《Measurement Systems：Application and Design》课程教学资源（扩展知识）Cross-Linked Gold Nanoparticles on Polyethylene：Resistive Responses to Tensile Strain and Vapors

ACSAPPLIED MATERIALS Research Article INTERFACES www.acsami.org Cross-Linked Gold Nanoparticles on Polyethylene:Resistive Responses to Tensile Strain and Vapors Natalia Olichwer,Elisabeth W.Leib,Annelie H.Halfar,'Alexey Petrov,and Tobias Vossmeyer* Institute of Physical Chemistry,University of Hamburg,Grindelallee 117,20146 Hamburg,Germany Supporting Information ABSTRACT:In this study,coatings of cross-linked gold nanoparticles (AuNPs)on flexible polyethylene(PE)substrates were prepared via layer-by- layer deposition and their application as strain gauges and chemiresistors was investigated.Special emphasis was placed on characterizing the influence of strain on the chemiresistive responses.The coatings were deposited using amine stabilized AuNPs (4 and 9 nm diameter)and 1,9-nonanedithiol (NDT) or pentaerythritol tetrakis(3-mercaptopropionate)(PTM)as cross-linkers.To prepare films with homogeneous optical appearance,it was necessary to treat the substrates with oxygen plasma directly before film assembly.SEM images revealed film thicknesses between ~60 and ~90 nm and a porous nanoscale morphology.All films showed ohmic I-V characteristics with conductivities ranging from 1 x 10+to 1 x 10-2cm depending on the structure of the linker and the nanoparticle size.When up to 3%strain was induced their resistance increased linearly and reversibly (gauge factors:~20).A comparative SEM investigation indicated that the stress induced formation and extension of nanocracks are important components of the signal transduction mechanism.Further,all films responded with a reversible increase in resistance when dosed with toluene,4-methyl-2-pentanone,1-propanol or water vapor (concentrations: 50-10 000 ppm).Films deposited onto high density PE substrates showed much faster response-recovery dynamics than films deposited onto low density PE.The chemical selectivity of the coatings was controlled by the chemical nature of the cross- linkers,with the highest sensitivities(10-s ppm)measured with analytes of matching solubility.The response isotherms of all film/vapor pairs could be fitted using a Langmuir-Henry model suggesting selective and bulk sorption.Under tensile stress (1%strain)all chemiresistors showed a reversible increase in their response amplitudes (30%),regardless of the analytes' permittivity.Taking into consideration the thermally activated tunneling model for charge transport,this behavior was assigned to stress induced formation of nanocracks,which enhance the films'ability to swell in lateral direction during analyte sorption. KEYWORDS:gold,nanoparticle,film,coating,polyethylene,chemiresistor,strain gauge ■INTRODUCTION fabricated from ligand-stabilized AuNPs as resistive sensing The conductivity o of thin films composed of ligand-stabilized elements have been studied intensively3-1 and pushed toward gold nanoparticles (AuNPs)is commonly described by specific applications in several laboratories.16 For discussing thermally activated tunneling of charge carriers.A widely the response characteristics of these sensors it is convenient to used mathematical representation of this model is given by the rearrange eq 1 into the following form7 following equation △R eBABeAE,/RT-1 G=doe-bhe-E,/RT (1) Ro (2) Here,go exp(-B)is the conductivity at infinite temperature,B Here,Ro is the baseline resistance and△R,△d,△E,are the is the tunneling decay constant,and 6 is the surface-to-surface changes in resistance,interparticle distance,and activation distance between neighboring metal cores,i.e.the tunneling energy,which could,for example,be caused by sorption of an distance.The Arrhenius term takes into account thermal analyte or stress. activation of charge carriers.Attempts have been made to Chemiresistors based on films of metal nanoparticles usually attribute the activation energy E,to the Coulomb charging respond with an increase in resistance to analyte sorption.This energy of the particles or to the reorganizational energy effect has been attributed to swelling of the films,leading to according to Marcus theoryImportantly,in both models increased tunneling distances.On the other hand,it has also is inversely proportional to the relative permittivity of the been shown that rigidly cross-linked nanoparticle films,which nanoparticles'environment and increases with increasing have only very limited freedom to swell,respond with a interparticle distance. Because of its exponential dependence on E,and the Received:August 24,2012 conductivity of these films is highly sensitive to any Accepted:November 5,2012 perturbation of these parameters.Thus,applications of films Published:November 5,2012 ACS Publications212 American chemical sey 6151 dx.doLorg/10.1021/am301780bl ACS Appl.Mater.Interfoces 2012,4,6151-6161

