ACS Applied Materials Interfaces Research Article by the error bars of the responses to toluene.The transients cracks and pores along the strain direction and the formation of show that the observed relative increase in response amplitudes new cracks,it seems more likely that tensile strain causes an was similar for all three vapors.Moreover,similar relative gains overall increase in void volume.This net increase in void of response amplitudes were also observed at lower vapor volume would,contrary to our observation,decrease the concentrations,e.g,at 100,500,and 1000 ppm (see the sensitivity of the strained sensors. Supporting Information,Figure S6).In the case of water vapor, Taken together,the enhancement in chemical sensitivity the influence of strain on the sensor response was ambiguous caused by inducing tensile strain in the sensors can qualitatively and we excluded this analyte from further investigations. be explained by rupture of the nanoparticle network and Figure 7b shows the effect of different strain levels on the formation of cracks,causing an increased ability of the film to responses of the AumNDT-film to toluene vapor (10 000 swell in lateral direction.The counteracting effect,which is due ppm).At 1%tensile strain the typical ~30%gain of the to the increase in permittivity via sorption of additional analyte transient amplitude is seen.Increasing the strain to 2%had a in the increased void volume,seems to have only a minor stronger effect with a~35%relative gain in response amplitude. impact. Further increasing the tensile strain to 3%,however,lowered As mentioned above,compressive strain tends to change the the strain-induced gain of the response amplitude. baseline resistance of the films irreversibly.Nevertheless,some When inducing tensile strain in the films Au4nmPTM and experiments were performed to test the influence of AumNDT,we observed a very similar increase in sensitivity, compressive strain on the chemiresistors'responses.When as was seen for the AuNDT film.Representative data are inducing 1%compressive strain,the baseline resistance provided as Supporting Information (Figures S7 and S8).We decreased by 14-26%,in reasonable agreement with the note that the observed increase in sensitivity caused by bending gauge factors presented in Table 1.Interestingly,compressive the films was not affected when varying the current(50-200 strain enhanced the chemiresistive response amplitudes (see nA)used to operate the sensors. the Supporting Information,Figure S9),but the effect was less The results reported here are qualitatively in agreement with pronounced than for films under tensile stress.According to observations reported by Zhong and co-workers.26 They our discussion above,the enhancement in sensitivity of films investigated strain gauge responses of cross-linked AuNP. under compressive stress suggests that compression reduces the films on PET substrates under nitrogen and in various vapor accessible void volume,and thus,diminishes the permittivity atmospheres.For vapors of analytes,which dissolved well increase during vapor sorption.At the same time,the ability of within the film matrix (e.g,hexane,ethanol),the strain gauge film swelling in the direction normal to the film surface remains responses were significantly stronger than those measured rather unaffected because concave bending compresses the film under nitrogen. only in lateral direction.Taking into account the inhomoge- In view of the activated tunneling model (eq 2),an neous nanoscale morphology of the films,it also appears enhancement in sensitivity can be caused by two effects. plausible that compressive stress induces structural dislocations First,if the ability of the sensor film to swell during analyte and rearrangements involving rupture of interparticle linkages. sorption increases under strain,the perturbation of charge This,again,would increase the ability of the film to swell,and transport via increased tunneling distances becomes more thus,would enhance the chemiresistive sensitivity. effective.As a consequence the sensitivity increases.Second,if the ability of the film to host guest molecules within voids SUMMARY AND CONCLUSIONS decreases,the increase in permittivity due to void filling In this study,we deposited cross-linked AuNPs onto plasma- becomes less pronounced.As a consequence,the decrease in treated HDPE substrates via layer-by-layer self-assembly. activation energy due to analyte sorption becomes weaker, Testing the films as strain gauges revealed gauge factors of resulting in gain of sensitivity. ~20 (Au4 mNDT,Au4 mPTM)and ~25(Au nmNDT).These To enhance the ability of the sensor films to swell,their values are lower than suggested by the tunneling model for freedom to extend in space must be raised.In principle,this can charge transport.To explain this difference we propose