A polymer based sensor for phosphate detection in water

Abstract

This paper presents the detection of phosphate (PO4-3) concentration in water by using a commercial quartz crystal microbalance (QCM) sensor that was coated with a novel polymeric affinity layer for the first time. The QCM sensor was exposed to PO4-3 and chloride (Cl-) solutions in various concentrations to investigate the sensitivity of the affinity layer. Changes in the viscosity of a solution, and weak binding of ions onto the polymeric layer eliminated the effect of PO4-3 binding to the polymer and hardened the concentration detection. Therefore, we have proposed a new method in which an intermediate Cl- solution is applied before introducing the PO4-3 solution to detect the concentration level in a continuous flow.

Full text

A polymer
b
ase
d
Faezeh Arab H
a
1,3
Dept. Civil and Structural
2
Dept. Materials Science and
1
f.a.hassani@sheffield.ac.uk
,
Abstract— This paper presents the dete
c
(PO
4
-3
) concentration in water b
y
usin
g
a
c
cr
y
stal microbalance (QCM) sensor that was
c
polymeric affinity layer for the first time. Th
e
exposed to PO
4
-3
and chloride (Cl
-
) sol
u
concentrations to investi
g
ate the sensitivit
y
o
f
Changes in the viscosit
y
of a solution, and w
e
onto the polymeric layer eliminated the effect
the
p
ol
y
mer and hardened the conce
n
Therefore, we have
p
ro
p
osed a new met
h
intermediate Cl
-
solution is a
pp
lied before in
t
solution to detect the concentration level in a c
o
Keywords—Quartz cr
y
stal microbalance
a
ff
init
y
la
y
er,
p
hos
p
hate detection, viscosit
y
, ma
s
I.
I
NTRODUCTION
Phosphorus is an essential nutrient for
p
that is available in rock and soil minerals, as
sewage and detergents [1]. The release
rivers’ water and lakes due to weathering
o
will results in the over enrichment of phos
p
This increase can cause massive growth
aquatic plants which leads to an eutrophicati
o
Wastewater discharge from sewage
contributes the largest proportion of the ph
[3]. For this reason, the presence of PO
4
-3
i
n
concentrations greater than 0.1 mg/L is no
t
European Water Framework Directive,
a
carefully monitored [4]. The currently availa
b
collecting water quality data from fields
s
samplers, portable and fixed site water qu
a
cumbersome and not accurate [5].
Cantilever-
b
ased and quartz crystal mi
c
sensors are the two mass-sensing options
detection, however, the damping of the
cantilever-
b
ased sensors in liquid made th
commercial use even with their high sensit
i
sensors are the best option for phosphate
d
and they are capable of rapid, sensitive,
a
detection of various types of biological and c
h
in liquid [8, 9]. A QCM sensor consists of a
with parallel circular electrodes
p
atterned
o
front side is in contact with the analyte
o
This work is financially supported by Innovate UK
Strategy Board), project number 36990-267249.
d
sensor for phosphate
d
water
a
ssani
1
, Nicola A Morley
2
, Maria Romero-Gonz
á
Engineering, The University of Sheffield, Sheffield,
U
Engineering, The University of Sheffield, Sheffield,
U
,
2
n.a.morley@sheffield.ac.uk,
3
m.e.romero-gonzalez
@
c
tion of
p
hos
p
hate
c
ommercial
q
uartz
c
oated with a novel
e
QCM sensor was
u
tions in various
f
the affinit
y
la
y
er.
e
ak binding of ions
of PO
4
-3
binding to
n
tration detection.
h
od in which an
t
roducin
g
the PO
4
-3
o
ntinuous flow.
sensor, polymeric
s
s bindin
g
p
lants and animals
well as fertilizers,
of phosphorus in
o
r waste pollution
p
hate ions (PO
4
-3
).
of plankton and
o
n process [2].
treatment plants
osphorus in rivers
n
river water with
t
permitted by the
a
nd needs to be
b
le instruments for
s
uch as automatic
a
lity monitors are
c
robalance (QCM)
[6] for phosphate
output signal of
em unsuitable for
i
vit
y
[6, 7]. QCM
d
etection in water,
a
nd cost effective
h
emical molecules
thin disk of quartz
o
n both sides. The
o
f interest and the
backside is exposed to air [10
]
applying a potential across t
h
shear force. The quartz crysta
l
standing waves are created [1
0
mass per unit area by meas
u
frequency, f
0
. The mass resp
o
calculated as following based
o
∆
∆




where Δm is the adsorbed
m
A
piezo
is the active sensing a
r
modulus and density of quartz,
The above equation is co
r
onto the QCM sensor, but in
such as polymer and
p
erformi
n
changes in the energy dissipati
o
violate the linear relation betw
following equation should be
c
viscosity,
η
l
, and liquid density
∆  

.

