Advances in Fingerprint Technology.pdf

nanocrystals. Photoluminescent semiconductor nanoparticles have recently
become the subject of intense investigation in the biotechnology arena,
mostly for labeling purposes. 19,20 DNA sequencing is an example. The salient
virtues that distinguish the nanoparticles (referred to also as quantum dots,
nanocrystals, and nanocomposites, depending on morphology) from ordinary
fluorescent labels are that their absorption and luminescence wavelengths
(colors) can be tuned by varying the particle size and that the luminescence
lifetimes are long, ranging from roughly 10 –8 to 10 –6 s, thus making
the nanoparticles suitable for time-resolved spectroscopy and time-resolved
imaging, with flexibility in terms of useful laser light sources. These features
are valuable from a biotechnology perspective and are, in principle, equally
pertinent to the fingerprint context, thus prompting a study of their potential
utility in fingerprint detection that commenced in 1999. 9 Because of financial
and manpower constraints, this investigation has to date only aimed at reduction
to practice 21 (i.e., demonstration of feasibility rather than work-up of
routine recipes). Nonetheless, a variety of nanoparticle systems and fingerprint
chemistries have been examined. Practical recipes will presumably follow
once the best general approaches have been delineated.
CdS Nanocrystals 22
Photoluminescent semiconductor nanocrystals may be expected to be used
for fingerprint detection in various ways, namely by incorporation into dusting
powder in a manner akin to fluorescent dye blending with magnetic
powder 9 by staining, especially once fingerprints have been exposed to
cyanoacrylate ester; or by chemical bonding to constituents of fingerprint
residue. In photoluminescence detection of fingerprints, the staining
approach generally tends to be more effective when applicable to the article
under examination, than dusting. Thus, the focus in this chapter section is
on staining with CdS nanocrystals. As a preface to this mode of fingerprint
detection, the photophysical properties of CdS quantum dots are examined,
primarily to assess suitability for phase-resolved imaging to suppress background
fluorescence, but also to determine suitable excitation wavelengths.
The employed CdS nanocrystal samples, prepared in inverse micelles 23,24
and capped with dioctyl sulfosuccinate (sodium salt), were obtained from
Professor John T. McDevitt (Chemistry Department, University of Texas,
Austin). Solubilization of the nanocrystals utilized heptane or a mixture of
hexanes (CH 3C 4H 8CH 3). Solutions had quantum dot concentrations of milligram/milliliter
order. Absorption spectra in these solvents exhibited band
edges at about 440 nm and very broad absorbance (with some structure, but
not absorption peaks as in typical atomic or molecular spectra) that increased
with decreasing wavelength to 300 nm, the lowest wavelength in our absorption
measurements. The spectra, fairly typical of semiconductor absorption