Polydiacetylene capacitive artificial nose - ACS Applied Materials

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Polydiacetylene capacitive artificial nose V. Kesava Rao, Nagappa L Teradal, and Raz Jelinek ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20930 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 7, 2019

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ACS Applied Materials & Interfaces

Polydiacetylene capacitive artificial nose

V. Kesava Rao,1 Nagappa L. Teradal1 and Raz Jelinek1,2*

1Department 2Ilse

of Chemistry, Ben Gurion University of the Negev, Beer Sheva 84105, Israel

Katz Institute for Nanotechnology, Ben Gurion University of the Negev, Beer Sheva

84105, Israel

*Corresponding author email: [email protected]

1 ACS Paragon Plus Environment

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Abstract Polydiacetylenes are a class of conjugated polymers exhibiting unique color and fluorescence properties and employed as useful sensing vehicles. Here we demonstrate for the first time that the dielectric properties of polydiacetylenes can be exploited for vapor sensing. Specifically, electrodes coated with polydiacetylenes, embedded within a porous polyvinylpyrrolidone (PVP) matrix, exhibit significant capacitance transformations upon exposure to different vapors. The capacitive response of the polydiacetylene/PVP films depended upon both the structures of the diacetylene monomers, and the extent of ultraviolet irradiation (i.e. polymerization), underscoring a unique sensing mechanism affected by conjugation, structure, and dielectric properties of the polydiacetylene/polymer matrix. Importantly, the variability of polydiacetylene structures allow vapor identification through an array-based pattern recognition (i.e. artificial nose). This study opens new avenues for applications of polydiacetylene systems, particularly pointing to their dielectric properties as powerful sensing determinants.

Keywords: polydiacetylene; dielectric properties; capacitive vapor sensing; artificial nose; polyvinylpyrrolidone


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1. Introduction Polydiacetylenes (PDAs) are π-conjugated polymers exhibiting interesting structural and optical properties, primarily related to their remarkable colorimetric and fluorescent transformations.1–7 The unique optical properties of PDA assemblies arise from the ene-yne topo-polymerization of the diacetylene monomers.8 PDA systems have attracted considerable interest as promising sensing platforms as their color and fluorescence transformations can be induced by varied external stimuli, including water-soluble analytes,9,10 organic solvents,11 mechanical stress12 and ligand-receptor interactions,13–15 and others.

Few reports have demonstrated PDA-based optical

sensing of vapors.16–20 Since the conjugated PDA network can be perceived as a quasi-one-dimensional (1D) π-conjugated polymer, charge carrier transport phenomena have been reported.21,22 Previous studies have shown, for example, that vacuum-deposited PDA films exhibit p-type semiconductor properties.23–25 PDA crystals, however, are nearly complete insulators (exhibiting electrical conductivity in the order of σ ≈ 10-12 S/cm). This feature make electronic applications of PDA systems rare;26,27 the insulating properties of PDA, however, may be exploited for dielectric-based applications, specifically capacitive-based sensing of vapor substances. Capacitive vapor sensors, which operate via modulation of the dielectric properties of the detection medium upon physical or chemical adsorption of target airborne molecules, are attractive due to their short response times, resilient operating conditions, low power consumption, and room temperature applicability.28–32 Since capacitive sensors have no static power consumption, they are suitable for applications such as low-power battery-operated systems 3 ACS Paragon Plus Environment


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and wireless vapor detection.33 An important advantage of capacitance-based gas sensing is the fact that detection properties are determined by dielectric modulation, generally exhibiting higher fidelity and sensitivity than charge effects which are dominant in resistance-based sensors.34,35 Here we show that PDAs embedded in a porous polyvinylpyrrolidone (PVP) polymer framework yield films exhibiting remarkable capacitance transformations, induced upon exposure to volatile molecules. Indeed, to the best of our knowledge this is the first study in which modulation of PDAs' dielectric properties are exploited for vapor sensing. Specifically, we demonstrate that capacitive sensors can be fabricated through deposition of PDA/PVP films upon inter-digitized electrodes (IDEs). Spectroscopic and microscopic analyses illuminate the capacitive response mechanisms, particularly dependence of the dielectric film properties upon diacetylene monomer structures and degree of polymerization. The PDA-PVP composite capacitance sensors exhibit excellent sensing properties, including high sensitivity, reproducibility, and rapid response and recovery times.

