(T1030-02-10) Enhancement of Optical Properties of Graphene Quantum Dots Using Nitrogen Doping: An Analytical Aspect

Purpose: Graphene quantum dots (GQDs) are of significant importance for label-free bioimaging and biosensing owing to the ultra-small size ( < 10 nm), tunable optical properties, good aqueous dispersion stability, high quantum yield, long-term photostability, high resistance against photobleaching and excellent biocompatibility. These properties make them superior over conventional organic dyes that are widely used for diagnostic purpose. These dyes hold several demerits such as low quantum yield (QY), photobleaching and toxicity. GQDs are also a potential drug nanocarrier and possess ability for photothermal therapy. To be able to deliver good performance for fluorescence based bioimaging and biosensing, high QY values with good photostability is crucial which can easily be obtained through nitrogen (N) doping. Hence, this study focuses on improvement of optical properties through N doping to showcase their potential for long-term bioimaging and biosensing applications.

Methods: Un-doped GQDs were prepared using hydrothermal treatment of glucose solution at 190°C for 4 hours. These GQDs were characterized using various analytical techniques: dynamic light scattering (DLS), high-resolution transmission electron microscopy (HR-TEM), X-ray Photon Spectroscopy (XPS), UV-Visible Spectrophotometer and Fluorescence Spectroscopy. N doped GQDs (N-GQDs) were synthesized from citric acid and urea as precursors. The solutions were hydrothermally treated at 170°C for 4 hours. N-GQDs were analyzed for their morphological and optical properties such as fluorescence and quantum yield (QY) values and compared with un-doped GQDs. Moreover, N-GQDs were assessed for their hemocompatibiliy and biosafety.

Results: HR-TEM and DLS studies displayed average diameter of 5.6 nm for glucose derived GQDs which increased to 8.30 nm post N doping (Figure 1). Un-doped GQDs showed maximum emission (λ_em) at 468 nm with QY value of 67.09%. In contrast to that, N-GQDs showed increased fluorescence intensity with blue shift of λ_em at 448 nm upon excitation at 365 nm. Bright blue fluorescence was observed under the influence of UV light (Figure 2). QY value of N-GQDs was increased to 72.03%. UV-Visible spectra of N-GQDs showed red shift of the peak depicting n to π* transition and increased absorption of peak representing π to π* transition as compared to un-doped GQDs. It speculated that aromatic sp² clusters may have increased post N doping which contribute to increased QY. XPS spectra confirmed N doping. The fluorescence integrity of N-GQDs remained up to 6 months against un-doped GQDs that showed fluorescence stability up to 3 months. Moreover, %hemolysis of N-GQDs was found to be 1.01+ 0.425% and considered hemocompatible as per ASTM E2524-08 standard ( < 5%). In vitro cytotoxicity studies also demonstrated safety of N-GQDs.

Conclusion: N doping enhanced the optical properties and offer long-term fluorescence stability as compared to un-doped GQDs. The hemocompatible and safe N-GQDs are suitable for bioimaging and biosensing.

References: 1. Gu J, Hu MJ, Guo QQ, Ding ZF, Sun XL, Yang J. High-yield synthesis of graphene quantum dots with strong green photoluminescence. RSC Advances. 2014;4(91):50141-4.
2. Qu D, Zheng M, Zhang L, Zhao H, Xie Z, Jing X, Haddad RE, Fan H, Sun Z. Formation mechanism and optimization of highly luminescent N-doped graphene quantum dots. Scientific reports. 2014;4(1):1.
3. Permatasari FA, Aimon AH, Iskandar F, Ogi T, Okuyama K. Role of C–N configurations in the photoluminescence of graphene quantum dots synthesized by a hydrothermal route. Scientific reports. 2016;6(1):1-8.

Acknowledgements:We are grateful to receive financial assistance under SHODH (ScHeme Of Developing High quality Research) from the government of Gujarat.

HR-TEM image (A) and size distribution (B) of N-GQDs

Emission spectra of un-doped GQDs and N-GQDs upon excitation at 365 nm