Cationic PolythiopheneâSurfactant Self-Assembly Complexes: Phase

Article pubs.acs.org/Langmuir

Cationic Polythiophene−Surfactant Self-Assembly Complexes: Phase Transitions, Optical Response, and Sensing Rachel C. Evans,*,† Matti Knaapila,‡ Niamh Willis-Fox,† Mario Kraft,§ Ann Terry,∥ Hugh D. Burrows,⊥ and Ullrich Scherf§ †

School of Chemistry, Trinity College Dublin, Dublin 2, Ireland Physics Department, Institute for Energy Technology, NO-2027 Kjeller, Norway § Fachbereich Chemie, Bergische Universität Wuppertal, D-42097 Wuppertal, Germany ∥ ISIS Facility, Science and Technology Facilities Council, Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot, OX11 0QX, U.K. ⊥ Departamento de Química, Universidade de Coimbra, 3004-535 Coimbra, Portugal ‡

S Supporting Information *

ABSTRACT: The absorption and photoluminescence spectra of t h e c a t io n ic c o n j ug a t e d p o ly e l ec t r o l y t e p o l y [ 3 - ( 6 trimethylammoniumhexyl)thiophene] (P3TMAHT) were observed to be dramatically altered in the presence of anionic surfactants due to self-assembly through ionic complex formation. Small-angle neutron scattering (SANS), UV/vis, and photoluminescence spectroscopy were used to probe the relationship between the supramolecular complex organization and the photophysical response of P3TMAHT in the presence of industrially important anionic surfactants. Subtle differences in the surfactant mole fraction and chemical structure (e.g., chain length, headgroup charge density, perfluorination) result in marked variations in the range and type of complexes formed, which can be directly correlated to a unique colorimetric and fluorimetric fingerprint. Our results show that P3TMAHT has potential as an optical sensor for anionic surfactants capable of selectively identifying distinct structural subgroups through dual mode detection.

1. INTRODUCTION

eration of both the photophysical properties and the solutionphase structure of the polymer in tandem. One strategy by which the conformation and/or aggregation state of conjugated polyelectrolytes may be manipulated is through their formation of novel heterostructures with inorganic oxides,11,12 other polymers,13,14 or surfactants.7,15−20 Surfactants are particularly attractive for this purpose since they enhance the water solubility of the polymer, facilitating the preparation of reproducible CPE-based devices from a green solvent using environmentally friendly solution processing techniques such as inkjet printing.21,22 The addition of nonionic and ionic surfactants to an aqueous CPE solution promotes the dispersion of weakly soluble polymer aggregates, which manifests itself as one or more of the following photophysical changes: (1) fluorescence enhancement, (2) a spectral shift in the position of the absorption and/or emission band, and (3) resolution of vibronic structure in the optical spectra. Spectral shifts can lead to significant changes in the solution color, socalled surfactochroism,8 the extent of which may be controlled simply by varying the surfactant fraction. This has further

Optical sensor platforms based on conjugated polyelectrolytes (CPEs) have attracted significant attention for chemo- and biosensing.1−4 Water solubility, which is particularly advantageous for environmental or biomedical monitoring, is achieved by the addition of hydrophilic or ionic side chains to the polymer backbone.3 On exposure to the analyte, these pendant chains may behave as triggers for the sensor response, either by providing selective receptor sites for analyte recognition1−4 or, alternatively, by undergoing conformational changes which disrupt the effective conjugation length of the polymer backbone.5 The sensory response initiated as a result of one or both of these triggers manifests itself as a change in the absorption or fluorescence properties, whose signal may be amplified due to efficient electronic transduction along the delocalized π-electron polymer backbone.2 Optical sensing relies on the photophysical properties of the CPE probe changing in a reliable and reproducible fashion on exposure to the analyte. However, water-soluble CPEs are well-known to form ill-defined aggregates in solution, resulting in spectral shifts and a reduction in the fluorescence quantum yield,6−10 which can lead to erroneous sensing results. The design of an effective CPE-based optical sensor therefore requires consid© 2012 American Chemical Society

