(41c) Wavelength-Tunable Light Scattering From Silver Nanoparticle Suspensions Enhances Microalgal Growth

Microalgae and photoactive bacteria have the potential to be used as active components in inexpensive biosensors that can detect environmental toxins. The metabolic activity of several photoactive microalgae is not uniform throughout the electromagnetic (EM) spectrum, e.g. green microalgae (MA) Chlamydomonas reinhardtii exhibit two peaks, one in the blue and the other in the red regions of the EM spectrum. This is because algal photoactive pigments work only in certain range of the spectrum, while other wavelengths of light may not be utilized by green microalgae. For example, light in the 520~680 nm range may cause photoinhibition for certain microalgae. So, it is advantageous to only enhance light of favorable wavelength available to green microalgae. The light selectivity would also reduce the growth of contaminant photosynthetic microorganisms that are photoactive in the wavelength range that is not suited for the microalgae.

Algal growth requires both sufficient light intensity and optimal wavelength. Various methods have been proposed to reduce the light intensity attenuation in the cell culture. However, to the best of our knowledge, wavelength specific of backscattering of light has not been utilized as means for promoting algal growth. The present approach takes advantage of localized surface plasmon resonance from metal nanoparticles (NPs). Plasmons refer to the collective oscillations of the free electron gas density at a metal-dielectric interface. Resonant interactions of light (photons) and surface plasmons can be used to amplify light absorption at specific wavelengths. Specifically, we show that strong backscattering of blue light from a suspension of Ag NPs to a microalgal culture significantly enhances the photosynthetic activity of the organisms leading to faster growth. In a broad sense, the NP suspension functions as a mirror but with three important advantages. First, the scattered total light flux to the microalgae culture can be precisely controlled by changing the NP concentration and size, so photoinhibition can be avoided. Second, the scattering spectrum can be tuned by NP shape and size. For example, spherical Ag NPs scatter light in the blue region of the EM spectrum which can be utilized for MA growth. Third, the fluid nature of NP suspensions allows shape flexible and light efficient backscattering in photobioreactors (PBRs). Such plasmonic mini-PBRs (PMPBRs) have potential applications in MA-based biosensors to detect toxic compounds in the natural environment, especially under limited ambient light.

Proof of concept experiments were performed in a PMPBR which consists of the culture medium for the algae placed on top of a compartment containing silver nanoparticles. The interface between the medium and the silver suspension is transparent, thereby allowing for the backscatter of blue light (due to plasmon resonance) into the algal culture. Measured increase in Chlamydomonas reinhardtii growth is predicted qualitatively by a mathematical model that couples the plasmonic field enhancement with algae growth kinetics. Growth enhancement is also observed for Cyanothece 51142, a photoactive bacterial species. The details of the mathematical model as well as the effects of nanoparticle size and shape on the performance of the photobioreactor will be presented.