Chemistry Reference
In-Depth Information
phospholipids to solubilise FeCo nanoparticles coated with a layer of graphite [69]. The hydrophobic fatty acid chains of the
phospholipids interact with the carbon layer on the nanoparticles while the polar charged head provides solubility in aqueous
media and can be modified, for example, with PeG molecules to reduce nonspecific interactions. In a different example,
sitharaman and co-workers used surfactant molecules to solubilise gadolinium loaded carbon nanotubes (CNTs) [102].
either sodium dodecylsulfate (sDs) or the more biocompatible surfactant pluronic F98 is used to keep their CNTs in
aqueous media for their measurements.
9.5.3
Inorganic coatings
The coating of prepared nanoparticles with a shell of a different inorganic material can also facilitate water solubilisation.
This method presents several advantages: First, the coating material is chosen to facilitate the chemical reactions needed
for the functionalisation of the particles, but at the same time it can protect the active core from oxidation or degradation
(further oxidation in the case of metal oxide nanoparticles). If the shell is porous, drugs can be encapsulated to facilitate
the nanoparticles' use as drug delivery agents as well as imaging probes. The addition of an outer layer of inorganic
material increases the total mass of the nanoparticles with redundant (from an imaging standpoint) material, but the overall
size increase facilitates the clearance of the particles
in vivo
by the reticulo-endothelial system.
Currently, there are three main options for the inorganic coating of MRI nanoprobes: gold, silica, or carbon. When
considering
in vivo
applications, silica is by far the most commonly used of the three. Although gold presents unbeatable
properties of stability and chemical versatility, its molecular weight is very high, so it dramatically increases the average
molecular weight of the nanoparticles. In contrast, carbon layers add a negligible amount of mass to the final nanoparti-
cles but feature complex preparation protocols.
(i) siO
2
shells: The coating of magnetic nanoparticles with silica shells is a popular option for a number of reasons:
silica is chemically inert and exceptionally stable, (especially in aqueous media), there are several well-established
and simple protocols for its preparation, and its porosity can be controlled [3]. From the range of synthetic method-
ologies available, the so-called stöber method [103] and the microemulsion approach [104, 105] are the most
popular ones. Both methods are based on the polymerisation of tetraethyl orthosilicate (TeOs) under basic condi-
tions. While the stöber approach uses polar solvents (alcohols or water-based solutions), microemulsion involves a
water-in-oil reverse emulsion. Thus, in general the second method is more useful because the starting nanoparticles
are usually not soluble in polar solvents. The addition of a silica shell generally comprises two steps: the polymeri-
sation of TeOs, followed by addition of an outer thin shell of a different silane bearing a functional group for further
functionalisation. The most common example of these silanes is 3-aminopropyl triethoxysilane (APTes) that, after
application, renders nanoparticles with terminal amine groups. There has been an increasing number of examples
reported in recent years, not only with iron oxide nanoparticles [70-72, 106-108], (Figure 9.9a) but also with other
materials, such as gadolinium oxide [49, 109].
(ii) Au shells: The application of gold-coated nanoparticles in the biomedical field is limited because of the high atomic
weight of gold atoms. Apart from that, the properties of gold make it ideal for
in vivo
applications. It is chemically
inert, extremely stable, and most importantly, easy to functionalise. Thiol-gold chemistry has been extensively studied
for the functionalisation of gold surfaces through self-assembled monolayers (sAMs) and can be applied in this case.
several synthetic protocols are available for the nanoparticle coating with gold. The thermal decomposition of gold
acetate over iron oxide nanoparticles immediately after their preparation and without further purification [110] is a
convenient approach that has been further modified to also be successful with manganese and cobalt ferrites nanopar-
ticles [111]. The thickness of the gold coating can be tuned by controlling the amount of the starting gold precursor.
A different strategy is the one described by Xu and co-workers [112] in which they coat Fe
3
O
4
nanoparticles with a
gold shell in chloroform at room temperature using hydrogen tetrachloroaurate hydrate (HAuCl
4
) as the gold source
and oleylamine as a mild reducing agent as well as a surfactant. In contrast, Lyon et al. [113] brought their magnetic
nanoparticles to water using citric acid and then coated them with a gold shell by the reduction of Au
3+
ions via itera-
tive hydroxylamine seeding. A more complex strategy was followed by Bouchard and co-workers to prepare what
they called gold-coated cobalt nanowontons (Figure 9.9b) [114]. Over silicon pillars they deposited sequentially four
metallic layers: 5 nm chromium, 10 nm gold, 10 nm cobalt, and finally 10 nm gold again. The silicon and the chro-
mium layers were etched away by immersion in 10% KOH solution at 80°C for 10 min. The result was a batch of
gold-coated nanoparticles with a size of around 60 nm featuring promising T
2
contrast properties.
(iii) Carbon shells: The addition of a carbon layer around the nanoparticle core is the less well explored option to date
due to the complexity of its preparation. However, graphitic carbon coating presents some outstanding properties,
such as increased chemical stability against acids and bases and reduced mass. so far, there is only one report on its
