4 nitrobenzoyl glycyl glycine

TNT solution dissolved in pure DMF (N,N-dimethylformamide) was obtained from Chugoku Kayaku, Co., Ltd., Kure, Japan. The concentration of the received TNT solution was 10.3 mg/mL and was freshly diluted as required with phosphate-buffered saline (PBS, 0.1 M, pH 7.4) containing 0.05% Tween 20 (T) and 5% DMF before each measurement. Five kinds of TNT analogues, research and development explosive (RDX) solution dissolved in pure DMF (10.3 mg/mL) were purchased from Chugoku Kayaku, Co., Ltd., Kure, Japan. 2,4-Dinitrophenyl glycine (DNP-glycine), 2,4-DNT, 2,6-DNT, and 4-nitrobenzoyl-glycyl-glycine were purchased from Tokyo Chemical Industry, Tokyo, Japan. 3-Aminopropyltriethoxysilane (APTES) 99% was purchased from Sigma-Aldrich, St. Louis, MO, USA. N-γ-maleimidobutyryl-oxysuccinimide ester (GMBS) was purchased from Thermo Fisher Scientific, Rockford, IL, USA. L-cysteine was purchased from Sigma-Aldrich. The SPR Au chip was purchased from Biacore, GE, Uppsala, Sweden. Sodium hydroxide solution (NaOH) and potassium hydroxide solution (KOH) were purchased from Wako Pure Chemical Industries, Tokyo, Japan. All other chemicals were purchased either from Tokyo Chemical Industry, Tokyo, Japan or Wako Pure Chemical Industries, Tokyo, Japan. All aqueous solutions were prepared by MilliQ deionized water (18 MΩ) from MilliQ-system (Millipore Corporation, Billerica, MA, USA).