Fiber-Optic pH Sensors Fabrication based on Selective
Deposition of Neutral Red
Carlos R. Zamarreño, Miguel Hernáez, Ignacio R. Matías, Francisco J. Arregui
Electric and Electronic Engineering Department
Public University of Navarre
31006, Pamplona, Spain
Email: carlos.ruiz@unavarra.es
Abstract— In this work, a novel application of the electric field
directed layer-by-layer self assembly (EFDLA) selective
deposition method for the fabrication of optical fiber pH sensors
is presented. Here, indium tin oxide (ITO) coated optical fibers
have been fabricated via a dip-coating deposition method. These
fibers are used as electrodes in the EFDLA protocol in order to
deposit selectively the sensitive layer. Neutral Red (NR)
colorimetric pH sensitive indicator and the polymers
poly(acrylic acid) (PAA) and poly(allylamine hydrochloride)
(PAH) are used in order to obtain a pH sensitive nanostructured
coating onto ITO coated optical fibers. The results obtained in
this work revealed that the LbL material adsorption on the
electrodes can be enhanced or even inhibited when applying a
specific direct current voltage between them under some other
specific fabrication parameters. Particularly, the response of
these sensors to variations of the pH in the surrounding medium
was studied when the pH of the solutions used for the
fabrication of the films was adjusted to 7.0 and the potential
applied between electrodes was set to 2.5 V. These sensors
showed fast response time and high repeatability.
I.
INTRODUCTION
The fabrication of ordered structures at the nanoscale level
has attracted the attention of many authors in the last decades
[1-3]. Among all the techniques used, Layer-by-Layer (LbL)
electrostatic self-assembly method has emerged as a very
simple and cost-effective method to create durable, highly
reproducible and molecularly ordered ultrathin multilayer
structures. This method consists on the consecutively
adsorption of oppositely charged molecules onto a previously
prepared substrate. The main governing parameters of this
fabrication method are well known and have been extensively
studied in literature [4–6]. As a consequence, LbL has
motivated numerous applications in many diverse areas such
as biological sensors, organo-electronic devices or
hydrophobic surfaces allowing the fabrication of multipurpose
structures [7–13].
Recently, a deposition technique based on LbL and
electrophoresis was described by Gao et al. [14]. This new
method called Electric Field Directed Layer-by-Layer
Assembly (EFDLA) consists on the application of an electric
field during the LbL process. Thus, LbL electrostatically
driven adsorption process can be enhanced or even inhibited
by means of the application of an electric field during the
adsorption processes varying the resultant multilayer
structures. Therefore, the amount of material adsorbed on the
substrate is controlled by the direct current (DC) voltage
applied between the electrodes. Additionally, this method
allows the selective deposition of materials onto the electrodes
by the appropriate adjustment of the fabrication parameters
[14-18].
Optical fiber sensors have been widely fabricated using the
Layer-by-Layer traditional method. However, the application
of the recently developed EFDLA technique to optical fibers
represents a new issue to the current state of the art with the
aim to obtain selectively deposited materials as well as the
possibility to enhance or even reduce the adsorption process of
diverse materials over optical fiber electrodes. The fabrication
of a transparent, conductive and homogeneous indium tin
oxide (ITO) film [19] onto the optical fibers core makes
possible the application of the EFDLA method to optical
fibers and the following application as an optical fiber
evanescent field sensor which represents an important
progress in this field.
This work is centered on the fabrication of optical fiber pH
sensors by the EFDLA method. The deposition conditions are
adjusted in order to obtain selective deposition between the
optical fiber electrodes. Additionally, the response of the
fabricated sensors was measured for changes in the pH of the
surrounding medium.
II.
EXPERIMENTAL
A.
Materials
All the chemicals used were obtained from Sigma-Aldrich
and used without any further purification. The materials used
in this study are represented in Fig. 1, composed basically by
Neutral Red as the pH sensitive molecule [12-13] and the
polymers poly(allylamine hydrochloride) (PAH) and
The authors acknowledge financial support to the Spanish Ministry of
Education and Science-FEDER TEC2006-12170.
978-1-4244-5335-1/09/$26.00 ©2009 IEEE
845
IEEE SENSORS 2009 Conference
poly(acrylic acid) (PAA) used as polycation and polyanion
respectively.
O
ONa
H 3C
N
H 2N
N
H
• HCl
NH 2
PAH
n
n
Cl
+
N
-
CH3
CH 3
NEUTRAL RED
PAA
Figure 1. Molecular structure of the chemicals used.
