High-resolution, flexible, and transparent nanopore thin film sensor enabled by cascaded Fabry-Perot effect.

This Letter reports a method to significantly improve the optical resolution of the anodic aluminum oxide (AAO) nanopore thin film sensor based on multi-cavity Fabry-Perot interference. The newly designed sensor is fabricated by bonding a layer of transparent polymer thin film (pTF), which is polydimethylsiloxane (PDMS), to a transparent AAO thin film to form a flexible pTF-nanopore sensor. In comparison with the AAO nanopore thin film sensor, the pTF-nanopore sensor shows a much-improved quality (Q) factor and optical resolution. Typical thicknesses of a PDMS layer and an AAO layer of the pTF-nanopore sensor are 80 µm and 2 µm, respectively. The pTF-nanopore sensor used for angle detection shows a sensitivity of 0.4 nm/deg with a resolution of 0.2 deg. The pTF-nanopore sensor can also be used for temperature monitoring with a sensitivity of 0.2 nm/°C and a resolution of 1°C.

Correction to: Microtissue size and cell-cell communication modulate cell migration in arrayed 3D collagen gels.

The original version of this article unfortunately contained a mistake. One line indicating statistical significance was improperly placed in Fig. 5.

Microtissue size and cell-cell communication modulate cell migration in arrayed 3D collagen gels.

Cells communicate through the extracellular matrix (ECM) in many physiological and pathological processes. This is particularly important during cell migration, where cell communication can alter both the speed and the direction of migration. However, most cell culture systems operate with large volumes relative to cell numbers, creating low cell densities and diluting factors that mediate cell communication. Furthermore, they lack the ability to isolate single cells or small groups of cells. Droplet forming devices allow for an ability to embed single or small groups of cells into small volume segregated 3D environments, increasing the cell density to physiological levels. In this paper we show a microfluidic droplet device for fabricating 3D collagen-based microtissues to study breast cancer cell motility. MDA-MB-231 cells fail to spread and divide in small, thin chambers. Cell migration is also stunted as compared to thick 3D gels. However, larger chambers formed by a thicker devices promote cell spreading, cell division and faster migration. In the large devices, both cell-ECM and cell-cell interactions affect cell motility. Increasing collagen density decreases cell migration and increasing the number of cells per chamber increases cell migration speed. Furthermore, cells appear to sense both the ECM-chamber wall interface as well as other cells. Cells migrate towards the ECM-chamber interface if within roughly 150 µm, whereas cells further than 150 µm tend to move towards the center of the chamber. Finally, while cells do not show enhanced movement towards the center of mass of a cell cluster, their migration speed is more variable when further away from the cell cluster center of mass. These results show that microfluidic droplet devices can array 3D collagen gels and promote cell spreading, division and migration similar to what is seen in thick 3D collagen gels. Furthermore, they can provide a new avenue to study cell migration and cell-cell communication at physiologically relevant cell densities.

Rapid multiplexed detection of beta-amyloid and total-tau as biomarkers for Alzheimer's disease in cerebrospinal fluid.

This paper reports the multiplexed monitoring of two promising biomarkers, beta-amyloid (Aß42) and total tau (T-tau), in both buffer and cerebrospinal fluid (CSF) for Alzheimer's disease (AD) using label-free optical nanosensors. It has been found that 7.8 pg/ml of Aß42 in buffer and 15.6 pg/ml of T-au in buffer can be readily detected with very good specificity. Based on our measurements, the purchased CSF itself contains Aß42, whose concentration is estimated to be about 400 pg/ml. Aß42 and T-tau in the mixtures of Aß42 and T-tau spiked in CSF have been detected successfully, indicating the feasibility of the optical nanosensors to detect these biomarkers in clinical samples. For the measurements, only a small amount (~1 µl) of the samples is required. This type of sensor is suitable for point-of-care application to diagnose the AD due to its low cost and ease-of-operation.

Nanopore thin film enabled optical platform for drug loading and release.

