Characterization of carbon quantum dots by capillary electrophoresis with laser-induced fluorescence detections

Recently, semiconductor quantum dots (QDs) have been established as a valuable tool for labeling and sensing [1-3]. However, semiconductor quantum dots possess certain limitations such as high toxicity due to the use of heavy metals in their production. It is known that heavy metals are highly toxic even at relatively low levels, which may prove prohibitive to any clinical studies. This prompted the creation of carbon-based fluorescent nanoparticles (CQDs) to replace semiconductor QDs due to their low toxicity, biocompatibility, low cost and chemical inertness in addition to having similar fluorescence properties [4]. Similarly to semiconductor QD, fluorescent carbon nanoparticles can employed for chemical sensing applications - monitoring of metal ion content [5,6], pH sensing [7], biosensing [8] and/or in vivo imaging [9,10].
Even though the capillary electrophoresis (CE) is an extremely valuable separation analytical method and in combination with laser-induced fluorescence detection provides exceptionally low limits of detection, its application to analysis of fluorescent carbon nanomaterials is relatively limited [11]. In contrast to stationary fluorescence spectrometry, CE is capable to reveal the presence of various species in the sample due to their different electrophoretic mobility.
In this work, preparation of CQDs from various precursors such as citric acid, sucrose and multiwall carbon nanotubes were synthetized, optically characterized by fluorescence spectrometry and investigated by capillary electrophoresis with laser-induced fluorescence detection.

Figure 1: Photographs of various types of CQDs solutions (MWCNT-CQDs, S-CQDs, PEG-CA- -CQDs) under UV (254 and 312 nm) and ambient light illumination.

Figure 2: (A) Absorbance spectra of synthetized CQDs. (B) Emission spectra of synthetizes CQDs after excitation by 480 nm.

Figure 3: CE-LIF of PEG-CA-CQDs. Separation conditions: internal diameter - 75 µm, length - 54/64.5 cm, separation voltage - 20 kV, hydrodynamic injection - 0.5 psi, 5 s, electrolyte - 20 mM sodium borate, pH 9.

Figure 4: CE-LIF of S-CQDs prepared by using 50, 60 and 70 °C. Separation conditions: internal diameter - 75 µm, length - 54/64.5 cm, separation voltage - 20 kV, hydrodynamic injection - 0.5 psi, 5 s, electrolyte - 20 mM sodium borate, pH 9. Inset: Dependence of the peak height on the preparation temperature.

Figure 5: CE-LIF of MWCNT-CQDs. Separation conditions: internal diameter - 75 µm, length - 54/64.5 cm, separation voltage - 20 kV, hydrodynamic injection - 0.5 psi, 5 s, electrolyte - 20 mM sodium borate, pH 9. Inset: Dependence of the peak height on the dilution by separation electrolyte.

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