Antimicrobial Resistance (AMR) undermines the effectiveness of currently available antibiotics; it poses a serious threat to both human health and the global health community. It is critically important to develop new tactics and effective antimicrobial agents in this constantly changing fight against harmful bacteria. One of the most promising alternatives is titanium dioxide nanoparticles (TiO₂ NPs) doped with metal; their exceptional antibacterial activity against a variety of microorganisms has drawn attention to them. This thorough analysis explores the intriguing world of these nanoparticles, elucidating their modes of action, demonstrating their effectiveness against a range of infections and critically evaluating their potential in the fight against AMR.¹ Because of its photocatalytic activity, pure titanium dioxide has been shown to have innate antibacterial qualities. When Ultraviolet (UV) light interacts with TiO₂ Nanoparticles (NPs), it produces Reactive Oxygen Species (ROS) such as Hydroxyl radicals (OH-) and superoxide anions (O₂-), which are potent antibacterial agents that damage and eventually kill micro, organisms by damaging their membranes, DNA and cell walls. However, the main limitation of pristine TiO₂ NPs is that it require UV light to activate, which limits their use in a lot of practical situations.² The increasing incidence of microbial attacks in various industries, including food, pharmaceuticals, textiles, water purification and food packaging, prompts a continuous quest for novel antimicrobial compounds. Interest in the production of inorganic antimicrobial chemicals has developed due to the increased bacterial resistance to specific antibiotics and the toxicity of certain organic antimicrobials agents to human health. Because of their broad-spectrum antibacterial properties, metal as well as several combinations of metal oxides have garnered a lot of attention among these substances. However, because of their superior qualities resulting from a high surface area-to-volume ratio, nanoscale materials are broadly acceptable. Antimicrobial nanoparticles have demonstrated better and unique activities from their bulk properties.³ Additionally, the greater surface area that nanoparticles have in touch with bacteria amplifies their antibacterial properties.⁴

It has been demonstrated that TiO₂ nanoparticles exhibit exceptional antifungal and antibacterial properties against different pathogens, like viruses, fungi, also Gram-positive and Gram-negative bacteria.⁵ Numerous investigations have demonstrated the increased antibacterial activity of TiO₂ nanoparticles via metal doping, including co-doping with nitrogen and silver and the creation of unique hollow TiO₂ nanospheres with antibacterial qualities.⁶ TiO₂ may be categorized as amorphous or crystalline depending on its crystallinity.⁷ Three polymorphs of Titanium dioxide (TiO₂) are known to be available: rutile, brookite and anatase. Because anatase has a higher photocatalytic activity than rutile and brookite, it is the one that is most frequently used for photocatalytic applications.⁸ While rutile has the smallest band gap (~3.0 eV), it often exhibits photocatalytic activity that is up to ten times lower than that of anatase. Even though pure brookite polymorphs may have more photocatalytic activity than the other two polymorphs, using them in heterogeneous photocatalysis is a difficult challenge because of their intricate production process. When compared to individual polymorphs, mixed phases of TiO₂ showed considerably better photocatalytic activity.⁹

TiO₂ has been extensively researched for its capacity to function as a photocatalyst, producing The light-induced inactivation of microorganisms caused by Reactive Oxygen Species (ROS).¹⁰ TiO₂ is the most effective photocatalyst among the many types. It has been shown that numerous organic contaminants may be efficiently degraded by photocatalytic disinfection using TiO₂. Because of its commercial availability and chemical stability, TiO₂ has been utilized extensively in photocatalysis.¹¹ Because of the randomly arranged particles, the amorphous form of titanium dioxide (TiO₂), sometimes referred to as anatase, has an uneven shape and typically develops around 350°C. Between 500 and 800°C, anatase and brookite permanently change into the stable rutile polymorph. Compared to rutile and brookite, anatase exhibits the highest degree of photosensitivity of the three polymorphs.^12–14

