Integrated microfluidic UV absorbance detector with attomol-level sensitivity for BSA

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.

Label-Free Physical Techniques and Methodologies for Proteins Detection in Microfluidic Biosensor Structures

Proteins in biological fluids (blood, urine, cerebrospinal fluid) are important biomarkers of various pathological conditions. Protein biomarkers detection and quantification have been proven to be an indispensable diagnostic tool in clinical practice. There is a growing tendency towards using portable diagnostic biosensor devices for point-of-care (POC) analysis based on microfluidic technology as an alternative to conventional laboratory protein assays. In contrast to universally accepted analytical methods involving protein labeling, label-free approaches often allow the development of biosensors with minimal requirements for sample preparation by omitting expensive labelling reagents. The aim of the present work is to review the variety of physical label-free techniques of protein detection and characterization which are suitable for application in micro-fluidic structures and analyze the technological and material aspects of label-free biosensors that implement these methods. The most widely used optical and impedance spectroscopy techniques: absorption, fluorescence, surface plasmon resonance, Raman scattering, and interferometry, as well as new trends in photonics are reviewed. The challenges of materials selection, surfaces tailoring in microfluidic structures, and enhancement of the sensitivity and miniaturization of biosensor systems are discussed. The review provides an overview for current advances and future trends in microfluidics integrated technologies for label-free protein biomarkers detection and discusses existing challenges and a way towards novel solutions.

Broad-Band Spectrum, High-Sensitivity Absorbance Spectroscopy in Picoliter Volumes

Picoliter-volume droplets within segmented flows can be probed in a rapid and efficient manner using optical detection methods. To date, however, most detection schemes for droplet content analysis have relied on the use of time-integrated fluorescence measurements. Despite its undoubted utility, the implementation of absorbance-based detectors is particularly challenging due to the reduced optical path lengths that are characteristic of microfluidic systems and deleterious scattering at droplet-oil interfaces. Unsurprisingly, efforts to develop sensitive absorbance-based detection schemes for the interrogation of rapidly moving droplets have primarily focused on ensuring adequate analytical sensitivity and, to date, have been exclusively limited to single-wavelength measurements. To address this limitation, and expand the information content associated with absorbance measurements on-chip, we herein describe a detection scheme for the extraction of broad-band absorbance spectra from pL-volume droplets with high sensitivity. The combination of a confocal optical system (that confines incident light to a reduced detection volume) and a postprocessing algorithm (that effectively removes the contribution of the carrier oil from the extracted spectra) engenders significant improvements in signal-to-noise ratios. Our system is initially calibrated by acquiring absorbance spectra from aqueous solutions of fluorescein isothiocyanate. These measurements confirm both excellent linearity over the studied range (from 0 to 100 μM) and a concentration limit of detection of 800 nM. The methodology is then used to monitor the salt-induced aggregation of gold nanoparticles with millisecond time resolution. This approach for small-volume absorbance spectroscopy allows for both high-throughput and high-information content measurements in subnanoliter volumes and will be highly desirable in a wide variety of bioanalytical applications where sensitivity and throughput are priorities.

High-throughput screening for high-efficiency small-molecule biosynthesis

Systems metabolic engineering faces the formidable task of rewiring microbial metabolism to cost-effectively generate high-value molecules from a variety of inexpensive feedstocks for many different applications. Because these cellular systems are still too complex to model accurately, vast collections of engineered organism variants must be systematically created and evaluated through an enormous trial-and-error process in order to identify a manufacturing-ready strain. The high-throughput screening of strains to optimize their scalable manufacturing potential requires execution of many carefully controlled, parallel, miniature fermentations, followed by high-precision analysis of the resulting complex mixtures. This review discusses strategies for the design of high-throughput, small-scale fermentation models to predict improved strain performance at large commercial scale. Established and promising approaches from industrial and academic groups are presented for both cell culture and analysis, with primary focus on microplate- and microfluidics-based screening systems.

