Due to their unusual physicochemical properties (e.g., high thermal stability, low volatility, high intrinsic conductivity, wide electrochemical windows and good solvating ability), ionic liquids have shown immense application potential in many research areas. Applications of ionic liquid in developing various sensors, especially for the sensing of biomolecules, such as nucleic acids, proteins and enzymes, gas sensing and sensing of various important ions, among other chemosensing platforms, are currently being explored by researchers worldwide. The use of ionic liquids for the detection of carbon dioxide (CO2) gas is currently a major topic of research due to the associated importance of this gas with daily human life. This review focuses on the application of ionic liquids in optical and electrochemical CO2 sensors. The design, mechanism, sensitivity and detection limit of each type of sensor are highlighted in this review.

Salt-added ionic liquid media have emerged as a versatile alternative to the conventional electrolytes in several applications. A lithium bis(trifluoromethylsulfonyl)imide (LiTfN)-added ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][TfN]) system up to a LiTfN mole fraction ( x) of 0.40 is investigated using a fluorophore-quencher pair of pyrene-nitromethane in the 298.15-358.15 K temperature range. Excited-state intensity decay of pyrene fits best to a single-exponential decay function irrespective of the concentration of nitromethane, x, and the temperature. Pyrene lifetimes decrease with increasing temperature at a given x with lifetime becoming more sensitive to temperature at higher LiTfN concentration. The pyrene-nitromethane fluorophore-quencher pair follows a simplistic Stern-Volmer formulation, indicating the quenching to be purely dynamic in nature affording dynamic quenching constants ( K) in the process. K along with the estimated bimolecular quenching rate constant ( k) within LiTfN-added [emim][TfN] first increases with increasing LiTfN until x ∼ 0.10, decreasing monotonically thereafter until x = 0.40. The decrease in K and k with increasing x is attributed to the exponentially increased dynamic viscosity with increasing x of the ([emim][TfN] + LiTfN) system. The initial increase in K and k is controlled by the structural changes within the system as LiTfN is added to [emim][TfN]. It is proposed that the presence of [Li(TfN)] anionic clusters stabilizes the partial positive charge that develops on excited pyrene during the electron/charge transfer to nitromethane during the quenching process. While the Stokes-Einstein formulation is not followed by the ([emim][TfN] + LiTfN) system in general, it is found to be obeyed at fixed x. The role of structural changes within the system beyond viscosity increase on the quenching process is amply highlighted.

In mixture of a deep eutectic solvent Reline with tetraethylene glycol, inter-species interactions are stronger than the intra-species interactions.

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Intrinsic fluorescence from l-tryptophan (l-Trp) is routinely used to obtain insight into the structural features and dynamics of proteins and enzymes. In contrast to aqueous enzymology, different parameters that control and influence the behavior of proteins and enzymes in nonaqueous media depend heavily on the solvent. Detailed analysis of the intrinsic fluorescence from l-Trp dissolved in two deep eutectic solvents (DESs), reline and glyceline, prepared by mixing salt choline chloride with H-bond donors urea and glycerol, respectively, in a 1:2 molar ratio within 298.15–358.15 K temperature range, is presented. Fluorescence emission maxima of l-Trp dissolved in DESs show bathochromic shift with increasing temperature. In comparison to water and several organic solvents, the fluorescence quantum yields of l-Trp in both DESs are significantly higher. While the rates of nonradiative decay in the two DESs are comparable and increase with increasing temperature, radiative decay rates are independent of temperature and are higher in glyceline than in reline, resulting in a higher fluorescence quantum yield of l-Trp in glyceline. Excited-state emission intensity decays of l-Trp fit best to a double exponential model irrespective of the identity of the DES and temperature. Average lifetime decreases with increasing temperature due to increased thermal deactivation; however, this decrease is much slower in DESs as compared to that in water. Both steady-state fluorescence anisotropy and rotational reorientation times for l-Trp are governed by the inherent complexity of the DESs as solubilizing milieu resulting in noncompliance to simple hydrodynamic treatment. Fluorescence quenching of l-Trp by acrylamide in reline is purely dynamic in nature. This is in contrast to the aqueous media where the decrease in fluorescence is a combined result of both dynamic and static quenching. The quenching within reline is fairly efficient considering the high viscosity of the medium. Significantly lower activation energy of the bimolecular quenching process as compared to the activation energy of the viscous flow indicates facilitation of the electron/charge transfer quenching of l-Trp by acrylamide within the ionic environment offered by reline. The effect of high viscosity is partly overcome by the strongly ionic environment of reline during the electron/charge transfer between l-Trp and acrylamide. The results highlight the structural complexity of these DESs especially within the cybotactic region of the probe, which is absent in common molecular solvents of similar high viscosity.