Titanium dioxide (TiO2) Nanoparticles (NPs) have drawn a great deal of attention due to their unique characteristics and uses, especially in the biomedical industry. Although a lot of research has been done on chemically manufactured TiO2 NPs, microbial production has a number of benefits over it, such as lower toxicity, biocompatibility and environmental friendliness. By biotransformation titanium precursors and using their metabolites, biomolecules such as enzymes as stabilizing and capping agents, microorganisms including bacteria, fungus and algae create TiO2 NPs.¹⁵ One major drawback is the effectiveness and efficiency of these microbially generated TiO₂ NPs as a nano-photocatalyst or antibacterial agent.¹⁶ This is because different microbial proteins that are capped on the microbially generated TiO₂ NPs (which have previously been shown in the literature) prevent the TiO₂ NPs from doing their intended function. There is less contact between pathogens and TiO₂ NPs or between pollutants and TiO₂ NPs as a consequence of the biomolecule masking the active sites of TiO₂ NPs during their antibacterial activity and photocatalytic impact. Furthermore, bigger biological macromolecules-up to several kilodaltons in size-are capped on the microbially produced TiO₂ NPs, increasing the total size of the particles. Because biological macromolecules are larger than TiO₂ NPs, the pathogens cannot be destroyed photocatalytically by them, which significantly reduces their entry into the pathogens. These TiO₂ NPs produced by microbiology are not as effective as TiO₂ NPs produced chemically or physically as antimicrobial agents or nano-photocatalysts.¹⁶

Depending on the synthesis technique used, Titanium dioxide (TiO₂) Nanoparticles (NPs) may have different sizes. TiO₂ NPs produced by microbial synthesis may have diameters ranging from 10-30 nm to over 100 nm, while NPs produced by chemical synthesis can have sizes between 10 and 100 nm.¹⁵ TiO₂ nanoparticles may be produced using different kind of methods, including the sol-gel method,, solvothermal method, immediate synthesis method, simple mixing method, method, microwave-assisted synthesis and precipitation method, according to the results from the cited publication.¹⁷ In general, the synthesis technique may affect the size of TiO₂ NPs, but other elements including surface chemistry and crystalline structure are also very important in defining their characteristics and uses.

It is seen that metal-doped titanium dioxide nanoparticles increase antibacterial effectiveness.^18–20 TiO₂’s photocatalytic activity is increased by doping it with different elements because new energy levels arise near to the conduction band. The heterogeneous process of photocatalysis using titanium dioxide depends critically on the catalyst’s surface.²¹ During the synthesis process, metal ions are added to the TiO₂ lattice to create metal-doped Titanium dioxide (TiO₂) Nanoparticles (NPs). Zn and Cd are examples of non-transition metals, while transition metals like Fe, Cu and Ni may be the dopants.²² TiO₂ NPs may be doped to increase their photocatalytic activity, stability and adjustable bandgap characteristics, which will increase their efficiency in breaking down pollutants and producing hydrogen.⁹ There are numerous approaches to synthesize metal-doped TiO₂ NPs, also solvothermal, sol-gel and hydrothermal approaches. The environmentally benign and biocompatible method of microbial synthesis has also been used to create metal-doped TiO₂ NPs.²³ However, TiO₂’s antibacterial activity may be improved by doping it with metals like Silver (Ag), Nickel (Ni), Copper (Cu), Manganese (Mn) and Sulfur (S), particularly when exposed to visible light. Under visible light, Ag-doped TiO² nanoparticles exhibited increased antibacterial activity against Staphylococcus aureus. TiO₂ NPs doped with Ni, Cu, Mn and S have also demonstrated antibacterial activity against various bacterial species. Table 1 show TiO₂-materials doped with metals has antibacterial properties against a variety of microorganisms. Moreover, it has been discovered that adding metal-doped TiO₂ nanoparticles to elastomers improves the photoinactivation effectiveness of visible light for surface cleaning. These results imply that metal-doped TiO₂ nanoparticles may find usage in a variety of applications as powerful antibacterial agents.

