4 n np

Spectra obtained from circular dichroism (CD) spectropolarimetry were used to observe the conformational change of the aptamer bound to 4-n-NP [32 (link),33 (link)]. The final aptamer concentration of 2 μM was formed using 6 μL of 100 μM aptamer (Bioneer Corp., Korea) dissolved into 264 μL of 0.1 M phosphate buffer saline buffer (PBS buffer, pH 7.4) containing 137 mM NaCl (Daejung, Korea), 2.7 mM KCl (Daejung), 10 mM Na₂HPO₄ (Junsei, Japan), and 2 mM KH₂PO₄ (Daejung). Then, 30 μL of ultrapure distilled water (DNase/RNase/Protease free, Thermo Fisher Scientific, Waltham, MA, USA) or 4-n-NP (Sigma-Aldrich, USA) were added to the solution to final concentrations of 0, 100, and 1000 nM 4-n-NP. Subsequently, the samples were incubated overnight at ambient temperature. The aptamer solution was transferred to a quartz cuvette with a 1-mm optical path length. The CD spectra were scanned from 350 to 220 nm at 25 °C using a Jasco J-1500 spectropolarimeter (Jasco Inc., Easton, MD, USA). The CD spectra were baseline (buffer only)-corrected and obtained from triplicate data accumulation.

Standards of 4-n-NP, BPA, BPF, MeP, PrP, ibuprofen, mefenamic acid, carbamazepine, diclofenac, allopurinol and paracetamol (all purity > 97%), NP technical grade (mixture of ring and chain isomers), N-(tert-butyldimethylsilyl)-N-methyl-trifluoroacetamide (MTBSTFA, purity ≥ 95%) with 1% tert-butyldimethylchlorosilane (TBDMCS) were acquired from Sigma-Aldrich (St. Louis, MO, USA). Ultrahigh purity water was obtained from a MilliQ water purification system (Millipore, Spain).
Acetonitrile (ACN), ethyl acetate (EtAc), and methanol (MeOH), for GC residue analysis, were supplied by Aldrich (Steinheim, Germany). Formic acid (purity ≥ 98%) was acquired from Acros Organics (Geel, Belgium). Ammonium hydroxide (NH₄OH, ≥32%) and primary secondary amine (PSA) bulk adsorbent were purchased from Scharlab (Barcelona, Spain).
Individual stock solutions (1 mg/mL) of each compound were prepared in ACN, except for diclofenac and paracetamol that were prepared in ACN:MeOH (90:10 v/v), allopurinol in dimethyl sulfoxide and mefenamic acid in EtAc. A working mixture solution at 1 μg/mL was prepared in ACN (except for carbamazepine and diclofenac that were at 4 μg/mL due to their low MS response). All the standard solutions were stored in amber vials at −20°C prior to use.