Quantum dots: a novel smart carrier for drug delivery

1SURENDRA SINGH SAURABH, 2Borkar Niyati, 3Rathore K.S., 1Solanki Renu, 1Nagori B.P.

1 Lachoo Memorial College of Science & Technology, Pharmacy Wing, Jodhpur (Raj.), India

2JSPM's Rajarshi Shahu College of Pharmacy and Research, Tathwade, Pune,India

3BN Institute of Pharmaceutical Sciences, Udaipur (Raj.), India

E- mail:surendra.s.saurabh@gmail.com

 Introduction

Quantum dots are tiny semiconductor crystals of size 1-10 nanometres made up of compounds e.g. Ag, Cd, Hg, Ln, P, Pb, Se, Te, and Zn etc. These fluorescent quantum dots are glow or fluorescence brightly in different colours such as Adirondack Green (520nm), Blue (514 nm), Greenish blue (544 nm), Green (559 nm), Yellowish green (571 nm), Yellow (577 nm), Yellowish orange (581 nm), Fort Orange (600nm), Orange (610 nm), Maple Red-Orange (620nm), depending on their size by a light source such as a laser. The communication with the cells by the researcher is done by using molecular photodetectors. Traceable drug delivery is a recent and promising application of quantum dots having potential which explains the pharmacokinetics and pharmacodynamics of drugs which helps in drug designing and discovery. Quantum dots are currently limited to cell and small animal uses in testing of drug candidate because of long-term in vivo toxicity and degradation. 

A quantum dot has all three dimensions in the nano range. Materials can be nanostructured for new properties and novel performance. Quantum dots consists of three parts i.e. core, shell and cap. 

Core is made up of semiconductor material i.e. CdSe. Shell is the coat of ZnS surrounds the semiconductor core for improving its optical properties and cap encapsulates the double layer quantum dots by different materials e.g. silica which helps in improving solubility in aqueous buffers. 

Fig. 1: Schematic Representation of Quantum Dot.

Fig. 2: Size of Quantum dots

 Advantages of Quantum Dots:

1. 1.      Physical stability: Quantum dots are more resistant to degradation than other optical imaging probes, allowing them to track cell processes for longer periods of time
2. 2.      Photostability: They have greater photostability than traditional dyes due its inorganic composition and its fluorescence intensity do not diminish with time while organic dyes lose their intensities in 20s.
3. 3.      Signal to noise ratio: Quantum dots have high signal to noise ratio compared to organic dyes.
4. 4.      Broader excitation and narrow emission: Quantum dots have broader excitation spectra and a narrow more sharply defined emission peak. Due to these properties, a single light source can be used to excite multicolor quantum dots simultaneously without overlap.
5. 5.      Brightness: The brightness of quantum dots compared to organic dyes is 10 to 20 times brighter
6. 6.      Fluorescent lifetime: They are highly photo-resistant with significantly longer fluorescence lifetimes. Researchers can use their intense fluorescence to track individual molecules.
7. 7.      Excitation by single or multiple sources: Quantum dot can be excited by the same source and multi-colour quantum dots allows the use of many probes to track several targetsin vivo simultaneously.
8. 8.      Sensitive and precise: Due to their large Stokes Shift and sharp emission spectra, our conjugates have high signal intensity with minimal background interference.
9. 9.      Shape flexibility: They can be moulded into different shapes and coated with a variety of biomaterials.
10. 10.  Imaging agent: As Quantum dots are nanocrystals, they provide good contrast for imaging with an electron microscope as scattering increases.

Limitations:

1. Quantum dots may kill the cells due to aggregation.
2. They have surface defects which can affect the recombination of electrons and holes by acting as temporary traps results in blinking and detoriates yield of the dots.
3. Biconjugation of quantum dots leads to delivery into the target difficult.
4. Building material of the quantum dots can be cytotoxic e.g. Hg.
5. Their metabolism and excretion is unknown so the accumulation in body tissues can leads to toxicity.

 Properties of quantum dots:

 1. Quantum dots "designer atoms" offer innumerable optical and electronic properties that can work around natural limits inherent in traditional semiconductors.

