292
Of carbonaceous nanomaterials, the first class of fullerenes (C 60 -atom hollow
sphere) originated in 1985. They are naturally non-ionogenic but gain charge under
selective conditions that possess a negative zeta potential and shows optical, elastic,
mechanical, and thermal properties (Brant et al. 2005). Further, in 1991, the carbon
nanotubes (CNTs), which are the cylindrical fullerene derivative, were synthesized.
Sheets of carbon atoms are linked covalently to form one-dimensional hollow cylindrical shape (Smart et al. 2006). CNTs are of two distinct types, namely, singlewalled nanotubes (SWNTs) and multiwalled nanotubes (MWNTs). The structure of
SWCNTs can be visualized as single-layered graphene sheets that are wrapped up
into seamless cylinder. In MWCNTs, two or more concentric layers of graphene
sheets with different length and sizes are found (Cao 2004). CNTs and fullerenes
find their application in various sectors like medical, plastics, catalysts, fuel electrodes, electrochemical capacitors, wastewater purification system, sensing appliance, etc. (Klaine et al. 2008).
Another type of ENPs comprises of metal-containing materials like metal oxides
and binary oxides. The two common methods of their preparation are precipitation
with stabilization and flame pyrolysis (Christian et al. 2008). In metal oxides, crystalline TiO 2 is an excellent band-gap semiconductor that has a large energy gap of
3.2 eV (Bellardita et al. 2007; Klaine et al. 2008; Lihitkar et al. 2007; Reijnders
2008). Another example of the same class is ZnO which finds application in cosmetics due to a band-gap energy of 3.36 eV, and high dielectric constant (Singh et al.
2007; Christian et al. 2008).
Quanta dots (QDs) semiconductors with nanocrystalline diameter (2–10 nm)
possesses unique magnetic and catalytic properties and constitute the third class of
ENPs (Schmid 2004). Examples include core type, core-shell type, or alloyed QDs
like chalcogenides of metals (Murray et al. 2001; Logothetidis 2006). They are
widely used in experimental medicines, attached to surface ligands or introduced
into live organisms for intracellular in vivo analysis, biomedical imaging, targeted
therapeutics, etc. (Alivisatos et al. 2005; Roszek et al. 2005; Logothetidis 2006;
Klaine et al. 2008).
Nanoscale zero-valent metals that are generally prepared by the reduction of
metal salts are also widely used. One such example is the synthesis of zero-valent
iron by reducing the ferric (Fe
3+)
or ferrous (Fe
2+
) salts with a sodium borohydride
(NaBH 4 ). Also, gold and silver NPs are synthesized chemically through metal or
metallic salt dissolution in a suitable solvent to reduce them to the zero-valent state
(Li et al. 2006). Further, these NPs exhibit unique optical properties called as surface plasmon resonance (SPR) (Noguez 2007).
The last class of ENPs is dendrimers, defined as a complex, highly branched
polymers of 1–10 nm diameter. They are asymmetrical and are transformed into
globular forms with increase in branching (Caminati et al. 1990). During synthesis
of dendrimers, in a process of emulsion polymerization, ammonium per sulfate is
used for initiating free radical polymerization. For example, an aqueous emulsion of
monomer like styrene or methyl acrylate is prepared using water and sodium
dodecyl sulfate or a sulfonate as a surfactant (Shim et al. 2004). Their diverse applications range from biomedicine to surface modification. Some uses of common
ENPs are enlisted in Table 11.1.
D. Kundu et al.
Of carbonaceous nanomaterials, the first class of fullerenes (C 60 -atom hollow
sphere) originated in 1985. They are naturally non-ionogenic but gain charge under
selective conditions that possess a negative zeta potential and shows optical, elastic,
mechanical, and thermal properties (Brant et al. 2005). Further, in 1991, the carbon
nanotubes (CNTs), which are the cylindrical fullerene derivative, were synthesized.
Sheets of carbon atoms are linked covalently to form one-dimensional hollow cylindrical shape (Smart et al. 2006). CNTs are of two distinct types, namely, singlewalled nanotubes (SWNTs) and multiwalled nanotubes (MWNTs). The structure of
SWCNTs can be visualized as single-layered graphene sheets that are wrapped up
into seamless cylinder. In MWCNTs, two or more concentric layers of graphene
sheets with different length and sizes are found (Cao 2004). CNTs and fullerenes
find their application in various sectors like medical, plastics, catalysts, fuel electrodes, electrochemical capacitors, wastewater purification system, sensing appliance, etc. (Klaine et al. 2008).
Another type of ENPs comprises of metal-containing materials like metal oxides
and binary oxides. The two common methods of their preparation are precipitation
with stabilization and flame pyrolysis (Christian et al. 2008). In metal oxides, crystalline TiO 2 is an excellent band-gap semiconductor that has a large energy gap of
3.2 eV (Bellardita et al. 2007; Klaine et al. 2008; Lihitkar et al. 2007; Reijnders
2008). Another example of the same class is ZnO which finds application in cosmetics due to a band-gap energy of 3.36 eV, and high dielectric constant (Singh et al.
2007; Christian et al. 2008).
Quanta dots (QDs) semiconductors with nanocrystalline diameter (2–10 nm)
possesses unique magnetic and catalytic properties and constitute the third class of
ENPs (Schmid 2004). Examples include core type, core-shell type, or alloyed QDs
like chalcogenides of metals (Murray et al. 2001; Logothetidis 2006). They are
widely used in experimental medicines, attached to surface ligands or introduced
into live organisms for intracellular in vivo analysis, biomedical imaging, targeted
therapeutics, etc. (Alivisatos et al. 2005; Roszek et al. 2005; Logothetidis 2006;
Klaine et al. 2008).
Nanoscale zero-valent metals that are generally prepared by the reduction of
metal salts are also widely used. One such example is the synthesis of zero-valent
iron by reducing the ferric (Fe
3+)
or ferrous (Fe
2+
) salts with a sodium borohydride
(NaBH 4 ). Also, gold and silver NPs are synthesized chemically through metal or
metallic salt dissolution in a suitable solvent to reduce them to the zero-valent state
(Li et al. 2006). Further, these NPs exhibit unique optical properties called as surface plasmon resonance (SPR) (Noguez 2007).
The last class of ENPs is dendrimers, defined as a complex, highly branched
polymers of 1–10 nm diameter. They are asymmetrical and are transformed into
globular forms with increase in branching (Caminati et al. 1990). During synthesis
of dendrimers, in a process of emulsion polymerization, ammonium per sulfate is
used for initiating free radical polymerization. For example, an aqueous emulsion of
monomer like styrene or methyl acrylate is prepared using water and sodium
dodecyl sulfate or a sulfonate as a surfactant (Shim et al. 2004). Their diverse applications range from biomedicine to surface modification. Some uses of common
ENPs are enlisted in Table 11.1.
D. Kundu et al.