Planar optofluidic chip for single particle detection, manipulation, and analysis

We present a fully planar integrated optofluidic platform that permits single particle detection, manipulation and analysis on a chip. Liquid-core optical waveguides guide both light and fluids in the same volume. They are integrated with fluidic reservoirs and solid-core optical waveguides to define sub-picoliter excitation volumes and collect the optical signal, resulting in fully planar beam geometries. Single fluorescently labeled liposomes are used to demonstrate the capabilities of the optofluidic chip. Liposome motion is controlled electrically, and fluorescence correlation spectroscopy (FCS) is used to determine concentration and dynamic properties such as diffusion coefficient and velocity. This demonstration of fully planar particle analysis on a semiconductor chip may lead to a new class of planar optofluidics-based instruments.     

Fluorescence cross-correlation spectroscopy using single wavelength laser

In this paper, we first introduced the basic principle of fluorescence cross-correlation spectroscopy (FCCS) and then established an FCCS setup using a single wavelength laser. We systematically optimized the setup, and the detection volume reached about 0.7 fL. The homebuilt setup was successfully applied for the study of the binding reaction of human immunoglobulin G with goat antihuman immunoglobulin G. Using quantum dots (745 nm emission wavelength) and Rhodamine B (580 nm emission wavelength) as labeling probes and 532 nm laser beam as an excitation source, the cross-talk effect was almost completely suppressed. The molecule numbers in a highly focused volume, the concentration, and the diffusion time and hydrodynamic radii of the reaction products can be determined by FCCS system. __________ Translated from Chemical Journal of Chinese Universities, 2008, 29(5) (in Chinese)     

Optofluidic variable-focus lenses for light manipulation

This paper presents a planar optofluidic lens for light manipulation utilizing a combination of optofluidic biconvex lens with micromixer. Three light manipulation techniques including tunable optical diverging, collimating and focusing are realized by altering the refractive index of the optofluidic variable-focus lenses formed by solid polydimethylsiloxane (PDMS) walls and tunable liquid lens body. The optical power from the laser input can be increased or decreased with the tuning of the variable-focus lenses' refractive indexes. The optical power adjustment capabilities are demonstrated and characterized. The combinations of benefits of all lens' optical manipulation capabilities, greater mechanical stability, significant increase of optofluidic device's life time and seamless integration with other lab-on-a-chip functionalities provide a promising and versatile optofluidic compartment to integrate with lab-on-a-chip excitation and sensing applications. Optofluidic lens-including system for tunable fluorescence sensing is demonstrated showing 186% increase in detected fluorescence intensity.     

Single wavelength excitation fluorescence cross-correlation spectroscopy with spectrally similar fluorophores: resolution for binding studies

It was shown recently that fluorescence cross-correlation spectroscopy (FCCS) can be performed using a single laser wavelength for excitation (SW-FCCS) [L. C. Hwang and T. Wohland, Chem. Phys. Chem 5, 549 (2004).]. This method simplifies the FCCS setup since it does not require the simultaneous alignment of two lasers to the same focal spot. But up to now the method was shown to work only with dyes possessing large Stokes' shifts, and thus was limited to the use of quantum dots and tandem dyes. In this work we show that standard organic dyes with overlapping emission spectra, for instance fluorescein and tetramethylrhodamine, can be used as fluorescent pairs in SW-FCCS. As a biological model system for ligand-receptor interactions we studied the binding of biotin to streptavidin. To investigate the applicability of SW-FCCS for binding studies we adapt the existing FCCS theory for SW-FCCS and calculate limits for the measurement of dissociation constants in dependence on sample concentration, sample purity, and spectral cross talk between the different detection channels.     

Integrated wavelength-selective optical waveguides for microfluidic-based laser-induced fluorescence detection

We demonstrate the fabrication and characterization of a novel, inexpensive microchip capable of laser induced fluorescence (LIF) detection using integrated waveguides with built-in optical filters. Integrated wavelength-selective optical waveguides are fabricated by doping poly(dimethysiloxane) (PDMS) with dye molecules. Liquid-core waveguides are created within dye-doped PDMS microfluidic chips by filling channels with high refractive index liquids. Dye molecules are allowed to diffuse into the liquid core from the surrounding dye-doped PDMS. The amount of diffusion is controlled by choosing either polar (low diffusion) or apolar (high diffusion) liquid waveguide cores. The doping dye is chosen to absorb excitation light and to transmit fluorescence emitted by the sample under test. After 24 h, apolar waveguides demonstrate propagation losses of 120 dB cm(-1) (532 nm) and 4.4 dB cm(-1) (633 nm) while polar waveguides experience losses of 8.2 dB cm(-1) (532 nm) and 1.1 dB cm(-1) (633 nm) where 532 and 633 nm light represent the excitation and fluorescence wavelengths, respectively. We demonstrate the separation and detection of end-labelled DNA fragments using polar waveguides for excitation light delivery and apolar waveguides for fluorescence collection. We demonstrate that the dye-doped waveguides can provide performance comparable to a commercial dielectric filter; however, for the present choice of dye, their ultimate performance is limited by autofluorescence from the dye. Through the detection of a BK virus polymerase chain reaction (PCR) product, we demonstrate that the dye-doped PDMS system is an order of magnitude more sensitive than a similar undoped system (SNR: 138 vs. 9) without the use of any external optical filters at the detector.     

