Nanomechanics
By combining the sensitivity of microcantilevers with the selectivity of IR spectroscopy, photothermal cantilever deflection spectroscopy (PCDS) opens up new opportunities for the detection, speciation, and quantification of explosive vapors in complex mixtures.
The Problem: Most vapor-sensing approaches based on immobilized chemical interfaces on sensor arrays fail to selectively identify target molecules when complex mixtures of molecules are present, such as in explosives.
The Background: Miniature sensors, such as microfabricated cantilevers, have very high sensitivity in the detection of adsorbed molecules, but do not have any intrinsic chemical selectivity. They require chemically selective interfaces to be immobilized on their surfaces to achieve selectivity in molecular detection.
But the nonspecific nature of chemical binding – especially those based on weak chemical interactions such as hydrogen bonds or van der Waals interactions – means that, despite their ability to detect molecules with extremely high sensitivity, miniature sensors, have had little market potential because of their failure to detect target molecules in mixtures.
The Solution: PCDS combines the sensitivity of microcantilevers and the selectivity of infrared (IR) spectroscopy. It delivers selective and sensitive detection of molecules even in the presence of interfering molecules with similar structures. In this technique, the physisorbed molecules on a thermally sensitive bi-material cantilever are excited with IR radiation in a sequential manner. IR energy absorption by the molecules results in a very small change in the temperature of the cantilever due to non-radiative decay, which in turn bends the cantilever.
A bi-material cantilever can detect extremely small changes in temperature in a way not dissimilar to a thermostat. Plotting cantilever bending as a function of illumination wavelength mimics the IR spectrum of adsorbed molecules. PCDS can successfully detect target molecules even in the presence of interfering molecules with similar molecular structures since vibrational spectra can be treated as linear combinations of spectra from individual members in the mixture. The resonance frequency change of the cantilever due to molecular adsorption can be used for quantification.
Our paper in Nature Scientific Reports (1) highlights successful implementation of PCDS for the selective and quantitative detection of ternary mixtures of explosive molecules, such as trinitrotoluene (TNT), cyclotrimethylene trinitramine (RDX), and pentaerythritol tetranitrate (PETN), with picogram levels of mass resolution. The approach could have immediate applications in areas such as national security, forensics, and humanitarian demining.
- S. Kim et al., “Molecular recognition using receptor-free nanomechanical infrared spectroscopy based on a quantum cascade laser”, Sci. Rep. 3, DOI: 0.1038/srep01111 (2013).
Thomas Thundat is a professor of chemical and materials engineering at the University of Alberta, Canada Excellence Research Chair in Oil Sands Molecular Engineering and also a research professor at the UT, Knoxville, a Visiting Professor at the University of Burgundy in France, and a Distinguished Professor at the Indian Institute of Technology, Madras. His research covers nanomechanics, solid-liquid interface, nanomechanical sensors for physical, chemical, and biological detection, scanning probe microscopy, quantum-confined atoms, and mid infrared spectroscopy of surface adsorbates. “My research currently focuses on developing chemical and biological sensors with extremely high sensitivity using micro and nanocantilever arrays.”