2. Quantum dots are made from tiny bits of metal about thousand times smaller than width of a hair.

3. Quantum dots can be molded into different shapes and coated with a variety of biomaterials.

4. Quantum dots luminescence under UV light, with the size of the dots controlling its colour. e.g. 2nm Quantum dots luminescence bright green,5 nm Quantum dots –luminescence red

5. Fluorescent quantum dots are usually compounds from group II to VI and III to V e.g. Ag, Cd, Hg, Ln, P, Pb, Se, Te, and Zn etc.

6. As size of quantum dots decreases, the wavelength it emits turns shorter.

7. Quantum dots have a broad excitation range.

8. Quantum dots have precise emission wavelength, so the spectra doesn't overlap in multiple fluorescent emission 6.

 Quantum Dot Products:

1. EviDots
2. EviComposites
3. EviTags
4.  EviFluors

 Future Prospective Of Quantum Dots:

1. Research is ongoing for designing hydrophilic quantum dots that are luminescent.

2. More selective and specific approach of labelling cells and biomolecules is undergoing research.

3. Work is being carried to study interference effect of quantum dots with normal physiology and Production of quantum dots with higher biosafety.

4. NASA scientist working on quantum dots as drug carrier for Mars expedition in near future.

5. Single quantum dots of compound semiconductors were successfully used as a replacement of organic dyes in various bio-tagging applications.

 Method of Characterization:

1. Fluorescent Microscopy:

Axiostar plus transmitted-light microscope was used to observe the fluorescenceof the particles. The sample was excited with a mercury short arc lamp (HBO 50),and image captured using the Evolution MP Cooled Camera Kit.

1. Confocal Laser Scanning Microscopy:

Three-dimensional image reconstructions of cells were obtained with a LeicaTCS SP2 confocal laser scanning microscope (Leica, Germany) equipped with acomputer-controlled, motorized scan stage. An argon laser for excitation at 488 nmwas used for imaging. A 510–525 nm band-pass filter for the emission signal wasplaced in the front of detectors. For each cell, 30 optical planes were scanned.

1. Transmission Electron Microscopy (TEM):

TEM measurement was carried out on JEOL 2010 transmission electronmicroscope operating at an acceleration voltage of 100 kV. The nanobead solutionwas diluted in an aqueous solution containing 5 wt% sodium dodecyl sulfate (SDS)and 2% phosphotungstic acid (PTA). After thorough mixing, a drop of solution wasput on a copper grid coated with a thin layer of Formvar.

Fig. 3: Overview TEM methods

1. Fourier Transform Infrared (FT-IR):

Fourier transform infrared (FT-IR) spectra were obtained using a Bio-RAD FTS135 FT-IR spectrometer. A small amount of the sample was milled with KBr and pressed into a disc for analysis.

1.  UV Spectrometry: UV-Vis absorption spectra were obtained using a Unicam 300 UV-Visrecording spectrometer. 3 ml of the sample were placed in 10 mm path length quartzcuvette (Cole Palmer, IL) and spectral analysis was performed in the 200-800 nmrange using a UNICAM UV 100 UV/Visible light spectrophotometer (ThermoSpectronic, WI).
2.  Fluorescent Spectrometry:

Fluorescence measurements were performed at room temperature using a RF5301 (Shimatsu) spectrofluorimeter. 3 ml of the sample were first placed in a 10 mmpath length quartz cuvette (Cole Palmer, IL). Excitation and emission spectra werethen acquired using a SpectroPro 2150i Fluorescence Spectrometer (Roper ScientificActon Research, MA) equipped with a 1200 g/mm grating (32 mm x 32 mm) and a75 W xenon arc lamp as the light source. Scans were carried out at a step size of 0.1and an integration time of 100 ms.

Fig. 4: Quantum dots after fluorescence in different nanometer wavelength

 4.      TYPES OF QUANTUM DOTS

1. Core-Type Quantum Dots:

Quantum dots can be single component materials with uniform internal compositions, such as chalcogenides (selenides or sulfides) of metals like cadmium or zinc, example, CdSe or CdS. The photo- and electroluminescence properties of core-type nanocrystals can be fine-tuned by simply changing the crystallite size.

