Integrated microfluidic system with chemiluminescence detection for single cell analysis after intracellular labeling

A chemiluminescence method induced by microplasma jet for nitrites detection and the miniature detection system using smartphone

A method of luminol-diazonium chemiluminescence (CL) induced by microplasma for hazardous substance detection is proposed. The luminol-diazonium CL is caused by microplasma jet, rather than hydrogen peroxide reagent or other oxidizing agents. The CL intensity is increased by the concentration of nitrites. Based on the process of microplasma generation and CL mechanism, the optimal work conditions of the method are obtained. The linear range for nitrites detection is 0.03-1 mmol L^-1 with the limit of detection (LOD) of 0.01 mmol L^-1. Furthermore, a miniature system using test paper and smartphone is designed for nitrites detection in emergency. The detection system is confined in the custom-tailored shell which is only 28 cm in length, 18 cm in width and 10 cm in height. After microplasma jet treatment, the color of the test paper changes with the NO₂^- concentration. The photographs of the test paper are taken by the built-in camera of smartphone and analyzed by visiting the website via smartphone. The LOD is 1 mmol L^-1 obtained by the CL miniature detection system based on test paper and smartphone. The accuracy, reliability and practicability of the proposed method is verified in this paper.

Electrochemical Nanoaptasensor for Continuous Monitoring of ATP Fluctuation at Subcellular Level

Monitoring the fluctuation of adenosine 5'-triphosphate (ATP) at the subcellular level is important for the study of cell energy metabolism. Herein, we fabricated an electrochemical nanoaptasensor for continuously monitoring ATP fluctuation at the subcellular level. A gold nanoelectrode with a diameter of 120 nm was fabricated, and ferrocene (Fc)-labeled anti-ATP aptamer was self-assembled onto the nanoelectrode surface to form a nanoaptasensor. In the presence of ATP, the ferrocene-labeled anti-ATP aptamer bound with two ATP units to form an ATP-aptamer conjugation, resulting in the close proximity of Fc to the nanoelectrode surface and then an increase of oxidation current of Fc. ATP can be detected with a detection limit of 26 μM within 2 min. Cell viability assays indicated that the nanoaptasensor was biocompatible with negligible biological effects. By taking advantage of the good biocompatibility of the nanoaptasensor, ATP fluctuation at the subcellular level was monitored under glucose starvation and Ca²⁺ induction. This work demonstrates that the nanoaptasensor is a useful tool for investigating ATP-relevant biological processes via the electrochemical method.

An ultrasensitive microchip electrophoresis chemiluminescence assay platform for detection of trace biomolecules

An ultrasensitive microchip electrophoresis chemiluminescence (MCE-CL) assay platform based on separation assisted cascade signal amplification was developed for detection of trace biomolecules. In this work, the aptamer was used as a target probe to bind target molecule and triggering cascade signal amplification reaction. The horseradish peroxide labeled DNA (HRP-DNA) was used as signal probe, utilizing nucleic acid hybridization and exonuclease cutting technology realized ultrasensitive detection of biomolecules on the MCE-CL assay platform. Taking gamma interferon (IFN-γ) as a model analyte, the linear range for IFN-γ detection is 8.0 × 10^-15-1.0 × 10^-8 M, the detection limit is 1.6 fM, which is six orders magnitude lower than that of without signal amplification. The proposed method was successfully applied for the quantification of IFN-γ in human plasma samples. It was demonstrated that the MCE-CL assay platform was quick, sensitive, and highly selective. It may serve as a tool for clinical analysis of IFN-γ to assist in the diagnosis of disease.

