Physical Vapor Deposited Thin Films of Lignins Extracted from Sugar

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Physical Vapor Deposited Thin Films of Lignins Extracted from Sugar Cane Bagasse: Morphology, Electrical Properties, and Sensing Applications Diogo Volpati,† Aislan D. Machado,† Clarissa A. Olivati,† Neri Alves,† Antonio A. S. Curvelo,‡ Daniel Pasquini,§ and Carlos J. L. Constantino*,† †

Faculdade de Ciências e Tecnologia, UNESP Univ Estadual Paulista, Presidente Prudente/SP, 19060-900, Brazil Instituto de Química de S~ao Carlos, USP Universidade de S~ao Paulo, S~ao Carlos/SP, 13560-970, Brazil § Instituto de Química, Universidade Federal de Uberlândia, Uberlândia/MG, 38400-902, Brazil ‡

ABSTRACT: The concern related to the environmental degradation and to the exhaustion of natural resources has induced the research on biodegradable materials obtained from renewable sources, which involves fundamental properties and general application. In this context, we have fabricated thin films of lignins, which were extracted from sugar cane bagasse via modified organosolv process using ethanol as organic solvent. The films were made using the vacuum thermal evaporation technique (PVD, physical vapor deposition) grown up to 120 nm. The main objective was to explore basic properties such as electrical and surface morphology and the sensing performance of these lignins as transducers. The PVD film growth was monitored via ultravioletvisible (UVvis) absorption spectroscopy and quartz crystal microbalance, revealing a linear relationship between absorbance and film thickness. The 120 nm lignin PVD film morphology presented small aggregates spread all over the film surface on the nanometer scale (atomic force microscopy, AFM) and homogeneous on the micrometer scale (optical microscopy). The PVD films were deposited onto Au interdigitated electrode (IDE) for both electrical characterization and sensing experiments. In the case of electrical characterization, current versus voltage (I vs V) dc measurements were carried out for the Au IDE coated with 120 nm lignin PVD film, leading to a conductivity of 3.6 1010 S/m. Using impedance spectroscopy, also for the Au IDE coated with the 120 nm lignin PVD film, dielectric constant of 8.0, tan δ of 3.9 103, and conductivity of 1.75 109 S/m were calculated at 1 kHz. As a proof-of-principle, the application of these lignins as transducers in sensing devices was monitored by both impedance spectroscopy (capacitance vs frequency) and I versus time dc measurements toward aniline vapor (saturated atmosphere). The electrical responses showed that the sensing units are sensible to aniline vapor with the process being reversible. AFM images conducted directly onto the sensing units (Au IDE coated with 120 nm lignin PVD film) before and after the sensing experiments showed a decrease in the PVD film roughness from 5.8 to 3.2 nm after exposing to aniline.

’ INTRODUCTION The environmental concern in relation to scarcity of natural resources and disposal of toxic and nonbiodegradable products has provided an advance in the development of biodegradable materials from renewable sources. In this context, lignins represent an important biomass source and an alternative for the polymer industry and other derivatives used as feedstock in chemical industries. Lignins are macromolecules found in vegetable tissues and formed mainly by phenylpropane units (C9 units) with different chemical groups linked in their structure depending on the plant species, representing one of the most abundant class of molecules in plant tissue.1,2 However, most of this renewable biomass resource is wasted from papermaking and sugar production processes, being burnt and used as energy source. Therefore, researches have been carried out obtain a better knowledge of the relationship between molecular structure and properties for this abundant material, allowing its use in technological applications. r 2011 American Chemical Society

The research via computational simulation is an alternative.3 The investigation through thin films has been another route, and some articles can be found in the literature,4 including Langmuir,59 LangmuirBlodgett (LB),914 layer-by-layer (LbL),1520 spin-coating,2123 dip-coating,24 casting,25,26 and electrodeposition either by electrophoresis27 or by chemical oxidation.28,29 These techniques have allowed us to obtain information at the molecular level for lignins, mainly regarding the morphology, monolayer thickness, and mean molecular area, besides thin film physicochemical properties such as surface energy, wettability, and electroactivity, in addition to possible applications such as surface coatings, composites, and transducer in sensing devices. These information can also contribute to a better understanding of the delignification mechanisms through Received: May 24, 2011 Revised: July 11, 2011 Published: July 18, 2011 3223

