A counterintuitive approach to atomic force microscopy-infrared spectroscopy (AFM-IR) means researchers can now “see” the structure and chemical composition of human cells with unmatched precision.
Cells are the smallest functional units in our body and have long attracted the interest of researchers wanting to find out what cells are made of and where each element is located. As technology advanced, structural imaging of cells at the nanoscale became possible; however, methods are lacking to obtain a direct record of the chemical composition. Knowing what is inside the cells and where it is would create a useful cellular blueprint for a variety of disciplines, including biology, chemistry and materials science.
researchers at the Beckman Institute for Advanced Science and Technology (IL, USA) led by Rohit Bhargava have built on previous chemical imaging studies to simultaneously record the structure and chemical composition of human cells at nanoscale resolution.
Optical microscopy uses visible light to illuminate surface features such as color and structure, while chemical imaging relies on infrared (IR) light to “look” at the inner workings of a cell. When a cell is exposed to IR light, its temperature increases, causing it to expand. Each molecule type absorbs IR light at a slightly different wavelength and emits a unique chemical signature. These absorption patterns can be analyzed to show what molecules the cell contains.
AFM-IR can capture both the physical structure and the chemical structure of a sample with high resolution. The thermal expansion of molecules can be detected locally using a signal detector, a nanoscale needle that scratches across the surface of a sample while being excited by an IR laser. A map of the cell and absorption patterns can then be generated.
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Over the past decade, innovations in spectroscopy have worked to increase the signal strength of the initial IR wavelengths. “It’s an intuitive approach because we’re conditioned to think bigger signals are better. We think, “The stronger the IR signal, the higher the temperature of a cell will be, the more it will expand and the more visible it will be,” Bhargava explained.
However, as the cell expands, the movement of the signal detector becomes more exaggerated and generates increased static (noise), preventing accurate measurements. This means that a stronger IR signal does not really improve the quality of the chemical images.
“It’s like turning the dial on a humming radio station — the music gets louder, but so does the hum,” said Seth Kenkel, the study’s lead author. “We need a solution to prevent the noise from growing alongside the signal.”
The researchers attempted to separate the IR signal from the movement of the detector, which would mean it can be amplified without being accompanied by additional noise. They used a small IR signal to test their method before amplifying it.
Using this method, high-resolution chemical and structural imaging of cells at the nanoscale was achieved. In addition, AFM-IR does not require fluorescent label or dye molecules to increase their visibility under a microscope.
“Now we can see the inside of cells at a much finer resolution and with significant chemical detail more easily than ever before,” Bhargava said. “This work opens up a number of possibilities, including a new way to study the combined chemical and physical aspects that drive human development and disease.”
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