Overview
Wang and colleagues at Wuhan University of Technology (National Engineering Research Center of Fiber Optic Sensing Technology and Networks) developed and characterized an optical fiber surface plasmon resonance (SPR) sensor for the detection of aqueous lead ions (Pb2+). The sensor uses a multimode-singlemode-multimode (MSM) hetero-core fiber architecture with a 15 mm single-mode sensing region coated with a 60 nm gold film, gold nanoparticles (AuNPs, approximately 20 nm diameter), and glutathione (GSH) as the Pb2+-selective ligand. This is a method-development and analytical characterization study; no food matrices were analyzed and no occurrence data are reported.
Study design
The study fabricated and evaluated a single sensor design across five sub-experiments: refractive index (RI) sensitivity measurement, Pb2+ concentration-response characterization, long-term stability, repeatability across three replicate sensors, and specificity against eight interfering metal ions. All experiments were conducted in aqueous solution (deionized water or phosphate-buffered saline, pH approximately 7.4) at 25 +/- 1 degrees C.
Optimization of the SMF sensing length was performed prior to main experiments by fabricating five MSM structures with SMF lengths of 5, 10, 15, 20, and 25 mm; 15 mm was selected as optimal based on RI sensitivity and stability (details in Tables S1 and S2 of the paper). The Au film thickness was fixed at 60 nm following a chromium adhesion layer of 5 nm deposited by magnetron sputtering. AuNPs were attached via a dithiothreitol (DTT) bridge; GSH was self-assembled onto the AuNPs through Au-S bonding.
Samples
No food samples, environmental samples, or biological matrices were analyzed. All analyte solutions were prepared from lead chloride (PbCl2) dissolved in deionized water, diluted from a 1 mM stock to concentrations spanning 10^-12 M to 10^-4 M (0.43 pM to 20.7 micromol/L, or approximately 0.09 pg/L to 20.7 microg/L as Pb). Sample n is not applicable in the food-occurrence sense; the study reports sensor response at defined concentration levels without replicate sampling of a population.
Results
All values are for aqueous Pb2+ solutions at 25 +/- 1 degrees C.
RI sensitivity. The AuNPs-modified sensor achieved an RI sensitivity of 2778.2 nm per refractive index unit (nm/RIU) compared to 2053.2 nm/RIU for the sensor coated with Au film only (no AuNPs), a 35% improvement. NaCl solutions at 0, 5, 10, 15, 20, and 25 weight percent (RI values 1.3320 to 1.3765) were used for calibration; total wavelength drift was 120.69 nm (AuNPs sensor) versus 98.33 nm (Au film only). Source: Section 4.2 and Figure 4.
Pb2+ detection performance. The linear concentration range was 10^-12 M to 10^-4 M (over eight orders of magnitude). The linear fitting function was y = 0.5908 lg(C) + 587.4641 (R2 = 0.9919), where y is wavelength in nm and C is Pb2+ concentration in mol/L. Maximum wavelength shift at the high end of the range (10^-4 M) was 5.39 nm. Sensitivity was 2.32 x 10^11 nm/(mol/L). The LOD was estimated as 0.43 pM (approximately 0.089 pg/L as Pb) using the formula C = delta-lambda / S, where delta-lambda is the minimum detectable wavelength shift and S is sensitivity. Source: Section 4.3 and Figure 5.
For comparison, the sensor without AuNPs but with GSH showed reduced sensitivity and narrower detection range, and the sensor without either AuNPs or GSH showed minimal or negligible response to Pb2+. Source: Section 4.3 and Figure S9.
Table 1. Key analytical performance metrics of the AuNPs/GSH-modified optical fiber SPR sensor
| Parameter | Value | Source |
|---|---|---|
| Linear range (mol/L) | 10^-12 to 10^-4 | Fig. 5b |
| Sensitivity (nm per mol/L) | 2.32 x 10^11 | Section 4.3 |
| LOD (pM) | 0.43 | Section 4.3 |
| LOD (pg/L as Pb) | approximately 0.089 | Calculated from 0.43 pM |
| R2 of linear fit | 0.9919 | Fig. 5b |
| Maximum wavelength shift (nm) | 5.39 | Section 4.3 |
| RI sensitivity, AuNPs sensor (nm/RIU) | 2778.2 | Section 4.2 |
| RI sensitivity, Au-only sensor (nm/RIU) | 2053.2 | Section 4.2 |
Stability. Measured over 90 continuous minutes at four Pb2+ concentrations (10^-12 M, 10^-9 M, 10^-6 M, 10^-4 M). Maximum deviations from initial wavelength were 0.09, 0.09, 0.07, and 0.03 nm respectively. Source: Section 4.4 and Figure 6.
