Abstract
Chemical imaging holds great promise for chemical, materials, and biological applications. However, its contrast often relies on subtle spectral differences arising from molecular-level changes. Here, we introduce label-free chemical imaging based on bond-specific coherent interference, which is highly sensitive to nanoscopic structural variations in collagen fibers. We demonstrated this idea using vibrational sum-frequency generation (VSFG) microscopy to identify lung tumors by detecting collagen I structural remodeling. Hyperspectral VSFG images reveal dramatic differences in the NH and CH(x) stretch regions between metastatic and tumor-free lung tissues. Based on these differences, key spectral signatures, such as the intensity ratio of NH(S)/CH(2,Ss) and CH(S)/CH(2,Ss) modes, were established to reliably distinguish metastatic tumor and tumor-free tissues with high fidelity. Theoretical modeling based on structural knowledge from Electron Microscopy indicates that distinctive interferences between VSFG vibrational modes make the intensity ratio directly related to interfibrillar distances at the 20-50 nm level. These findings suggest that collagen fibrils are more densely packed in tumors, corroborating the enhanced stiffness observed in tumor tissues. This result establishes an interference-based imaging mechanism that enhances chemical contrast and enables nanoscale structural readouts without labels. This method not only preserves sample integrity but also provides a powerful platform for fundamental biophysical studies and holds strong potential for future applications in oncology and pathology.