Received: May 28, 2012 Revised: July 23, 2012 Published: July 27, 2012 12348

dx.doi.org/10.1021/la302166a | Langmuir 2012, 28, 12348−12356

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internal and supramolecular organization of the PT−SDS complexes formed with each polymer, suggesting that the charge density of the ionic headgroup is a critical parameter in the self-assembly mechanism. Detailed understanding of the correlation between the solution structure of the CPE−surfactant complex and its optical properties will be crucial if the strategic design of a sensor directed at the identification of a specific surfactant class is to be achieved. Given that the chemical nature of the ionic group on the conjugated polyelectrolyte has been shown to significantly influence the binding mechanism, we now consider the role of the chemical composition, molecular shape, and charge density of the surfactant on this process. Here, we report the results of a systematic structural study of the self-assembly complexes formed between the cationic polymer P3TMAHT and two industrially important anionic surfactants: sodium octyl sulfate (SOS) and potassium heptadecafluoro-1-octanesulfonate (PFOS) (Figure 1). Subtle structural differences between

implications for the application of CPEs in sensing, where the surfactant enhances the polymer’s role as an emissive probe and may improve both sensitivity and selectivity of the sensor.3,16 Polythiophenes (PT), in particular, are well-known to exhibit dramatic colorimetric and/or fluorimetric responses on exposure to a variety of stimuli (e.g., temperature, 23 DNA,24,25 proteins,26 ions27−29), making them attractive candidates for sensing. Surfactants are themselves also important analytes, forming a major source of environmental pollution due to their extensive use in personal, home and car cleaning agents and industrial and agricultural processes.30 Single (colorimetric) and dual mode (colorimetric and fluorimetric) PT-based optical sensors for the selective detection of ionic surfactants have recently been reported.5,31−33 For example, colorimetric sensors based on the assembly-disassembly of poly(3-alkoxy-4-methylthiophene) (P3RO-4MeT) aggregates with 2-naphthalenesulfonate (NS)31 or 8-hydroxy-1,3,6-pyrenetrisulfonic acid (HPTS) in the presence of surfactants have been demonstrated.32 However, selectivity is limited to general identification of anionic, cationic, or nonionic surfactant classes, with the colorimetric response typically restricted to surfactant chains containing 12 carbon atoms or more. Moreover, little attention has been paid to the sensing mechanism. Complexation between a CPE and an oppositely charged surfactant is primarily driven by Coulombic interactions between the surfactant headgroup and the ionic polymer side chains.7 The role of secondary interactions (e.g., hydrophobic, van der Waals, hydrogen bonding, hydration) is less explicit and will undoubtedly determine the selectivity of the CPE toward structurally distinct subgroups of a given surfactant class. To date, however, there have been only few studies which attempt to correlate the photophysical properties of PT-based CPEs in aqueous surfactant solution with the structure and supramolecular organization of the electrostatic complexes formed in a systematic approach.18−20 Schosseler and coworkers recently investigated the binding of cationic surfactants with varying alkyl chain length (n = 6−16) to the conjugated polyanion poly(3-thiophene acetic acid) (PTAA) in dilute solution over a wide composition range (10−6 < x < 10−2), where x is the ratio of surfactant molecules to polymer repeat units (r.u.).20 At small x values, the molecularly dispersed PTAA chains are observed to collapse, which can be correlated with a blue-shift in the absorption maximum due to a decrease in the effective conjugation length. At very high x values, the absorption spectrum is only weakly sensitive to changes in the composition ratio, but the photoluminescence spectrum exhibits a new band that is considerably red-shifted from the parent PTAA emission. The origin of this new emission band is attributed to the formation of mixed PTAA−surfactant aggregates; however, the internal organization of these complexes remains unclear. We recently performed a systematic structural and photophysical study of the interaction between the cationic polythiophenes poly[3-(6trimethylammoniumhexyl)thiophene] (P3TMAHT)18 and poly[3-[6-(N-methylimidazoliumhexyl)thiophene] (P3ImiHT)19 and the anionic surfactant sodium dodecyl sulfate (SDS) in semiconcentrated aqueous solution (∼10 mg/mL). Although these two polymers differ only in the ionic terminal groups on their side chains, they exhibit quite distinct colorimetric and fluorimetric responses as a function of the SDS composition. Small-angle X-ray (SAXS) and neutron (SANS) scattering studies revealed significant differences in the

Figure 1. Chemical structures of the cationic polythiophene P3TMAHT (n = 40−50) and the anionic surfactants used in this study.

these surfactants have allowed us to investigate the effect of headgroup charge density, chain rigidity, and hydrophobicity on both the complex structure and optical properties using UV/vis absorption and photoluminescence (PL) spectroscopy and SANS. Structural and optical data for P3TMAHT upon binding to SDS have previously been reported.18 Key results are included here to facilitate discussion of the relationship between surfactant structure and the P3TMAHT response in the context of optical sensing.

