# The Concept of Using 2D Self-Assembly of Magnetic Nanoparticles for Bioassays
## 1 Introduction
One of the frequently used diagnostic methods is tests involving the analysis of spatial patterns formed from the deposit of a dried layer of body fluids, mucus, etc. These also can act as bioassays, where the reaction of a biofluid to the added chemical components is examined. The stains from dried biofluids carry a great deal of information, e.g., about the emergence of a disease state, a change in the nature of the acidity of the biofluid under study or salt concentration [14]. They often appear in the form of the characteristic shape of the so-called coffee ring [15]. On the other hand, for saliva, tear film or cervical mucus, fern-shaped crystallizing structures appear. They are usually associated with the presence of small salt crystals. An example can be found in [16], where the changes in the fern pattern for cervical mucus to determine the effect of pregnancy status and ovulation, among other things, were presented. A similar discussion can be found for saliva examination [17]. Ferring tests are also used in ophthalmology, and an example of one recent publication in this field is [18]. In such tests, the type of substrate surface may be important. It is easy to find in the literature examples of the stains from drying colloids that correlate with the crystallographic symmetry of the substrate, like the assembly of the collagen layer on the mica surface [19] or the assembly of caffeine crystallites on the mica surface [20]. These results may indicate the need for some standardization for medical diagnostics carried out using the analysis of stains from dried body fluids. It should be noted that chains of magnetic nanoparticles may also have great potential for biomedical applications. One example of preparing magnetic chains is the method discussed in paper [21], where the self-assembly of magnetic nanoparticles into 1D chain structures was stabilized in an external magnetic field and it was combined with simultaneous crystallization of silica using sol-gel processes. Other examples include papers [22; 23; 24]. Very promising is a method for growth of ganglion neurons by magnetic colloidal nanochains [25]. Other properties of magnetic nanoparticles that are useful for medical applications are their targeted functionalization [26] and their possible use in optical bioassays using surface-plasmon resonance (SPR) [2]. In this paper, the possibility is suggested of using a new type of materials represented by self-assembled magnetic iron-oxide nanoparticle-chain-like structures that may have great potential for controlled immobilization of enzymes and proteins, as well as DNA and RNA molecules. We demonstrate this using fluorescein as an example. At the same time, we show the possibility of chemical analysis of the material collected on such structures using Raman spectroscopy. The arrays of magnetic chains could serve as an analog of ELISA plates working along the magnetic-nanoparticle chains. Thus, some examples of magnetic nanoparticle applications discussed above could be exploited. The possibility of self-assembling magnetic nanoparticles into 2D structures during the drying of a magnetic-liquid droplet on a hydrophobic substrate was shown in our earlier paper [27]. In this case, the stains after the dried droplet formed an array of numerous concentric rings made of magnetic nanoparticles. It should be noted that similar experiments done by other groups were performed for non-magnetic particles [28; 29; 30].