Mid-infrared detection through ligand-driven local heating in lanthanide-doped nanoparticles

Domain: biology
Published: 2026-03-23
Editorial outcome: major_revision

Abstract
Mid-infrared photothermal detection, leveraging well-established visible/near-infrared detection techniques, holds significant potential for a wide range of scientific and industrial applications. Conventional mid-infrared photothermal methods show limited response at weak mid-infrared radiation due to insufficient thermal sensitivity. Here, we overcome this challenge by employing ligand-capped lanthanide nanoparticles, where organic molecules with strong and broadband mid-infrared absorption serve as localized heating sources under MIR irradiation. The increased temperature modulates the ratiometric photoluminescence of lanthanide ions via thermally sensitive energy transfer processes. This thermally mediated mechanism enables broadband (5–10 μm) mid-infrared detection at room temperature, achieving a detectivity of 4.8 × 108 Jones at 6.3 μm. The system exhibits a response time of approximately 2 ms and demonstrates spectral fidelity and gas sensing performance competitive with state-of-the-art Fourier-transform infrared systems. Our results establish a new design paradigm for mid-infrared photodetection that bridges molecular photonics and nanotechnology, paving the way for next-generation optical sensing platforms.

Introduction
Mid-infrared (MIR) detection is of particular interest and technological importance for various applications, including environment monitoring, homeland security, thermal imaging, and standoff sensing.1-5 However, typical MIR detector materials, such as InSb, HgCdTe, and quantum-well infrared photodetectors, face fundamental limitations in achieving the next generation of compact, low-cost, and highly sensitive MIR detectors under ambient conditions, primarily due to their reliance on cryogenic cooling and complex fabrication processing 6-8.

Upconversion detection, which leverages mature silicon-based visible/near-infrared (VIS/NIR) photodetectors by converting MIR photons into detectable VIS/NIR radiation, has attracted enormous interest and research efforts. However, conventional upconversion approaches based on nonlinear crystals suffer from strict phase-matching requirements, narrowband responses, and low conversion efficiencies9-12. Recently, researchers have innovatively utilized optomechanical nanocavities to enhance the coupling between vibrational and electronic states of organic molecules, achieving efficient upconversion MIR detection. However, the reliance on nanocavities introduces undesirable complexity to this strategy.13-15 In addition, taking advantage of the closely spaced electronic energy levels in lanthanide ions, researchers have achieved efficient modulation of NIR lanthanide luminescence using MIR light without the need for nanocavities, enabling MIR sensing and imaging applications.16 However, the parity-forbidden nature of 4f-4f transitions significantly restricts MIR absorption cross-sections of lanthanide ions compared to organic molecules, severely limiting this approach.

Similarly, MIR-induced photothermal (MIP) modulation serves as another efficient tool for MIR detection and imaging, by converting MIR absorption into a temperature rise that is read out through thermal-sensitive optical observables (e.g., refractive index, fluorescence intensity) in the VIS/NIR region. In scattering-type MIP, the signal scales with the refractive-index change Δn; for most dielectrics, the thermo-optic coefficient is small (typically ≲5×10−4 /K; PMMA is around 1.2×10−4 /K), which limits sensitivity in the weak illumination region.17 Fluorescence-based MIP addressed this by using thermosensitive dyes with ~1%/K fluorescence modulation, but it remains largely single-band and typically requires MIR lasers.18

Interestingly, the luminescence behavior of lanthanide ions is highly sensitive to their local environment. In particular, it plays a critical role by affecting spectral features through phonon-assisted processes, including changes in the population of emitting energy levels, energy transfer efficiency, non-radiative relaxation, and cross-relaxation pathways-principles that allow for luminescence thermometry. 19-21 Inspired by this, we propose that organic molecules, owing to their strong MIR absorption and rich vibrational modes, can locally heat the nanoparticles upon broadband MIR irradiation, thereby modulating lanthanide emission with high efficiency. The enriched energy level within the lanthanide ions provides multiple emission bands, enabling thermally modulated ratiometric readout that increases sensitivity while suppressing common-mode noise from pump fluctuations and ambient drift.

Nd3+/Yb3+ co-doped nanocrystals are well known for their strong near-infrared (NIR) downshifting luminescence. Notably, the energy transfer between Nd3+ and Yb3+ is highly efficient, with the back-transfer process from Yb3+ to Nd3+ exhibiting strong temperature dependence. Thus, we selected oleic acid (OA)-capped NaYF :Nd3+/Yb3+ nanoparticles to validate our hypothesis. (Figure. 1) We propose that the OA ligands act as effective MIR antennas for energy harvesting, generating localized heating under MIR irradiation. This localized thermal effect modulates the energy-transfer dynamics within the nanoparticles, significantly altering their luminescence behavior and enabling highly efficient MIR photothermal detection.

