Exploring the Cobalt-Histidine Complex for Wide-Ranging Colorimetric O2 Detection

Abstract

Developing a robust, cost-effective, and user-friendly sensor for monitoring molecular oxygen (O₂) ranging from a minute to a medically relevant level (85-100%) in a stream of flowing breathable gas is vital in various industrial domains. Here, we report an innovative application of the cobalt(l-histidine)₂ complex, a bioinspired model of O₂-carrying metalloproteins, for rapid and reliable sensing of O₂ from 0 to 100% saturation levels under realistic conditions. We have established two distinct colorimetric O₂ detection techniques, which can be executed with the use of a common smartphone camera and readily available color-detecting software. A series of spectroscopic experiments were performed to demonstrate the molecular-level alteration in cobalt(l-histidine)₂ following its exposure to oxygen, leading to an exclusive pink-to-brown color change. Therefore, this study establishes a template for designing bioinspired molecular complexes for O₂ sensing, leading to practical and straightforward solutions. This metal-amino acid complex's broad-spectrum sensing of O₂ has widened the scope of bioinspired model complexes for divergent applications in industrial sectors.

Figure 1

(A) Relative changes in the color of a neutral aqueous solution containing a 20 mM 1:2 Co(II)/l-histidine mixture under N₂ (Deoxy-M) and O₂ (Oxy-M) atmosphere. The corresponding RGB-corrected spots are also demonstrated in the bottom panel for the solutions. Here, the RGB correction is performed by subtracting the RGB readings of the initial Deoxy-M sample. (B) Five consecutive cycles demonstrating the reversibility of the reaction. (C) Relative alteration of Deoxy-M and Oxy-M absorbances recorded at 410 nm with different O₂/N₂ cycling.

Scheme 1. Formation of Deoxy-M and its conversion to the Oxy-M following its interaction with the molecular oxygen

Figure 2

(A) Visible color pattern/colorimetric response (top) of the prepared oxygen sensor M under different concentrations of oxygen and RGB-corrected colors in circle (bottom). (B) Absorption spectra of M, in equilibrium with different oxygen concentrations in the surrounding atmosphere. (C) Real and RGB-corrected colors of (1) Deoxy-M, (2) Oxy-M (under 100% O₂), (3) Oxy-M (under 90% O₂), and (4) Oxy-M (under 85% O₂) observed during O₂ detection via the direct colorimetric method experiment.

Figure 3

Serial changes in the coloration observed during the indirect O₂ measurement via the Co(His)₂ complex, involving the generation of stoichiometric amount of H₂O₂ and subsequent iodometric titration. The differentiation between samples exposed to 85–100% O₂ is highlighted with an appropriate RGB correction.

Figure 4

Comparative (A) optical and (C) corresponding CD spectra of the Deoxy-M (red trace) and Oxy-M (blue trace) complexes recorded in a neutral aqueous solution (pH 7.0), highlighting the respective LMCT and d-d transition bands and their changes following the oxygen exposure. The overlayed FTIR spectra for the Deoxy-M (red trace) and Oxy-M (blue trace) complexes measured in the (B) 450–1000 cm^–1 and (D) 1000–400 cm^–1 regions, highlighting the appearance of bridging peroxo stretching band and cobalt-bound l-histidine features, respectively. The FTIR data were recorded following the preparation of KBr pellets at room temperature.

Figure 5

Rate of interconversion of the (A) Deoxy-M complex to the Oxy-M complex on addition of 100 μL of oxygen with varying metal concentrations. The rate of interconversion of the (B) Oxy-M complex to the Deoxy-M complex (with nitrogen purging) with varying metal concentrations with time.