2. EXPERIMENTAL METHODS Materials. P3TMAHT with a bromide counterion was prepared as previously reported by McCullough.34 The number-averaged molecular weight, Mn, was 17 000 g mol−1, as measured for the polymer precursor with bromohexyl side chains (precursor 2 in Scheme 1 of ref 34). Direct molecular weight measurement of the cationic P3TMAHT is not possible due to strong interaction of the conjugated polyelectrolyte with the GPC column. Sodium octyl sulfate (SOS) and potassium heptadecafluoro-1octanesulfonate (PFOS) were purchased from Sigma-Aldrich, and D2O (>99.92% D) was purchased from Apollo Scientific. All chemicals were used as received. P3TMAHT(surfactant)x−D2O mixtures were prepared by mixing a 10 mg/mL P3TMAHT−D2O solution, where the concentration is given with respect to the P3TMAHT monomer units, with either SOS−D2O (13.10 mg/mL) or PFOS−D2O (5.65 mg/mL), so that the desired molar ratio x was reached. The value x = 1 corresponds to the stoichiometric charge balance. For SOS (similar to the SDS system18) 12349


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Figure 2. (a) Photograph of P3TMAHT(PFOS)x−D2O, P3TMAHT(SOS)x−D2O, and P3TMAHT(SDS)x−D2O as a function of composition x at room temperature. (b) and (c) show the normalized UV/vis absorption spectra for P3TMAHT(SOS)x−D2O and P3TMAHT(PFOS)x−D2O, respectively. The arrows indicate the direction of the spectral shifts and bandwidth reduction. we observe the coexistence of dissolved and precipitated phases at x ≈ 1/2−1. The precipitate redissolves on gentle agitation. The compositions of all P3TMAHT−surfactant complexes studied by small-angle neutron scattering and optical spectroscopy are given in Table S1 of the Supporting Information. Instrumentation. Photographs of the samples were taken using a Canon Ixus digital camera. UV/vis absorption spectra were recorded on a Shimadzu UV-2100 spectrometer at room temperature. Photoluminescence (PL) measurements were performed using a HORIBA Jobin Yvon Fluorolog-3 fluorimeter in the front face geometry at room temperature. Excitation and emission monochromator slit widths were both maintained at 2.0 nm for concentrated samples (∼10 mg/mL) and 5.0 nm for diluted samples (∼0.1 mg/ mL). Spectra were corrected for the wavelength response of the system using correction factors supplied by the manufacturer. Samples were measured in quartz cuvettes with an extremely short path length (0.1 mm) to avoid difficulties due to a saturated signal detector or selfabsorption caused due to the elevated sample concentration. For comparison, the samples were also diluted to ∼0.01 mg/mL in water. UV/vis absorption spectra and PL and excitation spectra were recorded on the same instruments as above in a 1 cm path length quartz cuvette. SANS measurements were carried out at the LOQ beamline at ISIS Facility, Rutherford Appleton Laboratory (UK).35 The LOQ instrument uses incident wavelengths between 2.2 and 10 Å sorted by time-of-flight with a sample-to-detector distance of 4.1 m, resulting in a q-range between 0.009 and 0.24 Å−1. The samples were placed in quartz cuvettes (Hellma) of 2 mm path length and maintained at 25.0 ± 0.5 °C during the measurements. The raw data were corrected for the transmission, D2O background, sample cell, and detector efficiency. The 2D scattering patterns were azimuthally averaged and converted to an absolute scale. Data Analysis. The SANS scattering functions I(q) were initially interpreted using scaling concepts.36 The scattering intensity I(q) may be considered to scale as 1 I(q) ≈ q−α c

which the scattering would also level off at low q. When the data showed a Guinier plateau, this simple interpretation was enhanced by numerical modeling to obtain the arbitrary particle shape using the indirect Fourier transformation (IFT) program GNOM.37 When the scattering curve scaled as q−2, the sample was expected to contain sheet-like particles and the data were fitted to

I(q) =

I(0) 1 + ξ q exp(q2L2 /12) 2 2

(2)

where ξ and L represent the lateral size and thickness of sheet, respectively.38

3. RESULTS AND DISCUSSION Optical Response of P3TMAHT−Surfactant Complexes. The addition of anionic surfactant to a solution of P3TMAHT in D2O (≈10 mg/mL) results in a series of colorimetric transitions. Figure 2a shows photographs of P3TMAHT(PFOS)x, P3TMAHT(SOS)x, and P3TMAHT(SDS)x as a function of the molar composition, x. Notably, the colorimetric response of P3TMAHT is highly sensitive to surfactant chain length. On addition of either PFOS or SOS, which both contain a C8 chain, a color transition from deep red to orange is observed only for x > 1. In contrast, P3TMAHT(SDS)x−D2O (C12 chain) exhibits a series of well-defined color transitions, from red (x = 0) to deep purple (x = 1/5−1), to orange (x = 2), and finally yellow (x = 5), such that the surfactant mole fraction may be estimated semiquantitatively using the naked eye alone. Counterion size has been shown to influence the water solubility, aggregation state, and chromic response of HT-2,5-poly(thiophene-3-propionic acid).29 Yao et al. reported a similarly dramatic colorimetric response for P3RO-4MeT-NS to SDS but were unable to visually detect color transitions for chain lengths shorter than C12.31 The colorimetric response can be correlated with changes in the UV/vis absorption spectra. The absorption spectrum of

