Multimodal smart sensing via wavelength-selective hydrochromism in zero-dimensional metal halides

Multimodal smart sensing via wavelength-selective hydrochromism in zero-dimensional metal halides | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Multimodal smart sensing via wavelength-selective hydrochromism in zero-dimensional metal halides Won Bin Im, Joo Hyeong Han, Jeong Min Seo, Sung Woo Jang Jang, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8728096/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Smart luminescent materials that are dynamically responsive to external stimuli are crucial for advanced sensing and encryption devices; however, integrating multimodal responsiveness into a single platform remains challenging. Herein, we present a versatile zero-dimensional metal halide, Cs 3 GdCl 6 , doped with Yb 3+ , Er 3+ , and Tb 3+ to achieve distinct sensitivities to UV, X-rays, temperature, and moisture. Characterization results and analyses allowed us to uncover a unique excitation-wavelength-dependent hydrochromic mechanism in Cs 3 Gd 0.8 Er 0.2 Cl 6 . The material retained its yellow emission without color variation under 980 nm excitation, whereas it underwent a rapid, green-to-red hydrochromic shift under 1550 nm excitation. Kinetic analysis confirmed that the absorption cross section is the decisive factor; sufficient absorption capability is a prerequisite for populating moisture-sensitive high-energy levels via energy transfer upconversion. By exploiting these properties, we developed two distinct application platforms: a dynamic pattern plate for qualitative multi-stimuli visualization and a power-free, sticker-type humidity dosimeter capable of quantitative analysis based on Fick’s second law. Physical sciences/Materials science/Materials for optics/Nonlinear optics Physical sciences/Optics and photonics/Optical techniques/Imaging and sensing photon upconversion zero-dimensional metal halides hydrochromism information encryption stimuli-responsive materials moisture dosimeter Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Smart luminescent materials play a pivotal role in enabling modern optoelectronic and security systems to actively communicate or conceal information by interacting with their environment 1 – 5 . These materials can dynamically alter their physicochemical properties in response to external stimuli, including light, temperature, pressure, and moisture; therefore they can be used in high-value applications such as anti-counterfeiting, information encryption, and data storage 6 , 7 . Recently, beyond the limitations of conventional materials that respond to only a single stimulus, there has been a growing demand for "one-source but multi-use" next-generation intelligent materials capable of performing complex functions simultaneously by tuning their reactivity to multiple stimuli 8 , 9 . Over the past few years, lead-based halide perovskites have garnered significant attention as innovative materials in various optoelectronic devices such as solar cells, LEDs, and photodetectors, owing to their high photoluminescence quantum yield, color purity, and facile bandgap tunability 10 – 12 . Additionally, their high sensitivity to moisture, which induces structural transformations, suggests their potential application as hydrochromic sensors 13 – 15 . However, lead is highly toxic for humans and the environment, posing severe challenges in manufacturing and disposal; this toxicity is a major barrier to commercialization owing to stringent international regulations designed to protect the environment and human health, such as the Restriction of Hazardous Substances Directive (RoHS) 16 – 18 . Furthermore, the moisture instability of perovskites, which is advantageous for sensing, is a critical drawback that compromises the long-term stability of these devices 19 – 21 . Consequently, the development of eco-friendly, lead-free metal halides that retain superior optical performances while addressing toxicity and stability issues has attracted considerable interest in materials science 22 , 23 . Among these, eco-friendly luminescent zero-dimensional (0D) lead-free metal halides are particularly highlighted because of their excellent chemical and thermal stabilities resulting from their unique isolated metal-ion structural characteristics 24 , 25 . In this study, we developed a multi-stimuli-responsive smart luminescent material by doping lanthanide ions (Tb 3+ , Yb 3+ , or Er 3+ ) into a Cs 3 GdCl 6 0D Pb-free metal halide host. The combination of chemically stable 0D metal halide lattices with upconversion-active lanthanide dopants provides a versatile platform for designing optical systems that are capable of interacting with multiple external stimuli. In particular, by integrating moisture-sensitive host characteristics with infrared-responsive lanthanide excitation pathways, we successfully demonstrated the feasibility of selectively modulating hydrochromic behavior within a single material using different excitation wavelengths. Moreover, we clarify the upconversion-mediated mechanism responsible for the hydrochromic response under 1550 nm excitation, which has not been coherently articulated in prior reports. This wavelength-dependent optical regulation offers a new route toward the rational design of multifunctional luminescent systems that can operate in distinct optical modes without material exchange. This study conceptually establishes an excitation-gated optical modulation platform for smart luminescent materials, enabling simultaneous responsiveness to multiple environmental and photonic inputs for advanced sensing and information-related technologies. Results Structural characterization and luminescence optimization of Ln 3+ -substituted Cs 3 GdCl 6 Cs 3 GdCl 6 feature a monoclinic structure characterized by a zero-dimensional framework in which [GdCl 6 ] 3− octahedra are isolated by Cs + cations (Fig. 1 a). Within this lattice, two crystallographically distinct octahedral sites (Gd1 and Gd2) serve as ideal accommodation sites for lanthanide ion substitution, as shown in Figs. 1 b and 1 c. The phase purity of the synthesized Cs 3 GdCl 6 was verified by Rietveld refinement of the X-ray diffraction (XRD) patterns. As shown in Fig. 1 d, the refinement results demonstrate excellent agreement between the observed and calculated profiles (χ 2 = 1.826, R wp = 7.63%), confirming a pure monoclinic structure free of secondary impurities. The detailed lattice parameters are listed in Supplementary Table 1. Subsequently, experiments were conducted to systematically substitute three lanthanide ions into the host lattice to detect various external stimuli. The Cs 3 GdCl 6 host was successfully doped with Yb 3+ , Er 3+ , or Tb 3+ . Strictly adhering to Vegard’s law 26 , the unit cell volume contracted linearly as the dopant concentration increased (Fig. 1 e). This monotonic decrease in the lattice volume supports the substitution of Gd 3+ ions with smaller lanthanide ions. The chemical compositions and electronic states of the materials were verified by X-ray photoelectron spectroscopy (XPS). Each spectrum (Fig. 1 f) exhibits characteristic binding energies corresponding to the respective orbitals, indicating that the substituted ions successfully exist in the trivalent oxidation state within the halide coordination environment. To optimize the luminescence efficiency following ion substitution in the Cs 3 GdCl 6 system, a comparative analysis of the luminescence intensity for various compositions was performed. Cs 3 Gd 1−x Er x Cl 6 exhibited the strongest luminescence intensity at an Er 3+ concentration of 20 mol% (Supplementary Fig. 1a). Additionally, for the Cs 3 Gd 1−x−y Yb x Er y Cl 6 material with the introduced Yb 3+ sensitizer, the strongest luminescence intensity was confirmed at an Er 3+ concentration of 2 mol% under a fixed concentration of 20 mol% Yb 3+ (Supplementary Fig. 1b and Fig. 2 ). Furthermore, in the Tb 3+ -substituted composition, the maximum luminescence efficiency (excitation at 280 nm and emission at 548 nm) was observed at 25 mol% Tb 3+ (Supplementary Fig. 1c and Fig. 3 ). Among these, Cs 3 Gd 0.8 Er 0.2 Cl 6 exhibited distinct emission pathways and spectral distributions depending on the wavelength of the excitation source. Excitation-wavelength-gated hydrochromic upconversion (1550 vs 980 nm) As shown in Figs. 2 a and 2 b, under 1550 nm laser excitation, green emission (525/550 nm) corresponding to the 2 H 11/2 → 4 I 15/2 and 4 S 3/2 → 4 I 15/2 transitions was dominant. In contrast, under 980 nm excitation, red emission (670 nm) originating from the 2 F 9/2 → 4 I 15/2 transition dominated. The specific excitation mechanisms of the upconversion photoluminescence are elucidated in the energy-level diagrams of Figs. 2 c and 2 d. Under 1550 nm excitation (Fig. 2 c), Er 3+ ions undergo a three-photon process involving the sequential absorption of three photons via ground-state absorption (GSA) and excited-state absorption (ESA). Through this process, electrons are promoted to the upper 2 H 11/2 and 4 S 3/2 levels, and the subsequent radiative decay from these levels induces green and red emissions. On the other hand, under 980 nm excitation (Fig. 2 d), a two-photon process acts as the primary pathway, where electrons absorb two photons to reach the 4 F 7/2 level. Electrons excited to this high energy level reach lower emissive levels via nonradiative relaxation, subsequently generating emissions at green and red wavelengths through electronic transitions. To evaluate the dynamic optical response to moisture, the time-dependent upconversion photoluminescence changes of Cs 3 Gd 0.8 Er 0.2 Cl 6 were monitored at a relative humidity (RH) of 22%. Figure 2 e presents photographs of the luminescence of the sample over time under 980 nm and 1550 nm laser excitation. Under 1550 nm excitation, the sample initially exhibited bright green luminescence but distinctly shifted to red within 3 min of exposure. In contrast, under 980 nm excitation, the sample displayed yellow luminescence, and no observable color change occurred during the same exposure period. Figure 2 f shows the changes in the emission spectra under 1550 nm excitation. As the exposure time increased, the intensity of the green emission bands (centered at 525 and 550 nm) decreased rapidly, whereas the red emission component remained relatively dominant, resulting in an overall color transition from green to red. On the other hand, under 980 nm excitation (Fig. 2 g), the emission spectra remained virtually unchanged, maintaining constant luminescence characteristics throughout moisture exposure. These observations demonstrate that hydrochromic color switching of this material can be selectively triggered depending on the excitation wavelength. Mechanistic origin of excitation-dependent hydrochromic upconversion To elucidate the origin of the hydrochromic behavior dependent on the excitation wavelength, we performed a comparative analysis of the optical properties and upconversion mechanisms of Cs 3 Gd 0.78 Yb 0.2 Er 0.02 Cl 6 and Cs 3 Gd 0.8 Er 0.2 Cl 6 under 980 nm excitation. We previously reported that Cs 3 Gd 0.78 Yb 0.2 Er 0.02 Cl 6 exhibits a distinct color transition from green to red upon moisture exposure under 980 nm irradiation 27 . In contrast, as observed in Fig. 2 , Cs 3 Gd 0.8 Er 0.2 Cl 6 shows no color change under identical conditions. This discrepancy is attributed to the differences in the light absorption capabilities and the resulting upconversion pathways of the two materials. Figures 3 a and 3 d compare the absorption spectra of the two samples. Cs 3 Gd 0.78 Yb 0.2 Er 0.02 Cl 6 displays strong absorption bands at 980 nm, corresponding to the 2 F 7/2 → 2 F 5/2 transition of Yb 3+ ions, and at 1550 nm, corresponding to the 4 I 15/2 → 4 I 13/2 transition of Er 3+ (Fig. 3 a). This strong absorption enables efficient energy harvesting. On the other hand, Cs 3 Gd 0.8 Er 0.2 Cl 6 exhibits weak absorption at 980 nm owing to the low oscillator strength of the 4 I 15/2 → 4 I 11/2 transition of Er 3+ , while showing a stronger absorption band at 1550 nm ( 4 I 15/2 → 4 I 13/2 ) (Fig. 3 d). This difference in absorption coefficients at 980 nm excitation plays a decisive role in determining moisture sensitivity. The kinetics of the upconversion