2.      Core-Shell Quantum Dots:

The luminescent properties of quantum dots arise from recombination of electron-hole pairs (exciton decay) through radiative pathways. However, the exciton decay can also occur through nonradiative methods, reducing the fluorescence quantum yield. One of the methods used to improve efficiency and brightness of semiconductor nanocrystals is growing shells of another higher band gap semiconducting material around them. These quantum dots with small regions of one material embedded in another with a wider band gap are known as core-shell quantum dots (CSQDs) or core-shell semiconducting nanocrystals (CSSNCs). For example, quantum dots with CdSe in the core and ZnS in the shell available from Aldrich Materials Science exhibit greater than 80% quantum yield. Coating quantum dots with shells improves quantum yield by passivizing nonradiative recombination sites and also makes them more robust to processing conditions for various applications. This method has been widely explored as a way to adjust the photophysical properties of quantum dots.

                                        Fig. 5: Quantum dots in core shell coating

 3.      Alloyed Quantum Dots:

The ability to tune optical and electronic properties by changing the crystallite size has become a hallmark of quantum dots. However, tuning the properties by changing the crystallite size could cause problems in many applications with size restrictions. Multicomponent quantum dots offer an alternative method to tune properties without changing crystallite size. Alloyed semiconductor quantum dots with both homogeneous and gradient internal structures allow tuning of the optical and electronic properties by merely changing the composition and internal structure without changing the crystallite size. For example- alloyed quantum dots of the compositions CdSxSe1-x/ZnS of 6nm diameter emits light of different wavelengths by just changing the composition. Alloyed semiconductor quantum dots formed by alloying together two semiconductors with different band gap energies exhibited interesting properties distinct not only from the properties of their bulk counterparts but also from those of their parent semiconductors. Thus, alloyed nanocrystals possess novel and additional composition-tunable properties aside from the properties that emerge due to quantum confinement effects.

Method of preparation of quantum dots  

Chemical synthesis of quantum dots represents a typical approach, which is generally divided into organic and water phase approaches.

1. Organic phase method:

a)       Colloidal synthesis:

The synthesis of colloidal quantum dots is based on a three-component system composed of precursors, organic surfactants, and   solvents. A reaction medium is heated to a sufficiently high temperature (e.g. 300°C) and under vigorous stirring the precursors are injected through syringe which chemically transform into monomers. Once the monomers reach a high enough super saturation level, the nanocrystal growth starts with a nucleation process. The solution immediately begins to change from colourless to colours like yellow, orange and red/brown, as the quantum dots increase in size by placing them under a "black light". This reaction results in the formation of monodispersed quantum dots. Surfactant is used to avoid aggregation, to make the quantum dots water-soluble.

Typical dots are made of binary alloys such as cadmium selenide, cadmium sulfide, indium arsenide, indium phosphide, ternary alloys such as cadmium selenide sulfide.

This method is the cheapest, less toxic and occurs at bench top condition.

 b)      Lithography:

By growing the quantum dots in a semiconductor heterostructure which refers to a plane of one semiconductor sandwiched between two other semiconductors. If this sandwiched layer is very thin i.e. about 10 nanometers or less, then the electrons can no longer move vertically and thus are confined to a particular dimension. This is called the quantum well. When a thin slice of this material is taken to create a narrow strip then it results in a quantum wire, as it gets trapped in a 2 dimensional area. Rotating this to 90 degrees and repeating the procedure results in the confinement of the electron in a 3 dimension which is called the quantum dot.

c)       Epitaxy:

Self-assembled dots can also be grown by depositing a semiconductor with larger lattice constant on a semiconductor with smaller lattice constant e.g. Germanium on Silicon. These self-assembled dots are then used to make quantum dot lasers. Hence, the quantum dots are actually formed when very thin semiconductor films buckle due to stress of having lattice structure slightly different in size from those on which the films are grown. The organic phase method produces quantum dots, which are generally capped with hydrophobic ligands (e.g. trioctylphosphine oxide or trioctylphosphine) and hence cannot be directly employed in bioapplications. To be used in biological applications, quantum dots need to be soluble in aqueous solutions and require surface modifications to achieve biocompatibility and stability.

