Flexible paper-based Ag dendritic SERS chips for rapid in situ detection of thiram residues on pear skin

Abstract

Surface-enhanced Raman scattering (SERS) is a powerful, highly efficient analytical technique capable of providing label-free, non-invasive, rapid, and ultrasensitive molecular detection down to the single-molecule level. Despite its advantages, SERS remains largely confined to laboratory settings due to the complexities of substrate fabrication and challenges in analyzing real-world samples. Developing flexible SERS substrates that achieve both high fabrication efficiency and high sensing performance, while being practical for field applications, is critical for advancing SERS toward broader, real-world use. In this study, we present a novel paper-based Ag dendritic SERS chip, fabricated via a simple chemical reduction process that directly forms Ag dendritic nanostructures on cellulose fibers. This chip substrate demonstrates exceptional sensitivity for the detection of thiram pesticide, with a detection limit as low as 7.76 × 10^-11 M. The chip substrate also exhibits outstanding reliability, with reproducibility and repeatability both less than 5%. Furthermore, the flexible nature of the paper substrate enables it to conform to curved surfaces and be in direct contact with analytes, exemplified by its ability to adhere to and retrieve thiram from pear skin using a novel "paste-and-peel-off" technique. The substrate shows remarkable performance for thiram detection on pear skin, with sharp recovery rates ranging from 90% to 105%. With its facile fabrication, excellent sensitivity, high reliability, and practical applicability in non-invasive sampling, the paper-based Ag dendritic SERS substrate offers significant potential as an advanced substrate to bring SERS out of the laboratory and closer to real-world applications.

Fig. 1. FE-SEM images of flexible Paper/Ag-d chips at different resolutions.

Fig. 2. EDX spectrum and EDX mapping analysis of flexible Paper/Ag-d chips.

Fig. 3. Raman spectra of the paper substrate, Paper/Ag-d chips, and the SERS spectrum of the thiram analyte on the Paper/Ag-d chips.

Fig. 4. SERS sensing performances of detecting thiram using Paper/Ag-d chips substrate (a), assessing the reliability of the Paper/Ag-d chips by collecting SERS signals at 5 different points on the same substrate (b) and across 5 substrates prepared at different times (c).

Fig. 5. Logarithmic plots of SERS intensity versus thiram concentration at 446 cm⁻¹ (slope: 0.69 ± 0.02, intercept: 8.27 ± 0.05) (a), 560 cm⁻¹ (slope: 0.25 ± 0.03, intercept: 5.12 ± 0.05) (b), 1150 cm⁻¹ (slope: 0.65 ± 0.05, intercept: 8.12 ± 0.05) (c), and 1386 cm⁻¹ (slope: 0.64 ± 0.05, intercept: 8.18 ± 0.05) (d).

Fig. 6. Protocol for the “paste and peel-off” technique for thiram collection on pear skin (a); SERS spectra of thiram obtained from Paper/Ag-d chips collected on pear skin at concentrations ranging from 10⁻⁷ to 10⁻¹⁰ M (b); comparison of SERS signals of thiram obtained using the “paste and peel-off” technique with standard solutions at concentrations of 10⁻⁷ M (c) and 10⁻⁸ M (d).