Smart nanomaterials for efficient microalgae harvesting: Bridging laboratory innovation and sustainable biorefinery

The economic feasibility of microalgal biorefineries is largely constrained by downstream processing, particularly biomass harvesting, which contributes approximately 25–30% of total production costs. Conventional harvesting techniques—including centrifugation, filtration, and chemical flocculation—are often limited by high energy demand, poor scalability, and environmental concerns. In response to these challenges, nanoparticle-assisted harvesting has emerged as a promising alternative, offering rapid separation, high harvesting efficiencies (>95%), reduced energy consumption, and potential recyclability. This review critically examines recent advances in the use of bare and surface-functionalized nanoparticles, with particular emphasis on magnetic nanoparticles (e.g., Fe₃O₄-based systems functionalized with PEI, chitosan, or urea), as well as selected non-magnetic metallic and organic nanomaterials. The underlying harvesting mechanisms—including electrostatic attraction, bridging flocculation, and Lewis acid–base interactions—are systematically analyzed in relation to key operational parameters such as algal species, growth phase, nanoparticle dosage, pH, salinity, and ionic composition. Distinct from previous reviews, this work provides a mechanistic and integrative recyclability, and downstream compatibility. Special attention is given to multifunctional nanoparticles that enable simultaneous harvesting, cell disruption, and lipid enhancement, thereby facilitating downstream processing. The review further evaluates recyclability, environmental safety, and scalability considerations, positioning nanoparticle-enabled harvesting within a circular biorefinery framework. Overall, this review bridges nanotechnology and microalgal process engineering, offering critical insights and design guidelines to advance sustainable and industrially viable microalgal biorefineries. perspective by linking nanoparticle–microalgae interactions with process performance.