Structuration and Integration of Magnetic Nanoparticles on Surfaces and Devices Elena Bellido , Neus Domingo , Isaac Ojea-Jiménez , and Daniel Ruiz-Molina *
Different experimental approaches used for structuration of magnetic nanoparticles on surfaces are reviewed. Nanoparticles tend to organize on surfaces through self-assembly mechanisms controlled by non-covalent interactions which are modulated by their shape, size and morphology as well as by other external parameters such as the nature of the solvent or the capping layer. Further control on the structuration can be achieved by the use of external magnetic fi elds or other structuring techniques, mainly lithographic or atomic force microscopy (AFM)-based techniques. Moreover, results can be improved by chemical functionalization or the use of biological templates. Chemical functionalization of the nanoparticles and/or the surface ensures a proper stability as well as control of the formation of a (sub)monolayer. On the other hand, the use of biological templates facilitates the structuration of several families of nanoparticles, which otherwise may be diffi cult to form, simply by establishing the experimental conditions required for the structuration of the organic capsule. All these experimental efforts are directed ultimately to the integration of magnetic nanoparticles in sensors which constitute the future generation of hybrid magnetic devices.
Assisted Self-Assembly and 3. Organization ...............................................6
Use of Biomolecular 4. Templates ................................................. 15
Integration into Hybrid 5. Devices ..................................................... 19
Summary and Perspectives 6. ......................23
From the Contents
small 2012, DOI: 10.1002/smll.201101456
E. Bellido et al.reviews
DOI: 10.1002/smll.201101456
Dr. E. Bellido , Dr. N. Domingo , Dr. I. Ojea-Jiménez , Dr. D. Ruiz-Molina Centro de Investigación en Nanociencia y Nanotecnología, (CIN2, ICN-CSIC)Esfera UAB. Campus UAB, Edifi cio CM7Cerdanyola del Vallès, 08193, Spain E-mail: [email protected]
Figure 1 . Schematic illustration of the experimental approaches used for the deposition of MNPs as self-assembled mono- and multi-layers on surfaces: a) drop-casting, b) substrate immersion into a colloidal solution, c) dip-coating, d) Langmuir–Blodgett deposition, e) spin-coating, and (f) tape-casting (also known as doctor-blade-casting). Images not drawn to scale.
1. Introduction
Due to their unique electrical, [ 1 ] optical, [ 2 , 3 ] and magnetic [ 4–6 ]
properties arising from their fi nite size and high surface area-
to-volume ratio, nanoparticles (NPs) have recently attracted
great interest for applications in different fi elds of nanotech-
nology. In particular, magnetic nanoparticles (MNPs) repre-
sent an important class of nanostructured materials [ 7 ] whose
exclusive properties have been extensively studied due to
their potential use in many applications such as magnetic
storage media, [ 8–10 ] spin transport devices, [ 11 ] medical drug
delivery, [ 12–14 ] and magnetic resonance imaging (MRI), [ 15 , 16 ]
among others. However, for such applications to become
a reality, the development of proper techniques for their
structuration on surfaces with control over their positioning,
geometry, and organization is a crucial issue. Such advances
can fuel the detailed understanding of their individual and
collective properties by having the possibility to study them
at the individual (or small number) level. [ 17–21 ] This would
allow, for instance, a detailed study of the shape- and size-
dependence of their magnetic properties as well as the mag-
netic interaction between MNPs at a desired interparticle
distance and organization. Moreover, from an applications
point-of-view, the continuous miniaturization of MNP-based
structures on surfaces has advantages in the development
of magnetic devices with higher storage densities and faster
speeds, [ 10 ] and the fabrication of new types of magnetoelec-
tronic devices. [ 22 , 23 ]
With the introduction of several micro- and nano-fabri-
cation techniques such as microcontact printing ( μ CP), [ 24 , 25 ]
electron beam lithography (EBL), [ 26 ] and atomic force micro-
scopy (AFM)-based lithographies, [ 27–30 ] scientists started the
fi rst steps to organize MNPs into ordered structures at the
nanometer scale. Since then, the number of methodologies
and approaches followed has been continuously increasing
so that, to date, one of the major challenges is the accurately
placing of a few or even a single MNP on the desired region
of a given surface. While there have been other comprehen-
sive reviews on various aspects of the nanoscale organization
of different building blocks on surfaces and interfaces [ 31–34 ]
as well as the nanofabrication of magnetic nanostructures for
their application in data storage media, [ 35 , 36 ] there is a need
for a comprehensive and detailed description of the different
strategies employed or developed for assembling MNPs into
ordered nanostructures. In this review, fi rst the different
approaches and factors infl uencing the self-assembly of
MNPs on surfaces are considered. Following this, examples of
self-assembly methods assisted by lithographic techniques, or
even external-fi eld- or biological template-assisted methods
are reviewed. Finally, in the last section, an overview of the
most representative experimental approaches so far, followed
for the integration of the MNPs on sensors and devices, is
given.
2. Un-assisted Self-Assembly
Self-assembly is the autonomous organization of pre-
existing entities through non-covalent interactions leading
Figure 2 . a) Scanning electron microscopy (SEM) image of a cobalt-platinum NP fi lm deposited by the Langmuir–Blodgett method on electron-beam lithographically patterned Au electrodes, b) magnifi ed view of the NP fi lm, c) schematic illustration of the device. Reproduced with permission. [ 42 ] Copyright 2008, American Chemical Society. d) Background SEM image of ∼ 11 nm Fe x O/CoFe 2 O 4 NPs on a Pt-covered Si substrate by tape-casting method. Cross-sectional transmission electron microscopy (TEM) images of a thin NP superlattice made of 11 nm spherical-shaped (e), and ∼ 11 nm cubic-shaped (f,g) Fe x O/CoFe 2 O 4 NPs. Reproduced with permission. [ 45 ] Copyright 2009, American Chemical Society.
over the isotropic ones, such as van der
Waals, steric repulsions, or temperature
diffusion effects.
The magnetic interaction energy for
two dipoles in units of thermal energy
k B T is:
VkBT = µo µ2
4πkBTr 2[̂S1 · ̂S2 − 3(̂S1·)(̂S2·)]
where ̂Sl are the orientation unit vectors
of the magnetic dipoles, separated by a
center-to-center distance of ̂r , and μ is
the magnetic moment of a particle. Con-
sidering a particle with fully saturated
magnetization, that is a single-domain
particle, of volume v p , and saturation
magnetization of the bulk m s , the mag-
netic moment can be approximated by:
μ = v p m s . Thus, the magnetic dipolar
interactions increase quadratically with
the particle magnetic moment, which
is also proportional to its volume. In
this sense, dipole–dipole interactions
become effective for big enough par-
ticles with high magnetic moments.
Still, the thermally fl uctuating mag-
netic moment of a superparamagnetic
nanoparticle can also be stabilized
when the particle is in the vicinity of
another particle and under the action
of dipole–dipole interaction. Puntes
et al., [ 48 ] for example, estimated that the
maximum dipolar fi eld created along the
axis between two Co NPs (with diameter
of ∼ 6 nm, magnetic moment of the order of
μ ≈ 10 4 μ B , and blocking temperature of
T B = 130 K) in contact, is of the order
of 7000 Oe. Thus, the energy of a Co NP
in such a fi eld is one order of magnitude
higher than its thermal energy at room
temperature and can also play an impor-
tant role in the self-assembly process of
superparamagnetic nanoparticles during
the evaporation process.
