An optical tweezer is a scientific instrument that uses a highly focused laser beam to hold and move microscopic objects, much like a science-fiction tractor beam. This tool allows researchers to control the smallest building blocks of life, and its development earned inventor Arthur Ashkin a share of the 2018 Nobel Prize in Physics for its application to biological systems.
How Light Exerts Force
Optical tweezers can manipulate matter because light, composed of particles called photons, carries momentum. When light strikes an object, this momentum is transferred, resulting in a tiny push. This transfer creates two forces, the first being the scattering force, which pushes a particle in the direction the light is traveling.
The second force, the gradient force, is what makes trapping possible. An optical tweezer uses a microscope objective to focus a laser beam into a tight spot, creating a region where light intensity is highest at the center. The gradient force is an attractive force that pulls a microscopic object toward this area of greatest light intensity.
Imagine a transparent bead placed in this focused beam. If the bead drifts from the center, more intense rays from the middle of the beam strike its surface and refract. Due to the conservation of momentum, as the light changes direction, it imparts an opposite push on the bead, guiding it back to the focal point. For a stable trap to form, this inward-pulling gradient force must be stronger than the outward-pushing scattering force, locking the particle in place in three dimensions.
Manipulating the Microscopic World
Scientists can use optical tweezers to trap and maneuver objects from the nanoscale up to the size of entire cells, providing a way to interact with the fundamental components of biological systems. This includes:
- Individual atoms
- Nanoparticles used in materials science
- Viruses
- Living cells such as bacteria, yeast, and red blood cells
A primary advantage of this technique is its non-invasive nature. Because the trap is made of light, it can hold delicate specimens without damaging physical contact. In 1987, Arthur Ashkin first demonstrated this by capturing living bacteria without harming them, which allowed for the study of biological processes in real-time.
Applications extend to the molecular level, allowing for the precise study of DNA, RNA, and proteins. This is often done by attaching a microscopic silica or plastic bead to the molecule of interest. The optical tweezers then grab this bead, using it as a handle to stretch, move, or exert controlled forces on the attached biological structure.
Breakthroughs Using Optical Tweezers
The ability to apply piconewton-scale forces has led to discoveries in cellular biology. For instance, scientists measured the force of molecular motors by attaching a bead to a single kinesin protein. They then used optical tweezers to observe it “walking” along a microtubule, a part of the cell’s skeleton. This allowed them to measure the motor’s 8-nanometer steps, showing how materials are transported within a cell.
Another research area involves studying the physical properties of DNA. By attaching beads to both ends of a DNA strand, researchers can use two optical tweezers to stretch the molecule. This technique measures the elasticity of the DNA double helix and reveals how it interacts with enzymes responsible for copying and repairing it, offering insights into genetic processes.
In physics and engineering, optical tweezers are used to build microscopic structures. Researchers use the laser trap’s precise control to pick up and place nanoparticles, assembling them into specific arrangements to create nanostructures and tiny devices. The ability to manipulate matter at this scale has opened new avenues for understanding both the biological and physical worlds.