Abstract
Microfluidic Technology
Paper-Based Ion-Selective Potentiometric Sensors
Novell et al. report a novel approach to developing ultra-low-cost, robust, rugged, and disposable potentiometric sensors. Conventional filter papers are turned into conductive papers by applying a suspension of carbon nanotubes in a water-surfactant mixture (carbon nanotubes ink). Modified filter papers with carbon nanotubes ink can then be used as substrates to build ion-selective electrodes. An ion-selective membrane cocktail is drop casted on a small circular area of the conductive paper to define the region of ion-elective membrane. As a result, the carbon nanotubes act as both electric conductors and ion-to-electron transducers to provide a potentiometric signal. Various sensors targeting different ions such as K+, NH4+, and pH were constructed and tested, and the results were compared with traditional solid-state ion-selective electrodes using carbon nanotubes as transducers and glassy carbon as a substrate. In all tests, the authors report similar analytical performance (sensitivity, linear ranges, limits of detection, selectivity, etc.) of these disposable paper electrodes to that obtained for the traditional ion-selective electrodes. The authors believe this approach opens new avenues for very low-cost chemical-sensing platforms. (Novell, M., et al., Anal. Chem.
Paper-Based Analytical Device for Particulate Metals
Mentele et al. report a paper-based analytical device (µPAD) designed to assess occupational exposure to metal-containing aerosols. The µPAD device is fabricated by wax printing on paper. A typical device includes components defined by wax such as the sample reservoir, detection and control reservoir, and pretreatment zone, all connected by channels. During detection, a punch is taken from an air-sampling filter, and the particulate metals trapped within the filter punch are digested using acid. Punches are then placed on the sample reservoir of the µPAD. Digested metals are transported to detection reservoirs upon addition of water via the capillary action of paper. Detection reservoirs are impregnated with the reagents for colorimetric detection of metals such as Fe, Cu, and Ni and dry buffer components to set the optimal pH in each detection reservoir. Precomplexation agents are also deposited in the pretreatment zone between the sample and detection zones to minimize interferences from competing metals. Metal concentrations can be quantified from analysis of the color intensity of the detection zone image obtained with a scanner. With the µPAD device, the authors report reproducible, log-linear calibration curves for each metal, with method detection limits ranging from 1.0 to 1.5 µg for each metal. Finally, a standard incineration ash sample is analyzed for the three metals of interest. The analysis results show good correlation with known amounts of the metals. The authors believe this technology shows great promise for dramatic cost reduction of the rapid assessment of particulate metal concentrations. (Mentele, M. M., et al., Anal. Chem.
Single-Cell Electroporation Using a Multifunctional Pipette
Electroporation is a commonly used technique in biology laboratories. It uses electric pulse to transiently open up nanometer-sized pores on cell membranes, allowing transport of genetic substances such as DNA/RNA into the cells. If performed correctly, cells should maintain their functions and viability and should quickly recover from the electroporation. Electroporation is a powerful tool that enables a wide variety of studies such as the investigation of gene expression and RNA interference. Ainla et al. report a very interesting way to perform single-cell electroporation using a microfluidic multifunctional pipette. This multifunctional pipette features a built-in microelectrode for electroporation and a rather sophisticated channel network within the pipette to allow localized solution delivery and switching. Such a fully integrated device configuration enables unique application for single-cell study. The microelectrode embedded at the probe tip makes it possible to individually probe single cells in a large cell population, whereas the localized solution delivery and switching ensure surrounding cells are not affected by the chemical/biological treatment. This unique combination provides versatility and performance that are far beyond traditional electroporation apparatuses. (Ainla, A., et al., Lab Chip
Sensing Technology
Toward CMOS Image Sensor-Based Glucose Monitoring
The complementary metal oxide semiconductor (CMOS) image sensor has being winning battles against its rival charge-coupled device (CCD) in digital imaging as it gains popularity in portal electronic devices such as smart phones and tablets. It appears that it might also be gaining favor in laboratory applications.
