Analyst PAPER Cite this: DOI: 10.1039/c6an00018e Received 4th January 2016, Accepted 15th February 2016 DOI: 10.1039/c6an00018e www.rsc.org/analyst An oligonucleotide-functionalized carbon nanotube chemiresistor for sensitive detection of mercury in saliva† Dawit N. Wordofa,‡ a Pankaj Ramnani,‡ a Thien-Toan Tran b and Ashok Mulchandani* a,c Divalent mercuric (Hg 2+ ) ion and monomethyl mercury (CH 3 Hg + ) are two forms of mercury that are known to be highly toxic to humans. In this work, we present a highly selective, sensitive and label-free chemiresistive biosensor for the detection of both, Hg 2+ and CH 3 Hg + ions using DNA-functionalized single-walled carbon nanotubes (SWNTs). The SWNTs were functionalized with the capture oligo- nucleotide, polyT, using a linker molecule. The polyT was hybridized with polyA to form a polyT:polyA duplex. Upon exposure to mercury ions, the polyT:polyA duplex dehybridizes and a T–Hg 2+ –T duplex is formed. This structure switch leads to the release of polyA from the SWNT surface and correspondingly a change in the resistance of the chemiresistive biosensor is observed, which is used to quantify the mercury ion concentration. The biosensor showed a wide dynamic range of 0.5 to 100 nM for the detec- tion of CH 3 Hg + ions in buffer solution with a sensitivity of 28.34% per log (nM) of CH 3 Hg + . Finally, real world application of the biosensor was demonstrated by the detection of Hg 2+ and CH 3 Hg + ions in simu- lated saliva samples spiked with a known concentration of mercury ions. Introduction Mercury (Hg) is a toxic metal-pollutant in the environment that can exist in elemental, inorganic and/or organic forms and each form is known to have a distinct toxicity profile. 1 While the inorganic form of mercury (HgCl 2 ) is toxic to kidneys, 2 organic forms such as methylmercury are primarily neurotoxins that affect the central nervous system. 3 Methyl- mercury is also known to have genotoxic effects and fetal exposure can cause intrauterine poisoning at lower exposure levels. 4 Methylmercury has a strong affinity to amino acids and eventually binds to proteins and polypeptide chains. 5 Humans are exposed to different forms of mercury due to natural causes such as off-gassing from the earth’s crust and anthro- pogenic sources such as environmental pollution caused by industrial utilization and the use of pesticides in agricultural practices. 6 Mercury is methylated by the microorganisms present in water and bioconcentrated via the aquatic food chain. 7 This eventually results in human exposure due to the consumption of fish and contaminated water. Incidents of epi- demic outbreaks due to mercury poisoning have been reported in the past. In the mid-1950s, waste containing methylmercury was discharged into the Minamata Bay in Japan by Chisso Corporation, a large acetaldehyde producing factory, leading to bioaccumulation of the toxic chemical in the shellfish and fish. When these fish were consumed by the local population, it resulted in large scale mercury poisoning among the resi- dents of Minamata Bay in Japan. 8 In 1972, a similar incident of mass poisoning by methylmercury was reported in Iraq due to direct contamination of food from the use of pesticides con- taining methylmercury in agriculture. 9 As a consequence, about 6500 patients were hospitalized and nearly 460 people died. Recent studies demonstrate that there is still a concern of exposure to methylmercury and elemental mercury from industrial activities. Silver/gold-mercury amalgam dental fill- ings are another potential source of mercury exposure receiv- ing scrutiny. 10 It is hypothesized that Hg 0 vapors emitted from amalgams are first converted to Hg 2+ and then transformed to CH 3 Hg + by oral bacteria. 11 Hence, the detection of mercury in several matrices including human saliva is an indispensable task. 12 The commonly used techniques used for mercury detection include cold-vapor atomic absorption spectroscopy, 13 atomic † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c6an00018e ‡ These authors contributed equally. a Department of Chemical and Environmental Engineering, University of California, Riverside, California 92521, USA. E-mail: adani@engr.ucr.edu; Tel: +1-951-827-6419 b Department of Bioengineering, University of California, Riverside, California 92521, USA c Material Science and Engineering Program, University of California, Riverside, California 92521, USA This journal is © The Royal Society of Chemistry 2016 Analyst Published on 15 February 2016. Downloaded by University of Nevada - Reno on 24/02/2016 07:32:39. View Article Online View Journal