Microfluidics and Raman microscopy: current applications and future challenges

Magnetic, Fluorescent, and Copolymeric Silicone Microspheres

Silicone microspheres are exceedingly difficult to make. Here, polydimethylsiloxane microspheres (≈1 μm diameter) are synthesized using ultrasonic spray pyrolysis, the first demonstration of a scalable synthetic procedure for crosslinked silicone microspheres. This continuous, aerosol process is also used to directly produce fluorescent, magnetic, and copolymeric derivatives; the potential biomedical applications of these microspheres are explored.

Composite Sensor Particles for Tuned SERS Sensing: Microfluidic Synthesis, Properties and Applications

Surface-enhanced Raman scattering (SERS) is a promising platform for particle-based sensor signaling, and droplet-based microfluidic systems are particularly advantageous for control of the size and composition of micro- and nanoparticles. For controlled sensing application, a high homogeneity of the sensor particles is a key requirement, and the particles with functional properties demand for the preparation in a minimum number of synthesis steps. Frequently used coflow and flow focusing arrangements, however, produce the microparticles of only larger size. To address such concern for downscaling of particle size, which is crucial for strong sensing outcome, we have used a peculiar micro cross-flow arrangement here for generating the polymer microparticles of broad size range between 30 and 600 μm along with in situ embedded silver nanoparticles. Embedded silver acts as nuclei for additional silver enforcement via silver-catalyzed silver deposition in order to realize the composite microparticles for SERS sensing. The homogeneous size and spatial distribution of silver nanoparticles inside the matrix and enforcement over the surface together with controlled pore size provides a high and homogeneous loading of polymer composite sensor. Moreover, different parameters such as analytes concentration and particles size have been studied here for SERS sensing application of biochemical molecules (amino acids and vitamins). Overall, the platform for size-tuned droplets generation, synthesis of composite microparticles, mechanism for synchronized photopolymerization-photoreduction, tuned silver enforcement, and the impacts of different analytes on differently composed microparticles are systematically investigated in this paper.

A multi-functional bubble-based microfluidic system

Recently, the bubble-based systems have offered a new paradigm in microfluidics. Gas bubbles are highly flexible, controllable and barely mix with liquids, and thus can be used for the creation of reconfigurable microfluidic systems. In this work, a hydrodynamically actuated bubble-based microfluidic system is introduced. This system enables the precise movement of air bubbles via axillary feeder channels to alter the geometry of the main channel and consequently the flow characteristics of the system. Mixing of neighbouring streams is demonstrated by oscillating the bubble at desired displacements and frequencies. Flow control is achieved by pushing the bubble to partially or fully close the main channel. Patterning of suspended particles is also demonstrated by creating a large bubble along the sidewalls. Rigorous analytical and numerical calculations are presented to describe the operation of the system. The examples presented in this paper highlight the versatility of the developed bubble-based actuator for a variety of applications; thus providing a vision that can be expanded for future highly reconfigurable microfluidics.

A new calibration concept for a reproducible quantitative detection based on SERS measurements in a microfluidic device demonstrated on the model analyte adenine

This study demonstrates a new concept of calibrating surface enhanced Raman scattering (SERS) intensities without using additional substances as an internal standard and explores factors such as laser fluctuation and different Ag substrates, which affect the results of quantitative analyses based on SERS. To demonstrate the capabilities of the concept, the model analyte adenine has been chosen. A lab-on-a-chip device is applied for the measurements to guarantee consistent data recording. In order to simulate varied measuring conditions, two varying silver colloids (batch 1 and 2) are utilized as a SERS substrate and two different laser power levels (25 or 55 mW) are applied on the sample. A concentration gradient was generated which allows the use of the analyte itself for the correction of the resulting SERS spectra regarding intensity deviations caused by different ambient conditions. In doing so, a vast improvement in the quantification using SERS, especially in view of the comparability, reproducibility and repeatability, is demonstrated.