Dipolar interactions have been
stated as responsible for the chain-like
assembly of ferrite NPs, both in their
reduced (Fe 3 O 4 ) and oxidized ( γ -Fe 2 O 3 )
forms, as well as for the assembly of Fe 3 S 4
and Fe 7 S 8 NPs inside magnetotactic
bacteria, which gives them the ability
to align themselves and swim along the
earth's magnetic fi eld. [ 49–51 ] Dipolar
interactions are thus at the origin of
magnetic anisotropy of chains of nano-
particles. In the case of an assembly of
aligned magnetosomes, it has been stated
that the origin of the magnetic anisot-
ropy of this system of aligned magnetic
inheim small 2012, DOI: 10.1002/smll.201101456
Magnetic Nanoparticles on Surfaces and Devices
Figure 3 . Cryo-TEM images of Fe NPs with different diameters (measured by TEM): a) ∼ 4 nm, b) ∼ 14 nm, and c) ∼ 16 nm. Mixture of Fe NPs with diameters of ∼ 4 and ∼ 14 nm in zero fi eld (d), and under external magnetic fi eld infl uence (e), with magnifi ed 4x image (inset). Reproduced with permission. [ 53 ] Copyright 2003, Macmillan Publishers Ltd: Nature Materials.
nanoparticles is the dipolar interactions (up
to 70%), while the contribution of the align-
ment of their individual easy axis is much
smaller (only of a 30%). [ 51 ]
Dipolar interactions have been fur-
ther investigated for Fe, Fe 3 O 4 , and Co
NPs. [ 48 , 49 , 51–53 ] While van der Waals inter-
actions and steric forces can be modulated
using different type of surfactants, dipolar
magnetic interactions can be modifi ed by
changing the size of the magnetic NPs.
By systematically increasing the MNP
diameter, and hence the magnetic dipole
moment (which is proportional to the
third power of the radius), Philipse et al.
tuned the dipole–dipole inter action of Fe
NPs coated with a thick surfactant layer. [ 53 ]
The self-assembly of such particles in the
liquid-phase, and in the absence of a mag-
netic fi eld, was studied with cryogenic
TEM (cryo-TEM), revealing the evolution
from isolated to randomly oriented chain-
like MNPs structures on going from ∼ 4 to
∼ 20 nm in diameter, respectively ( Figure 3 ).
To demonstrate that the chain-like struc-
tures formed arise from dominant dipolar
forces, an external magnetic fi eld (1.6 T)
was applied to the colloidal solution. By
contraposition with the previous results,
an alignment of the assemblies along the
fi eld direction in the form of parallel bun-
dles (Figure 3 d-e) was observed. This is the
characteristic assembly of weakly dipolar
MNPs that do not spontaneously organize
when orienting their magnetic moment
along an applied fi eld. [ 54 ] Similar results
were observed by Puntes et al. who observed disordered
magnetic aggregation and precipitation of 14 nm Co NP. [ 48 ] In
these cases, formation of ordered structures is prevented due
to too strong magnetic interactions.
In the absence of dipolar interactions, that is, for small
enough nanoparticles with low magnetic moments, isotropic
van der Waals interactions dominate the self-assembly
process and 2D or 3D arrays of NPs are formed. For instance,
Pileni et al. investigated the self-assembly of ∼ 10 nm γ -Fe 2 O 3
where the length of the capping layer is modifi ed by using
either octanoic (C 8 ) or dodecanoic acid (C 12 ). [ 55 ] The experi-
ments showed that the use of shorter alkyl chains favor the
presence of stronger van der Waals interactions, so directing
their self-assembly into spherical aggregates.
2.3. Geometry and Environmental Parameters
In addition to the type of interaction and/or deposition
methodology, there are other factors such as particle con-
centration, nature of the surfactant layer, and/or solvent, or
the morphology of the MNPs that have also been shown to
Particle shape and morphology . These parameters clearly
infl uence an optimum packed with minimized energies. A nice
example of the morphology infl uence on the self-assembly proc-
esses was obtained by drop-casting a diluted solution of (Fe x O)
NPs onto a carbon-coated TEM grid. Well-defi ned cubic-
ordered superlattices were obtained when cubic-shaped wüstite
(Fe x O) NPs (11–13 nm in size) were used. [ 39 ] This result is in
contrast with the use of spherical Fe x O analogues (8–10 nm) [ 39 ]
and Co NPs ( ∼ 10 nm) [ 56 ] that result in hexagonal superlattices.
Particle concentration. The infl uence of the particle con-
centration was investigated by the deposition of ∼ 10 nm γ -Fe 2 O 3 NPs on different surfaces by means of the dip-coating
process. [ 57 ] The adsorption of MNPs on a surface was shown to
be strongly dependent on the particle concentration in solu-
tion, which typically leads to an increase of particle coverage
with concentration until a maximum is reached. At low con-
centrations and after repeating several times the dip and dry
process, the particle coverage is signifi cantly increased though
no multiple layers are formed because the surfactant pre-
vents their formation. In contrast, multilayered assemblies of
MNPs (organized into 3D superlattices) have been obtained
using an initial higher concentration with the drop-casting
method [ 56 , 58 ] or, otherwise, immersing the substrate into a
5www.small-journal.combH & Co. KGaA, Weinheim
E. Bellido et al.reviews
Figure 4 . TEM images of multilayered assemblies of MNPs. a) fcc multilayers of ∼ 7 nm Co NPs. Reproduced with permission. [ 58 ] b) hcp of ∼ 9 nm Co NPs. Reproduced with permission. [ 4 ] Copyright 1999, American Institute of Physics.
solution containing the MNPs and allowing a slow evapora-
tion. [ 40 , 54 , 55 , 58 ] This is the case of Co NPs self-assembled into
face-centered cubic (fcc) ( ∼ 7 nm) ( Figure 4 a) [ 58 ] and hexa-
gonal close-packed (hcp) ( ∼ 9 nm) (Figure 4 b) [ 4 ] multi layers.
Nature of the solvent. The solvents are also found to
be important in the uniformity and morphology of the
MNP layer. In a self-explaining experiment, Wang et al.
demonstrated the importance of selecting a solvent with a
suitable evaporation rate in order to allow MNPs to diffuse
on the surface to form a monolayer. [ 59 ]
Effect of the temperature. The temperature is another
important external parameter to consider, which can be used
as a powerful tool for directing self-assembly of NPs toward
desired structures. From a thermodynamic point of view, the
fi nal ordering of large self-assembled patterns of MNPs is given
by minimization of the Gibbs free energy G = H – TS . The tem-
perature can be used as the weighting factor for the enthalpy
( H ) and entropy ( S ), so that the role of entropy can effectively
be increased by raising the temperature, while keeping all other
parameters constant. The entropy changes during the self-
assembly process are generally related to the vibrational and
confi gurational degrees of freedom. A study of the self-assembly
of Fe x O/CoFe 2 O 4 NPs in chloroform onto carbon-coated
TEM grids at different temperatures (between –40 and 45 ° C)
revealed that different superlattice structures were possible. [ 60 ]
Although the formation of fcc packing density was favored over
hcp at 45 ° C due to the higher entropy, the hcp structures were
progressively dominant with decreasing temperature (down
to –20 ° C). Finally, an hexagonal ordering forming non-close-
packed structures was reproducibly observed at –40 ° C. Similar
experiments with Co NPs deposited on silicon wafers, by means
of drop-casting or spin-coating techniques, have been per-
formed at different substrate temperature. [ 61 ] It was found that,
at low temperatures (25 ° C), instabilities such as capillary fl ows
causing ring-stains frequently appear, which destroy the self-
assembly. This contrasts with the great homogeneities observed
for large areas at high temperatures (150 ° C), which are due to
the large mobility of MNPs. Vanmaekelbergh's group recently
reported a systematic study of the thermodynamic driving
force behind the formation of binary nanocrystal superlattices,
carrie d out by inducing by the evaporation of the solvent at
layers. [ 63 , 64 ] By dipping fi rst a polymer-coated surface into a
solution containing FePt (4–6 nm) NPs the surface molecules
on the MNPs are replaced by the polymer, which forms a
strong monolayer on the substrate surface. Then, the thick-
ness of the multilayered assembly polymer/MNP was con-
trolled by adjusting the number of alternating coating steps.
Excess of surfactant molecules in the colloidal solution. Interestingly, the self-assembly of MNPs on surfaces has been
shown to be strongly dependent on the excess of surfactant
molecules freely dispersed in the colloidal solution (i.e., not
bound to the MNP surface). Kim et al. studied the infl uence
on the adsorption behavior of γ -Fe 2 O 3 ( ∼ 10 nm) NPs when
varying the concentration of surfactant molecules in the
colloidal solution. [ 57 ] In addition, to affect the viscosity of
the medium, a large excess of surfactant molecules can also
adsorb on the substrate surface and enter into competition
with the adsorption of MNPs. In contrast, a slight excess of
surfactant molecules may favor more uniform and long-range
ordering of MNPs with a relatively wide particle size distribu-
tion. For example, long-range self-assembly of ∼ 10 nm Fe 3 O 4
NPs coated with phospholipid molecules has been assisted
by the presence of free phospholipid molecules in the col-
loidal solution. [ 65 ] The authors suggested that due to the slight
excess of the lipid molecules, a phospholipid layer is formed
underneath the MNP monolayer, providing the necessary
binding force to hold them tied to the surface forming hexa-
gonal superlattices over extended areas of up to 2 cm × 2 cm.
