Sensors for Detecting Humic Substances and Heavy

Metal Complexes in WatersAlessandra Firmino" 2, Carlos E. Boratol2, Fabio L. Leite" 2, Osvaldo N. Oliveira Jr.3, Wilson T. L. Silva2 and Luiz H.C. Mattoso2.

I - Pos-Gradua,ao em Ciencia e Engenharia de Materiais - USP - Sao Carlos/SP - Brazil2 - Embrapa Instrumenta,co Agropecuaria - CNPDIA - Sao Carlos/SP - Brazil

3- Instituto de Fisica de Sao Carlos - USP - Sao Carlos/SP - Brazil

Abstract- The interaction between poly(o-ethoxyaniline)(POEA) adsorbed onto solid substrates and humic substances(HS) and Cu2+ ions has been investigated with UV-VIS.Spectroscopy and atomic force microscopy. Both HS and Cu2+cause the POEA film to be further doped, and alter the filmmorphology. This interaction was exploited in a sensor arraymade of nanostructured films of POEA, sulfonated lignin andHS, which could detect small concentrations of HS and CU2+ inwater.

I. INTRODUCTION

The detection of humic substances (HS) in waters is importantdue to their role in the interaction and complexation withheavy metals such as Cu2+. Heavy metals are not chemicallyor biologically degraded, and therefore pose a potential dangerto the environment [1,2]. Upon reacting with HS, heavymetals can be found either in solution or in the solid phase,being particularly toxic. Metal mobility in soils depends on theconcentration of the metal and on the acidity and buffercapacity of the soil. If metals bound to HS are released, waterswould acquire large amounts of free metal and have low pH,thus possessing high toxicity [3]. HS can also alter thecomplexation of metallic cations and immobilize them in theenvironment [4,5]. In this paper, we analyze the interaction ofsolutions containing HS and Cu2+ with nanostructured films ofpoly(o-methoxyaniline), a polyanilines derivative whoseoptical and electrical properties depend strongly on its degreeof protonation. Such interaction is exploited in a sensor arraymade of POEA and sulfonated lignin films adsorbed ontointerdigitated gold electrodes, which is able to detect traceamounts of HS and Cu2+ ions.

II. EXPERIMENTAL

The conducting polymers used in this work were poly(o-ethoxyaniline) (POEA) and sulfonated lignin (SL). Poly(o-ethoxyaniline), POEA was chemically synthesized accordingto the procedures in reference [6], in the emeraldine salt form.Sulfonated lignin (LS) is a polyelectrolytic lignin derivativeobtained from cellulose pulp extraction. Lignin is sulfonatedin acidic media by breaking the bonds in the a carbon of thephenol units and incorporating HSO3- groups, which causesthe solubility of the material in water to increase [7]. For thefilm fabrication, a POEA solution was prepared in aconcentration of 10-3M with pH adjusted to 5.0. The solution

of SL was prepared under the same conditions ofconcentration, volume and pH of the POEA solution. Theaquatic humic substances were extracted from water samplescollected in Joao Pereira River, in Bertioga, south littoral ofthe Sao Paulo state. They were prepared initially in aconcentration of 20 mg L-'. The CuS04 5H20 initial solutionwas prepared with a concentration of 50 mgL-'. The sensorarray was developed with nanostructured polymer filmsdeposited onto interdigitated gold electrodes using the layer-by-layer (LbL) technique [8-10]. The array contained 6sensing units made from different materials including poly(o-ethoxyaniline), sulfonated lignin and humic substances.Another sensor array, containing 5 sensorial units based onchrome electrodes with no films was also used. The chemicalinteraction between POEA (film and aqueous solution), HSand Cu2+ ions was evaluated by UV-Vis spectroscopy.Atomic force microscopy measurements were carried out inthe tapping mode using a Topometrix TMX 2010 atomicforce microscope to investigate the surface topography ofPOEA films after interacting with HS and Cu2+. The detectionof HS in water and the complexation with Cu2+ ions werecarried by additions of: I) HS in pure water; ii) CUS04 5H20in pure water and iii) CuS04 5H20 in HS solution (20 mg L-1). Impedance spectroscopy was employed to characterize thewater and HS samples. Measurements were taken at 1 kHzand 200 Hz for chrome electrodes, and the film capacitancecalculated using an equivalent electric circuit for thefilm/electrolyte system. The data were treated with PrincipalComponent Analysis (PCA).

III. RESULTS

Figure 1 shows the UV-Vis spectra of POEA in solution(a) and in the form of a one-layer film (b). Two peaks appearin the spectra, the first one at 300 nm due to n-Tt* transitionsof the aromatic rings, and the second at larger wavelengths,which may denote the doping state. POEA is already partiallydoped, particularly in solution. Both in solution and in thefilm, interaction with HS or Cu2+ causes further doping,which is indicated by the shift to higher wavelengths in thepolaronic band at - 720-770 nm. For HS the additionaldoping is explained by the acid groups (phenolic andcarboxylic). In the case of CuS04' 5H20 salt, doping may beattributed to a screening effect by the counterions of the

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positive charges in the conducting polymer, which facilitatesprotonation of POEA [ l 1].

