Resonance frequency shift of a zinc oxide- (ZnO-) functionalized microcantilever as a response to carbon monoxide (CO) gas has been investigated. Here, ZnO microrods were grown on the microcantilever surface by a hydrothermal method. The measurement of resonance frequency of the microcantilever vibrations due to the gas was carried out in two conditions, that is, gas flow with and without air pumping into an experiment chamber. The results show that the resonance frequency of the ZnO-functionalized microcantilever decreases because of CO in air pumping condition, while it increases when CO is introduced without air pumping. Such change in the resonance frequency is influenced by water vapor condition, and a possible model based on water-CO combination was proposed.
Microcantilever-based sensors could replace conventional sensors because of the ability to detect ultrasmall mass with fast response time. The working principle of this sensor is based on deflection of microcantilever (MC) due to an object attached on its surface (static mode) or resonance frequency shift of the MC vibration due to an object (dynamic mode). So far, the smallest mass detected using MC has been reported as femtogram (10−15 gram) level by Sone et al. [
On the other hand, in order to selectively detect chemical or biological molecules, a sensitive layer must be deposited on the microcantilever surface, such as polymer [
Generally, gas detection on metal oxide surfaces, such as zinc oxide (ZnO) which has a great potential in sensing applications, is especially influenced by the presence of humidity [
In this work, we have grown ZnO microrods on the microcantilever surface as sensitive layers for gas detection and studied the effect of carbon monoxide (CO) on resonance frequency of the MC at a room temperature. Here, we studied the effect of CO in different conditions, that is, with and without air flow. We experimentally observed a decrease in the resonance frequency due to gas flow with air pumping and an increase in the resonance frequency without air pumping. The possible model is also proposed to explain the experimental results.
Formation of the ZnO microrods consists of initial layer preparation and growth of rods. First, the MC was coated by an initial solution which was made of 0.3 M diethanolamine in ethylene glycol and stirred at 75°C for 1 hour. The coated cantilever was annealed at 100°C for 30 minutes.
The ZnO was then grown on the cantilever surface by dipping it in ZnO solution at 90°C for 2 hours. Here, the ZnO solution was prepared by using 0.02 M zinc nitrate tetrahydrate (Zn(NO3)2·6H2O) and hexamethylenetetramine (CH2)6N4 in ethanol and aqua bidest (1 : 3). Hexamethylenetetramine acts as a pH buffer of the solution and supply of OH− ions, as seen in the following reaction equations:
In this work, a commercial piezoresistive microcantilever (Seiko Instruments Inc.) was used, as shown in Figure
Schematic circuit system using piezoresistive microcantilever. The inset figure is a SEM image of the microcantilever.
Output of Wheatstone bridge (
When the actuated microcantilever vibrates in a certain frequency, we measured the resonance frequency of the MC vibration before and after gas detection. In gas measurements, we first put two cantilevers inside the chamber, as shown in Figure
Schematic view of chamber for gas measurement.
ZnO microrods were successfully grown on silicon microcantilevers as a sensitive layer. Figure
Scanning electron micrograph of ZnO sensitive layer. The inset is a zoomed-in image of hexagonal shape of ZnO. The yellow character “A” indicates EDS sensing position.
Next, the chemical composition of the ZnO microrods was characterized by EDS. Table
Energy dispersive spectroscopy (EDS) result for ZnO.
Element | Mass% | Error% | Atom% |
---|---|---|---|
O | 12.29 | 0.31 | 36.43 |
Si | 2.65 | 0.35 | 4.48 |
Zn | 79.63 | 1.54 | 57.78 |
Au | 5.43 | 1.04 | 1.31 |
|
|||
Total | 100.00 | 100.00 |
The initial frequencies of both microcantilevers were measured before measurement. The initial resonance frequency (
CO at concentrations of 30 ppm, 70 ppm, and 100 ppm was then introduced and ambient air was pumped into the chamber by using an air compressor pump before and after introducing the gas. The effect of CO at various concentrations is shown in Figure
Resonance frequency as a function of time for both microcantilevers (active cantilever: blue square markers for left axis, and reference cantilever: red circle markers for right axis) exposed to carbon monoxide gas in constant flow.
