Bioresponsive barcodes, capable of detecting molecules in a single liquid droplet, generated radiative energy that successfully converted dynamic biomolecular information into trillions of distinctive photonic barcodes. Microdroplet radiative energy that was transferred to binding biomolecules enabled the conversion. A team led by Yu-Cheng Chen of the Bio+Intelligent Photonics Laboratory at Nanyang Technological University demonstrated the process, introducing the concept of resonance energy transfer at the interface of the microcavity. The development overcomes the inability of optical barcodes to characterize what Chen called “dynamic changes” in response to analytes occurring over time. The work additionally expands on the idea that optical barcodes, historically, refer to a fixed spectral pattern’s correspondence to a single, unchanged target. Optical barcodes are used in/for multiplexed bioassays and cell tagging, as well. Via unique spectra fingerprints, for example, they support tracking and detection useful to enacting security measures. The system’s base is an active whispering-gallery mode resonator (WGMR). The resonator uses the present analyte as a gain medium, Chen explained, to support the excitation and subsequent collection of free space. That component of the process allows the resonator to acquire biological information from emission signals. Most WGMRs operate passively; they require evanescent wave coupling to achieve functionality, and operate based on mode changes. For mode changes to take place, perturbations must occur first. The factor created by the analytes’ mode occupation outside the cavity commonly causes a reduction to the effective Q-factor, as well as a poor signal-to-noise ratio (SNR). The Chen-led team was able to bypass this issue through resonant energy transfer — the very concept of which separates what are known as donor molecules from acceptor molecules at the interface of the cavity. It is at that interface that the transfer of radiative energy takes place, accompanied by electromagnetic radiation that permits the transfer of energy even when separation occurs between donor and acceptor molecules. Dynamic photonic barcodes enable molecular detection. Courtesy of Zhou et al. When mechanisms that include or are enhanced by a cavity, the efficient transfer of energy (as demonstrated) and donor/acceptor coupling can enhance light-matter interactions and the SNR, Chen said. A high concentration of dye from the donor molecules inside of the microdroplet triggered a cavity-enhanced energy transfer in the new system. This excited the acceptor molecules 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,” Chen said. By improving the SNR ratio after binding to target molecules, the system achieved dynamic spectral barcoding. The resulting encoding system increases the complexity of such systems. Applications, the researchers said, exist in biosensing and optical encryption. The research was published in Advanced Photonics (www.doi.org/10.1117/1.AP.2.6.066002).