Ultra-Pure Water and Extremophilic Bacteria interactions with Germanium Surfaces
Sah, Vasu R.
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Supported by a consortium of semiconductor industry sponsors, an international "TIE" project among 5 National Science Foundation (NSF) Industry/university Cooperative Research Centers discovered that a particular extremophilic microbe, Pseudomonas syzygii, persists in the UltraPure Water (UPW) supplies of chip fabrication facilities (FABs) and can bio-corrode germanium wafers to produce microbe-encased optically transparent crystals. Considered as potentially functional "biochips", this investigation explored mechanisms for the efficient and deliberate production of such microbe-germania adducts as a step toward later testing of their properties as sensors or switches in bioelectronic or biophotonic circuits. Recirculating UPW (Ultra-Pure Water) and other purified water, laminar-flow loops were developed across 50X20x1mm germanium (Ge) prisms, followed by subsequent examination of the prism surfaces using Multiple Attenuated Internal Reflection InfraRed (MAIR-IR) spectroscopy, Contact Potential measurements, Differential Interference Contrast Light Microscopy (DICLM), Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Analysis (EDS), and Electron Spectroscopy for Chemical Analysis (ESCA; XPS). P. syzygii cultures originally obtained from a working FAB at University of Arizona were successfully grown on R2A minimal nutrient media. They were found to be identical to the microbes in stored UPW from the same facility, such microbes routinely capable of nucleation and entrapment within GeO2 crystals on the Ge flow surfaces. Optimum flow rates and exposure times were 1 ml/minute (3.2 s-1 shear rate) for 4 days at room temperature, producing densest crystal arrays at the prism central zones 2-3 cm from the flow inlets. Other flow rates and exposure times have higher shear rate which induces a different nucleation mechanism and saturation of crystal formation. Nucleation events began with square and circular oxide deposits surrounding active attached bacteria, presumably in response to diffusing or spreading metabolic products. They germinated into amorphous germania moats around square crystalline growths incorporating bacteria in the ring centers, sometimes in multiples. Further distances of UPW flow along the prism faces showed both amorphous phase dissolution and crystal "ripening", followed by some crystal shedding and downstream secondary collections of crystal clusters. Microscopic viewing allowed micromanipulator-directed fine wire contacts with individual crystals to assess their electrical characteristics, with limited data due to the ceramic-like refractory properties of the germania crystals. A schematic is presented for the events of nucleation and crystal growth observed, offering the interpretation that Ge oxidation to GeO2 occurs from UV-dissociated water corroding the Ge surface while releasing protons that can drive the metabolic processes keeping the extremophilic organisms alive. It is likely that other extremophilic microorganisms can be similarly entrapped within semiconductor crystals. Further work is now required to discriminate between nucleation by microbial exudates and by the microbial surfaces directly, and to interrogate the crystals grown with advanced electronic and biophotonic probes.