process were further investigated using time-resolved photoluminescence decay curves. For Cs 3 Gd 0.78 Yb 0.2 Er 0.02 Cl 6 , a clear population buildup phase was observed in the green emission (550 nm) decay curve prior to moisture exposure (t = 0 min) (Fig. 3 b). This indicates that a rise time is required for the energy absorbed by the Yb 3+ ions acting as sensitizers to be transferred to the Er 3+ ions and for the population in the emissive state to accumulate, which is an inherent kinetic characteristic of the energy transfer upconversion (ETU) mechanism. However, as the moisture exposure time increased from 0 to 8 min, this rise-time characteristic gradually disappeared, and the overall lifetime decreased significantly. This implies that the high-energy OH − oscillations of absorbed water molecules effectively quench the 4 S 3/2 and 2 H 11/2 levels of Er 3+ , thereby hindering ETU efficiency. Meanwhile, the decay of the red emission (670 nm) showed relatively little change compared to the green emission, resulting in the dominance of the red emission (Fig. 3 c). In stark contrast, Cs 3 Gd 0.8 Er 0.2 Cl 6 exhibited distinctly different decay kinetics under 980 nm excitation. As shown in Figs. 3 e and 3 f, no population buildup was observed in either the green or red emission decay curves. The absence of such a rise time suggests that without the Yb 3+ sensitizer, the upconversion process in Cs 3 Gd 0.8 Er 0.2 Cl 6 proceeds primarily via GSA, followed by ESA, rather than through ETU. Importantly, the luminescence decay curves of Cs 3 Gd 0.8 Er 0.2 Cl 6 remained unchanged throughout the 8-min moisture exposure. These decay characteristics corroborate the results shown in Fig. 2 g, confirming that the ESA-mediated yellow emission pathway of the Cs 3 Gd 0.8 Er 0.2 Cl 6 system exhibited no color change due to moisture under 980 nm excitation. Based on these observations, we propose the mechanistic models illustrated in Figs. 3 g and 3 h. In Cs 3 Gd 0.78 Yb 0.2 Er 0.02 Cl 6 , the high absorption cross section of Yb 3+ facilitates efficient ETU, populating the green-emitting levels of Er 3+ . However, these populated levels are highly susceptible to nonradiative relaxation caused by the high-energy OH − oscillations (oscillation quenching, OQ) of adjacent water molecules, leading to a hydrochromic shift from green to red emission. In contrast, as depicted in Fig. 3 h, the Cs 3 Gd 0.8 Er 0.2 Cl 6 system under 980 nm excitation exhibits a limited population density owing to its low absorption coefficient, and the emission mechanism relies primarily on ESA. This pathway inherently favors stable red emission and is less affected by multiphonon relaxation induced by surface-adsorbed water, thereby maintaining spectroscopic stability. To confirm the hypothesis that hydrochromic behavior is fundamentally governed by the absorption coefficient, we further compared the responses under 1550 nm excitation (Supplementary Fig. 4). Unlike the 980 nm excitation, where only Cs 3 Gd 0.78 Yb 0.2 Er 0.02 Cl 6 exhibited strong absorption, 1550 nm excitation directly initiated the 4 I 15/2 → 4 I 13/2 transition of Er 3+ ions; thus, both samples possess high absorption coefficients regardless of the presence of Yb 3+ . Consistent with our expectations, both samples exhibited a rapid hydrochromic color transition from green to red upon moisture exposure under 1550 nm excitation. This contrasts with the results obtained under 980 nm excitation (Fig. 2 g) and strongly supports the premise that a sufficiently high absorption coefficient capable of populating sensitive high-energy levels is a prerequisite for inducing hydrochromic shifts. To further validate the upconversion mechanisms, we investigated the pump power dependence of the luminescence intensity (I ∝ P n ). Under 980 nm excitation, the intensity slope of approximately 2 confirmed a two-photon process, whereas intensity slopes between 2.5 and 3.0 under 1550 nm excitation indicated a three-photon process (Supplementary Fig. 5). A direct comparison reveals that the luminescence intensity of Cs 3 Gd 0.78 Yb 0.2 Er 0.02 Cl 6 at 980 nm is nearly two orders of magnitude higher than that of the Er 3+ -singly doped sample. This drastic difference confirms the low absorption cross-section of Er 3+ , which restricts the excited-state population. Consequently, this limited population prevents effective moisture-induced quenching, resulting in a stable yellow emission. Furthermore, to investigate the thermal stability and potential application of the synthesized materials as optical thermometers, we evaluated their temperature-dependent upconversion luminescence properties in the range of 20 to 200°C. As clearly illustrated in the contour maps in Supplementary Fig. 6, most compositions and excitation conditions (980/1550 nm excitation for Cs 3 Gd 0.8 Er 0.2 Cl 6 and 980 nm excitation for Cs 3 Gd 0.78 Yb 0.2 Er 0.02 Cl 6 ) exhibited typical thermal quenching behavior, where the overall luminescence intensity rapidly decreased with increasing temperature because of the activation of nonradiative relaxation induced by lattice vibrations. However, under 1550 nm excitation, Cs 3 Gd 0.78 Yb 0.2 Er 0.02 Cl 6 displayed distinctly different spectral evolution (Supplementary Fig. 7a). Specifically, the emission intensities at 525 nm ( 2 H 11/2 → 4 I 15/2 ) and 550 nm ( 4 S 3/2 → 4 I 15/2 ) exhibited divergent thermal responses (Supplementary Figs. 7b and 7c). While the 550 nm emission intensity significantly decreased as the temperature increased from 293 to 473 K, the 525 nm emission showed a relative increase, followed by stabilization. This phenomenon is attributed to thermal population redistribution following the Boltzmann distribution between the thermally coupled levels of 2 H 11/2 and 4 S 3/2 . Analysis of the fluorescence intensity ratio (FIR, I 525 /I 550 ) of these two transitions yielded a well-fitted curve (Supplementary Fig. 7d), confirming its validity as a ratiometric optical thermometer. The calculated maximum relative sensitivity (S r ) was 0.75% K − 1 , demonstrating that this material is a dual-functional material possessing both moisture and temperature sensing capabilities. Additionally, we confirmed that Tb 3+ -doped Cs 3 Gd 0.75 Tb 0.25 Cl 6 possessed efficient luminescence properties under both X-ray and ultraviolet (UV) excitations. Radioluminescence (RL) measurements revealed the characteristic 548 nm emission peak of Tb 3+ ions, which is identical to the photoluminescence characteristics. As shown in Fig. Supplementary Fig. 8a, this green emission spectrum matches well with the spectral sensitivity region of commercial CMOS image sensors. This high spectral matching implies that the sensor efficiency can be maximized by converting the photon signals generated by the scintillator into electronic signals, making it highly advantageous for applications in indirect X-ray imaging systems. The RL intensity also showed a linearly increasing trend with varying X-ray dose rates (Supplementary Fig. 8b). This linearity suggests that quantitative detection is possible over a wide range of X-ray exposure, from low to high doses. Qualitative visualization via a multi-stimuli-responsive pattern plate for security and sensing Based on the developed smart luminescent materials, we propose two application strategies: qualitative visualization and quantitative analysis. Figure 4 illustrates the implementation of a pattern plate that qualitatively demonstrates information security and sensing performance through immediate and intuitive visual responses to various external stimuli. We fabricated the pattern plate by arranging Cs 3 Gd 0.75 Tb 0.25 Cl 6 , Cs 3 Gd 0.78 Yb 0.2 Er 0.02 Cl 6 , and Cs 3 Gd 0.8 Er 0.2 Cl 6 in a specific configuration (Fig. 4 a). The fabricated pattern plate offers superior information security because the hidden patterns are completely indistinguishable to the naked eye in natural daylight (Fig. 4 b). However, upon the application of external stimuli, this pattern plate dynamically responded to multiple triggers, as shown in Fig. 4 c, thereby qualitatively visualizing hidden information. First, under 254 nm UV irradiation, Cs 3 Gd 0.75 Tb 0.25 Cl 6 positioned at the periphery was excited, revealing a vivid green circular pattern (Fig. 4 d). Using the same principle, when the plate was placed on top of a CMOS sensor array and exposed to X-rays, a circular pattern was imaged at an identical location (Fig. 4 e). This demonstrates that the material can be effectively used for X-ray detection and UV sensing. In particular, long-exposure imaging using 1550 nm and 980 nm infrared (IR) lasers clearly exhibits the wavelength-selective information display and moisture sensing capabilities of the pattern plate. Under 1550 nm excitation in a dry environment, both Cs 3 Gd 0.78 Yb 0.2 Er 0.02 Cl 6 and Cs 3 Gd 0.8 Er 0.2 Cl 6 emitted green light, revealing a central green comet-like pattern (Fig. 4 f). Upon exposure to atmospheric moisture in this state, the green emission is quenched, whereas the red emission dominates owing to the high moisture sensitivity of both materials under 1550 nm excitation, resulting in the entire pattern transitioning to red. This served as a qualitative indicator, allowing for the intuitive perception of moisture exposure through color change. On the other hand, distinctly different behavior was observed under 980 nm laser excitation. Under 980 nm excitation, the luminescence of Cs 3 Gd 0.8 Er 0.2 Cl 6 becomes prominent, forming a yellow, heart-shaped pattern (Fig. 4 g). Cs 3 Gd 0.8 Er 0.2 Cl 6 possesses high stability against moisture under 980 nm irradiation; thus, the heart pattern retains its yellow hue without significant color alteration, even after moisture exposure. These results strongly suggest its potential as a multi-security and sensing platform capable of selectively displaying or detecting distinct information within a single pattern plate, contingent upon the combination of the excitation wavelength and the presence of moisture. Quantitative analysis via a 1550 nm optical-key dosimetry platform We can extend the application scope of this material to sophisticated monitoring systems that require quantitative analyses. Specifically, by targeting sealed systems that demand the nondestructive diagnosis of internal states without external physical interference, we designed a monitoring platform possessing both optical security and precision analysis capabilities. The standard 980 nm excitation wavelength has a limitation in that it is perceived as visible light by the CMOS sensors of commercial smartphones, making it unsuitable for computer vision-based encryption technologies requiring security. In contrast, the 1550 nm excitation wavelength is not observed by general cameras, offering superior optical security. Notably, the Cs 3 Gd 0.8 Er 0.2 Cl 6 composition optimized in this study has a significantly higher Er concentration than the conventional Cs 3 Gd 0.78 Yb 0.2 Er 0.02 Cl 6 ; thus, the color change from green to red due to the moisture reaction under 1550 nm excitation can be identified with much higher sensitivity, clarity, and speed. The rapid and distinct moisture responsiveness of Cs 3 Gd 0.8 Er 0.2 Cl 6 suggests that the material can be utilized as a storage device to record moisture exposure history. To realize this potential as a practical application technology, we designed a power-free sticker-type humidity dosimeter system by combining the 1550 nm optical key characteristic with computer vision technology and quantitatively analyzed its performance. To overcome the limitations of qualitative visual observation and to precisely elucidate the reaction kinetics, we constructed an automated analysis framework that extracts frames from video data, automatically detects the region of interest (ROI), and converts the emission colors into a CIE L*a*b* coordinate system (Fig. 5 a). To ensure the accuracy of quantitative analysis, it is essential to clearly define the starting and ending points of the reaction. The a* value of the material gradually increased with continued moisture exposure and reached an equilibrium state without further changes once the interior of the crystal lattice became saturated with moisture. Based on this saturation behavior, we established the boundary