Researchers at MIT have developed a method that lets them read the signal from fluorescent sensors that are as deep as 5.5 centimeters in tissue. Previously, it was very difficult to get a good signal from a fluorescent sensor placed that deep, as fluorescence emitted by the tissue itself would muddy the signal. The new technique involves using a laser with an oscillating wavelength to illuminate the fluorescent sensor, resulting in the emitted fluorescence to double its frequency. This allows the researchers to easily pick out the fluorescence signal from the background noise. The new method could allow for new applications for fluorescent sensors, such as monitoring tumors.
Fluorescence is a useful modality, particularly in biomedical research, where scientists use it to label and track a variety of biological molecules and processes. This works well with cells in a Petri dish, but when it comes to medical applications, the natural fluorescence created by our tissues can confound things. Naturally occurring fluorescence is known as autofluorescence, and this phenomenon makes it difficult to distinguish between sensor fluorescence and background fluorescent noise, particularly for sensors that are buried deep within a tissue.
This new approach hopes to increase the clinical utility of fluorescence. “If you have a fluorescent sensor that can probe biochemical information in cell culture, or in thin tissue layers, this technology allows you to translate all of those fluorescent dyes and probes into thick tissue,” said Volodymyr Koman, one of the lead developers of the new technology.
The method is called wavelength-induced frequency filtering (WIFF), and to achieve it the researchers use a combination of three lasers to create a laser beam that has an oscillating wavelength. When this beam strikes a fluorescent sensor, it doubles the frequency of the emitted fluorescence. This makes it much easier to pick the fluorescent signal out amongst the background fluorescence, and the technique increases the sensor signal-to-noise ratio more than 50-fold.
So far, the MIT team tested the technique in animals, and have shown that they can read the signal from a sensor that is 5.5 cm deep within the brain. One possible application involves tracking how effective chemotherapy is by implanting a fluorescent sensor near a tumor.
“We are working on technology to make small sensors that could be implanted near the tumor itself, which can give an indication of how much drug is arriving at the tumor and whether it’s being metabolized,” said Michael Strano, another researcher involved in the study. “You could place a sensor near the tumor and verify from outside the body the efficacy of the drug in the actual tumor environment.”
Study in Nature Nanotechnology: A wavelength-induced frequency filtering method for fluorescent nanosensors in vivo