Dual-Responsive Lipid Nanotubes: Two-Way Morphology Control by pH and Redox Effects

Abstract

Lipid nanotubes are the preferred structures for many applications, especially biological ones, and thus have attracted much interest recently. However, there is still a significant need for developing more lipid nanotubes that are reversibly controllable to improve their functionality and usability. Here, we presented a two-way reversible morphology control of the nanotubes formed by the recently designed molecule AQUA (C25H29NO4). Because of its special design, the AQUA has both pH-sensitive and redox-active characters provided by the carboxylic acid and anthraquinone groups. Upon chemical reduction, the nanotubes turned into thinner ribbons, and this structural transformation was significantly reversible. The reduction of the AQUA nanotubes also switched the nanotubes from electrically conductive to insulative. Nanotube morphology can additionally be altered by decreasing the pH below the pKa value of the AQUA, at ∼4.9. Decreasing the pH caused the gradual unfolding of the nanotubes, and the interlayer distance in the nanotube's walls increased. This morphological change was fast and reversible at a wide pH range, including the physiological pH. Thus, the molecular design of the AQUA allowed for an unprecedented two-way and reversible morphology control with both redox and pH effects. These unique features make AQUA a very promising candidate for many applications, ranging from electronics to controlled drug delivery.

Smart Lipid Nanotubes for Easy Formation of Gold‐Lipid Hybrid Nanotubes and Tunable Gold Superstructures

“…At pH 1.0, AQua LNTs are transformed completely into sheets and spherical gold nanoparticle clusters can be observed, as shown in the TEM image of Sample 6 (Figure ). These 3D gold clusters have diameters of 40–160 nm.…”

“…A previous study revealed that the AQua LNT structure is disrupted below its pKa value, which is 4.9, and that the disruption degree increases with decreasing pH. At pH 3.0, most of the nanotubes have a somewhat unfolded, looser structure, but still have a tubular geometry whereas other LNTs are completely unfolded and exist as sheet‐type structures . However, as shown in Figure , the tubular geometry is almost completely destroyed and intermingled sheets are the dominant morphology.…”

“…We fabricated smart, stimuli‐responsive LNTs (Figure S1b) using a specially designed molecule (AQNH‐(CH 2 ) 10 COOH), containing anthraquinone and carboxylic acid functional groups, hereafter referred to as AQua (Figure S1a). AQua lipid nanotubes were used as the organic constituent to obtain hybrid structures due to their simple production, durability, multi‐functionality (both pH‐sensitive and redox active), self‐organization ability, high encapsulation performance, and low toxicity . In‐situ transformation of the Au(III) salt into gold nanoparticles on the LNT surfaces is used to obtain hybrid structures.…”

Electroactive Amphiphiles for Addressable Supramolecular Nanostructures

“…Further EAAs that demonstrate very elegant orthogonal switchability have also been prepared (see also Section 2.1.2 Drug delivery ). Specifically, an EAA based on anthraquinone, AQUA (AQNH(CH 2 ) 10 COOH, where AQ is anthraquinone), formed nanotubes in water in the presence of an equimolar amount of ethanolamine . Anthraquinone, a redox‐active group, can reversibly transition from a poor to a good conductor after reduction .…”

“…Specifically, an EAA based on anthraquinone, AQUA (AQNH(CH 2 ) 10 COOH, where AQ is anthraquinone), formed nanotubes in water in the presence of an equimolar amount of ethanolamine. [63] Anthraquinone, a redox-active group, can reversibly transition from a poor to a good conductor after reduction. [64] When chemically reduced, the nanotubes transformed into thin ribbons and exhibited an increase in conductivity.…”