F S L R C a b a A R R A A K L L S P A 1 f d e b t L d i i h 0 Applied Surface Science 416 (2017) 482–491 Contents lists available at ScienceDirect Applied Surface Science journa l h om epa ge: www.elsev ier .com/ locate /apsusc ull Length Article upramolecular architectures of iron phthalocyanine angmuir-Blodgett films: The role played by the solution solvents afael Jesus Gonç alves Rubiraa, Pedro Henrique Benites Aokib, arlos José Leopoldo Constantinoa, Priscila Alessioa,∗ São Paulo State University (UNESP), School of Technology and Applied Sciences, Presidente Prudente, SP, Brazil São Paulo State University (UNESP), School of Sciences, Humanities and Languages, Assis, SP, Brazil r t i c l e i n f o rticle history: eceived 23 February 2017 eceived in revised form 10 April 2017 ccepted 19 April 2017 vailable online 23 April 2017 eywords: angmuir angmuir-Blodgett upramolecular architecture ollutant trazine a b s t r a c t The developing of organic-based devices has been widely explored using ultrathin films as the trans- ducer element, whose supramolecular architecture plays a central role in the device performance. Here, Langmuir and Langmuir-Blodgett (LB) ultrathin films were fabricated from iron phthalocyanine (FePc) solutions in chloroform (CHCl3), dichloromethane (CH2Cl2), dimethylformamide (DMF), and tetrahydro- furan (THF) to determine the influence of different solvents on the supramolecular architecture of the ultrathin films. The UV–vis absorption spectroscopy shows a strong dependence of the FePc aggregation on these solvents. As a consequence, the surface pressure vs. mean molecular area (�-A) isotherms and Brewster angle microscopy (BAM) reveal a more homogeneous (surface morphology) Langmuir film at the air/water interface for FePc in DMF. The same morphological pattern observed for the Langmuir films is preserved upon LB deposition onto solid substrates. The Raman and FTIR analyses indicate the DMF- FePc interaction relies on coordination bonds between N atom (from DMF) and Fe atom (from FePc). Besides, the FePc molecular organization was also found to be affected by the DMF-FePc chemical inter- action. It is interesting to note that, if the DMF-FePc leads to less aggregated FePc either in solution or ultrathin films (Langmuir and LB), with time (one week) the opposite trend is found. Taking into account the N-Fe interaction, the performance of the FePc ultrathin films with distinct supramolecular architec- tures composing sensing units was explored as proof-of-principle in the detection of trace amounts of atrazine herbicide in water using impedance spectroscopy. Further statistical and computational analysis reveal not only the role played by FePc supramolecular architecture but also the sensitivity of the system to detect atrazine solutions down to 10−10 mol/L, which is sufficient to monitor the quality of drinking water even according to the most stringent international regulations. © 2017 Elsevier B.V. All rights reserved. . Introduction The design of supramolecular structures has been at the fore- ront of technological applications [1–5] paving the way for eveloping nanoarchitectonics [6]. Different strategies can be mployed to produce such structures, but the emphasis has een given to techniques that allow controlling molecular archi- ectures, including Layer-by-Layer [7] physical evaporation, [8] angmuir and Langmuir-Blodgett films [9–12]. In these cases, ifferent supramolecular architectures can be achieved by tun- ng experimental conditions, such as i) deposition methods [1] i) thermal annealing [13] iii) solvent interaction [14,15] among ∗ Corresponding author. E-mail address: priscila@fct.unesp.br (P. Alessio). ttp://dx.doi.org/10.1016/j.apsusc.2017.04.155 169-4332/© 2017 Elsevier B.V. All rights reserved. others. For instance, Xiao et al. [16] have shown the influ- ence of different solvents on the morphology and crystallinity of poly(3-hexylthiophene) (P3HT) thin films. The importance of the o-xylenesolvent to improve the Langmuir−Schaeffer (LS) deposi- tion of copper tetra-(tertbutyl)-phthalocyanine was presented by Kolker et al. [17–19]. Chiang et al. [14] have found the methyl sulfoxide