Two-Dimensional Cytometry Platform for Single-Particle/Cell Analysis with Laser-Induced Fluorescence and ICP-MS

Inertial and Deterministic Lateral Displacement Integrated Microfluidic Chips for Epithelial-Mesenchymal Transition Analysis

With the aim of efficiently sorting rare circulating tumor cells (CTCs) from blood and minimizing damage to CTCs during isolation, we constructed an inertia-assisted single-cell focusing generator (I-SCF) and a water droplet deterministic lateral displacement cell sorting (D-DLD) microfluidic system (IDIC) based on different sizes, the device is initially sorted by a continuous fluid swing and Dean flow-assisted helical micromixers, then flows through a droplet shaped DLD region, enabling single-cell focused sequencing and precise separation, improving cell separation efficiency (>95%) and purity, while ensuring a high single cells survival rate of more than 98.6%. Subsequently, breast cancer cell lines were run through our chip, and then the downstream epithelial-mesenchymal transition (EMT) process induced by TGF-β was detected, and the levels of three proteins, EpCAM, PD-L1, and N-cadherin, were analyzed to establish the relationship between PD-L1 and the EMT process. Compared with other analytical techniques such as the filtration method, the enrichment method and immunoaffinity capture methods, this method not only ensures the separation efficiency and purity, but also ensures the cell activity, and avoids missing the different results caused by the heterogeneity of CTCs due to the isolation of high purity (84.01%). The device has a high throughput processing capacity (5 mL of diluted whole blood/∼2.8 h). By using the chip, we can more easily and conveniently predict tumor stage and carry out cancer prevention and treatment in advance, and it is expected to be further developed into a clinical liquid biopsy technology in the future.

Compact Laser-Induced Fluorescence Detector with Adjustable Laser Focal Spot for Multiple Purposes

In many research fields, the demand for miniaturized laser-induced fluorescence (LIF) detection systems has been increasing. This work has developed a compact LIF detector, employing a laser diode as the excitation source and a photodiode as the photodetector with an adjustable laser focal spot, to meet the diverse requirements of various observation targets, such as capillaries, PCR tubes, and microfluidic chips. It features the functionalities of background fluorescence correction, the adaptive adjustment of the dynamic range, and constant power control for the laser. The influence of the excitation power on the detection limit was studied through experiments, and the configuration results for LED/LD as light sources and 487/450 nm wavelengths were compared and optimized. A fully integrated, compact, modular epifluorescence LIF detector was subsequently constructed, measuring 40 × 22 × 38 mm3 in total size, with a cost of USD 320, and achieving a detection limit of 0.4 nM for fluorescein sodium. Finally, the detector was integrated into a nucleic acid detection system with a microfluidic chip on the Chinese Space Station (CSS) and was also tested with PCR tubes and capillaries, proving its broad practicality and adaptability to various analytical systems.

Single-Cell Analysis of Elemental Contents by Laser Ablation-Inductively Coupled Plasma Mass Spectrometry

Elemental analysis at the single-cell level is an emerging technique in the field of inductively coupled plasma mass spectrometry (ICP-MS). In comparison to the analysis of cell suspensions by fast time-resolved analysis, single-cell sampling by laser ablation (LA) allows the discriminatory analysis of single cells according to their size and morphology. In this study, we evaluated the changes in elemental contents in a single cell through differentiation of rat adrenal pheochromocytoma into neuron-like cells by LA-ICP-MS. The contents of seven essential minerals were increased about 2-3 times after the differentiation. In addition, we evaluated the degree of differentiation at the single-cell level by means of imaging cytometry after immunofluorescence staining of microtubule-associated protein 2 (Map2), a neuron-specific protein. The fluorescence intensities of Alexa Fluor 488-conjugated secondary antibody against the anti-Map2 primary antibody showed large variations among the cells after the onset of differentiation. Although the average fluorescence intensity was increased through the differentiation, there were still less-matured neuron-like cells that exhibited a lower fluorescence intensity after 5 days of differentiation. Since a positive correlation between the fluorescence intensity and the cell size in area was found, we separately measured the elemental contents in the less-matured smaller cells and well-matured larger cells by LA-ICP-MS. The larger cells had higher elemental contents than the smaller cells, indicating that the essential minerals are highly required at a later stage of differentiation.

