Chromatin
is a DNA–protein complex that is densely packed
in the cell nucleus. The nanoscale chromatin compaction plays critical
roles in the modulation of cell nuclear processes. However, little
is known about the spatiotemporal dynamics of chromatin compaction
states because it remains difficult to quantitatively measure the
chromatin compaction level in live cells. Here, we demonstrate a strategy,
referenced as DYNAMICS imaging, for mapping chromatin organization
in live cell nuclei by analyzing the dynamic scattering signal of
molecular fluctuations. Highly sensitive optical interference microscopy,
coherent brightfield (COBRI) microscopy, is implemented to detect
the linear scattering of unlabeled chromatin at a high speed. A theoretical
model is established to determine the local chromatin density from
the statistical fluctuation of the measured scattering signal. DYNAMICS
imaging allows us to reconstruct a speckle-free nucleus map that is
highly correlated to the fluorescence chromatin image. Moreover, together
with calibration based on nanoparticle colloids, we show that the
DYNAMICS signal is sensitive to the chromatin compaction level at
the nanoscale. We confirm the effectiveness of DYNAMICS imaging in
detecting the condensation and decondensation of chromatin induced
by chemical drug treatments. Importantly, the stable scattering signal
supports a continuous observation of the chromatin condensation and
decondensation processes for more than 1 h. Using this technique,
we detect transient and nanoscopic chromatin condensation events occurring
on a time scale of a few seconds. Label-free DYNAMICS imaging offers
the opportunity to investigate chromatin conformational dynamics and
to explore their significance in various gene activities.
Light
absorption is a common phenomenon in nature, but accurate
and quantitative absorption measurement at the nanoscale remains challenging
especially in the application of widefield imaging. Here, we demonstrated
optical widefield interferometric photothermal microscopy that allowed
us to visualize and quantify the heat generation of single nanoparticles.
The working principle was to measure the scattering signal due to
the refractive index change of the surrounding media induced by the
dissipated heat (known as the thermal lens effect). The sensitivity
of our local heat measurement was a few nanowattsthe high
sensitivity made it possible to detect single gold nanoparticles,
as small as 5 nm. By changing the particle sizes, we found that, for
small metallic nanoparticles (gold and silver nanoparticles < 40
nm), the photothermal signal was determined by the amount of the dissipated
heat, independent of the particle size. A model was established to
explain our experimental results, indicating that the photothermal
signal was essentially contributed by the interferometric detection
of the scattered field of the thermal lens. Importantly, on the basis
of this model, we further investigated the photothermal signal of
large nanoparticles (40–100 nm for our setup) where the scattered
light of the particle was considerable relative to the probe light.
In this regime, the strong scattered field of the particle effectively
served as the main reference beam that interfered with the scattered
field of the thermal lens, resulting in an enhanced photothermal signal.
Our work illustrates an important fact that the measured photothermal
signal is fundamentally affected by the scattering property of the
sample. This finding paves the way to accurate and sensitive absorption-based
imaging in complex biological samples where the scattering is often
spatially heterogeneous.
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