MnO₂ Doped with Ag Nanoparticles and Their Applications in Antimicrobial and Photocatalytic Reactions

Abstract

A wide range of nanoparticles have been produced for photocatalysis applications. Nonetheless, degrading organic dyes requires nanoparticles that are efficient and excellent. As a photocatalyst, pure manganese oxide (MnO₂) was prepared via a sol–gel method using silver (Ag) nanoparticles of transition metal oxide. In addition to X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX), the crystal structure and elemental composition were analysed. According to XRD data, the transition metal of MnO₂ oxide is highly pure and has a small crystallite size. The presence of functional groups was confirmed and clarified using Fourier-transform infrared spectra (FTIR). By irradiating the transition pure and doped MnO₂ photocatalysts with visible light, the UV-vis, μ-Raman, and surface areas were determined. As a result, of using the photocatalysts with aqueous methylene blue (MB) solutions under visible light irradiation, the MnO₂ doped with Ag nanoparticles demonstrated high degradation efficiencies and were utilised to establish heterogeneous photocatalysis dominance. In this paper, we demonstrate that the photocatalytic efficiency of transition metal oxides is exclusively determined by the particle size and surface area of nano-sized materials. Due to their high surface charge ratio and different surface orientations, have the highest photocatalytic efficiency. Generally, MnO₂ doped with Ag nanoparticles is resistant to bacteria of both Gram-positive and Gram-negative types (B. sublittus and Escherichia coli). There is still a need for more research to be performed on reducing the toxicity of metal and metal oxide nanoparticles so that they can be used as an effective alternative to antibiotics and disinfectants, particularly for biomedical applications.

1. Introduction

As the century progresses, environmental degradation will become more and more prevalent. There is an increase in both the variety and quantity of pollutants, from nuclear waste to microplastics. As we move into the new era of science and technology, nanoparticles have indeed proven the ability of researchers to harness the ‘science of small’ to achieve greater industrial revolutions, especially in the area of electronics. In all spheres of life, it has a positive impact on every aspect of life due to its significant global market share and salutary impact [1]. When particles are reduced to the nano scale, their properties change drastically, both physically and chemically. By controlling the properties of materials at the nanoscale level rather than by changing their composition, new techniques are being developed to control the properties of materials. Due to their distinct optical and chemical properties, semiconductor nanoparticles have attracted much attention in recent years due to their exciting properties, including tunable bandgaps as a function of size, nonlinear optical properties, and luminescence [2]. As a result, these nanoparticles are suitable for a wide range of electronic applications, such as sensors, photocatalytic reactions, and optoelectronic devices.

In the environment we live in, there are many pollutants, such as alcohols, carboxylic acids, amines, herbicides, and aldehydes, that can be degraded with great efficiency by photocatalysis. Photocatalysis is a novel approach and a requisite tool that exhibits high-efficiency degradation of pollutants. Semiconductor nanoparticles play a significant role as photocatalysts, since their enhanced properties enable them to destroy pollutants through chemical reaction [3]. Thus, tuning the morphology of nanomaterials requires understanding of their growth process and parameters. In recent years, semiconductor nanoparticles have attracted much more attention due to their quantum confinement effect, since researchers across the globe are working on this in order to create a sustainable future. As a result of strong interactions between hole–electron (e-/h+) pairs generated by excited photons, semiconductors might exhibit quantum confinement effects when the size of the nanoparticles exceeds the Bohr radius exciton [4]. In biomedical sciences, the synthesis of transition metal and metal oxide nanoparticles is becoming more prominent [5]. Metal oxide-linked semiconductor photocatalysts have attracted a lot of press regarding their usage in environmental remediation, where they are utilised degrade contaminants in the air and water [6]. Coloured wastewater results in ecological effects, caused by increasing concentrations of dyes are substances with a wide range of applications, primarily for colouring the final products of textiles, precious stones, leather, paper, plastics, and food [7]. For years, semiconductor photocatalysts have been used to remove organic and inorganic pollutants from water. Regardless, many photocatalysts have been created, because of their better stability, non-toxicity, and antibacterial qualities, metal oxide semiconductors have attracted a lot of interest [8]. The quantum confinement effect causes the bulk properties of the particles to vanish as the particle size is lowered, and fresh features emerge because of the quantum confinement effect. Nanomaterials possess different characteristics with respect to size, structure, surface orientations, degradation efficiency and bacterial activity [9,10]. The high surface area-to-volume ratio of the particle improves as the particle size is lowered, resulting in improved attributes such as enhanced catalytic and antibacterial activities. Various metal oxide nanoparticles, such as MnO₂ doped with Ag nanoparticles, have attained remarkable growth in recent years [11,12]. The optical, catalytic, mechanical, and antibacterial applications of MnO₂ nanoparticles are driving intense research activity.

