Fig.1 a) TEM image of NaMgF3<9>.6%Ni@NaMgF3 NCs. Inset: enlarged TEM, HR-TEM, and SAED images, respectively. b) Ni K-edge XANES spectra of NaMgF3<9>.6%Ni, NiF2 and Ni-foil. c) Fourier-transformed Ni K-edge of EXAFS spectra of NaMgF3<9>.6%Ni and NiF2. d) Absorption (top) and emission (bottom) spectra of NaMgF3<9>.6%Ni@NaMgF3 in dry powder. Inset: proposed luminescence mechanism of Ni2+. e) Luminescence intensity (785 and 1320 nm excitation) and f) decay curves (1320 nm excitation) of NaMgF3:_x_Ni@NaMgF3 (∼5 nm shell) with variable Ni2+ concentration in the cyclohexane phase (x = 0–14.3%). g) Luminescence intensity and h) decay curves under 1320 nm excitation of NaMgF3<9>.6%Ni@NaMgF3 with different shell thickness in cyclohexane solution.#

Fig.2 a) Spatial distribution of Ni2+ 3d orbitals in an octahedral field and their interactions with F 2p orbitals. b) Elongation of the Ni–F bond weakens the orbital overlap. c) Reduced orbital overlap decreases the ΔE. d) Schematic of tuning Ni2+ absorption and emission by Ni–F bond length. e) Calculated PDOS of the Ni 3d t2g and eg orbitals and f) energy gap as a function of the theoretical Ni–F bond length in the four matrices. g) Two-dimensional electron-density maps of Ni–F interactions in different matrices. h) Bader charges of Ni2+ and F– as a function of the bond length in different matrices.#

为了验证晶体场环境对光谱的调控作用，研究团队系统地选择了具有不同局域晶格环境的氟化物基质。如图2所示，态密度计算和电子密度图表明，从NaMgF₃逐步替换为MgF₂、KMgF₃及NaMnF₃基质时，Ni–F平均键长从1.97 Å拉长至2.10 Å。随着键长的逐渐伸长，镍离子的3d轨道与氟离子的2p轨道之间的重叠削弱，晶体场分裂能随之降低，从而导致d–d跃迁能量减小。

Fig.3 a) TEM images of NaMgF3<9>.6%Ni, MgF2<9>.8%Ni, KMgF3<9>.8%Ni, and NaMnF3<10>%Ni NCs with similar particle size (∼10 nm). b) Normalized absorption spectra and c) emission spectra of Ni2+ for the above matrices in dry powder (1320 nm excitation). d) NIR-II emission energy as a function of Ni–F bond length. e) Emission fwhm as a function of Ni–F bond length. f) Comparison of absorption and emission wavelength of NLNCs with previously reported NIR-II luminescent materials. (31, 32, 38−48) In the quantum dot sample, the vertical lines represent the possible excitation wavelengths. Detailed optical properties of the materials are shown in Table S1.#

实验测试进一步印证了理论预测。如图3所示，随着Ni–F键长被系统性拉长，四种纳米晶的吸收峰从1318 nm单调红移至1414 nm，发射峰也相应地从1620 nm红移至1780 nm。吸收和发射能量与Ni–F键长之间呈现出良好的线性相关性，建立了明确的波长调控规律。同时，这四种基质纳米晶的发光半高全宽在240 nm至380 nm之间变化，量子产率保持在4.7%至20.7%的较高水平。

Fig.4 a) Optical transmittance spectra of intralipid and mouse tissues. b) Normalized attenuation of 980 and 1320 nm lasers in 1 wt % intralipid. c,d) Tissue penetration of 980 and 1320 nm lasers through mouse skin, muscle, and skull: c) transmitted-light images; d) corresponding intensity statistics. e) Schematic comparison of 980 nm laser-excited Er3+- or Tm3+-doped NCs and 1320 nm laser-excited NLNCs. f) In vivo dual-channel NIR-II-L imaging of mouse intestines and blood vessels using Er3+- or Tm3+-doped NCs (top) and NLNCs (bottom). g,h) SNR values (g) and fwhm values (h) of Er3+- or Tm3+-doped NCs and NLNCs along the dashed lines in f.#

得益于1320 nm处的强吸收，该探针开启了直接在NIR-II窗口内激发的全新路径。如图4所示，在离体组织和脂肪乳模拟组织实验中，1320 nm激发光展现出远高于传统980 nm激发光的穿透能力，大幅减少了组织散射与能量衰减。在活体小鼠成像测试中，经过聚乙二醇修饰的纳米探针成功实现了肠道和血管的高对比度成像。相比于传统镧系稀土探针，基于键长工程调控的纳米晶在活体多重成像中展现出极交叉干扰，使图像信噪比提升了1.1至1.2倍，精细结构的表观分辨率提升了约1.4倍。