Bio-Engineered TiO2 Nanoparticles for Circular Nanocomposites: Sustainable Synthesis, Enhanced Functional Performance, and Emerging Applications

Titanium dioxide (TiO 2 ) nanoparticles (NPs) exhibit polymorphic phases—anatase, rutile, and brookite—whose crystal structure critically governs their physicochemical behavior and functional performance. In recent years, bio-assisted (green) synthesis strategies employing plant extracts, microorganisms, and biopolymers as reducing and stabilizing agents have emerged as sustainable alternatives to conventional methods, enabling improved control over nucleation, crystal growth, and surface energetics. This review provides a critical and comparative analysis of these synthesis routes, highlighting their influence on particle size distribution, phase selectivity, and scalability. The resulting TiO 2 nanocrystals can be integrated into biodegradable or bio-derived polymer matrices to form hybrid inorganic–organic (circular) nanocomposites with enhanced structural, optical, and interfacial properties. Advanced characterization techniques, including X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM/TEM), Fourier-transform infrared spectroscopy (FTIR), dynamic light scattering (DLS), and UV–Visible spectroscopy, are discussed in relation to establishing clear structure–property correlations. Particular emphasis is placed on the relationship between TiO 2 crystalline phases and functional performance, especially in photocatalytic and antimicrobial applications. Furthermore, key challenges associated with scalability, cost-effectiveness, and environmental and toxicological impacts are critically evaluated. This paper critically examines bio-engineering methods for reprogramming TiO 2 nanoparticles for the creation of circular nanocomposite, sustainable synthesis, scalability, cost-effective methodologies, and determining their future directions and next-generation functional performance. • Mechanistic and comparative evaluation of bio-assisted TiO 2 nanoparticle synthesis routes. • Phase-controlled TiO 2 nanocrystals engineered via plant-, microbial-, and polymer-mediated approaches. • Structure–property relationships of anatase, rutile, and brookite phases analyzed using XRD and TEM. • Hybrid inorganic–organic TiO 2 nanocomposites exhibit enhanced photocatalytic and antimicrobial performance. • Scalability, sustainability, and real-world applications of TiO 2 nanocomposites critically assessed.