NADH Regeneration Promoted by Solar Light Using Gold Nanoparticles/Layered Double Hydroxides as Novel Photocatalytic Nanoplatforms

Source: NADH Regeneration Promoted by Solar Light Using Gold Nanoparticles/Layered Double Hydroxides as Novel Photocatalytic Nanoplatforms

The implication of enzyme catalysis for our word is tremendous. Enzymes act as catalysts and are involved in almost all the reactions which support life as digestion, respiration, photosynthesis, muscular contraction and other biological processes.[1] They are highly specific and can operate under mild conditions of pressure and temperature. Since their isolation, purification and characterization were possible, the enzymes started to gain interest as excellent eco-friendly catalysts for the industrial chemicals production.

Oxidoreductases are a large group of enzymes, that are attractive biocatalysts for the synthesis of valuable chemicals and pharmaceutical products,[4] however they must be used with complex and expensive cofactors such as nicotinamide adenine dinucleotide (NADH) or with its phosphorylated correspondent (NADPH). During the enzymatic reaction, cofactors are reduced or oxidized, while the enzyme remains unmodified. Since the enzyme catalyzes continuously the same process, the cofactor has to be always available in the system, thus their cost efficient regeneration processes are important.

NADH acts as hydrogen donor and leads to NAD+ formation. Because of high NADH price (approximatively 2600 $/mol), its regeneration by NAD+ reduction is the most adequate option.[6] In living cells, the conversion of cofactor from the reduced to oxidized one and reverse is carried out by a network of enzymatic reactions.[5] On the other hand, in situ regeneration process involves the use of secondary substrate and enzyme able to convert the NAD(P)+ to NAD(P)H or vice versa.[4] Despite their high selectivity for the active NAD(P)H, the enzyme-based cofactor regeneration in industrial processes is not economically viable, because of the expensive enzymes price.[7] Furthermore, the process effectiveness is limited by the production of a significant amount of byproducts, whose separation is also costly. Even so, because the lack of suitable solutions, at the industrial level, the cofactors have been regenerated only via enzymatic pathway. However, aspects as enzyme instability and deactivation, the need of supplementary reactants for maintaining the optimal enzyme operational conditions, as well as the complexity of product purification, highlight the necessity of finding new methods for cofactor regeneration.[4] Whole-cells NADH regeneration comes as an extent for the enzymatic route, but without the enzyme purification steps. When cofactors are regenerated via whole– cell pathway, the control of the primary and auxiliary enzyme production is difficult to be achieved.[8] Furthermore, cells contain more than one type of enzymes, so side reactions can take place.[4] Medium contamination with different components of the cell can also occur.[9] Additionally, in order to facilitate the substrate transport through cell membrane, organic detergents have been used to increase permeabilization.[10] Promising results were obtained with cell surface-displayed enzymes. This type of systems are able to solve enzyme stability problems and the mass transfer limitations.[11] The major barrier in surface display technology to industrial application is the low enzyme concentration in the cell wall.[9] Besides the enzymatic route, a broad range of non-enzymatic NAD+ reduction methods have been developed to overcome the shortcoming of the traditional regeneration way.[4,12]

The non-enzymatic cofactor regeneration by using catalytic, electrochemical or photochemical reactions can be associated with advantages as cleanliness, process simplicity and the use of cheap, clean and renewable energy. Nevertheless, comparing with the enzymatic route, some of these methods are mediator dependent, and in some situations, the selectivity might be also an issue.[7] In terms of NADH regeneration, these techniques presented good cofactor conversion yields, but the regeneration rates are much higher in case of photocatalytic reactions, compared, for instance with the electrocatalytic one.[4,13] Furthermore, finding an efficient mediator with high selectivity for the active NADH isomer is a good strategy to overcome the selectivity problems. Results have proven that the mediators as flavin mononucleotide (FMN) and [Cp*Rh(bpy) H2O]2+ were selective toward 1,4-NADH photoconversion.[14,15]

The irradiation, in special the solar one, is a low cost and clean energy source. The natural reduction of NAD+ to NADH initiated by the solar light during photosynthesis, inspired the photocatalytic regeneration of cofactors using simulated irradiation. Efforts have been focused in designing photoreactors as valuable tools in performing accurate and reproducible processes.[16,17] On the other hand, promising results were obtained also for photocatalytic reactions initiated directly by the natural sun irradiation.[18–20] Considering cofactor regeneration, by now, the most prospected catalyst is TiO2. [14,21,22] Generally, a photocatalytic system designed for cofactor regeneration contains a catalyst, an electron donor (triethanolamine – TEOA, ethylenediaminetetraacetic acid – EDTA, ascorbic acid) and a mediator (flavin mononucleotide – FMN, [Cp*Rh(bpy)H2O]2+), which facilitates the charge transfer.[14,23,24] Recently, K.A. Brown et al.[24] efficiently succeed in the regeneration of NADPH by using visible light and a combination between CdSe quantum dots and ferredoxin NADP+-reductase. Heterogeneous Pt based catalysts were as well investigated and permitted a conversion of more than 50% of the initial NAD+ concentration.[4,25] Additionally, organic semiconductors as g-C3N4, in presence of [Cp*Rh(bpy)H2O]2+ and TEOA as a mediator and electron donor, are good alternatives to photoreduce the NAD+ in a range of 50 to 100%.[26–29]

In order to decrease the process costs and also to limit the generation of additional wastes, Mifsud et al.[14] highlighted that water molecule can behave as sacrificial electron donor, driving biocatalytic redox processes with TiO2 based catalysts, under light irradiation. Similar observation was obtained in works of Aresta et al.[15] Those authors showed that a NADH photoregeneration can be initiated by a wide range of solid catalysts by using water as an electron donor.

Noble elements as Pt, Ag and Au nanoparticles (NPs) exhibit unique and versatile plasmonic properties allowing them to be used in light driven reactions. Despite their high potential in photocatalysis, the traditional synthesis methods for this kind of nanoparticles involve the use of organic compounds,[30] sometimes costly and environmentally-unfriendly. Doping different semiconductor materials with nanoparticles comes as an efficient way to obtain composites with improved photocatalytic properties.[31–34] Recently, we report a simple way to synthetize AuNPs self-assembled on layered double hydroxide (LDH) semiconductors, by using the unique property of these anionic clays to re-build their lamellar structure after thermal treatment. Furthermore, without using any organic surfactants and/or stabilizers for NPs, the results show that gold nanoparticles can interact intimately with the LDH matrix, leading to a highly preformat photocatalyst for organic pollutants solar photodegradation.[35] Moreover, the presence of the plasmonic NPs on semiconductor photocatalysts can increase the life time of the photo induced charge carriers, and the photocatalytic performance is enhanced.[36]

The LDH-2D materials, with their characteristics as high surface area, ionic exchange capacity, semiconductor properties and easy tunable textural and morphological features, are used many times in processes initiated by light.[37–40] However, as far as we know, no work deals with their potential application to regenerate the NADH from NAD+. Since AuNPs,[41] PtNPs[4,25] were successfully used for cofactors regeneration, the aim of this work is to be a proof of concept for the ability of supported AuNPs on ZnAl-LDH catalysts to photo-initiate the NAD+ reduction reaction. The photocatalytic regeneration is driven under solar simulated light, using water as a sacrificial electron donor and flavin mononucleotide (FMN) as a mediator.

References can be found in the full paper.

READ FULL PAPER HERE...