POLYMERIC FILM SENSORS BASED ON PAH-PAZO IONIC

SELF-ASSEMBLED MULTI-NANOLAYERS

Celso Ribeiro, Paulo J. Gomes, Paulo A. Ribeiro, Maria Raposo

Centro de Investigação em Física Tecnológica (CEFITEC), Physics Department, Faculdade de Ciências e Tecnologia

(FCT), New University of Lisbon (UNL), 2829-516, Caparica, Portugal

Hugo Águas

Department of Material Science and CINEMAT, Faculdade de Ciências e Tecnologia (FCT), New University of Lisbon

(UNL), 2829-516, Caparica, Portugal

Pedro Santos, Beatriz Borges

Department of Electrical and Computer Engineering, Instituto Superior Técnico (IST), 1049-001, Lisbon, Portugal

Pedro Brogueira

Department of Physics and ICEMS, Instituto Superior Técnico (IST), 1049-001, Lisbon, Portugal

Keywords: Sensors, Biosensors, Electronic Tongue, Lab-on-chip, Polymers, polyelectrolytes, PAH, PAZO, Layer-by-

Layer, Nanotechnology, Nanofilms, Thin films, Impedance Spectroscopy, AFM, Roughness, Ellipsometry.

Abstract: A sensor platform composed by interdigitated electrodes covered by a tailored nanofilm of 5 bilayers of

PAH and PAZO polyelectrolytes was successfully built and tested in NaCl solution at relative low

concentrations (10

-5

-10

-6

M). The effect of temperature on the electrical measurements was addressed and

indicated its importance for to sake accuracy reduction in about one order of magnitude. The film

morphology was studied by using ellipsometry and atomic force microscopy techniques. The first inferred

precise and consistent values for the film thickness about 50 nm. With respect to the second, relative low

values of the surface roughness of about few nanometers were measured and also micrometer diameter

agglomerates and channels with a bilayer size depth were identified.

1 INTRODUCTION

Sensing, monitoring, and control are natural tasks

performed by any living organism and also an

increasing part of the humankind activity in which

technology is directly or indirectly involved in day-

to-day life. Numerous sensors are constantly being

developed and the advent of the nanomaterials

increases even further their variety and applicability.

A new branch of potential platform for sensors is

being widely researched by using polyectrolytes

nano-thin films ionically self-assembled in bilayers

via what is so-called layer-by-layer (LbL) technique

(Decher and Hong, 1991, Oliveira Jr., 2001). The

production of a bilayer involves sequential

adsorption of two opposite charged polyelectrolytes

from an aqueous solution, one by one, onto an inert

substrate such as glass or metal. The process is

repeated for attaining multi-layers. This simple and

inexpensive technique is also expected to deliver

films with high thermal stability and robust to

critical solvent and alkaline environments which are

the ideal platform for sensors.

Among several polyeletrolytes bilayer pairs for

building this platform, the ones made by the

polyanic poly[1-[4-(3-carboxy-4-hydroxyphenylazo)

benzene sulfonamide]-1,2-ethanediyl, sodium salt]

(PAZO) and the polycationic poly (allylamine

hydrochloride) (PAH) have a great potential and

results have already been reported (Mermut and

Barrett, 2001 and Ferreira, 2007).

In addition, selective molecules (organic,

inorganic, enzymes, DNA, etc) can be equally

mobilized onto the last layers in order to react

chemically to a particular analyte of interest.

458

Ribeiro C., Gomes P., Ribeiro P., Raposo M., Águas H., Santos P., Borges B. and Brogueira P. (2009).

POLYMERIC FILM SENSORS BASED ON PAH-PAZO IONIC SELF-ASSEMBLED MULTI-NANOLAYERS.

In Proceedings of the International Conference on Biomedical Electronics and Devices, pages 458-461

DOI: 10.5220/0001830904580461

 SciTePress

The interplay of a broad set of parameters related

to the measurement such as the temperature, pH,

hysteresis, and their time-domain dynamic (drift)

should be carefully quantified aiming to control or

more likely to correct theirs effects via a posteriori

software analysis. This optimization towards

reliability can enhance the enormous commercial

potential of the LbL sensors mainly as a monitor in

an environment with no lab-control.

This work reports preliminary results on the

temperature effects on a LbL film sensor plataform

built with PAH/PAZO bilayers and tested in rather

simple aqueous solutions containing sodium choride

(NaCl) via impedance spectroscopy technique. The

polyelectrolytye PAZO which is at the last surface

of the multilayers should attract electrostatically the

cation Na

, so the global electrically properties of

the multilayer are expected to change according to

the concentration of this cation and therefore NaCl

concentration is sensed. In addition to this,

prelimilary analysis on the sensor film thickness and

morphology are presented by using ellipsometry and

atomic force microscopy (AFM) techniques.

