Chemiresistors represent a powerful class of chemical sensors that can be readily integrated into any electrical system, can be miniaturized, are readily multiplexed, and take nearly zero-power to operate. One of the greatest limitations to these sensors is a lack of selectivity, which is the electronic equivalent of noise. Interference from large varying background signals, such as humidity, can compromise the sensor signal to a point where there is no useful data. To address this challenge, new ways to integrate molecular constructs into carbon nanotube compositions that produce enhanced selectivity to certain molecules or classes of molecules were investigated to increase the signal to noise level in chemical sensors.

Figure 1. Chemical structures of metalloporphyrin complexes employed in the chemiresistive sensor array. Axial H₂O ligands have been omitted for clarity

To realize the full diversity of the first row transition metal sensors, metal porphyrin complexes were targeted. These materials have extended π-electron systems that make them ideal candidates to bind through non-covalent mechanisms to the surfaces of carbon nanotubes. To promote strong interactions, a porphyrin system containing fused pyrene systems was chosen. Although the synthetic procedures were reproducible and it was possible to make the regioisomeric Co+2 complexes of these molecules, it was found that the tendency to self-associate precluded their ability to form strong complexes with carbon nanotubes. As a result, it was found that functionalization of single walled carbon nanotubes (SWCNTs) with these molecules has minimal effects on their response to specific chemicals of interest.

To properly evaluate the ability to utilize porphyrins to create selectively modified surfaces for chemical sensing, a study was conducted wherein the first row transition metal series of complexes (Figure 1) based on tetraphenylporphyrin (tpp) were prepared and used to functionalize the SWCNTs. Previous studies on porphyrins in chemical sensing note that despite their promise for this application, a drawback that limits their usage is that they are relatively unselective. However, earlier studies with sensors fabricated from porphyrin-CNT composites measured chemiresistive responses of only 2-3 metal centers to only 4-5 different analytes. A more comprehensive study on the chemiresistive responses has been completed and it was found that the responses of metalloporphyrin-SWCNT-based sensors to vapors of various volatile organic compounds (VOCs) were strong and were subjected to statistical analyses that enabled the successful classification of representative VOCs into five different categories (aliphatic hydrocarbons, alcohols, ketones, aromatic hydrocarbons, and amines) with 98% accuracy.

Figure 2. Fabrication of chemiresistive chemical sensors by drawing. Sensing materials (SWCNT-based) and graphite as electrodes were both deposited by mechanical abrasion to yield fully-drawn, chemiresistive gas-sensors on various A) unmodified substrates such as adhesive tape and unpolished silicon wafer, and B) laser-etched substrates such as PMMA and weighing paper. C) Fabrication of the sensing material consists of mechanically mixing and compressing SWCNT composites into a pellet. D) Three-step fabrication of fully drawn chemiresistive sensors on PMMA: laser-etch PMMA, deposit SWCNTs by abrasion (sensing material), and deposit graphite by abrasion (electrodes).

With the exception of amines, which are capable of strong charge transfer interactions, the basis of classification appears to correlate with the differences in the solubility properties of the porphyrin compounds in the various VOCs as solvents. This feature suggests that solvent vapors modulate the strength of interactions between the porphyrins and the nanotubes. These results further demonstrate the potential for porphyrin- functionalized SWCNT-based electronic noses for applications in inexpensive, portable chemical sensors for the identification of VOCs.

The ability to create on demand chemical sensors with minimal infrastructure offers a useful capability in support of covert and/or battlefield applications. The object, therefore, is to develop a rapid, scalable, portable, and cost-effective approach for the fabrication of fully-drawn chemical sensing arrays on a variety of different substrates (e.g., paper, plastic, and undoped float zones silicon wafer). This approach is entirely solvent-free, requires only small amounts of sensory materials, and is capable of producing highly-sensitive chemical sensors.

This approach has been demonstrated in the context of sensing and differentiating a variety of vapors at ppm concentrations. The demonstration employs solid composites of single-walled carbon nanotubes (SWCNTs) and small molecules as the sensing material and graphite as electrodes utilizing a previously established method to generate sensing materials, or PENCILs (Process Enhanced NanoCarbon for Integrated Logic), by the mechanical mixing of SWCNTs with commercially available small molecules (solid or liquid). A technique called DRAFT (Deposition of Resistors with Abrasion Fabrication Technique) was then used to deposit these materials on a variety of substrates. Sequential deposition by mechanical abrasion of sensing materials and commercial graphite pencils on various etched and non-etched substrates yields precisely fabricated fully-drawn chemiresistive sensing arrays (Figure 2).

The performance of the arrays were benchmarked against those created using conventional metal based electrodes. It was found in all cases that the fully drawn sensors were able to match the performance of those on similar substrates with metal (Au) electrodes.

This work was done by Timothy M. Swager of Massachusetts Institute of Technology for the Army Research Office. ARL-0193

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Precision Assembly of Systems on Surfaces (PASS)

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This article first appeared in the June, 2016 issue of Aerospace & Defense Technology Magazine.

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