RESUMO
Plastic recycling strategies to combat rapidly increasing waste buildup are of utmost environmental importance. Chemical recycling to monomers has emerged as a powerful strategy that enables infinite recyclability through depolymerization. However, methods for chemical recycling to monomers typically rely on bulk heating of polymers, which leads to unselective depolymerization in complex polymer mixtures and the formation of degradation byproducts. Here, we report a selective chemical recycling strategy facilitated by photothermal carbon quantum dots under visible light irradiation. Upon photoexcitation, we found that carbon quantum dots generate thermal gradients that induce depolymerization of various polymer classes, including commodity and postconsumer waste plastics, in a solvent-free system. This method also provides selective depolymerization in a mixture of polymers, not possible by bulk heating alone, enabled by localized photothermal heat gradients and the subsequent spatial control imparted over radical generation. Photothermal conversion by metal-free nanomaterials facilitates chemical recycling to monomers, an important approach in addressing the plastic waste crisis. More broadly, photothermal catalysis enables challenging C-C bond cleavages with the generality of heating but without indiscriminate side reactions typical of bulk thermolysis processes.
RESUMO
Extracellular electron transfer (EET) could enable electron uptake into microbial metabolism for the synthesis of complex, energy dense organic molecules from CO2 and renewable electricity1-6. Theoretically EET could do this with an efficiency comparable to H2-oxidation7,8 but without the need for a volatile intermediate and the problems it causes for scale up9. However, significant gaps remain in understanding the mechanism and genetics of electron uptake. For example, studies of electron uptake in electroactive microbes have shown a role for the Mtr EET complex in the electroactive microbe Shewanella oneidensis MR-110-14, though there is substantial variation in the magnitude of effect deletion of these genes has depending on the terminal electron acceptor used. This speaks to the potential for previously uncharacterized and/or differentially utilized genes involved in electron uptake. To address this, we screened gene disruption mutants for 3667 genes, representing ≈99% of all nonessential genes, from the S. oneidensis whole genome knockout collection using a redox dye oxidation assay. Confirmation of electron uptake using electrochemical testing allowed us to identify five genes from S. oneidensis that are indispensable for electron uptake from a cathode. Knockout of each gene eliminates extracellular electron uptake, yet in four of the five cases produces no significant defect in electron donation to an anode. This result highlights both distinct electron uptake components and an electronic connection between aerobic and anaerobic electron transport chains that allow electrons from the reversible EET machinery to be coupled to different respiratory processes in S. oneidensis. Homologs to these genes across many different genera suggesting that electron uptake by EET coupled to respiration could be widespread. These gene discoveries provide a foundation for: studying this phenotype in exotic metal-oxidizing microbes, genetic optimization of electron uptake in S. oneidensis; and genetically engineering electron uptake into a highly tractable host like E. coli to complement recent advances in synthetic CO2 fixation15.