Microfluidic droplet-based liquid-liquid extraction and on-chip IR spectroscopy detection of cocaine in human saliva

Development and future of droplet microfluidics

Over the past two decades, advances in droplet-based microfluidics have facilitated new approaches to process and analyze samples with unprecedented levels of precision and throughput. A wide variety of applications has been inspired across multiple disciplines ranging from materials science to biology. Understanding the dynamics of droplets enables optimization of microfluidic operations and design of new techniques tailored to emerging demands. In this review, we discuss the underlying physics behind high-throughput generation and manipulation of droplets. We also summarize the applications in droplet-derived materials and droplet-based lab-on-a-chip biotechnology. In addition, we offer perspectives on future directions to realize wider use of droplet microfluidics in industrial production and biomedical analyses.

Saliva Analysis Based on Microfluidics: Focusing the Wide Spectrum of Target Analyte

Saliva is one of the most critical human body fluids that can reflect the state of the human body. The detection of saliva is of great significance for disease diagnosis and health monitoring. Microfluidics, characterized by microscale size and high integration, is an ideal platform for the development of rapid and low-cost disease diagnostic techniques and devices. Microfluidic-based saliva testing methods have aroused considerable interest due to the increasing need for noninvasive testing and frequent or long-term testing. This review briefly described the significance of saliva analysis and generally classified the targets in saliva detection into pathogenic microorganisms, inorganic substances, and organic substances. By using this classification as a benchmark, the state-of-the-art research results on microfluidic detection of various substances in saliva were summarized. This work also put forward the challenges and future development directions of microfluidic detection methods for saliva.

Microfluidic nanodevices for drug sensing and screening applications

The outbreak of pandemics (e.g., severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2 in 2019), influenza A viruses (H1N1 in 2009), etc.), and worldwide spike in the aging population have created unprecedented urgency for developing new drugs to improve disease treatment. As a result, extensive efforts have been made to design novel techniques for efficient drug monitoring and screening, which form the backbone of drug development. Compared to traditional techniques, microfluidics-based platforms have emerged as promising alternatives for high-throughput drug screening due to their inherent miniaturization characteristics, low sample consumption, integration, and compatibility with diverse analytical strategies. Moreover, the microfluidic-based models utilizing human cells to produce in-vitro biomimetics of the human body pave new ways to predict more accurate drug effects in humans. This review provides a comprehensive summary of different microfluidics-based drug sensing and screening strategies and briefly discusses their advantages. Most importantly, an in-depth outlook of the commonly used detection techniques integrated with microfluidic chips for highly sensitive drug screening is provided. Then, the influence of critical parameters such as sensing materials and microfluidic platform geometries on screening performance is summarized. This review also outlines the recent applications of microfluidic approaches for screening therapeutic and illicit drugs. Moreover, the current challenges and the future perspective of this research field is elaborately highlighted, which we believe will contribute immensely towards significant achievements in all aspects of drug development.

Advanced Waveguide Based LOC Biosensors: A Minireview

This mini review features contemporary advances in mid-infrared (MIR) thin-film waveguide technology and on-chip photonics, promoting high-performance biosensing platforms. Supported by recent developments in MIR thin-film waveguides, it is expected that label-free assimilated MIR sensing platforms will soon supplement the current sensing technologies for biomedical diagnostics. The state-of-the-art shows that various types of waveguide material can be utilized for waveguide spectroscopic measurements in MIR. However, there are challenges to integrating these waveguide platforms with microfluidic/Lab-on-a-Chip (LOC) devices, due to poor light-material interactions. Graphene and its analogs have found many applications in microfluidic-based LOC devices, to address to this issue. Graphene-based materials possess a high conductivity, a large surface-to-volume ratio, a smaller and tunable bandgap, and allow easier sample loading; which is essential for acquiring precise electrochemical information. This work discusses advanced waveguide materials, their advantages, and disease diagnostics with MIR thin-film based waveguides. The incorporation of graphene into waveguides improves the light-graphene interaction, and photonic devices greatly benefit from graphene's strong field-controlled optical response.

