1.       Introduction

Antimicrobial resistance (AMR) is recognized as one of the most pressing global health challenges, responsible for increasing morbidity, mortality, and healthcare costs worldwide (1). The rapid emergence of multidrug-resistant pathogens has significantly reduced the effectiveness of conventional antibiotics, necessitating the search for novel antimicrobial materials. In this context, inorganic nanomaterials, especially transition-metal molybdates, have gained considerable attention due to their multifunctional roles in photocatalysis, luminescence, and antimicrobial activity (2) The ternary transition-metal molybdate compound zinc molybdate (ZnMoO₄) has garnered a lot of attention lately because of its combination of special chemical and physical characteristics, such as chemical stability, biocidal activity, and photocatalytic behavior. Because molybdate-based materials may produce reactive oxygen species (ROS), which damage microbial membranes and prevent the development of harmful bacteria and other microbes, they are generally regarded as potential antibacterial agents.(3). One of the most researched ternary oxides in the AMoO₄ family is zinc molybdate (A is a transition element or a divalent metal from the alkaline earth column). It comes in two different crystallographic forms: β-ZnMoO₄, which is monoclinic with both zinc and molybdenum in octahedral geometry, and α-ZnMoO₄, which has a triclinic cell with zinc in an octahedral site and molybdenum in a tetrahedral site. ZnMoO₄ is less hazardous since it is insoluble in water. It serves as a white pigment. Photocatalysis, phosphor for light-emitting diodes, electrochemical anode material for lithium batteries, scintillating bolometers for double beta decay of 100Mo, humidity sensors, antibacterial, biological imaging of deep tumors, supercapacitors, and catalysis in the oxidation of propane and propene are just a few of the many uses for this material(4). Among molybdates, zinc molybdate (ZnMoO₄) stands out because of its chemical stability, tunable optical properties, and inherent antimicrobial potential. Recent studies demonstrated that nano-ZnMoO₄ particles, particularly with rod or hybrid morphologies, exhibit strong antibacterial activity against Staphylococcus aureus. Notably, nanosized ZnMoO₄ achieved minimum inhibitory concentration (MIC) values as low as 10 ppm, compared to 200 ppm for bulk particles, emphasizing the critical role of particle size and morphology in biological efficacy (5). Furthermore, ZnMoO₄ synthesized via green or ultrasonic methods has shown promising photocatalytic degradation of dyes and concurrent antibacterial effects, making it a multifunctional candidate for environmental and biomedical applications (6,7). Zinc itself is well established as an antimicrobial agent. Zn²⁺ ions destabilize bacterial membranes, disrupt enzymatic processes, and promote reactive oxygen species (ROS) generation, leading to cell death (8). For instance, zinc-doped hydroxyapatite ceramics demonstrated significant inhibitory effects against Escherichia coli and Staphylococcus aureus, with higher Zn content directly correlating with enhanced antimicrobial activity (9). Similarly, ZnO and ZnO-based nanostructures have been extensively studied for their antibacterial performance, where dopants, surface charge, and morphology critically determine their efficacy (10). These findings highlight the versatility of zinc-containing compounds in designing effective antimicrobial agents.

Despite these positive outcomes, ZnMoO₄'s inherent electronic structure and surface activity may restrict its effectiveness as an antibacterial agent. Doping with rare-earth elements like holmium (Ho) is one efficient way to increase its antibacterial efficacy. The structural, optical, and electrical characteristics of oxide and molybdate hosts can be changed by rare-earth dopants, which can then affect the production of ROS and interactions with microorganisms. Doping usually modifies charge carrier dynamics, surface imperfections, and crystallite size, increasing the number of active sites for ROS generation and enhancing biocidal effect. (11).

Despite these advantages, ZnMoO₄ still faces limitations, including stability under environmental stress, possible cytotoxicity, and limited antimicrobial efficiency under dark conditions. One promising approach to overcome these issues is rare-earth ion doping, which has been widely used to tune structural, electronic, and optical properties of host lattices. Rare-earth ions can introduce defect sites, alter band gaps, and enhance surface interactions, thereby potentially improving antimicrobial efficiency (12)(13). Holmium (Ho³⁺), a lanthanide element, has unique optical and magnetic properties, and its incorporation into oxide matrices has been shown to alter surface defects, band gap, and morphology. While direct reports on holmium-doped ZnMoO₄ are scarce, related studies indicate that Ho-doped ZnO nanostructures display modified structural and luminescent properties, suggesting possible enhancements in antimicrobial performance through ROS generation and surface modification (14). Moreover, rare-earth ions in general have demonstrated antibacterial, antifungal, and antibiofilm activities with relatively low cytotoxicity compared to heavy metals such as copper and silver (15,16). For example, Ho-containing bioactive glasses have shown selective cytotoxicity against osteosarcoma cells while supporting pre-osteoblast proliferation, highlighting biomedical safety and functional potential (17). The current study emphasizes zinc molybdate (ZnMoO₄) as a promising antimicrobial agent due to its chemical stability, tunable optical properties, and ability to generate reactive oxygen species (ROS) that facilitate microbial destruction. However, the intrinsic electronic structure, limited surface activity, and diminished performance under dark conditions restrict the efficacy of pristine ZnMoO₄. To overcome these limitations, rare-earth ion doping has emerged as an effective modification approach. Specifically, introducing holmium (Ho³⁺) ions into the ZnMoO₄ lattice can alter its crystal framework, create defect sites, tailor the band gap, and enhance charge carrier dynamics, thereby boosting ROS production and antimicrobial potency. While direct studies on Ho-doped ZnMoO₄ remain scarce, evidence from related rare-earth-doped oxide systems indicates strong potential for improved antimicrobial activity with favorable biocompatibility. Building on this, the present work undertakes the synthesis of Ho-doped ZnMoO₄ and a detailed investigation of its structural, optical, and morphological characteristics, alongside its antimicrobial effectiveness against clinically significant bacterial strains. This research seeks to address existing knowledge gaps and advance the development of multifunctional antimicrobial materials for biomedical and environmental applications.