A G-quadruplex/hemin DNAzyme-based microchip electrophoresis chemiluminescence assay for highly sensitive detection of biotin in flour

Quantitative analysis of biotin in biological fluids, foods, and pharmaceutical is important for diagnosis and treatment of biotin-related diseases and health maintenance. In this work, a novel G-quadruplex/hemin DNAzyme-based microchip electrophoresis chemiluminescence (CL) assay method was established for rapid and highly sensitive detection of biotin. This method is based on the specific binding between biotin and streptavidin, the catalytic CL characteristics of G-quadruplex/hemin DNAzyme to the oxidation-reduction reaction of hydrogen peroxide with luminol, and the on-line separation function of microchip electrophoresis. Under the optimal experimental conditions, on-chip biotin analysis was achieved within 1 min. The CL intensity is linearly proportional to the concentration of biotin in the range of 13-630 nM with a detection limit of 6.4 nM. The proposed method has been applied for the detection of biotin in flour, biotin contents in three flour samples are found in the range of 199-223 ng/g with a mean value of 214 ng/g. The recoveries were in the range of 94-103%. With excellent sensitivity and good selectivity, the proposed method could be applied in a wide range of biological fluids, foods, and pharmaceutical analysis.

Microfluidic single-cell analysis of oxidative stress in Dictyostelium discoideum

Microfluidic chemical cytometry is a powerful technique for examining chemical contents of individual cells, but applications have focused on cells from multicellular organisms, especially mammals. We demonstrate the first use of microfluidic chemical cytometry to examine a unicellular organism, the social amoeba Dictyostelium discoideum. We used the reactive oxygen species indicator dichlorodihydrofluorescein diacetate to report on oxidative stress and controlled for variations in indicator loading and retention using carboxyfluorescein diacetate as an internal standard. After optimizing indicator concentration, we investigated the effect of peroxide treatment through single-cell measurements of 353 individual cells. The peak area ratio of dichlorofluorescein to carboxyfluorescein increased from 1.69 ± 0.89 for untreated cells to 5.19 ± 2.72 for cells treated with 40 mM hydrogen peroxide. Interestingly, the variance of the data also increased with oxidative stress. While preliminary, these results are consistent with the hypothesis that heterogeneous stress responses in unicellular organisms may be adaptive.

Laser-treated glass platform for rapid wicking-driven transport and particle separation in bio microfluidics

In this work, we present a laser-based fabrication technique for direct patterning of micro-channels consisting of interconnected micro-cracks on soda-lime glass.

Sensitive analysis of multiple low-molecular-weight thiols in a single human cervical cancer cell by chemical derivatization-liquid chromatography-mass spectrometry

Simultaneous quantification of multiple low-molecular-weight thiols from a single HeLa cell was realized by chemical derivatization assisted LC-MS method.

Recent advances in the use of microfluidic technologies for single cell analysis

The inherent heterogeneity in cell populations has become of great interest and importance as analytical techniques have improved over the past decades. With the advent of personalized medicine, understanding the impact of this heterogeneity has become an important challenge for the research community. Many different microfluidic approaches with varying levels of throughput and resolution exist to study single cell activity. In this review, we take a broad view of the recent microfluidic developments in single cell analysis based on microwell, microchamber, and droplet platforms. We cover physical, chemical, and molecular biology approaches for cellular and molecular analysis including newly emerging genome-wide analysis.

Quantification of glutathione in single cells from rat liver by microchip electrophoresis with chemiluminescence detection

Glutathione (GSH) is a major endogenous antioxidant that has a central role in cellular defense against toxins and free radicals. Rapid and accurate detection of GSH content in single cells is important to the early diagnosis of disease and biomedical research. In this work, a novel method based on microchip electrophoresis chemiluminescence (MCE-CL) detection was developed for the quantification of glutathione (GSH) in single cells from rat liver. The detection of GSH is based on the strong sensitization of mercapto compound to luminol-H₂O₂CL system. The injection, localization, and membrane dissolution of single cell were simply and rapidly carried out on the microchip by direct electric field force, which did not require any additional membrane dissolution reagent. Under optimized experimental conditions, single cell assay was achieved within 2min. The peak area of the GSH was taken as quantification of GSH, and a good linear relationship of GSH concentration to peak area in the range of 3.0 × 10^-6M to 6.0 × 10^-4M was obtained. The detection limit for GSH is 9.6 × 10^-7M, calculated by S/N = 3. The measured GSH content in single cells from rat liver (n = 10) ranged from 7.8fmol to 13.fmol with a mean value of 10.8fmol.