solvent interferes with the PEDOT conformation, affect- ing the electrochemical properties of the films. PEDOT with linear conformation displays higher conductivity and reversibility for ion exchange, which increases the sensitivity of electrochromic devices [14]. Phthalocyanines (Pc) attract great attention due to their ther- mal and chemical stability, electrical and optical properties, and the possibility of thin film fabrication [20–24]. However, the strong �-� stacking interactions significantly reduce the solubility of metal Pc in organic solvents [25–28]. In this work, Langmuir and dx.doi.org/10.1016/j.apsusc.2017.04.155 http://www.sciencedirect.com/science/journal/01694332 http://www.elsevier.com/locate/apsusc http://crossmark.crossref.org/dialog/?doi=10.1016/j.apsusc.2017.04.155&domain=pdf mailto:priscila@fct.unesp.br dx.doi.org/10.1016/j.apsusc.2017.04.155 rface L i m i m a a t a w i i w n e i b [ m i e 2 2 f p d f S p s r g a 2 f m ( a T s W u m f m s T 1 t w c d a F d 0 f R.J.G. Rubira et al. / Applied Su angmuir-Blodgett (LB) films were fabricated from FePc solutions n chloroform (CHCl3), dichloromethane (CH2Cl2), dimethylfor- amide (DMF), and tetrahydrofuran (THF) to investigate the nfluence of these different solvents on both optical absorption and orphological properties of the films. The solvents were chosen ccording to their polarity (Fig. S1) and presence (or not) of N e O toms in their molecular structure. The FePc was chosen mainly due o the metal core (Fe), susceptible to coordination with nitrogenous nd phenolic groups [29–31]. As proof-of-principle, FePc LB films ith distinct supramolecular architectures were evaluated as sens- ng units in the detection of trace amounts of atrazine herbicide n water through impedance spectroscopy. The impedance data ere further analyzed using multidimensional projection tech- iques and principal component analysis (PCA) to demonstrate the ffect of the different LB supramolecular architectures on the sens- ng unit distinction ability. Atrazine belongs to the s-triazine family, eing widely used in combating weeds in many different crops 32], which can potentially lead to contamination of the environ- ent, especially the groundwater. Depending on the concentration, t can act as an inhibitor of photosynthesis [33,34] and may present ndocrine disruptor or carcinogenic effects [35–37]. . Materials and methods .1. Reagents and solutions Iron phthalocyanine (FePc, MW = 568.38 g/mol) was purchased rom Kodak and used as received. FePc solutions were pre- ared at 0.5 mg/mL (8.8 × 10−4 mol/L) using chloroform (CHCl3), ichloromethane (CH2Cl2), tetrahydrofuran (THF), and dimethyl- ormamide (DMF), whose molecular structures are given in Fig. 1. The atrazine pesticide (C8H14ClN5, MM = 215.68 g/mol, Fig. S1), urity = 98.8%, was purchased from Fluka Analytical-Brazil. The olutions were prepared by simply adding FePc powder to the eagents followed by 30 min of sonication. All solvents are HPLC rade acquired from Merck. The ultrapure water (18.2 M� cm) was cquired from a Milli-Q system, model Simplicity. .2. Langmuir and Langmuir-Blodgett (LB) films The FePc Langmuir and Langmuir-Blodgett (LB) films were abricated using a KSV Langmuir trough, model 2000. Lang- uir films were prepared by spreading 350 �L of FePc solutions 8.8 × 10−4 mol/L) onto the water subphase (1.3 L). The solvent was llowed to evaporate for ca. 20 min before the first compression. he Langmuir monolayers were characterized by surface pres- ure vs. mean molecular area (�–A) isotherms at 23 ◦C using the ilhelmy method. The monolayer was symmetrically compressed nder a constant barrier speed of 10 mm/min. Brewster angle icroscopy (KSV, model micro-BAM) images were also acquired or different areas during the Langmuir film compression. The LB films were obtained by transferring the FePc Langmuir onolayers from the air/water interface to different solid sub- trates depending on the characterization technique to be applied. he surface pressure was kept constant at 25 mN/m during ca. 5 min prior the LB deposition, controlled by the displacement of he barriers (1 mm of displacement every 5 min). Y-type LB films ere obtained