CLUSTER SYNTHESIS AND CHARACTERIZATION

The dimensions of the core and shell of the cluster are determined by the conditions of its synthesis. Figure 2 illustrates the alkanethiol-gold cluster synthesis. The two critical reagents are the gold chloride and the alkanethiol. They are suspended in a common medium, and a reducing agent, typically NaBH4, 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 the 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 the 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 the molecular chain length 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.

MIME SENSOR RESPONSE TO VAPORS

Figure 3 shows the response of an Au:C8(1:1) MIME sensor to five 60-s exposure-purge cycles of toluene at high and low vapor concentrations. The sensor response is a measured current change (right axis) that is electronically converted to a frequency (left axis) via precision current-to-voltage and voltage/ frequency converters to allow data acquisition over a wide dynamic range using a computerized frequency counter. 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 s of its 30-s exposure. The recovery is equally rapid and complete. The lower portion of Fig. 3 indicates that detection limits well below 1 ppm are achievable.

The MIME sensor response to toluene displays a dependece 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 to that of a thinner shell, while a thinner shell has less of an organic character to solvate the incoming toluene. In the former case, it is not clear why a variation from 1 to 5 nm would pass through an optimum in sensor sensitivity to toluene.

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