Fabrication of plasmon length-based surface enhanced Raman scattering for multiplex detection on microfluidic device

The length of bioreceptors plays an important role in signal enhancement of surface-enhanced Raman scattering (SERS) due to amplification of electromagnetic fields generated by the excitation of localized surface plasmons. Herein, intact antibodies (IgG) and Fab fragments conjugated onto gold nanostar were used to fabricate two kinds of immunosensors for measurement of their SERS signals. Using CA125 as the antigen and Rhodamine-6G (R6G)-conjugated immunogolds, a SERS immunosensor was self-assembled by antigen-antibody interaction. The results showed that the SERS signal from the Fab immunosensor was 2.4 times higher than that of the IgG immunosensor. Furthermore, increased hot-spots by silver atom deposition onto the IgG and Fab immunosensor showed 2.1 and 1.4 times higher signals than before enhancement, respectively. For application, based on the Fab immunosensor, a SERS-compatible microfluidic system was designed for multiplex assays to overcome the drawbacks of conventional assays. This system can measure biological specimens directly from bio fluids instead of using a complex microfluidic device containing separation and detection elements. Four approved biomarkers of breast cancer, including cancer antigen (CA125), HER2, epididymis protein (HE4), and Eotaxin-1, were detected from patient-mimicked serum with limits of 15 fM, 17 fM, 21 fM, and 6.5 fM, respectively. The results indicated that the lengths and geometry of the bioreceptors determined the intensity of SERS signal from the interface and cavity of the sandwich immunosensor. Silver atom deposition at the cavity of the immunosensor increased the SERS signal. Finally, the SERS immunosensor built-in microfluidic system improved the performance of multiplex diagnostics.

Surface enhanced Raman spectroscopy detection of biomolecules using EBL fabricated nanostructured substrates

Fabrication and characterization of conjugate nano-biological systems interfacing metallic nanostructures on solid supports with immobilized biomolecules is reported. The entire sequence of relevant experimental steps is described, involving the fabrication of nanostructured substrates using electron beam lithography, immobilization of biomolecules on the substrates, and their characterization utilizing surface-enhanced Raman spectroscopy (SERS). Three different designs of nano-biological systems are employed, including protein A, glucose binding protein, and a dopamine binding DNA aptamer. In the latter two cases, the binding of respective ligands, D-glucose and dopamine, is also included. The three kinds of biomolecules are immobilized on nanostructured substrates by different methods, and the results of SERS imaging are reported. The capabilities of SERS to detect vibrational modes from surface-immobilized proteins, as well as to capture the protein-ligand and aptamer-ligand binding are demonstrated. The results also illustrate the influence of the surface nanostructure geometry, biomolecules immobilization strategy, Raman activity of the molecules and presence or absence of the ligand binding on the SERS spectra acquired.

Analytical characterization using surface-enhanced Raman scattering (SERS) and microfluidic sampling

With the rapid development of analytical techniques, it has become much easier to detect chemical and biological analytes, even at very low detection limits. In recent years, techniques based on vibrational spectroscopy, such as surface enhanced Raman spectroscopy (SERS), have been developed for non-destructive detection of pathogenic microorganisms. SERS is a highly sensitive analytical tool that can be used to characterize chemical and biological analytes interacting with SERS-active substrates. However, it has always been a challenge to obtain consistent and reproducible SERS spectroscopic results at complicated experimental conditions. Microfluidics, a tool for highly precise manipulation of small volume liquid samples, can be used to overcome the major drawbacks of SERS-based techniques. High reproducibility of SERS measurement could be obtained in continuous flow generated inside microfluidic devices. This article provides a thorough review of the principles, concepts and methods of SERS-microfluidic platforms, and the applications of such platforms in trace analysis of chemical and biological analytes.

Integrated real-time optofluidic SERS via a liquid-core/liquid-cladding waveguide

This paper describes the realization of highly sensitive surface-enhanced Raman spectroscopy (SERS) via an integrated three-dimensional liquid-core/liquid-cladding waveguide.

Towards high-throughput microfluidic Raman-activated cell sorting

Raman-activated cell sorting (RACS) is a promising single-cell analysis technology that is able to identify and isolate individual cells of targeted type, state or environment from an isogenic population or complex consortium of cells, in a label-free and non-invasive manner.

A microfluidic chip for high resolution Raman imaging of biological cells

A microfluidic chip was designed, prepared and tested for integration with a confocal Raman imaging spectrometer with the specific purpose of enabling studies of individual biological cells.

On-chip monitoring of chemical syntheses in microdroplets via surface-enhanced Raman spectroscopy

An approach for inline monitoring of organic syntheses in a microfluidic droplet chip via surface-enhanced Raman spectroscopy is presented. In a proof of concept it was successfully applied to follow thiazole syntheses in real-time.

