SEM–EDS micrographs and elemental mapping

All fabrics were analyzed by SEM–EDS. It is important to note that to obtain nano size particles, an optimum Ti loading should be immobilized on the cotton fabrics so as not to lose Ti species in the washing process by leaching. The cotton fabrics shown in Table 1 have an optimum Ti loading and, therefore only the highest Ti loading samples showed SEM–EDS signal due to the sensibility of the technique. Since those nano particles would only be observed appropriately by the TEM. Therefore, the SEM technique helped to select the optimum Ti loading corresponding to NPs domain (reported in Table 1).

Figure 1 shows the micrographs and elemental mapping of selected impregnated cotton fabrics. Figure 1a corresponds to the impregnated lab coat fabrics by sonochemical method (CIU) at 100X resolution. From Fig. 1a can be suggested the correct synthesis method since the observed size of TiO2 corresponds to the NPs domain. Figure 1b shows the micrograph at 1000X resolution and it can be observed that the cotton threads are covered by TiO2-NPs (white dots) with an adequate particle size distribution on the cotton surface as it was also observed in Fig. 1a. Figure 1c shows the micrograph of cotton fabric without supported TiO2-NPs. Figure 1d shows the elemental mapping of Ti, confirming the adequate Ti distribution on the cotton fabric. For comparison purposes, Fig. 1e and 1f show a sample with high Ti loading than the cotton fabric shown in Fig. 1a and 1b. Both micrographs shown the microparticles domain of these samples with high Ti loading. Similar results (no shown) were obtained for all fabrics.

Fig. 1

Micrographs and elemental mapping of CIU (a–d) and fabrics with high Ti loading (e and f)

Tensile and textural properties of fabrics

The Table 2 shows the surface area determined by N2 physisorption, the lattice thread diameter, the grammage and the tensile strength of the cotton fabrics. The tensile strength of the fabrics was assessed using the bare cotton fabrics and after a first and a second treatments. The surface area of the cotton fabrics is too low, less than 1 m2g−1, which is a typical value for cotton fabrics. Also, due to this low value of surface area, the immobilization of the TiO2-NPs should be carried out in a proper way to avoid TiO2-NPs leaching due to the saturation of OH- surface groups on the cotton fabrics. According to the previous SEM–EDS results, the morphological characteristics suggested that TiO2-NPs was successfully synthetized and as will be seen later, the immobilization of TiO2-NPs was carried out satisfactorily.

Table 2 Tensile properties of the cotton fabrics

The lattice thread diameter of lab coat and Indiolino is very similar, the difference is only 0.08 mm, which indicates the great similarity of both cotton-derived fabrics. Also, this feature explains in great manner the low value of surface area since the cotton fabrics are made up of intertwined threads. Additionally, the thin structure of the cotton fabrics threads emphasizes that their surface can only be coated by very small structures such as nanoclusters or nanoparticles. The grammage represents the cotton fabric weight per unit area and as it can be observed, its value is greater for the lab coat cotton than for Indiolino. The grammage value difference is due to the density values that are slightly different due to the final preparation of the fabrics. Therefore, it could be inferred that lab coat cotton fabrics present a greater interaction with TiO2-NPs on their surface. The tensile strength of the cotton fabrics, without and with ultrasound treatments, is an important characteristic since the immobilization of TiO2-NPs could largely depend on this property. That is, in addition to the fact that TiO2-NPs immobilization on the cotton fabrics surface is a function of physisorption and to the chemisorption phenomena, probably TiO2-NPs interaction with OH- surface groups could also be influenced by the mechanical stability of the intertwined threads of cotton fabrics. Breaking loads of Indiolino and Lab coat fabrics were 9.90 and 8.33 N, respectively. After first treatment (the immersion of cotton fabrics in an ultrasound bath for 2 h and then a drying process at 65 °C), the fabrics were about 54 and 25% more resistant than the pristine one. The acidic condition of reaction solution (pH = 3.5) and ultrasonic irradiation did not cause destruction on the structure of cotton. After the second treatment (the cotton fabrics were maintained for 5 h under ultrasound irradiation and finally a dried process at 110 °C), the fabrics were about 10% less resistant than fabrics with only one first treatment but were more resistant than untreated fabrics. This loss on tensile strength fabric could be related to the ultrasonic irradiation and/or cleavage of the cellulosic chains by acid hydrolysis (Akhavan et al. 2014). In addition, shrinkage might occur while sonication causing a slight increase in breaking elongation from 5 to 6%. The results indicated that in-situ sonosynthesis of TiO2-NPs on the cotton fabric did not cause any significant damage to on the structure of cotton, as has also been stated by Akhavan et al. (2014).

