Jothirmayanantham Pichaandi

An Effective Polymer Cross-Linking Strategy To Obtain Stable Dispersions of Upconverting NaYF4 Nanoparticles in Buffers and Biological Growth Media for Biolabeling Applications

Ligands on the nanoparticle surface provide steric stabilization, resulting in good dispersion stability. However, because of their highly dynamic nature, they can be replaced irreversibly in buffers and biological medium, leading to poor colloidal stability. To overcome this, we report a simple and effective cross-linking methodology to transfer oleate-stabilized upconverting NaYF(4) core/shell nanoparticles (UCNPs) from hydrophobic to aqueous phase, with long-term dispersion stability in buffers and biological medium. Amphiphilic poly(maleic anhydride-alt-1-octadecene) (PMAO) modified with and without poly(ethylene glycol) (PEG) was used to intercalate with the surface oleates, enabling the transfer of the UCNPs to water. The PMAO units on the phase transferred UCNPs were then successfully cross-linked using bis(hexamethylene)triamine (BHMT). The primary advantage of cross-linking of PMAO by BHMT is that it improves the stability of the UCNPs in water, physiological saline buffers, and biological growth media and in a wide range of pH values when compared to un-cross-linked PMAO. The cross-linked PMAO-BHMT coated UCNPs were found to be stable in water for more than 2 months and in physiological saline buffers for weeks, substantiating the effectiveness of cross-linking in providing high dispersion stability. The PMAO-BHMT cross-linked UCNPs were extensively characterized using various techniques providing supporting evidence for the cross-linking process. These UCNPs were found to be stable in serum supplemented growth medium (37 °C) for more than 2 days. Utilizing this, we demonstrate the uptake of cross-linked UCNPs by LNCaP cells (human prostate cancer cell line), showing their utility as biolabels.

Upconverting core-shell nanocrystals with high quantum yield under low irradiance: On the role of isotropic and thick shells

Colloidal upconverter nanocrystals (UCNCs) that convert near-infrared photons to higher energies are promising for applications ranging from life sciences to solar energy harvesting. However, practical applications of UCNCs are hindered by their low upconversion quantum yield (UCQY) and the high irradiances necessary to produce relevant upconversion luminescence. Achieving high UCQY under practically relevant irradiance remains a major challenge. The UCQY is severely limited due to non-radiative surface quenching processes. We present a rate equation model for migration of the excitation energy to show that surface quenching does not only affect the lanthanide ions directly at the surface but also many other lanthanide ions quite far away from the surface. The average migration path length is on the order of several nanometers and depends on the doping as well as the irradiance of the excitation. Using Er3+-doped β-NaYF4 UCNCs, we show that very isotropic and thick (∼10 nm) β-NaLuF4 inert shells dramatica...

Photoluminescence dynamics in solid formulations of colloidal PbSe quantum dots: Three-dimensional versus two-dimensional films

Time-resolved photoluminescence spectroscopy is used to compare decay dynamics of colloidal PbSe quantum dots as (i) dilute dispersions in hexanes, (ii) thick, dense, emulsive films, and (iii) sub-monolayer films on silicon surfaces. Accounting for the effect of each dielectric environment on exciton radiative decay rates, we deduce the non-radiative decay rate of excitons increases going from solution to thick films by ≈6 fold and to sub-monolayer films by ≈7–16 fold. This is attributed to surface passivation degradation due to a lack of mobile ligands when the particles are taken out of solution.

Are upconverting Ln3+ based nanoparticles any good for deep tissue imaging with retention of optical sectioning?

An effective strategy is presented to make spherical Ln3+ doped NaYF4 nanoparticles that show upconversion, with the aim of deep-tissue optical imaging. Upconversion is the conversion of two or more low-energy photons into one of higher energy, e.g. 980 nm to 545 and 680 nm and 980 nm to 800 nm. In order to avoid the formation of nanoparticles with an aspect ratio, we developed a strategy in which subsequent shells were grown on spherical seed nanoparticles. The last shell is undoped in order to improve the optical properties. In addition, a simple intercalation strategy involving the oleate ligands on the surface has been developed to make the nanoparticles dispersible in aqueous solutions and physiological buffers. Two-photon upconversion laser scanning microscopy (TPULSM) and two-photon upconversion wide-field microscopy (TPUWFM) have been tested for their suitability in deep-tissue imaging with retention of lateral and depth resolution (also called optical sectioning). TPULSM can be used up to ~ 600 μm deep, but takes inordinately long times to acquire, which is due to the fact that the absorption cross section of Yb3+ is low, the quantum yield of the upconversion process are << 1%, and the Ln3+ excited states are up to several hundreds of μs. Hence UCNPs in general are not very bright (i.e. large emitted photon flux). The TPUWFM seems more promising because acquisition times are only several minutes, with depth profiling up to 400 μm. We show the first optical sectioning with this technique in the brain of a mouse, through a thin shaved skull.