Raman, Infrared and Brillouin Spectroscopies of Biofluids for Medical Diagnostics and for Detection of Biomarkers

This review surveys Infrared, Raman/SERS and Brillouin spectroscopies for medical diagnostics and detection of biomarkers in biofluids, that include urine, blood, saliva and other biofluids. These optical sensing techniques are non-contact, noninvasive and relatively rapid, accurate, label-free and affordable. However, those techniques still have to overcome some challenges to be widely adopted in routine clinical diagnostics. This review summarizes and provides insights on recent advancements in research within the field of vibrational spectroscopy for medical diagnostics and its use in detection of many health conditions such as kidney injury, cancers, cardiovascular and infectious diseases. The six comprehensive tables in the review and four tables in supplementary information summarize a few dozen experimental papers in terms of such analytical parameters as limit of detection, range, diagnostic sensitivity and specificity, and other figures of merits. Critical comparison between SERS and FTIR methods of analysis reveals that on average the reported sensitivity for biomarkers in biofluids for SERS vs FTIR is about 103 to 105 times higher, since LOD SERS are lower than LOD FTIR by about this factor. High sensitivity gives SERS an edge in detection of many biomarkers present in biofluids at low concentration (nM and sub nM), which can be particularly advantageous for example in early diagnostics of cancer or viral infections.HighlightsRaman, Infrared spectroscopies use low volume of biofluidic samples, little sample preparation, fast time of analysis and relatively inexpensive instrumentation.Applications of SERS may be a bit more complicated than applications of FTIR (e.g., limited shelf life for nanoparticles and substrates, etc.), but this can be generously compensated by much higher (by several order of magnitude) sensitivity in comparison to FTIR.High sensitivity makes SERS a noninvasive analytical method of choice for detection, quantification and diagnostics of many health conditions, metabolites, and drugs, particularly in diagnostics of cancer, including diagnostics of its early stages.FTIR, particularly ATR-FTIR can be a method of choice for efficient sensing of many biomarkers, present in urine, blood and other biofluids at sufficiently high concentrations (mM and even a few µM)Brillouin scattering spectroscopy detecting visco-elastic properties of probed liquid medium, may also find application in clinical analysis of some biofluids, such as cerebrospinal fluid and urine.

Ex-situ and in-situ rapid and quantitative determination of benzene derivatives in seawater using nanoconfined liquid phase nanoextraction

Benzene derivatives (BDs) constitute a class of environmental pollutants whose exposure poses a grave risk to human health. These compounds rapidly diffuse from the atmosphere to the marine ecosystem: for this reason, their monitoring in seawater is every day more compelling. In this work, nanoconfined liquid phase nanoextraction (NLPNE), a versatile extraction technique recently described, has been for the first time applied to the gas chromatographic mass spectrometry (GC/MS) analysis of BDs in seawater. Ex-situ and in-situ NLPNE procedures have been developed and optimized in terms of extraction capabilities, analysis time, precision, and accuracy. Compared to the traditional extraction procedures, based on solid-phase microextraction (SPME) and liquid-liquid extraction (LLE), the proposed NLPNE methods allowed a rapid on-site analysis of benzene compounds with low solvent consumption, higher enrichment factors, and improved automation grade. Determination coefficients ranging from 0.9929 to 0.9997 were obtained for all BDs in the range 0.10-500 ng mL^-1 and 5.00-500 ng mL^-1, for ex-situ and in-situ NLPNE, respectively. Ex-situ and in-situ limits of detection ranged from 0.2 to 7.6 ng mL^-1 and 0.04-1.00 ng mL^-1. Our results suggest that NLPNE coupled to GC-MS can be considered a powerful technique for high-throughput analyses of trace compounds in environmental, food and biological samples.

