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. 2021 Oct 28;6(44):29629–29640. doi: 10.1021/acsomega.1c03925

Catalytic Use toward the Redox Reaction of Toxic Industrial Wastes in Innocuous Aqueous Medium and Antibacterial Activity of Novel CuxAgxZn1–2xO Nanocomposites

Anik Sinha ‡, Sanjay Kumar Sahu †, Suman Biswas ‡, Manab Mandal §, Vivekananda Mandal §, Tanmay Kumar Ghorai †,*
PMCID: PMC8582044  PMID: 34778634

Abstract

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In this work, we report the redox properties in organic catalytic transformation and antibacterial activity of novel CuxAgxZn1–2xO nanocomposites. Cu- and Ag-doped ZnO [CuxAgxZn1–2xO (x = 0.1)] (CAZ), Cu-doped ZnO [CuxZn1–xO (x = 0.1)] (CZ), and Ag-doped ZnO [AgxZn1–xO (x = 0.1)] (AZ) were prepared via a chemical co-precipitation method. The synthesized nanocomposites were characterized using different spectroscopic techniques. The catalytic activity of CAZ, CZ, and AZ was examined for the reduction of 4-nitrophenol (4-NP) and 4-nitroaniline (4-NA) in the presence of NaBH4 in an aqueous medium. The photocatalytic oxidation efficiency of these catalysts was also observed against naphthol orange (NO) under ultraviolet light. It was found that the catalytic reduction and oxidation efficiency of CAZ is higher than that of CZ and AZ in 4-NP/4-NA and NO in a water solvent, respectively. The antibacterial property of CAZ was also studied against Gram-positive and Gram-negative bacteria by agar well diffusion and the minimum inhibitory concentration methods. It was found that CAZ shows better antimicrobial activity compared to its parental Cu(NO3)2·3H2O, AgNO3, and ZnO. Therefore, the incorporation of Cu and Ag into ZnO increases its catalytic and antimicrobial activity remarkably. Fourier-transform infrared and X-ray diffraction (XRD) studies of CAZ indicate the incorporation of Cu and Ag into the lattice of ZnO. The phase structure of CAZ was wurtzite hexagonal, and the average crystallite size was 93 ± 1 nm measured from XRD. The average grain size and particle size of CAZ were found to be 200 and 100 ± 5 nm originating from SEM and transmission electron microscopy studies, respectively. The optical energy band gap of CAZ is 3.15 eV, which supports the excellent photocatalyst under UV light. CAZ also exhibits good agreement for photoluminescence properties with a high intensity peak at 571 nm, indicating surface oxygen vacancies and defects which might be responsible for higher photocatalytic activity compared to others. The nanocomposite shows excellent reusability without any significant loss of activity.

Introduction

Nitroaromatic compounds (NACs) and azo dyes are considered the major industrial waste component due to their extensive use in chemical and agricultural industries. They are toxic and carcinogenic and a severe threat to human and aquatic life.1,2 4-Nitrophenol (4-NP) has a high toxicity level in water resources even at a very low concentration, and it is identified as a dangerous organic pollutant by the US Environmental Protection Agency.3 These NACs are highly soluble in water, and it is a challenge to remove them from water through biological and chemical methods due to their high stability.4 Thus, removal of the toxic pollutant becomes a significant target nowadays.5 A number of chemical and biological processes have been developed, such as detoxification, mineralization, and transformation of these toxic organic pollutants.6 Aqueous-phase conversion of this highly toxic 4-NP compound into 4-aminophenol (4-AP) compound is a very appealing strategy as water is the greenest solvent, and nonetheless, 4-AP is less toxic and readily biodegradable compared to 4-NP.7 One of the most popular methods of reducing 4-NP to 4-AP is using NaBH4 as a reducing agent. This reaction is thermodynamically favorable but kinetically unfavorable at room temperature due to the repelling of negative ions of 4-NP and BH4–.8 Thus, to overcome the large energy barrier of this reaction, a suitable catalyst is required, and nanocomposites are found to be suitable for this purpose. Most importantly, these aromatic amines act as an intermediate in producing various antipyretic and analgesic drugs, antioxidant and pharmaceutical dyes, photographic developers, corrosion inhibitors, and anti-corrosion lubricants.9−11 Therefore, scientists have paid attention to develop new catalysts which can readily convert these NACs into aromatic amine compounds.

In recent years, metal oxides were found to be very suitable in reducing NACs and degradation of dye’s which required mild reaction conditions and a decrease in products with high catalytic efficiency. Scientists are continuously trying to improve its reductive activity by doping it with foreign elements or altering reaction conditions. Various 3d/4d transition-metal oxide, such as SnO2,12 CdS,13 Fe2O3,14 ZrO2,15 CuO,16 TiO2,17 and ZnO18 have been reported in the reduction of 4-NP to 4-nitroaniline (4-NA) in the presence of NaBH4. Among these, ZnO attracts attention due to its low cost, high availability, low toxicity, and high stability and can be easily doped with foreign elements to improve catalytic and photocatalytic activity. Reduction of NAC in the presence of ZnO and NaBH4 is very slow, and reduction efficiency is very poor which leads to low yield. The reusability factor is also decreasing rapidly after each cycle. This is due to the large energy barrier of hydrogen transfer from BH4– to NAC. ZnO in the presence of NaBH4 is not able to overcome this barrier.19−22 Now to overcome this problem, ZnO is doped with various suitable foreign elements, especially metals which can easily cross the energy barrier and facilitate rapid hydride transfer to NAC and thus improve the reduction efficiency with relatively higher yield.