The optimization of all these parameters (i.e., effects
of MNP shape and concentration, type of solvent, and the
presence of surfactant molecules) led to the successful depo-
sition, by the spin-coating process, of highly uniform mono-
layers of ∼ 6 nm FePt NPs with different morphology and
over large areas (1.5 cm × 1.5 cm) of a Si/SiO 2 surface. [ 59 ]
3. Assisted Self-Assembly and Organization
3.1. Chemically Assisted Assembly
Chemical assistance is an attractive approach to fabricate
highly stable and well-ordered monolayer fi lms of MNPs
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Magnetic Nanoparticles on Surfaces and Devices
Figure 5 . Schematic represenation of the three main approaches used in the chemically-assisted assembly of MNPs on a substrate: a) functionalization of the NPs with linkers bearing terminal groups able to react with the surface, b) functionalization of the surface with linkers bearing terminal groups able to react with the NPs, and c) a combination of both these methods. (Images not drawn to scale).
over large substrate areas. With this approach the stability
of the fi lms is strongly improved by introducing appropriate
linkers with suitable terminal functionalities that anchor the
MNPs to the surface. Beyond the stability gain, this approach
clearly favors the monolayer distribution of the MNPs over
the substrate instead of the formation of multilayers. [ 66 , 67 ]
The use of the linkers can be done in three different ways:
(i) functionalization of the MNPs with linkers bearing ter-
minal groups able to react with the surface, (ii) functionaliza-
tion of the surface with linkers bearing terminal groups able
to react with the MNPs, and (iii) a combination of both of
these methods ( Figure 5 ).
3.1.1. Pre-functionalization of the MNPs
Usually, the coating layer of the as-synthesized MNPs is
not really suitable to react with bare substrates. For this
reason, one common strategy is to replace the coating lig-
ands by bifunctional molecules able to react both with the
surface and the MNPs (see Figure 5 a). For example, par-
tial exchange of the oleic acid coating of ∼ 7 nm Fe 3 O 4 NPs
with trimethoxy-7-octen-1-yl-silane, bearing trimethoxysi-
lane groups that interact with the Fe 3 O 4 surface, has been
achieved ( Figure 6 a). [ 66 ] The new ligand also bears free vinyl
groups that can be anchored to H-terminated Si surfaces
through a stable covalent C–Si bond (Figure 6 b), resulting
in homogeneous coverage of the substrate (Figure 6 c). Fur-
ther magnetization studies using a SQUID magnetometer
showed that the MNPs effectively bound to the silicon
surface without any signifi cant alteration of their magnetic
properties (i.e., zero-fi eld-cooled (ZFC) and fi eld-cooled
(FC) magnetizations, and hysteresis loop). Parallel studies
made use of 10-undecynoic acid as a cross-linker between
∼ 5 nm Fe 3 O 4 NPs and the H-terminated Si surface. [ 68 ] An
extensive characterization of the MNP layer showed a
(APTS), as a coupling layer. [ 74 ] The resulting FePt NP
fi lm was extremely robust and preserved its integrity after
in-situ annealing (up to 800 ° C and 10 − 8 Torr for 30 min)
with no apparent aggregation ( Figure 7 a,b). The annealing
process is needed to transform the as-synthesized NPs from
the chemically disordered fcc structure to an ordered face-
centered tetragonal (fct) structure (L1 0 phase) that exhibits
improved magnetic properties, such as larger coercivity,
required in applications such as high-density data storage
media, permanent magnetic nanocomposites, and biomag-
netic applications. [ 75 ]
This approach can also allow the formation of less closely
packed NP-decorated surfaces, if required. Mallah et al.
prepared a Si surface bearing cationic N 3 Ni(H 2 O) 3 groups
oriented out of the substrate, which interacted with the nega-
tively charged ∼ 6 nm CsNiCr NPs by means of electrostatic/
coordination bonds (Figure 7 c,d). [ 67 ] To reduce the surface
coverage, unreactive spacers with –CH 3 terminal groups were
intercalated among the anchoring functionalities. Therefore, a
rather large NP–NP distance was expected as a result of the
low concentration of anchoring groups on the surface. SQUID
7www.small-journal.com & Co. KGaA, Weinheim
E. Bellido et al.reviews
Figure 6 . Deposition of pre-functionalized MNPs with surface-reactive functional terminal groups. a) Schematic representation for the replacement of the oleic acid coating of ∼ 7 nm Fe 3 O 4 NPs by trimethoxy-7-octen-1-yl-silane, b) posterior immobilization of the NPs on an H-terminated Si substrate through reaction with the vinyl groups, and c) 3D AFM image of the resulting NP monolayer. Reproduced with permission. [ 66 ] d) Schematic illustration of the synthesis of ∼ 3 nm CoFe 2 O 4 NPs covered with vinyl-terminated groups ready to interact with H-terminated Si-substrates, and e) cross-section TEM image of the resulting NP monolayer. Reproduced with permission. [ 69 ] Copyright 2007, American Chemical Society.
characterization indicated that the structural and magnetic
integrity of the NPs were maintained upon grafting, being
consistent with average interparticle dipolar interactions sim-
ilar to those of the same MNPs diluted into a poly mer matrix.
Pichon et al. used a similar strategy to control the immobiliza-
tion of oleic-acid-coated ∼ 12 nm iron oxide NPs (composed of
Fe 3 O 4 / γ -Fe 2 O 3 ) on top of SAM-functionalized Au surfaces dis-
playing different functional groups (such as –COOH, –OH and
–CH 3 ). [ 73 ] They observed that SAMs decorated with –COOH
groups have the highest ability to interact with MNPs, while
non-polar terminal–CH 3 groups prevented the assembly.
3.1.3. Combined Functionalization of the MNP and the Substrate Surface
The chemical modifi cation of both features simultaneously
results in a more stable and specifi c MNP–substrate inter-
action (see Figure 5 c). Kinge et al. have recently employed
this strategy combined with click-chemistry. [ 76 ] The original
coating layer of as-synthesized ∼ 5 nm FePt NPs was replaced
by 5-hexanoic acid and 6-amino-1-hexyne groups to form
alkyne-substituted FePt NPs ( Figure 8 a). Subsequently, a
SiO 2 surface is functionalized with azide-terminated SAMs
that react with the alkyne-functionalized NPs through the
formation of a triazole group. SEM analysis showed the suc-
cessful assembly of the MNPs in monolayers covering the
substrate (Figure 8 b). The nanoparticulated monolayer was
further annealed to transform the NPs from their as-synthe-
sized disordered fcc phase into the ordered L1 0 phase.
A different approach consists of the use of a functional-
ized polymer surface for the covalent immobilization of MNPs.
For instance, a polystyrene polymer matrix containing –COOH
functional groups was used for the carbodiimide-mediated
anchoring of γ -Fe 2 O 3 particles ( ∼ 200 nm) coated with an
amino-decorated polymeric shell. [ 77 ] This procedure led to a
single-layer of randomly distributed particles on top of the poly-
meric fi lm. Further arrangement of the magnetic particles into
ordered assemblies over large areas could be performed by the
application of an external magnetic fi eld parallel to the polymer
surface during the reaction process. Under these conditions, the
magnetic dipole of the particles aligned in the direction of the
applied fi eld and led to the formation of 1D chain-like ordered
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Magnetic Nanoparticles on Surfaces and Devices
Figure 7 . a) High-resolution SEM image of a ∼ 3 nm FePt NP monolayer formed over a surface functionalized with an SAM displaying terminal amino groups (bright dots correspond to the MNPs), and b) typical high-resolution SEM image of an FePt NP monolayer after thermal annealing process up to 800 ° C. Reproduced with permission. [ 74 ] Copyright 2003, American Institute of Physics. c) AFM image of a monolayer of ∼ 6 nm CsNiCr NPs formed on a surface functionalized with a SAM bearing cationic N 3 Ni(H 2 O) 3 and CH 3 terminal groups, and d) height profi le along the blue line drawn on (c). Reproduced with permission. [ 67 ] Copyright 2008, American Chemical Society.