In this paper, a drug loading and release device fabricated using nanopore thin film and layer-by-layer (LbL) nanoassembly is reported. The nanopore thin film is a layer of anodic aluminum oxide (AAO), consisting of honeycomb-shape nanopores. Using the LbL nanoassembly process, the drug, using gentamicin sulfate (GS) as the model, can be loaded into the nanopores and the stacked layers on the nanopore thin film surface. The drug release from the device is achieved by immersing it into flowing DI water. Both the loading and release processes can be monitored optically. The effect of the nanopore size/volume on drug loading and release has also been evaluated. Further, the neuron cells have been cultured and can grow normally on the nanopore thin film, verifying its bio-compatibility. The successful fabrication of nanopore thin film device on silicon membrane render it as a potential implantable controlled drug release device.

Characterization of Field Effect Transistor Biosensors Fabricated Using Layer-by-Layer Nanoassembly Process.

In order to avoid the fabrication complexity involved with a single carbon nanotube (CNT) based immunosensor, herein we report an FET based biosensor, in which the channel is made out of Carbon Nanotube Thin Film (CNTF). The CNTF channel between the source and drain electrodes is assembled using a combination of photolithography and electrostatic layer-by-layer self-assembly (LbL). The fabricated device behaves like a p-type transistor. The bio-affinity interaction between Protein A and rabbit Immunoglobulin G (IgG) is used to model the immunosensing, and our initial results show the device is capable of detecting IgG concentrations as low as 1 pg/mL.

A nanostructured aluminum oxide-based microfluidic device for enhancing immunoassay's fluorescence and detection sensitivity.

A nanostructured aluminum oxide (NAO)-based fluorescence biosensing platform with a programmable sample delivery microfluidic interface is reported. The NAO-based fluorescence sensor can tremendously enhance the fluorescence signals, typically up to 100 × or more, over the glass substrate. The programmable sample delivery microfluidic interface, which is integrated with the NAO-based sensors, can automatically generate and deliver a series of different concentrations of the biological samples to each individual sensor. Hence it can facilitate the fluorescence-based biodetection and analysis for high throughput applications. Using Protein A and fluorophore-labeled Immunoglobulin G (IgG) as models, the binding between them on this platform have been demonstrated. It has been shown that the IgG of programmable concentrations can be delivered to individual sensor using the microfluidic interface and confirmed by the fluorescence images. Using current NAO-based fluorescence sensors without any optimization, the detectable concentration of IgG can be as low as 20 pg/mm(2) using a conventional fluorescence microscope. Due to its inexpensive fabrication process, this technology could provide a disposable technical platform for fluorescence-based sensing and analysis.

Assessment of the Behaviors of an In Vitro Brain Model On-Chip under Shockwave Impacts.

Herein we report the assessment of the effects of shockwave (SW) impacts on adult rat hippocampal progenitor cell (AHPC) neurospheres (NSs), which are used as in vitro brain models, for enhancing our understanding of the mechanisms of traumatic brain injury (TBI). The assessment has been achieved by using culture dishes and a new microchip. The microchip allows the chemicals released from the brain models cultured inside the cell culture chamber under SW impacts to diffuse to the nanosensors in adjacent sensor chambers through built-in diffusion barriers, which are used to prevent the cells from entering the sensor chambers, thereby mitigating the biofouling issues of the sensor surface. Experiments showed the negative impact of the SW on the viability, proliferation, and differentiation of the cells within the NSs. A qPCR gene expression analysis was performed and appeared to confirm some of the immunocytochemistry (ICC) results. Finally, we demonstrated that the microchip can be used to monitor lactate dehydrogenase (LDH) released from the AHPC-NSs subjected to SW impacts. As expected, LDH levels changed when AHPC-NSs were injured by SW impacts, verifying this chip can be used for assessing the degrees of injuries to AHPC-NSs by monitoring LDH levels. Taken together, these results suggest the feasibility of using the chip to better understand the interactions between SW impacts and in vitro brain models, paving the way for potentially establishing in vitro TBI models on a chip.

Fluorescence detection and imaging of biomolecules using the micropatterned nanostructured aluminum oxide.

