Sol-gel indicator (SGI) composites will be used by the Savannah River Technology Center to manufacture sensor elements to measure a variety of conditions and species of interest. The overall program includes SGIs for measuring solution pH, and components such as heavy metals (e.g. U, Hg) and selected halogenated organics. The technology is based on incorporation of indicator materials into specially prepared porous silica glass matrices. The SGIs change color in response to analyte concentration. The SGI composites are coated onto optical components, which will allow photometric measurements to be made.
The current task will involve the following:
Increased emphasis in recent years on environmental analyses has fueled a need for field-portable and remote monitoring instruments. An important contribution to this effort has been the development of portable fiber-optic diode array spectrophotometers (DAS) and associated fiber-optic probes. These probes contain the sensing element or function as a measurement cell, and are small, rugged and easily deployed.
Sol-gel technology has opened up a route to sensor element fabrication; the indicator may be trapped in a porous silica sol-gel matrix. Pore sizes of the sol gel can be controlled during synthesis to allow the desired analyte to react reversibly with the indicator molecule, while keeping the indicator trapped within the glass matrix. Sol-gel technology has the advantage of being able to distribute and incorporate the indicator in a chemically durable matrix at room temperature, and hence, avoid the high temperatures in conventional glass manufacture that degrade the organic indicator molecules. Figure 2.9a (below) shows how the sol-gel sensor element is integrated into a fiber-optic DAS system.
A sol-gel sensor has been prepared with the pH indicator bromophenol blue. This sensor has been tested in buffers of pH 1-10. The response of the bromophenol blue pH sensor is shown in the following, Figure 2.9b.
The response range of almost 4 pH units is broader than seen in aqueous solutions of the indicator, where the response range is generally restricted to about 2 pH units. The sol-gel pH sensor also responds to a more basic pH range than the aqueous solution of the indicator. The sensor in Figure 2.9b is responsive over the pH range 3.9-7.7. pH differences as small as 0.1 pH unit cause marked changes in the sensor spectrum. The pH sensor response time is on the order of a few seconds to half a minute. Sensor responses were modeled by multivariate techniques. For the bromophenol blue sensor, a principal component regression model based on a training set with intervals of 0.1 pH units gave predictions accurate to ±0.3 pH units on subsequent solutions.
A first generation uranium sol-gel sensor, based on the indicator arsenazo III, has been fabricated. The arsenazo III sol-gel product has been tested in dilute uranyl nitrate (97 ppm, pH = 2). Initial results have been encouraging, but sensor response time is slow. Modifications and optimization of the sol-gel formulation will be undertaken to improve the response time of the probe.
COLLABORATION/TECHNOLOGY TRANSFER
This technology will prove useful as a screening tool for in field measurements (wells, lakes, streams, etc.) of properties such as pH and dissolved heavy metals content. Measurements of this type are needed at many sites nationwide that require environmental remediation. Efforts towards technology transfer include the following:
FY 94 Contacts:
George Wicks
Westinghouse Savannah River Company
Building 773-A
Aiken, SC 29808
(803) 725-3190
John L. Steele
Technical Program Manager
(803) 725-1830
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CMST-IP Technology Summary, April 1994