To improve the redox properties, reductive activity, and reusability against the reduction of 4-NP and 4-NA in the presence of a small amount of NaBH4 and naphthol orange (NO) oxidation, Cu- and Ag-doped ZnO (CAZ) has been adopted. In this work, CAZ [CuxAgxZn1–2xO (x = 0.1)], separately Cu-doped ZnO [CuxZn1–xO (x = 0.1)] (CZ), and Ag-doped ZnO [AgxZn1–xO (x = 0.1)] (AZ) nanocomposites were synthesized by the co-precipitation method. The different charges of copper/silver/zinc ions are satisfied by oxygen vacancies or defects in the composition. The synthesized nanocomposite was checked for the reduction of 4-NP in an aqueous medium.

In addition to industrial waste, pathogenic microbes had been a significant threat to humankind and the environment. Although there is a significant amount of development in antimicrobial agents, many infectious diseases are complicated to treat because of bacterial resistance, transportation to cell membrane, decrease in activity in cells, toxicity of antimicrobial agents (causes health damage), and so on.23−26 Nano-oxides like ZnO are emerging as therapeutics, diagnostics, and nanomedicine-based antimicrobial agents to overcome this problem. Scientists are continuously working to improve its antimicrobial property. Doping ZnO with foreign elements, especially 3d transition metals, was very effective in improving its optical,27 photoluminescence (PL),28 antifungal,29 and antimicrobial properties.30,31 In this work, we also investigated the antimicrobial property of CAZ against the Gram-positive bacteria (Bacillus cereus MTCC 1272, Enterococcus faecalis MCC 2041T, Staphylococcus aureus MTCC 96, Streptococcus mutans MTCC 497, and Staphylococcus epidermidis MTCC 3086) and Gram-negative bacteria (Escherichia coli MTCC 723 and Salmonella enterica serovar typhimurium MTCC 98) that are known to cause microbial hazards. The antimicrobial potency was also compared with that of its parent Cu(NO3)2, AgNO3, and ZnO compounds.

Results and Discussion

FT-IR Analysis

Fourier-transform infrared (FT-IR) spectra have been used to analyze the nature of bonds present in the prepared nanocomposite. FT-IR spectra of the synthesized CZ, CAZ, and AZ nanocomposite were recorded in the range from 450 to 4000 cm–1 and are shown in Figure 1. The presence of a band at 3476 cm–1 indicates the stretching vibration OH group due to the absorbed water molecule on the surface of the nanocomposite. The weak band near 1627 cm–1 is assigned to H–O–H bending due to moisture adsorption. The peaks at 2430, 1435, and 1384 cm–1 are due to the atmospheric CO2 and C=O bonds present in the instrument. The absorption peak at 469 cm–1 may be related to the absorption peak of ZnO, which originates from the stretching mode of the Zn–O bond.32 The peak at 520 cm–1 may be due to the stretching mode of the Ag–O bond.33 The peak around 612 cm–1 may be due to the Cu–O bond.34

Figure 1.

Figure 1

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FT-IR spectrum of CZ, CAZ, and AZ nanocomposites.

XRD Analysis

The X-ray diffraction (XRD) technique was used to determine and confirm the crystal structure of CAZ, AZ, and CZ nanocomposites (Figure 2). Samples were heated at temperature (200 °C) for 6 h, then XRD was taken. The diffraction peaks detected in CAZ and AZ at 2θ = 38.20 and 64.72° are assigned to the (111) and (220) crystal planes corresponding to the face-centered cubic phase of Ag (JCPDS file no: 01-089-3722).35 Simultaneously, the peak at 2θ = 37.21° corresponds to the (111) plane of AgO in AZ (JCPDS no. 89-3081).35 The diffraction peaks observed in CAZ and CZ at 2θ = 35.82, 43.92, and 51.39° are due to the (111) and {(111), (200)} crystal planes of the face-centered cubic phase of Cu2O and Cu, respectively (PDF file no: 01-085-1326).36 The characteristics diffraction peaks observed in CAZ, CZ, and AZ at 2θ = 32.66, 34.55, 36.78, 47.11, 58.53, 68.37, and 69.65° corresponded to planes of the (100), (002), (101), (102), (110), (200), and (112), respectively, assigned to the hexagonal geometry of ZnO (JCPDS card no: 36-1451).37 The diffraction peaks of CAZ at 2θ = 23.59, 25.03, and 26.52° corresponded to planes of (111), (223), and (134) indicating to the minimal amount of unreacted Zn(OH)2 (JCPDS card no: 47-965).38 The crystalline size of CAZ is 93 ± 1 nm measured using the Scherer method at the (002) plane or the maximum intense peak.