Employing diacetylene

monomers displaying different headgroup and pendant sidechains, we show that target specificity can be accomplished through an array-based "artificial nose" sensing platform.

2. Results and discussions 2. 1. Sensor design and characterization Figure 1 schematically depicts the fabrication procedure of the PDA-based capacitive vapor sensor. As shown in Figure 1, the vapor sensing platform was prepared by spin-coating a mixture of diacetylene monomers and polyvinyl pyrrolidone (PVP) upon an inter-digitized electrode (IDE). We employed PVP as the host matrix 4 ACS Paragon Plus Environment

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in the PDA-based vapor sensor since this transparent polymer allows both efficient dispersion of the diacetylene monomeric units, as well as ultraviolet (UV) – induced polymerization. Moreover, PVP films exhibit enhanced porosity and high surface area,36,37 which are essential for effective vapor sensors. Following spin-coating of the diacetylene/PVP film, UV irradiation was applied (254 nm, 20 minutes), generating the conjugated polydiacetylene (PDA) network,38 giving rise to the typical blue color for most diacetylene derivatives employed here. Exposure of the PDA/PVP-IDE sensor to a target vapor induced a rapid change in the electrode capacitance which could be readily recorded in a computer-interfaced inductance-capacitance-resistance (LCR) meter (Figure 1, right).

Figure 1: Preparation of the polydiacetylene/PVP capacitive vapor sensor. The diacetylene and polymer constituents and mixed and spin-coated on an interdigitized electrode (IDE), followed by polymerization of the diacetylenes via UV irradiation. Scheme 1 depicts the chemical structures of the diacetylene monomers utilized in the sensor platforms. Monomers 1-5 contain different headgroups and pendant sidechains. Specifically, 1 and 2 display a single carboxylic terminus and different number of carbons in the sidechains, 3 exhibits two carboxylic termini, 4 displays a melamine headgroup, while 5 contains two phenoxy units in the sidechain termini. The use of different monomers allowed both investigation of the structural parameters affecting vapor-induced capacitance changes within the PDA/PVP-coated electrode, as 5 ACS Paragon Plus Environment


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well as generation of distinct PDA-dependent signals, thus facilitating vapor specificity (see below). O OH

1

O

2

O

OH

OH HO

O

3

O

N H

4 O O

H N

N N

NH2 N

NH2

5

Scheme 1: Molecular structures of the diacetylene monomers. 1. 10,12tricosadiynoic acid; 2. 10,12-Pentacosadiynoic acid; 3. Docosa-10,12-diynedioic acid; 4. N-(2-((4,6-diamino-1,3,5-triazin-2-yl)amino)ethyl)tricosa-10,12-diynamide; 5. 1,6diphenoxy-2,4-hexadiyne. Microscopic and spectroscopic characterization of the PDA/PVP films are presented in Figure 2.

The scanning electron microscopy (SEM) images of the

PDA/PVP films reveal variability in film morphologies (Figure 2A). The distinct surface features apparent in Figure 2A are due to the different molecular structures of 1-5, which affect organization and structure of the deposited films; distinct film morphologies were recorded also prior to UV-induced polymerization of the monomers (Figure S1). A notable aspect apparent in the SEM images in Figure 2A is the significant film-surface roughness, evident in all PDA/PVP film derivatives. This property is important as high surface area is essential for effective and sensitive vapor molecules adsorption and detection. Figure 2A indicates that PDA/PVP films generated from diacetylene derivatives 1-4 displayed an abundance of irregularly-shaped rectangular fragments (or sheets).


