Datenpaket: Redox-Active Polyindole-Modified Electrode for Electrochemical Quantification of Uric Acid

Uric acid (UA) is an important clinical biomarker whose reliable determination in complex biological media is often hampered by co-existing electroactive interferents. Intrinsically conducting polymers are versatile materials for electrochemical sensing, yet polyindole (PIn) has remained largely unexplored for this purpose. Here, we report a PIn-modified glassy carbon electrode for the non-enzymatic determination of UA in neutral phosphate buffer. PIn was synthesized by chemical oxidative polymerization in the presence of sulfonate dopants and deposited as a thin film on glassy carbon; physicochemical characterization confirmed a doped, conjugated, and nanostructured coating with a high effective surface area. The electrode showed a well-defined UA oxidation signal and, by chronoamperometry, a linear response over 0.01-0.10 mM with a sensitivity of 4.53 µA cm-2 mM-1 and a limit of detection of 4.0 M. Under the applied conditions, ascorbic acid, dopamine, glucose, and sodium chloride contributed only marginally to the response, and the sensor performed reliably in diluted human serum, with recoveries of 99.7-99.9% and relative standard deviations below 5%. These findings identify polyindole as a promising, still underexplored intrinsically conducting polymer for electrochemical biosensing.

Uric acid (UA) is an important clinical biomarker whose reliable determination in complex biological media is often hampered by co-existing electroactive interferents. Intrinsically conducting polymers are versatile materials for electrochemical sensing, yet polyindole (PIn) has remained largely unexplored for this purpose. Here, we report a PIn-modified glassy carbon electrode for the non-enzymatic determination of UA in neutral phosphate buffer. PIn was synthesized by chemical oxidative polymerization in the presence of sulfonate dopants and deposited as a thin film on glassy carbon; physicochemical characterization confirmed a doped, conjugated, and nanostructured coating with a high effective surface area. The electrode showed a well-defined UA oxidation signal and, by chronoamperometry, a linear response over 0.01-0.10 mM with a sensitivity of 4.53 µA cm-2 mM-1 and a limit of detection of 4.0 M. Under the applied conditions, ascorbic acid, dopamine, glucose, and sodium chloride contributed only marginally to the response, and the sensor performed reliably in diluted human serum, with recoveries of 99.7-99.9% and relative standard deviations below 5%. These findings identify polyindole as a promising, still underexplored intrinsically conducting polymer for electrochemical biosensing.