Wastewater treatment, solar energy conversion, dye degradation electronics, and catalysis are just a few of the applications for transition metal oxides [13,14,15,16,17]. Because of their non-toxicity, low cost, abundance, durability in aquatic environments, and modest bandgap of 2.0–2.2 eV, the metal oxide nanoparticles have particular interest among these oxide nanoparticles. The small bandgap of metal oxide makes the materials are suitable for photoconductive and photothermal applications. MnO₂ has also been employed in medicine delivery, biosensors, energy devices, photocatalysis, and antimicrobial applications in printed electronics [18,19,20,21,22]. Metal oxide nanoparticles are synthesised for antimicrobial uses, such as medicinal treatments, industry production, and widespread absorption into varied materials, such as cosmetics and apparel, are currently gaining popularity. Generally, metals of similar size to the parent ion provide effective property enhancement without significantly altering structural qualities. When different transition metals are doped into oxide nanoparticles, similar alterations have been found. Due to their low bandgap, chemical stability, and inertness, metal oxide semiconductor nanocrystals such as TiO₂, ZnO, MgO, WO₃, MnO₂, and SnO₂ have recently acquired favour as photocatalysts [23,24,25,26]. The use of metal ion doping in MnO₂ to achieve visible-range absorbance has been investigated. Another advantage of doping MnO₂ with metal ions in the correct amounts is reduced antibacterial energy transfer. Its chemical interactions, in particular, are responsible for various pharmacological and antibacterial activities.

We produced nanoparticles using the sol–gel process and explained their properties. X-ray diffraction (XRD) analysis, UV-vis absorbance spectroscopy, FTIR, μ-Raman spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and photocatalytic activity have all been described. As far as we know, no paper has been published on the antibacterial activity of MnO₂ doped with Ag nanoparticles. These investigations focus on changes in the crystalline structure, surface morphology, antibacterial activity, and photocatalytic activity of MnO₂ on methylene blue as photocatalysts for biological and environmental applications were reported in detail.

2. Results and Discussion

2.1. XRD Analysis

The rhombohedral (hexagonal) structure of hematite presents twelve pronounced peaks with crystal planes of (200), (202), (300), (211), (330), (301), and (321). Since the amount of Ag dopant in the produced compounds is so tiny, no phase matching to silver nanoparticles or other silver samples can be seen in the XRD pattern, indicating that silver centres are lodged in the MnO₂ lattice, as shown in Figure 1. The Debye–Scherrer expression can also be used to compute the crystallite size of synthetic substances from results, also corroborating these findings. Stiff narrow peaks without any notable shift indicate that the nanoparticle products were observed upon evaluation of MnO₂ doped with Ag nanoparticles under X-ray diffraction, signifying that the particles were pure, with a = ~89.83 nm; the crystallite size was determined using Debye–Scherrer equation, D = Kλ/βcosθ, where D represents the crystallite size, K represents the Scherrer constant (0.9), presents the X-ray wavelength, represents the full width at half maximum (FWHM), and represents the diffraction angle. The lattice parameters (a = b and c for a hexagonal unit cell) were determined using the following equation [27,28,29].

$$\frac{1}{d_{hkl}}=\frac{4}{3}.\frac{h^2+k^2+l^2}{a^2+b^2+c^2}$$

(1)

The inter-reticular distance is d_hkl, the lattice parameters are a, b and c, and the Miller indices are h, k, and l. For pure MnO₂ and MnO₂ doped with Ag nanoparticles, average crystalline sizes of 14 nm and 20.77 nm, respectively, were computed [30,31,32].