Figure 2.

Figure 2

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XRD of CAZ, AZ, and CZ nanocomposites.

SEM Analysis

Scanning electron microscopy (SEM) studies on CAZ were performed to gain information about the average grain size and morphology of the nanocatalyst. Figure 3A,B represents SEM images of CAZ at different resolutions at 10k× and 20k×, respectively. The average grain size of CAZ nanoparticles is found to be ∼200 nm at 20k× resolution. The SEM image shows that they are arranged in a well-ordered manner and seen as beautiful flex-type materials.

Figure 3.

Figure 3

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(A,B) SEM image of the CAZ nanocomposite at different magnifications.

TEM Analysis

Transmission electron microscopy (TEM) is a significant technique for directly imaging nanoparticles to obtain the quantitative measurement of particle size, size distribution, and shapes. The bright-field electron micrograph (model Philips TM-30, Philips Research Laboratories) of the CAZ is produced at 200 °C reflects an almost distorted spherical particle shape, with average particle sizes of 100 ± 5 nm (Figure 4a–c). The particle sizes of CAZ nanoparticles are distinct in shape, separated by well-defined boundaries, and are clearly visible, as shown Figure 4c. Figure 4d shows the enlarged view of one particle of Figure 4c which looks like having almost a square shape and of mesoporous nature; as a result, CAZ acts as a good nanocatalyst for the organic transformation reaction and antimicrobial activity.

Figure 4.

Figure 4

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TEM of the CAZ nanocomposite (a–c) in the normal mode and (d) enlarged single particle of (c).

Catalytic Activity

Reduction Reaction of Aromatic Nitro Compounds (4-NP and 4-NA)

The catalytic model reaction (reduction/hydrogenation) of 4-NP to 4-AP and 4-NA to 4-phenyldiamine (4-PDA) and schematic presentation of the conversion reaction 4-NP to 4-AP were performed using nanocatalysts and NaBH4, as shown in Figure 5A,B; C,D,E, respectively. The catalytic reaction was investigated using an UV–vis spectrophotometer. To study this reaction, the 0.02 g of the nanocatalyst and 0.02 g of NaBH4 were added into 30 mL of 4-NP/4-NA in each set of the reaction. Then, a small volume of reactant liquid was siphoned out in a quartz cell, and the diffusion rate of 4-NP/4-NA was measured by UV–vis spectroscopy at a constant time intervals. In an aqueous medium, 4-NP is dissolved slowly, forming a light yellow solution. After adding 0.02 g of NaBH4 into the aqueous solution of 4-NP, its color changes from light yellow to intense yellow due to the formation of a 4-nitrophenolate ion. Initially, the absorbance peak of 4-NP and 4-NA appeared at wavelengths 405 and 379 nm with absorbance values of 1.78 and 1.15, respectively. After the addition of the nanocatalyst, the yellow color of 4-NP and 4-NA fades away slowly with the progress of the reaction, and the rate of the reduction reaction is monitored by UV–vis spectrophotometry. The maximum absorbance peak of 4-NP and 4-NA successively decreases with time and disappears, simultaneously, a new characteristic peak appears at around 300 and 286 nm, which indicates the formation of 4-AP and 4-PDA, respectively. The reduction reactions of ANCs completed in the presence of CAZ within a few minutes, that is, 10 and 15 min were taken for conversion of 4-NP to 4-AP and 4-NA to 4-PDA (as shown in Figure 5A,B; C,D). The catalytic reduction of other composites was also studied in the same procedure, and it was found that CZ, AZ, and ZnO took much more time (Table 1). Therefore, the reduction efficiency of CAZ is 3 and 2 times higher than that of CZ and 5 and 3 times higher than that of AZ against 4-NP and 4-NA, respectively, and their corresponding reaction rate constant is illustrated in Table 1.

Figure 5.

Figure 5

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Plot of absorbance (Co/C) vs irradiation time, to determine the reaction rate constant and reduction efficiency of CAZ, CZ, AZ, and ZnO nanocatalysts in the presence of (A) 4-NP and NaBH4 and (B) corresponding UV–vis absorption of 4-NP to 4-AP at a small time interval (2 min) in the presence of CAZ and NaBH4; (C) 4-NA and NaBH4 and (D) corresponding UV–vis absorption of 4-NA to 4-PDA at a small time interval (5 min) in the presence of CAZ and NaBH4; and (E) schematic model conversion reaction of 4-NP to 4-AP.

Table 1. Reaction Rate Constant of Reduction of 4-NP and 4-NA and Oxidation of NO in Presence of CAZ, CZ, AZ, and ZnOa.
    reaction rate constant k (×10–3 min–1)
 complete degradation time (in min)
nanocomposite acronym 4-NP 4-NA NO MB 4-NP 4-NA NO MB
CuxAgxZn1–2xO (x = 0.1) CAZ 109.97 71.61 265.42   10 15 12 no change up to 120
CuxZn1–xO (x = 0.1) CZ 36.58 43.39 132.80   14 25 28  
AgxZn1–xO (x = 0.1) AZ 19.8 22.8 45.62   18 40 72  
ZnO   14.3 12.2 25.6          
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a

The reaction rate measured after 50% degradation of dyes.

