Optical biosensing: future possibilities 'How the construction of oximeter sensors has evolved over the last decade is a reflection of the advancement in light sources and detectors from the optoelectronics and microelectronics research fields.'

It is difficult to pinpoint the precise time atwhich optical biosensors came of age, but itis clear that the ‘omics’ era has been instru-mental in the recent need, evolution and sig-nificant expansion of this area of technology.Research involving genomics, proteomicsand metabolomics is reliantupon the evaluation of largenumbers of different bioana-lytes and the relative quantifi-cation of these within a settime frame, preferably in realtime. Although some of thesensing and analysis approaches used for cur-rent ‘omics’ studies are not optical ones,optical biosensing and analysis methods haveand will continue to contribute significantlyto providing ways for bioscience and bio-medical science questions to be answered.One can categorically state that research inoptical engineering topics has not beendriven by the biosciences – the major engi-neering advances have been made for farmore commercially lucrative reasons, forinstance telecommunications applications asexemplified by optical fibers, laser sourcesand integrated optical waveguides. However,these small-sized structures, which have beendeveloped for massively parallel processing ofoptical signals, are beginning to providesome of the optical biosensors, devices andmethodologies for the future. Most notablyfor the detection and quantification of small-sized bioanalytical samples – all in highthroughput and in formats suitable for fastdata collection.It is unclear, to me at least, when the firstoptical biosensor device was invented. How-ever, perhaps one of the first important opticalbiosensors was the oximeter for the evaluationof blood oxygenation. Indeed, optical oximetryis perhaps the most commonly used opticalsensor technology in use. Sucha system is based upon the dif-ferences in the light absorptionspectra of hemoglobin andhemoglobin–oxygen complex –the difference between arterialand venous blood. Two sen-sors are used – one at approximately 600 nm(red) and the other in the near-infrared(805–1000 nm). The number of photons of805 nm light absorbed per molecule of hemo-globin or oxygenated hemoglobin is the same,and the sensor at this wavelength is used as thereference one (providing a method to compen-sate for the variation of the blood volume).Thus, relative to this reference, the absorptionof the red light (600 nm) provides a method todirectly measure the level of oxygenation of theblood. This methodology has been effectivelyused and optimized, and pulsed oximetry isnow one of the most reliable methods for non-invasive blood oxygenation sensing. The basisof the pulsed method is to measure the changein light transmitted through the skin thatoccurs as a result of the arterial pulsation. Thelight transmitted through the skin at theinflow of the cardiac cycle is arterial blood.Most pulse oximeters are of the transmissiontype, where forward-scattered light throughthe fingertip or earlobe is analyzed.