Devadhasan and Kim report a glucose-monitoring application based on a CMOS image sensor. In this study, the CMOS image sensor is exploited for detecting glucose levels by simple photon count variation with high sensitivity. Various concentrations of glucose (ranging from 100 mg/dL to 1000 mg/dL) are added onto a simple polydimethylsiloxane (PDMS) membrane on top of the CMOS sensor, and glucose was oxidized via an enzymatic reaction catalyzed by glucose oxidase. Oxidized glucose yields a brown color with the help of chromogen during the enzymatic reaction, and the color density varies with the glucose concentration. An ordinary ceiling light is used as a light source for the experiment. Photons from the ceiling light pass through the glucose solution with varying color density and arrive at photo sensor pixels. Photons are counted by the CMOS image sensor, and the color density, and hence the glucose concentration, can be calculated based on the digital output of the sensor. By correlating the CMOS output with glucose concentration, the authors show how it can be possible to measure a wide range of blood glucose levels with great linearity. Based on these results, the authors claim the CMOS sensor can be used as a convenient point-of-care diagnosis device. (Devadhasan and Kim, Analyst
On-Chip Radiation Biodosimetry with Three-Dimensional Microtissues
In field applications, it is highly desirable to quickly and accurately determine radiation-induced living cell damage. Currently, several different types of radiation dosimeters such as ion chambers, quartz fiber dosimeters, film dosimeters, and polymer gel dosimeters are available for determining type and dose of radiation. However, none of these can provide information regarding the biological response to radiation. Therefore, there has been much interest in developing biological dosimeter based on biomaterials such as live cells. Biological dosimeter or biodosimeter can be a great complement to physical dosimeter in field applications as it can provide information related to radiation injury assessment and medical treatment.
Luo et al. report an image-based, on-chip microtissue radiation biodosimeter that is capable of simultaneously monitoring radiation responses of multiple mammalian cell types. The microtissue chip includes microwells fabricated by molding molten agarose gel. Different microwells are seeded with a variety of cell suspensions that are immobilized in agarose gel. Images of cell-containing microwells can be made with a cellphone camera, and the color changes of microtissues upon X-ray irradiation can be recorded and used as the basis for accurate determination of radiation-related cell death and radiation dose. Another advantage of the device is that the images can be wirelessly transmitted to a safe location, allowing remote sensing and monitoring of radiation exposure in field applications. (Luo, Y., et al., Analyst
A Fluorescein-Based Probe with High Selectivity to Cysteine over Homocysteine and Glutathione
Cysteine (Cys), homocysteine (Hcy), and glutathione (GSH) are a group of intracellular thiols, each of which plays an important and unique role in the regulation of important cellular processes. For example, GSH is the antioxidant, Hcy is critical for sulfur metabolism and the methionine cycle, and Cys is the precursor of GSH. Changes of intracellular thiol concentrations are relative to many diseased states. Therefore, the detection of these intracellular thiols is of great importance. One of the ways to detect intracellular thiols is with fluorescent probes, the dyes that yield fluorescence upon specific binding with certain intracellular thiol. Because of the similarity in both structure and reactivity, it is very difficult to develop a fluorescence dye that can discriminate these three thiols.
Wang et al. report the synthesis strategy of a fluorescence sensor probe that is very highly selective to Cys over Hcy and GSH. This new fluorescent probe is a derivative of commonly used fluorescent dye, fluorescein. In the study, fluorescein is reacted with acryloyl chloride in the presence of triethylamine in CH2Cl2 to introduce double acrylate moieties. During detection, the reaction of thiol to the acryloyl group in the probe followed by the cleavage of an ester bond to form the fluorescein is believed to be the cause of fluorescence enhancement. It is also speculated that double acrylate moieties significantly enhance the selectivity of probe to Cys. In experiments, strong fluorescent peaks specific to Cys in background of Hcy, GSH, and many other amino acids are demonstrated. Good cell permeability and cell staining also are demonstrated with live PC-12 cells in confocal microscopy. (Wang, H., et al., Chem. Comm.
