Improving Quantum Yield of Upconverting Nanoparticles in Aqueous Media via Emission Sensitization

Abstract

We demonstrate a facile method to improve upconversion quantum yields in Yb,Er-based nanoparticles via emission dye-sensitization. Using the commercially available dye ATTO 542, chosen for its high radiative rate and significant spectral overlap with the green emission of Er, we decorate the surfaces of sub-25 nm hexagonal-phase Na(Y/Gd/Lu)F:YbEr upconverting nanoparticles with varying dye concentrations. Upconversion photoluminescence and absorption spectroscopy provide experimental confirmation of energy transfer to and emission from the dye molecules. Upconversion quantum yield is observed to increase with dye sensitization, with the highest enhancement measured for the smallest particles investigated (10.9 nm in diameter); specifically, these dye-decorated particles are more than 2× brighter than are unmodified, organic-soluble nanoparticles and more than 10× brighter than are water-soluble nanoparticles. We also observe 3× lifetime reductions with dye adsorption, confirming the quantum yield enhancement to result from the high radiative rate of the dye. The approach detailed in this work is widely implementable, renders the nanoparticles water-soluble, and most significantly improves sub-15 nm nanoparticles, making our method especially attractive for biological imaging applications.

Sub-20 nm Core–Shell–Shell Nanoparticles for Bright Upconversion and Enhanced Förster Resonant Energy Transfer

“…[36][37][38] Further when UCNPs are coupled to a fluorescent dye via FRET, the upconversion quantum yield (UCQY) of the system has been shown to improve by nearly an order of magnitude. 23 Together, these studies inform design considerations for bright upconversion in a small nanoparticle for FRET: a passivating inert shell, a high concentration of Yb 3+ , a tuned Er 3+ concentration, and efficient coupling to a FRET acceptor.…”

“…), which dominate because of the high surfaceto-volume ratio. [19][20][21] Additionally, improved FRET coupling has been shown with smaller nanoparticles, 22,23 further driving the need for small and bright UCNPs.…”

Upconversion of low-energy photons in semiconductor nanostructures for solar energy harvesting

“…Major advances include: (i) the synthesis of doped cores surrounded by undoped shells, which separate the excited ions from surface states that mediate nonradiative recombination and energy loss, 13 (ii) the use of NaYF 4 host matrices that suppress phonon-mediated relaxation, 14 and (iii) sensitization of upconversion emission using dye molecules. 15 Some of these and other improvements 16,17 have been used to reach the current record internal upconversion quantum yield (UCQY) for lanthanides of 16.7%, 18 where the internal UCQY here is defined as the ratio of the number of emitted high-energy photons to the number of absorbed low-energy photons and can have a maximum value of 50%. In this review, the UCQY is equivalent to half the iUQE.…”

Physical Manipulation of Lanthanide‐Activated Photoluminescence

“…[27] Thanks to the robust chemical synthesis methodologies developed for both bulk and micro/nanoscale materials, it is comparatively easy to manipulate the luminescent properties by means of modulating the composition, crystallographic parameters, sensitizer and activator ions distribution, size, morphology, and surface defects of luminescent materials. By integrating organic dye, indocyanine green (ICG), onto the surface of NaYF 4 :Yb 3+ /X 3+ @NaYbF 4 @NaYF 4 :Nd 3+ (X = null, Er, Ho, Tm, or Pr) core/shell/shell nanocrystals, Shao et al [41] broadened successfully the excitation spectral range (700-860 nm) and increased a set of narrow band emissions located within 1000-1700 nm taking advantage of the large absorption cross section of organic dye and subsequently improved energy transfer efficiency to lanthanide ions; Wisser et al [42] utilized dye ATTO 542 to decorate the surfaces of hexagonal-phase Na(Y/Gd/Lu) 0.8 F 4 :Yb 0.18 Er 0.02 upconverting nanoparticles to enhance the luminescence intensity and also tune the emission color and lifetime, where the Förster resonance energy transfer from Er 3+ to dye and the high radiative rate of the dye play a key role for luminescence manipulation. For example, co-doping of Ce 3+ with Tb 3+ and Eu 3+ at different concentrations within ScPO 4 ·2H 2 O microparticles allows manipulation of the emission wavelengths, intensity, and lifetime [37] ; Increasing the concentration of Tm 3+ /Er 3+ in NaYbF 4 nanoparticles could impact the energy transfer process between sensitizer Yb 3+ and activator Tm 3+ /Er 3+ , leading to the upconversion color tuning from blue/green to red [38] ; Zhuo et al [39] distributed multiple activators (Tm 3+ , Er 3+ , and Ho 3+ ) into spatially separated layers in one single KSc 2 F 7 nanorod to effectively restrain the deleterious energy transfer between these activators and enhance their www.advancedsciencenews.com www.ann-phys.org upconversion luminescence.…”

“…The investigation of upconversion luminescence of Yb 3+ /Er 3+ -doped nanocrystals with average size ranging from 6 to 45 nm by Zhao et al, [40] displayed a reduced overall luminescence intensity and luminescence lifetime but an increasing red-to-green intensity ratio along with size decreasing, where the changes in intensity and lifetime are due to the incremental nonradiative excitation energy consumption paths (surface defects, vibration of surface ligands, and solvent) competing with radiative transition in smaller nanocrystals, and the color evolution is because the nonradiative transitions ( 4 I 11/2 → 4 I 13/2 and 4 S 3/2 → 4 F 9/2 ) of Er 3+ are more efficient for smaller particles. By integrating organic dye, indocyanine green (ICG), onto the surface of NaYF 4 :Yb 3+ /X 3+ @NaYbF 4 @NaYF 4 :Nd 3+ (X = null, Er, Ho, Tm, or Pr) core/shell/shell nanocrystals, Shao et al [41] broadened successfully the excitation spectral range (700-860 nm) and increased a set of narrow band emissions located within 1000-1700 nm taking advantage of the large absorption cross section of organic dye and subsequently improved energy transfer efficiency to lanthanide ions; Wisser et al [42] utilized dye ATTO 542 to decorate the surfaces of hexagonal-phase Na(Y/Gd/Lu) 0.8 F 4 :Yb 0.18 Er 0.02 upconverting nanoparticles to enhance the luminescence intensity and also tune the emission color and lifetime, where the Förster resonance energy transfer from Er 3+ to dye and the high radiative rate of the dye play a key role for luminescence manipulation. It is clear that all of these chemical approaches need cumbersome synthesis process, which is usually irreversible, and do not provide any dynamic information on luminescent property evolution.…”