Lately, many noncommunicable diseases (NCDs) caused by wide-ranging Gram-positive and Gram-negative bacteria have developed a serious health issues and concerns. According to the hospital reviews, most of the noncommunicable diseases effortlessly spread into immuno-compromised patients through polymeric bioimplant medical devices. Commonly, this bioimplant materials, which are embedded inside human veins, favor the breeding and adherence of broad-spectrum bacteria. Alternatively, an ideal antibacterial polymeric nanocomposite with biocidal properties are being fabricated to tackle the issue.
The destructive mechanisms of nanoparticles (NPs) on different microbes involve metal ions release, production of reactive oxygen species (ROS), internalization and attachment of NPs, and electrostatic interaction of NPs [1, 2, 3]. Therefore, comprehensive understanding of antibacterial mechanisms is required to improve the efficacy of inorganic oxides in disease treatment. Table 1 shows the different metal embedded into polypropylene (PP) polymers that have been evaluated for their bactericidal activity alongside their postulated destructive mechanism. Among these, suspensions of Cu and TiO2 NPs are efficient in tackling a wide range of bacteria. Although most researchers focus on producing antibacterial polymer materials using a single type of inorganic oxides, the bacterial sensitivity to NPs and efficacy of antibacterial polymers differ and depend on the type, size, shape, and amount of inorganic oxides [24, 25, 26] and the nature of the polymer matrix (amorphous or semicrystalline) [27, 28]. Also, the antibacterial performance of inorganic bactericidal agents on pathogens are greatly rely on the antibacterial activity of the types of metal oxide NPs and the types of bacteria. Generally, microorganisms have different tolerance or sensitivity against different types of antibacterial material. Some studies have reported the significant improvements of bactericidal efficiencies of inorganic antibacterial agents on microorganisms in multi-ionic systems, such as dual (Ag-Cu, Ag-Zn, and Zn-Cu) and ternary (Ag-Zn-Cu) ionic zeolite, which are incorporated in polyether-type thermoplastic polyurethane matrix [29].
The polymer matrix embedded with single inorganic oxides also exhibits antibacterial activity on certain types of microorganisms [29]. Normally, CuO NPs which was synthesized by aqueous precipitation method, exhibited broad spectrum of bactericidal activity against eight different types of microbial pathogens, such as Escherichia coli (E. coli), Pseudomonas aeruginosa (P. aeruginosa), Klebsiella pneumoniae (K. pneumoniae), Enterococcus faecalis (E. faecalis), Shigella flexneri (S. flexneri), Salmonella typhimurium (S. typhimurium), Proteus vulgaris (P. vulgaris), and Staphylococcus aureus (S. aureus). K. pneumoniae showed the least strain sensitivity to CuO NPs, whereas E. coli and E. faecalis are more sensitive [30]. The inhibitory effect of porous Cu towards Bacillus atrophaeus (B. atrophaeus) is greatly enhanced by a thin Ag surface coating. Several studies have highlighted that metal-ionic antibacterial agent incorporated PP polymer generates weak bacteriostatic effect towards Gram-positive and Gram-negative strains (Table 1). But in this work, PP modified with oval-shaped Cu-TiO2 NPs demonstrated strong bactericidal effect (≥ 3log10 CFU/mL reduction) towards S. aureus and E. coli. In the literature, a mixture of Cu and Ag ions demonstrates a strong synergistic bactericidal effect against P. aeruginosa and Acinetobacter baumannii (A. baumannii) and antagonistic effect against Stenotrophomonas maltophilia (S. maltophilia) [31].