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Sheet-like domains have been previously reported in composite PDA/polymer films, ascribed to formation of aligned PDA networks.38 Indeed, appearance of microscopic sheet-like anisotropic structures is a defining structural feature in diverse PDA assemblies, recorded in vesicular aggregates,27,39 Langmuir-Blodgett films,40 and spincoated films.41 Interestingly, the SEM image recorded in case of 5/PVP reveals that the surface morphology of the film was significantly different than the other derivatives (Figure 2A); specifically, no sharp-edged sheet-like surface domains are observed in case of the 5/PVP film, rather a smoother surface in which numerous circular depressions appeared. The distinctive surface morphology of the spin-coated 5/PVP film is likely related to interactions between the aromatic residues of 5 and hydrophobic PVP backbone.42,43 These van der Waals interactions are significantly different than the polar interactions / hydrogen bonding between the headgroups of 1-4 and pyrrolidone residues dictating the distinct organization and surface morphologies of 1-4-PDA/PVP films (Figure 2A).



A 1

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Figure 2: Characterization of the polydiacetylene/PVP films. A. Scanning electron microscopy (SEM) images of the PDA/PVP films comprising monomers 1-5. B. UVVis spectra, and C. Raman spectra of the films. Numbers 1-5 in both B and C correspond to the monomers employed for fil generation The spectroscopy data in Figure 2B-C further illuminate the structural features of the PDA/PVP films, confirming formation of polymerized, conjugated PDA networks. The UV-Vis absorbance spectra in Figure 2B, in particular, show that PDA/PVP films formed upon UV irradiation of derivatives 1-4 exhibit the PDA8 ACS Paragon Plus Environment

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associated peak at around 640 nm and shoulder at ~580 nm. While the UV-Vis absorbance spectra acquired for 1-4/PVP exhibit slight shifts and different intensity ratios between the ~640 nm and ~580 nm signals (Figure 2B), their appearance attest to formation of the "blue-phase" PDA framework;2,3 gradual increase in peak intensities upon the extent of UV irradiation was recorded (Figure S2), supporting this interpretation.

Figure 2B also shows that polymerized 5/PVP yielded a broad

absorption band between 300-450 nm, reflecting the yellow color of this film (photographs of the polymerized PDA/PVP films are presented in Figure S3). The distinct polymerization profile of 5/PVP, apparent in Figure 2B, is consistent with the different morphology of these films recorded in the SEM experiment (Figure 2A). The Raman scattering data in Figure 2C complement the UV-Vis spectroscopy results, confirming the formation of PDA networks in the PDA/PVP films, and further underscoring the distinct organization of 5 within the PVP film. Specifically, the Raman spectra of the UV-irradiated 1-4 PDA/PVP films in Figure 2C feature the typical peaks at 1450 cm-1 and 2085 cm-1, corresponding to the carbon-carbon double bond and triple bond, respectively,44 while the signal at around 695 cm-1 is ascribed to in-plane bending vibrations of the PDA backbone.45 In comparison, the Raman signals recorded for 5/PVP were significantly shifted to approximately 1600 cm-1 and 2260 cm-1, respectively, further appearing noticeably broader compared to the corresponding peaks in 1-4 PDA/PVP. The shifts in the Raman scattering signals corresponding to the carbon-carbon double bond (~1600 cm-1) and triple bond (~2250 cm-1) likely reflect the presence of the phenoxy units at both termini of 5. Similar positions of Raman peaks in PDA functionalized with aromatic residues have been reported.46 The broad Raman peak at around 3300 cm-1 in the spectrum of 5/PVP is ascribed to the aromatic C-H units (Figure S4).47



2.2. Capacitive vapor sensing by PDA/PVP films and mechanistic analysis The PDA/PVP electrodes exhibit remarkable capacitance transformations upon exposure to gas molecules. Capacitive vapor sensing properties of the electrodesupported PDA/PVP films and mechanistic analyses of the capacitance response are depicted in Figure 3. Figure 3A-B present the capacitive changes induced in the 4/PVP and 5/PVP film electrodes upon exposure to ethanol vapor (concentration 300 ppm), employed here as a representative target analyte.

