Surfactants as Quantum Dots used in Bio-Imaging
Dipti Bhave, Snehal Deshpande, Upendra Dabholkar*
B’Tech Oils, Oleochemicals and Surfactants Technology, at Institute of Chemical Technology, Mumbai.
*Corresponding Author E-mail: upendradabholkar1997@gmail.com
Abstract:
Quantum Dots (QDs) are semiconductor nanocrystals, which arose a new class for chemical analysis, molecular imaging, and biomedical diagnostics as fluorescent labels. There are traditional ways to synthesize QD. In QD micelle formation takes place, with hydrophobic tails and hydrophilic head. The core and the ligand play a key role in defining the properties of quantum dots. Colloidal Synthesis, Plasma Synthesis and Green Synthesis methods are followed to synthesize QD. The green synthesis method would be helpful in near future because of the use of oil and micro-organisms as raw material. In this paper, the size, shape of the QD, presence of ligand and other factors affecting the properties of QDs are discussed. Also, the toxic effects in vitro and in vivo imaging of QDs are highlighted along with the preventive measures to subjugate the difficulty. QDs overcome the limitations of organic dyes and fluorescent protein because of their ability to tune the size of fluorescence emission and generating any specific wavelength from UV through near infrared. QDs may provide the new class of biological values that can overcome the limitations of organic dyes and fluorescent protein. These properties are apposite for imaging and for multiplexed biomedical diagnostics at very high sensitivity. Hence, QDs are growing interest in medical field as an imaging tool of diseased tissues and organs.
KEY WORDS: Colloidal synthesis, Green synthesis, Imaging, Toxicity.
INTRODUCTION:
The quantum dots (QDs) are novel photo-stable, near spherical semiconductor nanocrystals possessing wide excitation spectra and narrow symmetrical emission spectra. The dot is a conducting island having shape and size which are comparable to the Fermi wavelength in all spatial directions, however the size is much bigger in nanometre. A quantum dot, also known as a synthetic atom, has different electronic energy levels, similar to electronic energy levels of molecule or atom, but in case of quantum dots, the experiments can very precisely alter the specific space of the electronic energy levels by variation in the size. These quantum dots are a captivating subject for a research in the laboratory. [1]. Quantum dots are fluorescent in nature. In QDs, the attractive forces are exerted by background charges while in atoms they are exerted by nuclei.
Furthermore QDs are highly tunable. They provide a very few possibilities to place interacting particles into a small volume giving permission to evaluate and verify fundamental concepts and encourage new uses. The dots use less energy than other sources. Because of these reasons for many treatments in medical fields these dots are used. QDs are growing interest in medical field as an imaging tool of diseased tissues and organs. There are some factors which impede its use in biomedical application like biocompatibility, toxicity and fluorescence instability [2].
The recent investigation of nano level particle probes for treatments like biomolecular imaging as well as diagnostics is nowadays an area of considerable interest. The main concept states that nanometer-sized particles have structural, other functional characteristics that are unavailable from either different molecules. When associated with biomolecular loving ligands, for example peptides or very small (micro level) molecules, these nanoparticles may be useful for the detection of molecular biomarkers and tumor cells at very high sensitivity with specific characteristics. The nanoparticles have a large surface areas for the attachment of more than one diagnostic (e.g., radioisotopic, or magnetic) and therapeutic (anticancer) agents. The increased research have led to the development of biodegradable, very small (nanostructures) for delivery of the injected drug [4] nanocrystals of iron oxide for magnetic resonance imaging.
Because of their highly tunable properties, QDs applications includes transistors, solar cells, LEDs, lasers(diode) and second-harmonic generation.[5] Furthermore, their very small size allows QD to be suspended in solution which is useful in spin-coating, ink jet printing.[6]
SYNTHESIS OF QUANTUM DOTS:
Colloidal Synthesis:
Hydroxylation of the surface of PbS nanocrystals passivated with oleic acid
Danylo Zherebetskyy1, Marcus Scheele1,2, Yingjie Zhang1,3, Noah Bronstein2, Christopher Thompson1,2, David Britt4, Miquel Salmeron1, Paul Alivisatos1,2, Lin-Wang Wang1, Science 20 Jun 2014:Vol. 344, Issue 6190, pp. 1380-1384 DOI: 10.1126/science.1252727
Colloidal semiconductor crystals are produced from solutions. The main discrepancy is the product does not form precipitate and not even remains in a dissolved state [7]. When we heat the solution at high temperature, it generates nanocrystals. Temperature is the crucial factor for the small sized crystal growth. Another crucial factor that needs to be controlled during this type of growth is concentration of monomer. Also many colloidal methods are available to produce various types of semiconductors: These dots are made of binary compounds.