2. Water phase method:

a)       Cap exchange:

The hydrophobic layer of organic solvent can be replaced with bifunctional molecules containing a soft acidic group (usually a thiol, e.g. mercaptoacetic acid, mercaptopropionic acid, mercaptoundecanoic acid or reduced glutathione (GSH), sodium thioglycolate) and hydrophilic groups (e.g. carboxylic or amino groups) which point outwards from the quantum dots surfaces to bulk water molecules. In fact, substitution of monothiols by polythiols or phosphines usually improves stability. From these ligands, GSH seems to be very perspective molecule, since provides an additional functionality to the Quantum dots due to its key function in detoxification of heavy metals in organisms.

b)       Native surface modification:

Adding a silica shell to the nanoparticles using a silica precursor during the polycondensation quantum dots are rendered water-soluble using several synthesis strategies, such as water soluble ligands, silanization, organic dendrons, cysteines, dihydrolipoic acid, encapsulation with block-copolymer micelles, with amphiphilic polymers, amphiphilic polymers conjugated with poly (ethylene glycol), and surface coating with phytochelatin-related peptides. All these synthesis strategies have effectively solubilized CdSe or CdSe/ ZnS quantum dots. In addition, quantum dots can be conjugated to biological molecules such as proteins, oligonucleids, small molecules, etc. which are used to direct binding of the quantum dots to areas of interest for biolabelling and biosensing.

 Application of quantum dots

• Quantum dots as carriers:

In quantum dot core, small molecule hydrophobic drugs can be embedded between the inorganic core and the amphiphilic polymer coating layer. Polymer coating of quantum dots is powerful tool toward diagnostics. Small size Quantum dots easily cleared from body by renal filtration.

• Quantum dots as tags for other drug carriers:

The research and development of various drug nanocarriers is an important part for the advance of nanomedicine. In traceable drug delivery – labelling a conventional drug carrier such as poly (lactic-co-glycolic acid) and polyethyleneimine (PEI) with quantum dots, which serve as photostable fluorescent reporters.

• Detecting Cell Death:

By combining a quantum dot with a novel carrier of the magnetic resonance imaging (MRI) agent gadolinium - a nanoparticle that can spot apoptosis, or programmed cell death, using both MRI and fluorescence imaging is designed.

• In vivo imaging:

Non-targeted near infrared emitting quantum dot (EviTags) as non-invasive optical molecular imaging probes will have a great impact on the early detection, diagnosis and treatment monitoring of cancer. No uptake in the tumor was observed, suggesting the next round of imaging to be done with tumor targeted EviTags will have minimal background signal within the tumor.

• Tumor Cell Markers:

In active targeting, quantum dots can be conjugated with tumor-specific active binding sites so as to attach themselves to tumor cells. Sequentially, immunofluorescent probes are manufactured with antibodies to detect these tumors.

In passive targeting, the quantum dot probes do not have the tumor-specific active binding sites. Instead, certain properties of the tumor cells are exploited. The growth rate of tumor cells greatly surpasses that of normal cells and thus the membranes of such cells are more permeable. This increased permeability sufficiently enables the absorption of nanocrystalline quantum dots. Through tumor cells' lymphatic drainage system deficiency and keen retention capabilities, further quantum dot absorption and multiplication can take place. In this way, tumor cells have bittersweet adaptations. Consequently, through tumor cells' abilities to efficiently take in and retain nanoparticles, passive targeting is made possible.

Fig. 6:

Left: Normal and cancer cells without quantum dot marking (top row) and with different quantum dot marking (middle and bottom)

Top Right: cancer cells marked with standard methods (white circle) compared with quantum dot tagged cancer cells (bright orange)

Bottom Right: comparison of different types of quantum dots injected beneath the skin of a mouse

• Immunoassay:

Immunoassay was carried out on a glass chip using a sandwich assay approach, where antibody covalently bound to a glass chip was allowed to capture antigen specially. The ZnS-coated CdSe quantum dots (ZnS/CdSe Quantum dots) were linked to a detection antibody. Antibody labeled with quantum dot was allowed to bind selectively to the captured antigen. The fluorescent signals of the sandwich conjugate were detected by a laser confocal scanner.

