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Posted: Aug 16th, 2007
Nanotechnology assembly with living materials
(Nanowerk Spotlight) Self-assembly is Nature's way of building stuff. This fundamental principle that governs natural structures on all scales, from molecules to galaxies, generates structural organization from pre-existing parts or components. In nanotechnology, self-assembly is seen as a key technique that will one day allow the fabrication of materials and devices from the bottom up. Still only tinkering with the basics, scientists so far have designed and created simple systems that could mimic natural functions by connecting biological components to abiotic materials to understand the workings of the biological system or to take advantage of the unique properties of the nonbiological components in a natural setting. Most nanotechnologist, even if they manage to self-assemble functional nanodevices, still operate exclusively at the nanoscale (it will be a while before you can order "Tea. Earl Grey. Hot" from your food replicator in the wall). Bridging the gap between the nano- and the macroworld has proven to be a huge hurdle. In a novel approach that merges material chemistry, biology and medicine, researchers in Germany have used living bacteria to show that self-assembly of functional materials and living systems is possible through a chemically programmed construction.
"We set out to realize the first step toward the exchange of specific information between synthetic systems and bacteria" Dr. Luisa De Cola explains to Nanowerk. "We have functionalized biocompatible artificial nanocontainers (the synthetic mineral zeolite L) and attached them to nonpathogenic bacteria (Escherichia coli; E. coli). The functional systems (the zeolite) can be filled with different molecules rendering it fluorescent or active for further actions. Owing to the particularly defined geometrical arrangement of the zeolite and bacteria, we are also able to self-organize two bacteria by using the nanocontainer as a junction."
De Cola, a professor in the Physikalische Institut and CeNTech (Center for Nanotechnology) at the University of Münster in Germany, and her research group are working with zeolites by loading their one-dimensional channels with suitable small molecules, giving rise to hybrid systems, opportunely designed to play an important role like photo- and electro-responsive materials, with interesting applications in nanophotonics and electronics, as well as combined with living systems, for molecular in-vivo imaging and drug delivery experiments.
De Cola's research was triggered by her close cooperation with Professor Gion Calzaferri at the University of Berne in Switzerland. Calzaferri's group is working with zeolite and develops highly organized dye-zeolite materials for nanosensors and photoelectronic devices.
"I was convinced that we could push the field in a different direction and we started a collaboration which is very successful" says De Cola. "One idea was to move into the nanomedicine field with these objects and we started with bacteria assembly, which led to the current research findings as reported in Angewandte Chemie ("Self-Assembling Living Systems with Functional Nanomaterials").
Zeolites L are crystalline aluminosilicates, a biocompatible material – basically a little piece of transparent volcanic rock – and the German research team is currently trying to use these nanocontainers (Zeolite L contains one-dimensional channels running through the whole crystal; a single crystal with a diameter of 550 nm consists of about 80,000 parallel channels) for in vivo applications. "I can foresee several applications in nanomedicine but also in other material fields" says De Cola.
She points out that the use of such materials versus the more conventional nanoparticles and quantum dots is appealing because of their tunable size, ranging from 30 nm up to several thousand nm, but also shape, their optical transparency, their biocompatibility and of course their porous character and easy covalent functionalization. "We have recently shown that we can assembly them in rod-type structures due to their selective channel entrances functionalization."
(top) Schematic depiction of a linear E.coli/zeolite L/E.coli assembly; (bottom) Assembly of 1:1 zeolite L/bacterium in PBS buffer solution. SEM image of the assembly after evaporation of the solvent and subsequent coating with silver. (Images: Dr. De Cola)
To construct their linear Zeolite - E.coli assembly, De Cola's team decided to explore an electrostatic-type binding between the negatively charged outer cell membrane and a positively charged zeolite L crystal.
"Interestingly we are able to position the charge only at the entrance of the channels so that the entire crystal retains its own character on the surface" says De Cola. "To realize the construction, we first loaded 1-µm long zeolite L with the green luminescent organic dye pyronine through an ion-exchange procedure. We then functionalized the channel entrances of the zeolite with thousands of amino derivatives, which under our conditions are protonated, leading therefore to the desired positively charged systems. Amino-functionalized zeolite crystals and bacteria in an estimated 1:1 ratio were then incubated together for 1 hour at 37°C in phosphate-buffered saline (PBS) solution."
The resulting geometrically linear assembly stimulated a perhaps obvious question: "Can we assemble living systems by using the nanocontainers as a junction?" asked De Cola.
To achieve this goal, the researchers changed the estimated ratio between the cells and the zeolite L by using a large excess of bacteria. "After mixing, we observe, under the microscope, a linear structure that does not correspond to a single cell" says De Cola. "An accurate analysis of the assemblies proved that now the ratio between the zeolite and the bacteria is 1:2. At this stage, we do not know if the zeolite can play an active role in the communication of the two linked systems. In fact, we wish to stress that our zeolite, used as a connector, is not only biocompatible and stable but also functional and modular."
This research opens the possibility of arranging living cells and nanomaterials in various combinations and have them self-assemble into complex structures. De Cola even believes that an exchange of specific information between the zeolite and/or the bacteria is possible. For instance, substances stored inside the Zeolite's nanochannels could be transferred to the living cells; vice versa, substances discharged from the bacteria could be captured by the crystals and analyzed.
"It will be fascinating to explore the consequences of this" says De Cola.