1.5 -

^.1.0 -

0c)

.0< 0.5 -

0.0 -

immersed in HS/Cu2+ complex in Figure 2(c). When only Cu2+interacts with POEA, in the absence of HS, there is apparentlythe formation of Cu2+ crystals over the film surface. Thisanalysis illustrates how the film morphology may be affectedby analytes interacting with the film.

(a) monolayer POEA(20x20 pm)

monolayer POEA 3D(2x2 p.m)

x (nm)

(a)

(b) POEA/HS

(c) POEA/HS/Cu2+

POEA/HS 3D

POEA/HS/Cu2+ 3D

(b)Fig - (a) UV-Vis absorption spectra for interaction POEA (a) solution and

(b) film.

Figure 2 shows AFM micrographs of POEA films, asdeposited and after being immersed for 20 min. into solutionscontaining HS, HS/Cu2+ and Cu2+ ions and then dried. For theas-deposited POEA film adsorbed during 3 minutes, the imagein Figure 2(a) displays a globular morphology with grainsdistributed all over the sample surface, similar to images fromthe literature [12]. The appearance of smaller domains inFigure 2(b) may be due to the incorporation of counterionsfrom HS in the polymeric matrix of POEA, causingmorphology changes [13]. The same occurs for the film

(d) POEA/Cu2+ POEA/Cu2+ 3DFig 2 - AFM images for as-deposited POEA film and of POEA films after

being immersed into solutions containing HS, HS/Cu2' and Cu2+.

421

ao

6i

.0

-0

k(nm)

J

,Vwwvw-------. i..rivp

X-

,,. 1 12.30so't

TABLE I - Average height (Z) and roughness for POEA films: as-depositedand after being immersed into solutions containing HS and HS/Cu2+ (20x20

Films Z (nm) Roughness (nm)POEA monolayer (pH =5) 19.4 3.5

POEA monolayer (pH = 5)/ HS (20 ppm; 272 20.7

pH =5)POEA monolayer (pH = 5)/ HS/Cu2+ (20 567 67.9

ppm; pH = 5)-. X ,POEA (pH = 5) + Cu2+ (I ppm) 8000 983

One could hypothesize that the changes in morphology shouldalso affect the electrical response of sensing units producedfrom nanostructured POEA films. In order to verify this point,we used a sensor array containing the following sensing units:bare gold electrode, POEA film, POEA/SL film, POEA-LS(complexed) film, POEA/HS film and HS film. Note that HSis now used in nanostructured films obtained by physicaladsorption. In a subsidiary experiment, we employed a sensor

array made of five bare chrome electrodes. Measurementswere carried out with the sensing units immersed into pure

water or aqueous solutions containing HS and Cu2+ ions. Theelectrical response was analyzed using an equivalent electriccircuit, whose film capacitance was taken as the figure ofmerit [14]. The PCA plot in Figure 3 shows that a distinctioncan be made of the capacitance data for solutions with variousconcentrations of HS (0.5; 1; 5; 10 and 20) mgL-, and purewater. One can note that the first principal componentincreases monotonically with the HS concentration.

1 Pare Water

1i I | |HS20magL

1-

0

= 065 =E

-2 l] HS HSSmgL

0

N

1 .5

AHS1mg/L ~

-2 -165 -1 -0.5 0 0.5 1 1.51st Principal Com ponent p8-4%)

Fig 3 - PCA for HS solutions and pure water.

4

° 2

cO0CL

E -20

i -4.-

u

N-10

-128

-14-6

Fig 4

0 Cu4mg/l

-4 -2 0 2 4 61st Principal Com ponent 97.74%)

- PCA for Cu2+ solutions and pure water.

The effects from adding Cu2+ into a HS aqueous solution can

also be monitored with electrical measurements, as indicatedby the PCA plot in Figure 5. The latter contains data forvarious aliquots of CUSO4 5H20 added to a HS solution (20mg L-l). Now, even a concentration of 1 mg LU' of Cu2+ couldbe distinguished.

0

E0

U0.

E0

R'I

3 1,,10Z zm

,HS20mg/L +CulO0mgIL HS20 mgIL C

2 +H

1 ~~~~~DHS 20 mg/L + Cu 8 mgL

HS20mg/L Cu 6mglL

HS20mgIL+C Img/L

-2HS 20mgIL Cu 2mgL

0 HS20mgW+Cu4fL

-6 -4 -2 0 2 4 6 8 1'

lst Principal Component 879A%)

Fig 5 - PCA plot for solutions containing Cu2+ and HS.

The data for the capacitance values when the sensing units areimmersed in CUSO4 5H2O solutions with final concentrationsof 1; 2; 4; 6; 8 and 10 mgL-1 of Cu2+ are shown in the PCAplot of Figure 4. With the exception of 1 mg L-1, whose signalalmost coincided with pure water, concentrations of 2 mgLU'or higher could be detected.