Results
Hexagonal-phase NaYF :Nd/Yb nanoparticles (~20-25 nm in diameter) were first synthesized via a co-precipitation method in 1-octadecene, with OA acting as surface-capping ligands anchored in situ (Supplementary Figures 1 and 2). The resulting lanthanide nanocrystals were then integrated into thin films and tested using a fluorescence measurement setup (Supplementary Figure 3), where a 730 nm NIR pump and an MIR stimulation beam were directed onto the film from opposite directions. The thicknesses of film are kept around 10 µm for all samples (Supplementary Figure 4). Under excitation with a 730 nm laser, prominent NIR luminescence bands originating from Nd3+ and Yb3+ ions can be observed. As shown in Supplementary Figure 5, the quantum yield (QY) for the samples used in the manuscript is Nd40Yb5: 5.1%, Nd40Yb15: 2.1%, and Nd40Yb30: 0.91%. As expected, higher doping concentration of Yb3+ ions decreased the 980 nm emission due to enhanced quenching pathways. Moreover, under MIR stimulation, the change in spectral ratio (806 nm/980 nm) becomes more significant in samples with lower Yb3+ doping concentrations (Supplementary Figure 6), where individual Yb3+ ions are predominantly surrounded by Nd3+ ions. However, we observe that Nd40Yb5 saturates under intense MIR irradiation, compressing the usable dynamic range. Balancing these trade-offs, we selected Nd40Yb15 for the proof-of-concept demonstrations, which provides adequate brightness (QY = 2.1 %), robust thermal modulation, and wider dynamic range. As shown in Fig. 2a, upon MIR illumination at 6.3 μm, the Yb3+ emission peak is drastically suppressed, approaching complete quenching under high-intensity MIR exposure. Simultaneously, the 806 nm emission undergoes substantial enhancement due to the population redistribution within the Nd3+ energy levels. This led to a remarkable enhancement in ratiometric emission, with the I /I ratio increasing from 0.03 to 5.22, representing an unprecedented 177-fold enhancement.

However, control experiments with ligand-free (LF) samples (Supplementary Figure 7) revealed only a minor reduction in 980 nm emission under similar MIR irradiation, indicating that the intrinsic MIR absorption of the ligand-free film is too weak to generate sufficient temperature change for luminescence modulation. Importantly, the presence of capping ligands boosts the nanoparticle film’s ability to attenuate incident MIR light by nearly two orders of magnitude (Fig. 2b top, from <1% to 36%). Even more intriguing, we observed that the nanoparticle film’s MIR absorption closely corresponds to its ability to modulate the NIR luminescence in that range (Fig. 2b bottom), indicating that the MIR absorption of OA ligand indeed leads to the spectral regulation of Nd3+/Yb3+ codoped nanoparticles. The enhanced MIR absorption and photothermal response arise from coordinated (oleate) OA, not free OA. As shown in Supplementary Figure 8, pure OA exhibits a strong C=O stretch at 1710 cm-1, whereas OA-capped films lose this band and display the COO- doublet at ~1580 (ν ) and ~1350 cm-1 (ν ).22 The MIR photothermal spectrum follows these carboxylate bands, showing pronounced modulation at 1580/1350 cm-1, while the 1710 cm-1 feature of unbound OA is negligible.

To evaluate the influence of temperature on fluorescence emission under MIR radiation, we measured the temperature of the film under MIR radiation (6 .3 μm, 6.5-35.8 mW) by attaching micro-thermocouple to the back of the BaF substrate. The resulting curve (Supplementary Figure 9) is monotonic, reaching around 165 °C at 35.5 mW. Subsequently, the 980 nm emission under different temperatures (no MIR) was recorded and compared to the MIR-power-dependent fluorescence changes. As shown in Fig. 2c and Supplementary Figure 10, the temperature-dependent and MIR-power-dependent traces overlap in trend; small offsets are expected from the through-substrate thermocouple geometry. These findings suggest that the observed MIR photothermal effect is attributable to thermally modulated PL in lanthanide nanoparticles. Furthermore, we quantified the thermal sensitivity. The 980-nm emission exhibits 0.6 %/K sensitivity, whereas the ratiometric 806/980 ratio readout reaches 2.3%/K, providing a substantially more responsive and noise-robust metric. (Table S1) We anticipate further performance improvement in lanthanide-based MIR photothermal detection by leveraging lanthanide platforms that exhibit substantially higher thermal sensitivity (on the order of 20–30% K⁻¹). Thanks to the enhanced efficiency, the lanthanide MIR photothermal system can function with both incoherent NIR and MIR sources. As shown in Supplementary Figure 11, under MIR-LED irradiation (Thorlab, IRHP103) the 980-nm emission decreases as expected from photothermal quenching. The ability to operate at very low MIR power with LED sources makes the approach attractive for scalable, wide-field sensing.

