Topics in Current Chemistry (2020) 378:35
1 3
2 QDs: Nature and Types
Despite the many problems encountered in the initial attempts to synthesize colloidal fluorescent semiconductor NPs, such as lack of reproducibility and reduced
optical quality, important advances have been achieved in this field. In pioneer studies on the routine preparation of colloidal QDs, the core of the QDs was usually
capped with an organic layer that coordinates with core-metal sites and stabilizes
the QD surface, thereby preventing an irreversible flocculation (aggregation) of the
nanocrystals [24]. Unfortunately, these protective ligands are also hydrophobic, and
thus nanocrystals capped with such coatings are not compatible with bioanalytical assay conditions. Consequently, the QD surface should be further modified by
attaching the appropriate hydrophilic functional groups to allow dispersion of the
QDs in aqueous solutions while maintaining their high photoluminescence quantum
yield.
Although fluorescence emission is the most exploited property of QDs, doping
the core of the nanocrystals with transition elements has been a strategy adopted
to provide the QDs with new improved multimodal characteristics for biomedical
applications. Additionally, during recent years, many research groups have tried
to overcome the problem of eventual cytotoxicity of the more conventional heavymetal based QDs. Approaches based on carbon-based nanomaterials are one of the
most promising strategies. In this section, we briefly review the nature and characteristics of QDs typically used in clinical and biomedical applications.
2.1 Semiconductor QDs
Quantum dots are spherical semiconductor photoluminescent NPs with a diameter
ranging between 2 and 10 nm. Since the dimensions of the NP are smaller than the
Bohr radius, the energy levels are quantized. As a consequence, the optoelectronic
properties of QDs depend on their size due to quantum confinement effects and differ from the properties observed for the same bulk material [25]. In fact, due to the
quantized energy levels, QDs generate an intense emission of photoluminescence:
when the semiconductor QD is irradiated with a light source, the absorption causes
an electron to move from the lower energy valence band to a higher energy conduction band, following which an electron–hole pair is generated, and its recombination
gives rise to the emission of intense photoluminescence. Other optical features that
make QDs very appealing for use in analytical applications include broad absorption
spectra, narrow and symmetric emission bands that can be tuned by changing the
composition and size of the NP (see Fig. 1), large Stoke shifts and high photostability [26, 27].
The energy band gap decreases with increases in QD diameter, and as a result
the emission wavelength shifts to longer wavelengths. Hence, for QDs with the
same composition, the emission can be tuned by just modifying the size of the
NP (referred to as size-dependent emission). In addition, the QD can be synthesized with different semiconductors, such as CdS, CdSe, CdTe, ZnS, ZnSe,
136
Reprinted from the journal
1 3
2 QDs: Nature and Types
Despite the many problems encountered in the initial attempts to synthesize colloidal fluorescent semiconductor NPs, such as lack of reproducibility and reduced
optical quality, important advances have been achieved in this field. In pioneer studies on the routine preparation of colloidal QDs, the core of the QDs was usually
capped with an organic layer that coordinates with core-metal sites and stabilizes
the QD surface, thereby preventing an irreversible flocculation (aggregation) of the
nanocrystals [24]. Unfortunately, these protective ligands are also hydrophobic, and
thus nanocrystals capped with such coatings are not compatible with bioanalytical assay conditions. Consequently, the QD surface should be further modified by
attaching the appropriate hydrophilic functional groups to allow dispersion of the
QDs in aqueous solutions while maintaining their high photoluminescence quantum
yield.
Although fluorescence emission is the most exploited property of QDs, doping
the core of the nanocrystals with transition elements has been a strategy adopted
to provide the QDs with new improved multimodal characteristics for biomedical
applications. Additionally, during recent years, many research groups have tried
to overcome the problem of eventual cytotoxicity of the more conventional heavymetal based QDs. Approaches based on carbon-based nanomaterials are one of the
most promising strategies. In this section, we briefly review the nature and characteristics of QDs typically used in clinical and biomedical applications.
2.1 Semiconductor QDs
Quantum dots are spherical semiconductor photoluminescent NPs with a diameter
ranging between 2 and 10 nm. Since the dimensions of the NP are smaller than the
Bohr radius, the energy levels are quantized. As a consequence, the optoelectronic
properties of QDs depend on their size due to quantum confinement effects and differ from the properties observed for the same bulk material [25]. In fact, due to the
quantized energy levels, QDs generate an intense emission of photoluminescence:
when the semiconductor QD is irradiated with a light source, the absorption causes
an electron to move from the lower energy valence band to a higher energy conduction band, following which an electron–hole pair is generated, and its recombination
gives rise to the emission of intense photoluminescence. Other optical features that
make QDs very appealing for use in analytical applications include broad absorption
spectra, narrow and symmetric emission bands that can be tuned by changing the
composition and size of the NP (see Fig. 1), large Stoke shifts and high photostability [26, 27].
The energy band gap decreases with increases in QD diameter, and as a result
the emission wavelength shifts to longer wavelengths. Hence, for QDs with the
same composition, the emission can be tuned by just modifying the size of the
NP (referred to as size-dependent emission). In addition, the QD can be synthesized with different semiconductors, such as CdS, CdSe, CdTe, ZnS, ZnSe,
136
Reprinted from the journal