Effect of Metal Cations on Polydiacetylene Langmuir Films - Langmuir

Article pubs.acs.org/Langmuir

Effect of Metal Cations on Polydiacetylene Langmuir Films Alexander Upcher,†,‡ Yevgeniy Lifshitz,†,‡ Leila Zeiri,‡,§ Yuval Golan,†,‡ and Amir Berman*,‡,∥,⊥ †

Department of Materials Engineering, ‡Ilse Katz Institute for Nanoscale Science and Technology, §Chemistry Department, Department of Biotechnology Engineering, and ⊥National Institute for Biotechnology, Negev (NIBN), Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel

∥

ABSTRACT: Polydiacetylene (PDA) Langmuir films (LFs) are a unique class of materials that couple a highly aligned conjugated backbone with tailorable pendant side groups and terminal functionalities. The films exhibit chromatic transitions from monomer to blue polymer and finally to a red phase that can be activated optically, thermally, chemically, and mechanically. The properties of PDA LFs are strongly affected by the presence of metal cations in the aqueous subphase of the film due to their interaction with the carboxylic head groups of the polymer. In the present study the influence of divalent cadmium, barium, copper, and lead cations on the structural, morphological, and optical properties of PDA LFs was investigated by means of surface pressure−molecular area (π−A) isotherms, atomic force microscopy, optical absorbance, and Raman spectroscopy. The threshold concentrations for the influence of metal cations on the film structure, stability, and phase transformation were determined by π−A analyses. It was found that each of the investigated cations has a unique influence on the properties of PDA LFs. Cadmium cations induce moderate phase transition kinetics with reduced domain size and fragmented morphology. Barium cations contribute to stabilization of the PDA blue phase and enhanced linear strand morphology. On the other hand, copper cations enhance rapid formation of the PDA red phase and cause fragmented morphology of the film, while the presence of lead cations results in severe perturbation of the film with only a small area of the film able to be effectively polymerized. The influence of the metal cations is correlated with the solubility product (Ksp), association strength, and ionic−covalent bond nature between the metal cations and the PDA carboxylic head groups.

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and the red PDA shifts are at 1515 and 2120 cm−1 for the double and triple bonds, respectively.20,21 The polymerization process of PDA LFs was monitored in situ using synchrotron grazing incidence X-ray diffraction (GIXD).22 The monomer to blue to red chromatic phase transitions were shown to be accompanied by variations in crystal structure and molecular alignment within the LF. Transition from monomer to the blue phase involves in-plane shear motion of the molecules, while the blue to red transition involves a decrease in the planar unit cell area, manifested in a shorter d-spacing between the polymer backbones, and accompanied with simultaneous movement of the alkyl residues to a near-upright position. To describe the rate of phase transitions in polydiacetylene films, reaction kinetic models were extracted from changes in the optical absorbance in the films.19 It was found that the reaction rate was strongly dependent on the film substrate and thickness. Polymerization directly at the air−water interface was found to be 2−3 orders of magnitude faster compared to that of solidsupported films of the same material. Gaining control over structural, morphological, and optical properties of PDA LFs bears both fundamental and potential technological interest. Possible routes to these objectives can be

INTRODUCTION Polydiacetylene Langmuir films (PDA LFs) are ultrathin organic layers produced on the air−liquid interface. These linear conjugated backbone polymers consist of alternating triple and double bonds in an “yne−ene” motif. PDA forms robust, crystalline polymer films with well-defined linear strand morphology, strong optical absorbance in the visible range, and good mechanical stability.1 There is growing interest and intense investigations of polyconjugated organic films which can be incorporated into novel multifunctional “molecular” devices.2−4 Long chain, amphiphilic PDA films can be chemically modified, a property that renders them attractive due to their optical5−8 and sensing9−12 properties, as well as their use as effective templates for oriented nucleation of calcite13,14 and semiconductor nanocrystals.15−18 Polydiacetylene LFs undergo topotactic polymerization reaction under UV irradiation, from the colorless monomer film to the metastable blue phase of the polymer. Upon further irradiation, transition from the transient blue to stable red phase of the polymer occurs, followed by the degradation stage of the polymeric film at high doses of radiation.19 Each chromatic phase has two absorbance peaks, vibronic and excitonic, positioned at 590 and 640 nm for the blue phase and 500 and 550 nm for the red phase, respectively.8 The conjugated backbone of PDA shows a strong Raman scattering signal: the PDA blue phase Raman shifts are positioned at 1455 and 2085 cm−1 © 2012 American Chemical Society

Received: December 1, 2011 Revised: January 26, 2012 Published: January 30, 2012 4248

