Polyethyleneimine-interlayered silica-core quantum dot-shell nanocomposites for sensitive detection of Salmonella typhimurium via a lateral flow immunoassay

Herein, we synthesized high-performance SiO2–core quantum dot (QD)–shell nanocomposites (SiO2@PEI-QDs) using the polyethyleneimine (PEI)-mediated adsorption method. Cationic PEI was used to form a positively charged interlayer on the SiO2 core, which achieved a dense adsorption of carboxylated QDs to form a shell of QDs and maintained a good dispersibility of the nanocomposite. The SiO2@PEI-QDs showed excellent stability and high luminescence, and served as high-performance fluorescent labels for the detection of bacteria when used with the lateral flow immunoassay (LFA) technique. An SiO2@PEI-QD-based LFA strip was successfully applied to rapidly detect Salmonella typhimurium in milk samples with a low limit of 5 × 102 cells per mL.


S1.2 Instruments
The TEM images and HR-TEM images of SiO 2 @PEI-QDs were recorded on a Hitachi H-7650 microscope at an accelerating voltage of 80 kV and a Philips Tecnai G2 F20 microscope at an accelerating voltage of 200 kV, respectively. Elemental mapping images were recorded by energy-dispersive X-ray spectroscopy (EDS) using a Philips Tecnai G2 F20 microscope equipped with a STEM unit. The zeta potentials of the synthesized nanocomposites were characterized using Nano-ZS90 Zeta Sizer.
Emission spectra of QD-based fluorescent LFA strip were recorded by using a FIC-S1 fluorescent strip reader (Suzhou Hemai, China).
Electronic Supplementary Material (ESI) for RSC Advances. This journal is © The Royal Society of Chemistry 2020

S1.3 Bacteria sample preparation
The standard strain of S. typhi, E. coli, S. aureus, P. aeruginosa, L. mono, and A. baumannii were provided by Beijing Institute of Radiation Medicine. Conventional plate counting method was used to determine the concentrations of bacteria. In brief, bacterial strain was cultivated in 5% sheep blood agar plates at 37 °C in an atmosphere containing 5% CO 2 . After 12 h culture, dozens of colonies were picked from the plate, and transferred into 1 mL of PBS solution (10 mM, pH 7.4) as initial bacterial solution. Then, 0.2 mL of bacterial solution was diluted in sterile water for 1 × 10 5 times, and coated on the blood agar plate at 37 °C. After overnight culture, the number of colony-forming units (CFUs) on the plate was counted. According to the result of CFU counting, the initial bacterial solution was adjusted to concentrations of 10 7 cells/mL for follow-up test.

S1.4 Preparation of SiO 2 @PEI-QDs
The monodispersed SiO 2 @PEI-QDs with a core-shell nanostructure were fabricated via a simple three-step reaction. First, 150 nm SiO 2 NPs were prepared according to a modified method. 1 In brief, 4 mL of ammonia solution (28%) and 6 mL of deionized water were added to 100 mL ethanol under magnetic stirring (600 rpm/min). Then, 3.7 mL of TEOS was added into the above mixture, and the reaction was kept at room temperature for 4h. Finally, the resulting SiO 2 NPs were centrifuged and redispersed in 20 mL of ethanol.
Second, the SiO 2 @PEI NPs were prepared via a PEI self-assembly process under sonication. 2 In brief, 0.25g PEI were dissolved in 100 mL of deionized water to prepare PEI solution. Then, 0.2 mL of prepared SiO 2 NPs (10 mg/mL) was dispersed in the PEI solution under sonication for 30 min, during which PEI gradually selfassembled on the silica cores. The resulting SiO 2 @PEI NPs were separated by centrifugation at 5500 rpm for 6 min. The SiO 2 @PEI NPs were washed for twice with deionized water and resuspended in 10 mL of deionized water for future us.
Third, the prepared SiO 2 @PEI NPs were added into 100 mL carboxylfunctionalized CdSe/ZnS QD solution (1 nM) and sonicated for 30 min. The process was repeated two times to ensure the full absorption of QDs on the SiO 2 @PEI surface.
Finally, the resulting SiO 2 @PEI-QDs were separated and dispersed in 10 mL ethanol.

S1.6 Preparation of fluorescent LFA strips
The SiO 2 @PEI-QD-based fluorescent LFA strip comprised an absorbent pad, an NC membrane with 15 mm pore size (CN95), a conjugate pad with SiO 2 @PEI-QD nanotags, and a sample loading pad (Scheme 1b). 0.6 mg/mL of anti-S. typhi (Catalog #ab8274) and polyclonal goat anti-mouse IgG antibody diluted with 10 mM PBS were spotted onto the NC membrane as the test and control lines, respectively, with a dispense rate of 0.1 μL/mm via a spraying platform (Biodot xyz5050). The asprepared NC membrane was dried in a constant temperature oven at 37 °C for 2 h.
Subsequently, the sample pad, conjugate pad, NC membrane, and absorbent pad were assembled onto a plastic backing card. Finally, the prepared strips were fitted into plastic cassettes and stored with desiccants at room temperature until use.

S1.7 Sample testing on SiO 2 @PEI-QDs based LFA strips
The assay was carried at room temperature. The sample pad of a test strip was dipped into an aliquot of the sample (70 μL) for 2 min and then placed on a horizontal surface.
The result was checked within 10 min after sample application. The fluorescence signal on the test line was recorded using a fluorescent reader (365 nm excitation).
For each test strip, the process of fluorescence reading was performed three times and the fluorescence data were collected and averaged before use.        typhi at a concentration range of 10 4 -10 7 cells/mL.