Compact fluorescence detection using in-fiber microchannels-its potential for lab-on-a-chip applications

In-fibre micro-channel: its potential for in-fibre detection

Micro-channels (μ-channels) in microstructure fibres can be regarded as natural in-fibre flow channels. Thus, the advent of microstructure fibres with μ-channels makes it possible to realize in-fibre integrated microfluidic devices, and microstructure fibres with μ-channels provide the possibility of creating new online monitoring systems and define new concepts. Herein, we developed a novel compact in-fibre detection platform, which is combined with a μ-channel in a new-type microstructure fibre, side-hole fibre, for in-fibre detection. The optical component of this proposed in-fibre detection platform is made of a simple cross-axial open-cavity Fabry-Perot interferometer. This miniaturized cross-axial open-cavity Fabry-Perot interferometer is formed by a 45°-angled side-hole fibre, which is fabricated by a simple end-face polishing process. For a 45°-angled fibre, the incident optical axis can be steered based on total internal reflection at the oblique fibre-air interface, and the reflected light will enter the side-hole μ-channel, and the front and rear inner μ-channel walls form the cavity; the natural in-fibre μ-channel functions as a (liquid/gas) flow channel. Experimentally, this proposed in-fibre detection platform was fabricated, and its spectral characteristics were investigated. Its relative humidity characteristics and potential application in breath sensing were calibrated by measuring the evolution of the reflection spectrum. As a whole, the proposed detection platform demonstrates the advantages of simple structure, easy fabrication without additional sensitive materials, and potential application in breath sensing or lab-in-fibre technology.

A hybrid silicon-PDMS optofluidic platform for sensing applications

A hybrid silicon-poly(dimethysiloxane) (PDMS) optofluidic platform for lab-on-a-chip applications is proposed. A liquid-core waveguide with a self-aligned solid-core waveguide and a microfluidic device are integrated with a multilayer approach, resulting in a three-dimensional device assembly. The optofluidic layer was fabricated with a hybrid silicon-polymer technology, whereas the microfluidic layer was fabricated with a soft lithography technique. The combination of different materials and fabrication processes allows a modular approach, enabling both the benefits from the high optical quality achievable with silicon technology and the low cost of polymer processing. The proposed chip has been tested for fluorescence measurements on Cy5 water solutions, demonstrating the possibility to obtain a limit of detection of 2.5 nM.

Optofluidic jet waveguide for laser-induced fluorescence spectroscopy

An optofluidic water-jet waveguide on chip for fluorescence analysis is presented. A high-speed water stream produced by means of a microchannel acts at the same time as the solution to analyze and as the collecting optical waveguide. The absence of solid walls and the very optically smooth surface of the liquid stream permits a strong increase of the signal-to-noise ratio. The device layout allows a self-alignment direct coupling of a water-jet waveguide with a multimode optical fiber connected to the detector. The performances of the integrated system are tested on Cy5 water solutions. For a 4.5 mm long water-jet waveguide, the measured limit of detection (LOD) is 2.56 nM and filter-free detection is possible with an LOD of 6.11 nM.

Microfluidic on-chip fluorescence-activated interface control system

A microfluidic dynamic fluorescence-activated interface control system was developed for lab-on-a-chip applications. The system consists of a straight rectangular microchannel, a fluorescence excitation source, a detection sensor, a signal conversion circuit, and a high-voltage feedback system. Aqueous NaCl as conducting fluid and aqueous glycerol as nonconducting fluid were introduced to flow side by side into the straight rectangular microchannel. Fluorescent dye was added to the aqueous NaCl to work as a signal representing the interface position. Automatic control of the liquid interface was achieved by controlling the electroosmotic effect that exists only in the conducting fluid using a high-voltage feedback system. A LABVIEW program was developed to control the output of high-voltage power supply according the actual interface position, and then the interface position is modified as the output of high-voltage power supply. At last, the interface can be moved to the desired position automatically using this feedback system. The results show that the system presented in this paper can control an arbitrary interface location in real time. The effects of viscosity ratio, flow rates, and polarity of electric field were discussed. This technique can be extended to switch the sample flow and droplets automatically.

Detection of fluorescence generated in microfluidic channel using in-fiber grooves and in-fiber microchannel sensors

In life sciences, the problem of very small volume of sample, analytes, and reagents is often faced. Micro-fluidic technology is ideal for handling costly and difficult-to-obtain samples, analytes, and reagents, because it requires very small volume of samples, in order of microL or even nL. Among many types of optical techniques commonly used for biosensing in microfluidic chip, fluorescence detection technique is the most common. The standard free-space detection techniques used to detect fluorescence emission from microfluidic channel often suffer issues like scattering noise, crosstalks, misalignment, autofluorescence of substrate, and low collection efficiency. This chapter describes two fluorescence detection methods, based on in-fiber microchannels and in-fiber grooves, which can solve those problems, as the techniques integrate the excitation and emission light paths, and the sensing part. Utilizing an optical fiber as a sensing component makes these detection techniques suitable for lab-on-a-chip or microTAS applications.

