Self Assembled Chemical Field Effect Transistors for Heavy Metal Detection

In this paper, we demonstrate improvements on semiconductor ChemFET sensors that have enabled fast sensor response times and an inexpensive method for the detection of mercury and other heavy metal concentrations. The ChemFET structure was built on a 200nm-thick silicon on insulator (SOI) layer. The exposed channel area is 100 microns by 100 microns. 500 micron by 500 micron ohmic contacts were patterned by lift-off for source and drain metal contacts. 100-nm-thick HybridShield^® was used to encapsulate the source/drain regions, with only the gate region open to allow the liquid solutions to cross the surface. For comparison, the chemFET sensors were modified in three ways, initially with the intent of targeting mercury and lead ions, respectively.

The first modification was designed for mercury selectivity and was achieved using a self-assembly system which includes poly(diallyldimethylammonium chloride) as an initial surface modification for the application of gold nanoparticles at the sensor gate. A layer-by-layer series of gold nanopaticle bilayers was added until a reasonable coverage was assumed based upon prior experience. The gold nanoparticles are functionalized with a self-assembled monolayer of thioglycolic acid. Any additional thioglycolic acid molecules were rinsed off with de-ionized water. The build-up of the thickness of the gold nanoparticle bilayers is approximately linear at a rate of 2nm/bilayer. For the initial chemFET sensor, nine bilayers were applied prior to testing of the sensor. A less than 10s response time to Hg ion solution was obtained for the self assembled ChemFET.  The Hg ion concentration detection limit for the thioglycolic acid functionalized ChemFET sensor is approximately 0.1 ppm. The drain current for the Cs ion remains unchanged during the exposure and the current is reduced after exposure to the Cr, Hg, Pb and As ions. The response for the Hg ion, with a normalized current change of -2.5%, is significantly higher than that for other ions. This is due to the fact that the thioglycolic acid molecules on the Au surface align vertically with the carboxylic acid functional group, toward the solution.

The second modification was designed for lead selectivity and was achieved by depositing the following chemicals to the ChemFET sensor.  It is noted that polyvinylchloride is a support matrix for the ionophore N,N,N′,N′-Tetradodecyl-3,6-dioxaoctanedithioamide which is sensitive primarily to lead. The modified chemFET sensors were then conditioned in a 1x10 ^-5 M solution of cadmium nitrate in deionized water over night. Again, the sensor was exposed to different target molecules with the same concentration of 10ppm. An increase in the hydrophilicity of the treated surface was confirmed by contact angle measurement. It is noted that the drain current for the Cs and Cr ions remains relatively unchanged during the exposure. The current decreased after exposure to the Hg and As ions. However, the drain current increased after exposure to the Pb ion. This is significant, because the N,N,N′,N′-Tetradodecyl-3,6-dioxaoctanedithioamide-functionalized chemFET sensor shows selectivity of Pb ion against other heavy metal ions.

The third modification is a hybrid study by combining 18 bilayers of self assembled gold nanoparticles and N,N,N′,N′-Tetradodecyl-3,6-dioxaoctanedithioamide ionophore. The sensor was exposed to different targets (Cs, Cd, Hg, Pb, As, Cr³⁺ and Cr⁶⁺) with the same concentration of 100ppm. It is noted that the responses for the Cr³⁺ and Cr⁶⁺, with the normalized current change of 2.9% for Cr³⁺ and 5.9% for Cr⁶⁺, are significantly higher than for other ions such as Cs, Cd, Hg, Pb and As. This demonstrates that by design, we can combine self-assembled gold nanoparticles and lead ionophore, and are able to produce a sensor that is sensitive and selective to not only chromium, but also Cr³⁺ and Cr⁶⁺. The self assembly process is critically important to obtaining the desired combined properties, due to of a high electron sheet carrier concentration channel induced by the strained semiconductor SOI layer and the self assembled bonds.