To further elucidate the energy transfer mechanism, we measured the transient PL spectra of the lanthanide thin film with and without MIR illumination. As shown in Fig. 2d, the lifetime of the Yb3+ 2F 5/2 state decreased markedly from 210 μs to 34 μs under MIR irradiation, while the Nd³⁺ 4F lifetime 5/2 increased from 18 μs to 54 μs (Supplementary Figure 12). This behavior aligns with previous reports,20 which suggest that higher temperatures (here induced by MIR radiation) enhance the energy back-transfer rate, consequently leading to a substantial suppression of the 980 nm emission and a concurrent enhancement of the Nd3+ emission at 806 nm. The lifetime of Nd3+ 4F remained nearly unchanged upon MIR radiation, consistent with the observation that the 866 nm emission peak remained stable. This is due to the Nd3+ populations in 4F and 4F following a Boltzmann distribution; thus, as temperature rises, Nd3+ ions are thermally populated from 4F to 4F . This redistribution increases the effective lifetime and enhances the emission associated with 4F . By contrast, the 4F level simultaneously depopulates toward 4F via thermalization and is replenished by Yb to Nd back transfer. These two temperature-dependent effects act in opposite directions on the 4F population. By carefully selecting dopant concentrations, the opposing trends can be balanced. In particular, an intermediate Nd/Yb ratio yields near cancellation, so the 4F lifetime and the 866 nm band remain essentially unchanged at a certain temperature. At lower Yb concentration , reduced back-transfer cannot compensate for the thermal drain from 4F , and the 866 nm emission decreases at elevated temperature. At higher Yb concentration, stronger back-transfer overcompensates the thermal drain, and the 866 nm emission increases with temperature. These behaviours are demonstrated by the MIR-power (thermal) sweeps in Supplementary Figure 13 for NaYF :(Nd40Yb5, Nd40Yb15, and Nd40Yb30). Across all compositions, the 806 nm Nd3+ band increases with MIR power (consistent with higher-temperature population of the upper Nd energy state), while the 980 nm Yb3+ band decreases (consistent with enhanced nonradiative pathways/back-transfer at higher temperature). Crucially, the 866 nm band for Nd40Yb15 increases at first and decreases at higher temperatures. This is due to the Boltzmann distribution within the Nd energy level, which shows exponential decay for Nd 4F .

The OA mass load is estimated by the thermogravimetric analysis (TGA). As shown in Supplementary Figure 14, the loading of the OA is around 12% in mass, which is consistent with that in the literature, demonstrating the reproducibility of the lanthanide MIR photothermal system.23 Converting to volume fraction, the film comprises ~61 vol% particles and ~39 vol% OA, confirming a substantial ligand content that enhances MIR absorptance and provides sufficient thermal insulation. Supplementary Figure 15 illustrates the change in NIR emission (ΔP) under MIR radiation. Remarkably, Nd3+-Yb3+ co-doped samples showed a significant increase in MIR response, with ΔP = 3 μW and ΔP = 9.2 μW under MIR radiation (35 mW at 6.3 μm). Considering that only 6.5% of the emission power was collected by the objective lens, the total emission change from the film was estimated to be 141 μW. The quantum efficiency (QE) of the device was estimated to be around 10-3, which is two orders of magnitude higher than previously reported systems.15

Then, a Si photodetector (Responsivity 7.5×105 V/W) coupled with a spectral filter to isolate the 980 nm emission was used to quantitatively assess the MIR detection performance at room temperature. Fig. 3a presents the photo-induced voltages (V ) as a function of varying NIR and MIR (6.3 μm) power levels. In this context, the NIR power acts as the equivalent 'bias voltage' for the photothermal detector. The MIR sensitivity was evaluated by the reduction in photovoltage (ΔV =V -V ). As the NIR power (P ) increased, V rose correspondingly, reaching a saturation point at high P (Supplementary Figure 16). Similarly, ΔV increased with P and exhibited a linear dependence on MIR power (P ) (Fig. 3b). The highest photoresponse was measured at 170 V/W for a P of 1.6 W. Fig. 3c presents ΔV for different MIR wavelengths at P = 1.6 W, with calculated photo-responses of 30, 170, 164, 133, and 103 V/W for 5.6, 6.3, 7.3, 8.1, and 10 μm, respectively.

The time-resolved photovoltage responses (Supplementary Figure 17) were recorded under periodic laser switches (MIR power: 35.8 - 2.5 mW), demonstrating rapid and stable V responses. Additionally, the response time of our device was investigated by a high-resolution time-resolved V response at 6.3 μm (Fig. 3d), yielding rise and fall times of 2.2 ms and 2.1 ms, corresponding to a 3dB cut-off frequency of around 160 Hz.24 Detectivity (D*) is a crucial figure-of-merit for photodetectors, defined by the noise equivalent power (NEP).25 Fig. 3e shows the frequency-dependent noise level with and without the NIR pumping at 730 nm. While the Si detector exhibited ultra-low noise, the introduction of 730 nm excitation introduced extra noise. Considering the response time of the system, the estimated noise level was around 3.1 nV at 160 Hz operation. Consequently, the detectivity was calculated as 4.8 × 108 Jones for 6.3 μm. Fig. 3f shows the detectivity of our system at different MIR wavelengths as well as the comparison with commercially available room temperature MIR detectors.26-30 Our MIR photothermal detection system shows superior detectivity, especially at the long-wave infrared (LWIR) region, which surpasses those of room temperature MCT detectors.

