Enzyme-responsive Fluorescent Ionic Liquid

A turn-on chemiluminescent assay for alkaline phosphatase using two-dimensional Fe-centered metal-organic frameworks as the signaling probe

Alkaline phosphatase (ALP) is an essential enzyme involved in cell phosphorus metabolism. Developing sensitive and accurate ALP quantitative assays is significant. In this study, a turn-on chemiluminescence (CL) analysis platform for the detection of ALP activity in human serum was established based on two-dimensional (2D) Fe-centered metal-organic frameworks with 1,3,5-benzene tricarboxylic acid as ligands (denoted as 2D Fe-BTC). The 2D Fe-BTC as the signaling probe reacts with ascorbic acid forming reduced Fe-BTC which catalyzes the luminol CL reaction producing a strong CL signal. The 2D Fe-BTC-based luminol CL reaction exhibited good CL responses when the concentration of ascorbic acid was in the range of 5-500 nM. By employing magnesium ascorbyl phosphate (MAP) as the substrate which can be hydrolyzed by ALP to generate ascorbic acid, a turn-on CL assay for the detection of ALP was established. Under optimal conditions, as low as 0.00046 U L-1 of ALP could be sensitively detected with a linear range of 0.001-0.1 U L-1. ALP in human serum can be detected after a simple dilution process without any other pretreatment.

Nanoemulsion-based silver ion-selective optode based on colorimetrically silver ion-responsive ionic liquid-based dye

In this study, we describe the fast-responsive nanoemulsion (NE)-based silver ion (Ag⁺)-selective optode based on colorimetrically silver ion-responsive ionic liquid-based dye (ILD). The ILD comprises purely functional sensing molecules, a protonated cationic merocyanine dye (KD-M13-H⁺) and an anionic Ag⁺ ionophore (BDM-SO₃^-), and thus, it can be used for highly sensitive silver ion (Ag⁺) sensing due to the extremely high content of dye in the organic phase (ionic-liquid phase). However, during the Ag⁺ sensing, the cationic merocyanine dye is converted into electrically neutral form by deprotonation of the dye, which leads to the conversion of liquified dye into solid form in the organic phase, which makes the response time slower when ILD is used for poly(vinyl chloride) (PVC) membrane-based ion-selective optode, especially for sensing of high Ag⁺ concentration. To solve this problem, we focused on the use of the nano-emulsification technique. The response time of the ILD-based nanoemulsion (NE) was considerably shorter (1 s) compared to that of the ILD-based PVC membrane (a few minutes) owing to the large surface area and excellent diffusivity of the emulsion. The ILD-based NE contained a very high dye concentration (833 mmol kg^-1) and exhibited approximately 12 times higher sensitivity than that of the plasticizer-based conventional NE. In the cation measurements, the ILD-based NE responded to Ag⁺ via a cation-exchange mechanism and demonstrated a highly selective response to Ag⁺ (log [Formula: see text] = - 3.0). ILD-NE was successfully applied to the detection of spiked Ag⁺ in a tap water sample with recoveries of 98 - 103% with a relative standard deviation (RSD) of less than 5%. In comparison with NE based on non-ionic ionophores without charge, NE based on BDM-SO₃^- responded to lower Ag⁺ concentrations owing to the effect of negative charge on the binding property. The novel ILD-based NE was capable of highly sensitive, rapid, and selective Ag⁺ sensing, providing potential for analytical devices applicable to high-performance on-site analysis.

Chloride ion-selective dye liquid nanoemulsion: improved sensor performance due to intermolecular interactions between dye and ionophore

Ionophore-based dye liquid nanoemulsion sensors exhibiting rapid response, high selectivity, and high sensitivity to chloride were developed. Intermolecular interactions within emulsion contributed to the background suppression.

Enzyme-responsive fluorescent nanoemulsion based on lipophilic dye liquid

An enzyme-responsive fluorescent nanoemulsion (NE) based on lipophilic dye liquid (LDL) was developed for alkaline phosphatase (ALP). The response mechanism of the NE involved enzymatic reactions and simultaneous extraction of anions. The LDL-based NE exhibited 3.8 times higher sensitivity than plasticizer-based conventional NE. Detection limit and response range were 2.7 (U L-1) and 5-50 (U L-1), respectively. The response time was reduced to less than half that of the LDL-based membrane.

