Facile Synthesis of L-Cysteine Functionalized Graphene Quantum Dots as a Bioimaging and Photosensitive Agent.

Mila Milenković,Svetlana Jovanović,Aurelio Bonasera,Dragana Jovanović,Gabriele Ciasca,Milena Stevanović,Jelena Pejić,Aleksandra Mišović,Marija Mojsin,Ana Popović Bijelić,Sabrina Romanò

doi:10.3390/nano11081879

Abstract

Nowadays, a larger number of aggressive and corrosive chemical reagents as well as toxic solvents are used to achieve structural modification and cleaning of the final products. These lead to the production of residual, waste chemicals, which are often reactive, cancerogenic, and toxic to the environment. This study shows a new approach to the modification of graphene quantum dots (GQDs) using gamma irradiation where the usage of reagents was avoided. We achieved the incorporation of S and N atoms in the GQD structure by selecting an aqueous solution of L-cysteine as an irradiation medium. GQDs were exposed to gamma-irradiation at doses of 25, 50 and 200 kGy. After irradiation, the optical, structural, and morphological properties, as well as the possibility of their use as an agent in bioimaging and photodynamic therapy, were studied. We measured an enhanced quantum yield of photoluminescence with the highest dose of 25 kGy (21.60%). Both S- and N-functional groups were detected in all gamma-irradiated GQDs: amino, amide, thiol, and thione. Spin trap electron paramagnetic resonance showed that GQDs irradiated with 25 kGy can generate singlet oxygen upon illumination. Bioimaging on HeLa cells showed the best visibility for cells treated with GQDs irradiated with 25 kGy, while cytotoxicity was not detected after treatment of HeLa cells with gamma-irradiated GQDs.

Highlights

Our previous study showed that PL quantum yields (QY) for p-graphene quantum dots (GQDs) was only 1.45% at the excitation wavelength of 340 nm [32]. These results suggest that gamma irradiation improved PL QY of GQDs and that the largest improvement was achieved at a dose of 25 kGy
The successful incorporation of N- atoms in the structure of GQDs was detected. These results proved that gamma irradiation in the presence of L-cysteine and isopropyl alcohol (IPA) in a reduction atmosphere leads to S,Ndoping of GQDs
The results showed that only the GQD-cys-25 dispersion can produce singlet oxygen when it is exposed to light

Summary

Introduction

Graphene quantum dots (GQDs) are a zero-dimensional (0D) nanomaterial that consists of graphene sheets with π-conjugated sp2 -hybridized carbon atoms. Since the discovery of GQDs in 2008 by Ponomarenko et al, they have attracted much attention [1]. They belong to 0D carbon nanomaterials because the movements of their excitons are confined in all three spatial directions [2]. On the basal plane and at the edges, GQDs have covalently bonded oxygen-containing functional groups [3,4].

Methods

Discussion

Conclusion