One-Pot Synthesis of Bright Blue Luminescent N-Doped GQDs: Optical Properties and Cell Imaging

Abstract

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.

Structural defects in graphene quantum dots: A review

“…Additionally, the presence of strong defects can be identified from the red‐shift of the D peak in the Raman spectra of GQDs compared to the normal graphite [78–81]. Wang et al [91] have aimed to introduce a hydrothermal synthesis of nitrogen‐doped GQDs (N‐GQDs) with l ‐Glumatic ( l ‐Glu) as a passivant. This aided in reducing the surface defects of GQDs followed by enhanced quantum yield [82, 83], stability and biocompatibility.…”

“…Additionally, the presence of strong defects can be identified from the red-shift of the D peak in the Raman spectra of GQDs compared to the normal graphite [78][79][80][81]. Wang et al [91] have aimed to introduce a hydrothermal synthesis of nitrogen-doped GQDs (N- [11] • Red-shifting of PL peaks from 280 to 340 nm as shown in Figure 5 [4] • Variation in bandgap with cyclic aromatic hydrocarbon-like conjugated structure [4] • Various defect contents • Distortion of the original structure of graphite precursors followed by introducing various groups • High defect GQDs lead to poor optical properties [11]. • Lead to polymorphic atomic defects in graphene layers [35,36] • High defects lead to some disordered stacking with sp 2 carbon structures and disordered peaks as D band of GQDs and graphitic peak and G band according to XRD and Raman characterization analysis • Red-shifting of G mode in Raman spectra due to presence of 2.08% of defect concentration [53] • Although the structure of GQDs withholds hexagonal shape, due to the existence of this defect, four adjacent hexagons turn out to two heptagon-pentagon (7-5) pairs [54].…”

“…It is based upon the crystallographic orientation of the edges. The edge on GQDs domains results in formation of different functional groups, which get transformed to new structural defects [91]. Liu et al [111] reported the in situ exfoliation of graphene on the surface of carbon fibers with edge defects, which resulted in enhanced electrolyte affinity and mass transport.…”

Research progress in the synthesis and biological application of quantum dots

“…The synthetic N-GQDs have an average particle size of 2.65 nm and a QY of 54%, showing excellent photobleaching resistance and stability, and showing great bio-imaging potential when successfully used in BV2 cell imaging. 246 In a recent study by Lin et al , N-GQDs were first prepared using a hydrothermal method, then coated with PEG and branched polyethyleneimine (bPEI) to obtain P@N-GQDs, and finally conjugated with anti-GD 2 antibody to obtain the Ab-GD 2 @P@N-GQDs composite vector. The P@N-GQDs achieved good imaging results in the IVIS system and were more suitable for biological imaging than the unmodified N-GQDs.…”

Self-Assembled Nanocomposites and Nanostructures for Environmental and Energic Applications

“…Yang et al prepared bright blue fluorescent N-doped graphene quantum dots (N-GQDs) via an ultrasonic-assisted hydrothermal method [ 10 ]. The obtained N-GQDs showed excellent photo-bleaching resistance and superior photo-stability at room temperature and in the pH range of 3–8, demonstrating great potential for bioimaging or biomarking applications.…”