A microchip electrophoresis-based fluorescence signal amplification strategy for highly sensitive detection of biomolecules

We have developed a microchip electrophoresis (MCE)-based fluorescence signal amplification strategy as a universal MCE method for the detection of trace biomolecules. This strategy exhibits high sensitivity and specificity for target molecules, and has been applied for the detection of interferon-gamma (IFN-γ) in human plasma with satisfactory results.

Novel label-free and high-throughput microchip electrophoresis platform for multiplex antibiotic residues detection based on aptamer probes and target catalyzed hairpin assembly for signal amplification

Novel label-free and multiplex aptasensors have been developed for simultaneous detection of several antibiotics based on a microchip electrophoresis (MCE) platform and target catalyzed hairpin assembly (CHA) for signal amplification. Kanamycin (Kana) and oxytetracycline (OTC) were employed as models for testing the system. These aptasensors contained six DNA strands termed as Kana aptamer-catalysis strand (Kana apt-C), Kana inhibit strand (Kana inh), OTC aptamer-catalysis strand (OTC apt-C), OTC inhibit strand (OTC inh), hairpin structures H1 and H2 which were partially complementary. Upon the addition of Kana or OTC, the binding event of aptamer and target triggered the self-assembly between H1 and H2, resulting in the formation of many H1-H2 complexes. They could show strong signals which represented the concentration of Kana or OTC respectively in the MCE system. With the help of the well-designed and high-quality CHA amplification, the assay could yield 300-fold amplified signal comparing that from non-amplified system. Under optimal conditions, this assay exhibited a linear correlation in the ranges from 0.001ngmL^-1 to 10ngmL^-1, with the detection limits of 0.7pgmL^-1 and 0.9pgmL^-1 (S/N=3) toward Kana and OTC, respectively. The platform has the following advantages: firstly, the aptamer probes can be fabricated easily without labeling signal tags for MCE detection; Secondly, the targets can just react with probes and produce the amplified signal in one-pot. Finally, the targets can be simultaneously detected within 10min in different channels, thus high-throughput measurement can be achieved. Based on this work, it is estimated that this detection platform will be universally served as a simple, sensitive and portable platform for antibiotic contaminants detection in biological and environmental samples.

Core-shell self-assembly triggered via a thiol-disulfide exchange reaction for reduced glutathione detection and single cells monitoring

A novel core-shell DNA self-assembly catalyzed by thiol-disulfide exchange reactions was proposed, which could realize GSH-initiated hybridization chain reaction (HCR) for signal amplification and molecules gathering. Significantly, these self-assembled products via electrostatic interaction could accumulate into prominent and clustered fluorescence-bright spots in single cancer cells for reduced glutathione monitoring, which will effectively drive cell monitoring into a new era.

Response of single leukemic cells to peptidase inhibitor therapy across time and dose using a microfluidic device

Single-cell methodologies are revealing cellular heterogeneity in numerous biological processes and pathologies. For example, cancer cells are characterized by substantial heterogeneity in basal signaling and in response to perturbations, such as drug treatment. In this work, we examined the response of 678 individual U937 (human acute myeloid leukemia) cells to an aminopeptidase-inhibiting chemotherapeutic drug (Tosedostat) over the course of 95 days. Using a fluorescent reporter peptide and a microfluidic device, we quantified the rate of reporter degradation as a function of dose. While the single-cell measurements reflected ensemble results, they added a layer of detail by revealing unique degradation patterns and outliers within the larger population. Regression modeling of the data allowed us to quantitatively explore the relationships between reporter loading, incubation time, and drug dose on peptidase activity in individual cells. Incubation time was negatively correlated with the number of peptide fragment peaks observed, while peak area (which was proportional to reporter loading) was positively correlated with both the number of fragment peaks observed and the degradation rate. Notably, a statistically significant change in the number of peaks observed was identified as dose increased from 2 to 4 μM. Similarly, a significant difference in degradation rate as a function of reporter loading was observed for doses ≥2 μM compared to the 1 μM dose. These results suggest that additional enzymes may become inhibited at doses >1 μM and >2 μM, demonstrating the utility of single-cell data to yield novel biological hypotheses.

