Liquid-environment NanoIR | Tian Research Group

Introduction

Infrared nanospectroscopy (nanoIR) has attracted broad attention since 2013. Early nanoIR systems, however, struggled to analyze samples in aqueous environments because water strongly absorbs infrared light and damps the oscillation of AFM probes. Characterizing samples in liquids, particularly at electrochemical interfaces, has therefore remained a long-standing challenge.

With support from a national scientific-instrumentation program, our team developed an advanced liquid-environment nanoIR platform over the past five years. We studied its underlying physical principles, measurement and control technologies, environmental control, and sample-preparation methods. These efforts led to three generations of nanoIR systems that progressively addressed key technical challenges.

The first-generation system established the technical foundation, including software, hardware, data acquisition, and online algorithms. The second generation introduced a bottom-excitation module and enabled nanoIR experiments in aqueous environments. The current third-generation system uses a customized electrochemical cell to detect in situ nanoIR signals from electrochemical reactions at solid/liquid interfaces for the first time. It also enables nanoscale multimodal imaging that correlates morphology, mechanics, electrical information, and chemical signatures.

Three generations of self-developed nanoIR instruments — Figure 1. Schematic diagram of three generations of self-developed nanoIR instruments and their corresponding technological breakthroughs.

The third-generation nanoIR system is built around a customized spectroelectrochemical cell. The cell consists of a BaF₂ substrate coated with a 10 nm gold film as the working electrode and is illuminated directly from the bottom through a reflective objective. The cell is mounted on a three-axis nanopositioning stage and integrated into a multi-axis cage system for stability and compatibility.

To support the system, we developed integrated measurement and control software covering mid-IR laser control, AFM operation, electrochemical functions, and data processing. The software includes algorithms for optimizing IR pulse excitation and transmitting nanoIR signals to the AFM controller for simultaneous display with correlated morphology. The nanopositioning stage enables sample scanning and can be combined with intelligent algorithms for high-speed nanoscale infrared hyperspectral imaging.

Third-generation nanoIR instrument design — Figure 2. Design of the third-generation instrument and spectroelectrochemical cell.

Performance

Polymethyl methacrylate (PMMA), polycarbonate (PC), polystyrene (PS), and PMMA-b-PS block-copolymer films are suitable benchmark samples for nanoIR characterization. AFM force curves measured on PMMA films show clear differences between air and aqueous environments: the free oscillation amplitude of the probe persists in air but decreases rapidly in solution, and the jump-to-contact point is much more pronounced in air.

The nanoIR spectra maintain a good signal-to-noise ratio in both air and solution, although the baseline is higher in solution. By imaging the C=O bond distribution at a fixed excitation wavenumber of 1730 cm^-1, we demonstrated stable high-performance nanoIR microscopy in aqueous environments. Correlated nanoscale imaging of morphology and chemical information revealed a heterogeneous C=O distribution and verified the effectiveness of the system.

NanoIR measurement of polymer films in water — Figure 3. NanoIR measurements of a standard polymer-film surface in an aqueous environment.

To demonstrate sensitivity, we prepared ultrathin samples on an ultrathin gold-film substrate using the Langmuir-Blodgett (LB) method. AFM topography and nanoIR images at 1630 cm^-1 showed strong correlation. Comparing the topography profile with the nanoIR intensity indicated a spatial resolution of 3.16 nm perpendicular to the edge of the LB film, exceeding the previously reported 6 nm resolution. We also imaged DNA double-helix chains at 1280 and 1700 cm^-1, observing helix-like structures at DNA-strand edges and demonstrating high sensitivity on a 1.2 nm thick sample.

High-resolution nanoIR imaging of ultrathin films — Figure 4. Ultrasensitive, high-spatial-resolution nanoIR imaging of ultrathin sample films.

In Situ NanoIR Spectroscopic Observation of Electrochemical Processes

We demonstrated in situ electrochemical measurements with the third-generation nanoIR system by monitoring aniline electropolymerization. The infrared beam passed through a BaF₂ substrate to reach the probe-sample nanogap. The reaction was carried out in an electrochemical cell containing 0.1 M H₂SO₄ and 30 mM aniline.

During polymerization, the reaction area at the electrode interface changed color from light yellow, reflected by the 10 nm gold film, to dark green polyaniline. Cyclic voltammetry showed shifts and increases in the first oxidation and reduction peaks of aniline over multiple cycles, indicating polymerization and electrochemical proton doping. Infrared spectra of the film after reaction showed three main peaks associated with benzene-ring and quinone structures. When the IR wavenumber was fixed at 1500 cm^-1, the vibrational signal of the benzene-ring skeleton became stronger during the reaction and showed a heterogeneous distribution correlated with morphology.

