Single Nanowire Sensors Research Articles

ZnO single nanowire gas sensor: A platform to investigate the sensitization of Pt

Platinum (Pt) has been extensively used as a catalyst in metal oxide semiconductor (MOS) gas sensors. However, a clear understanding on the size effect of Pt sensitization is still missing. Herein, we establish a ZnO single nanowire sensor as a platform to investigate the sensitization of Pt catalyst. Pt has been loaded on ZnO nanowires by both atomic layer deposition (ALD) and solution method to realize delicate control over the size and loading mount. With increasing ALD cycles, the Pt size increases from single atom to an average of 11.86 nm and the sensor resistance shows a decrease. The nanowire sensor with a medium Pt loading (0.61 at%) with a size of 3.95 nm shows the best response of 7.01 to 20 ppm NO2, revealing a 5-fold enhancement compared to pristine ZnO. The sensor operating temperature shows a remarkable decrease from 310 to 220 °C due to Pt sensitization. A control sensor with almost identical Pt loading of 0.59 at% and size of 4.98 nm prepared by solution impregnation displays a response of 5.04 to 20 ppm NO2, but showing much slower response and recovery. Our findings determinate the crucial effect of Pt size in chemical sensors.

A single nanowire sensor for intracellular glucose detection.

Glucose metabolism plays an important role in cell energy supply, and quantitative detection of the intracellular glucose level is particularly important for understanding many physiological processes. Glucose electrochemical sensors are widely used for blood and extracellular glucose detection. However, intracellular glucose detection cannot be achieved by these sensors owing to their large size and consequent low spatial resolution. Herein, we developed a single nanowire glucose sensor for electrochemical detection of intracellular glucose by depositing Pt nanoparticles (Pt NPs) on a SiC@C nanowire and further immobilizing glucose oxidase (GOD) thereon. Glucose was converted by GOD to an electroactive product H2O2 which was further electro-catalyzed by Pt NPs. The glucose nanowire sensor is endowed with a high sensitivity, high spatial-temporal resolution and enzyme specificity due to its nanoscale size and enzymatic reaction. This allows the real-time monitoring of the intracellular glucose level, and the increase of the intracellular glucose level induced by a novel potential hypoglycemic agent, reinforcing its potential application in lowering the blood glucose level. This work provides a versatile method for the construction of enzyme-modified nanosensors to electrochemically detect intracellular non-electroactive molecules, which is of great benefit for physiological and pathological studies.

Conduction mechanisms in SnO2 single-nanowire gas sensors: An impedance spectroscopy study

Results of studies on single and multiple SnO2 nanowire gas sensors with impedance spectroscopy are reported. Equivalent circuit modeling is used to draw fundamental conclusions about the dominant conduction mechanism in single-nanowire sensors, where the diameter of the nanowire is found to play a key role. This is then extended to multiple-nanowire sensors. For single-nanowire sensors, I–V measurements are also used to demonstrate that the contribution from the electrode-nanowire contact to the overall resistance changes with atmosphere and temperature. We find that for the randomly-orientated multiple-nanowire sensors, the main contribution to the resistance comes from the nanowire–nanowire junction.

A disposable breath sensing tube with on-tube single-nanowire sensor array for on-site detection of exhaled breath biomarkers.

This paper presents a novel disposable breath-sensing tube with an on-tube single-nanowire (NW) sensor array for noninvasive, simple, and on-site detection of exhaled breath biomarkers. Although various noninvasive detection methods for lung cancer biomarkers in breath samples exist, they are unsuitable for self-diagnostics and immediate detection because they entail complicated handling procedures and are time intensive. In this study, we simulated, fabricated, and characterized disposable nanosensors by using single TiO2 and Ag NWs in flexible plastic tubes. The proposed sensors simultaneously detect 2-propyl-1-pentanol (2-PP) lung cancer biomarkers and changes in the humidity, flow rate, and temperature of the flowing gas. The optimal dimension of the tubes was determined and verified through dynamic simulations and experiments. The current tube design decreases sensing variation and moisture interference by 43.28% and 78.77%, respectively, compared with previous designs. In the future, the proposed breath sensor, which has a response time of less than 10 s, can be used in a tube for simple and quick screening of lung cancer patients with the 2-PP concentration exceeding 100 ppb. The developed breath-sensing platform can be applied in noninvasive on-site disease screening.

A highly flexible platform for nanowire sensor assembly using a combination of optically induced and conventional dielectrophoresis.

The number and position of assembled nanowires cannot be controlled using most nanowire sensor assembling methods. In this paper, we demonstrate a high-yield, highly flexible platform for nanowire sensor assembly using a combination of optically induced dielectrophoresis (ODEP) and conventional dielectrophoresis (DEP). With the ODEP platform, optical images can be used as virtual electrodes to locally turn on a non-contact DEP force and manipulate a micron- or nano-scale substance suspended in fluid. Nanowires were first moved next to the previously deposited metal electrodes using optical images and, then, were attracted to and arranged in the gap between two electrodes through DEP forces generated by switching on alternating current signals to the metal electrodes. A single nanowire can be assembled within 24 seconds using this approach. In addition, the number of nanowires in a single nanowire sensor can be controlled, and the assembly of a single nanowire on each of the adjacent electrodes can also be achieved. The electrical properties of the assembled nanowires were characterized by IV curve measurement. Additionally, the contact resistance between the nanowires and electrodes and the stickiness between the nanowires and substrates were further investigated in this study.