Rapid Screening of Biomarkers in KYSE-150 Cells Exposed to Polycyclic Aromatic Hydrocarbons via Inkjet Printing Single-Cell Mass Spectrometry

Single-cell analysis by mass spectrometry (MS) is emerging as a powerful tool that not only contributes to cellular heterogeneity but also offers an unprecedented opportunity to predict pathology onset and facilitates novel biomarker discovery. However, the development of single-cell MS analysis techniques with a focus on sample extraction, separation, and ionization methods for volume-limited samples and complexity of cellular samples are still a big challenge. In this study, we present a high-throughput approach to inkjet drop on demand printing single-cell MS for rapid screening of biomarkers of polycyclic aromatic hydrocarbon (PAH) exposure at the KYSE-150 cell, aiming to elucidate the pathogenesis of PAH-induced esophageal cancer. With an analytical bulk KYSE-150 cell throughput of up to 51 cells per minute, the method provides a new opportunity for simultaneous single-cell analysis of multiple biomarkers. We screened 930 characteristic ions from 3,683 detected peak signals and identified 91 distinctive molecules that exhibited significant differences under various concentrations of PAH exposure. These molecules have potential as clinical diagnostic biomarkers. Additionally, the current study identifies specific biomarkers that behave completely opposite in single-cell and multicell lipidomics as the concentration of PAH changes. These biomarkers potentially subdivide KYSE-150 cells into PAH-sensitive and PAH-insensitive types, providing a basis for revealing PAH toxicity and disease pathogenesis from the heterogeneity of cellular metabolism.

Microfluidic Impedance Cytometry Enabled One-Step Sample Preparation for Efficient Single-Cell Mass Spectrometry

Single-cell mass spectrometry (MS) is significant in biochemical analysis and holds great potential in biomedical applications. Efficient sample preparation like sorting (i.e., separating target cells from the mixed population) and desalting (i.e., moving the cells off non-volatile salt solution) is urgently required in single-cell MS. However, traditional sample preparation methods suffer from complicated operation with various apparatus, or insufficient performance. Herein, a one-step sample preparation strategy by leveraging label-free impedance flow cytometry (IFC) based microfluidics is proposed. Specifically, the IFC framework to characterize and sort single-cells is adopted. Simultaneously with sorting, the target cell is transferred from the local high-salinity buffer to the MS-compatible solution. In this way, one-step sorting and desalting are achieved and the collected cells can be directly fed for MS analysis. A high sorting efficiency (>99%), cancer cell purity (≈87%), and desalting efficiency (>99%), and the whole workflow of impedance-based separation and MS analysis of normal cells (MCF-10A) and cancer cells (MDA-MB-468) are verified. As a standalone sample preparation module, the microfluidic chip is compatible with a variety of MS analysis methods, and envisioned to provide a new paradigm in efficient MS sample preparation, and further in multi-modal (i.e., electrical and metabolic) characterization of single-cells.

Multiplex Profiling of Biomarker and Drug Uptake in Single Cells Using Microfluidic Flow Cytometry and Mass Spectrometry

To perform multiplex profiling of single cells and eliminate the risk of potential sample loss caused by centrifugation, we developed a microfluidic flow cytometry and mass spectrometry system (μCytoMS) to evaluate the drug uptake and induced protein expression at the single cell level. It involves a microfluidic chip for the alignment and purification of single cells followed by detection with laser-induced fluorescence (LIF) and inductively coupled plasma mass spectrometry (ICP-MS). Biofunctionalized nanoprobes (BioNPs), conjugating ∼3000 6-FAM-Sgc8 aptamers on a single gold nanoparticle (AuNP) (Kd = 0.23 nM), were engineered to selectively bind with protein tyrosine kinase 7 (PTK7) on target cells. PTK7 expression induced by oxaliplatin (OXA) uptake was assayed with LIF, while ICP-MS measurement of 195Pt revealed OXA uptake of the drug in individual cells, which provided further in-depth information about the drug in relation to PTK7 expression. At an ultralow flow of ∼0.043 dyn/cm2 (20 μL/min), the chip facilitates the extremely fast focusing of BioNPs labeled single cells without the need for centrifugal purification. It ensures multiplex profiling of single cells at a throughput speed of 500 cells/min as compared to 40 cells/min in previous studies. Using a machine learning algorithm to initially profile drug uptake and marker expression in tumor cell lines, μCytoMS was able to perform in situ profiling of the PTK7 response to the OXA at single-cell resolution for tests done on clinical samples from 10 breast cancer patients. It offers great potential for multiplex single-cell phenotypic analysis and clinical diagnosis.

