Polymer–Silicon Flexible Structures for Fast Chemical Vapor Detection

Abstract

Although there are several aspects that contribute to an efficient chemical sensor system, the choice of responsive materials can help to optimize several key attributes critical for their ultimate performance, specifically high sensitivity, selectivity, fast response time, and wide dynamic range. Some common issues reported to date for sensors are limited detection range, slow response time, long recovery period, and fast saturation (limited dynamic range).[1] Most of these issues are directly related to the large volume of bulk porous material that is needed for measurable signal output, which leads to inherently slow sorption, diffusion, and desorption processes. One type of gas sensor that receives only limited attention despite being among the most frequently used sensors for measuring important environmental quantities is the humidity sensor.[2]Important applications of this type of sensor include for respiratory equipment, incubators, chemical gas purification, and surgical operations. The main requirements for humidity sensors are good sensitivity over a wide humidity range, low hysteresis, good reproducibility, and longevity. [2,3] Often sensors are required to be very small and suitable for integration into arrayed systems, such as odor-sensing arrays. Humidity sensors currently tend to exploit the characteristics of bulk materials with the required resistive and capacitive response properties. This type of sensor comprises a moisture sensitive ceramic, metal, or polymer material that undergoes changes in resistance or capacitance with variations in ambient humidity. [3,4] Water vapor sensors that use polymeric materials commonly incorporate polyimides, polycarbonates, cellulose acetates, or conductive polymers. [5,6] The typical vapor sensitivity of such sensors is of the order of tens of parts per million (or ±0.05 % relative humidity, RH), which is sufficient for most routine measurements. However, this is not acceptable if a fast, real-time monitoring of vapor content variation is required. Increased sensitivity, to a low ppm level, can be achieved using a porous material to maximize the specific surface area available for water vapor adsorption. [7][8][9] For example, 0.4 ppm v (parts per million by volume) water vapor detection was demonstrated by Salonen et al. using carbonized porous silicon.[9] Bruno et al. used a polymer-coated resonant device to obtain a sensitivity of 7 ppm v .[10] In these cases, although the sensors reported showed good sensitivity they suffered sluggish response times, often requiring a few minutes to completely recover. Currently, modest sensitivity and slow response are the main obstacles to the design and development of microcantilever-based sensor technology. [11,12] In this Communication, we present a bimaterial design for humidity sensing with a vapor-sensitive plasma-polymerized nanolayer coating on a silicon microcantilever with a low flexural rigidity (Fig. 1a,d). The coating acts as a sensitive mechanical actuator, which mediates high internal stresses, enabling...

Bimaterial Microcantilevers as a Hybrid Sensing Platform

“…21a and b). [237] These MCs exhibited a monotonous deflection response from 5 % to 70 % (total deflection > 200 lm) with very small hysteresis (< 2 %). The MCs also exhibited an extremely stable response to water vapor over long storage time (nearly 2 years) with less than 5 % variation.…”

“…Two approaches have been adapted for the detection of explosives using MCs: 1) static or dynamic mode of operation where MCs are functionalized with SAMs or polymer layers to achieve selective binding. [128,150,237,240] 2) Microdeflagration of the explosives on the MC surface. [241][242][243] Several approaches to detect dangerous chemicals are described in literature: photomechanical chemical microsensors based on adsorptioninduced and photo-induced stress changes due to the presence of diisopropyl methyl phosphonate (DIMP), which is a model compound for phosphorous-containing chemical warfare agents, and trinitrotoluene (TNT).…”

“…In recent study, Tsukruk and co workers have also demonstrated a plasma polymerized benzonitrile coated MCs exhibited extremely high static deflection with a low detection limit below 10 ppb to hydrazine, a potentially explosive vapor. [237] The second method involves a microexplosion of the molecules sticking to MC surface by an electrical pulse, which results in an exothermic spike in the static deflection signal of the MC as shown in Figure 23. [241] This spike was found to be related to the heat produced during deflagration.…”

“…c) Response to cycles of small variations of humidity. d) Deflection of cantilever to a sudden change in humidity (0.01 % step) [237].…”

Metalized Porous Interference Lithographic Microstructures via Biofunctionalization

“…3a,b). [46,47] The vapor-deposited films differ from the solution-deposited films in that the former form a highly crosslinked network with a fraction of the functional groups dissociated during the polymerization process. The fine details of the chemical composition (e.g., the density of functional groups) is expected to be different in both the cases, although the nominal chemistry is similar.…”