Fluorescent organic nanoparticles operating as cell tracers outperform existing methods for long-term tracking of living cells

September 3, 2013

Noninvasive organic quantum dots emit light to track biological processes

An Asian research team has developed a method for continually tracking biological processes for long periods of time, using noninvasive fluorescent organic tracers, overcoming the limitations of inorganic quantum dots and other methods.

Bin Liu and Ben Zhong Tang of the A*STAR Institute of Materials Research and Engineering in Singapore and their co-workers developed inorganic quantum dots (nanocrystals) composed of a small number of organic molecules.

These new tracers promise superior performance in tracking the history of carcinogenesis or the progress of interventions such as stem cell therapies, for example.

Organic quantum dots

On average, Tat-AIE dots (above) are by more than one order of magnitude brighter than Qtracker 655 inorganic quantum dots (bottom) and display a more narrow (precise) distribution of fluorescence lifetimes centered around 2-3 ns, while Qtracker 655 shows a broad distribution of lifetimes ranging from several nanoseconds to tens of nanoseconds.

The new tracers are organic “quantum dots,” composed of a small number of molecules with optical characteristics that rely on quantum-mechanical effects.

Technically, the new quantum dots are referred to as aggregation-induced emission dots (“AIE dots”) because they become photostable and highly efficient fluorescent emitters when their component molecules aggregate.*

Compared with existing inorganic quantum dots (which contain heavy metal ions that can be toxic for living systems), the team’s carbon-based tracers also show improved biocompatibility and greater chemical stability with cell biochemistry.

They are also more resistant to bleaching by light and do not interfere with normal biochemical processes, the fluorescent signals emitted by the new quantum dots do not overlap with the signal naturally emitted by cells, and rather than “blink,” they show steady fluorescence.

Green fluorescent protein replacement

AIE dots also improve on green fluorescent protein (GFP) and its variants,which have been used for genetic cell tagging to achieve long-term cell tracing.

GFP is vulnerable to damage by proteolytic enzymes, it has spectral overlap with biosubstrate autofluorescence and poor photostability, and can disrupt normal cell function.

AIE dots could track isolated human breast cancer cells in vitro for 10 to 12 generations and glioma tumor cells in vivo in mice for 21 days, says Liu. “They outperform existing commercial inorganic quantum dots, and open a new avenue in the development of advanced fluorescent probes for following biological processes such as carcinogenesis, stem cell transplantation, and other cell-based therapies.”

The researchers plan to broaden the application of the organic tracers for use with magnetic resonance and nuclear imaging techniques.

* The assembly began with the synthesis of an organic molecules, 2,3-bis(4-
(phenyl(4-(1,2,2-triphenylvinyl)phenyl)amino)phenyl)fumaronitrile (TPETPAFN), which the researchers then encapsulated in an insoluble lipid-based matrix. Next, the researchers attached small peptide molecules derived from the human immunodeficiency virus (HIV) to exploit the ability of these peptides to promote efficient uptake of AIE dots into living cells.

(Credit for images: Li, K., Qin et al./Scientific Reports)