单波长荧光交叉相关光谱单分子检测系统

基于激光共焦构型建立了一套单波长荧光交叉相关光谱检测系统, 并对检测系统进行了优化, 阐述了荧光交叉(相关)光谱基本理论. 以荧光量子点和有机染料为标记探针, 以人免疫球蛋白与羊抗人免疫球蛋白的结合反应为研究对象, 利用该系统成功地实现了在单分子水平上对免疫结合产物的分子数、浓度、特征扩散时间和动力学半径等参数的表征.     

Modulation-frequency encoded multi-color fluorescent DNA analysis in an optofluidic chip

We introduce a principle of parallel optical processing to an optofluidic lab-on-a-chip. During electrophoretic separation, the ultra-low limit of detection achieved with our set-up allows us to record fluorescence from covalently end-labeled DNA molecules. Different sets of exclusively color-labeled DNA fragments-otherwise rendered indistinguishable by spatio-temporal coincidence-are traced back to their origin by modulation-frequency-encoded multi-wavelength laser excitation, fluorescence detection with a single ultrasensitive, albeit color-blind photomultiplier, and Fourier analysis decoding. As a proof of principle, fragments obtained by multiplex ligation-dependent probe amplification from independent human genomic segments, associated with genetic predispositions to breast cancer and anemia, are simultaneously analyzed.     

Quantum dot enabled thermal imaging of optofluidic devices

Quantum dot thermal imaging has been used to analyse the chromatic dependence of laser-induced thermal effects inside optofluidic devices with monolithically integrated near-infrared waveguides. We demonstrate how microchannel optical local heating plays an important role, which cannot be disregarded within the context of on-chip optical cell manipulation. We also report on the thermal imaging of locally illuminated microchannels when filled with nano-heating particles such as carbon nanotubes.     

Dual-core optofluidic chip for independent particle detection and tunable spectral filtering

We present the first integration of fluidically tunable filters with a separate particle detection channel on a single planar, optofluidic chip. Two optically connected, but fluidically isolated liquid-core antiresonant reflecting optical waveguide (ARROW) segments serve as analyte and spectral filter sections, respectively. Ultrasensitive detection of fluorescent nanobeads with high signal-to-noise ratio provided by a fluidically tuned excitation notch filter is demonstrated. In addition, reconfigurable filter response is demonstrated using both core index tuning and bulk liquid tuning. Notch filters with 43 dB rejection ratio and a record 90 nm tuning range are implemented by using different mixtures of ethylene glycol and water in the filter section. Moreover, absorber dyes and liquids with pH-dependent transmission in the filter channel provide additional spectral control independent of the waveguide response. Using both core index and pH control, independent filter tuning at multiple wavelengths is demonstrated for the first time. This extensive on-chip control over spectral filtering as one of the fundamental components of optical particle detection techniques offers significant advantages in terms of compactness, cost, and simplicity, and opens new opportunities for waveguide-based optofluidic analysis systems.     

Novel tuneable optical elements based on nanoparticle suspensions in microfluidics

This work demonstrates the application of dielectrophoretic (DEP) control of silica nanoparticles to form tuneable optical elements within a microfluidic system. The implementation consisted of a microfluidic channel with an array of curved microelectrodes along its base. Various DEP conditions were investigated at alternating current voltage amplitudes, flow rates and frequencies from 5 to 15 V, 2 to 10 μL/min and 0 to 20 MHz, respectively. The fluid channel was filled with deionized water suspending silica particles with diameters of 230 and 450 nm. Experiments were conducted to demonstrate DEP concentration and deflection of the particles and the impact of these particles distributions on the optical transmission through the fluid channel. Both confinement and scattering of the light were observed depending on the particle dimensions and the parameters of the DEP excitation. The results of this investigation illustrate the feasibility of DEP control in an optofluidic system and represent a significant step toward the dynamic formation of electrically controlled liquid optical waveguides.     

