Control over the emerging chirality in supramolecular gels and solutions by chiral microvortices in milliseconds

Abstract

The origin of homochirality in life is a fundamental mystery. Symmetry breaking and subsequent amplification of chiral bias are regarded as one of the underlying mechanisms. However, the selection and control of initial chiral bias in a spontaneous mirror symmetry breaking process remains a great challenge. Here we show experimental evidences that laminar chiral microvortices generated within asymmetric microchambers can lead to a hydrodynamic selection of initial chiral bias of supramolecular systems composed of exclusively achiral molecules within milliseconds. The self-assembled nuclei with the chirality sign affected by the shear force of enantiomorphic microvortices are subsequently amplified into almost absolutely chirality-controlled supramolecular gels or nanotubes. In contrast, turbulent vortices in stirring cuvettes fail to select the chirality of supramolecular gels. This study reveals that a laminar chiral microflow can induce enantioselection far from equilibrium, and provides an insight on the origin of natural homochirality.

Fig. 1

Microfluidic design for self-assembly of BTAC building blocks in microvortices. a Schematic hypothesis of the emergence of supramolecular chirality after molecules pass through the submarine rock micropores (left), and the imitated design of microfluidic device consisting of ten pairs of inclined microchambers (right) to generate counter-rotating microvortices. The blue (red) streamlines refer to CCW (CW) microvortices in the left (right) microchambers. L (R) indicates the left (right) outlet along the flow direction. Scale bar, 500 μm. The inset is SEM image of the cross-section of microchambers. Scale bar in the inset, 100 μm. b Effect of the shear forces of enantiomorphic microvortices (CW and CCW rotating directions) on the mirror symmetry breaking process of achiral BTAC molecules in DMF/H₂O mixtures leading to the formation of BTAC gels. M (P) chirality refers to left-handed (right-handed) helical twists

Fig. 2

Microvortex selection of supramolecular chirality of BTAC gels. a UV-Vis spectra of BTAC gels. The blue (red) line refers to measurement from the gels collected from the L- (R-) outlet. b CD spectra indicate opposite chirality for BTAC gels collected from different outlets. c, d SEM characterizations show predominantly M and P twists for BTAC gels from the L- and R-outlet, respectively. Size of chiral twists is 295.2 ± 10.8 nm in width, and 869.3 ± 31.1 nm in length (mean ± s.e.m.; n = 40). Scale bars, 1 μm. e CD signals of 28 independent experiments indicate nearly absolute chirality control of BTAC gels by microvortices. The void blue (solid red) circles indicate CD signals of gels from the L- (R-) outlets

Fig. 3

TPPS₄ J-aggregates with opposite chirality signs selected by microvortices. a Formation of chiral supramolecular nanotubes by the self-assembly of achiral TPPS₄ building blocks (blue) and the C₂mim⁺ ionic stabilizer (red) within the microvortices. b UV-Vis spectra exhibit a monomer peak at 433 nm and a J-aggregate peak at 491 nm for TPPS₄ assemblies from the L- (blue line) and R-outlet (red line). c CD spectra indicate opposite chirality of the TPPS₄ assemblies from the L- (blue line) and R-outlet (red line). d CD signals of 28 independent experiments reveal nearly absolute chirality control over supramolecular TPPS₄ assemblies from the L- (void blue circles) and R-outlet (solid red circles). e, f Cryo-EM examinations of TPPS₄ aggregates from the L- and R-outlet show the hollow tubular nanostructures with an average diameter of 12.8 ± 0.1 nm (mean ± s.e.m.; n = 40). Scale bars, 50 nm. The insets are the enlarged cryo-EM images. Scale bars in the insets, 20 nm

Fig. 4

Analysis of laminar chiral microvortices. a CFD simulation shows that fluid can rotate upward (red curves) or downward (blue curves) to generate laminar P- or M-chiral microvortices in the left microchamber. The color bar indicates the magnitude of flow velocity. The channel height is 50 μm. Scale bar, 100 μm. b A majority (~84%) of microvortices rotate upward, suggesting a predominantly P chirality (red bar) in the left microchamber. c Shear rate gradient from P-chiral microvortices in the left microchamber. Insert: shear rate (black line) obtained from simulation results (void black circles). d Residence time (gray bars) of nuclei within the left microchamber from simulation results

Fig. 5

Laminar chiral microvortices versus turbulent vortices in stirring cuvettes. a Schematic mechanism of laminar chiral microvortex-selected chirality of BTAC and TPPS₄ nuclei. The rapidly formed nuclei (<1 ms) are aligned in the laminar flow, and twisted by a shear force (F_s) to give rise to an initial chiral bias dependent on the rotation sense of microvortices. These primary nuclei then serve as the templates for the subsequent growth into supramolecular assemblies with the predefined chirality. Red, blue, and black axes refer to the spatial distributions of shear rate gradient and viscous shear force along the x, y, and z axes. b CFD simulation of chaotic flows generated by CW stirring at ~1000 rpm in cuvettes. Left: 2D representation of the flow direction (black streamlines) and velocity (color). Right: 3D streamlines display a snapshot of chaotic flows. The color bar indicates the magnitude of flow velocity. Scale bar, 5 mm. c CW stirring in cuvettes shows 9 positive CD signals (P chirality, solid red circles) and 11 negative CD signals (M chirality, void blue circles) of BTAC gels