ACS Applied Materials Interfaces Research Article a perfectly ordered cubic lattice of particles and,further,that the strain exerted onto the nanoparticle film by geometric 1.0 -Au NDT 0 1020 30 10 Time [min] expansion of the underlying substrate is translated quantita- AuNDT (LDPE) 0.8 tively into a uniform increase in interparticle distance.For multilayered,disordered nanoparticle films,it seems more likely 06 that the film responds to strain with structural rearrangements, resulting in a significantly smaller increase in interparticle 0.4 distance.Recently reported results by Farcau et al.23 corroborate this assumption.They observed that the gauge 0.2 factors of monolayered AuNP films were three times larger than those of multilayer films.A smaller effective increase in 0.0 nanoparticle separation explains not only the low g-values 500 ppm 10000ppm observed but also the linear response characteristics,because -0.2 for an effective strain of only 0.5%,the first order Figure 4.Response transients of AuNP films deposited onto LDPE approximation ofeq3 is applicableo (dashed red lines)or HDPE (solid black lines)substrates to toluene vapor at concentrations of 500 ppm and 10 000 ppm. △R =gE Ro (4) together with the AuNP-coating.Recently,Faupel and co- workers2 observed similar response/recovery dynamics with Another factor that should be considered when interpreting the chemiresistors based on AuNP-layers,which were deposited strain gauge responses is the possible strain-induced formation onto various polymers via thermal evaporation.They explained of cracks.Taking into account the heterogeneous nanoscale the observed baseline drifts by entropy-driven embedding of morphology of the films it seems reasonable to expect that AuNPs at the polymer surface.Because HDPE is obviously the submicrometer cracks and pores are somewhat extended when better suited substrate for chemiresistor applications,all further bending the substrates.Indeed,a comparative SEM inves- investigations described herein were carried out with films tigation of a relaxed AuamNDT-film and of the same film deposited onto HDPE substrates. under 3%tensile strain(convex bending)indicated that cracks Figure 5a shows the transient responses of the film became somewhat more pronounced under strain,as shown in AunmNDT to the four analytes at various concentrations Figure 3. The response amplitudes decreased in the order toluene The crack hypothesis is further corroborated by another 4M2P>1-propanol water.The pronounced selectivity for observation:When bending the films for the first few times,we hydrophobic analytes reflects the nonpolar nature of the NDT- occasionally noticed some minor irreversible increases in linker and is in general agreement with the selectivity of AuNP/ resistance,especially when the strain exceeded 1.0%.In that alkanedithiol-chemiresistor films deposited onto glass or silicon strain range (>1.0%)the Aug nmNDT-film even showed substrates.Up to the concentration of 1000 Ppm the significant irreversible baseline drifts.Therefore,in the transients have almost ideal rectangular shapes.For toluene and experiments reported here,this particular material was 4M2P vapors a slow signal rise following the initial steep characterized only at lower strain,i.e.g<1.0%. increase in resistance becomes more and more visible at higher Resistive Responses to Vapors.The resistive responses vapor concentrations.In contrast,at high concentrations of 1- to vapors were investigated using toluene,4-methyl-2- propanol a slight decay of the signal intensity following the pentanone (4M2P),1-propanol,and water as analytes,with initial steep increase in resistance is observed.After switching concentrations ranging from 50 to 10000 ppm.These analytes back to carrier gas,transient overshooting is observed,which is have similar vapor pressures,but very different polarities. more pronounced for the more polar analytes 4M2P and Thus,their partition coefficients for sorption within the films especially,1-propanol.In view of the thermally activated are mainly controlled by their polarity and chemical nature. tunneling model for charge transport,overshooting can be In addition to the three films deposited onto HDPE explained by sorption of analyte within voids in the film,leading substrates (AumNDT,Au4 mPTM,Au NDT),the chem- to an increased permittivity of the matrix surrounding the iresistor responses of a film deposited onto a plasma-treated AuNPs.This increase in permittivity decreases the activation low density polyethylene(LDPE)substrate were measured,for energy and,thus,decreases the resistance.Taking into comparison.This film,which comprises NDT-cross-linked 4 consideration the temporal course of the transients,this nm sized AuNPs,is referred to as AuNDT(LDPE). interpretation implies that after switching from vapor back to Figure 4 shows typical responses of the sensors AumNDT carrier gas the initial interparticle distances are restored quickly and AumNDT(LDPE)to toluene vapor. (reversing film swelling),whereas desorption of analyte from At a concentration of 500 ppm both sensors displayed fast voids happens at a significantly slower rate.A more detailed and reversible response transients.In the case of the film knowledge about the porous micro-and nanostructure of the AumNDT(LDPE),increasing the vapor concentration to 10 film with its impact on the course of sorption processes remains 000 ppm resulted in a strong signal drift with very slow an open key issue for developing a more quantitative recovery of the baseline after switching back to carrier gas.In understanding of these transient features. contrast,the film AuNDT showed only a slight signal drift Figure Sb shows the response transients of the chemiresistor and fast recovery of the baseline signal.Because it is well-known AumPTM.Compared to the AuNDT-film the selectivity that LDPE is more permeable to solvent vapors than HDPE has changed with the response amplitudes decreasing in the we attribute the slow response/recovery dynamics of the order 1-propanol >4M2P toluene water (for analyte chemiresistor AunmNDT(LDPE)to significant sorption of concentrations >2000 ppm).This change in selectivity is due to toluene within the substrate causing the substrate to expand the more polar nature of the cross-linker PTM,with its 6156 dx.dolorg/10.1021/am301780bl ACS Appl.Mater.Interfaces 2012,4,6151-6161a perfectly ordered cubic lattice