Single-sized N-functionality graphene quantum dot in tunable dual-modality near infrared–I/II illumination detection and photodynamic therapy under multiphoton nonlinear excitation

Abstract

Doping sorted graphene quantum dots (GQDs) with heteroatoms and functionalizing them with amino acid could improve their radiative recombination and two-photon properties—including their excitation-wavelength-independent photoluminescence from the ultraviolet to the near-infrared-I (NIR-I) region, absorption, quantum yield, absolute cross section, lifetime, and radiative-to-nonradiative decay ratio—under two-photon excitation (TPE) at a low excitation energy and short photoexcitation duration, as determined using a self-made optical microscopy system with a femtosecond Ti–sapphire laser. Four types of sorted GQDs were investigated: undoped GQDs, nitrogen-doped GQDs (N-GQDs), amino-functionalized GQDs (amino-GQDs), and N-doped and amino-functionalized GQDs (amino-N-GQDs). Among them, the sorted amino-N-GQDs are effective as a two-photon photosensitizer and generate the highest quantity of reactive oxygen species for the elimination of multidrug-resistant cancer cells through two-photon photodynamic therapy (PDT). Larger amino-N-GQDs result in a greater number of C–N and N–functionalities, leading to a superior photochemical effect and more favorable intrinsic luminescence properties, making the dots effective contrast agents for tracking and localizing cancer cells during in-depth bioimaging in a three-dimensional biological environment under TPE in the NIR-II region. Overall, this study highlights the potential of large amino-N-GQDs as a material for future application to dual-modality two-photon PDT and biomedical imaging.