From (
Next, 70 ppm CO was inserted and the resonance frequency decreased to 28.420 kHz. The longer response time for reaching a stable state may be due to air humidity change during this measurement. When the gas was turned off and the air compressor was turned on, the resonance frequency of the MC increased to 28.840 kHz. Finally, the MC was exposed to 100 ppm CO. The resonance frequency shifted to 28.250 kHz and then increased to be 28.750 kHz due to gas OFF and air flow. The difference of 310 Hz in the resonance frequency between initial resonance frequency (29.060 kHz) and frequency after the gas measurement (28.750 kHz) is probably caused by remaining gas molecules of about 1.49 fg on the zinc oxide layer. Such difference was also found for gas concentrations of 30 ppm and 70 ppm. It is noted that the resonance frequency of the reference cantilever (red marker) was almost constant during the measurement. We previously found that the water vapor was directly adsorbed on the uncoated MC which reduced the resonance frequency of MC [
Figure
Resonance frequency shift (blue solid curve for left axis) and mass change (black dotted curve for right axis) versus CO concentration with average sensitivity of 8.27 femtogram/Hz.
The decrease of the active resonance frequency due to CO can be probably explained as follows. Mostly, metal oxides adsorb water vapor molecules at their surface in ambient air. In the present experiment, air pumping treatment provides water vapor spreading inside the experiment chamber. Water molecules in air are likely to be dissociated on the ZnO surface into OH− and H+. Such water dissociation on ZnO surface into OH− and H+ may produce the hydrogen-rich condition of the hydrogen ion, as discussed by Xu et al. [
Possible model of CO gas adsorption in the presence of water vapors.
In this experiment, the CO gas was introduced into the chamber at a constant rate of 1 ml/min at humidity of 65% RH, and then the gas was turned off without air pump until reaching a stable frequency. The initial resonance frequency (
Resonance frequency versus time for both microcantilevers exposure to carbon monoxide gas without air pump treatment (active cantilever: blue square markers for left axis; reference cantilever: red circle markers for right axis).
Next, the CO gas flow was turned off, resulting in a decrease in the resonance frequency of active MC (blue marker) to be 28.506 kHz. This frequency did not return back to its initial value. As the gas flow was turned on, the resonance frequency of the active cantilever increased again to 29.276 kHz with the resonance frequency shift of 0.77 kHz, and the frequency decreased to 28.316 kHz due to gas flow being turned off. As shown in Figure
Unlike the gas measurement with air pump treatment, the resonance frequency of the active MC increases due to effect of CO. The possible mechanism of this result may be due to the influence of water vapor which is explained in Figure
Possible model of CO gas adsorption in high humidity condition: (a) water vapors are present on the ZnO surface when CO is introduced; (b) CO gases produce the water vapor adsorption; (c) CO gases bind directly with zinc oxide layer.
To support the possible model in Figure
Relative humidity change due to introduced CO gas compared to resonance frequency change of the ZnO-coated MC.
In this study, the measurements were done in humid air at room temperature. Therefore, the mechanisms proposed above may not be appropriate for high temperature. If the substrate temperature is raised, the water vapors at the ZnO surface evaporate. In this case, CO will be directly absorbed on the ZnO surface, resulting in a decrease in the resonance frequency of the active MC. The same condition occurs if the ZnO surface is modified with a hydrophobic coating, in which the water vapor should not be adsorbed on the ZnO surface, and in case of CO adsorption, the gas will be adsorbed on the hydrophobic layer.
We have grown ZnO microrods on MC surfaces and studied the gas response to the ZnO rods in humid air at room temperature. The results showed that the resonance frequency of the ZnO-MC changed due to interaction with CO. It was found that the sensitivity for CO detection was about 8 femtogram/Hz. However, the response is highly influenced by the presence of humidity. The resonance frequency of the ZnO-MC decreased due to CO in rich water vapor condition, while the resonance frequency increased when CO was introduced in poor water vapor. We proposed the water-CO combination-based model to explain the results.
The authors declare that they have no conflicts of interest.
This paper was funded by an incentive research grant from Ministry of Research, Technology and Higher Education of the Republic of Indonesia which the authors gratefully acknowledge.