Sonja Draxler

Luminescence decay-time-based optical sensors: principles and problems

Sensors based on luminescence intensity measurements suffer from the fact that, during the operating time of the instrument, changes in source intensity, light throughput, detector sensitivity, indicator quantum yield and indicator concentration are inevitable and have to be overcome by extensive referencing and recalibration procedures. Decay-time sensors should not suffer from these drawbacks. In this contribution the advantages and problems of decay-time-based sensors are reviewed and the underlying molecular mechanisms are discussed.

Fluorescence properties of dyes adsorbed to silver islands, investigated by picosecond techniques

The fluorescence properties of dye molecules (rhodamine 6G and erythrosin) adsorbed on pure glass surfaces and on silver islands films are investigated by cw and picosecond time-resolved methods. On pure glass surfaces we observe concentration quenching below a critical intermolecular distance (reduction of the fluorescence power per molecule as well as shortened and non-exponential fluorescence decay). On silver islands films the shortening in fluorescence lifetime is more drastic and is nearly independent of the intermolecular distance. This behavior suggests an electrodynamic interaction between dye monomers and plasmons in the metal particles, modified by a damping influence of dye dimers.

Luminescence lifetime-based sensing: new materials, new devices

Abstract Advantages of luminescence-lifetime over intensity measurements in sensing applications include independence of variations in source intensity, detector sensitivity, light throughput and, most importantly, indicator concentration. Nevertheless, most researchers still believe that lifetime measurement needs highly sophisticated instrumentation and hence is unsuitable for practical applications. In this contribution it will be shown that this is no longer true. With the advent of more powerful blue light-emitting diodes, virtually the whole visible part of the spectrum can be covered by low-cost light sources. Typical singlet excited-state lifetimes are in the range of some nanoseconds. Recently, however, luminophores with lifetimes from hundreds of nanoseconds up to hundreds of microseconds have been introduced to optical sensing. Families of sensor dyes, all members being based on the same ‘long’ luminophore but covering a number of different analytes, have been developed. Lifetime sensing is hence no longer restricted to ultrashort times. Standard electronics as used in consumer circuits can be applied in low-cost lifetime instrumentation. Thus a whole range of analytes, from oxygen, pH and CO2 over cations and anions to glucose can be measured by cheap and reliable lifetime-based sensor devices.

Lifetime-based sensing: influence of the microenvironment.

The influence of the microenvironment on the fluorescence behavior of indicator molecules is investigated. A model is developed to describe the fluorescence decay of indicator molecules in a nonuniform medium. Its consequences for fluorescence lifetime-based chemical sensors are discussed and verified in two examples, namely, a pH sensor using a pyrene compound in a hydrogel and a ruthenium complex for oxygen sensing embedded in a polystyrene membrane.

pH sensors using fluorescence decay time

Optical pH sensors based on different fluorescence decay times of acid and base forms of suitable indicators have been developed. The indicator is incorporated in a hydrogel matrix providing the aqueous environment necessary for the acid-base reactions. Dramatic improvement of long-term stability and reproducibility compared to conventional intensity-based sensors is obtained.

Lifetime-based capillary waveguide sensor instrumentation

A small, portable, inexpensive instrument for measuring fluorescence lifetimes in optical sensors has been developed, which employs a super-bright blue or red light-emitting diode (LED) as excitation source and a photodiode with a fast high-gain amplifier for the detection of the fluorescence. A time resolution of down to 20 ns can be achieved with a total span of more than 5 μs. Evaluation of the raw data is accomplished by a laptop PC. Performance is demonstrated for an oxygen sensor.

Synthesis and characterization of fluorophore-absorber pairs for sensing of ammonia based on fluorescence

A new and simple preparation procedure for fluorophore absorber pairs which enable optical sensing of ammonia is reported. In ion pairs formed between organoruthenium complexes (fluorophore) and triphenylmethane dyes (absorber), a deprotonation of the absorber leads to an absorbance band which overlaps the emission of the fluorophore whereby both the fluorescence intensity and the fluorescence lifetime of the fluorophore are altered. Dissolving these ion pairs in polymer materials such as poly (vinyl chloride) or porous glass obtained by the sol‐gel process results in membranes which respond to ammonia. Plasticized PVC membranes containing the fluorophore-absorber pair and coated with a PTFE layer allow a continuous assay of dissolved ammonia in the range of 0.01 to 25 mg l ˇ1 . Membranes composed of the ion pair dissolved in a sol‐gel-based glass and coated with PTFE respond to ammonia with a similar sensitive range and a limit of detection of 0.01 mg l ˇ1 . # 1998 Elsevier Science B.V.

Energy transfer of dyes in Langmuir-Blodgett monolayers studied by picosecond time-resolved fluorimetry

Abstract Energy transfer via dipole-dipole interaction between chromophores in Langmuir-Blodgett monolayer assemblies is investigated by standard continuous wave (c.w.) and picosecond time-resolved fluorescence measurements. The dependence of the fluorescence decay function on the chromophore density in the monolayer is studied, together with the effects of adding non-fluorescing acceptors to the same layer and to a separate layer. Comparison of the resulting non-exponential decays with calculated model functions shows that the different cases can be distinguished by their degree of fit to a decay function valid for Forster-type energy transfer. For c.w. laser irradiation over a long period, different bleaching behaviour for chromophore densities corresponding to weak and strong energy transfer suggests that dye monomers and dye aggregates in the monolayer differ in their photochemical damage probability.

Capillary waveguide optrodes: an approach to optical sensing in medical diagnostics

Glass capillaries with a chemically sensitive coating on the inner surface are used as optical sensors for medical diagnostics. A capillary simultaneously serves as a sample compartment, a sensor element, and an inhomogeneous optical waveguide. Various detection schemes based on absorption, fluorescence intensity, or fluorescence lifetime are described. In absorption-based capillary waveguide optrodes the absorption in the sensor layer is analyte dependent; hence light transmission along the inhomogeneous waveguiding structure formed by the capillary wall and the sensing layer is a function of the analyte concentration. Similarly, in fluorescence-based capillary optrodes the fluorescence intensity or the fluorescence lifetime of an indicator dye fixed in the sensing layer is analyte dependent; thus the specific property of fluorescent light excited in the sensing layer and thereafter guided along the inhomogeneous waveguiding structure is a function of the analyte concentration. Both schemes are experimentally demonstrated, one with carbon dioxide as the analyte and the other one with oxygen. The device combines optical sensors with the standard glass capillaries usually applied to gather blood drops from fingertips, to yield a versatile diagnostic instrument, integrating the sample compartment, the optical sensor, and the light-collecting optics into a single piece. This ensures enhanced sensor performance as well as improved handling compared with other sensors.

Time-resolved fluorescence spectroscopy for chemical sensors.

A family of sensors is presented with fluorescence decay-time measurements used as the sensing technique. The concept is to take a single fluorophore with a suitably long fluorescence decay time as the basic building block for numerous different sensors. Analyte recognition can be performed by different functional groups that are necessary for selective interaction with the analyte. To achieve this, the principle of excited-state electron transfer is applied with pyrene as the fluorophore. Therefore the same instrumentation based on a small, ambient air-nitrogen laser and solid-state electronics can be used to measure different analytes, for example, oxygen, pH, carbon dioxide, potassium, ammonium, lead, cadmium, zinc, and phosphate.