Enantioselective manipulation of single chiral nanoparticles using optical tweezers

Abstract

We put forward a twofold enantioselective method for chiral nanoparticles using optical tweezers. First, we demonstrate that the optical trapping force in a typical, realistic optical tweezing setup with circularly-polarized trapping beams are sensitive to the chirality of core-shell nanoparticles, allowing for efficient enantioselection. Second, we propose another enantioselective method based on the rotation of core-shell chiral nanoparticles' equilibrium position under the effect of a transverse Stokes drag force. In this case, the chirality of the particle shell gives rise to an additional twist, thus leading to a strong enhancement of the optical torque driving the rotation. Both chiral resolution methods are shown to be robust against the variation of the size of the system and material parameters, suggesting it could be applied to a wide range of experimental situations, particularly those of biological interest for which optical tweezing is widely used. Our results provide alternative enantioselective mechanisms and pave the way for all-optical manipulation and enantiopure synthesis of chiral nanoparticles. In addition, they can also be applied in the characterization of chirality for testing existing methods of enantioselection.

Theory of optical tweezing of dielectric microspheres in chiral host media and its applications

“…Although the optical torque (OT) on a transparent isotropic microsphere centered along the beam symmetry axis vanishes, transfer of SAM to the center of mass can still be observed in this case from the analysis of Brownian fluctuations 24 or by driving the sample laterally so as to displace the equilibrium position from the beam axis 26 . OT is predicted to be significantly enhanced in the case of chiral particles, opening the way for enantioselective manipulation and characterization of the chiral response of individual nanoparticles with optical tweezers 36 .

Figure 2 ( a ) Schematic representation of the optical torque on a particle trapped in a chiral medium.

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“…The optical force on chiral particles has been employed for chirality sorting and recognition in optical traps [27][28][29][30] . Enantioselective optical manipulation schemes have been proposed [31][32][33][34] , including the possibility of employing optical torques 35,36 . The transfer of optical angular momentum dramatically changes when trapping particles in a chiral host medium 37 .…”

“…We consider a CP trapping beam and calculate the optical torque resulting from the transfer of SAM to the trapped particle. When a lateral Stokes drag force is simultaneously applied 26,36 , the equilibrium position is displaced sideways and rotates around the optical axis as a consequence of the optical torque. The angle of rotation is strongly chiral-dependent and very large when considering radii a few times larger than the wavelength.…”

Optical separation and discrimination of chiral particles by vector beams with orbital angular momentum

“…In recent years, optical forces on chiral objects have attracted considerable attention not only for the optical separation of enantiomers but also for the further identification of chirality. [23][24][25][26][27][28][29][30][31][32] However, most of these studies have been restricted to the dipolar or geometric optics regime, [23][24][25][26][27][28][29][30] and use scalar beams with spatially uniform states of polarization (SOP) as illumination. [23][24][25][26][27][28][29]31,32 Moreover, it is usually need to change the SOP of incident beam to realize the recognition of another enantiomer.…”

“…[23][24][25][26][27][28][29][30][31][32] However, most of these studies have been restricted to the dipolar or geometric optics regime, [23][24][25][26][27][28][29][30] and use scalar beams with spatially uniform states of polarization (SOP) as illumination. [23][24][25][26][27][28][29]31,32 Moreover, it is usually need to change the SOP of incident beam to realize the recognition of another enantiomer. [23][24][25]28,29 .…”