(157a) Enhanced Sensitivity of Resonating Nanocantilever Chemical Vapor Sensors Via Localized Sorptive Film Deposition

Sensors that are fast, small, sensitive, and can be integrated with standard microelectronics processing techniques are needed for emerging technologies such as wearable chemical vapor detectors. A promising type of sensor for these applications is the resonating cantilever, which has demonstrated detection of volatile organic compounds (VOCs), chemical warfare agents, and explosives, and has also been integrated into arrays for discrimination and identification of chemical vapors.¹ With the advent of nanoprocessing technologies, cantilever dimensions have been pushed to the nanoscale in pursuit of ever more sensitive detectors. For example, shrinking cantilever dimensions to the nanoscale greatly reduces viscous damping, allowing them to maintain high Q in ambient conditions where vapor detection occurs.² A high Q is required to measure small changes in the device's resonant frequency, which correspond to sorption of tiny quantities of analyte molecules. Additionally, nanocantilever sensors exhibit very high mass sensitivity, and are able to detect as little as 1 ag (10^-18 g) of sorbed vapor.² However, an examination of the cantilever response mechanism shows that even further increases in the sensitivity of cantilever based sensors may be possible by selecting the response regime via device fabrication and surface functionalization.

Cantilever resonance frequency is affected by the device spring constant and effective mass, both of which change upon sorption of an analyte vapor. The relative change in resonance frequency can be approximated by:

                                                                                 (1)

where Δf is the change in resonance frequency, f_o is the resonance frequency, m_e is the effective mass, and k is the spring constant respectively.³ Since sorption of analyte molecules generally causes both mass and spring constant to increase, the sum of these factors is smaller than their individual contributions to the sensor response. Eliminating sorption on the legs removes the influence of spring constant term, effectively increasing the observed response. Sensitivity can also be enhanced by allowing for vapor sorption only on the legs and relying instead solely on the spring constant response. To accomplish this, localized deposition of sorptive films is required, necessitating the use of bottom up approaches due to the size constraints of nanocantilever sensors.

Figure 1: Nanocantilever with chromium mask deposited on the tip over integrated thin gold film used for thermoelastic actuation and piezoresistive readout of the frequency signal.

Here, we demonstrate the selective deposition of sorptive films on nanocantilever sensors by utilizing the well-known self assembly of thiols onto gold surfaces.⁴ To prevent film growth on either the nanocantilever tip or legs, we first deposit an additional masking layer of chromium onto the cantilever surface. We pursue two strategies to selectively deposit sorptive films on only the exposed gold regions. Our first strategy is a self assembled multilayer scheme, wherein a film is built up via sequential monolayer assembly of 11-mercaptoundecanoic acid and copper ions from copper(II) perchlorate hexahydrate in a sandwich structure.⁵ Previously, this has been demonstrated as an advanced lithography technique,⁶ and we can grow multilayers in excess of 30 nm thick. Our second strategy is a one-step self assembly of thiol-terminated, long-chain polyethylene glycol (PEG) onto the exposed gold surface. In both cases, ellipsometry and x-ray photoelectron spectroscopy (XPS) are used to characterize the films, and changes in nanocantilever resonance frequency are used to verify film deposition onto devices. The functionalized cantilevers are then exposed to a series of analyte vapors under dry (nitrogen background flow) and mildly humid (laboratory air background flow) conditions to determine their performance. The performance of nanocantilevers with sorptive films deposited only on the device tip or on the legs is compared to the performance of uncoated cantilevers and to that of coated cantilevers without masked regions.

This work offers the first proof of concept for the benefit of localized sorptive film deposition on nanoscale cantilever chemical vapor sensors. From here, surface functionalization can be tailored to target specific vapor analytes of interest such as nerve agents or explosives. Also, localized deposition of chemically-varied films can be pursued toward the creation of nanocantilever arrays, such as have already been demonstrated with polymer coated devices.⁷ This work can also be extended to micro and nanoscale biosensors, where control over binding location of biomolecules could greatly enhance detection of very low concentration analytes.

References:

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4.    Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G., FORMATION OF MONOLAYER FILMS BY THE SPONTANEOUS ASSEMBLY OF ORGANIC THIOLS FROM SOLUTION ONTO GOLD. J. Am. Chem. Soc. 1989, 111 (1), 321-335.

5.    Evans, S. D.; Ulman, A.; Goppertberarducci, K. E.; Gerenser, L. J., SELF-ASSEMBLED MULTILAYERS OF OMEGA-MERCAPTOALKANOIC ACIDS - SELECTIVE IONIC INTERACTIONS. J. Am. Chem. Soc. 1991, 113 (15), 5866-5868.

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7.    McCaig, H. C., Myers, E. B.; Chi, D.; Zhang, X.; Lewis, N. S.; Roukes, M. L., Polymer-coated nanocantilever sensor array for discrimination of volatile organic compounds (VOCs). In American Chemical Society Spring Meeting, 2010.