In contrast, the fabric prepared by solvothermal method (CTS) from TiCl4 at 100 °C in the autoclave produces HCl. Therefore, strong acidic condition of reaction solution provokes the destruction of the structure of cotton. Even neutralizing the acid conditions, was not possible to preserve the structure of cotton and this preparation method was discarded.

Raman spectroscopy

Cotton fibers are composed of the biopolymer β-cellulose (Fig. 2), whose repeating units are β-D-glucose molecules and are linked by β-1,4-glucoside bonds (Pakdel and Daoud et al. 2013). Its fibers have a great amount of hydroxyl groups on the cellulose surface, so that extensive research have been made about the chemical functionalization of this material taking advantage of this high reactive functional group (OH-). The reactivity of cotton is a function of the considerable number of hydroxyl groups located on the C2, C3 and C6 carbons in each β-D-glupiranose or anhydroglucose unit (Pakdel and Daoud 2013; Al-Taweel and Saud 2016).

Fig. 2

Al-Taweel and Saud (2016) stated that the TiO2-NPs can interact strongly with surface hydroxyl and carboxyl groups of cotton cellulose. Additionally, Pakdel and Daoud (2013), Daoud and Xin (2004) and Daoud et al. (2005) propose an anchoring mechanism for TiO2/SiO2 composites on the cotton surface where nanoparticles are anchored on the hydroxyl groups of the C2, C3 and C6 carbons in cellulose. Therefore, it can be suggested that TiO2-NPs synthetized and immobilized by ultrasonic irradiation as in this work, could be anchored on the surface of cotton fabrics in a similar way as commented above. In this sense, the Fig. 3 shows the Raman spectra of Degussa P25, IIU and CIU. From Fig. 3a it can be observed the strong absorption at 151 cm−1 (Eg) corresponding to the vibrational mode of anatase TiO2 NPs and less intense bands at 398 cm−1 (B1g o A1g and B1g), also for this phase. The bands at 514 cm−1 (Eg) and 643 cm−1 (A1g) are assigned to rutile phase of TiO2, according to Qina et al. (2009), Burlacov et al. (2006) and Hana et al. (2018). The observed red shift for the bands is because the increase of critalinity of the sample and, also could be by the interaction of anatase and rutile TiO2 phases.

Fig. 3

Raman spectra of a Ti Degussa P25, b Indiolino prepared by ultrasound irradiation (IIU) and c cotton lab coat prepared by ultrasound irradiation (CIU)

The observed bands at 1094, 578, 516, 433 and 383 cm−1 for CIU (Fig. 3c) corresponding at vibrational modes of cellulose. According with Abid et al. (2017), the most intense band at 1094 cm−1 could be assigned to the C–O–C symmetric and assymetric stretching mode of the glycosidic bonds of the celulose`s glucopyranose ring. Figure 3c shows the vibrational modes at 144, 151, 383, 516, 513 and 638 cm−1, which are characteristics to TiO2-anatase nanoparticles, in agree accordance with published literature (Al-Taweel and Saud 2016). These bands are assigned to the extension of the Ti–O bond (Eg) at 639 cm−1, the bending of the Ti–O bond (A1g + B1g) at 517 cm−1 and the bending of the O-Ti–O bond (Eg) at 397 cm−1. It can be observed a red shift in the Eg mode frequencies for the TiO2-NPs supported on the cotton fabrics, i.e., the band at 151 cm−1 for the supported TiO2-NPs (see Fig. 3c) correspond to the red shift of the band at 145 cm−1 for TiO2-anatase nanoparticles.

