logo ban ban logo

Je hanba, že se někdo dovede namáhat po mnoho let, aby se stal dobrým lékařem, obhájcem, učitelem nebo geometrem, a přitom není ochoten namáhat se příslušně dlouhý čas, aby se stal dobrým člověkem.

Galénos z Pergamu

mendel

Výzkum

Capillary electrophoresis of quantum dots

Nanotechnology is widely spread technology applied it almost every field of science today [1-6]. Its backbone is hidden in principle of creating or engineering materials in the atomic or molecular scale. The year 1959 in history of nanotechnology can be marked as turning-point because of brilliant speech of Richard P. Feynman who gave the vision that science and technology can be based on nanoscale [7]. However, begin of nanotechnology or closely speaking of nanomaterials was given much earlier by Michael Faraday in 1857 by observing characteristics of gold nanoparticles produced in aqueous solution [8]. Nanotechnology roots are tightly connected with development of colloids and physical chemistry, thus, the great names such as Albert Einstein with his Brownian motion theory and Nobel prized Jean-Baptiste Perrin should not be neglected [9].The biggest member of the family of nanomaterials is a group of nanoparticles that covers metal nanoparticles, metal oxide nanoparticles, polymer nanoparticles and/or silica nanoparticles. One of the most interesting is group of gold nanoparticles with their well-known optical characteristics as absorption, fluorescence and stability. Iron nanoparticles also play important role due to their magnetic properties [10-12]. Beside nanoparticles carbon-based nanomaterials such as nanotubes, fullerenes, graphene which exhibit very good optical, electrochemical, and adsorptive properties are subject of constant investigation [13]. Other nanomaterials which cannot be neglected nor considered less important are liposomes [14, 15] and dendrimers [16, 17]. In this review, we focused on finding of articles containing phrases quantum dot and electrophoresis. We were interested mainly in the field of characterization of these unique materials using electrophoresis and further in biomolecules binding.

Quantum dots
Quantum dots belong to the family of nanoparticles and they are defined as semiconductor nanocrystals with size from 1-10 nm usually spherical shape, but they can also be cubic, rod-like or tetrapod-like [18]. Quantum dots are mostly made of elements of II-VI groups as CdSe, CdTe (Fig. 1), CdS and ZnSe or III-V groups as InP and InAs [19, 20] and their optical and electronic properties can be placed between those of bulk materials and isolated molecules or atoms [21]. Size-depending properties and quantum confinement give them unique characteristics such as symmetric and narrow emission (Fig. 1), continuous absorption spectra and high emission quantum yields. For biological and medical usage their photostability and resistance to chemical degradations are valuable [22, 23]. Core of quantum dots created of inorganic elements is toxic for living systems and cells, thus it is hydrophobic and this makes them unsuitable for working in aqueous environment or for any application in biological system. Two strategies of surface modification are usually used to solve this problem. One is exchanging existing ligand with others less toxic and acceptable for biological purposes and another one considers usage of amphylic polymer capping agents. Different surface modifications provide easy conjugations with biomolecules trough covalent or non-covalent attachment [24, 25]. Nowadays, when concern for environment is emphasized, more environment-friendly technologies are explored and used for quantum dots synthesis [26-28]. The newest discovery in this field is synthesis of quantum dots in earthworms using earthworm's metal detoxification pathway for it [29].
Quantum dots exhibit one specific phenomenon called “blinking” defined as fluctuations in luminescence. Besides blinking on quantum dots fluorescence effect environment, some molecules or regents can cause increasing or decreasing of fluorescent signal of quantum dots. The blinking and the environment influence can cause limitation and issues in quantum dots application or analysis and are considered as drawbacks. The mechanism and explanations of drawbacks remains unknown and unclear [30]. Nevertheless the topic which still occupies and worries science is the toxicity of quantum dots. On the other hand, one may mention that even large scale production of these materials cannot cope with the well-known sources of metals, which are much greater. Even modified quantum dots bring doubts because nobody can tell with certainty what will happen to the materials used for modification in biological environment [31]. Other opened question is environment itself and disposal of quantum dots in it [32]. Development and improving of quantum dots are equally followed with researching about their toxicity [33-35] and even methods for evaluation of their toxicity have been developed [36]. Based on these facts, there still remain un-answered questions concerning these materials and their well characterization is one of the keys to answer them.

