Overview
This Thai university study (Khon Kaen University, Rajamangala University of Technology Isan) reports the development and validation of a colorimetric sensor for cadmium ion (Cd2+) detection in environmental water. The sensor is fabricated by modifying silica sol (colloidal silica) with dodecyltrimethylammonium bromide (DTAB, a cationic surfactant) and dithizone (DZ, a chelating ligand). In the presence of Cd2+, the probe changes color from purple to orange owing to Cd2+-DZ complex formation, and detection is performed via UV-vis spectrophotometry at 435 nm and by a customized portable RGB-based cadmium (Cd) analyzer. No food matrix measurements are reported. All sample matrices are environmental water (tap, natural, and wastewater) from Khon Kaen Province, Thailand. The study is framed in the context of food-chain cadmium bioaccumulation in its introduction, and the WHO drinking-water guideline for cadmium (0.003-0.005 mg/L) is referenced as the regulatory baseline. This paper contributes no food contamination data to the wiki.
Study design
Sensor development study. The workflow comprised: (1) synthesis of silica sol by the Stober method (hydrolysis and condensation of TEOS and L-arginine); (2) optimization of probe conditions (pH, reaction time, silica sol volume, surfactant type and concentration); (3) characterization of probe morphology by FIB-SEM and surface charge by zeta potential; (4) selectivity testing against eight coexisting ions (Co2+, Mg2+, Fe2+, Cu2+, K+, Cl-, NO3-, SO42-); (5) calibration curve generation and LOD/LOQ calculation by UV-vis and customized Cd analyzer; (6) application to four spiked environmental water samples with recovery assessment and comparison to flame atomic absorption spectrometry (FAAS).
Instrument for calibration: UV-vis spectrophotometer (Cary 60 Agilent) scanning 380-490 nm in a 3.5 mL quartz cuvette. Customized Cd analyzer using RGB analysis (red channel selected for quantification). Zeta potential: Zetasizer Nano ZS (Malvern, UK). Morphology: FIB-SEM (Helios NanoLab G3 CX, FEI, USA).
Optimized conditions: phosphate buffer pH 3.0; 30-minute reaction time; 3.0 mL silica sol; 0.1 mol/L DTAB.
Samples
n = 4 environmental water samples: tap water (W1), natural water (W2), and two wastewater samples (W3, W4), all from Khon Kaen Province, Thailand. Samples were filtered through a 0.45-micron membrane prior to analysis. No food samples were collected or analyzed. The number of samples is minimal (n = 4), and the sampling constitutes a proof-of-concept application rather than a systematic survey.
Results
Analytical performance (Table 1 and Section 3.4)
| Parameter | UV-vis method | Customized Cd analyzer |
|---|---|---|
| Linear range | 0.01-0.25 mg/L | 0.01-0.25 mg/L |
| Regression equation | Y = 116.70X + 21.30 | Y = 103.15X + 65.44 |
| R2 | 0.9944 | 0.9984 |
| LOD (3s/m) | 5.0 microg/L (44.5 nM) | not separately stated |
| LOQ (10s/m) | 18 microg/L | not separately stated |
| Intra-day RSD (n=5, 0.09 mg/L) | less than 2.59% | less than 7.67% |
| Inter-day RSD (5 days, 0.09 mg/L) | less than 3.24% | less than 7.67% |
The LOD of 5 microg/L equals the WHO permissible level for Cd in drinking water (0.005 mg/L). No certified reference material (CRM) recovery is reported.
Comparison with other colorimetric Cd sensors (Table 1)
The paper benchmarks against four methods: chitosan-dithiocarbamate-modified gold nanoparticles (LOD 7.06 microg/L, tap water); 1-amino-2-naphthol-4-sulfonic-acid-functionalized silver nanoparticles (LOD 0.087 mM, milk powder, serum, and lake water); silver nanoparticles capped with chalcone carboxylic acid (LOD 0.13 mM, drinking water and lake water); and the authors’ previous TTAB/murexide/silica sol probe (LOD 0.21 mM, rice). The current DTAB/DZ/silica sol sensor achieves a lower LOD than any of these comparators.
Real water sample results (Table 2)
Cadmium was not detected in any of the four as-collected water samples (W1-W4). Spike recovery tests at three concentration levels (0.050, 0.125, and 0.250 mg/L) yielded recoveries of 96.8-105.6% with RSD less than 4.0% for the colorimetric method. FAAS recoveries ranged from 90.0-100.8% with RSD up to 4.2%. A paired t-test (p = 0.05) showed no significant difference between the two methods (t-cal = 1.35, t-table = 2.92).
Selectivity
Of eight tested coexisting ions (Co2+, Mg2+, Fe2+, Cu2+, K+, Cl-, NO3-, SO42-) at 0.09 mg/L, none produced a detectable orange complex with the DTAB/DZ/silica sol probe. When each interfering ion was mixed with Cd2+ at equal concentration, the effect on absorbance was described as negligible (Fig. 4).
Methods
Analyte species: total dissolved Cd2+ in aqueous solution. Measurement basis: mg/L (aqueous). No matrix digestion reported (aqueous samples only). Detection wavelength: 435 nm. Reagents: TEOS (GC grade, Sigma-Aldrich), CdCl2 (HIMEDIA), CTAB and DZ (Fluka), DTAB (Sigma-Aldrich). Silica sol synthesis by Stober method from TEOS and L-arginine at 60 degrees C, 300 rpm, 26 hours. Probe: 2000 microL silica sol + 0.09 mg/L DZ + 1000 microL DTAB in 10 mL phosphate buffer pH 3.0.
CRM and recovery
No certified reference material is used in this study. Recovery is assessed by spiked addition to environmental water samples only (see Results above). Recovery in food matrices is not reported.
Evidence fitness
This paper provides no food contamination data and cannot contribute to any ingredient or product contamination profile in the wiki. Its contribution is analytical: it establishes the performance characteristics of a portable, low-cost colorimetric platform for Cd detection in water matrices at concentrations relevant to the WHO drinking-water guideline. The comparison table (Table 1) references a prior silica sol method tested on rice (Charoensuk et al. 2021, Microchem. J.), which is a separate paper not included in this deposit.
Evidence tier assigned as B (corrected from the prior A assignment): the paper is method-focused, not a contamination survey; sample n is minimal (n = 4); no CRM is used; and RSC Advances is a broad chemistry journal without specialist food-safety peer review.
Limitations
Sampling is from a single province in Thailand (Khon Kaen). The study tests only four water samples; this is insufficient to characterize cadmium prevalence in environmental water. No food matrices were analyzed. No certified reference material was used. The LOD (5 microg/L) is at the boundary of the WHO drinking-water guideline and may not be adequate for lower-concentration environmental monitoring contexts. The linear range (0.01-0.25 mg/L) is relatively narrow.
Regulatory context
The WHO Guidelines for Drinking-Water Quality (4th edition, 2017) set a maximum cadmium level of 0.003-0.005 mg/L in drinking water (reference 6 in the paper). The paper’s LOD of 5 microg/L (0.005 mg/L) equals the upper end of this range. No food regulatory limits are cited or applicable in this paper.
Related evidence
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