Upconverting phosphors as an amplifier of a colorimetric signal in dry chemistry test strips for enzymatic measurement of glucose

We describe the use of upconverting phosphors (UCPs) as an amplifier of the colorimetric signal in enzymatic dry chemistry test strips for the determination of glucose. The indicator compound used in the strips attenuated both the emission and excitation wavelengths of the upconverting phosphors, thus taking advantage of the nonlinear relationship between the excitation and emission of these particles. By monitoring the emission of the UCPs under 978 nm excitation, we were able to obtain significantly higher signal response (a steeper calibration curve) from the colorimetric assay compared to a reflectance measurement. The limit of detection of the developed method was 2.1-fold lower than the reflectance-based reference method. The new method was able to determine glucose from a sample of water in the range of 5.7 μM to 22 mM, which covers the typical blood glucose range in newborns and adults. In low analyte concentrations we obtained up to 3-fold improvement in the slope of the assay calibration curve. This difference decreased with increasing analyte concentration and the effect was diminished by the highest glucose concentration. However, the analytical sensitivity (the ratio of slope and standard deviation) was practically identical between the two methods because of the higher deviation in the UCP emission signal. (paper)