reflection-based sensor for gaseous ammonia

We found, that sensors prepared on different oxide layers with the same indicator, show different signal change in the presence of the same concentration of ammonia gas.. In this work we

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Sensors and Actuators B: Chemical

j o u r n a l h o m e p a g e :w w w e l s e v i e r c o m / l o c a t e / s n b

Reflection-based sensor for gaseous ammonia

Ákos Markovicsa, Géza Nagyb, Barna Kovácsa,b,∗

aDepartment of General and Physical Chemistry, University of Pécs, Ifjuság 6, Pécs, Hungary

bSouth-Transdanubian Cooperative Research Center, Ifjuság 6, Pécs, Hungary

a r t i c l e i n f o

Article history:

Available online 31 March 2009

Keywords:

Anodized aluminum

Ammonia gas sensing

Optical sensor

Reflection

a b s t r a c t

In this work we describe the fabrication of an ammonia sensor on anodized aluminum substrate Pure aluminum was oxidized with direct current (DC) method at different voltages to obtain oxide layers with different porosity The adsorption capacities of the differently prepared layers were measured Bromophe-nol blue (BPB), bromocresol green (BCG) and bromocresol purple (BCP) indicators were immobilized by simple adsorption Sensor properties, such as detection limit (100 ppb, 5 and 50 ppm for BPB, BCG and BCP, respectively), dynamic range (0–80, 10–90, 100–600 ppm for BPB, BCG and BCP, respectively), response time and reversibility were investigated We found, that sensors prepared on different oxide layers with the same indicator, show different signal change in the presence of the same concentration of ammonia gas Sensors with optimal performance were selected for solving different tasks

© 2009 Elsevier B.V All rights reserved

1 Introduction

The sensing of gaseous ammonia is important in agriculture,

bio-process monitoring as well as in food-freshness testing In chicken

farming for instance, the presence of ammonia can cause eye and

respiratory irritation of the stock, which has a negative effect on

the egg production Cheap and reliable sensors are needed for

con-tinuous monitoring of the concentration of this compound mainly

in the ppm level [1] There are many different ways to detect

this volatile, basic gas Semiconductor-based solid-state sensors

[2], IR adsorption-based detection[3], or electrochemical

meth-ods[4] can be used for ammonia concentration measurements

Nowadays optical chemical sensors are in the spotlight, due to

their relatively low manufacturing and operational costs

Opti-cal chemiOpti-cal sensors are not affected by electromagnetic noise,

and often remote sensing is also possible by using optical fibers

[5]

Many ammonia-sensitive optical chemical sensors use the

acid–base properties of the indicator as well as of the ammonia In

this way ammonia can deprotonate triphenyl-methane type

indi-cators, which results in readily detectable optical changes[6] If the

indicator molecules are entrapped on the surface of a substrate, the

presence of the gas can be detected in reflection mode Bromocresol

purple (BCP), phenol red (PR), fluorescein (FL) and their

deriva-tives have already been used for sensor fabrication[7,8] The limit

∗ Corresponding author at: Department of General and Physical Chemistry,

Uni-versity of Pécs, Ifjusag 6, Pécs, Hungary Tel.: +36 72 503 600x4680; fax: +36 72 503

635.

E-mail address:kovacs1@gamma.ttk.pte.hu (B Kovács).

of detection depends mainly on the dissociation constants of the indicator, and hence on the matrix properties

In sensor preparation, the sensitive chemical compounds have

to be immobilized on the surface of a substrate Plasticized PVC and other polymer membranes, sol–gels are often used to form sensitive layers on the support materials, which are pla-nar waveguides, microscope slides, or optical fibers in many cases [9–11] The properties of the sensing films depend strongly on the ageing of the polymer matrices The lifetime of the sen-sor can be prolonged if the organic dye molecules are bonded directly to the surface A convenient and effective way of this direct binding is the entrapment of the indicator molecules in nanometer-sized pores on anodized alumina, with a simple adsorp-tion process[12,13] This technique is highly reproducible, cheap, and suitable for standardized production even in large quanti-ties The metal aluminum substrates provide a highly reflective background for reflection-based measurements, so the addition of reflection enhancers is not needed as it is at other types of sen-sors