2 EXPERIMENTAL

The sensor was produced from polyelectrolytes LbL

films deposited onto substrates of BK7 optical glass

where gold interdigitated electrodes were deposited

by vacuum evaporation. The sensor effective area

was about 2x5mm

, the interspace between the lines

was 20μm and their width and thickness were 2μm

and 0.2μm, respectively. These dimensions were

measured by a Dektak perfilomoter and an optical

microscopy Olympicus SZ-PT.

The polyelectrolytes poly [ 1-[4-(3-carboxy-4-

hydroxyphenylazo) benzenesulfonamido]-1,2-

ethanediyl, sodium salt] (PAZO) and the

poly(allylamine hydrochloride)(PAH) (average M

50,000-60,000g/mol) were acquired from Sigma-

Aldrich. The PAH polyelectrolyte aqueous solution

with concentration of 10

-2

M was prepared by

dissolving this polyectrolyte in deionised water with

a resistivity of 18.2MΩcm supplied by a Millipore

system (Milli-Q, Millipore GmbH). The PAZO

aqueous solution also with a concentration of 10

-2

was obtained by dissolving this polyelectrolyte on an

aqueous buffer solution of pH=10. The

polyelectrolyte concentrations were based on the

molecular weight repeat unit and the buffer solution

was prepared mixing a 0.05M sodium hydrogen

carbonate (NaHCO

) aqueous solution with a 0.1M

sodium hydroxide (NaOH) solution in a proportion

of 500:107(v/v) (Ferreira (a), 2007). The PAZO

solution was also filtered with a 5 mm thick and 50

μm porous diameter ceramic filter.

The LbL films were prepared by immersing the

substrate with the interdigitated electrodes into the

PAH solution for 5 minutes, washed 3 times into

water for a total of 10s, and then immersed into the

PAZO solution for the same 5 minutes and equally

washed but into the buffer solution instead of water.

This procedure leads to a production of a bilayer and

repeated until the 5 bilayers were obtained. Finally,

the thin film was dried with a nitrogen flux.

The sensor impedance measurements were

carried out by a Precision Impedance Analyser

Agilent 4294A (40Hz-110MHz, 1mHz resolution,

GPIB connection). The root mean square oscillator

voltage signal level was 50 mV.

The film thickness was measured using a

spectroscopic ellipsometer model HORIBA Jobin

Yvon UVISEL. A three layer model was used

assuming the sensor is composed by a film layer on

the top of a 1 mm thick BK7 glass substrate and

another on its back. The spectral range used was 1.5-

6.5 eV (531-2302 nm) with a 0.025 eV increment.

The AFM measurements were performed by a

Dimension 3100 SPM with a Nanoscope IIIa

controller from Digital Instruments (DI) under

ambient conditions in tapping mode

. A

commercial tapping mode etched silicon cantilever

probe from DI (constant force of 42N/m, resonance

frequency of 320kHz) and a 90x90 μm

scanner

were used. The scan rate was 1.51 Hz. The image

resolution was fixed to 256×256 pixels.

3 RESULTS AND DISCUSSION

The influence of the NaCl concentration on the

sensor impedance (Z=Z′+iZ″) were analysed. Real

(Z′) and imaginary (Z″) parts, module and phase of

Z, and Nyquist representation were all studied. The

clearest way for visualise that influence was by

using the Z′ part against the impedance analyser

frequencies which is presented in the graph of fig.1.

The sensor was immersed into solutions of different

NaCl concentrations from the lowest concentration,

virtually null corresponding to Milli-Q pure water,

to the highest of 1M. Impedance spectra

measurements within several weeks show systematic

drifts for all concentrations but not to a point to

overlap any two consecutive values, excepted at low

frequencies below 5 kHz.

POLYMERIC FILM SENSORS BASED ON PAH-PAZO IONIC SELF-ASSEMBLED MULTI-NANOLAYERS

459

100

10k

100k

possible values to

discriminate the

concentrations

Real component of the Impedance Z' (Ω )

frequency (Hz)

NaCl in M

5x10

-1

2x10

-1

-2

-3

-4

-5

-6

-7

milli-Q

water

Figure 1: Real part, Z′, of the sensor impedance versus the

impedance analyser frequencies for the sensor immersed

in NaCl aqueous solutions with different concentrations.

The overall Z′ curves show a complex behaviour

with the frequency and concentration leading to a

short window for an effective analysis which is

empirically delimited (e.g. by the two lines drawn in

this figure). In the lower limit at 8kHz, a

concentration up to 10

-5

M can be easily measured

and possibly 10

-6

M too if sufficient statistics and at

least temperature measurements of the aqueous

medium are available as will be seen further on.

At the upper frequency of 40 kHz the sensor is

robust since the Z′ values are well distinguished for

each concentration, in addition to be less sensitivity

to the frequency. Values of 10

-5

M can not be really

exceeded. However, in this region the sensor thermal

stability is very high as it is shown in the plot of

fig.2, where the impedance spectra are shown for

NaCl solutions of 10

-4

M and 10

-3

M concentrations

over a temperature range of 9 to 62°C and 9.0 to

26°C, respectively. Around 40kHz, Z′ variation is

almost the same for temperatures close to 9°C (Δ

)

or to 26°C (Δ

) for these concentrations. However, it

is clear that the 10

-4

M concentration can be

mistaken for the 10

-3

M if the right temperature

curve is not used. An overlap of these curves occurs

below 20kHz, even for small temperature variation.

