Multi-sensor system based on phase detection, an LED array, and luminophore-doped xerogels

Abstract

An optical multi-sensor system based on the excited-state lifetime of luminophore doped xerogels is described. The system is based on the novel concept of combining addressable surface-mount light-emitting diode (SM-LED) arrays with doped xerogel thin films; each SM-LED in the array is coated with a different xerogel-based sensor element. Each sensor element exhibits a unique response profile to the target analyte. A prototype system with a 2×2 LED array is presented, with three of the LEDs coated with xerogel-based sensing layers, each layer having a different sensitivity and response profile to oxygen (O₂).

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Multi-sensor system based on phase
detection, an LED array, and
luminophore-doped xerogels
V.P. Chodavarapu, R.M. Bukowski, S.J. Kim, A.H. Titus,
A.N. Cartwright and F.V. Bright
An optical multi-sensor system based on the excited-state lifetime of
luminophore doped xerogels is described. The system is based on the
novel concept of combining addressable surface-mount light-emitting
diode (SM-LED) arrays with doped xerogel thin films; each SM-LED
in the array is coated with a different xerogel-based sensor element.
Each sensor element exhibits a unique response profile to the target
analyte. A prototype system with a 2?2 LED array is presented, with
three of the LEDs coated with xerogel-based sensing layers, each layer
having a different sensitivity and response profile to oxygen (O2).
Introduction: Frequency-domain (phase angle) measurements provide
an attractive and practical approach for developing chemical sensors
for many environmental, medical and industrial applications [1, 2].
This detection modality yields sensors that are insensitive to various
operating parameters including luminophore photobleaching or wash-
out, variation in the excitation intensity, or light scattering [2]. In this
Letter, we demonstrate the use of an addressable LED array for an O2
sensitive multi-sensor system. This system provides three unique
response profiles for the same target analyte (i.e. diversified response).
The operating principle behind the O2 sensors is the quenching
of a luminescent molecule (tris(4,7-diphenyl-1,10-phenathroline)
ruthenium(II), ([Ru(dpp)3]2þ)) sequestered within a nanoporous
xerogel [3].
In the case where the luminescent molecules emit with a single
excited state lifetime (t0) in the absence of a quencher from within
the xerogel and all luminescing luminophores are equally accessible to
the quencher molecules, one can write [2]:
t0=t ¼ 1 þ t0kqQ
ð1Þ
which relates the excited state luminophore lifetime at any quencher
concentration (t) and t0 to the luminophore-quencher bimolecular
quenching constant (kq) and the quencher concentration (Q). When
luminophores are excited by sinusoidally modulated light the excited
state lifetime (t) is given by:
t ¼ ð1=2pf Þtany
ð2Þ
In this expression, f represents the linear modulation frequency (in Hz)
and y is the phase angle (in degrees) associated with the emission.
Thus, measurement of y provides a way to determine the quencher
concentration in a sample.
LIA / OSC
pre-AMP
PD
clear glass
chamber
outlet
inlet
optical filter
SMT LED
power supply
and
function
generator
Fig. 1 Block diagram of multi-sensor system
PD: photodetector; pre-AMP: preamplifier; LIA=OSC: lock-in-amplifier or
oscilloscope
Fig. 1 presents a simplified block diagram of the sensor system. The
individual LEDs (length¼1.6 mm, width¼800 mm) are arranged in a
2?2 array in a DIP package each of which can be accessed using an
external switch. Crosstalk is reduced by using epoxy between the
LEDs. The array consists of three blue LEDs (lpeak: 468 nm, Stanley
Electric Co., Inc.) and one amber LED (lpeak: 595 nm, Matsuhita
Electronics, Ltd.). The system also uses a silicon photodiode (Thorlabs,
Inc.) as the detector followed by a preamplifier. The output is recorded
by using a lock-in-amplifier, an oscilloscope or an appropriate phase
detection device. In the current embodiment the frequency range of the
system is 1–100 kHz.
Sensor materials and fabrication: Three sol–gel derived, xerogel-
based sensor elements were used in this work. Sol 1 was created by
mixing 1.026 ml of n-octyltriethoxysilane (C8-TriEOS), 0.724 ml of
triethoxysilane (TEOS), 1.35 ml of ethanol and 0.40 ml of 0.1 M HCl.
Sol2wascreatedbymixing0.598 mlofn-propyltrimethoxysilane(C3-
TriMOS), 0.500 ml of trimethoxysilane (TMOS), 1.35 ml of ethanol
and 0.40 ml of 0.1 M HCl. Sol 3 was created by mixing 0.684 ml of
methyltrimethoxysilane (C1-TriMOS), 0.500 ml of TMOS, 1.35 ml of
ethanol and 0.40 ml of 0.1 M HCl. Each sol solution was sonicated
for 1 h.
Sensor elements are formed by mixing 80 ml of each sol solution with
