【Adv.Mater.】基于荧光纳米金刚石的微尺度离子浓度与电压成像：实现 16 mV Hz⁻¹/² 电压灵敏度与 1.8% mM⁻¹ 盐浓度响应#

文章标题：Ion Concentration and Voltage Imaging With Fluorescent Nanodiamonds 文章作者：Patrick Voorhoeve, Alessandro Mameli, Hiroshi Abe, Takeshi Ohshima, Qiang Sun, Anita Quigley, Rob Kapsa, Nikolai Dontschuk, Philipp Reineck 文章链接：https://doi-org-s-3.proxy.itic-sci.com/10.1002/adma.74728

1. 研究背景与核心痛点#

金刚石中的氮-空位色心（Nitrogen-Vacancy center，简称 NV 色心）是一种原子级别的发光点缺陷。传统基于 NV 色心的量子传感主要依赖微波场来操控其电子自旋状态，用于测量磁场、温度或 pH 值。然而，微波场在水等液体环境中极易被强力吸收，这使得传统的自旋量子传感在生物细胞、电化学溶液等湿法环境中的应用受到很大限制。

为了摆脱对微波的依赖，全光学电荷态传感逐渐成为前沿替代方案。NV 色心主要存在三种电荷状态：带负电的 NV⁻、呈电中性的 NV⁰ 以及带正电的 NV⁺。其中，仅有 NV⁻ 和 NV⁰ 能够发出明亮的荧光，而 NV⁺ 处于不发光的“暗态”。当金刚石处于不同的电化学环境时，NV 色心能够可逆地得到或失去电子，从而改变荧光颜色和亮度。

以往的研究多局限于毫米级的大块单晶金刚石表面。而在粒径小于 30 nm 的荧光纳米金刚石（Fluorescent Nanodiamonds，简称 FNDs）中，由于所有 NV 色心都天然暴露在纳米级的表面附近，外界电荷态调控极其脆弱且难以捉摸，业内长期缺乏可靠、可逆的电荷态调控方法，限制了其在活体神经元与微流控离子成像中的大规模应用。

Fig.1 Overview of the fluorescent nanodiamonds (FNDs) and processing techniques used in this study and their envisaged application. (a–c) Schematic illustrations of the FND processing (a and b) and imaging substrate fabrication process (c). Oxidized FNDs (FND-Oxy) show strong NV0 photoluminescence (PL), which is reduced upon hydrogen surface termination (FND-Hyd) via annealing in forming gas due to the creation of NV+. The FND-Hyd particles exhibit a positive zeta potential in water at neutral pH and electrostatically attach to negatively charged substrates. (d and e) Confocal PL images of FND-Oxy (d) and FND-Hyd (e) on a quartz substrate and corresponding energy diagrams illustrating the surface-termination-induced band bending of the NV charge state transition levels for 20 nm FND-Oxy and FND-Hyd. (f) Illustration of the envisaged application of FND voltage imaging chips. Neurons are grown on a transparent imaging chip coated with FND-Hyd particles. The FND-Hyd PL switches on in response to a neuronal action potential and is very low otherwise.#

2. 核心技术突破与制备工艺#

本研究针对尺寸在 20 nm 及以下（sub-30 nm） 的超小荧光纳米金刚石，首次实现了基于表面化学修饰的“亮-暗”可逆电荷态开关，并成功拓展到毫克级粉末的大规模制备与芯片自组装工艺中。

研究团队利用表面能带弯曲理论，通过调控表面化学基团实现了对色心发光的精确控制：

氧化处理（FND-Oxy）：金刚石表面呈现正电子亲和势（PEA），能带向下弯曲，将近表面的 NV 色心牢固稳定在发光的 NV⁰ 态，表现出明亮稳定的荧光。
氢化处理（FND-Hyd）：采用 800°C 形成气（95% N₂ + 5% H₂）退火 1 小时，为金刚石构建负电子亲和势（NEA）的氢终端表面。这会诱导能带向上弯曲，促使近表面的 NV 色心向表面受体转移电子，转变为不发光的 NV⁺ 暗态，使颗粒整体荧光强度下降超过一个数量级。
紫外臭氧可逆恢复（FND-Ozo）：对氢化后的纳米颗粒仅需进行 15 分钟的常温紫外臭氧照射，即可重新构建氧终端表面，使平均荧光强度从 25 kcps 重新回升至 61 kcps，证实了该电荷态转变完全可逆且未破坏金刚石晶格。

