M.D. Fernández-Ramos

A new light emitting diode-light emitting diode portable carbon dioxide gas sensor based on an interchangeable membrane system for industrial applications.

A new system for CO(2) measurement (0-100%) based on a paired emitter-detector diode arrangement as a colorimetric detection system is described. Two different configurations were tested: configuration 1 (an opposite side configuration) where a secondary inner-filter effect accounts for CO(2) sensitivity. This configuration involves the absorption of the phosphorescence emitted from a CO(2)-insensitive luminophore by an acid-base indicator and configuration 2 wherein the membrane containing the luminophore is removed, simplifying the sensing membrane that now only contains the acid-base indicator. In addition, two different instrumental configurations have been studied, using a paired emitter-detector diode system, consisting of two LEDs wherein one is used as the light source (emitter) and the other is used in reverse bias mode as the light detector. The first configuration uses a green LED as emitter and a red LED as detector, whereas in the second case two identical red LEDs are used as emitter and detector. The system was characterised in terms of sensitivity, dynamic response, reproducibility, stability and temperature influence. We found that configuration 2 presented a better CO(2) response in terms of sensitivity.

Phosphorescent sensing of carbon dioxide based on secondary inner-filter quenching

A study of different strategies to prepare phosphorescence-based sensors for gaseous CO(2) determination has been performed. It includes the characterization of different configurations tested, a discussion of the results obtained and possibilities for the future. The optical sensor for gaseous CO(2) is based on changes in the phosphorescence intensity of the platinum octaethylporphyrin (PtOEP) complex trapped both on oxygen-insensitive poly(vinylidene chloride-co-vinyl chloride) (PVCD) membranes and PVCD microparticles, due to the displacement of the alpha-naphtholphthalein acid-base equilibrium with CO(2) concentration. A secondary inner-filter mechanism was tested for the sensor and a full range linearized calibration was obtained by plotting (I(100)-I(0))/(I-I(0)) versus the inverse of the CO(2) concentration, where I(0) and I(100) are the detected luminescence intensities from a membrane exposed to 100% nitrogen and 100% CO(2), respectively, and I at a defined CO(2) concentration. The different configurations tested included the use of membranes containing luminophore and pH-sensitive dye placed on two opposite sides of a transparent support to prevent the observed degradation of the PtOEP complex in the presence of the tetraoctylammonium hydroxide (TOAOH) phase transfer agent, which produced better results regarding stability and sensitivity. The CO(2) gas sensor based on PtOEP homogeneous membranes presented better properties in terms of response time and sensitivity than that based on PtOEP microparticles. With a detection limit of 0.02%, the response time (10-90% maximum signal) is 9 s and the recovery time (90-10%) is 115 s. The lifetime of the membranes for CO(2) sensing preserved in a 94% RH atmosphere and dark conditions is longer than at least 4 months.

Portable light-emitting diode-based photometer with one-shot optochemical sensors for measurement in the field

This report describes the electronics of a portable, low-cost, light-emitting diode (LED)-based photometer dedicated to one-shot optochemical sensors. Optical detection is made through a monolithic photodiode with an on-chip single-supply transimpedance amplifier that reduces some drawbacks such as leakage currents, interferences, and parasitic capacitances. The main instrument characteristics are its high light source stability and thermal correction. The former is obtained by means of the optical feedback from the LED polarization circuit, implementing a pseudo-two light beam scheme from a unique light source with a built-in beam splitter. The feedback loop has also been used to adjust the LED power in several ranges. Moreover, the low-thermal coefficient achieved (-90 ppm/degrees C) is compensated by thermal monitoring and calibration function compensation in the digital processing. The hand-held instrument directly gives the absorbance ratio used as the analytical parameter and the analyte concentration after programming the calibration function in the microcontroller. The application of this photometer for the determination of potassium and nitrate, using one-shot sensors with ionophore-based chemistries is also demonstrated, with a simple analytical methodology that shortens the analysis time, eliminating some calibrating solutions (HCl, NaOH, and buffer). Therefore, this compact instrument is suitable for real-time analyte determination and operation in the field.

LED–LED portable oxygen gas sensor

A portable instrument for oxygen determination, based on the quenching of phosphorescent octaethylporphyrin by gaseous O2, has been developed using the fluorimetric paired emitter–detector diode technique (FPEDD). The instrument configuration consists of two light-emitting diodes (LEDs) facing each other, with an interchangeable support containing a phosphorescent membrane in between, in which one of the LEDs is used as the light source (emitter LED) and the other, working in reverse bias mode, as the light detector. The feasibility of using a LED as a luminescence detector is studied. Its small size enables integration of the instrument into a portable measurement system. A systematic study of the system capabilities as a portable instrument was performed to optimize range, sensitivity, short term and long term stability, dynamic behaviour, effect of temperature and humidity, and temporal drift.