20 ml of 0.4 mM [Ru(dpp)3]2þdissolved in ethanol. Approximately
20 ml of the luminophore-doped sol solutions is deposited onto the face
of LEDs 1, 2 and 3, respectively. The xerogels were allowed to form and
age for two weeks under ambient conditions. Fig. 2 is a photograph,
taken through the optical filter (Newport: FSQ-OG550), of the sensor
array when LED 1 is powered with 3 Vand 6 mA. Each of the sensors
will operate satisfactorily over the entire range of O2concentrations, but
each sensor element exhibits a different sensitivity to O2. This differ-
ence in sensitivity is due to differences in kqwithin each xerogel [4, 5].
5mm
LED 2
LED 1
LED 3
amber
LED
Fig. 2 Photograph of emission from single xerogel-based sensor element
Experimental results and discussion: The sensor response was
measured against gaseous O2 concentration at room temperature.
The LED modulation frequency was 20 kHz. During the initial
experiments, only one LED was powered at a time. The amber LED
is used to generate the phase signal reference; eliminating the need for
two optical filters, which would be needed if just the blue LED were
used for the excitation and reference [1]. Each measurement cycle
consists of two phase measurements (a) blue LED turned on (ytotal)
and (b) amber LED turned on (yinstrument). The phase angle associated
with the luminescence from the xerogel sensor elements is
ytotal?yinstrument.
Fig. 3 shows the effects of O2concentration on the phase angle from
the three sensors. As expected the phase angle decreases ([Ru(dpp)3]2þ
lifetime decreases) as the O2concentration increases ((1) and (2)). These
data represent five separate sets of measurements that were recorded
over a one month period of time, indicating very good stability and
reproducibility. Fig. 4 presents the Stern-Volmer plots (i.e. t0=t against
O2concentration) for the three sensors. The solid lines passing through
the points are the best fits to the Stern-Volmer model (1) or Lehrer [6]
models (sensors 2 and 3). The error bars are a reflection of the
imprecision in multiple measurements over the course of a month.
Again, the different sensitivities (slopes) arise from differences in kq,
the transport properties of O2within the xerogels [4, 5].
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35
30
25
20
15
10
5
0
phase shift, deg
[%O ]
2
0 2040 60 80 100
sensor 1
sensor 2
sensor 3
- typical error bar
Fig. 3 Phase response of individual O2responsive sensor elements (typical
error bar shown)
18
16
14
12
10
8
6
4
2
0
t0/t
[%O ]
2
0 20406080100
sensor 1
fit to Stern-Volmer model
sensor 2
fit to Lehrer model
sensor 3
fit to Lehrer model
Fig. 4 Stern-Volmer plots for individual O2responsive sensor elements
Conclusion: An optical multi-sensor system based on phase detection
is reported. The system has an LED array, a single photodetector, a
phase measurement device, and three O2 responsive sensors with
different response profiles. Multiple sensitivities combined with phase
detection enables the development of a powerful sensing system that
improves accuracy and reduces uncertainty.
Acknowledgments: This work was generously supported by the
Johnson & Johnson Focused Giving Program and National Science
Foundation (BES-0330240 to A.H. Titus and CHE-0315129 to
F.V. Bright).
# IEE 2005
Electronics Letters online no: 20052142
doi: 10.1049/el:20052142
13 June 2005
V.P. Chodavarapu, S.J. Kim, A.H. Titus and A.N. Cartwright (Depart-
ment of Electrical Engineering, University at Buffalo, The State
University of New York, Buffalo, NY 14260, USA)
E-mail: ahtitus@eng.buffalo.edu
R.M. Bukowski and F.V. Bright (Department of Chemistry, University
at Buffalo, The State University of New York, Buffalo, NY 14260,
USA)
References
1McDonagh, C., Kolle, C., McEvoy, A.K., Dowling, D.L., Cafolla, A.A.,
Cullen, S.J., and MacCraith, B.D.: ‘Phase fluorometric dissolved oxygen
sensor’, Sens. Actuators B, 2001, 74, pp. 124–130
Lakowicz, J.R.: ‘Principles of fluorescence spectroscopy’ (Kluwer
Academic=Plenum, New York, 1999, 2nd edn.)
Cho, E.J., and Bright, F.V.: ‘Optical sensor array and integrated light
source’, Anal. Chem., 2001, 73, pp. 3289–3293
McDonagh, C., Bowe, P., Mongey, K., and MacCraith, B.D.:
‘Characterization of porosity and sensor response times of sol–gel
derived thin films for oxygen sensor applications’, Non-Cryst. Solids,
2002, 306, pp. 138–148
Tang, Y., Tehan, E.C., Tao, Z., and Bright, F.V.: ‘Sol–gel-derived sensor
materialsthat yieldlinear calibrationplots, highsensitivity,andlong-term
stability’, Anal. Chem., 2003, 75, (10), pp. 2407–2413
Carraway, E.R., Demas, J.N., and DeGraff, B.A.: ‘Luminescence
quenching mechanism for microheterogeneous systems’, Anal. Chem.,
1991, 63, (4), pp. 332–336
2
3
4
5
6
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