A simple chemical-etching approach is used to prepare the silicon carbide quantum dots (QDs). The raw materials of silicon carbide (SiC) with homogeneous nanoparticles fabricated via self-propagating combustion synthesis are corroded in mixture etchants of nitric and hydrofluoric acid. After sonication and chromatography in the ultra-gravity field for the etched products, aqueous solution with QDs can be obtained. The microstructure evolution of raw particles and optical properties of QDs were measured. Different organophilic groups on the surface like carboxyl, oxygroup, and hyfroxy were produced in the process of etching. Fluorescent labeling and imaging for living cells of Aureobasidium pulluans were investigated. The results indicated that SiC QDs were not cytotoxic and could stably label due to the conjugation between organophilic groups of QDs and specific protein of cells, it can be utilized for fluorescent imaging and tracking cells with in vivo and long-term-distance. Moreover, mechanism and specificity of mark were also analyzed.
Quantum dots (QDs) have attracted considerable interest over the past two decades due to their remarkable luminescent properties which are known to offer several unique advantages such as sizeand composition-induced tunable emission, high quantum yield, and low photobleaching. The huge interest to QDs of recent researchers is the potential application for fluorescence microscopy allowing the functional study of various molecules that have been identified in living cells through developing new probes for tagging molecules and observing changes in their cellular concentrations and activities. Moreover, further important application prospect has been expected in the scientific fields like fixed cell imaging, bioanalytical assays, biosensors, Ex vivo live cell imaging, in vivo animal targeting, and so on [
However, recent research results indicated that the widely used II-VI semiconductor QDs, e.g. CdSe, CdTe, CdS, and ZnSe and III-V, e.g. InP and InAs, were found to be cytotoxic to living cells through the release of free metallic cadmium ions and arsenics, even if a protective shell ZnS or a polymer on its surface were systematically and carefully added. It is just the one of the major limiting factors for the applications of II-VI and III-V QDs in efficient living cell imaging because of their cytotoxicity strongly influencing biological cell functioning [
At present, silicon carbon (SiC) nanocrystals have been investigated by many researchers because of their excellent biocompatibility (in particular blood compatibility), low density, and high rigidity, which are potentially useful in biology and medicine as well, for example, in bio-labeling [
In this paper, aqueous solution with SiC QDs was fabricated via a simple chemical-etching method. After microstructure and optical properties were investigated, the bio-application of the SiC QDs was studied.
Raw materials are homogeneous SiC nanoparticles (100 - 500 nm in diameter) fabricated via self-propagating combustion synthesis (SHS), from Technical Institute of Physics, Chinese Academy of Sciences [
Micro-morphologies and microstructures of the β-SiC nanoparticles were inspected by transmission electron microscopy (TEM, JEM-2100, JEOL, Japan) and scanning electronic microscopy (SEM, HITACHIS-4300, Japan).
Aureobasidium pulluans was used for fluorescent markers and long-term-distance imaged by the present SiC QDs. The culture conditions and method were listed like: 1) the strain is stored in a fungus incubator on the slant, scraped for two loops, and inoculated in sterilized seed media under sterile conditions for reactivation in a thermostatic shaker at 28˚C and 200 rpm on a thermostatic shaker for two days; 2) the amount of the activated strain with 8% (volume fraction) to mould fermentation medium with 10% (volume fraction) of the SiC QDs solution was inoculated and cultured at 28˚C and 200 rpm on a thermostatic shaker, and 3) the solution of Aureobasidium pulluans cultured for 2 d and 7 d were dropped onto slides and observed under a fluorescence microscope.
Raw materials of SiC nanoparticles of 100 - 500 nm in diameter prepared via SHS method, as shown in
FirstlySiC + 2HNO3 + 2H2O → 2HNO2 + 4OH− + SiC4+ (1)
4OH− + SiC4+ → SiO2 + CO2 + 2H2 (2)
In the beginning of the reaction process, due to a large amount of CO2 and H2 present at the same time, and release of plenty of the reaction heat, this step was a drastic action. Plastic mixing of the miscible liquids and external temperature controlling can decrease the action.
