Guomin Zuo

High-mode resonant piezoresistive cantilever sensors for tens-femtogram resoluble mass sensing in air

According to the demand for an ultrasensitive mass sensor for bio/chemical molecular detection, resonant cantilever sensors are developed for detection in an air environment. Both a piezoresistive bridge and a metal coil are integrated in the cantilever for signal sensing and Lorentz-force resonance excitation, respectively. Compared with conventional first flexure mode resonance, measurement results for the second mode resonance show an improved mass-sensing resolution from 0.17 pg to 0.06 pg due to the higher quality factor. For further improving the resolution, an optimized electromagnetic excitation method specifically for the second resonant mode is proposed and developed. The optimized method provides a two-point excitation that matches the second mode shape function of the cantilever deflection and excites the second mode more efficiently. Compared to a cantilever with a conventional excitation method, the optimally excited cantilever improves the quality factor from 307 to 857. Based on the experimental results for the optimally excited second mode resonant sensor, 29 × 10−15 g resolution for in-air mass sensing is achieved. The developed second mode resonant cantilever sensor with piezoresistive sensing element integrated on-chip is promising for use in high-performance portable biological/chemical sensing 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.

Silicon dioxide microcantilever with piezoresistive element integrated for portable ultraresoluble gaseous detection

Surface-stress sensing microcantilevers with on-chip signal readout are demanded for on-the-spot ultrasensitive biological/chemical detection. For enlarging the bending of the cantilever under surface stress induced by specific reaction, a novel SiO2 cantilever is developed which features much lower Young’s modulus than conventional Si or SiNx cantilevers. Thin single-crystalline-silicon piezoresistors are integrated with the SiO2 cantilevers for electric readout. For improving resolution, the piezoresistors are fully encapsulated by SiO2. Thus, the piezoresistors with SiO2 isolation show much lower leakage-related noise than those with p-n junction isolation. Following the description of microfabrication process, this letter gives the sensing model and discusses the thermal mechanism of the piezoresistive SiO2 cantilever. With a specific self-assembled monolayer functionalized on the cantilever surface, on-chip detection for the vapor of trinitrotoluene is performed with a resolution of about 20ppt (part...

Integrated cantilever sensors with a torsional resonance mode for ultraresoluble on-the-spot bio/chemical detection

Torsion-mode resonance is built in an integrated cantilever sensor for ultraresoluble detection of specifically bio/chemical mass adsorption. The superior mass resolution of the torsion-mode cantilever to a conventional bending-mode one is verified by energy-dissipation analysis and Q-factor simulation. With integrated transverse piezoresistance for frequency-shift signal readout and Lorentz force for resonance excitation, the torsion-mode sensor is optimally designed for high sensitivity. The microfabricated torsion-mode sensor is measured with a high close-loop Q factor in air. By Allan-variance analysis for the measured frequency stability, 23fg resolution is obtained for the torsion-mode sensor, which is much improved compared to the 313fg for the conventional flexure-mode sensor. The torsional sensor is used to recognize biotin-avidin specific combination, resulting in 443Hz frequency shift for 50μM streptavidin solution.

AFM probes fabricated with masked-maskless combined anisotropic etching and p + surface doping

The paper presents a newly developed high-yield micro-fabrication technology for single-crystalline silicon atomic force microscope (AFM) probes. Both the tips and the cantilevers are simultaneously formed by a masked?maskless combined anisotropic etching process. Compared to a conventional tip-to-cantilever sequential fabrication scheme, this tip-and-cantilever simultaneous formation can effectively increase fabrication yield by avoiding the tips damaged during the following processed photolithographic steps for defining the cantilevers. By heavy boron doping at the surface, the conductive AFM probe provides an electrical path to the electric ground of the AFM that helps to eliminate the electrostatic accumulation of charges and, therefore, eliminate undesirable electrostatic forces between the probes and the samples. A fabrication yield as high as 90% has been obtained for the AFM probes for 4 inch wafers. The tips after oxidation-sharpening treatment generally have a radius of 10?30 nm. The cantilever spring constant can be well controlled in the range of 0.025?40 N m?1. High-quality sample scanning results with the formed AFM probes are obtained with a slightly better resolution than that from commercial probes without surface conductive treatment.

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.

A three-layer model of self-assembly induced surface-energy variation experimentally extracted by using nanomechanically sensitive cantilevers

This research is aimed at elucidating surface-energy (or interfacial energy) variation during the process of molecule-layer self-assembly on a solid surface. A quasi-quantitative plotting model is proposed and established to distinguish the surface-energy variation contributed by the three characteristic layers of a thiol-on-gold self-assembled monolayer (SAM), namely the assembly-medium correlative gold/head-group layer, the chain/chain interaction layer and the tail/medium layer, respectively. The data for building the model are experimentally extracted from a set of correlative thiol self-assemblies in different media. The variation in surface-energy during self-assembly is obtained by in situ recording of the self-assembly induced nanomechanical surface-stress using integrated micro-cantilever sensors. Based on the correlative self-assembly experiment, and by using the nanomechanically sensitive self-sensing cantilevers to monitor the self-assembly induced surface-stressin situ, the experimentally extracted separate contributions of the three layers to the overall surface-energy change aid a comprehensive understanding of the self-assembly mechanism. Moreover, the quasi-quantitative modeling method is helpful for optimal design, molecule synthesis and performance evaluation of molecule self-assembly for application-specific surface functionalization.