Plasma synthesis:
Plasma synthesis is a prominent gaseous phase approach for the manufacturing of quantum dots, specifically QD forming covalent bonds. Non-thermal plasma method is used to synthesized silicon (Si) and germanium (Ge) quantum dots.
The factors like shape, size and composition of these quantum dots can be controlled in non-thermal plasma method [8]. QD are in form of powder which are synthesized by plasma, so surface modification may be required. Therefore, QDs can be excellently dispersed in organic solvents [9] or water [10].
Bulk manufacture:
“High temperature dual injection” is the process on which QD manufacturing depends. This is produces by many companies for the commercial applications which requires large quantities (kilogram to tonnes) of QD. Such reproducible manufacturing method may be applied to wide range of quantum dot composition.
“Molecular seeding” is the alternative process for synthesis of quantum dots. It provides high quality quantum dots production in large scale. Same molecules of a molecular cluster compound is used as the coagulation sites for nanoparticle growth. Hence, we can avoid the step of “high temperature injection step” [11].
Heavy metal free QD:
Use of heavy metals in household utensils are banned, as a result of this many quantum dots which are based on cadmium are not used for goods application. For industrial and commercial use, QDs (free from heavy metal) are synthesized, which show emission in the near infrared as well as in visible region of spectrum, and show similar properties like CdSe QDs. Potential quantum dot material is peptide. [12]. Such QD are nontoxic and easily biodegradable as peptides are occurs naturally in all organisms.
Quantum dots which are used in bio-applications are (not inclusively) colloidal crystals of very small size.[13] The available QD fluorophores which helps in different microbiological applications are made up of CdSe cores; applying a coating of ZnS since this chemical process is refined. The ZnS layer passivates the core surface, prevents oxidation.[14]
Quantum Dots which are prepared using high-temperature paths are not soluble in aqua, thus for the phase transfer to aqueous solution surface functionalization with hydrophilic ligands has to be there. It can be done in two ways; the first is ‘cap exchange’, it is a process which is primarily carried out by mass action.[15]
These ligands mediate both the colloid’s solubility and serve as an exact point where chemical attachment of biomolecules takes place. Synthesis of new caps forms an ever-increasing library which are suitable for obtaining aqueous QD dispersions; nevertheless, a lot of them were created for nothing but special purposes and thus their applications are restricted. [16, 17]
The first uses ‘cap exchange’ and comprises the substitution of the native TOP/TOPO with dual functional ligands, the groups which are opposing having hydrophilic end (for example: hydroxyl, carboxyl) to achieve hydrophilicity. The second method involves manufacturing of polymerized silica shells functionalized with polar groups, which provides insulation the hydrophilic. QD.
The next method preserves the native TOP/TOPO on the Quantum Dots and applies variants of amphiphilic ‘diblock’ as well as ‘triblock’ copolymers to tightly interdigitate the ligands such as alkylphosphine through lyophilic attraction, whereas the hydrophilic outer block give permission to disperse in aqua phase and further derivatization . But the advantages of each method have to be carefully weighed against the problems. For example, compact mono-mercapto ligands, although not difficult to produce, have short shelf life encapsulation, gives a good stability over a pH range, For example, phospholipid and block copolymer coatings tend to change(increase) the diameter of CdSe–ZnS QDs (before encapsulation) from 4–8 nm to 20–30 nm, a size which is of course smaller than most mammalian cells ; still limits intracellular mobility and may preclude.
Inorganic–biological hybrids are made by conjugating inorganic nanostructures (nanoparticles, nanorods) along with biomolecules (proteins, DNAs) and the resulting conjugates combined the properties, that is, the spectroscopic characteristics of the nanocrystal and the biomolecular function of the surface-attached things. Owing to its measurable size (comparable to or slightly larger than that of many protein) a single QD can be conjugated to many proteins at the same time.