2.2. UV Absorbance Analysis

By recording the UV-visible absorbance spectrum of nanomaterial suspensions, the optical bandgap was estimated. Using the UV-vis spectra, the Tauc-plot relation was then employed to establish bandgaps. The absorption coefficient (α) is given by hν = (hν − E_g)^m according to the Tauc relation, αhν = β (hυ − E_g)^m; where β is a constant with variable values for different transitions, E_g is the bandgap energy, h is the energy of a photon, and m is an exponent with varying weights depending on the electronic change [33,34,35]. All samples were evaluated in the UV-vis region of 200 nm to 600 nm. Optical analysis was performed using a dual-beam 60 UV-visible NIR spectrophotometer. Figure 2 shows the absorption spectrum of bare manganese oxide (MnO₂) and MnO₂ doped with Ag nanoparticles, with absorption at 311 nm are confined in bare MnO₂ is due to charge transfer of ions and absorption band obtained at 324 nm are found in MnO₂ doped with Ag nanoparticles, which explains charge transfer via direct ligand-to-metal transition in the optical absorption spectrum. The straight bandgaps of pure MnO₂ and MnO₂ doped with Ag nanoparticles were 2.79 eV and 2.66 eV, respectively, as shown in Figure 3.

2.3. FTIR Analysis

The FTIR spectra were collected in the 4000–500 cm⁻¹ range. The FTIR spectra of pure MnO₂ and MnO₂ doped with Ag nanoparticles are shown in Figure 4. Significant peaks can be observed at 3366 cm⁻¹, 2359 cm⁻¹, 1611 cm⁻¹, 1122 cm⁻¹, 641 cm⁻¹ and 515 cm⁻¹. The stretching mode of the -OH group corresponds to a broad band at 3366 cm⁻¹. The H-O-H bending vibration of water contained in moisture received from the atmosphere is allocated to a bar near 1611 cm⁻¹. Ag ions may be responsible for the bands at 1478 cm⁻¹. The distinctive absorption of Mn-O stretching at around 641 cm⁻¹ indicates the establishment of Mn-O bonds in the produced nanomaterials. The FTIR spectra of doped metal oxide nanoparticles are also shown in Figure 4. The curves contain comparable peaks, with the Mn-O rise shifting to 515 cm⁻¹. On doping, the strength of the Mn-O height was similarly reduced. This modest shift in band validates doping in nanomaterials, but lower intensity indicates minimal doping and causes a minor skeletal perturbation in the Mn-O structure [36]. This decline is easily explained by Hook’s law, which asserts that the vibration frequency is indirectly proportional to mass reduction, with the vibrational frequency shifting to a lower value with increasing condensed mass.

2.4. Scanning Electron Microscopy and EDX Analysis

The particle grains of MnO₂ nanoparticles of a few nanometres are presented in Figure 5a,b, indicating that pure nanostructures and those doped with Ag were successfully synthesised, as reported in Figure 5c,d. Due to the increased density and reduction in grain size with doping, the smaller nanoparticles are visible and less agglomerated in the SEM image of MnO₂ doped with Ag nanoparticles, resulting in a large surface area that is ideal for photocatalytic activity. It is also evident that the MnO₂ particles formed in nature are porous and have a grain size of roughly 24 nm [37,38,39]. The elemental compositions of the generated distinct samples are also confirmed using EDX, and their spectra are presented in the EDX spectrum for pure and doped MnO₂, respectively. Mn, Ag, and O components are visible in the EDX spectrum. The concentration of Ag in the doped samples is determined to be 4.29 percent. The chemical compositions of Mn and O in pure Mn-O₂ nanoparticles are 54.66% and 42.37%, respectively, as shown in Figure 5e. The iron proportion in MnO₂ doped Ag nanoparticles, on the other hand, fell, while the oxygen percentage increased somewhat [40].