Novel Analytical Methods
In Situ Molecular Analysis of Plant Tissues by Live Single-Cell Mass Spectrometry
Metabolite profiling of plants is critical for understanding the biochemistry of plants. The difficulty of performing plant metabolite profiling is that plant metabolites are extremely diverse, and many of them are easily oxidized. Therefore, the extraction process and the equipment can affect the success of a study. Traditionally, bulk plant extracts have been the main resource in plant biochemistry. This method, however, is limited as it provides information on only a mixture of all the tissues. Ideally, a good sample extraction method should make it possible to analyze a few cells in the same tissue or even single cells.
Tejedor et al. report the development of a rapid, direct molecular analysis of live, single plant cells. The extraction is performed with live plant tissues viewed under a video microscope in their natural environment. The authors use a nanoelectrospray tip to extract the contents of a single leaf, stem, or petal cell from Pelargonium zonale, and samples are analyzed with a mass spectrometer by nanoelectrospray ionization. About 1000 m/z peaks belonging to metabolites and other compounds in each sample are obtained. Statistical tools are used to identify the cell-specific molecular peaks. Hybrid high-resolution mass spectrometry analysis is performed to confirm the structure of specific metabolites from the analyzed samples. The authors believe this is a useful method for identifying specific molecules in live single cells from plant tissue and allow different cell types and stages from different sites in the plant to be compared with morphological observations. (Tejedor, M. L., et al., Anal. Chem.
Microarray of Human P450 with an Integrated Oxygen Sensing Film for High-Throughput Detection of Metabolic Activities
As a crucial part of drug and dietary material metabolism in the human body, the cytochrome P450 enzyme super family has been a main target in many studies ranging from identification of toxicity in drug candidates to food and environmental safety inspections. Currently, there is a major gap between the need of metabolic inspections and the capability that current instruments, such as HPLC-MC, can provide. Therefore, there is a huge need for a high-throughput assay for P450 metabolic profiling.
To address this issue, various high-throughput microarray-based P450 assay platforms have been developed. However, one of the key challenges is that most of these assays are based on fluorogenic substrates, which are applicable to only a limited number of P450 isoforms. To overcome this disadvantage, Chang et al. propose a new P450-sensing platform for P450 activity monitoring with oxygen-sensing films. The authors argue that oxygen-sensing film-based detection should be universal for all P450 isoforms as oxygen is involved in all metabolic reaction catalyzed by P450.
In the study, the authors construct a human P450 microarray that contains multiple microwells for parallel assay of P450 metabolic activities. Microwells contain oxygen-sensing films at the bottom. These oxygen-sensing films are made of organically modified silica films (ORMOSIL) doped with tris(4,7-diphenyl-1,10-phenanthroline) ruthenium dichloride (Ru(dpp)3Cl2). Various types of human P450s are mixed in the agarose matrix and deposited on top of the oxygen-sensing films. Activities of P450s can be determined via the fluorescence intensity enhancement of the oxygen sensors due to the oxygen consumption by the metabolic reaction. By analyzing the florescence enhancement pattern, reaction of various P450 isoforms and substrate material can be analyzed.
For example, the authors discovered that the fluorescence enhancement patterns obtained from two psoralen derivatives resembled each other, whereas a structurally different substrate (capsaicin) resulted in a distinct pattern, suggesting the potential of the microarray to analyze the activities of diverse P450 isoforms in a high-throughput fashion. In addition, the authors also find that the P450 microarray with an oxygen sensor can also be used to detect mechanism-based inactivation (BMI) by chemicals commonly found in food. Additional benefits using a smaller microwell format, compared with a commercial plate, include much less sample volume and shorter assay time. (Chang, G., et al., Anal. Chem.
