Quantum dots as nanoshells

Dr Pradnya Palekar Shanbhag

Quantum dots (QDs) are tiny semiconductor nanocrystals with a quantum confinement property that enables them to emit fluorescence from visible to infrared wavelengths on excitation. The size of QDs ranges within the nanometre scale, normally 2-10 nm in diameter. Typically, a single QD contains a total of approximately 100-1, 00,000 atoms in its crystal core. Recent advances in nanomaterial have produced a new class of fluorescent labels by conjugating semiconductor quantum dots with biomolecules. These nanometre-sized conjugates are water-soluble and biocompatible, and provide important advantages over organic dyes. In particular, the emission wavelength of quantum-dot nanocrystals can be continuously tuned by changing the particle size, and a single light source can be used for simultaneous excitation of all different-sized dots. High-quality dots are also highly stable against photo bleaching and have narrow and symmetric emission spectra. These novel optical properties render quantum dots ideal fluorophores for multicolour, imaging therapy, tumour detection, tissue imaging, immunohistochemistry, infectious agent detection and multiplexed diagnostics in nanomedicine.

Mechanism of fluorescence

Structure of a quantum dot

In the bulk form of the semiconductor material the electrons exist in a range of energy levels described as continuous. At the nanoscale, these levels become discrete owing to the effects of quantum confinement. Following a stimulus, the electron jumps from the valence to the conduction band across the band gap leaving behind a positively charged hole. After being excited to the conduction band, the valence electron drops back to its valence position emitting electromagnetic radiation which is different from the original stimulus. This emission frequency is perceived as fluorescence and depends on the size of the band gap which can be altered by changing the size of the QD as well as changing the surface chemistry. It is important to note that the smaller the QD the higher the band gap energy. This size tunable absorption and emission property of QDs is an extremely valuable property for biological imaging as they can be tuned all the way from the UV to the NIR of the spectrum.

Biocompatibility

In order to utilise QDs in a biological environment they need to be made hydrophilic. The main strategies to make QDs biocompatible include salinisation and surface exchange with bi-functional molecules. Also by method of encapsulation of QDs within phospholipids micelles, polymer beads or shells, amphiphilic polysaccharides or block– copolymer micelles, which are composed of synthetic polymers containing hydrophilic and hydrophobic parts. Mercaptohydrocarbonic acids such as mercaptoacetic acid can be used for coating QDs to make them biocompatible presents the advantages and limitations of the main methods for making QDs biocompatible.

Quantum dots as carriers with integrate functionalities

The size of QDs can be continuously tuned from 2 – 10 nm which, after polymer encapsulation, generally increases to 5 – 20 nm in diameter. Particles smaller than 5 nm are quickly cleared by renal filtration whereas bigger particles are more likely to be uptake, by the reticuloendothelial system before reaching the targeted disease sites. Additionally, larger particles have limited penetration depth into solid tissues. Hydrophilic therapeutic agents (including small interfering RNA [siRNA] and antisense oligodeoxynucleotide [ODN]) and targeting biomolecules (such as antibodies, peptides and aptamers) in turn can be immobilised onto the hydrophilic side of the amphiphilic polymer via either covalent or non-covalent bonds. This fully integrated nanostructure may behave like a magic bullet that will identify, bind to and treat diseased cells and emit detectable signals for real time monitoring of its trajectory.

Applications

One additional feature of QDs is that they can emit in the infrared and near-infrared regions. This makes them suitable for imaging and diagnostic applications in cells deep within tissues as the absorption of tissues is minimal in this region QDs whose composition includes hybrids of heavier metals e.g. CdTe or HgSe or PbSe are being considered for such applications due the extension of their emission spectra into the near-infrared region. Also the stability of QDs and their resistance to metabolic degradation in live cells would allow long-term imaging studies and several studies have indicated lack of cytotoxicity for periods up to four months.

• In vitro nanodiagnostics immunohistochemistry
• A QD-based assay for the detection of the ovarian cancer
• QDs survive typical tissue-mounting procedures
• Immunoassays
• Nucleic acid detection
• Detection of genetic polymorphisms
• Single-molecule detection
• Stem cell tracking
• Neurotransmitter detection
• Imaging
• Diagnostic imaging
• Proteomic and genomic applications
• Drug delivery
• QDs are important for delivering drugs
• Photodynamic therapy
• Drug discovery

Quantum dots catch cancer early

Cancer-related DNA strands stick to single quantum dot like arms extending from an Octopus

Using an ultraviolet light or laser light shines on a quantum dot. The dot quickly passes energy to nearby molecules that use the energy to emit a fluorescent glow. Cancer-related DNA strands lights up and identify them. Up to 60 of the targeted DNA strands stick themselves to a single quantum dot like arms extending from an octopus.

Limitations

• Ability of QDs to be conjugated to several biomolecules where a QD can hold up to ten 150-kDa proteins which would seriously hinder both the mobility of QDs and the functionality of the conjugated protein molecules unless its valency is reduced
• QDs may also be subject to reduced luminescence activity due to their relatively large surface areas
• The transport of a large volume (due to multiple attachments of drug molecules to a single QD) across the membrane will be more difficult than the single molecule
• Large scale use of QDs is their toxicity
• The fact that QDs have basic components that are highly toxic to humans e.g. Cadmium raises serious safety issues
• For in vivo studies the main concern is the robustness of the surface coating. An unstable surface coating could expose the CdSe core of the QD to UV damage or air oxidation and result in release of cadmium ions from the QD core the ZnS capping would protect the core from air oxidation but not from UV damage

Commercialisation and regulatory issues

The next challenge associated with QD-based medical applications is the commercialisation of the products and development of the appropriate regulations. Again toxicity issues are a major concern. Although research is going on and QDs for life sciences research applications are commercially available applications involving QDs for in vivo imaging, drug deliveries and therapy have a long way to go before they can hit the market. In general, drug development is an extremely rigorous and costly process with delivery times for preclinical, clinical studies and approval increasing from 11.6 years in the 1970s to 14.1 years in the 1990s. Nano-biotechnology market is rapidly expanding and several establishments including the National Institute Health (NIH) and the National Nanotechnology Initiative (NNI) are investing into nanomedicine in general and resolving QD toxicity issues for medical applications in particular. The NIH expects that over half of the biomedical advances by 2010 will be in the nanotechnology sector and by that time the projected market growth for molecular imaging is $45 billion.

Conclusion

The potential applications of QDs in nanomedicine are numerous spanning the areas of imaging, therapy, drug delivery and nanodiagnostics. In the latter area the most promising applications are tumour detection, tissue imaging, intracellular imaging, immunohistochemistry, infectious agent detection, multiplexed diagnostics and fluoroimmunoassays. The National Heart Lung and Blood Institute Nanotechnology Working Group concluded that using QDs as tools for diagnostics and biosensor development is a promising domain and recommended its follow up. Despite all their promise QDs are still away from large scale use in nanomedicine pending the resolution of toxicity concerns and regulatory and commercialisation issues. The in vitro applications are likely to expand quickly in the coming few years as the technology is rapidly utilised and optimised to reach the sensitivity and overall needed clinical performance level and the toxicity issues are much less pressing than for in vivo application, which will probably take over a decade to be used commercially with confidence. It must also be emphasised that the concerns and issues associated with the QD technology must be investigated. This will ultimately enhance the chances of QDs coming into regular use in the medical practice and avoid running into roadblocks with their application later on. All in all, the potential of QDs is immense and would shed a new light on various medical applications.

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(With inputs from Sujata S Gaikwad, Manasi M Chogale, Sneha V Jog)