Cross-Linked Gold Nanoparticles on Polyethylene: Resistive Responses to Tensile Strain and Vapors Natalia Olichwer, Elisabeth W. Leib, Annelie H. Halfar,† Alexey Petrov, and Tobias Vossmeyer* Institute of Physical Chemistry, University of Hamburg, Grindelallee 117, 20146 Hamburg, Germany *S Supporting Information ABSTRACT: In this study, coatings of cross-linked gold nanoparticles (AuNPs) on flexible polyethylene (PE) substrates were prepared via layer-by￾layer deposition and their application as strain gauges and chemiresistors was investigated. Special emphasis was placed on characterizing the influence of strain on the chemiresistive responses. The coatings were deposited using amine stabilized AuNPs (4 and 9 nm diameter) and 1,9-nonanedithiol (NDT) or pentaerythritol tetrakis(3-mercaptopropionate) (PTM) as cross-linkers. To prepare films with homogeneous optical appearance, it was necessary to treat the substrates with oxygen plasma directly before film assembly. SEM images revealed film thicknesses between ∼60 and ∼90 nm and a porous nanoscale morphology. All films showed ohmic I-V characteristics with conductivities ranging from 1 × 10−4 to 1 × 10−2 Ω−1 cm−1 , depending on the structure of the linker and the nanoparticle size. When up to 3% strain was induced their resistance increased linearly and reversibly (gauge factors: ∼20). A comparative SEM investigation indicated that the stress induced formation and extension of nanocracks are important components of the signal transduction mechanism. Further, all films responded with a reversible increase in resistance when dosed with toluene, 4-methyl-2-pentanone, 1-propanol or water vapor (concentrations: 50−10 000 ppm). Films deposited onto high density PE substrates showed much faster response-recovery dynamics than films deposited onto low density PE. The chemical selectivity of the coatings was controlled by the chemical nature of the cross￾linkers, with the highest sensitivities (∼1 × 10−5 ppm−1 ) measured with analytes of matching solubility. The response isotherms of all film/vapor pairs could be fitted using a Langmuir−Henry model suggesting selective and bulk sorption. Under tensile stress (1% strain) all chemiresistors showed a reversible increase in their response amplitudes (∼30%), regardless of the analytes’ permittivity. Taking into consideration the thermally activated tunneling model for charge transport, this behavior was assigned to stress induced formation of nanocracks, which enhance the films’ ability to swell in lateral direction during analyte sorption. KEYWORDS: gold, nanoparticle, film, coating, polyethylene, chemiresistor, strain gauge ■ INTRODUCTION The conductivity σ of thin films composed of ligand-stabilized gold nanoparticles (AuNPs) is commonly described by thermally activated tunneling of charge carriers. A widely used mathematical representation of this model is given by the following equation1 σ σ = − − βδ e e E RT 0 /a (1) Here, σ0 exp(−βδ) is the conductivity at infinite temperature, β is the tunneling decay constant, and δ is the surface-to-surface distance between neighboring metal cores, i.e. the tunneling distance. The Arrhenius term takes into account thermal activation of charge carriers. Attempts have been made to attribute the activation energy Ea to the Coulomb charging energy of the particles or to the reorganizational energy according to Marcus theory.1,2 Importantly, in both models Ea is inversely proportional to the relative permittivity of the nanoparticles’ environment and increases with increasing interparticle distance. Because of its exponential dependence on Ea and δ, the conductivity of these films is highly sensitive to any perturbation of these parameters. Thus, applications of films fabricated from ligand-stabilized AuNPs as resistive sensing elements have been studied intensively3−11 and pushed toward specific applications in several laboratories.12−16 For discussing the response characteristics of these sensors it is convenient to