Measurement of Two‐Photon Absorption of Silicon Nanocrystals in Colloidal Suspension for Bio‐Imaging Applications

Abstract

Two‐photon absorption (2PA) of ligand‐passivated silicon nanocrystals (ncSi) in liquid suspensions is difficult to measure directly because of their low 2PA cross section and competing nonlinear optical processes in the suspension medium at high light intensity. Here we overcome these difficulties for small (diameter d < 5 nm) ncSi by measuring background‐free, 2PA‐induced photoluminescence (PL) at intensities below the threshold for high‐order processes, and at wavelengths shorter than the excitation wavelength λ(2) = 810 nm. We calibrate the instrument response by measuring PL induced by one‐photon absorption, and confirm previously reported 2PA cross sections σ(2) for a control rhodamine B dye solution. We find σ(2) = (0.28 ± 0.05, 1.25 ± 0.17, and 18 ± 3) × 10−50 cm4 s/photon, respectively, for d = 2.0, 2.6, and 5.1 nm diameter ncSi suspended in toluene. For d = 5.1 nm ncSi, the PL spectrum overlaps λ(2), and PL scales sub‐quadratically with intensity at lower intensities in our range, indicating one‐photon excitation of the upper PL level. Thus the indirect method reported here appears best suited for ncSi with d < 5 nm. 2PA confocal microscopy of mouse cells incubated with d = 2.7 nm ncSi is demonstrated.

Hierarchical Nanostructuring of Porous Silicon with Electrochemical and Regenerative Electroless Etching

“…The application of two-photon fluorescence imaging for obtaining higher spatial resolution, contrast and the suppression of tissue autofluorescence has received significant interest. , In this regard, we tested the excitation of the COOH-RaPSi particles with near-infrared photons of different wavelengths up to 900 nm, with the results indicating an excitation wavelength of ca . 800 nm to be most optimal for the particles used in this experiment (Figure S9).…”

“…Through use of filters, both the RaPSi particles and the FAM-labeled peptide can be observed simultaneously as shown in Figure S8, due to the distinct emission maxima of the label and the particles indicating that covalent conjugation of a dye or cell internalization do not disturb or alter the initial PL properties of the RaPSi nanoparticles The application of two-photon fluorescence imaging for obtaining higher spatial resolution, contrast and the suppression of tissue autofluorescence has received significant interest. 48,57 In this regard, we tested the excitation of the COOH-RaPSi particles with near-infrared photons of different wavelengths up to 900 nm, with the results indicating an excitation wavelength of ca. 800 nm to be most optimal for the particles used in this experiment (Figure S9).…”

Characterization of Mechanochemical Modification of Porous Silicon with Arginine

“…The visible photoluminescence (PL) of mesoporous silicon arises from an interplay of quantum confinement and surface passivation effects [9]. Its optical properties facilitate applications in biological imaging [10] excited with one or two photons [11,12], in theranostics [13] and photodynamic therapy [14]. In addition, porous silicon has many advantages in pharmaceuticals and drug delivery, as its pore size, pore volume, and specific surface area are all tunable, which allow for the loading of a wide variety of organic species, such as small molecule drugs [15], RNA [16], insulin [17], and peptides [18].…”

Crystallinity and Size Control of Colloidal Germanium Nanoparticles from Organogermanium Halide Reagents

“…Group IV semiconducting materials, such as silicon (Si) and Ge, are good examples for observing and investigating the size-dependent properties that arise due to quantum confinement. − In their bulk form, group IV materials have an indirect band gap, but quantum confinement induces electronic perturbations that force the system to exhibit direct band gap transitions, resulting in excitons displaying optical transitions in the visible spectral region. ,− The favorable electronic properties and narrow band gap of bulk Ge (0.67 eV at a temperature of 300 K), coupled with its large Bohr radius ( a 0 ≈ 24 nm) , renders quantum confinement effects observable at relatively large particle sizes. This allows exploitation of the effects arising due to quantum confinement, such as decoupling the thermal and electrical conductivity − or tuning light emission − at relatively large sizes, making Ge nanoparticles (NPs) particularly attractive for many applications, such as bioimaging and thermoelectric devices, among others. − …”