The use of one-shot sensors with a dedicated portable electronic radiometer for nitrate measurements in aqueous solutions

A simplified procedure for the in situ determination of nitrate in waters is presented based on ionophore–chromoionophore one-shot sensors measured in a simple form by a portable radiometer designed by us. The colour change in the sensing film is detected by measuring the transmitted intensity with a solid state radiometer. A light-emitting diode (LED), with a dominant wavelength of 660 nm, was used as the illumination source. Negative feedback for LED bias and thermal correction were included to improve system stability. The procedure is based on the use of one-shot sensors pretreated with NaOH and the measurement of an absorbance ratio as analytical parameter. The one-shot sensors are used directly without any prior conditioning and the absorbance is measured with the portable radiometer before and after equilibration with the sample. The results obtained show that the procedure has good sensitivity with a range between 0.002 and 1000 mM using a sigmoidal calibration function, and a precision around 4% expressed as the logarithm of the nitrate concentration. The performance of the optical one-shot sensor was tested for the analysis of nitrate in different types of natural water (tap, river, well and sea), validating the results against a reference procedure.

An IUPAC-based approach to estimate the detection limit in co-extraction-based optical sensors for anions with sigmoidal response calibration curves

An approach based on IUPAC methodology to estimate the limit of detection of bulk optode-based analytical methods for anions has been developed. The traditional IUPAC methodology for calculating the detection limit was modified to be adapted to particular cases where the calibration curves have a sigmoidal profile. Starting from the different full theoretical models for every co-extraction mechanism of the analyte in the membrane in bulk optodes, several particular simplified models at low analyte concentration were obtained and validated. The slope of the calibration curve at low analyte concentration was calculated from the first derivative of the simplified equation and, subsequently, the detection limit was estimated. This fitted-for-purpose estimation strategy was applied to anion quantification for in-house bulk optode-based analytical methods, and the estimated limits of detection were compared with those obtained by applying classical geometrical methodology. This way of establishing the detection limit yields values that maintain their true statistical and probabilistic aspects. It can be easily applied to any analytical system which yields non-linear calibration curves at low analyte concentration.

Optical sensor for carbon dioxide gas determination, characterization and improvements.

A study of different alternatives to improve the stability and lifetime of sensors for the determination of gaseous CO2 has been performed. It includes the characterization of different sensing membranes, a discussion of the results obtained and possibilities for the future. The solid sensor membrane for gaseous CO2 based on changes in the luminiscence of a luminophore immobilized on O2-insensitive film, concurrent with the displacement of a pH indicator, has some drawbacks, such as the loss of efficiency over time and the need to maintain the sensor in special atmospheric conditions. As a solution to these drawbacks, two alternatives were tested, the first alternative was replacing the newly proposed tetraoctyl ammonium hydroxide (TOAOH ) phase transfer agent with other basic agents that did not undergo a Hoffman degradation reaction, and the second alternative was the use of hydrophilic polymers that could retain water needed for CO2 sensing more efficiently. The different membranes tested indicated that the use of tetramethyl ammonium (TMAOH) instead of TOAOH as the phase transfer agent produced better results regarding stability and sensitivity. In addition, replacing the membrane polymer with hydrophilic polymers improved the sensing characteristics in terms of response time and stability over hydrophobic polymers. With a detection limit of 0.006%, the response time is 19s and the recovery time is 100s. The lifetime of the sensing membranes, which do not need to be held in any special atmosphere other than darkness, is longer than at least 300 days for membranes with TMAOH in hydrophilic polymer and 515 days for membranes with TMAOH in ethyl cellulose.

Calcium selective test strip for water and milk.

We have developed a selective and reversible test strip based on an ion-exchange mechanism to determine calcium. The optical test strip contains a polymeric film of plasticised PVC that contains all of the reagents necessary to produce a response to calcium, among them the new ionophore, 4,13-bis[(N-adamantylcarbamoyl)propionyl]-1,7,10,16-tetraoxa-4,13-diazacyclooctadecane. The measurement of the absorbance at 655 nm in a standard photometer makes it possible to determine calcium activities. The composition of the membrane and reaction conditions have been adjusted to obtain adequate selectivity. The test strip responded linearly to calcium between 0.050 and 135 mM in activities. The reproducibility intermembrane at a medium level of the range was 6.2%, as RSD, of log a(Mg(2+)), and 3.4% as RSD intramembrane. The procedure was applied to the determination of calcium ion in different types of waters (tap, well, spring and mineral) and milks (whole, skimmed, skimmed with calcium added, special types) validating the results against a reference procedure.

Membrane preconcentration of nalidixic and piromidic acids and simultaneous determination using a synchronous solid-phase phosphorescence approach

Abstract A preconcentration membrane for the retention and phosphorimetric determination of pharmaceuticals nalidixic and piromidic acids is proposed. The membrane has a circular zone of plasticized PVC adhered to the surface of a polyester strip, which constitutes the preconcentration area where the analytes are retained by absorption from a solution that contains it. Measurement of intrinsic phosphorescence in the synchronous phosphorescence spectra performed at 281/371 nm for nalidixic acid and 331/471 nm for piromidic acid make the resolution of the mixture possible. The applicable concentration range, detection limit, and precision (as relative standard deviation) are from 0.3 to 3.0 mg · l−1, 0.08 mg · l−1, and 5.1% for nalidixic acid and from 0.03 to 0.15 mg · l−1, 0.002 mg · l−1, and 5.2% for piromidic acid. The method was applied to the determination of residues of both pharmaceuticals in cow milk and human urine.