Introducing cymantrene labels into scattering scanning near-field infrared microscopy

Abstract

In this paper we investigate metal–organic compounds as infrared (IR) active labels by scattering scanning near-field infrared microscopy (IR s-SNOM, often also abbreviated as s-SNIM) with a lateral resolution of 90 × 90 nm². Tailor-made IR spectroscopic probes based on cymantrene (CpMn(CO)₃ with Cp = η⁵-C₅H₅) conjugated to a cysteine-modified pseudoneurotensin (pNT-Cys-OH) peptide were prepared by automated microwave-assisted solid phase peptide synthesis (SPPS) and characterized by HPLC, ESI-MS and IR. Well-defined patterned self-assembled monolayers on a gold surface were prepared by microcontact printing of 1-octadecanethiol (ODT) followed by additional incubation in ethanolic solution of the cymantrene–peptide derivative. The self-assembled monolayers have been evidenced by infrared reflection absorption spectroscopy (IRRAS) and AFM. CO laser source radiation was tuned (1944, 1900, 1798 and 1658 cm⁻¹) for imaging contrast with good matching correlation between spectroscopic and topographic patterns at specific characteristic metal carbonyl and amide bands (1944 cm⁻¹ (λ = 5.14 μm) and 1658 cm⁻¹ (λ = 6.03 μm)). Cymantrene probes provide an attractive method to tag a unique spectroscopic feature on any bio(macro)molecule. Introducing such probes into super-resolution IR s-SNOM will enable molecular tracking and distribution studies even in complex biological systems.