An alphabet too small to read without a microscope
At the University of California, Los Angeles, researchers have developed a method for producing colloidal dispersions of letter-shaped microscopic particles. The structures may have application in cell biology and in studies of thermodynamic self-assembly, and the technique can be modified to create nanoscale particles as well.
To manufacture the structures — called LithoParticles — the scientists used automated stepper UV lithography. Past efforts to create monodisperse colloids with other bottom-up techniques produced a limited number of geometric shapes, but none as complex as letters. Movable structures for microelectromechanical systems applications also tend to remain attached to the wafer’s surface, rather than being freed completely into solution.
First the scientists created masks for the 26 letters in the English alphabet using electron beam lithography. Then they used an Ultratech i-line stepper at 365 nm — an automated lithographic projection system — with the mask to expose photoresist mixed with red, blue and green fluorescent dye. The method yielded up to 250 million particles per minute, as opposed to the hundreds of particles per second obtainable with continuous-flow lithography.
A fluorescence micrograph of a colloidal dispersion showsletter-shaped lithographic particles colored with fluorescent dyes.
They imaged the letters under multiwavelength excitation with a Leica confocal microscope and examined their uniformity and definition with a JEOL scanning electron microscope. The 1-μm-thick characters were about 7 μm tall and 4 μm wide — the same height and width as a 1/100-point font.
Other particles the researchers made assumed a variety of configurations, from square doughnuts to triangular prisms. They also manufactured multilayered particles that incorporated magnetic components and organic dyes.
If their surfaces were modified with biopolymers, the particles could serve as fluorescent probes for marking cells and for studying intracellular microstructural changes, according to the team. With deep-UV steppers, submicron particles of similar quality could be produced. Manipulated with laser tweezers, such structures might be placed on specific cells as a label. They also could prove useful in studies of thermal-driven self-assembly of differently shaped components.
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