dx.doi.org/10.1021/bm200704m | Biomacromolecules 2011, 12, 3223–3231

Biomacromolecules analysis of lignin solubilized under different experimental conditions5,7,10,30 because the extraction process is another parameter that strongly interferes in the lignin molecular structure.31,32 The importance of fabricating thin films comes from the fact that a large portion of technological devices based on organic materials applies these compounds in the form of thin films. Furthermore, the molecular architecture of these thin films, that is, thickness, morphology, molecular organization, and crystallinity, plays an important role in their own electrical and optical properties and consequently on the performance of the device.33,34 Considering the lack of works reported in the literature concerning thin films of lignins, most of them quoted above, it is clear that an extensive effort has yet to be done in this field. The physical vapor deposition technique (PVD)35 could be another possibility of fabricating lignin thin films. The PVD technique refers to a set of methods where the materials are vaporized from solid sources, being transported under vacuum (low pressure) and solidified when in contact with the substrate, forming films with thicknesses varying from nano- to micrometers. Normally, the PVD methods allow high deposition rates without causing damage to the substrate surface due to low energy of incident species. Indeed, as far as we know, we are the first and unique group of applying this approach to lignins.36 In the latter, lignins extracted from sugar cane bagasse were deposited by vacuum thermal evaporation on solid substrates with thickness varying from 20 to 140 nm being monitored by quartz crystal microbalance and ultravioletvisible (UVvis) absorption spectroscopy. The thermal stability was verified by thermogravimetric analysis (TGA) and Fourier transform infrared (FTIR) absorption spectroscopy. Besides, optical microscopy revealed a homogeneous surface on the micrometer scale and FTIR (reflection and transmission modes) revealed a random molecular organization of the lignins in the PVD film. Here, as a complementary work, we have investigated the electrical properties of these lignin PVD films through dc and ac measurements. Besides, the lignin PVD films were applied as sensing units toward aniline as a proof-of-principle considering the affinity between both materials36 and the importance of detecting aniline considering its toxicity.37,38 For instance, in Alessio et al.,36 lignin PVD films were deposited onto quartz plates, exposed to vapors of aniline, phenol, toluene, chloroform, ammonia, and formaldehyde, and investigated through UVvis absorption spectroscopy. Here the sensing unit is formed by the PVD film deposited onto Au interdigitated electrode (IDE), which was characterized through dc current versus time measurements and impedance spectroscopy (capacitance vs frequency) in the presence and absence of aniline vapor. The latter is an adaptation of the concept of the electronic tongue.39 Besides, the morphology of these PVD films was monitored on a nanometer scale through atomic force microscopy (AFM) before and after the sensing experiments.

’ EXPERIMENTAL PROCEDURE The lignins used here were extracted from sugar cane bagasse via modified organosolv process using a solvent mixture of ethanol/water associated with supercritical CO2 (organosolvCO2 supercritical pulping process).40 The latter may give different chemical characteristics from that extracted via the traditional pulping process. In the organosolv process, the exclusive use of organic solvent/water mixtures eliminates

