3.2. Study of the Synthesis on the Surface of the Wool
illustrates the FTIR spectra of treated and untreated samples. These IR spectra show signals of wool and HKUST-1.
HKUST-1 FTIR, Figure 2
b, spectra exhibited MOF formation bands [35
]; see Table 1
. Pure keratin comprised 90% of the wool, yielding the behavior of the FTIR for a protein. According to Figure 2
a, wool FTIR spectra coincided with keratin polymer bands [37
]; however, the spectra also showed bands at 1735, 1453, and 1393 cm−1
consistent with terminal COOH and NH2
functionalities. These bands allow the wool to serve as a prospective anchoring fiber for the growth of HKUST-1 [38
Due to the low pH used in the synthesis, the protonated groups will not coordinate directly with copper. As the MOF/fabric and samples after treatment spectra nearly corresponded, identifying the fabric group responsible for MOF anchoring is unlikely. Studies have shown that acid and oxidant treatments (such as the presence of nitrate and pH 3 used in this work) break S–S bonds and form new terminal functional groups such as –SH, –SO–S–, and SO3
]; consequently, new bands, absent in untreated wool, appear in the region between 1000 and 1120 cm−1
. When observing the spectra of the treated and washed fabric, with low superficial MOF loss, small bands appear in the region between 1000 and 1120 cm−1
, and the intensity at 3400 cm−1
increases, indicating S–S bond cleavage [44
According to Zhang et al. [45
], S–H functional groups and increased fabric hydrogen bonds affect these regions of the FTIR spectra. Therefore, the observation of the FTIR disorients the identification of the functional group related to MOF anchoring. In addition, the literature shows that the acid treatment increases the number of coordination sites proportionally to the hydrogen interactions; and the absence of shifts for C=C aromatic vibration frequencies, pointing to the lack of the influence of the aromatic ring in the MOF–wool interaction [21
The observation of FTIR spectra for all samples, in Figure 2
d,f, allows concluding that washing samples reduced the intensity of HKUST-1 bands and removed MOF from the wool. SEM images of all samples, Figure 3
, show the same amount of MOF particles, demonstrating that copper and terminal function linking are non-exclusive interactions between MOF; non-covalent weak interactions like hydrogen binding occur.
displays the surface morphology of wool with the incorporation of HKUST-1 with visible nanoparticles on the fiber before and after washing.
The morphology of the MOFs, Figure 3
b, resembles an octahedral structure, as presented by Lin and Hsieh [33
], Hosseini, Zeinali, and Sheikhi [46
], and Toyao et al. [47
]. Figure 4
compares the EDS spectra of [email protected]
prepared for 24 h (a), 24 h after washing (b), 48 h (c), and 48 h after washing (d). These spectra reveal that the metal organic framework HKUST-1 was added on the wool fibers’ surface, and its presence after washing was proven by cooper signals (near 1.0, 8.0, and 9.0 KeV). These results indicate the performance of the synthesis on the surface of the fabric, as indicated by the thermal analysis and PXRD.
PXRD analysis determined the crystallinity of the structures and confirmed the presence of HKUST-1 on the surface of the wool. Figure 5
shows the diffractograms of non-treated wool fabric, MOF HKUST-1 (conventional method), and treated wool before and after washing. A direct relation is observed concerning time and the presence of HKUST-1 on the surface. The charts for wool and MOF, shown in Figure 5
, exhibit two characteristic peaks, 2θ ≈ 10° for the wool and 2θ ≈ 13° for the HKUST-1.
The diffractogram of synthesized MOF in [28
] and [41
] shows the same peaks, especially the one at 2θ ≈ 13°. In general, before the washing, the behavior was similar to the MOF diffractogram (a), with sharp peaks close to 2θ ≈ 13° and 2θ ≈ 20°’; however, once samples were washed, as in the diffractogram in (d), they showed similar wool peaks in the 2θ ≈ 10° region and from the 20 to 2θ ≈ 25° range, resulting from the fabric peaks overlap of HKUST-1, Figure 5
c,d. This difference shows that washing causes a slight decrease in the number of crystalline structures over the fabric.
Nevertheless, Figure 5
e,f shows the contrary. After 48 h of synthesis, the washing effects decreased, with more MOF observed on the surface of the fabric. In general, it is possible to see the peak at 2θ ≈ 13° indicating effective incorporation of MOF.
3.3. Evaluation of the Finish
The color analysis verified the presence of HKUST-1 on the wool surface; copper present in the fabric changed its color from a blue to a green hue. Table 2
shows the color data before and after samples were washed. The values of L*, a*, and b* refer to the luminosity, coordinates red/green, and coordinates yellow/blue, respectively.
The luminosity value of the untreated sample was 83.94 ± 0.12. Regarding the values of the treated samples before and after washing (in this order), the 24 h synthesis presented the values: 56.35 ± 0.67 and 54.25 ± 0.32; the 48 h synthesis presented the values: 55.81 ± 0.41 and 48.55 ± 0.29. This reveals that the samples lost luminosity, getting darker because of the MOF presence [6
]. MOF and fiber macromolecules interacted and changed the color of the fiber from blue (Cu2+
) to green: coordination of copper (II) with the functional groups of wool. The absorption band shifted from the visible to near-infrared region explained by (1) HKUST-1 water ligand substitution from wool functional groups, such as carboxyl and thiol, and MOF–wool interaction removing electronic density from copper (II), increasing the ionic character. Thus, Ligand-Metal Charge Transition (LMCT) shifted to lower energy regions. (2) There was a decrease in the Crystal Field Stabilization Energy (CFSE) due to the substitution of water ligand possibly by wool functional groups, once they displayed more intense-donors than the first [48
shows the antimicrobial activity for untreated and treated wool fabric before and after washing.
The untreated woolen fabric and the positive control showed an expected growth of microorganisms, and the untreated fabric had a reduction of 27.73%, as shown in Table 3
The presence of MOF in the treated samples attributes them antimicrobial effects, as presented by Wyszogrodzka et al. [12
]. The presence of copper in the MOF structure eliminates microorganisms due to the intrinsic property of this metal. The [email protected]
fabric completely inhibited the tested microorganism. The results after washing showed a slight decrease in the level of inhibition of growth of the microorganisms; nevertheless, the inhibition remained almost complete: 99.97% after 24 h and 99.99% after 48 h of treatment. The fabrics treated with HKUST-1 during the synthesis of 48 h are noteworthy, with better antimicrobial performance.