422

Cu 2mML +CulOmglI

Pure Water

Culmgit = = CulSuegL

u Img IC ==

11Cud &i

-

6

8

0

0

2

1-S

0

E00

Q

C)

u

c.

C4

X 10124 ~-ICul

1 Pure Water 1

1' -

Cu 2 muL

- 1- +

0 Pure Water2

-2 O -

-3 X 3

11Pure Water 3

-1 -0.5 0 0.5 1 1.5 2 2.5 31 st P rincipal Corm ponent 95Mf9%) x 10

Fig 6 - PCA for Cu2+ solutions and pure water, obtained withthe sensor array made up of 5 chrome electrodes (no film

deposited).

Interestingly, distinction among the various samples couldalso be obtained using a sensor array with 5 sensing unitsmade of bare chrome electrodes. The electrical response ofthese electrodes differed from each other owing to the distinctmorphologies of the electrodes. Fig. 6 illustrates this findingfor detection of Cu2+ ions down to a concentration of 2 mgLJ1.That a sensor array with no films could be equally efficient indetecting trace amounts of ions indicates the predominance ofinterfacial phenomena governing the electrical response. It isclear therefore that as far as the sensing ability is concerned,one may merely employ bare metal electrodes. However, thedistinct interactions with POEA films demonstrated here pointto a further avenue to explore, in which non-specificinteractions may be combined with specific interactions.Indeed, this appears to be the case of highly efficient sensorsmade of nanostructured films of humic acid, which arecapable of detecting pentachlorophenol down to 10-10 M [15].

IV. CONCLUSIONS

The interaction between a POEA film in the sensing units andHS/Cu2+ complexes in liquid samples was investigated usingatomic force microscopy and UV-VIS. spectroscopy. With thelatter we observed that the absorption spectra of POEA filmsare altered when in contact with HS and Cu2+ ions. PCA plotsindicated that the sensor array could distinguish between thewater samples containing humic substances and differentconcentrations of Cu2+.

ACKNOWLEDGMENTS

This work was supported by Embrapa InstrumentacaoAgropecuairia, Sao Carlos, Fapesp, CNPq, Rede Nanobiotecand CT-HIDRO (Brazil).

REFERENCES

[1] Branco, S.M., In: A Agua e o Homem. In Porto, R.L.L. EtAl., Hidrologia Ambiental. Editora Da Universidade De SaoPaulo: Associacao Brasileira De Recursos Hidricos: V.3.414p., (1991).

[2] Meybeck, M., River Quality Global Ranges, Time AndSpace Variabihtes, Proposal For Some Redefinitions. VerthInternat. Verein Lininol. Stuttgart. P. 81-96, (1996).

[3] Zhang, M. and Florence, T.M., Anal. Cuim. Acta, V. 197,P. 137 -148, (1987).

[4] Lombardi, A. T. and Jardim, W.F., Chem. Speciat.Bioavail, 9, P. 1-8 (1997).

[5] da Silva, W. T. L.; Thobie-Crouter, C.; Rezende, M.O.O.,Electroanalysis, 14(1), p. 71-77, (2002).

[6] Mattoso, L.H.C.; Manohar, S.K.; MacDiarmid, A.G.;Epstein, A.J., J. Polym. Sci: Part A: Polym. Chem., 33, 1227-1234 (1995).

[7] Instituto de Pesquisas Tecnologicas de Sao Paulo. Celulosee Papel. Tecnologia de fabrica,co de pasta celulosica, ed.2,Sao Paulo, 1, 84 (1988).

[8] Paterno, L. G.; Mattoso, L. H. C.; Oliveira Jr., 0. N.,Quimica Nova, 24 (2), P. 228-235, (2001).

[9] Paterno, L. G.; Mattoso, L. H. C., Polymer, 42, P. 5239-5245, (2001).

[10] Decher, G.; Hong, J. D., Ber. Bunsengs. Phys. Chem., 95(11), P.1430-1434, (1991).

[11] Patemo, L. G.; Constantino, C. J. L.; Oliveira Jr., 0. N.;Mattoso, L. H. C., Colloids And Surfaces B: Biointerfaces, 23,P. 257-262, (2002).

[12] Paterno, L. G.; Mattoso, L. H. C., Journal of AppliedPolymer Science, 83, P.1309-1316, (2002).

[13] Xie, L. K.; Josefowicz, L. J., Journal of MaterialsScience, 29, P. 4200-4204, (1994).

[14] Riul Jr., A.; Dos Santos, D. S.; Wohnrath, K.; DiTommazo, R.; Carvalho, A. C. P. L. F.; Fonseca, F. J.;Oliveira Jr., 0. N.; Taylor, D. M.; Mattoso, L. H. C.,Langmuir, 18, P. 239-245, (2002).

[15] Crespilho, F.N.; Zucolotto, V.; Siqueira Jr., J.R.;Constantino, C.J.L.; Nart, F.C.; Oliveira Jr., O.N.;Environmental Science & Technology, in press.

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