Enhancement of Photosynthetic Productivity by Quantum Dots Application

Abstract

The challenge of climate change promotes use of carbon neutral fuels. Biofuels are made via fixing carbon dioxide via photosynthesis which is inefficient. Light trapping pigments use restricted light wavelengths. A study using the microalga Botryococcus braunii (which produces bio-oil), the bacterium Rhodobacter sphaeroides (which produces hydrogen), and the cyanobacterium Arthrospira platensis (for bulk biomass) showed that photosynthetic productivity was increased by up to 2.5-fold by upconverting unused wavelengths of sunlight via using quantum dots. For large scale commercial energy processes, a 100fold cost reduction was calculated as the break-even point for adoption of classical QD technology into large scale photobioreactors (PBRs). As a potential alternative, zinc sulfide nanoparticles (NPs) were made using waste H 2 S derived from another process that precipitates metals from mine wastewaters. Biogenic ZnS NPs behaved identically to ZnS quantum dots with absorbance and emission maxima of 290 nm (UVB, which is mostly absorbed by the atmosphere) and 410 nm, respectively; the optimal wavelength for chlorophyll a is 430 nm. By using a low concentration of citrate (10 mM) during ZnS synthesis, the excitation wavelength was redshifted to 315 nm (into the UVA, 85% of which reaches the earth's surface) with an emission peak of 425 nm, i.e., appropriate for photosynthesis. The potential for use in large scale photobioreactors is discussed in the light of current PBR designs, with respect to the need for durable UV-transmitting materials in appropriate QD delivery systems.

Nanotechnology as a vital science in accelerating biofuel production, a boon or bane

“…A study by Murray and coworkers demonstrated the use of zinc sulfide (ZnS) QDs in the biohydrogen production process, which resulted in around 1.1‐fold enhancement in the generation of biohydrogen in comparison to the PF process without using ZnS QDs. This was caused by the photoluminescence feature of the QDs, which allows solar light to be reformed by turning the inactive wavelengths into a usable wavelength where it can be used by the bacteriochlorophylls for the photosynthesis process 127 . The PNSB are essentially active and utilize light energy under the near‐infrared range between wavelengths 790 nm to 940 nm.…”

“…The PNSB are essentially active and utilize light energy under the near‐infrared range between wavelengths 790 nm to 940 nm. Hence, the properties of QDs, such as wavelength absorbance and emissions, may be changed and varied by altering the synthesis parameters, which can allow the use of size‐modified QDs with optical properties that favor the growth of PNSB and consecutively the PFB generation process 127,128 …”

“…This was caused by the photoluminescence feature of the QDs, which allows solar light to be reformed by turning the inactive wavelengths into a usable wavelength where it can be used by the bacteriochlorophylls for the photosynthesis process. 127 The PNSB are essentially active and utilize light energy under the near-infrared range between wavelengths 790 nm to 940 nm. Hence, the properties of QDs, such as wavelength absorbance and emissions, may be changed and varied by altering the synthesis parameters, which can allow the use of size-modified QDs with optical properties that favor the growth of PNSB and consecutively the PFB generation process.…”

“…Hence, the properties of QDs, such as wavelength absorbance and emissions, may be changed and varied by altering the synthesis parameters, which can allow the use of size-modified QDs with optical properties that favor the growth of PNSB and consecutively the PFB generation process. 127,128 Overall, the production of photo-biohydrogen is a very complicated process that is significantly affected by several variables such as temperature, pH of starting medium, type of substrate used and its concentration, presence of trace metals, and the ratio of carbon to nitrogen in the substrate. To increase the generation of hydrogen, it is imperative to optimize these parameters.…”

Coupled Biohydrogen Production and Bio-Nanocatalysis for Dual Energy from Cellulose: Towards Cellulosic Waste Up-Conversion into Biofuels

“…The above discussion indicates that the energy products generated by the thermalhydrolysis of cellulose (model compound used in this work) are currently incapable to provide, at the least, the energy needed for the overall process. In addition, in the valorisation of real lignocellulosic wastes, cellulose extraction from lignocellulosic biomass requires high 'parasitic' energy demand of upstream comminution, which is required to obtain a suitable particle size for cellulose extraction, which can then be efficiently achieved via delignification using environmentally friendly thermal subcritical water-ethanol-CO 2 mediated hydrolysis (P initial 55 bar; T final 200 • C) [58,59]. Nontrivially, the energy consumption to mill Miscanthus (moisture content of 15%) to <4 mm was 184 kJ/kg of dry matter [60], which can be compared to a hydrogen energy content of only 10 kJ/litre at 1 atm and 15 • C and, hence, 18.4 L of H 2 would be 'consumed' per kilo of Miscanthus source material.…”