Phosphor/fluorescent molecules/particles have been widely used in various applications
for quite some time. Typically, light with longer wavelength(s) is emitted when excited by shorter
wavelength light. The opposite effect also exists, where a phosphor particle is excited with an
infrared or red light and emits color(s) of shorter wavelengths, a process called up-conversion.
Materials with upconverting properties have narrower absorption and line emission spectra than
their down-converting counterparts. Because most non-target materials in a complex mixture do
not possess such photon up-conversion properties, a dramatically improved S/N ratio is expected
in sensing and luminescence reporting applications. This makes photon upconverting materials
ideal for identification of trace amounts of target molecules. Here we report the synthesis,
characterization and DNA detection application based on NaYF4:Yb3+, Er3+ photon upconverting
nanoparticles. The design of a nucleotide sensor for the detection of point mutation associated
with sickle cell disease is described. The underlying principle for the detection is luminescence
resonance energy transfer (LRET), with the photon upconverting nanoparticle as the donor and a
dye, N,N,N',N'-tetramethyl-6-carboxyrhodamine (TAMRA), as the acceptor. The detection
scheme is based on a sandwich-type hybridization format. The presence of the target DNA is
indicated by the increase of the normalized acceptor's emission. Based on photon upconverting
nanoparticles, which display high S/N ratio and no photobleaching, the DNA sensor demonstrates
high sensitivity and specificity. The results demonstrate great potential of such nanomaterials as
oligonucleotide sensors.
KEYWORDS: Near field scanning optical microscopy, Proteins, Quantum dots, Spectroscopy, Near field, Sensors, Polymers, Near field optics, Luminescence, Avalanche photodetectors
We investigated the engineered bioconjugate of cadmium selenide core/zinc sulfide shell, (CdSe)ZnS, quantum dots (QDs) with genetically modified proteins using near-field scanning optical microscopy (NSOM). A genetically engineered protein polymer was expressed and purified from E. coli. The protein polymer was allowed to self-assemble to the bacterial microcrystalline cellulose surface through the cellulosic binding domain. QDs were then conjugated to the protein/cellulose assembly through interaction with the 6x-histidine tag on the protein. The transmitted near-field optical signals are collected and detected by both a PMT (near-field scanning optical microscopy, NSOM) and a spectrometer (near-field scanning optical spectroscopy, NSOS). Results from the sample containing the QDs/protein/cellulose assemblies suggest that QDs were arrayed along the cellulose surface. The near-field spectroscopic study also showed that the slight change of spectroscopic properties of the QDs upon bioconjugation, indicating the strong interaction between the constructed protein and QDs.
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