Advances in Fingerprint Technology.pdf

ZnS
encapsulant
CdSe
conjugating
ligand
Figure 6.21 Structure of encapsulated and functionalized CdSe nanocrystal.
biotechnology arena, and the functionalization has tended to involve carboxylate
and amino groups. Given the above-described dendrimer investigation,
we have in place a chemistry that should lend itself to the fingerprint context.
Because of the high cost and low availability of CdSe nanocrystals, we have to
date only addressed fingerprint development with carboxylate-functionalized
ones (obtained from Quantum Dot Corp., Palo Alto, CA). Diimide mediation
of the chemical fingerprint labeling was essential (fingerprints 1-month-old on
aluminum foil). Heating was not useful, destroying the nanocrystals. Nanocrystals
are generally stored at low temperature (about 4°C) to prevent flocculation.
No fingerprint development was obtained at that temperature, even with
sample immersion for up to 5 days. At room temperature, 24-hr fingerprint
immersion in a water solution containing in 1 µM concentration the watersoluble
CdSe/ZnS/carboxylate functionalized nanocrystals and in 8 µM concentration
the water-soluble 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide
hydrochloride produced development of luminescent fingerprints. An example
is shown in Figure 6.22. The carbodiimide relative concentration was kept low
(one would normally use higher concentration) to prevent flocculation of
nanocrystals. The low nanocrystal concentration itself was dictated by the
limited nanocrystal supply.
In keeping with semiconductor characteristics (i.e., broad band-structure
characteristics in absorption), the utilized nanocrystals luminesced under
excitation that could range from the ultraviolet to the red. This makes for
greater flexibility in terms of the excitation light source. The comparatively
rather sharp red luminescence peaked at 635 nm and had full-width at halfmaximum
of 35 nm (comparable to molecular fluorescence); the absorption/luminescence
spectra are shown in Figure 6.23. Luminescence lifetime
measurements were best interpreted by two-component fits. These yielded