Oxidation of NO

The photocatalytic efficiency of CAZ, CZ, AZ, and ZnO was investigated against oxidation/degradation of NO and methylene blue (MB) under UV light with a fixed wavelength of 265 nm. It was observed that in the presence of catalysts and UV light that the oxidation reaction occurs on the anionic dye NO, whereas no degradation was observed on the cationic dye MB up to 120 min examined from UV–vis spectroscopy measurements. Therefore, the UV–visible spectra of NO were measured at constant interval time after irradiation with nanocatalysts and UV light, and the corresponding absorption peak at 484 nm decreased with the progress of the reaction time. The degradation rate of NO in the presence of CAZ is very much faster compared to other catalysts, within 12 min it completely degraded, as shown in Figure 6A, and corresponding changes in concentrations of NO were exposed, as shown in Figure 6B. It was found that the photocatalytic degradation efficiency of CAZ is almost 2 times faster than CZ and 5.81 times faster than AZ, and the rate constant of the dyes in the presence of the catalyst is shown in Table 1. The results interpret that anionic dyes in diverse wastewater followed the Freundlich pathway, and the mechanism of anionic dye elimination has been stuck by hydrophobic interactions on account of the organic phase designed by cationic dyes in the interlayer.

Figure 6.

Figure 6

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(A) Reaction rate constants of NO in the presence of CAZ, CZ, AZ, and ZnO and UV light. (B) Changes in concentrations of NO in the presence of CAZ at constant time interval at 484 nm.

Optical Analysis

The electronic absorption spectrum of CAZ, AZ, and CZ nanocomposites in the UV–vis range enables us to characterize the absorption edge and band structure shown in Figure 7A. The optical band gap energy of the photocatalyst was calculated using Tauc’s formula according to eq 1

graphic file with name ao1c03925_m001.jpg 1

where α denotes the absorption coefficient, A is a constant, Eg indicates the band gap energy, exponent n depends on the types of transition, h denotes Planck’s constant, and υ is the frequency of light.

Figure 7.

Figure 7

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(A) UV–visible absorption spectra of CAZ, CZ, and AZ, and the inset figure presents the binding energy plot of CAZ and (B) PL spectra of CAZ, CZ, and AZ.

The calculated energy band gap value of the CAZ photocatalyst is 3.15 eV, and it is 3.20 and 3.23 eV for CZ and AZ, respectively (Figures S1 and S2). This result is in agreement with our experimental results for photo-oxidation of NO, which indicates that CAZ is photocatalytically more active than CZ and AZ.

To investigate the effects of CAZ, emission spectra show PL properties at room temperature with an excitation wavelength at 284 nm for all the composites of CAZ, AZ, and CZ (Figure 7B). A single emission band was observed at 571 nm that is in the visible region for all nanocomposites. Several intrinsic and extrinsic defects could be responsible for this visible emission. The intensity of peaks is different for all composites. CAZ nanocomposites have a higher PL intensity peak compared to CZ and AZ, which indicates greater content of surface oxygen vacancies with defects, and might be responsible for higher photocatalytic activity.39 Many emission peaks of very small intensity appeared in the spectra, which are due to the incorporation of Cu and Ag in the ZnO lattice.

Proposed Mechanism for Oxidation and Reduction Reactions

The proposed mechanism for the oxidation of NO and reduction of NACs in the presence of nanocomposites is schematically represented in Figure 8. It is well known that NaBH4 acts as a hydrogen source that liberates hydrogen gas by hydrolysis at room temperature, and this process can be catalytically accelerated by CAZ. In the present work, the possible mechanism for the reduction of 4-NP/4-NA is hydride transfer from BH4– to NACs via CAZ nanocomposites.40,41 The intense color of 4-NP is increased after adding NaBH4 into the aqueous solution because of the formation of a nitrophenolate ion. As CAZ may act as a cationic surface, which is capable of adsorbing BH4® and nitrophenolate ions on its surface, the catalyst helps this reduction process by conveying electrons from the donor BH4® ions to the acceptor 4-nitrophenolate ions. The product 4-amino phenol/4-phenylenediamine is neutrally charged. Therefore, it gets quickly released from the surface of the CAZ nanocatalyst. Notably, the reaction rate is much higher in the case of CAZ compared to CZ, AZ, and ZnO. Simultaneously, NO is a negative dye that interacts with the surface of the CAZ photocatalyst, and it oxidizes NO to degradation products through the Freundlich Phenton pathway (Figure 8).

Figure 8.

Figure 8

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Schematic representation of oxidation of NO and reduction of NACs by the CAZ nanocomposite.