Millikan’s Oil Drop Experiment in Nanometer Scale: Measuring the Size and Charge of Single Nanoscale Objects in Solution Using an Electrostatic Fluidic Trap
Millikan’s oil drop experiment was an experiment performed by Millikan and Fletcher in the 1900s to determine the charge of a single electron. This experiment is a classic example of genius experimental designs of modern physics. The experiment is based on the balance of gravity, viscous drag, and electric force of a tiny charged oil droplet suspended in water between two electrodes. Knowing the density and size of the droplets, the charge of the oil droplet can be calculated based on the equilibrium of gravity, drag, and electric force. After multiple experiments with many different droplets, it was found that the charges of droplets were always multiples of a certain fundamental value, which is the charge of a single electron.
This principle is reapplied in namometer scale by Mojarad and Krishnan, who report a novel method to measure the size and charge of single nanoscale objects in solution using an electrostatic fluidic trap. Measuring the size and charge of tiny objects in solution, such as dispersions of colloids or macromolecules, has been very challenging. Traditional size measurement based on light scattering does not work well for nanometer scale objects because the light-scattering intensity decreases rather quickly as the size of objects shrinks: it scales as the six power of size. In terms of charge measurement, techniques based on collective migration of species in an externally applied field suffer from low accuracy and poor resolution.
In this study, the authors propose a new method to measure the size and charge of single nanoscale objects via high-throughput analysis of their thermal motion. Objects in suspension are introduced in a nanometer-sized electrostatic fluidic trap, where they remain trapped for a period of time, during which the viscous drag on the object and the stiffness of its confinement can be measured. The viscous drag yields information on object’s size, whereas confinement stiffness data allow calculation of the charges that objects carry. (Mojarad, N., and Krishnan, M., Nat. Nanotech.
Tissue Engineering
Neuroscience Goes on a Chip
Soe et al. provide a review of lab-on-a-chip system applications in neuroscience. Advances in microelectronics, microfluidics, polymers, and microfabrication make it possible to create lab-on-a-chip systems sophisticated enough to study neuroscience. Examples of applications that have been enabled by lab-on-a-chip systems include cellular and molecular biochemical experimentations, morphological observations, and electrophysiological investigations. The advantages of integrating miniaturized components are a high level of automation, rapid analysis, and flexibility in device operation. With these features, the authors see possibilities to replace bulky and expensive bench-top instruments with their lab-on-a-chip counterparts to perform genomic, proteomic, epigenomic, peptidomic, connectomic, and electrophysiological studies. In addition, it is also possible to use lab-on-a-chip systems to perform even higher-level studies, including developmental neurobiology and behavioral investigations. In this review, the authors discuss the functions and features of existing bench-top neuroscience instruments, followed by a review of their lab-on-a-chip counterparts. Finally, perspectives on the opportunities for lab-on-a-chip systems in neuroscience research are provided. (Soe, A. K., et al., Biosen. Bioe.
Portable Electronics
Biofuel Cell as a Power Source for Electronic Contact Lenses
Falk et al. report an interesting application of a biofuel cell in portable electronics. In their study, they demonstrate experimental proof that microscale membraneless, direct electron transfer–based enzymatic fuel cells can produce sufficient amounts of electrical energy in human lachrymal liquid (tears) to power electronic contact lenses. This biofuel cell uses 100 μm diameter gold wires, covered with 17 nm gold nanoparticles, as three-dimensional nanostructured microelectrodes. The microelectrodes are modified with Corynascus thermophilus cellobiose dehydrogenase and Myrothecium verrucaria bilirubin oxidase as anodic and cathodic bioelements, respectively. Operating in human tears and using the glucose as biofuel and oxygen as biooxidant, this miniature biofuel cell is capable of generating 0.57 V open-circuit voltage, about 1 µW cm−2 maximum power density at a cell voltage of 0.5 V with a more than 20 h operational half-life. The authors perform the theoretical calculation of maximum recoverable electrical energy that can be extracted from a human tear based on the amounts of glucose and oxygen that are available and show that it is possible to use such a biofuel cell to power smart contact lenses using just human tears. (Falk, M., et al., Biosen. Bioe.