FTIR spectroscopy analysis Fourier transform infrared (FTIR) spectroscopy is commonly used to learn more better understand the local nano-microenvironment of the ligands at the QD surface. In some cases, it has proven to be the most important technique for the characterization of the interactions between the ligand and the quantum dot [35, 44]. The FTIR spectrum of chitosan copolymer (Additional file 1: Figure S1) presents absorption peaks at 1,645 and 1,560 cm-1 which are Epigenetic Reader Domain inhibitor assigned to the carbonyl stretching of the secondary amides (amide I band) and the N-H bending vibrations of the deacetylated primary amine
(-NH2) and amide II band, respectively. NH vibrations (stretching) also occur within the 3,400 to 3,200 cm-1 region overlapping the OH stretch from the carbohydrate ring. In addition, the absorptions at 1,030
to 1,040 cm-1 and 1,080 to 1,100 cm-1 indicate the C-O stretching vibration in chitosan, which are associated with the C6-OH primary alcohol and the C3-OH secondary alcohol, respectively [6, 19, 45]. These amine, amide and hydroxyl groups are the most reactive KU55933 sites of chitosan and are involved in the chemical modifications of this carbohydrate and in the interactions of chitosan with cations and anions [46, 47]. After conjugating the quantum dots with the capping biopolymer (curves (b) in Figure 5 and Additional file 2: Figure S2), there were several bands of chitosan in the FTIR spectra (curves (a) in Figure 5 and Additional file 2: Figure S2) that exhibited changes in their energies (i.e. wavenumber). These changes can be mainly attributed to the interactions occurring between the functional groups of the chitosan ligand (amine/acetamide and hydroxyls) and the ZnS pheromone QDs. For example, in the spectra of the bioconjugated QDs (Figure 5), the amide I band (1,650 cm-1) shifted to a lower wavenumber by 7 cm-1 for the ZnS nanoconjugates synthesised at pH 4.0 and 6.0. The amine band (bending NH, at 1,560 cm-1) was ‘red-shifted’ (i.e. shifted to a lower energy) by approximately 6 cm-1 for QD_ZnS_6 and 9 cm-1 for QD_ZnS_4. A significant change was also observed in the region from 1,000 to 1,200 cm-1, which was
essentially associated with -OH groups (alcohol groups). The band associated with the primary alcohol (C6-OH) vibration was red-shifted by 13 cm-1 for QD_ZnS_6 and 18 cm-1 for QD_ZnS_4. The peak assigned to C3-OH (secondary alcohol) stretching shifted its position to a lower energy by 38 cm-1 for QD_ZnS_6 and 15 cm-1 for QD_ZnS_4. Figure 5C summarises the red shift of bands related to functional groups of chitosan after bioconjugation as a function of pH. Additionally, at all the pH concentrations under evaluation, the wide peak of chitosan at 3,385 cm-1 (Additional file 3: Figure S3), corresponding to the stretching vibration of -NH2 and -OH groups, became significantly narrower after stabilisation of the quantum dots. This peak narrowing indicates the reduction of ‘free’ amine groups after quantum dot stabilisation [35].
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