4 research outputs found
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> UVāvis 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 MoTe<sub>2</sub>
Molybdenum telluride (MoTe<sub>2</sub>) has emerged as a special member in the family of two-dimensional transition metal dichalcogenide semiconductors, owing to the strong spināorbit 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><sup>ā</sup>), neutral (<i>X</i><sup>0</sup>), and positively charged (<i>X</i><sup>+</sup>) excitons in monolayer MoTe<sub>2</sub> <i>via</i> photoluminescence spectroscopy. The binding energies of <i>X</i><sup>+</sup> and <i>X</i><sup>ā</sup> were measured to be ā¼24 and ā¼27 meV, respectively.The exciton binding energy of monolayer MoTe<sub>2</sub> was measured to be 0.58 Ā± 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 CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> Perovskite from Photoluminescence
Spectrally resolved photoluminescence
is used to measure the band-to-band
absorption coefficient Ī±<sub>BB</sub>(āĻ) of organicāinorganic
hybrid perovskite methylammonium lead iodide (CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>) 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<sup>ā14</sup> cm<sup>ā1</sup> 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 Ī±<sub>BB</sub>(āĻ) is calculated
from the photoluminescence spectra of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> in the temperature range 80ā360 K. Based on
the temperature-dependent Ī±<sub>BB</sub>(āĻ), the
product of the radiative recombination coefficient and square of the
intrinsic carrier density, <i>B</i>(<i>T</i>)
Ć <i>n</i><sub><i>i</i></sub><sup>2</sup>, 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 1.7ā1.8 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