Abstract

Constructing micro-/nanostructure-modulated photofields in upconversion devices to absorb low-energy photons and emit high-energy light is revolutionary for bioimaging, lasers, and photovoltaics, with proven capability to boost upconversion luminescence (UCL) by orders of magnitude. However, photoenergy dissipation and inadequate absorption result in excitation thresholds exceeding 1 mW/cm², which exceeds retinal safety limits and hinders wearable upconversion optics. Here, we report the use of upconversion core-shell microsphere-induced infrared field convergence, NaYF₄:Yb,Er shell-based resonant cavities for multiple reflection-absorption-upconversion and photonic crystal amplifiers to improve UCL intensity three orders of magnitude, and achieve ultralow threshold (0.0025 mW/cm²). The 500 nm upconversion core-shell microspheres generated 1200-fold stronger electric field through concentrated photofield and attained 8-fold infrared absorption with a forward/backward emission ratio of 150. Fabricated upconversion contact lenses significantly improved dark-light imaging clarity and vision restoration in retinal degeneration rabbits. Microsphere-mediated directional upconversion strategy maximizes photoenergy utilization, paving the way for high-performance wearable upconversion devices.

Introduction

Lanthanide-doped upconversion phosphors are capable of absorbing low-energy photons to emit high-energy photons through a nonlinear anti-Stokes process¹, providing immense value for applications in bio-imaging^2,3, optogenetics⁴, laser modulation⁵, solar cells⁶ and infrared vision^7,8. Typically, high luminous intensity upconversion phosphors are prepared by combining Yb³⁺ ions with large absorption cross sections as the sensitizers to absorb infrared light, Er³⁺ ions with multiple energy levels as the activators to emit visible light and low phonon energy NaYF₄ as the substrates⁹. However, the inherent low optical absorption and response of Yb- and Er-doped NaYF₄ make exciting upconversion require a powerful light source to enable adequate amounts of dopant jumping from the ground state to the excited state¹⁰. Multilayered core-shell structures^11,12,13,14, dye sensitization^15,16 and surface passivation^17,18 have been proposed to improve the sensitizer-to-activator energy transfer efficiency and address the high doping concentration quenching effect, improving the luminous intensity by several orders of magnitude, but still remain the challenge of high excitation thresholds. Recent studies demonstrated promising approaches to lower the excitation threshold by employing externally arranged nano- and micro- assembled structures, such as plasma-coupled nanocavities¹⁹, dielectric nanoantennas²⁰ and photonic crystal microspheres^21,22. Meng et al. achieved detected upconversion luminescence of individual upconversion nanoparticle (UCNP) under 0.45 W/cm² excitation via coupling plasmonic nanocavity²³. Liang et al. improved the luminous intensity up to 5 orders of magnitude by modulating the optical field of UCNPs through an array of dielectric microbeads, and realized imaging at 0.012 W/cm² infrared light²¹. However, these external enhancement units limited the application scenarios and adaptability of upconversion phosphors, and inevitably incurred optical scattering and photoenergy dissipation, which were detrimental to the efficient utilization of photons, especially in low-power stimulation.

Ma and co-workers inventively injected UCNPs for direct connection with retinal photoreceptor cells to empower mammals with infrared vision through converting invisible infrared light into visible light⁸. This proximity stimulation of photoreceptor cells diminished the energy loss during upconversion fluorescence transmission and effectively utilized the optical sensitivity of retinal rod cells, thereby reducing the upconversion luminescent photons required for infrared vision in the mammalian eyes. The approach showed the feasibility of upconversion-driven infrared vision under 1.62 mW/cm² infrared light, however, the light utilization efficiency was 10^-5 less than visible vision due to the quantum yield below 0.5% and its application was limited by injection surgical risks such as infection, inflammation, or retinal damage²⁴. Commonly, inadequate retinal photoreception induced dull vision or even night blindness under dark conditions^25,26. For certain irreversible retinal degenerative diseases in which partially functional retinal rod cells and photoreceptor neurons remain, it is desirable to realize revolutionary therapeutic strategies beyond gene and stem cell therapies^27,28. The development of enhanced absorption and more efficient upconversion materials is essential and urgent for the opportunities presented by visual restoration and night vision in mammals. Considering that light exposure above a power of 0.3 mW/cm² could impair the retina by thinning the outer nuclear layer (ONL) which contains photoreceptor cells^29,30, it is necessary to further lower the excitation threshold and improve the upconversion efficiency.

