Sensing with photoluminescent semiconductor quantum dots

Abstract

Fluorescent sensors benefit from high signal-to-noise and multiple measurement modalities, allowing for a multitude of applications and flexibility of design. Semiconductor nanocrystal quantum dots (QDs) are excellent fluorophores for sensors because of their extraordinary optical properties. They have high thermal and photochemical stability compared to organic dyes or fluorescent proteins and are extremely bright due to their large molar cross-sections. In contrast to organic dyes, QD emission profiles are symmetric, with relatively low full width half maxima. In addition, the size tunability of their emission color, which is a result of quantum confinement, make QDs exceptional emitters with high color purity from the ultraviolet to near infrared wavelength range. The role of QDs in sensors ranges from simple fluorescent tags, as used in immunoassays, to intrinsic sensors that utilize the inherent photophysical response of QDs to fluctuations in temperature, electric field or ion concentration. In more complex configurations, QDs and biomolecular recognition moieties like antibodies are combined with a third component to transduce the optical signal via energy transfer. QDs can act as donors, acceptors, or both in energy transfer-based sensors using Förster resonance energy transfer (FRET), nanometal surface energy transfer (NSET), or charge or electron transfer. The changes in both spectral response and photoluminescent lifetimes have been successfully harnessed to produce more sensitive sensors and multiplexed devices. While technical challenges related to biofunctionalization and the high cost of laboratory-grade fluorimeters have thus far prevented broad implementation of QD-based sensing in clinical or commercial settings, improvements in bioconjugation methods and detection schemes, including using simple consumer devices like cell phone cameras, are lowering the barrier to broad use of more sensitive QD-based devices.

Ligand-induced chirality and optical activity in semiconductor nanocrystals: theory and applications

“…A multitude of studies consider semiconductor NCs as promising components of optical sensors [93][94][95]. Chiral ligands further extend the sensing capabilities of NCs via chiral recognition, enabling enantiomer recognition and separation [21,22].…”

A Förster Resonance Energy Transfer‐Based Ratiometric Sensor with the Allosteric Transcription Factor TetR

“…Adding multiple acceptor molecules has the benefit of increasing the FRET efficiency compared to a single acceptor at the same donor–acceptor distance. [ 23,24 ] Titrating the Cy5–DNA FRET acceptors against the TetR C –tdTomato or QD–TetR C donors elucidates the donor:acceptor ratio that maximizes energy transfer with the donor concentration adjusted to keep the TetR C concentration constant at 200 × 10 −9 m . When the tetO sequence is used, we observe the characteristic nonlinear response to increasing acceptor concentration, which is fit to a modified Hill equation (Table S2, Supporting Information).…”

In Vivo Biosensing Using Resonance Energy Transfer

“…Each of the FRET examples above relies on changes in the luminescence intensity as the signal output. Photoluminescence lifetime is a complimentary and powerful tool for imaging because the lifetime is dependent on the environment and excitation mechanism of the fluorophore, but is independent of dye concentration, assuming the fluorophore is dilute enough to avoid homoFRET and other near-field effects [132,133]. Each fluorophore exhibits its own characteristic PL lifetime, which can range from ps for organic dyes to µs for lanthanide-based emitters [134].…”

“…Bound ligands exhibited a shortened donor PL lifetime, while the lifetime was not shortened by FRET for unbound ligands. Measuring the PL lifetime rather than emission intensity has a distinct advantage as the fluorescence intensity can be decreased by changes in local concentration and environmental effects (e.g., pH, temperature) as well as FRET [132]; the use of PL lifetime is more reliable for quantification. Nude mice inoculated with palpable, orthotopic T47D breast tumors were dosed with tail vein injections of AF700- and AF750-conjugated Tfn at acceptor:donor ratios of 0:1 and 2:1.…”