Otto S. Wolfbeis

Fiber-optic remote detection of pesticides and related inhibitors of the enzyme acetylcholine esterase

Abstract We present a novel method for the remote detection of inhibitors (e.g., pesticides) of the enzyme acetylcholine esterase (AChE) with a fiber-optic photometer based exclusively on the use of solid-state opto-electronic components including light-emitting diodes and photodiodes. The method employs a yellow synthetic enzyme substrate which is hydrolyzed by the enzyme to give a blue product. In the presence of an inhibitor, the rate of formation of this blue product is reduced. The resulting signal change is detected via fiber optics and may serve as an alarm.

Optical sensor for on-line determination of solvent mixtures based on a fluorescent solvent polarity probe

Abstract A new optical sensor for determination of solvent mixtures (such as water in organic solvent) is based on a fluorescent solvent polarity (SP) probe immobilized on an ion-exchange membrane. The fluorescent probe responds to changes in SP by both a shift in the fluorescence emission maximum and a change in fluorescence quantum yield. In this sensor, the analytical information is the relative fluorescence intensity measured at 620 nm (500 nm excitation). The sensor can be applied over a wide range of solvents. The response time (t90) is of the order of 15 s. Given the unique applications of fibre optic sensors (e.g., for remote sensing and sensing in explosive areas), the new sensor is expected to be applicable in process control.

Fiber optic biosensing based on molecular recognition

Abstract A review is given on the use of specific biorecognition elements (such as enzymes or optically active receptors) in optical sensing of chemical species. In enzyme-based sensors various options exist: depending on which species is immobilized, assays for substrates (such as glucose, lactate or cholesterol), enzymes (such as carboxylesterase) or inhibitors (such as organophosphates) can be designed. Respective examples are given. In the second part it is discussed how the intrinsic optical properties of certain enzymes or metabolites can be utilized for sensing purposes. In contrast to enzymes which digest their substrates, the use of optically active receptors which do not metabolize their target molecules offers a scheme for specific recognition of substrates by a receptor. If the receptor is enantio-selective (i.e., preferentially binds one species out of a pair of optical isomers), a fairly specific recognition of enantiometers of biogenic amines (such as the β-blocker propranolol) becomes possible. In contrast to enzyme-based sensing where steady-state response is a result of kinetic equilibration, this type of substrate binding results in thermodynamic equilibria. Specific examples are given for each sensor type, and their respective limitations are discussed.

Optical sensor instrumentation using absorption- and fluorescence-based capillary waveguide optrodes

An analytical instrument comprising absorption- and fluorescence-based capillary waveguide optrodes (CWOs) is described. Glass capillaries with a chemically sensitive coating on the inner surface are used for optical chemical sensing in gaseous and liquid samples. In case of absorption-based CWOs, light from a LED is coupled into and out of the capillary under a defined angle via a rigid waveguide and an immersion coupler. The coated glass capillary forms an inhomogeneous waveguide, in which the light is guided in both the glass and the coating. The portion of the light which is absorbed in the chemically sensitive coating is proportional to a chemcial concentration or activity. This principle is demonstrated with a pCO2-sensitive inner coating. Typical relative light intensity signal changes with this type of optical interrogation are 98%, with an active capillary length of 10 mm. For fluorescence- based CWOs, the excitation light from an LED is coupled diffusely into the glass capillary and the optical sensor layer. A major portion of the excited fluorescence light is then collected within the coated capillary, and guided to the photodiode, which is located on the distal end of the capillary waveguide. Hereby, the excitation light is separated very efficiently from the fluorescent light. As an example, a CWO for pO2 is described. By applying this optical geometry, it was possible to utilize fluorescence decay time of the sensor layer as the transducer signal even when using solid state components (LEDs and photodiodes).

A New Type Of Fiber Optic Biosensor Based On The Intrinsic Fluorescence Of Immobilized Flavoproteins

We describe a new biosensor for monitoring the concentration of enzyme glucose, lactate, and other substrates that are metabolized by an oxidation process. The method is based on the finding that enzymes having FAD as a prosthetic group change their fluorescence during interaction with a substrate. Typical enzymes that have been studied include glucose oxidase (GOD), lactate mono-oxygenase (LMO), and cholesterol oxidase (ChOD). Their fluorescence is monitored via fiber optic light guides at wavelengths above 500 nm, following fluorescence excitation at around 410 - 450 nm. The relative fluorescence intensities of the enzymes vary to a large extent, being highest for LMO, and rather low for ChOD. Typical detection limits are in the 0.5 mM range for lactate and 1.5 mM for glucose at ambient oxygen pressure. A characteristic feature of this sensor is the narrow dynamic range which usually does not exceed 3 mM. This can be explained in terms of enzyme kinetics and diffusional processes. Unlike optical biosensors based on measurement of the intrinsic fluorescence of NADH, this sensor type has the advantages of full reversibility (because reduced FAD-based enzymes accept oxygen as a second substrate) and analytical wavelengths that are compatible with plastic or glass fiber optics. It is fairly simple in construction because the enzyme acts as both the recognition and transduction element. The method also has been applied successfully in an flow injection analysis-like type of arrangement.