This paper reports the design, fabrication, characterization, and performance evaluation of a glassy carbon electrode modified with ZnO-doped porous silica nanoparticles (ZnO@SiO2) for electrochemical detection of Cd2+ ions. The authors synthesized ZnO@SiO2 by a sol-gel route in which a porous SiO2 shell is formed around ZnO nanoparticles using tetraethyl orthosilicate (TEOS) at 40 degrees C. The sensor was validated against four environmental water samples by standard addition. No food matrix samples were analyzed. The analyte is total dissolved Cd2+; no speciation was performed.
Study design
Laboratory materials and analytical chemistry study. The ZnO@SiO2 nanocomposite was fabricated and deposited (3 to 20 µL of a 1 mg/mL ethanol suspension) on a glassy carbon electrode (GCE, diameter 3 mm). Electrochemical performance was characterized under optimized conditions: 7 µL casting volume, 2 h drying time, pH 5 (0.1 mol/L acetate buffer), and 5 min accumulation time at -1 V versus Ag/AgCl (3 mol/L KCl). Detection used differential pulse voltammetry (DPV). Accumulation reduced Cd2+ to Cd0 on the electrode surface; re-oxidation during the DPV potential sweep (-1 V to -0.4 V) generated the analytical signal. The instrument was an Autolab PG Potentiostat/Galvanostat (AUT 83965) with Autolab Nova 2.1.6 software (Metrohm, Switzerland). A three-electrode cell was used: ZnO@SiO2/GCE working electrode, platinum counter electrode, and Ag/AgCl (3 mol/L KCl) reference electrode.
Selectivity was tested against five divalent ions (Mn2+, Ni2+, Pb2+, Fe2+, Co2+) at 10-5 mol/L alongside Cd2+ concentrations of 10-9 mol/L and 7.5 x 10-11 mol/L. Repeatability was assessed across six independently fabricated electrodes. Recovery cycling used 1 mol/L EDTA solution cleaning. Real-sample validation used four water sources from Tunisia spiked at 1.75 x 10-9 mol/L and 1.5 x 10-9 mol/L Cd2+.
Key numbers
Sensor performance
| Parameter | Value | Conditions |
|---|---|---|
| Detection limit (LOD) | 4.4 x 10-11 mol/L | S/N = 3; 0.1 mol/L ABS pH 5; p. 8 |
| Linear range 1 (low, diffusion-controlled) | 2.5 x 10-11 to 1.75 x 10-10 mol/L | R2 = 0.993; p. 8 |
| Linear range 2 (high, surface-controlled) | 2.0 x 10-10 to 1.75 x 10-9 mol/L | R2 = 0.986; p. 8 |
| Sensitivity (low range slope) | 28.72 +/- 1.196 µA/nmol/L | Figure 8b |
| Sensitivity (high range slope) | 17.05 +/- 0.99 µA/nmol/L | Figure 8b |
| Oxidation peak potential | -0.78 V versus Ag/AgCl | p. 8 |
| Peak current (1.75 x 10-9 mol/L Cd2+) | 4.75 µA | Figure 7; p. 8 |
| Repeatability RSD | 6.28% | n = 6 independently fabricated electrodes; p. 10 |
| Recovery cycles before significant loss | 5 cycles (95.2% retained) | 1 mol/L EDTA cleaning; Figure 10; p. 11 |
Real-sample spike recovery (Table 2, p. 11)
| Sample | Spiked (mol/L) | Measured (mol/L) | Recovery (%) |
|---|---|---|---|
| Tap water (laboratory) | 1.75 x 10-9 | 1.57 x 10-9 | 89.71 |
| Tinted water (local store) | 1.75 x 10-9 | 1.6119 x 10-9 | 92 |
| Seawater (Skanes) | 1.75 x 10-9 | 1.7411 x 10-9 | 99.49 |
| Seawater (Ksibet Elmediouni) | 1.5 x 10-9 | 1.788 x 10-9 | 102.2 |
Recovery range: 89 to 102%.
Comparison with related sensors (Table 1, p. 9)
The authors compared with eleven published Cd2+ sensors. Selected entries:
| Electrode | Method | Linear range (mol/L) | LOD (mol/L) |
|---|---|---|---|
| Chitosan-carbon nanotubes/GCE | SWASV | 1.3 x 10-5 to 3.9 x 10-5 | 7.1 x 10-6 |
| LAL-AuNPs/GCE | SWASV | 3.0 x 10-7 to 1.4 x 10-6 | 3.0 x 10-7 |
| Porous carbon-PdNPs/GCE | DPV | 5.0 x 10-7 to 5.5 x 10-6 | 4.1 x 10-8 |
| MSK-NPs@GRCPE | DPASV | 5 x 10-11 to 2 x 10-6 | 5.44 x 10-9 |
| SiO2/GCE (undoped) | DPV | 5 x 10-7 to 2.5 x 10-6 | 5 x 10-7 |
| ZnO@SiO2/GCE (this work) | DPV | 2.5 x 10-11 to 1.75 x 10-10 and 2 x 10-10 to 1.75 x 10-9 | 4.4 x 10-11 |
SWASV: square wave anodic stripping voltammetry; DPASV: differential pulse anodic stripping voltammetry; DPV: differential pulse voltammetry.
