Celebrating Earth Day: Portable Raman Spectroscopy for a SERS-Based Screening of Freshwater Contamination

As we celebrate Earth Day 2021 we would like to focus on Raman studies done to protect our environment. There is a need for capabilities to measure contaminants in our water supply, as they aid in developing means of mitigation, and prevention of such contamination in the future. Heavy metal contamination of water can have a marked health impact on plants and animals.

Zinc (Zn²⁺) is an element that can be used as an indicator of heavy metal contamination in freshwater bodies. Recently, research groups at the Université Paris Diderot and Université de Lille have collaborated to develop a low-cost and simple SERS-based approach to the quantitation of Zn²⁺ in freshwater samples using an i-Raman Plus portable Raman spectrometer.¹ They published their findings in 2020 in Scientific Reports.

Both deficiency and overabundance of Zn²⁺ are harmful to plant and animal health. Monitoring of Zn²⁺ contamination from industrial waste is important, and can also indicate trace-levels of contamination from elements such as cadmium, lead, and mercury. These heavy metals negatively affect human health and the ecosystem more than Zn²⁺, but are significantly less abundant than Zn²⁺. When time and cost do not permit the analysis of these toxic metals, Zn²⁺ levels may be used as an indicator for their levels.

Water treatment analysis requires high sample volumes to assess the extent of pollution, safety of drinking water, and the health of aquatic ecosystems. For these analyses to be effective, the development of portable techniques is highly preferred compared to the past “gold standard” laboratory-based analytical techniques. These traditional techniques can reach low limits of detection, but are expensive, require highly-skilled operators, and are slow in analysis time. Thus, these techniques are difficult to implement in developing countries where tools for monitoring water quality are needed the most. Field testing methods for rapid screening can expand environmental testing, and potentially protect people from polluted water.

Surface-enhanced Raman spectroscopy (SERS), a modified version of Raman spectroscopy, was chosen as a technique to measure Zn²⁺ in freshwater samples due to its ability to increase Raman scattering by orders of magnitude. Although the adoption of SERS as a routine analysis for monitoring of water quality has been slow due to the complexities of SERS sample preparation, the researchers have developed a technique that employs a SERS material which can be synthesized in a single step on a large scale by non-chemists, thus demonstrating the usefulness of SERS as a field-technique in testing of water quality.

The SERS substrates can be easily prepared by exploiting the binding properties of Zn²⁺ to a Raman-active chelating agent.  Silver nanoparticles were aggregated and linked with the Zn²⁺ ligand Xylenol Orange (XO), by a cross-linking agent. The aggregate particles are then exposed to the analyte solution and the XO is modified in the presence of the bound Zn²⁺.

A series of titrations were carried out to add increasing amounts of Zn²⁺ to the aggregates loaded with XO ranging from 0-2.23 µM and their SERS spectra measured using the i-Raman Plus 785 nm Raman system. Both univariate and multivariate (partial least squares) methods were explored for quantification of the Zn²⁺. In particular, the multivariate analysis showed higher recovery rates, better accuracy and precision, and a lower LOD (~200 nM) and LOQ than the univariate methods. Although the LOD for the technique is still an order of magnitude higher than traditional lab-based instrumentation like atomic absorption spectroscopy and inductively coupled plasma-optical emission spectroscopy, it is a suitable economic alternative for rapid testing of freshwater samples to recognize trends and abnormalities. It can be an effective screening method as the typical Zn²⁺ concentrations in some areas may be well above the LOD, at harmful levels.

This development of a portable, low-cost, and sensitive SERS-based method for the determination of Zn²⁺ in freshwater demonstrates the value and usefulness of portable Raman and SERS in environmental testing.

Read the full open-access journal article here.

Microplastics are also significant contaminants in water. Read our application note on using portable Raman microscopy to identify microplastics here.

References

1. G. Brackx, D. Guinoiseau, L. Duponchel, A. Gélabert, V. Reichel, S. Zrig, J-M. Di Meglio, M.F. Benedetti, J. Gaillardet and G. Charron, Sci Rep. 10, 1883 (2020).