Importantly, ethanol-induced

capacitance transformations were also recorded in the PDA/PVP films, UV-irradiated for different durations prior to vapor exposure, aimed at assessing the effect of UVinduced structural transformations of PDA upon the capacitive response of the film electrodes. The graphs in Figure 3A-B reveal dramatic differences in capacitance modulation, both between 4/PVP (Figure 3A) and 5/PVP (Figure 3B), and also when the same film was pre-treated with different lengths of UV irradiation. 4/PVP, for example, exhibited very small capacitive response in the non-polymerized state (black curve in Figure 3A), while considerably more pronounced capacitance increase was induced upon ethanol exposure when the film was UV-irradiated for 20 minutes prior to the measurement (red curve, Figure 3A). Interestingly, longer UV irradiation of the 4/PVP film (for 40 minutes), resulting in the blue-red transformation of the PDA, effectively quenched the capacitive response of the film to ethanol vapor (green curve, Figure 3A). A similar trend was apparent in case of PDA/PVP electrodes comprising derivatives 1-3 (Figure S5).


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A. 4/PVP

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Figure 3: Vapor-induced capacitive response of PDA/PVP film electrodes and mechanistic analyses. Capacitance changes induced upon exposure of 4/PVP electrode (A) and 5/PVP electrode (B) to ethanol vapor (300 ppm). Curves in both panels: nonpolymerized films (black curves); 20-minute UV-irradiation (red curves); 40-minute UV irradiation (green curves). C-D: Dielectric spectra of ac-capacitance (C), and tan() (D) of 4/PVP (black curves) and 5/PVP (red curves) electrodes at room temperature. Solid curves – films prior to UV-irradiation, dashed curves – films after 20-min UV-irradiation. The vertical arrows indicate the changes in capacitance and loss of both films at low frequencies following UV-irradiation. Significantly different capacitive responses to ethanol vapor were recorded in case of 5/PVP (Figure 3B).

In contrast to the 4/PVP film (Figure 3A), 5/PVP

experienced the highest ethanol-induced capacitance increase in the non-polymerized state, i.e. in the as-prepared film assembly without UV irradiation (black curve in Figure 3B). In fact, UV irradiation of the 5/PVP film significantly reduced the capacitive response to ethanol vapor; small capacitive response was recorded when the film was 11 ACS Paragon Plus Environment


UV irradiated for 20 minutes prior to measurement (Figure 3B, red curve), and the capacitance change was completely eliminated when the 5/PVP film electrode was preirradiated for 40 minutes (Figure 3B, green curve). To elucidate the distinct capacitive sensing properties of the PDA/polymer films and the dramatic differences between the capacitance responses of 1-4-PDA/PVP vs 5PDA/PVP films (Figure 3A-B), we carried out a frequency sweep at room temperature (Figure 3C-D). The graphs in Figure 3C-D depict the real component of the complex capacitance and loss [tan (𝛿)(𝜔) = 𝐶′′(𝜔)/𝐶′(𝜔)] for 4/PVP (black) and 5/PVP (red) (detailed description of the dielectric measurements is provided in Figure S6). In addition, we also compared (within each plot) the dielectric profiles prior to UV irradiation (solid lines), and after 20-minute UV-irradiation at 254 nm (broken lines). The graphs in Figure 3C-D reveal significant differences in the dielectric properties between the two films. While 4/PVP underwent a decrease in the real part of both the complex capacitance and loss (emphasized by the black arrows, Figure 3CD), the behavior of the 5/PVP film was the opposite – both the real capacitance and particularly the loss increased significantly upon UV irradiation (red arrows in Figure 3C-D). Moreover, the magnitude of the change at low frequencies was much more pronounced for 5/PVP compared to 4/PVP (red vs black arrows in Figure 3C-D). Importantly, the differences between the curves were predominantly manifested in the low frequency regime which reflects the macroscale organization of the films, while in high frequencies, corresponding to nano- and meso-scale film features,48,49 the curves were very close to each other (both the curves of 4/PVP compared to 5/PVP, as well as non-UV-irradiated vs. UV-irradiated films); curve proximity is clearly apparent, for example, in the high frequency relaxation maxima at around 20 kHz (magnified region in Figure 3D).


