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1. INTRODUCTION

Optical sensors belong to the most important types of chemical sensors that have been extensively studied in recent years for the continuous and real-time monitoring of analytes. Depending on the origin of the optical signals, these types of sensors may be roughly classified into absorbance and luminescence-based sensors, the latter mainly utilizing the principle of fluorescence and chemiluminescence (CL) detection. Although they offer many advantages such as high sensitivity, good selectivity, and fast response time, fluorescence-based sensors require an excitation light source and spectral separation of exciting and emitted light, leading to relatively sophisticated equipment producing high background signals.

However, these drawbacks do not occur in the case of CL detection since the energy required for CL emission does not originate from an exciting light beam but is produced in a chemical reaction. Significant advances in design and applications of CL sensors and biosensors were recorded in the last few years [1–3]. The most utilized type of sensor is the flow-through one [4]. The reliability of this type of sensor made them suitable for utilization as detectors in flow injection analysis (FIA) systems [5]. The first CL sensor for hydrogen peroxide analysis was reported by Freeman and Seitz in 1978 [6] and since then various types of CL-based sensors have been extensively studied for inorganic, organic, and biological/pharmaceutical compounds. In this chapter, the development of CL sensors is reviewed and the advantages and limitations when applying the technique to routine analysis are discussed.

2. TYPES OF CL SENSORS

A CL sensor can be defined as an analytical device incorporating an active material with a transducer, with the purpose of detecting in a continuous, selective,

as well as organic vapors, etc. Most reports, however, involve the analysis of compounds using CL sensors in liquids, which include enzyme-based CL sensors, non-enzyme-based CL sensors as well as CL immunosensors, etc.

3.CL SENSORS FOR THE DETERMINATION OF ANALYTES IN AIR OR VAPORS

3.1 NO2-Air Sensors

Different instrumental variations, based upon the oxidation of luminol-producing CL, have been developed for the detection of gas-phase oxidants. Maeda et al. [7] designed a CL sensor for the detection of NO2 that comprised a pool of alkaline luminol solution directly below a photomultiplier tube (PMT) introducing the sampled airstream into the region above the solution. This system was very sensitive to movements of the compartment and had a relatively slow time response. An alternate design for CL detection of NO2 was subsequently presented by Wendel et al. [8], wherein a length of filter paper was positioned adjacent to a PMT, and a flow of alkaline luminol solution was directed down the paper in a fine film. A simpler optrode for CL detection of NO2 was reported by Yin et al. [9] wherein a piece of filter paper soaked in alkaline luminol solution was positioned to one end of an optical fiber. The CL signal transmitted through the optical fiber could be detected by a PMT positioned at another end of this fiber. A further variation of the instrument as described by Wendel et al. [8] led to the development of a commercially available instrument devoted to the measurement of NO2; a detailed description of this instrument was presented by Schiff et al. [10]. The reaction cell consists of a fabric wick positioned in front of a PMT that is continually wetted with fresh luminol solution delivered via a peristaltic pump, and whose surface is exposed to a stream of the ambient air pumped through the cell. In recent years, this instrument has been utilized in combination with various pretreatment stages for trace detection of organic nitrates.

High sensitivities can be achieved by utilizing the CL biosensors proposed by Spicer et al. [11]: the first one is based on the reduction of NO2 to NO followed by the detection of NO by the CL produced from its reaction with O3 while the second one is based on the detection of CL produced from the reaction of NO2 with luminol solution. The working concentration range for the O3 CL sensor (0–800 g/L NO2) is larger than the working concentration range obtained using the luminol CL sensor (0–50 g/L). The main disadvantage of these types of CL sensors is nonselectivity with respect to several substances (e.g., nitrous acid).

