Keywords

1 Introduction

The rise in the demand for synthetic dyes in pulp and paper, distilleries and tanneries, leather, paint and textile industries led to elevation in synthetic dyes production, and ~1–1.5 tons of the dyes are globally released into the environment [1], in which textile industry is the major contributor [2,3,4]. This has caused serious concerns on the surface and ground waters, and it is affecting the fauna and flora [5]. Further, the presence of pathogenic microorganisms in the contaminated water causes serious gastrointestinal diseases [6]. Moreover, most of the synthetic dyes are stable organic molecules, and they may be persisting in the environment for longer time, which can even pass through food chain to become carcinogenic due to their chemical nature [7,8,9].

The anthraquinone, arylmethane, azo, indigoid, phthalocyanine, and xanthane class of dyes are recurrently used for industrial purposes [10]. Among them, malachite green, a triarylmethane group, and anionic dye is used in aquaculture, food additive, cosmetics, leather, paint, and textile industries for its fungicidal, parasiticide, antiprotozoal, and antibacterial nature. However, the industrial malachite green enters the body in ionic form which is aided by their high lipid solubility to cause severe toxicity and bioaccumulation in the humans. With a tolerance limit of 0.1 mg/kg in the human body, malachite green acts a multi-organ toxin having undesirable effects on liver, spleen, kidney, urinary bladder, heart, and thyroid follicles [11]. Furthermore, malachite green is known to cause teratogenicity, mutagenesis, chromosomal fracture, and respiratory toxicity [12, 13]. To aggravate, the toxicity of the malachite green increases with the raise in the temperature [11]. However, high-frequency usage of malachite green continues despite being banned in many countries due to its highly effective and economical nature.

In the context of above, the conventional methods for removal of dyes are based on chemical, physical, or biological procedures [14,15,16,17,18]; however, these are having several limitations such as high-priced, low efficiency, inferiorly versatile, and their waste-disposal (wherein the treatment is incompetent for full degradation of stable synthetic dyes) [19, 20]. One of the most effective methods developed recently includes titanium dioxide (TiO2)-mediated degradation, which is an active photocatalyst, and known to be biologically/chemically inert [21,22,23]. Furthermore, the nanotechnological-based solutions are the state-of-the-art resolutions to degradation problems due to the large surface area and unique features influenced by the size, surface structure, and shape of the synthesized nanoparticles.

In recent years, the incorporation of metal nanoparticles into polymer matrices have gained much attention for their remarkable applications, and silver (Ag) nanoparticles are at the forefront due to their unique catalytic, optical, electrical, and antibacterial properties. Ag nanoparticle-based catalytic systems are gaining great attention because of their stability, selectivity, and reactivity [24]. The efficiency of the degradation can be further improved in the visible region by fine-tuning their physicochemical properties [25,26,27].

Therefore, in the present study, different films have been developed by incorporating TiO2, and Ag nanoparticles in PVA scaffold. PVA is a hydrophilic polymer, and it acts as a membrane to entrap TiO2, Ag nanoparticles, and other pristine molecules. This newly prepared hybrid membrane composite materials are active combination of polymer, TiO2 and nanoparticles, and they are expected to improve the mechanical properties, fire resistance capabilities, biocompatibility, and other factors of the polymer. The hybrid polymer matrices are found to have better catalytic activity than colloidal Ag nanoparticles due to stability and uniform dispersity [26], and they are reported to degrade many organic pollutants including synthetic dyes [28,29,30,31]. The pollutants degradation and antimicrobial potential of hybrid polymer matrices are due to a series of reactions caused and mediated by hydroxyl radicals [32, 33].

Therefore, we have evaluated the photocatalytic degradation of malachite green dye, antibacterial property against Escherichia coli, and assessed the electrical and dielectric properties in the frequency range from 50 to 8 MHz at room temperature for conductance capacity of the prepared hybrid films.

2 Materials and Methods

2.1 Materials

Potassium hydroxide (KOH), l-tyrosine, poly vinyl alcohol (PVA, MW 893.

