Near-IR sensors

The majority of currently deployed IR sensors operate in the near-IR. Although near-IR sensors suffer from limited selectivity and sensitivity due to the relatively unstructured broadband absorptions in this frequency range, the easy availability of waveguides and other instrumentation give this spectral range a significant advantage over the mid-IR. Main application areas involve substance identification and process control. 

In the near-IR, sensors almost exclusively rely on silica fibres (standard or low-OH) as they are accepted as industrially fully applicable32, 33 Silica-based glass fibres are chemically and mechanically robust, easy to handle, inexpensive, available with various core and outer diameters, a core-clad transfer fibres or bare sensing fibres, and have successfully been optimised to their theoretical attenuation limit.34. The spectral window allows application up to 2,5 pm. 

Near-IR sensors are also a topic of increased interest for the analysis of biological materials, from food quality control to bioactivity measurements. First applications... 

Ajayaghosh A (2005) Chemistry of squaraine-derived materials near-IR dyes, low band gap systems, and cation sensors. Acc Chem Res 38 449 159... 

Nonetheless, near-IR is the most widely used IR technique. Less intense water absorptions permit to increase the sampling volume to compensate, to some extent, for the lower near-IR absorption coefficients and the inferior specificity of the absorption bands can for many applications be overcome by application of advanced chemometric methods. Miniaturised light sources, various sensor probes, in particular based on transmission or transflectance layouts, and detectors for this spectral range are available at competitive prices, as are (telecommunications) glass or quartz fibres. 

In practice, very few applications of FEWS sensors can be found outside laboratory applications and demonstration systems. In the near-IR, suitable fibres are readily available but usually there is no real necessity to use them. Possible transmission pathlengths are sufficiently large to allow using standard transmission probes, while turbid samples can be measured using transflection or diffuse reflection probes. In the mid-IR, high intrinsic losses, difficulties in fibres handling and limited chemical and mechanical stability limit the applicability of optical fibres as sensor elements. 

Consequently, mirror optics are more common, in particular in the mid-IR. The mirrors used are usually aluminium- or gold-coated flat or curved substrates. While near-IR mirrors are usually protected by thin SiO-layers, in the mid-IR unprotected mirrors have to be used. Disadvantages of mirror optics are the elevated space consumption and the higher prices in comparison to refractive optics, especially comparing non-standard mirrors against non-standard lens. In total, mirror optics are so preferable to fibres and refractive optics, at least in the mid-IR, that in some technical applications they are used to replace waveguides to transport IR radiation between source, sensor head and spectrometer. 

Similar to IR sensors, process analysis is the prevalent application area. Due to the applicability of standard VIS instrumentation, Raman probes have been used for more than two decades65, 66. Typically, Raman probes are applied where near-IR probes fail and hence are in direct competition to mid-IR probes. 

In this section we will review the application of near-IR system instrumentation to the most commonly encountered fluorescence measurements such as steady-state spectra, excited state lifetimes, anisotropy, microscopy, multiplexing, high-performance liquid chromatography (HPLC), and sensors. 

The near-IR offers many attractions when working with sensors owing to low transmission losses in optical fibers, compatibility with the wealth of optoelectronics... 

Nonconventional fluorimeters and sensors that incorporate optical waveguide coupling possess less optical attenuation in the near-IR as compared with the UV/vis-ible because of the reduction in Rayleigh scattering. The temporal dispersion is also reduced in the near-IR for the same reason, e.g., 80 psec/nm/km at 900 nm as compared with 1 nsec/nm/km at 400 nm for a single-mode fiber. 

The opportunities for near-IR fluorescence sensors are of course not only limited to analytical chemistry. Physical parameters such as temperature can also be measured. For example, Grattan and Palmer have used the fluorescence lifetime quenching of neodymium glass fluorescence at 1054 nm, excited at 810 nm with a gallium-alumi-... 

In conclusion, it is probably fair to say that the true potential for near-IR fluorescence sensors has yet to be exploited. 

J. Lin and C.W. Brown, Near-IR fiber-optic temperature sensor, Appl. Spectrosc., 47, 62-68 (1993). 

Details are given of the development of an on-line sensor using near IR spectroscoy for monitoring carbon dioxide concentration in polymeric extrusion foaming processes. The calibration curve relating the absorbance spectrum at 2019 nm to the dissolved gas concentration was derived so as to infer dissolved carbon dioxide gas concentration... 

Optical properties light-emitting diodes, resonance absorption of near IR-radiation Physical and chemical properties large specific surface and possibihty of surface chemical modification, adsorbents, catalysts, chemical sensors, materials for electrodes, chemical batteries, fuel elements and super condensers. 

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