<i>Operando</i> Monitoring of the Polymerization Process of Lignin Monomer and Oligomer Surrogates with Microstructured Fiber Grating Sensor

The enzymatic depolymerization is a promising route to valorize the lignin polymers by turning the cross-linked polymers into monomers or oligomers. However, the lignin polymers cannot be effectively converted into small chemicals, as the oligomers are prone to polymerization, which is particularly challenging to monitor and thus regulate. Here, we develop a microstructured fiber Bragg grating (mFBG) sensor to probe the dynamic polymerization process of typical lignin oligomer surrogates─guaiacol (monomer) and guaiacylglycerol-β-guaiacyl ether (GBG, dimer)─catalyzed by laccase in an <i>operando</i> way. The mFBG sensor was developed with its reliability well validated by control experiments at first. Further, <i>operando</i> monitoring of the polymerization reaction process of the typical lignin monomer (i.e., guaiacol) and dimer (guaiacylglycerol-β-guaiacyl ether, GBG) was demonstrated under various conditions with the mFBG sensor. The GC-MS and UV-vis absorption measurements were carried out as a further check. Finally, the specific polymerization characteristics and reaction mechanism were studied. The mFBG sensor enables <i>operando</i> monitoring of the heterogeneous polymerization process of lignin monomers and oligomers and can potentially be tailored to probe more complex lignin depolymerization processes and unveil enzymatic synergistic mechanisms for the biological transition of biomass.