dx.doi.org/10.1021/la204735t | Langmuir 2012, 28, 4248−4258

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area per molecule correlated well with the degree of ionic versus covalent bonding as estimated by the Pauling electronegativity. BaA2 > MnA2 > CdA2 > PbSt2 is ordered in increased ionic bond character, with BaA2 being the most ionic. The lattice parameters, symmetry, and area per molecule were found to be independent of the length of the fatty acid alkane chains for all investigated cations; hence, they are the outcome of the metal ion identity. Novel properties of organized molecular films of 10,12tricosadiynoic acid (TRCDA), modulated by transition-metal ions, were also investigated.33 It was found that Cu2+, Zn2+, Ni2+, Cd2+, and Ag+ metal ions at uniform 1 mM concentration in the subphase can greatly affect the monolayer formation of TRCDA and the properties of the subsequently deposited LB films, particularly in the case of Ag+, Zn2+, and Cu2+ ions. TRCDA LB film from the subphase containing Ag+ ion could not be photopolymerized. Zinc ion coordinated TRCDA film could be photopolymerized into a blue polydiacetylene film, which showed a reversible thermochromism between blue and purple color upon thermal stimulation. On the other hand, copper ion coordinated TRCDA film could only be photopolymerized to a red PDA film, which showed supramolecular chirality although TRCDA itself is achiral. Our own studies of the zinc cation influence on the properties of PDA LFs showed that zinc cations stabilize the blue phase of PDA and inhibit its transformation into the red phase, withstanding prolonged exposure to UV irradiation.34 The increased stability was accompanied with loss of the linear strand morphology and the appearance of a new, transient purple phase before the final red phase stage. It was found that the most significant differences in film properties occur when the zinc cations are already present in solution during the polymerization stage at sufficient concentration. In such a case zinc ions penetrate into the headgroup interlayer in the trilayer organization and are implicated in the film’s chromatic properties. In cases where polymerization was carried out prior to introduction of the cations, the properties of PDA/Zn2+ films are similar to those of films produced on a pure water subphase. Here we report the modification of the PDA optical and structural properties by introduction of barium, cadmium, copper, and lead divalent cations to the LF aqueous subphase. These properties were systematically investigated by various analytical techniques including π−A isotherms, atomic force microscopy (AFM), optical absorbance, and resonance Raman scattering (RRS). Unique behavior was observed for each of the investigated cations that was correlated with the association strength between the metal cations and the PDA carboxylic head groups.

achieved by chemical modifications of the PDA pendant groups or by introduction of different side chains. Studies of the interactions of metal cations with organic lipid monolayers at the air−water interface showed that, under certain conditions, the properties of the organic films were modified and enhanced. For example, metal cations were shown to improve the stability of floating monolayers.23,24 To get the most dense octadecanoyl monohydroxamate (C18N) monolayer at the air−water interface, the influence of various ions (Ca2+, Sr2+, Ba2+, and Fe3+) at various subphase pH values was studied by pressure−area (π−A) isotherms and Brewster angle microscopy (BAM).25 It was shown that monolayers developed on metal ion containing subphases were more compact than those on pure water due to interaction between C18N molecules at the air−solution interface, and the ions dissolved in the subphase in the order Sr2+ > Fe3+ > Ca2+ > Ba2+. Another report concerns Langmuir−Blodgett (LB) multilayers of 4-(10,12-pentacosadiynamidomethyl)pyridine, which could only be built up when the subphase contained Cu2+ or Cd2+ ions.26 Structural investigations of lipid monolayers with metal cations were carried out using different characterization techniques. Surface pressure versus molecular area (π−A) isotherms of stearic acid monolayers were systematically studied on aqueous subphases containing monovalent (Na) or different divalent (Mg, Ca, Ba, Zn, and Cd) cations at various pH values.27 Increased pH in the subphase was found to lower the transition surface pressure from the tilted to upright (LC to S) phases. GIXD studies of behenic acid (BA) Langmuir monolayers on cadmium, lead, magnesium, and manganese aqueous subphases showed a threshold in subphase concentration, above which a superlattice inorganic organized layer is formed.28 This threshold value also strongly depends on the subphase pH, where high pH enhances superlattice formation. Below the threshold, the ions are no longer organized but induce a condensing effect on the BA film that improves LB transfer. Another study showed using GIXD an ordered organization of a double crystalline layer of cadmium cations under arachidic monolayers.29 Infrared reflection− absorption spectroscopy (IRRAS) measurements of octadecanoic acid monolayers on aqueous subphases containing 1 mM BaCl2, CuCl2, NiCl2, and ZnCl2 provided insight into the molecular order and the coordination complexes of the alkanoic acid/bivalent cation system.30 It was shown that each of the investigated cations stabilized different periodic structures of the organic films. Ba2+ cations induce chain packing mainly in the orthorhombic structure, Cu2+ in the hexagonal structure, and Zn2+ in the triclinic structure. Variation of the dissociation of a Langmuir monolayer of arachidic acid at the air/water interface as a function of the subphase pH and for Cd2+, Ca2+, Mg2+, and Na+ cations by the polarization-modulated infrared reflection−absorption spectroscopy (PM-IRRAS) method was also investigated. The infrared spectra give access to the relative concentration of acid and salt molecules and allowed determination of the influence of the subphase pH on the acid dissociation reaction for each cation. It was found that Na+ obeys the purely electrostatic Gouy−Chapman theory quite well. On the contrary, ions with a high number of electrons such as Ca2+ and Cd2+ have a specificity and an aptitude to form directed interactions with the carboxylate and, thus, to form a complex at the interface.31 The structures of LB films of manganese arachidate (MnA2), lead stearate (PbSt2), cadmium arachidate (CdA2), and barium arachidate (BaA2) monolayers were investigated using atomic force microscopy.32 The limiting

