Cyclic Denaturation and Renaturation of Double-Stranded DNA by Redox-State Switching of DNA Intercalators

Hybridization of complementary nucleic acid strands is fundamental to nearly all molecular bioanalytical methods ranging from polymerase chain reaction and DNA biosensors to next generation sequencing. For nucleic acid amplification methods, controlled DNA denaturation and renaturation is particularly essential and achieved by cycling elevated temperatures. Although this is by far the most used technique, the management of rapid temperature changes requires bulky instrumentation and intense power supply. These factors so far precluded the development of true point-of-care tests for molecular diagnostics. To overcome this limitation we explored the possibility of using electrochemical means to control reversible DNA hybridization by using the electroactive intercalator daunomycin (DM). We show that redox-state switching of DM altered its properties from DNA binding to nonbinding, under otherwise constant conditions, and thus altered the thermodynamic stability of duplex DNA. The operational principle was demonstrated using complementary synthetic 20mer and 40mer DNA oligonucleotides. Absorbance-based melting curve analysis revealed significantly higher melting temperatures for DNA in the presence of oxidized compared to chemically reduced DM. This difference was exploited to drive cyclic electrochemically controlled denaturation and renaturation. Analysis with <i>in situ</i> UV–vis and circular dichroism spectroelectrochemistry, as two independent techniques, indicated that up to 80% of the DNA was reversibly hybridized. This remarkable demonstration of electrochemical control of five cycles of DNA denaturation and renaturation, under otherwise constant conditions, could have wide-ranging implications for the future development of miniaturized analytical systems for molecular diagnostics and beyond

Robust Excitons and Trions in Monolayer MoTe₂

Molybdenum telluride (MoTe₂) has emerged as a special member in the family of two-dimensional transition metal dichalcogenide semiconductors, owing to the strong spin–orbit coupling and relatively small energy gap, which offers new applications in valleytronic and excitonic devices. Here we successfully demonstrated the electrical modulation of negatively charged (<i>X</i>^–), neutral (<i>X</i>⁰), and positively charged (<i>X</i>⁺) excitons in monolayer MoTe₂ <i>via</i> photoluminescence spectroscopy. The binding energies of <i>X</i>⁺ and <i>X</i>^– were measured to be ∼24 and ∼27 meV, respectively.The exciton binding energy of monolayer MoTe₂ was measured to be 0.58 ± 0.08 eV <i>via</i> photoluminescence excitation spectroscopy, which matches well with our calculated value of 0.64 eV

Ultralow Absorption Coefficient and Temperature Dependence of Radiative Recombination of CH₃NH₃PbI₃ Perovskite from Photoluminescence

Spectrally resolved photoluminescence is used to measure the band-to-band absorption coefficient α_BB(ℏω) of organic–inorganic hybrid perovskite methylammonium lead iodide (CH₃NH₃PbI₃) films from 675 to 1400 nm. Unlike other methods used to extract the absorption coefficient, photoluminescence is only affected by band-to-band absorption and is capable of detecting absorption events at very low energy levels. Absorption coefficients as low as 10^–14 cm^–1 are detected at room temperature for long wavelengths, which is 14 orders of magnitude lower than reported values at shorter wavelengths. The temperature dependence of α_BB(ℏω) is calculated from the photoluminescence spectra of CH₃NH₃PbI₃ in the temperature range 80–360 K. Based on the temperature-dependent α_BB(ℏω), the product of the radiative recombination coefficient and square of the intrinsic carrier density, <i>B</i>(<i>T</i>) × <i>n</i>_<i>i</i>², is also obtained

Light and Electrically Induced Phase Segregation and Its Impact on the Stability of Quadruple Cation High Bandgap Perovskite Solar Cells

Perovskite material with a bandgap of 1.7–1.8 eV is highly desirable for the top cell in a tandem configuration with a lower bandgap bottom cell, such as a silicon cell. This can be achieved by alloying iodide and bromide anions, but light-induced phase-segregation phenomena are often observed in perovskite films of this kind, with implications for solar cell efficiency. Here, we investigate light-induced phase segregation inside quadruple-cation perovskite material in a complete cell structure and find that the magnitude of this phenomenon is dependent on the operating condition of the solar cell. Under short-circuit and even maximum power point conditions, phase segregation is found to be negligible compared to the magnitude of segregation under open-circuit conditions. In accordance with the finding, perovskite cells based on quadruple-cation perovskite with 1.73 eV bandgap retain 94% of the original efficiency after 12 h operation at the maximum power point, while the cell only retains 82% of the original efficiency after 12 h operation at the open-circuit condition. This result highlights the need to have standard methods including light/dark and bias condition for testing the stability of perovskite solar cells. Additionally, phase segregation is observed when the cell was forward biased at 1.2 V in the dark, which indicates that photoexcitation is not required to induce phase segregation