Optical trapping uses a highly focused laser beam to hold and move microscopic objects like cells, bacteria, and DNA strands. This non-invasive technique provides physical control over the microscopic world using only light, making it a significant tool in both biology and physics. It allows for the precise manipulation of objects mere nanometers in size.
The Gentle Grip of Light: How Optical Traps Work
Optical trapping works because light carries momentum and can exert force. When a laser beam interacts with a microscopic particle, two primary forces are involved. The first is the scattering force, where photons reflecting off the particle’s surface push it in the direction of the beam, much like a stream of water pushing a small ball.
The second is the gradient force. A focused laser beam is most intense at its center, creating an energy gradient. This gradient attracts dielectric particles—materials that do not conduct electricity easily—pulling them toward the brightest point. This acts as a restoring force, pulling the particle back to the center if it drifts.
For a stable trap to form, the gradient force must overcome the scattering force. This is achieved by focusing the laser beam very tightly to create a steep intensity gradient. The result is a three-dimensional trap where the particle is held slightly downstream from the laser’s focal point, at a position where the forward-pushing scattering force is balanced by the pull of the gradient force.
Building Blocks of an Optical Trap
An optical trapping system, often called “optical tweezers,” is constructed with a few core components, and the entire apparatus is often built upon a standard optical microscope platform.
- A laser to provide the focused beam of light. Near-infrared lasers are common in biology as their wavelength is less likely to damage living cells.
- Optics, such as mirrors and lenses, to steer and expand the beam before it enters the objective.
- A high-quality microscope objective that focuses the laser to create the trap and allows the scientist to view the specimen.
- A detection system, such as a camera, to observe and record the experiment. Position detectors can also be included to measure the particle’s movement and calculate forces.
Exploring the Microworld: What Can Be Trapped and Manipulated
Optical traps can manipulate a wide range of objects, from nanoscopic to microscopic. In biology, they can safely trap living organisms like bacteria, yeast, and various cells, as well as smaller entities like viruses and proteins. They can also manipulate non-biological items like polystyrene spheres and small metal particles.
Beyond simply holding an object, optical tweezers can move items with sub-micron accuracy. This allows scientists to draw paths for particles in a liquid medium or probe the elasticity of a cell membrane. It is also possible to measure incredibly small forces on the scale of piconewtons.
Revolutionizing Science: Applications of Optical Trapping
The unique ability to manipulate the microscopic world has led to significant scientific advancements. In biology, optical tweezers are used to study molecular motors like kinesin and myosin. Researchers can attach a bead to a single motor protein and trap it to measure the forces and stepping motions as the motor moves. The elasticity of DNA can also be studied by tethering a strand between two trapped beads and pulling them apart.
In nanotechnology, optical trapping is a microfabrication tool used to assemble nanostructures by placing components like nanowires with precision. The technique also enables physics research, allowing for direct observation of processes like Brownian motion and the interaction between light and matter.
Applications are also being explored in medicine for diagnostics and drug delivery. Optical tweezers could sort cells based on their properties or study the interactions between a drug-carrying nanoparticle and a target cell. Trapping individual cells allows scientists to explore complex interactions, like how immune cells respond to pathogens.
A Nobel Idea: The Story of Optical Tweezers
The concept originated with Arthur Ashkin at Bell Laboratories, who in 1970 published the first observation of forces from laser light trapping microscopic particles. His initial experiments used two counter-propagating laser beams to levitate tiny transparent spheres.
This work culminated in 1986 with the single-beam gradient force trap, which he named “optical tweezers.” This breakthrough showed a single focused laser beam could create a stable three-dimensional trap. A year later, Ashkin’s team used optical tweezers to trap living bacteria and viruses safely, showcasing the technology’s potential for biology.
For his work on optical tweezers and their application to biological systems, Arthur Ashkin was awarded a share of the 2018 Nobel Prize in Physics. His invention provided scientists with a tool to interact with the machinery of life on a previously inaccessible scale.