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Quantum Dots


To cite this article use: Michalska, M., Coy, E., Flak, D., Przysiecka, L. Quantum Dots. J. Nano Sc. Tech, 3(2015)36-43

Reducing the gap between research and applications

M. Michalska

L. E. Coy

D. Flak

L. Przysiecka

NanoBioMedical Centre

Adam Mickiewicz University

Email: m.mich@amu.edu.pl

The past few decades have witnessed an almost exponential growth in the application of nanomaterials in everyday life products. The gap between scientific developments and real life applications has been reduced significantly. Nowadays, many outstanding properties of commercially available electronic devices, cosmetics, automotive products, medicaments, clothes and even food products are attributed to the results derived from nanotechnology and nanomaterials research [1]. Nanomaterials have come a long way in their production and application, and some of their most interesting applications still have a long way to go and are under thorough and intensive investigation and development. Nanomaterials come in many “flavors” according to their constituents (i.e: metallic, oxides, organic and polymeric), and in appearance according to their morphology and shape (i.e: nanotubes, thin films, nanowires, nanosheets, nanocomposites and nanoparticles). During the last few years, the old known friend, the NANOPARTICLE, has been widely studied in terms of the so called bandgap tailoring, gaining potential but also achievable applications in several fields of research like biomedicine and health care, energy and environment, photo-chemistry and catalysis, textiles, food and agriculture, just to mention a few main areas.

Basic concepts

In order to talk about the bandgap tailoring, it is important to explain first in general what actually the bandgap is? Readers familiar with the topic should keep in mind that in this text we will specifically focus on the optical bandgap of solid materials. The bandgap is a complex subject on its own, however, for the sake of the simplicity and in order to guide the Reader along this article, it can be described as the amount of energy (e.g. heat, light), that has to be given to an electron, in a given material, to make it “freely available”. In other words it is the energy gap/space between the valence band, where the electrons are not able to move, and the conduction band, where the electrons are delocalized and free to move, Figure 1. In fact, this space is the main characteristic which allows classifying a given material as conductor, insulator or semiconductor. When the bandgap is zero, the materials need almost no energy in order to make its electrons free. Therefore, they can move freely within the solid materials (within the atomic lattice) and hence these materials are called conductors. In the case of an insulator, this space is much bigger and therefore such materials show a high resistance to delocalize its electrons. The final case is placed right in the middle of the both cases aforementioned – semiconductors. These are materials in which this gap is rather small but not zero. Therefore, part of electrons “naturally” jumps this gap, although not all of them are available and require some energy to make them fully available. It is important to mention, that semiconductor materials are the keystone of modern electronics, due to their susceptibility to the bandgap tailoring. Now, having briefly described what the bandgap of materials is, we can easily address the question of, What is the bandgap tailoring? As the name probably suggests, it is an engineering process, in which the bandgap can be altered or tuned according to the requirements of a specific application. In electronics, a semiconductor closer to an insulator is called P-Type, while the one who is closer to a conductor is called N-Type. In these materials, the tuning process is performed by adding impurities (so called dopants) to the materials, which can alter its bandgap energy value and structure by introducing new electron states into the band gap. Among the most commonly used doping materials for silicon technology are boron, arsenic, phosphorus, and in some cases gallium and aluminum. The capability of generating P-Type and N-type semiconductors allowed creating logic gates and memory storage devices in late 60s and 70s and had indubitably transform our society in the late 80s and90s with the major commercialization of computers.

As it was briefly showed before, bandgap tailoring is not a new subject. However, if considered in case of nanomaterials, it appears to be a very exciting, as new physical phenomenon and their mechanisms are being discovered and described.

In fact, most of the materials when reduced to the nanoscale exhibits new properties unseen before, even at micrometric scale. Among the most notable and easy to relate to are: changes in color, (metallic nanoparticles [2]) and enhancement of mechanical and electrical properties (nanostructured carbon: nanotubes, fullerenes, etc…[3]). In case of superconductor materials, an unexpected effect was found when materials were reduced to the nanoscale and shaped as particles, this is the case of Quantum Dots.