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the need of burning the liquor, allowing the isolation of the lignins by distillation of the organic solvent. In organosolvCO2 pulping employed in this work, an additional advantage is the application of pressurized (liquid) carbon dioxide as an important part of the pulping liquor (50% alcohol/water mixture and 50% carbon dioxide). This process combines the employment of a lower amount of organic solvent, helps the lignin recovering, and produces pulp with lower strength properties but in similar yields and in shorter time when compared with the industrial processes. The molecular weight distributions were determined by size exclusion chromatography (SEC) using three Plgel columns (500, 103 and 104 Å), THF as solvent at 1.0 mL/min, and polystyrene standards leading to Mn = 1185 g/mol, Mw = 1971 g/mol, and polydispersivity Mw/Mn = 1.66. Further information regarding the molecular structure of the ethanol lignin (C9-formula) obtained by elemental analysis, 13C NMR, and FTIR can be found elsewhere.14,40 The lignin PVD films were grown using the vacuum thermal evaporation technique in a Boc Edwards model Auto 306 machine. The growth process consists of placing the lignin powder in a metallic boat (Ta in this case, melting point of 3017 °C), where an electric current is passed through, heating the Ta boat. The substrate and the quartz crystal microbalance are placed parallel and positioned ca. 15 cm above the Ta boat. The evaporation process is performed within a vacuum chamber under 107 Torr. The electric current was adjusted slowly from 0.0 to 1.9 A (10 V), which corresponds to the value that the evaporation process started. This is a key step and must be conducted very carefully to avoid missing the control of the evaporation rate, which could lead the powder to blow up.41,42 When the evaporation rate was stabilized between 0.1 and 0.3 nm/s, the quartz crystal microbalance was brought to zero value, and the shutter that protects the substrate is opened, allowing the growth of the lignin PVD film until the desirable mass thickness. It is important to mention that the PVD films were grown in steps, where each step corresponds to a mass of ∼5.5 mg of lignin that was placed in the Ta boat and evaporated. Lignin PVD films with thicknesses from 20 to 120 nm were grown onto quartz substrates, being monitored in situ by the quartz crystal microbalance placed within the evaporated vacuum chamber and ex situ by UVvis absorption spectroscopy using a spectrophotometer Varian model Cary 50 between 190 and 1100 nm. The dc electrical measurements (current vs voltage, I vs V) were carried out using a Keithley equipment model 238 between 10.0 and +10.0 V in steps of 1.0 V for 120 nm PVD films deposited onto Au IDE. The latter was fabricated by photolithography technique, presenting the following dimensions: height = 100 nm, length = 8 mm, and width = 100 μm, with a total of 25 pairs of digits, which are spaced 100 μm from each other. The ac electrical measurements were carried out using a Solartron equipment model 1260A for the same 120 nm PVD films deposited onto the Au IDE. The ac measurements were analyzed in terms of the complex capacitance as a function of frequency with 100 mV of amplitude and frequency from 1 to 106 Hz. AFM measurements were carried out in a Nanosurf Easy Scan 2 microscope. Images with 10.0 10.0 μm2 and 5.0 5.0 μm2 were collected with resolution of 512 points/line using silicon recovered with aluminum probe in tapping mode. The images were collected, and the statistical analyses were processed using Gwyddion software. The sensing experiments toward aniline vapor were conducted through dc (I vs time) and ac (capacitance vs frequency) electrical measurements using the same experimental set up. Basically, the sensing unit composed of the Au IDE coated with the 120 nm lignin PVD film was placed inside a bottle (7 cm of diameter and 5.5 cm of height), and the measurements were conducted with and without aniline vapor atmosphere. For these measurements, 20 mL of aniline was dropped in a beaker inside the bottle with the sensing unit fixed ca. 5 cm away 3224

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Figure 1. (a) UVvis absorption spectra for lignin PVD films deposited through six consecutive evaporation steps. (b) Absorbance at 280 nm versus PVD film thickness. (c) Absorbance at 280 nm versus lignin mass (initially weighted). from the aniline liquid (facing the liquid). The measurements started immediately after insert aniline in the bottle for both ac and dc techniques. As previously mentioned, the sensing experiments toward aniline vapor were carried out as a proof-of-principle. However, it is possible to estimate the aniline concentration into the bottle using Antoine’s equation43 when ac and dc electrical measurements reach maximum values (saturation). Antoine’s equation states the relation between vapor pressure and temperature for pure components. Considering temperature of 23 °C and pressure of 760 mmHg, the pressure vapor calculated for aniline is between 0.45 and 0.55 mmHg. This interval comes from different values found in the literature for the constants in the Antoine’s equation,4345 leading to molar fractions between 0.06 and 0.07% or 600 and 700 ppm (part per million) of aniline vapor inside the bottle. AFM images were also recorded directly from the sensing units before and after exposing to aniline vapor.