Fast quantification of amino acids by microchip electrophoresis-mass spectrometry

A fast microchip electrophoresis-nano-electrospray ionization-mass spectrometric method (MCE-nanoESI-MS) was developed for analysis of amino acids in biological samples. A glass/poly(dimethylsiloxane) hybrid microchip with a monolithic nanoESI emitter was used in the platform. The proposed MCE-nanoESI-MS analytical method showed high separation efficiency for amino acids. Baseline separation of an amino acid mixture containing Lys, Arg, Val, Tyr, and Glu was completed within 120 s with theoretical plate numbers of >7,500. The method was applied to study cellular release of excitatory amino acids (i.e., aspartic acid (Asp) and glutamic acid (Glu)) under chemical stimulations. Linear calibration curves were obtained for both Asp and Glu in a concentration range from 1.00 to 150.0 μM. Limits of detection were found to be 0.37 μM for Asp and 0.33 μM for Glu (S/N = 3). Assay repeatability (relative standard deviation, n = 6) was 4.2 and 4.5%, for Asp and Glu at 5.0 μM, respectively. In the study of cellular release, PC-12 nerve cells were incubated with alcohol at various concentrations for 1 h. Both extra- and intracellular levels of Asp and Glu were measured by the proposed method. The results clearly indicated that ethanol promoted the release of both Asp and Glu from the cells.

Recent advances in the development of single cell analysis--a review

Development of techniques for the analysis of the content of individual cells represents an important direction in modern bioanalytical chemistry. While the analysis of chromosomes, organelles, or location of selected proteins has been traditionally the domain of microscopic techniques, the advances in miniaturized analytical systems bring new possibilities for separations and detections of molecules inside the individual cells including smaller molecules such as hormones or metabolites. It should be stressed that the field of single cell analysis is very broad, covering advanced optical, electrochemical and mass spectrometry instrumentation, sensor technology and separation techniques. The number of papers published on single cell analysis has reached several hundred in recent years. Thus a complete literature coverage is beyond the limits of a journal article. The following text provides a critical overview of some of the latest developments with the main focus on mass spectrometry, microseparation methods, electrophoresis in capillaries and microfluidic devices and respective detection techniques for performing single cell analyses.

Single cell detection using a magnetic zigzag nanowire biosensor

A magnetic zigzag nanowire device was designed for single cell biosensing. Nanowires with widths of 150, 300, 500, and 800 nm were fabricated on silicon trenches by electron beam lithography, electron beam evaporation, and lift-off processes. Magnetoresistance measurements were performed before and after the attachment of a single magnetic cell to the nanowires to characterize the magnetic signal change due to the influence of the magnetic cell. Magnetoresistance responses were measured in different magnetic field directions, and the results showed that this nanowire device can be used for multi-directional detection. It was observed that the highest switching field variation occurred in a 150 nm wide nanowire when the field was perpendicular to the substrate plane. On the other hand, the highest magnetoresistance ratio variation occurred in a 800 nm wide nanowire also when the field was perpendicular to the substrate plane. Besides, the trench-structured substrate proposed in this study can fix the magnetic cell to the sensor in a fluid environment, and the stray field generated by the corners of the magnetic zigzag nanowires has the function of actively attracting the magnetic cells for detection.

