Application and modeling of frequency-domain lifetime spectroscopy for microsphere-based optical glucose sensors

A new glucose affinity sensor based on a homogeneous fluorescence resonance energy transfer (FRET) assay system was developed to monitor the competitive binding between concanavalin A (ConA) and dextran. The FRET quenching kinetics of the donor were analyzed from frequency-domain (FD) measurements as functions of both glucose and acceptor-protein concentrations using a F?rster-type decay kinetics model. The results showed that the FD measurements and donor decay kinetics can indicate quantitative changes in the presence of glucose at concentrations ranging from 0 to 224 mg/dL. The second set of experiments proved the feasibility of performing analyte sensing with FD lifetime spectroscopy using microsphere-based sensors in multiple scattering solutions. A well characterized pH-sensitive fluorophore was entrapped in poly(ethylene glycol) microspheres. The particles were then immersed in a buffered polystyrene solution of various pH. Measurements of phase shift and modulation of the generated and multiply scattered fluorescent light were acquired as the modulation frequency of the incident excitation light varied from 10 to 120 MHz. After the measured data were analyzed with the coupled diffusion equations, the obtained lifetimes from the scattering measurements matched values from non-scattering measurements. Lastly, a new two-speed Monte Carlo (MC) simulation was developed to predict light propagation through the sensors and thus was used to evaluate these sensors and to design these sensors for implantation. The model used random packing structure and considered geometric optics and two light propagation speeds. Experimental measurements of phase-shift and modulation of excitation light were made on a cubic phantom with non-fluorescent resin microspheres of 74 ?m diameter, and compared to those computed from the MC simulation. The results showed that the mean phase shift (PS) deviation was 0.736? and the mean amplitude deviation was 42%. Quantitative changes in detected fluorescence phase-shift and modulation were investigated for microsphere diameter, volume fraction, refractive index, and fluorophore lifetime. We also found that even though the sensitivity of PS change in the presence of scattering was the same as the value without scattering, the values of PS were magnified due to the scattering effects.