’ RESULTS AND DISCUSSION PVD Films - Growth. The growth of the lignin PVD films was monitored by UVvis absorption and quartz crystal microbalance for 23, 44, 66, 81, 109, and 120 nm thicknesses. Figure 1a shows the UVvis spectra recorded for six consecutive depositions (evaporation steps) and Figure 1b,c gives, respectively, the absorbance at 280 nm versus the thickness measured with the microbalance and the absorbance at 280 nm versus the mass of lignin (originally weighted) used in each evaporation step measured with an analytical balance. The absorption band with maximum at 280 nm in Figure 1a is assigned to the π f π* electronic transition from phenyl groups.46,47 The linear increase in the maximum at 280 nm with both thickness PVD film (Figure 1b) and lignin mass (Figure 1c) allows one to conclude that a similar amount of lignin is deposited in each evaporation step, leading to an average thickness of ca. 20 nm for each 5.5 mg of lignin mass (initially weighted). The values of PVD film thickness per lignin mass are in good agreement with the relation reported in the

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previous work for the same lignin,36 that is, (19 ( 1) nm for each 5.0 mg of lignin. It is worth mentioning that the UVvis absorption spectra shown in Figure 1a are pretty similar to those reported in a previous work14 for LB films of lignin extracted from sugar cane bagasse using buthanol as the organic solvent. Despite the fact that the buthanol lignin presents lower molecular weight (Mn = 1060 g/mol and Mw = 1590 g/mol),14 both ethanol and buthanol lignins have their molecular structure dominated by phenylpropane units and then leading to similar UVvis spectra. PVD Films - Morphology. The surface morphology of the lignin PVD film deposited onto Au IDE was analyzed by AFM technique. Figure 2a shows a 2D image of the bare Au electrode surface, Figure 2b shows the profile extracted for this surface, and Figure 2c shows a 3D plot for the same area shown in Figure 2a. Complementarily, Figure 2d shows a 2D image of the 120 nm lignin PVD film coating the Au IDE, and Figure 2e,f shows the profile extracted for this surface and a 3D image for the same area shown in Figure 2d, respectively. In a general way, it is seen that the surface of the bare electrode (Figure 2ac) is visibly smooth, which is changed when the Au electrode is coated with the 120 nm PVD film, presenting some aggregates. The latter is observed for both digit surface and valley (Figure 2df). In terms of quantitative analysis, the root-mean-squared roughness (rms) and average roughness (Ra) were extracted from the AFM images shown in Figure 2a,d for digit surface and valley areas. In the case of bare Au electrode, we found rms = 0.8 nm and Ra = 0.6 nm for an area of 1.0 μm 1.0 μm and rms = 0.7 nm and Ra = 0.6 nm for an area of 4.0 μm 4.0 μm. Similar values were found when valley areas were analyzed, revealing a smooth surface for both digit (Au) and valley (glass). Considering the 120 nm lignin PVD film coating the Au IDE, we found rms = 6.7 nm and Ra = 5.5 nm for an area of 1.0 μm 1.0 μm and rms = 8.5 nm and Ra = 6.3 nm for an area of 4.0 μm 4.0 μm. Similar values were found when valley areas were analyzed. Comparing both bare and coated electrodes, the increase in rms and Ra roughness in the presence of the coating lignin PVD film is evident. In Pasquini et al.,13 LB films with five layers were fabricated with two types of lignin, one extracted using acetone (SAC) and the other extracted with ethanol (EL), which were morphologically analyzed using AFM. The rms values found were 3.8 nm for SAC-lignin and 0.24 nm for EL-lignin in areas with 1.0 μm 1.0 μm. The differences between these rms values were related to the molecular organization found for both lignins, which was determined using FTIR. Comparing the rms values for both LB and PVD lignins, the differences can be attributed to the thin film fabrication technique, substrate, and film thickness. The techniques LB and PVD involve a different physical process to produce the thin films, inducing some morphological differences. The LB films were deposited onto mica, whereas the PVD films were evaporated onto Au IDE. Lastly, the thickness can also be related to the differences in rms values. For five-layer LB films, a few tens of nanometers is expected, whereas the PVD film was grown with 120 nm of thickness. If the rms is considered in relation to the film thickness, then we would find 6.7/120 ≈ 0.06 (6% for 1.0 μm 1.0 μm) for PVD film, which is a very reasonable value in terms of smooth surface. PVD Films - Electrical Properties. The electrical conductivity for the lignin was determined through dc electrical measurements using a 120 nm PVD film deposited onto Au IDEs. Figure 3 shows the I versus V measurement between 10 and +10 V carried out in the dark. It is known that the charge 3225


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Figure 2. (a) AFM images for a bare Au IDE, (b) surface profile, and (c) 3D plot of the AFM image shown in part a. (d) AFM image for a 120 nm lignin PVD film deposited onto the Au IDE, (e) surface profile, and (f) 3D plot of the AFM image shown in part d. The edge presented in all 2D and 3D AFM images is the step between one valley and one digit surface of the IDE.

