In this paper we describe a wireless passive pH sensor for high-resolution remote pH monitoring. The sensor is based on a passive LC coil resonator whose resonant frequency is monitored remotely by measuring the change in impedance of an interrogator coil coupled to the sensor coil. The sensor resonator consists of an inductive coil connected in parallel with a voltage dependent capacitor and a pH combination electrode. Change in the electrode potential in response to variations of the pH of the solution changes the capacitance, and therefore the resonant frequency of the sensor. A linear response with a 0.1 pH resolution is achieved over a 4–10 pH dynamic range. The response time of the sensor is demonstrated to be less than 30 s and is limited by the response time of the pH combination electrode. The described sensor technology is suitable for long-term remote pH monitoring in numerous fields such as biomedical sensing, environmental monitoring, industrial and chemical processing, and structural health monitoring.

1.
Engels
,
J. M. L.
, and
Kuypers
,
M. H.
, 1983, “
Medical Application of Silicon Sensors
,”
J. Phys. E
0022-3735,
16
(
10
), pp.
987
994
.
2.
Grant
,
S. A.
, and
Glass
,
R. S.
, 1997, “
A Sol-Gel Based Fiber Optic Sensor for Local Blood pH Measurements
,”
Sens. Actuators B
0925-4005,
45
(
1
), pp.
35
42
.
3.
Wolthuis
,
R.
,
McCrae
,
D.
,
Saaski
,
E.
,
Hartl
,
J.
, and
Mitchell
,
G.
, 1992, “
Development of Medical Fiber-Optic pH Sensor Based on Optical Absorption
,”
IEEE Trans. Biomed. Eng.
0018-9294,
39
(
5
), pp.
531
537
.
4.
Papeschi
,
G.
,
Bordi
,
S.
,
Carlagrave
,
M.
,
Criscione
,
L.
, and
Ledda
,
F.
, 1981, “
An Iridium-Iridium Oxide Electrode for In Vivo Monitoring of Blood pH Changes
,”
J. Med. Eng. Technol.
0309-1902,
5
(
2
), pp.
86
87
.
5.
Soller
,
B. R.
,
Cingo
,
N.
, and
Khan
,
T.
, 2002, “
Fiber Optic Sensing of Tissue pH to Assess Low Blood Flow States
,”
Proceedings of IEEE Sensors 2002
, Vol.
1
, pp.
266
269
.
6.
Ativanichayaphong
,
T.
,
Wang
,
J.
,
Huang
,
W. -D.
,
Rao
,
S.
,
Tibbals
,
H. F.
,
Tang
,
S. -J.
,
Spechler
,
S. J.
,
Stephanou
,
H.
, and
Chiao
,
J. -C.
, 2007, “
Development of an Implanted RFID Impedance Sensor for Detecting Gastroesophageal Reflux
,”
Proceedings of the IEEE International Conference on RFID 2007
, pp.
127
133
.
7.
Ativanichayaphong
,
T.
,
Tang
,
S. -J.
,
Hsu
,
L. -C.
,
Huang
,
W. -D.
,
Seo
,
Y. -S.
,
Tibbals
,
H. F.
,
Spechler
,
S.
, and
Chiao
,
J. -C.
, 2010, “
An Implantable Batteryless Wireless Impedance Sensor for Gastroesophageal Reflux Diagnosis
,”
Proceedings of the IEEE 2010 MTT-S International Microwave Symposium Digest
, pp.
608
611
.
8.
Gill
,
E. I.
,
Arshak
,
A.
,
Arshak
,
K.
, and
Korostynska
,
O.
, 2009, “
Investigation of Thick-Film Polyaniline-Based Conductimetric pH Sensors for Medical Applications
,”
IEEE Sens. J.
1530-437X,
9
(
5
), pp.
555
562
.
9.
Sridhar
,
V.
, and
Takahata
,
K.
, 2009, “
A Hydrogel-Based Passive Wireless Sensor Using a Flex-Circuit Inductive Transducer
,”
Sens. Actuators, A
0924-4247,
155
(
1
), pp.
58
65
.
10.
Wilson
,
K.
, and
Walker
,
J.
, 2010,
Principles and Techniques of Biochemistry and Molecular Biology
,
7th ed.
,
Cambridge University Press
,
New York
.
11.
Jain
,
M. K.
,
Cai
,
Q.
, and
Grimes
,
C. A.
, 2001, “
A Wireless Micro-Sensor for Simultaneous Measurement of pH, Temperature, and Pressure
,”
Smart Mater. Struct.
0964-1726,
10
(
2
), pp.
347
353
.
12.
Lei
,
M.
,
Baldi
,
A.
,
Pan
,
T.
,
Gu
,
Y.
,
Siegel
,
R. A.
, and
Ziaie
,
B.
, 2004, “
A Hydrogel-Based Wireless Chemical Sensor
,”
Proceedings of the Micro Electro Mechanical Systems
, pp.
391
394
.
13.
Horton
,
B. E.
,
Pereles
,
B. D.
,
Ruan
,
C.
,
Tan
,
E. L.
, and
Ong
,
K. G.
, 2009, “
Wireless Passive pH Sensor Based on Magnetic Higher-Order Harmonic Fields
,”
Sens. Lett.
1546-198X,
7
(
4
), pp.
599
604
.
14.
Robinson
,
M. P.
, and
Clegg
,
J.
, 2005, “
Improved Determination of Q-Factor and Resonant Frequency by a Quadratic Curve Fitting Method
,”
IEEE Trans. Electromagn. Compat.
0018-9375,
47
(
2
), pp.
399
402
.
15.
Ong
,
J. B.
,
You
,
Z.
,
Mills-Beale
,
J.
,
Tan
,
E. L.
,
Pereles
,
B. D.
, and
Ong
,
K. G.
, 2008, “
A Wireless, Passive Embedded Sensor for Real Time Monitoring of Water Content in Civil Engineering Materials
,”
IEEE Sens. J.
1530-437X,
8
(
12
), pp.
2053
2058
.
16.
Tesche
,
F. M.
,
Ianoz
,
M. V.
, and
Karlsson
,
T.
, 1997,
EMC Analysis Methods and Computational Models
,
Wiley
,
New York
.
You do not currently have access to this content.