Monitoring and controlling combustion-related emissions are key issues for applications in mechatronics. The exhaust gas from an internal combustion engine, besides water (H₂O) and carbon dioxide (CO₂), also contains unburned and partially-oxidized hydrocarbons (HCs), carbon monoxide (CO) and particulate matter (PM). During combustion, the temperature is so high that the endothermic equilibrium between nitrogen and oxygen to form nitrogen oxide (NO) is established, according to the following scheme: (1) Fuel ( C x H y ) + Air → CO 2 + H 2 O + CO + SO x + NO x + PM + HCs

Subsequent rapid cooling and ejection of combusted gases into the exhaust system freeze this equilibrium, so exhaust gases also contain significant levels of NO, NO₂ and N₂O (generally referred to as NO_x).

Vehicle exhaust emissions are undesirable and can cause serious pollution products due to complex photo-initiated reactions between NO, HCs and O₂, resulting in the production of ozone and other reactive oxygen species causing the photochemical smog that is plaguing many major cities. Controlling the HCs and NO_x vehicles' emissions can have a direct impact on the effective reduction of the urban photochemical smog formation.

The US regulations signed in Dec. 2000, introduced very stringent NO_x emission standards for all in-use vehicles. In Japan, diesel emission standards require that in-use on-road light commercial vehicles should meet NO_x emission of 0.25 g/km starting from the end of 2005 and achieve full implementation by 2011 [1]. The European Commission has also introduced a series of regulations, the so called EURO Emission directives: from Euro I in 1991, to tighter limitations in 2009 (Euro V 80 g/km) and 2014 (Euro VI 0.08 g/Km), to meet the air quality standards stated by the international agencies [2]. Figure 1 illustrates the progression of the Euro and USA emission standard regulations in order to stepwise tighten the limit NO_x values.

Extensive research and development efforts have focused on the fabrication of NO_x sensors. While the world production of vehicles is presently suffering from the current economic crisis and will presumably reach saturation by 2010, the automotive exhaust gas sensors market is still predicted to grow and reach the level of over 100 million sensors per year in the next few years. High sensitivity, reliability to degradation in harsh environments, and fast response and recovery times are some of the desirable features of such devices. The objective of the present paper is to provide an up-to-date overview of current production and emerging state-of the-art NO_x sensors based on optical and nanomaterials technologies. The objective of the present paper is to provide an up-to-date overview of current production and emerging state-of the-art NO_x sensors based on optical and nanomaterial technologies.

Infrared laser absorption spectroscopy is a powerful tool for sensitive and selective trace gas detection for mechatronic applications. Up to now sensitivities in the range of the parts per million/trillion by volume (ppmv/pptv) have been demonstrated [10].

In the absence of optical saturation and particulate-related scattering, the intensity of light I(x) propagating in a homogeneous gas of sample length L is described by: (2) I ( λ ) = I 0 ( λ ) exp [ − L σ ( λ ) c ]where I₀(λ) represents the initial optical intensity, I(λ) the intensity of the radiation after it passes a gas sample of length L where the species to be measured is present at the concentration c, σ(λ) is the absorption cross-section.

In the mid-infrared region of the electromagnetic spectrum (2–14 μm) most molecular species exhibit a unique spectral signature, i.e. a characteristic series of fundamental absorption lines due to transitions between rotational-vibrational states, characterized by very large cross-sections. Hence, mid-infrared spectroscopy is in principle the best choice for the qualitative and quantitative measurements of molecular species in air. Very recently, the development of mid-ir detection techniques have received a significant boost from the invention and development of efficient mid-infrared semiconductor laser sources which promise to substitute optical methods based on the study of overtones and combination of lines falling in the near-infrared spectral region where the absorption cross sections drop by orders of magnitudes.

Among the absorption techniques, direct absorption spectroscopy [10] and cavity enhancement approaches [11] take advantage of long optical path length absorption in multi-pass cells and high finesse optical cavities, respectively. However, in spite of the high sensitivities, these techniques need sophisticated and cumbersome equipments not suitable in mechatronic applications which require compact and transportable sensors.

These limitations can be overcome by fully exploiting the advantages offered by photoacoustic (PA) spectroscopy, which is one of the most effective tools for exhaust gas detection, due to the high sensitivity (parts per billion, ppbv, detection limits), compact set-up, fast response time and portability. PA sensors are based on the generation of an acoustic wave in a specially designed gas cell that arises from the absorption of modulated light of appropriate wavelength by a selected target compound. The amplitude of this sound wave is directly proportional to the gas concentration and can be detected using a sensitive microphone if the laser beam is modulated in the audio frequency range.