Simultaneous particle counting and detecting on a chip

This paper reports a lab-on-a-chip device that performs particle detection and number counting by coupling the fluorescent detection and particle counting simultaneously. The particle number counting is realized by a resistive pulse sensor (RPS) and fluorescent particle detection is achieved by a miniaturized laser-fiber optic detection system. By using a single microfluidic channel with two detecting arm channels placed at the two ends of the sensing section, the RPS signal-to-noise ratio is improved significantly. Two-stage differential amplification is used to further increase the signal-to-noise ratio for both the RPS and fluorescent signals. This method is also highly sensitive, so that we were able to realize the RPS and fluorescent detection of 0.9 microm (mean diameter) fluorescent particles. Excellent agreement was achieved by comparing the results obtained by our system with the results from a commercial flow cytometer for a variety of samples of mixed fluorescent and non-fluorescent particles. The method described in this paper is simple and can be applied to develop a compact device without the need of lock-in amplifier or similar bulky supplemental equipment.

Numerical and experimental study of microfluidic devices in step-index optical fibers

Microfluidic devices composed of microslits in step-index optical fibers are thoroughly investigated. Numerical simulations are performed to explain scattering and power loss in such devices. Experimental results based on microslits fabricated by femtosecond laser processing corroborate theoretical data. Dependency of the device performance on the refractive index of fluid in the slit is further utilized to construct a refractive index sensor and an in-fiber attenuator.

Polymer waveguide backplanes for optical sensor interfaces in microfluidics

A polymer optical backplane capable of generic luminescence detection within microfluidic chips is demonstrated using large core polymer waveguides and vertical couplers. The waveguides are fabricated through a new process combining mechanical machining and vapor polishing with elastomer microtransfer molding. A backplane approach enables general optical integration with planar array microfluidics since optical backplanes can be independently designed but still integrated with planar fluidic circuits. Fabricated large core waveguides exhibit a loss of 0.1 dB cm(-1) at 626 nm, a measured numerical aperture of 0.50, and a collection efficiency of 2.86% in an n = 1.459 medium, comparable to a 0.50 NA microscope objective. In addition to vertical couplers for out-of-plane collection and excitation, polymer waveguides are doped with organic dyes to provide wavelength selective filtering within waveguides, further improving optical device integration. With large core low loss waveguides, luminescence collection is improved and measurements can be performed with simple LEDs and photodetectors. Fluorescein detection via fluorescence intensity with a limit of detection (3sigma) of 200 nM in a 1 microL volume is demonstrated. Phosphorescence lifetime based oxygen detection in water in an oxygen controllable microbial cell culture chip with a limit of detection (3sigma) of 0.08% or 35 ppb is also demonstrated utilizing the waveguide backplane. Single waveguide luminescence collection performance is equivalent to a back collection geometry fiber bundle consisting of nine 500 microm diameter collection fibers.

Optofluidic integration for microanalysis

This review describes recent research in the application of optical techniques to microfluidic systems for chemical and biochemical analysis. The "lab-on-a-chip" presents great benefits in terms of reagent and sample consumption, speed, precision, and automation of analysis, and thus cost and ease of use, resulting in rapidly escalating adoption of microfluidic approaches. The use of light for detection of particles and chemical species within these systems is widespread because of the sensitivity and specificity which can be achieved, and optical trapping, manipulation and sorting of particles show significant benefits in terms of discrimination and reconfigurability. Nonetheless, the full integration of optical functions within microfluidic chips is in its infancy, and this review aims to highlight approaches, which may contribute to further miniaturisation and integration.

Optical filtering technologies for integrated fluorescence sensors

Numerous approaches have been taken to miniaturizing fluorescence sensing, which is a key capability for micro-total-analysis systems. This critical, comprehensive review focuses on the optical hardware required to attenuate excitation light while transmitting fluorescence. It summarizes, evaluates, and compares the various technologies, including filtering approaches such as interference filters and absorption filters and filterless approaches such as multicolor sensors and light-guiding elements. It presents the physical principles behind the different architectures, the state-of-the-art micro-fluorometers and how they were microfabricated, and their performance metrics. Promising technologies that have not yet been integrated are also described. This information will permit the identification of methods that meet particular design requirements, from both performance and integration perspectives, and the recognition of the remaining technological challenges. Finally, a set of performance metrics are proposed for evaluating and reporting spectral discrimination characteristics of integrated devices in order to promote side-by-side comparisons among diverse technologies and, ultimately, to facilitate optimized designs of micro-fluorometers for specific applications.