conditions. Specifically, by defining the a* value in the initial moisture-free state (OH − 0%) as the minimum and the a* value at equilibrium in the moisture-saturated state (OH − 100%) as the maximum (Fig. 5 a, right), we normalized the entire reaction progress to a saturation level ranging between 0% and 100%. Experiments were conducted under various RH environments precisely controlled from 11% to 93% using saturated salt solutions (Figs. 5 b and 5 c), allowing for real-time tracking of color changes due to moisture infiltration. The fitted time-resolved response curves in Fig. 5 d show the changes in a* values according to humidity conditions. Under all humidity conditions, the a* values tended to increase over time and converged to the set saturation boundary value, and the rate of change increased exponentially in higher-humidity environments. Recognizing that this behavior was a physical phenomenon of moisture molecules diffusing into the crystal lattice, we fitted the data using the long-time approximation solution of Fick’s second law 30 , 31 . The experimental data showed very high agreement (R 2 > 0.97) with the theoretical model under high-humidity conditions, and the derived diffusion coefficients (D) across the entire humidity range exhibited an exponentially increasing trend with a high correlation coefficient of R 2 > 0.96 with respect to the relative humidity (Fig. 5 e). This proves that the material exhibits predictable physical behavior according to external environmental variables, implying that it can operate as a precision sensor with a dynamic range rather than as a simple discoloration indicator. Based on the established model, we propose a smartphone-based dosimeter reading system equipped with an inverse analysis based on Fick’s second law (Fig. 5 f). Unlike conventional binary indicators such as CoCl 2 28,29 , this system quantifies the color coordinates of images captured under a 1,550 nm laser and performs an inverse calculation by substituting them into a master curve. Thus, it is possible to derive the saturation level, which represents the current accumulated moisture exposure state, regardless of fluctuating exposure rates. By providing not only simple warnings but also quantitative Figs. (e.g., saturation level: 68%) through a smartphone interface, it offers an effective solution for the nondestructive diagnosis of internal moisture integrity using only an attached sticker for sealed packaging systems, where external interference must be minimized. These characteristics demonstrate the potential of doped Cs 3 GdCl 6 as a sensing material for highly reliable monitoring platforms in various industrial fields that require strict environmental control, such as precision chemicals, pharmaceutical storage, and cultural heritage management. Discussion In summary, we achieved versatile responsiveness to UV, X-rays, infrared, temperature, and moisture through lanthanide ion substitution (Tb 3+ , Yb 3+ , and Er 3+ ) in Cs 3 GdCl 6 . The most significant contribution of this study is the discovery of excitation wavelength-dependent hydrochromic behavior in single-phase Cs 3 Gd 0.8 Er 0.2 Cl 6 . Unlike conventional Yb 3+ /Er 3+ systems, Cs 3 Gd 0.8 Er 0.2 Cl 6 exhibits a unique characteristic: it displays green-to-red switching under 1550 nm excitation and maintains high spectroscopic stability under 980 nm excitation. Through in-depth spectroscopic analysis, we determined that the hydrochromic sensitivity is fundamentally governed by the absorption coefficient at the excitation wavelength. We demonstrated that a high absorption coefficient is a prerequisite for populating moisture-sensitive high-energy levels via ETU or GSA/ESA, and that these levels are subsequently quenched from green to red emission by the high-energy OH − oscillations of the adjacent water molecules. These mechanistic insights provide new design rules for controlling the environmental sensitivities of upconversion smart luminescent materials. Furthermore, we demonstrated the utility of this material by developing two different application platforms. First, we fabricated a multi-stimuli-responsive dynamic sensing pattern plate capable of qualitative information encryption and visualization under UV, X-ray, and IR stimuli. Second, we propose a smartphone-based quantitative humidity dosimeter concept, integrating an inverse analysis based on Fick’s second law as a moisture exposure readout for pharmaceuticals and cultural heritage. The findings from this study not only deepen the understanding of upconversion photophysics but also pave the way for the development of next-generation smart optical sensors that simultaneously offer high security and precise environmental monitoring. Methods Raw materials All reagents were used without purification. Cesium chloride (CsCl, 99.9%), and gadolinium chloride hexahydrate (GdCl 3 ∙6H 2 O, 99.9%) and other lanthanide precursors (LnCl 3 ∙6H 2 O, 99.9%; Ln = Tb, Er, Yb) were purchased from Sigma-Aldrich. All chemicals were used without any further purification. Synthesis of Cs 3 Gd 1− x Ln x Cl 6 All chemicals were used without any further purification. Stoichiometric amounts of starting materials were ground in agate mortar, placed in alumina crucibles, and fired at 450°C for 24 h in a tubular furnace using Ar gas. Structural and morphology characterizations The structures of the as-synthesized samples were characterized by XRD (Miniflex 600) using a diffracted beam monochromator set for Cu-Kα radiation (λ = 1.54056 Å). The 2θ scan range was 10°–100° with a step size of 0.01°. Structural information was derived from Rietveld refinement using the GSAS software suite 32 . A three-dimensional visualization system for electronic and structural analysis (VESTA) was used to draw the crystal structures 33 . The phase purity of the as-synthesized samples was estimated via Rietveld refinement of the XRD results with the consideration of full refinement of the crystallographic and instrumental parameters in the GSAS program suite. XPS (K-Alpha) was used to analyze the chemical composition of the samples. Optical characterizations Steady-state PL spectra of the samples were recorded using a Hitachi F-7000 fluorescence spectrophotometer and Horiba FluoroMax. Upconversion luminescence properties of the samples were characterized using 980 nm laser diode (IRM980TA-1000FC, Shanghai laser & optics century) and 1550 nm laser diode (EB21387, CNI laser). The variation in the PL intensity during heating was measured by connecting a Hitachi F-7000 fluorescence spectrometer to an integrated heater, temperature controller, and thermal sensor. Controlled humidity environments Humidity-dependent kinetic experiments were conducted inside a custom-designed sealed chamber. Precise control over the relative humidity (RH) was achieved using various saturated salt solutions at room temperature. Specifically, Saturated Lithium Chloride (LiCl), Calcium Chloride (CaCl 2 ), Calcium Nitrate (Ca(NO 3 ) 2 ), Sodium Chloride (NaCl), and Potassium Nitrate (KNO 3 ) solutions were prepared to establish RH levels of 11%, 32%, 51%, 75%, and 93%, respectively. Automated video processing and region of interest detection To minimize observer bias and ensure high-throughput analysis of the moisture reaction, we developed a custom automated analysis pipeline using Python (v3.11). Video recordings of the sensor response under controlled RH conditions were processed using the OpenCV library, where files were decomposed into individual frames at a sampling rate of 1 Hz to generate a time-series image dataset. To automatically identify the active crystal area, we implemented an adaptive detection algorithm that first converts the initial frame to grayscale and generates a binary mask using a dynamic threshold. This threshold was determined as the median of three statistical parameters derived from pixel intensity distributions: the mean intensity plus one standard deviation, 95% of the 85th percentile intensity, and twice the mean intensity. Following morphological operations to remove noise, a fixed window of 50 x 50 pixels was established around the centroid of the detected sensor contour to ensure consistent spatial sampling across all timeframes. Colorimetric analysis and kinetic modeling For quantitative kinetic analysis, the RGB values within the defined ROI were averaged and converted to the CIE L*a*b* color space using the skimage.color module, specifically tracking the a* coordinate which indicates the green-red color transition. The raw a* values were normalized to a saturation percentage (S(t)) ranging from 0% to 100% based on pre-determined boundary conditions, where the values − 33.21 and 20.37 were defined as the initial anhydrous state and the fully saturated equilibrium state, respectively. The resulting time-resolved saturation data were analyzed using the long-time approximation of Fick's second law 30 , 31 . $$\:\frac{\partial\:\text{C}}{\partial\:\text{t}}\:=\:\text{D}\:\frac{{\partial\:}^{2}\:\text{C}}{\partial\:{\text{x}}^{2}}$$ For the long-time approximation, the experimental data were fitted to the following exponential solution using the Levenberg-Marquardt algorithm 34 via the SciPy library: $$\:S\left(t\right)\:=\:{S}_{{\infty\:}}\:-\:({S}_{{\infty\:}}\:-\:\text{S}₀)\:\text{e}\text{x}\text{p}(-Dt)$$ This exponential model provided a superior fit (R 2 > 0.97) for high humidity conditions compared to square-root models, allowing for the precise extraction of diffusion coefficients (D) to characterize the moisture infiltration kinetics. Data availability The data that support the findings of this study are available from the corresponding author upon reasonable request. Declarations Acknowledgments This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (RS-2024-00411892, RS-2024-00425883, RS-2024-00424047, RS-2024-00419831). Author contributions J.H.H designed the experiments and wrote the original draft. J.H.H., J.M.S., S.W.J., Y.M.P., S.H.C., and J.H.C. did the measurements. W.B.I revised and supervised the manuscript. All authors reviewed the manuscript. Competing interests The authors declare that they have no known competing financial interests or personal relationships that could influence the work reported in this study. References Chen, J. K. et al. Ultrafast and multicolor luminescence switching in a lanthanide-based hydrochromic perovskite. J. Am. Chem. Soc. 144 , 22295 (2022). Riaz, Z. & Khan, K. A. Next-generation programmable materials: multifunctionality, smart integration, and industrial frontiers. ES Mater. Manuf. 29 , 1755 (2025). Nie, F. & Yan, D. 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Mater. 35 , 2212022 (2023). Chen, D. et al. Metal halide perovskite LEDs for visible light communication and lasing applications. Adv. Mater. 37 , 2414745 (2025). Zhang, L. et al. Advances in the application of perovskite materials. Nano-Micro Lett. 15 , 177 (2023). Li, M. et al. Acceleration of radiative recombination for efficient perovskite LEDs. Nature 630 , 631 (2024). Stoumpos, C. C. & Kanatzidis, M. G. Halide perovskites: poor Man’s high-performance semiconductors. Adv. Mater. 28 , 5778–5793 (2016). Gong, R. et al. Hydrochromic effect of perovskite-polymer composites. ACS Nano 18 , 33097 (2024). Li, S. et al. Emerging hydrochromic metal halide perovskites. Adv. Opt. Mater. 13 , e01080 (2025). Samanta, T. et al. Recent progress in lanthanide-based metal halide perovskites: synthesis, properties, and applications. Opt. Mater. X 18 , 100238 (2023). Chen, C. H. et al. Toxicity, leakage, and recycling of lead in perovskite photovoltaics. Adv. Energy Mater. 13 , 2204144 (2023). Ren, M. et al. Potential lead toxicity and leakage issues on lead halide perovskite photovoltaics. J. Hazard. Mater. 426 , 127848 (2022). Kore, B. P. et al. The impact of moisture on the stability and degradation of perovskites in solar cells. Mater. Adv. 5 , 2200–2217 (2024). Shamsi, J. et al. Metal halide perovskite nanocrystals: synthesis, post-synthesis modifications, and their optical properties. Chem. Rev. 119 , 3296 (2019). Grandhi, G. K. et al. Strategies for improving luminescence efficiencies of blue-emitting metal halide perovskites. J. Korean Ceram. Soc. 58 , 28 (2021). Han, J. H. et al. Highly stable zero-dimensional lead-free metal halides for X-ray imaging. ACS Energy Lett. 8 , 545 (2022). Cho, H. B. et al. Three-dimensional lead halide perovskites embedded in zero-dimensional lead halide perovskites: synthesis, stability, and applications. ACS Appl. Electron. Mater. 