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The dielectric response of the materials, reflected in the dielectric spectroscopy data in Figure 3(C-D) is due to the interactions of the electric field with induced charge carriers in the samples.48,50 Importantly, the two distinct relaxation processes observed in Figure 3D (e.g. low frequency vs high frequency) are related to film organization in different length-scales. Accordingly, the high frequency results indicate that charge separation at small distances (nanometer-to-sub-micrometer scale) is similar for the two film samples at both irradiation conditions, an observation which likely corresponds to the formation of localized conjugated polydiacetylene domains in both 4/PVP and 5/PVP. However, in the long-range interaction regime (e.g. low frequency), dramatic variations are apparent both between the two films, and between irradiated and nonirradiated samples. These divergences (indicated by the arrows in Figure 3(C-D) echo the significant morphological differences between 1-4/PVP films and 5/PVP, recorded in the SEM images in Figure 2 and Figure S1. Notably, the low-frequency dielectric data in Figure 3C-D account for the significantly different effects of UV irradiation upon vapor capacitance responses of the 4/PVP film vs 5/PVP film (Figure 3A-B).

Specifically, while Figure 3D

demonstrates that in case of 4/PVP the loss angle decreased following UV irradiation (i.e. the amplitude of the relaxation decreased), thus facilitating greater capacitance signal in the UV-irradiated film following vapor adsorption (Figure 3A), for 5/PVP the loss angle increased considerably (red arrow in Figure 3D), much higher than the capacitance increase (red arrow in Figure 3C), effectively quenching the capacitance response, as indeed apparent in Figure 3B.



2.3. Vapor sensing properties and selectivity via an "artificial nose" concept Figures 4 and 5 present practical applications of the PDA/PVP electrodes for capacitive sensing of gases. Figure 4 depicts capacitive responses of 4/PVP (UV irradiated for 20 minutes to ensure maximal sensitivity, e.g. Figure 3A) to ethanol, (Figure 4A-B), and to humidity (water vapor, Figure 4C). Capacitance response cycles induced by ethanol vapor (300 ppm) shown in Figure 4A attest to sensing stability and signal reproducibility. Indeed, Figure 4A indicates a small, 5% standard deviation of the ethanol-induced capacitance values. The capacitive response data in Figure 4A also demonstrate rapid sensor response and recovery times (100 sec and 10 sec, respectively), which are comparable or better than many capacitive sensors reported.33,51–53 Similar capacitance cycle reproducibility was observed in PDA/PVP films comprising the other diacetylene derivatives (Figure S9). A

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Figure 4. (A) 4/PVP sensor response upon on/off ethanol (300 ppm) vapor recycling; (B) Graphical representation of the percentage capacitance response with 4/PVP, highlighting the linearity in the range of 30–300 ppm for the ethanol; (C) Humidity response of 4/PVP thin film. Figure 4B presents the capacitive responses of the 4/PVP film electrode to different ethanol vapor concentrations.

