Rational Design and Engineering of Quantum‐Dot‐Sensitized TiO₂ Nanotube Arrays for Artificial Photosynthesis

Abstract

Redox enzymes can catalyze complex synthesis reactions under mild conditions but conventional catalysts rarely accomplish this task. Despite the high potential of redox enzymes for the synthesis of valuable compounds (e.g., chiral alcohols and drug intermediates), [1][2][3][4][5] their application is hampered by the high cost of enzyme-specifi c cofactors that are required as a redox equivalent, such as nicotinamide adenine dinucleotide (NAD(P) H) and fl avin adenine dinucleotide (FADH). Thus, numerous efforts have been made over the past decades to accomplish in situ cofactor regeneration from their oxidized counterpart. [6][7][8][9] For example, researchers found that NAD(P)H can be successfully regenerated by introducing secondary enzymes [10][11][12] that reduce its oxidized counterpart (i.e., NAD(P) + ) or electrodes [13][14][15] with an external power supply into reaction media. However, these approaches present intrinsic drawbacks (e.g., by-product formation and requirement of secondary enzymes for biocatalytic regeneration, as well as extremely low yield and high overpotential for electrochemical regeneration) that hindered their practical application beyond the laboratory scale. [6][7][8] Herein, we report on the development of quantum-dotsensitized TiO 2 nanotube arrays for redox enzymatic synthesis coupled with the photoregeneration of nicotinamide cofactors via inspiration from natural photosynthesis. In natural photosynthesis, [ 16 , 17 ] incident light electronically excites a membranebound protein-pigment complexes called a photosystem. The photogenerated electrons are rapidly delivered to reaction centers along the electron transport chain for regenerating NADPH cofactors. These cofactors drive redox enzymatic reactions to synthesize organic compounds in the Calvin cycle. Its unique features (e.g., environmental compatibility and nearunity quantum yield) have fascinated scientists and provided inspiration to improve the effi ciency of solar cells and photoelectrochemical hydrogen production systems. [18][19][20][21][22][23][24] In the present study, the photosystem for in situ NAD(P)H regeneration consisted of TiO 2 -CdS nanotubes as a photoelectrode, triethanolamine (TEOA) as an electron donor, and pentamethylcyclopentadienyl rhodium bipyridine ([Cp * Rh(bpy)(H 2 O)] 2 + ) as an electron mediator and a hydride transfer catalyst ( Figure S1, Supporting Information). As a photoelectrode for non-enzymatic regeneration of NAD(P)H, a nanotubular TiO 2 -CdS fi lm has many advantages that include easy synthesis and morphology control, [ 25 , 26 ] effi cient charge separation, [ 24 , 27 ] and better diffusion of reaction species through nanotube channels. Due to the small size and more negative position of the conduction band (CB) edge of CdS compared to TiO 2 (at least 0.2 V more negative), photogenerated electrons can be rapidly injected from CdS to TiO 2 in a thermodynamically favorable manner ( Figure S1, Supporting Information). This injection suppresses electron-hole recombination, which is more...

Photobiokatalyse: Aktivierung von Redoxenzymen durch direkten oder indirekten Transfer photoinduzierter Elektronen

“…Hierdurch ergeben sich in der Folge meist sehr komplexe Gesamtreaktionen. [132][133][134] Park et al beschrieben ein selbstorganisiertes Hybridsystem aus Peptidnanorçhren und einem Photosensibilisator unter Einsatz von Diphenylalanin (Phe-Phe,F F) als molekularem Baustein. Dahingegen wird die elektrochemische Oxidation von NADP(H) als sauberer Prozess betrachtet, der keine Nebenprodukte liefert.…”

“…Dahingegen wird die elektrochemische Oxidation von NADP(H) als sauberer Prozess betrachtet, der keine Nebenprodukte liefert. [132] Der Einsatz photosynthetisierender Mikroorganismen wie Cyanonbakterien stellt eine Mçglichkeit der photogetriebenen Regeneration von NADH dar,i nd er der Einsatz zusätzlicher Elektronendonoren oder Redoxmediatoren vermieden werden kann. [123] Das zur Oxidation von NAD(P)H nçtige Potential kann durch Photosensibilisation erreicht werden.…”

Product Selectivity in Homogeneous Artificial Photosynthesis Using [(bpy)Rh(Cp*)X]n+-Based Catalysts

“…A broad variety of photosensitizers have been applied in these systems, ranging from small organic dyes [85,[89][90][91] to functionalized graphenes [92][93][94][95], photoredoxactive polymers [96,97], red light absorbing tin porphyrines from Knör and coworkers [98], ruthenium polypyridine complexes [32,97,99], and different solid photoactive materials [100][101][102][103][104][105][106][107][108][109][110].…”

Photobiocatalysis: The Power of Combining Photocatalysis and Enzymes

“…[62] Not surprisingly, the concept of TiO 2 structuring as nanotubes has also been applied for the development of a photobiocatalyst. [63] Specifically, CdS quantum dots have been deposited on TiO 2 nanotubes to develop a heterojunction nanostructure for photoelectrocatalytic NAD + reduction using a mediator. The narrow band gap CdS semiconductor absorbs visible light and injects one electron into the conduction band of TiO 2 that due to its morphology allows fast migration of the electron through the nanotube over long distances, favoring charge separation.…”

“…CdS quantum dot-sensitized TiO 2 nanotube arrays for photocatalytic reduction of NAD + allowing the operation of a coupled enzymatic system. (Taken with permission from ref [63] ).…”