5 , 66-76. (2022). Li, M. Z. & Xia, Z. G. Recent progress of zero-dimensional luminescent metal halides. Chem. Soc. Rev. 50 , 2626 (2021). Samanta, T. et al. Thermally stable self-trapped assisted single-component white light from lead-free zero-dimensional metal halide nanocrystals. Adv. Opt. Mater. 11 , 2202744 (2023). Vegard, L. Die konstitution der mischkristalle und die raumfüllung der atome. Z. Phys. 5 , 17 (1921). Han, J. H. et al. Intense hydrochromic photon upconversion from lead-free 0D metal halides for water detection and information encryption. Adv. Mater. 35 , 2302442 (2023). Kuzubasoglu, B. A. Recent studies on the humidity sensor: a mini review. ACS Appl. Electron. Mater. 4 , 4797 (2022). Zhang, Y. P. et al. Structured color humidity indicator from reversible pitch tuning in self-assembled nanocrystalline cellulose films. Sens. Actuators B Chem. 176 , 692 (2013). Fick, A. Ueber diffusion. Ann. Phys. 94 , 59 (1855). Fick, A. On liquid diffusion. Philos. Mag. 10 , 30 (1855). Toby, B. H. EXPGUI, a graphical user interface for GSAS. J. Appl. Crystallogr. 34 , 210–213 (2001). Momma, K. & Izumi, F. VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data. J. Appl. Crystallogr. 44 , 1272–1276 (2011). Ranganathan, A. The Levenberg–Marquardt algorithm. (2004). Additional Declarations There is NO Competing Interest. Supplementary Files 1550SupplementaryInformationNC.docx Supplementary information Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8728096","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":582993380,"identity":"6f0aa214-c9b7-49ce-bc29-dc9033a9a2ab","order_by":0,"name":"Won Bin 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07:05:13","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8728096/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8728096/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":101628225,"identity":"9cb5036e-657a-4504-9c3b-f42346c18752","added_by":"auto","created_at":"2026-02-02 04:43:35","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1116709,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eStructural analysis of Ln\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e3+\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003e-doped Cs\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eGdCl\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e6\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e Cs\u003csub\u003e3\u003c/sub\u003eGdCl\u003csub\u003e6\u003c/sub\u003e crystal structure. \u003cstrong\u003eb\u003c/strong\u003e GdCl\u003csub\u003e6\u003c/sub\u003e octahedral sites. \u003cstrong\u003ec\u003c/strong\u003e Rietveld refined XRD pattern of Cs\u003csub\u003e3\u003c/sub\u003eGdCl\u003csub\u003e6\u003c/sub\u003e. \u003cstrong\u003ed\u003c/strong\u003e Cell parameters of Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e1-\u003c/sub\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eLn\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e (Ln\u003csup\u003e3+\u003c/sup\u003e = Yb\u003csup\u003e3+\u003c/sup\u003e, Er\u003csup\u003e3+\u003c/sup\u003e, Tb\u003csup\u003e3+\u003c/sup\u003e) were obtained after full-pattern Rietveld refinement. \u003cstrong\u003ee\u003c/strong\u003e XPS survey spectra of Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e1-\u003c/sub\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eLn\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8728096/v1/1af0600c3ddb365d71faa138.png"},{"id":101628227,"identity":"c2f8476c-2d82-4dfb-b601-ca622be4a756","added_by":"auto","created_at":"2026-02-02 04:43:35","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1226480,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eExcitation-wavelength-dependent upconversion and hydrochromic photoluminescence in Cs\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eGd\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e0.8\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eEr\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e0.2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eCl\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e6\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e (980 vs 1550 nm).\u003c/strong\u003e PL spectra of Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.8\u003c/sub\u003eEr\u003csub\u003e0.2\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e under \u003cstrong\u003ea\u003c/strong\u003e 1550 nm and \u003cstrong\u003eb\u003c/strong\u003e 980 nm excitation. Upconversion photoluminescence scheme under \u003cstrong\u003ec\u003c/strong\u003e 980 nm excitation and \u003cstrong\u003ed\u003c/strong\u003e 1550 nm excitation. \u003cstrong\u003ee\u003c/strong\u003e Hydrochromic upconversion photoluminescence photographs of Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.8\u003c/sub\u003eEr\u003csub\u003e0.2\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e: Exposure time in atmospheric moisture under 1550 nm and 980 nm excitation. Time-dependent upconversion photoluminescence spectra under \u003cstrong\u003ef\u003c/strong\u003e 1550 nm and \u003cstrong\u003eg\u003c/strong\u003e 980 nm excitation of Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.8\u003c/sub\u003eEr\u003csub\u003e0.2\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8728096/v1/8fd42a8ece7c48f9c6e273db.png"},{"id":101628229,"identity":"932b753f-808d-4191-99cc-717134bcc241","added_by":"auto","created_at":"2026-02-02 04:43:35","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1422736,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSpectroscopic and kinetic evidence for hydrochromic upconversion pathways: Yb/Er co-doped vs Er-only systems.\u003c/strong\u003e\u0026nbsp;\u003cstrong\u003ea\u003c/strong\u003e Normalized absorbance spectra of Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.78\u003c/sub\u003eYb\u003csub\u003e0.2\u003c/sub\u003eEr\u003csub\u003e0.02\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e. Time-resolved photoluminescence decay curves of\u0026nbsp;Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.78\u003c/sub\u003eYb\u003csub\u003e0.2\u003c/sub\u003eEr\u003csub\u003e0.02\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e monitored at \u003cstrong\u003eb\u003c/strong\u003e 550 nm and \u003cstrong\u003ec\u003c/strong\u003e 670 nm. \u003cstrong\u003ed\u003c/strong\u003e Normalized absorbance spectra of Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.8\u003c/sub\u003eEr\u003csub\u003e0.2\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e. Time-resolved photoluminescence decay curves of\u0026nbsp;Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.8\u003c/sub\u003eEr\u003csub\u003e0.2\u003c/sub\u003eCl\u003csub\u003e6 \u003c/sub\u003emonitored at \u003cstrong\u003ee\u003c/strong\u003e 550 nm and \u003cstrong\u003ef\u003c/strong\u003e 670 nm. Schematic illustration of hydrochromic upconversion mechanism under 980 nm excitation for \u003cstrong\u003eg\u003c/strong\u003e Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.78\u003c/sub\u003eYb\u003csub\u003e0.2\u003c/sub\u003eEr\u003csub\u003e0.02\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e and \u003cstrong\u003eh\u003c/strong\u003e Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.8\u003c/sub\u003eEr\u003csub\u003e0.2\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8728096/v1/e93c4d0fe7441e66b4af869f.png"},{"id":101628226,"identity":"6f41920a-d998-4e79-b091-e94f7111a356","added_by":"auto","created_at":"2026-02-02 04:43:35","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":705306,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMulti-stimulus dynamic sensing pattern plate.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e Pattern plate conceptual image. \u003cstrong\u003eb\u003c/strong\u003e Daylight image. \u003cstrong\u003ec\u003c/strong\u003e Multi-stimulus dynamic sensing pattern plate schematic. Pattern images under various stimuli: Green circular pattern under \u003cstrong\u003ed\u003c/strong\u003e 254 nm and \u003cstrong\u003ee\u003c/strong\u003e X-ray irradiation. \u003cstrong\u003ef\u003c/strong\u003e Green comet-shaped pattern long exposure under 1550 nm irradiation. \u003cstrong\u003eg\u003c/strong\u003e Yellow heart-shaped pattern under 980 nm excitation.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8728096/v1/3d9d0a656c9fc11918a33e4c.png"},{"id":101628228,"identity":"f14995a9-926e-4307-9fb0-ac4c7d9a3998","added_by":"auto","created_at":"2026-02-02 04:43:35","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":788437,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eHumidity dosimetry quantification and diffusion-based readout.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e Video data analysis framework for determining saturation level. \u003cstrong\u003eb\u003c/strong\u003e Experimental setup and \u003cstrong\u003ec\u003c/strong\u003e dosimeter images at various RH values. \u003cstrong\u003ed\u003c/strong\u003e Time-resolved a* value evolution. \u003cstrong\u003ee\u003c/strong\u003e Diffusion coefficient versus RH. \u003cstrong\u003ef\u003c/strong\u003e Smartphone-based dosimetric readout system based on Fick’s second law.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8728096/v1/1ecb3203eafed2789d0e5063.png"},{"id":104397676,"identity":"1d42d4a5-52ef-4370-80a1-c997c4d0a344","added_by":"auto","created_at":"2026-03-11 11:54:24","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":5924216,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8728096/v1/83fcdc8e-e5eb-4fcc-b327-58c3e1886109.pdf"},{"id":101628230,"identity":"b4e23487-bd87-4261-a4ec-f2ca426efb0c","added_by":"auto","created_at":"2026-02-02 04:43:36","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":2773028,"visible":true,"origin":"","legend":"Supplementary information","description":"","filename":"1550SupplementaryInformationNC.docx","url":"https://assets-eu.researchsquare.com/files/rs-8728096/v1/3099b2d8eca2b593f87d670b.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Multimodal smart sensing via wavelength-selective hydrochromism in zero-dimensional metal halides","fulltext":[{"header":"Introduction","content":"\u003cp\u003eSmart luminescent materials play a pivotal role in enabling modern optoelectronic and security systems to actively communicate or conceal information by interacting with their environment\u003csup\u003e\u003cspan additionalcitationids=\"CR2 CR3 CR4\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e. These materials can dynamically alter their physicochemical properties in response to external stimuli, including light, temperature, pressure, and moisture; therefore they can be used in high-value applications such as anti-counterfeiting, information encryption, and data storage\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e,\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e. Recently, beyond the limitations of conventional materials that respond to only a single stimulus, there has been a growing demand for \"one-source but multi-use\" next-generation intelligent materials capable of performing complex functions simultaneously by tuning their reactivity to multiple stimuli\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e,\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eOver the past few years, lead-based halide perovskites have garnered significant attention as innovative materials in various optoelectronic devices such as solar cells, LEDs, and photodetectors, owing to their high photoluminescence quantum yield, color purity, and facile bandgap tunability\u003csup\u003e\u003cspan additionalcitationids=\"CR11\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e. Additionally, their high sensitivity to moisture, which induces structural transformations, suggests their potential application as hydrochromic sensors\u003csup\u003e\u003cspan additionalcitationids=\"CR14\" citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. However, lead is highly toxic for humans and the environment, posing severe challenges in manufacturing and disposal; this toxicity is a major barrier to commercialization owing to stringent international regulations