Both AC and DC current can be used for electrochemical prepa-ration of aluminum-oxide layers In direct current methods, the substrate is connected as anode, against an aluminum cathode, while as electrolyte most often sulfuric acid, phosphoric acid,

or some other organic acids are selected [12,13] The formed aluminum-oxide has a porous structure Its surface morphology strongly depends on the fabrication parameters The size, number and surface density of the pores can be controlled by the composi-tion and temperature of the electrolyte, the current density, voltage and the electrolysis time applied[14–16] The preparation and char-acterization of anodized alumina surfaces has become very popular

in the recent years It was established, that over a certain voltage a

0925-4005/$ – see front matter © 2009 Elsevier B.V All rights reserved.

self-ordering process can be observed, hexagonal cells occur in the

oxide-layer[17–21]

Most of the optical ammonia sensors reported contain thin

plas-ticized polymeric membranes with embedded chemical sensing

molecules In their case the ageing of the soft polymer layer could

result in continuous drift of the sensor signal and in a limited

oper-ational lifetime of the optical sensor To overcome the problem,

plasticizer free polymer and sol–gel matrices were developed In

this work we describe the fabrication of different ammonia sensors

on anodized aluminum substrates All the sensors were prepared

on aluminum sheets, oxidized at different voltages to obtain oxide

layers of different porosity Triphenyl-methane dyes of different

pKawere immobilized on the oxide layers by simple adsorption

The analytical properties, such as detection limit, dynamic range,

response time and reversibility of the sensors were investigated

We found, that sensors prepared of the same indicator on different

oxide-layers have different relative signal change when exposing

to the same concentration of analyte, and also different dynamic

range

2 Experimental

2.1 Materials

Aluminum plates (99.5%) with a thickness of 0.5 mm were

pur-chased from Köbal Ltd (Budapest, Hungary) Triphenyl-methane

dyes, such as bromophenol blue (BPB), bromocresol green (BCG)

and bromocresol purple (BCP), all indicator grades, were purchased

from Reanal Ltd (Budapest, Hungary)

Dodecylbenzenesulfonic-acid (H-DBS) was obtained from Fluka (Buchs, Switzerland) Acids

and the other chemicals used for preparing the anodizing bath

were Riedel de Haen products All the chemicals were analytical

grade and used as received Solutions were prepared with deionized

water; its specific conductivity was less than 0.8␮S cm−1

Cali-brating gas (93.7 ppm NH3in nitrogen) was purchased from Linde

(Répcelak, Hungary)

2.2 Sensor fabrication

Pure 1.5 cm× 4 cm aluminum sheets were used as substrates in

the sensor fabrication process Their surfaces were electropolished

in a 4:1 ethanol–perchloric acid (60%) mixture until the natural

oxide-layer and other impurities were totally removed and a shiny,

smooth surface was formed (3.3 A, 1 min) The plates were rinsed

thoroughly with deionized water

The anodizing of the substrates was carried out in an

electrolyz-ing cell; the sensor plates were connected as anodes and a U-shaped

aluminum block was used as cathode The cell voltage was

con-trolled by a CAI 20-1084 laboratory power-supply within the 0–30 V

range The 5% sulfuric-acid electrolyte solution was continuously

stirred at 150 rpm during the whole oxidation process The time

of the electrolytic oxide layer formation was 10 min in most of the

experiments

The anodized substrates were rinsed, and sonicated in deionized

water for 2 min to remove the excess of the anodizing solution, then

they were dried at room temperature for 10–20 min After drying,

the chemical sensing layer was prepared by immersing the plates

for 10 min in 0.1% solutions of different triphenyl-methane type

indicator dyes The indicator solutions were made by dissolving

0.1 g of a selected dye in 1 ml of ethanol (96%) and then diluted

to 100 ml in a volumetric flask

After dyeing, the sensors were rinsed with distilled water and

were protonated with a diluted (1%) aqueous solution of H-DBS for

5 s by immersion, and finally dried at room temperature

2.3 Determination of the adsorption capacity of the anodized alumina films

The anodized plates were cut into 3 cm× 1 cm pieces, and they were immersed into BCG containing solutions of known volume (5.00 ml) The concentration of the dye solution ranged from 0.01

to 1 mM The absorption spectra of the dye solution were measured prior the soaking procedure The aluminum plates were kept for

