# The Concept of Using 2D Self-Assembly of Magnetic Nanoparticles for Bioassays
## 1 Introduction  
Magnetic nanoparticles find numerous applications in the development of new technologies related to environmental protection (pollution reduction, electromagnetic noise shielding/absorption) [1], medicine and biotechnology [2; 3; 4; 5], the pharmaceutical industry [6; 7], agriculture [8] and some micro/nanoelectromechanical devices (MEMS/NEMS) [9] or magnetoelectronics [10]. In this study, we limit ourselves to only one type of nanoparticles, magnetic iron-oxide nanoparticles, and we show the possibility of using their property to self-assemble into the 2D structure of nanoparticle chains in applications for medical diagnostics, including the form of various types of bioassays. Iron-oxide magnetic nanoparticles, due to their biocompatibility and low toxicity, are becoming increasingly used in medical diagnostics and medical treatment. The best-known examples of magnetic nanoparticle applications in this regard relate to the magnetic-hyperthermia method [3; 4]. This method takes advantage of the magnetic-nanoparticle's special property that it can absorb radio-frequency magnetic-field energy, which is then released to the nanoparticle's surroundings in the form of heat. It is this emitted heat that is used in magnetic-hyperthermia methods to destroy cancer cells. The method of magnetic hyperthermia using iron-oxide nanoparticles has been perfected for many years (for example, see the pioneering paper [11] in 1957, where the effect of magnetic hyperthermia on cancer cells in lymph nodes was studied). Today, it is one of the recognized techniques for destroying cancer cells, which in many cases can be used alongside chemotherapy and conventional irradiation techniques [3]. Another widely known area of magnetic-nanoparticle applications is the use of magnetic nanoparticles in medical imaging. An example is magnetic resonance imaging (MRI), where magnetic nanoparticles are used as contrast agents [5]. This applies to very smallmagnetic nanoparticles with diameters in the order of a few nanometers, such as dextran-stabilized iron-oxide nanoparticles of 4.8 to 5.6 nm [5]. These are single-domain nanoparticles in which the heat-power dissipation \(P_{e}\) from the eddy currents can be ignored because it is several orders of magnitude smaller than the heat-power dissipation \(P\) from the absorbed electromagnetic radiation [12]. The same applies to magnetic nanoparticles synthesized for magnetic hyperthermia, where they are mainly single-domain iron-oxide nanoparticles with sizes up to 20 nm. Further areas of medical applications include controlled targeted drug delivery, treatment and diagnostics [2]. For this purpose, magnetic nanoparticles are appropriately functionalized with, among other things, biological components, which gives them additional diagnostic function, and dispensing of the drug to be released [2]. At the same time, the polymer coating of the nanoparticle prevents the agglomeration of magnetic nanoparticles due to magnetic dipole-dipole interactions. The placement of various functional amino and carboxyl groups on the surface of nanoparticles allows us to use them in a new type of immunoassay becoming competitive with conventional ELISA methods. In an earlier publication [13] of two co-authors of this work, it was shown that the presence of magnetic nanoparticles of iron oxide significantly increases the capacity of antibodies to recognize antigens by means of the formation of paratope-epitope-binding complexes. It was explained that the mechanism causing such an effect is a local change in pH in the surrounding nanoparticles. In addition, appropriately dosed use of remote heating of magnetic nanoparticles by an alternating magnetic field permitted control of this pH change. In recent years, antibodies have become a more and more important component of the functionalized nanoparticle surface used in different kinds of bioassays and for targeted drug delivery. In paper [2], there is attached a very broad review of papers on chemical electrobiosensors.