To evaluate the MIR sensitivity in a full optical system, we measured the 806/980 nm emission ratio under very weak MIR radiation and derived the SNR=1 threshold as a separate figure of merit. As expected, this methodology aggregates additional electronic/optical noise from the instrument and fibre coupling. Supplementary Figure 18 shows that we can measure the ratio signal for very weak MIR radiation (6.3 μm, 2.4 μW), and the SD calculated for the noise floor is around 2.86E-5. Then, the calculated NEP is around 51 nV, corresponding to a power density of 7.5 pW/μm2, which is much enhanced compared with previous MIR photothermal works.15,16,31,32 We believe this dual-metric approach provides the comprehensive assessment of our device, showing that while the transducer itself is highly sensitive, the practical system performance is consistent with the experimental SNR limits.

To evaluate the upconversion module for practical spectroscopy applications, we compared its performance with a commercially available pyroelectric sensor in a gas detection system. As shown in Fig. 4a, MIR light was transmitted through a 2-inch gas cell filled with SO of different concentrations. The transmitted signal was detected by either a pyroelectric sensor or our photothermal module. Fig. 4b exhibits the absorptance spectrum of SO obtained using the 980 nm signal, which closely aligns with the HITRAN database (Supplementary Figure 19), demonstrating the high spectral fidelity of the system. The intensity of the strongest absorption peak (1376 cm-1) was plotted as a function of SO concentration for three detection methods: pyroelectric, emission at 980 nm, and ratio of 806/980 signal. All methods exhibited a linear response with slopes of 1.84× 10-4, 1.8× 10-4, and 1.77× 10-4, respectively. To evaluate the limit of detection (LOD), we analyzed blank spectra obtained using pure N (Fig. 4d-e). The standard deviations (SD) at 1376 cm-1 are 0.0122 and 0.0107 for the pyroelectric and 980nm signal, respectively, which exhibit comparable detection limits. Moreover, the ratiometric signal can realize noise rejection, which is robust to pump power and ambient drift. As shown in the detector noise spectra (Fig. 3e), NIR pumping introduces excess low-frequency noise in single-band detection for 980 nm. By using the 806/980 ratio, common-mode fluctuations can be largely cancelled. As shown in Supplementary Figure 20, the log-normalized signals at 806 and 980 nm co-fluctuate, whereas their ratio is notably flatter. Fig. 4f shows that the SD of the blank sample obtained with the ratio signal at 1376 cm-1 is around 0.00248. Therefore, the LOD, calculated as 3.3*SD/slope, was estimated to be 218.87 ppm, 196.02 ppm, and 46 ppm, respectively. This indicates that our lanthanide MIR photothermal system (using the 806/980 nm signal) outperforms commercial pyroelectric sensors, which are widely used in FTIR spectroscopy.

In conclusion, we have developed a highly efficient MIR photothermal detection system that exploits temperature-modulated PL in Nd3+/Yb3+ co-doped lanthanide nanoparticles. By utilizing the strong MIR absorption of organic ligands, this system converts thermal effects induced by MIR radiation into measurable changes in emission intensity. This thermally driven modulation leads to a 177-fold enhancement in ratiometric emission and achieves an impressive detectivity of 4.8×108 Jones at 6.3 μm, surpassing the performance of conventional room-temperature MIR detectors, while offering a fast response time of ~2.2 ms. Compared to commercial pyroelectric sensors, our detector demonstrates a lower LOD of 46 ppm for SO and high spectral fidelity with the HITRAN database. Moreover, reproducibility controls can make ligand loading and film thickness measurable, stable, and normalized at the point of use, enabling practical deployment of the sensing platform. These results highlight the potential of lanthanide-based systems for MIR detection through thermally modulated PL, offering a compact and highly sensitive platform for next-generation MIR sensing and imaging technologies.

Methods
Materials: Sodium hydroxide (NaOH; >98 %), ammonium fluoride (NH F; >98 %), 1-octadecene (90 %), oleic acid (90 %), yttrium acetate hydrate (Y(CH CO ) ⋅x H O; 99.9 %), neodymium acetate hydrate (Nd(CH CO ) ⋅x H O; 99.9 %), ytterbium acetate hydrate (Yb(CH CO ) ⋅x H O; 99.9 %), cyclohexane (>98 %), and methanol (>99 %) were all purchased from Sigma–Aldrich and used as received without further purification.

Synthesis. Specially designed Nd3+-Yb3+ doped nanocrystals were synthesized by a modified co-precipitation method. In a typical procedure, an aqueous solution (4.0 mL) of Nd(CH CO ) , Yb(CH CO ) and Y(CH CO ) in various molar ratios was added to a 50-mL flask containing 6.0 mL of oleic acid and 14.0 mL of 1-octadecene. The mixture was heated to 150 °C and kept for 60 min to form lanthanide-oleate coordination complexes. After cooling down to 50 °C, a methanol solution (12.0 mL) containing NH F (3.2 mmol) and NaOH (2.0 mmol) was added under stirring at 50 °C for 60 min. The solution was then heated to 100 °C and kept under vacuum for 20 min to remove the volatile residue. The remaining mixture was heated to 290 °C and maintained at this temperature for three hours under an inert nitrogen environment. After cooling to room temperature, the resulting nanocrystals were collected by centrifugation, washed several times with ethanol, and dispersed in 4.0 mL of cyclohexane for further treatment.

