Application of the Quantum dots

Sandros (2005) present a facile, reagentless method for generating protein-based semiconducting nanoparticle sensors for small molecules.

Previous author reported that Maltose binding protein (MBP) is typically for biosensor method development. As a result of MBP undergo a reversible, ligand-dependent conformational change. These movements have been harnessed for maltose sensing either by differentially opening/closing motion on the protein surface [37-39], or by using lever-action molecular displacement [40-41]. Hence, Sandros employs the lever-action strategy to alter the interaction between an MBP attached [(tetraamine)(5-maleimido-phenanthroline)ruthenium]- [PF6]2 (1) and the surface of a water-soluble CdSe nanoparticle. The system provides reagentless selective detection of maltose by changing the interaction between 1 and CdSe nanoparticle surface in a distance-dependent fashion (as shown in Figure 1.13) [42]. The valence-band hole of the CdSe nanoparticle would be occupied on electron transfer from (1), when source excited on the CdSe.

Alternatively, electron transfer from (1) to the valence band of the CdSe excited state, forming a nonfluorescent CdSe anion, is consistent wuth the decrease in CdSe emission intensity. A 1.4-fold increase in fluorescence intensity was observed upon maltose addition (Figure 1.14). The phenomenon indicated a decrease in the (1)-CdSe electron transfer quenching.

Figure 1.13 Scheme of Maltose-Dependent Change in CdSe Emission [42].

Figure 1.14 Maltose-dependent fluorescence of complex 1 modified-K46C MBP-MT attached to 3.0-3.5 nm diameter THDA capped CdSe nanoparticles under 363 nm excited) at pH 7.5 (20 mM 3-(N-morpholino)propanesulfonic acid, MOPS): fluorescence emission spectra of a solution (5 nM biosensor) without (solid line) and with 1 mM maltose (dashed line) [42].

Shi (2006) reported the development of quantum dots FRET-based protease sensors and their application for the measurement of extracellular matrix metalloproteinases (MMPs) activity in normal and cancerous breast cells. The quantum dots based probes were prepared by exchanging the TOPO capping ligands of CdSe/ZnS QDs with tetrapeptide RGDC molecules. The acceptors are rhodamine which be labeled on the peptide molecules by covalent bonding. Upon enzymatic cleavage of the peptide molecules, the rhodamine (acceptor) molecules no longer provided an efficient energy transfer channel to QDs.

Therefore, the emission color of the QDs changed back to green ( the scheme show in Figure 1.5). The QDs FRET-based enzymatic activity probes were first used to determine the activity of collagenase in solution to test the analytical capabilities of the QDs FRET-based probes in a model system. FRET measurements of the QDs at increasing levels of collagenase in solution are shown in Figure 1. The fluorescence intensity (545 nm) of the QDs increased while the fluorescence intensity of the rhodamine molecules decreased due to the enzymatic cleavage of the RGDC peptide by collagenase [43].

Figure 1.15 Principle of Quantum Dots-Based Enzymatic Activity Probes [43].

Figure 1.16 Emission spectra of rhodamine-labeled peptide-coated quantum dots 15 min following the addition of collagenase of increasing concentration. (a) 0 µg/mL (black), (b) 0.5 µg/mL (red), (c) 2.5 µg/mL (green), and (d) 5.0 µg/mL (blue) [43].

1.3 Motivations

In this work, we aim to achieve several goals as below: First at all, modifying the CdSe/ZnS QDs surface contains carboxyl group and water soluble in aqueous. As we have discussed previously, modifiable ligand on the QDs surface, such as ligand exchange by thio group, encapsulation with amphiphilic polymer, and combinations of layers with thio group modified on biomolecular, are also effective for water-soluble QDs and play an important role in the biological or biosensor application area. Among the semiconductor NPs, CdSe or CdSe/ZnS QDs have more advantages in their optical properties, such as tunable photoexcitation depend on the size, high photostability, narrow and symmetric luminescence spectra, and high quantum yield.

Therefore, QDs charm many researchers to choose them to apply in the bio-labeling, biosensor (bioindicator), nanodevice, and solar-cell, etc.

Recently, the biosensors (bioindicators) are popular in the clinic or portable. However, in the composition of biosensor, they also need complex component, such as sensing area, enzyme immobilized, and circuit designing. For this reason, we propose easily bioindicators, which are urea, glucose, by luminescence intensity of QDs under photoluminescence spectrum instrument monitored. Second, to improve the quantum efficiency of water-soluble QDs (MSA-QDs) suit to assay lipid (triglyceride), where the MSA-QDs must be photoactivated through fluorescent lamp and support more hydrophobic site on the QDs surface to soluble the triglyceride. As we discussed above, current reviewers suggest that the luminescence intensity of QDs would be

increase around 5000% after photoactivation by UV lamp. However, the QDs could not easily control to keep away the aggregation. In this work, the fluorescent lamp is employed as photoactivation source, which contains wide band spectra, and easily gets high performance luminescence POD-QDs with low aggregation phenomena. The POD-QDs is also employed to apply in the triglyceride sensing through visualizing under 365 nm excited. Third, to fabricate and utilize the PDDA-QDs interaction forces to assemble protein molecular assay the bimolecular through fluorescent resonance energy transfer (FRET).

As we have discussed previous, the water-soluble QDs has lethal shortcoming in the luminescence through ligand exchange, and is not easily applied in the biological. Hence, we propose an easily method to improve QDs luminescence, which contains the photoactivation and polymer capping under the fluorescent lamp irradiation. The high performance water-soluble QDs would be fabricated and realized into the biosensing system through fluorescent resonance energy transfer (FRET).