Researchers Use Optical Fibers to Detect Lead Ions

By Surbhi JainReviewed by Susha Cheriyedath, M.Sc.Jul 27 2022

In an article recently published in the journal Optical Fiber Technology, researchers discussed the utility of functionalized D-shaped optical fibers for measuring heavy metal ion pollution.

Heavy Metals Ions as Pollutants

Heavy metal ions are thought to be one of the components of the Earth's crust. However, several sources are now replenishing their natural abundance. As a result, their concentration frequently exceeds acceptable levels.

When the concentration of heavy metal ions rises above these permissible levels, they become poisonous and endanger their home. These heavy metal ions enter the human body through food, air, and water.

Heavy metal ions are a significant contributor to water pollution. Industrial and agricultural wastes heavily contaminate groundwater and nearby bodies of water.

Heavy Metal Ions Detection Using Optical Fiber-Based Sensors

It is crucial to analyze these contaminants and their abundance in drinking water on a quantitative and qualitative level.

A wide range of sensors is being developed to detect and remove heavy metal ions.

Most dangerous heavy metal ions can be detected using various standard routine methods. Among these, atomic absorption spectroscopy (AAS), inductively coupled plasma mass spectroscopy (ICP-MS), and atomic fluorescence spectroscopy (AFS) deserve special attention.

However, because of their large size and high maintenance costs, these procedures are inappropriate for field analysis.

As a result, sensors with quick on-site detection and great sensitivity gradually replace these standard procedures. It is important to note that optical fiber-based sensors are replacing many earlier solutions. Their excellent resistance to electromagnetic interference is the reason.

When the shape of the optical fiber probes is changed, such as D-shaped, U-shaped, and tapered, the sensors produce adjusted throughput.

D-shaped fiber probes are extremely useful for sensing.

D-Shaped Fiber Probes for Lead Ion Detection

The authors demonstrated a simple technique for detecting lead ions in water in the study. This was accomplished by adding aspartic acid and functionalizing gold nanoparticles to a D-shaped optical fiber.

The throughput in terms of intensity through the functionalization of fiber was dependent on the aggregation of gold nanoparticles caused by Pb²⁺ ions.

The modulated intensity was subsequently evaluated using an optical detector. The obtained results demonstrated an impressive response down to the ppb range, particularly for the lead ion with the lowest selectivity to other ions.

The team used a D-shaped optical fiber as the sensing unit as it was simpler to fabricate a D-shaped optical probe than a tapered or U-shaped optical probe.

Compared with their counterparts, D-shaped probes offered improved output response stability due to a broader cross-section of the interaction area. In contrast to other shapes, the D-shaped showed a more extensive engagement area and higher throughput.

Preparation of Gold Nanoparticles Functionalized D-Shaped Optical Fiber Probe

The researchers introduced a D-shaped optical fiber probe that utilized the functionalization of gold nanoparticles (AuNPs) to detect lead ions.

The synthesis of gold nanoparticles (AuNPs) and the construction of the probe by functionalizing the gold nanoparticles (AuNPs) were examined, followed by a sensitivity analysis.

The team used a multimode optical fiber with a 125 µm core radius and a 0.22 numerical aperture.

Gold chloride, trisodium citrate, and medium-molecular-weight chitosan were the necessary ingredients for the functionalization. Gold nanoparticles (AuNPs) were created using a conventional process for chemical reduction. A 1 mM gold chloride solution was warmed to 60 °C for functionalization.

The authors created a chitosan solution in acetic acid to functionalize gold nanoparticles. After that, a solution containing aspartic acid in the double-distilled water was prepared for the final functionalization process of the gold nanoparticles.

A 12 cm optical fiber was used to create the sensor probe. The side polished fiber's 2 cm of half-cladding was then carefully peeled off with a sharp blade to create a D-shape optical fiber.

Once ready, the gold nanoparticles post functionalization were used to sensitize the exposed area, which made them susceptible to injection of heavy metal ions using a micropipette.

The optical detector was set up for readout. The samples were subsequently delivered through a micropipette to the fiber's sensing region.

Characterization of AuNPs functionalized D-Shaped Optical Fiber Probe

Both the pure gold nanoparticles (AuNPs) and the AuNPs underwent an ultraviolet-visible (UV-vis) characterization after functionalization.

Unmodified AuNPs produced higher absorption values than functionalized ones. The red shift of the absorption peak was also plainly visible. This could be explained by either a reduction in plasmon oscillation frequencies or a change in the permittivity of the environment around the AuNPs.

Conduction electron resonance oscillations resulted from the evanescent field's interaction with aspartate acid-functionalized gold nanoparticles (AuNPs) in the sensing region.

One of the -COOH groups bound to the surface of the AuNPs, whereas the other, which was free, could easily interact with lead ions.

The lead ion's interaction with -COOH changed the refractive index around the AuNPs due to the external analyte's presence.

Due to the direct relationship between modulated intensity and refractive index, modulated light intensity resulted when lead ion concentrations were changed.

Output growth was observed up to a certain point, followed by a zone of saturation. The sensor probe's output response increased as the lead ions' concentration rose.

However, saturation happened in the sensor probe, and there was no longer any binding with lead ions after the passage of a specific amount of time and concentration.

Functionalization of gold nanoparticles (AuNPs) was responsible for the saturation. Here, it was implied that the interaction between the lead ions and the carboxylic group stopped, which resulted in the least number of changes in the refractive index and saturated output.

The obtained results demonstrated that lead ions produced more intensely regulated outputs than Cd²⁺, As³⁺, and Hg²⁺, other heavy metal ions.

The much higher affinity of the Pb²⁺ ion for the -COOH group could be the cause of this more robust response. Larger throughput eventually resulted from increased refractive index fluctuation in lead ion abundance.

A Straightforward and Cost-Effective Sensing Approach for Assessing Pb²⁺

To summarize, this study described a simple and affordable sensing strategy for evaluating Pb²⁺, a dangerous aquatic pollutant.

A unique D-shaped optical fiber probe was employed to detect Pb²⁺. The sensing area of the D-shaped fiber was coated with aspartic acid-functionalized AuNPs, allowing lead ions to be measured down to ppb levels.

The obtained results demonstrated an impressive response down to the ppb scale, particularly for lead ions having the least selectivity to other palpable ions.

The authors mentioned that the reported methodology provides a compact, cost-effective method for detecting heavy metal ions contaminants in water.

Reference

Biswas, R., Bhuyan, R., Boruah, B. S., et al. (2022) Assessing heavy metal ion contamination through functionalized d-shaped optical fiber. Optical Fiber Technology 102996. https://www.sciencedirect.com/science/article/abs/pii/S1068520022001791

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