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bionanalytes determination using biosensors. This
will be achieved with the inclusion, in a unique
instrument, of the photodiode, the potentiostat and
the reconfigurable electronics which adapts the
analog processing to the ECL signal. Main
advantages of our design lie on portability, low cost
because of the use of a photodiode instead of a
costly or bulky photomultiplier, and the use of a few
microliters of sample analysis.
2 MATERIALS
The disposable cell consists of a screen printed three
electrodes cell where we can found a round-shaped
graphite working electrode, a graphite counter
electrode and a silver pseudo-reference electrode.
The screen-printed electrode was covered by a thick
overlapping plastic layer with a 50-μl volume hole
in the electrode area to place the sample. The
sensing layer was cover by two types of solutions,
one formed of luminol dissolved in 0.25 M NaCl
and pH 9.0 phosphate buffer 0.2 M with H
2
O
2
as
analyte and another for luminophore Ru(bpy)
3
2+
with
0.25 M NaCl, pH 8.5 Tris buffer 0.2 M and
triethylamine (TEA) as analyte.
3 PORTABLE INSTRUMENT
DESCRIPTION
The system is composed by a photodiode with an
operational amplifier integrated in the same chip, an
electronic amplification stage, a potentiostat and a
microcontroller that controls all elements (Figure 1).
In this instrument, the PIC18F2553 of Michochip
Technologies (USA) has been chosen.
3.1 Instrument Overview
In order to avoid the external illumination
interferences, the electrode is placed into a dark little
drawer attached to the instrument housing. The
output current of the photodiode needs to be
converted into voltage by an I/V amplifier. This
converter can be included into the photodiode case,
in the same chip, or can be external. Before
amplification, the output voltage of the I/V converter
was filtered in order to reduce the electrical grid
interference (Figure 1). The filtered output is
amplified by a Programmable Gain Amplifier
(PGA). The gain of this amplifier can be configured
by the microcontroller, and as consequence, the
dynamic range can be adapted depending on the
emission intensity of the ECL reaction. We used the
PGA103 of Texas Instruments with 1, 10 and 100
gain factors that can be selected easily. Finally the
results are storage in an EEPROM memory.
Microcontroller
ADC
EEPROM
memory
I2C
PGA
Low pass
filter
Electrode
Low pass
filter
Photodiode and
I/V converter
Feedback
resistor net
DAC
Potentiostat
Amplification and
conditioning
circuit
Figure 1: Block diagram of the luminometer.
The potentiostat is the electronics in charge of
starting the ECL reaction by applying voltage
pulses. Basically, it consists of a Digital-to-Analog-
Converter (DAC) and analog circuitry to amplify
and to shift the voltage output. In section 3.2 the
pulse configuration will be described in detail.
Finally, the presented luminometer in this work can
be controlled via USB by a computer. An ad hoc
software application has been developed for
allowing the remote configuration and downloading
of results.
3.2 Selection of the Photodiode
We studied two photodiodes without an integrated
I/V converter: S1227-66BR and S1227-1010R
(Hamamatsu, Japan), with an active area of 33 and
100 mm
2
respectively. To achieve a gain factor high
enough, a T net resistor was used as the feedback
converter resulting in an effective resistance of 5.2
GΩ. Others three photodiodes with a built-in I/V
amplifier were tested: the S9269 and S9270
(Hamamatsu, Japan), with an active area of 33 and
100 mm
2
respectively; and the OPT301 (Texas
Instruments, USA), with an area of 5.2 mm
2
. The
S9269 and the S9270 have an internal resistance of
1GΩ and the OPT301 internal resistance is 1MΩ. In
the last one, an additional external T net resistance
was added to reach a value of 5.2 GΩ. The output
currents of the different photodiodes are plotted in
the Figure 2.
As expected, the most sensitive photodiodes
were the S9270 and the S1228-1010BR due to their
bigger sensitive area. We selected the most sensitive
photodiode, the S9270. In addition, this device
includes a built-in I/V converter, showing better
interference immunity.
BIODEVICES 2011 - International Conference on Biomedical Electronics and Devices
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