Upconverted excitation energy lock-in for deep- ultraviolet enhancement

Qianqian Su E401C, 99 Shanghai University, Baoshan https://orcid.org/0000-0002-8706-6760 Han-Lin Wei Shanghai University Shuai Wang Shanghai University Chaohao Chen Institute for Biomedical Materials and Devices (IBMD), Faculty of Science, University of Technology Sydney, NSW 2007 https://orcid.org/0000-0003-4620-7771 Guan Ming Southern University of Science and Technology Yan Su Genome Institute of Singapore Haifang Wang Shanghai University Zhigang Chen Donghua University Dayong Jin (  dayong.jin@uts.edu.au ) University of Technology Sydney https://orcid.org/0000-0003-1046-2666


Introduction
Multiphoton upconversion processes that convert NIR excitation into visible emissions have attracted considerable attention owing to broad technical applications of anti-Stokes shifts [1][2][3][4][5][6] . Though UV upconversion luminescence can be a powerful tool for applications in biomedical [7][8][9] , environmental 10,11 , and industrial elds 12,13 , their practical implementations have been hindered by low emission intensities and di culties in achieving large shifts into the deep-UV region. NIR-to-deep UV upconversion is signi cantly in uenced by many deleterious factors, such as concentration quenching, surface quenching, cross-relaxation between lanthanide ions, and competitive energy harvesting from low-lying energy levels.
Attempts have been made to enhance the emission intensity in the UV range, for instance, by controlling the particle phase and size 14 , the pulse width of excitation beams 15 , dopant composition 16 , and nanoparticle core-shell structures 12,[17][18][19] . However, severe cross-relaxation between lanthanide ions and unwanted energy consumption by interior and surface quenchers drastically depopulates the excited states at high-lying emitting levels, thus mitigating upconverted UV emission 20 .
Deep-UV upconversion emission is particularly useful for drug release in deep tissues, since NIR excitation penetrates deeper through tissue than visible light and localized UV generation can trigger drug release with high spatial and temporal precisions [21][22][23][24][25] . Compared with Yb 3+ -sensitized upconversion nanoparticles (UCNPs), Nd 3+ -sensitized UCNPs offer deep penetration depths and minimal over-heating effect, owing to low coe cients of water absorption under 800-nm excitation 26 . Despite enticing prospects, deep-UV emission from Nd 3+ -sensitized UCNPs has been challenging because of densely packed excited states of Nd 3+ and dominant cross-relaxation within the nanoparticle systems 27,28 .
Our mechanistic investigation reveals upconverted excitation lock-in (UCEL) mode in which Gd 3+sensitized excitation energy can be retained by using an optical inert NaYF 4 interlayer. This nanostructure preserves the upconversion energy within the core domain and effectively suppresses energy dissipation by interior traps, enabling six-photon-upconverted UV emission at 253 nm under 808-nm excitation.

Results
Heterogeneous nanostructural design. In our experiment, we designed a heterogeneous core-multishell structure to suppress surface quenching and achieve tunable emissions. In a conventional design 29,30 , under 808-nm excitation, Nd 3+ sensitizers harvest excitation photons and subsequently pass them to Yb 3+ ions with an excited state at ~ 10,000 cm -1 . Energy migration through a network of high concentration Yb 3+ ions promotes back-energy transfer of the NIR excitation to Tm 3+ emitters with ladderlike metastable intermediate states, facilitating sequential upconversion processes from NIR to visible/UV. By doping of Gd 3+ , upconverted UV emission from high-lying states of Tm 3+ can be further transferred to Gd 3+ ions as deep-UV energy reservoirs.
The key to our design is the optical inert NaYF 4 layer locating in the rst shell layer of NaGdF 4 :Yb,Tm@ NaGdF 4 :Yb@NaGdF 4 :Yb,Nd@NaGdF 4 (Gd-CS Gd S 2 S 3 ) nanoparticle (Scheme 1). This inert layer can lock-in the upconverted excitation energy of Gd 3+ ions. The Gd 3+ network can reuse the upconverted excitation energy and prevent depopulation by deleterious energy traps within the nanoparticles. The NaYF 4 layer plays a key role in interdicting detrimental energy transfer between Gd 3+ and interior traps, enhancing veand six-photon-upconverted UV emissions.
