Highly biocompatible,
excellently photostable, nitrogen- and sulfur-containing
novel zwitterionic carbon dots (CDs) were synthesized by microwave-assisted
pyrolysis. The size of CDs were 2–5 nm, with an average size
of 2.61 ± 0.7 nm. CDs were characterized by UV/vis spectroscopy,
fluorescence spectroscopy, zeta potential, Fourier-transform infrared
spectroscopy, X-ray diffraction, and time-resolved fluorescence spectroscopy.
CDs were known to emit blue fluorescence when excited at 360 nm, that
is, UV region, and emit in the blue region of visible spectrum, that
is, at 443 nm. CDs showed excitation-independent photoluminescence
behavior and were highly fluorescent even at lower concentration under
UV light. These CDs were highly fluorescent in nature, with the quantum
yield being as high as 80%, which is comparable to that of organic
dyes. The CDs were further used to image two different oral cancer
cell lines, namely, FaDu (human pharyngeal carcinoma) and Cal-27 (human
tongue carcinoma). The cell viability assay demonstarted that
CDs were highly biocompatible, which was further confirmed by the
side scattering studies as no change in the granularity was observed
even at the highest concentration of 1600 μg/mL. The generation
of reactive oxygen species (ROS) was also investigated and negligible
generaton of ROS was detected. In addition to that, the uptake
phenomenon, cell cycle analysis, exocytosis, and cellular uptake at
4 °C and in the presence of ATP inhibitor were studied. It was
found that CDs easily cross the plasma membrane without hampering
the cellular integrity.
An optical sensing platform for the
detection of an important mycotoxin,
aflatoxin B1 (AFB1), in the absence of a bioactive environment is
explored. In this work, a fluorescence-based sensing technique was
designed by combining graphene quantum dots (GQDs) and AFB1 via fluorescence quenching, where AFB1 acts as the quencher
of GQD fluorescence. GQDs were synthesized through a single-step hydrothermal
reaction from the leaves of “curry tree” (Murraya Koenigii) at 200 °C. The fluorescent
GQDs were quenched by AFB1 (quencher), which itself is detecting the
analyte. Hence, this study reports the direct sensing of the mycotoxin
AFB1 without the involvement of inhibitors or biological entities.
The possible mode of quenching is the nonradiative resonance energy
transfer between the GQDs and the AFB1 molecules. This innovative
sensor could detect AFB1 in the range from 5 to 800 ng mL–1 with a detection limit of 0.158 ng mL–1. The interferent
study was also carried out in the presence of different mycotoxins
and carbohydrates (d-fructose, cellulose, and starch), which
demonstrated the high selectivity and robustness of the sensor in
the complex sample matrix. The recovery percentage of the spiked samples
was also calculated to be up to 106.8%. Thus, this study reports the
first GQD based optical sensor for AFB1.
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