Figure 5. In situ nanoIR observation of aniline electropolymerization.

引言

红外纳米光谱（nanoIR）自 2013 年以来受到广泛关注。然而，早期 nanoIR 系统难以分析水环境中的样品，因为水会强烈吸收红外光，并阻尼 AFM 探针的振荡。因此，在液体中，特别是在电化学界面处表征样品，长期以来一直是一个重要挑战。

在国家重大科研仪器项目支持下，我们团队在过去五年中发展了先进的液体环境 nanoIR 平台。我们研究了其基础物理原理、测量与控制技术、环境控制以及样品制备方法。这些工作形成了三代 nanoIR 系统，逐步解决关键技术难题。

第一代系统建立了技术基础，包括软件、硬件、数据采集和在线算法。第二代系统引入底部激发模块，使水环境中的 nanoIR 实验成为可能。目前的第三代系统采用定制电化学池，首次从固/液界面电化学反应中检测到原位 nanoIR 信号。该系统还实现了纳米尺度多模态成像，可关联形貌、力学、电学信息和化学特征。

第三代 nanoIR 系统以定制谱学电化学池为核心。该电化学池由镀有 10 nm 金膜作为工作电极的 BaF₂ 衬底构成，并通过反射式物镜从底部直接照明。电化学池安装在三轴纳米定位平台上，并集成于多轴笼式系统中，以获得稳定性和兼容性。

为支持该系统，我们开发了集成测量与控制软件，涵盖中红外激光控制、AFM 操作、电化学功能和数据处理。该软件包括用于优化红外脉冲激发的算法，并可将 nanoIR 信号传输至 AFM 控制器，以与关联形貌同步显示。纳米定位平台可实现样品扫描，并可结合智能算法进行高速纳米尺度红外高光谱成像。

性能

聚甲基丙烯酸甲酯（PMMA）、聚碳酸酯（PC）、聚苯乙烯（PS）以及 PMMA-b-PS 嵌段共聚物薄膜是适合 nanoIR 表征的标准样品。在 PMMA 薄膜上测得的 AFM 力曲线显示，空气和水环境之间存在明显差异：探针的自由振荡振幅在空气中可以保持，而在溶液中会迅速降低，且跳入接触点在空气中更为明显。

nanoIR 光谱在空气和溶液中均保持良好的信噪比，尽管溶液中的基线更高。通过在固定激发波数 1730 cm^-1 下成像 C=O 键分布，我们证明了水环境中稳定的高性能 nanoIR 显微成像。形貌与化学信息的关联纳米尺度成像揭示了 C=O 分布的异质性，并验证了系统的有效性。

水环境中聚合物薄膜的 nanoIR 测量 — 图 3. 水环境中标准聚合物薄膜表面的 nanoIR 测量。

为展示灵敏度，我们采用 Langmuir-Blodgett（LB）方法在超薄金膜衬底上制备了超薄样品。AFM 形貌和 1630 cm^-1 处的 nanoIR 图像显示出强相关性。比较形貌轮廓与 nanoIR 强度表明，在垂直于 LB 膜边缘的方向上空间分辨率为 3.16 nm，优于此前报道的 6 nm 分辨率。我们还在 1280 和 1700 cm^-1 下成像 DNA 双螺旋链，在 DNA 链边缘观察到类似螺旋的结构，并证明了对 1.2 nm 厚样品的高灵敏度。

超薄膜的高分辨 nanoIR 成像 — 图 4. 超薄样品薄膜的超灵敏、高空间分辨 nanoIR 成像。

电化学过程的原位 nanoIR 光谱观察

我们通过监测苯胺电聚合，展示了第三代 nanoIR 系统的原位电化学测量能力。红外光束穿过 BaF₂ 衬底到达探针-样品纳米间隙。反应在含有 0.1 M H₂SO₄ 和 30 mM 苯胺的电化学池中进行。

在聚合过程中，电极界面反应区域的颜色从 10 nm 金膜反射的浅黄色变为聚苯胺的深绿色。循环伏安结果显示，苯胺的第一氧化峰和还原峰在多次循环中发生位移并增强，表明聚合和电化学质子掺杂的发生。反应后薄膜的红外光谱显示三个主要峰，分别与苯环和醌式结构相关。当红外波数固定在 1500 cm^-1 时，苯环骨架振动信号在反应过程中逐渐增强，并表现出与形貌相关的异质分布。