Red emitting fluorogenic dye as an efficient turn-on probe for milk allergen

Supramolecular recognition, fluorescence, and sensing: Two trilobal dyes and casein in milk

We describe here two trilobal dyes called aniline blue (AB) and light green SF yellowish (LGSF) for their supramolecular recognition with casein, the principal protein in milk. The trilobal dyes bind con-covalently with casein as demonstrated by various approaches including docking and isothermal titration calorimetry. The supramolecular interaction triggers fluorescence enhancement. Specifically, AB and LGSF bind to casein micelles and emit blue (AB) and red (LGSF) fluorescence. The fluorescence intensities of AB-casein and LGSF-casein are linearly correlated in the range of 0-300 μg mL-1 (AB) and 0-400 μg mL-1 (LGSF) with the limit of detection 6.23 μg mL-1 (AB) and 6.83 μg mL-1 (LGSF). Actual milk samples were examined using the proposed probes, and the spiked recoveries ranged from 95.0 % to 107.1 % (AB) and from 93.7 % to 105.1 % (LGSF). Finally, AB and LGSF exhibit good selectivity and anti-interference capability, and are potentially excellent new fluorescent casein probes for milk or other relevant products.

Fluorometric and colorimetric dual sensor for the quantification of cancer biomarker in complex biological fluid via dissociation of host assisted dye aggregate assembly

Present study explores the utilization of a host assisted aggregate assembly of a DNA intercalater cationic dye, ethidium bromide (EtBr) with polyanionic macrocyclic host, sulfated cyclodextrin (SCD) as colorimetric and fluorometric dual mode sensor for a cancer biomarker, spermine (Sp). The cationic dye, EtBr at the vicinity of the multi-negative SCD portal experiences strong electrostatic attraction, favoring their aggregation in an end to end fashion. Consequently the solution changes its color from orange to brick red with substantial reduction in the fluorescence intensity. Interestingly, in the presence of multi-cationic Sp, the dye aggregates are dissociated which restores the original color and fluorescence intensity of the dye solution. Such changes in the optical features been calibrated with the added Sp concentration to detect and quantify Sp with an astonishingly low detection limit of 0.28 μM in water, 0.59 μM in a 1 % human serum matrix and 5.9 µM in 10 % urine matrix. Furthermore, the ability of the system to undergo visible colour change enhances its reliability as a dual sensor, functioning as a fluorometric as well as a colorimetric tool for spermine detection with enhanced sensitivity, selectivity and rapid response.

Multiple recognition-powered abiotic bimetallic oxide nanozyme-based colorimetric platform for the selective detection of allergen β-lactoglobulin

β-lactoglobulin (β-lg) is the major allergen in dairy products, and poses a significant threat to special people including infants and young children. Therefore, a convenient, cost-efficient, and aptamer-free colorimetric sensing platform is developed for β-lg assay based on a germanium/zinc bimetallic oxide nanozyme (GeO2/ZnO). The CTAB-assisted intercalation growth of ZnO nanoparticles between GeO2 layers endows GeO2/ZnO with enhanced peroxidase-mimicking activity and β-lg affinity. By virtue of multiple recognition-meditated specific protein corona formation, the catalytic activity of GeO2/ZnO is greatly deteriorated in the presence of β-lg. The selective discrimination and accurate detection of β-lg in dairy samples is achieved with a detection limit of 14.1 nM, and a portable kit is further established for point-of-care testing and visual colorimetric analysis. This study not only provides a promising strategy for allergen assay, but paves an avenue for engineering the abiotic protein affinity sensors for food safety analysis.

Characterization of the effects of bridging linker on the β-Lactoglobulin binding mechanism on the nanoscale metal-organic frameworks

Revealing the interaction modes between nanoscale metal-organic frameworks (NMOFs) and food matrix is crucial for functional release but it still remains largely unknown to date. This study specifically focused on the milk protein adsorption mechanism of NMOFs using UiO66/UiO66-NH2 and β-lactoglobulin (β-LG) as models. UiO66 and UiO66-NH2 quenched the fluorescence of β-LG via static mechanism. Due to the enhanced electrostatic forces caused by NH2, UiO66-NH2-β-LG (2.83 × 105 mol·L-1) exhibited higher binding constant than UiO66-β-LG (2.61 × 105 mol·L-1), while UiO66 with higher hydrophobicity adsorbed more β-LG. The defects of UiO influenced the binding sites on the β-LG, and the higher the defect degree, the higher the binding energy. For the stability of the system, the H-bonding between UiO66 and SER30/PRO38, and the hydrophobic interaction between UiO66-NH2 and LYS101 played important roles. Furthermore, the secondary structure content of β-LG changed after interacting with both UiO, resulting in reduced density of β-LG.

