TG–DTA spectra of the silica–urease composites are shown in
Fig. 3. TG curves of silica–urease composites show obvious weight
200 150 100 50 0
ppm
50 0 -50 -100 -150
ppm
B
A
Q2:Q3:Q4 =
2.6:20.5:76.9
Q4
Q3
Q2
Urease
-SiO-CH2-CH3
Fig. 4. Solid state 13C CP MAS NMR (A) and 29Si MAS NMR (B) for the silica particles
synthesized under the reaction conditions of 50 mM urea at 25 ◦C. From the peak
area of 29Si MAS, relative intensities of Qn groups were estimated.
loss stages. The weight loss between 500 and 600 ◦C was assigned to
the decomposition of urease protein. A major weight loss (∼93 wt%)
is observed from the composite prepared from 5 mM urea at 25 ◦C
condition (Fig. 3A), indicating that urease was trapped with quite
a small amount of silica. The weight loss percentage of the composite
produced from 20 mM urea at 25 ◦C (Fig. 3B) is
∼55%, which
agrees closely to the value (approximately 40–50%) corresponding
to the weight of urease inside the silica. The lower weight loss
percentage (below 10%) was obtained for the composite produced
from 50 mM urea at 25 ◦C (Fig. 3C), and agreed with the calculated
value for urease entrapped in silica materials (10 mg urease/110 mg
total mass of silica). From these results, we observed that the initial
weight of urease (10 mg) was trapped inside the silica gel networks
regardless of the concentration of urea used as an enzyme substrate