Instantaneous ballistic velocity of suspended Brownian nanocrystals measured by upconversion nanothermometry


Brownian motion is one of the most fascinating phenomena in nature(1,2). Its conceptual implications have a profound impact in almost every field of science and even economics, from dissipative processes in thermodynamic systems(3,4), gene therapy in biomedical research(5), artificial motors(6) and galaxy formation(7) to the behaviour of stock prices(8). However, despite extensive experimental investigations, the basic microscopic knowledge of prototypical systems such as colloidal particles in a fluid is still far from being complete. This is particularly the case for the measurement of the particles' instantaneous velocities, elusive due to the rapid random movements on extremely short timescales(9). Here, we report the measurement of the instantaneous ballistic velocity of Brownian nanocrystals suspended in both aqueous and organic solvents. To achieve this, we develop a technique based on upconversion nanothermometry. We find that the population of excited electronic states in NaYF4: Yb/Er nanocrystals at thermal equilibrium can be used for temperature mapping of the nanofluid with great thermal sensitivity (1.15% K-1 at 296 K) and a high spatial resolution (< 1 mu m). A distinct correlation between the heat flux in the nanofluid and the temporal evolution of Er3+ emission allows us to measure the instantaneous velocity of nanocrystals with different sizes and shapes.



subject category

Science & Technology - Other Topics; Materials Science


Brites, CDS; Xie, XJ; Debasu, ML; Qin, X; Chen, RF; Huang, W; Rocha, J; Liu, XG; Carlos, LD

our authors


We acknowledge V.S. Amaral for helpful discussion and Q. Sun for assistance with the nanorod synthesis. This work is supported by the CICECO-Aveiro Institute of Materials (FCTUID/CTM/50011/2013), financed by Portuguese funds through the FCT/MEC and when applicable co-financed by FEDER under the PT2020 Partnership Agreement. X.L. thanks the Agency for Science, Technology and Research (A*STAR) for support under contracts 122-PSE-0014 and 1231AFG028 (Singapore). W.H. is grateful for support from the National Basic Research Program of China (973, Grant 2015CB932200) and National Natural Science Foundation of China (61136003). C.D.S.B. and M.L.D. thank the FCT for postdoctoral research training (under grants SFRH/BPD/89003/2012 and SFRH/BPD/93884/2013).

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