(1)

where c is the concentration and the exponent α may be interpreted in terms of an arbitrary particle shape: α = 1 for separated rods; α = 2 for sheet-like particles; α = 4 for smooth 3-dimensional particles, for 12350


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P3TMAHT(SDS)x−D2O exhibits similarly pronounced spectral transitions, with the absorption maximum (λabs) initially red-shifting from 438 nm (x = 0) to 500 nm (x = 1/5) to 590 nm (x = 1), before blue-shifting to 420 nm for higher surfactant fractions (x = 5).18 Figures 2b and 2c show the corresponding UV/vis absorption spectra for P3TMAHT(PFOS)x and P3TMAHT(SOS)x, respectively. For both systems, the absorption spectrum is characterized by a broad, featureless band; however, despite exhibiting similar colorimetric transitions, subtle differences in the spectral shift of the absorption maximum as a function of surfactant composition are discernible. P3TMAHT(SOS)x−D2O exhibits an analogous trend with x to SDS, first red-shifting to longer wavelengths for mole fractions below the charge compensation point (i.e., x < 1) and subsequently blue-shifting to shorter wavelengths for x > 1. In contrast, λabs remains essentially unchanged for P3TMAHT(PFOS)x−D2O, except at the highest surfactant fractions (x = 5). In addition, both systems show a significant reduction in the absorption bandwidth with increasing x. Bandwidth reduction has previously been attributed to increased molecular order and improved packing in J-aggregates of a cyanine dye upon addition of zwitterionic surfactants below the critical micelle concentration (cmc).39 Longer surfactant chains were shown to exhibit greater steric hindrance, thus weakening the intermolecular approach between dye and surfactant molecules, yielding less rigid aggregates. Notably, on dilution of P3TMAHT(PFOS)x−D2O and P3TMAHT(SOS)x−D2O by a factor of 100 (c ∼ 0.1 mg/mL), the UV/vis absorption spectra exhibit comparable trends to the parent samples in all cases (Figure S1). Dilution leaves the relative P3TMAHT−surfactant mole fraction unchanged but dilutes the surfactant concentration below the cmc for the SOS system. The photoluminescence (PL) properties of P3TMAHT are also highly sensitive to surfactant structure, with distinct fluorescence “fingerprints” obtained in response to chain length, ionic headgroup, and perfluorination. Addition of either SOS or SDS (x = 1/5 to 1) triggers both narrowing of the emission band and the observation of more resolved vibronic structure (shown in Figure 3a for P3TMAHT(SOS)x−D2O), which is assigned to the vibronic progression of the CC stretching mode (ΔE ≈ 0.15 eV).40 The effect is more pronounced for P3TMAHT(SDS)x−D2O, where it is also

accompanied by a significant red-shift in the emission maximum.18 The vibronic structure along with the narrowing emission band suggests that P3TMAHT adopts a more planar, ordered conformation in this concentration regime, which prevents free rotation of the polymer backbone. For x > 1, further distinctions can be identified. For P3TMAHT(SDS)x−D2O, the emission band loses its vibronic structure, broadens, and is blue-shifted, suggesting the return to a more twisted conformation along the polymer backbone.18 Notably, for x = 5, the emission maximum is considerably blueshifted compared with the P3TMAHT−D2O system, which was attributed to both a reduction in interchain interactions due to screening of polymer aggregates in this phase regime where SDS is present in (charge) excess and also the improved solubilization of P3TMAHT into isolated chains.18 In contrast, for P3TMAHT(SOS)x−D2O in this charge regime, the emission band exhibits significant red-edge broadening, indicating the some contribution from larger, less well-defined aggregates. However, neither loss of vibronic structure nor significant spectral shifts are observed, suggesting that no substantial structural reorganization takes place in this regime. We note that these systems are likely to exhibit a reasonable degree of polydispersity. The PL spectrum of P3TMAHT is notably insensitive to PFOS (Figure 3b). Addition of excess PFOS (x = 5) is required to induce even a moderate blue-shift (≈20 nm) in the emission maximum. The markedly different photoluminescence profiles for the SDS, SOS, and PFOS systems suggests that the structural organization of the P3TMAHT−surfactant complexes formed may be quite different in this concentration regime. Structure and Supramolecular Organization of P3TMAHT−Surfactant Complexes. SANS was used to study the nanoscale organization of P3TMAHT−surfactant aggregates. Here, differences in the scattering length densities (SLDs) of individual components in the P3TMAHT(surfactant)x−D2O system allow us to probe different “zones” of the P3TMAHT−surfactant complex. The SLDs of the surfactants differ significantly: SOS (and SDS) is closer to P3TMAHT, and PFOS is closer to D2O (Figure 4). Assuming that P3TMAHT is electrostatically complexed with the surfactant, such that surfactant molecules are physically located in its vicinity, the sequence of SLDs means that the P3TMAHT(SOS)x complex appears to neutrons predomi-

