Max E. Lippitsch

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.

Novel aspects of fluorescence lifetime for molecules positioned close to metal surfaces

On metal surfaces with submicroscopic corrugations, surface-enhanced optical processes can be observed. Results obtained by picosecond time-resolved fluorescence spectroscopy for dye molecules in the proximity (0–50 nm) of silver islands films are reported. It is demonstrated how the rather complex dependence of the integral fluorescence intensity on the distance dye-islands, can be resolved in the contributions of different mechanisms by analysing the fluorescence decay curves at various distances. It turns out, that the enhancement of absorption influences only the peak fluorescence intensity without changing the decay time, while the enhancement of emission and dissipative losses reduces the decay time. Thus time-resolved spectroscopy opens the possibility to test theoretical concepts on surface enhancement and provides basic data for tailoring molecule-metal structures with well-defined surface-enhancement properties.

Response characteristics of luminescent oxygen sensors

Abstract Nonlinea Stern-Volmer quenching characteristics are a common feature of oxygen sensors containing Ru(II) complexes immobilized in solid-state matrices. We discuss the origins of this behaviour and address the implications of the molecular processes on sensor performance with the help of two different sensor systems: (1) For tris(2,2′-bipyridyl)Ru(II) dichloride (Rubpy) adsorbed on silica-gel the influence of both the absorption isotherm and heterogeneity of the indicators environment significantly contributes to the intensity and lifeime Stern-Volmer behaviour. A modified method to determine an average liftime from parameters received by a multi-exponential decay analysis is employed to show that static quenching is absent in this system. (2) For tris(4,7′-diphenyl-1,10′-phenanthroline)Ru(II) diperchlorate (RuPh 2 phen) dissolved in polystyrene a two-component approach is shown to be adequate for the description of the intensity and mean lifetime Stern-Volmer behaviour, as well as the excited state decay. The origin of this behaviour is discussed in consideration of lifetime distributions present in disordered media. The pre-exponentially weighted mean lifetime is used to show that static quenching is also absent in this system. The importance of the thermal history of the sample is noted.

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.

On raman scattering in molecular complexes involving charge transfer

Abstract A model is presented attributing the enhancement of Raman intensities in molecular complexes involving charge transfer to different displaceabilities of the electronic charge transferred between the donor and the acceptor. During certain vibrations the extent of charge transfer is changed due to alteration of the donor-acceptor distance, leading to an additional change in polarizability during the vibrational motion. An estimation of the order of magnitude of this effect is in good agreement with experimental values. Particularly, this model allows explanation of considerable Raman intensity enhancement found in molecules adsorbed on a metallic surface.

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.

Surface enhanced raman spectra of biliverdine and pyrromethenone adsorbed to silver colloids

Abstract Adsorption of certain bile pigments to silver colloids yields an enormous enhancement in Raman intensity, while fluorescence remains more or less unaffected. It is argued that this may be caused by (weak) chemisorption.

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.