Here, we report a directional upconversion microsphere with consecutive β-phase NaYF₄:Yb, Er shell, which was incorporated into a photonic crystal contact lens amplifier to achieve an ultralow excitation threshold for night blindness treatment. Using upconversion microsphere (UCM) as the heart of this device, we developed an approach that exploited the resonant cavity based on the NaYF₄:Yb, Er shell and the optical convergence effect for directional upconversion enhancement. The core-shell structure is constructed with photoactive NaYF₄:Yb, Er exposed on the outside and inert SiO₂ as the inner core (Fig. 1a), as opposed to the conventional inert SiO₂ layer on the outside^17,31. This designed UCM achieved bright upconversion luminescence under low-power excitation by promoting Yb³⁺ absorbing infrared photons and transferring energy to enable sufficient quantity of Er³⁺ jumping to the excited state, thus increasing the radiative jump (Fig. 1a). The excitation spectrum exhibited broadband upconversion with substantially pure green light emission in the range of 800-1100 nm in the simulated solar spectrum (Fig. 1b). Moreover, as the particle size increased, finite-difference time-domain (FDTD) analysis of the UCMs revealed that 980 nm infrared light gradually to produce directionality and an enhanced photofield and electric field (Fig. 1c, Supplementary Fig. 1). The upconversion luminescence intensity of 500 nm UCM in the forward direction of the incident light is approximately 150 times greater than that in the backward direction (Fig. 1d) and three orders of magnitude greater than that of upconversion nanoparticles (Supplementary Fig. 2). To obtain a wearable efficient upconversion device, we designed an upconversion contact lenses (UCCL) with a 4-layer structure consisting of an antireflective layer (ARL), an upconversion luminescence layer (UCL) contained UCMs, and green and infrared light reflective layers (GRL and IRL) assembled from photonic crystals (Fig. 1e, Supplementary Fig. 3). Under the illumination of infrared light retained from AM 1.5 G (Supplementary Fig. 4), the light modulation of UCCL significantly improved the image clarity compared to a conventional contact lens, demonstrating powerful upconversion capability.

Fig. 1: Directional luminescence enhancement of upconversion core-shell microspheres and the application in a 4-layer upconversion contact lens.

Mechanistic investigation

SiO₂@NaYF₄:Yb,Er microspheres was conceptualized to increase the infrared absorption by lengthening the travel length of light within the shell through the reflection-absorption effect. As light travelled from the light-dense NaYF₄:Yb,Er shell³² into the light-sparse medium of SiO₂ core^33,34, the differential refractive indices and the shell shape induced multiple internal reflections (Fig. 2a). The NaYF₄:Yb, Er shell acts as both a resonant cavity and an upconversion layer, and the infrared absorbance of the 500 nm microspheres increased 8-fold over that of the nanoparticles due to the multiple reflection-absorption-upconversion effects (Fig. 2b). In addition, the long path of the infrared light within the upconversion shell, enhanced the interaction of the light with the photoactive ions (Yb³⁺ and Er³⁺) and improved the energy transfer efficiency between Yb³⁺ and Er³⁺ ions, as manifested by an increased fluorescence lifetime (Fig. 2c). This is the reason that UCM exhibited broadband absorption property under infrared light and significantly enhanced green luminescence by transferring more energy to the higher energy levels of ⁴I_11/2 and ⁴F_7/2 of Er and then improving the probability of the radiative jump of ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 (Fig. 2d). Surprisingly, the UCM exhibited a significant directional upconversion luminescence, especially under low power excitation (Fig. 2e). FDTD simulations revealed that the UCM accumulated infrared flux on one side of the microsphere to generate a strong electric field, which was enhanced by 1200-fold of 500 nm UCM compared to 40 nm UCNP along the direction of incident light (Fig. 1c, Supplementary Fig. 1). This strengthened electric field environment dramatically increased the incidence of electronic energy level radiative jumps of lanthanide atoms under low-power light, raising the enhancement factor up to three orders of magnitude (Fig. 2f). Moreover, we additionally synthesised β-phase NaYF₄: Yb,Er block with the size of about 500 nm, however, the luminous intensity is much lower than that of 500 nm UCM, confirming that the size effect was not the main reason for the upconversion enhancement (Supplementary Fig. 5). To integrate UCM into wearable device and further enhance the directional upconversion luminescence, we designed sandwich-shaped photonic crystal structures to increase the utilization efficiency of infrared light (Fig. 2g). The IRL with high reflectivity for infrared light at approximately 1000 nm facilitated secondary utilization of the incident infrared light by the UCL, and propagated the generated green light along the incident direction with 32% synergistic reflection of GRL (Fig. 2h). With the cooperative efforts of the upconversion core-shell microsphere-induced directional convergence, multiple reflection-absorption-upconversion by the NaYF₄:Yb, Er shell resonant cavity and photonic crystal amplifiers, 4L-UCCL achieves an ultralow threshold of 0.0025 mW/cm² at 980 nm excitation (Fig. 2i).

