Rhodopseudomonas palustris-based conversion of organic acids to hydrogen using plasmonic nanoparticles and near-infrared light

Abstract

The simultaneous elimination of organic waste and the production of clean fuels will have an immense impact on both the society and the industrial manufacturing sector. The enhanced understanding of the interface between nanoparticles and photo-responsive bacteria will further advance the knowledge of their interactions with biological systems. Although literature shows the production of gases by photobacteria, herein, we demonstrated the integration of photonics, biology, and nanostructured plasmonic materials for hydrogen production with a lower greenhouse CO₂ gas content at quantified light energy intensity and wavelength. Phototrophic purple non-sulfur bacteria were able to generate hydrogen as a byproduct of nitrogen fixation using the energy absorbed from visible and near-IR (NIR) light. This type of biological hydrogen production has suffered from low efficiency of converting light energy into hydrogen in part due to light sources that do not exploit the organisms' capacity for NIR absorption. We used NIR light sources and optically resonant gold-silica core-shell nanoparticles to increase the light utilization of the bacteria to convert waste organic acids such as acetic and maleic acids to hydrogen. The batch growth studies for the small cultures (40 mL) of Rhodopseudomonas palustris demonstrated >2.5-fold increase in hydrogen production when grown under an NIR source (167 ± 18 μmol H₂) compared to that for a broad-band light source (60 ± 6 μmol H₂) at equal light intensity (130 W m^-2). The addition of the mPEG-coated optically resonant gold-silica core-shell nanoparticles in the solution further improved the hydrogen production from 167 ± 18 to 398 ± 108 μmol H₂ at 130 W m^-2. The average hydrogen production rate with the nanoparticles was 127 ± 35 μmol L^-1 h^-1 at 130 W m^-2.

Fig. 1. Experimental scheme and reaction pathway. (a) NIR illumination was used as a light source for growing PNSB (R. palustris). The silica–gold core–shell plasmonic nanoparticles further enhanced the biohydrogen production in solution through light scattering and surface plasmon effects. (b) Overview of the metabolic process of R. palustris under photoheterotrophic nitrogen fixing conditions. Abbreviations: LH1, LH2 = light harvesting complexes; RC = reaction center; Fd = ferredoxin (oxidized, reduced); adapted from (13). (c) 3D drawing of bacteria and nanoparticles in suspension with evolved CO₂ and H₂ in the headspace.

Fig. 2. Cell growth and extinction spectra under illuminated and dark conditions. (a) Anaerobic growth of R. palustris in 20 mL minimal media at 30 °C using 70 mM acetate as a carbon source and white LED (20 W m⁻²) for samples grown in light. Samples grown in dark were covered in an aluminum foil. (b) Sample extinction spectrum for R. palustris grown anaerobically under dark and light conditions after 90 hours. Only the samples grown in light conditions exhibited bacteriochlorophyll absorption peaks near 800 and 850 nm. Bacteriochlorophyll (*) = B800 (**) = B850, B875.

Fig. 3. Enhancement of hydrogen production under near IR illumination. (a) Batch growth in 40 mL PTFE/silicone septa vials illuminated with an NIR LED array (850 nm) or a tungsten lamp (peak 660 nm, broadband) for 90 hours at an intensity of 130 W m²; n = 4 vials per illumination source. All vials were incubated at 30 °C and shaken at 100 rpm in an orbital shaker. The cultures illuminated with the NIR LED array showed increased cell growth and hydrogen generation compared to the cultures illuminated with a broad wavelength lamp. (b) Extinction spectra for cultures grown (a) under two light sources. (c) Light intensity spectra for the two light sources used in (a). (d) R. palustris cultured in a 300 mL reactor under a broad light source (tungsten lamp), then switched to illumination using an 850 nm LED array for 2 days, and switched back to the broad light source. The illumination intensity for both sources was adjusted to the range of 82–91 W m⁻², and the culture was maintained at 22 °C. Hydrogen production represents the headspace collection over 24 h; n = 3 headspace measurements (e) membrane reactor (300 mL) with the R. palustris culture. Error bars represent standard deviations.

Fig. 4. CO₂, H₂, production, and cell mass of R. palustris using two substrates. Cultures of R. palustris (300 mL) grown anaerobically at 22 °C using 850 nm LED array illumination adjusted to 82–91 W m⁻² (a) Acetate (70 mM) as a carbon source with the gas concentration in the headspace collected over 24 h after 48 h growth. (b) Malate (70 mM) as a carbon source; for each measurement, the headspace was flushed with N₂.

Fig. 5. Role of gold-shell nanoparticles on the enhancement of hydrogen production. (a) The silica-core, gold-shell nanoparticle cross-section. The scale bar is 50 nm and the shell thickness is approximately 18 nm. The particles were coated with mPEG (methoxy polyethylene glycol) and had a 188 nm hydrodynamic diameter. (b) Extinction cross-section simulation and measurement. The inset shows the modeled near-field enhancement. (c) Dry cell mass and hydrogen production from batch growth in 40 mL PTFE/silicone septa vials illuminated with an NIR LED array (850 nm) for 90 hours at an intensity of 130 W m⁻² (n = 4 vials per illumination source). For the NP vials, the culture media contained 2.64 × 10⁸ particles per mL. All vials were incubated at 30 °C and shaken at 100 rpm in an orbital shaker. The dry cell mass was estimated by correlating the optical density at 660 nm. Error bars represent standard deviations. (d) Extinction spectra for the control and nanoparticle-dosed cultures from (c) as well as the nanoparticle stock in DI H₂O (2.9 × 10⁹ particles per mL). (e) Light spectra for the NIR LED light used for both sets of cultures in (c).