About the Instrument:
About fifteen years ago some researchers were playing around with laser beams when they discovered that a tightly focused beam could suck a microscopic object into its focal point and trap it there. When they moved the laser, the object moved with it. They had discovered optical tweezers. The modern instrument is based around an optical microscope. It uses the microscope objective to focus a laser beam onto the microscope slide where the objects to be trapped sit in solution. A CCD detector is used to capture an image of the microscope slide and display it on a computer screen. You can use the mouse to scroll around the slide, find objects, trap them and move them around – it’s hard to remember you’re moving real objects and not just playing a computer game. The new instrument at Massey has three extra functions that make it extremely sensitive and versatile. Firstly, a ‘spatial light modulator’ splits the laser beam up to create multiple traps of different shapes and sizes – large ring traps, small oval traps etc. The second additional feature is a ‘piezo-electric stage’ which shifts the microscope slide side to side with nanometre precision. This allows you to easily scan around to find objects that have wandered out of view or measure very small position changes. Thirdly, the instrument has a probe laser for making extremely sensitive position measurements.
Optical tweezers are a physicist’s ticket into the realm of biology. At the microscopic scale you can view a biological organism as a machine performing mechanical processes. With optical tweezers a physicist like Bill can start to uncover the mechanical properties of these systems. One of Bill’s main aims is to use the tweezers to stretch biological molecules such as DNA and pectin. As he explains: “In a lot of the processes in molecular biology, transcription for example, DNA has to unwind.” The process is similar to a spring being stretched so it can be treated like a classical physics problem. Bill stretches molecules by attaching one end to the microscope slide and the other end to a microscopic silica bead. He then traps the bead with the tweezers and uses it as a handle to pull the two ends apart. By looking at how far the bead has been pulled away from the centre of the trap he can work out what force it takes to stretch the molecule – just like a mini force-meter. Understanding the physics of these processes will provide a new level of insight for biologists. Bill is also interested in measuring the microscopic properties of fluids. By trapping tiny silica beads and pulling them through different fluids you get a direct, almost tactile, experience of their texture and viscosity. In a thick jelly-like fluid the bead gets dragged out of the trap if you move it suddenly, whereas in a thin liquid it stays with the trap as you move it quickly from side to side. The research possibilities here are endless. You could, for example, insert a bead into a biological cell to measure the fluid properties of the cytoplasm. Or try to drag a bead through an interface. You could trap two beads next to each other, shake one and see what happens to the other. Tweezers can also be used to analyse emulsions, which consist of microscopic droplets of one fluid immersed in another and have great relevance to food technology. “You can basically grab emulsion drops like you can grab a particle,” Bill explains.
“The cool thing about optical tweezers,” says Bill, “is that you could sit around with almost any scientist in the Institute and think of something you could do with them. It’s pretty exciting.” “I’m a physicist interested in biology,” he says. “I think that it’s often looking over the fence into someone else’s area and thinking ‘Oh, maybe we could do this’ that something comes out of it.” Bill is already working with Juliet Gerrard’s biology group at Canterbury University and several plant biology groups in Europe and the United States. To help get research and collaborations started, the MacDiarmid Institute have funded a postdoctoral fellow. When Bill advertised for the position, he received applications from three top optical tweezers groups in Europe. The chosen candidate, Steve Keen from Glasgow University says “The optical tweezers were a big attraction. After spending my PhD aligning optical tweezers and testing out the use of high-speed cameras within optical tweezers setups I was keen to use my skills in a new project. I had never done single-molecule stretching, so it sounded like an exciting project to get my teeth into! So far I’m really enjoying the project and New Zealand!” Steve is happy to do ‘look-see’ experiments for potential collaborators.
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