The schematic of the QCM
affinity layer is shown in Fig. 1
II. M
E
T
We aim to provide detectio
n
by using a polymeric affinity
sensor. The commercial gold
c
sensors, Biolin Scientific) [
experiments.
Fig. 1. The schematic of the added p
o
sensor.
(former Technology
d
etection in
á
lez
3
U
nited Kingdom
U
nited Kingdo
m
@
sheffield.ac.uk
]
. The QCM sensor operates by
h
e electrodes and generating a
l
resonates as electromechanical
0
]. The QCM sensor measures a
u
ring the change in resonance
o
nsivity of the QCM sensor is
o
n the Sauerbrey equation [11]:






.



(1)
m
ass, Δf is changes in frequency,
r
ea, and 

and 

are shear
respectively.
r
rect for the added solid layers
the case of adding a soft layer
n
g a measurement in liquid, the
o
n of the oscillating system will
een Δf and Δm. In this case, the
c
onsidered to include the liquid
,
ρ
l
[11]:









(2)
sensor and the added polymeric
.
T
HODOLOGY
n
of PO
4
-3
concentration in water
layer spin coated onto a QCM
c
oated 5 MHz crystals (Q-Sense
12] have been used in our
o
lymeric affinity layer on top of a QCM
978-1-4799-8203-5/15/$31.00 ©2015 IEEE
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At the start of the experiment, the pol
y
sensor is exposed to ultra high quality (UHQ
)
the baseline. This is followed by an inj
e
solution, and this process is repeated for a
Other solutions including diluted hydroc
h
0.1M), Sodium Chloride (NaCl, 0.1M), and
(CaCl
2
, 0.05M)) have also been applied
t
p
olymer response to changes in solution ioni
c
The
p
olymeric layer has been spin coa
t
sensors at 3000 rpm for 20 sec. Prior to th
e
crystals were cleaned by sonication u
s
isopropyl alcohol followed by rinsing in
u
(UHQ) water, and nitrogen drying.
III. R
ESULTS AND DISCUSS
The measurement setup (Fig. 2) consi
s
constantly fed using solutions of PO
4
-3
o
r
different concentrations with the help of
a
Two flow rates of 1.9 and 3.9 mL/min hav
e
the experiments. A QCM instrument (eQ
C
Instruments) [13] was used for reading the
f
versus time.
A. Phosphate-water cyclic experiment
Fig. 3 shows the change in frequenc
y
response to the cyclic feeding of UHQ fo
l
solution at concentrations of 0.001 and 0.
1
The grey areas in the figures represent the p
e
was left ON with the flow rate of 1.9 mL/
m
room temperature and hydration of the
solution causes a decreasing slope through
o
for both phosphate concentrations. By t
h
phosphate solution, the Cl
-
functional grou
p
by PO
4
-3
with larger molar mass via an ion
The introduction of a 0.001 mg/L PO
4
-3
sol
u
water cycle results in negative changes
frequency due to the binding of PO
4
-3
to th
e
N
o slope changes were observed whe
n
performed using a 0.1 mg/L PO
4
-3
solution.
B.
I
ntroducing a new method for phosphate
continous flow
To confirm
the effect of the PO
4
-3
molec
u
the frequency increase solely, we introduce
d
solutions (i.e. HCl, NaCl, and CaCl
2
) keepi
n
Cl
-
ions constant. No slope changes were ex
p
Cl
-
solutions as the polymer surface is ter
m
ions.
Fig. 2. The measurement setup.
y
mer coated QCM
)
water to measure
e
ction of a PO
4
-3
number of cycles.
h
loric acid (HCl,
Calcium Chloride
t
o investigate the
c
strength.
t
ed onto the QCM
e
coating step, the
s
ing acetone and
u
ltra high quality
ION
s
ts of a flow cell
r
HCl in water at
a
peristaltic pump.
e
been used during
C
M 10M, Gamry
f
requency changes
y
of the sensor in
l
lowed by a PO
4
-3
1
mg/L over time.
e
riod that the pump
m
i
n
. Changes in the
polymer layer in
o
u
t
the experiment
h
e introduction of
p
s will be replaced
exchange process.
u
tion after a UHQ
in the resonance
e
polymer surface.
n
the cycle was
detection in a
u
les interaction on
d
different chloride
n
g the molarity of
p
ected by applying
m
inated by chloride
(
(
Fig. 3. Cyclic UHQ-PO
4-3
solution e
x
sensor for: (a) 0.001 mg/L, and (b) 0.1
m
Fig. 4 shows the response
o
solutions in cyclic UHQ-Cl
-
s
o
was left ON with the flow rat
