Chemistry Reference
In-Depth Information
be possible to have a clear distinction between the measurements. By focusing on
the constant c
1
, the glucose concentration
can be correlated with a Bragg peak
shift. The constant c
1
is generally related to the complexion capacity of the cis diol
groups of 3-APB with glucose molecules.
When a Bragg peak shift in time (0
ʲ
5 min) is measured, the rate of Bragg peak
shift decreases as the time increases based on the decrease of cis diol groups in the
tetrahedral coordination form. As the time lapses, the binding mechanism trans-
forms from tetrahedral from to trigonal planar form, which is kinetically slower.
Therefore, the rate of change of bound molecules n(t) is proportional to the total
number of glucose Nf:
g
molecules and the total number of cis diol groups Nf:
f
:
-
N
f
¼
N
f
dn
ð
t
Þ
dt
/
N
g
nt
ðÞ
nt
ðÞ
aN
g
n
ð
t
Þ
n
ð
t
Þ
ð
5
11
Þ
:
When the Bragg peak shift is proportional to the amount of binding of cis diol
groups to glucose molecules, this expression can be reduced to:
C
f
D
k
¼
C
f
D
k
d
k
ðÞ
dt
/
t
C
g
D
k
ðÞ
t
ðÞ
t
aC
g
D
k
ðÞ
t
ðÞ
t
ð
5
12
Þ
:
where C
g
and C
f
are constants with proportionality to cis diol and glucose groups
present. The solution of this equation is nontrivial to be
fitted with the experimental
data; instead, the numerical solution of d
(t)/dt has been obtained from the data.
Thus, two values were plotted, and a quadratic solution was
ʻ
fitted to the equation to
obtain the variables Cf.
g
and C
f
. As the solution was commutable (changing the order
of the operands does not change the result), the minimum value was considered as C
g
.
Other factors such as a potential decrease in the elasticity of the polymer matrix might
in
uence the projected decrease of the Bragg peak shift. Figure
5.17
shows three
independent measurements with urine samples of diabetic patients at different glu-
cose concentrations. The increase in the slope of the
fl
fit can be correlated with the
concentration of carbohydrate (mainly glucose) in the urine samples.
5.7 Quanti
cation of Glucose Concentration in Urine
Anonymised urine samples (n = 33) were collected from diabetic patients attending
the Wolfson Diabetes and Endocrine Clinic (Addenbrooke
s Hospital, Cambridge,
UK) in February 2014 under the Human Tissue Act 2004 (c 30) of the UK. Urine
samples were frozen immediately for glucose, fructose and lactate testing. Samples
were thawed and kept at 4
'
C, and centrifuged (1 min, 10,000 rpm) to remove any
precipitates prior to testing. For holographic glucose sensor measurements, the
urine samples had a pH of 5.96
°
0.11 and it was adjusted to 7.40 by adding NaOH
(2.0 M) while monitoring the pH using an electrochemical pH meter. The mea-
surements of Bragg peak shifts from the holographic sensor allowed inferring the
±