Fabrication of perforated isoporous membranes via a transfer-free strategy: enabling high-resolution separation of cells

Thin perforated membranes with ordered pores are ideal barriers for high-resolution and high-efficiency selective transport and separation of biological species. However, for self-assembled thin membranes with a thickness less than several micrometers, an additional step of transferring the membranes onto porous supports is generally required. In this article, we present a facile transfer-free strategy for fabrication of robust perforated composite membranes via the breath figure process, and for the first time, demonstrate the application of the membranes in high-resolution cell separation of yeasts and lactobacilli without external pressure, achieving almost 100% rejection of yeasts and more than 70% recovery of lactobacilli with excellent viability. The avoidance of the transfer step simplifies the fabrication procedure of composite membranes and greatly improves the membrane homogeneity. Moreover, the introduction of an elastic triblock copolymer increases the interfacial strength between the membrane and the support, and allows the preservation of composite membranes in a dry state. Such perforated ordered membranes can also be applied in other size-based separation systems, enabling new opportunities in bioseparation and biosensors.

Optoelectrochemical biorecognition by optically transparent highly conductive graphene-modified fluorine-doped tin oxide substrates

Both optical and electrochemical graphene-based sensors have gone through rapid development, reaching high sensitivity at low cost and with fast response time. However, the complex validating biochemical operations, needed for their consistent use, currently limits their effective application. We propose an integration strategy for optoelectrochemical detection that overcomes previous limitations of these sensors used separately. We develop an optoelectrochemical sensor for aptamer-mediated protein detection based on few-layer graphene immobilization on selectively modified fluorine-doped tin oxide (FTO) substrates. Our results show that the electrochemical properties of graphene-modified FTO samples are suitable for complex biological detection due to the stability and inertness of the engineered electrodic interface. In addition, few-layer immobilization of graphene sheets through electrostatic linkage with an electrochemically grafted FTO surface allows obtaining an optically accessible and highly conductive platform. As a proof of concept, we used insulin as the target molecule to reveal in solution. Because of its transparency and low sampling volume (a few microliters), our sensing unit can be easily integrated in lab-on-a-chip cell culture systems for effectively monitoring subnanomolar concentrations of proteins relevant for biomedical applications.

Microfluidic mixing of nonpolar liquids by contact charge electrophoresis

We present a simple and effective ratcheted microfluidic mixer that uses contact charge electrophoresis (CCEP) of a micron-scale particle to rapidly mix nonpolar liquids. CCEP combines contact charging and electrostatic actuation to drive the continuous oscillatory motion of a conductive particle between two electrodes subject to a constant (DC) voltage. We show how this oscillatory motion can be harnessed to mix laminar flows by using dielectric "ramps" to direct the particle along non-reciprocal, orbital trajectories, which repeatedly stretch and fold the flowing streams. Complete mixing requires that the speed of the particle is much larger than the fluid velocity such that the particle completes many orbits as the fluid flows through the mixing region. The extent of mixing also depends strongly on the size of the particle and the shape of its trajectory; effective mixers relied on larger particles (comparable to the size of the channel) moving along non-reciprocal orbits. While the present study uses mineral oil as a convenient nonpolar liquid, we also screened fifteen common solvents to determine the applicability of CCEP for mixing other organic liquids. Owing to its simple design and low power requirements (~100 nW), the orbital mixer presented here demonstrates the utility and versatility of ratcheted electrostatic actuation in powering active microfluidic operations.

Phase-specific Raman spectroscopy for fast segmented microfluidic flows

An intensifier based Raman measuring strategy is introduced which allows for a phase-specific signal detection of one single phase in segmented flows at droplet generation frequencies of potentially up to several kHz.

High resolution scanning electron microscopy of cells using dielectrophoresis

Ultrastructural analysis of cells can reveal valuable information about their morphological, physiological, and biochemical characteristics. Scanning electron microscopy (SEM) has been widely used to provide high-resolution images from the surface of biological samples. However, samples need to be dehydrated and coated with conductive materials for SEM imaging. Besides, immobilizing non-adherent cells during processing and analysis is challenging and requires complex fixation protocols. In this work, we developed a novel dielectrophoresis based microfluidic platform for interfacing non-adherent cells with high-resolution SEM at low vacuum mode. The system enables rapid immobilization and dehydration of samples without deposition of chemical residues over the cell surface. Moreover, it enables the on-chip chemical stimulation and fixation of immobilized cells with minimum dislodgement. These advantages were demonstrated for comparing the morphological changes of non-budding and budding yeast cells following Lyticase treatment.