2.       Materials and Methods

2.1 Material

The Ho-doped ZnMoO₄ nanomaterial was synthesized using high-purity reagents to ensure reliable results. Zinc sulphate heptahydrate (ZnSO₄⋅7H₂O) and sodium molybdate dihydrate (Na₂MoO₄⋅2H₂O), both 99.8% pure and sourced from Merck, served as zinc and molybdenum precursors. Holmium nitrate hexahydrate (Ho(NO₃)₃⋅6H₂O) with 99.99% purity was used as the dopant to incorporate Ho ions into the ZnMoO₄ lattice.

Acetone and ethylene glycol (Merck) were used as solvents and reaction media to ensure uniform mixing and controlled nanomaterial growth. Urea (HiMedia) served as a precipitating and complexing agent, regulating nucleation and particle development. Methylene blue dye (S.D. Fine) was employed as a model organic pollutant in photocatalytic activity studies.

For antibacterial studies, pure cultures of Escherichia coli and Staphylococcus aureus were used as representative Gram-negative and Gram-positive bacterial strains, respectively. Natural agar media obtained from Hi-Media were used for bacterial culturing and antimicrobial assays.

All chemicals used were of analytical grade, purchased commercially, and applied without further purification to preserve their properties. Triply distilled water, prepared in an all-glass apparatus, was used throughout to remove ionic and organic impurities, ensuring a pure reaction medium and reliable experimental results.

2.2   Antibacterial Activity

Overall, two E. coli (gram negative) and S. aureus (gram positive), were analysed to show antibacterial activity of ZnMoO₄based nano material. The strains were already

isolated from patients with urinary tract infections and sewage water. Antibacterial activity was well diffused assay on nutrient agar medium (NAM), which put into Petri dish under sterile conditions and kept it up to 1 h for solidification. After that, fresh cultured E. coli and S. aureus (100 µg/mL) used in two different agar media. Both used media left for 15–20 min for complete absorption. Wells were prepared by gel puncture (7–8 mm) under aseptic conditions. At different concentration (10, 20, and 30 µg/mL), samples of HoZnMoO₄, nano material added into wells. Those treated dishes with nano materials kept up to 30 min at room temperature and allow the diffusion of extracts. After that incubated it at 37up to 24 h for maximum growth of micro-organisms. Nano materials with antibacterial activity showed inhibition of microorganism growth via clear zone of inhibition (ZOI) around the well after incubation.

3         Result and Discussion

3.1   Antibacterial Activities

The antibacterial property of Ho doped ZnMoO₄ nano material, prevented the further growth of two bacterial strains like E. coli and S. aureus. It was processed as inhibiting protein synthesis (18).

According to Fig 1, showed that the different ZOI (zones of inhibition) for antibacterial activity obtained through holmium doped zinc molybdate (Ho-ZnMoO₄) with different concentrations (10, 20 and 30 µg/mL) in methanol (Table 1).

Here, it was clearly showed that HoZnMoO₄ nano material produced quite better ZOI for E. coli then S. aureus (Fig 1). S. aureuspoorly showed ZOI (Fig 1). Where, E. coli showed small clear area around sample, showed complete inhibition. The space surrounded with ZOI, which called partial zone of inhibition, where smaller activity observed.

Table 1: Zones of inhibition (ZOIs) of antimicrobial activity for Ho-ZnMoO₄ nano material (10, 20, and 30 µg/mL) with different concentrations in methanol solvent

Figure 1: (a) E. coli; (b) S. aureus treated with Ho-ZnMoO₄ nano material in different concentration in methanol (10, 20 and 30 µg/mL).

4.       Conclusion

The present study demonstrated the successful synthesis and evaluation of holmium-doped zinc molybdate (Ho–ZnMoO₄) nanomaterials for their antimicrobial activity. Antimicrobial resistance continues to pose a critical global challenge, and the development of multifunctional nanomaterials provides a promising alternative to conventional antibiotics. In this work, Ho–ZnMoO₄ exhibited significant inhibitory effects against both Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) bacterial strains, with performance strongly influenced by concentration. The nanomaterial produced notable zones of inhibition (ZOIs) against E. coli, achieving a maximum of 3.7 mm at 30 µg/mL, while S. aureus showed comparatively weaker inhibition, reaching 0.8 mm at the same concentration. These results highlight the greater susceptibility of Gram-negative bacteria to the synthesized material, likely due to differences in cell wall structure and surface interactions. The findings also underscore the role of holmium doping in enhancing the antibacterial performance of ZnMoO₄, possibly through defect engineering, reactive oxygen species generation, and improved surface reactivity. Overall, Ho–ZnMoO₄ emerges as a promising nanomaterial for antimicrobial applications, particularly for targeting Gram-negative pathogens. Future studies should further investigate its cytotoxicity, mechanism of action, and potential incorporation into biomedical and environmental systems.