24 h in the dye solution to achieve complete adsorption After then, the plates were removed from the dyeing solution, and the absorp-tion spectra of the soluabsorp-tions were measured again The adsorbed dye amount was then calculated by knowing the volume of the dye solution, and its concentrations before and after the adsorption:

nad= V(c0− ceq)

where V is the volume of the dye solution, c0and ceqare the initial and the equilibrium dye concentrations (before and after adsorp-tion, respectively)

2.4 Instrumentation

The sensors were tested and calibrated in a home-built flow-through cell, prepared on a 0.5 cm thick, stable aluminum base An approximately 2 mm× 50 mm2gasket was formed in a 2 mm thick thin rubber layer, which was pressed against the aluminum plate by

a Plexiglas cover The gas in- and outlets were prepared of 20 mm long, 0.5 mm inner diameter stainless steel tubes, taken from medi-cal needles,Fig 1shows the instrumental setup used for the sensor characterization

During the measurements the sensor was placed face down between the Plexiglas cover and the rubber layer The chemically sensitive layer was illuminated by a halogen light source (Avantes Ava-Hal) through the central fiber of a seven-fiber bundle The other six fibers were used to guide the reflected light to a two-channel fiber optic, diode array photometer (Avantes, Avaspec-2048-2) Spectra were taken in reflection mode, in a concentration range from 0 to 93 ppm ammonia in air The 100% reflection was set by using a home-prepared reflective element, made of electropolished aluminum, the same quality as the substrate material

The calibrating gas mixture was prepared by using three inde-pendent Cole–Parmer flow meters, an air-pump and a flask of

93 ppm ammonia in nitrogen On two flow meters the rate of the air flow, on the third one the amount of the added ammonia could

Fig 1 Experimental setup (A) screw, (B) plexiglas cover, (C) sensor, (D) rubber

gasket with gas inlet and outlet, (E) aluminum base and (F) fiber bundle with one

be controlled This way the calibrating gas was diluted with air, and

the concentration of the mixture could be adjusted in the 0–93 ppm

range, with a resolution of 1.56 ppm The flow rate in the cell was

kept constant (33 ml/min) during the experiments

In order to investigate the surface morphology of the different

layers prepared, scanning electron microscope images were taken

at 20 kV by a Jeol-100 SEM device

3 Result and discussion

3.1 Anodizing of the aluminum support

The pure aluminum sheets, used as substrates in the sensor

fab-rication process, were electropolished, and then DC anodized in two

ways:

• Constant current mode: the current density was adjusted to

16 mA/cm2by using a stabilized power supply During the layer

formation, the initial 12 V potential dropped to 4 V

• Constant voltage mode: porous oxide layers were prepared using,

12, 18 and 24 V DC No significant change of the current (except

some transient fluctuation in the first 10 s) was observed during

the layer fabrication, except when 24 V cell voltage was selected

In this case, the temperature of the solution increased by 15–20◦C,

since the higher current resulted in more heat to absorb (the

elec-tric power is proportional to I2) At this temperature the mobility

of the ions is higher and a current drift can be observed

By increasing the anodizing time, thicker oxide layer with lower

reflexivity can be prepared After 10 min of anodizing (in DC mode),

an average of 20␮m was measured for layer thickness, using

micro-scopic methods As it can be seen inFig 2, the reflection of the

Fig 2 Reflection of the aluminum at 600 nm as a function of the anodizing time.

