Abstract:

Vibrational tags are small chemical motifs that exhibit optically active modes in the cell’s silent region of the vibrational spectrum. Such tags have emerged as powerful tools for bio-imaging, providing researchers with the ability to track and visualize biomolecules with high specificity. Among these, alkyne tags are particularly effective due to their narrow bandwidth, strong Raman response, spectral tunability, and inertness in biological samples. These properties make alkyne-derived vibrational tags ideal for chemically selective imaging of biological tissues and cells.

In this study, we designed a new set of alkyne-derived vibrational tags aimed at boosting the Raman response and/or improving their infrared absorption signatures. Our design strategy involved linking alkyne-based moieties to the biomolecular target via a sulfur linker. This connection was strategically chosen to increase the polarizability of the alkyne through n − π bonding. We also explored the impact of different substituent groups on the terminal carbon of the alkyne, including aromatic groups to enhance the Raman intensity through π − π interactions and silicon-based groups to increase the transition dipole moment of the alkyne unit, thereby making the tags IR-active. The result is a versatile collection of vibrational tags with a robust vibrational response, narrow vibrational resonances, and spectral tunability.

To evaluate the effectiveness of these tags, we linked them to palmitic acid as the exemplar target and studied their uptake and metabolic conversion in cell cultures. The cells were examined using a custom-built coherent Raman scattering (CRS) microscope and a Fourier transform infrared (FTIR) microscope. This approach allowed us to successfully visualize the cellular uptake and metabolic processing of the tagged compounds, demonstrating the tags’ practical application in bio-imaging. Our work sets the stage for further enhancement of vibrational tag performance, as well as applying them to solve complex biological problems.