Figure 3. Normalized PL data of P3TMAHT(SOS)x and P3TMAHT(PFOS)x in D2O at room temperature (λex = 430 nm): x = 0 (solid black line), x = 1/3 (dashed black line), x = 2/3 (dot-dashed black line), x = 1 (solid blue line), x = 2 (dashed blue line), and x = 5 (solid red line).

Figure 4. Estimated neutron scattering length densities (SLDs) for the polymers and surfactants investigated. The scattering length densities for sodium dodecyl sulfate (SDS) and deuterated SDS (SDS-d25) are given for comparison. 12351


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GNOM37 or a sheet model (for q−2).38 The structural parameters obtained from the fits are given in Table 1. Addition of SOS to P3TMAHT−D2O leads first to a rapid increase (up to x = 1) and a subsequent decrease in scattering intensity (x > 1) (Figure 5a). The particle shape varies from cylindrical for x = 1/5 (q−1), to sheet-like for x = 1/3−1/2 (q−2), to larger, arbitrary-shaped aggregates for x = 1 (q−2.8), and back to sheets for x = 2−5. The lateral sheet dimension is >640 Å, the thickness increasing from ≈15 to ≈25 Å with x. The surfactant concentration is significantly below the cmc (130 mM (≈32 mg/mL) in D2O at 25 °C41), and as such, we do not anticipate the coexistence of isolated surfactant micelles here. This is supported by the SANS data for SOS in this concentration regime (Figure S4). This behavior is somewhat comparable to that of P3TMAHT(SDS)x−D2O,18 which shifts from cylindrical (x = 1/5−1/2) to sheet-like aggregates (x = 1/ 2−1), and finally to interacting particles that are essentially polymer-modified SDS micelles (x > 2). The difference between these systems seems plausible, since (i) the cmc of SDS is lower (8.2 mM ≈ 2.4 mg/mL42) and is exceeded at higher mole fractions (x > 1/2) and (ii) SOS is shorter, which results in less steric crowding and thus poorer screening of the polymer charge. The scattering intensity of P3TMAHT(PFOS)x−D2O is 10 times lower than observed for P3TMAHT(SOS)x−D2O (Figure 5b). Although the PFOS concentration is above the cmc (2 mM (≈1.1 mg/mL) in H2O at 25 °C43), contrast matching between the solvent and the surfactant means that the observed scattering arises primarily from surfactant-modified P3TMAHT. The data initially resemble pure P3TMAHT but start to manifest a turning point at ∼0.026 Å−1 and a slope q−2 in the region q < 0.026 Å−1 for x = 1/3−1. We interpret this behavior as coexisting spherical and sheet-like P3TMAHT particles within polymer−surfactant complexes. The spherical particles (Dmax = 80−100 Å) transmute into sheets with a lateral size >640 Å at higher surfactant mole fractions (x > 2). When the PFOS fraction is increased to x = 5, the data decay as q−4, which is indicative of the formation of much larger, yet compact, P3TMAHT particles within the complex. Implications for Sensing. In Figure 6, we rationalize the relationship between surfactant mole fraction and chemical structure, the P3TMAHT−surfactant complex organization, and optical response, as determined from SANS and optical spectroscopy experiments. The spectral window of the colorimetric response is critically dependent on the surfactant chain length. Longer (n = 12) alkyl chain lengths generate a series of distinct color transitions as the surfactant mole fraction is varied; the colorimetric changes are much less obvious for the SOS and PFOS systems, which both contain an eight-carbon chain. Counterion tailoring has a major influence on the water solubility of polythiophenes and has been used previously to modulate the absorption properties of PT solutions.29 McCullough et al.29 showed that with increasing counterion size a significant color change from a red aggregated phase to a yellow solution with disrupted aggregates is observed, which was attributed to a transition from an ordered rod-like phase to a disordered coil-like phase. Here, exchange of the P3TMAHT counterion changes the effective steric bulk of the substituent, promoting the disassembly of P3TMAHT aggregates and the concomitant formation of P3TMAHT−surfactant complexes and supramolecular assemblies. Longer surfactant chains introduce greater steric bulk, and consequently the order−

