Nano fluidic devices and carbon nanohorns

The distinct structural properties of carbon nanoparticles, in particular their high aspect ratio and propensity to functional modification and subsequent use as carrier vectors, as well as their potential biocompatibility, make them useful for pharmaceutical nanodelivery. Carbon nanotubes (CNTs) have the added advantage of being potential nanofluidic devices for controlled drug delivery.

Great interest has been generated in fullerenes in general, but especially in CNTs and carbon nanohorns (CNHs) as biologically compatible materials and drug carriers mainly because of their distinct architecture, hollow interior, and cagelike structures.

Application of CNTs in biological systems depends on their compatibility with hydrophilic environments; therefore, the solubilization of CNTs in pharmaceutical solvents is essential. Furthermore, because it is becoming increasingly important
that the relevant chemical, physiochemical, and pharmaceutical properties of CNTs be identified, we have prepared a “mini-monograph” of CNTs that compiles their pertinent properties.





.......MWNTs generally have a larger outer diameter (2.5–100 nm) than SWNTs (0.6–
2.4 nm) and consist of a varying number of concentric SWNT layers, with an interlayer separation of about 0.34 nm. SWNTs have a better defined diameter, whereas
MWNTs are more likely to have structural defects, resulting in a less stable nanostructure. CNTs combine high stiffness with resilience and the ability to buckle and collapse reversibly. The high C-C bond stiffness of the hexagonal network produces an axial Young's modulus (measure of stiffness) of approximately 1 TPa and a tensile strength of 150 GPa, 17 making CNTs one of the stiffest materials known, yet with the capacity to deform (buckle) elastically under compression.

CNT dispersion and solubility
The solubility of CNTs in aqueous solvents is a prerequisite for biocompatibility; hence, CNT composites in therapeutic delivery should meet this basic requirement.
Similarly, it is important that such CNT dispersions be uniform and stable to obtain accurate concentration data. In this regard, the solubilization of pristine CNTs in aqueous solvents remains an obstacle to realizing their potential as pharmaceutical excipients because of the rather hydrophobic character of the graphene sidewalls, coupled with the strong p-p interactions between the individual tubes, which causes CNTs to assemble as bundles. To successfully disperse CNTs the dispersing medium should be capable of both wetting the hydrophobic tube surfaces and modifying the tube surfaces to decrease tube aggregation. Four basic approaches have been used to obtain a dispersion: (1) surfactant-assisted dispersion, (2) solvent dispersion, (3) functionalization of CNT sidewalls, and (4) biomolecular dispersion.


Summary of pharmaceutically relevant properties

Even though pharmaceutical excipients have been regarded as inert or nonactive components of dosage forms, they are essential and necessary components of the
formulation. Hence, it is becoming increasingly important that the pharmaceutically relevant properties of CNTs be identified.

Organoleptic properties refer to the appearance and physical description of a substance. Both SWNTs and MWNTs appear as granular or fluffy black powders, although
SWNT samples may also have a shiny metallic appearance. Aligned CNTs (also known as vertically aligned nanotubes or VANTs) appear as velvety sheets. EM images of SWNTs and MWNTs show CNTs in aggregated bundles, whereas in VANTs the CNTs are ordered in an array. Raman spectral analysis of CNTs is useful to distinguish samples of SWNTs
from those of MWNTs and/or VANTs, because only SWNTs have the diagnostic RBM peak. Raman spectra are also useful in estimating the diameter of individual CNTs in a SWNT sample.

We have investigated the dispersibility of CNTs in a series of pharmaceutical solvents and present visible, microscopic, and SEM images of CNTs in five of the most
important simple solvents using a three-category assessment of dispersibility: insoluble, swollen, and soluble. SWNTs are insoluble in water and ethanol, and they aggregate and sediment soon after sonication, seen as black sediments at the bottom of the vials....... For propylene glycol and DMSO dispersions, the inset photographs, light micrographs, and SEMs show swollen or intermediate dispersions of SWNTs in solution. Here, the SWNT clusters appear to be smaller and more
loosely aggregated. For the sodium dodecyl sulfate (SDS) dispersion, the inset photograph shows the black/brown uniform color characteristic of a homogeneous dispersion and is consistent with what is seen in the light micrograph, which shows an even distribution with few aggregates of CNTs. The SEM image of CNTs in SDS shows debundled CNTs and very small SWNT bundles.

