Quantum Dots in Biological Imaging

Structural and physical properties of InAlAs quantum dots grown on GaAs

Tremendous research efforts have been devoted to nanoparticles for applications in optoelectronics and biomedicine. Over the past decade, quantum dots (QDs) have become one of the fastest growing areas of research in nanotechnology because of outstanding photophysical properties, including narrow and symmetrical emission spectrum, broad fluorescence excitation spectrum, the tenability of the emission wavelength with the particle size and composition, anti-photobleaching ability and stable fluorescence. These characteristics are suitable for optical imaging, drug delivery and other biomedical applications. Research on QDs toxicology has demonstrated QDs affect or damage the biological system to some extent, and this situation is generally caused by the metal ions and some special properties in QDs, which hinders the further application of QDs in the biomedical field. The toxicological mechanism mainly stems from the release of heavy metal ions and generation of reactive oxygen species (ROS). At the same time, the contact reaction with QDs also cause disorders in organelles and changes in gene expression profiles. In this review, we try to present an overview of the toxicity and related toxicity mechanisms of QDs in different target organs. It is believed that the evaluation of toxicity and the synthesis of environmentally friendly QDs are the primary issues to be addressed for future widespread applications. However, considering the many different types and potential modifications, this review on the potential toxicity of QDs is still not clearly elucidated, and further research is needed on this meaningful topic.

Variation of the steady-state optical properties of quantum dots (QDs) caused by photoinduced charge transfer is a widely known phenomenon which hinders QD applications requiring long-term stability of the optical signal, for example, bioimaging, single-photon sources, molecular tracking, and lasing. Intense light irradiation may provoke QD photodarkening, when their photoluminescence (PL) quantum yield (QY) is reduced, or photobrightening, when the QY increases and QD PL becomes brighter. In order to understand the nature of these processes and to determine the optimal operating conditions for QDs with different core sizes and shell thicknesses, we have systematically studied the photodarkening and photobrightening phenomena for different wavelengths and intensities of irradiation. The results have shown that changes in the core size only slightly alter the effect of light irradiation on the QD optical properties, whereas the irradiation wavelength and shell thickness are the major factors that determine the variation of the QD PL QY. We have found that the observed photoinduced processes are independent of the irradiation intensity, which proves the single-photon origin of these processes. We have also used comparative data on irradiation of QD solution and QDs embedded in polymer matrices to develop a general model describing the mechanism of the QD sensitivity to intense irradiation and elucidating the role of surface ligands in this mechanism. Our findings determine the strategy for increasing QD optical stability and optimization of operational conditions for QD-based devices.

Nanocrystal quantum dots (QDs) have been applied to molecular biology because of their greater and longer fluorescence. Here we report the potential cytotoxicity of our characterized QDs modified with various molecules. Surface modification of QDs changed their physicochemical properties. In addition, the cytotoxicity of QDs was dependent on their surface molecules. These results suggested that the properties of QDs are not related to those of QD-core materials but to molecules covering the surface of QDs.

In the twenty-first century, nanotechnology has become cutting-edge technology. It is interdisciplinary and multidisciplinary, covering numerous fields such as medicine, engineering, biology, physics, material sciences, and chemistry. The present work aims to cover the optical properties, method of preparations, surface modifications, bio-conjugation, characterization, stability, and cytotoxicity of quantum dots (QDs). Articles were reviewed in English literature reporting the pharmaceutical and bio-pharmaceutical aspects of QDs which were indexed in Scopus, web of science, google scholar and PubMed without applying the year of publication criterion. One significant value of utilizing nanotechnology is that one can alter and control the properties in a genuinely unsurprising way to address explicit applications' issues. In science and biomedicine, the usage of functional nanomaterials has been broadly investigated and has become one of the quick-moving and stimulating research directions. Different types of nanomaterial (silicon nanowires, QDs, carbon nanotubes, nanoparticles of gold/silver) were extensively utilized for biological purposes. Nanomedicine shows numerous advantages in the natural characteristics of targeted drug delivery and therapeutics. For instance, protection of drugs against degradation, improvement in the drug's stability, prolonged circulation time, deceased side effects, and enhanced distribution in tissues. The present review article deals with the quantum dots, their optical properties, method of preparations, surface modifications, bio-conjugation, characterization, stability, and cytotoxicity of quantum dots. The review also discusses various biomedical applications of QDs. The QDs-based bio-nanotechnology will always be in the growing list of unique applications, with progress being made in specialized nanoparticle development, the detection of elegant conjugation methods, and the discovery of new targeting ligands.

