Metal-Insulator-Metal Ensemble Gold Nanocluster ... - ACS Publications

Chapter 3

Metal-Insulator-Metal Ensemble Gold Nanocluster Vapor Sensors 1

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Arthur W. Snow , Hank Wohltjen , and N. Lynn Jarvis 1

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Naval Research Laboratory, 4554 Overlook Avenue, SW, Washington, DC 20375 Microsensor Systems, Inc., 62 Corporate Court, Bowling Green, K Y 42103

Downloaded by COLUMBIA UNIV on August 4, 2012 | http://pubs.acs.org Publication Date: January 21, 2005 | doi: 10.1021/bk-2005-0891.ch003

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A nanocluster Metal-Insulator-Metal Ensemble (MIME) chemical vapor sensor is a solid-state sensor composed of nanometer size gold particles encapsulated by a monomolecular layer of alkanethiol surfactant deposited as a thin film on an interdigital microelectrode. The principle by which this sensor operates is that vapors reversibly absorb into the organic monolayer which causes a large modulation in the electrical conductivity of the film. The tunneling current through the monolayer between gold particle contacts is extremely sensitive to very small amounts of monolayer swelling and dielectric alteration caused by absorption of vapor molecules. The nanometer scale of the particle domains and correspondingly large surface area translate into a very large vapor sensitivity range extending to sub-ppm concentrations. Selectivity of the sensor is regulated by incorporation of chemical functionalities in the structure of the alkanethiol surfactant or substitution of the entire alkane structure. The current focus of research is in mapping the selectivity and sensitivity of sensor elements made by incorporating these functionalities into the shell of the nanocluster. Targeted applications include detection of chemical warfare agents and explosives, and residual life indication of carbon filters and protective clothing.

© 2005 American Chemical Society

In Defense Applications of Nanomaterials; Miziolek, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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Introduction Detecting hazardous chemical vapors with highly miniaturized analytical devices is a present and future capability that is assuming an increasing importance in many DOD and civilian scenarios. These scenarios involve chemical weapons, concealed explosives, volatile organics in breathing air, residual life indication of gas mask filters and protective clothing, and electronic nose functions (identification of unknown substances by odor). Until 25 years ago detection of hazardous chemicals relied on large scale laboratory configured instrumentation such as mass and optical spectroscopies, gas chromatography, and to a smaller extent on colorimetric schemes utilizing wet chemistry. Since then, the impetus has been toward reduction in instrumentation size and field deployment of analytical instrumentation. Also during this time sensors as minimal-component, highly miniaturized, stand-alone devices emerged. These devices generally incorporated a chemically active surface interfaced to an electronic substrate. Examples include metal oxide semiconductor (MOS) devices, miniature electrochemical cells, surface acoustic wave (SAW) devices and chemiresistors. For vapor sensing these devices rely of a partitioning of an analyte vapor between the gas phase and that sorbed onto/into the chemically active surface. The chemically active surface then acts as a transducer. A property change caused by sorption of a vapor generates an electronic signal that may be processed into analytical information regarding the concentration of the analyte vapor. NRL has been active in microsensor research since 1981 with SAW and chemiresisor devices. Features which govern the sensitivity of these devices are the transduction mechanism, vapor partitioning, and the surface area to volume ratio of the chemically active adsorbent coating. The first feature is determined by the coupling between the electronic substrate and the chemically active surface or coating and is varied mostly by the substrate design. The second feature is determined by the chemical interaction between the analyte vapor and the adsorbent surface's chemical structure and is varied by the design and synthesis of this component. Over the past 20 years much research at NRL and elsewhere has focused on optimizing these features. The third feature (surface to volume ratio of the chemically responsive coating) has received very little attention as the thickness of the chemically active coatings were optimized to be thick enough for good sensitivity but thin enough for a fast response. A nanoscale materials approach changed this radically. A new type of chemical microsensor with distinct advantages derived from nanometer scale material domains emerged. The objective of this research is to understand the operating principles of this new sensor and exploit its advantages in practical applications.


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M I M E Sensor Concept The new sensor is based on a metal nanocluster encapsulated by a single layer of organic molecules. Our original report (I) has been followed by other examples (2-9). The materials concept is illustrated in Figure 1. A single cluster, depicted in the upper left comer, is composed of a gold core encapsulated by a shell of alkanethiol molecules. The gold core may range from 1 to 5 nm in diameter, and from it originates the electronic properties of this material. The alkanethiol molecules of the shell are bonded to the surface of the gold core by a gold-tosulfur bond. This organic shell forms a very thin insulating barrier, and its thickness, which may vary from 0.4 to 1.0 nm, has an enormous effect on electron tunneling between adjacent clusters. The organic shell also imparts an organic character to the cluster which promotes solubility in organic solvents such that a cluster as much as 90% gold by weight will dissolve in toluene. This

Individual Nanoclusters

Sorption of Vapor

DC VOLTAGE 3 E

Micron Scale Coated Electrode

Chemiresistor Device

Figure 1. Schematic of the Metal-Insulator-Metal ensemble (MIME) sensor concept. A micron-scale interdigital electrode is coated with a film of alkanethiol stabilized gold nanoclusters and exposed to toluene vapor. The toluene adsorbs into the alkanethiol monolayer shell, and the consequent swelling causes an increase in the separation distance between gold cores and a reduction of electron tunneling between them. (Reproduced from NRL Review. U.S. Government work in the public domain.) (See page 2 of color insert)



