(Invited) Hybrid Plasmonic Nanomaterials for Uranyl Sensing
Authors: Haes Amanda J , Phan Hoa 
Published: May 2020

Introduction

Reproducible detection of uranyl, an important biological and environmental contaminant, from complex matrices by surface-enhanced Raman scattering (SERS) is successfully achieved using hybrid plasmonic nanoparticles. Traditionally, non-specific binding of interfering species limits detects of molecules such as uranyl. Herein, this is overcome using materials design and rigorous sample analysis workflow design. Synergistic approaches for uranyl isolation and SERS detection is promising for real-world sample detection and eliminates the need of radioactive tracers and extensive sample pretreatment steps.
Uranium, a radioactive material with a long half-life, accumulates in the environment in its oxidative form uranyl, which can contaminate soil and water [1]. Importantly, uranyl forms complexes with anions and cations thereby influencing solubility, toxicity, and fate of these heavy metal species. Furthermore, uranyl speciation varies with pH. The resulting complex speciation complicates detection and/or requires significant sample pretreatment. As such, methods that are capable of identifying trace uranyl species in complex samples are needed.
Herein, hybrid plasmonic nanomaterials and experimental workflow implementation will be used to establish a rigorous protocol for uranyl detection [2,3]. Namely, localized surface plasmon resonance spectroscopy (LSPR), Raman spectroscopy [4], and surface enhanced Raman scattering (SERS) [5,6] serve as label-free and near real-time methods for identifying uranium species in complex aqueous solutions.

Experimental Approaches

A rigorous protocol for spectral analysis will be shown using Raman spectroscopy and aqueous uranium samples. Raman excitation wavelength, pH, and coordinating ions are systematically varied. The spectral analysis results are rigorously validated using uranyl speciation models. Next, plasmonic nanomaterials are used to enhance the Raman signals for trace detection of low (and high) abundant species. Finally, an approach that promotes the reproducible detection of uranyl using SERS will be shown. All in all, the developed protocol provides an accurate and routine analysis of Raman spectra for uranyl species identification and relative abundance elucidation. These advances are expected to provide a straight-forward approach for uranium species identified using hybrid plasmonic nanomaterials.

Results and Conclusions

Plasmonic nanomaterials offer many advantages over traditional methodologies in applications ranging from catalysis, sensing, and imaging. Despite these strengths, challenges arise including for both direct and indirect SERS detection of uranyl include nanoparticle stability, SERS spectral complexity, and variations in SERS intensities and vibrational frequencies. All of these challenges depend on intra- and intermolecular interactions at the plasmonic metal interface as well as the plasmonic nanomaterial stability in complex matrices.
Uranyl, unfortunately, exhibits poor affinity to traditional SERS substrates. Methods to promote molecule-metal interactions often lead to nanoparticle instability. Herein, gold nanostars and gold coated silver nanospheres are used. Implications of nanoparticle architecture are investigated for maximizing SERS responses and detectability of uranyl. Nanoparticle architecture and surface potential drive these interactions.
To promote detectability in complex matrices, both polymer and silica surface chemistries will be evaluated to initially “screen” unwanted interfering species for adsorbing to the metal surface while also permitted the adsorption and detection of uranyl for SERS detection. Finally, solution conditions are used to identify key features that rationally promote uranyl-surface interactions without compromising the physical stability of the nanostructures. Solution composition induces changes in nanoparticle architecture and/or adsorption processes, yet systematic responses are observed. As a result, we expect these studies will broaden the scope of SERS and plasmonic-based assays as small molecules with weak affinity to metal nanostructures will be more readily detected.
While uranyl is a difficult molecule to detect in complex matrices, hybrid nanomaterial design and carefully designed experimental approaches facilitate detectability in a reproducible manner. By using polymers and silica hybrid materials, influences from interfering species are minimized and solution impacts reduced. Future advances in further reducing detection limits and integrating these materials with rapid sampling platforms have the potential to lead to a revolutionary sensor for heavy metal detection.

Acknowledgments

Research reported in this publication was supported by the National Institute of Environmental Health Sciences of the National Institutes of Health under award number R01ES027145 and the National Science Foundation, (CHE-1707859).

References

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