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

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 (r(0)). 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 (lambda (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 lambda (eff). When lambda (eff) <= 2.96, lamellar structures (e.g., nanosheets and bowl-like assemblies) were captured by varying R-Q and Q(tot). When lambda (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.