nantly as a single entity in D2O, enabling scattering from the entire polymer−surfactant complex to be observed. In contrast, scattering from P3TMAHT(PFOS)x originates primarily from the polymer, the surfactant appearing less visible in D2O, enabling the polymer organization within the complex to be studied. In principle, the scattering length density of PFOS matches the polymer backbone so it could be possible to see backbone−side chain−surfactant−solvent interfaces if the parts were sharply segregated. However, it is more plausible that polymer and surfactant are mixed. Also, these structures, even if real, would be smaller than observable by small-angle scattering. We note that these systems are likely to be highly polydisperse, and the shapes and/or supramolecular organization of the P3TMAHT−surfactant complexes described below are representative of the universal aggregate population only. Small quantities of less well-defined P3TMAHT aggregates are also anticipated to be present, as observed in the PL data described above, but their specific contribution may not be resolved from the SANS data. Figures 5a and 5b plot the SANS curves of P3TMAHT(SOS)x and P3TMAHT(PFOS)x complexes in D2O alongside

Figure 5. SANS data of P3TMAHT−D2O (black squares) and (a) P3TMAHT(SOS)x−D2O and (b) P3TMAHT(PFOS)x−D2O for x = 1/5 (red circles), x = 1/3 (blue triangles), x = 1/2 (cyan triangles), x = 2/3 (magenta diamonds), x = 1 (dark yellow stars), x = 2 (green crosses) and x = 5 (wine hexagons). Solid lines represent fits to the data as described in the text. Dotted lines show q−1, q−2, and q−4 for comparison. The overall concentration was about 10 mg/mL. T = 25 °C.

appropriate fits to the data (see below). In the absence of surfactant, scattering from pure P3TMAHT is observed, the data leveling off at low q and showing a broad maximum at 0.03−0.04 Å−1 (which is better resolved in the semilogarithmic presentationsee Figure S3), which we have previously assigned to interacting, predominantly spherical polymer particles (diameter, Dmax < 80 Å).18 SANS data were initially interpreted using scaling concepts to provide an indication of the aggregate shape (q = 0.01−0.1 Å−1).36 When the data showed a Guinier plateau, numerical modeling was applied using either the indirect Fourier transformation (IFT) program 12352


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Table 1. Selected Structural Parameters Obtained from the SANS Data for P3TMAHT−D2O, P3TMAHT(SOS)x−D2O, and P3TMAHT(PFOS)x−D2Oa material

α

α −1

0.01−0.026 Å P3TMAHT P3TMAHT(SOS)x 1/5 1/3 1/2 2/3 1 2 5 P3TMAHT(PFOS)x 1/5 1/3 1/2 2/3 1 2 5

model

0.01−0.1 Å

spheres (interacting) 1.19 1.93 1.93 2.18 2.79 2.18 2.15

± ± ± ± ± ± ±

Dmax (Å)

Rg (Å)

I(0) (cm−1)

80

23.6 ± 0.6

0.79 ± 0.01

ξ (Å)

L (Å)

958 ± 4 964 ± 3 1486 ± 4

>640 >640 >640

∼15 ∼15 22 ± 1

1225 ± 2 829 ± 2

>640 >640

23 ± 1 25 ± 1

>640

∼15

−1

0.02 0.01 0.01 0.02 0.03 0.02 0.01

1.68 ± 0.08 1.80 ± 0.11 2.15 ± 0.04 2.00 ± 0.03 3.54 ± 0.25

cylinders sheets sheets sheets emerging larger aggregates sheets sheets spheres spheres spheres + sheets spheres + sheets spheres + sheets sheets large aggregates

80 100

24.5 ± 0.6 29.6 ± 1.3

0.68 ± 0.01 0.71 ± 0.02

240 ± 3

α, Dmax, and Rg, are respectively the scattering power, considered maximum size of the particle, and the radius of gyration of the arbitrary-shaped particle. I(0) is the estimated scattering intensity at zero angle. ξ and L represent lateral size and thickness of sheet-like particles.