Figure 3. dc electrical measurements I V for a 120 nm lignin PVD film deposited onto Au IDE. Inset: cartoon of the Au IDE coated with 120 nm PVD film.

transport mechanisms depend on the nature of the electrical contacts in the sample. For instance, a linear relationship between the applied voltage and the measured current is observed in Figure 3, which is characteristic of ohmic behavior.48,49 The electrical conductance was calculated from a linear adjustment of the I versus V data using the model developed by Olthuis et al.50 The fitting for a function type y = ax + b provided the value a = 6.97 1011 S (b = 0). Therefore, using the cell constant of the electrode, it is possible to estimate the electrical conductivity of the material leading to 3.6 1010 S/m. The cell constant is

determined by taking into account the width and length of the digit, the total number of digits, and the free space between each digit.50,51 In our case, the cell constant used is 5.1 m1. Advancing in the electrical characterization of the lignin PVD films, the relativity permittivity, also called the dielectric constant, was determined through ac electrical measurements for the 120 nm PVD film deposited on Au IDE. The dielectric constant is indeed the real component of the complex dielectric function (ε* = ε0 r + jε00 r) and can be obtained from the complex capacitance C* (C* = C0 + jC00 ) in impedance measurements.52 For instance, the real component of capacitance (C0 ) can be expressed by the capacitor equation defined as C0 = ε0 rε0k, where εo is the vacuum permittivity (εo = 8.85 1012 F/m) and k is the geometric parameter of the Au IDE. (k is the inverse value of cell constant used to calculate the dc conductivity previously described.) In our case, the value of C0 was extracted from the impedance measurements discussed in Figure 4 for the PVD film in the dark and exposed to air. Taking into account the C0 value at 1 kHz (C0 = 1.39 1011 F), we found a dielectric constant of ε0 r = 8.0 for the lignin, which is consistent with insulating materials. For instance, organic materials such as cyanoethylpullulan (CYEPL) with 1200 nm of thickness and poly(vinyl alcohol) (PVA) with 500 nm present ε0 r = 12 and 10, respectively,53 and ε0 r = 4 for poly(4-vinyl phenol) (PVP) with 380 nm of thickness.54 For inorganic compounds, ε0 r = 17.3 for barium zirconate titanate (BZT) film with 122 nm55 and ε0 r = 8.4 for aluminum oxide (Al2O3) with 93 nm.56 Another parameter extracted from ac electrical measurements for the 120 nm PVD film deposited onto Au IDE was the Tan δ). When an electric field is applied in a material, some energy can 3226


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Figure 4. ac electrical measurements of capacitance magnitude versus frequency for a 120 nm lignin PVD film deposited onto Au IDE (a) exposing to vapor of aniline and (b) removing the aniline vapor.

Figure 5. dc experimental measurements of I versus t when for 5 V is applied to a 120 nm lignin PVD film coating the Au IDE exposed to air and aniline vapor. Inset: cartoon of the Au IDE coated with 120 nm lignin PVD film.

be lost because of internal motions. This energy loss is expressed by the imaginary component ε00 r of the complex dielectric constant, whereas the real component ε0 r quantifies the energy (from the applied electric field) that is elastically stored. So, the dissipation factor Tan δ is obtained by the ratio between ε00 r and ε0 r: Tan δ = ε00 r/ε0 r = C00 /C0 . Using the values of C0 (C0 = 1.39 1011 F) and C00 (C00 = 5.47 1014 F) at 1 kHz extracted from the same impedance measurements carried out in air and dark in Figure 4, the Tan δ calculated is 3.9 103. This means that for our measurement system, the lignin PVD film presents a low energy loss for an alternating electric field, consistent with insulating materials. For instance, this value (at 1 kHz) is equivalent to other insulating polymers found in the literature,57 such as poly(vinyl cyclohexane) (PVCH) with Tan δ = 2.0 103 and poly(cyclohexyl methacrylate) (PCHMA) with Tan δ = 4.7 103. Finally, using the 120 nm PVD film deposited onto Au IDE, the conductivity of the lignin was also calculated via ac measurements by obtaining the conductance from impedance measurements. 52 The conductance calculated at 1 kHz is 3.45 1010 S, and applying the cell constant, the electrical conductivity found was 1.75 10 9 S/m,