Semiconductor quantum dots are usually compared to artificial atoms, because their electronic structure consists of discrete energy levels as for natural atoms. These artificial systems are integrated in solid materials and can be localized with a spatial precision of the order of nanometers. Besides, they conserve their quantum properties even at quite high temperatures (∼ 10 K). These properties make quantum dots one of the most suitable systems for the realization of quantum devices and computers. However, the energy states and the optical properties of quantum dots are much more complicated than for atoms, because a quantum dot is never an isolated potential well. Instead, its electronic structure depends on the crystallin structure of the semiconductor material and on the Coulomb ion-electron and electron-electron correlations. In particular, the presence of several valence bands and their mixing, induced by quantum confinement, gives rise to novel properties which are still not completely understood and exploited in applications. To get a major advance in this field, a full deterministic control of the spatial shape of the quantum confinement is needed, combined with a deeper understanding of the connections between electronic and optical properties. This thesis work has these two main objectives. We realized and experimentally studied different quantum dot systems, in pyramidal hetero-structures grown with MOCVD techniques. These systems allowed the realization of several different geometries for the carrier confining potential, with a precision in the order of nanometers. The optical characterization has been obtained in particular by means of polarization-resolved microphotoluminescence, magneto-photoluminescence, excitation photoluminescence (PLE), and interferometry techniques. For single quantum dots, we have observed and characterized for the first time new excitonic complexes, arising from excited hole states. This allowed a full caracterizatioon of the valence band hole states in our peculiar system. By means of photon correlation measurements, we have also experimentally demonstrated that, even in presence of a large family of exciton states, these quantum dot systems can emit single photons. We have then realized much more complex quantum dot structures, double dot systems (quantum dot molecules) and a completely new system called Dot-in-Dot (DiD). This latter is composed by a small inner dot surrounded by an electrostatic potential well (which can be considered as an outer elongated dot). Such a composite system is characterized by a strong valence band mixing. This state superposition is however very sensitive to small variations of the confining potential. Therefore the degree of valence band mixing can be easily switched by the introduction of a week external field. Since the valence band mixing determines the polarization properties of the emitted light, the DiD changes the polarization properties of its emission spectrum under the action of an external field. In particular, we have experimentally demonstrated this effect for an external static magnetic field, while we have numerically predicted a very similar effect for a static electric field. In the latter case, the polarization switching is a direct consequence of the quantum confined Stark effect induced in the DiD. Hence the DiD appears to be an ideal candidate for realizing emitters of single photons with tunable and controllable polarization.

ABSTRACTThe influence of a nanoengineering procedure on the properties of single- and multi-layer self-assembled InAs quantum dot (QDs) has been studied using photoluminescence, transmission electron microscopy (TEM), and electroluminescence. Optical properties of QDs were optimized by shape engineering via adjustment of a GaAs overlayer thickness prior to a heating (truncation) step. TEM micrographs have confirmed that the employed growth procedure results in truncated pyramidal QDs covered entirely by AlAs with smooth top interfaces. Truncated QDs with two-monolayer-thick AlAs capping have demonstrated a strong blue shift of ground state (GS) energies and up to 10 meV larger separation between GS and first excited state energy levels as compared to non-capped QDs. We believe that AlAs capping, in combination with truncation procedure, results in significant suppression of carrier transport between QDs within the same layer as well as between QD layers. A record high characteristic temperature for GS lasing threshold, T0 = 380 K up to 55 °C, as well as a maximum saturated gain of 16 (5.3 per layer) cm-1, were measured for a 1.22 μm edge-emitting lasers with this truncated triple layer QD gain medium. A maximum saturated gain, 27 (3.9 per layer) cm-1, and T0 = 110 K have been demonstrated in a 1.19 μm lasers with truncated seven layer QD medium.

