Digital phosphorimeter with frequency domain signal processing: Application to real-time fiber-optic oxygen sensing

Abstract

An instrument to measure the excited-state lifetimes of phosphorescent materials in real time is described. This apparatus uses pulsed and frequency-doubled Nd:YAG solid-state laser for excitation, sampler for data acquisition, and frequency domain methods for data fitting. The instrument amplifies the ac components of the detector output and band limits the signal to 25 kHz. The fundamental frequency of the excitation is then set to obtain a desired number of harmonics. This band limited signal is sampled and averaged over few thousand cycles in the time domain. The frequency domain representation of the data is obtained by employing fast Fourier transform algorithms. The phase delay and the modulation ratio of each sampled harmonic is then computed. Ten to a hundred values of the phase and modulations are averaged before computing the sensor lifetime. The instrument is capable of measuring precise and accurate excited-state lifetimes from subpicowatt luminescent signals in 100 μm optical fibers. To monitor oxygen for biomedical applications the response time of the system is decreased by collecting only 8 or 16 harmonics. A least-squares fit yields the lifetimes of single exponentials. A component of zero lifetime is introduced to account for the backscatter excitation. The phosphorescence lifetimes measured reproducibly to three parts in a thousand are used to monitor oxygen. Oxygen concentrations are computed employing empirical polynomials. The system drift is less than 1% over 100 h of continuous operation. This instrument is used to measure oxygen concentrations in vitro and in vivo with 2 s update times and 90 s full response times. Examples of measurements in saline solutions and in dogs are presented.

Opto-Chemical and Opto-Immuno Sensors

“…Since the necessary equipment is expensive, the phase-modulation method is preferred [268], measuring the phase shift between a frequency-modulated excitation source and the emission. Research news: materials for NIR optical fiber [33] SERS chem. and biochem.…”

Nanoparticle-Based in Vivo Investigation on Blood−Brain Barrier Permeability Following Ischemia and Reperfusion

“…Oxygen-dependent quenching of phosphorescence can provide rapid and quantitative measurements of tissue oxygen levels (i.e., two-dimensional imaging of oxygen distribution throughout the tissue). , The recent development of this method now allows noninvasive assessment of tissue oxygenation in vivo using a multifrequency system to perform time-resolved phosphorescence analysis . The principle of phosphorescence quenching has been described in detail elsewhere. − To real-time monitor cerebral oxygenation after middle cerebral artery occlusion (MCAO) and reperfusion, and after accounting for the weak collected phosphorescent signal and the high scattering properties of tissue, the phosphorescence lifetime measurements in the frequency domain are found to be preferable to those in the conventional time domain. − …”

“…[21][22][23] To real-time monitor cerebral oxygenation after middle cerebral artery occlusion (MCAO) and reperfusion, and after accounting for the weak collected phosphorescent signal and the high scattering properties of tissue, the phosphorescence lifetime measurements in the frequency domain are found to be preferable to those in the conventional time domain. [24][25][26] In the present study, we developed an analytical method that combined the in vivo microdialysis sampling technique and the phosphorescence lifetime measurement to simultaneously monitor changes of extracellular nanosphere concentrations and oxygen levels in the brain. This method was applied to study correlations between oxygen concentration and blood-brain barrier permeability to extravasated nanospheres, following cerebral ischemia and reperfusion.…”

Construction of a Fourier-transform phase-modulation fluorometer

“…For example, frequency-domain fluorometers utilizing a laser beat noise or harmonics of a mode-locked laser have been reported and successfully applied to determine rotational correlation times of molecules [17][18][19]. Furthermore, a PMF using a UV light-emitting diode (LED) [7,20], compact frequency-domain fluorometers [21] and phosphometers incorporating a green or a blue LED [22][23][24] have been reported.…”