Fluorescent liquid-core/air-cladding waveguides towards integrated optofluidic light sources

We have demonstrated fluorescent liquid-core/air-cladding (LA) waveguides suitable for use as integrated optofluidic light sources. These waveguides were fabricated by conventional soft lithography using poly(dimethylsiloxane) (PDMS). Two-phase stratified flows of air and ethylene glycol with fluorescent dye were generated along the PDMS channel. Compared to the liquid-core/liquid-cladding (L(2)) waveguide, the larger refractive index contrast of the LA waveguide resulted in stronger optical confinement. Specifically, the larger refractive index contrast led to experimentally achievable captured fractions (the amount of light to be coupled into the liquid core) as high as 22.8% and the measured propagation losses as low as 0.14 dB cm(-1). Furthermore, in our LA waveguides, diffusional mixing of the core and cladding fluids did not occur and the size of the core stream could be reversibly tuned simply by adjusting the flow rates of the two contiguous phases.     

Frontiers of optofluidics in synthetic biology

The development of optofluidic-based technology has ushered in a new era of lab-on-a-chip functionality, including miniaturization of biomedical devices, enhanced sensitivity for molecular detection, and multiplexing of optical measurements. While having great potential, optofluidic devices have only begun to be exploited in many biotechnological applications. Here, we highlight the potential of integrating optofluidic devices with synthetic biological systems, which is a field focusing on creating novel cellular systems by engineering synthetic gene and protein networks. First, we review the development of synthetic biology at different length scales, ranging from single-molecule, single-cell, to cellular population. We emphasize light-sensitive synthetic biological systems that would be relevant for the integration with optofluidic devices. Next, we propose several areas for potential applications of optofluidics in synthetic biology. The integration of optofluidics and synthetic biology would have a broad impact on point-of-care diagnostics and biotechnology.     

Time-resolved fluorescence spectroscopy and imaging of proteinaceous binders used in paintings

The differentiation of proteins commonly found as binding media in paintings is presented based on spectrally resolved and time-resolved laser-induced fluorescence (LIF) and total emission spectroscopy. Proteins from eggs and animal glue were analysed with pulsed laser excitation at 248 nm (KrF excimer) and 355 nm (third harmonic of Nd:YAG) for spectrally resolved measurements, and at 337 nm (N₂) and 405 nm (N₂ pumped dye laser) for spectrally resolved lifetime measurements and fluorescence lifetime imaging (FLIM). Total emission spectra of binding media are used for the interpretation of LIF spectra. Time-resolved techniques become decisive with excitation at longer wavelengths as fluorescence lifetime permits the discrimination amongst binding media, despite minimal spectral differences; spectrally resolved measurements of fluorescence lifetime have maximum differences between the binding media examined using excitation at 337 nm, with maximum observed fluorescence at 410 nm. FLIM, which measures the average lifetime of the emissions detected, can also differentiate between media, is non-invasive and is potentially advantageous for the analysis of paintings. Figure The fluorescence of solid ox glue extracted from collagen can be visualised in this Total Fluorescence Spectrum; three different peaks from multiple fluorophores are present and allow the discrimination between collagen- and non-collagen proteinaceous binding media found in paintings     

Discretely tunable optofluidic compound microlenses

We report a novel method to fabricate high zoom-ratio optofluidic compound microlenses using poly(dimethylsiloxane) with multi-layer architecture. The layered structure of deformable lenses, biconvex and plano-concave, are self-aligned as a group. The refractive index contrast of each lens, which is controlled by filling the chambers with a specific medium, is the key factor for determining the device's numerical aperture. The chip has multiple independent pneumatic valves that can be digitally switched on and off, pushing the liquid into the lens chambers with great accuracy and consistency. This quickly and precisely tunes the focal length of the microlens device from centimetres to sub-millimetre. The system has great potential for applications in portable microscopic imaging, bio-sensing, and laser beam configuration.     

An optofluidic prism tuned by two laminar flows

This paper presents a tunable optofluidic prism based on the configuration of two laminar flow streams with different refractive indices in a triangular chamber. The chambers with 70° and 90° apex angles are designed based on simulation results, which provide the optimum working range and avoid recirculating flows in the chambers. In addition, a hydrodynamic model has been developed to predict the tuning of the prisms by the variation in the flow rates. Prisms with different refractive indices are realized using benzyl alcohol and deionized (DI) water as the inner liquids, respectively. The mixture of ethylene glycol and DI water with an effective refractive index matched to that of the microchannel is used as the outer liquid. The apex angle of the prism is tuned from 75° to 135° by adjusting the ratio of the two flow rates. Subsequently, the deviation angle of the output light beam is tuned from -13.5° to 22°. One of the new features of this optofluidic prism is its capability to transform from a symmetric to an asymmetric prism with the assistance of a third flow. Two optical behaviours have been performed using the optofluidic prism. First, parallel light beam scanning is achieved with a constant deviation angle of 10° and a tuning range of 60 μm using the asymmetric prism. The detected output light intensity is increased by 65.7%. Second, light dispersion is experimentally demonstrated using 488-nm and 633-nm laser beams. The two laser beams become distinguishable with a deviation angle difference of 2.5° when the apex angle of the prism reaches 116°.     