Therefore, the possibility to measure lower

concentrations (e.g. down to 10

-6

M) really demands

the precise knowledge of the temperature and the

statistical values of the concentration in a laboratory

environment. A more feasible alternative to this

might be to increase the sensor sensitivity by

increasing the number of bilayers, for example.

Ellipsometry measurements inferred a precise

and consistent sensor film average thickness of

49±4nm. The ellipsometric measurements were

20k 30k 40k 50k 60k 70k 80k 90k 100k

400.0

600.0

800.0

1.0k

2.1k

2.2k

2.3k

2.4k

2.5

possible value

to discriminate

the concentrations

Real component of the Impedance Z' (Ω )

frequency (Hz)

Temperature (

at 10

- 4

8.8

------------------------

26 (10

- 5

26 (10

- 3

9.0 (ibidem)

Figure 2: The real part, Z′, of the sensor impedance versus

the impedance analyser frequencies for the sensor

immersed in NaCl aqueous solutions with few different

concentrations and temperatures.

carried out about 8 mm away form the interdigitated

electrodes where only the PAH/PAZO film

deposited onto the glass substrate was presented.

Topography measurements of the PAH/PAZO

films were obtained by AFM scanning over the film

sensor at 2 mm and 8 mm away from the

interdigitated electrodes, again in regions where only

the PAH/PAZO film deposited onto the glass

substrate was presented. In the fig.3 two typical 3-D

reconstructions of the topography, both with a scan

window of 5x5μm

, are presented in a sensor used in

a NaCl aqueous solution. In the fig.3a, a relative

smooth surface is observed except for the presence

of agglomerates with a micrometer size diameter and

at least 60 nm height. These structures are possibly

made or a result of NaCl attachment to the surface.

The root mean square roughness (Rrms) varied over

7-13nm for this type of scan window. These values

further reduce to 3-4nm for scan windows of

1×1μm

. In the fig3b, clear channels were shown at

the surface possibly due to the effect of the electrical

currents, since the electrical field created was

relatively high, i.e. of the order of 2.5kV/m. The

creation of such structure should be avoided in order

to keep the multilayer integrity and thus the sensor

reliability. A worthwhile attempt to minimise this

might be the reduction of oscillator voltage applied

by the impedance analyser. In addition, the reduction

of the upper frequency could also help since no

useful information can be extracted for values much

beyond the optimum frequency (by 40kHz in this

case) where Z′ substantially reduces and the current

flow through the sensor increases accordingly.

The channels depth can be roughly estimated as

10nm from the topographic AFM measurements

BIODEVICES 2009 - International Conference on Biomedical Electronics and Devices

460

shown in fig.3b. This corresponds to the size of one

bilayer assuming a linear scaling between the

number of bilayers with the film thickness.

Figure 3: AFM 3-D topography of the PAH/PAZO film:

(a) in a typical region; (b) where channels were formed.

As a remark, the AFM morphology of samples

that have not been immersed into a NaCl aqueous

solution showed no agglomerates such as the ones

discussed earlier and their Rrms values are lower,

regardless the scan window values used, i.e. they are

3-5nm, 1.5nm, and 1.6nm for 5×5μm

, 2×2μm

, and

1×1μm

, respectively. The particular value of 1.6nm

here is about one magnitude smaller from that of

~13nm, obtained from previous studies (Ferreira,

2008). This indicates that filtering the polyetrolytes

solution such as for the PAZO here may play an

important role for reducing the roughness.

4 CONCLUSIONS

A sensor platform composed by an interdigitated

electrode covered by a tailored film of 5 bilayers of

polyelectrolytes PAH-PAZO was successful built

and tested in NaCl aqueous solutions at relative low

concentrations around few micro moles per litre.

The effect of temperature on the impedance

measurements was addressed in a limited range of

this variable. Nevertheless, the preliminary results

indicate that the knowledge of this parameter is vital

for attaining accuracy close to one order of

magnitude in useful range of concentrations.

The film thickness and morphology were

characterized by using ellipsometry and atomic force

microscopy techniques. The first allowed to obtain

precise and consistent values for the multilayer film

thickness of about few tens of nanometers. From the

second, relative low values of surface root mean

square roughness, about few nanometers, were

measured and channels with a bilayer size depth and

micrometer diameter agglomerates were also

identified.

ACKNOWLEDGEMENTS

The authors thanks the plurianual financial support

from "Fundação para a Ciência e Tecnologia-FCT",

Portugal. CR is supported by the programme

“Compromisso com a Ciência’’ also from FCT.

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Ferreira, Q., Gomes, P.J., Raposo, M., Giacometti, J. A.,

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(a)

(b)

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