Recyclability of the CAZ Nanocomposite

The recyclability of the CAZ nanocomposite was studied to perform the reusing ability toward reduction of 4-NP and 4-NA and degradation of NO. After the first cycle, the nanocomposite was recovered through centrifugation, and the collected nanocomposite was extensively washed with deionized water and ethanol to remove the trace amounts of 4-NA, 4-NP, and NO. The collected nanocomposite was dried in a hot air oven for 8 h, it was then allowed to react for the second cycle, and the procedure was repeated for the next cycle. The percentage of reduction in the efficiency of the catalyst was found to be from 94 to 85, 92 to 88, and 90 to 83 for 4-NP, 4-NA, and NO, respectively, after reusing it four times (Figure 9). This result interprets that the CAZ nanocomposite not only has an excellent reduction efficiency but also significant degradation and reusing ability.

Figure 9.

Figure 9

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Recycling of the CAZ nanocomposite.

To investigate the stability or any structural change of nanocomposites after the reaction, XRD and FT-IR of the CAZ nanocomposites after the fourth catalytic cycle were carried out. As shown in Figure 10A,B, there is no notable change observed after the fourth catalytic cycle from XRD and FTIR analyses, which indicates the stability of nanocomposites.

Figure 10.

Figure 10

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(A) XRD and (B) FT-IR spectra of CAZ before and after the fourth catalytic cycle.

Antibacterial Activity

Antibacterial Potentiality of the Nanocomposite

It was found that doping Cu and Ag increases the antibacterial property of ZnO (Tables 2 and 3, Figure 11). Furthermore, the nanocomposite has significantly lower minimum inhibitory concentrations (MIC) values than the parental compound and even with the ZnO nanoparticles (Table 3). The dose-dependent time-kill kinetics of the CAZ nanocomposite against E. coli and S. mutans, at MIC doses of 7.5 and 5.0 μg/mL, respectively, has reduced the cfu values to a significantly low amount compared to the untreated one (Table 4).

Table 2. Spectrum of Antimicrobial Activity of the CuxAgxZn1–2xO (x = 0.1) Nanocomposite against the Gram-Positive and Gram-Negative Bacterial Strainsa.
SI no. bacterial strains strain’s properties growth condition CAZ nanocomposite (10 μg/mL) antibiotics streptomycin (50 μg/mL) activity index
1 B. cereus (Gram-positive) pathogen of food-borne illnesses TSB, 37 °C 15.0 ± 0.46 19.0 ± 0.70 0.789
2 E. coli (Gram-negative) GP, colicin indicator strain TSB, 37 °C 15.5 ± 0.38 20.0 ± 0.76 1.333
3 E. faecalis(Gram-positive) gastrointestinal, causing abdominal infections NB, 37 °C 14.6 ± 0.41 18.5 ± 0.61 0.789
4 S. aureus (Gram-positive) pathogenic, antibiotic-sensitive strain TSB, 37 °C 13.0 ± 0.49 20.0 ± 0.76 0.65
5 S. epidermidis (Gram-positive) air-borne skin-infecting pathogen TSB, 37 °C 15.0 ± 0.45 19.0 ± 0.69 0.789
6 S. mutans (Gram-positive) dental caries pathogen BHI, 37 °C 13.4 ± 0.37 19.5 ± 0.70 0.687
7 S. enterica serovar typhimurium (Gram-negative) gastroenteritis pathogenic prototrophic strain TSB, 37 °C 16.0 ± 0.28 20.0 ± 0.76 0.80
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a

The values indicate the diameter of inhibition zone (mm).

Table 3. MIC of the Synthesized CAZ Nanocomposite and Parental Compounds [Silver Nitrate (AgNO3), Copper Nitrate (CuNO3), and ZnO] at Different Concentrations against the Gram-Positive and Gram-Negative Bacterial Strainsa.
  concentration of CAZ nanocomposite (μg/mL)
 concentration of AgNO3(μg/mL)
 concentration of Cu(NO3)2·3H2O (μg/mL)
 concentration of ZnO (μg/mL)
strains 7.5 5.0 2.5 7.5 5.0 2.5 7.5 5.0 2.5 7.5 5.0 2.5
B. cereus 13.0 ± 0.42 11 ± 0.41 10 ± 0.22 8.5 ± 0.12 6.5 ± 0.22 0 10 ± 0.22 8 ± 0.12 6 ± 0.12 11 ± 0.12 9 ± 0.22 8 ± 0.32
E. faecalis 12.5 ± 0.12 10 ± 0.42 7 ± 0.31 9.5 ± 0.11 0 0 10 ± 0.12 8 ± 0.08 0 10 ± 0.12 0 0
S. aureus 11 ± 0.22 10 ± 0.45 0 8 ± 0.42 7 ± 0.42 6 ± 0.08 0 0 0 0 0 0
S. epidermidis 14.5 ± 0.12 10 ± 0.18 8 ± 0.24 10 ± 0.32 8 ± 0.12 6 ± 0.32 10 ± 0.12 8 ± 0.32 6 ± 0.12 16 ± 0.22 15 ± 0.44 13 ± 0.41
S. mutans 14 ± 0.48 12 ± 0.37 10 ± 0.41 10 ± 0.32 8 ± 0.14 7 ± 0.09 11 ± 0.14 8 ± 0.13 0 10 ± 0.24 8 ± 0.14 6 ± 0.10
E. coli 14 ± 0.12 10 ± 0.32 8 ± 0.12 9 ± 0.17 7 ± 0.15 0 0 0 0 9 ± 0.14 7 ± 0.14 0
S. enterica serovar typhimurium 15 ± 0.24 11.5 ± 0.14 9 ± 0.24 9 ± 0.14 7.5 ± 0.32 0 10 ± 0.14 7 ± 0.24 0 0 0 0
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a

The values indicate the diameter of inhibition zone (mm).

