Photonic Barcodes Record Energy Transfer

Illustration of dynamic photonic barcodes which enable molecular detection. (Source: Zhou et al., SPIE)

Optical barcodes enable detection and tracking via unique spectral fingerprints. They’ve been widely applied in areas ranging from multiplexed bioassays and cell tagging to anticounter­feiting and security. Yu-Cheng Chen of the Bio+Intelli­gent Photonics Laboratory at Nanyang Techno­logical University notes that the concept of optical barcodes typically refers to a fixed spectral pattern corresponding to a single target. “Optical barcodes have lacked the capability to charac­terize dynamic changes in response to analytes through time,” says Chen. Thanks to Chen’s research, that’s about to change.

Chen’s group recently developed bioresponsive dynamic barcodes, intro­ducing the concept of resonance energy transfer at the interface of the micro­cavity. Now, the team demons­trated the barcode experi­mentally to detect molecules in a droplet. The radiative energy from a single micro­droplet is transferred to binding biomo­lecules, converting dynamic biomo­lecular infor­mation into more than trillions of distinc­tive photonic barcodes. The system is based on a whispering-gallery mode resonator (WGMR). The majority of WGMRs are classified as passive. As such, they require evanescent wave coupling and operate based on mode changes induced by pertur­bations. “In contrast,” explains Chen, “active resonators that utilize the analyte as a gain medium can support free-space excitation and collection to acquire more biological infor­mation from emission signals.”

According to Chen, the trouble when considering molecular detection is the mode occu­pation factor of the analyte outside the cavity: It is only a few tenths from that inside the cavity, leading to a reduced effective Q-factor and unsatis­factory signal-to-noise ratio. The concept of resonant energy transfer separates donor molecules and acceptor molecules at the cavity interface, where radiative energy transfer happens. Radiative energy transfer is accompanied by electro­magnetic radiation – unlike conven­tional non-radiative fluores­cence resonance energy transfer, FRET. Because of that radiation, energy transfer can occur even in situa­tions where the donor and acceptor are separated.

“In the presence of cavity-enhanced mechanisms, efficient energy transfer and coupling between donors and acceptors may lead to enhanced light-matter interactions and signal-to-noise ratio,” says Chen. The developed system takes advantage of an effect whereby the high concen­tration of dye (donor) inside the micro­droplet triggers a cavity-enhanced energy transfer to excite the molecules (acceptor) attached to the cavity interface.

“When biomo­lecules bind to the cavity interface, the number of binding molecules alters the amount of energy transfer, resulting in distinc­tive modulated fluores­cence emission peaks,” says Chen. Dynamic spectral barcoding was achieved by a significant improvement in the signal-to-noise ratio upon binding to target molecules. According to the researchers, this biomo­lecular encoding system illu­minates a beacon for real-time inter­molecular inter­action and can greatly increase the complexity of an encoding system. They believe the concept can be widely applied in many biosensing appli­cations and optical encryption. (Source: SPIE)

Reference: Y. Zhou et al.: Dynamic photonic barcodes for molecular detection based on cavity-enhanced energy transfer, Adv. Phot. 2, 066002 (2020); DOI: 10.1117/1.AP.2.6.066002

Link: Bio+Intelligent Photonics Laboratory, Nanyang Technological University, Nanyang, China

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