# Potentiometric Biosensor Based on Artificial Antibodies for an Alzheimer Biomarker Detection
## 3 Results and Discussion
### Raman Spectroscopy Studies  
The chemical profile of the different materials was evaluated by analysing the Raman spectra. The analysis was performed for the following materials: (i) SWCNT, (ii) SWCNT-COOH and (iii) MIP and NIP. RAMAN spectra give information about the efficiency of the nanotube carboxylation, polymerization and template removal [65].  
In general, the structure of the spectrum highlights four distinctive peaks (Figure 2): the first peak in the region around 1300 cm\({}^{-1}\) (D), the second in the region around 1600 cm\({}^{-1}\) (G), the third in the high frequencies around 2600 cm\({}^{-1}\) (\(G^{\prime}\)) and the fourth in the region around 1930 cm\({}^{-1}\). Figure 2 shows the Raman spectra for each sample separately, and the ratio of the intensity between G and D band, which is useful for determining the purity of and the degree of the chemical modification of the material. As a result, we observe a significant difference in terms of the peak intensity and shift between different materials. The shifts are essentially perceptible for the G and D bands.  
Carbon nanotubes are rolled-up graphene sheets that have been wound into tubes. These nanomaterials are cylindrical tubes with a single outer wall and a diameter typically around 1-2 nm. Due to their exceptional mechanical, electrical and thermal assets, carbon nanotubes are widely used in the field of carbon nanotechnology nowadays.  
The Raman spectrum of an SWCNT is similar to graphene, which is to be expected as it is a rolled-up sheet of graphene. In Figure 2, in the Raman spectrum of SWCNT, prominent G and G\({}^{\prime}\) peaks are visible. The band relative to the frequency of tangential displacement (G) is close to 1590 cm\({}^{-1}\), and the band associated with the second order peak (G\({}^{\prime}\)) is related to the charge transfer between the nanotubes and the reaction mixture [66]. It is also possible to see a well-defined band near 1350 cm\({}^{-1}\). This band is denominated as the D band. The D band is ascribed to hybridized vibrational mode, which is related to the graphene edges, and it designates the presence of some disorder to the carbon structure [67].  
Comparing the RAMAN spectra of SWCNT and SWCNT-COOH, the latter shows two additional bands near 1540.7 and 267.7 cm\({}^{-1}\), and a peak shift of the G\({}^{\prime}\) and G bands approximately 60 and 9 cm\({}^{-1}\) to the right. These changes are associated with defects in the walls of the nanotubes caused by chemical oxidation by introducing groups (OH, CHO and COOH) on their surface. Additionally, a decrease in the I\({}_{\text{G}}\)/I\({}_{\text{D}}\) of the SWCNT after chemical modification from 7.9 to 2.1 was observed, confirming the presence of additional defects in the nanomaterial.  
Figure 2: Raman spectra of SWCNT, CNT-COOH and MIP and NIP.  
Regarding the RAMAN spectra of MIP and NIP materials, the most relevant information involves changes at the level of the G and D bands and the respective peak ratio (Figure 2). The \(\mathrm{I_{G}/I_{D}}\) for MIP and NIP were 4.4 and 4.7, respectively. These values are very similar, which would be expected, since the polymeric matrix is identical. Besides, this ratio increased in comparison to SWCNT-COOH, indicating the presence of a crystalline structure, since the G band is the Raman signature for sp2 carbons. Additionally, we can observe a shift of the peaks G', G and D to the right compared to the previous step of the chemical modification of SWCNT-COOH, and an enhancement of an additional band at approximately 1900 cm\({}^{-1}\). The shift of these bands was approximately 13.9 and 12 cm\({}^{-1}\), respectively. Overall, this result confirms the SWCNT oxidation and MIP and NIP polymerization.