Novel Inroads to Highly Sensitive and Selective Liquid and Gas Phase Sensors
James L. Gole, Georgia Inst. of Technology
The development of MEMS/NEMS nanotechnology portends of a revolution in sensors driven by the potential for the smallest size, lowest power, and highest sensitivity enabled by utilizing a novel nanostructure-driven configuration. We have been concerned with a new platform for the detection of gases, organic vapors and their liquids 1,2 .
The objective of our research effort is the development of low cost, low power, sensitive, ambient air sensor arrays suitable for identification of the quality of breathable air, the signatures of disease (ex: asthma, non-invasive biomarkers of disease and lung inflammation), the detection of hazardous gases and odors and, the ability to perform selective measurements related to actionable information that improves human health. The key here is to provide an approach that has sufficient analytical reliability, which bridges the gap between lower cost and higher performance, while significantly reducing complexity and cost. The systematics are designed to provide a general portable format for use with minor modifications to all gases. Further they can operate in a wireless mode.
We have extended our sensing platform to detect low ppb NH
3
and NO x
levels focused on the simultaneous detection of the signatures of asthma. Asthma is characterized by a complex pattern of airway inflammation on respiratory surfaces, which results in poor lung function. The onset of an asthma attack is marked by an exponential increase in NO which combines with O
2
to form
NO
2.
NO and NO
2
provide distinct and opposite conductometric signals, thus forming a combined means of identifying the sequence of an asthma attack. In addition, NH
3
is an important buffer of endogenous airway acids formed in response to rhinovirus infection and other pro-inflammatory stimuli.
Available laboratory systems to assess inflammatory biomarkers in breadth require technical assistance, are not portable, and are costly. An easy to use yet accurate device to assess the level of airway inflammation in the ambulatory setting by asthma sufferers could greatly enhance disease management. A simple array- based sensor could be used for both NO x
and
NH
3
,monitoring simultaneously NH
3
,NO x
, and NO. This would represent a portable and easy to use device, patients with asthma can potentially use on a regular basis to monitor lung inflammation and thereby greatly improve their application of medications. The goal is to develop an encompassing detection system which can simultaneously test for NO, NO
2
, and NH
3 in a device that can be maintained in the room of an asthmatic and used to detect the rapid increase
Ошибка! Закладка не определена.
of NO and the subsequent formation of NO
2
during an attack, allowing the patient to take medication early on. This can save a $6,000 overnight hospital stay.
The potential extension of our technology to other deleterious gases including H
2
S, SO
2, and CO, has also been demonstrated 1,2 . These gases have been detected selectively in the presence of the
BTEX compounds (benzene, toluene, and xylene) and exceeds 1000/1 which exceeds the best industry standards.

Outline of concept
It is possible to develop nanostructure-based gas sensor systems, which represent a significant advance in sensitivity and selectivity versus traditional metal oxide systems. Using nanostructure directed modification of an extrinsic semiconductor interface, we have developed a method to control the degree of electron transduction (and therefore electron transfer) between analytes and a doped semiconductor interface. Based on hard/soft acid/base concepts, the careful selection of nanostructure directed modifications enhance or impair electron transduction to and from the interface, This can be made to precisely control the increase or decrease sensor response to a given analyte.
A general road map, the Inverse Hard/Soft Acid Base (IHSAB) concept, provides a novel and versatile means of sensor design 1 . The developing sensor system design represent a robust technology for a wide range of conductometric detection applications and demonstrate considerably higher sensitivities than traditional metal oxide sensors. The sensor platform does not require film-based technology, operates at room temperature , and can be obtained without the use of time-consuming assembly processes
2
. The sensors are rapidly responding (with an initial response time of 2 sec.), reversible, sensitive, and selective, have been operated for extended periods (6 months or longer) and can be readily rejuvenated.
With a highly efficient electrical contact formed to a modified nanopore, selective coatings create an environment that forces physisorption. The sorption even causes shifts in the quantum levels for conduction that can be used to interpret the amount and type of adsorption [selectivity].
Nanopore coated micropores allow rapid Fickian diffusion of analytes to nanostructure directing metal oxide sites. The nanoporous coating (1) provides a “phase match” for the application of nanostructured metal oxide island sites which provide for notably higher sensitivities and selectivity and prevents the subsequent sintering of these distinct nanometer-based island sites.
We require only that the island sites be deposited at sufficiently low concentration so as not to interact electronically with each other. This novel mode of construction provides for the detection of analytes in an array-based format at the sub-ppm level. The nanostructured metal oxide sites and their in-situ functionalized counterparts are made to create a broad range of distinct responses 3 for a given analyte which form the basis for operation in an array-based format. Observed sensitivities and sensor system reversibility can be predicted from the recently developing IHSAB model formulated to facilitate significant and predictable changes in sensor interface sensitivity for a variety of gases
1,2
.
Conductometric nanostructured array-based sensor devices are constructed by first establishing the acidity/basicity of the gas to be detected and subsequently developing nanostructured island sites which encompass as wide a range of acidity and basicity as possible. The deposited metal oxide island sites can be coupled with the p or n-type character of a PS semiconductor interface to produce a range of reversible interactions with the majority charge carriers, either electrons (ntype) or holes (p-type) to achieve a maximum reversible response.
1. James L. Gole and William I. Laminack Nanostructure Directed Chemical Sensing: The IHSAB Principle and the
Dynamics of Acid/Base- Interaction,” James L. Gole and William I. Laminack Invited submission, Thematic Issue-
Nanostructure Directed Chemical Sensing, Beilstein Journal of Nanotechnology, 4 , 20-31(2013).

2.
J.L. Gole Increasing Energy Efficiency and Sensitivity with Simple Sensor Platforms, J.L. Gole, Talanta, 132,
87-95 (2015). DOI: 10.1016/j.talanta.2014.08.038.
3 .
Direc t in-situ Nitridation of Nanostructured Metal Oxide Deposited Semiconductor Interfaces: Tuning the
Response of Reversibly Interacting Sensor Sites, W. I. Laminack and James L. Gole,ChemPhysChem, DOI:
10.1002/cphc.201402108