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EXPERIMENTAL SECTION

Chemicals. 10,12-Pentacosadiynoic acid, PCDA (≥97%, HPLC grade, Fluka), chloroform stabilized with amylene (HPLC grade, Bio Lab, Ltd., Israel), and the metal salts CdCl2, BaCl2, PbCl2, and CuCl2·2H2O (Sigma, analytical grade) were used as received. Distilled water was obtained using a Millipore filtration system with a resistivity of 18.2 MΩ·cm. Langmuir Film Preparation. A Teflon Langmuir trough (model 611, Nima Coventry, U.K.) was filled with ultrapure water (18.2 MΩ·cm, Millipore), and PCDA spreading solution (50 μL, 2 mM in chloroform) was carefully spread onto the water surface. After spreading, 15 min was allowed for solvent evaporation and surface diffusion. Slow surface compression (20 cm2·min−1) was performed at ambient conditions (room temperature, approximately 25 °C) on aqueous subphases of the 4249


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Figure 1. Evolution π−A compression isotherms with metal cation concentration in the aqueous subphase and limiting area (AL) vs surface pressure at the collapse point (πc) determined for each isotherm on different subphases for divalent (a, b) barium, (c, d) cadmium, (e, f) copper, and (g, h) lead cations. The representative concentrations for low and high regimes used in this work are marked with bold lines and full circles. metal cation solutions. For each metal salt 7−8 different concentrations were used, spanning the metal cation range 0 < [M2+] < 50 mM. A UV Pen-Ray lamp (λ = 254 nm, 4 W, UVP) was used to polymerize the LFs. The lamp was mounted at a distance of 8 cm from the floating film, corresponding to an irradiation power density of 1.1 × 10−4 W·cm−2. Characterization Methods. AFM. PDA LFs were transferred onto precleaned glass slides using the Langmuir−Schäffer (LS) method and characterized at ambient conditions using a Dimension 3100 (Veeco) AFM instrument. The instrument was mounted on an active antivibration table and operated in tapping mode using a 100 μm scanner. Typically, 5 μm scans at a scan rate of 1 Hz were taken. Optical Absorbance. Measurements were carried out on a Jasco V-550 spectrophotometer equipped with a double-beam system including a single monochromator, halogen lamps, and a photomultiplier detector. For these measurements the film was deposited on a precleaned glass substrate by the LS technique. RRS. Measurements were performed on a Jobin-Yvon LabRam HR 800 micro-Raman system, equipped with a liquid nitrogen cooled detector and 50× microscope objective lens. A He−Ne (633 nm) laser was used for excitation. This wavelength closely corresponds to the main absorbance peaks of the blue phase of PDA, giving rise to the resonance Raman (RR) effect. Most measurements were taken using 600 gratings/mm and a microscope confocal hole setting of 100 μm corresponding to a resolution of about 4 cm−1. The incident intensity on the sample was attenuated using neutral density filters. Sample preparation was as described for optical absorbance measurements.

properties of the Langmuir films during compression. PCDA monolayers compressed on pure water under ambient conditions (T = 25 °C) collapse at a surface pressure (πc) of ca. 13 mN m−1 and limiting area (AL) of 27 Å2 molecule−1, followed by rearrangement to a trilayer morphology.35−37 AL is defined as the intersection of the tangent line at the isotherm’s steepest linear region to the A-axis. Appearance of the equilibrium phases in the surface π−A isotherms, e.g., gaseous, liquid, and solid phases, assured that the compression rate (20 cm2/min) was sufficiently slow. The presence of metal cations in the aqueous subphase causes significant changes in the isotherm profiles, suggesting different packing modes of the PCDA. Each of the investigated metal cations had a unique influence on the π−A isotherms profiles. Compression π−A isotherms for various concentrations of metal cations of cadmium, barium, copper, and lead are presented in Figure 1. Metal cation concentrations were generally divided into two meaningful ranges, low and high, apart from cadmium and barium, which also have a medium concentration (Table 1). At the low concentration range isotherms and correspondingly AL were similar to those obtained on the pure water subphase. On the other hand, at the medium and high concentration range each metal cation shows its unique effect on the isotherm profile. For increasing concentrations of Ba2+, the only change in the π−A isotherms was an increase in the surface pressure at the collapse point without significant reduction in AL. Only at very high Ba2+ concentrations (10 mM < [Ba2+]), isotherms showed larger AL with decreased collapse pressure. This increase in AL could be attributed to the large

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RESULTS π−A Isotherms. π−A isotherms of PCDA molecules obtained on different aqueous subphases reflect the structural 4250


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Table 1. Summary of Concentration Ranges, Threshold Concentrations, Surface Pressures, and Limiting Areas at the Collapse Stage for Different Metal Cations Based on π−A Compression Isotherm Analysis Cd2+ Ba low

conc. range (mM) AL (Å2 molecule−1) πc (mN m−1) actual conc. used in this studya (mM) medium conc. range (mM) AL (Å2 molecule−1) πc (mN m−1) actual conc. used in this studya (mM) high conc. range (mM) AL (Å2 molecule−1) πc (mN m−1) actual conc. used in this studya (mM) threshold conc. (mM)

2+

0 ≤ [Ba ] ≤ 0.15 27 13−22 0.05 0.15 < [Ba2+] ≤ 10 27−26.2 22−28 5b 10 < [Ba2+] ≤ 50 26.2−33 28−21 2+