According to Abid et al. (2017), the red shift could be attributed to the phonon confinement effect due to the minor size of the TiO2-NPs and the crystallinity increase. Even, the interaction between TiO2-NPs and -OH groups on the cotton fabric could contributed to this effect. For Indiolino prepared by ultrasound irradiation (IIU) (Fig. 3b) it can be observed an intense band at 152 cm−1 and less intense bands at 650 cm−1, 1741 cm−1, 2913 cm−1. The first two bands could be related to anatase TiO2-NPs and the bands at 1741 cm−1, 2913 cm−1 could be assigned to the characteristic vibrational modes of cellulose and TiO2 phases. As for CIU, the red shift in the IIU Raman spectrum can be explained by the minor size of the TiO2-NPs and/or the interaction between surface -OH groups of cotton fabrics and TiO2-NPs. From these results, for both IIU and CIU, the presence of TiO2-NPs intense bands allows to suggest the adequate functionalization of the cotton fabrics.

Attenuated total reflectance infrared spectroscopy (FTIR-ATR)

Figure 4 shows the FTIR-ATR spectra of the cotton lab coat fabric and CIU. The bands at 1161 and 1030 cm−1 correspond to the tension vibration of the functional groups C–C and C-O of cellulose, respectively. For all samples, a broad band between 3336 and 3280 cm−1 is observed, which is characteristic of O–H functional groups in cellulose (Ugur et al. 2010).

Fig. 4

FTIR-ATR spectra of: a cotton fabric, b CIU without washing, and c CIU after 5 washing cycles

These bands, characteristic of cellulose and observed in cotton fabric, persistent in CIU without and with washing treatment showing that the textile was not modified appreciably. Only was observed to shift of these bands to high wavelengths values, it could be attributed to the interaction of TiO2-NPs to cotton surface.

From 800 cm−1 toward longer wavelength, it can be observed the evolution of a broad region. According to Praveen et al. (2014) the region from 1000 cm−1 to 400 cm−1 correspond to the Ti–O stretching and Ti–O-Ti bridging stretching modes of the TiO2-NPs. Also, it is reported that around 450 cm−1 presents absorption band which is attributed to the TiO2 NPs. The lack of definition in Fig. 4 below 800 cm−1 is due to FTIR-ATR was carried out with a diamond crystal with a range 4000–550 cm−1, which does not allow a good definition of this region. However, the obtained results allow to establish the presence of TiO2-NPs (crystalline TiO2-anatase phase) on the cotton fabric surface. Figure 4 shows the FTIR-ATR spectra of the CIU before and after washing processes (Fig. 4b-c). As reference, the spectrum of the alone cotton fabric is shown (Fig. 4a). It can be observed that the FTIR-ATR spectrum of the cotton fabric alone does not show the increasing absorption band below 800 cm−1, which allowed to corroborate that this region can be attributed to the presence of TiO2-NPs.

UV–Vis spectra of reaction and washing solutions

Figure 5 shows the UV–Vis spectra of the preparation and washing solutions of the synthetized fabrics. TiO2 spectrum presents an absortion band at ~ 270—280 nm and therefore it is possible the identification of the TiO2-NPs in the preparation and washing effluents (Akhavan et al. 2014). It is important to note that TiO2-NPs physically adsorbed on the fabric surface are likely to leach into the washing solution. The spectrum of CIU prepared with an excess of IPT and the CIU prepared with an adequately amount of IPT are shown in Fig. 5 (a and b, respectively). The CIU prepared with an excess of IPT showed the greatest absorbance of the all analized samples. The UV–Vis spectrum of CIU prepared with an adequately amount of IPT shown an absortion band around of ~ 270—280 nm, which is less intense than the sample with an excess of IPT. CBH, IBH and IIU samples showed even lower values of absortion. Also, the UV–Vis analysis of subsequent washing solutions showed no absorption bands suggesting that the TiO2-NPs do not leach from the fabric surface. The above results indicated that the good immobilization of TiO2-NPs on the cotton fibers.