+V Rinse
Step
Cationic
Solution
Figure 3. Optical measuring setup
- V+
Rinse
Step
Ultrapure
Anionic
water
Solution
Repeat
III.
Ultrapure
water
Figure 2. EFDLA fabrication process protocol
B. Fabrication process
Firstly, we utilize a modified dip-coating technique to
create a homogenous thin conductive transparent film of
Indium Tin Oxide (ITO) layer onto a short segment of a silica
fiber, 200 microns of diameter and around 2 cm in length [19].
This coated fiber acts as an optical fiber core where the core is
the original silica fiber and the cladding is the ITO layer.
Later on, these fibers served both as electric field sources and
substrates. EFDLA fabrication process was used in order to
deposit selectively the NR colorimetric pH indicator. The
EFDLA deposition process consisted on the application of a
DC voltage between electrodes during the LbL deposition
method. To favor the deposition on the same electrode during
the fabrication process, the polarity of the electrodes was
reversed at the same time that the solutions were changed.
The electrode where the positive potential is first applied is
denoted as positive electrode while the electrode where
negative potential is first applied is denoted as negative
electrode.
The final structure was composed by the sequential
adsorption of the polyelectrolytes up to 20 bilayers with an
immersion time of 2 min. in the cationic and anionic solutions
respectively followed by a rinse in ultrapure water between
each one as it is schematically represented in Fig. 2. The pH
of the cationic and anionic solutions was adjusted to 7.0 and
the potential between electrodes was chosen to 2.5 V after
several probes.
C. Measuring setup
All the optical measurements were carried out at room
conditions (25ºC). A typical reflection-type setup was
disposed in order to measure the reflected optical power from
the sensor tips as it is depicted in Fig. 3.
RESULTS AND DISCUSSION
ITO coated optical fibers served as electrodes in order to
obtain the NR-based sensitive coating with the aid of the
EFDLA selective deposition method. The resulting sensitive
layer surrounds the previously deposited ITO film. In Fig. 4 is
shown a cross-section of the optical fiber tip after the
fabrication process, where it can be appreciated an inner ITO
layer of 100 nm approximately and an outer coating of around
150 nm which is basically composed of NR and other
polymer. Hence, the potential applied between the ITO coated
optical fiber electrodes allows the fabrication of the multilayer
structure onto the fiber, which would not be possible by using
the LbL method [13]. Additionally, EFDLA technique enables
the selective deposition of the NR molecule onto the positive
electrode as explained in previous works [15]. In the next
paragraphs we will only refer to the multilayered structures
fabricated onto the positive electrodes.
SENSITIVE COATING
ITO COATING
100nm
1μm
Figure 4. SEM micrographs of the optical fiber tips.
The sensing properties of these nanostructured devices
were monitored by integrating the reflected power from the
fiber between 475 nm and 525 nm when the sensor tips were
immersed in different pH buffer solutions. Firstly, the
dynamical response of the sensors was obtained when the pH
of the solutions was sequentially varied from pH 3 to pH 7
consecutively. These results are represented in Figure 5,
showing a fast response time (below 30 seconds). These
sensors showed an almost negligible drift after several cycles
in the range studied, which suggests a high robustness of the
films.
846
Absorbance (arb. units)
[2]
5
pH 3
[3]
4.5
[4]
[5]
4
pH 7
[6]
3.5
0
5
10
15
20
25
30
35
40
45
Time (min)
[7]
Absorbance (arb. units)
Figure 5. Dynamical response of the sensors when they are immersed in
different pH buffer solutions.
[8]
5
[9]
4.5
[10]
4
[11]
[12]
3.5
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
7.5
pH units
[13]
Figure 6. Transfer function of the sensors between pH 3 and pH 7.
Additionally, the transfer function of the sensors was
obtained when the sensitive tips were immersed in three
different buffer solutions of pH 3, pH 5 and pH 7 for several
cycles as it is displayed in Fig 6.
IV.
[15]
CONCLUSION
A pH sensitive layer has been deposited selectively onto
ITO coated optical fibers. Moreover, the fabricated optical
fiber pH sensors showed fast response times, good
repeatability and high durability.
Finally, the results obtained showed the feasibility of using
this approach for the fabrication of different kinds of sensing
coatings. To our knowledge this is the first time that the
EFDLA technique has been used for the fabrication of optical
fiber sensors.
[16]
[17]
[18]
[19]
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