPs,
respectively; k is the number of individual nanoparticles in clusters
or aggregates.
fraction of individual NPs. Dmax
clusters or aggregates in the system.
bVolume fraction of individual NPs.
1 is the maximum size of the
cNumber
Figure 6. Examples of structures of clusters and aggregates in different
groups of specimens: I (NP1-P3, NP2-P4, and NP3-P4), II (NP1-
PMAcOD, NP1-P1, and NP1-P2), and III (NP1-G2 and NP1-G3).
Hydrophilization of Magnetic NPs with Copolymers
J. Phys. Chem. C, Vol. xxx, No. xx, XXXX E