Mid-IR sensing platform for trace analysis in aqueous solutions based on a germanium-on-silicon waveguide chip with a mesoporous silica coating for analyte enrichment

A novel platform based on evanescent wave sensing in the 6.5 to 7.5 µm wavelength range is presented with the example of toluene detection in an aqueous solution. The overall sensing platform consists of a germanium-on-silicon waveguide with a functionalized mesoporous silica cladding and integrated microlenses for alignment-tolerant back-side optical interfacing with a tunable laser spectrometer. Hydrophobic functionalization of the mesoporous cladding allows enrichment of apolar analyte molecules and prevents strong interaction of water with the evanescent wave. The sensing performance was evaluated for aqueous toluene standards resulting in a limit of detection of 7 ppm. Recorded adsorption/desorption profiles followed Freundlich adsorption isotherms with rapid equilibration and resulting sensor response times of a few seconds. This indicates that continuous monitoring of contaminants in water is possible. A significant increase in LOD can be expected by likely improvements to the spectrometer noise floor which, expressed as a relative standard deviation of 100% lines, is currently in the range of 10^-2A.U.

Perspective on Thin Film Waveguides for on-Chip Mid-Infrared Spectroscopy of Liquid Biochemical Analytes

Miniaturized spectrometers offering low cost, low reagent consumption, high throughput, sensitivity and automation are the future of sensing and have significant applications in environmental monitoring, food safety, biotechnology, pharmaceuticals, and healthcare. Midinfrared (MIR) spectroscopy employing complementary metal oxide semiconductor (CMOS) compatible thin film waveguides and microfluidics shows great promise toward highly integrated and robust detection tools and liquid handling. This perspective provides an overview of the emergence of thin film optical waveguides used for evanescent field sensing of liquid chemical and biological samples for MIR absorption spectroscopy. The state of the art of new material and waveguide systems used for spectroscopic measurements in the MIR is presented. An outlook on the advantages and future of waveguide-based MIR spectroscopy for application in clinical settings for point-of-care biochemical analysis is discussed.

Spectroelectrochemistry, the future of visualizing electrode processes by hyphenating electrochemistry with spectroscopic techniques

Reviewing the future of electrochemistry combined with infrared, Raman, and nuclear magnetic resonance spectroscopy as well as mass spectrometry.

A Bottom-Up Approach for Developing Aptasensors for Abused Drugs: Biosensors in Forensics

Aptamer-based point-of-care (POC) diagnostics platforms may be of substantial benefit in forensic analysis as they provide rapid, sensitive, user-friendly, and selective analysis tools for detection. Aptasensors have not yet been adapted commercially. However, the significance of the applications of aptasensors in the literature exceeded their potential. Herein, in this review, a bottom-up approach is followed to describe the aptasensor development and application procedure, starting from the synthesis of the corresponding aptamer sequence for the selected analyte to creating a smart surface for the sensitive detection of the molecule of interest. Optical and electrochemical biosensing platforms, which are designed with aptamers as recognition molecules, detecting abused drugs are critically reviewed, and existing and possible applications of different designs are discussed. Several potential disciplines in which aptamer-based biosensing technology can be of greatest value, including forensic drug analysis and biological evidence, are then highlighted to encourage researchers to focus on developing aptasensors in these specific areas.

Biomedical applications of mid-infrared quantum cascade lasers - a review

Mid-infrared spectroscopy has been applied to research in biology and medicine for more than 20 years and conceivable applications have been identified. More recently, these applications have been shown to benefit from the use of quantum cascade lasers due to their specific properties, namely high spectral power density, small beam parameter product, narrow emission spectrum and, if needed, tuning capabilities. This review provides an overview of the achievements and illustrates some applications which benefit from the key characteristics of quantum cascade laser-based mid-infrared spectroscopy using examples such as breath analysis, the investigation of serum, non-invasive glucose monitoring in bulk tissue and the combination of spectroscopy and microscopy of tissue thin sections for rapid histopathology.

A porous organic polymer with magnetic nanoparticles on a chip array for preconcentration of platinum(IV), gold(III) and bismuth(III) prior to their on-line quantitation by ICP-MS

