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 2012,4,6151-6161Cross-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