One-pot microwave synthesis of upconverting nanoparticles for biosensingAna Egatz-Gomez1, Sonia Melle2, Oscar Calderon2, Loreto Parra3, Amador Guzman4

1 Chemical and Biomolecular Engineering Department, University of Notre Dame, Notre Dame, Indiana, USA2 Department of Optics, School of Optics, Complutense University of Madrid, Madrid, Spain

3 Department of Chemical and Bioprocesses Engineering, Pontifical Catholic University of Chile, Santiago, Chile3 Department of Mechanical and Metallurgic Engineering, Pontifical Catholic University of Chile, Santiago, Chile

• Up-converting nanoparticles are man-made materials that absorb two ormore photons of infrared light and emit in the visible range.

• Infrared excitation light can penetrate deeper and does not produce background fluorescence in biological matrices.

• These nanoparticles are robust do not photobleach.• They have very large anti-Stokes shifts and narrow emission bands. • Application areas: multimodal imaging, biosensing. • Microwave heating provides fast, fairly homogeneous heating.

Background

Microwave-Assisted Synthesis

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Cubic NaYF4COD 96‐151‐7677

Hexagonal NaYF4PDF 00‐028‐1192 

Figure 4. Nanocrystals’structure.X-ray powder diffraction pattern ofUCNPs synthesized at 315°C for 10minutes (cubic phase) and 60 minutes(mixed cubic and hexagonal phases)(D8 Advance DaVinci, Bruker).

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Figure 1. Schematic representation of the one-pot synthesis method.Trifluoroacetate (TFA) salts of sodium,

yttrium, ytterbium, and erbium weredissolved in a mix of oleic acid and di-butyl-phthalate at different ratios by firstheating the solution at 120 °C for 10minutes, followed by heating at 315 °Cfor 10 to 120 minutes. Trifluoroacetatesalts and di-n-butyl phthalate are goodmicrowave absorbers and allow the

microwave synthesis. Oleic acid is used as a ligand to control the size andshape of the nanoparticles. The commonly used non-coordinating solvent 1-octadecene is a very low-microwave absorber and thus not ideal for thisapplication.

Figure 3. Morphology. SEM of NaYF4:Yb:Er nanoparticles synthesized at 315°C for 10 minutes(10.8 ± 0.6 nm), 30 minutes (11.0 ± 0.6 nm) and 60 minutes (21.5 ± 1.9 nm).

Figure 5. Surface ligands.FTIR of UCNPs synthesized at315°C for 30 minutes in amixture of oleic acid and di-n-butyl phthalate (FT/IR 6300,Jasco). The characteristic peaksof oleate ligands ( 3004, 2934,and 2853 cm-1) are absent fromthis pattern. Inset: di-n-butylphthalate (DBP)

Figure 6. UCNPs as temperature sensors in glass capillaries. (a) Schematic of the experimental setup. (b) Measurements of the fluorescence intensity peak ratio Peak542 nm / Peak 527 nm

Conclusions and future work This is a very simple and fast (15 to 150 minutes reactor time) one-pot synthesis method.

Due to the fast and homogeneous microwave heating it is possible to obtain narrow nanoparticlesize distribution of 10 to 20 nm.

The nanoparticles form stable colloidal dispersions in isopropanol at concentrations as high as 2mg/ml.

The well-established alternative synthesis method in oleic acid/octadecene yields oleate-cappedparticles dispersible in non-polar solvents such as cyclohexane. To make nanoparticles water-dispersible, further processing such as ligand exchange and reverse-emulsion silica coatings arenecessary. In contrast, the particles synthesized by the method presented here can be coated byhydrolysis of silicates in isopropanol to obtain ultrathin coatings without nanoparticle aggregation.

• This particles can be used as very robust temperature sensors, since they do not depend o theintensity of the luminescence, but on a relative intensity of 542 nm and 527 nm peaks. Our resultssuggest a resolution of 1°C or lower is possible.

Figure 1: Stokes and anti-Stokes shifts. Fluorescein, left; quantumdots, center; upconverting nanoparticles, right. a) Full width at half maximum(FWHM). b) Overlap between excitation and emission at half maximum.

temperature to rise from 120°C to 315 °C in approximately 5 minutes. Themicrowave reactor was a CEM Discover using single mode and continuouspower at 2.45 GHz.

Table 1. Solvent properties.The high boiling point, good chemicalstability at high temperature, andmoderately polar character makes di-n-butyl-phthalate well suited for

Figure 2. Upconversionluminescence spectra andlifetime. Colloidalnanoparticle dispersions in 2-propanol at 5mg/ml wereexcited with a 500 mW, 980 nmlaser. The lifetimes of the green(540 nm) and the red (655 nm)emission were obtained byexciting the samples with10 mslaser pulses.

Acknowledgements. The authors thank the Luksic Grants Program (Chile), Notre Dame Office of the Vice President for Research, and the Ministry of Economy of Spain (MINECO FIS2013-41709-P) for funding; and Notre Dame Energy Materials Characterization Facility for the use of the microwave reactor, SEM, FTIR and XRD instrumentation.

The emitted light was focused on a Jobin-Yvon HR spectrometer, and detectedusing a photomultiplier and an averager oscilloscope. The lifetime of thesenanoparticles ranges from 200 to 150 μs at 540 nm, and from 320 to 250 μs at655 nm, and follows a decreasing trend with increasing synthesis time.

Characterization

Temperature sensing in microcapillaries

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DBP

References. Nüchter, M., et al. "Microwave assisted synthesis–a critical technology overview." Green chemistry 6.3 (2004): 128-141.Bilecka, Idalia, and Markus Niederberger. "Microwave chemistry for inorganic nanomaterials synthesis." Nanoscale 2.8 (2010): 1358-1374.Naccache, R., Yu, Q., & Capobianco, J. A. (2015). The Fluoride Host: Nucleation, Growth, and Upconversion of Lanthanide‐Doped Nanoparticles. Advanced Optical Materials, 3(4), 482-509.Liu, Baoxia, Hongliang Tan, and Yang Chen. "Upconversion nanoparticle-based fluorescence resonance energy transfer assay for Cr (III) ions in urine."Analytica chimica acta 761 (2013): 178-185.

Solvent Boiling point

1-octadecene 315 °C 2.1 oleic acid 350 °C 2.46 di-n-butyl-phthalate 340 °C 6.43

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