34 solubility makes the processing of these clusters into thin films very facile. The sensor is fabricated by depositing a film of these clusters onto a micron scale interdigital electrode substrate. When connected to a small bias (50 to 500 mV), a nanoamp current flows through the film. Exposure to vapors causes very large changes in the conductivity of the film. This results from a sorption of the vapor into the very thin shell, and the consequent swelling of the shell results in a small but very significant increase in the distance between cores of adjacent metal clusters. The tunneling current is extremely sensitive to the distance between cores. A final feature of significance in Figure 1 is the packing of the clusters in the film. Being spherically shaped clusters, any type of packing will have nanometer scale voids within its matrix. The size differential between a typical vapor molecule such as toluene and a nanocluster is approximately a factor of 10. As such, this network of voids in the cluster matrix makes for a rapid ingress and egress of vapors, much more so than the slow diffusion into polymer films used on typical microsensors. This provides a pathway for a much faster response and recovery for sensors based on a metal cluster ensemble. Combined with the cluster ensemble's fast kinetics for sorption and desorption is an extremely large surface to volume ratio which translates into a highly enhanced sensitivity for this MIME sensor. The MIME sensor derives its name from the Metal-Insulator-Metal Ensemble character of the cluster film. The critical features in its design are the dimensions of die core and shell and the chemical composition of the molecules composing the shell. These features are described in the following paragraphs.

Cluster Synthesis and Characterization The dimensions of the core and shell of the cluster are determined by the conditions of its synthesis which are modifiedfromthose originally reported by Brust et al (10). The alkanethiol-gold cluster synthesis is illustrated in Figure 2. The two critical reagents are the gold chloride and the alkanethiol. They are suspended in a common medium, and a reducing agent, typically NaBH » is added. The trivalent gold is reduced to neutral gold, and the gold atoms aggregate to form a particle nucleus. The gold particle grows by addition of gold atoms and smaller particles. As a competing process, the alkanethiol reacts with the neutral gold surface to form a sulfur to gold bond. The gold particle growth is terminated when its surface is encapsulated by complexation with the alkanethiol. The relative rates of gold particle growth and alkanethiol surface complexation are dependent on the concentrations of the respective gold chloride and alkanethiol reagents. Thus, the molar ratio of these reagents is a simple way to regulate the core size of die cluster. Typically, this ratio ranges from 1:3 to 8:1 and causes the corresponding core diameters to vary from 1 to 5 nm The shell thickness of the cluster is determined by molecular chain length 4



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Figure 2. Schematic of the alkanethiol stabilized cluster synthesis. After reduction of the gold ions, the competitive processes of gold particle growth and alkanethiol surface complexation determine the size of the gold nanocluster. (See page 3 of color insert)

of the alkanethiol selected as the reagent. The number of carbon atoms in the chain length typically ranges from 4 to 16 and generates a corresponding shell thickness ranging from 0.4 to 1.0 nm. A matrix of these clusters has been synthesized as is depicted in Table 1 where the varying core and shell relative sizes are represented pictorially by concentric circles. As a shorthand to designate individual clusters, the general abbreviation AwCn(X:Y) is used where X:Yis the gold chloride.alkanethiol synthesis stoichiometric ratio that correlates with core size and n is the number of carbon atoms in the alkanethiol chain that correlates with shell thickness. The number beneath each cluster in Table 1 is the corresponding bulk DC electrical conductivity. An appreciation for the respective effects of the shell thickness and core size on the electrical conductivity can be obtained by examining the magnitude of conductivity variation down the column headed (1:1) and across the row headed C i , For the former the variation is on the order of 10 which illustrates why such minute swelling effects on the shell have such dramatic effects on the conductivity. 2

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M I M E Sensor Response to Vapors The response of a Au:C5(/:/) MIME sensor to five 60 sec exposure-purge cycles of toluene at high and low vapor concentrations is presented in Figure 3.



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SOURCE: NRL Review. U.S. Government work in the public domain. See page 3 of the color insert.

The sensor response is a measured current change (right axis) which is electronically converted to a frequency (left axis) via precision current-tovoltage and voltage/frequency converters to allow data acquisition over a wide dynamic range using a computerizedfrequencycounter. The toluene vapor causes a very large and rapid decrease in conductance of the sensor. Greater than 90% of the signal response occurs within 1 sec of its 30 sec exposure. The recovery is equally rapid and complete. The lower portion of Figure 3 indicates that detection limits well below 1 ppm are achievable. The MIME sensor response to toluene displays a dependence on both the core and shell dimensions of its cluster component. When the matrix of clusters depicted in Table 1 is investigated, optimum sensitivities are found for both the core diameter and the shell thickness in the midrange of the matrix. Clearly two effects operate in each case. In the latter, a thicker shell requires more sorbed vapor to achieve an amount of swelling comparable swelling to that of a thinner shell, while a thinner shell has less of an organic character to solvate the


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Figure 3. Response of Au:C

Metal-Insulator-Metal Ensemble Gold Nanocluster ... - ACS Publications

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