ACS Applied Materials Interfaces Research Article decrease in resistance to analyte sorption.This effect has been size had only little influence on the strain gauge properties attributed to a decrease in the activation energy,caused by the However,the chemical selectivity of the films clearly changed increase in permittivity due to sorption of the analyte within toward more polar analytes when using a more polar cross- voids.Taken together,the net responses of chemiresistors linker.Third,and most importantly,we investigated the based on ligand-stabilized metal nanoparticles is the result of influence of strain on the chemiresistive responses.We show two counteracting effects:Swelling causes an increase in that the sensitivity of the films was significantly enhanced when resistance due to increased tunneling distances,whereas void- the films were under strain,regardless of the analytes's filling-or displacement of ligands by analytes of higher permittivity.Supported by a comparative SEM study,we permittivity -decreases the resistance by augmenting the attribute this effect to strain induced formation of nanoscale ca prime的uomnr cracks,which enhances the ability of the film to swell during vapor sorption. However,results reported by Lewis and co-workers21 suggest that a more quantitative description of the sensing mechanism EXPERIMENTAL SECTION requires a deeper understanding of the structural rearrange- ments induced by vapor sorption.The most important Materials.Chemicals were purchased from Aldrich,Fluka,Th. Geyer GmbH Co.KG and Grtissing GmbH.All chemicals and achievements of investigations into the sensing mechanism solvents except oleylamine (70 wt %)were of analytical grade and used and new trends for applications of these chemiresistors have as received.Deionized water (resistivity:18.2 M cm)was purified recently been summarized in a comprehensive review by Ibafez using a Millipore Simplicity system.Polyethylene substrates with a and Zamborini.22 diameter of 40 mm and a thickness of 0.56 mm were prepared by Another interesting application which uses the dependence injection molding (HAAKE MiniJet system)using low-density polyethylene (LDPE,Aldrich)with a melt index of 25 g/10 min ugeriemenaeth晋SotcteRarticdedstanGesestg produced strain gauges by depositing (190 C/2.16 kg)and high-density polyethylene (HDPE,Aldrich) AuNPs on inkjet transparencies and reported sensitivities 2 with a melt index of 42 g/10 min (190C/2.16 kg).Dodecylamine- orders of magnitude higher than that of conventional metal foil stablied AuNPswithveragediameter ofweresythesid gauges.Similar results were obtained by us and by Farcau et as described previously." who investigated strain gauges fabricated from wire. olluing theme设E and using a reaction temperature of 95 C.After purification via patterned monolayers and multilayers of AuNPs.Kulkarni and fractionated precipitation with ethanol used as nonsolvent,the AuNPs co-workers reported gauge factors up to 390 for strain sensors were dissolved in toluene and stored in a refrigerator.TEM images and based on micromolded Pd-nanoparticle-carbon u-stripes. size-histograms of AuNPs used in this study are provided as While the studies referenced above focused either on the Supporting Information(Figure S1). application of AuNP-films as chemical sensors or strain sensors, Film Preparation.AuNP-films were prepared via the layer-by-layer one pioneering study by Zhong and co-workers26 investigated self-assembly method Prior to film deposition,the polyethylene the concerted influence of both strain and vapor sorption on substrates were treated with oxygen plasma (Plasma Prep II,SPI Supplies)for 20 min.The oxygen pressure was 0.67 mbar and the the resistivity of these films.Interestingly,this study showed current 40 mA.Immediately after the plasma treatment,the oxidized that the surrounding vapor atmosphere can significantly affect substrates were immersed into the solution of AuNPs in toluene with the strain gauge responses.The quite complex data sets the concentration adjusted to an absorbance of 1.2 at the plasmon presented were interpreted qualitatively by taking into account absorption maximum,at 1 cm path length.Taking into account the analyte partitioning and the analyte's permittivity.Clearly,the cubic scaling of the extinction coefficient of AuNPs with particle size, rational development of sensors based on flexible AuNP- the particle concentration of the solution containing the larger coatings,which are robust against unwanted signal interferences oleylamine-stabilized AuNPs was approximately 1 order of magnitude when operated under harsh conditions,as well as the design of lower than that of the smaller dodecylamine-stabilized AuNPs.After 5 min the substrates were washed with toluene and then immersed into novel dual chemical/strain sensors,requires further research a solution of the linker in toluene (6 mmol/L)for 5 min.As the last efforts.In addition,the possibility to induce structural changes step of one deposition cycle the substrates were washed again with within the sensitive coatings by straining the substrates provides toluene.The film preparation was completed after finishing 22 highly interesting opportunities