Micropatterns of the nanostructured aluminum oxide (NAO) with sizes from 5 to 200 µm have been successfully fabricated on the indium tin oxide (ITO) glass substrate by simply combining a lift-off process and a one-step anodization process for the first time. The detection of fluorescent dyes and biomolecules tagged with fluorescent dyes on the NAO has been investigated and demonstrated successfully. Experiments reveal that the micropatterned NAO substrates can increase the fluorescence signals up to 2 or 3 orders of magnitude compared to the glass substrate, suggesting a possibility to significantly reduce the consumption of the biosamples for fluorescence-based sensing, imaging, and analysis. The stability of the NAO substrates for fluorescence enhancement has also been evaluated by monitoring the fluorescence signals after the fluorophores applied on the substrates for a period of time and reusing the same NAO substrates many times. It was found that this type of substrate has very good stability. Because the micropatterned NAO can be easily integrated with microsensors or microfluidic chips, a simple and inexpensive fluorescence enhancement platform can be developed for a variety of applications, such as microarray technology and single-cell imaging, facilitating the construction of the on-chip fluorescence-based micro- or nanosystems.

Ultrasensitive thin film infrared sensors enabled by hybrid nanomaterials.

IR sensing is an important technology with applications in renewable energy, environmental science and medical engineering. Herein environment-friendly IR sensors based on the single-walled carbon nanotube-copper sulfide nanoparticle (SWNT-CuS NP) hybrid nanomaterials are reported. The IR response in the photocurrent of a SWNT-CuS NP hybrid thin-film sensor is significantly enhanced when the IR light illuminates the thin-film device asymmetrically. We show that the change of photocurrent is up to 300%, which is 10× to 100× larger than those of other reported nanomaterial-based IR sensors. The detection limit can be as low as 48 µW mm(-2), among the lowest of the previously reported IR nanosensors. The dramatically enhanced sensitivity and detection limit are due to the temperature difference between the two junctions formed by the nanohybrid thin film and Cu-wire electrodes under asymmetric IR illumination, and the difference between the effective Seebeck coefficient of the nanohybrid thin film and that of the Cu wire. The IR sensor embedded in polydimethylsiloxane (PDMS) layers has been fabricated and tested, indicating its potential application as a flexible IR sensor.

Facile Process for Fabrication of Silicon Micro-Nanostructures of Different Shapes as Molds for Fabricating Flexible Micro-Nanostructures and Wearable Sensors.

We report a method to fabricate silicon micro-nanostructures of different shapes by tuning the number of layers and the sizes of self-assembled polystyrene beads, which serve as the mask, and by tuning the reactive ion etching (RIE) time. This process is simple, scalable, and inexpensive without using any sophisticated nanomanufacturing equipment. Specifically, in this work, we demonstrate the proposed process by fabricating silicon micro- or nanoflowers, micro- or nanobells, nanopyramids, and nanotriangles using a self-assembled monolayer or bilayer of polystyrene beads as the mask. We also fabricate flexible micro-nanostructures by using silicon molds with micro-nanostructures. Finally, we demonstrate the fabrication of bandage-type electrochemical sensors with micro-nanostructured working electrodes for detecting dopamine, a neurotransmitter related to stress and neurodegenerative diseases in artificial sweat. All these demonstrations indicate that the proposed process provides a low-cost, easy-to-use approach for fabricating silicon micro-nanostructures and flexible micro-nanostructures, thus paving a way for developing wearable micro-nanostructures enabled sensors for a variety of applications in an efficient manner.

An integrated microfluidic chip for studying the effects of neurotransmitters on neurospheroids.