that the be achieved by loosening the particle network via strain- films are not strained evenly when bending the substrates.More induced rupture of interconnects and crack formation likely,structural reorganization,reversible extension of existing Supporting this hypothesis,it has been shown previously that cracks and the formation of new cracks on the nanometer scale chemiresistors consisting of noninterlinked nanoparticles are have to be taken into account for better understanding the more sensitive than those comprising cross-linked nano- strain sensing mechanism. particles.20 This effect has been assigned to an enhanced ability An AuNP-film deposited onto LDPE showed very slow of the noninterlinked material to swell during analyte sorption. chemiresistor responses when dosed with concentrated toluene Further,as pointed out above,SEM investigations indicated vapor.We assign this effect to significant sorption of toluene that cracks became somewhat more pronounced when straining within the substrate.Much faster responses were obtained for the film Aus amNDT by convex bending (Figure 3).Taking into AuNP-films on HDPE substrates,which are more resistant to account the reversibility of the strain gauge responses,our organic solvents.The chemical sensitivity of the film interpretation requires that rupture of the particle network AuNDT decreased with the polarity of the analytes:toluene and/or crack formation and crack widening are reversible 4M2P>1-propanol water.Films comprising differently processes. sized AuNPs (4 and 9 nm core diameter)showed similar To enhance the chemical sensitivity by diminishing the chemical selectivity and sensitivities,especially at higher vapor permittivity increase during analyte sorption,the accessible void concentrations (>4000 ppm).Using the more polar PTM as volume must be decreased.In principle,the Poisson effect cross-linker selectively enhanced the sensitivities to 1-propanol combined with structural rearrangements may lead to some and water.The response isotherms of all three films for vapors reduction of void volume when straining the films.However, in the concentration range from 50 to 10000 ppm were well taking into consideration the expansion of already existing reproduced using a Langmuir-Henry sorption model. 6159 dx.doLorg/10.1021/am301780bl ACS Appl.Mater.Interfaces 2012,4,6151-6161by the error bars of the responses to toluene. The transients show that the observed relative increase in response amplitudes was similar for all three vapors. Moreover, similar relative gains of response amplitudes were also observed at lower vapor concentrations, e.g., at 100, 500, and 1000 ppm (see the Supporting Information, Figure S6). In the case of water vapor, the influence of strain on the sensor response was ambiguous and we excluded this analyte from further investigations. Figure 7b shows the effect of different strain levels on the responses of the Au4 nmNDT-film to toluene vapor (10 000 ppm). At 1% tensile strain the typical ∼30% gain of the transient amplitude is seen. Increasing the strain to 2% had a stronger effect with a ∼35% relative gain in response amplitude. Further increasing the tensile strain to 3%, however, lowered the strain-induced gain of the response amplitude. When inducing tensile strain in the films Au4 nmPTM and Au9 nmNDT, we observed a very similar increase in sensitivity, as was seen for the Au4 nmNDT film. Representative data are provided as Supporting Information (Figures S7 and S8). We note that the observed increase in sensitivity caused by bending the films was not affected when varying the current (50−200 nA) used to operate the sensors. The results reported here are qualitatively in agreement with observations reported by Zhong and co-workers.26 They investigated strain gauge responses of cross-linked AuNP- films on PET substrates under nitrogen and in various vapor atmospheres. For vapors of analytes, which dissolved well within the film matrix (e.g., hexane, ethanol), the strain gauge responses were significantly stronger than those measured under nitrogen. In view of the activated tunneling model (eq 2), an enhancement in sensitivity can be caused by two effects. First, if the ability of the sensor film to swell during analyte sorption increases under strain, the perturbation of charge transport via increased tunneling distances becomes more effective. As a consequence the sensitivity increases. Second, if the ability of the film to host guest molecules within voids decreases, the increase in permittivity due to void filling becomes less pronounced. As a consequence, the decrease in activation energy due to analyte sorption becomes weaker, resulting in gain of sensitivity. To enhance the ability of the sensor films to swell, their freedom to extend in space must be raised. In principle, this can be achieved by