The QD thus acts as a nanoscaffold for attachment of different proteins, or other biomolecules, creating a multifunctional nanoparticle–biological hybrid. Experiments have shown that 15–20 maltose binding protein can be attached to each 6-nm-diameter QD. In these assemblies, conjugate dimensions depend on key parameters, such as Surface proteins that do not have their recognition site exposed away from the QD surface may lose their quality to bind a target.
Conjugation schemes for attaching proteins to QDs are divided into three categories (i) Use of EDC, surface to amines; (ii) direct binding to the QD surface by using thiolated peptides or polyhistidine (HIS) residues; and (iii) adsorption or noncovalent self-assembly using engineered proteins. They produce intermediate aggregates due to bad QD stability in neutral/acidic buffers.
By using metal-affinity coordination, HIS-expressing proteins or peptides can be directly attached to Zn on the QD-surface. This (very) strong interaction (Zn2+-HIS) has a dissociation constant, KD, only slightly less than that measured for 6- HIS to NTA-Ni2+ but stronger than most antibody bindings [18].
Green Synthesis:
Nowadays there are two synthetic methods (which can be slightly varied) to produce QDs. Each method exploits diverse aspects of chemistry to produce QDs.
The first method is an organometallic colloidal synthesis (OCS) operating at higher temperature, which makes use of high boiling point organic solvents [19]. The easier and greener approaches for OCS have been developed by replacing Cd precursor that is highly toxic and volatile with more stable cadmium sources that is also cheaper, for example Cd(Ac)2 ,CdO and CdCl2
The greenest approach is using environmentally renewable raw materials, oleic acid, castor, and olive oil, as solvents and coordinating agents in OCS method. These three raw materials is noteworthy in QD synthesis avoiding the need for the use of air-sensitive, toxic, and expensive chemicals, such as tri-n-octylphosphine oxide (TOPO), trioctylphosphine (TOP), or hexadecyl amine (HAD).
The second method is an aqueous colloidal synthesis (ACS). For biological applications ACS appears in response to the needs of TOP/TOPO-QDs solubilization. The greenest approach to produce highly fluorescent QDs is through plant or micro-organism mediated biosynthesis at ambient conditions using appropriate solvents. Plant based bio-synthesis of CdS QDs is an efficient and environmentally method. It uses hairy root culture[19] of Linaria maroccana L (flowering plant) [20].
Several studies show that synthesis of nanoparticle could be mediated by natural products such as such as alkaloids, terpenoids, phenols, flavanoids, tannins, and quinines using redox processes. Synthesis of CdSe, CdTe, and CdS QDs have been engaged by microorganisms such as fungi (Fusarium oxysporum and Saccharomyces cerevisiae) [21] or bacteria (Escherichia coli) respectively.
FACTORS AFFECTING QUANTUM DOTS:
Size:
QDs have highly compact sizes, which helps reserves the superior optical properties of the nanocrystals. They are multifunctional, multidentate polymers ligands. These multidentate polymer ligands displaces the prevailing ligands present on the QD and tightly binds to the nanocrystals surface forming a closed “loops-and-trains” conformation, producing unusually thin polymer shell. This conformation eliminates the hydrophobic barrier layer, excellent colloidal stability, not affected by photobleaching, large quantum yield and small overall particle size are the phenomenal properties because of the multidentate binding of the polymer.
High Throughput Quantum Dot Based LEDs
P. Amini1, M. Dolatyari1, G. Rostami1 and A. Rostami1
[1] OIC Research Group, School of Engineering-Emerging Technologies, University of Tabriz, Tabriz, Iran
Presence of ligands:
Methodic change of surface ligands is carried out, in order to disperse QDs in aqueous solutions, Type of exchanging ligands determines overall properties of QDs including hydrodynamic radius, fluorescent quantum yield and colloidal stability. The resuspended solution is usually kept at high temperature (60–90oC) for 5–18 hours to facilitate exchange and enable sufficient surface coverage.[23]
After ligand exchange, the QDs display polar functional groups that impart solubility in aqueous media. These ligands are bound to the QD surface by non-covalent bonds. Several designs of these ligands have been tuned to increase bond strength, stability and solution properties of QDs. Similarly, the procedure for ligand exchange may additionally influence the polydispersity of the water-solubilized QDs [24].