在规模化制备上，团队处理了 150 mg 的 18 nm 与 120 nm 纳米金刚石粉末。红外光谱（FTIR）与 X 射线光电子能谱（XPS）证实，氢化后颗粒表面的氧信号占比由 9.9% 骤降至 1.4%，同时颗粒的 Zeta 电位从 -25 mV 反转为 +31 mV。利用这种强正电荷特性，研究人员无需复杂化学交联，仅通过简易的静电自组装即可在带负电的玻璃或石英基底上快速沉积出均匀的 FND-Hyd 传感功能薄膜。

3. 全光学宽场电压成像性能#

在电化学测试池中，将沉积有 18 nm FND-Hyd 颗粒的透明氧化铟锡（ITO）玻璃电极浸入 170 mM NaCl 溶液中，施加外加电压并利用 532 nm 激光宽场显微镜记录荧光强度变化（ $\Delta PL$ ）。

实验呈现出高度一致的响应规律与关键指标：

电压调制与灵敏度：在 -1 V 至 +1 V 的电位扫描下，表现最优的单个 FND 聚集体在 -1 V 下荧光增益达 +42%，在 +1 V 下降低 -23%；在零偏压附近的散粒噪声极限电压灵敏度高达 16 mV Hz⁻¹/²。
群聚颗粒一致性：视场内全部 98 个 被分析的纳米金刚石团聚体均呈现出完全一致的极性响应趋势，在 +1 V 和 -1 V 脉冲下的平均发光响应幅度分别达到 -11% 和 +15%。
超高光稳定性：在连续 25 分钟、经历 36 次脉冲循环的长时间测试中，信号脉冲间标准差仅为 4.2% 至 5.3%，基础荧光强度漂移小于 4.0%，无光漂白衰减现象。相比之下，未经氢化的 FND-Oxy 对外加电压的荧光波动始终低于 5%，证明了氢终端对电荷态传感的决定性作用。

Fig.2 Reversible switching of the NV charge state in individual small FND aggregates. (a–c) Confocal PL images of FNDs on a marked substrate. The same FNDs were imaged in their oxidized form (a), after hydrogenation (b) and after a UV-ozone treatment (c). The top images show a 25 × 25 µm2 area on the substrate, and the bottom image a zoomed-in image. (d) Box plot of the PL intensity of 95 FNDs after the different processing steps. Small dots represent individual measurements, the box plot represents the average (horizontal line) and the upper and lower quartiles, and the whiskers the minimum and maximum PL values. (e) Typical PL spectra of a small FND aggregate after the different processing steps. (f) AFM z-height images of the zoomed-in image region in the bottom row of panels a–c.#

Fig.3 Characterization of FND powders before and after functionalization. (a) FTIR spectra of 18 nm and 120 nm FND-Oxy and FND-Hyd powders. (b and c) Zeta potential distribution (b) and dynamic light scattering particle size distributions (c) of 18 nm (top) and 120 nm (bottom) FNDs suspended in water before and after hydrogenation. (d) PL spectra of 18 and 120 nm FNDs suspended in water at 1 and 0.1 mg mL−1, respectively. (e) Schematic illustrations of the NV charge state distribution (top) and energy diagrams of the near-surface band bending (bottom) in 18 and 120 nm FNDs.#

4. 微尺度离子浓度梯度动态成像#

当纳米金刚石与电极脱离物理接触、自组装于两个间距为 160 µm 的铂电极之间的石英表面时，施加电压将在溶液中建立微观的离子浓度梯度。

结合 COMSOL 有限元时域扩散模型与光学成像，研究揭示了离子浓度传感的核心规律：

时空动态吻合：电极极性反转后，颗粒荧光强度随时间的变化轨迹与理论计算的局部 NaCl 浓度演变高度重合，证实发光变化受控于局域盐浓度调制。
离子响应灵敏度：所有响应颗粒的荧光变化均与局域盐浓度增减呈正相关，平均响应灵敏度集中在 0.31% 至 0.45% mM⁻¹ NaCl，最高灵敏度可达 1.8% mM⁻¹ NaCl。
纯溶液环境验证：在无外加电压的纯水溶液中逐步滴加 NaCl 溶液（0 至 2 M），无电场干扰下 FND-Hyd 的荧光强度单调递增，单次加盐最高产生 4.9% 的直接发光增强，直接证实了离子浓度改变能够独立调控金刚石-电解质双电层并驱动 NV 电荷态转变。

