Steering Acoustically Propelled Nanowire Motors toward Cells in a Biologically Compatible Environment Using Magnetic Fields

Abstract

The recent discovery of fuel-free propulsion of nanomotors using acoustic energy has provided a new avenue for using nanomotors in biocompatible media. Crucial to the application of nanomotors in biosensing and biomedical applications is the ability to remotely control and steer them toward targets of interest, such as specific cells and tissues. We demonstrate in vitro magnetic steering of acoustically powered nanorod motors in a biologically compatible environment. Steering was accomplished by incorporating (40 ± 5) nm thick nickel stripes into the electrochemically grown nanowires. An external magnetic field of 40-45 mT was used to orient the motors, which were acoustically propelled along their long axes. In the absence of a magnetic field, (300 ± 30) nm diameter, (4.3 ± 0.2) μm long nanowires with (40 ± 5) nm thick magnetic stripes exhibit the same self-acoustophoretic behavior, including pattern formation into concentric nanowire circles, aligned spinning chains, and autonomous axial motion, as their non-magnetic counterparts. In a magnetic field, these wires and their paths are oriented as evidenced by their relatively linear trajectories. Coordinated motion of multiple motors and targeting of individual motors toward HeLa cells with micrometer-level precision was demonstrated.

Fabrication of Micro/Nanoscale Motors

“…As an example, the magnetic guidance of nanowires toward live HeLa cells in biocompatible buffers was demonstrated ( Figure 13C). 202 Because of interactions between the nickel parts, nanowires with a thin nickel segment at one end can assemble into geometrically regular dimers, trimers, and higher multimers while being levitated in a fluid acoustic cell. 205 The relative number of various assemblies depends on the density of the nanowire suspensions and the speed of their axial motion.…”

Synthetic Nano- and Micromachines in Analytical Chemistry: Sensing, Migration, Capture, Delivery, and Separation

“…Other forms of motion driven by self-generated gradients have more recently been demonstrated. For example, motors driven by self-diffusiophoresis (Figure 2b) are propelled by self-generated chemical concentration gradients (60,(76)(77)(78), and self-acoustophoretic motors (Figure 2c) are propelled by asymmetric steady streaming of the fluid around them in an acoustic field (63,(79)(80)(81). Temperature gradients can also be used by microparticles to create motion, a mechanism referred to as self-thermophoresis (65)(66)(67).…”

“…The motors can capture specific analytes via numerous mechanisms and then transport them to a specific destination guided by a magnetic field (15,63,85,134,135) or through chemotaxis. Capture of analytes can be divided into two main categories.…”

“…It is only in the past decade that researchers have managed to design self-powered synthetic nano-and micromachines (1,2) that convert chemical energy into mechanical motion. The initial discoveries have sparked tremendous interest in this emerging field , and a wide repertoire of synthetic machines that can be powered by chemical fuels (5,12,18,(38)(39)(40)(41)(42)(43)(44)(45)(46)(47) or externally applied fields that deliver magnetic (48)(49)(50)(51)(52)(53)(54)(55)(56), electrical (14,57), light (58)(59)(60)(61), acoustic (62)(63)(64), or thermal (65)(66)(67) energy have been designed.…”

Interplay Between Mechanochemistry and Sonochemistry

“…The fuel-free propulsion of a nanomotor by means of acoustic energy can be additionally oriented in the presence of a magnetic field, thus targeting specific cells and tissues. As a proof of concept, a ferromagnetic nickel stripe has been electrochemically grown between diamagnetic Au and Ru segments, which then respond to a weak external magnetic field (40-45 mT) used to orient nanowires that move along their long axes, propelled by SAWs (3.7 MHz) [129]. These features enable m-level precision toward living human cervical cancer cells (HeLa cells) cultured in an aqueous phosphate buffer ( Figure 24).…”