rearrange eq 1 into the following form9,17 Δ = − R β δ Δ Δ R e e 1 E RT 0 /a (2) Here, R0 is the baseline resistance and ΔR, Δδ, ΔEa are the changes in resistance, interparticle distance, and activation energy, which could, for example, be caused by sorption of an analyte or stress. Chemiresistors based on films of metal nanoparticles usually respond with an increase in resistance to analyte sorption. This effect has been attributed to swelling of the films, leading to increased tunneling distances.3,9 On the other hand, it has also been shown that rigidly cross-linked nanoparticle films, which have only very limited freedom to swell, respond with a Received: August 24, 2012 Accepted: November 5, 2012 Published: November 5, 2012 Research Article www.acsami.org © 2012 American Chemical Society 6151 dx.doi.org/10.1021/am301780b | ACS Appl. Mater. Interfaces 2012, 4, 6151−6161

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 2012,4,6151-6161

decrease 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

ACS Applied Materials Interfaces Research Article resistance measurements,a Keithley multimeter 2000 was employed. deposition process.Carboxylate groups,for example,have some The I-V curves were recorded using a Keithley sourcemeter 2601A. affinity to gold nanoparticles as known from citrate-stabilized Unless otherwise indicated,the measurements were carried out at AuNPs.Also,spontaneous self-assembly of nanoparticles at ambient conditions (~298 K,~50%relative humidity). Electron Microscopy.TEM images of AuNPs were measured polar/nonpolar interfaces has been reported by Lin et al.36 with a JEOL 1011,100 kV,LaBs microscope.To stabilize the 4 nm previously. AuNPs for TEM investigations the dodecylamine ligands were In our present study,we prepared three different film exchanged by dodecanethiol,as described previously.3 SEM images materials,two of them,AunmNDT and Au nmPTM,both of the films were recorded using a LEO-1550 (Carl Zeiss)field. consisted of 4 nm sized AuNPs,but different cross-linkers.The emission scanning electron microscope structures of the cross-linkers,1,9-nonanedithiol (NDT)and To examine possible changes in the film morphology caused by pentaerythritol tetrakis(3-mercaptopropionate)(PTM),are tensile strain,we measured selected areas of the film AumNDT first shown in Scheme 1. under no strain and then under 3%tensile strain.For these measurements a rather low acceleration voltage of 1 kV was used in order to avoid any deformation of the sample due to energy input.The Scheme 1.Gold Nanoparticle Film Based Resistor Equipped images were taken with 25 000-fold magnification.Increasing the with Gold Electrodes(typical dimensions);Zoom:Cross- magnification led to blurred images and carbon contaminations Linked Gold Nanoparticles Swollen with Solvent;NDT and making it difficult to observe the subtle structural changes induced by PTM were Used As Linker Molecules strain.To better recognize subtle structural differences,we used the backscatter detector to enhance the Z-contrast. Crosslinker For imaging cross-sections of the films,the sensors were frozen in HS SH liquid nitrogen and then cleaved. Vapor Sensing Measurements.Test vapors were generated NDT using a commercial programmable calibration system (Kalibriersystem Model CGM 2000,Umwelttechnik MCZ).As carrier gas nitrogen 5.0 was used.The flow through the sensor test chamber (glass,~40 mL) was adjusted to a rate of 400 mL/min.The samples were placed into holders (Teflon)which allowed for controlled bending.According to the curvature of these holders it was possible to induce a strain of-1, PTM 0,1,2,and 3%in the sensor films.Unless otherwise indicated,the chemiresistor signals were acquired by supplying a constant current of 100 nA (Keithley Sourcemeter 2601A)and measuring the change in voltage (Keithley Multimeter 2002).All experiments were carried out at room temperature. RESULTS AND DISCUSSION 25 mm Deposition of Cross-Linked AuNP Films onto Poly- ethylene Substrates and Their Morphology.Our method NDT is a simple,hydrophobic alkanedithiol.In contrast, for depositing cross-linked AuNPs onto high density poly- PTM comprises four polar ester groups,capable of acting as ethylene (HDPE)substrates is based on layer-by-layer self- hydrogen-bond acceptors,and four thiol groups for cross- assembly.Before depositing the first layer of AuNPs,we linking AuNPs.Both compounds are commercially available opupmana and were chosen to study the