A label-free fluorimetric detection of biothiols based on the oxidase-like activity of Ag+ ions

Enlightening the Path to Protein Engineering: Chemoselective Turn-On Probes for High-Throughput Screening of Enzymatic Activity

Many successful stories in enzyme engineering are based on the creation of randomized diversity in large mutant libraries, containing millions to billions of enzyme variants. Methods that enabled their evaluation with high throughput are dominated by spectroscopic techniques due to their high speed and sensitivity. A large proportion of studies relies on fluorogenic substrates that mimic the chemical properties of the target or coupled enzymatic assays with an optical read-out that assesses the desired catalytic efficiency indirectly. The most reliable hits, however, are achieved by screening for conversions of the starting material to the desired product. For this purpose, functional group assays offer a general approach to achieve a fast, optical read-out. They use the chemoselectivity, differences in electronic and steric properties of various functional groups, to reduce the number of false-positive results and the analytical noise stemming from enzymatic background activities. This review summarizes the developments and use of functional group probes for chemoselective derivatizations, with a clear focus on screening for enzymatic activity in protein engineering.

In situ reaction-based ratiometric fluorescent assay for alkaline phosphatase activity and bioimaging

Alkaline phosphatase (ALP) is an important biomarker, it is of great significance to develop a sensitive and efficient analytical method for ALP. In this study, an in situ reaction based ratiometric fluorescence assay for ALP was proposed. l-ascorbic acid-2-phosphate (AA2P) was used as a substrate for ALP, and Cu²⁺/o-phenylenediamine (OPD) were involved in this system. Cu²⁺ can oxidize OPD to 2,3-diaminophenazine (OPDox) with an emission centered at 566 nm. The presence of ALP can catalyze the hydrolysis of AA2P to ascorbic acid (AA), which will inhibit the production of OPDox and reduce the corresponding fluorescence intensity, and AA will react with OPD to generate 3-(dihydroxyethyl)furan[3,4-b]quinoxalin-1-one (DFQ) with an emission peak at 447 nm. The fluorescence ratio of F447/F566 has a linear relationship with ALP activity. The proposed method is highly sensitive, finely selective, cost efficiency and easy to operate, it exhibits good linearity in the range of 0.5-22 and 22-40 mU·mL^-1, with a detection limit as low as 0.06 mU·mL^-1. The excellent applicability of this strategy in human serum samples and MCF-7 cells imaging suggests that this method has promising prospects for biomedical research.

A highly sensitive and selective ratiometric sensing platform based on 7-amino-4-methylcoumarin for naked-eye visual fluorescence sensing of Cu2

Cu²⁺ is a major environmental pollutant. An efficient measurement for Cu²⁺ is urgently needed. In this report, we have developed a new sensitive and selective ratiometric sensing platform using 7-amino-4-methylcoumarin (AMC) for detecting Cu²⁺ in real samples. In the presence of Cu²⁺, o-phenylenediamine (OPD) could be catalytically oxidized to 2,3-diaminophenazine (DAP), which could react with AMC, leading to quench the fluorescence intensity of AMC at 438 nm. Meanwhile, DAP provided a new emission peak at 557 nm. Based on the efficient overlapped spectrum of AMC and DAP, a ratiometric sensing platform through fluorescence resonance energy transfer (FRET) was carried out. Furthermore, the as-proposed strategy displayed the linear relationship in the wide range from 6 to 250 μM with a low detection limit of 0.059 μM, and the recoveries of the spiked samples in real samples ranged from 86.5% to 110.1%. Moreover, comparing the visual fluorescence colors of the real samples with the standard colorimetric card, we used the as-proposed strategy as a solid-based platform for realizing an efficient semi-quantitative detection of Cu²⁺ via naked-eye visual fluorescence mode without any complicated instrument and operation. The above results implied that the as-proposed strategy could be used in the practice determination of Cu²⁺.