Upconverted excitation lock-in (UCEL) mode. The UCEL mode requires both an optical inert NaYF 4 interlayer and a network of Gd 3+ ions to recycle upconversion energy for deep-UV emission ampli cation. Fig. 1 illustrates a typical upconversion process in heterogeneous, core-multishell nanoparticles upon 808-nm excitation. In brief, 808 nm photons are rst sensitized by Nd 3+ sensitizer ions, being populated at the 4 F 5/2 energy state and quickly relaxed to the 4 F 3/2 energy state of Nd 3+ . The excited Yb 3+ ions serve as an energy migrator to populate the 3 P 2 state of Tm 3+ through a ve-photon upconversion process. The 6 D J state of Gd 3+ is further populated by the appropriate energy matching of the following transitions of Tm 3+ : 3 P 0,1 → 1 D 2 (~7600 cm -1 , ~8200 cm -1 ), 3 F 2,3 → 3 H 5 (~6700 cm -1 , ~6200 cm -1 ), and 1 G 4 → 3 H 4 (~8600 cm -1 ), via an energy transfer process 31 . However, the probability of nonradiative relaxation of 6 I J → 6 P J is larger than that of the radiative transition of 6 I J → 8 S 7/2 , resulting in an e cient population of the 6 P 7/2 state, commonly observed in Gd-based homogeneous nanostructures 18 . In our design, the NaYF 4 -based rst shell layer selectively blocks the energy transfer from Gd 3+ to interior energy traps (e.g., lattice defects and impurities). It preserves and recycles the excitation energy within the core region, leading to increased populations in the 6 D J , 6 I J , and 6 P J states of Gd 3+ and intense UV emission of Gd 3+ .
Controlled synthesis. We used a layer-by-layer epitaxial growth method 20 to synthesize a batch of Gd-CS Y S 2 S 3 nanoparticles with optimized concentrations of co-dopants 30 following the design of NaGdF 4 :49%Yb,1%Tm@NaYF 4 :20%Yb@NaGdF 4 :10%Yb,50%Nd@NaGdF 4 (Fig. 2a). Transmission electron microscopy (TEM) images of obtained Gd-CS Y S 2 S 3 nanoparticles show the average size of ~29 nm with each layer ~2.5 nm in thickness ( Supplementary Fig. S1). High-resolution TEM shows the singlecrystalline structure of the as-synthesized core-multishell nanoparticles ( Fig. 2b inset), and X-ray powder diffraction result (XRD, JCPDS le number 27-0699, Supplementary Fig. S2) con rms the hexagonal phase of the as-prepared nanoparticles. High-angle annular dark eld scanning TEM identi ed the formation of the heterogeneous core-multishell structures (Fig. 1b), in which the brighter regions correspond to heavier elements (Gd, Yb, and Nd) and the darker parts correspond to lighter ones (Y).
We further studied the excitation power dependence of luminescence intensity from higher-lying 6 D J , 6 I J and 6 P J excited states of Gd 3+ (Fig. 2f). The number of photons (n) required to populate the upper emitting state can be calculated by the luminescence intensity I f , and the pump power of laser P following relation of I f ∝P n32 . The output slope for 253 nm emission band was calculated as 5.92, indicating that six 808 nm photons needed to populate the 6 D J level, following a six photon upconversion process (Fig. 2g), while n values obtained for 276 and 311 nm emissions were 5.07 and 5.09, indicating ve-photon processes (Fig. S13). Gd 3+ energy recycling above 6 P J . To probe the role of NaYF 4 layer in locking-in and recycling Gd 3+ excitation energy, we have compared the excited state lifetime of Gd 3+ . As shown in Fig. 3 and Supplementary Fig. S14, a signi cant prolonged (~4 times) lifetime of Gd 3+ emission from the 6 P 7/2 level was achieved when the NaYF 4 rst layer was applied. In contrast, there were negligible changes in the Gd 3+ lifetimes for emissions from 6 D J and 6 I J energy levels, indicating the energy loss from Gd 3+ to interior energy traps was mainly through 6 P 7/2 energy level of Gd 3+ due to small energy gap between 6 D J , 6 I J and 6 P J (Supplementary Fig. S15). In addition, the emission intensities of Nd 3+ at 893 nm ( 4 F 3/2 → 4 I 9/2 ), 1057 nm ( 4 F 3/2 → 4 I 11/2 ), and 1330 nm ( 4 F 3/2 → 4 I 13/2 ) and Tm 3+ at ~1460 nm ( 3 H 4 → 3 F 4 ) in the near-infrared range were essentially unaltered ( Supplementary Fig. S16). These results indicate that the NaYF 4 -assisted UCEL mechanism favors the upconversion emissions from high-lying energy levels.
The role of the rst layer of NaYF 4 shell. To further verify the role of NaYF 4 layer in enhancing the deep-UV emissions, we synthesized a group of Gd-CS Y S 2 S 3 , Gd-CS 1 S Y S 3 and Gd-CS 1 S 2 S Y heterogeneous nanoparticles, in which NaGdF 4 was selectively replaced by NaYF 4 host lattice in the rst, second and third layer, respectively (Fig. 4a). The intense deep UV emission was only observed in Gd-CS Y S 2 S 3 nanoparticles. The emission pro les of Gd-CS 1 S Y S 3 and Gd-CS 1 S 2 S Y were quite similar to Gd-CS Gd S 2 S 3 nanoparticles. Moreover, when the optically inert Y 3+ ions in the rst layer were replaced by half of the Gd 3+ ions, a drastic reduction of the Gd 3+ emission was observed, indicating that the optical inert NaYF 4 layer can effectively prevent the back energy transfer from Gd 3+ (Supplementary Fig. S17).
