_applied sciences_  
_Article_  
**Investigation of Antibacterial Activity of Carob-Mediated Calcium Hydroxide Nanoparticles against Different Aerobic and Anaerobic Bacteria**  
[PERSON] 1,2\({}^{\copyright}\) [PERSON] 3, [PERSON] 3,*\({}^{\copyright}\), [PERSON] 4\({}^{\copyright}\), [PERSON] 4 and [PERSON] 5\({}^{\copyright}\)  
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solution in biomedical research to counteract infections and the continuously growing antibiotic resistance of microorganisms [3]. Green synthesis of nanomaterials has become the current trend in nanotechnology, providing a cleaner alternative to physical and chemical routes [5]. It provides an eco-friendly, economic, rapid, and scalable synthetic method [6]. [PERSON] et al. reported that utilizing the green synthesis method to synthesize silver NPs using _Ceratonia siliqua_ extract provided simplicity, rapidity, cost-effectiveness, and eco-friendliness compared to other methods [7]. Moreover, using certain plants containing active biological compounds in green synthesis could enhance or add to the properties of NPs (e.g., antibacterial activity) [8; 9; 10]. For instance, the green synthesis of silver NPs using various plant extracts has demonstrated enhanced antibacterial activity compared to commercially available silver and standard antibiotics [11; 12]. In addition, [PERSON] et al. reported that the green synthesis of chromium oxide NPs using _Abutilon indicum_ leaf extract exhibited superior antibacterial potency compared to chemically synthesized NPs [13]. Furthermore, [PERSON] et al. reported that zinc oxide NPs synthesized using _Atalantia monophylla_ leaf extract had a better antibacterial effect than standard drugs [14]. Therefore, using plant sources in the green synthesis of NPs can produce superior antibacterial agents that are suitable for various biomedical applications [5; 6]. Carob (_Ceratonia siliqua_ L.; _Alkharroub_ in Arabic) is a distinguished plant that has been used in various medical, pharmaceutical, and cosmetic applications [15]. It is an evergreen tree that belongs to the legume family (Fabaceae) and is well known for its rich nutritive value and its mineral and antioxidant contents [16]. Polyphenols are the primary phytochemicals in carob and are linked to its many capabilities, including its antibacterial capacity [15; 16; 17]. Over the years, several studies have investigated the antibacterial effect of carob and its different extracts and showed clear susceptibility of Gram-negative and Gram-positive strains [15; 18; 19; 20]. In addition, recent studies have combined carob extracts with NPs (i.e., iron oxide and sliver) and reported a pronounced antibacterial activity compared to bulk materials [21; 22]. This indicates that carob presents a valuable resource in the search for new antibacterial nano-agents. Calcium hydroxide (CaOH\({}_{2}\)) has gained ample recognition in dentistry and medicine as an eco-friendly, biocompatible material with numerous applications, including intracanal medication, tissue regeneration, and antibacterial coatings of metal implants [23; 24; 25]. Its antibacterial activity is a valuable quality that stems from the dissolution of CaOH\({}_{2}\) into calcium (Ca\({}^{+}\)) and hydroxyl (OH\({}^{-}\)) ions [4]. This dissolution promotes a highly alkaline environment and inhibits enzymatic activities at the cell membrane, which are required for metabolism, growth, and proliferation of microorganisms [4]. Hence, CaOH\({}_{2}\) has proven to have a wide range of antibacterial activity against different bacteria [4]. As nanoparticles, CaOH\({}_{2}\) antibacterial efficacy is enhanced by the nanometer size, high surface area, and highly charged density of the nanoparticles that enable greater interaction and penetration of bacterial cells membranes [26; 27; 28].