Flow cytometry is arguably the most powerful optical method routinely used in the diagnosis of health disorders and disease monitoring. We enhance an optical flow cytometer with a photon number statistics measurement. Our technique allows for an absolute measurement of biomarker concentration and proves sensitivity to a single biomarker.
We invented a flow cytometer that can self-calibrate in an absolute way that can reach the sensitivity for quantification and enumeration of a single and small number of biological markers per a biological entity. The cytometer uses a photon-number resolving detector (or a detector arrangement capable of resolving photon numbers, such as Hanbury-Brown Twiss arrangement), accumulating times of arrival and photon number statistics and using first principles of quantum optics that provide a number of participating emitters. There are two aspects to this invention. One describes a hardware modification, and the other describes measurement methods. Specifically:
--The absolute calibration. With our setup, at a given concentration of fluorophores (per cell), we accumulate the optical signal from many exposures (many cells) and statistically find second-order correlation functions and their dependence on various time delays. We also measure average photon flux as a function of time and time delays. This process is repeated for several concentrations. The second-order correlation functions relate to concentrations via laws of quantum optics, this dependence does not require the prior knowledge of transmittance losses and detection inefficiencies, even with an unknown optical background present. This establishes an optical flux to moles calibration from first principles. The dependence of second-order correlation functions on time gives the most advantageous signal processing algorithm for a "sorting" mode of operation. This signal processing algorithm improves signal to noise.
The implications of this method are twofold. This invention solves the problem of calibration of flow cytometers for the purposes of intercomparing and quantification. Note that intercomparison with flow cytometers has not yet been achieved. Second, this invention teaches how to use the calibration information to optimize a sensitivity of a real-time cell-to-cell measurement to a level of a single fluorophore.
Light produced by a system of few optically active biomarkers (fluorescent molecules, quantum dots) exhibits quantum behavior. Therefore, quantum measurement methods can be applied. Measurement of photon number statistics can unambiguously resolve the number of emitters in the system and it is resilient to loss. This unambiguous measurement, therefore, enables self-calibration of a flow cytometer. We start with a sample of an unknown concentration of fluorophores, and dilute it by known factors to reach a range of concentrations with an average of fluorophores ranging from approximately .1 (or lower) to approximately 10 per interrogation volume. Then, we acquire photon number statistics for each of the diluted samples. Finally, we combine the measurements and fit them to theory. The initial unknown concentration is then extracted from the fit. Although a cw-laser can be used to optically excite the fluorophores, we believe that using a pulsed laser will significantly improve sensitivity. In addition to an absolute calibration, because the complete list of all photoelectronic detections is retained, the time-resolving statistical analysis identifies temporal properties of signal (nonclassical) and noise (classical).
During a normal operation, a flow cytometer collects signal from each biological entity for only a short time (lO's of microseconds to a millisecond). Therefore, the number of collected photons from trace concentrations of biomarkers is limited. Therefore, shot noise is norma11y the limit for separating signal from background. Because accurate time of single photon detections are available, and because temporal properties of signal and noise are independently identified, the separation of signal from background can be made sub-shot noise.
This invention will provide fundamentally better sensitivity and specificity than currently state-of-the-art flow cytometer technologies, where the saturation is reached, after which further increase in sensitivity does not result in improved specificity. This method will allow to enumerate the biological targets, such as antibodies, microRNA and exosomes, at low concentrations.
In addition, this method will provide an absolute scale for such enumeration in situ, and will open the possibility of intercomparison.