0.2

first collapse 0 ≤ [Cd ] < 0.10 27 13−20 0.05 0.2 ≤ [Cd2+] < 0.6 26 23

second collapse

2+

0.6 ≤ [Cd2+] ≤ 10 22 22 5 0.1

Cu2+

Pb2+

2+

0 ≤ [Cu ] ≤ 0.01 27 14−16 0.01

0 ≤ [Pb2+] ≤ 0.005 27 13−25 0.0002

0.01 < [Cu2+] ≤ 10 27−21.2 16−23 5 0.01

0.005 < [Pb2+] ≤ 10 27−22.5 25−34 5 0.005

0.2 ≤ [Cd2+] < 0.6 23 25−28 0.6 ≤ [Cd2+] ≤ 10 19−20 41−45 5

a

These are the actual concentrations representing the low and high concentration regimes that were used in this study. Further in the paper the low and high notations refer to these concentrations. bMedium and high concentrations of Ba2+ showed the same properties; hence, a 5 mM concentration will be referred to as high later in the text to be consistent with other metal cations.

Figure 2. AFM images of PDA LS films compressed on low and high concentrations of (a, b) barium, (c, d) cadmium, (e, f) copper, and (g, h) lead cations.

for Pb2+ cations low concentrations were needed, compared to other metal cations, due to the high PDA LF sensitivity to Pb2+ cations. On the basis of this analysis of concentration dependence for metal cations, their morphological and optical properties were further investigated. AFM. The morphology of PDA LS films was investigated by means of atomic force microscopy (Figure 2). PDA LS films on pure water15 and at low concentration of metal cations exhibit a distinct domain morphology of parallel organized linear strands, several micrometers long and ca. 10 nm wide, which coincide with the conjugated polymer direction (Figure 2a,c,e,g).22

cation radius of barium (1.34 Å), combined with its high concentration in the aqueous subphase under the PCDA film. The accumulation of Ba2+ under the floating film could lead to a larger distance between the compressed PCDA molecules; hence, a larger area per molecule is observed in the isotherm profile. On the other hand, increasing metal cation concentration in the aqueous subphase for Cd2+, Cu2+, and Pb2+ promoted an increase in surface pressure (πc) and reduction of AL. Only in the case of high concentrations of cadmium do the isotherms exhibit two pronounced collapsed stages. The reduction of AL indicates closer packing of the compressed film at medium and high concentrations for Cd2+, Cu2+, and Pb2+ cations. Furthermore, 4251


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All the peaks in the spectrum are shifted to lower wavelengths, particularly the blue phase peaks which are shifted from 600 to 578 nm and from 640 to 615 nm for vibronic and excitonic peaks, respectively. Furthermore, the fraction of the blue phase

Metal ions at high subphase concentrations had a distinct impact on the film morphology. The presence of barium cations at high concentrations stabilizes the linear strand morphology and induces the formation of larger PDA domains (Figure 2b). PDA/Cd2+ LS films exhibit reduced domain size, loss of the linear “strand morphology”, and drastic fragmentation of the domains (Figure 2d). Increased Cu2+ concentration in the aqueous subphase leads to smaller domain size, loss of the linear morphology, and film fragmentation (Figure 2f). Lead cations severely perturb the PDA film at the liquid interface, and only disorganized and broken fragments of the film are found on the surface (Figure 2h). The changes in PDA LS film morphology in the presence of various metal cations are correlated with the changes of the film compression properties, as observed by π−A isotherm analysis. These changes show that closer packing, indicated by lower AL values, is accompanied by loss of the linear morphology (Cd, Cu, and Pb) of the PCDA molecules at the liquid−air interface. The same was observed for Zn2+.34 In contrast, barium ions stabilize the PDA film’s strand morphology even at high concentration. Optical Absorbance. Optical absorbance properties of PDA LS films with low and high concentrations of metal cations in the aqueous subphase were investigated in detail. The chromatic transition from the blue to red phase within the films was monitored and correlated with doses of UV irradiation. The spectral evolution of the films with increasing dosage of irradiation is shown in Figure 3. For low metal cation concentrations the blue to red transition occurs at low doses of UV irradiation (Figure 3a,b,g,h,m,n), as in the case for PDA LS films which were compressed and polymerized on pure water.19 Both chromatic phases show two absorbance peaks, vibronic and excitonic, at 590 and 640 nm for the metastable blue phase and at 500 and 550 nm for the red phase, respectively. At the initial stage of polymerization, the film is visibly blue and the presence of both chromatic phases can be detected in the spectrum. Subsequently, excess UV irradiation transforms the film completely to the stable red phase. At the high concentration range of metal cations, absorbance spectra are prominently different. High concentrations of barium cations stabilize PDA LS films in the blue phase (Figure 3d,e). The dose at which total conversion from the blue to red phase is complete is considerably higher (62 mW·s·cm−2), compared to that with the low barium concentration (19 mW·s·cm−2); thus, high barium concentration appears to stabilize the blue phase. This is also corroborated by the AFM morphology of this film (Figure 2b). Optical absorbance analysis of PDA LS films with high concentration of cadmium cations shows that higher irradiation doses are required to induce the blue to red transition (Figure 3j,k). In this case, a stable red PDA phase is achieved only at a dose of 150 mW·s·cm−2, compared with 20−25 mW·s·cm−2 on pure water19 and low cadmium concentration. In the presence of high Cd2+ concentration, the blue phase peaks shift to longer wavelengths, 600 → 608 nm and 640 → 650 nm for vibronic and excitonic peaks, respectively, while the red phase peaks remain at their positions at 500 and 550 nm. PDA/Cu2+ LS films at high concentrations have much lower absorbance intensity, compared to those at low concentrations at the same UV dosage (Figure 3p,q). Combined with the AFM morphology information, this reduction can be associated with the film breakdown at high copper concentrations (Figure 2f).