ThenSiO2 + 6HF → H2SiF6 + 2H2O (3)
Due to the formation characteristics of material preparation, the self-propagating combustion synthesis of the rapid reaction and fast cooling can lead to the non-equilibrium crystallization conditions. So, many defects on the surface of productions (crystal lattice distortion, dislocation, grain boundary and so on) will be formed, which results in the higher surface energy and lower corrosion activation energy. Rapidly corrosion can be developed without any additional energy like an electric current as reported in literature [
The hollow grid-like structure plays very important role in the formation process of SiC QDs. As we know, the ultrasonic cavitation effect can generate strong shock waves and micro-jet in solution [
After the aqueous solution has been treated by ultrasonic cavitation, size distribution of the SiC particles was inhomogeneous in the size scope of <10 nm to 200 nm. In an ultra-gravity field (at least 2000 g), centrifugation chromatography for the aqueous phase solution was used to collect the top part of the suspension; containing the uniformly dispersed SiC nanoparticles. TEM imaging and size distribution of the SiC QDs were listed in the Figures 2(a) and (b), respectively. Yellow exhibition of suspension in the aqueous solution under daylight exciting was also displayed in
As can be seen, the SiC QD dimensions are below 4 nm, obeying the center of the distribution approximately, the average diameter of 2.5 nm, which is similar to the former results [
and size distribution of the QDs are the most important factors on its optical properties. Many studies verified that the core-shell spherical structure of QDs with size of less than its exciton Bohr radius will display a strong quantum confinement effect.
As the same results of the former work [
The Aureobasidium pullulans is aureobasidium of imperfect fungi and second-class fungi with the shape of yeast and hypha. Its pullulan production has excellent physicochemical and biological properties such as filmforming, fibrous, resistance oxygen, and easy decomposition [
The Aureobasidium pulluans cells in the culture medium solution with SiC QDs for different times were dropped onto slides and observed under a fluorescence microscope with 340 nm exciting wavelengths, as shown. in
The photoluminescence effect will happen and cause strong fluorescence emission when excited by appropriate light wavelength. At the early stage of cells culturing, 3 days, for example, the Aureobasidium pulluans will be stably marked on the cell membrane, displaying in the left-top position of
Many results indicate that biological function bondings including carboxyl (COO−), oxygroup (O−), and hyfroxy (OH−) on the surface can be formed after the SiC particles’ etching in mixture acid of HF and HNO3
[
The fabrication and application of the fluorescent SiC QDs for labeling and imaging living cells were demonstrated. The process of chemical etching, the optical properties of QDs, cell penetration, and accumulation were highlighted. This study used a homogeneous nanometer β-SiC powders which was prepared using the selfpropagating combustion. After chemical etching, sonication, and chromatography in the ultra-gravity field, the aqueous solution with SiC QD products of 2.5 nm in diameter was obtained, which had a remarkable photoluminescence effect. The surface of the etched cubic β-SiC powder indicates that different organophilic groups were produced like carboxyl (COO−), oxygroup (O−), and hyfroxy (OH−). The most significant among these organophilic groups may conjugate SiC QDs and specific protein. The experimental results from the treatment of Aureobasidium pulluans showed that the SiC QDs were not cytotoxic, and it can be utilized for long-term and long-distance fluorescent imaging of living cells. The main advantage of the elaborated SiC QDs, over conventionally used QDs based on II-VI semiconductors, is of course, their lack of cytotoxicity for in vitro analysis and their potential biocompatibility for in vivo studies.
This work was supported by a grant from the Fundamental R&D Program for Core Technology of Materials (10037206) funded by the Ministry of Knowledge Economy, Korea. Y. P. Song acknowledges that this study was funded by China Postdoctoral Science Foundation (2013M531636), Shandong Postdoctoral Innovative program (20120310), Special project for independent innovation of Shandong province(2013CXC90201), Special fund of modern agricultural technology system of Shandong Province-Fruit innovation team, Program of Agricultural Data Industry Technology Innovation Strategic Alliance (75007).