Lanthanide nanotransducer films were prepared using a simple drop-casting method. Specifically, 20 mL of ethanol solution (10 mg/mL) was dropped onto a BaF substrate placed in a Petri dish. This process is repeated to increase the thickness of the film.

Characterization. The sizes and morphologies of the as-prepared nanocrystals were characterized using a transmission electron microscope (JEOL-1400 TEM) operating at an acceleration voltage of 100 kV. Powder X-ray diffraction (XRD) patterns were recorded by an X-ray diffractometer (ADDS) with Cu/Ka radiation (40 kV, 40 mA, l=1.54184 Å).

Customized fluorescence system was utilized to examine the MIR radiation response of the lanthanide nanotransducers. A multimode laser operating at 740 nm (CNI-MDL-III) was used as the excitation process. This light was directed by a dichroic long-pass filter with a cut-off at 760 nm (Newport HPD760, Newport) and concentrated onto the nanotransducer film via a 50X objective lens (Mitutoyo, NA 0.55). The tunable quantum cascade laser (QCL) from 4.4 to 10 micrometers (Daylight Solutions, MIRcat), was applied as the MIR source, with the beam precisely focused onto the film by a zinc selenide (ZnSe) lens with a 5 cm focal length (Thorlabs)

The lifetime of the lanthanide particle was measured in the same system; a Si APD was used to collect NIR emission (APD430A, 400MHz). The band pass filter around 805, 860 and 980 nm was used to filter the emission band. An oscilloscope (Tektronix, TBS1000C) was connected to the APD to get the electrical signal. The function generator provides the analog signal. The luminescence of the film was recorded by a CCD spectrometer (Witec) with a 750-nm long-pass filter. The integration time was set as 10 ms. The emission power can be detected and measured with a Si amplified detector (Thorlabs, PDA36A2). The 950 nm long-pass filter was placed in front of the detector to filter the emission for Yb3+. The output voltage was connected to a high-sensitivity digital source meter (Keysight, 2912A). The MIR laser power was measured using a thermal power meter (OPHIR, Nova display-ROHS). The noise spectra are recorded with a Keysight PXA Signal Analyzer (N9030B).