The discovery of Quantum Dots

In 1981 Dr Alexey Ekimov at the Ioffe Institute in St. Petersburg in Russia discovered the first quantum dots (QDs), while in 1982 Prof. Louis E. Brus at AT&T Bell Laboratories, was the first one who obtained them as a colloidal solution. The amazing properties of QDs they observed were not recognized until early 90’s, when their research gained main stream scientific recognition. It was found that semiconductor materials exhibit an unique property when their dimensions are reduced below 10 nm and are in crystalline form, which essentially means that the atoms that constitute them are well arranged in a perfect matrix. In this scenario, the bandgap of QDs exhibits a never observed before direct relationship to their size, namely it gets bigger as QD particle size decreases. Another exciting observation was the fluorescence of the particles which emitted light after the excited electrons returned to their lower energy state. In formal terms, the energy needed to overcome the bandgap is inversely proportional to wavelength of the fluorescence emitted (the color of light). This means that the larger the QD, the smaller is the bandgap energy, and therefore its fluorescence emission is shifted toward the red region of the visible spectrum. In general, their optical properties in terms of fluorescence are strongly dependent upon their size and therefore, can be tailored for specific applications. Even more interesting is the fact, that their color emission is solely governed by their size, so in this case the material that they are made from is irrelevant for their electronic properties.

 

Fig1

Figure 1: Visual representation of bandgap, excitation states and emission in Quantum Dots. CB stands for conduction band, VB for valence band.

At the beginning, the properties mentioned before where though to revolutionize the electronics industry and transistors design. However, as Prof. Louis E. Brus has openly stated, “the basic research scientists, who invent something, are not the best judges of where it is useful”[5], it turned out that due to such unusual properties QDs found their significant role in biomedical and later on in solar energy applications. Today, QDs are considered as important photonic tools and are being the subject of intensive studies in many industrial and academic environments. Their applications can be classified accordingly to the characteristic optical properties. For example, light-emitting devices such as illuminators, displays, lasers, LEDs [6,7] or fluorescent probes for Life Science as fluorescent probes [8, 9].

Figure3

Figure 2: a) Emission profile of CuInS2/ZnS QDs, inset shows the influence of the shell thickness on the emission profile, far left bottle is uncoated core; b) schematic representation of QDs for biological imaging; c) HR-TEM images of QDs of different shape and size.

Biomedical applications

One of the most interesting applications of QDs appeared when their unique light emitting properties have been discovered. Commercially available organic dyes were traditionally used in life sciences as fluorescence probes for tracking different processes of cellular activity. These dyes, although routinely used in fluorescence imaging and spectroscopy, they have some deficiencies, that made them insufficient in the rapidly growing field of bio-imaging. The main limitation of their use are classified as: limited life time (few ns), progressive reduction of fluorescence (photobleaching), low contrast in some applications, emission spectra with red tail and low emission time, to mention a few. The need for new biomarkers was quickly supplied by the novel properties of QDs, ultimately allowing them to make their way into the mainstream biomedical research. As biomarkers, QDs show many superior properties compared to organic dyes. In order to make a short contrast between them, some of these properties will be mentioned: photostability (around 100 times larger), higher brightness (in some cases 20 times higher), narrower emission line width allowing more chromatic contrast, larger fluorescence lifetime (from 5 to 10 times larger), and finally their continuous and broad absorption spectra (a single excitation source may simultaneously excite of multicolor nanocrystals).

Among the most commonly used semiconductor materials as QDs we can find core/shell structures of CdSe/ZnS, CdTe/CdS, ZnSe/ZnS and CuInS2/ZnS. The core refers to the intrinsic and photoactive compound, while the shell to the external material. Such combination shows how the bandgap engineering has come a step ahead making bilayered quantum dots, in which the electronic properties of two semiconductor materials are coupled and confined in a limited space. The advantages of this type of structure are diverse, including further tunable emission properties and both, electro- and chemical surface stability. Second of all, the use of cadmium (Cd) in some of the particles is worth of some attention. Cadmium is an excellent electronic material with many desirable properties; however its toxicity has been the object of criticism. Therefore, in order to reduce both, environmental and health hazards, like in the case of lead (Pb), the international community is promoting research in Cd-free QDs.