According to Haque et al. (2023) the compound’s synthesis has concluded. For the procedure TiO₂ nanoparticles were produced via sol-gel using titanium isopropoxide as the titania precursor. A homogeneous 1:1 (v/v) mixture of water and 2-proponal was combined with a solution of titanium isopropoxide (0.1 M) in 2-proponal (25 mL), added drop by drop and constantly stirred. Turbidity in the solution would emerge as a consequence, indicating the hydrolysis of titanium isopropoxide. Following the full addition, the mixture was shaken for the whole night before being filtered and repeatedly cleaned with water. In order to dope TiO₂ particles with nickel, a solution including water and 2-proponal was mixed with a known concentration of NiCl₂ (0.1-0.8%) before being added to the titanium isopropoxide solution. First, the undoped TiO₂ and Ni-TiO₂ were both dried for 2-3 hr at 100 C. After the dry residue was manually ground in an agate mortar, it was heated to various temperatures-300, 400 and 500°C-for 4 hr. Figure 1 demonstrate the sol-gel method’s synthesis of metal-doped TiO₂/Ag.²⁸ Figure 1 show the Synthesis of metal-doped TiO₂/Ni by sol-gel method.

Using a range of analytical methods, the structural, morphological and elemental properties of TiO2/Ni nanocomposites prepared by the sol-gel process are examined. Numerous techniques, including as X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), energy dispersive X-ray Spectroscopy (EDX), Fourier Transform Infrared Spectroscopy (FTIR), etc., are often used to study TiO2/Ni nanocomposites.²⁹ SEM, XRD and UV-vis spectroscopy were among the traditional analytical techniques used to characterize the generated catalysts. To study the particle morphology, a Hitachi S-5000 FEG scanning electron microscope running at 20 kV was used. Using Cu Kα radiation (h=1.5418 ù) from a Rigaku Miniflex II device running at 30 kV and 15 mA, X-ray Diffraction (XRD) was used to structurally characterize the undoped TiO₂ and Ni-TiO₂ nanoparticles in the 20-80° range. Using a Shimadzu UV-vis spectrophotometer (Model 1601), the UV-Vis spectra in the 300-800 nm range was obtained at room temperature ²⁸

Staphylococcus aureus is strongly inhibited by Ni-doped TiO₂ nanoparticles. The antibacterial activity was measured using the well diffusion method. After being prepared, cleaned and put into a petri plate, Mueller-Hinton agar (Hi-Media) was used. After allowing the medium to solidify, the aseptic placement of the prepared inoculum onto the plate occurred. Using a cotton swab or an L-shaped glass spreader that had been sterilized, the inoculum was applied. A sterilized test tube was used to create a well with a 6 mm diameter, which was then filled with a 2.5 L suspension of various diluted products to be tested. At 28 degrees, the plates were incubated for three days. The zone of inhibition diameter (mm) is used to represent the activity. Figure 2 shows how different concentrations of the Ni dopant TiO₂ affected antibacterial activity. With an increase in dopant concentration, the zone of inhibition widened. However, it was shown that agglomeration caused the zone of inhibition to narrow at a concentration of 2%.³⁰

Nanoparticles composed of Ag-TiO₂ were synthesized through a range of diverse methodologies as described in the investigations conducted by Fan et al.,³¹ de Oliveira et al.,¹⁸ Wani et al. and Ren et al.³² Fan and colleagues conducted a synthesis of a composite material that consisted of Polyamino Acid (PAA) and TiO₂/Ag nanoparticles via in situ melting polycondensation.³³ Ag-doped TiO₂ nanoparticles were synthesized by de Oliveira et al. using the sol-gel process followed by Ag doping. Wani et al. looked into the use of ion beam irradiation to create N and Ag-modified nanocrystalline TiO₂. Ren et al.’s attempt to create PPDO-coated Ag loading TiO₂ nanoparticles was unsuccessful. These studies used a variety of approaches, including ion beam irradiation, sol-gel process and in situ melting polycondensation, to create Ag-TiO₂ nanoparticles with different compositions and characteristics. Figure 3 show TiO₂, sol-gel TiO₂/Ag synthesis. The synthesis of Ag-TiO₂ nanocomposites can be accomplished through a modified sol-gel technique, wherein the incorporation of silver ions takes place in the course of the preparation and undergoes reduction to generate Ag nanoparticles on the surface of TiO₂.^34,35