Ratiometric fluorescence and smartphone dual-mode detection of glutathione using carbon dots coupled with Ag+-triggered oxidation of o-phenylenediamine

Developing ratiometric fluorescence and smartphone dual-mode bioanalysis methods is important but challenging. A ratiometric fluorescence method for determining glutathione (GSH) using carbon dots (CDs) and Ag⁺-triggered o-phenylenediamine (OPD) oxidation is described here. Ag⁺oxidizes OPD to give 2,3-diaminophenazine (oxOPD), which effectively quenches CD fluorescence at 436 nm through the inner filter effect and causes a new emission peak at 561 nm. GSH chelates with Ag⁺and prevents the Ag⁺oxidizing OPD and therefore effectively preserves CD emission at 436 nm (blue) and allows only weak oxOPD fluorescence at 561 nm (orange) to occur. The oxOPD to CD fluorescence intensity ratio decreased linearly as the GSH concentration increased in the range 0-150 nM, and the detection limit was 15 nM. The ratiometric fluorescence probe lit with an ultraviolet lamp clearly changed color from orange to blue as the GSH concentration increased. An image was acquired using a smartphone camera and converted into digital values. The blue and red channel ratio was calculated and used to quantify GSH. The method therefore allows dual-mode detection of GSH.

Anderson polyoxometalates with intrinsic oxidase-mimic activity for "turn on" fluorescence sensing of dopamine

Anderson-type polyoxometalate containing Fe³⁺ and Mo⁶⁺, (NH₄)₃[H₆Fe(III)Mo₆O₂₄] (FeMo₆), was found to work as an oxidase-mimicking nanoenzyme for the first time, exhibiting the ability of catalytic oxidation of o-phenylenediamine (OPD), 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTs), and 3,3',5,5'-tetramethylbenzidine (TMB), which features easy synthesis, low cost, simple operation, and low consumption. Attributed to the nature of FeMo₆ and Fenton-like effect, a novel sensor based on two consecutive "turn on" fluorescence was developed for detecting dopamine (DA) by employing the FeMo₆-OPD system, and the linear range was from 1 to 100 μM with the detection limit 0.0227 μM (3σ/s). Moreover, to increase oxidase-mimic activity of FeMo₆, reduced graphene oxide (rGO) loading FeMo₆ composites (FeMo₆@rGO (n), n = 5%, 10%, 15%) was fabricated, and results show that oxidase-like activities of FeMo₆@rGO (n) are dependent on the mass ratio of FeMo₆/rGO, and FeMo₆@rGO (10%) exhibits the highest oxidase-mimic activity and the fastest respond time (4 min) among all reported oxidase mimic of DA to date. Graphical abstract Anderson-type Mo-POMs FeMo₆ was found to work as an oxidase-mimicking nanoenzyme for the first time and was used to detect DA for two consecutive "turn on" fluorescence sensor modes.

Fluorescent and Colorimetric Sensors Based on the Oxidation of o-Phenylenediamine

o-Phenylenediamine (OPD) can be readily oxidized by several types of oxidants to generate fluorescent 2,3-diaminophenazine (oxidized OPD, OPDox). The unique fluorescence response process during the oxidation of OPD provides an important model for the design of novel sensors. In recent years, a series of fluorescent and colorimetric sensors have been developed based on the oxidation of OPD. In this review, fluorescent and colorimetric sensors for the detection of metal ions and small organic molecules are discussed. These sensing processes exhibit distinguishable and prominent fluorescent and colorimetric responses. The sensing systems include autocatalytic reactions and using nanomaterials, carbon dots, or fluorophore labeled DNA as reference fluorophore for fluorescent and colorimetric detection.

Convenient and sensitive colorimetric determination of alendronate sodium with Ce4+-triggered oxidation of TMB

A facile colorimetric assay for the sensitive and selective detection of alendronate sodium has been developed based on Ce⁴⁺-triggered oxidation of TMB for the first time.

One-pot synthesis of dual carbon dots using only an N and S co-existed dopant for fluorescence detection of Ag. SPECTROCHIMICA ACTA. PART A, MOLECULAR AND BIOMOLECULAR SPECTROSCOPY 2019;208:162-171. [PMID: 30312843 DOI: 10.1016/j.saa.2018.10.004]

Luminescent carbon-based nanoparticles, named often as carbon dots (CDs), were synthesized from citric acid (CA) and guanidine thiocyanate (GITC) via an N and S co-doped hydrothermal procedure. In the present structure characterization, N and S elements could be sufficiently doped by means of the heteroatom or the functional groups bonded on the surface of CDs. The as-prepared CDs solution showed blue color fluorescence under ultraviolet excitation, yet the PL spectra exhibited a repetitive emission process from excitation-independent to excitation-dependent. In view of the triexponential feature of fluorescence lifetimes of CDs, one possibility was proposed to be co-existence of two types of CDs with different surface states. Additionally, the as-prepared CDs were used as a sensing probe for the detection of Ag⁺ taking into consideration of the possible interactions between Ag⁺ and various fluorophores attached to the CD surface. As expected, the changes of fluorescence intensities were linearly proportional to the different concentration ranges of Ag⁺, which suggests the complex nature of the quenching mechanism. And for the first time, the SCN group was found to accelerate the quenching of CDs towards Ag⁺, promising a new approach for efficient detection of Ag⁺ for the application in industrial pollutants.

