Spectroscopic monitoring of NO traces in plants an

Authors: S M Cristescu D Marchenko J Mandon K Hebelstrup G W Griffith L A J Mur F J M Harren

Publish Date: 2012/05/24

Volume: 110, Issue: 2, Pages: 203-211

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Abstract

Optical methods based on quantum cascade lasers QCLs are becoming popular in many life science applications We report on two trace gas detection schemes based on continuous wave QCLs for online detection of nitric oxide NO at the subpartperbillion level by volume ppbv 110−9 using wavelength modulation spectroscopy WMS and Faraday rotation spectroscopy FRS at 1894 cm−1 and 187573 cm−1 respectively Several technical incremental steps are discussed to further improve the sensitivity of these methods Examples are included to demonstrate the merits of WMSbased sensor direct monitoring of NO concentrations in exhaled breath and from plants under pathogen attack A simple handheld breath sampling device that allows single breath collection at various exhalation flows 15 50 100 and 300 mL/s respectively is developed for offline measurements and validated in combination with the WMSbased sensor Additionally the capability of plants to remove environmental NO is presentedIn 1890 Alfred Nobel’s physicians prescribed nitroglycerine a key component of dynamite as a remedy for his heart disease For more than 100 years this was used in the treatment of angina pectoris although nobody knew the physiological mechanism of action The pioneering work of Robert Furchgott Louis Ignarro and Ferid Murad awarded the Nobel Prize in Physiology or Medicine in 1998 has shown that nitroglycerine acts by releasing nitric oxide NO as the therapeutic agent and demonstrated that NO is involved in blood pressure control by triggering the blood vessels to relax and widenStarting in 1992 when NO was named the ‘molecule of the year’ the scientific community has realized the biological significance of NO not only its role as atmospheric pollutant After the first report of exhaled NO in animals and humans 1 the interest in NO monitoring for clinical applications has tremendously increased with over 1200 articles in the field of asthma and other pulmonary diseases This fast progress of NO as a potential noninvasive test for disease evaluation and treatment monitoring was possible because the technology for NO detection was already available on the market the chemiluminescence analysers were used for measuring NO in air pollution and were easily adopted for exhaled NO analysis Currently chemiluminescence devices are considered as the ‘gold standard’ providing sufficient accuracy and precision However they are bulky and expensive have high running costs and require technical expertise in calibration therefore limiting their use in routine patient care A convenient alternative is offered by the electrochemical sensors but despite their lower cost and possibility for development of portable or handheld devices they suffer from a lack of sensitivity detection limit 5 ppbv and efficiency in NO multiple flow analysis 2 3 4The quantum cascade laser QCL technology has rapidly advanced during the last years offering narrow linewidth high power at room temperature and continuous wave cw operation at midIR wavelengths 3–24 μm 5 6 These features are nowadays combined with a wide collection of noninvasive optical methods for selective sensitive and accurate analysis of NO concentrations 6 7 8 9Novel midIR gas sensors especially using QCLs as the source are gaining popularity within the medical/clinical community Several medical reports have shown their high performances in terms of sensitivity selectivity and accuracy in comparison to other technologies 10 Another great advantage offered by optical sensors is the possibility to perform multiple flow rate analysis exhaled NO concentration is flow dependent without modification of the sensor 4Although most reported research on NO detection is performed in animals and humans nitric oxide is also a signalling molecule in plants The last decade has brought much new insight into the pathways of NO production and function in plants Nevertheless there are still many details of the NO story to be elucidated This lack of knowledge is partially due to the technical difficulties in measuring and quantifying NO Conventional chemiluminescence detection requires higher flow rates 12 L/h or greater to achieve a typical detection limit of 1 ppbv This implies that the NO production from the plant material should be least 12 ppbv otherwise the NO concentration is barely visible due to the dilution effect With the optical sensors this limitation is overcome lower flows of a few litres per hour are easily possible An updated review describing the advantages and disadvantages of several available methods to detect NO in plants including laserbased spectroscopy has been recently published 11In this paper two techniques are presented in detail using QCL in combination with wavelength modulation spectroscopy WMS and Faraday rotation spectroscopy FRS for NO detection in both humans and plants The WMSbased sensor was applied for online monitoring of NO concentration during single exhalation Possibilities for offline measurements are also considered a simple handheld device for sampling breath in a standard way is developed and validated Moreover the suitability of WMSbased sensors for accurate detection of NO in planta is shown in two important plant research areas pathogen infection and NO removal from the atmosphereContinuous wave QCLs cwQCLs are becoming increasingly popular sources for IR chemical sensing Nowadays operating at ambient temperatures with high output powers and excellent spectral quality thermoelectrically cooled cwQCLs have created a range of novel midIR gas sensors offering high sensitivity selectivity and fastresponse concentration measurements Moreover new applications for gas sensing are emerging due to their compact size robust construction and low power requirements

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