Using a nickel filter, powder X-ray Diffraction (XRD) spectra of anode Cu-WL 1 radiation with a wavelength of 1.5406 nm were acquired from PAN Analytical in order to assess the crystalline structure of the photocatalysts. The voltage was 40 kV and the current was 40 milliamperes. The average crystallite size of anatase was determined by using the Scherrer equation in combination with Full Width at Half Maximum (FWHM) data from the chosen peak. The surface area and porosity of the material were determined using the Micrometrics Gemini VII surface area analyzer. The nitrogen adsorption/desorption isotherms were monitored two or three times to get repeatable results. The data came from the surface/volume mesopore study of BJH. By using the Frenkel-Halsey-Hill isotherm equation, the micropores’ volume may be calculated. For duration of 2 hr, every sample was degassed at 300°C. A Transmission Electron Microscope (TEM) equipped with JEOL/JEM 2100 and running at 200 kV was used to measure the catalyst’s size. Using a Scanning Electron Microscope (SEM), the catalyst’s form and underlying elemental composition were examined.³⁶ The researchers are Imandi Manga Raju, Siva Rao Tirukkovalluri and Sankara Rao Miditana.

The following reaction rate kinetic constants were determined using the pseudo-first-order kinetic model:

Where,

t is the irradiation time,

k is kinetic constant (min-1) and,

C is the concentration of the MB (mg/L).

In compliance with reference methods of the Clinical and Laboratory Standards Institute (CLSI) for the determination of MICs of aerobic bacteria by broth microdilution, the antibacterial activity of amorphous and crystalline TiO₂:Ag0, TiO₂a: Ag0, TiO₂:Ag0/Ag+, TiO₂:Ag0/Ag+ and TiO₂:Ag0/ Ag0 nanospheres was evaluated using Minimal Inhibitory Concentration (MIC) and Minimal Bactericidal Concentration (MBC) values. The lowest concentration of silver on a microplate that prevented bacterial growth was known as the Minimum Inhibitory Concentration (MIC) and it was determined after 16-19 hr of incubation at 37°C. The lowest extract concentration that was found in MBC eliminated 99.9% of the germs.³⁸ Two important microbiological indicators that indicate how well antimicrobial medications work against bacteria are the Minimum Bactericidal Concentration (MBC) and least Inhibitory Concentration (MIC) values. Lower MIC and MBC values, which indicate the lowest doses at which 99.9% of bacteria are killed and bacterial growth is inhibited, respectively, indicate better antibacterial efficacy. The Figure 4 show graphical statics of Antibacterial Action of Ni/TiO₂ against Staphylococcus. Mueller Hinton Broth (MHB) and Mueller Hinton Agar (MHA) were utilized as the bacterial growth media throughout the experiment. 200 ml was dispensed each spot for testing and the antimicrobial agent stock solution was diluted by published methods. The Figure 4 show graphical statics of Antibacterial Action of Ni/TiO₂ against Staphylococcus. A 0.5 McFarland standard containing around 1-2×10^8 Colony-Forming Units per milliliter (CFU/mL) was altered to generate the inoculum. To get the intended final inoculum concentration of 10^4 CFU per spot, this standard was then diluted at a ratio of 1:10 in sterile saline. Spots devoid of nanoparticles containing medium and cultures were included as controls to guarantee precise evaluation of the antimicrobial agent’s ability to impede bacterial growth and kill it.³⁵

Figure 1:
Synthesis of metal-doped TiO₂/Ni by sol-gel method.

TiO₃-NPs, or nanoparticles of doped titanium dioxide, have drawn a lot of interest because of their special qualities and possible uses. The physicochemical characteristics of titanium dioxide nanoparticles vary from those of their fine particle analogs, potentially modifying their bioactivity Because of their stability and photocatalytic qualities, TiO₂ NPs are extensively employed in many different sectors, including solar cells, water treatment and catalytic processes. Inhalation is the main way that people are exposed to TiO₂ NPs at work, however, consumer goods like sunscreen and makeup may also expose people to these particles.³⁹

TiO₂ NPs may move from the lung and gastrointestinal tract to systemic organs, according to studies, which raises questions regarding possible health repercussions.⁴⁰

Additionally, studies have concentrated on creating nitrogen-doped titanium dioxide nanoparticles, which exhibit greater absorbance in the visible spectrum than plain ones.⁴¹ There are many ways to synthesize titanium dioxide nanoparticles, one of which is green synthesis, which uses plant extracts and is safe for the environment and non-toxic. Applications in wastewater treatment and antibacterial activity have also been investigated for the microbial production of titanium dioxide nanoparticles.¹⁵