influence of the linker polarity on the chemical selectivity of the films.As the nanoparticle the exposure of polyethylene to oxygen plasma leads to the component,we used dodecylamine-stabilized AuNPs.During formation of functional groups(e.g,-OH,-CHO,-COOH) film assembly,the amine ligands are quickly replaced by and at the same time to the deterioration of hydrocarbon thiolated cross-linkers.On the macroscopic scale both films, chains.After the plasma treatment,we observed an increase in AumNDT and AunmPTM,had a homogeneous optical the wettability of the substrates by water.The advancing appearance with a purple-bluish color in transmission and contact angle decreased from ~83 to ~39.This observation metallic-like reflection(see the Supporting Information,Figure indicates the formation of polar surface groups.Attenuated S3).The SEM images presented in Figure 1a-d reveal quite total reflectance Fourier transform infrared (ATR-FTIR) continuous coatings on the micrometer scale.On the spectroscopy performed before and after plasma treatment submicrometer scale,however,the AuNDT-film showed revealed the decomposition of some functional surface groups cracks with lengths up to ~300 nm and widths up to ~50 nm. (see the Supporting Information,Figure S2),which we In striking contrast the Aus amPTM-film showed a porous attribute to surface contaminants and/or the presence of a morphology,with pore diameters up to ~50 nm. slip agent.s It was,however,impossible to detect the formation The approximate thicknesses of these coatings were of new oxidized carbon species,showing that the surface determined from profile SEM images (see the Supporting density of polar groups generated by plasma treatment was Information,Figure S4)and are presented in Table 1. rather low.Directly after plasma treatment the AuNP-films To investigate any possible influence of the particle size on were deposited onto the substrates. the sensing properties we prepared a third film,AumNDT Comparative experiments revealed that omitting the plasma using oleylamine-stabilized AuNPs with a core diameter of 9 treatment resulted in the formation of rather inhomogeneous nm and NDT as cross-linker.Attempts to deposit these and thinner AuNP coatings (see the Supporting Information, particles directly onto plasma-treated PE-substrates failed in Table S1).Thus,plasma etching of the substrate was indeed that the assembly process was significantly slowed down.We necessary to deposit homogeneously appearing coatings. explain this observation by the longer chain length of the Plasma-generated polar surface groups probably accelerate the oleylamine ligands,which provides the particles with more 6153 dx.doLorg/10.1021/am301780bl ACS Appl.Mater.Interfaces 2012,4,6151-6161

resistance measurements, a Keithley multimeter 2000 was employed. The I−V curves were recorded using a Keithley sourcemeter 2601A. Unless otherwise indicated, the measurements were carried out at ambient conditions (∼298 K, ∼50% relative humidity). Electron Microscopy. TEM images of AuNPs were measured with a JEOL 1011, 100 kV, LaB6 microscope. To stabilize the 4 nm AuNPs for TEM investigations the dodecylamine ligands were exchanged by dodecanethiol, as described previously.30 SEM images of the films were recorded using a LEO-1550 (Carl Zeiss) field￾emission scanning electron microscope. To examine possible changes in the film morphology caused by tensile strain, we measured selected areas of the film Au4 nmNDT first under no strain and then under 3% tensile strain. For these measurements a rather low acceleration voltage of 1 kV was used in order to avoid any deformation of the sample due to energy input. The images were taken with 25 000-fold magnification. Increasing the magnification led to blurred images and carbon contaminations making it difficult to observe the subtle structural changes induced by strain. To better recognize subtle structural differences, we used the backscatter detector to enhance the Z-contrast. For imaging cross-sections of the films, the sensors were frozen in liquid nitrogen and then cleaved. Vapor Sensing Measurements. Test vapors were generated using a commercial programmable calibration system (Kalibriersystem Model CGM 2000, Umwelttechnik MCZ). As carrier gas nitrogen 5.0 was used. The flow through the sensor test chamber (glass, ∼40 mL) was adjusted to a rate of 400 mL/min. The samples were placed into holders (Teflon) which allowed for controlled