DNA Gold Nanoparticle Nanocomposite Films for Chemiresistive Vapor Sensing Kan Fu, † Shihui Li, § Xiaoqiang Jiang, ‡ Yong Wang, § and Brian G. Willis* ,‡ † Department of Materials Science and Engineering, University of Connecticut, Storrs, Connecticut 06269, United States ‡ Department of Chemical and Biomolecular Engineering, University of Connecticut, Storrs, Connecticut 06269, United States § Department of Bioengineering, Pennsylvania State University, University Park, Pennsylvania 16802, United States ABSTRACT: Chemiresistive vapor sensors combining func- tionalized gold nanoparticles and single-stranded DNA oligomers are investigated to enhance specificity in chemical sensing. Sensors are made by depositing DNA-functionalized gold nanoparticles onto microfabricated electrodes using four distinct sequences. Sensor performance is evaluated for response to relative humidity and exposure to vapor analytes including ethanol, methanol, hexane, dimethyl methylphosph- onate, and toluene under different relative humidity. It is found that sensors display a nonmonotonic resistance change toward increasing humidity due to the combined effects of hydration induced swelling and ionic conduction. Responses to vapor analytes show sequence-dependent patterns as well as a strong influence of humidity. Overall, the findings are encouraging for using DNA oligomers to enhance specificity in chemical sensing. ■ INTRODUCTION Chemiresistive sensing of vapors using chemically function- alized gold nanoparticle (AuNP) films has been intensively investigated in recent years. The level of interest is based on several favorable characteristics including response speed, sensitivity, reversibility, potential for miniaturization, and ease of integration with microelectronics. 1-4 Typically, an electric current through a nanoparticle film is measured under steady ac or dc voltage bias, and the current response is sensitive to the sorption of vapors, which may yield a sensitive probe of the vapor composition. The simplicity of chemiresistor designs provides practical benefits compared to other devices that use more complex transduction mechanisms such as chemitransis- tors, 5,6 fluorescence-based detectors, 7 surface acoustic wave (SAW) sensors, 8 fiber-optic vapor sensors, 9 and surface- enhanced Raman scattering (SERS) sensors. 10 One of the most attractive aspects of AuNPs as chemiresistor elements is their high flexibility toward surface modification with thiol-functionalized organic molecules, enabling a large library of monolayer-protected gold metal clusters (Au-MPCs). Partitioning of volatile organic compounds into the organic capping layers 2,3 leads to a series of effects, including changes in electron hopping distance, alteration of dielectric constants, and possible activation of mobile charged species. 2 Because of differences of sorption parameters for different analytes, variations in electrical responses are observed. In this way, vapors have been identified by principal component analysis of data from arrays of Au-MPC sensors with different function- alities. 3,11 These arrays are sometimes referred to as “electronic noses” because they mimic mammalian olfaction. A limitation of current Au-MPCs is the general difficulty with achieving specificity of sensor response characteristics. Specific- ity is limited by the range of solubility characteristics that can be achieved with organic capping layers. A new direction that may enable enhanced specificity is to incorporate nucleic acid oligomers as molecular recognition elements. The interest in nucleic acid oligomers is partly motivated by the immense number of base sequences that can be created. The number of possible unique receptor configurations scales with the length N of the oligomer as 4 N , giving rise to a large diversity of sensing elements. In addition, DNA oligomers may be used to develop aptamers, which are short olignulceotides that bind target analytes with high specificity. For example, aptamers have been developed for small molecule targets including cocaine, ATP, 12 and trinitrotoluene (TNT). 13 Though oligomeric nucleotide sequences are most com- monly used in solution, recent reports have included fluorescence-based solid-state sensors that respond to target vapor analytes through sorption-induced changes of fluores- cence intensity. 10 White et al. used 29 different single-stranded (ss) DNA sequences tagged with dye molecules to create sensor arrays and measured their response to volatile compounds. The number of distinct sequences employed was roughly twice as many as other MPC-based sensor arrays reported to date. 14 Others have proposed modifying nucleobases with fluorescent groups to give fluorescence Received: July 11, 2013 Revised: September 19, 2013 Published: October 10, 2013 Article pubs.acs.org/Langmuir © 2013 American Chemical Society 14335 dx.doi.org/10.1021/la402626p | Langmuir 2013, 29, 14335-14343