In recent years, the development of new mid infrared laser sources has given a new impulse to infrared laser-based trace gas sensors. In particular, single mode quantum cascade lasers (QCLs) have become very attractive for mid-infrared gas sensing techniques thanks to single-frequency operation, narrow linewidth, high powers at mid-IR wavelengths (3 to 24 μm), room temperature and continuous wave (CW) operation [12]. They overcome some of the major drawbacks of other traditional mid-IR laser sources, i.e. lack of continuous wavelength tunability and large size and weight of gas lasers (e.g. CO and CO₂), large size and cooling requirement of lead salt diode lasers, complexity and low power of nonlinear optical sources.

Recently quantum cascade lasers, working in the 5 – 8 μm range, have been largely used for the spectroscopic detection of NO_x with measured high sensitivities in the ppbv range. In the following section, we report on a quantum cascade laser-based photoacustic sensor for the detection of nitric oxide, showing a detection limit of 450 ppbv.

III-V compound semiconductors, such as GaAs, have been chemically functionalized with porphyirins molecules and proposed as a method of for selective detection of NO [42]. Among the various porphyrin compounds, hemin (Fe-porphyrin) has been shown to be ideal for sensing NO due to the strong binding affinity it has to this small molecule [43,44]. Generally, there are a host of porphyrin compounds available with distinctly unique properties, and specificity to various gas molecules, that can be exploited for sensor applications. Of scientific interest is the integration of an organic compound, such as the porphyrin molecules, with an inorganic material and the capability for engineered sensor attributes.

The III-V compounds are especially interesting as transduction platforms for reasons of chemical inertness and wide ranging electronic properties. Detection of NO by way of hemin functionalized to GaAs has been shown to decrease its surface potential, which caused an increase in current throughout the GaAs device [45]. As such, another favorable property of these semiconductors is its sensitivity to adsorbates.

In recent years, the authors have evaluated InAs and AlGaN/GaN HFET structures functionalized with hemin to study chemical bonding state and associated sensor function when exposed to various analytes including NO. The important aspect of n-InAs is its characteristic surface accumulation layer which is modified based on changes in surfaces states due to adsorption of analyte molecules. Similarly, the buried 2-dimensional electron gas in AlGaN/GaN heterojunction field effect transistor (HFET) structures is also coupled to surface charge states and sensitive to modifications in charge carrier concentration upon analyte capture. We have used surface-sensitive techniques in our work, such as spectroscopic ellipsometry, to determine the thickness and/or coverage of the functionalizing layer; x-ray photoelectron spectroscopy, to reveal the hemin/semiconductor binding chemistry; and atomic force microscopy (AFM) to evaluate surface topology of the III-V compounds functionalized by hemin [46]. Herein, we present an overview of surface chemistry and sensor function results for hemin functionalized surfaces and its response to NO. The approach we devised for obtaining electrical data for NO gas detection is based on the established four-contact van der Pauw (VDP) method for characterizing sheet resistivity [47, 48]. This methodology differs from current chemical sensors in a variety of ways. One such differentiation is the enhanced accuracy of the four point probe averaging technique reducing the effects due to contact resistance. Another benefit is the relative simplicity of the device and ease of manufacture.

Hemin chloride was acquired from Frontier Scientific, Inc. and 1–2 mM concentrations in dimethylformamide were used. InAs (001) and AlGaN/GaN(0001) surfaces were immersed into the hemin solution for 1–3 h. The samples were subsequently rinsed with a 5% by volume chloroform/hexane solution and dried with N₂ gas. The goal of this procedure was to obtain a uniform monolayer of dispersed hemin molecules chemically attached onto the sample surface. The presence and efficacy of the procedure was evaluated by XPS and some typical data are seen in Figure 8. One notable feature of the C 1s core level XPS scan is a peak at ∼ 284 eV observed for the functionalization chemistries, which is attributed to C=N bonding within the porphyrin molecule [49]. Furthermore, for hemin-functionalized InAs surfaces, a strong N1s peak from the hemin molecule seen for the functionalized surface provides evidence for the semiconductor hemin functionalization. Interestingly, this work also revealed chemical bonding of the carboxylic acid ligands on the hemin to the anion of the semiconductor. The AFM images of InAs and GaN functionalized with hemin are shown in Fig. 9. The 500×500 nm2 images show the appearance of grain-like features whose size and density increases with the increase of hemin concentration from 1 to 2 mM. Those features are due to hemin aggregates [46], which are inactive from the sensing perspective and therefore, their formation has to be avoided. AFM and ellipsometry data have been used to perfect the functionalization chemistry procedure and determined an optimal concentration <1mM to yield homogeneous porphyrinated monolayers with a thickness of 17 Å [42].

The sheet resistivity response of a hemin functionalized AlGaN/GaN HFET structure is shown in Figure 10. These early, initial results show that the functionalized samples show significant changes in sheet resistivity when exposed to 70 ppm of NO in argon. The plot shows attempts to purge the NO gas from the sensor environment for evaluating reversibility and response times. Upon exposure to NO there is a response with an easily measured change in sheet resistivity. The removal of the sample from the sample cell, exposure to dry nitrogen and then reinsertion into the cell did decrease the sheet resistivity to near its initial value. The re-exposure of the structure to NO again changed the sheet resistivity to near its previous NO exposed value. The resistivity response of functionalized InAs to NO and NO₂ is shown in Figure 11 in terms of the normalized sheet resistivity response ((Rsh – R₀)/R₀). The graphic depicts increasing levels of both gases exposed to the sensor followed by decreasing levels of NO and NO₂ (10 to 80 ppm).