Microfluidic chemical cytometry of peptide degradation in single drug-treated acute myeloid leukemia cells

Microfluidic systems show great promise for single-cell analysis; however, as these technologies mature, their utility must be validated by studies of biologically relevant processes. An important biomedical application of these systems is characterization of tumor cell heterogeneity. In this work, we used a robust microfluidic platform to explore the heterogeneity of enzyme activity in single cells treated with a chemotherapeutic drug. Using chemical cytometry, we measured peptide degradation in the U937 acute myeloid leukemia (AML) cell line in the presence and absence of the aminopeptidase inhibitor Tosedostat (CHR-2797). The analysis of 99 untreated cells revealed rapid and consistent degradation of the peptide reporter within 20 min of loading. Results from drug-treated cells showed inhibited, but ongoing degradation of the reporter. Because the device operates at an average sustained throughput of 37 ± 7 cells/h, we were able to sample cells over the course of this time-dependent degradation. In data from 498 individual drug-treated cells, we found a linear dependence of degradation rate on amount of substrate loaded superimposed upon substantial heterogeneity in peptide processing in response to inhibitor treatment. Importantly, these data demonstrated the potential of microfluidic systems to sample biologically relevant analytes and time-dependent processes in large numbers of single cells.

Evaluation of a microchip electrophoresis-mass spectrometry platform deploying a pressure-driven make-up flow

Integration of a pressure-driven make-up flow (MUF) into a microchip electrophoresis (MCE) platform in order to facilitate its coupling with electrospray ionization-mass spectrometric detection (ESI-MS) is described. In the glass/PDMS hybrid microchip, a MUF channel was made to intersect with the MCE separation channel at an angle of 45°. The MUF was generated by a syringe pump. Microscopic image results from simulation studies showed that the pressure-driven MUF and the potential-driven electroosmotic flow in the MCE separation channel could be run separately without interfering with each other and mixed well at the joint point by adjusting either the MUF flow rate or the potential applied for MCE separation. The MUF had several desirable functions, including making the start of electrospray easy and cleaning the nanoESI emitter continuously when not spraying. High separation efficiency was achieved with the proposed MCE-nanoESI-MS system in separating an amino acid mixture containing glutamine, serine, threonine, phenylalanine, and glutamic acid. All of them were baseline separated from each other within 3 min. Plate numbers of >10,000 (on a 2.5 cm MCE separation channel) were obtained. The analytical platform also showed a linear response for quantification of DOPA with a detection limit (S/N=3) of 0.10 μM. In addition, on-line derivatization of MCE elutes in order to enhance MS detection sensitivity was easily carried out by adding the tagging reagent into the MUF. These results indicated that the present system might have a good potential in MCE-MS applications.

Detection of electrochemiluminescence from floating metal platelets in suspension

We present a generation of electrochemiluminescence (ECL) signal, based on square shaped gold electrodes with a size of 50 μm positioned inside a fused silica capillary. The ECL was generated using electric pulses with duration in the range from 100 ms to 5 s and an electrical field strength from 300 V cm(-1) to 500 V cm(-1). We have demonstrated that the electrochemical reaction with detectable optical output can be produced using freely moving and thus disposable electrodes.

GOLD NANOPARTICLES-BASED BIOSENSORS FOR BIOMEDICAL APPLICATION

Gold nanoparticles are the most extensively studied nanomaterials for biomedical application due to their unique properties, such as rapid and simple synthesis, large surface area, strong adsorption ability and facile conjugation to various biomolecules. The remarkable photophysical properties of gold nanoparticles have provided plenty of opportunities for the preparation of gold nanoparticles-based optical biosensors, while the excellent biocompatibility, conductivity, catalytic properties and large surface-to-volume ratio have facilitated the application of gold nanoparticles in the construction of electrochemical biosensors. In this review, we mainly detail the gold nanoparticles-based optical and electrochemical biosensors for biomedical application in the recent two years, which have exhibited greatly enhanced analytical performances in the detection of DNA, proteins and some important small molecules.

