Sensors and Actuators B 150 (2010) 402–405 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Quartz crystal tuning fork photoacoustic point sensing Charles W. Van Neste a,c,∗ , Marissa E. Morales-Rodríguez a , Larry R. Senesac a,b , Satish M. Mahajan c , Thomas Thundat a,b a Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA b Department of Physics, University of Tennessee, Knoxville, TN 37996, USA c Department of Electrical Engineering, Tennessee Technological University, Cookeville, TN 38505, USA article info Article history: Received 17 February 2010 Received in revised form 8 June 2010 Accepted 22 June 2010 Available online 30 June 2010 Keywords: Quartz crystal tuning fork Photoacoustic Spectroscopy Quantum cascade laser Point Sensing abstract Achieving chemical specificity in trace detection of small molecules is a challenge. Here we describe a highly selective method for detection of trace chemicals using photoacoustic spectroscopy of adsorbed molecules on a quartz crystal tuning fork. The technique is demonstrated in an open environment without the need of a resonant cavity. Mid-infrared quantum cascade lasers are used as the light source with cyclotrimethylenetrinitromine (RDX) residue being the target analyte. The results show absorption peaks matching those of earlier literature. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Most point sensors, where target molecules are allowed to adsorb on the sensor surface for detection, achieve chemical speci- ficity by using chemically selective interfaces. These chemical interfaces are often based on weak chemical binding for reversible detection of target molecules. The generality of the weak chem- ical interactions, that can be broken at room temperature (k B T) for regeneration, makes the detection less selective. Increasing the number of sensors in the array does not increase the selec- tivity [1–3]. However, this challenge can be addressed by using chemical interfaces that interact with target molecules with higher chemical binding energy. Examples of higher binding energy inter- actions include Lewis donor–acceptor, Bronsted acid–base, and charge transfer reactions that have heat of adsorption in the range of 40–100 kJ/Mol. Selectivity is proportional to the equilibrium con- stant (K) defined in Eq. (1): K = e (G/RT ) , (1) where G is the Gibbs free energy, R is the ideal gas constant (8.314 J/K mol), and T is the temperature in degrees Kelvin. As seen in Eq. (1), higher G results in higher selectivity due to the ∗ Corresponding author at: Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA. E-mail address: vannestecw@ornl.gov (C.W. Van Neste). increased concentration of target analytes interacting with a chem- ically specific interface at equilibrium. However, this binding is irreversible at room temperature and regeneration of the device is possible only by increasing the temperature of the sensor. One way of achieving high chemical selectivity with point sensors is by combining them with spectroscopic techniques. Microcantilever point sensors, when fashioned properly, show very high sensitivity to extremely small changes in temperature. For example, high selectivity can be achieved by using photothermal spectroscopy of adsorbed chemicals on microfabricated bimaterial cantilever beams [4–7]. In photothermal deflection spectroscopy (PDS), a bimaterial microcantilever with adsorbed chemicals is sequentially exposed to infrared light from a monochromator. The deflection of the cantilever as a function of the illumination wave- length shows infrared absorption peaks of the adsorbed chemicals. The cantilever deflection in PDS is monitored using an optical beam readout. The PDS sensitivity is proportional to the cantilever’s abil- ity to bend (higher bending generates a greater deflection signal at the readout) which is directly related to the stiffness of the cantilever. Lower stiffness yields greater bending but reduces the quality factor of the cantilever. For this reason, most cantilevers used in PDS have very low Q which increases signal noise when exposed to an open environment. Attempts to use piezoresistive readouts (which may be fabricated with higher quality factors) resulted in the loss of sensitivity due to internal heating of the cantilever desorbing the adsorbed materials, reducing the signal strength. In addition, increased background temperature of the can- 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.06.045