To improve our understanding of how the central nervous system functions in health and disease, we report the development of an integrated chip for studying the effects of the neurotransmitters dopamine and serotonin on adult rat hippocampal progenitor cell (AHPC) neurospheroids. This chip allows dopamine or serotonin located in one chamber to diffuse to AHPC neurospheroids cultured in an adjacent chamber through a built-in diffusion barrier created by an array of intentionally misaligned micropillars. The gaps among the micropillars are filled with porous poly(ethylene glycol) (PEG) gel to tune the permeability of the diffusion barrier. An electrochemical sensor is also integrated within the chamber where the neurospheroids can be cultured, thereby allowing monitoring of the concentrations of dopamine or serotonin. Experiments show that concentrations of the neurotransmitters inside the neurospheroid chamber can be increased over a period of several hours to over 10 days by controlling the compositions of the PEG gel inside the diffusion barrier. The AHPC neurospheroids cultured in the chip remain highly viable following dopamine or serotonin treatment. Cell proliferation and neuronal differentiation have also been observed following treatment, revealing that the AHPC neurospheroids are a valuable in vitro brain model for neurogenesis research. Finally, we show that by tuning the permeability of diffusion barrier, we can block transfer of Escherichia coli cells across the diffusion barrier, while allowing dopamine or serotonin to pass through. These results suggest the feasibility of using the chip to better understand the interactions between microbiota and brain via the gut-brain axis.

Integrated Sensing Chip for Ultrasensitive Label-Free Detection of the Products of Loop-Mediated Isothermal Amplification.

Loop-mediated isothermal amplification (LAMP) is a nucleic acid amplification technique that has been widely used for the detection of pathogens in many organisms. Current LAMP-based sensors usually require the LAMP products to be labeled in order for them to be detected. Here, we present a novel label-free LAMP chip, which consists of a nanopore thin-film sensor embedded inside a LAMP reaction chamber. A fraction of LAMP primers is immobilized on the sensor surface, allowing the LAMP products to be synthesized and bound to the sensor surface via immobilized primers. After the LAMP reaction components are removed from the reaction chamber, the amplified LAMP products bound to the sensor surface give rise to significantly increased transducing signals, which can be measured by a portable optical spectrometer through an optical fiber probe. As a demonstration, we used the LAMP chip to detect the causal agent of late blight, Phytophthora infestans, which is one of the most devastating plant pathogens and poses a major threat to sustainable crop production worldwide. We show that this chip can detect as low as 1 fg/µL of P. infestans DNA in 30 min, which corresponds to an attomolar level of 1.6 × 10-6 attomole/µL and is at least 10 times more sensitive than the currently available methods. This label-free sensing technology holds great promise to open up a new avenue for ultrasensitive, highly specific, rapid, and cost-effective point-of-care diagnostics of plant, animal, human, and foodborne pathogens.

Development of a Ratiometric Tension Sensor Exclusively Responding to Integrin Tension Magnitude in Live Cells.

Integrin tensions are critical for cell mechanotransduction. By converting force to fluorescence, molecular tension sensors image integrin tensions in live cells with a high resolution. However, the fluorescence signal intensity results collectively from integrin tension magnitude, tension dwell time, integrin density, sensor accessibility, and so forth, making it highly challenging to specifically monitor the molecular force level of integrin tensions. Here, a ratiometric tension sensor (RTS) was developed to exclusively monitor the integrin tension magnitude. The RTS consists of two tension-sensing units that are coupled in series and always subject to the same integrin tension. These two units are activated by tension to fluoresce in separate spectra and with different activation rates. The ratio of their activation probabilities, reported by fluorescence ratiometric measurement, is solely determined by the local integrin tension magnitude. RTS responded sensitively to the variation of integrin tension magnitude in platelets and focal adhesions due to different cell plating times, actomyosin inhibition, or vinculin knockout. At last, RTS confirmed that integrin tension magnitude in platelets and focal adhesions decreases monotonically with the substrate rigidity, verifying the rigidity dependence of integrin tensions in live cells and suggesting that integrin tension magnitude could be a key biomechanical factor in cell rigidity sensing.

Aluminum oxide nanostructure-based substrates for fluorescence enhancement.

A new fluorescence enhancement technical platform based on anodic aluminum oxide (AAO) nanostructure substrate is reported for the first time. Several fluorophores have been examined on the AAO nanostructure substrates. Systematic experiments found that the enhancement factor can be up to two orders of magnitude compared to the fluorescence signals on a glass substrate, indicating its great potential for ultrasensitive fluorescence detection. Given the simple and cost-effective fabrication process of lithographically patterned AAO nanostructure, this type of AAO nanostructure platform has great potential applications, especially its integration with microdevices and microfluidic devices for fluorescence-based biological analysis.

