Go aqueous solution

A 4 ​mg/mL GO aqueous solution (Sigma-Aldrich, 777676) was first diluted with deionized water to 300 ​μg/mL and placed in an oven at 80 ​°C for 3, 5, and 9 days. The solution was then dispersed using an ultrasonic probe sonicator and filtered using a 1.2 ​μm syringe filter to obtain the aGO solution. For the synthesis of aGO-COL composites, carboxyl groups of the aGO solution were activated through the EDC/NHS coupling reaction for 1 ​h; it was then mixed with type I collagen from rat tails (Thermo Fisher Scientific, A1048301) for 1 ​h. Three different aGO weight percentages in aGO-COL were used in the experiments: 16%, 32%, and 64% (w/w, aGO/COL).

GO aqueous solution (40%), MGO aqueous solution (40%), o-phenylenediamine (OPD), o-phthaldialdehyde (OPA), ethylenediaminetetracetic acid (EDTA), gallic acid, 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) diammonium salt, 2,4,6-tris(2-pyridyl)-s-triazine (TPTZ), aminoguanidine (AG) hydrochloride, glucose, L-serine, sodium azide, acetic acid and formic acid were purchased from Sigma-Aldrich (St. Louis, MO). Hydrochloric acid (HCl, 37%), methanol HPLC-grade, acetonitrile HPLC- and LC-MS-grades, ammonium formate LC-MS-grade, 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox) and potassium persulfate were obtained from Thermo Fisher Scientific (Waltham, MA). Sodium dihydrogen phosphate dihydrate (NaH₂PO₄·2H₂O), di-sodium hydrogen phosphate dihydrate (Na₂HPO₄·2H₂O), sodium acetate, Folin-Ciocalteu′s phenol reagent and iron (III) chloride hexaydrate (FeCl₃ 6H₂O) were acquired from Merck (Darmstadt, Germany). Analytical standards N-ε-carboxyethyllysine (CEL) and N-ε-carboxyethyllysine-d4 (CEL-d4), N-ε-carboxymethyllysine (CML), N-ε-carboxymethyllysine-d4 (CML-d4), methylglyoxal hydroimidazolone trifluoroacetic acid salt (MG-H1), methylglyoxal-hydroimidazolone-d3 (MG-H1-d3) and furosine were purchased from Iris Biotech GmbH (Marktredwitz, Germany).