• Gene technology:

A number of studies have revealed that quantum Dot-conjugated oligonucleotide sequences (attached via surface carboxylic acid groups) may be targeted to bind with DNA or mRNA. Using precise la beling like red, green and blue Quantum dots in a number of combinations, identification of target sequences of DNA can be achieved.

• Pathogen and toxin detection:

Several different pathogens have been targeted so far, including Cryptosporidium parvumand Giardia lamblia, Escherichia coli and Salmonella Typhi and Listeria monocytogenes. Simultaneous multiplexed labelling of both C. parvum and G. lamblia using immunofluorescent staining methods with quantum dots fluorophores with better photostability and brightness compared with two commonly used commercial staining kits.

• Detection of viral infections:

Quantum dots bind to molecular structures that are unique to the virus coat and the cells that it infects. Rapid and sensitive diagnosis of Respiratory Syncytial Virus (RSV) is important for infection control and development of antiviral drugs. Antibody- conjugated Quantum dots rapidly and sensitively detects RSV. As a result, when Quantum dots come in contact with either viral particles or infected cells they stick to their surface and they illuminate bright fluorescence.

• Neuroscience:

Recent studies using quantum dots in neuroscience illustrate the potential of this technology. Antibody functionalized quantum dots are used to track the lateral diffusion of glycine receptors in cultures of primary spinal cord neurons. They were able to track the trajectory of individual glycine receptors for tens of minutes at spatial resolutions of 5–10 nm, demonstrating that the diffusion dynamics varied depending on whether the receptors were synaptic, persynaptic, or extrasynaptic.

• Drug discovery:

The features of quantum dots such as their multiplexing potential, photostability, and inorganic nature make them of value for drug discovery. For example, they would allow monitoring of multiple drug candidates over extended time periods in cell culture simultaneously, thus saving time and cost.

• Biosensor and biolabels:

A number of analytical tools have been developed with application of this smart and potential technology. These tools are employed for determination of various pathological proteins and physiological-biochemical indicator associated with disease or disrupted metabolic conditions of body.

• Surgical guidance:

Quantum dots also have a potential surgical utility by providing optical guidance that can result in reduction of cancer metastases. Scientists such utility by mapping sentinel lymph nodes at 1 cm tissue depth using oligomeric phosphine-coated quantum dots that emit in the nearinfrared region. The sensitivity and stability were superior to conventional dyes and thus this approach could improve the sensitivity of surgical lymphatic resectioning.

Table 1 Summary of Application Areas for Nanoscale Pharmaceuticals and Medicine in Diagnostics

Material/technique

Property

Applications

Timescale (to market launch)

Diagnostics Nanosized markers,

i.e. the attachment of

nanoparticles to molecules of interest.

Minute quantities of a substance can be detected, down to individual molecules.

Detection of cancer cells to allow early treatment.

----------

‘Lab-on-a-chip' technologies

Miniaturisation and

speeding up of the analytical process

The creation of miniature, portable diagnostic laboratories for uses in the food, pharmaceutical and chemical industries; in disease prevention and control; and in environmental monitoring.

Although chips currently cost over £125

(US$2085) each to make, within three years the costs should fall dramatically, making these tools widely available.

Quantum dots

Quantum dots can be tracked very precisely when molecules are ‘bar coded' by their unique light spectrum.

Diagnosis

In early stage of development, but there is enough interest here for some commercialization.

 Conclusion

In the area of nanomedicine, quantum dots add to the expansion of new diagnostic and delivery systems. As they are well defined in size, shape, provide sole optical properties for highly sensitive detection and can be customized with various targeting principles. It has created powerful impact in various fields of disease diagnosis, intracellular tagging as photo sensitizer for treatment of cancer, biotechnology and bioassays. Current advancement in the surface chemistry of quantum dots expanded their use in biological applications, reduced their cytotoxicity and rendered quantum dots a powerful device for the research of distinct cellular processes, like uptake, receptor trafficking and intracellular delivery.

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About the Author

Dr.Kamal Singh Rathore

Reader, Bhupal Nobles' Girls' College of Pharmacy, Udaipur-Raj.313002 INDIA Email: kamalsrathore@yahoo.com kamalsrathore@gmail.com Mobile:... (show bio)