crystals are biocompatible, the nanocrystals need to be water soluble and possess
pendent groups at the surface that bind to biomolecules like proteins. These
changes can be brought about by tailoring the ligand shell with small DNA fragments or mercapto acids. Several in vivo and in vitro fluorescence biochemical assays have been carried out with nanocrystalline markers [25, 26, 84, 171]. A few
studies have sought to exploit the dependence of the plasmon absorption band
on the dielectric constant of the surrounding medium in metal nanocrystals to
detect binding events taking place at the ligand shell. Thus, Au nanocrystals could
colorimetrically determine the successful hybridization of oligonucleotide strands
bound to its surface [99, 103]. It has been proposed that colorimetric sensing of
heavy metal ions could be obtained by the use of carboxylic acid terminated bifunctional thiols bound to metal nanocrystals [172, 173]. The changes in the electronic absorption spectra of @5 nm Ag nanocrystals capped with lipoic acid, following the addition of the heavy ions, Cu
2þ and Fe
2þ is shown in Figure 4.26.
Such a dampening also brings about a change in color. It is apparent that Cu
2þ
ions dampen the plasmon band more effectively than Fe
2þ . It is hoped that mesoscalar organizations could provide useful substrates consisting of ordered nanocrystals that are required to carry out the above experiments in the solid state.
Nanocrystals are thought of as important in single electron devices, operating at
room temperature, such as supersensitive electrometers and memory devices [161,
174]. Capped nanocrystals of both metal and semiconductors by virtue of their size,
possess capacitance in the range of aF (10
À18 F). Charging a nanocrystal with an
extra electron perturbs it to such an extent that the next electron requires an appreciable change in the charging potential. This is often seen as a ‘Coulomb staircase’ in the current–voltage tunneling spectra (see Chapter 1). Indeed, the charging energy varies linearly with the inverse of the diameter of the nanocrystal [175,
176]. The sensitivity of a nanocrystal to single electron charging makes it an ideal
candidate for use in single electron transistors and memory devices.
It is well known that in the nanometric domain, the coercivity of magnetic
nanocrystals tends to zero [115, 177]. Thus, the nanocrystals behave, as superparamagnets with no associated coercivity or retentivity. The blocking temperature
which marks the onset of this superparamagnetism increases with the nanocrystal
size. This scenario however changes in the case of interacting nanocrystals, where
the interparticle interaction, and hence its magnetic properties, can be tuned by
varying the interparticle distance. Thus, lattices of interacting magnetic nanocrystals are considered important in the future magnetic storage devices. Further,
the magnetic moment per atom is seen to increase as the size of a particle decreases [178].
Several polymer/polyelectrolyte–nanocrystal hybrid devices have been fabricated
seeking to exploit the electro and photoluminescent properties of such material
[179–188]. Device fabrication in all these cases is by low-cost self-assembly based
techniques. These devices utilize thin films of these hybrids obtained either by
multilayer deposition or drop/spin casting methods. Thus, ‘solar cells’ have been
made from poly(2-hexylthiophene)aCdSe nanorod multilayers, lasers from drop
cast films of CdSeatitania composites and an infrared emitter from multilayers
4 Mesoscopic Assembly and Other Properties of Metal and Semiconductor Nanocrystals
80
pendent groups at the surface that bind to biomolecules like proteins. These
changes can be brought about by tailoring the ligand shell with small DNA fragments or mercapto acids. Several in vivo and in vitro fluorescence biochemical assays have been carried out with nanocrystalline markers [25, 26, 84, 171]. A few
studies have sought to exploit the dependence of the plasmon absorption band
on the dielectric constant of the surrounding medium in metal nanocrystals to
detect binding events taking place at the ligand shell. Thus, Au nanocrystals could
colorimetrically determine the successful hybridization of oligonucleotide strands
bound to its surface [99, 103]. It has been proposed that colorimetric sensing of
heavy metal ions could be obtained by the use of carboxylic acid terminated bifunctional thiols bound to metal nanocrystals [172, 173]. The changes in the electronic absorption spectra of @5 nm Ag nanocrystals capped with lipoic acid, following the addition of the heavy ions, Cu
2þ and Fe
2þ is shown in Figure 4.26.
Such a dampening also brings about a change in color. It is apparent that Cu
2þ
ions dampen the plasmon band more effectively than Fe
2þ . It is hoped that mesoscalar organizations could provide useful substrates consisting of ordered nanocrystals that are required to carry out the above experiments in the solid state.
Nanocrystals are thought of as important in single electron devices, operating at
room temperature, such as supersensitive electrometers and memory devices [161,
174]. Capped nanocrystals of both metal and semiconductors by virtue of their size,
possess capacitance in the range of aF (10
À18 F). Charging a nanocrystal with an
extra electron perturbs it to such an extent that the next electron requires an appreciable change in the charging potential. This is often seen as a ‘Coulomb staircase’ in the current–voltage tunneling spectra (see Chapter 1). Indeed, the charging energy varies linearly with the inverse of the diameter of the nanocrystal [175,
176]. The sensitivity of a nanocrystal to single electron charging makes it an ideal
candidate for use in single electron transistors and memory devices.
It is well known that in the nanometric domain, the coercivity of magnetic
nanocrystals tends to zero [115, 177]. Thus, the nanocrystals behave, as superparamagnets with no associated coercivity or retentivity. The blocking temperature
which marks the onset of this superparamagnetism increases with the nanocrystal
size. This scenario however changes in the case of interacting nanocrystals, where
the interparticle interaction, and hence its magnetic properties, can be tuned by
varying the interparticle distance. Thus, lattices of interacting magnetic nanocrystals are considered important in the future magnetic storage devices. Further,
the magnetic moment per atom is seen to increase as the size of a particle decreases [178].
Several polymer/polyelectrolyte–nanocrystal hybrid devices have been fabricated
seeking to exploit the electro and photoluminescent properties of such material
[179–188]. Device fabrication in all these cases is by low-cost self-assembly based
techniques. These devices utilize thin films of these hybrids obtained either by
multilayer deposition or drop/spin casting methods. Thus, ‘solar cells’ have been
made from poly(2-hexylthiophene)aCdSe nanorod multilayers, lasers from drop
cast films of CdSeatitania composites and an infrared emitter from multilayers
4 Mesoscopic Assembly and Other Properties of Metal and Semiconductor Nanocrystals
80