Biological fabrication of photoluminescent nanocomb structures by metabolic incorporation of germanium into the biosilica of the diatom Nitzschia frustulum

Diatoms are single-celled algae that make microscale silica shells or frustules with intricate nanoscale features such as two-dimensional pore arrays. In this study, the metabolic insertion of low levels of germanium into the frustule biosilica of the pennate diatom Nitzschia frustulum by a two-stage cultivation process induced the formation of frustules which strongly resembled double-sided nanocomb structures. The final product from the two-stage cultivation process contained 0.41 wt % Ge in biosilica and consisted of an equal mixture of parent frustule valves possessing a normal two-dimensional array of 200 nm pores and daughter valves possessing the nanocomb structure. The nanocomb structures had overall length of 8 mu m, rib width of 200 nm, rib length of 500 nm, and slit width of 100 nm. Each slit of the nanocomb was most likely formed by a directed morphology change of a row of 200 nm pores to a single open slit following Ge incorporation into the developing frustule during the final cell division. The frustules possessed blue photoluminescence at peak wavelengths between 450 and 480 nm, which was attributed to contributions from nanostructured biosilica in both the parent valves and in the nanocomb daughter valves. This is the first reported study of using a cell culture system to biologically fabricate a photoluminescent nanocomb structure.