Multiplexed Online Monitoring of Microfluidic Free-Flow Electrophoresis via Mass Spectrometry

Fabrication of µFFE Devices in COC via Hot Embossing with a 3D-Printed Master Mold

The fabrication of high-performance microscale devices in substrates with optimal material properties while keeping costs low and maintaining the flexibility to rapidly prototype new designs remains an ongoing challenge in the microfluidics field. To this end, we have fabricated a micro free-flow electrophoresis (µFFE) device in cyclic olefin copolymer (COC) via hot embossing using a PolyJet 3D-printed master mold. A room-temperature cyclohexane vapor bath was used to clarify the device and facilitate solvent-assisted thermal bonding to fully enclose the channels. Device profiling showed 55 µm deep channels with no detectable feature degradation due to solvent exposure. Baseline separation of fluorescein, rhodamine 110, and rhodamine 123, was achieved at 150 V. Limits of detection for these fluorophores were 2 nM, 1 nM, and 10 nM, respectively, and were comparable to previously reported values for glass and 3D-printed devices. Using PolyJet 3D printing in conjunction with hot embossing, the full design cycle, from initial design to production of fully functional COC µFFE devices, could be completed in as little as 6 days without the need for specialized clean room facilities. Replicate COC µFFE devices could be produced from an existing embossing mold in as little as two hours.

Investigating peak dispersion in free-flow counterflow gradient focusing due to electroosmotic flow

Free-flow electrophoresis (FFE) has the ability to continuously separate charged solutes from complex biological mixtures. Recently, a free-flow counterflow gradient focusing mechanism has been introduced to FFE, and it offers the potential for improved resolution and versatility. However, further investigation is needed to understand the solute dispersion at the focal position. Therefore, the goal of this work is to model the impact of electroosmotic flow, which is found to produce a pressure-driven backflow to maintain the fixed counterflow inputs. Like the counterflow, this backflow has a parabolic velocity profile that must be considered when predicting the concentration distribution of a given solute. After the model is established, preliminary experimental results are presented for a qualitative comparison. Results demonstrate a reasonable agreement at low applied voltages and provide a strong framework for future experimental validation.

Antifouling Electrochemical Biosensor Based on Conductive Hydrogel of DNA Scaffold for Ultrasensitive Detection of ATP

As an energy supplier, ATP plays an important role in various life activities, and there is an urgent need to develop an effective means of detecting ATP. However, the traditional sensors face serious nonspecific adsorption. In this work, an antifouling electrochemical biosensor based on the interpenetrating network of Y-DNA scaffold and polyaniline hydrogel was designed for ATP detection. The polyaniline hydrogel was conducive to the transport of electrons and ions, the structure of Y-DNA cross-linked by ATP aptamers in the polyaniline hydrogel achieved the effect of signal amplification. Super hydrophilic cellulose nanocrystals (CNCs) and zwitterion polypeptide sequence (Pep) were doped to play a synergistic antifouling effect. The hydrogel sensor we have built has a wide linear range of 0.1 pM-1 μM for ATP detection and a low detection limit of 0.025 pM (S/N = 3). For ATP detection in actual serum samples, the recovery of this sensor was 99.5%-106%, and the relative standard deviation was 0.4%-2.88%. It is proven that the sensor has good ATP detection performance, and it will provide a certain reference value for the detection of other biological small molecules.

Microfluidic Sampling and Biosensing Systems for Foodborne Escherichia coli and Salmonella

