An off-on fluorescent nanosensor for the detection of cadmium ions based on APDC-etched CdTe/CdS/SiO2 quantum dots

Overview

This Heliyon paper from Guangdong Ocean University (Fang, corresponding author) describes the development and matrix validation of a fluorescent “OFF-ON” nanosensor for cadmium ion (Cd2+) detection based on ammonium pyrrolidine dithiocarbamate (APDC)-etched CdTe/CdS/SiO2 quantum dots (QDs). The sensor exploits APDC-induced fluorescence quenching followed by Cd2+-triggered fluorescence recovery: APDC corrodes the mercaptopropionic acid (MPA) passivation layer on the QD surface, suppressing luminescence; addition of Cd2+ displaces APDC and reforms the Cd-thiol complex, restoring fluorescence. The paper is a sensor development and analytical chemistry study. No unspiked occurrence measurements of cadmium in milk, seawater, or freshwater are reported; all matrix measurements use spiked samples with known Cd2+ additions of 10, 20, and 40 micromol per liter. The milk matrix application demonstrates that the sensor can tolerate complex food matrices. The sensor achieved an LOD of 0.3451 micromol per liter, two linear response ranges covering 0 to 160 micromol per liter, and average Cd2+ recoveries of 90.73 to 111.60 percent in milk with RSD at or below 8.26 percent.

Study design

The study follows a standard analytical sensor development workflow: (1) synthesis and characterization of CdTe/CdS/SiO2 core-shell-shell QDs; (2) comparative evaluation of three fluorescence quenching agents (EDTA, 1,10-phenanthroline (Phen), and APDC) to identify the most effective quencher; (3) optimization of reaction conditions (APDC concentration, pH, reaction time) using the selected quencher; (4) characterization of analytical performance (linearity, LOD, LOQ, selectivity, interference); and (5) matrix validation in spiked seawater (Pacific Ocean), freshwater (Huguang Lake), and whole milk samples. All fluorescence measurements used a fixed excitation wavelength of 365 nm. All experiments were performed at minimum in triplicate; data are expressed as mean plus or minus standard error. Statistical analysis used IBM SPSS Statistics 25; group comparisons used one-way ANOVA; significance threshold was P less than 0.05. The study reports no field collection of environmental or food samples for occurrence purposes.

Analytes

Cadmium ion (Cd2+), total dissolved cadmium. The sensor does not speciate cadmium; it responds to free Cd2+ in solution. No other metals are targeted for quantification, though selectivity is assessed against 15 competing cations: K+, Na+, Ba2+, Mg2+, Ca2+, Hg2+, Fe2+, Fe3+, Pb2+, Zn2+, Cu2+, Al3+, Mn2+, Co2+, and Ni2+.

Instrument and analytical methods

QD synthesis. CdTe core QDs were synthesized by thermally decomposing cadmium (Cd) precursors in the presence of a Te precursor; MPA served as the surface ligand and stabilizer. The CdS shell was grown by adding CdCl2-2.5H2O and MPA to the prepared CdTe cores, with TAA providing the sulfur source (reflux at 100 degrees Celsius for 1 hour). CdTe/CdS/SiO2 (core-shell-shell) QDs were synthesized by the Stober method: tetraethyl orthosilicate (TEOS) acted as the silica precursor, and 3-aminopropyltriethoxysilane (APS) polymerized the silicone matrix around the QD core. The final QD suspension was dispersed in 10 mL PBS (pH 7.0) at 1 mg/mL.

QD characterization. UV-vis and fluorescence spectra: multifunctional enzyme spectrometer (Shimadzu, Japan). FTIR: Fourier infrared spectrometer (FRIT). TEM: Thermo Fisher Scientific Talos F200X G2 TEM at 200 kV. Particle size distribution: Nicomp N3000 dynamic light scattering instrument (Nicocomp, USA). pH: Mettler-Toledo pH meter. UV lamp images: SONY Alpha7 mark III camera under 365 nm UV light.