2.5. Raman Spectroscopy

Raman spectroscopy is a powerful technique for investigating complex structures and chemical bonds. Figure 6 depicts the creation of a rhombohedral phase constructed of pure MnO₂ and MnO₂ doped with Ag nanoparticles, as revealed by Raman analysis. The Mn, Ag and O stretching vibrational modes of metal–oxygen interaction between Mn and O atoms are responsible for three significant peaks at 74 cm⁻¹, 437 cm⁻¹, and 1054 cm⁻¹ in MnO₂ nanoparticles treated by a simple sol–gel process. The Ag and Mn-O modes of MnO₂ nanoparticles produce a couple of soft peaks at 940 cm⁻¹ and 1427 cm⁻¹. The production of MnO₂ nanoparticles is responsible for the substantial high-intensity rise at 1054 cm⁻¹ [41,42,43]. The Raman peak intensities and peak positions match the face-centred MnO₂ doped with Ag nanoparticles, as previously reported [44,45,46].

2.6. Photocatalytic Analysis

In photodegradation reactions, the intensity of incident light, the beginning concentration of pollutants, and the concentration of the photocatalyst are all significant characteristics. Two main approaches are important during photocatalytic experiments in the presence of transition metal ions: (a) the effect of transition metal ions on photocatalysis reaction rate, and (b) the concentration of metal ions into less toxic species or the deposition of ions on catalyst surface to recover the expensive and useful metal ions. A photocatalysis experiment was performed on generated all transition metal oxide nanoparticle samples [47,48]. Methylene blue (MB), an organic dye, was used to track the performance of photocatalysis. The solution was placed in the dark to maintain adsorption and desorption equilibrium at room temperature after 50 mm of 5 ppm MB solution and 2 mg of catalyst were dissolved. After defined intervals of time (0, 30, 60, 90, 120, min), the rate of absorption was measured using a Carry 60 UV-visible spectrophotometer.

Degradation = η = [1 − C_o/C_t] × 100

(2)

The relation given above was used to calculate the percentage of methylene blue degradation. Photo-degradation of metal oxides MnO2 is depicted in Figure 7a–b. With time, the distinct colour of organic dye (MB) faded. Methylene blue’s transparent hue indicates that the dye has completely degraded. Figure 8 shows the degradation rate of nano-photocatalysts made from various cycles. The reactions throughout the experiment follow pseudo-first-order kinetics, as seen in the graphs. The simple oxides exhibited good catalytic activity, as evidenced by degradation percentages and rate constant values of distinct transition metal oxides. The formation of excitations on UV visible light exposure is required for the photocatalytic activities of transition metal oxides. When a photocatalyst is subjected to UV-vis light, electrons from the valence band are stimulated to the conduction band, and electron–hole pairs are formed. The resulting pairs can recombine or interact with other molecules [49]. To precede oxidation–reduction processes, electron–hole pairs at CB and VB migrate towards the photocatalyst surface, which immediately interacts with the dye solution. The degradation efficiency of the synthesised MnO₂ doped with Ag nanomaterials was found to be 90%, meaning that the material can employed as an excellent photocatalyst. Figure 9 shows the reaction mechanism occurring during the photocatalytic activity of the MnO₂ doped with Ag nanoparticle photocatalyst under visible light.

2.7. Screening of Antibacterial Activity

Two bacterial strains (Bacillus subtilis and Escherichia coli) were used throughout the investigation. All the bacterial cultures were obtained from the microbial type culture collection (MTCC), institute of microbial technology, Chandigarh, India. The young bacterial broth cultures were prepared before the screening procedure as follows: (i) preparation of inoculums: stock cultures were maintained at 4 °C on slopes of nutrient agar. Active cultures were prepared for the experiment by transferring a loopful of cells from the stock cultures to test tube of Muller–Hinton broth (MHB) for bacteria that were incubated without agitation for 24 h at 37 °C and 25 °C, respectively. The cultures were diluted with fresh Muller–Hinton broth to achieve optical densities corresponding to 2.0 × 10⁶ colony forming units (CFU/mL) for bacteria.