Methods
Synthesis: 0.5 g ZnO nanoparticles (Sigma-Aldrich, particle size under 100 nm) dispersed in 50 mL ethanol and 10 mL water by ultrasonic bath (30 min). Then 1.6 mL ammonia solution (1 mol/L) and 8 mL TEOS were added with vigorous stirring at 40 degrees C for 6 h. Product washed three times with ethanol and distilled water, dried at 60 degrees C for 12 h.
Electrode preparation: GCE polished with 0.05 µm alumina, rinsed, and ultrasonically cleaned in ethanol (5 min). Then 1 mg ZnO@SiO2 dispersed in 1 mL ethanol by sonication (30 min); 3 to 20 µL applied and dried at room temperature. Optimal was 7 µL with 2 h drying time.
Characterization: XRD (Philips PanalyticalX’Pert, Cu Ka radiation, 2-theta 5 to 80 degrees); SEM (JEOL JEM-2100 FX); TEM (JEOL 2100 at 200 kV); FTIR (PerkinElmer Spectrum IR version 10.6.2); UV-Vis (Beckman DU640). XRD confirmed hexagonal wurtzite ZnO phase (space group P63mc, JCPDS 01-079-2205). SiO2 showed amorphous broad band at approximately 2-theta = 23 degrees. FTIR identified SiO2 bands at 451, 794, and 1065 cm-1; ZnO absorption at 359 cm-1; additional Si-O-Zn bond peak at 556 cm-1 in ZnO@SiO2.
Electrochemical measurements: DPV in 0.1 mol/L acetate buffer (pH 5), accumulation at -1 V versus Ag/AgCl for 5 min, potential sweep -1 to -0.4 V. Supporting electrolyte ABS 0.1 mol/L (1:1 acetic acid and sodium acetate). pH adjusted with NaOH or HCl.
Instrument and CRM recovery
Instrument: Autolab PG Potentiostat/Galvanostat AUT 83965 with Nova 2.1.6 software (Metrohm, Switzerland).
CRM recovery: not reported. The authors used spike-recovery in real environmental water samples as the validation approach; no certified reference materials were used.
Evidence fitness
This is an electrochemical materials and analytical chemistry paper. The sensor measures total dissolved Cd2+ in aqueous solution; no speciation (inorganic versus organic) is reported. The paper does not report unspiked food or ingredient cadmium (Cd) concentrations. The recovery data (89 to 102%) are for environmental water samples only. This source does not support dietary exposure estimates, food occurrence tabulations, or regulatory limit comparisons for cadmium in food. Its contribution to the wiki is contextual: it documents a sensitive electrochemical platform capable of trace Cd2+ measurement at concentrations well below regulatory thresholds and demonstrates that ZnO doping of SiO2 significantly improves electron transfer and sensitivity compared to SiO2 alone (LOD improvement from 5 x 10-7 mol/L for undoped SiO2/GCE to 4.4 x 10-11 mol/L for ZnO@SiO2/GCE, approximately four orders of magnitude).
Limitations
No food matrix samples were analyzed; all real-sample data are environmental water. No certified reference material was used for accuracy verification. The dual linear range reflects a change in the electrochemical oxidation mechanism at approximately 2 x 10-10 mol/L, attributable to transition from diffusion-controlled to surface-controlled Cd2+ oxidation; this non-linearity requires careful calibration in routine use. Sample dilution (1:99 ratio with ABS) was required to reduce chloride interference from seawater, which may limit direct applicability to high-salinity matrices without dilution. Recovery of 89.71% for tap water is at the low end for a validated analytical method. Only one replicate measurement per sample type is reported for the spike-recovery experiment (n = 1 per water source); no uncertainty is stated for the real-sample measurements.
Regulatory context
The US Environmental Protection Agency maximum contaminant level for Cd2+ in drinking water is 3.0 µg/L (approximately 2.7 x 10-8 mol/L). The sensor LOD of 4.4 x 10-11 mol/L (approximately 4.9 x 10-3 µg/L) is roughly three orders of magnitude below this limit, indicating substantial sensitivity reserve for environmental monitoring applications. No food regulatory limits are addressed in the paper.
Related evidence
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