Fig. 5

UV–Vis spectra of washing effluents: a CIU prepared with excess of IPT, b CIU, c CBH, d IBH, e IIU andf water solution as reference

It is important to mention that TiO2-NPs could be anchored on hydroxyl and/or carboxyl groups by covalent linkages (Akhavan et al. 2014). The chemically anchored TiO2-NPs on the hydroxyl and carboxyl groups of the cotton surface must be the preponderant species that persist after the washing tests, in agreement with Akhavan et al. (2014) and Montazer and Seifollahzadeh (2011).

According to Ugur et al. (2010), the simultaneous synthesis of TiO2-NPs and the functionalization of cotton fibers in a one-step process by sonochemical irradiation can be represented by a hydrolysis reaction (Eq. 1) and a condensation reaction (Eq. 2) as follow:

$$\mathrm}_}_\right)}_+}_\mathrm\to -\mathrm\right)}_}+}_}_\mathrm$$

(1)

$$-\mathrm\right)}_}\to -\mathrm-\mathrm\right)}_}+}_\mathrm$$

(2)

The formation of TiO2-NPs can be explained in two steps as follow: 1) The hydrolytic Ti species, formed by the IPT hydrolysis in water, are condensed in many gel nuclei units and then are aggregated in large clusters. 2) The ·OH and ·H radicals, formed by the water sonolysis due to the cavitation process by the ultrasound irradiation, promote the hydrolysis of IPT and the polycondensation of Ti–OH groups, generating the TiO2-NPs. According to Ugur et al. (2010), the sonochemical formation of TiO2-NPs is carried out in the liquid phase, around the collapsing bubble and not within the collapsing cavity. The TiO2-NPs generation is a function of the microturbulence and the shock waves. The microturbulence is due to the radial liquid movement by the cavitation of the bubbles. The shock waves are a result of the high-pressure waves emitted by collapsing bubbles. The conjunction of microturbulence and the shock waves phenomena generates a localized heat and pressure transient zones. Large temperature and pressure gradients cause water sonolysis, therefore ·OH and ·H radicals are generated and promoting the Ti–OH condensation. It is proposed that nucleation ratio increases by the shock waves and the growth rate of the nanocrystals is a function of the microturbulence.

Photocatalytic activity

The photocatalytic activity of the TiO2-NPs on cotton fabrics was assessed to corroborate the self-cleaning properties and the antiviral and antibacterial features. Since as it has been established the ROS generation is a key factor. In this sense, the photocatalytic degradation of model molecules such as dyes has been carried out as elsewhere (Hosseini‑Sarvari et al. 2022; Skiba and Vorobyova 2020). In this sense, 5 × 5 cm squares of the fabrics were impregnated with MB (methylene blue) solution (10 ppm) and subsequently irradiated with UV (320 nm) or solar irradiation. The adsorbed amount of MB did not exceed 5% for all cotton samples, which is indicative of the low adsorption capacity of the material, due mainly to the low surface area of the materials (see Table 2). Due to the characteristic blue color of the Indiolino cotton fabric it was not possible to determine the MB degradation by DRS. For this reason, white color cotton indiolino was used for the photodegradation tests. Prior to irradiation, the 5 × 5 cm squares were cut off in four parts (2.5 × 2.5 cm) and were placed in a set-up in such a way that all samples were irradiated with the same intensity. Further, the set-up was stored in the darkness for 1 h to discard any possible change in the concentration, resulting from the photolysis process (Abid et al. 2016). Figure 6 shows the MB degradation on IBH under solar (Fig. 6b to 6e) and under UV irradiation (Fig. 6g to 6j). It can be observed a noticeable decrease of the blue color on the cotton fabrics after UV and solar irradiation. The discoloration observed was due to the photocatalytic degradation of MB by the TiO2-NPs presence, showing the efficiency of the functionalized cotton fabrics.