Figure 8 . Assembly of ∼ 5 nm alkyne-substituted FePt NPs on an azide-terminated SAM using click-chemistry: a) schematic illustration of the procedure, and b) SEM image of the resulting FePt NP monolayer. Reproduced with permission. [ 76 ] Copyright 2011, American Chemical Society.
structures. The infl uence of an external magnetic fi eld on the
MNP assembly is further discussed in the next section.
3.2. Magnetic-Field-Assisted Assembly
The presence of an applied external magnetic fi eld allows
nanostructures to be grown in a specifi c directional manner,
either perpendicular or parallel to the substrate. [ 54 , 78 , 79 ] The
MNPs tend to align according to the direction of the applied
magnetic fi eld to minimize their magnetic energy, which is
strongly affected by the strength of the applied fi eld. [ 55 , 80 , 81 ]
The intensity of this force is directly proportional to a particle’s
magnetic moment and thus, to its magnetization properties.
On the other hand, magnetic moments feel a force under
magnetic fi eld gradients that tend to move the moment to
the region with higher magnetic fi eld density. So, in the pres-
ence of an external magnetic fi eld, the infl uence of the dipolar
Figure 9 . SEM images of patterns obtained by immersing a HOPG substrate into an ∼ 8 nm Co NP solution under an applied magnetic fi eld perpendicular to the substrate surface until complete evaporation of the solvent. The strength of the applied fi eld was: a) 0 T, b–d) 0.27 T, e) 0.45 T, and f) 0.78 T. The magnifi cation of the hexagonal network obtained at 0.27 T is viewed: c) from above, and d) tilted. Reproduced with permission. [ 81 ]
Figure 10 . Assembly of MNPs under the infl uence of an applied magnetic fi eld parallel to the surface. TEM images of linear patterns made of ∼ 8 nm Co NPs obtained under: a) 0.27 T, and b) 0.56 T. Reproduced with permission. [ 18 ] c) Optical microscope image of needle-like structures made of cubic-shaped Fe x O NPs (11–13 nm in size) in the presence of a weak parallel magnetic fi eld. Reproduced with permission. [ 39 ] Copyright 2004, American Chemical Society. d) TEM images of cigar-shaped γ -Fe 2 O 3 NPs (length and width of ∼ 325 and ∼ 49 nm, respectively) deposited in the absence (d), and presence (e), of an external parallel magnetic fi eld (0.14 T). Reproduced with permission. [ 83 ] Copyright 2002, IOP Publishing Ltd.
all the cases. Pileni et al. later demonstrated that
the size distribution of the MNPs is key to deter-
mine the morphology of the 3D superlattices
obtained under both the absence and presence of
an applied magnetic fi eld. [ 82 ] Small distributions
(≤13%) of ∼ 6 nm Co NPs assembled into well-
defi ned columns (3.05 μ m in height and 1.7 μ m
in width) with a fcc structure when applying a
magnetic fi eld (0.25 T) perpendicular to the sur-
face during the evaporation process. By contrast,
an increase of the size distribution from 13% to
18% induced the formation of longer (3.8 μ m)
and thinner (0.7 μ m) columns with lower com-
pactness, which tended to aggregate in the form
of worm-like and labyrinth structures.
Application of magnetic fi elds parallel to
the substrate leads to longer linear patterns. For
Co NPs ( ∼ 8 nm), an increase of the strength of
the in-plane applied fi eld from 0.27 to 0.56 T
also infl uenced the 3D assembly, increasing the
order and compactness of the stripe structures
( Figure 10 a,b). [ 18 , 81 ] A similar behavior was
observed for ∼ 10 nm γ -Fe 2 O 3 NPs, [ 18 ] which
evolve from needle-like structures to homoge-
neous tube-like structures when increasing the
strength of a parallel external magnetic fi eld. The
shape of the MNP also plays an important role in
the 3D ordering. For instance, cubic-shaped wüs-
tite Fe x O NPs (11–13 nm in size) align parallel to
the substrate in the direction of the applied mag-
netic fi eld, forming needle-like structures made of
Figure 11 . a) Schematic illustration of the indirect-patterning approach by means of μ CP. b) Optical image of a polystyrene-coated ∼ 10 nm Fe 3 O 4 NPs into ring-shaped structures. Reproduced with permission. [ 25 ] Copyright 2005, Elsevier. c) Schematic illustration of the direct-patterning approach by means of μ CP.
small 2012, DOI: 10.1002/smll.201101456
structures was also feasible by the fabrication of patterned
hydrophobic and hydrophilic SAMs using μ CP, followed by
the dewetting of a polystyrene-coated ∼ 10 nm Fe 3 O 4 NPs
solution on top of the pattern (Figure 11 b). [ 25 ] The patterned
surface was fi rst used to direct the condensation of water
droplets onto the hydrophilic areas after cooling below the
dew point. Then, a liquid layer of chloroform containing the
NPs was deposited on top of the water-patterned surface
by dip-coating, which after the complete evaporation of the
solvents resulted in an ordered pattern of magnetic rings of
approximately 10 μ m in diameter and 450 nm in height.
μ CP technique is also suitable for the fabrication of SAM
templates that induce the selective attachment of the MNPs
by means of ligand-place-exchange reactions, leaving highly
stable monolayers of MNPs. For example, Al 2 O 3 substrates
were patterned using aminobutylphosphonic acid (ABP)
SAMs, whose NH 2 -terminated groups are known to effectively
interact with FePt NPs by ligand-place-exchange reactions. [ 84 ]
After dip-coating the template into the colloidal solution,
SEM images revealed the selective formation of monolayers
of ∼ 10 nm FePt NPs on top of NH 2 -terminated micrometric
areas while leaving all the non-patterned areas uncovered.
Direct patterning . By combining both μ CP and click-
chemistry, Kinge et al. directly generated patterns of highly
packed ∼ 5 nm FePt NPs monolayers with an elastomeric
stamp that was directly immersed into a solution containing
the MNPs (Figure 11 c). [ 76 ] The stamp can also been func-
tionalized with a LB fi lm, technique known as patterned
Langmuir– Blodgett (pLB) ( Figure 12 a). [ 85–87 ] Following this
11www.small-journal.comH & Co. KGaA, Weinheim
Figure 12 . a) Schematic illustration of pLB procedure followed by additional thermal treatment. SEM images of the obtained FePt microstructures after thermal annealing: b) lines (1 μ m wide), c) squares (2 μ m wide), and d) meshes (10 μ m dot diameter). Reproduced with permission. [ 86 ]
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Figure 13 . Pattern fabrication by means of MIMIC. a) Schematic illustration of the procedure. Details of the last steps of the process when using: b) high, or c) low colloidal solution concentration. AFM image of patterns of ∼ 13 nm Fe 3 O 4 NPs monolayers showing the variations in the morphology depending on the solution volume inside the micrometer-channels, which are: d) full (100%), and e) ∼ 25% of the volume. Reproduced with permission. [ 89 ] Copyright 2008, IOP Publishing Ltd.
approach Yang et al. prepared LB fi lms on top of a stamp of
γ -Fe 2 O 3 NPs ( ∼ 13 nm) [ 85 ] or core–shell ∼ 4 nm Pt@Fe 2 O 3 NPs
patterns [ 86 ] by pLB technique that were later transferred
to silicon or other substrates (Figure 12 b). Uniform MNP
patterns with different shapes could be printed including
micrometer size lines, squares and meshes, with heights of
40 ± 3 nm corresponding to six MNPs (considering the length
of the coating layer and the NP diameter). These MNP pat-
terns were then converted into L1 0 phase FePt fi lms upon
thermal annealing, after which the original pattern design
was left unmodifi ed.