Notably, the ethanol-induced capacitance

values of PDA/PVP demonstrate striking linearity in the analytically-important 30 300 ppm concentration range (Table S1).54,55 Linear response graphs were similarly recorded for other vapor molecules and other diacetylene derivatives (Figure S10). Linearity of the PDA/PVP capacitive response is important, as this feature facilitates, in principle, construction of calibration curves that can be employed for determining concentrations of target vapors. Application of the 4/PVP electrode for humidity monitoring is depicted in Figure 4C. Interestingly, the capacitive response of the film to relative humidity was not linear, displaying no capacitance change up to a relative humidity (RH) level of approximately 55%, while increasing significantly thereafter. The capacitive response profile in Figure 4C can be attributed to the distinct modes of water vapor adsorption onto the 4/PVP film. The negligible capacitance response up to 55% RH likely reflects chemisorption of water monolayer upon the sensor surface, whereas the pronounced increase beyond this RH value is probably due to physisorption of water multilayers.56,57 Inspection of Figure 4C also reveals that the dynamic range [(C/C0)*10-4] of the 4/PVP sensor for humidity measurements was approximately 1.7, which is better than most humidity sensors reported in the literature.56,57 Similar capacitive sensing curves were recorded in case of the other PDA derivatives (Figure S7). The observation of virtually negligible capacitive response to water vapor up to 55% RH in the 15 ACS Paragon Plus Environment


PDA/PVP film electrodes is significant, since it points to minimal interference from humidity when employing the PDA/PVP sensor as a practical sensor for varied gas targets. Figure 5 illustrates the application of the PDA/PVP capacitive sensor for analysis of different gases; in particular, Figure 5 demonstrates that vapor selectivity can be attained by employing an array of diacetylene derivatives, effectively creating an "artificial nose". The capacitance response curves in Figures 5A-B show the principles of the sensing strategy. In essence, Figure 5A demonstrates that PVP films embedding different PDA derivatives (i.e. 1-5) undergone different degrees of capacitance change upon exposure to the same molecule (ethanol) at the same concentration (300 ppm). The significantly different capacitance transformations in Figure 5A reflect the distinct affinities of ethanol molecules to the films, each comprising different diacetylene units exhibiting different functional units and molecular properties. The contributions of distinct molecular interactions dictating the degree of capacitance responses recorded in the PDA/PVP electrodes are similarly apparent in Figure 5B, illustrating the dramatic variability in capacitance response of 4/PVP to different vapor molecules. Note that the capacitive response analyses in Figure 5A-B reveal variations in both the extent of capacitance increase as well as the response time of the sensor, reflecting the significance of interactions between the vapor molecules and the PDA constituents as core parameters for sensor response. The degree of capacitance increase can be partially attributed to characteristics of the vapor molecules, specifically dielectric constant and polarity (Table S1). For example, the pronounced response of 4/PVP film electrode to DMF (Figure 5B) is likely due to its high dielectric constant and high polarizability. Similarly, in case of the alcohols (ethanol, n-propanol, n-


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butanol, n-pentanol, isopropanol), the decrease in capacitance response upon adsorption onto the 4/PVP electrode and longer response times (Figure 5B) can be ascribed to the trend of dielectric constants of these molecules, decreasing with hydrocarbon chain length of the alcohols (Table S1).

A

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Figure 5. Array-based capacitive artificial nose for vapor detection. A. Capacitance responses of ethanol (300 ppm) vapor added to 1-5-PDA/PVP composite thin films. B. Capacitive response curves of 4/PVP following addition of different vapors. C. Array-



based color code identification of different vapors using 1-4-PDA/PVP and 5-DA/PVP capacitive sensors; the color code on the right indicates degrees of capacitance change. Figure 5C depicts a color scheme summarizing the capacitive responses of the different PDA-derivative/PVP electrodes to several target vapor molecules (concentrations 300 ppm). The colors of the array elements reflect the extent of capacitance increase / decrease according to the color-code shown on the right. Importantly, Figure 5C demonstrates that distinguishing vapor molecules can be accomplished via the array-based "artificial nose" approach. Specifically, each tested molecule can be assigned a distinctive "fingerprint" determined by the combination of capacitance changes induced upon exposure to the different PDA derivatives. For example, ethanol shows the maximal capacitance response upon interaction with 4/PVP, and the succeeding order is 1/PVP~2PVP > 3/PVP > 5/PVP, while acetone gave rise to the highest capacitance response in case of 5/PVP, whereas no capacitance changes were apparent in film electrodes comprising PDA derivatives 1-4 (Figure S8). Several notable features are apparent in Figure 5C. 5/PVP, for example, underwent particularly low capacitance changes upon exposure to most vapor molecules (bottom row in Figure 5C). This result is likely due to the significantly lower polarity of the aromatic termini and distinct polymerization properties of this derivative compared to the other diacetylenes employed (i.e. Figures 2-3). It should be also noted that the apparent non-polar interactions between 5 and acetone or THF are less pronounced than the polar interactions between the other vapors tested and PDA derivatives 1-4, accounting for the low capacitive responses recorded. Another noteworthy observation in Figure 5C is the negligible capacitance changes induced in the 1-4/PVP film sensors by acetone (column vii in Figure 5C). This result may be attributed to lesser adsorption of acetone onto PVP films,58 and the absence of hydroxyl