Continuous Online Protein Quality Monitoring during Perfusion Culture Production Using an Integrated Micro/Nanofluidic System

We demonstrate a new micro/nanofluidic system for continuous and automatic monitoring of protein product size and quantity directly from the culture supernatant during a high-cell-concentration CHO cell perfusion culture. A microfluidic device enables clog-free cell retention for a bench-scale (350 mL) perfusion bioreactor that continuously produces the culture supernatant containing monoclonal antibodies (IgG₁). A nanofluidic device directly monitors the protein size and quantity in the culture supernatant. The continuous-flow and fully automated operation of this nanofluidic protein analytics reduces design complexity and offers more detailed information on protein products than offline and batch-mode conventional analytics. Moreover, chemical and mechanical robustness of the nanofluidic device enables continuous monitoring for several days to a week. This continuous and online protein quality monitoring could be deployed at different steps and scales of biomanufacturing to improve product quality and manufacturing efficiency.

Monitoring the endocytosis of bovine serum albumin based on the fluorescence lifetime of small squaraine dye in living cells

Bovine serum albumin (BSA) has a wide range of physiological functions involving the binding, transportation, and delivery of fatty acids, porphyrins, bilirubin, steroids, etc. In the present study, we prepared a small squaraine dye (SD), which can selectively detect BSA using fluorescence lifetime imaging microscopy (FLIM), to monitor the endocytosis of BSA in live cultured cells in real time. This approach revealed that BSA uptake is concentration-dependent in living cells. Furthermore, we used paclitaxel (PTX), a chemotherapeutic drug, to influence the endocytosis of BSA in living cells. The results demonstrated that the endocytic rate was clearly reduced after pretreatment with 0.4 µM PTX for 2 h. The present study demonstrates the potential value of using the fluorescence lifetime of SD to detect BSA concentration and study the physiological mechanism of chemotherapeutic drugs.

A graphene/Si Schottky diode for the highly sensitive detection of protein

Herein, a graphene/Si-based device was introduced for bovine serum albumin (BSA) sensing.

Real-Time Intrinsic Fluorescence Visualization and Sizing of Proteins and Protein Complexes in Microfluidic Devices

Optical detection has become a convenient and scalable approach to read out information from microfluidic systems. For the study of many key biomolecules, however, including peptides and proteins, which have low fluorescence emission efficiencies at visible wavelengths, this approach typically requires labeling of the species of interest with extrinsic fluorophores to enhance the optical signal obtained - a process which can be time-consuming, requires purification steps, and has the propensity to perturb the behavior of the systems under study due to interactions between the labels and the analyte molecules. As such, the exploitation of the intrinsic fluorescence of protein molecules in the UV range of the electromagnetic spectrum is an attractive path to allow the study of unlabeled proteins. However, direct visualization using 280 nm excitation in microfluidic devices has to date commonly required the use of coherent sources with frequency multipliers and devices fabricated out of materials that are incompatible with soft lithography techniques. Here, we have developed a simple, robust, and cost-effective 280 nm LED platform that allows real-time visualization of intrinsic fluorescence from both unlabeled proteins and protein complexes in polydimethylsiloxane microfluidic channels fabricated through soft lithography. Using this platform, we demonstrate intrinsic fluorescence visualization of proteins at nanomolar concentrations on chip and combine visualization with micron-scale diffusional sizing to measure the hydrodynamic radii of individual proteins and protein complexes under their native conditions in solution in a label-free manner.

Time-lapse contact microscopy of cell cultures based on non-coherent illumination

Video microscopy offers outstanding capabilities to investigate the dynamics of biological and pathological mechanisms in optimal culture conditions. Contact imaging is one of the simplest imaging architectures to digitally record images of cells due to the absence of any objective between the sample and the image sensor. However, in the framework of in-line holography, other optical components, e.g., an optical filter or a pinhole, are placed underneath the light source in order to illuminate the cells with a coherent or quasi-coherent incident light. In this study, we demonstrate that contact imaging with an incident light of both limited temporal and spatial coherences can be achieved with sufficiently high quality for most applications in cell biology, including monitoring of cell sedimentation, rolling, adhesion, spreading, proliferation, motility, death and detachment. Patterns of cells were recorded at various distances between 0 and 1000 μm from the pixel array of the image sensors. Cells in suspension, just deposited or at mitosis focalise light into photonic nanojets which can be visualised by contact imaging. Light refraction by cells significantly varies during the adhesion process, the cell cycle and among the cell population in connection with every modification in the tridimensional morphology of a cell.