An important advantage of the pLB technique is that it
ensures a thickness control of the fi lm due to the formation
of single-layered MNP structures. [ 87 , 88 ] For instance, the for-
mation of patterned monolayers of ∼ 7 nm FePt and ∼ 25 nm
FePt@SiO 2 NPs organized in highly ordered hexagonal close-
packed assemblies was reported by Korgel and co-workers. [ 88 ]
For this, LB fi lms made of MNP monolayers were transferred
to elastomeric stamps patterned with dots, squares and lines
of 1.5 to 20 μ m in size and stamped onto a silicon substrate.
Moreover, the fabrication of strictly controlled multilayer
process for the controlled positioning of spherical-shape
∼ 18 nm core–shell FeO@CoFe 2 O 4 and cubic-shaped ∼ 50 nm
Fe 3 O 4 NPs ( Figure 14 ). [ 26 ] The Si surface was pre-patterned
by means of either EBL or via charged-particle nanopat-
terning (CHARPAN) to create round- or square-shaped
nano metric pits, respectively. The MNPs were then depos-
ited on the template by drop-casting and, after complete
evaporation of the solvent, organized along the surface in
a close-packed assembly. A mechanical polishing process
was subsequently applied in order to remove the MNPs
from the fl at parts of the substrate, leaving only those
MNPs already placed at the lithographically fabricated
motives. Interestingly, this method successfully controlled
the number of MNPs down to a single NP per unit pit by
accurately controlling the pit and the MNP size, as well as
the MNP concentration. Therefore, pit diameters of 90 nm
yielded a hexagonal arrangement of 7 closely packed NPs,
while for pit diameters of 25 nm, single NP occupation was
successfully obtained (Figure 14 b–d).
An alternative approach is the use of masks that guide the
self-assembly of the MNPs exclusively onto the bare areas of
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Magnetic Nanoparticles on Surfaces and Devices
Figure 14 . Patterns of ∼ 18 nm FeO/CoFe 2 O 4 core–shell and ∼ 50 nm Fe 3 O 4 NPs fabricated by means of the Damascene process. a) Large-area SEM image of an array with 110 nm pit diameter fi lled with NPs. b–d) SEM images showing the control over the number of MNPs per pit down to the single NP level, simply by adjusting the pit diameter from 90 to 25 nm. Reproduced with permission. [ 26 ]
the substrate, while the lift-off of the mask removes the rest of
the MNPs. The masks are prepared by spin-coating a polymer
on a surface and posterior pattern with the desired motive by
means of photolithography. [ 92 , 93 ] Using this strategy, 3–4 nm
FePt NPs were covalently anchored on a H-terminated sil-
icon surface and patterned into ordered arrays of 1 μ m 2 dots
with heights of 15–30 nm, which corresponded to 2–4 layers
of NPs (considering the surfactant coating of the MNP was
∼ 2 nm). [ 92 ]
3.4. AFM-Based Lithographies
AFM-based lithography techniques have attracted much
interest because of their exceptional capabilities for
ment of the tip by a conductive stamp permitted the struc-
turation of MNPs into parallel stripes or grids (width of 200
nm) applied along vast areas of 1 cm × 1 cm. The number of
embedded NPs depended on the MNP concentration of the
initial solution.
3.4.3. Dip-Pen Nanolithography (DPN)
The two techniques previously described follow the indirect-
patterning method, that relies fi rst on the fabrication of tem-
plates to later on assist the assembly of MNPs. In addition
to this, DPN is an AFM-based nanolithographic technique
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Figure 15 . Pattern of ∼ 9 nm γ -Fe 2 O 3 fabricated by means of bias-assisted AFM nanolithography. a) Schematic illustration of the procedure. b) AFM image of the magnetic pattern, whose height scale is 100 nm, and c) height profi le along the white dashed line in (b). Reproduced with permission. [ 95 ] Copyright 2007, American Chemical Society.
that can also allow direct-writing to directly deliver materials
(ink) to nanoscopic regions of a substrate simply by using the
tip as a nanoscale pen ( Figure 16 a). [ 99–101 ] The possibility to
directly write MNPs at specifi c positions of a substrate in a
MNPs on surfaces. [ 27 , 28 , 105 ] One possibility
is to fabricate templates/motives made of
SAMs bearing COOH-terminal groups
on a Au surface while the rest of the sur-
face is passivated with an unreactive layer
of CH 3 -terminated SAM. [ 27 ] For this, alkanethiol molecules
were fi rst coated and dried onto the tip and then brought
in contact with the Au substrate. The resulting template was
then dip-coated into a basic solution containing either Fe 3 O 4
, Weinheim small 2012, DOI: 10.1002/smll.201101456
Magnetic Nanoparticles on Surfaces and Devices
Figure 16 . a) Schematic illustration of the DPN process. Reproduced with permission. [ 102 ] Copyright 2004, Elsevier. b) AFM image of an array made of lines accommodating individual Fe@C NPs and, c) a higher magnifi cation of the same array. Reproduced with permission. [ 28 ] Copyright 2008, American Chemical Society.
or MnFe 2 O 4 NPs (both of ∼ 10 nm) coated with positively
charged tetramethylammonium ligands in order to favor the
electrostatic interaction with the negatively charged COO − -
pattern. Single-layer MNPs structures of dots and stripes—
with high control of the feature size (from few micrometers
to 45 nm)—and homogeneous arrays of 21 × 22 dots with a
fi xed dot diameter (85 ± 5 nm) were achieved. Further studies
with the DPN deposition method were directed towards
accommodating single MNPs per feature of a nanoarray. [ 28 ]
This was accomplished by using a solvent that selectively
wets the carboxylic pattern but not the methyl-passivated
regions. The effect of the template feature size on the MNP
assembly was studied with dots of sizes in the range of 50 to
120 nm, and it was found that feature sizes of 60 nm were
optimal to generate single Fe@C NP ( ∼ 5 nm) per feature,
which demonstrates the high control on the deposition at a
single particle level (Figure 16 b,c).
3.4.4. Thermal Dip-Pen Nanolithography (t-DPN)
Finally, t-DPN is a variation of the DPN technique that
makes use of a cantilever with integrated resistive heaters.
Initially the ink deposited on the tip is solid at room tem-
perature, but upon heating the melted ink fl ows from the
tip to the surface. [ 106 ] Depending on the scan speed and the
tip temperature, the deposition can be switched on and off
Figure 17 . a) Schematic illustration of the fabrication of MNP-based motives by means t-DPN. After deposition of the composite ink (made of ∼ 7 nm Fe 3 O 4 NPs and PMMA); the polymer can be removed by oxygen plasma treatment. AFM and SEM images of the fabricated pattern before (b), and after (c, d) the elimination of the polymer, resulting in single MNP lines. Reproduced with permission. [ 107 ] Copyright 2010, American Chemical Society.
Figure 18 . Schematic illustration of the structure of ferritin molecule. Reproduced with permission. [ 130 ] Copyright 2010, Elsevier.
Metallic and semiconductor NPs have also been synthesized,
such as Cu, [ 124 ] Pd, [ 125 ] Ni, [ 126 ] Co, [ 126 ] CoPt, [ 127 ] CdSe, [ 128 ] and
ZnSe. [ 129 ]
Interestingly, the external protein shell can be modi-
fied to guide the assembly of ferritin on a given substrate
while preserving all the inner characteristics. This prop-
erty allows the use of ferritin as a vehicle (template) for
guiding and positioning a large variety of internal inor-
ganic NPs to desired positions on a surface by using in
all the cases the same experimental conditions. Indeed,
since proteins are more easily decomposed than inor-
ganic materials, once structured on a surface the organic
protein shell can be selectively eliminated by means of
heat or UV/ozone treatment, while the inorganic mate-
rial preserves its original integrity and location on the
surface. [ 131 , 132 ] In view of these unique properties, ferritin
emerges as a fascinating scaffold for the nanostructu-
ration of MNPs on a substrate. Representative examples
for the structuration of these materials both assisted and
un-assisted are considered next. The representative exam-
ples have been divided following the same experimental
sections as those previously described without biological
templates.
