According to a recent study published in Biosensors, a team of researchers has produced an optical sensor for uranyl detection. The researchers used gold-plated plastic optical fiber and a receptor made of the synthetic molecule 11-mercapto-undecyl phosphonic acid (MUPA) to complex the target ion and fix the molecule at the gold resonant surface as a molecular layer in an easy and repeatable manner.
The researchers demonstrated the sensor's performance by analyzing the uranyl content response in aqueous solutions of various compositions and real-world samples, such as tap water and seawater.
Harmful Effects of Uranium
Uranium is a radioactive element with an estimated mean concentration of 2.7 mg kg-1 in the Earth's crust and 3.3 g L-1 in seawater. It exists in the environment in various chemical species and is radioactively and chemically dangerous. The predominant form of uranium is uranyl (UO2 2+). Uranyl is a double-charged ion in which uranium is at a +6 oxidation state.
Uranium has some significant industrial use in the realm of defense. Due to its high density and hardness, a by-product of the uranium enrichment process is used as armor-piercing ammunition in several global military activities. Using such ammunition has increased the spread of harmful 238U into the environment by releasing DU into the atmosphere. As a result, there has recently been a lot of public interest in the potential increased presence of uranium in environmental waters.
Current Method to Detect Uranium in Environmental Waters
Traditional metal detection methods for detecting uranium in water at low concentration levels is a successful method. The out-of-lab analysis is another method for detecting uranium, saving time and money.
In the past few decades, optical sensing techniques have garnered much attention due to their low detection limits by utilizing a DNAzyme specialized for the uranyl compound as a molecule-recognizing element.
Plastic Optical Fiber for Specific Detection of Chemical Species
Optical sensors utilizing the surface plasmon resonance phenomena on multimode optical fibers are used to precisely detect chemical species of biological relevance. A surface plasmon resonance (SPR) optical platform based on a multimode plastic optical fiber can be manufactured easily and only needs a small instrument.
The distinctive D-shaped profile of plastic optical fiber is created by erasing a multimode plastic optical fiber with a multilayer plasmonic resonant surface. The combination of plasmonic resonance surface and molecule recognizing element (MRE) mounted as a layer in close contact with the surface is used to produce the sensor.
Development of D-shaped SPR Optical Platform (SPR-POF) for Uranyl Detection in Water
A specialized sensor for uranyl detection in water is produced by Alberti et al. using the D-shaped SPR optical platform (SPR-POF) to demonstrate the capability of the sensing strategy for the determination in actual aqueous samples in the g L-1 range.
The MRE used is 11-mercaptoundecanphosphonic acid (MUPA), a highly effective uranyl receptor in an electrochemical sensor due to the presence of the phosphonic moiety. The -SH group allows it to self-assemble, making it a simple synthetic molecule that is less expensive and more stable in out-of-lab applications. At the same time, it can be quickly connected to the gold surface of the optical platform.
The sensing device suggested by Alberti et al. is intriguing as it can be used immediately in the field. This technique provides an analytical response in a quick and reasonably priced manner. Given that a suitable MRE is established at the SPR interface, the marker-free SPR transduction approach can be applied to various metal ions, including those that are not electroactive as in the case of electrochemical transduction.
Polymeric Optical Fiber Platform Preparation
The optical sensor platform was created using a polymeric optical fiber with a poly-methylmethacrylate (PMMA) core and a fluorinated polymer cladding placed in resin support. A D-shaped zone was created on the plastic optical fiber by removing the cladding and a portion of the core around the half circle and polishing with two different types of polishing sheets. A spin coater machine was then spun on the exposed plastic optical fiber core to create a layer between the core and the metal. This layer can significantly boost performance because its refractive index is higher than the plastic optical fiber core.
In this research, a D-shaped optical platform on a multimode plastic optical fiber is appropriate as an optical sensing instrument for uranyl detection in waters of environmental interest at concentrations typically present in these samples.
The MRE under consideration, MUPA, is a molecule that can firmly bind uranyl ions while also being steadily connected to the platform of the gold surface via a straightforward process. For low detection limits and excellent selectivity, the choice of this chemical recognition element is essential. A sufficient low detection limit for the direct detection of uranyl in environmental waters can be achieved with only a very mild treatment of the sample, such as the addition of an ionic strength buffer, owing to the good selectivity of the platform for uranyl in comparison to other metal ions that are widely present in environmental waters.
Alberti, G., Pesavento, M., De Maria, L., Cennamo, N., Zeni, L., & Merli, D. (2022). An Optical Fiber Sensor for Uranium Detection in Water. Biosensors, 12(8), 635. https://www.mdpi.com/2079-6374/12/8/635