Hydrogen tetrachloroaurate 3 hydrate

Hydrogen tetrachloroaurate(III) hydrate (HAuCl₄∙xH₂O, 99.9%), sodium citrate, AgNO₃, sodium borohydride (NaBH₄), L-ascorbic acid, 2NT, and cysteamine hydrochloride were purchased from Sigma-Aldrich. Amphiphilic diblock copolymer PSPAA (PS₁₅₄-b-PAA₄₉, M_n = 16000 for the PS block and M_n = 3500 for the PAA block, M_w/M_n = 1.15) was obtained from Polymer Source. Milli-Q water (resistance > 18.2 MΩ·cm⁻¹) was used throughout the whole experiment.

Hydrogen tetrachloroaurate(III) hydrate, sodium borohydride, 4,4-bis(4-hydroxyphenyl)valeric acid (BPA-valeric acid), N-hydroxysuccinimide (NHS) and N,N′-dicyclohexylcarbodiimide (DCC), thimerosal, dimethyl sulfoxide (DMSO), bovine serum albumin (BSA), biotin, avidin-horseradish peroxidase (HRP), 3,3′,5,5′-tetramethylbenzidine (TMB), and Tween-20 were purchased from Sigma-Aldrich (Sydney, Australia). Absolute ethanol (EtOH), methanol (MeOH), dimethylformamide (DMF), tri-sodium citrate, sulphuric acid, hydrochloric acid, nitric acid, and hydrogen peroxide were obtained from Ajax Finechem (Sydney, Australia). Soybean protein (SBP) was purchased from Nature’s Way (Sydney, Australia). For the preparation of buffers, chemicals used were sourced from either BDH Chemicals (Melbourne, Australia) or Ajax Finechem (Sydney, Australia). Maxisorp polystyrene 96-well plates were obtained from Nunc (Roskilde, Denmark). Deionized water was prepared from a Millipore Milli-Q Academic system (18.2 MΩ·cm). Reverse osmosis (RO) water (17.6 MΩ·cm) of 99.0% purity was sourced from Sartorius Arium 61316/611VF. The preparation of cysteamine-BPA-valerate (cysBPAv), AuNP, the rabbit anti-BPA antibody (Ab-BPA-V2#4), and the conjugation of an antibody and biotin (Ab-BPA-V2#4-biotin) were described in the previous publications [12 (link),17 (link),18 (link)].

Hydrogen
tetrachloroaurate (III) hydrate
(HAuCl₄·3H₂O) and 2-mercaptoethanol (2-ME)
were purchased from Sigma-Aldrich. Sodium chloride and trisodium citrate
dihydrate were purchased from Sinopharm Chemical Reagent Co., Ltd.
Phosphate-buffered saline (10 × PBS) and thiazole orange (TO)
were obtained from Solarbio Science & Technology Co., Ltd. Deionized
(DI) water and oligonucleotides were purchased from Sangon Biotech.
The sequences and modifications of the DNAs are shown in Table 1. All chemicals were
used as received without further purification, and DI water was applied
in all of the procedures in experiments.

Gold nanoparticles were synthesized using the chemical reduction method with dextran acting as the stabilizing agent. In short, 5 g of Dextran DEAE (Sigma) was dissolved in distilled water (100 mL) to prepare 5% (w/v) to be used as a stabilizing solution. The solution was heated until boiling, and 50 mL of hydrogen tetrachloroaurate (III) hydrate (Sigma, 291566458) stock solution (0.5 g/mL) was added thereafter. The reaction mixture was boiled for 20 min until the colour of the mixtures turned deep-violet. The reaction mixture was then cooled to room temperature. Finally, Au/dextran nanocomposites were filtered and redispersed in 10 mL distilled water and stored at 4 °C. UV-Vis spectroscopy was performed to confirm the presence of the nanocomposites by obtaining the absorbance spectra.