The reflections of three sensors prepared in different batches are presented.

surface decreased by 30% during the preparation This change was measured at 600 nm, which wavelength corresponds to the previ-ously determined absorption maxima of BPB and BCG solutions Scanning electron microscopy (SEM) images prove that the anodizing voltage has a great influence on the surface morphology

of the alumina layers (Fig 3) By increasing the potential in DC mode, the diameter, number and density of the pores changed signifi-cantly At 6 V the layer has very small pores in the nanometer range

At 12 V the number of greater pores (0.1␮m) increased, while at 18 V the pore diameter and the wall thickness became comparable By reaching the so-called self-ordering voltage (approximately 24 V)

Fig 4 Adsorption capacities of layers prepared at different potentials The adsorbed

dye was BCG (a) 6 V, (b) 12 V, (c) 18 V and (d) 24 V.

hexagonal pores[18]were growing, the surface showed cellular

order with reduced wall thickness The high electric field strength

at the barrier layer of the porous films is the main controlling factor

of this phenomenon

We note that by using controlled current electrolysis, the

sur-face morphology became similar to that obtained with controlled

voltage mode electrolysis at 12 V It can be explained by the

obser-vation that for 16 mA/cm2current density 12 V starting potential

has to be set Although the potential drops during the process, the

oxide layer grows inward (towards the internal parts of the

sub-strate), thus the structure of the upper layer is developed during

the first few minutes of the electrolysis

As another important parameter, the adsorption capacities of the

different layers were measured spectrophotometrically by using

BCG indicator, as it was written in Section2 By plotting number

of the adsorbed moles against the equilibrium concentration of

the dye at constant temperature, adsorption isotherms could be

obtained (Fig 4) Interestingly, the adsorption capacity of a layer

prepared at 12 V was found eight times higher than that of the layer

prepared at 6 V

This is in good agreement with the morphology comparison By

increasing the pore number (12 V—approximately 10 pore/␮m2) a

higher adsorption capacity was found Over 12 V cell voltage, the

diameter of the pores starts to increase dramatically (to

approx-imately 0.5␮m), that leads to the decrease of their inner surface

and the adsorption capacity

Higher concentration of indicator results in higher signal change,

as well as in better signal to noise ratio We could conclude that

Fig 5 Calibration graphs of sensors prepared with different triphenyl-methane

Fig 6 Difference spectra of a BCG-based sensor anodized at 12 V for 10 min The

spectra were taken at 10 different ammonia concentrations between 0, 9, 19, 28, 37,

47, 56, 65, 74, 84, 93 ppm.

very small and very large pores are not advantageous for sensing purposes This way the optimal electrolysis conditions were found

3.2 Comparison of sensors prepared with constant current

To examine how the pKaof the indicator affects the calibration curves of the sensors, alumina layers were prepared with constant

Fig 7 Calibration plots of BCG-based sensors Sensors were prepared at 12 V (A) and

at 18 V (B).

Table 1

Parameters of BCG-based ammonia sensors made on substrates anodized at different potentials.

Anodizing potential (V) Change in reflection (%) Sensitivity a (ppm) Response time (s) Reverse response, t50 (s)

a The concentration which causes 50% relative change in reflection at 600 nm.

current anodizing method (10 min) and they were then soaked in

three different dye solutions The sensors were protonated, dried,

put into the flow cell, and calibrations were made at different

ammonia concentrations The reflection changes were measured at

600 nm that corresponds to the absorption maxima of the

depro-tonated form of the dyes (Fig 5)

In order to compare the sensitivities of the different sensors, we

investigated which concentration of gaseous ammonia results in a

25% relative signal change For the 100% reference, the saturation

(total deprotonation) of the sensors with 1% (V/V) ammonia gas was

taken As it is plotted inFig 6, BPB, BCG and BCP have significantly

different calibration curves For the 25% relative signal change in

case of these three dyes, in order: 25, 50 and 400 ppm ammonia

concentrations were measured These decreasing sensitivities

cor-respond to the increasing pKavalues of the dyes (pKa= 3.8 for BPB;