Optical and thermal response of single-walled carbon nanotube-copper sulfide nanoparticle hybrid nanomaterials.

This paper reports the optical and thermal response of a single-walled carbon nanotube-copper sulfide nanoparticle (SWNT-CuS NP) hybrid nanomaterial and its application as a thermoelectric generator. The hybrid nanomaterial was synthesized using oleylamine molecules as the linker molecules between SWNTs and CuS NPs. Measurements found that the hybrid nanomaterial has significantly increased light absorption (up to 80%) compared to the pure SWNT. Measurements also found that the hybrid nanomaterial thin-film devices exhibit a clear optical and thermal switching effect, which can be further enhanced up to 10 ×  by asymmetric illumination of light and thermal radiation on the thin-film devices instead of symmetric illumination. A simple prototype thermoelectric generator enabled by the hybrid nanomaterials is demonstrated, indicating a new route for achieving thermoelectricity.

A micromachined carbon nanotube film cantilever-based energy cell.

This paper reports a new type of energy cell based on micromachined carbon nanotube film (CNF)-lead zirconate titanate cantilevers that is fabricated on silicon substrates. Measurements found that this type of micro-energy cell generates both AC voltages due to the self-reciprocation of the microcantilevers and DC voltages due to the thermoelectric effect upon exposure to light and thermal radiation, resulting from the unique optical and thermal properties of the CNF. Typically the measured power density of the micro-energy cell can be from 4 to 300 µW cm(-2) when it is exposed to sunlight under different operational conditions. It is anticipated that hundreds of integrated micro-energy cells can generate power in the range of milliwatts, paving the way for the construction of self-powered micro- or nanosystems.

Studies of self-reciprocating characteristic of carbon nanotube film-based cantilevers under light and thermal radiation.

Self-reciprocating characteristic of carbon nanotube film (CNF)-Cu cantilevers upon exposure to light and thermal radiation was observed. This unique characteristic offers an attractive technical platform for harvesting solar and thermal energies on a single chip, which has been demonstrated recently. This paper reports the detailed experimental studies of this phenomenon. It reveals that the low-frequency self-reciprocation, sensitive to the thicknesses of CNF and Cu and the intensity of the light and thermal radiation, is mainly attributed to the electrostatic interaction among randomly connected carbon nanotubes (CNTs) in CNF. This is due to the fact that electrical currents in CNF induced by light and thermal radiation also exhibit an oscillating characteristic, similar to the self-reciprocating characteristic of the CNF-Cu cantilevers. The mechanism for this observed phenomenon is also discussed by relating the optical, thermal, electrical, elastic and mechanical properties of the CNF.

Fabrication and characterization of lithographically patterned and optically transparent anodic aluminum Oxide (AAO) nanostructure thin film.

Optically transparent anodic aluminum oxide (AAO) nanostructure thin film has been successfully fabricated from lithographically patterned aluminum on indium tin oxide (ITO) glass substrates for the first time, indicating the feasibility to integrate the AAO nanostructures with microdevices or microfluidics for a variety of applications. Both one-step and two-step anodization processes using sulfuric acid and oxalic acid have been utilized for fabricating the AAO nanostructure thin film. The optical properties of the fabricated AAO nanostructure thin film have been evaluated and analyzed.

Drug effects analysis on cells using a high throughput microfluidic chip.

Usually cell-based assay is performed using titer plates. Because of the large library of chemical compounds, robust and rapid methods are required to find, refine and test a potential drug candidate in an efficient manner. In this article, the drug effects analysis on human breast cancer cells with a droplet microfluidic chip is reported. Each droplet serves as a nanoliter-volume titer plate and contains a human breast cancer cell MDA-MB-231, Cytochalasin D drug solution and cell viability indicator such as Calcein AM, which emits cytoplasmic green fluorescence. The drug effects on each cell are monitored in real time using a fluorescence microscope and by analyzing the fluorescence image of each cell. Clear change of the cell shape and size has been observed after the drug treatment, which is similar to that of conventional petri dish technique, suggesting this approach is a potential viable technical platform for drug effect analysis and for high throughput drug screen and discovery.