Direct measurements of xanthine in 2000-fold diluted xanthinuric urine with a nanoporous carbon fiber sensor.

High selectivity and sensitivity is reported in the measurements of xanthine in urine by fast scan cyclic voltammetry (FSV) with a nanostructured carbon fiber sensor of 3.5 +/- 0.4 mum radius. Fabrication of the sensors for the measurements is described. Fabrication of the nanostructure at the carbon fiber sensor surface exposes surface pores. SEM images confirm the formation of the nanostructure. The results indicate that the nanostructure improves the sensitivity and limit of detection (LOD) in the measurements of xanthine and uric acid. The sensors allow rapid direct measurements of xanthine in 2000-fold diluted xanthinuric urine and of uric acid in 2000-fold diluted normal urine. The sensitivity and the LOD of xanthine is 0.40 +/- 0.02 nA microM(-1) (0.995) and 1 microM, respectively, and 0.99 +/- 0.01 nA microM(-1) (0.998) and 500 nM for uric acid. The concentration of xanthine in 2000-fold diluted xanthinuric urine is 1.6 +/- 0.2 muM from FSV and from HPLC. The concentration of xanthine and uric acid in urine can be determined by pre- or post-calibration of the sensor in buffer or by the method of standard addition.