by using an upstroke and downstroke speed of a. 2.0 mm/min. FePc LB films containing up to 21 layers were eposited onto quartz substrate for ultraviolet-visible (UV–vis) bsorption spectroscopy and onto ZnSe for FTIR measurements. ePc LB films containing five layers were deposited onto Pt inter- igitated electrodes (IDE, 50 pairs of digits with 10 �m width, .5 mm length and 100 nm height, and 10 �m apart each other) or impedance spectroscopy. Science 416 (2017) 482–491 483 2.3. Characterization techniques The UV–vis absorption spectra were recorded for FePc solutions and LB films using a Varian spectrophotometer, model Cary 50, from 190 to 1100 nm. The micro-Raman scattering experiments were conducted with a micro-Raman Renishaw spectrograph, model in- Via, with laser excitation at 633 nm. The system is equipped with a Leica microscope, whose 50x (NA 0.75) objective lens allows collecting spectra with ca. 1 �m2 spatial resolution. Single-point spectra were recorded with ca. 4 cm−1 resolution and 10 s accumu- lation time using a computer-controlled motorized stage (XY). The FTIR spectra were taken on a Bruker spectrometer, model Tensor 27, between 4000 and 500 cm−1. FTIR measurements were performed using the transmission mode with 128 scans and spectral resolu- tion of 4 cm−1. The surface morphology was characterized through AFM using a Nanosurf Instrument, model easyScan 2, with a tip of silicon nitride and using the tapping mode. All topographic images were collected with a resolution of 512 lines per scan at a scan rate of 0.5 Hz. The images were processed with the software Gwyddion 2.19. Impedance spectroscopy measurements were performed using a Solartron analyzer model 1260 A. Impedance spectra were acquired from 1 Hz to 1 MHz using 50 mV of amplitude. The sens- ing array is composed of four sensing units: a bare Pt interdigitated electrode (IDE) and three Pt IDEs coated with 5 LB layers built up from FePc solutions in CH2Cl2, DMF/freshly prepared and DMF/aged for one week, as illustrated in Fig. S2. The bare Pt electrode was used to monitor any change in the electrical response caused by the ultrathin films. The measurements were performed with sensing units immersed into ultrapure water (18.2 M� cm) used as a refer- ence and into atrazine aqueous solutions at 0.1 × 10−9, 1.0 × 10−9, 15 × 10−9, 50 × 10−9, and 100 × 10−9 mol/L. The course of measure- ments started from the lowest (0.1 × 10−9 mol/L) to the highest (100 × 10−9 mol/L) atrazine concentrations. Between each set of measurements involving a specific concentration of atrazine, the sensing units were carefully washed with ultrapure water. The sensing units were left soaking 20 min inside the solutions before data acquisition to enable a stable reading. The impedance data were further analyzed using multidimensional projection tech- niques, whose details can be found in Oliveira et al. [38] and Paulovich et al. [39]. 3. Results and discussion 3.1. FePc Langmuir films 3.1.1. �-A isotherms Fig. 1 shows the �-A isotherms for FePc in CHCl3, CH2Cl2, THF, and DMF. Similar profiles are found for both FePc/CH2Cl2 and FePc/CHCl3 �-A isotherms, considering all the compression stages. The corresponding liquid-phase (up to 15 mN/m) of the FePc/THF is displaced towards larger molecular areas, which suggests an inter- action between FePc and THF, unperceived for CHCl3 and CH2Cl2 solvents. There is a possibility of a coordination of the DMF oxy- gen with the FePc iron, leading to the formation of dimeric species of FePc, as reported by Barbosa et al. [30] working with Mg–Al hydrotalcite-like materials used as support for the immobiliza- tion of Fe(III) tetrasulfonated phthalocyanine (FePcTs). However, at higher surface pressures (above 15 mN/m) the FePc/THF �-A isotherm tends to overlap the FePc/CH2Cl2 and FePc/CHCl3 �-A isotherms. The later indicates the FePc-THF interaction is not strong enough, and the final packaging of FePc molecules is substan- tially the same as in CHCl3 and CH2Cl2. Therefore, the formation of FePc dimeric species through coordination interaction can be min- imized. Besides, the FePc solutions in CHCl3, CH2Cl2, and THF have 484 R.J.G. Rubira et