Fast exciton−exciton annihilation occurring at a few 10s ps time scale possesses a potential hurdle to the successful utilization of a multiple exciton generation (MEG) process. MEG produces over 100% quantum efficiency of exciton generation and thereby a dramatic improvement in device performance. Successful implementation of MEG would require a faster charge separation than the exciton annihilation time. In this work we showed < 1 ps photoinduced electron transfer (PET) time scale at graphene quantum dot (GQD)/N,N-dimethylaniline (DMA) interface that would probably allow electron−hole separation much before it annihilates. Modern experimental techniques, including ensemble-based femtosecond fluorescence upconversion and single molecule sensitive fluorescence correlation spectroscopy (FCS), are employed for an in-depth study of PET kinetics. Former technique asserts the ultrafast nature of interfacial PET kinetics, while FCS reveals weak molecular interactions resulting in short-lived (∼4−6 μs) GQD-DMA complex formation in water. A few microseconds binding time allowed us to measure accurately the much faster (<1 ps) intrinsic PET time scale in GQD-DMA complex before it dissociates.

Blinking of freely diffusing CsPbBr 3 nanocrystals (NCs) is studied using fluorescence lifetime correlation spectroscopy (FLCS). Emitted photons from each NCs are assigned to an emission state (exciton or trap) based on their lifetime. Subsequently, an intrastate autocorrelation function (ACF) and an interstate cross-correlation function (CCF) are constructed. Fitting of the AFCs with an analytical model shows that, at low excitation power, the microsecond blinking timescale of the exciton state matches well with that of the trap state. Most interestingly, both of those timescales further correlate with the microsecond growth timescale of the CCF. The strong anti-correlation of the CCF along with the stretched exponential nature of the blinking kinetics confirms the involvement of carrier diffusion and dispersive trap states in NC blinking. At high excitation power, enhanced sample heterogeneity causes a more dispersive blinking. To the best of our knowledge, this is the first report of a NC blinking study using a single-molecule-based FLCS technique.

Peptide nucleic acids (PNAs) are getting prodigious interest currently in the biomedical and diagnostic field as an extremely powerful tool because of their potentiality to hybridize with natural nucleic acids. Although PNA has strong affinity and sequence specificity to DNA/RNA, there is a considerable ongoing effort to further enhance their special chemical and biological properties for potential application in numerous fields, notably in the field of therapeutics. The toolbox for backbone modified PNAs synthesis has been extended substantially in recent decades, providing a more efficient synthesis of peptides with numerous scaffolds and modifications. This paper reviews the various strategies that have been developed so far for the modification of the PNA backbone, challenging the search for new PNA systems with improved chemical and physical properties lacking in the original aegPNA backbone. The various practical issues and limitations of different PNA systems are also summarized. The focus of this review is on the evolution of PNA by its backbone modification to improve the cellular uptake, sequence specificity, and compatibility of PNA to bind to DNA/RNA. Finally, an insight was also gained into major applications of backbone modified PNAs for the development of biosensors.

Ultrafast photoinduced electron transfer (PET) from a photoexcited graphene quantum dot (GQD*) to an electron-deficient molecule 2,4-dinitrotoluene (DNT) is studied in a water−methanol mixture (1:1 by volume) at different temperatures (5 °C−60 °C). The temperature-dependent study reveals that quenching of GQD emission by DNT is a complex process, where use of collisional or static quenching alone in the fitting model cannot fit the entire time regime of the kinetics. Irrespective of temperature, the collisional quenching rate obtained from the TCSPC lifetime quenching study appears at the upper limit of the bimolecular diffusion-controlled rate of the medium. The weak solubility of DNT in the polar solvents leads to GQD−DNT complex formation through hydrophobic interactions, allowing one to obtain a diffusion-free ultrafast PET timescale of the complex using a femtosecond upconversion setup. GQD−DNT complex formation is manifested by the heat change in the isothermal titration calorimetry (ITC) study upon mixing of DNT with GQD. Findings of our work would reinforce the understanding of the interfacial charge transfer process of GQD and thereby expand the promises to its real applications, especially in sensing and photovoltaics where materials with ultrafast PET are highly desirable.