Quaternary Amine-Terminated Quantum Dots Induce Structural Changes to Supported Lipid Bilayers

Abstract

The cytoplasmic membrane represents an essential barrier between the cytoplasm and the environment external to cells. Interaction with nanomaterials can alter the integrity of the cytoplasmic membrane through the formation of holes and membrane thinning, which can ultimately lead to adverse biological impacts. Here we use supported lipid bilayers as experimental models for the cytoplasmic membrane to investigate the impact of quantum dots functionalized with the cationic polymer poly(diallyldimethylammonium chloride) (PDDA) on membrane structure. Using a quartz crystal microbalance with dissipation monitoring we show that the positively charged quantum dots attach to and induce structural rearrangement to zwitterionic bilayers in solely the liquid-disordered phase and in those containing phase-segregated liquid-ordered domains. Real-time atomic force microscopy imaging revealed that PDDA-coated quantum dots and, to a lesser extent, PDDA itself induced the disappearance of liquid-ordered domains. We hypothesize this effect is due to an increase in energy per unit area caused by collisions between PDDA-coated quantum dots at the membrane surface. This increase in free energy per area exceeds the approximate free-energy change associated with membrane mixing between the liquid-ordered and liquid-disordered phases and results in the destabilization of membrane domains.

Preferential interactions of primary amine-terminated quantum dots with membrane domain boundaries and lipid rafts revealed with nanometer resolution

“…Supported lipid bilayers were formed on the mica surface, as previously described. 25 Briefly, the mica was equilibrated in 3 mL of 0.150 M NaCl and 0.005 M CaCl 2 buffered to pH 7.4 with 0.010 M HEPES, incubated in 3 mL of 0.03125 mg mL −1 of 60/20/20 mol% DOPC/SM/Chol + 0.1 mol% TopFluor® PC at 45°C for 1 h, and allowed to cool to room temperature. The solution was exchanged with vesicle and calcium-free solution to remove loosely bound vesicles and calcium (15 mL in three 5 mL aliquots).…”

“…Mica has been demonstrated previously to be a suitable substrate for imaging phase-segregated domain containing SLBs. 25,51 Mica also provides the optical transparency needed for fluorescence imaging, which was necessary for performing correlative AFM and fluorescence imaging on the same area of the bilayer. Modifying a previous approach, 41 Fig.…”

“…24 Recent work by our group showed that quaternary amine-terminated quantum dots (Qdots) caused the collapse of the L o phase. 25 Furthermore, experimental 16 and computational 26 studies suggested that primary amineterminated gold NPs interact favorably at the L o /L d phase boundary, but direct observation of such interaction has not yet been achieved. While these unique interactions with phase-segregated bilayers have been explored for a very small subset of NP types, the generalizability of these results to NPs with other surface functionalizations remains elusive.…”

“…Atomic force microscopy (AFM) has been used previously to study interactions between NPs and phase-segregated bilayers, providing Ångström resolution and a three-dimensional analysis of the surface. 21,22,25 Fluorescence microscopy has also been used to examine interactions between NPs and phase-segregated model membranes. 20,24,27 While not attaining the resolution of AFM, fluorescence microscopy provides high detection selectivity enabled by fluorescence tagging.…”

“…The primary amine surface termination was chosen as part of a systematic effort to decipher the impact of distinct positive functional groups at NP surfaces on the membrane structure. 16,25 Further, primary amine-terminated NPs have been hypothesized previously to interact with the L o /L d phase boundary, 16,26 but direct observations have not been achieved. We chose the trout gill epithelial cell line, an established model system for studying environmental exposures on aquatic organisms.…”