a

The P3TMAHT photoluminescence is sensitive to differences in chain length, perfluorination, backbone rigidity, and headgroup charge density. The emergence of vibronic structure is observed for both SDS and SOS for x > 1/2, suggesting the formation of rigid, compact planar aggregates. This effect is more pronounced and the emission significantly more redshifted for the SDS. The SANS data indicate the formation of two-dimensional sheet-like assemblies in this concentration regime, where the sheets are comprised of P3TMAHT− surfactant rods interspersed by surfactant molecules (Figure 6). More striking distinctions are observed between the PL spectra for SOS and PFOS, which both contain a C8 chain, but differ in the chemical nature of their headgroups and hydrophobic tails. It is well-known that perfluorinated alkanes are more rigid than their hydrocarbon analogues44 and typically form lyotropic liquid crystalline phases with low curvature (e.g., lamellar).45 The cmc for perfluorinated surfactants is usually lower than for hydrogenated surfactants with the same chain length (as seen here for PFOS and SOS), and van der Waals interactions tend to be weaker.46 Theoretical studies have also shown that sulfate and sulfonate head groups differ significantly in their charge density (δ = −1.13 and −0.66, respectively).47 In addition, the inductive effect of the fluorine atoms on the chain will reduce this still further, suggesting that the electrostatic interaction between P3TMAHT and the surfactant headgroup will be weaker for PFOS. Aggregation may also be adversely affected by PFOS having the more rigid fluorocarbon chain. The PL data indicate that the P3TMAHT aggregation state shows little variation for x = 0−1 for P3TMAHT(PFOS)x−D2O, suggesting that the presence of PFOS does not disrupt the conformation of intrinsic P3TMAHT aggregates. This idea is supported by the SANS data, where spherical and sheet-like P3TMAHT aggregates are observed in this concentration range. The SANS data also show that PFOS exhibits a reduced range of structural transitions compared to both SOS and SDS, preferring to adopt a low-curvature, sheet-like conformation within P3TMAHT−surfactant aggregates. We propose that the

Figure 6. Schematic representation of the correlation between the structure and supramolecular organization of P3TMAHT−surfactant complexes and the observed optical response. (I) Fluorimetric response represented by changes in emission band shape or wavelength relative to isolated P3TMAHT (black line). Blue- and red-shifts in λem are indicated by the emission band color. (II) Observed colorimetric response as a function of x. (III) Proposed P3TMAHT−surfactant complex structures in aqueous solution. The red and yellow areas refer to P3TMAHT and surfactant, respectively.

disorder transition is induced at lower mole fractions and to an apparently greater extent in our concentration regime. 12353


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difference anionic surfactants, P3TMAHT may find potential application in dual-mode optical sensors that enable discrimination between different structural motifs within a general surfactant class. Since the spectral profile of free P3TMAHT is known, a ratiometric approach could be employed to correlate the concentration of the free and bound forms of the polymer with the surfactant concentration, thus providing a means of internal referencing and self-calibration. We note that the structural organization of the P3TMAHT−surfactant complexes formed, and therefore the optical response, is likely to be sensitive to the solvent polarity, and this factor should be considered if designing a sensor platform for application in nonaqueous media. Clearly, practical application of P3TMAHT in optical sensing requires transfer of this approach to a solidstate sensor platform. Responsive surfaces based on poly(3,4ethylenedioxythiophenes) containing imidazolium pendant groups have recently been demonstrated to exhibit changes in color, oxidation state, and wetting behavior due to anion exchange.51 We propose that a similar approach could be exploited here to design a sensor based on array of sensor chips comprised of P3TMAHT films coated onto microtiter wells or deposited as inkjet printed “dots”. Anion exchange and pattern recognition based on spectral changes could subsequently be utilized to identify and analyze anionic surfactants present in complex multianalyte environments.52

