shows the 13C CP MAS NMR spectra for the silica–urease
composite prepared from 50 mM urea at 25 ◦C. Strong signals
at 16.3 and 58.0, assigned to the carbons in the Si-OCH2CH3
groups, were observed on analyzing the samples. This result confirms
the presence of the ethoxy groups inside the frameworks
of silica–urease composite. The peak at 58.0 ppm is attributed to
the carbon from the surface methylene groups in Si-OCH2CH3. The
methyl groups, which might be formed by esterificaition of surface
SiOH groups with ethanol derived from TEOS, in the silica
matrix could possibly be responsible for the resonance at 16.3 ppm.
The broad and small signal in the 60–80 ppm range arises from
the urease carbons, indicating that urease was encapsulated in
the sol–gel silica networks. The solid-state 29Si MAS NMR spectra
(Fig. 4B) of the silica–urease composite obtained by enzymatic
reactions shows peaks corresponding to the Q2, Q3, and Q4 silicons
(Qn = Si (OSi)n(OH)4−n). The Q2:Q3:Q4 ratio is calculated to be
2.6:20.5:76.9, suggesting that a number of Q2, ( SiO)2Si(OH)2 and
Q3, ( SiO)3SiOH sites are located on the surface of the composites.
In addition, enzymes can produce highly condensed silica particles.