A chip-based array is described for magnetic solid-phase microextraction (MSPME) of the ions of Pt, Au and Bi. Magnetic porous organic polymers (MOPs) prepared from magnetite nanoparticles and 1,3,5-tris(4-aminophenyl)benzene are introduced as a novel adsorbent. Eight solid phase extraction columns packed with MOPs were integrated in parallel on a microfluidic chip for array microextraction. After elution with a 12% (m/v) solution of cysteamine hydrochloride (pH 8.0), the eluent is introduced into an ICP-MS instrument for quantification. Under the optimized conditions, the limits of detection for Pt, Au and Bi are 8.6, 4.4 and 3.4 ng L^-1, respectively. The sample throughput is 7 h^-1, and the adsorption capacities are 32, 24 and 24 μg mg^-1 for Pt, Au and Bi, respectively. The method was validated by the determination of Bi in a certified reference material (GSH-1A; human hair), and the values obtained coincided with the certified value. This method was also applied to the determination of Pt, Au and Bi in (spiked) urine and cell samples, and good recoveries (85.8-113%) were achieved. The method is highly sensitive and has a high throughput and a low sample/reagent consumption (with 500 HeLa cells consumed). Graphical abstract Schematic presentation of the magnetic packed column, microfluidic chip, and online chip-based MSPME-ICPMS system. Design sketch of the online system: microextraction unit (blue lines), microvalves (black lines), outlet channels (yellow lines), permanent magnets (red), urine and cell samples.

Local surface modification at precise position using a chemical pen

Push-pull cannula system, which was first proposed by Gaddum, has grown to be an important method for the perfusion of brain and region-selective surface treatment. However, reported push-pull cannula systems only concerned on single reagent applications. Microfluidic system was then an exciting tool for multi-reagent treatment on substrate in closed microchannels. Nowadays, it is still a challenge to apply online mixing and reaction for surface pattern in an open environment. Here, we present a novel method using a chemical pen that enables region-selective online chemical reactions for the micro-surface modification/patterning. We utilized this method to fabricate labeling protein array using an online labeling strategy. Moreover, the device was applied for local modification of biomaterials surface by using a three-component reaction at precise position. This tool was the first demonstration of design to perform online reaction of two different reagents on a real solid sample in an open environment. It was demonstrated a useful method for protein array fabrication with online labeled protein.

Facile Design of Phase Separation for Microfluidic Droplet-Based Liquid Phase Microextraction as a Front End to Electrothermal Vaporization-ICPMS for the Analysis of Trace Metals in Cells

The issue of quantifying trace metals in cells has drawn widespread attention but is threatened with insufficient sensitivity of the instruments, complex cellular matrix and limited cell consumption. In this study, microfluidic droplet-based liquid phase microextraction (LPME), as a miniaturized platform, was developed and combined with electrothermal vaporization (ETV)-inductively coupled plasma mass spectrometry (ICPMS) for the analysis of trace Cd, Hg, Pb, and Bi in cells. A novel and facile design of phase separation region was proposed, which made the phase separation very easily for subsequent ETV-ICPMS detection. Mechanism of the phase separation was carefully discussed using the incompressible formulation of the Navier-Stokes equations. The developed microfluidic droplet-based LPME system exhibited much higher extraction efficiency to target metals than microfluidic stratified flow-based LPME. Under the optimized conditions, the limits of detection of the proposed microfluidic droplet-based LPME-ETV-ICPMS system were 2.5, 3.9, 5.5, and 3.4 ng L^-1 for Cd, Hg, Pb, and Bi, respectively. The accuracy of the developed method was well validated by analyzing the target metals in Certified Reference Materials of GBW07601a human hair. Finally, the proposed method was successfully applied to the analysis of target metals in HeLa and HepG2 cells with the recoveries for the spiked samples ranging from 83.5 to 112.3%. Overall, the proposed design is a simple and reliable solution for the phase separation on droplet-chip and the microfluidic droplet-based LPME-ETV-ICPMS combination strategy shows great promise for trace elements analysis in cells.