,000–98,000), silver nitrate (AgNO3), and titanium dioxide (TiO2) were purchased from Sigma-Aldrich, and malachite green was purchased from Qualigens Fine Chemicals. All the chemicals were of analytical grade and were used as received. Milli-Q water of 18.2 MΩ cm resistivity at 25 °C was used to prepare all the solutions and growth media. All the glass-wares were thoroughly washed with aqua regia (mixture of HCl and HNO3) before use.

2.2 l-tyrosine Synthesized Silver (Ag) Nanoparticles

The protocol followed for the preparation of Ag nanoparticles is as described in [34], wherein a 100 ml solution consisting of l-tyrosine (0.1 mM) and KOH (1 mM) was allowed to warm at 70 °C. The AgNO3 (0.2 mM) was added drop by drop, and the heating was turned off when the solution turned yellow. The nanoparticles solution was kept under continuous stirring at room temperature, followed by slow heating to evaporate water from the nanoparticles solution to increase the Ag metal concentration 5X. The concentrated Ag nanoparticles solution was used to incorporate in PVA scaffold (Fig. 1, F4 and F6).

Fig. 1
figure 1

Schematic representation of different PVA scaffold-based films (F1–F6) with various compositions incorporating AgNO3, TiO2, Ag nanoparticles (AgNPs) and tyrosine amino acid (Panel A). The UV–visible spectroscopic analysis of tyrosine synthesized Ag nanoparticles and their optical image showing characteristic yellow color of Ag nanoparticles (Panel B). The digital photographs of various composite films, wherein, the incorporation of AgNO3, TiO2, Ag nanoparticles, and tyrosine have imparted various colors (Panel C)

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2.3 Preparation of PVA and PVA Based Scaffolds by Incorporation of AgNO3, l-tyrosine, AgNPs, and TiO2

In a typical experiment, 1.5 g of PVA powder was dissolved in MilliQ water, and the volume was made up to 30 mL. The solution was continuously stirred at 40 °C for 30 min, and the viscous solution was poured into a circular plate. After oven dry for 4 h, the obtained film was separated from the circular plate and named as F1 (Fig. 1).

Likewise, in the viscous solution of PVA, 2.5 mL of 0.1 M TiO2 (Fig. 1, F2) or 2.5 mL of 1 mM AgNO3/2 drops of l-tyrosine (0.01 mM) (Fig. 1, F3) or 2.5 mL of Ag nanoparticles (Fig. 1, F4) or 2.5 mL of 100 mM TiO2/2.5 mL of 1 mM AgNO3/few drops of l-tyrosine (0.01 mM) (Fig. 1, F5) or 2.5 mL of 100 mM TiO2/2.5 mL of Ag nanoparticles (Fig. 1, F6) were added to secure different films. The total volume of the reaction solutions in all the cases was maintained 30 mL.

2.4 Electrical and Dialectical Properties of the Films

The dielectric properties and AC electrical conductivities of the pure PVA scaffold (F1) and composites-PVA scaffolds (F2-F5) were measured using high precision LCR meter [Make: Hioki, Japan; Model: IM3536] in the frequency range from 50 Hz to 8 MHz using two-probe technique. The samples were cut in the form of circular disks of diameter 13 mm, and silver electrodes were provided on both sides of the sample for the measurement of their electrical and dielectric properties. The calculations of the dielectric constant, dielectric loss, loss tangent, and AC conductivity are presented elsewhere [35].

2.5 Evaluation of Dye Degradation Capacity

The dye degradation properties of the PVA based scaffolds (F1–F6) were assessed in time-dependent manner for malachite green through spectroscopic measurements. In a typical experiment, 3 mL of the dye malachite green, with the initial concentration of 10 mg/L was taken, and 1cm2 respective scaffolds were placed into the vials. The dye solution and dye vials containing scaffolds were irradiated with a UV light of 270 nm, followed by collection of 200 µl of solutions at 0, 1, 3, 5, and 7 h, respectively. The absorbance was measured at a maximum wavelength of 618 nm for malachite green by using UV–vis spectrophotometer (Bio Mate 3S, Thermo Scientific, Massachusetts, USA), and the % degradation was calculated.

$$ {\text{Percentage }}\left( \% \right){\text{ dye degradation }} = \frac{{\left( {A - A_{0} } \right)}}{{A_{0} }} \times 100 $$
(1)

where A0 is initial absorbance and A is absorbance after treatment. The experiments were performed in triplicates and error expressed as mean ± SE.

