Evaluation of the Antimicrobial Properties and Shear Bond Strength of Conventional Orthodontic Adhesive Modified by Green-Synthesized Titanium Dioxide Nanoparticles Admixed With Calotropis gigantea Flowers: An In Vitro Study

Background Titanium dioxide (TiO₂) nanoparticles (NPs) have been incorporated into orthodontic adhesives to enhance antibacterial performance. However, limited data exist on their biologically synthesized forms derived from herbal sources. Objective This study aimed to evaluate the antimicrobial activity and shear bond strength (SBS) of a conventional orthodontic adhesive modified with green-synthesized (GS) TiO₂ nanoparticles prepared using <i>Calotropis gigantea</i> flower extract. Methods Forty premolars extracted for orthodontic reasons were randomly allocated to five groups (n = 8). Group I served as control (unmodified adhesive). Experimental groups were as follows: Group II, adhesive + 1% TiO₂ NPs; Group III, adhesive + 5% TiO₂ NPs; Group IV, adhesive + 1% GS-TiO₂ NPs; and Group V, adhesive + 5% GS-TiO₂ NPs. Green synthesis employed ethanolic extracts of dried <i>C. gigantea</i> flowers as reducing and stabilizing agents. Nanoparticle formation was confirmed by color change and characterized by Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), ultraviolet (UV)-visible spectrophotometry, energy-dispersive X-ray spectroscopy (EDS), particle-size analysis, and zeta potential measurement. Antimicrobial activity was tested using agar well and disc agar diffusion (DAD) assays, and SBS was measured with a universal testing machine. Results The incorporation of TiO₂ NPs reduced microbial growth compared to the control adhesive. The mean SBS for the control (Group I) was 14.79 ± 1.51 MPa. SBS decreased with increasing NP concentration: Group II = 13.66 ± 1.63 MPa, Group III = 10.33 ± 1.08 MPa, Group IV = 9.09 ± 0.92 MPa, and Group V = 8.15 ± 0.99 MPa (p < 0.001). Conclusions Both antimicrobial activity and SBS declined with higher TiO₂ concentrations, and the green-synthesized formulations showed slightly lower values than conventionally synthesized TiO₂ adhesives. Nevertheless, all SBS values remained within the clinically acceptable range for orthodontic bonding.