Hybrid nanoparticles (NPs) consisting of an n-type CdSe/ZnS quantum dot (QD) as a core and p-type π-conjugate molecules as a shell were fabricated, and their nanoscale photoluminescence (PL) and photoresponsive molecular electronic characteristics were investigated. π-Conjugated macromolecular dioctyloxybenzodithiophene-based polythiophene (P3000) or a single carbazole (CB) molecule with insulating molecular blocks were attached to the QD surface. The nanoscale PL characteristics for the single QD were drastically changed through close contact with P3000 owing to energy and charge transfer effects. However, for the hybrid QD-CB NP, the PL of the QD was dominant because of weak energy transfer resulting from the relatively longer insulating molecular block between the QD and the CB molecule. From time-resolved PL spectra, the exciton lifetimes of the QD in the hybrid QD-P3000 and QD-CB NPs were clearly different because of a variation in the energy transfer rate. The photocurrents of the single hybrid QD-P3000 NP were considerably higher and actively responded to both forward and reverse biases due to the energy and charge transfer effects, while those of the single QD-CB NP exhibited diode characteristics. The QD-based hybrids show distinct nanoscale PL features and photoresponsive molecular electronic characteristics depending on the structures of π-conjugated molecules. For n–p-junctioned QD-P3000 hybrid NPs, the PL intensity from the green QD drastically decreased and that of the π-conjugated P3000 was weakly changed due to energy and charge transfer effects. The photocurrent of the QD-P3000 was much higher and more symmetric than that of the QD-CB NP, which had an insulating molecular block, suggesting more active charge transfer between the p-type P3000 and the n-type QD. The nanoscale PL and molecular optoelectronic properties of the hybrids of QDs and π-conjugated molecules could be tuned by their relative distance and the degree of spectral overlapping. An enhanced control over the optical properties of quantum dots, achieved by attaching organic molecules, has been demonstrated by Kwang-Sup Lee and Jinsoo Joo from Hannam University/Korea University and their collaborators in South Korea and the United States. The addition of nanometer-sized quantum dots dramatically alters the properties of a material, which is of use in applications such as solar cells. At the same time, it is important to exert effective control over these modified properties. This can be achieved by surrounding the quantum dots with other materials that interact with them. In their study of cadmium selenide quantum dots, the researchers use organic molecules for this purpose and demonstrate that the light emission from the quantum dots depends strongly on the type of molecule attached to them. This enables a broader flexibility when designing quantum dots for specific electronic or optical applications.

Semiconductor quantum dots (QDs) exhibit exciting photophysical properties for a wide variety of applications in the field of energy conversion, lighting, and more recently quantum communication. The photophysics underpinning these applications at ambient conditions strongly depends on atomistic details of their dynamic surface. Neutral organic ligands used in surface passivation play a critical role in determining the structural and optoelectronic properties of these QDs. Small sizes and irregular atomic arrangements make these QD surface–ligand interfaces challenging to explore at the atomistic level. Here, we combine several computational simulation techniques to study thermally induced geometrical fluctuations in stable cadmium selenide (CdSe) QDs with and without ligand passivation. We find that structural fluctuations of surface atoms significantly depend on passivating molecules. The bulky and strongly binding ligands such as phosphine oxides induce a higher extent of interfacial dynamics. Though these stoichiometric QDs do not possess any permanent in-gap states, significant thermal distortion of QD–ligand interfaces can induce fluctuating defectlike states near band edges. Such vibronic dynamics in these QDs also modifies band-edge state positions on sub-picosecond timescale, impacting their functional properties. Our results further suggest that primary amine ligands are optimal choices for QD passivation. These insights may be helpful to make design principles for screening and optimizing passivating ligands best suited for various QD applications, such as for quantum communication technologies.

In a hybrid structure consisting of a nanorod and a core/shell quantum dot, the interparticle interactions of elements and the effect of the optical response of nanorod on the third-order nonlinear optical response of quantum dot are investigated. The effective parameters in these interactions such as the enhancement factors, type of structural configuration, dimensional ratio, type of background medium, quantum dot size and interparticle distance are studied in the structure for engineering the system to an optimum response. The enhancement factors of field due to the presence of nanorod are obtained in the site of quantum dot, followed by the third-order optical nonlinear response quantum dot is calculated by using the local field theory. It is seen for the situation where quantum dot is perpendicular to the axis of nanorod, the interparticle interactions and enhancement factors have maximum amount at a given aspect ratio of nanorod (about 2.5), depending on the type of surrounding environment. Therefore, the nanorod can produce great nonlinearities atthe quantum dot compared to the spherical nanoparticle into the hybrid structure . As well, it is found that the larger sizes of quantum dot and less interparticle distances cause to the stronger coupling and thereby larger optical nonlinearity in the quantum dot.