Targeted isolation and analysis of single tumor cells with aptamer-encoded microwell array on microfluidic device

Microfluidic-based single cells analysis has been of great interest in recent years, promising disease diagnosis and personalized medicine. Current technologies are challenging in bioselectively isolating specific single cells from complex matrices. Herein, a novel microfluidic platform integrated with cell-recognizable aptamer-encoded microwells was specifically developed to isolate single tumor cells with satisfied single-cell occupancy and unique bioselectivity. In this work, the designed microwell-structures enable us to encourage strong 3D local topographic interactions of the target cell surface with biomolecules and regulate the single-cell resolution. Under the optimized size of microwells, the single-cell occupancy was significantly enhanced from 0.5% to 88.2% through the introduction of the aptamer. Analysis of the target cells was directly performed in short time periods (<5.0 min) with small volumes (4.5 μL). Importantly, such an aptamer-enabled microfluidic device shows an excellent selectivity for target single cells isolation compared with three control cells. Subsequently, targeted isolation and analysis of single tumor cells were demonstrated by using artificial complex cell samples at simulated conditions, and various cellular carboxylesterases were studied by time-course measurements of cellular fluorescence kinetics at individual-cell level. Thus, our technique will open up a new opportunity in single-cell level-based disease diagnosis and personalize medicine screening.

Quantification of amino acids in a single cell by microchip electrophoresis with chemiluminescence detection

Analyzing individual cells allows detecting a minor group of abnormal cells present in a large population of normal cells. This ability can be essential to understanding diseases, such as cancer and diabetes. Microchip electrophoresis (MCE) is the technique of choice for single-cell analysis. However, since the channels in microfluidic devices are very small, achieving the desired assay sensitivity on a microfluidic platform remains a challenge. Here, we describe an MCE method with highly sensitive chemiluminescence detection for simultaneous determination of multiple amino acids present in single cells.

A magnetically active microfluidic device for chemiluminescence bioassays

Highly active horseradish peroxidase functionalized magnetic nanoparticles were prepared and packed into a microfluidic channel, producing an in-line bioreactor that enabled a sensitive chemiluminescence assay of H(2)O(2). The proposed magnetically active microfluidic device proved useful for chemiluminescence assays of biomedically interesting compounds.

Electrokinetic gated injection-based microfluidic system for quantitative analysis of hydrogen peroxide in individual HepG2 cells

A microfluidic system to determine hydrogen peroxide (H(2)O(2)) in individual HepG2 cells based on the electrokinetic gated injection was developed for the first time. A home-synthesized fluorescent probe, bis(p-methylbenzenesulfonate)dichlorofluorescein (FS), was employed to label intracellular H(2)O(2) in the intact cells. On a simple cross microchip, multiple single-cell operations, including single cell injection, cytolysis, electrophoresis separation and detection of H(2)O(2), were automatically carried out within 60 s using the electrokinetic gated injection and laser-induced fluorescence detection (LIFD). The performance of the method was evaluated under the optimal conditions. The linear calibration curve was over a range of 4.39-610 amol (R(2)=0.9994). The detection limit was 0.55 amol or 9.0×10(-10) M (S/N=3). The relative standard deviations (RSDs, n=6) of migration time and peak area were 1.4% and 4.8%, respectively. With the use of this method, the average content of H(2)O(2) in single HepG2 cells was found to be 16.09±9.84 amol (n=15). Separation efficiencies in excess of 17,000 theoretical plates for the cells were achieved. These results demonstrated that the efficient integration and automation of these single-cell operations enabled the sensitive, reproducible, and quantitative examination of intracellular H(2)O(2) at single-cell level. Owing to the advantages of simple microchip structure, controllable single-cell manipulation and ease in building, this platform provides a universal way to automatically determine other intracellular constituents within single cells.