reduced charge density of the sulfonate headgroup means that PFOS is less able to penetrate existing spherical P3TMAHT aggregates than SOS, such that the electrostatic interactions occur predominantly at the aggregate surface. Since this will not alter the effective conjugation length of the P3TMAHT backbone, the optical and electronic properties are essentially unchanged. The results indicate that the colorimetric and fluorimetric response of P3TMAHT can be correlated with a series of transitions related to the self-organization of P3TMAHT− surfactant complexes. Subtle differences in the surfactant chain length, headgroup charge density, and rigidity/hydrophobicity of the alkyl/perfluoroalkyl chain result in marked variations in the range and type of self-assembly complexes formed. Clearly, the electrostatic interaction between the conjugated polyelectrolyte and the surfactant is not the only parameter driving supramolecular organization. Hydrophobic packing, hydrogen bonding, and/or ion-pairing also play a significant role in determining the nature and packing of the aggregates formed, which subsequently determines the optical response. Thermodynamic considerations are also important. It has been shown for poly(3-thiopheneacetic acid) in the presence of cationic surfactants at low mole fractions that spontaneous exchange of the Na counterions on the polyelectrolyte and subsequent surfactant binding are favored due to both enthalpic and entropic gains.20 As the surfactant mole fraction is increased, cooperative binding occurs, where the presence of bound surfactant facilitates and drives further binding. The binding between saturated polyelectrolytes and oppositely charged surfactants may be either endothermic48−50 or exothermic.48 A hydrophobic backbone favors exothermic binding, whereas endothermic binding occurs with polyelectrolytes with hydrophilic backbones for entropic reasons. It seems likely, therefore, that exchange of the P3TMAHT bromide counterion with the anionic surfactant will also result in thermodynamically favored binding. At low surfactant fractions, we anticipate that the observed spectroscopic response will contain a contribution from both free and bound P3TMAHT; at higher mole fractions, however, the optical spectra are expected to arise predominantly from bound P3TMAHT−surfactant complexes. Based on the thermodynamic considerations described above, surfactant binding should be favored, and we do not anticipate a contribution from free surfactant molecules at mole fractions below the charge compensation point. To enable direct correlation between the structure and supramolecular organization of P3TMAHT−surfactant complexes in solution and the observed colorimetric and photoluminescence trends, SANS and optical spectroscopy studies were performed on the same samples. Clearly, the concentrations required for SANS (∼10 mg/mL) are somewhat elevated compared to the optically dilute regime required for quantitative UV/vis absorption and photoluminescence measurements. The method used here, therefore, means that we are unable to directly quantify absorbance or PL quenching effects or PL quantum yields. However, on dilution of P3TMAHT(surfactant)x−D2O to the typical concentrations used for optical sensing, comparable trends are observed (Figures S1 and S2). In addition, pure surfactant solutions exhibit no significant UV/vis absorption or photoluminescence across the concentration regime and spectral window investigated. Given the observed variations of both the colorimetric and photoluminescence response of P3TMAHT in the presence of

4. CONCLUSIONS The absorption and photoluminescence spectra of the cationic conjugated polyelectrolyte poly[3-(6trimethylammoniumhexyl)thiophene] (P3TMAHT) were observed to be dramatically altered in the presence of anionic surfactants due to self-assembly through ionic complex formation, whose internal and supramolecular organization varies with both the surfactant mole fraction and chemical structure. At room temperature, the observed phase transitions are from charged P3TMAHT aggregates with interparticle order to rod-like and sheet-like particles with embedded polymer bundles or sheet-like polymer associations. These structural transitions are accompanied by a series of colorimetric shifts in the solution color, which are more distinctive for longer surfactant chain lengths. The P3TMAHT photoluminescence profile is also modified by the formation of P3TMAHT−surfactant aggregates, resulting in increased resolution of vibronic structure and a red-shift in the emission band for SOS and SDS. In contrast, in the presence of PFOS, the PL spectrum remains essentially unchanged. Subtle differences in the surfactant mole fraction and chemical structure (e.g., chain length, headgroup charge density, perfluorination) therefore give rise to marked variations in the range and type of complexes formed, which can be directly correlated to a unique colorimetric and fluorimetric fingerprint. Our results suggest that P3TMAHT has potential as an optical sensor for anionic surfactants capable of selectively identifying distinct structural subgroups through dual mode detection. We believe that detailed investigation of the specific nature of the polymer−analyte interaction is therefore crucial if the mechanism of CPE-based sensors is to be understood. This knowledge can be harnessed to enable the strategic design of CPE sensors with a tailored spectral response and selectivity. Inspired by these results, we are now investigating the development of a quantitative dual-mode optical sensing assay for anionic surfactants based on P3TMAHT. 12354


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ASSOCIATED CONTENT

S Supporting Information *

SANS data of pure surfactants and P3TMAHT in D2O, sample compositions, and UV/vis absorption and PL spectra after dilution. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +353 (0)1 896 4215. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research project has been supported by the European Commission under the seventh Framework Programme through the “Research Infrastructures” action of the “Capacities” programme (Contract CP-CSA_INFRA-2008-1.1.1 Number 226507-NM13). We thank ISIS and STFC for the allocation of beamtime and the Irish Research Council for Science, Engineering and Technology (IRCSET) for the award of a postgraduate scholarship (NWF).

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REFERENCES

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