bending. According to the curvature of these holders it was possible to induce a strain of −1, 0, 1, 2, and 3% in the sensor films. Unless otherwise indicated, the chemiresistor signals were acquired by supplying a constant current of 100 nA (Keithley Sourcemeter 2601A) and measuring the change in voltage (Keithley Multimeter 2002). All experiments were carried out at room temperature. ■ RESULTS AND DISCUSSION Deposition of Cross-Linked AuNP Films onto Poly￾ethylene Substrates and Their Morphology. Our method for depositing cross-linked AuNPs onto high density poly￾ethylene (HDPE) substrates is based on layer-by-layer self￾assembly.11,29 Before depositing the first layer of AuNPs, we cleaned the substrates and functionalized them by exposing them to oxygen-plasma. As reported by several authors,31−34 the exposure of polyethylene to oxygen plasma leads to the formation of functional groups (e.g., −OH, −CHO, −COOH) and at the same time to the deterioration of hydrocarbon chains. After the plasma treatment, we observed an increase in the wettability of the substrates by water. The advancing contact angle decreased from ∼83° to ∼39°. This observation indicates the formation of polar surface groups. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy performed before and after plasma treatment revealed the decomposition of some functional surface groups (see the Supporting Information, Figure S2), which we attribute to surface contaminants and/or the presence of a slip agent.35 It was, however, impossible to detect the formation of new oxidized carbon species, showing that the surface density of polar groups generated by plasma treatment was rather low. Directly after plasma treatment the AuNP-films were deposited onto the substrates. Comparative experiments revealed that omitting the plasma treatment resulted in the formation of rather inhomogeneous and thinner AuNP coatings (see the Supporting Information, Table S1). Thus, plasma etching of the substrate was indeed necessary to deposit homogeneously appearing coatings. Plasma-generated polar surface groups probably accelerate the deposition process. Carboxylate groups, for example, have some affinity to gold nanoparticles as known from citrate-stabilized AuNPs. Also, spontaneous self-assembly of nanoparticles at polar/nonpolar interfaces has been reported by Lin et al.,36 previously. In our present study, we prepared three different film materials, two of them, Au4 nmNDT and Au4 nmPTM, both consisted of 4 nm sized AuNPs, but different cross-linkers. The structures of the cross-linkers, 1,9-nonanedithiol (NDT) and pentaerythritol tetrakis(3-mercaptopropionate) (PTM), are shown in Scheme 1. NDT is a simple, hydrophobic alkanedithiol. In contrast, PTM comprises four polar ester groups, capable of acting as hydrogen-bond acceptors, and four thiol groups for cross￾linking AuNPs. Both compounds are commercially available and were chosen to study the influence of the linker polarity on the chemical selectivity of the films. As the nanoparticle component, we used dodecylamine-stabilized AuNPs. During film assembly, the amine ligands are quickly replaced by thiolated cross-linkers.8 On the macroscopic scale both films, Au4 nmNDT and Au4 nmPTM, had a homogeneous optical appearance with a purple-bluish color in transmission and metallic-like reflection (see the Supporting Information, Figure S3). The SEM images presented in Figure 1a−d reveal quite continuous coatings on the micrometer scale. On the submicrometer scale, however, the Au4 nmNDT-film showed cracks with lengths up to ∼300 nm and widths up to ∼50 nm. In striking contrast the Au4 nmPTM-film showed a porous morphology, with pore diameters up to ∼50 nm. The approximate thicknesses of these coatings were determined from profile SEM images (see the Supporting Information, Figure S4) and are presented in Table 1. To investigate any possible influence of the particle size on the sensing properties we prepared a third film, Au9 nmNDT using oleylamine-stabilized AuNPs with a core diameter of 9 nm and NDT as cross-linker. Attempts to deposit these particles directly onto plasma-treated PE-substrates failed in that the assembly process was significantly slowed down. We explain this observation by the longer chain length of the oleylamine ligands, which provides the particles with more Scheme 1. Gold Nanoparticle Film Based Resistor Equipped with Gold Electrodes (typical dimensions); Zoom: Cross￾Linked Gold Nanoparticles Swollen with Solvent; NDT and PTM were Used As Linker Molecules ACS Applied Materials & Interfaces Research Article 6153 dx.doi.org/10.1021/am301780b | ACS Appl. Mater. Interfaces 2012, 4, 6151−6161