2.7.1. Antimicrobial Susceptibility Test

The disc diffusion method was used to screen the antimicrobial activity. In vitro antimicrobial activity was screened by using Muller–Hinton Agar (MHA) obtained from Himedia (Mumbai). The MHA plates were prepared by pouring 15 mL of molten media into sterile petriplates. The plates were allowed to solidify for 5 min, and 0.1% inoculum suspension was swabbed uniformly, and the inoculums were allowed to dry for 5 min. The concentration of extracts is 40 mg/disc was loaded on a 6 mm sterile disc. The loaded disc was placed on the surface of medium and the extract was made it possible to diffuse for 5 min, and the plates were kept for incubation at 37 °C for 24 h. At the end of incubation, inhibition zones formed around the disc were measured with transparent ruler in millimetres is shown in Figure 10a,b. Antibacterial activity of Ag nanoparticles was seen at ultra-low concentrations; however, the antibacterial activity of MnO₂ nanoparticles was dependent on concentration and surface area. The antibacterial activity of pure and doped MnO₂ nanoparticles against Gram-negative bacteria (E. coli) and Staphylococcus aureus was examined. The zones of inhibition of MnO₂ doped with Ag nanoparticles against Gram-positive and Gram-negative bacteria strains are shown in Figure 11a,b. The materials are viewed at the impact of particle size reduction and concentration on the antibacterial activity of MnO₂ nanoparticles. The antibacterial activity of MnO₂ nanoparticles improved significantly with increasing Ag powder content, according to our findings [50,51].

2.7.2. Antifungal Activity

Fungi Tested

Two fungal strains (Aspergillus niger and Candida albicans) were used throughout the investigation. All the fungal cultures were obtained from the microbial type culture collection (MTCC), institute of microbial technology and Chandigarh, India. The young fungal broth cultures were prepared before the screening procedure. The agar well diffusion method was modified. Sabouraud’s dextrose agar (SDA) was used for fungal cultures. The culture medium was inoculated with the fungal strains separately suspended in Sabouraud’s dextrose broth. A total of 8 mm wells were punched into the agar and filled with plant extracts and solvent blanks (methanol, ethyl acetate and hexane). A standard antibiotic (Fucanazole, concentration 1 mg/mL) was used as a positive control, and fungal plates were incubated at 37 °C for 72 h. The diameters of the zones of inhibition observed were measured, as shown in Figure 12a,b. The sizes of the inhibitory zones that were detected were measured, as shown in Figure 13a,b. MnO₂ nanoparticles show good antifungal action against the C-albicans fumiga bacterium, according to the findings. The ability of silver nanoparticles to give strong antifungal efficacy was validated, indicating that MnO₂ nanoparticles has high potential activity, as evidenced by the zone of inhibition with the growth of tested bacteria, as shown in

Table 1. Antibacterial and Antifungal activity of MnO_{2 doped with Ag} against Escherichia coli, Bacillus subtilis, Aspergillus niger and Candida albicans strains.

Organism’s	Control	Concentration (%)
		Pure MnO2		MnO2 doped with Ag
		60	80	60	80
Bacillus subtilis	19	12	21	18	25
Escherichia coli	22	13	20	19	24
Aspergillus niger	20	14	16	17	20
Candida albicans	21	13	19	20	23

. The findings suggest that oxide nanoparticles containing metals like Ag could be helpful in the creation of antimicrobial agents [52].

3. Experimental Methods

MnO₂ doped with Ag nanoparticles was made utilizing [MnSO₄·H₂O], AgNO₃, and KMnO₄ as raw materials in a straightforward sol–gel technique. [MnSO₄·H₂O] (0.9 g) was first dissolved in water (25 mL) using a sonicator. The aqueous dispersion of KMnO₄ (1.25 mg/mL) was then added to the solution under magnetic stirring. To make a gel, five percent AgNO₃ was added to the MnO₂ precursor and agitated for 8 h at 90 °C. The gel was then dried at 60 °C in a vacuum oven for later use. Finally, grey-hued MnO₂ doped with Ag nanoparticles was created. Pure MnO₂ was created using the same experimental method. All prepared samples were placed in an alumina crucible with a cover and cooked in a muffle furnace for 5 h at 500 °C. Fine nanoparticles of bare MnO₂ doped with Ag nanoparticles were collected and analysed.

