Bias-free multiparametric luminescence sensing by a single upconverting particle

. Control over particles is key for bio-applications. Rotation dynamics analysis allows the medium characterization. An optically trapped and rotated microparticle is presented as sensor to characterize the properties of a liquid medium at the microscale..


Introduction
Sensing at the micro/nano scale is one of the most challenging topics that the scientific community is nowadays facing and are becoming increasingly important for applications spanning biomedical diagnosis, biosensing, environmental monitoring or nanostructure assembly.It should be ideally done in a contactless way for a twofold reason: a) to reduce or avoid the risk of cross-contamination; and b) to keep the external perturbation over the sample at minimum values.
In this context, biophotonics has emerged as a possible alternative to traditional methods.A variety of optical methods has been developed based on spectroscopy and microscopy approaches that exploit the light-matter interaction.Different types of inorganic micro/nanoparticles have raised great expectations over the traditional organic fluorophores because of their attractive optical and chemical features.Among the different particles used in biophotonics, upconverting particles have attracted great attention due to their capability of infrared-to-visible optical conversion through sequential multistep absorption of infrared photons.This makes possible the acquisition of high resolution and low background bioimages.One of the most commonly used upconverting particle for thermal sensing is NaYF4 doped with Er 3+ ions.The ratio of the relative intensities, from the radiative de-excitation of its thermally coupled states, is temperature-dependent on excitation since the emitted intensities are proportional to the population of the corresponding excited states.[1,2] Optical trapping has emerged as a reliable technique to achieve precise translation and rotational control over micro/nanostructures.It is a contactless technique that has been already used for long-term studies of single cells and bacteria.[3,4] Very recently, it has been demonstrated how single laser beams can not only trap but also induce rotation of birefringent upconverting particles.[5] The rotation dynamics of these microparticles has been found to be strongly dependent on the rotating mass and environmental conditions (e.g., viscosity, temperature, etc.) so that they have been introduced to the scientific community as a potential high-sensitivity sensors.[6,7] In this work, we have taken advantage of the sensing properties of NaYF4:Er,Yb micron-sized particles to develop a contactless temperature and viscosity sensor, by analyzing its emission spectrum.

Methods
The NaYF4:Er,Yb microparticles were synthesized by hydrothermal method and dispersed in D2O at a very low concentration to avoid multiple particle trapping.[5] The solution was introduced into a 120 µm height microchamber.A linearly polarized 808 nm, singlemode, fiber coupled diode laser was used as the optical excitation source.A quarter-wave plate placed afterwards converted the laser beam into circularly polarized light.Optical excitation at 808 nm was selected as it keeps at minimum the laser-induced thermal loading of the microspinner and surrounding medium.The NaYF4:Er,Yb microparticles used here show an intense visible emission when optically excited by 808 nm radiation thanks to a multiphoton excitation process already described elsewhere.[8] The visible emission spectra generated by our NaYF4:Er,Yb microparticles is collected by a spectrometer.

Result and discussion
The visible emission spectra, after 808 nm laser excitation, generated by our NaYF4:Er,Yb microparticles is shown in Figure 1  Finally, we want to demonstrate the capacity of the NaYF4:Yb 3+ , Er 3+ upconverting microparticle for local and remote sensing at the microscale by luminescence signal analysis.With that purpose, the dispersion was introduced in a microchannel and a single microparticle was trapped (Figure 2(a)).After that, a viscous solution was also introduced.The luminescent signal was monitored in real time, and the temperature and viscosity were obtained.Under our experimental conditions, the temperature remains constant during mixing (Figure 2

Conclusions
We demonstrate a novel strategy to achieve the precise in situ temperature and viscosity sensing during a mixture process based on a single upconverting microparticle, which allows to precisely monitor the local temperature and viscosity at the microscale.This study will be relevant for further analysis of different bio-systems or temperature-changing events such as reaction processes.This work has been partially supported by the Ministerio de Ciencia e Innovación de España (PID2019 105195RA I00, CNS2022-135495, and TED2021-129937B-I00). E.O.R gratefully acknowledges the financial support provided by the Spanish Ministerio de Universidades, through the FPU program (FPU19/04803) (a).The green emission band, centered around 540 nm, corresponds to the 4 S3/2 ( 2 H11/2) 4 I15/2 transitions of Er 3+ ions, and the red emission band, centered around 660 nm, to the 4 F9/2  4 I15/2 transitions of Er 3+ ions.The two excited states, 4 S3/2 ( 2 H11/2), are thermally coupled, their emission intensities will change as a function of temperature (Figure1(b)).On the other hand, the red emission band is polarized.It depends on the polarization direction of excitation.The intensity ratio, between the red peaks centered around 650 and 670 nm, varies with time, depending on the angular velocity of the microparticle, whose rotation depends on the viscosity of the liquid.[9]Figure1(c) shows the calibration of liquid viscosity as a function of the angular velocity

Fig. 1 .
Fig. 1.(a) Emission spectra of an optically trapped single microparticle when excited at 808 nm.The two spectra correspond to different detection angle (0º and 90º).(b) Temperature-dependent ratio between the two green bands.(c) Viscosity dependence as a function of the angular velocity, extracted from the red emission band.
(b)), and the medium viscosity has increased (Figure 2(c)) up to 5 mPas.

Fig. 2 .
Fig. 2. (a) Schematic representation of a trapped and rotated particle with an 808 nm laser.After three minutes, a viscous solution is introduced.b) Ratio between the green emissions and temperature evolution with time.c) Viscosity evolution with time.