to study structure/sensitivity deposition cycles.We note that shortening the time of plasma relationships and to improve our current understanding of the treatment to 30 s and reducing the current to 35 mA had only little underlying sensing mechanisms. influence on the visual appearance of prepared AuNP-coatings (see the In our previous contribution,we reported on the Supporting Information,Table S1).In order to deposit the preparation and the charge transport properties of non- oleylamine-stabilized AuNPs,we first had to deposit dodecylamine- anedithiol (NDT)cross-linked AuNP-coatings deposited onto stabilized AuNPs as an adhesion layer (three deposition cycles).The low density polyethylene (LDPE)substrates.It was demon- oleylamine-stabilized AuNPs were then deposited as explained above (19 deposition cycles).After completing the assembly process the strated that these coatings are mechanically robust and very films were dried in ambient air and then stored under nitrogen until well suited for strain gauge applications.Motivated by this needed. result we have now significantly extended our investigations: ATR-FTIR Spectroscopy.IR spectra of the substrate surfaces were First,we investigated if polyethylene(PE)substrates allow for recorded using a Bruker Equinox 55 spectrometer equipped with a the application of cross-linked AuNP-coatings as flexible diamond crystal as ATR-IR element. chemiresistors.As shown here,high density polyethylene Resistance Measurements.Gold electrode pairs of nominal 100 (HDPE)provides an excellent substrate for this purpose, nm thickness and 400 um spacing were deposited onto the gold whereas LDPE has some drawbacks due to its higher vapor nanoparticle films by vacuum evaporation (Pfeiffer,Classic 250) through a shadow mask.To measure strain induced resistance changes permeability.Second,we varied the composition of the films in the devices were placed into sample holders made of acrylic glass order to test the influence of different particle sizes and allowing for defined convex bending of the substrates.The tensile different linker structures on both,the strain gauge and strain,determined by the substrate thickness and the radius of chemiresistive responses.Surprisingly,the variation in particle curvature of the holders,was 0.25,0.5,1,1.5,2,2.5,and 3%.For the 6152 dx.doLorg/10.1021/am301780bl ACS Appl.Mater.Interfaces 2012,4,6151-6161decrease in resistance to analyte sorption.18 This effect has been attributed to a decrease in the activation energy, caused by the increase in permittivity due to sorption of the analyte within voids. Taken together, the net responses of chemiresistors based on ligand-stabilized metal nanoparticles is the result of two counteracting effects: Swelling causes an increase in resistance due to increased tunneling distances, whereas void- filling - or displacement of ligands by analytes of higher permittivity - decreases the resistance by augmenting the effective permittivity in the nanoparticles’ environment. In fact, several experimental studies confirm this interpretation.9,19,20 However, results reported by Lewis and co-workers21 suggest that a more quantitative description of the sensing mechanism requires a deeper understanding of the structural rearrange￾ments induced by vapor sorption. The most important achievements of investigations into the sensing mechanism and new trends for applications of these chemiresistors have recently been summarized in a comprehensive review by Ibañ ez and Zamborini.22 Another interesting application which uses the dependence of the resistance on changes in interparticle distances are strain gauges. Herrmann et al.10 produced strain gauges by depositing AuNPs on inkjet transparencies and reported sensitivities 2 orders of magnitude higher than that of conventional metal foil gauges. Similar results were obtained by us11 and by Farcau et al.23,24 who investigated strain gauges fabricated from wire￾patterned monolayers and multilayers of AuNPs. Kulkarni and co-workers25 reported gauge factors up to 390 for strain sensors based on micromolded Pd-nanoparticle-carbon μ-stripes. While the studies referenced above focused either on the application of AuNP-films as chemical sensors or strain sensors, one pioneering study by Zhong and co-workers26 investigated the concerted influence of both strain and vapor sorption on the resistivity of these films. Interestingly, this study showed that the surrounding vapor atmosphere can significantly affect the strain gauge responses. The quite complex data sets presented were interpreted qualitatively by taking into account analyte partitioning and the analyte’s permittivity. Clearly, the rational development of sensors based on flexible AuNP￾coatings, which are robust against unwanted signal interferences when operated under harsh conditions, as well as the design of novel dual chemical/strain sensors, requires further research efforts. In addition, the possibility to induce structural changes within the sensitive coatings by straining the substrates provides highly interesting opportunities to study structure/sensitivity relationships and to improve our current understanding of the underlying sensing mechanisms. In