Quantum dots are the serendipitous outcome of materials research. It is the tiny carbonaceous nanoparticles with diameters ranging from 1 to 10 nm. This review is a brief discussion of the synthesis, properties, and biomedical applicability of quantum dots, especially in herbal therapy. As quantum dots are highly polar, they can be surface decorated with several kinds of polar functionalities, such as polymeric molecules, small functional molecules, and so on. The review also consists of the basic physical and optical properties of quantum dots and their excitation―dependent properties in the application section. We focus on therapeutics, where quantum dots are used as drugs or imaging probes. Nanoprobes for several diagnostics are quite new in the biomedical research domain. Quantum dot―based nanoprobes are in high demand due to their excellent fluorescence, non-bleaching nature, biocompatibility, anchoring feasibility for several analytes, and fast point―of―care sensibility. Lastly, we also included a discussion on quantum dot―based drug delivery as phytomedicine.

Cu-In-S/ZnS:Gd3+ quantum dots with isolated fluorescent and paramagnetic modules for dual-modality imaging in vivo

The utilisation of quantum dots (QDs) as promising materials for next-generation photovoltaics is a recent development. The optical properties of QDs can undergo tuning, and they enhance energy conversion efficiencies. In this study, CuLaSe2 and Zn-doped CuLaSe2 (ZnCuLaSe2) QDs, specifically tailored for quantum dot-sensitized solar cells (QDSSCs). These QDs, which are environmentally friendly as they are free of toxic Cd and Pb elements, exhibit adjustable energy band gaps and improved photoluminescence quantum yields. The incorporation of Zn into CuLaSe2 QDs led to a significant blue shift in optical properties and enhanced photovoltaic performance. The highest power conversion efficiency (PCE) achieved was 2.52% for ZnCuLaSe2 QDs, compared to 1.94% for CuLaSe2 QDs. This improvement is attributed to Zn doping, which enhances charge separation, suppresses surface trap states, and facilitates better electron transfer by modifying the energy band alignment. The synthesis methods have been developed in such a way that they are scalable, and are also compatible with low-cost, eco-friendly production processes; this underscores their feasibility for industrial applications. It can be concluded that the present study fulfills a vital function within the global energy research landscape by identifying two QDs that have the potential to be key components in advancing photovoltaic technology.

Wang, L. and Shi, Y., 2019. Determination of heavy metal ions in seawater by water-soluble quantum dot fluorescence probe. In: Li, L.; Wan, X.; and Huang, X. (eds.), Recent Developments in Practices and Research on Coastal Regions: Transportation, Environment and Economy. Journal of Coastal Research, Special Issue No. 98, pp. 146–150. Coconut Creek (Florida), ISSN 0749-0208.In recent years, many analytical methods have been applied to the detection of toxic metal ions. However, the interference caused by a matrix will make the measurement method less precise, and some drugs are difficult to determine. We must find a highly sensitive analytical method; only in this way can we effectively monitor the content of heavy metal ions. Quantum dot (QD) technology is an interdisciplinary technology with high sensitivity. The photoluminescence properties of QDs are closely related to their surface modification. Many metal ions can significantly affect the optical properties of QDs through different mechanisms of action. QDs with wide excitation, narrow emission, and high-fluorescence quantum yield have broad application prospects in the field of fluorescence imaging in biological analysis. At present, the application of QDs to detect metal ions in complex substrates is a promising method. The difficulty of using QDs to detect heavy metals lies in how to improve their anti-interference performance in complex substrates.

High luminescence quantum yield water-soluble CdTe/ZnS core/shell quantum dots (QDs) stabilized with thioglycolic acid were synthesized. QDs were chemically coupled to fully humanized antivascular endothelial growth factor165 monoclonal antibodies to produce fluorescent probes. These probes can be used to assay the biological affinity of the antibody. The properties of QDs conjugated to an antibody were characterized by ultraviolet and visible spectrophotometry, fluorescent spectrophotometry, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, transmission electron microscopy and fluorescence microscopy. Cell-targeted imaging was performed in human breast cancer cell lines. The cytotoxicity of bare QDs and fluorescent probes was evaluated in the MCF-7 cells with an MTT viability assay. The results proved that CdTe/ZnS QD-monoclonal antibody nanoprobes had been successfully prepared with excellent spectral properties in target detections. Surface modification by ZnS shell could mitigate the cytotoxicity of cadmium-based QDs. The therapeutic effects of antivascular endothelial growth factor antibodies towards cultured human cancer cells were confirmed by MTT assay.