Digitally synthesized beat frequency-multiplexed fluorescence lifetime spectroscopy

Direct frequency domain fluorescence lifetime imaging using simultaneous ultraviolet and visible excitation

Due to the complexity, limited practicality, and cost of conventional fluorescence lifetime imaging/microscopy (FLIM) instrumentation, FLIM adoption has been mostly limited to academic settings. We present a novel point scanning frequency-domain (FD) FLIM instrumentation design capable of simultaneous multi-wavelength excitation, simultaneous multispectral detection, and sub-nanosecond to nanosecond fluorescence lifetime estimation. Fluorescence excitation is implemented using intensity-modulated CW diode lasers that are available in a selection of wavelengths spanning the UV-VI-NIR range (375-1064 nm). Digital laser intensity modulation was adopted to enable simultaneous frequency interrogation at the fundamental frequency and corresponding harmonics. Time-resolved fluorescence detection is implemented using low-cost, fixed-gain, narrow bandwidth (100 MHz) avalanche photodiodes, thus, enabling cost-effective fluorescence lifetime measurements at multiple emission spectral bands simultaneously. Synchronized laser modulation and fluorescence signal digitization (250 MHz) is implemented using a common field-programmable gate array (FPGA). This synchronization reduces temporal jitter, which simplifies instrumentation, system calibration, and data processing. The FPGA also allows for the implementation of the real-time processing of the fluorescence emission phase and modulation at up to 13 modulation frequencies (processing rate matching the sampling rate of 250 MHz). Rigorous validation experiments have demonstrated the capabilities of this novel FD-FLIM implementation to accurately measure fluorescence lifetimes in the range of 0.5-12 ns. In vivo endogenous, dual-excitation (375nm/445nm), multispectral (four bands) FD-FLIM imaging of human skin and oral mucosa at 12.5 kHz pixel rate and room-light conditions was also successfully demonstrated. This versatile, simple, compact, and cost-effective FD-FLIM implementation will facilitate the clinical translation of FLIM imaging and microscopy.

Extremely High-Throughput Parallel Microfluidic Vortex-Actuated Cell Sorting

We demonstrate extremely high-throughput microfluidic cell sorting by making a parallel version of the vortex-actuated cell sorter (VACS). The set-up includes a parallel microfluidic sorter chip and parallel cytometry instrumentation: optics, electronics and control software. The result is capable of sorting lymphocyte-sized particles at 16 times the rate of our single-stream VACS devices, and approximately 10 times the rate of commercial cell sorters for an equivalent procedure. We believe this opens the potential to scale cell sorting for applications requiring the processing of much greater cell numbers than currently possible with conventional cell sorting.

Full-field fluorescence lifetime dual-comb microscopy using spectral mapping and frequency multiplexing of dual-comb optical beats

Fluorescence lifetime imaging microscopy (FLIM) is a powerful tool for quantitative fluorescence imaging because fluorescence lifetime is independent of concentration of fluorescent molecules or excitation/detection efficiency and is robust to photobleaching. However, since most FLIMs are based on point-to-point measurements, mechanical scanning of a focal spot is needed for forming an image, which hampers rapid imaging. Here, we demonstrate scan-less full-field FLIM based on a one-to-one correspondence between two-dimensional (2D) image pixels and frequency-multiplexed radio frequency (RF) signals. A vast number of dual-comb optical beats between dual optical frequency combs are effectively adopted for 2D spectral mapping and high-density frequency multiplexing in the RF region. Bimodal images of fluorescence amplitude and lifetime are obtained with high quantitativeness from amplitude and phase spectra of fluorescence RF comb modes without the need for mechanical scanning. The parallelized FLIM will be useful for rapid quantitative fluorescence imaging in life science.

Direct frequency domain fluorescence lifetime imaging using field programmable gate arrays for real time processing

Frequency domain (FD) fluorescence lifetime imaging (FLIM) involves the excitation of the sample of interest with a modulated light source and digitization of the fluorescence emission for further analysis. Traditional FD-FLIM systems use heterodyne or homodyne detection, where the excitation light source and detector are modulated at specific frequency(s). More recently, FD-FLIM systems that use reflection of the light source as a trigger or phase reference for lifetime calculations have been developed. These detection schemes, however, require extra components that increase the cost and complexity of the FD-FLIM system. Here, we report a novel FD-FLIM detection scheme whereby the light source modulation and emission digitization are implemented using Field Programmable Gate Arrays (FPGAs), and fixed gain avalanche photodiodes are used for fluorescence detection. The reported FD-FLIM system was designed for probing nanosecond lifetime fluorophores (2-10 ns) at three emission bands simultaneously. The system utilizes a 375 nm diode laser for excitation at multiple simultaneous modulation frequencies (between 1 MHz and 83 MHz, bandwidth limited intentionally by using a lowpass filter) and three fixed gain avalanche photodiodes for simultaneous detection of three emission bands: 405/20 nm, 440/40 nm, and 525/50 nm (center/FWHM). Real-time computation of the modulation and phase lifetimes is simply performed by direct application of the discrete Fourier transform (max. of 10 frequencies) to the digitized fluorescence emission signals. The accuracy and sensitivity of this novel FD-FLIM detection scheme was demonstrated by imaging standard fluorophores and ex vivo unfixed human coronary artery tissue samples.

Cell sorting actuated by a microfluidic inertial vortex

The sorting of specific cell populations is an established tool in biological research, with new applications demanding greater cell throughput, sterility and elimination of cross-contamination. Here we report 'vortex-actuated cell sorting' (VACS), a new technique that deflects cells individually, via the generation of a transient microfluidic vortex by a thermal vapour bubble: a novel mechanism, which is able to sort cells based on fluorescently-labelled molecular markers. Using in silico simulation and experiments on beads, an immortal cell line and human peripheral blood mononuclear cells (PBMCs), we demonstrate high-purity and high-recovery sorting with input rates up to 104 cells per s and switching speeds comparable to existing techniques (>40 kHz). A tiny footprint (1 × 0.25 mm) affords miniaturization and the potential to achieve multiplexing: a crucial step in increasing processing rate. Simple construction using biocompatible materials potentially minimizes cost of fabrication and permits single-use sterile cartridges. We believe VACS potentially enables parallel sorting at throughputs relevant to cell therapy, liquid biopsy and phenotypic screening.