CHINESE JOURNAL OF CHEMICAL PHYSICS VOLUME 24, NUMBER 3 JUNE 27, 2011

ARTICLE

Adsorption of Cationic Laser Dye onto Polymer/Surfactant Complex Film

Pabitra Kumar Paula∗, Syed Arshad Hussainb, Debajyoti Bhattacharjee b, Mrinal Palc

a. Department of Physics, Jadavpur University, Jadavpur, Kolkata-700032, West Bengal, Indiab. Department of Physics, Tripura University, Suryamaninagar-799130, Tripura West, Indiac. CSIR-Central Mechanical Engineering Research Institute, Durgapur-713209, India

(Dated: Received on January 20, 2011; Accepted on May 8, 2011)

Fabrication of complex molecular films of organic materials is one of the most importantissues in modern nanoscience and nanotechnology. Soft materials with flexible propertieshave been given much attention and can be obtained through bottom up processing fromfunctional molecules, where self-assembly based on supramolecular chemistry and designedassembly have become crucial processes and technologies. In this work, we report thesuccessful incorporation of cationic laser dye rhodamine 6G abbreviated as R6G into thepre-assembled polyelectrolyte/surfactant complex film onto quartz substrate by electrostaticadsorption technique. Poly(allylamine hydrochloride) (PAH) was used as polycation andsodium dodecyl sulphate (SDS) was used as anionic surfactant. UV-Vis absorption spec-troscopic characterization reveals the formation of only H-type aggregates of R6G in theiraqueous solution and both H- and J-type aggregates in PAH/SDS/R6G complex layer-by-layber films as well as the adsorption kinetics of R6G onto the complex films. The ratio ofthe absorbance intensity of two aggregated bands in PAH/SDS/R6G complex films is merelyindependent of the concentration range of the SDS solution used to fabricate PAH/SDS com-plex self-assembled films. Atomic force microscopy reveals the formation of R6G aggregatesin PAH/SDS/R6G complex films.

Key words: Layer-by-layer self assembly, Cationic dye, Electrostatic adsorption, UV-Visabsorption spectroscopy

I. INTRODUCTION

Layer-by-layer (LbL) electrostatic self-assembly tech-nique is a versatile approach for the fabrication ofnanoscale thin films of organic dyes onto solid sub-strates. However, the aggregation and other physico-chemical properties of the adsorbed dye can be con-trolled with ease by the incorporation of some oppo-sitely charged surfactant in the assembly. In recenttimes polyelectrolyte-surfactant complexation and theirinteractions with some cationic laser dye at the solid-liquid interface has drawn much attention in the field ofmaterial chemistry [1]. Such complexes possess grow-ing commercial relevance as materials for separation-membranes, solubilization, and compatibilization [2].

In the present work, we have addressed the forma-tion of polymer-surfactant complex architectures ontoquartz substrate and thereby successful incorporationof a cationic laser dye onto this complex assembliesto fabricate polymer/surfactant/dye complex molecu-lar films by electrostatic LbL self-assembly technique.Poly(allylamine hydrochloride) (PAH) was taken as

∗Author to whom correspondence should be addressed. E-mail:pabitra tu@yahoo.co.in, FAX: +91-33-24138917

the cationic polyelectrolyte and sodium dodecyl sul-phate (SDS) was taken as anionic surfactant. The dyemolecule studied here was Rhodamine 6G (R6G) andit is a cationic laser dye. R6G is a dye from the xan-thene family. R6G has been used extensively as a sensor[3], nonlinear optical material [4], and photosensitizer[5]. R6G has also been utilized as a probe moleculein the field of extremely sensitive detection, such assingle-molecule detection using the surface enhancedresonance Raman effect [6, 7], nonlinear vibrational de-tection using hyper-Raman scattering [8], and nanome-ter scale detection using near-field Raman spectroscopy[9].

Therefore, it is extremely important to understandthe photophysical and photochemical behavior of theadsorbed dye to the polymer-surfactant complex archi-tectures onto solid substrates. It is worthy to men-tion that dye-surfactant interactions are of great inter-est in dyeing and photographic industries [10] in bio-logical and medicinal photosensitization [11]. Owing tothe displacement of small counterions, polyelectrolyte-surfactant association can be both entropically and elec-trostatically driven with a modest contribution fromhydrophobic interactions [12]. The expulsion of smallcounterions into the solvent during ion-pair complex-ation is the key driving force for many ionic aggre-gation processes involving macromolecules. The ionic

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Chin. J. Chem. Phys., Vol. 24, No. 3 Adsorption of Dye onto Polymer/Surfactant Film 349

character of these aggregates favors the adsorptionof oppositely charged dye molecules to form poly-mer/surfactant/dye complex molecular systems ontothe solid support.