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residues capable of hydrogen bond formation with the PDA headgroups. Overall, the color matrix in Figure 5C nicely demonstrates that vapor selectivity can be accomplished in the PDA/PVP capacitive sensor system through an artificial nose concept – i.e. the creation of detector arrays in which each element comprises of a distinct molecular unit.

3. Conclusions This work presents a new, powerful sensing application utilizing polydiacetylenes, specifically employing modulation of the dielectric properties of PDA/PVP films for capacitive vapor sensing. The experiments demonstrate that electrode-deposited PVP films embedding PDAs displaying different pendant sidechains undergo significant, vapor-dependent capacitance transformations; the capacitance changes are fully reversible upon discarding the vapor molecules, underscoring the potential usage of the system for practical vapor sensing. Spectroscopic and microscopic analyses illuminate the distinct organization of the mixed PDA/PVP films and the likely mechanisms responsible for capacitance transformations of the mixed films.

In particular, we investigated the factors

underlining the intriguing dependence of the capacitance changes upon diacetylene monomer structures and degree of polymerization.

Finally, we demonstrate an

"artificial nose" sensing application using PDA/PVP films comprising diacetylene monomers displaying different headgroup and pendant sidechains. The PDA/PVP capacitive films constitutes a rapid, easy to produce, robust, and inexpensive vapor sensor. The films can be readily integrated with conventional portable inductancecapacitance-resistance (LCR) readers to provide on-site detection of varied analyte



targets. The considerable versatility of PDA functional units should allow identification of diverse vapor targets through the array-based artificial nose concept.

4. Experimental Section Materials: Poly (vinyl pyrrolidone) (PVP, average molecular weight: 1300 kDa; Alfa aesar), 10,12-tricosadiynoic acid (TrC-DA, Alfa aesar), N-(2-((4,6-diamino-1,3,5triazin-2-yl)amino)ethyl)tricosa-10,12-diynamide (TrMeI-DA, prepared using the reported procedure),39 10,12-Pentacosadiynoic acid (Pentacosa-DA, Alfa aesar), Docosa-10,12-diynedioic acid, (Docosa-DA, ABCR-GmbH & Co.KG), 1,6diphenoxy-2,4-hexadiyne (Phenoxy-DA, Alfa aesar), 1,2-dichlorobenzene (DCB, Sigma Aldrich), lithium chloride (Merck), cobalt chloride (Sigma Aldrich), potassium acetate (Sigma Aldrich), potassium iodide (Merck), potassium carbonate (Merck), potassium sulfate (Merck), phosphorus pentoxide (Alfa aesar), absolute ethanol (Carlo erba), n-propanol (Merck Millipore), n-butanol (Sigma-Aldrich), n-pentanol (SigmaAldrich), isopropanol (Bio-Lab Ltd, Israel), dimethyl formamide (DMF, Bio-Lab Ltd, Israel), acetone (Gadot, Israel), tetrahydrofuran (THF, Bio-Lab Ltd, Israel) and IDEAu (electrode width: 10 µm, electrode gap: 10 µm, Number of fingers: 90 pairs, MicruX Technologies).

