Chemiresistive Vapor Sensing with Microscale Films of Gold Monolayer Protected Clusters

Charged Metal Nanoparticles for Chemoelectronic Circuits

Although metal nanoparticles (NPs) stabilized with self-assembled monolayers (SAMs) of various organic ligands have proven useful in applications ranging from chemical sensing, to bionanotechnology, to plasmonics and energy conversion, they have not been widely considered as suitable building blocks of electronic circuitry, largely because metals screen electric fields and prevent electrically tunable conductivity. However, when metal nanoparticles a few nanometers in size are stabilized by charged ligands and placed under bias, the counterions surrounding the NPs can redistribute and establish local electric fields that feed back into the electronic currents passing through the nanoparticles' metallic cores. Herein, the manner in which the interplay between counterion gradients and electron flows can be controlled by using different types of SAMs is discussed. This can give rise to a new class of nanoparticle-based "chemoelectronic" logic circuits capable of sensing, processing, and ultimately reporting various chemical signals.

Imparting sensitivity and selectivity to a gold nanoparticle chemiresistor through thiol monolayer functionalization for sensing acetone

Chemiresistor-based gas sensors for detection of target volatile organic compounds (VOCs) in air face common challenges of poor sensitivity and selectivity as well as suffering from interference by other constituent gases and/or humidity. This work demonstrates that functionalizing gold nanoparticles (AuNPs) with a designed thiol monolayer improves sensitivity and selectivity of the derived AuNPs gas sensor. We report the synthesis and application of a thiol ligand fitted with both a urea motif and a tert-butyl end group for functionalizing AuNPs. The AuNPs sensor prepared using the urea thiol ligand demonstrated significantly increased acetone sensing in comparison with tested commercially available thiol-functionalized AuNPs. The sensor worked under ambient temperature and high humidity conditions, and demonstrated a linear relationship between the sensor response and the common logarithm of analyte concentration.

Polymer nanocomposites using click chemistry: novel materials for hydrogen peroxide vapor sensors

Functionalization of nano graphite platelets with polystyrene through click reaction.

Cross-linked gold nanoparticles on polyethylene: resistive responses to tensile strain and vapors

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.

Gold nanoparticle chemiresistors operating in biological fluids

Functionalised gold nanoparticle (Au(NP)) chemiresistors are investigated for direct sensing of small organic molecules in biological fluids. The principle reason that Au(NP) chemiresistors, and many other sensing devices, have limited operation in biological fluids is due to protein and lipid fouling deactivating the sensing mechanism. In order to extend the capability of such chemiresistor sensors to operate directly in biofluids, it is essential to minimise undesirable matrix effects due to protein and lipidic components. Ultrafiltration membranes were investigated as semi-permeable size-selective barriers to prevent large biomolecule interactions with Au(NP) chemiresistors operating in protein-loaded biofluids. All of the ultrafiltration membranes protected the Au(NP) chemiresistors from fouling by the globular biomolecules, with the 10 kDa molecular weight cut-off size being optimum for operation in biofluids. Titrations of toluene in different protein-loaded fluids indicated that small molecule detection was possible. A sensor array consisting of six different thiolate-functionalised Au(NP) chemiresistors protected with a size-selective ultrafiltration membrane successfully identified, and discriminated the spoilage of pasteurised bovine milk. This proof-of-principle study demonstrates the on-chip protein separation and small metabolite detection capability, illustrating the potential for this technology in the field of microbial metabolomics. Overall, these results demonstrate that a sensor array can be protected from protein fouling with the use of a membrane, significantly increasing the possible application areas of Au(NP) chemiresistors ranging from the food industry to health services.

