To
address the growing demand for simultaneous imaging of multiple
biomarkers in highly scattering media such as organotypic cell cultures,
we introduce a new type of photoluminescent nanomaterial termed “tau-ruby” composed of ruby
nanocrystals (Al2O3:Cr3+) with tunable
emission lifetime. The lifetime tuning range from 2.4 to 3.2 ms was
achieved by varying the Cr3+ dopant concentration from
0.8% to 0.2%, affording facile implementation of background-free detection.
We developed inexpensive scalable production of tau-ruby characterized
by bright emission, narrow spectrum (693 ± 2 nm), and virtually
unlimited photostability upon excitation with affordable excitation/detection
sources, noncytotoxic and insensitive to microenvironmental fluctuations.
By functionalizing the surface of tau-rubies with targeting antibodies,
we obtained different biomarkers suitable for multiplexed lifetime
imaging. As a proof of principle, three tau-ruby bioprobes, characterized
by three mean lifetimes, were deployed to label three μ-opioid
receptor species expressed on transfected cancer cells, each fused
to a unique epitope, so that three types of cells were lifetime-encoded.
Robust decoding of photoluminescent signals that report on each cell
type was achieved by using a home-built lifetime imaging system and
resulted in high-contrast multiplexed lifetime imaging of the cells.
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