# Metal–organic biohybrid

Metal–organic biohybrids (MOBs) are a family of materials containing a metal component, such as copper, and a biological component, such as the amino acid dimer cystine. One of the MOB families first described was the copper-high aspect ratio structure called CuHARS. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) of CuHARS revealed linear morphology and smooth surface texture. SEM, TEM and light microscopy showed that CuHARS composites had scalable dimensions from nano- to micro-, with diameters as low as 40 nm, lengths exceeding 150 microns, and average aspect ratios of 100.

## Structure

MOBs are composed of two major components: a metal ion or cluster of metal ions and a biological molecule. Examples are:
* CuHARS which contain copper as the metal ion and cystine as the biological molecule
* The use of silver as the metal ion in combination with cystine. Cystine is the dimer form of the amino acid cysteine. Cobalt has also been used in combination with cystine to form CoMOBs.
When combined with copper to form CuHARS, the cystine may provide a linker function leading to a linear, high-aspect ratio structure that gives CuHARS its name: copper high-aspect ratio structures. In contrast to CuHARS, MOBs formed with silver and cystine result in silver nanoparticles with spherical, rounded structure. These have been named AgCysNPs.
Figure 1 shows comparative electron microscopy of CuHARS and AgCysNPs.

### Synthesis

MOBs under reducing conditions using sodium hydroxide (NaOH) can be self-assembled at body temperature (37 degrees Celsius). In the case of copper CuHARS, MOBs can be produced by transforming copper nanoparticles to provide the copper source, or using copper(II) sulfate.

## Physical Characteristics

CuHARS have been shown to completely degrade under physiological conditions (cell culture media at 37 °C), even in the absence of cells; this is possibly due to the metal chelating properties of typical cell culture medias. These may include the copper-binding properties of cerulosplasmin and of albumin. Additionally, CuHARS have been shown to polarize light using inverted microscopy. Cobalt-containing MOBs (CoMOBs) have been shown to be susceptible to an externally applied magnetic field as shown in Figure 2.

### Uses and Applications

MOBs have been incorporated into composites including cellulose. Additionally, MOBs composed of the copper-containing CuHARS have been shown to provide catalytic function to produce nitric oxide (NO).

### Nitric Oxide production

This production of NO was shown to impart anti-microbial activity, and the CuHARS in this case were incorporated into a biodegradable, biocompatible, and renewable resource material, namely cellulose. The release of NO catalzyed by copper from CuHARS may have beneficial biomedical applications.

### Anti-Cancer Effects

Both copper- and silver-containing MOBs were shown to have anti-cancer effect on cells in vitro. In the case of possible uses for CuHARS, copper may have a potential role in tumor immunity and for antitumor therapy. Since CuHARS are 100% biodegradable under physiological conditions, copper metabolism of CuHARS may have benefits as an approach for treating glioma.

### MOBs as Green Materials using Self-Assembly

Green nanomedicine has been suggested as a path to the next generation of materials for diagnosing brain tumors and for therapeutics, including the use of CuHARS.

### Angiogenic Effects

CuHARS embedded into nanofiber aerogels have been shown to have angiogenic effects.

### Antibacterial Effects

CuHARS embedded into nanofiber aerogels and via CuHARS-mediated nitric oxide generation have both been examples of antibacterial effects.