Dietmar Kieslinger

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 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.

Azomethines with nonlinear optical properties and polyesters covalently functionalized with them. Syntheses with nitriles IC

Abstract Polyesters with covalently bound azomethine and carbonanalogous dye moieties were obtained by polymer analogous reactions. Their nonlinear optical properties were examined by electric field induced second harmonic generation measurements and compared with corresponding monomeric dyes. All of them had maximum absorption wavelengths between 400 and 546 nm. Values for μβ (dipole mement μ, molecular hyperpolarizability β) reached 290×10 −68 Cm 5 /V. Quantum chemical calculations indicate the existence of various conformers of the dyes with large differences in their nonlinear optical properties. Furthermore, the calculations also point to a profound sensitivity of the molecular hyperpolarizability to environmental (solvent) effects amounting to an order of magnitude.

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.

Electric field induced second harmonic generation (EFISH) experiments in the swivel cell: New aspects of an established method

The EFISH technique is well known and widely used to determine the efficiency of the second harmonic generation (SHG) of liquid samples. In the first part of this paper, a simple and convenient experimental setup for EFISH measurements is presented. A standard photometric cell was used and the cell was swiveled during the experiments. The mathematical treatment of the data included corrections for the SHG of the optical cells and the solvent. The second part of the paper deals with the measurements of compounds which showed notable absorption at the SHG wavelength. In addition to the contributions of the solvent and the cell to the SHG signal the molar absorption coefficient of the compound and the refractive indices for the ground and the second harmonic wave of the solution were included in the processing of the EFISH data. Results of these measurements were in good agreement with those which others have obtained by independent methods. Thus the swivel method allowed estimation of the SHG (in terms of μβ where μ is the dipole moment of the molecule and β is the second-order polarizability) of non-absorbing, weakly absorbing and notably strongly absorbing compounds by easy-to-perform EFISH measurements.

Capillary Waveguide Optrodes for Medical Applications

Glass capillaries with a chemically sensitive coating on the inner surface are used as optical sensors for medical diagnostics. The capillary simultaneously serves as a sample compartment, a sensor element, and an inhomogeneous optical waveguide. Different optical setups have been investigated and compared regarding its waveguiding properties.

Nonlinear Optical Properties of Dicyanomethylene-Derived Heteroaromatic Dyes: Semiempirical Molecular Orbital Calculations and Experimental Investigations

The effect of conformation (E/Z isomerism), nature (donor/acceptor) of substituents, and endgroups (indandione, pyrazolone, pyrazoledione) on the molecular hyperpolarizabilityvec of dicyanomethylene (hetero)aromatic dyes is investigated by means of semiempirical (AM1, ZINDO) molecular orbital calculations. Unless Z isomers are stabilized by intramolecular hydrogen bonding, generally E conformers have largervecs. Replacement of one nitrile group of the dicyanomethylene moiety by p-aminoaryl rather than p-R-arylamino (RD NMe2 ,M eO, H, NO 2 )i s found to be advantageous. Increasing the acceptor strength of 29 by successively replacing the carbonyl with dicyanovinyl groups leads to a maximum ofvec for the derivative with one rather than two C(CN)2 groups. With respect to endgroups, the indandione moiety generally is the least active group. Solvent effects are treated within the framework of the self-consistent reaction field approximation. In most cases gas-phase tendencies are either parallel or even reinforced if solvent effects are taken into account. The calculated results are compared with electric field induced second harmonic generation (EFISH) measurements. c 2000 John Wiley & Sons, Inc. Int J Quantum Chem 79: 253-266, 2000

Nonlinear optic properties of p-tricyanovinyl, p-dicyanovinyl, and p-dicyano-methyleneamino-anilines and poly(methylmethacrylate)s covalently functionalized with them. Syntheses with nitriles C☆

Abstract Copolymers of methyl methacrylate and chromophores with methacrylate end groups were obtained by radical polymerization. Their nonlinear optical properties ware examined by electric field induced second-harmonic generation (EFISH) measurements and compared with corresponding monomeric dyes. All of them had maximum absorption wavelengths between 480 and 510 nm. Values for μβ (dipole moment μ, molecular hyperpolarizability β) were remarkably high and reached 530×10 −68 Cm 5 V −1 . According to quantum chemical calculations ( ab initio and semiempirical) increasing the acceptor strength in going from the dicyanovinyl to the dicyanomethyleneamino moiety also leads to an increase in the molecular hyperpolarizability. In contrast, futher increasing the acceptor strength to the tricyanovinyl group is accompanied by a levelling off or, even, a decrease in the calculated β-value.

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).

Lifetime-based portable instrument for blood gas analysis

A portable, compact device for measuring blood gases by using the fluorescence decay time as the information carrier is presented. The instrument is based on solid state technology only, thus using LEDs for excitation and a photodiode as detector. A capillary coated on its inner surface with different sensing membranes serves as a sample compartment and an optical sensor element simultaneously. Furthermore, due to inhomogeneous waveguiding in the capillary walls, only the fluorescent light is guided. Technical details of the electronic circuit, the optical design and the instrumental performance will be discussed.