Modeling single molecule detection probabilities in microdroplets. Final report Page: 4 of 4
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indd&r nftey (N/ aqattFigure 2: Detection efficiency and signal to noise ratio as
a. function of incident intensity,
actions with the droplet on the detection probability, and
4) suggest experimental variations in which the detection
efficiencies may be increased.
For the results shown, the incident waves propagate
in the tz directions. The collection lens is centered on
the x axis and has a NA of 0.42. Results are shown for
two classes of droplets: water droplets with ~ 0.75 pm
diameters, and glycerol/water (85 % glycerol and 15 %
water) droplets with 6.5 pm diameters.
Figure 1 illustrates the number of photons collected
per droplet from a sequence of 1600 glycerol/water (85 %)
droplets. Each has a diameter of 6.5 pm. The average
number of R6G molecules per droplet is 0.02. Droplets
having 0 or I molecules are observed. The lower curve
shows counts from the blank (no R6G) droplets. The up-
per curve has been offset by 200 counts to allow compar-
ison. The illumination intensity is 40,000 W/cn9, and
the illumination/collection time is 100 ms. The Raman
scattering from the droplet is more important than the
scattering from gasses in the cell.
A primary figure of merit for a single molecule de-
tection scheme, is the molecular detection efficiency, the
probability that the photons detected from a single mole-
cule exceed some threshold. A typical value for the thresh-
old, the value of the threshold used here, is the sum of the
background ("blank") signal and three times the standard
deviation of background.
Figures 2 and 3 show the molecular detection effi-
ciency and the signal-to-noise ratio as a function of the
incident intensity. In Figs. 2 and 3 the droplets are glyc-
erol/water (85%), are in the beam for 100 ma, and have 6.5
pm diameters. The incident intensities in Fig. 3 were cho-
sen to illustrate the detection efficiencies within the range
of intensities typically used in experiments. The intensi-
ties in Fig. 3 were chosen to illustrate the range where
the detection efficiency is largest and where the detectionFigure 3: Detection efficiency and signal to noise ratio as
a function of incident intensity.
efficiency and S/N show the most variation.
Figures 2 and 3 suggest that the models can help in
opimizing experimental arrangements for single molecule
detection in microdroplets.
References
[1] M.D. Barnes, W. B. Whitten, and J. M. Ramsey, "De-
tecting single molecules in liquids," Anal. Chem. 67,
418A-423A (1995).
[2] S. C. Hill, H. L Saleheen, M. D. Barnes, W. B. Whit-
ten, and J. U. Ramsey, "Modeling fluorescence col-
lection of fluorescence from single molecules in mi-
crospheres; effects of position, orientation and fre-
quency," Appl. Opt. 35, 6278-6288 (1996).
[3] S. C. Hill, M. D. Barnes, W. B. Whitten, and J. M.
Ramsey, "Fluorescence from single molecules inside
of droplets: effects of illumination geometry," Appl.
Opt., in press.
[4] M. D. Barnes, N. Lermer, C.-Y. Kung, W. B. Whitten,
J. M. Ramsey, and S. C. Hill, "An approach to single-
molecule quantum optics using microdroplet streams
in a linear quadrupole," submitted to Opt. Lett.
[5] C.-Y. Kung, M. D. Barnes, N. Lermer, W. B. Whit-
ten, and J. M. Ramsey, "Molecular confinement
in sub-femtoliter volumes: Real-time observation of
single-molecule fluorescence in 1-pm diameter water
droplets," submitted to Science.4
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Hill, S.C. Modeling single molecule detection probabilities in microdroplets. Final report, report, May 22, 1997; United States. (https://digital.library.unt.edu/ark:/67531/metadc684621/m1/4/: accessed March 28, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.