# Selective Visualization of Type II Collagen Using Sum‐Frequency Generation (SFG)

## TL;DR

This paper introduces a new microscopy technique that can distinguish collagen Type II from Type I without labels, revealing its unique structural organization in cartilage.

## Contribution

The study introduces a polarization-resolved SFG method to specifically visualize and differentiate collagen Type II from Type I.

## Key findings

- Collagen Type II can be selectively identified using the XXY tensor element at 2860cm−1 in SFG measurements.
- Collagen Type II in rat auricular cartilage forms pocket-like domains of aligned fibrils rather than a uniform network.
- Polarization-resolved SFG microscopy provides label-free discrimination between collagen Types I and II.

## Abstract

Collagen Types I and II share highly conserved triple‐helical backbones and similar C—H stretch vibrational spectra, which limits the ability of conventional spectroscopic or second‐harmonic generation methods to unambiguously distinguish between them in native and engineered matrices. By combining polarization‐resolved sum‐frequency generation (SFG) measurements with tensor‐based simulations of the C—H stretch response, this work identifies collagen's asymmetric CH2 mode measured via the XXY tensor element at 2860cm−1 as a robust optical marker that exhibits distinct spatial symmetries for collagen Type I and Type II. In rat auricular cartilage, analysis of the polarization‐resolved SFG signatures combined with vertex component analysis reveals pocket‐like domains of differently oriented collagen Type II fibrils rather than a uniformly aligned network. These findings establish polarization‐resolved SFG microscopy as a structurally specific tool for mapping collagen Type II architecture and label‐free discrimination of collagen Types I and II.

Polarization‐resolved sum‐frequency generation microscopy enables the label‐free registration of collagen Type II, while discriminating it from Type I. The technique further reveals that in elastic cartilage, collagen Type II is organized into small domains of aligned fibrils.