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Diffusive Gradients in Thin Gels (
DGT
)
While not considered metal sensors or analytical systems, diffusive equilibrium based sampling
devices provide a low cost means to obtain temporally integrative samples of metal distributions
over a range of spatial scales.
DGT
devices are comprised of a metal chelating resin underlying
a thin (<1mm) hydrogel film. Loading of metal ions onto the resin is limited by its rate of
diffusion through the hydrogel (a function of metal species diffusivity, gel pore size and
thickness). For a given deployment time, accumulated metal concentration in the resin will be
proportional to the average concentration of the labile metal fraction in the ambient water or
sediment, and can be derived using the Fick equation (Davison et al. 2000). Concentration
measurements based on diffusive metal accumulation are comparable to direct measurement by
ASV representing the dynamic metal fraction. While metal analysis is performed off site
generally using spectroscopic methodology, these sampling devices do provide a low cost,
sensitive and reliable method to passively obtain multi-element samples. As their deployment
does not require expert users, these devices may be ideal to incorporate into regional volunteer
WQ monitoring efforts (
see B. Hoover presentation
) and enable much higher spatial and temporal
sampling schemes. DGT samplers are commercially available (
www.windsor-
ltd.co.uk/dgt.html
), and are relatively straightforward to construct (Davison et al, 2000; Twiss and
Moffett, 2002). It would also appear that specificity of the samplers could also benefit from
recommended IS coating developments as well as metal species specific ligands.
Biosensors
This represents a nascent technology area but one with great promise, as it leverages the precise
molecular recognition interactions inherent in biological systems and tremendous advances in site
directed mutagenesis and
in vitro
protein expression protocols to manipulate binding site
characteristics. Sensor development strategies include manipulating native metal binding site
properties to alter metal selectivity or developing molecular mimics of binding site structure.
Transduction of metal-macromolecule interactions can be achieved by reporters responding to
metal-induced conformational changes via quenching or enhancement of site-specific fluorescent
or luminescent signals. Metaloproteins provide obvious targets for metal biosensor development.
Zinc-finger proteins or their consensus metal binding motifs and human carbonic anhydrase have
provided highly selective binding sites with picomolar affinities for Zn(II), Cu(II) and low
interference from abundant Mg(II) and Ca(II) cations in biological or environmental fluids
(Thompson et al. 1999). Site directed covalent attachment of a variety of fluorophores near the
native metal binding sites can yield a signal transducing system yielding metal concentration
enhanced or quenched fluorescence intensity, fluorescence lifetime shifts and polarization shifts
depending upon the metal dye-combination and molecular proximity. These molecular
transduction signals report free metal ion binding and provide a means to measure free metal ion
concentrations
in situ
, a parameter generally beyond the detection limits of current analytical
techniques in absence of preconcentration. Use of time resolved fluorescence signals to monitor
bind site activity makes this sensor design compatible with fiber optic measurement systems,
enabling spatially precise free metal ion measurements in near real time. In terms of cross-over
into WQ monitoring applications, proof of concept for the human carbonic anhydrase based fiber
optic sensor has been reported for picomolar detection of cupric ion (Cu(II)) in natural seawater
(Zheng
et al
. 2003). Current technological impediments to operational deployment of these true
biosensors relate to slower metal ion off rates and either denaturation or poisoning / fouling of
ACT Workshop on Trace Metal Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10