which is ∼1 order of magnitude higher than that calculated via dc electrical measurements. The ac conductivity was also calculated at 100 Hz being 2.5 10 10 S/m, showing its tendency to decrease for lower frequencies, reaching 3.6 10 10 S/m in the dc experiment (Figure 3). Sensing Experiments. A 120 nm, lignin PVD film deposited onto Au IDE using six evaporation steps, as previously described, was tested in sensing experiments toward aniline vapor. The same linear increase in the maximum absorbance at 280 nm with both thickness PVD film and lignin mass was found, corroborating the reproducibility of this evaporation process. Figure 4a shows the capacitance magnitude (C*) versus frequency curves for the lignin sensing unit exposed to aniline vapor from 0 to 16 min. The latter corresponds to the period of time for which a maximum of capacitance variation was detected. Tests regarding the recovery of the sensing units were performed, as shown in Figure 4b. It is seen that 8 min after the aniline vapor atmosphere is removed, the capacitance curves turn over to the initial values obtained in the absence of aniline vapor, revealing the reversibility of the process. As noted in Figure 4a,b, the maximum variation of the capacitance magnitude (C*) was observed at low-frequency values (above 102 Hz, no changes were observed; data not shown), and the process is reversible. Supported by this fact, an experiment was carried out using dc measurements to check the adsorption/ desorption of aniline to/from lignin PVD film forming sensing unit. Figure 5 shows the dc measurement of the current as a function of the time that the sensing device is exposed to air and aniline atmosphere (I vs t) when 5 V is applied to the sensing unit. The same experimental conditions as those of the ac measurements were kept, as previously discussed. The I versus t curve from 0 to 180 s (3 min) shows current values of ca. 6.4 1010 A in air (before exposing to aniline), which is in good agreement with the current value obtained for the sensing device previously prepared to calculate conductivity (Figure 3), confirming the reproducibility of the system. After 180 s, the lignin film was exposed to aniline vapor inside the same bottle used in the ac measurements (Figure 4). From this point, it is possible to observe the increase in the current until reaching a maximum at ca. 1.9 107 A after 5 min in the presence of aniline vapor. Then, the current decreases and, after 20 min in aniline vapor, it reaches a stable value at ca. 1.2 107 A. When aniline is removed, then an abrupt decay in the current is observed, stabilizing at ca. 7.9 1010 A, which is pretty close to the initial 3227


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Figure 6. AFM images collected from an area of 10.0 μm 10.0 μm of the sensing unit (a) before and (b) after exposing to aniline vapor.

Figure 7. Optical images of the Au IDE coated with 120 nm lignin PVD film recorded with (a) 500 and (b) 50 magnifications before exposure to aniline atmosphere. Optical images of the same interdigitated electrode are presented after exposing to aniline with (c) 500 and (d) 50 magnifications.

value measured before being exposed to aniline. At the end of the measurement, the current value is 7.25 1010 A, evidencing the reversibility of the process adsorption/desorption and the suitability of this system for sensing applications. In Alessio et al.,36 PVD films of the same lignin revealed an interaction with vapors of aniline, phenol, and toluene, as indicated by the increase in the intensity of the band with maximum at 206 nm. The measurements were carried out through UVvis absorption spectroscopy before and after being exposed to those vapors (besides chloroform, ammonia, and formaldehyde). The absorption band at 206 nm was assigned to ππ* transitions in the benzene rings, which is present not only in the lignin molecular structure but also in the solvents aniline, phenol, and toluene. The desorption of the aniline from the lignin PVD film in Alessio et al.36 was also checked via UVvis

