New research optimizes quantum dot probe size for single-receptor imaging

8/19/2020

Huan Song

Illinois researchers have created the smallest quantum dots (QDs) to date to evaluate individual receptors in living cells. The team published their findings in a paper titled, “Optimizing quantum dot probe size for single-receptor imaging” in ACS Nano. 

Quantum dots are fluorescent nanoparticles that can be used in single-molecule imaging. Their unique physiochemical and photophysical properties at this size makes quantum dots particularly suited to imaging applications. Specifically, QDs can be used in cell receptor imaging, a widely applied biotechnology test performed in life science labs and used for clinical diagnostics of diseases like cancer. 

"Many diseases are diagnosed by determining the number of specific molecules in tissues or blood from a patient," said Andrew Smith, bioengineering professor and a member of the research team.  Different diseases might exhibit different concentrations of proteins and mRNA molecules. Sometimes, diseased cells might have higher or lower concentrations of particular molecules than normal cells. At other times, there might be an absence of a normal molecule or the presence of an abnormal one. 

"QDs are a way that these can be measured because they emit light that can be detected, as the molecules themselves do not have an intrinsic signal," said Smith, who is also affiliated with the Holonyak Micro & Nanotechnology Lab, Carle Illinois College of Medicine, department of materials science and engineering and the Cancer Center at Illinois

In a previous study through the Illinois Mayo Alliance, Smith has worked with a team to develop a quantum dot-based FISH method to image prostate cancer cells. 

The research team in this study analyzed the impact of nanocrystal size on receptor labeling on live and fixed cells. Three QD sizes were tested: 9.2 nm, 13.3 nm and 17.4nm. The work focused on GluR2 membrane receptors on fixed and permeable HeLa cells and in the live hippocampal neuron synapses. To determine whether QDs are binding to the correct cells, researchers counted how many QDs are bound to cells expressing the enhanced green fluorescent protein (eGFP). 

Similar to FISH technique, this study used a light microscope to image individual cells. "The QDs are much smaller than the cells and we can see individual ones because they emit light. We then use computer algorithms to determine their number from images collected in the light microscope," Smith said. 

Even with reduced photophysical properties, the paper still finds that QD9.2 (3.2mn (core)shell alloyed nanocrystal) was the best size probe at least for the GluR2 membrane receptors. Smith said, "this study shows that there is substantial merit in further engineering to shrink the size of QD labels which is an ongoing project in our lab in collaboration with other labs on the paper." 

This multidisciplinary team included four principal investigators, Andrew Smith and Pablo Perez-Pinera from the department of bioengineering, Paul Selvin from the department of physics and Hee Jung Chung from the School of Molecular and Cellular Biology. The first author of this study, Phuong Le, is a 2020 Schmidt Science Fellow. The National Institutes of Health supported this research.

The paper “Optimizing quantum dot probe size for single-receptor imaging” is available online: https://doi.org/10.1021/acsnano.0c02390