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Droplet Detection: Fluorescence ' The detection of microfluidic droplets with fluorescence spectroscopy offers a low cost, highly stable method of optical detection. The bright signals and rest response times generated by fluorescence assays are beneficial for droplet microfluidics. This system consists of an optical assembly for receiving and processing light signals from a light source and directing this light onto a microfluidic channel. The fluorescent components in the droplet fluorophores are detected by the fluorescence produced following light excitation. The emitted light is then processed by the optical system and analyzed, allowing quantification of the concentrations or presence of a certain fluorophore. Methods for detection include avalanche photobodies (APD) and photomultiplier tubes (PMT); PMTs are also described briefly in this article in the section below.

Photomultiplier tubes (PMT) Photomultiplier tubes (PMT) are photodetectors and popular in microfluidics for their optic capabilities. PMTs receive emitted light photons and produce a signal based on the corresponding level of analyte concentration. PMTs are commonly used to process fluorescence images produced by high-speed cameras capturing the fluorescence intensities produced by droplets in a non-destructive way. PMTs improve the detection sensitivity of the signal to noise ratio with spatial and software filtering. The fluorescence signal is regulated by the amount of control voltage of the PMT. As the control voltage increases, the fluorescence signal increases. Varying concentrations of fluorophores (producing varied levels of fluorescence intensity) in micro-droplets can be identified using this system. In a droplet-based microfluidic device, each microfluidic droplet emits a light fluorescence signal, and the PMT then sends these signals to a computer for data analysis.

Fluorescent Dyes (Fluorescein) Fluorescent combinations of dyes can be used for detecting cell recognition and act as cell/particle markers within droplet populations. Different color and concentrations of dyes are encapsulated in droplet populations. The dye concentrations are proportional to the fluorescence light emitted. Many fluorescent assays require two simultaneous dye colors for detection. Fluorescein derivatives, including fluorescein isothiocyanate (FITC), are commonly used fluorescent reagents for biological research and droplet-based microfluidics due to the high absorptivity, fluorescence quantum yield, and water solubility (Thermo Fisher). Fluorescein exhibits light excitation at 490 nm and emission at 514 nm.