Time-correlated single-photon counting on a chip
Recent years have seen the development of a variety of integrated analytical systems known as “lab on a chip” systems. Researchers have shown, for example, that functional optical structures such as waveguides can be incorporated into the same platform as microfluidic channels — so that the ends of the waveguides form interfaces with the channels, allowing them to probe a sample using light from the waveguides.
The technique known as time-correlated single-photon counting has been demonstrated for a range of applications. In analytical biochemistry, investigators have used it for fluorescence lifetime analysis in macroscopic samples as well as for DNA analysis. Still, to date, only a few groups have been working to implement it in an integrated optical microfluidic system.
Researchers have described an integrated lab-on-a-chip system in which they implemented time-correlated single-photon counting in an optical microfluidic platform. The fabrication method they used eliminated the need to deposit upper cladding silica layers — simplifying the fabrication process — and enabled them to create complex optical structures.
In the Aug. 13 issue of Applied Physics Letters, researchers with the University of Glasgow in the United Kingdom, with Institut National d’Optique in Sainte Foy, Quebec, Canada, with Heriot-Watt University in Edinburgh, UK, and with the University of Toronto described an integrated lab-on-a-chip system in which they incorporated very sensitive detectors, low-loss optical waveguides and a microfluidic platform for time-correlated single-photon counting of fluorescent molecules.
“We have worked in the field of integrating optics and microfluidics for the last five years,” said principal investigator Jonathan M. Cooper. “The previous work involved etching the glasses to create waveguides, which was lengthy and expensive. Using buried waveguides, written with an electron beam writer and a polydimethysiloxane (PDMS) gasket certainly makes the platform easier to fabricate.”
Using electron beam densification of planar silica on silicon to fabricate the waveguides eliminated the need to deposit upper cladding silica layers; with other techniques, this additional step is required before microfluidic channels can be integrated. The fabrication method also allowed the researchers to produce complex optical structures such as the Y-structure that enabled continuous monitoring of the waveguide input.
The researchers photolithographically defined channels that were 9 × 80 μm and an analytical chamber that was 100 × 50 × 9 μm and then produced them by dry-etching through the waveguide core and undercladding. Finally, they sealed the device using a PDMS gasket.
A 630-nm pulsed diode laser made by PicoQuant GmbH of Berlin provided excitation and was launched into the input waveguide by a 20× objective. Fluorescence emission was collected by a 100-μm-core fiber coupled to the output waveguide and sent to a single-photon avalanche diode for detection. This very sensitive detector provided measurements of both the intensity and the decay time of the dye.
Using the device, the researchers measured fluorescence intensities and decay times for Nile blue dye in methanol, with concentrations of 5 mM to 750 pM. They noted that the decay time remains constant across these changes in dye concentration, thus demonstrating the potential of the device for detecting particular dyes across a wide range of concentrations.
Cooper noted that the device could contribute to a number of applications, from low-cost multiplexed diagnostics for health care to homeland security. He and his colleagues plan to develop the device for a particular application, including the low-level multiplexed detection of DNA probes associated with, for example, the detection of microorganisms.
“The time-correlated single-photon counting technique has significant potential in identifying fluorophores uniquely, and we hope in the future to demonstrate the multiplexed measurement system,” he said.
Contact: Jonathan M. Cooper, University of Glasgow; e-mail: firstname.lastname@example.org.
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