EVALUATION OF THE ROLE PLAYED BY A SPACER IN THE FUNCTIONALIZATION OF MAGHEMITE (-Fe2O3) NANOPARTICLES WITH A NITRO-DENDRON

J. PAEZ; M. ANGELLELI; P. FROIMOWICZ; K. LANDFESTER; V. BRUNETTI; M. STRUMIA

Simposio; XII Simposio Latinoamericano de Polímeros y el X Congreso Iberoamericano de Polímeros; 2010

Maghemite (g-Fe₂O₃) magnetic nanoparticles (MNPs) having appropriate surface chemistry have been employed for many biomedical (1) and non-biomedical applications (i.e. magnetic sensors (2), heavy-metal remediation (3), and catalysis (4)). Some advantages of these MNPs are: good chemical stability, biocompatibility, strong superparamagnetic behavior, and they have hydroxyl groups at their surface that can be conveniently modified to improve desired properties (5,6). It has been proven that nanoparticles can be chemically modified using dendrons (7). We have recently reported the modification of gold and carbon surfaces using a dendron bearing nitro as end-groups (DNO₂) (8,9) and the generation of conductive surfaces with electroactive chemical functions by self-assembly. These results encouraged us to link together the reported functionalization strategies and the aforementioned advantages of maghemite (g-Fe₂O₃) MNPs to explore novel platform sensors (10). Herein, we set up the construction of tailored hybrid nanostructures of nitro dendronized-MNPs and their use to modify carbon surfaces.

Firstly, MNPs were dendronized via covalent binding of DNO₂. Two different synthetic routes, involving the presence and the absence of 3-aminopropyl triethoxysilane (APS) as spacer were followed (Fig. 1). Thus, two dendronized-MNPs were obtained: with and without APS generating MNPs-APS-DNO₂ and MNPs-DNO₂, respectively (route 1 and 2 in Fig. 1). These two systems helped us to gain a deeper insight about the influence of using APS (as spacer) on the properties of the dendronized-MNPs.

Fig. 1. Two different routes followed to synthesize nitro-containing dendronized-MNPs.

Modified-MNPs were then characterized. Diffuse reflectance IR spectroscopy (DR-IR) studies showed the success of the functionalization trough the characteristic absorption bands of the incorporated organic molecules in each synthetic step. TGA measurements and TEM micrographs were in agreement confirming the DR-IR results. TGA also permitted to calculate the grafting density of the resulting materials, where route 2 (without APS) was more efficient than route 1 since a higher incorporation of dendron onto MNPs was achieved. Moreover, an increased dispersability and stability in organic solvents (DMSO, acetonitrile, DMAc) was observed after the derivatization in both cases.

Secondly, the dendronized-MNPs were used to modify glassy carbon electrodes (GCE) via self-assembly. Only dendron-decorated MNPs self-assembled onto GCE, suggesting that DNO₂ acts as a “linker” between the dendronized-MNPs and GCE. The immobilization of dendronized-MNPs onto GCE was evaluated by cyclic voltammetry (CV), following the electrochemical signal of aryl-nitro as reporting group.

Lastly, we evaluated the role played by the spacer taking into account the ratio between the incorporated DNO₂ onto MNPs (MNPs-APS-DNO₂ and MNPs-DNO₂) and the electrochemical response of GCE/dendronized-MNPs. Preliminary results show that a higher charge density related to nitro groups is observed for MNPs-APS-DNO₂ even when they incorporated less DNO₂ than MNPs-DNO₂. One reason for this could be that APS provides a short flexible segment between the dendron and the MNP improving the self-assembly onto the carbon surfaces. Probably this is due to an enhanced dispersability in the organic solvent or because the higher mobility the dendrons on the modified-MNPs have, the better the immobilization onto carbon surfaces is.

In conclusion, we prepared tailored dendronized-MNPs and used them to modify carbon surfaces. By using a straightforward and rapid method, we immobilize magnetic material onto carbon electrodes and evaluated their electrochemical behavior. These functionalized carbon surfaces are electrochemically active and expected to be applicable to biosensor platform development.   

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