A Curated Graphene Quantum Dot-Porphyrin-Based Photosensitizer for Effective Singlet Oxygen Generation through Energy Transfer

Porphyrin-based photosensitizers are proven generatorsof reactiveoxygen species (ROS), such as singlet oxygen, and used as anti-cancertherapeutic agents. However, most of these compounds suffer from potentialdrawbacks due to limited photostability, hydrophobicity, aggregationpropensity, and low cellular uptake. Ultrasmall fluorescent graphenequantum dots (GQDs) have emerged as the next-generation carriers fordrugs and have gained reputation in the pharmaceutical domain. Consideringthe various limiting factors in porphyrin-based ROS generation andcellular internalization, here, we have developed a method to generatetetrakis(m-nitrophenyl) porphyrin (TNPP)-GQD exciplexes.This allows resonance energy transfer (RET) from GQDs to TNPP. Thecalculated overlap integrals for GQD-TNPP and AGQD-TNPP (1.001 x10(18) and 1.257 x 10(17) M-1 cm(-1) nm(4), respectively) assured 95and 71% energy transfer. The optimum donor-acceptor distancesin these couples are 59.82 and 62.65 & ANGS;, respectively, which yieldedthe rate constant of RET as 4.09 and 0.56 ns(-1),respectively. The efficient RET helped in subsequent generation ofsinglet oxygen. The singlet oxygen quantum yields (SOQY) of around0.435 and 0.464 for GQD-TNPP and AGQD-TNPP, respectively, are comparableto those of different porphyrin derivatives where the SOQY rangesfrom 0.55 to 0.70 when used with Triton X-100. The data show thatnon-conjugated amine- and amide-functionalized GQDs (AGQDs) are bettercandidates in this case because of the special properties of the aminegroups. The systems could be excited at 450 nm for FRET, which favorsbiological usage.

Automated summary

Resonance energy transfer from graphene quantum dots (GQDs) to tetrakis(m-nitrophenyl) porphyrin (TNPP) was achieved, leading to efficient generation of singlet oxygen with high quantum yield.