Dynamic upconversion multicolour editing enabled by molecule-assisted opto-electrochemical modulation

Controlling nonlinear optical signals electrically offers many opportunities for technological developments. Lanthanide-activated nanoparticles have recently emerged as leading platforms for nonlinear upconversion of infra-red excitation within nanometric volumes. However, manipulation of upconversion emission is restricted to varying percentages of component materials, nanocrystal structure, and optical pumping conditions. Here, we report temporal modulation of anti-Stokes luminescence by coupling upconversion nanoparticles with an electrochemically responsive molecule. By electrically tailoring orbital energy levels of the molecules anchored on nanoparticle surfaces, we demonstrate reversible control of molecular absorption, resulting in dynamic colour editing of anti-Stokes luminescence at single-particle resolution. Moreover, we show that a programmable logic gate array based on opto-electrochemical modulation can be constructed to convert information-encrypted electrical signals into visible patterns with millisecond photonic readout. These findings offer insights into precise control of anti-Stokes luminescence, while enabling a host of applications from low-threshold infrared logic switches to multichannel, high-fidelity photonic circuits.

N onlinear photonic devices capable of detecting and modulating infrared signals as a communication medium are essential for technological developments in thermography, night vision, medical diagnosis, information encryption, and optical communication [1][2][3][4][5][6] . Such optoelectronic applications can be accomplished by using nonlinear nanocrystals that enable frequency conversion of invisible infrared radiation to visible luminescence. The ability to dynamically control optical functionalities of the upconversion medium by modulating external stimuli, such as temperature, pressure, magnetic or electric field, will facilitate development of multifunctional, nonlinear optoelectronic devices [7][8][9] . In particular, there is strong demand for electrical control over optical properties of upconversion nanomaterials, as such control will not only allow monolithic integration of photonic elements with optoelectronic functionalities but will also lead to many intriguing phenomena in nonlinear regimes [10][11][12] .
Compared with conventional nonlinear nanocrystals, lanthanideactivated upconversion nanoparticles (UCNPs) allow more-efficient frequency upconversion owing to abundant, physically existing energy states. UCNPs also exhibit excellent photostability, large anti-stokes shift, low pumping threshold, and broadly tunable multicolour emission 13,14 . Temporal modulation of upconversion emission can be realised by varying excitation wavelengths or by adjusting excitation power densities [15][16][17][18][19] . These methods, however, require multiple excitation sources and stringent control over crystalline phase, chemical composition, and surface ligands. Variations in the thermal field also allow reversible luminescence modulation of UCNPs, though these are impractical for device configuration 20,21 . External stimulus-responsive hosts, such as liquid crystal polymer networks, ferroelectric or optomagnetic materials, have been studied to manipulate emission dynamically under external electrical or magnetic stimulation [22][23][24][25] . However, high phonon energy and low upconversion efficiency of these systems, as well as a limited range of emission colours, hinder their practical utility for multicolour switching. Despite enormous efforts, implementation of reversible, dynamic, full-colour emission modulation of UCNPs through electric field stimulation has not been achieved, largely owing to the non-conducting nature of conventional lanthanide-doped host materials and shielding effects of 4f electrons against external electrostatic perturbation.
In this work, inspired by electrochromic organic molecules for optoelectronic applications, such as rewritable, optical labels, and colour-changing metasurfaces 26,27 , we reasoned that fast, multicolour switching of upconversion luminescence should be possible by electrically controlling energy transfer from UCNPs to electrochemically responsive organic molecules. Viologen molecules are the choice of materials for optical switching owing to their good photostability, fast temporal responses, high absorption coefficients, and tailorable absorption wavelengths, as well as ease of structural modification [28][29][30] . Dynamic modulation of upconversion emission colour and intensity can be achieved with molecular-assisted surface electrochemical tuning (MASET) (Fig. 1a). When pumped with a 980 nm laser, UCNPs exhibit steady luminescence. Upon stimulation with an external electric field, viologen molecules receive electrons and convert from an oxidised state to a reduced state via a redox electrochemical process, resulting in electrically controlled energy transfer from UCNPs to viologen molecules owing to spectral overlap between nanocrystal emission and dye absorption. A recovery of upconversion emission can be rapidly achieved through a reversible redox reaction under a reverse electric field (Fig. 1b).
