How a laser beam can grab and pull a single molecule

Health · Published 18 October 2026 Swipe more like this →

In short: Imagine trying to manipulate a single strand of DNA or a microscopic motor using only light. Scientists use a technology called optical tweezers to exert piconewton-level forces. This allows them to grab, hold, and move tiny objects like atoms or cells to study the delicate mechanics of life.

Optical tweezers use highly focused laser beams to hold and move microscopic objects, such as atoms, nanoparticles, or biological molecules like DNA and proteins. This is achieved through radiation pressure, where the light provides an attractive or repulsive force depending on the refractive index of the object relative to its surrounding medium. These forces are typically on the order of piconewtons, making them ideal for studying the delicate mechanics of life at the molecular scale.

The technology was pioneered by Arthur Ashkin, who first reported the detection of optical scattering and gradient forces in 1970. By 1986, a single-beam gradient force trap was demonstrated for dielectric particles, a foundational step that led to the modern method of single-molecule manipulation. Ashkin's work was so significant that he was awarded the 2018 Nobel Prize in Physics for its development.

In biophysics, these 'light tweezers' allow researchers to measure forces, displacements, torques, and angles of molecular motors like kinesin and myosin. For example, researchers used them to directly measure the 8 nm step size of the kinesin motor protein. They can also study protein-nucleic acid interactions and the folding of RNA and proteins under specific tensions, such as identifying the overstretching transition of B-DNA at approximately 65 pN.

Beyond basic manipulation, the field is moving toward automation and multi-modal sensing. New platforms like SmartTrap aim to remove human operator bias by integrating 3D particle tracking and microfluidics for autonomous, large-scale studies. Additionally, optical tweezers are being combined with Raman spectroscopy and fluorescence to simultaneously monitor the mechanical properties and molecular states of cells, such as red blood cells, to aid in disease diagnosis and drug evaluation.

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