Supplementary Information

A Flexible and Superhydrophobic Upconversion-Luminescence-Membrane as

an Ultrasensitive Fluorescence Sensorfor Single Droplet DetectionKuichao Liu,a Zhenyi Zhang,a* Chongxin Shan,b, c Zhiqing Feng,a Jiasu Li,a Chunlei Song,a Yanan

Bao,a Xiaohui Qi,a and Bin Donga*

aKey Laboratory of New Energy and Rare Earth Resource Utilization of State Ethnic Affairs Commission, Dalian Nationalities University, 18 Liaohe West Road, Dalian 116600, P. R. ChinabState Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, ChinacSchool of Physics and Engineering, Zhengzhou University, Zhengzhou 450001, China.

Corresponding author: zhangzy@dlnu.edu.cn; Tel: +8641187658872; Fax: +8641187658872;dong@dlnu.edu.cn; Tel: +8641187556959; Fax: +8641187556959.

Figure S1. High magnification TEM image of NaYF4:Yb3+, Tm3+/PS hybrid nanofiber.

Figure S2. (A) SEM image of the as-electrospunNaYF4:Yb3+, Er3+/PS hybrid nanofibers; (B) TEM image of an individual NaYF4:Yb3+, Er3+/PS hybrid nanofiber; (C) XRD pattern of the as-fabricated samples NaYF4:Yb3+, Er3+/PS hybrid nanofibers; (D) SAED pattern of an individual NaYF4:Yb3+, Er3+/PS hybrid nanofiber.

Figure S3. Transmittance spectrum of the pure PS film.

Figure S4. Excitation power density dependence of UC emissions of the as-synthesized samples: (A)NaYF4:Yb3+, Er3+ nanoparticles; (B) NaYF4:Yb3+, Er3+/PS HFM; (C) NaYF4:Yb3+, Tm3+

nanoparticles; (D) NaYF4:Yb3+, Tm3+/PS HFM.

In order to study the UC mechanism for the Ln3+-doped NaYF4 nanoparticles after coating

the PS, the number of photons (n) involved in the UC process was investigated in Figure S4. In

the case of NaYF4:Yb3+, Er3+/PS hybrid membrane, the n values for green and red emissions

decrease as compared to the pure NaYF4:Yb3+, Er3+nanoparticles.[1] On the other hand, compared

with those in pure NaYF4:Yb3+, Tm3+nanoparticles, the n value for blue emission decreases, but

for red emission, n increases in NaYF4:Yb3+,Tm3+/PS hybrid membrane.[2] The changes for n

values suggest that the UC mechanism of Ln3+-doped NaYF4 nanoparticles embedded in the

polymer PS is different from that of pure Ln3+-doped NaYF4 nanoparticles due to the interaction

between the Ln3+-doped NaYF4nanoparticles and PS.

[1] P. Zou, X. Hong, Y. Ding, Z. Zhang, X. Chu, T. Shaymurat, C. Shao, Y. Liu, J. Phys. Chem. C 2012, 116, 5787.[2] F. Vetrone, V.Mahalingam, J. A.Capobianco, Chem. Mater 2009, 21, 1847.

Figure S5. (A) Fourier transform infrared (FT-IR) spectra of the as-electrospun (a) PS fibrous membrane, (b) NaYF4:Yb3+, Er3+/PS hybrid fibrous membrane, and (c) NaYF4:Yb3+, Tm3+/PS hybrid fibrous membrane; (B) Schematic diagram of the energy transfer between Ln3+ co-doped NaYF4 nanoparticles and PS nanofiber; upconversion mechanisms of (C) NaYF4:Yb3+, Tm3+/PS hybrid fibrous membrane and (D) NaYF4:Yb3+, Er3+/PS hybrid fibrous membrane.

FT-IR spectra of the as-electrospun PS FM, NaYF4:Yb3+, Er3+/PS HFM, and NaYF4:Yb3+,

Tm3+/PS HFM are shown in Figure S5. It can be seen that the PS FM exhibiting the vibration

peaks at 754, 1451, 1490, 1600, 3024, and 3060 cm-1 correspond to the phenyl group, the peaks

at 2850 and 2923 cm-1 to the methenyl and methylene groups, respectively, and the peak at 1028

cm-1 to the C-phenyl bond.[3] After embedding the UCLNPs into the PS fibrous matrix, small part

of these characteristic peaks show tiny changes in profile, which may be attributed to the weak

interactions between PS molecules and UCLNPs occurring in the thermal dissolution process of

PS in DMF solution. Under 980-nm excitation, the relaxation energies of some neighboring

levels in the UCLNPs may match the above characteristic vibration transitions of PS, which

could lead to the changes in some populating channels through nonradiative energy transfer from

the excited UCLNPs to the adjacent PS.[4] In the case of NaYF4:Yb3+, Er3+/PS HFM, the

vibrational absorption peaking around 3000 cm-1 match the relaxation energy from 4I11/2to4I13/2 of

Er3+ ions. Thus, the phenyl group in the PS may absorb this relaxation energy to reduce the

population of 4F9/2 from the 980-nm-excited 4I13/2, resulting in the decreased red-emission from

the 4F9/2-4I15/2 of Er3+ ions.[1] That is why the intensity ratio of green to red emissions was

increased after coating the PS onto NaYF4:Yb3+, Er3+ NPs surface in our work. For the

NaYF4:Yb3+, Tm3+/PS HFM, the relaxation energy from 3H5 to 3F4 of Tm3+ ions(~2700 cm-1) may

match the vibration transition of methenyl or methylene groups(~2850 cm-1), leading to the

depopulation on 3F2, 3 transferred from 3F4. Thus, the 3F2-3H6, and 3F3-3H6 transitions of Tm3+ ions

were decreased as compared to pure NaYF4:Yb3+, Tm3+ NPs. Meanwhile, the nonradiative

relaxation process from 3F2 to 3H4 of Tm3+ ions (~1680 cm-1) may be also weakened due to its

matching energy with the vibration transition of phenyl group peaking around 1600 cm -1.[2]

Accordingly, the population on 1G4, 1D2, and 1I6 would be decreased, which therefore leads to the

emission decreases of 1I6-3F4, 1D2-3H6, 1D2-3F4, and 1G4-3H6in our work.

[3] P. Muthiah, S-H. Hsu, W. Sigmund, Langmuir 2010, 26, 12483.[4] B. Dong, H. Song, H. Yu, H. Zhang, R. Qin, X. Bai, G. H. Pan, S. Lu, F. Wang, L. Fan, Q. Dai, J. Phys. Chem. C 2008, 112, 1435.

Figure S6. Normalized UC emission spectra of the NaYF4-based nanoparticles dispersed in the aqueous solution containing different kinds of fluorescence molecule: (A) NaYF4:Yb3+,Tm3+

nanoparticles in avidin solution; (B)NaYF4:Yb3+, Er3+ nanoparticles in RhB solution.

Figure S7. Normalized UC emission spectra of NaYF4:Yb3+, Tm3+/PS HFM loading with a single water droplet containing different concentration of RhB.