Analysis of cellular response to drugs with a microfluidic single-cell platform based on hyperspectral imaging

BACKGROUND

Cellular response to pharmacological action of drugs is significant for drug development. Traditional detection method for cellular response to drugs normally rely on cell proliferation assay and metabolomics examination. In principle, these analytical methods often required cell labeling, invasion analysis, and hours of co-culture with drugs, which are relatively complex and time-consuming. Moreover, these methods can only indicate the drug effectiveness on cell colony rather than single cells. Thus, to meet the requirements of personal precision medicine, the development of drug response analysis on the high resolution of single cell is demanded.

RESULTS

To provide precise result for drug response on single-cell level, a microfluidic platform coupled with the label-free hyperspectral imaging was developed. With the help of horizontal single-cell trapping sieves, hundreds of single cells were trapped independently in microfluidic channels for the purposes of real-time drug delivery and single-cell hyperspectral image recording. To significantly identify the cellular hyperspectral change after drug stimulation, the differenced single-cell spectrum was proposed. Compared with the deep learning classification method based on hyperspectral images, an optimal performance can be achieved by the classification strategy based on differenced spectra. And the cellular response to different reagents, for example, K+, Epidermal Growth Factor (EGF), and Gefitinib at different concentrations can be accurately characterized by the differenced single-cell spectra analysis.

SIGNIFICANCE AND NOVELTY

The high-throughput, rapid analysis of cellular response to drugs at the single-cell level can be accurately performed by our platform. After systematically analyzing the materials and the structures of the single-cell microfluidic chip, the optimal single-cell trapping method was proposed to contribute to the further application of hyperspectral imaging on microfluidic single-cell analysis. And the hyperspectral characterization of single-cell with cancer drug stimulation proved the application potential of our method in personal cancer medication.

Interaction of Cellular Uptake of Nanosilver and Metallothionein Stress Expression Elucidated by 2D Single-Cell Analyses Based on LIF and ICP-MS

The exploration of cytology mechanisms of nanosilver uptake, toxicity, and detoxification has become an important issue due to its widespread applications. Previous studies have shown differences in the toxic response of mammalian cells to nanosilver. However, the analysis results based on cell populations ignore the impact of cell uptake heterogeneity on the expression of associated stress proteins and cellular physiological activities. In this respect, this work investigated the interaction between silver uptake and metallothionein (MT) expression in individual cells. In addition, we have also preliminarily elucidated the sensitivity variation to AgNPs by using five cell lines, e.g., LX-2, HepG-2, SK-HEP-1, Huh-7, and MDA-MB-231, by adopting a two-dimensional (2D) high-throughput single-cell analysis platform coupling laser-induced fluorescence (LIF) and inductively coupled plasma mass spectrometry (ICP-MS). We developed a 2D data analysis method for one-to-one unification of fluorescence-mass spectrometry signals corresponding to a specific single cell. It indicated that there is no obvious correlation between cellular silver uptake and cell size, and the low MT expression of cells is more sensitive to silver nanoparticles. For each cell line, significant heterogeneity in MT expression was observed. This provides important information for understanding the potential heterogeneous effects of nanosilver on mammalian biological systems. Overall, detoxified cells are more tolerant to nanosilver and normal cells are more tolerant than cancer cells.

Two-Dimensional Multi-parameter Cytometry Platform for Single-Cell Analysis

A 2D flow cytometry platform, known as CytoLM Plus, was developed for multi-parameter single-cell analysis. Single particles or cells after hydrodynamic alignment in a microfluidic unit undergo first-dimension fluorescence and side scattering dual-channel optical detection. They were thereafter immediately directed to ICP-MS by connecting the microfluidic unit with a high-efficiency nebulizer to facilitate the second-dimension ICP-MS detection. Flow cytometry measurements of fluorescent microspheres evaluated the performance of CytoLM Plus for optical detection. 6434 fluorescence bursts were observed with a valid signal proportion as high as 99.7%. After signal unification and gating analysis, 6067 sets of single-particle signals were obtained with 6.6 and 6.2% deviations for fluorescence burst area and height, respectively. This is fairly comparable with that achieved by a commercial flow cytometer. Afterward, CytoLM Plus was evaluated by 2D flow cytometry measurement of Ag+-incubated and AO-stained MCF-7 cells. A program for 2D single-cell signal unification was developed based on the algorithm of screening in lag time window. In the present case, a lag time window of -4.2 ± 0.09 s was determined by cross-correlation analysis and two-parameter optimization, which efficiently unified the concurrent single-cell signals from fluorescence, side scattering, and ICP-MS. A total of 495 sets of concurrent 2D signals were screened out, and the statistical analysis of these single-cell signals ensured 2D multi-parameter single-cell analysis and data elucidation.

A sample-preparation-free, point-of-care testing system for in situ detection of bovine mastitis

We present a highly integrated point-of-care testing (POCT) device capable of immediately and accurately screening bovine mastitis infection based on somatic cell counting (SCC). The system primarily consists of a homemade cell-counting chamber and a miniature fluorescent microscope. The cell-counting chamber is pre-embedded with acridine orange (AO) in advance, which is simple and practical. And then SCC is directly identified by microscopic imaging analysis to evaluate the bovine mastitis infection. Only 4 μL of raw bovine milk is required for a simple sample testing and accurate SCC. The entire assay process from sampling to result in presentation is completed quickly within 6 min, enabling instant "sample-in and answer-out." Under laboratory conditions, we mixed bovine leukocyte suspension with whole milk and achieved a detection limit as low as 2.12 × 104 cells/mL on the system, which is capable of screening various types of clinical standards of bovine milk. The fitting degrees of the proposed POCT system with manual fluorescence microscopy were generally consistent (R2 > 0.99). As a proof of concept, four fresh milk samples were used in the test. The average accuracy of somatic cell counts was 98.0%, which was able to successfully differentiate diseased cows from healthy ones. The POCT system is user-friendly and low-cost, making it a potential tool for on-site diagnosis of bovine mastitis in resource-limited areas.

Open flow cytometer with the combination of 3D hydrodynamic single cell focusing and confocal laser-induced fluorescence detection

Flow cytometry is among the most powerful tools for single-cell analysis, while the high cost and mechanical complexity of the commercial instrumentation limit the applications in personalized single-cell analysis. For this issue, we hereby construct an open and low-cost flow cytometer. It is highly compact to integrate the functions of (1) single cell aligning by a lab-made modularized 3D hydrodynamic focusing device, and (2) fluorescence detection of the single cells by a confocal laser-induced fluorescence (LIF) detector. The ceiling cost of the entire hardware for the LIF detection unit and 3D focusing device is $ 3200 and $ 400 respectively. A sheath flow velocity of 150 μL/min produces a focused sample stream of 17.6 μm × 14.6 μm at sample flow of 2 μL/min, based on the LIF response frequency and the laser beam spot diameter. The assay performance of the flow cytometer was evaluated by characterizing fluorescent microparticles and acridine orange (AO) stained HepG2 cells, producing throughputs of 40.5/s and 6.2/s respectively. Favorable assay precision and accuracy were demonstrated by the agreement of frequency histogram with imaging analysis, and good Gaussian-like distributions of fluorescent microparticles and AO-stained HepG2 cells. Practically, the flow cytometer was successfully applied for the evaluation of ROS generation in single HepG2 cells.

Data analysis for nanoparticles characterization with single particle inductively coupled plasma mass spectrometry: From microsecond to millisecond dwell times

Single particle-inductively coupled plasma-mass spectrometry (SP-ICP-MS) has become a powerful technique for the characterization of nanoparticles (NPs). However, the accuracy of the characterization of NPs by SP-ICP-MS is greatly affected by the data acquisition rate and the way of data processing. For SP-ICP-MS analysis, ICP-MS instruments typically apply microsecond to millisecond dwell times (10 μs-10 ms). Considering the duration of one nanoparticle event in the detector is 0.4-0.9 ms, NPs will show different data forms when working with microsecond and millisecond dwell times. In this work, the effects of dwell times from microsecond to millisecond (50 μs, 100 μs, 1 ms and 5 ms) on the data forms in SP-ICP-MS analysis are discussed. The data analysis and data processing for different dwell times is discussed in detail, including the measurement of transport efficiency (TE), the distinction of signal and background, the evaluation of diameter limit of detection (LOD_d) and the quantification of mass, size and particle number concentration (PNC) of NPs. This work provides data support for the data processing process and aspects to be considered in the characterization of NPs by SP-ICP-MS, which is expected to provide guidance and reference for researchers in SP-ICP-MS analysis.

[Determination of glutathione in cells by capillary electrophoresis-laser induced fluorescence]

Glutathione (GSH) is vital for oxidative stress resistance and heavy metals detoxification. It is significant to develop a sensitive and accurate quantitative GSH approach for the toxicity mechanism for studying heavy metals in cells. A high-sensitive capillary electrophoresis-laser induced fluorescence (CE-LIF) detection approach was proposed in this study to detect GSH content in cells. The approach employed HepG2 cells as an object and 2,3-naphthalenedicarboxaldehyde (NDA) with the active group of aromatic o-dialdehyde as a labeling reagent. The effects of buffer solution types, pH, additives on the GSH reaction rate with NDA, and the sensitivity of NDA-GSH were systematically investigated. The sensitivity of NDA-GSH and the reaction rate of GSH with NDA were compared in tris(hydroxymethyl)aminomethane (Tris) buffer solution at pH 7.4 or 9.2 and borate-Tris buffer solution at pH 9.2. The results revealed that the NDA-GSH sensitivity was the highest and the reaction rate of GSH and NDA was the fastest in borate buffer solution at pH 9.2. The effects of the four additives on the sensitivity of NDA-GSH were further compared. The best additive was revealed to be β-cyclodextrin (β-CD). GSH reacted with NDA to reach equilibrium within 5 min under the optimal experimental conditions, and the electrophoretic signal of NDA-GSH could be seen in 3 min. Quantitative analysis of GSH in HepG2 cells was performed using an external standard approach by determining a series of GSH standard solutions. The results revealed that the approach had a good linear relationship with the peak area vs. concentration (0.01-20.00 mmol/L) of GSH. The limit of detection (LOD) and limit of quantification (LOQ) of GSH were determined using signal-to-noise ratios of 3 (S/N=3) and 10 (S/N=10), which were 0.006 μmol/L and 0.020 μmol/L, respectively. The approach's spiked recoveries were 95.7%-112.6%, with relative standard deviations of the approach being 3.8%-5.0% (n=3). This approach offers high sensitivity, good stability, accuracy, and reliability. To study the relationship between the toxicity of arsenic and chromium on HepG2 cells and the content of GSH in HepG2 cells, the effects of arsenic and chromium with different valences on cell viability were analyzed. The results illustrated that the cytotoxicity of potassium dichromate (Cr(Ⅵ)) was the strongest. The variations of GSH content in HepG2 cells stimulated with arsenite (As(Ⅲ)), arsenate (As(Ⅴ)), chromium chloride (Cr(Ⅲ)), and Cr(Ⅵ) were analyzed by the proposed approach and analysis of intracellular GSH imaging. The results revealed that the stimulation group i. e. analyzed doses (low-dose 2 mg/L, high-dose 5 mg/L) of As(Ⅲ), As(Ⅴ), and Cr(Ⅲ) had no obvious effect on GSH content in HepG2 cells compared with the control group, whereas high-dose Cr(Ⅵ) can significantly reduce GSH content in HepG2 cells. Considering the analysis of cytotoxicity of As(Ⅲ), As(Ⅴ), Cr(Ⅲ), and Cr(Ⅵ), it shows that the content of GSH in HepG2 cells is related to cytotoxicity, and the content of GSH will decrease with the increase in cytotoxicity.

Inertial microfluidics: current status, challenges, and future opportunities

Inertial microfluidics uses the hydrodynamic effects induced at finite Reynolds numbers to achieve passive manipulation of particles, cells, or fluids and offers the advantages of high-throughput processing, simple channel geometry, and label-free and external field-free operation. Since its proposal in 2007, inertial microfluidics has attracted increasing interest and is currently widely employed as an important sample preparation protocol for single-cell detection and analysis. Although great success has been achieved in the inertial microfluidics field, its performance and outcome can be further improved. From this perspective, herein, we reviewed the current status, challenges, and opportunities of inertial microfluidics concerning the underlying physical mechanisms, available simulation tools, channel innovation, multistage, multiplexing, or multifunction integration, rapid prototyping, and commercial instrument development. With an improved understanding of the physical mechanisms and the development of novel channels, integration strategies, and commercial instruments, improved inertial microfluidic platforms may represent a new foundation for advancing biomedical research and disease diagnosis.

Microfluidic flow cytometry for blood-based biomarker analysis

Flow cytometry has proven its capability for rapid and quantitative analysis of individual cells and the separation of targeted biological samples from others. The emerging microfluidics technology makes it possible to develop portable microfluidic diagnostic devices for point-of-care testing (POCT) applications. Microfluidic flow cytometry (MFCM), where flow cytometry and microfluidics are combined to achieve similar or even superior functionalities on microfluidic chips, provides a powerful single-cell characterisation and sorting tool for various biological samples. In recent years, researchers have made great progress in the development of the MFCM including focusing, detecting, and sorting subsystems, and its unique capabilities have been demonstrated in various biological applications. Moreover, liquid biopsy using blood can provide various physiological and pathological information. Thus, biomarkers from blood are regarded as meaningful circulating transporters of signal molecules or particles and have great potential to be used as non (or minimally)-invasive diagnostic tools. In this review, we summarise the recent progress of the key subsystems for MFCM and its achievements in blood-based biomarker analysis. Finally, foresight is offered to highlight the research challenges faced by MFCM in expanding into blood-based POCT applications, potentially yielding commercialisation opportunities.

Living in a transient world: ICP-MS reinvented via time-resolved analysis for monitoring single events

After 40 years of development, inductively coupled plasma-mass spectrometry (ICP-MS) can hardly be considered as a novel technique anymore. ICP-MS has become the reference when it comes to multi-element bulk analysis at (ultra)trace levels, as well as to isotope ratio determination for metal(loid)s. However, over the last decade, this technique has managed to uncover an entirely new application field, providing information in a variety of contexts related to the individual analysis of single entities (e.g., nanoparticles, cells, or micro/nanoplastics), thus addressing new societal challenges. And this profound expansion of its application range becomes even more remarkable when considering that it has been made possible in an a priori simple way: by providing faster data acquisition and developing the corresponding theoretical substrate to relate the time-resolved signals thus obtained with the elemental composition of the target entities. This review presents the underlying concepts behind single event-ICP-MS, which are needed to fully understand its potential, highlighting key areas of application (e.g., single particle-ICP-MS or single cell-ICP-MS) as well as of future development (e.g., micro/nanoplastics).

Multiplex DNA Walking Machines for Lung Cancer-Associated miRNAs

Biomimetic DNA walking machines have gained great success in scrutinizing the microscopic world and sensitive biosensing of disease biomarkers. Despite superb achievements, the research on DNA walking machines for simultaneous detection of multiple analytes is still rare, while the design and realization of multiplexing are considered as an important bottleneck. The multiplex detection of biomarkers can not only improve the specificity of bioassays but also avoid the squander of valuable biological specimens. Herein, we reported multiplex three-dimensional (3D) DNA walking machines based on high-resolution inductively coupled plasma mass spectrometry (HR-ICPMS) for lung cancer-associated miRNA detection. In the presence of lung cancer-associated target miRNAs (miR-21, miR-141, and miR-125b), DNA walking machines were stimulated and operated to liberate a large number of lanthanide elements (Tb, Ho, and Tm), and then the signals were collected simultaneously by HR-ICPMS. The recovery test of target miRNAs in human serum and the simultaneous monitoring experiment of three miRNAs in human lung cancer cell line (A549) and normal cell line (HBE) specimens display satisfactory analysis capabilities for complex biological samples. Thanks to the vast potential of lanthanide tags and the modular design, the proposed bioassay might flexibly detect different miRNA combinations with corresponding sets of DNA walking machines to meet the requirements of various tasks.