in the spectrum at low doses of UV irradiation decreases. The small fraction of the blue phase during the polymerization process suggests that the blue to red transition is very fast. Therefore, it can be clearly seen that the presence of copper cations in the aqueous subphase stabilizes the PDA red phase. High concentrations of lead cations show very low, if any, absorbance signal; hence, no meaningful optical absorbance analysis was possible. Pb2+ cations destroy the PDA film as indicated by the AFM images (Figure 2h). Phase transition kinetic analysis, based on the optical absorbance data, was performed using the unidirectional kinetic model (UKM), which is described in detail elsewhere.19 Briefly, the UKM describes phase transitions from the monomer phase to the blue phase, with further irradiation to the red phase and finally to the degradation of the polymer (expression 1). Note that the degradation process is allowed to proceed from both blue and red phases with the same kinetic rate. The amounts of the experimental blue and red phases data are calculated after detailed deconvolution of the optical absorbance spectra. Both phases are represented by the sum of their corresponding excitonic and vibronic peaks. To obtain the fractions of the blue and red phases throughout the transformation stages, the absorbance signal of each phase after every UV dose increment was normalized by the maximum value of polymer absorbance (maximum value of total absorbance of the film). Note that the monomer and degradation stages are represented only by the model curves. The fit of the model curves to the experimental data is performed by optimization of the kinetic constants. Resulting kinetic constants for the different metals and concentration regimes are presented in Table 2. Constant kb describes the transition rate from the monomer to blue phase, kr the transition from the blue to red phase, and kd the rate of the degradation of the film.19 The χ2 parameter validates the goodness of fit between the experimental points and UKM model curves. Lower values of χ2 indicate better fit of the model.19 The results of the UKM fit presented above are shown in Figure 3c,f,i,l,o,r. In these diagrams the measured and calculated relative fractions of the monomer, blue, red, and degradation phases are compared for the different PDA LS films as a function of UV exposure. The results are plotted on a logarithmic exposure scale (x-axis) to underline the fast kinetics at low exposure doses for low concentrations of metal cations. For all metal cations at low concentrations (Figure 3c,i,o), the polymerization reactions advance rapidly from the monomer to red phase, thus underlining the metastable nature of the blue phase. In these cases the values of kb and kr are high, reflecting the fast transition from the monomer to red phase via the metastable blue phase. On the other hand, the rate of the degradation process under high UV doses is similar for all the PDA films at low concentrations. 4252


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Figure 3. Optical absorbance spectra, deconvolution analysis of the optical spectra (marked with bold dashed lines in the spectra), and fit of the experimental data with the UKM19 for PDA LS films with low (a−c, g−i, m−o) and high (d−f, j−l, p−r) metal cation concentrations of (a−f) barium, (g−l) cadmium, and (m−r) copper.

tion from the blue to red phase (kr = 85 (W·s·cm−2)−1), which results in the stabilization of the blue phase. Due to these factors, a greater fraction of the blue phase is accumulated in the film under

A high barium concentration range stabilizes the blue phase of PDA. It is manifested by rapid transition from the monomer to blue phase (kb = 135 (W·s·cm−2)−1) and relatively slow transi4253


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Table 2. Reaction Kinetic Constants Obtained by Fitting the Experimental Data with UKM Curves for PDA LS Films with Different Metal Cationsa Cd

2+

Ba2+ Cu2+ H2O19

subphase

kb [(W·s·cm−2)−1]

kr [(W·s·cm−2)−1]

kd [(W·s·cm−2)−1]

χ2

low high low high low high

400 25 325 135 290 80 374

280 30 200 85 80 170 140

1.0 0.1 1.2 7.0 0.5 1.0 0.88

0.0339 0.0411 0.0236 0.0171 0.0385 0.0267 0.0087

χ is defined as χ2 = ∑(xi − xi̅ )2/(N − p), where xi are the experimental data, xi̅ are the calculated data from model equations, N is the number of data points, and p is the number of fit parameters.19 a 2