Under appropriate electric field manipulation, PV molecules displayed reversible transformation between their di-cation oxidised form (PV 2+ ) and cation-radical reduced form (PV ·+ ) (Fig. 2f). Notably, in contrast to the oxidised state, which is transparent to visible and NIR radiation, the reduced state displays a broad absorption band, overlapping considerably with emission bands of NaYF 4 :Yb/Er nanoparticles ( Fig. 2g and Supplementary Fig. 11). Under 980 nm irradiation, the hybrid device gave rise to dominant yellow emission peaks at 540 and 654 nm, corresponding to 2 H 11/2 → 4 I 15/2 and 4 S 3/2 → 4 I 15/2 transitions of Er 3+ ions. However, yellow emission was significantly suppressed upon applying an electric field of −3 V. By monitoring voltage-dependent absorbance of PV molecules at 654 nm, we found that electrochemical conversions of PV molecules can be initiated by applying an electric field of −2V. The absorption intensity of PV molecules reached its maximum as the applied voltage was set at −3 V ( Fig. 2h and Supplementary Fig. 12). As a result, photoquenching of UCNPs was precisely controlled under an electric field, by which emission intensity gradually decreased with increasing absorbance of PV molecules (Fig. 2h).
The UCNP/PV hybrid system exhibited good durability and fast switching speed with a rise time of 400 ms and a decay time of 300 ms, which are orders of magnitude faster than those of photochromic molecule-mediated upconversion tuning 18,37 . Moreover, no noticeable degradation was observed after 47 writing/erasing cycles, and the "on" and "off" states for each cycle remained the same ( Fig. 2i and Supplementary Fig. 12), indicating reversible and stable electric-switching characteristics with excellent fatigue resistance. On the other hand, lifetime decay curves measured for NaYF 4 :Yb/Er nanoparticles in electrochemical cells with or without the electric field remained unchanged ( Supplementary Fig. 13), revealing that energy transfer from UCNPs to PV molecules is dominated by a radiative reabsorption process instead of nonradiative Förster resonance energy transfer 38 . Our theoretical investigations suggest that the maximum absorption of oxidised PV molecules is assigned to the electronic transition of HOMO-3→HOMO+3 at 272 nm. In comparison, reduced PV molecules exhibit three dominant electronic transitions of β-HOMO-2 → α-HOMO+1 at 387 nm, α-HOMO → α-HOMO at 521 nm and α-HOMO-2 → α-HOMO+1 at 645 nm ( Fig. 2j and Table S1), in good agreement with experimental results.  Fig. 1 Molecular-assisted surface electrochemical tuning (MASET). a Schematic illustration of electrochemically controlled energy transfer (ET) from UCNPs to viologen molecules (R represents a functional group). An applied electric field induces electrochemical conversion of viologen molecules. For a given set of nanocrystals and viologen molecules, no energy transfer occurs upon NIR laser excitation (λ exc ), owing to lack of spectral overlap between nanocrystal emission and dye absorption. Applying an external electric field drives the oxidised state (Oxi.) of viologen molecules to the reduced state (red.) through a redox process, resulting in selective quenching of luminescence owing to spectral resonance between nanocrystals and molecules. A reverse electric field oxidises the reduced viologen molecules, leading to a recovery of upconversion emission. b Simplified energy level scheme of lanthanide-activated UCNPs and schematic energy level diagram of viologen molecules in oxidised (Black) and reduced (Blue) forms, illustrating electrically controlled energy transfer from UCNPs to dye molecules. Solid and dashed lines represent electronic transitions. Upconversion multicolour editing. To further demonstrate the versatility of the MASET strategy to realise multicolour switching (Fig. 3a), we designed a molecule, 1,1'-bis(3,4-dicarboxybenzyl)-4,4'-bipyridinium dichloride (CV), using density functional theory simulations (Table S1 and Supplementary Figs. [14][15][16][17][18][19]. Theoretical predictions and experimental investigations revealed that this molecule features three distinct redox states (CV 2+ , CV +· , and CV ·· ) under different applied voltages, yielding diverse tunable absorption profiles (Fig. 3b and Supplementary Figs. 20, 21). Electrochemical cells based on CV molecules and NaYF 4 :Yb/Er nanoparticles displayed dominant yellow emission peaks at 540 and 654 nm upon 980 nm excitation, owing to negligible absorption of visible and NIR radiation by the CV molecule in the oxidised state (CV 2+ ). However, applying an electric field of −2.5 V markedly suppressed the red emission, but with a slight effect on the green emission at 540 nm. This can be attributed to spectral overlap between the red emission of NaYF 4 :Yb/Er nanoparticles and the absorption band of CV molecules in the first reduced state (CV +· ) at~660 nm. Further increasing the voltage to −3 