Stimuli-Responsive Self-Assembled Multilayer Azo Thin Films: Effect of Aggregates and Salt on Significant Spectral Shifts Nasir M. Ahmad, 1 Syed J. Ali, 1 Mudassara Saqib 2 1 School of Chemical and Materials Engineering, National University of Sciences and Technology (NUST), NUST H-12 Campus, Islamabad 44000, Pakistan 2 Federal Postgraduate Medical Institute, Sheikh Zayed Hospital, Lahore, Pakistan Correspondence to: N. M. Ahmad (E-mail: nasir.ahmad@scme.nust.edu.pk) Received 29 October 2011; accepted 16 January 2012; published online 28 February 2012 DOI: 10.1002/pola.25968 KEYWORDS: adsorption; aggregates; azobenzene; blue shift; methyl orange dye; polyelectrolyte; self-assembly; thin films INTRODUCTION Fabrication of polyelectrolyte (PEL) layer- by-layer (LBL) self-assembled multilayer (SAMU) thin films has been extensively explored in recent years to obtain mo- lecular level control in numerous applications in the area of nanotechnology, biomedical, and many others. 1,2 Considering the versatility of SAMU film technique, it has also been now widely used to incorporate wide range of materials such as PELs, nanoparticles, clay, biomacromolecules, and colloids. In addition to these materials, it has also been demonstrated that low-molecular weight azobenzene chromophore dyes can also be adsorbed onto suitable substrate from aqueous solution via SAMU films. 3,4 Azobenzene dyes have long been considered as optically sensitive materials because of their strong absorbance in the UV–visible region. Because of such characteristics, azo compounds are widely used as colorants, stains, and markers. Among the various azo dyes, use of methyl orange (MO) in research has attracted much attention in recent years because of its roles such as an indicator in acid–base titrations, spectroscopy changes, dye oxidation, and loading/release studies. 5 In addition, research work has also carried out to investigate the self-assembly and aggrega- tion of MO to understand the state of interactions of dye molecules at the interfaces. 6–8 It is observed that in certain cases, very strong interactions of cationic surfactants and an- ionic MO dye occur at concentrations, which are much lower than the critical micelle concentration of the individual sur- factants and are characterized by as much as 80 nm lower wavelength shift of the main absorbance band of the dye. 9 Dye aggregation of the H-type has been suggested to account for the spectral changes. 6–8 Dilution of the dyes over micelles occurs upon increasing the surfactant concentration, and under these conditions, absorbance shift becomes similar to that in an organic environment. It is also observed that dye interaction and subsequent aggregation are responsible for the decreasing value of absorbance maxima, k max of MO in the presence of cationic polymers, surfactant, and poly- soaps. 10–12 Similarly, interactions between the chromophores are found to be responsible for the blue shift of azo- dendrimers and in cast films of azobenzene-containing amphiphiles. 13 In another related study, Langmuir–Blodgett films formed from the pair of MO and cationic surfactant and dye molecules oriented in a more or less parallel fashion to exhibit a blue shift band. 14 The aforementioned studies of MO suggest that although MO has been used widely in self-assembly, however, its direct use as an anion to build SAMU films has not been explored in much detail to the best of our knowledge. Considering this, it is interesting to investigate the behavior of smaller molecular dye of MO with anionic charge as component for alternate as- sembly and correlate charge density, solubility, and nature of aggregation of MO to the properties of the SAMU films. In this direction, our previous work has clearly highlighted the vari- ous factors including aggregation, which control the charac- teristics of the LBL SAMU thin films. 15–18 Therefore, in contin- uation of our previous work, in this work, formation of sequential LBL-SAMU thin films from the aqueous solutions of polycation poly(diallyldimethylammonium)chloride (PDAC) with anionic MO dye were used to investigate the magnitude and type of interactions of azo dye molecules. The MO dye is a low-molecular weight molecule; nonetheless, it is expected to possess sufficient charge density to allow charge compensa- tion for the polycation of PDAC to adsorb via electrostatic interaction between counter ions. In addition, aggregations of MO with PDAC in solutions with salt and in salt-free aqueous solutions were also examined. It was anticipated that electro- static interaction between PDAC/MO pairs might result in the increment in the interactions to produce aggregate to reflect V C 2012 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 1881–1889 1881 JOURNAL OF POLYMER SCIENCE WWW.POLYMERCHEMISTRY.ORG RAPID COMMUNICATION