The dosage at which the red frequencies (1515 and 2120 cm−1) begin to evolve is considerably higher than that at the low range. The double bond frequency maintains an almost constant value during the irradiation process (1453.5−1454.2 cm−1), while the triple bond frequency shifts from 2087 to 2078 cm−1 under UV irradiation (Figure 5b). This behavior is very similar to that observed at low concentrations of metal cations; hence, barium cations have no significant influence on Raman frequencies. RRS spectra of PDA/Cd2+ LS films at high concentrations (Figure 4d) initially show a blue phase spectrum. The frequencies of both double and triple bonds move upon UV irradiation (from 1450.6 to 1455.8 cm−1 and from 2081.5 to 2085.7 cm−1). Red phase typical peaks (1515 and 2120 cm−1) appear only at a higher dose of UV irradiation, contrary to low Cd2+ concentration (Figure 4c). High concentration of cadmium cations inhibits the complete phase transition from the blue to red phase. Moreover, the double bond of the blue phase shows a shift from 1450.6 cm−1 at the initial stage of irradiation to 1455.8 cm−1 at the final stage, whereas the triple bond frequency shifts from 2081.5 to 2085.7 cm−1 (Figure 5d). The Raman shifts of the blue phase to higher frequencies for both double and triple bonds are correlated with optical absorbance analysis which shows shifts of vibronic and excitonic peaks to higher wavelengths (600 → 608 nm and 640 → 650 nm, respectively). These shifts can be attributed to the presence of Cd2+ cations in the films and a change of the molecular packing in the blue PDA phase. PDA/Cu2+ LS films at high concentrations showed mainly double and triple bond blue phase frequencies at 1450.4− 1455.1 and 2070.5−2077.9 cm−1, respectively (Figure 4f). The double and triple bonds showed a continuous shift toward higher frequencies during irradiation (Figure 5f). The initial position of the triple bond is shifted to lower frequencies, compared to that for pure water or low concentration of metal cations (∼2071 cm−1 vs 2086 cm−1). The shift to lower frequency of the triple bond is consistent with the shift of optical absorbance peaks to lower wavelengths (600 → 578 nm and 640 → 615 nm). As in the case of cadmium cations, shifts of Raman and absorbance peaks suggest the formation of a different PDA structure on high concentrations of copper cations in the aqueous subphase. We have observed that the triple bond frequency is more sensitive to phase transitions within the PDA films. The ratios of blue and red phases in the films, as indicated by the optical absorbance analysis, are quite accurately represented also by the blue and red triple bond frequency ratio. Additionally, at high doses of irradiation (when most of the PDA film is in the red phase) the signal-to-noise ratio of the Raman spectrum is lower (Figure 4b,d). This could be attributed mainly to the lack of

these conditions (Figure 3f). It should be noted again that PDA LF at this regime is structurally reminiscent of, and according to the kinetic constants is more stable than, PDA LF formed on water. PDA LS films formed on high Cd2+ concentration exhibit a slow rate of phase transition for all phases (Figure 3l). The transition from the monomer to blue phase, denoted by the kb constant, is low (25 (W·s·cm−2)−1) and similar in magnitude to the blue to red transition (kr = 30 (W·s·cm−2)−1). This fact can explain the prolonged coexistence of the blue and red phases, despite the high UV irradiation doses and extension of the full transition to the red phase to higher UV doses. The slow kinetics of the PDA LFs on high cadmium concentration can be attributed to the formation of a cadmium cation superlattice in the film vicinity.28 This superlattice appears to stabilize and inhibit structural rearrangements that are associated with phase transitions.22 Copper cations at high concentrations stabilize the red phase of PDA LS films (Figure 3r). Kinetic analysis shows that the combination of relatively low polymerization rate (kb = 80 (W·s·cm−2)−1) and rapid blue to red transition, denoted by the kr constant (170 (W·s·cm−2)−1), results in a relatively small blue phase fraction and red phase dominance during the entire irradiation range. RRS Spectroscopy. RRS spectroscopy was used to follow the chromatic PDA LS film phase evolution with increasing UV irradiation doses (Figure 4). PDA excitation was performed with a He−Ne (633 nm) laser, which closely corresponds to the main absorption peak of the PDA blue phase (640 nm), thus giving rise to RRS. The RRS spectra were normalized with respect to the maximum intensity of each Raman spectrum. Hence, only relative changes in the blue or red phase fraction can be observed, as well as shifts in the peak positions of the blue phase. These shifts appear to be manifestations of structural changes in the metastable blue phase during the irradiation process under different metal cation environments. The Raman signal of PDA films deposited on a subphase with low metal cation concentration shows a gradual transition of the double and triple bond frequencies from the blue phase at 1452.5−1456 and 2086−2078 cm−1 to the red phase at 1515 and 2120 cm−1, respectively, for the entire range of UV irradiation dosage (Figure 4a,c,e). The blue to red chromatic phase transition occurs at low doses of irradiation, as in the case of pure water.19 The double bond frequency is nearly constant except for the very low irradiation doses that can be associated with the presence of unpolymerized film. The triple bond frequency is down shifted from 2086 to 2078 cm−1 during film irradiation (Figure 5a,c,5e). Barium cations at high concentrations stabilize the blue phase, compared to PDA LS films at low concentrations (Figure 4a,b). 4254


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Figure 4. Raman spectra excited with a 633 nm laser for PDA LS films with low and high concentrations of (a, b) barium, (c, d) cadmium, and (e, f) copper cations.

Figure 5. Blue PDA phase double and triple bond Raman frequency center analysis for low and high concentrations of (a, b) barium, (c, d) cadmium, and (e, f) copper cations. Note the different y-axis scale for the blue phase triple bond in (f). 4255