2. Characterization of quantum dots with capillary electrophoresis
Progress in technology of CE is going still further. Basic CE method was modified and developed according to different separation mechanisms and conditions. Based on these advances, CE methods have been used for characterization and separation of quantum dots capillary zone electrophoresis (CZE), micellar electrokinetic chromatography (MEKC) and capillary gel electrophoresis (CGE), as we discussed on the following paragraphs.
Characterization of quantum dots was done by Song et al. with CGE employing LIF detector for the first time [41]. This group used linear polyacrylamid (PAA) as sieving media as successful choice for characterization and separation of different size quantum dots, also providing valuable information of QDs behavior in wide pH range. Very important is a study of peak broadening in sieving media as well as percentage of used sieving media. A challenge to perform characterization of quantum dots with CZE was fulfilled by Pereira et al. [42]. Measurements included characterization of commercially available QDs with UV detection, LIF detection and sodium phosphate as background electrolyte. Very interesting insight in QDs separation was given by Pyell and co-workers on CdSe/ZnS/SiO2 core/shell/shell nanocrystals [43]. Pyell’s group was using, previously given by Ohshima group [44], formula of electrophoretical mobility µ independence of zeta potential ?, particle radius r and ionic strength I. The theory matched with practice and it proofed that mobility and size of nanoparticles are in nonlinear function. This is also used for calculation of the size distribution of nanoparticles and was confirmed by TEM method.
In addition, CZE was used for monitoring synthesis conditions which determine size of quantum dots with CdTe in core and with thioglycolic acid as capping agent by Clarot et al. [45]. In following work Li and co-workers used CE method with added polymer additives as sieving medium to BGE for size determination of CdSe/ZnS quantum dots and observing the influence of different concentration of polymer to separation resolution, concentration of BGE and pH. Novelty of this work was mathematical formula for size calculation relied on correlation between electrophoretic mobility and QD size. Confirmation of formula accuracy was done by TEM method [46]. Interesting work was done by Oswaldowski group [47], where they have used micellar electrokinetic chromatography and CZE to separate mixture of CdSe quantum dots coated with cationic, anionic and non-ionic surfactants. Method is relied on formation bilayer between hydrophobictrioctylphosphineoxide (TOPO) and ionic and non-ionic surfactants and gives a possibility of monitoring interactions between these layers. Oswaldowski group have also done researches using preconcentration and micellar plug as a new method for analysis of quantum dots surface modified with amphiphilic, bidentate ligands and biologically active molecules, which give quantum dots neutral or charged surface [48-50]. Besides, the role of ligands on growth rate and size distribution of CdTe quantum dots was monitored by micellar electrokinetic CE with LIF detector and it was confirmed that proper ligands give quantum dots bigger size, but narrower size-distribution [51].
An interesting analysis of BSA-coated quantum dots was done by Buecking et al. [52]. BSA attracted attention as possible coating because of solubility in concentrated salt solutions, low cost and variety of functional groups capable of interaction. Characterization was done in combination of agarose gel electrophoresis, dynamic light scattering, laser Doppler electrophoresis and isotachophoresis methods [52]. However, the best resolution for QDs separation, whose surface was modified with trioctylphosphine oxide/trioctylphosphine (TOPO/TOP) and sodium dodecyl sulfate (SDS), was done by Carrollio-Carrion et al. using micellar electrokinetic chromatography. Authors separated QDs with 0.5 nm difference in diameter and 19 nm difference in fluorescence emission maximum [53]. The summary of CE conditions employed for QD analysis is given in Tab. 1.

CE of bioconjugated quantum dots
As it was aforementioned, after preparation quantum, their surface has to be capped, functionalized and/or bioconjugated. Applying surface modification and bioconjugation on QDs certainly affects their characteristics important for CE analysis, such as size, charge and therefore electrophoretic mobility.
Pioneers in this work were Huang et al., who observed quantum dots capped with mercaptoprotionic acid as ligand and coupled with BSA and horseradish peroxidase (HRP). The authors efficiently separated bioconjugated QDs and free QDs adjusting buffer’s pH foreseeing that CE- LIF will be used in further investigations of bioconjugated QDs [54]. Application of quantum dots as fluorescent label in immunoassay was reported for the first time by Feng et al. [55]. Vincente et al. using CE-LIF showed on three differently bioconjugated QDs, with streptavidin, biotin and immunoglobulin G, of which electrophoretical mobility is dependent on biomolecule attached on QDs and using polymeric additives can improve the resolution of bioconjugates. The group was also observing separation of bioconjugated quantum dots with different emission maxima using one excitation source. Besides, this was first reported separation of three differently bioconjugated quantum dots, because, only separation of quantum dots and bioconjugated quantum dots has been performed since than [56].

Práce je spojená s projektem CZ.1.07/2.3.00/30.0039

Zemědělská 1/1665
613 00 Brno
Budova D		Tel.: +420 545 133 350
Fax.: +420 545 212 044