High fluorescent graphene quantum dots (GQDs) are promising in bioimaging and optoelectronics. In this paper, bright blue fluorescent N-doped GQDs were synthesized using a ultrasonic-assisted hydrothermal method. The morphology, structure, surface chemistry, optical properties, and stability subject to photo-bleaching, temperature, pH and preservation period for the N-GQDs were investigated in detail using various microscopy and spectroscopy techniques. The results showed that the N-GQDs possessed an average size of 2.65 nm, 3.57% N doping, and up to 54% quantum yield (QY). The photoluminescence (PL) spectra of the N-GQDs are excitation dependent when excited in the range of 300–370 nm and excitation independent in the range of 380–500 nm for the core and surface states emission. The N-GQDs showed excellent photo-bleaching resistance and superior photo-stability. At room temperature and in the pH range of 3–8, the fluorescence of the N-GQDs was almost invariable. The N-GQDs can be stably preserved for at least 40 days. The average decay lifetime of the N-GQDs was 2.653 ns, and the radiative and nonradiative decay rate constants were calculated to be 2.04 × 108 s−1 and 1.73 × 108 s−1, respectively. The PL mechanism was qualitatively explained. The N-GQDs was used for cell imaging, and it showed good results, implying great potential applications for bioimaging or biomarking.
Fluorescent N-doped carbon nanodots (CNDs) are a type of environmentally friendly nanomaterial that is promising for application in cell imaging and optoelectronics. In this paper, a natural amino acid (L-glutamic acid) was used as a precursor, and two different morphological and structured N-doped carbon quantum dots (CQDs) were synthesized via a one-step ultrasonic-assisted hydrothermal method at 230 and 250 °C. Various microscopy and spectroscopy techniques were employed to characterize the morphology, structure, optical properties, and stability of the CQDs. The results showed that N-CQDs-1 are new CNDs composed of amorphous carbon with a large amount of pyroglutamic acid, and N-CQDs-2 are composed of pure amorphous carbon. The CQDs exhibit excellent optical properties, such as 40.5% quantum yield, strong photobleaching resistance, and superior photostability. Combining the fluorescence lifetimes and radiative and non-radiative decay constants, the photoluminescence mechanism of the CQDs was qualitatively explained. The two CQDs were used for BV2 cell imaging and showed good results, implying the ultrasonic-assisted hydrothermal approach as a facile method to obtain structure-and morphology-controllable N-doped CQDs with prospect for application in cell imaging.
The increasing levels of environmental estrogens are causing negative effects on water, soil, wildlife, and human beings; label-free immunosensors with high specificities and sensitivities are being developed to test estrogeneous chemicals in complex environmental conditions. For the first time, highly fluorescent graphene quantum dots (GQDs) were prepared using a visible-Fenton catalysis reaction with graphene oxide (GO) as a precursor. Different microscopy and spectroscopy techniques were employed to characterize the physical and chemical properties of the GQDs. Based on the fluorescence resonance energy transfer (FRET) between amino-functionalized GQDs conjugated with anti-lipovitellin monoclonal antibodies (Anti-Lv-mAb) and reduced graphene oxide (rGO), an ultrasensitive fluorescent “ON-OFF” label-free immunosensor for the detection of lipovitellin (Lv), a sensitive biomarker derived from Paralichthys olivaceus for environmental estrogen, has been established. The immunosensor has a wide linear test range (0.001–1500 ng/mL), a lower limit of detection (LOD, 0.9 pg/mL), excellent sensitivity (26,407.8 CPS/(ng/mL)), and high selectivity and reproducibility for Lv quantification. The results demonstrated that the visible-Fenton is a simple, mild, green, efficient, and general approach to fabricating GQDs, and the fluorescent “ON-OFF” immunosensor is an easy-to-use, time-saving, ultrasensitive, and accurate detection method for weak estrogenic activity.
In this note we give a short introduction to the Duflo-Zuker shell model mass formula which has been shown to have great prediction power but is mostly used as a black box due to its complex nature.
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