A selective, long-lived deep-red emissive ruthenium(II) polypyridine complexes for the detection of BSA

A selective, label free luminescence sensor for bovine serum albumin (BSA) is investigated using ruthenium(II) complexes over the other proteins. Interaction between BSA and ruthenium(II) complexes has been studied using absorption, emission, excited state lifetime and circular dichroism (CD) spectral techniques. The luminescence intensity of ruthenium(II) complexes (I and II), has enhanced at 602 and 613 nm with a large hypsochromic shift of 18 and 5 nm respectively upon addition of BSA. The mode of binding of ruthenium(II) complexes with BSA has analyzed using computational docking studies.

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.

FRET for lab-on-a-chip devices - current trends and future prospects

This review focuses on the use of Förster Resonance Energy Transfer (FRET) to monitor intra- and intermolecular reactions occurring in microfluidic reactors. Microfluidic devices have recently been used for performing highly efficient and miniaturised biological assays for the analysis of biological entities such as cells, proteins and nucleic acids. Microfluidic assays are characterised by nanolitre to femtolitre reaction volumes, which necessitates the adoption of a sensitive optical detection scheme. FRET serves as a strong 'spectroscopic ruler' for elucidating the tertiary structure of biomolecules, as the efficiency of the non-radiative energy transfer is extremely sensitive to nanoscale changes in the separation between donor and acceptor markers attached to the biomolecule of interest. In this review, we will review the implementation of various microfluidic assays which employ FRET for diverse applications in the biomedical field, along with the advantages and disadvantages of the various approaches. The future prospects for development of microfluidic devices incorporating FRET detection will be discussed.

Modular integration of electronics and microfluidic systems using flexible printed circuit boards

Microfluidic systems offer an attractive alternative to conventional wet chemical methods with benefits including reduced sample and reagent volumes, shorter reaction times, high-throughput, automation, and low cost. However, most present microfluidic systems rely on external means to analyze reaction products. This substantially adds to the size, complexity, and cost of the overall system. Electronic detection based on sub-millimetre size integrated circuits (ICs) has been demonstrated for a wide range of targets including nucleic and amino acids, but deployment of this technology to date has been limited due to the lack of a flexible process to integrate these chips within microfluidic devices. This paper presents a modular and inexpensive process to integrate ICs with microfluidic systems based on standard printed circuit board (PCB) technology to assemble the independently designed microfluidic and electronic components. The integrated system can accommodate multiple chips of different sizes bonded to glass or PDMS microfluidic systems. Since IC chips and flex PCB manufacturing and assembly are industry standards with low cost, the integrated system is economical for both laboratory and point-of-care settings.

Gold nanoparticle enlargement coupled with fluorescence quenching for highly sensitive detection of analytes

We report a versatile and facile route for highly sensitive detection of analytes through coupling the enlargement of gold nanoparticles with fluorescence quenching. The fluorescence intensity of dye molecules (e.g., fluorescein or rhodamine B) significantly decreased with the increasing concentration of reducing agents, such as hydrogen peroxide and hydroquinone. The sensitivity for the detection of reducing agents was much higher than that of other methods based on the absorbance measurement of enlarged gold nanoparticles or quantum-dot-enzyme hybridization. We could successfully detect acetylthiocholine with the detection limit of several nanomolar concentration using an enzymatic reaction by acetylcholine esterase, a key route for the detection of toxic organophosphate compounds. The fluorescence quenching approach described in this report requires only a simple addition of fluorescence dye to the reaction solution without any chemical modification. The strategy of fluorescence quenching coupled with nanoparticle growth would provide a new horizon for the development of highly sensitive optical biosensors.

Determination of ciprofloxacin with functionalized cadmium sulfide nanoparticles as a fluorescence probe

A novel assay of ciprofloxacin with a sensitivity at the microgram level is proposed based on the measurement of enhanced fluorescence intensity signals resulting from the interaction of functionalized nano-CdS with ciprofloxacin. The CdS nanoparticles was synthesized by thioacetamide (TAA) and cadmium nitrate (Cd(NO(3))(2)) in the alkaline solution. At pH 7.4, the fluorescence signals of functionalized nano-CdS were greatly enhanced by ciprofloxacin with the increase concentration of ciprofloxacin. Linear relationship can be established between the enhanced fluorescence intensity and ciprofloxacin concentration in the range of (1.25-8.75)x10(-4) mg mL(-1) ((3.77-26.4) x 10(-4)mmol L(-1)) or (8.75-1200) x 10(-4)mg mL(-1) ((26.4-3625) x 10(-4) mmol L(-1)). The limit of detection is 7.64 x 10(-6) mg mL(-1) (2.31 x 10(-5)mmol L(-1)). Based on this, a new direct quantitative determination method for ciprofloxacin in human serum samples without separation of foreign substances was established. The contents of ciprofloxacin in human serum samples were determined with recovery of 95-105% and relative standard deviation (R.S.D.) of 1.5-2.5%. This method was proved to be very sensitive, rapid, simple and tolerance of most interfering substances.