Chemiresistive sensing with chemically modified metal and alloy nanoparticles

This review describes the use of chemically modified pure and alloyed metal nanoparticles for chemiresistive sensing applications. Chemically modified metal nanoparticles consist of a pure or alloyed metallic core with some type of chemical coating. Researchers have studied the electronic properties of 1D, 2D, and 3D assemblies of chemically modified metal nanoparticles, and even single individual nanoparticles. The interaction with the analyte alters the conductivity of the sensitive material, providing a signal to measure the analyte concentration. This review focuses on chemiresistive sensing of a wide variety of gas- and liquid-phase analytes with metal nanoparticles coated with organothiols, ions, polymers, surfactants, and biomolecules. Different strategies used to incorporate chemically modified nanoparticles into chemiresistive sensing devices are reviewed, focusing on the different types of metal and alloy compositions, coatings, methods of assembly, and analytes (vapors, gases, liquids, biological materials), along with other important factors.

Characterization of dense arrays of chemiresistor vapor sensors with submicrometer features and patterned nanoparticle interface layers

The performance of arrays of small, densely integrated chemiresistor (CR) vapor sensors with electron-beam patterned interface layers of thiolate-monolayer-protected gold nanoparticles (MPNs) is explored. Each CR in the array consists of a 100-μm(2) interdigital electrode separated from adjacent devices by 4 μm. Initial studies involved four separate arrays, each containing four CRs coated with one of four different MPNs, which were calibrated with five vapors before and after MPN-film patterning. MPNs derived from n-octanethiol (C8), 4-(phenylethynyl)-benzenethiol (DPA), 6-phenoxyhexane-1-thiol (OPH), and methyl-6-mercaptohexanoate (HME) were tested. Parallel calibrations of MPN-coated thickness-shear-mode resonators (TSMR) were used to derive partition coefficients of unpatterned films and to assess transducer-dependent factors affecting responses. A 600-μm(2) 4-CR array with four different patterned MPN interface layers, in which the MPN derived from 7-hydroxy-7,7-bis(trifluoro-methyl)heptane-1-thiol (HFA) was substituted for HME, was then characterized. This is the smallest multi-MPN array yet reported. Reductions in the diversity of the collective response patterns are observed with the patterned films, but projected vapor discrimination rates remain high. The use of such arrays as ultralow-dead-volume detectors in microscale gas chromatographic analyzers is discussed.

Densely integrated array of chemiresistor vapor sensors with electron-beam patterned monolayer-protected gold nanoparticle interface films

Use of electron-beam induced crosslinking to pattern films of monolayer-protected gold nanoparticles (MPNs) onto a chemiresistor (CR) sensor array is described. Each of the four CRs comprises a 100 µm(2) set of interdigital electrodes (IDEs) with 100 nm widths and spaces, separated from adjacent devices by 4 µm. Films of four MPNs, each with a different thiolate monolayer, were successively patterned on the IDEs. Vapor exposures yield rapid, reversible changes in CR resistances and differential vapor sensitivities comparable to those reported for larger CRs with unpatterned MPN films. The array response patterns facilitate vapor discrimination. This is the smallest MPN-coated CR array yet reported. The advantages of using such an array as the detector in microfabricated gas chromatographic analyzers are considered.

Electrochemical size discrimination of gold nanoparticles attached to glass/indium-tin-oxide electrodes by oxidation in bromide-containing electrolyte

Here we describe the electrochemical oxidation of an assembly of gold nanoparticles (Au NPs) attached to glass/indium-tin-oxide (ITO) electrodes as a function of particle size. We synthesized Au NP arrays with NP diameters ranging from 8 to 250 nm by electrodeposition of Au from HAuCl(4) in H(2)SO(4) at potentials of -0.2 to 0.8 V versus Ag/AgCl using chronocoulometry to keep the amount of Au deposited constant. The average Au NP size increased with increasing deposition potential. The chemical reduction of HAuCl(4) by NaBH(4) in trisodium citrate solution led to 4 nm average diameter Au NPs, which we chemisorbed to the glass/ITO electrode. Linear sweep voltammograms (LSVs) obtained on the glass/ITO/Au NP (4 to 250 nm) electrodes (with a constant coverage of Au in terms of Au atoms per cm(2)) from 0.5 to 1.1 V in 0.01 M potassium bromide plus 0.1 M HClO(4) showed a positive shift in oxidation potential from 734 +/- 1 mV to 913 +/- 19 mV with increasing Au NP diameter. The shift agrees qualitatively with that predicted by a shift in the redox potential based on a difference in free energy associated with a change in surface energy as a function of particle size. On the basis of the charge during Au deposition versus the charge during oxidation, the oxidation process produces a mixture of Au(III)Br(4)(-) (25%) and Au(I)Br(2)(-) (75%). A glass/ITO electrode coated with a mixture of 4 and 250 nm Au NPs revealed 2 oxidation peaks, consistent with the two Au NP size populations present on the surface.