Self-assembly of S,N-codoped Ce/Cu bimetallic nanoparticles for fluorescence and visual detection of hexavalent chromium

Ce2(SO4)3 was doped into 4,6-diamino-2-mercaptopyrimidine (DAMP)-encapsulated copper nanoclusters (CuNCs) via a facile, rapid, low-temperature, and green self-assembly synthesis method to obtain fluorescent S,N-codoped Cu/Ce-DAMP nanoparticles (S,N-codoped Cu/CeNPs) for the detection of Cr(VI). The prepared Cu/CeNPs exhibit double emission peaks at 470 nm and 610 nm. The fluorescence emission at 610 nm is significantly enhanced due to the aggregation-induced emission (AIE) effect, and the quantum yield is as high as 20.19%. The fluorescence emission at 610 nm can be selectively quenched by Cr(VI) due to the internal filter effect (IFE) and dynamic quenching, whereas the weak fluorescence at 470 nm remains almost stable. On this basis, a fluorescence assay method for Cr(VI) was established, with good linearity in the concentration range 0.5-120 µM and a detection limit (LOD) of 134 nM. Using a smartphone to take photos of the fluorescence signals changes caused by Cr(VI) rapid visual detection is achieved with a linear range of 10-130 μM and a LOD of 2.35 μM. The proposed method was successfully applied to the detection of Cr(VI) in actual water samples.

Surfactant-based supramolecular dye assembly: A highly selective and economically viable platform for quantification of heparin antidote

Herein, we have employed a supramolecular assembly of a cationic dye, LDS-698 and a common surfactant sodium dodecyl sulfate (SDS) as a turn-on fluorescent sensor for protamine (Pr) detection. Addition of cationic Pr to the solution of dye-surfactant complex brings negatively charged SDS molecules together through strong electrostatic interaction, assisting aggregation of SDS way before its critical micellar concentration (CMC). These aggregates encapsulate the dye molecules within their hydrophobic region, arresting non-radiative decay channels of the excited dye. Thus, the LDS-698•SDS assembly displays substantial enhancement in fluorescence intensity that follows a nice linear trend with Pr concentration, providing limit of detection (LOD) for Pr as low as 3.84(±0.11) nM in buffer, 124.4(±6.7) nM in 1% human serum and 28.3(±0.5) nM in 100% human urine. Furthermore, high selectivity, low background signal, large stokes shift, and emission in the biologically favorable deep-red region make the studied assembly a promising platform for Pr sensing. As of our knowledge it is the first ever Pr sensory platform, using a very common surfactant (SDS), which is economically affordable and very easily available in the market. This innovative approach can replace the expensive, exotic and specialized chemicals considered for the purpose and thus showcase its potential in practical applications.

Quinaldine Red as a fluorescent probe for determining the melting temperature (Tm) of proteins: a simple, rapid and high-throughput assay

Proteins play an important role in biological systems and several proteins are used in diagnosis, therapy, food industry etc. Thus, knowledge about the physical properties of the proteins is of utmost importance, which will aid in understanding their function and subsequent applications. The melting temperature (Tm) of a protein is one of the essential parameters which gives information about the stability of a protein under different conditions. In the present study, we have demonstrated a method for determining the Tm of proteins using the supramolecular interaction between Quinaldine Red (QR) and proteins. Using this method, we have determined the Tm of 5 proteins and compared our results with established protocols. Our results showed good agreement with the other methods and published values. The method developed in this study is inexpensive, quick, and devoid of complex instruments and pre/post-treatment of the samples. In addition, this method can be adopted for high throughput in multi-plate mode. Thus, this study projects a new methodology for Tm determination of various proteins with user friendly operation.

In house synthesized novel distyryl-BODIPY dye and polymer assembly as deep-red emitting probe for protamine detection

In this contribution, we designed and synthesized a deep-red emitting distyryl-BODIPY dye (dye 3) which is non-fluorescent in aqueous solution due to the formation of non-emissive aggregates. However, in presence of an amphiphilic polymer (polystyrene sulfonate, PSS), the aggregated dye molecules de-aggregate and form dye 3-PSS complex, which significantly modulates the optical features of the bound dye. Interestingly, the dye 3-PSS complex shows turn-on fluorescence response in deep-red region in presence of protamine (Pr) due to the formation of dye 3-PSS-Pr ternary complex. Such enhancement follows a linear trend in the dynamic range of 0-8.75 μM of Pr which has been utilized to determine Pr with limit of detection (LOD) of 15.04(±0.5) nM in phosphate buffer. Furthermore, excellent selectivity of the dye 3-PSS system towards Pr allows us to determine Pr even in complex biological matrix like 1% human serum. Thus, dye 3-PSS system can be applied as a very effective tool for the detection and quantification of Pr in deep-red region, overcoming several limitations encountered with the probes in the shorter wavelength region. This is the first report on BODIPY dye based supramolecular assembly for sensing and quantification of protamine.