As it was mentioned above, QDs are being used in life sciences as molecular markers. The main advantage of this application is the capability of functionalization of QDs with selective molecules, which will attach exclusively to a given target (Figure 2). The biofunctionalization allows the application of several types of QDs at the same time, specific to different organisms, cells or structures in a living cell. Due to their size dependent emission and reduced size (always below 10 nm) particles find way to enter the living cell and attach to the target. This results in effective staining the area of interest, and ensures superior resolution and contrast for bio-imaging. Another important feature is that QDs can be irradiated with a single ultraviolet source, emitting in well-defined and specific wavelengths.

Figure 3. Specific functionalization of QDs (CuInS2/ZnS, red) according to breast cancer cell line imaging. .a)  the structure of CuInS2/ZnS quantum dots targeted with HER-2 receptor-specific peptide and schematic mechanism of their action.; b), c) cellular localization of functionalized quantum dots . Cells nuclei are stained in blue,  membranes stained green. Cells were visualized by confocal microscope.

When combined and tuned all QDs properties according to the specific requirements, researchers might obtain a multimodal system. Now, let’s imagine that a QD is a nanocarrier, which can be readily functionalized by coupling it with a drug, for instance. Then it serves as nanoplatform with both therapeutic and imaging function at the same time, simplifying the work of the reader and additionally because of its high resistance to photobleaching, a long-term imaging is possible. In other words they can be used as a tracking system for a drug testing. However, since the aim is to study the drug influence on an organism, it is necessary to evaluate, whether the QD itself is inert to the organism. Thus, their toxicity in general is a remaining issue and a big challenge for scientists. Quantum dots are not the exceptions, as for all the other nanomaterials there is no a standard procedure to assess “nanotoxicity”. As long as there are not sufficient results obtained in this field, the clinical trials cannot be easily reached. Therefore, much attention is also focused on the interaction of QDs with in vivo and in situ biological entities, such cells, luckily their light emission properties allow the direct observation of their interactions, speeding up this process and giving a promising future.

Among the most remarkable applications of QDs are the ones in cancer research. According to the American Cancer Society, cancer remains the second cause of death in USA. Statistics are alarming, and the diagnosis of the cancer is often perceived by patients as a death sentence. Classical cancer treatment methods such as surgery, chemotherapy, radiotherapy often turn to be insufficient, as their results are transient, and involve numerous undesirable side effects. Therefore, significant progress in the basic and applied cancer research and treatment methods is one of the greatest challenges for modern medicine. New cancer treatment methods are being searched and developed not only towards higher effectiveness, but also towards improvement of the quality of the treatment.

One of the promising strategies might be a QDs-guided surgery. Many scientists have been working on QDs functionalization with a unique molecule, which could be specifically recognized by a particular cancer cell. Once such a system is internalized, the light-emitted from QDs makes this area visible and accurately recognizable. It is important to remember that cancer cells show a rapid growth and therefore efficient removal of damaged tissue is needed, in this case. Moreover, being able to remove just the affected region would bring not only higher probability of complete recuperation, but will imply less harm to the patients being treated.

Here, we show different images of breast cancer being imaged by QDs of CuInS2/ZnS, Figure 4, the core of QDs is tuned to emit in the red spectra and the shell is engineered to provide a small blue shift. Please notice that nuclei and cytoplasm are stained with conventional imaging molecule and the broad spectra of emission of the green molecule, tend to overlap with the QDs fine tuning, however, the narrow band emission of the QDs made it quite easy to recognize once their spectra was determined.  In this study QDs are used as contrast agent for cancer cells membrane. This is achieved by targeting a membrane receptor called “HER2”, a receptor that is overexpressed in up to 30% of all breast cancer types, and therefore it is the perfect indicator of their presence. The particles used are as small as 3 nm and, as shown in the images, are fully crystalline. The confocal images show their successful attachment to the HER2 receptor and also their internalization into the cells.

Another, type of functionalization of QD, which can help to get QD into the target cells are so called cell penetrating peptides (CPPs). In recent years, CPPs received great attention as efficient cellular delivery vectors due to their intrinsic ability to enter cells and mediate uptake of a wide range of macromolecular cargo, such as plasmid DNA, anticancer drugs, as well as nanoparticles, like QDs. CPP act as a enhancer of internalization of QD into the cells, and allows to reduce needed concentration of QDs, simultaneously maintaining high efficiency as bioimaging agent (Figure. 4).