In this work, we studied the photophysical behavior ofthe PAH/SDS/R6G complex LbL films and the adsorp-tion kinetics of R6G to the PAH/SDS complex archi-tectures in a narrow concentration range of SDS wherethe system may be sensitive to surfactant concentra-tion. This system was selected because of the interest-ing photophysical and photochemical properties of themonomeric and the aggregated dye [13]. Both H- and J-type aggregates of R6G are formed in PAH/SDS/R6Gcomplex LbL films as evidenced from their spectro-scopic characterization.

II. EXPERIMENTS

A. Materials

The cationic laser dye R6G (C28H31N2O3Cl,MW=479.0) and cationic polymer poly(allylamine hy-drochloride) (PAH) (purity>99%, MW=7×104) werepurchased from Aldrich Chemical Co., USA andwere used without any further purification. An-ionic surfactant sodium dodecyl sulphate (SDS)(CH3(CH2)11OSO3Na, MW=288.38, purity>99%) waspurchased from BDH Chemical Co., England. Thethree compounds are shown in Fig.1. The film depo-sitions were done onto the thoroughly cleaned fluores-cence grade quartz substrates. The electrolytic depo-sition baths were prepared with 1 mmol/L (based onrepeat units for the polyion) aqueous solutions. Purewater was taken from a Millipore system comprising re-verse osmosis followed by ion exchange and filtrationsteps.

B. Preparation of PAH/SDS/R6G complex LbL films

The detailed experimental procedure for fabricat-ing LbL electrostatic self-assembled films has been de-scribed elsewhere [14, 15]. In this experiment, PAHwas deposited onto quartz substrate for 15 min. Afterrinsing with pure water, the film containing PAH layerwas immersed into anionic SDS solutions with differentconcentrations (0.01−10 mmol/L) for 15 min. PAH wasused here to fix the anionic surfactant SDS moleculesonto the top of the PAH layer so that the terminal sur-face becomes negatively charged. Then each of theseLbL self-assembled films of PAH/SDS systems ontoquartz substrates were immersed into the R6G aque-ous solutions with dye concentration of 0.01 mmol/L.Sufficient time was allowed to adsorb the dye moleculesonto PAH/SDS LbL films and thus the PAH/SDS/R6Gcomplex LbL self assembled films were prepared. Theadsorption of the R6G molecules on the PAH/SDS sys-

FIG. 1 Molecular structure of (a) R6G, (b) PAH, and (c)SDS.

tems as well as the morphology of the complex filmwere studied by of UV-Vis absorption spectroscopy andatomic force microscopy (AFM, Veeco, Digital Instru-ment CP II Microscope) respectively.

III. RESULTS AND DISCUSSION

Figure 2 shows the normalized UV-Vis absorptionspectra of R6G aqueous solution. It is observed thatthe electronic absorption spectra of R6G aqueous solu-tions have the maximum absorption band at around526 nm due to R6G monomer and a higher energyshoulder band around 495 nm due to the aggregates ofR6G (sometimes called H dimer) [16]. These dimers areformed through van der Waals dye-dye interactions andRhodamine-water (counterions) interactions [17]. In or-der to check the contribution of H-dimer we measuredthe absorption spectra at high dye concentration in so-lution. Little increase of relative intensity of 495 nmband in comparison to 526 nm band was observed (fig-ure not shown). This may be due to the contribu-tion of aggregate and consequent formation of H-dimer.Peyratout et al. also observed the high energy shoulderof H-dimer along with monomeric band with R6G dye[18].

The molar extinction coefficient (ε) was calculatedas 8.1×104 mol−1cm−1 at wavelength 526 nm. Thepresence of two bands in the absorption spectra of thedye solution agreed completely with existing conceptof the splitting of energy levels upon combination ofthe dye molecules into H-type species such as dimer,trimer or higher order aggregates with the increase in

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350 Chin. J. Chem. Phys., Vol. 24, No. 3 Pabitra Kumar Paul et al.

FIG. 2 Normalized UV-Vis absorption spectrum of R6G inaqueous solution.

molar concentration. Baranova and Levshin reportedthat R6G aggregates in the dimerization stage at con-centration bellow 2 mmol/L but higher aggregates areformed at higher concentration [19]. At the low con-centration range the formation of the dimer and theirhypsochromic shift compared to their monomeric bandhas already been interpreted by DeVoe in terms of weakcoupling theory [20].

