Flow hydrodynamics-dependent assembly of polymer-tethered gold nanoparticles in microfluidic channels

Abstract

Recently, self-assembly of polymer-tethered inorganic nanoparticles (NPs) in microfluidic chips has been employed as an effective approach to fabricate novel assemblies. Herein, we systematically studied the self-assembly of polystyrene-grafted gold NPs (AuNP@PS) in microfluidic chips into various structures by changing the organic/aqueous flow rate ratio (R_Q), the total flow rate (Q_tot), the molecular weight of PS ligands (M_n), and the radius of the AuNP core (r₀). Morphological transitions of the assemblies were demostrated to be dependent on the hydrodynamic conditions, i.e., R_Q and Q_tot. We then employed the effective softness (λ_eff) to describe the structural features of AuNP@PS and the formation of assemblies. The results indicated that the morphology of the AuNP@PS assemblies was affected by λ_eff. When λ_eff ≤ 2.96, lamellar structures (e.g., nanosheets and bowl-like assemblies) were captured by varying R_Q and Q_tot. When λ_eff > 2.96, only spherical assemblies were obtained with the increase of R_Q and Q_tot. We further found that the AuNP@PS assemblies showed an excellent photothermal conversion performance. These findings not only demonstrate the kinetics dependent assembly behavior of polymer-tethered NPs in microfluidic chips, but also provide a convenient way to prepare NP assemblies with well-ordered structures, potentially useful in drug delivery, photothermal therapy, and others.