Aida Ebrahimi

High sensitivity interdigited capacitive sensors using branched treelike carbon nanotubes on silicon membranes

Branched treelike carbon nanotubes on silicon substrate have been exploited for the realization of high sensitivity interdigital capacitive pressure sensors. The interdigital structure has been realized using a micromachining technique on silicon membranes, whereas the growth of nanotubes has been achieved using a direct-current plasma enhanced chemical vapor deposition method. A sequential growth and hydrogenation has led to the formation of multiple branched structures of nanotubes. The growth in an interdigital manner results in a high overlap between neighboring fingers and consequently a magnified response to mechanical variations in the membrane as a result of applying an external pressure is observed. An oscillatory behavior has been observed which may be attributed to the vibration of nanotubes on thinned membranes.

Droplet-based Biosensing for Lab-on-a-Chip, Open Microfluidics Platforms.

Low cost, portable sensors can transform health care by bringing easily available diagnostic devices to low and middle income population, particularly in developing countries. Sample preparation, analyte handling and labeling are primary cost concerns for traditional lab-based diagnostic systems. Lab-on-a-chip (LoC) platforms based on droplet-based microfluidics promise to integrate and automate these complex and expensive laboratory procedures onto a single chip; the cost will be further reduced if label-free biosensors could be integrated onto the LoC platforms. Here, we review some recent developments of label-free, droplet-based biosensors, compatible with “open” digital microfluidic systems. These low-cost droplet-based biosensors overcome some of the fundamental limitations of the classical sensors, enabling timely diagnosis. We identify the key challenges that must be addressed to make these sensors commercially viable and summarize a number of promising research directions.

Evaporation-induced stimulation of bacterial osmoregulation for electrical assessment of cell viability

Significance Conventional bacterial viability assays rely on cell multiplication until they are detectable by optical, electrical, or other sensors. Consequently, the assay time of classical growth-based techniques ranges from hours to weeks depending on bacteria type. In contrast, we present a fundamentally different paradigm based on bacterial osmoregulation to identify viable cells in minutes. Our label-free platform relies on two key advances: (i) the fact that osmoregulation is as universal as cell division and (ii) the ability to create a microliter-sized environment on a specially designed multifunctional structure. Our contribution advances the field with a collage of ideas from diverse disciplines (e.g., biology, microfluidics, surface energetics, and impedance spectroscopy) and therefore, will attract a broad audience of physicists, material scientists, biologists, and engineers. Bacteria cells use osmoregulatory proteins as emergency valves to respond to changes in the osmotic pressure of their external environment. The existence of these emergency valves has been known since the 1960s, but they have never been used as the basis of a viability assay to tell dead bacteria cells apart from live ones. In this paper, we show that osmoregulation provides a much faster, label-free assessment of cell viability compared with traditional approaches that rely on cell multiplication (growth) to reach a detectable threshold. The cells are confined in an evaporating droplet that serves as a dynamic microenvironment. Evaporation-induced increase in ionic concentration is reflected in a proportional increase of the droplet’s osmotic pressure, which in turn, stimulates the osmoregulatory response from the cells. By monitoring the time-varying electrical conductance of evaporating droplets, bacterial cells are identified within a few minutes compared with several hours in growth-based methods. To show the versatility of the proposed method, we show detection of WT and genetically modified nonhalotolerant cells (Salmonella typhimurium) and dead vs. live differentiation of nonhalotolerant (such as Escherichia coli DH5α) and halotolerant cells (such as Staphylococcus epidermidis). Unlike the growth-based techniques, the assay time of the proposed method is independent of cell concentration or the bacteria type. The proposed label-free approach paves the road toward realization of a new class of real time, array-formatted electrical sensors compatible with droplet microfluidics for laboratory on a chip applications.

Evaporation-enhanced impedance sensing for highly-sensitive differentiation of dsDNA from ssDNA

In this paper, we adopt a radically different approach to beat the diffusion limit through time-dependent evaporation of microliter-sized droplets (containing DNA molecules) pinned onto an array of electroplated nickel electrodes, and thereby achieve highly sensitive, statistically-robust, label-free, non-faradaic impedance-based differentiation of ssDNA vs. dsDNA at femtomolar (~fM) concentration. The proposed approach is ideally suited for integration in a portable, array format, and could be used in real-time point-of-care applications for which highly sensitive, yet selective detection of DNA hybridization is desired.

Incubation-free detection of bacteria cells by using droplet-based impedance sensing

In this study, we have demonstrated the capability of droplet-based impedance sensor as a cost-effective, simple and rapid bacterial quantification assay. By using evaporation to concentrate and amplify the ions released by bacteria cells, the detection limit of classical non-Faradic impedance sensors is improved by two orders of magnitude. Further, the time-multiplexing capability of DNFIS (achieved by continuous impedance monitoring as the droplet evaporates) significantly reduces the data variability. We showed that through capturing the conductance modulation inherited to bacteria cells suspended in a low conductivity solution, they can be quantified within ca. 20 min which is ~ 10-50 times shorter than the growth-based assays, as summarized in Fig. 3(b).

Fabrication of Silicon-Based Actuators Using Branched Carbon Nano-Structures

We report a novel interdigital sensor and actuator device based on branched tree-like carbon nanotubes (CNT) on silicon-based membranes with a high capacitance value. The presence of tree-like CNTs leads to a high overlap between interdigital fingers owing to their three dimensional nanometric features, hence increasing the value of the capacitance from 0.2 to 15 pF. The electromechanical behavior of the device has been investigated with monitoring capacitive characteristics of the sensor. An almost linear rise in the capacitance value by applying the external voltage is observed which could be due to the modulated overlapping between neighboring electrodes. The response of the sensor/actuator device to various frequencies has been studied.

Photoluminescence from SiO 2 nanostructures prepared by a sequential RIE process

SiO<inf>2</inf> nanostructures have been fabricated by sequential etching/passivation treatment of an amorphous SiO<inf>2</inf> layer, using mixture of SF<inf>6</inf>, H<inf>2</inf> and O<inf>2</inf> gases. Photoluminescence measurements depicts that nano-textured SiO<inf>2</inf> films have emission spectra between 530 and 580 nm depending on the details of the sample treatment. The effects of sequence numbers, post-RIE-process hydrogenation treatment, and annealing temperature on the PL characteristics were explored. It is observed that annealing the porous SiO<inf>2</inf> nanostructures significantly increases the PL intensity. Increasing either the plasma current or hydrogenation time will cause a redshift in the PL spectrum. The morphology of the SiO<inf>2</inf> nanostructures has also been studied using field-emission scanning electron microscopy.

Highly Sensitive Capacitive Transducer based on Comb-Like Array of Branched Carbon Nanotubes

Abstract In this paper we report a highly sensitive capacitive pressure sensor and transducer, with ultra high capacitance values, based on branched carbon nanotubes on ultra thin silicon membranes. The presence of tree-like CNTs is believed to be responsible for high overlap between fingers which results in a significant capacitance change of the fabricated pressure sensor by applying pressure. Also, by applying an external voltage between fingers at one side of the ultra thin membrane an observable actuation was obtained. This actuation is sensed by a capacitive interdigital sensor at the other side of the same membrane.