Measurement of the Intensity of Long-Lifetime Luminophores in the Presence of Background Signals Using Phase-Modulation Fluorometry

Self-Referenced Multifrequency Phase-Resolved Luminescence Spectroscopy

Phase-resolved luminescence chemical sensors provide the analyte determination based on the estimation of the luminescence lifetime. The lifetime is estimated from an analysis of the amplitudes and/or phases of the excitation and emission signals at one or several modulation frequencies. This requires recording both the excitation signal (used to modulate the light source) and the emission signal (obtained from an optical transducer illuminated by the luminescent sensing phase). The excitation signal is conventionally used as reference, in order to obtain the modulation factor (the ratio between the emission and the excitation amplitudes) and/or the phase shift (the difference between the emission and the excitation phases) at each modulation frequency, which are used to estimate the luminescence lifetime. In this manuscript, we propose a new method providing the luminescence lifetimes (based either on amplitudes or phases) using only the emission signal (i.e., omitting the excitation signal in the procedure). We demonstrate that the luminescence lifetime can be derived from the emission signal when it contains at least two harmonics, because in this case the amplitude and phase of one of the harmonics can be used as reference. We present the theoretical formulation as well as an example of application to an oxygen measuring system. The proposed self-referenced lifetime estimation provides two practical advantages for luminescence chemical sensors. On one hand, it simplifies the instrument architecture, since only one analog-to-digital converter (for the emission signal) is necessary. On the other hand, the self-referenced estimation of the lifetime improves the robustness against degradation of the sensing phase or variations in the optical coupling, which reduces the recalibration requirements when the lifetimes are based on amplitudes.

Plasmon-controlled fluorescence towards high-sensitivity optical sensing

Fluorescence spectroscopy is widely used in chemical and biological research. Until recently most of the fluorescence experiments have been performed in the far-field regime. By far-field we imply at least several wavelengths from the fluorescent probe molecule. In recent years there has been growing interest in the interactions of fluorophores with metallic surfaces or particles. Near-field interactions are those occurring within a wavelength distance of an excited fluorophore. The spectral properties of fluorophores can dramatically be altered by near-field interactions with the electron clouds present in metals. These interactions modify the emission in ways not seen in classical fluorescence experiments. Fluorophores in the excited state can create plasmons that radiate into the far-field and fluorophores in the ground state can interact with and be excited by surface plasmons. These reciprocal interactions suggest that the novel optical absorption and scattering properties of metallic nanostructures can be used to control the decay rates, location, and direction of fluorophore emission. We refer to these phenomena as plasmon-controlled fluorescence (PCF). An overview of the recent work on metal-fluorophore interactions is presented. Recent research combining plasmonics and fluorescence suggest that PCF could lead to new classes of experimental procedures, novel probes, bioassays, and devices.

A novel method for monitoring monoclonal antibody production during cell culture

We describe a new format for surface-based fluoroimmunoassays that allows detection of biomolecule interactions without separation steps. The bioactive layer was immobilized on the surface of a glass substrate covered with silver islands that provide optical amplification of the distinctive fluorescence signal from bound probes when compared to unbound probes. The technique used was phase-modulation fluorometry that allows sensitive detection of bound probes with a very short lifetime in the presence of excess free probes in solution. The new method was applied to assay monoclonal antibody production during cell culture. Excellent agreement was found between the new method and ELISA analysis of hybridoma cell culture samples. It is predicted that the near real time monitoring of protein products during bioprocessing will be possible with the described technology.

Analysis of data obtained from a frequency-multiplexed phase-modulation fluorometer using an autoregressive model

In order to derive plural values of fluorescence lifetimes simultaneously from a multi-component sample, we formulate a mathematical method for analyzing data obtained from a frequency-multiplexed phase-modulation fluorometer (FM-PMF) using an autoregressive (AR) model. Various parameter settings necessary for performing accurate data analysis based on the AR model are studied through numerical simulations. Measurement results of fluorescence lifetimes of real samples, 10 ppm quinine sulfate in 0.1 N H(2)SO(4), 10 ppm rhodamine 6G in ethanol, and their mixture with a volume ratio of 1:1, demonstrate that the proposed method works quite well.

Self-referencing intensity measurements based on square-wave gated phase-modulation fluorimetry

An adaptation of square-wave gated phase-modulation (GPM) fluorimetry allows for self-referenced intensity measurements without the complexity of dual excitation or dual emission wavelengths. This AC technique utilizes square-wave excitation, gated detection, a reference emitter, and a sensor molecule. The theory and experimental data demonstrating the effectiveness and advantages of the adapted GPM scheme are presented. One component must have an extremely short lifetime relative to the other. Both components are affected identically by changes in intensity of the excitation source, but the sensor intensity also depends on the concentration of the analyte. The fluctuations of the excitation source and any optical transmission changes are eliminated by ratioing the sensor emission to the reference emission. As the concentration of the analyte changes, the corresponding sensor intensity changes can be quantified through several schemes including digitization of the signal and digital integration or AC methods. To measure pH, digital methods are used with Na3[Tb(dpa)3] (dpa = 2,6-pyridinedicarboxylic acid) as the long-lived reference molecule and fluorescein as the short-lived sensor molecule. Measurements from the adapted GPM scheme are directly compared to conventional ratiometric measurements. Good agreement between the data collection methods is demonstrated through the apparent pKa. For the adapted GPM measurements, conventional measurements, and a global fit the apparent pKa values agree within less than 2%. A key element of the adapted GPM method is its insensitivity to fluctuations in the source intensity. For a roughly 8-fold change in the excitation intensity, the signal ratio changes by less than 3%.