The chemical groups, having very high affinity for the QD surfaces include, carboxylic acids, phosphonic acids and amines. Monothiol-based ligands are extensively used for ligand exchange of QDs. However, they have their own limitation, owing to very high negative charge density at physiological pH, DHLA (dihydrolipoic acid) -coated QDs often show high non-specific adsorption of proteins that in turn affect their biological applicability.
In order to specifically address this type of issue, variants of DHLA have been developed. These variants often include a passivating layer of polyethylene glycol (PEG) with the dithiol [23]. Another variant of DHLA-based small molecule ligand includes zwitterions instead of PEG that impart hydrophilicity and antifouling properties to the QDs [23].
Polymeric ligands:
Liang HuChang Zhang
Many polymeric ligands have been synthesized, using the idea from small molecule ligand synthesis, another substitute to this comprises manufacturing of a polymer backbone and then conjugation of hydrophilic entities as well as functionalizing entities on this scaffold [109].
Once this is done (production of polymers), these preparations are added either directly to the QDs (in the case of ligands based on PEG, etc.) or to Quantum dots pre-exchanged with MPA or other small ligands. Based on the nature of manufacturing of QDs, types of ligands and nature of reducing agents, variable protocols are optimized [26]. The exchange procedures and type of ligands tends to affect the state of agglomeration of the capped Quantum Dots.
Toxicity:
QDs are condemned for toxic ions present in their core. The ions are Cd2+, Hg2+, Pb2+, and As3−, which limits their application in biomedical fields. One way to overpower this drawback is to coat with ZnS, introduced to afford core/shell nanostructures. They reduce the surface defects and increases the biocompatibility of QDs, Thus augmenting the luminescence. But, the problem due to ZnS coating inevitably increases the overall size, which is not desirable in biomedical application.
To circumvent these limitations another promising alternatives are I–VI-based QDs (Ag2S, Ag2Se, and Ag2Te). Ag2S QDs as a near infra-red II probe for molecular imaging of living cells. It is also important to account that the ligand exchange process does not significantly alter the photoluminescence properties. Cell proliferation, cell apoptosis/necrosis, production of reactive oxygen species, and comet test for DNA damage; all the mentioned tests indicated that Ag2S QDs have negligible cytotoxicity [26]. Also, Si and ZnO QDs are nontoxic and environment friendly.
Even for in vitro applications, QDs disposal into the environment is a major concern. One of the major concern is the strength of the surface coating, the core of the QD can be damaged due to air oxidation and UV damage, if the surface coating is not proper. This could lead to the release of toxic ions from the core. To protect the core from air oxidation ZnS capping should be done also to protect from UV damage QDs can be coated with poly ethylene glycol or encapsulating with micelles. [27, 28]
QUANTUM DOTS VS. ORGANIC FLUROPHORES:
Due to characteristic high sensitivity of detection and easily use, fluorescent dyes are used in the study of biological phenomenon. QDs may provide the newer class of biological values that can overcome the boundaries of organic dyes and fluorescent protein. With ability to tune the size of fluorescence emission, QDs can be introduced for any specific wavelength, from UV through near infrared.[29]
Chemical analysis and cellular imaging with quantum dots
Andrew M. Smith and Shuming Nie, Departments of Biomedical Engineering and Chemistry, Emory University and Georgia Institute of
Technology, 1639 Pierce Drive Suite 2001, Atlanta, GA 30322, USA Received 24th March 2004, Accepted 20th April 2004 First published as an Advance Article on the web 10th June 2004
QDs emission peaks are hypothetical for application involving the simultaneous detection of multiple fluorophores, because the QDs emission peaks are narrower (FWHM typically 25–35 nm) and symmetric compared to organic fluorophores .[ 30] In addition ,the excitation of multiple fluorophores with single light source, at any wavelength shorter than the emission peak wavelength permit by the broad absorption spectra of QDs. QDs are impeding to photo bleaching. It is the characteristic problem for organic fluorophores, hence making them useful for continuous controlling of fluorescence.
Quantum dots shows very high molar excitation coefficients and high quantum yield, results in bright fluorescent probe in aqueous solution. Fluorescence lifetime of QDs is 20-50 ns, Which may allow them to distinguish from background and other fluorophores for elevated sensitivity of detection . However QDs are unlikely replace organic dyes.