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Table 3. Ksp (25 °C) Values of Metal Hydroxides in the Aqueous Subphase,42,43 Association Constant (Ka) of Metal Carboxylate,41 Electronegativity Values of Metal Elements,44 and Electronegativity Difference with Respect to Oxygena metal cation

hydroxide

Ba Zn Cd Cu Pb

Ba(OH)2·8H2O Zn(OH)2 Cd(OH)2 Cu(OH)2 Pb(OH)2

metal hydroxide solubility product Ksp42,43 2.55 1.00 5.27 2.00 1.43

× × × × ×

association constant Ka of metal carboxylate salt41

electronegativity44

electronegativity difference with respect to oxygen

0.50 0.91 1.30 1.76 1.93

0.89 1.65 1.69 1.90 2.33

2.61 1.85 1.81 1.60 1.17

10−4 10−16 10−15 10−20 10−20

a The association strength K is defined as K = [ML+]/[M2+][L−], and the stability constant is defined as log K, where [M2+] is the concentration of free metal cations, [L−] is the concentration of free carboxylate anions, and [ML+] is the 1:1 metal−ligand complex concentration.41 The influence of the ions increases as one moves down the table.

chain orientation, unlike the structure of PDA films formed on water.22 Indeed, the film loses its linear strand morphology and appears fragmented (Figure 2d). Absorption spectra obtained after irradiation with increased UV doses show slow phase transition rates for both polymerization and blue to red transition. Raman frequencies for both double and triple bonds showed a continuous shift toward higher frequencies (1450.6 → 1455.8 cm−1 and 2081.5 → 2085.7 cm−1) during the irradiation process. Additionally, optical absorbance deconvolution analysis showed a shift of blue phase peaks to higher wavelengths (600 → 608 nm and 640 → 650 nm). These features indicate that PDA LS films under exposure to cadmium cations exhibit structural changes which have a profound effect on their properties. This is supported by our previous in situ synchrotron GIXD measurements.39 In contrast to cadmium, PDA films formed on copper show a gradual shift in AL with increased subphase concentration, yet similar to the effect of cadmium, a transition from linear strand morphology at low copper concentrations to a fragmented film at high concentrations is observed. The presence of copper in the subphase induces a slow polymerization rate but rapid transition from the blue to red phase, thus resulting in formation of predominantly red PDA films. Unlike any other PDA blue phase films, the absorbance peaks of the PDA/Cu2+ blue phase are strongly shifted to lower wavelengths, compared with PDA on water: 600 → 578 nm and 640 → 615 nm for the vibronic and excitonic absorptions, respectively. Moreover, Raman peaks also showed a continuous shift of double and triple bonds to higher frequencies together with a shift of the initial position of the triple bond of the blue phase to lower frequency of ca. 2071 cm−1. The lower absorbance intensity (Figure 3q) and disordered morphology (Figure 2f) put PDA/Cu2+ in an intermediate situation between cadmium and lead and are also in agreement with their properties listed in Table 3. Hence, our interpretation is that the “blue shift” of the blue phase PDA/ Cu2+ system is associated with increasing disorder due to stronger association with copper ions. Note that the PDA red phase is not resonant Raman active with the excitation laser (λ = 633 nm); hence, the relative intensity of the Raman peaks associated with the blue and red PDA peaks (Figure 4) is strongly biased in favor of the blue phase. We observe that all PDA/metal2+ films formed on low metal ion concentration exhibit attributes similar to those of PDA formed on water. These include similar AL (Figure 1), strand morphology (Figure 2a,c,e,g), optical properties (Figure 3c,i,o), and also Raman spectral behavior (Figures 4a,c,e and Figure 5a,c,e). The typical Raman spectral behavior of these films is characterized by constant frequency associated with the double

resonance between the He−Ne (633 nm) laser and PDA red phase (550 nm) and the beginning of the degradation process in the film. High concentrations of lead cations destroy PFA LS films; therefore, no Raman analysis was possible.

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DISCUSSION AND CONCLUSIONS In the current study we have characterized the optical and morphological properties of PDA films formed on metal ions containing aqueous subphases and irradiated with variable UV dosage. Each of the metal cations used in this study had a unique spectral and structural effect which is mainly governed by its concentration in the solution. Analysis of the π−A isotherms indicated the threshold concentration of cations in solution, at which the transition in the film properties take place. AL for PCDA on a pure water subphase is ca. 27 Å2 molecule−1.35,36 When AL drops below this value, the structure of the film is denser. The change of the film structure, and the associated metal cations, affects the optical and morphological properties of PDA LFs. Compression isotherms on high barium concentrations showed only negligible reduction of AL, but were manifested in elevated surface pressure collapse. Only at very high Ba2+ concentrations was an increase in AL observed, which could be attributed to expansion of the PCDA packing. It follows that no significant change in the PDA film structure and consequently in its properties is expected. Indeed, at high concentrations, PDA LS films show linear strand morphology as in the case on pure water or low barium concentration. Optical absorbance indicated that the UV irradiation dose required for blue to red transition is higher than for PDA on water,19 but lower than for PDA on similar zinc concentrations.34 Notably, PDA films formed on low and high barium concentrations are very similar in their absorbance and Raman spectra, confirming the assumption that no significant change in film structure takes place in the presence of barium cations. Compression isotherms on cadmium exhibit three regimes of subphase concentrations: low ([Cd2+] ≤ 0.05 mM), medium (0.2 mM < [Cd2+] < 0.4 mM), and high ([Cd2+] > 0.6 mM) according to the monolayer AL, summarized in Table 1. In contrast to other ions, PDA LFs on medium and high cadmium subphase levels have two collapse stages. The AL values of the first collapse are 27 and 22 Å2 molecule−1 for the medium and high regimes, respectively. Similarly, the AL values of the second collapse are 22 (medium) and 19 (high) mN m−1. These features suggest that cadmium cations have a great influence on the obtained PDA LF structure, forming a highly dense monolayer with area comparable to the maximum packing density obtained for single-chain lipids.38 This suggests upright 4256