Novel detection schemes of nuclear magnetic resonance and magnetic resonance imaging: applications from analytical chemistry to molecular sensors

Nuclear magnetic resonance (NMR) is a well-established analytical technique in chemistry. The ability to precisely control the nuclear spin interactions that give rise to the NMR phenomenon has led to revolutionary advances in fields as diverse as protein structure determination and medical diagnosis. Here, we discuss methods for increasing the sensitivity of magnetic resonance experiments, moving away from the paradigm of traditional NMR by separating the encoding and detection steps of the experiment. This added flexibility allows for diverse applications ranging from lab-on-a-chip flow imaging and biological sensors to optical detection of magnetic resonance imaging at low magnetic fields. We aim to compare and discuss various approaches for a host of problems in material science, biology, and physics that differ from the high-field methods routinely used in analytical chemistry and medical imaging.

Fully integrated microfluidic separations systems for biochemical analysis

Over the past decade a tremendous amount of research has been performed using microfluidic analytical devices to detect over 200 different chemical species. Most of this work has involved substantial integration of fluid manipulation components such as separation channels, valves, and filters. This level of integration has enabled complex sample processing on miniscule sample volumes. Such devices have also demonstrated high throughput, sensitivity, and separation performance. Although the miniaturization of fluidics has been highly valuable, these devices typically rely on conventional ancillary equipment such as power supplies, detection systems, and pumps for operation. This auxiliary equipment prevents the full realization of a "lab-on-a-chip" device with complete portability, autonomous operation, and low cost. Integration and/or miniaturization of ancillary components would dramatically increase the capability and impact of microfluidic separations systems. This review describes recent efforts to incorporate auxiliary equipment either as miniaturized plug-in modules or directly fabricated into the microfluidic device.

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.

A hybrid microdevice with a thin PDMS membrane on the detection window for UV absorbance detection

We exploited a PDMS-quartz hybrid microchip with a thin PDMS membrane on the concave detection window for UV absorbance detection. The thickness of the PDMS membrane is about 100 mum, with high UV transmittance. As compared to a PDMS-quartz hybrid chip with a common detection window, the proposed one exhibited over an one order of magnitude sensitivity enhancement, and an about two orders of magnitude S/N increase for gastrodin (p-hydroxymethylphenyl-beta-D-glucopyranoside). In addition, the limit of the detection wavelength has been extended from 240 to 210 nm, which is otherwise impossible for a traditional PDMS-quartz hybrid microchip. This kind of microchip has the potential for a large range of applications in an integrated microfluidic system with UV detection.

Time-of-flight flow imaging of two-component flow inside a microfluidic chip

Here we report on using NMR imaging and spectroscopy in conjunction with time-of-flight tracking to noninvasively tag and monitor nuclear spins as they flow through the channels of a microfluidic chip. Any species with resolvable chemical-shift signatures can be separately monitored in a single experiment, irrespective of the optical properties of the fluids, thereby eliminating the need for foreign tracers. This is demonstrated on a chip with a mixing geometry in which two fluids converge from separate channels, and is generally applicable to any microfluidic device through which fluid flows within the nuclear spin-lattice relaxation time.

Integrated isotachophoretic preconcentration with zone electrophoresis separation on a quartz microchip for UV detection of flavonoids

A quartz microchip integrated isotachophoretic (ITP) preconcentration with zone electrophoresis (ZE) separation was fabricated using a novel multi-point pressure method featured in normal temperature and lower pressure during bonding process. ITP followed by subsequential ZE of two flavonoids, quercetin and isorhamnetin on the microchip was performed consecutively on the homemade microfluidic workstation with UV detection, resulting in a decreased detectable concentration of 32-fold, compared to the ZE mode only, and their detection limits decreased down to 0.2 microg/mL and 1.2 microg/mL, respectively.