Liposome-Based Biosensor for Rare Earth Element Detection

Liposomal structures have been used to encapsulate and enable targeted delivery of pharmaceuticals, nutrients, and particles for therapeutic purposes. In targeted drug delivery applications, the lipid bilayer can be engineered to fuse with the target cells lipid bilayer. Liposomes have also been used as a biosensor to detect bacteria, metals, toxins, and pathogens. In this work, we utilized liposomes embedded with adsorptive nanoparticles and encapsulating bacteriophage as a biosensing system for rare earth elements (REEs), which are critical to development of cutting-edge technologies, yet onerous to identify, separate, and extract.

REEs are critical to production of high-tech materials and industrial development in the areas of green energy and communications. Efficient REE extraction at a feasible cost is challenging due to similar physical and chemical properties that make these elements difficult to separate. In that work, we utilized a nanoparticle (NP) that was highly absorptive, non-toxic, and had a magnetic core for easy collection post-absorption. The NP was originally designed to absorb Cu and remain environmentally benign. The NP consists of a magnetic core for collection, a hydroxyapatite outer layer for biocompatibility, and a titanium dioxide functionalized surface to improve adsorption efficacy. We termed these NPs titanium dioxide functionalized hydroxyapatite magnetite NPs (TiHAMNPs). As a preliminary study, we also evaluated if the TiHAMNPs could separate REEs. At 60 min, TiHAMNPs removed between 93-100 % Gd, Eu, and Pr while leaving 24.5 % lanthanum, indicating that the NP may be an effective adsorber and separator material for REEs as well.

In this work, we have designed a liposomal REE detection system, whereby, TiHAMNPs were embedded in the bilayer of liposomes. Multilaminar 1,2-didodecanoyl-sn-glycero-3-phosphocholine (DLPC) liposomes were formed via a modified ethanol injection method and stabilized with plant cholesterol. The liposomes were separated into single bilayer liposomes via an Avanti Mini-Extruder, and NPs were incorporated via agitation by sonication as used by Zhang et al. (2006). When in contact with various REEs, embedded TiHAMNPs became coated in the REEs thereby disrupting the liposomal bilayer and “popping” the liposome. As the liposome popped, internally incorporated bacteriophages were released and could be quantified via filtration and subsequent plaque assay, with increased plaque counts indicating higher levels of REE present. Bacteriophage incorporation was accomplished by methods adapted from Colom et al. (2015) who demonstrated the method for producing oral antibacterial therapies.

At present, we are working to refine our methods to improve consistency of producing single bilayer liposomes and to improve bacteriophage incorporation and TiHAMNP embedding techniques. As a next step, we are also genetically modifying Escherichia coli with the firefly luciferase gene via CRISPR/Cas9 so that upon phage release and bacterial infection, a luminescent signal may be quantified to determine REE concentration.