Thin film assemblies of molecularly-linked metal nanoparticles and multifunctional properties

The use of metal nanoparticles as building blocks toward thin film assembly creates intriguing opportunities for exploring multifunctional properties. Such an exploration requires the ability to engineer the size, shape, composition, and especially interparticle properties in nanoparticle assemblies for harnessing the collective properties of the nanoscale building blocks. This article highlights some of the important findings of our investigations of thin film assemblies of molecularly linked nanoparticles for exploiting their multifunctional and collective properties in molecular recognition and chemical sensing. The thermally activated processing approach presents a viable pathway for nanoengineering metal, alloy, and core-shell nanoparticles as building blocks. The molecular mediator-templating approach offers an effective strategy to thin film assemblies of the nanoscale building blocks that impart multifunctional properties. In such thin film assemblies, the interparticle interactions and structures dictate the correlation between the nanostructural parameters and the optical and electrical properties. By highlighting selected examples involving ligand-framework binding of ionic species at the film/liquid interface and electrical responses to molecular sorption at the film/gas interface, the multifunctional properties of the thin film assemblies are further discussed in terms of interparticle covalent, hydrogen bonding, ionic, or van der Waals interactions in a framework-type architecture for the creation of molecular recognition and chemical sensing sites that can be tuned chemically or electrochemically. Implications of these insights to expanding the exploration of nanoparticle thin film assemblies for a wide range of technological applications are also discussed.

Chemiresistive sensing of volatile organic compounds with films of surfactant-stabilized gold and gold-silver alloy nanoparticles

Here we describe the chemiresistive sensing of volatile organic compounds (VOCs) with films of chemically synthesized approximately 4 nm diameter Au and AuAg alloy nanoparticles (NPs) stabilized by a surfactant, tetraoctylammonium bromide (TOABr). The chemiresistive sensing properties were measured over a concentration range of 100 to 0.04% saturation for methanol (MeOH), ethanol (EtOH), 2-propanol (IPA), and toluene (Tol) vapor analytes and compared directly to the chemiresistive sensing properties of films of 1.6 nm diameter hexanethiolate (C6S)-coated Au monolayer-protected clusters (MPCs). Films of TOABr-stabilized Au NPs exhibit the opposite response compared to those of C6S-coated Au MPCs. The details are unclear, but the mechanism likely involves changes in capacitive charging in the film or improved conductive pathways through the Au NPs upon incorporation of VOCs into the film for the former as opposed to the well-known change in electron hopping conductivity for the latter. This leads to a decrease in resistance in the presence of VOCs for TOABr Au as opposed to an increase for C6S Au. The TOABr Au sensors are more sensitive, especially for polar analytes, and have greater long-term stability compared to C6S Au. The limit of detection (LOD) for films of TOABr-coated Au NPs is 3, 2, 12, and 37 ppm for IPA, MeOH, EtOH, and Tol, respectively, as compared to 106, 326, 242, and 48 for C6S Au. Films of TOABr-stabilized AuAg alloy NPs exhibit the same type of response, but the sensitivity decreases dramatically with increasing Ag content, showing that the metal composition of the NPs in the film plays a role in the sensing properties, which has not been well-recognized in the literature.