Numerous applications for doped-TiO₂ nanoparticles exist in biomedicine. They have been investigated as possible temperature nanoprobes, between the doped nanoparticles’ thermally sensitive radiative transitions, the fluorescence intensity ratio establishes the sensors’ relative sensitivity.⁴² Doped or undoped TiO₂ nanoparticles have shown significant use in enhancing chemical, biological and physical behavior. They have been used to improve several facets of the environment, agriculture and health, such as enhancing nanomedicine, eliminating pollutants from the environment and enhancing agricultural qualities.⁴³ Potential optical nanothermometers using Nd³⁺ doped TiO₂ nanocrystals have been studied. Temperature is determined by the fluorescence intensity ratio of emissions at various wavelengths.⁴⁴ The Figure 5 shows various applications of Various Uses of Doped-TiO₃ in Biomedicine.

Additionally, doped-TiO₂ nanoparticles have been used to produce novel biocompatible materials with distinct physical-chemical characteristics, such as coatings of magnetite nanoparticles with noble metals.⁴⁵ Nitrogen-doped nanoparticles have demonstrated enhanced actions that include cytotoxic, antifungal, antioxidant, antidiabetic and inhibit protein kinase when compared to undoped TiO₂ nanoparticles. As such, they represent a strong contender for biomedical applications.⁴⁶

Current research on the use of doped-TiO3 in biomedical applications is centered on its utilization in ferroelectrics and luminous phosphors, among other functional materials. One well-known example is the development of a Mn- and Yb-doped Ba TiO3-(Na0.5Bi0.5) TiO3 ferroelectric relaxor material; the Mn doping results in better dielectric characteristics. This work suggests that Mn dopant functions as an acceptor, enhancing electrical resistivity in the vicinity of the Curie temperature and decreasing oxygen vacancies, which enhances insulation resistance.⁴⁷

A further investigation delves into the creation and description of (Ba, Sr) TiO3 phosphors doped with Eu³⁺ and Dy³⁺, demonstrating that these dopants provide adjustable photoluminescence characteristics. The emission intensities of the phosphors change when Sr²⁺ is substituted for Ba²⁺, causing a phase shift from tetragonal to cubic structures.⁴⁸

Figure 2:
Antibacterial effect of (a) 1% Ni/TiO₂ (b) 1.5% Ni/TiO₂ and (c) 2% Ni/TiO₂ against Staphylococcus aureus. The figure is taken from.³⁰

It is further shown that the structural, electrical and optical characteristics of the resultant materials may be studied by doping Y at the Ba and Ti sites of BaTiO₂.⁴⁹ Rare-earth element doping (Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Ho³⁺, Er³* and Yb³⁺) in lead-free ferroelectric Bi1/2Na1/2TiO3 materials has also been investigated to get adjustable photoluminescent characteristics.⁵⁰

Figure 3:
TiO₂, sol-gel TiO₂/Ag synthesis.

Doped TiO₂ nanoparticles have shown antimicrobial efficacy against a variety of bacteria under a range of lighting conditions. Under visible light, methicillin-resistant and susceptible Staphylococcus aureus was more effectively inhibited by Ag-doped TiO₂ nanoparticles.¹⁸ Table 2 shows possible mechanisms and Doped Materials.

Escherichia coli and B. subtilis are two examples of the Gram-positive and Gram-negative bacteria that the ZnO-doped TiO₂ nanocomposites shown increased antibacterial activity against.⁵¹ Furthermore, yttrium-doped TiO₂ nanostructures showed exceptional antibacterial properties against E. coli.⁵² One can decrease electron-hole recombination, boost resistance to microorganisms and dope TiO₂ with various ions or compounds to expand photocatalytic activity to visible light.²⁹ Under visible light, it has been shown that metal-doped TiO₂, e.g., TiO₂/Ni, TiO₂/Cu and TiO₂/Mn/S, inhibits a variety of bacteria, including B. coagulans, K. pneumonia, E. coli and S. aureus.⁵³

