Thermal lens detection device

Droplet-based μChopper device with a 3D-printed pneumatic valving layer and a simple photometer for absorbance based fructosamine quantification in human serum

The development of microfluidic systems for biological assays presents challenges, particularly in adapting traditional optical absorbance assays to smaller volumes or to microfluidic formats. This often requires assay modification or translation to a fluorescence version, which can be impractical. To address this issue, our group has developed the μChopper device, which uses microfluidic droplet formation as a surrogate for an optical beam chopper, allowing for lock-in analysis and improved limits of detection with both absorbance and fluorescence optics without modifying the optical path length. Here, we have adapted the μChopper to low-cost optics using a light-emitting diode (LED) source and photodiode detector, and we have fabricated the pnuematically valved devices entirely by 3D printing instead of traditional photolithography. Using a hybrid device structure, fluidic channels were made in polydimethylsiloxane (PDMS) by moulding onto a 3D-printed master then bonding to a prefabricated thin layer, and the pneumatic layer was directly made of 3D-printed resin. This hybrid structure allowed an optical slit to be fabricated directly under fluidic channels, with the LED interfaced closely above the channel. Vacuum-operated, normally closed valves provided precise temporal control of droplet formation from 0.6 to 2.0 Hz. The system was validated against the standard plate reader format using a colorimetric fructosamine assay and by quantifying fructosamine in human serum from normal and diabetic patients, where strong correlation was shown. Showing a standard benefit of microfluidics in analysis, the device required 6.4-fold less serum volume for each assay. This μChopper device and lower cost optical system should be applicable to various absorbance based assays in low volumes, and the reliance on inexpensive 3D printers makes it more accessible to users without cleanroom facilities.

Recent Progress and Applications of Thermal Lens Spectrometry and Photothermal Beam Deflection Techniques in Environmental Sensing

This paper presents recent development and applications of thermal lens microscopy (TLM) and beam deflection spectrometry (BDS) for the analysis of water samples and sea ice. Coupling of TLM detection to a microfluidic system for flow injection analysis (μFIA) enables the detection of microcystin-LR in waters with a four samples/min throughput (in triplicate injections) and provides an LOD of 0.08 µg/L which is 12-times lower than the MCL for microcystin-LR in water. μFIA-TLM was also applied for the determination of total Fe and Fe(II) in 3 µL samples of synthetic cloudwater. The LODs were found to be 100 nM for Fe(II) and 70 nM for total Fe. The application of µFIA-TLM for the determination of ammonium in water resulted in an LOD of 2.3 µM for injection of a 5 µL sample and TLM detection in a 100 µm deep microfluidic channel. For the determination of iron species in sea ice, the BDS was coupled to a diffusive gradient in the thin film technique (DGT). The 2D distribution of Fe(II) and total Fe on DGT gels provided by the BDS (LOD of 50 nM) reflected the distribution of Fe species in sea ice put in contact with DGT gels.

Digital plasmonic nanobubble detection for rapid and ultrasensitive virus diagnostics

Rapid and sensitive diagnostics of infectious diseases is an urgent and unmet need as evidenced by the COVID-19 pandemic. Here, we report a strategy, based on DIgitAl plasMONic nanobubble Detection (DIAMOND), to address this need. Plasmonic nanobubbles are transient vapor bubbles generated by laser heating of plasmonic nanoparticles (NPs) and allow single-NP detection. Using gold NPs as labels and an optofluidic setup, we demonstrate that DIAMOND achieves compartment-free digital counting and works on homogeneous immunoassays without separation and amplification steps. DIAMOND allows specific detection of respiratory syncytial virus spiked in nasal swab samples and achieves a detection limit of ~100 PFU/mL (equivalent to 1 RNA copy/µL), which is competitive with digital isothermal amplification for virus detection. Therefore, DIAMOND has the advantages including one-step and single-NP detection, direct sensing of intact viruses at room temperature, and no complex liquid handling, and is a platform technology for rapid and ultrasensitive diagnostics.

An Optical Configuration of Crossed-Beam Photothermal Lens Spectrometer Operating at High Flow Velocities and Its Application for Cysteine Determination in Human Serum and Saliva

Photothermal lens spectrometry (TLS) is a high sensitive technique for trace determination of nonfluorescent materials. Previous photothermal lens spectrometers suffer from operating limitations at high flow velocities, arising from taking the heated element off the probe beam direction, which results in a decrease in the thermal lens (TL) signal. Herein, we describe an optical configuration of the crossed-beam photothermal lens in transversal flow mode in which the propagating direction of the probe beam and liquid sample flow azimuth are concentric (CBTC). The system consists of a microfluidic cell with a volume of lower than 3 μL. In the current optical configuration, using 1-(2-pyridylazo)-2-naphthol (PAN) in ethanol as a test solution, by increasing the sample flow velocity and without increasing chopping frequency, the reduction in sensitivity is less pronounced. Under a 15 Hz chopping frequency, the optimum sample flow velocity is about 2 cm s^-1, which is among the highest reported values achieved to date for photothermal lens spectrometers. Although the system operates at higher flow velocities and lower chopping frequencies compared to the collinear configuration, it provides a comparable analytical limit of detection. This optical configuration has been successfully employed for highly sensitive and selective determination of cysteine in human serum and saliva samples through a competitive complexation reaction with Cu-PAN as a colorimetric probe. The detection limit of this method (9.5 nM) shows a significant enhancement (726-times) in comparison to UV-vis measurements.