Figure5

Figure 4. Cells internalization by QDs (CuInS2/ZnS, red) f unctionalized with cell-penetrating peptides.  Cells were treated with A) QD only, b) complexes of QD and non-covalently attached CPP. Cells were visualized by confocal microscope. The cells organelles are stained with fluorescent dyes : nucleus – blue, mitochondria – green.

Near future market?

It is expected that a steady increment on QDs global market, by 2016 and the market will generate $7.5 billion dollars in revenue. These numbers could sound overoptimistic, but QDs have found another novel application, as light emitting sources for electronics, making these projections more plausible and maybe even short in sight. Quantum dot based displays have joined the market with outstanding results and reception. As it was mentioned before, the unique properties of QDs are desirable in many fields, in the case of electronic displays, low energy consumption, narrow light emission and easy stabilization in liquid and solid systems, makes them ideal for such application [10]. The easy stabilization of the particles in different media allows the simplification of the manufacture process and mass production of such devices. This is a crucial feature in the industry and a decisive parameter in technology transfer from research to industrial environments.

The first of this QD-based screens was developed by Sony and placed in the marked in 2013 [11]. One of advantages, that this technology is offering, is the possibility of having light on demand, which essentially means, that pixels that are not needed, or need to be black at a given moment, receive no energy and therefore lower their energy consumption. This couple also with the superior efficiency of QDs, which translates in 30-50% saving in energy, when compared with commercial LCD screens. Moreover, the aforementioned narrow emission of QDs allows for much more vibrant and intense colors, getting outstanding images and even getting superior quality results than other competitor technologies such as LCD and OLED. The outstanding results of QD technology have eclipsed other technologies such the OLED. Samsung, the giant of electronics, has announced in November 2014 their decision of stop marketing and developing of OLED devices, in pro of QDs technology [12]. This shows how far QDs have come since their discovery in the 80s and how their unique properties are revolutionizing both industry and research. It is important to remark, that there are many other electronic applications in which QDs are being successfully applied. Technologies such as lasers, quantum computing, photovoltaics and photodetectors are a few of the ones extensively considered and studied.

To conclude, QDs have found their way into mainstream scientific fields after almost 30 years of extensive research. Their wide range of applications makes them one of the most versatile nanostructures to date. Moreover, their massive success can be attributed to the interdisciplinary approach, which researchers followed, after the nanotechnology boom in the 90s, pushing a development in semiconductor technology into a wide brochure of applications, touching all the main fields of science and finding industrial applications.

References

[1]   http://www.theguardian.com/nanotechnology-world/nanotechnology-in-everyday-life

[2]  Catherine J. Murphy et al. J. Phys. Chem. B 2005, 109, 13857-13870

[3]   Baughman et al., Science 297 (5582): 787-792 (2002)

[4]  Felice C. Frankel © http://chemistry.mit.edu/bawendi-quantum-dots & http://www.felicefrankel.com/

[5]  The Invention of Quantum Dot – http://youtu.be/MzoSDLL7NTA

[6]   Aldakov, D. et al. J. Mater. Chem. C 2013, 1, 3756

[7]  Aboulaich, A. et al. ACS Appl. Mater. Interfaces 2014, 6, 252–258

[8]   Hsu, J.-C et al. J. Mater. Chem. 2011, 21, 19257

[9]  Guo, W. et al. Theranostics 2013, 3, 99–108

[10]  QDs Are Entering the Mainstream, http://www.photonics.com/Article.aspx?AID=53053

[11]   Quantum Dot technology may smother OLED TVs in the crib (Sept 2014) http://www.computerworld.com/article/2607120/quantum-dot-technology-may-smother-oled-tvs-in-the-crib.html

[12]   Samsung slams door on OLED TVs, makes QUANTUM dot LEAP (Nov 2014) http://www.theregister.co.uk/2014/11/10/samsung_slams_door_on_oled_tvs_goes_to_quantum_dot_next/

 

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