al. / Applied Surface Science 416 (2017) 482–491 F shly p o p fi m e i o d b 3 m f ( F d F L a F s 3 C p ( a b a h i l m c w e t C i f ig. 1. �-A isotherms for Langmuir films of FePc in CHCl3, CH2Cl2, THF, and DMF (fre nto ultrapure water subphase at 23 ◦C. roven to be stable with time, providing reproducible Langmuir lms even after months of the prepared solution. On the other hand, considering the DMF solvent, the displace- ent of the FePc/DMF isotherm to smaller molecular areas is vidence of a packed monolayer. However, the FePc/DMF �-A sotherm was significantly displaced to larger molecular areas after ne week of the solution preparation. The later indicates a time- ependent interaction between FePc and DMF, which is going to e discussed in detail in further sections. .1.2. Morphology (BAM) Fig. 2a shows BAM images recorded for FePc/CH2Cl2 Lang- uir film at 20 mN/m. The results obtained for FePc/DMF reshly prepared and after one week of the solution preparation FePc/DMFaged) are given in Fig. 2b. As the time goes on, the ePc/DMF film becomes rougher, with the coalescence of molecular omains. Nevertheless, the surface morphology for FePc/DMF and ePc/DMFaged is still more homogeneous than that for FePc/CH2Cl2 angmuir films. These effects might be explained taking into ccount the interaction established while in the solutions between ePc and solvents, as we shall demonstrate with UV–vis absorption pectroscopy. .1.3. Solution UV–vis absorption Fig. 3a shows UV–vis absorption spectra of FePc in CHCl3, H2Cl2, THF, and DMF 8.8 × 10−4 mol/L solutions (freshly pre- ared). In general, the phthalocyanine spectra display a Soret band or B-band) between 300 and 400 nm and a Q-band between 550 nd 800 nm, which are assigned to � → �* transition between onding and antibonding molecular orbitals [40]. The Q-band usu- lly displays two maxima of absorption at lower (blue-shift) and igher (red-shift) wavelengths. It is well established the blue-shift s due to the absorption of dimers (and higher orders of molecu- ar aggregates [41]) and the red-shift is due to the absorption of onomeric species of phthalocyanine molecules [25,28,40]. In our ase, more than two maxima are frequently seen for FePc (Fig. 3a), hose difference in energy among these maxima (within tenths of V) suggests transitions from the fundamental to different vibra- ional levels of the electronic excited state [40]. Similar spectra were recorded for FePc solutions in CHCl3, H2Cl2, and THF, suggesting the FePc molecules are found with sim- lar aggregation level in these media. The broad Q-band observed or FePc in these solvents is similar to the spectrum profile found repared and aged solutions) obtained spreading 350 �L of 8.8 × 10−4 mol/L solution for FePc in the solid state, whose absorption is assigned to exci- ton coupling between adjacent macrocycles [40]. Indeed, the FePc spectra from 8.8 × 10−4 mol/L CH2Cl2 solution (Fig. 3a) and LB film (Fig. 4d) are pretty similar (broader). However, in the case of DMF, the 8.8 × 10−4 mol/L solution spectrum (sharper − Fig. 3a) is quite different from the LB film spectrum (broader − Fig. 4d). In this sense, Fig. S3a shows UV–vis absorption spectra for FePc in CH2Cl2 diluted solutions (8.8 × 10−5–5.5 × 10−6 mol/L), whose sharp profiles are similar to 8.8 × 10−4 mol/L DMF solution, revealing the potential of DMF to dissolve FePc even at high concentrations, leading to less aggregation. Regarding the maximum of the absorption bands, they are shifted from 796 nm for FePc/THF to 773 nm for FePc/CHCl3 and 768 nm for FePc/CH2Cl2 (Fig. 3b), which must be related to differ- ent levels of FePc aggregation. The UV–vis absorption spectrum for FePc in CH2Cl2 was not affected over time, as shown in Fig. S3b. The profile of the FePc UV–vis absorption spectrum is signif- icantly affected when in DMF solution (Fig. 3a), with maximum absorption at 662 nm assigned to dimers (and higher orders of molecular aggregates [41]), 682 nm assigned to monomers [25,28,40], and a broad band at 771 nm. This profile change sug- gests the FePc-DMF interaction is distinct that observed for CHCl3, CH2Cl2, and THF, consistent with the �-A isotherms for Langmuir films. The �-A isotherms also revealed the FePc-DMF interaction is time-dependent, which can be correlated with significant varia- tions in the relative intensities of the UV–vis absorption spectrum observed after one week, as shown in Fig. 3b and Fig. S3c. The maxi- mum at 662 nm (dimers and higher orders of molecular aggregates) increases with time while the maximum at 682 nm (monomers) and the broad band at 771 nm decrease. The latter (771 nm) could also be assigned to monomers due to its opposite trend compar- ing to the maximum at 662 nm. These spectral changes are clearly assigned to aggregation of FePc molecules over time, i.e., if the DMF leads to less aggregated FePc in fresh solution, with time (one week) the opposite trend is found. However, this increase in aggre- gation is not as much as for the other solvents. For instance, the sharp UV–vis spectrum of FePc/DMFaged solution (Fig. 3b) is far from the broad spectrum for FePc in the other solvents (Fig. 3a). Leznoff and Lever [40] have reported the coordination established between FePc and DMSO (dimethyl sulfoxide) or DMA (dimethy- lacetamide) solvents. Interactions between amine groups and iron have already been reported for layer-by-layer (LbL) films of FePc and poly(allylamine hydrochloride) [29,42]. Therefore, it is likely the N atom of DMF establishes an interaction with the Fe atom of R.J.G. Rubira et al. / Applied Surface Science 416 (2017) 482–491 485 Fig. 2. BAM images recorded for Langmuir films of (a) FePc/CH2Cl2, (b) FePc/DMF, and (c) FePc/DMFaged prepared from 8.8 × 10−4 mol/L FePc solutions. Inset: BAM image recorded for ultrapure water subphase used as a reference. F prep w F a t a 3 3 o A o a U t f e g ig. 3. (a) UV–vis absorption spectra of FePc in CHCl3, CH2Cl2, THF, and DMF (freshly eek) solutions. Concentration of all solutions: 8.8 × 10−4 mol/L. ePc, which is time dependent. Therefore, the FePc-solvent inter- ction established while in solution might be incorporated into he Langmuir films, affecting their supramolecular structure at the ir/water interface. .2. Langmuir-Blodgett (LB) films .2.1. Optical property and growth LB films of FePc/CH2Cl2, FePc/DMF, and FePc/DMFaged (aged for ne week) were grown at a constant surface pressure of 25 mN/m. t this value, the FePc Langmuir films are already well packed nto the water subphase, regardless the solvent used. The optical bsorption and the growth of the LB films were monitored using V–vis absorption spectroscopy. The spectra were recorded every wo layers up to 21 layers (Fig. S4), and the maximum of absorbance or each deposited layer is shown in Fig. 4a–c. Despite the pres- nce of molecular aggregates, it is interesting to note the linear rowth of the absorbance with the number of LB layers (in aver- ared) solutions. (b) UV–vis absorption spectra for FePc/DMF and FePc/DMFaged (one age), which indicates a controlled growth of the LB films. Fig. 4d shows the UV–vis absorption spectra for 21-layer LB films from CH2Cl2, DMF, and DMFaged, which are out of scale in absorbance to stress the direct relationship between the molecular aggregates and the UV–vis band shape. The FePc/DMFaged band shape seems to be an intermediate stage between FePc/DMF and FePc/CH2Cl2, as a consequence of the FePc aggregation when growing LB films from these solvents. 3.2.2. Morphology at micrometer scale (micro-Raman) Fig. 5 shows the micro-Raman images collected for LB films containing 21 layers of FePc/CH2Cl2, FePc/DMF, and FePc/DMFaged. Optical images show the morphology of the LB films agrees with the BAM images recorded for the Langmuir films, i.e., larger FePc molecular domains (aggregates) for FePc/CH2Cl2 film and, conse- quently, higher homogeneity (surface morphology) for FePc/DMF LB films. The characterization of such aggregates was performed by Raman spectroscopy collecting spectra point-by-point along a line 486 R.J.G. Rubira et al. / Applied Surface Science 416 (2017) 482–491 F FePc/ c o t d t a t R a l b a t p l a a F 3 F e a r t w t s ig. 4. Absorbance vs number of deposited layers for LB films of (a) FePc/CH2Cl2, (b) ontaining 21 layers of FePc/CH2Cl2, FePc/DMF, and FePc/DMFaged. f 100 �m with a 1 �m step. The