Stimuli-responsive Behaviors for Room-temperature Fluorescent Liquid Materials based on N-Heteroacenes and their Mixtures in Response to HCl Vapor and their Facile Synthesis

In this paper, we report on stimuli-responsive behaviors for room temperature fluorescent liquid materials based on N-heteroacene frameworks in response to HCl vapor. These liquid materials as well as their mixtures prepared by varying the combination can provide various emission colors and stimuli-responsive properties in liquid states. Also, we achieved an improvement in total synthetic yield (>40%) by redesigning the molecular structures of liquid materials as compared to previous liquid materials (<10%).

Lipophilic Fluorescent Dye Liquids: Förster Resonance Energy Transfer-Based Fluorescence Amplification for Ion Selective Optical Sensors Based on a Solvent Polymeric Membrane

Optical sensors based on solvent polymeric membranes have the potential to measure analytes present in an aqueous solution through the development of a tailored method for a specific target. However, limits in the concentrations of the component dyes have prevented improvements in sensitivity. We propose a Förster resonance energy transfer (FRET)-based fluorescence amplification system for ion-selective optical sensors using a highly fluorescent liquid material composed of a lipophilic phosphonium cation and a pyrene modifying sulfonate anion ([P₆₆₆₁₄][HP-SO₃]), as both the plasticizer and donor, in addition to a combination of the lipophilic phosphonium cation and the fluorescein dodecyl ester anion ([P₆₆₆₁₄][12-FL]) as the fluorescent sensing dye acceptor. For ion extraction-based sensing, the donor and acceptor were retained in the plasticized PVC membrane with negligible leaching upon exposure to acidic and basic aqueous solutions. Systematic investigation of the donor and acceptor ratios clarified the effect of the amplification factor and the sensitivity of the sensor. At an acceptor doping level of 0.5 mol % (vs donor), an approximately 22-fold higher sensitivity was obtained compared to that of a conventional PVC membrane optical sensor. During ion measurement based on the coextraction of protons and anions, selectivity following the Hofmeister order was observed, which was controlled by the addition of ionophores. The proposed FRET system based on a lipophilic fluorescent liquid material has the potential to significantly improve the sensitivities of optical sensors using solvent polymeric membranes with high selectivities for various target analytes.

Highly sensitive optical ion sensor with ionic liquid-based colorimetric membrane/photonic crystal hybrid structure

An ionic liquid-based thin (~ 1 µm) colorimetric membrane (CM) is a key nano-tool for optical ion sensing, and a two-dimensional photonic crystal slab (PCS) is an important nano-platform for ultimate light control. For highly sensitive optical ion sensing, this report proposes a hybrid of these two optical nano-elements, namely, a CM/PCS hybrid. This structure was successfully fabricated by a simple and rapid process using nanoimprinting and spin-coating, which enabled control of the CM thickness. Optical characterization of the hybrid structure was conducted by optical measurement and simulation of the reflection spectrum, indicating that the light confined in the holes of the PCS was drastically absorbed by the CM when the spectrum overlapped with the absorption spectrum of the CM. This optical property obtained by the hybridization of CM and PCS enabled drastic improvement in the absorption sensitivity in Ca ion sensing, by ca. 78 times compared to that without PCS. Experimental and simulated investigation of the relation between the CM thickness and absorption sensitivity enhancement suggested that the controlled light in the PCS enhanced the absorption cross-section of the dye molecules within the CM based on the enhanced local density of states. This highly sensitive optical ion sensor is expected to be applied for micro-scale bio-analysis like cell-dynamics based on reflectometric Ca ion detection.

A lipophilic ionic liquid-based dye for anion optodes: importance of dye lipophilicity and application to heparin measurement

Herein, a fully lipophilic ionic liquid (IL) comprising a lipophilic fluorescein anion and a trihexyltetradecylphosphonium cation was synthesized and used as the plasticizer for a plasticized poly(vinyl chloride) (PVC) membrane optode. Systematic investigation of the alkyl chain length of the fluorescein anion proved the significance of lipophilicity for obtaining the reversible absorbance measurements. A PVC membrane fabricated with the synthesized lipophilic IL was observed to comprise an unusually high dye concentration (915 mmol kg^-1) and exhibited good sensitivity as well as response time in its sensor performance. The sensitivity of the presented PVC membrane was 26-fold higher than that of a conventional optode membrane with the same membrane thickness and the same lipophilic dye of typical dye content (1 wt%). The response time was observed to be >120-fold faster by using a significantly thinner PVC membrane (approx. 140 nm). Heparin is known to be a polyanionic anticoagulant, and the presented PVC membrane exhibited an extremely fast response (20-150 seconds) to the heparin in diluted serum within the required concentration region. Thus, the lipophilic IL-based dye could significantly improve the sensor performance in conventional optodes, especially for an analyte showing slow diffusion, such as macromolecular heparin.