N methylurea

Lithium cobalt oxide (LCO,
99.9%), N-methyl urea (NMU, 97%), acetamide (A, 99%),
urea (U, ACS reagent), N,N′-dimethylurea
(NN′DMU, 99%), N-methylacetamide (NMA, 99%), N-methyl-2-pyrrolidone (NMP), potassium nitrate (ACS reagent),
nitric acid, methanol (HPLC grade), and acetic acid (glacial) were
all purchased from Sigma-Aldrich and used without any purification
process unless otherwise mentioned. Gold-coated silicon wafers were
purchased from Sigma-Aldrich. NMU was recrystallized from methanol.
Ultrapure-grade water (resistivity 18.2 MΩ), purified using
the Milli-Q gradient water filtration system (Millipore), was used
for all purposes wherever necessary. Prior to usage, the Au thin films
were immersed in a piranha solution (3:1, H₂O₂ (30%)/H₂SO₄ (conc.) v/v) for 30 s, washed with 5 mL of water five times (fresh
water was replaced each time), dried flushed with N₂ and
immediately used for analysis.

Solvents used for extraction, analytical thin-layer chromatography (TLC), column chromatography (CC) and fast centrifugal partition chromatography (FCPC), such as diethyl ether (Et₂O), ethyl acetate (EtOAc), n-hexane (Hex), dichloromethane (DCM) and ethanol (EtOH) were of analytical grade and were purchased from VWR Chemicals (Prolabo^®, Fontenay-sous-Bois, France). Water (H₂O), used for chromatographic separations, was of ultrapure grade and was produced by Milli-Q water (1.8 MΩ) equipment (Merck^®, KGaA, Darmstadt, Germany). Acetonitrile (ACN) and methanol (MeOH), used for high-performance liquid chromatography (HPLC), and chloroform, used to determine optical rotation values, were of HPLC grade and were purchased from VWR^® (Madrid, Spain). Deuterated chloroform (CDCl₃) was used to prepare solutions of isolated compounds for nuclear magnetic resonance (NMR) analysis and were purchased from VWR^® (Madrid, Spain). Acetic acid (AcOH), used for HPLC, was purchased from VWR^® (Madrid, Spain). Silica gel 60 F240 precoated aluminum sheets were purchased from Merck^® (Darmstadt, Germany). N-methylurea, used to prepare N-methyl-N-nitrosourea (see Supplementary Material), was purchased from Sigma-Aldrich^® (Madrid, Spain). Sodium hydroxide was purchased from VWR Chemicals^® (Prolabo, Fontenay-sous-Bois, France).

The selectivity of the enzyme-functionalized hydrogel system was tested with thiourea (Gruessing, Filsum, Germany), N-methylurea (Sigma Aldrich) and N,N,N’,N’-tetramethylurea (Merck, Darmstadt, Germany), which are chemically similar species to urea. Therefore, PBS buffer solutions with 20 mmol/L of urea, thiourea, N-methylurea and N,N,N’,N’-tetramethylurea were produced. Now the swelling kinetics of the gels was recorded in dependence on the substrate solutions as described in Section 2.3.