Developments of portable biosensors for field-deployable detections have been increasingly important to control foodborne pathogens in regulatory environment and in early stage of outbreaks. Conventional cultivation and gene amplification methods require sophisticated instruments and highly skilled professionals; while portable biosensing devices provide more freedom for rapid detections not only in research laboratories but also in the field; however, their sensitivity and specificity are limited. Microfluidic methods have the advantage of miniaturizing instrumental size while integrating multiple functions and high-throughput capability into one streamlined system at low cost. Minimal sample consumption is another advantage to detect samples in different sizes and concentrations, which is important for the close monitoring of pathogens at consumer end. They improve measurement or manipulation of bacteria by increasing the ratio of functional interface of the device to the targeted biospecies and in turn reducing background interference. This article introduces the major active and passive microfluidic devices that have been used for bacteria sampling and biosensing. The emphasis is on particle-based sorting/enrichment methods with or without external physical fields applied to the microfluidic devices and on various biosensing applications reported for bacteria sampling. Three major fabrication methods for microfluidics are briefly discussed with their advantages and limitations. The applications of these active and passive microfluidic sampling methods in the past 5 years have been summarized, with the focus on Escherichia coli and Salmonella. The current challenges to microfluidic bacteria sampling are caused by the small size and nonspherical shape of various bacterial cells, which can induce unpredictable deviations in sampling and biosensing processes. Future studies are needed to develop rapid prototyping methods for device manufacturing, which can facilitate rapid response to various foodborne pathogen outbreaks.

Multi-channel contactless conductivity detection device for online detection of free-flow electrophoresis separation

现有自由流电泳(FFE)装置因不具备在线检测功能,其实用性仍然存在明显不足。针对这一问题,该工作发展了一种多通道电容耦合式非接触电导检测(MC-C⁴D)装置并开发了自动测量软件。MC-C⁴D装置采用了并行分时的非接触电导检测技术,即由多个同样的非接触电导检测模块并行排列,而单个电导检测模块又由多个非接触电导检测池组成,采用模拟开关切换这些检测池,能够分时检测流经相应检测池溶液的电导率。多个电导检测模块的检测池总数等于FFE的组分数,它们分别串行接入到FFE各流路中,这样MC-C⁴D装置就可在线并行分时在线测量各组分溶液的电导率。为验证所设计MC-C⁴D装置的检测性能,采用配制的氯化钾标准溶液作为检测对象对MC-C⁴D装置进行了标定和测试。实验数据表明,MC-C⁴D装置电导率检测范围为0.015~2.5 mS/cm,检出限(LOD)为0.002 mS/cm,日内相对标准偏差(RSD, n=3)为2.31%,测量相对误差(RE)为3.03%和通道间测量相对偏差为1.60%,这些参数表明该装置检测范围较大,LOD低,重复性好,准确性高,通道间测量相对偏差小。另外,将MC-C ⁴D装置应用于往复式自由流等电聚焦电泳(RFFIEF)在蛋白质聚焦过程中对各组分溶液电导率进行实时在线检测,结果表明,所开发的MC-C⁴D装置不仅可实现对FFE各组分溶液电导率的实时在线检测,而且还可在RFFIEF实验中辅助掌握分离的实验进度,提高FFE装置的实用性。

Investigating peak dispersion in free-flow counterflow gradient focusing

Free-flow electrophoresis (FFE) enables the continuous separation and collection of charged solutes, and as a result, it has drawn interest as both a preparative and an analytical tool for biological applications. Recently, a free-flow counterflow gradient focusing (FF-CGF) mechanism has been proposed with the goal of improving the resolution and versatility of FFE. To realize this potential, the factors that influence solute dispersion deserve further attention, including the gradient strength and the parabolic profile of the counterflow. Therefore, the goal of this work is to develop a theoretical model to study the interplay between these factors and molecular diffusion. Overall, an asymmetric solute distribution emerges for a wide range of parameters, and this behavior can be characterized with an exponentially modified Gaussian function. Results show that FF-CGF can achieve high-resolution separations, with the potential for high-throughput protein purification. Moreover, this work provides a practical guide for optimizing experimental conditions, as well as a strong framework for understanding and developing FF-CGF further.