Fig. 6

MB degradation on IBH samples under solar irradiation (b to e; at 15, 30, 60 and 120 min, respectively) and under UV irradiation (g to j; at 15, 30, 60 and 120 min, respectively). a bare IBH and f IBH impregnated with MB solution

Also, the changes of the MB concentration on the fabrics were determined by UV–Vis DRS absorption at 0, 15, 30, 60 and 120 min of light exposure. According to Uddin et al. (2007), the observed band at 664 nm is assigned at dimeric and trimeric MB species adsorbed on the surface. The MB degradation was determined as the total MB conversion, by the C/C0 relationship. Figure 7 shows the MB degradation, using IBH under UV irradiation in the photoreactor. It can be observed that MB was degraded almost 70% in 60 min. It is important to mention that the MB degradation of CIU (not shown) was near to 99% in 60 min under UV irradiation. Also, it was observed negligible change in the MB concentration in the presence of bare cotton fabric under UV or solar irradiation, which confirmed the effective role of the TiO2-NPs in the photocatalytic activity. Based on these results it is possible to infer that IPT is a better option as Ti precursor than BuOT.

Fig. 7

DRS results of MB degradation on IBH, obtained by UV irradiation

The MB degradation data were well fitted with a pseudo-first order kinetics. Table 3 shows the kinetic results of MB degradation using the lab coat and Indiolino fabrics prepared by sonochemical or hydrothermal methods. Under UV irradiation, for the fabrics synthesized by different method and Ti precursor, it can be observed that pseudo-first order kinetic constants for Indiolino fabrics (IBH and IIU) are higher than for the cotton fabrics (CBH and CIU); 0.01 vs. 0.0067/min and 0.0117 vs. 0.0075 cm−1 (respectively). Meanwhile, the pseudo-first order kinetic constants under solar irradiation are higher for Lab coat than Indiolino fabrics. The kinetic constants are: 0.0165 for CIU vs. 0.0142 for IIU and 0.0154 for CBH vs. 0.0117 for IBH.

Table 3 Kinetic results of MB photo degradation of NPs-TiO2 on cotton fabrics by solar light (sun) or UV irradiation (uv)

The higher pseudo-first order kinetic constants values for Indiolino fabrics compared with cotton fabrics, under UV irradiation, probably could be because the lightly longer size of TiO2 crystals for Indiolino fabrics, since as it has been stated the TiO2 particles of larger size are more photoactive under shorter wavelengths, UV spectrum (Senić et al. 2011; Uddin et al. 2007). Contrarily, the higher photoactivity for cotton fabrics, under solar irradiation, it can be explained by two effects or by the combination of both. The first one could be due to the shorter size of TiO2 crystals for cotton fabrics which could absorb longer wavelength photons, in visible spectrum. The second one could be due to the greater amount of the irradiated photons under solar irradiation. Figure 8 shows the MB degradation using CIU under sunlight irradiation. It can be observed that almost a complete MB degradation was achieved in 120 min. Additionally, the Table 3 show that the pseudo-first order kinetic constant for CIU under solar irradiation was higher than pseudo-first order kinetic constants for IBH and IIU (1.4 and 1.2 times, respectively). The aforementioned results confirmed the higher photoactivity of functionalized cotton lab coat fabrics compared to the functionalized Indiolino fabrics under solar irradiation.

Fig. 8

DRS results of MB degradation on CIU, obtained by sunlight irradiation

The characteristic degradation time (τ) is the time needed to degrade half of the contaminant species present in the experiment. Figure 9 shows the τ of MB under UV or solar irradiations. For all samples (CIU, CBH, IBH and IBU), τ was reached faster under solar irradiation than under UV irradiation. That is, the same cotton fabric (cotton lab coat or Indiolino) prepared by the same method and Ti precursor and used under solar irradiation showed a greater MB degradation than when UV radiation was used on the same fabrics. These results are according to the reported pseudo-first order kinetic constants in Table 3 discussed earlier. This behavior could be due to the simultaneous effect of the shorter size of TiO2 crystals for cotton fabrics (which absorb visible irradiation) and the greater amount of the irradiated photons under solar irradiation. In general, the functionalized cotton samples synthetized in this work shown an adequately structure and a successful photocatalytic behavior as has been discussed previously.