V leads to the conversion of CV molecules from the first (CV +· ) to the second (CV ·· ) reduced state, which features absorption in the blue−green spectral range, but negligible absorption in the red region. As a result, the hybrid device showed red emission ( Fig. 3c and Supplementary Fig. 22). These results suggest that by adjusting the voltage within the range of −3 to 3 V, dynamic multicolour editing from yellow to green to red emission is possible using CV molecule-modified NaYF 4 :Yb/Er nanoparticles through modulation of the ratio of red/green emission under different voltages. For example, a low voltage of −2.5 V results in a red/green ratio of 1, giving rise to green emission. Increasing the voltage to −3 V yields intense red emission with a red/green ratio of 35 (Fig. 3d). Based on the MASET strategy, a wide range of colours can be realised at the single-particle level by coupling upconversion nanocrystals possessing tunable emission bands (e.g., NaGdF 4 :Yb/Tm@NaYF 4 : Eu and NaGdF 4 :Yb/Tm@NaYF 4 :Tb) with viologen molecules having variable absorption profiles (Fig. 3e, f  Nonlinear logic switching circuits. The ability to electrically modulate anti-Stokes luminescence enables the development of multifunctional nonlinear optoelectronic devices. As a proof-ofconcept, electrochemical cells based on PV molecules and NaGdF 4 : Yb/Tm@NaYF 4 :Eu nanoparticles (Eu-UCNPs) were fabricated as opto-electrochemical logic gates for signal processing. In our design, the 980 nm pumping beam (on or off) is considered the optical input (In1). Upon 980 nm excitation, Eu-UCNPs exhibit dominant blue and red emission peaks at 454 and 615 nm, respectively. An alternating electric field (positive or negative) serves as the second input (In2), which can induce reversible spectral resonance between Eu-UCNPs and PV molecules. As such, an upconversion logic AND gate can be constructed with red emission as the output signal. Photonic readout signals, on (1) and off (0), are programmable by modulating NIR excitation and alternating electrical field (Fig. 4a, b).
We next demonstrated this principle by transmitting Roman letters using binary codes of standard eight-bit ASCII characters to digitally encoded optical outputs. Each letter is represented with an eight-digit binary code comprising different combinations of zeros and ones. For instance, the word "NUS" with three capital letters in a specific sequence is composed of data strings of 01001110, 01010101, and 01010011 in binary code. Figure 4c shows a schematic of the Eu-UCNPs/PV based logic gate array with eight pixels representing binary codes of standard eight-bit ASCII characters. Working electrodes (with UCNPs) and counter electrodes (ITO glass) are arranged orthogonally. Cross-sectional components are considered pixels of the matrix, which are controlled individually and independently. In our experimental setting, writing and erasing were performed by applying voltages of −3.0 and +3.0 V. When electrical signals carrying the word "NUS" were transmitted to the eight-pixeled UCNP/PV platform, the device converted electrical signals into upconverted emitting photons, encoding optical outputs of "NUS" in binary digits. Binary codes of "0" or "1" can be expressed by monitoring red emission intensity at 615 nm upon applying voltages of −3.0 and +3.0 V, respectively (Fig. 4d). Moreover, binary digital codes, "0" and "1", can also be represented with blue and red colours by applying alternating electrical pulses. We further demonstrated conversion of electrical signals "Just Do It" to high-fidelity photonic output through upconversion logic AND gate (Fig. 4e). Compared with DNA-and quantum dot-based switching circuits with readout based on changes in structural configuration or monochromatic emission intensity 39,40 , the multicolour switching characteristic of our system has advantages such as direct visualisation and easy identification of encrypted information.

Discussion
In summary, we have experimentally demonstrated precise control over anti-Stokes luminescence, based on coupling of lanthanide-activated nanoparticles and electrochemically responsive viologen molecules. By tailoring orbital energy levels of molecules under electric field modulation, the absorption profile of viologen molecules can be adjusted on-demand, enabling electrically tunable spectral resonance between UCNPs and dye species and ultimately resulting in dynamic multicolour editing of anti-Stokes luminescence. This molecularly engineered UCNP platform also enables opto-electrochemical signal processing for rapid, high-fidelity, and far-field communication.
These results not only allow active control over anti-Stokes luminescence under low electric fields and pumping thresholds, but will also benefit the future development of nonlinear photonic devices for applications in quantum computing, multiplexed sensing, data encryption, and potentially many others.