A convenient fluorescent assay for quinolones based on their inhibition towards the oxidase-like activity of Cu2+

This work reports on a novel and convenient fluorescent assay for four quinolones including nalidixic acid, cinoxacin, ciprofloxacin and moxifloxacin, with Cu²⁺-triggered and quinolone-inhibited oxidation of o-phenylenediamine.

MoS2 nanosheets with peroxidase mimicking activity as viable dual-mode optical probes for determination and imaging of intracellular hydrogen peroxide

The authors describe a dual-mode (colorimetric-fluorometric) nanoprobe for H₂O₂ that was fabricated by covering molybdenum disulfide nanosheets (MoS₂ NS) with ortho-phenylenediamine (OPD). The probe (OPD-MoS₂ NS) was applied to the optical determination of H₂O₂, to the quantitation of cell numbers, and to the detection of intracellular concentrations of H₂O₂. Oxidation by H₂O₂ leads to a colored and fluorescent product (oxidized OPD) with absorption/excitation/fluorescence peaks at 450/450/557 nm. The nanoprobe can detect H₂O₂ in down to 500 nM concentrations, and HeLa cells at levels of 100 cells mL^-1. The detection limit for intracellular H₂O₂ is in the 5.5 to 12.6 μM concentration range when the method is applied to cells at levels of 10²-10⁶ cells mL^-1. Due to its good biocompatibility and easy cell uptake, the nanoprobe also permits sensitive fluorometric imaging of intracellular H₂O₂. It can also comparatively discriminate the change of intracellular oxidation state in living cancerous and normal cells. Graphical abstract Editor, we provided image with high resolution. Please find it in a folder name "MIAC-D-18-00081" in the FTP site. A dual-mode (colorimetric-fluorometric) detection nanoplatform based on OPD-modified MoS₂ nanosheets is used to quantitatively detect H₂O₂, cell numbers and intracellular H₂O₂. The MoS₂ nanoprobes also permit sensitive fluorescence imaging of intracellular H₂O₂, and can discriminate intracellular oxide states in living cancerous and normal cells.

Supramolecular interactions of oxidative stress biomarker glutathione with fluorone black

Oxidative stress biomarkers, including glutathione (GSH) and related compounds, are involved in a variety of interactions enabling redox potential maintenance in living cells and protection against radicals. Since the oxidative stress is promoting and, in many cases, inducing serious illnesses, monitoring of GSH levels can aid in diagnostics and disease prevention. Herein, we report on the discovery of the formation of a supramolecular ensemble of GSH with fluorone black (9-phenyl fluorone, FB) which is optically active and enables sensitive determination of GSH by resonance elastic light scattering (RELS). We have found that supramolecular interactions of GSH with FB can be probed with spectroscopic, RELS, and electrochemical methods. Our investigations show that RELS intensity for FB solutions increases with GSH concentration while fluorescence emission of FB is not affected, as quenching begins only above 0.8mM GSH. The UV-Vis difference spectra show a positive peak at 383nm and a negative peak at 458nm, indicating a higher-energy absorbing complex in comparison to the non-bonded FB host. Supramolecular interactions of FB with GSH have also been corroborated by electrochemical measurements involving two configurations of FB-GSH ensembles on electrodes: (i) an inverted orientation on Au-coated quartz crystal piezoelectrode (Au@SG-FB), with strong thiolate bonding to gold, and (ii) a non-inverted orientation on glassy carbon electrode (GCE@FB-GS), with weak π-π stacking attachment and efficient charge mediation through the ensemble. The formation of a supramolecular ensemble with hydrogen bonding has also been confirmed by quantum mechanical calculations. The discovery of supramolecular FB-GSH ensemble formation enables elucidating the mechanisms of strong RELS responses, changes in UV-Vis absorption spectra, and the electrochemical reactivity. Also, it provides new insights to the understanding of the efficient charge-transfer in redox potential homeostasis which is likely based on an intermediate formation of a similar type of supramolecular ensembles.

Simple and label-free fluorescence detection of ascorbic acid in rat brain microdialysates in the presence of catecholamines

A facile and ultrasensitive sensor was constructed successfully for AA sensing based on the synergistic effect of reducing capability of AA and IFE.