4.1. Un-assisted Self-Assembly
To date, several strategies to organize ferritin mol-
ecules into well-ordered mono- and multi-layers,
including the transference of a ferritin fi lm from an air–
liquid interface onto a solid support [ 131 , 132 ] as well as the use
of scratching [ 133 ] and spread-coating [ 134 ] methods, have been
described. Yamashita et al. reported a method called Bio
Nano Process for the fabrication of 2D FeO MNP mono-
layers on substrates. [ 131 , 132 ] In this process, the ferritin mole-
cules fi rst self-assemble into well-ordered 2D hexagonal
close-packed arrays by means of a charged polypeptide
layer spread at an air–liquid interface. Then, the MNP fi lm
was transferred onto a hydrophobic Si surface (pre-treated
with 1,1,1,3,3,3-hexamethyldisilazane) by laying the sub-
strate on the air–liquid interface ( Figure 19 a). Finally, the
erlag GmbH & Co. KGaA, Weinheim
Figure 19 . High-resolution SEM images of a 2D array of ferritin on a Si substrate before (a) and after (b) heat treatment at 450 ° C under nitrogen fabricated by means of the Bio Nano Process method. White spots correspond to the iron oxide cores of ferritin molecules. b) A tilted high-resolution SEM image where secondary electron intensity has been transformed into a height profi le to form a pseudo 3D image, showing the achieved high-order of the 2D array of FeO NPs. Reproduced with permission. [ 132 ] Copyright 2001, Elsevier.
small 2012, DOI: 10.1002/smll.201101456
Magnetic Nanoparticles on Surfaces and Devices
Figure 20 . a) Schematic illustration of the fabrication of 2D arrays of ferritin molecules which were genetically modifi ed in order to selectively promote their adsorption onto the droplet liquid surface. The nucleation takes place in a controlled way along the contact line at the liquid surface. b) SEM image of the single-domain 2D array where the white spots represent the ferritin cores and the dark spots are considered to be apoferritin molecules, invisible in SEM. Inset corresponds to an FFT image of the SEM image confi rming the hexagonally close-packed arrangement of the ferritin molecules. Reproduced with permission. [ 140 ] Copyright 2008, American Chemical Society.
protein shell was selectively removed by subjecting the
sample to either heat treatment (450 ° C under nitrogen
for 1 h) or ozone treatment, while preserving the order of
the array; thus leaving inorganic cores with diameters less
than 6 nm (Figure 19 b). XPS analysis after heat treatment
indicated that the original core was transformed into FeO
mixed with a small proportion of Fe 2 N.
4.2. Assisted Self-Assembly and Organization
4.2.1. Chemically Assisted Self-Assembly
The interaction of the ferritin with the substrate can be tai-
lored through specifi c covalent or electrostatic bonds to
induce the formation of more stable ferritin networks. Sev-
eral strategies have been developed in this direction, such
as modifi cation of the substrate surface with SAMs bearing
metic layer-by-layer assembly (BioLBL), [ 149 ] and AFM-based
lithographies, [ 150 , 151 ] among others. The great majority of
the developed approaches are based on indirect-patterning
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Figure 21 . a) SEM image of Ti pattern on SiO 2 fabricated by means of NSL and EBL technique. Inset shows an AFM image of the pattern. b) SEM image after site-selective adsorption of ferritin (displaying Ti-binding peptides on their surface). The maximum number of adsorbed ferritins per Ti pattern was four. Three imaginary polystyrene particles used for the Ti patterning by NSL technique were drawn. Reproduced with permission. [ 153 ]
deposition techniques, which fi rst fabricate pre-modifi ed
chemical patterns as templates that guide the posterior site-
selective immobilization of the protein onto pre-defi ned areas.
Following this strategy, well-ordered ferritin arrays have been
successfully fabricated bearing few or even a single ferritin
molecule immobilized on each feature of the array.
A genetically modifi ed ferritin with target-specifi c affi nity
peptides was used to guide the immobilization of the modi-
fi ed protein selectively onto lithographically pre-patterned
areas (i.e., Au [ 152 ] or Ti [ 149 , 153 , 154 ] ). For example, Yamashita
et al. made use of a modifi ed ferritin displaying a peptide
with a high affi nity for Ti, named TBP-1, which selectively
adsorbs onto a hexagonal pattern made of triangular 40 nm
Ti dots. [ 153 , 154 ] The Ti template was fabricated on a SiO 2 sur-
face using NSL and EBL techniques, using a 2D hexa gonally
close-packed array of poly styrene (150 nm) particles as a
mask for the deposition of the Ti fi lm ( Figure 21 a). This
was then immersed in a ferritin solution for a given time
at room temperature followed by intensive washing and
drying (Figure 21 b). After ferritin immobilization, the pro-
tein shell could be selectively removed by UV/ozone treat-
ment (at 110 ° C for 40 min) while preserving the hexagonal
periodic array, leading to well-ordered NP arrays on the Ti
pattern. The specifi c binding of the modifi ed protein was not
affected by the inner NPs, and therefore, the procedure could
be reproduced for ferritin displaying two different NPs syn-
thesized inside its cavity, ferrihydrite and Co 3 O 4 cores. More
recently, Sano et al. made use of the peptide (TBP-1) to
genetically modify the ferritin molecules and fabricate mul-
tilayered structures selectively adsorbed on nanopatterned Ti
stripes fabricated by EBL. [ 149 ] This method, called BioLBL,
employed the target-specifi c affi nity peptide not only as
a binding molecule to immobilize the ferritins but also as
a mediator for the biomineralization of a thin fi lm of the
target material (i.e. TiO 2 ) from a precursor reagent present
in solution. Moreover, the TiO 2 fi lm could be used in turn
as a target for the adsorption of a second ferritin layer. By
repeating the process, multilayered structures with different
NP-containing ferritins (fi lled with Fe and Co oxides) depos-
ited between TiO 2 layers were selectively built on top of the
original strip pattern and characterized by means of SEM and
energy dispersive spectro scopy (EDS) mapping.
In further studies, Yamashita and co-workers used elec-
trostatic interactions as a driving force to make ferritins
selectively adsorb individually onto specifi c regions of a lith-
ographically pre-patterned substrate. [ 155 , 156 ] They fabricated
a pattern made of circular APTES disks with two different
sizes ( ∼ 15 and ∼ 45 nm) on a SiO 2 substrate by means of EBL,
vapor-phase APTES deposition and lift-off processes. Ferritin
was fi rst genetically modifi ed to increase its surface charge
density and, therefore, enhance the electrostatic interaction
with APTES disks while avoiding secondary ferritin adsorp-
tion ( Figure 22 a). The patterned substrate was then incubated
in the ferritin solution for a given time at room temperature
and studied by means of SEM. The suitable tailoring of the
electrostatic interactions (supported by numerical calcula-
tion) allowed the placement of a single ferritin molecule on
top of each APTES disk of the array, whose disk diameter
(up to 45 nm) was much larger than the size of a ferritin
( ∼ 12 nm) (Figure 22 b). Additionally, the obtained single fer-
ritin array on 15 nm APTES disks was subjected to rapid
thermal annealing (RTA) under O 2 gas at 500 ° C for 10 min
in order to selectively eliminate not only the protein shell but
also the APTES pattern; thus leaving independent iron oxide
NPs located on the designated positions of the array. Interest-
ingly, the obtained periodic single NP array has recently been
used as a catalyst for the position-controlled vertical growth
of individual carbon nanotubes (CNTs). [ 157 ]
4.2.3. AFM-Assisted Self-Assembly
Most of the examples described up to know are based on the
indirect writing of ferritins on tip-prefunctionalized areas.