Another variation on the luminol CL sensor for NO2 was introduced by Collins and Ross-Pehrsson [12] where a solid-phase reagent was positioned below a PMT, across which the air under test is pumped. Of the hydrogel or polymeric sorbents investigated, a Waterlock superabsorbing polymer (hydrogel)

Figure 3 Schematic diagram of a solid-phase NO2 sensor. The sensor consists of a small cell supporting the polymer-coated, glass substrate behind a glass window in full view of a PMT. The CL reagent is immobilized on the hydrogel substrate. The gel is sandwiched between the glass window and a Teflon PTFE membrane. The purpose of the Teflon membrane is to permit the diffusion of NO2 from the airstream into the gel while preventing the loss of water from the hydrogel. Inlet and outlet tubes (PTFE) allow a vacuum pump to sample air (2 L/min) directly across the surface of the chemical sensor. (Adapted with permission from Ref. 12.)

incorporating luminol and Cu(II) proved most suitable for the selective determination of NO2. The sensitivity was 0.46 ppb. The sensors showed drift during continuous exposure due to the irreversibility of the luminol reaction. Figure 3 shows the schematic diagram of a typical NO2-air sensor.

3.2 O2- and O3-Air Sensors

The first CL sensor for oxygen analysis was reported by Freeman and Seitz in 1978 [6]. Collins and Ross-Pehrsson [12] investigated the effect of polymer type, pH, and metal catalyst incorporated within the film. Oxygen levels as low as 2.4 ppm in nitrogen have been detected using the oligomer fluoropolyol as the support matrix for immobilizing luminol, KOH, and the metal catalyst Fe2(SO4)3. A sensor

for monitoring atmospheric ozone was proposed by Schurath et al. [13] by coating a dye, Coumarin 47 on a thermostatized commercially available TLC plate of dry silica. Ozone was determined by the CL reaction with the surface-absorbed dye. The flow rate, pressure, temperature, and humidity dependence of the detector sensitivity were measured. The lightest balloon-borne version of the detector, including batteries, weighed 1.3 kg.

3.3 Sensor for Carbon Dioxide

Lan and Mottola [14] have presented two continuous-flow-sensing strategies for the determination of CO2 in gas mixtures using a special reaction cell. Both approaches are based on the effect of the complex of Co(II) with phthalocyanine as a rate modifier of the CL emission generated by luminol in the absence of an added oxidant agent, which is enhanced by the presence of CO2 in the system. This enhancement allows the fast and simple determination of carbon dioxide at ppm levels (v/v) in atmospheric air and in human breath. In the first case, a continuous monitoring system was applied; however, because the flow of expired gas is not constant, a discrete sample introduction approach was used in the analysis of CO2 in breath.

3.4Sensors for Chlorideand Ammonia-Containing Compounds

A sensor for organic chloride-containing compounds was constructed by immobilization of luminol or tris-(2,2′-bipyridyl)ruthenium(III) between a PMT and a poly(tetrafluoro)ethylene (PTFE) membrane [15], through which a stream of air was sampled by diffusion. A heated Pt filament incorporated in the gas line leading to the CL cell was used to oxidize the analytes prior to diffusion across the PTFE membrane. Detection limits for CCl4, CHCl3, and CH2Cl2 were 1.2–4 ppm. A similar device could also be used for the determination of hydrazine and its monomethyl and dimethyl derivatives or NH3 vapor. The detection limit for hydrazine was only 0.42 ppb [16].

3.5 Sensors for Sulfur-Containing Compounds

Meng et al. [17] described a CL sensor based on tris(2,2′-bipyridylruthenium(II) permanganate for the SO2 assay. SO2 can be sampled if air is purged through a 0.1% triethanolamine absorbing solution. Furthermore, the slope of the calibration graph is constant for a given triethanolamine solution and it was stepwise linear from 1 10 7 to 1.25 10 5 mol/L of SO2 in the triethanolamine solution. The recovery of SO2 in air samples is between 94.6 and 105.4%.

A high-speed sensor for the assay of dimethyl sulfide in the marine troposphere based on its CL reaction with F2 was recently reported [18]. Sample air and F2 in He were introduced at opposite ends of a reaction cell with a window at one end. The production of vibrationally excited HF and electronically excited fluorohydrocarbon (FHC) produced CL emission in the wavelength range 450– 650 nm, which was monitored via photon counting. Dimethyl sulfide could be determined in the 0–1200 pptv (parts per trillion by volume) concentration range, with a 4-pptv detection limit.