designed to protect the environment and human health, such as the Restriction of Hazardous Substances Directive (RoHS)\u003csup\u003e\u003cspan additionalcitationids=\"CR17\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. Furthermore, the moisture instability of perovskites, which is advantageous for sensing, is a critical drawback that compromises the long-term stability of these devices\u003csup\u003e\u003cspan additionalcitationids=\"CR20\" citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e. Consequently, the development of eco-friendly, lead-free metal halides that retain superior optical performances while addressing toxicity and stability issues has attracted considerable interest in materials science\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e,\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e. Among these, eco-friendly luminescent zero-dimensional (0D) lead-free metal halides are particularly highlighted because of their excellent chemical and thermal stabilities resulting from their unique isolated metal-ion structural characteristics\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e,\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eIn this study, we developed a multi-stimuli-responsive smart luminescent material by doping lanthanide ions (Tb\u003csup\u003e3+\u003c/sup\u003e, Yb\u003csup\u003e3+\u003c/sup\u003e, or Er\u003csup\u003e3+\u003c/sup\u003e) into a Cs\u003csub\u003e3\u003c/sub\u003eGdCl\u003csub\u003e6\u003c/sub\u003e 0D Pb-free metal halide host. The combination of chemically stable 0D metal halide lattices with upconversion-active lanthanide dopants provides a versatile platform for designing optical systems that are capable of interacting with multiple external stimuli. In particular, by integrating moisture-sensitive host characteristics with infrared-responsive lanthanide excitation pathways, we successfully demonstrated the feasibility of selectively modulating hydrochromic behavior within a single material using different excitation wavelengths. Moreover, we clarify the upconversion-mediated mechanism responsible for the hydrochromic response under 1550 nm excitation, which has not been coherently articulated in prior reports. This wavelength-dependent optical regulation offers a new route toward the rational design of multifunctional luminescent systems that can operate in distinct optical modes without material exchange. This study conceptually establishes an excitation-gated optical modulation platform for smart luminescent materials, enabling simultaneous responsiveness to multiple environmental and photonic inputs for advanced sensing and information-related technologies.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eStructural characterization and luminescence optimization of Ln\u003csup\u003e3+\u003c/sup\u003e-substituted Cs\u003csub\u003e3\u003c/sub\u003eGdCl\u003csub\u003e6\u003c/sub\u003e\u003c/h2\u003e \u003cp\u003eCs\u003csub\u003e3\u003c/sub\u003eGdCl\u003csub\u003e6\u003c/sub\u003e feature a monoclinic structure characterized by a zero-dimensional framework in which [GdCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e octahedra are isolated by Cs\u003csup\u003e+\u003c/sup\u003e cations (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). Within this lattice, two crystallographically distinct octahedral sites (Gd1 and Gd2) serve as ideal accommodation sites for lanthanide ion substitution, as shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb and \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec. The phase purity of the synthesized Cs\u003csub\u003e3\u003c/sub\u003eGdCl\u003csub\u003e6\u003c/sub\u003e was verified by Rietveld refinement of the X-ray diffraction (XRD) patterns. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed, the refinement results demonstrate excellent agreement between the observed and calculated profiles (χ\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;1.826, R\u003csub\u003ewp\u003c/sub\u003e = 7.63%), confirming a pure monoclinic structure free of secondary impurities. The detailed lattice parameters are listed in Supplementary Table\u0026nbsp;1. Subsequently, experiments were conducted to systematically substitute three lanthanide ions into the host lattice to detect various external stimuli. The Cs\u003csub\u003e3\u003c/sub\u003eGdCl\u003csub\u003e6\u003c/sub\u003e host was successfully doped with Yb\u003csup\u003e3+\u003c/sup\u003e, Er\u003csup\u003e3+\u003c/sup\u003e, or Tb\u003csup\u003e3+\u003c/sup\u003e. Strictly adhering to Vegard\u0026rsquo;s law\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e, the unit cell volume contracted linearly as the dopant concentration increased (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee). This monotonic decrease in the lattice volume supports the substitution of Gd\u003csup\u003e3+\u003c/sup\u003e ions with smaller lanthanide ions. The chemical compositions and electronic states of the materials were verified by X-ray photoelectron spectroscopy (XPS). Each spectrum (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef) exhibits characteristic binding energies corresponding to the respective orbitals, indicating that the substituted ions successfully exist in the trivalent oxidation state within the halide coordination environment. To optimize the luminescence efficiency following ion substitution in the Cs\u003csub\u003e3\u003c/sub\u003eGdCl\u003csub\u003e6\u003c/sub\u003e system, a comparative analysis of the luminescence intensity for various compositions was performed. Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e1\u0026minus;x\u003c/sub\u003eEr\u003csub\u003ex\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e exhibited the strongest luminescence intensity at an Er\u003csup\u003e3+\u003c/sup\u003e concentration of 20 mol% (Supplementary Fig.\u0026nbsp;1a). Additionally, for the Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e1\u0026minus;x\u0026minus;y\u003c/sub\u003eYb\u003csub\u003ex\u003c/sub\u003eEr\u003csub\u003ey\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e material with the introduced Yb\u003csup\u003e3+\u003c/sup\u003e sensitizer, the strongest luminescence intensity was confirmed at an Er\u003csup\u003e3+\u003c/sup\u003e concentration of 2 mol% under a fixed concentration of 20 mol% Yb\u003csup\u003e3+\u003c/sup\u003e (Supplementary Fig.\u0026nbsp;1b and Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Furthermore, in the Tb\u003csup\u003e3+\u003c/sup\u003e-substituted composition, the maximum luminescence efficiency (excitation at 280 nm and emission at 548 nm) was observed at 25 mol% Tb\u003csup\u003e3+\u003c/sup\u003e (Supplementary Fig.\u0026nbsp;1c and Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Among these, Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.8\u003c/sub\u003eEr\u003csub\u003e0.2\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e exhibited distinct emission pathways and spectral distributions depending on the wavelength of the excitation source.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eExcitation-wavelength-gated hydrochromic upconversion (1550 vs 980 nm)\u003c/h3\u003e\n\u003cp\u003eAs shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb, under 1550 nm laser excitation, green emission (525/550 nm) corresponding to the \u003csup\u003e2\u003c/sup\u003eH\u003csub\u003e11/2\u003c/sub\u003e \u0026rarr; \u003csup\u003e4\u003c/sup\u003eI\u003csub\u003e15/2\u003c/sub\u003e and \u003csup\u003e4\u003c/sup\u003eS\u003csub\u003e3/2\u003c/sub\u003e \u0026rarr; \u003csup\u003e4\u003c/sup\u003eI\u003csub\u003e15/2\u003c/sub\u003e transitions was dominant. In contrast, under 980 nm excitation, red emission (670 nm) originating from the \u003csup\u003e2\u003c/sup\u003eF\u003csub\u003e9/2\u003c/sub\u003e \u0026rarr; \u003csup\u003e4\u003c/sup\u003eI\u003csub\u003e15/2\u003c/sub\u003e transition dominated.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe specific excitation mechanisms of the upconversion photoluminescence are elucidated in the energy-level diagrams of Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed. Under 1550 nm excitation (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec), Er\u003csup\u003e3+\u003c/sup\u003e ions undergo a three-photon process involving the sequential absorption of three photons via ground-state absorption (GSA) and excited-state absorption (ESA). Through this process, electrons are promoted to the upper \u003csup\u003e2\u003c/sup\u003eH\u003csub\u003e11/2\u003c/sub\u003e and \u003csup\u003e4\u003c/sup\u003eS\u003csub\u003e3/2\u003c/sub\u003e levels, and the subsequent radiative decay from these levels induces green and red emissions. On the other hand, under 980 nm excitation (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed), a two-photon process acts as the primary pathway, where electrons absorb two photons to reach the \u003csup\u003e4\u003c/sup\u003eF\u003csub\u003e7/2\u003c/sub\u003e level. Electrons excited to this high energy level reach lower emissive levels via nonradiative relaxation, subsequently generating emissions at green and red wavelengths through electronic transitions.\u003c/p\u003e \u003cp\u003eTo evaluate the dynamic optical response to moisture, the time-dependent upconversion photoluminescence changes of Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.8\u003c/sub\u003eEr\u003csub\u003e0.2\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e were monitored at a relative humidity (RH) of 22%. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee presents photographs of the luminescence of the sample over time under 980 nm and 1550 nm laser excitation. Under 1550 nm excitation, the sample initially exhibited bright green luminescence but distinctly shifted to red within 3 min of exposure. In contrast, under 980 nm excitation, the sample displayed yellow luminescence, and no observable color change occurred during the same exposure period. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ef shows the changes in the emission spectra under 1550 nm excitation. As the exposure time increased, the intensity of the green emission bands (centered at 525 and 550 nm) decreased rapidly, whereas the red emission component remained relatively dominant, resulting in an overall color transition from green to red. On the other hand, under 980 nm excitation (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eg), the emission spectra remained virtually unchanged, maintaining constant luminescence characteristics throughout moisture exposure. These observations demonstrate that hydrochromic color switching of this material can be selectively triggered depending on the excitation wavelength.\u003c/p\u003e\n\u003ch3\u003eMechanistic origin of excitation-dependent hydrochromic upconversion\u003c/h3\u003e\n\u003cp\u003e \u003c/p\u003e \u003cp\u003eTo elucidate the origin of the hydrochromic behavior dependent on the excitation wavelength, we performed a comparative analysis of the optical properties and upconversion mechanisms of Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.78\u003c/sub\u003eYb\u003csub\u003e0.2\u003c/sub\u003eEr\u003csub\u003e0.02\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e and Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.8\u003c/sub\u003eEr\u003csub\u003e0.2\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e under 980 nm excitation. We previously reported that Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.78\u003c/sub\u003eYb\u003csub\u003e0.2\u003c/sub\u003eEr\u003csub\u003e0.02\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e exhibits a distinct color transition from green to red upon moisture exposure under 980 nm irradiation\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e. In contrast, as observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.8\u003c/sub\u003eEr\u003csub\u003e0.2\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e shows no color change under identical conditions. This discrepancy is attributed to the differences in the light absorption capabilities and the resulting upconversion pathways of the two materials.\u003c/p\u003e \u003cp\u003eFigures\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea and \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed compare the absorption spectra of the two samples. Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.78\u003c/sub\u003eYb\u003csub\u003e0.2\u003c/sub\u003eEr\u003csub\u003e0.02\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e displays strong absorption bands at 980 nm, corresponding to the \u003csup\u003e2\u003c/sup\u003eF\u003csub\u003e7/2\u003c/sub\u003e \u0026rarr; \u003csup\u003e2\u003c/sup\u003eF\u003csub\u003e5/2\u003c/sub\u003e transition of Yb\u003csup\u003e3+\u003c/sup\u003e ions, and at 1550 nm, corresponding to the \u003csup\u003e4\u003c/sup\u003eI\u003csub\u003e15/2\u003c/sub\u003e \u0026rarr; \u003csup\u003e4\u003c/sup\u003eI\u003csub\u003e13/2\u003c/sub\u003e transition of Er\u003csup\u003e3+\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). This strong absorption enables efficient energy harvesting. On the other hand, Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.8\u003c/sub\u003eEr\u003csub\u003e0.2\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e exhibits weak absorption at 980 nm owing to the low oscillator strength of the \u003csup\u003e4\u003c/sup\u003eI\u003csub\u003e15/2\u003c/sub\u003e \u0026rarr; \u003csup\u003e4\u003c/sup\u003eI\u003csub\u003e11/2\u003c/sub\u003e transition of Er\u003csup\u003e3+\u003c/sup\u003e, while showing a stronger absorption band at 1550 nm (\u003csup\u003e4\u003c/sup\u003eI\u003csub\u003e15/2\u003c/sub\u003e \u0026rarr; \u003csup\u003e4\u003c/sup\u003eI\u003csub\u003e13/2\u003c/sub\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed). This difference in absorption coefficients at 980 nm excitation plays a decisive role in determining moisture sensitivity. The kinetics of the upconversion process were further investigated using time-resolved photoluminescence decay curves. For Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.78\u003c/sub\u003eYb\u003csub\u003e0.2\u003c/sub\u003eEr\u003csub\u003e0.02\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e, a clear population buildup phase was observed in the green emission (550 nm) decay curve prior to moisture exposure (t\u0026thinsp;=\u0026thinsp;0 min) (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). This indicates that a rise time is required for the energy absorbed by the Yb\u003csup\u003e3+\u003c/sup\u003e ions acting as sensitizers to be transferred to the Er\u003csup\u003e3+\u003c/sup\u003e ions and for the population in the emissive state to accumulate, which is an inherent kinetic characteristic of the energy transfer upconversion (ETU) mechanism. However, as the moisture exposure time increased from 0 to 8 min, this rise-time characteristic gradually disappeared, and the overall lifetime decreased significantly. This implies that the high-energy OH\u003csup\u003e\u0026minus;\u003c/sup\u003e oscillations of absorbed water molecules effectively quench the \u003csup\u003e4\u003c/sup\u003eS\u003csub\u003e3/2\u003c/sub\u003e and \u003csup\u003e2\u003c/sup\u003eH\u003csub\u003e11/2\u003c/sub\u003e levels of Er\u003csup\u003e3+\u003c/sup\u003e, thereby hindering ETU efficiency. Meanwhile, the decay of the red emission (670 nm) showed relatively little change compared to the green emission, resulting in the dominance of the red emission (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec). In stark contrast, Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.8\u003c/sub\u003eEr\u003csub\u003e0.2\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e exhibited distinctly different decay kinetics under 980 nm excitation. As shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee and \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef, no population buildup was observed in either the green or red emission decay curves. The absence of such a rise time suggests that without the Yb\u003csup\u003e3+\u003c/sup\u003e sensitizer, the upconversion process in Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.8\u003c/sub\u003eEr\u003csub\u003e0.2\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e proceeds primarily via GSA, followed by ESA, rather than through ETU. Importantly, the luminescence decay curves of Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.8\u003c/sub\u003eEr\u003csub\u003e0.2\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e remained unchanged throughout the 8-min moisture exposure. These decay characteristics corroborate the results shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eg, confirming that the ESA-mediated yellow emission pathway of the Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.8\u003c/sub\u003eEr\u003csub\u003e0.2\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e system exhibited no color change due to moisture under 980 nm excitation. Based on these observations, we propose the mechanistic models illustrated in Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eg and \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eh. In Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.78\u003c/sub\u003eYb\u003csub\u003e0.2\u003c/sub\u003eEr\u003csub\u003e0.02\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e, the high absorption cross section of Yb\u003csup\u003e3+\u003c/sup\u003e facilitates efficient ETU, populating the green-emitting levels of Er\u003csup\u003e3+\u003c/sup\u003e. However, these populated levels are highly susceptible to nonradiative relaxation caused by the high-energy OH\u003csup\u003e\u0026minus;\u003c/sup\u003e oscillations (oscillation quenching, OQ) of adjacent water molecules, leading to a hydrochromic shift from green to red emission. In contrast, as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eh, the Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.8\u003c/sub\u003eEr\u003csub\u003e0.2\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e system under 980 nm excitation exhibits a limited population density owing to its low absorption coefficient, and the emission mechanism relies primarily on ESA. This pathway inherently favors stable red emission and is less affected by multiphonon relaxation induced by surface-adsorbed water, thereby maintaining spectroscopic stability. To confirm the hypothesis that hydrochromic behavior is fundamentally governed by the absorption coefficient, we further compared the responses under 1550 nm excitation (Supplementary Fig.\u0026nbsp;4). Unlike the 980 nm excitation, where only Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.78\u003c/sub\u003eYb\u003csub\u003e0.2\u003c/sub\u003eEr\u003csub\u003e0.02\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e exhibited strong absorption, 1550 nm excitation directly initiated the \u003csup\u003e4\u003c/sup\u003eI\u003csub\u003e15/2\u003c/sub\u003e \u0026rarr; \u003csup\u003e4\u003c/sup\u003eI\u003csub\u003e13/2\u003c/sub\u003e transition of Er\u003csup\u003e3+\u003c/sup\u003e ions; thus, both samples possess high absorption coefficients regardless of the presence of Yb\u003csup\u003e3+\u003c/sup\u003e. Consistent with our expectations, both samples exhibited a rapid hydrochromic color transition from green to red upon moisture exposure under 1550 nm excitation. This contrasts with the results obtained under 980 nm excitation (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eg) and strongly supports the premise that a sufficiently high absorption coefficient capable of populating sensitive high-energy levels is a prerequisite for inducing hydrochromic shifts.\u003c/p\u003e \u003cp\u003eTo further validate the upconversion mechanisms, we investigated the pump power dependence of the luminescence intensity (I \u0026prop; P\u003csup\u003en\u003c/sup\u003e). Under 980 nm excitation, the intensity slope of approximately 2 confirmed a two-photon process, whereas intensity slopes between 2.5 and 3.0 under 1550 nm excitation indicated a three-photon process (Supplementary Fig.\u0026nbsp;5). A direct comparison reveals that the luminescence intensity of Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.78\u003c/sub\u003eYb\u003csub\u003e0.2\u003c/sub\u003eEr\u003csub\u003e0.02\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e at 980 nm is nearly two orders of magnitude higher than that of the Er\u003csup\u003e3+\u003c/sup\u003e-singly doped sample. This drastic difference confirms the low absorption cross-section of Er\u003csup\u003e3+\u003c/sup\u003e, which restricts the excited-state population. Consequently, this limited population prevents effective moisture-induced quenching, resulting in a stable yellow emission.\u003c/p\u003e \u003cp\u003eFurthermore, to investigate the thermal stability and potential application of the synthesized materials as optical thermometers, we evaluated their temperature-dependent upconversion luminescence properties in the range of 20 to 200\u0026deg;C. As clearly illustrated in the contour maps in Supplementary Fig.\u0026nbsp;6, most compositions and excitation conditions (980/1550 nm excitation for Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.8\u003c/sub\u003eEr\u003csub\u003e0.2\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e and 980 nm excitation for Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.78\u003c/sub\u003eYb\u003csub\u003e0.2\u003c/sub\u003eEr\u003csub\u003e0.02\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e) exhibited typical thermal quenching behavior, where the overall luminescence intensity rapidly decreased with increasing temperature because of the activation of nonradiative relaxation induced by lattice vibrations. However, under 1550 nm excitation, Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.78\u003c/sub\u003eYb\u003csub\u003e0.2\u003c/sub\u003eEr\u003csub\u003e0.02\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e displayed distinctly different spectral evolution (Supplementary Fig.\u0026nbsp;7a). Specifically, the emission intensities at 525 nm (\u003csup\u003e2\u003c/sup\u003eH\u003csub\u003e11/2\u003c/sub\u003e \u0026rarr; \u003csup\u003e4\u003c/sup\u003eI\u003csub\u003e15/2\u003c/sub\u003e) and 550 nm (\u003csup\u003e4\u003c/sup\u003eS\u003csub\u003e3/2\u003c/sub\u003e \u0026rarr; \u003csup\u003e4\u003c/sup\u003eI\u003csub\u003e15/2\u003c/sub\u003e) exhibited divergent thermal responses (Supplementary Figs.\u0026nbsp;7b and 7c). While the 550 nm emission intensity significantly decreased as the temperature increased from 293 to 473 K, the 525 nm emission showed a relative increase, followed by stabilization. This phenomenon is attributed to thermal population redistribution following the Boltzmann distribution between the thermally coupled levels of \u003csup\u003e2\u003c/sup\u003eH\u003csub\u003e11/2\u003c/sub\u003e and \u003csup\u003e4\u003c/sup\u003eS\u003csub\u003e3/2\u003c/sub\u003e. Analysis of the fluorescence intensity ratio (FIR, I\u003csub\u003e525\u003c/sub\u003e/I\u003csub\u003e550\u003c/sub\u003e) of these two transitions yielded a well-fitted curve (Supplementary Fig.\u0026nbsp;7d), confirming its validity as a ratiometric optical thermometer. The calculated maximum relative sensitivity (S\u003csub\u003er\u003c/sub\u003e) was 0.75% K\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, demonstrating that this material is a dual-functional material possessing both moisture and temperature sensing capabilities.\u003c/p\u003e \u003cp\u003eAdditionally, we confirmed that Tb\u003csup\u003e3+\u003c/sup\u003e-doped Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.75\u003c/sub\u003eTb\u003csub\u003e0.25\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e possessed efficient luminescence properties under both X-ray and ultraviolet (UV) excitations. Radioluminescence (RL) measurements revealed the characteristic 548 nm emission peak of Tb\u003csup\u003e3+\u003c/sup\u003e ions, which is identical to the photoluminescence characteristics. As shown in Fig. Supplementary Fig.\u0026nbsp;8a, this green emission spectrum matches well with the spectral sensitivity region of commercial CMOS image sensors. This high spectral matching implies that the sensor efficiency can be maximized by converting the photon signals generated by the scintillator into electronic signals, making it highly advantageous for applications in indirect X-ray imaging systems. The RL intensity also showed a linearly increasing trend with varying X-ray dose rates (Supplementary Fig.\u0026nbsp;8b). This linearity suggests that quantitative detection is possible over a wide range of X-ray exposure, from low to high doses.