LON [ 94 ] has been used for the fabrication of oxide dots and
stripes of only few nanometers that can serve as templates
for the selective attachment of ferritin molecules. [ 30 , 158 , 159 ]
This method applies a suitable bias voltage to a conduc-
tive tip, which makes the water meniscus formed in the tip-
substrate gap dissociate and the oxidative ions to react with
the substrate, leaving localized oxide nanostructures with a
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Figure 22 . a) Schematic illustration of the adsorption of a single ferritin molecule onto an APTES disk, where the repulsive fi elds between the substrate surface and the fi rst adsorbed molecule avoids the approach of a second ferritin. b) SEM image of an APTES array showing the adsorption of single ferritin molecules on each disk. Reproduced with permission. [ 156 ] Copyright 2009, American Institute of Physics.
resolution down to 10 nm. Yoshinobu et al., explored two
different protein patterning routes based on the LON tech-
nique. [ 158 ] In the case of the negative patterning, a Si substrate
was modifi ed with positively charged APTES molecules
ready to electrostatically interact with the negatively charged
ferritins. Then, oxide patterns were formed on this surface by
LON and subsequently the ferritins were selectively immo-
bilized outside the oxide patterns. In the case of the positive
patterning, the Si substrate was fi rst protected with OTS to
introduce unreactive long alkyl groups on the surface. Then,
LON was applied to fabricate oxide patterns which in turn
were modifi ed with APTES to promote the immobilization
of the ferritin molecules. Interestingly, the use of electrostatic
interactions presents additional advantages such as control-
ling the attachment/removal of this protein on the surface by
changing the pH of the solution. [ 30 , 159 ] The deposition at the
single particle level reported by Coronado and co-workers
was possible due to the high spatial resolution of LON, which
allows accurate matching of the size of the oxide pattern to
the protein size ( ∼ 12 nm). [ 30 ] Thus, individual ferritin mol-
ecules could be positioned into chain-like structures over
sub-micrometer areas ( Figure 23 a). By means of print-based
methods such as lithography-controlled dewetting instead
of LON, this approach could be successfully extended to
pattern ferritin particles over macroscopic regions (1 cm 2 )
(Figure 23 b).
Recently, our group has reported the deposition of fer-
ritin-based NPs using DPN technique via the direct-pat-
terning approach, which avoids previous functionalization
of either the substrate or the molecule, and does not require
the application of external stimuli. [ 160 ] Ferritin nanostruc-
tures were fabricated by coating a tip with an aqueous fer-
ritin solution (ink). Afterwards the tip was brought in contact
with the substrate surface to dispensie femtoliter volumes of
the protein solution with control of the shape and size, mini-
aturizing the dot features down to 100 nm. Moreover a wide
variety of substrates of technological interest such as Au, Si/
SiO 2 , Nb, and Al have been successfully used ( Figure 24 a).
More interesting is the fact that ferritin molecules deposited
on each dot were organized as sub-monolayers. After ferritin
deposition, the ferritin patterns were subjected to heat treat-
ment (450 ° C for 1 h in an inert atmosphere) to selectively
remove the organic protein shell. SEM characterization of the
heated array revealed that the order, dimensions, and spacing
of the DPN-generated dot-like nanostructures, as well as the
NP location, were maintained after heating. The presence of
iron oxide NPs on these dot-like features and the complete
elimination of the protein portion were confi rmed by time-of-
fl ight secondary-ion mass spectrometry (TOF-SIMS) analysis.
In addition, the direct-write capabilities of DPN provides the
potential not only to fabricate miniaturized arrays but also to
have fi ne control of the number of ferritins deposited at spe-
cifi c locations on a surface. We have demonstrated by TEM
analyses that this could be afforded by calculating the contact
angle between the ink ferritin solution and the substrate, and
controlling the initial protein concentration and the dot-like
feature diameter (Figure 24 b). [ 29 ]
5. Integration into Hybrid Devices
Up to now, we have reviewed the structuration of MNPs on
surfaces from 3D down to 1D organization. A new aspect of
the structuration fi eld must be addressed when site-selectivity
with control at the nano scale level is required. This is the case
for the integration of single or a few nanoparticles into nano-
devices whose objectives rely on either, using the MNPs as a
subject of study to understand the magnetization relaxation
at the individual NP level, like in nanoHall sensors or nano-
SQUIDs, or by using the single MNPs as analysers, polarizers
or fi lters, as for example in spin valve nanodevices. In this
section we will review the different approaches followed to
integrate MNPs on different sensors and briefl y overview the
magnetic and transport measurements that can be derived
from them.
5.1. Randomized Integration into Devices
The simplest method initially used to integrate MNPs into
nanoscale devices consists of the random deposition of the
sample by using some of the deposition techniques previ-
ously described in section 2.1. Representative examples are
drop-casting, by placing a drop of the solution with dispersed
19www.small-journal.com & Co. KGaA, Weinheim
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Figure 23 . a) Schematic illustration of the ferritin patterning by combining LON and surface functionalization with OTS and APTES. Local oxide pattern made of parallel lines with widths of 10–15 nm before (b) and after (c) adsorption of ferritin molecules into single molecule chains. d,e) Parallel patterning of ferritin molecules by means of lithography-controlled dewetting. Insets correspond to phase images indicating the lines widths. Reproduced with permission. [ 30 ]
target particles on a chip containing hundreds of devices, [ 161 ]
or by spray pyrolisis of nanodroplets of a solution containing
the sample nanoparticles. [ 162 ] After drying, the particles are
physisorbed onto the chip, but only the ones falling in the
sensitive zones of the sensors such as the nanobridge of the
SQUID loop, have enough fl ux coupling with the sensor to
be detected. Those are selected for the magnetic measure-
ments. In all these cases, the position and morphology of the
particles is determined by SEM only after magnetization
measurements are carried out. Typically, parameters such
as concentration of MNPs in the solution or evaporation
time are adjusted to ensure a dilute submonolayer distribu-
tion of MNPs on the sensors array, so that MNPs are more
or less isolated on the surface and only one or few of them
can statistically reach the target position on the nano devices.
The advantages of random integration are mainly the
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Figure 24 . a) 3D AFM image of a ferritin array generated by direct-write DPN on Au substrate with spacing between dot-like features of 500 nm and dot size of 100 nm. Reproduced with permission. [ 160 ] b) TEM images of a ferritin array fabricated on a carbon-coated TEM grid. The number of ferritin molecules deposited on each dot of the array could be clearly determined and studied due to the high contrast of the inorganic core of the protein. Reproduced with permission. [ 29 ]
straightforward application of 2D structuration techniques
already known without any modifi cations. The structura-
tion step is simple and many nanodevices can be function-
alized in a single experiment. The major drawback of this
methodology relies on the implicit poor control for the
exact particle position that hinders the experimental work
for the nanodevice operation characterization. Tens or even
hundreds of experiments must be done before an optimal
nanodevice with one or very few MNPs placed in the right
position is found. While this fact favors the obtaining of sta-
tistics over the operational properties, it can also be very
expensive and time-consuming.
The development of magnetic sensor devices such as
SQUIDs has been closely linked with the aim of measuring
the magnetization reversal of an individual magnetic par-
ticle. [ 163 ] For this, the SQUIDs not only have to increase their
sensitivity up to a single particle level, diminishing its char-
acteristic size down to few nanometers, but also MNPs must
be integrated on the areas of the device that show highest
sensitivity. To this end, Wernsdorfer and co-workers used a
randomized structuration methodology for the successful
Figure 25 . a) TEM image of (iron)–(iron oxide) core–shell nanocubes ( ∼ 18 nm in size) attached to a single multiwall carbon nanotube (MWCNT). b) SEM image of an Au electrode array. c) SEM image of a single MWCNT decorated with (iron)-(iron oxide) nanocubes (light spots) and connected to two Au electrodes through wet deposition method. Reproduced by permission. [ 168 ] Copyright 2009, IOP Publishing Ltd.
Figure 26 . a) SEM image of the SQUID sensor showing the four rectangular shaped pick-up coils. b) Schematic illustration of the ferritin nanoparticle deposition process by DPN on the most active areas of the SQUID sensor. c,d) SEM images of a sensor directly after generating ferritin nanostructures on the desired positions of the sensor. Some of the obtained nanostructures are highlighted in image (d) with red circles. Reproduced with permission. [ 174 ] Copyright 2011, American Institute of Physics.
cles, with a magnetic moment of μ < 300 μ B , into a
nanoSQUID (a step further on the miniaturization)
and detected its magnetization reversal. [ 175 ] In this
case, to optimize the fl ux coupling, a small number
of ferritin nanoparticles were directly attached over
the nanojunction with a method combining mono-
layer self-assembly with EBL ( Figure 27 ). [ 176 ] For
this, PMMA is deposited onto the Nb nanoSQUID
that has been previously covered with gold. After-
wards a 200 nm × 200 nm window in the PMMA
resistor is opened directly over the nanojunction by
EBL. Finally, organic linker molecules are deposited
directly over the exposed part of the gold and used
to attach the ferritin magnetic molecules to the Nd
nanoSQUID device. Even this approximation has
high accuracy, it shows two main disadvantatges:
i) the presence of the linker between the magnetic
material and the sensor surface diminishes their
effective coupling, and ii) surface functionalization
can modify the sensor characteristics.
