Zhenxing Cheng

CuO Nanostructures As Quartz Crystal Microbalance Sensing Layers for Detection of Trace Hydrogen Cyanide Gas

In this work, quartz crystal microbalance (QCM) sensors for detection of trace hydrogen cyanide (HCN) gas were developed based on nanostructural (flower-like, boat-like, ellipsoid-like, plate-like) CuO. Responses of all the sensors to HCN were found to be in an opposite direction as compared with other common volatile substances, offering excellent selectivity for HCN detection. The sensitivity of these sensors is dependent on the morphology of CuO nanostructures, among which the plate-like CuO has the highest sensitivity (2.26 Hz/μg). Comparison of the specific surface areas of CuO nanostructures shows that CuO of higher surface area (9.3 m(2)/g) is more sensitive than that of lower surface area (1.5 m(2)/g), indicating that the specific surface area of these CuO nanostructures plays an important role in the sensitivity of related sensors. On the basis of experimental results, a sensing mechanism was proposed in which a surface redox reaction occurs between CuO and Cu(2)O on the CuO nanostructures reversibly upon contact with HCN and air, respectively. The CuO-functionalized QCM sensors are considered to be a promising candidate for trace HCN gas detection in practical applications.

Dual-SAM functionalization on integrated cantilevers for specific trace-explosive sensing and non-specific adsorption suppression

Two self-assembled monolayers (SAMs), 6-mercaptonicotinic acid (6-MNA) and hydrophobic heptadecafluorodecyltrimethoxysilane (FAS-17), are used to specifically modify the two surfaces of a piezoresistive SiO2 cantilever for functionalizations of both specific explosive-sensing improvement and non-specific molecular-adsorption suppression. With the dual-SAM modification technique, the on-chip-integrated ultra-sensitive microcantilever sensor behaves more sensitively and has quicker sensing properties to trace trinitrotoluene (TNT) than the previously reported cantilever functionalized with 4-MBA SAM, as well as being able to significantly suppress the cross-talk influence from environmental air humidity. Measurement results show that the high-performance sensor achieves a rapid, reversible and reproducible response to TNT vapour, with a detecting resolution of tens of ppt.

Single crystal WO3 nanoflakes as quartz crystal microbalance sensing layer for ultrafast detection of trace sarin simulant

Tungsten oxide (WO(3)) nanoflakes were synthesized, and characterized by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. Thermogravimetry and X-ray photoelectron spectroscopy demonstrate the existence of strongly bound surface water molecules on the surface of tungsten oxide nanoflakes. WO(3) nanoflake functionalized quartz crystal microbalance sensors were fabricated, and firstly used for detection of trace sarin simulant, dimethyl methylphosphonate (DMMP). The sensors have better reproducibility and stability as well as much shorter response (30s) and recovery time (73s) than those functionalized by conventional hydrogen-bond acidic branched copolymers. The strongly bound surface water molecules on the surface of WO(3) nanoflakes are believed to play a key role in achieving such excellent DMMP sensing characteristics.

Hyper-branched sensing polymer directly constructed on a resonant micro-cantilever for the detection of trace chemical vapor

A hyper-branched polymer is layer-by-layer self-assembled on a resonant micro-cantilever and, then, functionalized with sensing-terminals for the specific detection of the trace chemical vapor of dimethyl methylphosphonate (DMMP, a typical simulant for nerve agents). The hyper-branched polymer is directly constructed on the SiO2 surface of the cantilever via an A2 + B4 layer-by-layer route, where A2 and B4 are complementary interacting groups which undergo coupled linking. After modification with 4-(2-(4-(allyloxy)phenyl)-1,1,1,3,3,3-hexafluoropropan-2-yl)phenol (APHFPP) groups specific to DMMP, the high specific-surface-area hyper-branched polymer provides very dense sensing sites to adsorb a great number of DMMP molecules for micro-gravimetric detection. Moreover, the sensing polymer possesses a “more branches but fewer roots” configuration on the cantilever surface to depress the cross-talk effect caused by adsorption induced cantilever spring-stiffening. Experimental results indicate that, self-assembled with the hyper-branched sensing polymer, the resonant cantilevers exhibit rapid and reproducible detection of trace DMMP (with the detection limit lower than 7.2 ppb) and effectively depressed parasitic frequency-shift from the cantilever spring stiffening effect. In addition, the sensor features satisfactory selectivity in the presence of water and organic solvents. When an alternative sensing-group of 2-allylhexafluoroisopropanol (AHFIP) is modified on the hyper-branched architecture, the cantilever becomes specifically sensitive to trace explosive vapor. Therefore, the developed technique for the functionalization of hyper-branched polymer directly grown on a cantilever provides a widely usable micro/nano sensing-platform for the detection of trace chemical vapors.