Fig. 2: Mechanism of upconversion luminescence enhancement and directionality.

Discussion

In conclusion, we demonstrated that microsphere-mediated directional photofield enhancement can achieve ultralow-threshold upconversion infrared vision. The designed upconversion core-shell microsphere with a consecutive β-phase dense NaYF₄:Yb, Er resonant cavity shell enables efficient infrared utilization through directional photofield convergence, the multiple reflection-absorption-upconversion effect and broadband absorption in the range of 800-1100 nm. Compared with UCNP, 500-nm UCM offers an 8-fold improvement in 980-nm absorption, with a directional emission index of 150. By integrating UCMs into a photonic crystal contact lens amplifier, the device achieves a record-low threshold (0.0025 mW/cm²) with a high quantum yield (9.4%) under 980 nm irradiation, which is 600 times lower and 30 times higher than those of the UCNPs, respectively. Moreover, we demonstrated that upconversion infrared vision substantially improved the clarity of dark-light imaging and significantly restored the impaired dark vision of the eyes of rabbits with retinal degeneration. This directional-enhanced upconversion strategy addresses the bottleneck of photoenergy dissipation and insufficient infrared absorption and provides a practical path for optimizing a variety of functionalities for low-threshold wearable devices. Meanwhile, there is further potential to improve luminescence intensity and modulate fluorescence color by designing the optically inert shell, multiple core-shell structure and chemical composition. Besides optical imaging systems, we anticipate that the efficient utilization of light energy is also applicable for increasing the photovoltaic efficiency of solar cells and treating incurable diseases such as retinal degeneration via directional photostimulation.

Methods

Materials

Y(NO₃)₃·6H₂O (99.9%), Yb(NO₃)₃·5H₂O (99.99%), Er(NO₃)₃·5H₂O (99.9%), NaF (99.99%), ethylene glycol, polyethylene glycol 400, tetraethyl orthosilicate, ethanol, hydroxyethyl methacrylate (HEMA; 99%), methyl methacrylate (MMA; 99.5%), poly(ethylene glycol)methyl ether methacrylate (PGMEMA; Mn~475) and 2,2’-azobis(2,4-dimethyl)valeronitrile (ABVN; >98%) were supplied by Aladdin Chemistry Co., Ltd. Hollow silicon dioxide was obtained from Ningbo Tegran Technology Co., Ltd. Ethylene glycol dimethacrylate (EGDMA; >98%) was purchased from Sigma-Aldrich. HEMA, PGMEMA and MMA were used after further purification through vacuum distillation, and ABVN was recrystallized.

Synthesis of upconversion core-shell microspheres

We prepared a series of SiO₂ microspheres ( ~ 200, 300, 400 nm) via the classical Stöber method⁴⁴ as follows: 170 mL of anhydrous ethanol, 4.5 mL of deionized water and ammonia were mixed and heated for 15 min at 30 °C in a water bath, and silica sphere emulsion was obtained by dropwise addition of tetraethyl orthosilicate for a 12 h reaction. Silica spheres powder was obtained by freeze drying and well ground by using the mortar (Supplementary Fig. 13). An upconversion shell with a chemical composition of NaYF₄:Yb,Er was synthesized via a hydrothermal method, substituting ethylene glycol instead of a portion of water as the solvent. A total of 2.5 mL of an aqueous solution of 0.4 M 78% Y(NO₃)₃, 20% Yb(NO₃)₃, and 2% Er(NO₃)₃, 3 mL of deionized water, 10 mL of ethylene glycol and 16 mL of polyethylene glycol-400 were mixed with stirring for 0.5 hours at room temperature, forming solution A. SiO₂ microspheres were added to solution A and dispersed uniformly via a mechanical ultrasonic probe, forming solution B. 300 mg NaF was dissolved in 10 mL of deionized water and 10 mL of ethylene glycol to form solution C, and then, solution C was added dropwise into solution B under vigorous stirring. After aging for 2 h, the mixture was transferred to a Teflon-lined autoclave, sealed and heated at 200 °C for 12 h. As the autoclave was naturally cooled to room temperature, the precipitate was collected in a bottle and washed with ethanol three times. The products were centrifuged and dried in air at 70 °C for 10 h.