Apart from these two previous methodologies, a
range of other techniques based on the use of tips can
also be applied. [ 173 , 177–179 ] Pakes et al. demonstrated
great control of the positioning of ferritin by mechan-
ical pushing of a single particle with the tip of an
AFM. [ 173 ] Garno et al. reviewed other methods based
on nanoshaving processes of SAM to deposit Au
nano particles onto the exposed areas. [ 178 ] Hao et al.
used a SEM together with a sharpened probe (carbon
fi bre) to lift and position a FePt nanobead directly
onto a nanoSQUID loop in a reversible way. [ 179 ]
Finally, one of the main candidates of interest
for the future in the fi eld of magnetic sensors is
graphene. It has shown itself to be a promising
material for use in different types of improved
devices based on its fascinating structural, elec-
trical and magnetic features, namely high crystal-
line order, mass-less Dirac fermion-like charge
carriers, or long spin coherence length. [ 180–182 ] It
& Co. KGaA, Weinheim small 2012, DOI: 10.1002/smll.201101456
Magnetic Nanoparticles on Surfaces and Devices
Figure 27 . a) Schematic diagram of the covalent attachment of a ferritin particle to the Au surface through the use of organic ligands [EDC refers to 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and NHS to N -hydroxysuccinimide]. b) AFM image of a 700 nm × 700 nm area of the SQUID sensor. The positions of the ferritin entities are indicated with arrows. The square on the image indicates the approximate location of the PMMA window used for attachment. Reproduced by permission. [ 176 ] Copyright 2008, IOP Publishing Ltd.
has recently been shown that Hall sensors made on graphene
are very promising for nanoscale detection of small mag-
netic signals. [ 183 ] The intrinsic high mobility of graphene
results in a high magnetic fi eld sensitivity while the structure
of graphene also provides an atomically thick sensor layer
located right at the surface that reduces the loss of magnetic
fl ux throughout the sensor thickness; therefore increasing the
resolution and the sensitivity. [ 184 ] However, a delicate step in
the integration on this device is the proper manipulation and
the controlled deposition of a magnetic material while pre-
venting damage of the graphene fl akes. Moreover, nanoHall
gradiometry technique for example, requires that the mate-
rial is positioned exclusively over one of the cross-sections of
the device, that can be of the order of a few square nano-
meters, while the other is kept empty. This situation obviously
requires the use of an accurate and nanometer site-selective
structuration technique. In this case, direct-write AFM nano-
structuration technique has been shown to be appropriate
since the probe tip can be used to both visualize the sensor
with nanometric resolution without damaging the surface to
locate the nanometric target region, and afterwards directly
deposit nanomaterials on it with site-specifi city, using the tip
to deliver controlled quantities of an ink by capillary trans-
port. Domingo and co-workers have successfully used this
selective and direct integration method to deposit some
tens of Co MNPs of ∼ 8 nm diameter on the 1 μ m 2 cross-
section of a graphene-based nanoHall sensor with accurate
resolution and without damage of the graphene fl ake (see
Figure 28 ). [ 185 ]
6. Summary and Perspectives
A detailed consideration of the main scientifi c approaches
developed over the last years for the deposition and structur-
ation of magnetic nanoparticles on surfaces has been given in
this review. For this, the manuscript has been divided into four
main areas we consider crucial and which, therefore, require
special attention: (i) un-assisted self-assembly, (ii) assisted
self-assembly and organization, (iii) use of biomolecular tem-
plates, and (iv) integration into hybrid devices.
As expected, the fi rst two sections involving the devel-
opment of surface structuration techniques is where sci-
entists started to develop their work, and consequently,
where most examples have been described. Such numerous
examples clearly reveal the large number of experimental
techniques used for: i) the structuration of MNPs on sur-
faces that range from a few nanometers up to several mil-
limeters, as well as ii) controling the number of MNPs
deposited from complete layers down to the deposition
of a single nanoparticle. A high degree of control can be
achieved simply by considering structural parameters such
as shape, size, and morphology of the nanoparticles. Special
attention has also been given to the different non-covalent
interactions that may infl uence the self-assembly proc-
esses, where dipolar interactions play an important role.
Whereas van der Waals interactions and steric forces can
be modulated by using different type of surfactants, dipolar
magnetic interactions can be modifi ed by changing the
size of the magnetic NPs since the magnetic dipolar inter-
actions increase quadratically with the particle magnetic
moment, which is also proportional to its volume. In this
sense, dipole–dipole interactions become the most effective
supramolecular interaction for large enough particles with
high magnetic moments.
The deposition methods, from the most simple drop-
casting or spin-coating techniques, to lithographically assisted
methodologies, have also been shown to have a crucial infl u-
ence. For instance, if the deposition is assisted by lithographic,
AFM, or similar techniques, structured arrays of diverse
motives can be achieved with control of the number, as well
as the morphology, size, and pitch of the dots. Moreover,
improvement of the resulting arrays is achieved by chemical
functionalization of either the nanoparticle and/or the sur-
face, allowing in most cases excellent control of the formation
of a (sub)monolayer. To improve the effectiveness of the
techniques previously described it is possible to use bio-
logical templates such as viruses or ferritins. The advantages
are twofold. First, once the conditions for the structuration
of the organic capsule are established, these can be used for
the structuration of a broad range of inorganic nanoparticles
23www.small-journal.comH & Co. KGaA, Weinheim
E. Bellido et al.reviews
Figure 28 . a) 3D AFM image of a graphene-based nanoHall sensor with cross-sections of 1 μ m × 1 μ m in size. b) Hall cross indicated in image (a) was functionalized with Co NPs ( ∼ 8 nm) by means of AFM-based nanolithography. c) Height profi le along the white dashed line in image (b). [ 185 ]
that can be held inside, enormously simplifying the process.
Second, the organic capsule can be modifi ed by means of a
biochemical approach to ensure and/or improve the resulting
arrays.
In spite of the large number of techniques already
described, new examples of techniques appear daily, trying to
fulfi l the high demands to build magnetic nanoscale devices.
Although still necessary, it would be of rather higher benefi t
to the scientifi c community to focus the development of these
techniques towards integrating the nanoparticles on devices
rather than simply describing new structuration procedures.
From the different techniques reported, those that have been
shown to have more potential are self-assembly or dip-pen
nanotechnology (DPN). Remarkable advances have been
obtained especially with DPN, which has allowed the inte-
gration of the MNPs on the most sensitive areas of 1 μ m ×
1 μ m graphene-based sensors. These results are very impor-
tant since integration techniques must, in the future, fulfi ll the
requirements presented by the continuous miniaturization
of devices, such as ensuring high precision of the deposition
process and careful manipulation, in order to minimize the
effect over the sensor at any time. Unfortunately, there is
a long way to go before this technique can be applied, due
to the main limitations of reproducibility and scalability.
Improving both limitations will require full control over the
self-assembly processes of the molecules, which is not an easy
task. A good solution to overcome these limitations could be
the combination of self-assembly with AFM-assisted lithog-
raphies in solution. While the integration of the molecular
materials on the sensors and devices could represent a real
challenge, the maintenance of the magnetic properties upon
integration is a must. Perturbations arising from surface
interactions or other chemical modifi cations (i.e., redox proc-
esses, chemical decomposition, etc.) that alter the magnetic
properties must also be strongly taken into consideration.
These factors, which have been shown to be crucial for softer
materials such as molecular magnetic materials, are at the
present moment not always considered.
Acknowledgements
We thank the Spanish government for the project MAT2009-13977-C03-03. N.D and E.B. thank the Spanish government for a Ramon y Cajal and an F.P.I. grant, respectively.
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Received: July 18, 2011 Revised: November 7, 2011Published online:
