Direct Infusion ICP-qMS of Lined-up Single-Cell Using an Oil-Free Passive Microfluidic System

Label-free single-cell analysis in microdroplets using a light-scattering-based optofluidic chip

Droplet-based single-cell analysis is a very powerful tool for studying phenotypic and genomic heterogeneity at single-cell resolution for a variety of biological problems. In conventional two-phase droplet microfluidics, due to the mismatch in optical properties between oil and aqueous phases, light scattering mainly happens at the oil/water interface that disables light-scattering-based cell analysis confined in microdroplets. Detection and analysis of cells in microdroplets thus mostly rely on the fluorescence labeling of cell samples, which may suffer from complex operation, cytotoxicity, and low fluorescence stability. In this work, we propose a novel light-scattering-based droplet screening (LSDS) that can effectively detect and characterize single cells confined in droplets by adjusting the optical properties of droplets in a multiangle optofluidic chip. Theoretical and simulated calculations suggest that refractive index (RI) matching in droplet two-phase materials can reduce or eliminate droplets' scattered signals (background signal), enabling the differentiation of scattered signals from single cells and particles within droplets. Furthermore, by using a set of multiangle (from -145° to 140°) optical fibers integrated into the optofluidic chip, the scattered light properties of droplets with the RI ranging from 1.334 to 1.429 are measured. We find that the smaller the RI and size of microparticles inside droplets are, the smaller the RI difference between two-phase materials Δn is required. Especially, when Δn is smaller than 0.02, single cells in droplets can be detected and analyzed solely based on light scattering. This capability allows to accurately detect droplets containing one single cell and one single gel bead, a typical droplet encapsulation for single-cell sequencing. Altogether, this work provides a powerful platform for high-throughput label-free single-cell analysis in microdroplets for diverse single-cell related biological assays.

Design of a dual Ir-Eu tag for fluorescent visualization and ICP-MS quantification of SIRPα and its host cells

With the expansion of ICP-MS application into the field of bioanalysis, there is an urgent need for novel element tags today. Here, we report the design of a dual-element Ir-Eu tag, opening the door to simultaneous fluorescent imaging and ICP-MS quantification. The ratio of 153Eu/193Ir may serve as a precision control of the labeling process, allowing internal validation of the quantitative results obtained. As for SIRPα and its host cell analysis exemplified here, the Ir-Eu tag demonstrated superior figures of ICP-MS quantification with the LOD (3σ) down to 0.5 (153Eu) and 1.1 (193Ir) pM SIRPα and 220 (153Eu) and 830 (193Ir) RAW264.7 cells more than 130 times more sensitive compared with the LOD (3σ) of 65.2 pM SIRPα at 612 nm using fluorometry. Not limited to these demonstrations, we believe that the design ideas of the dual Ir-Eu tags should be applicable to various cases of bioanalysis when dual optical profiling and ICP-MS quantification are indispensable.

Single-cell ICP-MS to address the role of trace elements at a cellular level

The heterogeneity properties shown by cells or unicellular organisms have led to the development of analytical methods at the single-cell level. In this sense, considering the importance of trace elements in these biological systems, the inductively coupled plasma mass spectrometer (ICP-MS) configured for analyzing single cell has presented a high potential to assess the evaluation of elements in cells. Moreover, advances in instrumentation, such as coupling laser ablation to the tandem configuration (ICP-MS/MS), or alternative mass analyzers (ICP-SFMS and ICP-TOFMS), brought significant benefits, including sensitivity improvement, high-resolution imaging, and the cell fingerprint. From this perspective, the single-cell ICP-MS has been widely reported in studies involving many fields, from oncology to environmental research. Hence, it has contributed to finding important results, such as elucidating nanoparticle toxicity at the cellular level and vaccine development. Therefore, in this review, the theory of single-cell ICP-MS analysis is explored, and the applications in this field are pointed out. In addition, the instrumentation advances for single-cell ICP-MS are addressed.

Zero-Interfacing μHPLC to ICPMS

The chromatography-mass spectrometry hyphenated technique is the most widely adopted tool for quantifying trace analytes in a complex biosample. One issue we frequently encountered, however, is that the separated analyte-containing chromatographic peaks broaden and even remix prior to mass spectrometric quantification due to the inevitable molecular diffusion within the dead-volume introduced by hyphenation. We developed a zero-interfacing approach for coupling microbore (μ) HPLC with inductively coupled plasma mass spectrometry (ICPMS). Zero-interfacing μHPLC to ICPMS has been achieved by a column-nebulizer assembly (COL-NEB) of a self-designed glass framework with a tapered nozzle, in which a capillary chromatographic column can be harbored while an Ar gas flow is blown through the nozzle mouth. The COL-NEB can be positioned just before the base of the Ar-ICP serving as the central sampling channel of a conventional Ar-ICP torch for online nebulization and transportation of the analytes separated on μHPLC into ICPMS, maintaining the molecular resolution obtained on μHPLC and the limit of detection (LOD) of ICPMS. For example, the full width at half-maximum of a SLUGT peptide chromatographic peak was reduced to 1.71 ± 0.07 s (n = 5) with a 0.72 fg LOD (3σ) of ⁸⁰Se. Moreover, at least 32 Se-containing peptides were determined in the trypsin lysate of the water-soluble fraction (≥3000 MW) from Se-enriched yeast CRM SELM-1 within a 10 min run, the highest record to date. We believe such an approach paves the way to determining accurate information on a heteroatom and its binding biomolecules that play key roles during life processes.

A chip-based supersonic microfluidic nebulizer for efficient sample introduction into inductively coupled plasma - Mass spectrometry

As the use of microfluidic chips for handling biological samples is increasing, so is the need for combining them with powerful analytical techniques for metal determination such as inductively-coupled plasma mass spectrometry (ICP-MS). So far, coupling a microfluidic chip to an ICP-MS has been demonstrated mainly through the use of conventional pneumatic micro-flow nebulizers. However, disadvantages associated with the use of such nebulizers entail dead volume issues and liquid suction exerted on the outlet channel of the chip. Herein, we propose that a microfluidic chip, bearing a pneumatic nozzle for liquid nebulization, has the potential to advance metal determination in chip-based ICP-MS. More specifically, we demonstrate for the first time that the coupling of a chip-based supersonic microfluidic nebulizer (chip-μf-Neb) to an ICP-MS can be conveniently achieved through the use of a spray chamber with a laminar flow makeup gas. Operation of the combined system was evaluated at low liquid flow rates across 0.5-20 μL min^-1, while nebulization and makeup argon (Ar) gas flow rates were optimized with respect to maximizing indium (In) sensitivity and minimizing oxide formation; a maximum sensitivity of 40000 cps (μg L^-1)^-1 was achieved at 10 μL min^-1. The system was further evaluated for its performance in single-particle analysis, featuring a transport efficiency of 46% for Ag nanoparticles. Finally, the capabilities for conducting single-cell analysis were demonstrated with the detection of ⁸⁰Se and ⁷⁵As in individual Chlamydomas reinhardtii cells, which were previously incubated in 20 μM of selenate and 300 μM of arsenate, respectively. Efficient operation at low liquid flow rates along with the absence of self-aspiration render this nebulizer a promising tool for combining the powerful field of microfluidics with metal quantitation by means of ICP-MS.

Multifunctional microfluidic chips for the single particle inductively coupled plasma mass spectrometry analysis of inorganic nanoparticles

This study aimed at exploiting the so far unexploited potential of carrying out on-line sample pretreatment steps on microfluidic chips for single particle inductively coupled plasma mass spectrometry (spICP-MS) measurements, and demonstrating their ability to practically facilitate most of the simpler tasks involved in the spICP-MS analysis of nanoparticles. For this purpose, polydimethylsiloxane microfluidic chips, capable of high-range dilution and sample injection were made by casting, using high-precision, 3D-printed molds. Optimization of their geometry and functions was done by running several hydrodynamic simulations and by gravimetric, fluorescence enhanced microscope imaging and solution-based ICP-MS experiments. On the optimized microfluidic chips, several experiments were done, demonstrating the benefits of the approach and these devices, such as the determination of nanoparticle concentration using only a few tens of microliters of sample, elimination of solute interferences by dilution, solution-based size calibration and characterisation of binary nanoparticles. Due to the unique design of the chips, they can be linked together to extend the dilution range of the system by more than a magnitude per chip. This feature was also demonstrated in applications requiring multiple-magnitude dilution rates, when two chips were sequentially coupled.

Standard-free single magnetic bead evaluation: a stable nanoplatform for prostate disease differentiation

Explicit interpretation of heterogeneity between prostate-specific antigen (PSA) subtypes is essential for prostate cancer differentiation during different disease courses, whereas a universal protocol with uniform criteria is still lacking across the globe. In this work, a standard-free single magnetic bead (SMB) nanoplatform utilizing metal nanoparticles with optimal diameters was proposed for prostate disease differentiation in a 134-donor model. The inaccuracy of detection in absolute quantification was diminished via evaluations of metal intensities on the single magnetic bead. The intrinsic proportion of fPSA in tPSA was successfully evaluated by direct use of the Pt to Au intensity ratio (Pt/Au ratio), exhibiting better differentiation between healthy and unhealthy, benign prostatic hyperplasia (BPH) and cancer individuals compared with solo fPSA or tPSA. We generated thresholds respectively for prostate disease differentiation, envisioning that this standard-free SMB nanoplatform would establish a standardized methodology with uniform criteria worldwide in cancer diagnosis, staging, and postoperative assessments.

Negative Magnetophoresis Focusing Microchips Online-Coupled with ICP-MS for High-Throughput Single-Cell Analysis

High-throughput single-cell analysis is critical to elucidate the cell heterogeneity. Recently, droplet microchips using oil/gas phases to generate single-cell encapsulated droplets have been combined with inductively coupled plasma-mass spectrometry (ICP-MS) for determination of trace elements in single cells with a throughput of dozens of cells per min. To improve the sample throughput and avoid the oil phase introduced into ICP-MS, herein, a negative magnetophoresis focusing microchip was established and online-coupled to ICP-MS for single-cell analysis. MCF-7 cells in the paramagnetic salt solution were introduced into the designed focusing microchannel, in which they were focused into a single stream under both the magnetic repulsion force and inertial lift force, and then were introduced into ICP-MS for online single-cell analysis. The important parameters including the chip design, the concentration of the paramagnetic salt solution, flow rate, cell density, and dwell time were optimized. Under the optimal conditions, a high sample throughput of 1390 cells min^-1 was obtained. The established online analytical system was applied to study the uptake behaviors of MCF-7 cells for Zn²⁺ and ZnO nanoparticles (NPs) at a single-cell level. The single-cell analysis results indicate that MCF-7 cells displayed more remarkable heterogeneity when they were treated with ZnO NPs, and the uptake content of ZnO NPs by MCF-7 cells was less than that of Zn²⁺. Compared with other droplet microdevice-ICP-MS analysis systems, the developed system has the advantages of simple design and fabrication, no organic phase, a high throughput, and a low sample consumption (only 5 μL).

Droplet-assisted electrospray phase separation using an integrated silicon microfluidic platform

We report a silicon microfluidic platform that enables monolithic integration of transparent micron-scale microfluidic channels, an on-chip segmentation of analyte flows into picoliter-volume droplets, and a nano-electrospray ionization emitter that enables spatial and temporal separation of oil and aqueous phases during electro-spray for subsequent mass spectrometry analysis.

Quantification of active selenols in cells: a selenol-specific recognition europium-switched signal-amplification ICP-MS approach

Selenium (Se) is a mysterious thus tempting element playing a dual bio-chemical function, mainly through selenol, during life processes. Quantification of the selenols is thus of great significance for understanding the biological roles of Se, but remains a big challenge. Herein we report a selenol-specific recognition-mediated and europium (Eu) signal-switched amplification inductively coupled plasma mass spectrometry (ICP-MS) approach for quantifying the free active selenols (act-SeH) in cells. A bifunctional molecule, 2,4-dinitrobenzenesulfonyl-piperidin-4-yl-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic europium (DNBS-DOTA-Eu), was designed and synthesized for the specific recognition and highly sensitive quantification of act-SeH via switching Se to more sensitive Eu ICP-MS signals. The limit of detection (LOD, 3σ) of 3.41 pg/mL (22.43 pmol/L), corresponding to the absolute mass LOD of 6.82 ag act-SeH per cell, is almost 25 times lower than 83.76 pg/mL (1.06 nmol/L), 167.52 ag, when monitoring ⁸⁰Se. The results indicate that act-SeH in the selenite-precultured cancerous HepG2 and paracancerous HL7702 cells are 0.090 ± 0.002 pg/cell (n = 7) and 0.021 ± 0.006 pg/cell (n = 7), more than 4.28 times higher in HepG2 than in HL7702. Preliminary application of this approach to the cells from real hepatic tissue samples suggested that act-SeH has a positive relationship with the degree of hepatic disease. act-SeH in cells appears to be a very promising relevant index for understanding the biochemical functions of Se, besides the total Se in cells and blood serum and/or plasma.

Faster, better, and cheaper: harnessing microfluidics and mass spectrometry for biotechnology

High-throughput screening technologies are widely used for elucidating biological activities. These typically require trade-offs in assay specificity and sensitivity to achieve higher throughput. Microfluidic approaches enable rapid manipulation of small volumes and have found a wide range of applications in biotechnology providing improved control of reaction conditions, faster assays, and reduced reagent consumption. The integration of mass spectrometry with microfluidics has the potential to create high-throughput, sensitivity, and specificity assays. This review introduces the widely-used mass spectrometry ionization techniques that have been successfully integrated with microfluidics approaches such as continuous-flow system, microchip electrophoresis, droplet microfluidics, digital microfluidics, centrifugal microfluidics, and paper microfluidics. In addition, we discuss recent applications of microfluidics integrated with mass spectrometry in single-cell analysis, compound screening, and the study of microorganisms. Lastly, we provide future outlooks towards online coupling, improving the sensitivity and integration of multi-omics into a single platform.

Chip-based microfluidics on-line with inductively coupled plasma - mass spectrometry for standard dilution analysis

Microfluidics coupled on-line with ICP-MS detection can be combined with powerful quantitation procedures that take advantage of internal standardization and standard additions, such as the recently introduced Standard Dilution Analysis (SDA). Although so far used at mL min^-1 flow rates, here we demonstrate that SDA can be conveniently employed with a microfluidic chip-based ICP-MS system to improve determination accuracy for various sample types, including water, biological and cell digest samples, analyzed at μL min^-1 flow rates. The efficient coupling of a microfluidic chip to ICP-MS was accomplished using a combination of commercially available components, including a pneumatic high-efficiency nebulizer and a spray chamber designed to allow for the addition of a laminar flow makeup gas. The addition of the makeup gas was crucial in order to avoid detrimental suction effects that can disrupt the operation of the microfluidic chip and cause signal instability, while it still allowed for the highly sensitive detection of metal isotopes by using ICP-MS. All mixing and dilution operations of the sample with the two calibration solutions required for SDA were performed in an automated and highly reproducible fashion on the microfluidic chip with the assistance of an external distributor valve. High average recoveries (97.4-100.1%) and low average relative standard deviations (2.9-4.8%) were achieved for the determined elements (Cd, Co, Pb, Cr) across several spiked matrices and certified reference materials, whereas only 140 μL of sample is required for SDA in triplicate or 40 μL for a single analysis. Hence, accuracy, precision, limited sample consumption, and the elimination of the need for manual sample dilution and mixing manipulations are some of the advantages of this newly developed chip-based microfluidic SDA ICP-MS technique.

Single-cell fucosylation breakdown: Switching fucose to europium

Fucosylation and its fucosidic linkage-specific motifs are believed to be essential to understand their distinct roles in cellular behavior, but their quantitative information has not yet been fully disclosed due to the requirements of ultra-sensitivity and selectivity. Herein, we report an approach that converts fucose (Fuc) to stable europium (Eu) isotopic mass signal on hard ionization inductively coupled plasma mass spectrometry (ICP-MS). Metabolically assembled azido-fucose on the cell surface allows us to tag them with an alkyne-customized Eu-crafted bacteriophage MS2 capsid nanoparticle for Eu signal multiplication, resulting in an ever lowest detection limit of 4.2 zmol Fuc. Quantitative breakdown of the linkage-specific fucosylation motifs in situ preserved on single cancerous HepG2 and paracancerous HL7702 cells can thus be realized on a single-cell ICP-MS platform, specifying their roles during the cancering process. This approach was further applied to the discrimination of normal hepatocellular cells and highly, moderately, and poorly differentiated hepatoma cells collected from real hepatocellular carcinoma tissues.

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Switching facile fucose to stable Eu mass signal on a single-cell ICP-MS platform

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Ever lowest LOD of 4.2 zmol FucAz was achieved using a Eu-decorated MS2 nanoparticle

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Single-cell breakdown of fucosidic linkage-specific motifs

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Discrimination of highly, moderately, and poorly differentiated HCC from normal ones

The up-to-date strategies for the isolation and manipulation of single cells

Due to the large cellular heterogeneity, the strategies for the isolation and manipulation of single cells have been pronounced indispensable in the fields of disease diagnostics, drug delivery, and cancer biology at the single-cell resolution. Herein, an overview of the up-to-date techniques for precise manipulation/separation and analysis of single-cell is accomplished, these include the various approaches for the isolation and detection of individual cells in flow cytometry, microfluidic systems, micromodule systems, and others. In addition, the advanced application of these protocols is discussed. In particular, a few designs are highlighted for visualization, non-invasion, and intelligentization in single cell analysis, i.e., imaging flow cytometry, label-free microfluidic platform, single-cell capillary probe, and other related techniques. At the present, the main barriers in the various schemes for single cell manipulation which limited their practical applications are their cumbersome construction and single-functionality. The future opportunities and outstanding challenges in the isolation/manipulation of single cells are depicted.