Doped-TiO₂ has antibacterial action via a variety of methods. TiO₂ may be doped with various ions or chemicals and compounded with polymers to increase its photocatalytic activity and range to visible light, hence improving its antibacterial effectiveness.¹⁸ ZnO-doped TiO₂, one kind of etal-doped TiO₂, has shown improved antibacterial activity against a range of microbes., such as Escherichia coli and Staphylococcus aureus.²⁹ Figure 6 illustrates the Mechanism of actions of doped-TiO₂. Doped-TiO₂ has antibacterial properties because light exposure produces Reactive Oxygen Species (ROS), which may harm the cell membranes of bacteria and prevent bacterial development.⁵¹ To ensure the uniformity and control over mass distribution of doped TiO₂ nanoparticles, the sol-gel production process is often used. TiO₂ has strong antibacterial properties because of the synergistic effects of photocatalysis and the targeted activities of dopant compounds. Doped-TiO₂ produces Reactive Oxygen Species (ROS) such as superoxide anions and hydroxyl radicals when exposed to light. These ROS led to oxidative stress in microbial cells, which harms proteins, lipids and DNA and finally causes cell death. Dopants such as zinc and copper also contribute extra antibacterial mechanisms. For example, copper ions may physically harm cell membranes and interfere with cellular metabolic processes. Doped-TiO₂ is a very powerful substance that can effectively fight a variety of bacteria, viruses and fungi due to the combined effects of these methods.⁵⁸

Figure 4:
Antibacterial Action of Ni/TiO₂ against Staphylococcus.

Figure 5:
Various Uses of Doped-TiO₃ in Biomedicine.

Figure 6:
Mechanism of actions of doped-TiO₂

Numerous studies place a strong emphasis on the generation of Reactive Oxygen Species (ROS). A promising method for LDs-mediated photodynamic treatment was created by Wang et al. using a photosensitizer-based system that preferentially accumulates into Lipid Droplets (LDs) and produces ROS upon light activation.⁵⁹ Palladium (Pd) nanocrystals with various crystalline forms were examined for their cytotoxicity by Wu et al. They discovered that facet-dependent ROS production, which is facilitated by Pd nanocrystals, is a critical component of cytotoxicity.⁶⁰ A potential therapeutic approach has been presented by Ling et al.’s development of a biomimetic nanosheet material that can create ROS via a cascade of intracellular biological events and provide H₂O₂ in an acidic environment.⁶¹

Photon energy higher than or equal to the doped TiO₂’s modified bandgap are absorbed by the substance. In the Valence Band (VB), a positive hole (h+) remains because an electron (e-) is promoted to the Conduction Band (CB).

The formula is as follows:

Dopants add trap sites that catch holes or electrons, preventing their recombination and boosting the effectiveness of following processes that generate ROS.⁶³

Holes react when they come into contact with hydroxide ions or adsorbed water molecules.

Electron reactions: Electrons cut down on molecules of dissolved oxygen.

Metal-doped titanium dioxide nanoparticles have demonstrated amazing potential as an antibacterial agent against a range of illnesses. Doping metals on top of nanoparticles can change their size and magnetic characteristics by mixing spins and electrical charges to reduce the energy band gap. Additionally, it has been demonstrated that because of their larger surface-to-volume ratio and enhanced ability to interact with bacterial cells, smaller metal-doped nanoparticles exhibit stronger antibacterial action. TiO₂ NPs have remarkable antifungal and antibacterial properties. Their primary mode of action against bacteria is attributed to their strong oxidizing ability, which generates radicals such as superoxide and hydroxyl anion. TiO₂ NPs doped with metals, namely silver, nickel, copper, manganese and sulfur, have shown increased antibacterial activity against several kinds of bacteria. Sol-gel, solvothermal and hydrothermal techniques are some of the ways that may be used to synthesize metal-doped TiO₂ NPs. It is possible to assess the antibacterial activity of metal-doped TiO₂ NPs utilizing methods like the well diffusion technique. All things considered, metal-doped titanium dioxide nanoparticles have the potential to be an effective antibacterial agent in a variety of fields, such as medicine, agriculture and water treatment.

I would like to express my gratitude to the Co-authors specifically Saniya Makkad, Sanskruti Shende, and Mrunal Deshmukh, for their comprehensive review on the antimicrobial efficacy of metal-doped titanium dioxide nanoparticles.