4.7 for BCG; 6.0 for BCP) Since the most important concentration

range in environmental monitoring is below 100 ppm (even much

lower for dissolved ammonia), BCP is practically useless for that

sensor purposes

The response time and reversibility of these sensors were tested

by switching the concentration from 0 to 93 ppm and back All the

sensors responded in 4–10 s, however the reverse response times

(t50) were much longer: 20, 9 and 3 min for BPB, BCG and BCP made

sensors, respectively

Comparing to other reflection-based optical chemical ammonia

sensors[8], the presented three sensors cover a wide concentration

range For a desired application sensor can be prepared by choosing

an indicator with a proper pKa

3.3 Comparison of sensors prepared with constant voltages

To examine the effect of the surface morphology on the

analyti-cal properties of the sensors four different potentials (6, 12, 18 and

24 V) were selected to prepare four differently porous oxide layers

as sensor substrates

Ammonia sensors were prepared by immersing the anodized

plates into bromocresol green (BCG) solution, washed and finally

dried The sensors were tested in the flow-through cell in

reflec-tion mode Typical differential reflecreflec-tion spectra are shown in

Fig 6, that were obtained in the 0–93 ppm concentration range

One can see that the shape of the spectra slightly differs from that

could be measured in transmission or absorption mode in

solu-tions or in polymeric membranes: a wave is superimposed on the

absorption bands This is caused by interference effect The incident

light reflected by the surface of the layer interferes with the light

reflected by the lower aluminum layer

Surprisingly the sensitivities of the differently prepared layers

were also different As it is shown inFig 7A and B, the sensor

pre-pared at 12 V is more sensitive (30%) than that made at 18 V The

measurements were completed also for membranes prepared at

6 and 24 V; the results are summarized inTable 1 Interestingly

the sensitivities of the sensing layers are in good agreement with

the adsorption capacity of the films The higher is the capacity, the

higher is the sensitivity of the film The highest differences in the

sensitivities were calculated between the 6 and 12 V made sensor,

this latter showed a 2-fold increase in the sensitivity The obtained

sensitivities, dynamic ranges are determined mainly by the pK of

the indicator, although the surface morphology also affects these parameters

It was expected from the SEM-results, that larger pores obtained

at higher potentials could affect the diffusion processes in the sen-sor layer, and as a result the recovery time decreases As it is listed

inTable 1, no relation was found between the pore diameter and the recovery time Presumably, the thickness of the layer and the amount of immobilized indicator determine that parameter; the clarification requires further investigations

4 Conclusions

The results show that the sensitivity, the limit of detection, and the dynamic range of the sensors were significantly affected by the layer-porosity hence the applied potential Highest sensitivity (18 ppm) was obtained with the sensors prepared at 12 V with con-trolled potential method and BCG, or at 16 mA/cm2current density with controlled current film formations and BPB indicator These sensors showed also the highest signal change (with different sen-sitivity) in the 1–50 ppm range that is typical in stockyards and hutches in poultry breeding Response times for increasing ammo-nia concentration were similar (3–7 s) while the reverse processes took 8–20 min depending on the oxide layer

Anodized aluminum has excellent reflection property that makes it suitable for remote measurements The anodizing process could be easily controlled which results in reproducible sensing layer thickness that could be produce in large quantities

Acknowledgments

The authors are thankful to F Kaposvari for his kind assistance

in the SEM measurements

This work was supported by the Hungarian Research Foundation (OTKA T046798)

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Biographies

Ákos Markovics received his MSc in physics in 2004 from the University of Pécs

Dur-ing these 5 years, his interest turned towards chemistry and started further studies.

He received his MSc in chemistry in 2007, and currently he is a PhD student at the Department of General and Physical Chemistry.

Géza Nagy is a full professor of physical chemistry at the University of Pécs He

obtained MSc from Kossuth Lajos University Debrecen, Hungary, PhD from Technical University of Budapest, DSc from Hungarian Academy of Sciences He worked as postdoc fellow with G.G Guilbault at LSUNO (New Orleans, LA), with R.N Adams (KU, Lawrence), as visiting scholar at UF (Gainesville, FL) with Roger Bates, at UNC (Chapel Hill) with R.P Buck, at TU (Austin, TX) with A.J Bard He is author of more than 200 scientific papers.

Barna Kovács studied chemistry at the University of Szeged and obtained his

diploma in 1989 After finishing his doctoral work in 1991 on potentiometric surfac-tant sensitive electrodes, he moved to Graz and worked as postdoc in the group of O.S Wolfbeis From 1994 to 1999 he has been working at the University of Pécs as assistant In 2000 he received associate professor position From 2003 he is head of the analytical department of the South-Trans-Danubian Cooperative Research Cen-ter His main interests are luminescent-based analytical techniques and sensors for environmental analysis.