Fabrication of PDA-PVP thin film: A 5 mg of DA (TrC-DA, Pentacosa-DA and Docosa-DA) or 15 mg of DA (TrMeI-DA and Phenoxy-DA) dissolved in 0.2 ml of DCB was mixed with 20 mg of PVP dissolved in 0.2 ml of ethanol and sonicated for 20 min. The solution mixture was spin-coated on the substrate (glass/IDE-Au) using a Laurell Technologies Corporation Model WS-650HZ-23NPP/LITE Photoresist


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Spinner at 500 rpm for 10 s followed by 2000 rpm for 10 s. The film was exposed to UV lamp (254 nm) for 20 min for PDA cross-linking under ambient atmosphere.

Characterization of the PVP–DA and PVP-PDA composite thin films: UV-Visible spectra of the composite thin films were recorded on an Evolution 220 UV-Visible spectrometer (Thermo Scientific). Scanning electron microscopy (SEM) images were recorded using a Jeol JSM-7400F Scanning electron microscope (JEOL LTD, Tokyo, Japan) operated and analyzed using the instrument software. Raman scattering measurements were performed on a LabRam HR-high resolution analytical Raman (excitation source was 633 nm laser and 50× long-focal-length objective lenses). Dielectric spectroscopy measurements were performed at a wide frequency range of 1 Hz to 1 MHz at room temperature on a broadband dielectric spectrometer (Novocontrol BDS 80; Novocontrol Technologies GmbH).

Capacitance measurements were conducted using the E4980A Precision LCR Meter connected to computer.

Gas sensing studies: In gas sensing experiments, we used dry nitrogen as a carrier and flow was split into two components: one carrier flow bubbling through the volatile organic compound (in this case, ethanol, n-propanol, n-butanol, n-pentanol, isopropanol, DMF, acetone and THF) at a variable rate. The sensor consists of samples spin-coated with DA-PVP and PDA-PVP thin films on IDEs-Au were placed inside the detection chamber, connected to the LCR meter to detect the capacitance change. The change in capacitance was calculated using the formula, (C/C0)*10-4. We worked with the concentrations ranging from 30 to 500 ppm. The indicated vapor concentrations 21 ACS Paragon Plus Environment


were confirmed by using a standard commercially available VOC sensor (MiniRAE Lite system, USA). Different relative humidity (RH) environments were generated by saturated aqueous solutions of lithium chloride, potassium acetate, potassium carbonate, cobalt chloride, potassium iodide and potassium sulfate in an airtight glass vessel at a temperature of 25oC, which yielded RHs of 11%, 25%, 43%, 65%, 78% and 97% respectively. Furthermore, a solid fine powder of phosphorus pentoxide was used to establish the 0% RH at a temperature of 25oC. The relative humidity values designated were also confirmed by a standard humidity and temperature sensor (TH 210, KIMO, Instruments, France).

Supporting Information The supporting information includes SEM images of nonpolymerized (DA/PVP) thin films, UV-Vis spectra and Raman spectra of thin films with different lengths of UV irradiation, photographs of composite thin films before and after UV irradiation, vapor-induced capacitive response of 1-3-PDA/PVP film electrodes with different lengths of UV irradiation, evaluation of dielectric data, humidity capacitance responses with composite thin film electrodes, capacitance response of various vapors with different electrodes, optimum sensor response upon on/off chemical vapors recycling, graphical representation of linear capacitance response with optimum sensor with varying the concentrations of different vapors, and different characteristics of vapors.

Acknowledgements


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We are grateful to Prof. Yuri Feldman and Dr. Anna Greenbaum from Department of Applied Physics, The Hebrew University of Jerusalem, for their assistance with the dielectric spectroscopy experiments. We also thankful to Dr. Zeiri Leila for help with Raman analysis.

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Table of Contents (TOC) Graphic R

2

R

Diacetylene -4

1

N

Polyvinyl pyrrolidone

(C/C0)*10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O

n

2.4 1.8 1.2 0.6 0.0

0

500

1000 1500

Time (sec)


Polydiacetylene capacitive artificial nose - ACS Applied Materials

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