2.6 Assessment of Antimicrobial Potential of the Composite Scaffolds

The antimicrobial potency of the PVA based scaffolds (F1–F6) were assessed toward Gram-negative bacteria Escherichia coli. Typically, 100 µl culture from the overnight grown fresh culture with the absorbance of 1.48 was inoculated into the 30 mL nutrient broth, and 1 cm2 respective films were placed into the flask. The flask from the same bacterial culture without any treatment was considered control under similar experimental conditions. The absorbance at 600 nm was recorded at an interval of 2 h until the stationary phase was attained, and bacterial growth curve was constructed.

3 Results and Discussion

In the present work, PVA has been chosen as the scaffold because it is hydrophilic, biodegradable, and relatively stable polymer [36]. To this, tyrosine-capped Ag nanoparticles and TiO2 were introduced to develop PVA based composite scaffolds. As shown in Fig. 1c, various free-standing films were prepared, and all of them have different color due to variety of compositions. The pure PVA membrane (Fig. 1c, F1) appears transparent, whereas TiO2 containing film emerged as semi-transparent milky-white color (Fig. 1c, F2). The PVA polymer chains are most likely bound to the free TiO2 through hydrogen bonds [37, 38]. The Ag nanoparticles or Ag+ comprising films appeared in distinct colors ranging from light brown to dark brown due to differential composition of the membrane components. The films F3 and F5 developed yellow and dark yellow colors due to in-situ formation of the Ag nanoparticles by the reduction of Ag+ through tyrosine, respectively. The similar in-situ Ag nanoparticles synthesis has been reported with polyetherimide using dimethylformamide as a reducing agent to improve the mechanical and electrical properties of the film [39]. The ex-situ formed tyrosine-capped Ag nanoparticles were also incorporated into the PVA scaffolds (Fig. 1c, F4 and F6). Previously, we have reported that phenolic group of tyrosine acts as a reducing agent to form Ag nanoparticles, which are capped with the oxidized tyrosine molecules [40,41,42,43]. Moreover, it has been reported that the PVA nanofibers interact with the Ag nanoparticles through the OH groups present in the PVA [44]. When tyrosine-coated Ag nanoparticles were added to the PVA, the interaction might be stimulated through the formation of hydroxy-1,4-benzoquinone like structures. Further, it has been indicated that island like structures will be developed on the addition of citrate-coated Ag nanoparticles within the TiO2 sheets which are in nanometric size [45]. Since the interaction between Ag nanoparticles and TiO2 is directed by the characteristics of the surface coating of the nanoparticles, we can expect similar interaction between tyrosine and TiO2 which may influence the appearance of the PVA based films. In this study, the scaffolds were found to be stable at 50 °C for more than a year. Further, no visible aggregation was observed indicating the uniform distribution of the nanoparticles and TiO2 moieties within the scaffolds. During the incorporation, it was observed that polymer matrix thereby prevented the nanoparticles from aggregation [26].

The UV–visible absorbance spectra of Ag nanoparticles showed an SPR band with maxima at 424 nm, which is consequent to metallic Ag as shown in Fig. 1b. An additional absorbance feature at 273 nm is originating due to the molecular transitions within Ag-attached oxidized tyrosine. The UV–visible spectroscopy of Ag nanoparticles further indicated that there was no aggregation in the particles, and they were found to be stable over a year in the solutions under standard laboratory conditions (Fig. 1b). As shown in Fig. 1b, the solution color of the Ag nanoparticles was yellow, which is a typical characteristic of Ag nanoparticles. This further provided visible evidence that the nanoparticles are stable in nature, and there is no visible indication on aggregation.