Preparation of a 4-layer upconversion contact lens

Substratal acrylate solution was mixed at a weight ratio of HEMA:PGMEMA:EGDMA:ABVN = 10:1:0.3:0.05. Hollow silicon dioxide, 124-nm PHEMA-MMA microspheres (synthesis details in the Supplementary Information), 500-nm UCM, and 420-nm SiO₂ were added at weight ratios of 10%, 30%, 5%, and 33%, respectively, and dispersed equably to the acrylate mixture for the preparation of ARL, GRL, UCL and IRL of the 4-layer upconversion contact lens. Through integral-molding routes, UCL was synthesized and placed in the middle, as shown in Fig. 3e, and acrylate monomer dispersions of PHEMA-MMA microspheres and SiO₂ microspheres were added dropwise to the lower and upper layers. Lamination was carried out by molding and heat curing at 60 °C for 4 h, enabling the microspheres to be assembled in an orderly manner as the GRL and IRL. Finally, the hollow SiO₂ disperse mixture was dropped onto the surface of the GRL, cured under 365 nm UV light to form the ARL and fully solidified in the mold at 100 °C.

Characterization and optical properties

The structural morphologies were obtained via a field emission scanning electron microscope (JSM-7600F, JEOL Ltd., Japan), a field emission transmission electron microscope (Talos-F200X, FEI, Netherlands) and an ultradepth-of-field microscope (EasyZoom5 3D, Ltd., China). The crystal structure of UCM was obtained via an X-ray diffractometer (Philips, X’pert3 powder, Cu Kα, λ = 1.5406 Å). The UV-Vis reflectivity and absorptivity were recorded with a UV 1801 spectrophotometer (SolidSpec-3700, SHIMADZU, Japan). Microsphere sizes were measured by a laser particle size analyzer (Zetasizer Nano ZS90, Malvern, UK). Photoluminescence (PL), PL decay spectra and the PL quantum yield (PLQY) were measured via a QuantaMaster 8000 (HORIBA Scientific, Canada) with an integrating sphere.

Finite-difference time-domain (FDTD) analysis

We simulated the photic and electric fields of upconversion core-shell microspheres with the software package Ansys Lumerical FDTD Solutions. The SiO₂ core and NaYF₄:Yb, Er shell were modeled with constant refractive indices of n = 1.46 and 1.48, respectively. The diameter of the core ranged from 200 to 400 nm, and the thickness of the shell was 50 nm. The incident light was set at 980 nm in the same direction. The photic and electric field intensities around the microspheres were recorded with a photic and electric field monitor.

In vivo pupillary light reflex of rabbits

In vivo tests were conducted on the basis of the guidelines of the National Institutes of Health for the care and use of laboratory animals and with the approval of the Institute of Animal Care and Use Committee of Hubei Innovation and Transformation Medical Research Institute Co., Ltd. (TJYY23100B). The Institute of Animal Care and Use Committee of Hubei Innovation and Transformation Medical Research Institute Co., Ltd. was the ethics review committee. Three male New Zealand white rabbits (three months aged) were head fixed and sedated with Zoletil 50 (10 mg/kg; Virbac, Carros, France) after their pupils were dilated with atropine (100 μg/mL, Sigma-Aldrich). With the right eye of the rabbit completely occluded, the left eye was propped open with a pupil dilator and saline drops were added at 10-minute intervals to keep the cornea moist. A halogen light source (100 W, 64637 HLX, OSRAM GmbH.) was placed 10 cm from the cornea and the left eye was irradiated for 120 min, then the rabbits were transferred to a dark environment for 1 h and returned to the animal room for rearing. The pupillary light reflex (PLR) was used to measure light perception in rabbit eyes. All the PLR experiments were performed during the day with 4-hour dark adaptation, after which the rabbits were maintained on 12/12-hour light/dark cycles. Contact lenses were sterilized by the moist heat method before wearing. To measure the PLR of the rabbit eye, we built a pupillometer with an infrared CCD camera for video recording. A 532 nm LED (0.2 mW) and a 980 nm LED (2 mW) with attenuation pieces were placed 3 cm away in front of the eye. Videos of the experimental and control eyes were recorded via the same experimental method, with 5 min intervals between each light stimulus. The pupil area was calculated via graphics processing software (ImageJ, National Institutes of Health).

In vivo electroretinography of rabbits

Three male New Zealand white rabbits (three months aged) were anesthetized with Zoletil 50, and the hair on the skull and ears of each rabbit was removed to attach the electrodes after 12 hours of dark adaptation. During the experiment, the anesthetized rabbits were placed into a Faraday cage, and a disposable electrode for electroretinography (ERG-Jet, Fabrinal, Switzerland) with a tip diameter of 12 mm was placed tightly against the center of the cornea. A 532 nm laser and a 980 nm laser with 10,000x and 1000x attenuation pieces were placed in front of the pupil for stimulation. The interval between each stimulation test was 15 minutes. Data acquisition was carried out via a visual electrodiagnostic system (LKC Technologies, Inc.). The data were analyzed via custom routines in Origin 9.1 (Origin Lab Corp.). ERG b-wave amplitudes were measured for every recording and averaged from the three animals of each experimental group.