Data for Figures in the Manuscript
Figure 1:
File names:
Figure_1a_FTIR_PIn
Figure_1b_xrd_PIn
Description:
FTIR spectrum (a) and X-ray diffraction pattern (b) of chemically synthesized, doped polyindole (PIn).
Conditions:
Sample: chemically synthesized polyindole doped with H₂SO₄ and DBSA, FTIR: ATR mode; spectral range 400–3500 cm⁻¹; resolution 2 cm⁻¹; temperature: room temperature; XRD: Bruker D8 ADVANCE with Cu Kα radiation (λ = 1.54 Å); tube settings 40 kV and 40 mA; 2θ range 10–70°; step size 0.01°; scan speed 2 steps s⁻¹; temperature: room temperature.
Figure 2:
File name: Figure_2_SEM_PIn
Description:
SEM images of polyindole at different magnifications showing granular, approximately spherical particles and rough surface morphology.
Conditions:
Sample: polyindole powder deposited on a conductive carbon substrate; instrument: SEM (Supra 55VP, ZEISS FEGSEM) with field-emission gun; imaging in high-vacuum mode; temperature: 20 °C
Figure 3:
File names
Figure_3a_CV_glassy_carbon
Figure_3b_CV_PIn
Description:
Cyclic voltammograms of (a) bare glassy carbon (GC) electrode and (b) PIn-modified GC electrode in 1 M H₂SO₄ at different scan rates, showing featureless response for bare GC and a well-defined redox couple for PIn/GC.
Conditions:
Electrochemical cell: three-electrode configuration; working electrode: bare GC (a) or PIn-modified GC (b); reference electrode: Ag/AgCl (KCl-saturated, aqueous); counter electrode: gold sheet; electrolyte: 1.0 M H₂SO₄ (aqueous); temperature: 25 °C; technique: cyclic voltammetry; scan rates: 10–150 mV s⁻¹; potential window -0.2 V to 0.8 V vs. Ag/AgCl
Figure 4:
File names:
Figure_4a_CV_PIn_buffer
Figure_4b_linearity_sqrt(nu)_vs_jp
Figure_4c_CV_UA_concentration_variation
Figure_4d_concentration_vs_peak_current_densities
Description:
(a) CVs of the PIn-modified GC electrode in 0.1 M PBS (pH 7.0) at various scan rates without uric acid; (b) plot of peak current density vs. square root of scan rate; (c) CVs at 50 mV s⁻¹ in PBS for different uric acid concentrations; (d) calibration plot of anodic peak current density vs. uric acid concentration.
Conditions:
Electrochemical cell: three-electrode configuration; working electrode: PIn-modified GC; reference electrode: Ag/AgCl (KCl-saturated); counter electrode: gold coil; electrolyte: 0.1 M phosphate buffer solution (PBS), pH 7.0; temperature: 25 °C; (a,b) uric acid-free PBS; scan rates: 10–110 mV s⁻¹; potential window: -0.2 to +0.8 V vs. Ag/AgCl; (c,d) uric acid in PBS at concentrations spanning 0.2–3.5 mM; scan rate: 50 mV s⁻¹; currents normalized to electrochemically active surface area (ECSA).
Figure 5:
File names:
Figure_5a_CA_UA_concenctration
Figure_5b_UA_concentration
Figure_5c_cross_sensitivity
Figure_5d_long_term_stability
Description:
(a) Chronoamperometric current–time curves for successive additions of uric acid at a fixed potential; (b) calibration plot of steady-state current density vs. uric acid concentration; (c) chronoamperometric responses to uric acid and common interferents; (d) long-term stability of the uric acid response over 9 days.
Conditions:
Electrochemical cell: three-electrode configuration; working electrode: PIn-modified GC; reference electrode: Ag/AgCl (KCl-saturated); counter electrode: gold coil; electrolyte: 0.1 M PBS, pH 7.0; temperature: 25 °C; technique: chronoamperometry; applied potential: 0.5 V vs. Ag/AgCl; (a,b) uric acid additions in PBS, concentration range 0.2–3.5 mM; currents evaluated at quasi-steady state; (c) analytes: 0.3 mM uric acid, 5 mM glucose, 1 mM NaCl, 0.1 mM ascorbic acid, 0.001 mM dopamine, each in 0.1 M PBS at 0.5 V; (d) repeated uric acid measurements once per day over 9 days at fixed uric acid concentration (within linear range) and 0.5 V vs. Ag/AgCl; currents expressed as (mu)A cm_ECSA⁻² and normalized to initial response.
Figure 6:
File name:
Figure_6a_DPV
Figure_6b_linearity_c_vs_jp
Figure_6c_mixed_interferences
Description:
(a) DPV of PIn with difference UA concentrations
(b) Plot of UA concentrations vs peak current densities jp
(c) bar plot of UA with different concentrations and mixtures of interferences
Conditions:
Electrochemical cell: three-electrode configuration; working electrode: PIn-modified GC; reference electrode: Ag/AgCl (KCl-saturated); counter electrode: gold coil; electrolyte: 0.1 M PBS, pH 7.0; temperature: 25 °C; technique: differential pulse voltammetry (DPV); applied potential: 0.1 V to 0.65 V vs. Ag/AgCl, step size 2 mV, amplitude 5 mV; (a,b) uric acid additions in PBS, concentration range 0.03–0.25 mM; (c) analytes: 1 mM uric acid, 1 mM glucose, 1 mM NaCl, 0.1 mM ascorbic acid, 0.001 mM dopamine, each in 0.1 M PBS at 0.5 V; currents expressed as (mu)A cm_ECSA⁻² and normalized to initial response.