3.1. Characterisation Studies

The sample was characterised by X-ray diffraction (Brucker D8 Advance) with CuKα radiation (λ = 1.5481 nm) with 2θ range from 10° to 90° at a scanning rate 10 deg/min to confirm the crystalline structure and to measure the particle size. A double beam UV–vis–NIR spectrometer was used to analyse the absorption properties of the prepared samples (Model No. UH5300). It was verified that moieties existed using an FTIR Spectrophotometer (SHIMADZU, 8400S). An optical Raman spectrometer (STR 300, AIRIX, Japan) was used to collect Raman spectrums with a diode laser source of 352 nm wavelength. For the surface morphology analysis, a scanning electron microscope (FESEM) (Carl Zeiss (ZEISS, Sigma)) was used, and for elemental analysis, an energy dispersive X-ray spectrometer (EDS) was used.

3.2. Photocatalytic Dye Degradation Test

When visible light was used to break down methylene blue (MB), the photocatalytic abilities of the photocatalysts were checked. One of the lights was a 500 W halogen lamp that can make visible light (λ > 420 nm) that can be seen by the people who are in the background. Fifty milligrams of the as-made photocatalysts added to 100 mL of water with constant magnetic stirring and water circulation. There were no lights or ventilation for 30 min. This allowed the adsorption and desorption equilibrium of MB on the photocatalysts to be reached. Then, the light was turned on. At 30-min intervals, 5 mL of the suspension was taken and centrifuged at 6000 rpm to remove the remaining photocatalyst. Then, the UV-vis spectrometer was used to look at the results. The decrease in the intensity of the characteristic band absorption at 662 nm was used to figure out how much MB had changed. The rate at which MB dyes break down when exposed to sunlight was determined by keeping track of the amount of withdrawn solutions absorbed at different times, λ_max~662 nm.

$$\mathrm{Dye}\mathrm{degradation}\mathrm{efficiency}(\%)={\frac{C_0-C_t}{C_0}}_{}\times 100$$

C_o is the concentration at the start of the reaction, and C_t is the concentration at the time of the reaction.

4. Conclusions

In the present work, pure and doped MnO₂ nanoparticles were synthesised using a sol–gel process and were characterised using XRD, UV-vis, FTIR, SEM, EDX, Raman spectroscopy, photocatalytic studies, and with respect to antimicrobial activity. According to the XRD data, rhombohedral phase structures with crystalline sizes of 19.3 and 20.4 nm were formed. Optical analysis was used to determine the bandgap energies of MnO₂ nanostructures, which were determined to be 2.79 eV and 2.66 eV, respectively. FTIR and Raman spectroscopy shows the confined functional groups of pure and doped synthesised materials. The crystalline diameters were 80–90 nm, as determined by SEM and EDX. The photocatalytic characteristics of MB dye were investigated for MnO₂ doped with Ag, after 180 min, the degradation efficiency was assessed to be 90%, providing evidence that the created nanomaterials could potentially serve as good nanophotocatalysts. Even after five cycles of degradation testing, the cyclic degradation study revealed that nanoparticles were more stable. Pure and doped MnO₂ nanoparticles were found to obtain vigorous antibacterial activity against E. coli (Gram-negative bacteria) and Bacillus subtilis (Gram-positive bacteria) and a more potential antifungal activity against Aspergillus niger and Candida albicans. The synthesised MnO₂ doped with Ag nanoparticles shows that low-toxicity metal oxide nanoparticles could be widely used to eradicate a range of infectious diseases shortly. They could be employed as effective antibacterial, anticancer, and antioxidant agents in commercial biomedical applications, according to the findings.