Figure 11.

Figure 11

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Antibacterial potency of the nanocomposite over its parental molecules. Here, the figures in the parental and synthetic compounds are the diameter of inhibition zones (mm).

Table 4. Dose-Dependent Time-Kill Assay of the CAZ Nanocompositea.
  bacterial strains
  E. coli MTCC 723
 S. mutans MTCC 497
treatment concentration 2× 4× 6× 8× 2× 4× 6× 8×
treated viable cells (log cfu/mL) after 10 h 280 × 104 198 × 103 122 × 102 55 × 102 1400 × 104 424 × 103 272 × 102 18 × 102
untreated viable cells (time) 30 min 90 min 300 min 600 min 30 min 90 min 300 min 600 min
untreated viable cells (cfu/mL) 1043 × 104 1443 × 104 124 × 106 451 × 108 843 × 104 1013 × 104 154 × 106 651 × 108
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a

Here, time—3 h. x = MIC dose. For E. coli and S. mutans, MIC doses are 7.5 and 5.0 μg/mL, respectively.

MIC and Dose-dependent Time-Kill Kinetics of the CAZ Nanocomposite

Multidrug-resistant microorganisms are a growing challenge, and the use of nanoparticles is among the most promising strategies to overcome microbial drug resistance. Studies observed that the nanoparticles could overcome drug resistance mechanisms, like decreased uptake of antibacterials, increased efflux of drug from microbial cells, and biofilm formation. In a study of multidrug-resistant Gram-positive and Gram-negative bacteria from clinical sources, Cavassin et al. observed that among the silver nanoparticles stabilized with citrate, chitosan, and polyvinyl alcohol (PVA), and commercial silver nanoparticles, better results were achieved with citrate and chitosan silver nanoparticles than PVA-stabilized particles at a concentration of 6.75 μg mL–1.42 The reason behind such higher efficacy might be due to the lower stability of those citrate and chitosan silver nanoparticles and, consequently, the release of Ag+ ions to exert their action. However, Kar et al. studied the Cu/Ag nanoparticle-loaded mullite nanocomposite system for antibacterial potentiality (S. aureus MTCC-96 and E. coli MTCC-1652).43 They observed that these nanocomposites at a 1–10 mg/mL concentration have excellent antibacterial activity against Gram-positive and Gram-negative bacteria. The treatment causes high intracellular oxidative stress in the bacterial cells exposed. Moreover, the nanocomposites exhibit good cytocompatibility at a concentration of 1 mg/mL (MBC) with wound healing characteristics in mouse fibroblast cell line (L929). Recently, Jang et al. synthesized bimetallic nanoparticles on a graphene oxide (GO) surface (Ag/Cu/GO) using a chemical reduction method and tested their antibacterial effects against several bacterial species like Methylobacterium spp., Sphingomonas spp., and Pseudomonas aeruginosa and also in vitro cytotoxicity in human dermal fibroblasts.44 The study revealed that the synthesized Ag/Cu/GO nanocomposite could kill all three bacterial species at a harmless concentration to human cells. In addition, it also successfully removed a biofilm of P. aeruginosa in a microchannel with a dynamic flow and healed the skin wound quickly and efficiently by the topical application. They conclude that the Ag/Cu/GO nanocomposites could be used to effectively remove antibiotic-resistant bacteria and treat diseases in the skin or wound due to bacterial infections and biofilm formation. Thus, in comparison to similar findings, it appears that the present CAZ nanocomposite could be trialed for such in vitro and in vivo studies in aid of humankind. It could be trialed in nanotechnology-based drug delivery systems to treat emerging multi-drug resistant pathogens and several invasive diseases like pneumonia, tuberculosis, deep mycoses, and even viral diseases.45−50 In a study, Gavel et al. (2018)51 reported that the nanofibrillar Amoc (9-anthracenemethoxycarbonyl)-capped dipeptide hydrogels have antibacterial properties against Gram-positive Bacillus subtilis and S. aureus and Gram-negative E. coli, P. aeruginosa, and Salmonella typhi bacteria with the MIC50 in the range of 10–200 μM concentrations. In addition, the preparation has wound healing properties in the experimental mice model (Gavel et al.).52 However, compared to these studies, our present investigation shows that the CAZ nanocomposite had MIC doses against E. coli and S. mutans of 7.5 and 5.0 μg/mL, respectively. Thus, this nanocomposite has superior antibacterial efficacy against both enteric and topical pathogenic bacteria.