The gas cell is a cylinder cell with a 1-inch diameter and 6-inch length (optical path length 15.24 cm). Both ends are sealed with CaF₂ windows (ambient pressure). Total flow is 100 sccm, and the stream passes a small static mixer before entering the cell. The MIR light is modulated by an optical chopper with a frequency set as 100-200 Hz. For comparison, the transmitted light is tested with a commercial pyroelectric sensor (Shimadzu, MTG-22). Alternatively, the light is focused on the lanthanide film. The emission goes through the band pass filter (980 nm or 805 nm), and is measured with the Si detector (Thorlabs, PDA36A2). The output signal is demodulated by the lock-in amplifier (Zurich Instrument, HF2LI).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this work, Nd/Yb co-doped nanoparticles were introduced into the detection of mid-infrared light. The emission spectrum of nanoparticles was changed with the intensity of MIR light, allowing for the detection of energy variations in MIR light. Overall, the topic is interesting, and the presented data support that the as-prepared material has such detection ability. However, there are a few questions that need to be answered before acceptance: This article suggests that OA ligands enhance absorption of MIR light, thereby amplifying the thermal effect. However, there is no further evidence to support this claim. 1.It is necessary to confirm whether the absorption of MIR light is attributed to oleic acid or coordination of OA with lanthanide ions. 2.If the change in emission spectrum is due to the heating effect caused by MIR light, then the trend of emission spectrum changes caused by the external heating and MIR power should be consistent. The MIR power in Figure 2C is only up to 2.4 mW and needs to be expanded to 35 mW to correspond with Figure 2A. 3. In addition, the proposed sensing mechanism is difficult to be used actually. During the synthesis procedure, it is hard to control the concentration of OA coordinated to the surface of nanoparticles. The aggregation state will also influence the temperature under irradiation of MIR light because of the complex heat dissipation process. The authors should comment more about this. Regarding the energy transfer mechanism, theoretically, energy back-transfer process involves the 2F7/2 state of Yb transferring energy to the 4F3/2 state of Nd, directly affecting the lifetime of the 4F3/2 state of Nd. However, the lifetime of the 4F3/2 state of Nd increased by 17%, with a slight change in intensity, while the lifetime of the 4F5/2 state increased by 200%, with a significant intensity variation. There is a discrepancy between this phenomenon and the theory. It is recommended to detect the lifetime under different MIR power to verify if it is consistent with theoretical predictions. In addition, there are some formatting issues： 1. Figure 2C does not show error bars. 2. Figure S3 does not show the CCD spectrometer, and is the central cube a dichroic beam splitter? 3. Figure S7B image is blurry.", "reviewer_2": "This work employs oleic acid ligand-capped NaYF₄:Nd/Yb nanoparticles for high-sensitivity, broadband mid-infrared (MIR) detection. The oleic acid ligands are proposed to possess strong and broad MIR absorption, acting as localized heat sources upon MIR irradiation. This thermal effect modulates the emissions of Yb at 980 nm and Nd at 806 nm. The concept is intriguing and has potential for publication. However, the overall coherence of the manuscript requires improvement. Specifically, the emission intensity ratio changes between Yb (980 nm) and Nd (806 nm) discussed in Figures 1 and 2 are not referenced or utilized in Figures 3 and 4. Additionally, the underlying mechanism remains insufficiently explained—the general assumption is that MIR absorption by oleic acid leads to localized heating, which alters lanthanide emission intensity, forming the basis of the MIR detection strategy. However, clear and compelling experimental evidence are lacked. Specific comments come as follows: 1. The quantum yield (QY) of the lanthanide nanoparticles is a critical parameter that affects detector sensitivity. Please provide the QY of the lanthanide nanoparticles used, and discuss its influence on device performance. 2. The sensitivity of the system heavily depends on the MIR absorption and photothermal conversion efficiency of the surface-bound ligands. However, essential information is missing, including: (i) absorption spectrum of oleic acid (free and bound), (ii) molar extinction coefficient, (iii) ligand density per nanoparticle, and (iv) photothermal conversion efficiency. These parameters are vital for evaluating the effectiveness of the ligand-mediated heat generation. 3. Figure 2a. The enhanced 806 nm emission likely results from thermal redistribution of populations between the 4F5/2 and 4F3/2 levels according to Boltzmann statistics. MIR irradiation increases local temperature, thus modifying the population distribution. However, the invariant intensity at 860 nm is anomalous and may involve changes in back energy transfer (BET). Notably, Figure S4 shows that the emission can either increase or decrease under MIR irradiation. Does this depend on MIR power? A deeper mechanistic explanation is needed. 4. It is interesting that the 4F5/2 lifetime increases under MIR irradiation while the 4F3/2 lifetime remains unchanged. Given that BET is expected to affect the 4F3/2 state first, followed by the 4F5/2state via thermal redistribution, this observation is counterintuitive. Please elaborate on the mechanism responsible for this behavior. 5. Figure 2b. Please clarify whether the FTIR spectrum originates from the oleic acid ligands alone, the nanoparticle film, or both. Including a control sample of ligand-only coated nanoparticles would strengthen the interpretation. Also, clearly define what is meant by \"MIR sensitivity. 6. Provide data or discussion on the back energy transfer (BET) and energy transfer (ET) rates at various temperatures. While both rates typically increase with temperature, their respective sensitivities to thermal changes may differ significantly. 7. The method for calculating quantum efficiency (QE) is unclear. What does PIR and Pvis refer to? Please specify. 8. In Figure 3c, what is the exact value of incident MIR power (P_NIR, in watts)? In Figure 3d, please explain the significance of the ~160 Hz 3 dB bandwidth, and illustrate how this number was calculated. The caption for Figure 3e does not correspond accurately with the presented data—please revise accordingly. 