Salivary Exosome and Cell-Free DNA for Cancer Detection

Liquid biopsies are easier to acquire patient derived samples than conventional tissue biopsies, and their use enables real-time monitoring of the disease through continuous sampling after initial diagnosis, resulting in a paradigm shift to customized treatment according to the patient’s prognosis. Among the various liquid biopsy samples, saliva is easily obtained by spitting or swab sucking without needing an expert for sample collection. In addition, it is known that disease related biomarkers that exist in the blood and have undergone extensive research exist in saliva even at a lower concentration than the blood. Thus, interest in the use of saliva as a liquid biopsy has increased. In this review, we focused on the salivary exosome and cell-free DNA (cfDNA) among the various biomarkers in saliva. Since the exosome and cfDNA in saliva are present at lower concentrations than the biomarkers in blood, it is important to separate and concentrate them before conducting down-stream analyses such as exosome cargo analysis, quantitative polymerase chain reaction (qPCR), and sequencing. However, saliva is difficult to apply directly to microfluidics-based systems for separation because of its high viscosity and the presence of various foreign substances. Therefore, we reviewed the microfluidics-based saliva pretreatment method and then compared the commercially available kit and the microfluidic chip for isolation and enrichment of the exosome and cfDNA in saliva.

Simple, low-cost fabrication of acrylic based droplet microfluidics and its use to generate DNA-coated particles

Hydrogel microparticles were copolymerized with surface-immobilized DNA. Particles derived from a microfluidic device and particles derived from mechanical homogenization were compared. The hypothesis was tested that a controlled droplet generation mechanism would produce more homogeneous particles. Surprisingly, the DNA content of both particle types was similarly inhomogeneous. To make this test possible, a simple, low cost, and rapid method was developed to fabricate a microfluidic chip for droplet generation and in-line polymerization. This method used a low-cost laser cutter ($400) and direct heat bonding (no adhesives or intermediate layers). The flow focusing droplet generator produced droplets and hydrogel particles 10-200 μm in diameter.

Electrified Soft Interface as a Selective Sensor for Cocaine Detection in Street Samples

A straightforward, direct, and selective method is presented for electrochemical cocaine identification in street samples. The sensing mechanism is based on a simple ion transfer reaction across the polarized liquid–liquid interface. The interfacial behavior of a number of cutting agents is also reported. Interfacial miniaturization has led to improved electroanalytical properties of the liquid–liquid interface based sensor as compared with the macroscopic analogue. The reported method holds great potential to replace colorimetric tests with poor selectivity for on-site street sample analysis.

Infrared spectroscopy based on broadly tunable quantum cascade lasers and polycrystalline diamond waveguides

Recently emerging broadly tunable quantum cascade lasers (tQCL) emitting in the mid-infrared (MIR) are a versatile alternative to well established thermal emitters in combination with interferometers as applied in Fourier transform infrared (FTIR) spectroscopy.

10 nm deep, sub-nanoliter fluidic nanochannels on germanium for attenuated total reflection infrared (ATR-IR) spectroscopy

Nanoslits with a depth of ∼10 nm were manufactured on a germanium internal reflection element for attenuated internal reflection infrared spectroscopy.

Quantum cascade lasers (QCLs) in biomedical spectroscopy

This review focuses on the recent applications of QCLs in mid-IR spectroscopy of clinically relevant samples.

Advances in Mid-Infrared Spectroscopy for Chemical Analysis

Infrared spectroscopy in the 3-20 μm spectral window has evolved from a routine laboratory technique into a state-of-the-art spectroscopy and sensing tool by benefitting from recent progress in increasingly sophisticated spectra acquisition techniques and advanced materials for generating, guiding, and detecting mid-infrared (MIR) radiation. Today, MIR spectroscopy provides molecular information with trace to ultratrace sensitivity, fast data acquisition rates, and high spectral resolution catering to demanding applications in bioanalytics, for example, and to improved routine analysis. In addition to advances in miniaturized device technology without sacrificing analytical performance, selected innovative applications for MIR spectroscopy ranging from process analysis to biotechnology and medical diagnostics are highlighted in this review.

Capillary electrophoresis for automated on-line monitoring of suspension cultures: Correlating cell density, nutrients and metabolites in near real-time

Increasingly stringent demands on the production of biopharmaceuticals demand monitoring of process parameters that impact on their quality. We developed an automated platform for on-line, near real-time monitoring of suspension cultures by integrating microfluidic components for cell counting and filtration with a high-resolution separation technique. This enabled the correlation of the growth of a human lymphocyte cell line with changes in the essential metabolic markers, glucose, glutamine, leucine/isoleucine and lactate, determined by Sequential Injection-Capillary Electrophoresis (SI-CE). Using 8.1 mL of media (41 μL per run), the metabolic status and cell density were recorded every 30 min over 4 days. The presented platform is flexible, simple and automated and allows for fast, robust and sensitive analysis with low sample consumption and high sample throughput. It is compatible with up- and out-scaling, and as such provides a promising new solution to meet the future demands in process monitoring in the biopharmaceutical industry.