Proposed Mechanism for Antibacterial Activity

The most popular method of disinfection of bacteria is the generation of reactive oxygen species in the medium. In aqueous solvent, the CAZ nanocomposite reacts with water molecules to form various reactive oxygen species through a Fenton-type reaction (Figure 12). These species irreversibly damage bacterial cell membranes, DNA, and proteins, which causes bacterial death.47,48 In addition to that, CAZ nanocomposites generate Zn2+, Cu, Cu+, Cu2+, and Ag+ ions in the medium, which act as cytotoxin to bacteria. By altering the DNA synthesis, these ions inhibit the enzymatic system of the respiratory chain.49 These ions also enter in the cells, causing damage to their membranes, DNA, and other species, which causes bacterial death.50

Figure 12.

Figure 12

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Multiple possible pathways via CAZ shows antibacterial action.

Conclusions

In summary, we report the facile and green method of synthesizing CAZ, CZ, and AZ nanocomposites, via the co-precipitation method. The structure, morphology, and particle sizes of the synthesized CAZ NPs were characterized by XRD, FTIR, SEM, and TEM analyses. The XRD pattern indicates well crystalline and wurtzite hexagonal structures of nanocomposites. SEM analysis reveals the size of CuxAgxZn1–2xO (x = 0.1) in the range of 200 nm with a beautiful flex-type structure, and from TEM measurements, CAZ nanoparticles are in distinct spherical shape with 100 ± 5 nm size. The catalytic activity of the CAZ catalysts against the reduction of 4-NP and 4-NA was observed in the presence of NaBH4, and it has been found that CAZ shows the highest reduction efficiency in comparison to other nanocomposites. The reduction efficiency of CAZ is 94.26% in 4-NP and 92.27% in 4-NA, and the degradation efficiency is 90.2% in NO. The energy band gap of CAZ is 3.15 eV, which shows excellent photocatalytic activity under UV light compared to CZ and AZ. CAZ also exhibits good agreement for PL properties with a high-intensity peak at 571 nm, indicating surface oxygen vacancies and defects. The antibacterial activity of CAZ was also investigated, and it showed excellent antimicrobial activity against several Gram-positive and Gram-negative bacteria. Our results show that CAZ can be a promising material of choice for various practical applications in catalysis and industrial and antibacterial applications. Furthermore, we have also investigated the typical organic transformation reaction like Knovenegal, hydrogenation, and so on using this catalyst.

Experimental Section

Materials and Characterizations

Cu(NO3)2·3H2O (analytical reagent), AgNO3 (analytical reagent), ZnO(analytical reagent), NaBH4 (analytical reagent), dilute HNO3, NH4OH, EtOH, 4-NP, 4-NA, and NO (analytical grade) were important analytic reagents for this experiment. All aqueous solutions were prepared using distilled water.

Organic catalytic reduction and photo-oxidation of the various synthesized nanocatalysts were compared, and based on the highest activity, CAZ was used for further characterization studies. The crystal structure of CAZ was measured by XRD at room temperature using PANalytical X’Pert3 Powder, equipped with Cu Kα (1.54060 Å) as the incident radiation. The Scherrer equation D = Kλ/β cos θ was used for the calculation of crystallite size, where K = 0.9, D = crystal size (Å), λ = wavelength of Cu Kα radiation, and β = corrected half-width of the diffraction peak. FT-IR spectra analysis was carried out at room temperature using a PerkinElmer Paragon 1000 FT-IR spectrometer. The fine structure of the prepared sample was analyzed using a field emission scanning electron microscope (Carl Zeiss Germany, model Supra-40) and a transmission electron microscope (model Philips TM-30, Philips Research Laboratories). A UV–visible spectrometer (UV-1800, Shimadzu) was used for the measurement of absorption spectra. A fluorescence spectrophotometer (PerkinElmer FL 6500) was used to study PL measurements at room temperature.

Synthesis of CuxAgxZn1–2xO (x = 0.1)(CAZ), CZ and AZ Nanocomposites

The CuxAgxZn1–2xO (x = 0.1) nanocomposite were prepared by the co-precipitation method. For this purpose, the prerequisite amount of Cu(NO3)2·3H2O and AgNO3 was dissolved in 25 mL of distilled water separately. Since ZnO is sparingly soluble in water thus dropwise, a small amount of HNO3 was added to the solution until it completely dissolved. Then, ZnO, Cu(NO3)2·3H2O and AgNO3 solution were poured in a beaker, and the resulting mixture was stirred for 5 min with the help of a magnetic stirrer. Now, 6 mL of ethanol was added to the solution and stirred well. After that, an aqueous ammonia solution was added dropwise to the mixture until complete precipitation appears with constant stirring at pH 8–9. The precipitate was then filtered and washed with deionized water until it is free from alkali and then dried at 200 °C in a hot air oven for 6 h. The resulting solid was crushed in a pestle. In the same way, we prepared CuxZn1–xO (x = 0.1) and AgxZn1–xO (x = 0.1) nanocomposites for comparative analysis. The total synthesis for the preparation of CuxAgxZn1–2xO (x = 0.1) is presented in the flowchart diagram (Figure S3), and corresponding color images of nanocomposites are shown in Figure S4.