3.6 Organic Vapor Sensors

Ethanol and acetone in air could also be detected by a vapor sensor that was constructed from a sintered layer (0.5 mm) thick of γ-Al2O3 or an α-Al2O3 substrate on a ceramic substrate (3 1.5 mm) with a heater layer of Pt thin film [19, 20]. It was placed in a quartz tube (8.4 mm id) through which the gaseous sample was passed. The CL produced on oxidation of the sample was measured at 380–420 nm using the photocounting technique, and at 450–650°C with a flow rate of 400 mL/min. It was possible to determine up to 400 ppm of ethanol and 200 ppm of acetone in gaseous mixtures. A similar design was also reported for discriminating and determining constituents in mixed gases [21]. Ethanol and butanol spectra were similar, but they differed from propanol and butyric acid spectra. Detection limits were around 1 ppm for the vapors.

Recently ‘‘cataluminescence’’ has been used as detection technique in a gas sensor for recognizing organic vapors [22]. Cataluminescence is termed CL emitted during catalytic oxidation of a combustible gas. In this new approach, spectroscopic images of cataluminescence intensity, which reflect the type and concentration of organic vapor as function of wavelength and temperature, can be measured continuously. Alcohol and ketone or different kinds of alcohols could be discriminated and determined using this method.

4.CL SENSORS FOR THE DETERMINATION OF ANALYTES IN LIQUIDS

4.1 Enzyme-Based CL Sensors

4.1.1Hydrogen Peroxide

Freeman and Seitz [6] developed one of the first enzyme-based CL sensors with convincing performance. They immobilized horseradish peroxidase (HRP) at the end of an optical fiber and achieved a detection limit of 2 10 6 mol/L H2O2. Preuschoff et al. [23] developed a fiberoptic flow cell for H2O2 detection with long-term stability, suitable for fast FIA. Different peroxidases were covalently

immobilized on an affinity membrane and compared with respect to the catalytic luminol oxidation, achieving high sensitivity and best long-term stability with microbial peroxidase. The operational stability of the sensor is longer than 10 weeks and hydrogen peroxide can be determined in the range between 10 3 and 10 8 M. Higher sensitivity was obtained using a different approach for a H2O2 biosensor based on the luminol-peroxidase system [24]. CL intensity was increased by using a high-salt-concentration medium, such as 3 M KCl or NaCl, and the enhancement phenomenon was just as effective on soluble peroxidase and on immobilized peroxidase, its magnitude depending on the hydrogen peroxide concentration.

More recently a CL biosensor was developed using silicate glasses obtained by the sol-gel method [25], which represents an alternative for immobilization of biological entities owing to its low-temperature preparation, providing an adequate supporting matrix in which HRP is immobilized by microencapsulation. This sol-gel biosensor based on the CL reaction of the hydrogen peroxide-lumi- nol-HRP system permits the determination of hydrogen peroxide in the range of 0.1–3.0 mM by measuring CL in a cuvette and through an optical fiber modified at its end with immobilized HPR gel, providing a detection limit of 6.7 10 4 M. The method was satisfactorily applied to hydrogen peroxide determinations in disinfectant solutions for contact lenses.

4.1.2Glucose

Several types of CL biosensors are described for glucose assays. Some of them are based on the utilization of glucose oxidase for enzymatic reaction in the coupling with luminol for the CL reaction. Aizawa et al. [26] and Blum [27] developed the CL bienzyme sensors for glucose determination on the basis of the HRPcatalyzed H2O2/luminol reaction and coimmobilized glucose oxidase. A fiberoptic was incorporated in the FIA system for the analysis of glucose using the above-mentioned technique [28]. The working concentration range is of mM magnitude order, with a detection limit of 1 µM. High sensitivity was obtained in the analysis of glucose in plasma using the luminol reaction with H2O2 produced by immobilized pyranose oxidase within a flow-through cell containing immobilized peroxidase [29]. The enzyme catalyzes the reaction

D-Glucose O2 H2O → D-glucosone H2O2

This enzyme oxidizes α- and β-anomers of D-glucose to the same extent and shows excellent stability and sensitivity about twice that for the methods with immobilized glucose oxidase. In this approach, pyranose oxidase is immobilized on tresylate-poly(vinylalcohol) beads and packed into a stainless column and peroxidase is immobilized on tresylate-hydrophylic vinyl polymer beads and packed into a transparent PTFE tube that is used as the CL flow cell. The H2O2