We further prepared a group of Gd-CS Y S 2 S 3 nanoparticles doped with Tb 3+ or Eu 3+ ions in the rst layer (Gd-CS Y-15%Tb S 2 S 3 or Gd-CS Y-15%Eu S 2 S 3 ), which can extract the excitation energy from Gd 3+ to emit green and red upconversion emissions through the scheme of energy migration upconversion (EMU) 18 . Upon excitation at 808 nm, the characteristic emissions of Tb 3+ and Eu 3+ (highlighted in color) were observed ( Fig. 4b-c and Supplementary Fig. S18), but no enhancement in deep-UV emission. Doping with 15% Tb 3+ or Eu 3+ in the outmost layer only led to weak emission of Tb 3+ or Eu 3+ (Supplementary Fig. S19). The weak Tb 3+ and Eu 3+ emissions were attributed to the interior energy trapping of the excitation energy in the Gd 3+ sublattice. Together, these results indicate that an e cient energy transfer pathway (Nd 3+ →Yb 3+ →Tm 3+ →Gd 3+ ) occurs 33 , and the excitation energy of Gd 3+ can be easily dissipated through the emission of Tb 3+ , Eu 3+ , or interior traps without the NaYF 4 rst-shell layer.
Determination of the interior traps. An e cient energy transfer can occur between Gd 3+ and Nd 3+ ions 34 .
However, in our design, the energy transfer between these two ions did not happen. To preclude the possibility of the interior Nd 3+ energy trapping, we prepared a series of Gd-CS Gd S 2 S 3 nanoparticles with and without Nd 3+ dopant (Gd-CS Gd S 50%Nd S 3 and Gd-CS Gd S 0%Nd S 3 ). The lifetimes of Gd 3+ ( 6 D J , 6 I J , 6 P J ) and Tm 3+ ( 1 I 6 , 1 D 2 ) were virtually unchanged after removing Nd 3+ dopants in nanoparticles ( Fig. 4d and Supplementary Fig. 20). These results con rm that intense deep-UV emission from Gd 3+ is enabled by obstructing the energy transfer from Gd 3+ to interior lattice defects or impurities.
Furthermore, we compared the amount of light-to-heat conversion in Gd-CS Y S 2 S 3 and Gd-CS Gd S 2 S 3 nanoparticles by using an infrared thermal imaging camera. As a higher concentration of Nd 3+ in nanoparticles would generate more heat under single-beam infrared laser excitation 35 , we measured the concentrations of Nd 3+ in these two types of nanoparticles (Supplementary Table 1). The measured temperature rises of the solution of Gd-CS Y S 2 S 3 and Gd-CS Gd S 2 S 3 are 5.0 o C and 7.2 o C under irradiation at 808 nm light, 1.9 o C and 3.2 o C under irradiation at 980 nm light, respectively (Fig. 5). These results suggest that less excitation energy is converted to lattice heating in heterogeneous core-multishell structures than in conventional nanoparticles. Enhancement in highly doped single nanoparticles. To further evaluate UCEL mode in enhancing the highorder upconversion emissions in the heterogenous core-multishell structures, we implemented the similar design in the highly doped UCNP core, e.g. Gd-C 8%Tm S Y S 2 S 3 and Gd-C 8%Tm S Gd S 2 S 3 , and quantify the brightness of single UCNPs using a purpose-built confocal microscopy system (see Supplementary Fig.  S21). Due to the signi cant UV absorption by the optical components, including the objective lens and mirrors, instead of a direct quanti cation of the deep UV emissions at a single nanoparticle level, we monitored the amount of the blue band emissions from a single nanoparticle. Under the same excitation power from both 808 nm and 976 nm lasers, the emission intensities of Gd-C 1%Tm S Y S 2 S 3 and Gd-C 1%Tm S Gd S 2 S 3 nanoparticles under the 808 nm excitation were ~4 times and ~5 times higher than those under the 976 nm excitation, respectively ( Supplementary Fig. S22). In contrast, much higher enhancement factors of the highly doped Gd-C 8%Tm S Y S 2 S 3 (~25 times) and Gd-C 8%Tm S Gd S 2 S 3 (~15 times) nanoparticles were achieved under the 808 nm v.s. 976 nm excitations. These results suggest UCEL mode could be broadly applied to a variety of UCNP core concentrations 36

Discussion
In this study, we demonstrated an UCEL approach through the core-multishell heterogeneous structure design to regulate the energy transfer pathway in lanthanide-doped UCNPs for deep UV generation by 808 nm excitation. The key to this design is the utilization of an optical inert NaYF 4 interlayer between multiple cascade NIR photon sensitization shells and emitting core. Therefore, the sensitized NIR excitation energies can be transferred inbound and upconverted at the core domain of NaGdF 4 :Yb,Tm, where high-concentration Gd 3+ ions can recycle among the higher-lying excited energy states above 6 P J to realize intense deep UV upconversion emissions. We believe this approach will advance the design rationale for enhancing the NIR sensitized deep UV upconversion emissions towards the potential areas of biomedicine, information technology, photocatalysis, environmental science and many other emerging elds.
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Supplementary Files
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