Sensor operation. For Cd2+ detection, 10 microliters of CdTe/CdS/SiO2 QDs were mixed with 30 microliters of APDC (1 mmol/L) and a fixed volume of Cd2+ in a series of micropores at room temperature. The mixture was diluted with PBS (pH 7.0) to 200 microliters total volume. Fluorescence spectra were recorded under UV irradiation (excitation 365 nm) in the dark for spectroscopic analysis.

Quenching efficiency was calculated as: quenching efficiency = (1 minus F/F0) times 100 percent, where F/F0 is the fluorescence intensity ratio of the sensor system in the presence of APDC at different times and F0 is the fluorescence intensity of the QDs alone.

F/F0 recovery ratio was used as the analytical signal for Cd2+: F is the fluorescence intensity of the QDs-APDC-Cd2+ system and F0 is the fluorescence intensity of the bare QDs.

LOD/LOQ

The limit of detection (LOD) was calculated by the formula LOD = 3*delta/S, where delta is the standard deviation of the fluorescence intensity of the APDC/CdTe/CdS/SiO2 QDs system without Cd2+ (calculated from 20 repeated measurements) and S is the slope of the calibration curve (Figure 6D). The LOD of the QDs-APDC sensor for Cd2+ was 0.3451 micromol per liter, in the linear range of 0 to 20 micromol per liter. The limit of quantification (LOQ) was 1.150 micromol per liter. These values were obtained under optimized detection conditions (APDC 300 micromol/L, pH 7.0, reaction time 10 minutes).

CRM and recovery

No certified reference material (CRM) was used for Cd2+ analysis in this study. Matrix validation relied entirely on spiked recovery experiments. QD synthesis and characterization procedures referenced reagent-grade materials purchased from commercial suppliers (McLean, Zhanyan Chemical Co., Sinoform Chemical Co.) without independent analytical certification of Cd content in reagents. Recovery results for spiked matrix experiments are reported in Table 2 of the paper.

Results

QD characterization

The CdTe/CdS/SiO2 QDs exhibited maximum UV-vis absorption at 608 nm and maximum fluorescence emission at 622 nm under excitation at 365 nm (Figure 2A). The QDs emitted bright red fluorescence under UV light. FTIR analysis (Figure 2B) confirmed: 1647 cm-1 (C=O and C=N vibrations, indicating MPA covalent bonding), 675 cm-1 (Cd-S covalent bond), 2131 cm-1 (C triple bond N functional group), and 3417 cm-1 (N-H or O-H functional groups). TEM analysis (Figure 2C-D) showed that 95 percent of particle sizes were in the range of 6 to 10 nm, with an average particle size of 7.74 nm. Hydrated particle size was in the range of 15 to 40 nm (Figure 2E). Fluorescence stability: QD fluorescence intensity decreased by only 30 percent after storage at 4 degrees Celsius for 90 days (Figure 2F). Quantum yield: 87.26 percent.

Quencher selection

The quenching efficiencies of EDTA, Phen, and APDC on the CdTe/CdS/SiO2 QDs were compared (Figure 3A). APDC exhibited the most pronounced quenching effect at 94.06 percent, superior to EDTA and Phen. APDC was selected as the optimal quenching agent.

Optimization of detection conditions

APDC concentration: Fluorescence intensity of the QDs decreased gradually with increasing APDC concentration up to 300 micromol/L, where the quenching efficiency reached 94.06 percent (Figure 3B, 3C). The optimum APDC concentration was 300 micromol/L.

Reaction time: Quenching efficiency of APDC increased rapidly within the first 10 minutes and remained stable at a high level after 10 minutes (Figure 3D). The optimal reaction time for Cd2+ detection was established at 10 minutes.

pH: Fluorescence of the QDs-APDC system was suppressed at low pH (4 to 6) and improved with increasing pH from 7 to 10 (Figure 3E, Figure 6A). pH 7.0 was selected as the optimal condition. At pH 7 to 10, QDs-APDC-Cd2+ fluorescence was notably higher than QDs-APDC, confirming Cd2+ restoration of fluorescence.

