Chemistry Reference
In-Depth Information
where I
0
and
ʻ
0
represent the asymptotes of the curve and
ʻ
max
is 2n
ʛ
cos(
θ
),
obeying Bragg
'
is law, in which n is the effective index of refraction, and
θ
is the
angle of
illumination from the normal. Additionally,
the displacement of
the asymptotes regarding the
first order could be attributed to other factors such as
the scattering strength of each Ag
0
NP, which increased at Mie plasmon resonances
in the blue/green region, hence the total amount of scattering decreased as the Bragg
resonance shifted to longer wavelengths. The Bragg peak was
565 nm for
*
glucose-free arti
cial urine, and additions of up to 10.0 mM glucose shifted this
peak systematically by 21, 81, 356, 379 and 420 nm, at pH values of 7.00, 7.25,
7.40, 7.75 and 8.00 (Fig.
5.10
b); with the limit of detections of 0.61, 0.50, 0.41,
0.36, 0.26 mM, respectively. The limit of detection represents three times average
standard deviation divided by the slope. The diffraction exhibited green, yellow,
orange and red light before moving into the near-infrared region with further
increases in glucose concentration (inset in Fig.
5.10
b). Therefore, at an apparent
pK
a
of 8.5, the glucose bound with the tetrahedral form with degrees of ionisation
of 0.4, 3.2, 4.9, 17.8 and 25.7 % at pH values of 7.00, 7.25, 7.40, 7.75 and 8.00 and
subsequently reached equilibrium. At low concentrations, glucose bound with the
boronic acid groups in a tetrahedral coordination form, in which the binding
transformed, although kinetically slower, to a trigonal planar form at higher
concentrations [
46
]. A decrease in the slope was consequently observed at the
higher concentration range. Another explanation for the change in the slope was
that as the hydrogel matrix expanded, the rate of swelling slowed down due to a
decrease in the elasticity. The potential clinical utility of the sensor in detecting
hypoglycosuria associated with urinary tract infections was tested by quantifying
glucose concentrations below 1.0 mM. When bacteria are present in urine, they
metabolise existing glucose, decreasing its concentration below 1.0 mM [
47
].
Therefore, measuring low concentrations of urine glucose can be used as a surro-
gate for rapid screening of urinary tract infections [
33
]. The detection and early
treatment of urinary tract infections may reduce the risk of chronic kidney failure
due to renal scarring [
48
,
49
]. Figure
5.10
c shows quanti
cation of the glucose
concentration from 0.0 to 1.0 mM in arti
cial urine solutions (pH 7.40) by the
systematic shift of the Bragg peak to longer wavelengths, displaying a typical
narrow-band spectral readout. Figure
5.10
d illustrates quanti
cation of the glucose
concentration from 0.0 to 1.0 mM in arti
cial urine solutions at different pH values
(see inset in Fig.
5.10
d for colorimetric response). A Bragg peak shift from 3 to
120 nm with a systematic increase in pH from 7.00 to 8.00 provided concentration
detection limits ranging from 240 to 90
μ
M, respectively.
5.5.3 Lactate and Fructose Interference
The cis diol groups of 3-APB can competitively bind to lactate and fructose. Lactate
is present at low concentrations in urine 0.00
0.25 mmol/L [
50
] and it may increase
-
during physical activity. Through its
ʱ
-hydroxy acid, lactate can competitively bind
