Two-Photon Excitation Spectroscopy of Silicon Quantum Dots and Ramifications for Bio-Imaging

Abstract

Two-photon excitation in the near-infrared (NIR) of colloidal nanocrystalline silicon quantum dots (nc-SiQDs) with photoluminescence also in the NIR has potential opportunities in the field of deep biological imaging. Spectra of the degenerate two-photon absorption (2PA) cross section of colloidal nc-SiQDs are measured using two-photon excitation over a spectral range 1.46 < ℏω < 1.91 eV (wavelength 850 > λ > 650 nm) above the two-photon band gap E g (QD)/2, and at a representative photon energy ℏω = 0.99 eV (λ = 1250 nm) below this gap. Two-photon excited photoluminescence (2PE-PL) spectra of nc-SiQDs with diameters d = 1.8 ± 0.2 nm and d = 2.3 ± 0.3 nm, each passivated with 1-dodecene and dispersed in toluene, are calibrated in strength against 2PE-PL from a known concentration of Rhodamine B dye in methanol. The 2PA cross section is observed to be smaller for the smaller diameter nanocrystals, and the onset of 2PA is observed to be blue shifted from the two-photon indirect band gap of bulk Si, as expected for quantum confinement of excitons. The efficiencies of nc-SiQDs for bioimaging using 2PE-PL are simulated in various biological tissues and compared to efficiencies of other quantum dots and molecular fluorophores and found to be comparable or superior at greater depths.

Silicon quantum dots: surface matter, what next?

“…So it is very noticeable when we tune the particle size from ϕ ∼ 2.5 nm to ϕ ∼ 15 nm; PL-max is redshift gradually from λ max 590 nm to λ max 1100 nm. 4,6,11,12,17,29–37,39–52,56–71,77,95,98–107…”

“…SiQDs for bioimaging efficiency compared with other quantum dots and molecular fluorophores and found comparable. 100 Later, Sarwat et al (2022) conducted experiments to synthesize metal-functionalized SiQDs for bioimaging applications. Sc-SiQDs, Cu-SiQDs, and Zn-SiQDs were successfully synthesized, and their optimal PL and cytotoxicity were investigated.…”

Effect of the surface coverage of an alkyl carboxylic acid monolayer on waterborne and cellular uptake behaviors for silicon quantum dots

“…Other way for emerging red-to-NIR emission is the use of rare-earth doped nanocrystals 34 – 36 , or Pb-free perovskite nanocrystals such as CeSnI 3 , Sb-doped Cs 2 SnCl 4 Br 2 , and lanthanide-doped CsMnBr 3 37 – 40 . Most of these QDs and nanocrystals exhibit the downshifting luminescence, a single-photon process that converts absorbed high-energy photons to low-energy ones while those with excitation maxima in the short-wavelength region are efficiently photoexcited for red-to-NIR emission via upconversion techniques 41 – 44 . Nevertheless, the red-to-NIR-light emitter used in the clinical spot for surgical medical treatments is limited to indocyanine green (ICG).…”

“…Their superior photoluminescence (PL) properties include the narrow red-to-NIR PL spectra having no long emission tails 48 , continuously tunable over the 650–1000 nm wavelength range 48 , 49 , and decay times on the scale of 100 μsec 48 . Furthermore, by taking advantage of multiphoton absorption or fluorescence resonance energy transfer (FRET) processes, SiQDs emit the red-to-NIR light through the sequential absorption of photons from the red-to-NIR light 41 , 48 , 50 , 51 .…”

Activated Two-Photon Near-Infrared Ratiometric Fluorescent Nanoprobe for ONOO– Detection and Early Diagnosis and Assessment of Liver Injury

“…However, these methods have some drawbacks such as complicated sample pretreatment, cumbersome operation, nonquantitative detection, and especially, unsuitability for noninvasive and real-time detection of ONOO – in vivo. Recently, fluorescent probe-based detection techniques have emerged as a prevalent method to detect diverse analytes in living organisms due to their distinctive benefits of simple operation, high selectivity, and low detection limit. − More importantly, these probes can be employed for noninvasive real-time detection and imaging analysis in vivo and generally constructed from small-molecule organic fluorescent dyes, quantum dots, carbon nanotubes, metallic materials, , etc. However, most of these probes suffer from disadvantages like poor water solubility and biocompatibility, thus limiting their application in vitro and in vivo.…”