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Optical tweezers beyond refractive index mismatch using highly doped upconversion nanoparticles


Optical tweezers are widely used in materials assembly1, characterization2, biomechanical force sensing3,4 and the in vivo manipulation of cells5 and organs6. The trapping force has primarily been generated through the refractive index mismatch between a trapped object and its surrounding medium. This poses a fundamental challenge for the optical trapping of low-refractive-index nanoscale objects, including nanoparticles and intracellular organelles. Here, we report a technology that employs a resonance effect to enhance the permittivity and polarizability of nanocrystals, leading to enhanced optical trapping forces by orders of magnitude. This effectively bypasses the requirement of refractive index mismatch at the nanoscale. We show that under resonance conditions, highly doping lanthanide ions in NaYF4 nanocrystals makes the real part of the Clausius–Mossotti factor approach its asymptotic limit, thereby achieving a maximum optical trap stiffness of 0.086 pN μm–1 mW–1 for 23.3-nm-radius low-refractive-index (1.46) nanoparticles, that is, more than 30 times stronger than the reported value for gold nanoparticles of the same size. Our results suggest a new potential of lanthanide doping for the optical control of the refractive index of nanomaterials, developing the optical force tag for the intracellular manipulation of organelles and integrating optical tweezers with temperature sensing and laser cooling7 capabilities.

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Fig. 1: Comparison of optical trapping of low-refractive-index nanoparticles with or without doping by lanthanide ions.
Fig. 2: Investigation of ytterbium, erbium and neodymium doping in enhancing the optical gradient force.
Fig. 3: Effect of oscillating ion concentration on optical trapping.
Fig. 4: Trap stiffness measurements of lanthanide-doped nanoparticles of different volumes.
Fig. 5: Escape velocity measurements to quantify the trap stiffness for HeLa cells with and without lanthanide-doped nanoparticles.

Data availability

The data that support Figs. 25 can be found in the Source Data, and the data that support the other findings of this study are available within the article and its Supplementary Information. Additional data are available from the corresponding author upon request. Source data are provided with this paper.

Code availability

All custom code is available from the corresponding author upon request.


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The authors thank L. Zhang for polymer synthesis. The authors acknowledge financial support from a UTS Chancellor’s Postdoctoral Research Fellowship (PRO18-6128), an Australian Research Council (ARC) DECRA fellowship (DE200100074, F.W.), the ARC Discovery Project (DP190101058, F.W), the National Natural Science Foundation of China (NSFC, 61729501), the Major International (Regional) Joint Research Project of the NSFC (51720105015), the Science and Technology Innovation Commission of Shenzhen (KQTD20170810110913065) and the Australia–China Science and Research Fund Joint Research Centre for Point-of-Care Testing (ACSRF658277, SQ2017YFGH001190). X.S., D.W., X.D., C.C., J.L., Y.L. and L.D. acknowledge the financial support from China Scholarship Council scholarships (X.S., 201708200004l; D.W, 201706170027; X.D., 201706170028; C.C., 201607950009; J.L., 201508530231; Y.L., 201607950010; L.D., 201809370076).

Author information




F.W. and D.J. conceived the project and designed the experiments. X.S., F.W., C.C., Y.L., and L.D. constructed the optical setup and performed the optical experiments. F.W., P.J.R. and Y.L. built the theoretical simulation and analytical model. S.W. and J.L. synthesized the nanoparticles. D.W. and X.D. conducted the cell biology experiments. X.S., P.N. and F.W. developed the trap stiffness detection method. F.W., X.S., P.J.R. and D.J. analysed the results, prepared the figures and wrote the manuscript. D.J. and F.W. supervised the project.

Corresponding authors

Correspondence to Fan Wang or Peter J. Reece or Dayong Jin.

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The authors declare no competing interests.

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Peer review information Nature Nanotechnology thanks Onofrio Marago and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

Supplementary Discussion, Figs. 1–23 and Tables 1–9.

Source data

Source Data Fig. 2

Statistical source data.

Source Data Fig. 3

Statistical source data.

Source Data Fig. 4

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Source Data Fig. 5

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Shan, X., Wang, F., Wang, D. et al. Optical tweezers beyond refractive index mismatch using highly doped upconversion nanoparticles. Nat. Nanotechnol. (2021).

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