\u003c/p\u003e\n\u003ch3\u003eQualitative visualization via a multi-stimuli-responsive pattern plate for security and sensing\u003c/h3\u003e\n\u003cp\u003eBased on the developed smart luminescent materials, we propose two application strategies: qualitative visualization and quantitative analysis. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e illustrates the implementation of a pattern plate that qualitatively demonstrates information security and sensing performance through immediate and intuitive visual responses to various external stimuli. We fabricated the pattern plate by arranging Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.75\u003c/sub\u003eTb\u003csub\u003e0.25\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e, Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.78\u003c/sub\u003eYb\u003csub\u003e0.2\u003c/sub\u003eEr\u003csub\u003e0.02\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e, and Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.8\u003c/sub\u003eEr\u003csub\u003e0.2\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e in a specific configuration (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). The fabricated pattern plate offers superior information security because the hidden patterns are completely indistinguishable to the naked eye in natural daylight (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb). However, upon the application of external stimuli, this pattern plate dynamically responded to multiple triggers, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec, thereby qualitatively visualizing hidden information. First, under 254 nm UV irradiation, Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.75\u003c/sub\u003eTb\u003csub\u003e0.25\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e positioned at the periphery was excited, revealing a vivid green circular pattern (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed). Using the same principle, when the plate was placed on top of a CMOS sensor array and exposed to X-rays, a circular pattern was imaged at an identical location (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee). This demonstrates that the material can be effectively used for X-ray detection and UV sensing.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn particular, long-exposure imaging using 1550 nm and 980 nm infrared (IR) lasers clearly exhibits the wavelength-selective information display and moisture sensing capabilities of the pattern plate. Under 1550 nm excitation in a dry environment, both Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.78\u003c/sub\u003eYb\u003csub\u003e0.2\u003c/sub\u003eEr\u003csub\u003e0.02\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e and Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.8\u003c/sub\u003eEr\u003csub\u003e0.2\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e emitted green light, revealing a central green comet-like pattern (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef). Upon exposure to atmospheric moisture in this state, the green emission is quenched, whereas the red emission dominates owing to the high moisture sensitivity of both materials under 1550 nm excitation, resulting in the entire pattern transitioning to red. This served as a qualitative indicator, allowing for the intuitive perception of moisture exposure through color change.\u003c/p\u003e \u003cp\u003eOn the other hand, distinctly different behavior was observed under 980 nm laser excitation. Under 980 nm excitation, the luminescence of Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.8\u003c/sub\u003eEr\u003csub\u003e0.2\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e becomes prominent, forming a yellow, heart-shaped pattern (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eg). Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.8\u003c/sub\u003eEr\u003csub\u003e0.2\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e possesses high stability against moisture under 980 nm irradiation; thus, the heart pattern retains its yellow hue without significant color alteration, even after moisture exposure. These results strongly suggest its potential as a multi-security and sensing platform capable of selectively displaying or detecting distinct information within a single pattern plate, contingent upon the combination of the excitation wavelength and the presence of moisture.\u003c/p\u003e\n\u003ch3\u003eQuantitative analysis via a 1550 nm optical-key dosimetry platform\u003c/h3\u003e\n\u003cp\u003e \u003c/p\u003e \u003cp\u003eWe can extend the application scope of this material to sophisticated monitoring systems that require quantitative analyses. Specifically, by targeting sealed systems that demand the nondestructive diagnosis of internal states without external physical interference, we designed a monitoring platform possessing both optical security and precision analysis capabilities. The standard 980 nm excitation wavelength has a limitation in that it is perceived as visible light by the CMOS sensors of commercial smartphones, making it unsuitable for computer vision-based encryption technologies requiring security. In contrast, the 1550 nm excitation wavelength is not observed by general cameras, offering superior optical security. Notably, the Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.8\u003c/sub\u003eEr\u003csub\u003e0.2\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e composition optimized in this study has a significantly higher Er concentration than the conventional Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.78\u003c/sub\u003eYb\u003csub\u003e0.2\u003c/sub\u003eEr\u003csub\u003e0.02\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e; thus, the color change from green to red due to the moisture reaction under 1550 nm excitation can be identified with much higher sensitivity, clarity, and speed. The rapid and distinct moisture responsiveness of Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.8\u003c/sub\u003eEr\u003csub\u003e0.2\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e suggests that the material can be utilized as a storage device to record moisture exposure history. To realize this potential as a practical application technology, we designed a power-free sticker-type humidity dosimeter system by combining the 1550 nm optical key characteristic with computer vision technology and quantitatively analyzed its performance.\u003c/p\u003e \u003cp\u003eTo overcome the limitations of qualitative visual observation and to precisely elucidate the reaction kinetics, we constructed an automated analysis framework that extracts frames from video data, automatically detects the region of interest (ROI), and converts the emission colors into a CIE L*a*b* coordinate system (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea). To ensure the accuracy of quantitative analysis, it is essential to clearly define the starting and ending points of the reaction. The a* value of the material gradually increased with continued moisture exposure and reached an equilibrium state without further changes once the interior of the crystal lattice became saturated with moisture. Based on this saturation behavior, we established the boundary conditions. Specifically, by defining the a* value in the initial moisture-free state (OH\u003csup\u003e\u0026minus;\u003c/sup\u003e 0%) as the minimum and the a* value at equilibrium in the moisture-saturated state (OH\u003csup\u003e\u0026minus;\u003c/sup\u003e 100%) as the maximum (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea, right), we normalized the entire reaction progress to a saturation level ranging between 0% and 100%.\u003c/p\u003e \u003cp\u003eExperiments were conducted under various RH environments precisely controlled from 11% to 93% using saturated salt solutions (Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec), allowing for real-time tracking of color changes due to moisture infiltration. The fitted time-resolved response curves in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed show the changes in a* values according to humidity conditions. Under all humidity conditions, the a* values tended to increase over time and converged to the set saturation boundary value, and the rate of change increased exponentially in higher-humidity environments. Recognizing that this behavior was a physical phenomenon of moisture molecules diffusing into the crystal lattice, we fitted the data using the long-time approximation solution of Fick\u0026rsquo;s second law\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e,\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e. The experimental data showed very high agreement (R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;\u0026gt;\u0026thinsp;0.97) with the theoretical model under high-humidity conditions, and the derived diffusion coefficients (D) across the entire humidity range exhibited an exponentially increasing trend with a high correlation coefficient of R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;\u0026gt;\u0026thinsp;0.96 with respect to the relative humidity (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee). This proves that the material exhibits predictable physical behavior according to external environmental variables, implying that it can operate as a precision sensor with a dynamic range rather than as a simple discoloration indicator.\u003c/p\u003e \u003cp\u003eBased on the established model, we propose a smartphone-based dosimeter reading system equipped with an inverse analysis based on Fick\u0026rsquo;s second law (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ef). Unlike conventional binary indicators such as CoCl\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e28,29\u003c/sup\u003e, this system quantifies the color coordinates of images captured under a 1,550 nm laser and performs an inverse calculation by substituting them into a master curve. Thus, it is possible to derive the saturation level, which represents the current accumulated moisture exposure state, regardless of fluctuating exposure rates. By providing not only simple warnings but also quantitative Figs. (e.g., saturation level: 68%) through a smartphone interface, it offers an effective solution for the nondestructive diagnosis of internal moisture integrity using only an attached sticker for sealed packaging systems, where external interference must be minimized. These characteristics demonstrate the potential of doped Cs\u003csub\u003e3\u003c/sub\u003eGdCl\u003csub\u003e6\u003c/sub\u003e as a sensing material for highly reliable monitoring platforms in various industrial fields that require strict environmental control, such as precision chemicals, pharmaceutical storage, and cultural heritage management.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eIn summary, we achieved versatile responsiveness to UV, X-rays, infrared, temperature, and moisture through lanthanide ion substitution (Tb\u003csup\u003e3+\u003c/sup\u003e, Yb\u003csup\u003e3+\u003c/sup\u003e, and Er\u003csup\u003e3+\u003c/sup\u003e) in Cs\u003csub\u003e3\u003c/sub\u003eGdCl\u003csub\u003e6\u003c/sub\u003e. The most significant contribution of this study is the discovery of excitation wavelength-dependent hydrochromic behavior in single-phase Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.8\u003c/sub\u003eEr\u003csub\u003e0.2\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e. Unlike conventional Yb\u003csup\u003e3+\u003c/sup\u003e/Er\u003csup\u003e3+\u003c/sup\u003e systems, Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.8\u003c/sub\u003eEr\u003csub\u003e0.2\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e exhibits a unique characteristic: it displays green-to-red switching under 1550 nm excitation and maintains high spectroscopic stability under 980 nm excitation. Through in-depth spectroscopic analysis, we determined that the hydrochromic sensitivity is fundamentally governed by the absorption coefficient at the excitation wavelength. We demonstrated that a high absorption coefficient is a prerequisite for populating moisture-sensitive high-energy levels via ETU or GSA/ESA, and that these levels are subsequently quenched from green to red emission by the high-energy OH\u003csup\u003e\u0026minus;\u003c/sup\u003e oscillations of the adjacent water molecules. These mechanistic insights provide new design rules for controlling the environmental sensitivities of upconversion smart luminescent materials. Furthermore, we demonstrated the utility of this material by developing two different application platforms. First, we fabricated a multi-stimuli-responsive dynamic sensing pattern plate capable of qualitative information encryption and visualization under UV, X-ray, and IR stimuli. Second, we propose a smartphone-based quantitative humidity dosimeter concept, integrating an inverse analysis based on Fick\u0026rsquo;s second law as a moisture exposure readout for pharmaceuticals and cultural heritage. The findings from this study not only deepen the understanding of upconversion photophysics but also pave the way for the development of next-generation smart optical sensors that simultaneously offer high security and precise environmental monitoring.