The development of cost-efficient, portable, and sensitive detection systems for volatile organic compounds (VOCs) has become increasingly important. VOC sensors have extensive utility in environmental monitoring, health care, agriculture, food safety, defense, and homeland security applications. Many sensing technologies have been used for detection of VOCs, including differential ion mobility spectrometry, gas chromatography-mass spectrometry, photoionization, laser desorption mass spectrometry, nanowire coatings, and cantilever detectors. Devices based on the sorption of gas molecules are increasingly being adopted because of their simplicity, compactness, and amenability to use with sensor arrays. Sorption-based sensors have a chemosensitive coating that selectively and reversibly sorbs analytes of interest. To achieve optimal measurement of the analyte, sensing materials are often immobilized directly onto the surface of a transducer that converts the binding event into an electronic signal. Mechanical oscillators, chemicapacitors, and chemiresistors are often the transducers of choice for analyses of a broad range of chemical vapors.
A quartz crystal microbalance (QCM) is a common piezoelectric transducer that can be used as a sorption-based sensor. The operating principle of this sensor is based on the alteration of the characteristics of acoustic shear waves propagating through the piezoelectric material. A QCM typically comprises a thin slice of AT-cut quartz wafer that is sandwiched between two electrodes. When an oscillating electric voltage is applied perpendicular to the surface of the quartz resonator, an acoustic shear wave is produced that propagates across the thickness of the crystal. As the sorbent coating interacts with the analyte, its mass and mechanical properties are altered, which in turn leads to a phase shift and attenuation of the shear wave propagating through the film adhering to the electrode surface. This phase shift leads to a change in resonance frequency, which depends on the mass of added material on the surface of the resonator. The frequency shift, Δf, and the added mass, Δm, of the analyte are related according to the Sauerbrey equation:
                              Δ          ⁢                                          ⁢          f                =                              -                                          2                ⁢                                                                  ⁢                Δ                ⁢                                                                  ⁢                                  mf                  2                                                                              A                  ⁡                                      (                                          μ                      ⁢                                                                                          ⁢                                              ρ                        q                                                              )                                                                    1                  2                                                              =                                    -                              C                f                                      ⁢            Δ            ⁢                                                  ⁢            m                                              (                  Equation          ⁢                                          ⁢          1                )            
where f is the intrinsic frequency of the quartz crystal, A is the active vibrating area, μ is the shear modulus of quartz (2.95×1011 dyne/cm2), ρg is the density of quartz (2.65 g/cm3), and Cf is the integrated QCM sensitivity (i.e., collecting all the factors except Δm). The Sauerbrey equation is valid in the regime where the adsorbed mass is small as compared to the overall mass of the crystal, and the additional mass is rigidly bound and evenly distributed over the electrode's surface. Attenuation of the shear wave is due to dissipation of energy during oscillation. This attenuation can be estimated by measuring an electrical property called motional resistance, R, of the QCM. Thin and rigid films display less dissipation and hence produce a small increase in R, while thick and viscoelastic films exhibit high dissipation and a correspondingly large increase in R.
Various sensing materials, including polymers, inorganic oxides, polymer/carbon black composites, carbon nanotubes, graphite microparticles, amino acids, TiO2-porphyrin nanocomposites, calixarenes, lipids, and room temperature ionic liquids (RTILs), have been used in formulating chemosensitive QCM coatings to detect and identify a range of VOCs.
Ionic liquids (ILs) are usually defined as organic salts that melt below 100° C. ILs that are liquid at or below room temperature are commonly known as RTILs, whereas those in the solid state, i.e. room temperature to 100° C., are often referred to as ‘frozen’ ILs.
The use of RTILs as sensing materials is relatively recent. The unique combination of thermal and chemical stability with tunable physico-chemical properties has led to the use of RTILs for gas sensing applications. Short response time and high reversibility are properties of QCM/RTIL sensors. However, the use of RTILs as gas sensors has two major limitations: de-wetting of a film coating to form macroscopic drops; and the well-known viscosity-density effect. The absorption of organic vapor into RTILs causes a decrease in the density and viscosity of the liquid, leading to an increase in frequency—the so-called viscosity-density effect. To overcome these drawbacks, very thin RTIL coatings have sometimes been used. The thin coatings behave as quasi-rigid layers, and hence exhibit a decrease in frequency upon analyte sorption. The use of thin films, however, limits the sensitivity of the sensor, because the amount of vapor that can be absorbed depends on the quantity of the sorbent material deposited on the surface.
“GUMBOS” are compounds from a Group of Uniform Materials Based on Organic Salts. The acronym GUMBOS includes both frozen ILs (those with melting points from 25° C. to 100° C.) and analogous organic salts that melt from 100° C. to 250° C. See A. Tesfai, B. El-Zahab, A. T. Kelley, M. Li, J. C. Garno, G. A. Baker, I. M. Warner, ACS Nano 2009, 3, 3244; and published international patent application WO 2009/082618. “GUMBOS” is defined to mean an organic salt having a melting point between 25° C. and 250° C. (The word “GUMBOS” may be either singular or plural.)
A. F. Holloway, A. Nabok, M. Thompson, A. K. Ray, D. Crowther, J. Siddiqi, Sensors 2003, 3, 187 reported using calyx[4]resorcinarene films with QCM to measure Δf and ΔR, distinguishing between hexane and toluene vapors. While the authors reported measurements of Δf and ΔR, they did not report any correlation between Δf/ΔR and the physico-chemical properties of the analytes.