results are shown in Fig. 5, where he 101 spectra collected point-by-point are exhibited in 2 and 3- imensions. The line mapping exhibited in 2-dimensions shows he intensity of the band at 1517 cm−1 assigned to CNC stretching nd C H deformation [42–44], where the brighter spots are related o more intense Raman signals. The same analysis is made for the aman mapping exhibited in 3-dimensions, where the strong vari- tion of the whole Raman spectrum is observed along the mapped ine. The Raman mappings (chemical information) are compati- le with optical images (morphological information). Regions with ggregates present stronger Raman signal, confirming the FePc in he aggregate composition, which is in agreement with what was reviously observed by Alessio et al. [42] working with layer-by- ayer (LbL) films of FePc/poly(allylamine hydrochloride) (PAH). The uthors found the FePc molecular aggregates play a central role, llowing the LbL film growth through NH2 (from PAH) and Fe (from ePc) interactions. .2.3. Morphology at the nanometer scale (AFM) AFM measurements for 21 layers of FePc/CH2Cl2, FePc/DMF, and ePc/DMFaged LB films were carried out to evaluate the solvent ffect on the morphology of the LB films at the nanometric scale, s shown in Fig. 6. Also, AFM results were analyzed through RMS oughness, given by the standard deviation following the equa-√√√√√ n∑( Zn−Z )2 ion Rrms = n=1 N−1 , where Z̄ is the mean of the Z values ithin the given area, Zn is the height of the nth pixel, and N is he number of pixels considered within the given area. The mea- urements were taken from areas of 10 �m × 10 �m and the RMS DMF, (c) FePc/DMFaged. (d) UV–vis absorption spectra (out-of-scale in Y) of LB films roughness for FePc/DMF and FePc/DMFaged LB films was found to be ca. 2.3 and 5.3 nm, respectively. The RMS roughness has sig- nificantly increased to ca. 23.3 nm considering the FePc/CH2Cl2 LB film. The latter indicates the FePc/DMF and FePc/DMFaged LB films present higher homogeneity (surface morphology) at nanometric scale when compared to FePc/CH2Cl2, in agreement to what was previously observed at micrometric level via BAM (Langmuir films) and micro-Raman (LB films). 3.2.4. FePc-solvent chemical interaction (vibrational spectroscopy) Fig. 7 shows the Raman spectra of FePc/DMF and FePc/DMFaged collected for both aggregates and smooth regions of the LB films. A similar profile is observed for the spectra collected from the aggre- gates (fresh and aged). Small differences arise because the FePc molecular aggregates are less susceptible to the solvent action. Comparing the spectra of the smooth region, the bands at 680 and 749 cm−1 (macrocycle vibration, C N C N Fe stretching, benzene and C H deformation [42,44]) showed a small decrease in rela- tive intensity, and the bands between 1400 and 1540 cm−1 were shifted. The bands at 1307 cm−1 (C H in-plane bending, isoin- dole stretching N–Fe) and 1401 cm−1 (C N C stretching, pyrrole expansion and C H in-plane bending) [42–44] were significantly affected (decrease in relative intensity and a shift in wavenum- ber) for FePc/DMFaged LB film. These spectral changes confirm the coordination bonds between N-Fe, as initially discussed for �-A isotherms and UV–vis absorption. The FePc/CH2Cl2 Raman spectra taken from both aggregates and smooth regions presented profiles similar to the FePc/DMF Raman spectra collected from aggregates (Fig. S5). R.J.G. Rubira et al. / Applied Surface Science 416 (2017) 482–491 487 Fig. 5. Raman mapping (two and three dimensions) built collecting spectra point-by-point along a line of 100 �m with a step of 1 �m for 21 layers of FePc/CH2Cl2, FePc/DMF, and FePc/DMFaged LB films. The two-dimensional Raman mapping is superposed to the optical image where the brighter spots represent more intense Raman signals for the band at 1517 cm−1. The color of the optical image in 5c is not the real color of the FePc/DMFaged LB film. nd Fe e m i i Fig. 6. AFM topographic images for 21 layers of FePc/CH2Cl2, FePc/DMF, a Complementary, FTIR spectroscopy was carried out for 21 lay- rs of FePc/CH2Cl2 FePc/DMF, FePc/DMFaged LB films (transmission ode) and 21 layers of FePc/DMF LB film (reflection mode), shown n Fig. 8. The bands at 1331 cm−1 (pyrrole stretching, C N stretch- ng, and in-plane C H bending) and 1338 cm−1 (C N stretching), Pc/DMFaged LB films deposited onto quartz plates, whose RMS is 0.89 nm. both related to the FePc ring vibration [42–44], had inverted their relative intensities. Other minor changes are observed for the band at 751 cm−1 (benzene deformation, isoindole deformation [43,44]), which had decreased its relative intensity for the FePc/DMFaged LB 488 R.J.G. Rubira et al. / Applied Surface Science 416 (2017) 482–491 Fig. 7. Raman spectra of FePc/DMF and FePc/DMFaged collected for both aggregates and smooth regions of the LB films. MF, an fi s 3 ( F s s Fig. 8. FTIR spectra (transmission mode) for 21 layers of FePc/CH2Cl2, FePc/D lm. These changes corroborate the results obtained from Raman pectroscopy (Fig. 7). .2.5. Molecular organization analyses of the molecular organization were performed by FTIR transmission and reflection modes − Fig. 8) for 21 layers of ePc/DMF, FePc/DMFaged, and FePc/CH2Cl2 LB films (Fig. 8). The pectral profile of the FePc/DMF LB film (freshly prepared) is pretty imilar for both transmission and reflection modes (Fig. 8). Besides, d FePc/DMFaged LB films. Reflection mode for 21 layers of FePc/DMF LB Film. both FTIR spectra are also similar to the FePc powder spectrum (transmission mode) shown in the work of Volpati et al. [1]. Because the powder spectrum is a reference to the random molecular orga- nization, one can conclude the FePc molecules are isotropically organized in the FePc/DMF LB film. Considering the similarity of the FTIR spectra (transmission mode) in Fig. 8 for both FePc/DMF and FePc/DMFaged, one can also conclude the aging process does not affect the isotropic molecular organization found for FePc/DMF LB film. On the other hand, the FTIR spectrum (transmission mode) R.J.G. Rubira et al. / Applied Surface Science 416 (2017) 482–491 489 Fig. 9. IDMAP projection obtained using the real capacitance vs. frequency (Fig. S6) for the sensing array immersed into atrazine solutions. The sensing units are identified by the colors in the inset. 501 H f s s F t f L e 3 i s s Fig. 10. PCA obtained using the real capacitance vs. frequency at or FePc/CH2Cl2 (Fig. 8) is pretty similar to the FePc/CHCl3 LB film pectrum (transmission mode) obtained by Volpati et al. [1]. This imilarity led the authors to conclude their LB films are with the ePc molecules organized with the macrocycle plane preferentially ilted between 0 and 45◦ in relation to the substrate surface. There- ore, we can also conclude the FePc molecules forming FePc/CH2Cl2 B film are organized in the same way, i.e., macrocycle plane prefer- ntially tilted between 0 and 45◦ in relation to the substrate surface. .3. Sensing of atrazine herbicide The performance of the FePc LB films as transducers in sens- ng applications was evaluated by immersing them into aqueous olutions containing atrazine at different molar concentrations. The ensor array was composed by one bare Pt IDE, and three Pt IDEs z (Fig. S6) for the sensing array immersed into atrazine solutions. coated with 5 layers of FePc/DMF, FePc/DMFaged, and FePc/CH2Cl2 LB films. This array allows direct comparison among the sensing units regarding the effect of the film supramolecular structures, achieved by the different solvents, on the impedance response. The real capacitance vs. frequency curves collected for the sensing units immersed into 0.1 × 10−9, 1.0 × 10−9, 15 × 10−9, 50 × 10−9, and 100 × 10−9 mol/L atrazine solutions are shown in Fig. S6. A visual inspection of Fig. S6 does not allow one to distinguish the samples with different atrazine concentrations. The distinc- tion ability is demonstrated by treating the capacitance data in Fig. S6 using the IDMAP [39] multidimensional projection technique, whose results are given in Fig. 9. The electrical responses from each sensing unit and for each solution are plotted individually. Circle colors identify the sensing unit, while the atrazine concentration is shown next to each circle in the plot. Because the closer the cir- 4 rface c f s o p e o ( t e E u a i c 9 s t ( 4 e t B L d t i fi