The study of electrical and dielectric properties of such nanoparticles and TiO2 moieties incorporated in to the PVA scaffolds is important, and therefore, we have investigated these physical properties as well. The frequency dependence of dielectric constant, ε′, of pure PVA and PVA-nanocomposite scaffolds are presented in Fig. 2. All the scaffolds show higher values of ε′ at lower frequencies, and the values decreases with increasing frequency. This behavior is due to the interfacial relaxation. The ε′ values of PVA-nanocomposite scaffolds are greater than the pure PVA at all frequencies due to dipole–dipole interaction on addition of nanofillers. This also shows that the dielectric properties of PVA-nanocomposites can be tuned by varying the type and concentration of nanofiller. F3 sample has higher values of ε′ at all frequencies among the studied samples. However, the lower values of ε′ of F5 may be attributed to the restriction in the mobility of polymer chains due to the introduction of AgNO3-tyrosine-TiO2 in the PVA matrix. This shows the inability of polarizable units in nanocomposites to orient fast enough to keep up with the oscillations of an alternating electric field [46]. The variation of dielectric loss, ε′, and tangent loss, tan δ, with frequency is shown in Fig. 2b, c, respectively. It can be seen that both ε′ and tan δ decrease with increasing frequency, reach minimum, and again increase with further increase of frequency. The higher values of ε′ and tanδ at higher frequencies may be due to the space charge polarization with addition of nanofillers, macroscopic distortion, and higher interfacial polarization.

Fig. 2
figure 2

Variation in dielectric constant, ε′, with frequency of the pure PVA film and PVA based hybrid scaffolds (Panel A), variation in dielectric loss, ε′, with frequency of the pure PVA film and PVA based hybrid scaffolds (Panel B), Variation in loss tangent, tanδ, with frequency of the pure PVA film and PVA based hybrid scaffolds (Panel C), and variation in AC electrical conductivity, σAC, with frequency of the pure PVA film and PVA based hybrid scaffolds (Panel D)

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The frequency dependence of AC electrical conductivity, σAC, of pure PVA and PVA-nanocomposites is shown in Fig. 2d. The σAC increases linearly with increasing frequency of the applied electric field for all the samples. The combined effect of intrinsic dipole polarization and Maxwell–Wagner–Sillars (MWS) polarization explains the frequency dependence of σAC [47]. The sample F6 shows relatively higher values of σAC at all frequencies compared to the remaining composite scaffolds.

The photocatalytic degradation of malachite green in the presence of various scaffolds composition under the influence of UV light was investigated by observing the optical and spectroscopic changes in the dye solutions. The absorbance maxima in malachite green can be noted at 618 nm due to the existence of the linked benzene rings. When these benzene rings are simplified into by-products owing to the photocatalytic degradation effect by the F1-F6 films, the absorbance of the dye decreases, and therefore, the degradation of the dyes can be noted. The chemical structure of malachite green is displayed in the inset in Fig. 3a. The decrease in the absorbance is recorded, and values are represented in comparison with the original dye solution measured in time-dependent manner as shown in Fig. 3a. The malachite green was directly degraded under the UV light and was recorded as control. Other parameters that have an influence on degradation of dye are initial concentration of dyes, pH, type of photocatalyst, concentration of photocatalyst, and the reaction temperature, which were kept constant for comparison purpose [48].

Fig. 3
figure 3

Chemical structure of malachite green (MG) dyes. The optical image of malachite green at 0 h and post-treatment with different films after 7 h (Panel A). Percentage of malachite green degradation in the presence of PVA based scaffolds under UV treatment. The error bars are representative of the standard error. The scaffold dimension used is 1 * 1 cm2, and the dye concentration is 10 mg/L) (Panel B)

