Modification and functionalisation of nanomaterials

There are limitations in the applications of nanomaterials because of their restricted behaviour in different solvents. Surface modifications of nanomaterials help to tune their properties to suit different applications in the field of nanotechnology, because surface properties determine the interaction among the components, as well as the solubility and agglomeration behaviour in different solvents. This decides the stability of nanocrystals as nanobuilding blocks for the design of nanocomposites or for self organizing nanodevices or to couple electronic, photonic, or catalytic properties of quantum-size nanoparticles with molecular functionalities. For example, nucleic acid-functionalized Au nanoparticles, exhibiting a characteristic red color originating from the single-particle plasmon exciton, turns blue upon hybridization with the target DNA due to the formation of an interparticle-coupled plasmon exciton.

surface modification methods

There are different methods to modify the surfaces of inorganic nanomaterials to make organic-inorganic nanocomposites. The simplest approach is to entrap an inorganic component in an organic host, or vice versa. Examples include inorganic-organic hybrid polymers with interpenetrating but otherwise nonconnected inorganic and organic networks, polymers filled with inorganic nanoparticles, or layered inorganic compoundes interclated with organic molecules. In the second class of inorganic-organic hybrid materials, strong covalent or ionic bonds connect the constituents with each other.

organic modification

To connect the inorganic nanomaterials to the organic moiety in hybrid materials by strong covalent or ionic interactions, reactive organic groups have to be attached onto the surface of inorganic component. There are two main strategies for the preparation of organically modified nanoparticles. The organic groups can either be grafted to preformed nanoparticles by post synthesis functionalization or introduced during the nanoparticles synthesis by in-situ functionalization. While former has the advantage of offering a means to alter the interfacial properties without affecting the bulk characteristics of materials, the significance of the latter method lies in the self-limiting organization process of the inorganic and organic building blocks, where organic ligand controls, the growth, size and crystallintiy of nanomaterials.

common method for the synthesis

The most common, and general method for the synthesis of metal nanoparticles is the reduction of metal compounds in the presence of stabilizing ligands or templates. Traditionally, the ligands that have been used to produce stable nanoparticles by reduction method are phosphines, thiols and amines. Block copolymers or dendrimers have also been used to provide cavities within which the reduction of metal like gold compound can be carried out.

Functionalization of carbon

Nanotubes with organic molecules can be used to tune their physical and chemical properties. For example, depending on the pattern of hydrogen atom coverage, while a metallic armchair of SWNT can be transformed to a wide band gap semiconductor, a semiconducting zigzag tube may become a metal with very high state density. A free SWNT, which is normally nonmagnetic, becomes magnetic with unpaired spins upon the adsorption of oxygen molecules or specific transition metal atoms. A recent study demonstrates that a semiconducting zigzag tube becomes both, a magnetic and a highly conducting wire as a result of Ti coating. Functionalization of carbon nanotubes can be achieved easily by oxidizing the surface, which results in the formation of carboxylic groups on the surface of a nanotube. Using the reactive functional groups surface can be tailored easily. This method is useful to improve the physical properties of nanotubes like solubility but has very low impact on the electrical or mechanical properties of nanotubes. Other methods used to functionalize carbon nanotubes are derivatization of carbon nanotubes with metal containing molecular coordination complexes, fluorination followed by nucleophilic substitution and chemically induced exfoliation of larger tubular bundles into smaller aggregates.