absorption, and the results showed that it was necessary to induce desorption by heating (50 °C during 30 min). In a similar work, Silina et al.58 showed a special sensibility of vegetal extract films (mycelium mushroom) to aniline and, mainly, phenol vapors detected via piezoelectric sensor. Stergiou et al.59 applied lignin casting films as ozone sensor via cyclic voltammetry and electrochemical impedance spectroscopy. To advance on the principle-of-detection of aniline using lignin PVD films, we recorded AFM images for the 120 nm PVD film before and after being exposed to aniline vapor, as shown in Figure 6a,b, respectively, for areas of 10 10 μm2 collected directly from the sensing unit. Before the vapor exposition, the lignin film presents aggregates at the surface, rms = 5.8 nm and Ra = 4.0 nm, measured for the 10 10 μm2 area, which is consistent with the data discussed in Figure 2b. 3228


Biomacromolecules After the vapor exposition, larger and less spread aggregates and a decrease in the surface roughness are observed, with rms = 3.2 nm and Ra = 1.9 nm. In Stergiou et al.,59 scanning electron microscopy (SEM) images revealed the degradation of the lignin casting films due to exposition to ozone after 10 min. In the case of lignin PVD films, Alessio et al.36 showed via FTIR and UVvis absorption spectroscopies that the lignin is not degraded by aniline. A possible chemical interaction between aniline vapor and lignin could involve the lone electron pair from N (aniline) with the benzene rings (lignin).60 However, it was also discarded in Alessio et al.36 because no bathochromic shift was observed for the band at 206 nm, as it would be expected for this kind of interaction. The latter strongly suggests a physisorption process of the aniline vapors within the matrix of lignin, which was also confirmed by FTIR measurements before and after exposing the lignin PVD films to aniline vapors.36 The possibility of desorption of the aniline observed in ref 36 and here is consistent with the physisorption mechanism. Therefore, this physisorption might be closely related to the changes in the lignin PVD film morphology observed on a nanometer scale, leading also to the changes observed in the electrical measurements. The presence of larger and less spread aggregates on a nanometer scale (AFM images) due to aniline vapor exposition was also observed on a micrometer scale. Optical images were collected for the sensing unit before and after exposure to aniline vapor. Figure 7a,b shows the Au IDE coated with the 120 nm lignin PVD film with magnifications of 500 and 50, respectively, before aniline exposition. Figure 7c,d shows the same electrode with the same magnification, however, after exposure to aniline atmosphere. A homogeneous lignin PVD film is observed on a micrometer scale before exposure to aniline vapor, consistent with Alessio et al.;36 however, this morphology is changed by the presence of dark spots after exposing to aniline. It is important to comment that micro-Raman and surfaceenhanced Raman scattering (SERS) spectra were collected directly from the sensing units after aniline vapor exposition trying to find some aniline molecule adsorbed in the film surface. However, no characteristic aniline Raman scattering signal was found. In the case of conventional Raman, the adsorbed aniline concentration might be too low to be detected. In the case of SERS, the measurement required an evaporation of Ag (6 nm thickness) onto the lignin PVD film surface. This process must be made under vacuum; therefore, as discussed in Figures 4b and 5, the aniline adsorbed onto the lignin PVD film might have been removed in a way that even SERS was not able to detect it.

’ CONCLUSIONS PVD films of lignin extracted from sugar cane bagasse were fabricated under vacuum thermal evaporation. Lignin PVD films with 120 nm were deposited onto Au IDE for dc and ac electrical characterization. From current versus voltage (I vs V) dc measurement, a conductivity of 3.6 1010 S/m was determined. From impedance spectroscopy, a dielectric constant of 8.0, Tan δ of 3.9 103, and conductivity of 1.75 109 S/m were determined at 1 kHz. Besides, as a proof-of-principle, the Au IDE coated with 120 nm lignin PVD film was applied as a sensing unit by exposing to aniline vapor. Both dc and ac electrical measurements revealed the sensitivity of lignin toward aniline; the aniline adsorption and desorption processes are reversible. The morphology of the 120 nm lignin PVD film was followed by AFM directly onto the sensing unit before and after the sensing

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experiment. The results showed a decrease in the film surface roughness from 5.8 to 3.2 nm after exposure to aniline, with the small aggregates found all over the surface changing to larger and less spread aggregates at surface. The latter is also consistent with morphological changes observed on a micrometer scale via optical microscopy.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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