ACS Applied Materials Interfaces Research Article a um um 200nm 200nm 50nm Figure 1.SEM images in two different magnifications of the films (a,b)AuNDT,(c,d)AumPTM,(e,f)AuNDT. Table 1.Characteristics of AuNP Films Deposited onto oleylamine-stabilized AuNPs could be deposited within HDPE reasonable reaction times (5 min per deposition step). Aus NDT AuPTM Auy aNDT Compared to films prepared with the smaller AuNPs,SEM images e and f in Figure 1 reveal a less continuous coating with sheet resistance 18.5±33 352±84 10.7±1.9 (MQ2) granular structures consisting of clustered nanoparticles.A thickness(nm)4 statistical analysis of several SEM images showed that ~75%of 90 70 60 the substrate surface was covered by the film.As determined by conductivity (Q-cm-i) 6X1×400±1209±1× 10N SEM profile images (see the Supporting Information,Figure gauge factor 21(21) 18(16) 24(32) S4)the height of the granular features was ~60 nm(Table 1). "The thickness was estimated from the SEM images of the breaking We assume that the granular morphology of the AugmNDT- edge (Figure S4,Supporting Information).The standard deviations film with its incomplete coverage resulted from an inhomoge- were calculated from 3 to 6 measured values.Gauge factors in neous structure of the underlying adhesion layer,which parentheses were measured under nitrogen atmosphere. provided the template for subsequent deposition of the 9 nm AuNPs.Supporting this assumption,it was reported efficient sterical stabilization.In order to accelerate the previously37 that the layer-by-layer deposition of dodecyl- deposition of oleylamine-stabilized AuNPs we first deposited amine-stabilized AuNPs typically forms an island structure a thin adhesion layer using the less stable dodecylamine- during the first few deposition cycles. stabilized AuNPs (3 deposition cycles).After this layer was Electrical Properties and Responses to Tensile Strain. functionalized with thiol groups of the NDT linker,the The simple architecture and the dimensions of the resistor a) b)60 0.8- 0% 0% 0.6- 0.5% 1% Au % 2% AuNP-Film HDPE 02 3% 0. 30- -0.2 20 10- -0.6 9=17.95 -0.8 R2=0.9960 -10 0 10 0.0 0.5 1.01.5 2.0 25 3.0 Bias [V] Tensile strain [% Figure 2.(a)I-V curves of the film AuPTM (on HDPE)at initial state and under different strain,as indicated.(b)Relative change in resistance vs tensile strain.The plotted data points were acquired in a set of 4 consecutive strain-relaxation cycles in which the AuNP-film was strained by convex bending of the substrate as shown in the inset.The solid line is a linear fit to the data,with the slope being the gauge factor g. 6154 dx.doLorg/10.1021/am301780bl ACS Appl.Mater.Interfaces 2012,4,6151-6161

efficient sterical stabilization. In order to accelerate the deposition of oleylamine-stabilized AuNPs we first deposited a thin adhesion layer using the less stable dodecylamine￾stabilized AuNPs (3 deposition cycles). After this layer was functionalized with thiol groups of the NDT linker, the oleylamine-stabilized AuNPs could be deposited within reasonable reaction times (5 min per deposition step). Compared to films prepared with the smaller AuNPs, SEM images e and f in Figure 1 reveal a less continuous coating with granular structures consisting of clustered nanoparticles. A statistical analysis of several SEM images showed that ∼75% of the substrate surface was covered by the film. As determined by SEM profile images (see the Supporting Information, Figure S4) the height of the granular features was ∼60 nm (Table 1). We assume that the granular morphology of the Au9 nmNDT- film with its incomplete coverage resulted from an inhomoge￾neous structure of the underlying adhesion layer, which provided the template for subsequent deposition of the 9 nm AuNPs. Supporting this assumption, it was reported previously37 that the layer-by-layer deposition of dodecyl￾amine-stabilized AuNPs typically forms an island structure during the first few deposition cycles. Electrical Properties and Responses to Tensile Strain. The simple architecture and the dimensions of the resistor Figure 1. SEM images in two different magnifications of the films (a, b) Au4 nmNDT, (c, d) Au4 nmPTM, (e, f) Au9 nmNDT. Table 1. Characteristics of AuNP Films Deposited onto HDPE