Trends in Biotechnology
ReviewsOptical sensor systems for bioprocess monitoring
Section snippets
Optical-density probes
For measuring cell growth and biomass concentration, the on-line or off-line optical density (OD) are often used as the reference method. Commercially available in situ optical sensors for the on-line determination of microbial growth measure light absorption (turbidity) or scattering (nephelometry) continuously in the visible and near-infrared (NIR) ranges.
These devices allow accurate on-line determination of cell density with no need for intermittent calibration procedures. Commercially
In situ microscopy
On-line characterization of cell populations in bioreactors can also be performed by in situ microscopy and, indeed, cell-concentration measurements are already performed in this way. A microscope can be mounted directly in a port of a bioreactor to generate in situ images from the agitated broth using pulsed illumination. This technique was successfully tested during yeast fermentations and gave results that correlated well with those obtained from a hemocytometer. This technique also allows
Optical biosensors
In an optical biosensor, a biological ‘receptor’ (e.g. an enzyme, microorganism or antibody) produces an optical signal (e.g. chemiluminescence or NADH fluorescence); this signal is converted by an electronic transducer into an electrical signal12. Bioluminescent sensors have also been developed, which consist of a bioluminescent enzyme and an optical transducer. Optical biosensors provide a rapid and highly selective detection system.
In most cases, the analysis must be performed outside the
Fiber-optic sensors
Fiber-optic sensors (known as optodes) are based on a change in the optical properties (such as absorption or luminescence) of particular indicator. Fiber-optic oxygen sensors are produced by the immobilization of suitable oxygen-sensitive dyes at the tip of an optical fiber.
The principle of these measurements is the decrease in fluorescence intensity of an organometallic dye, caused by the interactions with oxygen. The light of a blue light-emitting-diode (LED) is guided through an optical
Near-infrared sensors
NIR spectroscopy can be used to measure the concentration of certain organic species, even in complex media. Biologically important bonds (aliphatic CH, aromatic or alkene CH, amine NH and OH) absorb in the NIR range, at 2.0 to 2.5 μm. Each chemical structure is related to a specific position, shape and size of the analytebsorption bands. Because the absorption bands are very similar, advanced data-analysis algorithms are required to extract the analytical information in a reliable manner.
Fluorescence sensors
The most common fluorescence sensors are based on the fluorescence measurement of the reduced form of nicotinamide adenine dinucleotide (phosphate) [NAD(P)H], which was first used by Duysenz and Amesz for in vivo measurements27. NAD(P)H-dependent fluorescence at 450 nm is measured after excitation at 360 nm. During the growth phase of a cultivation process, the NAD(P)H-fluorescence signal has often been found to show a good correlation with biomass concentration. This correlation is only found
Future prospects
Optical sensor systems are currently of increasing interest because of their rapid response, high sensitivity and easy maintenance. Their potential for combining with modern data-evaluation systems such as chemometrics enables continuous monitoring during cultivation or downstream processing. Through the use of intelligent data-processing models, problems of interactions can be excluded and non-invasive, flexible and detailed monitoring for improved processing, documentation and control becomes
References (35)
- et al.
Trends Biotechnol.
(1994) - et al.
Trends Biotechnol.
(1997) - et al.
Trends Biotechnol.
(1994) - et al.
J. Biotechnol.
(1992) Appl. Biosens.
(1993)J. Biotechnol.
(1994)- et al.
Sens. Actuat. B
(1997) - et al.
Sens. Actuat. B
(1996) - et al.
Biochim. Biophys. Acta
(1957) - et al.
Trends Biotechnol.
(1995)