Shadow masking for nanomaterial-based biosensors incorporated with a microfluidic device

Integrating PDMS channels and chips containing pre-functionalized biosensors into microfluidic devices with irreversible sealing has been challenging because the integration process requires use of an O2 plasma treatment that usually destroys the biosensors. In this study, we examined the usefulness of introducing a shadow mask into the process as a method of protecting the pre-functionalized biosensors. Single nanowire sensors were pre-functionalized with fluorescently labeled biomolecules and then subjected to O2 plasma with and without the shadow mask. Results from the two groups were then compared. Those sensors without a shadow mask were destroyed, giving the sensor an infinite resistance and reduced fluorescence intensity. In contrast, the sensors with the shadow mask were protected and exhibited little changes in the resistance and fluorescence intensity. Then two different nanowires, aptamers incorporated polypyrrole nanowire (entrapment) and antibodies immobilized polyaniline nanowire (surface covalent binding), were used to investigate their detection performance before and after the plasma treatment in the presence of shadow mask. The protected samples showed a good sensitivity to the targets with a slight reduction in response compared with the as-prepared samples. After the O2 plasma treatment, the microfluidic channels were integrated with single nanowire biosensors. This microfluidic biosensor showed a high sensitivity with about ~0.5% change in conductance at the lowest IgE protein concentration of 10 pM.

A highly selective chemical sensor array based on nanowire/nanostructure for gas identification

Real-time detection and identification of multiple gases based on a single-nanowire sensor array have been investigated. We present the fabrication, characterization, and sensing performance of a single-nanowire array consisting of four different materials from three categories, which are metal (palladium), conducting polymer (polypyrrole and polyaniline), and semiconductor (zinc oxide), on a chip via a site-specific electrochemical deposition process. The presented nanowire array on a single chip has been used to detect and identify four target gases including hydrogen, methanol, carbon monoxide, and nitrogen dioxide. Each single nanowire shows an excellent sensitivity at room temperature, and a sub-ppm (parts per million) detection limit is achieved. The identification of these four targets has been successfully demonstrated with the help of principal component analysis. Our study shows that this single-nanowire sensor array is able to not only accurately distinguish four targets but also roughly estimate the target concentration, forming a basis for an electronic nose with far-reaching applications.

Nanoscale sensor analysis using the immersed molecular electrokinetic finite element method

The concentration and detection of molecular biomarkers remain as a challenge to develop point-of-care diagnostic devices. An electric field induced concentration has been studied for such purposes but with limited success due to limited efficacy. This paper presents a computational study for investigating the molecular concentration and retention efficacy of single nanowire (SNW) and dendritic nanotip (DNT) sensors. Our computational results indicate that compared to a DNT, the SNW sensor produces higher dielectrophoretic (DEP) forces in the vicinity of the terminal end of the tip. Furthermore, the magnitude of the DEP force increases exponentially as the diameter of the SNW is decreased, resulting in a further improved retention efficacy of NPs. However, the SNW sensor's concentration efficacy was not much improved for NPs smaller than 10 nm diameter when the nanowire diameter was reduced from 500 to 50 nm. Compared to the SNW, the DNT sensor showed improved concentration efficacy due to multiple points of electric field concentrations, which retard the exponential decay of the DEP force resulting in a greater widespread region where the DEP force dominates the Brownian motion forces. When oligonucleotides are used as a target particle, the DEP force can be used to elongate oligonucleotides to further enhance the concentration and retention efficacy.

Hierarchical Nano Heterostructures of SnO2-WOX: Growth and Sensing Studies

SnO2-WOX nanowire heterostructure were grown by physical deposition method. Characterization by XRD, UV-Vis and TEM shows that both the nanowire systems do not interact with each other, and retain their individual characteristic. Sensors fabricated from both, mat films and single isolated nanowire shows response to chlorine as well as H2S gas at room temperature. Single nanowire sensors are found to be better sensitive as well as selective Cl2 sensor.

Nanojunction effects in multiple ZnO nanowire gas sensor

The effects of nanojunctions in multiple ZnO nanowire sensors were investigated in terms of the electrical properties, the sensor response, and the rate characteristic of sensor response based on a comparison against single nanowire sensors. Due to the effects of nanojunctions, the multiple nanowire devices showed reduced gating effects as an electrical characteristic and an enhanced sensor response in hydrogen detection as compared to single nanowire device. On the other hand, it was found that the sensor response rate in multiple nanowires was affected by gas diffusion inside the nanojunctions. The effect of the nanojunctions on the rate characteristics of the sensor response was quantified from simplified kinetic-diffusion modeling based on the kinetic information of the single nanowire.

Photoconductivity of vertically aligned ZnO nanoneedle array

A high-density vertically well-aligned ZnO nanoneedle array was fabricated on a ZnO-buffer film on silicon substrates by metal-organic chemical vapor deposition at a growth temperature of 480–500°C. Highly crystalline ZnO nanoneedle arrays showed a strong near-bandedge emission at 380nm in room-temperature photoluminescence. A simple ultraviolet (UV) sensor was fabricated by evaporating a Ag electrode on the ZnO nanoneedle array. The photoresponse results showed very high photocurrent (∼10−4A) from ZnO nanoneedles compared to a single-nanowire sensor (∼10−8A), indicating high sensitivity of the photosensor. It also showed fast rise and decay times in UV-on/off switching measurements.