Creatinine is an important biomarker of kidney function
and muscular
metabolism. In this paper, we developed the β-lead dioxide/single-walled
carbon nanotube (β-PbO2/CNT) and the β-PbO2/CNT ion-selective electrode (β-PbO2/CNT/ISE),
which were used as highly sensitive potentiometric sensors for creatinine
detection. The fabricated electrodes exhibited highly pH-sensitive
characteristics due to the synergistic effect of the electrochemical
properties of CNT and β-PbO2. Moreover, an ammonium-ion-selective
membrane coating allowed the β-PbO2/CNT electrode
to be NH4+-selective for direct detection of the ammonium
ion. By exploiting the electrochemical characteristics of these electrodes,
the creatinine assay was established through the one-step selective
conversion of creatinine by creatinine deiminase, in which the OH– and NH4+ generated by the enzymatic reaction
were detected using β-PbO2/CNT and β-PbO2/CNT/ISE electrodes as pH- and NH4+-responsive
sensors, respectively. The total creatinine assay can be completed
within ∼5 min. The assay results from β-PbO2/CNT and β-PbO2/CNT/ISE showed excellent sensitivity
values of −75.56 and 64.62 mV in the detection range of 10–400
μM with a fast response (20 s), and the limits of detection
were calculated to be 0.06 and 0.13 μM, respectively. Moreover,
the developed creatinine sensor showed high selectivity against 11
interfering bio/chemical species with negligible interferences (selectivity
coefficient <10–4) and excellent repeatability
(>97% within 25 cycles) and long-term stability for 4 weeks of
storage.
In addition, the feasibility and practicality of the device were successfully
demonstrated in human serum tests, with recoveries of 95–104%
for PbO2/CNT and 92–110% for PbO2/CNT/ISE.