Carbon nanotubes as functional excipients for nanomedicines: I. pharmaceutical properties; science direct, 2008




Stacking in biology

In DNA, pi stacking occurs between adjacent nucleotides and adds to the stability of the molecular structure. The nitrogenous bases of the nucleotides are made from either purine or pyrimidine rings, consisting of aromatic rings. Within the DNA molecule, the aromatic rings are positioned nearly perpendicular to the length of the DNA strands. Thus, the faces of the aromatic rings are arranged parallel to each other, allowing the bases to participate in aromatic interactions. Through aromatic interactions, the pi bonds, extending from atoms participating in double bonds, overlap with pi bonds of adjacent bases. This is a type of non-covalent chemical bond. Though a non-covalent bond is weaker than a covalent bond, the sum of all pi stacking interactions within the double-stranded DNA molecule creates a large net stabilizing energy.

Uses in materials

Many discotic liquid crystals can form columnar structures by π-π interactions. In addition, π-π interactions are an important factor in molecular self-assembly techniques in bottom-up nanotechnology.

Aromatic stacking interaction

Aromatic stacking interaction, sometimes called phenyl stacking, is a phenomenon in organic chemistry that affects aromatic compounds and functional groups. Because of especially strong Van der Waals bonding between the surfaces of flat aromatic rings, these groups in different molecules tend to arrange themselves like a stack of coins. This bonding behavior affects the properties of polymers as diverse as aramids, polystyrene, DNA, RNA, proteins, and peptides. The effect can be exploited in gas sensors to detect the presence of aromatic chemicals.

T-stacking

A related effect called T-stacking is often seen in proteins where the partially positively charged hydrogen atom of one aromatic system points perpendicular to the center of the aromatic plane of the other aromatic system.






Pi bond

In chemistry, pi bonds (π bonds) are covalent chemical bonds where two lobes of one involved electron orbital overlap two lobes of the other involved electron orbital. Only one of the orbital's nodal planes passes through both of the involved nuclei.

The Greek letter π in their name refers to p orbitals, since the orbital symmetry of the pi bond is the same as that of the p orbital when seen down the bond axis. P orbitals usually engage in this sort of bonding. D orbitals are also assumed to engage in pi bonding but this is not necessarily the case in reality, although the concept of bonding d orbitals still accounts well for hypervalence.

Pi bonds are usually weaker than sigma bonds because their (negatively charged) electron density is farther from the positive charge of the atomic nucleus, which requires more energy. From the perspective of quantum mechanics, this bond's weakness is explained by significantly less overlap between the component p-orbitals due to their parallel orientation.

Although the pi bond by itself is weaker than a sigma bond, pi bonds are often components of multiple bonds, together with sigma bonds. The combination of pi and sigma bond is stronger than either bond by itself. The enhanced strength of a multiple bond vs. a single (sigma bond) is indicated in many ways, but most obviously by a contraction in bond lengths. For example in organic chemistry, carbon-carbon bond lengths are ethane (154 pm), ethylene (133 pm) and acetylene (120 pm).

Wikipedia



Employing Raman spectroscopy to qualitatively evaluate the purity of carbon single-wall nanotube materials.


Raman spectroscopy may be employed to differentiate between metallic and semi-conducting nanotubes, and may also be employed to determine SWNT diameters and even the nanotube chirality. Single-wall carbon nanotubes are generated in a variety of ways, including arc-discharge, laser vaporization and various chemical vapor deposition (CVD) techniques. In all of these methods, a metal catalyst must be employed to observe SWNT formation. Also, all of the current synthesis techniques generate various non-nanotube carbon impurities, including amorphous carbon, fullerenes, multi-wall nanotubes (MWNTs) and nano-crystalline graphite, as well as larger micro-sized particles of graphite. For any of the potential nanotube applications to be realized, it is, therefore, necessary that purification techniques resulting in the recovery of predominantly SWNTs at high-yields be developed. It is, of course, equally important that a method for determining nanotube wt.% purity levels be developed and standardized.

Dillon AC et al, Nanosci Nanotech, 2004 Sep;4(7):691-703

http://www.ncbi.nlm.nih.gov/pubmed/15570946?dopt=abstract