Dual-Color Peak Force Infrared Microscopy

Toward the Next Frontiers of Vibrational Bioimaging

Chemical imaging based on vibrational contrasts can extract molecular information entangled in complex biological systems. To this end, nonlinear Raman scattering microscopy, mid-infrared photothermal (MIP) microscopy, and atomic force microscopy (AFM)-based force-detected photothermal microscopies are emerging with better chemical sensitivity, molecular specificity, and spatial resolution than conventional vibrational methods. Their utilization in bioimaging applications has provided biological knowledge in unprecedented detail. This Perspective outlines key methodological developments, bioimaging applications, and recent technical innovations of the three techniques. Representative biological demonstrations are also highlighted to exemplify the unique advantages of obtaining vibrational contrasts. With years of effort, these three methods compose an expanding vibrational bioimaging toolbox to tackle specific bioimaging needs, benefiting many biological investigations with rich information in both label-free and labeling manners. Each technique will be discussed and compared in the outlook, leading to possible future directions to accommodate growing needs in vibrational bioimaging.

Lock-in amplifier based peak force infrared microscopy

Nanoscale infrared (nano-IR) microscopy enables label-free chemical imaging with a spatial resolution below Abbe's diffraction limit through the integration of atomic force microscopy and infrared radiation. Peak force infrared (PFIR) microscopy is one of the emerging nano-IR methods that provides non-destructive multimodal chemical and mechanical characterization capabilities using a straightforward photothermal signal generation mechanism. PFIR microscopy has been demonstrated to work for a wide range of heterogeneous samples, and it even allows operation in the fluid phase. However, the current PFIR microscope requires customized hardware configuration and software programming for real-time signal acquisition and processing, which creates a high barrier to PFIR implementation. In this communication, we describe a type of lock-in amplifier-based PFIR microscopy that can be assembled with generic, commercially available equipment without special hardware or software programming. We demonstrate this method on soft matters of structured polymer blends and blocks, as well as biological cells of E. coli. The lock-in amplifier-based PFIR reduces the entry barrier for PFIR microscopy and makes it a competitive nano-IR method for new users.

Principle and applications of peak force infrared microscopy

Peak force infrared (PFIR) microscopy is an emerging atomic force microscopy (AFM)-based infrared microscopy that bypasses Abbe's diffraction limit on spatial resolution. The PFIR microscopy utilizes a nanoscopically sharp AFM tip to mechanically detect the tip-enhanced infrared photothermal response of the sample in the time domain. The time-gated mechanical signals of cantilever deflections transduce the infrared absorption of the sample, delivering infrared imaging and spectroscopy capability at sub 10 nm spatial resolution. Both the infrared absorption response and mechanical properties of the sample are obtained in parallel while preserving the surface integrity of the sample. This review describes the constructions of the PFIR microscope and several variations, including multiple-pulse excitation, total internal reflection geometry, dual-color configuration, liquid-phase operations, and integrations with simultaneous surface potential measurement. Representative applications of PFIR microscopy are also included in this review. In the outlook section, we lay out several future directions of innovations in PFIR microscopy and applications in chemical and material research.