The experimental data suggest that the absorp-tion of R6G associates has a minimum in the range525−544 nm; therefore the contribution of absorptionof molecular aggregates of this compound to the over-all electronic absorption spectrum of the solution wassmall in this spectral range for very low dye concentra-tion [21]. This is why the LbL film fabrication was per-formed using very low concentration i.e. 0.01 mmol/Lof R6G in water.

In the present work, we have basically exam-ined the successful incorporation of the cationic laserdye R6G onto the pre-assembled PAH/SDS molecu-lar films. Figure 3(a) shows the absorption spec-tra of PAH/SDS/R6G complex LbL self-assembledfilms onto quartz substrate for different concentrations(0.01−10 mmol/L) of SDS aqueous solutions. In all thecases 15 min were considered for the deposition of SDSand R6G in the complex LbL films. From the figureit is observed that the two bands have been shifted to509 and 540 nm respectively compared to their solu-tion absorption spectrum. Also the spectrum is over allbroadened with respect to their solution counter part.The occurrence of 509 and 540 nm band in complex LbLfilms and the resultant red shift of the absorption spec-tra of LbL films compared to absorption spectrum ofR6G solution are due to the formation of greater num-ber of H-type dimer aggregates along with formation ofJ-type aggregates. It is these J-type aggregates whichcause spectral broadening and this red shift [22].

R6G microcrystal spectrum (Fig.3(b)) also showsband system with peaks at around 509 and 540 nm,red shifted with respect to the solution spectrum. Alsothe microcrystal spectrum shows a broadened spectralprofile. This red shift along with the broadening is dueto the presence of microcrystalline aggregate of R6G in

FIG. 3 (a) UV-Vis absorption spectra of PAH/SDS/R6Gcomplex LbL self-assembled films for different concentra-tions of SDS aqueous solution (0.01−10 mmol/L of SDS).Concentration of R6G aqueous solution was 0.01 mmol/L.(b) R6G microcrystal absorption spectrum (normalized).

the microcrystal films.It is worthwhile to mention that Arbeloa et al. re-

ported the adsorption R6G molecules onto Laponiteclay films at various concentrations and they showedthat various H- and J-type dimer aggregates would formand coexist within the film [23]. Grauer et al. foundred shift of about 10 nm for R6G in laponite surface[24]. Estevez and co-workers reported a red shift ofthe monomer absorption of R6G by 11 and 24 nm inlaponite [25, 26].

In the present case the red shift of about 24 nm ofR6G main absorption peak with respect to the corre-sponding solution spectrum and the close similarity tothe microcrystal absorption spectrum is surely due tothe formation of aggregation in the LbL films. Thesedifferent aggregates may be possibly due to the elec-trostatic interaction between cationic parts of the dyemolecules and the anionic parts of the surfactant SDSmolecules i.e. R6G molecules were subjected to morepolar environments in the complex films [27, 28]. Theratio of the absorption intensity of these two bands (fig-ure not shown) is merely independent of concentrationrange of SDS solution used to fabricate PAH/SDS com-plex assemblies onto the solid substrates. In this casethe electrostatic interaction dominate non-cooperativeassociation between PAH and SDS, where SDS be-ing hydrophobic moiety may favor aggregation by co-operative binding to the polymer. This also controls

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Chin. J. Chem. Phys., Vol. 24, No. 3 Adsorption of Dye onto Polymer/Surfactant Film 351

FIG. 4 (a) UV-Vis absorption spectra of PAH/SDS/R6Gcomplex LbL self-assembled films for different depositiontime of Rhodamine 6G onto PAH/SDS complex film, Arrowdirection shows the dye deposition time change as 20, 30,40, 50, 60, 300, 600, 900, 1200, 1500, 1800, 2400, 3000, and3600 s, respectively. Concentration of SDS and dye solutionwere 0.1 and 0.01 mmol/L respectively. (b) Absorbance in-tensity at 540 nm of PAH/SDS/R6G complex film vs. de-position time.

the overall aggregation of the dye molecules in theLbL films. This complexation may also depend uponthe overall charge distribution of the polymer onto thequartz substrates.

The internal structure of the polyelectrolyte-surfactant complex in self-assembled films is an impor-tant feature governing the further adsorption of the dyemolecules to the terminal surface of complex film ontoquartz substrates. In particular surfactant content andthe degree of association between surfactant and PAHwill determine the adsorption of R6G molecules as ev-idenced from the sequential increase in the absorbancein the main absorption peak (540 nm) with increasingconcentration of SDS in PAH/SDS complex molecularfilms. The effective adsorption requires available bind-ing sites on the terminal layer. In such case the cationicpart of the R6G molecules should interact with theavailable anionic binding sites of SDS in the PAH/SDScomplex architectures.