of particles and, further, that the strain exerted onto the nanoparticle film by geometric expansion of the underlying substrate is translated quantita￾tively into a uniform increase in interparticle distance. For multilayered, disordered nanoparticle films, it seems more likely that the film responds to strain with structural rearrangements, resulting in a significantly smaller increase in interparticle distance. Recently reported results by Farcau et al.23 corroborate this assumption. They observed that the gauge factors of monolayered AuNP films were three times larger than those of multilayer films. A smaller effective increase in nanoparticle separation explains not only the low g-values observed but also the linear response characteristics, because for an effective strain of only 0.5%, the first order approximation of eq 3 is applicable10 ε Δ = R R g 0 (4) Another factor that should be considered when interpreting the strain gauge responses is the possible strain-induced formation of cracks. Taking into account the heterogeneous nanoscale morphology of the films it seems reasonable to expect that submicrometer cracks and pores are somewhat extended when bending the substrates. Indeed, a comparative SEM inves￾tigation of a relaxed Au4 nmNDT-film and of the same film under 3% tensile strain (convex bending) indicated that cracks became somewhat more pronounced under strain, as shown in Figure 3. The crack hypothesis is further corroborated by another observation: When bending the films for the first few times, we occasionally noticed some minor irreversible increases in resistance, especially when the strain exceeded 1.0%. In that strain range (>1.0%) the Au9 nmNDT-film even showed significant irreversible baseline drifts. Therefore, in the experiments reported here, this particular material was characterized only at lower strain, i.e. ε ≤ 1.0%. Resistive Responses to Vapors. The resistive responses to vapors were investigated using toluene, 4-methyl-2- pentanone (4M2P), 1-propanol, and water as analytes, with concentrations ranging from 50 to 10 000 ppm. These analytes have similar vapor pressures, but very different polarities.18 Thus, their partition coefficients for sorption within the films are mainly controlled by their polarity and chemical nature. In addition to the three films deposited onto HDPE substrates (Au4 nmNDT, Au4 nmPTM, Au9 nmNDT), the chem￾iresistor responses of a film deposited onto a plasma-treated low density polyethylene (LDPE) substrate were measured, for comparison. This film, which comprises NDT-cross-linked 4 nm sized AuNPs, is referred to as Au4 nmNDT(LDPE). Figure 4 shows typical responses of the sensors Au4 nmNDT and Au4 nmNDT(LDPE) to toluene vapor. At a concentration of 500 ppm both sensors displayed fast and reversible response transients. In the case of the film Au4 nmNDT(LDPE), increasing the vapor concentration to 10 000 ppm resulted in a strong signal drift with very slow recovery of the baseline after switching back to carrier gas. In contrast, the film Au4 nmNDT showed only a slight signal drift and fast recovery of the baseline signal. Because it is well-known that LDPE is more permeable to solvent vapors than HDPE,41 we attribute the slow response/recovery dynamics of the chemiresistor Au4 nmNDT(LDPE) to significant sorption of toluene within the substrate causing the substrate to expand together with the AuNP-coating. Recently, Faupel and co￾workers42 observed similar response/recovery dynamics with chemiresistors based on AuNP-layers, which were deposited onto various polymers via thermal evaporation. They explained the observed baseline drifts by entropy-driven embedding of AuNPs at the polymer surface. Because HDPE is obviously the better suited substrate for chemiresistor applications, all further investigations described herein were carried out with films deposited onto HDPE substrates. Figure 5a shows the transient responses of the film Au4 nmNDT to the four analytes at various concentrations. The response amplitudes decreased in the order toluene ≈4M2P > 1-propanol > water. The pronounced selectivity for hydrophobic analytes reflects the nonpolar nature of the NDT￾linker and is in general agreement with the selectivity of AuNP/ alkanedithiol-chemiresistor films deposited onto glass or silicon substrates.18,38,43 Up to the concentration of 1000 ppm the transients have almost ideal rectangular shapes. For toluene and 4M2P vapors a slow signal rise following the initial steep increase in resistance becomes more and more visible at higher vapor concentrations. In contrast, at high concentrations of 1- propanol a slight decay of the signal intensity following the initial steep increase in resistance is observed. After switching back to carrier gas, transient overshooting is observed, which is more pronounced for the more polar analytes 4M2P and, especially, 1-propanol. In view of the thermally activated tunneling model for charge transport, overshooting can be explained by sorption of analyte within voids in the film, leading to an increased permittivity of the matrix surrounding the AuNPs. This increase in permittivity decreases the activation energy and, thus, decreases the resistance. Taking into consideration the temporal course of the transients, this interpretation implies that after switching from vapor back to carrier gas the initial interparticle distances are restored quickly (reversing film swelling), whereas desorption of analyte from voids happens at a significantly slower rate. A more detailed knowledge about the porous micro- and nanostructure of the film with its impact on the course of sorption processes remains an open key issue for developing a more quantitative understanding of these transient features. Figure 5b shows the response transients of the chemiresistor Au4 nmPTM. Compared to the Au4 nmNDT-film the selectivity has changed with the response amplitudes decreasing in the order 1-propanol >4M2P ≈ toluene > water (for analyte concentrations >2000 ppm). This change in selectivity is due to the more polar nature of the cross-linker PTM, with its Figure 4. Response transients of AuNP films deposited onto LDPE (dashed red lines) or HDPE (solid black lines) substrates to toluene vapor at concentrations of 500 ppm and 10 000 ppm. ACS Applied Materials & Interfaces Research Article 6156 dx.doi.org/10.1021/am301780b | ACS Appl. Mater. Interfaces 2012, 4, 6151−6161