*2.1. Materials*

All the reagents were of analytical grade. Acetonitrile (ACN) HPLC grade was supplied by Prolabo (Fontenay-sous-Bois, France). Ethanol, acetone, and DPA were purchased from Scharlau (Barcelona, Spain). Stock standard solution of DPA (10 μg/mL) was prepared by dissolving an adequate amount of DPA in acetonitrile. Working solutions of this compound were prepared by dilution of the stock solution with water. Ultrapure water was obtained from a Nanopure II system (Sybron, Barnstead, UK). All solutions were stored in the dark at 4 ◦C.

Cotton swabs (100% cotton; 0.03 g of the amount of cotton on each tip) from a local market, double-sided carbon adhesive tape (8 mm wide × 0.16 mm thick × 1 cm long; Ted Pella Inc. Redding, CA, US), and tape lift kits (Adhesive Lifts GRA 200, Sirchie Finger Print Laboratories, Youngsville, NC, USA) were employed as sample collectors. Polydimethylsiloxane (PDMS) Sylgard® 184 Silicone Elastomer Kit containing Sylgard® 184 silicone elastomer base and Sylgard® 184 silicone elastomer curing agent, provided by Dow Corning (Midland, MI, USA) and tetraethyl orthosilicate (TEOS) purchased from Sigma–Aldrich (St. Louis, MO, USA), PDMS, and TEOS were used to prepare several samplers. Polydimethylsiloxane base was mixed with TEOS under vigorous magnetic stirring for 10 min at room temperature. Then, a PDMS curing agen<sup>t</sup> was added with a weight ratio of 1:10 to the PDMS base under magnetic stirring for 10 min at room temperature. Finally, 0.02 g of that blend was deposited on well-plates, and then was cured at 30 ◦C for hours or a day, depending on the film composition (as TEOS increases, curing time increases too). Several weight ratios of PDMS/TEOS were tested (100/0, 50/50, 30/70). The thickness of the film was 1 mm and the diameter was 15 mm.

#### *2.2. Apparatus and Chromatographic Conditions*

The capillary chromatographic system used consisted of a capillary liquid chromatography pump (Agilent 1100 Series, Waldbronn, Germany), a high-pressure six-port valve (7725 Reodhyne, Rohnert Park, CA, USA), an on-line degasser, and an UV-Vis photodiode array detector (Agilent, 1260 Series) equipped with an 80-nL flow cell. The detector was linked to a data system (Agilent, HPLC ChemStation) for data acquisition and calculation. The absorption spectra were recorded between 190 and 400 nm and the chromatograms were monitored at 280 nm. A Zorbax SB-C18 capillary analytical column (150 mm × 0.5 mm i.d., 5 μm particle diameter) was employed for the chromatographic separation (Agilent, Waldbronn, Germany). The mobile phase used was a mixture of acetonitrile:water in gradient elution mode: the initial acetonitrile content was 70% during 1 min, increased to 100% until 12 min, and maintained at 16 min, and then from 16 min to 20 min at 70% acetonitrile. The mobile phase flow rate was 10 μL min−1. All solutions were filtered with 0.45-μm nylon membranes (Teknokroma, Barcelona, Spain) before use.

An ultrasonic bath (300 W, 40 kHz, Sonitech, Guarnizo, Spain) and a ZX3 vortex mixer (40 Hz) from VELP Scientifica (Usmate Velate, Italy) were employed for the lixiviation of the DPA from the sample collectors. An optical microscope (ECLIPSE E200LED MV Series, Nikon Corporation, Tokyo, Japan) under bright-field illumination and using a 10× objective was used to see the collection of inorganic particles on the cotton swab. Nis Elements 4.20.02 software (Nikon Corporation) was used for acquiring the images. In order to test the presence and morphology of IGSRs, scanning electron microscopy (SEM) images were obtained with Hitachi S-4800 FEG (Tokyo, Japan) and Philips XL30 operating at 20 Kv for tape lift kit and cotton swab samples. Au/Pd coating was required. Elemental analysis was performed by an EDX analysis system incorporated into the microscope.