Development of a conductivity-based photothermal absorbance detection microchip using polyelectrolytic gel electrodes

The development and application of polyelectrolytic gel electrodes (PGEs) for a microfluidic photothermal absorbance detection system is described. The PGEs are used to measure changes in conductivity based on heat generation by analytes absorbing light and changing the solution viscosity. The PGEs are suitable for direct contact conductivity measurements since they do not degrade with exposure to high electric fields. Both a 2-electrode system with DC voltages and a 3-electrode system with AC voltages were investigated. Experimental factors including excitation voltage, excitation frequency, laser modulation frequency, laser power, and path length were tested. The limits of detection for the 3-electrode and 2-electrode systems are 500nM and 0.55nM for DABSYL-tagged glucosamine, respectively. In addition, an electrokinetic separation of a potassium, DABSYL-tagged glucosamine, Rhodamine 6G, and Rhodamine B mixture was demonstrated.

Determination of cattle foot-and-mouth disease virus by micro-ELISA method

The development of foot-and-mouth disease virus (FMDV) detection methods is crucial for animal food security, tackling regional FMDV epidemic, and global FMDV prognostic control. For these purposes, a fast and sensitive analysis method is required. In this study, we developed a microchip-based ELISA (enzyme-linked immunosorbent assay), micro-ELISA, to realize FMDV detection. Nickel(II) chelating chemistry was utilized to immobilize recombinant protein (antigen) on polystyrene micro-beads in order to determine FMDV antibodies in cattle serum samples. In addition, reaction protocol and conditions were investigated. As a result, the FMDV detection was successfully demonstrated with only a 10-μL sample volume in 25-minute assay time. Analytical sensitivity was evaluated by a maximum nominal positiveness percentage value (NPPV) of 303 and a dilution factor of 32×. The method's inter-run and intra-run CV (coefficients of variance) values were 15.5 and 17.1%, respectively, which were fully compatible with the OIE (World Organization for Animal Health) principle of validation of diagnosis assays for infectious diseases. The developed method should become a powerful tool for determining other animal contagious diseases and/or zoonosis.

Optofluidic differential spectroscopy for absorbance detection of sub-nanolitre liquid samples

We present a novel optofluidic differential method for carrying out absorbance spectroscopy of sub-nanolitre volumes of liquid samples on a microfluidic chip. Due to the reduction of liquid volume, the absorbance detection in microfluidics is often hindered by either low sensitivity or complex fabrication. To address this issue, we introduced an optofluidic modulator which can be easily integrated into a PDMS (polydimethylsiloxane) based microfluidic chip. The modulator was controlled by the fluid pressure and the absorbance spectrum of the analyte was obtained by taking differential measurements between the analyte and reference medium. An advantage is that this method doesn't need a complicated fabrication step. It is compatible with conventional microfluidic chips and measurements can be carried out on a normal transmission microscope. The performance of the device was tested by measuring solutions containing methylene blue, with concentrations as low as 13 μM.

Self-regulated, droplet-based sample chopper for microfluidic absorbance detection

Akin to optical beam chopping, we demonstrate that formation and routing of aqueous droplets in oil can chop a fluidic sample to permit phase sensitive detection. This hand-operated microfluidic sample chopper (μChopper) greatly reduces the detection limit of molecular absorbance in a 27 μm optical path. With direct dependence on path length, absorbance is fundamentally incompatible with microfluidics. While other microfluidic absorbance approaches use complex additions to fabrication, such as fiber coupling and increased optical paths, this self-regulated μChopper uses opposing droplet generators to passively alternate sample and reference droplets at ~10 Hz each. Each droplet's identity is automatically locked-in to its generator, allowing downstream lock-in analysis to nearly eliminate large signal drift or 1/f noise. With a lock-in time constant of 1.9 s and total interrogated volume of 59 nL (122 droplets), a detection limit of 3.0 × 10(-4) absorbance units or 500 nM bromophenol blue (BPB) (29 fmol) was achieved using only an optical microscope and a standard, single-depth (27 μm) microfluidic device. The system was further applied to nanoliter pH sensing and validated with a spectrophotometer. The μChopper represents a fluidic analog to an optical beam chopper, and the self-regulated sample/reference droplet alternation promotes ease of use.