Spectrophotometry and Optical Biosensor
In the previous chapter we learned about optoelectronic light sensors, especially photodiode. Although many photodiodes are made to be specifically sensitive to a certain range of colors, e.g., UV-blue, red-IR, etc., they cannot tell the exact color or co
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Spectrophotometry and Optical Biosensor
In the previous chapter we learned about optoelectronic light sensors, especially photodiode. Although many photodiodes are made to be specifically sensitive to a certain range of colors, e.g., UV-blue, red-IR, etc., they cannot tell the exact color or color combination of light signal. Photodiodes (and most other optoelectronic light sensors) simply inform us the intensity of light signals. Detailed information on color combination can be obtained by using a technique known as spectrophotometry, which involves an instrument spectrophotometer. Spectrophotometers are quite bulky and relatively expensive and have not been considered as a topic for sensors or biosensors. Recent years, however, very small spectrophotometers (a size comparable to a mobile phone) have become commercially available. In addition, spectrophotometers are becoming an integral part of many sensor and biosensor devices.
8.1
Spectrophotometry
A spectrophotometer measures the light intensity that is absorbed or transmitted through (sometimes scattered or reflected from) a sample material, which is typically liquid solution in a container, but sometimes gas in a container. This intensity measurement can be made at a specific color (wavelength), or scanned through a range of colors. If the measurement is made at a specific color (wavelength), it is called photometry. If the measurement is made for a range of colors (wavelengths), it is called spectrometry, and the resulting light intensity—wavelength curve is called a spectrum (plural: spectra). Therefore you can see that spectrophotometry is a collective term for both spectrometry and photometry. Of course, most commercial instruments are capable of conducting both spectrometry and photometry, hence they are called spectrophotometers. Most spectrophotometers measure absorbance, thus they are specifically called absorption spectrophotometers, and the resulting spectrum is called the absorption spectrum. J.-Y. Yoon, Introduction to Biosensors: From Electric Circuits to Immunosensors, 121 DOI 10.1007/978-1-4419-6022-1_8, # Springer Science+Business Media New York 2013
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8 Spectrophotometry and Optical Biosensor
Fig. 8.1 Left: a UV/Vis spectrophotometer. Right: an FT-IR spectrophotometer. Picture taken by Tim Vickers in July 2008 (left) and S. Levchenkov in June 2008 (right) and placed in public domain. Accessed March 2012 from http://commons.wikimedia.org/wiki/File:DU640_spectrophotometer.jpg (left) and http://commons.wikimedia.org/wiki/File:IR_spectrometer.jpg (right)
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monochromator Vin
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PD – cuvette
Fig. 8.2 A simple spectrophotometer
Many different types of light can be used for spectrophotometry, including, but not limited to: X-ray, ultraviolet (UV), visible (Vis), infrared (IR), etc. The most common types of spectrophotometers are the ones that use both UV and Vis (called UV/Vis spectrophotometer) and IR (typically with Fourier transform feature, hence FT-IR spectrophotometer) (Fig. 8.1). Figure 8.2 is a schematic illustration fo
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