To understand the adsorption kinetics of R6Gmolecules onto the PAH/SDS complex assemblies, wehave taken the absorption spectra of PAH/SDS/R6Gcomplex films for different deposition times of R6G.Concentration of dye solution was 0.01 mmol/L of R6G.

700600500

400

300200

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0 100 200 400 600300 500 700X / nm

Y / n

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0.00 1.00 2.00 µm

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(b)

FIG. 5 (a) Atomic force micrograph of PAH/SDS/R6Gcomplex film deposited onto glass substrate. R6G depo-sition time was 15 min. (b) Atomic force micrograph ofPAH/SDS film deposited onto glass substrate.

Here in all the cases the PAH and SDS deposition timewas considered as 15 min but the dye deposition timevaried as 20, 30, 40, 50, 60, 300, 600, 900, 1200, 1500,1800, 2400, 3000, and 3600 s. After each immersion thesubstrate was washed off with an HCl aqueous solutionand then dried by blowing nitrogen gas. It is observedfrom Fig.4 that the main absorption peak (540 nm) in-creases sharply up to 600 s and then remained merelyconstant for the deposition time greater than 600 s.This is also evidenced from the plot of the the 540nm band versus deposition time. It is important tomention that the interaction of R6G molecules to thePAH/SDS films was occurred so rapidly that almost66% (as calculated from the change in absorbance in-tensity) of R6G molecules were deposited within first20 s and then saturated after 600 s. This observa-tion reveals that the electrostatic adsorption of the dyemolecules onto PAH/SDS complex film was completedafter 600 s, because of unavailability of the anionic bind-ing sites in the complex architectures. Non-uniformityin the absorption maxima as evidenced from Fig.4(b)occurs after 600 s of deposition as the electrostatic in-teraction doesn’t work as well and some repulsive typeof interaction plays between the cationic part of the dyemolecules in complex LbL films and the cationic groupof the R6G dye molecules in the solution [29].

To visualize and investigate the formation ofR6G domains as well as surface roughness in the

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PAH/SDS/R6G complex LbL self-assembled films weuse atomic force microscopy (AFM). Figure 5 showthe AFM pictures of the PAH/SDS/R6G complex LbLfilm and PAH/SDS LbL films deposited onto glass sub-strate. Concentration of R6G aqueous solution was0.01 mmol/L and its deposition time was 15 min. SDSconcentration was 0.1 mmol/L. From the Fig.5(a) weobserve closed packed conformation of dye moleculeswith their sharp edge and also their aggregates in theLbL films. This observation also confirms that the R6Gmolecules cover the whole area of the film after the in-teraction with SDS is completed. The RMS roughnessof the film was calculated as 2.157 nm. Figure 5(b)shows the complex molecular assemblies of PAH/SDSonto glass substrate.

IV. CONCLUSION

In summary, we have demonstrated the formationof polymer/surfactant complex molecular architecturesonto the quartz substrates and the successful incorpora-tion of cationic laser dye R6G onto the complex molec-ular system. These complexation and formation of dyeaggregates are due to the electrostatic interaction be-tween the oppositely charged species. Our observationreveals that in aqueous solution R6G molecules formedH-type aggregates (H-dimer) and these associates startto dominate on increasing the molar concentration asevidenced form their UV-Vis absorption spectroscopicdata. However, in case of PAH/SDS/R6G complexLbL films the absorption bands have been shifted to540 and 509 nm due to the formation of both H andJ-type aggregates. Electrostatic interactions betweenthe cationic dye and the anionic binding sites in thePAH/SDS facilitates complexation in the film. The ra-tio of the absorbance intensity of two bands in LbL filmis merely independent of the concentration range of SDSsolution used in the present work. Adsorption of R6Gmolecules to the PAH/SDS complex film occurred sorapidly that 66% R6G molecules deposited within 20 sand then saturated after 600 s of deposition as the un-availability of the anionic binding sites in the complexfilm. AFM picture also confirms the presence of R6Gand their aggregates in the complex LbL film.

V. ACKNOWLEDGMENTS

This work was supported by the Jadavpur Univer-sity Research (No.R-11/A/47/09) and the excellent in-frastructural facilities. S. A. Hussain is grateful toDST for financial support of DST Fast-Track project(No.SE/FTP/PS-54/2007).

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