2.1. Reagents and Materials

Bovine serum albumin (BSA), thrombin, immunoglobin G (IgG), hCG, KH₂PO₄, and K₂HPO₄ were purchased from Sigma-Aldrich (Shanghai, China). Recombinant human erythropoietin (rHuEPO) was provided by BioVision, Inc. (Milpitas, CA, USA). Beta-subunit of hCG (β-hCG) was obtained from YuduoBio Co., Ltd. (Shanghai, China). Single layer GO with oxygen content of 35%–40% was provided by Nanjing XFNANO Materials Tech Co., Ltd. (Nanjing, China). The Atomic Force Microscope (AFM) image shows that the thickness of GO is 0.8–1.2 nm and the lateral size is 0.5–5 μm (Figure 1A). The peptide probe labeled with fluorescein isothiocyanate (FITC) (a well-known fluorescent reporter for design of GO-based fluorescent chem/bio-sensors) in the N-terminal (FITC-PPLRINRHILTR) was obtained from Synpeptide Co., Ltd. (Shanghai, China). The serum sample containing 16.14 mIU/mL follicle-stimulating hormone (FSH), 11.82 mIU/mL luteotropic hormone (LH) and 3.41 mIU/mL thyroid stimulating hormone (TSH) from one woman donor (35 years old) and the urine samples from one woman donor (33 years old) were provided by Anyang Maternal and Child Heath Care Hospital (Anyang, China) and were stored at −18 °C for use. Amino acids used in this work were provided by Sangon Biotech. Co., Ltd. (Shanghai, China). The peptide at the concentration of 1 mM was dissolved with deionized-water and diluted to the desired concentration with a phosphate-buffered saline solution (PBS buffer, 20 mM, pH 7.2).

(A) AFM image and lateral size of GO; (B) Fluorescence spectra of 50 nM FITC-PPLRINRHILTR in the presence of different concentrations of GO; (C) Fluorescence intensity of FITC-PPLRINRHILTR versus the concentration of GO.