Quantum dots (QDs) that emit light at the same brightness regardless of color could help bring fluorescence imaging into the realm of quantifiable data. Researchers at the University of Illinois at Urbana-Champaign engineered the QDs to have tunable emission wavelengths, extinction coefficients and quantum yields. Fluorescing at the same brightness but at different colors, the QDs could be used to measure concentrations of different molecules in tissue. "Previously light emission had an unknown correspondence with molecule number," said Andrew M. Smith, an assistant professor of bioengineering at Illinois. "Now it can be precisely tuned and calibrated to accurately count specific molecules. This will be particularly useful for understanding complex processes in neurons and cancer cells to help us unravel disease mechanisms, and for characterizing cells from diseased tissue of patients." Conventional fluorescent materials like quantum dots and dyes have mismatched brightness between different colors (left). When these materials are administered to a tumor to measure molecular concentrations, the signals are dominated by the brighter fluorophores. Brightness-equalized quantum dots yield evenly matched signals (right), allowing measurement of many molecules at the same time. Courtesy of the University of Illinois. Fluorescent dyes are common in bioimaging, but it has been difficult to use them for quantitative measurements because the amount of light emitted from a single dye is unstable and often unpredictable. The brightness of these fluorophores also varies drastically between different colors, which complicates the use of multiple dye colors at the same time. Brightness-equalized QDs, on the other hand, provide a consistent and tunable number of photons per tagged biomolecule. The QDs also maintain brightness over time, while conventional QDs with mismatched brightness become more mismatched over time. Other potential applications include precise color matching in light-emitting devices and displays, and for photon-on-demand encryption applications. The brightness-equalization principle should be applicable across a range of semiconducting materials, the researchers said. The research was published in Nature Communications (doi: 10.1038/ncomms9210).