The resistivity response of porphyrinated InAs to NO and NO₂ generally shows an exponential increase with analyte exposure and decrease with diminishing concentrations, eventually reaching a constant level. This exponential behavior is attributed to Langmuir adsorption kinetics common in semiconductors with responsive surface space-charge regions [52]. Adsorption of NO by the porphyrin molecule tethered to the III-V semiconductor causes conductivity changes through polarization induced bond formation or through van der Waals interaction leading to valence band-bending and/or charge transfer. The InAs surface accumulation layer, with its Fermi level above the conduction band minimum (CBM) balances its highly mobile free-electron concentration with positive surface states [53]. During the adsorption of NO and NO₂, the space charge region at the InAs surface is affected and has a direct impact on the conductivity change because as the electrons are depleted from the accumulation layer [54].

Strict emission regulations and deeper environmental awareness have led to intense research into emissions reduction by various engine manufacturers and research institutes. Under these circumstances, NO_x detection is becoming a key aspect, and several sensing and transduction mechanisms have been investigated in the recent literature. In the present study, quantum cascade laser based photoacoustic (QCL-PA) detectors, gold nanoparticle-based field-effect transistor sensors (Au-NP-FET) and functionalized III-V semiconductor based devices have been developed, characterized and explored, in terms of sensitivity and selectivity. In Table 3 the sensing properties of the reported NO_x sensors have been summarized and compared with commercial YSZ lambda gauges.

The QCL-based PA sensor consists of an amplitude modulated QCL, a PA cell, and a signal acquisition and processing equipment. A resonant configuration of the PA cell, excited in its first longitudinal mode, has been chosen in order to increase the sensitivity, with a detection limit of 450 ppb (SNR =3). The actual device scheme makes the reported PA sensor more suitable for stationary exhaust gas testing than for on-line applications. Future developments will be the implementation in the device scheme of: i) a QC laser operating at room temperature in continuous mode, in order to avoid the thermal chirping of pulsed operation, thus increasing the selectivity; ii) optical microphone, characterized by high sensibility (5 V/Pa) to minimize the electromagnetic noise; iii) fiber coupling with collimating system between the QC laser and the PA cell to take rid of any optical alignment issue and thus obtaining a more compact, robust and portable PA sensor.

Metal Insulator Semiconductor field effect devices used as gas sensors can be of different types: transistors, Schottky diodes or capacitors, with transistors being preferred for commercial devices. The gas sensing principle for field effect sensors is based on molecules adsorbing and dissociating on a catalytically active gate material on the sensor. These interactions create a change in the electric charges on the semiconductor surface, which in turn results in a shift in the sensor output voltage.

The interactions of the gas molecules with the gate material depend on the operating temperature and the morphology and chemical characteristics of the gate material. Electrochemically synthesized gold-nanoparticles, were used as catalytically active materials in field-effect transistor (FET) sensors capable to monitor NO_x in the concentration range comprised between 50 and 200 parts per million. The use of nanostructured films as gate material has the potential to give sensors with increased sensitivity, and faster response and recovery due to the larger surface area available for interaction with the gas molecules, as compared to conventional thin film sensing layers. These sensors displayed an appreciable selectivity towards NO_x, providing small responses to NH₃ and H₂ and not responding at all to CO and C₃H₆.

Reliable NO sensor could be fabricated with a porphyrin as the functional group and a GaN-based HFET as the semiconductor material. Among the various porphirins, hemin was proposed for the detection of NO because of its chemical affinity, affording for a preferential binding to NO, when exposed to a mixture of gases such as NO, O₂, and CO. The reported sensing results represent initial data for porphyrinated III-V semiconductor based sensors and show (i) unambiguous response to NO and NO₂, (ii) proof of Langmuir –type adsorption operative in the sensor function, and (iii) the reversibility and sensitivity of the sensors. Analyte selectivity experiments are underway, but our proof-of-concept devices showed obvious selectivity to NO,compared to O₂, CO₂, N₂, using hemin porphyrin. We have to underlined that InAs and AlGaN/GaN porphyrins functionalized sensors represent completely new systems, that is no previous study on this type of systems have been reported so far. Only GaAs functionalized with hemin based sensor have been reported in literatore as NO sensor. And even in that case the system was neither optimized from functionalization or mechanism point of views.

Further studies are in progress for the optimization of the InAs and AlGaN/GaN porphyrins sensors with respect to sensitivity and selectivity. Nevertheless, since their novelty and promising results they are worth of consideration.