Methods
Nanocrytal synthesis. Upconversion nanoparticles were synthesised using a coprecipitation method. Upconversion microrods were synthesised by a hydrothermal method. Detailed experimental procedures for the preparation of different types of nanoparticles/microrods are provided in the Supplementary Methods.   Fig. 4 Demonstration of opto-electrochemical logic AND gate using PV-modified UCNPs. a, b Basic operating principle of logic AND gate based on the Eu-UCNP/PV platform. The nonlinear output signal is programmed by a NIR laser and an alternating electric field. Under NIR illumination, the photonic readout is controlled by alternating the electric field, which initiates the redox reaction of PV molecules. c Schematic of an eight-pixeled, UCNP/PV logic gate array, in which each pixel is controlled individually by applying voltages of ±3.0 V. d Information processing of electrical inputs carrying the word "NUS" into optical signals using upconversion logic gating. The measured emission intensity of the Eu-UCNP/PV thin film at 615 nm below the 30% or above the 70% threshold represents binary codes, "0" or "1", respectively. e Multiplexed data processing through opto-electrochemical modulation. A string of electrical signals carrying encrypted information of "Just Do It" is converted into visible patterns for signal output.
Preparation of ligand-free NaLnF 4 nanocrystals. As-synthesised oleic acidcapped upconversion nanoparticles/microrods were dispersed in the mixture containing 1 mL of ethanol and 1 mL of 2-M HCl solution. The resulting mixture was ultrasonicated for 10 min to remove oleic acid ligands. Ligand-free nanoparticles were collected by centrifugation, washed with ethanol/deionized water several times, and re-dispersed in deionized water.
Preparation of NaLnF 4 @SiO 2 @TiO 2 nanoparticles. Fabrication of opto-electrochemical devices. In brief, the prepared NaLnF 4 @-SiO 2 @TiO 2 nanoparticles were mixed with a TiO 2 paste, and the mixture was dispersed in ethanol containing 5 wt.% PEG to form a stock slurry under stirring overnight. The fluorine-doped tin oxide (FTO) (15 Ω cm −2 ) glass was cleaned by oxygen plasma for 20 min. A compact TiO 2 layer was formed on the FTO glass by spin-coating the mixed solution (0.1 M) of titanium diisopropoxide bis(acetylacetonate) in 1-butanol on the substrate and then annealed at 400°C for 30 min. The UCNPs@SiO 2 @TiO 2 hybrid film was prepared on the TiO 2 -coated FTO glass by a doctor blade method. The hybrid film was then dried at room temperature for 30 min and then heated at 80°C for another 30 min, followed by annealing at 400°C for 20 min in a vacuum tube furnace. The hybrid film was immersed in a 50-mM aqueous solution of viologen molecules for 12 h. The substrate was washed with ethanol and then dried in a vacuum oven overnight. To construct the electrochemical cell, UCNPs@SiO 2 @TiO 2 nanoparticle-based working electrode and another ITO counter electrode (6 Ω cm −2 ) were joined using double-sided tape. A solution of 0.5 M LiClO 4 in propylene carbonate was injected as the electrolyte into the cell after removal of air.
Physical measurements. Powder X-ray diffraction data were obtained from a Bruker D8 Advance diffractometer using graphite-monochromatized CuKα radiation (λ = 1.5406 Å). Transmission electron microscopy images were collected from a JEOL-JEM 2100 F electron microscope. Scanning electron microscopy images were taken on a JEOL-JSM-6701F electron microscope. 1 H and 13 C NMR spectra were recorded on a Bruker Advance 500 spectrometer at ambient temperature. UV-vis absorption spectra were recorded with a SHIMADZU ultraviolet-3600 spectrophotometer. Electrochemical characterisation of the viologen molecule-modified electrode employed a CS310 potentiostat with a Ag/AgCl/1 M KCl (aq) reference electrode.
Optical characterisations. Optical characterisations were carried out using the custom-built microscope capable of luminescence imaging and spectroscopy. For photoluminescence measurements, a 980 nm continuous-wave excitation laser is coupled to an optical microscope and focused onto the samples. The 980 nm excitation laser with the power intensity of~200 W/cm 2 was used for all the experiments. The photoluminescence is collected through a microscope objective and passed through an 800 nm short-pass fluorescence filter and sent onto a fibrecoupled Ocean Spectrometer.