9. In Figures 1 and 2, the analysis focuses on the emission intensity ratio (980 nm / 806 nm), while in Figure 3 (and Figure 4), the detector performance is evaluated based solely on the reduction of 980 nm intensity. The manuscript should reconcile these two approaches and clearly define their relationship.", "reviewer_3": "Wang et al. employed oleic acid -capped Nd³⁺/Yb³⁺ co-doped nanocrystal films for mid-infrared photon detection, where ligand-induced heating effects were investigated through spectroscopic studies. The detection system demonstrated MIR detectivity of 4.8x108 jones at 6.3 μm and was further validated for gas sensing, showing a LOD comparable to that of commercial sensors. The sensing approach based on MIR upconversion presented in this work is novel and interesting. The following comments are intended to strengthen the manuscript by enhancing data reproducibility and scientific clarity. It would be beneficial to include more detailed characterizations of the material structures, as well as clearly defined spectroscopic excitation conditions. In addition, a more in-depth elaboration on the energy transfer mechanism would greatly enhance the reader’s understanding of the underlying photophysical processes. My detailed comments are as follow: 1. It is recommended to provide TEM images for all samples, including NaYF₄:Nd40Yb5, NaYF₄:Nd40Yb15, and NaYF₄:Nd40Yb30. Also, it would be great to provide characterization data on the thickness and the surface status of the thin films prepared from those nanoparticles. When comparing their PL properties, it is important to ensure that the samples having the comparable parameters to avoid structural effects influencing the results. 2. clarification is needed regarding the observed change in the behavior of the 866 nm emission peak in Fig. S4 with varying Yb³⁺ doping concentrations. For instance, at 5% Yb doping, the 866 nm emission decreases upon IR excitation, whereas at higher Yb³⁺ concentrations, this emission appears to increase. The underlying mechanism for this doping-dependent transition behavior requires further explanation. 3. In Fig.2c, the excitation condition for the red dots is unclear. What are various MIR radiations? Various power? Or 730 nm excitation with varied environmental temperatures? All the spectroscopic data should provide the excitation condition including wavelength, power density etc. e.g, what are the excitation wavelength and power density for lifetime measurements. 4. It is not clear how the power of NIR emissions in Fig. S9 were measured. And what is the definition of the change in NIR emission? 5. What does this mean? --- “Considering that only 6.5% of the emission power was collected the objecttive lens, the total emission change from the film was estimated to be 141 μW.” 6. It is recommended to elaborate on the reasons behind the two orders of magnitude difference in QE observed in this work compared to previously reported systems. 7. In line 126, why does the 6.3 μm wavelength yield the maximum photo-response? A more detailed explanation of the correlation between these excitation wavelengths and the energy levels of Nd³⁺ and Yb³⁺ would help clarify the underlying energy transfer mechanism. 8. Why not use the 806 nm and 980 nm ratio to perform the sensing? Would it further lower the LOD? Some minor points: 1. Line 104, Fig.2e should be Fig. 2d 2. Line 105, from 32 us to 54 us. Should it be 18 us to 54 us? According to the data shown in Fig. S8 3. The illustration in Fig. 4a is too simplistic to convey the actual positioning of the gas cell within the detection system. It is unclear whether the cell needs to be firmly attached to the sensor or the upconversion film. Including a real image of the complete detection setup would greatly enhance clarity and understanding. 4. LOD is normally calculated to be 3.3 SD/slope rather than SD/slope. 5. 66.3 ppm and 59.4 ppm are at comparable levels in terms of LOD. Therefore, it would be more appropriate to describe the technology as having performance comparable to that of a commercial pyroelectric sensor, rather than stating that it \"outperforms\" it.", "reviewer_4": "The manuscript presents a method for mid-infrared (MIR) detection using lanthanide-doped nanoparticles (NaYF₄:Nd³⁺/Yb³⁺) functionalized with MIR-absorbing ligands. The authors claim to achieve “MIR upconversion” detection based on the modulation of near-infrared (NIR) emission by localized heating induced by MIR absorption. While the concept of thermally mediated photoluminescence modulation is known, and the integration with ligand-capped UCNPs is technically interesting, the manuscript contains conceptual issues and lacks novelty at warrants publication at Nature Communications. The manuscript repeatedly refers to the process as \"MIR upconversion,\" which is fundamentally incorrect. There is no anti-Stokes or photon energy upconversion observed here. Instead, the MIR light perturbs the local temperature via ligand absorption, which in turn modulates the Stokes-shifted emission of lanthanide ions (Nd³⁺ and Yb³⁺) under 730 nm excitation. This mechanism is better described as MIR-induced thermal modulation or perturbation of luminescence, not upconversion. The terminology must be corrected throughout the text to avoid misleading interpretation. The principle of using MIR-induced local heating to modulate optical properties is well established: DOI: 10.1126/sciadv.aav7127; DOI: 10.1126/sciadv.abg1559; doi.org/10.1038/s41592-025-02689-0 and similar principles have been applied in plasmonic crystals using CW MIR sources: DOI: 10.1126/sciadv.adn8255; doi.org/10.1038/s41377-022-00709-8. Thus the present work does not provide a new physical mechanism or sensing paradigm beyond ligand-enhanced heating, which is incremental in scope. The detectivity estimation presented (D* ~ 4.8 × 10⁸ Jones) lacks clarity in terms of assumptions and equations. When calculating based on: MIR power = 1 mW, Sensor area = 100 × 100 μm² (Fig. 1a), Time constant = 2 ms ⇒ bandwidth ≈ 250 Hz, The resulting D* are closer to <100 Jones, far below commercial pyroelectric sensors. This discrepancy must be resolved with explicit equations, assumptions, and parameter definitions. The authors claim local temperature increases of up to 10 K under 2.6 mW MIR illumination (Fig. 2c), but do not specify the film thickness or thermal insulation. Thermal cameras are not reliable at this length scale (sub-mm films). Independent verification (e.g., via embedded thermocouples) is required for robust temperature profiling. The manuscript suggests a change in energy back-transfer from Yb³⁺ to Nd³⁺ under MIR illumination. However, an alternative explanation could be: MIR-driven heating changes the relaxation dynamics in Nd³⁺ levels directly, Suppressing energy transfer to Yb³⁺ without invoking back-transfer. The distinction is not rigorously established. Transient PL measurements alone are insufficient and new experiments are needed The SO₂ detection presented is passive, as MIR absorption occurs in a gas cell external to the nanoparticle film. There is no description of: gas cell construction and flow control or ppm-level calibration."