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It is interesting to observe that the UV-irradiated dye without the treatment of the films itself degraded to ~13% in first one hour, which increased post-treatment. It is interesting to observe that after 7 h, scaffold containing photocatalyst TiO2 (F2, F5, and F6) indicates higher degradation compared to those films which hold only Ag nanoparticles (F3 and F4). It is important to state that the scaffold F6 has the highest dye degradation capability, which may be attributed to photocatalyst TiO2 and pre-synthesized Ag nanoparticles composition. The degradation of malachite green by F6 is also clearly seen in the optical image where the dye solution is almost clear after 7 h treatment as shown in the insert of Fig. 3a. The pre-synthesized Ag nanoparticles incorporation with TiO2 and PVA (F6) has higher photocatalytic effect than pure PVA (p < 0.05), PVA-Ag+-tyrosine (p < 0.05), and PVA-TiO2 (p < 0.05). The basic mechanism of photocatalysis using TiO2 under UV irradiation is well documented [32, 49,50,51]. When UV light falls on the TiO2, electron-hole pairs are formed, and the positively charged holes encounter water molecules to produce H+ and hydroxy radicals (OH.), and the negatively charged electrons react with dissolved oxygen to form superoxide (O2−) ions, which sequentially reacts with water molecules to yield hydroxide ions (OH−) and peroxide radicals (OOH.). The holes oxidize OH− to OH,. and the peroxide radicals combine with H+ to form OH. and OH−. Due to the high energy of the UV, the recombination of the electron–hole pairs is prevented. Therefore, each species of the radicals (OH., OH−, O2−, OOH.) attacks the chromophores and degrades them [21]. The higher surface area and the lower binding energy of TiO2 may be the major contributor to dye degradation. It was captivating to observe that the degradation by F3 and F4 was comparable at different hours of treatment, which indicated that the in-situ synthesized (F3) and ex-situ synthesized (F4) Ag nanoparticles could not take part efficiently in malachite green degradation. However, the degradation efficiency increased due to the collaborative efforts of Ag nanoparticles and TiO2 as seen in raise in degradation efficiency in F5 and F6, respectively.

Since Ag+, Ag nanoparticles, and TiO2 are known to have antibacterial potential, we have further evaluated their antimicrobial capabilities toward Gram-negative bacteria (Escherichia coli). The antimicrobial efficiency of the polymeric scaffolds containing PVA, AgNO3, TiO2, tyrosine, and tyrosine-capped Ag nanoparticles is assessed in time-dependent manner, and the bacterial growth curve is constructed as shown in Fig. 4. The films containing Ag+ in the formulation are found to be the most effective, and a complete inhibition in the growth of Escherichia coli is observed. AgNO3 is a well-known for its antibacterial activity [52]; however, its usage is limited due to its extensive toxicity through bioaccumulation [53]. They exert their toxicity through depletion of glutathione and surpass the ROS capacity of the cell thereby leading to secondary damages to lipids, proteins, and DNA. Alternatively, Ag+ ions also activate the Fenton reactions leading to proliferation of ROS generation [54, 55]. The Ag nanoparticles embedded in the PVA scaffolds (F4 and F6) could not control the bacterial growth, which may be attributed to the restricted dissolution of Ag nanoparticles to Ag+ ions. Nevertheless, this work demonstrates that the suitable formulation of scaffolds with integration of active nanomaterials may provide opportunities in controlling industrial dye pollution, preventing the dissemination of pathogenic bacteria, and in energy storage or flexible electronics. Such scaffolds may additionally find their utility in water remediation due to their potential as photocatalyst and antimicrobials, which needs further investigations.

Fig. 4
figure 4

Bacterial cell growth for Escherichia coli after treatment with PVA based scaffolds and controls

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4 Conclusions

In summary, we report formulation of various PVA scaffolds integrated with Ag nanoparticles, TiO2, or reducing agent/metal ions, respectively. The importance of scaffold-integrated particles is established by investigating the dielectric properties, AC electrical conductivity, dye-degradation capacity, and antibacterial potential of these scaffolds. The results of the study suggest that appropriate homogenous amalgamation of nanoparticles or functionally active molecules within the PVA scaffolds can open new opportunities toward dye degradation, antimicrobials, energy storage, and flexible electronics.