Self-Assembly of Synthetic and Biological Polymeric Systems of ...

Also, it is necessary to note that fibril integrity and stability
was perfectly maintained under these reaction conditions
(see Figure S4 in the Supporting Information).
With more addition steps carried out the fibrils became
progressively covered with more magnetic material without
any apparent increase in nanoparticle size. The continuous
fibril coverage is a result of the fusion of adjacent as-synthesised
magnetic nanoparticles (see Figure S5 in the Supporting
Information). When a progressive and sufficient
excess of Fe was added, the nanowire widths increased and
their surfaces also appeared rougher, which might indicate
that more than one layer of magnetic material had been deposited
onto the template (Figure 2 a–b). In this regard, the
Figure 2. TEM images showing the progressive increases in the widths of
the magnetic wires and subsequent bundling after a) three, b) four or
c) five sequential additions of Fe ions at MR = 100. d) After 5 sequential
additions at MR = 100 in the presence of sodium citrate as stabiliser.
widths of the magnetic wires could be modified from 6 to
50 nm by increasing the number of sequential additions of
Fe ions onto the biotemplate. These values depend on the
type of fibril used as the template, the [Fe]/ACHTUNGTRENUNG[protein] molar
ratio and the number of sequential additions performed (see
Figure 1 and Figure S2 in the Supporting Information). Nevertheless,
constant width along the whole nanofibrils could
not be achieved, due to the non-uniform deposition of the
ions onto the template. We are currently modifying the synthetic
protocol with the goals both of improving uniformity
in fibril coverage and of achieving perfect control over the
magnetic nanowire dimensions and their resulting magnetic
properties. In addition, under conditions of excessive Fe
ions in solution an increasing presence of fibril networks as
a consequence of fibril bundling was observed, leading to
precipitation of the nanowires after several days (see Figure
2 c). This can be a result either of magnetic interaction
between nanowires or of inefficient stabilisation of the 1D
7368
P. Taboada et al.
nanostructures. To prevent these phenomena, we decided to
stabilise the magnetic nanowires further by the addition of
sodium citrate, which has been shown to be an effective stabiliser
for water-soluble magnetic nanoparticles. [22] In this
process, no fibril precipitation was observed for at least one
month and a lower level of bundling was confirmed, probably
as a consequence of the generation of a surface organic
layer that moderately screens the magnetic interactions between
magnetic covered fibrils (Figure 2 d).
On the other hand, the morphologies and sizes of the
fully covered fibrils made of HSA and Lys present some differences
irrespective of the method used to create the magnetic
nanowires. This is a consequence of the inherent structural
differences between the fibrils assembled from the two
proteins. In this respect, HSA-derived nanowires have shorter
lengths (between 0.1 and 2 mm) and widths (4–10 nm),
and have a curly morphology as a direct consequence of the
template structure (see Figure S6 in the Supporting Information).
[16a,b] In contrast, Lys-derived wires appear to be
straighter and longer (lengths and widths of 0.4–8 mm and
12–20 nm, respectively). [16c,d] This observation additionally
confirms that the overall structures of the fibril templates
remain intact after surface modification.
XRD analysis, selected area electron diffraction (SAED)
patterns, high-resolution TEM (HR-TEM) images and
FTIR spectroscopic examination of the fully covered fibrils
confirmed the formation and nature of the magnetic coverage
on the template surface. The XRD pattern showed reflections
indexed to (111), (220), (311), (222), (400), (422),
(511), (440) and (533) planes, corresponding to the cubic
spinel crystal structure of iron oxides (see Figure 3 a, JPCDS
file No. 19-0629, magnetite, or JCPDS file 39-1346, maghemite).
In addition, XRD demonstrated a polycrystalline
structure with cell constant a 0 = 0.8381 nm, which is in good
agreement with the standard card of iron oxides. The peak
broadening of the XRD pattern also indicates that the resulting
iron oxide crystallites were rather small. The crystallite
sizes calculated from the modified Scherrer relation [23]
for the magnetic fibrils of both Lys and HSA were about
(6.2 2) nm, consistent with the size distributions measured
from TEM images. SAED, HR-TEM and FTIR confirm the
predominance of the magnetite phase on the biotemplate.
The SAED pattern consisted of spots on the rings corresponding
to the diffraction planes (111), (220), (311), (400),
(511) and (440) characteristic of the magnetite phase, which
also revealed the polycrystalline nature of the nanocomposites
(see Figure 3 b). The difference between the polycrystalline
electron diffraction patterns of magnetite and maghemite
phases is solved by the presence of the (111) diffraction
plane, distinctive of the magnetite fcc structure. [24] From the
HR-TEM images (Figure 3 c) we can observe that the obtained
magnetite nanoparticles are single crystalline, as indicated
by the well-resolved lattice fringes. The distance between
two adjacent planes is about 0.255 nm, which corresponds
to the (311) lattice plane in the spinel structure of
Fe 3O 4. [25] The strong peak at 580 cm 1 in the FTIR plot (Figure
3 d) additionally confirms that the phase is magnetite
www.chemeurj.org 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2011, 17, 7366 – 7373