Fig.4 Voltage response of hydrogenated and oxidized FNDs in an aqueous electrochemical cell. (a) Graphical representation of the electrochemical cell setup and FNDs dispersed on an ITO-coated glass substrate. (b) Change in FND-Hyd (red trace) and FND-Oxy (blue trace) PL as a function of applied voltage. (c) Applied voltage versus time for the experiments shown in (d and e). (d and e) Change in PL as a function of time for each FND type as voltage pulses are applied. (f) Change in PL for both FND types and calculated NaCl concentration at the position of FNDs as a function of time for one pulse sequence. (g) Wide-field PL images of ΔPL of FND-Hyd particles at the end of a +1 V (left) and −1 V pulse (right). (h) Histogram of ΔPL for all particles shown in panel (g) for +1 V and −1 V applied. (i) Scatter plot of ΔPL for all particles in (g) as a function of the substrate area covered by the FND particle aggregate.#

Fig.5 Modulation of the PL of hydrogenated nanodiamonds by voltage-induced microscale salt concentration gradients. (a) Schematic illustration of the experimental setup. (b) Wide-field PL image of the change in PL of FND-Hyd particles dispersed on a quartz substrate between two platinum electrodes with +1 V applied. (c) ΔPL binned along the direction of the electrodes in (b) as a function of time as the polarity of the applied voltage changes from +1 V (0–1 s) to −1 V (1–5 s). (d) Calculated change in NaCl concentration (Δ[NaCl]) between the electrodes as a function of time for the applied voltages and duration shown in (c). Δ[NaCl] was calculated using FDTD simulations. (e) ΔPL (left axes) and calculated Δ[NaCl] (right axes) a as a function of time for locations I–IX indicated in panel (b) as the polarity of the applied voltage changes. (f) ΔPL of all FND-Hyd particles between the electrodes as a function of the minimum and maximum calculated change in NaCl concentration (Δ[NaCl]max and Δ[NaCl]min, respectively) during +1 V (purple dots) and −1 V pulses (green dots). (g) Histogram of ΔPL normalized by the calculated Δ[NaCl] for all particles. (h) PL intensity averaged over 156 × 156 µm2 of a quartz substrate covered with FND-Hyd particles versus time as the salt concentration is increased stepwise via the addition of a concentrated NaCl solution.#

5. 研究价值、局限性与未来展望#

该项研究开创性地证实了亚 30 纳米金刚石能够在无微波介入的条件下，仅通过常规光学显微镜实现高灵敏度、亚微米空间分辨率的实时电压与局域离子浓度定量成像。这一低成本、可规模化组装的传感层，为未来在生理溶液中原位监测神经元动作电位传播及微流控芯片离子梯度扩散提供了全新的全光学检测平台。

本研究目前仍存在一定的实验局限与待解机制：

灵敏度差异机制待解：在电极间动态离子梯度场中测得的灵敏度（约 0.4% mM⁻¹）显著高于宏观稳态加盐测试中的灵敏度（低于 0.01% mM⁻¹），其背后的固液界面双电层微观微扰机制尚需进一步的理论与谱学验证。
荧光寿命与暗态归属：现有荧光检测手段尚无法严格区分暂时性的基态 NV⁺ 转变与激发态 NV⁰ 向表面受体发生非辐射电子转移的过程，需结合超快光谱进一步厘清电荷动力学过程。
后续优化方向：优化纳米颗粒的单分散性与表面均一性，避免因团聚体尺寸差异导致响应不对称，并推动该平台在活体神经组织与芯片实验室中的原位集成应用。

【Adv.Mater.】基于荧光纳米金刚石的微尺度离子浓度与电压成像：实现 16 mV Hz⁻¹/² 电压灵敏度与 1.8% mM⁻¹ 盐浓度响应