The Ag/Cu porous material was discovered to release Ag+ and Cu2+ at a concentration capable of rendering an antibacterial efficacy [32]. The mixture of [Zn]/[Fe] metal oxide NPs with weight proportion higher than 1:1 exhibited an improved colonial inhibition effect on Gram-positive S. aureus and, to a slighter extent, on Gram-negative E. coli. The electrostatic attraction and penetration of metal oxide ions and free radicals of [Zn]/[Fe] NPs, is fully responsible for microbial cell death [33]. However, investigations regarding the synergistic impact and potential bactericidal mechanism of dual inorganic metal oxides, such as Cu-TiO2, remain inadequately elucidated. It is postulated that the combined action of Cu-TiO2 nanoparticles (NPs) exerts a more potent bactericidal effect than the mere sum of their individual actions. This heightened efficacy can be attributed to the direct attraction and penetration of copper NPs, coupled with the ROS generation by photocatalytic TiO2 NPs, directed towards bacterial cell membranes. Moreover, the incorporation of Cu-TiO2 NPs into a polypropylene (PP) matrix has been observed to confer long-lasting antibacterial properties, ensuring their sustained effectiveness over time. The gradual and controlled release of copper ions from the composite material guarantees a continuous supply of antibacterial agents to the surrounding environment. Given the escalating concern surrounding the development of antibiotic resistance with conventional antibacterial agents, Cu-TiO2 NPs present a promising solution. They employ multiple mechanisms of action, rendering it challenging for microorganisms to develop resistance. As a result, the current study aims to investigate the structural characteristics of Cu-TiO2-PP composites and evaluate the biocidal activity of Cu-TiO2 particles within the PP matrix against E. coli and S. aureus.
Table 1
Antibacterial activity of inorganic oxides incorporated into PP polymers.
Biocide | Killing efficacy | Postulated killing mechanism | Application | Refs |
Cu2O | Reduction growth %: S. aureus (88%) > E. coli (87%) > C. albicans (85%) | Release copper ions | Textile industry | [4] |
Cu-N-doped TiO2 | CFUs: E. coli (81%), S. aureus (98%) and MRSA (97%) | Release of ROS | Biomedical | [5] |
ZnO | Viability: E. coli: 71% and S. aureus: 66% | Nil | Urinary stent | [6] |
ZnO | Reduction gowth %: E. coli (99.9%) | Active oxygen species | Antibacterial | [7] |
ZnO | Reduction gowth %: E. coli (99.9%) | Nil | Active food packaging | [8] |
TiO2 | CFU/mL: S. aureus (7 x 106) and E. coli (8 x 106) | OH, radicals and ROS | Antibacterial | [9] |
CuO | CFU/mL: S. aureus (< 10), E. coli (< 10) and C. albicans (< 10) | Release copper ions | Antimicrobial | [10] |
ZnO | CFU/mL: S. aureus (> 99%) and E. coli (> 99%) | Electrostatic adhesion and ROS | Antibacterial | [11] |
ZnO | Reduction growth %: S. aureus (99.5%) and E. coli (99.9%) | Electrostatic adhesion and ROS | Photodegradation and antimicrobial | [12] |
ZnO | ISO 20743: antibacterial activity index > 3-strong for S. aureus, E. coli and K. pneumoniae | Nil | Antibacterial | [13] |
Table 1
Antibacterial activity of inorganic oxides incorporated into PP polymers (continued).
Biocide | Killing efficacy | Postulated killing mechanism | Application | Refs |
Nd-doped TiO2 | Antibacterial rate: 94.75% | Nil | Antibacterial | [14] |
TiO2 | Sharp reduction of E. coli CFU/mL | Generated-OH radicals | Antibacterial | [15] |
ZnO | Reduction of E. coli colonies: <log2 | Release zinc ions | Antibacterial | [16] |
Cu | Antibacterial rate: P. aeruginosa (100%) and S. aureus (100%) | Release copper ions | Antimicrobial | [17] |
Ag/TiO2 | Biostatic efficiency against S. aureus (> 99%) | Nil | Antibacterial | [18] |
CuO | Reduction in E. coli colonies: >log3 | Release copper ions | Antibacterial | [19] |
Se | ZOI: S. aureus: 20.9 mm, B. cereus: 22.7 mm, E. coli: 23.2 mm and P. aeruginosa: 11.3 mm | Release of ions, ROS generation and attachment of NPs | Multifunctional fabrics | [20] |
Ag | 100% biocide activity against P. aeruginosa and S. aureus | Release of silver ions | Biocidal | [21] |
ZnO-CA-THY | Antibacterial rate: S. aureus (95%) and E. coli (90%) | Release of zinc ions and hydrogen peroxide | Antibacterial | [22] |
M-T-ZnOw@Ag | Antibacterial rate: E. coli (100%) and S. aureus (100%) | Release of silver and zinc ions, electrostatic interactions, and ROS | Antistatic and antibacterial | [23] |
Cu-TiO2 | Sharp ≥ 3log10 reduction CFU/mL of E. coli and S. aureus (99.9%) after day 3 | Steady and slow release of copper ions and ROS | Antibacterial | “This work” |