Repeatability. Six repeated measurements at four concentrations (10^-12, 10^-9, 10^-6, and 10^-4 M) on three independently fabricated sensors (sensors cleaned with DI water and alcohol between measurements). Wavelength deviations for one sensor were 0.03, 0.06, 0.07, and 0.04 nm. Source: Section 4.5 and Figure 7.
Specificity. Tested against Ca2+, Al3+, Hg2+, Ba2+, Mg2+, Fe3+, Co2+, and Na+ at 10^-4 M (the highest Pb2+ test concentration). The sensor showed minimal wavelength shift for all eight interferents relative to Pb2+. Additional specificity tests at 10^-10, 10^-9, 10^-8, 10^-7, 10^-6, and 10^-5 M are in Figure S11. The selectivity mechanism is attributed to deprotonation of the carboxyl groups in GSH forming specific chelates with Pb2+ while the protonated amino group (NH2) sterically prevents binding by other metal ions. Source: Section 4.6 and Figure 8.
Instrument and analytical method
Fiber fabrication used an FSM-60s fusion splicer (Fujikura Ltd., Tokyo, Japan). Au film deposition used magnetron sputtering (Bestec GmbH, Berlin, Germany). AuNPs were characterized by UV-visible spectrophotometry (AvaSpec-ULS2048L, Avantes Co., Ltd., Beijing, China). Fiber surface morphology was characterized by scanning electron microscopy (SEM, Zeiss G300, Carl Zeiss AG, Oberkochen, Germany) and energy dispersive spectroscopy (EDS, Zeiss Sigma HD, Germany). Pb2+ binding was confirmed by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA, USA). The SPR measurement system used a halogen lamp light source (Ocean Optics HL-2000, Dunedin, FL, USA) and an optical fiber spectrometer (Ocean Optics USB 2000+, Dunedin, FL, USA).
No certified reference material (CRM) was used and no CRM recovery data are reported. The study does not report an LOQ value; only the LOD of 0.43 pM is stated.
Statistical analysis
Sensor performance curves (RI sensitivity and Pb2+ concentration response) were fitted by least-squares linear regression. The R2 value is reported for the Pb2+ calibration (R2 = 0.9919). No formal statistical comparison tests or uncertainty estimates are reported beyond the wavelength deviation values noted under stability and repeatability.
Evidence fitness
Evidence tier A. This is a peer-reviewed article published in MDPI Materials (CC BY license) with fully documented fabrication, characterization, and performance data. All key performance claims are supported by data presented in the figures and tables of the paper.
For the purposes of this wiki, the paper is relevant only as a methodological reference for Pb2+ sensing technology and not as an occurrence study. It does not contribute to any food-matrix contamination profile, dietary exposure estimate, or regulatory compliance assessment. The LOD of 0.43 pM (approximately 0.09 pg/L as Pb) is well below any regulatory or health-based threshold for lead (Pb) in drinking water or food, confirming the sensor operates at trace and ultra-trace levels. It does not, however, demonstrate performance in complex food matrices; all tests were conducted in simple aqueous or PBS solution.
Limitations
No food matrices, biological samples, or certified reference materials were analyzed. Sensor performance was validated only in deionized water or PBS solution; no spiked recovery tests in food extracts, beverage matrices, or complex environmental samples were conducted. The paper acknowledges in Section 4.7 that “the detection performance of the sensors is yet to be verified in various complex environments for practical applications.” No LOQ value is reported; only the LOD is provided. CRM recovery data are absent. The repeatability data cover three identical sensors but no inter-laboratory comparison is included. The study was conducted entirely within two research institutions in China; no independent replication by another group is reported in this paper.
Regulatory context
The paper cites two regulatory limits as motivation for the sensor’s development. The WHO recommends that lead concentration in drinking water should not exceed 10 ppb (approximately 48.3 nmol/L). The U.S. Environmental Protection Agency sets a maximum allowable lead concentration in food of 72 nmol/L (approximately 14.9 microg/L as Pb). The paper notes that the sensor’s LOD of 0.43 pM is significantly below the WHO drinking water limit. Neither limit is directly evaluated against experimental data; these are cited as the basis for the analytical need. No EU, Codex, or food-specific regulatory limits are discussed.
Implications
Certification: This source is not suitable as a basis for setting Pb maximum levels in any food or personal care product category. No food occurrence data are reported.
Courses: Useful for illustrating optical fiber SPR sensing as an emerging detection technology for heavy metal ions; demonstrates how biocompatible chelating agents (glutathione) can be used as selective recognition elements in lieu of toxic chelators; illustrates the principle of LSPR amplification via gold nanoparticles.
App: Does not contribute to any food contamination profile; no consumption or occurrence data.
Microbiome: No microbiome endpoint measured.
Related evidence
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