Clay Nanotube Liquid Marbles Enhanced with Inner Biofilm Formation for the Encapsulation and Storage of Bacteria at Room Temperature

Reversing the organization of oil-in-water Pickering emulsion formed with halloysite nanoclay, liquid marbles were created with water droplets encapsulated by a layer of clay nanotubes. After incorporating microorganisms inside the spherical liquid marble of 0.5 mm in diameter, the simple clay capsule has potential as a storage vehicle for bacterial cultures. Halloysite nanotubes are naturally formed biocompatible and widely available aluminosilicate clay. The inherently hydrophilic halloysite is rendered hydrophobic (contact angle >90°) through silane grafting of long alkane groups onto the external surface. Because of the water repellency, the nanotubes are able to trap water inside a thin shell making a stable interface between inner liquid and air, thus forming marbles with reversed emulsion architecture. The halloysite with its alkane modifications encourages the growth of selected bacteria inside the marble; Alcanivorax borkumensis is one such example. The biofilm produced at the inner walls of the halloysite shell by such bacteria strengthens the marbles’ structure and reduced evaporation, keeping the bacteria viable for a period of up to 4 days after drying. A symbiotic correlation between the halloysite external shell and bacteria creates stable liquid marble structures, paving the way for a strategy to encapsulate and store bacteria at room temperature.