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Highly
sensitive and highly selective tests are important for the early
detection of disease, the detection of environmental toxins, or for the
detection of explosives at airports. Increased selectivity for the
target analytes helps to avoid false-positive results. In the journal
Angewandte Chemie, Indian scientists have now introduced a specific
detection method for the explosive TNT that can be used to detect even a
single molecule.
Thalappil
Pradeep, Ammu Mathew, and P. R. Sajanlal at the Indian Institute of
Technology Madras use an ingenious combination of micro- and
nanostructures as sensors: gold mesoflowers, flower-shaped gold
particles about 4 µm in size, act as supports for silver clusters, tiny
clumps of exactly 15 silver atoms embedded in the protein bovine serum
albumin. When irradiated with light of the right wavelength, the silver
clusters luminesce, giving off red light. The gold of the mesoflower
supports intensifies the fluorescence. Their unique shape is a
particular advantage, because it is easy to unambiguously identify under
an optical microscope, unlike spherical particles.
If
a drop of a solution containing TNT is applied, it reacts with the
amino groups of the bovine serum albumin to make a Meisenheimer
complex—a reaction specific to TNT. This extinguishes the red glow of
the silver clusters. In order to make this reaction even more distinct,
the researchers also embedded a green fluorescing dye, which was
adsorbed on a silicon dioxide layer grown on the gold flowers. As long
as the silver clusters glow red, the green fluorescence is suppressed.
When TNT molecules switch off the red light, the green dye begins to
glow. The color change from red to green can be observed with a
fluorescence microscope.
A
TNT concentration of one ppb (part per billion) extinguishes the
fluorescence, one ppt (part per trillion), reduces it markedly. The
researchers supplemented their luminescence technique with a second
analytical method, SERS (surface-enhanced Raman scattering), which also
functions extremely well with a version of the flower-shaped sensors.
“This allowed us to attain detection limits as low as the sub-zeptomole
range (10-21 mol),” explains Pradeep. Just one “flower” is enough to
operate as a sensor. It reacts when it comes into contact with as few as
nine molecules. A device based on this principle is under development.
The
researchers were also able to detect mercury with similar sensitivity
by using the same sensor strategy. Says Pradeep: “Our concept could also
be used for the ultra-trace analysis of other substances through the
incorporation of specific ligands on the sensors.”
Selective Visual Detection of TNT at the Sub-Zeptomole Level
Source: Wiley
