Converting CO2 to biofuels with photosynthetic microalgae immobilized in cellulose nanofibrils

We created versatile templates for biofuel prodn. by using TEMPO-oxidized cellulose nanofibrils (TEMPO CNF) to immobilize photosynthetically active microalgae. In contrast to conventional suspension cultures, entrapping microalgae within a fibrillar matrix greatly enhance cell fitness and viability. The innate hydrophilic and hygroscopic nature and high surface area of TEMPO CNF create a system with favorable water transition properties, while the nanoscale fibrillar network provides mech. stability as well as control over the d./porosity structure. Furthermore, its transparency in the photosynthetically active radiation (PAR) region ensures optimal light utilization and enhanced photosynthetic productivity. To demonstrate this, we entrapped H2-producing microalgae in TEMPO CNF via three immobilization approaches: a pure TEMPO CNF hydrogel, a Ca2+ -stabilized TEMPO CNF hydrogel (Fig. 1A) and a chem. cross-linked TEMPO CNF film (Fig. 1B). All approaches showcased excellent biol. compatibility, as well as good H2 prodn. capacity with similar or higher values compared to the conventionally used alginate-based matrixes. The surface interactions between microalgae and TEMPO CNF were also revealed using Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D). We found that the anionic surface charges of both microalgae cells and TEMPO CNF prevent surface attachment, leading to immobilization via passive entrapment. Conversely, microalgae cells clearly attach to cationized TEMPO CNF surface. Thus, a simple surface modification can further tailor the matrix towards direct cell attachment. These findings will facilitate the development of new approaches for targeted biofuels and chems. prodn. in bio-industry.