# The Concept of Using 2D Self-Assembly of Magnetic Nanoparticles for Bioassays
## 4 Discussion  
The presented results of using the ordered 2D structure of the magnetic-nanoparticle chains to capture fluorescein from a drying droplet of a solution containing fluorescein may suggest the possibility of extending this method to other molecular systems, such as proteins, enzymes and others. In this work, fluorescein acted only as a marker for the deposition of molecules on nanostructures, but it shows the possibility of testing the chemical reactions of the materials under study at multiple points simultaneously. At the same time, the example of the application of Raman spectroscopy shown in the paper demonstrates the possibility of analyzing the results of these chemical reactions. Moreover, magnetic nanoparticles can further catalyze such reactions by locally changing the pH or further functionalizing their surfaces. Iron-oxide nanoparticles can acquire a pH-dependent electric charge on their surface when they come into contact with an aqueous solution. Similar to other metal oxides, both magnetic and non-magnetic, there is a specific value of pH that is called the point of zero charge (PZC), at which the sum of all positive and negative surface charges equals zero. The charge is positive for pH \(<\) PZC and negative for pH \(>\) PZC. In this work, the PZC of iron-oxide magnetic nanoparticles is around 6.2. If we denote the change in pH at the surface of the nanoparticles by \(\Delta\)pH = pH\({}_{0}-\) pH, where pH\({}_{0}\) represents the surface-pH value, then \(\Delta\)pH defines the surface electric potential \(\psi_{0}\) as the following:  
\[\psi_{0}=2.3026\frac{RT}{F}\Delta\text{pH}\,, \tag{1}\]  
where \(F\) is the Faraday's constant, \(T\) is the temperature and \(R\) is the gas constant. This electric potential depends on the fraction of protonated and deprotonated hydroxyl groups present on the nanoparticle surface. The resulting change in pH could be used to control certain chemical reactions directly on the magnetic-nanoparticle chains. It should be added that the linear part of \(\psi_{0}\) is the well-known form of the Nernst potential [32]:  
\[\psi_{0}^{N}=2.3026\frac{RT}{F}(\text{PZC}-\text{pH})\,. \tag{2}\]  
The greater the pH deviation from PZC, the greater will be the change in \(\Delta\)pH around the nanoparticles (Equation (1)). Depending on the pH value of the solution, the presence of nanoparticles will lower the pH value or increase it in their immediate vicinity. Referring to paper [13] on catalyzing antibody-antigen reactions with magnetic nanoparticles, the catalytic capabilities of magnetic-nanoparticle chains can be extended by using alternating magnetic fields at radio frequencies. Once the energy of the alternating magnetic field absorbed by magnetic nanoparticles is released to the surrounding nanoparticles in the form of heat it can affect the pH level. In [12] it was shown, using the example of the silica  
Figure 5: Plots of Raman spectra for pure fluorescein, LDPE film and a fluorescein-coated magnetic-nanoparticle chain, the latter measured at point A marked in Figure 4: **(a)** Fragments of the Raman spectra in the range of values 200–1000 cm\({}^{-1}\) are shown. **(b)** The full Raman spectrum of LDPE, the red-framed section of which is shown in panel **(a)**. Because of the fluorescence effect, a laser with wavelength \(\lambda=785\) nm was used for fluorescein. For LDPE, the wavelength \(\lambda=532\) nm was used.  
magnetic-nanoparticles interface, that such a process may lead to increased deprotonation in the surroundings of the nanoparticles.  
This work mainly focuses on ordered chains of magnetic nanoparticles in the form of concentric rings. However, noteworthy are rectilinear magnetic nanowires (as in panel (e) of Figure 3), which could enrich electrophoresis techniques by being able to add magnetic-field effects on separation in nano and microfluidics. It should be noted that magnetic nanoparticles are intensively used in immunoassays, due to their stability and ability to functionalize [2].