Chemical analysis and cellular imaging with quantum dots
Andrew M. Smith and Shuming Nie, Departments of Biomedical Engineering and Chemistry, Emory University and Georgia Institute of
Technology, 1639 Pierce Drive Suite 2001, Atlanta, GA 30322, USA Received 24th March 2004, Accepted 20th April 2004 First published as an Advance Article on the web 10th June 2004
QDs are expensive than that of organic dyes even though QDs are commercially available. It also changing the system of biological detection from dyes to QDs. Other than this, in order of magnitude QDs are larger than organic dyes. As QDs are smaller than 1nm are inherently unstable, therefore application like real time monitoring of biomolecular interaction may require to use of organic dyes. In addition, fluorophores of different emission are similar sterically, compared to large difference in QDs size, which is required to tune their wavelength as most organic dyes are of similar size.it is found that, the emission wavelength of alloy QDs may be tuned by keeping size constant and changing the alloy consumption.[29]
CANCER DIAGNOSTICS:
ER/PR/Her2 is one of the most common biomarker panels employed by oncologists and pathologists. To diagnose breast cancer and to elicit the most cogent treatment strategy for breast cancer patients this biomarker panel is used. Immunoassays such as Hercep Test and traditional IHC techniques, all of which rely on the subjective assessment of protein marker expression anticipate by standard chromagen measured with the markers. The simultaneous staining and measurement of these biomarkers in both cultured human breast cancer cells and in fixed (FFPE) clinical tissue specimens by using multiplexed QDs have been demonstrated by Yezhelyev et al.
QD based methods for the quantification of ER/PR/Her2 proteins:
A clinical tissue specimen is bombarded by the five QD colors simultaneously to identify five unique markers (ER/mTOR/PR/EGFR/Her2), further, the molecular profiling potential of these nanoparticles is demonstrated in complex tissue sample. The capability of QD immunostaining and comparisons with current clinical methods have been investigated by Chen et al., who detected the caveolin-1 (proliferating cell nuclear antigen) using lung cancer tissue microarrays. It is noticed that QD based immunostaining methods have a higher detection sensitivity in comparison to antecedent clinical techniques. Li and colleagues performed a detailed examination of QD staining for the Her2 protein and also demonstrated accuracy and sensitivity.
Marker detection using QDs was more sensitive and accurate than the standard techniques, it is stated by comparing conventional IHC and FISH. Especially in cases with moderate marker expression. On basis of all this research, further studies particularly in cases with moderate marker expression, in which subjective assessment using accepted methods are often problematic and can introduce error or bias. Li and colleagues proposed in a new indicator (total Her2 load) to assess prognosis in breast cancer patients By connecting the increased accuracy and sensitivity of QD-based immunostaining with another key parameter (total tumor size). This results in identifying more patients in the poor prognosis group than did Her2 gene amplification. Other method was identified with the total Her2 load parameter. This results improves the breast cancer diagnosis and QD-based immunostaining assays is used for patient classification and also and demonstrate the potential for individualized diagnostics.[31]
CdSe/ZnS type Self-assembly of QD and protein due to electrostatic attraction between the negatively charged DHLA cap on the QD and positively charged leucine zipper tail of the protein. The application is An alternative conjugation strategy for conjugating proteins to the surface of CdSe/ZnS DHLA-capped QDs other than using EDC (1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide) crosslink.[32]
CONCLUSION:
Considering future aspects, fundamental studies and pragmatic applications for semiconductor nanocrystals would have major advancement. In core research, synthesis of new nanocrystals, having desired chemical, electronic, and optical properties will produce enormous success. On the other hand in biomedical application, our major concern should be to synthesize nanocrystals that minimize the steric hindrance, non-specific protein adsorption, bright single molecule imaging, and to have complete knowledge about the toxic effects caused due to semiconductor materials.
Hence we conclude that semiconductor QDs is a major breakthrough in today’s world with respect to in vivo and in vitro imaging. The capability to image in real time the single-cell migration is chief to several research areas such as embryogenesis, cancer metastasis, stem cell therapeutics, and lymphocyte immunology. Production of high-resolution three-dimensional image is augmented by improved photostability of quantum dots.
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Received on 27.05.2018 Modified on 28.06.2018 Accepted on 25.07.2018 ©A&V Publications All right reserved Research J. Science and Tech. 2018; 10(3):188-196. DOI: 10.5958/2349-2988.2018.00026.8 |
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