Au4 nmNDT Au4 nmPTM Au9 nmNDT sheet resistance (MΩ) 18.5 ± 3.3 352 ± 84 10.7 ± 1.9 thickness (nm)a ∼90 ∼70 ∼60 conductivity (Ω−1 cm−1 ) b 6 × 10−3 ± 1 × 10−3 4 × 10−4 ± 1 × 10−4 2 × 10−2 ± <1 × 10−2 gauge factorc 21 (21) 18 (16) 24 (32) a The thickness was estimated from the SEM images of the breaking edge (Figure S4, Supporting Information). b The standard deviations were calculated from 3 to 6 measured values. c Gauge factors in parentheses were measured under nitrogen atmosphere. Figure 2. (a) I−V curves of the film Au4 nmPTM (on HDPE) at initial state and under different strain, as indicated. (b) Relative change in resistance vs tensile strain. The plotted data points were acquired in a set of 4 consecutive strain−relaxation cycles in which the AuNP-film was strained by convex bending of the substrate as shown in the inset. The solid line is a linear fit to the data, with the slope being the gauge factor g. ACS Applied Materials & Interfaces Research Article 6154 dx.doi.org/10.1021/am301780b | ACS Appl. Mater. Interfaces 2012, 4, 6151−6161

ACS Applied Materials Interfaces Research Article 300nm Figure 3.SEM images of the film AuNDT under no strain (left)and under 3%tensile strain induced in horizontal direction (right).Under strain the cracks are somewhat more pronounced.In order to better recognize the subtle changes of the structure,the images were measured using the backscatter detector to enhance the Z-contrast(see Experimental Section ) devices investigated are depicted in Scheme 1.In the10 V presented in Figure 2b.These data refer to the film AumPTM range (+250 V cm-)we observed ohmic current-voltage (I- with a gauge factor of ~18.Very similar results were obtained V)characteristics.Figure 2a shows I-V characteristics of the in case of the other two films,AusnmNDT and Aug nmNDT Au4mPTM-film as a representative example. (see the Supporting Information,Figure S5).The gauge factors Table 1 presents the sheet resistances and the estimated of all three films are listed in Table 1.These data were conductivities of the three films investigated.The conductivity measured at ambient conditions.Conducting the experiments of the AumNDT-film is of the same order of magnitude as under nitrogen resulted in similar gauge factors(see Table 1). values reported for the same material deposited on glasss,3s or Both the linear increase in resistance with tensile strain and the LDPE substrates.1 In contrast,the conductivity of the values of the gauge factors are in agreement with data reported Aus mPTM-film is 1 order of magnitude lower,as can be for Au4 nmNDT-films on LDPE substrates.Compared to explained by the larger size of the PTM-linker.In PTM the conventional strain gauges based on metal wire grids,the gauge molecular backbone between the thiol groups is two atoms factors reported here are about 1 order of magnitude higher. longer than in NDT.As shown previously,the conductivity of Taking into consideration the dominating tunnel term of eq AuNP-films cross-linked with alkanedithiols decreases by 2,Herrmann et al.proposed the following model to describe approximately 1 order of magnitude when increasing the the strain gauge response of films made from ligand-stabilized length of the cross-linker by three methylene units. In metal nanoparticles addition,the branched structure of PTM,with more than twice the molecular weight of NDT,most likely shields the △R=ee-1 nanoparticles more efficiently than NDT. Ro (3) The inhomogeneous morphology makes it difficult to with g=B(d 6o) estimate the conductivity of the AuNDT-film.Assuming Here,g is the gauge factor,d is the diameter of the metal an average film thickness of 60 nm with a surface coverage of cores,8o is the edge-to-edge distance between neighboring 75%suggests a conductivity of ~2x 102cm.This is metal cores at zero strain,and g is the strain (ie.,the relative approximately three times as large as that of the AumNDT- elongation Al/lo of the film under stress).Contrary to our film. observation,this model suggests an exponential increase in The strain sensitivity of the films'conductance was measured resistance with increasing strain.Further,using 10 nm- by convex bending of the substrates.It should be noted that in (for alkane chains values reported range from9 to 13 nm-)9 this study we investigated the film responses primarily to tensile and1nm the model claims higher gauge factors:~50 for strain (convex bending),because previous work indicated that particles with diameters of 4 nm,and ~100 for particles with compressive strain (concave bending)tends to irreversibly diameters of 9 nm. affect the baseline resistance.A set of typical data showing the To explain the differences between our experimental data relative increase in resistance with increasing tensile strain is and eq 3,we need to take into account that the model assumes 6155 dx.doLorg/10.1021/am301780bl ACS Appl.Mater.Interfaces 2012,4,6151-6161

