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The ability to detect optical signals form a cellular target depends upon the amount of optical energy that can be
generated by this target as the signal. Given that the sensitivity of optical detectors has some finite limit, further increase
of the sensitivity of optical diagnostic methods requires increasing the energy of target-generated signal. Usually this
energy is converted by the cellular target upon its optical excitation and is limited by many factors such as: cell and
target damage thresholds, efficiency of excitation energy conversion, size of the target etc. All these put principal
limitation on sensing small targets (like molecules) in living cells with any optical method because the energy that can
be safely converted by the target into a signal is limited. To overcome this limitation and to improve the sensitivity of
optical microscopy of living cells (and cytometry in general) we propose the concept of intracellular amplification of the
optical signal. This concept includes two major steps. First, primary (pump) optical radiation interacts with the target (a
probe molecule) to generate a transient target. Second, the transient target is sensed with additional optical radiation that
does not interact strongly with primary target or the cell, and, hence, may have high enough energy to increase the
signal from transient target even above the energy of pump radiation, which is limited by cell and target damage
thresholds. We propose to use optical scattering from clusters of gold nanoparticles (the target) that are selectively
formed in specific cells through antibody-receptor interaction and through endocytosis. To amplify this optical signal
we propose to generate photothermal bubbles (the transient target) around those clusters. In experiments with water
suspensions and with individual tumor K562 cells we have achieved optical signal amplification in individual cells
(relatively to the scattering signal from intact cells): with gold nanorod intracellular clusters, 14.8 times, with
photothermal bubbles, generated around those clusters, more than 100 times. Those signals were much higher than
corresponding fluorescent signals and were obtained from living cells.
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