One of the emerging goals of nanotechnology is to functionalize inert and biocompatible materials to impart precise biological functions. Several novel materials including quantum dots, polymers, and magnetic or magnetofluorescent nanoparticles have been described for diagnostic or therapeutic use. Significant research efforts have been made to tailor the nanoparticle surfaces and in order to modulate pharmacokinetic properties, toxicity, immunogenicity and efficient targeting. Targeting has generally been achieved by conjugating nanoparticle surfaces to antibodies. Although this approach was quite successful for in vitro sensing, its in vivo application has proved more challenging because the administration of therapeutic drugs that selectively reach the desired targets with marginal or no collateral damage. Most striking is the recognition that only a small fraction of intravenously administered monoclonal antibodies reach their parenchymal targets in vivo.
There are several synergistic goals that may be pursued to increase the efficacy per dose of any diagnostic or therapeutic formulation: (i) to increase its targeting selectivity, and (ii) to endow the agent(s) comprising the therapeutic formulation with the means to overcome the biological barriers that prevent it from reaching its target.
Nanomaterials are attractive probe candidates because of their (1) small size (1-50 nm) and correspondingly large surface-to-volume ratio, (2) chemically tailorable physical properties which directly relate to size, composition, and shape, (3) unusual target binding properties, and (4) overall structural robustness. The size of a nanomaterial can be an advantage over a bulk structure, simply because a target binding event involving the nanomaterial can have a significant effect on its physical and chemical properties, thereby providing a mode of signal transduction not necessarily available with a bulk structure made of the same material.
In terms of the studies on biofunctional magnetic nanoparticles, there are several interesting challenges that are yet to be met. First, we need a robust surface chemistry to attach bioactive molecules onto magnetic nanoparticles without laborious synthetic efforts. Second, we need more precise control of the numbers and orientations of the molecules on the surfaces of magnetic nanoparticles. Third, we need multifunctional and (perhaps) multimodal magnetic nanoparticles for biological applications. By virtue of their magnetic properties iron oxide nanoparticles can not only be used for efficient drug targeting but also for detecting tumors using Magnetic Resonance Imaging (MRI).
Protein Separation
There is a strong demand for identifying or evolving new industrial enzymes capable of catalyzing processes under a range of different conditions. Examples include enzymes that are stable and active over long periods of time, those that are active in non-aqueous solvents, or that can accept and catalyze efficient turnover of various unnatural substrates. The identification and "rational" re-design of these proteins require their separation. Magnetic nanoparticles would be ideally suited for protein separation in a "positive" sense where the magnetic beads bind in a highly selective fashion to an analyte target. In favourable cases this allows separation of the particle-bound proteins by magnetic decantation using a permanent magnet. To date, magnetic particles have been used primarily to separate and concentrate analytes for off-line detection in a non-specific manner. We have also used magnetic nanoparticles specifically functionalized for protein separation.
Here polymer coated superparamagnetic γ-Fe2O3 nanoparticles were derivatized with a synthetic double-stranded RNA [poly(IC)], a known allosteric activator of the latent (2-5)A synthetase, to separate a single 35 kDa protein from a crude extract which cross reacted with antibodies, raised against the sponge enzyme.
Drug Transport
When the immune system goes to war against invading pathogens or the internal attack of cancer cells, it can deploy an arsenal of weapons. Regardless of the individual target, successful immunization results in activation of adaptive immunity, which might be accomplished, in part, through stimulation of the so-called Toll-like receptors (TLRs), a family of pattern-recognition receptors that recognize structural components shared by many bacteria, viruses and fungi. One example of such a component is viral single- or double-stranded RNA, recognized by the TLR3 receptor. Polyinosinic-polycytidylic acid [poly(IC)], is a synthetic double-stranded RNA (dsRNA). dsRNAs (except dsRNA stretches in RNA stem loops) are not normally found in mammalian cells, but they are present in cells infected by some viruses, and they can mimic viral infections. Among the most potent dsRNAs is the synthetic dsRNA, poly(IC), which consists of a pair of strands of poly-inosinic and poly-cytidylic acids. Poly(IC) shows antitumor and antiviral activity and recently entered into phase II clinical trials for patients with malignant gliomas. We have immobilized the abnormal nucleic acid polyinosinic-polycytidylic acid [poly(IC)] on γ-Fe2O3 maghemite nanoparticles via the phosphor-amidate route using a multifunctional polymer is reported. This approach may open new routes for magnetic drug delivery applications and solve various bottlenecks in immunology.
Multifunctional magnetic nanoparticles
Multifunctional magnetic nanoparticles functionalized with poly(IC) as target molecules and carrying fluorescent ligands may be prepared either by coating magnetic nanoparticles with an SiO2 layer, a polymer film or a thin Au mantle. These inorganic and organic protective coatings may be functionalized additionally with suitable target and fluorescent detector molecules.