loosening the particle network via strain￾induced rupture of interconnects and crack formation. Supporting this hypothesis, it has been shown previously that chemiresistors consisting of noninterlinked nanoparticles are more sensitive than those comprising cross-linked nano￾particles.20 This effect has been assigned to an enhanced ability of the noninterlinked material to swell during analyte sorption. Further, as pointed out above, SEM investigations indicated that cracks became somewhat more pronounced when straining the film Au4 nmNDT by convex bending (Figure 3). Taking into account the reversibility of the strain gauge responses, our interpretation requires that rupture of the particle network and/or crack formation and crack widening are reversible processes. To enhance the chemical sensitivity by diminishing the permittivity increase during analyte sorption, the accessible void volume must be decreased. In principle, the Poisson effect combined with structural rearrangements may lead to some reduction of void volume when straining the films. However, taking into consideration the expansion of already existing cracks and pores along the strain direction and the formation of new cracks, it seems more likely that tensile strain causes an overall increase in void volume. This net increase in void volume would, contrary to our observation, decrease the sensitivity of the strained sensors. Taken together, the enhancement in chemical sensitivity caused by inducing tensile strain in the sensors can qualitatively be explained by rupture of the nanoparticle network and formation of cracks, causing an increased ability of the film to swell in lateral direction. The counteracting effect, which is due to the increase in permittivity via sorption of additional analyte in the increased void volume, seems to have only a minor impact. As mentioned above, compressive strain tends to change the baseline resistance of the films irreversibly. Nevertheless, some experiments were performed to test the influence of compressive strain on the chemiresistors’ responses. When inducing 1% compressive strain, the baseline resistance decreased by 14−26%, in reasonable agreement with the gauge factors presented in Table 1. Interestingly, compressive strain enhanced the chemiresistive response amplitudes (see the Supporting Information, Figure S9), but the effect was less pronounced than for films under tensile stress. According to our discussion above, the enhancement in sensitivity of films under compressive stress suggests that compression reduces the accessible void volume, and thus, diminishes the permittivity increase during vapor sorption. At the same time, the ability of film swelling in the direction normal to the film surface remains rather unaffected because concave bending compresses the film only in lateral direction. Taking into account the inhomoge￾neous nanoscale morphology of the films, it also appears plausible that compressive stress induces structural dislocations and rearrangements involving rupture of interparticle linkages. This, again, would increase the ability of the film to swell, and thus, would enhance the chemiresistive sensitivity. ■ SUMMARY AND CONCLUSIONS In this study, we deposited cross-linked AuNPs onto plasma￾treated HDPE substrates via layer-by-layer self-assembly. Testing the films as strain gauges revealed gauge factors of ∼20 (Au4 nmNDT, Au4 nmPTM) and ∼25 (Au9 nmNDT). These values are lower than suggested by the tunneling model for charge transport. To explain this difference we propose that the films are not strained evenly when bending the substrates. More likely, structural reorganization, reversible extension of existing cracks and the formation of new cracks on the nanometer scale have to be taken into account for better understanding the strain sensing mechanism. An AuNP-film deposited onto LDPE showed very slow chemiresistor responses when dosed with concentrated toluene vapor. We assign this effect to significant sorption of toluene within the substrate. Much faster responses were obtained for AuNP-films on HDPE substrates, which are more resistant to organic solvents. The chemical sensitivity of the film Au4 nmNDT decreased with the polarity of the analytes: toluene ≈4M2P > 1-propanol > water. Films comprising differently sized AuNPs (4 and 9 nm core diameter) showed similar chemical selectivity and sensitivities, especially at higher vapor concentrations (>4000 ppm). Using the more polar PTM as cross-linker selectively enhanced the sensitivities to 1-propanol and water. The response isotherms of all three films for vapors in the concentration range from 50 to 10 000 ppm were well reproduced using a Langmuir−Henry sorption model. ACS Applied Materials & Interfaces Research Article 6159 dx.doi.org/10.1021/am301780b | ACS Appl. Mater. Interfaces 2012, 4, 6151−6161