The decoration of virus with luminescent QDs has also been demonstrated.45 The QDs fluorescence characteristic offers the opportunity to label the myosin46 or any protein undergoing conformational changes in multiple locations, so that a three-dimensional picture of the structure dynamics can be obtained at the molecular level in real time. Moreover, QDs conjugated with a paramagnetic lipidic coating enable the use both for optical imaging and magnetic resonance imaging (MRI).47

The QD-tagged microbeads are emerging as a new class of fluorescent labels with enhanced photostability and brightness, which are expected to open new opportunities in nanotechnology and biology.48

 

QDs can also be used as valuable analytical tools for various detection applications. With surface modification by different ligands, QDs as a chemical sensor have the luminescence responding selectively to many physiologically important metal cations, e.g., Zn2+, Cu2+,50, 51 Ca2+, Mg2+, Mn2+, Ni2+, Co2+,52 and Ag+,53 etc. The QDs conjugated with a certain receptor also exhibited the fluorescence tunability with the presence of some anions, such as F-, Cl-, Br-, HS-, and CH3COO-, etc.54 Based on the proximity-induced fluorescence resonance energy transfer (FRET), the luminescent QDs conjugated to antibody fragments and emission receptors were developed as solution-phase nano-scale sensing assemblies. As a specific example, the explosive 2,4,6-trinitrotoluene (TNT) in aqueous environments were detected by monitoring the increase of QD emission, as indicated in Fig. 2.6.49 When TNB-BHQ-10 is bound to the QD-TNB2-45 conjugate, the QD fluorescence is quenched (left). As TNT is added to the assay, it competes for binding to the antibody fragment and the QD fluorescence increases following TNB-BHQ-10 release from the conjugate (right). The photo-induced electron transfer between quenchers and QDs has also been utilized to probe protein-ligand interactions with luminescence measurements.55

Furthermore, the QDs linked to a photosensitizer were used for photodynamic cancer therapy.56 And studies for drug delivery have benefited from the use of QDs due to their ability to accumulate in tumors by enhanced permeability and retention at tumor sites or by antibody binding to cancer-specific cell surface biomarkers.57 It was indicated that QDs can bind with bacteria and impair the functions of a cell’s antioxidative system, which have the potential to be a novel antimicrobial material with excellent optical properties.58

Besides photoluminescence, QDs can also be excited by an electric field and emit photons, or absorb photons and generate electric potential difference. Significant amount of research has been carrying out to utilize QDs in displaying and solar cells based on their electroluminescence59-63 and photovoltaic64-67 properties. The integration of organic and inorganic materials at the nanometer scale into hybrid optoelectronic structures enables active devices that combine the diversity of organic materials with the high-performance electronic and optical properties of QDs.68 A light-emitting device (LED) with QDs as the luminescent source usually consists of a QDs layer sandwiched between organic thin films and electrodes on indium tin oxide (ITO) coated glass substrate.59 Compared with other light source, QDs can generate purer color because they emit light in a very specific wavelength, and require less power input since they are not color filtered. The multilayer structure can also be used in optoelectronic applications with QDs as the light absorber and electric carrier source.

Many other interesting phenomena have been explored and investigated to understand the behavior and properties of QDs for their wide spreading applications. It was reported that QDs could initiate current bursts in lipid bi-layer membranes upon application of a bias voltage, as a large permanent dipole moment of the QDs resulted in insertion into the lipid bi-layer in the presence of an electric field.69, 70 Due to the large surface-to-volume ratio, nanometer-sized QDs colloids can be used to degrade water pollutants as efficient photocatalyst in environmental remediation technologies.71