Topino: A Graphical Tool for Quantitative Assessment of Molecular Stream Separations

In molecular-stream separation (MSS), a stream of a multicomponent mixture is separated into multiple streams of individual components. Quantitative evaluation of MSS data has been a bottleneck in MSS for decades as there was no conventional way to present the data in a reproducible and uniform fashion. The roots of the problem were in the multidimensional nature of MSS data; even in the ideal case of steady-state separation, the data is three-dimensional: intensity and two spatial coordinates. We recently found a way to reduce the dimensionality via presenting the MSS data in a polar coordinate system and convoluting the data via integration of intensity along the radius axis. The result of this convolution is an angulagram, a simple 2D plot presenting integrated intensity vs angle. Not only does an angulagram simplify the visual assessment, but it also allows the determination of three quantitative parameters characterizing the quality of MSS: stream width, stream linearity, and stream deflection. Reliably converting an MSS image into an angulagram and accurately determining the stream parameters requires an advanced and user-friendly software tool. In this technical note, we introduce such a tool: the open-source software Topino available at https://github.com/Schallaven/topino. Topino is a stand-alone program with a modern graphical user interface that allows processing an MSS image in a fast (<2 min) and straightforward way. The robustness and ruggedness of Topino were confirmed by comparing the results obtained by three users. Topino removes the analytical bottleneck in MSS and will be an indispensable tool for MSS users with varying levels of experience.

Coupling Miniaturized Free-Flow Electrophoresis to Mass Spectrometry via a Multi-Emitter ESI Interface

We present a novel multi-emitter electrospray ionization (ESI) interface for the coupling of microfluidic free-flow electrophoresis (μFFE) with mass spectrometry (MS). The effluents of the μFFE outlets are analyzed in near real-time, allowing a direct optimization of the electrophoretic separation and an online monitoring of qualitative sample compositions. The short measurement time of just a few seconds for all outlets even enables a reasonable time-dependent monitoring. As a proof of concept, we employ the multi-emitter ESI interface for the continuous identification of analytes at 15 μFFE outlets via MS to optimize the μFFE separation of important players of cellular respiration in operando. The results indicate great potential of the presented system in downstream processing control, for example, for the monitoring and purification of products in continuous-flow microreactors.

Cell Analysis on Microfluidics Combined with Mass Spectrometry

Cell analysis is of great significance for the exploration of human diseases and health. However, there are not many techniques for high-throughput cell analysis in the simulated cell microenvironment. The high designability of the microfluidic chip enables multiple kinds of cells to be co-cultured on the chip, with other functions such as sample preprocessing and cell manipulation. Mass spectrometry (MS) can detect a large number of biomolecules without labelling. Therefore, the application of the microfluidic chip coupled with MS has represented a major branch of cell analysis over the past decades. Here, we concisely introduce various microfluidic devices coupled with MS used for cell analysis. The main functions of microfluidic devices are described first, followed by introductions of different interfaces with different types of MS. Then, their various applications in cell analysis are highlighted, with an emphasis on cell metabolism, drug screening, and signal transduction. Current limitations and prospective trends of microfluidics coupled with MS are discussed at the end.

Microfluidics for core-shell drug carrier particles - a review

Core-shell drug-carrier particles are known for their unique features. Due to the combination of superior properties not exhibited by the individual components, core-shell particles have gained a lot of interest. The structures could integrate core and shell characteristics and properties. These particles were designed for controlled drug release in the desired location. Therefore, the side effects would be minimized. So, these particles' advantages have led to the introduction of new methods and ideas for their fabrication. In the past few years, the generation of drug carrier core-shell particles in microfluidic chips has attracted much attention. This method makes it possible to produce particles at nanometer and micrometer levels of the same shape and size; it usually costs less than other methods. The other advantages of using microfluidic techniques compared to conventional bulk methods are integration capability, reproducibility, and higher efficiency. These advantages have created a positive outlook on this approach. This review gives an overview of the various fluidic concepts that are used to generate microparticles or nanoparticles. Also, an overview of traditional and more recent microfluidic devices and their design and structure for the generation of core-shell particles is given. The unique benefits of the microfluidic technique for core-shell drug carrier particle generation are demonstrated.