Transformation optofluidics for large-angle light bending and tuning

Transformation optics is a new art of light bending by designing materials with spatially variable parameters for developing wave-manipulation devices. Here, we introduce a transformation optofluidic Y-branch splitter with large-angle bending and tuning based on the design of a spatially variable index. Differing from traditional splitters, the optofluidic splitter is achieved in an inhomogeneous medium by coordinate transformation. The designed bidirectional gradient index (GRIN) distribution can be achieved practically by the convection-diffusion process of liquid flowing streams. The transformation optofluidic splitter can achieve a much larger split angle with little bend loss than the traditional ones. In the experiments, a large tunable split angle up to 30° is achieved by tuning the flow rates, allowing optical signals to be freely transferred to different channels. Besides the symmetrical branch splitting, asymmetrical Y-branch splitting with approximately equal power splitting is also demonstrated by changing the composition of the liquids. The optofluidic splitter has high potential applications in biological, chemical and biomedical solution measurement and detection.     

Dynamic manipulation of modes in an optical waveguide using dielectrophoresis

The emergence of optofluidics has brought a high degree of tuneability and reconfigurability to optical devices. These possibilities are provided by characteristics of fluids including mobility, wide range of index modulation, and abrupt interfaces that can be easily reshaped. In this work, we created a new class of optofluidic waveguides, in which suspended mesoparticles were employed to greatly enhance the flexibility of the system. We demonstrated tuneable quasi single mode waveguides using spatially controllable mesoparticles in optofluidics. The coupling of waveguiding modes into the assembly of mesoparticles produces strong interactions and resonant conditions, which promote the transitions of the waveguiding modes. The modal response of the system depends on the distribution of packed particles above the polymeric rib waveguide which can be readily controlled under the appropriate combination of dielectrophoresis and hydrodynamic forces.     

Diffusion driven optofluidic dye lasers encapsulated into polymer chips

Lab-on-a-chip systems made of polymers are promising for the integration of active optical elements, enabling e.g. on-chip excitation of fluorescent markers or spectroscopy. In this work we present diffusion operation of tunable optofluidic dye lasers in a polymer foil. We demonstrate that these first order distributed feedback lasers can be operated for more than 90 min at a pulse repetition rate of 2 Hz without fluidic pumping. Ultra-high output pulse energies of more than 10 μJ and laser thresholds of 2 μJ are achieved for resonator lengths of 3 mm. By introducing comparatively large on-chip dye solution reservoirs, the required exchange of dye molecules is accomplished solely by diffusion. Polymer chips the size of a microscope cover slip (18 × 18 mm(2)) were fabricated in batches on a wafer using a commercially available polymer (TOPAS(?) Cyclic Olefin Copolymer). Thermal imprinting of micro- and nanoscale structures into 100 μm foils simultaneously defines photonic resonators, liquid-core waveguides, and fluidic reservoirs. Subsequently, the fluidic structures are sealed with another 220 μm foil by thermal bonding. Tunability of laser output wavelengths over a spectral range of 24 nm on a single chip is accomplished by varying the laser grating period in steps of 2 nm. Low-cost manufacturing suitable for mass production, wide laser tunability, ultra-high output pulse energies, and long operation times without external fluidic pumping make these on-chip lasers suitable for a wide range of lab-on-a-chip applications, e.g. on-chip spectroscopy, biosensing, excitation of fluorescent markers, or surface enhanced Raman spectroscopy (SERS).     

Two-color two-photon excitation using femtosecond laser pulses

The use of two-color two-photon (2c2p) excitation easily extends the wavelength range of Ti:sapphire lasers to the UV, widening the scope of its applications especially in biological sciences. We report observation of 2c2p excitation fluorescence of p-terphenyl (PTP), 2-methyl-5-t-butyl-p-quaterphenyl (DMQ) and tryptophan upon excitation with 400 and 800 nm wavelengths using the second harmonic and fundamental wavelength of a mode-locked Ti:sapphire femtosecond laser. This excitation is energetically equivalent to a one-photon excitation wavelength at 266 nm. The fluorescence signal is observed only when both wavelengths are spatially and temporally overlapping. Adjustment of the relative delay of the two laser pulses renders a cross correlation curve which is in good agreement with the pulse width of our laser. The fluorescence signal is linearly dependent on the intensity of each of the two colors but quadratically on the total incident illumination power of both colors. In fluorescence microscopy, the use of a combination of intense IR and low-intensity blue light as a substitute for UV light for excitation can have numerous advantages. Additionally, the effect of differently polarized excitation photons relative to each other is demonstrated. This offers information about different transition symmetries and yields deeper insight into the two-photon excitation process.