*2.6. Semi-Quantitative Analysis of Three Pesticides and Determination of Intra-Batch and Inter-Batch Precision*

Firstly, malathion and phoxim stork solutions with a concentration of 1000 mg L−<sup>1</sup> were prepared by dissolving 1 μL of malathion and phoxim in 1.230 mL and 1.176 mL acetonitrile, respectively. Standard solutions with sixteen different concentrations (100, 50, 20, 10, 5, 2, 1 mg L−<sup>1</sup> and 500, 200, 100, 50, 20, 10, 5, 2, 1 μg L<sup>−</sup>1) were prepared by diluting the stork solution with corresponding amounts of solvent. Similarly, thiram stock solution was prepared by first weighing 10 mg thiram, dissolved in 10 mL methanol and diluted into solutions with sixteen different concentrations. Mixed solutions were prepared by mixing three standard solutions at different concentrations.

For semi-quantitative analysis of the three pesticides, SERS spectra of different concentrations of standard solutions were acquired with the following parameter setting: laser power 1 mW, integration time 5 s. In order to evaluate the intra-batch precision of this method, AgNPs-PDMS substrates fabricated in the same batch were used to record SERS spectra of 1000 μg L−<sup>1</sup> thiram, malathion, and phoxim, respectively. Six SERS spectra were collected for each pesticide, and the relative standard deviation (RSD) of SERS intensity was calculated as the intra-batch precision. For inter-batch precision, six batches of AgNPs-PDMS substrates were prepared, then SERS spectra were recorded for each pesticide at the concentration of 1000 μg L−1. The inter-batch precision was determined by calculation of the RSD of SERS signals generated from different batches of AgNPs-PDMS substrates.

### *2.7. Detection of Three Pesticides on Orange Surfaces*

Thiram, malathion, and phoxim were tested on the surfaces of oranges from a local market to examine the performance of the AgNPs-PDMS substrate. Bought oranges were washed with water to clean the surface. Then, 10 μL mixed solution of pesticides at different concentrations (100 μg L−<sup>1</sup> for thiram, 500 μg L−<sup>1</sup> for malathion, 200 μg L−<sup>1</sup> for phoxim) was spiked on the orange peel. After solvent evaporation, the spot was sprayed with a small amount of ethanol, pressed, and gently swabbed by the AgNPs-PDMS film to transfer pesticide residues onto the substrate. The SERS signal of the swabbed substrate was acquired for qualitative and semi-quantitative analysis. The Raman parameter setting was as follows: excitation wavelength and scanning range remained the same as before, laser power 3 mW, integration time 10 s.