our previous contribution,11 we reported on the preparation and the charge transport properties of non￾anedithiol (NDT) cross-linked AuNP-coatings deposited onto low density polyethylene (LDPE) substrates. It was demon￾strated that these coatings are mechanically robust and very well suited for strain gauge applications. Motivated by this result we have now significantly extended our investigations: First, we investigated if polyethylene (PE) substrates allow for the application of cross-linked AuNP-coatings as flexible chemiresistors. As shown here, high density polyethylene (HDPE) provides an excellent substrate for this purpose, whereas LDPE has some drawbacks due to its higher vapor permeability. Second, we varied the composition of the films in order to test the influence of different particle sizes and different linker structures on both, the strain gauge and chemiresistive responses. Surprisingly, the variation in particle size had only little influence on the strain gauge properties. However, the chemical selectivity of the films clearly changed toward more polar analytes when using a more polar cross￾linker. Third, and most importantly, we investigated the influence of strain on the chemiresistive responses. We show that the sensitivity of the films was significantly enhanced when the films were under strain, regardless of the analytes’s permittivity. Supported by a comparative SEM study, we attribute this effect to strain induced formation of nanoscale cracks, which enhances the ability of the film to swell during vapor sorption. ■ EXPERIMENTAL SECTION Materials. Chemicals were purchased from Aldrich, Fluka, Th. Geyer GmbH & Co. KG and Grü ssing GmbH. All chemicals and solvents except oleylamine (70 wt %) were of analytical grade and used as received. Deionized water (resistivity: 18.2 MΩ cm) was purified using a Millipore Simplicity system. Polyethylene substrates with a diameter of 40 mm and a thickness of 0.56 mm were prepared by injection molding (HAAKE MiniJet system) using low-density polyethylene (LDPE, Aldrich) with a melt index of 25 g/10 min (190 °C/2.16 kg) and high-density polyethylene (HDPE, Aldrich) with a melt index of 42 g/10 min (190 °C/2.16 kg). Dodecylamine￾stabilized AuNPs with an average diameter of 4 nm were synthesized as described previously.8,27 Oleylamine-stabilized AuNPs with a core size of 9 nm were synthesized following the method of Shen et al.28 and using a reaction temperature of 95 °C. After purification via fractionated precipitation with ethanol used as nonsolvent, the AuNPs were dissolved in toluene and stored in a refrigerator. TEM images and size-histograms of AuNPs used in this study are provided as Supporting Information (Figure S1). Film Preparation. AuNP-films were prepared via the layer-by-layer self-assembly method.8,29 Prior to film deposition, the polyethylene substrates were treated with oxygen plasma (Plasma Prep II, SPI Supplies) for 20 min. The oxygen pressure was 0.67 mbar and the current 40 mA. Immediately after the plasma treatment, the oxidized substrates were immersed into the solution of AuNPs in toluene with the concentration adjusted to an absorbance of 1.2 at the plasmon absorption maximum, at 1 cm path length. Taking into account the cubic scaling of the extinction coefficient of AuNPs with particle size, the particle concentration of the solution containing the larger oleylamine-stabilized AuNPs was approximately 1 order of magnitude lower than that of the smaller dodecylamine-stabilized AuNPs. After 5 min the substrates were washed with toluene and then immersed into a solution of the linker in toluene (6 mmol/L) for 5 min. As the last step of one deposition cycle the substrates were washed again with toluene. The film preparation was completed after finishing 22 deposition cycles. We note that shortening the time of plasma treatment to 30 s and reducing the current to 35 mA had only little influence on the visual appearance of prepared AuNP-coatings (see the Supporting Information, Table S1). In order to deposit the oleylamine-stabilized AuNPs, we first had to deposit dodecylamine￾stabilized AuNPs as an adhesion layer (three deposition cycles). The oleylamine-stabilized AuNPs were then deposited as explained above (19 deposition cycles). After completing the assembly process the films were dried in ambient air and then stored under nitrogen until needed. ATR-FTIR Spectroscopy. IR spectra of the substrate surfaces were recorded using a Bruker Equinox 55 spectrometer equipped with a diamond crystal as ATR-IR element. Resistance Measurements. Gold electrode pairs of nominal 100 nm thickness and 400 μm spacing were deposited onto the gold nanoparticle films by vacuum evaporation (Pfeiffer, Classic 250) through a shadow mask. To measure strain induced resistance changes the devices were placed into sample holders made of acrylic glass allowing for defined convex bending of the substrates.11 The tensile strain, determined by the substrate thickness and the radius of curvature of the holders, was 0.25, 0.5, 1, 1.5, 2, 2.5, and 3%. For the ACS Applied Materials & Interfaces Research Article 6152 dx.doi.org/10.1021/am301780b | ACS Appl. Mater. Interfaces 2012, 4, 6151−6161