# The Concept of Using 2D Self-Assembly of Magnetic Nanoparticles for Bioassays
## 3 Results
### Preparation of 2D Array of Magnetic-Nanoparticle Chains  
Low-density polyethylene (LDPE) film was chosen as the substrate, which is hydrophobic, is a non-polar, flexible material and is chemically resistant to both acidic and basic solutions. It is commonly used for packaging and as an insulating material but isalso of increasing importance in biomedical applications [31]. However, another hydrophobic substrate with axial asymmetry, e.g., with a parallel-fiber structure, can be used. In the case of thin LDPE film, such an asymmetry is obtained by stretching the film in one direction--say, the \(x\)-direction, as shown in Figure 1a,b. In the examples shown, 1 cm of the film has been stretched to 5 cm. The process of self-assembling magnetic nanoparticles on an unstretched film is very random and the ring structures are very fuzzy. The method of preparing a multi-ring structure of magnetic nanoparticles [27] has been extended to include the ability to remove the core (the spot in the center part of the droplet deposit in Figure 1c) formed from excess magnetic nanoparticles in the drying droplet, so that only chains of nanoparticles remain on the substrate (Figure 1d). In addition, Figure 1e shows the possibility of preparing rectilinear chains of magnetic nanoparticles. It should be noted that the removal of the excess magnetic liquid in the drying droplet can be done at any time. For this purpose, this excess can be removed with a pipette or perilstatic pump (the methods used in this work) and suitable hygroscopic material. The time needed for the formation of subsequent nanoparticle chains as the magnetic-liquid droplet dries is several seconds (see also [27]) under ambient conditions. Figure 1: 2D printing of spatial structures from magnetic iron-oxide nanoparticles functionalized with APTES on a thin film of LDPE: (**a**) Schematic showing the preparation of the LDPE substrate for the sample by stretching it in the x-direction. (**b**) AFM image of the surface of the stretched LDPE. (**c**) Typical optical-microscopy image of a multi-ring magnetic-nanoparticle deposit formed by a drying droplet of magnetic colloid with a circular stain inside. (**d**) Analogous to (**c**) but the circular stain has not been created, due to a different printing procedure involving the removal of excess magnetic colloid during the final stage of the droplet drying process. (**e**) Optical-microscope image of  
the magnetic nanoparticle deposit in the form of a structure of evenly spaced rectilinear chains. Note: images of spatial nanostructures obtained by optical microscopy are visible due to the light-scattering phenomenon, as are numerous dark spots caused by the irregularities inside the LDPE. Figure 2 shows AFM images with details of the topography of the magnetic-nanoparticle chain arrangement in the 2D structure, and TEM-microscopy images enriched with elemental analysis of the prepared magnetic nanoparticles. The nanoparticles were coated with APTES. Elemental composition analysis was performed on a copper grid with the help of the EDX method. The formation of elongated structures from nanoparticles is a result of the mechanism of the coffee-ring phenomenon [15]. It usually means the appearance of a single-ring or several-rings structure of precipitate from the material of the drying colloid droplet. In this work, the phenomenon concerns a magnetic colloid droplet drying on a hydrophobic substrate, where during drying the droplet's contact line shrinks while agglomerating magnetic nanoparticles on it, which slows down this shrinking by interacting with the substrate, so that the growing agglomerates take on a ring shape consistent with that of the contact line and pin the contact line to the substrate.