p a F m a t d t t T c r s A A t 1 R [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ 90 R.J.G. Rubira et al. / Applied Su les the more similar the electrical responses, it is observed the ollowing: (i) the bare Pt electrode is positioned apart from the ensing units, revealing the film effect on the electrical response f the IDEs, in spite of being ultrathin; (ii) the sensing units com- osed of FePc/DMF and FePc/DMFaged LB films are more similar to ach other than the FePc/CH2Cl2 LB film, highlighting the effect f the supramolecular structure of the films in this sort of sensor; iii) the sensing units can distinguish atrazine solutions even down o 10−10 mol/L, sufficient to monitor the quality of drinking water ven according to the most stringent international regulations [45]. vidently, the sensing array was probed as proof-of-principle in ltrapure water. A real system would require measurements that re out of the scope of this article. Fig. 9 is consistent with the PCA analysis shown in Fig. 10, reveal- ng the sensing units achieve their purpose, i.e. distinguish atrazine oncentrations. This result was obtained with a correlation of ca. 8% to PC1 for atrazine concentration, which shows the combined ensing units can perform the role that is expected of a sensor: dis- inguish (classify) the different concentrations of certain analyte atrazine). . Conclusion A systematic study was performed to investigate the influ- nce of different solvents (CHCl3, CH2Cl2, DMF, and THF) on he supramolecular architecture of FePc Langmuir and Langmuir- lodgett (LB) films. Besides the FePc molecular organization in B films, different morphological and optical properties are also ependent on the solvent. Coordination bonds between N-Fe lead o stronger DMF-FePc interaction, providing less FePc aggregation n solutions (for a certain concentration), and Langmuir and LB lms with more homogeneous (surface morphology) when com- ared to the films built up from FePc solutions in CHCl3, CH2Cl2, nd THF. The DMF-FePc interaction also leads to LB films with the ePc molecules isotropically organized, while for CH2Cl2 the FePc olecules are with the macrocycle preferentially tilted between 0 nd 45◦. Nevertheless, the DMF-FePc interaction has shown to be ime-dependent, increasing the FePc aggregation with time. Such ifferences in the supramolecular architecture are also reflected in he electric response of FePc LB films deposited onto Pt interdigi- ated electrodes when immersed into atrazine aqueous solutions. he role played by the different FePc supramolecular architectures ould be accessed by information visualization methods, which also evealed a high sensitivity of the sensor array to detect atrazine olutions down to 10−10 mol/L. cknowledgments This work was supported by FAPESP, CNPq and CAPES. ppendix A. Supplementary data Supplementary data associated with this article can be found, in he online version, at http://dx.doi.org/10.1016/j.apsusc.2017.04. 55%20001 eferences [1] D. Volpati, P. Alessio, A.A. Zanfolim, F.C. Storti, A.E. Job, M. Ferreira, A. Riul, O.N. Oliveira, C.J.L. Constantino, Exploiting distinct molecular architectures of ultrathin films made with iron phthalocyanine for sensing, J. Phys. Chem. B 112 (2008) 15275–15282, http://dx.doi.org/10.1021/jp804159h. [2] P. 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http://ec.europa.eu/food/plant/protection/evaluation/existactive/list_atrazine.pdf Supramolecular architectures of iron phthalocyanine Langmuir-Blodgett films: The role played by the solution solvents 1 Introduction 2 Materials and methods 2.1 Reagents and solutions 2.2 Langmuir and Langmuir-Blodgett (LB) films 2.3 Characterization techniques 3 Results and discussion 3.1 FePc Langmuir films 3.1.1 π-A isotherms 3.1.2 Morphology (BAM) 3.1.3 Solution UV–vis absorption 3.2 Langmuir-Blodgett (LB) films 3.2.1 Optical property and growth 3.2.2 Morphology at micrometer scale (micro-Raman) 3.2.3 Morphology at the nanometer scale (AFM) 3.2.4 FePc-solvent chemical interaction (vibrational spectroscopy) 3.2.5 Molecular organization 3.3 Sensing of atrazine herbicide 4 Conclusion Acknowledgments Appendix A Supplementary data References