Bond-selective imaging by optically sensing the mid-infrared photothermal effect

Abstract

Mid-infrared (IR) spectroscopic imaging using inherent vibrational contrast has been broadly used as a powerful analytical tool for sample identification and characterization. However, the low spatial resolution and large water absorption associated with the long IR wavelengths hinder its applications to study subcellular features in living systems. Recently developed mid-infrared photothermal (MIP) microscopy overcomes these limitations by probing the IR absorption–induced photothermal effect using a visible light. MIP microscopy yields submicrometer spatial resolution with high spectral fidelity and reduced water background. In this review, we categorize different photothermal contrast mechanisms and discuss instrumentations for scanning and widefield MIP microscope configurations. We highlight a broad range of applications from life science to materials. We further provide future perspective and potential venues in MIP microscopy field.

Nanosecond-resolution photothermal dynamic imaging via MHZ digitization and match filtering

“…For this reason, PDI can largely benefit the mid-IR photothermal microscope with a powerful optical parametric oscillator (OPO) source 20 , 47 , which has a pulse duration of few nanoseconds and a fixed low repetition rate of tens kilohertz. Such a short-pulsed and high peak power excitation source is highly preferred for generating large modulation depth of small objects on a thermo-conductive substrate or in an aqueous environment, where heat dissipation is relatively rapid 7 . In such a case, the photothermal signal has a duty cycle of less than 1% and the lock-in amplifier can only capture a tiny portion of modulation or balanced detection for reducing the large laser noise is required at such low modulation frequency 47 .…”

“…This new imaging modality not only enriches the photothermal techniques with enormous molecular fingerprint information, but also overcomes the limitations in conventional mid-IR absorption microscopy and near-field IR approaches. MIP microscopy is emerging as a valuable tool for biological and material science 7 . It enables IR spectroscopic imaging at the submicron spatial resolution, empowering a broad spectrum of applications in both research and industry, including but not limited to non-contact material characterization 8 – 10 , biomolecular mapping 11 , 12 , protein dynamics and aggregation in neurons 13 , 14 , bacterial response to antibiotics 15 , single virus detection 16 and imaging of metabolism in living cells and other organisms 5 , 17 , 18 .…”

Bond-selective intensity diffraction tomography

“…This oscillation creates "Hot" and "Cold" states, respectively, where the chemical-specific RI variations are present or absent in the sample. The sufficiently fast and sensitive pulsed IDT imaging system with multiple off-resonant probe beams can capture this information within a microsecond time scale to recover the chemical-specific RI variations of the object quantitatively by a subtraction between "Hot" and "Cold" states 18,38 .…”

“…To realize chemical-specific label-free microscopy, vibrational spectroscopic imaging techniques have been developed to chemically image cellular morphology based on signals from intrinsic chemical bond vibrations 18,19 . Among numerous technical realizations, coherent Raman scattering microscopy has been developed for high-speed vibrational imaging and applied to address various biomedical problems [20][21][22] .…”

“…Furthermore, IR imaging can be implemented without a tight beam focus, featuring higher chemical sensitivity and reduced photodamage risk. The emerging mid-infrared photothermal (MIP) microscopy 18 inherits IR absorption spectroscopy's advantages but circumvents conventional IR micro-spectroscopy's low-resolution and slow speed limit. MIP microscopy provides diffraction-limited resolution at the visible band using a visible probe beam and is compatible with both point-scanning [25][26][27][28] and wide-field [29][30][31][32][33][34] configuration.…”