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003eRaw materials\u003c/h2\u003e \u003cp\u003eAll reagents were used without purification. Cesium chloride (CsCl, 99.9%), and gadolinium chloride hexahydrate (GdCl\u003csub\u003e3\u003c/sub\u003e∙6H\u003csub\u003e2\u003c/sub\u003eO, 99.9%) and other lanthanide precursors (LnCl\u003csub\u003e3\u003c/sub\u003e∙6H\u003csub\u003e2\u003c/sub\u003eO, 99.9%; Ln\u0026thinsp;=\u0026thinsp;Tb, Er, Yb) were purchased from Sigma-Aldrich. All chemicals were used without any further purification.\u003c/p\u003e \u003cp\u003e \u003cb\u003eSynthesis of Cs\u003c/b\u003e \u003csub\u003e \u003cb\u003e3\u003c/b\u003e \u003c/sub\u003e \u003cb\u003eGd\u003c/b\u003e \u003csub\u003e \u003cb\u003e1\u0026minus;\u003c/b\u003e \u003cb\u003ex\u003c/b\u003e \u003c/sub\u003e \u003cb\u003eLn\u003c/b\u003e \u003csub\u003e \u003cb\u003ex\u003c/b\u003e \u003c/sub\u003e \u003cb\u003eCl\u003c/b\u003e \u003csub\u003e \u003cb\u003e6\u003c/b\u003e \u003c/sub\u003e \u003c/p\u003e \u003cp\u003eAll chemicals were used without any further purification. Stoichiometric amounts of starting materials were ground in agate mortar, placed in alumina crucibles, and fired at 450\u0026deg;C for 24 h in a tubular furnace using Ar gas.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eStructural and morphology characterizations\u003c/h2\u003e \u003cp\u003eThe structures of the as-synthesized samples were characterized by XRD (Miniflex 600) using a diffracted beam monochromator set for Cu-Kα radiation (λ\u0026thinsp;=\u0026thinsp;1.54056 \u0026Aring;). The 2θ scan range was 10\u0026deg;\u0026ndash;100\u0026deg; with a step size of 0.01\u0026deg;. Structural information was derived from Rietveld refinement using the GSAS software suite\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e. A three-dimensional visualization system for electronic and structural analysis (VESTA) was used to draw the crystal structures\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e. The phase purity of the as-synthesized samples was estimated via Rietveld refinement of the XRD results with the consideration of full refinement of the crystallographic and instrumental parameters in the GSAS program suite. XPS (K-Alpha) was used to analyze the chemical composition of the samples.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eOptical characterizations\u003c/h2\u003e \u003cp\u003eSteady-state PL spectra of the samples were recorded using a Hitachi F-7000 fluorescence spectrophotometer and Horiba FluoroMax. Upconversion luminescence properties of the samples were characterized using 980 nm laser diode (IRM980TA-1000FC, Shanghai laser \u0026amp; optics century) and 1550 nm laser diode (EB21387, CNI laser). The variation in the PL intensity during heating was measured by connecting a Hitachi F-7000 fluorescence spectrometer to an integrated heater, temperature controller, and thermal sensor.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003eControlled humidity environments\u003c/h2\u003e \u003cp\u003eHumidity-dependent kinetic experiments were conducted inside a custom-designed sealed chamber. Precise control over the relative humidity (RH) was achieved using various saturated salt solutions at room temperature. Specifically, Saturated Lithium Chloride (LiCl), Calcium Chloride (CaCl\u003csub\u003e2\u003c/sub\u003e), Calcium Nitrate (Ca(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e), Sodium Chloride (NaCl), and Potassium Nitrate (KNO\u003csub\u003e3\u003c/sub\u003e) solutions were prepared to establish RH levels of 11%, 32%, 51%, 75%, and 93%, respectively.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003eAutomated video processing and region of interest detection\u003c/h2\u003e \u003cp\u003eTo minimize observer bias and ensure high-throughput analysis of the moisture reaction, we developed a custom automated analysis pipeline using Python (v3.11). Video recordings of the sensor response under controlled RH conditions were processed using the OpenCV library, where files were decomposed into individual frames at a sampling rate of 1 Hz to generate a time-series image dataset. To automatically identify the active crystal area, we implemented an adaptive detection algorithm that first converts the initial frame to grayscale and generates a binary mask using a dynamic threshold. This threshold was determined as the median of three statistical parameters derived from pixel intensity distributions: the mean intensity plus one standard deviation, 95% of the 85th percentile intensity, and twice the mean intensity. Following morphological operations to remove noise, a fixed window of 50 x 50 pixels was established around the centroid of the detected sensor contour to ensure consistent spatial sampling across all timeframes.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003eColorimetric analysis and kinetic modeling\u003c/h2\u003e \u003cp\u003eFor quantitative kinetic analysis, the RGB values within the defined ROI were averaged and converted to the CIE L*a*b* color space using the skimage.color module, specifically tracking the a* coordinate which indicates the green-red color transition. The raw a* values were normalized to a saturation percentage (S(t)) ranging from 0% to 100% based on pre-determined boundary conditions, where the values\u0026thinsp;\u0026minus;\u0026thinsp;33.21 and 20.37 were defined as the initial anhydrous state and the fully saturated equilibrium state, respectively. The resulting time-resolved saturation data were analyzed using the long-time approximation of Fick's second law\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e,\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e.\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:\\frac{\\partial\\:\\text{C}}{\\partial\\:\\text{t}}\\:=\\:\\text{D}\\:\\frac{{\\partial\\:}^{2}\\:\\text{C}}{\\partial\\:{\\text{x}}^{2}}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eFor the long-time approximation, the experimental data were fitted to the following exponential solution using the Levenberg-Marquardt algorithm\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e via the SciPy library:\u003cdiv id=\"Equb\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e\n$$\\:S\\left(t\\right)\\:=\\:{S}_{{\\infty\\:}}\\:-\\:({S}_{{\\infty\\:}}\\:-\\:\\text{S}₀)\\:\\text{e}\\text{x}\\text{p}(-Dt)$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eThis exponential model provided a superior fit (R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;\u0026gt;\u0026thinsp;0.97) for high humidity conditions compared to square-root models, allowing for the precise extraction of diffusion coefficients (D) to characterize the moisture infiltration kinetics.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003eData availability\u003c/h2\u003e \u003cp\u003e \u003cb\u003eThe data that support the findings of this study are available from the corresponding author upon reasonable request.\u003c/b\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (RS-2024-00411892, RS-2024-00425883, RS-2024-00424047, RS-2024-00419831).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eJ.H.H designed the experiments and wrote the original draft. J.H.H., J.M.S., S.W.J., Y.M.P., S.H.C., and J.H.C. did the measurements. W.B.I revised and supervised the manuscript. All authors reviewed the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could influence the work reported in this study.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eChen, J. K. et al. Ultrafast and multicolor luminescence switching in a lanthanide-based hydrochromic perovskite. \u003cem\u003eJ. Am. Chem. Soc.\u003c/em\u003e \u003cstrong\u003e144\u003c/strong\u003e, 22295 (2022).\u003c/li\u003e\n \u003cli\u003eRiaz, Z. \u0026amp; Khan, K. A. Next-generation programmable materials: multifunctionality, smart integration, and industrial frontiers. \u003cem\u003eES Mater. 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Mag.\u003c/em\u003e \u003cstrong\u003e10\u003c/strong\u003e, 30 (1855).\u003c/li\u003e\n \u003cli\u003eToby, B. H. EXPGUI, a graphical user interface for GSAS. \u003cem\u003eJ. Appl. Crystallogr.\u003c/em\u003e \u003cstrong\u003e34\u003c/strong\u003e, 210\u0026ndash;213 (2001).\u003c/li\u003e\n \u003cli\u003eMomma, K. \u0026amp; Izumi, F. VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data. J. Appl. Crystallogr. \u003cstrong\u003e44\u003c/strong\u003e, 1272\u0026ndash;1276 (2011).\u003c/li\u003e\n \u003cli\u003eRanganathan, A. The Levenberg\u0026ndash;Marquardt algorithm. (2004).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":false,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"photon upconversion, zero-dimensional metal halides, hydrochromism, information encryption, stimuli-responsive materials, moisture dosimeter","lastPublishedDoi":"10.21203/rs.3.rs-8728096/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8728096/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSmart luminescent materials that are dynamically responsive to external stimuli are crucial for advanced sensing and encryption devices; however, integrating multimodal responsiveness into a single platform remains challenging. Herein, we present a versatile zero-dimensional metal halide, Cs\u003csub\u003e3\u003c/sub\u003eGdCl\u003csub\u003e6\u003c/sub\u003e, doped with Yb\u003csup\u003e3+\u003c/sup\u003e, Er\u003csup\u003e3+\u003c/sup\u003e, and Tb\u003csup\u003e3+\u003c/sup\u003e to achieve distinct sensitivities to UV, X-rays, temperature, and moisture. Characterization results and analyses allowed us to uncover a unique excitation-wavelength-dependent hydrochromic mechanism in Cs\u003csub\u003e3\u003c/sub\u003eGd\u003csub\u003e0.8\u003c/sub\u003eEr\u003csub\u003e0.2\u003c/sub\u003eCl\u003csub\u003e6\u003c/sub\u003e. The material retained its yellow emission without color variation under 980 nm excitation, whereas it underwent a rapid, green-to-red hydrochromic shift under 1550 nm excitation. Kinetic analysis confirmed that the absorption cross section is the decisive factor; sufficient absorption capability is a prerequisite for populating moisture-sensitive high-energy levels via energy transfer upconversion. By exploiting these properties, we developed two distinct application platforms: a dynamic pattern plate for qualitative multi-stimuli visualization and a power-free, sticker-type humidity dosimeter capable of quantitative analysis based on Fick\u0026rsquo;s second law.\u003c/p\u003e","manuscriptTitle":"Multimodal smart sensing via wavelength-selective hydrochromism in zero-dimensional metal halides","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-02 04:43:30","doi":"10.21203/rs.3.rs-8728096/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"a5be1904-cc33-4a8b-9837-67c376d3cff5","owner":[],"postedDate":"February 2nd, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":62024874,"name":"Physical sciences/Materials science/Materials for optics/Nonlinear optics"},{"id":62024875,"name":"Physical sciences/Optics and photonics/Optical techniques/Imaging and sensing"}],"tags":[],"updatedAt":"2026-04-22T23:05:07+00:00","versionOfRecord":[],"versionCreatedAt":"2026-02-02 04:43:30","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8728096","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8728096","identity":"rs-8728096","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}