Paper coated with silver nanoflower composite, a dual substrate for SERS and PS-MS

Recently a novel dual substrate for both SERS and PS-MS based on the deposition of silver nanoflowers (AgNFs) onto a paper-based analytical device (PAD) has been developed. Although the obtention of the substrate requires several steps, it is possible to synthesize a bunch of them at the same time. Figure 1 illustrates the different stages involved in the synthesis process of the so-called ny-AgNF PADs (further details are included in the full article). To obtain the silver nanoflowers, an already described protocol was followed, which is based on a simple reaction involving silver nitrate and ascorbic acid as the reducing agent (Figure 1, steps 4-7). Furthermore, previous modification of the paper was needed to anchor the nanoflowers to its surface (Figure 1, steps 1-3, and 8). First, a coating of nylon-6 was needed to assure the surface availability of the analytes in Surface-enhanced Raman spectroscopy (SERS). Next, a surface of metallic silver was acquired by the deposition of silver nitrate and subsequent reduction with UV light. The PAD was then incubated in ethylenediamine, which acted as a chemical bridge to anchor the nanoflowers, thus providing the substrate with mechanical stability. Finally, it was immersed in the AgNFs solution, and the final ny-AgNF PAD was obtained.

Figure 1. Diagram of the different steps of the synthesis of ny-AgNF PADs.

The resulting substrate was characterized in different stages of the synthesis in terms of scanning electron microscopy (SEM) and infrared spectroscopy. Regarding SEM, a uniform coating of the nanoflowers was observed. This coating led to an enhancement of the signal not only in SERS, but also in paper-spray mass spectrometry (PS-MS).

The enhancement properties of the AgNFs immobilized at the functionalized PADs were evaluated via SERS studies using crystal violet (CV) as a SERS reporter, obtaining an enhancement factor of 1.97·10⁴ compared to raw filter paper. Once the potential was demonstrated, the SERS effect was further evaluated using ketoprofen as analyte, in which the coating of nylon-6 was of vital importance to assure the surface availability of the compound.

The substrate also showed potential in PS-MS, enhancing the sensitivity compared to bare paper, the enhancement arising from the presence of the AgNFs. This aspect broadens the application scope of the substrate since MS admits more complex matrices where SERS presents a problem due to the lower selectivity. The ny-AgNF PAD was then applied for the determination of ketoprofen in dried saliva samples, providing similar results to those obtained with standards.

We encourage everyone who comes across this to read the full paper, which can be found at the Analytical and Bioanalytical Chemistry webpage.

Reference

Díaz-Liñán, M. C., García-Valverde, M. T., López-Lorente, A. I., Cárdenas, S., & Lucena, R. (2020). Silver nanoflower-coated paper as dual substrate for surface-enhanced Raman spectroscopy and ambient pressure mass spectrometry analysis. Analytical and Bioanalytical Chemistry. https://doi.org/10.1007/s00216-020-02603-x

About the author

María del Carmen Díaz-Liñán is a PhD student at the Department of Analytical Chemistry at the University of Córdoba (UCO, Spain), where she obtained her bachelor’s degree in chemistry (2017). Her current research is focused on the development of sensors based on planar substrates such as paper combined with the use of nanomaterials, which are coupled to different spectroscopic techniques such as Raman spectroscopy and mass spectrometry.