Processing waste corncob into NIR transparent optical biofilter

Near infrared (NIR) transparent materials, featuring ultraviolet-visible (UV-vis) light blocking and high NIR light transmittance, are widely applied in night vision, privacy protection, and medicolegal identification. However, current NIR transparent materials poses the problems with safety, sustainability, and poor balance between UV-vis and NIR transmittance, urgently requiring effective engineering strategies to fabricate high-performance NIR transparent filters with environmental friendliness.

Considered the color of lignin and the fascinating optical property of cellulose, lignin−cellulose composites can theoretically be processed into the films that have low haze and dark color, meeting the requirements of NIR transparent filters. However, lignin in cellulose easily aggregates, thereby compromising interior compactness, producing optical haze, and reducing transmittance.

To address these challenges, the mainly efficient strategy is to eliminate gaps in lignin−cellulose composites. Professors Qinghua Feng, Yuning Zhong, and Chaoji Chen proposed a lignin capturing−fusing approach to fabricate a lignocellulose-based NIR transparent optical biofilter from waste corncob for the first time, which was published in Research, entitled “Lignocellulose-Based Optical Biofilter with High Near-Infrared Transmittance via Lignin Capturing–Fusing Approach”.

In this work, cellulose and acetic acid lignin were extracted from waste corncob. Then, the regenerated cellulose hydrogels were employed to capture lignin colloidal spheres based on solvent/antisolvent assembly of lignin. After preliminary drying and hot-pressing, the captured lignin could be fused to flow into the gaps in cellulose network and hold the fibers tightly via abundant hydrogen bonds, resulting a homogeneously dense structure.

The authors first provided a comprehensive analysis on the chemical and physical structure and suggested that the homogeneously dense structure and smooth surface were the keys to the unique optical properties. With the designed structure, the resulting biofilter featured high NIR transmittance of ~90%, ultralow haze of close to 0%, and strong UV-vis light blocking (~100% at 400 nm and 57.58% to 98.59% at 550 nm).

In addition, the authors mentioned that dense structure and lignin incorporation offered the biofilter with excellent comprehensive stability, including water stability, solvents stability, thermal stability, and environmental stability, which enabled the biofilter to be practically applied as NIR transparent materials.

Given the excellent properties of the biofilter, the researchers finally exhibited the applications of proposed biofilter in NIR region. The biofilter could act as a window for NIR diffuse spectroscopy to determine the diffuse reflection of an apple, prevent the NIR night vision monitor from strong light exposure, and protect private information. The authors hope that, in future, the proposed lignin capturing–fusing approach will prompt other high-performance optical biofilter fabrication to broaden the applications of lignocellulose-based biomass in optics.