While the global demand for high technologies rises, their production still widely relies on unsustainable and environmentally unfriendly production techniques, and mining of critical raw materials. Photonic crystals are among highly demanded technological components, as they can manipulate light at the nanoscale with implications ranging from optoelectronics, telecommunication and information technologies, opto-medical and pharmaceutical devices, towards emerging quantum logic technologies. Achieving high quality of these structured optical materials relies on specialized micro- and nanofabrication techniques, as advanced as electron-beam lithography, chemical etching and thin-film deposition, requiring highly controlled environments up to cleanroom standard. With this project we will introduce microalgae as the bio-factory for high quality photonic nanostructures production to serve as platforms in optical/plasmonic sensing devices. With this approach we will be paving the way to photonic production through clean, safe and cost-effective natural methods in which the manipulation of the growth conditions of diatom microalgae can turn their biomass into as-grown photonic devices with a wide range of applications such as water quality monitoring (1). We recently demonstrated that high-quality photonic crystals are available through natural, environmentally friendly production techniques (2). In detail, single cell microalgae called diatoms grow photonic crystals as part of an extracellular bio-silica shell, with photonic properties in the visible and near infra-red spectral range. In fact, these structures are easily accessible through large fossil deposits all around the globe (known as diatomaceous earth). We found that the photonic properties are highly reproducible at low refractive index contrast conditions, e.g. when immersed in water. This is due to highly reproducible material properties and structural parameters, such as
the unit cell period; however, other parameters with implications upon the photonic properties at higher refractive index contrast, like the pore filling fraction, could vary between individual structures. This observation gave rise to question the biological implications of the photonic properties in diatoms, given that conservation of the properties coincide with effective refractive index conditions present under natural conditions in water. But it is also pertinent to ask can we modify these properties on-demand and during cell growth with a particular application in mind? Preliminary explorations suggest the answer is yes. The technological potential of the bio-silica nano-structures has been validated in our experiments based on reproducibility and sample purification techniques. We identified routines to provide high purity photonic crystals and concluded that they could serve as an alternative to cleanroom-based nanofabrication techniques (3). But actual manipulation of the nanostructure has not yet been developed mostly due to two main bottlenecks: i) a natural variation of pore filling fraction causes different optical results, and ii) as the underlying biological process of biosilicification is widely unknown, there are currently no routines available for manipulating the structural parameters in order to control the photonic properties. In order to control the lattice parameters of the bio-silica structures from diatoms this project will tackle those two bottlenecks with two methodologies: 1) We will use in vivo doping during the cell growth (4), to alter the refractive index of the bio-silica and to induce teratological alteration to the structure (5). 2) We will use bio-functionalisation to fabricate composite structures with chemical functionalisation and metal sputtering for inducing plasmonic properties. Our interdisciplinary approach will render outputs at different levels and fields of research. On the fundamental biological level we will identify the potential implications of photonic nanostructures upon solar energy harvesting and/or other physiological implications in diatoms. On the other hand we will obtain high quality nanostructure materials based on porous bio-silica matrixes produced during cell growth and with ad-hoc photonic properties. Moreover, we propose that by understanding the modifications of the bio-silica photonic properties during cell growth it will be possible to use diatoms as natural biomonitoring organism. Therefore, our project has the potential to impact scientific and technological fields alike, paving the way to cost-effective, bio-based photonics.