Fig. 9

Characteristic degradation time (τ) of MB for all synthetized cotton fabrics under UV or solar (sun) irradiation

Bactericidal activity of TiO2-NPs on fabrics

The bactericidal activity of the fabrics synthesized was not possible to study by Kirby-Bauer technique because these ones do not form inhibition region, which has also been previously reported with fabrics impregnated with TiO2-NPs (Singhal et al. 2021). For this reason, fabrics squares of 0.5 cm × 0.5 cm from IIU, IBH, CIU and CBH were inoculated with the E. coli bacteria and were evaluated in Petri dishes with LB-agar medium. After the inoculation without UV irradiation, it was found that the IIU sample, TiO2-NPs impregnated Indiolino fabric by sonosynthesis and using isopropoxide as Ti precursor, has a greater capacity to inhibit E. coli, (see Fig. 10, sample 1). Also, CIU showed good antibacterial capacity (sample 3). The fabrics prepared by hydrothermal method, IBH (sample 2) and CBH (sample 4), showed the shorter bacterial inhibition capacity. It is important to note that Indiolino without TiO2-NPs (sample 6) showed bactericidal capacity, in a minor amount of its counterparts that contain TiO2-NPs. The halo of bacteria around the Indiolino fabric in sample 6 is more homogeneous than the other TiO2-NPs treated fabrics.

Fig. 10

Antibacterial activity of the fabrics on petri dish LB-agar. Sample 1: IIU, sample 2: IBH, sample 3: CIU, sample 4: CBH. Controls samples (5 and 6) correspond to cotton lab coat and Indiolino, respectively

Based on the bactericidal results obtained in the Petri dishes, the in-situ antimicrobial activity analysis was carried out by SEM only for IIU sample and its corresponding control. The E. coli and B. pumilus bacteria was grown on the surface of the Indiolino fabric (control sample), which can be observed in the SEM micrographs in Fig. 11a and Fig. 11c. In both cases it was possible to observe the bacteria at 2000X magnification (see Fig. 11b y 11d and it can be observed that the bacteria cover most of the Indiolino surface. The E. coli (Fig. 11b) and B. pumilus (Fig. 11d) bacteria show no apparent damage, and it is observed a normal morphology and size, in agree with (Percival et al. 2014; Grutsch et al. 2018). In the case of TiO2-NPs treated Indiolino fabric, the bactericidal activity was significantly appreciated by means of SEM, see Fig. 12. Fig 12a, b show the fabrics at low magnification (1300X) where the homogeneous deposition of the TiO2 and bacteria can be observed. In Figure b and Figure d, the bacteria E. coli and B. pumilus can be seen in greater detail (see insets). In both cases, the areas without TiO2-NPs showed the presence of bacteria, that is the bacteria grew in zones without TiO2 deposits, which demonstrates the ability to inhibit both bacteria. When the samples are irradiated with UV irradiation for 2.5 min on both sides, it is possible to observe a greater bactericidal capacity on the fabric surface, see Fig. 13. The Fig. 13a shows the E. coli biofilm with abnormal morphology or damage. From Fig. 13b it can be observed that bacteria only grew on areas without NPs, as was analyzed for the other Indiolino fabrics. In the case of the fabric inoculated with B. pumilus, the bacteria were totally eliminated (see Fig. 13c) under UV irradiation. Additionally, bacteria were not found on the fabric surface, even the areas without NPs are free of bacillus. When the Indolino fabrics were exposed for 5 min on both sides, both bacteria were no longer found in the fabric with and without TiO2-NPs.

Fig. 11

SEM micrographs of Indiolino fabric inoculated with E. coli (a) and B. pumilus (c). Micrographs at 2000X magnification for E. coli (b) and B. pumilus (d)

Fig. 12