ABSTRACT
2. Theory and Background
QDs as a smart photo-electrochemically active material is composed mainly of semiconductor cores, which often coated with one or more shell(s) consisting of semiconductor material (e.g., CdSe/ZnS core/shell or CdSe/
CdS/ZnS core/shell/shell QDs) (Fig. 1) (Murray et al. 2000, Medintz et al. 2005). Because of surface defects and the surrounding medium, the shell passivates the QD core from quenching effects and increases the photoluminescence quantum yields of QDs (Esteve-Turrillas and Abad-
Fuentes 2013). Owing to the suitable lattice parameters and a relatively small band gap or higher band-gap energy between the v alence band and conduction band, QDs can be behaving like insulators at ambient conditions and exhibiting electrical conductivity only under external stimulation (Kamat 2007). The optical properties of QDs can be described by conventional semiconductor physics and quantum mechanics. When a semiconductor is optically or electrically excited, static electrons (electrons located in the valence band) become mobile (electrons located in the conduction band) within the semiconductor matrix and after a certain period of time the electrons and holes r ecombine (Li et al. 2012). The quantum confi nement effect will occur only when the size of the nanostructure is on the order of the exciton Bohr radius (< 10 nm) (Brus 1986), and the excitation and emission peaks of QDs can be modulated easily by changing the nanoparticles diameters or engineering the QD core−shell structures,
Scheme 1. Graphical abstract of macrocyclic molecules coated quantum dots for fl uorescence sensing.
Figure 1. Schematic illustration of the quantum dots structure.
which results in wide ultraviolet−visible (UV−vis) absorption spectra, and narrow and symmetric emission bands (Medintz et al. 2003) (Fig. 2).
So far, fl uorescence-quenching and enhancement effects account for the main mechanisms of QDs as optical sensors. The mechanism of quantum dots fl uorescent quenching is usually explained by electron transfer (ET) and fl uorescence resonance energy transfer (FRET). Electron and energy transfer processes can therefore be designed to switch the luminescence of QDs in response to molecular recognition events, providing extremely sensitive probes or effi ciently sensitize the electrodes for solar cell applications. The electron transfer (ET ) quenching of photoexcited QDs is a versatile useful photophysical mechanism to follow spatially-restricted close interactions between electron donor–acceptor sites. It may occur over long distances and be associated with major dipole moment changes, making the process particularly sensitive to the microenvironment of the QDs (Freeman and Willner 2012). Thus, it can be expected that electrostatic interaction between the donor and the acceptor moieties with different charge (i.e., positive charge or negative charge) will change the photophysical properties of the functionalized QDs.
Fig ure 2. Photo demonstrating the size-tunable fl uorescence properties and spectral range of QD dispersions plotted. Reproduced with permission from ref. (Medintz et al. 2005) Copyright
© 2005 Nature Publishing Group.
Fluorescence resonance energy transfer (FRET) is a distance-dependent physical process by which energy is transferred to non-radiative from an excited molecular fl uorophore (the donor) to another fl uorophore (the acceptor) by means of intermolecular long-range dipole–dipole coupling (Bagalkot et al. 2007). It mainly infl uenced by three factors: the distance between the donor and the acceptor, the extent of spectral overlap between the donor emission and acceptor absorption spectrum and the relative orientation of the donor emission dipole moment and acceptor absorption moment. As is required, the distance between the donor and the acceptor is smaller than a critical radius, known as the Fö rster radius.
This returns the donor to its electronic ground state, and emission may then occur from the acceptor center. QDs can be served both as FRET donors and acceptors, which have been treated in several comprehensive reviews in the recent literature. As donors, QDs can be combined with a large variety of acceptors (e.g., organic dyes or fl uorescent proteins). Using QDs as acceptors is less common because of their broad excitation spectra, which will cause QD excitation at almost any wavelength (independent of the donor). This will lead to many QD acceptors in excited states, which is very counter productive for FRET (the acceptor must be in the ground state).
The photophysical properties of QDs can be controlled by their nanocrystal core sizes, the shell thickness, and the composition of the semiconductor materials of cores and shell(s) and partly by their surface ligands (Green 2010). The superior photophysical features of semiconductor QDs (high fl uorescence quantum yields and stability against photobleaching) are usually observed in organic solvents, and their introduction into aqueous media is usually accompanied with a drastic decrease in the luminescence yields of the QDs (Gao et al. 2005). Thereafter, one of the main problems encountered in the development of QD-based systems is the modifi cation of suitable capping ligands on the surface of QDs to improve the selectivity, sensitivity and biocompatibility of these systems with enhanced brightness and stability. To address this issue, various strategies have proposed which including (a) attaching a certain metal-specifi c ligand based on an electron transfer process; (b) exchanging of the ligands with more thiol groups by host-guest chemistry on the surfaces of the particles; (c) capped the surface with amphiphilic molecules or comb like polymers possessed a hydrophilic backbone and hydrophobic side chains. As a result, QDs are intrinsically soluble in non-polar media and suitable surface modifi cation will be performed.