Photochemical transformations accelerated in continuous-flow reactors: basic concepts and applications

Continuous-flow photochemistry is used increasingly by researchers in academia and industry to facilitate photochemical processes and their subsequent scale-up. However, without detailed knowledge concerning the engineering aspects of photochemistry, it can be quite challenging to develop a suitable photochemical microreactor for a given reaction. In this review, we provide an up-to-date overview of both technological and chemical aspects associated with photochemical processes in microreactors. Important design considerations, such as light sources, material selection, and solvent constraints are discussed. In addition, a detailed description of photon and mass-transfer phenomena in microreactors is made and fundamental principles are deduced for making a judicious choice for a suitable photomicroreactor. The advantages of microreactor technology for photochemistry are described for UV and visible-light driven photochemical processes and are compared with their batch counterparts. In addition, different scale-up strategies and limitations of continuous-flow microreactors are discussed.

Microfluidics-based in vivo mimetic systems for the study of cellular biology

The human body is a complex network of molecules, organelles, cells, tissues, and organs: an uncountable number of interactions and transformations interconnect all the system’s components. In addition to these biochemical components, biophysical components, such as pressure, flow, and morphology, and the location of all of these interactions play an important role in the human body. Technical difficulties have frequently limited researchers from observing cellular biology as it occurs within the human body, but some state-of-the-art analytical techniques have revealed distinct cellular behaviors that occur only in the context of the interactions. These types of findings have inspired bioanalytical chemists to provide new tools to better understand these cellular behaviors and interactions.

What blocks us from understanding critical biological interactions in the human body? Conventional approaches are often too naïve to provide realistic data and in vivo whole animal studies give complex results that may or may not be relevant for humans. Microfluidics offers an opportunity to bridge these two extremes: while these studies will not model the complexity of the in vivo human system, they can control the complexity so researchers can examine critical factors of interest carefully and quantitatively. In addition, the use of human cells, such as cells isolated from donated blood, captures human-relevant data and limits the use of animals in research. In addition, researchers can adapt these systems easily and cost-effectively to a variety of high-end signal transduction mechanisms, facilitating high-throughput studies that are also spatially, temporally, or chemically resolved. These strengths should allow microfluidic platforms to reveal critical parameters in the human body and provide insights that will help with the translation of pharmacological advances to clinical trials.

In this Account, we describe selected microfluidic innovations within the last 5 years that focus on modeling both biophysical and biochemical interactions in cellular communication, such as flow and cell–cell networks. We also describe more advanced systems that mimic higher level biological networks, such as organ on-a-chip and animal on-a-chip models. Since the first papers in the early 1990s, interest in the bioanalytical use of microfluidics has grown significantly. Advances in micro-/nanofabrication technology have allowed researchers to produce miniaturized, biocompatible assay platforms suitable for microfluidic studies in biochemistry and chemical biology. Well-designed microfluidic platforms can achieve quick, in vitro analyses on pico- and femtoliter volume samples that are temporally, spatially, and chemically resolved. In addition, controlled cell culture techniques using a microfluidic platform have produced biomimetic systems that allow researchers to replicate and monitor physiological interactions. Pioneering work has successfully created cell–fluid, cell–cell, cell–tissue, tissue–tissue, even organ-like level interfaces. Researchers have monitored cellular behaviors in these biomimetic microfluidic environments, producing validated model systems to understand human pathophysiology and to support the development of new therapeutics.

Ion-driven photoluminescence modulation of quasi-two-dimensional MoS2 nanoflakes for applications in biological systems

Quasi-two-dimensional (quasi-2D) molybdenum disulfide (MoS2) is a photoluminescence (PL) material with unique properties. The recent demonstration of its PL, controlled by the intercalation of positive ions, can lead to many opportunities for employing this quasi-2D material in ion-related biological applications. Here, we present two representative models of biological systems that incorporate the ion-controlled PL of quasi-2D MoS2 nanoflakes. The ion exchange behaviors of these two models are investigated to reveal enzymatic activities and cell viabilities. While the ion intercalation of MoS2 in enzymatic activities is enabled via an external applied voltage, the intercalation of ions in cell viability investigations occurs in the presence of the intrinsic cell membrane potential.