Cd2+ fluorescence restoration and linear response

After addition of 500 micromol/L Cd2+, the fluorescence intensity of the QDs-APDC-Cd2+ system was restored (Figure 4C). Fluorescence intensity stabilized after approximately 10 minutes of reaction time (Figure 6B). Two linear dose-response relationships were observed across the full 0 to 160 micromol per liter Cd2+ concentration range (Figure 6C, 6D):

• Low range (0 to 20 micromol/L): F/F0 = 0.37725 times C(Cd2+) minus 3.20073; R2 = 0.983
• High range (20 to 160 micromol/L): F/F0 = 0.00518 times C(Cd2+) plus 3.8823; R2 = 0.984

Where F and F0 are the fluorescence intensities of the QDs-APDC system in the presence and absence of Cd2+, respectively.

Selectivity and anti-interference

The selectivity of QDs-APDC for Cd2+ was assessed by measuring the fluorescence recovery in the presence of Cd2+ (150 micromol/L) and 15 coexisting metal ions (150 micromol/L each): K+, Na+, Ba2+, Mg2+, Ca2+, Hg2+, Fe2+, Fe3+, Pb2+, Zn2+, Cu2+, Al3+, Mn2+, Co2+, and Ni2+ (Figure 5A). QDs-APDC exhibited the highest recovery efficiency for Cd2+ (89.22 percent) compared to all other tested ions, demonstrating high selectivity. Hg2+ and Co2+ showed apparent interference with Cd2+ detection: Hg2+ and Co2+ could potentially coordinate with the C=O bond and O-H bond on the QD surface (Figure 5B). Zn2+ showed minor interference, attributed to similar electron configuration and chemical properties to Cd2+.

Matrix validation (spiked recovery)

Table 2 of the paper reports recovery data for n = 3 spiked samples per matrix at added concentrations of 10, 20, and 40 micromol per liter Cd2+:

Average recoveries: seawater 95.27 to 110.68 percent; freshwater 92.00 to 106.47 percent; milk 90.73 to 111.60 percent. All RSDs were at or below 8.26 percent. The authors attribute higher average RSD in seawater relative to freshwater to complex soluble components in seawater such as chloride, carbonate, and fluoride salt.

Comparison with other QD sensors for Cd2+

Table 1 of the paper compares the analytical performance of the QDs-APDC sensor against five previously reported quantum dot or fluorescent probe systems for Cd2+ determination:

The CdTe/CdS/SiO2 QD sensor LOD (0.3451 micromol/L) is lower than most previously reported nanomaterials in the table, though N,S co-doped carbon QDs/Au nanoclusters (LOD 0.062 micromol/L) and CQDs/CdTe QDs (LOD 0.018 micromol/L) achieve lower detection limits over narrower linear ranges.

Evidence fitness

This is an A-tier analytical chemistry paper published in a peer-reviewed journal (Heliyon, Elsevier, CC BY license) with well-characterized methods and reproducible validation data. The study design is appropriate for its stated purpose (sensor development and matrix validation) and follows established protocols for QD sensor characterization. All experiments were conducted in at least triplicate. LOD calculation follows the standard 3-sigma method with 20 repeated blank measurements, which is methodologically sound. The core analytical performance data (LOD, linear range, recovery) are internally consistent.

The paper’s utility for the Heavy Metal Index is restricted to the analytical methods domain. No unspiked occurrence measurements of cadmium in milk, seawater, or freshwater are reported; all matrix tests use spiked samples. The spiked concentrations (10, 20, and 40 micromol per liter) are orders of magnitude above typical environmental and food cadmium concentrations: the paper itself cites milk cadmium content of up to 0.27 micrograms per liter (0.0024 micromol per liter) and drinking water cadmium at 0.002 to 0.008 mg/L (0.018 to 0.071 micromol per liter). The sensor’s LOD (0.3451 micromol per liter) substantially exceeds these typical environmental concentrations, limiting its near-term applicability for routine environmental monitoring at background levels. Recovery experiments at spiked concentrations that greatly exceed natural levels are standard practice in sensor validation but do not demonstrate performance at environmentally relevant concentrations.