devices investigated are depicted in Scheme 1. In the ±10 V range (±250 V cm−1 ) we observed ohmic current−voltage (I− V) characteristics. Figure 2a shows I−V characteristics of the Au4 nmPTM-film as a representative example. Table 1 presents the sheet resistances and the estimated conductivities of the three films investigated. The conductivity of the Au4 nmNDT-film is of the same order of magnitude as values reported for the same material deposited on glass8,38 or LDPE substrates.11 In contrast, the conductivity of the Au4 nmPTM-film is 1 order of magnitude lower, as can be explained by the larger size of the PTM-linker. In PTM the molecular backbone between the thiol groups is two atoms longer than in NDT. As shown previously, the conductivity of AuNP-films cross-linked with alkanedithiols decreases by approximately 1 order of magnitude when increasing the length of the cross-linker by three methylene units.38 In addition, the branched structure of PTM, with more than twice the molecular weight of NDT, most likely shields the nanoparticles more efficiently than NDT. The inhomogeneous morphology makes it difficult to estimate the conductivity of the Au9 nmNDT-film. Assuming an average film thickness of 60 nm with a surface coverage of 75% suggests a conductivity of ∼2 × 10−2 Ω−1 cm−1 . This is approximately three times as large as that of the Au4 nmNDT- film. The strain sensitivity of the films’ conductance was measured by convex bending of the substrates. It should be noted that in this study we investigated the film responses primarily to tensile strain (convex bending), because previous work indicated that compressive strain (concave bending) tends to irreversibly affect the baseline resistance.11 A set of typical data showing the relative increase in resistance with increasing tensile strain is presented in Figure 2b. These data refer to the film Au4 nmPTM with a gauge factor of ∼18. Very similar results were obtained in case of the other two films, Au4 nmNDT and Au9 nmNDT (see the Supporting Information, Figure S5). The gauge factors of all three films are listed in Table 1. These data were measured at ambient conditions. Conducting the experiments under nitrogen resulted in similar gauge factors (see Table 1). Both the linear increase in resistance with tensile strain and the values of the gauge factors are in agreement with data reported for Au4 nmNDT-films on LDPE substrates.11 Compared to conventional strain gauges based on metal wire grids, the gauge factors reported here are about 1 order of magnitude higher. Taking into consideration the dominating tunnel term of eq 2, Herrmann et al.10 proposed the following model to describe the strain gauge response of films made from ligand-stabilized metal nanoparticles Δ = − R ε R e 1 g 0 (3) with g = β(d + δ0) Here, g is the gauge factor, d is the diameter of the metal cores, δ0 is the edge-to-edge distance between neighboring metal cores at zero strain, and ε is the strain (i.e., the relative elongation Δl/l0 of the film under stress). Contrary to our observation, this model suggests an exponential increase in resistance with increasing strain. Further, using β ≈ 10 nm−1 (for alkane chains β values reported range from 9 to 13 nm−1 ) 39 and δ0 ≈ 1 nm40 the model claims higher gauge factors: ∼50 for particles with diameters of 4 nm, and ∼100 for particles with diameters of 9 nm. To explain the differences between our experimental data and eq 3, we need to take into account that the model assumes Figure 3. SEM images of the film Au4 nmNDT under no strain (left) and under 3% tensile strain induced in horizontal direction (right). Under strain the cracks are somewhat more pronounced. In order to better recognize the subtle changes of the structure, the images were measured using the backscatter detector to enhance the Z-contrast (see Experimental Section ). ACS Applied Materials & Interfaces Research Article 6155 dx.doi.org/10.1021/am301780b | ACS Appl. Mater. Interfaces 2012, 4, 6151−6161