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bond (1454 ± 1 cm−1) and a gradual frequency shift of the triple bond to lower frequencies with increasing irradiation dose. Similar Raman frequency shifts in PDA were induced by increasing the pressure. Vardeny and colleagues have applied external pressure to a PDA derivative and observed a Raman scattered frequency increase with increased applied pressure.40 Hence, internal strain is probably manifested in increased Raman frequency. Conversely, a decrease in Raman frequency may be interpreted as internal strain relaxation. The UV irradiation dose corresponds to the full range of existence of the blue phase (Figure 3c,i,o), almost until its full conversion to the red phase. Hence, the observed shift is associated with the decreasing strain in the conjugated blue PDA during the phase transition to the red phase. We have recently published the crystal structures of PDA formed on water in the blue and red phases.22 Notably, the red phase unit cell is smaller than the blue cell. Hence, upon the progression of the blue to red transition, the residual blue phase become progressively less strained due to the available film’s area gained by the accumulation of the more condensed red phase. In contrast to the shifts observed in the low regime, PDA films formed on high metal cation concentration in the subphase exhibit shifts of peaks associated with both the double (ca. 1450 cm−1) and triple (ca. 2080 cm−1) bonds to higher frequencies. As is the case for the low concentration regime, the irradiation doses correspond to the full range of the blue phase existence, until its complete conversion to the red phase (compare parts f, l, and r of Figure 3). Notably, blue to red transformation requires higher irradiation doses than in the low regime. This, together with the smaller AL (Figure 1) and the loss of the typical PDA strand morphology (Figure 2), indicates that the PDA structure on a high concentration of metal ions in the subphase deviates significantly from that on water and low metal concentration.22 This suggests that during the blue to red transition no relaxation of the blue phase takes place, as is the case in pure water and low concentration metal ions in the subphase. The Raman shift to higher frequency upon UV irradiation indicates accumulated strain in the conjugated system that is not relaxed when the blue phase transforms to the red phase and may be associated with different PDA film structures formed in the presence of metal cations.39 The increased strain slows the transformation kinetics and is manifested in increased UV doses required for the phase transformation. Note that the effect of lead cations on the Raman spectral behavior is not reported here. The harsh influence of lead on the integrity of the compressed PDA film, even at very low concentrations (threshold value of [Pb2+] = 5 μM) above which the film is disordered and torn (Figure 2h), results in very weak spectral signals. We observe that the threshold concentrations that mark the shift between the low and high regimes for the different metal cations examined in this work (Table 1) increase as [Pb] < [Cu] < [Cd] < [Zn]34 < [Ba]. This order agrees with the association constants41 between PCDA carboxylate moieties and the various metal cations (Table 3), which result in increasingly greater influence on the PDA LF properties. This order correlates also with both the electronegativity difference with oxygen for these elements and the solubility product (Ksp) for their respective hydroxides, shown in Table 3. Order of magnitude calculation indicates that a subphase concentration of [M2+] = 0.25 μM is equivalent to the LF headgroup charges. Our results show that for lead ions, where the strongest interaction between the metal cations and PDA LFs takes place,

the threshold concentration is 1 order of magnitude higher than the subphase concentration calculated above. Metal cations with weaker association constants (Tables 1 and 3) exhibit much higher thresholds, e.g., up to 3 orders of magnitude for barium. The Ksp values for the metal hydroxides were selected as reference values due to the similarity between the metal−oxygen interaction in the hydroxides and the metal−carboxylates in PCDA. The underlying origin of the observed differences in metal cation behavior is the Pauling electronegativities of the examined metal cations. The nature of the metal−oxygen bond varies from more covalent in lead to more ionic in barium. This suggests that the covalent bond nature results in a stronger influence of the cations on the PDA LF properties. It is consistent with the conclusions of Schwartz et al.,32 who showed that AL per molecule was primarily governed by the tendency for covalent bonding. The current study gives important insights into ways to control and obtain desired morphological and optical properties of PDA LFs. PDA film properties can be adjusted for specific needs and tuned to the desired level of response by manipulating the metal cation identity and concentration. PDA LF applications could include use as templates for nucleation of semiconductor nanocrystals.13,15−18,35 PDA LFs with low concentration of metal cations promote very orderly linear strand morphology. For that reason they can be used as effective templates for nucleation of ordered arrays of minerals and semiconductor nanocrystals such as PbS, CdS, ZnS, and others. Another potentially important aspect emerging from this work is the use of the Raman frequency shift as an estimate of the relative degree of strain in the conjugated backbone. We claim that this property can be a better and more sensitive indicator for the “degree of interaction” in PDA-based sensors. It follows that, instead of quantifying the degree of blue to red “chromatic transition” or measuring fluorescence intensity which is unique to the red phase, we offer to probe changes in Raman shift that are associated with the strain at the conjugated backbone. This measure has the advantage of directly probing the structural cause for the various chromatic properties of PDA. Unlike the chromatic property which is binary, blue or red,19 the Raman shift response is continuous and detectable already in the blue phase, hence providing a better estimate for the degree of strain that precedes the chromatic probe. This property can be exploited for the design of more sensitive PDA-based sensors.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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