}, "author_response": "Responses to the referees’ comments (NCOMMS-25-37842) We are grateful for the reviewers’ constructive comments and support for our work. In the revision, we have included a detailed point-by-point response and substantially strengthened the manuscript. Changes in the revised manuscript as a response to the reviewers’ comments are highlighted in red color and clarifications regarding the reviewers’ comments are provided in blue color. [Detailed responses to Reviewers 1-4 follow, including figures R1-R22 and revised manuscript text]"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The manuscript is revised well according to the reviewers' comments, and the supplemental data can support the conclusions well. Now the manuscript is acceptable.", "reviewer_2": "This revised manuscript has addressed most of my concerns, and could be recommended for publication as it stands.", "reviewer_3": "The authors have provided additional data and descriptions that satisfactorily address my comments. I support its publication.", "reviewer_4": "The authors have addressed few of my earlier points but have not resolved the main conceptual issues. The response does not change my overall assessment and significant mechanistic ambiguities and methodological inconsistencies remain. The work still lacks the novelty and rigor required for publication in Nature Communications, so I recommend for rejection. My detailed comments follow. The authors now agree that the observed signal is not upconversion and photothermal modulation as seen in many other systems. Consequently, the approach offers no fundamental advantage over existing photothermal detection schemes that use fluorescence thermometry or scattering-based MIR photothermal detection. The signal change (2–3% K⁻¹) and overall sensitivities are within the range of conventional fluorophores and previously reported thermometric nanocrystals. Thus, the claim of a factor-of-two improvement is not meaningful, as this variation lies within the noise and sample-to-sample variability. Moreover, the proposed method remains entirely dependent on local heating, without any new physical mechanism for background-free MIR detection. The authors state that artefacts can be removed using the 806/980 nm ratio, yet Fig. R16 and the main text (Fig. 4d–f) consistently show a ratio of zero or undefined response. The data in Figs. R20 and S7 obtained with the MIR-LED are interesting but remain highly preliminary. The MIR powers used are very high, and the contribution of non-resonant heating from broadband emission of the LED is not quantified. Without separating these effects, it is difficult to assess the relevance of these results for true narrow-band MIR detection due to vibrational absorption in the ligands. Detectivity and NEP estimation: This remains the central quantitative flaw. The detectivity (D*) and noise-equivalent power (NEP) are incorrectly derived and overestimated by several orders of magnitude. The reported responsivity of ~170 V W⁻¹ is obtained under simultaneous NIR (1.6 W at 785 nm) and MIR illumination, with the NIR beam acting as an optical bias enabling MIR modulation. However, the voltage noise (3.1 nV) used in the NEP calculation originates from the NIR-driven readout channel and not much relevant for MIR detection. Hence, the derived NEP (~1.4 × 10⁻¹² W Hz⁻½) and D* (~4.8 × 10⁸ Jones) represent a conditional detectivity under optical pumping, not the true MIR sensitivity of the detector. This interpretation also contradicts the experimental data, since the minimum measurable MIR power is ~1 mW, which is several orders of magnitude higher than the NEP-inferred detection limit (~10⁻¹¹ W). A credible assessment of MIR sensitivity would require measuring the MIR-induced signal amplitude versus MIR power (at fixed NIR bias) and defining the SNR = 1 threshold directly from the data. Temperature change and photothermal mechanism: The claimed temperature rise has increased from ~10 K in the original version to >100 K in the revision, which is implausibly high for the reported laser powers. This discrepancy raises serious concerns about the underlying mechanism. Experiments on strongly absorbing plasmonic nanoparticles show ΔT ≈ 50 K for 1 mW μm⁻² at 500 nm excitation (DOI: 10.1021/nn901144d). In contrast, 6 μm MIR photons carry ten times lower energy and typically produce much weaker absorption in vibration of ligands. It is difficult to reconcile how a few surface ligands could convert 35 mW of MIR light into >100 K heating. For photothermal mechanism, the cited reference (ACS Appl. Nano Mater. 2023, 3c01579) reports temperatures below 50 °C, not above 150 °C as claimed here. The quantum yield of only ~5 % also implies that >95 % of the NIR excitation is converted to heat, which should itself dominate the thermal modulation. Yet the manuscript suggests that 30 mW of NIR laser produces negligible heating, whereas 30 mW of MIR light causes >100 K temperature rise, this is physically inconsistent."}, "author_response": "Responses to the referees’ comments. We are grateful for the reviewers’ constructive comments. In the revision, we have included a detailed point-by-point response and substantially strengthened the manuscript. Clarifications regarding the reviewers’ comments are provided in red. Changes in the revised manuscript as a response to the reviewers’ comments are highlighted in blue in the response below. [Detailed responses to Reviewer 4 follow, including figures R1-R6 and revised manuscript text]"}, {"round": 3, "reviewer_comments": {"reviewer_4": "The authors have substantially revised the manuscript and addressed the key conceptual and quantitative concerns raised in earlier rounds, particularly regarding the mechanism, terminology, and NEP/detectivity estimation. While some physical interpretations remain open to discussion, the revised work is now internally consistent and appropriately scoped as a MIR photothermal detection platform rather than upconversion. I am satisfied that the major issues have been addressed to some extent and would support publication in its current form."}, "author_response": ""}]

Decision letter