Community Sewage Sensors towards Evaluation of Drug Use Trends: Detection of Cocaine in Wastewater with DNA-Directed Immobilization Aptamer Sensors

Illicit drug use has a global concern and effective monitoring and interventions are highly required to combat drug abuse. Wastewater-based epidemiology (WBE) is an innovative and cost-effective approach to evaluate community-wide drug use trends, compared to traditional population surveys. Here we report for the first time, a novel quantitative community sewage sensor (namely DNA-directed immobilization of aptamer sensors, DDIAS) for rapid and cost-effective estimation of cocaine use trends via WBE. Thiolated single-stranded DNA (ssDNA) probe was hybridized with aptamer ssDNA in solution, followed by co-immobilization with 6-mercapto-hexane onto the gold electrodes to control the surface density to effectively bind with cocaine. DDIAS was optimized to detect cocaine at as low as 10 nM with a dynamic range from 10 nM to 5 μM, which were further employed for the quantification of cocaine in wastewater samples collected from a wastewater treatment plant in seven consecutive days. The concentration pattern of the sampling week is comparable with that from mass spectrometry. Our results demonstrate that the developed DDIAS can be used as community sewage sensors for rapid and cost-effective evaluation of drug use trends, and potentially implemented as a powerful tool for on-site and real-time monitoring of wastewater by un-skilled personnel.

Selective extraction and determination of fluoroquinolones in bovine milk samples with montmorillonite magnetic molecularly imprinted polymers and capillary electrophoresis

A sensitive and selective method for separating fluoroquinolones (FQs) from bovine milk samples was successfully developed using montmorillonite magnetic molecularly imprinted polymers (MMMIPs) as adsorbents. MMMIPs were prepared using montmorillonite as carrier, fleroxacin (FLE) as template molecule, and Fe3O4 magnetite as magnetic component. MMMIPs possessed high adsorption capacity of 46.3 mg g(-1) for FLE. A rapid and convenient magnetic solid-phase extraction procedure coupled with capillary electrophoresis was established with MMMIPs as adsorbents for simultaneous and selective extraction of four FQs in bovine milk samples. Limits of detection ranged between 12.9 and 18.8 μg L(-1), and the RSDs were between 1.8% and 8.6%. The proposed method was successfully applied to spike bovine milk samples with recoveries of 92.7%-108.6%.

The enhancement of liquid–liquid extraction with high phase ratio by microfluidic-based hollow droplet

We developed a novel method to enhance the liquid–liquid extraction by a microfluidic-based hollow droplet structure. A one-step microfluidic device is used for the generation of gas-in-oil-in-water double emulsions.

An overview of the use of microchips in electrophoretic separation techniques: fabrication, separation modes, sample preparation opportunities, and on-chip detection

This chapter is intended as a basic introduction to microchip-based capillary electrophoresis to set the scene for newcomers and give pointers to reference material. An outline of some commonly used setups and key concepts is given, many of which are explored in greater depth in later chapters.

Electrokinetics for sample preparation of biological molecules in biological samples using microfluidic systems

Sample preparation is the first part of every analytical method, but is often considered only after the optimization of the method. It is traditionally performed using a range of techniques requiring extensive manual handling, with solid-phase extraction, liquid–liquid extraction, protein precipitation and ultracentrfiguation, among others, being used depending on the targets and the application. In this article, we will focus on alternatives based on electrokinetics for applications including sample clean-up, concentration and derivatization of large biological molecules (DNA, peptides and proteins) of diagnostic importance, as well as small molecules as a tool for therapeutic drug monitoring. This article describes these approaches in terms of mechanisms, applicability and potential to be integrated into a lab-on-a-chip device for directly processing biological samples. Examples dealing with treated or clean samples have been excluded except where they show exceptionally high value.