e
intervals in Fig. 4. The positiv
e
≈150 Hz has been seen for all
t
owing to molecular interactio
n
the sensor that result in reduct
i
due to shear thinning in the int
e
p
ositive ions to the surface
o
p
resents the frequency chang
e
solutions in Fig. 4. As Δf/Δt
r
s
adding Cl
-
solution, Δt
r
p
resen
t
time of applying Cl
-
solution
a
to reduce. Similarly, Δf is the
fr
two points. Table I shows simi
solutions.
A cyclic experiment perf
o
sensor exposed to HCl soluti
o
positive frequency change o
b
polymer is reduced to ≈40 Hz
w
of the viscosity changes at the
happen on the sensor surface
w
A similar frequency chang
e
happened for the 0.1 mg/L
(
a)
(
b)
x
periment of the polymer coated QCM
m
g/L phosphate concentrations.
o
f the coated QCM sensor to Cl
-
o
lution experiments. The pump
e
of 1.9 mL/mi
n
during the grey
e
resonance frequency change of
t
he Cl
-
solutions. This change is
n
s in the solid-liquid interface of
i
on of the viscosity of the liquid
e
rface [14], and weak binding of
o
f the senso
r
[15, 16]. Table I
e
over time (Δf/Δt
r
) for all Cl
-
s
hows the maximum slope after
t
s the time interval between the
a
nd the time that the slope starts
fr
equency changes between these
lar level of sensitivity for all Cl
-
o
rmed using an uncoated QCM
o
n (Fig. 5), it showed that the
b
served in the presence of the
w
ith no polyme
r
. This is because
interface, as no binding should
w
hen there is no polymer coating.
e
for the HCl solution in Fig. 5
PO
4
-3
solution in Fig. 3. This
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frequency change in different direction has
effect of PO
4
-3
b
inding to the polymer
applying intermediate Cl
-
solutions to th
e
before introducing PO
4
-3
solution will impro
v
changes in frequency.
Fig. 6 shows the response of QCM sen
s
experiment using UHQ-HCl-PO
4
-3
solut
i
particular order. The pump was left ON wi
t
about 3.9 mL/min during the experiment.
mg/L PO
4
-3
solution results in the sensitivity
sensitivity has been calculated as explaine
d
required time for the polymer to be saturate
d
ions, Δt
s
, has been calculated from Fig. 6 to
b
sensor response in Fig. 6 demonstrates that
c
HCl solution previous to introducing the
essential to detect mass binding to the pol
y
sensor.
(a)
(b)
(c)
Fig. 4. The response of polymer coated QCM sens
o
solutions: (a) 0.1M HCl, (b) 0.1M NaCl, and (c) 0.05M
cancelled out the
layer. Therefore,
e
polymer surface
v
e the detection of
s
or to a full cyclic
i
o
n
fed in that
t
h the flow rate of
Injecting the 0.01
of Δf/Δt
r
=45. The
d
for Fig. 4. The
d
by the phosphate
b
e about 627 s. The
c
onditioning using
PO
4
-3
solution is
y
mer coated QCM
o
r to various chloride
CaCl
2
.
TABLE I. T
HE SENSIT IVITY
O
CHLORIDE
Solution
0.1M HCl
0.1M NaCl
0.05M CaCl
2
Fig. 5. Cyclic UHQ-HCl
experiment o
f
Fig. 6. The response of polymer c
o
solution experiment for 0.01 mg/L
p
ho
s
C
ONC
L
This paper presents a new
characterize the sensitivity o
f
concentrations of phosphate
p
olymeric affinity layer spu
n
introduction of intermediate
surface before introducing PO
4
-
of changes in frequency in th
e
This technique will be applie
d
b
ased sensor for sensing va
r
Increasing the sensitivity of th
e
while keeping the saturation
considered in our future work.
A
CKNOW
L
Authors would like to ex
p
preparing the polymer layer.
O
F THE POLYMER LAYER TO VARIOUS
SOLUTION S
Δf/Δt
r
13.45
9.81
2
10.63
f
the uncoated QCM sensor.
o
ated QCM sensor to UHQ-HCl-PO
4-3
s
phate concentrations.
L
USION
cyclic experiment technique to
f
the QCM sensor to different
in water by using a novel
n
coated onto the sensor. The
Cl
-
solutions to the polymer
-
3
solution will ease the detection
e
presence of a continuous flow.
d
to our future designed QCM-
r
ious pollu
t
ants in river water.
e
polymer layer to the pollutants
time as long as possible is
L
EDGMENT
p
ress thanks to Tom Turton for
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