Redox Reaction of the Catalytic Organic Transformation Experiment

The redox phenomenon of CAZ, CZ, and AZ was studied against reduction and oxidation efficiency of (i) 4-NP and (ii) 4-NA in the presence of NaBH4 and (iii) NO under UV light, respectively. All the experiments are examined at room temperature. Reaction solutions of each set were prepared by adding the nanocatalyst (0.02 g) with 0.02 g of NaBH4 into 30 mL of 4-NP and 30 mL of 4-NA solution. For the oxidation of NO, about 0.05 g of the prepared nanocatalyst was added to 30 mL of standard NO dye solution and exposed to UV light. In different cases, a small volume (2 mL) of reactant liquid was siphoned out at a regular interval time of 2, 5, and 2 min of 4-NP, 4-NA, and NO, respectively, to determine the concentration of organic compounds along with dye measured by UV visible spectrophotometry at its wavelength of maximum absorption. All the reactions follow first-order kinetics. The following equation, Co/C = kt, determines the rate constant for this reaction (where Co = initial concentration at “0” time and C = concentration at a time “T”). The value of k was calculated from the graph between ln Co/C and time interval where Co and C denote the concentration of NO dye at time t = 0 and t = t, respectively.

Antibacterial Activity

Determination of the Antibacterial Activity of the Nanocomposite by the Agar Well Diffusion Method

The antibacterial potentiality of the CAZ nanocomposite was measured by the agar well diffusion method53 against some common Gram-positive bacteria (B. cereus MTCC 1272, E. faecalis MCC 2041T, S. aureus MTCC 96, S. mutans MTCC 497, and S. epidermidis MTCC 3086) and Gram-negative bacteria (E. coli MTCC 723 and S. enterica serovar typhimurium MTCC 98), procured from Microbial Type Culture Collection (MTCC), IMTECH, Chandigarh and Microbial Culture Collection (MCC), Pune, India. The strains were cultured in their respective media. For the antibacterial assay, respective bacterial strains were grown in the sterilized nutrient broth medium (NB) for 15 h 37 °C in a BOD incubator. 10 μL of active culture was suspended in 200 μL of sterile water and spread on the pre-poured nutrient agar (NA) plates using a sterile spreader. Five mm diameter agar cups were cut using a sterile cork borer, and 30 μL of the test samples [CAZ nanocomposite (10 μg/mL)] was loaded into the well and incubated for 24 h at 37 °C and observed for the zone of growth inhibition. The assay was compared with a standard antibiotic, streptomycin (50 μg/mL), as a positive control. The activity index of the compound was calculated by the following formula: activity index = (zone of inhibition of extract/zone of inhibition of antibiotic).

Determination of MIC and Dose-Dependent Time-Kill Assay

The different concentrations ranging from 7.0 and 5.0, to 2.5 μg/mL of the nanocomposite sample CAZ and its parental compounds comprising copper nitrate [Cu(NO3)2·3H2O], silver nitrate (AgNO3), and zinc oxide (ZnO) were prepared using deionized water as diluents and were used for MIC determination following the protocol of Pathak et al.54 on the sensitive bacterial strains. 30 μL of the test samples was loaded into the well and incubated for 24 h at 37 °C in a BOD incubator and observed for the zone of growth inhibition. The MIC is defined as the lowest concentration where a positive growth inhibition was observed.

The dose-dependent bactericidal kinetic assay was performed in NA plates. The nanocomposite samples at a multitude higher concentration (2× to 8×) of the MIC values were applied to the actively grown bacterial strains (log phase growth) of E. coli MTCC 723 and S. mutans MTCC 497 and incubated in a BOD incubator at 37 °C for 10 h. At different times of incubation (0–24 h, 3 h interval), 100 μL of the treated culture was taken out and serially diluted at 10–2 or 10–4 times and spread on NA plates in triplicate, incubated at 37 °C for 24 h, and observed for no viable colony development. The viable counts were calculated to give log cfu/mL (cfu is a colony-forming unit), and kill curves were plotted with time (up to 10 h) against the logarithm of the viable count. A constant decrease in the logarithmic rate of kill has been defined as a bactericidal effect.

Acknowledgments

This work was fully supported by the Madhya Pradesh Council of Science & Technology, Govt. of India, Madhya Pradesh (file no. A/R&D/RP-2/Phy&Engg./2017-18/271), Indira Gandhi National Tribal University, Amarkantak, Madhya Pradesh, India, the University of Gour Banga, Malda, WB, India, West Bengal State University, Barasat, Kolkata, India, and the University of Calcutta, WB, India. The authors are also thankful to DST-FIST [file no. SR/FST/CS-I/2017/2(C)] for financial support to the development of the Instrumental facility at Department of Chemistry, IGNTU and CIF of IGNTU.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c03925.

  • Energy band gap plot, flowchart diagram, and images of CAZ, AZ, and CZ nanocomposites (PDF)

Author Contributions

All experiments and applications were performed by A.S., S.K.S., and M.M. under supervision of T.K.G. and V.M. The manuscript was written by A.S. and designed and corrected by T.K.G. and S.B.

The authors declare no competing financial interest.

Supplementary Material

ao1c03925_si_001.pdf (211KB, pdf)

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