Interference from Hg2+ and Co2+ is noted; for samples containing these metals, additional sample pretreatment or confirmatory analysis may be required. No CRM was used; recovery was assessed only by spiked addition.

Limitations

Limitations noted by the authors or apparent from the methods: (1) The spiked concentrations in matrix validation (10 to 40 micromol/L) are substantially higher than typical environmental cadmium levels in milk (less than 0.003 micromol/L) and drinking water (less than 0.1 micromol/L), so recovery data at environmentally realistic concentrations are not provided; (2) Hg2+ and Co2+ interfere with Cd2+ detection, as these metals accumulate on the QD surface and reduce fluorescence recovery, limiting sensor selectivity in samples where these metals are present; (3) no CRM was used to independently verify sensor accuracy; (4) the LOD (0.3451 micromol/L, equivalent to approximately 38.9 micrograms per liter) is higher than the WHO guideline value for cadmium in drinking water (3 micrograms per liter), making the sensor unsuitable for direct regulatory compliance monitoring in water at guideline levels; (5) fluorescence of the QDs decreased by 30 percent after irradiation at 365 nm for 2 hours, indicating some photostability limitations; (6) the QD core contains cadmium telluride, raising potential concerns about cadmium leakage, though the authors state that the SiO2 coating and MPA surface modification reduce leakage and toxicity relative to bare QDs.

Regulatory context

No regulatory limits for cadmium are established or discussed as primary targets in this paper. The introduction cites background cadmium levels: 0.002 to 0.008 mg/L (0.018 to 0.071 micromol/L) in drinking water (referencing Chinese published data), and up to 0.27 micrograms per liter (0.0024 micromol/L) in milk. The WHO guideline value for cadmium in drinking water is 3 micrograms per liter; the EU maximum level for cadmium in raw cow milk is 0.020 mg/kg (Regulation EC 1881/2006). The sensor’s LOD of 0.3451 micromol/L (approximately 38.9 micrograms per liter) substantially exceeds these regulatory thresholds, so the sensor in its reported form is not applicable to direct regulatory compliance testing at current guideline levels for water or milk. The paper frames the sensor’s utility in the context of rapid screening rather than regulatory compliance monitoring.

Key conclusions

A core-shell-shell CdTe/CdS/SiO2 QD sensor using APDC as the fluorescence quenching agent achieves an OFF-ON response to Cd2+ with an LOD of 0.3451 micromol/L and two linear ranges of 0 to 20 and 20 to 160 micromol/L. The sensor demonstrated satisfactory spiked recoveries in milk (90.73 to 111.60 percent, RSD at or below 8.26 percent), seawater (95.27 to 110.68 percent), and freshwater (92.00 to 106.47 percent) at added concentrations of 10 to 40 micromol/L. The sensor achieved rapid detection (10 minutes) with visual confirmation under UV light. The quantum yield of 87.26 percent and particle size below 10 nm contribute to high surface-to-volume sensitivity. Hg2+ and Co2+ are the principal interfering ions. The sensor LOD is above typical background cadmium concentrations in food and water, limiting its use to applications where elevated cadmium levels are expected rather than routine environmental surveillance at regulatory guideline levels.

Implications

Certification: No contribution to cadmium contamination profiles for milk or any food product category. The study reports no occurrence data. The analytical method operates at concentrations well above regulatory thresholds for cadmium in milk or water, and no HMTc threshold-relevant concentration data are generated.

Courses: Useful as an example of fluorescence-based QD sensor development and matrix validation in dairy and water matrices. Illustrates the OFF-ON sensor design principle and the role of surface chemistry in QD-based heavy metal detection.

Related evidence

• Cadmium
• Milk and Dairy

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