Review on optofluidic microreactors for artificial photosynthesis

Abstract

Artificial photosynthesis (APS) mimics natural photosynthesis (NPS) to store solar energy in chemical compounds for applications such as water splitting, CO₂ fixation and coenzyme regeneration. NPS is naturally an optofluidic system since the cells (typical size 10 to 100 µm) of green plants, algae, and cyanobacteria enable light capture, biochemical and enzymatic reactions and the related material transport in a microscale, aqueous environment. The long history of evolution has equipped NPS with the remarkable merits of a large surface-area-to-volume ratio, fast small molecule diffusion and precise control of mass transfer. APS is expected to share many of the same advantages of NPS and could even provide more functionality if optofluidic technology is introduced. Recently, many studies have reported on optofluidic APS systems, but there is still a lack of an in-depth review. This article will start with a brief introduction of the physical mechanisms and will then review recent progresses in water splitting, CO₂ fixation and coenzyme regeneration in optofluidic APS systems, followed by discussions on pending problems for real applications.

Keywords: artificial photosynthesis; carbon dioxide fixation; coenzyme regeneration; microfluidics; optofluidics; water splitting.

Figure 1

(A) A typical plant leaf. (B) Chloroplasts inside the plant cells. The average size of the chloroplasts is 6 µm (ranging from 3 to 10 µm). (C) Plant cell chloroplast structure. Adapted from [22], copyright BioMed Central Ltd. 2014. (D) Thylakoid membrane containing photosystem II reaction centers P680 and photosystem I reaction centers P700. Adapted from [23], copyright 2013 Michelet, Zaffagnini, Morisse, Sparla, Pérez-Pérez, Francia, Danon, Marchand, Fermani, Trost, and Lemaire.

Figure 2

Basic principles and reactions in artificial photosynthesis, in which the processes of water splitting, CO₂ reduction and coenzyme regeneration all utilize electrons on the reduction site.

Figure 3

Schematic diagrams of heterojunction and Z-scheme systems. Transfer of charge carriers in (A) the heterojunction-type photocatalytic system, (B) the Z-scheme PS-A/D-PS system, (C) the Z-scheme PS-C-PS system, and (D) the Z-scheme electron PS-PS system. Adapted from [68], copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA. PS stands for photosystem.

Figure 4

(A) Schematic of the high-surface-area optofluidic microreactor with micropillar structure. (B) Staggered micropillars in the reaction chamber. (C) Fabrication procedure of the optofluidic microreactor with the catalyst-coated micropillars. Reprint with the permission from [74], copyright 2014 Elsevier Ltd.

Figure 5

(A) Schematic of the high-surface-area optofluidic microreactor with micro-grooved structure. (B) Fabrication procedure of the optofluidic microreactor with the catalysts on the PDMS substrate. Adapted from [36], copyright 2015 Elsevier Ltd.

Figure 6

Schematic of the optofluidic membrane microreactor for photocatalytic CO₂ reduction. Adapted from [76], copyright 2016 Elsevier Ltd.

Figure 7

(A) Bacterium–CdS hybrid system that has CdS nanoparticles on the bacterium membrane (yellow particles). (B) Pathway diagram for the light harvesting and the photosynthetic conversion of CO₂ to acetic acid with the bacterial enzyme system. Adapted from [79], copyright 2016 American Association for the Advancement of Science.

Figure 8

Microfluidic APS platform that incorporates quantum dots and redox enzymes for photoenzymatic synthesis. (A) Concept and (B) besign of microreactor in which the cofactor regeneration takes place in the light-dependent reaction zone and the enzymatic synthesis in the light-independent zone. Adapted from [88], copyright 2011 Royal Society of Chemistry.

Figure 9

Microfluidic chip based artificial photosystem I. (A) Schematic illustration of the PS I reaction center. (B) One-step fabrication process of the immobilized artificial PS I (IAPSI) in the form of the g-C₃N₄-M film. (C) Simple procedures to fabricate the IAPSI microreactor. (D) Photograph of the as-fabricated IAPSI microreactor, in which the inset presents the leaf-like shape of g-C₃N₄-M. The scale bar is 2 mm. Adapted from [89], copyright 2016 The Royal Society of Chemistry.