Hydrothermal synthesis of nanostructured flower-like Ni(OH)2 particles and their excellent sensing performance towards low concentration HCN gas

Hierarchically structured Ni(OH)2 particles with a well-defined flower-like morphology were synthesized via a hydrothermal route. The molar ratio of SDBS/Ni, hydrothermal temperature and reaction time were found to have a profound influence on the size and morphology of the resulting products. These hierarchically structured flower-like Ni(OH)2 were employed on quartz crystal microbalance resonators for HCN sensing. The flower-like Ni(OH)2 modified QCM resonators exhibited excellent sensing performance. The sensitivity of flower-like Ni(OH)2 modified QCM resonators has reached 5.14 Hz (μg ppm)−1, nearly 40-fold as high as previous reports. The high sensitivity is attributed to its high specific surface area (61 m2 g−1), special morphology and especially to its surface structure. A sensing mechanism that involves surface vacancy sites and their adsorption and activation of oxygen molecules was proposed on the basis of experimental results. Effects of relative humidity show high tolerance of the sensors to relative humidity. The prefect linear relationship between response signal (ΔF) and HCN concentration, and a fast and sensitive response to low concentration HCN coupled with high selectivity show a promising future for these Ni(OH)2 modified QCM resonators in the detection of trace gaseous HCN.

Trace TNT Vapor Detection with an SAM-functionalized Piezoresistive SiO2 Microcantilever

Presented is an ultra-sensitive piezoresistive SiO2 microcantilever sensor for trace TNT detection. A thin single crystalline silicon piezoresistor is encapsulated in a SiO2 cantilever for on-chip electric readout. The cantilever can lead to a more sensitive characteristic compared with conventional silicon cantilever because of a much lower Youngs modulus of SiO2. Moreover, fully covered by SiO2, the piezoresistor behaves a much lower current leakage relative noise comparing to those with p-n junction isolation. In addition to the performance improvement of the cantilever sensor, an optimized self-assembled monolayer (SAM) of 6-mercaptonicotinic acid (6-MNA) is immobilized on the cantilever as specifically sensitive coating to capture TNT molecules and, then, causes surface stress for piezoresistive detection. Experimental results shows that the sensor yields a rapid, reversible and reproducible response to TNT vapor, and can achieve a detection limit of about 20-30 ppt.

Hyper-branched sensing polymers self-assembled on resonant micro-cantilever sensors for ultra-low concentration DMMP vapor detection

The paper reports a hyper-branched polymer functionalized on resonant micro-cantilevers to detect trace-level chemical vapor of dimethyl methylphosphonate (DMMP, simulant of sarin). The hyperbranch-polymer with functional-groups is directly constructed onto cantilever-surface by layer-to-layer step-growing monomers with an A2+B4 composition, where A2 and B4 are complementary interaction-groups for coupled linking. Experimental results verify that the hyperbranch-polymer-equipped resonant cantilevers are capable of detecting 5ppb ultra-low concentration DMMP.

Four-Cantilever Trace Explosive Sensors with Dual SAMs Functionalized for Specific-Sensing Improvement and Nonspecific-Adsorption Depression

A top-down formed four-cantilever micro-sensor structure is equipped with self-assembled dual monolayers on the surfaces for specific detection to trace explosive and depression of environmental cross-talk. Sensor design of the four-lever scheme, micro-fabrication, surface modification of the dual monolayers and experimental results for ultra-sensitive detection of explosive are sequentially related. The testing results confirm that the sensor can detect TNT at the resolution level of several tens of ppt and can effectively depress the influences from environmental temperature and humidity change.