Fluorimetric sensors

Papkovsky D.B., Smiddy M.A., Papkovskaia N.Y., Kerry J.P., Nondestructive Measurement Of Oxygen In Modified Atmosphere Packaged Hams Using A Phase-Fluorimetric Sensor System, J. Food Set 2002 67 3164-3169. 

Aldakov D, Anzenbacher P Jr (2003) Dipyrrolyl quinoxalines with extended chromophores are efficient fluorimetric sensors for pyrophosphate. Chem Commun 12 1394—1395... 

Figure 4.5 — Generic manifold for implementation of methods using fluorimetric sensors for directly measuring the intrinsic fluorescence of the analyte (singlechannel system) or a reaction product (by including the zone bound by the dotted line for reagent supply).

Figure 5.22 — Reversible flow-through fluorimetric sensor for the determination of potassium in human blood plasma based on the mechanism shown in Fig. 5.21.3. (A) Flow-cell containing the lipophilic membrane. (B) Flow injection conflguration. P pump IV injection valve W waste. For details, see text. (Reproduced from [86] with permission of Elsevier Science Publishers).

Vetrichelvan M, Nagarajan R, Vahyaveettil S. Carbazole-containing conjugated copolymers as colorimetric/fluorimetric sensor for iodide anion. Mactomolecules 2006 39 8303-8310. 

Canizares P, Castro L de, Valcarcel M. 1994. Flow-through fluorimetric sensor for the determination of aluminum at the nanogram per milliliter level. Anal Lett 27 247-262. 

Dipyrrolylquinoxalines with extended ehromophores are efficient fluorimetric sensors for pyrophosphate. The Stille coupling of 5,8-dibromo-2,3-di(pyrrol-2-yl) quinoxaline with aryItributylstannane produced 5,8-diaryl substituted quinoxalines 205 as possible sensors [100]. 

Another fluorimetric sensor, based on energy transfer, was recently developed [44]. The fluorophore (eosin) and an absorber (Phenol Red) are co-immobilized. An argon laser (A, = 488 nm) excites the eosin at a wavelength at which Phenol Red is non-absorbent. The fluorescence emitted by the fluorophore (donor) is modulated in the presence of the absorber (acceptor) either by the inner fibers effect (decrease in fluorescence) or by the energy transfer. These two effects generate an overlap of the fluorescence and absorption emissions. The measurement at 546 nm is taken between pH 4.S and pH 8.6 (65 l o decrease in intensity). When optimized for the physiological range, an accuracy of 0.01 pH units is obtained. 

In phase-fluorimetric oxygen sensors, active elements are excited with periodically modulated light, and changes in fluorescence phase characteristics are measured. The delay or emission (phase shift, ( ), measured in degrees angle) relates to the lifetime of the dye (x) and oxygen concentration as follows ... 

A number of optical chemical sensor systems have been developed for food and packaging applications and proven their utility. Some sensors, such as phase-fluorimetric oxygen sensor, have already reached high degree of maturity and demonstrated their potential for use on a mass scale. While the others still require extensive research, development and search for new solutions, so as to match basic practical requirements for such sensors. The experience and lessons learned with current optical sensor systems must be... 

Many methods including photometric, fluorimetric, chromatographic, and electrochemical methods have been used to detect the antioxidants so far. Recently, electrochemical methods have intensively been used for antioxidant detection. Among the electrochemical methods, the detection of antioxidant based on the direct redox transformation of cyt c has been studied over the decade. Since cyt c can act as an oxidant of superoxide, the superoxide level in solution can be detected as an oxidation current at the sensor electrode due to electron transfer from the radical via cyt c to the electrode. 

The equipment required to develop this type of sensor is very simple and resembles closely that used to implement ordinary liquid-solid separations in FI manifolds. The only difference lies in the replacement of the packed reactor located in the transport-reaction zone with a packed (usually photometric or fluorimetric) flow-cell accommodated in the detector. Whether the packing material is inert or active, it should meet the following requirements (a) its particle diameter should be large enough (< 80-100 fim) to avoid overpressure (b) it should be made of a material compatible with the nature of the integrated detection system e.g. almost transparent for absorbance measurements) and, (c) the retention/elution process should be fast enough to avoid kinetic problems. 

The cyanide sensor developed by the authors group is based on the formation of an addition product between cyanide ion and pyridoxal-5-phosphate, and its subsequent retention in the sensor (a fluorimetric flow-cell packed with QAE-Sephadex resin). The eluent is not injected, but merged with a stream of 0.05 M HCl after the reactor that is used both to acidify the complex and elute it after measurement. The calibration graph for the target analyte was linear from 50 ng/mL to 3.0 pg/mL, and the relative standard deviation and sample throughput were 1.4% (for 2 pg CN7mL) and... 

The analytes typically determined by using this type of sensor are those usually addressed by gas-diffiision systems, viz. ammonia (or ammonium ion), carbon dioxide (or carbonates) and oxygen. The detection system used is most frequently photometric, fluorimetric or potentiometric, and can be integrated with or connected to the sensing microzone. The description below is based on the two choices shown in Fig. 5.4. 

Wolfbeis et al. [105-107] developed several fluorimetric flow-through sensors based on a sensing microzone consisting of a multilayer lipid membrane formed on a glass support by using the Langmuir-Blodgett (LB)... 

Figure 5.24 — Enantioselective fluorimetric flow-through optrode using a lipophilic tartrate (di-/er/-butyrotartrate) as ion-carrier for the determination of (protonated) 1 -phenylethylamine. (A) Sensor response to increasing concentrations of the analyte [(—) S enantiomer (- - -) R enantiomer]. (B) Stem-Volmer calibration plot. and F denote fluorescence intensities = 520, = 600...

Colorimetric and fluorimetric NH3 sensors contain mixtures of pH indicators having suitable dissociation constants at the tip of the fiber bundle. The measuring solution is separated from this indicator layer by an NH3 gas-permeable membrane covered by an immobilized de-aminating enzyme, e.g. urease (Wolfbeis, 1987 Arnold, 1987). The fluorimetric indication of NADH has been used in optical biosensors for lactate, pyruvate, and ethanol, where the respective dehydrogenase is immobilized at the tip of an optical NADH sensor (Arnold, 1987 Wangsa and Arnold, 1988). 

Laval etal. (1984) bound LDH covalently to electrochemically pretreated carbon. The enzyme was fixed by carbodiimide coupling simultaneously with anodic oxidation of the electrode surface. The total amount of immobilized LDH was determined fluorimetrically after removal from the electrode and hydrolysis. The authors found that at a maximal enzyme loading of 13 pmol/cm2 six enzyme layers are formed. The immobilization yield was about 15%. The kinetic constants, pmax and. Km, were not affected by the immobilization. The obtained enzyme loading factor of 10-3 indicates that diffusion in the enzyme layer was of minor influence on the response of the sensor. The layer behaved like a kinetically controlled enzyme membrane, i.e., the NADH oxidation current was proportional to the substrate concentration only far below Km- With increasing enzyme loading the sensitivity for NADH decreased due to masking of the electrode surface. 

Arnold et al. (1987) described an optoelectronic ethanol sensor based on fluorimetric detection of NADH formed in the reaction catalyzed by ADH. The enzyme was fixed to the inner surface of a membrane permeable to volatile substances, which separated the sample from the internal sensor solution. This solution contained NADH and semicarbazide, so that no reagent had to be added to the sample. The arrangement was named an internal optical enzyme sensor . 

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