Michael Bassler

Spatially modulated fluorescence emission from moving particles

An optical detection technique for a flow cytometer is described, which delivers high signal-to-noise discrimination without precision optics to enable a flow cytometer that can combine high performance, robustness, compactness, low cost, and ease of use. The enabling technique is termed “spatially modulated emission” and generates a time-dependent signal as a continuously fluorescing bioparticle traverses a predefined pattern for optical transmission. Correlating the detected signal with the known pattern achieves high discrimination of the particle signal from background noise. The technique is demonstrated with measurements of fluorescent beads flowing through a microfluidic chip.

Fluorescence spectrometer-on-a-fluidic-chip

A chip-size spectrometer is realized by combining a linear variable band-pass filter with a CMOS camera. The filter converts the spectral information of the incident light into a spatially dependent signal that is analyzed by the camera. A fluidic platform is integrated onto the spectrometer for analyzing the fluorescence from moving objects. The target is continuously excited within an anti-resonant waveguide, and its fluorescence spectrum is recorded as the object traverses the detection area.

Guiding light in fluids

In contrast to conventional waveguides with high-refractive-index core, we describe a concept for guiding light within a low-refractive-index medium surrounded by a medium with higher refractive index. This concept is especially compatible with fluidic sensors because the channel itself can be used as the core of the waveguide and thereby enabling a strong light-analyte interaction over an extended distance. The concept is based on antiresonant modes, and experiments have shown that these modes can be excited by collimating excitation light under an appropriate angle onto the waveguide structure.

Performance of chip-size wavelength detectors

Chip-size wavelength detectors are composed from a linear variable band-pass filter and a photodetector array. The filter converts the incident spectral distribution into a spatial distribution that is recorded by the detector array. This concept enables very compact and rugged spectrometers due to the monolithic integration of all functional components on a single chip. This type of spectrometer reveals its most convincing advantages through appropriate systems integration. We discuss the advantages of this concept for spectroscopy of light distributions that are hard to focus onto the entrance slit of a conventional spectrometer, namely large light emitting areas and moving point-like light sources. The excellent spectral performance of the system is demonstrated for both light input geometries.

CLASS IDENTIFICATION OF BIO-MOLECULES BASED ON MULTI-COLOR NATIVE FLUORESCENCE SPECTROSCOPY

Laser-induced native fluorescence is measured on a set of bio-molecules from different classes (bacteria, proteins, fungi) for excitation at 266nm and 355nm. A method of preprocessing the spectra to obtain an inherently normalized set of data is described. Class identification on the normalized data set is demonstrated.

Enhanced light-target interaction using a novel anti-resonant waveguide concept

In optical biosensors waveguides are a good choice to deliver light to the area used for sensing. In traditional optical waveguides the light is confined by total internal reflection inside of a high index layer surrounded by regions of low refractive index. Since many sensing applications are based on liquids, it is necessary to guide the light within the liquid. Liquids usually have a lower refractive index than their surroundings. Hence, conventional waveguides provide only a weak interaction between light and target molecules. In order to improve the interaction we are using a novel anti-resonant waveguide concept, in which the core region has a lower refractive index than the cladding layers. With this concept the light can be guided within the target-containing medium, thereby enabling an extended interaction length. An anti-resonant waveguide is especially compatible with a fluidic biosensor because the fluidic channel itself can be used as the core of the anti-resonant waveguide. The light propagation and coupling mechanism of an anti-resonant waveguide is reviewed and is demonstrated with large area fluorescence excitation. By coupling the excitation light into a liquid film between two glass slides we are able to excite fluorescence within a 5 cm long channel. The measured fluorescence intensity per unit area is equal to that obtained by focusing the total excitation power onto a small spot. From analyzing the angular intensity distribution at the end facet of the waveguide we gain a better understanding of the guiding mechanism.

Compact and Fast Interrogation Unit for Fiber Bragg Grating Sensors

We present a compact and fast wavelength monitor capable of resolving pm wavelength changes. A photosensor array or position detector element is coated with a linear variable filter, which converts the wavelength information of the incident light into a spatial intensity distribution on the detector. Differential read-out of two adjacent elements of the photosensor array or the position detector is used to determine the centroid of this distribution. A wavelength change of the incident light is detected as a shift of the centroid of the distribution. The performance of this wavelength detector was tested with a wavelength tunable light source. We have demonstrated that our device is capable of detecting wavelength changes as small as ~0.1 pm. The wavelength monitor can be used as read-out unit for any optical sensor that produces a wavelength shift in response to a stimulus. In particular, changes in the reflection properties of one and two-dimensional photonic crystals can been detected. The performance of this interrogation method has been tested for the case of temperature and strain sensors based on Fiber Bragg Gratings (FBG).

COMPACT OPTICAL CHARACTERIZATION PLATFORM FOR DETECTION OF BIO-MOLECULES IN FLUIDIC AND AEROSOL SAMPLES

An optical characterization unit based on fluorescence spectroscopy-on-a-chip is described. It comprises a compact fluidic platform that is integrated onto a chip-size spectrometer. The analyte is continuously excited within a novel waveguide. Fluorescence spectra are recorded as the analyte traverse the detection area. In order to achieve a strong interaction between excitation light and analyte we use an anti-resonant waveguide, in which the light is guided within the target-containing medium, thereby enabling a continuous excitation of a large volume. The excitation light is guided in the lower-refractive-index fluid when the light is coupled into the waveguide at an appropriate angle. Compact spectrometers can be integrated along the fluidic channel. The spectrometers are composed of a detector array which is coated with a linear variable band-pass filter. The filter converts the spectral fluorescence information into a spatially dependent signal that is analyzed by the detector array. These chip-size spectrometers are especially applicable for characterization of moving analytes.

Microfluidic-based detection platform for on-the-flow analyte characterization

While commercial flow cytometers are sophisticated analytical instruments extensively used in research and clinical laboratories, they do not meet the challenging practical requirements for point-of-care (POC) diagnostics. In this paper we will describe and illustrate a new detection technique that will enable a compact, microfluidic-based flow cytometer that satisfies POC specifications for performance, robustness, compactness, cost, reagent consumption, and ease of use. The technology has been demonstrated with CD4 counts in whole blood.

Micro-fluidic-based optical detection platform for characterizing fluorescing objects with integrated wavelength detection

This presentation will give a brief overview on on-the-flow analyte detection based on native fluorescence spectroscopy. This is a very promising approach that does not require specific binding or tagging of the analyte. However, the variety of cells is large compared to the number of basic molecular building blocks. Therefore, the fluorescence spectra of different species are often very similar, and sophisticated detection methods are required to reveal differences. The specificity of this approach can be further improved by implementing high spectral resolution and using multiple excitation wavelengths. In order to test class identification based on multi-color native fluorescence we have measured, with a conventional laboratory set-up, the laser-induced fluorescence spectra of various biological building blocks and recorded fluorescence spectra for representative analyte simulants.