Environmental monitoring of hydrocarbons: a chemical sensor perspective

Assessing the environmental impact of organic pollutants requires reliable analytical tools that can rapidly screen them with minimal sample handling. Chemical sensors are expected to play an increasing role in environmental monitoring, and recent technological advances are certain to facilitate the application of chemical sensing devices. The search for highly selective, sensitive, low cost, stable, and robust sensors for hydrocarbons is an area of interest that is reflected by many publications on this topic. This report surveys some of the work that has been undertaken using sensors to detect hydrocarbons in the gas and liquid phase. The analytical capabilities of various sensors are compared and discussed in terms of their selectivity, sensitivity, and detection limit. It was found that the sensitivity is highly dependent on the experimental conditions used in the preparation of the sensing surface. Many sensors display acceptable sensitivity under controlled laboratory conditions; however, very few are selective enough to distinguish among several hydrocarbons in complex mixtures. Selectivity is still a challenge that is hindering the widespread application of chemical sensors for environmental monitoring of hydrocarbons and a number of strategies have been proposed to help overcome some of these problems.

Array of Molecularly Mediated Thin Film Assemblies of Nanoparticles: Correlation of Vapor Sensing with Interparticle Spatial Properties

The ability to tune interparticle spatial properties of nanoparticle assemblies is essential for the design of sensing materials toward desired sensitivity and selectivity. This paper reports findings of an investigation of molecularly mediated thin film assemblies of metal nanoparticles with controllable interparticle spatial properties as a sensing array. The interparticle spatial properties are controlled by a combination of alpha,omega-difunctional alkyl mediators (X-(CH(2))(n)-X) such as alkyl dithiols, dicarboxylate acids, and alkanethiol shells capped on nanoparticles. Alkanethiolate-capped gold and gold-silver alloy nanoparticles (2-3 nm) were studied as model building blocks toward the thin film assemblies, whereas the variation of alkyl chain length manipulates the interparticle spacing. The thin films assembled on an interdigitated microelectrode array platform are characterized for determining their responses to the sorption of volatile organic compounds (VOCs). The correlation between the response sensitivity and the interparticle spacing properties revealed not only a clear dependence of the sensitivity on alkyl chain length but also the occurrence of a dramatic change of the sensitivity in a region of chain length for the alkyl mediator comparable with that of the capping alkyl chains. This finding reflects a balance between the interparticle chain-chain cohesive interdigitation and the nanostructure-vapor interaction which determines the relative change of the electrical conductivity of the inked nanoparticle thin film in response to vapor sorption. The results, along with statistical analysis of the sensor array data in terms of sensitivity and selectivity, have provided important insights into the detailed delineation between the interparticle spacing and the nanostructured sensing properties.

Ozone- and thermally activated films of palladium monolayer-protected clusters for chemiresistive hydrogen sensing

Here we describe the chemiresistive H2-sensing properties of drop-cast films comprised of 3.0 nm average diameter hexanethiolate-coated Pd monolayer-protected clusters (C6 Pd MPCs) bridging a pair of electrodes separated by a 23 microm gap. The gas-sensing properties were measured for 9.6-0.11% H2 in a H2/N2 mixture. The sensing mechanism is based on changes in the resistance of the film upon reaction of Pd with H2 to form PdH(x), which is known to be larger in volume and more resistive than pure Pd. As-prepared Pd MPC films are highly insensitive to H2, requiring O3 and thermal treatment to enhance changes in film resistance in the presence of H2. Exposure to O3 for 15 min followed by activation in 100% H2 leads to an increase in film conductivity in the presence of H2, with a detection limit of 0.11% H2. When exposed to temperatures of 180-200 degrees C, the conductivity of the film increases and a decrease in conductivity occurs in the presence of H2 with a detection limit of 0.21%. The sensing behavior reverses after further heating to 260 degrees C, exhibiting an increase in conductivity in the presence of H2 as in O3-treated films and a detection limit of 0.11%. The sensitivity of the variously treated films follows the order O3 > high temp > low temp, and the response times at 1.0% H2 range from 10 to 50s, depending on the treatment. FTIR spectroscopy, Raman spectroscopy, and atomic force microscopy provide information about the C6 monolayer, Pd metal, and film morphology, respectively, as a function of O3 and heat treatment to aid in understanding the observed sensing behavior. This work demonstrates a simple chemical approach toward fabricating a fast, reversible sensor capable of detecting low concentrations of H2.